Patent ID: 8365411

Claim:
A method of producing a metal closed-section member formed in a cross-sectionally closed shape, from a metal plate workpiece, comprising: a first press-forming step of providing a first forming die assembly, and, under a condition that the plate workpiece is clamped between a die and a blank holder of the first forming die assembly, causing relative movement of a punch of the first forming die assembly with respect to the die to allow the punch to be moved into a forming space of the die, so that the plate workpiece is press-formed into a shape convexly protruding in a direction of the movement of the punch relative to the die, and a convex wall portion is formed in the top wall of the press-formed plate workpiece to protrude in a direction opposite to the convexly protruding direction of the press-formed plate workpiece; and a second press-forming step of providing a second forming die assembly, and, after the first press-forming step, under a condition that the press-formed plate workpiece is positioned between a receiving die and a core die of the second forming die assembly, wherein the core die is disposed on the side opposite to the convexly protruding direction of the press-formed plate workpiece, and that the press-formed plate workpiece is held on the receiving die of the second forming die assembly while allowing the convex wall portion of the press-formed plate workpiece to be placed on a concave portion of the receiving die, causing relative movement of the core die with respect to the receiving die to allow the convex wall portion of the press-formed plate workpiece to be flattened and formed as a press-formed wall, by an interaction between the core die of the second forming die assembly and the concave portion, so that two sidewalls of a convex-shaped portion of the press-formed plate workpiece formed on respective ones of both sides of the top wall during the first press-forming step are displaced toward an inward side of the press-formed plate workpiece and cross-sectionally closed together, and wherein a cross-sectional length of at least one of the sidewalls changes in a longitudinal direction of the closed-section member, and wherein the first press-forming step includes forming the convex wall portion to divide the top wall into two top wall portions along a longitudinal direction of the press-formed plate workpiece, in such a manner that a cross-sectional length of at least one of the top wall portions of the top wall divided by the convex wall portion changes in the longitudinal direction; and the receiving die of the second forming die assembly for use in the second press-forming step is configured such that a cross-sectional length of a side surface of the concave portion of the receiving die is approximately equal to a shortest one of a plurality of cross-sectional lengths of the at least one top wall portion of the top wall.