MICROSTRUCTURE EVOLUTION OF HIGH MN STEEL THROUGH ART ANNEALING

The various experimental studies are conducted on the medium-Mn steels with Mn content ranged in 3-9% and C content ranged from ultra low carbon to 0.4% were presented. It was shown that long time austenite reverse transformation (ART) annealing of the hardened steels results in an ultrafine ferrite/austenite duplex structure and a substantially improved mechanical property with the product of tensile strength to total elongation about 30-40GPa%. Also, the ART annealing for both short time and long time could be applied to produce the ultrafine duplex structure in the studied steels and the mechanical properties were further improved. The improved mechanical properties of these type of steels were attributed to the ultrafine grains and the large fractioned austenite (30%). Therefore, the stability of austenite is considered as a result of a combination of the dominant role of fine grain austenite size and some contribution of chemical stabilization by Mn and carbon partitioned in the austenite that can be achieved only at specific intercritical temperatures. So, these type of steel presented a premium combination of strength and ductility, Rm×A≥30GPa%. The performances of steel sheets subject to welding, cold forming, cycling loads, and hydrogen fitted well with the existed fabrication technologies for auto steel sheets. It was also found that carbides precipitated during initial annealing stage and gradually dissolved during following annealing process for all annealing temperatures; and interestingly, the fresh martensite was found, indicating that part of the newly forming austenite was unstable and would transform into martensite when quenching Based on the microstructural analysis and the calculation by Thermo-Calc software, it was proposed that the different evolution behaviours of the microstructure during annealing were not only controlled by the thermal kinetics of the austenite, but also affected significantly by the thermal stability of the austenite.