JPH0967663A - Aluminum extrusion die and nitriding method thereof - Google Patents

Abstract

(57) [Summary] [Objective] By forming a white layer and a nitrogen diffusion layer having specified thicknesses, an extrusion die without damage during extrusion is obtained. [Structure] This die has a thickness of 1 mm on the surface of a die steel for extrusion processing in contact with an aluminum alloy as an extruded material.
Thickness of 4 to 8 μm through a nitrogen diffusion layer of 0 to 150 μm
White layer is formed. (White layer + nitrogen diffusion layer area)
The ratio of / (total area of cross section) is preferably 70% or less with respect to the cross section of the die part perpendicular to the extrusion direction.
In the nitriding process, the die steel is 300
The temperature is maintained at ˜520 ° C., and then at 450˜520 ° C. in an NH ₃ —RX mixed gas atmosphere. In addition, when the white layer disappears due to wear, renitriding is performed under the same conditions. [Effect] Since the hard wear-resistant nitride layer can be formed on the die surface while ensuring the toughness of the die steel, damage to the small cross-sectional area such as the mandrel section with a small cross-sectional area is prevented, and the die life is improved. Life is extended.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum extrusion die having excellent durability and a method for producing the die.

[0002]

2. Description of the Related Art Ultra-thin hollow materials manufactured by extrusion molding are used in aluminum heat exchangers for automobile radiators and air conditioners. As for the die used for this type of extrusion, it is required that the die for extrusion is also complicated and has high shape accuracy. This tendency is not limited to the extrusion of thin hollow materials, and is the same in the field of producing other extrusion molded products. In an extrusion die used for manufacturing a solid extruded material, a bearing surface 2 that defines a die hole is formed in a die body 1 as shown in FIG. The aluminum alloy that is the material to be extruded passes through the die hole in the flow 3 indicated by the arrow, and becomes the extruded material 4 having a cross section corresponding to the die hole. When manufacturing a hollow extruded material, a female die 5 and a male die 6 are used in combination as shown in FIG. The mandrel 7 is fed into the die hole of the female die 5 in the male die 6.
Is formed, and the aluminum alloy that is the material to be extruded is extruded as a flow 3 that surrounds the mandrel 7. The result is an extruded profile 4 with a hollow cross section as shown. The die used for such extrusion processing is nitrided by salt bath nitriding, gas nitriding, or the like. Further, in order to improve the wear resistance of the nitride layer, in JP-A-3-44457, the nitride layer is formed after forming the fluoride film on the surface of the die.

[0003]

DISCLOSURE OF THE INVENTION In extrusion processing,
The surface of the die such as the bearing surface 2 has a great influence on the surface properties of the extruded profile obtained, but the aluminum alloy is in contact with the extruded profile, and the surface of the die is easily worn by the flow of the aluminum alloy. is there. Therefore, how to keep the surface of the die in the same state as when starting extrusion is important for stabilizing the quality of the extruded material. For example, as extrusion continues, the bearing surface wears and the hard surface layer partially falls off. As a result, the dimensions of the extruded profile change, and defects such as streaks that deteriorate the surface quality are likely to appear on the surface of the extruded profile. Therefore, in actual operation, after a certain amount is extruded, the shape of the bearing surface is corrected by polishing, renitriding is performed, and the bearing is repeatedly used for the next extrusion.

The die is thinned by repeated shape modification of the die surface, and is finally discarded. However, since the die is expensive, the larger the number of times it can be repeatedly used by renitriding before it is discarded, the lower the production cost. Especially in the case of die parts that have a precise and small cross-sectional shape according to fine extrusion, such as the male die mandrel used for extrusion of ultra-thin hollow materials, increasing the number of times of repeated use offsets the cost of the die. Becomes important above. However, in a die component having a precise and small cross-sectional shape, the hard nitride layer formed on the surface is brittle, so that an accident often occurs during the extrusion process. Therefore, in actual operation, even if it is in a state where it can be sufficiently used by re-nitriding to avoid damage accidents, it is thrown away after the predetermined number of times of re-nitriding treatment, which reduces the manufacturing cost. Had become a neck in. The present invention has been devised to solve such a problem. By controlling the layer structure of the surface of the die that has been subjected to the nitriding treatment, the extrusion amount by one nitriding treatment is increased, and the extrusion processing is performed. The purpose of the present invention is to prevent the damage of die parts in the inside and to obtain an extruded profile having a good surface quality.

