Characterization and modelling of microstructure evolution during partitioning in Q&P steels

Abstract

Development of a simulation methodology and model which combines the analytical estimation of the microstructure at different points of the Q&P route with the numerical modelling of the main physical phenomena during partitioning was done. Detailed characterization of microstructure was performed at different stages of the Q&P process: at the quenching temperature (QT), at different interrupted conditions of partitioning and after final cooling down to room temperature. In addition, some investigations during the heating step from quenching to partitioning temperature was done by measuring the RA fraction. Then, the evolution of carbon free bainite and retained austenite phase fraction during the partitioning and different processing conditions was analyzed. The austenite grain size distribution produced by the first quenching was experimentally characterized and implemented in the model. Modelling approach to describe the main phenomena taking place during the partitioning step, i.e. carbon diffusion and bainite formation, was proposed. The built one-dimensional model predicts the microstructure evolution during partitioning step in the Q&P processing: starting from quenching temperature and going to the final cooling to room temperature. Finally, model performance was evaluated using two experimental datasets: one for testing and another for the validation.