The stretch flangeability of tailor-welded blank depends on the angle between weld line and the tangential line of blank edge, which is generally the tensile direction in stretch flanging. This effect was studied by uniaxial tensile testing of tailor-welded high-strength-steel specimens with a weld line at 45, 60, or 90 degrees from the tensile direction. The 60-degree-weld-line specimen provides the highest elongation at the maximum load. This is the angle at which the strain component in the weld-line direction is close to zero. At the other angles, the weld lines, of which hardness sufficiently increases by welding, play a role as a constraint for vicinal base metal. This leads to early strain localization and low elongation. Furthermore, the elongation at the maximum load can, in some cases, exceed the uniform elongation of base metal. This is pronounced by a base metal with a low n-value.

To decrease car body weight and improve crash safety, tailor-welded blanks are often used for the car body materials. In the past, little research has been conducted on the formability of the weld metal (WM). This study focuses on the bendability of the WM and evaluates it by the bending test. The main results are as follows. (1) The bendability of the laser-welded specimens was better than that of the plasma-welded specimens. (2) The bendability of the weld line (WL) was related to the homogeneity of the WM structure, and it increased with increasing homogeneity. (3) The surface roughness of the WL has no effect on the bendability of the WL. (4) The deformation of the bent surface of the WL changed with the WL width, and the bendability increased as the WL narrowed because the bent surface showed increasing uniaxial stretching deformation.

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We developed a hot stamping method for panel parts with a step-shaped wall. The die-set for the developed method divides the punch at a step-shaped wall into an outer punch and an inner punch. The outer punch is placed first. In addition, an opening is added to the blank of the developed method. The developed method makes it possible to change the main forming type from draw forming to cylindrical stretch flange forming in the middle of a forming stroke. The stretch flange forming can prevent a fall in temperature at a hole edge, because the hole edge does not come into contact with the die-set during forming and because the high ductility of high-temperature materials can be used, making it advantageous for hot stamping. If the preceding amount of the outer punch is large with the developed method, the effect of suppressing wrinkles is large but the sheet thickness reduction that can cause cracking rises. If the preceding amount of the outer punch is small, the rate of sheet thickness reduction ratio is greatly restrained, but the wrinkles become larger. By setting an appropriate preceding amount of the outer punch with the developed method, forming without cracks or wrinkles is possible.