Titanium material

Abstract

The present invention is a titanium material in which when a cluster of α grains in which: the orientation difference between the c-axes of adjacent α grains is 20° or less, and the equivalent circle diameter is 50 μm or more is taken as microtexture, in the metal structure of a surface layer portion, the area fraction of the microtexture is 30.0% or less, the average equivalent circle diameter of the microtexture is 300 μm or less; and when two microtexture domains are adjacent to each other, and the angle formed by the average direction of the c-axes of α grains constituting one microtexture domain of said two microtexture domains and the average direction of the c-axes of α grains constituting the other microtexture domain is denoted as angle θ0, the percentage of the area fraction of microtexture contained in two microtexture domains satisfying angle θ0 of 70° or more with respect to the area fraction of the microtexture is 20.0% or less.

Description

Titanium material

[0001] The present disclosure relates to titanium materials.

[0002] Titanium is a lightweight and high-strength material and is used in parts for aircraft, automobiles, golf clubs, etc. Among the above-mentioned parts, when titanium is used in high-speed rotating parts for automobiles, such as motor sleeves, Dwell fatigue properties are required.

[0003] Dwell fatigue is a type of fatigue caused by a high stress load close to the yield stress that continues for a certain period of time. Normal fatigue is evaluated using a typical triangular or sinusoidal load cycle. Dwell fatigue, on the other hand, is evaluated using a trapezoidal load cycle, and generally, high stress acts for a longer period of time than in normal fatigue, resulting in fracture after a shorter number of cycles.

[0004] Titanium materials are prone to fracture in environments where Dwell fatigue occurs. Therefore, improvements in Dwell fatigue properties have been desired. For example, Patent Documents 1 and 2 disclose titanium materials with improved Dwell fatigue properties.

[0005] JP 2021-167448 A JP 2021-167449 A

[0006] The titanium materials disclosed in Patent Documents 1 and 2 improve Dwell fatigue properties by controlling the size of aggregates of crystal grains with locally similar crystal orientations, known as microtexture. However, in recent years, improvements in engine fuel efficiency and higher motor output have been progressing even in automobiles, and vehicles are now in environments where high stress loads continue, so there is a demand for better Dwell fatigue properties, specifically a Dwell fatigue life of 15,000 cycles or more.

[0007] In light of the above, an object of the present disclosure is to provide a titanium material with improved Dwell fatigue properties, specifically, a titanium material with a Dwell fatigue life of 15,000 cycles or more.

[0008] The present disclosure has been made to solve the above-mentioned problems, and is summarized as follows: a titanium material and a manufacturing method thereof.

[0009] (1) When a microtexture is an aggregate of α grains in which the misorientation between the c-axes of adjacent α grains is 20° or less and the equivalent circle diameter is 50 μm or more, in the metal structure of the surface layer, the area ratio of the microtexture is 30.0% or less, the average equivalent circle diameter of the microtexture is 300 μm or less, two microtextures are adjacent to each other, and the angle between the average direction of the c-axes of the α grains constituting one of the two microtextures and the average direction of the c-axes of the α grains constituting the other microtexture is an angle θ ０ When the angle θ is ０ A titanium material, wherein the area ratio of the microtexture contained in the two microtextures satisfying an angle of 70° or more is 20.0% or less.

[0010] (2) The titanium material according to (1) above, wherein in the metal structure of the surface layer portion, the average equivalent circle diameter of α grains is 20.0 μm or less.

[0011] (3) In the metal structure of the surface layer portion, the angle formed by the average direction of the c-axis of the α grains constituting one microtexture and the average direction of the c-axis of the α grains adjacent to the one microtexture is defined as angle θ １ When the angle θ is １ The titanium material according to (1) or (2) above, wherein the area ratio of the microtexture satisfying the condition of 45° or more is 15.0% or less.

[0012] (4) The titanium material according to any one of (1) to (3) above, which is an α type or an α+β type.

[0013] (5) The titanium material according to any one of (1) to (4) above, having a chemical composition, in mass%, of Al: 4.40 to 6.75%, Fe: 0.05 to 3.10%, O: 0.05 to 0.40%, V: 0 to 4.50%, Mo: 0 to 5.5%, Si: 0 to 0.40%, and the balance: Ti and impurities.

[0014] (6) The titanium material according to any one of (1) to (4) above, having a chemical composition, in mass%, of Al: 4.40 to 5.50%, Fe: 1.40 to 2.50%, Mo: 1.50 to 5.50%, O: 0.05 to 0.25%, and the balance: Ti and impurities.

[0015] (7) The titanium material according to any one of (1) to (4) above, having a chemical composition, in mass%, of Al: 5.50 to 6.75%, V: 3.50 to 4.50%, Fe: 0.05 to 0.40%, O: 0.05 to 0.25%, and the balance: Ti and impurities.

[0016] (8) The titanium material according to any one of (1) to (4) above, having a chemical composition, in mass%, of Al: 4.70 to 6.50%, Fe: 0.05 to 3.10%, Si: 0.15 to 0.40%, O: 0.05 to 0.40%, and the balance: Ti and impurities.

[0017] (9) The titanium material according to any one of (1) to (4) above, having a chemical composition, in mass%, of Al: 4.40 to 5.50%, Fe: 0.50 to 1.40%, Si: 0 to 0.10%, O: 0.05 to 0.40%, and the balance: Ti and impurities.

[0018] (10) The titanium material according to any one of (1) to (9) above, which is a rod material.

[0019] According to the present disclosure, a titanium material with improved Dwell fatigue properties can be obtained.

[0020] The present inventors have investigated methods for improving the Dwell fatigue properties of titanium materials and have obtained the following findings.

[0021] (a) The above-mentioned microtexture is an aggregate of α grains in which the misorientation between the c-axes of adjacent α grains is 20° or less, and behaves as a single α grain at least with respect to Dwell fatigue characteristics, and is affected by the average direction of the c-axes of the α grains that make up the microtexture.

[0022] (b) Dwell fatigue characteristics can be improved by reducing the proportion of the microtexture and further reducing the size of the microtexture. That is, it is effective to reduce the area ratio of the microtexture to 30.0% or less and further reduce the average equivalent circle diameter of the microtexture to 300 μm or less.

[0023] However, in titanium materials with multiple microtextures, even if the microtextures have similar equivalent circle diameters, they can sometimes have a short life, making it difficult to further improve the Dwell fatigue properties. We have discovered that the cause of this is a pair of adjacent microtextures that form a specific orientation relationship, leading to early fracture.

