Arm component manufacturing method and the arm parts for vehicles for vehicles

A while having a predetermined following torsional stiffness, to provide a manufacturing method of the arm part for a vehicle having a sufficient tensile strength over the entire circumferential direction. For An X-direction and the workpiece W is a flat plate that extends in the XY plane Y directions forms, two sides W1, W2 of the workpiece in the XZ plane formed by the X and Z directions is a gap G as opposed through a manufacturing method for a vehicle arm component for the manufacture of a vehicle arm part 1 having a stepwise pressing to be manufactured, a hollow shape open cross section workpiece, list inner to like die 110 projections 113 which extend in the X direction, is arranged in the gap between the opposing two sides, the two sides from the outer circumference of the workpiece while in contact with the protrusion having a restriking step of pressure molding the circumference. .The 12