Titanium material

Abstract

A titanium material in which, in a case where a collection of α grains in which an orientation difference between c-axes of adjoining α grains is 20° or less and an equivalent circle diameter is 50 μm or more is taken as a microstructure, in a metallographic structure of a surface layer portion, an area ratio of the microstructure is 30.0% or less, an average equivalent circle diameter of the microstructure is 300 μm or less, in two microstructures adjoining, an angle θ0 is set as an angle between an average direction of c-axes of α grains constituting one of the two microstructures and an average direction of c-axes of α grains constituting the other microstructure, and a proportion of an area ratio of the microstructure included in the two microstructures in which the angle θ0 is 70° or more or more with respect to the area ratio of the microstructure is 20.0% or less.

Description

Technical Field

[0001] This disclosure relates to titanium materials. Background Technology

[0002] Titanium is a lightweight and high-strength material used in components of aircraft, automobiles, golf clubs, and more. In these components, particularly high-speed rotating parts used in automobiles, such as motor sleeves, load-bearing fatigue resistance is crucial.

[0003] Hold-load fatigue refers to fatigue caused by a high-stress load close to the yield stress sustained for a certain period of time. In typical fatigue, evaluation is based on a general triangular or sinusoidal load cycle. Conversely, hold-load fatigue is evaluated using a trapezoidal load cycle. Generally, compared to typical fatigue, the higher stress acts for a longer period, resulting in fracture with fewer cycles.

[0004] Titanium materials are prone to failure under load-bearing fatigue conditions. Therefore, there is a need to improve load-bearing fatigue properties. Consequently, titanium materials with improved load-bearing fatigue properties are disclosed, for example, in Patent Documents 1 and 2.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2021-167448

[0008] Patent Document 2: Japanese Patent Application Publication No. 2021-167449 Summary of the Invention

[0009] The problem the invention aims to solve

[0010] Patent documents 1 and 2 disclose methods for controlling the size of aggregates of locally similar crystal orientations in titanium materials, known as microtexture, to improve load-bearing fatigue characteristics. However, in recent years, the development of improved engine fuel efficiency and higher motor output in automobiles has led to a demand for even better load-bearing fatigue characteristics, specifically a load-bearing fatigue life of over 15,000 cycles, under continuous high-stress load conditions.

[0011] In view of the above, the purpose of this disclosure is to provide titanium materials with improved load-bearing fatigue characteristics, specifically, to provide titanium materials with a load-bearing fatigue life of 15,000 cycles or more.

[0012] Solution for solving the problem

[0013] This disclosure was made to solve the above-mentioned problems, and its main purpose is the following titanium material and manufacturing method.

[0014] (1) A titanium material, wherein, when the aggregate of α grains with an orientation difference of less than 20° between the c-axis of adjacent α grains and an equivalent circle diameter of more than 50 μm is used as the microtexture.

[0015] In the metallographic structure of the surface layer

[0016] The area fraction of the microtexture is below 30.0%.

[0017] The average equivalent circle diameter of the microtexture is less than 300 μm.

[0018] When two microtextures are adjacent, and the angle between the average direction of the c-axis of the α-grain constituting one of the two microtextures and the average direction of the c-axis of the α-grain constituting the other microtexture is set as angle θ0,

[0019] The area ratio of the microtexture contained in the two microtextures where the angle θ0 is 70° or higher is less than 20.0% relative to the area ratio of the microtexture.

[0020] (2) According to the titanium material described in (1) above, in the metallographic structure of the surface layer, the average equivalent circle diameter of the α grains is less than 20.0 μm.

[0021] (3) According to the titanium material described in (1) or (2) above, when the angle between the average direction of the c-axis of the α grain constituting a micro-texture and the average direction of the c-axis of the α grain adjacent to the micro-texture is set as angle θ1, the area ratio of the micro-texture with an angle θ1 of 45° or more is 15.0% or less relative to the area ratio of the micro-texture.

[0022] (4) The titanium material according to any one of (1) to (3) above is of type α or type α+β.

[0023] (5) The titanium material according to any one of (1) to (4) above has the following chemical composition in mass%:

[0024] Al: 4.40~6.75%, Fe: 0.05~3.10%, O: 0.05~0.40%, V: 0~4.50%, Mo: 0~5.5%, Si: 0~0.40%, with the balance being Ti and impurities.

[0025] (6) The titanium material according to any one of (1) to (4) above has the following chemical composition in mass%: Al: 4.40~5.50%, Fe: 1.40~2.50%, Mo: 1.50~5.50%, O: 0.05~0.25%, balance: Ti and impurities.

[0026] (7) The titanium material according to any one of (1) to (4) above has the following chemical composition in mass%: Al: 5.50~6.75%, V: 3.50~4.50%, Fe: 0.05~0.40%, O: 0.05~0.25%, balance: Ti and impurities.

[0027] (8) The titanium material according to any one of (1) to (4) above has the following chemical composition in mass%: Al: 4.70~6.50%, Fe: 0.05~3.10%, Si: 0.15~0.40%, O: 0.05~0.40%, balance: Ti and impurities.

[0028] (9) The titanium material according to any one of (1) to (4) above has the following chemical composition in mass%: Al: 4.40~5.50%, Fe: 0.50~1.40%, Si: 0~0.10%, O: 0.05~0.40%, balance: Ti and impurities.

[0029] (10) The titanium material according to any one of (1) to (9) above is a rod.

[0030] The effects of the invention

[0031] According to this disclosure, titanium materials with improved load-bearing fatigue properties can be obtained. Detailed Implementation

[0032] The inventors have studied methods to improve the load-bearing fatigue properties of titanium materials and obtained the following insights.

[0033] (a) The above-mentioned microtexture is an aggregate of α grains with an orientation difference of less than 20° between the c-axis of adjacent α grains. It is at least related to the load-bearing fatigue characteristics, acts as an α grain, and is affected by the average direction of the c-axis of the α grains constituting the microtexture.

[0034] (b) By reducing the proportion of the aforementioned microtexture, and thus making the microtexture smaller, the load-bearing fatigue characteristics can be improved. That is, it is effective to reduce the area ratio of the microtexture to below 30.0% and to set the average equivalent circle diameter of the microtexture to below 300 μm.

[0035] However, in titanium materials with multiple microtextures, even those consisting of microtextures with equivalent circle diameters of the same degree sometimes exhibit short lifespans, making it difficult to further improve load-bearing fatigue characteristics. Moreover, the cause was found to be a pair of adjacent microtextures with a specific orientation relationship, leading to early failure.

