US20240018629A1 - Titanium alloy sheet, titanium alloy coil, method for manufacturing titanium alloy sheet, and method for manufacturing titanium alloy coil - Google Patents

Titanium alloy sheet, titanium alloy coil, method for manufacturing titanium alloy sheet, and method for manufacturing titanium alloy coil Download PDF

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US20240018629A1
US20240018629A1 US18/036,033 US202118036033A US2024018629A1 US 20240018629 A1 US20240018629 A1 US 20240018629A1 US 202118036033 A US202118036033 A US 202118036033A US 2024018629 A1 US2024018629 A1 US 2024018629A1
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titanium alloy
rolling
cold rolling
alloy sheet
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Tomonori Kunieda
Genki TSUKAMOTO
Yoshiki Koike
Toshiyuki OKUI
Hidenori Takebe
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Nippon Steel Corp
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Nippon Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C14/00—Alloys based on titanium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0268—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment between cold rolling steps
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/16—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of other metals or alloys based thereon
    • C22F1/18—High-melting or refractory metals or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/16—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of other metals or alloys based thereon
    • C22F1/18—High-melting or refractory metals or alloys based thereon
    • C22F1/183—High-melting or refractory metals or alloys based thereon of titanium or alloys based thereon

Definitions

  • the present disclosure relates to a titanium alloy sheet, a titanium alloy coil, a method for manufacturing a titanium alloy sheet, and a method for manufacturing a titanium alloy coil.
  • Titanium is a material that is lightweight and has high strength and excellent corrosion resistance, and a material that can be applied to the field of aircrafts from the viewpoint of reduction in weight and improvement in fuel efficiency. For that reason, titanium alloys have been actively developed in accordance with characteristics required for each of constituent members of aircrafts.
  • Patent Document 1 discloses an ⁇ + ⁇ 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 a remainder of titanium and impurities.
  • Patent Document 2 discloses an ⁇ + ⁇ type titanium alloy bar containing 0.5% or more and less than 1.4% Fe, 4.4% or more and less than 5.5% Al, and a remainder of titanium and impurities.
  • Patent Document 3 discloses a method for manufacturing a Ti—6Al—4V alloy sheet by pack rolling characterized in that, in a method for manufacturing a sheet in which a pack-rolled material is formed by covering one or a plurality of sheet-shaped core materials with spacer materials and cover materials and the pack material is rolled to reduce thicknesses of the core materials, initial sheet thicknesses of each material are set by setting sheet thicknesses of the cover materials such that the ratio of the core materials to the pack material is at least 0.25 or more.
  • Patent Document 4 discloses a method for manufacturing a Ti—6Al—4V alloy sheet by pack rolling characterized in that, in a method for manufacturing a sheet in which a pack material is formed by covering one or more sheet-shaped core materials with spacer materials and cover materials, and the pack material is rolled to reduce the thicknesses of the core materials, the rolling rate per pass is set to 15% or more for rolling in which the sheet thickness reduction ratio between before and after reduction in thickness of the pack material is 3 or more.
  • Patent Document 5 discloses a method for manufacturing a titanium alloy sheet characterized in that a hot-rolled and annealed titanium alloy sheet containing, in weight percent, Al: 2.5 to 3.5%, V: 2.0 to 3.0%, and a remainder of Ti and ordinary impurities is cold-rolled in the same direction as a hot rolling direction at a total rolling rate of 67% or more, and then annealed at a temperature between 650 and 900° C.
  • Patent Document 6 discloses a method for manufacturing an ⁇ + ⁇ type titanium alloy sheet characterized by performing intermediate annealing after cold rolling in a manufacturing process of an ⁇ + ⁇ type titanium alloy cold-rolled sheet under conditions of an annealing temperature: a temperature range of [ ⁇ transformation point ⁇ 25° C.] or higher and less than the ⁇ transformation point, an annealing time: 0.5 to 4 hours, a cooling rate after heating and holding: 0.5 to 5 ° C./sec, and a temperature range for cooling at the above cooling rate: 300° C. or lower.
  • an annealing temperature a temperature range of [ ⁇ transformation point ⁇ 25° C.] or higher and less than the ⁇ transformation point
  • an annealing time 0.5 to 4 hours
  • a cooling rate after heating and holding 0.5 to 5 ° C./sec
  • a temperature range for cooling at the above cooling rate 300° C. or lower.
  • Patent Document 7 discloses an a+ ⁇ type titanium alloy sheet characterized by containing at least one complete solid-solution type ⁇ -stabilizing element at 2.0 to 4.5% by mass in Mo equivalent, at least one eutectoid-type ⁇ -stabilizing element at 0.3 to 2.0% by mass in Fe equivalent, at least one ⁇ -stabilizing element at more than 3.0% by mass and 5.5% by mass or less in Al equivalent, and a remainder of Ti and unavoidable impurities, in which the average grain size of an ⁇ -phase is 5.0 ⁇ m or less, the maximum grain size of the ⁇ -phase is 10.0 ⁇ in or less, the average aspect ratio of the ⁇ -phase is 2.0 or less, and the maximum aspect ratio of the ⁇ -phase is 5.0 or less.
  • Patent Document 8 discloses an ⁇ + ⁇ type titanium alloy sheet having excellent cold rolling properties and cold handling properties characterized in that an ⁇ + ⁇ type titanium alloy hot-rolled sheet is formed such that, when (a) a normal direction (a sheet thickness direction) of a hot-rolled sheet is defined as ND, a hot rolling direction is defined as RD, a width direction of the hot-rolled sheet is defined as TD, a normal direction of a (0001) plane of an ⁇ -phase is defined as a c axis orientation, an angle formed by the c axis orientation and ND is defined as ⁇ , and an angle formed by a surface including the c axis orientation and ND and a surface including ND and TD is defined as ⁇ , (b1) the strongest intensity among (0002) reflection relative intensities of X-rays of crystal grains in which 0 is 0 degrees or more and 30 degrees or less and ⁇ falls within the entire circumference ( ⁇ 180 degrees to 180 degrees) is defined as XND, and (b2) the strongest intensity among (000
  • Patent Document 10 discloses a method for manufacturing an ⁇ + ⁇ type titanium alloy sheet characterized in that an ⁇ + ⁇ type titanium alloy sheet manufactured by rolling or forging is cold-rolled with a rolling reduction of 20% or more and then annealed at a temperature of 700° C. or higher and a ⁇ transformation point or lower, thereby obtaining a sheet having a fine equiaxed ⁇ structure.
  • Non-Patent Document 1 discloses an a+13 titanium alloy sheet having anisotropy in strength in a rolling direction and in a direction perpendicular to the rolling direction.
  • Non-Patent Document 2 discloses an ⁇ + ⁇ titanium alloy sheet obtained by hot rolling at a temperature higher than ⁇ transformation point to reduce anisotropy in strength in a rolling direction and in a direction perpendicular to the rolling direction.
  • titanium materials used for members requiring higher strength among constituent elements of aircrafts contain a large amount of Al, but have a high deformation resistance in hot rolling or cold rolling, and thus an allowable load of a rolling mill may be exceeded in the case of manufacturing a sheet. For that reason, it is difficult to manufacture a high-strength titanium alloy sheet by known hot rolling methods or cold rolling methods.
  • the present disclosure has been made in view of the above problems, and an object of the present disclosure is to provide a titanium alloy sheet and a titanium alloy coil which have high strength, a method for manufacturing the same titanium alloy sheet, and a method for manufacturing the same titanium alloy coil.
  • the present inventors have found that, when a titanium alloy sheet contains a predetermined amount of Al and has a texture in which there is a peak of intensity of crystal grains within a predetermined angle with respect to a width direction of final rolling in a (0001) pole figure in a sheet thickness direction, it has high strength and excellent workability.
  • the present inventors have found a method for manufacturing the titanium alloy sheet by cold rolling, which can achieve such a chemical composition and texture at the same time, and reached the present disclosure.
  • a titanium alloy sheet and a titanium alloy coil which have high strength, a method for manufacturing the titanium alloy sheet, and a method for manufacturing the titanium alloy coil.
  • FIG. 1 is an example of a (0001) pole figure in a sheet thickness direction (ND) of a titanium alloy sheet according to an embodiment of the present disclosure.
  • FIG. 2 is a diagram showing an angle formed between a direction indicating a peak of intensity and a width direction thereof.
  • FIG. 3 is a diagram showing an example of an optical microscope photograph of the titanium alloy sheet according to the same embodiment.
