Mechanical behavior and stability of dispersed retained austenite in thermomechanically rolled and isothermally-treated TRIP-aided multiphase steel

Abstract

The effect of thermomechanical processing and cooling conditions on the microstructure and mechanical properties during multi-step interrupted tensile tests was investigated in 0.24C–1.55Mn-0.87Si-0.40Al multiphase steel. Two temperature variants of isothermal bainitic holdings at 450 and 350 °C were applied. The tensile tests were interrupted at strain levels of 5, 10, 15%. The microstructure was assessed using light and SEM microscopy. EBSD analysis was performed to determine the retained austenite (RA) distribution and size. Detailed calculations concerning the mechanical performance were performed including the Jaoul-Crussard's analysis. Multiphase steels with the high retained austenite fraction (ca. 15%) of different γ phase sizes below 4 μm in a ferritic-bainitic matrix (with an average size of <6 μm) have been obtained. The research showed that during martensitic transformation of RA, the grain undergoes fragmentation. It has been shown that for the proper use of the TRIP effect, it is necessary to produce the dispersed RA of different sizes in the structure instead of striving for maximum fragmentation, which is a commonly used practice. Obtaining too small grains in the microstructure blocks their transformation during deformation due to their too high stability. Obtaining grains of one size leads to the intensification of the TRIP effect at a given stage of deformation only, reducing the plasticity in the others. The corresponding different mechanical stability of the γ phase ensures a gradual martensitic transformation and beneficial strain hardening progress.