Enhancement of the tensile properties and impact toughness of a medium-Mn steel through the homogeneous microstrain distribution

In this study, we reveal that strain partitioning control is an essential technique to improve the mechanical response of medium-Mn steels by the micro digital image correlation analysis. An Fe-12Mn-0.06C-3Al (wt%) steel, which shows transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP), was used as a model alloy. Two alternative austenitization treatments at 850 °C (LA) or 1100 °C (HA) for 10 min were conducted on cold-rolled steels to achieve different sizes of prior γ grains before intercritical annealing (IA) (600–660 °C for 1 h). The samples, which were annealed at the same temperature, showed two-phase nanolaminate microstructures of tempered martensite (α'_temp) and retained austenite (γ_R) with similar phase fraction regardless of the austenitization temperature. When annealed at the same temperature, the LA specimen showed smaller colonies of γ_R grains with the same crystallographic orientation and spatial alignment as compared to the HA specimen. The LA specimen revealed higher strength, ductility, and toughness than the HA specimen. The main reason for this mechanical response is related to microstrain localization during deformation. Due to the appearance of colonies of γ_R with the same crystallographic orientation and spatial alignment, the microstrain tends to be localized in colonies with favorable orientation and alignment to the tensile axis apart from strain partitioning to the γ_R. The LA specimen with smaller colonies of γ_R exhibits a more homogeneous microstrain distribution relative to the HA specimen. This results in more global TRIP and TWIP behaviors during plastic deformation of LA specimen, leading to the improved mechanical responses.