[0005]

In order to achieve the object, the extrusion die of the present invention has a thickness of 100 to 150 μm of nitrogen on the surface of the extrusion die steel with which an aluminum alloy as an extruded material comes into contact. Thickness 4 through diffusion layer
It is characterized in that a white layer of ˜8 μm is formed. Especially for a die part with a small cross section perpendicular to the extrusion direction, such as a mandrel, in the cross section of the die part perpendicular to the extrusion direction, the ratio of (white layer + nitrogen diffusion layer area) / (total area of cross section) It is preferably 70% or less. The extrusion die having the nitrided layer holds the die steel at 300 to 520 ° C. in a fluorine-based gas atmosphere, and then N
It is manufactured by holding at 450 to 520 ° C. in a mixed gas atmosphere of H ₃ gas or NH ₃ gas and another non-nitriding gas or a mixed gas of a gas containing a carbon source and NH ₃ gas. Fluorine-based gases include NF ₃ , CF ₄ , SF
One or two or more of the compound gas containing F component such as ₆ , C ₂ F ₆ and BF ₃ and F ₂ are used. NH ₃
Instead of the gas alone, it is also possible to use NH ₃ gas mixed with a non-nitriding gas such as N ₂ or H ₂ . After being used for extrusion, when the white layer disappears due to wear, it can be reused for the next extrusion by subjecting it to a renitriding treatment under the same conditions.

As shown in FIG. 3, the die surface layer near the bearing surface subjected to the nitriding treatment has a layer structure composed of a nitrogen diffusion layer 9 in which nitrogen is diffused in the base steel 8 and a white layer 10 on the outermost surface. . White layer 10 is ε-F in which N and Fe are combined.
e _2-3 N, γ'-Fe ₄ N, Fe ₃ N, etc. are contained,
It is a very hard layer with a hardness of HV1000 or more. When this white layer 10 is exposed to a stream 3 of aluminum metal, it protects the die steel and prevents wear. According to the study by the present inventors, it was found that the action and effect of the white layer 10 became remarkable when the thickness was 4 to 8 μm. If the thickness of the white layer 10 does not reach 4 μm, the damage due to abrasion is severe, and the amount of extrusion until renitriding is small, resulting in a decrease in productivity. However, in the case of a thick white layer 10 having a thickness of more than 8 μm, on the contrary, partial peeling of the white layer 10 is likely to occur during the extrusion process, which causes a surface defect in the obtained extruded material. This is because as the white layer 10 becomes thicker, the adhesion between the white layer 10 and the nitrogen diffusion layer 9 deteriorates, and the white layer 10 easily separates from the nitrogen diffusion layer 9. Further, in order to thicken the white layer 10, it is necessary to increase the time and temperature of the nitriding treatment. Treatment for a long time is not economical, and treatment at a high temperature causes the die steel to change its structure and softens, so that sufficient strength as a die cannot be secured.