  • FIG. 4 is an optical microscope photograph showing an example of a band structure.
  • FIG. 5 is a schematic diagram showing a method for measuring the average sheet thickness.
  • FIG. 1 is an example of a (0001) pole figure in a sheet thickness direction (ND) of the titanium alloy sheet according to the present embodiment.
  • FIG. 2 is a diagram showing an angle formed between a direction indicating a peak of intensity and a width direction thereof.
  • the (0001) pole figure in the sheet thickness direction (ND) in FIG. 2 is the same as in FIG. 1 .
  • FIG. 3 is a diagram showing an example of an optical microscope photograph of the titanium alloy sheet according to the present embodiment.
  • FIG. 4 is an optical microscope photograph showing an example of a band structure.
  • FIG. 5 is a schematic diagram showing a method for measuring the average sheet thickness.
  • the titanium alloy sheet according to the present embodiment can be manufactured by a method including a cold rolling process.
  • the titanium alloy sheet according to the present embodiment contains, in % by mass, Al: more than 4.0% and 6.6% or less, Fe: 0% or more and 2.3% or less, V: 0% or more and 4.5% or less, Si: 0% or more and 0.60% 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 0.40% 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 of Ti and impurities. Also, hereinafter, in the description of the chemical components, the notation “%” represents “% by mass” unless otherwise specified.
  • Al is an ⁇ -phase stabilizing element and an element with high solid-solution strengthening ability.
  • tensile strength at room temperature increases. If the Al content is more than 4.0%, high tensile strength can be obtained. Further, a hot-rolled sheet before cold rolling can maintain high cold rolling properties.
  • the Al content is preferably 4.5% or more and more preferably 4.6% or more.
  • the Al content is more than 6.6%, the cold rolling properties of the hot-rolled sheet before cold rolling significantly deteriorate, and a region in which Al is excessively dissolved is locally generated due to solidification segregation or the like, and thus Al is ordered. This Al-ordered region reduces impact toughness of the titanium alloy sheet. Accordingly, the Al content is 6.6% or less, preferably 6.5% or less, and more preferably 6.4% or less.
  • Fe is a ⁇ -phase stabilizing element.
  • Fe is an element with high solid-solution strengthening ability, and thus when the Fe content increases, tensile strength at room temperature increases.
  • ail-phase has higher workability than an ⁇ -phase, and thus when the Fe content increases, workability of the titanium alloy sheet improves, making it possible to improve dimensional accuracy.
  • Fe is not essential in the titanium alloy sheet, a lower limit of its content is 0%.
  • the Fe content is preferably 0.5% or more.
  • the Fe content is more preferably 0.7% or more.
  • Fe is an element that is very prone to solidification segregation, and thus, when Fe is excessively contained, Fe segregates locally, which may cause variations in properties between a portion in which Fe is segregated and a portion in which Fe is not segregated. Further, when Fe is excessively contained in the titanium alloy sheet, fatigue strength may be lowered. Accordingly, the Fe content is preferably 2.3% or less. The Fe content is more preferably 2.1% or less, and still more preferably 2.0% or less. Also, Fe is less expensive than ⁇ -phase stabilizing elements such as V or Si or the like.
  • V is a complete solid-solution type ⁇ -phase stabilizing element and an element with solid-solution strengthening ability. Since V is not essential in the titanium alloy sheet, a lower limit of its content is 0%. However, in order to obtain solid-solution strengthening ability equivalent to that of Fe described above, the V content is preferably 2.5% or more. The V content is more preferably 3.0% or more. Replacing Fe with V increases costs, but since V is less likely to segregate than Fe, variations in properties due to segregation are inhibited. As a result, it becomes easier to obtain stable properties in a longitudinal direction and a width direction of the titanium alloy sheet. In order to inhibit variations in properties due to V segregation, the V content is preferably 4.5% or less. As described above, since V is less likely to segregate than Fe, V is preferably contained in a titanium material in the case of manufacturing a large ingot.
  • Si is a ⁇ -phase stabilizing element, it also dissolves in the ⁇ -phase and exhibits high solid-solution strengthening ability.
  • Fe may segregate when more than 2.3% thereof is contained in the titanium alloy sheet, and thus Si may be contained to increase strength of the titanium alloy sheet, if necessary.
  • Si has a segregation tendency opposite to that of O described below, and is less likely to solidify and segregate than O, and thus, by including appropriate amounts of Si and O in the titanium alloy sheet, it can be expected to achieve both high fatigue strength and tensile strength.
  • an intermetallic compound of Si called a silicide is formed, which may reduce fatigue strength of the titanium alloy sheet.
  • the Si content is 0.60% or less, generation of a coarse silicide is inhibited, and a decrease in fatigue strength is inhibited. Accordingly, the Si content is preferably 0.60% or less.
  • the Si content is more preferably 0.50% or less, and still more preferably 0.40% or less. Since Si is not essential in the titanium alloy sheet, a lower limit of its content is 0%, but the Si content may be, for example, 0.10% or more.
  • the C content is preferably less than 0.080%. Since C is not essential in the titanium alloy sheet, a lower limit of its content is 0%. Also, C is an unavoidably incorporated substance, and its substantial content is usually 0.0001% or more. The C content is more preferably or less.
  • an upper limit of the N content is preferably 0.050%. Since N is not essential in the titanium alloy sheet, a lower limit of its content is 0%. Also, N is an unavoidably incorporated substance, and its substantial content is usually 0.0001% or more. The N content is more preferably 0.04% or less.
  • an upper limit of the O content is preferably 0.40%, more preferably 0.38%, and still more preferably 0.35%. Since O is not essential in the titanium alloy sheet, a lower limit of its content is 0%. Also, O is an unavoidably incorporated substance, and its substantial content is usually 0.01% or more.
  • Ni is an element that improves tensile strength and workability.
  • the Ni content is preferably less than 0.15%.
  • the Ni content is more preferably 0.14% or less, or 0.12% or less, and still more preferably 0.11% or less. Since Ni is not essential in the titanium alloy sheet, a lower limit of its content is 0%, but the Ni content may be, for example, 0.01% or more.
  • Cr is an element that improves tensile strength and workability.
  • the Cr content is preferably less than 0.25%.
  • the Cr content is more preferably 0.24% or less, or 0.21% or less. Since Cr is not essential in the titanium alloy sheet, a lower limit of its content is 0%, but the Cr content may be, for example, or more.
  • Mn is an element that improves tensile strength and workability.
  • the Mn content is preferably less than 0.25%.
  • the Mn content is more preferably 0.24% or less, and still more preferably 0.20% or less. Since Mn is not essential in the titanium alloy sheet, a lower limit of its content is 0%, but the Mn content may be, for example, 0.01% or more.
  • the titanium alloy sheet according to the present embodiment contains, as optional elements, either Fe: 0.5 to 2.3% or V: 2.5 to 4.5%, and Si: 0 to 0.60%, and preferably contains C: less than 0.080%, N: 0.050% or less, and O: 0.40% or less.
  • the titanium alloy sheet according to the present embodiment contains either Fe: 0.5 to 2.3% or V: 2.5 to 4.5%, it preferably contains one element or two or more elements selected from the group including Ni: less than 0.15%, Cr: less than and Mn: less than 0.25% in place of a part of Fe or V.
  • the titanium alloy sheet according to the present embodiment contains Fe
  • the total amount of Fe, Ni, Cr, and Mn is preferably 0.5% or more and 2.3% or less. If the total amount of Fe, Ni, Cr, and Mn is 0.5% or more, high tensile strength is obtained. In addition, if the total amount of Fe, Ni, Cr, and Mn is 0.5% or more, the ⁇ -phase having good workability at room temperature is maintained to improve workability of the titanium alloy sheet, and thus it is possible to improve dimensional accuracy. Further, if the total amount of Fe, Ni, Cr, and Mn is 2.3% or less, segregation of these elements is inhibited, which makes it possible to inhibit variations in properties of the titanium alloy sheet.
  • the titanium alloy sheet according to the present embodiment contains V
  • when it contains one element or two or more elements selected from the group including Ni: less than 0.15%, Cr: less than 0.25%, and Mn: less than the total amount of V, Ni, Cr, and Mn is preferably 2.5% or more and 4.5% or less. If the total amount of V, Ni, Cr, and Mn is 2.5% or more, high tensile strength is obtained. In addition, if the total amount of V, Ni, Cr, and Mn is 2.5% or more, the ⁇ -phase having good workability at room temperature is maintained to improve the workability of the titanium alloy sheet, and thus it is possible to improve dimensional accuracy. Further, if the total amount of V, Ni, Cr, and Mn is 4.5% or less, segregation of these elements is inhibited, which makes it possible to inhibit variations in the properties of the titanium alloy sheet.
