Formation of internal cracks during soft reduction in rectangular bloom continuous casting

Abstract

To investigate the formation of internal cracks in GCr15 bearing steels during the soft reduction process in rectangular bloom continuous casting, fully coupled thermomechanical finite element models were developed using the commercial software MSC.MARC, and microstructures and fractographs were also observed. With the finite element models, the contours of temperature, equivalent plastic strain, and equivalent von Mises stress were simulated. It is observed that the fracture surfaces of internal cracks are covered by cleavage or quasi-cleavage facets. The region of internal cracks in the intergranular brittle fracture mode is in the mushy zone between the zero ductility temperature (ZDT) and the zero strength temperature (ZST). The simulated equivalent plastic strain in the crack region is 2.34%–2.45%, which is larger than the critical strain (0.4%–1.5%), and the equivalent von Mises stress is 1.84–5.05 MPa, which is within the range of critical stress (3.9–7.2 MPa), thus resulting in the occurrence of internal cracks. Reducing the soft reduction amount from 3 to 2 mm can lower the stress under the critical value.

Keywords

bearing steel / rectangular blooms / soft reduction / finite element method / internal cracks

References

[1] Scholes A. Segregation in continuous casting. Ironmaking Steelmaking, 2005, 32(2): 101

[2] Flemings M.C. Our understanding of macrosegregation: Past and present. ISIJ Int., 2000, 40(9): 833

[3] Thome R., Harste K. Principles of billet soft-reduction and consequences for continuous casting. ISIJ Int., 2006, 46(12): 1839

[4] Bülte R., Bleck W. Improvement of center segregation in high-carbon steel billets using soft reduction. J. Univ. Sci. Technol. Beijing, 2006, 13(6): 490

[5] Li G., Zhang K., Han C., et al. SMART/ASTC dynamic soft reduction technology and its application on the bloom continuous caster at Pangang. J. Univ. Sci. Technol. Beijing, 2006, 13(2): 121

[6] Liu M., Li J., Li S., et al. Application of soft reduction technique to continuous-casting billets. J. Univ. Sci. Technol. Beijing, 2004, 11(4): 302.

[7] Mörwald K., Thalhammer M., Federspiel C., et al. Benefits of SMART® segment technology and ASTC strand taper control in continuous casting. Steel Times Int., 2002, 27(1): 17.

[8] Brimacombe J.K., Sorimachi K. Crack formation in the continuous casting of steel. Metall. Trans. B, 1977, 8(2): 489

[9] Won Y.M., Yeo T.J., Seol D.J., et al. A new criterion for internal crack formation in continuously cast steels. Metall. Mater. Trans. B, 2000, 31(4): 779

[10] Chen W., Zhang Y.Z., Zhang C.J., et al. Numerical simulation of the thermo-mechanical process for beam blank continuous casting. Acta Metall. Sin., 2007, 20(4): 241

[11] Kim K., Han H.N., Yeo T., et al. Analysis of surface and internal cracks in continuously cast beam blank. Ironmaking Steelmaking, 1997, 24(3): 249.

[12] Wang W.J., Ning L.X., Bulte R., et al. Formation of internal cracks in steel billets during soft reduction. J. Univ. Sci. Technol. Beijing, 2008, 15(2): 114

[13] Incropera F.P., Dewitt D.P. Fundamentals of Heat and Mass Transfer, 1990, New York, John Wiley & Sons

[14] Voller V.R., Swaminathan C.R. General source-based method for solidification phase change. Numer. Heat Transfer Part B, 1991, 19(2): 175

[15] Chen L. Continuous Casting Steel, 1993, Beijing, Metallurgical Industry Press, 66.

[16] Won Y.M., Thomas B.G. Simple model of microsegregation during solidification of steels. Metall. Mater. Trans. A, 2001, 32(7): 1755

[17] Won Y.M., Kim K.H., Yeo T.J., et al. Effect of cooling rate on ZST, LIT and ZDT of carbon steels near melting point. ISIJ Int., 1998, 38(10): 1093

[18] Shin G., Kajitani T., Suzuki T., et al. Mechanical properties of carbon steels during solidification. Tetsu-to-Hagane, 1992, 78(4): 587.

[19] Seol D.J., Won Y.M., Oh K.H., et al. Mechanical behavior of carbon steels in the temperature range of mushy zone. ISIJ Int., 2000, 40(4): 356

[20] Cornelissen M.C.M. Mathematical model for solidification of multicomponent alloys. Ironmaking Steelmaking, 1986, 13(4): 204.

[21] Yamanaka A., Nakajima K., Okamura K. Critical strain for internal crack formation in continuous casting. Ironmaking Steelmaking, 1995, 22(6): 508.

[22] Kobayashi S. Relationships of fraction solid with zero ductility and zero strength temperatures during solidification. Tetsu-to-Hagane, 1987, 73, S896.

[23] Nakagawa T., Umeda T., Murata J., et al. Deformation behavior during solidification of steels. ISIJ Int., 1995, 35(6): 723

[24] Narita K., Nozaki T., Mori T., et al. On the internal cracks caused by the bending test of small ingot: Mechanism of the occurrence of internal cracks in continuously cast bloom III. Tetsu-to-Hagane, 1980, 66, S806.

[25] Hiebler H., Zirngast J., Wolf M.M., et al. Inner crack formation in continuous casting: Stress or strain criterion?. Steelmaking Conf. Proc., 1994, 77, 405.