Investigations On Sheared Edge Crack Susceptibility Of The Third Generation Automobile Steels

High strength is the development trend of automobile steel.However,with the increase of strength level,the plasticity of steel usually decreases and the susceptibility of shear edge crack increases.The third generation automotive steel has remarkably improved the plasticity of high strength steel,but still has the phenomenon of shear edge crack susceptibility during the forming process.In this paper,the medium manganese steel of third generation automotive steel is studied.The effect of shear clearance,austenite phase,material strength and cutting methods on sheared edge quality of medium manganese steel was studied by means of sheared edge tensile test,hardness test and microstructure characterization.The shear process is simulated by Deform software.The edge of the blanking has the features of rollover zone,burnish zone and fracture zone when the clearance is 2%t,5%t,6.6%t,10%t and 15%t(t is the thickness of sheet metal).When clearance is greater than 10%t,obvious burr defects are produced.With the increase of die clearance,rollover zone,fracture zone,burr area increased,the number and area of the micro voids increase near the shear edge,he degree and range of the work hardening increase,and the plasticity of the sample is gradually decreased.Sheared tensile sample occurs necking and approaches the smooth sample,when die clearance is 5%t.The shear edge is poor quality and the sample does not appear necking during shearing,when die clearance above 6.6%t.It is recommended that the die clearance be 2%t-5%t.The sheared edges with metastable austenite content of 15% and 0 were compared.Martensitic transformation takes place during the shearing process of metastable austenite.The range of hardness and hardness of the sheared edge is relatively larger than without austenite steel.The automobile steel of 235 MPa,500MPa and 700 MPa yield strength was studied.The shear edge of high strength steel is more affected by the die clearance,and the reduction of shear clearance is beneficial to improve the flanging formability of high strength steel.The influences of three methods,laser cutting,Wire Electrical Discharge Machining(WEDM)and blanking,are studied.The result shows that WEDM has the least influence on the hardness and plasticity of cutting edge.In the laser cutting,the microstructure of the sample edge appears quenched martensite,which leads to the highest hardness.Sheared samples found deformation induced martensitic transformation,which resulted in the largest impact region.The plasticity of the sheared edge in small clearance or grinding can reach the level equivalent to laser processing.The shearing process of medium manganese steel under three shear clearances of 6.6%t,10%t and 15%t is numerically simulated.For medium manganese steel,the ayada fracture criterion is used,and the simulation results are better.Under these conditions,the simulation results and test results are uniformity.when the clearance is greater than 10%t,the burr has obviously grown.With the increase of shear clearance,the stress affected zone of sheet increases.