Method of manufacturing automobile door hinge

A method of manufacturing an automobile door hinge through a process including hot forging and punching, the automobile door hinge being made of steel and including a strip-shaped mounting portion, a strip-shaped arm portion that is bent away from the mounting portion, and an engagement portion that is disposed at an end of the arm portion and that has a cylindrical shape, the engagement portion having a shaft hole into which a hinge pin is to be inserted, the method comprising:

• a hot forging step of hot forging a round steel bar to form a strip-shaped mounting portion, a strip-shaped arm portion that is bent away from the mounting portion, and a column portion that is disposed at an end of the arm portion and that has a horizontal cross section having a circular or elliptical shape;
• a shaft hole forming step of forming a shaft hole, into which a hinge pin is to be inserted, in the column portion formed in the hot forging step by using a first punch and a first die so that the shaft hole extends through an axial center of the column portion;
• and
• a shaft hole finishing step of punching the shaft hole formed in the shaft hole forming step by using a second punch and a second die from a side of the shaft hole on which a punching end point exists at which punching with the first punch has been finished,
• wherein, in the hot forging step, a round steel bar, which has been heated to 950° C. to 1350° C., is placed on a lower die, an upper die is dropped onto the round steel bar a plurality of times, and thereby a forged workpiece having a predetermined shape is formed with the dies,
• wherein the lower die has a concave portion for receiving a protruding surface on a protruding side of a bent portion of the forged workpiece to be formed in the hot forging step so that the bent portion of the forged workpiece protrudes downward,
• wherein the upper die has a convex portion whose shape matches that of an opposite surface of the bent portion of the forged workpiece opposite to the protruding surface,
• wherein a height of the shaft hole formed in the shaft hole forming step in the column portion is 2.0 times a diameter of the shaft hole or larger,
• wherein the first punch used in the shaft hole forming step includes a tip having a conical shape having a vertex angle in a range of 70° to 120°,
• wherein the first die has an inner side wall that is separated from an outer periphery of the column portion of the forged workpiece with a gap therebetween, and a gap volume that is provided between the outer periphery of the column portion and the inner side wall has such a size that, during punching of the column portion of the forged workpiece using the first punch, a slug is not generated and the column portion expands outward when the first punch is pressed into the column portion from a punching start point to a predetermined dimension, and a slug is generated and discharged when the first punch is pressed into the column portion from the predetermined dimension to the punching end point,
• wherein the second die has substantially the same shape as the first die, and
• wherein the second punch includes a tip having a frusto-conical or conical shape having a vertex angle in a range of 70° to 120°, and the second punch has a maximum diameter that is larger than a maximum diameter of the first punch by 0.1 to 0.3 mm.