MICROSTRUCTURE EVOLUTION IN COLD ROLLED MEDIUM MANAGANESE STEEL DURING TWO STEP INTERCRITICAL ANNEALING

Microstructure evolution during two step intercritical annealing of 0.1C-5Mn cold rolled Steel processed by austenisation at 6500C with different holding time was examined by SEM, TEM and XRD. The experimental results indicate that the TRIP steel with Mn of 5% could form a considerable amount of retained austenite with good TRIP effect. The evolution of microstructure during the second annealing was analyzed. Austenite reverted transformation (ART) was observed during Intercritical annealing. It was shown that a complex matrix microstructure composed of three phases (retainedaustenite/martensite/ferrite) was formed and two types of morphologies were detected (lath like and polygonal).Furthermore, a high volume fraction of retained austenite (22%), which was stabilized at room temperature, was the origin of a TRIP effect. A good balance between strength and ductility can be achieved by optimizing the heat treatment.