Method for producing a high strength bolt
United States Patent [191 Shimizu et al.  
 [451 Apr. 15, 1975 [75] Inventors: Kiyoshi Shimizu; Toshihiro Minami,  
 both of Kobe; Eiji Niina, Nishinomiya, all of Japan [73] Assignee: Kobe Steel, Ltd., Kobe, Japan [22] Filed: Sept. 4, 1973 [21] Appl. No.: 394,134  
 [30] Foreign Application Priority Data 3,674,570 7/1972 Hallstrom et al. 148/12 Primary ExaminerLowell A. Larson Attorney, Agent, or Firm-Oblon, Fisher, Spivak, McClelland &amp; Maier [57] ABSTRACT A method for producing a high strength bolt from a carbon steel or a low alloy steel material, which comprises the steps of subjecting said material, in turn, to cold working at the reduction-of-area percentage of 10 percent and over rapid heating to a temperature range from 450C to A transformation point, warmforming to a bolt shape and air-cooling or cooling at a cooling rate higher than that of the air-cooling. The steel material adapted for use herein includes a steel having a pearlite structure or a tempered martensite structure, and particularly the steel having the latter structure presents excellent resistance to the delayed rupture phenomenon with an accompanied high tensile strength of over 100 kg/mm particularly, in the range from 130 to 140 kg/mm and over.  
 10 Claims, 5 Drawing Figures C(AISI I027 STEEL) 0F BOLTS SUBJECTED T0 WARM FORMING AT 550C PATENTEDAPR I 51975 3,877 281 SHEET 1 0T 3 FIG. 1A  
 MATERIAL SUBJECTED T0 43% WIRE- DRAWING (YP76.5 TS88.4 EL|7.4 RAGLO) FIG. 1B  
 MATERIAL SUBJECTED T0 43% WIRE- DRAW- ms AND THEN STRENGTHENING TREATMENT AT 550% (YPns T5925 EL2|5 mgmismzsms mfg? 3 3.877. 281  
 HEAD PORTION SHANK PORTION TEMPERATURE m WARMFORMING (c) FIG. 2  
 METHOD FOR PRODUCING A HIGH STRENGTH BOLT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for producing a high strength bolt from a carbon steel or a low alloy steel material which has been subjected to hot-rolling, normalizing or annealing, thereby presenting a ferritepearlite structure or which has been subjected to hardening and tempering, thereby presenting a tempered martensite structure, and more particularly to a method which comprises the steps of subjecting said steel material, in turn, to cold working at a reductionof-area percentage of percent and over, rapid heating to a temperature range from 450 C to A transformation point, hot forming to a bolt shape and air cooling or cooling at a cooling rate higher than that of the air-cooling, thereby presenting a high strength bolt having a tensile strength of 70 kg/mm and over. (Meant by the term, strengthening treatment as used herein is a combination of working and heat-treatment which includes a cycle of cold working, rapid heating and rapid cooling.)  
 2. Description of the Prior Art I-Iithereto, a method for producing a high strength bolt has recourse to the steps wherein, to form a bolt shape, a cold-or-hot-working or machining operation is used, followed by a refining treatment such as hardening and tempering, thus obtaining a desired strength and toughness.  
  However, such a method for producing a high strength bolt tends to incur problems in quality assurance of products. The conventional method necessarily dictates the use of a refining treatment at an elevated temperature, such as hardening and tempering after forming into a bolt shape, such that such a refining treatment results in the necessity for close adjustment of the atmosphere used in a furnace. Unless such an atmosphere furnace is used, there will result an oxidation and decarbonization phenomena on the surface layer of the bolt, thereby bringing about a wider range of variation in strength with the resultant failure to present a stable level of quality for the bolts produced.  
  In general, the production of a high strength bolt having a tensile strength exceeding the range of 120 kg/mm to 130 kg/mm has been deemed difficult, because when such a bolt is subjected to tension under a substantial static load for a certain period of time, there tends to occur a delayed rupture phenomenon due to a sudden occurrence of embrittlement, with the appearance thereof exhibiting no plastic deformation.  
  Accordingly, it remains desirable to have a method for producing a high strength bolt which has a high tensile strength but does not create a delayed rupture phenomenon due to the sudden occurrence of embrittlement.  