[0024] (c) There is a correlation between the angle φ formed by the average direction of the c-axes of the α grains that make up the microtexture and the direction of the acting stress, and the deformation resistance; the closer the angle φ is to 0°, the more difficult the deformation, and the closer it is to 90°, the easier it is to deform. Therefore, if the difference in the average directions of the c-axes of the α grains that make up a pair of adjacent microtextures is 70° or more, the stress distribution resulting from the difference in deformability will become large, leading to early fracture.

[0025] In other words, if the average direction of the c-axes of the α-grains constituting one of the two microtextures is 70° or more with respect to the average direction of the c-axes of the α-grains constituting the other microtexture, early fracture will occur. Therefore, it is effective to reduce the area ratio of the microtexture having such an orientation relationship, specifically to make the area ratio 20.0% or less.

[0026] (d) In addition, from the viewpoint of design, titanium materials are required to have a mirror finish, i.e., the ability to be polished to a good mirror finish. The inventors have clarified that the microtexture also affects the mirror finish. For example, if there is a large difference in orientation between the microtexture and the crystal grains formed around it, a difference in unevenness occurs during polishing, resulting in a decrease in the mirror finish.

[0027] Therefore, in order to obtain better specularity, it is effective to set the area ratio of microtextures in which the angle between the average direction of the c-axes of the α-grains constituting one microtexture and the average direction of the c-axes of the α-grains adjacent to that microtexture is 45° or more to 15.0% or less.

[0028] An embodiment of the present invention has been made based on the above findings. Each requirement of the titanium material of this embodiment will be described in detail below.

[0029] 1. Metal structure 1-1. Definition of microtexture In the titanium material of this embodiment, the dwell characteristics are improved by controlling the microtexture structure. Here, microtexture refers to an aggregate of α grains in which the misorientation between the c-axes of adjacent α grains is 20° or less and the aggregate has a circle equivalent diameter of 50 μm or more. The α grains are crystalline grains of the α phase.

[0030] When the misorientation between the c-axes of adjacent α grains exceeds 20°, the aggregate is less likely to deform. Since the threshold misorientation of the c-axes of α grains, which are the starting point of fatigue fracture, is 0-20°, the lower limit of the misorientation between the c-axes of adjacent α grains in the microtexture is preferably 0°. Furthermore, aggregates of adjacent α grains with a circle equivalent diameter of less than 50 μm do not significantly affect the Dwell characteristics. Therefore, the definition of the texture of the microtexture is as described above. The circle equivalent diameter of the microtexture refers to the diameter of a circle having the same area as the microtexture. Furthermore, the c-axis refers to the direction perpendicular to the base of the hcp structure.

[0031] In the titanium material of this embodiment, microtexture is controlled in the metallographic structure of the surface layer. Here, the term "surface layer" refers to a position at a depth of r / 2 from the side surface of a cylindrical titanium material whose cross section perpendicular to the longitudinal direction is a circle with a radius r. For a prismatic titanium material whose cross section is a rectangle with a side length d, the term "surface layer" refers to a position at a depth of d / 4 from the side surface having the side length d. Note that if the requirements of this embodiment are satisfied at a depth of r / 2 or d / 4 from the surface, it can also be said that the requirements of this embodiment are satisfied at positions shallower than the depth of r / 2 or d / 4 from the surface. Therefore, in the titanium material of this embodiment, the metallographic structure is observed at a depth of r / 2 or d / 4 from the surface. Furthermore, with regard to fatigue properties, including Dwell fatigue, it is generally believed that the initiation of fatigue fracture occurs in the surface layer, including the surface, where the maximum stress often acts. Therefore, the surface layer, including the surface, is the target.

[0032] 1-2. Microtexture Area Ratio In the titanium material of this embodiment, the microtexture area ratio is 30.0% or less. If the microtexture area ratio is 30.0% or less, the frequency of fatigue crack occurrence is reduced and the simultaneous formation of facet fracture surfaces corresponding to the size of the microtexture is suppressed. As a result, crack initiation and propagation are reduced and the Dwell fatigue life is improved. The microtexture area ratio is preferably 25.0% or less, and more preferably 20.0% or less. A smaller microtexture area ratio is preferable, but since the formation of microtexture is unavoidable, the practical lower limit is 1.0%.

[0033] 1-3. Average Equivalent Circular Diameter of Microtexture In the titanium material of this embodiment, the average equivalent circular diameter of the microtexture is 300 μm or less. Here, the average equivalent circular diameter of the microtexture refers to the average value of the circle-equivalent diameters of the individual microtextures. If the average equivalent circular diameter of the microtexture is 300 μm or less, stress distribution due to fatigue can be reduced, and the occurrence of fatigue cracks can be reduced, thereby improving the Dwell fatigue characteristics. The average equivalent circular diameter of the microtexture is preferably 250 μm or less, and more preferably 200 μm or less. Note that, based on the definition of microtexture above, the practical lower limit of the average equivalent circular diameter of the microtexture is 50 μm.

[0034] 1-4. Area Ratio of Paired Microtextures In the titanium material of this embodiment, the Dwell fatigue life can be improved to 15,000 cycles or more by reducing the area ratio of pairs of adjacent microtextures having a specific orientation relationship, which are the starting points for fracture.

[0035] Specifically, in the titanium material of this embodiment, two microtextures are adjacent to each other, and the angle formed by the average direction of the c-axes of the plurality of α-grains constituting one of the two microtextures and the average direction of the c-axes of the plurality of α-grains constituting the other microtexture is defined as angle θ ０ When the angle θ ０ The area ratio of the microtextures contained in two microtextures (referred to as "paired microtextures" in this specification) that satisfy an angle of 70° or more (hereinafter, for simplicity, referred to as "paired area ratio") is 20.0% or less.

[0036] For example, three microtextures (referred to as microtextures a, b, and c) may be adjacent to each other. If microtextures a and b, microtextures b and c, and microtextures a and c are paired microtextures, microtextures a, b, and c are included in the paired microtextures. In other words, the total area ratio of microtextures a, b, and c is the area ratio of the microtexture included in the paired microtextures.

[0037] When the pair area ratio is 20.0% or less, the proportion of paired microtextures that serve as fatigue initiation points is reduced, making it difficult for cracks to occur early. As a result, Dwell fatigue characteristics are improved. Therefore, the pair area ratio is preferably 18.0% or less, and more preferably 16.0% or less. Since the formation of paired microtextures with large misorientation is unavoidable, the substantial lower limit of the pair area ratio is 1.0%. Note that the pair area ratio is substantially 6.0% or less of the total observed area. Furthermore, when the misorientation exceeds 90°, 90° is subtracted from the misorientation, so orientations with a misorientation of 90° or less are the target of measurement.

