Formation and evolution of nonmetallic inclusions in 27SiMn steel
SONG Bao-min1, SHI Shu-guang1, LIU Kun-long2, LÜ Ming2
1. Shagang Group, Anyang Yongxing Special Steel Co., Ltd., Anyang 455113, Henan, China; 2. School of Metallurgical Engineering,Xi’an University of Architecture and Technology, Xi’an 710055,Shaanxi,China
Abstract:In order to study the formation and evolution of non-metallic inclusions in 27SiMn steel, the production status of a steel plant was systematically analyzed. The evolution behavior of inclusions in each stage of “LF inlet→before calcium treatment→after soft blowing→tundish→slab” was explored through industrial experiments. According to the current situation of calcium treatment of 27SiMn steel, the Al-Ca equilibrium reaction in the molten steel treated with calcium was calculated by thermodynamics. The results show that the inclusions before calcium treatment are mainly CaO-MgO-Al2O3 ternary inclusions, and the mass fraction of Al2O3 is about 67%. After soft blowing, in the ternary phase diagram of CaO-MgO-Al2O3 system, the mass fraction of Al2O3 was reduced by about 35%, and the inclusions moved to the side with high CaO mass fraction on average. In the ternary phase diagram of CaS-CaO-Al2O3 system, the inclusion composition does not enter the liquid phase region because of the high CaS mass fraction in the inclusion composition, which is about 21%. In the casting process, most of the inclusions were transformed into spherical CaO-MgO-Al2O3-SiO2 composite inclusions, and a large number of CaS inclusions were observed. Through thermodynamic analysis, at 1 873 K, when w([Al])=0.022% in steel, w([Ca]) is controlled between 0.001 3% and 0.003 5%, which is in the region conducive to the formation of 12CaO·7Al2O3, which can improve the liquid modification effect of inclusions. At the same time, in order to prevent the precipitation of pure CaS inclusions in molten steel, w([S]) should be controlled below 0.004 8%.
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