Methods of producing large steel ingots
United States Patent [1 1 Cooper 1 1 METHODS OF PRODUCING LARGE STEEL INGOTS [75] Inventor: Lloyd R. Cooper, Pittsburgh, Pa.  
 [73] Assignee: l-leppenstall Company, Pittsburgh.  
 [22] Filed: Oct. 9, 1973 [2]) Appl. No.: 404,245  
 [52] U.S. C1 164/52; 164/154 [51] Int. Cl. 822d 27/02 [58] Field of Search 164/4, 521, 154, 252; 29/5265 [56] References Cited UNITED STATES PATENTS 3,603,374 9/1971 Cooper 164/52 FOREIGN PATENTS OR APPLICATIONS 1504.348 12/1967 France 164/52 [II] 3,875,990 [451 Apr. 8, 1975 2.001607 8/1971 Germany 164/52 Primary E.raminerFrancis S. Husar Assistant Examiner.1ohn E. Roethel Anorney, Agent, or Firm-Buell, Blenko and Ziesenheim [57] ABSTRACT A method is provided for producing large steel ingots with increased yields by casting a steel ingot to the desired final size, removing metal to form an axial central cavity, melting a steel electrode under a fused slag in the axial cavity while simultaneously remelting and refining a proportionate controlled portion of the ingot wall surrounding the central cavity by action of the fused slag and solidifying the combined melted electrode and melted metal from the ingot wall integral with the remaining ingot shell to form a composite solid ingot mass.  
 8 Claims, 4 Drawing Figures Liquid Metal Pool I l Rasolldified Central Zone l PATENTEU APR 81875 SzziET 1 UP 3 METHODS OF PRODUCING LARGE STEEL INGOTS This invention relates to methods of producing large steel ingots and particularly to a method of producing a large steel ingot of controlled axial composition and physical character.  
  In my U.S. Pat No. 3,603,374 I have described a method for producing large steel ingots by combining the steps of pouring an ingot in the conventional manner, solidifying the same, removing an axial core lengthwise of said ingot to form a central cavity and filling the cavity with metal from an electrode by electroslag remelting.  
  The present invention is an improvement on that patent wherein the process of electroslag remelting within the central cavity of an ingot is controlled, by an improved method, so that a determined amount of ingot metal from the internal zone around the central cavity is remelted at the same time as the metal is remelted from the electrode. In this manner, the total amount of metal that is remelted, refined and resolidified in the internal portion of the initial ingot is controlled according to the intended use for the ingot, and the desired physical, chemical and mechanical characteristics in the interior of the ingot and its end use product.  
  As will be described, the principles of this control make it feasible and practical to remelt and refine a major zone of the initial ingot, along with a controlled amount of an electrode of similar chemical composition for the purpose of producing a complete ingot of essentially common chemical composition with a central refined zone equal to as much as 50 per cent, or more, of the total cross-sectional area (70 per cent or more of the diameter of the ingot).  
  The same principles of control allow for remelting a lesser amount of metal from the ingot wall, and a predominantly greater proportion of metal from the electrode, even though the electrode may have a distinctly different chemical composition and a higher liquidus (melting) temperature from that of the metal. In this manner a new ingot is formed, with one chemical composition defining the outer zone or ingot wall, and an entirely different chemical composition defining the central zone of the ingot. Such an arrangement of differing chemical compositions makes possible the manufacture of heavy steel forgings or other wrought products with a hard, strong outer (working) surface and a tough, more ductile interior, thereby extending the ca pabilities of the composite product.  
  The method of this invention coordinates (l) the electrical energy applied to the electrically conductive slag through the electrode, (2) the thermal energy in the ingot surrounding the central cavity and (3) the amounts of metal melted from the electrode and from the ingot wall. Calculations, as well as trials of test ingots weighing more than 50 tons, have shown that with a temperature in the ingot wall in the order of 900C, the weight of the metal melted from the ingot wall may easily be more than times the weight ofmetal melted from the electrode itself, even though both are of the same chemical composition, and thereby have equal heat contents at the same temperature of the liquid metal (e.g., 1600C. or 29l2F.).  
  FIG. I is a fragmentary section of ingot and electrode showing their relationship to the liquid metal pool;  
  FIG. 2 is a graph of electrode melt rate to radius at different ingot temperatures;  
  FIG. 3 is&#39;a graph of electrode melt rate to radius at different ingot temperatures.  
