JP7633542B2 - Titanium alloy plate, titanium alloy coil, and method for producing titanium alloy plate and method for producing titanium alloy coil - Google Patents

Description

The present disclosure relates to titanium alloy plates and titanium alloy coils, as well as methods for manufacturing titanium alloy plates and titanium alloy coils.

Titanium is a lightweight, high-strength, corrosion-resistant material that can be applied to the aircraft industry from the perspective of reducing weight and improving fuel efficiency. For this reason, titanium alloys that meet the characteristics required for each component of an aircraft are being actively developed.

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

Patent Document 2 discloses an α+β type titanium alloy bar material containing 0.5% or more and less than 1.4% Fe, 4.4% or more and less than 5.5% Al, the remainder being titanium and impurities.

Patent Document 3 discloses a method for manufacturing Ti-6Al-4V alloy thin plate by pack rolling, in which one or more plate-shaped core materials are covered with spacer material and cover material to form a packed rolled material, and this packed material is rolled to reduce the thickness of the core material, characterized in that the thickness of the cover material is set so that the ratio of the core material to the pack material is at least 0.25 for the initial thickness of each plate.

Patent Document 4 discloses a method for manufacturing thin plate by pack rolling, in which one or more plate-shaped core materials are covered with a spacer material and a cover material to form a pack material, and this pack material is rolled to reduce the thickness of the core material, characterized in that the rolling reduction ratio per pass is 15% or more for rolling in which the reduction ratio of the plate thickness before and after reduction of the pack material is 3 or more.

Patent Document 5 discloses a method for producing a titanium alloy sheet, which comprises cold rolling a hot-rolled annealed sheet of a titanium alloy consisting of, by weight, 2.5-3.5% Al, 2.0-3.0% V, with the balance being Ti and ordinary impurities, in the same direction as the hot rolling direction at a total rolling reduction of 67% or more, and then annealing the sheet at a temperature between 650 and 900°C.

Patent Document 6 discloses a method for producing an α+β type titanium alloy thin sheet, characterized in that in the manufacturing process of an α+β type titanium alloy cold-rolled sheet, intermediate annealing is carried out after cold rolling under the following conditions: annealing temperature: a temperature range of [β transformation point - 25°C] or higher and lower than the β transformation point, annealing time: 0.5 to 4 hours, cooling rate after heating and holding: 0.5 to 5°C/sec, and temperature range for cooling at the above cooling rate: up to 300°C.

Patent Document 7 discloses an α+β type titanium alloy thin plate that contains at least one fully dissolved β stabilizing element in an Mo equivalent of 2.0 to 4.5 mass%, at least one eutectoid β stabilizing element in an Fe equivalent of 0.3 to 2.0 mass%, at least one α stabilizing element in an Al equivalent of more than 3.0 mass% and not more than 5.5 mass%, with the balance being Ti and unavoidable impurities, and that is characterized in that the average grain size of the α phase is 5.0 μm or less, the maximum grain size of the α phase is 10.0 μm or less, the average aspect ratio of the α phase is 2.0 or less, and the maximum aspect ratio of the α phase is 5.0 or less.

Patent Document 8 describes an α+β type titanium alloy hot-rolled sheet, in which (a) the normal direction (sheet thickness direction) of the hot-rolled sheet is defined as ND, the hot-rolling direction is defined as RD, and the width direction of the hot-rolled sheet is defined as TD, and the normal direction of the (0001) plane of the α phase is defined as the c-axis orientation, the angle that the c-axis orientation makes with ND is defined as θ, and the angle that the plane including the c-axis orientation and ND makes with the plane including ND and TD is defined as Φ, and (b1) θ is 0 degrees or more and 30 degrees or less, and Φ is uniform throughout the entire circumference (-180 degrees to 180 degrees). (b2) among the (0002) reflection relative intensities of X-rays caused by crystal grains in which θ is 80 degrees or more and less than 100 degrees and Φ is within ±10 degrees, the strongest intensity is defined as XND; (b3) among the (0002) reflection relative intensities of X-rays caused by crystal grains in which θ is 80 degrees or more and less than 100 degrees and Φ is within ±10 degrees, the strongest intensity is defined as XTD; and (c) an α+β type titanium alloy sheet having excellent cold rollability and cold handleability is disclosed, characterized in that XTD/XND is 5.0 or more.

