JP2914771B2 - Forging method for cylindrical parts with irregular cross section - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forging a cylindrical part having an irregular cross-section, and more particularly to a method for manufacturing a large number of parts at once instead of welding a plurality of divided parts. In addition, the present invention relates to a warm forging method capable of obtaining a high-precision, high-quality cylindrical forged part.

[0002]

2. Description of the Related Art For forming a deep cylindrical part having an irregular cross section, a method of manufacturing by welding a plurality of divided parts or a method of forming a hole by cutting is employed. But,
The welding method has problems not only in the stability of quality and dimensional accuracy but also in the processing time and the mass productivity is poor.
In addition, the cutting method requires a long processing time, is expensive, and is not suitable for mass production.

[0003] For the molding of these parts, forging of the molding method is desired from the viewpoints of quality, stability of dimensional accuracy, mass productivity and cost. However, in cold forging for high-strength components using medium to high carbon alloy steels, the processing load is increased and the punch is broken, so that forging, especially drilling, becomes difficult. On the other hand, if forging is performed warmly or hotly in order to reduce the processing load, mass production cannot be performed due to damage such as softening and abrasion of the punch, deterioration of the quality and dimensional accuracy of the forged product, and the cost increases.

[0004] In order to solve the problems of these technologies, technical development has been actively carried out, and the following methods have been proposed.

[0005] As a first method, there has been proposed a method of using a graphite-based lubricant as a lubricant in warm or hot forging to prevent punch damage and the like.

As a second method, a "warm forging method" using boron oxide, which is a low-melting metal oxide, as a lubricant for warm forging (Japanese Patent Application Laid-Open No. 60-210338).
JP-A-63-268530) has been proposed.

As a third method, a lubricant composed of boron oxide or a substance mainly containing boron oxide is applied to the surface of a billet for forging and heated to 600 to 900 ° C. "Lubrication method for warm forging" in which metal soap is applied and forged (Japanese Patent Laid-Open No. 61-180634)
Publication).

[0008]

However, in the first method, in the hot or hot drilling, seizure is liable to occur due to severe frictional conditions between the punch and the workpiece (billet). However, even if a graphite-based lubricant is applied, this seizure cannot be sufficiently prevented, and the mold tends to soften and wear. Therefore, the method has a problem that the punch is damaged early in such a drilling process, and good forging cannot be performed. Further, when a large amount of graphite is applied to enhance the lubrication performance, there is a problem that the working environment is remarkably deteriorated.

It is also said that the second method exhibits an excellent anti-seizure ability even in the drilling of deep cylindrical parts, compared to the graphite. However, in deep drilling of irregular cross-sections, the punch softens because the punch temperature rises due to repetition in a short cycle time,
There was a problem that it was worn or deformed.

Further, in the third method, there is a problem in that, in deep drilling of a deformed cross section, the boron oxide film is cut off, so that seizure of the punch cannot be prevented. Furthermore, in order to prevent malfunction of the mold sliding part, a large amount of metal soap must be applied,
There was a problem that a burning flame of the metal soap was generated, which was not preferable for work.

The inventors of the present invention have made intensive studies to solve the above-mentioned problems of the prior art, and have conducted various systematic experiments. As a result, the present invention has been accomplished.

An object of the present invention is to provide a warm forging method capable of obtaining a high-precision and high-quality forged part even with a cylindrical part having an irregular cross section.

The present inventors have focused on the following with respect to the above-mentioned problems of the prior art. That is, first, attention was paid to the mechanism of punch damage generation when boron oxide was used as a lubricant in warm drilling of a deep container having an irregular cross section. In other words, in warm drilling, heat conduction from the workpiece and frictional heating at the interface between the workpiece and the punch cause
A large amount of heat energy is supplied to the punch, and the punch temperature rises significantly. In the method using boron oxide of the prior art, the rise in punch temperature in each forging operation is small compared to the method using graphite, but the punch temperature is gradually increased due to heat accumulation in the punch in multiple continuous forging processes. Rises to
The punch softens and wear progresses. In addition, in order to forcibly cool the punch, in the method of spraying a metallic soap, which has been made into an aqueous dispersion in combination with boron oxide of the prior art, onto a mold, the punch is burned into the punch by cutting the boron oxide film on the perforated surface. Cause sticking. This is because the amount of boron oxide captured at the tip of the punch at the start of drilling decreases due to the metallic soap applied to the punch, and the boron oxide captured at the tip of the punch is uniform in the irregular cross-section as in a circular cross section. Because it cannot be leaked.

