Study On The Regulation Of Austenite Stability And Mechanical Properties In0.2C-5Mn Steels

Developing high strength steels is an inevitable trend for automobile steels. However,the tensile strength usually increases with a significant loss of ductility. On the basis ofmicrostructure control theory characterized by Multiphase, Metastable and Multiscale(called M~3for short), the ideas of microstructure control characterized by ultrafinelamellar duplex structure for the third generation automobile steels are carried out, whichare new type C-Mn steels with medium manganese content and austenite revertedtransformation annealing.In this study, the new type medium manganese steels were used as experimentalmaterials, which were ART-annealed to obtain ultrafine ferrite/austenite lamellar duplexstructure. Microstructures and mechanical properties of medium manganese steelsprocessed under different heat treatment conditions were examined by SEM, TEM, XRDand EBSD. It was found that high temperature austenization (above Ac3) resulted in a fullmartensite structure after quenching, which gradually transformed into an austenite-ferriteduplex structure during the following annealing process. However, austenization in theintercritical region (between Ac1and Ac3) gave a duplex structure after quenching, whichwas nearly not affected by following annealing process. The ultrahigh strength~1000MPaand total elongation~40%were only obtained in the specimens with6hours annealing at650°C under both heat treatment conditions. Otherwise, it was found that tempering at100-600°C doesn’t result in a significant change of the austenite plus ferrite duplexstructure, which was developed in the previous annealing through austenite revertedtransformation, whereas significant decreasing of the austenite fraction and carbonconcentration was found in the specimens tempered at200°C and500°C due to theprecipitation of carbides. Correspondingly tempering slightly deteriorates the ductilitywhen the specimens were tempered at200°C and500°C without effects on mechanicalproperties around400°C. Furthermore, the optimal mechanical properties with tensilestrength1000MPa and total elongation40%could be obtained after tempering at400°Cwith1hour, which means that galvanization is feasible in the0.2C-5Mn steel with balanceof high strength and ductility. Based on the analysis on the relationship between microstructure and mechanicalproperties, it was found that the total elongation was strongly dependent on the austenitefraction, which was ascribed to the phase transformation induced plasticity of the largevolume fraction of austenite. And the comprehensive mechanical properties (Rm*AT)increase with austenite fraction, which implies that TRIP effect is the key factor for highstrength and ductility. Otherwise, the dependence of yield stress on the austenite grain sizeaccords with Hall-Petch equation, which implies that the austenite is the soft phase.