Relationship of microstructure and mechanical properties of TRIP-aided steel processed by press forging

The main emphasis of this study has been placed on thermomechanical processing simulations performed using press forging. In order to develop a comprehensive understanding of the effect of large deformation of bulk specimens, introduced in the austenite recrystallized, nonrecrystallized and intercritical temperature regions, on the microstructure evolution and mechanical properties, the C–Si–Mn transformation induced plasticity (TRIP)-aided steel was used for experimental. The choice of applied strain and thermal parameters had strong impact on austenite transformation kinetics and obtained final multiphase structure, which clearly modified the mechanical properties of TRIP steel. To rationalize the retained austenite volume fraction employing different thermomechanical schedules, the effect of austenite conditioning on ferrite transformation was considered at the first step and the subsequent isothermal bainite transformation treatment was optimized in the second step. The microstructures of treated samples were characterized by optical metallography and scanning electron microscopy. The tensile tests were conducted to characterize the development of TRIP effect with respect to volume fraction of retained austenite in multiphase structure. The volume fraction of retained austenite was measured using X-ray and neutron diffraction methods. The specific design of thermomechanical schedule with varying of strain and temperature values affected multiphase structure development in TRIP bulk steel and yielded in good combination of high tensile strength over 900 MPa and elongation over 20% nearly at all experimental schedules. The complex relationship among the volume fraction of the retained austenite, the morphology and distribution of phases present in the microstructure, and mechanical properties of TRIP steel were revealed. These findings suggested that the intercritical treatment conditions prior austenite transformation appeared to have decisive effect on steel refinement and forming of convenient multiphase structure, which should provide both, the high strength and sufficient ductility of bulky products.