Numerical Calculation Method Of Microstructure Evolution Of Metal Materials And Its Application In Hot Stamping

Sheet metal hot stamping technology is a thermo-plastic forming processes,which is widely used for production of components and parts in automotive body,aerospace and equipment manufacturing industries.The final microstructures and size accuracy of thermal forming parts will determine the macro performance of parts.Precise prediction of microstructure evolution and deformation processes is of significance for the forming process design.The calculation approaches for microstructure evolution are mostly based on the isothermal transformation curve(Time-Temperature-Transformation TTT)data and the additivity hypothesis,which is feasible and effective for microstructure evolution of some materials.However,concerning the high-strength alloy steel(22Mn B5),the microstructure evolution is multiphase phase transformation at slow cooling rate,in addition,the incubation time of phase transformation is very short and difficult to be measured.It,nevertheless,has significant effects on start time and the process of phase transformations.The computational accuracy of traditional calculation method is not enough for this kind of issues.Therefore,for the simulation of hot stamping process,it is important to improve the computational accuracy of traditional calculation method of microstructure evolution,to innovate an effective new calculation method of microstructure evolution,to employ the calculation method of microstructure evolution for the hot stamping process,and to excavate the effects of microstructure evolution for the enhancement of the computational accuracy.Based on the numerical calculation method,this thesis is first focused on the traditional calculation method of microstructure evolution;and then a new calculation method of microstructure evolution has been developed based on the previous researches;in addition,a mechanical response model relate d with the microstructure evolution of metallic materials subjected to external loads is built based on the heat treatment deformation experiment and the simulation;finally,the actual hot stamping forming simulation processes are discussed using the built model show n in this thesis.The detailed content and major results are listed as follows:(1)The experimental material used in this thesis is the high strength boron steel(22Mn B5).The incubation time of diffusional phase transformation is calculated according to the experimental results.At the particular temperature range,two virtual values of incubation time of the ferrite transformation and the bainite transformation were pre-set,and then,the virtual were determined by solving the continuous cooling transformation process.A material subroutine(USDFLD)has been built based on the traditional calculation method of microstructure evolution and considering the influence of incubation time in the calculation model.The phase transformation processes of high strength boron steel have been calculated using above approach.Finally,an end quenching experiment and the corresponding calculation of the high strength boron steel have been carried out in this thesis,and the comparison results show that the incubation times have an important effect on the calculation of phase transformation process;and the calculation model can well predict the microstructure evolution of phase transformation with arbitrary cooling path of high strength boron steel.(2)Two calculation methods that can be applied to calculate the microstructure evolution of phase transformation process with ar bitrary cooling path have been built and improved in this study,based on the data of continuous cooling transformation(CCT)curve.First,about the inverse calculation method of parameter,the parameters of isothermal transformation can be obtained by us ing this method based on the CCT data,and then,the microstructure evolution of phase transformation with arbitrary cooling path can be calculated by using tranditional calculated method of microstructure evolution and the isothermal transformation kineti cs parameters obtained by using the parameter inverse calculation method.Then,about the generalized calculation method,a generalized contribution coefficient is proposed and applied to describe the influences of current temperature and cooling rate on t he incubation time of nucleation of grain,which improves the accuracy of results in the generalized calculation.Furthermore,a new generalized calculation method of grain growth of phase transformation has been built in this study.Finally,the two calculation methods have been verified by the experiment s using a single-phase bainite steel G17 Gr Mo9-10.The results show that the isothermal transformation kinetics parameters obtained from experiment s and the inverse method are in good agreement.The calculation accuracy of this method is higher than traditional inverse calculation method.And the generalized contribution coefficient p lays an important role in the generalized calculation.The microstructure evolution processes can be calculated by using the generalized calculation method.(3)A mechanical response calculation model is built considering the effects of microstructure evolution on the calculation model.The influences of four calculation parameters on the simulation results of heat treatment deformation are discussed in this study.The results show that a small variation of the thermal parameters has little effect on the heat treatment deformation;similarly,a small variation of the data of stress-strain curves at low temperature has little effect on the heat treatment deformation,however,the same variation of the stress-strain curve data at high temperature is very sensitive to the heat treatment deformation;there is no plastic deformation for air cooling condition,which will not lead to hot process deformation,however,the oil and water cooling condition s are very sensitive to the heat treatment deformation;the phase transformation related parameters,especially the transformation plasticity,have significant effects on the heat treatment deformation calculation results.(4)Finally,the calculation model built in this study are applied to the simulation of hot stamping processes.On the one hand,based on the tailored hot stamping processes,the robustness of four conditions to calculat e results were investigated;on the other hand,the influences of phase transformation and transformation plasticity related parameters on the calculation of springback and residual stress have been discussed in this study.The results show that the proposed method is more accurate than the traditional calculation method,and only the tool temperature,the interface heat transfer coefficient and the transformation latent heat have significant effects on the tailored hot stamping processes;and the phase transf ormation and transformation plasticity-related parameters have significant effects on the calculation of springback and residual stress of traditional hot stamping processes.