Thermodynamic assessment of CaO-SiO2-Al2O3-MgO-Cr2O3-MnO-FetO slags for refining chromium-containing steels

The slag system of CaO-SiO2-Al2O3-MgO-Cr2O3-MnO-FetO relevant to refining chromium-containing steels such as bearing steel is thermodynamically assessed at 1873 K. The activity coefficient of FetO shows an initially rapid increment followed by a gradual reduction according to Cr2O3 content at a constant basicity, and decreases with increasing slag basicity. gamma(MnO) is decreased abruptly by increasing Cr2O3 content and thereafter, maintains a nearly constant level. From the standpoint of inclusion control, the Cr2O3 presence in ladle refining slags is thermodynamically harmful in that it minimizes the inclusion level by inducing the increment of gamma(FetO) even though Cr2O3 exists in extremely small amounts. However, it is beneficial in that it diminishes Al reoxidation by decreasing gamma(MnO). The presence of carbon in slag decreases gamma(FetO) and gamma(MnO), which turns out to be favourable for the reduction of Al reoxidation. The thermodynamic equilibria of chromium and manganese are quantified in terms of FetO and Cr2O3 content as well as slag basicity by using multiple regression analysis. L-Cr and L-Mn are increased by the presence of Cr2O3, indicating a low recovery efficiency of Cr and Mn in the treatment of ferroalloy addition. In determining L-S values, Cr2O3 is not sa important as the basicity of slags.