Therefore, in the deep warm drilling process of the irregular cross section, the billet is coated with boron oxide and the boron oxide is sealed and lubricated, so that the boron oxide can be sufficiently supplied in the deep drilling process. Focusing on the application of a metallic soap in a water dispersion to the punch for cooling, thereby preventing breakage of the film of boron oxide during perforation, preventing sticking to the punch, The softening and wear of the punch can be suppressed by reducing the thermal load on the punch.

Further, the boric acid and boron oxide applied to the billet partially remain in the container after forging.
The inner surface of the container not only comes into contact with the billet (forged product) but also slides with the knockout punch. Therefore,
The increase in the amount of boron oxide deposited due to the repetition of the forging process
There is a problem that the malfunction of the knockout punch is caused. Therefore, on the container side, we focused on preventing the knockout punch from malfunctioning by realizing low friction in a wide temperature range and reducing the amount of lubricant accumulated, rather than preventing seizure and reducing thermal load. .

As described above, in deep warm drilling having a deformed cross section, a forging method is developed in which a more excellent lubricating performance is exhibited and a rapid and safe lubricating performance is exhibited while utilizing the advantages of the conventionally known method using boron oxide. Reached.

[0017]

SUMMARY OF THE INVENTION According to the present invention, a method for forging a cylindrical part having a deformed cross section has a concave portion on a drilled surface, and at least the boring surface contains a pretreatment agent containing boric acid as a main component and an oxidizing agent. A billet formed with a lubricating film composed of a lubricant containing boron as a main component, and a punch formed with a lubricating film formed by applying an aqueous dispersion containing metal soap or boric acid as a main component on at least the contact surface with the billet; Preparing a container coated with an aqueous dispersion of graphite, and disposing a billet having the lubricating film on the perforated surface in the container coated with the aqueous dispersion of graphite, and forming a billet on the perforated surface of the billet. A warm forging step in which a punch having the lubricating film is brought into contact with the formed concave portion to perform extrusion perforation, and a forged product of high quality can be obtained with high accuracy even in a deep cylindrical part.

[0018]

The mechanism by which the method for forging a cylindrical part having an irregular cross-section of the present invention exerts an excellent effect is not necessarily clear yet, but is considered as follows.

In the method for forging a cylindrical part having an irregular cross section according to the present invention, a perforated surface has a concave portion, and at least the perforated surface has a pretreatment agent containing boric acid as a main component and boron oxide as a main component. A billet having a lubricating film formed of a lubricant to be formed, a punch having a lubricating film formed by applying an aqueous dispersion containing metal soap or boric acid as a main component on at least the contact surface with the billet, and an aqueous dispersion of graphite. Prepare a container with the body applied. Next, a billet having the lubricating film is arranged on the perforated surface in the container coated with the graphite aqueous dispersion, and a punch having the lubricating film is brought into contact with a concave portion formed on the perforated surface of the billet. By performing extrusion perforation, even if an aqueous dispersion of metallic soap or boric acid is applied on the boron oxide transferred from the billet to the punch to forcibly cool the punch, the film is formed,
It is possible to capture a necessary and sufficient amount of boron oxide that does not cause film breakage during the perforation process. Thereby, the performance of preventing seizure of boron oxide and the performance of reducing thermal load can be exhibited. In addition, since the punch can be cooled by applying the aqueous dispersion to the punch before processing, an increase in punch temperature can be suppressed.

As described above, the punch life is remarkably improved,
Not only has it become possible to forge deep cylindrical parts made of medium to high carbon alloys with irregular cross-sections that were not possible before,
Forged parts with high precision and quality can be obtained.

Further, by applying a water dispersion of graphite to the container surface, not only can the lubrication between the container being drilled and the billet outer surface be improved, but also the sliding between the processed container and the knockout punch can be achieved. Dynamic resistance can be reduced,
Malfunctions disappear. Further, it is not necessary to apply boric acid to the container surface side (side surface) of the billet. Further, trouble during the forging operation can be eliminated, and automation can be performed.

