Titanium alloy member and manufacturing method thereof

Abstract

To provide a titanium alloy member excellent in fatigue characteristic without applying water cooling, when processed in a β single phase region and a manufacturing method thereof.SOLUTION: A titanium alloy member having a longitudinal direction, in which a metal structure in a surface layer portion of a cross section perpendicular to the longitudinal direction is an acicular structure containing α colonies having a plurality of α phase crystal grains, and an average diameter of the α colonies is 10 μm or more and less than 300 μm, and the α phase has a hcp structure, and, in the region made of the α colonies, an area rate of a region of the α phase where an angle between a c axis and the longitudinal direction in the hcp structure is 0 to 45° is 30% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a titanium alloy member and a method for manufacturing the same. [Background technology]

[0002] Driving members of automobiles, such as engine valves and connecting rods, are subjected to repeated loads during use, and are therefore required to have high strength and excellent fatigue properties.

[0003] The strength of α+β titanium alloys, which are also used in drive components, can be increased by controlling manufacturing conditions such as processing and heat treatment. However, when attempting to improve fatigue properties by controlling manufacturing conditions, the alloy suffers from the problem of reduced productivity. Specifically, in order to control the metal structure to an equiaxed structure with good fatigue properties, hot working in the α+β two-phase region is necessary, which increases deformation resistance. As a result, productivity is reduced.

[0004] Therefore, Non-Patent Document 1 and Patent Documents 1 and 2 investigate methods for improving the fatigue properties of alloys even when hot working or heat treatment is performed in the β single-phase region rather than the α+β two-phase region. Normally, when hot working or heat treatment is performed in the β single-phase region, a coarse acicular structure is formed instead of an equiaxed structure, resulting in a decrease in fatigue properties.

[0005] Non-Patent Document 1 discloses a titanium alloy member in which the acicular structure is made finer and fatigue properties are improved by water cooling after solution treatment in the β single-phase region. Patent Document 1 also discloses a titanium alloy member in which the fatigue properties are improved by heating at a temperature above the β transformation point and then rapidly cooling from a temperature above the Ms point to control the structure to a fine martensite structure. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2-213453 [Patent Document 2] Japanese Patent Application Publication No. 6-184683 [Non-patent literature]

[0007] [Non-Patent Document 1] G. Lutjering, A. Gysler: Titanium Science and Technology, 4(1985), p.2065 Summary of the Invention [Problem to be solved by the invention]

[0008] In the above-mentioned Non-Patent Document 1 and Patent Document 1, water cooling is required when manufacturing a titanium alloy part. However, water cooling generates residual stress, which causes deformation such as bending. As a result, there is a problem that the process load for subsequent cutting and polishing increases.

[0009] Patent Document 2 discloses a titanium alloy member in which the acicular structure is controlled without water cooling, but there is room for further improvement in terms of fatigue properties and alloy costs.

[0010] The present invention aims to solve the above problems and to provide a titanium alloy part that has excellent fatigue properties when processed in the β single phase region without water cooling, and a method for manufacturing the same. [Means for solving the problem]

[0011] The present invention has been made to solve the above-mentioned problems, and is summarized as the following titanium alloy member and a method for manufacturing the same.

[0012] (1) A titanium alloy component having a longitudinal direction, The metal structure in the surface layer portion of the cross section perpendicular to the longitudinal direction is an acicular structure including α colonies having a plurality of α phase crystal grains; the average diameter of the α colonies is 10 μm or more and less than 300 μm; The α phase has an hcp structure, and in the region consisting of the α colonies, the area ratio of the α phase region in which the angle between the c axis of the hcp structure and the longitudinal direction is 0 to 45° is 30% or less.

[0013] (2) Chemical composition, in mass %, Al: 4.4-5.5% Fe: 1.4-2.5% Mo: 1.5-5.5% O: 0.05-0.25%, The titanium alloy member according to (1) above, wherein the balance is Ti and impurities.

[0014] (3) Chemical composition, in mass %, Al: 5.50~6.75% V: 3.5-4.5%, Fe: 0.05 to 0.40% O: 0.05-0.25%, The titanium alloy member according to (1) above, wherein the balance is Ti and impurities.

