5th International High Manganese Steel Conference - HMNS 2022

3rd generation AHSS (advanced high-strength steel) family encompasses different innovating microstructure concepts, among which medium Mn steels. They have ground-breaking properties due to their particular microstructures containing a nanostructured ferritic matrix and a large amount of metastable retained austenite. Under mechanical solicitation, strain induced martensitic transformation of austenite will occur, leading to an efficient TRIP effect (transformation induced plasticity).

The purpose of this work is to evaluate the stability of austenite during mechanical loading, accounting for stress partitioning between phases, the strain induced martensitic transformation and the mechanical instabilities (Lüders banding and Portevin-Le Chatelier effect) that are commonly reported in those steels. In situ High Energy X-Ray Diffraction (HEXRD) on synchrotron beamline during quasi-static tensile tests have been performed. The presented experiments have been realized on Petra-III P07 line at DESY (Hamburg). The high energy (100 keV) coupled with a high-performance detector enable high acquisition rates (10 Hz) and thus in situ time-resolved investigations. Mechanically induced martensitic transformations kinetics are determined using Rietveld refinements. The in situ evolution of 3D stress partitioning between the ferritic matrix and retained austenite was characterized by the sin²ψ method. Stress in fresh martensite is obtained through a micromechanical calculation. Digital image correlation has been carried out with 3D stereographic cameras to obtain the local strain near the diffracting volume and follow the mechanical instabilities.

Four microstructures produced by intercritical annealing between 680°C to 750°C have been studied. The samples have the same nominal composition, but the composition and fraction of each phase differ depending on the thermal treatment. The current study gives new insights into the effects of chemistry, grain size, initial phase fractions and mechanical instability on austenite stability.