A defect in the dimensional accuracy caused by springback is a major problem in sheet metal forming of high-strength steel sheets. In particular, ensuring dimensional accuracy of a long curved part is difficult owing to twist springback, which leads to complex three-dimensional surface variation. In this study, a new forming method for reducing twist springback was developed by focusing on stress at the flange, which is a cause of twist springback. The developed forming consists of two forming processes. In the first process, the flange part of the blank is sloped owing to the gap between the upper die and the flange-forming die, which are set at a lower position at height h relative to the punch. In the second process, the flange part of the blank is formed into the target shape by the flange-forming die, and the slight reverse strain that contributes to stress reduction at the flange. Experiments of press forming of a simple curved part and an actual automotive part using high-strength steel sheets were carried out. The experimental results showed that twist springback varied with the height h of the flange forming die. It was experimentally proven that twist springback can be minimized by the developed forming.
A new press-forming technique using in-plane shear deformation was developed for suppressing fractures and wrinkles of curved shapes in the height direction, for automobile body parts such as the front-side member rear. The developed technique is a two-step processes, draw-forming to induce in-plane shear strain and bending into a curved shape. From experiments on a simplified curve part, in-plane shear deformation which is effective for suppressing fractures and wrinkles occurred in the first process when the press stroke changed along the longitudinal direction of the part. When the induced in-plane shear deformation was increased, a part with a higher curved angle in the height direction could be formed in the second process. Then, we conducted press trials of a front-side member rear model using a 1180 MPa-grade ultrahigh-strength steel sheet to evaluate the effectiveness of the developed technique. The results suggested that a complex curved part could be formed without fractures or wrinkles when in-plane shear deformation was induced in the first process.
Further weight reduction is required for the automotive body from the viewpoint of global environment protection. Therefore, steel sheets for automotive body parts are becoming increasingly thin and strong. Generally, as the tensile strength of a steel sheet becomes higher, formability deteriorates owing to reduced ductility and deformation localization during forming. Then, we worked on developing a forming technology utilizing deformation dispersion for forming high-strength parts with either a flange-down or flange-up formed portion at the longitudinal edge. As a result, proposed forming method, which enables deformation dispersion, improved the formability, wrinkle resistance in case of flange-down forming and fracture resistance in case of flange-up forming. In addition, the proposed methods make it possible to compensate for decreases in strain dispersion characteristics of high-tensile-strength steel sheets by means of deformation dispersion. As a result, the proposed methods compensate for decreases in flange-down and flange-up formability each due to deteriorations of material properties in high-tensile-strength steel sheets.
We developed a hot stamping method for panel parts with a step-shaped wall. The die-set for the developed method divides the punch at a step-shaped wall into an outer punch and an inner punch. The outer punch is placed first. In addition, an opening is added to the blank of the developed method. The developed method makes it possible to change the main forming type from draw forming to cylindrical stretch flange forming in the middle of a forming stroke. The stretch flange forming can prevent a fall in temperature at a hole edge, because the hole edge does not come into contact with the die-set during forming and because the high ductility of high-temperature materials can be used, making it advantageous for hot stamping. If the preceding amount of the outer punch is large with the developed method, the effect of suppressing wrinkles is large but the sheet thickness reduction that can cause cracking rises. If the preceding amount of the outer punch is small, the rate of sheet thickness reduction ratio is greatly restrained, but the wrinkles become larger. By setting an appropriate preceding amount of the outer punch with the developed method, forming without cracks or wrinkles is possible.
Unlike mild steels, cold rolling of high-strength steels gives rise to various problems, the causes of which are related to the high tensile strength, which is a property of the material, the high degree of work hardening, and the fine-scale composition. Furthermore, these characteristics make it easy to generate slip during the rolling of high-strength steel. In this report, the simulation of this rolling slip and its influence on slip occurrence when changing the conditions of rolling oil, which is the lubrication factor, were investigated. On the basis of these conditions, we developed a rolling oil for high-strength steel with excellent slip resistance.