JP4865438B2 - Surface treatment method of aluminum extrusion die and aluminum extrusion die - Google Patents

Description

  The present invention relates to a surface treatment method for an aluminum (including an alloy thereof) extrusion die having both wear resistance and peel resistance, and an aluminum extrusion die obtained thereby.

  Aluminum materials are widely used for building members because they are lightweight and have excellent corrosion resistance. Among them, molding processing by extrusion has been widely adopted for members that require glitter. In such an extrusion die, tool steel based on JIS (Japanese Industrial Standards) SKD61 and SKD62 is mainly used as a die base material.

Further, when wear proceeds on the bearing surface of the die due to the extrusion of the aluminum alloy, the size and surface properties of the obtained extruded material are greatly affected. In particular, when the aluminum alloy is of the 3000 series, wear tends to occur on the bearing surface of the die. Therefore, various nitriding treatments are performed on the tool steel in order to suppress wear (see, for example, Patent Document 1).
Japanese Patent Laid-Open No. 10-30164

In the nitriding treatment as described above, a nitride compound layer mainly composed of Fe ₂ N and Fe ₃ N generally called a compound layer is formed on the outermost surface by nitriding the extrusion die, and nitrogen is formed on the base material side. A diffusion hardened layer in which is diffused is formed.

  At this time, since the outermost compound layer has a high hardness of 1000 MHv or higher and has a low affinity with the aluminum alloy that is in contact with the base metal, the effect of suppressing wear including adhesion due to extrusion of the aluminum alloy is suppressed. There is. Further, the diffusion hardened layer on the base material side functions as an intermediate layer for preventing peeling due to a large hardness difference between the compound layer and the base material.

  However, in the aluminum extrusion process, the final shape is obtained only by extrusion from an aluminum alloy billet, and therefore the stress applied to the die surface is considerably large. On the other hand, a diffusion hardened layer whose hardness has been increased by diffusion of nitrogen tends to be brittle. For this reason, the diffusion hardened layer may not be able to withstand the stress such as extrusion, and the diffusion hardened layer may be cracked to cause peeling of the diffusion hardened layer. When peeling occurs in the diffusion hardened layer in this manner, the shape of the extruded material and the surface property are poor, and in some cases, a trouble that the peeled piece is mixed into the extruded material may occur. Therefore, the extrusion die subjected to nitriding treatment is required to have a property having peeling resistance.

  That is, in a gas nitriding process in which a carbon source is not added to a nitriding gas (nitriding gas), nitrogen (N) is diffused from the surface of the base material, so that carbon (C) dissolved in the material is converted into the base material. As a result, the C concentration in the diffusion hardened layer is lowered and the N concentration is increased, so that the nitride layer is easily embrittled. Such embrittlement of the diffusion hardened layer is easy to cause cracking and progress when a large stress is applied during the extrusion process. As a result, a large surface defect such that most of the diffusion hardened layer peels off. Is likely to occur.

  Here, in order to suppress the brittleness of the nitrided layer, carbon not only nitrogen but also carbon is simultaneously diffused, so that carbonitriding is not a nitride alone but a compound having a slightly lower hardness and higher toughness than nitride. A method of forming a product or carbide is also effective. As the method, there are gas soft nitriding treatment and salt bath nitriding treatment in which nitriding treatment is performed in an atmosphere containing a carbon source. In view of this, as shown in Patent Document 1, a technique for improving the peelability from a base material by carbonitriding in a nitriding gas atmosphere to which a gas having a carbon source is added has been proposed.

However, the extrusion temperature of the aluminum alloy is usually 450 ° C. or higher, and in particular, the die bearing portion, which is heated to a higher temperature when the extruded aluminum alloy is subjected to strong processing, often has a temperature of 500 ° C. or higher. On the other hand, the compound layer mainly composed of Fe ₂ N and Fe ₃ N has a property of dissolving carbon in a large amount. However, when the above carbonitriding treatment is performed, it has become clear from the results of research that the higher the solid solution amount of carbon in the compound layer, the easier the decomposition occurs at high temperatures (about 500 ° C. or higher). .

That is, FIG. 1 (a) shows a sample subjected to gas soft nitriding in a mixed gas atmosphere of 50% NH ₃ + 50% carburizing gas (RX gas) under the conditions of a heating temperature of 570 ° C. and a holding time of 3 hours. The results of EPMA analysis are shown, and FIG. 1 (b) shows the results of EPMA analysis of a sample subjected to salt bath nitriding under the conditions of a heating temperature of 580 ° C. and a holding time of 2 hours. In these figures, “white layer” refers to a compound layer. In this method, the compound layer formed on the outermost surface is also a nitride containing a large amount of carbon. Here, the reason why the compound layer containing a large amount of carbon is likely to be decomposed at a high temperature is considered to be that the binding force with Fe is weaker than C as compared with N.

