Study of Deformation and Fracture of High Strength Steel Sheet during Conventional and Robust Trimming by Conducting Partial Trimming Tests

Abstract High-strength steels are used in the automotive industry for weight reduction and improved vehicle crashworthiness. In this work, an instrumented trimming die equipped with D2 steel inserts with sharp edges was used to trim DP980 sheets with clearances of 10%, 20% and 30% of the sheet thickness. Trimming trials were conducted either without a support pad under the scrap piece (conventional trimming) or with it (robust trimming). The deformation and fracture mechanisms in the shear affected zone (SAZ) at the sheared edge of DP980 sheet specimens were investigated by performing partial trimming tests and using a new metallographic preparation method as well as 3D surface profilometry. For 10% clearance under conventional trimming, the damage distribution within the SAZ was practically symmetrical and the shearing mechanism consisted of cracks propagating simultaneously from the upper and lower surfaces of the sheet toward each other. However, the damage distribution became asymmetrical and the shearing mechanisms changed when the clearance was increased to 30%: the crack was initiated from the upper surface and propagated toward the lower surface resulting in the formation of a tensile-type burr. At 30% clearance, the robust trimming process (with a support pad) prevented the bending of the scrap piece resulting in a symmetrical damage distribution within the SAZ. In this case, a crack was initiated from the bottom surface of the sheet near the lower trim edge, and subsequently, another crack initiated from the upper surface, result in a decrease in upper die penetrations at the onset of fracture and at the end of trimming progress compare with conventional trimming. By using sharp die edge under robust trimming process, an improved sheared edge quality is achieved at larger clearances and this will help to reduce the production costs associated with dulling the die edge and the alignment of trim dies when the clearance is tight.