Analysis of Side Wall Curl in Draw-bending of High Strength Steel Sheet

Abstract

Geometrical defects observed in applying high strength steel sheet to auto-body structural members with hat-shaped cross-section were studied using a simple experimental draw-bend die. One of the typical troubles in forming of these parts is the side wall curl which appears along side walls from a bottom to a top (or a flange) and deflects walls outward.
It was found that the side wall curl was eliminated by choosing the die profile radius about twice as large as sheet thickness under the optimum die clearance.
A rather small die profile radius was advantageous for producing the reverse bending against bending at the die profile.
This reverse bending contributed to reducing the side wall curl by cancelling the residual bending moment in side walls which had been subjected to bending at the die profile. Such behaviour of material between a die and a punch was an important fact pointed out in this investigation.
The other interesting observation was that the dual phase steel had a larger tendency to the outward side wall curl than the precipitation hardening steel with the similar tensile strength. This difference was also discussed in this paper.