High-nitrogen ferritic heat-resisting steel with high niobium content and method of production thereof
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a ferritic heat-resisting steel, more particularly to a high-nitrogen ferritic heat-resisting steel containing chromium, and being appropriate for use in a high-temperature, high-pressure environment, and to a method of producing the same. 
     2. Description of the Prior Art 
     Recent years have seen a marked increase in the temperatures and pressures under which thermal power plant boilers are required to operate. Some plans already call for operation at 566° C. and 314 bar and it is expected that operation under conditions of 650° C. and 355 bar will be implemented in the future. These are extremely severe conditions from the viewpoint of the boiler materials used. 
     At operating temperatures exceeding 550° C., it has, from the viewpoints of oxidation resistance and high-temperature strength, been necessary to switch from ferritic 2·1/4 Cr-1 Mo steel to high-grade austenitic steels such as 18-8 stainless steel. In other words, it has been necessary to adopt expensive materials with properties exceeding what is required. 
     Decades have been spent in search of steels for filling in the gap between 2·1/4 Cr-1 Mo steel and austenitic stainless steel. Medium Cr (e.g. 9 Cr and 12 Cr) steel boiler pipes are made of heat-resisting steels that were developed against this backdrop. They achieve high-temperature strength and creep rupture strength on a par with austenitic steels by use of a base metal composition which includes various alloying elements for precipitation hardening and solution hardening. 
     The creep rupture strength of a heat-resisting steel is governed by solution hardening in the case of short-term aging and by precipitation hardening in the case of prolonged aging. This is because the solution-hardening elements initially present in solid solution in the steel for the most part precipitate as stable carbides such as M 23  C 6  during aging, and then when the aging is prolonged these precipitates coagulate and enlarge, with a resulting decrease in creep rupture strength. 
     Thus, with the aim of maintaining the creep rupture strength of heat-resisting steels at a high level, a considerable amount of research has been done for discovering ways for avoiding the precipitation of the solution hardening elements and maintaining them in solid solution for as long as possible. 
     For example, Japanese Patent Public Disclosures No. Sho 63-89644, Sho 61-231139 and Sho 62-297435 teach ferritic steels that achieve dramatically higher creep rupture strength than conventional Mo-containing ferritic heat-resisting steels by the use of W as a solution hardening element. 
     While the solution hardening by W in these steels may be more effective than by Mo, the precipitates are still fundamentally carbides of the M 23  C 6  type, so that it is not possible to avoid reduction of the creep rupture strength with prolonged aging. 
     Moreover, the use of ferritic heat-resisting steels at up to 650° C. has been considered difficult because of their inferior high-temperature oxidation resistance as compared with austenitic heat-resisting steels. A particular problem with these steels is the pronounced degradation of high-temperature oxidation resistance that results from the precipitation of Cr in the form of coarse M 23  C 6  type precipitates at the grain boundaries. 
     The highest temperature limit for use of ferritic heat-resisting steel has therefore been considered to be 600° C. 
     The need for heat-resisting steels capable of standing up under extremely severe conditions has grown more acute not only because of the increasingly severe operating conditions mentioned earlier but also because of plans to reduce operating costs by extending the period of continuous power plant operation from the current 100 thousand hours up to around 150 thousand hours. 
     Although ferritic heat-resisting steels are somewhat inferior to austenitic steels in high-temperature strength and anticorrosion property, they have a cost advantage. Furthermore, for reasons related to the difference in thermal expansion coefficient, among the various steam oxidation resistance properties they are particularly superior in scale defoliation resistance. For these reasons, they are attracting attention as a boiler material. 
     For the reasons set out above, however, it is clearly not possible with the currently available technology to develop ferritic heat-resisting steels that are capable of standing up for 150 thousand hours under operating conditions of 650° C. and 355 bar, that are low in price and that exhibit good steam oxidation resistance. 
     Based on the foregoing knowledge and as described in Japanese Patent Application No. Hei 2-37895, the inventors earlier disclosed that a high-nitrogen ferritic heat-resisting steel estimated by linear extrapolation to exhibit a creep rupture strength of not less than 147 MPa under operating conditions of 650° C. and 355 bar for 150 thousand hours can be obtained by using a pressurized atmosphere to add nitrogen exceeding the solution limit and thus inducing precipitation of the excess nitrogen in the form of fine nitrides and carbo-nitrides. The gist of their disclosure was a ferritic heat-resisting steel characterized in comprising, in weight per cent, 0.01-0.30% C, 0.02-0.80% Si, 0.20-1.00% Mn, 8.00-13.00% Cr, 0.50-3.00% W, 0.005-1.00% Mo, 0.05-0.50% V, 0.02-0.12% Nb and 0.10-0.50%N and being controlled to include not more than 0.050% P, not more than 0.010% S and not more than 0.020% O, and optionally comprising (A) one or both of 0.01-1.00% Ta and 0.01-1.00% Hf and/or (B) one or both of 0.0005-0.10% Zr and 0.01-0.10% Ti, the balance being Fe and unavoidable impurities, and a method of producing the steel wherein the steel components are melted and equilibrated in an atmosphere of a mixed gas of a prescribed nitrogen partial pressure or nitrogen gas, and the resulting melt is thereafter cast or solidified in an atmosphere controlled to have a nitrogen partial pressure of not less than 1.0 bar and a total pressure of not less than 4.0 bar, with the relationship between the partial pressure p and the total pressure P being 
     
         10.sup.p &lt;P.sup.0.37 +log.sub.10.sup.6 
    
     thereby obtaining good quality ingot free of blowholes. 
     Based on the results of tests for determining the creep rupture strength of the steel taught by Japanese Patent Application No. Hei 2-37895 up to 50 thousand hours, the inventors discovered that the creep rupture strength of the steel at 150 thousand hours, as estimated by linear extrapolation, is no more than 176 MPa and, in particular, that the steel experiences a marked decrease in creep rupture strength between 30 and 50 thousand hours. Further studies showed that the reason for the decrease in creep rupture strength was that during the creep test large Fe 2  W grains measuring 1 μm or more in diameter precipitated in large amounts, principally at the grain boundaries, leading to large-scale loss of W as a solid solution element from the steel. 
     Moreover, they further discovered that by limiting the W content to not more than 1.5% so as to prevent precipitation of W as Fe 2  W and by adding Nb in excess of 0.12% so that NbN and (Nb, V)N, the most stable of all nitrides, become the principal precipitation hardening factors, it is possible to obtain a ferritic heat-resisting steel exhibiting a creep rupture strength at 650° C., 355 bar and 150 thousand hours of not less than 200 MPa, as estimated by linear extrapolation. 
     In addition they discovered that owing to the increase in the N solution limit resulting from the addition of a large amount of Nb the pressurized atmosphere conditions required for casting of sound ingot become a total pressure of not less than 2.5 bar and a nitrogen partial pressure of not less than 1.0 bar, with the relationship between the total pressure P and the nitrogen partial pressure p being 
     
         P&gt;2.5p. 
    
     There have been few papers published on research into high-nitrogen ferritic heat-resisting steels and the only known published report in this field in Ergebnisse der Werkstoff-Forschung, Band I, Varlag Schweizerische Akademie der Werkstoffwissenschaften &#34;Thubal-Kain&#34;, Zurich, 1987, 161-180. 
     However, the research described in this report is limited to that in connection with ordinary heat-resisting steel and there is no mention of materials which can be used under such severe conditions at 650° C., 355 bar and 150 thousand hours continuous operation. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a high-nitrogen ferritic heat-resisting steel which overcomes the shortcomings of the conventional heat-resisting steels, and particularly to provide such a steel exhibiting outstanding creep rupture strength and capable of being used under severe operating conditions, wherein the decrease in creep rupture strength following prolonged aging and the degradation of high-temperature oxidation resistance caused by precipitation of carbides are mitigated by adding nitrogen to supersaturation so as to precipitate fine nitrides and/or carbo-nitrides which suppress the formation of carbides such as the M 23  C 6  precipitates seen in conventional steels. 
     This invention was accomplished in the light of the aforesaid knowledge and, in one aspect, pertains substantially to a high-nitrogen ferritic heat-resisting steel with high niobium content comprising, in weight percent 0.01-0.30% C, 0.02-0.80% Si, 0.20-1.00% Mn, 8.00-13.00% Cr, 0.005-1.00% Mo, 0.20-1.50% W, 0.05-1.00% V, over 0.12 up to 2.00% Nb and 0.10-0.50% N and being controlled to include not more than 0.050% P, not more than 0.010% S and not more than 0.020% O, and optionally comprising (A) one or both of 0.01-1.00% Ta and 0.01-1.00% Hf and/or (B) one or both of 0.0005-0.10% Zr and 0.01-0.10% Ti, the balance being Fe and unavoidable impurities. 
     Another aspect of the invention pertains to a method of producing such a high-nitrogen ferritic heat-resisting steel with high niobium content wherein the steel components are melted and equilibrated in an atmosphere of a mixed gas of a prescribed nitrogen partial pressure or nitrogen gas, and the resulting melt is thereafter cast or solidified in an atmosphere controlled to have a total pressure of not less than 2.5 bar and a nitrogen partial pressure of not less than 1.0 bar, with the relationship between the nitrogen partial pressure p and the total pressure P being 
     
         P&gt;2.5p. 
    
