Abstract: In order to study the influence of Al₂O₃ content in medium-basicity refining slag on steel cleanness and inclusions, the equilibrium experiment between CaO-SiO₂-Al₂O₃-MgO top slag with initial basicity of 4 and different Al₂O₃ content and Al-killed steel was carried out by MgO crucible at 1 873 K. The results show that after slag-steel reaction, the inclusion type changes from MgO-Al₂O₃-MnS to complex centered with MgO-Al₂O₃ and surrounded by MgO-Al₂O₃-SiO₂-CaO(-MnO) system. The shape changes from block and cluster shape with angular boundary to near-spherical shape with smooth boundary. With the increase of Al₂O₃ content in slag, Al₂O₃ content in inclusions increases significantly, MgO content decreases, and other components have little change. The inclusion size tends to increase. The number density of inclusions increases which indicates that the cleanliness of steel deteriorates. Under present experimental conditions, when the Al₂O₃ mass fracton is about 20 %, the number density of inclusions is the lowest, and the steel cleanliness is the best. Based on the experimental results, thermodynamic analysis was done to discuss the composition change of slag and steel as well as the transformation of inclusions.

Key words: refining slag； Al-killed steel；medium basicity；Al₂O₃ , content；non-metallic inclusion