Study On Q&P Heat Treatment Process And Deformation Mechanism Of Super High Strength Medium Manganese Steel

Q&P process is a new heat treatment process,which has been successfully used in TRIP steel and other low-carbon low manganese steel,and obtained high strength and high plasticity.The microstructure and mechanical properties of a medium manganese Quenching and Partitioning(Q&P)steel for automobile were characterized by means of optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD)and mechanical property techniques.The content includes the microstructure evolution and mechanical behavior during the Q&P process and the process prior to Q&P heat treatment,mechanism of carbon and manganese partitioning,and the stability of retained austenite,the tensile deformation and in-situ tensile tests were carried out on the manganese Q&P steels to study the deformation mechanism during the deformation process.The main conclusions are as follows:The tested steel was cold rolled prior to Q&P heat treatment.It is constitute of deformed ferrite with low content of carbon and manganese,deformed martensite with high content of carbon and manganese and a small amount of residual austenite.The quenching and partitioning process was studied by thermal dilatiometer.It is showed from the expansin curve that the initial martensitic transformation was not clearly displayed and austenite transformed to martensite partially in final quenching.Annealing temperature has a greater influence on the microstructure which includes grain size,recovery degree and the volume of austenite,etc.The tensile strength of 1084MPa,elongation of 34.2%,and product of strength and elongation(37089MPa ×%)are obtained for the 5Mn steel of the mechanical properties;7Mn steel available yield strength of 1172MPa,tensile strength of 1243MPa,the elongation of 36.2%%,and product of strength and elongation(40472MPa ×%)of the mechanical properties.During the critical annealing process of Medium-Manganese Q&P steel,the Mn element distribution occurred.The enrichment of Mn in retained austenite improves the volume fraction of retained austenite in Q&P steel.It is shown that Mn element can be divided into austenite and ferrite even in the short time of annealing in the critical region by the thermal expansion method.It is found the thermostability of the retain austenite is quite high,the retain austenite doesn't decompose even at a very low heat rate,which is caused by the lack of the dynamic condition.On the other hand,the research on the mechanical stability found that the stability increase with the inter-critical heat temperature decreasing.When the annealing temperature is too low,the austenite content is relatively small,the stability of the austenite is good,the deformation may not occur during the stretching process,and no contribution is made to the plasticity of the test steel.When the annealing temperature is increased,the stability of austenite is poor.The stress state has an effect on the stability of retained austenite.Further,though the deformation test with different state of stress,it is found that the mechanical stability of retain austenite is less in the condition of plain strain,compared with the condition of single dimension tensile stress.Determination of a medium-manganese Q&P steel with the characteristics of a slip in the process of plastic deformation,phase transformation induced plasticity,and twinning induced plasticity.It is considerd that Luders deformation plays a bigger part when lower annealing temperature is used,and TRIP effect when higher annealing temperature used.Under the medium temperature annealing condition,the deformation occurs in the form of Luders deformation first,and then continues at a relatively low strain hardening rate before fracture.