[0036] (c) There is a correlation between the angle φ formed by the average direction of the c-axis of the α grains constituting the microtexture and the direction of the applied stress, and the deformation resistance. The closer the angle φ is to 0°, the more difficult the deformation; the closer it is to 90°, the easier the deformation. Therefore, it can be considered that if the difference in the average direction of the c-axis of an adjacent pair of α grains constituting the microtexture is greater than 70°, the stress distribution caused by the difference in deformation capacity will increase, leading to early failure.

[0037] That is, when the average direction of the c-axis of the α grains constituting one of the two microtextures is more than 70° away from the average direction of the c-axis of the α grains constituting the other microtexture, premature failure occurs. Therefore, reducing the area fraction of microtextures with such orientation relationships, specifically, making the area fraction below 20.0%, is effective.

[0038] (d) Building on this, from a design perspective, titanium materials also require a mirror finish, meaning they can be polished to a good mirror-like state. Furthermore, the inventors have clarified that microstructure also affects mirror finish. This is because, for example, when there is a large orientation difference between the microstructure and the grains formed around it, unevenness is generated during polishing, reducing mirror finish.

[0039] Therefore, in order to obtain better mirror properties, it is effective to have an area ratio of 15.0% or less for microtextures in which the average direction of the c-axis of the α grains constituting a microtexture and the average direction of the c-axis of the α grains adjacent to the microtexture form an angle of 45° or more.

[0040] One embodiment of the present invention was made based on the above insights. Hereinafter, the various features of the titanium material of this embodiment will be described in detail.

[0041] 1. Metallographic structure

[0042] 1-1. Definition of Microtexture

[0043] In the titanium material of this embodiment, load-bearing properties are improved by controlling the microstructure. Here, the microstructure is an aggregate of α grains whose c-axis orientation difference between adjacent α grains is 20° or less and whose equivalent circle diameter is 50 μm or more. It should be noted that α grains are α-phase grains.

[0044] When the orientation difference between the c-axis of adjacent α grains exceeds 20°, it is difficult for them to deform as a unified aggregate. It should be noted that since the threshold value for the orientation difference between the c-axis of α grains, which becomes the starting point of fatigue failure, is 0~20°, the lower limit of the orientation difference between the c-axis of adjacent α grains in the microtexture is preferably 0°. Furthermore, an aggregate of adjacent α grains with an equivalent circle diameter less than 50 μm will not significantly affect the load-bearing properties. Therefore, the definition of microtexture is as described above. It should be noted that the equivalent circle diameter of the aforementioned microtexture refers to the diameter of a circle with the same area as the microtexture. Additionally, the aforementioned c-axis refers to the direction perpendicular to the bottom surface of the hcp structure.

[0045] In the titanium material of this embodiment, the microstructure is controlled in the metallographic structure of the surface layer. Here, for a cylindrical titanium material with a circular cross-section of radius r perpendicular to its long side, the surface layer refers to a depth of r / 2 from the side surface of the titanium material; for a prismatic titanium material with a rectangular cross-section of side length d, the surface layer refers to a depth of d / 4 from the side surface of that side length. It should be noted that if the depth of r / 2 or d / 4 from the surface satisfies the requirements of this embodiment, it can be said that even a depth shallower than r / 2 or d / 4 from the surface satisfies the requirements of this embodiment. 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. In addition, regarding fatigue characteristics including load-bearing fatigue, it is generally believed that the surface layer, which contains the surface with the greatest stress, is the starting point of fatigue failure. Therefore, the surface layer including the surface is considered.

[0046] 1-2. Area ratio of microtexture

[0047] In the titanium material of this embodiment, the area fraction of the microtexture is 30.0% or less. If the area fraction of the microtexture is 30.0% or less, the frequency of fatigue crack occurrence decreases, and the simultaneous and frequent formation of faceted fracture surfaces of similar size to the microtexture is suppressed. As a result, crack initiation and propagation are reduced, and the fatigue life under load is improved. The area fraction of the microtexture is preferably 25.0% or less, more preferably 20.0% or less. A smaller area fraction of the microtexture is more preferred, but the formation of microtexture cannot be completely avoided; therefore, the practical lower limit is 1.0%.

[0048] 1-3. Average equivalent circle diameter of microtexture

[0049] In the titanium material of this embodiment, the average equivalent circle diameter of the microtexture is 300 μm or less. Here, the average equivalent circle diameter of the microtexture refers to the average of the equivalent circle diameters of each microtexture. If the average equivalent circle diameter of the microtexture is 300 μm or less, stress distribution caused by fatigue can be reduced, fatigue crack formation can be reduced, and thus load-bearing fatigue characteristics are improved. The average equivalent circle diameter of the microtexture is preferably 250 μm or less, and more preferably 200 μm or less. It should be noted that, according to the above definition of microtexture, the practical lower limit of the average equivalent circle diameter of the microtexture is 50 μm.

[0050] 1-4. Area ratio of paired microtextures

[0051] In the titanium material of this embodiment, by reducing the area ratio of a pair of adjacent microtextures with a specific orientation relationship that become the starting point of failure, the fatigue life under load can be increased to more than 15,000 cycles.

[0052] Specifically, in the titanium material of this embodiment, when two microtextures are adjacent to each other, and the angle between the average direction of the c-axis of the plurality of α grains constituting one of the two microtextures and the average direction of the c-axis of the plurality of α grains constituting the other microtexture is set as angle θ0, the ratio of the area ratio of the microtextures contained in the two microtextures (referred to as "paired microtextures" in this application specification) to the area ratio of the microtextures described above (hereinafter, for simplicity, referred to as "paired area ratio") is 20.0% or less.

[0053] It should be noted that, for example, the case where three microtextures (let's call them microtextures a, b, and c) are adjacent to each other can also be considered. Furthermore, when microtextures a and b, microtextures b and c, and microtextures a and c are pairs of microtextures, microtextures a, b, and c become the microtextures contained within the pairs of microtextures. That is, the total area ratio of microtextures a, b, and c becomes the area ratio of the microtextures contained within the pairs of microtextures.

[0054] If the paired area ratio is 20.0% or less, the proportion of paired microtextures that become the fatigue initiation point is smaller, making early crack formation less likely. As a result, the load-bearing fatigue characteristics are improved. Therefore, the above-mentioned paired area ratio is preferably 18.0% or less, more preferably 16.0% or less. Since the formation of paired microtextures with large orientation differences is inevitable, the practical lower limit of the above-mentioned paired area ratio is 1.0%. It should be noted that the above-mentioned paired area ratio is practically 6.0% or less relative to the observed total area. In addition, when the orientation difference is greater than 90°, 90° is subtracted from the orientation difference, so the orientation difference of 90° or less becomes the measurement target.