 SUMMARY OF THE INVENTION It is accordingly an object of the present invention to provide a method for producing a high strength bolt which dispenses with the use of a refining treatment using an atmosphere furnace as well as heat-treatment at an elevated temperature, yet providing the quality equivalent or superior to that of a high strength bolt which is produced using a conventional refining treatment.  
  Briefly, according to one embodiment of the present invention, there is provided a method for producing a high strength bolt from a carbon steel or low alloy steel material, wherein, for the production of a high strength bolt having a tensile strength ranging from to kglmm a steel wire (rod) as rolled or as normalized and annealed is in turn subjected to cold working at a reduction-of-area percentage of 10 percent and over, heating at a heating rate of over 50C/min. to a temperature range from 450 C to A, transformation point by using heating means such as resistance heating under air, high frequency induction heating, flame heating, or the like, and then hot-forming to a bolt shape, followed by air cooling or cooling at a cooling rate higher than that of the air cooling. For the production of a high strength bolt having a tensile strength of over 100 kg/mm a carbon steel or low alloy steel material in a wire or rod form is subjected in turn to a refining treatment of hardening and tempering, cold working at a reduction-of-area percentage of 10 percent and over, heating at a heating rate of over 50 C/min. to a temperature range from 450 C to A transformation point by using the aforesaid heating means, Warm-forming to a bolt shape and finally air-cooling or cooling at a cooling rate higher than that of the air cooling. The level of tensile strength referred to herein is not necessarily limited to that exceeding 110 kg/mm A high strength bolt, which is made of carbon steel or low alloy steel material and which is produced according to the method of the present invention and has a tensile strength of over 110 kg/mm presents excellent resistance to the delayed rupture phenomenon due to a sudden occurrence of embrittlement, and particularly presents a bolt which has a tensile strength of a range from to kg/mm and over, affording excellent resistance to the delayed rupture pheonomenon, as compared with conventional bolts subjected to the refining treatment.  
  BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an electron-microscopic picture at 22,50OX, of the structure of a steel A as rolled and the structure of the steel A which has been subjected to 43 percent wire-drawing, followed by a strengthening treatment at 550C;  
  FIG. 2 is a plot showing the relationship of the temperature used in hot-forming a rolled steel A to a bolt shape versus the hardness of a bolt at bolt head and shank;  
  FIG. 3 is a diagram showing distribution of hardness at bolt head and shank, in connection to the bolt produced according to the present invention and the bolt produced according to the prior art (cold-forming); and  
  FIG. 4 is a diagram showing distribution of hardness at bolt head and shank of a bolt produced from the refined steel C according to the present invention.  
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The types of steels applicable to the method of the present invention should preferably be carbon steels having a carbon content of no more than 0.5 percent and low alloy steels.  
  The reason why the content of carbon should be no more than 0.5 percent is as follows: Although a high strength bolt having a tensile strength of 100 to 120 kg/mm may be produced of rolled or tempered steels, the use of low and medium carbon steels rather than high carbon steels is best suited and advantageous for the method of the present invention using the strengthening treatment from the viewpoints of tensile strength and toughness. Carbon is an essential component for a material for use in producing a high strength bolt having a tensile strength of over 110 kg/mm for the purpose of applying a refining treatment of hardening and tempering as well as for achieving arequired hardenability and strength. However, from the viewpoint of the delayed rupture phenomenon, the content of the carbon should preferably be no more than 0.5 percent. Although the content of other elements to be added are not specifically limited, it is preferable that a small amount of elements such as Al, N, Ti, Nb, etc. may be added for achieving a finer grain size of austenite crystals.  
  The steel material, of which the high strength bolt having a tensile strength of over 110 kg/mm is made, has a tempered martensite structure which has been subjected to hardening and annealing treatments. In this respect, it should be noted that the tempering to be used depends on desired strength and toughness of the steel, and the workability of the subsequent working steps of the steel material, and it should also be recognized that the hardening treatment is effective for achieving the desired resistance to the delayed rupture pheonomenon, if the grain size of the austenite crystals over ASTM No. is obtained by utilizing a rapid heating for achieving finer grain size, in addition to the ordinary heat-treatment.  
