- Email: [email protected]
Continuous casting of steels in Japan
S C I E N C E A N D TECHNOLOGY OF
Science and Technology of Advanced Materials 2 (2001) 59±65
ADVANCED MATERIALS
www.elsevier.com/locate/stam
Continuous casting of steels in Japan Ken-ichi Miyazawa* Steelmaking Research Laboratory, Nippon Steel Corporation, 20-1 Shintomi, Futtsu, Chiba-Ken 293-8511, Japan Received 17 June 1999
1. Introduction In Japan, conventional continuous casting with an oscillating mold started to be used for the practical production of steels in 1960. Since 1970, there has been a remarkable propagation of continuous casting for the mass production of steels to aim for higher yield and quality of products, resulting in a rapid increase in the CC percentage from 5% in 1970, 50% in 1979 to 90% in 1985. The technology has really contributed as the driving force for ef®cient steel production in Japan. Before the middle of the 1980s, the basic technologies were well established with views of high productivity, saving energy, enlargement of castable steels and high quality of products. Since then, further innovative developments have been actively done for higher productivity, higher quality of steels and new casting processes. 2. Solidi®cation phenomena in continuous casting process of steels As shown in Table 1, various phenomena are taking place in the continuous casting of steels, which need to be understood and controlled for creating better technologies of continuous casting. As the basic phenomena, there are undercooling and nucleation phenomena, crystal growth, solute redistribution and enrichment at the solid±liquid interface, transport phenomena of heat, momentum and solutes in molten steel, solid±liquid region and solidifying shell, deformation of the solidifying shell, agglomeration and ¯otation of non-metallic inclusions, and so on. So far, the complex phenomena during solidi®cation, which consist of the basic phenomena, have been very much elucidated both experimentally and theoretically, especially regarding the formation of solidi®cation structure, micro- and macrosegregation, surface formation of cast steels and initial solidi®cation, cracking and embrittlement at high * Tel.: 181-439-80-2150; fax: 181-439-80-2742. E-mail address: [email protected] (K.-i. Miyazawa).
temperature, the behavior of inclusions and bubbles in molten steel, and the precipitation and growth of inclusions. The elucidation and control of solidi®cation phenomena have basically contributed not only to progress in the technologies of the conventional continuous casting of steels for higher productivity and higher quality of products, but also to the development of near net shape casting processes.
3. Control of initial solidi®cation and casting technologies for high productivity In this decade, various technologies have been developed to meet higher productivity under increasing demands for various steel grades, small lot of products and high quality, and to promote ef®cient connection between casting and rolling such as hot charge rolling (HCR) and hot direct rolling (HDR). Especially, the trend to high speed casting is obvious. From the latter half of the 1980s, developments of high speed casting has been actively done [1]. Recently, the speed is 0.8±1.8 m/min in the usual slab casters attaching importance to high quality and products-mix, and 2.0± 3.0 m/min for recent high speed slab casting of 220± 250 mm thickness. The through-put is 3±8 t/min/strand. Further, higher casting speed up to 5 m/min has been achieved for the casting of middle thickness slab of around 100 mm. In addition to higher casting speed, hitherto the increases in the section size of casts and in the number of sequential casting have contributed very much to the higher productivity of continuous casting. The other effective technologies are as follows; the intensive casting of similar steel grade and change in section size of casts with change of mold width during casting or in the sizing press after solidi®cation; a long life casting nozzle to prevent nozzle clogging and wear on the basis of understanding of the chemical reaction between molten steel and refractory, or quick exchange of the nozzle; sequential casting of different steel grade using electromagnetic braking in the mold; hot repeating use of the tundish; speedy preparing and repair of
1468-6996/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 1468-699 6(01)00026-2
60
K.-i. Miyazawa / Science and Technology of Advanced Materials 2 (2001) 59±65
Table 1 Solidi®cation phenomena in the continuous casting process
caster and tundish; and fore-detection of brake-out of the solidifying shell in the mold. In high speed casting, understanding and control of various metallurgical phenomena taking place in the casting process are basically indispensable. Namely, technologies for preventing casting troubles such as brake out or bulging below the mold, precise controls for surface and internal cracks, non-metallic inclusions, center segregation and porosity, are necessary. Especially, control of initial solidi®cation in the mold, as well as longitudinal cracks, is essential. Thus, many studies have been done for understanding the homogeneous growth of initial solidi®cation shells without cracks, and furthermore, the counter-measures to realize homogeneous heat transfer and lubrication in the mold have been developed intensively. The main technologies developed for high speed casting are: (1) for mold deformation due to large heat ¯uximprovements of mold structure to secure suf®cient cooling ability; (2) for break out, longitudinal and corner cracksimprovements of mold ¯ux, oscillation conditions and mold taper; (3) for entrainment of mold ¯ux and meniscus ¯uctuation-electromagnetic ¯ow control and nozzle design; (4) for internal cracks-multipoint bending and unbending, smaller roll pitch.