[0015] (4) Chemical composition, in mass %, Al: 5.50~6.50%, Sn: 1.75~2.25% Zr: 3.5-4.5% Mo: 1.8-2.2% Si: 0.10% or less, Fe: 0.02 to 0.25% O: 0.02 to 0.15%, The titanium alloy member according to (1) above, wherein the balance is Ti and impurities.

[0016] (5) The titanium alloy member according to any one of (1) to (4) above, which is an engine valve.

[0017] (6) The titanium alloy part according to any one of (1) to (4) above, which is a connecting rod.

[0018] (7) A method for producing a titanium alloy member according to any one of (1) to (6) above, A heat treatment process is performed in which the material is heated to a temperature of (Tβ+20)°C or higher and 1240°C or lower, and then air-cooled, so that the total heating time at (Tβ+20)°C or higher is 10 minutes or less. A method for producing a titanium alloy member, wherein in the heat treatment step, the temperature rise rate in the temperature range of (Tβ-10) to (Tβ+20)°C is 3°C / s or more.

[0019] (8) The method for producing a titanium alloy part according to (7) above, further comprising a cutting step of performing cutting before the heat treatment step.

[0020] (9) A method for producing a titanium alloy part as described in (8) above, further comprising a forming process step in which, prior to the cutting process step, the part is heated to a temperature of (Tβ + 70)°C or higher but lower than (Tβ + 270)°C, and processed at a temperature range of (Tβ + 50)°C or higher with an area reduction rate of 10% or higher, and then processed at a temperature range of (Tβ - 100)°C or higher but lower than (Tβ + 50)°C with an area reduction rate of 30% or higher but lower than 85%. [Effects of the Invention]

[0021] According to the present invention, when processed in the β single phase region, a titanium alloy part having excellent fatigue properties can be obtained without water cooling. [Brief explanation of the drawings]

[0022] [Figure 1] Figure 1 is a micrograph showing the acicular structure. [Figure 2] Figure 2 is a microstructure photograph showing the equiaxed structure. [Figure 3] FIG. 3 is a tissue photograph showing an α colony. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present inventors have investigated the acicular structure formed when processing is performed in the β single phase region, and have obtained the following findings (a) to (c) regarding the fatigue properties of titanium alloy members.

[0024] (a) The coarse α phase in the acicular structure becomes the starting point of fatigue fracture. As a result, fatigue strength decreases. Therefore, the inventors focused on this coarse α phase. They then revealed that this coarse α phase is an aggregate (hereinafter referred to as "α colony") of acicular α phase crystal grains (hereinafter referred to as "α phase grains") with the same crystal orientation.

[0025] (b) Refining the α colony size is effective in improving fatigue properties. The inventors also found that the crystal grains that make up the α colony, which is the starting point of fracture, have a specific crystal orientation. Therefore, controlling the crystal orientation of the crystal grains that make up the α colony and suppressing the formation of α colonies that can be the starting point of fracture is also effective.

[0026] (c) In light of the above, it is preferable to control the manufacturing conditions in order to control the size of the α colonies and the crystal orientation of the crystal grains that make up the α colonies. In order to achieve the above-mentioned structural control without water cooling, it is effective to perform heat treatment after processing in the β single-phase region at a faster heating rate than usual, followed by air cooling.

[0027] An embodiment of the present invention has been made based on the above findings. Each requirement of this embodiment will be described in detail below.

[0028] 1.Metal structure of titanium alloy components 1-1. Spicular tissue and alpha colonies Since fatigue strength is affected by the metal structure near the surface, the size of the α colonies in the surface layer of the titanium alloy part of this embodiment is controlled.

[0029] The titanium alloy part of this embodiment is a titanium alloy material having a longitudinal direction, and the metal structure in the surface layer portion of the cross section perpendicular to the longitudinal direction is controlled. Specifically, the metal structure in the surface layer portion of the cross section perpendicular to the longitudinal direction is made to have an acicular structure.

[0030] Here, the surface layer portion refers to a region extending from the surface of the titanium alloy component to a depth of 1 mm in a cross section perpendicular to the longitudinal direction. Note that the structure of the surface layer portion only needs to be observed and measured within the measurement field of view described below, and it is not necessary to observe and measure the entire range.

[0031] The acicular structure is a structure formed when cooling from the β single-phase region, and is a microstructure containing plate-like or needle-like α-phase grains (hereinafter also referred to as "acicular α grains"), as shown in Figure 1. Figure 2 shows the equiaxed structure formed when cooling from the α + β two-phase region. It can be seen that the shape of the crystal grains in the acicular structure is different from that in the equiaxed structure.