     The above and other features of the present invention will become apparent from the following description made with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an ingot and the manner in which it is to be cut. 
     FIG. 2 is a graph showing the relationship between the steel nitrogen content and the weight percentage of the total of M 23  C 6  +M 6  C+NbC+Cr 2  N+NbN among the precipitates in the steel accounted for by M 23  C 6  +M 6  C+NbC and the relationship between the steel nitrogen content and the weight percentage of the total of M 23  C 6  +M 6  C+NbC+Cr 2  N+NbN among the precipitates in the steel accounted for by Cr 2  N+NbN. 
     FIG. 3 is a graph showing conditions under which blowholes occur in the ingot in terms of the relationship between the total pressure and nitrogen partial pressure of the atmosphere during casting. 
     FIG. 4 is a schematic view showing the manner in which creep test pieces are taken from a pipe specimen and a rolled plate specimen. 
     FIG. 5 is a graph showing the relationship between steel nitrogen content and estimated creep rupture strength at 650° C., 150 thousand hours. 
     FIG. 6 is a graph showing the relationship between steel Nb content and estimated creep rupture strength at 650° C., 150 thousand hours. 
     FIG. 7 is a graph showing the relationship between steel W content and estimated creep rupture strength at 650° C., 150 thousand hours. 
     FIG. 8 is a graph showing an example of creep test results in terms of stress vs rupture time. 
     FIG. 9 is a graph showing the relationship between steel nitrogen content and Charpy impact absorption energy at 0° C. following aging at 700° C. for 10 thousand hours. 
     FIG. 10 is a graph showing the relationship between steel nitrogen content and the thickness of the oxidation scale formed on the surface of a test piece after oxidation at 650° C. for 10 thousand hours. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The reasons for the limits placed on the components of the high-nitrogen ferritic heat-resisting steel with high Nb content according to this invention will now be explained. 
     C is required for achieving strength. Adequate strength cannot be achieved at a C content of less than 0.01%, while at a C content exceeding 0.30% the steel is strongly affected by welding heat and undergoes hardening which becomes a cause for low-temperature cracking. The C content range is therefore set at 0.01-0.30%. 
     Si is important for achieving oxidation resistance and is also required as a deoxidizing agent. It is insufficient for these purposes at a content of less than 0.02%, whereas a content exceeding 0.80% reduces the creep rupture strength. The Si content range is therefore set at 0.02-0.80%. 
     Mn is required for deoxidation and also for achieving strength. It has to be added an amount of at least 0.20% for adequately exhibiting its effect. When it exceeds 1.00% it may in some cases reduce creep rupture strength. The Mn content range is therefore set at 0.20-1.00%. 
     Cr is indispensable to oxidation resistance. It also contributes to increasing creep resistance by combining with N and finely precipitating in the base metal matrix in the form of Cr 2  N, Cr 2  (C, N) and the like. Its lower limit is set at 8.00% from the viewpoint of oxidation resistance. Its upper limit is set at 13.00% for maintaining the Cr equivalent value at a low level so as to realize a martensite phase texture. 
     W produces a marked increase in creep rupture strength by solution hardening. Its effect toward increasing creep rupture strength over long periods at high temperatures of 550° C. and higher is particularly pronounced. Its upper limit is set at 1.50% because at contents higher than this level it precipitates in large quantities in the form of carbide and intermetallic compounds which sharply reduce the toughness of the base metal. The lower limit is set at 0.20% because it does not exhibit adequate solution hardening effect at lower levels. 
     Mo increases high-temperature strength through solution hardening. It does not exhibit adequate effect at a content of less than 0.005% and at a content higher than 1.00% it may, when added together with W, cause heavy precipitation of Mo 2  C type oxides which markedly reduce base metal toughness. The Mo content range is therefore set at 0.005-1.00%. 
     V produces a marked increase in the high-temperature strength of the steel regardless of whether it forms precipitates or, like W, enters solid solution in the matrix. When it precipitates, the resulting VN and (Nb, V)N serve as precipitation nuclei for Cr 2  N and Cr 2  (C, N), which has a pronounced effect toward promoting fine dispersion of the precipitates. It has no effect at below 0.05% and reduces toughness at higher than 1.00%. The V content range is therefore set at 0.05-1.00%. 
     Nb is an element which increases high-temperature strength by precipitating as NbN, (Nb, V)N, Nb(C, N) and (Nb, V) (C, N). Also, similarly to V, it promotes fine precipitate dispersion by forming precipitation nuclei for Cr 2  N, Cr 2  (C, N) and the like. For it to disperse in the steel as the primary precipitation hardening factor it has to be added in excess of 0.12%. However, its upper limit is set at 2.00% because when present at higher levels it reduces strength by causing precipitate coagulation and enlargement. 
     N dissolves in the matrix and also forms nitride and carbo-nitride precipitates. As the form of the precipitates is mainly Cr 2  N and Cr 2  (C, N), there is less precipitate-induced consumption of Cr and W than in the case of the M 23  C 6 , M 6  C and other such precipitates observed in conventional steels. N thus increases oxidation resistance and creep rupture strength. At least 0.10% is required for precipitation of nitrides and carbo-nitrides and suppressing precipitation of M 23  C 6  and M 6  C. The upper limit is set at 0.50% for preventing coagulation and enlargement of nitride and carbo-nitride precipitates by the presence of excessive nitrogen. 
     P, S and O are present in the steel according to this invention as impurities. P and S hinder the achievement of the purpose of the invention by lowering strength, while O has the adverse effect of forming oxides which reduce toughness. The upper limits on these elements is therefore set at 0.050%, 0.010% and 0.020%, respectively. 
     The basic components of the steel according to this invention (aside from Fe) are as set out above. Depending on the purpose to which the steel is to be put, however, it may additionally contain (A) one or both of 0.01-1.00% Ta and 0.01-1.00% Hf and/or (B) one or both of 0.0005-0.10% Zr and 0.01-0.10% Ti. 
     At low concentrations Ta and Hf act as deoxidizing agents. At high concentrations they form fine high melting point nitrides and carbo-nitrides and, as such, increase toughness by decreasing the austenite grain size. In addition, they also reduce the degree to which Cr and W dissolve in precipitates and by this effect enhance the effect of supersaturation with nitrogen. Neither element exhibits any effect at less than 0.01%. When either is present at greater than 1.00%, it reduces toughness by causing enlargement of nitride and carbo-nitride precipitates. The content range of each of these elements is therefore set at 0.01-1.00%. 
     Acting to govern the deoxidation equilibrium in the steel, Zr suppresses the formation of oxides by markedly reducing the amount of oxygen activity. In addition, its strong affinity for N promotes precipitation of fine nitrides and carbo-nitrides which increase creep rupture strength and high-temperature oxidation resistance. When present at less than 0.0005% it does not provide an adequate effect of governing the deoxidation equilibrium and when present at greater than 0.10% it results in heavy precipitation of coarse ZrN and ZrC which markedly reduce the toughness of the base metal. The Zr content range is therefore set at 0.0005-0.10%. 
     Ti raises the effect of excess nitrogen by precipitating in the form of nitrides and carbo-nitrides. At a content of less than 0.01% it has no effect while a Ti content of over 0.10% results in precipitation of coarse nitrides and carbo-nitrides which reduce toughness. The Ti content range is therefore set at 0.01-0.10%. 
     The aforesaid alloying components can be added individually or in combinations. 
     The object of this invention is to provide a ferritic heat-resisting steel that is superior in creep rupture strength and high-temperature oxidation resistance. Depending on the purpose of use it can be produced by various methods and be subjected to various types of heat treatment. These methods and treatments in no way diminish the effect of the invention. 
     However, in view of the need to supersaturate the steel with nitrogen, it is necessary during casting to raise the total pressure of the atmosphere to not less than 2.5 bar and to control the relationship between the total pressure P and the nitrogen partial pressure p to satisfy the inequation P&gt;2.5 p. As an auxiliary gas to be mixed with the nitrogen gas it is appropriate to use an inert gas such as Ar, Ne, Xe or Kr. These casting conditions were determined by the following experiment. 
     Steel of a chemical composition, aside from nitrogen, as indicated in the present invention was melted in an induction heating furnace installed in a chamber that could be pressurized up to 150 bar. A mixed gas of argon and nitrogen having a nitrogen partial pressure adequate for achieving the target nitrogen content was introduced into the furnace and maintained at a pressure which was varied from test to test. After the nitrogen and molten metal had reached chemical equilibrium, the molten metal was cast into a mold that had been installed in the chamber beforehand, whereby there was obtained a 5-ton ingot. 
     The ingot was cut vertically as shown in FIG. 1 and the ingot 1 was visually examined for the presence of blowholes. 
     Following this examination, a part of the ingot was placed in a furnace and maintained at 1180° C. for 1 hour and then forged into a plate measuring 50 mm in thickness, 750 mm in width and 4,000 mm in length. 
     This plate was subjected to solution treatment at 1200° C. for 1 hour and to tempering at 800° C. for 3 hours. The steel was then chemically analyzed and the dispersion state and morphology of the nitrides and carbo-nitrides were investigated by observation with an optical microscope, an electron microscope, X-ray diffraction and electron beam diffraction, whereby the chemical structure was determined. 
     Among the precipitates present within the as-heat-treated steel, FIG. 2 shows how the proportion of the precipitates in the steel accounted for by M 23  C 6  type carbides and M 6  C or NbC type carbides and the proportion thereof accounted for by Cr 2  N type nitrides and NbN type nitrides vary with nitrogen concentration. At a nitrogen concentration of 0.10%, nitrides account for the majority of the precipitates in the steel of the invention, while at a nitrogen concentration of 0.15%, substantially 100% of the precipitates are nitrides with virtually no carbides present whatsoever. Thus for the effect of this invention to be adequately manifested it is necessary for the nitrogen concentration of the steel to be not less than 0.1%. 
     The graph of FIG. 3 shows how the state of blowhole occurrence varies depending on the relationship between the total and nitrogen partial pressures of the atmosphere. For achieving a nitrogen concentration of 0.10% or higher it is necessary to use a total pressure of not less than 2.5 bar. Equilibrium calculation based on Sievert&#39;s law shows that in this case the nitrogen partial pressure in the steel of this invention is not less than 1.0 bar. 
     Moreover, where for controlling the amount of nitride and carbo-nitride precipitation the nitrogen partial pressure is maintained at 1.0-6.0 bar (nitrogen concentration within the steel of approximately 0.5 mass %), it becomes necessary to vary the total pressure between 2.5 and about 15 bar, the actual value selected depending on the nitrogen partial pressure. Namely, it is necessary to use a total pressure falling above the broken line representing the boundary pressure in FIG. 3. 
     When the boundary line of FIG. 3 is determined experimentally it is found to lie at 
     
         P=2.5p 
    
     meaning that the steel according to this invention can be obtained by selecting an atmosphere of a pressure and composition meeting the condition of the inequality 
     