The nitrogen diffusion layer 9 below the white layer 10 is 10
It is necessary to have a thickness of 0 to 150 μm. The nitrogen diffusion layer 9 is a layer in which N forms a solid solution with Fe without forming a compound, and has a hardness greater than that of the base steel 8. However, since the diffusion amount of N becomes smaller from the surface toward the inside of the die, the hardness distribution in the thickness direction approaches the hardness of the base steel as shown in FIG. On the other hand, near the surface, the hardness is close to that of the white layer 10. The nitrogen diffusion layer 9 having such an inclined hardness distribution has a function of backing up the hard white layer 10. The thicknesses of the white layer 10 and the nitrogen diffusion layer 9 are determined by the nitriding conditions. The white layer 10 becomes thinner as the extrusion process is continued, but when the white layer 10 is lost due to wear, a renitriding treatment is performed. This point can be caught by the dimensional change of the extruded profile produced by extrusion. Since the N atoms are pushed into the iron crystal lattice by the renitriding treatment, it is estimated that the white layer swells from the starting surface of the white layer formation at a rate of about 70% of the formed thickness. As shown in FIG. 6, the decrease of (4) is about (4 to 8) μm × 30% = 1.2 to 2.4 μm per one side from the start before renitriding. That is, the cross section of the extruded material is thickened by about 2 to 6 μm.

Prior to the nitriding of the die steel, it is carried out at 300 to 520 ° C. (preferably 300 to 350 ° C.) for 5 to 60 minutes (preferably, in the above-mentioned fluorine-containing gas atmosphere).
Hold for 15 to 45 minutes) is effective. The holding time varies depending on the oxidation state of the die surface, the concentration of fluorine gas, or the mass flow rate per hour, but 10 to 30 minutes is usually sufficient. By this pretreatment, cleaning for removing the passivation film on the surface of the steel material is performed, and the surface of the steel material is activated by fluorine. Therefore, the diffusion of nitrogen is promoted in the subsequent nitriding step, and the nitriding treatment time is shortened as compared with the case where no pretreatment is performed. The fluorination temperature is preferably 300 to 400 ° C., and at 250 ° C. or lower, the difference from the nitriding temperature is too large, and therefore 520 ° C.
At a high temperature exceeding 10 ° C., the fluorinated film becomes unstable, and thus the effect of the fluorination treatment becomes relatively small. The die steel having a surface activated by a fluorine-based gas is subsequently heated to 450 to 520 ° C. (preferably 480 to 520 ° C.) in a mixed gas atmosphere containing RX such as butane and propane having NH ₃ and a carbon source. The temperature of 2) is maintained by heating for 2 to 10 hours (preferably 2 to 4 hours). At this time, since the activated steel surface is subjected to nitriding treatment, the nitriding reaction proceeds at a relatively low temperature as compared with the conventional method, and the tempering phenomenon (structural change) of the die steel is suppressed. This is one of the features of the invention. That is, even after the nitriding treatment, carbide does not grow, and the base steel of the die steel retains the original hard structure having excellent flex resistance.

The nitrogen diffusion layer having a thickness of 100 to 150 μm and 4 to 8 formed on the surface of the working die, which are the main points of the present invention, are included.
There is a close correlation between the nitriding treatment temperature and the treatment time for ensuring the white layer of μm. If the nitriding temperature is too high or the treatment time is too short, the desired hardened layer cannot be obtained. The combination of temperature and time that satisfies the above correlation is important. For example, at a treatment temperature of higher than 520 ° C., changes in the metal structure of the die steel, particularly growth of carbide-based precipitates are observed, and the strength of the die steel as a whole cannot be secured due to softening. As a result, the die easily bends during extrusion, and the operation becomes unstable. In addition, in the nitriding treatment for a long time, the white layer grows excessively, partial peeling is likely to occur during the extrusion process, and the resulting extruded material tends to increase surface defects. In the present invention, gas nitriding is adopted. Gas nitriding is particularly effective for nitriding a die having a portion with a small cross-sectional area, such as a die for hollow material extruded material having a large number of thin mandrels. That is, in gas nitriding, the nitriding reaction proceeds more uniformly than in the salt bath method, and a nitride layer is formed in detail. Further, since the treatment time is short and the treatment temperature is low, softening of the die steel itself is suppressed.
In this respect, in the salt bath method, it is necessary to heat the die steel to 550 to 570 ° C, and softening of the die steel cannot be avoided.

