Influence of the cutting edge on the formability of steel

The sensitivity of sheared Advanced High-Strength Steels (AHSS) to fail at the sheared edge is a critical problem for the automotive industry. The problem is exacerbated by the fact that this edge failure can not be predicted by conventional methods. To understand the edge formability of AHSS under different edge preparation conditions, the hole expansion ratio λ (HER), a measure of the engineering strain obtained by a ISO 16630 Hole Expansion Test (HET) (see figure a) is used as a comparison tool. Comparing sheared edges, 15 % cutting clearance provided the best HER. The strain induced during the shearing operation plays a key role in the subsequent reduced formability. This strain, if calculated from the hardness values at the cut edge using hardness to stress conversion rules, such as Levy, Tabor etc., and mapped onto the edge of the blank in the forming simulation allows for an accurate prediction of edge failure (see figure b). However, it is observed that the HET results are highly sensitive to the tesing apparatus and conditions being used and it is essential to make the process more robust (see figure c-d).