In making the die of automotive outer panels, the most difficult process is fixing surface deflection. To fabricate high-quality outer panels without modifying dies, it is important to develop an evaluation method and a numerical analysis method for the surface deflection of outer panels. In this study, we developed a new evaluation method that uses the maximum value of curvature, calculated using reflecting curves in the surface. This made the examiner evaluation conform to digital evaluation. The method shows better agreement with the examiner evaluation than the conventional method. We proposed a new analysis method of predicting surface deflection correctly. By the proposed method, plastic deformation is calculated in consideration of stress in the thickness direction, and restriking conditions are examined. We applied our methods to the fabrication of automotive outer panels and verified that they were useful and practical.

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Metallic glass (MG) has unique mechanical properties, combining high strength and low Young’s modulus. By applying MG to the fabrication of fastening bolts, high resistance against bolt loosening is expected. However, MG components are thought to be brittle because MG exhibits poor ductility when subjected to uniaxial loading at room temperature. We have developed hexagonal cap bolts made of zirconium-based MG by cold thread rolling. The MG bolt showed a 1.6% plastic strain with a tensile strength of more than 1550 MPa. In addition, the load-strain curve was similar to that of a strain hardening material although MG itself is free of strain hardening. In this study, we attempt to clarify the reasons for the behavior, which are advantageous for bolts in terms of toughness and reliability. Various experiments and numerical analysis indicate that residual stress plays an important role in the behavior.

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Methods by which smooth and high-precision cut surfaces of high strength steel sheets with strength of up to 980MPa class have been investigated using a servo press in which the average punching speed can be varied from 2 to 140mm·s^-1. A method called finish blanking has been performed, and the other method called press shaving has also been conducted to obtain either smooth cut surfaces or highly precise product shapes and dimensional sizes. The results show that both methods can yield good cut surfaces if appropriate working parameters including punch/die clearance, edge radius and punching speed are used. However, as regards shape and dimensional accuracies, the press shaving method yielded the most satisfactory results.

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In the roll forming of high-strength electric-resistance-welded pipes, the work hardening and springback of strips should be clarified. In this study, the effects of roll type, roll diameter and sheet strength on such operations in a single bending stand are examined by finite element analysis and experiment. Two kinds of roll are used. One (Type A) is a roll stand that pinches strips entirely in the circumferential direction by top and bottom rolls. The other (Type B) is a roll stand that pinches strips only at the edge part of the strips. The springback in Type A is smaller than that in Type B. However, in the case for Type B, overbending occurs owing to the hollow of the bottom roll, and the final circumferential curvature is almost the same as that in the case for Type A. The amount of work hardening in the case for Type A is larger than that in the case for Type B because of the transfer of the longitudinal top roll shape.

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The fracture toughness of cemented carbide for cold forging dies is necessary for predicting the fracture of a die insert. It is generally measured by the hardness indentation method. In the present study, it was measured by the three-point bending test using a test piece with a precrack generated by the EC method. Seven types of cemented carbide were used. It was found that the fracture toughness increases with an increase in WC grain size or Co mass percent. It was also found that the bending test using a test piece with a precrack gives good results compared with the hardness indentation method. It was shown that cemented carbides could be destroyed macroscopically under a 160 MPa tensile stress as analyzed by FEM simulation using a test piece model without a precrack.

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Continuous blanking was carried out using polycrystalline diamond (PCD) tools without a lubricant with the aims of developing the technologies required to apply these tools to shearing and of putting them to practical use. The adhesion of an A5052 aluminum alloy sheet subjected to blanking to the PCD tools was markedly reduced compared with that in the case of conventional SKD11 tools. It was also clarified that a high-precision sheared surface with properties similar to those obtained in the first blanking can be obtained even after 10,000 times of blanking. In addition, the adhesion of a SUS304 stainless-steel sheet to the PCD tools was prevented during the blanking, and the wear resistance of the PCD tools was markedly improved as compared with that of conventional tools. The properties of the sheared surface obtained after blanking 200,000 times were similar to those of the sheared surface obtained in the first blanking. The burr height and inner and outer diameters of the workpiece obtained after blanking 200,000 times did not change, demonstrating that high-precision sheared surfaces can be stably obtained. The above results show that PCD tools can be practically used in blanking.

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