[0032] In the titanium alloy part of this embodiment, the acicular structure includes an α colony. An α colony is a structure having a plurality of α phase grains, specifically, acicular α grains. Figure 3 shows a structural photograph of an α colony. The area surrounded by the dashed line in Figure 3 corresponds to the α colony. An α colony is an aggregate of α phase grains formed when the β phase transforms to the α phase, and is formed within the prior β phase grains. Furthermore, the acicular α grains within an α colony have the same crystal orientation and orientation. Specifically, the misorientation between the α grains within an α colony is within ±5°.

[0033] In addition to the alpha colonies, the acicular structure also contains grain boundary alpha grains that precipitate at the beta grain boundaries. When air-cooled, the area ratio of the grain boundary alpha grains is small, less than 5%, and the width of the grain boundary alpha grains is 5 μm or less, so they are not coarse. For this reason, there is no significant impact on fatigue properties, so there is no need to particularly control the grain boundary alpha grains.

[0034] 1-2.Average diameter of α colonies In the titanium alloy structural member of this embodiment, the average diameter of the α colonies is 10 μm or more and less than 300 μm. When a titanium alloy structural member is manufactured without water cooling, the average diameter of the α colonies is 10 μm or more. Therefore, the average diameter of the α colonies is 10 μm or more. On the other hand, if the average diameter of the α colonies exceeds 300 μm, the fatigue strength decreases. Therefore, the average diameter of the α colonies is 300 μm or less. The average diameter of the α colonies is preferably 200 μm or less, more preferably 150 μm or less, and even more preferably 20 μm or less.

[0035] 1-3. Low fatigue strength area ratio in α colonies The α phase is known to have an hcp structure. An α colony is an aggregate of multiple α grains, but they have the same crystal orientation and orientation, and the c-axis direction in the hcp structure is also the same. The α colonies that are the starting points for fatigue fracture have a crystal orientation in which the c-axis is at an angle of 0 to 45° with respect to the stress axis (the longitudinal direction in an alloy member). Therefore, it is necessary to reduce α colonies with this orientation, specifically, an orientation in which the angle between the c-axis in the hcp structure and the longitudinal direction of the alloy material is 0 to 45° (hereinafter referred to as "low fatigue strength orientation"). In other words, it is preferable to reduce the α phase that constitutes α colonies with low fatigue strength orientation.

[0036] Therefore, the area ratio of the α-phase region having a low fatigue strength orientation in the region consisting of α colonies (hereinafter referred to as the "low fatigue strength area ratio") is set to 30% or less. It is preferable to reduce the low fatigue strength area ratio as much as possible.

[0037] The acicular structure, the average diameter of α colonies, and the area ratio of low fatigue strength can be measured by the following procedure. First, a test specimen is taken from a cross section perpendicular to the longitudinal direction of the titanium alloy component so as to include the surface layer. Next, the obtained test specimen is observed with a scanning electron microscope (SEM), with the cross section including the surface layer as the observation surface. For the observation, five or more locations in a rectangular region measuring 0.8 mm in length and 0.8 mm in width can be observed at a magnification of 50x, from the surface to a depth of 1 mm.

[0038] Next, measurements are taken using an EBSD (Electron Backscatter Diffraction) device attached to the SEM, and the measurement results are analyzed using OIM (crystal orientation analysis software manufactured by TSL Solutions, Inc.). In the EBSD analysis, the angle difference between the crystal orientations of adjacent α colonies is set to 5° or less to identify the α colonies. The circle-equivalent diameter of the identified α colonies is calculated and used as the average diameter.

[0039] The area corresponding to the α colony is identified from all the observation areas, and within this area, the α phase area with low fatigue strength orientation is identified and its area ratio (%) is calculated. The EBSD measurement conditions are a measurement interval of 2.0 μm and an acceleration voltage of 15 kV.

[0040] 2.Chemical composition In the titanium alloy part of this embodiment, the type, chemical composition, etc. of the titanium alloy are not particularly limited as long as it is an α+β type titanium alloy. Note that an α+β type titanium alloy is a titanium alloy having a metal structure in which the α phase is the main phase and the β phase is the secondary phase at 25°C. Below, as an example, a preferred chemical composition is described.

