Titanium alloy sheet and method for producing same

Abstract

A titanium alloy sheet includes a predetermined chemical composition, in a case where a crystal orientation of an α-phase is expressed by Euler angles g={φ1, ϕ, φ2} according to Bunge notation, a maximum integration orientation expressed by a crystal orientation distribution function f(g) is in a range of φ1: 0° to 30°, ϕ: 60° to 90°, and φ2: 0° to 60°, a maximum integration in the maximum integration orientation is 10.0 or more, a maximum integration in ranges of φ1: 70° to 90°, ϕ: 70° to 90°, and φ2: 0° to 60° and φ1: 70° to 90°, ϕ: 10° to 30°, and φ2: 0° to 60° is 2.5 or less, and YR in a sheet width direction is 0.99 or less.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a National Phase under 35 U.S.C. 371 of PCT / JP2022 / 028491, filed on Jul. 22, 2022, and designated the U.S., which claims priority to Japanese Patent Application No. 2022-073153, filed on Apr. 27, 2022. The contents of each are wholly incorporated herein by reference.TECHNICAL FIELD

[0002] The present disclosure relates to a titanium alloy sheet and a method for producing the titanium alloy sheet.

[0003] Priority is claimed on Japanese Patent Application No. 2022-073153, filed on Apr. 27, 2022, the content of which is incorporated herein by reference.RELATED ART

[0004] Titanium alloys are used in fields such as aircrafts, automobiles, and consumer products such as golf clubs as materials having a light weight and a high strength. The alloy that is commonly used among titanium alloys is an α+β type titanium alloy mainly including an α-phase and a β-phase, and known examples thereof include a Ti-6Al-4V alloy, a Ti-6Al-2Sn-4Zr-2Mo alloy, and a Ti-5Al-1Fe alloy.

[0005] Titanium alloys are widely used in golf club applications among the above applications, especially in drivers. The characteristics required for the titanium alloys for use in golf clubs are a high strength and a low specific gravity in order to achieve large capacity (weight reduction), and a high Young's modulus so that it is possible to avoid repulsion regulation even in a case where further thinning is carried out. In golf applications, the vertical direction of a face greatly affects a repulsive force and the like. Therefore, as the above-described strength and Young's modulus, the strength (particularly, 0.2% PS) and the Young's modulus in a sheet width direction are generally required. In addition, workability is also required in working into golf clubs.

[0006] In consideration of the golf club applications, a Ti-6A1-4V alloy needs to contain about 4% of V, that is an expensive element, and thus this is not preferable. Therefore, a titanium alloy that can obtain equivalent characteristics without containing V is required.

[0007] Regarding such a problem, for example, Patent Document 1 discloses a high specific strength α+β type titanium alloy including, by mass %, Al: 7.1% to 10%, Fe: 0.1% to 3.0%, C: 0.5% or less, 0: 0.05% to 0.5%, N: 0.5% or less, and a remainder of Ti and unavoidable impurities. Patent Document 1 describes that by substituting V, that is a β-stabilizing element, with Fe and adding Al, that is an α-stabilizing element and has a low specific gravity, the specific strength of the high specific strength α+β type titanium alloy can be increased and the cost can be lowered.

[0008] However, in the alloy of Patent Document 1, it is considered that since the Al content is high, a hot deformation resistance is high, a possibility of decreased producibility due to the formation of a regular phase (Ti3Al) in the producing process, or the like, and thus the coil producibility is low (it is difficult to produce coils).

[0009] In addition, without containing expensive V, a Ti-5Al-1Fe alloy can obtain mechanical characteristics equivalent to or greater than those of a Ti-6A1-4V alloy.

[0010] For example, Patent Document 2 discloses, as a Ti-5Al-1Fe alloy, an α+β type titanium alloy including 1.4% or more and less than 2.1% of Fe, 4.4% or more and less than 5.5% of Al, and a remainder of titanium and impurities.

[0011] However, it is considered that, in the titanium alloy of Patent Document 2, the texture and the like are not specified and the characteristics in a sheet width direction required for golf applications are not necessarily satisfied.

[0012] In addition, Patent Document 3 discloses a titanium alloy sheet having a high strength and a high Young's modulus in one direction in a sheet surface, and having excellent fatigue properties and / or impact toughness and good hot workability. In Patent Document 3, the tensile strength and the Young's modulus in a sheet width direction are increased by developing a texture called a transverse-texture in which the c-axis of a titanium α-phase is strongly oriented in the sheet width direction.

[0013] However, in Patent Document 3, due to the strength of the hot-rolled sheet without annealing and with residual strain, there were cases where the strength significantly decreased when hot working was performed during working into a product. Furthermore, since the Al content was 5.5% or less and the Fe content was 1.3% or less, there were cases where the strength could not be obtained after working into a product. Meanwhile, even in a case where the working was performed at room temperature, YR was large due to residual strain, resulting in poor workability in some cases.

[0014] In addition, since a Ti-6A1-2Sn-4Zr-2Mo alloy needs to contain a large amount of elements heavier (having a larger atomic weight) than Ti, such as Sn, Zr, and Mo, the specific gravity increases, and thus there is a problem that the alloy cannot be made lightweight.RELATED ART DOCUMENTPatent Document

[0015] Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2007-239030

[0016] Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H7-62474

[0017] Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2014-224301SUMMARYProblems to be Solved

[0018] As described above, titanium alloys of the related art cannot simultaneously satisfy a high strength, a high Young's modulus, a low specific gravity, and high workability, that have been required in recent years for titanium alloys suitable for golf club applications. Therefore, an object of the present disclosure is to provide a titanium alloy sheet having a high strength, a high Young's modulus, a low specific gravity, and high workability. In a case where the golf club applications are taken into consideration, the titanium alloy sheet is a hot-rolled sheet having a sheet thickness of more than 2.5 mm.Means for Solving the Problem

[0019] The present inventors have conducted studies on a strength (0.2% PS (0.2% proof stress)), a Young's modulus, a specific gravity, and workability of an α+β type titanium alloy in which expensive elements such as V is not essential.

[0020] As a result, it has been found that a high strength, a high Young's modulus, a low specific gravity, and a high workability can be obtained by controlling the chemical composition, texture, and the like in appropriate ranges.

[0021] The present disclosure has been completed based on the above findings, and the gist thereof is as follows.

[0022] [1] A titanium alloy sheet according to one aspect of the present disclosure is a titanium alloy sheet including, as a chemical composition, by mass %: Al: 5.0% to 6.6%; Fe: 0.7% to 2.3%; Si: 0.20% to 0.30%; 0: 0.10% to 0.20%; C: less than 0.050%; N: 0.050% 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 a remainder: Ti and impurities, in which in the chemical composition, in a case where an Al content is denoted by [% Al], an Fe content is denoted by [% Fe], a Si content is denoted by [% Si], and an O content is denoted by [% O], by mass %, Expressions (1) and (2) are satisfied, in a case where a crystal orientation of an α-phase is expressed by Euler angles g={φ1, Φ, φ2} according to Bunge notation, a maximum integration orientation expressed by a crystal orientation distribution function f(g) is in a range of φ1: 0° to 30°, Φ: 60° to 90°, and φ2: 0° to 60°, a maximum integration in the maximum integration orientation is 10.0 or more, a maximum integration in ranges of φ1: 70° to 90°, Φ: 70° to 90°, and φ2: 0° to 60° and φ1: 70° to 90°, Φ: 10° to 30°, and φ2: 0° to 60° is 2.5 or less, and YR in a sheet width direction is 0.99 or less.11.5<[%⁢ Al]+2×[%⁢ Fe]+8×[%⁢ Si]+18×[%⁢ O]<15.5.(1)-4.5<[%⁢ Fe]-0.9×[%⁢ Al]+1.3×[%⁢ O]+1.8×[%⁢ Si]<-2.4.(2)[2] In the titanium alloy sheet according to [1], a 0.2% proof stress in the sheet width direction at 25° C. may be 1,000 MPa or more, a Young's modulus in the sheet width direction may be 135 GPa or more, and a specific gravity may be 4.45 g / cm3 or less.

[0024] [3] In the titanium alloy sheet according to [1] or [2], the half width of a diffraction peak at 2θ=53.3±1° detected by an X-ray diffraction method using CuKα as a line source may be 0.20° or less.

[0025] [4] In the titanium alloy sheet according to any one of [1] to [3], a band structure having an aspect ratio of more than 3.0 and elongated in a longitudinal direction of the sheet may be provided, and the area ratio of the band structure may be 70% or more.

[0026] [5] In the titanium alloy sheet according to any one of [1] to [3], the YR in the sheet width direction may be 0.85 or more and 0.97 or less.

[0027] [6] In the titanium alloy sheet according to [4], the YR in the sheet width direction may be 0.85 or more and 0.97 or less.

[0028] [7] In the titanium alloy sheet according to [1] or [2], the sheet thickness may be more than 2.5 mm.

[0029] [8] In the titanium alloy sheet according to [3], the sheet thickness may be more than 2.5 mm.

[0030] [9] In the titanium alloy sheet according to [4], the sheet thickness may be more than 2.5 mm.

[0031]

[10] In the titanium alloy sheet according to [5], the sheet thickness may be more than 2.5 mm.

[0032]

[11] In the titanium alloy sheet according to [6], the sheet thickness may be more than 2.5 mm.

