Closed structure parts, method and press forming apparatus for manufacturing the same

A method of manufacturing a closed structure part having a closed section using a blank made from a metal plate by pressing the blank using a press forming die and fixing a pair of joint ends of the blank to each other, the method comprising:

• a pre-forming step of processing the blank into an interim part having a cross section corresponding to the closed structure part;
• a closing step of, after the pre-forming step is completed, moving the joint ends of the interim part toward each other and urging, against one of the joint ends, a concave flange processing portion formed in an upper end portion of one of a pair of press forming surfaces of the press forming die wherein each of the press forming surfaces is connected to each other at an edge end adjacent to the other press forming surface and the press forming surfaces extend downward from the connected edge ends;
• wherein the concave flange processing portion linearly extends in the depth direction of the press forming die along a portion of the one of a pair of press forming surfaces and a bottom surface of the concave flange processing portion is substantially parallel to and adjacent to the one of a pair of press forming surfaces extending from the lower edge of the flange processing portion;
• and
• a press joining step of, after the closing step is completed, further moving the press forming die in a pressing direction, bending the one of the joint ends using a pressing force applied from the flange processing portion so that the joint end overlaps an outer surface of the other joint end and a latch flange portion that latches the other joint end is formed and, simultaneously, pressing the blank using the connected pair of press forming surfaces and press-forming outer portions of the pair of the joint ends of the blank into final predetermined shapes with overlapping joint ends.