Method of forming a closed cross-sectional structure

A method of forming a closed cross-sectional structure by bending a plate-shaped workpiece at positions of a plurality of bend lines extending in a longitudinal direction, the structure including a bottom portion formed in a central part of the workpiece in a width direction and left and right side wall portions located on both sides of the bottom portion in the width direction, comprising:

• a first step of:
• press-forming the plate-shaped workpiece into a shape including portions corresponding to the bottom portion and the left and right side wall portions such that the plurality of bend lines are formed at boundaries therebetween;
• and
• providing bend-facilitating lines at the plurality of bend lines;
• a second step of bending the workpiece, which has been formed in the first step, in a direction that the portions corresponding to the left and right side wall portions approach each other;
• and
• a third step of bending the portions corresponding to the bottom portion and the left and right side wall portions along the bend-facilitating lines by pressing the portions corresponding to the bottom portion and the left and right side wall portions against an outer periphery of a plug having an outer peripheral shape the same as a final shape of the closed cross-sectional structure while the plug is placed only on end portions in the longitudinal direction of the portion of the workpiece corresponding to the bottom portion, which has been formed in the second step.

An apparatus that forms a closed cross-sectional structure by bending a plate-shaped workpiece at positions of a plurality of bend lines extending in a longitudinal direction, the structure including a bottom portion formed in a central part of the workpiece in a width direction and left and right side wall portions located on both sides of the bottom portion in the width direction, comprising:

• a pressing die including an upper die and a lower die that press-forms the plate-shaped workpiece into a shape including portions corresponding to the bottom portion and the left and right side wall portions such that the plurality of bend lines are formed at boundaries therebetween and to provide bend-facilitating lines at the plurality of bend lines;
• a bending die that bends the workpiece, which has been formed using the pressing die, in a direction that the portions corresponding to the left and right side wall portions approach each other by pressing a punch into a space between a pair of dies while clamping the portion corresponding to the bottom portion between the punch and a pad in a plate thickness direction;
• and
• a final-closed-cross-section bending die including a plug, a pair of pressure cams and support pad, the plug having an outer peripheral shape the same as a final shape of the closed cross-sectional structure and disposed on the portion of the workpiece corresponding to the bottom portion, which has been formed using the bending die, the support pad supporting the portion of the workpiece corresponding to the bottom portion, the pair of pressure cams being disposed outside of the plug in the width direction, the final-closed-cross-section die bending the portions corresponding to the bottom portion and the left and right side wall portions along the bend-facilitating lines by pressing the portions corresponding to the bottom portion and the left and right side wall portions against an outer periphery of the plug using the support pad and the pair of pressure cams.