Continuous Cooling Transformation Behaviour of C-Si-Mn TRIP Steel

Thermomechanical simulations were performed by a Gleeble 1500 thermomechanical simulator in C-Si-Mn TRIP steel. The continuous cooling transformation (CCT) behaviors of this steel were investigated in the undeformed and deformed conditions, respectively, and the corresponding static (without hot deformation) and dynamic (with hot deformation) CCT diagrams were constructed. The microstructures were observed by scanning electron microscopy (SEM). The microstructural evolution obtained from the thermal simulation specimens revealed that the fastest cooling rate produced fully martensitic microstructure. As the cooling rate decreased, granular bainite, polygonal ferrite and pearlite were formed. A rapid cooling rate depressed the formation of ferrite and pearlite, which resulted in higher hardness. Alloying elements of Si and Mn made the pearlite and bainite curves of CCT diagrams move to the right. Hot deformation shifted the ferrite, pearlite and bainite transformation left to high temperature region in dynamic CCT diagram, because deformation increased nucleation sites and energy and accelerated the transformation. Polygonal ferrite, granular bainite and retained austenite could be obtained at cooling rates of 3.3–16.7°C/s in the deformed condition, and this might contribute to a satisfactory TRIP effect due to the presence of retained austenite in the microstructure.