Forming Quality And Microstructure Control Of Current Auxiliary Hot Forming Process Of High-strength Steel Slender Structure Parts

With the increasing requirement of lighter vehicle body, lower fuel consumption and higher security, high-strength steel has been used more and more widely in automobile manufacturing field. Due to the high yield strength and high tensile strength of high-strength steel, the formability of stamping parts become very poor, the defects of springbacks and crack could arise on the stamping parts which severely restrict the application of high-strength steel. The objective forming parts of this research is an automobile door anti-collision beam,which is a kind of high strength steel slender structure through deep drawing. When using the traditional hot forming method, it will lead to low efficiency, high cost and huge equipment loss. Current auxiliary hot forming could improve those shortcomings and disadvantages of the traditional hot forming effectively, and achieve the forming of high strength steel sheet.In this research, firstly, the test of the electric and thermal property parameters and the mechanical property parameters of the high-strength steel DP1180 in order to getting the heating curves under different current density was carried out. The test preliminarily reached a conclusion that the temperature of the sheet could be controlled between 750°C ~ 900°C, when the current density was between 10 A/mm2 ~ 10.8 A/mm2. Meanwhile in order to find out the influence of different resistance heating temperature to the microstructure, the microstructure observation of the heating sheet under different current density was performed.In order to obtain the change rules of mechanical performance of the high strength steel material under the effect of current, the current auxiliary tensile experiment of the high strength steel sheet was designed. The tensile experiment under different parameters has been carried out by using the designed electric heating tensile fixtures. Through the experiments, the change rules of the high-strength steel stress-strain curves under the influence of the current density, strain rate, tensile rate and the specimen gage length were got. And through the analysis of the fracture and the microstructure, the microscopic mechanism of the current auxiliary tensile experiments was summed up.The modeling of the forming dies for the slender structure part was designed by 3D drawing software, and through the preliminary experiments and finite element simulations, the parameters of dies were optimized. In the process of hot stamping of the slender structure parts, the forming experiments in five different temperatures between 600°C and 850°C respectively were carried out, and the satisfied forming parts were got finally. After the hot forming experiment, the analysis of the surface quality, crack, thickness, tensile strength and microstructure of the forming parts was performed. These experiments reached a conclusion that it would be a relative ideal forming process when the high-strength steel was heated at 1450 A, kept in 900°C and formed when cooled to 800°C. At last, the problems in the forming process and the quality control methods were analyzed and estimated.