Titanium alloy plate

Abstract

This titanium alloy plate has a chemical composition comprising, in mass%: 4.5% or more and 6.6% or less of Al; 0.3% or more and 2.3% or less of Fe; 0.2% or more and 2.0% or less of Cu; 0.05% or more and 0.50% or less of Si; 0% or more and less than 0.080% of C; 0% or more and 0.050% or less of N; 0% or more and 0.25% or less of O; 0% or more and less than 0.15% of Ni; 0% or more and less than 0.25% of Cr; 0% or more and less than 0.25% of Mn; and a remainder of Ti and impurities. The titanium alloy plate satisfies expression (1).　Expression (1): 35.0 ≤ 5 × AL + 5 × Cu + 10 × Fe + 20 × Si ≤ 60.0. The elements shown in expression (1) each indicate the content of each element in unit mass%.

Description

[Technical field]

[0001] The present invention relates to a titanium alloy plate. [Background technology]

[0002] In the aircraft industry, titanium is widely used to reduce aircraft weight and improve fuel efficiency, and among these, Ti-6Al-4V (64 alloy), a general-purpose α+β type titanium alloy, is often used for components that require high strength.

[0003] However, since high-strength α+β titanium alloys such as alloy 64 have poor cold rollability, a pack rolling method is usually used to manufacture thin sheets of α+β titanium alloys, in which the material is covered with a relatively thick iron plate and hot-rolled while keeping the material warm. Furthermore, when titanium alloys are hot-rolled in one direction at high speed at a temperature in the β region or the α+β region with a high β phase ratio, a texture (T-texture) in which the c-axis of the hexagonal close-packed structure (hcp) is oriented in the sheet width direction is formed due to variant selection during the transformation from β phase to α phase. In this case, the titanium alloy sheet has anisotropy in which the mechanical properties in the longitudinal direction and the width direction of the sheet are significantly different. Therefore, when a thin sheet with small anisotropy in the sheet plane is required, the titanium material needs to be cross-rolled or rolled at a low speed at a temperature in the α+β region, which is a factor in increasing costs.

[0004] In addition, when titanium is cold-rolled, a texture (B-texture) is formed in which the hcp c-axis is oriented in the sheet thickness direction, reducing the anisotropy in the sheet plane. In particular, in titanium alloys containing Al, twin deformation is less likely to occur, making it easier to form a stronger texture. On the other hand, the formation of the above texture reduces cold rollability. Therefore, to manufacture titanium alloy thin sheets, cold rolling with a small reduction ratio and annealing must be repeated multiple times, which increases costs. In addition, as the cold rolling rate decreases, it becomes more difficult to improve the texture, and the anisotropy in the sheet plane remains. In order to solve the above problems, development of titanium alloys that can be rolled in one direction is underway.

[0005] Patent Document 1 discloses a casting α+β type titanium alloy having a specified chemical composition, a tensile strength of 890 MPa or more in the as-cast state, and a melting point of 1650° C. or less.

[0006] Patent Document 2 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, and the balance being titanium and impurities.

[0007] Patent Document 3 discloses an α+β type titanium alloy bar material 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.

[0008] Patent Document 4 discloses a method for producing a titanium alloy sheet, which comprises cold rolling a hot-rolled annealed sheet of a titanium alloy consisting of, by weight, 2.5-3.5% Al, 2.0-3.0% V, the balance being Ti and ordinary impurities, in the same direction as the hot-rolling direction at a total rolling reduction of 67% or more, and then annealing the sheet at a temperature between 650 and 900°C.

[0009] Patent Document 5 discloses a method for producing an α+β type titanium alloy thin plate, which is characterized in that in the manufacturing process of an α+β type titanium alloy cold-rolled plate, intermediate annealing after cold rolling is performed under the following conditions: annealing temperature: temperature range of [β transformation point -25°C] or higher and lower than the β transformation point, annealing time: 0.5 to 4 hours, cooling rate after heating and holding: 0.5 to 5°C / sec, and temperature range for cooling at the above cooling rate: up to 300°C or lower.

[0010] Patent Document 6 discloses an α+β type titanium alloy thin plate that contains at least one fully dissolved β stabilizing element in an Mo equivalent amount of 2.0 to 4.5 mass%, at least one eutectoid β stabilizing element in an Fe equivalent amount of 0.3 to 2.0 mass%, at least one α stabilizing element in an Al equivalent amount of more than 3.0 mass% and not more than 5.5 mass%, with the balance being Ti and unavoidable impurities, wherein the average grain size of the α phase is 5.0 μm or less, the maximum grain size of the α phase is 10.0 μm or less, the average aspect ratio of the α phase is 2.0 or less, and the maximum aspect ratio of the α phase is 5.0 or less.

[0011] Patent Document 7 describes an α+β type titanium alloy hot-rolled sheet, in which (a) the normal direction (sheet thickness direction) of the hot-rolled sheet is defined as ND, the hot-rolling direction is defined as RD, and the width direction of the hot-rolled sheet is defined as TD, and the normal direction of the (0001) plane of the α phase is defined as the c-axis orientation, and the angle that the c-axis orientation makes with ND is defined as θ, and the angle that the plane including the c-axis orientation and ND makes with the plane including ND and TD is defined as Φ, and (b1) θ is 0 degrees or more and 30 degrees or less, and Φ is the angle that the entire circumference (-180 degrees to 18 (b2) among the (0002) reflection relative intensities of X-rays caused by crystal grains having a θ of 80 degrees or more and less than 100 degrees, and a Φ of ±10 degrees, the strongest intensity is defined as XND, and (c) among the (0002) reflection relative intensities of X-rays caused by crystal grains having a θ of 80 degrees or more and less than 100 degrees, and a Φ of ±10 degrees, the strongest intensity is defined as XTD, and an α+β type titanium alloy sheet having excellent cold rollability and cold handleability is disclosed, characterized in that XTD / XND is 5.0 or more.

[0012] Patent Document 8 describes a high-strength α+β type titanium alloy hot-rolled sheet containing, by mass%, 0.8-1.5% Fe, 4.8-5.5% Al, and 0.030% or less N, and containing O and N in a range satisfying Q(%)=0.14-0.38, defined as Q(%)=[O]+2.77·[N], where [O] is the content (mass%) of O and [N] is the content (mass%) of N, with the balance being Ti and unavoidable impurities, and wherein (a) the normal direction of the hot-rolled sheet is defined as ND, the hot-rolling direction is defined as RD, and the width direction of the hot-rolled sheet is defined as TD, the normal direction of the (0001) plane of the α phase is defined as the c-axis orientation, and the angle between the c-axis orientation and ND is defined as The present invention discloses a high-strength α+β type titanium alloy sheet having excellent cold coil (strip) handleability, characterized in that (b1) θ is 0 degrees or more and 30 degrees or less, and φ is the angle between the plane including the c-axis orientation and the ND direction and the plane including ND and TD, (b2) θ is 80 degrees or more and less than 100 degrees, and φ is the strongest intensity of the (0002) reflection of X-rays by crystal grains within the full circumference (-180 degrees to 180 degrees), and (b3) XTD / XND is 4.0 or more.

[0013] Patent Document 9 discloses a Ti alloy with excellent heat resistance, which is characterized by containing, by weight, one or more of Al: 10% or less, Sn: 15% or less, and Zr: 15% or less, and further containing 0.01 to 15% Ga, with the balance being Ti and impurities. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] Japanese Patent Publication No. 2010-7166 [Patent Document 2] Japanese Patent Publication No. 7-62474 [Patent Document 3] Japanese Patent Publication No. 7-70676 [Patent Document 4] Japanese Patent Publication No. 61-147864 [Patent Document 5] Japanese Patent Publication No. 1-127653 [Patent Document 6] Japanese Patent Application Publication No. 2013-227618 [Patent Document 7] International Publication No. 2012 / 115242 [Patent Document 8] International Publication No. 2012 / 115243 [Patent Document 9] Japanese Patent Publication No. 4-41635 Summary of the Invention [Problem to be solved by the invention]

[0015] However, the techniques disclosed in Patent Documents 1 to 9 sometimes lacked sufficient strength in the medium temperature range of 200 to 300°C required in the aircraft field, and sometimes did not provide sufficient ductility at room temperature. Furthermore, the same is true for strength in the high temperature range of over 300°C, which is beyond the medium temperature range.

[0016] The present invention has been made in consideration of the above problems, and an object of the present invention is to provide a titanium alloy plate having high strength, particularly a titanium alloy plate having excellent strength in the intermediate temperature range. [Means for solving the problem]

[0017] The present inventors have found that by adding Al to a titanium alloy, the strength of the titanium alloy at temperatures above the medium temperature range (for example, strength at about 300°C or higher) is increased. On the other hand, the present inventors have found that if the Al content is excessive, the cold workability is significantly reduced due to the precipitation of intermetallic compounds such as Ti3Al. The present inventors have found that by setting the Al content to a level at which the cold workability is not significantly reduced and further utilizing Cu and Si, it is possible to improve the strength of a titanium alloy plate in the medium temperature range, assuming excellent cold workability.

