Progcerrc~ in the Methods of Steel-Prwluction
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FOFaTSOHRITlE IN DEN VERFAHREN DER STAHLERLEUQUNQ (Progcerrc~in the Methods of Steel-Prwluction) ABSTRACT . By Shinsaku ONODERA*) and Koreaki SUZUKI**) The steel making and ingot making processes have made a great pro- gress in the past two decades, which has been one of the direct supports for developing those megalo-technologies as in iron and steel making, petrochemical, fossil and nuclear power ,generation, and other industries. In discussing steel quality, large or small, everything is to the ingot, an in order to reduce miscellaneous imperfections in steel ingots, for largest ones in particular, tremendous efforts have been made so far. In addition to the improvements in conventional ingot making method, a number of innovational techniques were developed such as ESR, PESC. &R, BEST, etc ::with some i-esults :.in.:respective -fields of applica- tt:on:: - -:. ,: , :. Nevertheless, for largest sizes of ingot from 250 to 500 tons, the conventional method was successful enough. The history of conventional method is summarized for the past quarter century. The improved quality of large ingots could bring some more merits in the final forged products, which are: 1) Improved isotropy of mechanical properties ' 2) Sufficient effect of forging with much less forging ratio than ever, e.g. 1.5.; *) Dr. Eng., Associate Director, Engineering, Tokyo Head Office, The Japan Steel Works, Ltd. **) Dr. Eng., General Manager, Research Laboratory, Muroran Plant, The Japan Steel Works, Ltd. 1. INTRODUCTION In the past two decades, the steel making and ingot making processes have made a great progress in every step of them, such as raw materials, high efficiency production methods and fac'ilities , accurate control of chemical elements, 'ladle refining, vacuum treatment, controlled solid- ification of ingots etc., thus making an epoch in steel age. The contribution made by the progress was indeed outstanding to every field of usage of steels. To the topics of the'present Seminar, in particular, the progress has been one of the direct support in realiz- ing the reliable materials and components. There has been so far a number of papers reviewing the steel making technologies and the qualities of the products achieved, in details and in historical development1-'l). Also, the same sort of works, cqntributed by the steel makers as well as steel users 8-13), covered both. ingot making and semi-processed block making methods with which the quality of the large forgings are essentially determined. Recently, the steel fqrgings for nuclear steam supply system (NSSS) are characterized in the way below. 1) Use of larger size, more integral type and reduced weld type pieces, primarily to have merit for in-service inspection (ISI). 2) Higher quality: less segregation, less defects, more uniform and isotropic mechanical properties. In .spite of diversities of innova- tional techniques for manufacturing blocks for large forgings, it \ seems, to the best knowledges and experiences of the authors, that the improved conventional method of ingot making is'still to be applied to the actual manufacturing of NSSS materials. It is therefore the intention of the authors, within the limited space given, that the present paper reviews the development and the states-of-the-art of the steel making of large ingots, say 250 to 500 tons a piece, together with some of the subsequent processes associated with the quality of the forgings. 2. DEVELOPMENT OF LARGE STEEL INGOTS MADE BY CONVENTIONAL METHOD: A REVIEW . The imperfections of the large steel ingots, in terms of segrega- tion, loose structures in the central zone, non-metallic inclusions of large size or in clusters etc. , originate from the differential 'solidi- fication of ingots. The amekioration of quality of the forgings through the successive forging and heat treatment operations is relatively a minor part, thus'leading to the common belief of forge masters that "everything is to the ingot". 2.1 Heterogeneity and Imperfections in Large Steel Ingots To stand on a common basis of discussions, we would first reproduce' some of the features of the heterogeneity of large steel ingot. Fig. 1 is the sulphur print of a 75 ton ingot cast into vacuo in 1959, a typical one observed in a normal ingot14). Fig. 2 is a schematical drawing 15> showing. macro segregations,. including the negative. qegregation zone . These segregations might not be necessarily taken for real defects in ingots. However, it is true 'that the imperfections in large steel ingots are primarily located in macro segregations zones,, particularly along A- and V-segregates and in negative segregation zone. The cause of defects in large steel ingot has been consideraply clarified, however, the counter measures ,against them' are not fully established. 