  FIG. 4 is a graph of electrode melt rate to radius at different ingot temperatures.  
  In order to make the proper changes in the electrical energy applied through the electrode, as well as thermal energy in the ingot metal in the vicinity of the electrically conductive liquid slag, it is necessary to know, at all times during the course of the remelting cycle, the extent to which the melted slag has penetrated into the ingot wall. The method of determining this penetration, and therefore the diameter of the remelted ingot interior, is shown in FIG. 1.  
  This figure shows the profile through the ingot during the course of remelting, refining and resolidifying the central zone, with the layer of liquid, electrically conductive slag on top of the liquid metal pool. The metal in the liquid metal pool is made up of remelted metal from the electrode and remelted metal from the adjacent ingot wall. The proportionate amount of metal remelted from the ingot wall is determined by the extent to which the layer of liquid slag penetrates into the ingot wall.  
  In FIG. 1, the central axial hole, of diameter D&#34; (or radius r&#34;)is formed in the previously teemed and solidified ingot. The electrode makes electrical contact with the liquid slag and provides the electrical energy for heating the slag and melting the metal in contact with the liquid slag. The slag of volume V&#34; has a thickness 1&#34; and a diameter The radius of the slag layer extends from radius r at its top surface, to radius p where it rests on top of the liquid metal pool.  
  Measurements on partially remelted ingots, interrupted during the course of the remelting process indicate that the curvature of the line .A&#34; where the liquid slag contacts the original ingot wall agrees very closely with the curve for an ellipse. The geometric equation for this line is thereby represented by )r /p y&#39; /t l, for the ellipse (.\&#39;r)&#39;/p y /r =l Applying the principles for volume of a solid of revolution, about the central (y&#34;) axis, the volume of the slag is expressed as From this relationship, (I) knowing the volume of liquid slag, from the weight of slag, and its liquid density, (2) knowing the radius of the central hole r, and (3) measuring the depth of the liquid slag r, the penetration p of the liquid slag into the ingot wall can be determined continuously during the course of the remelting operation.  
  With the determination of the extent of this penetration into the ingot wall, and thus the size of the total remelted central zone of the ingot, it is possible to control the electrical power input and the rate at which the electrode is melted, in coordination with the thermal energy in the ingot wall.  
  The amount of electrical energy required to melt iron can be determined from calculations for the increase in the heat content from a starting temperature (e.g., am-  
 Electrical Energy to Heat to Liquid Starting Temperature State at I600C (2912F.)  
  kwh/metric ton kWh/short ton 25C. (77F) 385.9 350.8 600C (1ll2F.| 287.8 261.6 700C. 1292F.) 270.2 245.6 800C. (1472F.) 243.3 221.2 900C. (l652F.I 222.0 201.8  
  These electrical requirements can be related to the total weight of steel melted, according to the total electrical energy applied, and the electrical efficiency of the facility. Thus, for example, with a power input of 1560 kw 104 volts at 15,000 amps), if the facility requires 400 kwh to melt a ton (metric) of steel (slightly more than the theoretical value), a total weight of 3.9 metric tons (or 3900 kg.) will be melted per hour. If the system is less efficient, and requires a more normal amount of 1200 kWh/metric ton, the total melted weight will be 1300 kg. per hour.  
  ln the conventional electroslag remelting furnace, with a water-cooled crucible, all of the steel melted by this electrical energy comes from the electrode, and the rate of melting the electrode reveals the total requirement for the system in kWh/ton. A value of 1200 kWh/ton is considered to be normal for some industrial electroslag remelting installations.  
  In this invention, however, the electrical energy serves to melt some of the steel from the ingot wall as well as steel from the electrode according to the energy balance (electrical and thermal energy) as shown here.  
  The relationship of the amount of ingot versus electrode melted can be controlled by controlling the temperature of the ingot. A major amount of the ingot is melted if the ingot is preheated whereas a lesser or minor amount is melted if the temperature of the ingot is limited or if the ingot is cooled.  
  FlG. II shows graphically the relationship, for a given power input of 1560 kw, between the steel melted from the electrode and the steel melted from the ingot body according to the representative energy requirement of 1200 kWh/metric ton. In this example, the ingot has an outside diameter of 2000 mm (78.7 inches) and an inside diameter of 600 mm (23.6 inches). The electrode diameter is 410 mm {16.l inches).  
