JP7799181B2 - Titanium alloy member and manufacturing method thereof - Google Patents

Description

The present invention relates to a titanium alloy member and a manufacturing method thereof.

Drive components in automobiles, such as engine valves and connecting rods, are subjected to repeated loads during use. For this reason, these drive components are required to have high strength as well as excellent fatigue properties.

Alpha + beta titanium alloys, which are also used in drive components, can have increased strength by controlling manufacturing conditions such as processing and heat treatment. However, when attempting to improve fatigue properties by controlling manufacturing conditions, these alloys suffer from the problem of reduced productivity. Specifically, in order to control the metal structure to an equiaxed structure with good fatigue properties, hot working must be performed in the alpha + beta two-phase region, which increases deformation resistance. As a result, productivity decreases.

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 reduced fatigue properties.

Non-Patent Document 1 discloses a titanium alloy member in which the acicular structure is refined 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 the material to a temperature above the β transformation point and then rapidly cooling it from a temperature above the Ms point to control the formation of a fine martensite structure.

Japanese Patent Application Publication No. 2-213453 Japanese Unexamined Patent Publication No. 6-184683

G. Lutjering, A. Gysler: Titanium Science and Technology, 4 (1985), p. 2065

The above-mentioned Non-Patent Document 1 and Patent Document 1 require water cooling when manufacturing titanium alloy parts. Such water cooling generates residual stress, which can lead to deformation such as bending. This results in increased process loads for subsequent cutting and polishing, among other processes.

Patent Document 2 discloses a titanium alloy component 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.

The present invention aims to solve the above problems and provide a titanium alloy component that exhibits excellent fatigue properties when processed in the β single-phase region without water cooling, as well as a method for manufacturing the same.

The present invention was made to solve the above problems, and is summarized as follows: The titanium alloy member and its manufacturing method.

(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 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) 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.

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

(4) Chemical composition, in mass%,
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 titanium alloy member according to (1) above, wherein the balance is Ti and impurities.

(5) A titanium alloy component according to any one of (1) to (4) above, which is an engine valve.

(6) The titanium alloy component described in (1) to (4) above, which is a connecting rod.

(7) A method for producing a titanium alloy part according to any one of (1) to (6) above,
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.

(8) The method for manufacturing a titanium alloy component according to (7) above, further comprising a cutting process step in which cutting is performed before the heat treatment process.

(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, the material is heated to a temperature of (Tβ + 70)°C or higher but lower than (Tβ + 270)°C, processed at a temperature range of (Tβ + 50)°C or higher with an area reduction of 10% or more, and then processed at a temperature range of (Tβ - 100)°C or higher but lower than (Tβ + 50)°C with an area reduction of 30% or higher but lower than 85%.

According to the present invention, when processed in the β single-phase region, titanium alloy components with excellent fatigue properties can be obtained without water cooling.

FIG. 1 is a photograph of the structure showing the acicular structure. FIG. 2 is a structural photograph showing the equiaxed structure. FIG. 3 is a tissue photograph showing α colonies.

The inventors investigated the acicular structure that forms when processing is performed in the single-phase β region, and obtained the following findings (a) to (c) regarding the fatigue properties of titanium alloy components.

(a) The coarse α phase in the acicular structure becomes the starting point for fatigue fracture, resulting in a decrease in fatigue strength. Therefore, the inventors focused on this coarse α phase. They discovered that this coarse α phase is an aggregate (hereinafter referred to as an "α colony") of acicular α phase crystal grains (hereinafter referred to as "α phase grains") with the same crystal orientation.

(b) In order to improve fatigue properties, it is effective to make the α colonies finer. The inventors have also clarified that the crystal grains that make up the α colonies, which are the starting points for fracture, have a specific crystal orientation. Therefore, it is also effective to control the crystal orientation of the crystal grains that make up the α colonies and suppress the formation of α colonies that can serve as the starting points for fracture.

(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 heat the material at a faster rate than usual during heat treatment after processing in the β single-phase region, followed by air cooling.

One embodiment of the present invention was developed based on the above findings. Each requirement of this embodiment will be explained in detail below.

1. Metal structure of titanium alloy member 1-1. Acicular structure and α colonies Fatigue strength is affected by the metal structure near the surface. For this reason, in the titanium alloy member of this embodiment, the size of the α colonies in the surface layer is controlled.

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

Here, the surface layer refers to the 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 only needs to be observed and measured within the measurement field of view described below; it is not necessary to observe and measure the entire area.

Furthermore, an acicular structure is a structure that forms when cooling from the β single-phase region, and refers to a microstructure that contains 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 that forms when cooling from the α + β two-phase region. It can be seen that the shape of the crystal grains in an acicular structure is different from that in an equiaxed structure.