  The reason why the reduction-of-area percentage of the cold working should be over 10 percent is that an increase in strength of a bolt shank portion, which has been warm-worked, may be expected from a rapid heating by using air-atmosphere resistance-heating and high frequency induction heating during the warmworking, whereas the strength in the bolt head portion will be decreased due to the aforesaid warm-working. As a result, in order to accommodate such incompatibility, the reduction-of-area percentage of 10 percent and over is required for achieving uniformity in the percent is difficult to apply to a martensite steel. For this reason, the reduction-of-area percentage of cold working of over 10 percent is suited for rolled steel material or normalized steel material, while the reductionof-area percentage of cold working of 10 percent to percent is suited for a hardened and tempered steel. Cold working as used herein should preferably be cold drawing (wire drawing), but is not limited thereto. For instance, the cold workingused may include roller-dies working.  
  The temperature as used for heating in the present invention should fall in a range from 450 C to A transformation point. This is because the heating temperature of a range from 449 C to 250 C tends to cause cracking in the head portion of a bolt, since the heating temperature is within the range of blue shortness temperatures, while heating temperatures of over the A, transformation point will lead to a poor strengthening effect. Meant by the heating temperature is the temperature obtained by using electric-resistance heating means, high frequency induction heating means, etc.  
  Alternatively, in case the strength of a bolt thus produced can not satisfy the value required, then the bolt may be rapidly heated in a salt bath, lead bath, air atmosphere furnace, or the like, which has been maintained at a temperature ranging from 450C to 700C, and soaked for no more than 5 minutes therein, followed by air cooling or cooling at a cooling rate higher than that of the air cooling, thereby achieving the desired strengthening effect.  
  As is apparent from the foregoing description, the method of the present invention can obviate the use of heat treatment at an elevated temperature, such as hardening and tempering, after forming to a bolt shape as in the conventional method. In addition, since the temperature for warm-forming or re-heating treatment is limited to 700C, which is much lower than the hardening temperatures, there can be achieved improvements in the skin condition as well as dimensional accuracy for the bolts, without incurring the possibility of oxidation or decarbonization, thereby enabling the production of consistent high quality bolts.  
  The present invention will now be described in more detail with reference to the ensuing examples.  
  Table 1 shows the chemical composition of steel wires tested.  
 Table 1 Chemical Composition of Samples Si Mn P S C Cr Mo B A A151 1024 Steel 0.22 0.28 1.55 0.021 0.020 A151 1030 Steel 0.38 0.23 0.75 0.022 0.016 A151 1037 Steel 0.25 0.23 1.51 0.017 0.015  
 AlSl 1035 Steel 0.35 0.24 0.84 0.016 0.024 0.13 0.23 0.0016 A181 4510 Steel 0.37 0.26 0.76 0.014 0.020 1.07 0.20  
 strength of a bolt. Furthermore, although the efficacy of the strengthening treatment, according to the method of the present invention, depends on the types of the materials to be worked, the increase in the strength of a bolt is lower in the case of the reductionof-area percentage of below 10 percent, while the working at the reduction-of-area percentage of over 40 In the above Table l, steels A and B represent the samples which were subjected in turn to cold wiredrawing at a reduction-of-area percentage of 5 to 43 percent, heating at a heating rate of 40C/sec. by using an electric heating means, warm-forming to M10 bolt (shank: 48 L) and water-quenching immediately thereafter. In addition to thisQthe bolts thus produced were again placed in a salt bath which had been heated to a temperature of 550C and held therein for 20 seconds, followed by water quenching.  
  The shank portions of the M bolts were machined to a diameter of 8.0 and then subjected to No. 4 tensile test applicable to bolts. The results of the test are shown in Table 2.  
