Competitive Precipitation Mechanism Between Ti _xO _y and TiN Inclusions Dependent on Nucleation Pathways in Ti-Contained Steel Solidification Process

TiN and Ti_xO_y (Ti₃O₅, Ti₂O₃, TiO₂) are commonly encountered inclusions, which significantly affect the process control and the final mechanical properties of micro-alloyed steels containing Ti. Thermodynamic calculation indicates that the competitive precipitation between Ti_xO_y species depends on [pct O] and temperature at a fixed [pct Ti]₀. The critical value of [pct O]₀ for the competitive precipitation of Ti_xO_y species decreases with precipitate order from TiO₂ → Ti₃O₅ → Ti₂O₃ and decreases with decreasing temperature. The preferential precipitation sequence between Ti_xO_y and TiN depend on the closed relation with temperature, [pct N]₀ or [N]/[O] ratio. The critical values of [pct N]₀ ratio for the preferential precipitation of TiN inclusion decrease with the decreasing of temperature, and the critical [N]/[O] ratio increases with the increasing of [pct O]₀. Furthermore, DFT calculation indicated that the competitive precipitation between TiN and Ti_xO_y depend on the nucleation pathways and the relative stability of pre-nucleation clusters (Ti_xO_y)_n vs (TiN)_n. The result shows that the required [pct N] for (Ti_xO_y)_n cluster transforming into TiN inclusion increases with increasing (Ti_xO_y)_n cluster size. The critical value of [N]/[O] ratio for bulk Ti_xO_y transforming into TiN inclusion is higher than that for (Ti_xO_y)_n cluster transforming into TiN inclusion. The reaction of (TiN)_n cluster with [O] to form Ti_xO_y inclusion is facilitated by (TiN)_n cluster being highly unstable in steel.