Coupling structure, coupling member having coupling structure, and method for manufacturing coupling member having coupling structure

A method for manufacturing a coupling member including a hollow rod section and a fastening section integrally formed with the hollow rod section, comprising:

• a slit-forming process of forming a pair of wall sections by forming two slits extending in an axial direction of a metal pipe at an end section of the metal pipe;
• a first plastic deformation process of plastically deforming the pair of the wall sections using a press tool that relatively moves in the axial direction of the steel pipe so that the slits are separated from each other from a base end side to a front end side;
• and
• a second plastic deformation process of plastically deforming the pair of the wall sections using the press tool so that each of the pair of the wall sections which have been plastically deformed through the first plastic deformation process is formed with (I) a pair of base end sections which are each connected with a peripheral wall section forming a closed cross-section at the end section of the hollow rod section, and are separated from each other, and (II) a pair of front end sections each including (II-i) a bottom wall including a flat surface connected to the pair of the base end sections, and (II-ii) a pair of side walls curving from at least widthwise end sections of the bottom wall on a base end section side of one of the pair of front end sections, toward the other of the pair of front end sections.