  • the balance of the chemical composition of the titanium alloy sheet according to the present embodiment may be Ti and impurities.
  • the impurities include, for example, H, Cl, Na, Mg, Ca, and B that are mixed in during a refining process and Zr, Sn, Mo, Nb, Ta, and Cu that are mixed from scraps or the like. If the total amount of the impurities is 0.5% or less, it is a level of not causing problems. Also, the H content is 150 ppm or less. There is a risk that B may form coarse precipitates in an ingot. For that reason, even in a case in which B is contained as an impurity, it is preferable to inhibit the B content as much as possible.
  • the B content is preferably 0.01% or less.
  • V contained in the titanium alloy sheet may be contained in an amount considered as an impurity
  • Fe contained in the titanium alloy sheet may be contained in an amount considered as an impurity
  • the titanium alloy sheet according to the present embodiment may contain various elements instead of Ti as long as it has high strength and excellent dimensional accuracy can be obtained.
  • the elements provided as exemplary impurities if the titanium alloy sheet has high strength and excellent dimensional accuracy, it may contain more than the amount considered as an impurity.
  • the titanium alloy sheet according to the present embodiment can have the above chemical components. More specifically, the chemical composition of the titanium alloy sheet according to the present embodiment may be, for example, Ti—6Al-4V, Ti—6Al-4V ELI, or Ti—5Al—1Fe.
  • the titanium alloy sheet according to the present embodiment has a texture in which, in a (0001) pole figure in the sheet thickness direction, an angle formed between the sheet thickness direction and a direction indicating a peak of intensity calculated by texture analysis in a case in which a series rank is 16 and the Gaussian half width is 5° for an inverse pole figure using a spherical harmonics method of an electron backscatter diffraction (EBSD) method is 65° or less.
  • EBSD electron backscatter diffraction
  • a texture in which a c axis of a hexagonal close-packed (hcp) structure is oriented in a width direction perpendicular to a longitudinal direction on a rolled surface according to variant selection rules is formed during phase transformation from the ⁇ -phase to the ⁇ -phase.
  • hcp hexagonal close-packed
  • the angle formed between the direction in which the c axis of hcp is most oriented (the direction indicating the peak of intensity) and the sheet thickness direction is 65° or less, which makes it possible to reduce the anisotropy, ensure high workability, and improve the dimensional accuracy.
  • the angle formed between the direction in which the c axis of hcp is most oriented and the sheet thickness direction is preferably 60° or less, more preferably 55° or less, and still more preferably 35° or less.
  • the lower limit of the angle formed between the direction in which the c axis of hcp is most oriented and the sheet thickness direction is not particularly limited, but is 0° or more. In a case in which the titanium alloy sheet is manufactured by rolling in one direction, the lower limit of the angle formed between the direction in which the c axis of hcp is most oriented and the sheet thickness direction is 20° or more.
  • the texture in which the c axis of hcp axes may be tilted in the width direction (TD) (a split-TD type texture).
  • the split-TD type texture is excellent in moldability, particularly in bendability. Accordingly, the angle formed between the direction indicating the peak of intensity and the sheet thickness direction is preferably or more and 65° or less, which is the split-TD type texture.
  • the (0001) pole figure is obtained by chemically polishing an observation surface of a sample of the titanium alloy sheet and analyzing its crystallographic orientation using EBSD. Specifically, a cross-section (an L cross-section) obtained by cutting the titanium alloy sheet in the sheet thickness direction along the longitudinal direction at a central position in the width direction (TD) is chemically polished, and crystallographic orientation analysis is performed by the EBSD method at 2 to 10 locations at intervals of 1 to 2 ⁇ m in a region of (total sheet thickness) ⁇ 2 mm of the cross-section, so that the (0001) pole figure can be drawn.
  • Peak position data of intensity of a specific orientation in the (0001) pole figure is calculated by the texture analysis of the inverse pole figure using the spherical harmonics method using OIM AnalysisTM software (Ver.8.1.0) manufactured by TSL Solutions.
  • the peak position of intensity is a position of the highest contour line, and a value of the highest intensity at the peak position is defined as the maximum intensity.
  • an intensity of a specific orientation in the (0001) pole figure indicates how many times a frequency of presence of crystal grains having that orientation is with respect to a structure having a completely random orientation distribution (an intensity of the structure is 1).
  • the L cross-section at the central position in the width direction is set to the observation surface, but a crystallographic orientation of the titanium alloy sheet is uniformly distributed in the width direction, and thus an L cross-section at an arbitrary sheet width position may be set to the observation surface.
  • FIG. 1 shows an example of the (0001) pole figure in the sheet thickness direction (ND) of the titanium alloy sheet according to the present embodiment.
  • detected poles of each crystallographic orientation are accumulated in accordance with inclinations in a final rolling direction (RD) and a final rolling width direction (TD), and contour lines of intensity are drawn in the (0001) pole figure.
  • a peak P 1 of crystal grains is located at the highest contour line in the figure. Accordingly, in the present embodiment, the angle formed between the direction indicating the peak P 1 of the crystal grains and ND is 65° or less. Normally, the maximum intensity is the intensity of the peak P 1 of the crystal grains.
  • the angle formed between the width direction and the direction indicating the peak of intensity calculated by the texture analysis in the case in which a series rank is 16 and the Gaussian half width is 5° for the inverse pole figure using the spherical harmonics method of the electron backscatter diffraction method may be 10° or less.
  • the angle formed between the direction indicating the peak of intensity and the width direction is an angle ⁇ 2 formed between the width direction (TD) and a direction from a center of the (0001) pole figure in the sheet thickness direction to a position indicating the peak of intensity. From the viewpoint of manufacturing and a method for observing the structure, the angle is preferably 5° or less, and more preferably 3° or less.
  • the angle formed between the sheet thickness direction and the direction indicating the peak of intensity calculated by the texture analysis in the case in which a series rank is 16 and the Gaussian half width is 5° for the inverse pole figure using the spherical harmonics method of the electron backscatter diffraction method may be 35° or less.
  • an area ratio of the ⁇ -phase is 80% or more.
  • the titanium alloy sheet according to the present embodiment contains a large amount of the a-stabilizing element in order to increase the strength. For that reason, when an amount of addition of the ⁇ -stabilizing element is further increased, the strength becomes too high, and thus manufacturing by cold rolling becomes impossible. Accordingly, in the titanium alloy sheet according to the present embodiment, the area ratio of the ⁇ -phase is 80% or more.
  • the area ratio of the ⁇ -phase may be, for example, 82% or more.
  • the upper limit of the area ratio of the ⁇ -phase is not particularly limited, and the area ratio of the ⁇ -phase may be, for example, 100% or less, or 98% or less.
  • a structure of the titanium alloy sheet according to the present embodiment includes the ⁇ -phase and the balance structure, and the balance structure contains the ⁇ -phase, TiFe, Ti 3 Al, and a silicide.
  • the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more is more than 53%.
  • the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or less is high, ductility at room temperature may be poor, and thus the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more is more than 53%.
  • the area ratio of the ⁇ -phase having the diameter of 1 ⁇ m or more may be 55% or more, or may be 60% or more.
  • An upper limit of the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more is not particularly limited, and the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more may be, for example, 98% or less.
  • the microstructure of the titanium alloy sheet according to the present embodiment is, for example, as shown in FIG. 3 .
  • the upper limit of the equivalent circle diameter of the ⁇ -phase is not particularly limited, and the equivalent circle diameter of the ⁇ -phase is, for example, 20 ⁇ m or less.
  • the area ratio of the ⁇ -phase and the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more are measured by the following method.
  • the cross-section (L cross-section) obtained by cutting the titanium alloy sheet in the thickness direction along the longitudinal direction at the central position in the width direction (TD) is chemically polished, and the crystallographic orientation analysis is performed by the EBSD method for the region of (total sheet thickness) ⁇ 200 ⁇ m in the cross-section while targeting about 2 to 5 fields of view at steps of 1 to 5 ⁇ m.
  • the ⁇ -phase is identified through the crystallographic orientation analysis by this EBSD.
  • An area ratio of the ⁇ -phase to an area of the region is defined as the area ratio of the ⁇ -phase.