         P&gt;2.5p 
    
     It is therefore necessary to use furnace equipment enabling pressure and atmosphere control. Without such equipment, it is difficult to produce the steel of the present invention. 
     There are no limitations whatever on the melting method. Based on the chemical composition of the steel and cost considerations, it suffices to select from among processes using a converter, an induction heating furnace, an arc melting furnace or an electric furnace. 
     The situation regarding refining is similar. Insofar as the atmosphere is controlled to a total pressure of not less than 2.5 bar and a nitrogen partial pressure of not less than 1.0 bar, it is both possible and effective to use a ladle furnace, an electro-slag remelting furnace or a zone melting furnace. 
     After casting under a pressurized atmosphere of a total pressure of not less than 2.5 bar and a nitrogen partial pressure of not less than 1.0 bar, it is possible to process the steel into billet, bloom or plate by forging or hot rolling. Since the steel of this invention includes finely dispersed nitrides and carbo-nitrides, it is superior to conventional ferritic heat-resisting steels in hot-workability. This is also one reason for employing nitrides and carbo-nitrides obtained by adding nitrogen to beyond the solution limit. 
     For processing the steel into products, it is possible to first process it into round or rectangular billet and then form it into seamless pipe or tube by hot extrusion or any of various seamless rolling methods. Otherwise it can be formed into sheet by hot and cold rolling and then made into welded tube by electric resistance welding. Alternatively, it can be processed into welded pipe or tube by use of TIG, MIG, SAW, LASER and EB welding, individually or in combination. Moreover, it is possible to expand the size range of products to which the present invention can be applied by following any of the aforesaid processes by hot or warm stretch reduction or sizing. 
     The steel according to the invention can also be provided in the form of plate or sheet. The plate or sheet can, in its hot-rolled state or after whatever heat treatment is found necessary, be provided as a heat-resisting material in various shapes, without any influence on the effects provided by the invention. 
     The pipe, tube, plate, sheet and variously shaped heat-resisting materials referred to above can, in accordance with their purpose and application, be subjected to various heat treatments, and it is important for them to be so treated for realizing the full effect of the invention. 
     While the production process ordinarily involves normalizing (solution heat treatment)+ tempering, it is also possible and useful additionally to carry out one or a combination of two or more of quenching, tempering and normalizing. It is also possible, without influencing the effects of the present invention in any way, to repeatedly carry out one or more of the aforesaid processes to whatever degree is necessary for adequately bringing out the steel properties. 
     The aforesaid processes can be appropriately selected and applied to the manufacture of the steel according to the invention. 
     WORKING EXAMPLES 
     The steels indicated in Tables 1-14, each having a composition according to the present invention, were separately melted in amounts of 5 tons each in an induction heating furnace provided with pressurizing equipment. The resulting melt was cleaned by ladle furnace processing (under bubbling with a gas of the same composition as the atmosphere) for reducing its impurity content, whereafter the atmosphere was regulated using a mixed gas of nitrogen and argon so as to satisfy the conditions of the inequality P&gt;2.5 p. The melt was then cast into a mold and processed into a round billet, part of which was hot extruded to obtain a tube 60 mm in outside diameter and 10 mm in wall thickness and the remainder of which was subjected to seamless rolling to obtain a pipe 380 mm in outside diameter and 50 mm in wall thickness. The tube and pipe were subjected to a single normalization at 1200° C. for 1 hour and were then tempered at 800° C. for 3 hours. 
     In addition, a 5 ton ingot was cast and forged into a slab which was hot rolled into 25 mm and 50 mm thick plates. 
     As shown in FIG. 4, creep test pieces 6 measuring 6 mm in diameter were taken along the axial direction 4 of the pipe or tube 3 and along the rolling direction 5 of the plates and subjected to creep test measurement at 650° C. Based on the data obtained, a linear extrapolation was made for estimating the creep rupture strength at 150 thousand hours. A creep rupture strength of 150 MPa was used as the creep rupture strength evaluation reference value. The creep rupture strength at 650° C., 150 thousand hours is hereinafter defined as the linearly extrapolated value at 150 thousand hours on the creep rupture strength vs rupture time graph. 
     Toughness was evaluated through an accelerated evaluation test in which aging was carried out at 700° C. for 10 thousand hours. JIS No. 4 tension test pieces were cut from the aged steel and evaluated for impact absorption energy. Assuming a water pressure test at 0° C., the toughness evaluation reference value was set at 10 J. 
     High-temperature oxidation resistance was evaluated by suspending a 25 mm×25 mm×5 mm test piece cut from the steel in 650° C. atmospheric air in a furnace for 10 thousand hours and then cutting the test piece parallel to the direction of growth of the scale and measuring the oxidation scale thickness. 
     The 650° C., 150 thousand hour creep rupture strength, the Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours and the oxidation scale thickness after oxidation at 650° C. for 10 thousand hours are shown in Tables 2, 4, 6, 8, 10, 12, and 14. 
     For comparison, steels of compositions not falling within the present invention were melted, processed and tested in the same way as described above. Their chemical compositions and the evaluation results are shown in Tables 15 and 16. 
     FIG. 5 shows the relationship between the nitrogen content of the steels and the estimated creep rupture strength at 650° C., 150 thousand hours. It will be noted that the creep rupture strength attains high values exceeding 150 MPa at a steel nitrogen content of 0.1% or higher but falls below 150 MPa and fails to satisfy the evaluation reference value that was set at a steel nitrogen content of less than 0.1%. 
     FIG. 6 shows the relationship between the Nb content of the steels and the estimated creep rupture strength at 650° C., 150 thousand hours. It will be noted that the creep rupture strength attains values exceeding 150 MPa at a steel Nb content exceeding 0.12% but at a Nb content of 2.0% or higher the creep rupture strength is instead lowered owing to the precipitation of coarse NbN and Fe 2  Nb type Laves phase at the melting stage. 
     FIG. 7 shows the relationship between the W content of the steels and the estimated creep rupture strength at 650° C., 150 thousand hours. The creep rupture strength is below 150 MPa at a W content of less than 0.2% and is 150 MPa or higher in a content range of 0.2-1.5%. When the W is present in excess of 1.5%, the creep rupture strength falls below 150 MPa owing to coarse Fe 2  W precipitating at the grain boundaries. 
     FIG. 8 shows the results of the creep test in terms of stress vs rupture time. A good linear relationship can be noted between stress and rupture time at a steel nitrogen content of not less than 0.1%. Moreover, the creep rupture strength is high. On the other hand, when the steel nitrogen content falls below 0.1%, the relationship between stress and rupture time exhibits a pronounced decline in creep rupture strength with increasing time lapse. Either the linearity is not maintained, or the slope of the creep rupture curve is steep, with the short-term side creep rupture strength being high but the long-term creep rupture strength being low, or the creep rupture strength is low throughout. This is because W and the other solution hardening elements precipitate as carbides whose coagulation and enlargement degrades the creep rupture strength property of the base metal. In contrast, at a nitrogen content of 0.1% or higher, fine nitrides are preferentially precipitated so that the formation of carbides is greatly delayed. Therefore, since the dissolution of the solution hardening elements into carbides was suppressed and also because the finely precipitated nitrides remained present in a stable state without coagulating and enlarging during the long-term high-temperature creep test, a high creep rupture strength was maintained in the long-term creep test. 
     FIG. 9 shows the relationship between Charpy impact absorption energy at 0° C. following aging at 700° C. for 10 thousand hours and steel nitrogen content. When the steel nitrogen content falls within the range of 0.1-0.5%, the impact absorption energy exceeds 10 J. In contrast, when it falls below 0.1%, there is little or no suppression of grain growth by residual high melting point nitrides during solution treatment and, as a result, the impact absorption energy decreases, and when it exceeds 0.5%, the impact absorption energy is reduced by heavy nitride precipitation. 
     FIG. 10 shows the relationship between the thickness of the oxidation scale formed on the surface of a test piece after oxidation at 650° C. for 10 thousand hours and the steel nitrogen content. Although the oxidation scale thickness is between 400 and 900 μm when the steel nitrogen content falls below 0.1%, it decreases to 50 μm or less when the steel nitrogen content is 0.1% or higher. 
     Reference is now made to the comparison steels shown in Table 5. Nos. 161 and 162 are examples in which insufficient steel nitrogen content resulted in a low estimated creep rupture strength at 650° C., 150 thousand hours and also to poor high-temperature oxidation resistance. Nos. 163 and 164 are examples in which excessive steel nitrogen content caused heavy precipitation of coarse nitrides and carbo-nitrides, resulting in a Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours of not more than 10 J. No. 165 is an example in which a low W concentration resulted in a low creep rupture strength at 650° C., 150 thousand hours owing to insufficient solution hardening notwithstanding that the steel nitrogen content fell within the range of the invention. No. 166 is an example in which a high W concentration led to low rupture strength and toughness owing to precipitation of coarse Fe 2  W type Laves phase at the grain boundaries during creep. No. 167 is an example in which a low Nb content resulted in a low estimated creep rupture strength at 650° C., 150 thousand hours. No. 168 is an example in which a high Nb content caused profuse precipitation of coarse Fe 2  Nb type Laves phase during creep, which in turn lowered both the estimated creep rupture strength at 650° C., 150 thousand hours and the Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours. No. 169 is an example in which heavy precipitation of coarse ZrN caused by a Zr concentration in excess of 0.1% resulted in a Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours of less than 10 J. Nos. 170, 171 and 172 are examples similar to the case of No. 169 except that the elements present in excess were Ta, Hf and Ti, respectively. As a result, heavy precipitation of coarse TaN, HfN and TiN resulted in a Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours of less than 10 J. No. 173 is an example in which, notwithstanding that the steel composition satisfied the conditions of the present invention, since the nitrogen partial pressure was 2.2 bar and the total pressure was 2.5 bar, values not satisfying the inequality P&gt;2.5 p, many large blowholes formed in the ingot, making it impossible to obtain either a sound ingot or a plate and leading to a reduction in both the estimated creep rupture strength at 650° C., 150 thousand hours and the Charpy impact absorption energy at 0° C. after aging at 700° C. for 10 thousand hours. 
     
                       TABLE 1                                                     
______________________________________                                    
(mass %) Invention steels                                                 
No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                      
______________________________________                                    
 1   0.195  0.251  0.801                                                  
                        0.048                                             
                             0.008 0.666                                  
                                        0.094                             
                                              9.52                        
                                                  0.586                   
 2   0.070  0.475  0.632                                                  
                        0.013                                             
                             0.005 0.132                                  
                                        0.370                             
                                             10.38                        
                                                  0.038                   
 3   0.192  0.709  0.261                                                  
                        0.042                                             
                             0.009 1.185                                  
                                        0.174                             
                                              8.70                        
                                                  0.707                   
 4   0.032  0.316  0.231                                                  
                        0.022                                             
                             0.008 0.418                                  
                                        0.487                             
                                             12.80                        
                                                  0.767                   
 5   0.219  0.220  0.299                                                  
                        0.018                                             
                             0.004 1.064                                  
                                        0.496                             
                                             11.18                        
                                                  0.427                   
 6   0.096  0.190  0.620                                                  
                        0.022                                             
                             0.003 0.608                                  
                                        0.066                             
                                              9.82                        
                                                  0.865                   
 7   0.288  0.762  0.546                                                  
                        0.038                                             
                             0.008 1.308                                  
                                        0.402                             
                                              9.66                        
                                                  0.283                   
 8   0.270  0.515  0.216                                                  
                        0.014                                             
                             0.001 0.680                                  
                                        0.462                             
                                              9.07                        
                                                  0.843                   
 9   0.098  0.369  0.583                                                  
                        0.039                                             
                             0.003 0.927                                  
                                        0.105                             
                                             11.51                        
                                                  0.748                   
10   0.259  0.477  0.995                                                  
                        0.039                                             
                             0.002 0.582                                  
                                        0.089                             
                                             10.40                        
                                                  0.179                   
11   0.083  0.594  0.404                                                  
                        0.021                                             
                             0.007 1.824                                  
                                        0.050                             
                                             11.70                        
                                                  0.820                   
12   0.206  0.478  0.961                                                  
                        0.035                                             
                             0.004 0.336                                  
                                        0.073                             
                                              8.84                        
                                                  0.366                   
13   0.100  0.663  0.812                                                  
                        0.026                                             
                             0.003 1.163                                  
                                        0.469                             
                                             12.74                        
                                                  0.504                   
14   0.113  0.515  0.514                                                  
                        0.043                                             
                             0.004 1.837                                  
                                        0.418                             
                                             11.87                        
                                                  0.416                   
15   0.152  0.577  0.217                                                  
                        0.033                                             
                             0.007 0.823                                  
                                        0.237                             
                                             12.21                        
                                                  0.804                   
16   0.127  0.600  0.467                                                  
                        0.024                                             
                             0.002 0.392                                  
                                        0.454                             
                                              9.62                        
                                                  0.843                   
17   0.092  0.722  0.203                                                  
                        0.022                                             
                             0.006 1.747                                  
                                        0.246                             
                                             12.49                        
                                                  0.704                   
18   0.243  0.320  0.395                                                  
                        0.036                                             
                             0.007 1.583                                  
                                        0.165                             
                                              9.38                        
                                                  0.613                   
19   0.060  0.713  0.388                                                  
                        0.013                                             
                             0.001 0.194                                  
                                        0.285                             
                                             11.20                        
                                                  0.732                   
20   0.030  0.087  0.722                                                  
                        0.020                                             
                             0.009 0.790                                  
                                        0.465                             
                                              8.83                        
                                                  0.751                   
21   0.210  0.544  0.354                                                  
                        0.026                                             
                             0.004 1.791                                  
                                        0.341                             
                                             10.73                        
                                                  0.363                   
22   0.213  0.281  0.237                                                  
                        0.036                                             
                             0.008 1.714                                  
                                        0.451                             
                                             10.38                        
                                                  0.756                   
23   0.101  0.227  0.520                                                  
                        0.041                                             
                             0.008 0.897                                  
                                        0.200                             
                                              8.70                        
                                                  0.117                   
24   0.067  0.128  0.544                                                  
                        0.045                                             
                             0.001 0.640                                  
                                        0.348                             
                                              9.08                        
                                                  0.625                   
25   0.172  0.142  0.473                                                  
                        0.038                                             
                             0.002 0.340                                  
                                        0.307                             
                                             12.53                        
                                                  0.482                   
______________________________________                                    
 
    
     