The thicknesses of the white layer and the diffusion layer defined in the present invention are particularly effective for nitriding a die having a thin mandrel. A thick mandrel has no problem because there is a thick base steel in the center. However, as shown in FIG. 5, the thickness A is 1 m.
In the mandrel 11 of about m, the depth B of the surface nitrided layer affects the toughness of the entire mandrel 11. That is, as the surface nitriding layer becomes deeper, the amount of the base steel 8 decreases, and the mandrel 11 as a whole has sufficient hardness but becomes brittle, and is likely to be damaged during extrusion. When an experiment was conducted by the present inventors,
(Area of white layer 10 + nitrogen diffusion layer 9) / (mandrel 11
It has been found that the breakage of the mandrel 11 is suppressed during the extrusion by controlling the ratio of the total cross-sectional area orthogonal to the extrusion direction) of 70% or less. For example, in FIG. 5, when the cross section of the mandrel 11 orthogonal to the extrusion direction is a regular quadrangle having a side length of 2 C (C = 500 μm),
The white layer 10 has a thickness of 8 μm and the nitrogen diffusion layer 9 has a thickness of 150 μm.
If it is set to μm, the area of (white layer 10 + nitrogen diffusion layer 9) becomes 5.3 × 10 ⁵ μm ² , which is 70% of the total cross-sectional area of the mandrel 11, that is, 1000 μm × 1000 μm × 7.
0% = 7 × 10 ⁵ μm ² or less. Under these conditions, the mandrel portion is not damaged. Also, mandrel 1
The thickness of (white layer 10 + nitrogen diffusion layer 9) corresponding to 70% of the total cross-sectional area of 1 is 225 μm, and if the surface nitriding layer is formed thicker than this, it is easily damaged. Therefore, it is necessary to set the nitriding conditions such as temperature and time in consideration of the cross-sectional area of the mandrel, and particular attention should be paid to the condition setting during the renitriding process. On the other hand, when the thickness of the nitride layer exceeds 150 μm, the white layer 10 inevitably grows to a thickness of 8 μm or more, so that surface defects due to cracking, peeling, and the like of the white layer easily occur in the extruded material. Therefore, it is important to set the nitriding conditions so that the thickness of the white layer 10 does not exceed 8 μm and the surface nitrided layer area of the cross section does not exceed 70% in the case of a small mandrel cross section.

The relationship between the renitriding treatment and the cross-sectional dimension of the extruded shape member is as follows. At the start of extrusion, as shown in FIG.
As shown in FIG. 6A, the nitride layer in which the white layer 10 and the nitrogen diffusion layer 9 had thicknesses of 8 μm and 150 μm, respectively, disappeared as the extrusion continued, as shown in FIG. 6B. To do. Therefore, it is necessary to perform re-nitriding at the time when the white layer disappears. The time when renitriding is required is judged from the fact that the cross-sectional dimension of the extruded material increases by 4 to 8 μm per side. As a result of the renitriding, the nitride layer swells as shown in FIG. 6 (c), and becomes a nitride layer whose height is 2.4 μm lower than the initial height.
Specifically, the thickness of (4 to 8) μm × 70% is recovered,
A white layer of 4 to 8 μm is formed again. The renitriding is a repetition of this, and the maximum cross-section of the extruded profile is 2.4 for each renitriding.
Since the size on one side increases by μm, the extrusion length is shortened even if a billet having the same weight is extruded. At the stage of performing renitriding 10 times, 2.4 μm per side × 10 times = 24 μm
The extruded material becomes thicker by m, and the extruded material having a rectangular cross section becomes 24 μm × 2 = 48 μm thicker than the initial thickness. Therefore, re-nitriding is repeated until the thickness exceeds the allowable range of thickness determined by design specifications, and the die is repeatedly used.
As a result, the die cost is reduced.

