Springback is one of the most serious problems in high-strength steel-sheet forming to produce automotive body parts. It is, therefore, important to control and suppress springback in the use of high-strength steels. Against this background, two springback-root-cause analysis methods were developed to identify the areas of stresses at the bottom dead point, which are the most influential in springback. One method uses stresses at the bottom dead point, and the other method uses springback driving stresses, that is, the difference between stresses before and after springback. The two methods were applied to the forming of an actual part. The areas of stresses that have a major impact on springback were identified by the two methods and were almost the same. From the viewpoint of accuracy, however the method of using springback driving stresses was more accurate than that using stresses at the bottom dead point. A countermeasure based on these results was applied to the actual part, enabling the amount of springback to be reduced substantially.

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To improve fuel economy and crash safety in the automotive industry, three-dimensional hot bending and direct quench (3DQ) technology, which enables the formation of hollow automotive parts with a tensile strength of 1470 MPa or more, has been developed. In this paper, the forming characteristics in 3DQ are investigated by experiments and FEM analysis. The main results are as follows. (1) Measured and calculated bending force and twisting torque are low and within the load capacity of general robots. (2) The constraint at the tube top by the robot in 3DQ can increase the accuracy of products even in inhomogeneous coiling. (3) A narrow deformation area suppresses the change in the product cross section, and it is clarified that the calculated maximum value of σ_θ/σ_l is closer to 0.5 in a narrow deformation area, where σ_θ is circumferential stress, and σ_l is longitudinal stress. (4) The effects of the maximum heating temperature and the feeding speed on the deformation area are small.

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Recently, interest in global environment has been increasing. Demands for improved structural components and lightweight parts are increasing. It is further necessary to increase the strength of high-carbon steel wires. Delamination is a critical problem of high-strength steel wire. A major factor in the delamination is strain aging by the heat generated during wire drawing. Therefore, cryogenic drawing is effective for avoiding strain aging. In this study, the heat generated during wire drawing is focused on, and the drawing force in cryogenic drawing with various types of lubricants, i.e., liquid or powder lubricants, is discussed. Cryogenic drawing was realized by holding the wire in a box filled with liquid nitrogen prior to drawing, and the drawing force was measured using a load cell. An appropriate lubricant in cryogenic drawing was found to be powder lubricant.

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The seizure resistance in ironing of stainless steel cups was improved by using a TiCN-based cermet die having a surface making fine lubricant pockets on polished surfaces by shot peening. The shape of the lubricant pockets was controlled to improve the seizure resistance. The seizure resistance for a die having lubricant pockets of an appropriate surface shape became higher than that for a polished surface having very fine surface roughness, because the friction was reduced by the liquid lubricant being squeezed out from the pockets during ironing, and the ironing load became smaller. The mechanism of the superior seizure resistance of the die having an appropriate surface shape was examined from the remaining amount of lubricant in ironed cups. It was found that the use of the TiCN-based cermet die having lubricant pockets is effective in preventing seizure in the ironing of stainless steel drawn cups.

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The sizing press process has been developed to achieve extensive width reduction and improvement of the yield rate. The slab head and tail shapes after sizing pressing are important in crop loss reduction and stable operation. This study was carried out to investigate the deformation behavior of slab heads upon sizing pressing with back pressure. Finite element analysis and a model experiment were conducted to study the deformation behaviors of the slab head thickness profile and a plan view pattern. It was found that width reduction by the back-pressure method can enable the control of the slab head shape. The maximum thickness of the central part of the slab head was reduced and the plan view pattern changed from a fishtail shape to a tongue shape by applying back pressure during sizing pressing.

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The frictional behavior of galvannealed steel sheets (GA) depending on tool materials was investigated. GA having different ζ / δ₁ phase intensity ratios was prepared as specimens. In addition, lubrication-treated GA was also prepared using these specimens as the base material. 4 kinds of tools composed of zinc alloy die casting (ZAS), ductile cast iron (FCD), alloy tool steel (SKD) and Cr-coated FCD were used as sliding tools in order to simulate actual press tools used in trials and mass-production. When the tool material was harder than those of the test specimens, the friction coefficient of GA increased with increasing amount of the ζ phase and the effect of lubrication treatment was clearly observed. However when the tool material was softer than those of the test specimens, the friction coefficient was constant independent of the existence of the ζ phase and lubrication treatment. The change in the frictional behavior was discussed from the viewpoint of the change in the frictional mechanism depending on the comparative hardness between the surfaces of the specimens and the tool materials.

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Recent experimental findings have suggested that the slide motion of sheet metal forming driven by servo-drive presses enhances its formability and forming accuracy. Previous studies have focused primarily on the relationship between the lubricated condition and formability in sheet metal forming. However, little attention has been paid to the relationship between flow stress and formability under slide motions. In this study, we have found that flow stress, which depends on the strain rate, affects the thickness reduction. Stress-strain curves under various strain rates were measured, and the stress-strain curves were evaluated using the Tanimura-Mimura model, which is defined as the sum of a function based on a static tensile test and a function of strain rate. The parameters in the Tanimura-Mimura model and the Cowper-Symonds model were identified using the stress-strain curves in the entire measurable strain range by the nonlinear least- squares method, and the stress-strain curves calculated using the Tanimura-Mimura model exhibited better correspondence with the experimental stress-strain curves. Then, FEM analysis of sheet metal forming under slide motion was performed using this model. As a result, the strain rate of the punch shoulder in circular cup drawing was increased, and the localization of thickness reduction was relieved by adopting the slide motion.

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