Article ID: ISIJINT-2018-584
The inclusion of CaO–Al2O3 is occasionally observed during the secondary refining of Al-killed steel. In this study, MgO and CaO saturated slag was reacted with Al-killed steel in a MgO crucible, and the dissolution behaviors of Mg and Ca from the slag and the change in composition of inclusions were considered. Both the MgO and CaO in the slag were reduced from the presence of Al in the steel, and the concentration of dissolved Mg and Ca increased with the reaction time and Al concentration. When the Al concentration in the steel was 0.25 mass%, the concentration of dissolved Mg was 30 ppm or higher, whereas that of Ca was only 0.3 ppm. The initial Al2O3 inclusions transformed into a MgO·Al2O3 spinel, and finally changed into MgO inclusions rather than CaO–Al2O3 type inclusions. When the Al concentration was 0.75 mass%, the dissolved Ca concentration increased to 0.9 ppm, and the CaO–Al2O3 type inclusion with MgO was observed at 120 min. When the Al concentration further increased to 2.5 mass%, the concentrations of dissolved Mg and Ca increased to 110 and 3 ppm, respectively. The initial Al2O3 inclusions transformed into MgO inclusions within 5 min, and a CaO-Al2O3-type inclusion with MgO was observed after a 10 min reaction period. Finally, all inclusions transformed into a CaO-Al2O3-type inclusion with MgO at 120 min.