5th International High Manganese Steel Conference - HMNS 2022

The effects of starting microstructure and intercritical annealing (IA) temperature on the phase transformation kinetics and microstructural evolution of a prototype Fe-0.15C-5.56Mn-1.1Si-1.89Al medium-Mn third-generation advanced high strength steel (3G AHSS) were determined. The starting microstructures comprised i) an as-received cold rolled (CR) microstructure containing a significant fraction of ferrite, tempered martensite and cementite and ii) an austenitized and quenched martensite and ferrite (approximately 10 vol%, MF) microstructure. Based on the microstructural observations, two different scenarios for austenite formation during intercritical annealing have been proposed based on the starting CR or MF microstructures. In the case of the CR starting microstructure, isolated cementite at the ferrite grain boundaries and ferrite/cementite interfaces were austenite nucleation sites. However, the MF starting microstructure, which contained thin films of inter-lath retained austenite with very little cementite, showed a different mechanism. In this case, austenite either formed on the martensite lath boundaries or grew directly from the existing inter-lath retained austenite this case. The studies of austenite growth and solute partitioning during intercritical isothermal holding were interpreted using the DICTRA module of Thermo-Calc. In the case of the CR starting microstructures, pre-existing cementite particles dis not dissolve during the 120 s intercritical annealing isothermal hold, whereas cementite particles formed during heating in the MF microstructure and then were largely re-dissolved at higher temperatures. The sluggish cementite dissolution observed during CR intercritical annealing was found to be the result of Mn partitioning to cementite during isothermal holding. Simulations indicate that the time for achieving the same volume fraction of austenite is longer in the presence of microstructural cementite, explaining the accelerated austenite reversion kinetics observed for the MF starting microstructure samples. This paper will present and discuss the experimental and modelling efforts used to come to this conclusion.