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 c hromium-containing steels such as bearing steel is thermodynamically assess ed at 1873 K. The activity coefficient of FetO shows an initially rapid inc rement followed by a gradual reduction according to Cr2O3 content at a cons tant basicity, and decreases with increasing slag basicity. gamma(MnO) is d ecreased abruptly by increasing Cr2O3 content and thereafter, maintains a n early constant level. From the standpoint of inclusion control, the Cr2O3 p resence in ladle refining slags is thermodynamically harmful in that it min imizes the inclusion level by inducing the increment of gamma(FetO) even th ough 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 equilibr ia of chromium and manganese are quantified in terms of FetO and Cr2O3 cont ent as well as slag basicity by using multiple regression analysis. L-Cr an d L-Mn are increased by the presence of Cr2O3, indicating a low recovery ef ficiency of Cr and Mn in the treatment of ferroalloy addition. In determini ng L-S values, Cr2O3 is not sa important as the basicity of slags.