Titanium alloy sheet and method for manufacturing a titanium alloy sheet

Abstract

The titanium alloy sheet disclosed herein contains a specified chemical composition. When the crystal orientation of the α phase is represented by Euler angles g = {φ1, Φ, φ2} using the Bunge-based notation method, and in texture analysis using the spherical harmonic function method in electron backscatter diffraction, with the expansion coefficient set to 16 and the Gaussian half-width at half-maximum set to 5°, the maximum aggregation orientation shown by the crystal orientation distribution function f(g) is within the range of φ1: 0–30°, Φ: 60–90°, and φ2: 0–60°. The aggregation degree of the maximum aggregation orientation is 10.0 or higher. The 0.2% yield strength in the width direction of the titanium alloy sheet at 25°C is 800 MPa or higher. The Young's modulus in the width direction of the titanium alloy sheet is 125 GPa or higher. The average thickness of the titanium alloy sheet is 2.5 mm or less.

Description

Technical Field

[0001] This disclosure relates to a titanium alloy sheet and a method for manufacturing the titanium alloy sheet. Background Technology

[0002] Titanium is a lightweight, high-strength, and highly corrosion-resistant material, making it suitable for the aerospace industry from the perspectives of weight reduction and improved fuel efficiency. Therefore, the development of titanium alloys that meet the required properties for various aircraft components is gaining momentum.

[0003] For example, Patent Document 1 discloses an α+β type titanium alloy wire containing 1.4% or more and less than 2.1% Fe, 4.4% or more and less than 5.5% Al, the balance being titanium and impurities.

[0004] Patent document 2 discloses an α+β type titanium alloy rod containing 0.5% or more and less than 1.4% Fe, 4.4% or more and less than 5.5% Al, the balance being titanium and impurities.

[0005] Patent document 3 discloses a method for manufacturing a Ti-6Al-4V alloy thin plate based on lamination rolling. The method involves covering one or more plate-shaped core materials with spacer materials and cover materials to form a lamination plate, and rolling the lamination plate to reduce the thickness of the core material. The method is characterized in that the thickness of the cover material is set such that the ratio of the core material to the lamination plate is at least 0.25 with respect to the initial plate thickness of each material.

[0006] Patent document 4 discloses a method for manufacturing a Ti-6Al-4V alloy thin plate based on stacked rolling. The method involves covering one or more plate-shaped core materials with spacer materials and covering materials to form a stacked plate, and rolling the stacked plate to reduce the thickness of the core material. The method is characterized in that, for rolling of the stacked plate where the reduction ratio of the plate thickness before and after decompression is 3 or more, the rolling rate of each pass is set to 15% or more.

[0007] Patent document 5 discloses a method for manufacturing a titanium alloy sheet, characterized in that a hot-rolled annealed sheet of titanium alloy is cold-rolled in the same direction as the hot rolling direction with a total rolling ratio of 67% or more, and then annealed at a temperature between 650 and 900°C. The titanium alloy contains, by weight %: Al: 2.5-3.5%, V: 2.0-3.0%, balance Ti and common impurities.

[0008] Patent document 6 discloses a method for manufacturing α+β type titanium alloy thin sheet, characterized in that, in the manufacturing process of α+β type titanium alloy cold-rolled sheet, an intermediate annealing is performed after cold rolling under the following conditions: annealing temperature: above and below the β phase transformation point (-25°C); annealing time: 0.5 to 4 hours; cooling rate after heating and holding: 0.5 to 5°C / second; and cooling temperature range at the above cooling rate: up to below 300°C.

[0009] Patent document 7 discloses an α+β type titanium alloy sheet, characterized in that the α+β type titanium alloy sheet contains: at least one fully solid-solution type β stabilizing element in the amount of 2.0 to 4.5% by mass of Mo equivalent, at least one eutectoid type β stabilizing element in the amount of 0.3 to 2.0% by mass of Fe equivalent, at least one type α stabilizing element in the amount of more than 3.0% by mass and less than 5.5% by mass of Al equivalent, with the balance being Ti and unavoidable impurities, wherein the average particle size of the α phase is less than 5.0 μm and the maximum particle size of the α phase is less than 10.0 μm, the average aspect ratio of the α phase is less than 2.0 and the maximum aspect ratio of the α phase is less than 5.0.

[0010] Patent document 8 discloses an α+β type titanium alloy plate with excellent cold rolling and cold working properties. This α+β type titanium alloy plate is an α+β type hot-rolled titanium alloy plate, characterized in that: (a) the normal direction (thickness direction) of the rolled surface of the hot-rolled plate is set as ND, the hot rolling direction is set as RD, the width direction of the hot-rolled plate is set as TD, the normal direction of the (0001) plane of the α phase is set as the c-axis orientation, the angle between the c-axis orientation and ND is set as θ, and the plane containing the c-axis orientation and ND is... The angle between the surfaces of ND and TD is set as Φ. (b1) The strongest intensity of the relative intensity of X-ray (0002) reflection caused by grains with θ above 0 degrees and below 30 degrees and Φ falling within the entire circumference (-180 degrees to 180 degrees) is set as XND. (b2) The strongest intensity of the relative intensity of X-ray (0002) reflection caused by grains with θ above 80 degrees and below 100 degrees and Φ falling within ±10 degrees is set as XTD. (c) XTD / XND is 5.0 or higher.

[0011] Patent document 9 discloses a high-strength α+β type titanium alloy plate with excellent cold workability in cold-rolled coils. It is a hot-rolled α+β type titanium alloy plate containing, by mass%, Fe: 0.8–1.5%, Al: 4.8–5.5%, and N: less than 0.030%, and containing O (mass%) set as [O] and N (mass%) set as [N], satisfying Q(%) = 0.14–0.38 as defined by Q(%) = [O] + 2.77·[N], with the balance being Ti and unavoidable impurities. The plate is characterized by: (a) the normal direction of the hot-rolled plate being set as ND, the hot-rolling direction being set as RD, and the hot-rolled plate... The width direction is set as TD, the normal direction of the (0001) plane of the α phase is set as the c-axis orientation, the angle between the c-axis orientation and ND is set as θ, the angle between the plane containing the c-axis orientation and ND and the plane containing ND and TD is set as φ, (b1) the strongest intensity of the relative intensity of X-ray (0002) reflection caused by grains with θ above 0 degrees and below 30 degrees and φ falling within the full circumference (-180 degrees to 180 degrees) is set as XND, (b2) the strongest intensity of the relative intensity of X-ray (0002) reflection caused by grains with θ above 80 degrees and below 100 degrees and φ falling within ±10 degrees is set as XTD, (c) XTD / XND is 4.0 or above.

[0012] Patent document 10 discloses an α+β type titanium alloy plate with high strength and Young's modulus in the width direction. This α+β type titanium alloy plate is a high-strength α+β type titanium alloy cold-rolled annealed plate, containing 0.8–1.5% Fe and less than 0.020% N by mass. It contains O, N, and Fe with the content (mass%) set as [O], N (mass%) as [N], and Fe (mass%) as [Fe], satisfying Q(%) = 0.34–0.55 as defined by Q(%) = [O] + 2.77 × [N] + 0.1 × [Fe]. The balance is Ti and unavoidable impurities. The key feature is that, when analyzing the texture in the plate surface direction, the rolling surface method of the cold-rolled annealed plate is used... The line direction is set as ND, the plate length direction is set as RD, the plate width direction is set as TD, the normal direction of the (0001) plane of the α phase is set as the c-axis orientation, the angle between the c-axis orientation and ND is set as θ, and the angle between the projection line of the c-axis orientation on the plate surface and the plate width direction (TD) is set as φ. When the strongest intensity of the relative intensity of X-ray (0002) reflection caused by grains with an angle θ of 0 degrees or more and less than 30 degrees and φ falling within -180 degrees to 180 degrees is set as XND, and the strongest intensity of the relative intensity of X-ray (0002) reflection caused by grains with an angle θ of 80 degrees or more and less than 100 degrees and φ falling within the range of ±10 degrees is set as XTD, the ratio of XTD / XND is 5.0 or more.

[0013] Non-patent document 1 discloses an α+β titanium alloy sheet that exhibits anisotropy in strength in the rolling direction and in the direction perpendicular to the rolling direction.

[0014] Non-patent document 2 discloses an α+β titanium alloy sheet that is hot-rolled at a temperature higher than the β phase transformation point, thereby reducing the anisotropy of strength in the rolling direction and the direction perpendicular to the rolling direction.

[0015] Existing technical documents

[0016] Patent documents

[0017] Patent Document 1: Japanese Patent Application Publication No. 7-62474

[0018] Patent Document 2: Japanese Patent Application Publication No. 7-70676

[0019] Patent Document 3: Japanese Patent Application Publication No. 2001-300603

[0020] Patent Document 4: Japanese Patent Application Publication No. 2001-300604

[0021] Patent Document 5: Japanese Patent Application Publication No. 61-147864

[0022] Patent Document 6: Japanese Patent Application Publication No. 1-127653

[0023] Patent Document 7: Japanese Patent Application Publication No. 2013-227618

[0024] Patent Document 8: International Publication No. 2012 / 115242

[0025] Patent Document 9: International Publication No. 2012 / 115243

[0026] Patent Document 10: International Publication No. 2015 / 156356

[0027] Non-patent literature

[0028] Non-patent literature 1: KOBE STEEL ENGINEERING REPORTS / Vol.59, No.1 (2009), pp.81-84

[0029] Non-patent literature 2: KOBE STEEL ENGINEERING REPORTS / Vol.60, No.2 (2010), pp.50-54 Summary of the Invention

[0030] The problem the invention aims to solve

[0031] For structural components in aircraft that require higher strength, alloys with a high content of Al, an α-phase solid solution strengthening element, are often used, such as Ti-6Al-4V (64 alloy), an α+β type titanium alloy. α+β type titanium alloys like Alloy 64, which contain a large amount of Al and are high-strength, are generally considered to lack machinability and are difficult to cold roll.

