Effect of Slag Adjustment on Inclusions and Mechanical Properties of Si-Killed 55SiCr Spring Steel

The effects of the Al2O3 content and basicity of CaO–SiO2–Al2O3–10 wt.% MgO refining slag on inclusions removal in 55SiCr spring steel were investigated. The viscosity of slag was studied using a viscometer, while the microstructure investigation involved using a water-quenching furnace and a Fourier-transform infrared spectrometer. The influence mechanism of the slag adjustment on inclusions was explored through thermodynamic calculations and kinetic analysis. The results indicated that the viscosity of the molten slag increased gradually with the content of Al2O3 increasing due to it increasing the degree of polymerization of the slag network structure, especially the [AlO4]5− and [Si-O-Si] structures. In contrast, the viscosity of molten slag experienced the opposite pattern, with the basicity of molten slag increasing. This was due to the fact that Ca2+ can significantly reduce the degree of polymerization of a slag network structure, especially the percentages of the [SiO4]4−, [AlO4]5− and [Si-O-Si] network structures. Finally, the changes in physical properties and structure of slag significantly affected the removal effect of the inclusions in molten steel. As a result, the number, size distribution, composition distribution and morphology of the inclusions displayed significant changes when the content of Al2O3 increased from 3 wt.% to 12 wt.% and the basicity of the slag gradually increased from 0.5 to 1.2.