  • Crystal grains of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more include a band structure, which will be described later.
  • the area ratio of the ⁇ -phase and the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more are measured on the basis of the L cross-section at the central position in the width direction, but the ⁇ -phase is uniformly distributed in the width direction, and thus the area ratio of the ⁇ -phase and the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more may be measured on the basis of the L cross-section at an arbitrary sheet width position.
  • the titanium alloy sheet according to the present embodiment has a microstructure including an equiaxed structure with an aspect ratio of 3.0 or less and a longitudinally elongated band structure with an aspect ratio of more than 3.0, in which the average grain size of the equiaxed structure is 0.1 ⁇ m or more and 20.0 ⁇ m or less, and the area ratio of the band structure to an area of the microstructure is preferably or less.
  • a microstructure including an equiaxed structure with an aspect ratio of 3.0 or less and a longitudinally elongated band structure with an aspect ratio of more than 3.0, in which the average grain size of the equiaxed structure is 0.1 ⁇ m or more and 20.0 ⁇ m or less, and the area ratio of the band structure to an area of the microstructure is preferably or less.
  • a titanium alloy When a titanium alloy is hot-rolled at a temperature in an ⁇ + ⁇ range or a ⁇ range, it may form a structure called a “band structure” as shown in FIG. 4 .
  • the band structure mentioned here is, for example, a longitudinally elongated structure as shown in FIG. 4 .
  • the band structure indicates crystal grains having an aspect ratio of more than 3.0, which is represented by the major axis/minor axis of a crystal grain.
  • the titanium alloy sheet according to the present embodiment may have a longitudinally elongated band structure. Formation of the band structure may cause strength anisotropy or defects during molding. For that reason, it is preferable that the band structure be as small as possible.
  • the area ratio of the band structure to the area of the microstructure is preferably 10.0% or less.
  • the area ratio of the band structure is more preferably 8.0% or less.
  • it is better not to have this band structure its lower limit is 0%.
  • the aspect ratio and the area ratio of the band structure can be calculated as follows.
  • the cross-section (L cross-section) obtained by cutting the titanium alloy sheet in the sheet thickness direction along the longitudinal direction at the central position in the width direction (TD) is chemically polished, and the crystallographic orientation analysis is performed by the EBSD method for the region of (total sheet thickness) ⁇ 200 in the cross-section while targeting about 2 to 5 fields of view at steps of 1 to 5 ⁇ m. From results of the crystallographic orientation analysis by this EBSD, the aspect ratio is calculated for each crystal grain. After that, the area ratio of crystal grains with an aspect ratio exceeding 3.0 is calculated.
  • the aspect ratio and the area ratio of the band structure are calculated on the basis of the L cross-section at the central position in the width direction, but the band structure is uniformly distributed in the width direction, and thus the aspect ratio and the area ratio of the band structure may be calculated on the basis of the L cross-section at an arbitrary sheet width position.
  • the balance of the microstructure other than the band structure is preferably an equiaxed structure formed by recrystallization.
  • the titanium alloy sheet preferably has the equiaxed structure, and in particular, the titanium alloy sheet preferably has fine grains because it may be formed by utilizing its superplasticity.
  • the average grain size of the equiaxed structure is preferably 20.0 ⁇ m or less.
  • the average grain size of the equiaxed structure is more preferably 15.0 ⁇ m or less, still more preferably 10.0 ⁇ m or less, and even more preferably 8.0 ⁇ m or less.
  • the average grain size of the equiaxed structure is less than 0.5 ⁇ m, the strength may become too large due to a grain refinement effect, and the ductility may be significantly lowered. As a result, in particular, cold (room temperature) workability may deteriorate. For that reason, the average grain size of the equiaxed structure is preferably 0.5 ⁇ m or more. The average grain size of the equiaxed structure is more preferably 1.0 ⁇ m or more.
  • more than 80% of the equiaxed structure and the band structure are ⁇ -phases, and there are ⁇ -phases between the ⁇ -phases.
  • Presence or absence of recrystallized grains can be determined by measuring aspect ratios (ratios of the major axis/minor axis) of crystal grains. If the aspect ratio is 3.0 or less, the grain can be determined to be a recrystallized grain. Also, the lower limit of the aspect ratio of the equiaxed structure is 1.0.
  • the average grain size of the equiaxed structure can be calculated as follows.
  • the smaller of a 0.2% proof stress in the longitudinal direction at 25° C. and a proof stress in the width direction at 25° C. of the titanium alloy sheet according to the present embodiment is preferably 700 MPa or more.
  • the smaller of the 0.2% proof stress in the longitudinal direction and the 0.2% proof stress in the width direction is simply referred to as a 0.2% proof stress.
  • tensile strength close to the tensile strength at 25° C. of Ti—6Al—4V which is a general-purpose ⁇ + ⁇ type titanium alloy, is often required. If the 0.2% proof stress of the titanium alloy sheet is 700 MPa or more, it can be used for applications requiring high strength.
  • the 0.2% proof stress at 25° C. of the titanium alloy sheet is preferably 1200 MPa or less.
  • the 0.2% proof stress at 25° C. of the titanium alloy sheet is more preferably 1150 MPa or less. Further, if the 0.2% proof stress at 25° C.
  • the 0.2% proof stress at 25° C. of the titanium alloy sheet is even more preferably 1100 MPa or less.
  • the 0.2% proof stress can be measured by a method based on JIS Z2241:2011. That is, the 0.2% proof stress in the longitudinal direction and the 0.2% proof stress in the width direction can be measured by a method based on JIS Z2241:2011.
  • the longitudinal direction mentioned here is the final rolling direction. For those skilled in the art, the final rolling direction is easy to identify and the final rolling direction is obvious.
  • a proof stress ratio ⁇ T/ ⁇ L which is a ratio of a 0.2% proof stress ⁇ in the width direction at 25° C. to a proof stress ⁇ L the longitudinal direction at 25° C. is preferably 0.85 or more and 1.18 or less. Since ⁇ + ⁇ type titanium has an hcp-phase ( ⁇ -phase) as described above, it exhibits higher anisotropy in a hcp direction. As described above, since the anisotropy increases when the T-texture is formed, it is sometimes desired to reduce the anisotropy as much as possible, especially in the field of aircrafts.
  • the proof stress ratio ⁇ T/ ⁇ L is more preferably 1.16 or less, even more preferably 1.15 or less, and still more preferably 1.14 or less.
  • the proof stress ratio ⁇ T/ ⁇ L can be set to 0.85 or more and 1.10 or less.
  • the proof stress ratio ⁇ T/ ⁇ L of the titanium alloy sheet manufactured by cold cross-rolling is preferably 0.90 or more, and more preferably 0.95 or more.
  • the proof stress ratio ⁇ T/ ⁇ L of the titanium alloy sheet manufactured by cold cross-rolling is preferably 1.05 or less. In the case of cold rolling in one direction in the longitudinal direction, it is difficult to make the proof stress ratio ⁇ T/ ⁇ L less than 1.05, and it is possible to set it to 1.05 or more. Also, since the titanium alloy sheet having a proof stress ratio ⁇ T/ ⁇ L of greater than 1.18 can be manufactured by cold rolling in one direction, ⁇ T/ ⁇ L may be greater than 1.18.
  • the average sheet thickness of the titanium alloy sheet according to the present embodiment is 2.5 mm or less.
  • the average sheet thickness of the titanium alloy sheet can be reduced to 2.5 mm or less by using the titanium material containing the above chemical components by a method for manufacturing the titanium alloy sheet, which will be described later.
  • a titanium material having an Al content of more than 4.0% and 6.6% or less has large deformation resistance, and thus in a general rolling mill, an allowable load of the rolling mill may be exceeded when a sheet is manufactured. For that reason, it is difficult to manufacture the titanium alloy sheet containing the above chemical components and having a sheet thickness of 2.5 mm or less.
  • a temperature drops sharply, and thus deformation resistance increases.
  • the allowable load of the rolling mill may be exceeded, and it is difficult to reduce the average sheet thickness to 2.5 mm or less.
  • the average sheet thickness of the titanium alloy sheet according to the present embodiment is preferably 0.1 mm or more.
  • the thickness of the titanium alloy sheet according to the present embodiment is preferably 2.0 mm or less, and more preferably 1.5 mm or less.
  • the average sheet thickness of the titanium alloy sheet according to the present embodiment is more preferably 0.2 mm or more.
