Research On The Evaluation Of Shearing Edge Quality And Edge Formability For Various Ultra High Strength Steels

Ultra High Strength Steel(UHSS)has the advantages of high strength and good forming performance,which is one of the ideal materials to achieve the goal of automobile lightweight.Edge cracking of UHSS is an urgent problem to be solved in stamping process.The defects and damages of the cutting edge reduce the subsequent forming performance seriously,which make the materials crack in the cutting edge first.Therefore,it is necessary to study the edge cracking of UHSS.With the support of National Key Reserch and Development Project(NO.2017YFB0304403),the influence of blanking and materials parameters on the sheared edge quality and the influence of edge quality on the forming performance have been investigated.Besides,the cracking mechanism of different edge quality was revealed.All the research work will provide a certain guide for the development of a new generation of UHSS and the design of the edge forming process.The main research contents are as follows:The blanking experimental platform has been built to explore the influence of blanking and material parameters on the quality of shearing surface for UHSS.The results showed that the percentage of burnished zone was the largest for HSLA800,reaching more than 26%,but the unevenness of the fracture increased seriously when the clearance was 13.1% of sheet thickness.Compared to DP980 with the same thickness,the burnished zone was smaller and the burr was larger under the same blanking parameters.QP1180 and QP980 were most sensitive to the change of blanking clearances,and when the blanking clearance was 23.3% of the sheet thickness,the burr increased significantly and the quality of the shearing surface was deteriorated seriously.With the change of blanking clearances,for MS1180,the burnished zone increased first and the decreased,and the burr height was small and basically maintained at the same level.The Sheared Edge Tension(SET)test based on Digital Image Correlation(DIC)technology was used to investigate the influence of blanking parameters and cutting processes on the tensile property of the edge.Combined with the observation of fracture morphology,the microscopic mechanism of crack initiation and fracture during the tensile process of different edge quality was revealed.The results showed that the sheared edge quality had a certain influence on the total elongation and the fracture strain,as well as the fracture pattern.Compared to DP980 and MS1180,QP1180 was more sensitive to the change of blanking clearances.What's more,the tensile property of the edge was improved when the the sheared edge was sanded slightly,and the property was likely to be superior to the cut edge by wire electrical discharge machining(WEDM).Compared with the necking fracture and edge-cracking fracture,there were mainly a large number of dimples in the micro-morphology of the oblique fracture.The lateral fracture of the edge-cracking fracture also showed equal-axis dimples and a small amount of cleavage facet which indicated that the edge damage caused a decrease in plasticity.Based on the Hole Expansion(HE)test,the influence of the burr direction,the punching clearances and the shape of the hole expansion punch on the hole expanding performance was investigated.The results showed that when the specimen was placed in the reverse burr direction,the conical punch contacted the burr to produce a pressing action first which reduced the possibility of crack initiation from the burr edge,so the hole expansion ratio was greatly improved.When the punching clearance varied between 10% to 15% of sheet thickness,the hole expansion ratio was larger.When the conical punch was used,the contact area between sheet and punch was smaller,and the edge of the hole could be largely deformed.Comparing the results of the SET test and the HE test,the similarities and differences of tension and hole expansion fracture are summarized.The total elongation in SET test was generally smaller than the hole expansion ratio obtained in HE test.Further more,comparing the morphology of the two fracture modes,it was found that both of the two kinds of fracture morphology were a large number of dimples.However,the dimple size of the tension fracture was smaller,and some large dimples appeared in the hole expansion fracture.In particular,the tensile fracture of MS1180 also showed a small amount of river pattern,which was characteristic of brittle fracture.