4. Improvement of casting qualities 4.1. Control of inclusions and casting technologies for clean steels Non-metallic inclusions are formed by deoxidation in the
secondary re®ning ladle, reoxidation of molten steel by contact with air and slag, and entrainment of slag and mold ¯ux into the molten steel during transport in the ladle, tundish and mold. Thus, the oxide inclusions in cast steels mainly consist of deoxidation and reoxidation products, suspended slag and mold ¯ux. Both the elimination of inclusions by ¯ow control of the molten steel and the prevention of contaminations of the molten steel such as reoxidation and entrainment in each process from the ladle to the mold are important. The metallurgical elucidation of inclusion behavior in the ladle, tundish and mold have much advanced with the help of evaluation techniques of inclusions and theoretical predictions using physical and mathematical simulations of 3-dimensional ¯uid ¯ow [2]. Recently, the mathematical simulation of inclusion behavior in the ladle, tundish and mold as well as the behavior of Ar bubbles in the mold, which are taking account of the agglomeration of inclusions, the ¯otation and removal, the entrapment of inclusions by Ar bubbles or by the solidi®cation front are further progressing to supply reliable information for better technologies of continuous casting. Regarding the research of cleanness of molten steel consistent from re®ning to casting, recently in Japan a research forum of the Iron and Steel Institute of Japan for ultra clean steels has been actively conducted to investigate the physical chemistry of formation and modi®cation, fundamentals of agglomeration and removal, and assessment technique of inclusions [3]. Regarding tundish technologies, tundish capacity has been increased. Now the capacity is usually 40±80 t in slab casters. The large capacity tundish such as the Hshaped tundish is designed to provide a long path of molten
K.-i. Miyazawa / Science and Technology of Advanced Materials 2 (2001) 59±65
61
Fig. 3. Entrapment of an inclusion in the solidi®cation front. Fig. 1. Schematic representation of M-EMS in the continuous casting of steel slab.
steel pro®table for the removal of inclusions and better teeming operating at the ladle exchange [2]. Furthermore, high-capacity heating of molten steel in the tundish by plasma or induction heaters have been developed for keeping the teeming temperature constant and the positive elimination of inclusions. A tundish with a high-capacity heater has been recognized to be effective also for the casting of small-lot products. It has been con®rmed that it is important for clean steels to suppress the reoxidation of molten steel by air and tundish slag, the entrainment of ladle slag into the tundish and to control the ¯ow of molten steel for promoting the removal of inclusions. Taking account of these features, recently the hot repeating use of the tundish has been adopted for refractory life and energy saving. As another approach, the centrifugal ¯ow of molten steel by electromagnetic force has been applied in the inlet area of the tundish to improve the cleanness of the molten steel [4]. With regard to ¯ow control in the mold, many developments have been done. A vertical-bending type caster has
Fig. 2. Effects of EMS in the mold on the size distribution of alumina clusters.
been found effective for the removal of inclusions and Ar bubbles. Besides advances in nozzle design and meniscus level control, a recent in¯uential trend is the development of electromagnetic ¯ow control. Electromagnetic stirring in the mold (M-EMS) by Traveling Magnetic Field [5]. In the stirring, the traveling magnetic ®elds that are generated by AC induction coils installed in the mold induce electromagnetic body force in the molten steel. Two kinds of the stirring have been developed. One drives the molten steel horizontally across wide faces of the mold, whilst the other accelerates or decelerates the teeming stream. The former M-EMS developed in NSC [6], a schematic illustration of which is shown in Fig. 1, has been found very effective not only for suppressing CO blowholes but also for decreasing the entrapment of inclusions near to the surface layer of slabs and in longitudinal cracks [7]. Fig. 2 shows an example on the reduction of alumina cluster by EMS in the mold. As shown in Fig. 3, by making the ¯ow of molten steel along the solidi®cation front, inclusions with larger diameter separate from the solidi®cation front in the turbulent boundary layer by lift and possibly in the solute boundary layer by gradient of interfacial tension, although smaller inclusions are entrapped by the solidi®cation front. Electromagnetic braking (EMBr) in the mold. Interest in the braking effect is growing with the increase in casting speed. By applying a DC magnetic ®eld to control the ¯ow of the molten steel, electromagnetic body force is induced to damp down the ¯ow. Two kinds of braking, shown in Fig. 4, have been developed [5]. One is the application of a local DC magnetic ®eld near to the nozzle outlet for decreasing the stream velocity, whilst the other is the application of a uniform DC magnetic ®eld over the whole width of the mold, called level magnetic ®eld (LMF). The LMF has been found effective for reduction in the penetration depth of the teeming stream and for the suppression of the mixing of different steel grades in the mold at the ladle exchange [8]. Also, an application of two DC magnetic ®elds near to the meniscus in the mold and below the nozzle outlet have
62
K.-i. Miyazawa / Science and Technology of Advanced Materials 2 (2001) 59±65
Fig. 4. Schematic views on applications of electromagnetic braking.
been developed for practical use to control both the meniscus ¯ow and the penetration ¯ow in the mold [9].
disk rolls to provide higher ef®ciency of reduction, and hard reductions with forging dies and with large diameter rolls. These reductions have been applied under equi-axied solidi®cation with electromagnetic stirring. The technology of soft reduction has much advanced through both elucidation of segregation phenomena and optimum conditions to reduce the maximum size of segregation spots to less than 1±2 mm [11]. Also in billet casting, soft reduction has been applied with electromagnetic stirring to improve the internal qualities. Recently, reduction of the solidifying shell during solidi®cation is applying to the reduction in the slab thickness during solidi®cation in thin slab casting and to the prevention of macro-segregation in middle thickness slab castings.