                       TABLE 2                                                     
______________________________________                                    
                                          CS   VE   TO                    
No.  W      Zr     Ta   Hf  Ti  N    O    MPa  J    μm                 
______________________________________                                    
 1   1.154  --     --   --  --  0.375                                     
                                     0.002                                
                                          231  19.4 46                    
 2   0.716  --     --   --  --  0.242                                     
                                     0.010                                
                                          214  17.7 46                    
 3   1.169  --     --   --  --  0.422                                     
                                     0.019                                
                                          230  55.3 38                    
 4   1.439  --     --   --  --  0.187                                     
                                     0.013                                
                                          195  38.7 28                    
 5   0.497  --     --   --  --  0.210                                     
                                     0.007                                
                                          183  54.3 47                    
 6   1.187  --     --   --  --  0.376                                     
                                     0.002                                
                                          209  77.9 19                    
 7   0.496  --     --   --  --  0.153                                     
                                     0.012                                
                                          243  18.8 13                    
 8   0.608  --     --   --  --  0.217                                     
                                     0.012                                
                                          153  64.1 36                    
 9   0.473  --     --   --  --  0.440                                     
                                     0.006                                
                                          157  16.9 36                    
10   0.329  --     --   --  --  0.301                                     
                                     0.017                                
                                          226  31.2 20                    
11   0.420  0.050  --   --  --  0.270                                     
                                     0.009                                
                                          226  14.1 16                    
12   0.999  0.092  --   --  --  0.427                                     
                                     0.006                                
                                          178  49.6 11                    
13   1.006  0.031  --   --  --  0.445                                     
                                     0.019                                
                                          201  45.7 33                    
14   1.328  0.035  --   --  --  0.197                                     
                                     0.016                                
                                          161  15.1 50                    
15   0.280  0.073  --   --  --  0.164                                     
                                     0.010                                
                                          179  41.0 27                    
16   0.686  0.015  --   --  --  0.218                                     
                                     0.012                                
                                          175  32.4 35                    
17   1.464  0.026  --   --  --  0.228                                     
                                     0.017                                
                                          213  35.1 38                    
18   0.323  0.028  --   --  --  0.276                                     
                                     0.002                                
                                          166  64.9 46                    
19   1.363  0.040  --   --  --  0.438                                     
                                     0.005                                
                                          202  28.6 26                    
20   0.280  0.080  --   --  --  0.183                                     
                                     0.013                                
                                          240  45.7 26                    
21   0.574  --     0.291                                                  
                        --  --  0.190                                     
                                     0.019                                
                                          246  56.8 32                    
22   1.196  --     0.414                                                  
                        --  --  0.134                                     
                                     0.018                                
                                          242  42.0 12                    
23   1.086  --     0.275                                                  
                        --  --  0.293                                     
                                     0.013                                
                                          195  13.6 34                    
24   0.480  --     0.038                                                  
                        --  --  0.132                                     
                                     0.017                                
                                          226  55.8 25                    
25   0.926  --     0.039                                                  
                        --  --  0.139                                     
                                     0.006                                
                                          200  75.2 13                    
______________________________________                                    
 CS: Creep rupture strength at 650° C., 150 thousand hours;        
 VE: Charpy impact absorption energy at 0° C. after aging at       
 700° C. for 10 thousand hours;                                    
 TO: Oxidation scale thickness after 650° C., 10 thousand hour     
 hightemperature oxidation                                                
 
    
     
                       TABLE 3                                                     
______________________________________                                    
(mass %) Invention steels                                                 
No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                      
______________________________________                                    
26   0.027  0.796  0.445                                                  
                        0.045                                             
                             0.005 1.306                                  
                                        0.157                             
                                             12.42                        
                                                  0.859                   
27   0.273  0.684  0.616                                                  
                        0.036                                             
                             0.006 1.054                                  
                                        0.209                             
                                             12.23                        
                                                  0.954                   
28   0.093  0.727  0.740                                                  
                        0.018                                             
                             0.007 1.824                                  
                                        0.100                             
                                             10.41                        
                                                  0.459                   
29   0.120  0.290  0.555                                                  
                        0.017                                             
                             0.004 1.164                                  
                                        0.363                             
                                             10.34                        
                                                  0.055                   
30   0.118  0.298  0.215                                                  
                        0.023                                             
                             0.006 0.328                                  
                                        0.177                             
                                             10.47                        
                                                  0.914                   
31   0.144  0.520  0.282                                                  
                        0.041                                             
                             0.008 0.648                                  
                                        0.087                             
                                              9.32                        
                                                  0.756                   
32   0.248  0.694  0.487                                                  
                        0.019                                             
                             0.007 0.572                                  
                                        0.391                             
                                             10.20                        
                                                  0.241                   
33   0.055  0.208  0.602                                                  
                        0.046                                             
                             0.001 1.432                                  
                                        0.286                             
                                              9.19                        
                                                  0.080                   
34   0.198  0.752  0.361                                                  
                        0.048                                             
                             0.002 1.631                                  
                                        0.296                             
                                             12.13                        
                                                  0.939                   
35   0.292  0.519  0.709                                                  
                        0.013                                             
                             0.005 1.583                                  
                                        0.056                             
                                              9.91                        
                                                  0.011                   
36   0.106  0.449  0.217                                                  
                        0.049                                             
                             0.003 1.617                                  
                                        0.334                             
                                             11.12                        
                                                  0.494                   
37   0.127  0.685  0.219                                                  
                        0.023                                             
                             0.002 0.190                                  
                                        0.304                             
                                             11.57                        
                                                  0.563                   
38   0.118  0.576  0.459                                                  
                        0.022                                             
                             0.006 0.492                                  
                                        0.278                             
                                              9.73                        
                                                  0.406                   
39   0.279  0.286  0.703                                                  
                        0.014                                             
                             0.001 1.016                                  
                                        0.344                             
                                             11.55                        
                                                  0.220                   
40   0.237  0.746  0.301                                                  
                        0.027                                             
                             0.003 1.137                                  
                                        0.138                             
                                              8.01                        
                                                  0.691                   
41   0.036  0.137  0.987                                                  
                        0.028                                             
                             0.004 0.275                                  
                                        0.408                             
                                             12.45                        
                                                  0.993                   
42   0.024  0.558  0.524                                                  
                        0.023                                             
                             0.001 1.619                                  
                                        0.351                             
                                             10.85                        
                                                  0.824                   
43   0.111  0.602  0.658                                                  
                        0.043                                             
                             0.006 1.759                                  
                                        0.272                             
                                             11.70                        
                                                  0.878                   
44   0.063  0.416  0.740                                                  
                        0.013                                             
                             0.007 1.035                                  
                                        0.252                             
                                             10.85                        
                                                  0.331                   
45   0.220  0.528  0.523                                                  
                        0.040                                             
                             0.009 0.723                                  
                                        0.380                             
                                              9.33                        
                                                  0.379                   
46   0.203  0.303  0.544                                                  
                        0.013                                             
                             0.010 0.266                                  
                                        0.060                             
                                             10.46                        
                                                  0.195                   
47   0.211  0.280  0.956                                                  
                        0.039                                             
                             0.006 0.957                                  
                                        0.055                             
                                             12.74                        
                                                  0.470                   
48   0.158  0.227  0.728                                                  
                        0.025                                             
                             0.005 0.334                                  
                                        0.227                             
                                              9.64                        
                                                  0.408                   
49   0.241  0.279  0.798                                                  
                        0.020                                             
                             0.004 1.557                                  
                                        0.210                             
                                             12.12                        
                                                  0.195                   
50   0.078  0.197  0.915                                                  
                        0.030                                             
                             0.008 0.262                                  
                                        0.310                             
                                              8.54                        
                                                  0.524                   
______________________________________                                    
 
    
     
                                           TABLE 4                                 
__________________________________________________________________________
(mass %) Invention steels                                                 
                            CS  VE TO                                     
No. W  Zr  Ta Hf  Ti N   O  MPa J  μm                                  
__________________________________________________________________________
26  1.374                                                                 
       --  0.611                                                          
              --  -- 0.377                                                
                         0.001                                            
                            238 15.8                                      
                                   45                                     
27  0.756                                                                 
       --  0.221                                                          
              --  -- 0.329                                                
                         0.016                                            
                            160 30.8                                      
                                   49                                     
28  1.272                                                                 
       --  0.295                                                          
              --  -- 0.468                                                
                         0.007                                            
                            232 63.1                                      
                                   13                                     
29  1.225                                                                 
       --  0.134                                                          
              --  -- 0.266                                                
                         0.015                                            
                            225 55.9                                      
                                   23                                     
30  0.834                                                                 
       --  0.227                                                          
              --  -- 0.199                                                
                         0.007                                            
                            156 21.9                                      
                                   27                                     
31  1.018                                                                 
       0.017                                                              
           0.053                                                          
              --  -- 0.149                                                
                         0.013                                            
                            194 79.6                                      
                                   33                                     
32  1.356                                                                 
       0.007                                                              
           0.337                                                          
              --  -- 0.379                                                
                         0.019                                            
                            159 32.3                                      
                                   29                                     
33  1.460                                                                 
       0.012                                                              
           0.494                                                          
              --  -- 0.378                                                
                         0.019                                            
                            221 68.5                                      
                                   21                                     
34  0.246                                                                 
       0.003                                                              
           0.098                                                          
              --  -- 0.411                                                
                         0.016                                            
                            194 59.9                                      
                                   37                                     
35  1.227                                                                 
       0.017                                                              
           0.325                                                          
              --  -- 0.353                                                
                         0.005                                            
                            200 21.0                                      
                                   26                                     
36  1.401                                                                 
       0.001                                                              
           0.118                                                          
              --  -- 0.284                                                
                         0.017                                            
                            158 69.4                                      
                                   13                                     
37  0.337                                                                 
       0.084                                                              
           0.681                                                          
              --  -- 0.476                                                
                         0.009                                            
                            186 77.7                                      
                                   47                                     
38  0.899                                                                 
       0.031                                                              
           0.079                                                          
              --  -- 0.399                                                
                         0.002                                            
                            152 24.2                                      
                                   15                                     
39  0.726                                                                 
       0.013                                                              
           0.549                                                          
              --  -- 0.135                                                
                         0.019                                            
                            176 28.7                                      
                                   26                                     
40  0.722                                                                 
       0.046                                                              
           0.062                                                          
              --  -- 0.163                                                
                         0.009                                            
                            227 32.3                                      
                                   45                                     
41  0.450                                                                 
       --  -- 0.844                                                       
                  -- 0.148                                                
                         0.001                                            
                            171 48.4                                      
                                   41                                     
42  1.484                                                                 
       --  -- 0.732                                                       
                  -- 0.168                                                
                         0.010                                            
                            180 55.9                                      
                                   42                                     
43  1.463                                                                 
       --  -- 0.509                                                       
                  -- 0.301                                                
                         0.019                                            
                            172 67.8                                      
                                   14                                     
44  1.266                                                                 
       --  -- 0.36                                                        
                  -- 0.423                                                
                         0.014                                            
                            165 26.7                                      
                                   46                                     
45  0.495                                                                 
       --  -- 0.954                                                       
                  -- 0.202                                                
                         0.007                                            
                            226 50.2                                      
                                   43                                     
46  0.695                                                                 
       --  -- 0.516                                                       
                  -- 0.116                                                
                         0.002                                            
                            227 47.5                                      
                                   35                                     
47  1.456                                                                 
       --  -- 0.215                                                       
                  -- 0.439                                                
                         0.012                                            
                            235 66.8                                      
                                   49                                     
48  0.429                                                                 
       --  -- 0.239                                                       
                  -- 0.254                                                
                         0.013                                            
                            221 16.1                                      
                                   33                                     
49  1.116                                                                 
       --  -- 0.775                                                       
                  -- 0.229                                                
                         0.007                                            
                            152 48.8                                      
                                   13                                     
50  1.143                                                                 
       --  -- 0.511                                                       
                  -- 0.241                                                
                         0.008                                            
                            195 71.6                                      
                                   18                                     
__________________________________________________________________________
 CS: Creep rupture strength at 650° C., 150 thousand hours;        
 VE: Charpy impact absorption energy at 0° C. after aging at       
 700° C. for 10 thousand hours;                                    
 TO: Oxidation scale thickness after 650° C., 10 thousand hour     
 hightemperature oxidation                                                
 
    
     