 Table 2 i Reduction-of-area percentage of cold wire-drawing versus mechanical properties Reduction Yield Tensile Elonga- Final Type Production -of-area point strength tion reduction of 7: of cold 4\/ -of-area Steel condition wire dra- (kg/mm (kglmm (71) percenwing (71) tage Hot-forming 10 69.6 77.2 27.0 62.3  
 AlSl 76.5 87.0 24.0 61.5  
 Hot-forming 66.7 75.6 28.0 64.2  
 and reheat- 74.8 81.0 25.5 62.0  
 ing treat 20 77.8 87.5 25.0 59.5  
 ment 77.8 86.5 23.8 58.7  
 Hot-forming 10 69.6 81.8 18.0 42.7  
 Hot-forming 5 67.1 77.8 20.0 45.0  
 and reheat- 10 70.6 82.0 18.0 43.7  
 ing treat- 72.4 83.6 18.5 41.5  
 ment 20 77.2 90.9 15.0 38.6  
  As can be seen from Table 2, cold wire-drawing prior to warm-forming exerts a great effect on the mechanical properties of bolts which have been warm-formed. The tensile strength and yield point show an increase with an increase in the reduction-of-area percentage of cold working, despite the heating used for warmforming. On the other hand, elongation, and toughness required for the final reduction-of-area percentage show a tendency to slightly decreasewith the increase in strength. Another fact is that bolts, which have been re-heated, present improved strength, as compared with those which have not been subjected to re-heating treatment.  
  FIG. 1 shows the relationship of the mechanical properties and the electron-microscopic pictures of materials which are made of rolled steel of sample A and have been subjected to 43 percent cold wirepared with a cold worked steel. Thus, the aforesaid fact is considered to have bearing on the increase in strength of bolts, because the shank portion of a bolt does not undergo the influence by hot-forming and hence retains improved properties given by the strengthening treatment of the present invention.  
 Table 3 Heating Rate Versus Mechanical Properties Tpye of Heating Yield Tensile Elonga- Final Reduc- Steel Rate Point Strength tion tion-of-area (kglmm (kg/mm 4 A percentage(%) C/sec. 83.7 97.3 21.0 42.0 83.3 96.5 21.3 42.3 40C/sec. 83.0 97.0 21.0 41.8 82.8 96.3 21.5 42.5  
 Table 3 Continued Heating Rate Versus Mechanical Properties Yield FIG. 2 shows the relationship of a warm-forming temperature versus hardness of head portions and shank portions of bolts which were produced by subjecting steel A, in turn, to cold wire-drawing at a reduction-ofarea of 20 percent, rapid heating at a heating rate of 40C/sec., and forming to M10 bolt, followed by water quenching.  
  FIG. 2 reveals that the relationship of the difference in reduction-of-area percentage is maintained in fact for the difference in hardness for the bolts which have been cold formed, because the head portion of a cold formed bolt has been subjected to cold working at about 75 percent reduction-of-area percentage, whereas the shank portion of the bolt has been subjected to cold wire drawing of only percent reduction-of-area percentage. In such cold working, the head least difference in hardness between the shank and head portions of the bolt. With such a bolt, there occurs normal rupture at the thread portion, when the bolt ruptures due to a great load. Accordingly, such a bolt is preferable. The cooling after warm-forming should terminate as rapidly as possible. In other words, if a bolt is slowly cooled, then the bolt will be annealed, thereby failing to present the tensile strength of over 80 kg/mm which is essential for the high strength bolt.  
  Table 4 shows the tensile test results of bolts which have been produced by heating the steel B to 550C at a heating rate of C/sec. and hot-forming the same to M10 bolt, and subjecting the bolt to three types of cooling, i.e., water-quenching, air-cooling and slow cooling. This test reveals that the slow cooling results in the failure to obtain a high strength bolt.  
 Table 4 Cooling Conditions After Hot-Forming Versus Mechanical Properties portion of the bolt gives extremely great hardness and high strength as compared with those of the shank portion, while presenting reduction in toughness therewith. Accordingly, it is not preferable that the rupture occurs at the neck portion of the bolt, in case the bolt .is subjected to a great load and broken, in contrast to the normal rupture at the thread portion. As the forming temperature is increased, there results a lesser difference in hardness between the head and shank por- &#34;tions of the bolt. Although the increase in hardness of the shank portion of the bolt depends on the reductionof-area percentage of cold working, soaking time at a heating temperature and cooling rate, such an increase shows a peak at 400C and thereafter gradually goes down as the temperature becomes higher than the peak temperature. On the other hand, the hardness of the head portion of the bolt varies, to some extent, with the varying reduction-of-area percentage of cold-wiredrawing and compression percentage of the head portion of the bolt, while the hardness increases in temperature range from 200C to 300C with the increase in warm-forming temperatures, but decreases thereafter, showing a sharp decrease at 400C thus presenting the In such cooling, air cooling may be suitably used for a bolt of a small diameter, whereas a bolt of a great diameter should be subjected to oil cooling or water cooling.  