[0038] 1-5. Total Equivalent Circle Diameter of Paired Microtextures In the titanium material of this embodiment, the average value of the total equivalent circle diameter of the paired microtextures (hereinafter, for simplicity, referred to as the "paired equivalent circle diameter") is preferably 430 μm or less. The total equivalent circle diameter is the circle equivalent diameter calculated from the cross-sectional areas of the two microtextures. In other words, it is the circle equivalent diameter calculated when the two microtextures are considered as a single aggregate.

[0039] When the paired circle equivalent diameter is 430 μm or less, stress distribution between the two microtextures is reduced, improving the Dwell fatigue characteristics. The paired circle equivalent diameter is preferably 400 μm or less, more preferably 380 μm or less, and even more preferably 350 μm or less. Note that, based on the definition of the microtexture described above, the substantial lower limit of the paired circle equivalent diameter is 70 μm.

[0040] 1-6. Area ratio of microtextures with large misorientation from adjacent α grains To improve specularity, it is effective not only to suppress coarse microtextures but also to reduce the misorientation between the microtextures and the crystal grains formed around them. This is because such structural control can suppress the occurrence of irregularities during polishing.

[0041] Therefore, in the titanium material of this embodiment, the angle formed by the average direction of the c-axes of the α-grains constituting one microtexture and the average direction of the c-axes of the α-grains adjacent to that microtexture is defined as an angle θ １ When the angle θ １ It is preferable that the ratio of the area ratio of the microtexture satisfying the condition of 45° or more (hereinafter referred to as "adjacent grain area ratio") is 15.0% or less. １ When the angle is less than 45°, the misorientation between the microtexture and the crystal grains formed around it is sufficiently small, and the difference in unevenness is small. Therefore, the difference in unevenness is not apparent, and the effect on specularity can be ignored. Furthermore, if the adjacent grain area ratio is 15.0% or less, unevenness is unlikely to occur between one microtexture and adjacent α grains, and specularity can be improved. The adjacent grain area ratio is preferably 13.0% or less, more preferably 10.0% or less, and even more preferably 8.0% or less. It is preferable to reduce the adjacent grain area ratio as much as possible, and most preferably 0%. Note that when the misorientation exceeds 90°, 90° is subtracted from the misorientation, so orientations with a misorientation of 90° or less are the target for measurement.

[0042] 1-7. Average Equivalent Circular Diameter of α Grains In the titanium material of this embodiment, the average equivalent circular diameter of α grains is preferably 20.0 μm or less. If the average equivalent circular diameter of α grains is 20.0 μm or less, the average equivalent circular diameter of the microtexture can inevitably be reduced. As a result, stress distribution due to the difference in orientation between the microtexture and the surrounding crystal grains is reduced, making fatigue cracks less likely to occur. In other words, Dwell fatigue characteristics are improved. The average equivalent circular diameter of α grains is preferably 16.0 μm or less, more preferably 12.0 μm or less, and even more preferably 10.0 μm or less.

[0043] 1-8. Observation of Metallic Structure The above-mentioned metallic structure can be observed by measuring using an EBSD (Electron Backscatter Diffraction) device attached to a scanning electron microscope (hereinafter also referred to as "SEM") in the following manner.

[0044] The observation area was a rectangular region measuring 3 mm long x 3 mm wide in the surface layer of the titanium material. For cylindrical titanium materials with a circular cross section perpendicular to the longitudinal direction and a radius of r, the observation surface was a plane parallel to the longitudinal direction of the titanium material, perpendicular to the radial direction of the cross section, and centered at a depth of r / 2 from the side surface. For prismatic titanium materials with a rectangular cross section perpendicular to the longitudinal direction and a side length of d, the observation surface was a plane parallel to the longitudinal direction of the titanium material and the side surface with the same side length, and located at a depth of d / 4 from the side surface with the same side length. For fatigue properties, including Dwell fatigue, measurements are preferably taken on a plane perpendicular to the maximum applied stress. While the direction of the applied stress depends on the operating environment and test method, the surface layer was observed because fatigue fracture is likely to occur in the surface layer, including the surface where the maximum stress is often applied. Strain introduced when cutting out the observation surface was removed by polishing. At this time, the surface is polished with colloidal silica to prevent new strain from being introduced by polishing.

[0045] The measurement interval is 2.0 μm in all measurement areas, and the orientation is measured using EBSD at an acceleration voltage of 15 kV. The obtained measurement results are analyzed using OIM (crystal orientation analysis software "OIM Analysis v8.1.0" manufactured by TSL Solutions Co., Ltd.). At this time, a Partition targeting only the α phase is created and used as the object of analysis. Note that, since it is usually unlikely that the entire periphery of the microtexture is adjacent to β grains, it is sufficient to target only α grains.

[0046] Then, α-grains are determined as those for which the difference in the crystal orientation angle (misorientation angle) between adjacent EBSD measurement points is 15° or more, and the circle-equivalent diameter of each α-grain is calculated from the number of measurement points of the α-grains, and the average circle-equivalent diameter of the α-grains is calculated. Furthermore, the c-axis misorientation between adjacent EBSD measurement points is determined from the obtained measurement results (Euler angles ph1, PH, ph2), and an aggregate of α-grains with a c-axis misorientation of 0 to 20° and an equivalent circle diameter of 50 μm or more is determined to be microtexture.

[0047] Among the structures determined to be microtextures, two adjacent microtextures are identified, and the average direction of the c-axis of the α grains constituting each microtexture is calculated. Next, the difference between the average directions of the c-axes of the two microtextures, i.e., the angle θ ０ Then, calculate the angle θ ０ Two microtextures that satisfy the angle of 70° or more, i.e., microtextures included in a pair of microtextures, are extracted, the area ratio of the microtexture in the observation area is found, and this is divided by the area ratio of all microtextures to calculate the pair area ratio.Furthermore, when calculating the pair circle equivalent diameter of a microtexture, similarly, the pair of microtextures are extracted, the total cross-sectional area of ​​the pair of microtextures is measured, and the pair circle equivalent diameter is calculated from the average value of the cross-sectional areas of all the extracted pair of microtextures.

[0048] In addition, when calculating the adjacent grain area ratio, the angle formed by the average direction of the c-axis of the α-grains constituting the region determined to be microtexture and the average direction of the c-axis of the α-grains adjacent to the microtexture so as to surround the microtexture, that is, the angle θ１ Then, calculate the angle θ １ The area ratio of the microtexture in the observation area is calculated by dividing it by the area ratio of all microtextures to calculate the adjacent grain area ratio.

[0049] 2. Type The type of titanium material in this embodiment is not particularly limited. For example, it may be a titanium alloy.