                       TABLE 5                                                     
______________________________________                                    
(mass %) Invention steels                                                 
No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                      
______________________________________                                    
51   0.013  0.514  0.430                                                  
                        0.043                                             
                             0.001 0.562                                  
                                        0.385                             
                                              9.86                        
                                                  0.024                   
52   0.244  0.202  0.686                                                  
                        0.027                                             
                             0.008 1.922                                  
                                        0.074                             
                                              9.51                        
                                                  0.451                   
53   0.251  0.594  0.652                                                  
                        0.029                                             
                             0.003 0.881                                  
                                        0.135                             
                                             10.99                        
                                                  0.969                   
54   0.229  0.256  0.857                                                  
                        0.036                                             
                             0.002 0.636                                  
                                        0.203                             
                                             10.17                        
                                                  0.274                   
55   0.091  0.135  0.271                                                  
                        0.016                                             
                             0.001 0.590                                  
                                        0.138                             
                                              8.04                        
                                                  0.047                   
56   0.032  0.745  0.624                                                  
                        0.017                                             
                             0.007 1.097                                  
                                        0.471                             
                                             12.99                        
                                                  0.466                   
57   0.252  0.293  0.474                                                  
                        0.026                                             
                             0.007 1.554                                  
                                        0.160                             
                                              8.45                        
                                                  0.225                   
58   0.081  0.484  0.769                                                  
                        0.030                                             
                             0.009 1.403                                  
                                        0.416                             
                                             12.86                        
                                                  0.661                   
59   0.011  0.299  0.870                                                  
                        0.018                                             
                             0.008 0.988                                  
                                        0.430                             
                                             12.23                        
                                                  0.017                   
60   0.093  0.653  0.510                                                  
                        0.015                                             
                             0.004 0.628                                  
                                        0.051                             
                                              9.89                        
                                                  0.914                   
61   0.236  0.532  0.650                                                  
                        0.031                                             
                             0.006 1.657                                  
                                        0.267                             
                                             11.83                        
                                                  0.408                   
62   0.207  0.470  0.638                                                  
                        0.032                                             
                             0.001 0.660                                  
                                        0.493                             
                                             11.53                        
                                                  0.187                   
63   0.060  0.620  0.630                                                  
                        0.026                                             
                             0.007 1.736                                  
                                        0.134                             
                                              9.91                        
                                                  0.926                   
64   0.138  0.327  0.757                                                  
                        0.021                                             
                             0.005 0.544                                  
                                        0.396                             
                                             12.80                        
                                                  0.318                   
65   0.061  0.155  0.791                                                  
                        0.024                                             
                             0.004 1.198                                  
                                        0.245                             
                                             11.28                        
                                                  0.057                   
66   0.175  0.617  0.869                                                  
                        0.022                                             
                             0.008 0.296                                  
                                        0.436                             
                                             11.21                        
                                                  0.317                   
67   0.095  0.311  0.345                                                  
                        0.023                                             
                             0.005 1.922                                  
                                        0.377                             
                                             11.03                        
                                                  0.788                   
68   0.169  0.165  0.971                                                  
                        0.015                                             
                             0.007 0.839                                  
                                        0.384                             
                                              9.23                        
                                                  0.673                   
69   0.018  0.714  0.898                                                  
                        0.045                                             
                             0.002 0.192                                  
                                        0.314                             
                                              9.23                        
                                                  0.154                   
70   0.271  0.143  0.664                                                  
                        0.020                                             
                             0.004 0.309                                  
                                        0.471                             
                                              9.42                        
                                                  0.402                   
71   0.119  0.553  0.823                                                  
                        0.029                                             
                             0.003 0.583                                  
                                        0.261                             
                                              8.20                        
                                                  0.605                   
72   0.033  0.126  0.712                                                  
                        0.036                                             
                             0.003 0.506                                  
                                        0.447                             
                                              8.99                        
                                                  0.944                   
73   0.240  0.227  0.929                                                  
                        0.013                                             
                             0.003 0.416                                  
                                        0.228                             
                                             10.50                        
                                                  0.658                   
74   0.054  0.575  0.388                                                  
                        0.045                                             
                             0.009 1.583                                  
                                        0.129                             
                                             10.93                        
                                                  0.898                   
75   0.116  0.159  0.450                                                  
                        0.041                                             
                             0.009 1.533                                  
                                        0.233                             
                                              8.81                        
                                                  0.361                   
______________________________________                                    
 
    
     
                                           TABLE 6                                 
__________________________________________________________________________
(mass %) Invention steels                                                 
                            CS  VE TO                                     
No. W  Zr  Ta Hf  Ti N   O  MPa J  μm                                  
__________________________________________________________________________
51  0.750                                                                 
       0.065                                                              
           -- 0.132                                                       
                  -- 0.363                                                
                         0.010                                            
                            171 28.4                                      
                                   13                                     
52  0.200                                                                 
       0.047                                                              
           -- 0.829                                                       
                  -- 0.469                                                
                         0.007                                            
                            249 38.4                                      
                                   49                                     
53  0.378                                                                 
       0.073                                                              
           -- 0.297                                                       
                  -- 0.243                                                
                         0.011                                            
                            247 78.5                                      
                                   31                                     
54  0.882                                                                 
       0.066                                                              
           -- 0.709                                                       
                  -- 0.348                                                
                         0.009                                            
                            234 57.7                                      
                                   48                                     
55  1.307                                                                 
       0.043                                                              
           -- 0.767                                                       
                  -- 0.271                                                
                         0.002                                            
                            178 23.3                                      
                                   26                                     
56  0.583                                                                 
       0.022                                                              
           -- 0.647                                                       
                  -- 0.277                                                
                         0.004                                            
                            199 23.7                                      
                                   14                                     
57  0.908                                                                 
       0.072                                                              
           -- 0.033                                                       
                  -- 0.242                                                
                         0.010                                            
                            213 36.2                                      
                                   18                                     
58  0.573                                                                 
       0.064                                                              
           -- 0.619                                                       
                  -- 0.444                                                
                         0.003                                            
                            243 40.8                                      
                                   47                                     
59  1.144                                                                 
       0.064                                                              
           -- 0.603                                                       
                  -- 0.359                                                
                         0.016                                            
                            205 14.2                                      
                                   27                                     
60  0.883                                                                 
       0.042                                                              
           -- 0.150                                                       
                  -- 0.257                                                
                         0.015                                            
                            217 39.2                                      
                                   46                                     
61  0.699                                                                 
       --  0.207                                                          
              0.955                                                       
                  -- 0.218                                                
                         0.012                                            
                            200 25.1                                      
                                   33                                     
62  1.497                                                                 
       --  0.536                                                          
              0.229                                                       
                  -- 0.117                                                
                         0.002                                            
                            222 19.8                                      
                                   33                                     
63  0.258                                                                 
       --  0.341                                                          
              0.135                                                       
                  -- 0.227                                                
                         0.007                                            
                            201 35.9                                      
                                   16                                     
64  0.468                                                                 
       --  0.360                                                          
              0.355                                                       
                  -- 0.220                                                
                         0.002                                            
                            207 51.8                                      
                                   27                                     
65  0.441                                                                 
       --  0.396                                                          
              0.595                                                       
                  -- 0.377                                                
                         0.009                                            
                            157 10.9                                      
                                   15                                     
66  0.405                                                                 
       --  0.897                                                          
              0.534                                                       
                  -- 0.168                                                
                         0.004                                            
                            172 30.7                                      
                                   48                                     
67  1.254                                                                 
       --  0.097                                                          
              0.849                                                       
                  -- 0.272                                                
                         0.010                                            
                            167 54.5                                      
                                   12                                     
68  0.646                                                                 
       --  0.016                                                          
              0.264                                                       
                  -- 0.363                                                
                         0.001                                            
                            166 61.3                                      
                                   16                                     
69  1.421                                                                 
       --  0.143                                                          
              0.434                                                       
                  -- 0.318                                                
                         0.019                                            
                            184 28.0                                      
                                   26                                     
70  1.206                                                                 
       --  0.178                                                          
              0.542                                                       
                  -- 0.404                                                
                         0.001                                            
                            191 73.5                                      
                                   11                                     
71  0.881                                                                 
       0.045                                                              
           0.012                                                          
              0.273                                                       
                  -- 0.301                                                
                         0.017                                            
                            202 32.9                                      
                                   15                                     
72  1.365                                                                 
       0.019                                                              
           0.047                                                          
              0.337                                                       
                  -- 0.272                                                
                         0.004                                            
                            242 29.6                                      
                                   11                                     
73  0.980                                                                 
       0.015                                                              
           0.162                                                          
              0.924                                                       
                  -- 0.223                                                
                         0.013                                            
                            227 74.0                                      
                                   43                                     
74  0.592                                                                 
       0.051                                                              
           0.010                                                          
              0.719                                                       
                  -- 0.404                                                
                         0.019                                            
                            227 75.1                                      
                                   20                                     
75  1.100                                                                 
       0.046                                                              
           0.178                                                          
              0.898                                                       
                  -- 0.101                                                
                         0.018                                            
                            209 79.9                                      
                                   28                                     
__________________________________________________________________________
 CS: Creep rupture strength at 650° C., 150 thousand hours;        
 VE: Charpy impact absorption energy at 0° C. after aging at       
 700° C. for 10 thousand hours;                                    
 TO: Oxidation scale thickness after 650° C., 10 thousand hour     
 hightemperature oxidation                                                
 
    
     
                       TABLE 76                                                    
______________________________________                                    
(mass %) Invention steels                                                 
No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                      
______________________________________                                    
76   0.038  0.278  0.848                                                  
                        0.046                                             
                             0.008 1.558                                  
                                        0.167                             
                                             12.46                        
                                                  0.708                   
77   0.168  0.707  0.952                                                  
                        0.030                                             
                             0.009 1.092                                  
                                        0.253                             
                                             10.45                        
                                                  0.161                   
78   0.271  0.415  0.904                                                  
                        0.020                                             
                             0.005 0.466                                  
                                        0.344                             
                                             11.15                        
                                                  0.886                   
79   0.253  0.712  0.448                                                  
                        0.022                                             
                             0.010 0.980                                  
                                        0.296                             
                                             12.97                        
                                                  0.574                   
80   0.027  0.196  0.545                                                  
                        0.045                                             
                             0.003 1.114                                  
                                        0.203                             
                                             11.93                        
                                                  0.423                   
81   0.228  0.180  0.543                                                  
                        0.036                                             
                             0.006 1.228                                  
                                        0.370                             
                                             11.44                        
                                                  0.614                   
82   0.184  0.028  0.988                                                  
                        0.022                                             
                             0.005 0.696                                  
                                        0.428                             
                                             12.41                        
                                                  0.834                   
83   0.092  0.657  0.819                                                  
                        0.012                                             
                             0.006 1.776                                  
                                        0.235                             
                                              8.22                        
                                                  0.755                   
84   0.296  0.277  0.206                                                  
                        0.031                                             
                             0.003 1.997                                  
                                        0.107                             
                                             12.43                        
                                                  0.591                   
85   0.275  0.590  0.894                                                  
                        0.049                                             
                             0.006 1.727                                  
                                        0.127                             
                                             10.93                        
                                                  0.521                   
86   0.082  0.182  0.627                                                  
                        0.013                                             
                             0.009 1.291                                  
                                        0.375                             
                                              9.62                        
                                                  0.223                   
87   0.201  0.362  0.750                                                  
                        0.049                                             
                             0.006 1.062                                  
                                        0.487                             
                                              9.47                        
                                                  0.886                   
88   0.116  0.513  0.228                                                  
                        0.026                                             
                             0.002 0.437                                  
                                        0.326                             
                                              8.93                        
                                                  0.710                   
89   0.161  0.761  0.800                                                  
                        0.011                                             
                             0.002 0.717                                  
                                        0.195                             
                                             11.38                        
                                                  0.866                   
90   0.254  0.099  0.223                                                  
                        0.031                                             
                             0.003 0.383                                  
                                        0.187                             
                                             12.86                        
                                                  0.066                   
91   0.299  0.227  0.243                                                  
                        0.015                                             
                             0.002 0.718                                  
                                        0.155                             
                                             11.73                        
                                                  0.969                   
92   0.063  0.509  0.608                                                  
                        0.049                                             
                             0.001 0.188                                  
                                        0.184                             
                                              8.92                        
                                                  0.777                   
93   0.179  0.315  0.673                                                  
                        0.020                                             
                             0.006 1.041                                  
                                        0.360                             
                                             10.41                        
                                                  0.680                   
94   0.269  0.573  0.589                                                  
                        0.018                                             
                             0.006 0.808                                  
                                        0.317                             
                                             12.14                        
                                                  0.863                   
95   0.239  0.643  0.497                                                  
                        0.048                                             
                             0.006 0.746                                  
                                        0.137                             
                                             12.49                        
                                                  0.502                   
96   0.142  0.665  0.549                                                  
                        0.019                                             
                             0.008 1.266                                  
                                        0.357                             
                                              9.91                        
                                                  0.796                   
97   0.182  0.791  0.892                                                  
                        0.018                                             
                             0.005 1.684                                  
                                        0.152                             
                                             12.81                        
                                                  0.418                   
98   0.260  0.786  0.889                                                  
                        0.031                                             
                             0.008 0.605                                  
                                        0.300                             
                                             12.25                        
                                                  0.083                   
99   0.298  0.405  0.687                                                  
                        0.049                                             
                             0.009 0.167                                  
                                        0.197                             
                                             11.60                        
                                                  0.461                   
100  0.198  0.566  0.429                                                  
                        0.017                                             
                             0.001 1.715                                  
                                        0.383                             
                                             11.77                        
                                                  0.241                   
______________________________________                                    
 
