A 2-stage plate forging process consisting of drawing and compression of tailored blanks having local thickening for the deep drawing of square cups was developed. A sheet having uniform thickness is drawn into a cup having a flange, and then is compressed with a punch and a flat die under fixture of edges of the flange to thicken the inclined portions. The degree of thickening was increased by controlling the punch stroke in the drawing stage, because the degree of thickening is small for a small stroke and folding occurs for a large stroke. The tailored blank having local thickening was drawn into a square cup. The limiting height of the drawn cup without fracture using the tailored blank was increased to 34 mm, whereas the height limit using the uniform blank was 18 mm. The limiting height of the cups for the tailored blanks formed by the drawing-compression having the 2 stages was almost similar to that formed by the bending-compression having the 4 stages, because the regions except for the target ones are thickened for the bending-compression. 2 shock lines appearing in the tailored blank were eliminated with a counter punch in the compression stage.
To meet the requirements for the weight reduction of automobiles, high-tensile-strength steel sheets have been applied extensively to the manufacture of auto parts. But their relatively lower press formability has obstructed their further application, and it is strongly demanded that the stretch flange ability of such steel sheets be increased, and cracks that occur when stretching edges during the press forming of such steel sheets be suppressed. Thus the authors have carried out a study to improve the stretch flange ability of high-tensile-strength steel sheets by optimizing the punching method and obtained the following conclusions. Work hardening on a pierced edge is mainly caused by deformation by the punch motion while shearing the material. By attaching a hump of certain shape at the bottom of the punch, the ductile fracture in the material caused by the punch corner is accelerated by the hump′s effect to increase stress triaxiality, which reduces the extent of total deformation through shearing, thereby improving the stretch flange ability of the material.
The optimum geometry of a specimen used for an in-plane compression test on sheet metal was investigated by FEM. Geometrical parameters were determined to minimize the stress measurement error, i.e., the difference between the mean stress, which is calculated from the measured load and compressive strain data assuming a constant volume, and the Cauchy stress at the center of the specimen, where a strain gauge is mounted. The stress measurement error decreases with increasing aspect ratio of the gauge area, where L and W are the length and width of the gauge area, respectively. A maximum error of 1% was achieved by taking an aspect ratio 2.2. Furthermore, the effects of the radius of the fillet, R, connecting the chucking area and gauge area, and the width of the chucking area, B, on the accuracy of stress measurement were investigated. It was found that 1.6 and 1.4 minimize the stress measurement error. Moreover, the optimum geometrical parameters for successfully applying a compressive strain of over to the specimen without in-plane buckling were clarified. The chucking area should be fully covered by chucking jigs to prevent the in-plane buckling of the gauge area.