On the role of chemical heterogeneity in phase transformations and mechanical behavior of flash annealed quenching & partitioning steels

Abstract

The microstructure of advanced high-strength steels (AHSSs) is usually designed via adjusting austenite decomposition behavior upon cooling, while relatively less attention was paid to austenite formation upon heating. Here we explore the potentials of flash heating in tuning the microstructure and mechanical behavior of Quenching & Partitioning (Q&P) steels with an emphasis on the role of chemical heterogeneity. Besides substantially refining intercritical austenite grains (e.g. austenite formed during intercritical annealing), it was interestingly found that flash heating can also allow intercritical austenite to inherit Mn heterogeneity in the original pearlite-ferrite microstructure due to the kinetic mismatch between the sluggish diffusion of Mn and the rapid austenite formation. Chemical heterogeneity can to a large extent alter the decomposition of intercritical austenite and carbon partitioning upon cooling, and plays a notable role in enhancing thermal stability of austenite. The role of chemical heterogeneity in austenite decomposition and carbon partitioning behavior was explained via phase field simulations. The flash treated Q&P (FQP) steels have a broad range of tensile strength (from 980 MPa to 1180 MPa) and good ductility, which outperforms the conventional Q&P (CQP) steels. The current study demonstrates that flash heating opens alternative routes to create unique microstructures and improve the mechanical performance of AHSSs.