  As described above, a compound layer in which carbon is dissolved has a drawback that it is more easily decomposed than a compound layer containing only nitride when exposed to a high temperature during the extrusion process. Therefore, the thickness reduction of the compound layer due to decomposition that is not simple wear during the extrusion process becomes significant.

  Thus, since the compound layer containing carbon tends to disappear by decomposition, the die described in Patent Document 1 improves the toughness of the diffusion hardened layer, but the compound layer decomposes and disappears and diffuses during the extrusion process. There is a problem that the hardened layer is easily exposed. As described above, when the compound layer disappears and the diffusion hardened layer is exposed, although the hardness of the diffusion hardened layer is as high as 900 to 1100 MHv, the affinity with the aluminum alloy is increased in the same manner as the base material. Will progress. As a result, a trouble that the surface property of the extruded material is greatly deteriorated occurs.

  The present invention has been made in view of the above circumstances, and a surface treatment method of an aluminum extrusion die having both wear resistance and peel resistance, an aluminum extrusion die obtained thereby, and aluminum It aims at providing the manufacturing method of the die | dye for extrusion.

  The present invention provides the following means.

[1] First nitriding treatment for forming a diffusion hardened layer containing carbon and nitrogen on the surface layer of the die body by heating and holding the die body made of tool steel in a nitriding gas atmosphere containing a carburizing gas Process,
The die body that has undergone the first nitriding treatment step is heated and held in a nitriding gas atmosphere that does not contain a carburizing gas, thereby forming a compound layer that substantially does not contain carbon on the surface of the diffusion hardened layer. A second nitriding step;
A surface treatment method of an aluminum extrusion die, characterized by comprising:

  [2] The surface treatment method for an aluminum extruding die as recited in the aforementioned Item 1, wherein the heating temperature is 500 to 580 ° C. in the first nitriding treatment step.

  [3] The surface treatment method for an aluminum extruding die as recited in the aforementioned Item 1 or 2, wherein the holding time is 1 to 5 hours in the first nitriding treatment step.

[4] In the first nitriding treatment step, the carburizing gas is a modified gas of butane or propane, and the nitriding gas is NH ₃ gas or a mixed gas containing NH ₃ gas and non-nitriding gas. The surface treatment method for a die for aluminum extrusion according to any one of items 1 to 3.

  [5] The surface treatment method for an aluminum extruding die as recited in the aforementioned Item 4, wherein the carburizing gas is RX gas.

  [6] The surface treatment method for an aluminum extruding die as recited in any one of the aforementioned Items 1 to 5, wherein in the first nitriding step, the diffusion hardened layer is formed to a thickness of 50 μm or more.

  [7] The surface treatment method for an aluminum extruding die as recited in any one of the aforementioned Items 1 to 6, wherein the heating temperature is 500 to 580 ° C. in the second nitriding treatment step.

  [8] The surface treatment method for an aluminum extruding die as recited in any one of the aforementioned Items 1 to 7, wherein the holding time is 1 to 5 hours in the second nitriding treatment step.

[9] The aluminum according to any one of items 1 to 8, wherein in the second nitriding treatment step, the nitriding gas is NH ₃ gas or a mixed gas containing NH ₃ gas and non-nitriding gas. Surface treatment method for extrusion dies.

  [10] The surface treatment method for an aluminum extruding die as recited in any one of the aforementioned Items 1 to 9, wherein in the second nitriding treatment step, the compound layer is formed to a thickness of 2 to 10 μm.

  [11] When the thickness of the compound layer formed in the second nitriding process is reduced or disappears by extrusion, the die is subjected to a renitriding process including the second nitriding process. The surface treatment method of the die | dye for aluminum extrusion of any one of these.

  [12] The surface treatment method for an aluminum extruding die as recited in any one of the aforementioned Items 1 to 11, wherein the die is used for extruding a 3000 series aluminum alloy.

  [13] The surface treatment method for an aluminum extruding die as recited in any one of the aforementioned Items 1 to 12, wherein the material of the die body is hot tool steel that has been quenched and tempered.

  [14] An aluminum extrusion die processed by the surface treatment method for an aluminum extrusion die according to any one of items 1 to 13.

  [15] A diffusion hardened layer containing carbon and nitrogen is formed on the surface layer portion of the die body made of tool steel, and a compound layer substantially free of carbon is formed on the surface of the diffusion hardened layer. An aluminum extrusion die characterized by the above.