[0033]

[12] A method for producing a titanium alloy sheet according to another aspect of the present disclosure is a method for producing the titanium alloy sheet according to [1], the method including: heating a titanium material including, as a chemical composition, by mass %, Al: 5.0% to 6.6%, Fe: 0.7% to 2.3%, Si: 0.20% to 0.30%, 0: 0.10% to 0.20%, C: less than 0.050%, N: 0.050% 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 a remainder: Ti and impurities, to a heating temperature; hot-rolling the titanium material after the heating in one direction to obtain a hot-rolled sheet; cooling the hot-rolled sheet after the hot rolling to a coiling temperature of 400° C. or lower at a rate of 8.0° C. / s or higher and coiling the hot-rolled sheet at the coiling temperature; and performing annealing on the hot-rolled sheet after the coiling, in which in the heating, the heating temperature is Tβ° C. or higher and (Tβ+150)° C. or lower, where Tβ is a β-transformation temperature in the unit of ° C., in the hot rolling, a rolling reduction is 85% or more, and a finishing temperature is (Tβ−170)° C. or higher and (Tβ−100)° C. or lower, and in the annealing, an annealing temperature T during the annealing is 600° C. or higher and Tβ or lower, and the annealing temperature T and a holding time t in the unit of second at the annealing temperature satisfy Expression (3).(T+2⁢7⁢3.1⁢5)×(Log10(t)+2⁢0)<27<semantics definitionURL="">,<annotation encoding="Mathematica">TagBox[",", "NumberComma", Rule[SyntaxForm, "0"]]< / annotation>< / semantics>0.Expression⁢ (3)

[13] In the method for producing the titanium alloy sheet according to

[12] , in the annealing, the annealing temperature T during the annealing may be 600° C. or higher and Tβ or lower, and the annealing temperature T and the holding time t in the unit of second at the annealing temperature may satisfy Expression (3′).22<semantics definitionURL="">,<annotation encoding="Mathematica">TagBox[",", "NumberComma", Rule[SyntaxForm, "0"]]< / annotation>< / semantics>000≤(T+273.15)×(Log10(t)+20)<27<semantics definitionURL="">,<annotation encoding="Mathematica">TagBox[",", "NumberComma", Rule[SyntaxForm, "0"]]< / annotation>< / semantics>0.Expression⁢ (3’)EffectsAccording to the aspect of the present disclosure, it is possible to provide a titanium alloy sheet that is an α+β type titanium alloy sheet having a high strength, a high Young's modulus, a low specific gravity, and high workability, and a method for producing the titanium alloy sheet.

[0036] The titanium alloy sheet is suitable for golf club applications.BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG. 1 is an explanatory diagram showing a crystal orientation of an α-phase crystal grain of a titanium alloy sheet by Euler angles according to Bunge notation.

[0038] FIG. 2 is an example of a crystal orientation distribution function of the titanium alloy sheet according to one embodiment of the present disclosure obtained by an electron backscatter diffraction method.

[0039] FIG. 3 is an optical microscope photograph (magnification: 200 times) showing an example of a band structure.DETAILED DESCRIPTION

[0040] A titanium alloy sheet according to an embodiment of the present disclosure (a titanium alloy sheet according to the present embodiment) will be described.

[0041] A titanium alloy sheet according to the present embodiment includes, as a chemical composition, by mass %, Al: 5.0% to 6.6%, Fe: 0.7% to 2.3%, Si: 0.20% to 0.30%, 0: 0.10% to 0.20%, C: less than 0.050%, N: 0.050% 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 a remainder: Ti and impurities, in which in the chemical composition, in a case where an Al content is denoted by [% Al], an Fe content is denoted by [% Fe], a Si content is denoted by [% Si], and an O content is denoted by [% O], by mass %, 11.5<[% Al]+2×[% Fe]+8×[% Si]+18×[% O]<15.5 and −4.5<[% Fe]−0.9×[% Al]+1.3×[% O]+1.8×[% Si]<−2.4 are satisfied,

[0042] in a case where a crystal orientation of an α-phase is expressed by Euler angles g={φ1, Φ, φ2} according to Bunge notation, a maximum integration orientation expressed by a crystal orientation distribution function f(g) is in a range of φ1: 0° to 30°, Φ: 60° to 90°, and φ2: 0° to 60°,

[0043] a maximum integration in the maximum integration orientation is 10.0 or more,

[0044] a maximum integration in ranges of φ1: 70θ to 90°, Φ: 70° to 90°, and φ2: 0° to 60° and φ1: 70° to 90°, Φ: 10° to 30°, and φ2: 0° to 60° is 2.5 or less, and

[0045] YR in a sheet width direction is 0.99 or less.<Chemical Composition>

[0046] The chemical composition of the titanium alloy sheet according to the present embodiment will be described. In the following description of alloying element contents, unless otherwise specified, the notation “%” indicates “mass %”.Al: 5.0% to 6.6%

[0047] Al is an element having a high solid solution strengthening ability and is an effective element for increasing the strength at room temperature. In addition, Al is an element lighter than Ti and contributes to a decrease in specific gravity. In order to ensure the strength (particularly, 0.2% PS (0.2% proof stress)) at a predetermined room temperature and to reduce the specific gravity, the Al content is set to 5.0% or more. The Al content is preferably 5.2% or more, more preferably 5.3% or more, and still more preferably 5.4% or more.

[0048] Meanwhile, in a case where the Al content is more than 6.6%, there is a concern that the alloy sheet may embrittle and be broken during rolling and transportation. In addition, a hard region is locally generated due to solidifying segregation or the like, leading to excessive solid solution strengthening of the α-phase, and the toughness decreases. Therefore, the Al content is set to 6.6% or less. The Al content is preferably 6.5% or less, more preferably 6.3% or less, and still more preferably 6.0% or less.Fe: 0.7% to 2.3%

[0049] Fe is an inexpensive element but is an element having a high solid solution strengthening ability. Therefore, Fe is an effective element for increasing the strength at room temperature at low cost. In addition, Fe is a β-stabilizing element and is an element having an effect of expanding a high temperature two-phase temperature range required to obtain a T-texture that is developed to increase a Young's modulus and a strength in a sheet width direction, which will be described later.

[0050] In order to obtain these effects, the Fe content is set to 0.7% or more. The Fe content is preferably 0.9% or more, and more preferably 1.2% or more.

[0051] Meanwhile, Fe is an element that is very likely to segregate by solidifying segregation. Therefore, in a case where the content is large, the performance varies greatly, and the fatigue strength decreases depending on the location. Furthermore, since Fe is heavier than Ti, the specific gravity increases in a case where Fe is added excessively. Therefore, the Fe content is set to 2.3% or less. The Fe content is preferably 2.1% or less, and more preferably 1.9% or less.Si: 0.20% to 0.30%

[0052] Si is a β-stabilizing element but is an element that is solid-solubilized also in the α-phase and exhibits a high solid solution strengthening ability. As described above, it is difficult to contain Fe in an amount of more than 2.3% due to a problem of segregation. Therefore, in the titanium alloy sheet according to the present embodiment, the strength is further increased by solid solution strengthening of Si. Si has an opposite segregation tendency to O which will be described below, and is less likely to segregate by solidifying segregation than O. Therefore, in a case where an appropriate amount of Si is contained with O, a high strength (0.2% PS) can be obtained. In a case where the Si content is less than 0.20%, sufficient effects cannot be obtained. Therefore, the Si content is set to 0.20% or more.

[0053] Meanwhile, in a case where the Si content is high, an intermetallic compound called a silicide may be formed, and thus the fatigue strength may decrease or cracks may be likely to occur during rolling. In addition, differences in characteristics in a sheet thickness direction may be increased due to segregation. Therefore, the Si content is set to 0.30% or less.O: 0.10% to 0.20%

[0054] O is an effective element for increasing the strength. In order to obtain this effect, the O content is set to 0.10% or more. As described above, the effect is large in a case where O is contained with Si.

[0055] Meanwhile, in a case where the O content is large, the ductility and the workability may decrease, or breaking may be likely to occur in the production of a hot-rolled sheet, resulting in a decrease in producibility. In particular, in a case where O coexists with Al, the formation of Ti3Al is promoted. The formation of Ti3Al causes a significant decrease in ductility and workability at room temperature. In addition, O is an interstitial element, and the specific gravity increases in a case where the O content is large. Therefore, it becomes necessary to adjust Expressions (1) and (2) to be described later. Therefore, the O content is set to 0.20% or less.C: less than 0.050%N: 0.050% or less

[0056] C and N are elements that decrease the ductility and the workability of the titanium alloy sheet. Therefore, the C content is set to less than 0.050%, and the N content is set to 0.050% or less.

[0057] C and N are elements that contribute to an increase in strength similar to O. However, in the titanium alloy sheet according to the present embodiment, the strength is improved by O, and thus C and N are not essential.

