The Impact Of Uniaxial Pre-strain To The Forming Limit Of The High Strength Steel For Auto-body Under Electromagnetic Forming

Since the21th century, the global automobile industry has been developingrapidly. As the global amount of automobile increasing year by year, automobile hasgradually become the necessity that bringing convenience for our daily life. Becauseof this, however, the resulting environmental and energy problems are getting moreand more serious. Due to the increasingly strict policies in environmental protectionand energy saving, automobile lightweighting has become the inevitable trend in thedevelopment of the world automobile industry. At the premise of vehicle’s safety andstrength performance, automobile lightweighting requires to reduce the weight ofvehicle as much as possible, so as to improve the mobility, reduce the fuelconsumption and exhaust pollution.The application of high strength steel and aluminum alloy in automobile industryhas become an important way to realize the automobile lightweighting. For a longtime, steel has been the most important material in auto manufacturing. Consideringthe lower material price and cost in manufacturing and processing of steel comparedto aluminum alloy, the world major steel manufactures developed lots of highstrength steel for auto manufacturing that making the application proportion of highstrength steel in auto body increasing by a certain scale. For some automotivecovering parts and components, high strength steel can ensure the strength of autobody with the reduction of thickness. However, with the higher Tensile Strength(b)than ordinary low carbon steel, springback and rupture defects are easily observed inthe processing of high strength steel. Therefore, how to improve the formability ofadvanced high strength steel has become one of the most important problems in theprocess of automobile lightweighting.Research has shown that the high strain rate forming process, such aselectromagnetic forming(EMF) whose strain rate is over103/s, can improve theformability of material that has difficult forming properties. The compound formingprocess, which combine the common stamping process with high strain rate formingprocess, can better reflect its advantages that has great potential in industrialmanufacture. However, material mechanics performance under high strain rate condition is significantly different from that under quasi-static condition and differentstrain paths in materials processing have great influence to materials plasticdeformation behavior and materials forming limit. Meanwhile, high strength steel hasstrong sensitivity to strain rate and the change of plastic deformation behavior andmechanical performance of high strength steel under compound forming process arenot yet clear, that limits the application of high strength steel in industrialmanufacture.In this thesis, we regarded the compound forming process as quasi-staticpre-deformation and high strain rate (EMF) final-deformation and then someexperiments, including quasi-static uniaxial tensile, stretch forming, high strain rateforming, compound forming process(quasi-static uniaxial tensile pre-deformation andEMF final-deformation), were conducted with the typical high strength steel ofDP590and B280VK. The forming limit diagram were established respectively of thetwo steels that under different forming conditions. The result showed that theelectromagnetic forming can significantly improve the forming limit of DP590andB280VK that under the uniaxial tensile strain condition, while the forming limit thatunder the biaxial stretch forming condition were improved slightly. Besides, theforming limit of DP590and B280VK that under the uniaxial tensile strain conditionwere increased at first and then decreased with the introduction of different unaxialpre-strain levels, but the turning point was distinct. The results of the research havecertain reference significance for the development of advanced forming technologyfor high strength steel.