[0055] 1-5. Total equivalent circle diameter of paired microtextures

[0056] In the titanium material of this embodiment, it is preferable that the average value of the total equivalent circular diameter of the paired microtextures (hereinafter, for simplicity, referred to as "paired equivalent circular diameter") is 430 μm or less. It should be noted that the total equivalent circular diameter refers to the equivalent circular diameter calculated from the cross-sectional areas of the two microtextures. That is, the equivalent circular diameter calculated when the two microtextures are considered as a single aggregate.

[0057] If the diameter of the paired equivalent circles is 430 μm or less, the stress distribution between the two microtextures becomes smaller, and the load-bearing fatigue characteristics are improved. The diameter of the paired equivalent circles is preferably 400 μm or less, more preferably 380 μm or less, and even more preferably 350 μm or less. It should be noted that, according to the definition of microtexture above, the practical lower limit of the diameter of the paired equivalent circles is 70 μm.

[0058] 1-6. Area ratio of microtextures with large orientation differences from adjacent α grains

[0059] To improve mirror finish, it is effective not only to suppress coarse microtextures but also to reduce the orientation difference between the microtextures and the surrounding grains. This is because such microstructure control can suppress the formation of unevenness during polishing.

[0060] Therefore, in the titanium material of this embodiment, when the angle between the average direction of the c-axis of the α grain constituting a microtexture and the average direction of the c-axis of the α grain adjacent to the microtexture is defined as angle θ1, the ratio of the area ratio of the microtexture with an angle θ1 of 45° or more to the area ratio of the microtexture (hereinafter referred to as "adjacent grain area ratio") is preferably 15.0% or less. It should be noted that when the angle θ1 is less than 45°, the orientation difference between the microtexture and the surrounding grains is sufficiently small, and the difference in unevenness is small. Therefore, the difference in unevenness is not significant, and the influence on mirror finish can be ignored. Moreover, if the adjacent grain area ratio is 15.0% or less, unevenness is less likely to occur between a microtexture and the adjacent α grain, thereby improving mirror finish. 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. The adjacent grain area ratio is preferably minimized as much as possible, and most preferably 0%. It should be noted that when the orientation difference is greater than 90°, 90° is subtracted from the orientation difference. Therefore, the orientation difference is less than 90° and is the object of measurement.

[0061] 1-7. Average equivalent circle diameter of α grains

[0062] In the titanium material of this embodiment, the average equivalent circle diameter of the α grains is preferably 20.0 μm or less. If the average equivalent circle diameter of the α grains is 20.0 μm or less, the average equivalent circle diameter of the microtexture can also be reduced. As a result, due to the different orientations of the microtexture and the surrounding grains, the stress distribution becomes smaller, and fatigue cracks are less likely to occur. That is, the load-bearing fatigue characteristics are improved. The average equivalent circle diameter of the 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.

[0063] 1-8. Observation of metallographic structure

[0064] The above-mentioned metallographic structure was observed using an EBSD (Electron Backscatter Diffraction) device attached to a scanning electron microscope (hereinafter also referred to as "SEM"), and the measurements were performed according to the following steps.

[0065] In the surface layer of the titanium material, a rectangular area of ​​3mm x 3mm is used as the observation area. For observation of the surface layer, for cylindrical titanium materials with a circular cross-section of radius r perpendicular to the long side, the observation surface is a plane parallel to the long side, radially perpendicular to the aforementioned cross-section, and centered at a depth of r / 2 from the side surface. For prismatic titanium materials with a rectangular cross-section of side length d perpendicular to the long side, the observation surface is a plane parallel to the long side and the side surface of that side length, and centered at a depth of d / 4 from the side surface of that side length. It should be noted that fatigue characteristics, including load-bearing fatigue, are preferably measured on the plane perpendicular to the maximum applied stress. Furthermore, the direction of the applied stress depends on the usage environment and testing method, but the surface layer, including the surface where the maximum stress occurs most frequently, is prone to the initiation of fatigue failure; therefore, the surface layer is observed. It should be noted that the strain introduced when cutting out the observation surface is removed by grinding. At this point, in order to avoid introducing new strain due to grinding, colloidal silica grinding and finishing is performed.

[0066] The measurement interval was set to 2.0 μm across the entire measurement area, and orientation was determined using EBSD at an accelerating voltage of 15 kV. The measurement results were analyzed using OIM (OIM Analysis v8.1.0, a crystal orientation analysis software manufactured by TSL Solutions, Inc.). At this time, a Partiton focusing solely on the α phase was fabricated as the object of analysis. It should be noted that it is generally difficult to assume that the entire surrounding microtexture is adjacent to β grains; therefore, focusing only on the α grains is sufficient.

[0067] Then, α grains are determined by setting the angular difference (orientation difference angle) of crystal orientation between adjacent EBSD measurement points to 15° or more. The equivalent circle diameter of each α grain is calculated based on the number of measurement points for that α grain, and the average equivalent circle diameter of the α grains is calculated. In addition, based on the obtained measurement results (Euler angles ph1, ph2), the c-axis orientation difference with adjacent EBSD measurement points is calculated. Aggregates of α grains with a c-axis orientation difference of 0~20° and an equivalent circle diameter of 50μm or more are identified as microtexture.

[0068] In an organization identified as having microtexture, 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 in the average direction of the c-axis of the two microtextures, i.e., the angle θ0, is calculated. Then, the microtextures contained in pairs of microtextures (where θ0 is 70° or greater) are extracted, and the area ratio of this microtexture in the observation region is calculated. This area ratio is then divided by the area ratio of all microtextures. Furthermore, having calculated the paired equivalent circle diameter of each microtexture, the paired microtextures are similarly extracted, and the total cross-sectional area of ​​the paired microtextures is measured. The paired equivalent circle diameter is calculated from the average of the cross-sectional areas of all extracted paired microtextures.

[0069] Furthermore, after calculating the area ratio of adjacent grains, the angle θ1 is calculated between the average direction of the c-axis of the α-grains constituting the region determined as a microtexture and the average direction of the c-axis of the α-grains adjacent to it in a manner that surrounds the microtexture. Then, microtextures with an angle θ1 of 45° or greater are extracted, the area ratio of the microtexture in the observation region is calculated, and the area ratio of adjacent grains is calculated by dividing it by the area ratio of all microtextures.

[0070] 2. Types

[0071] The type of titanium material used in this embodiment is not particularly limited. For example, any titanium alloy is acceptable.

