Strain Hardening and Forming Limits of Automotive Steels

The formability window of a material depends upon the forming limit and its strain distribution ability. For two materials with the same forming limit, the formability performance is governed by their strain distribution ability. In this study, strain hardening behavior of different strength steels was investigated using a uniaxial tension test and the forming limit was studied using both Marciniak cup and dome tests. The n-value of steels varies with strain. Different strain hardening behaviors are found between mild steels and high strength steels. Strain distribution ability of steels increases with the increase in the overall n-value. The peak n-value at a low strain level enhances the strain distribution ability of the steel. It has been shown from both theoretical and experimental studies that a constant thickness strain line exists on the left side of the forming limit curve. The conventional equation for the calculation of the FLC_o based on the n-value and material thickness is valid only for in-plane deformation. The FLC_o determined from the equation agrees very well with the experimental data for two IF, one AKDQ, one BIW HSS and one BH steels.