JP2000005840A - Forging piston and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a light weighting of a piston by making a pin boss part thickness small without cutting a fiber flow, wherein an outer face of the piston bottom end part is made so as to be along with a cylindrical shape of the cylinder inner face as a whole or longer in its circular direction so that a rigid ity at the piston bottom part is improved, and reduces a side pressure from the cylinder inner face, which interacts with a skirt part, and also a pin boss part undercut shape is formed by forging. SOLUTION: This piston 1 makes a metal block material 10 into a product by applying a finish machinery processing after the block material 10 is forged by a forging die to a forged molding material 11, which being equipped with a head part 2, a pin boss part 3 and a skirt part 4. In this case, at a stage where it is in the forged molding material 11, an outside face of the pin boss part 3 is made inwardly concaved, leaving its bottom part being untouched.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piston used for an internal combustion engine, a compressor, and the like, and a method of manufacturing the piston. More particularly, the invention relates to a molding material formed by forging a metal lump material with a forging die. The present invention relates to a forged piston that is commercialized by performing finishing machining, and a method for manufacturing such a forged piston.

[0002]

2. Description of the Related Art In a lightweight aluminum alloy piston used for an internal combustion engine such as a vehicle engine, if the piston is formed by casting, the solidified structure of the material becomes coarse and disadvantageous in strength. By using a metal block of alloy as a piston material and forging it with a forging die having a mold part that moves forward and backward in one direction, a substantially piston-shaped molding material with a head part, a pin boss part, and a skirt part After that, the forged molding material is commercialized by finishing machining such as drilling a pin hole for inserting a piston pin and cutting a ring groove for mounting a piston ring. Such forged pistons have been conventionally developed.

[0003]

By the way, in the forged piston for an internal combustion engine as described above, in order to improve the rigidity of the lower end of the piston and to reduce the side pressure acting on the skirt from the inner surface of the cylinder, the lower end of the piston is required. If you try to make the outer surface of the pin boss longer or longer along the entire circumference or in the circumferential direction, the entire or a part of the pin boss becomes a cylindrical surface whose outer surface is continuous with the outer surface of the skirt. Then, the pin boss portion becomes thicker in the axial direction of the piston as a whole, and the weight of the piston becomes excessive. The inertia force of the motor increases, and the engine vibration increases.

[0004] Therefore, after forming by forging, each pin boss portion is machined away from the outside by a predetermined amount except for the lower end portion, so that the outer surface of the lower end portion of the piston is extended longer in the entire circumference or circumferential direction. It has been considered that the thickness of the pin boss portion is reduced while keeping the shape along the cylindrical shape of the cylinder to reduce the weight of the entire piston.However, the fiber flow formed during forging causes the fiber flow of the pin boss portion to decrease. Cutting is performed at the cutting portion (undercut portion), and as a result, there is a possibility that the pin boss portion may be damaged when the operation of the engine is performed for a long time.

[0005] That is, during the operation of the engine, an explosive force acts on the piston or an inertial force acts on the connecting rod or the crankshaft, so that a bending force acts on the piston pin and the pin bosses of both pin bosses are actuated. The force that expands and contracts the distance between the distal ends acts on each pin boss.However, if the fiber flow is cut at the pin boss, the force that expands and contracts the distance between the distal ends of both pin bosses by prolonged operation of the engine By working repeatedly, cracks are formed in the cut portion of the fiber flow, and the pin boss portion is damaged.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems. Specifically, in a forged piston, the outer surface of the lower end portion of the piston is made longer in the entire circumference or circumferential direction. By following the shape, it is possible to improve the rigidity of the lower end of the piston and reduce the side pressure acting on the skirt from the inner surface of the cylinder, and to form the undercut shape of the pin boss by forging It is an object of the present invention to reduce the thickness of the pin boss portion and reduce the weight of the piston without cutting the fiber flow formed at the time of forging.

[0007]

In order to solve the above-mentioned problems, the present invention comprises a head, a pin boss, and a skirt by forging a metal lump material with a forging die. In a forged piston that is made into a product by applying finishing machining to a forged molding material, the outer surface of the pin boss is concave inward at the stage of the forged molding material, leaving its lower end at the lower end. It is characterized by having been done.