[0072] As titanium alloys, α-type titanium alloys or α+β-type titanium alloys are preferred. It should be noted that α-type titanium alloys include, for example, highly corrosion-resistant alloys (JIS standards types 11-13, 17, 19-22, and titanium alloys specified in ASTM standards Grades 7, 11-14, 17, 31, and 34, and further, 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-0.05~0.2Pd, etc.

[0073] Generally, higher strength leads to better load-bearing fatigue characteristics. Therefore, materials with a tensile strength of 950 MPa or higher are preferred. It should be noted that α+β type titanium alloys, due to their high strength, are used as motor sleeves and other components in automobiles. For example, α+β type titanium alloys are preferably those with the α phase as the main phase and the β phase as the second phase at room temperature (25°C).

[0074] It should be noted that the chemical composition of the α+β type alloy is not particularly limited. For example, the preferred composition by mass percentage is Al: 4.40~6.75%, Fe: 0.05~3.10%, O: 0.05~0.25%, V: 0~4.50%, Mo: 0~5.50%, Si: 0~0.40%, with the balance being Ti and impurities.

[0075] More preferably, by mass % Al: 4.40~5.50%, Fe: 1.40~2.50%, Mo: 1.50~5.50%, O: 0.05~0.25%, balance: Ti and impurities (commonly referred to as "Ti-5Al-2Fe-3Mo"). More preferably, by mass % Al: 5.50~6.75%, V: 3.50~4.50%, Fe: 0.05~0.40%, O: 0.05~0.25%, balance: Ti and impurities (commonly referred to as "Ti-6Al-4V").

[0076] Furthermore, the preferred composition by mass% is Al: 4.70~6.50%, Fe: 0.05~3.10%, Si: 0.15~0.40%, O: 0.05~0.40%, with the balance being Ti and impurities (commonly referred to as "Ti-5.5Al-1.5Fe-0.25Si"). More preferably, the composition by mass% is Al: 4.40~5.50%, Fe: 0.50~1.40%, Si: 0~0.10%, O: 0.05~0.40%, with the balance being Ti and impurities (commonly referred to as "Ti-5Al-1Fe").

[0077] The titanium materials of Ti-6Al-4V, Ti-5Al-2Fe-3Mo, Ti-5.5Al-1.5Fe-0.25Si, and Ti-5Al-1Fe have tensile strengths of over 950 MPa, and are therefore suitable for the applications described in this application. Alternatively, general-purpose alloys as specified in AMS4975, etc., may also be used.

[0078] It should be noted that common impurities contained in the aforementioned titanium alloys include N, C, and H, which may be present as long as their content is less than 0.08% by mass for N, less than 0.08% by mass for C, and less than 0.015% by mass for H. Other elements besides those mentioned above include, for example, Ni, Cr, Mn, Nb, and Cu. Preferably, the content of these elements is less than 0.1% each, and the total content is less than 0.3%.

[0079] 3. Shape

[0080] The shape of the titanium material in this embodiment is not limited to wire, bar, or sheet. Here, the bar-shaped titanium material, including wire, is referred to as a bar. The area of ​​the cross-section perpendicular to the long side of the bar is preferably 7900 mm². 2 Below, 5200mm is preferred. 2 The preferred size is 79mm. 2 The above is preferred, with 177mm being even better. 2 The above is further preferred to be 310mm. 2 The above. For example, a bar with a circular cross-section and a diameter of 10 to 100 mm is preferred, and a bar with a diameter of 15 to 80 mm is more preferred.

[0081] 4. Characteristics

[0082] 4-1. Load-holding fatigue characteristics

[0083] The titanium material of this embodiment meets the above requirements in terms of metallographic structure and exhibits excellent load-bearing fatigue characteristics. In this disclosure, the load-bearing fatigue characteristics are evaluated by performing tensile tests and fatigue tests as described below.

[0084] First, tensile and fatigue test pieces are collected from the surface of the titanium material, with the long side of the titanium material aligning with the long side of the test piece. In the case where the titanium material is a cylinder with a circular cross-section perpendicular to its long side and a radius of r, tensile and fatigue test pieces are collected centered at a depth of r / 2 from the side surface of the titanium material. Alternatively, in the case where the titanium material is a prism with a rectangular cross-section perpendicular to its long side and a side length of d, tensile and fatigue test pieces are collected centered at a depth of d / 4 from the side surface of that side length, along the center of the aforementioned side length.

[0085] In the tensile test, a tensile test piece with a parallel section of φ5×30mm was used. The distance between gauge points (gauge length) was set to 25mm, and the strain rate was set to 8.3×10⁻⁶. -5 / s, other conditions according to ASTM E8M, thus determining the 0.2% conditional yield strength.

[0086] In addition, in the fatigue test, a fatigue test piece with a parallel section of φ5.08mm×15.24mm was used. The distance between the gauge points (gauge length) was set to 12mm, the axial force, pulsation, and stress ratio were set to 0.05, and the maximum stress was set to 95% of the 0.2% conditional yield strength determined in the above tensile test. Furthermore, the stress waveform was set to a trapezoidal wave load cycle of 1s loading, 120s holding, and 1s unloading, thereby determining the fatigue life under load.

[0087] In this embodiment, if the fatigue life measured by the above-mentioned fatigue test is more than 15,000 cycles, it is judged to have excellent fatigue characteristics.

[0088] 4-2. Mirror effect

[0089] In the titanium material of this embodiment, good mirror finish is preferred when design flexibility is required. Mirror finish is influenced by the metallographic structure near the surface, i.e., the surface layer. In this embodiment, as described above, good mirror finish can be obtained by setting the adjacent grain area ratio to 15.0% or less. This results in a homogeneous material from which parts can be manufactured with excellent mirror finish regardless of where the parts are manufactured. In this invention, mirror finish is evaluated using the following method.

[0090] First, test pieces are collected from the surface of the titanium material, with an area of ​​8mm x 8mm serving as the evaluation surface. In the case where the titanium material is a cylinder with a circular cross-section of radius r perpendicular to its long side, the evaluation surface is the surface centered at a depth of r / 2 from the side surface, parallel to the long side and radially perpendicular to the aforementioned cross-section. Alternatively, in the case where the titanium material is a prism with a rectangular cross-section of side length d perpendicular to its long side, the evaluation surface is the surface parallel to the side surface of that side and at a depth of d / 4 from the side surface of that side.

