Estimation of Cutting Parameters in Two-Stage Piercing to Reduce Edge Strain Hardening

Edge fracture is a common problem when forming advanced high strength steels (AHSS). A particular case of edge fracture occurs during a collar forming/hole extrusion process, which is widely used in the sheet metal forming industry. This study attempts to relate the edge stretchability in collar forming to the strain hardening along the pierced edge; thus, Finite Element (FE) simulations can be used to reduce the number of experiments required to improve cutting settings for a given material and thickness. Using a complex-phase steel, CP-W 800 with thickness of 4.0 mm, a single-stage piercing operation is compared with a two-stage piercing operation, so called shaving, in terms of strains along the pierced edge, calculated by FE simulation. Results indicated that strains were reduced along the pierced edge by shaving. These results are correlated with experimental observations made at the Technical University of Munich (TUM) where better hole expansion ratios (HERs) were obtained using shaving as opposed to single-stage piercing. Moreover, in this study, the combination of cutting parameters that would generate the least edge strain hardening is evaluated based on simulation results. This approach could reduce the need for hardness measurements along the pierced edge as well as the number of experiments required to improve the shaving process.