4.2. Control of macrosegregation
5. Oxide metallurgy
Macrosegregation is induced by macroscopic ¯ow of inter-dendritic liquid, which is enriched in solute concentration by micro-segregation in the mushy zone, mainly due to bulging of the solidifying shell and solidi®cation shrinkage. The elucidation of the macro-segregation mechanism and the practical behavior under counter-measures to prevent the inter-dendritic ¯uid ¯ow have been much progressed. Soft reduction of the solidifying strand to suppress the solidi®cation shrinkage ¯ow, as shown in Fig. 5, is well established. In slab casting, soft reduction has been applied under dendritic solidi®cation to reduce segregation size. Through experimental and theoretical advances on elucidating the optimum conditions for the location and rate of soft reduction, segregation control has so highly progressed in NSC that the size of segregation spots is less than about 0.2± 0.5 mm [10]. Regarding bloom casting, in Japan there have been developed soft reductions with crown rolls, convex or
The control of non-metallic inclusions is divided into three categories: (1) elimination from molten steels mentioned above; (2) modi®cation to make inclusions harmless, an example of which is Ca treatment of Al deoxidized steels to change alumina to Ca-aluminate for preventing nozzle clogging; (3) utilization of ®ne inclusions of less than a few microns for improving steel properties through re®nement of crystal grains. The utilization and modi®cation of inclusions are called `Oxide Metallurgy' [12], which has been actively investigated in NSC and used practically for Ti deoxidized steels for plates with high HAZ toughness, microalloyed steels for hot forging and other steels. In the Ti deoxidized steels, the ®ne oxides act as precipitation sites of nitride, carbide and sul®de to promote the formation of ®ne intra-granular ferrite (IGF) grains with resultant high HAZ toughness. The wide concept of oxide metallurgy is expected to create a new soft
Fig. 5. Inter-dendritic ¯uid ¯ow due to solidi®cation shrinkage in continuous casting.
K.-i. Miyazawa / Science and Technology of Advanced Materials 2 (2001) 59±65
63
Fig. 6. Precipitation and growth of oxides during solidi®cation.
technology for the drastic improvement of steel properties [12]. Fig. 6 shows a schematic representation of the precipitation and growth of oxides during solidi®cation. So far, many researches have been done to understand the phenomena during solidi®cation [13]. As one of the results, Fig. 7 shows the estimated growth of ®ne oxides during solidi®cation in the continuous casting of Ti deoxidized steel. 6. New continuous casting processes In this decade, the innovative developments on near net shape castings and new applications of electromagnetic forces to create unique functions of steel products or new principles of continuous casting are remarkable. 6.1. Thin slab casting In the 1980s, developments of thin slab castings became active in Europe and USA with oscillating narrow mold for thin slabs of 50±80 mm at casting speeds of 3±6 m/min. On
Fig. 8. Solidi®cation and ¯uid ¯ow in twin roll casting.
the other hand, in Japan, the casting processes with a moving mold have been developed [14]: inclined twin belt casting for slabs of 50 mm thickness at speed of around 5 m/ min; vertical twin belt castings for slabs of 30 mm at a speed up to 10 m/min and for slabs of 50±75 mm at a speed of 5± 10 m/min. Various technologies have been established to control the teeming into a narrow space between belts, the ¯uctuation of meniscus level, and the deformations of belts and solidifying shells for preventing cracks. The production rate of vertical twin belt casting (around 1.9±5.7 t/min/m per slab width) is higher than that for narrow mold CC with oscillation (roughly 1.4±2.3 t/min/m), and dif®culties in preventing surface defects are more relaxed in the moving mold CC. Although thin slab casting is attractive, it is not used practically Japan because the conventional CC processes are still working with higher productivity. 6.2. Strip casting
Fig. 7. Estimated change in the diameter of oxides during solidi®cation.
The strip casting of steels has been developed intensively in this decade to pursue the merits of near net shape casting and increase in steel properties due to rapid solidi®cation [15,16]. Since the ®rst half of the 1980s, research with laboratory scale twin roll casters on the fundamental phenomena in the process, shown in Fig. 8, have been done [16]. From the latter half of 1980s, industrial scale developments have become very active, mainly for stainless steels; vertical twin roll casting for strips of about 1±5 mm thickness and 600±1330 mm width in 1±10 t heat of molten steel and for thinner strips of around 0.5 mm thickness and 500 mm width in 3 t; and unequal diameter twin roll casting for 650 mm width in 4 t. The main technical subjects were
64
K.-i. Miyazawa / Science and Technology of Advanced Materials 2 (2001) 59±65
Fig. 9. The principle of clad slab casting.