  • Sheet thicknesses at each of the central position in the width direction (TD) and positions at a distance of 1 ⁇ 4 of a sheet width from both ends in the width direction are measured at five or more locations at intervals of 1 m or more in the longitudinal direction using X-rays, a micrometer, or a vernier caliper, and the average value of the measured sheet thicknesses is set to the average sheet thickness.
  • Dimensional accuracy of the sheet thickness of the titanium alloy sheet according to the present embodiment is preferably 5.0% or less with respect to the average sheet thickness.
  • sheet thickness dimensional accuracy is preferably 5.0% or less with respect to the average sheet thickness.
  • a titanium alloy sheet is manufactured by hot rolling titanium materials that are laminated in multiple layers and wrapped by steel materials, but deformation resistance of the titanium materials laminated in multiple layers varies greatly depending on a temperature distribution, and thus it is difficult to manufacture a sheet with a uniform sheet thickness.
  • the titanium alloy sheet according to the present embodiment is manufactured through the cold rolling, which will be described later, it becomes a titanium alloy sheet having excellent sheet thickness dimensional accuracy.
  • the dimensional accuracy of the titanium alloy sheet according to the present embodiment is more preferably 4.0% or less with respect to the average sheet thickness, and even more preferably 2.0% or less with respect to the average sheet thickness.
  • the sheet thickness dimensional accuracy is measured by the following method.
  • the sheet thicknesses at each of the central position in the width direction (TD) and the positions at a distance of 1 ⁇ 4 of the sheet width from both ends in the width direction are measured at five or more locations at intervals of 1 m or more in the longitudinal direction using X-rays, a micrometer, or a vernier caliper.
  • the maximum value of a′ calculated by the following formula (101) using an actually measured sheet thickness d and the average sheet thickness dave is defined as the sheet thickness dimensional accuracy a.
  • the titanium alloy sheet according to the present embodiment has been described above. Since the titanium alloy sheet according to the present embodiment has the above chemical components and metal structure, it has high strength.
  • the titanium alloy sheet according to the present embodiment described above may be manufactured by any method and can also be manufactured, for example, by a method for manufacturing the titanium alloy sheet according to the present embodiment described below.
  • the method for manufacturing the titanium alloy sheet according to the present embodiment includes: a slab manufacturing process of manufacturing a titanium alloy slab; a hot rolling process of hot rolling the titanium alloy slab; a cold rolling process of cold rolling the titanium material after the hot rolling process; and a temper rolling or tension levelling process of temper rolling or tension levelling the titanium material after the cold rolling process depending on needs.
  • a slab manufacturing process of manufacturing a titanium alloy slab includes: a hot rolling process of hot rolling the titanium alloy slab; a hot rolling process of hot rolling the titanium alloy slab; a cold rolling process of cold rolling the titanium material after the hot rolling process; and a temper rolling or tension levelling process of temper rolling or tension levelling the titanium material after the cold rolling process depending on needs.
  • a temper rolling or tension levelling process of temper rolling or tension levelling the titanium material after the cold rolling process depending on needs.
  • the titanium alloy slab is manufactured.
  • the method for manufacturing the titanium alloy slab is not particularly limited, and for example, it can be manufactured according to the following procedure.
  • an ingot is produced from sponge titanium by various melting methods such as a vacuum arc melting method, an electron beam melting method, a hearth melting method such as a plasma melting method, and the like.
  • the titanium alloy slab can be obtained by hot forging the obtained ingot at a temperature in a ⁇ -phase high-temperature range, an ⁇ + ⁇ two-phase range, or a ⁇ -phase single phase range.
  • the titanium alloy slab may be subjected to pretreatment such as cleaning treatment and cutting, if necessary.
  • the manufactured titanium alloy slab contains, in % by mass, Al: more than 4.0% and 6.6% or less, Fe: 0% or more and 2.3% or less, V: 0% or more and 4.5% or less, Si: 0% or more and 0.60% or less, C: 0% or more and less than N: 0% or more and 0.050% or less, and O: 0% or more and 0.40% or less, Ni: 0% or more and less than 0.15%, Cr: 0% or more and less than 0.25%, and Mn: 0% or more and less than 0.25%.
  • the titanium alloy slab is heated and then hot-rolled.
  • the titanium alloy slab may be heated to a temperature range equal to or higher than a ⁇ transformation point T ⁇ ° C., and then rolled such that the total rolling reduction is 80% or higher.
  • hot rolling is started at a temperature below an ⁇ + ⁇ phase temperature range, cracks may occur in the titanium alloy slab, or the metal structure described above may not be obtained even if cracks do not occur. For that reason, in the present process, hot rolling is started from a ⁇ -phase temperature range.
  • a finishing temperature which is a temperature immediately after the hot rolling, is in the ⁇ + ⁇ phase temperature range and varies depending on a composition of the titanium alloy slab, but it may be set to, for example, (T ⁇ ⁇ 250)° C. or higher and (T ⁇ ⁇ 50)° C. or lower, and hot rolling may be performed such that the rolling reduction is the above rolling reduction in one hot rolling, or may be performed multiple times to achieve the above rolling reduction.
  • the titanium material after the present hot rolling process contains, in % by mass, Al: more than 4.0% and 6.6% or less, Fe: 0% or more and 2.3% or less, V: 0% or more and 4.5% or less, Si: 0% or more and 0.60% 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 0.40% or less, Ni: 0% or more and less than 0.15%, Cr: 0% or more and less than and Mn: 0% or more and less than 0.25%.
  • the “ ⁇ transformation point” is a boundary temperature at which an ⁇ -phase begins to form when a titanium alloy is cooled from a ⁇ -phase single phase range.
  • the ⁇ transformation point can be obtained from a phase diagram.
  • the phase diagram can be obtained, for example, by a computer coupling of phase diagrams and thermochemistry (CALPHAD) method.
  • CALPHAD phase diagrams and thermochemistry
  • the phase diagram of the titanium alloy is obtained by the CALPHAD method using Thermo-Calc, which is an integrated thermodynamic calculation system manufactured by Thermo-Calc Software AB, and a predetermined database (TI3), so that the ⁇ transformation point can be calculated.
  • the titanium alloy slab can be continuously hot-rolled using known continuous hot rolling equipment.
  • the titanium alloy slab is hot-rolled and then coiled by a coiling machine to form a titanium alloy hot-rolled coil.
  • the titanium material after the hot rolling process includes a sheet-shaped titanium material and a coil-shaped titanium material more elongated than the sheet-shaped titanium material.
  • the titanium material after the hot rolling process may be subjected to annealing by a known method, removal of oxide scale and the like by pickling or cutting, or cleaning treatment and the like, if necessary.
  • the titanium material after the hot rolling process is annealed at a temperature of 650° C. or higher and 800° C. or lower for a time of 20 minutes or longer and 90 minutes or shorter.
  • the annealing may be performed in an air atmosphere, an inert atmosphere, or a vacuum atmosphere.
  • the titanium material after the hot rolling process corresponds to the titanium material according to the present disclosure.
  • the titanium material after the hot rolling process is subjected to one or more cold rolling passes in the longitudinal direction.
  • a rolling rate per cold rolling pass in the cold rolling process is more than 30%, and the total rolling rate is 60% or more.
  • the present cold rolling process causes the c axis of hcp to approach the ND.
  • the crystallographic orientation hardly changes, and the angle formed between the direction indicating the peak of intensity and the sheet thickness direction does not become 65° or less. In this case, anisotropy of the titanium alloy sheet is not improved.
  • the band structure described above is formed by hot rolling, but when the cold rolling rate per cold rolling pass and the total cold rolling rate in the cold rolling after the hot rolling are small, the band structure remains in the titanium material without being destroyed. Accordingly, the rolling rate per cold rolling pass in the cold rolling process is more than 30%, and the total rolling rate is 60% or more. The total rolling rate is preferably 70% or more.
  • a cold rolling pass mentioned here indicates continuously performed cold rolling. Specifically, a cold rolling pass indicates cold rolling from after the hot rolling process until the titanium material reaches a final product thickness or from after the hot rolling process to before a temper rolling process, which will be described later, in the case of performing the temper rolling process after the hot rolling process.
  • cold rolling from after the hot rolling process to the intermediate annealing treatment and cold rolling from the intermediate annealing treatment until the titanium material reaches the final product thickness or to before the temper rolling process are respectively called a cold rolling pass.
  • cold rolling from the previous intermediate annealing treatment to the subsequent intermediate annealing treatment is also called a cold rolling pass.