  After being subjected to the method for producing a high strength bolt according to the present invention, if a bolt is still short of the strength required, then the bolt may be re-heated to achieve the required strength.  
  For reheating, a bolt is placed in a salt bath or lead bath which has been heated to a temperature of 450C to 700C, then allowed to stand therein for 5 minutes,  
 and quenched.  
  In this respect, the re-heating should be carried out at a high heating rate and thus the bolt should be directly charged into a heating bath. The soaking temperature of the bolt should be limited to a range between 450C and 700C, because a temperature over 450C can avoid the blue shortness, while a temperature below 700C can prevent a decrease in strength. While the soaking time depends on the type of heating furnace, size of bolt, heating temperatures and the like, the shorter soaking time is preferable because of its greater effectiveness, and thus limited up to 5 minutes.  
  Table shows the relationship of the mechanical properties and soaking time when M bolt is reheated to 450C in an air atmosphere furnace, the M10 bolt being produced from a steel wire B according to the method of the invention.  
  In this connection, cooling after re-heating treatment should be accelerated to a cooling rate higher than that of air cooling, because slow cooling results in an annealed bolt having a low strength, as in the case of hotforming.  
  The steel wires, as used in the present invention, should include those as rolled or normalized, and the type of steel should cover carbon steel or low alloy steels which are generally used as materials for producing bolts. v  
  The rolled steels of two types, i.e., steel A and steel B, as shown in Table l, were cold-drawn&#39;at a reductionof-area percentage of 5 to 20 percent, then heated at a heating rate of 40C/sec. in an electric furnace, warm-formed to M10 bolt and water-quenched immediately thereafter. On the other hand, part of the bolts thus treated were directly charged into a salt bath which has been heated to 550C for re-heating treatment, allowed to stand therein for 20 seconds, and then water quenched. Those M10 bolts thus prepared were subjected toa-tensile test using a wedge. The results thereof are shown in Tables 6 and 7.  
 Table 6 Results of Tensile Test Using A Wedge for Bolts of Steel A&#39; Production Condition Wedge Angle 0 Wedge Angle l0 Remarks Reductionof-area percentage Tensile Tensile in wire draw- Strength Rupture Strength Rupture ing (7r) Forming (kg/mm&#34;) Position (kg/mm). Position 5 74.1 Thread 72.6 Thread Comparative 74.5 Portion 72.0 Portion Example Warm l0 &#39;Forming 80.0 79.2 This lnven- (550C) 80.8 78.6 tion 20 86.7 83.0 This lnven- 86.l 83.9 tion 5 77.2 76.0 Comparative Warm 76.8 75.5 Example Forming l0 .(550C) 83.9 81.0 &#34;lnven- This invenand Re- 83.0 80.6 tion Heating 20 (550C) 89.9 t 87.5 This lnven&#39; 89.4 88.0 tion 10 Cold 75.0 71.5 Prior Forming 74.5 70.5 Art Table 7 Results of Tensile Test With Wedge for Bolts of Steel Production Condition Wedge Angle 0 Wedge Angle l0 Remarks Reductionof-area percentage Tensile Tensile in wire draw- Strength Rupture Strength Rupture ing Forming (kglmm Position (kg/mm) Position 5 75.3 Thread 74.2 Thread Comparative 85.8 Portion 73.3 Portion Example Warm l0 Forming 83.3 81.6 This lnven- (550C) 83.0 82.2 tion 20 90.5 88.9 This Inven- 9l.3 89.9 tion 5 Warm 79.4 77.8 Comparative Forming 78.8 76.8 Example (550C) 10 and Re- 84.6 83.0 This Inven- Heating 85.3 82.7 tion (550C) 20 93.0 90.9 This Inven- 92.3 i f 91.5 tion l0 Cold 76.5 72.5 Prior Forming 76.0 73.0 Art FIG. 3 shows the hardness distribution at the heads and shanks of M10 bolts of two groups, one of which has been subjected in turn to cold water-drawing at a reduction-of-area percentage of 20 percent and warm- Table 8 Continued Heating Rate Versus Mechanical Properties (Sample steel C&#39;) forming at 550C according to the present invention, 5 Heating Yield Tensile Elonga- Final Reductionand the other of which has been subjected to cold Rate Point Strength Of-area P working according to the conventional method. The (kglmmz) (kg/ninja) 4 A age test results reveal that the high strength bolts produced, HOT/min. 851 1005. 