[0012]

【Example】

Example 1: A male die for producing a hollow extruded material was made of hot work tool steel SKD61. As shown in FIG. 7, this male die has a height of 3 mm and a side of a triangle of 1.5 m.
16 mandrels 11 each having m and a slit width of 0.3 mm are set. This die steel was subjected to a fluorination treatment and a nitriding treatment to form a nitride layer on the die surface. In the fluorination treatment, NF ₃ gas was introduced into a furnace heated to 300 to 520 ° C. to activate the die steel surface for 5 to 60 minutes. Then, the temperature is raised to 450 to 520 ° C. in an N ₂ atmosphere, and NH
_{2 to 3} in an atmosphere in which 30 to 35% RX gas is mixed
By performing the treatment for 10 hours, a white layer of 4 to 8 μm and a nitrogen diffusion layer of 100 to 150 μm were formed as shown in FIG. Further, in a die re-nitrided under the same conditions where the white layer disappeared due to repeated use for extrusion, the nitrogen diffusion layer and the white layer were formed on the sound matrix even after several re-nitriding treatments. Had been formed. On the other hand, in the case where the salt bath nitriding was performed at 570 ° C. for 2 hours, the growth of carbide-based precipitates was observed as shown in FIG. 8B, and the matrix was softened. Therefore, in the present embodiment, fluorination treatment and gas nitriding were adopted.

The nitriding treatment with various temperatures and times was applied to the male die, and the effect of the ratio of the nitriding layer to the cross-sectional area ratio on the mandrel damage was investigated. In the actual operation, a plurality of male dies having 16 mandrels were set in a die in one extrusion, and if any one of the male dies was damaged, it was evaluated that the male dies were damaged. . FIG. 9 shows the relationship between the nitride layer ratio and the breakage rate when an aluminum alloy JIS A1050 billet having a diameter of 152 mm and a length of 331 mm is extruded into a flat tube using such a die. The breakage rate indicates the ratio of the male die in which the mandrel portion was broken to the plurality of male dies set in one extrusion. As shown in FIG. 9, when the nitriding ratio is 50%, about a few% of the male die is damaged, but when the nitriding ratio is 90%, the mandrel portion is very easily damaged. The mandrel portion was broken in the 60% male die. The breakage was likely to occur when the extrusion amount in the initial stage of extrusion was small. It is considered that this is because the nitriding ratio was too large and the mandrel portion was damaged by the rapid application of pressure in the initial stage of extrusion.

On the other hand, when the nitriding ratio was in the range of 50 to 70%, the damage ratio was less than 10% and the ratio of damage occurrence decreased. Also, the time of occurrence of breakage changed not to the initial stage of extrusion but to the point of time when the average extrusion amount was exceeded. Therefore, the damage rate in this case is due to the original die life due to wear and the like, and this is the reason why the damage occurs even at the nitriding ratio of 50 to 70%. In this case, since the average extrusion amount is exceeded, the breakage rate is allowable in operation. Also, a healthy male die can be reused in the next extrusion process by simply replacing the damaged male die. Further, when the area ratio of nitriding is 40% or less, it can be considered that the trouble caused by the damage does not substantially occur. In this way, control the thickness of the nitride layer by the cross-sectional area ratio,
Further, by maintaining the cross-sectional area ratio even in the re-nitriding treatment, nitriding can be performed in a cycle longer than the conventional nitriding cycle, and the extrusion amount becomes larger than in the conventional one in one nitriding cycle. Therefore, the die can be used for a long period of time, and the productivity is improved and the processing cost is reduced.