  [16] A method for producing an aluminum extruding die, comprising the surface treatment method for an aluminum extruding die as recited in any one of the aforementioned items 1 to 13.

  The present invention has the following effects.

  In the invention of [1], since the compound layer on the surface of the die is relatively difficult to decompose and the diffusion hardened layer is hardly peeled off, the carburizing gas having a carbon source is used in the first nitriding treatment step. After performing the nitriding treatment (first nitriding treatment) in the nitriding gas atmosphere including the nitriding treatment (second nitriding treatment), the nitriding treatment (second nitriding treatment) is subsequently performed in the nitriding gas atmosphere containing no carburizing gas in the second nitriding treatment step. Thereby, by diffusing both nitrogen and carbon in the diffusion hardened layer, embrittlement of the diffusion hardened layer is suppressed and peeling is prevented. In addition, by reducing the carbon concentration of the compound layer formed on the outermost surface as much as possible, it suppresses the formation of carbides and carbonitrides, which are compounds that have a lower binding force with Fe than iron nitride alone, especially in the high temperature region A compound layer that is difficult to be decomposed is formed. In this way, a die for aluminum extrusion having a nitride layer structure having both wear resistance and peel resistance is obtained. As described above, the aluminum extrusion die that has been surface-treated by the method of the present invention has a nitrided layer structure that has both wear resistance and peel resistance, and is particularly suitable for aluminum extruded pipe processing with a relatively high extrusion temperature. The use of this die is effective in reducing the amount of wear, and it is extremely difficult to cause troubles due to the occurrence of peeling. Therefore, improvement in the quality of the extruded material and further improvement in productivity can be expected.

  In the invention of [2], the softening of the die can be prevented, and the necessary thickness and high productivity of the diffusion hardened layer can be reliably ensured.

  In the invention of [3], a diffusion hardened layer having a sufficient thickness can be reliably formed, and high productivity can be reliably ensured.

  In the invention of [4], the diffusion hardened layer can be reliably formed.

  In the invention of [5], the diffusion hardened layer can be more reliably formed.

  In the invention of [6], it is possible to provide a die that is excellent not only in wear resistance but also in peel resistance.

  In the invention of [7], softening of the die can be prevented, and the necessary thickness of the compound layer and high productivity can be ensured reliably.

  In the invention of [8], the C concentration in the compound layer can be reliably reduced, and stirring of C in the diffusion hardened layer to the base material side can be reliably prevented.

  In the invention of [9], the compound layer can be reliably formed.

  In the invention of [10], high productivity can be ensured.

  In the invention of [11], at the timing when the compound layer is reduced or disappeared by extrusion, the diffusion hardened layer is exposed, and the wear progresses rapidly, a compound layer having a very low carbon concentration and hardly decomposed is formed again. Can be reused.

  In this case, when the renitriding process includes the first nitriding process and the second nitriding process, the depth of the diffusion hardened layer is increased by the renitriding process. Can improve the function as an inclined layer that supports the compound layer of the diffusion hardened layer and gradually lowers the hardness to the base material, and can make the structure less prone to peeling.

  In the invention of [12], it is possible to provide a die having excellent wear resistance for extruding a 3000 series aluminum alloy which is an alloy in which wear is likely to occur among various aluminum alloys.

  In the invention of [13], the base material can provide a die having higher strength.

  In the inventions of [14] and [15], both nitrogen and carbon are diffused in the diffusion hardened layer to suppress embrittlement of the diffusion hardened layer and prevent peeling. In addition, by reducing the carbon concentration of the compound layer formed on the outermost surface as much as possible, it suppresses the formation of carbides and carbonitrides, which are compounds that have a lower binding force with Fe than iron nitride alone, especially in the high temperature region Suppresses the decomposition of the compound layer. Thus, the aluminum extrusion die surface-treated by the method of the present invention has a nitrided layer structure that has both wear resistance and peeling resistance, and is particularly suitable for aluminum extruded pipe processing with a relatively high extrusion temperature. Dies are effective in reducing the amount of wear, and it is very difficult to cause troubles due to peeling. Therefore, the quality of the extruded material can be improved, and further improvement in productivity can be expected.

  The invention [16] has the same effects as the invention of any one of [1] to [13].

  Next, the preferable form for implementing this invention is demonstrated below.

  FIG. 5 is a schematic cross-sectional view of an aluminum (including an alloy thereof) extrusion die according to an embodiment of the present invention. This die is a porthole die 1 in detail. The porthole die 1 is used to extrude a pipe material (not shown) having a circular cross section as an extruded material, and a female die 1A and a male die 1B are used in combination at the time of extrusion. .