[0058] From the viewpoint of ductility and workability, the C content and the N content are preferably small and may be 0%. However, since C and N may be mixed as impurities from raw material sponge titanium, scraps, and alloying element raw materials, the C content may be set to 0.0001% or more, and the N content may be set to 0.0001% or more.Remainder: Ti and Impurities

[0059] The remainder of the chemical composition of the titanium alloy sheet according to the present embodiment may consist of Ti and impurities. Examples of the impurities include H, Cl, Na, Mg, Ca, and B that are mixed during a refining step and the like, and Zr, Sn, Mo, Nb, Ta, and V that are mixed from scraps and the like, and are not limited thereto. The level of the impurities is acceptable as long as the amount of each element is 0.1% or less and the total amount of the elements is 0.5% or less. In addition, the H content is preferably 150 ppm or less. There is a concern that B may change into coarse precipitates in the ingot. Therefore, the B content is preferably suppressed as much as possible even in a case where B is contained as impurities. In the titanium alloy sheet according to the present embodiment, the B content is preferably set to 0.01% or less.

[0060] However, in the titanium alloy sheet according to the present embodiment, Ni, Cr, and Mn may be contained in the following ranges in place of a part of the remainder in order to improve the strength and the workability. However, since these elements may not be contained (optional elements), the lower limits thereof are 0%. In addition, the above elements may be contained as impurities.Ni: Less than 0.15%

[0061] Ni is an element that improves the tensile strength and the workability. Therefore, Ni may be contained. In obtaining these effects, the Ni content is preferably 0.01% or more.

[0062] Meanwhile, in a case where the Ni content is 0.15% or more, an intermetallic compound Ti2Ni that is an equilibrium phase may be generated, and the fatigue strength and the ductility at room temperature of the titanium alloy sheet may deteriorate. Therefore, in a case where Ni is contained, the Ni content is set to less than 0.15%. The Ni content is preferably 0.14% or less, and more preferably 0.11% or less.Cr: Less than 0.25%

[0063] Cr is an element that improves the tensile strength and the workability. Therefore, Cr may be contained. In obtaining these effects, the Cr content is preferably 0.01% or more.

[0064] Meanwhile, in a case where the Cr content is 0.25% or more, an intermetallic compound TiCr2 that is an equilibrium phase may be generated, and the fatigue strength and the ductility at room temperature of the titanium alloy sheet may deteriorate. Therefore, in a case where Cr is contained, the Cr content is set to less than 0.25%. The Cr content is preferably 0.24% or less, and more preferably 0.21% or less.Mn: Less than 0.25%

[0065] Mn is an element that improves the tensile strength and the workability. Therefore, Mn may be contained. In obtaining these effects, the Mn content is preferably 0.01% or more.

[0066] Meanwhile, in a case where the Mn content is 0.25% or more, an intermetallic compound TiMn that is an equilibrium phase may be generated, and the fatigue strength and the ductility at room temperature of the titanium alloy sheet may deteriorate. Therefore, in a case where Mn is contained, the Mn content is set to less than 0.25%. The Mn content is preferably 0.24% or less, and more preferably 0.20% or less.

[0067] In the titanium alloy sheet according to the present embodiment, in a case where the Al content is denoted by [% Al], the Fe content is denoted by [% Fe], the Si content is denoted by [% Si], and the O content is denoted by [% O], by mass %, it is necessary to satisfy Expressions (1) and (2) below, while each element satisfies the above range.11.5<[%⁢ Al]+2×[%⁢ Fe]+8×[%⁢ Si]+18×[%⁢ O]<15.5.(1)-4.5<[%⁢ Fe]-0.9×[%⁢ Al]+1.3×[%⁢ O]+1.8×[%⁢ Si]<-2.4.(2)

[0068] In a case where the value in the middle of Expression (1) is 11.5 or less, or the value in the middle of Expression (2) is −4.5 or less, sufficient 0.2% PS cannot be obtained.

[0069] Meanwhile, in a case where the value in the middle of Expression (1) is 15.5 or more, or the value in the middle of Expression (2) is −2.4 or more, the specific gravity increases.

[0070] The value in the middle of Expression (1) is preferably 12.0 or more and 15.0 or less.<Texture>

[0071] In a case where a titanium alloy is hot-rolled in one direction at a temperature of a β-region or an α+β-high temperature region having a high β-phase ratio, a texture (T-texture) in which the C-axis of hcp is oriented in the sheet width direction is formed by the variant selection rule when phase transformation occurs from the β-phase to the α-phase. The T-texture is a texture formed when the unrecrystallized p-phase that has undergone rolling deformation transforms into the α-phase. The T-texture improves the strength such as 0.2% PS and tensile strength and the Young's modulus in the sheet width direction.

[0072] In a case where the crystal orientation of the α-phase is expressed by Euler angles g={φ1, Φ, φ2} according to Bunge notation, a structure in which a T-texture is developed is formed when a maximum integration orientation expressed by a crystal orientation distribution function f(g) is in a range of φ1: 0° to 30°, Φ: 60° to 90°, and φ2: 0° to 60° (hereinafter, may be referred to as region 1) and the maximum integration in the maximum integration orientation is 10.0 or more.

[0073] In a case where the maximum integration in the region 1 is less than 10.0, the strength such as tensile strength and the Young's modulus in the sheet width direction do not sufficiently increase.

[0074] The upper limit of the maximum integration in the region 1 is not limited, but may be 40.0 or less or 30.0 or less.

[0075] In addition, in the titanium alloy sheet according to the present embodiment, in a case where the crystal orientation of the α-phase is expressed by the Euler angles g={φ1, Φ, φ2} according to Bunge notation, a maximum integration in ranges of φ1: 70° to 90°, Φ: 70° to 90°, and φ2: 0° to 60° and φ1: 70° to 90°, Φ: 10° to 30°, and φ2: 0° to 60° (hereinafter, may be referred to as region 2), expressed by a crystal orientation distribution function f(g), is 2.5 or less.

[0076] In a case where the maximum integration in the region 2 is more than 2.5 (in a case where the maximum integration is more than 2.5 in any of the ranges of φ1: 70° to 90°, Φ: 70° to 90°, and φ2: 0° to 60° and φ1: 70° to 90°, Φ: 10° to 30°, and φ2: 0° to 60°), the workability decreases. In addition, in a case where the degree of integration in the region 2 increases, the degree of integration in the region 1 decreases and this leads to a decrease in strength and Young's modulus in the sheet width direction. A high degree of integration in the region 2 means that a large number of textures in which the C-axis of hcp is oriented in the longitudinal direction of the titanium alloy sheet are formed.

[0077] Here, the Euler angles g={φ1, Φ, φ2} according to Bunge notation will be described with reference to FIG. 1. FIG. 1 is an explanatory diagram showing a crystal orientation of an α-phase crystal grain of the titanium alloy sheet by Euler angles according to Bunge notation. As a sample coordinate system, three coordinate axes of RD (rolling direction), TD (sheet width direction), and ND (normal direction of the rolling surface) having a relationship orthogonal to each other are shown. In addition, as a crystal coordinate system, three coordinate axes of an X-axis, a Y-axis, and a Z-axis having a relationship orthogonal to each other are shown. In addition, each of the coordinate axes is disposed so that the origins of the coordinate systems coincide with each other, and a hexagonal column indicating hcp is shown so that the center of the (0001) plane of hcp that is an α-phase of titanium coincides with the origin. In FIG. 1, the X-axis coincides with a [10-10]direction of the α-phase, the Y-axis coincides with a [−12-10] direction, and the Z-axis coincides with a

[0001] direction (C-axis direction).

[0078] In the Bunge notation, a state in which the RD, TD, and ND of the sample coordinate system and the X-axis, Y-axis, and Z-axis of the crystal coordinate system coincide with each other is first considered. Then, the crystal coordinate system is rotated by an angle φ1 around the Z-axis and is rotated by an angle Φ around the X-axis after the φ1-degree rotation (the state in FIG. 1). Finally, the system is rotated by an angle φ2 about the Z-axis after the Φ-degree rotation. By the three angles φ1, Φ, and φ2, the crystal or crystal coordinate system shows a specific inclined state with respect to the sample coordinate system. That is, the crystal orientation is uniquely determined using the three angles φ1, Φ, and φ2. The three angles φ1, Φ, and φ2 are referred to as Euler angles according to Bunge notation. The crystal orientation (C-axis direction and the like) of the α-phase crystal grain of the titanium alloy sheet is specified by the Euler angles according to Bunge notation.

[0079] In FIG. 1, φ1 is an angle formed by an intersection line between the RD-TD plane (rolling plane) of the sample coordinate system and the [10-10]-[−12-10]plane of the crystal coordinate system and the RD (rolling direction) of the sample coordinate system. (is an angle formed by the ND (normal direction of the rolling surface) of the sample coordinate system and the

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

[0080] The maximum integration orientation and the maximum integration can be obtained as follows.

[0081] A cross section (L-cross section) of the titanium alloy sheet perpendicular to the sheet width direction is chemically polished at a center position in the width direction (TD), and crystal orientation analysis is performed using an electron backscatter diffraction (EBSD) method.

[0082] Specifically, the L-cross section is wet-polished using emery paper, and then mirror-polished using colloidal silica to provide a mirror surface. In the mirror surface, about 5 visual fields are measured in steps of 1 μm for a rectangular region in a range in the thickness direction excluding 1,000 μm from each of the front and rear surfaces in the sheet thickness direction and in a range of 1,000 μm in the longitudinal direction of the sheet ((entire sheet thickness—front and rear surfaces of 1,000 m)×1,000 m)). In a case where the sheet thickness is less than 2.5 mm, the measurement range in the sheet thickness direction is set to a range of 30% of the sheet thickness centered on the center portion in the sheet thickness.