in the early stage of the developments, breaking off and large cracks of strips, and after that, precise controls of strip thickness, surface wrinkles and ®ne cracks, internal porosity, solidi®cation and crystal structures. Now most of the problems have been solved by understanding the phenomena and process control [15]. Since Henry Bessemer in the middle of the 19th century, strip casting has been the ambition for many. The ®rst practical production of stainless steel strips started at Hikari Works of NSC in 1997. 6.3. New waves on continuous casting processes (1) Clad slab casting [17]. In this process, as shown in Fig. 9, a uniform DC magnetic ®eld (LMF) was applied to suppress the mixing of two different molten steels separately poured into the upper and lower spaces in the mold. The surface layer is formed by solidi®cation of the molten steel above the DC magneto. The industrial possibility of the casting process has been con®rmed by experiments with clad slabs of 170 mm thickness, 800 mm width, changing the combination of the steels: for example, the surface layer of stainless steel and the inside of carbon steel. Because various combinations of steels are possible, it is highly expected to create new functions of steel products. (2) Hollow billet casting [18]. In the process, a cylindrical copper mold with ultrasonic vibration was applied as a core to form the inner surface of a pipe, while hot-top casting technologies with a ceramic mold and induction heating were applied to achieve stable pouring of the molten steel. The process has been con®rmed by pilot scale experiments for thin-wall hollow carbon steel billets of 160 mm and 100 mm outer and inner diameters, respectively, to be hopeful for the near net shape casting of pipes. (3) New application of electromagnetic forces. In the casting of molten steel, electromagnetic force has such functions as shape control of the free interface, stirring or driving of the molten steel, suppression of ¯ow, heating of the molten steel and separation of inclusions from the molten steel [19]. Controls of initial solidi®cation by new applications of electromagnetic pressure induced by AC electromagnetic ®elds of high or low frequencies have been studied
to suppress oscillation marks and hooks of solidi®ed meniscus, which cause lateral cracks, local segregation and entrapments of inclusions and bubbles. The decreases in the depth of oscillation marks and Ni segregation at the marks in stainless steels have been con®rmed by plant scale experiments under the application of a low frequency AC electromagnetic ®eld [5]. Recently, new applications of the functions of electromagnetic have been investigated actively, especially in a research forum of the Iron and Steel Institute of Japan. Also, a national project on electromagnetic casting to control initial solidi®cation and ¯uid ¯ow in the mold using superconductive magnets for defect-free casting is being conducted [20]. 7. Conclusions In the future, new steel production system optimizing a global cycle of materials and energy will be constructed for the utilization of various iron resources and energy, the recyclic use of materials, and generic technology that is gentle for the earth. In the area of continuous casting, technological development of conventional continuous casting for higher productivity and higher quality of steel products, new applications of electromagnetic force, the spreading of practical thin slab casting and strip casting, and clean steel technology consistent from re®ning to casting, will further proceed with understanding and control of solidi®cation phenomena under world-wide cooperation. References [1] M. Oji, Proceedings of 153rd Nishiyama Memorial Technical Lectures, 1995, pp. 1±39 (ISIJ). [2] H. Kimura, A. Uehara, M. Mori, H. Tanaka, R. Miura, T. Shirai, K. Sugawara. Nippon Steel Technical Report, No. 61, 1994, pp. 22±28. [3] H. Suito, Symposium Proceeding of the Clean Steel Research Forum, Tokyo, May 16, 1997 (ISIJ). [4] Y. Miki, H. Kitaoka, N. Bessho, T. Sakuraya, S. Ogura, M. Kuga, Tetsut-to-Hagane 82 (1996) 498±503. [5] E. Takeuchi, M. Zeze, T. Toh, S. Mizoguchi, Magnetohydrodynamics in Process Metallurgy, 1991, pp. 189±202, 261±266 (Warrendale, The Mineral, Metals and Materials). [6] J. Fukuda, Y. Ohtani, A. Kiyose, T. Kawase, K. Tsutsumi, Proceedings of the 3rd European Conference on Continuous Casting, Madrid, Spain, October 20, 1998, pp. 437±445. [7] A. Kiyose, K. Miyazawa, J. Fukuda, Y. Ohtani, J. Nakashima, Current advance in materials and processes, ISIJ 7 (4) (1994) 1193±1195. [8] H. Harada, E. Takeuchi, M. Zeze, T. Ishii, A. Uehara, T. Okazaki, Current Advance in Materials and Processes, ISIJ 9 (1) (1996) 205. [9] K. Kariya, Y. Kitano, M. Kuga, A. Idogawa, K. Sorimachi, Steelmaking Conf. Proc. 77 (1994) 53±58. [10] T. Matsuzaki, H. Misumi, S. Mizoguchi, S. Ogibayashi, M. Zeze, T. Shirai, T. Inaba, S. Nagata, M. Yamada, Current advance in materials and processes, ISIJ 2 (4) (1989) 1150±1153. [11] S. Sugimaru, K. Miyazawa, M. Uchimura, H. Takahashi, H. Gotoda, H. Mihara, Current advance in materials and processes, ISIJ 7 (1) (1994) 175±178. [12] J. Takamura, S. Mizoguchi, Proceedings of 6th International Iron Steel Congress, Nagoya, Japan, 1990, pp. 591±597 (ISIJ).
K.-i. Miyazawa / Science and Technology of Advanced Materials 2 (2001) 59±65 [13] H. Goto, K. Miyazawa, W. Yamada, K. Tanaka, ISIJ Int. 35 (1995) 708±714. [14] Y. Sugitani, Proceedings of 153rd Nishiyama Memorial Technical Lectures, 1994, pp. 225±250 (ISIJ). [15] H. Takeuchi, Proceedings of 153rd Nishiyama Memorial Technical Lectures, 1994, pp. 251±275 (ISIJ). [16] K. Miyazawa, T. Mizoguchi, Y. Ueshima, S. Mizoguchi, New Smelting Reduction and Near Net Shape Casting Technologies for Steels, Preprint 2, 1990, pp. 745±753 (The Korean Inst. Metals and The Inst. Metals, UK).
65
[17] E. Takeuchi, H. Kajioka, M. Zeze, H. Tanaka, S. Mizoguchi, I. Miyoshino, Current Advance in Materials and Processes, ISIJ 7 (4) (1994) 895±897. [18] H. Harada, E. Anzai, E. Takeuchi, Near Net Shape Casting in the Minimills, 1995, pp. 193±204 (The Canadian Inst. of Mining, Metallurgy and Petroleum, Montreal). [19] S. Asai, Proceedings of 153rd and 154th Nishiyama Memorial Technical Lectures, 1994, p. 89±102 (ISIJ). [20] K. Ayata, K. Miyazawa, H. Uesugi, et al., CAMP-ISIJ 10 (1997) 828.
