Hole widening method, forming tool, and formed product

A hole widening and a stretched flange forming method comprising:

• providing a forming tool which has a diameter-increasing portion increasing in diameter from a front end side toward a rear end side and a line-shaped projection formed to protrude outward from a surface of the diameter-increasing portion and having a spiral shape and continuously extending at least one round in a circumferential direction of the forming tool in a case of being seen from the front end side, and a workpiece in which a pilot hole is formed;
• and
• successively widening the pilot hole by pushing the forming tool into the pilot hole such that the line-shaped projection of the forming tool comes into point contact with a part of a circumferential edge portion of the pilot hole in the workpiece two times or more, and forming a stretched flange,
• wherein a number of a thread of the line-shaped projection is 2 to 7, and
• wherein a pitch of the line-shaped projection is 5.0 to 30.0 mm.