[0041] For example, the chemical composition of the titanium alloy member is preferably Al: 4.4-5.5%, Fe: 1.4-2.5%, Mo: 1.5-5.5%, O: 0.05-0.25%, the balance: Ti and impurities. For example, like AMS4928, the chemical composition of the titanium alloy member is preferably Al: 5.50-6.75%, V: 3.5-4.5%, Fe: 0.05-0.40%, O: 0.05-0.25%, the balance: Ti and impurities. For example, the chemical composition of the titanium alloy member is preferably Al: 5.50-6.50%, Sn: 1.75-2.25%, Zr: 3.5-4.5%, Mo: 1.8-2.2%, Si: 0.10% or less, Fe: 0.02-0.25%, O: 0.02-0.15%, and the balance: Ti and impurities, as in AMS 4976. Note that in the contents of each of the above elements, "%" means "% by mass."

[0042] Furthermore, "impurities" refer to components that are mixed in due to various factors in the raw materials and manufacturing process when industrially manufacturing a titanium alloy member, and are acceptable within a range that does not adversely affect the present embodiment. Examples of impurities that may be contained other than those mentioned above include N, C, H, etc. In this case, it is preferable that N is 0.08% or less, C is 0.08% or less, and H is 0.015% or less. Furthermore, Ni, Cr, Mn, Nb, and Cu may be contained as impurities. The content of each of these elements is preferably 0.1% or less, and the total content of these elements is preferably less than 0.5%.

[0043] In order to increase the fatigue strength, it is better for the strength itself to be high, and a titanium alloy having a tensile strength of 950 MPa or more is preferable.

[0044] 3.Applications The titanium alloy part of this embodiment is preferably a driving part for an automobile, etc. The driving part is, for example, an engine valve, a connecting rod, etc.

[0045] 4. Manufacturing method A preferred method for producing the titanium alloy part of this embodiment will now be described. The titanium alloy part of this embodiment can be stably produced by the following production method.

[0046] A titanium alloy ingot or titanium alloy bar is prepared as a processing material. The dimensions of this processing material are not particularly limited, as long as they are sized to be able to be handled in the forming process described below. The chemical composition and tensile strength of the processing material are preferably within the ranges described above.

[0047] 4-1. Molding process The above-mentioned material is preferably hot-worked and formed into the desired shape. In the hot working, it is preferable to first heat the material to a temperature of (Tβ + 70)°C or higher and lower than (Tβ + 270)°C. After the heating, it is preferable to carry out working with an area reduction of 10% or higher in a temperature range of (Tβ + 50)°C or higher. After the working, it is preferable to carry out working with an area reduction of 30% or higher and lower than 85% in a temperature range of (Tβ - 100)°C or higher and lower than (Tβ + 50)°C. This forming process is a one-heat process, i.e., a process carried out in a single heating, and the temperature is controlled by the surface temperature (object temperature) of the material.

[0048] It is preferable to heat the workpiece to a temperature of (Tβ + 70)°C or higher but lower than (Tβ + 270)°C. Here, if the heating temperature during hot working is lower than (Tβ + 70)°C, if there are areas in the heating furnace where the temperature is uneven, or if the size of the workpiece itself is uneven, it is difficult for the entire workpiece to reach a temperature of Tβ°C or higher. For this reason, it is preferable that the heating temperature during hot working be higher than (Tβ + 70)°C. On the other hand, if the heating temperature during hot working is higher than (Tβ + 270)°C, the surface layer of the workpiece is likely to oxidize. In addition, the metal structure of the workpiece is likely to become coarse. For this reason, it is preferable that the heating temperature during hot working be lower than (Tβ + 270)°C.

[0049] Furthermore, after heating to the above-mentioned temperature, it is preferable to carry out working with an area reduction rate of 10% or more in a temperature range of (Tβ + 50)°C or higher. If the area reduction rate in a temperature range of (Tβ + 50)°C or higher during working is less than 10%, it becomes difficult to obtain the effect of recrystallization to refine the β phase grains. As a result, the average diameter of the α colonies also tends to increase. For this reason, it is preferable that the area reduction rate during working in a temperature range of (Tβ + 50)°C or higher be 10% or more.

