Evolution of non-metallic inclusions in 316L stainless steel during refining and casting process

The formation and evolution of non-metallic inclusions in 316L stainless steel produced by AOD-LF-Continuous Casting (CC) process were investigated through systematic sampling of liquid steel and slabs. Analysis methods included total oxygen and nitrogen contents, optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results show that total oxygen content presents a downward trend except at the start and end of ladle pouring, which indicates that there exists re-oxidation during ladle change practice. The morphology and composition of inclusions had significantly changed during the AOD, ladle treatment and casting process. After Fe-Si addition, spherical complex inclusions mainly consisting of SiO2-CaO were observed. The content of MgO and Al2O3 in the calcium silicate type inclusions continuously increases as the steel melts transfer from AOD to LF process and then, the spinel phase could be precipitated in the calcium silicate matrix during solidification process from tundish to slabs. Thermodynamic calculations were carried out using FactSage commercial software, which agreed well with the experimental results.