Diffusional and Partitionless Ferrite-to-Austenite Phase Transformations during Intercritical Annealing of Medium-Mn Steels

Experimental research on intercritical annealing of a cold-rolled Fe-7Mn steel with ultralow carbon concentration is presented, providing insight into the efficacy of generating Mn-enriched austenite during relatively short (approximate to 1000 s) heat treatments compared with batch annealing, as well as the phase transformation mechanisms that can occur during intercritical annealing. It is shown through both bulk characterization and electron microscopy in conjunction with energy-dispersive X-ray spectroscopy that Mn partitioning to austenite occurs via diffusional, partitioning growth of austenite during these shorter treatments and that the intercritical temperature influences the level of Mn enrichment in austenite. Evidence of massive (partitionless) and diffusional (partitioning) transformations of ferrite to austenite occurring simultaneously in a single sample are also shown along with a thermodynamic rationale for this occurrence, which is suggested to be related to residual Mn-banding that develops during solidification.