[0018] The gist of the present invention, which has been completed based on the above findings, is as follows. [1] The titanium alloy plate according to one embodiment of the present invention has a chemical composition, in mass%, of Al: 4.5% or more and 6.6% or less, Fe: 0.3% or more and 2.3% or less, Cu: 0.2% or more and 2.0% or less, Si: 0.05% or more and 0.50% or less, C: 0% or more and less than 0.080%, N: 0% or more and 0.050% or less, O: 0% or more and 0.25% 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%, and the balance: Ti and impurities, and satisfies the following formula (1). 35.0≦5×Al+5×Cu+10×Fe+20×Si≦60.0 …(1) formula The elements shown in the formula (1) above each represent the content of each element in unit mass %. [2] The titanium alloy sheet according to the above item [1] may have an angle of 30° or less between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α phase and the sheet width direction. [3] The titanium alloy plate according to the above item [1] may have an angle of less than 75° between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α phase and the plate thickness direction. Effect of the Invention

[0019] According to the above-mentioned aspects of the present invention, it is possible to provide a titanium alloy plate having high strength, particularly a titanium alloy plate having excellent strength in the intermediate temperature range. [Brief description of the drawings]

[0020] [Figure 1] 1 is an example of a (0001) pole figure in the thickness direction (ND) of a titanium alloy hot-rolled sheet according to a first embodiment of the present disclosure. [Diagram 2] FIG. 2 is a schematic diagram for explaining a method for measuring an average plate thickness. [Diagram 3] FIG. 11 is an example of a (0001) pole figure in the sheet thickness direction (ND) of the titanium alloy cold rolled sheet according to the second embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] A preferred embodiment of the present invention will be described in detail below with reference to the accompanying drawings. In the following, a hot-rolled titanium alloy sheet (hot-rolled titanium alloy sheet) will be described as a first embodiment of the present disclosure, and a cold-rolled titanium alloy sheet (cold-rolled titanium alloy sheet) will be described as a second embodiment. In the present disclosure, the titanium alloy sheet includes hot-rolled and cold-rolled titanium alloy sheets. Furthermore, the hot-rolled titanium alloy sheet includes a hot-rolled annealed sheet, and the cold-rolled titanium alloy sheet includes a cold-rolled annealed sheet.

[0022] <<First embodiment>> <Chemical composition> First, the chemical components contained in the titanium alloy hot-rolled sheet according to this embodiment will be described. In the following description of the chemical components, unless otherwise specified, the notation "%" will represent "mass %".

[0023] The titanium alloy hot-rolled sheet according to this embodiment has a chemical composition, in mass%, of Al: 4.5% or more and 6.6% or less, Fe: 0.3% or more and 2.3% or less, Cu: 0.2% or more and 2.0% or less, Si: 0.05% or more and 0.50% or less, C: 0% or more and less than 0.080%, N: 0% or more and 0.050% or less, O: 0% or more and 0.25% 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%, and the balance: Ti and impurities.

[0024] [Al:4.5% or more, 6.6% or less] Al is an α-phase stabilizing element and has a high solid solution strengthening ability. When the Al content increases, the tensile strength at room temperature and in the medium temperature range increases. In order to increase the tensile strength in the medium temperature range, the Al content is set to 4.5% or more. The Al content may be more than 4.5% and 4.6% or more. On the other hand, when the Al content exceeds 6.6%, the tensile strength in the medium temperature range increases, but the cold rollability decreases significantly, and the α-phase is excessively solid solution strengthened by solidification segregation, etc., resulting in the generation of locally hard regions, and the impact toughness decreases. Therefore, the Al content is 6.6% or less. The Al content is preferably 6.5% or less, more preferably 6.4% or less.

[0025] [Fe: 0.3% or more, 2.3% or less] Fe is a β-stabilizing element and has high solid solution strengthening ability. Therefore, increasing the Fe content increases the tensile strength at room temperature and in the intermediate temperature range. In addition, the β phase is a phase with excellent workability at room temperature. In addition, when the Fe content is low, T-texture is difficult to form by hot rolling. Therefore, the Fe content is set to 0.3% or more. The Fe content may be more than 0.4% and 0.5% or more. On the other hand, since Fe is an element that is very prone to solidification segregation, if the Fe content is too high, the performance varies greatly depending on the position in the titanium alloy plate, and some parts have reduced fatigue strength depending on the position. Therefore, the Fe content is set to 2.3% or less. The Fe content is preferably 2.1% or less, more preferably 2.0% or less, and even more preferably 1.9% or less. In addition, Fe is an inexpensive element among the β-stabilizing elements.

[0026] [Cu: 0.2% or more, 2.0% or less] Cu, like Fe, is a β-stabilizing element and has high solid solution strengthening ability. In addition, Cu has excellent solid solution strengthening ability in the medium temperature range around 300°C. Furthermore, unlike Al, Cu does not hinder workability, so it is an extremely effective element for inexpensive production of titanium alloy hot-rolled sheets. In addition, when the Cu content is low, T-texture is difficult to form. In order to obtain tensile strength mainly in the medium temperature range, the Cu content is set to 0.2% or more. The Cu content is preferably 0.3% or more. On the other hand, if the titanium alloy hot-rolled sheet contains an amount of Cu that significantly exceeds the solid solubility limit of Cu in the α phase, the β phase fraction becomes high, and conversely, the strength at 300°C decreases. In addition, if the Cu content is too high, Ti2Cu may precipitate, significantly decreasing the workability at room temperature. Therefore, the Cu content is set to 2.0% or less. The Cu content is preferably 1.8% or less, more preferably 1.7% or less, and even more preferably 1.6% or less. Although Cu is more expensive than Fe, it is relatively inexpensive among the β stabilizing elements.

[0027] [Si: 0.05% or more, 0.50% or less] Although Si is a β-stabilizing element, it also dissolves in the α-phase and exhibits high solid-solution strengthening ability. Si is particularly excellent in solid-solution strengthening ability at high temperatures, improving tensile strength in the medium temperature range. In order to obtain the effect of improving tensile strength in the medium temperature range, the Si content is set to 0.05% or more. The Si content may be 0.10% or more. In addition, Si has a tendency to segregate in the opposite direction to O (oxygen) described below, and is less likely to solidify and segregate than O. Therefore, by containing appropriate amounts of Si and O in the titanium alloy hot-rolled sheet, high fatigue strength and tensile strength can be achieved at the same time. On the other hand, if the Si content is too high, an intermetallic compound called silicide is formed, and the fatigue strength is reduced. If the Si content exceeds 0.50%, coarse silicide is generated during the manufacturing process of the titanium alloy hot-rolled sheet, and the fatigue strength is reduced. Therefore, the Si content is set to 0.50% or less. The Si content is preferably 0.40% or less.

[0028] [C: 0% or more, less than 0.080%] If a large amount of C is contained in the titanium alloy hot-rolled sheet, the ductility or workability may be reduced. Therefore, the C content is less than 0.080%. The C content may be 0.070% or less, 0.050% or less, or less than 0.050%. On the other hand, since C does not have to be contained in the titanium alloy hot-rolled sheet, the C content is preferably low, and is 0% or more. However, since C is an impurity that is inevitably mixed in, and therefore an element that is unavoidably contained in the titanium alloy hot-rolled sheet, the C content may be more than 0%, may be 0.0001% or more, or may be 0.005% or more.

[0029] [N: 0% or more, 0.050% or less] If a large amount of N is contained in the titanium alloy hot-rolled sheet, the ductility or workability may be reduced. Therefore, the N content is 0.050% or less. The N content may be less than 0.040%. On the other hand, since N does not have to be contained in the titanium alloy hot-rolled sheet, the N content is preferably low, and is 0% or more. However, since N is an impurity that is inevitably mixed in, and is an element that cannot be avoided in the titanium alloy hot-rolled sheet, the N content may be more than 0%, may be 0.001% or more, or may be 0.005% or more.

[0030] [O: 0% or more, 0.25% or less] If a large amount of O is contained in the titanium alloy hot-rolled sheet, the ductility or workability may decrease. Therefore, the O content is 0.25% or less. The O content is preferably less than 0.25%, more preferably 0.23% or less, and further preferably 0.22% or less. On the other hand, since O does not have to be contained in the titanium alloy hot-rolled sheet, the O content is preferably low, and is 0% or more. However, since O is an impurity that is inevitably mixed in, and therefore an element that is unavoidably contained in the titanium alloy hot-rolled sheet, the O content may be more than 0%, may be 0.01% or more, or may be 0.05% or more.

[0031] [Ni: 0% or more, less than 0.15%] If a titanium alloy hot-rolled sheet contains a large amount of Ni, an equilibrium intermetallic compound (Ti2Ni) may be generated, and at least one of the fatigue strength and room temperature ductility may deteriorate. Therefore, the Ni content is less than 0.15%. The Ni content is preferably 0.14% or less. On the other hand, since Ni does not have to be contained in the titanium alloy hot-rolled sheet, the Ni content is preferably low, and is 0% or more.

[0032] [Cr: 0% or more, less than 0.25%] If Cr is contained in a large amount in the titanium alloy hot-rolled sheet, an intermetallic compound (TiCr2) which is an equilibrium phase may be generated, and at least one of fatigue strength and room temperature ductility may be deteriorated. Therefore, the Cr content is less than 0.25%. The Cr content is preferably 0.24% or less, and more preferably 0.21% or less. On the other hand, since Cr does not have to be contained in the titanium alloy hot-rolled sheet, the Cr content is preferably low, and is 0% or more.

[0033] [Mn: 0% or more, less than 0.25%] If Mn is contained in a large amount in the titanium alloy hot-rolled sheet, an intermetallic compound (TiMn) which is an equilibrium phase may be generated, and at least one of the fatigue strength and room temperature ductility may be deteriorated. Therefore, the Mn content is less than 0.25%. The Mn content is preferably 0.24% or less, and more preferably 0.21% or less. On the other hand, since Mn does not have to be contained in the titanium alloy hot-rolled sheet, the Mn content is preferably low, and is 0% or more.

[0034] [35.0≦5×Al+5×Cu+10×Fe+20×Si≦60.0] The titanium alloy plate according to this embodiment satisfies the following formula (1). 35.0≦5×Al+5×Cu+10×Fe+20×Si≦60.0 …(1) formula The elements shown in the above formula (1) each indicate the content of each element in unit mass %.

[0035] When the value of 5×Al+5×Cu+10×Fe+20×Si is 35.0 or more, the tensile strength in the medium temperature range can be 520MPa or more. The value of 5×Al+5×Cu+10×Fe+20×Si is more preferably 37.0 or more, and even more preferably 39.0 or more. On the other hand, when the value of 5×Al+5×Cu+10×Fe+20×Si is 60.0 or less, the tensile strength in the medium temperature range can be 520MPa or more while preventing excessive hardening and maintaining manufacturability. The value of 5×Al+5×Cu+10×Fe+20×Si is more preferably 55.0 or less, and even more preferably 53.0 or less.