  Under the stated conditions, if no ingot wall is melted, the electrode will melt at the rate of 1300 Heat No. 413822 kg/hour, or a length of 1262 mm/hour. If the ingot is at the same ambient temperature of 25C., and the electrode melts at a slower rate, the remaining electrical energy will melt a determined amount of the ingot wall. At the indicated electrode melt rate of 450 mm/hour,  
 463 kg/hour will melt from the electrode, and 837 kg/hour from the ingot wall. This will produce a melted central zone of 1006 mm diameter (Point A).  
  In like manner, if the ingot wall is at 600C. the remaining electrical energy (after melting the 463 kg/hour from the electrode) will melt more of the ingot wall: 1 122 kg/hour and the melted central zone will be 1 100 mm in diameter (Point B).  
  If the ingot wall is at 900C, under the same conditions 1455 kg/hour will melt from the ingot wall and the melted central zone will be 1220 mm in diameter (Point C).  
  The family of curves in FIG. ll, is determined in this manner, with different selected electrode melt rates, as  
 Similar sets of curves are drawn. in the same manner, for system power requirements higher and lower than the 1200 kWh/metric ton example outlined above.  
  For convenience and rapid control of the remelting operation, the family of curves showing the effect of ingot temperature are transferred to a clear plastic slide which can then be positioned on the logarithmic curve according to the electrical power input and the electrical energy requirement of the specific facility.  
  In the application of the method disclosed here, the ingot blank, with its central axial hole, is heated to the temperature range that corresponds to the size of central melted zone that is desired, with the power input and the electrode melt rate. By constant monitoring of the zone of penetration, the factors of power input (volts, amperes), electrode melt rate, and temperature of the ingot, are adjusted throughout the melt cycle. By controlling these parameters according to the details described here, the zone of remelted ingot can be held within the desired large diameter, or small diameter, with the original ingot blank.  
  This invention can perhaps best be understood by reference to the following examples illustrating the practice of my invention.  
 EXAMPLE I A 50-ton (metric) ingot measuring 2130 mm (83.9 inches) diameter X 2370 mm (93.3 inches) high with a 700 mm (27.6 inches) diameter axial hole, was processed according to this invention, using an electrode whose diameter was 430 mm (16.9 inches).  
  The ingot and electrode were of the same compositron:  
 230 kg (507 lbs.) of slag composed of calcium oxide, alumina, and calcium fluoride were used in the melting cluding the hot-topping&#34; cycle. During the course of the melting operation, the power applied to the electrode was adjusted to 64 volts at 15,000 amperes, or 960 kw, and the rate at which the electrode was melted averaged 478 mm (18.8 inches) per hour, correspond- 5 ing to 544 kg per hour (or 1200 lbs. per hour) from the electrode.  
  The control for this example is shown in FIG. 111, with the penetration into the ingot wall providing a melt zone of 444 mm radius, or 888 mm (35.0 inches) diameter (Point D on graph). With the known rate of melting the 430 mm diameter electrode (478 mm/hr.) causing a rate of rise of the melt zone with the 700 mm diameter hole equivalent to 180 mm (1.1 inches) per hour, the total zone melted corresponds to an overall melt rate of 869 kg per hour (or 1915 lbs. per hour).  
 EXAMPLE ll C Heat No. 413 I97 0.85% 1.869?  
 and the electrode of plain low carbon steel:  
 , C Mn 51 11.10% 0.31% 0.25%  
 450 kg (992 lbs.) of slag composed of calcium oxide, alumina, and calcium fluoride were used in the melting and refining process for this composite ingot.  
  The ingot blank was preheated to 600C. (1112F.) before starting the process. The power applied to the electrode was adjusted to 72 volts at 15,000 amperes, or 1080 kw, with the electrode melting at a steady rate of 262 mm (10.3 inches) per hour, corresponding to 271 kg per hour (or 598 lbs. per hour) from the electrode.  
  FIG. 1V shows the penetration of the melt zone into the ingot wall (Point E&#34;) equal to 610 mm radius. or 1220 mm (48.0 inches) diameter. The 262 mm per hour melting rate of the 410 mm diameter electrode corresponds to a 122 mm (4.8 inches) per hour rate of rise within the 600 mm diameter axial hole. Thus the total melt zone of 1220 mm (48.0 inches) diameter rising at the same rate corresponds to an overall melt rate, in the system, of 1118 kg per hour (or 2465 lbs. per hour). This is a melt rate 28.7% greater than that of Example 1. although the kw power input was only 12.5% higher.  
