| High strength steel hot-stamping technique is an effective way to reduce the automobiles weight and achieve energy saving. Compared with the conventional cold stamping process, hot stamping structural parts have good formability, less springback, high shape accuracy. During hot stamping process, the temperature of high strength steel varies from 600℃ to 900 ℃. However, the damage evolution becomes more complicated during hot stamping process simultaneously. So it is necessary to predict damage evolution model and mechanism accurately, which will be able to guide the optimization of forming process effectively, and save the cost of trial and increase productivity. Therefore, it is imperative to study metal sheet damage behavior under elevated temperature deformation, which helps to predict high strength steel hot stamping simulation effectively.GTN damage model has been widely used to predict ductile materials’ necking or cracking during the forming simulation process, which is the link of material tensile process and damage evolution. So it is great significant that take GTN damage model into the forming simulation. In the thesis, uniaxial tensile tests were conducted from room temperature(RT) to 900 ℃,which obtained high-strength boron steel 22 Mn B5 blank mechanic properties in the thermal conditions. Secondly, the scanning electron microscopy(SEM) was selected to observe the damage characteristics of tensile specimens. Furthermore, based on GTN damage model and combined with experimental design and response surface center complex, damage parameters’ experiment level has been designed. Through employing the finite element software combined with genetic algorithm(GA) to inverse identification the characteristic damage parameters of high-strength boron steels from RT to 900 ℃. The influence of damage parameter variation on macroscopic mechanical behavior was analyzed. And the effect of temperature on the damage parameters was shown.Finally, to obtain hot state FLD of high strength boron steel 22 Mn B5 blanks, a rigid hemispherical punch bulging test was conducted and the forming limit curve(FLC) at 700 ℃ was executed. Establishing finite element analysis of bulging test coupled GTN damage model and predicting the cracking of sheet accurately in the hot stamping process. Because of material damage variable associated deformation history that make damage parameter Cf as hot stamping metal sheet forming limit basis. The results were compared with bulging forming limit test and obtained good fitting which can provide theoretical guidance for numerical simulation method of hot stamping. |
