Research On Reverse Transformation Annealing And Mechanical Properties Of Cold Rolled Medium Manganese Steel

Developing high strength, high ductility and low cost steels is an inevitable trend for auto steel materials, however, the tensile strength (Rm) usually increases with a significant loss of ductility.Motivated by the theory of microstructure control characterized by Multi-phase, Meta-stable and Multi-scale(called M3 simply), summary and review of the automobile steels home and abroad were carried out, which led to the idea microstructure controlling characterized by ultrafine grained matrix and metastable austenite phase and the approaches of low cost medium manganese alloying and austenite reverted transformation annealing for the research and development of the 3 rd generation automobile steel.Austenite reverted transformation is a effective method to obtain metastable austenite at room temperature, which is dependent on full or partial ex-quenched martensite transforming to austenite with enriched solute elements during the following annealing, then gets the multiphase microstucture. In this study, the hot rolled new type of manganese steels were applied as experimental material, which was first soft annealed at 550℃and 650℃for 6 hours, then, deformation 50% at room temperature. After treatment by these two conditions, the plates were annealed at different temperatures (550℃-700℃) with varied holding time (from1minutes to 6 hours). The Purpose of these heat treatment is to explore the annealing technique obtaining a duplex steel with large volume metastable and stable austenite phase and high-strength matrix. Microstructure evolution of these steels during annealing process were characterized by means of scanning electron microscopy(SEM) and electron back scattered diffraction (EBSD)techniques. The residual austenite contents in the steel were determined by X-ray diffraction analysis. Mechanical properties of the annealed steels were studied by tensile tests and Vickers hardness meter.By the processing of soft annealing, cold rolling plus ART-annealing at 650℃, the steels with grain size of 0.3-0.6μm and metastable austenite with fraction of 25%were obtained.The results of SEM and EBSD show that not only the austenite reverted transformation took place but also recovery occurred in the annealing process. Maximum values of A (-46%) and Rm×A (-46GPa%) were obtained when ART-annealed at 650℃with-10minutes. Compared with conventional steel (IF steel, DP steel, Martensitic steel, conventional TRIP steel, nanobainitic steel,15-20Mn-3Si-3A1, and TWIP and other austenitic steel), it can be seen that the Rm* Aproduct of our designed C/Mn steels is about twice that of conventional TRIP steel and even close to that of the high alloyed steels. Moreover, it is interesting to note that Rm* A increases almost linearly with increasing austenite fraction, with a slope 0.6-0.7 GPa%/(1%-γ). It was concluded that the enhanced mechanical properties of the processed Mn-TRIP steel is attributed to the enhanced TRIP effects of the large fractioned metastable austenite and the superfine grain size.