Patent Document 9 describes a high-strength α+β-type titanium alloy hot-rolled sheet containing, by mass%, 0.8-1.5% Fe, 4.8-5.5% Al, and 0.030% or less N, and containing O and N in a range that satisfies Q(%) = 0.14-0.38, where Q(%) = [O] + 2.77 × [N], where the O content (mass%) is [O] and the N content (mass%) is [N], with the balance being Ti and unavoidable impurities, and (a) the normal direction of the hot-rolled sheet is ND, the hot-rolling direction is RD, and the width direction of the hot-rolled sheet is TD, and the normal direction of the (0001) plane of the α phase is the c-axis orientation, and the c-axis orientation is ND. The angle is θ, the angle between the plane including the c-axis orientation and the ND direction and the plane including the ND and TD is φ, (b1) θ is 0 degrees or more and 30 degrees or less, and the strongest intensity of the (0002) reflection relative intensity of X-rays due to crystal grains in which φ is in the full circumference (-180 degrees to 180 degrees) is XND, (b2) θ is 80 degrees or more and less than 100 degrees, and the strongest intensity of the (0002) reflection relative intensity of X-rays due to crystal grains in which φ is in the ±10 degree range is XTD, and (c) XTD/XND is 4.0 or more. A high-strength α+β type titanium alloy plate with excellent cold coil (band) handling properties is disclosed.

Patent Document 10 discloses a method for producing an α+β type titanium alloy thin plate, which is characterized in that an α+β type titanium alloy thin plate produced by rolling or forging is subjected to cold rolling at a reduction rate of 20% or more, and then annealed at a temperature of 700°C or higher and lower than the β transformation point, thereby obtaining a plate having a fine equiaxed α structure.

Non-patent document 1 discloses an α+β titanium alloy thin plate having anisotropy in strength in the rolling direction and in the direction perpendicular to the rolling direction.

Non-patent document 2 discloses an α+β titanium alloy thin plate that has been hot rolled at a temperature higher than the β transformation point to reduce the anisotropy of strength in the rolling direction and in the direction perpendicular to the rolling direction.

Japanese Patent Application Publication No. 7-62474 Japanese Patent Application Publication No. 7-70676 Japanese Patent Publication No. 2001-300603 Japanese Patent Application Publication No. 2001-300604 Japanese Patent Application Publication No. 61-147864 Japanese Patent Publication No. 1-127653 Japanese Patent Application Publication No. 2013-227618 International Publication No. 2012/115242 International Publication No. 2012/115243 Japanese Patent Publication No. 62-33750

KOBE STEEL ENGINEERING REPORTS/Vol. 59, No. 1 (2009), P. 81-84 KOBE STEEL ENGINEERING REPORTS/Vol. 60, No. 2 (2010), P. 50-54

Titanium materials used in aircraft components that require higher strength contain a lot of Al, but because of their high deformation resistance during hot rolling or cold rolling, the allowable load of the rolling mill may be exceeded when manufacturing thin plates. For this reason, it is difficult to manufacture high-strength titanium alloy thin plates using conventional hot rolling or cold rolling methods.

The present disclosure has been made in consideration of the above problems, and an object of the present disclosure is to provide a titanium alloy plate and a titanium alloy coil having high strength, as well as a method for manufacturing the same.

The inventors have discovered that a titanium alloy sheet has high strength and excellent workability when it contains a predetermined amount of Al and has a texture in which the peak of the crystal grain density in the (0001) pole figure from the sheet thickness direction is within a predetermined angle with respect to the width direction of the final rolling. They have then discovered a method for manufacturing a titanium alloy sheet by cold rolling that can simultaneously achieve such a chemical composition and texture, and have come to the present disclosure.

