The thermodynamic equilibria of manganese and phosphorus between liquid iron and CaO-MgOSat-SiO2-FetO-MnO-P2O5-Al2O3 (0–33%) ladle slag have been investigated at 1873 K from the viewpoint of Mn and P yields for the production of high-strength steels. The equilibrium distribution ratios of Mn and P were found to increase with increasing FetO content; however, these ratios vary with basicity, but they do this the other way round. The addition of alumina into slag at a fixed basicity and FetO content decreases both the equilibrium manganese and phosphorus distributions. The equilibrium distribution ratios were discussed in terms of the variation of activity coefficients of FetO, MnO and PO2.5, according to the slag basicity and Al2O3 content. The quantitative contributions of basicity and (%FetO + %MnO) on LMn and LP were empirically determined and their usefulness was discussed with the aid of plant data:
To improve Mn and P yields in the practical RH operation, it is strongly recommended that Fe-Mn and Fe-P alloys be added after Al deoxidation treatment inducing relatively high Al2O3 in slag and maintaining low FetO content. In addition, a ladle slag composition for the targeted Mn and P contents in liquid iron was substantially estimated using the empirical relationships.