Estimation of Cutting Parameters in Two-Stage Piercing to Reduce Edge Strain Hardening
To be published on April 2, 2019 by SAE International in United States
Edge cracking is a common problem when forming advanced high strength steels (AHSS). A particular case of this issue is during collar forming (or hole extrusion) process, which is widely used in the stamping industry. This study attempts to relate the edge stretchability in collar forming to strain hardening along the pierced edge, as suggested by the Institute of Metal Forming (UTG) of the Technical University of Munich (TUM). The study intends to illustrate how Finite Element simulations can be used to reduce the number of experiments required to improve cutting settings for a given material and thickness. Using CP800 of 4.0 mm thickness, a single-stage piercing operation is compared with a two-stage cutting operation (shaving) in terms of strains (consequently strain hardening) along the pierced edge, calculated by finite element simulation. Results indicated that strains were reduced along the pierced edge by shaving. These results are in agreement with experimental observations made at the Technical University of Munich, 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 proposed based on simulation results. This approach could reduce or eliminate the need for hardness measurements along the pierced edge as well as the number of experiments required to improve the shaving process.