Production of Steel by the

Transcription

1 Production of Steel by the Bessemer Process By E. F. Ketterer* THE pneumatic steelmaking process invented by William Kelly and Henry Bessemer, and commonly known as the bessemer process, has been in use in the United States since The early utilization of acid bessemer steel in this country involved production of a considerable quantity of rail steel, and for many years this process was the principal use for the production of steel. At the present time, the acid bessemer process in the United States is used primarily in the production of steel for butt-welded pipe, seamless pipe, free-machining bars, wire, and blown metal for the duplex process. Fully killed acid bessemer steel was used fo,r the first time commercially by National Tube Division of United States Steel Corporation in the production of seamless pipe: Raw Materials In the production of bessemer steel, the most important area of quality is in the selection and control of raw materials for the production of bessemer blast furnace iron. The raw materials involved are: ore, coal for the production of coke, limestone, and scrap. Uniformity of analysis of ore is not left to chance. Each lot of ore is sampled at the mine as the cars are loaded. Rapid and accurate analysis of determining elements are made and each lot is assigned to a particular group, so that variations of composition within a group are held to a minimum. In addition, when ore is delivered to its destination, it is sampled by a predetermined plan to verify the classification designated by the mine. New methods of ore beneficiation and concentration have been developed, which tend further to reduce the variation in composition. This uniformity is demonstrated by the following data, which present the average and range in * Division Manager-Quality Control, National Tube Division, United States Steel Corporation, Lorain, Ohio. ' References are on page 438.

2 Bessemer Process 423 chemical analysis of all cargoes of bessemer ore delivered to one steel plant during one.year: Constituent Average Range Silica, pct Iron, pct Manganese, pct Phosphorus, pct One example of the effort extended to improve raw materials is the development of taconite sinter. Chemical composition of this material produces a low-phosphorus, low-sulphur iron for the manufacture of bessemer products. The quality of coke is also of major importance in the production of blast furnace iron for the manufacture of quality bessemer steel. Satisfactory metallurgical coke for this purpose requires low ash and low sulphur. In order to secure the highest quality coke, the United States Steel Corporation has installed coal-washing equipment, which reduces extraneous material in mined coal to a minimum. Here again extensive sampling, grading, and blending take place in order to secure low-ash and low-sulphur coal. Electronic computers are employed in order to determine quickly and accurately any departure from specified average or variability of coal composition. Averages and standard deviations for ash and sulphur in coal and coke are presented in Table 1. Other blast furnace burden materials such as limestone, sinter, and scrap are also subject to close control with respect to elements which have an adverse effect on iron product. Blast Furnace Practice Blast furnace practice is also a factor in the production of bessemer steel. Furnace-charging practices have been developed to take advantage of various types of charge materials such as screened coke, sized and sintered ores, which can be varied to meet the quality requirements of iron for the manufacture of various types of bessemer product. This flexibility assists in the maintenance and control of the proper chemical composition of iron in order to produce a satisfactory blowing practice. Facilities are constantly being provided to insure proper storage and blending of ores in order to maintain uniformity of chemical analysis. Modern methods of statistical quality control are employed to insure that iron Table 1-Deviations Deviations Average Standard

3 424 Proceedings, Open Hearth and Basic Oxygen Steel Conference, 1963 Fig 1-Distributions of manganese and phosphorus in bessemer iron. quality with respect to chemical analysis and temperature is controlled at the proper level for the products being made. Frequency distributions of manganese, sulphur, silicon, and phosphorus, showing the close control maintained on these elements for a three-month production period, are presented in Figs 1 and 2. Bessemer Practice National Tube's bessemer plant is designed to insure maximum *iformity of product. The first step in mdntaining uniformity is the iron mixer. in which three or four blast furnace casts can be held at one time. Fig 2-Distributions of sulphur and silicon in bessemer iron.

