Hole expansion performance of a medium manganese advanced high-strength steel after hot rolling and intercritical annealing

Abstract. This paper investigates the hole expansion behavior of a medium manganese advanced high-strength steel. The study began with vacuum-cast 0.3C–1Si–6Mn–2Al steel, which was subsequently laboratory hot-rolled to a thickness of 4 mm. Intercritical annealing treatments (IAT) were then carried out at temperatures of 650 °C and 700 °C to produce varying fractions of tempered martensite/ferrite and retained austenite (RA). Tensile tests were performed in the longitudinal direction, while hole expansion tests following ISO 16630 were conducted to assess the stretch-flangeability of the IAT materials. Test holes were prepared using punching and wire electrical discharge machining (W-EDM) to examine the effects of different edge conditions. Tensile strength of 1027 MPa and 911 MPa corresponding to a microstructure consisting of RA fraction of 7.5% and 43.3% were obtained at IA temperatures of 650 °C and 700 °C, respectively. Mean hole expansion ratios (HER) with punched holes were relatively low for the given strength levels (3% and 17% for IAT 650 °C and IAT 700 °C, respectively). W-EDM edge condition considerably improved HER but not as much as expected, potentially due to the high inhomogeneity of the microstructure. Further research on optimization of processing and microstructure is required to enhance the poor hole expansion performance of intercritically annealed medium manganese steels.