[0050] The titanium alloy is preferably, for example, an α-type titanium alloy or an α+β-type titanium alloy, including, for example, highly corrosion-resistant alloys (titanium alloys specified in JIS standards 11 to 13, 17, 19 to 22, and ASTM standards Grades 7, 11 to 14, 17, 31, and 34, as well as titanium alloys containing small amounts of various elements), Ti-0.5Cu, Ti-1.0Cu, Ti-1.0Cu-0.5Nb, Ti-1.0Cu-1.0Sn-0.3Si-0.25Nb, Ti-0.05 to 0.2Pd, etc.

[0051] Generally, the higher the strength, the better the Dwell fatigue properties. For this reason, materials with a tensile strength of 950 MPa or more are preferred. Alpha-beta titanium alloys have high strength and are used for automobile parts such as motor sleeves. The alpha-beta titanium alloy is preferably an alpha-beta titanium alloy that has, for example, an alpha phase as the main phase and a beta phase as the secondary phase at room temperature (25°C).

[0052] The chemical composition of the α+β type alloy is not particularly limited, but is preferably, for example, in mass %, Al: 4.40 to 6.75%, Fe: 0.05 to 3.10%, O: 0.05 to 0.25%, V: 0 to 4.50%, Mo: 0 to 5.50%, Si: 0 to 0.40%, and the balance: Ti and impurities.

[0053] More preferably, the composition is, by mass%, 4.40 to 5.50% Al, 1.40 to 2.50% Fe, 1.50 to 5.50% Mo, 0.05 to 0.25% O, and the balance being Ti and impurities (usually also referred to as "Ti-5Al-2Fe-3Mo"). More preferably, the composition is, by mass%, 5.50 to 6.75% Al, 3.50 to 4.50% V, 0.05 to 0.40% Fe, 0.05 to 0.25% O, and the balance being Ti and impurities (usually also referred to as "Ti-6Al-4V").

[0054] Furthermore, preferably, by mass%, Al: 4.70 to 6.50%, Fe: 0.05 to 3.10%, Si: 0.15 to 0.40%, O: 0.05 to 0.40%, the balance: Ti and impurities (usually also referred to as "Ti-5.5Al-1.5Fe-0.25Si"). More preferably, by mass%, Al: 4.40 to 5.50%, Fe: 0.50 to 1.40%, Si: 0 to 0.10%, O: 0.05 to 0.40%, the balance: Ti and impurities (usually also referred to as "Ti-5Al-1Fe").

[0055] The titanium materials Ti-6Al-4V, Ti-5Al-2Fe-3Mo, Ti-5.5Al-1.5Fe-0.25Si, and Ti-5Al-1Fe have tensile strengths of 950 MPa or more and are therefore suitable for the application of this application. Other general-purpose alloys such as those specified in AMS 4975 may also be used.

[0056] Common impurities contained in the titanium alloy include N, C, H, etc., and may be contained as long as the contents are 0.08 mass% or less of N, 0.08 mass% or less of C, and 0.015 mass% or less of H. Elements other than those mentioned above include, for example, Ni, Cr, Mn, Nb, and Cu. It is preferable that the contents of these elements are each 0.1% or less, and that the total content is less than 0.3%.

[0057] 3. Shape The shape of the titanium material in this embodiment is not limited to wire, bar, plate, etc. Here, rod-shaped titanium material, including wire, is referred to as bar. The area of ​​the cross section perpendicular to the longitudinal direction of the bar is 7900 mm. ２ Preferably, it is 5200 mm or less. ２More preferably, it is 79 mm or less. ２ It is preferable that the length is 177 mm or more. ２ More preferably, it is 310 mm or more. ２ For example, the rod preferably has a circular cross section and a cross-sectional diameter of 10 to 100 mm, and more preferably a cross-sectional diameter of 15 to 80 mm.

[0058] 4. Properties 4-1. Dwell fatigue properties The titanium material of this embodiment has excellent Dwell fatigue properties because the metal structure satisfies the above-mentioned specifications. In this disclosure, the Dwell fatigue properties are evaluated by conducting the tensile test and fatigue test described below.

[0059] First, tensile test specimens and fatigue test specimens are taken from the surface layer of a titanium material so that the longitudinal direction of the titanium material is the longitudinal direction of the test specimen. At this time, if the titanium material is cylindrical and the cross section perpendicular to the longitudinal direction is a circle of radius r, the tensile test specimens and fatigue test specimens are taken so that their centers are located at a depth of r / 2 from the side surface of the titanium material. On the other hand, if the titanium material is prismatic and the cross section perpendicular to the longitudinal direction is rectangular with a side length d, the tensile test specimens and fatigue test specimens are taken so that their centers are located at a depth of d / 4 from the side surface with the side length, and at the center in the side length direction.

[0060] In the tensile test, a tensile test piece with a parallel section of φ5 × 30 mm was used, the gauge length was 25 mm, and the strain rate was 8.3 × 10 －５ / s, and other conditions are in accordance with ASTM E8M, and the 0.2% proof stress is measured.

[0061] In the fatigue test, a fatigue test piece with a parallel portion of φ5.08 mm × 15.24 mm was used, with a gauge length of 12 mm, axial force, pulsating force, and stress ratio of 0.05, and the maximum stress was set to 95% of the 0.2% yield strength measured in the above tensile test. The stress waveform was a trapezoidal load cycle of 1 s for loading, 120 s for holding, and 1 s for unloading, and the Dwell fatigue life was measured.

[0062] In this embodiment, if the Dwell fatigue life measured by the above-mentioned fatigue test is 15,000 cycles or more, it is determined that the Dwell fatigue characteristics are excellent.

[0063] 4-2. Specularity When design is required, the titanium material of this embodiment preferably has good specularity. Specularity is affected by the metal structure near the surface, i.e., the surface layer. In this embodiment, as described above, good specularity is obtained by setting the adjacent grain area ratio to 15.0% or less. This results in a homogeneous material that can be used to manufacture parts with excellent specularity, regardless of the position of the titanium material from which the parts are manufactured. In this disclosure, specularity is evaluated using the following method.

[0064] First, a test piece is taken from the surface layer of the titanium material, so that a region of 8 mm length x 8 mm width serves as the evaluation surface. In this case, if the titanium material is cylindrical and the cross section perpendicular to the longitudinal direction is circular with a radius of r, the evaluation surface is a surface that is parallel to the longitudinal direction of the titanium material, perpendicular to the radial direction of the cross section, and has its center at a depth of r / 2 from the side surface. In addition, if the titanium material is prismatic and the cross section perpendicular to the longitudinal direction is rectangular with a side length d, the evaluation surface is a surface that is parallel to the longitudinal direction of the titanium material and the side surface with that side length, and is located at a depth of d / 4 from the side surface with that side length.