[0032] On the other hand, if titanium alloys are subjected to unidirectional high-speed hot rolling at temperatures in the α+β high-temperature domain or the β-phase high-proportion domain, a hexagonal close-packed (hcp) texture with a c-axis orientation in the width direction is formed during the transformation from the β-phase to the α-phase, depending on the variable selection. Titanium's c-axis direction has a higher Young's modulus and strength compared to other directions, making the T-texture suitable for achieving high strength and high Young's modulus in the width direction. However, when hot rolling is used to manufacture thin titanium alloy sheets, the reduction in sheet thickness leads to a sharp drop in material temperature during hot rolling. This significantly increases the deformation resistance of the titanium alloy, which has increased high-strength α-phase and decreased high-temperature strength β-phase, sometimes exceeding the mill's allowable load. Therefore, it is difficult to manufacture thin sheets with a thickness of less than 2.5 mm using only hot rolling. Furthermore, this texture tends to disappear when a recrystallization texture is formed through high-temperature annealing to soften work hardening during cold rolling. Therefore, this texture has not been effectively utilized in thin sheets with a thickness of less than 2.5 mm. Based on these circumstances, it is believed that it has been difficult to manufacture high-strength, high-Young's modulus thin titanium alloy sheets containing a large amount of Al and with a well-developed T-texture.

[0033] This disclosure was made in view of the above-mentioned problems. The purpose of this disclosure is to provide an Al-containing titanium alloy sheet with a thickness of less than 2.5 mm that utilizes T-texture, has high strength in the width direction and high Young's modulus in the width direction, and a method for manufacturing the titanium alloy sheet.

[0034] Solution for solving the problem

[0035] Originally, the T-texture in titanium with an hcp structure is difficult to cold roll in the same direction due to the deformation caused by the expected sliding in the hot rolling direction. The inventors have conducted in-depth and detailed research on the manufacture of thin sheets less than 2.5 mm using Al-containing titanium alloys with well-developed T-textures through hot rolling and cold rolling.

[0036] This disclosure is based on the above understanding, and its main contents are as follows.

[0037] [1] One aspect of this disclosure relates to a titanium alloy sheet containing, by mass percent, Al: greater than 4.0% and less than 6.6%, Fe: greater than 0% and less than 2.3%, V: greater than 0% and less than 4.5%, Si: greater than 0% and less than 0.60%, Ni: greater than 0% and less than 0.15%, Cr: greater than 0% and less than 0.25%, Mn: greater than 0% and less than 0.25%, C: greater than 0% and less than 0.080%, N: greater than 0% and less than 0.050%, and O: greater than 0% and less than 0.40%, with the balance being Ti and impurities.

[0038] When the crystal orientation of the α phase is represented by Euler angles g = {φ1, Φ, φ2} using the Bunge-based notation method, in the texture analysis using the spherical harmonic function method in electron backscattering diffraction, with the expansion coefficient set to 16 and the Gaussian full width at half maximum (FWHM) set to 5°, the maximum aggregation orientation shown by the crystal orientation distribution function f(g) is within the range of φ1: 0–30°, Φ: 60–90°, and φ2: 0–60°. The aggregation degree at the maximum aggregation orientation is 10.0 or higher.

[0039] The 0.2% yield strength of this titanium alloy sheet in the width direction at 25°C is above 800 MPa.

[0040] The Young's modulus of this titanium alloy sheet in the width direction is above 125 GPa.

[0041] The average thickness of this titanium alloy sheet is less than 2.5 mm.

[0042] [2] The titanium alloy sheet described in [1] above may contain, by mass %

[0043] Fe: 0.5% or more and 2.3% or less or V: 2.5% or more and 4.5% or less.

[0044] [3] The titanium alloy sheet described above [2] may contain, by mass %, selected free radicals.

[0045] Ni: less than 0.15%

[0046] Cr: less than 0.25%, and

[0047] Mn: less than 0.25% of one or more of the group consisting of Fe or V in place of a portion of the Fe or V.

[0048] [4] In the titanium alloy sheet described in [2] or [3] above, when one or more of the group consisting of O, N, Fe and V are used to replace a portion of the Ti, and the content of O in mass% is set to [O], the content of N is set to [N], the content of Fe is set to [Fe] and the content of V is set to [V], Q shown in the following formula (1) can be 0.340 or less.

[0049] Q=[O]+(2.77×[N])+(0.1×[Fe])+(0.025×[V])…Equation (1)

[0050] [5] In any one of the titanium alloy thin plates described above [1] to [4], the half-width at half-maximum of the diffraction peak at 2θ = 53.3 ± 1° detected by X-ray diffraction with CuKα as the X-ray source can be 0.20° or higher.

[0051] [6] The titanium alloy sheet according to any one of [1] to [5] above may have a banded structure with an aspect ratio greater than 3.0 and extending along the length of the sheet.

[0052] The area ratio of the band-like tissue is over 70%.

[0053] [7] In any one of the titanium alloy sheets described in [1] to [6] above, the dimensional accuracy of the sheet thickness relative to the average sheet thickness may be less than 5.0%.

[0054] [8] Another aspect of this disclosure relates to a method for manufacturing titanium alloy sheet, which is a method for manufacturing titanium alloy sheet as described in any one of [1] to [7] above, comprising the following steps:

[0055] The heating process involves heating a titanium billet, which contains, by mass percent, Al: greater than 4.0% and less than 6.6%, Fe: greater than 0% and less than 2.3%, V: greater than 0% and less than 4.5%, Si: greater than 0% and less than 0.60%, Ni: greater than 0% and less than 0.15%, Cr: greater than 0% and less than 0.25%, Mn: greater than 0% and less than 0.25%, C: greater than 0% and less than 0.08%, N: greater than 0% and less than 0.05%, and O: greater than 0% and less than 0.40%, with the balance being Ti and impurities.

[0056] The hot rolling process involves hot rolling the titanium billet after the heating process in one direction; and

[0057] The cold rolling process involves performing at least one cold rolling pass along the length of the titanium billet after the hot rolling process.

[0058] Let the β phase transition point be T. βAt (°C), the heating temperature of the titanium billet in the heating process is T. β ℃ and above (T) β +150)℃ or below,

[0059] The reduction rate in the hot rolling process is 80.0% or more.

[0060] The final temperature in the hot rolling process is (T) β -250)℃ or above and (T β -50)℃ or below,

[0061] In the cold rolling process, when the average rolling rate per cold rolling pass is less than 40% and multiple cold rolling passes are performed, an intermediate annealing process is included.

[0062] The annealing conditions for the intermediate annealing process are as follows:

[0063] The annealing temperature is above 500°C and below 750°C, and the annealing temperature T (°C) and the holding time t (seconds) at the annealing temperature satisfy the following formula (2).

[0064] 18000≤(T+273.15)×(Log 10 (t)+20)<22000…Equation (2)

[0065] [9] In the manufacturing method of titanium alloy sheet described in [8] above, after the last cold rolling pass, a final annealing with an annealing temperature of 500°C or higher and 750°C or lower, and satisfying the formula (2) can also be carried out.

[0066] The effects of the invention

[0067] According to this disclosure, an Al-containing titanium alloy sheet with a thickness of less than 2.5 mm, which has high strength and high Young's modulus in the width direction and utilizes a T-texture, can be provided, along with a method for manufacturing the titanium alloy sheet. Attached Figure Description

[0068] Figure 1 This is an illustration of the crystal orientation of the α-phase grains in a titanium plate obtained by Euler angles based on the Bunge marking method.

[0069] Figure 2 This is an example of a crystal orientation distribution function of a titanium alloy sheet obtained by electron backscatter diffraction, according to one embodiment of this disclosure.

[0070] Figure 3 An optical microscope photograph illustrating an example of band-like tissue.

[0071] Figure 4This is an example of an optical microscope photograph of the titanium alloy sheet involved in this embodiment.

[0072] Figure 5 This is a schematic diagram illustrating the method for determining the average plate thickness. Detailed Implementation

[0073] <1. Titanium Alloy Sheet>

[0074] First, the titanium alloy sheet involved in this embodiment will be described with reference to the accompanying drawings.

[0075] (1.1. Chemical Composition)

[0076] The chemical composition of the titanium alloy sheet involved in this embodiment is described below. The titanium alloy sheet involved in this embodiment contains, by mass%, Al: greater than 4.0% and less than 6.6%, Fe: 0% and less than 2.3%, V: 0% and less than 4.5%, Si: 0% and less than 0.60%, Ni: 0% and less than 0.15%, Cr: 0% and less than 0.25%, Mn: 0% and less than 0.25%, C: 0% and less than 0.08%, N: 0% and less than 0.05%, and O: 0% and less than 0.40%, with the balance being Ti and impurities. It should be noted that, unless otherwise specified, "%" in the following description of the chemical composition refers to "mass %".

