Formation Mechanism of CaS-Al₂O₃ Inclusions in Low Sulfur Al-Killed Steel After Calcium Treatment

The laboratory experiments of alumina inclusions modified by calcium treatment in Al-killed steel were carried out at 1873 K (1600 °C), and the inclusions in steel samples were characterized at 1, 5, and 10 minutes after calcium addition. The results show that the type of inclusions after calcium treatment was determined by the sulfur and T.O contents of steel. CaS-Al₂O₃ inclusions were obtained in steels with high sulfur and low T.O contents. The mass ratio between CaS and Al₂O₃ was determined by T.Ca and T.O contents of steel. The influence of holding time after calcium addition on the composition of inclusions was negligible. The thermodynamics for the formation of CaS-Al₂O₃ inclusions after calcium treatment was discussed, and a simple formation mechanism was proposed. Moreover, the CaO, Al₂O₃, and CaS contents in the inclusions were predicted through the sulfur, total calcium (T.Ca), and T.O contents, and it was found that the CaO content decreases with increasing S/T.O, while (pctCaS)/(pctAl₂O₃)^1/3 increases with increasing T.Ca/T.O.