[0065] The test piece is embedded in epoxy resin, and the evaluation surface is polished using abrasive paper, starting from coarse to fine grits, in a wet polishing process. The evaluation surface is then dry buffed to a mirror finish. The distinctness of image (DOI) of the evaluation surface is then measured.

[0066] The DOI is measured using an appearance analyzer Rhopoint IQ Flex20 manufactured by Rhopoint Instruments, with the angle of incident light set to 20° and other conditions conforming to ASTM D 5767.

[0067] DOI is a parameter that represents image clarity and is an index of specularity. The higher the DOI, the better the specularity. In this embodiment, a DOI of 60 or more is considered to be excellent in specularity.

[0068] 5. Manufacturing Method The titanium material of this embodiment can be stably manufactured, for example, by the following manufacturing method. Note that, for the sake of simplicity, the following description will be given taking the case of a bar stock as an example.

[0069] The method for manufacturing a titanium material according to this embodiment involves the following first, second, and third steps, carried out in this order. The first step involves heating the titanium ingot to a temperature of (Tβ+20)°C or higher but lower than (Tβ+270)°C, and then hot forging the ingot at a cross-sectional area reduction rate of 40% or higher in a temperature range of Tβ (°C) or higher while rolling it down in a direction perpendicular to the longitudinal direction, followed by cooling. Tβ (°C) refers to the β transformation point and is calculated using the following formula: Tβ=900+20×Al+120×O-10×Mo-18×Fe-15×V+5×Si

[0070] In this process, the titanium ingot is forged by rolling it down from a direction perpendicular to its longitudinal direction along the longitudinal direction, and then the titanium ingot is rotated around its longitudinal direction and forged again in the same way. This process is repeated. Furthermore, forging is performed with a difference in strain rate between each forging. By performing the first step, strain is introduced uniformly along the longitudinal direction of the bar and in the direction perpendicular to it, suppressing the formation of coarse microtextures and changing the stable crystal orientation, thereby reducing the misorientation of adjacent microtextures.

[0071] The second step involves heating the forged titanium material to a temperature of (Tβ-150)°C or higher but lower than (Tβ-50)°C, hot forging it with a cross-sectional area reduction rate of 20% or higher, and then cooling it. As with the first step, a strain rate difference is applied between each forging step. By performing the second step, the microtexture structure can be controlled, as with the first step, and the amount of β phase can be controlled to achieve the desired strength.

[0072] The third step is a step of heating at a temperature of (Tβ-100)°C or higher and lower than (Tβ-20)°C, rolling at a cross-sectional area reduction rate of 75% or higher, and then cooling. By performing the third step, the α grains are refined and the microtexture is also refined.

[0073] Unlike the manufacturing method of this embodiment, if a typical bar manufacturing method, such as the forging process in the first step, is attempted in which the bar is not rotated in the circumferential direction but is reduced in only one direction, the c-axes of the α grains rotate in a direction perpendicular to the longitudinal direction of the bar, leading to refinement of the structure and the development of a texture. When the c-axes of the α grains are aligned perpendicular to the longitudinal direction of the bar, deformation converges in that direction, resulting in a coarse microtexture elongated in the longitudinal direction. As a result, the microtexture is less likely to change its c-axis orientation even during the subsequent rolling process in the third step, making it difficult to obtain the desired microtexture. Each step will be described in detail below.

[0074] 5-1. First Step: Manufacture an ingot of titanium material (hereinafter simply referred to as "titanium ingot") adjusted to a desired chemical composition. The method for manufacturing the titanium ingot is not particularly limited, but it may be manufactured by, for example, vacuum arc remelting (VAR) or electron beam remelting (EBR), and the ingot may be cut to a desired size.

[0075] The obtained titanium ingot is heated to a temperature of (Tβ + 20)°C or higher but lower than (Tβ + 270)°C, forged at a temperature of Tβ°C or higher so that the total area reduction is 40% or higher, and then cooled. This process is called the first process, and the titanium ingot after the first process is used as a titanium forging material.

[0076] In the first step, if the heating temperature during forging is less than (Tβ + 20)°C, uneven temperatures will occur in the heating furnace, or if the titanium ingot is large, the entire ingot will be below Tβ°C. As a result, in alloys that contain β phase, it will be difficult to form a sufficient amount of β phase. On the other hand, if the heating temperature is (Tβ + 270)°C or higher, the surface layer of the titanium rolled material will be more likely to oxidize, or the metal structure will become coarser. Therefore, the heating temperature for the titanium ingot in the first step should be (Tβ + 20)°C or higher but lower than (Tβ + 270)°C.

[0077] Furthermore, if the total area reduction rate at Tβ°C or higher in the first forging step is less than 40%, the crystal grains cannot be refined by utilizing the recrystallization of the β phase that occurs during the subsequent cooling. As a result, coarse microtextures tend to form. In order to make the average equivalent circle diameter of the microtexture 300 μm or less, the total area reduction rate at Tβ°C or higher in the first forging step is set to 40% or more. If it is desired to make the average equivalent circle diameter of the microtexture 250 μm or less, the total area reduction rate at Tβ°C or higher is preferably 55% or more.

[0078] The "total area reduction rate" in the first step is the average area of ​​the cross section perpendicular to the longitudinal direction of the titanium ingot before forging in the first step, which is calculated as "S １ｓ ", and the average area of ​​the cross section perpendicular to the longitudinal direction of the titanium forged material after the first process is "S １ｆ ", then {(S １ｓ -S １ｆ ) / S １ｓ} × 100. When upset forging is performed as described below, the average area of ​​the cross section perpendicular to the longitudinal direction of the titanium ingot after upset forging is referred to as "S １ｓ However, the average area of ​​the cross section perpendicular to the longitudinal direction of a titanium ingot or forged titanium material means the average value of the area of ​​one end of the titanium ingot or forged titanium material in the longitudinal direction, the cross-sectional area of ​​the center, and the area of ​​the other end.

[0079] In the first step, when the titanium ingot is reduced, the titanium ingot is first fed in the longitudinal direction and forged in multiple passes across the length of the ingot using a pair of anvils in a direction perpendicular to the longitudinal direction. In this specification, this multiple passes across the length of the ingot is referred to as a single forging. During each forging, the titanium ingot is not rotated around its length, but is only moved in the longitudinal direction. After the first forging, the titanium ingot is rotated 30 to 45 degrees around its length, the reduction direction is changed, and a second forging is performed in the same manner as the first forging.

[0080] Note that there may be cases where no rotation is performed between one forging and the next. However, the titanium ingot is rotated and the rolling direction is changed six or more times, and the total number of forgings is six or more. Furthermore, the direction in which the titanium ingot is rotated is always the same in the first step. If the aforementioned adjacent grain area ratio is to be 15.0% or less, it is preferable that the rolling direction be changed eight or more times.

