Thermodynamics for the influence of slag composition on the inclusion control in semi-killed liquid steels

Abstract

Thermodynamic equilibria between CaO-Al2O3-SiO2-CaF2-MgO (-MnO) slag and Fe-l.5%Mn-0.5%Si-0.5%Cr steel was investigated at 1873 K to understand the effect of slag composition on the concentration of Al2O3 in the inclusions in Si-Mn-killed steels. The composition of the inclusions were mainly equal to (MnOy(SiO2)=0.8(±0.06) (mole ratio) with various Al2O3 content which increased by increasing the basicity of slag. The concentration ratio of the inclusion components, (MnOx Al2O3/SiO2) and the activity ratio of the steel components showed a good linear relationship in a logarithmic scale, indicating that the activity coefficient ratio of the inclusion components was not significantly changed. From the slag-metalinclusion multiphase equilibria, the concentration of Al2O3 in the inclusions was expressed as a linear function of the activity ratio of the slag components in a logarithmic scale. Consequently, the compositional window of the slag for obtaining the inclusions with low melting point in the Si-Mn-killed steel treated in an alumina refractory was recommended.