| Safty and environment-friendly become the main topics of automobile industry recently. Using ultra-high strength steel sheets to make body-in-white can lighten mass, reduce emission, and enhance crashworthiness. Forming defects, such as fracture, springback, often arise in the traditional cold stamping process, but the hot stamping of boron steel can solve the problems effectively. There are some main steps in the hot stamping process:heating the blank of boron steel to austenization temperature and keeping the temperature constant for a period of time, swiftly transferring the blank to a set of dies with a cooling system mounted on a press machine, forming and quenching the blank by using the set of dies, holding the pressure to accomplish the martensite transformation of blank and to obtain accurate dimensions of parts. The parts formed by hot stamping process have a high tensile strength of approximately1500MPa.The boron steel hot stamping process is a typical strength-enhanced integrated process:the process can improve the strength of parts and obtain the shape and dimension of parts simultaneously, which is the frontier of research area in part forming. There are many issues of engineering science need to address in hot stamping process:the effect of hot stamping process on the mechanical properties and microstructure of blank, the transformation of blank in heating and die quenching, the formability and forming limits of blank at high temperature, the heat transfer between blank and tools, the tribological behavior between blank and tools, etc. This study use the thermophysical simulation to investigate the process problems of hot stamping, and focus on the damage evolution, modeling and simulation of blank in hot forming.Three aspects of hot stamping process were investigated by using a Gleeble thermophysical simulation system:the influences of heating history curves on the high-temperature formability of22MnB5and on the room-temperature mechanical properties of quenched22MnB5, conducting continuous heating transformation experiment to determine the Johnson-Mehl-Avrami equation of22MnB5austenitization, the influence rules of process parameters on the mechanical properties and microstructure of quenched22MnB5. The results of experiments are the experimental basis and theoretical guidance for the design of process and the selection of process parameters, and indicate the microstructure evolution of22MnB5in hot stamping process.The deformation behavior of22MnB5at high temperature were studied by high temperature tensile testing. An Arrhenius-type constitutive equation that is expressed by hyperbolic sine function and includes the effect of thermal activation in hot deformation was established, and described the relation of true stress-strain well. A dislocation-based unified viscoplastic constitutive model was established, of which parameters were optimized and determined by Genetic Algorithm. The intrinsic relationships of stress, dislocation density, strain-rate, and deformation temperature were considered in the model. The model can reflect the essential rule of deformation behavior of22MnB5at high temperature.The microstructure near fracture and fractography were observed by scanning electron microscope (SEM) in order to analyze the mechanism of damage nucleation and evolution. The damage evolution process of22MnB5during the hot deformation was indicated:inclusions separation, micro-voids nucleation, micro-voids coalescence, bigger micro-voids, micro-cracks formation, and material failure. Damage-coupled unified viscoplastic constitutive model was established on the basis of continuum damage mechanics, which depicted three procedures during hot tensile deformation:work hardening, stable-state flow, damage-induced failure. A VUMAT subroutine was programmed and embedded into ABAQUS for the numerical simulation of uniaxial tensile testing, in order to verify the veracity of model and the validity of subroutine.The forming limit curves (FLC) of22MnB5at high temperature was obtained from the hot Nakazima testing. A multi-axial factor was introduced into the damage-coupled unified viscoplastic constitutive model for deducing the model from uniaxial state to planer stress state. The model parameters was determined and optimized according to the FLC data. The FLCs of22MnB5was predicted at different forming conditions by taking the optimal model parameters into the model, and the effects of model parameters on the prediction of model was analyzed and discussed. A VUMAT subroutine was programmed and embedded into ABAQUS for the numerical simulation of biaxial proportional tensile testing, in order to verify the veracity of model and the validity of subroutine.The elastic prediction and plastic iterative correction algorithm was used to program the subroutine of damage-coupled viscoplastic constitutive model. The uniaxial tensile testing at high temperature was simulated by using the developed VUMAT subroutine, and the results of numerical simulation was compared with experimental data. The finite element (FE) simulation and testing of hot cup deep drawing were conducted in order to confirm the prediction of model on the fracture during the hot cup deep drawing. The effects of process parameters on the forming limits of hot cup deep drawing were investigated by the FE simulation. The hot stamping process of an M-shape section part was investigated by conducting FE simulation. The distribution and evolution of strain, stress, temperature and damage during hot forming was analyzed and discussed, and the computed thickness using FE simulation was compared with the actual thickness measured from the part. The results of numerical simulations indicated that the established damage-coupled unified viscoplastic constitutive model can compute the damage evolution and predict the fracture during hot stamping effectually. The model is capable of predicting the forming limits of actual parts and avoiding the fracture defects in actual production. |
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