2L] 6&amp;8 according to the present invention,&#39;as shown in Tables 59C/rpi 83.5 98-3 21.5 09.7 6 and 7 and FIG. 3, present a lesser hardness difference 10 5 C/mm&#39; 22D at the head and shank portions of the bolts, as compared with bolts produced according to the conventional method as well as the bolts produced for comparison purpose, while the former exhibits normal rupture. As is apparent from Table 8, the greater the heating The sample steels, C, D and E, as shown in Table 1, l5 rateat warm-forming, the higherwill be the strength. were subjected to a refining treatment of hardening and Accordingly, a heating rate of at least over 50C/min. tempering, thereby attaining the tensile strength of should be adopted for attaining the desired increase in over 100 kg/mm to obtain high strength bolts. strength. 1  
  The sample steel C was oil-hardened at 870C and Sample steel C was subjectedin turn to a refining subjected to a refining treatment of tempering at treatment of oil hardening at 870C, then tempering at 570C, thus attaining mechanical properties as a re- 570C, then: to the prelimiary cold working at a reducfined steel, as shown in Table 4. Thereafter, the steel tion-of-area percentageof 20 percent, heating to 550C C was subjected to preliminary cold wire drawing at a at a heating rate of 75C/min. in a direct electricreduction-of-area percentage of 20 percent, heated at current-flowing heating means placed immediately a heating rate of 5C/min. to 40C/sec. to a tempera- 25 ahead of a press forming machine, and hot-forming to ture of 550C, allowed to stand thereat for 5 seconds, M10 bolt 5 minutes thereafter. In this connection, the and water quenched. The heating rates used and the cooling after warm-forming should be carried out as mechanical properties obtained are shown in Table 8. .rapidly as possible, since slow cooling results in an an- Table 2 shows the mechanical properties including nealed bolt due to self-retained heat of the bolt given the tensile strength of 10 steel wire which has been subduring-hotforming, thus failingto achieve the intended jected in turn to preliminary cold wire-drawing, rapid strengthening effect. heating and water quenching, while&#39;the strength of the Table 9 shows the tensile test results of bolts which bolt shown therein represents that of the shank portion have been subjected to three types of cooling after hotof the bolt. Accordingly, the 10 wire should present forming, i.e., water quenching air cooling and slow properties the same as shown in Table 8, because the cooling, revealing that slow cooling does not give a high sample steel C was subjected, like a 10 steel wire to the strength.  
 Table 9 Cooling Condition After Hot-Forming I And Mechanical Properties Type of Cooling Yield Tensile Elonga- Final Reduc- Steel Condition Point Strength tion tion-of-area (kg/mm (kglmm 4 A percentage(%) Water 94.3 110.0 19.8 67.5 S tCeel Quenching (Al Sl Air 1027 Cooling 84.0 100.0 21.5 68.9 Steel) Slow Cooling 71.4 85.4 23.5 77.5  
 preliminary cold wire drawing, rapid heating for hot- The intended properties may be obtained for a bolt forming, and rapid cooling. of small diameter by air cooling. However, a bolt of Table 8 great diameter requires oil-cooling or water quenching.  
  The sample steel C was sub ected to a refining treatment of oil-hardening at 870C and then tempering at figiliiiigffigli 570C, then preliminary cold working at a reductionof-area percentage of 5 to 40 percent, then heating at gzf f&#39; g f f g f 550C at a heating rate of C/sec., warm-forming to (kg/mum (kg/mm 4 A age 65 M10 bolt 5 seconds thereafter, and finally waterquenching. Table 10 shows the influence of the prelimigg ffii 23:3 182:; 13:; 23:8 nary cold working on the mechanical properties ob- 180C/min. 88.3 102.0 20.0 68.5 tained.  