4 Table 2-Comparative Results Bessemer Process 425 Results Manganese, Pct Silicon, Pct Phosphorus, Pct Beginning of day.. End of day I Lorain Works has two such mixers, thus insuring minimum variability of chemical constituents in iron delivered to the blowing vessels. Iron analysis taken from a random day's production at the beginning and end of the day are shown in Table 2. A photograph of an iron mixer pouring into a bessemer transfer ladle is shown in Fig 3. In addition to blast furnace iron, a part (approximately 10 pct) of each normal bessemer charge consists of steel scrap. Additional scrap, up to a total of 12 pct, can be employed if the blast is enriched with oxygen.' National Tube Divfsion usually is self-sustaining with respect to scrap used in all melting operations. There is no contamination of the bessemer charge F lg 3-Bessemer iron mixer pwrlng into transfer ladle.

5 426 Proceedings, Open Hearth and Basic Oxygen Steel Conference, 1963 Fig 4-Bessemer converters blowing. Fig 5-Bessemer blowers pulpit.

6 Bessemer Process 427 with purchased scrap. All scrap used in the bessemer is selected so that only the very best and most suitable scrap is used in this process. Lorain Works of National Tube Division, includes a modern bessemer plant with three bottom-blown vessels or converters, each with 30 tons capacity. Two vessels are used simultaneously in the production of each heat. The use of three vessels in the production unit permits minor repairs such as changing bottom, blocking off tuyeres, and vessel maintenance without disrupting production. This type of vessel operation further promotes uniformity of quality in the finished product. Variations in iron analysis and blowing practice are thus reduced to a minimum. Additions of ferromanganese and other elements necessary to meet specification requirements are made in the steel ladle. In its early history, blowing a heat of bessemer steel was considered an art, with control of pressure, temperature, blowing cycle, and so forth performed on the judgment of the blower. Modern converters are equipped with Fig 6-Instrument panel in blowing engine room.

7 428 Proceedings, Open Hearth and Basic Oxygen Steel Conference, 1963 scientific instruments that afford much more positive control of this operation. A bessemer converter blowing, the blower's pulpit and instrument panel, the recording instruments for wind pressure, steam, and other operating variables, together with a modern blowing engine, are presented in Figs 4, 5, 6, and 7. Limited success in controlling the blowing cycle has been achieved with the use of spectrographic instruments; however, most bessemer plants still depend on observation by the blower to determine the progress and end point of the blow, While instrumentation and modern equipment assist immeasurably in controlling the blowing operation, the role of the blower is still of paramount importance in the production of quality bessemer steel. There are at present many classes of bessemer product, and within each product class there may be several grades, each of which requires different treatment at the end of the blow, Additions are made in the vessel, in the ladle, and in the mold, depending on the type of steel being produced. Nozzles, stoppers, and stopper-rod assemblies may last through four or five 60-ton pours. However, on most grades, surface quality requires that this assembly be restricted to two pours. Ingot molds may be open top, bottle top or hot-topped, also depending on quality requirements of the semifinished or finished product. Emphasis is placed on mold inspection, cleaning, and coating. Lorain Works has recently installed a modern facility for this purpose where molds are inspected for cracks and worn surfaces, following Fig 7-Bessemer blowing engine.

