Formation process of slag rim during hypo-peritectic steel continuous casting based on full-sectional microstructures

This study developed a full-sectional microstructure characterization method to investigate the formation process of coarse slag rims during the continuous casting of hypo-peritectic steel. Using this method, cross-sectional microstructural analysis of typical slag rims for two highly crystalline powders (Powder A and Powder B) revealed that their formation is primarily driven by the solidification of liquid slag. Distinct differences were observed in the microstructures of the two powders. Powder A (characterized by a higher breaking temperature and higher viscosity) displayed alternating lamellar microstructures of coarse and fine phases, with the coarse phases composed of akermanite-gehlenite transition phases. In contrast, Powder B (with a lower breaking temperature and lower viscosity) comprised predominantly regular akermanite-gehlenite crystals interspersed with a certain amount of glassy phases. Numerical simulation of three-phase fluid flow coupled with heat transfer indicated that slag rim formation correlates with mold oscillation. Solidification of liquid slag at the slag rim front predominantly occurs during the negative stroke of mold oscillation. The average heating rate during the mold’s ascending stage reaches approximately 100 K·s⁻¹, while the average cooling rate during the descending stage attains 400 K·s⁻¹. This temperature variation leads to the formation of lamellar microstructures, whereas the ascending stage promotes coarse structures and thicker slag rims. Due to powder properties, two distinct formation pathways exist for highly crystalline mold powders. For powders with a higher breaking temperature, higher viscosity, and narrower solidification range (Powder A), coarse microstructures and thicker slag rims preferentially form. For powders with lower breaking temperature and viscosity, and wider solidification range (Powder B), the liquid slag resists rapid solidification, and the extended mushy zone allows partial liquid slag to persist at the slag rim front, promoting the formation of a thin slag rim. This study enhances the understanding of slag rim formation in highly crystalline mold powders and provides critical insights for control of longitudinal surface cracks in hypo-peritectic steel.