JP4398831B2 - Surface treatment method of metal material made of titanium or titanium alloy - Google Patents

Description

  The present invention relates to a surface treatment method in which a shot material is sprayed onto the surface of a metal material made of titanium or a titanium alloy to harden it by refining crystal grains in a surface layer portion, and another layer is formed. It is.

  Conventionally, in order to improve the fatigue strength of metal products such as springs, cast steel products, and forged products, a surface hardening method for hardening the surface layer portion of the metal products has been used. Such surface hardening methods include, for example, shot peening treatment in which a part or all of the surface of a steel product or non-ferrous metal product that has been subjected to treatment such as quenching and tempering, or martensitic transformation. Surface heat treatment method for causing martensitic transformation by raising the surface temperature of the steel product to the A3 transformation point or higher by performing fine particle shot peening on the steel product containing carbon of a level capable of producing carbon Etc. are known.

  In addition, as a surface hardening treatment method for increasing the hardness of the surface of a metal product, Patent Document 1 discloses that the formation of a hardened structure in the surface layer portion of a metal product is shot under a predetermined condition regardless of martensitic transformation. A method is disclosed that is realized by nanocrystallizing metallographic crystal grains in the surface layer portion by shot peening treatment in which a material is injected. According to this method, it is possible to obtain a metal product having an extremely high surface hardness because the range of material selection is wider than that of the surface processing heat treatment method described above and the surface layer portion is nanocrystallized.

JP 2004-124227 A

  However, when a method of nano-crystallizing the metal structure of the surface layer by performing shot peening treatment on the surface of the metal product, the surface hardness of the metal product becomes extremely high. It was a problem because noise at a frequency might occur. That is, for example, when the surface microstructure of the engine valve for automobiles is nanocrystallized by performing shot peening treatment, the engine valve has the advantage that the durability against friction and impact becomes very high. Although it is obtained, when the engine valve is driven to come into contact with surrounding parts, high frequency noise may be generated from the engine valve, which is a problem.

  Therefore, the present invention not only generates extremely high surface hardness by nanocrystallizing the metal structure of the surface layer portion, but also makes it difficult to generate high-frequency noise even when applied to driving parts such as engine valves. An object of the present invention is to provide a surface treatment method for a metal material made of titanium or a titanium alloy.

The inventors have made the surface layer part by subjecting the surface of the metal material made of titanium or titanium alloy to nano-crystallization of the metal structure of the surface layer part by performing shot peening treatment, and then subjecting the metal material to heat treatment. As a result, it was found that a β layer was formed as an intermediate layer between the nanocrystal layer formed on the substrate and the base material, and that this β layer was particularly effective in reducing high-frequency noise. completed.
(1) provide Reinforced shot peening treatment to the surface of titanium or a metal material made of titanium alloy, the surface portion of the metal structure and shot peening step of nanocrystallisation, the shot peening step and simultaneously with or subsequent to the metal material a heating step of heating, was closed,
In the heating step, by heating the metal material, a β layer is formed between the nanocrystal layer formed on the surface of the metal material and the base material in the shot peening step,
The shot material used in the shot peening process is a shot material containing a β-stabilizing element, and in the shot peening process, while allowing the β-stabilizing element contained in the shot material to enter the inside of the metal structure, A surface treatment method for a metal material, characterized by nanocrystallizing a metal structure of a surface layer portion of the metal material.
(2) The surface treatment method for a metal material according to (1), wherein the metal material is an α + β type titanium alloy.
(3) A surface treatment method for a metal material according to (1) or (2),
In the heating step, the metal material is heated at a temperature of 700 ° C. or higher.
(4) The surface treatment method for a metal material according to any one of (1) to (3),
In the heating step, the metal material surface treatment method is characterized in that the metal material is heated at a temperature of 700 ° C. or more for 0.5 hour or more.
( 5 ) A metal material comprising titanium or a titanium alloy surface-treated by the surface treatment method for a metal material according to any one of (1) to ( 4 ).
( 6 ) An engine valve made of titanium or a titanium alloy surface-treated by the surface treatment method for a metal material according to any one of (1) to ( 4 ).
(7) a metal material composed of titanium or titanium alloy, wherein a β layer is formed between the grain size was formed with the following nanocrystal layer and the substrate 100nm on the surface.

