| A-pillar reinforcement is an important part of white body parts. It directly relates to safety of driver and passengers. Thus, components must have high strength, stiffness and resistance performances. Under the background of promoting emission reduction such as green manufacturing, safty and lightweight, automobile parts are made by thin thickness of ultra-high strength steel, which is an effective way to realize lightweight of car body. Complex structure parts were made difficultly by a single hot forming because they were tended to wrinkle and crack. Therefore, we put forward to a new method-double hot forming. By this method, blank was initially heated by hot forming to obtain preformed parts, and preformed part was put on the furnace to heat again. Then, hot preformed part was placed on mould and pressure quenching. Eventually, hot stamping parts met the requirements of shape size and the strength reached to higher than 1400 MPa. In this paper, the following key techniques of hot forming were studied comprehensively: hot forming technology process and hot forming theory, hot forming process parameters on performance of ultra high strength steel and its optimization of process parameters, high temperature mechanical properties and thermal coupling constitutive equation of ultra-high strength steel(BR1500HS), numerical simulation of hot forming process and defect diagnosis, cooling system design of hot forming mold and numerical simulation analysis of conjugate heat transfer, hot forming experiment of ultra high strength steel, and et al. Several aspects were researched as following:① Hot forming principle and process parameters controlledThe principle of hot forming was analyzed and process parameters were controled, based on the complex structure by a single hot forming of car body, the characteristics of the forming performance was poor, the hot forming process was put forward.② Effects of process parameters of ultra high strength steel BR1500 HS and its performance and optimizationBR1500HS, ultra high strength steel, was regarded as the research material. The key technical problems in car complicated structure of double hot forming process was studied. After lots of physics experiments, some influence laws were obtained: hot stamping process parameters of quenching medium, austenitizing temperature, deformation temperature, strain rate, holding time, and cooling rate influenced microstructure, fracture morphology, hardness, tensile strength and martensite phase transformation point.③ High temperature mechanical properties and constitutive model building of BR1500HSMechanical properties of BR1500 HS at high temperatures were studied; different strains, strain rates, and temperatures influenced on the law of stress and strain were analyzed. Based on these experimental data of high temperature mechanical properties, coupled thermo-mechanical constitutive models were established at the stage of forming in high temperature and pressure hardening.④ Numerical simulation of double hot forming process.1) Based on material constitutive model, hot forming process were studied by finite element method. Furthermore, temperature field, preforming force, preforming stroke and stress field distribution of complicated car structure were studied. Finally, hot forming parameters were chosen to meet the production process. By using numerical simulation software, bursting of defect formation were studied for complex structure in double hot forming. We proposed to stamp end to open and avoid making pressure on the side. We combined the method of numerical simulation with production test to solve bursting of defects in hot forming.2) Inserting cooling system design was optimized, and layout of mould inserting circuit was set reasonably. Numerical simulation of conjugate heat transfer for key part of automobile pressure quenching phase was studied. The temperature field change of forming pieces of the key parts and mould were analyzed by this numerical simulation. The temperature field change law of forming part and mould was studied, and phase change equalization of forming was controlled by changing temperature. ⑤ Double hot forming experiments of BR1500HSComplicated structure of car body was manufactured by double hot forming technology. The thickness of hot forming at important position under different flow rates and temperatures was measured, and microstructure, tensile strength and hardness of the important position on forming part were tested. Problems of uneven martensite transformation in hot forming were solved by adjusting process parameters and load control strategy. Finally, the size stability and strength of 1400 MPa hot forming parts were obtained. |
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