Effect of CaO/Al₂O₃ in refining slag on the cleanliness of high-carbon chromium bearing steel

The chemical composition of refining slag is of crucial importance in determining the cleanliness of all steels. The effect of the CaO to Al2O3 mass ratio in refining slag (C/Aslag) on the cleanliness of high-carbon chromium bearing steel was systematically investigated via steel-slag equilibrium experiments in a high-temperature Si-Mo tubular resistance furnace. The C/Aslag ratio in slag was designed at the values of 1.3 to 1.6, 2.0, and 2.3, while the CaO to SiO2 mass ratio was maintained at 5.0. The C/Aslag ratio firstly influenced the activities of its components: as the C/A ratio increased, the activities of CaO and MgO rose while that of Al2O3 declined. These changes in slag component activities enhanced the slag's capacity to absorb inclusions and altered the steel-slag reaction equilibrium, thereby modifying the steel composition, specifically increasing Ca and Mg contents and decreasing acid-soluble Al. Consequently, Al2O3-rich inclusions in the steel were transformed into lower-melting-point phases, which helped mitigate nozzle clogging during continuous casting. Consistent with this mechanism, experimental results showed that both total oxygen content (0.0012 wt.%, 0.0009 wt.%, 0.0007 wt.%, and 0.0006 wt.%) and inclusion number density (14.2 #/mm2, 12.2 #/mm2, 5.0 #/mm2, and 5.2 #/mm2) decreased with increasing C/Aslag ratio. These experimental observations were also explained by systematic thermodynamic calculations. Comprehensively considering the fluidity of a refining slag and its effect on the cleanliness of molten steel, the C/Aslag ratio should be controlled at 2.0 when the CaO to SiO2 mass ratio is at 5.0.