5th International High Manganese Steel Conference - HMNS 2022

The growing demands of weight saving, emission reduction and passenger safety in the automotive industry have promoted new concepts for medium-Mn steels design to achieve an excellent combination of high strength and superior ductility. Tailoring the microstructural evolution for mechanical enhancement requires deep understanding of austenite reversion mechanism. The previous research work on the transformation kinetics revealed many discrepancies between experimental measurements and the theoretic prediction by the classical diffusive transformation theory. The theory also predicts a sharp Mn concentration gradient in austenite, i.e. Mn spike, near the austenite/ferrite phase interface after a short period of reverse transformation, which have not been experimentally confirmed until now.
In this study, we applied cyclic partial-transformation annealing in medium-Mn steel to reserve the transit elemental partitioning features of migration interface and to produce multi Mn spike in front of the 𝛼/𝛾 phase boundary in Fe-5Mn-2Al-0.1C (wt. %) medium-Mn steel. By DICTRA simulation, the annealing parameters are optimized in the homogenized samples, in order to include the region of interest in atom probe tomography (APT) tip. The qualitative and quantitative analyses of microstructure are achieved by EBSD and synchrotron X-ray diffraction technique (SYXRD). SYXRD technique detected a significant amount of austenite formed during transit cyclic partial-transformation annealing, showing high transformation kinetics. The C concentrations in constitution phases are in good consistency between APT and SYXRD analysis. Mn concentration profile reveals multi-spike characteristics, similar as predicted by DICTRA simulation. Extremely high austenite reversion kinetics is found to be controlled under the negligible partitioning local equilibrium (NPLE) state and be correlated with the preserved carbon enrichment.