  According to the present invention, the metal structure of the surface layer is nanocrystallized and the surface hardness is extremely high, and even when applied to driving parts such as engine valves, high-frequency noise is unlikely to occur. It is possible to provide a surface treatment method for a metal material made of titanium or a titanium alloy.

Hereinafter, embodiments of the present invention will be described in detail.
As the metal material to be surface-treated by the method of the present invention, a metal material made of titanium or a titanium alloy is used. The term “titanium or titanium alloy” as used herein means that it may be a metal of titanium alone or an alloy of titanium and another metal element. Therefore, as the metal material to be surface-treated in the present invention, not only a titanium simple metal (for example, JIS type 1 to 4 types of pure titanium metal) but also, for example, Ti-6Al-4V, Ti-6Al-2Sn-4Zr- Various titanium alloys such as 2Mo, Ti-3Al-2.5V, Ti-6Al-2Sn-4Zr-6Mo can be used. Titanium alloys can be classified into three alloy systems, α-type, β-type, and α + β-type. However, as a metal material to be surface-treated in the present invention, α + β has a good balance between strength and toughness and excellent corrosion resistance. A titanium alloy of a type can be particularly preferably employed. Examples of the α + β type titanium alloy include the various titanium alloys described above.

  The surface treatment method for a metal material according to the present invention is roughly constituted by the following two steps. The first step is a “shot peening step” in which shot peening is performed on the surface of a metal material made of titanium or a titanium alloy. Through this shot peening process, a nanocrystal layer having a metal structure crystal grain size of 100 nm or less is formed on the surface layer of the metal material. This nanocrystalline layer is a thermally stable and extremely hard layer. The second step is a “heating step” in which the metal material is heated simultaneously with or after the shot peening step. By this heating step, a β layer as an intermediate layer is formed between the nanocrystal layer and the base material in the surface layer portion. This β layer is a layer containing a relatively larger amount of β-type titanium alloy than the metal structure before heating. Since the β-type titanium alloy has a smaller Young's modulus than the α-type and α + β-type titanium alloys, it is possible to reduce the restitution coefficient of the metal material and prevent the generation of high-frequency noise.

FIG. 1 is a schematic explanatory view of a shot peening process and a heating process.
As shown in FIG. 1A, in the shot peening process, a shot material 12 made of metal powder or the like is sprayed onto the surface 10 of the metal material using, for example, an air jet type shot peening apparatus. Thereby, a large deformation can be caused in the surface layer portion of the metal material at a high strain rate. When the dislocation density of the surface layer portion of the metal material reaches a critical level, the crystal structure of the surface layer portion is recrystallized (recrystallized during strong processing without increasing the temperature), and the crystal grain size is 100 nm or less. At the same time, the hardness changes to, for example, about twice as much as Vickers hardness. By this shot peening process, as shown in FIG. 1B, a nanocrystal layer 14 having a nanocrystal grain structure of 100 nm or less is formed on the surface layer portion of the metal material.

  Various conditions in the shot peening process are not particularly limited as long as the surface layer portion of the metal material can be nanocrystallized. For example, the injection speed of the shot material can be set to 100 m / second or more. The diameter of the shot material can be set within a range of 40 μm to 200 μm (more preferably 40 μm to 100 μm). The time for performing the shot peening process can be set within a range of 30 to 60 seconds. As the material of the shot material, for example, cast iron, steel or the like can be adopted. By setting various conditions in the shot peening process within such a range, the metal structure in the surface layer portion can be more reliably nanocrystallized and hardened. It is also possible to adjust the thickness and hardness of the nanocrystal layer 14 by adjusting various conditions in the shot peening process (for example, the time of the shot peening process).

