In this study, the hole expansion ratio and fatigue properties of high-strength steel sheets pierced using a ”press working (PW) punch” were investigated. The PW punch, which was developed to prevent seizing between a punch and a hole surface, was reported to have the effect of smoothing a pierced edge. The PW punch increased the hole expansion ratio at a minute clearance compared with a conventional punch, and at larger clearances, the PW punch decreased it. The reason for this is considered to be the fact that the PW punch suppresses work hardening in the vicinity of the pierced surface and induces work hardening at the inner part of the pierced edge with an increase in clearance. Regarding the fatigue properties, the hole made using the PW punch had a higher fatigue limit than that made using conventional punches at large clearances. This is because a PW punch reduces roughness in the fracture zone.
A hot stamping process of quenchable steel sheets using bypass resistance heating for forming ultra-high strength steel products having a strength distribution was developed. In the resistance heating of the sheet, non-electrified portions by contact with copper bypasses with a low resistance were not heated, whereas portions not in contact with the bypasses were heated by the electrification and were quenched by holding tools at the bottom dead center. The bypass resistance heating was stable as partial heating, and the electrical power loss was small. The hardenability for the bypass resistance heating was first examined by sandwiching a partially heated sheet between large steel blocks without deformation. Next, hot hat-shaped bending using the bypass resistance heating was performed to form a product having a high strength around the corners. A hat-shaped product having a tensile strength of approximately 1.5 GPa at the corners was formed, and the input energy and punching load in the bottom portion were considerably smaller than those for the whole heating.
A new draw bending process using a double-action punch, consisting of inner and outer punches, has been developed to control the springback of the formed parts in high-strength steel sheets. In this process, the draw bending by the inner punch is followed by the reverse draw bending by the outer punch. By performing hat-shaped draw bending experiments, it was confirmed that the above-mentioned sequential punch actions, with the application of an appropriate stretching force to the sheets by a controlled blank holding force, allow us to obtain completely flat flange and noncurled side walls of the hat-shaped part in 980MPa high-strength steel sheets. Such a marked decrease in springback is due to the reduction in bending moment by sequential bending and unbending under a large stretching force.
Surface deflection is caused by a small local out-of-plane displacement on exterior automotive body panels, such as the handle portion of door panels and the fuel filler lid mounting area of side outer panels. Surface deflection is influenced by many factors, such as punch geometry, material properties, sheet thickness, and forming conditions. In this study, surface deflection is investigated by press forming experiments and their numerical simulations of the exterior door panel model. Emphasis is placed on the effects of mechanical properties on surface deflection, and the mechanisms of surface deflection are discussed. It is revealed that the nonuniform distribution of stress around the handle portion during press forming causes surface deflection. Yield strength, plastic anisotropy and Bauschinger effect have significant effects on surface deflection.
To evaluate the 3D springback regarding the rear member model tool for high-strength steel up to a 980 MPa tensile strength, twisting and camber were observed in forming samples. The springback simulation of the rear-member model was carried out using isotropic and mixed hardening models. Analytical results were compared with experimental ones under different lubricant conditions. The simulation results of the mixed hardening model showed good correlation with the experimental results for twisting and camber because the Bauschinger effect is considered. To investigate the mechanism of the 3D springback, theoretical evaluation of the simulation results before springback is carried out. The torsion angle theoretically calculated using torsion moment is in good agreement with experimental one. The main reason for the increasing torsion moment is the increasing wall stress. It is thought that bending moment in the longitudinal direction affects camber distribution. The bending moments of the web and flange areas were larger than the other areas especially. Moreover, rear-member model tests of the countermeasures of twisting and camber were carried out. Twisting and camber were reduced by placing partial beads in proper areas. In this part, the effects of partial beads become larger in the case of placing beads in stretch flange areas on wide width side. The effects of reducing springback were also obtained by two forming methods. The forming methods entailing a decrease in punch width in a curved area of the stretch flange on a large width side is effective because it can control torsion moment.
Forming limit curves (FLCs) of high-strength steel (HSS) sheets under proportional and nonproportional deformations were obtained by performing stretch forming tests. To predict such FLCs, Marciniak-Kuczynski (M-K) type analysis was carried out considering void growth at the neck of a sheet. From the experimental observations of the necked fracture zone of sheets and the corresponding M-K analysis results, it was clarified that HSS sheets fracture by localized necking. M-K analysis predicts FLCs of HSS sheets reasonably well. The forming limit of an equi-biaxially prestrained HSS sheet is markedly low compared with the original FLC under proportional deformation and, in this case, analytical predictions show good agreement with the experimental results. On the other hand, for a uniaxially prestrained HSS sheet, M-K analysis overestimates FLC when the strain path abruptly changes from uniaxial tension to equi-biaxial tension. A simple method of determining FLCs for nonproportional strain paths is presented.