The friction behavior of ultrahigh-tensile-strength steel sheets such as a 980MPa-grade tensile strength sheet has been investigated to improve the formability of steel sheets. The results of sliding friction tests showed that the friction coefficients of high-tensile-strength steel sheets decrease as the contact pressure between tools and sheets and sliding velocity increases. The results of sliding friction tests with a long bar tool showed that friction coefficients increase proportionally to sliding length. It is clarified that the effects of contact pressure, sliding velocity and sliding length on the frictional behavior of high-tensile-strength steel sheets are the same as those on the frictional behavior of mild steels. The friction coefficients are closely associated with the flat area ratio on sheet surfaces, which results from the deformation of a sheet surface convex portion. Finally, the friction model for FE forming analysis is proposed.
A numerical control (NC) servo press can be set to function with a variable motion in a single operation. In this study, we investigated the drawability of a cup formed by an NC servo press by measuring both sheet thickness and limiting drawing ratio. We also developed and built a new TZP test capable of evaluating NC servo press slide motion within a short time. The testing apparatus includes a sheet clamp with an NC die cushion, which eliminates the need for a dedicated die or a testing machine. As a result, the drawability evaluation with the NC servo press based on sheet thickness differed from that based on limiting drawing ratio. The proposed TZP test showed a similar trend to that of the sheet thickness evaluation. The evaluation based on the TZP test also revealed that a drawability higher than that attainable by link motion can be attained by detaching the die surfaces from the sheet at the proper position.
A cutting tool is designed for the shear cutting of thin-walled tubes. Cutting force is evaluated by changing cutting parameters such as shear strength, wall thickness, punch top angle and side length, and is found to be similar to that used in the shearing of flat sheets with a shear angle. Surface strain is observed during cutting and is found to vary depending with cutting conditions, which affects punch distortion. To prevent this deformation, the gap between the punch and the guide should be zero, which cannot be easily achieved using a conventional punch guide. To achieve zero clearance easily, a new guide using rollers is developed. This roller guide is always in contact with the punch, which is also available for cutting rectangular tubes. When designing the punch guide without distortion of the punch, a punch with a thickness of 2mm for a wall thickness of 2.3mm and another punch with a thickness of 1mm for a wall thickness of 1.6mm are available. This leads to a reduction in the amount of cut scraps.