Role of retained austenite homogeneity in suppressing flow discontinuities in a medium-Mn steel

Abstract

Medium-Mn steels (MMnS) achieve attractive strength–ductility combinations through retained-austenite-assisted deformation, but their plastic flow can be interrupted by yield-point phenomena and dynamic strain aging. Although retained austenite heterogeneity is often considered beneficial for progressive transformation-induced plasticity, its role in controlling flow stability remains unclear. Here, a cold-rolled Fe–0.4C–6Mn–2Al–1Si–0.05Nb MMnS was subjected to a two-step intercritical annealing route to systematically vary the fraction, stability, and spatial distribution of retained austenite. The first annealing step produced a ferritic matrix containing retained austenite, part of which transformed to strain induced martensite during subsequent cold rolling. During the second annealing step, deformation-inherited austenite (γpre) and newly formed austenite (γnew) developed in different fractions. X-ray diffraction, electron microscopy, tensile testing, and quasi in-situ magnetic measurements show that the tensile response is governed not only by retained austenite fraction, but also by its stability and spatial distribution. Heterogeneous, relatively stable retained austenite containing both γpre and γnew delays strain-induced martensitic transformation and promotes discontinuous yielding. In contrast, a more homogeneous and less stable γnew-dominated microstructure activates transformation-induced plasticity earlier and more gradually, leading to continuous yielding and stable plastic flow while maintaining approximately 850 MPa yield strength, 1 GPa ultimate tensile strength, and 30% total elongation. At excessive effective annealing severity, achieved either by higher annealing temperature or longer holding at intermediate temperatures, the least mechanically stable retained austenite, together with fresh martensite, promotes serrations in the stress-strain curve associated with dynamic strain aging. These results demonstrate that mitigating flow discontinuities in MMnS requires a balanced retained-austenite architecture, combining sufficient retained-austenite fraction with controlled stability and chemical–morphological homogeneity.