Optimizing intercritical annealing treatment for hot-rolled medium-manganese steels

Abstract

The aim of this master’s thesis was to determine the optimal intercritical annealing treatment (IAT) parameters and alloying composition for hot-rolled medium-manganese steels to achieve the best possible combination of strength and elongation using a simple process route of hot-rolling followed by single-step IAT. Three different compositions (in wt.%) were studied: 1) 6Mn–0.3C, 2) 6Mn–0.4C, and 3) 8Mn–0.4C(–2Al–1Si–0.05Nb–Fe). The materials were laboratory hot-rolled to a thickness of 6 mm. IAT was simulated using Gleeble 3800 and Linseis DIL L78 DQT/RITA dilatometer. The IAT variations included four annealing temperatures (650 °C, 675 °C, 700 °C, and 725 °C) and three holding times (3, 10, and 30 minutes). The heating rate was 50 °C/s, and the cooling rate was 10 °C/s for all variations. Quasi-static tensile tests were performed parallel to the rolling direction. XRD and EBSD phase mappings were conducted to evaluate the effect of IAT parameters on the volume fraction of retained austenite. The 6Mn–0.4C composition annealed at 700 °C exhibited the most promising mechanical properties, with a product of strength and elongation exceeding 40,000 MPa%. However, the investigated materials, particularly 6Mn–0.4C, appeared to be highly sensitive to IAT temperature, potentially posing challenges for industrial-scale production. Also, all materials exhibited some level of serrations during tensile testing, a phenomenon frequently encountered in medium-manganese steels. Further research is required on the evolution of strain-induced martensitic transformation with increasing strain to assess the role of austenite stability in the mechanical behavior of these materials. Moreover, the effects of varying heating and cooling rates on the mechanical properties should be investigated further, as they could have significant effect.