  • the rolling rate of each cold rolling mill may be any rate as long as the rolling rate per pass is more than 30%.
  • a 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 crystal grains are refined during cold rolling, making it easier to develop superplasticity.
  • the cold rolling temperature is more preferably 400° C. or lower.
  • the lower limit of the cold rolling temperature is not particularly limited, and the cold rolling temperature can be, for example, room temperature or higher. The room temperature here is intended to 0° C. or higher.
  • the cold rolling process preferably includes an intermediate annealing process for annealing the titanium material between the plurality of cold rolling passes.
  • an intermediate material in the cold rolling process is preferably annealed such that an annealing temperature T is 600° C. or higher and (T ⁇ ⁇ 50)° C. or lower, and an annealing temperature T (° C.) and a holding time t (seconds) at the annealing temperature T satisfy the following formula (102).
  • (T+273.15) ⁇ (Log 10 (t)+20) in the following formula (102) is a Larson-Miller parameter.
  • T ⁇ is the ⁇ transformation point (° C.).
  • a final annealing process is a process of annealing the titanium material after a final cold rolling pass.
  • Annealing conditions in the final annealing process are not particularly limited, but in order to improve moldability of the titanium alloy sheet, an annealing temperature T is 600° C. or higher and (T ⁇ ⁇ 50)° C. or lower, and an annealing temperature T (° C.) and a holding time t (seconds) at the annealing temperature T preferably satisfy the above formula (102).
  • the annealing temperature is just below the ⁇ transformation point, a bimodal structure in which the equiaxed structure and the acicular structure are mixed is formed.
  • Acicular and bimodal structures may cause internal cracks and edge cracks during cold rolling.
  • the acicular or bimodal structure often has coarse grains, making it difficult to develop superplasticity.
  • the annealing temperature T is 600° C. or higher and (T ⁇ ⁇ 50)° C.
  • the annealing temperature T and the annealing time t satisfy the above formula (102)
  • the c axis of the ⁇ -phase is caused to approach the ND direction due to the recrystallization, so that the anisotropy of the titanium alloy sheet can be further reduced and the band structure in the microstructure can be further reduced.
  • the annealing temperature T and the annealing time t by determining the annealing temperature T and the annealing time t such that the annealing temperature T is 600° C. or higher and (T ⁇ ⁇ 50)° C.
  • the titanium alloy sheet is manufactured through the above cold rolling process, but the titanium alloy sheet after the cold rolling process is preferably subjected to temper rolling for adjusting mechanical properties or tension levelling for correcting its shape, if necessary.
  • the rolling reduction in the temper rolling is preferably 10% or less, and the elongation in the tension levelling is preferably 5% or less. Also, the temper rolling and the tension levelling may not be performed if unnecessary.
  • the rolling rate per pass in the cold rolling is more than 30% and the total rolling rate is 60% or more, so that the titanium alloy sheet can be obtained in which, in the (0001) pole figure in the sheet thickness direction, the angle formed between the sheet thickness direction and the direction indicating the peak of intensity calculated by the texture analysis in the case in which a series rank is 16 and the Gaussian half width is 5° for the inverse pole figure using the spherical harmonics method of the EBSD method is 65° or less.
  • the average sheet thickness of the titanium alloy sheet can be set to 2.5 mm or less, and the dimensional accuracy of the sheet thickness can be set to 5.0% or less with respect to the average sheet thickness.
  • the metal structure of the titanium alloy sheet has the microstructure including the equiaxed structure with an aspect ratio of 3.0 or less and the longitudinally elongated band structure with an aspect ratio of more than 3.0, the average grain size of the equiaxed structure is 0.1 ⁇ m or more and 20.0 ⁇ m or less, and the area ratio of the band structure to the area of the microstructure is 10.0% or less.
  • the anisotropy of the titanium alloy sheet is further reduced.
  • the ratio of the 0.2% proof stress in the width direction to the 0.2% proof stress in the longitudinal direction can be set to 1.05 or more and 1.18 or less.
  • the crystal grains become finer, making it easier to exhibit superplasticity, and thus the titanium alloy sheet has excellent workability in molding a sheet.
  • the method for manufacturing the titanium alloy sheet according to the present embodiment since it includes the unidirectional cold rolling process, it is possible to manufacture elongated titanium alloy sheets and titanium alloy coils. Accordingly, the above manufacturing method can also be said to be a method for manufacturing a titanium alloy coil. Thus, needless to say, the titanium alloy coil manufactured by the above manufacturing method has the same features as the titanium alloy sheet of the present disclosure.
  • the titanium alloy coil of the present disclosure contains, in % by mass, Al: more than 4.0% and 6.6% or less, Fe: 0% or more and 2.3% or less, V: 0% or more and 4 .5% or less, Si: 0% or more and 0.60% 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 0.40% or less, Ni: 0% or more and less than 0.15%, Cr: 0% or more and less than Mn: 0% or more and less than 0.25%, and a remainder of Ti and impurities, in which the area ratio of the ⁇ -phase is 80% or more, the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more is more than 53%, and in the (0001) pole figure in the sheet thickness direction, the angle formed between the sheet thickness direction and the direction indicating the peak of intensity calculated by the texture analysis in the case in which a series rank is 16 and the Gaussian half width is
  • the “longitudinal direction” corresponds to a longitudinal direction of the titanium alloy coil
  • the “width direction” corresponds to a direction perpendicular to a longitudinal direction of a rolled surface of the titanium alloy coil.
  • the second manufacturing method is different in the cold rolling process from the first manufacturing method, and other processes are the same as those of the first manufacturing method. For that reason, the cold rolling process will be described in detail here, and description of other processes will be omitted.
  • the cold rolling process in the second manufacturing method is a cold cross-rolling process in which the titanium material after the hot rolling process is subjected to cold rolling passes in the longitudinal direction and the width direction.
  • the total rolling rate including both rolling in the longitudinal direction and rolling in the width direction in the present process is 60% or more.
  • a final rolling direction in the present process is the longitudinal direction, and a direction orthogonal to the longitudinal direction is the width direction. If the total rolling rate is 60% or more, the c axis of hcp is more oriented in the ND direction, so that the titanium alloy sheet with small anisotropy can be manufactured. As the rolling rate increases, the c axis of the ⁇ -phase of the titanium alloy sheet approaches the sheet thickness direction and intensity increases, and thus an upper limit of the rolling rate is not restricted.
  • a cross-rolling ratio is not particularly limited and is, for example, 0.05 or more and 20.00 or less.
  • the cross-rolling ratio mentioned here indicates a rolling rate in the longitudinal direction to a rolling rate in the width direction (longitudinal rolling rate/widthwise rolling rate) performed until the sheet thickness reaches a target thickness from 4 mm. If the cross-rolling ratio is 0.05 or more and 20.00 or less, the c axis of hcp is more oriented toward ND, and the sheet with small anisotropy can be manufactured. In addition, it is possible to reduce excessively generated band structures.
  • the cross-rolling ratio is more preferably 0.07 or more and 15.00 or less.
  • a rolling rate per cold rolling pass is not particularly limited as long as the total rolling rate is 60% or more.
  • one cold rolling pass indicates cold rolling in the longitudinal direction or cold rolling in the width direction continuously performed on the hot-rolled sheet. Accordingly, in the present cold cross-rolling process, in a case in which each of the cold rolling in the longitudinal direction and the cold rolling in the width direction is performed a plurality of times for the hot-rolled sheet, the total number of times is the number of cold rolling passes. For example, in the case of performing the cold rolling in the longitudinal direction once and the cold rolling in the width direction once for the hot-rolled sheet, the number of cold rolling passes is two. In the second manufacturing method, the rolling in the longitudinal direction or rolling in the width direction may be performed a plurality of times. Also, even if the sheet thickness is 4 mm or less, reheating or the like may be performed. Further, hot rolling in the width direction may be performed whenever hot rolling in the longitudinal direction is performed once or several times.
  • the rolling in the width direction may be performed at any timing.
  • the rolling rate per cold rolling pass is not particularly limited and can be, for example, 5% or more.
  • the rolling rate per cold rolling pass is preferably 10% or more, more preferably 20% or more.
  • the rolling rate per cold rolling pass may be 80% or less, or may be 75% or less.
  • a rolling temperature in the cold cross-rolling process is preferably 500° C. or lower. If the rolling temperature is 500° C. or lower, high dimensional accuracy can be obtained, and crystal grains are refined during rolling.
