Hierarchical microstructure design of a bimodal grained twinning-induced plasticity steel with excellent cryogenic mechanical properties

Combined nanoprecipitation and grain refinement were introduced in a bimodal grained (BG) twinning-induced plasticity (TWIP) high manganese steel to achieve high strength-ductility combinations. Hierarchical microstructural characteristics of heterogeneous grain size (0.2 μm - 4 μm) and precipitates (κ-carbides and Nb-rich carbides) distribution was obtained by a controlled thermo-mechanical treatment. Compared with the as-received states, the BG-TWIP steels showed a significant improvement in yield strength (YS) with little loss in plasticity whether at room temperature (RT) or liquid nitrogen temperature (LNT). The multiple strengthening contributions to YS mainly originate from the combination of solid solution and grain refinement strengthening. When deformed at RT, some deformation twins nucleated in the coarse grains (CG) of the BG-TWIP steels while only numerous stacking faults formed in the fine grains (FG) at a true strain of 0.14. The twin density remained nearly unchanged with progressive deformation and the dislocation strengthening dominated in the later deformation stage. In the early deformation stage at LNT, the twin amount of the BG-TWIP steel was still small. However, the volume fraction of twins increased greatly in both the FGs and CGs when deformed to a true strain of 0.26. The occurrence of high density of nano-twins in the later deformation stage at LNT not only contributes to a strength increment of 220 MPa, but also largely increases the geometrically necessary dislocations (GND) density to enhance the forest hardening effect, corresponding to a significant higher back stress hardening at large strains.