Effect of inclusions on the solidification structures of ferritic stainless steel: Computational and experimental study of inclusion evolution

The effect of oxide and nitride inclusions in a steel melt on the formation of the equiaxed grain structure during solidification of ferritic stainless steel has been investigated. The solidified grain size decreased with an increasing content of titanium. In steel samples with large solidified grains, the inclusions were generally a two-phase system in which the titanium oxide was precipitated in the liquid TiOx–Cr2O3–SiO2 matrix during cooling. Alternatively, in steel samples with fine equiaxed grains, single TiN and MgAl2O4–TiN complex particles were observed. MgO–Al2O3–TiOx ternary compounds formed in molten steel, and the spinel crystals grew at the expense of the liquid phase as the temperature decreased. Concurrently, the TiN nucleated on the surface of the MgAl2O4 particles because the lattice disregistry between MgAl2O4 and TiN was low. The formation behaviors of non-metallic compounds were successively predicted via thermochemical computation. Single mode log-normal distributions with mode particle diameters (dmode) were observed in many samples, whereas a bimodal distribution was obtained in solidified samples with a fine-grained equiaxed structure. The grain sizes of the solidified samples decreased when the mean diameter of the inclusions increased. Consequently, the solidification structure can be interpreted based on the effectiveness of TiN and MgAl2O4–TiN complex inclusions as inoculants for the nucleation of δ-Fe. ► Steel composition was controlled by flux/metal equilibrium before solidification. ► Inclusion evolution and the phase equilibria in FSS were thoroughly computed and experimentally proved. ► The role of TiN/MgAl2O4 single and/or complex solids in grain refinement was elucidated. ► Bimodal distribution of inclusions was found in fine-grained equiaxed structure.