Decreasing the amount of heating energy in hot forging has been required to alleviate global warming. Some forming conditions of low-friction and high-aspect-ratio billets are effective in reducing load to exchange hot forging to cold forging. It is helpful to decrease the amount of hot forging if other conditions are found to reduce the forming load of high-strength steel. In this study, the effect of the elastic deformation of a forging die on the load is investigated. The load increased when a high-strength material is compressed using a die made of tool steel. Experimental and analytical results show that the flat tool surface has a deep elastic concave, that inhibits the free flow of the material and increases the hydrostatic pressure component, resulting in increases in the surface pressure and load. Therefore, when a cylindrical billet of bearing steel is compressed by 70% using a die made of carbide, the load is reduced by 20% compared with that when using the tool made of tool steel. The tool made of carbide is effective for suppressing the load increase caused by the concave due to elastic deformation.

The Keisha-Tanmen (KT) method was developed to improve the corrosion resistance of the cut-edge surface of Zn-alloy-coated steel sheets. The dies used in the KT method have a triangular convex blade. Using the upper and lower blades, a Zn-alloy-coated steel sheet was pinched and broken off. The cut-edge surface resulting from the KT method consists of three areas: roll over, inclined plane, and fracture surface. After using the KT method, a coating could be spread effectively over the inclined plane, and the fracture surface area was decreased compared with that obtained after a conventional cutting operation, namely, shearing. The cutting behavior in the KT method was simulated by the finite element method (FEM). The reason behind the decreased fracture surface area was that the negative stress triaxiality around blade edges prevented crack generation and delayed breakage. The rate of red rust formation on the cut-edge surface after the KT method was approximately 20 % and that after shearing was approximately 60-80 % after 10-200 days of atmospheric corrosion tests. The KT method was confirmed to yield excellent corrosion resistance on the cut-edge surface of a Zn-alloy-coated steel sheet compared with that observed after a shearing operation.

The look of a car has become more complicated, and a concern is the sharpness of character lines. However, a sharp character line causes the surface defect called a skid line. Although it is well known that skid lines are caused by inhomogeneous tension, the mechanism of skid line generation is not yet fully understood. In this study, the skid line defect was quantitatively evaluated by a draw forming experiment. The mechanism of skid line formation was clarified by comparing the experimental results with the results of a finite element analysis. The results revealed the effects of the tensile forces caused by blank holder forces on the change in the amount of displacement of the skid line. The results suggest that the skid line surface defect detected could be a concave shape caused in the area initially formed as a character line that shifts from its original location as a result of inhomogeneous tension, and then is subjected to reverse bending by the die.