[0083] With the obtained data, a crystal orientation distribution function (ODF) f(g) is calculated using OIM Analysis™ software (Ver. 8.1.0) manufactured by TSL Solutions. The crystal orientation distribution function f(g) is calculated with an expansion index of 16 and a Gaussian half width of 5° in Texture analysis using a spherical harmonics method of the EBSD method. In that case, the calculation is performed so that line symmetry is achieved with respect to the sheet thickness direction, the rolling direction, and the sheet width direction, taking into consideration the symmetry of the rolling deformation.

[0084] The ODF is a distribution function representing a three-dimensional distribution in which the measured crystal orientation is plotted in a three-dimensional space (Euler space) of φ1-Φ-φ2. FIG. 2 is an example of a crystal orientation distribution function f(g) of the titanium alloy sheet according to the present embodiment obtained by the electron backscatter diffraction method. In FIG. 2, in order to two-dimensionally represent the Euler space, the Euler space is sliced horizontally at intervals of 5° in the direction of the angle φ2, and the obtained cross sections are arranged. By the ODF, the maximum integration orientation and the maximum integration can be calculated. In FIG. 2, the maximum integration orientation is confirmed at φ1=0°, Φ=90°, and φ2=30° (point A), and the maximum integration is 36.1. In the above description, the maximum integration orientation and the maximum integration are obtained based on the L-cross section at the center position in the sheet width direction. However, since the texture of the titanium alloy sheet is substantially uniform in the sheet width direction except for the end portions (edge portions), the maximum integration orientation and the maximum integration may be obtained based on an L-cross section at any position in the sheet width, excluding the edge portions. However, the edge portions are trimmed in many cases in the production of product sheets. Therefore, in that case, the maximum integration orientation and the maximum integration may be obtained based on an L-cross section at any position in the sheet width of the product sheet. In a case where it is not known whether the trimming is performed, the maximum integration orientation and the maximum integration may be obtained from a position more than 50 mm away from the end portion.<Half Width of Diffraction Peak at 2θ=53.3±1°>

[0085] In the titanium alloy sheet, in a case where water cooling is performed after hot rolling in the producing process of the titanium alloy sheet, dislocation accumulated by hot rolling remains or strain during transformation from the β-phase to the a phase remains. Although such strain increases the strength of the hot-rolled sheet, there is a concern that the strain may cause a decrease in strength when heat is applied in subsequent working. In addition, in a case where strain remains, products may be deformed in subsequent working. In addition, in a case where the strain remains, the amount of work hardening during working is reduced, so that YR increases and the workability decreases. Therefore, it is desirable that the strain (dislocation) of the hot-rolled sheet is small in order to suppress deformation during processing, obtain a high strength regardless of working conditions, and reduce YR.

[0086] As a method for estimating the amount of remaining strain (dislocation density), there is a method for estimating the dislocation density from the half width of a diffraction peak obtained by an X-ray diffraction (XRD) method. The larger the half width of the diffraction peak, the larger the amount of remaining strain. In the titanium alloy sheet according to the present embodiment, the amount of remaining strain is evaluated by a half width of a diffraction peak of the (10-12) plane appearing at a position of 2θ=53.3±1° detected by the X-ray diffraction method using CuKα as a line source. In the titanium alloy sheet according to the present embodiment, the half width of the diffraction peak at 2θ=53.3±1° detected by the X-ray diffraction method using CuKα as a line source is preferably 0.200 or less. The half width of the diffraction peak is more preferably 0.17° or less, and still more preferably 0.12° or less.

[0087] It is preferable that the half width of the diffraction peak be low. However, the lower limit thereof is substantially 0.05° since dislocation is present to some extent even in a stable phase state and it is difficult to completely remove the dislocation.

[0088] In the measurement of the half width, the surface of the titanium alloy sheet is wet-polished using emery paper, and then mirror-polished using colloidal silica to provide a mirror surface. XRD measurement is performed on the surface of the titanium alloy sheet that has been mirror-finished. The XRD measurement is performed with CuKα as a line source at a measurement pitch of 0.01° and a measurement rate of 2° / min in a range of 20 of 50.0° to 55.0°. The half width is calculated by integrated powder X-ray analysis software PDXL manufactured by Rigaku Corporation using X-ray diffraction data measured by SmartLab manufactured by Rigaku Corporation.<Microstructure>

[0089] The titanium alloy sheet according to the present embodiment preferably has a band structure 1 having an aspect ratio of more than 3.0 and elongated in the longitudinal direction of the sheet (rolling direction), and the area ratio of the band structure 1 is preferably 70% or more. The band structure 1 mentioned here is, for example, a structure elongated in the longitudinal direction of the sheet as shown in the optical microscope photograph of FIG. 3. Specifically, the band structure 1 refers to crystal grains of which the aspect ratio represented by major axis / minor axis of the crystal grains is more than 3.0.

[0090] In a case where a titanium alloy is hot-rolled at a temperature in an α+β-region immediately below a transformation point or a β-region, a band structure 1 elongated in the longitudinal direction of the sheet is formed. The band structure 1 has many crystal grain boundaries perpendicular to the sheet thickness direction. Therefore, the strength and the Young's modulus in the sheet width direction are increased by forming the band structure 1. Therefore, in the titanium alloy sheet according to the present embodiment, the area ratio of the band structure 1 is preferably 70% or more. The area ratio is more preferably 75% or more, and still more preferably 80% or more. In addition, all crystal grains may be a band structure, and the upper limit is 100%.

[0091] The area ratio of the band structure is obtained by the following method.

[0092] A cross section obtained by cutting each sample in a direction perpendicular to the sheet width direction at a center position in the sheet width is wet-polished using emery paper, and then mirror-polished using colloidal silica to provide a mirror surface. For a rectangular region of the mirror-polished cross section in a range in the thickness direction excluding 1,000 m from each of the front and rear surfaces in the sheet thickness direction and in a range of 1,000 m in the longitudinal direction of the sheet, about 5 visual fields are subjected to crystal orientation analysis using an EBSD method in steps of 1 m, and an aspect ratio of each of crystal grains is calculated on the assumption that high-tilt boundaries with an orientation difference of 150 or more, corresponding to prior p-grain boundaries, are defined as grain boundaries, and ranges surrounded by the grain boundaries are defined as crystal grains. A portion of the crystal grains having an aspect ratio of more than 3.0 is regarded as a band structure, and an area ratio thereof is calculated.<Characteristics>

[0093] The titanium alloy sheet according to the present embodiment has a high strength, a high Young's modulus, a low specific gravity, and high workability by controlling the chemical composition, the texture, and the microstructure as described above.

[0094] In consideration of demands for a titanium alloy sheet for a golf club application in recent years, the titanium alloy sheet according to the present embodiment aims to satisfy the following as indicators of the strength, Young's modulus, specific gravity, and workability.YR (0.2% PS / TS) in Sheet Width Direction: 0.99 or Less

[0095] In the titanium alloy sheet according to the present embodiment, excellent workability can be obtained by controlling the texture. However, even in that case, in a case where YR (0.2% PS / TS (tensile strength)) is high, the workability decreases depending on the kind of working. In a titanium alloy sheet that is generally worked by cold rolling, the workability is improved in a case where the working is performed at a high temperature, but the strength decreases due to heating, which is not preferable.

[0096] Specifically, even in a case where the above-described texture is provided, in a case where the YR is more than 0.99, there is no amount of work hardening during working, so that breaking is likely to occur, resulting in a decrease in workability. Therefore, in the titanium alloy sheet according to the present embodiment, YR in the sheet width direction is set to 0.99 (99%) or less.

[0097] Furthermore, in a case where workability in the sheet width direction is required, the YR in the sheet width direction is preferably 0.98 or less. The YR in the sheet width direction is more preferably 0.97 or less, still more preferably 0.95 or less, and yet still more preferably 0.93 or less.

[0098] The YR changes depending on the amount of residual strain and other factors. However, since the amount of strain has a large influence, it is preferable to decrease the amount of strain in a case where the YR is decreased.

[0099] Meanwhile, in a case where the YR is less than 0.85, it is suggested that the target texture cannot be formed. Therefore, the target value of the YR is 0.85 or more.0.2% PS in Sheet Width Direction at Room Temperature: 1,000 MPa or More

[0100] In recent years, a titanium alloy sheet for a golf club application has been required to have a higher strength than that in the past in order to further reduce the weight, and specifically, it is required that 0.2% PS in a sheet width direction is 1,000 MPa or more. Therefore, in the titanium alloy sheet according to the present embodiment, the target value of 0.2% PS in the sheet width direction at room temperature is 1,000 MPa or more.

[0101] The 0.2% PS of the titanium alloy sheet in the sheet width direction at room temperature is preferably 1,010 MPa or more, and more preferably 1,030 MPa or more.

[0102] Meanwhile, the 0.2% PS is preferably high, but in a case where the 0.2% PS is too high, the risk of breaking of the alloy sheet increases from the viewpoint of notch sensitivity. Therefore, the 0.2% PS in the sheet width direction at room temperature is preferably 1,200 MPa or less. The 0.2% PS in the sheet width direction at room temperature is more preferably 1,150 MPa or less.

