High strength-high ductility medium Mn steel obtained through CALPHAD based alloy design and thermomechanical processing

There is considerable research interest in developing medium Mn steels as part of the 3rd generation of advanced high strength steels, mainly due to the possibility to achieve high tensile strength-high ductility combination at an affordable cost. In the present work, we have designed a steel chemistry and its thermomechanical processing route based on a computational approach based on CALPHAD with an objective to achieve tensile strength and uniform elongation in excess of 1000 MPa and 20%, respectively. The influence of alloying elements on factors such as weldability, coatability and formability are also taken into account while designing the steel chemistries. The alloy chemistry was optimised to achieve at least 50% of retained austenite and a stacking fault energy in the range 12?20 mJ m- 2 to activate transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) effects. The designed steel was cast and thermomechanically processed to produce ultrafine-grained ferrite and austenite microstructure with similar to 50% of retained austenite. Analysis of local composition by atom probe tomography revealed preferential partitioning of Mn and C to austenite during intercritical annealing, thereby enhancing its stability. The optimised microstructure resulted in tensile strength and uniform elongation in excess of 1300 MPa and 26%, respectively. The stress-strain curves revealed serrations and a staircase type of strain hardening. A detailed study of the strain hardening behaviour showed that this can be attributed to the occurrence of discontinuous TRIP effect and deformation twinning in the austenite. This was further corroborated by transmission electron microscopy of the deformed samples which showed the presence of nano-twins in the austenite phase while the XRD and EBSD of the deformed samples showed a significant drop in the austenite fraction post deformation.