  • the rolling temperature is more preferably 400° C. or lower.
  • the lower limit of the cold rolling temperature is not particularly limited, and the cold rolling temperature can be, for example, room temperature or higher. The room temperature here is intended to 0° C. or higher.
  • the titanium alloy sheet can be obtained in which, in the (0001) pole figure in the sheet thickness direction, the angle formed between the sheet thickness direction and the direction indicating the peak of intensity calculated by the texture analysis in the case in which a series rank is 16 and the Gaussian half width is 5° for the inverse pole figure using the spherical harmonics method of the electron backscatter diffraction method is 35° or less, and the ratio of the proof stress in the width direction to the 0.2% proof stress in the longitudinal direction is 0.85 or more and 1.10 or less.
  • the titanium material contains a large amount of ⁇ -phase stabilizing elements such as V
  • the T-texture is easily formed, and the anisotropy of the titanium alloy sheet tends to increase.
  • the second manufacturing method since the cold cross-rolling is performed, formation of the T-texture is inhibited even in the case in which the titanium material contains the ⁇ -phase stabilizing elements such as V. As a result, the titanium alloy sheet with small anisotropy can be manufactured.
  • the metal structure of the titanium alloy sheet has the microstructure including the equiaxed structure with an aspect ratio of 3.0 or less and the longitudinally elongated band structure with an aspect ratio of more than 3.0, the average grain size of the equiaxed structure is 0.1 ⁇ m or more and 20.0 ⁇ m or less, and the area ratio of the band structure to the area of the microstructure is 10.0% or less.
  • the anisotropy of the titanium alloy sheet is further reduced.
  • a titanium alloy ingot serving as a material for titanium alloy sheets having the chemical components shown in Table 1 was manufactured by any one of vacuum arc remelting (VAR), electron beam remelting (EBR), and plasma arc melting (PAM), and 150 mm thick ⁇ 800 mm wide ⁇ 5000 mm long titanium alloy slabs were then manufactured by blooming or forging. After that, these titanium alloy slabs were subjected to hot rolling, hot-rolled sheet annealing, shot blasting, and pickling to obtain hot-rolled sheets having a thickness of 4 mm. In the hot rolling, the titanium alloy slabs were heated to 1050 to 1100° C.
  • a phase diagram of a titanium alloy was obtained by the CALPHAD method using Thermo-Calc, which is an integrated thermodynamic calculation system manufactured by Thermo-Calc Software AB, and a predetermined database (TB) to calculate the ⁇ transformation point T ⁇ .
  • Inventive Examples 1 to 18, 30, and Comparative Example 3 in Table 2 are examples obtained by repeating cold rolling at a rolling rate per cold rolling pass of 35 to 60% and intermediate annealing under the conditions shown in Table 2, and performing cold rolling until the total rolling rate reaches 70 to 94%.
  • Inventive Example 19 is an example obtained by repeating cold rolling at a rolling rate per cold rolling pass of 35% and intermediate annealing under the conditions shown in Table 2, and performing cold rolling until the total rolling rate reaches 60%.
  • Inventive Example 20 is an example obtained by performing cold rolling at a cold rolling temperature of 300° C.
  • Inventive Example 21 is an example obtained by repeating cold rolling at a rolling rate per cold rolling pass of 40% and intermediate annealing under the conditions shown in Table 2, and performing cold rolling until the total rolling rate reaches 78%.
  • the intermediate annealing process in Inventive Example 21 is an example of not satisfying the above formula (102).
  • Inventive Examples 22 and 23 are examples obtained by performing cold rolling respectively at rolling rates of 75% and 60% without intermediate annealing.
  • Inventive Examples 24 to 26 are examples obtained by performing cold rolling at a rolling rate in a first cold rolling pass of 75%, then performing intermediate annealing under the conditions shown in Table 2, and subsequently performing cold rolling at a rolling rate in a second cold rolling pass of 50%, thereby adjusting the total rolling rate to 88%.
  • Inventive Examples 27 to 29 are examples obtained by performing cold rolling at a rolling rate in the first cold rolling pass of 50%, then performing a first intermediate annealing under the conditions shown in Table 2, performing cold rolling at a rolling rate in the second cold rolling pass of 50%, performing a second intermediate annealing under the conditions shown in Table 2 after the second cold rolling pass, and performing cold rolling at a rolling rate in a third cold rolling pass of 60%, thereby adjusting the total rolling rate to 90%.
  • the reference example is a hot-rolled sheet that was not subjected to the cold rolling process.
  • Comparative Example 1 is an example in which a rolling rate per pass is 20% and the total rolling rate is 59%.
  • Comparative Example 2 is an example in which the total rolling rate is 50%.
  • Comparative Example 4 using a titanium material O with a high Al content, surface cracks and severe edge cracks occurred during cold rolling after hot rolling. For that reason, in Comparative Example 4, intermediate annealing and final annealing were not performed.
  • T ⁇ is the ⁇ transformation point
  • “Larson-Miller parameter” is the value of (T+273.15) ⁇ (Log 10 (t)+20).
  • attern A indicates a cold rolling pattern in which cold rolling was performed at a rolling rate in the first cold rolling pass of 75% and a rolling rate in the second cold rolling pass of 50%.
  • “Pattern B” in Table 2 indicates a cold rolling pattern in which cold rolling was performed at a rolling rate in the first cold rolling pass of 50%, a rolling rate in the second cold rolling pass of 50%, and a rolling rate in the third cold rolling pass of 60%.
  • An observation surface of a sample of the titanium alloy sheet according to each of the inventive examples, the reference example, and the comparative examples was chemically polished, and crystallographic orientation analysis was performed using the electron backscatter diffraction method, thereby obtaining a (0001) pole figure.
  • an L cross-section is chemically polished at a central position in a width direction (TD) of each sample, and in the cross-section, crystallographic orientation analysis was performed by the EBSD method targeting about 2 to 10 fields of view at intervals of 1 to 2 ⁇ m in a region of (total sheet thickness) ⁇ 2 mm, and thus the (0001) pole figure was drawn.
  • the area ratio of the ⁇ -phase and an area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more were measured by the following method.
  • the titanium alloy sheet was chemically polished at its cross-section cut perpendicularly to the width direction at the central position in the width direction (TD), and crystallographic orientation analysis was performed by the EBSD method targeting about 2 to 5 fields of view at steps of 1 to 5 ⁇ m in a region of (total sheet thickness) ⁇ 200 ⁇ m in the cross-section.
  • An area ratio of the ⁇ -phase with respect to an area of the region was defined as the area ratio of the ⁇ -phase.
  • the sample of each of the titanium alloy sheets was chemically polished at the cross-section cut perpendicularly to the width direction at the central position in the width direction (TD), and crystallographic orientation analysis is performed by the EBSD method targeting about 2 to 5 fields of view at steps of 1 to 5 ⁇ m in the region of (total sheet thickness) ⁇ 200 ⁇ m in the cross-section. From results of the crystallographic orientation analysis by the EBSD, an aspect ratio was calculated for each crystal grain.
  • the 0.2% proof stress ⁇ at 25° C. of the titanium alloy sheet according to each of the inventive examples, the reference example, and the comparative examples was measured based on JIS Z 2241:2011.
  • the average sheet thickness dave of the titanium alloy sheet according to each of the inventive examples, the reference example, and the comparative examples was measured by the following method. Sheet thicknesses at each of a central position in the width direction and positions at a distance of 1 ⁇ 4 of a sheet width from both ends in the width direction of each of the manufactured titanium alloy sheets were measured using X-rays, a micrometer, or a vernier caliper at 5 or more locations at intervals of 1 m or more in the longitudinal direction, and the average value of the measured sheet thicknesses was set to the average sheet thickness dave.
  • the sheet thickness dimensional accuracy a of the titanium alloy sheet according to each of the inventive examples, the reference example, and the comparative examples is obtained such that, using a sheet thickness d actually measured by the above method and the average sheet thickness dave, the maximum value of a′ calculated by the following formula (101) was defined as the dimensional accuracy a.
  • ⁇ shown in Table 3 is the angle formed between the sheet thickness direction and the direction indicating the peak of intensity calculated by the texture analysis in the case in which a series rank is 16 and the Gaussian half width is 5° for the inverse pole figure using the spherical harmonics method of the electron backscatter diffraction method in the (0001) pole figure in the sheet thickness direction.