[0103] The reason for increasing the 0.2% PS in the sheet width direction is that in golf applications, the vertical direction of a face greatly affects a repulsive force and the like, and the titanium alloy sheet is usually worked so that the sheet width direction is the vertical direction of the face.

[0104] In the present embodiment, the room temperature is 25° C.

[0105] The 0.2% PS and TS in the sheet width direction are measured in accordance with JIS Z 2241: 2011.

[0106] Specifically, a No. 13B tensile test piece (width of parallel portion: 12.5 mm, gauge length: 50 mm) specified in JIS Z 2241:2011 is produced so that a tensile direction is a sheet width direction of the titanium alloy sheet, and a tensile test is performed at a strain rate of 0.5% / min for measurement.Young's Modulus in Sheet Width Direction: 135 GPa or More

[0107] As described above, in recent years, there is repulsion regulation in golf clubs, and from that viewpoint, a titanium alloy sheet for a golf club application is required to have a high Young's modulus. Since the repulsion regulation is satisfied in a case where the Young's modulus in the sheet width direction is 135 GPa or more, the target value of the Young's modulus is 135 GPa or more. The target value is more preferably 137 GPa or more. Meanwhile, since the Young's modulus is preferably as high as possible, there is no need to limit the upper limit thereof. However, from the viewpoint of structure control and chemical composition, the upper limit is substantially about 150 GPa.

[0108] In the titanium alloy sheet, the Young's modulus in the sheet width direction can be increased by controlling the texture or forming a band structure.

[0109] The Young's modulus in the sheet width direction is obtained by the following method.

[0110] A No. 13B tensile test piece (width of parallel portion: 12.5 mm, gauge length: 50 mm) specified in JIS Z 2241:2011 is produced so that a tensile direction is a sheet width direction of the titanium alloy sheet, and a strain gauge is attached thereto. Loading-unloading is repeated 5 times in a stress range of 100 MPa to half of 0.2% PS at a strain rate of 10.0% / min, the inclination thereof is obtained, and the average of three values excluding a maximum value and a minimum value is defined as a Young's modulus.Specific Gravity: 4.45 g / cm3 or Less

[0111] In a case where the specific gravity of a titanium alloy sheet for a golf club application is too high, the face cannot be made lighter even in a case where the strength is increased. Therefore, the specific gravity is important from the viewpoint of weight reduction. In the titanium alloy sheet according to the present embodiment, the target value of the specific gravity is 4.45 g / cm3 or less from the viewpoint of weight reduction. The specific gravity is preferably low, but the lower limit thereof is substantially 4.38 / cm3 from the viewpoint of chemical composition.

[0112] The specific gravity may be measured by a dry method using a gas or a wet method using a liquid.

[0113] As the dry method, specifically, AccuPyc II manufactured by Micromeritics Instrument Corporation is used. The container size is 1 to 100 cm3, and any container may be used according to the sample size. As the gas, N2, Ar, and He gases, that are inert gases, are used.<Sheet Thickness>

[0114] The sheet thickness of the titanium alloy sheet according to the present embodiment is not limited.

[0115] The sheet thickness is, for example, more than 2.5 mm in consideration of application to golf clubs. The sheet thickness is preferably 3.0 mm or more.

[0116] Meanwhile, in a case where the thickness is more than 6.0 mm, the load excessively increases in a step such as annealing of the hot-rolled sheet or uncoiling after pickling, whereby the thickness may be set to 6.0 mm or less.<Producing Method>

[0117] If the titanium alloy sheet according to the present embodiment has the above-described features, it is possible to obtain the effects thereof. Therefore, the producing method is not limited. However, a producing method including the following steps is preferable since the production can be stably performed.

[0118] That is, the titanium alloy sheet according to the present embodiment can be produced by a producing method including:

[0119] (I) a heating step of heating a titanium material including, as a chemical composition, by mass %, Al: 5.0% to 6.6%, Fe: 0.7% to 2.3%, Si: 0.20% to 0.30%, 0: 0.10% to 0.20%, C: less than 0.050%, N: 0.050% 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 a remainder: Ti and impurities, to a heating temperature;

[0120] (II) a hot rolling step of hot-rolling the titanium material after the heating step in one direction to obtain a hot-rolled sheet;

[0121] (III) a coiling step of cooling the hot-rolled sheet after the hot rolling step to a coiling temperature of 400° C. or lower at an average cooling rate of 8° C. / s or higher and coiling the hot-rolled sheet at the coiling temperature; and

[0122] (IV) an annealing step of performing annealing on the hot-rolled sheet after the coiling step.

[0123] Preferable conditions for each step will be described. Known conditions can be applied to steps and conditions not described.[Heating Step]

[0124] In the heating step, a titanium material such as a titanium alloy slab including, as a chemical composition, by mass %, Al: 5.0% to 6.6%, Fe: 0.7% to 2.3%, Si: 0.20% to 0.30%, 0: 0.10% to 0.20%, C: less than 0.050%, N: 0.050% 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 a remainder: Ti and impurities is heated to a heating temperature.

[0125] The heating temperature is set to Tβ° C. or higher and (Tβ+150)° C. or lower, where Tβ is a β-transformation temperature in the unit of ° C.

[0126] In a case where the heating temperature is lower than Tβ° C., the titanium material is rolled in a state in which the proportion of the α-phase is high, and thus the rolling in a state in which the proportion of the-β phase is high is not sufficient. Therefore, the T-texture does not sufficiently develop.

[0127] In addition, in a case where the rolling reduction is low in a state in which the proportion of the β-phase is high, it may be difficult to form a band structure. Therefore, in a case where the proportion of the band structure is increased, the heating temperature is preferably (Tβ+20° C.) or higher.

[0128] Meanwhile, in a case where the heating temperature is higher than (Tβ+150° C.), there is a very high possibility that the β-phase will recrystallize during rolling. In this case, since variant selection does not occur during phase transformation from the β-phase to the α-phase, the T-texture is difficult to develop. Furthermore, the surface of the titanium material is severely oxidized, and the surface of the hot-rolled sheet is likely to be scratched or damaged after the hot rolling.

[0129] In the present embodiment, the β-transformation temperature Tβ means a boundary temperature at which the α-phase starts to be generated when the titanium alloy is cooled from the β-phase single phase region. Tβ can be obtained from a state diagram. The state diagram can be acquired, for example, by a computer coupling of phase diagrams and thermochemistry (CALPHAD) method. Specifically, a state diagram of the titanium alloy can be acquired by the CALPHAD method using an integrated thermodynamic calculation system Thermo-Calc manufactured by Thermo-Calc Software AB and a predetermined database (TI3), and Tβ can be calculated.[Hot Rolling Step]

[0130] In the hot rolling step, the titanium material after the heating step is hot-rolled in one direction to obtain a hot-rolled sheet. In that case, the rolling reduction is set to 85% or more, and the finishing temperature is set to (Tβ−170)° C. or higher and (Tβ−100)° C. or lower.

[0131] It has been known that in a hot-rolled sheet of a titanium alloy, a T-texture can be formed by heating to a β-region or an α+β-two phase high-temperature region immediately below a β-transformation point and hot rolling over an α+β region (for example, Japanese Unexamined Patent Application, First Publication No. 2012-149283).

[0132] However, as a result of studies by the present inventors, it has been found that the texture of the titanium alloy sheet according to the present embodiment is not necessarily obtained under the above conditions.

[0133] In addition, in Japanese Unexamined Patent Application, First Publication No. 2012-149283, it is necessary to carry out 85% or more of rolling at up to 900° C. in an α+β-two phase high-temperature region. However, 900° C. in the two phase region is a very high temperature, and furthermore, in a case where the heating is performed to the β-region or immediately below the β-transformation point, the center portion in the sheet thickness is at a higher temperature, and in some alloy species, there may occur a large temperature difference from the β-transformation point. Therefore, in a case where the holding time is long, recrystallization may occur due to strain accumulated by hot rolling.

[0134] The present inventors have conducted studies, and as a result, found that, in a case where a hot rolling completion temperature (finishing temperature) is set to a temperature of (Tβ−170)° C. or higher and (Tβ−100)° C. or lower and subsequent steps are then performed under predetermined conditions, a predetermined texture can be obtained.

[0135] In a case where the finishing temperature is lower than (Tβ−170)° C., the texture develops in a range of φ1: 70° to 90°, Φ: 10° to 30°, and φ2: 0° to 60°, and thus the texture in the above-described target range (region 1) does not develop, and the strength and the Young's modulus in the sheet width direction are not sufficiently improved. In addition, the workability at room temperature also decreases.

[0136] Meanwhile, in a case where the finishing temperature is higher than (Tβ−100)° C., the texture at φ1: 70° to 90°, Φ: 70° to 90°, and φ2: 0° to 60° develops. Therefore, the target texture does not develop, and thus target characteristics cannot be obtained.[Coiling Step]

[0137] In the coiling step, the hot-rolled sheet after the hot rolling step is cooled to a coiling temperature of 400° C. or lower at an average cooling rate of 8.0° C. / s or higher and coiled at the temperature (coiling temperature).

[0138] In a case where the coiling temperature (cooling stop temperature) is higher than 400° C., the sheet is cooled in a coil shape. That is, the shape is frozen due to the influence of the solid-solubilizing of Al, O, and the like described above or the precipitation of a compound. Accordingly, subsequent uncoiling is difficult to perform, and a sheet or coil cannot be produced. The coiling temperature is preferably 300° C. or lower, and more preferably 100° C. or lower.

