Pipe, die, and method for manufacturing pipe

A metal pipe including a pipe axis line and a curved portion produced by bending, wherein

• an outer circumferential surface of the curved portion comprises an inside portion closer to a center of a curvature of the curved portion than the pipe axis line,
• a plurality of non-bending surfaces on the inside portion;
• and
• a plurality of bent portions on the inside portion, at which the non-bending surfaces are connected in a bending manner, and wherein
• the pipe axis line of the curved portion bends, and
• a plane on the pipe axis line and the bent portions intersects each of the non-bending surfaces to form a line of intersection, and each line of intersection between the plane and each of the non-bending surfaces is interconnected to an adjacent line of intersection by a straight line,
• wherein the bent portions include a pair of sub-bent portions provided at both ends thereof in a direction along the pipe axis line and a plurality of main bent portions provided between the pair of sub-bent portions, and
• at the positions where the intersections between the plane and the main bent portions are defined, straight lines interconnecting the adjacent intersections serve as a part of a regular polygon having the intersections as apices thereof and having the center of the curvature side as a center thereof;
• wherein a number of corners of the regular polygon is six or larger;
• and
• further comprising a straight portion continuously connected to the curved portion, wherein the number of corners of the regular polygon is a natural number equal to or smaller than a value (x) derived by Expression (1) below, where (d) denotes an outside diameter of the straight portion, (r) denotes a radius of curvature of the curved portion, and (t) denotes a wall thickness of the straight portion:
• x=180/sin−1(t/(2r−d)). [Expression (1)]