Enhanced ductility and toughness in an ultrahigh-strength Mn-Si-Cr-C steel: The great potential of ultrafine filmy retained austenite

Three heat-treatment routes incorporating bainite formation, namely bainite-based quenching plus tempering, bainite austempering and bainite-based quenching plus partitioning (BQ&P), were applied to a medium-carbon Mn-Si-Cr alloyed steel. An optimum combination of strength, ductility and toughness was achieved after the BQ&P treatment (ultimate tensile strength: 1688 MPa; total elongation: 25.2%; U-notch impact toughness at −40 °C: 48 J cm−2). The enhanced mechanical properties were attributed to an increased amount of refined filmy retained austenite (22 vol.%, nanometer width range: <100 nm and submicron width range: 100-500 nm). The formation process of the bainitic microstructure as well as martensite and retained austenite was revealed by conducting dilatometry, X-ray diffraction, scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy (TEM). The effect of the retained austenite on mechanical properties was discussed in terms of its size and morphology.