    
     
                                           TABLE 8                                 
__________________________________________________________________________
(mass %) Invention steels                                                 
                            CS  VE TO                                     
No. W  Zr  Ta Hf  Ti N   O  MPa J  μm                                  
__________________________________________________________________________
76  0.770                                                                 
       0.069                                                              
           0.562                                                          
              0.644                                                       
                  -- 0.178                                                
                         0.011                                            
                            242 32.0                                      
                                   36                                     
77  1.183                                                                 
       0.039                                                              
           0.104                                                          
              0.666                                                       
                  -- 0.321                                                
                         0.002                                            
                            195 14.8                                      
                                   30                                     
78  1.261                                                                 
       0.024                                                              
           0.157                                                          
              0.136                                                       
                  -- 0.200                                                
                         0.014                                            
                            187 40.8                                      
                                   19                                     
79  1.250                                                                 
       0.042                                                              
           0.071                                                          
              0.233                                                       
                  -- 0.487                                                
                         0.013                                            
                            235 53.2                                      
                                   49                                     
80  0.762                                                                 
       0.065                                                              
           0.019                                                          
              0.660                                                       
                  -- 0.464                                                
                         0.003                                            
                            212 31.2                                      
                                   29                                     
81  0.676                                                                 
       --  -- --  0.082                                                   
                     0.204                                                
                         0.014                                            
                            183 57.2                                      
                                   26                                     
82  0.581                                                                 
       --  -- --  0.081                                                   
                     0.380                                                
                         0.013                                            
                            167 38.9                                      
                                   46                                     
83  0.716                                                                 
       --  -- --  0.090                                                   
                     0.264                                                
                         0.004                                            
                            184 59.2                                      
                                   38                                     
84  0.695                                                                 
       --  -- --  0.083                                                   
                     0.134                                                
                         0.018                                            
                            202 62.3                                      
                                   10                                     
85  1.407                                                                 
       --  -- --  0.076                                                   
                     0.268                                                
                         0.001                                            
                            241 61.1                                      
                                   29                                     
86  1.129                                                                 
       --  -- --  0.084                                                   
                     0.137                                                
                         0.007                                            
                            218 67.7                                      
                                   21                                     
87  1.415                                                                 
       --  -- --  0.082                                                   
                     0.415                                                
                         0.017                                            
                            152 61.7                                      
                                   16                                     
88  1.211                                                                 
       --  -- --  0.040                                                   
                     0.183                                                
                         0.008                                            
                            213 20.0                                      
                                   19                                     
89  0.758                                                                 
       --  -- --  0.065                                                   
                     0.309                                                
                         0.012                                            
                            168 26.2                                      
                                   40                                     
90  0.956                                                                 
       --  -- --  0.039                                                   
                     0.473                                                
                         0.004                                            
                            238 58.4                                      
                                   24                                     
91  1.400                                                                 
       0.002                                                              
           -- --  0.069                                                   
                     0.349                                                
                         0.006                                            
                            190 60.0                                      
                                   41                                     
92  1.017                                                                 
       0.024                                                              
           -- --  0.037                                                   
                     0.210                                                
                         0.002                                            
                            243 79.0                                      
                                   16                                     
93  1.367                                                                 
       0.039                                                              
           -- --  0.050                                                   
                     0.133                                                
                         0.016                                            
                            217 58.3                                      
                                   39                                     
94  0.736                                                                 
       0.020                                                              
           -- --  0.080                                                   
                     0.195                                                
                         0.014                                            
                            179 46.5                                      
                                   36                                     
95  1.317                                                                 
       0.083                                                              
           -- --  0.052                                                   
                     0.466                                                
                         0.016                                            
                            198 55.2                                      
                                   34                                     
96  1.405                                                                 
       0.051                                                              
           -- --  0.075                                                   
                     0.218                                                
                         0.010                                            
                            194 64.5                                      
                                   41                                     
97  0.565                                                                 
       0.037                                                              
           -- --  0.073                                                   
                     0.132                                                
                         0.015                                            
                            232 49.8                                      
                                   45                                     
98  0.412                                                                 
       0.066                                                              
           -- --  0.023                                                   
                     0.482                                                
                         0.009                                            
                            237 28.1                                      
                                   14                                     
99  0.479                                                                 
       0.004                                                              
           -- --  0.027                                                   
                     0.347                                                
                         0.003                                            
                            223 45.0                                      
                                   34                                     
100 0.394                                                                 
       0.069                                                              
           -- --  0.016                                                   
                     0.396                                                
                         0.001                                            
                            162 79.7                                      
                                   14                                     
__________________________________________________________________________
 CS: Creep rupture strength at 650° C., 150 thousand hours;        
 VE: Charpy impact absorption energy at 0° C. after aging at       
 700° C. for 10 thousand hours;                                    
 TO: Oxidation scale thickness after 650° C., 10 thousand hour     
 hightemperature oxidation                                                
 
    
     
                       TABLE 9                                                     
______________________________________                                    
(mass %) Invention steels                                                 
No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                      
______________________________________                                    
101  0.019  0.256  0.274                                                  
                        0.023                                             
                             0.004 0.876                                  
                                        0.266                             
                                             11.27                        
                                                  0.616                   
102  0.100  0.392  0.765                                                  
                        0.014                                             
                             0.001 1.758                                  
                                        0.380                             
                                             10.96                        
                                                  0.270                   
103  0.061  0.732  0.450                                                  
                        0.038                                             
                             0.008 1.892                                  
                                        0.357                             
                                             11.79                        
                                                  0.383                   
104  0.249  0.253  0.688                                                  
                        0.043                                             
                             0.008 1.646                                  
                                        0.284                             
                                              9.74                        
                                                  0.154                   
105  0.282  0.275  0.824                                                  
                        0.036                                             
                             0.007 1.357                                  
                                        0.470                             
                                             10.74                        
                                                  0.763                   
106  0.212  0.148  0.595                                                  
                        0.044                                             
                             0.007 0.453                                  
                                        0.098                             
                                              8.14                        
                                                  0.897                   
107  0.099  0.043  0.977                                                  
                        0.015                                             
                             0.006 0.226                                  
                                        0.380                             
                                              8.02                        
                                                  0.757                   
108  0.284  0.478  0.899                                                  
                        0.017                                             
                             0.004 1.955                                  
                                        0.499                             
                                              8.69                        
                                                  0.513                   
109  0.237  0.048  0.922                                                  
                        0.017                                             
                             0.004 1.291                                  
                                        0.312                             
                                             11.17                        
                                                  0.856                   
110  0.167  0.070  0.793                                                  
                        0.043                                             
                             0.004 0.482                                  
                                        0.135                             
                                             11.37                        
                                                  0.489                   
111  0.123  0.680  0.262                                                  
                        0.021                                             
                             0.003 1.753                                  
                                        0.495                             
                                             12.72                        
                                                  0.084                   
112  0.156  0.085  0.278                                                  
                        0.020                                             
                             0.009 1.696                                  
                                        0.356                             
                                             11.17                        
                                                  0.827                   
113  0.038  0.770  0.994                                                  
                        0.047                                             
                             0.002 0.663                                  
                                        0.475                             
                                             10.15                        
                                                  0.416                   
114  0.108  0.533  0.552                                                  
                        0.029                                             
                             0.009 1.814                                  
                                        0.485                             
                                             10.58                        
                                                  0.526                   
115  0.274  0.302  0.304                                                  
                        0.014                                             
                             0.005 1.769                                  
                                        0.473                             
                                              8.42                        
                                                  0.851                   
116  0.015  0.624  0.410                                                  
                        0.030                                             
                             0.006 1.807                                  
                                        0.300                             
                                             12.12                        
                                                  0.587                   
117  0.255  0.467  0.614                                                  
                        0.024                                             
                             0.003 0.392                                  
                                        0.272                             
                                             10.10                        
                                                  0.910                   
118  0.177  0.373  0.265                                                  
                        0.023                                             
                             0.004 1.667                                  
                                        0.476                             
                                              8.30                        
                                                  0.017                   
119  0.272  0.483  0.721                                                  
                        0.036                                             
                             0.002 1.254                                  
                                        0.323                             
                                             10.19                        
                                                  0.590                   
120  0.156  0.164  0.520                                                  
                        0.029                                             
                             0.001 1.393                                  
                                        0.118                             
                                              9.17                        
                                                  0.815                   
121  0.030  0.730  0.614                                                  
                        0.030                                             
                             0.005 1.490                                  
                                        0.395                             
                                             11.15                        
                                                  0.877                   
122  0.239  0.382  0.759                                                  
                        0.046                                             
                             0.009 1.912                                  
                                        0.116                             
                                              9.62                        
                                                  0.474                   
123  0.111  0.044  0.701                                                  
                        0.026                                             
                             0.007 1.124                                  
                                        0.489                             
                                             10.50                        
                                                  0.281                   
124  0.283  0.645  0.990                                                  
                        0.036                                             
                             0.001 0.417                                  
                                        0.250                             
                                             12.20                        
                                                  0.493                   
125  0.226  0.762  0.575                                                  
                        0.024                                             
                             0.005 0.131                                  
                                        0.157                             
                                             10.70                        
                                                  0.298                   
______________________________________                                    
 
    
     
