Abstract:

The non-metallic inclusions in Ti-IF steel smelted by semi-steel was studied by thermal test to improve the cleanliness of molten steel and reduce the size of inclusions for process optimization. The research shows that the inclusions are spherical FeO-MnO in LF inlet, the main inclusions are wrapped and clustered MgO-FeO-MnO-Al₂O₃ in LF outlet. The main inclusions are mainly spherical, spindle-shaped and irregular FeO-Al₂O₃ after RH decarburization, and are mainly triangular, quasi-spherical and clustered Al₂O₃ after RH aluminum deoxidation for 3 minutes. After Ti alloying for 3 minutes, the main shapes of inclusions are hexagonal TiO₂-Al₂O₃, spindle-shaped and almost diamond-shaped Al₂O₃. After leaving the RH station，the inclusions are spherical TiO₂-Al₂O₃, spindle-shaped and nearly diamond-shaped Al₂O₃ inclusions. The inclusions in the slab are mainly diamond-shaped TiN, quasi-spherical Al₂O₃ and irregular-shaped Al₂O₃-TiN. The non-metallic inclusions in the original process that affect the surface quality of Ti-IF steel and the purity of molten steel are mainly large-sized Al₂O₃ (≥15μm). By optimizing the oxygen lance nozzle, RH process parameters and tundish flow field, the T.O mass fraction in slab is reduced to 16.3×10^-6, the maximum non-metallic inclusion size is reduced to 9.5 μm. So the cleanliness of Ti-IF molten steel is significantly improved.

Key words: semi-steel, Ti-IF steel, nonmetallic inclusions, cleanliness, slab