We developed a new press forming method by which steel sheets of uniform thickness are press-formed into a hat cross-sectional shape with differential thickness, without using semifinished sheet products such as tailored blanks. In the proposed method, the sheet thickness can be continuously reduced by tensile-bending deformation during draw bending; therefore, the wall thickness can be reduced beyond the limitation of sheet thickness in tensile deformation. The effect of the forming die-set shapes on the inflow resistance of the material and the thinning behavior were investigated using forming CAE. It was found that the thickness of steel sheets with a tensile strength of 590 MPa can be reduced by up to 41% with one-stroke forming. In addition, we demonstrated experimentally that the sheet thickness can be reduced by 37% by adapting this proposed method. Furthermore, it was clarified that this forming method can reduce springback deformation.

In the present paper, we describe a tube drawing method with diameter expansion for the effectively formation of thin-walled tubes. The proposed method makes the tube thickness easier to reduce than the conventional method, because the tube wall stretches biaxially in the axial and hoop directions. The effects of the plug half-angle on the forming limit, drawing load, thickness reduction, the precision of the inner diameter, and the strength of the formed tube were investigated by a series of experiments and finite element method (FEM) analysis. As a result, the forming limit was the highest at a plug half-angle α of 36˚, and then the maximum thickness reduction ratio γ _max of 32 % was obtained by increasing the expansion ratio. In addition, the thickness reduction ratio increased with the plug half-angle owing to the increase in drawing load. On the other hand, the dimensional precision of the tube inner diameter deteriorated with the increase in plug half-angle because of an overshoot of the tube wall, indicating a gap between the tube wall and the plug. The effect of the plug half-angle on the strength was small, while elongation changed with the plug half-angle. The above results suggest that the plug half-angle should be selected appropriately to obtain the target dimension and mechanical properties.

The surface finishing process has been evaluated in terms of its comprehensive performance as well as it has also been evaluated by its productivity and environmental effect. Burnishing is a surface finishing process for metal products after machining. This process utilizes the plastic deformation of the workpiece surface and yields a smooth surface. In addition, residual stress occurs in the processed workpiece surface. Therefore, the fatigue strength and abrasion resistance of the workpiece are improved. However, friction heating is a severe problem in maintaining the workability of a diamond tip burnishing tool. To estimate the temperature rise caused by burnishing, a new burnishing tool made of boron-doped chemical vapor deposition (CVD) diamond was developed. Its temperature and other characteristics were determined by the tool-workpiece thermocouple method. The thermoelectromotive force was measured and calibrated to temperature using the obtained calibration curve. The tool temperature was found to increase with the rotation speed. The tool temperature obtained under experimental conditions using the highest rotation speed was about 950-960 ℃. Moreover, black scratches as seen in tool wear were observed.