Thermodynamic Analysis of Mn-depleted Zone Near Ti Oxide Inclusions for Intragranular Nucleation of Ferrite in Steel

A series of thermodynamic analyses have been performed to elucidate possible mechanism of Mn-depleted zone (MDZ) development near Ti oxide inclusions as nucleation sites of Intragranular Acicular Ferrite (IGF) transformation in steels, using a computational thermodynamic approach (CALPHAD). It is demonstrated that thermodynamic calculations are able to reproduce experimentally known inclusions evolution in the steels. The MDZ development near the Ti2O3 inclusions is shown to be a result of Mn absorption into the Ti2O3. Moreover, from thermodynamic analysis of phase equilibria in the Mn Ti complex oxide system, it is proposed that a phase change of inclusion from (Ti3O5) s.s. (pseudobrookite structure solid solution dissolving Mn) to (Ti2O3) s.s. (ilmenite structure solid solution dissolving Mn) accelerate the Mn absorption into the Ti2O3 inclusion in steel. From a series of thermodynamic calculations, optimum thermal heating condition to enhance Mn absorption into Ti2O3 inclusions as well as IGF formation is discussed.