Microstructures and mechanical properties of hot-rolled Nb-microalloyed TRIP steels by different thermo-mechanical processes

The microstructural evolution and mechanical properties of hot-rolled Nb-microalloyed TRIP steel with different states of austenite (recrystallized/pancaked) prior to the transformation of austenite to ferrite were investigated under the thermo-mechanical process based on controlled cooling or based on dynamic transformation of undercooled austenite (DTUA) by hot compression tests using a Gleeble-1500 hot simulator, in combination with OM, SEM, TEM, XRD and ICP-OES. The results indicated that the influence of the states of austenite prior to the transformation of austenite to ferrite on the multi-phase microstructures and the mechanical properties of the tested steel became weak in the case of the process based on DTUA, in comparison with the case of the process based on controlled cooling. The much refined and uniform microstructures with smaller sizes of ferrite and bainitic packets and the more stable retained austenite were obtained by the process based on DTUA, resulting in the markedly improved combination of strength and ductility of the tested steel.