Mechanism of simultaneous increase of strength and ductility with grain refinement in Si-added high-Mn austenitic steel

We systematically investigated the effect of grain refinement on mechanical properties and fracture behavior of Si-added high-Mn austenitic steel. A 22Mn-0.6C–3Si steel (wt. %) with single-phase FCC structure was investigated in the present study. By applying high pressure torsion (HPT) and subsequent annealing process, specimens with fully recrystallized microstructures having various mean grain sizes ranging from 0.9 μm to 113 μm were obtained. The present steel exhibited a unique mechanical behavior that both strength and ductility simultaneously improved with grain refinement. Coarse-grained (dmean = 113 μm) and medium-grained (dmean = 13 μm) specimens fractured before reaching the plastic instability condition, whereas ultrafine-grained specimen (dmean = 0.9 μm) fractured after satisfying the plastic instability condition. Deformation microstructures of tensile-fractured specimens were investigated by using EBSD phase map. Obtained results revealed that the fraction of deformation induced ε-martensite significantly decreased with the grain refinement. At the same time, fracture surfaces of the different grain-sized specimens showed that the fraction of the fracture surfaces having step-like ridge pattern decreased with the grain refinement. Micro-cracks were observed to characterize a crack initiation site, and it was found that micro-cracks formed and propagated along the prior austenite grain boundaries where ε-martensite impinged. By applying the fracture surface topography analysis (FRASTA) to the tensile-fractured specimens, it was revealed that the crack initiated from the step-like ridge pattern, where each step was considered to be ε-martensite plate. The results consistently suggested that the step-like ridge pattern was grain boundary fracture surface related to ε-martensite, and grain refinement suppressed the ε-martensitic transformation and avoid the fracture before reaching the plastic instability condition, leading to the enhanced ductility as well as the high strength.