Austenite formation in a medium-Mn steel during intercritical annealing via in situ high-energy X-ray diffraction

The microstructural evolution of a prototype Fe-0.15C-5.56Mn-1.1Si-1.89Al medium-Mn third-generation advanced high strength steel (3G AHSS) with a martensite-ferrite (MF) starting microstructure during intercritical annealing was determined in situ using high energy X-ray diffraction (HEXRD). Intercritical annealing was carried out at 665 °C and 710 °C for 120 s and 240 s, followed by cooling to room temperature. HEXRD data were analyzed to monitor the austenite fraction and lattice parameters throughout the thermal cycle. During isothermal holding, the austenite fraction increased, up to 31% and 45% for the samples annealed for 120 s, and up to 33% and 46% for the samples annealed for 240 s at 665 °C and 710 °C, respectively. Observed changes in lattice parameters during isothermal holding were attributed to composition changes due to the partitioning of C between austenite and ferrite. Diffusion simulations using DICTRA were used to calculate solute partitioning during intercritical annealing, providing further insights into both austenite growth and the distribution of alloying elements within the austenite.