Effect of Draw Beads on the Mechanical Properties of Sheel Sheet

Draw beads are used in many stamping dies to control the flow of metal into a die cavity. The multiple bends imposed by the draw beads cause a change in the mechanical properties of the sheet before it enters the die cavity. Since the necessary data to completely characterize the full deformation history of a sheet passing through draw beads are not available, it is not possible to use a fundamental approach to determine the effect on subsequent mechanical properties. In the present investigation data from a prior study [1] are used to develop empirical relationships to predict the yield strength, uniform elongation and tensile strength as a function of the cumulative maximum effective bending strain (ε̅_max-cum) due to draw beads and the entry radius to a die cavity. The objective of the present paper is to develop the empirical relationships for several grades of sheet steels and use these relationships to provide insight about the mechanical properties of these steels as they enter the die cavity. For this case, it is found that ε̅_max-cum can be used to predict yield strength, uniform elongation, and tensile strength for AKDQ, BH210, HSLA350, and DP600 steels as they enter the die cavity. It is also found that for, ε̅_max-cum > 0.15, the resulting uniform elongation, which in true strain is the terminal n-value, is independent of steel grade. It has also been found that the tensile strength in the wall of the die cavity can be used to provide insights on the potential for springback for the four steel grades that were investigated.