Science and Technology of Advanced Materials 2 (2001) 59±65
ADVANCED MATERIALS
www.elsevier.com/locate/stam
Continuous casting of steels in Japan Ken-ichi Miyazawa* Steelmaking Research Laboratory, Nippon Steel Corporation, 20-1 Shintomi, Futtsu, Chiba-Ken 293-8511, Japan Received 17 June 1999
1. Introduction In Japan, conventional continuous casting with an oscillating mold started to be used for the practical production of steels in 1960. Since 1970, there has been a remarkable propagation of continuous casting for the mass production of steels to aim for higher yield and quality of products, resulting in a rapid increase in the CC percentage from 5% in 1970, 50% in 1979 to 90% in 1985. The technology has really contributed as the driving force for ef®cient steel production in Japan. Before the middle of the 1980s, the basic technologies were well established with views of high productivity, saving energy, enlargement of castable steels and high quality of products. Since then, further innovative developments have been actively done for higher productivity, higher quality of steels and new casting processes. 2. Solidi®cation phenomena in continuous casting process of steels As shown in Table 1, various phenomena are taking place in the continuous casting of steels, which need to be understood and controlled for creating better technologies of continuous casting. As the basic phenomena, there are undercooling and nucleation phenomena, crystal growth, solute redistribution and enrichment at the solid±liquid interface, transport phenomena of heat, momentum and solutes in molten steel, solid±liquid region and solidifying shell, deformation of the solidifying shell, agglomeration and ¯otation of non-metallic inclusions, and so on. So far, the complex phenomena during solidi®cation, which consist of the basic phenomena, have been very much elucidated both experimentally and theoretically, especially regarding the formation of solidi®cation structure, micro- and macrosegregation, surface formation of cast steels and initial solidi®cation, cracking and embrittlement at high * Tel.: 181-439-80-2150; fax: 181-439-80-2742. E-mail address: [email protected] (K.-i. Miyazawa).
temperature, the behavior of inclusions and bubbles in molten steel, and the precipitation and growth of inclusions. The elucidation and control of solidi®cation phenomena have basically contributed not only to progress in the technologies of the conventional continuous casting of steels for higher productivity and higher quality of products, but also to the development of near net shape casting processes.
3. Control of initial solidi®cation and casting technologies for high productivity In this decade, various technologies have been developed to meet higher productivity under increasing demands for various steel grades, small lot of products and high quality, and to promote ef®cient connection between casting and rolling such as hot charge rolling (HCR) and hot direct rolling (HDR). Especially, the trend to high speed casting is obvious. From the latter half of the 1980s, developments of high speed casting has been actively done [1]. Recently, the speed is 0.8±1.8 m/min in the usual slab casters attaching importance to high quality and products-mix, and 2.0± 3.0 m/min for recent high speed slab casting of 220± 250 mm thickness. The through-put is 3±8 t/min/strand. Further, higher casting speed up to 5 m/min has been achieved for the casting of middle thickness slab of around 100 mm. In addition to higher casting speed, hitherto the increases in the section size of casts and in the number of sequential casting have contributed very much to the higher productivity of continuous casting. The other effective technologies are as follows; the intensive casting of similar steel grade and change in section size of casts with change of mold width during casting or in the sizing press after solidi®cation; a long life casting nozzle to prevent nozzle clogging and wear on the basis of understanding of the chemical reaction between molten steel and refractory, or quick exchange of the nozzle; sequential casting of different steel grade using electromagnetic braking in the mold; hot repeating use of the tundish; speedy preparing and repair of
1468-6996/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 1468-699 6(01)00026-2
60
K.-i. Miyazawa / Science and Technology of Advanced Materials 2 (2001) 59±65
Table 1 Solidi®cation phenomena in the continuous casting process
caster and tundish; and fore-detection of brake-out of the solidifying shell in the mold. In high speed casting, understanding and control of various metallurgical phenomena taking place in the casting process are basically indispensable. Namely, technologies for preventing casting troubles such as brake out or bulging below the mold, precise controls for surface and internal cracks, non-metallic inclusions, center segregation and porosity, are necessary. Especially, control of initial solidi®cation in the mold, as well as longitudinal cracks, is essential. Thus, many studies have been done for understanding the homogeneous growth of initial solidi®cation shells without cracks, and furthermore, the counter-measures to realize homogeneous heat transfer and lubrication in the mold have been developed intensively. The main technologies developed for high speed casting are: (1) for mold deformation due to large heat ¯uximprovements of mold structure to secure suf®cient cooling ability; (2) for break out, longitudinal and corner cracksimprovements of mold ¯ux, oscillation conditions and mold taper; (3) for entrainment of mold ¯ux and meniscus ¯uctuation-electromagnetic ¯ow control and nozzle design; (4) for internal cracks-multipoint bending and unbending, smaller roll pitch.
