Tailoring thermal stability of retained austenite in thermomechanically processed medium-Mn steel via quenching and partitioning process

Tailoring thermal stability of austenite through a Q&P (quenching and partitioning) thermal cycle is essential for obtaining a beneficial strength-ductility balance. In this study, the thermal stability of austenite during the Q&P process applied for thermomechanically processed 0.17C-4.2Mn-0.98Al-0.87Si-Nb steel was analyzed. Q&P thermal cycles in which the partitioning steps were carried out in a temperature range from 350 to 450 degrees C for time from 90 to 900 s were investigated. The diffusion simulations using DICTRA (TM) were performed to assess carbon partitioning during the Q&P process providing further insights into distribution of carbon at austenite/martensite interface. The microstructure evolution during partitioning step was quantitative and qualitative investigated using techniques with different resolution. The thermal stability of austenite was tailored by optimizing its fraction during a quenching step and by minimizing the formation of undesirable carbides and bainite during the partitioning step. As a consequence, the formation of undesired secondary martensite (SM) was significantly reduced. The thermal stability of RA resulting from an increase in carbon content can be enhanced by increasing the partitioning temperature to 450 degrees C (90 s) or by extending the process duration at 400 degrees C to 300-900 s. This improvement is associated with an increase in carbon content in RA as partitioning temperature and time increase. The final microstructures consist of low-C martensite and up to 16 vol.% of lath-type retained austenite (RA) with minimal fraction of SM.