Work hardening behavior in medium MN trip steels

International audienceContinued efforts of the automotive industry to develop high strength, high formability steels have led to increased interest in so-called medium manganese steels and low density steels. This research characterized the mechanical properties and deformations of several alloys of these two families with ultra-fine-grained ferrite + austenite + (martensite) microstructures. Samples of steels with varying amounts of Mn and Al were annealed in the intercritical ferrite + austenite domain. Annealing temperatures were varied to produce different levels of retained austenite stability, which in turn determined whether transformation-induced plasticity (TRIP) or twinning-induced plasticity (TWIP) would be favored. Unidirectional tensile tests using in-situ magnetic saturation and neutron diffraction measurements were performed to characterize the plastic response, the active deformation modes, and the kinetics of the TRIP phenomena. Metallographic observations by SEM and TEM confirmed the insitu results and explained the micromechanical aspects of the martensite formation