Modeling of Surface Crack Defects Developed on Shear Edge in High-strength Automotive Steel Sheets

Surface crack defects developed on the shear edge cause a problem in shearing of high-strength steels. The surface crack formation mechanism was clarified by microstructural examinations and numerical simulation. Two types of 780 MPa grade hot-rolled steel sheets with a thickness of 2.6 mm were chosen for the evaluations because the materials show different surface crack susceptibilities. Cleavage fracture was responsible for the surface cracks, and micro-ductile cracks with a length of 30 µm to 40 µm were detected in the interrupted punching samples. A numerical simulation demonstrated that a tensile stress was developed in the direction of the micro-ductile cracks opening during punching process. The critical length of the micro-ductile crack for cleavage fracture as a crack initiation site was given by linear fracture mechanics; for example, the critical length is 23 µm or longer under the applied tensile stress of 910 MPa. The tensile stress causing cleavage fracture decreased by reducing the tool clearances, and it was shown experimentally that surface crack defects can be prevented by controlling the clearance appropriately.