[0036] [impurities] The balance of the chemical composition of the titanium alloy hot-rolled sheet according to this embodiment is Ti and impurities. The impurities are, for example, H, Cl, Na, Mg, Ca, B mixed in during the refining process, and Zr, Sn, Mo, Nb, Ta, V, etc. mixed in from scrap, etc. The impurities are each 0.1% or less, and the total amount is 0.5% or less, so there is no problem. The H content is 150 ppm or less. There is a concern that B will become coarse precipitates in the ingot. Therefore, even if it is contained as an impurity, it is preferable to suppress the B content as much as possible. In the titanium alloy hot-rolled sheet according to this embodiment, the B content is preferably 0.01% or less.

[0037] <Metal structure> Next, the metal structure of the titanium alloy hot-rolled sheet according to this embodiment will be described. The titanium alloy hot-rolled sheet according to this embodiment may have an area ratio of α phase in the metal structure of 80% or more. Furthermore, in the titanium alloy hot-rolled sheet according to this embodiment, the area ratio of elongated grains, which are crystal grains having an aspect ratio of more than 3.3, may be 70.0% or more. This will be explained in detail below.

[0038] [Microstructure] (Area ratio of α phase is 80.0% or more) Since the Young's modulus of the β phase is low, if the β phase ratio is high, the Young's modulus decreases. Also, if the α phase ratio is small, the cold rollability may decrease. Therefore, it is preferable that the α phase ratio is high, and the area ratio of the α phase is preferably 80.0% or more. The area ratio of the α phase is more preferably 82.0% or more, and further preferably 85.0% or more. The β phase inhibits grain growth, so it is preferable that the area ratio of the β phase is 1.0% or more. The area ratio of the β phase is more preferably 2.0% or more. This area ratio of the β phase is a value at room temperature. If this is expressed as the area ratio of the α phase of the titanium alloy hot-rolled sheet, the upper limit of the α phase is substantially 99.0%, preferably 98.0%. The measurement method will be described in detail later. Note that silicide may be present in addition to the α phase and the β phase, but even if it is present, the area ratio is less than 0.5%, and since it is fine, it does not have a significant effect on the characteristics. The area ratio obtained by the measurement method described later is substantially the same as the volume ratio.

[0039] To observe the microstructure, a titanium alloy hot-rolled sheet is taken at the center of the sheet width direction, and a cross section (L cross section) including the rolling direction (RD) and thickness direction (ND) of the sheet is mirror-polished, then etched using a nitric hydrofluoric acid solution. An optical microscope is then used to observe 10 fields of view at 50 to 100 magnifications to cover the entire sheet thickness in order to obtain average values ​​for the entire material. The cross section (L cross section) including the rolling direction (RD) and the sheet thickness direction (ND) of the titanium alloy hot-rolled sheet is specified by the following method. That is, in titanium, the texture formed when rolling in one direction accumulates at a position where the hcp c-axis is inclined to the rolling width direction (sheet width direction). Therefore, the maximum accumulation direction of the c-axis is the sheet width direction, and the direction perpendicular to the sheet width direction is defined as the rolling direction.

[0040] The area ratios of the α and β phases are measured by SEM (Scanning Electron Microscopy) / EPMA (Electron Probe Micro Analyzer). The region where the content of β stabilizing elements (total content of Fe and Cu) by EPMA is 2 mass% or more is considered to be the β phase, and the region where the content of β stabilizing elements is less than 2 mass% is considered to be the α phase. After mirror polishing the L cross section, a total of five measurement ranges of 500 μm × 500 μm are measured on a surface (L cross section) parallel to the rolling direction of the material and parallel to the plate thickness direction. The measurement step in each measurement range is 1 μm. The area ratios of the α and β phases in each measurement range are calculated, and the arithmetic average of each value is evaluated as the area ratio of the α phase and the area ratio of the β phase.

[0041] (The area ratio of expanded grains is 70.0% or more) The microstructure of the titanium alloy hot-rolled sheet according to the present embodiment has crystal grains with an aspect ratio of more than 3.3. Hereinafter, crystal grains with an aspect ratio of more than 3.3 may be referred to as elongated grains. Also, crystal grains with an aspect ratio of 3.3 or less may be referred to as equiaxed grains. When titanium alloy is hot-rolled at a temperature in the α+β region or β region, elongated grains elongated in the rolling direction are formed. If the area ratio of these elongated grains is 70.0% or more, the propagation of cracks generated from the end of the sheet in the sheet width direction can be slowed. The area ratio of the elongated grains is more preferably 75.0% or more, and even more preferably 80.0% or more. Also, all the crystal grains may be elongated grains, and the upper limit of the area ratio of the elongated grains is 100%. The microstructure of the titanium alloy hot-rolled sheet according to this embodiment is made up of equiaxed grains and the remainder of precipitates, other than the elongated grains. Examples of precipitates include TiC and silicide. The combined area ratio of the elongated grains and equiaxed grains is 95% or more.

[0042] The aspect ratio of crystal grains and the area ratio of extended grains are measured by electron backscatter diffraction (EBSD). A titanium alloy plate is cut in the thickness direction along the longitudinal direction at the center of the plate width direction (TD), and a cross section (L cross section) is observed at a magnification of 500x or more over an area of ​​1 mm in the rolling direction and the entire thickness in the plate thickness direction, and electron backscatter diffraction (EBSD) is performed with measurement steps of 0.5 to 1.0 μm. The aspect ratio of each crystal grain is calculated from the ratio of the long axis to the short axis of one crystal grain. The sum of the areas of crystal grains with an aspect ratio of more than 3.3 relative to the total measured area is defined as the area ratio of extended grains. The aspect ratio is calculated using the OIM Analysis software from TSL Solutions Co., Ltd. TM (Ver.8.1.0), crystal grains are classified by regarding a grain boundary as one in which the crystal orientation difference is 15° or more. In the above, the L-section at the center position in the sheet width direction is used as the observation surface, but since the crystal orientation of a titanium alloy hot-rolled sheet is uniformly distributed in the sheet width direction, the L-section at any sheet width position may be used as the observation surface.

[0043] [Collective tissue] In the titanium alloy hot-rolled sheet according to the present embodiment, the angle between the maximum accumulation direction of the c-axis of the hexagonal close-packed structure constituting the α phase and the rolling perpendicular direction (sheet width direction), which is the direction perpendicular to the rolling direction on the rolled surface, is 40° or less. In titanium alloys, when high-speed hot-rolling is performed in one direction in the β region or the α+β high-temperature region where the β phase ratio is high, a texture (T-texture) in which the c-axis of the hexagonal close-packed structure (hcp) is oriented in the sheet width direction is formed by variant selection during transformation from the β phase to the α phase. When the T-texture is formed, the strength and Young's modulus in the sheet width direction are particularly high. Since the titanium alloy hot-rolled sheet according to the present embodiment has a high Young's modulus in the sheet width direction, it can be used, for example, in the face part of the head of a golf club, which requires a material that is less likely to bend. In addition, the titanium alloy sheet having the T-texture is easy to cold-roll. If the angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α-phase and the sheet width direction is 30° or less, the strength and Young's modulus of the titanium alloy hot-rolled sheet in the sheet width direction will be further increased. Also, if the angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α-phase and the sheet width direction is 30° or less, cold rolling can be easily performed. If the angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α-phase and the sheet width direction is 30° or less, the Young's modulus of the titanium alloy hot-rolled sheet in the sheet width direction at 25°C will be further increased, and the Young's modulus in the sheet width direction at 25°C will be 130GPa or more. Therefore, the angle is preferably 30° or less. The angle is more preferably 20° or less, and even more preferably 15° or less.

[0044] The angle between the maximum integration direction of the c-axis of the close-packed hexagonal structure that constitutes the α phase and the sheet width direction is calculated using a (0001) pole figure. A (0001) pole figure can be obtained by chemically polishing the observation surface of a titanium alloy sheet sample and analyzing the crystal orientation using EBSD. Specifically, the titanium alloy sheet is cut in the thickness direction along the longitudinal direction at the center position in the sheet width direction (TD), and a cross section (L cross section) is chemically polished. A (0001) pole figure can be created by performing crystal orientation analysis using the EBSD method on two locations on the cross section, an area of ​​(total sheet thickness) × 2 mm, spaced 1 to 2 μm apart. The peak position of the integration degree of a specific orientation in the (0001) pole figure can be determined by using the data from OIM Analysis software provided by TSL Solutions. TM The calculation is performed by texture analysis of the inverse pole figure using the spherical harmonic function method using software (Ver. 8.1.0). In this case, the position where the contour line is the highest is the peak position of the concentration, the value of the highest concentration among the peak positions is the maximum concentration, and the direction showing the peak of the maximum concentration from the center of the (0001) pole figure in the plate thickness direction is the maximum concentration direction. The concentration of a specific orientation in the (0001) pole figure indicates how many times the frequency of the existence of crystal grains with that orientation is compared with a structure with a completely random orientation distribution (concentration degree 1). The concentration of a specific orientation in the (0001) pole figure is calculated using a theoretical value in the above software. In addition, in the above, the L cross section at the center position in the plate width direction is used as the observation surface, but since the crystal orientation of a titanium alloy hot-rolled plate is uniformly distributed in the plate width direction, the L cross section at any plate width position may be used as the observation surface.

[0045] FIG. 1 shows an example of a (0001) pole figure from the thickness direction (ND) of the titanium alloy hot-rolled sheet according to this embodiment. In FIG. 1, the poles of each detected crystal orientation are accumulated according to the inclination in the rolling direction (RD) and the sheet width direction (TD), and the (0001) pole figure is drawn with the contour line of the accumulation degree. The part where the contour line in the figure is the highest is the peak P1 of the crystal grain. In this embodiment, the angle θ between the direction showing the accumulation degree peak P1 from the center of the (0001) pole figure from the sheet thickness direction (maximum accumulation direction) and the sheet width direction is preferably 30° or less. In FIG. 1, the direction showing the accumulation degree peak P1 from the center of the (0001) pole figure from the sheet thickness direction and the sheet width direction coincide with each other, so the angle θ is 0°.

[0046] <0.2% yield strength at 300℃ in the rolling direction is 520MPa or more> The titanium alloy hot-rolled sheet according to the present embodiment has a 0.2% yield strength in the rolling direction at 300°C of 520 MPa or more, and can be used in parts exposed to high temperatures. The yield strength of the titanium alloy hot-rolled sheet according to the present embodiment is preferably 530 MPa or more, and more preferably 530 MPa or more. On the other hand, since the higher the yield strength, the better, it is not particularly limited, and may be, for example, 750 MPa or less, 700 MPa or less.

