Strain-induced martensite formation during punching of a medium manganese advanced high-strength steel

Abstract

Partitioning of carbon and manganese during intercritical annealing of medium manganese steels is known to enhance the thermal stability of austenite by retarding the transformation to martensite during cooling, thereby promoting the retention of austenite at room temperature. The presence of retained austenite (RA) is particularly crucial in high-strength steel applications that demand an optimal balance between strength and ductility. This study investigates the mechanical stability of RA and the strain-induced martensitic transformation in a medium manganese advanced high-strength steel during the punching process. A vacuum-cast steel with a composition of 0.3C–1Si–6Mn–2Al (in wt.%) was laboratory hot-rolled to a thickness of 4 mm. Subsequently, the intercritical annealing treatments (IAT) were carried out at temperatures of 650 °C and 700 °C to stabilize varying RA fractions within a tempered martensite/ferrite matrix. The tensile strengths of 1027 MPa and 911 MPa were obtained for microstructures with RA fractions of 7.5% and 43.3% at IAT temperatures of 650 °C and 700 °C, respectively. Significant differences in strain hardening of the material were observed near the punched hole edges. The IAT 700 °C material exhibited higher maximum hardness near the edge, and a greater depth of the shear-affected zone compared to the IAT 650 °C material. This indicates that strain-induced martensitic transformation under the localized deformation during the punching process is a key factor in increasing edge hardness value of the punched IAT materials. Notably, the transformation behaviour of austenite during tensile testing differed from that observed during punching, due to variation in material flow, deforming mode and localized strain.