A study of the effect of prior austenite grain size on the microstructure of Quenching and Partitioning steels with Phase-Field Modelling

Abstract

Quenching and partitioning (Q&P) is a novel processing route that can create microstructures that combine high strength, ductility and easy formability without the excess use of alloying elements by also keeping the production cost low. Therefore, this method is ideal to be applied to steels that are used in the automotive industry. The material is initially annealed at a high temperature to form austenite. Then it is quenched to a temperature below the martensite start temperature to form a controlled fraction of martensite. Second annealing follows, at a lower temperature than the first one, that stabilises the austenitic phase at room temperature. It has been found that the austenite grain size at the end of the first annealing (prior austenite grain size) has a significant impact on the processing method, the final microstructure and consequently on the final properties of the material. In this thesis, phase-field modelling was employed to investigate the role of prior austenite grain size on the quenching and partitioning process. Simulations of the processing were performed for three different prior austenite grain sizes (6, 25 and 67 μm). Microstructures consisting of about 10.5% austenite and 89.5% martensite were created by simulating the first quenching of the Q&P process. Then, the evolution of the microstructure and carbon distribution were investigated for partitioning at 400 ◦C for times up to 4000 s, and quenching to room temperature. The results gave insights into the effect of the prior austenite grain size on the morphology of the microstructure and the kinetics of carbon during partitioning. It was found that a finer microstructure was derived with decreasing PAGS. Moreover, partitioning becomes more efficient because of the shorter distance that carbon has to diffuse, and the even spatial distribution of austenite. Finally, wide grain size distributions of the austenite that is present during partitioning can lead to significant variations in the carbon content among the austenite grains of the final microstructure, especially for short partitioning times. This affects the efficiency of the partitioning process and has an impact on the mechanical stability of the austenite in the final microstructure and consequently on the TRIP phenomenon.