Forming Limit Curve Determination of a DP-780 Steel Sheet

Abstract

In the present work, the forming limit curve (FLC) of a 1.1 mm thick DP-780 steel sheet is evaluated. The full FLC determination involved tensile tests of planar samples with different notch geometries ? providing data in the tension-compression range ? and Nakajima tests with a 40 mm diameter hemispherical punch and different notch geometries, covering the full loading zone of the forming limit diagram. The proposed Nakajima tool geometry reduces the standard test dimensions (ISO 12004-2) by 60%. Results show that the reduced diameter Nakajima test, in the particular case of Dual-Phase steels, overestimates limit strains when compared to the tensile test. This observed difference is explained in terms of the presence of a strain gradient through the sheet thickness imposed by the 40 mm diameter Nakajima punch, leading to the conclusion that this reduced sample type is not suitable for FLC determination when the DP-780 steel sheet is more than a millimeter thick.