[0047] The 0.2% proof stress at 300°C in the rolling direction is measured by the following method. That is, a tensile test piece is taken whose longitudinal direction is parallel to the rolling direction, whose parallel part width is 10 mm, whose parallel part length and gauge length are 35 mm, and whose thickness is the full thickness of the product plate. The above tensile test piece is subjected to a tensile test with a strain rate of 0.3% / min up to a strain of 1.5%, and then 7.5% / min until fracture. The test is performed in air at 300°C, and the tensile test piece is held in the test atmosphere for 10 minutes to allow the test temperature to be sufficiently reached, after which the tensile test is performed.

[0048] <0.2% yield strength at 25℃ in the rolling direction is 800MPa or more> The titanium alloy hot-rolled sheet according to the present embodiment has a 0.2% yield strength of 800 MPa or more at 25°C in the rolling direction. Therefore, high strength is required at around room temperature. The yield strength of the titanium alloy hot-rolled sheet according to the present embodiment is preferably 825 MPa or more. On the other hand, since the higher the yield strength, the better, it is not particularly limited, but may be, for example, 1050 MPa or less.

[0049] <Young's modulus in the width direction at 25℃ is 130GPa or more> The titanium alloy hot-rolled plate according to the present embodiment has a Young's modulus of 130 GPa or more at 25° C. in the sheet width direction, and can be used in applications requiring a high Young's modulus (such as golf clubs). The Young's modulus of the titanium alloy hot-rolled plate according to the present embodiment at 25° C. in the sheet width direction is preferably 135 GPa or more. On the other hand, since the higher the Young's modulus in the sheet width direction at 25° C. is, the more preferable it is, there is no particular restriction, and it may be, for example, 155 GPa or less, 150 GPa or less. The Young's modulus at 25°C in the sheet width direction can be measured by the following method: A 13B tensile test piece (parallel part width 12.5 mm, gauge length 50 mm) specified in JIS Z 2241:2011 is prepared so that the tensile direction is the sheet width direction of the titanium alloy thin sheet, a strain gauge is attached, and loading and unloading are repeated five times at a strain rate of 10.0% / min in the stress range from 100 MPa to half of the 0.2% proof stress, the slope is calculated, and the average value of three times excluding the maximum and minimum values ​​is taken as the Young's modulus at 25°C in the sheet width direction.

[0050] <Average plate thickness: 2.0mm or more, 6.0mm or less> The average thickness of the titanium alloy hot-rolled sheet according to this embodiment is not particularly limited, but in terms of production, it is substantially 2.0 mm or more and 6.0 mm or less.

[0051] Here, a method for measuring the average thickness will be described with reference to Fig. 2. Fig. 2 is a schematic diagram for explaining the method for measuring the average thickness. The thickness at each position is measured at 5 or more positions at intervals of 1 m or more in the longitudinal direction using an X-ray, a micrometer, or a vernier caliper, at the center position in the lateral direction and at positions at a distance of 1 / 4 of the lateral length from each of the lateral ends, and the average value of the measured thicknesses is taken as the average thickness. So far, the titanium alloy hot-rolled sheet according to this embodiment has been described.

[0052] <Method of manufacturing titanium alloy hot-rolled sheet> The titanium alloy hot-rolled sheet according to the present embodiment has the above-mentioned chemical composition and satisfies the formula (1). Therefore, the manufacturing method of the titanium alloy hot-rolled sheet according to the present embodiment is not particularly limited. However, for example, a titanium alloy hot-rolled sheet can be manufactured by carrying out a slab manufacturing process for manufacturing a titanium alloy slab that is a raw material (titanium material) of the titanium alloy hot-rolled sheet, a heating process for heating the titanium alloy slab, a hot rolling process for hot rolling the titanium alloy slab after the heating process, and, if necessary, a temper rolling and tensile straightening process for temper rolling or tensile straightening the titanium material after the hot rolling process. By adopting this manufacturing method, a titanium alloy hot-rolled sheet can be obtained in which the angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α phase and the sheet width direction is 30° or less. Below, as an example of the manufacturing method of the titanium alloy hot-rolled sheet according to the present embodiment, each step of the manufacturing method of the titanium alloy hot-rolled sheet including the slab manufacturing process, the heating process, the hot rolling process, and the temper rolling and tensile straightening process will be described. However, as described above, the manufacturing method of the titanium alloy hot-rolled sheet according to the present embodiment is not particularly limited. A titanium alloy hot-rolled sheet that satisfies the above requirements is considered to be a titanium alloy hot-rolled sheet according to this embodiment, regardless of its manufacturing method. The manufacturing method described below is merely a preferred example, and does not limit the titanium alloy hot-rolled sheet according to this embodiment.

[0053] (Slab manufacturing process) In the slab manufacturing process, a titanium alloy slab is manufactured. As the raw material, a material having the above-mentioned chemical composition and manufactured by a known method can be used. The manufacturing method of the titanium alloy slab is not particularly limited, and for example, it can be manufactured by the following procedure. For example, an ingot is produced from sponge titanium by various melting methods such as a vacuum arc melting method, an electron beam melting method, or a hearth melting method such as a plasma melting method. Next, the obtained ingot is hot forged at a temperature in the α-phase high temperature region, the α + β two-phase region, or the β-phase single-phase region to obtain a titanium alloy slab. Note that the titanium alloy slab may be subjected to pretreatment such as cleaning treatment and cutting as necessary. Alternatively, when the titanium alloy slab is made into a rectangular shape that can be hot rolled by the melting method, it may be subjected to hot rolling without hot forging or the like. The produced titanium alloy slab has a chemical composition, in mass%, of Al: 4.5% or more and 6.6% or less, Fe: 0.3% or more and 2.3% or less, Cu: 0.2% or more and 2.0% or less, Si: 0.05% or more and 0.50% or less, C: 0% or more and less than 0.080%, N: 0% or more and 0.050% or less, O: 0% or more and 0.25% 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%, and the balance: Ti and impurities.

[0054] (Heating process) In this process, the titanium alloy slab is heated to the β transformation point T β ℃ or more (T β +150℃). β By setting the heating temperature at ℃ or higher, the reduction is performed with a high ratio of β phase, and T-texture develops. β If the heating temperature is below (T +150℃), the recrystallization of the β phase during rolling is suppressed, variant selection is likely to occur during the phase transformation from the β phase to the α phase, and the T-texture is likely to develop. β +150°C) or less, excessive oxidation of the titanium alloy slab surface is suppressed, and the occurrence of scabs and scratches on the hot-rolled sheet surface after hot rolling is suppressed. The temperature of the titanium alloy slab here is the surface temperature, which is measured by a radiation thermometer. The emissivity of the radiation thermometer is a value calibrated to match the temperature measured by a contact thermocouple immediately after the slab comes out of the heating furnace.

[0055] In this specification, the β transformation point T β T is the boundary temperature at which the α phase begins to form when a titanium alloy is cooled from the β phase single phase region. β can be obtained from a phase diagram. The phase diagram can be obtained, for example, by the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) method. Specifically, a phase diagram of a titanium alloy is obtained by the CALPHAD method using Thermo-Calc, an integrated thermodynamic calculation system from Thermo-Calc Software AB, and a specified database (TI3), and T β can be calculated.

[0056] (Hot rolling process) When titanium alloys are hot-rolled in one direction at high speed at a temperature in the β region or the high-temperature side of the α+β region where the β phase fraction is high, the β phase transforms into α phase, forming a T-texture. β By starting hot rolling at a temperature of -50)°C or higher, T-texture can be sufficiently developed. Although the β transformation point differs depending on the composition of the titanium alloy slab, for example, hot rolling is started at a temperature of 950°C or higher. In order to develop T-texture, it is also important to perform rolling at a high reduction rate in a temperature range with a high β phase ratio, develop the texture of the β phase, and suppress recrystallization of the β phase. In order to form and develop T-texture, a hot rolling process is provided in which a titanium alloy slab is hot rolled in one direction, the reduction rate of the titanium alloy slab in the hot rolling process is set to 80% or more, and the finishing temperature is set to (T β -250)℃ or more (T β-50)°C or lower. As a result, T-texture is formed in the titanium alloy hot-rolled sheet obtained by hot rolling the slab. T-texture is excellent in cold rolling properties and is effective in increasing strength and Young's modulus in the sheet width direction.

[0057] Finishing temperature (T β By setting the finishing temperature at (T -250)℃ or higher, the reduction is performed with a high proportion of β phase, which makes it easier for T-texture to develop. β By setting the temperature at or above -250°C, a rapid increase in hot deformation resistance is suppressed and hot workability is maintained. This suppresses the occurrence of edge cracks and the like, and suppresses a decrease in yield.

[0058] Finishing temperature is (T β If the temperature is below −50°C, the recrystallization of the β phase during hot rolling is suppressed, variant selection occurs during the phase transformation from the β phase to the α phase, and the T-texture becomes more likely to develop.

[0059] When the rolling reduction is 80% or more, a large amount of processing strain is introduced and the strain is uniformly introduced throughout the sheet thickness, making it easy for T-texture to develop.

[0060] In order to make the texture of the titanium alloy hot-rolled sheet a strong T-texture and ensure high in-plane anisotropy, it is preferable to heat the titanium alloy slab to the above heating temperature and hold it for 30 minutes or more. By holding the titanium alloy slab at the above heating temperature for 30 minutes or more, the crystal phase of the titanium alloy slab becomes a β single phase, and the T-texture is more easily formed and developed.

[0061] The heating temperature and the finishing temperature are the surface temperatures of the titanium alloy slab and can be measured by a known method. For example, the heating temperature and the finishing temperature can be measured using a radiation thermometer.

[0062] In the hot rolling process, the titanium alloy slab can be continuously hot rolled using a known continuous hot rolling equipment. When using the continuous hot rolling equipment, the titanium alloy slab is hot rolled and then wound by a winder to become a titanium alloy hot rolled coil.

