Composition Change of Inclusions in High Carbon Steel before and after Addition of Aluminum

The effect of Al addition at the midpoint of the metal-slag reaction on the formation of MgO center dot Al2O3 spinel-type inclusions was investigated by laboratory-scale experiments and a kinetic model calculation with the aim of reducing spinel-type inclusions in high-carbon steel. As results of the experiments, the total Mg content in the steel and the average content of MgO in the inclusions were relatively low before Al addition, and formation of spinel-type inclusions was rare. After Al addition, spinel-type inclusions formed when CaO/SiO2 and CaO/Al2O3 in the slag were high, and the total Mg content in the steel and the average MgO content in the inclusions were also high. On the other hand, formation of spinel-type inclusions was suppressed at lower CaO/SiO2 and CaO/Al2O3 in the slag. Therefore, the experimental results indicated that Al addition at the midpoint of the reaction and control of the slag composition are effective for suppression of spinel-type inclusions. However, spinel-type inclusions formed soon after Al addition in slag with higher CaO/SiO2 and higher CaO/Al2O3. To evaluate the effect of midpoint addition of Al in the actual process, a kinetic model calculation was carried out. According to the calculation, the increase in the Mg content in the steel under actual-scale conditions was lower than that in the laboratory, and formation of spinel-type inclusions could be avoided.