Constitutive Equation And Damage Evolution Model Of Boron Steel At High Temperature

Recent years, more and more people pay more attention to the energy saving and environmental protection and they hope the security, reliability, environmental protection, comfortable and other requirements of the automobile could increase gradually. In order to improve product quality, the automobile manufacturing company continue to innovation and improvement the new manufacturing method and manufacturing process, reducing vehicle weight by using the new structure design and new materials. In automotive manufacturing, the use of ultra high strength steel can meet the requirements of lightweight and the vehicle’s safety performance. Therefor, some researchers focused on the hot forming of boron steel. Compared to the traditional experimental methods, the way of numerical simulation is easy to implement and money-saving, the results have guiding significance in some extent. By the analysis of the constitutive model, we can provide some useful method for the selection of the material constitutive model and the research ideas. In this thesis, B1500HS steel be selected as a subject, the tensile process in the high-temperature thermal stretching and fracture process were studied, and the tensile and fracture constitutive model at different temperatures and different strain rate are analyzed.Through the isothermal uniaxial tensile test, the true stress-strain curves at the relative conditions were obtained. Analysis the effects of the deformation temperature and strain rate on the flow stress. Studying Arrhenius constitutive model of boron steel B1500HS at austenitic state. The results show that the effects of deformation temperature and strain rate on flow stress are dominating and the computational data using the constitutive relationships are consistent with the experimental data. By analysing existed constitutive model, the flow behavior of boron steel B1500HS was described using Hensel-Spittel model, Nemat-Nasser model, Tong-Wahlen model and Norton-Hoff model, respectively. The predictions of these constitutive models are compared with each other using statistical measures like correlation coefficient, average absolute relative error and its root mean square error. According to the characteristics of the flow stress curve of boron steel, Norton-Hoff model was modified. The results show that the modified Norton-Hoff model is better to predict the flow stress of B1500HS boron steel.The evolution process of Lemaitre ductile damage criterion is derived. Simulating the process of boron steel hot tensile test by using FORGE software at different temperature and rate, the fracture process be described by Lemaitre ductile fracture criterion. F racture behavior of boron steel was described by Lemaitre ductile damage. The parameters of the Lemaitre ductile fracture criterion were optimized by modeFRONTIER software. Compared with the experimental data, the result show that Lemaitre ductile damage criterion has good accuracy and reliability to simulate the tensile fracture process of boron steel at high temperature.