[0077] Al is an α-phase stabilizing element and has high solid solution strengthening ability. Increasing the Al content increases the tensile strength at room temperature and the strength at relatively high temperatures. Furthermore, Al increases Young's modulus. In addition, if the Al content is greater than 4.0%, the hot-rolled sheet before cold rolling can maintain high cold-rollability. The Al content is preferably 4.5% or more. On the other hand, when the Al content is greater than 6.6%, the cold-rollability of the hot-rolled sheet before cold rolling decreases significantly, and due to solidification segregation, Al becomes excessively enriched locally, leading to Al ordering. This region of Al ordering results in a decrease in the impact toughness of the titanium alloy sheet. Therefore, the Al content is 6.6% or less, preferably 6.5% or less, 6.3% or less, and more preferably 6.2% or less.

[0078] Fe is a β-phase stabilizing element. Fe has high solid solution strengthening ability, therefore increasing the Fe content increases the tensile strength at room temperature. Furthermore, the β phase has higher workability compared to the α phase, thus increasing the Fe content improves the workability of the titanium alloy sheet. To maintain good workability of the β phase at room temperature and obtain the desired tensile strength, the Fe content is preferably 0.5% or more. Fe is not essential in titanium alloy sheets, therefore its lower limit is 0%. The Fe content is more preferably 0.7% or more. On the other hand, Fe is an element that is very prone to solidification and segregation, therefore, when the Fe content is too high, Fe will segregate locally, sometimes resulting in characteristic deviations between the segregated and unsegregated portions. Additionally, excessive Fe in titanium alloy sheets can sometimes reduce fatigue strength. Therefore, the Fe content is preferably 2.3% or less. The Fe content is more preferably 2.1% or less, and even more preferably 2.0% or less. It should be noted that Fe is cheaper than β-phase stabilizing elements such as V or Si.

[0079] In this embodiment, the titanium alloy sheet may contain Fe, which can be replaced by V. V is a fully solid-solution β-phase stabilizing element and has solid-solution strengthening ability. To obtain the same solid-solution strengthening ability as Fe, the V content is preferably 2.5% or more. The V content is more preferably 3.0% or more. V is not essential in the titanium alloy sheet, so its content has a lower limit of 0%. Although replacing Fe with V increases the cost, V is less prone to segregation than Fe, thus suppressing the deviation in properties caused by segregation. As a result, stable properties are easily obtained in both the length and width directions of the titanium alloy sheet. To suppress the deviation in properties caused by V segregation, the V content is preferably 4.5% or less. It should be noted that, as mentioned above, V is less prone to segregation than Fe, therefore, when manufacturing large ingots, it is preferable to include V in the titanium billet.

[0080] Si is a β-phase stabilizing element, but it can also be dissolved in the α-phase and exhibits high solid solution strengthening ability. As mentioned above, Fe sometimes segregates when it contains more than 2.3% in titanium alloy sheets, so Si can be included as needed to achieve high strength in titanium alloy sheets. In addition, Si has the opposite segregation tendency to O, and it is difficult to solidify and segregate like O. Therefore, by including appropriate amounts of Si and O in titanium alloy sheets, it is expected to achieve both high fatigue strength and tensile strength. On the other hand, when the Si content is high, Si intermetallic compounds called silicides can sometimes form, which reduces the fatigue strength of the titanium alloy sheet. If the Si content is 0.60% or less, the formation of coarse silicides is suppressed, and the reduction in fatigue strength is suppressed. Therefore, the Si content is preferably 0.60% or less. The Si content is preferably 0.50% or less, more preferably 0.40% or less, and even more preferably 0.30% or less. Si is not essential in titanium alloy sheets, so its content has a lower limit of 0%, but the Si content can be, for example, 0.10% or more, or 0.15% or more.

[0081] Similar to Fe or V, Ni is an element that improves tensile strength and processability. However, when the Ni content is 0.15% or more, the intermetallic compound Ti₂Ni, which forms as an equilibrium phase, can sometimes be formed, leading to a deterioration in the fatigue strength and room temperature ductility of the titanium alloy sheet. Therefore, the Ni content is preferably less than 0.15%. More preferably, the Ni content is 0.14% or less, or 0.11% or less. Ni is not essential in titanium alloy sheets, so its lower limit is 0%, but the Ni content can, for example, be 0.01% or more.

[0082] Similar to Fe or V, Cr is an element that improves tensile strength and processability. However, when the Cr content is 0.25% or more, the intermetallic compound TiCr2, which forms as an equilibrium phase, can sometimes be formed, leading to a deterioration in the fatigue strength and room temperature ductility of the titanium alloy sheet. Therefore, the Cr content is preferably less than 0.25%. More preferably, the Cr content is 0.24% or less, and even more preferably 0.21% or less. Cr is not essential in titanium alloy sheets, so its content has a lower limit of 0%, but the Cr content can, for example, be 0.01% or more.

[0083] Similar to Fe or V, Mn is an element that improves tensile strength and processability. However, when the Mn content is 0.25% or more, the intermetallic compound TiMn, which forms as an equilibrium phase, sometimes forms, deteriorating the fatigue strength and room temperature ductility of the titanium alloy sheet. Therefore, the Mn content is preferably less than 0.25%. More preferably, the Mn content is 0.24% or less, and even more preferably 0.20% or less. Mn is not essential in titanium alloy sheets, so its content has a lower limit of 0%, but the Mn content can, for example, be 0.01% or more.

[0084] When considering the effects of the above chemical composition, the titanium alloy sheet involved in this embodiment preferably contains either Fe: 0.5 to 2.3% or V: 2.5 to 4.5% as an arbitrary element.

[0085] Furthermore, considering the effects of the aforementioned chemical composition, when the titanium alloy sheet involved in this embodiment contains either Fe: 0.5 to 2.3% or V: 2.5 to 4.5%, it is preferable to contain one or more of the following selected from the group consisting of Ni: less than 0.15%, Cr: less than 0.25%, and Mn: less than 0.25% to replace a portion of Fe or V.

[0086] In the case of the titanium alloy sheet according to this embodiment containing Fe, when it contains one or more elements selected from the group consisting of Ni: less than 0.15%, Cr: less than 0.25%, and Mn: less than 0.25%, the total amount of Fe, Ni, Cr, and Mn is preferably 0.5% or more and 2.3% or less. When the total amount of Fe, Ni, Cr, and Mn is 0.5% or more, high tensile strength can be obtained, and the β phase with good workability at room temperature can be maintained, thus improving the workability of the titanium alloy sheet. In addition, when the total amount of Fe, Ni, Cr, and Mn is 2.3% or less, the segregation of these elements is suppressed, and deviations in the properties of the titanium alloy sheet can be suppressed.

[0087] Furthermore, when the titanium alloy sheet according to this embodiment contains V, when it contains one or more elements selected from the group consisting of Ni: less than 0.15%, Cr: less than 0.25%, and Mn: less than 0.25%, the total amount of V, Ni, Cr, and Mn is preferably 2.5% or more and 4.5% or less. When the total amount of V, Ni, Cr, and Mn is 2.5% or more, high tensile strength can be obtained, and the β phase with good workability at room temperature can be maintained, thus improving the workability of the titanium alloy sheet. In addition, when the total amount of V, Ni, Cr, and Mn is 4.5% or less, the segregation of these elements is suppressed, and deviations in the properties of the titanium alloy sheet can be suppressed.

[0088] In this embodiment, the titanium alloy sheet is preferably limited to C: less than 0.080%, N: less than 0.050%, and O: less than 0.40%. The content of each element will be explained below. It should be noted that C, N, and O are not essential in the titanium alloy sheet, therefore their content is limited to 0%.

[0089] When titanium alloy sheets contain a large amount of carbon (C), it can sometimes reduce their ductility or workability. Therefore, the C content is preferably less than 0.080%. It should be noted that C is an unavoidable contamination, and its actual content is usually above 0.0001%.

[0090] Similar to carbon (C), a high nitrogen (N) content in titanium alloy sheets can sometimes reduce their ductility or workability. Furthermore, while N is an infiltrating element that penetrates the α-phase to strengthen the titanium through solid solution, a high N content can sometimes reduce its cold-rollability. Therefore, the N content is preferably 0.050% or less. It should be noted that N is an unavoidable contamination, and its actual content is typically 0.0001% or more.

[0091] Similar to carbon (C), a high content of oxygen (O) in titanium alloy sheets can sometimes reduce their ductility or workability. Furthermore, similar to nitrogen (N), oxygen is an infiltrating element that penetrates the α phase to strengthen the titanium material through solid solution treatment; however, a high content can sometimes reduce its cold rollability. Therefore, the O content is preferably 0.40% or less, more preferably 0.35% or less, and even more preferably 0.30% or less. It should be noted that O is an unavoidable contamination, and its content is typically 0.0001% or more.

[0092] When the titanium alloy sheet involved in this embodiment contains one or more substances selected from the group consisting of O, N, Fe and V, and the content of O (in mass percent) is set as [O], the content of N is set as [N], the content of Fe is set as [Fe], and the content of V is set as [V], the Q value shown in the following formula (1) is preferably 0.340 or less. There is no particular limitation on the lower limit of the Q value, but since O and N are unavoidably mixed substances, the Q value is actually greater than 0.