[0139] Meanwhile, in a case where the average cooling rate up to the coiling temperature is lower than 8.0° C. / s, a predetermined texture does not sufficiently develop. The average cooling rate is preferably 10.0° C. / s or higher, more preferably 12.0° C. / s or higher, and still more preferably 15.0° C. / s or higher. In order to obtain the cooling rate, the cooling is preferably controlled by water cooling, oil cooling, gas blowing, or the like. For the atmosphere, an inert gas is preferably used, that is expected to suppress the oxidation. However, the atmosphere may be air in a case where descaling is performed.[Annealing Step]

[0140] In the annealing step, the hot-rolled sheet after the coiling step is annealed.

[0141] In the hot-rolled sheet that has undergone the above-described steps, not only working strain but also transformation strain remain.

[0142] Therefore, the strength is high, but the workability in the sheet width direction is significantly low. Therefore, in the method for producing the titanium alloy sheet according to the present embodiment, the strain is removed by annealing (hot-rolled sheet annealing) of the hot-rolled sheet.

[0143] In order to remove the strain and reduce the YR, the annealing temperature T (° C.) is set to 600° C. or higher, and the annealing is performed so that the annealing temperature T in the unit of ° C. and a holding time t in the unit of second at the annealing temperature satisfy Expression (3) below.

[0144] In a case where the annealing temperature T is lower than 600° C., the strain is not sufficiently removed. Meanwhile, in a case where (T+273.15)×(Log10(t)+20) is more than 27,000, phase transformation occurs or the crystal grains coarsen more than necessary, whereby a predetermined texture cannot be obtained.(T+2⁢7⁢3.1⁢5)×(Log10(t)+2⁢0)<27<semantics definitionURL="">,<annotation encoding="Mathematica">TagBox[",", "NumberComma", Rule[SyntaxForm, "0"]]< / annotation>< / semantics>0.Expression⁢ (3)

[0145] In a case where the YR is further low, e.g., 0.97 or less, it is preferable that the annealing temperature T (° C.) be set to 600° C. or higher and the annealing be performed so that the annealing temperature T in the unit of ° C. and the holding time t in the unit of second at the annealing temperature satisfy Expression (3′) below.22<semantics definitionURL="">,<annotation encoding="Mathematica">TagBox[",", "NumberComma", Rule[SyntaxForm, "0"]]< / annotation>< / semantics>000≤(T+273.15)×(Log10(t)+20)<27<semantics definitionURL="">,<annotation encoding="Mathematica">TagBox[",", "NumberComma", Rule[SyntaxForm, "0"]]< / annotation>< / semantics>0.Expression⁢ (3’)

[0146] That is, (T+273.15)×(Log10(t)+20) is preferably 22,000 or more. (T+273.15)×(Log10(t)+20) is more preferably 23,000 or more.

[0147] Meanwhile, in a case where the heating is performed up to the β-region, β→α transformation occurs and a needle-like structure is formed. In such a structure, the texture changes, and the strength and the Young's modulus in the sheet width direction decrease. Therefore, the annealing temperature T is set to Tβ or lower.

[0148] In addition, it is concerned that a bimodal structure (a mixed structure of the equiaxed structure and the needle-like structure) may be formed even immediately below Tβ (β-transformation temperature) (° C.). Therefore, the annealing temperature T is preferably (Tβ−50)° C. or lower.

[0149] Here, the holding time t at the annealing temperature is a time at annealing temperature±20° C., and also includes a time during which the temperature fluctuates in the temperature range.

[0150] The temperature of the titanium material and the like described in the producing steps is a surface temperature, and is measured by a radiation type thermometer after each step is performed. As the emissivity of the radiation type thermometer, a value calibrated to match the temperature measured using a contact type thermocouple on the slab immediately after taken out of the heating furnace is used. Instead of the radiation type thermometer, a contact type thermometer may be used.

[0151] In a case where the measurement is performed a plurality of times without changing the producing device and it is confirmed that the target temperature has been achieved as a result value of the measurement, taking into consideration fluctuations due to seasonal factors, the measurement may be omitted unless there is a special reason such as a change of the device or significant deterioration.EXAMPLES

[0152] Hereinafter, the embodiments of the present disclosure will be specifically described with reference to Examples. Examples shown below are merely examples, and are not limited to the following examples.

[0153] First, titanium alloy ingots serving as materials of titanium alloy sheets shown in A to M and R to T in Table 1 were produced by vacuum arc remelting (VAR). Then, slabs of 150 to 200 mm thickness×1,000 mm width×5,000 mm length were produced by blooming or forging. In addition, a titanium alloy ingot serving as a material of a titanium alloy sheet shown in N in Table 1 was produced by electron beam remelting (EBR). Then, a slab of 160 mm thickness×1,000 mm width×5,000 mm length was produced by blooming or forging.

[0154] A titanium alloy ingot serving as a material of a titanium alloy sheet shown in O in Table 1 was produced by plasma arc melting (PAR). Then, a slab of 160 mm thickness×800 mm width×5,000 mm length was produced by blooming or forging.

[0155] A titanium alloy slab serving as a material of a titanium alloy sheet shown in P in Table 1 was produced into 160 mm thickness×800 mm width×5,000 mm length by EBR.

[0156] A titanium alloy slab serving as a material of a titanium alloy sheet shown in Q in Table 1 was produced into 160 mm thickness×800 mm width×5,000 mm length by PAM.

[0157] After production, the surfaces and side surfaces of all the slabs were subjected to milling.

[0158] Regarding the chemical composition of the slab, Al, Fe, Si, Ni, Cr, and Mn were measured by ICP emission spectrochemical analysis. O and N were measured by inert gas melting and a thermal conductivity-infrared absorption method using an oxygen-nitrogen simultaneous analyzer. C was measured by an infrared absorption method using a carbon-sulfur simultaneous analyzer. The chemical compositions of the produced hot-rolled sheets were the same as the chemical compositions of the titanium alloy slabs shown in Table 1. In addition, regarding the titanium materials A to T shown in Table 1, state diagrams of the titanium alloys were acquired by a CALPHAD method using an integrated thermodynamic calculation system Thermo-Calc manufactured by Thermo-Calc Software AB and a predetermined database (TI3), and (β-transformation temperatures Tβ were calculated.TABLE 1Chemical Components (mass %) with Remainder[% Al] + 2 ×[% Fe]− 0.9 ×AlloyConsisting of Ti and Impurities[% Fe] + 8 × [%[% Al] + 1.3 × [%TβTypeAlFeSiCNONiCrMnSi] + 18 × [% O]O] + 1.8 × [% Si](° C.)A5.51.50.250.0070.0050.15———13.2−2.81005B5.01.10.200.0080.0070.17———11.9−2.81007C6.01.00.250.0050.0070.15———12.7−3.81025D5.31.20.220.0060.0050.14———12.0−3.01004E6.71.20.320.0080.0050.18———14.9−4.01043F5.32.30.250.0070.0050.15———14.6−1.8987G5.31.00.210.0070.0080.13———11.3−3.21006H5.71.30.220.0070.0050.30———15.5−3.01038I5.70.60.220.0070.0050.17———11.7−3.91031J5.81.90.220.0300.0300.11———13.3−2.8993K6.41.30.250.0080.0050.15———13.7−3.81027M5.11.70.250.0100.0030.19———13.9−2.2992N5.91.20.220.0050.0070.16———12.9−3.51021O6.00.80.230.0050.0070.17———12.5−4.01029P5.61.50.250.0050.0070.15———13.3−2.91007Q5.31.30.280.0050.0070.18———13.4−2.71007R5.51.40.220.0070.0050.170.13——13.1−2.91008S5.51.30.220.0070.0050.17—0.20—12.9−3.01009T5.51.30.220.0070.0050.16——0.2012.7−3.01008The underlined parts indicate that they are outside the ranges of the disclosure.

[0159] Next, under the conditions shown in Table 2, these slabs were heated, hot-rolled in one direction, cooled and coiled, and annealed to obtain titanium alloy sheets. The coiling temperature was set to 100° C. or lower to enable subsequent uncoiling and unfolding. However, only in Comparative Example 7, coiling was rapidly performed at 500° C.