  The port hole die 1 has a bearing hole 2 having a cross-sectional shape (that is, a circular cross-section) corresponding to the cross-sectional shape of the extruded material, and a plurality of port holes 3. In this die 1, after the billet flows in from the port hole 3, the extruded material is pushed out from the bearing hole 2.

  At least the bearing surface 2a of the die 1 is treated by the surface treatment method according to the present invention. Further, the port surface 3a of the die 1 is also treated by the surface treatment method according to the present invention. Specifically, in the present embodiment, the entire surface of the die 1 is surface-treated.

  However, in the present invention, the aluminum extrusion die is not limited to being a port hole die, but may be a bridge die, a solid die, a loader die, a semi-holoder die, a flat die, or the like.

  FIG. 6 is an enlarged schematic cross-sectional view of the bearing portion of the die 1. The die body 5 of the die 1 is made of tool steel. More specifically, the die body 5 is made of hot tool steel that has been quenched and tempered. A diffusion hardened layer 6 is formed on the surface layer portion of the die body 5, and a compound layer 7 is further formed on the surface of the diffusion hardened layer 6. The cross-sectional structure of the port portion of the die 1 is the same as that shown in FIG.

  Next, the surface treatment method of the aluminum extrusion die according to the present invention will be described below.

The surface treatment method of the aluminum extrusion die according to the present invention,
(1) A first nitriding treatment for forming a diffusion hardened layer containing carbon and nitrogen on the surface layer portion of the die body by heating and holding the die body made of tool steel in a nitriding gas atmosphere containing a carburizing gas. Process,
(2) Forming a compound layer substantially free of carbon on the surface of the diffusion hardened layer by heating and holding the die body subjected to the first nitriding treatment process in a nitriding gas atmosphere not containing a carburizing gas. A second nitriding treatment step,
including.

  In the present invention, various types of tool steel can be used as the die base material. For example, Cr-W-V alloy steel represented by JIS (Japanese Industrial Standard) SKD4, 5, etc., Cr-Mo-V alloy steel represented by JIS SKD6, 61, 7, etc., JIS SKD62, 8 etc. And Cr—W—Mo—V alloy steel represented by JIS SKT3, Ni—Cr—Mo alloy steel represented by 4 and the like. In particular, the tool steel is desirably hot tool steel that has been subjected to quenching and tempering treatment. By doing so, the strength of the die base material can be increased.

  In the first nitriding step, the die body made of tool steel is heated and held in a nitriding gas atmosphere containing a carburizing gas, thereby forming a diffusion hardened layer containing carbon and nitrogen on the surface layer portion of the die body.

The nitriding gas used as an atmosphere gas (nitriding gas), can be used NH ₃ Gasuka or NH ₃ gas and a mixed gas of non-nitriding gas such as N _2,. Carburizing gas (for example, RX gas) such as butane or propane metamorphic gas is mixed into the nitriding gas.

  The heating temperature in the first nitriding treatment step is preferably set to 500 to 580 ° C. This is because if the heating temperature is less than 500 ° C., the retention time becomes too long in order to ensure a sufficient thickness of the compound layer and the diffusion hardened layer that supports the compound layer, and the productivity may deteriorate. On the contrary, if the heating temperature exceeds 580 ° C., the die base material (ie, the die body) is softened at a high rate and the durability life of the die may be shortened.

  In this way, by making the temperature of the tens of tens of degrees lower than the tempering temperature of the die body as the first nitriding temperature, the softening of the die can be prevented, and the necessary diffusion hardened layer thickness and high productivity can be secured. can do.

  Moreover, about the holding time (namely, processing time) of the heating temperature in a 1st nitriding process process, 1 to 5 hours are suitable. By so doing, a diffusion hardened layer having a sufficient thickness can be reliably formed, and high productivity can be reliably ensured. In other words, if the holding time is less than 1 hour, there is a possibility that sufficient C cannot be diffused to alleviate the brittleness of the diffusion hardened layer. When the holding time is 5 hours or less, a sufficiently thick diffusion hardened layer can be reliably formed. On the other hand, if the holding time exceeds 5 hours, the productivity may deteriorate.

  However, in the present invention, the heating temperature and the holding time in the first nitriding treatment step are not limited to the above ranges.

  The holding time is changed depending on the nitriding temperature, and the nitriding temperature is optimized in consideration of the characteristics of the dies, and the holding time is sufficient to obtain a diffusion hardened layer having a thickness sufficient to withstand the extrusion conditions. Can be selected.

