Recently, for the weight reduction of automotive bodies, steel sheets used in automobiles have been becoming thinner owing to the application of high-strength steels. Press forming of high-strength steels, however, has many problems. Wrinkle formation is one of the most serious problems in press forming, especially in the application of highstrength steels. The cause of wrinkle formation has not yet been completely understood, particularly in the case of bending flange wrinkles, which are common in automotive parts forming. There are, therefore, few well-known methods of satisfactorily preventing bending flange wrinkles. In order to prevent bending flange wrinkles, it is important to understand the effect of press conditions on wrinkle formation. In this study, cylindrical forming experiments simulating bending flange wrinkle formation in press forming of automotive body parts were conducted to clarify the forming limit for wrinkles, that is, the critical circumferential strain, which is the threshold of bending flange wrinkle formation. In addition, a new method of wrinkle suppression by die shoulder shape optimization was developed on the basis of the results of the above experiments. A die shoulder shape was designed such that the edge of the blank remains in contact with the die shoulder. The superiority of the developed design method was verified by forming experiments.

In the 2nd report, the evaluation factors for void closure under the condition of multipass forging revealed that true strain ε_z in the z direction is appropriate for the evaluation factors for void closure. However, although the effectiveness of the hydrostatic stress ratio σ_m /σ_eq for void closure was clear in the 1st report, the reasons were not clarified. In this paper, we investigated the effects of stress factors on void closure and its handling through numerical experiments. The following results were obtained. 1) By using the true stress in each direction, the deformation behavior of voids can be evaluated as a dimensional change of the diameter on the basis of the frictionless model. 2) In the z stroke direction, the void is easier to close than in the frictionless model when σ_z /σ_eq is -1 or less, and harder to close than in the frictionless model if not. 3) The void diameter in the x direction, d_x , is smaller than in the frictionless model when σ_x /σ_eq is 0 or less, and larger than in the frictionless model if not. 4) When there is a void on the axis of symmetry, σ_m /σ_eq can be used to evaluate ε_z for void closure and the dimensional change of d_x . In addition, we clarified the influences of material size, void size, and mechanical properties of the material on void closure.

In this study, we investigated the influence of various bending conditions on cracks and microstructures during the V bending test. The AZX811 alloy used in this research was in the form of two kinds of sheets: high-strength large-elongation rolled sheets and static recrystallization sheet (annealed at 450 °C for 2h). Under the V bending test conditions, the processing temperature was changed from room temperature to 300 °C. The ratio (R/t) of the punch radius R to the sheet thickness t was 1 to 3. During the bending test, cracks occurred outside the punch subjected to tensile stress. Under the condition of R/t = 1, it was possible to bend the static recrystallized sheet at 300 °C and the rolled sheet at 200 °C. It is considered that in the rolled sheet, in which strain remains, dynamic recrystallization occurs at lower temperatures than in the static recrystallized sheet.