 Table Preliminary Cold Working and Mechanical Properties As can be seen from Table 10, the application of preliminary cold working results in the increase in strength, whereas the toughness thereof decreases. The  
 preliminary cold working at a reduction-of-area percentage of over 40 percent causes transverse cracking in the refined steel material, thus interrupting further working. a  
  The sample steel C was in turn subjected to a refining treatment of oil-hardening at 870C and then tempering at 570C, then preliminary cold working at a reduction-of-area percentage of 20 percent, heating to 550C at a heating rate of 75C/sec. in a direct current-flowing heating means positioned immediately ahead of a press forming machine, warm-forming to a M10 bolt 5 seconds thereafter, and then water-quenching. FIG. 4  
 shows hardness distributions at the head and shank portions of bolts.  
  In general, the head of a bolt has been subjected to 75 percent working, whereas the shank portion has been subjected to only 20 percent preliminary working,  
 such that the difference in such working percentage is reflected in tact to the hardness difference. As a result, the bolt head exhibits abnormally high hardness (strength) as compared with those of the shank portion, while the toughness thereof decreases accordthe present invention presents lowered hardness for the head portion, because, when warm-formed as shown in FIG. 2, the deformation resistance of the sample steel will decrease with the accompanied lesser workhardening, while the shank portion of the bolt presents a hardness increase due to the strengthening treatment of the invention, i.e., a cycle of the preliminary cold working at a reduction-of-area percentage of 20 percent, rapid heating used for warm-forming and rapid cooling thereafter, thereby lessening the difference in hardness between the head and shank portions of the bolt.  
  The sample steels D (boron steel) and E (Cr-Mo steel) were subjected twice to a cycle of hardening treatment by a high-frequency heating means, then a tempering treatment in a head bath, preliminary cold working at a reduction-of-area percentage of 20 percent, heating to 550C at a heating rate of C/sec. and warm forming to M10 bolt.  
  M10 bolt was formed with a notch of 0.05R (stress concentration factor 6) and was subjected to a socalled loop type delayed rupture test in a highhumidity atmosphere, in which the bolt was immersed in a water bath, with the tension being applied thereto. Table 11 shows the results of such a test.  
  As is clear from Table 11, in case a bolt is produced by hot-forming according to the strengthening treatment of the invention, after the austenite crystal grain size has been rendered finer by means of the refining treatment in combination with the high frequency induction heating treatment, the bolt thus produced presents excellent resistance to delayed rupture, as compared with a conventional bolt having the same strength.  
  As is apparent from the foregoing description, a high strength bolt according to the present invention presents a high tensile strength of over kg/mm without impairing the toughness thereof, in contrast to the conventional heat-treated bolt.  
  On the other hand, since rapid heating in the high frequency induction heating means used for the refining treatment for preliminary cold working can render finer the austenite crystal grain size, the bolts produced according to the present invention can present excellent resistance&#39;to delayed rupture, as compared with conventional high strength bolts.  
 Table 1 1 Results of the Delayed-Rupture Test Mechanical Properties Sample Steel Heat-treatment Yield Tensile Elonga- Final Reduc- Austenite Notch Rupture Point Strength tion tion-of-area Grain Size Strength Time (kglmm (kglmm *1 percentage *2 (kg/mm *3 (HV) D-O (compara- .Formed to a bolt tive material) and then heat- 130.8 141.3 17.6 55.2 7.0 208.5 50  
  treatment, 850C 0.0. 420C A.C. D-l (this High frequency ininvention) duction heating heat 135.6 137.6 18.3 59.1 11.8 220.3 3256 treatment 20% preliminary working W.l-l. 850C 00. 380C A.C. 500C W.l-l.  
 Table .11 -Continued Results of the Delayed-Rupture Test Mechanical Properties Sample Steel Heat-treatment Yield Tensile Elonga- Final Reduc- Austenite Notch Rupture Pomt Strength tion (72) tion-of-area Grain Size Strength Time (kg/mm) (kg/mm) *1 percentage (71) *2 (kglmm *3 (HV) E-O (compara- Formed to bolt and tivc material) then heat-treatment, 128.2 138.3 15.6 53.4 8.0 207.0 43  
  850C 0.0. 490C A.C. E-l (this High frequency ininvention) duction heating heat 137.8 138.1 18.3 58.0 12.0 225.0 8634 treatment 209? preliminary working W.H. 850C 0.0. 490C A.C. 500C W.H.  