[0091] The test pieces were embedded in epoxy resin, and the evaluation surfaces were polished using wet grinding, proceeding sequentially from coarse to fine grit sandpaper. Next, the evaluation surfaces underwent dry polishing and finishing to achieve a mirror finish. Finally, the Distinctness of Image (DOI) was measured on the evaluation surfaces.

[0092] DOI was determined using a Rhopoint IQFlex20 appearance analyzer manufactured by Rhopoint Instruments, with the incident light angle set at 20° and other conditions in accordance with ASTM D 5767.

[0093] DOI is a parameter representing image sharpness and an indicator of specularity. A higher DOI indicates better specularity. In this embodiment, a DOI of 60 or higher is considered to indicate excellent specularity.

[0094] 5. Manufacturing method

[0095] The titanium material of this embodiment can be stably manufactured by, for example, the manufacturing method described below. It should be noted that, for the sake of simplicity, the following explanation uses a bar stock as an example.

[0096] The titanium material manufacturing method of this embodiment sequentially performs the following first, second, and third steps. The first step involves heating the titanium ingot to a temperature of (Tβ+20)°C or higher and lower than (Tβ+270)°C, pressing the ingot down from a direction perpendicular to its long side, hot forging it at a reduction of area of ​​40% or higher in the temperature range above Tβ (°C), and then cooling it. It should be noted that Tβ (°C) is the β phase transformation point, calculated using the following formula.

[0097]

[0098] At this point, forging is performed by pressing the titanium ingot down along its long side in a direction perpendicular to the long side. Then, the ingot is rotated around its long side and the same forging process is repeated. Furthermore, a difference in strain rate is set for each forging operation. By performing this first process, strain is uniformly introduced along the long side and perpendicular to the ingot, suppressing the formation of coarse microtextures and stabilizing crystal orientation changes, thus reducing the orientation difference between adjacent microtextures.

[0099] The second process involves heating the titanium forging billet to a temperature above (Tβ-150)℃ and below (Tβ-50)℃, hot forging it with a reduction of area of ​​20% or more, and then cooling it. Similar to the first process, a strain rate difference is set for each forging operation. By performing this second process, the microstructure is controlled in the same way as in the first process, and the amount of the β phase is also controlled to achieve the desired strength.

[0100] The third process involves heating at a temperature above (Tβ-100)℃ and below (Tβ-20)℃, rolling to a reduction of area of ​​75% or more, and then cooling. This third process refines the α-grains and the microstructure.

[0101] It should be noted that, unlike the manufacturing method of this embodiment, in general bar manufacturing methods, such as the forging process in the first step, if it is desired to manufacture the bar by pressing it down in only one direction without rotation in the circumferential direction, the c-axis of the α grains is rotated in a direction perpendicular to the long side direction of the bar to refine the microstructure and develop texture. If the c-axis of the α grains is perpendicular to the long side direction of the bar, the deformation converges in that orientation, resulting in a coarse microstructure extending along the long side direction. As a result, it is difficult to produce a change in the c-axis orientation even in the subsequent rolling process in the third step, making it difficult to obtain the desired microstructure. It should be noted that each step will be described in detail below.

[0102] 5-1. First Process

[0103] The process involves manufacturing ingots of titanium billets with the desired chemical composition (hereinafter referred to as "titanium ingots"). It should be noted that there are no particular limitations on the manufacturing method of titanium ingots. For example, they can be manufactured by vacuum arc remelting (VAR) or electron beam remelting (EBR), in which case the desired size can be set.

[0104] The obtained titanium ingot is heated to a temperature above (Tβ+20)℃ but below (Tβ+270)℃, and then forged to a total reduction of area of ​​more than 40% above Tβ℃ before cooling. This process is called the first process, and the titanium ingot after the first process is used as the titanium forging billet.

[0105] In the first process, if the heating temperature during forging is below (Tβ+20)℃, uneven temperature distribution will occur within the furnace, or, in the case of a large titanium ingot, the overall ingot temperature will be below Tβ℃. Consequently, in the case of alloys containing the β phase, it will be difficult to form a sufficient amount of β phase. On the other hand, if the heating temperature is above (Tβ+270)℃, the surface of the titanium rolled billet is easily oxidized, or coarsening of the metallographic structure will occur. Therefore, the heating temperature of the titanium ingot in the first process is above (Tβ+20)℃ but below (Tβ+270)℃.

[0106] Furthermore, if the total reduction of area above Tβ℃ is less than 40% during the first forging process, the recrystallization of the β phase generated during subsequent cooling cannot be effectively utilized to refine the grains. As a result, coarse microtextures are more likely to form. To ensure that the average equivalent circle diameter of the microtexture is 300 μm or less, the total reduction of area above Tβ℃ during the first forging process is 40% or more. If the average equivalent circle diameter of the microtexture is to be 250 μm or less, the total reduction of area above Tβ℃ is preferably 55% or more.

[0107] It should be noted that the "total reduction of area" in the first process refers to the average area of ​​the cross-section perpendicular to the long side of the titanium ingot before forging, which is performed in the first process, perpendicular to the long side of the titanium ingot. "S" is defined as the area of ​​the cross-section perpendicular to the long side of the titanium ingot. 1s "The average area of ​​the cross-section perpendicular to the long side of the titanium forging billet after the first process is defined as "S". 1f "When, by {(S 1s -S 1f ) / S 1s The value represented by}×100. It should be noted that, in the case of upsetting as described later, the average area of ​​the cross-section perpendicular to the long side of the upset titanium ingot is defined as "S". 1s "The average area of ​​the cross section perpendicular to the long side of the titanium ingot or titanium forging billet refers to the average of the area of ​​one end, the cross-sectional area of ​​the central part, and the area of ​​the other end along the long side of the titanium ingot or titanium forging billet."

[0108] In the first process, when pressing down the titanium ingot, the ingot is first conveyed along its long side while a pair of hammers and anvils perform multiple passes of forging along the long side, pressing it down in a direction perpendicular to the long side. In this application specification, this multiple passes of pressing down along the long side are referred to as one forging. During the first forging, the titanium ingot is not rotated around its long side, but only moved along that direction. After the first forging, the titanium ingot is rotated 30-45° around its long side, changing the pressing direction, and a second forging is performed in the same manner as the first forging.

[0109] It should be noted that rotation may not be performed between forging cycles. However, the titanium ingot is rotated to change the pressing direction at least six times, and the total number of forging cycles is at least six. Furthermore, the titanium ingot is rotated in the same direction throughout the first process. If the adjacent grain area ratio is to be set to 15.0% or less, the number of times the pressing direction is changed is preferably eight or more.

