Formation and Evolution of Oxide Inclusions in Titanium-Stabilized 18Cr Stainless Steel

During Ti-stabilized stainless steelmaking process, oxide inclusions in steel generally cause the clogging of submerged entry nozzle and surface defects of cold-rolled products. Therefore, the evolution mechanism of oxide inclusions in Ti-stabilized 18Cr stainless steel was investigated by industrial experiments. The characteristics of inclusions in specimens were analyzed by scanning electron microscopy and energy dispersive spectroscopy. After Al deoxidation, the main inclusions were irregular MgO-Al2O3 spinel. After calcium treatment, MgO-Al2O3 inclusions were modified to be spherical multilayer CaO-MgO-Al2O3 inclusions consisting of spinel crystal embedded in CaO-Al2O3 liquid matrix. Thermodynamic calculation indicated that several ppm Ca could significantly expand the liquid oxides phase in Mg-Al-O phase diagram. After Ti addition, multilayer CaO-MgO-Al2O3-TiOx inclusions were formed. The compositions of steel were located close to Al2O3-TiOx liquid oxide phase, which would help to reduce oxide inclusions and increase titanium yield. Titanium addition has modified spinel inclusions to multilayer MgO-Al2O3-Ti3O5 inclusions containing solid spinel inner layer and MgO-Al2O3-Ti3O5 liquid oxide outer layer. As for improving the cleanliness of molten steel, the contents of magnesium, aluminum, and titanium could be considered simultaneously to liquefy oxide inclusions during Ti-stabilized stainless steelmaking process.