[0063] The hot-rolled titanium alloy sheet obtained through the above-mentioned hot rolling process may be annealed by a known method, pickled or cut to remove oxide scale, or cleaned, if necessary.

[0064] In the hot rolling process, the titanium material after the last rolling pass may be subjected to a final annealing treatment. The final annealing treatment may be performed as appropriate and is not essential. The conditions for the final annealing treatment are that the annealing temperature is 700°C or higher and 950°C or lower, and the annealing temperature T (°C) and the holding time t (seconds) at the annealing temperature satisfy the following formula (2). In addition, the following formula (2) (T+273.15)×(Log 10 (t)+20) is the Larson-Miller parameter. 22000≦(T+273.15)×(Log 10 (t)+20)≦27000 …(2) formula

[0065] By carrying out the final annealing treatment under the above conditions, recrystallization is suppressed and the T-texture is maintained.

[0066] <<Second embodiment>> Next, a description will be given of a titanium alloy cold-rolled sheet according to one embodiment of the present invention. The chemical composition of the titanium alloy cold-rolled sheet according to this embodiment is the same as that of the titanium alloy hot-rolled sheet described above. In addition, the titanium alloy cold-rolled sheet according to this embodiment satisfies the formula (1) as in the titanium alloy cold-rolled sheet described above. Therefore, detailed description of the chemical composition of the titanium alloy cold-rolled sheet and the formula (1) will be omitted here.

[0067] <Metal structure> [Microstructure] The metal structure of the titanium alloy cold-rolled sheet according to this embodiment will be described. The cold-rolled titanium alloy sheet according to this embodiment may have an α-phase area ratio of 80% or more in the metal structure. Furthermore, the titanium alloy cold-rolled sheet according to this embodiment may have an area ratio of equiaxed grains, which are crystal grains having an aspect ratio of 3.3 or less, of 50.0% or more. The titanium alloy cold-rolled sheet according to the present embodiment may have a microstructure having equiaxed grains, which are crystal grains with an aspect ratio of 3.3 or less, the average grain size of the equiaxed grains being 1.0 μm or more and 20.0 μm or less, and the area ratio of the elongated grains with an aspect ratio of more than 3.3 to the area of ​​the microstructure may be 30% or less. This will be explained in detail below.

[0068] [Microstructure] (Area ratio of α phase is 80.0% or more) Since the Young's modulus of the β phase is low, if the β phase ratio of the titanium alloy cold-rolled sheet is high, the Young's modulus will decrease. Also, if the α phase ratio is low, the cold rollability may decrease. Therefore, it is preferable that the α phase ratio is high, and the area ratio of the α phase is preferably 80.0% or more. The area ratio of the α phase is more preferably 82.0% or more, and further preferably 85.0% or more. The β phase inhibits grain growth, so it is preferable that the area ratio of the β phase is 1.0% or more. The area ratio of the β phase is more preferably 2.0% or more. This area ratio of the β phase is a value at room temperature. Moreover, when this is expressed as the area ratio of the α phase of the titanium alloy cold-rolled sheet, the upper limit of the α phase is substantially 99.0%, preferably 98.0%. The measurement method is the same as the measurement method for the titanium alloy hot-rolled sheet according to this embodiment.

[0069] The titanium alloy cold-rolled sheet according to this embodiment has a microstructure having equiaxed grains with an aspect ratio of 3.3 or less.

[0070] (Area ratio of equiaxed grains is 50.0% or more) In the titanium alloy cold-rolled sheet according to the present embodiment, the area ratio of equiaxed grains is preferably 50.0% or more. When the area ratio of equiaxed grains is 50.0% or more, the titanium alloy cold-rolled sheet has excellent ductility. When the area ratio of equiaxed grains is 50.0% or more, the in-plane anisotropy can be reduced. The area ratio of equiaxed grains is more preferably 60.0% or more, and even more preferably 70.0% or more.

[0071] (The area ratio of expanded grains is 30.0% or less) In the titanium alloy cold-rolled sheet according to the present embodiment, the area ratio of the extended grains is preferably 30.0% or less. If extended grains are formed, they may cause anisotropy, deterioration of hot and cold formability, and even deterioration of fatigue properties. Since the titanium alloy cold-rolled sheet is subjected to various forming processes, it is better that the number of extended grains in the titanium alloy cold-rolled sheet is as small as possible. The area ratio of the extended grains is more preferably 15.0% or less, even more preferably 10.0% or less, and even more preferably 8.0% or less. On the other hand, since it is better that the extended grains are not present in the titanium alloy cold-rolled sheet, the lower limit of the area ratio of the extended grains is 0%. In the microstructure of the titanium alloy cold-rolled sheet according to this embodiment, the structure other than the equiaxed grains is composed of elongated grains and the remainder is precipitates. Examples of precipitates include Ti2C and silicide. The total area ratio of the equiaxed grains and the elongated grains is 95% or more.

[0072] (Average aspect ratio of equiaxed grains) If the average aspect ratio of the crystal grains is large, anisotropy of strength occurs in the sheet surface of the titanium alloy cold-rolled sheet, so it is preferable that the average aspect ratio of the equiaxed grains is small, and the average aspect ratio of the equiaxed grains is preferably 2.5 or less, more preferably 2.0 or less.

[0073] The area ratio of the elongated grains and equiaxed grains and the average aspect ratio of the equiaxed grains can be calculated as follows. A cross section (L cross section) of a titanium alloy plate cut in the thickness direction along the longitudinal direction at the center position of the width direction (TD) and a cross section cut perpendicular to the width direction are chemically polished, and a crystal orientation analysis is performed by the EBSD method on an area of ​​(total plate thickness) × 200 μm of the cross section, with steps of 1 to 5 μm, for about 2 to 5 fields of view. From the results of this EBSD crystal orientation analysis, the aspect ratio of each crystal grain is calculated. The aspect ratio of each crystal grain is determined from the ratio of the long axis to the short axis of one crystal grain. Then, the area ratio of crystal grains with an aspect ratio of more than 3.3 is calculated, and this is the area ratio of the elongated grains. In addition, the sum of the areas of crystal grains with an aspect ratio of 3.3 or less relative to the total measured area is the area ratio of the equiaxed grains. In the above, the area ratios of the extended grains and the equiaxed grains and the average aspect ratio of the equiaxed grains are calculated based on the L-section at the center position in the sheet width direction. However, since the extended grains and the equiaxed grains are uniformly distributed in the sheet width direction, the area ratios of the extended grains and the equiaxed grains and the average aspect ratio of the equiaxed grains may be calculated based on the L-section at any sheet width position.

[0074] The average aspect ratio of equiaxed grains referred to here is the arithmetic mean of the aspect ratios of crystal grains having an aspect ratio of 3.3 or less.

[0075] (The average grain size of equiaxed grains is 1.0 μm or more and 20.0 μm or less) In the titanium alloy cold-rolled sheet according to the present embodiment, the equiaxed grains have an average grain size of 1.0 μm or more and 20.0 μm or less. Since titanium alloys are sometimes formed by utilizing superplastic properties, fine grains are preferable. If the average grain size of the equiaxed grains is 20.0 μm or less, the superplastic properties are improved and the workability is improved. From the viewpoint of formability and superplasticity, the average grain size of the equiaxed grains is more preferably 18.0 μm or less. The average grain size of the equiaxed grains is further preferably 15.0 μm or less. On the other hand, if the grain size of the equiaxed grains is less than 1.0 μm, the strength becomes too high due to the crystal grain refining effect, and the ductility is significantly reduced. In particular, the formability in the cold (room temperature) is reduced. Therefore, from the viewpoint of formability, the average grain size of the equiaxed grains is more preferably 1.0 μm or more. The average grain size of equiaxed grains is determined by calculating the circle equivalent grain size (area A = π × (grain size D / 2)2) from the crystal grain area measured by EBSD, and the average value based on the number of these is taken as the average grain size of equiaxed grains.

[0076] [Collective tissue] In the titanium alloy cold-rolled sheet according to the present embodiment, the angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α-phase and the sheet thickness direction is less than 85°. If the angle is less than 75°, the anisotropy of the strength is small. As a result, it is possible to ensure high workability, and it is possible to improve the dimensional accuracy when the titanium alloy cold-rolled sheet is processed. In other words, if the angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α-phase and the sheet thickness direction is less than 75°, the material design can be made easier. Therefore, the angle is preferably less than 75°. The angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α-phase and the sheet thickness direction is more preferably 70° or less, even more preferably 65° or less, and even more preferably 60° or less. The lower limit of the angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α-phase and the sheet thickness direction is not particularly limited, and may be 0°. When a cold-rolled titanium alloy sheet is produced by unidirectional rolling, the angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α phase and the sheet thickness direction is, for example, 20° or more.

[0077] The angle between the maximum integration direction of the c-axis of the close-packed hexagonal structure constituting the α phase and the plate thickness direction is calculated using a (0001) pole figure. The (0001) pole figure is drawn in the same manner as in the first embodiment. The position of the integration peak of the c-axis of the close-packed hexagonal structure constituting the α phase in the (0001) pole figure is calculated by using the data of OIM Analysis software by TSL Solutions. TM Using software (Ver. 8.1.0), the inverse pole figure is calculated by texture analysis using the spherical harmonic function method. Fig. 3 is an example of a (0001) pole figure from the sheet thickness direction (ND) of the titanium alloy cold rolled sheet according to this embodiment.

[0078] <0.2% yield strength at 300℃ in the rolling direction is 520MPa or more> Titanium alloy cold-rolled sheets are often used in applications requiring higher strength than industrially pure titanium. When high tensile strength at room temperature is required, Ti-6Al-4V, an α+β type titanium alloy, is often used. In particular, in the aircraft field, when titanium alloys are used in areas close to engines, they may be exposed to 200 to 300 ° C., so strength at temperatures close to the above temperatures is required. If the 0.2% proof stress in the rolling direction at 300 ° C. is 520 MPa or more, it can be used in areas exposed to high temperatures as described above. In the titanium alloy cold-rolled sheet according to the present disclosure, the 0.2% proof stress in the rolling direction at 300 ° C. is 520 MPa or more. The proof stress of the titanium alloy cold-rolled sheet according to the present embodiment is preferably 530 MPa or more, more preferably 540 MPa or more. On the other hand, the higher the proof stress, the better, so there is no particular limitation, and it may be, for example, 750 MPa or less.

