Press-forming method

1. A press-forming method for a plate-shaped workpiece that uses a second-stage forming punch, a first-stage forming punch disposed along an outer periphery of the second-stage forming punch, an axial pressing punch disposed along an outer periphery of the first-stage forming punch, and a die including: from a leading end side where the first-stage and second-stage forming punches are inserted, a first die shoulder portion that expands outward as it goes to the leading end; a first die hole portion parallel to a central axis; a second die shoulder portion that faces the first-stage forming punch and the axial pressing punch in a central axis direction and expands outward as it goes to the leading end; a second die hole portion connected to the second die shoulder portion, central axes of all of which are disposed coaxially; and a counter punch disposed to face the second-stage forming punch in the central axis direction, wherein the second die hole portion is disposed along an outer periphery of the counter punch, the method comprising:

a step S 1 of moving the first-stage and second-stage forming punches relatively to the die and inserting the first-stage and second-stage forming punches into the first die hole portion, to thereby push the workpiece into the first die hole portion through the first die shoulder portion to form a first cup-shaped workpiece having a cup bottom portion and a circumferential cup sidewall portion rising from a circumferential edge portion of the cup bottom portion;

a step S 2 of moving the axial pressing punch relatively to the die to abut on a cup opening end of the cup sidewall portion, and while constraining the cup opening end, moving the second-stage forming punch relatively to the die to insert the second-stage forming punch into the second die hole portion and pushing a part of the first cup-shaped workpiece into the second die hole portion through the second die shoulder portion, to thereby reduce the cup sidewall portion in diameter and thicken the cup sidewall portion; wherein

the step S 2 includes a step S 21 of, while constraining the cup opening end by the axial pressing punch and while constraining deformation of the cup sidewall portion on the cup bottom portion side in a sheet thickness direction by the second-stage forming punch and the second die hole portion, bringing the counter punch close to the second-stage forming punch to compress the cup bottom portion in the second die hole portion by the counter punch and the second-stage forming punch, to thereby form a cup shoulder portion connecting the cup bottom portion and the cup sidewall portion into an orthogonal shape and cause a material of the cup bottom portion to flow into the cup shoulder portion to thicken the cup shoulder portion; and

a step S 3 of, in a state of the constraint of the cup opening end by the axial pressing punch being released, moving the second-stage forming punch relatively to the die and further pushing the first cup-shaped workpiece deeper in the second die hole portion, to thereby form a second cup-shaped workpiece smaller in diameter than the first cup-shaped workpiece.

2. The press-forming method according to claim 1 , wherein

in the step S 3 , the first cup-shaped workpiece is pushed into the second die hole portion until the cup opening end is positioned on the second die shoulder portion, to thereby form the second cup-shaped workpiece provided with an expanded opening portion that expands outward as it goes to the cup opening end side.

3. A press-forming method for a plate-shaped workpiece that uses a second-stage forming punch, a first-stage forming punch disposed along an outer periphery of the second-stage forming punch, an axial pressing punch disposed along an outer periphery of the first-stage forming punch, a counter punch disposed to face the second-stage forming punch in a central axis direction, and a die including, from a leading end side where the first-stage and second-stage forming punches are inserted, a first die shoulder portion that expands outward as it goes to the leading end; a first die hole portion parallel to a central axis; a second die shoulder portion that faces the first-stage forming punch and the axial pressing punch in the central axis direction and expands outward as it goes to the leading end; and a second die hole portion disposed along an outer periphery of the counter punch, central axes of all of which are disposed coaxially, the method comprising:

a step SA 1 of moving the first-stage and second-stage forming punches relatively to the die and inserting the first-stage and second-stage forming punches into the first die hole portion, to thereby push the workpiece into the first die hole portion through the first die shoulder portion to form a first cup-shaped workpiece having a cup bottom portion and a circumferential cup sidewall portion rising from a circumferential edge portion of the cup bottom portion;

a step SA 2 of bringing the axial pressing punch to abut on a cup opening end of the cup sidewall portion, and while constraining the cup opening end, moving the axial pressing punch relatively to the die and pushing a part of the first cup-shaped workpiece into the second die hole portion through the second die shoulder portion, to thereby reduce the cup sidewall portion in diameter and thicken the cup sidewall portion;

a step SA 3 of, while constraining the cup opening end by the axial pressing punch, bringing the second-stage forming punch and the counter punch close to each other, compressing the cup bottom portion in the second die hole portion by the second-stage forming punch and the counter punch to form the flat cup bottom portion, and causing a material of the cup bottom portion to flow into a cup shoulder portion connecting the cup bottom portion and the cup sidewall portion to thicken the cup shoulder portion; and

a step SA 4 of moving the second-stage forming punch relatively to the die and further pushing the first cup-shaped workpiece deeper in the second die hole portion, to thereby form a second cup-shaped workpiece smaller in diameter than the first cup-shaped workpiece.

4. The press-forming method according to claim 3 , wherein

in the step SA 4 , the second cup-shaped workpiece is pushed into the second die hole portion until the cup opening end is positioned in the second die hole portion, the method further comprising:

a step SA 5 of while compressing the cup bottom portion by the second-stage forming punch and the counter punch and while constraining deformation of the cup sidewall portion on the cup bottom portion side in a sheet thickness direction by the second-stage forming punch and the second die hole portion, moving the second-stage forming punch and the counter punch relatively in a direction to come close to the first-stage forming punch to cause the cup opening end to butt against the first-stage forming punch, to thereby form the cup shoulder portion into an orthogonal shape.

5. The press-forming method according to claim 4 , wherein

in the step SA 4 , the first cup-shaped workpiece is pushed into the second die hole portion until the cup opening end is positioned on the second die shoulder portion, to thereby form the second cup-shaped workpiece provided with an expanded opening portion that expands outward as it goes to the cup opening end side.

6. The press-forming method according to claim 3 , wherein

in the step SA 4 , the first cup-shaped workpiece is pushed into the second die hole portion until the cup opening end is positioned on the second die shoulder portion, to thereby form the second cup-shaped workpiece provided with an expanded opening portion that expands outward as it goes to the cup opening end side.