Dimensional scattering is a severe problem in press forming of ultrahigh-strength steels (UHSS), because of material strength scattering in mass production. In this study, camber back which occurs in longitudinally curved parts was focused on and a new forming method whereby dimensional scattering of camber back can be suppressed by the Bauschinger effect was developed. The new method consists of two processes. In the 1st process, a blank is formed to with a small radius of curvature compared with the 2nd-process radius of curvature. In the 2nd process, the 1st-formed part is formed to a large radius of curvature compared with the 1st-process radius of curvature. Then the Bauschinger effect works to decrease the amount of camber back. The new method was applied to a hat-shaped model of 590, 980, and 1180 MPagrade steels, where the radius of curvature in the longitudinal direction was 1600 mm. The experimental results showed that the difference in the amount of camber back between 590 MPa and 1180 MPa steels formed by the developed method decreased 95 % compared with the case of the conventional method.
Dimensional accuracy defects caused by springback behavior are one of the serious problems in press forming for automobile parts using ultrahigh-strength steel sheets. In particular, in hat-shaped parts curved in the longitudinal direction, camber back, which is a kind of springback behavior, occurs. In this study, in order to suppress camber back, a new press-forming method that can impart in-plane compression stress was investigated using parts with a simple hatshaped section and a W-shaped section simulating automotive parts. As a result, it was found that the amount of camber back decreases on applying compression stress in the longitudinal direction, because the bending moment caused by the difference between stress in the punch bottom area and that in the flange area was reduced. It was also clarified that the sensitivity of the amount of camber back to tensile strength was lowered when this forming method was used. Furthermore, a forming method in which buckling in both ends under a high compression condition can be prevented was developed.
In the manufacturing process of large forged steel products, internal quality must be improved by introducing stress and strain. The effect of the forging pass schedule on the consolidation behavior of internal defects can be evaluated using consolidation parameter Q-value calculated from the hydrostatic stress and the equivalent strain. In this study, we performed the actual steel ingot test to clarify the consolidation behavior and threshold of Q-value. Subsequently, the effect of the void distribution on the Q-value was investigated. The density distributions of initial defects were determined by solidification simulation calculation and the change in density distribution with increasing Q-value was investigated by forging simulation. After conducting this preliminary study, we report the result of verifying void closure using a tool for actual forging process design.
We studied the effect of the bending method on the hydroforming of a tube in a subsequent process, using experiments and the finite element method. Two bending methods, rotary draw bending and intrusion bending, were investigated. A tube was bent into an S shape by each bending method, and a hydroforming test was carried out with the same hydroforming die. The wall thickness distribution was measured after the hydroforming. The wall thickness of the tube was smallest on the compression side when it was bent by the rotary draw bending method and on the tension side or the intermediate side when it was bent by the intrusion bending method. In both bending cases, burst occurred on the intermediate side during axial feeding. In rotary draw bending, a burst occurred on the compression side or at the R stop nearby. However, in intrusion bending, burst did not occur on the compression side but occurred on the R stop nearby during the final increase of internal pressure. Intrusion bending left a larger hydroforming allowance than rotary draw bending. This is because the wall thickness after intrusion bending was greater than that after rotary draw bending.
In this work, by focusing on multiprocess tube flaring for eccentric parts, we studied the effect of punch shape on deformation behavior by FEM analysis. In the case of using only the eccentric punch, thickness deviation occurs in the circumferential direction and thickness reduction is enhanced because the tube is greatly flared at the circumferential position of 180 degrees. However, it is found that the thickness deviation and thickness reduction are improved by evenly flaring both sides using a concentric punch. Furthermore, although the punch shoulder radius has a negligible effect on formability, it is confirmed that the deformation near the tube edge transitions from uniaxial tension to pure shear as the punch semiangle is increased. The above result clarify that by using concentric punches with punch semiangle larger than the taper angle of the parts shape from the initial process, the thickness reduction is drastically minimized compared with other forming methods.