Effect of deformation temperature on the mechanical behavior and stability of retained austenite in TRIP-assisted medium-C multiphase steel

The temperature-dependent microstructural evolution and corresponding mechanical stability of retained austenite in medium-C TRIP-assisted 0.43C-1.45Mn-0.98Si-1Al-0.033Nb-0.01Ti steel obtained by thermomechanical processing was investigated using static tensile tests and microstructural studies. The light microscopy, image analysis, XRD diffraction and the Jaoul-Crussard analysis were applied to reveal relationships between microstructure and mechanical properties. Specimens were deformed in the static tensile tests in a temperature range of-20-140 °C. It was found that an increase in deformation temperature resulted in the reduced intensity of the TRIP effect due to the higher stability of retained austenite. An increase in the retained austenite stability along with a smaller grain size and a change from its blocky morphology to thin layers was also indicated. The impact of strengthening mechanisms at different temperatures was analyzed. The best combination of strength and ductility was obtained in the samples deformed at 20 and 60 °C, which is associated with the moderate work hardening in this temperature range. The Jaoul-Crussard analysis showed much less strengthening during the second phase of deformation at 100 and 140 °C due to the high stability of retained austenite. The higher C content in the investigated TRIP steel resulted in substantial volume fractions of retained austenite stable after completing deformation.