Example 2 The extrusion amount of a flat tube for a heat exchanger was compared between the nitriding method according to the present invention and a conventional salt bath nitriding method. The changes in the extrusion length and the extrusion amount were different depending on the nitriding method, as shown in FIG. The numerical values in FIG. 10 indicate the number of times of nitriding treatment in the conventional method. In the method of the present invention, renitriding was performed up to 3 times, and only subsequent cleaning was performed without renitriding 4 to 8 times thereafter. From FIG. 10, in the conventional salt bath nitriding, about 3.2 tons of billet was extruded in 10 nitriding treatments, whereas in the nitriding treatment according to the present invention, approximately the same extrusion amount was obtained in 3 nitriding treatments. I understand that it was done.
Further, it can be seen that in the nitriding treatment according to the present invention, the reduction rate of the extrusion length is small, that is, the die wear is less than that of the conventional product. From this, according to the present invention, a large amount of extrusion can be performed with a small number of times of nitriding treatment as compared with the conventional method, the production efficiency is improved by extending the life of the die, the die manufacturing cost is reduced, and the nitriding treatment cost is reduced. It was confirmed that such effects as described above can be obtained.

Example 3: The same hot work tool steel SK as in Example 1
D61 was salt-nitrided to form a white layer and a nitrogen diffusion layer on the surface of the die. In salt bath nitriding, the thicknesses of the white layer and the nitrogen diffusion layer are determined depending on the temperature and time during the nitriding treatment, and therefore the respective thicknesses cannot be controlled independently. The nitriding condition for forming the white layer of 8 μm and the nitrogen diffusion layer of 150 μm was 570 ° C. × 3 hours. Also,
In salt bath nitriding at 570 ° C. for 1.5 hours, a white layer having a thickness of 4 μm and a nitrogen diffusion layer having a thickness of 100 μm were formed. Moreover, since the steel material was heated to a high temperature during the nitriding treatment, the matrix of the die steel tended to be softened. When nitriding the die steel by fluorination and gas nitriding according to the present invention, a treatment of 500 ° C. × 4 hours was required to obtain a white layer of 8 μm and a nitrogen diffusion layer of 150 μm. 5
By the treatment at 00 ° C. for 6 hours, the thickness of the white layer and the nitrogen diffusion layer increased to 13 μm and 200 μm, respectively. When the white layer became thicker than this, a large amount of surface defects occurred in the extruded material as the extrusion amount increased, and the extrusion amount of non-defective products became unacceptably small in operation. 500 ° C
× 2 hours of nitriding treatment, a white layer with a thickness of 4 μm and a thickness of 1
A 00 μm nitrogen diffusion layer was formed. In the nitriding treatment at 500 ° C. for 1 hour, a white layer with a thickness of 2 μm and a nitrogen diffusion layer with a thickness of 50 μm were formed, but although the surface defects of the extruded material did not occur, the cross-sectional dimension of the extruded material was early after the start of extrusion. Became thicker, and the amount of extrusion as a good product became unacceptably small in production. Thus, in the method according to the present invention, the thickness of the white layer and the nitrogen diffusion layer can be adjusted with a high degree of freedom by controlling the conditions of the nitriding treatment, and the extrusion amount can be increased with a small number of nitriding treatments. Became.

[0017]

As described above, according to the present invention, by controlling the thickness of the white layer and the nitrogen diffusion layer formed on the surface of the steel material by the nitriding treatment, the extrusion process can be performed while ensuring the toughness of the die. It makes it possible to give the necessary wear resistance to the die surface. As a result, especially in the die portion having a small cross-sectional area such as the mandrel portion, abrasion and damage due to the pressure during extrusion are suppressed, and a die that can be used for a large amount of extrusion in a long renitriding cycle compared to the conventional method can be obtained. Improves productivity, reduces processing cost, and extends die life.

[Brief description of drawings]

FIG. 1 A state in which a solid extruded material having a rectangular cross section is extruded.

FIG. 2 A state in which a hollow extruded material having a rectangular cross section is extruded.

FIG. 3 A nitride layer formed on the bearing surface of the extrusion die.

FIG. 4 is a hardness distribution in the thickness direction of a die having a nitrided layer.

FIG. 5 is a nitride layer formed on the mandrel.