                                           TABLE 10                                
__________________________________________________________________________
(mass %) Invention steels                                                 
                            CS  VE TO                                     
No. W  Zr  Ta Hf  Ti N   O  MPa J  μm                                  
__________________________________________________________________________
101 0.381                                                                 
       --  0.089                                                          
              --  0.094                                                   
                     0.310                                                
                         0.017                                            
                            170 39.6                                      
                                   40                                     
102 1.346                                                                 
       --  0.057                                                          
              --  0.054                                                   
                     0.160                                                
                         0.016                                            
                            217 43.3                                      
                                   21                                     
103 0.263                                                                 
       --  0.646                                                          
              --  0.046                                                   
                     0.218                                                
                         0.012                                            
                            221 68.1                                      
                                   43                                     
104 0.645                                                                 
       --  0.097                                                          
              --  0.044                                                   
                     0.164                                                
                         0.007                                            
                            152 57.9                                      
                                   15                                     
105 1.425                                                                 
       --  0.213                                                          
              --  0.019                                                   
                     0.273                                                
                         0.005                                            
                            227 25.4                                      
                                   23                                     
106 0.904                                                                 
       --  0.941                                                          
              --  0.034                                                   
                     0.213                                                
                         0.019                                            
                            216 19.2                                      
                                   15                                     
107 1.275                                                                 
       --  0.589                                                          
              --  0.060                                                   
                     0.268                                                
                         0.006                                            
                            189 17.4                                      
                                   20                                     
108 0.827                                                                 
       --  0.257                                                          
              --  0.068                                                   
                     0.170                                                
                         0.007                                            
                            211 32.1                                      
                                   43                                     
109 1.104                                                                 
       --  0.440                                                          
              --  0.031                                                   
                     0.210                                                
                         0.015                                            
                            155 77.8                                      
                                   34                                     
110 1.060                                                                 
       --  0.545                                                          
              --  0.056                                                   
                     0.450                                                
                         0.016                                            
                            236 62.2                                      
                                   38                                     
111 0.271                                                                 
       0.039                                                              
           0.030                                                          
              --  0.024                                                   
                     0.298                                                
                         0.004                                            
                            225 67.8                                      
                                   45                                     
112 0.296                                                                 
       0.052                                                              
           0.119                                                          
              --  0.043                                                   
                     0.234                                                
                         0.010                                            
                            192 38.7                                      
                                   20                                     
113 0.729                                                                 
       0.085                                                              
           0.011                                                          
              --  0.084                                                   
                     0.334                                                
                         0.019                                            
                            202 57.0                                      
                                   13                                     
114 1.148                                                                 
       0.009                                                              
           0.066                                                          
              --  0.073                                                   
                     0.466                                                
                         0.016                                            
                            176 31.0                                      
                                   29                                     
115 1.156                                                                 
       0.041                                                              
           0.174                                                          
              --  0.073                                                   
                     0.108                                                
                         0.005                                            
                            182 79.5                                      
                                   17                                     
116 0.997                                                                 
       0.052                                                              
           0.030                                                          
              --  0.012                                                   
                     0.139                                                
                         0.017                                            
                            241 76.1                                      
                                   25                                     
117 0.921                                                                 
       0.007                                                              
           0.023                                                          
              --  0.086                                                   
                     0.432                                                
                         0.009                                            
                            219 32.3                                      
                                   20                                     
118 1.171                                                                 
       0.032                                                              
           0.149                                                          
              --  0.086                                                   
                     0.262                                                
                         0.011                                            
                            187 67.8                                      
                                   28                                     
119 1.341                                                                 
       0.089                                                              
           0.057                                                          
              --  0.030                                                   
                     0.105                                                
                         0.010                                            
                            212 19.7                                      
                                   39                                     
120 0.364                                                                 
       0.049                                                              
           0.199                                                          
              --  0.024                                                   
                     0.232                                                
                         0.010                                            
                            213 76.1                                      
                                   39                                     
121 1.085                                                                 
       --  -- 0.518                                                       
                  0.085                                                   
                     0.321                                                
                         0.002                                            
                            158 40.8                                      
                                   35                                     
122 0.507                                                                 
       --  -- 0.911                                                       
                  0.096                                                   
                     0.245                                                
                         0.010                                            
                            168 31.1                                      
                                   16                                     
123 0.813                                                                 
       --  -- 0.693                                                       
                  0.072                                                   
                     0.482                                                
                         0.010                                            
                            161 61.2                                      
                                   49                                     
124 0.958                                                                 
       --  -- 0.058                                                       
                  0.032                                                   
                     0.307                                                
                         0.015                                            
                            204 73.3                                      
                                   40                                     
125 1.176                                                                 
       --  -- 0.171                                                       
                  0.035                                                   
                     0.389                                                
                         0.018                                            
                            217 45.3                                      
                                   39                                     
__________________________________________________________________________
 CS: Creep rupture strength at 650° C., 150 thousand hours;        
 VE: Charpy impact absorption energy at 0° C. after aging at       
 700° C. for 10 thousand hours;                                    
 TO: Oxidation scale thickness after 650° C., 10 thousand hour     
 hightemperature oxidation                                                
 
    
     
                       TABLE 11                                                    
______________________________________                                    
(mass %) Invention steels                                                 
No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                      
______________________________________                                    
126  0.283  0.545  0.806                                                  
                        0.037                                             
                             0.009 0.633                                  
                                        0.067                             
                                             11.89                        
                                                  0.372                   
127  0.079  0.145  0.236                                                  
                        0.044                                             
                             0.003 0.416                                  
                                        0.070                             
                                             11.77                        
                                                  0.918                   
128  0.101  0.490  0.613                                                  
                        0.011                                             
                             0.009 0.723                                  
                                        0.246                             
                                             11.22                        
                                                  0.617                   
129  0.112  0.546  0.487                                                  
                        0.046                                             
                             0.001 1.964                                  
                                        0.230                             
                                             11.21                        
                                                  0.157                   
130  0.168  0.078  0.806                                                  
                        0.016                                             
                             0.007 0.672                                  
                                        0.102                             
                                             10.14                        
                                                  0.713                   
131  0.176  0.388  0.916                                                  
                        0.029                                             
                             0.008 1.932                                  
                                        0.406                             
                                             11.45                        
                                                  0.488                   
132  0.048  0.090  0.586                                                  
                        0.013                                             
                             0.008 0.474                                  
                                        0.325                             
                                              8.67                        
                                                  0.178                   
133  0.274  0.484  0.754                                                  
                        0.035                                             
                             0.005 1.318                                  
                                        0.165                             
                                             12.68                        
                                                  0.954                   
134  0.180  0.543  0.766                                                  
                        0.042                                             
                             0.006 1.076                                  
                                        0.457                             
                                             11.88                        
                                                  0.494                   
135  0.258  0.024  0.373                                                  
                        0.023                                             
                             0.007 1.989                                  
                                        0.391                             
                                              9.73                        
                                                  0.029                   
136  0.130  0.645  0.536                                                  
                        0.044                                             
                             0.006 0.611                                  
                                        0.278                             
                                             12.22                        
                                                  0.208                   
137  0.277  0.191  0.985                                                  
                        0.033                                             
                             0.009 0.139                                  
                                        0.415                             
                                             10.17                        
                                                  0.214                   
138  0.181  0.384  0.681                                                  
                        0.017                                             
                             0.004 1.865                                  
                                        0.224                             
                                             12.09                        
                                                  0.745                   
139  0.181  0.587  0.978                                                  
                        0.044                                             
                             0.006 1.433                                  
                                        0.235                             
                                              8.94                        
                                                  0.500                   
140  0.044  0.160  0.419                                                  
                        0.022                                             
                             0.003 0.142                                  
                                        0.082                             
                                             11.61                        
                                                  0.652                   
141  0.116  0.041  0.761                                                  
                        0.018                                             
                             0.003 1.168                                  
                                        0.072                             
                                             11.08                        
                                                  0.397                   
142  0.015  0.763  0.554                                                  
                        0.019                                             
                             0.001 1.115                                  
                                        0.076                             
                                              8.29                        
                                                  0.928                   
143  0.231  0.128  0.741                                                  
                        0.033                                             
                             0.004 1.269                                  
                                        0.393                             
                                              9.13                        
                                                  0.422                   
144  0.237  0.626  0.679                                                  
                        0.028                                             
                             0.005 1.969                                  
                                        0.434                             
                                              9.59                        
                                                  0.072                   
145  0.236  0.452  0.514                                                  
                        0.011                                             
                             0.001 1.700                                  
                                        0.059                             
                                              8.08                        
                                                  0.062                   
146  0.135  0.148  0.803                                                  
                        0.030                                             
                             0.007 1.742                                  
                                        0.254                             
                                              8.08                        
                                                  0.083                   
147  0.129  0.624  0.481                                                  
                        0.031                                             
                             0.002 0.580                                  
                                        0.425                             
                                             10.43                        
                                                  0.812                   
148  0.279  0.092  0.512                                                  
                        0.044                                             
                             0.007 0.434                                  
                                        0.209                             
                                              8.66                        
                                                  0.454                   
149  0.200  0.253  0.237                                                  
                        0.034                                             
                             0.009 0.723                                  
                                        0.138                             
                                             11.16                        
                                                  0.182                   
150  0.237  0.466  0.610                                                  
                        0.049                                             
                             0.009 1.793                                  
                                        0.296                             
                                             11.37                        
                                                  0.184                   
______________________________________                                    
 
    
     
                                           TABLE 12                                
__________________________________________________________________________
(mass %) Invention steels                                                 
                            CS  VE TO                                     
No. W  Zr  Ta Hf  Ti N   O  MPa J  μm                                  
__________________________________________________________________________
126 1.059                                                                 
       --  -- 0.733                                                       
                  0.010                                                   
                     0.207                                                
                         0.004                                            
                            219 60.8                                      
                                   11                                     
127 0.872                                                                 
       --  -- 0.945                                                       
                  0.019                                                   
                     0.237                                                
                         0.014                                            
                            229 63.5                                      
                                   46                                     
128 1.484                                                                 
       --  -- 0.472                                                       
                  0.033                                                   
                     0.156                                                
                         0.015                                            
                            214 30.4                                      
                                   49                                     
129 1.198                                                                 
       --  -- 0.523                                                       
                  0.070                                                   
                     0.432                                                
                         0.018                                            
                            246 10.9                                      
                                   50                                     
130 0.892                                                                 
       --  -- 0.752                                                       
                  0.055                                                   
                     0.346                                                
                         0.001                                            
                            190 51.7                                      
                                   34                                     
131 1.487                                                                 
       0.008                                                              
           -- 0.873                                                       
                  0.050                                                   
                     0.167                                                
                         0.003                                            
                            166 61.5                                      
                                   11                                     
132 0.625                                                                 
       0.059                                                              
           -- 0.091                                                       
                  0.075                                                   
                     0.180                                                
                         0.018                                            
                            245 19.4                                      
                                   47                                     
133 0.490                                                                 
       0.011                                                              
           -- 0.559                                                       
                  0.099                                                   
                     0.453                                                
                         0.013                                            
                            193 77.2                                      
                                   49                                     
134 1.317                                                                 
       0.029                                                              
           -- 0.965                                                       
                  0.049                                                   
                     0.263                                                
                         0.011                                            
                            230 73.7                                      
                                   26                                     
135 1.413                                                                 
       0.081                                                              
           -- 0.038                                                       
                  0.034                                                   
                     0.482                                                
                         0.005                                            
                            175 57.5                                      
                                   24                                     
136 1.023                                                                 
       0.035                                                              
           -- 0.412                                                       
                  0.068                                                   
                     0.392                                                
                         0.007                                            
                            195 75.0                                      
                                   32                                     
137 0.370                                                                 
       0.012                                                              
           -- 0.798                                                       
                  0.080                                                   
                     0.257                                                
                         0.012                                            
                            211 70.9                                      
                                   16                                     
138 1.018                                                                 
       0.078                                                              
           -- 0.887                                                       
                  0.040                                                   
                     0.360                                                
                         0.015                                            
                            240 22.6                                      
                                   25                                     
139 0.601                                                                 
       0.053                                                              
           -- 0.365                                                       
                  0.056                                                   
                     0.174                                                
                         0.010                                            
                            202 13.2                                      
                                   25                                     
140 1.318                                                                 
       0.055                                                              
           -- 0.912                                                       
                  0.010                                                   
                     0.373                                                
                         0.018                                            
                            214 55.7                                      
                                   24                                     
141 1.461                                                                 
       --  0.893                                                          
              0.227                                                       
                  0.099                                                   
                     0.433                                                
                         0.005                                            
                            197 59.3                                      
                                   30                                     
142 1.291                                                                 
       --  0.389                                                          
              0.493                                                       
                  0.025                                                   
                     0.233                                                
                         0.005                                            
                            219 36.6                                      
                                   31                                     
143 1.031                                                                 
       --  0.277                                                          
              0.404                                                       
                  0.029                                                   
                     0.442                                                
                         0.010                                            
                            199 70.7                                      
                                   14                                     
144 0.928                                                                 
       --  0.128                                                          
              0.139                                                       
                  0.039                                                   
                     0.339                                                
                         0.013                                            
                            155 69.5                                      
                                   45                                     
145 0.579                                                                 
       --  0.089                                                          
              0.880                                                       
                  0.072                                                   
                     0.199                                                
                         0.003                                            
                            206 11.7                                      
                                   34                                     
146 0.875                                                                 
       --  0.348                                                          
              0.329                                                       
                  0.098                                                   
                     0.297                                                
                         0.017                                            
                            221 49.5                                      
                                   27                                     
147 1.417                                                                 
       --  0.113                                                          
              0.662                                                       
                  0.045                                                   
                     0.100                                                
                         0.013                                            
                            168 21.4                                      
                                   11                                     
148 1.142                                                                 
       --  0.247                                                          
              0.075                                                       
                  0.020                                                   
                     0.494                                                
                         0.004                                            
                            217 10.9                                      
                                   15                                     
149 0.288                                                                 
       --  0.022                                                          
              0.243                                                       
                  0.040                                                   
                     0.149                                                
                         0.013                                            
                            185 64.9                                      
                                   25                                     
150 0.401                                                                 
       --  0.372                                                          
              0.357                                                       
                  0.076                                                   
                     0.337                                                
                         0.005                                            
                            242 58.3                                      
                                   10                                     
__________________________________________________________________________
 CS: Creep rupture strength at 650° C., 150 thousand hours;        
 VE: Charpy impact absorption energy at 0° C. after aging at       
 700° C. for 10 thousand hours;                                    
 TO: Oxidation scale thickness after 650° C., 10 thousand hour     
 hightemperature oxidation                                                
 