[0081] Furthermore, the reduction amount ΔH per pass (each pass) is set to 15% or more of the distance between flats of the titanium ingot in the reduction direction. This is to uniformly introduce strain, refine the microtexture, and set the microtexture area ratio to 30.0% or less. To set the microtexture area ratio to 25.0% or less, the reduction amount ΔH per pass (each pass) is preferably set to 20% or more of the distance between flats of the titanium ingot in the reduction direction.

[0082] The pair of anvils used above have a longitudinal width of 100 mm or more on the rolling face, because the use of such wide anvils allows strain to be introduced uniformly over a wide range.

[0083] Here, in the first step, in the six or more forgings described above, it is necessary to repeatedly perform forging at a low strain rate and forging at a high strain rate. In this step, the strain rate in the low strain rate forging (hereinafter also referred to as the "low side strain rate") is 0.10 / s or less, and the strain rate in the high strain rate forging (hereinafter also referred to as the "high side strain rate") is 1.4 / s or more. The low side strain rate is preferably 0.05 / s or less, and the high side strain rate is preferably 2.5 / s or more.

[0084] In addition, the reduction of area during forging when the strain rate is changed is set to 10% or more. By repeatedly performing the above-mentioned low strain rate forging and high strain rate forging, and setting the reduction of area during forging when the strain rate is changed to 10% or more, it becomes possible to reduce the pair area ratio. The reduction of area during forging when the strain rate is changed is preferably 15% or more.

[0085] The reduction of area in forging when the strain rate is changed means the total reduction of area in forging when the forging is changed from a low strain rate to a high strain rate, and when the forging is changed from a high strain rate to a low strain rate, among all forgings. A more specific calculation method will be explained below.

[0086] For example, assume that five forgings are performed under the following forging conditions: First forging: Slow strain rate, cross-sectional area S １ｓ From S １１ Second forging: The cross-sectional area is S at a low strain rate. １１ From S １２ Forging 3rd time: High strain rate and cross-sectional area S １２ From S １３ Forging is performed so that the cross-sectional area becomes S at a high strain rate. １３ From S １４ Forging is performed so that the cross-sectional area becomes S at a low strain rate. １４ From S １ｆ In this example, the forgings when the strain rate is changed are the third and fifth forgings. The reduction in area of ​​the forgings when the strain rate is changed is {1 - (S１３ / S １２ ) × (S １ｆ / S １４ )} × 100.

[0087] By repeatedly performing forging at a low strain rate and forging at a high strain rate, it is possible to introduce uniform strain not only in the direction perpendicular to the longitudinal direction but also in the direction parallel to the longitudinal direction, thereby changing the crystal orientation. As a result, the microtexture becomes finer, the α grains become finer, and the pair area ratio can be reduced.

[0088] After this forging, the material is cooled. Cooling is preferably carried out by rapid cooling. Rapid cooling is generally carried out by immersing the forged titanium material in a sufficient amount of water. This method may be used if a cooling rate equivalent to or faster than water cooling can be obtained. Rapid cooling is preferably continued until the surface temperature of the forged titanium material drops to 300°C or below.

[0089] In the first step, upset forging may be performed prior to forging, which involves rolling down the titanium ingot from a direction perpendicular to its longitudinal direction. Upset forging is forging in which the titanium ingot is rolled down from both the top and bottom, i.e., both sides of the longitudinal direction. Upset forging makes it possible to introduce strain more uniformly into the interior of the titanium ingot. In addition, upset forging can increase the cross-sectional area of ​​the titanium ingot, which makes it easier to increase the total reduction in area during the forging in the first step.

[0090] After the first step, the forged titanium material is heated to a temperature between (Tβ-150)°C and (Tβ-50)°C, forged to a total area reduction of 20% or more, and then cooled. This step is called the second step, and the forged titanium material after the second step is called rolled titanium material.

[0091] If the heating temperature in the second step is less than (Tβ-150)°C, workability may decrease and cracks may occur. Therefore, the heating temperature is set to (Tβ-150)°C or higher. On the other hand, if the heating temperature is set to (Tβ-50)°C or higher, the β phase ratio increases and strain cannot be effectively imparted to the α grains, making it impossible to refine the microtexture and achieving the desired structure. Therefore, the heating temperature is set to less than (Tβ-50)°C.

[0092] Furthermore, if the total area reduction rate in the forging of the second step is less than 20%, the α grains cannot be refined. Furthermore, the crystal orientation cannot be randomized, and the microtexture is not refined. Therefore, the total area reduction rate is set to 20% or more. If the above-mentioned paired circle equivalent diameter is desired to be 430 μm or less, the total area reduction rate is preferably 30% or more.

[0093] The "total area reduction rate" in the second process is the average area of ​​the cross section perpendicular to the longitudinal direction of the forged titanium material before the second process, expressed as "S ２ｓ ", and the average area of ​​the cross section perpendicular to the longitudinal direction of the titanium rolled material after the second process is "S ２ｆ ", then {(S ２ｓ -S ２ｆ ) / S ２ｓ} × 100. After forging in this manner, the material is cooled. Cooling is preferably carried out in the same manner as in the first step. However, the average area of ​​a cross section perpendicular to the longitudinal direction of the forged or rolled titanium material refers to the average value of three values: the area of ​​one end of the forged or rolled titanium material in the longitudinal direction, the cross-sectional area of ​​the center, and the area of ​​the other end.

[0094] As in the first step, the second step also requires repeated forging at a low strain rate and a high strain rate. In this step, the low-side strain rate is set to 0.10 / s or less, and the high-side strain rate is set to 1.4 / s or more. The low-side strain rate is preferably 0.05 / s or less, and the high-side strain rate is preferably 2.5 / s or more.

[0095] By alternately repeating forging with a slow strain rate and forging with a fast strain rate, it is possible to introduce uniform strain not only in the direction perpendicular to the longitudinal direction but also in the direction parallel to the longitudinal direction, thereby changing the crystal orientation. As a result, the microtexture becomes finer, the α grains become finer, and the pair area ratio can be reduced. In the second step, as in the first step, it is preferable to forge six or more times. In the second step, as in the first step, the titanium forged material may be forged while being rotated around its longitudinal direction, or it may be forged in only one direction without rotation.

[0096] After the first step or the second step, cutting may be carried out as necessary to shape the steel sheet into a desired shape. This cutting is carried out for the purpose of removing defects that occur during hot working, etc.