Further, in the method for manufacturing such a forged piston, a movable die capable of opening and closing a portion corresponding to an outer surface of a pin boss portion of a forging die in a direction intersecting with a pressing direction at the time of forging. Thus, the forged molding material forged such that the outer surface of the pin boss portion is depressed inward leaving the lower end portion thereof can be taken out from the forging die.

According to the forged piston as described above, a forged molding material is used for a piston body in which the outer surface of the lower end portion of the piston is longer along the entire inner circumference or circumferential direction of the inner surface of the cylinder. At this stage, the undercut portion of the pin boss portion is formed by forging, so that the weight of the piston can be reduced without forming the undercut portion of the pin boss portion by cutting after forging.

Further, according to the above-described method for manufacturing a forged piston, a piston main body having an outer surface at the lower end portion of the piston which is formed along the entire inner circumference or circumferential direction of the cylinder inner surface is used as a mold. In practice, it is possible to form an undercut portion of the pin boss portion by forging according to (1).

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a forged piston of the present invention and a method for manufacturing the same will be described with reference to the drawings.

FIGS. 1 and 2 schematically show the structure of a piston for an internal combustion engine according to one embodiment of the forged piston of the present invention, and FIG. 1 (A) shows an appearance viewed from a side. , FIG. 1B shows an external view seen from below, and FIG.
(A) shows a cross section along the line AA in FIG. 1 (B),
FIG. 2B shows a state of a fiber flow in a cross section taken along line BB of FIG. 1B.

The piston body 1 has a head portion 2 whose upper surface is exposed to the combustion chamber, a pin boss portion 3 for supporting a piston pin, and a skirt portion 4 whose side surface is in sliding contact with the inner surface of the cylinder.
A ring groove 5 for fitting a piston ring is formed on the outer peripheral surface of the head portion 2, and a pin hole 6 for inserting a piston pin is formed in each of a pair of pin boss portions 3 opposed to each other.
Are formed.

In such a piston body 1, each of the pin boss portions 3 has a pin hole 6 formed by cutting after forging.
The outer peripheral surface of the pin boss portion 3 leaves the lower end portion of the outer peripheral surface of the piston (virtual cylindrical surface including the outer peripheral surface of the skirt portion 2).
It is formed integrally with the head portion 2 and the skirt portion 4 by forging so as to be depressed inward, that is, so that the undercut portion 7 is formed on the outer surface side of the pin boss portion 3.

In the piston body 1 in which the pin boss portion 3 is made thin by the undercut portion 7 formed by such forging, the ring groove 5 and the pin hole 6 formed by cutting after forging are removed. Except for Fig. 2 (B)
As shown in FIG. 5, the fiber flow formed at the time of forging is left uncut by the undercut portion 7.

According to the forged piston for an internal combustion engine having the above-described structure, the outer peripheral surface of the lower end portion of the piston main body 1 has a partially short cylindrical surface along the cylindrical shape of the inner surface of the cylinder. Therefore, the rigidity of the lower end of the piston can be ensured to be high, the distortion at the undercut portion 7 can be reduced, and the pin boss portion 3 can be prevented from being damaged.
Further, the side pressure acting on the skirt portion 4 from the inner surface of the cylinder can be reduced.

In a state where the fiber flow formed at the time of forging is not cut except for the ring groove 5 and the pin hole 6, an undercut portion 7 is formed outside the pin boss portion 3 so that the piston body 1 Reduced weight reduces vibration caused by the inertia of the piston during engine operation, and avoids damage to the pin boss due to fiber flow cutting even when the engine is operating for a long time. Can be.

FIG. 3 schematically shows a manufacturing process of the piston body 1 as described above. First, a piston material (buret) 10 made of a thick disk-shaped aluminum alloy is used.
On the other hand, a mold release agent is applied to the outer circumference, then heated to improve moldability, and the heated material is sandwiched between a pair of upper and lower mold parts and forged so as to strongly press the head. A substantially piston-shaped molding material 11 having the portion 2, the pin boss portion 3, and the skirt portion 4 is obtained.

After the forged molding material 11 is subjected to a heat treatment to increase the strength, a ring groove 5 for fitting a piston ring and a pin hole 6 for inserting a piston pin are formed. After performing a machining process such as scraping off a suitable portion, and then, if necessary, performing a surface treatment such as plating on the side surface of the skirt portion 4 in order to improve the sliding characteristics and abrasion resistance. Thus, the piston body 1 is finished.

