To understand the thermodynamic characteristics of the melting process for high manganese twinning-induced plasticity steel, the activity coefficient of FeO in molten slag of MnO–CaO–SiO2–FeO–MgO slag system with up to 52 mass% MnO was measured at 1450°C by the experiments of the iron equilibrium between liquid silver and molten slag under the mixed gas atmosphere of CO, CO2 and Ar. The effects of MnO, FeO and the basicity on the activity coefficient of FeO in molten slag were discussed. The relationships between the activity coefficients of FeO, the concentrations of FeO and MnO and the basicity ((mass% CaO + mass% MgO)/mass% SiO2) were investigated by the regression analysis method. The results show that the activity coefficient of FeO was proportional to the reciprocal of FeO concentration, and increased slightly with the increasing MnO, but slowly decreased by increasing the basicity under the research conditions.
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