  As shown in FIG. 1B, a nanocrystal layer 14 is formed on the surface layer portion of the metal material by performing shot peening treatment on the surface of the metal material. This nanocrystal layer 14 is a layer having a crystal grain size of a metal structure of 100 nm or less, and is a layer newly formed on the surface layer portion of the metal material. In this nanocrystal layer, the crystal grain size of the metal structure is “100 nm or less” when, for example, the state of the metal structure is observed with an electron microscope, a region of 50% or more of the metal structure (preferably 80% In the above region), it means that the crystal grain size in the minor axis direction is distributed within a range of approximately 100 nm or less.

  In the metal material surface treatment method of the present invention, the heating step is performed simultaneously with or after the above-described shot peening step. More preferably, the heating step is performed after the nanocrystal layer 14 is formed by the shot peening step. In this heating step, as shown in FIG. 1C, the metal material after the shot peening treatment is heated under predetermined conditions, whereby the base material 16 and the nanocrystal layer 14 of the metal material are heated. The β layer 18 is formed as an intermediate layer. The mechanism by which the β layer 18 is formed will be described. In the nanocrystal region, grain growth due to heating is extremely slow compared to a normal work-hardened region and a region on the base material 16 side. In contrast, in the region on the base material 16 side made of titanium or a titanium alloy, the metal structure starts to be transformed into a β-type titanium alloy by recrystallization by heating (annealing). As a result, a β layer 18 is newly formed by recrystallization by heating between the nanocrystal layer 14 formed on the surface layer portion and the base material 16, and a fine crystal grain size is maintained. The boundary between the nanocrystal layer 14 and the β layer 18 is clearly recognized by an electron microscope or the like.

  As described above, the heat treatment is performed after the shot peening treatment is performed on the surface of the metal material made of titanium or the titanium alloy, so that the heating is performed between the nanocrystal layer 14 formed on the surface layer portion and the base material 16. A β layer 18, which is a layer containing more β-type titanium alloy than before, is newly formed. Note that the “β layer” newly formed in the heating process means that all regions of the β layer are made of β-type titanium alloy when, for example, the metal structure of the β layer is observed with an electron microscope. Does not mean. The “β layer” referred to in the present invention includes more β-type titanium alloy than before being heated in the heating step by promoting transformation to a β-type titanium alloy by heating at a predetermined temperature or higher. It means the layer that became. Therefore, the β-type metal structure newly formed in the heating process contains a relatively large amount of β-type titanium alloy. The titanium alloy may be slightly contained.

  In the metal material surface treatment method according to the present invention, the heating temperature of the metal material in the heating step is preferably 700 ° C. or higher, more preferably 800 ° C. or higher, and further preferably 850 ° C. or higher. By heating the metal material in this temperature range, the β layer can be more reliably formed between the nanocrystal layer formed on the surface layer portion and the substrate. The heating time of the metal material in the heating step is preferably 0.5 hours or longer while maintaining a temperature of 700 ° C. or higher, more preferably 1 hour or longer while maintaining a temperature of 700 ° C. or higher. . The heating means in the heating process is not particularly limited, but a metal material made of titanium or a titanium alloy can be heated by, for example, an electrothermal heater.

  By forming a nanocrystal layer having a high hardness on the surface portion of the metal material, the impact resistance and wear resistance of the metal material can be dramatically improved. For example, after manufacturing an automobile engine part (for example, an engine valve) using a metal material made of an α + β type titanium alloy, the surface of the engine part is subjected to the surface treatment method of the present invention. The wear resistance, impact resistance, etc. can be dramatically improved.

In the present invention, the shot material used in the shot peening process is preferably a shot material containing a β-stabilizing element. For example, it is preferable to use a shot material containing a β-stabilizing element such as Fe, Mo, Nb, V, Ta, Cr, Ru.
By using a shot material containing these β-stabilizing elements as a shot material in the shot peening process, the β-stabilizing element contained in the shot material can be penetrated into the metal structure. Thereby, in the heating process performed after the shot peening process, it is possible to more stably form a β layer as an intermediate layer between the nanocrystal layer formed on the surface layer portion and the base material.

  On the contrary, by using a shot material containing an α-stabilizing element as a shot material in the shot peening process, the α-stabilizing element contained in the shot material can be penetrated into the metal structure. Examples of the α stabilizing element include Al, C, O, N, Sn and the like. Thereafter, the α layer can be formed by heat treatment. That is, by selectively using a shot material containing an α-stabilizing element and a shot material containing a β-stabilizing element, it is possible to arbitrarily select the type of element that enters the metal material.