[0097] After the second step, the rolled titanium material is heated to a temperature of (Tβ-100)°C or higher but lower than (Tβ-20)°C, rolled to a total area reduction of 75% or higher, and then cooled. This step is called the third step.

[0098] If the heating temperature in the third step is less than (Tβ-100)°C, it is difficult for the α grains to become fine and equiaxed. As a result, the microtexture also becomes coarse. For this reason, the heating temperature is set to (Tβ-100)°C or higher. On the other hand, even if the heating temperature is (Tβ-20)°C or higher, it is difficult for the α grains to become fine and equiaxed. As a result, the microtexture also becomes coarse. For this reason, the heating temperature is set to less than (Tβ-20)°C.

[0099] Furthermore, even if the total area reduction rate of the rolling in the third step is less than 75%, the microtexture will become coarse. Therefore, the total area reduction rate is set to 75% or more. After this forging, the steel is cooled. Cooling is preferably performed in the same manner as in the first and second steps.

[0100] In the third step, the type of rolling is not particularly limited, but groove rolling is usually preferred.

[0101] After the third step, if necessary, a heat treatment may be performed for the purpose of strain removal, structural stabilization, etc. For example, when strain removal and structural stabilization are the objectives, it is preferable to hold the material at 750°C for 1 hour and then air-cool it.

[0102] Furthermore, for the purpose of material quality control, solution aging treatment may be performed after the third step, if necessary. In solution aging treatment, solution treatment is first performed by holding the titanium material at a temperature of 800°C or higher but lower than Tβ°C for at least one minute, followed by rapid cooling. Next, aging treatment is performed by holding the titanium material at a temperature of 400°C or higher but lower than 600°C for at least one minute, followed by air cooling. The holding time may be any time long enough to achieve uniform heating of the titanium material, and may be adjusted to, for example, 5 minutes or more or 10 minutes or more depending on the size of the titanium material and the performance of the furnace. Solution aging treatment reduces the area ratio of microtexture and α-grains, which are prone to crack initiation, and further improves Dwell fatigue properties. The heating temperatures and solution aging treatment temperatures in the first to third steps refer to the temperature inside the furnace.

[0103] The titanium material according to the present invention will be described in more detail below with reference to examples, but the titanium material according to this embodiment is not limited to these examples.

[0104] Titanium ingots (400 mm in diameter) having the chemical compositions shown in Table 1 were produced by vacuum arc melting (VAR). In Table 1, "-" indicates that no elements were intentionally added.

[0105]

[0106] The obtained titanium ingot was subjected to the first process under the conditions shown in Table 2, followed by cooling to 300°C or below, the second process, followed by cooling to 300°C or below, and the third process, followed by cooling to 300°C or below. If necessary, the ingot was subjected to solution aging treatment under the conditions shown in Table 2. The "Heat Treatment" column in Table 2 lists the heating temperature, holding time, and cooling method, in that order. Titanium material (bar material with a circular cross section) with a diameter of 35 mm was used. Other conditions not listed were within the range of preferred manufacturing conditions. In Table 2, underlines indicate conditions outside the range of preferred conditions.

[0107] In the first step, the ΔH per pass / flat distance in the rolling direction was the same for each pass, and the strain rate was the same for each forging. When forging was performed with different strain rates, the strain rate was changed between the slow strain rate and the fast strain rate shown in Table 2. In the first step, the workpiece was rotated 30 to 45° around the longitudinal direction the number of times shown in Table 2, and the rolling direction was changed for rolling. The total area reduction in the first step refers to the total area reduction at a temperature of Tβ°C or higher. Furthermore, in the second step, the workpiece was repeatedly forged with different strain rates between the slow strain rate and the fast strain rate shown in Table 2 for each forging. In the second step, the workpiece was rotated 30 to 45° around the longitudinal direction for each forging, and the rolling direction was changed for rolling.

[0108]

[0109] The microtexture area ratio, average equivalent circle diameter, paired area ratio, paired equivalent circle diameter, adjacent grain area ratio, and average equivalent circle diameter of α grains were calculated for the obtained titanium materials using the following methods. The Dwell fatigue properties and specularity of the titanium materials were also evaluated using the following procedures.

[0110] (Observation of Metal Structure) Observation of the metal structure was carried out using an EBSD device attached to the SEM.

[0111] Test pieces for metallographic observation were taken from the surface layer of the titanium material, with a length of 3 mm and a width of 3 mm as the observation area. The observation surface was parallel to the longitudinal direction of the titanium material, perpendicular to the radial direction of the circular cross section perpendicular to the longitudinal direction of the titanium material, and centered at a depth of r / 2 from the side surface of the titanium material. Strain introduced during the collection of the test pieces was removed by polishing. During polishing, the specimens were polished with colloidal silica to prevent the introduction of new strain.

[0112] The measurement interval was 2.0 μm in all measurement areas, and the orientation was measured using EBSD at an acceleration voltage of 15 kV. The obtained measurement results were analyzed using OIM (crystal orientation analysis software "OIM Analysis v8.1.0" manufactured by TSL Solutions Co., Ltd.). In this case, a Partition targeting only the α phase was created and used as the subject of analysis. Note that, since it is usually unlikely that the entire periphery of the microtexture is adjacent to β grains, only α grains were targeted.

[0113] Then, α-grains were determined as those for which the angular difference in crystal orientation (misorientation angle) between adjacent EBSD measurement points was 15° or more, and the circle-equivalent diameter of each α-grain was calculated from the number of measurement points of the α-grains, thereby calculating the average circle-equivalent diameter of the α-grains. Furthermore, the c-axis orientation difference between adjacent EBSD measurement points was determined from the obtained measurement results (Euler angles ph1, PH, ph2), and cases in which the c-axis orientation difference was 0 to 20° and the circle-equivalent diameter was 50 μm or more were determined to be microtexture.

[0114] Among the structures judged to be microtextures, two adjacent microtextures were used, and the difference in the average direction of the c-axis of the α grains constituting each microtexture was calculated. Next, the difference in the average direction of the c-axis of the two microtextures, i.e., the angle θ ０ Then, the angle θ ０ The area ratio of the microtexture in the observation area was calculated by extracting the microtexture contained in the pair of microtextures whose angle was 70° or more, and dividing this by the area ratio of all the microtextures to calculate the pair area ratio. Similarly, the pair of microtextures was extracted, the total cross-sectional area of ​​the pair of microtextures was measured, and the pair circle equivalent diameter was calculated from the average value of the cross-sectional areas of all the extracted pair of microtextures.