2..2 Transition of Steel Making and Large Conventional Ingot Making Fig. 3 shows a brief'summary of development of making conventional 16 steel ingot of largest sizes in the Japan Steel Works, Ltd. (JSW) . Even with the lack of full control of segregation, the demand for larger size steel ingot was continuously increased from industries, nuclear power generation in particular. In 1967, the 250 ton ingot, largest at that time, was only sufficient to manufacture the generator rotor forg- ings of about 100 tons for two loop PWR. In order to meet the quality of 200 ton class generator forgings, which are required for 1,000 to 1,300 MWe LWR, the manufacture of 400 ton ingot was started in 1969. As an extension, the manufacture of: 500 ton ingot was decided in 1972. Now a total of 154 pieces of 400 ton and 500 ton ingots was manufactured by JSW as of August, 1979. Needless to say, the increase of the size. had to be well preceded . by 'the quality of products, and the progresses of each respective techn&,que.were the bases' supporting them as describe~'below, 1) Melting 6 1 Before the invention of vacuum casti,ng of ingot by Dr. A. Tix , decrease of gases in molten'steel was most important to minimize the' danger of fLaking in large forgings.. For this purpose, the' acid open hearth furnace steel was believedto be best, even'with inferior cleanliness of steel. However, after the introduction of vacuum casting technique in 1959, consideration of gases, hydrogen in particular, was no more of first . concern. The combination of basic electric furnace steel and vacuum treatment could now provide us with better cleanliness and much reduced gases and defects in steel. The increased quantity of molten steel required was another point - of concern. The increased number of electric furnaces was the. solution, firstly for the purpose of controlling segregation by..mult-i-pguring, (MP) process, and secondly for economy. In the early stage of vacuum casting, a vacuum of 3 to 8 Torr during the pouring was normal by the use of mechanical booster system. In 1968, the vacuum eqiripment was replaced with fine stage steam ejecter boosters, the vacuum graded up to 0.1 to 0.5 Torr. For the control of segregation in largest size ingots, MP process with differential chemidal composition between the heats of steel was started with success. 3) Deoxidization The orthodox deoxidization by Si and Mn, and in addition by 'Al, . has been the standard process of deoxidization. In 1969, the vacuum carbon deoxidization (VCD) process was started operation for the ingot of large i rotor shaft. several years thereafter, the VCD process has been success- fully applied to the forgings of turbo generator rotor shaft, chemical vessel, large hydro-turbine shaft etc. The process is ready to be applied to NSSS materials17 . 41, Other Major Factors ; a Summary Furhter factors attributable to the improvement of large steel ingots ire as follows. (1) Configuration of ingot mould Mould ratio, taper, cross section with corrugations were changed. (2) Proper pouring temperature (3) Limitation of tramp elements Summarizing, the proper combination of all the processes and factors could give us an unexpectedly excellent internal quality of large steel ingots, which has been achieved in a trial-and-error process by the accumulation of experiences. 2.3 Typical Examples of Improvement 1) Sulphur print of rotor forgings Fig. 4 is an interesting comparison, of sulphur prints of two Ni-Cr- Mo-V rotor forgings, taken along their center axes. The distinctions are. Upper one: Steel - Basic electric furnace steel. Ingot - 140 ton, low-Si, VCDed. Year of pour - 1975 Lower one: Steel -Acid open hearth furnace steel. Ingot - 75 ton, poured in air. Year of pour - 1952 It is noticed here that the sulphur content for the older forging was very low as acid open hearth furnace steel in more than two decades ago and that, even with the same sulphur content and larger ingot size, the newer forging shows almost "white" sulphur print. The "white" sulphur print could easily give us an image of uniform- ity of the forgings. A paper written by one of the authors will make clear some of the theoretical background of the mechanism of low-Si, VCD process18 . Fig. 5 is the comparison between VCDed and Si-deoxidized 35 ton ingot of 20MnMoNi55 steel. 2) 'Segregation in vessel flange forgings Next to sulphur and phosphorus, carbon is prone to segregate. In Fig. 6, distribution of carbon in two vessel flanges is shown, well falling in 0.20 f 0.02%~even started from a.400 ton or a 500 ton ingot19 . 2.4 Miscellaneous Methods of Large Steel Block Making In addition to the improvements in conventional ingot making method described above, a number of innovational techniques were developed such a$ ESR, PESC , MHKW, BEST, ABR1-2 ) , which brought some results in re- spective field of application. However, the size of the blocks thus obtained seems to be limited to 100 tons to 160 tons.