In the manufacturing process of the piston body 1 as described above, at the stage of the forged molding material 11, the outer surfaces of the pin boss portions 3 have already been left on the pair of pin boss portions 3, respectively, except for the lower ends thereof. An undercut portion 7 is formed so as to be depressed inward from the outer peripheral surface of the cylinder of the piston. In the machining after forging, although a pin hole 6 is formed in the pin boss portion 3, the pin boss portion 3 is formed. There is no attempt to reduce the thickness and weight by shaving off the undercut portion.

By the way, the piston blank 10 shown in FIG. 3 is made of, for example, aluminum (Al) as a base material, and silicon (Si) of 10 to 25% by weight and iron (Fe) of 1 in the whole. Weight% or less, copper (Cu) 0.5 ~
7% by weight, 0.1 to 2% by weight of magnesium (Mg),
Manganese (Mn) 1.5% by weight or less, nickel (N
An aluminum alloy containing 1.5% by weight or less of i) and 1.5% by weight or less of chromium (Cr) cut from an ingot continuously cast into a cylindrical rod-shaped body is used. .

Further, an aluminum alloy having the above composition is drawn out from the bottom of the melting furnace as a continuous cast having a columnar shape, and a stirrer comprising an electromagnet or an ultrasonic oscillator is provided on the outer periphery of a portion where solidification starts from the melting furnace. Is arranged and solidified while stirring and mixing the central portion and the outer peripheral portion of the cylindrical continuous cast body, thereby suppressing the growth of precipitated crystal particles to reduce the particle size, and from the outer peripheral portion to the central portion. A solid cylindrical aluminum alloy rod-like body in which crystal grains are uniformly dispersed may be cut into an appropriate size and used as the piston material 10.

According to the piston material 10 in which the central portion and the outer peripheral portion of the columnar continuous cast body are solidified while stirring and mixing, the crystal grains are small and uniformly dispersed. Since cracks are less likely to occur during forging, the yield during forging can be improved, and the fatigue strength of the skirt portion 4 when the piston body 1 is operated with the piston body 1 incorporated in the engine can be increased.

Further, as the piston material 10, an aluminum alloy obtained by solidifying a rapidly solidified powder, which contains silicon (Si) having an average primary crystal particle diameter of 10 μm or less in a range of 10 to 22% by weight, is used. You may.

As such a rapidly solidified powdered aluminum alloy, for example, aluminum (Al) is used as a base material, silicon (Si) is 10 to 22% by weight, and iron (Fe) is contained in the whole.
1 to 10% by weight, copper (Cu) 0.5 to 5% by weight, magnesium (Mg) 0.5 to 5% by weight, manganese (M
n) is 1% by weight or less, nickel (Ni) is 1% by weight or less, chromium (Cr) is 1% by weight or less, zirconium (Z
r) in a range of 2% by weight or less and molybdenum (Mo) in a range of 1% by weight or less.

Among the components contained in such a rapidly solidified powdered aluminum alloy, silicon (Si) has abrasion resistance and seizure resistance by crystallizing hard primary crystals and eutectic silicon grains in the metal structure. Added to enhance iron (Fe)
Is added to increase the strength at 200 ° C. or higher by dispersing and strengthening the metal structure, and copper (Cu) and magnesium (Mg) are added to increase the strength at 200 ° C. or lower. So, about their addition amount,
Outside the above range, the desired wear resistance and seizure resistance and the required strength at high temperatures cannot be obtained.

In the piston material 10 made of the rapidly solidified powdered aluminum alloy as described above, the melted aluminum alloy is sprayed in the form of a mist and rapidly solidified to be powderized and then solidified. About 10 in particle size
0 μm, and the silicon (S
i) shows that the hard primary crystal silicon crystallized in the metal structure of the aluminum alloy which solidifies while being powdered has an average particle size of 10 μm.
It is finely divided as follows, and is dispersed for each aluminum alloy particle.

Since silicon (Si) is finely dispersed as described above, when the piston material 10 is forged into a substantially piston-shaped forming material 11, the material is particularly thinly drawn at the skirt portion 4. Even if it is forged so as to be extended, the primary crystal silicon particles do not crack at that portion and cracks do not occur. As a result, the piston body 1 formed by forging has a fatigue strength at the skirt portion 4. Will be higher.