    
     
                       TABLE 13                                                    
______________________________________                                    
(mass %) Invention steels                                                 
No.  C      Si     Mn   P    S     Nb   V    Cr   Mo                      
______________________________________                                    
151  0.157  0.497  0.978                                                  
                        0.037                                             
                             0.010 0.234                                  
                                        0.321                             
                                             12.72                        
                                                  0.652                   
152  0.154  0.087  0.687                                                  
                        0.026                                             
                             0.008 1.123                                  
                                        0.289                             
                                              8.62                        
                                                  0.764                   
153  0.144  0.189  0.303                                                  
                        0.023                                             
                             0.001 1.021                                  
                                        0.072                             
                                             10.12                        
                                                  0.231                   
154  0.176  0.143  0.360                                                  
                        0.015                                             
                             0.009 0.282                                  
                                        0.193                             
                                              8.02                        
                                                  0.213                   
155  0.159  0.104  0.608                                                  
                        0.010                                             
                             0.002 1.169                                  
                                        0.288                             
                                             10.81                        
                                                  0.401                   
156  0.010  0.619  0.814                                                  
                        0.016                                             
                             0.008 1.219                                  
                                        0.230                             
                                              9.53                        
                                                  0.516                   
157  0.069  0.270  0.320                                                  
                        0.020                                             
                             0.007 0.757                                  
                                        0.317                             
                                             10.02                        
                                                  0.178                   
158  0.246  0.373  0.476                                                  
                        0.049                                             
                             0.010 0.142                                  
                                        0.186                             
                                             10.76                        
                                                  0.205                   
159  0.159  0.680  0.631                                                  
                        0.032                                             
                             0.008 0.675                                  
                                        0.143                             
                                             10.13                        
                                                  0.649                   
160  0.152  0.556  0.529                                                  
                        0.035                                             
                             0.004 1.745                                  
                                        0.275                             
                                              9.10                        
                                                  0.748                   
______________________________________                                    
 
    
     
                                           TABLE 14                                
__________________________________________________________________________
(mass %) Invention steels                                                 
                            CS  VE TO                                     
No. W  Zr  Ta Hf  Ti N   O  MPa J  μm                                  
__________________________________________________________________________
151 0.429                                                                 
       0.091                                                              
           0.090                                                          
              0.113                                                       
                  0.055                                                   
                     0.338                                                
                         0.002                                            
                            206 37.3                                      
                                   16                                     
152 0.610                                                                 
       0.090                                                              
           0.715                                                          
              0.686                                                       
                  0.085                                                   
                     0.421                                                
                         0.017                                            
                            197 51.9                                      
                                   12                                     
153 0.554                                                                 
       0.012                                                              
           0.131                                                          
              0.772                                                       
                  0.091                                                   
                     0.269                                                
                         0.017                                            
                            183 43.9                                      
                                   24                                     
154 0.478                                                                 
       0.071                                                              
           0.497                                                          
              0.062                                                       
                  0.047                                                   
                     0.350                                                
                         0.014                                            
                            231 26.6                                      
                                   40                                     
155 1.283                                                                 
       0.091                                                              
           0.492                                                          
              0.726                                                       
                  0.013                                                   
                     0.340                                                
                         0.003                                            
                            212 13.8                                      
                                   29                                     
156 1.301                                                                 
       0.090                                                              
           0.202                                                          
              0.490                                                       
                  0.053                                                   
                     0.120                                                
                         0.005                                            
                            245 53.5                                      
                                   16                                     
157 0.741                                                                 
       0.085                                                              
           0.361                                                          
              0.510                                                       
                  0.048                                                   
                     0.164                                                
                         0.015                                            
                            199 19.2                                      
                                   31                                     
158 1.158                                                                 
       0.046                                                              
           0.140                                                          
              0.980                                                       
                  0.015                                                   
                     0.147                                                
                         0.010                                            
                            242 28.4                                      
                                   20                                     
159 0.833                                                                 
       0.091                                                              
           0.089                                                          
              0.591                                                       
                  0.015                                                   
                     0.339                                                
                         0.009                                            
                            192 24.2                                      
                                   31                                     
160 0.668                                                                 
       0.038                                                              
           0.034                                                          
              0.026                                                       
                  0.020                                                   
                     0.218                                                
                         0.008                                            
                            218 54.3                                      
                                   10                                     
__________________________________________________________________________
 CS: Creep rupture strength at 650° C., 150 thousand hours;        
 VE: Charpy impact absorption energy at 0° C. after aging at       
 700° C. for 10 thousand hours;                                    
 TO: Oxidation scale thickness after 650° C., 10 thousand hour     
 hightemperature oxidation                                                
 
    
     
                                           TABLE 15                                
__________________________________________________________________________
Comparison steels (mass %)                                                
No. C  Si  Mn P   S  Nb  V  Cr  Mo W                                      
__________________________________________________________________________
161 0.02                                                                  
       0.06                                                               
           0.45                                                           
              0.014                                                       
                  0.008                                                   
                     0.051                                                
                         0.20                                             
                            8.53                                          
                                0.592                                     
                                   0.87                                   
162 0.07                                                                  
       0.07                                                               
           0.57                                                           
              0.012                                                       
                  0.004                                                   
                     0.066                                                
                         0.22                                             
                            8.99                                          
                                0.424                                     
                                   0.88                                   
163 0.23                                                                  
       0.10                                                               
           0.52                                                           
              0.016                                                       
                  0.004                                                   
                     0.074                                                
                         0.18                                             
                            9.05                                          
                                0.560                                     
                                   0.86                                   
164 0.15                                                                  
       0.13                                                               
           0.45                                                           
              0.011                                                       
                  0.002                                                   
                     0.033                                                
                         0.19                                             
                            9.23                                          
                                0.550                                     
                                   0.77                                   
165 0.29                                                                  
       0.12                                                               
           0.98                                                           
              0.008                                                       
                  0.001                                                   
                     0.127                                                
                         0.08                                             
                            12.1                                          
                                0.320                                     
                                   0.11                                   
166 0.15                                                                  
       0.09                                                               
           0.24                                                           
              0.009                                                       
                  0.001                                                   
                     0.290                                                
                         0.45                                             
                            12.7                                          
                                0.007                                     
                                   1.80                                   
167 0.18                                                                  
       0.09                                                               
           0.44                                                           
              0.009                                                       
                  0.001                                                   
                     0.014                                                
                         0.31                                             
                            11.4                                          
                                0.989                                     
                                   0.99                                   
168 0.11                                                                  
       0.12                                                               
           0.74                                                           
              0.004                                                       
                  0.001                                                   
                     2.880                                                
                         0.22                                             
                            11.8                                          
                                0.679                                     
                                   0.67                                   
169 0.09                                                                  
       0.21                                                               
           0.81                                                           
              0.044                                                       
                  0.002                                                   
                     0.321                                                
                         0.20                                             
                            10.5                                          
                                0.814                                     
                                   0.23                                   
170 0.10                                                                  
       0.23                                                               
           0.99                                                           
              0.024                                                       
                  0.001                                                   
                     0.545                                                
                         0.19                                             
                            10.0                                          
                                0.333                                     
                                   0.77                                   
171 0.22                                                                  
       0.21                                                               
           0.11                                                           
              0.012                                                       
                  0.001                                                   
                     1.227                                                
                         0.40                                             
                            9.76                                          
                                0.545                                     
                                   1.21                                   
172 0.07                                                                  
       0.27                                                               
           0.07                                                           
              0.013                                                       
                  0.009                                                   
                     1.621                                                
                         0.49                                             
                            9.00                                          
                                0.512                                     
                                   1.09                                   
173 0.23                                                                  
       0.23                                                               
           0.09                                                           
              0.002                                                       
                  0.010                                                   
                     1.998                                                
                         0.07                                             
                            8.86                                          
                                0.533                                     
                                   1.15                                   
__________________________________________________________________________
 
    
     
                       TABLE 16                                                    
______________________________________                                    
Comparison steels (mass %)                                                
                                        CS   VE   TO                      
No.  Zr     Ta     Hf   Ti   N     O    MPa  J    (μm)                 
______________________________________                                    
161  0.007  0.65   --   0.034                                             
                             0.072 0.007                                  
                                        120  70   760                     
162  0.008  0.77   --   0.031                                             
                             0.081 0.009                                  
                                        107  28   660                     
163  0.002  0.78   --   0.044                                             
                             0.872 0.007                                  
                                        205  6    50                      
164  0.012  0.71   0.66 0.100                                             
                             0.525 0.002                                  
                                        185  3    35                      
165  0.011  0.76   0.87 0.010                                             
                             0.164 0.002                                  
                                         65  95   35                      
166  --     --     0.81 --   0.128 0.002                                  
                                         90  2    20                      
167  --     --     0.59 --   0.154 0.001                                  
                                         70  60   25                      
168  --     --     --   0.060                                             
                             0.332 0.002                                  
                                        116  4     5                      
169  0.145  0.89   --   0.071                                             
                             0.425 0.002                                  
                                        187  4    25                      
170  --     1.21   --   0.032                                             
                             0.202 0.002                                  
                                        153  7    40                      
171  0.011  0.32   1.13 --   0.191 0.008                                  
                                        220  7    45                      
172  0.540  0.05   0.22 0.29 0.103 0.012                                  
                                        210  8    15                      
173  0.880  --     0.10 --   0.200 0.006                                  
                                         24  2    30                      
______________________________________                                    
 CS: Creep rupture strength at 650° C., 150 thousand hours;        
 VE: Charpy impact absorption energy at 0° C. after aging at       
 700° C. for 10 thousand hours;                                    
 TO: Oxidation scale thickness after 650° C., 10 thousand hour     
 hightemperature oxidation                                                
 
    
     The present invention provides a high-nitrogen ferritic heat-resisting steel with high Nb content exhibiting a high rupture strength after prolonged creep and superior high-temperature oxidation resistance and, as such, can be expected to make a major contribution to industrial progress.