8 ~ Ultimate Bessemer Process 429 which they are dipped while hot into a water bath and then thoroughly coated with a prescribed mold coating. Pouring practice and steel temperature are checked by a metallurgical observer, who also serves as a liaison man between the pouring platform and the blower's pulpit, relaying pertinent information to the blower that will assist in the maintenance of practices to produce a steel of the required quality. 1. Low-carbon capped steel for continuous welded standard pipe. 2. High-sulphur semikilled steel for free-machining bars and screw stock. 3. High-sulphur leaded steel for free-machining bars and screw stock. 4. Fully deoxidized steel for seamless pipe and tubing. i I Continuous welded standard pipe comprises the largest single outlet for bessemer steel, amounting to approximately 60 pct of the production at Lorain Works. Pipe is produced on a modern continuous weld mill, which results in a product comparable in quality to that of any other steelmaking process. Pipe sizes range from 1/2 in. to 4 in. standard. A summary of chemical analysis and mechanical properties is presented in Table 3. The ability of this pipe to withstand severe bending and forming operations is demonstrated in Figs 8 to 12. Fig 13 presents frequency distributions of chemical elements in the final pipe product. Resulphurized bessemer steel for screw stock and free-machining bar stock comprises the next largest proportion of bessemer tonnage produced at Lorain Works. This accounts for approximately 21 pct of the total production. This steel is used for the manufacture of automatic screw machine products. To economically produce this class of product, very close control of temperature during blowing and teeming, along with chemistry of the finished product are mandatory. Some automatically machined items require an even higher degree of machinability and surface finish than are provided by ordinary resulphurized Table 3-Chemical and Mechanical Propertles of CW Pipe 3/4-Inch Standard Black Pipe Yield point, psi: Amount Carbon, pct: Amount Average... 37,900 Range... 33,500-42,700 Specification, Minimum. 30,000 strength, psi: Average... 59,800 Range... 51,500-69,900 Specification, Minimum. 48,000 Elongation, pct: Average Range Manganese, pct: Average Range Phosphorus, pct: Average Range Sulphur, pct: Average Range Average I Specification, Minimum Range

9 Fig 8-Continuous welded pipe flattened to specif iccrtion requirements. Weld at 90 degrees. Fig 9-Continuous welded pipe flattened In excess of specification requirements. Weld at 45 degrees.

10 Fig 10-Halt-inch contlnwus welded pipe coiled on 4-l/2-inch diameter mandrel.

11 ~ i 11-Standard g pipe of 1-1/4-lmh colled on 8-1/4-inch diameter mandrel. Flg 12-U-bend on 2-inch standard continuws welded pipe.

12 Bessemer Process 433 Fig 13-Distributions of ladle chemistry on continuous weld product. steel. To meet this need, leaded resulphurized bessemer steel ha9 been developed. Samples of product that can be produced from regular resulphurized and leaded resulphurized steel are shown in Fig 14. Many customers believe that the leaded free-machining bessemer steels are superior to comparable open hearth grades. Leaded steel requires the utmost care in adding the lead to the steel in order to insure uniform distribution throughout the ingot. This operation is p e r f o r m e d by introducing the lead in the form of small pellets directly into the steam of molten metal as the ingot is being poured. This procedure is presented in Fig 15.5 The product from each ingot of leaded steel is subjected to chemical, macroetch, and lead exudation tests to insure its compliance with the high standard of quality required. A third important product category for bessemer steel is seamless pipe and tubing. This material is made from fully deoxidized acid bessemer steel which is recarburized with molten pig iron in the vessel. This unique method of recarburization permits a complete blowing cycle and the attain-

13 434 Proceedings, Open Hearth and Basic Oxygen Steel Conference, 1963 Fig 14-Representative types of products produced from freemachining bessemer steel. Fig 15-Teeming and making lead addition on a heat of leaded bessemer steel.

14 Table 4-Chemical Data Yield point, psi: Average... Range... Ultimate strength, psi: Average... Range... Elongation, pct: Average... Range... Carbon, pct: Average... Range... Manganese, pct: Average... Range... Phosphorus, pct: Average... Range... Sulphur, pct: Average... Range... Bessemer Process and Mechanical Properties of Bessemer Standard Pipe and Tubing Standard Pipe I API Grade J-55 Tubing ment of the proper carbon content of the finished product without the use cf a recarburizing ladle addition. Partial deoxidation is also achieved from the molten iron addition. Full deoxidation is obtained by the addition of aluminum in the ladle. Recarburization and deoxidation by this method results in the "fixation" of nitrogen which prevents its otherwise embrittling effect on this type of steel.' With respect to improved control of phosphorus and nitrogen in conventional acid bessemer steel, it has been demonstrated that with the use of a taconite burden on the blast furnace andside blowing with a specially designed bottom during the last few minutes of the blow, steels meeting pct max. phosphorus with nitrogen contents of pct or less can be produced. Extensive correlation studies of the effect of chemical analysis on mechanical properties have also shown that there is no difference between fully deoxidized bessemer and open hearth steel that cannot be explained by chemical compo~ition.~ A summary of chemical and mechanical properties on two grades of seamless pipe is presented in Table 4. Quality Control Successful production of quality products in any manufacturing operation depends on an effective quality-control program. At Lorain Works, the manufacturing practices used to produce bessemer steel are followed by statistical quality-control methods which are applied to raw materials, blowing practice, deoxidation, teeming of ingots, ingot delivery time, and product analysis. A system of daily reporting to management is maintained. Quality-control charts, showing performance by production turn,