[0050] After the above processing, it is preferable to further perform processing at a temperature range of (Tβ-100)°C or higher but lower than (Tβ+50)°C with an area reduction rate of 30% or higher but lower than 85%. If the area reduction rate during processing at a temperature range of (Tβ-100)°C or higher but lower than (Tβ+50)°C is less than 30%, the α colonies tend to become coarse. For this reason, it is preferable that the area reduction rate during processing at a temperature range of (Tβ-100)°C or higher but lower than (Tβ+50)°C be 30% or higher. On the other hand, if the area reduction rate during processing at a temperature range of (Tβ-100)°C or higher but lower than (Tβ+50)°C is 85% or higher, the α phase tends to accumulate in a specific orientation. As a result, in the heat treatment process described below, the β phase grains and α colonies coarsen, and the β phase grains and α colonies accumulate in a specific crystal orientation, which tends to reduce fatigue strength. For this reason, it is preferable that the area reduction rate during working in the temperature range of (Tβ-100)°C or higher and lower than (Tβ+50)°C is less than 85%.

[0051] 4-2.Cutting process The intermediate material that has undergone the above-mentioned forming process may be further subjected to cutting, if necessary. For example, if defects occur on the surface due to hot working, the defects can be removed by cutting. Cutting may also be performed to form a desired shape. Note that the titanium alloy component may be processed into a desired shape by only cutting, without undergoing the forming process. The cutting method is not particularly limited; conventional machining may be performed. Hereinafter, the intermediate material that has undergone the forming process and / or cutting process will be referred to as a processed material.

[0052] 4-3.Heat treatment process The processed material is subjected to heat treatment. In this heat treatment, the heat treatment temperature is set to (Tβ+20)°C or higher and 1240°C or lower, and the material is heated so that the total heating time at (Tβ+20)°C or higher is 10 minutes or less, followed by air cooling. In addition, in the heat treatment, the temperature rise rate in the temperature range of (Tβ-10) to (Tβ+20)°C is set to 3°C / s or higher.

[0053] The heat treatment temperature is set to (Tβ + 20)°C or higher. If the heat treatment temperature is lower than (Tβ + 20)°C, the entire processed material cannot be sufficiently heated to a temperature of Tβ°C or higher, regardless of the state of the heating furnace or the shape of the processed material. For this reason, the heat treatment temperature is set to (Tβ + 20)°C or higher. On the other hand, if the heat treatment temperature exceeds 1240°C, the average diameter of the α colonies increases and the fatigue strength decreases. For this reason, the heat treatment temperature is set to 1240°C or lower. It is preferable that the heat treatment temperature be 1200°C or lower.

[0054] Furthermore, if the total heating time at (Tβ + 20)°C or higher exceeds 10 minutes during heat treatment, the β phase grains will coarsen, making it difficult to form prior β grain boundaries of the desired size after air cooling. As a result, the average diameter of the α colonies will also coarsen. For this reason, the total heating time at (Tβ + 20)°C or higher is set to 10 minutes or less. The total heating time at (Tβ + 20)°C or higher is the total time maintained in the temperature range at (Tβ + 20)°C or higher.

[0055] After heating, the material is air-cooled. Air-cooling is a cooling method in which the material is cooled naturally in air, with a cooling rate of approximately 0.1 to 5°C / s. If the material is cooled at a slower rate than air-cooling, for example, by furnace cooling, the time spent in the temperature range above Tβ°C will be longer. This results in coarsening of the β grains and α colonies, and the precipitation of α grains at the grain boundaries. This reduces fatigue strength. On the other hand, if the material is cooled at a faster rate than air-cooling, for example, by water cooling, deformation such as bending will occur, increasing the load on subsequent processes such as cutting and polishing. Therefore, air-cooling is performed after heating.

[0056] Here, the heating rate during heat treatment is set to 3°C / s or higher in the temperature range of (Tβ-10) to (Tβ+20)°C. If the heating rate in the temperature range of (Tβ-10) to (Tβ+20)°C is less than 3°C / s, the grain growth rate will exceed the nucleation rate, resulting in coarsening of the β grain size and the accumulation of β phase crystal orientations in specific directions. As a result, the α colonies that precipitate during the β→α transformation during subsequent cooling will become coarse and accumulate in specific directions. This increases the area ratio of low fatigue strength and reduces fatigue strength.