[0160] In Comparative Examples 1, 4, and 7, it was difficult to uncool the titanium alloy sheet, and the sheet was broken. Therefore, the subsequent steps were stopped (annealing was not performed), and the evaluation of characteristics to be described later was not performed.TABLE 2Heating StepHot Rolling StepHeatingFinishingSlabRollingSheetTβTemperatureTemperatureThicknessRatioThicknessNo.Alloy Type(° C.)(° C.)(° C.)(μm)(%)(mm)Example 1A1005105088022097.75.0Example 2B1007105088020098.04.0Example 3C1025110088020098.53.0Example 4D1004103085020098.04.0ComparativeE1043105088020097.55.0Example 1ComparativeF987105088020097.55.0Example 2ComparativeG1006105088020097.55.0Example 3ComparativeH1038105088016096.95.0Example 4ComparativeI1031105088022097.75.0Example 5Example 5J993105088016096.36.0Example 6K1027110088015097.34.0ComparativeA1005 95080016096.95.0Example 6ComparativeA1007105088020098.04.0Example 7ComparativeA1005105085016096.36.0Example 8ComparativeA1005110088012095.85.0Example 9ComparativeM992105087020098.04.0Example 10ComparativeA1005120095018097.84.0Example 11Example 7A1005105088020097.55.0ComparativeA1005105088020097.55.0Example 12Example 8N1021105088016097.54.0Example 9O1029105088016098.13.0Example 10P1007105088016097.54.0Example 11Q1007105088016097.54.0Example 12A1005101084016097.54.0Example 13R1008105087020097.55.0Example 14S1009105087020097.55.0Example 15T1008105087020097.55.0Annealing StepCoiling StepPresence orCoolingCoilingAbsence ofTemperature(T + 273.15) ×RateTemperatureHeatTTime t(Log10(t) +No.(° C. / s)(° C.)Treatment(° C.)(s)20)Example 110.0≤100Presence80030024121Example 210.0≤100Presence7506022282Example 310.0≤100Presence80030024121Example 410.0≤100Presence80030024121Comparative10.0≤100————Example 1Comparative10.0≤100Presence80030024121Example 2Comparative10.0≤100Presence80030024121Example 3Comparative10.0≤100————Example 4Comparative10.0≤100Presence80030024121Example 5Example 550.0≤100Presence93018026776Example 630.0≤100Presence75060023305Comparative10.0≤100Presence80030024121Example 6Comparative10.0   500————Example 7Comparative 0.5≤100Presence80030024121Example 8Comparative10.0≤100Absence———Example 9Comparative20.0≤100Presence8506024460Example 10Comparative20.0≤100Presence80060024444Example 11Example 750.0≤100Presence65030020750Comparative50.0≤100Presence98030028167Example 12Example 820.0≤100Presence83030024796Example 920.0≤100Presence83030024796Example 1020.0≤100Presence83030024796Example 1120.0≤100Presence83030024796Example 1220.0≤100Presence80030024121Example 1320.0≤100Presence83060025128Example 1420.0≤100Presence83060025128Example 1520.0≤100Presence83060025128The underlined parts indicate that they deviate from the preferable manufacturing conditions.The symbol “—” indicates that no heat treatment is performed.

[0161] From the obtained titanium alloy sheets (except for Comparative Examples 1, 4, and 7, as described above. The same applies hereinafter.), the orientation in which the degree of integration of the titanium alloy sheet was maximum and the maximum degrees of integration in regions 1 and 2 were obtained. In the measurement, a cross section perpendicular to a sheet width direction (TD) of the titanium alloy sheet at a center position in the sheet width direction was wet-polished using emery paper, and then mirror-polished using colloidal silica to provide a mirror surface. A crystal orientation analysis was performed thereon using EBSD.

[0162] For a region in a range in a thickness direction excluding 1,000 μm from each of front and rear surfaces in the sheet thickness direction and in a range of 1,000 μm in the longitudinal direction of the sheet, about 5 visual fields were measured in steps of 1 m, and an ODF was calculated from data of the measurement using OIM Analysis™ software (Ver. 8.1.0) manufactured by TSL Solutions. The peak position of the degree of integration and the maximum integration were calculated from the ODF.

[0163] The ODF was calculated with an expansion index of 16 and a Gaussian half width of 5° in Texture analysis using a spherical harmonics method of the EBSD method. In that case, the calculation was performed so that line symmetry was achieved with respect to the sheet thickness direction, the rolling direction, and the sheet width direction, taking into consideration the symmetry of the rolling deformation.

[0164] In any case, in a case where the crystal orientation of the α-phase was expressed by Euler angles g={φ1, Φ, φ2} according to Bunge notation, a maximum integration orientation expressed by a crystal orientation distribution function f(g) was in a range of φ1: 0° to 30°, Φ: 60° to 90°, and φ2: 0° to 60°.

[0165] In addition, 0.2% PS (0.2% proof stress), a tensile strength (TS), YR, and a Young's modulus of the obtained titanium alloy sheet in the sheet width direction were obtained.

[0166] The 0.2% PS and the tensile strength were measured in accordance with JIS Z 2241: 2011. Specifically, a No. 13B tensile test piece (width of parallel portion: 12.5 mm, gauge length: 50 mm) specified in JIS Z 2241:2011 was produced so that a tensile direction was a sheet width direction of the titanium alloy sheet, and a tensile test was performed at 25° C. and a strain rate of 0.5% / min for measurement.

[0167] In addition, the YR was calculated from the 0.2% PS and the tensile strength.

[0168] The Young's modulus in the sheet width direction was obtained by the following method.

[0169] A No. 13B tensile test piece (width of parallel portion: 12.5 mm, gauge length: 50 mm) specified in JIS Z 2241:2011 was produced so that a tensile direction was a sheet width direction of the titanium alloy sheet, and a strain gauge was attached thereto. Loading-unloading was repeated 5 times in a stress range of 100 MPa to half of 0.2% proof stress at a strain rate of 10.0% / min, an inclination thereof was obtained, and the average of three values excluding the maximum value and the minimum value in that case was defined as a Young's modulus.

[0170] In addition, the half width of a diffraction peak ((10-12) peak) at 2θ=53.3±1° detected by an X-ray diffraction method using CuKα as a line source, of the obtained titanium alloy sheet, was calculated.

[0171] Specifically, the surface of the titanium alloy sheet was wet-polished using emery paper, and then mirror-polished using colloidal silica to provide a mirror surface. XRD measurement was performed on the surface of the titanium alloy sheet that has been mirror-finished. The XRD measurement was performed with CuKα as a line source at a measurement pitch of 0.01° and a measurement rate of 2° / min in a range of 2θ of 50.0° to 55.0°. The half width was calculated by integrated powder X-ray analysis software PDXL manufactured by Rigaku Corporation using X-ray diffraction data measured by SmartLab manufactured by Rigaku Corporation.

[0172] In addition, of the obtained titanium alloy sheet, the area ratio of a band structure was obtained.

[0173] To obtain the area ratio of the band structure, a cross section obtained by cutting in a direction perpendicular to the sheet width direction at a center position in the sheet width was wet-polished using emery paper, and then mirror-polished using colloidal silica to provide a mirror surface; and for a rectangular region of the cross section in a range excluding 1,000 m from each of the front and rear surfaces in the sheet thickness direction x a range of 1,000 μm in the longitudinal direction of the sheet, about 5 visual fields were subjected to crystal orientation analysis using an EBSD method in steps of 1 m, an aspect ratio of each of crystal grains was calculated, and the area ratio of crystal grains having an aspect ratio of more than 3.0 was calculated.

[0174] In addition, of the obtained titanium alloy sheet, a specific gravity was measured by a dry method using a gas. In that case, the measurement was performed using AccuPyc II manufactured by Micromeritics Instrument Corporation with a container size of 1 cm3 or 10 cm3 and an N2 gas as the gas.

[0175] As workability evaluation, first, the obtained titanium alloy sheet was subjected to evaluation by cold rolling.

[0176] Specifically, the hot-rolled sheet after the annealing step (the hot-rolled sheet after the coiling step in Comparative Example 9, since no annealing was performed) was subjected to shot blasting and scarfing with a nitric hydrofluoric acid solution by 100 to 150 μm to remove the oxidized phase on the surface, and then cold-rolled for the purpose of quantitatively evaluating the workability (cold workability) of the hot-rolled sheet, and cracks (edge cracks) on the surface and side surfaces were evaluated. A case where surface and edge cracks were generated in the cold rolling at a rolling reduction of 38% or less was evaluated as NG, and a case where no surface and edge cracks were generated was evaluated as OK (high workability).

[0177] In addition, as described above, in a case where various workings are taken into consideration, it is preferable that YR be low together with the above-described cold workability evaluation. Therefore, in a case where YR was 0.97 or less on the premise that the cold workability was evaluated as OK, it was determined that the workability was more excellent (workability considering YR is denoted by Ex in the table). A case where the cold workability was evaluated as OK, but YR was more than 0.97 was evaluated as GOOD.

[0178] The results are shown in Table 3.TABLE 3Mechanical CharacteristicsTexture(sheet width direction)MaximumMaximumHalf WidthMicrostructure0.2%IntegrationIntegrationof (10-12)Area ratio ofProofTensileYoung'sWorkabilityDegree inDegree inPeakBand StructureStressStrengthModulusSpecificColdConsideringNo.Region 1Region 2(°)(%)(MPa)(MPa)YR(GPa)GravityWorkabilityYRExample 115.01.00.1090107111510.931424.44OKExExample 213.31.10.1790104510750.971394.44OKExExample 314.01.10.1090104311230.931414.41OKExExample 411.02.10.1090100410840.931354.43OKExComparative————————4.41——Example 1Comparative13.01.10.1090115012300.931384.46OKExExample 2Comparative12.01.10.109096810480.921374.43OKExExample 3Comparative————————4.43——Example 4Comparative11.01.10.109098710670.931374.41OKExExample 5Example 512.51.20.1090107512050.891394.44OKExExample 616.01.00.1090113111810.961444.41OKExComparative 8.03.00.1065102111010.931294.44NGNGExample 6Comparative————————4.44——Example 7Comparative 6.52.70.109099110710.931254.44NGNGExample 8Comparative11.01.10.2790114911511.001374.44NGNGExample 9Comparative14.00.90.1090105811380.931404.46OKExExample 10Comparative 7.02.00.1070104111210.931254.44OKExExample 11Example 715.01.10.2590113111560.981404.44OKGOODComparative 9.02.20.1850112112010.931294.44OKExExample 12Example 814.01.10.1080105811580.911394.42OKExExample 912.01.50.1075103111310.911374.41OKExExample 1014.01.10.1085107711770.921394.44OKExExample 1114.01.10.1085107611760.911394.44OKExExample 1211.01.70.1060105611360.931364.44OKExExample 1320.01.30.1073105411440.921404.43OKExExample 1420.01.30.1075104311330.921404.43OKExExample 1520.01.30.1071103411240.921404.43OKExThe underlined parts indicate that they are outside the ranges of the disclosure.The symbol “—” indicates that no evaluation is performed due to breaking after coiling.