[0093] Q=[O]+(2.77×[N])+(0.1×[Fe])+(0.025×[V])…Equation (1)

[0094] The Q value is an indicator for estimating the cold rollability of titanium materials. When the Q value is greater than 0.340, the cold rollability can sometimes be significantly reduced. As mentioned above, the cold rollability decreases when there are large amounts of O and N. In particular, in systems containing more than 4.0% by mass of Al, O sometimes undergoes ordering with Al to form intermetallic compounds, resulting in a decrease in cold rollability. Fe and V are β-phase stabilizing elements and generally have the effect of improving cold rollability, but when they are present in excess, the strength of the α and β phases increases, impairing ductility, and therefore sometimes reducing cold rollability. The coefficients of [N], [Fe], and [V] are determined considering their influence on the reduction of cold rollability.

[0095] The balance of the chemical composition of the titanium alloy sheet involved in this embodiment can be Ti and impurities. Impurities include, for example, H, Cl, Na, Mg, Ca, and B introduced during refining processes, and Zr, Sn, Mo, Nb, and Ta introduced from scrap. A total impurity content of 0.5% or less is considered acceptable. Furthermore, the H content is 150 ppm or less. B can potentially form large precipitates within the ingot. Therefore, even when B is present as an impurity, it is preferable to suppress the B content as much as possible. In the titanium alloy sheet of this embodiment, the B content is preferably set to 0.01% or less.

[0096] It should be noted that when the titanium alloy sheet involved in this embodiment contains 0.5 to 2.3% Fe, the V contained in the titanium alloy sheet is sometimes only an amount considered as an impurity. When the titanium alloy sheet involved in this embodiment contains 2.5 to 4.5% V, the Fe contained in the titanium alloy sheet is sometimes only an amount considered as an impurity.

[0097] Furthermore, as long as the titanium alloy sheet involved in this embodiment can achieve high strength and high Young's modulus in the width direction, it can of course contain various elements instead of Ti. Similarly, as for elements exemplified as impurities, as long as the titanium alloy sheet has high strength and excellent machinability, it can contain an amount or more that is considered an impurity.

[0098] As explained above, the titanium alloy sheet involved in this embodiment may have the aforementioned chemical composition. More specifically, the chemical composition of the titanium alloy sheet involved in this embodiment may be, for example, Ti-6Al-4V, Ti-6Al-4V ELI, or Ti-5Al-1Fe.

[0099] (1.2. Metallographic Structure)

[0100] Next, the metallographic structure of the titanium alloy sheet involved in this embodiment will be described.

[0101] [Texture]

[0102] First, the crystal orientation of the texture of the titanium alloy sheet will be explained. If the titanium alloy is subjected to unidirectional high-speed hot rolling at a temperature in the α+β high-temperature region with a high proportion of β phase, a texture (T-texture) with the c-axis orientation of hcp in the width direction of the sheet will be formed during the transformation from β phase to α phase by variable selection. T-texture is the texture formed when the unrecrystallized β phase transforms into the α phase after rolling deformation. T-texture increases the strength and Young's modulus in the width direction of the sheet. When the crystal orientation of the α phase is represented by Euler angles g = {φ1, Φ, φ2} based on the Bunge notation method, if the maximum aggregation orientation shown by the crystal orientation distribution function f(g) is in the range of φ1: 0~30°, Φ: 60~90°, φ2: 0~60°, and the aggregation degree of the maximum aggregation orientation is 10.0 or more, then it is a structure with well-developed T-texture. The titanium alloy sheet involved in this embodiment has a structure with well-developed T-texture and contains a large amount of unrecrystallized structure.

[0103] Here, refer to Figure 1 The Euler angles g = {φ1, Φ, φ2} based on the Bunge-based notation method are explained. Figure 1 This is an explanatory diagram illustrating the crystal orientation of the α-phase grains in a titanium alloy sheet obtained using Euler angles based on the Bunge marking method. As the sample coordinate system, three mutually orthogonal coordinate axes are shown: RD (rolling direction), TD (plate width direction), and ND (normal direction of the rolled surface). Additionally, as the crystal coordinate system, three mutually orthogonal coordinate axes are shown: the X-axis, Y-axis, and Z-axis. Furthermore, the coordinate axes are arranged with their origins aligned, and the hexagonal prism representing hcp is shown with the center of the (0001) face of hcp, representing the α-phase of titanium, aligned with the origin. Figure 1 In the equation, the X-axis is aligned with the [10-10] direction of phase α, the Y-axis is aligned with the [-12-10] direction, and the Z-axis is aligned with the

[0001] direction (C-axis direction).

[0104] In Bunge's marking method, the first consideration is that the RD, TD, and ND of the sample coordinate system are aligned with the X, Y, and Z axes of the crystal coordinate system, respectively. Then, the crystal coordinate system is rotated by an angle φ1 around the Z-axis, and then rotated around the rotated X-axis (…). Figure 1The state is then rotated by an angle Φ. Finally, the Z-axis is rotated by an angle φ2 after rotating by Φ. These three angles, φ1, Φ, and φ2, represent the specific tilt of the crystal or crystal coordinate system relative to the sample coordinate system. That is, the crystal orientation is uniquely determined using these three angles. These three angles, φ1, Φ, and φ2, are called Euler angles based on the Bunge notation method. The crystal orientation (C-axis direction, etc.) of the α-phase grains in the titanium alloy sheet is specified using these Euler angles based on the Bunge notation method.

[0105] exist Figure 1 In the diagram, φ1 is the angle between the intersection of the RD-TD plane (rolling plane) of the sample coordinate system and the [10-10]-[-12-10] plane of the crystal coordinate system, and the RD (rolling direction) of the sample coordinate system. Φ is the angle between the ND (normal direction of the rolling surface) of the sample coordinate system and the

[0001] direction (normal direction of the (0001) plane) of the crystal coordinate system. φ2 is the angle between the intersection of the RD-TD plane (rolling surface) of the sample coordinate system and the [10-10]-[-12-10] plane of the crystal coordinate system, and the [10-10] direction of the crystal coordinate system.

[0106] The maximum aggregation orientation and maximum aggregation degree can be determined as follows. For titanium alloy thin plates, at the center of the width direction (TD), a section perpendicular to the width direction (L section) is chemically ground, and crystal orientation analysis is performed using electron backscatter diffraction (EBSD). For the lower surface and the central part of the thickness of the titanium alloy thin plate, approximately five fields of view are measured at 1 μm intervals for a region of (whole plate thickness) × 200 μm. The data are analyzed using OIM Analysis software manufactured by TSL Solutions. TM The software (Ver. 8.1.0) is used to calculate the crystal orientation distribution function f(g) (ODF). In the texture analysis using the spherical harmonic function method of the EBSD method, the expansion coefficient is set to 16 and the Gaussian half-width at half-maximum is set to 5° to calculate the crystal orientation distribution function f(g). At this time, considering the symmetry of rolling deformation, the calculation is performed in a way that is linearly symmetric with respect to the plate thickness direction, rolling direction, and plate width direction, respectively. The ODF represents the three-dimensional distribution of the measured crystal orientation marked in the three-dimensional space (Eulerian space) of φ1-Φ-φ2 using the distribution function. Figure 2 This is an example of the crystal orientation distribution function f(g) of the titanium alloy sheet involved in this embodiment, obtained by electron backscattering diffraction. Figure 2In order to represent Eulerian space in two dimensions, the Eulerian space is horizontally sliced ​​every 5 degrees along the angular direction φ2, and the resulting cross-sections are arranged. Using this ODF, the maximum clustering orientation and maximum clustering degree can be calculated. It should be noted that... Figure 2 At point A, where φ1 = 0°, Φ = 90°, and φ2 = 30°, the location of maximum aggregation can be identified, with a maximum aggregation degree of 36.3. It should be noted that although the maximum aggregation location and maximum aggregation degree were determined above based on the L-section at the center of the width direction, since the texture of the titanium alloy sheet is uniform in the width direction, the maximum aggregation location and maximum aggregation degree can also be determined based on the L-section at any position across the sheet width.

[0107] [Dislocation Density]

[0108] Metallic materials typically undergo work hardening, which involves the introduction of dislocations. In titanium alloy sheets, the higher the dislocation density, the higher the strength. The titanium alloy sheet involved in this embodiment has a well-developed T-texture structure, and therefore contains a large amount of unrecrystallized structure. Unrecrystallized structure is a structure that can introduce a large number of dislocations. As a method for estimating this dislocation density, there is a method that estimates the dislocation density based on the full width at half maximum (FWHM) of the diffraction peaks obtained by X-ray diffraction (XRD). The larger the FWHM of the diffraction peaks, the higher the dislocation density. In order to obtain sufficient work hardening, it is preferable that the FWHM of the diffraction peaks on the (102) plane, represented by the position of 2θ = 53.3 ± 1° detected by X-ray diffraction using CuKα as the X-ray source, is 0.20° or more. On the other hand, when the dislocation density is too high, the strength is too high, the notch sensitivity becomes high, and the sheet may break. Therefore, the FWHM of the diffraction peaks on the (102) plane is preferably 1.00° or less, and more preferably 0.80° or less.

[0109] The dislocation density was calculated using the following method. After wet grinding of the surface of a titanium alloy sheet with sandpaper, a mirror finish was achieved by mirror polishing the surface with colloidal silica. XRD measurements were then performed on the mirror-finished titanium alloy sheet surface. In the XRD measurements, CuKα was used as the X-ray source, and measurements were performed on 2θ in the range of 50.0° to 55.0° at a measurement interval of 0.01° and a measurement speed of 2° / min. The full width at half maximum (FWHM) was calculated using the Rigaku-manufactured comprehensive powder X-ray analysis software PDXL, based on X-ray diffraction data measured using Rigaku's Smartlab software.