[0057] For this reason, the heating rate in the temperature range of (Tβ-10) to (Tβ+20)°C is set to 3°C / s or more, preferably 8°C / s or more. The heating rate is controlled by measuring the temperature with a thermocouple attached to the processed material. In order to achieve a heating rate of 3°C / s or more in the temperature range of (Tβ-10) to (Tβ+20)°C, high-frequency heating or electrical heating can be used, for example. Tβ (β transformation point) is the temperature at which the α → β transformation ends when the material is heated to the β single-phase region, and is a value obtained through prior experiments.

[0058] The preliminary experimental method for investigating Tβ is as follows: First, observe the structure of the titanium alloy material. If the initial structure of the material is an acicular structure, it is subjected to forming processing. The forming processing is carried out after heating to a temperature of 950°C or less, so that the area reduction rate is 30% or more. The above-mentioned forming processing results in the initial structure of the material being different from an acicular structure. If the initial structure of the material is different from an acicular structure, forming processing does not need to be carried out.

[0059] The materials for the preliminary experiments prepared in this way are cut or milled as necessary to form rods (bars) with a longitudinal direction and a circular or rectangular cross section perpendicular to the longitudinal direction. If the cross section perpendicular to the longitudinal direction of the bar is circular, the diameter should be 30 mm or less. If the cross section perpendicular to the longitudinal direction of the bar is rectangular, the length of the short side should be 30 mm or less.

[0060] By forming the rod into the specified shape, it is possible to heat the material sufficiently to the inside in the subsequent heat treatment. The obtained rod is heated, held for 30 minutes, and then water-cooled for heat treatment. The temperature of the heat treatment is controlled by the surface temperature of the rod. After the above heat treatment, the structure in the center of the cross section perpendicular to the longitudinal direction of the rod is observed. The heating temperature of the above heat treatment is changed in 5°C increments, and the lowest temperature at which the structure after the above heat treatment consists only of an acicular structure is defined as Tβ.

[0061] The titanium alloy member will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples. [Example]

[0062] A titanium alloy workpiece (round bar material: φ30 mm) having the chemical composition listed in Table 1 was prepared. The prepared workpiece was heated under the conditions listed in Table 2, and then hot-rolled using a round grooved roll to obtain an intermediate material with the diameter listed in Table 2. The obtained intermediate material was machined (on a lathe) into the shape of a fatigue test specimen. The machined shape conformed to JIS Z 2274:1978 (rotating bending fatigue test), with the parallel section dimensions standardized to φ6 x 12.5 mm / gripping section φ12 mm, and the specimen was taken from the center of the intermediate material. Subsequently, heat treatment was performed under the conditions listed in Table 2 to obtain a titanium alloy part.

[0063] During the heat treatment, the specimen was heated electrically (in a vacuum atmosphere) and the temperature was controlled by a thermocouple attached to the parallel part of the specimen. After the heat treatment, the surface of the parallel part was polished with abrasive paper (#1000) to remove any traces of the thermocouple before observing the structure, etc.

[0064] [Table 1]

[0065] [Table 2]

[0066] For each of the obtained titanium alloy members, the metal structure of the surface layer was observed by the following method, and the average diameter of α colonies and the area ratio of low fatigue strength were measured. The fatigue strength of the titanium alloy members was also measured by the following procedure.

[0067] (Structural observation, average diameter of α colonies, and low fatigue strength area ratio) The acicular structure, average diameter of α colonies, and area ratio of low fatigue strength were measured using the following procedure. First, the specimen was observed using a scanning electron microscope (SEM) to cover a depth of 1 mm from the surface in the thickness direction. Ten rectangular regions measuring 0.8 mm in length and 0.8 mm in width were observed at a magnification of 50x, from the surface to a depth of 1 mm, to determine whether or not they had an acicular structure.

[0068] Next, measurements were performed using an EBSD (Electron Backscatter Diffraction) device attached to the SEM, and the obtained measurement results were analyzed using OIM (crystal orientation analysis software manufactured by TSL Solutions Co., Ltd.). In the EBSD analysis, the angle difference between the crystal orientations of adjacent α colonies was set to 5° or less to identify α colonies. The circle-equivalent diameter of the identified α colonies was calculated and used as the average diameter.

[0069] The area corresponding to the α colony was identified from all the observation areas, and within this area, the α phase area with low fatigue strength orientation was identified and its area ratio was calculated as the low fatigue strength area ratio. The EBSD measurement conditions were a measurement interval of 2.0 μm and an accelerating voltage of 15 kV.