[0115] In addition, the angle formed by the average direction of the c-axes of the α-grains constituting the region determined to be a microtexture and the average direction of the c-axes of the α-grains adjacent to the microtexture so as to surround the microtexture, i.e., the angle θ １ The angle θ １The area ratio of the microtextures in the observation area was calculated by extracting the microtextures in which the angle was 45° or more, and dividing the area ratio by the area ratio of all the microtextures to calculate the adjacent grain area ratio.

[0116] (Dwell fatigue properties) Dwell fatigue properties were evaluated by conducting tensile tests and fatigue tests. Tensile test specimens and fatigue test specimens were taken from the obtained titanium material so that the longitudinal direction of the titanium material was the longitudinal direction of the test specimen and the position of the depth of r / 2 was the center. In the tensile test, a tensile test specimen with a parallel portion of φ5 × 30 mm was used, and the gauge length (gauge length) was 25 mm and the strain rate was 8.3 × 10 －５ / s, and other conditions were in accordance with ASTM E8M, and the 0.2% proof stress was measured.

[0117] In the fatigue tests, fatigue test specimens with a parallel section of φ5.08 mm × 15.24 mm were used, and the gauge length (gauge length) was 12 mm, the axial force was pulsating, the stress ratio was 0.05, the maximum stress was 95% of the 0.2% yield strength of the same material (same direction), and the stress waveform was a trapezoidal waveform load cycle of 1 s loading, 120 s holding, and 1 s unloading, to measure the Dwell fatigue life.

[0118] As a result of the measurement, when the Dwell fatigue life was 15,000 cycles or more, the Dwell fatigue characteristics were deemed to be good.

[0119] (Specularity) Specularity was further evaluated for some titanium materials. Specularity was evaluated by measuring DOI. Test pieces for DOI measurement were taken from the obtained titanium materials so that an area of ​​8 mm length x 8 mm width was the evaluation area. The evaluation surface was a surface that was parallel to the longitudinal direction of the titanium material, perpendicular to the radial direction of the circular cross section perpendicular to the longitudinal direction of the titanium material, and centered at a depth of r / 2 from the side surface of the titanium material.

[0120] The test specimen was embedded in epoxy resin, and the evaluation surface was wet polished using abrasive paper, from coarse to fine. The evaluation surface was then dry buffed to a mirror finish. The DOI of the evaluation surface was then measured. The DOI was measured using a Rhopoint IQ Flex 20 appearance analyzer manufactured by Rhopoint Instruments, with the incident light angle set at 20°, and other conditions in accordance with ASTM D 5767.

[0121] As a result of the measurement, the specularity was determined to be good when the DOI was 60 or more. In the "DOI" column of Table 3, "-" indicates that the DOI was not measured.

[0122] The results of the metal structure observation and property evaluation are summarized in Table 3. In Table 3, underlined items indicate items outside the scope of this embodiment.

[0123]

[0124] Test Nos. 1 to 27, which satisfied the requirements of this embodiment, had good Dwell fatigue properties. However, among these, Test Nos. 3 and 14, which had excessive adjacent grain area ratios, had DOIs of less than 60, resulting in reduced specularity. On the other hand, Test Nos. 28 to 38, which did not satisfy the requirements of this embodiment, had poor Dwell fatigue properties.

[0125] Specifically, in Tests No. 28 and 37, the low-speed strain rate in the first step was too fast, making it impossible to reduce the paired area ratio, resulting in a deterioration in the Dwell fatigue properties. In Test No. 29, the total area reduction rate in the first step was low, causing the average equivalent circle diameter of the microtexture to exceed the specified range, resulting in a deterioration in the Dwell fatigue properties. In Test No. 30, the low-speed strain rate in the second step was too fast, making it impossible to reduce the paired area ratio, resulting in a deterioration in the Dwell fatigue properties.

[0126] In Test No. 31, the heating temperature in the second step was too high, causing the average equivalent circle diameter of the microtexture to exceed the specified range, resulting in a deterioration in Dwell fatigue properties. In Test No. 32, the reduction in area of ​​the forged section when the strain rate was changed in the first step was low, making it impossible to reduce the paired area ratio, resulting in a deterioration in Dwell fatigue properties. In Tests No. 33 and 34, the high-speed strain rate in the first step was too slow, making it impossible to reduce the paired area ratio, resulting in a deterioration in Dwell fatigue properties.

[0127] In Test Nos. 35 and 38, the high-speed strain rate in the second step was too slow, making it impossible to reduce the paired area ratio, resulting in a deterioration in Dwell fatigue properties. In Test No. 36, the reduction amount ΔH per pass in the first step was small relative to the distance between flats of the titanium ingot in the reduction direction, causing the average circle-equivalent diameter of the microtexture to exceed the specified range, resulting in a deterioration in Dwell fatigue properties.

[0128] As described above, according to the present disclosure, a titanium material with improved Dwell fatigue properties can be obtained.

Claims

1. When a microtexture is an aggregate of α grains in which the misorientation between the c-axes of adjacent α grains is 20° or less and the equivalent circle diameter is 50 μm or more, in the metal structure of the surface layer, the area ratio of the microtexture is 30.0% or less, the average equivalent circle diameter of the microtexture is 300 μm or less, two microtextures are adjacent to each other, and the angle between the average direction of the c-axes of the α grains constituting one of the two microtextures and the average direction of the c-axes of the α grains constituting the other microtexture is an angle θ ０ When the angle θ is ０ A titanium material, wherein the area ratio of the microtextures contained in the two microtextures satisfying an angle of 70° or more is 20.0% or less.

2. The titanium material according to claim 1, wherein the average equivalent circle diameter of α grains in the metal structure of the surface layer is 20.0 μm or less.

3. In the metal structure of the surface layer, the angle between the average direction of the c-axis of the α grains constituting one microtexture and the average direction of the c-axis of the α grains adjacent to the one microtexture is defined as angle θ １ When the angle θ is １ The titanium material according to claim 1, wherein the area ratio of the microtexture satisfying the condition of 45° or more is 15.0% or less.

4. In the metal structure of the surface layer, the angle between the average direction of the c-axis of the α grains constituting one microtexture and the average direction of the c-axis of the α grains adjacent to the one microtexture is defined as angle θ １ When the angle θ is １ The titanium material according to claim 2, wherein the area ratio of the microtexture satisfying the condition of 45° or more is 15.0% or less.

5. The titanium material according to any one of claims 1 to 4, which is an α type or an α+β type.

6. A titanium material according to any one of claims 1 to 4, having a chemical composition, in mass%, of Al: 4.40 to 6.75%, Fe: 0.05 to 3.10%, O: 0.05 to 0.40%, V: 0 to 4.50%, Mo: 0 to 5.50%, Si: 0 to 0.40%, and the balance being Ti and impurities.

7. The titanium material according to any one of claims 1 to 4, which is in the form of a rod.

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