[0110] [Zone-like tissue]

[0111] The titanium alloy sheet involved in this embodiment has a banded structure with an aspect ratio greater than 3.0 and extending along the length of the sheet, and the area ratio of this banded structure is preferably 70% or more. The banded structure referred to here is, for example,... Figure 3 A banded structure, as shown in an optical microscope photograph, is a structure extending along its length. Specifically, it refers to grains with an aspect ratio greater than 3.0, expressed as the ratio of the major axis to the minor axis. For example... Figure 4 As shown in the optical microscope image of the titanium alloy sheet according to this embodiment, the titanium alloy sheet according to this embodiment has a banded structure extending along the length direction of the sheet. When hot-rolled at temperatures in the α+β and β domains, a banded structure extending along the length direction of the sheet is formed in the titanium alloy. The banded structure has numerous grain boundaries perpendicular to the thickness direction of the sheet. If the area ratio of the banded structure is 70% or more, the propagation of cracks originating from the sheet surface into the thickness direction can be slowed. More preferably, the area ratio of the banded structure is 75% or more, and even more preferably 80% or more. Furthermore, all grains may be banded, with an upper limit of 100%.

[0112] The aspect ratio and the area ratio of the banded structure can be calculated as follows. For titanium alloy sheets, a cross-section (L-section) perpendicular to the width direction is chemically ground at the center of the width direction (TD). Within any five fields of view of this cross-section, a region of (whole sheet thickness) × 200 μm is measured at 1 μm increments, and crystal orientation analysis is performed using the EBSD method. Based on the EBSD crystal orientation analysis results, the aspect ratio of each grain is calculated. Then, the area ratio of grains with an aspect ratio greater than 3.0 is calculated. It should be noted that although the aspect ratio and the area ratio of the banded structure are calculated based on the L-section at the center of the width direction, since the banded structure is uniformly distributed in the width direction, the aspect ratio and the area ratio of the banded structure can also be calculated based on the L-section at any position within the width of the sheet.

[0113] (1.3. 0.2% yield strength in the width direction of the plate)

[0114] The titanium alloy sheet described in this embodiment has a 0.2% yield strength in the width direction of 800 MPa or more at room temperature. In the aerospace field and other applications, tensile strength close to that of the commonly used α+β type titanium alloy Ti-6Al-4V at room temperature is generally required. If the titanium alloy sheet has a 0.2% yield strength in the width direction of 800 MPa or more at room temperature, it can be used for applications requiring high strength. The 0.2% yield strength in the width direction at room temperature is preferably 850 MPa or more. On the other hand, if the strength is too high, the strength of the hot-rolled sheet before cold rolling is also high, so it is sometimes difficult to cold roll the hot-rolled sheet, requiring more cold rolling passes and increasing costs. In addition, if the strength is too high, the cut sensitivity becomes higher, and sheet breakage may occur. Therefore, the 0.2% yield strength in the width direction at room temperature is preferably 1300 MPa or less. The 0.2% yield strength in the width direction at room temperature is more preferably 1250 MPa or less. The 0.2% yield strength can be determined using the method according to JIS Z2241:2011. Specifically, tensile test specimen No. 13B (12.5 mm width of parallel section and 50 mm distance between marks) as specified in JIS Z 2241:2011 can be prepared by changing the tensile direction to the width direction of the titanium alloy sheet, and a tensile test can be performed at a strain rate of 0.5% / min to determine the tensile strength.

[0115] (1.4. Young's modulus in the width direction of the plate)

[0116] The titanium alloy sheet involved in this embodiment has a Young's modulus in the width direction of 125 GPa or higher. If the Young's modulus is 125 GPa or higher, it can be used in applications requiring high precision, such as aerospace components, automotive parts, and consumer goods. In particular, if the Young's modulus in the width direction is 125 GPa or higher, it offers the advantage of a weight reduction of approximately 3-4% compared to conventional methods. While an excessively high Young's modulus is not problematic, the practical upper limit for titanium is around 150 GPa. The Young's modulus in the width direction can be determined using the following method. That is, tensile test specimen No. 13B (12.5 mm width of parallel section and 50 mm distance between marks) as specified in JIS Z2241:2011 is prepared by changing the tensile direction to the width direction of the titanium alloy sheet. A strain gauge is attached and the load-unload is repeated 5 times at a strain rate of 10.0% / min in a stress range of 100 MPa to half of 0.2% of the yield strength. The slope is calculated and the average of the three tests after removing the maximum and minimum values ​​is taken as Young's modulus.

[0117] (1.5. Vickers Hardness HV)

[0118] The Vickers hardness HV of the titanium alloy sheet involved in this embodiment is 330 or higher. Regarding the Vickers hardness HV, according to JIS Z 2244:2009, at the center of the width direction (TD) of the rolled sheet, a section perpendicular to the width direction (TD (Transverse direction) is mirror-polished. A load of 500g is applied for 15 seconds, and the Vickers hardness HV is measured at seven locations on this section. The average of the five points after removing the maximum and minimum values ​​is taken as the Vickers hardness HV. The Vickers hardness HV of the titanium alloy sheet involved in this embodiment can be 340 or higher, or 350 or higher. Alternatively, the Vickers hardness HV of the titanium alloy sheet involved in this embodiment can be 430 or lower, or 420 or lower. It should be noted that a Vickers hardness HV of 330 or higher for the titanium alloy sheet involved in this embodiment is equivalent to a tensile strength of 1 GPa or higher as measured according to the method of JIS Z2241:2011. It should be noted that although the TD surface at the center of the length direction is used as the measurement surface for Vickers hardness HV in the above description, since the deviation of Vickers hardness HV of titanium alloy thin plates in the length direction is small, the TD surface at any position in the length direction can also be used as the measurement surface for Vickers hardness HV.

[0119] (1.6. Average plate thickness)

[0120] The titanium alloy sheet according to this embodiment has an average thickness of 2.5 mm or less. In conventional hot rolling, as the sheet thickness decreases, the deformation resistance increases due to the rapid drop in temperature. Therefore, when hot rolling high-strength materials, the allowable load of the rolling mill may sometimes be exceeded, making it difficult to achieve an average thickness of 2.5 mm or less. On the other hand, as will be described in detail later, the titanium alloy sheet according to this embodiment is manufactured by a method including a cold rolling process, thus enabling an average thickness of 2.5 mm or less. Furthermore, while there is no particular limitation on the lower limit of the average thickness of the titanium alloy sheet according to this embodiment, in practice, the average thickness of titanium alloys with the aforementioned strength is mostly 0.1 mm or more. Therefore, the average thickness of the titanium alloy sheet according to this embodiment is preferably 0.1 mm or more. The average thickness of the titanium alloy sheet according to this embodiment is more preferably 0.3 mm or more.

[0121] Here, refer to Figure 5 The method for determining the average plate thickness is explained. Figure 5 This is a schematic diagram illustrating the method for measuring the average plate thickness. Using X-rays, a micrometer, or a vernier caliper, plate thickness is measured at at least five locations along the length of the plate, spaced at intervals of at least 1 meter, at locations 1 / 4 of the plate width at the center of the plate width direction and at locations at the ends of the plate width direction. The average plate thickness is then taken as the average plate thickness.

[0122] (1.7. Plate thickness dimensional accuracy)

[0123] The dimensional accuracy of the titanium alloy sheet according to this embodiment is preferably 5.0% or less relative to the average sheet thickness. In lamination rolling, titanium alloy sheets are manufactured by hot rolling multiple layers of titanium material encased in steel. However, due to the significant variation in deformation resistance caused by temperature distribution in the multiple layers of titanium material, it is difficult to manufacture sheets with uniform thickness. However, the titanium alloy sheet according to this embodiment is manufactured by cold rolling as described later, and therefore has excellent dimensional accuracy. More preferably, the dimensional accuracy of the titanium alloy sheet according to this embodiment is 4.0% or less relative to the average sheet thickness, and even more preferably, 2.0% or less relative to the average sheet thickness.

[0124] The plate thickness dimensional accuracy is determined by the following method. Using X-rays, a micrometer, or a vernier caliper, plate thickness is measured at at least five locations along the length direction at intervals of at least 1 m, at locations 1 / 4 of the plate width at the center of the width direction and at locations 1 / 4 of the width from both ends of the width direction. The maximum value of a', calculated using the actual measured plate thickness d and the aforementioned average plate thickness dave, is taken as the plate thickness dimensional accuracy a.

[0125] a'=(d-dave) / dave×100...Formula (101)

[0126] The titanium alloy sheet according to this embodiment has been described above. The titanium alloy sheet according to this embodiment described above can be manufactured by any method, for example, it can also be manufactured by the titanium alloy sheet manufacturing method described below.

[0127] <2. Manufacturing method of titanium alloy sheet>

[0128] The method for manufacturing titanium alloy sheet according to this embodiment includes: a slab manufacturing process, in which a titanium alloy slab (titanium billet) is manufactured as a blank to become a titanium alloy sheet; a heating process, in which the titanium alloy slab is heated; a hot rolling process, in which the titanium alloy slab after the heating process is hot rolled; a cold rolling process, in which the titanium billet after the hot rolling process is cold rolled; and a surface finishing and stretching straightening process, in which the titanium billet after the cold rolling process is surface finished or stretched straightened as needed. The following describes each step of the method for manufacturing titanium alloy sheet according to this embodiment.

