A form rolling analysis model generally needs massive elements for solving large plastic deformation of three- dimensional problems with sufficient accuracy. We have developed a unique and compact analysis model. The smallangle-cut model can be a small model but it will not be sufficient to analyze stress concentration near the contact surface between the roll and the blank. We have developed a compact model composed of small-angle one-layer solid elements given a certain angular velocity field with oblique roll contact. The computation time using this model was only about 1/2000 that when using the full-size model. This model will shorten both the model generation period and the span of design change leading to refined performances of the product prepared with the form rolling process.

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The edge of a punched hole was thickened by burring processes to improve the fatigue strength of punched ultra-high strength steel sheets. In this process, the sheet was punched by the bottom of the punch, and subsequently the hole edge was thickened by the taper of the punch and the corner step of the die. The fatigue strength of the hole edge thickened by burring was higher than that of the punched hole edge. The punching and the burring were combined into the punching process including thickening. The step height of the die was optimised to increase the height of the hole edge. The quality of the sheared edge for the thickened punched sheet was improved by ironing with the taper of the punch, and compressive residual stress was generated around the hole edge. It was found that the punching process including thickening is effective in improving the fatigue strength of the hole edge in ultra-high strength steel sheets and in preventing the occurrence of delayed fracture.

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Springback prediction is necessary for the application of high-strength steel sheets to automotive parts. The accuracy of springback prediction depends on the material model, which describes the deformation behavior of steel sheets. In this research, material model taking into consideration important material behaviors (Bauschinger effect, average Young’s modulus, elastic anisotropy and plastic anisotropy) was developed and implemented into FEM software. Moreover, springback analyses were carried out for curved hat-shaped parts made of high-strength steel sheets. As a result, the effects of each material behavior on springback were clarified. It was found that not only the Bauschinger effect and average Young’s modulus but also elastic and plastic anisotropies influenced the result of springback prediction, particularly in the case of anisotropic material. Springback analysis taking into consideration all four material behaviors yielded better springback prediction accuracy than those of conventional analyses.

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The effects of die dimensions on curvature-extrusion curved rectangular bars were investigated by experiments and numerical simulations. We conducted simulations and experiments of rectangular bar production using various die dimensions. The calculated trend of curvature showed good agreement with the experiment results, thereby validating the numerical simulation results. The curvature increases concomitantly with increasing exit height and exit width. The effect of die thickness on curvature is slight. Regarding the conditions of die dimensions, the exit height is the most important factor that affects the curvature. The curvature increases when the difference in exit velocity becomes large. The generation of a dead metal area, along with container configurations, affects the curved rectangular bar curvature. Results show that the curvature is determined by the difference in exit velocity, in conjunction with the die dimensions.

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