Effects of FeO and CaO/Al2O3 Ratio in Slag on the Cleanliness of Al-Killed Steel

Abstract

The slag composition plays a critical role in the formation of inclusions and the cleanliness of steel. In this study, the effects of FeO content and the C/A (CaO/Al2O3) ratio in the slag on the formation of inclusions were investigated based on a 10-minute slag–steel reaction in a MgO crucible. The FeO content in the top slag was shown to have a significant effect on the formation of MgO·Al2O3 spinel inclusions, and critical content exists; when the initial FeO content in the slag was less than 2 pct, MgO·Al2O3 spinel inclusions formed, and the T.O (total oxygen) was 20 ppm; when the initial FeO content in the slag was more than 4 pct, only Al2O3 inclusions were observed and the T.O was 50 ppm. It was clarified that the main source of Mg for the MgO·Al2O3 spinel inclusion formation was the top slag rather than the MgO crucible. In addition, the cleanliness of the steel increased as the initial FeO content in the top slag decreased. As regards the effects of the C/A ratio, the MgO amount in the observed inclusions gradually increased, whereas the T.O content decreased gradually with the increasing C/A ratio. Slag with a composition close to the CaO-saturated region had the best effect on the inclusion absorption.