Research On Hot Stamping Process For BR1500HS High Strength Square Boxes By Thermal-mechanical-phase Transformation Model

The using of high strength steel in automotive industry can reduce the body weight and improve the car shock resistance and crashworthiness effectively. However, with the increase of the strength of steel, the deformation resistance of material will increase, which not only has a greater requirement for device tonnages but springback of stamping becomes serious, aggravating the wear and tear of mold. So the forming difficulty of cold stamping process is improved. Nowadays hot forming technology has begun to receive attention in the automotive manufacturing industry because of being able to solve above problems. Hot forming technology is a new technology, which is designed specifically to form high strength steel plate including blanking, heating, anti-oxidation, stamping, quenching and shot peening treatment etc as a whole. In hot forming process, the temperature of sheet at high temperature state will decrease rapidly after encountering cold mold. So the actual hot stamping process of sheet metal is not a isothermal deformation but a non-isothermal deformation of continuous cooling. However, due to the complexity of the forming process and the control difficulty of the process of material microstructure changes, researches are developed mainly based on isothermal deformation at home and abroad, so the research results have certain limitations.In this paper, high strength steel BR1500HS was take as the research object and firstly isothermal deformation and non-isothermal deformation of high strength steel plate was carried out by using the thermal simulator Gleeble1500. The rheological behavior of the material was investigated through experimental studies and the effects of temperature and stress fields on mechanical properties and microstructure of materials were analysised. The study found that microstructure and flow stress of isothermal and non-isothermal deformation are different significantly. When the cooling rate is at 10-20℃/s, the micro structures of isothermal deformation are both a mixture of martensite and bainite. However, martensite did not appear in microstructures of non-isothermal deformation. The microstructures obtained from the non-isothermal deformation are a mixture of ferrite and pearlite when high temperature and the others are mainly composed by ferrite and bainite.The isothermal deformation is plastic deformation with the type of recrystallization and recovery, while non-isothermal deformation is plastic deformation with the type of work-hardening.Then experimental data was analysised and dealed with by using Multiple linear regression. Afterwards the high temperature constitutive equation of experimental material was established.Based on the established high temperature constitutive equation, the accuracy of the constitutive equations was verified by means of numerical simulation analysis of thermal simulation experiment by finite element simulation software Deform. Finally, the numerical simulation of hot stamping of the square boxes were studied and the effects of flow rate of water in the mold and die clearances on microstructure of square boxes were predicated. The results found that flow rate of water in the mold and die clearances have an obvious influence on microstructure composition of square boxes, especially on content of martensite. Further hot stamping experiments of square boxes were carried out to verify the suit of high temperature constitutive equation for hot stamping of square boxes, providing practical guidance for hot stamping of square boxes in the future.