The rapidly solidified powdered aluminum alloy used as the piston raw material 10 is not limited to the above-mentioned ones.
For example, in order to further increase the wear resistance, silicon carbide (SiC), which is a component harder than silicon (Si),
Is also used.

An example of a rapidly solidified powdered aluminum alloy containing silicon carbide (SiC) is aluminum (A).
l) as a base material, 10 to 22% by weight of silicon (Si), 1 to 10% by weight of iron (Fe), and 0.5% of copper (Cu).
-5% by weight, 0.5-5% by weight of magnesium (Mg), 1% by weight or less of manganese (Mn), nickel (N
i) not more than 1% by weight, chromium (Cr) not more than 1% by weight,
2% by weight or less of zirconium (Zr), molybdenum (M
In some cases, o) is contained in a range of 1% by weight or less and further, silicon carbide (SiC) is contained in a range of 1 to 10% by weight.

Piston material 10 made of such a rapidly solidified powdered aluminum alloy containing silicon carbide (SiC)
In this case, silicon (Si) is finely contained so that the primary crystal silicon has an average particle diameter of 10 μm or less. In addition, in order to further enhance wear resistance and seizure resistance, silicon (Si) is used. Silicon carbide (SiC), which is a hard, insoluble, non-metallic material, is contained in a metal structure dispersed in a finely divided state so that the average particle size is 10 μm or less. In the forged piston body 1, fine silicon carbide (SiC) is evenly dispersed in the aluminum alloy structure, whereby high wear resistance can be obtained.

In the manufacture of the piston material 10 from the above-mentioned rapidly solidified powdered aluminum alloy, first, an aluminum (Al) base material containing aluminum (Al) containing necessary components (silicon, silicon carbide and the like) in advance. Prepare an alloy ingot, melt it at about 700 ° C or more, spray it in a mist, cool it rapidly at a cooling rate of 100 ° C / sec or more to solidify it into a powder, or Aluminum alloy powder formed by melting and quenching and solidifying an aluminum alloy that does not have an average particle size of 1 to 10 μm
An aluminum alloy powder before solidification is obtained by, for example, mixing a predetermined amount of powder of a necessary component that has been refined so as to obtain an aluminum alloy powder before solidification.

Then, with respect to such an aluminum alloy powder, put the aluminum alloy powder in a mold,
(A temperature of less than 700 ° C.) and directly pressurize to form a piston material of a desired size and shape,
Alternatively, after the aluminum alloy powder is heated to 400 to 500 ° C. and solidified as a round bar by extruding, the round bar is cut into a thick disk having an appropriate amount corresponding to one piston. And a thick disk-shaped piston material 10.

The piston material 1 is made of aluminum alloy powder.
In addition, to mold 0, 40
After being guided between a pair of rolling rolls while being heated to 0 to 500 ° C. and rolled, it is formed as a thick disk-shaped piston material by punching out with a press, or cut into a desired size by shearing to obtain a rectangular shape. It is also possible to mold it as a piston material, and it is also possible to form such a rectangular shape and then pre-forge it to form a thick disk-shaped piston material.

FIG. 4 shows an example of a die for forging a piston material (burette) 10 manufactured by an appropriate method as described above into a substantially piston-shaped forming material 11. The upper mold 21 corresponds to the shape of the inner surface (including the lower surface of the head) of the piston body.
And a pair of left and right frame molds 22a and 22b corresponding to the shape of the outer surface of the piston main body, and a lower mold 23 corresponding to the shape of the upper surface of the piston main body (the upper surface of the head portion).

Each mold 21, 22a, 22b, 23
Are held individually in each mold holder,
By these mold holders, the upper mold 21 is moved up and down, the frame molds 22a and 22b are moved up and down and opened and closed in left and right directions, and the lower mold 23 is always fixed. I have.

In the forging using such a mold, first, a pair of left and right frame molds 22a and 22b are opened, and the piston blank 1 is opened.
0 is placed on the lower mold 23, and then the frame molds 22a, 22b
By closing, in the state as shown in FIG.
After accommodating the piston material 10 in the mold, the upper mold 21 is strongly moved downward, and the frame molds 22a and 22b are interlocked with the upper mold 21, thereby bringing the piston material 10 into a state as shown in FIG. From 10, a substantially piston-shaped forming material 11 is formed by forging.