  • ⁇ 2 shown in Table 3 is an angle formed between the sheet width direction and a direction from a center of the pole figure indicating the peak of intensity calculated by the texture analysis in the case in which a series rank is 16 and the Gaussian half width is 5° for the inverse pole figure using the spherical harmonics method of the electron backscatter diffraction method in the (0001) pole figure in the sheet thickness direction.
  • the angle ⁇ formed between the direction indicating the peak of intensity in the (0001) pole figure and the sheet thickness direction was 65° or less
  • the angle ⁇ 2 formed between the direction indicating the peak of intensity and the width direction was 0°
  • the average grain size of the equiaxed structure was 0.1 ⁇ m or more and 20.0 ⁇ m or less
  • the area ratio of the band structure was 10% or less.
  • the area ratio of the ⁇ -phase was 80% or more in all cases, and the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more was more than 53%.
  • the average sheet thickness was 1.0 to 1.2 mm, and the dimensional accuracy was 0.8 to 4.5%.
  • the 0.2% proof stress in the longitudinal direction at 25° C. was 700 Mpa or more
  • the proof stress ratio ⁇ T/ ⁇ L which is the ratio of the 0.2% proof stress GT in the width direction at 25° C. to the 0.2% proof stress ⁇ L in the longitudinal direction at 25° C., was 1.05 or more and 1.18 or less.
  • the angle ⁇ formed between the direction indicating the peak of intensity in the (0001) pole figure and the sheet thickness direction is 49°
  • the angle ⁇ 2 formed between the direction indicating the peak of intensity and the width direction was 0°
  • the average grain size of the equiaxed structure was 1.8 ⁇ m
  • the area ratio of the band structure was 5.0%
  • the area ratio of the ⁇ -phase was 88% or more
  • the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 or more was 88%.
  • the average sheet thickness was 0.9 mm, and the dimensional accuracy was 2.0%.
  • the 0.2% proof stress at 25° C. was 805 Mpa
  • the proof stress ratio ⁇ T/ ⁇ L was 1.12.
  • the angle ⁇ formed between the direction indicating the peak of intensity in the (0001) pole figure and the sheet thickness direction was 50°
  • the angle ⁇ 2 formed between the direction indicating the peak of intensity and the width direction was 0°
  • the average grain size of the equiaxed structure in Inventive Example 22 was 3.5 ⁇ m
  • the average grain size of the equiaxed structure in Inventive Example 23 was 10.5 ⁇ m.
  • the area ratios of the band structure were and 20.0%, respectively.
  • the area ratios of the ⁇ -phase were 80% or more, and the area ratios of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more were more than 53%.
  • the average sheet thickness was 1.0 mm and 1.6 mm, and the dimensional accuracy was 2.0% and 2.5%.
  • the 0.2% proof stress at was 700 MPa or more
  • the proof stress ratio ⁇ T/ ⁇ L was 1.11 and 1.15.
  • the angle ⁇ formed between the direction indicating the peak of intensity in the (0001) pole figure and the sheet thickness direction was 65° or less
  • the angle ⁇ 2 formed between the direction indicating the peak of intensity and the width direction was 0°
  • the average grain size of the equiaxed structure was 0.1 ⁇ m or more and 20.0 ⁇ m or less
  • the area ratio of the band structure was 10% or less.
  • the area ratio of the ⁇ -phase was 80% or more in all cases, and the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more was more than 53%.
  • the average sheet thickness was all 0.5 mm, and the dimensional accuracy was all 1.0.
  • the 0.2% proof stress in the longitudinal direction at 25° C. was 700 MPa or more
  • the proof stress ratio ⁇ T/ ⁇ L was 1.05 or more and 1.18 or less.
  • the angle ⁇ formed between the direction indicating the peak of intensity in the (0001) pole figure and the sheet thickness direction was 65° or less
  • the angle ⁇ 2 formed between the direction indicating the peak of intensity and the width direction was 0°
  • the average grain size of the equiaxed structure was 0.1 ⁇ m or more and 20.0 ⁇ m or less
  • the area ratio of the band structure was 10% or less.
  • the area ratio of the ⁇ -phase was 80% or more in all cases, and the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more was more than 53%.
  • the average sheet thickness was all 0.4 mm, and the dimensional accuracy was all 1.0% or less.
  • the 0.2% proof stress in the longitudinal direction was 700 MPa or more
  • the proof stress ratio ⁇ T/ ⁇ L was 1.05 or more and 1.18 or less.
  • the angle ⁇ formed between the direction indicating the peak of intensity in the (0001) pole figure and the sheet thickness direction was 45°
  • the angle ⁇ 2 formed between the direction indicating the peak of intensity and the width direction was 0°
  • the average grain size of the equiaxed structure was 3.5 ⁇ m
  • the area ratio of the band structure was 5.0%
  • the area ratio of the ⁇ -phase was 85% or more
  • the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more was 80%.
  • the average sheet thickness was 1.0 mm, and the dimensional accuracy was 1.5%.
  • the 0.2% proof stress was 800 MPa
  • the proof stress ratio ⁇ T/ ⁇ L was 1.14.
  • the angle ⁇ formed between the direction indicating the peak of intensity in the (0001) pole figure and the sheet thickness direction was more than 65°. For that reason, the proof stress ratio ⁇ T/ ⁇ L exceeded 1.18, indicating strong anisotropy.
  • the rolling rate per pass was as small as 20%, and the total rolling rate was also as small as 59%.
  • the angle ⁇ formed between the direction indicating the peak of intensity in the (0001) pole figure and the sheet thickness direction was more than 65°.
  • the proof stress ratio ⁇ T/ ⁇ L exceeded 1.18, indicating strong anisotropy.
  • the rolling rate per pas was 50%, intermediate annealing and cold rolling were not repeated, and the total rolling rate was as small as 50%. For that reason, the angle ⁇ formed between the direction indicating the peak of intensity in the (0001) pole figure and the sheet thickness direction was more than 65°.
  • Inventive Examples 31 to 37 in Table 2 a plurality of cold rolling passes were performed such that the rolling rate per cold rolling pass was set to 5% or more to obtain the total rolling rate shown in Table 4.
  • Inventive Examples 31 to 35 in Table 4 are examples obtained by repeating a plurality of cold rolling passes at a rolling temperature of 25° C. and intermediate annealing under the conditions shown in Table 2, and performing cold cross-rolling until the total rolling rate reaches 60 to 75%.
  • the intermediate annealing was performed at a temperature of 680 to 900° C. for 60 to 28800 s, and final annealing was performed at a temperature of 650 to 930° C. for 120 to 28800 s.
  • the cross-rolling ratios of Inventive Examples 32 to 36 were set to 0.4 to 7.0.
  • Inventive Example 36 is an example obtained by repeating a plurality of cold rolling passes at a rolling temperature of 400° C. and intermediate annealing under the conditions shown in Table 4, and performing cold cross-rolling until the total rolling rate reaches 75%.
  • the intermediate annealing was performed at a temperature of 800° C. for 120 s, and the final annealing was performed at a temperature of 850° C. for 120 s.
  • the cross-rolling ratio of Inventive Example 36 was set to 13.0.
  • Inventive Example 37 is an example obtained by repeating a plurality of cold rolling passes at a rolling temperature of 25° C. and intermediate annealing under the conditions shown in Table 4, and performing cold cross-rolling until the total rolling rate reaches 62%.
  • the intermediate annealing was performed at a temperature of 800° C. for 120 s, and the final annealing was performed at a temperature of 850° C. for 120 s.
  • the cross-rolling ratio of Inventive Example 37 was set to 0.17. In this case, rolling in a cross direction was performed while cutting is appropriately performed in accordance with the width of a rolling roll to obtain a rollable size.
  • the angle ⁇ formed between the direction indicating the peak of intensity in the (0001) pole figure and the sheet thickness direction was 35° or less.
  • the average grain size of the equiaxed structure was 0.1 ⁇ m or more and 10.0 ⁇ m or less, and the area ratio of the band structure was 10% or less.
  • the area ratio of the ⁇ -phase was 80% or more in all cases, and the area ratio of the ⁇ -phase having an equivalent circle diameter of 1 ⁇ m or more was more than 53%.
  • the average sheet thickness was 1.0 to 1.8 mm, and the dimensional accuracy was 1.5 to 3.5% or less.
  • the proof stress ratio ⁇ T/ ⁇ L which is the ratio of the 0.2% proof stress GT in the width direction at 25° C. to the 0.2% proof stress GL in the longitudinal direction at 25° C., was 0.85 or more and 1.10 or less.

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