  In the second nitriding treatment step, the die body subjected to the first nitriding treatment step is heated and held in a nitriding gas atmosphere containing no carburizing gas, so that the surface of the diffusion hardened layer substantially contains carbon. No compound layer is formed.

The nitriding gas used as an atmosphere gas, it is possible to use NH ₃ Gasuka or NH ₃ gas and a mixed gas of non-nitriding gas such as N _2,.

  The heating temperature in the second nitriding treatment step is preferably set to 500 to 580 ° C. If the temperature is less than 500 ° C., the retention time becomes too long in order to ensure a sufficient thickness of the compound layer, and the productivity may deteriorate. On the other hand, when the temperature exceeds 580 ° C., the die base material (die body) is softened at a high rate, and the durability of the die may be shortened.

  Thus, by making the temperature lower by about several tens of degrees C. than the tempering temperature of the die base material as the second nitriding temperature, it is possible to prevent the softening of the die and to secure the necessary thickness and productivity of the nitrided layer. Can do.

  Moreover, about the holding time (namely, processing time) of the heating temperature in a 2nd nitriding process process, 1 to 5 hours are suitable. By doing so, the C concentration in the compound layer can be reliably reduced, and stirring of the C in the diffusion hardened layer toward the base material (that is, the die body) side can be reliably prevented. That is, if the holding time is less than 1 hour, the C concentration in the compound layer may not be sufficiently lowered. On the other hand, if the holding time exceeds 5 hours, C in the diffusion hardened layer may be diffused to the base material side by being pushed by N entering from the surface.

  However, in the present invention, the heating temperature and the holding time in the second nitriding treatment step are not limited to the above ranges.

  The holding time is changed depending on the nitriding temperature, and the nitriding temperature is optimized in consideration of the characteristics of the die, and the holding time is sufficient to obtain a compound layer having a thickness sufficient to withstand the extrusion conditions. Can be selected.

  As described above, in the present invention, since the surface compound layer is relatively difficult to be decomposed and the diffusion hardened layer is hardly peeled, a nitriding treatment is performed in a nitriding gas atmosphere containing a carburizing gas having a carbon source. After performing (first nitriding treatment), nitriding treatment (second nitriding treatment) is subsequently performed in a nitriding gas atmosphere not containing carburizing gas. By doing so, both nitrogen and carbon are diffused in the diffusion hardened layer to suppress embrittlement of the diffusion hardened layer and prevent peeling. In addition, by reducing the carbon concentration of the compound layer formed on the outermost surface as much as possible, it suppresses the formation of carbides and carbonitrides, which are compounds that have a lower binding force with Fe than iron nitride alone, especially in the high temperature region A compound layer that is difficult to be decomposed is formed. In this way, a die for aluminum extrusion having a nitride layer structure having both wear resistance and peel resistance is obtained.

  As described above, the aluminum extrusion die that has been surface-treated by the method of the present invention has a nitrided layer structure that has both wear resistance and peel resistance, and is particularly suitable for aluminum extruded pipe processing with a relatively high extrusion temperature. Since the die for use is effective in reducing the amount of wear and it is extremely difficult to cause troubles due to peeling, the quality of the extruded material can be improved, and further improvement in productivity can be expected.

  In the aluminum extrusion die that has been surface-treated as described above, a diffusion hardened layer containing carbon and nitrogen is formed on the surface layer portion of the die body made of tool steel, and the surface of the diffusion hardened layer is substantially A compound layer not containing carbon is formed. The tool steel is a hot tool steel that has been subjected to quenching and tempering. However, the effect of the present invention can be obtained even with hot tool steel that has not been quenched and tempered or other tool steel.

  The compound layer substantially free of carbon (C) is mainly composed of an Fe—N compound (iron nitride), and its C concentration is less than 1% by mass (particularly desirably less than 0.8% by mass). Preferably there is. By doing in this way, since this compound layer becomes difficult to thermally decompose than the compound layer containing many C, the outstanding abrasion resistance is obtained.

  Moreover, it is preferable to set the thickness of this compound layer to 2-10 micrometers, More preferably, it is 4-8 micrometers. This is because if the thickness is less than 2 μm, the amount that can be extruded once is reduced and the renitriding cycle described later is shortened, so that the productivity of extrusion may be deteriorated. On the other hand, if the thickness exceeds 10 μm, high temperature and long time treatment is required to obtain the compound layer, causing problems such as deterioration of surface treatment productivity and reduction of the number of re-nitriding due to softening of the die base material. It is because there is a possibility of doing.

  However, in the present invention, the thickness of the compound layer is not limited to the above range.