As described above, it is possible to supply a required amount of lubricant and to apply each lubricant to a portion where the performance is maximized, so that the lubricating film is not broken and the cross section is irregular. Deep forging of cylindrical parts is possible, and the application amount can be significantly lower than using a single type of lubricant in all parts. Thereby, not only the cost reduction effect but also the working environment can be improved.

[0023]

According to the method of the present invention, a high-precision and excellent-quality forged product can be obtained even for a deep cylindrical part having an irregular cross section. As a result, compared with the conventional welded product, it is possible to produce a large number of molded products with high accuracy and high quality at a low cost in a short time, and further, it is possible to improve the working environment.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a specific example of the present invention will be described.

In the method of forging a cylindrical part having an irregular cross section according to the present embodiment, first, a billet having a concave portion on a perforated surface is prepared in order to sufficiently capture boron oxide between the punch and the initial stage of processing. . The shape of the concave portion is preferably a shape such that when the punch comes into contact with the perforated surface, boron oxide does not flow out from the punch, for example, a shape that does not produce a gap with the contour of the punch. By adopting such a shape, it is possible to suppress the outflow of boron oxide from the gap portion at the initial stage when the punch comes into contact with the billet and perforate, thereby reducing the amount of the boron oxide used. At the same time, the required amount of boron oxide can be reliably held. Further, as the processing proceeds, the boron oxide appropriately flows out of the punch tip surface to a necessary portion such as the punch tip R portion. In order to capture sufficient boron oxide on the entire tip surface of the punch, the shape of the concave portion is substantially similar to the cross-sectional shape of the tip portion of the punch. preferable. Further, the volume of the concave portion is preferably large enough to capture a necessary and sufficient amount of boron oxide that does not cause film breakage during the perforation process.

Next, a pretreatment agent containing boric acid as a main component is applied to the perforated surface of the billet in a necessary and sufficient amount to prevent oxidation of the perforated surface during heating, and the billet is heated. The boric acid becomes boron oxide after heating. More specifically, a mixture of boric acid and a binder such as sodium carboxymethylcellulose is used as the pretreatment agent. Note that boron oxide may be used instead of boric acid. Alternatively, after a binder such as sodium carboxymethylcellulose is first applied to the billet-perforated surface, a powder of a substance containing boric acid or boron oxide as a main component may be applied onto the binder.

In this case, it is desirable that the heating be performed rapidly up to a predetermined temperature. This is disclosed in JP-A-63-268.
This is because, as shown in Japanese Patent Publication No. 530, the longer the heating time, the applied boric acid deteriorates. The heating temperature is
The coating temperature and the heating time are preferably controlled so as to be 600 to 850 ° C. so that the applied boric acid does not deteriorate.

Next, before heating and before drilling, a lubricant containing boron oxide as a main component is applied to the drilled surface of the billet to form a lubricating film, and at least the billet having the lubricant is formed on the drilled surface. Prepare The lubricant may be boron oxide, or phosphorus pentoxide, sodium oxide,
Using a powder to which potassium oxide or the like is added, the powder may be applied to the perforated surface by spraying, showering, or the like, or a lubricant containing boron oxide in the form of a sheet as a main component may be placed on the perforated surface. . The amount of the lubricant to be applied may be an amount necessary and sufficient to fill the concave portion provided on the perforated surface.

Before or after the preparation of the billet, an aqueous dispersion composed of metallic soap or boric acid is applied to the punch, the punch is forcibly cooled, and moisture is evaporated to form a lubricating film. A punch having the lubricant on the contact surface with the billet is prepared. As described above, by applying an aqueous dispersion of metallic soap or boric acid to the surface of the punch, it is possible to reduce the temperature of the punch raised by the perforation when performing continuous processing, and to maintain the temperature of the punch at an appropriate level. Can be.

In the case of performing the continuous processing, a film of metallic soap or boric acid is formed on the surface of the punch on the boron oxide transferred from the billet surface to the punch by the previous perforation processing due to evaporation of moisture. The lubricating film on the surface of the punch has a problem that the amount of boron oxide captured between the billet and the punch in the initial stage of processing is reduced as compared with a film containing only boron oxide, and the film tends to be cut. However, in this specific example, such a problem does not occur because the amount of the trapped lubricant can be increased and the required amount can be maintained by providing a concave portion on the contact surface of the billet with the punch.

