Numerical Simulation Of Multi-physical Field For Microalloyed Forging Steel During 3D Complex Hot Forging

It is in urgent need for development of modern science and technology to transform the forging technology of complex geometry into intelligent manufacturing technique. The interactions among deformation, heat transfer and microstructural evolution during hot forging processes of materials are so complicated that the finite element method (FEM) is difficult to deal with the forging processes which care for microstructure. In order to investigate the microstructure evolution during hot forging processes, a integrated system which fit to simulate the microstructural evolution during multistage forging and cooling processes was developed by integrating the microstructural evolution models of two microalloyed forging steels into a commercial FEM code. Base on the simulation, scientific foundation for determining the forging parameters and controlling the forging quality during complex forging processes can be realized.Based on the tested data, the flow stress models of F40MnV and 38MnVS6(Ti) steel were determined in dynamic recovery region and dynamic recrystallization region, respectively. The characteristic parameters of flow stress models were obtained using compression tests and described as functions of Zener-Hollomon parameter. The validity of the flow stress models was demonstrated by comparing the tested data to calculated results. By using the established flow stress models, hot work processing technology and FEM research of homogeneous steels can be developed.A microstructural evolution model for F40MV steel was established in this paper based on physical simulation experiments. The dominant microstuctural evolution processes including dynamic recrystallization, static recrystallization and grain growth were taken into consideration in this model. Therefore, the effect of thermomechanical parameters and deformation history on microstructual evolution and grain size can be evaluated quantitatively. The predictions using the model were in agreement with tests. In addition, the dynamic recrystallization model of 38MnVS6(Ti) steel was established based on hot compression tests. A static recrystallization model which fit to 38MnVS6(Ti) steel was determined when temperature is higher than 850℃.In order to investigate phase transformation in austenite during cooling process after hot deformation. Considering the influence of hot deformation and based on the thermodynamic basis of superelement mode, the thermodynamics parameters of microalloyed forging steels were calculated. Based on the above thermodynamic parameters calculation, an integral model for studying isothermal kinetics of pro-eutectoid ferrite formation and pearlite transformation was deduced by referring modes proposed by Cahn. According to Scheil's additivity rule and experimental result obtained by thermal dilation method, a prediction model of transformation for un-deformed and hot-deformed austenite to ferrite and pearlite in microalloyed forging steel, which can be applied to continuous cooling process, was developed in the present paper. Taking the F40MnV steel as an example, the calculated transformation start temperature and transformation volume fraction of ferrite and pearlite are in agreement with the measured values.A set of computation methods were proposed to make sure the application of constant strain rate and temperature microstructral evolution models to changing strain rate and temperature conditions, which were based on continuity of recrystallization fraction increasing and grain evolution. The methods could be used in FEM easily. By integrating the microstructural evolution models into a commercial MSC.Superform FEM code with the computation methods during unsteady hot deformation and during cooling process, a two dimensional numerical simulation system was developed to simulate the whole unsteady hot compression and following cooling process of a column of F40MnV steel. Compared with the experimental data, the simulated austenite grain size and phase volume fraction were exciting.A three dimensional numerical simulation system for analyzing deformation, heat transfer and microstructural evolution in multistage complex hot forging and following cooling process were developed. Making using of the developed system, whole multistage forging processes and following cooling process of 38MnVS6(Ti) steel piston were simulated. The state of microstructural evolution and grain size in the forging both ferrite and pearlite transformation after deformation were revealed in detail. The simulation results agree well with the experimental results, which show the numerical simulation system can be used to design and optimize complex forging processing technics .