[0129] (2.1. Slab manufacturing process)

[0130] In the slab manufacturing process, titanium alloy slabs are manufactured. As raw materials, blanks having the aforementioned chemical composition and manufactured using known methods can be used. There are no particular limitations on the manufacturing method of the titanium alloy slab; for example, it can be manufactured in the following order. For example, an ingot can be made from sponge titanium using various melting methods such as vacuum arc remelting, electron beam melting, or plasma melting. Then, the obtained ingot can be hot-forged at a temperature in the α-phase high-temperature domain, the α+β two-phase domain, or the β-phase single-phase domain to obtain the titanium alloy slab. It should be noted that pretreatments such as cleaning and cutting can be applied to the titanium alloy slab as needed. Furthermore, when hot-rollable rectangles are formed using the furnace melting method, hot forging can be omitted before hot rolling. The manufactured titanium alloy slab contains Al: greater than 4.0% and less than 6.6%, Fe: greater than 0% and less than 2.3%, V: greater than 0% and less than 4.5%, Si: greater than 0% and less than 0.60%, Ni: greater than 0% and less than 0.15%, Cr: greater than 0% and less than 0.25%, Mn: greater than 0% and less than 0.25%, C: greater than 0% and less than 0.080%, N: greater than 0% and less than 0.050%, and O: greater than 0% and less than 0.40%.

[0131] (2.2. Heating process)

[0132] In this process, the titanium alloy slab is heated to the β phase transformation point T. β ℃ and above (T) β Temperatures below +150℃. Heating temperatures less than T. β At ℃, titanium alloy slabs are pressed down in the state with a high proportion of α phase, but the pressing becomes insufficient in the state with a high proportion of β phase. Therefore, the T-texture is not well developed. Furthermore, the heating temperature is greater than (T... β At +150°C, the likelihood of β-phase recrystallization during rolling becomes very high. Under these conditions, no variable selection occurs during the transformation from the β-phase to the α-phase, thus hindering the development of the T-texture. Consequently, oxidation of the titanium alloy slab surface becomes intense, easily leading to scabs or cracks on the hot-rolled plate surface. The temperature of the titanium alloy slab mentioned here is the surface temperature, measured using a radiation thermometer. Regarding the emissivity of the radiation thermometer, a value calibrated to match the temperature measured using a contact thermocouple is used for slabs immediately after heating.

[0133] It should be noted that, in this specification, the β phase transition point T β This refers to the boundary temperature at which the α phase begins to form when titanium alloys are cooled from the β-phase single-phase domain. T βPhase diagrams can be obtained. These diagrams can be obtained, for example, using the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) method. Specifically, the Thermo-Calc integrated thermodynamic calculation system from Thermo-Calc Sotware AB, along with the specified database (TI3), can be used to obtain the phase diagram of titanium alloys via the CALPHAD method, and calculate the T... β .

[0134] (2.3. Hot rolling process)

[0135] Generally, if titanium alloys are subjected to unidirectional high-speed hot rolling at temperatures on the high-temperature side of the α+β domain with a high proportion of β phase or β domain, a T-texture will form during the transformation from the β phase to the α phase. This T-texture is achieved by applying heat to the α+β domain at temperatures with a high proportion of β phase or β domain single phase, for example (T...). β Hot rolling at temperatures above -50°C allows for sufficient development of the T-texture. The β-phase transformation point varies depending on the composition of the titanium alloy slab, but hot rolling can, for example, begin at temperatures above 950°C. Furthermore, to develop the T-texture, rolling with a high reduction rate in a temperature range with a high β-phase proportion is important to develop the β-phase texture and to suppress β-phase recrystallization. In order to form and develop the T-texture, the method for manufacturing titanium alloy sheets according to this embodiment includes a hot rolling process in which the titanium alloy slab is hot rolled unidirectionally, with a reduction rate of 80% or more, and a final temperature of (T... β -250)℃ or above and (T β Temperatures below -50℃ are achieved. Consequently, a T-texture is formed in the hot-rolled titanium alloy sheet obtained after hot rolling of the slab. The T-texture exhibits excellent cold-rollability and is effective for increasing the strength and Young's modulus in the width direction of the sheet.

[0136] When the final temperature is less than (T) β At -250°C, titanium alloy slabs are pressed down in states with a high proportion of α-phase, but the pressing becomes insufficient in states with a high proportion of β-phase. Therefore, the T-texture is not well developed. Furthermore, the final temperature is less than (T... β At -250℃, the resistance to thermal deformation increases sharply and the hot workability decreases, which makes it easy to produce end cracks and reduce the yield.

[0137] The final temperature is greater than (T) β At -50℃, the possibility of β-phase recrystallization during hot rolling becomes very high. Under these conditions, no variable selection occurs during the transformation from β-phase to α-phase, thus hindering the development of T-texture.

[0138] When the reduction rate is less than 80.0%, the introduced processing strain is insufficient, and sometimes the strain is not uniformly introduced to the entire plate thickness, and the T-texture is not well developed.

[0139] To ensure high anisotropy and the formation of a strong T-texture in the hot-rolled titanium alloy sheet, it is preferable to heat the titanium alloy slab to the aforementioned heating temperature and hold it for at least 30 minutes. By holding the titanium alloy slab at the aforementioned heating temperature for at least 30 minutes, the crystalline phase of the titanium alloy slab can become a β single phase, making it easier to form and develop the T-texture.

[0140] In addition, the heating temperature and the final temperature are the surface temperatures of the titanium alloy slab, which can be determined using known methods. For example, a radiation thermometer can be used to determine the heating temperature and the final temperature.

[0141] In the hot rolling process, titanium alloy slabs can be continuously hot rolled using known continuous hot rolling equipment. When using continuous hot rolling equipment, the titanium alloy slabs are wound into hot-rolled titanium alloy coils by a coiler after hot rolling.

[0142] The titanium alloy hot-rolled plates obtained after the above hot rolling process can be subjected to annealing based on known methods, removal of oxide scale based on pickling or cutting, or cleaning treatment, as needed.

[0143] (2.4. Cold rolling process)

[0144] In this process, the titanium billet after the hot rolling process is subjected to at least one cold rolling pass along its length. The average rolling rate of each cold rolling pass is less than 40%. If the average rolling rate of each cold rolling pass is less than 40%, recrystallization is less likely to occur during subsequent intermediate and final annealing, thus maintaining the T-texture.

[0145] It should be noted that the cold rolling passes mentioned here refer to continuously performed cold rolling. Specifically, for cold rolling passes, in the case of the period from the hot rolling process to the point where the titanium billet reaches the final product thickness, or in the case of the surface finishing process described later after the hot rolling process, it refers to the cold rolling from the hot rolling process to the point where the surface finishing process is performed. Where intermediate annealing is performed during the cold rolling process, the cold rolling from the hot rolling process to the intermediate annealing process, and the cold rolling from the intermediate annealing process to the point where the titanium billet reaches the final product thickness, or to the point where the surface finishing process is performed, are respectively referred to as cold rolling passes. Furthermore, in the case of multiple intermediate annealing processes, the cold rolling from the previous intermediate annealing process to the next intermediate annealing process is also referred to as a cold rolling pass.

[0146] The operating temperature for cold rolling passes can be, for example, below 500°C or below 400°C. There is no particular limitation on the lower limit of the operating temperature for cold rolling passes; for example, the operating temperature can be above room temperature. Room temperature here refers to above 0°C.

[0147] In the cold rolling process, the titanium billet after the last cold rolling pass can also undergo final annealing. Final annealing is only necessary when performed appropriately. The conditions for intermediate and final annealing are: annealing temperature above 500℃ and below 750℃, and the annealing temperature T (℃) and the holding time t (seconds) at the annealing temperature satisfy the following equation (102).

[0148] It should be noted that in the following equation (102), (T+273.15)×(Log 10 (t)+20) is the Larsen-Miller parameter.

[0149] 18000≤(T+273.15)×(Log 10 (t)+20)<22000…(102)

[0150] By performing intermediate or final annealing under the above conditions, recrystallization can be suppressed and T-texture can be maintained. When the annealing temperature is less than 500°C or the annealing temperature or holding time does not satisfy the above formula (102), the recovery of the metallographic structure becomes insufficient, which can lead to internal or end cracks during cold rolling. In addition, the accumulated strain increases, and recrystallization may occur. On the other hand, when the annealing temperature is greater than 750°C, recrystallization occurs, and T-texture is lost. In the intermediate and final annealing processes, by determining the annealing temperature T and annealing time t in a manner that the annealing temperature is 500°C or higher and 750°C or lower, and the annealing temperature T (°C) and the holding time t (seconds) at the annealing temperature satisfy the following formula (102), T-texture can be maintained and internal and end cracks during cold rolling can be suppressed.

[0151] (2.5. Surface rolling and stretching straightening process)

[0152] Titanium alloy sheets are manufactured through the aforementioned cold rolling process. However, the titanium alloy sheets after the cold rolling process can preferably undergo surface rolling for adjusting mechanical properties or tensile straightening for correcting shape, as needed. The reduction rate in surface rolling is preferably 10% or less, and the elongation of the cold-rolled titanium alloy sheet in tensile straightening is preferably 5% or less. It should be noted that surface rolling and tensile straightening may be omitted if not required. The method for manufacturing titanium alloy sheets according to this embodiment has been described above.

