Effect of Slag Composition on the Concentration of Al2O3 in the Inclusions in Si-Mn-killed Steel

The thermodynamic equilibria between CaO-Al2O3-SiO2-CaF2-MgO(-MnO) slag and Fe-1.5 mass pct Mn-0.5 mass pct Si-0.5 mass pct Cr melt was investigated at 1873 K (1600 A degrees C) in order 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 (mol pct MnO)/(mol pct SiO2) = 0.8(+/- 0.06) with Al2O3 content that was increased from about 10 to 40 mol pct by increasing the basicity of slag (CaO/SiO2 ratio) from about 0.7 to 2.1. The concentration ratio of the inclusion components, , and the activity ratio of the steel components, , showed a good linear relationship on a logarithmic scale, indicating that the activity coefficient ratio of the inclusion components, , was not significantly changed. From the slag-steel-inclusion multiphase equilibria, the concentration of Al2O3 in the inclusions was expressed as a linear function of the activity ratio of the slag components, on a logarithmic scale. Consequently, a compositional window of the slag for obtaining inclusions with a low liquidus temperature in the Si-Mn-killed steel treated in an alumina ladle is recommended. (C) The Minerals, Metals & Materials Society and ASM International 2013