[0079] <0.2% yield strength at 25℃ in the rolling direction is 800MPa or more> The titanium alloy hot-rolled sheet according to the present embodiment has a 0.2% yield strength at 25°C in the rolling direction of 800 MPa or more, and can be used in areas where high strength is required near room temperature or where temperature changes up to 300°C occur. The yield strength of the titanium alloy hot-rolled sheet according to the present embodiment is preferably 825 MPa or more. On the other hand, since the higher the yield strength, the better, there is no particular limit, and it may be, for example, 1050 MPa or less.

[0080] <Anisotropy 1.00 or more and less than 1.16> The titanium alloy cold-rolled sheet according to this embodiment has an anisotropy (0.2% proof stress in the sheet width direction / 0.2% proof stress in the rolling direction) of 1.00 or more and less than 1.16. Therefore, the anisotropy is small, and it can be applied to aircraft applications where isotropic properties are often required. The anisotropy may be less than 1.16, preferably 1.15 or less. On the other hand, since the anisotropy of the titanium alloy cold-rolled sheet according to this embodiment is preferably low, the lower limit is preferably closer to 1.00, but it may be 1.03 or more, 1.05 or more. The anisotropy is determined by the following measurement method. The 0.2% proof stress can be measured by a method conforming to JIS Z2241: 2011. That is, the 0.2% proof stress in the rolling direction and the 0.2% proof stress in the sheet width direction can be measured by a method conforming to JIS Z2241: 2011.

[0081] <Average plate thickness: 3.0mm or less> The average thickness of the titanium alloy cold-rolled plate according to the present embodiment is not particularly limited, but can be, for example, 3.0 mm or less. Usually, when hot rolling is performed without using the pack rolling method, the temperature drops rapidly as the plate thickness becomes thinner, and the deformation resistance increases. Therefore, when hot rolling high-strength material by the pack rolling method, it is difficult to make the plate thickness as thin as that obtained by cold rolling. In hot rolling of high-strength titanium alloy, the plate thickness is limited to about 3.0 mm. On the other hand, in cold rolling, the plate thickness can be made thinner. Therefore, the average thickness of the titanium alloy cold-rolled plate according to the present embodiment can be, for example, 3.0 mm or less. The average thickness of the titanium alloy cold-rolled plate according to the present embodiment is preferably 2.0 mm or less. On the other hand, the lower limit of the plate thickness is not particularly limited, but in the case of high-strength titanium alloy, the practical limit is about 0.1 mm. Therefore, the average thickness of the titanium alloy cold-rolled plate according to the present embodiment is, for example, 0.1 mm or more.

[0082] The average thickness of the cold-rolled titanium alloy sheet is measured in the same manner as that for the hot-rolled titanium alloy sheet.

[0083] <Thickness variation: 5.0% or less of average thickness> The thickness variation of the titanium alloy cold rolled plate according to the present embodiment is preferably 5.0% or less with respect to the average thickness. In pack rolling, a titanium alloy thin plate is manufactured by hot rolling a titanium material that is laminated in multiple layers and wrapped with a steel material, but since the deformation resistance of the laminated titanium material varies greatly depending on the temperature distribution, it is difficult to obtain a uniform thickness. However, since the titanium alloy cold rolled plate according to the present embodiment is manufactured through cold rolling, it can be a titanium alloy cold rolled plate with excellent thickness dimensional accuracy. The thickness variation of the titanium alloy cold rolled plate according to the present embodiment is more preferably 4.0% or less with respect to the average thickness, and even more preferably 2.0% or less with respect to the average thickness. Naturally, it is preferable that the thickness variation is small, so that it is most preferable that the thickness variation is 0% with respect to the average thickness.

[0084] The plate thickness variation is measured using the following method. The plate thickness is measured at five or ten locations at intervals of 1 m or more in the longitudinal direction, including the center position in the lateral direction and positions at a distance of 1 / 4 of the lateral length from each of the lateral ends, using X-rays, a micrometer or a vernier caliper. Using the actually measured plate thickness d and the above average plate thickness dave, the maximum value of a' calculated using the following formula (3) is taken as the plate thickness variation a. a'=(d-dave) / dave×100 …(3) formula

[0085] <Method of manufacturing titanium alloy cold rolled sheet> The titanium alloy cold-rolled sheet according to the present embodiment has the above-mentioned chemical composition and satisfies the formula (1). Therefore, the manufacturing method of the titanium alloy cold-rolled sheet according to the present embodiment is not particularly limited. However, for example, the titanium material after the hot rolling process can be cold-rolled in one direction with a cold rolling rate of 30% or more per roll and a total cold rolling rate of 60% or more to manufacture the titanium alloy cold-rolled sheet. By adopting this manufacturing method, it is possible to obtain a titanium alloy cold-rolled sheet in which the angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α phase and the sheet thickness direction is less than 75°. Hereinafter, the above-mentioned cold rolling process will be described as an example of the manufacturing method of the titanium alloy cold-rolled sheet according to the present embodiment. However, as described above, the manufacturing method of the titanium alloy cold rolled sheet according to the present embodiment is not particularly limited. The titanium alloy cold rolled sheet that satisfies the above requirements is considered to be the titanium alloy cold rolled sheet according to the present embodiment, regardless of its manufacturing method. The manufacturing method described below is merely a suitable example, and does not limit the titanium alloy cold rolled sheet according to the present embodiment.

[0086] [Cold rolling process] In titanium, the c-axis of hcp changes to the sheet thickness direction (ND) due to recrystallization during cold rolling and the subsequent heat treatment. However, if the rolling reduction is too small or if the recrystallization during the intermediate annealing or final annealing described below is insufficient, the crystal orientation may hardly change and the anisotropy may not be improved. In particular, in the case of unidirectional cold rolling in which the rolling direction is one direction, since cross rolling is not performed, it is important to increase the cold rolling ratio in order to minimize the anisotropy. Therefore, in the manufacturing method of the titanium alloy cold rolled sheet according to this embodiment, it is preferable that the cold rolling ratio per run is 30% or more and the total cold rolling ratio is 60% or more. More preferably, the total cold rolling ratio is 70% or more. The cold rolling reduction rate per pass referred to here refers to the cold rolling reduction rate from the initial stage (start of cold rolling) to intermediate annealing, the cold rolling reduction rate from intermediate annealing to the end of cold rolling, and, when intermediate annealing is performed multiple times, the total value of the cold rolling reduction rates between each intermediate annealing, and the cold rolling reduction rate per pass during this period may be any ratio. Also, in this process, the titanium material is cold rolled in one direction.

[0087] The cold rolling temperature is preferably 500°C or lower. If the cold rolling temperature is 500°C or lower, high dimensional accuracy can be obtained, and the crystal grains are refined during cold rolling, making it easier to develop superplastic properties. The cold rolling temperature is more preferably 400°C or lower. There is no particular lower limit to the cold rolling temperature, and the cold rolling temperature can be, for example, room temperature or higher. Here, room temperature means 0°C or higher.

[0088] (Intermediate annealing) In order to reduce anisotropy, it is important to perform intermediate annealing and final annealing so that recrystallization occurs. In addition, recrystallization during intermediate annealing is important in order to reduce elongated grains in the microstructure. On the other hand, when heating is performed to a temperature in the β region, transformation from the β phase to the α phase occurs, and the metal structure becomes an acicular structure. In addition, even just below the β transformation point, a bimodal structure (a mixed structure of equiaxed grains and acicular structures) is formed. Such a structure causes internal cracking and edge cracking at low reduction rates. Furthermore, the structure becomes coarse, making it difficult to exhibit the superplastic properties required for titanium alloy sheets. The occurrence of recrystallization is determined by the annealing temperature and annealing time. To manufacture the titanium alloy cold-rolled sheet according to this embodiment, for example, the annealing temperature T (°C) in intermediate annealing is 600°C or higher, and T β The annealing temperature is -50°C or lower, and the annealing is performed so that the annealing temperature T (°C) and the annealing time t (seconds) satisfy the following formula (4). 22000≦(T+273.15)×(Log10(t)+20)≦27000 …(4) formula Here, T β is the β transformation point (℃).

[0089] (Final annealing) Final annealing is a process in which the titanium material is annealed after the last cold rolling pass. The annealing conditions in the final annealing are not particularly limited, but in order to reduce the anisotropy of strength and improve the formability of the titanium alloy sheet, the annealing temperature T is preferably 600°C or higher (T β It is preferable that the annealing temperature T (°C) and the holding time t (seconds) at the annealing temperature T satisfy the above formula (4).

[0090] By carrying out intermediate annealing and final annealing under the above conditions, the uncrystallized grains are recrystallized and the c-axis of the α phase approaches the plate thickness direction (ND). This makes it possible to reduce the anisotropy of the titanium alloy plate. In addition, the recrystallization eliminates the excessive amount of elongated grains in the microstructure. On the other hand, the annealing temperature is higher than the β transformation point T βIf the annealing temperature is above 600°C, a phase transformation from the β phase to the α phase occurs, and the α phase thus produced becomes an acicular structure. Even if the annealing temperature is just below the β transformation point, a bimodal structure in which equiaxed grains and acicular structures are mixed is formed. The acicular structure and bimodal structure may cause internal cracks and edge cracks during cold rolling. Furthermore, the acicular structure or bimodal structure often becomes coarse grains, making it difficult to develop superplastic properties. In the intermediate annealing and final annealing, the annealing temperature T is 600°C or higher (T β By determining the annealing temperature T and the annealing time t so that the annealing temperature T and the annealing time t satisfy the above formula (4), the c-axis of the α phase approaches the ND direction due to recrystallization, and the anisotropy of the titanium alloy sheet can be further reduced, and the number of elongated grains in the microstructure can be further reduced. Furthermore, in the intermediate annealing process and the final annealing process, the annealing temperature T is 600°C or higher (T β By determining the annealing temperature T and annealing time t so that the annealing temperature T and annealing time t are equal to or lower than −50° C. and satisfy the above formula (4), fine equiaxed grains are increased, internal cracks and edge cracks during cold rolling are suppressed, and superplastic properties are easily exhibited.