4. Improvement of casting qualities 4.1. Control of inclusions and casting technologies for clean steels Non-metallic inclusions are formed by deoxidation in the
secondary re®ning ladle, reoxidation of molten steel by contact with air and slag, and entrainment of slag and mold ¯ux into the molten steel during transport in the ladle, tundish and mold. Thus, the oxide inclusions in cast steels mainly consist of deoxidation and reoxidation products, suspended slag and mold ¯ux. Both the elimination of inclusions by ¯ow control of the molten steel and the prevention of contaminations of the molten steel such as reoxidation and entrainment in each process from the ladle to the mold are important. The metallurgical elucidation of inclusion behavior in the ladle, tundish and mold have much advanced with the help of evaluation techniques of inclusions and theoretical predictions using physical and mathematical simulations of 3-dimensional ¯uid ¯ow [2]. Recently, the mathematical simulation of inclusion behavior in the ladle, tundish and mold as well as the behavior of Ar bubbles in the mold, which are taking account of the agglomeration of inclusions, the ¯otation and removal, the entrapment of inclusions by Ar bubbles or by the solidi®cation front are further progressing to supply reliable information for better technologies of continuous casting. Regarding the research of cleanness of molten steel consistent from re®ning to casting, recently in Japan a research forum of the Iron and Steel Institute of Japan for ultra clean steels has been actively conducted to investigate the physical chemistry of formation and modi®cation, fundamentals of agglomeration and removal, and assessment technique of inclusions [3]. Regarding tundish technologies, tundish capacity has been increased. Now the capacity is usually 40±80 t in slab casters. The large capacity tundish such as the Hshaped tundish is designed to provide a long path of molten
K.-i. Miyazawa / Science and Technology of Advanced Materials 2 (2001) 59±65
61
Fig. 3. Entrapment of an inclusion in the solidi®cation front. Fig. 1. Schematic representation of M-EMS in the continuous casting of steel slab.
steel pro®table for the removal of inclusions and better teeming operating at the ladle exchange [2]. Furthermore, high-capacity heating of molten steel in the tundish by plasma or induction heaters have been developed for keeping the teeming temperature constant and the positive elimination of inclusions. A tundish with a high-capacity heater has been recognized to be effective also for the casting of small-lot products. It has been con®rmed that it is important for clean steels to suppress the reoxidation of molten steel by air and tundish slag, the entrainment of ladle slag into the tundish and to control the ¯ow of molten steel for promoting the removal of inclusions. Taking account of these features, recently the hot repeating use of the tundish has been adopted for refractory life and energy saving. As another approach, the centrifugal ¯ow of molten steel by electromagnetic force has been applied in the inlet area of the tundish to improve the cleanness of the molten steel [4]. With regard to ¯ow control in the mold, many developments have been done. A vertical-bending type caster has
Fig. 2. Effects of EMS in the mold on the size distribution of alumina clusters.
been found effective for the removal of inclusions and Ar bubbles. Besides advances in nozzle design and meniscus level control, a recent in¯uential trend is the development of electromagnetic ¯ow control. Electromagnetic stirring in the mold (M-EMS) by Traveling Magnetic Field [5]. In the stirring, the traveling magnetic ®elds that are generated by AC induction coils installed in the mold induce electromagnetic body force in the molten steel. Two kinds of the stirring have been developed. One drives the molten steel horizontally across wide faces of the mold, whilst the other accelerates or decelerates the teeming stream. The former M-EMS developed in NSC [6], a schematic illustration of which is shown in Fig. 1, has been found very effective not only for suppressing CO blowholes but also for decreasing the entrapment of inclusions near to the surface layer of slabs and in longitudinal cracks [7]. Fig. 2 shows an example on the reduction of alumina cluster by EMS in the mold. As shown in Fig. 3, by making the ¯ow of molten steel along the solidi®cation front, inclusions with larger diameter separate from the solidi®cation front in the turbulent boundary layer by lift and possibly in the solute boundary layer by gradient of interfacial tension, although smaller inclusions are entrapped by the solidi®cation front. Electromagnetic braking (EMBr) in the mold. Interest in the braking effect is growing with the increase in casting speed. By applying a DC magnetic ®eld to control the ¯ow of the molten steel, electromagnetic body force is induced to damp down the ¯ow. Two kinds of braking, shown in Fig. 4, have been developed [5]. One is the application of a local DC magnetic ®eld near to the nozzle outlet for decreasing the stream velocity, whilst the other is the application of a uniform DC magnetic ®eld over the whole width of the mold, called level magnetic ®eld (LMF). The LMF has been found effective for reduction in the penetration depth of the teeming stream and for the suppression of the mixing of different steel grades in the mold at the ladle exchange [8]. Also, an application of two DC magnetic ®elds near to the meniscus in the mold and below the nozzle outlet have
62
K.-i. Miyazawa / Science and Technology of Advanced Materials 2 (2001) 59±65
Fig. 4. Schematic views on applications of electromagnetic braking.
been developed for practical use to control both the meniscus ¯ow and the penetration ¯ow in the mold [9].
disk rolls to provide higher ef®ciency of reduction, and hard reductions with forging dies and with large diameter rolls. These reductions have been applied under equi-axied solidi®cation with electromagnetic stirring. The technology of soft reduction has much advanced through both elucidation of segregation phenomena and optimum conditions to reduce the maximum size of segregation spots to less than 1±2 mm [11]. Also in billet casting, soft reduction has been applied with electromagnetic stirring to improve the internal qualities. Recently, reduction of the solidifying shell during solidi®cation is applying to the reduction in the slab thickness during solidi®cation in thin slab casting and to the prevention of macro-segregation in middle thickness slab castings.
