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.