  According to the surface treatment method for a metal material according to the present invention, it is possible to form a nanocrystal layer with extremely high hardness on the surface layer portion of a metal material made of titanium or a titanium alloy. In addition, a β layer having a low hardness can be formed between the nanocrystal layer and the base material. Therefore, since the hardness of the surface layer portion is high, it has excellent wear resistance and impact resistance, and it is difficult to generate high-frequency noise by reducing the coefficient of restitution when the β layer with low hardness collides with other members. A metal material can be obtained.

The surface treatment method for a metal material according to the present invention can be particularly preferably applied to a component for which high frequency noise is desired to be reduced. In particular, by applying the surface treatment method of the present invention to an automobile engine valve made of titanium alloy, it becomes possible to reduce high-frequency noise that is generated when the engine valve is driven.
The surface treatment method of the present invention can also be applied to other engine component surface treatment methods, for example, a surface treatment method for shafts, cams, housings, etc. used in engines. It is.

  In addition, the surface treatment method for a metal material of the present invention has applicability as a surface treatment method for other parts other than automobile parts. For example, by applying the surface treatment method of the present invention to the head of a golf club, the restitution coefficient when the ball hits the face surface is adjusted by utilizing the fact that the restitution coefficient is reduced by the β layer. Is also possible.

[Shot peening process]
FIG. 2 is a schematic configuration diagram of the shot peening apparatus. In this embodiment, as shown in FIG. 2, the shot material is shot by using an air jet type shot peening apparatus 20 on the surface of a test piece 22 (metal material) of φ13 mm × L5 mm, Ti-6Al-4V alloy. Sprayed. As the shot material, a shot material made of 0.05 mm and made of Fe-1.0C (cast iron) was used. The injection speed of the shot material was set to 190 m / s. The coverage (C) indicating the degree of shot peening processing was set to 6000% (injection time 60 seconds). The coverage (C) is defined as C = B / A × 100% from the total processing area (A) of the test piece 22 and the total area (B) of the indentation generated by shot peening. In this embodiment, in order to more easily express the number of shot material collisions on the surface of the test piece, the coverage is expressed in proportion to the injection time with reference to the injection time required for processing with 50% coverage. . For example, when the time required for shot peening with 50% coverage is 0.5 seconds, the case where the injection time is 10 seconds is expressed as 1000% coverage.

[Heating process]
The test piece 22 subjected to the shot peening process by the shot peening apparatus 20 was subjected to a heating process by an electrothermal heater. The heating temperature at this time was set to 850 ° C., and the heating time was set to 1 hour.

[Structural observation by electron microscope and composition analysis by EDX]
The state near the surface of the test piece 22 subjected to the shot peening treatment and the heat treatment was observed with an electron microscope, and elemental analysis of the test piece 22 was performed with an energy dispersive X-ray microanalyzer (EDX). The result is shown in FIG.
In addition, as a comparative example, the state in the vicinity of the surface of the test piece 22 that has been subjected only to the shot peening treatment and before the heat treatment is observed with an electron microscope, and Elemental analysis was performed. The result is shown in FIG.

As shown in FIG. 3, in the vicinity of the surface of the test piece 22 subjected to the shot peening treatment and the heat treatment, between the nanocrystal layer (circled numbers 1 and 2) and the base material (circled numbers 5 and 6). In addition, it was confirmed that a β layer (circled numbers 3 and 4) of a β-type titanium alloy was newly formed. Moreover, it has confirmed that the boundary line of each layer (a nanocrystal layer, (beta) layer, a base material layer) appeared clearly. In addition, the results of elemental analysis by EDX confirmed that the layer formed between the nanocrystal layer and the base material is a β-type titanium alloy layer (circled number 3, 4).
On the other hand, as shown in FIG. 4, in the vicinity of the surface of the test piece 22 subjected to only shot peening treatment, a nanocrystal layer (circled number 1) and a base material (circled numbers 2 to 9) In the meantime, no β layer was formed.

