Microstructure evolution and strain behavior of a medium Mn TRIP/TWIP steel for excellent combination of strength and ductility

The structure-property relationship in a low carbon medium Mn steel subjected to annealing at temperatures from 700 to 850 degrees C was elucidated. The phase transformation kinetics, thermal stability, and strain behavior of reverted austenite were studied to obtain excellent combination of strength and ductility. The recovery and recrystallization of deformed bainite were observed. Reverse transformation from bainite to austenite occurred during the intercritical annealing process. With an increasing annealing temperature, the volume fraction of austenite first increased and reached the maximum of 65% at 775 degrees C, it then decreased sharply to 21% at 850 degrees C due to the weak Mn enrichment and coarsened grain size. Mn enrichment played a significant role in enhancing the thermal stability of reverted austenite. The maximum product of strength and ductility of 44.1 GPa% was obtained at 725-750 degrees C by adjusting the volume fraction and mechanical stability of austenite. The cooperative TRIP and TWIP effects contributed to the continuous strain hardening and local stress relaxation of this steel. In this way, the uniform elongation was greatly improved and resulted in a ductile fracture mode.