4.2. Control of macrosegregation
5. Oxide metallurgy
Macrosegregation is induced by macroscopic ¯ow of inter-dendritic liquid, which is enriched in solute concentration by micro-segregation in the mushy zone, mainly due to bulging of the solidifying shell and solidi®cation shrinkage. The elucidation of the macro-segregation mechanism and the practical behavior under counter-measures to prevent the inter-dendritic ¯uid ¯ow have been much progressed. Soft reduction of the solidifying strand to suppress the solidi®cation shrinkage ¯ow, as shown in Fig. 5, is well established. In slab casting, soft reduction has been applied under dendritic solidi®cation to reduce segregation size. Through experimental and theoretical advances on elucidating the optimum conditions for the location and rate of soft reduction, segregation control has so highly progressed in NSC that the size of segregation spots is less than about 0.2± 0.5 mm [10]. Regarding bloom casting, in Japan there have been developed soft reductions with crown rolls, convex or
The control of non-metallic inclusions is divided into three categories: (1) elimination from molten steels mentioned above; (2) modi®cation to make inclusions harmless, an example of which is Ca treatment of Al deoxidized steels to change alumina to Ca-aluminate for preventing nozzle clogging; (3) utilization of ®ne inclusions of less than a few microns for improving steel properties through re®nement of crystal grains. The utilization and modi®cation of inclusions are called `Oxide Metallurgy' [12], which has been actively investigated in NSC and used practically for Ti deoxidized steels for plates with high HAZ toughness, microalloyed steels for hot forging and other steels. In the Ti deoxidized steels, the ®ne oxides act as precipitation sites of nitride, carbide and sul®de to promote the formation of ®ne intra-granular ferrite (IGF) grains with resultant high HAZ toughness. The wide concept of oxide metallurgy is expected to create a new soft
Fig. 5. Inter-dendritic ¯uid ¯ow due to solidi®cation shrinkage in continuous casting.
K.-i. Miyazawa / Science and Technology of Advanced Materials 2 (2001) 59±65
63
Fig. 6. Precipitation and growth of oxides during solidi®cation.
technology for the drastic improvement of steel properties [12]. Fig. 6 shows a schematic representation of the precipitation and growth of oxides during solidi®cation. So far, many researches have been done to understand the phenomena during solidi®cation [13]. As one of the results, Fig. 7 shows the estimated growth of ®ne oxides during solidi®cation in the continuous casting of Ti deoxidized steel. 6. New continuous casting processes In this decade, the innovative developments on near net shape castings and new applications of electromagnetic forces to create unique functions of steel products or new principles of continuous casting are remarkable. 6.1. Thin slab casting In the 1980s, developments of thin slab castings became active in Europe and USA with oscillating narrow mold for thin slabs of 50±80 mm at casting speeds of 3±6 m/min. On
Fig. 8. Solidi®cation and ¯uid ¯ow in twin roll casting.
the other hand, in Japan, the casting processes with a moving mold have been developed [14]: inclined twin belt casting for slabs of 50 mm thickness at speed of around 5 m/ min; vertical twin belt castings for slabs of 30 mm at a speed up to 10 m/min and for slabs of 50±75 mm at a speed of 5± 10 m/min. Various technologies have been established to control the teeming into a narrow space between belts, the ¯uctuation of meniscus level, and the deformations of belts and solidifying shells for preventing cracks. The production rate of vertical twin belt casting (around 1.9±5.7 t/min/m per slab width) is higher than that for narrow mold CC with oscillation (roughly 1.4±2.3 t/min/m), and dif®culties in preventing surface defects are more relaxed in the moving mold CC. Although thin slab casting is attractive, it is not used practically Japan because the conventional CC processes are still working with higher productivity. 6.2. Strip casting
Fig. 7. Estimated change in the diameter of oxides during solidi®cation.
The strip casting of steels has been developed intensively in this decade to pursue the merits of near net shape casting and increase in steel properties due to rapid solidi®cation [15,16]. Since the ®rst half of the 1980s, research with laboratory scale twin roll casters on the fundamental phenomena in the process, shown in Fig. 8, have been done [16]. From the latter half of 1980s, industrial scale developments have become very active, mainly for stainless steels; vertical twin roll casting for strips of about 1±5 mm thickness and 600±1330 mm width in 1±10 t heat of molten steel and for thinner strips of around 0.5 mm thickness and 500 mm width in 3 t; and unequal diameter twin roll casting for 650 mm width in 4 t. The main technical subjects were
64
K.-i. Miyazawa / Science and Technology of Advanced Materials 2 (2001) 59±65
Fig. 9. The principle of clad slab casting.