In the titanium alloy component of this embodiment, the acicular structure includes α colonies. An α colony is a structure containing multiple 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 that form when the β phase transforms to the α phase, and forms within prior β phase grains. Furthermore, the acicular α grains within an α colony have similar crystal orientation and orientation. Specifically, the misorientation between α grains within an α colony is within ±5°.

In addition to 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 need to particularly control the grain boundary alpha grains, as they do not have a significant impact on fatigue properties.

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

1-3. Area ratio of low fatigue strength in α colonies
The α phase is known to have an hcp structure. While α colonies are aggregates of multiple α grains, they have similar crystal orientations and orientations, including the c-axis direction in the hcp structure. Furthermore, α colonies that are the starting point of fatigue fracture have a crystal orientation in which the c-axis is in the range of 0 to 45° relative to the stress axis (the longitudinal direction in alloy components). 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 advisable to reduce the α phase that constitutes α colonies with low fatigue strength orientation.

Therefore, within the region consisting of alpha colonies, the area ratio of alpha phase regions with low fatigue strength orientation (hereinafter referred to as the "low fatigue strength area ratio") should be 30% or less. It is preferable to reduce the low fatigue strength area ratio as much as possible.

The acicular structure, average diameter of α-colonies, and area ratio of low fatigue strength can be measured using the following procedure. First, a test specimen is taken from a cross section perpendicular to the longitudinal direction of the titanium alloy component, including the surface layer. Next, the obtained test specimen is observed using a scanning electron microscope (SEM), with the cross section including the surface layer as the observation surface. For 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, up to 1 mm deep from the surface.

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 Co., Ltd.). In the EBSD analysis, the angle difference between the crystal orientations of adjacent alpha colonies is set to 5° or less to identify alpha colonies. The circle-equivalent diameter of the identified alpha colonies is calculated and used as the average diameter.

Identify the area corresponding to the alpha colony from all observation areas, and within this area, identify the area of the alpha phase with low fatigue strength orientation, and calculate its area percentage (%). The EBSD measurement conditions should be a measurement interval of 2.0 μm and an acceleration voltage of 15 kV.

2. Chemical Composition In the titanium alloy member 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.

As an example, the chemical composition of the titanium alloy part is preferably Al: 4.4-5.5%, Fe: 1.4-2.5%, Mo: 1.5-5.5%, O: 0.05-0.25%, balance: Ti and impurities. As an example, like AMS4928, the chemical composition of the titanium alloy part is preferably Al: 5.50-6.75%, V: 3.5-4.5%, Fe: 0.05-0.40%, O: 0.05-0.25%, balance: Ti and impurities. As an example, the chemical composition of a titanium alloy component, such as AMS4976, 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. Note that in the content of each of the above elements, "%" means "mass %."

The term "impurities" refers to components that are mixed in due to various factors in the raw materials and manufacturing process when titanium alloy components are industrially manufactured, and are acceptable within a range that does not adversely affect this embodiment. Examples of impurities that may be contained other than those mentioned above include N, C, and H. In this case, it is preferable that N be 0.08% or less, C be 0.08% or less, and H be 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%.

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

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.

4. Manufacturing Method A preferred manufacturing method for the titanium alloy part of this embodiment will be described. The titanium alloy part of this embodiment can be stably manufactured by the following manufacturing method.

Prepare a processing material such as a titanium alloy ingot or titanium alloy bar. There are no particular restrictions on the dimensions of this processing material, as long as it is large enough to be handled in the forming process described below. It is also preferable that the chemical composition and tensile strength of the processing material be within the ranges described above.

4-1. Forming Process It is preferable to hot-work the above-mentioned material to be processed and formed into the desired shape. In hot processing, it is preferable to first heat the material to a temperature of (Tβ + 70) ° C or higher and lower than (Tβ + 270) ° C. Subsequently, after the above heating, it is preferable to carry out processing with an area reduction rate of 10% or higher in a temperature range of (Tβ + 50) ° C or higher. Furthermore, after the above processing, it is preferable to carry out processing with an area reduction rate of 30% or higher and lower than 85% in a temperature range of (Tβ - 100) ° C or higher and lower than (Tβ + 50) ° C. Note that this forming process is a one-heat process, i.e., a process carried out in one heating operation, and the temperature is controlled by the surface temperature (object temperature) of the material to be processed.

It is preferable to heat the workpiece to a temperature of (Tβ + 70)°C or higher and lower than (Tβ + 270)°C. If the heating temperature during hot working is lower than (Tβ + 70)°C, the entire workpiece will be less likely to reach a temperature of Tβ°C or higher if there are areas in the heating furnace where the temperature is uneven, or if the workpiece itself is uneven in size. For this reason, it is preferable to set the heating temperature during hot working to (Tβ + 70)°C or higher. On the other hand, if the heating temperature during hot working is higher than (Tβ + 270)°C, the surface of the workpiece will be more likely to oxidize. In addition, the metal structure of the workpiece will be more likely to become coarse. For this reason, it is preferable to set the heating temperature during hot working to lower than (Tβ + 270)°C.