  On the other hand, it is necessary for the diffusion hardened layer not to be peeled off due to the shear stress during extrusion. However, in the portion where the concentration of diffused N is greatly reduced and the hardness decreases from about 900 to 600 MHv in terms of Vickers hardness, there is a high possibility that the tensile stress is amplified. In consideration of the reduction, it is desirable to position the diffusion hardened layer as much as possible inside.

  Therefore, in order to improve the peel resistance, it is necessary to consider that the magnitude of the load stress varies depending on the extrusion conditions, but at least the thickness of the diffusion hardened layer is 50 μm or more, more preferably 80 μm or more. Is desirable. Such an aluminum extrusion die is excellent not only in wear resistance but also in peel resistance. The upper limit value of the thickness of the diffusion hardened layer is not limited, but is preferably 200 μm or less, particularly 150 μm or less, for example.

  However, in the present invention, the thickness of the diffusion hardened layer is not limited to the above range.

  When the compound layer formed in the second nitriding treatment step is reduced or disappears by extrusion, the aluminum extrusion die can be subjected to a renitriding treatment including the second nitriding treatment step. .

  That is, the thickness of the compound layer decreases due to wear and decomposition during extrusion. However, especially when the compound layer on the surface of the bearing portion that becomes high temperature disappears and the diffusion hardened layer is exposed, the wear rate increases due to the strong affinity with aluminum, and the shape of the bearing surface changes. In addition to affecting the dimensional accuracy and surface roughness of the extruded material, it may be difficult to maintain the surface of the die after extrusion, making it impossible to reuse the die.

Therefore, by performing the renitriding treatment before and after the compound layer disappears, more preferably before the compound layer disappears, it is possible to prevent such trouble and extend the life of the die. By doing so, the compound layer mainly composed of Fe ₂ N and Fe ₃ N is reduced or disappeared by the extrusion, and the carbon concentration is once again at the timing when the diffusion hardened layer is exposed and the wear easily proceeds. However, it is possible to form a compound layer that is extremely low and difficult to decompose, so that the die can be reused.

  In this case, as the renitriding treatment, when the treatment including the first nitriding treatment step and the second nitriding treatment step is performed, the thickness of the diffusion hardened layer is further increased by the renitriding treatment, so that the diffusion The hardened layer has a function as an inclined layer that supports the compound layer and gradually decreases the hardness to the base material, and can have a structure that is less likely to cause peeling.

  In addition, about the method of determining the compound layer remaining at the time of renitriding, the cupric chloride aqueous solution is dripped on the surface of the die after lightly polishing the surface of the die with emery paper or the like. It can be easily identified by the presence or absence of precipitation. Therefore, it is possible to construct an optimal renitriding cycle using this method.

  As described above, according to the present invention, nitrogen and carbon are simultaneously diffused by performing nitriding treatment (first nitriding treatment) in a nitriding gas atmosphere containing a carburizing gas having a carbon source in the previous stage of nitriding treatment. By forming a diffusion hardened layer that serves as an inclined layer that relaxes the brittleness of the diffusion hardened layer and further supports the compound layer and gradually lowers the hardness to the base material, a structure in which peeling is difficult to occur is obtained. Further, a compound layer having a low C concentration is formed on the outermost surface by performing nitriding treatment (second nitriding treatment) in a nitriding gas atmosphere not containing a carbon source at a later stage. This makes it possible to form an ideal nitrided layer on an aluminum extrusion die having both excellent wear resistance and peel resistance. Therefore, this die can be suitably used for extruding a 3000 series (for example, JIS 3000 series) aluminum alloy which is an alloy in which the die is likely to be worn among various aluminum alloys.

  In addition, the heating temperature, extrusion speed, etc. vary depending on the type of aluminum alloy to be extruded, the shape of the mold, etc., and the required balance of wear resistance and peel resistance will differ depending on the respective, so extrusion is particularly important. It is necessary to optimize the thickness of the compound layer and diffusion hardened layer for those with high temperature, strict dimensional accuracy and surface quality requirements, etc., but the gas composition during nitriding treatment By optimizing the temperature, time, etc., the process can sufficiently cope with them.

  Moreover, the manufacturing method of the die | dye for aluminum extrusion which concerns on this invention contains the surface treatment method of the said die | dye for aluminum extrusion. Therefore, the manufacturing method of the aluminum extrusion die according to the present invention has the same effect as described above.

  Next, specific examples will be described below.

[Example 1]
A material (base material) made of hot tool steel JIS SKD61 was sequentially quenched and tempered, and tempered to a hardness of HRC49. A plurality of 10 × 10 × 30 mm samples were prepared from this material, and the surface was polished with emery paper # 600 to obtain an analysis sample.

