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In the current study, the reduction of CaO-Al₂O₃ inclusions in steel by the subsequent addition of aluminum is revealed through industrial trials, laboratory experiments, and thermodynamic calculations. During the industrial production of the Al-killed Ca-treated steel, the Al₂O₃ content in CaO-Al₂O₃ inclusions increased from 72.05% to 79.83%, after the aluminum addition. In laboratory experiments, the addition of aluminum increased the Al₂O₃ content, and meanwhile lowered the contents of CaS and CaO in the inclusions. The average diameter of the inclusions decreased and the number density increased after the Al addition in steel, due to the formation of tiny inclusions. Based on the thermodynamic consideration, the transformation of the inclusions was attributed to the re-equilibrium of the reaction between steel and the inclusions after aluminizing. The Al₂O₃-riched calcium aluminate with a high melting point increased the possibility of the nozzle clogging during the continuous casting process, reducing the effect of the calcium treatment. Therefore, the addition of aluminum after calcium treatment should be avoided as far as possible.