Furthermore, after heating to the above-mentioned temperatures, it is preferable to carry out processing 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 processing 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 processing in a temperature range of (Tβ + 50)°C or higher be 10% or more.

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

4-2. Cutting Process The intermediate material that has undergone the above-mentioned forming process may be further cut, if necessary. For example, if defects occur on the surface due to hot working, these defects can be removed by cutting. Cutting may also be performed to form the desired shape. Note that the titanium alloy component may be processed into the desired shape by only cutting, without going through 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 the processed material.

4-3. Heat Treatment Step 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 at (Tβ + 20) ° C or higher for a total heating time of 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.

The heat treatment temperature must be (Tβ + 20)°C or higher. If the heat treatment temperature is below (Tβ + 20)°C, the entire processed material cannot be sufficiently heated to a temperature of Tβ°C or higher, regardless of the condition of the heating furnace or the shape of the processed material. For this reason, the heat treatment temperature must be (Tβ + 20)°C or higher. On the other hand, if the heat treatment temperature exceeds 1240°C, the average diameter of the alpha colonies will increase and fatigue strength will decrease. For this reason, the heat treatment temperature must be 1240°C or lower. A heat treatment temperature of 1200°C or lower is preferable.

Furthermore, if the total heating time at (Tβ + 20)°C or higher during heat treatment exceeds 10 minutes, 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 should be 10 minutes or less. Note that 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.

After heating, the material is air-cooled. Air-cooling is a cooling method in which the material is left to cool in the 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 temperature will be held at Tβ°C or higher for a longer period of time. 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.

Here, when heating during heat treatment, the heating rate in the temperature range of (Tβ-10) to (Tβ+20)°C is set to 3°C/s or more. 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 accumulation of the β phase crystal orientation in a specific direction. As a result, the α colonies that precipitate during the β→α transformation during the subsequent cooling will become coarse and accumulate in a specific direction. This increases the low fatigue strength area ratio and reduces fatigue strength.

For this reason, the heating rate in the temperature range of (Tβ - 10) to (Tβ + 20)°C should be 3°C/s or higher, and preferably 8°C/s or higher. 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 higher 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 experimentation.

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 acicular, 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.

The materials for the preliminary experiments prepared in this way are cut, milled, etc. 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.

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

The following describes the titanium alloy member in more detail using examples, but the present embodiment is not limited to these examples.

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 produce an intermediate material with the diameter listed in Table 2. The resulting 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 a standardized parallel section dimension of φ6 x 12.5 mm and grip section φ12 mm, and the specimen was taken from the center of the intermediate material. The intermediate material was then heat-treated under the conditions listed in Table 2 to obtain a titanium alloy part.

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

For each of the obtained titanium alloy components, the metal structure of the surface layer was observed using the following method, and the average diameter of alpha colonies and the area ratio of low fatigue strength were measured. The fatigue strength of the titanium alloy components was also measured using the following procedure.

(Structural observation, average diameter of α colonies, and area ratio of low fatigue strength)
The acicular structure, the average diameter of α colonies, and the area ratio of low fatigue strength were measured by the following procedure. First, the test specimen was observed using a scanning electron microscope (SEM) so as to include 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.

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 alpha colonies was set to 5° or less to identify alpha colonies. The circle-equivalent diameter of the identified alpha colonies was calculated and used as the average diameter.

Areas corresponding to alpha colonies were identified from all observation areas, and within these areas, areas of the alpha phase with low fatigue strength orientation were identified and their area ratio was calculated, which was taken as the low fatigue strength area ratio. The EBSD measurement conditions were a measurement interval of 2.0 μm and an acceleration voltage of 15 kV.

(Fatigue test)
The alloy materials were subjected to a rotating bending fatigue test to evaluate their fatigue strength. The test was carried out in accordance with JIS Z 2274:1978, and the fatigue strength ( ¹⁰⁷ cycles time strength) was evaluated. A fatigue strength of over 600 MPa was considered to be the best fatigue strength and marked with a ◎. A fatigue strength of 520 to 600 MPa was considered to be good fatigue strength and marked with a ○. A fatigue strength of less than 520 MPa was considered to be poor fatigue strength and marked with an ×. The results are shown in Table 3 below.

Nos. 1 to 17, which satisfy the requirements of this embodiment, exhibited good fatigue strength. On the other hand, Nos. 18 to 20, which do not satisfy the requirements of this embodiment, exhibited reduced fatigue strength. In No. 18, the heat treatment temperature rise rate was low, resulting in a large average diameter of α colonies and a large area ratio of low fatigue strength. In No. 19, the heat treatment temperature was too high, resulting in a large average diameter of α colonies. In No. 20, the heat treatment heating time was too long, resulting in a large average diameter of α colonies.

Claims (7)

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