This analytical sample was placed in a heat treatment furnace. Then, the first nitriding treatment was performed under the conditions of a heating temperature of 560 ° C. and a holding time of 90 minutes in an atmosphere of a mixed gas of 50% NH ₃ + 50% RX gas as a nitriding gas containing a carburizing gas. Thereafter, a second nitriding treatment is performed under the conditions of a heating temperature of 560 ° C. and a holding time of 90 minutes in an atmosphere of a mixed gas of 50% NH ₃ + 50% N ₂ gas as a nitriding gas substantially free of carburizing gas. went. In addition,% is all volume%.

  The EPMA analysis result of the cross section of this sample is shown in FIG. In the figure, the “white layer” means a compound layer. As shown in the figure, the C concentration in the portion corresponding to the compound layer is about 0.5% by mass, that is, the gas soft nitriding treatment and salt bath nitriding treatment shown in FIGS. 1 (a) and 1 (b). Compared with about 2% by mass, which is the C concentration in the sample, it was greatly reduced. Therefore, according to this processing method, it is predicted that the wear resistance is improved as compared with the conventional example.

[Example 2]
As Example 2, an aluminum extrusion die (surface hole die in detail) that has been surface-treated in the same manner as in Example 1 above, and a pipe material as an extruded material from a billet of JIS 3003 aluminum alloy is used. Extruded. And about this die, the surface roughness of the bearing surface before and behind extrusion was measured. The result is shown in FIG.

[Comparative Examples 1 and 2]
As Comparative Example 1, a die subjected to gas soft nitriding treatment was prepared in a mixed gas atmosphere of 50% NH ₃ + 50% RX gas under conditions of a heating temperature of 560 ° C. and a holding time of 3 hours. Further, as Comparative Example 2, a die was prepared by gas nitriding treatment under the conditions of a heating temperature of 560 ° C. and a holding time of 3 hours in an atmosphere of a mixed gas of 50% NH ₃ + 50% N ₂ gas. The base material (die main body) of each die is the same as that of the second embodiment. And the pipe material was extruded like Example 1 using these dies. Next, the surface roughness of the bearing surface before and after extrusion was measured for these dies. The results are shown in FIG.

  As shown in FIG. 3, in Comparative Example 1 subjected to the gas nitrocarburizing treatment, the surface roughness was significantly deteriorated due to the progress of wear at an early stage. Similarly, in Comparative Example 2 subjected to gas nitriding treatment, the surface roughness was deteriorated due to the progress of wear. On the other hand, in Example 2, no conspicuous wear could be confirmed, and the surface roughness due to the increase in the extrusion amount was very little. Therefore, it could be confirmed that Example 2 has excellent wear resistance.

  Further, in a die after extruding 36 pieces, a 10% by volume aqueous solution of cupric ammonium chloride (II) was dropped onto the bearing surface and the port surface, and the presence or absence of copper precipitation was examined. The results are shown in Table 1.

  As apparent from Table 1, it was confirmed that, on the bearing surface, copper precipitation occurred only in Comparative Example 1 subjected to gas soft nitriding treatment, and the compound layer had already disappeared. From this, it can be seen that the compound having a low C content in the compound layer as in Example 2 is also excellent in decomposition resistance due to heat load. Further, in Comparative Example 2 subjected to gas nitriding treatment, it was confirmed that peeling of the nitrided layer had already occurred on the port surface into which the billet of aluminum alloy was introduced at the stage after 12 extrusions. From this, it has been confirmed that brittleness becomes a problem particularly in the portion of the die where the temperature is relatively low.

[Example 3]
As Example 3, a nitriding treatment step similar to that of Example 1 was repeated, and a sample was repeatedly subjected to nitriding treatment 10 times at the maximum. Among these, samples with nitriding treatment repetition times of 1, 5, and 10 were embedded in the resin, respectively, and 4,5, 26, 27, and 28 μm depth positions from the surface using a micro Vickers hardness tester. An indentation was made with a load of 9 N, and the brittleness was examined by whether or not cracking occurred. The result is shown in FIG.

[Comparative Example 3]
As Comparative Example 3, a sample that was repeatedly subjected to nitriding treatment at most 10 times by the same method as the gas nitriding treatment of Comparative Example 2 was prepared. The brittleness was examined by the same method as in Example 3. The results are shown in FIG.