[0110] Furthermore, the reduction amount ΔH per pass (each pass) is set to be 15% or more relative to the distance between opposite sides of the titanium ingot in the reduction direction. This is to uniformly introduce strain, refine the microtexture, and ensure that the area fraction of the microtexture is 30.0% or less. To ensure that the area fraction of the microtexture is 25.0% or less, it is preferable to set the reduction amount ΔH per pass (each pass) to be 20% or more relative to the distance between opposite sides of the titanium ingot in the reduction direction.

[0111] It should be noted that, as the pair of hammer anvils mentioned above, a hammer anvil with a width of 100 mm or more along its long side is used. This is because, by using a wide hammer anvil like the one described above, strain can be uniformly introduced over a wide range.

[0112] Here, in the first process, during the aforementioned six or more forging operations, it is necessary to repeatedly perform forging at low strain rates and forging at high strain rates. In this process, the strain rate during forging at low strain rates (hereinafter also referred to as "low-speed side strain rate") is set to 0.10 / s or less, and the strain rate during forging at high strain rates (hereinafter also referred to as "high-speed side strain rate") is set to 1.4 / s or more. The low-speed side strain rate is preferably 0.05 / s or less, and the high-speed side strain rate is preferably 2.5 / s or more.

[0113] Based on this, the reduction of area during forging at varying strain rates is 10% or more. By repeatedly performing forging at both low and high strain rates, and ensuring a reduction of area of ​​10% or more during forging at varying strain rates, the ratio of paired areas can be reduced. Preferably, the reduction of area during forging at varying strain rates is 15% or more.

[0114] The reduction of area in forging at varying strain rates refers to the total reduction of area in all forging processes, specifically when forging is changed from a low strain rate to a high strain rate, and vice versa. The calculation method is explained in more detail below.

[0115] For example, five forging processes are performed, with the following forging conditions for each process.

[0116] First: with low strain rate and cross-sectional area from S 1s Change to S 11 Forging in this way

[0117] Second: with low strain rate and cross-sectional area from S 11 Change to S 12 Forging in this way

[0118] Third: with high strain rate and cross-sectional area from S 12 Change to S 13 Forging in this way

[0119] Fourth time: with high strain rate and cross-sectional area from S 13 Change to S 14 Forging in this way

[0120] Fifth: with low strain rate and cross-sectional area from S 14 Change to S 1f Forging in this way

[0121] In this example, the forging with the strain rate changed was performed as a third and fifth forging. Therefore, the reduction of area in the forging with the changed strain rate can be expressed as {1-(S 13 / S 12 )×(S 1f / S 14 )}×100 is obtained.

[0122] By repeatedly performing forging at low and high strain rates, strain can be uniformly introduced not only in the direction perpendicular to the long side but also in the direction parallel to the long side, thus altering the crystal orientation. As a result, the microstructure becomes finer, the α grains become finer, and the ratio of paired areas can be reduced.

[0123] After forging, the forged billet is cooled. Cooling is preferably performed by quenching. Quenching is typically water cooling, which is achieved by immersing the titanium forging billet in a sufficient amount of water. This method can also be used if a cooling rate equivalent to or greater than water cooling is obtained. Quenching preferably continues until the surface temperature of the titanium forging billet reaches below 300°C.

[0124] It should be noted that in the first process, upsetting can also be performed before forging, which involves pressing down in a direction perpendicular to the long side of the titanium ingot. Upsetting involves pressing down on both sides of the titanium ingot along its long side, from top to bottom. By performing upsetting, stress can be uniformly introduced into the interior of the titanium ingot. Furthermore, upsetting increases the cross-sectional area of ​​the titanium ingot, thereby facilitating an increase in the total reduction of area during the first forging process.

[0125] 5-2. Second process

[0126] Following the first process described above, the titanium forging billet is heated to above (Tβ-150)℃ but below (Tβ-50)℃, forged with a total reduction of area of ​​more than 20%, and then cooled. This process is referred to as the second process, and the titanium forging billet after the second process is used as the titanium rolling billet.

[0127] When the heating temperature in the second process is below (Tβ-150)℃, the workability sometimes decreases and cracks occur. Therefore, the heating temperature is (Tβ-150)℃ or higher. On the other hand, when the heating temperature is above (Tβ-50)℃, the β phase increases, making it impossible to effectively impart strain to the α grains, thus preventing the microstructure from being refined and the target microstructure from being obtained. Therefore, the heating temperature is below (Tβ-50)℃.

[0128] Furthermore, if the total reduction of area in the forging process of the second step is less than 20%, it is impossible to refine the α grains. Additionally, the crystal orientation cannot be randomized, and the microtexture cannot be refined. Therefore, the total reduction of area is set to 20% or more. When it is desired that the diameter of the paired equivalent circles is 430 μm or less, the total reduction of area is preferably 30% or more.

[0129] It should be noted that the "total reduction of area" in the second process refers to the average area of ​​the cross-section perpendicular to the long side of the titanium forging billet before the second process, defined as "S". 2s "The average area of ​​the cross-section perpendicular to the long side of the titanium rolled billet after the second process is defined as "S". 2f "When, by {(S 2s -S 2f ) / S 2s The value is represented by}×100. After forging, cooling is performed. Cooling is preferably performed in the same manner as the first process. The average area of ​​the cross-section perpendicular to the long side of the titanium forged or rolled billet refers to the average of the area of ​​one end, the cross-sectional area of ​​the central portion, and the area of ​​the other end along the long side of the titanium forged or rolled billet.

[0130] Similar to the first process, the second process also requires repeated forging at low and high strain rates. In this process, the low-speed strain rate is set to below 0.10 / s, and the high-speed strain rate is set to above 1.4 / s. Preferably, the low-speed strain rate is below 0.05 / s, and the high-speed strain rate is preferably above 2.5 / s.

[0131] By alternating and repeatedly performing forging at slow and fast strain rates, strain can be uniformly introduced not only in the direction perpendicular to the long side but also in the direction parallel to the long side, thus altering the crystal orientation. As a result, the microstructure becomes finer, the α-grains become finer, and the paired area ratio is reduced. It should be noted that, similar to the first process, in the second process, the number of forgings is preferably set to six or more. Furthermore, in the second process, as in the first process, forging can be performed while rotating the titanium forging billet around its long side, or it can be performed without rotation but only in one direction.

[0132] It should be noted that after the first or second process, cutting may be performed as needed to adjust the desired shape. This cutting is performed to remove defects generated during hot working, etc.