Before and after the preparation of the billet and the punch, a container coated with an aqueous dispersion of graphite is prepared. The lubrication performance required for the container surface is 100 ° C to 850
It has low friction in a wide temperature range up to ° C. Lubricants that satisfy this condition include graphite and molybdenum disulfide. Any of these can be used, but graphite is more preferable because of its low cost and high heat resistance. At this time, the graphite is applied by spraying or the like as an aqueous dispersion, also serving as cooling of the container surface. As the water evaporates, graphite adheres to the container surface. Due to the presence of the graphite, the malfunction of the knockout punch can be reduced.
In addition, there is no flame from the start to the end of the processing, and the operation can be performed safely.

The operation of the knockout punch due to the accumulation of the lubricant applied to the billet due to the repetition of processing cannot be completely prevented only by applying graphite to the container surface. In this specific example, by applying the lubricant mainly composed of the boron oxide to the necessary perforated surface, the amount of the lubricant adhering to the container surface can be reduced. Can be completely prevented. Further, the container surface is a portion where the lubrication performance of graphite is maximized, and the amount of application may be small. Therefore, the application amount can be remarkably reduced as compared with the case where warm forging is performed using only graphite, so that the working environment is not deteriorated.

Next, a billet having the lubricating film on the perforated surface is disposed in a container coated with the graphite aqueous dispersion, and a punch having the lubricating film in a concave portion formed on the perforated surface of the billet. And extruded perforation. The forged product obtained therefrom is air-cooled or water-cooled as it is, or is subjected to post-processing such as bottoming, and then air-cooled or water-cooled. However, even in the case of air cooling, it is better to wash with water while the forged product is at least 100 ° C. This is because by immersing the forged product at 100 ° C. or higher in water, the attached boron oxide can be easily removed. The forged product from which boron oxide has been removed has a metal surface without oxidation, and is excellent in quality and dimensional accuracy.

By performing forging as described above,
Deep drilling of irregular cross-sections becomes possible. Furthermore, the method enables the use of a hard-coated punch that could not be used in the conventional warm and hot forging, thereby further improving the punch life. That is,
In the conventional warm or hot forging, VC by the TRD method is used.
The film, the TiC film formed by the CVD method, and the like were deteriorated or lost by oxidation, or peeled off by softening of the base material, and could not sufficiently exhibit functions such as anti-seizure and improvement in wear resistance. . However, in the method of forging a cylindrical part having a deformed cross section of this specific example, the hard film can be prevented from being oxidized and peeled by the small temperature rise of the punch and the lubricating film of boron oxide covering the surface. The function of the hard film such as prevention of seizure and improvement of abrasion resistance can be sufficiently exhibited.

An embodiment of the present invention will be described below.

First Embodiment

After a billet cut into a predetermined length from a rolled square bar of SUP6 was subjected to an oxide film removal treatment, a concave portion was coined on a perforated surface. At this time, the shape of the concave portion is a rectangle whose cross section is similar to the cross section of the perforated punch, a pyramid shape in which the area decreases as the depth increases, and the surface has the largest cross sectional area and the area is equal to or equal to the cross sectional area of the punch. It was formed to be slightly larger.

Next, boric acid was applied to the perforated surface of the billet and its vicinity. The coating amount was about 50 g / m ² .
Next, the billet is heated in a high frequency induction heating furnace at 700 to 85.
After heating to 0 ° C., the billet was additionally coated with boron oxide. The boron oxide melted immediately upon application and filled the billet recesses.

Next, as shown in FIG. 1, the billet 30 is inserted into the container 10 having the inner surface 11 coated with the graphite 12, and the recess formed on the perforated surface of the billet 30 and having the surface coated with the lubricating film 32. A punch 20 coated with a metallic soap 21 made of calcium stearate was brought into contact with 31 to perform perforation. At this time, the perforation depth is about twice the punch cross-section long side, 48 mm, and the area reduction rate is 31%.
No problem such as seizure occurred on the punch 20. No inflammation occurred. Furthermore, the amount of lubrication used was significantly less than in the case where a single type of conventional lubricant was used for all parts, and the cost was reduced and the working environment was significantly improved. Thus, FIG.
As shown in (a) and (b), a cup-shaped forged product was obtained. FIG. 2A is a longitudinal sectional view of the forged product, and FIG.
(B) shows a cross-sectional view of the central portion thereof.

