Research And Numerical Simulation On Hot Forming Limit Of 22MnB5 High Strength Steel Sheet

High strength steel has high strength and excellent anti-collision perfor-mance,compared with light materials such as aluminum magnesium alloy,it has lower cost.Besides,high strength steel can meet the demand of automobile lightweight.Therefore,it has become the preferred material for automobile structural parts of many automobile enterprises.The hot stamping technology of high-strength steel can avoid the disadvantages of easy wrinkling and cracking of materials at room temperature,poor forming quality of workpieces,high requirements for equipment and easy damage to molds.And the strength of materials has been strengthened after hot forming.Therefore,the hot forming technology of high strength steel has attracted special attention of many scholars in recent years.In order to obtain the forming limit of high-strength steel,this subject takes 22Mn B5 high-strength steel sheet as the research object,the main research contents include:1.Through the unidirectional hot tensile test of 22Mn B5 high strength steel sheet under the deformation conditions of forming temperature of600～900℃and strain rate of 0.1～10s^-1,analyzed the changes of necking and fracture strain,thermoplastic index,fracture morphology and metallographic structure with the deformation conditions,and established the constitutive equation and fracture criterion in the process of hot deformation.The results show that:The necking and fracture strain,thermoplastic indexes such as elongation,and the diameter of fracture dimple increased with the increase of deformation temperature and strain rate when the deformation temperature is less than 800℃and the strain rate is less than 10s^-1,in addition,the grains in the metallographic structure become finer,and the plasticity and strength of the material are improved.The strain compensated constitutive model based on Arrhenius equation can accurately describe the flow stress in the process of material deformation.The improved Cockroft Latham criterion with Z parameter（zener Hollomon parameter）can accurately predict the failure behavior of 22Mn B5 high strength steel.2.The Nakazima experiment of 22Mn B5 high strength steel（20～180mm）under different sheet widths was simulated by DYNAFORM finite element simulation software to analyze the applicable conditions of instability criteria.This experiments were carried out on the same size sheet under different deformation conditions to analyze the influence of deformation conditions on sheet metal forming performance.Establish the thermoforming limit diagram at the initial sheet temperature of 700℃.The results show:the maximum punch force criterion is applicable to the sheet width of 20～80mm and the maximum strain path transition criterion is applicable to 100～180 mm.The sheet metal formability decreases with the increase of deformation speed and increases with the increase of initial deformation temperature,reaching the maximum at the initial temperature of 700℃,and then decreases with the increase of temperature.By fitting the established thermoforming limit diagram of 22Mn B5high strength steel sheet,the partial primary and secondary strains with minor strain less than zero are linear,and the part with minor strain greater than zero can be fitted by binomial equation.3.Expansion experiments were carried out with self-made mould to verify the sheet metal rupture state and the punch force after the experiments were finished.Comparing with the maximum principal strain cloud diagram and the punch force in the simulation results,it is found that the simulation results obtained by the finite element software are reliable.4.Using 22Mn B5 hot forming limit diagram analyzed the influence of different process parameters on the forming quality of simple anti-collision beam structure,and compared the workpieces of cold forming and hot forming.The results show that the forming quality of workpieces is better when the blank holder force is 20KN,the fillet of female die is 5mm and the die gap is 1.1 times the sheet thickness;The forming quality of workpiece is obviously improved by using hot forming technology.