[0070] (Fatigue test) The alloy material was subjected to a rotating bending fatigue test to evaluate its fatigue strength. The test was carried out in accordance with JIS Z 2274:1978, and the fatigue strength (10 7 The fatigue strength (cycle time strength) was evaluated. When the fatigue strength was over 600 MPa, the fatigue strength was considered to be the best, and marked with a ◎. When the fatigue strength was 520 to 600 MPa, the fatigue strength was considered to be good, and marked with a ○. When the fatigue strength was less than 520 MPa, the fatigue strength was evaluated to be poor, and marked with an ×. The results are shown in Table 3 below.

[0071] [Table 3]

[0072] Nos. 1 to 17, which satisfied the requirements of this embodiment, exhibited good fatigue strength. On the other hand, Nos. 18 to 20, which did not satisfy the requirements of this embodiment, exhibited reduced fatigue strength. No. 18 had a low heating rate in the heat treatment, which resulted in an increase in the average diameter of α colonies and a high low fatigue strength area ratio. No. 19 had a large average diameter of α colonies because the heat treatment temperature was too high. No. 20 had a large average diameter of α colonies because the heating time in the heat treatment was too long.

Claims

1. The chemical composition, in mass %, is Al: 4.4-5.5%, Fe: 1.4-2.5%, Mo: 1.5-5.5%, O: 0.05-0.25%, The balance is Ti and impurities. A titanium alloy component having a longitudinal direction, The metal structure in the surface layer portion of the cross section perpendicular to the longitudinal direction is an acicular structure including α colonies having a plurality of α phase crystal grains; the average diameter of the α colonies is 10 μm or more and less than 300 μm, The α phase has a hcp structure, and in the region consisting of the α colonies, the area ratio of the α phase region in which the angle between the c axis of the hcp structure and the longitudinal direction is 0 to 45° is 30% or less.

2. The chemical composition, in mass %, is Al: 5.50-6.75%, V: 3.5-4.5%, Fe: 0.05-0.40%, O: 0.05-0.25%, The balance is Ti and impurities. A titanium alloy component having a longitudinal direction, The metal structure in the surface layer portion of the cross section perpendicular to the longitudinal direction is an acicular structure including α colonies having a plurality of α phase crystal grains; the average diameter of the α colonies is 10 μm or more and less than 300 μm, The α phase has a hcp structure, and in the region consisting of the α colonies, the area ratio of the α phase region in which the angle between the c axis of the hcp structure and the longitudinal direction is 0 to 45° is 30% or less.

3. The chemical composition, in mass %, is Al: 5.50-6.50%, Sn: 1.75-2.25%, Zr: 3.5 to 4.5%, Mo: 1.8-2.2%, Si: 0.10% or less, Fe: 0.02-0.25%, O: 0.02-0.15%, The balance is Ti and impurities. A titanium alloy component having a longitudinal direction, The metal structure in the surface layer portion of the cross section perpendicular to the longitudinal direction is an acicular structure including α colonies having a plurality of α phase crystal grains; the average diameter of the α colonies is 10 μm or more and less than 300 μm, The α phase has a hcp structure, and in the region consisting of the α colonies, the area ratio of the α phase region in which the angle between the c axis of the hcp structure and the longitudinal direction is 0 to 45° is 30% or less.

4. The titanium alloy member according to any one of claims 1 to 3, which is an engine valve.

5. The titanium alloy member according to any one of claims 1 to 3, which is a connecting rod.

6. The method for producing a titanium alloy member according to any one of claims 1 to 5, a molding process in which the sheet is heated to a temperature of (Tβ+70)°C or higher but lower than (Tβ+270)°C, and processed at a temperature range of (Tβ+50)°C or higher with an area reduction rate of 10% or higher, and then processed at a temperature range of (Tβ-100)°C or higher but lower than (Tβ+50)°C with an area reduction rate of 30% or higher but lower than 85%; a heat treatment step of heating to a temperature of (Tβ + 20) ° C. or higher and 1240 ° C. or lower and then air-cooling so that the total heating time at (Tβ + 20) ° C. or higher is 10 minutes or less; In the heat treatment step, the temperature rise rate in the temperature range of (Tβ-10) to (Tβ+20)°C is 3°C / s or more.

7. The method for producing a titanium alloy part according to claim 6, further comprising a cutting step of performing cutting between the forming step and the heat treatment step.

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