Effect of refining slag basicity on transformation and removal of inclusions in Al-killed S-containing steel

A set of chemical equilibrium experiments between an Al-killed S-containing steel Cf53 and CaO-6 wt% MgO-Al 2 O 3 -SiO 2 slag were conducted at 1873 K using a tubular resistance furnace to analyze the impacts of slag basicity on the transformation mechanisms of inclusions in the steel. The results indicated that increasing the slag basicity from 3 to 5 and 7 while maintaining CaO/Al 2 O 3 mass ratio in slag at 1.6 led to a reduction in T.O and T.S contents, along with an increase in dissolved [Ca], [Al], and [Mg] contents of the liquid steel. Both the number density and average size of inclusions decreased as the slag basicity increased, and the major inclusion type was transformed from CaO-MgO-Al 2 O 3 -SiO 2 into CaO-MgO-Al 2 O 3 . Thermodynamic analysis indicated that [Si] in steel could reduce a part of Al 2 O 3 within CaO-MgO-Al 2 O 3 inclusions to form CaO-MgO-Al 2 O 3 -SiO 2 when slag basicity was 3. However, as slag basicity increased to 5 and 7, the elevated [Ca] and [Mg] contents promoted the reduction of SiO 2 within CaO-MgO-Al 2 O 3 -SiO 2 inclusions, facilitating their transformation into CaO-MgO-Al 2 O 3 inclusions. The increased [Mg] content also led to MgO saturation within the inclusions and pure MgO phase precipitated. CaS-containing inclusions were only generated in the steels with slag basicity of 5 and 7, which formed during steel solidification process according to thermodynamic analysis. Additionally, a mathematical model was developed to describe the inclusion movement characteristics at the slag-steel interface, effectively explaining the variations in inclusion number density and average size under different slag basicity conditions.