Formation mechanism of large-sized CaO-SiO2-CaF2-Al2O3-MgO composite inclusions in super 304H stainless steel rods

Abstract

Employing silicon deoxidation in the production of Super 304H stainless steel, large-sized CaO-SiO2-CaF2-Al2O3-MgO (CSFAM) composite inclusions are frequently observed in the rods, posing a substantial threat to product quality. This study delves into an analysis of the constituent phases of these large-sized inclusions, identifying the coexistence of liquid phase and spinel phase. Leveraging industrial experiments, systematic sampling of Super 304H stainless steel, and a comprehensive analysis involving dynamic and thermodynamic calculations, this study investigates the formation mechanism underlying such inclusions. The composition of inclusions in the sample before AOD tapping mirrors that of the slag, with high CaF2 content, indicating that they come from slag entrapment. Through kinetic calculations, it is indicated that the time of existence of the liquid film during the ascent of these inclusions is significantly longer than the collision time. This suggests that they are not easily removed and tend to persist in the steel. Despite LF refining, the predominant types of inclusions remain unchanged, and the decrease in temperature leads to the precipitation of solid-phase MgO within the inclusions. Solubility calculations for solid-phase MgO show a rising trend with temperature. Post-casting, MgO and Al2O3 content in inclusions increase, the solid-phase MgO region disappears, and in the subsequent cooling process, inclusions precipitate the stable spinel phase (MgAl2O4). Solubility calculations for solid-phase MgAl2O4 show a temperature-dependent increase. After the completion of casting, the steel ingot undergoes a deformation process through rolling, leading to the transformation of inclusions into large-sized, elongated, composite CSFAM inclusions.