The Research On Optimization Of Heat Treatment Process Parameters And Microstructure Of TWIP Steel

Twinning Induced Plasticity(TWIP)steel has comprehensive mechanical properties such as high strength and high plasticity,which has a broad development prospect in the automotive industry.In recent years,domestic and foreign made some achievements in the study of TWIP steel,but improving the product of ultimate tensile strength and total elongation(Rm×A value)of TWIP steel through heat treatment process parameters is still a problem to be solved.The neural network and genetic algorithm approaches are used to simulated and optimized the heat treatment process parameters and research on the change of microstructure during deformation of Fe-Mn-C-Al TWIP steel in this thesis,which can provide reference for practical application of TWIP steel.Firstly,using the three heat treatment process parameters of annealing temperature,holding time and cooling method as the factors.The heat treatment experiment program of TWIP steel was designed by orthogonal test and obtain the mechanical properties.Based on the experiment,the neural network model was constructed to describe the nonlinear relationship between three heat treatment process parameters and Rm×A value of TWIP steel.The genetic algorithm was used to optimize the heat treatment process parameters of TWIP steel.The results show that the constructed BP neural network can better describe the relationship between process parameters and Rm×A value.The optimal heat treatment process parameters are annealing temperature of 863?,holding time of 26 min and cooling method of furnace cooling.The deviation of RmxA value between actual value and the predicted result of the network is small.Then,the influence of heat treatment process parameters on the RmxA value was analyzed by BP neural network.The results show that with the increase of annealing temperature and the prolongation of holding time,The RmxA value shows a trend of increasing first and then decreasing.When the cooling method was furnace cooling.which was better than the other three cooling methods.Finally,the mechanical properties and microstructure of TWIP steel with optimal RmxA value are studied.The results show that TWIP steel maintains stable single-phase austenite structure during tensile deformation.There is no obvious yield platform on the true stress-strain curve,and the tensile strength and elongation are 845 MPa and 73%,respectively.As the degree of deformation increases,the density of deformation twins increases continuously and the intersecting "secondary twins" began to appear in the grain,which slows down the rate of work hardening rate and gradually increases to the maximum value,thus The TWIP steel exhibits excellent Work hardening ability.Meanwhile,the parameter optimization process fracture morphology of the TWIP steel compared to the original samples,which have number of the deeper and larger dimples,and thus has good plasticity.