Formation mechanism and control of MgO·Al₂O₃ inclusions in non-oriented silicon steel

Abstract

On the basis of the practical production of non-oriented silicon steel, the formation of MgO·Al₂O₃ inclusions was analyzed in the process of “basic oxygen furnace (BOF) → RH → compact strip production (CSP)”. The thermodynamic and kinetic conditions of the formation of MgO·Al₂O₃ inclusions were discussed, and the behavior of slag entrapment in molten steel during RH refining was simulated by computational fluid dynamics (CFD) software. The results showed that the MgO/Al₂O₃ mass ratio was in the range from 0.005 to 0.017 and that MgO·Al₂O₃ inclusions were not observed before the RH refining process. In contrast, the MgO/Al₂O₃ mass ratio was in the range from 0.30 to 0.50, and the percentage of MgO·Al₂O₃ spinel inclusions reached 58.4% of the total inclusions after the RH refining process. The compositions of the slag were similar to those of the inclusions; furthermore, the critical velocity of slag entrapment was calculated to be 0.45 m·s⁻¹ at an argon flow rate of 698 L·min⁻¹, as simulated using CFD software. When the test steel was in equilibrium with the slag, [Mg] was 0.00024wt%–0.00028wt% and [Al]s was 0.31wt%–0.37wt%; these concentrations were theoretically calculated to fall within the MgO·Al₂O₃ formation zone, thereby leading to the formation of MgO·Al₂O₃ inclusions in the steel. Thus, the formation of MgO·Al₂O₃ inclusions would be inhibited by reducing the quantity of slag entrapment, controlling the roughing slag during casting, and controlling the composition of the slag and the MgO content in the ladle refractory.

Keywords

silicon steel / steelmaking / inclusions / deoxidation / slag entrapment / thermodynamics / kinetics

References

[1] Song YQ, Li JS, Li J, Wang ZF, Yang SF. MgO·Al₂O₃ based inclusions in gas cylinder steel during refining. J. Univ. Sci. Technol. Beijing, 2009, 31(Suppl.1): 130

[2] Yang J, Wang XH. Thermodynamics analysis and control of formation of magnesia-alumina spinel during refining of ultra-low oxygen. Iron Steel, 2011, 46(7): 26

[3] Beskow K, Tripathi NN, Nzotta M, Sandberg A, Du SC. Impact of slag refractory lining reactions on the formation of inclusions in steel. Ironmaking Steelmaking, 2004, 31(6): 321.

[4] Itoh H, Hino M, Banya S. Thermodynamics on the formation of spinel nonmetallic inclusion in liquid steel. Metall. Mater. Trans. B, 1997, 28(5): 953.

[5] Park JH, Kim DS. Effect of CaO-A1₂O₃-MgO slags on the formation of MgO-A1₂O₃ inclusions in ferritic stainless steel. Metall. Mater. Trans. B, 2005, 36(4): 495.

[6] Park JH. Formation mechanism of spinel-type inclusions in high-alloyed stainless steel melts. Metall. Mater. Trans. B, 2007, 38(4): 657.

[7] Steel Res. Int., 2006, 77(1p.785)

[8] Seo CW, Kim SH, Jo SK, Suk MO, Byun SM. Modification and minimization of spinel (Al₂O₃·_xMgO) inclusions formed in Ti-added steel melts. Metall. Mater. Trans. B, 2010, 41(4): 790.

[9] Jiang M, Wang XH, Chen B, Wang WJ. Laboratory study on evolution mechanisms of non-metallic inclusions in high strength alloyed steel refined by high basicity slag. ISIJ Int., 2010, 50(1): 95.

[10] Jiang M, Wang XH, Wang WJ. Control of non-metallic inclusions by slag-metal reactions for high strength alloying steels. Steel Res. Int., 2010, 81(9): 759.

[11] Yang SF, Li JS, Wang ZF, Li J, Lin L. Modification of MgO·A1₂O₃ spinel inclusions in Al-killed steel by Ca-treatment. Int. J. Miner. Metall. Mater., 2011, 18(1): 18.

[12] Deng ZY, Zhu MY. Evolution mechanism of non-metallic inclusions in Al-killed alloyed steel during secondary refining process. ISIJ Int., 2013, 53(3): 450.

[13] Jiang M, Chen B, Yang W, Wang XH. A study on thermodynamics of spinel inclusions formation in an alloy structural steel. Spec. Steel, 2008, 29(1): 16

[14] Jiang M, Wang XH, Chen B, Wang WJ. Formation of MgO·Al₂O₃ inclusions in high strength alloyed structural steel refined by CaO-SiO₂-Al₂O₃-MgO slag. ISIJ Int., 2008, 48(7): 885.

[15] Todoroki H, Mizuno K. Effect of silica in slag on inclusion compositions in 304 stainless steel deoxidized with aluminum. ISIJ Int., 2004, 44(8): 1350.

[16] Park JH, Todoroki H. Control of MgO·Al₂O₃ spinel inclusions in stainless steels. ISIJ Int., 2010, 50(10): 1333.

[17] Fujii K, Nagasaka T, Hino M. Activities of the constituents in spinel solid solution and free energies of formation of MgO, MgO·Al₂O₃. ISIJ Int., 2000, 40(11): 1059.

[18] Lei H, Zhu MY, He JC. Mathematical modeling of slag entrapment in continuous casting mould. Acta Metall. Sin., 2000, 36(10): 1113

[19] Okuyama G, Yamaguchi K, Takeuchi S, Sorimachi K. Effect of slag composition on the kinetics of formation of Al₂O₃-MgO inclusions in aluminum killed ferritic stainless steel. ISIJ Int., 2000, 40(2): 121.