FIG. 6 is a die surface (a) subjected to a nitriding treatment, a die surface (b) in which a white layer has disappeared by extrusion, and a die surface (c) in which a white layer has been formed by renitriding.

FIG. 7: Male die with multiple mandrels used in the examples

FIG. 8 is a metallographic photograph of a cross section near the die surface showing a nitride layer (a) formed on the die surface according to the present invention and a nitride layer (b) formed by salt bath nitriding.

FIG. 9 is a graph showing the relationship between the nitriding ratio of the mandrel and the damage ratio.

FIG. 10 is a graph showing the relationship between the extrusion length and the extrusion amount when extrusion is performed using a renitriding die.

[Explanation of symbols]

1: Die body 2: Bearing surface 3: Metal flow 4: Cross section of extruded material 5: Female die 6: Male die 7: Mandrel
8: Base steel 9: Nitrogen diffusion layer 10: White layer 1
1: Mandrel A: Mandrel thickness B: Surface nitrided layer thickness
C: Half the length of the cross section

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[Procedure amendment]

[Submission date] July 10, 1996

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

 ─────────────────────────────────────────────────── --- Continuation of the front page (72) Inventor Reiko Takazawa 1-34-1 Kambara, Kambara-cho, Anbara-gun, Shizuoka Prefecture, Japan Group Technology Center, Japan Light Metal Co., Ltd. (72) Toshio Kawai 1 Kamaba-hara, Anbara-gun, Shizuoka Prefecture 34-1-1, Nippon Light Metal Co., Ltd. Group Technology Center (72) Inventor Kenji Usui 20-1 Shimoda-cho, Sakai City, Osaka Prefecture Japan Light Metal Co., Ltd., Osaka Plant (72) Inventor, Teruhiko Hanaoka, Osaka, Japan No. 20-1 Tamachi Nippon Light Metal Co., Ltd. Osaka Plant (72) Inventor Shigeki Aoyama Shimotacho 20-1 Sakai City, Osaka Prefecture Japan Light Metal Co. Ltd. Osaka Plant (72) Inventor Kenzo Kitano Naka Amagasaki, Hyogo Prefecture 1-8 Hamacho Daido Hokusan Co., Ltd. Amagasaki Research Institute Co., Ltd. (72) Inventor Takeshi Mori 1-8 Nakahamacho Amagasaki-shi Hyogo Daido Hoku In Amagasaki Research Institute Co., Ltd. (72) Naohisa Shimada 1-8 Nakahama-cho, Amagasaki-shi, Hyogo Daido Hokusan Co., Ltd. Inside Amagasaki Research Laboratory (72) Toshiko Yokoyama 1-8 Nakahama-cho, Amagasaki-shi Daigo Hokusan Co., Ltd. Amagasaki Institute

Claims (5)

[Claims] 1. A thickness of 100 to 15 is formed on a surface of a die steel for extrusion processing which is in contact with an aluminum alloy which is a material to be extruded.
An aluminum extrusion die for which a white layer having a thickness of 4 to 8 μm is formed via a 0 μm nitrogen diffusion layer. 2. In a cross section of the die part perpendicular to the extrusion direction, (white layer + area of nitrogen diffusion layer) / (total area of cross section)
The die for aluminum extrusion according to claim 1, wherein the ratio is 70% or less. 3. The die steel is 300 times in a fluorine-based gas atmosphere.
To 520 ° C., and then at 450 to 520 ° C. in a nitriding gas atmosphere having a [N] source, to form a die for aluminum extrusion forming a white layer and a nitrogen diffusion layer according to claim 1 or 2. Nitriding method. 4. The nitriding gas is NH ₃ alone, a mixed gas of NH ₃ and another non-nitriding gas, or a mixed gas of NH ₃ and a gas having a carbon source. A method for nitriding an aluminum extrusion die as described. 5. A method for nitriding an aluminum extrusion die, wherein the renitriding treatment according to claim 3 is performed when the white layer disappears due to abrasion.