[0153] According to the manufacturing method of titanium alloy sheet according to this embodiment, a T-texture is generated and developed through the above-described hot rolling process, and a titanium alloy sheet that can maintain the T-texture state is obtained through the above-described cold rolling process. Specifically, when the crystal orientation of the α phase is represented by Euler angles g = {φ1, Φ, φ2} based on the Bunge notation method, a titanium alloy sheet can be obtained in which the maximum aggregation orientation shown by the crystal orientation distribution function f(g) is in the range of φ1: 0~30°, Φ: 60~90°, φ2: 0~60°, and the aggregation degree of the maximum aggregation orientation is 10.0 or more. The titanium alloy sheet contains, by mass percent: Al: greater than 4.0% and less than 6.6%, Fe: greater than 0% and less than 2.3%, V: greater than 0% and less than 4.5%, Si: greater than 0% and less than 0.60%, Ni: greater than 0% and less than 0.15%, Cr: greater than 0% and less than 0.25%, Mn: greater than 0% and less than 0.25%, C: greater than 0% and less than 0.080%, N: greater than 0% and less than 0.050%, and O: greater than 0% and less than 0.40%. The titanium alloy sheet has a 0.2% yield strength in the width direction of 25°C of ≥800 MPa and a Young's modulus in the width direction of ≥125 GPa.

[0154] Furthermore, according to the manufacturing method of titanium alloy sheet according to this embodiment, the thickness dimensional accuracy of the sheet can be less than 5.0% relative to the average sheet thickness.

[0155] Furthermore, according to the manufacturing method of titanium alloy sheet according to this embodiment, since it is unidirectional rolling, it can also manufacture coils, and titanium alloy sheet can be manufactured with high productivity.

[0156] Example

[0157] The embodiments of this disclosure will be described in detail below. It should be noted that the embodiments shown below are merely examples of this disclosure, and this disclosure is not limited to the following examples.

[0158] (Example 1)

[0159] 1. Manufacturing of titanium alloy thin plates

[0160] First, titanium alloy ingots, as blanks for titanium alloy sheets as shown in Table 1, are manufactured by vacuum arc remelting (VAR). Then, slabs with a thickness of 150 mm × width of 800 mm × length of 5000 mm are manufactured by primary rolling or forging. It should be noted that, in addition to the elements listed in Table 1, the others are Ti and impurities. Furthermore, "Q" in Table 1 is a value calculated using the following formula (1).

[0161] Q=[O]+(2.77×[N])+(0.1×[Fe])+(0.025×[V])…Equation (1)

[0162] It should be noted that in the formula, [O] is the O content in mass% (%), [N] is the N content in mass% (%), [Fe] is the Fe content in mass% (%), and [V] is the V content in mass% (%).

[0163] The chemical composition of the slabs was determined by ICP emission spectroscopy analysis of Al, Fe, Si, Ni, Cr, Mn, and V. O and N were determined using an oxygen-nitrogen simultaneous analysis apparatus via inert gas melting, thermal conductivity, and infrared absorption. C was determined using a carbon-sulfur simultaneous analysis apparatus via infrared absorption. The chemical composition of each hot-rolled plate was the same as that of the titanium alloy slabs shown in Table 1. Furthermore, for the titanium billets A through P shown in Table 1, the phase diagram of the titanium alloys was obtained using the Thermo-Calc integrated thermodynamic calculation system (TI3) from Thermo-Calc Sotware AB and the specified database, and the β-phase transformation point T was calculated using the CALPHAD method. β .

[0164] Table 1

[0165]

[0166] Next, these slabs were hot-rolled under the conditions shown in Tables 2-1 and 2-3, followed by hot-rolled plate annealing, shot peening, and pickling to produce hot-rolled plates with a thickness of 4 mm. Hot rolling began after the heating temperature had decreased by approximately 50°C. Then, the resulting hot-rolled plates were cold-rolled under the conditions shown in Tables 2-2 and 2-4. It should be noted that in Tables 2-1 and 2-3, "T" indicates the thickness of the slab. β "T" represents the β phase transition point, and the "Larsen-Miller parameter" is (T+273.15)×(Log). 10 The value of (t)+20).

[0167] [Table 2-1]

[0168]

[0169] [Table 2-2]

[0170]

[0171] [Table 2-3]

[0172]

[0173] [Table 2-4]

[0174]

[0175] 2. Evaluation

[0176] The following items were evaluated for the titanium alloy sheets involved in each of the invention examples and comparative examples.

[0177] 2.1. Texture

[0178] The following describes the measurement and calculation of the maximum orientation and maximum aggregation of the titanium plates involved in each invention example and comparative example. At the center of the titanium alloy sheet in the width direction (TD), a section perpendicular to the width direction was chemically ground, and crystal orientation analysis was performed using EBSD. Approximately five fields of view were measured in a region of (whole sheet thickness) × 200 μm at 1 μm increments. The data were analyzed using OIM Analysis software manufactured by TSL Solutions. TM The software (Ver. 8.1.0) is used to calculate the ODF, from which the peak position and maximum aggregation degree are derived. In the texture analysis using the spherical harmonic function method of the EBSD method, the expansion factor is set to 16 and the Gaussian half-width at half-maximum (FWHM) is set to 5° to calculate the ODF. Considering the symmetry of rolling deformation, the calculation is performed in a manner that ensures linear symmetry with respect to the plate thickness direction, rolling direction, and plate width direction.

[0179] 2.2. Dislocation density

[0180] Dislocation density is related to the full width at half maximum (FWHM) of the diffraction peaks detected by XRD. Therefore, in this embodiment, the FWHM of the diffraction peaks of the (102) plane at the position of 2θ = 53.3 ± 1° detected by XRD using CuKα as the X-ray source is calculated. Specifically, after wet grinding of the surface of the titanium alloy sheet with sandpaper, the surface is mirror-polished using colloidal silica to form a mirror surface. XRD measurements are performed on the surface of the titanium alloy sheet with the mirror surface. In the XRD measurement, CuKα is used as the X-ray source, and the 2θ range from 50.0° to 55.0° is measured under the conditions of a measurement interval of 0.01° and a measurement speed of 2° / min. The FWHM is calculated using the comprehensive powder X-ray analysis software PDXL manufactured by Rigaku, based on the X-ray diffraction data measured by Smartlab manufactured by Rigaku. If the FWHM is 0.20° or higher, the dislocation density is sufficient to obtain a degree of work hardening.

[0181] 2.3. Area ratio of banded tissue

[0182] For each sample, at the center of the plate width, a section perpendicular to the plate width direction was chemically ground. The area of ​​this section (total plate thickness) × 200 μm was divided into 5 fields of view with a step size of 1 μm. Crystal orientation analysis based on EBSD method was performed to determine the aspect ratio of each grain and calculate the area ratio of grains with an aspect ratio greater than 3.0.

[0183] 2.4.0.2% Yield Strength σT

[0184] The 0.2% yield strength σT in the width direction of the titanium alloy sheet at 25°C, as described in the various inventive examples, reference examples, and comparative examples, was measured according to JIS Z 2241:2011. Specifically, a tensile test piece No. 13B (12.5 mm width of the parallel portion and 50 mm distance between markings) as specified in JIS Z 2241:2011 was prepared with the tensile direction changed to the width direction of the titanium alloy sheet, and a tensile test was performed at a strain rate of 0.5% / min to measure the yield strength.

[0185] 2.5. Young's modulus E in the width direction of the plate

[0186] The Young's modulus E in the width direction of the titanium alloy sheet involved in each of the inventive examples, reference examples, and comparative examples is determined by the following method. That is, a tensile test piece No. 13B (12.5 mm width of the parallel part and 50 mm distance between the marks) as specified in JIS Z2241:2011 is prepared with the tensile direction changed to the width direction of the titanium alloy sheet. A strain gauge is attached, and the load-unload cycle is repeated 5 times at a strain rate of 10.0% / min within a stress range of 100 MPa to half of 0.2% of the yield strength. The slope is calculated, and the average of the three tests after removing the maximum and minimum values ​​is taken as the Young's modulus.

[0187] 2.6. Vickers Hardness (HV)

[0188] Regarding Vickers hardness HV, according to JIS Z 2244:2009, at the center position in the length direction (RD), the cross section (TD (Transverse direction)) of the rolled surface perpendicular to the width direction of the plate is mirror polished. The cross section is measured at 7 locations with a load of 500g and a load time of 15 seconds. The average value of the 5 points after removing the maximum and minimum values ​​is taken as the Vickers hardness HV.

[0189] 2.7. Average plate thickness (dave)

[0190] The average thickness of the titanium alloy sheet involved in each of the invention examples, reference examples, and comparative examples was determined by the following method. For each manufactured titanium alloy sheet, at the center position in the width direction and at a distance of 1 / 4 of the width direction from both ends, the sheet thickness was measured at 5 or more positions along the length direction at intervals of 1 m or more using X-rays or a vernier caliper. The average value of the measured sheet thicknesses was taken as the average sheet thickness.

[0191] 2.8. Plate thickness dimensional accuracy a

[0192] For the thickness dimensional accuracy of the titanium alloy sheet involved in each of the invention examples, reference examples and comparative examples, the maximum value of a' calculated by the following formula (101) is used as the dimensional accuracy a, using the actual sheet thickness d measured by the above method and the above average sheet thickness dave.

