Die, method of manufacturing stepped metal pipe or tube, and stepped metal pipe or tube

A method of manufacturing a stepped metal pipe or tube, comprising:

• pushing a metal pipe or tube into a die in an axial direction, said die having a through hole for use in an extrusion process to reduce the diameter of a metal pipe or tube,
• said through hole having an inside surface including a bell portion, an approach portion, and a bearing portion from the entrance side formed in a continuous manner, wherein
• a diameter of said bell portion gradually decreases from the entrance part of said bell portion to the exit part of said bell portion of the hole,
• a diameter of said approach portion is D1 on the entrance part of said approach portion and D2 on the exit part of said approach portion, the diameter of said approach portion gradually decreasing from the entrance part of said approach portion to the exit part of said approach portion to satisfy Equation (1):
• 0.7≦D2/D1<0.97 (1),
• a die half angle of a first part of said approach portion having a diameter D3 is not less than a die half angle of a second part of said approach portion nearer to the exit part of said approach portion than the first part, the diameter D3 being D2/0.97,
• the axial length LR between the first part and the exit part of said approaching portion satisfies Equation (2):
• 20≦LR/((D3−D2)/2)≦115 (2),
• a diameter of said bearing portion is fixed at D2, and a length of said bearing portion is LB, the length LB satisfying Equation (3):
• 0.3≦LB/D2≦10 (3),
• said method comprising,
• extruding an end of said pushed metal pipe or tube to protrude a prescribed length from the exit side of said die, thereby making the metal pipe or tube into a stepped metal pipe or tube;
• and
• stopping extruding and pushing back the stepped metal pipe or tube in the direction opposite to the direction of pushing the metal pipe or tube.