[0133] 5-3. Third process

[0134] After the second process, the titanium billet is heated to above (Tβ-100)℃ but below (Tβ-20)℃, rolled to a total reduction of area of ​​more than 75%, and then cooled. This process is called the third process.

[0135] If the heating temperature in the third process is below (Tβ-100)℃, the α grains are difficult to become fine and equiaxed. As a result, the microtexture also becomes coarse. Therefore, the heating temperature is set to (Tβ-100)℃ or higher. On the other hand, even if the heating temperature is above (Tβ-20)℃, the α grains are difficult to become fine and equiaxed. As a result, the microtexture also becomes coarse. Therefore, the heating temperature is set to below (Tβ-20)℃.

[0136] Furthermore, even if the total reduction of area in the rolling process in the third step is less than 75%, the microtexture becomes coarse. Therefore, the total reduction of area is set to 75% or more. After this forging, cooling is performed. Cooling is preferably carried out in the same manner as the first and second steps.

[0137] It should be noted that there are no particular restrictions on the type of rolling in the third process, but roll forming is usually preferred.

[0138] Following the third step described above, heat treatment can be performed as needed to relieve strain or stabilize the microstructure. For example, when the purpose is to relieve strain and stabilize the microstructure, it is preferable to hold at 750°C for 1 hour followed by air cooling.

[0139] Alternatively, for material control purposes, solution aging treatment can be performed after the third process as needed. In solution aging treatment, firstly, a solution treatment is performed at a temperature above 800℃ and below Tβ℃ for at least 1 minute, followed by rapid cooling. Next, an aging treatment is performed at a temperature above 400℃ and below 600℃ for at least 1 minute, followed by air cooling. It should be noted that the holding time is sufficient to achieve homogenization of the titanium material and can be adjusted, for example, to 5 minutes or more, or 10 minutes or more, depending on the size of the titanium material and the performance of the furnace. Through solution aging treatment, the microtexture and α-grain area ratio, which are prone to becoming crack initiation points, can be reduced, further improving fatigue resistance. It should be noted that the heating temperatures in the first to third processes and the solution aging treatment temperatures refer to the furnace temperature.

[0140] The titanium material of the present invention will be described in more detail below through examples, but the titanium material of this embodiment is not limited to these examples.

[0141] Example

[0142] Titanium ingots (400 mm in diameter) with the chemical composition shown in Table 1 were manufactured using vacuum arc melting (VAR). "-" in Table 1 indicates that no elements were intentionally added.

[0143] [Table 1]

[0144]

[0145] The obtained titanium ingots are subjected to the following conditions: after the first process, cooling to below 300°C; after the second process, cooling to below 300°C; and then after the third process, cooling to below 300°C. Solution aging treatment is then performed as needed, under the conditions described in Table 2. The heating temperature, holding time, and cooling method are listed sequentially in the "Heat Treatment" column of Table 2. Titanium materials (circular cross-section bars) with a diameter of 35 mm are produced. Other conditions not described are within the range of preferred manufacturing conditions. In Table 2, underlines indicate conditions outside the preferred range.

[0146] It should be noted that in the first process, the ΔH / distance between opposite sides in the pressing direction is set to the same value for each pass, and the strain rate is set to the same value for each forging. When changing the strain rate during forging, it is implemented using the low-speed and high-speed strain rates listed in Table 2. In the first process, the forging direction is changed by rotating 30~45° around the long side only as shown in Table 2. Furthermore, the total reduction of area in the first process is the total reduction of area at a temperature above Tβ℃. Then, in the second process, the low-speed and high-speed strain rates listed in Table 2 are changed for each forging pass. In the second process, the forging direction is changed by rotating 30~45° around the long side for each forging pass.

[0147] [Table 2]

[0148]

[0149] For the obtained titanium material, the area ratio, average equivalent circle diameter, paired area ratio, paired equivalent circle diameter, adjacent grain area ratio, and average equivalent circle diameter of the α grains of the microtexture were calculated using the following methods. Furthermore, the load-bearing fatigue characteristics and mirror finish of the titanium material were evaluated according to the following steps.

[0150] (Metallographic structure observation)

[0151] Metallographic structures were observed using an EBSD device attached to the SEM.

[0152] Metallographic specimens were collected from the surface of the titanium material, with an observation area of ​​3mm x 3mm. An observation plane was constructed, centered at a depth of r / 2 from the side of the titanium material, perpendicular to the radial direction of a circular cross-section parallel to the long side of the titanium material and perpendicular to the radial direction of the long side. Strain introduced during specimen collection was removed by grinding. During grinding, colloidal silica polishing was performed to remove any new strain.

[0153] The measurement interval was set to 2.0 μm across the entire measurement area, and orientation was determined using EBSD at an accelerating voltage of 15 kV. The measurement results were analyzed using OIM (OIM Analysis v8.1.0, a crystal orientation analysis software manufactured by TSL Solutions, Inc.). At this time, a Partiton focusing solely on the α phase was fabricated as the object of analysis. It should be noted that it is generally difficult to assume that the microtexture is entirely adjacent to the β grains; therefore, only the α grains were considered.

[0154] Then, the α-grain is determined by setting the angle difference (orientation difference angle) of crystal orientation between adjacent EBSD measurement points to 15° or more. The equivalent circle diameter of each α-grain is calculated from the number of measurement points for that α-grain, and the average equivalent circle diameter of the α-grain is calculated. In addition, the c-axis orientation difference between adjacent EBSD measurement points is calculated from the obtained measurement results (Euler angles ph1, ph2). Cases with a c-axis orientation difference of 0~20° and an equivalent circle diameter of 50μm or more are identified as microtexture.

[0155] In microstructures identified as microtextures, the difference in the average direction of the c-axis of the α-grains constituting each microtexture, which are adjacent to each other, is calculated. Next, the difference in the average direction of the c-axis of the two microtextures, i.e., the angle θ0, is calculated. Then, the microtextures contained in pairs of microtextures with an angle θ0 greater than 70° are extracted, and the area ratio of this microtexture in the observation region is calculated. This area ratio is then divided by the area ratio of all microtextures to determine the paired area ratio. Similarly, paired microtextures are extracted, and the total cross-sectional area of ​​these paired microtextures is measured. The diameter of the paired equivalent circle is calculated from the average of the cross-sectional areas of all extracted paired microtextures.

[0156] In addition, the angle θ1 is calculated between the average direction of the c-axis of the α-grains constituting the region determined to be a microtexture and the average direction of the c-axis of the α-grains adjacent to it in a manner that surrounds the microtexture. Then, microtextures with an angle θ1 of 45° or greater are extracted, the area ratio in the observation region is calculated, and the area ratio of the adjacent grains is calculated by dividing by the area ratio of all microtextures.

