Study Of Microstructure Evolution And Mechanical Properties Of Fe-11Mn Cold-rolled TRIP Steels

With the rapid development of automotive industry, the new appeal of safety, environmental protection and energy-efficiency has been proposed in automotive field. The emergence of the third generation advanced high strength steel make the call coming true. Transformation induced plasticity (TRIP) steels which have attractive combinations of strength and ductility have been regarded as the potential candidates for automotive application and have a good prospect for application.There is a great deal of research that shows that Mn plays a very important role in enhancing the volume fraction and stability of retained austenite. Therefore, the dissertation focuses on the TRIP steels with the Mn mass contents of11%.The influence of austenite stability on TRIP effect has been investigated by using OM, XRD, SEM and TEM techniques, and deformation mechanism and mechanical properties of the experiment steels at different quenching temperatures are analyzed. At the same time, the influence of mechanical stability of the experiment steels on TRIP effect in different temperature condition has been researched. And the Mσs temperature of the experiment steels has been found through analysing the variation of yield strength at different temperatures. The influence factors of Mσs temperature of TRIP steel were discussed.The research results demonstrate that:(1) The constituents of the experiment steel at different quenching temperatures are mostly ferrite and austenite. The sample quenched at750℃and held on5minutes is characterized by an excellent combination of a tensile strength of979MPa, a elongation of64%and a product strength and elongation of62.7GPa%.(2) There are a lot of austenite with different degrees of stability contributing to TRIP effect timely and properly. At the same time, stable austenite relieves the stress concentration at the crack tip, and blunts crack growth. The elongation of the experiment steel was increased, and microvoid formnd in ferite.(3) Yield strength of the experiment steel increases with increasing temperature, whereas it suddenly falls at15℃so that Mσs temperature could be in the range of10～15℃. It has the best mechanical properties because of perfect mechanical stability of retained. The occurrence of strain induced mastensitic transformation could be identified on the basis of the variation of the morphologies of the retained austenite.(4) According to the modeling of austenite stability, the Mσs temperature depends on the following factors:chemical composition of the austenite particles; austenite particle size; yield strength of the matrix; stress state. The Mσs temperature increases and mechanical stability of retained austenite decreases as the yield strength of the steel and the mean austenite particle size increase. It is confirmed that the range of mean austenite particle size is from0.68to0.69μm.