[0091] [Temper rolling and tensile straightening process] The cold-rolled titanium alloy sheet after the cold rolling process is preferably subjected to temper rolling to adjust mechanical properties or tensile straightening to straighten the shape, as necessary. The rolling reduction in temper rolling is preferably 10% or less, and the elongation of the cold-rolled titanium alloy sheet in tensile straightening is preferably 5% or less. On the other hand, if temper rolling or tensile straightening is excessive, the elongation of the titanium material becomes significant, making it difficult to form. Note that temper rolling and tensile straightening do not have to be performed if they are not necessary. The manufacturing method of the cold-rolled titanium alloy sheet according to this embodiment has been described above.

[0092] The titanium alloy plate according to this embodiment is manufactured by hot rolling or cold rolling. In this embodiment, the metal structure of the titanium alloy plate manufactured by hot rolling and the metal structure of the titanium alloy plate manufactured by cold rolling are different from each other, but both have tensile strength of 520MPa or more in medium temperature range.

[0093] The titanium alloy plate according to the present disclosure has a chemical composition consisting of, in mass%, Al: 4.5% or more and 6.6% or less, Fe: 0.3% or more and 2.3% or less, Cu: 0.2% or more and 2.0% or less, Si: 0.05% or more and 0.50% or less, C: 0% or more and less than 0.080%, N: 0% or more and 0.050% or less, O: 0% or more and 0.25% or less, and the balance: Ti and impurities, and satisfies the formula (1). Therefore, the 0.2% proof stress at 300 ° C in the rolling direction is 520 MPa or more, and as described above, whether it is a titanium alloy hot-rolled plate or a titanium alloy cold-rolled plate, it has excellent tensile strength in the medium temperature range.

[0094] In the titanium alloy plate according to the present disclosure, when the angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α-phase and the plate width direction is 30° or less, the Young's modulus of the titanium alloy hot-rolled plate in the plate width direction at 25°C can be further increased, and the Young's modulus in the plate width direction at 25°C can be 130 GPa or more.

[0095] In addition, when the angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α-phase of the titanium alloy plate according to the present disclosure and the plate thickness direction is less than 75°, the anisotropy of strength in the plate plane can be reduced. EXAMPLES

[0096] Hereinafter, the embodiment of the present invention will be described in detail with reference to examples. Note that the examples shown below are merely examples of the present invention, and the present invention is not limited to the following examples.

[0097] Example 1 Titanium alloy ingots having the chemical compositions shown in Table 1 were produced by either vacuum arc melting, electron beam melting, or plasma melting, and then titanium alloy slabs of 200 mm thick x 1000 mm wide x 5000 mm long were produced by blooming or forging. These titanium alloy slabs were then hot rolled under the conditions shown in Table 2 to have the thicknesses shown in Table 2, and then subjected to the heat treatments, shot blasting, and pickling shown in Table 2 to produce hot-rolled sheets.

[0098] [Table 1]

[0099] [Chemical composition] The chemical composition of the titanium alloy cold-rolled sheet was measured for Al, Fe, Cu, Si, Ni, Cr, and Mn by ICP emission spectrometry. C was measured by infrared absorption using a carbon and sulfur simultaneous analyzer. O and N were measured by inert gas fusion, thermal conductivity, and infrared absorption using an oxygen and nitrogen simultaneous analyzer. In addition, "-" in Table 1 indicates that it was not intentionally added. Elements other than those listed in Table 1 are Ti and impurities.

[0100] [Maximum accumulation direction of c axis] The observation surface of each titanium alloy cold-rolled sheet sample was chemically polished, and the crystal orientation was analyzed using electron backscatter diffraction to obtain (0001) pole figures. Specifically, the L-section was chemically polished at the center of the sheet width direction (TD) of each sample, and crystal orientation analysis was performed using the EBSD method on the section, targeting 2 to 10 fields of view at intervals of 1 to 2 μm in an area of ​​(total sheet thickness) × 2 mm, to draw (0001) pole figures. The maximum accumulation direction of the c-axis in the (0001) pole figures was determined by analyzing the data using TSL Solutions' OIM Analysis software (OIM Analysis TM (Ver.8.1.0)) was used to perform texture analysis of the inverse pole figure using the spherical harmonics method. In the texture analysis, the expansion index was set to 16 and the Gaussian half-width was set to 5°.

[0101] [Area ratio of spread grains] The aspect ratio and the area ratio of elongated grains were measured by electron backscatter diffraction (EBSD). A titanium alloy plate was cut in the thickness direction along the longitudinal direction at the center of the plate width direction (TD) and a cross section (L cross section) was observed at a magnification of 500x or more over an area of ​​1 mm in the rolling direction and the entire thickness in the plate thickness direction, and electron backscatter diffraction (EBSD) was performed with measurement steps of 0.5 to 1.0 μm. The aspect ratio of each crystal grain was calculated from the ratio of the long axis to the short axis of one crystal grain. The sum of the areas of crystal grains with an aspect ratio of more than 3.3 relative to the total measured area was taken as the area ratio of elongated grains. The aspect ratio was calculated using the OIM Analysis software from TSL Solutions Co., Ltd. TM (Ver.8.1.0), crystal grains were classified by regarding crystal orientation differences of 15° or more as grain boundaries.

[0102] [Area ratio of α phase and β phase] The α-phase and β-phase were measured by SEM (Scanning Electron Microscopy) / EPMA (Electron Probe Micro Analyzer). The region where the β-stabilizing element was 2% by mass or more by EPMA was defined as the β-phase, and the region where the β-stabilizing element was less than 2% by mass was defined as the α-phase. After mirror polishing the L-section, a total of five measurement ranges of 500 μm × 500 μm were measured on a surface (L-section) parallel to the rolling direction of the material and parallel to the plate thickness direction. The measurement step in each measurement range was 1 μm. The average value of the area ratio of the α-phase and the β-phase in each measurement range was calculated, and the arithmetic mean value of each value was evaluated as the area ratio of the α-phase and the area ratio of the β-phase. The L cross section was specified by the following method: In the measurement of the maximum accumulation direction of the c axis described later, the maximum accumulation direction of the c axis was the sheet width direction, and the direction perpendicular to the sheet width direction was the rolling direction.

[0103] [0.2% proof stress in the rolling direction at 25℃ (room temperature)] The 0.2% at 25°C of each titanium alloy cold-rolled sheet was measured by the following method. That is, a tensile test piece with a longitudinal direction parallel to the rolling direction, a parallel part width of 12.5 mm, a parallel part length and a gauge length of 50 mm was taken from the above thin sheet. The strain rate was 0.5% / min up to a strain of 1.5%, and then 30% / min up to fracture, and the above tensile test piece was subjected to a tensile test.

[0104] [0.2% proof stress in rolling direction at 300℃] 0.2% of each titanium alloy cold-rolled sheet was measured by the following method. That is, for the tensile test at high temperature, a tensile test piece was taken from the above thin sheet, the longitudinal direction of which was parallel to the rolling direction, and the parallel part width was 10 mm, the parallel part length was 35 mm, and the gauge length was 35 mm. The above tensile test piece was subjected to the tensile test with a strain rate of 0.3% / min up to a strain of 1.5%, and then 7.5% / min until breakage. The test atmosphere was 300°C in air, and the tensile test was performed after holding the tensile test piece in the test atmosphere for 10 minutes so that the test temperature was sufficiently reached. When the tensile strength was 520 MPa or more, it was determined that the tensile strength in the medium temperature range was excellent and the test piece was judged to be passed.

[0105] [Young's modulus at 25°C in the plate width direction] The Young's modulus at 25°C in the sheet width direction was measured by the following method: A tensile test piece No. 13B (parallel part width 12.5 mm, gauge length 50 mm) specified in JIS Z 2241:2011 was prepared so that the tensile direction was the sheet width direction of the titanium alloy thin sheet, a strain gauge was attached, and loading and unloading were repeated five times at a strain rate of 10.0% / min in the stress range from 100 MPa to half of the 0.2% proof stress, the slope was calculated, and the average value of three times excluding the maximum and minimum values ​​was taken as the Young's modulus at 25°C in the sheet width direction.

[0106] [result] The above evaluation results are shown in Table 2. Note that "θ1" in Table 2 is the angle between the sheet width direction and the direction showing the peak of the degree of accumulation calculated by texture analysis in a (0001) pole figure from the sheet thickness direction using the spherical harmonic function method of electron backscatter diffraction when the expansion exponent is 16 and the Gaussian half-width is 5°.

[0107] [Table 2]

[0108] In all of Inventive Examples 1 to 12 and Comparative Example 1, the chemical composition of the produced titanium alloy hot-rolled sheet was equal to the chemical composition of the titanium alloy ingot used therein.

[0109] Inventive Examples 1 to 12 are titanium alloy hot-rolled sheets manufactured using steel types A to F having a chemical composition of Al: 4.5% or more and 6.6% or less, Fe: 0.3% or more and 2.3% or less, Cu: 0.2% or more and 2.0% or less, Si: 0.05% or more and 0.50% or less, C: 0% or more and less than 0.080%, N: 0% or more and 0.050% or less, O: 0% or more and 0.25% 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%, and the balance being Ti and impurities, and satisfying 35.0≦5×Al+5×Cu+10×Fe+20×Si≦60.0, and the 0.2% proof stress in the rolling direction at 300°C was 520MPa or more. Inventive Examples 1 to 10 and 12, θ1 was 30° or less, and thus had a larger Young's modulus than Inventive Example 11, in which θ1 exceeded 30°.

[0110] On the other hand, Comparative Example 1 is a titanium alloy hot-rolled sheet manufactured using steel type G that does not contain Cu or Si and does not satisfy 35.0≦5×Al+5×Cu+10×Fe+20×Si≦60.0. Since the effect of improving tensile strength in the medium temperature range due to these elements was not obtained, the 0.2% proof stress in the rolling direction at 300°C was less than 520 MPa.

