Characterization On Local Formability Of Sheared Edge And Prediction Of Edge Cracking For High Strength Steel Sheet

High-strength steel plate with high strength and good processing bending performance,is widely used in forming automotive longitudinals,seat beams,threshold,bumper,B-pillar and other key security components.The main processing methods include cutting,punching and other material separation process and subsequent bending,flanging,rolling and other subsequent forming process.With the increase of strength,the strength of high strength steel decreases.The edge crack problem of the material after the shear process,such as blanking,punching,etc.,in the subsequent forming process is serious,resulting in a significant increase in the rate of rejection.These forming defects are often retained in the final forming parts,thus directly affecting the safety and fatigue properties of the components.Edge cracking of the High-strength steel plates during the shearing and forming process is different from the necking.Global Formability indicators,which are simply based on the material yield strength,tensile strength and elongation and other overall shape performance indicators,can not be accurately evaluated.In addition,the material hardness,limit hole expansion rate and other local formability are closely related.The overall and local shape of the material together determine the high-strength steel edge cracking failure limit.With the increase of the strength of the material and the increase of the thickness,the edge cracking problem is more prominent,and the forming limit curve(FLC)suitable for the necking failure can not accurately predict the cracking of the forming edge of the high strength steel sheet.It is urgent to develop the evaluation criteria of the edge cracking of the sheet forming simulation and forecasting method,and effectively guide the shear and forming process design of high strength steel body parts.The bottlenecks that need to be solved include the evaluation index and its characterization of the local forming performance of the shear edge of the high strength steel plate,the edge cracking simulation modeling technique of the local forming performance of the shear edge,and the shear process control.In this paper,the research on the prediction and control method of edge cracking is carried out from the aspects of the overall ductile fracture and the local shear edge forming performance of high strength steel plate.In the aspect of the ductile fracture of the cold-rolled high-strength steel,the ductile fracture criterion of anisotropy is established.In the aspect of the local forming performance and evaluation of the shear edge,the key indicators of performance and its characterization method are established.Finally,the shear and forming process control measures are put forward according to the problem of cutting edge cracking.Main contents and innovations of this dissertation are given as follows:(1)Effect of material anisotropy on ductile fractureBased on the phenomenological MMC ductile fracture criterion,the anisotropy of the material is introduced into the ductile fracture criterion model,and the ductile fracture criterion considering the anisotropy of the material is established by theoretical analysis.Quantitative description of the effect of material anisotropy on the overall ductile fracture behavior is established.The parameter identification method of the ductile fracture criterion considering the anisotropy of the material is studied.The fracture surface of the high-strength steel is obtained by using the fracture surface of the equivalent strain in the stress ratio and anisotropic parameter space.(2)Study on local performance characterization of shear edge of high strength steelThe experimental system for doing the shearing tests of HSS is established.The local performance evaluation index of the cutting edge of the QSTE,BS series of hotrolled high-strength steel and DP and MS series of cold-rolled high-strength steel are studied in this paper.The influence of cutting parameters on local performance index is also studied.In this paper,the hardness distribution of the shear section is proposed as a quantitative index to characterize the local performance of the shear edge.Aiming at the nonuniformity of the hardness distribution in the thickness direction of hot rolled high strength steel,the method of hardness distribution of two-dimensional shear section is proposed to quantify the local performance of the shear edge,and the hardness range of the boundary zone between the burnished zone and the fracture zone is accurately expressed.Based on the experimental results,the relationship between the edge hardness distribution and the material parameters and the shear process parameters was established,and the quantitative performance of the local performance of the high strength steel sheet under different shear processes is realized,which lays the foundation for the prediction and control of forming edge cracking considering the shear edge state.(3)Prediction and control method of forming edge cracking considering local hardening of shear edgeAccording to different forming methods in the production between the cold and hot-rolled high-strength steel,the tensile test of specimens with center hole for cold-rolled high-strength steel plate and the three-point bending test method for hot-rolled high-strength steel plate are designed.And the establishment of shear edge local forming performance testing system is established.The influence of the geometrical shape of the shear section and the local hardening on the edge cracking is studied.It is found that the peak hardness area of the shear section is the starting region of the edge cracking.At the same time,the relationship between the hardness distribution of the shear edge and the constitutive parameters of the high-strength steel plate is established,and the edge cracking simulation method is proposed to introduce the local hardening characteristic of the shear edge.Based on the influence of shear process parameters on the local performance of the edge,a cutting method control method based on forming edge cracking is proposed.In this paper,the anisotropy of cold-rolled high-strength steel and the non-uniform hardening of the cross-section of high-strength steel are introduced into the local forming performance of the shear edge,and the accuracy of edge cracking simulation is improved.The influence of the shear process parameter on the local performance also provides a quantitative guide for the development of the shear separation process for high strength steel components.The experimental data covers QSTE,BS series of hotrolled high-strength steel and DP,MS series of cold-rolled high-strength steel.It develops an important engineering application value for passenger cars and commercial vehicle structural design and processing technology.