[0193] a'=(d-dave) / dave×100...Formula (101)

[0194] 2.9. Cold rolling properties

[0195] The cold rollability of the titanium alloy sheets involved in the various inventive examples, reference examples, and comparative examples is evaluated by the following method: The maximum value of the end crack after cold rolling is used for evaluation. Furthermore, if the maximum value of the end crack after cold rolling is 1 mm or less, the cold rollability is rated as "Very Good" (A); if the maximum value of the end crack after cold rolling is greater than 1 mm but less than 2 mm, the cold rollability is rated as "Good" (B); and if the maximum value of the end crack after cold rolling is greater than 2 mm, the cold rollability is rated as "Unacceptable" (C).

[0196] 3. Results

[0197] The evaluation results are shown in Tables 3-1 and 3-2. It should be noted that "φ1", "Φ", and "φ2" in Table 3 refer to angles based on the Bunge labeling method.

[0198] [Table 3-1]

[0199]

[0200] [Table 3-2]

[0201]

[0202] In any of Invention Examples 1 to 49, the maximum aggregation orientation is within the range of φ1: 0–30°, Φ: 60–90°, and φ2: 0–60°, and the maximum aggregation degree is 10.0 or higher. Furthermore, in Invention Examples 1 to 26, 28 to 35, and 37 to 49, the half-width at half-maximum is 0.20° or higher, and the area fraction of the banded structure is 70% or higher. Additionally, in any of Invention Examples 1 to 49, the 0.2% yield strength σT in the width direction at 25°C is 800 MPa or higher, and the Young's modulus in the width direction is 125 GPa or higher. The final average plate thickness dave is 1.2–1.9 mm, and the dimensional accuracy a is 5.0% or lower. In Comparative Example 10, the Al content is low, therefore the 0.2% yield strength is as low as 692 MPa, and the Young's modulus in the width direction is as low as 122 GPa. In Comparative Example 11, the high Al content resulted in surface cracks and severe end cracks during cold rolling after hot rolling. In Comparative Example 12, the temperature dropped significantly in the later stages of hot rolling, causing cracks in the hot-rolled plate, making it impossible to manufacture a 2.5 mm thick plate.

[0203] In Invention Examples 1-6, 9-20, and 25-49, the Q value is 0.340 or less. Compared with Invention Examples 7, 8, and 21-24, which have a Q value greater than 0.340, these invention examples show good cold rollability.

[0204] On the other hand, Comparative Examples 1 to 10 deviate from the manufacturing conditions of the manufacturing method of the titanium alloy sheet involved in this disclosure, the maximum aggregation orientation or the aggregation degree of the maximum aggregation orientation does not meet the requirements of this application, and the Young's modulus E in the width direction of the sheet is less than 125 GPa.

[0205] The preferred embodiments of this disclosure have been described in detail above, but this disclosure is not limited to the examples described. Obviously, those skilled in the art to which this disclosure pertains can conceive of various modifications or variations within the scope of the technical concept recorded in the claims, and these examples naturally also fall within the technical scope of this disclosure.

Claims

1. A titanium alloy sheet, which contains, by mass % Al: Greater than 4.0% and less than 6.6% Fe: 0% or more and 2.3% or less V: Above 0% and below 4.5% Si: 0% or more and 0.60% or less Ni: 0% or more and less than 0.15% Cr: 0% or more and less than 0.25% Mn: 0% or more and less than 0.25% C: 0% or more and less than 0.080% N: 0% or more and 0.050% or less, and O: Above 0% and below 0.40%, The balance consists of Ti and impurities. When the crystal orientation of the α phase is represented by Euler angles g = {φ1, Φ, φ2} using the Bunge-based notation method, in the texture analysis using the spherical harmonic function method in electron backscattering diffraction, with the expansion coefficient set to 16 and the Gaussian half-width at half-maximum (FWHM) set to 5°, the maximum aggregation orientation shown by the crystal orientation distribution function f(g) is within the range of φ1: 0–30°, Φ: 60–90°, and φ2: 0–60°. The aggregation degree of the maximum aggregation orientation is 10.0 or higher. The titanium alloy sheet has a 0.2% yield strength in the width direction of the sheet at 25°C of over 800 MPa. The Young's modulus of the titanium alloy sheet in the width direction is above 125 GPa. The average thickness of the titanium alloy sheet is less than 2.5 mm.

2. The titanium alloy sheet according to claim 1, wherein it contains, by mass%, Fe: 0.5% or more and 2.3% or less or V: 2.5% or more and 4.5% or less.

3. The titanium alloy sheet according to claim 2, wherein it contains, by mass%, selected from […]. Ni: less than 0.15%, Cr: less than 0.25%, and Mn: One or more species in a group consisting of less than 0.25% When the titanium alloy sheet contains Fe, the total amount of Fe, Ni, Cr, and Mn is 0.5% or more and 2.3% or less. When the titanium alloy sheet contains V, the total amount of V, Ni, Cr and Mn is more than 2.5% and less than 4.5%.

4. The titanium alloy sheet of claim 2 or 3, wherein, When Ti is used to replace a portion of the substance with one or more of the elements selected from the group consisting of O, N, Fe, and V, and the content of O (in mass%) is set as [O], the content of N is set as [N], the content of Fe is set as [Fe], and the content of V is set as [V], Q as shown in the following formula (1) is 0.340 or less. Q=[O]+(2.77×[N])+(0.1×[Fe])+(0.025×[V])…Equation (1).

5. The titanium alloy sheet of any one of claims 1-3, wherein, The half-width at half-maximum (FWHM) of the diffraction peak at 2θ = 53.3 ± 1°, detected by X-ray diffraction using CuKα as the X-ray source, is greater than 0.20°.

6. The titanium alloy sheet of claim 4, wherein, The half-width at half-maximum (FWHM) of the diffraction peak at 2θ = 53.3 ± 1°, detected by X-ray diffraction using CuKα as the X-ray source, is greater than 0.20°.

7. The titanium alloy sheet according to any one of claims 1 to 3, wherein it has a banded structure with an aspect ratio greater than 3.0 and extending along the length of the sheet. The area ratio of the banded tissue is over 70%.

8. The titanium alloy sheet according to claim 4, wherein it has a banded structure with an aspect ratio greater than 3.0 and extending along the length of the sheet. The area ratio of the banded tissue is over 70%.

9. The titanium alloy sheet according to claim 5, wherein it has a banded structure with an aspect ratio greater than 3.0 and extending along the length of the sheet. The area ratio of the banded tissue is over 70%.

10. The titanium alloy sheet according to claim 6, wherein it has a banded structure with an aspect ratio greater than 3.0 and extending along the length of the sheet. The area ratio of the banded tissue is over 70%.

11. The titanium alloy sheet of any of claims 1-3, wherein, The dimensional accuracy of the plate thickness is less than 5.0% relative to the average plate thickness.

12. The titanium alloy sheet of claim 4, wherein, The dimensional accuracy of the plate thickness is less than 5.0% relative to the average plate thickness.

13. The titanium alloy sheet of claim 5, wherein, The dimensional accuracy of the plate thickness is less than 5.0% relative to the average plate thickness.

14. The titanium alloy sheet of claim 6, wherein, The dimensional accuracy of the plate thickness is less than 5.0% relative to the average plate thickness.

15. The titanium alloy sheet of claim 7, wherein, The dimensional accuracy of the plate thickness is less than 5.0% relative to the average plate thickness.

16. The titanium alloy sheet of any of claims 8-10, wherein, The dimensional accuracy of the plate thickness is less than 5.0% relative to the average plate thickness.

17. A method for manufacturing a titanium alloy sheet according to any one of claims 1 to 16, comprising the following steps: The heating process involves heating the titanium billet, which contains, by mass %: Al: greater than 4.0% and less than 6.6%, Fe: greater than 0% and less than 2.3%, V: greater than 0% and less than 4.5%, Si: greater than 0% and less than 0.60%, Ni: greater than 0% and less than 0.15%, Cr: greater than 0% and less than 0.25%, Mn: greater than 0% and less than 0.25%, C: greater than 0% and less than 0.08%, N: greater than 0% and less than 0.05%, and O: greater than 0% and less than 0.40%, with the balance being Ti and impurities. The hot rolling process involves hot rolling the titanium billet after the heating process in one direction; and The cold rolling process involves performing at least one cold rolling pass along the length of the titanium billet after the hot rolling process. The β phase transformation point is set to T β (°C), the heating temperature of the titanium blank in the heating process is T β °C or higher and (T β + 150) °C or lower, The reduction rate in the hot rolling process is above 80.0%. The final temperature in the hot rolling process is (T β -250) °C or higher and (T β -50) °C or lower, In the cold rolling process, when the average rolling rate per cold rolling pass is less than 40% and multiple cold rolling passes are performed, intermediate annealing is included. The annealing conditions for the intermediate annealing process are as follows: The annealing temperature is above 500°C and below 750°C, and the annealing temperature T (°C) and the holding time t (seconds) at the annealing temperature satisfy the following formula (2). 18000 ≤ (T + 273.15) x (Log 10 (t) + 20) < 22000... Equation (2).

18. The method of manufacturing a titanium alloy sheet according to claim 17, wherein, After the final cold rolling pass, a final annealing is performed at an annealing temperature of 500°C or higher and 750°C or lower, satisfying the formula (2).

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