15 436 Proceedings, Open Hearth and Basic Oxygen Steel Conference, 1963 Fig 16-Control chart for sulphur in blast furnace iron used in bessemer steel. Fig 17-Control chart for finish pour to move time on high-sulphur bessemer steel.

16 Bessemer Process 437 Fig 18-Control chart for manganese in highsulphur bessemer steel. Fig 19-Control chart for pouring temperature of high-sulphur bessemer steel. are posted at the various processing locations. All relevant -data pertaining to quality practices are punched on tabulating cards from which'summaries and statistical studies can be made for the purpose of further improving product quality. Examples of control charts maintained daily are presented in Figs 16 to 19. Modern methods of data processing, including the use of computers, permits the relatively rapid processing of multiple factor problems which are used as search techniques for the isolation of controllable factors that affect quality and yield. This in turn results in improvement in finished product quality. A recent study of this type, pertaining to surface quality of resulphurized steel, has brought about an improvement in surface quality of semifinished product through the control of ladle analysis, pouring practice, and ingot delivery time.

17 438 Proceedings, Open Hearth and Basic Oxygen Steel Conference, 1963 References 1. Wright, E. C.: Manufacture and Properties of Killed Bessemer Steel. Trans. AIME (1944) Rogers, W. T. and L. T. Sanchez: Use of Oxygen in the Bessemer Converter. J. of Metals (September 1952). 3. Garvey, T. M.: How to Select Free-Machining Steel. Metalworking (October 1961). 4. Wilder, A. B.: Bessemer Converter Process. J. of Metals (Nov. and Dec. 1949). 5. Rogers, W. T.: Statistical Analysis of the Effect of Alloying Elements on Mechanical Properties of Seamless Steel Tubes. Trans. Am. Soc. for Metals (1950) 40. Role of Methane and Other Factors in Controlling Emissions from Steelmaking Processes By George W. P. Rengstorff* THIS is the third year in which the current status of the research supported by the American Iron and Steel Institute on controlling emissions from steelmaking processes has been presented at the Open Hearth Conference.',' The basic thesis of the research has been that it should be possible to eliminate iron oxide smoke from steelmaking processes at its source in the furnace. The research has, therefore, been aimed at understanding the fundamentals of the smoke-forming process both in bottomblown furnaces and in the top-blown converter or open hearth. This paper presents the results of successful smoke suppression in laboratory experiments. It must be emphasized at the outset that, although the laboratory tests show highly promising results, the successful application is still far removed from use inproductionfurnaces. Much more information is required before it can be known whether the technique can be usefully employed in commercial production. The research on control of emissions was first studied in bessemer-type ~onverters.',~,' It was based on the operational knowledge that when steam was added during normal converter practice, the rate of emissions appeared to be substantially reduced. From there the research at Battelle showed that other hydrogen-containing gases (methane, hydrogen, and ammonia) also suppressed smoke formation in bottom-blown converters. Last year, studies of the fundamentals of smoke formation in the topblowing processes were discussed. ' At that time, suppression had not been achieved with additions of hydrogen-containing gases. Additions of water or steam were only partially successful in suppressing smoke. Hydrogen and * Research Associate, Process Metallurgy Division, Battelle Memorial Institute, Columbus, Ohio. References are on page 452.