Thereafter, the other dies 21 and 22 are removed from the lower die 23.
a, 22b and the molding material 11 are moved upward integrally, as shown in FIG.
From the state shown in (3), the upper die 21 is further moved upward to open the frame dies 22a and 22b to the left and right, thereby forming a forged state in the state shown in FIG. 4 (4). The material 11 is separated from the dies 21, 22a, 22b and 23 and taken out of the dies.

FIG. 5 shows that the piston blank 10 is used to form the molding blank 1.
1 shows another example of a die for forging 1. The forging die includes a pair of left and right parts corresponding to the shapes of the outer surface and the upper surface (upper surface of the head) of the piston body. It is composed of upper dies 24a and 24b and a lower die 25 corresponding to the shape of the inner surface (including the lower surface of the head) of the piston body.

The molds 24a, 24b and 25 are individually held by respective mold holders, and the left and right upper dies 24a and 24b are separated by the mold holders.
It is moved up and down and opened and closed in the left and right direction,
The lower mold 25 is always fixed.

In such forging with a die, first, the pair of left and right upper dies 24a, 24b is opened, and the piston blank 1 is opened.
0 on the lower mold 25, and then a pair of left and right upper molds 24.
By closing the a and 24b, the piston material 10 is accommodated in the mold in the state as shown in FIG. 5A, and then both upper dies 24a and 24b are simultaneously strongly downwardly moved.
The state shown in FIG.
The molding material 11 having a substantially piston shape is formed by forging.

Thereafter, both upper dies 24a, 24b and the molding material 11 are moved upward integrally to obtain the state shown in FIG. 5 (3), and then the left and right upper dies 24a, 24b are further opened. Then, as shown in FIG. 5 (4), the forged molding material 11 is separated from the dies 24a, 24b and 25 and taken out of the dies.

In the case of forging using any of the dies described above, the piston blank 1 accommodated in the dies can be used.
0 is heated between 400 and 500 ° C. by a heater provided for at least one of the molds, and then hot forged to fully utilize the ductility of the aluminum alloy. Thus, the molding material 11 can be forged with high dimensional accuracy.

Regarding the hot forging, the piston blank 10 may be heated to 400 to 500 ° C. before being housed in the forging die, and then housed in the die and immediately forged. Even in that case, the die is forged while being preheated to 400 to 500 ° C. By setting the heating process of the piston blank 10 from the forging process to another parallel process, the time of the forging process can be reduced.

According to the forged piston manufacturing method of this embodiment in which the substantially piston-shaped forming material 11 is formed from the piston material 10 by forging with the above-described die, the die used for forging has a pin boss. Portions corresponding to the shape of the outer surface of the portion 3 (frame molds 22a, 22b,
Alternatively, a pair of left and right upper dies 24a, 24b shown in FIG. 5).
Is a movable type that can be opened and closed in a direction (left-right direction) intersecting with a pressing direction (up-down direction) during forging.

Thus, the shape of the molding material 11 is changed so that the outer surface of the pin boss portion 3 remains at the lower end so that the pin boss portion 3 can be thinned without shaving the pin boss portion 3 after forging. The molding material 11 having such a shape may be recessed inward from the outer peripheral surface.
Can be easily taken out of the mold after forging, and forging of the molding material 11 having such a shape by the mold can be practically performed.

The embodiments of the forged piston and the method of manufacturing the same according to the present invention have been described above. However, the present invention is not limited to the above-described embodiment. For example, the present invention is not limited to a piston for an internal combustion engine. The present invention can be applied to a piston of a reciprocating engine such as a compressor, and the structure of the piston body 1 is not limited to the specific structure as shown in the above embodiment.

For example, FIGS. 6A and 6B show a piston for an internal combustion engine according to another embodiment of the forged piston of the present invention. In the embodiment shown in FIG. In the embodiment shown in FIG. 6, the lower end of the pin boss portion 3 is left in a state where the outer surface of the portion is continuous as a cylindrical outer peripheral surface in the entire circumferential direction, whereas in the embodiment shown in FIG. Since the lower end of the pin boss portion 3 is left in a state where the underfilled portion 8 is formed inside the outer peripheral surface, the weight of the piston body 1 is further reduced.

