A model for predicting austenite to bainite phase transformation in producing dual phase steels

Based on the theory of free energy change between austenite and bainite phases during the cooling period, a material model is developed to predict the incubation time of the transformation from austenite to bainite for 50CrNiMo deformed at different strain levels and austenite states. Further constitutive equations are developed to model the growth of the nucleated bainite phase. These enable the bainite volume fraction to be predicted for different material cooling conditions after hot forming. To determine the material constants within the phase transformation model, an objective function is formulated to quantify the relative errors between experimental and computed data. The phase transformation model and the objective functions are implemented into an Evolutionary Programming (EP) based software package through a user defined subroutine for the optimisation. The experimental data of 50CrNiMo is used to evaluate the capability of the model in prediction of TTT diagrams and bainite phase growth for pre-plastic deformations of 0, 25 and 50%.