The effect of mechanical properties of sheet steels on the wrinkling behaviour during deep drawing of conical shells

Abstract

Wrinkling is a common defect in press-formed parts, particularly in those with sloping walls. Its elimination often presents considerable technological problems. It is known that wrinkling can be prevented by stretching the material in the shell wall. This can be achieved by increasing the blankholder load but this is, of course, limited by the ductility of hte material in the cup wall. Only if the blankholder load is within a narrow range above that necessary to stretch out wrinkles on the one hand, and below that producing wall fracture on the other, will pressing be defect-free. The size of this workable range of blankholder loads determines the criticality of the pressing. The wider the range, the less critical the draw. It has been shown that for a given cup with sloping walls, the workable blankholder-load range cannot be significantly increased by modifying such forming conditions as blank size, die-profile radius and lubrication. A minor improvement can be achieved by increasing punch-profile radius, or by selected lubrication in the punch-nose area. In practice this means that, if the design or material thickness cannot be changed, the wrinkling problem can only be alleviated by using material with better mechanical properties. In the present investigation, the effect of mechanical properties on wall wrinkling in conical cups has been studied using cold-rolled and annealed stabilised steel, interstitial-free steel and high-strength low-alloy steel. The material with a higher n-value, higher r-value and a lower yield stress has generally been considered to be more wrinkle-resistant. The present investigation indicates that the effect of these properties is more complex. It has been found that the n-value influences the plastic-strain distribution during forming. In conical cups a high- n-value material develops higher compressive strains in the cup wall than a low- n-value material when pressed to the same depth, thus leading to earlier wrinkling. A high- r-value material has been found to be more wrinkle-resistant than a low- r-value material. Consequently, when selecting steel sheets for pressed parts where wrinkling is a problem, consideration has to be given to the effect of both r- and n-values. A material with high r m -and low Δr-values should be specified. A high- n value material will not however alleviate the wrinkling problem in those cases where wrinkling is caused by excessive draw-in of material into the die orifice, such as in simple tapered cups, unless other changes in forming conditions, e.g., increased blankholder load, are also made. In complex parts, the effect of n-value on strain distribution should be evaluated with the grid-strain technique.