Role of grain size on deformation microstructures and stretch-flangeability of TWIP steel

The present study reports the effects of grain size on deformation microstructures and stretch-fiangeability of the fully austenitic cold-rolled and annealed Fe-17Mn-0.6C-1.5Al (wt. %) twinning-induced plasticity (TWIP) steel. Mechanical properties of the materials annealed at various annealing temperatures were investigated by uniaxial tensile tests and hole expansion tests. Grain refinement led to an increase in both tensile strength and hole expansion ratio. Higher deformation twin fraction was observed with increasing grain size. The martensitic transformation was observed at the hole edge of the hole expansion tested specimen, where severe deformation was concentrated. Main crack initiation sites were identified as twin boundaries, grain boundaries and phase boundaries between martensite and austenite. Therefore, the coarse-grained TWIP steel could show a high probability of crack initiation due to its high density of crack initiation sites. We suggest that grain refinement is the unique way to develop high strength and damage tolerant TWIP steels which can be applied for complex structural parts.