[0157] (Load-holding fatigue characteristics)

[0158] Load-bearing fatigue characteristics were evaluated through tensile and fatigue tests. Tensile and fatigue test specimens were collected from the titanium material, with the long side of the titanium material as the long side of the test piece and the depth of r / 2 as the center. In the tensile test, a parallel section φ5×30mm tensile test specimen was used, with the gauge length (distance between gauge points) set to 25mm and the strain rate set to 8.3×10⁻⁶. -5 / s, other conditions according to ASTM E8M, thus determining the 0.2% conditional yield strength.

[0159] In addition, in the fatigue test, a fatigue test piece with a parallel section of φ5.08mm×15.24mm was used. The distance between the gauge points (gauge length) was set to 12mm, the axial force, pulsation, and stress ratio were set to 0.05, and the maximum stress was set to 95% of the conditional yield strength of the material (in the same direction). In addition, the stress waveform was set to a trapezoidal wave load cycle of 1s loading, 120s holding, and 1s unloading, thereby determining the fatigue life under load.

[0160] Based on the test results, cases with a fatigue life of more than 15,000 cycles are considered to have good fatigue characteristics.

[0161] (Mirror effect)

[0162] A portion of the titanium material was further evaluated for its mirror finish. Mirror finish was evaluated by measuring the DOI (Depth of Indication). Test pieces for DOI measurement were collected from the obtained titanium material, with an evaluation area of ​​8 mm x 8 mm. Additionally, an evaluation surface was defined as a circular cross-section parallel to the long side of the titanium material, radially perpendicular to the direction perpendicular to the long side, and centered at a depth r / 2 from the side surface of the titanium material.

[0163] The test pieces were embedded in epoxy resin, and the evaluation surfaces were polished using wet grinding, proceeding sequentially from coarse to fine grit sandpaper. Next, the evaluation surfaces were dry-polished to achieve a mirror finish. Then, the DOI (Difference of Indication) was measured on the evaluation surfaces. The DOI was measured using a Rhopoint IQ Flex20 appearance analyzer manufactured by Rhopoint Instruments, with the incident light angle set at 20°, and other conditions conforming to ASTM D 5767.

[0164] The results were used to classify a DOI of 60 or higher as having good mirror properties. It should be noted that in the "DOI" column of Table 3, a "-" indicates that DOI was not measured.

[0165] The results of the above metallographic observations and property evaluations are summarized in Table 3. In Table 3, underlines indicate items outside the scope of this embodiment.

[0166] [Table 3]

[0167]

[0168] In tests No. 1 to 27 that meet the requirements of this embodiment, the load-holding fatigue characteristics are good. However, in tests No. 3 and 14, where the adjacent grain area ratio is excessive, the result is a DOI of less than 60 and reduced mirror finish. On the other hand, in tests No. 28 to 38 that do not meet the requirements of this embodiment, the load-holding fatigue characteristics are poor.

[0169] Specifically, in tests No. 28 and 37, the low-speed lateral strain rate in the first process was too high, thus failing to reduce the paired area ratio, resulting in decreased load-holding fatigue characteristics. In test No. 29, the total reduction of area in the first process was low, therefore the average equivalent circle diameter of the microtexture exceeded the specified range, resulting in decreased load-holding fatigue characteristics. In test No. 30, the low-speed lateral strain rate in the second process was too high, thus failing to reduce the paired area ratio, resulting in decreased load-holding fatigue characteristics.

[0170] In Test No. 31, the heating temperature in the second process was too high, resulting in the average equivalent circle diameter of the microtexture exceeding the specified range and a decrease in load-holding fatigue characteristics. In Test No. 32, the reduction of area during forging at different strain rates in the first process was low, making it impossible to reduce the paired area ratio, thus resulting in a decrease in load-holding fatigue characteristics. In Tests No. 33 and 34, the high-speed side strain rate in the first process was too slow, making it impossible to reduce the paired area ratio, thus resulting in a decrease in load-holding fatigue characteristics.

[0171] In tests No. 35 and 38, the high-speed lateral strain rate in the second process was too slow, thus failing to reduce the paired area ratio, resulting in decreased load-holding fatigue characteristics. In test No. 36, in the first process, the reduction ΔH per pass was small relative to the distance between opposite sides of the titanium ingot in the reduction direction, therefore the average equivalent circle diameter of the microtexture exceeded the specified range, further reducing load-holding fatigue characteristics.

[0172] Industrial availability

[0173] As described above, according to this disclosure, titanium materials with improved load-bearing fatigue properties can be obtained.

Claims

1. A titanium material, wherein, When considering an aggregate of α grains with an orientation difference of less than 20° between the c-axis of adjacent α grains and an equivalent circle diameter of more than 50 μm as the microtexture... In the metallographic structure of the surface layer The area fraction of the microtexture is below 30.0%. The average equivalent circle diameter of the microtexture is less than 300 μm. When two microtextures are adjacent, and the angle between the average direction of the c-axis of the α-grain constituting one of the two microtextures and the average direction of the c-axis of the α-grain constituting the other microtexture is set as angle θ0, The area ratio of the microtexture contained in the two microtextures where the angle θ0 satisfies 70° or more is less than 20.0% relative to the area ratio of the microtexture.

2. The titanium material of claim 1, wherein, In the metallographic structure of the surface layer, the average equivalent circle diameter of the α grains is less than 20.0 μm.

3. The titanium material of claim 1, wherein, In the metallographic structure of the surface layer, when the angle between the average direction of the c-axis of the α grain constituting a microtexture and the average direction of the c-axis of the adjacent α grain of the microtexture is set as angle θ1, the area ratio of the microtexture with an angle θ1 of 45° or more is 15.0% or less relative to the area ratio of the microtexture.

4. The titanium material of claim 2, wherein, In the metallographic structure of the surface layer, when the angle between the average direction of the c-axis of the α grain constituting a microtexture and the average direction of the c-axis of the adjacent α grain of the microtexture is set as angle θ1, the area ratio of the microtexture with an angle θ1 of 45° or more is 15.0% or less relative to the area ratio of the microtexture.

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

6. The titanium material according to any one of claims 1 to 4, wherein its chemical composition, in mass % is: Al：4.40~6.75%、 Fe: 0.05~3.10%, O：0.05~0.40%、 V：0~4.50%、 Mo: 0~5.50%, Si: 0~0.40%, Balance: Ti and impurities.

7. The titanium material according to any one of claims 1 to 4, wherein it is a rod.

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