Numerical Prediction Of Formability And Microstructural Mechanical Properties In Hot Stamping Of High Strength Steel

In order to reduce the weight of automobiles, decrease fuel consumptions while maintaining crash performance, hot stamping technology has been widely used to manufacture automotive structural parts. Different from conventional cold stamping process, hot stamping process includes heating, forming and quenching processes and it is a complex coupled thermo-mechanical-metallurgical process. During this process, the temperatures of blank and tools vary, the formability of blank may deteriorate due to the unsuitable processing parameters and the design of die-face, and the microstructure of blank evolves. Therefore, modeling and numerical prediction for hot stamping process are of great significance for the actual industrial production. This thesis is focused on the numerical prediction and experimental verification of temperature field, formability and microstructure evolution during hot stamping of high strength steel, and the research work mainly covers the following aspects:(1) The coupled thermo-mechanical-metallurgical constitutive equation was established based on the interactions between the temperature field, the mechanical field and the microstructure evolution during hot stamping process. Hot tensile tests of 22MnB5 steel were conducted to obtain the flow property under austenite state, and the effects of temperature and strain rate on flow property were investigated. By combining the flow properties of the studied material under ferrite, pearlite, bainite, and martensite conditions and taking into account the effect of equivalent stress of deformed austenite on diffusional transformation kinetics, the finite element software King Mesh Analysis System_Hot Stamping (KMAS_HS) dedicated to hot stamping simulation of high strength steel was developed based on the dynamic explicit algorithm.(2) The analysis module for three dimensional (3D) temperature field in hot stamping process was developed. The heat transfer in the whole hot stamping process was outlined. The heat conduction differential equation for hot stamping was established by taking into account the transformation latent heat release of blank. Based on the initial and boundary temperature conditions for hot stamping, the Galerkin weighted residual method was employed to solve the differential equation and the general finite element form for 3D transient temperature field of hot stamping was deduced. The blank and tools with cooling system were modeled by temperature shell and 3D tetrahedral elements respectively. The temperature distribution of blank, solid tools, and cooling pipes in the hot stamping process of a U-shaped specimen were calculated by this analysis module, and the numerical results were compared with the experiment ones. The good agreement between the numerical and experimental results is shown.(3) The analysis module for formability of blank in hot stamping process was developed. Four ductile fracture criteria under the category of "uncoupled phenomenological criterion" and the "fully-coupled damage criterion", i.e. the continuum damage mechanics (CDM)-based Lemaitre model, were employed to the finite element software KMAS_HS to compare their prediction capability on ductile failure prediction in hot stamping process. Hot forming limit tests of 22MnB5 steel were conducted and the forming limit curves (FLCs) at different temperatures were obtained. The temperature dependent critical values of these four ductile fracture criteria were determined by fitting the numerical FLCs to the experimental ones. Hot tensile tests of 22MnB5 steel were simulated and the damage parameters depend on temperature and strain rate were identified by an optimization-based robust, joint numerical and experimental procedure. Numerical results of the applications of these calibrated models to the hot stamping process simulation of an automotive B-pillar were compared with the experimental ones. It is concluded that thermo-mechanical finite element analysis in conjunction with CDM-based Lemaitre model can be used as a reliable tool to predict ductile damage and fracture of 22MnB5 steel in hot stamping process. And then the effects of blank holding force and friction on formability, damage evolution and punch force were analyzed by means of the CDM model.(4) The analysis module for microstructure evolution in hot stamping process and mechanical properties of the hot stamped parts was developed. The iterative Newton-Raphson technique was used to solve the diffusional transformation kinetic equations, and the Scheil additivity rule was employed to apply isothermal diffusional transformation kinetic models to non-isothermal condition. And the effect of deformation of austenite on incubation time of the diffusional transformations has been taken into account. For hot stamping process, the diffusional transformation models, Li and Akerstrom models, and non-diffusional transformation model, Koistinen-Marburger (K-M) model, were employed to simulate the microstructure evolution during the hot stamping process of an automotive B-pillar. And the mechanical property of the B-pillar was calculated by the Maynier hardness model. And the prediction results by using Li and Akerstrom models were compared to the experiment. It is concluded that the combination of Li model and K-M model provides the better prediction. For tailored hot stamping process, Lee diffusional transformation model and Yu non-diffusional transformation model were employed to calculate the microstrucute evolution during the tailored hot stamping process of an S-rail. The material parameters in Lee model were identified based on the continuous cooling transformation (CCT) curves of the studied material under free stress condition and a hardness model related to cooling rate was established and calibrated. The verification experiment was conducted, and the results show the hot stamped parts with tailored properties can be obtained by means of partially heated and cooled dies. And the agreement between the numerical and experiment results confirms the validity of the coupled thermo-mechanical-metallurgical constitutive equation, the transformation models and the hardness model.