[0111] Example 2 Titanium alloy ingots having the chemical compositions shown in Table 1 were produced by either vacuum arc remelting (VAR), electron beam remelting (EBR), or plasma arc melting (PAM), and then titanium alloy slabs of 200 mm thick x 1000 mm wide x 5000 mm long were produced by blooming or forging. These titanium alloy slabs were then subjected to hot rolling, hot-rolled sheet annealing, shot blasting, and pickling. Inventive Examples 13 to 28 and Comparative Examples 2 to 5, hot-rolled sheets were produced with a thickness of 4 mm, and in Inventive Examples 29 to 31, hot-rolled sheets were produced with a thickness of 6 mm. The hot rolling was carried out when the temperature of the titanium alloy slab was raised to the β transformation point T β The steel is heated to 1050 to 1150°C so that the temperature is equal to or higher than the β transformation point T β The temperature was set to 800 to 950°C so as to satisfy the following.

[0112] Next, the obtained hot-rolled sheet was subjected to a cold rolling process under the conditions shown in Table 3. In Table 3, the "Larson-Miller parameter" is (T + 273.15) × (Log 10 The value of the reduction ratio is (t)+20). In addition, "Pattern A" in Table 3 indicates a cold rolling pattern in which the reduction ratio of the first cold rolling pass is 75%, and the reduction ratio of the second cold rolling pass is 50%. In Table 3, "Pattern B" indicates a cold rolling pattern in which the reduction ratio of the first cold rolling pass is 50%, the reduction ratio of the second cold rolling pass is 50%, and the reduction ratio of the third cold rolling pass is 60%.

[0113] [Table 3]

[0114] [Chemical composition] The chemical composition of the titanium alloy cold-rolled sheet was measured for Al, Fe, Cu, Si, Ni, Cr, and Mn by ICP emission spectrometry. C was measured by infrared absorption using a carbon and sulfur simultaneous analyzer. O and N were measured by inert gas fusion, thermal conductivity, and infrared absorption using an oxygen and nitrogen simultaneous analyzer. In addition, "-" in Table 1 indicates that it was not intentionally added. Elements other than those listed in Table 1 are Ti and impurities.

[0115] [Maximum accumulation direction of c axis] The observation surface of each titanium alloy cold-rolled sheet sample was chemically polished, and the crystal orientation was analyzed using electron backscatter diffraction to obtain (0001) pole figures. Specifically, the L-section was chemically polished at the center of the sheet width direction (TD) of each sample, and crystal orientation analysis was performed using the EBSD method on the section, targeting 2 to 10 fields of view at intervals of 1 to 2 μm in an area of ​​(total sheet thickness) × 2 mm, to draw (0001) pole figures. The maximum accumulation direction of the c-axis in the (0001) pole figures was determined by analyzing the data using TSL Solutions' OIM Analysis software (OIM Analysis TM (Ver.8.1.0)) was used to perform texture analysis of the inverse pole figure using the spherical harmonics method. In the texture analysis, the expansion index was set to 16 and the Gaussian half-width was set to 5°.

[0116] The chemical composition of each cold-rolled sheet, the 0.2% yield strength in the rolling direction at 25°C (room temperature), the 0.2% yield strength in the rolling direction at 300°C, and the angle θ2 between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α phase and the sheet thickness direction were measured in the same manner as in Example 1.

[0117] The aspect ratio of the crystal grains and the area ratio of the extended grains were calculated as follows. Each titanium alloy plate was cut in the thickness direction along the longitudinal direction at the center position of the width direction (TD) and the cross section (L cross section) cut perpendicular to the width direction was chemically polished, and the crystal orientation analysis was performed by the EBSD method on the area of ​​(total plate thickness) × 200 μm of the cross section, with steps of 1 to 5 μm, for about 2 to 5 fields of view. From the results of the crystal orientation analysis by EBSD, the aspect ratio of each crystal grain was calculated. The aspect ratio of each crystal grain was obtained from the ratio of the long axis to the short axis of one crystal grain. Then, the area ratio of crystal grains with an aspect ratio of more than 3.3 was calculated, and this was taken as the area ratio of the extended grains. In addition, the sum of the areas of crystal grains with an aspect ratio of 3.3 or less relative to the total measured area was taken as the area ratio of the equiaxed grains. In addition, the average value of the crystal grains with an aspect ratio of 3.3 or less was taken as the average aspect ratio of the equiaxed grains.

[0118] [Average grain size of equiaxed grains] The average grain size of equiaxed grains was determined by calculating the circle equivalent grain size (area A = π × (grain size D / 2)2) from the crystal grain area of ​​the equiaxed grains measured by EBSD, and the average value based on the number of these was taken as the average grain size of the equiaxed grains.

[0119] [Area ratio of α phase and β phase] The α-phase and β-phase were measured by SEM (Scanning Electron Microscopy) / EPMA (Electron Probe Micro Analyzer). The region where the β-stabilizing element was 2% by mass or more by EPMA was defined as the β-phase, and the region where the β-stabilizing element was less than 2% by mass was defined as the α-phase. After mirror polishing the L-section, a total of five measurement ranges of 500 μm × 500 μm were measured on a surface (L-section) parallel to the rolling direction of the material and parallel to the plate thickness direction. The measurement step in each measurement range was 1 μm. The average value of the area ratio of the α-phase and the β-phase in each measurement range was calculated, and the arithmetic mean value of each value was evaluated as the area ratio of the α-phase and the area ratio of the β-phase. The L cross section was specified by the following method: In the measurement of the maximum accumulation direction of the c axis described later, the maximum accumulation direction of the c axis was the sheet width direction, and the direction perpendicular to the sheet width direction was the rolling direction.

[0120] [Average thickness dave] The average thickness dave of each titanium alloy cold-rolled sheet was measured by the following method. The thickness of each produced titanium alloy cold-rolled sheet was measured at 5 or more positions at intervals of 1 m or more in the longitudinal direction, at the center position in the transverse direction and at a distance of 1 / 4 of the transverse length from each end in the transverse direction, using an X-ray, a micrometer, or a vernier caliper, and the average of the measured thicknesses was taken as the average thickness dave.

[0121] [Thickness dimension accuracy a] The thickness dimensional accuracy a of each titanium alloy cold rolled sheet was determined as the maximum value of a' calculated by the following formula (101) using the sheet thickness d actually measured by the above method and the average sheet thickness dave. a'=(d-dave) / dave×100...Equation (101)

[0122] [result] The above evaluation results are shown in Table 4. Note that "θ2" shown in Table 4 is the angle between the thickness direction and the direction showing the peak of the degree of accumulation calculated by texture analysis when the expansion index is 16 and the Gaussian half-width is 5° in the inverse pole figure using the spherical harmonic function method of the electron backscatter diffraction method in the (0001) pole figure from the thickness direction. The value shown in the "anisotropy" column in Table 4 is the value of (0.2% proof stress in the width direction at 25°C (room temperature)) / (0.2% proof stress in the rolling direction at 25°C (room temperature)).

[0123] [Table 4]

[0124] In all of Inventive Examples 13 to 31 and Comparative Examples 2 and 3, the chemical compositions of the produced titanium alloy cold-rolled sheets were equal to the chemical compositions of the titanium alloy ingots used therein.

[0125] Inventive Examples 13 to 31 are titanium alloy cold-rolled sheets manufactured using steel types A to F, J, and K having a chemical composition of Al: 4.5% or more and 6.6% or less, Fe: 0.3% or more and 2.3% or less, Cu: 0.2% or more and 2.0% or less, Si: 0.05% or more and 0.50% or less, C: 0% or more and less than 0.080%, N: 0% or more and 0.050% or less, O: 0% or more and 0.25% 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%, and the balance being Ti and impurities, and satisfying 35≦5×Al+5×Cu+10×Fe+20×Si≦60, and the 0.2% proof stress in the rolling direction at 300°C was 520MPa or more. Inventive Examples 13 to 26, 29, and 30, θ2 was less than 75°, and the anisotropy was smaller than that in Inventive Examples 27, 28, and 31, in which θ2 was 75° or more.

[0126] On the other hand, Comparative Example 2 is a titanium alloy cold-rolled sheet manufactured using steel type G that does not contain Cu or Si and does not satisfy 35.0≦5×Al+5×Cu+10×Fe+20×Si≦60.0. Since the effect of improving tensile strength in the medium temperature range due to these elements was not obtained, the 0.2% proof stress in the rolling direction at 300°C was less than 520 MPa. Comparative Example 3 is a titanium alloy cold-rolled sheet produced using steel type H in which the Al content is too low and which does not satisfy 35.0≦5×Al+5×Cu+10×Fe+20×Si≦60.0. Since the effect of improving tensile strength in the medium temperature range due to these elements was not obtained, the 0.2% proof stress in the rolling direction at 300°C was less than 520 MPa. Comparative Example 4 is a titanium alloy cold-rolled sheet produced using steel type L having excessive Al and Si contents and not satisfying 35.0≦5×Al+5×Cu+10×Fe+20×Si≦60.0. The titanium material after hot rolling became too hard, and cracks occurred during cold rolling. In Comparative Example 5, the O content was excessive, which reduced the workability, and therefore cracks occurred during cold rolling.

[0127] Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited to such an example. It is clear that a person having ordinary knowledge in the technical field to which the present invention pertains can conceive of various modified or altered examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

Claims

1. In mass percent, Al: 4.5% or more, 6.6% or less, Fe: 0.3% or more, 2.3% or less, Cu: 0.2% or more, 2.0% or less, Si: 0.05% or more, 0.50% or less, C: 0% or more and less than 0.080%; N: 0% or more, 0.050% or less, O: 0% or more, 0.25% 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%; and The balance is Ti and impurities. having a chemical composition consisting of A titanium alloy plate satisfying the following formula (1): 35.0≦5×Al+5×Cu+10×Fe+20×Si≦60.0…(1) formula The elements shown in the formula (1) above each represent the content of each element in unit mass %.

2. 2. The titanium alloy plate according to claim 1, wherein the angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α-phase and the plate width direction is 30° or less.

3. 2. The titanium alloy plate according to claim 1, wherein the angle between the maximum accumulation direction of the c-axis of the close-packed hexagonal structure constituting the α-phase and the plate thickness direction is less than 75°.

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