Next, the bottom of the obtained forged product was punched out and formed into a cylindrical shape. This molded product exhibited an unoxidized metal surface and a scratch-free smooth surface on the inner surface, and had excellent dimensional accuracy even when repeated many times.

Next, a performance evaluation test of the molded product was performed by a strength test. That is, a V-shaped wedge was inserted into the cylinder of the molded body, and the relationship between the width of the mouth of the molded body and the wedge pushing load when the molded body was broken was examined. FIG. 3 shows the obtained results. Incidentally, the sample was performed using respectively the ones of varying the heat treatment conditions, in the figure, "A1" is the case where the hardness of the sample was H _R C = 57, hereinafter similarly "A2" hardness the case where the H _R C = 54, "A3" is a case of a H _R C = 52.5 hardness, "A4" indicates the case where the hardness and H _R C = 48.

Further, after knocking out, an aqueous dispersion of metallic soap was spray-applied to the punch 20 to cool the punch 20 and to apply metallic soap to the surface of the punch 20. At the same time, an aqueous dispersion of graphite was spray-applied to the inner surface 11 of the container 10 to cool the container, and graphite was applied to the surface of the inner surface 11 of the container. At this time, the concentration and application amount of the aqueous dispersion of the metallic soap on the punch 20 side and the aqueous dispersion of graphite on the container side were adjusted so that the water was completely evaporated during the forging cycle and the required amount was adhered. . Next, the billet 30 having the lubricating film in the concave portion of the perforated surface was inserted into the container as described above, and the billet was perforated in the same manner as described above. Further, this method was repeated to perforate about 4000 billets. As a result, even if a large number of billets, such as about 4,000, were drilled, a good-quality forged product could be obtained without causing problems such as seizure.

For comparison, as shown in FIG. 4, two U-shaped parts (material: S
UP6) was overlapped and welded to produce a cylindrical part having a rectangular cross section on the inside (Comparative Example 1: Sample No. C1). FIG.
Fig. 4 is a longitudinal sectional view (Fig. 4 (a)) of the obtained comparative part and a transverse sectional view (Fig. 4 (b)) of a central portion of the welded product. The strength test of the obtained comparative component was performed in the same manner as in the above-mentioned Example. The results are shown in FIG. In the figure, “C1” indicates the result of Comparative Example 1. As is apparent from FIG. 3, the strength of the comparative part was much lower than １ ／ of that of the forged product of the above example. Further, in the obtained comparative part, there were many problems that cracks occurred in the welded portion, and the quality and dimensional accuracy were not stable.

As Comparative Example 2, a forged product for comparison was prepared in the same manner as in Example 1 except that an aqueous dispersion of graphite was applied to all of the billet 30, punch 20 and container 10. Manufactured. However, in this case, seizure to the punch occurred, and further, the punch was significantly softened, and the punch was softened, deformed, and cracked after processing about 20 pieces.

As Comparative Example 3, a forged product for comparison was produced in the same manner as in Example 1 except that boron oxide was applied to the entire billet and metallic soap was applied to the punch and the container. . As a result, seizure occurred on the punches, and the dimensional accuracy was lost with about 20 punches.

Further, as Comparative Example 4, in the same manner as in the above-described Example, except that boron oxide was applied to the entire billet, an aqueous dispersion of metallic soap was applied to the container, and the punch was forcibly air-cooled. To produce a forged product for comparison.
As a result, a good forged product was obtained, but the punch was softened and deformed after about 400 processing steps. At this time,
The knockout punch did not drop by its own weight from the number of processing of about 100 pieces.

Second Embodiment

In the first embodiment, the punch is replaced with a TRD.
A billet was perforated in the same manner as above except that the punch was coated with VC by the method, to obtain a forged product. Than this,
The punch life was 2-3 times longer, and a cylindrical part having good quality and dimensional accuracy could be stably manufactured at low cost.

Third Embodiment

In the warm forging of a shaped part as shown in FIG. 5 using conventional graphite as a lubricant, the life of a punch for punching a central portion is extremely short, and a large amount of graphite is applied. It was bad. Therefore, the present invention was applied to the processing of such parts.

