Abstract:

For the process of steelmaking and continuous casting of ultra-low carbon steel with different oxygen contents, the evolution law of inclusions in steel were analyzed. The most common inclusions in steel change from the Al₂O₃-CaO-MgO-SiO₂ composite inclusions at the end of converter to the Al₂O₃-MnO inclusions at the RH inlet and at the end of decarburization, and then to the Al₂O₃ inclusions after aluminum deoxidation and at the RH outlet. Because the slow cooling rate in continuous casting slab is conducive to the full precipitation of TiN, the size of TiN in tundish steel sample is generally smaller than 2 μm, while the size of TiN in continuous casting slab is generally larger than 2 μm. The composite inclusions of Al₂O₃-MnS are often found in tundish, while the composite inclusions of Al₂O₃-MnS-TiN are often found in continuous casting slab. When the size of the central Al₂O₃ inclusion is large, the shape of the composite inclusion of Al₂O₃-TiN is irregular；when the size of the central Al₂O₃inclusion is small, the shape of the composite inclusion of Al₂O₃-TiN is regular. Because the oxygen mass fraction of heat O31 (31×10^-6) is higher than that of heat O22 (22×10^-6) in tundish at liquid steel quantity of ladle is 100 t, the number densities of the composite inclusions containing Al₂O₃ in tundish are higher than those of heat O22 at liquid steel quantity of ladle is 100 t and 50 t. Because of the final solidification in the center of the slab, the segregation of S, Mn, Ti, N and other elements results in the formation of a large number of fine MnS, TiN and TiN-MnS precipitates, the number density of inclusions at 1/2 of the slab width is the highest.

Key words: ultra-low carbon steel, evolution of inclusions, thermodynamic calculation, oxygen content, slag