Research On The Optimization Of Hot Stamping Process And Die For Ultra High Born Steel

With the haze problem and the energy shortage problem becoming increasingly prominent, reducing vehicle weight has been an important development direction for the automobile manufacturing industry. For the automotive lightweight, ultra-high strength steel can effectively reduce the thickness of the body steel plate due to its high strength and high hardness. In addition, the application of the ultra-high strength steel can further improve the strength and impact resistance of the body structure. However, due to the increase of yield strength and tensile strength, the steel’s stamping performance has decreased, and using cold stamping process easily leads to a lot of defects. Therefore, hot stamping technology of ultra-high strength steel has been rapidly applied. In the hot stamping process of ultra-high strength steel, the steel sheet is heated above the recrystallization temperature and kept in the temperature for a period of time for sufficient austenization, and the steel sheet is transferred to a die with cooling system to be formed and quenched, and then ultra-high strength stamping part will be obtained after martensitic transformation. This technology can effectively solve the contradiction between the forming performance and the strength of ultra-high strength steel. In this paper,22MnB5 is chosen as research material, and the U shaped part is chosen as research object. Furthermore, the design of hot stamping die, the simulation of hot stamping process, the optimization of die cooling system and the topology optimization of die structure are studied.The hot stamping die for U shaped part is designed by CATIA, and the design includes the selection of die materials, the design of overall structure of the die, the design of the cooling system and the design of the sealing devices. Based on the design results, a finite element model based on thermal-mechanical coupled method is set up through the software ABAQUS, and numerical simulation of ultra-high born steel about U shaped part’s hot forming and cooling process is conducted.In order to improve the cooling capacity of the hot stamping die cooling system, the key parameters of the cooling system are optimized. The main design parameters of the hot stamping die cooling system include the cooling pipe diameter D, the sidewall distance S between adjoining pipes, the distance H between pipe top and die surface, and the cooling water flow velocity V. Determining the optimal value of the 4 parameters is the key point to design the die cooling system. In this paper,16 levels are taken from the 4 factors of D, S, V and H in the cooling system, and uniform design experimentation scheme is established, and then corresponding hot stamping finite element simulation is carried out according to the experimentation scheme. According to the simulation results of uniform design experimentation and the method of regression analysis, quadratic regression model between the maximum temperature of the die at the end of quenching with the four parameters is established. Genetic algorithm is used to get the optimal value of the regression model, and the optimal value of the key parameters of cooling system is determined. Finally, the optimal results are verified by finite element simulation model.In order to improve the utilization rate of die material and reduce the die structure weight, the topology optimization of hot stamping die structure is carried out. The die structure is selected as the research object, and the contact stresses between blank and die at critical case are selected as the pressure load for the analysis model of the die structure. Moreover, minimizing the volume of the die structure is chosen as the goal, and the nodes displacement of key area of the die is chosen as the constraint, and the units relative density is chosen as the design variables. The topology optimization model of die structure is established, and topology optimization on hot stamping die structure is carried out. Consequently, the weight of die structure has been reduced by 25.2%, and die’s deformation is almost kept with the previous result during hot stamping process. The research provides a certain reference value for the topology optimization of the hot stamping die structure.