Strain-induced transformation behaviour of retained austenite and tensile properties of TRIP-aided steels with different matrix microstructure

The effects of strain rate and temperature on the strain induced transformation behaviour of retained austenite and hence mechanical properties of TRIP-aided steels with annealed martensite (AM), bainitic ferrite (BF) and polygonal ferrite (PF) matrix microstructures were examined. For this, tensile tests were carried out at four different strain rates varying from 3.33×10^-5 s^-1 to 3.33×10^-2 s ^-1 at room temperature and at 150°C. Additional intermittent tests were carried out at intermediate strain rates to estimate the extent of strain induced transformation of austenite to martensite using X-ray diffraction analysis. Tensile strength (TS) decreased with strain rate at both temperatures, though the TS at 150°C was lower for all steels except for BF steel tested at 3.33×10^-5s^-1. The total elongation (TEL) for PF steel significantly improved at 150°C at all strain rates, with a maximum of 78% obtained at 3.33×10^-3 s^-1, the corresponding TS being 930MPa. A similar effect was seen in AM steels tested at 3.33×10^-3s^-1 and 3.33×10^-4s ^-1. However, in BF steel, higher temperature had a detrimental effect on TEL, especially at strain rates of 3.33×10^-2 s^-1 and 3.33×10^-3s^-1.