[Restitution coefficient measurement test]
In order to confirm that the restitution coefficient is reduced by forming the β layer between the nanocrystal layer and the base material, a restitution coefficient measurement test was performed in the following manner.
First, as shown in Table 1 below, five types of test pieces with different surface treatment methods were prepared. The material and shape of the test piece are the same as those of the test piece 22 of the first embodiment.

  In Table 1, test piece number 1 is a test piece whose surface has not been subjected to any treatment. For this test piece number 1, three types of test pieces having different surface roughnesses were prepared. Test piece number 2 is a test piece subjected to only shot peening (coverage 6000%). Test piece number 3 is a test piece having a surface subjected to buffing after shot peening (coverage 6000%). Test piece number 4 is a test piece that has been subjected to heat treatment (820 ° C. for 1 hour) after being subjected to shot peening treatment (coverage 6000%). Test piece number 5 is a test piece subjected to only heat treatment (820 ° C. for 1 hour).

  After measuring the roughness ([Rz]) of the surface (upper surface) of the five types of test pieces in Table 1, the five types of test pieces in Table 1 were placed horizontally, and 1 m A ball of SUSJ2 (high carbon chromium bearing steel) was dropped from the height. The height of the first rebound ([m]) of this ball was measured as the coefficient of restitution (= h1 / h0: h0 is the drop height, and h1 is the rebound height). The result is shown in FIG.

  As can be seen from the results shown in FIG. 5, for the test piece (test piece number 4) subjected to shot peening treatment and heat treatment on the surface, the test piece (test piece number 2) subjected to only shot peening treatment and heat treatment It was confirmed that the coefficient of restitution is reduced to about ½ or less when compared with the test piece (test piece number 5) subjected only to the above. That is, from the result of Example 2, the metal material surface treatment method according to the present invention can reduce the coefficient of restitution of the metal material (in other words, reduce the noise at the time of collision with other members). It has been demonstrated that

It is typical explanatory drawing about a shot peening process and a heating process. It is a schematic block diagram of a shot peening apparatus. It is an electron micrograph of the state of the surface vicinity of the test piece which performed the shot peening process and the heat processing. Moreover, it is the result of the elemental analysis of this test piece by EDX. It is an electron micrograph of the state of the surface vicinity of the test piece before giving only a shot peening process and heat-processing. Moreover, it is the result of the elemental analysis of this test piece by EDX. It is a graph which shows the result of a restitution coefficient measurement test.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Surface 12 Shot material 14 Nanocrystal layer 16 Base material 18 Beta layer 20 Shot peening apparatus 22 Test piece

Claims (7)

Provide Reinforced shot peening treatment to the surface of the metal material composed of titanium or titanium alloy is heated and shot peening step of a surface layer portion of the metal structure to nanocrystallization, the shot peening step and simultaneously with or subsequent to the metal material heated and the step, possess,
In the heating step, by heating the metal material, a β layer is formed between the nanocrystal layer formed on the surface of the metal material and the base material in the shot peening step,
The shot material used in the shot peening process is a shot material containing a β-stabilizing element, and in the shot peening process, while allowing the β-stabilizing element contained in the shot material to enter the inside of the metal structure, A surface treatment method for a metal material, characterized by nanocrystallizing a metal structure of a surface layer portion of the metal material.   The metal material surface treatment method according to claim 1, wherein the metal material is an α + β type titanium alloy. A surface treatment method for a metal material according to claim 1 or 2,
In the heating step, the metal material is heated at a temperature of 700 ° C. or higher. It is the surface treatment method of the metal material of any one of Claims 1-3,
In the heating step, the metal material surface treatment method is characterized in that the metal material is heated at a temperature of 700 ° C. or higher for 0.5 hour or longer.   A metal material made of titanium or a titanium alloy surface-treated by the surface treatment method for a metal material according to any one of claims 1 to 4.   An engine valve made of titanium or a titanium alloy surface-treated by the metal material surface treatment method according to any one of claims 1 to 4. Metallic material composed of titanium or titanium alloy, wherein a β layer is formed between the grain size was formed with the following nanocrystal layer and the substrate 100nm on the surface.