Also in such an embodiment, since the outer surface of the lower end of the piston is longer in the circumferential direction and conforms to the cylindrical shape of the inner surface of the cylinder, the lateral pressure acting on the skirt portion 4 from the inner surface of the cylinder is reduced. The surface pressure due to the load can be reduced, the wear of the skirt portion 4 due to long-term operation can be reduced, and the rigidity of the lower end portion of the piston is also inferior to that shown in FIG. Sufficient rigidity is ensured by forming a state in which a rib 9 continuous with the skirt part 4 is formed at the lower end of the part 3 in parallel with the head part 2.

[0050]

According to the forged piston of the present invention as described above, the outer surface of the lower end portion of the piston extends along the cylindrical shape of the inner surface of the cylinder longer in the entire circumference or in the circumferential direction. Can improve the rigidity of the part and reduce the side pressure acting on the skirt from the cylinder inner surface.Moreover, by forming the undercut part of the pin boss part by forging, the fiber flow formed during forging can be reduced. Without cutting, the thickness of the pin boss portion can be reduced to reduce the weight of the piston, so that, for example, when the piston is installed in an engine and operated, engine vibration due to the inertia of the piston can be suppressed. In addition, even if the engine is running for a long time, it is possible to avoid damage to the pin boss due to cutting the fiber flow. .

Further, according to the method for manufacturing a forged piston of the present invention, the portion corresponding to the shape of the outer surface of the pin boss portion of the forging die is opened and closed in a direction intersecting the pressing direction during forging. By using a movable die that can be formed, it becomes practically possible to form a molding material having an undercut portion formed so that the outer surface of the pin boss portion is depressed inward by forging with a mold.

[Brief description of the drawings]

FIG. 1A is a side view and FIG. 1B is a bottom view of a piston for an internal combustion engine according to an embodiment of the forged piston of the present invention.

FIGS. 2A and 2B are cross-sectional views of the piston body shown in FIG. 1 taken along the line AA in FIG. 1;
Explanatory drawing which shows typically the state of the fiber flow in the cross section along the B line.

FIG. 3 is an explanatory view schematically showing a manufacturing process of the piston main body shown in FIG.

FIG. 4 is an explanatory sectional view showing an example of an operating state of a mold in a forging process for manufacturing the piston main body shown in FIG. 1;

FIG. 5 is an explanatory sectional view showing another example of an operating state of a mold in a forging process for manufacturing the piston body shown in FIG. 1;

6A and 6B are a side view and a bottom view, respectively, of a piston for an internal combustion engine according to another embodiment of the forged piston of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Piston main body (piston for internal combustion engine) 2 Head part 3 Pin boss part 4 Skirt part 7 Undercut part 10 Piston material (material of metal lump) 11 Forged molding material 22a Frame type (outside of pin boss part of forging die) 22b Frame die (portion corresponding to the outer surface of the pin boss of the forging die) 24a Upper die (portion corresponding to the outer surface of the pin boss of the forging die) 24b Upper die (for the forging) The part corresponding to the outer surface of the pin boss of the mold)

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3J044 AA01 AA06 BA04 BC01 CA27 DA09 DA10 EA10 4E087 AA02 AA10 BA04 BA15 BA24 CA13 CA21 CA33 CB01 DB01 DB15 DB24 EC12 EC17 EC22 EC37 EC46 HA62

Claims (2)

[Claims] 1. A metal lump material is forged by a forging die to form a forged molding material having a head portion, a pin boss portion, and a skirt portion. In a forged piston as described above, the outer surface of the pin boss is recessed inward at the stage of the forged molding material except for the lower end thereof. 2. Forging a metal lump material with a forging die to produce a forged molding material having a head portion, a pin boss portion, and a skirt portion, and then commercializing the material by performing finish machining. In such a method of manufacturing a forged piston, a portion corresponding to an outer surface of a pin boss portion of a forging die is formed as a movable die that can be opened and closed in a direction intersecting with a pressing direction at the time of forging. Characterized in that a forged molding material forged so that the outer surface thereof is recessed inward while leaving the lower end portion thereof can be taken out of a forging die.