Analysis of γ → α transformation in a Nb micro-alloyed C-Mn steel by phase field modelling

The phase field model is used to simulate the γ → α transformation in a Nb micro-alloyed C-Mn steel during cooling from different austenitisation temperatures. The initial austenitic microstructure and the nucleation conditions, derived by metallographic tests, are set as input data of the model. The interface mobility is taken as representative for the effect of NbC on the phase transformation. It is used as a fitting parameter to optimise the agreement between the experimental ferrite fraction curve, obtained by dilatometry, and the simulated ferrite fraction curve. A decrease of the γ/α interface mobility is found when the austenitisation temperature decreases from 1373 to 1173 K as a consequence of the presence of NbC precipitates during austenitisation. The presence of NbC has a distinct effect on the nature of the phase transformation with respect to the interface-controlled or diffusion-controlled mode.