In this study, we investigated the effect of shot blasting on the corrosion properties after electrodeposition and the fatigue properties of the arc welds in automotive steel sheets. The corrosion properties of arc welds after electrodeposition are significantly improved by shot blasting owing to the removal of slag and fume, which can cause painting defects. Also, the fatigue strength of welded joints was enhanced by shot blasting, because the mean stress effect due to compressive residual stress improved the fatigue properties. These results suggest that shot blasting has the potential to reduce the thickness of arc welded steel parts in automobile chassis.
Flange-up forming in the longitudinal edge of press-formed parts is one of the most difficult problems for materials with poor ductility, such as ultrahigh-strength steel. The authors investigated the effects of the shape and material properties on flange-up formability in previous studies, and it was clarified that the bending behavior at the flange root during forming was the key to improving the stretch flange formability. In this study, we propose a new pad, which is called the lower pad and which can control the bending behavior at the root, and the effect of the lower pad structure on the flange-up formability is examined numerically as well as experimentally. It is found that the lower pad improves the formability, especially in the case of materials with poor ductility, and enables flange-up forming even for 980-MPa-grade ultrahigh-strength steels. Furthermore, it is clarified that an optimum lower pad stroke exists. The formability is improved as the lower pad stroke becomes large. However, too large a stroke causes localized large curvature at the root and excessive thinning at the edge.
Blanking vibration is a cause of noise pollution and also degrades the precision and shortens the life expectancy of press machines and dies. Recently, a servo press has been developed to alleviate these problems, wherein low speed blanking and pulse motion blanking are used. However, such methods lengthen the blanking time, necessitating the proper control of the servo press slide in the case of blanking. In this study, we propose an active vibration control method for the reduction of blanking vibration. We used a crank servo press and a pulse motion in an experiment. We conducted a simulation and an experiment of blanking using an active vibration control method, which we named “counter pulse blanking.” As a result of the simulation and the experiment, we confirmed that counter pulse blanking reduced blanking vibration by 40%.
Sheet metal parts with many slots are used for filters, and decreases in the pitch between adjacent slots are demanded. However, in punching narrow slots with a small pitch, the bridge portion between slots tends to be twisted, causing functional problems. In this paper, countermeasures for this defect are proposed and demonstrated using a stainless steel sheet of 0.25mm thickness. When the pitch was reduced to the same value as the sheet thickness, in the conventional punching process, the defect could not be prevented by increasing the blank holding force. At the same pitch, the punch with an inclined face was effective for reducing the defect if the proper angle and direction of inclination were selected. From the viewpoint of supporting the side face of the bridge portion, the method of using punched scraps as guide tools was tested. By this method, the defect of the bridge portion at a small pitch of less than the sheet thickness could be prevented although an additional stage was needed for removing the scraps.
The tensile properties of a Japanese-sword-type steel sheet, which has a sandwich-type layered structure consisting of low and medium carbon steels, were clarified. The hard and soft center sheets were prepared by stacking 0.05%C steel as the soft layer and 0.44%C steel as the hard layer with different combinations of thickness, and plane-strain compression at 1000 °C was conducted for bonding. The yield stress and the tensile strength of the Japanese-sword-type steel sheets were independent of the layer structure regardless of the position of the hard center sheet or the soft center sheet, whereas the strength changed with a strong dependence on the volume fraction of the hard layers. When the hard-layer volume fraction was larger than 50%, the measured strength was better than the estimation based on the rule of linear mixture. On the other hand, the area reduction at a fracture of the hard-center sheets was larger than that of the soft-center sheet at any volume fraction of hard layers.