Micro- to Nanoscale Microstructural Differences Induced by Intercritical Annealing in a Hot-Rolled Medium Manganese Steel

Abstract

The effect of austenite-reverted transformation (ART) annealing process on the chemical and microstructural properties of medium Mn steel (7 wt% Mn) is evidenced by a combination of electron probe micro-analysis, electron back-scattering diffraction, and scanning transmission electron microscopy. Hot-rolled medium Mn steels with distinct dual-phase austenite/martensite microstructures and identical bulk composition can be obtained via direct austenite reverted transformation (D-ART) as compared to a reaustenized quenched (RAQ) + ART route. The different annealing sequences applied to the steel have an effect on the fraction of austenite, its morphology, chemical composition, and distribution. The D-ART route produces a core–shell structure of Mn-rich shell and Mn-lean core inside the austenite grains, while the RAQ + ART route did not. Such systematic characterizations using a combination of different microscale and nanoscale techniques are essential as chemistry and morphological differences at the individual grain level provide a key understanding to the microstructure–property relationship of such steels.