in the early stage of the developments, breaking off and large cracks of strips, and after that, precise controls of strip thickness, surface wrinkles and ®ne cracks, internal porosity, solidi®cation and crystal structures. Now most of the problems have been solved by understanding the phenomena and process control [15]. Since Henry Bessemer in the middle of the 19th century, strip casting has been the ambition for many. The ®rst practical production of stainless steel strips started at Hikari Works of NSC in 1997. 6.3. New waves on continuous casting processes (1) Clad slab casting [17]. In this process, as shown in Fig. 9, a uniform DC magnetic ®eld (LMF) was applied to suppress the mixing of two different molten steels separately poured into the upper and lower spaces in the mold. The surface layer is formed by solidi®cation of the molten steel above the DC magneto. The industrial possibility of the casting process has been con®rmed by experiments with clad slabs of 170 mm thickness, 800 mm width, changing the combination of the steels: for example, the surface layer of stainless steel and the inside of carbon steel. Because various combinations of steels are possible, it is highly expected to create new functions of steel products. (2) Hollow billet casting [18]. In the process, a cylindrical copper mold with ultrasonic vibration was applied as a core to form the inner surface of a pipe, while hot-top casting technologies with a ceramic mold and induction heating were applied to achieve stable pouring of the molten steel. The process has been con®rmed by pilot scale experiments for thin-wall hollow carbon steel billets of 160 mm and 100 mm outer and inner diameters, respectively, to be hopeful for the near net shape casting of pipes. (3) New application of electromagnetic forces. In the casting of molten steel, electromagnetic force has such functions as shape control of the free interface, stirring or driving of the molten steel, suppression of ¯ow, heating of the molten steel and separation of inclusions from the molten steel [19]. Controls of initial solidi®cation by new applications of electromagnetic pressure induced by AC electromagnetic ®elds of high or low frequencies have been studied
to suppress oscillation marks and hooks of solidi®ed meniscus, which cause lateral cracks, local segregation and entrapments of inclusions and bubbles. The decreases in the depth of oscillation marks and Ni segregation at the marks in stainless steels have been con®rmed by plant scale experiments under the application of a low frequency AC electromagnetic ®eld [5]. Recently, new applications of the functions of electromagnetic have been investigated actively, especially in a research forum of the Iron and Steel Institute of Japan. Also, a national project on electromagnetic casting to control initial solidi®cation and ¯uid ¯ow in the mold using superconductive magnets for defect-free casting is being conducted [20]. 7. Conclusions In the future, new steel production system optimizing a global cycle of materials and energy will be constructed for the utilization of various iron resources and energy, the recyclic use of materials, and generic technology that is gentle for the earth. In the area of continuous casting, technological development of conventional continuous casting for higher productivity and higher quality of steel products, new applications of electromagnetic force, the spreading of practical thin slab casting and strip casting, and clean steel technology consistent from re®ning to casting, will further proceed with understanding and control of solidi®cation phenomena under world-wide cooperation. References [1] M. Oji, Proceedings of 153rd Nishiyama Memorial Technical Lectures, 1995, pp. 1±39 (ISIJ). [2] H. Kimura, A. Uehara, M. Mori, H. Tanaka, R. Miura, T. Shirai, K. Sugawara. Nippon Steel Technical Report, No. 61, 1994, pp. 22±28. [3] H. Suito, Symposium Proceeding of the Clean Steel Research Forum, Tokyo, May 16, 1997 (ISIJ). [4] Y. Miki, H. Kitaoka, N. Bessho, T. Sakuraya, S. Ogura, M. Kuga, Tetsut-to-Hagane 82 (1996) 498±503. [5] E. Takeuchi, M. Zeze, T. Toh, S. Mizoguchi, Magnetohydrodynamics in Process Metallurgy, 1991, pp. 189±202, 261±266 (Warrendale, The Mineral, Metals and Materials). [6] J. Fukuda, Y. Ohtani, A. Kiyose, T. Kawase, K. Tsutsumi, Proceedings of the 3rd European Conference on Continuous Casting, Madrid, Spain, October 20, 1998, pp. 437±445. [7] A. Kiyose, K. Miyazawa, J. Fukuda, Y. Ohtani, J. Nakashima, Current advance in materials and processes, ISIJ 7 (4) (1994) 1193±1195. [8] H. Harada, E. Takeuchi, M. Zeze, T. Ishii, A. Uehara, T. Okazaki, Current Advance in Materials and Processes, ISIJ 9 (1) (1996) 205. [9] K. Kariya, Y. Kitano, M. Kuga, A. Idogawa, K. Sorimachi, Steelmaking Conf. Proc. 77 (1994) 53±58. [10] T. Matsuzaki, H. Misumi, S. Mizoguchi, S. Ogibayashi, M. Zeze, T. Shirai, T. Inaba, S. Nagata, M. Yamada, Current advance in materials and processes, ISIJ 2 (4) (1989) 1150±1153. [11] S. Sugimaru, K. Miyazawa, M. Uchimura, H. Takahashi, H. Gotoda, H. Mihara, Current advance in materials and processes, ISIJ 7 (1) (1994) 175±178. [12] J. Takamura, S. Mizoguchi, Proceedings of 6th International Iron Steel Congress, Nagoya, Japan, 1990, pp. 591±597 (ISIJ).
K.-i. Miyazawa / Science and Technology of Advanced Materials 2 (2001) 59±65 [13] H. Goto, K. Miyazawa, W. Yamada, K. Tanaka, ISIJ Int. 35 (1995) 708±714. [14] Y. Sugitani, Proceedings of 153rd Nishiyama Memorial Technical Lectures, 1994, pp. 225±250 (ISIJ). [15] H. Takeuchi, Proceedings of 153rd Nishiyama Memorial Technical Lectures, 1994, pp. 251±275 (ISIJ). [16] K. Miyazawa, T. Mizoguchi, Y. Ueshima, S. Mizoguchi, New Smelting Reduction and Near Net Shape Casting Technologies for Steels, Preprint 2, 1990, pp. 745±753 (The Korean Inst. Metals and The Inst. Metals, UK).
65
[17] E. Takeuchi, H. Kajioka, M. Zeze, H. Tanaka, S. Mizoguchi, I. Miyoshino, Current Advance in Materials and Processes, ISIJ 7 (4) (1994) 895±897. [18] H. Harada, E. Anzai, E. Takeuchi, Near Net Shape Casting in the Minimills, 1995, pp. 193±204 (The Canadian Inst. of Mining, Metallurgy and Petroleum, Montreal). [19] S. Asai, Proceedings of 153rd and 154th Nishiyama Memorial Technical Lectures, 1994, p. 89±102 (ISIJ). [20] K. Ayata, K. Miyazawa, H. Uesugi, et al., CAMP-ISIJ 10 (1997) 828.