 *1 Elongation was measured by gauge length 4 A; &#34;2 Grain size after heat-treatment:  
 &#34;3 The rupture time in the delayed rupture test is carried out under the nominal load of 175 kg/mm and using the water immersion at room temperature.  
 treatment of oil-hardening at 870C and tempering at 650C cold wire-drawing at a reduction-of-area percentage of percent, heating to 550C at a heating rate of 240C/sec., holding at said temperature for 5 seconds and water quenching.  
  The material thus treated was then subjected twice to a strengthening treatment consisting of cold working at a r&#39;eduction-of-area percentage of 20 percent, heating to 500C at a heating rate of 240C/sec., holding at said temperature for 5 seconds, and water quenching.  
  The materials, which have been subjected to one and two cycles of strengthening treatment of the invention as described, were further subjected in turn to a wire drawing at a reduction-of-area percentage of 20 percent, heating to 500C at a heating rate of 75C/sec., warm-forming to M10 bolt 5 seconds thereafter and water quenching. Y  
  The mechanical properties obtained are shown in Table 12.  
 Table 12 Mechanical Properties Material Worked Yield Tensile Elonga- Final Re- Point Strength tion duction-of- (kg/mm) (kg/mm) 4 A area percentage Refined Material One cycle of strengthening treatment Two cycles of strengthening treatments M10 bolt of refined material M10 bolt subjected to one cycle of strengthening treatment M10 bolt subjected to two cycles of strengthening treatments As can be seen from Table 12, in case the refined tensile strength and yield point. In addition, the test results show that the repeated strengthening treatments can present further increased tensile strength for bolts which have been warm-formed.  
  Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is to be understood therefore that within the scope of the appended claims, the present invention -may be practiced otherwise than as specifically de- 4 the steps of:  
 subjecting said material to cold working at a reduction-of-area percentage of 10 percent and over;  
 rapidly heating said material thus worked to a temperature range from 450C to the A transformation point and at a rate of at least 50C/min.;  
 warm-forming said material to a bolt shape; and  
 air cooling said material or cooling said material at a cooling rate higher than that of said air cooling, said cooling rate being at least 50C/min.  
  2. A method as defined in claim 1, wherein a carbon steel or a low alloy steel material is subjected at least once to the strengthening treatment consisting of cold working at a reduction-of-area percentage of 10 percent and over, rapid heating to a temperature range from 250C to the A transformation point, and rapid cooling, after which said material thus treated is subjected in turn to cold working at a reduction-of-area percentage of 10 percent and over, rapid heating to a temperature range from 450C to the A transformation point; warm-forming to a bolt shape, and air cooling or cooling at a cooling rate higher than that of said air cooling.  
  3. A method as defined in claim 1, wherein the high strength bolt produced according to claim 1 is subjected in turn to reheating to a temperature range from 450C to the A transformation point, air-cooling or cooling at a cooling rate higher than that of said air cooling.  
  4. A method as defined in claim 3, wherein the heating time for bolts at the reheating temperature during the reheating treatment is within 5 minutes.  
  5. A method as defined in claim 1, wherein said steel material is that which is hot-rolled or has been subjected to a normalizing treatment or annealing treatment after hot-rolling, thereby presenting a pearlite structure.  
  6. A method as defined in claim 1 wherein said steel material is that which has been subjected to hardening and tempering treatments after hot-rolling, thereby presenting a tempered martensite structure and wherein the reduction-of-area percentage for cold working used ranges from to 40 percent.  
  7. A method as defined in claim l, wherein said steel material is subjected at least once to a cycle of hotrolling, rapid heating and hardening, thereby rendering finer the austenite crystal grain size to over No. 10 and inclusive according to ASTM grain size, after which said material is subjected to a tempering treatment, thereby presenting a tempered martensite structure, and wherein the reduction-of-area percentage of cold working ranges from 10 to 40 percent.  
  8. A method as defined in claim 1, wherein the content of carbon is 0.5 percent, or lower.  
  9. A method as defined in claim 1 wherein the cold working is wire-drawing.  
  10. A method as defined in claim 1, wherein the heating time for bolts at the heating temperature during the strengthening treatment is within 3 minutes.