Formation Mechanism and Nucleation Effect of Ti₂O₃–TiN Complex Nucleus at Solidification Front of 18Cr Ferritic Stainless Steel

Formation mechanism of Ti₂O₃–TiN complex nucleus and its nucleation effect on δ-Fe at solidification front of Fe-18Cr ferritic stainless steel was investigated through experimental and theoretical methods. According to the equilibrium calculation, Ti₂O₃ will crystallize in the molten steel before solidification and TiN will not form until the solid phases appear. However, complex nucleus and equiaxed grains exist in the edge of Ti-bearing ingot, where the solidification velocity and temperature gradient are high. Therefore, complex nucleus is indicated to be formed at solidification front and a non-equilibrium solidification model for multi-component alloy was established to investigate the dendrite tip interface phenomenon. The calculation results exhibit an element enrichment at solidification front and explain well with the formation of complex nucleus before solidification. It is also found that there is approximate lattice matching between TiN, and Ti₂O₃, δ-Fe on the basis of disregistry theory. These results are consistent with the observed complex nucleus and the increased equiaxed zone ratio (EZR) of Ti-bearing ferritic stainless steel.