  As shown in FIG. 4, in Comparative Example 3, the occurrence of indentation cracks becomes clearer as the number of repetitions increases. For example, a sample with a repetition count of 5 causes cracks, and in particular, with a sample with a repetition count of 10 times, the indentation at a depth of 25 μm collapses (that is, defrosts) and cannot be hit normally. ing. On the other hand, in Example 3, no clear indentation cracking occurred even in the sample with 10 repetitions. Therefore, in Example 3, it was confirmed that even when repeated nitriding was performed, the structure was low in brittleness and difficult to cause peeling.

  The present invention can be applied to a surface treatment method of a die for hot extrusion of aluminum, and can be preferably applied particularly to a surface treatment method of a die for processing an aluminum extruded pipe having a high processing temperature.

It is a figure (graph) which shows the EPMA analysis result of the cross section of the sample which performed the gas soft nitriding process. It is a figure (graph) which shows the EPMA analysis result of the cross section of the sample which performed the salt bath nitriding process. It is a figure (graph) which shows the EPMA analysis result of the cross section of the sample of Example 1. FIG. It is a figure (graph) which shows the measurement result of the surface roughness before and behind extrusion performed about the die of Example 2 and comparative examples 1 and 2. FIG. It is a figure (photograph) which shows the evaluation result of the brittleness by the micro Vickers indentation test done about the sample of Example 3 and the comparative example 3. FIG. It is a schematic sectional drawing of the die | dye for aluminum extrusion which concerns on one Embodiment of this invention. It is an expansion schematic sectional drawing of the bearing part of the same die.

Explanation of symbols

1 ... Dice
1A ... Female type
1B ... Male type 2 ... Bearing hole
2a ... Bearing surface 3 ... Port hole
3a… Port side 5… Die body (base material)
6 ... diffusion hardened layer 7 ... compound layer

Claims (15)

A first nitriding treatment step of forming a diffusion hardened layer containing carbon and nitrogen on a surface portion of the die body by heating and holding the die body made of tool steel in a nitriding gas atmosphere containing a carburizing gas;
The die body that has undergone the first nitriding treatment step is heated and held in a nitriding gas atmosphere that does not contain a carburizing gas, thereby forming a compound layer that substantially does not contain carbon on the surface of the diffusion hardened layer. A second nitriding step;
A surface treatment method of an aluminum extrusion die, characterized by comprising:   The surface treatment method for a die for aluminum extrusion according to claim 1, wherein in the first nitriding treatment step, a heating temperature is 500 to 580 ° C.   The surface treatment method for an aluminum extruding die according to claim 1 or 2, wherein in the first nitriding treatment step, the holding time is 1 to 5 hours. In the first nitriding treatment step, the carburizing gas is a modified gas of butane or propane, and the nitriding gas is NH ₃ gas or a mixed gas containing NH ₃ gas and non-nitriding gas. The surface treatment method of the die | dye for aluminum extrusion of any one of 1-3.   The surface treatment method for an aluminum extrusion die according to claim 4, wherein the carburizing gas is RX gas.   The surface treatment method for an aluminum extruding die according to any one of claims 1 to 5, wherein, in the first nitriding treatment step, the diffusion hardened layer is formed with a thickness of 50 µm or more.   The surface treatment method for an aluminum extruding die according to any one of claims 1 to 6, wherein in the second nitriding treatment step, a heating temperature is 500 to 580 ° C.   The method for surface treatment of an aluminum extrusion die according to any one of claims 1 to 7, wherein in the second nitriding treatment step, the holding time is 1 to 5 hours. The aluminum extrusion process according to any one of claims 1 to 8, wherein in the second nitriding treatment step, the nitriding gas is NH ₃ gas or a mixed gas containing NH ₃ gas and non-nitriding gas. Die surface treatment method.   The surface treatment method for an aluminum extruding die according to any one of claims 1 to 9, wherein in the second nitriding treatment step, the compound layer is formed to a thickness of 2 to 10 µm.   The compound layer formed in the second nitriding treatment step is subjected to a renitriding treatment including the second nitriding treatment step on the die when the thickness is reduced or disappears by extrusion. A method for surface treatment of an aluminum extrusion die according to claim 1.   The surface treatment method for an aluminum extruding die according to any one of claims 1 to 11, wherein the die is for extruding a 3000 series aluminum alloy.   The surface treatment method for a die for aluminum extrusion according to any one of claims 1 to 12, wherein the material of the die body is hot tool steel that has been subjected to quenching and tempering treatment.   An aluminum extruding die processed by the surface treatment method for an aluminum extruding die according to any one of claims 1 to 13. A method for producing an aluminum extruding die, comprising the surface treatment method for an aluminum extruding die according to any one of claims 1 to 13.