| Summary: | Medium- and high-Mn steels have excellent properties but are very difficult to be commercially produced because of the high content of some alloy elements. To enhance the understanding of the reaction between medium/high-Mn steel and refining slag which is significantly different from the conventional steels, steel and slag composition and the inclusions were investigated by equilibrium reaction between Fe-xMn (x = 5, 10, 15, and 20 mass pct) and CaO-SiO<sub>2</sub>-Al<sub>2</sub>O<sub>3</sub>-MgO top slag at 1873 K in the laboratory. Furthermore, the effect of Mn content on inclusion transformation and steel cleanliness was also explored. After slag–steel reaction, both contents of MnO in slag and Si in steel increased. Most MnO inclusions in master steel transformed to MnO-SiO<sub>2</sub> and MnO-Al<sub>2</sub>O<sub>3</sub>-MgO. With the increase in Mn content, the amount share of MnO type inclusions decreased and that of MnO-Al<sub>2</sub>O<sub>3</sub>-MgO type increased. In addition, both the number density of observed inclusions and the calculated oxygen content in inclusions increased. Thermodynamic analysis indicates that the composition change of steel and slag and the transformation of inclusions are mainly the consequence of the reaction between Mn in molten steel and SiO<sub>2</sub> and MgO in top slag. The dissolved Mn in medium/high-Mn steel presents a strong reactivity.
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