Influence of manganese on deformation behavior of lightweight steel at different strain rate

Tensile deformation behavior of Fe–0.25C–3.5(10)Mn–5Al lightweight steel was studied in a large range of strain rate (0.001–1200 s−1) by using a universal material testing machine, intermediate strain rate tensile testing apparatus and rotation disk bar-bar tensile impact apparatus. Microstructures of the two steels before and after tension were observed by means of Optical Microscope (OM), Scanning Electron Microscope (SEM), X-ray diffraction (XRD) and Transmission Electron Microscope (TEM). The results show that the two lightweight steels have a high strength and plasticity and exhibit excellent combinations of specific strength and ductility at the strain-rate of 0.001 s−1 after annealing at 850 °C for 5 min then directly quenching into water. During the tensile deformation process, the austenite in 3.5 Mn steel is transformed to α′-martensite. While in 10 Mn steel, the austenite is transformed to twinning. With an increase in strain rate from 0.001 to 1200 s−1, tensile strength of the two steels increase, whilst the elongation initially decrease, and then increase. At the strain-rate of 450 s−1, the elongation of the two lightweight steels are minimal, and the energy absorption capacity are the lowest. With the deformation progresses, the value of n increases from small to large, the strain hardening effect becomes high. Uniform deformation of 10 Mn lightweight steel was suppressed at high strain-rate. Comparing with 10 Mn lightweight steel, the austenite in 3.5 Mn lightweight steel is obviously unstable and cannot provide progressive phase transition.