Multiphysics Field And Microstructural Coupled Simulation Of High Strength Steel In Hot-stamping

In recent years, for the demand of energy conservation and security improvement, high-strength steels are increasingly being used to produce the parts of an automobile. As an innovative process, hot stamping has become one of the main methods to process high-strength steel high strength because of its good formability and high strength obtained. In the hot stamping process, first the material was heated to obtain the uniform austenite and then form and quench the material at the same time.In this paper, analysis is made on the whole hot stamping process based on the numerical simulation and experiments.Based on the material thermodynamics theory and Fick diffusion Law, one cellular automata model is built to simulate the transformation process from ferrite and pearlite to austenite, the austenite coarsening and the carbon diffusion process of one HSS during the holding process in the furnace. The verification test is made after the simulation, and the conclusions are as follows:(1) The high strength steel blank could be fully austenitized within 13s after its temperature reached the austenite transformation start temperature, while it is heated in the furnace at constant 950℃. However, the carbon concentration distribution at this time is uneven, nor is the austenite grain size. High carbon concentration and high temperature can accelerate the growth rate of austenite grain. As a result, the growing up process shows a fast-slow- accelerating trend.(2) From the calculating results, which comply with the basic theory of metallography, we can get the austenite grain coarsening process motivated by grain boundary curvature growth and the distribution regulation of grain size. The ignorance of solute drag and the segregation could account for the lager calculating results.As for the hot stamping process, one material model suitable for the finite element analysis is built based on the martensitic transformation kinetics, heat transfer, and thermal elastic-plastic theory. The relevant formulas are derived and subroutines are developed to make multi- physics coupling simulation of the high strength steel hot stamping process. The simulation results are as follows:(1) The new constitutive model is used to predict the U-shaped hot-forming and the simulation result were coMPared with experimental results, and find that in the U-shaped parts the fastest cooling part is the flange, and the second is the side, while the cooling rate of the bottom is the slowest, so the austenite transformation is possibly not so sufficient at this part.(2) CoMPared with the Johnson-Cook model, the new constitutive model which takes the phase transformation-induced martensite expansion into consideration gives a larger calculation result and more close to the measured values.(3) Combination of the numerical simulation and experimental analysis about the hot stamping springback angle, it is found that the martensite volume expansion is the main reason of the opposite between the springback angle of hot stamping and cold forming.Finally, based on the above theoretical analysis, the process parameters of the hot stamping of Collision beam with high strength steel is developed and completed the sample trial, combined with the multi-physics coupling simulation analysis. The analysis results shows that because of the later contact with the mould, the temperature of the connection part between the bulge and dimple of the car door collision beams is higher than the neighboring areas, while the cooling and contraction of the surrounding area will produce a certain tension which result in the thinning of the region, and affects the car door anti-collision beam forming accuracy.