A boric acid powder was applied to the surface of a billet cut into a predetermined length from the bar of S45C. The boric acid was applied only to the upper half of the cut billet. The application amount is
It was about 50 g / m ² . 700 to high frequency induction heating furnace
After heating to 850 ° C., preliminary upsetting is first performed in the first step. Next, at the time of extruding the shaft portion in the second step, on the upper surface of a portion corresponding to the drilling surface in the next step,
A recess having a cross section substantially similar to the cross-sectional shape of the punch was formed. The shape is a substantially pyramid shape in which the cross-sectional area decreases while maintaining the shape as the depth increases. The size of the cross-sectional area on the perforated surface was equal to or slightly larger than the punch cross-sectional area.
When the sheet-shaped boron oxide was supplied onto the recess, the boron oxide immediately melted and filled the recess. Then
In the third step, the hole was formed by piercing. In this perforation process, there was no breakage of the lubricating film on the perforated surface, and there was no problem such as seizure of the punch. In each of the above steps, after molding, an aqueous dispersion of metallic soap was applied to the punch side and an aqueous dispersion of graphite was applied to the container side for cooling. The concentration and application amount of the aqueous dispersion of metal soap on the punch side and the aqueous dispersion of graphite on the container side were adjusted so that the water was completely evaporated during the forging cycle and the required amount was attached.

As a result, the softening and abrasion of the punch were small, and the processing could be performed without any problem up to the number of processing of 5,000 pieces, and no trouble during the processing such as a knockout failure and a transportation failure occurred. Further, as in the present embodiment, the present invention can be applied to multi-step warm forging, and the working environment here is also improved.

[Brief description of the drawings]

FIG. 1 is an explanatory view illustrating a forging process of a cylindrical part according to a first embodiment.

FIG. 2 is a longitudinal sectional view (FIG. 2 (a)) of a forged product obtained in a first embodiment, and a transverse sectional view of a central portion of the forged product (FIG. 2 (b)).

FIG. 3 is a diagram showing a performance evaluation test result of a forged product obtained in the first example. In a strength test, a relationship between a width of a mouth of a molded body and a wedge pushing load when the forged product is broken is shown. FIG.

FIG. 4 is a longitudinal sectional view (FIG. 4 (a)) of a comparative part obtained in Comparative Example 1, and a transverse sectional view (FIG. 4 (b)) of a central part of the comparative part.

FIG. 5 is an explanatory diagram illustrating a forging process of a cylindrical part in a third embodiment.

[Explanation of symbols]

Reference Signs List 10 container 11 inner surface of container 12 lubricating film (graphite) 20 punch 21 lubricating film (metal soap) 30 billet 31 concave portion on billet upper surface 32 lubricating film (boron oxide) 40 knockout punch A1 Example 1 (sample hardness after heat treatment: H _R C = 57) A2 example 1 (sample hardness after heat treatment: H _R C = 54) A3 example 1 (sample hardness after heat treatment: H _R C = 52.
5) A4 Example 1 (Sample hardness after heat treatment: H _R C = 48) C1 Comparative Example 1

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Toru Yamada 3950 Fushida, Kochi, Kochi, Taiyo Forging Co., Ltd. (72) Inventor Tomiji Oguri 3950, Fushida, Kochi, Kochi, Taiyo Forging Co., Ltd. (72) Invention Person Taizo Yamamoto 3950 Fushida, Kochi City, Kochi Prefecture Taiyo Forging Co., Ltd.Examiner Osamu Kawabata (58) Field surveyed (Int.Cl. ⁶ , DB name) B21J 3/00

Claims (1)

(57) [Claims] 1. A billet having a concave portion on a perforated surface and having a lubricating film formed on at least the perforated surface and comprising a pretreatment agent containing boric acid as a main component and a lubricant containing boron oxide as a main component; A punch in which a lubricating film is formed by applying an aqueous dispersion containing metal soap or boric acid as a main component to a contact surface with the billet, and a step of preparing a container to which an aqueous dispersion of graphite is applied; and A billet having the lubricating film is arranged on the perforated surface in the container coated with the water dispersion, and a punch having the lubricating film is brought into contact with a concave portion formed on the perforated surface of the billet to perform extrusion perforation. A forging method for a cylindrical part having an irregular cross-section, comprising a warm forging step, whereby a high quality forged part can be obtained with high accuracy even for a deep cylindrical part.