Coupled strengthening in a medium manganese lightweight steel with an inhomogeneously grained structure of austenite

The deformation mechanism of a medium-Mn lightweight steel with an inhomogeneously grained structure of austenite was investigated as a function of annealing temperature. All annealed specimens exhibited three different phases: γ-austenite, δ-ferrite and α'-martensite. Specimens annealed at temperatures below 1000 °C exhibited high strain-hardening rates (SHRs) and good combinations of ultimate tensile strength and ductility (∼35,000-37,000 MPa%) due to the sequential occurrence of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) during tensile deformation. The SHR-true strain curves of the annealed specimens are divided into four different stages: dynamic recovery of dislocations (stage I), active TRIP (stage II), slow TRIP (stage III) and mechanical twinning (stage IV). However, the specimen annealed at 1000°C did not exhibit stage IV, most likely due to its coarse grain size. Whereas the TRIP occurred in coarse γ-austenite grains at small tensile strains, the TWIP took place in fine γ-austenite grains with a size of less than ∼10 μm at large tensile strains. This result indicates that grain refinement induced the transition in deformation mechanism from the TRIP to the TWIP. The inhomogeneously grained structure of γ-austenite in the annealed specimens consists of coarse grains pre-existing in the hot-rolled state and fine grains newly formed during annealing.