  Examination of the cross section through forged blooms of Example 11 confirmed the size of the melt zone to be 1200 mm (47.25 inches) diameter as compared with the 1220 mm (48.0 inches) indicated by the control diagram of FIG. Ill. The chemical analysis through the section showed a uniform lower carbon content of 0.66 0.70% in the central melt zone as compared with the 0.85% carbon content in the outside shell of the ingot body. indicating the diluting effect of melting the 0.10% carbon steel electrode within the high carbon steel ingot body to produce the lower carbon steel core of the composite ingot.  
  Within the description of this invention, the rate of melting the electrode can be adjusted to provide a greater or lesser difference in chemical composition between the ingot wall and the ingot central zone, according the desired characteristics of the composite final ingot.  
  While I have illustrated and described certain pre ferred practices of my invention in the foregoing specification, it will be understood that this may be otherwise embodied within the scope of the following claims.  
 1 claim:  
  1. The improved process of producing large steel ingots with increased yield from poured weight to weight of wrought product comprising the steps of a. Casting a steel ingot to the final desired size, pref erably with all of the cast steel weight in the ingot body, with no sinkhead on top of the ingot body.  
 b. Cooling the ingot to solidify the same,  
 c. Removing axial metal from said cast ingot to form an axial central cavity for the full length of the in got,  
 d. Melting a steel electrode within said central cavity under a fused slag by passing an electrical current through said electrode within the cavity to the ingot,  
  Mo 0.3m  
 e. Simultaneously remelting and refining a proportionate controlled portion of the ingot wall sur rounding the central cavity by action of the fused slag,  
 f. Measuring the penetration of fused slag into the ingot wall during remelting;  
 g. Simultaneously controlling the temperature of the ingot body and the proportion of electrical current used to melt the electrode and to melt the ingot wall so as to maintain a selected penetration ofslag in the ingot wall, and  
 h. Solidifying the combined melted electrode metal and melted metal from the ingot wall, integral with the remaining ingot shell to form a solid ingot mass.  
  2. An improved method of controlling the process outlined in claim 1, wherein the electrical current is controlled so that the applied electrical energy melts metal from the steel electrode within the central cavity under the fused slag, and at the same time melts a portion of the ingot wall and refines the metal by reaction with the fused slag, the proportion of electrical energy used to melt the electrode and the portion of the ingot wall being governed by the electrical and thermal energy of the electrode and ingot body in the equipment system.  
  3. An improved method for controlling the process outlined in claim 1, wherein the relative proportion of the steel melted from the electrode and the steel melted from the ingot wall is determined by the material balance in the melting process, and the depth of slag that is positioned on top of the liquid steel within the central cavity.  
  4. An improved method of controlling the process outlined in claim 1, wherein a major portion of the cross-section of the initially cast steel ingot is subjected to the refining reaction of the fused slag, by permitting a major portion of the electrical energy input to be used for melting the steel from the ingot wall. while a minor portion of the electrical energy is used to melt the steel electrode, in the course of filling the central cavity.  
  5, An improved method of controlling the process outlined in claim 1, wherein a major portion of the cross-section of the initially cast steel ingot is subjected to the refining action of the fused slag, by preheating the ingot blank containing the central cavity, thereby requiring less total electrical energy input to accomplish the results of claim 5.  
  6. An improved process of producing large steel ingots as outlined in claim 1, wherein the steel electrode is of a different chemical composition from the ingot wall. and the process is controlled so that a minor portion of the ingot wall is remelted, and the melted central zone results mostly from the remelting of the steel electrode.  
  7. An improved method for controlling the process outlined in claim 6, wherein a minor portion of the cross-section of the initially cast steel ingot is remelted, by limiting the temperature of the ingot during the remelting process, so that the major portion of the elec trical input is used to remelt the steel electrode.  
  8. An improved method for controlling the process outlined in claim 6, wherein a minor portion of the cross-section of the initially cast steel ingot is remelted by cooling of the ingot during the remelting process, so that the major portion of the electrical input is used to remelt the steel electrode.