Burring processing method, burring processing mold, burring processing device, and burring processed product

A burring processing method, which is a method for forming a burring processed portion including a raised portion and a curved portion in a metal component having a pilot hole formed therein, the metal component being a steel member having a thickness of 1.8 to 4.2 mm at a location the burring processing method is performed, using a burring processing mold including:

• a first die that includes a first die hole and a first support surface perpendicular to an axis of the first die hole, and a second die that includes a second die hole and a second support surface perpendicular to an axis of the second die hole;
• a holder that includes a third support surface facing the first support surface and the second support surface and holds the metal component between the first die and the second die;
• and
• a punch that includes a shaft portion and is movably provided along the axis of the first die hole and the axis of the second die hole,
• in which the first support surface, the second support surface, and the third support surface are disposed parallel to each other,
• a diameter of the second die hole is smaller than a diameter of the first die hole, and an outer diameter of the second support surface is smaller than the diameter of the first die hole,
• the method comprising:
• a preforming step of enlarging a diameter of the pilot hole, moving an edge portion of the pilot hole relative to the metal component in a first direction of a thickness direction of the metal component in a first range around the pilot hole of the metal component, and forming a whole first range into a preformed portion raised from the metal component in the first direction;
• and
• a main forming step of deforming the preformed portion in a second direction opposite to the first direction, forming a second range on an outer diameter side of the preformed portion to have a same height as the first range in the first direction, and forming part of a third range on an inner diameter side of the preformed portion from the second range to be part of the curved portion and the raised portion,
• wherein an outer diameter of the curved portion is smaller than an outer diameter of the preformed portion,
• a maximum radius of curvature of the curved portion is smaller than the minimum radius of curvature of the preformed portion in a cross-sectional view parallel to the first direction and passing through a center of the pilot hole,
• the preformed portion is formed between the punch and the first die by holding the metal component between the first support surface of the first die and the third support surface of the holder and moving the punch relative to the first die in the first direction to insert the punch through the first die hole,
• the burring processed portion is formed between the second die, the punch, and the holder by moving the second die relative to the holder in the second direction to insert part of the second die between the punch and the first die in a state in which the metal component is held between the first support surface and the third support surface,
• when a difference between a radius of the first die hole and a radius of the second die hole is defined as U, a diameter of the shaft portion of the punch is defined as P, and the diameter of the pilot hole of the metal component is defined as A, Expression 1 below is satisfied, and
• when a height of the edge portion of the pilot hole of the metal component is defined as t, and a height of an outer surface of the curved portion in the first direction is defined as h, Expression 2 below is satisfied,
• 0.5×(P−A)/2<U<20×(P−A)/2 Expression 1
• 0.2<h/t<0.6 Expression 2.