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

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

• a pre-hemming step of bending a hemming prong protruding from a top end of one of the flange portions towards another flange portion;
• a closing step of, after the pre-hemming step is completed, urging a pair of insert guide surfaces formed on the press forming die against a top portion of the one of the flange portions having the hemming prong therein, moving the press forming die in a predetermined pressing direction so that the two flange portions are brought closer to each other due to a force component perpendicular to the pressing direction generated by each of the insert guide surfaces, and guiding the pair of the flange portions into a slit clearance formed between the pair of insert guide surfaces of the press forming die;
• and a hemming press step of, after the closing step is completed, further moving the press forming die in the pressing direction, inserting the pair of flange portions into the slit clearance, simultaneously bending the hemming prong with a pressing force transferred from an inner surface portion of the slit clearance to a top portion of the hemming prong so that an uppermost portion of the another flange portion is sandwiched between the hemming prong and the one of the flange portions, and the one of the flange portions is fixed to the another flange portion, and, simultaneously, pressing the metal plate using a press forming surface formed outside of each of the insert guide surfaces of the press forming die and press-forming outer portions of the pair of the flange portions of the metal plate into predetermined shapes.