Microstructures and mechanical properties of the third generation automobile steels fabricated by ART-annealing

The mechanical properties dependence on the microstructure was reviewed and analyzed, and the ultrafine grained duplex microstructure of BCC matrix and large fractioned austenite was given as one of the optimum structures to develop the third generation steel with high strength and high ductility. The medium-Mn steels with different carbon contents processed by austenite reverted transformation (ART-annealing) were studied thoroughly to fabricate the ultrafine duplex steels with large fractioned austenite. The lamellar typed ultrafine structure, the granular typed ultrafine duplex structure and the corresponding mechanical properties of the medium-Mn steels processed by ART-annealing were demonstrated in this paper. It was revealed that the duplex structure with large fraction of austenite and ultrafine grain structure is capable of producing steels with excellent combination of strength and ductility, i.e., RmxA about 30-50 GPa%, which is about two times of that of the conventional automobile steels and close to that of the TWIP steels. It was concluded that the ART-annealing of the medium-Mn steels would be at least one of the promising ways to fabricate the third generation automobile steels in the near future.