Phase field modelling of the interfacial condition at the moving interphase during the γ → α transformation in C-Mn steels

The phase field model is used to simulate the γ → α transformation in an Nb microalloyed C-Mn steel during cooling from different austenitisation temperatures. The model provides qualitative information on the developing microstructure, as well as quantitative information on both the ferrite fraction formed and the carbon concentration distribution in the remaining austenite. The initial austenitic microstructure (grain morphology and dimensions) and the nucleation conditions (nucleus density and nucleation undercooling) are set as input data of the model; they are derived by metallographic and dilatometric tests. The interface mobility is used as a fitting parameter to optimise the agreement between the experimental ferrite fraction curve, obtained by dilatometry, and the simulated ferrite fraction. A strong decrease of the γ/α interface mobility is found when the austenitisation temperature decreases from 1373 to 1173 K. A good agreement is obtained between the simulated microstructure and the actual microstructure observed after cooling.