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

摘要

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 Ti_(2)O_(3) inclusions is shown to be a result of Mn absorption into the Ti_(2)O_(3). Moreover, from thermodynamic analysis of phase equilibria in the Mn–Ti complex oxide system, it is proposed that a phase change of inclusion from (Ti_(3)O_(5)) s.s. (pseudobrookite structure solid solution dissolving Mn) to (Ti_(2)O_(3)) s.s. (ilmenite structure solid solution dissolving Mn) accelerate the Mn absorption into the Ti_(2)O_(3) inclusion in steel. From a series of thermodynamic calculations, optimum thermal heating condition to enhance Mn absorption into Ti_(2)O_(3) inclusions as well as IGF formation is discussed.