Austenite Nucleation and Growth as a Function of Starting Microstructure for a Fe–0.15C–5.56Mn–1.1Si–1.89Al Medium‐Mn Steel

The effects of starting microstructure and intercritical annealing 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 are examined. The starting microstructures comprise 1) an as‐received cold‐rolled (CR) microstructure containing a significant fraction of ferrite and tempered martensite and 2) an austenitized and quenched martensite–ferrite (MF) microstructure. Based on the microstructural observations, two different scenarios for austenite formation during intercritical annealing are proposed. For the CR starting microstructure, austenite can nucleate at ferrite/cementite interfaces, and at ferrite grain boundaries. In the case of the MF starting microstructure, which contains thin films of interlath retained austenite (RA), austenite forms on the martensite lath boundaries or grows directly from the existing interlath RA. The studies are interpreted using the DICTRA module of Thermo‐Calc. At 665 °C, the austenite reversion kinetics of the MF samples are faster than the transformation kinetics in the CR structure. At higher temperatures where most of the cementite has dissolved, the austenite fraction in both starting microstructures is very similar.