A new experimental steel containing in weight percent 0.3C-2.0Mn-0.5Si-1.0Al-2.2Cr and 0.3C-1.9Mn-1.0Si-1.0Cr was hot rolled in a laboratory rolling mill and directly quenched within the martensite start and finish temperature range. It was then partitioned without reheating during slow furnace cooling to achieve tensile yield strengths over 1100 MPa with good combinations of strength, ductility and impact toughness. Gleeble thermomechanical simulations led to the selection of the partitioning at the temperatures 175 and 225 °C, which produced the desired microstructures of lath martensite with finely divided retained austenite in fractions of 6.5% and 10% respectively. The microstructures were analyzed using light and scanning electron microscopy in combination with electron backscatter diffraction and X-ray diffraction analysis. The mechanical properties were characterized extensively using hardness, tensile and Charpy V impact testing. In tensile testing a transformation induced plasticity mechanism was shown to operate with the less stable, carbon-poorer retained austenite, which transformed to martensite during straining. The auspicious results in respect to microstructures and mechanical properties indicate that there are possibilities for developing tough ductile structural steels through thermomechanical rolling followed by the direct quenching and partitioning route.

In order to find optimal intercritical annealing treatment (IAT) temperature and alloy composition for simple process route of hot rolling followed by single-step IAT, the effects of IAT on three different medium-manganese steels were investigated. Nominal chemical compositions in wt.% were 1) 6Mn–0.3C, 2) 6Mn–0.4C and 3) 8Mn–0.4C(–2Al–1Si–0.05Nb–Fe). Materials were laboratory hot rolled to a thickness of 6 mm, and IAT was simulated with Gleeble 3800 and Linseis DIL L78 DQT / RITA dilatometer. Different variations of IAT included annealing temperatures of 650 °C, 675 °C, 700 °C and 725 °C, with holding time of 10 minutes, heating rate of 50 °C/s and cooling rate of 10 °C/s. Quasi-static tensile tests were performed parallel to rolling direction. XRD and EBSD phase mappings were performed to assess IAT temperatures effect on volume fraction of retained austenite. Most promising mechanical properties were obtained with material 6Mn–0.4C annealed at 700 °C. Product of strength and elongation well exceeded 40 000 MPa% for above-mentioned IAT-material variation, being distinguishable higher compared to other variations. However, investigated materials, especially 6Mn–0.4C, seems to be very sensitive to IAT temperature, which could inflict some challenges in industrial scale production. Also, all materials experienced some level of serrations during tensile testing, which is frequently encountered phenomena with medium-manganese steels. Further research is required, to evaluate the role of austenite stability on mechanical behavior of these materials and to determine effects of heating and cooling rates.