Experimental Investigation On The Influence Of Edge Quality On Tensile Property And Fracture Modes Of High Strength Steel

The lightweight design demand of automobile has made high strength steel widely used in car body structures,but the higher strength of steel also brings more challenges in its forming process.During the stamping process of high strength steel,it is inevitable to use blanking or other cutting processes.However,the edge defects caused by shearing process easily become the source of cracks in subsequent forming process,which makes the material crack at the sheared edge under very low strain state and severely reduces the formability of the material.As a result,the stability of production is affected.With the support from National Key Research and Development Project(No.2017YFB0304403),high-quality shearing and non-defective edge forming processes for high strength steel were explored in this paper.The edge blanking test and sheared edge tension test based on digital image correlation(DIC)technology were used to investigate the influences of blanking parameters on sheared edge quality and subsequently tensile property of TRIP780 and QP980 steels.In addition,a comparative study was conducted on the representative cutting techniques such as laser cutting,blanking and wire electrical discharge machining(Wire-EDM).Based on the tracking of the fracture process with DIC system and the analysis of fracture surface morphology,the fracture modes and micro-mechanisms of high strength steel with different edge qualities were also revealed.The major achievements obtained are as follows:(1)The effects of blanking parameters on sheared edge quality and subsequently tensile property of TRIP780 and QP980 were studied experimentally.The results show that for both materials,as the blanking clearance increases,the sheared edge quality and tensile performance gradually decrease.The blanking speed has few effects when velocity value varies between 8.25mm/s and 57.75mm/s.When using 5° oblique edge blanking,the ratio of burnish zone and burr zone decreases simultaneously,and the subsequently tensile performance increases significantly.With the further increase of punch edge angle,the enhancement of sheared edge quality and tensile performance is modest.(2)The influence of several typical cutting processes such as laser cutting,Wire-EDM,blanking and blanking with polishing on cut edge tensile property of QP980 were comparative studied.The results show that the level blade blanked specimens have the lowest elongation and fracture strain,followed by laser-cut specimens.The specimens cut by 10° oblique edge blanking,Wire-EDM,level blade blanking with polishing have the highest elongation and fracture strain.However,in contrast,oblique edge blanking can not only guarantee the cutting quality,but also be suitable for mass production.Oblique edge blanked surface has less void damage,meanwhile,the burr height is small and the edge is smooth,which makes the sheared edge have superior tensile properties.(3)Two typical fracture modes were found in sheared edge tension test for specimens with different edge qualities.They are fracture by necking and fracture by edge cracking.In this paper,crack evolution process of above two fracture modes was revealed based on the tracking of the fracture process with DIC system.The microstructure of the fracture surface was analyzed by scanning electron microscope(SEM),and the microcosmic mechanism of the two types of fracture was analyzed.