Synopsis : The effect of Al addition in the middle of the metal-slag reaction on the formation of MgO·Al 2 O 3 spinel-type inclusions was investigated by laboratory-scale experiments and a kinetic model calculation in order to reduce the spinel-type inclusions in high-carbon steel. As results of the experiments, the total Mg content in the steel and average content of MgO in the inclusions were relatively low before Al addition, and spinel-type inclusions were hardly formed. After Al addition,

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... pinel-type inclusions formed when CaO/SiO 2 and CaO/Al 2 O 3 in the slag were high, and the total Mg content in the steel and average MgO content in the inclusions were also higher. On the other hand, formation of spinel-type inclusions was suppressed at lower CaO/SiO 2 and CaO/Al 2 O 3 in the slag. Therefore, the experimental results indicated that addition of Al at the midpoint in the reaction and control of the slag composition were effective for suppression of spinel-type inclusions. However, spinel-type inclusions formed soon after Al addition in slag with higher CaO/SiO 2 and higher CaO/Al 2 O 3 . To evaluate the effect of midpoint addition of Al on the actual process, a kinetic model calculation under virtual conditions was carried out. According to the calculation, the increase in the content of Mg in the steel under actual-scale conditions was slower than that in the laboratory, and formation of spinel-type inclusions could be avoided.