[0179] As can be seen from Tables i to 3, in Examples i to 15, the chemical composition and the texture are within the ranges of the present disclosure, the 0.2% proof stress in the sheet width direction at 25° C. is 1,000 MPa or more, the Young's modulus in the sheet width direction is 135 GPa or more, the specific gravity is 4.45 g / cm3 or less, and the YR is 0.99 or less. Whereby, the examples have a high strength, a high Young's modulus, a low specific gravity, and high workability.

[0180] In contrast, in Comparative Examples 2, 3, 5, 6, and 8 to 12, at least one of the chemical composition and the texture is outside the range of the present disclosure, and any of the 0.2% proof stress, the Young's modulus, and the specific gravity does not meet the target.

[0181] In Comparative Examples 1, 4, and 7, the alloy sheet was broken in midcourse, and it was not possible to obtain a predetermined titanium alloy sheet.BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS1: band structure

[0183] 2: equiaxed portion

Claims

1. A titanium alloy sheet comprising, as a chemical composition, by mass %:Al: 5.0% to 6.6%;Fe: 0.7% to 2.3%;Si: 0.20% to 0.30%;O: 0.10% to 0.20%;C: less than 0.050%;N: 0.050% 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%; anda remainder: Ti and impurities,wherein in the chemical composition, in a case where an Al content is denoted by [% Al], an Fe content is denoted by [% Fe], a Si content is denoted by [% Si], and an O content is denoted by [% O], by mass %, Expressions (1) and (2) are satisfied,in a case where a crystal orientation of an α-phase is expressed by Euler angles g={φ1, ϕ, φ2} according to Bunge notation, a maximum integration orientation expressed by a crystal orientation distribution function f(g) is in a range of φ1: 0° to 30°, ϕ: 60° to 90°, and φ2: 0° to 60°,a maximum integration in the maximum integration orientation is 10.0 or more,a maximum integration in ranges of φ1: 70° to 90°, ϕ: 70° to 90°, and φ2: 0° to 60° and φ1: 70° to 90°, ϕ: 10° to 30°, and φ2: 0° to 60° is 2.5 or less, andYR in a sheet width direction is 0.99 or less,11.5<[%⁢ Al]+2×[%⁢ Fe]+8×[%⁢ Si]+18×[%⁢ O]<15.5(1)-4.5<[%⁢ Fe]-0.9×[%⁢ Al]+1.3×[%⁢ O]+1.8×[%⁢ Si]<-2.4.(2)2. The titanium alloy sheet according to claim 1,wherein a 0.2% proof stress in the sheet width direction at 25° C. is 1,000 MPa or more,a Young's modulus in the sheet width direction is 135 GPa or more, anda specific gravity is 4.45 g / cm3 or less.

3. The titanium alloy sheet according to claim 1,wherein a half width of a diffraction peak at 2θ=53.3±1° detected by an X-ray diffraction method using CuKα as a line source is 0.20° or less.

4. The titanium alloy sheet according to claim 3,wherein a band structure having an aspect ratio of more than 3.0 and elongated in a longitudinal direction of the sheet is provided, andan area ratio of the band structure is 70% or more.

5. The titanium alloy sheet according to claim 3,wherein the YR in the sheet width direction is 0.85 or more and 0.97 or less.

6. The titanium alloy sheet according to claim 4,wherein the YR in the sheet width direction is 0.85 or more and 0.97 or less.

7. The titanium alloy sheet according to claim 1,wherein a sheet thickness is more than 2.5 mm.

8. The titanium alloy sheet according to claim 3,wherein a sheet thickness is more than 2.5 mm.

9. The titanium alloy sheet according to claim 4,wherein a sheet thickness is more than 2.5 mm.

10. The titanium alloy sheet according to claim 5,wherein a sheet thickness is more than 2.5 mm.

11. The titanium alloy sheet according to claim 6,wherein a sheet thickness is more than 2.5 mm.

12. A method for producing a titanium alloy sheet comprising, as a chemical composition, by mass %:Al: 5.0% to 6.6%;Fe: 0.7% to 2.3%;Si: 0.20% to 0.30%;O: 0.10% to 0.20%;N: 0.050% 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%; anda remainder: Ti and impurities,wherein in the chemical composition, in a case where an Al content is denoted by [% Al], an Fe content is denoted by [% Fe], a Si content is denoted by [% Si], and an O content is denoted by [% O], by mass %, Expressions (1) and (2) are satisfied,in a case where a crystal orientation of an α-phase is expressed by Euler angles α={φ1, ϕ, φ2} according to Bunge notation, a maximum integration orientation expressed by a crystal orientation distribution function f(g) is in a range of φ1, 0° to 30°, ϕ: 60° to and φ1: 70° to 90°, ϕ: 10° to 30°, and φ2: 0° to 60° is 2.5 or less, andYR in a sheet width direction is 0.99 or less.11.5<[%⁢ Al]+2×[%⁢ Fe]+8×[%⁢ Si]+18×[%⁢ O]<15.5(1)-4.5<[%⁢ Fe]-0.9×[%⁢ Al]+1.3×[%⁢ O]+1.8×[%⁢ Si]<-2.4(2)the method comprising:heating a titanium material including, as a chemical composition, by mass %, Al: 5.0% to 6.6%, Fe: 0.7% to 2.3%, Si: 0.20% to 0.30%, 0: 0.10% to 0.20%, C: less than 0.050%, N: 0.050% 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 a remainder: Ti and impurities, to a heating temperature;hot-rolling the titanium material after the heating in one direction to obtain a hot-rolled sheet;cooling the hot-rolled sheet after the hot rolling to a coiling temperature of 400° C. or lower at a rate of 8.0° C. / s or higher and coiling the hot-rolled sheet at the coiling temperature; andperforming annealing on the hot-rolled sheet after the coiling,wherein in the heating, the heating temperature is Tβ° C. or higher and (Tβ+150)° C. or lower, where Tβ is a β-transformation temperature in the unit of ° C.,in the hot rolling, a rolling reduction is 85% or more, and a finishing temperature is (Tβ−170)° C. or higher and (Tβ−100)° C. or lower, andin the annealing, an annealing temperature T during the annealing is 600° C. or higher and Tβ or lower, and the annealing temperature T and a holding time t in the unit of second at the annealing temperature satisfy Expression (3),(T+2⁢7⁢3.1⁢5)×(Log10(t)+2⁢0)<27<semantics definitionURL="">,<annotation encoding="Mathematica">TagBox[",", "NumberComma", Rule[SyntaxForm, "0"]]< / annotation>< / semantics>0.Expression⁢ (3)13. The method for producing the titanium alloy sheet according to claim 12,wherein in the annealing, the annealing temperature T during the annealing is 600° C. or higher and Tβ or lower, and the annealing temperature T and the holding time t in the unit of second at the annealing temperature satisfy Expression (3′),22<semantics definitionURL="">,<annotation encoding="Mathematica">TagBox[",", "NumberComma", Rule[SyntaxForm, "0"]]< / annotation>< / semantics>000≤(T+273.15)×(Log10(t)+20)<27<semantics definitionURL="">,<annotation encoding="Mathematica">TagBox[",", "NumberComma", Rule[SyntaxForm, "0"]]< / annotation>< / semantics>0.Expression⁢ (3’)14. The titanium alloy sheet according to claim 2,wherein a half width of a diffraction peak at 2θ=53.3±1° detected by an X-ray diffraction method using CuKα as a line source is 0.20° or less.

15. The titanium alloy sheet according to claim 2,wherein a sheet thickness is more than 2.5 mm.

16. The titanium alloy sheet according to claim 14,wherein a band structure having an aspect ratio of more than 3.0 and elongated in a longitudinal direction of the sheet is provided, andan area ratio of the band structure is 70% or more.

17. The titanium alloy sheet according to claim 14,wherein the YR in the sheet width direction is 0.85 or more and 0.97 or less.

18. The titanium alloy sheet according to claim 17,wherein the YR in the sheet width direction is 0.85 or more and 0.97 or less.

19. The titanium alloy sheet according to claim 14,wherein a sheet thickness is more than 2.5 mm.

20. The titanium alloy sheet according to claim 16,wherein a sheet thickness is more than 2.5 mm.

21. The titanium alloy sheet according to claim 17,wherein a sheet thickness is more than 2.5 mm.

22. The titanium alloy sheet according to claim 18,wherein a sheet thickness is more than 2.5 mm.

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