The gist of the present disclosure, which has been completed based on the above findings, is as follows.
(1) A titanium alloy plate according to one embodiment of the present disclosure contains, by mass%, Al: more than 4.0% and not more than 6.6%, Fe: 0% or more and not more than 2.3%, V: 0% or more and not more than 4.5%, Si: 0% or more and not more than 0.60%, C: 0% or more and less than 0.080%, N: 0% or more and not more than 0.050%, O: 0% or more and not more than 0.40%, 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%, with the balance being Ti and and impurities, the area ratio of the α phase is 80% or more, the area ratio of the α phase having a circle equivalent diameter of 1 μm or more is more than 53%, in a (0001) pole figure from the plate thickness direction, the inverse pole figure using the spherical harmonic function method of electron backscatter diffraction has an expansion index of 16, and the angle between the direction showing the peak of the accumulation degree calculated by texture analysis when the Gaussian half width is 5° and the plate thickness direction is 65° or less, and the average plate thickness is 2.5 mm or less.
(2) The titanium alloy plate described in (1) above has a microstructure consisting of an equiaxed structure with an aspect ratio of 3.0 or less and a band structure with an aspect ratio of more than 3.0 extending in the longitudinal direction, and the average crystal grain size of the equiaxed structure is 0.5 μm or more and 20.0 μm or less, and the area ratio of the band structure to the area of the microstructure may be 10.0% or less.
(3) The titanium alloy plate according to (1) or (2) above may contain, by mass%, either Fe: 0.5% or more and 2.3% or less, or V: 2.5% or more and 4.5% or less.
(4) The titanium alloy plate according to any one of the above (1) to (3) may contain, in mass%, one or more selected from the group consisting of Ni: less than 0.15%, Cr: less than 0.25%, and Mn: less than 0.25%, instead of a part of the Fe or V.
(5) The titanium alloy plate according to any one of the above (1) to (4) may have a smaller one of 0.2% yield strength in the longitudinal direction at 25 ° C. or 0.2% yield strength in the width direction at 25 ° C. of 700 MPa or more and 1200 MPa or less.
(6) In the titanium alloy plate according to any one of the above (1) to (5), in the (0001) pole figure from the plate thickness direction, the inverse pole figure using the spherical harmonic function method of the electron backscattering diffraction method has an expansion index of 16, and the angle between the direction showing the peak of the degree of accumulation calculated by the texture analysis when the Gaussian half-width is 5° and the width direction is 10° or less, and the ratio of the 0.2% proof stress in the width direction to the 0.2% proof stress in the longitudinal direction may be 1.05 or more and 1.18 or less.
(7) In the titanium alloy plate according to any one of the above (1) to (5), in the (0001) pole figure from the plate thickness direction, the inverse pole figure using the spherical harmonic function method of the electron backscattering diffraction method has an expansion index of 16, and the angle between the direction showing the peak of the degree of accumulation calculated by the texture analysis when the Gaussian half-width is 5° and the plate thickness direction is 35° or less, and the ratio of the 0.2% proof stress in the width direction to the 0.2% proof stress in the longitudinal direction may be 0.85 or more and 1.10 or less.
(8) The titanium alloy plate according to any one of the above (1) to (7) may have a thickness dimensional accuracy of 5.0% or less with respect to the average thickness.

(9) A titanium alloy coil according to another aspect of the present disclosure contains, by mass%, Al: more than 4.0% and not more than 6.6%, Fe: 0% or more and not more than 2.3%, V: 0% or more and not more than 4.5%, Si: 0% or more and not more than 0.60%, C: 0% or more and not more than 0.080%, N: 0% or more and not more than 0.050%, O: 0% or more and not more than 0.40%, Ni: 0% or more and not more than 0.15%, Cr: 0% or more and not more than 0.25%, and Mn: 0% or more and not more than 0.25%, with the balance being Ti. and impurities, the area ratio of the α phase is 80% or more, the area ratio of the α phase having a circle equivalent diameter of 1 μm or more is more than 53%, in a (0001) pole figure from the thickness direction, the inverse pole figure using the spherical harmonic function method of electron backscatter diffraction has an expansion index of 16, and the angle between the direction showing the peak of the accumulation degree calculated by texture analysis when the Gaussian half width is 5° and the thickness direction is 65° or less, and the average thickness is 2.5 mm or less.

(10) A method for producing a titanium alloy plate according to yet another embodiment of the present disclosure is a method for producing a titanium alloy plate according to any one of the above (1) to (8), comprising the steps of: 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 0.050% or less; The method includes a cold rolling process in which one or more cold rolling passes are performed in the longitudinal direction of a titanium material containing less than 1.15%, Cr: 0% or more but less than 0.25%, and Mn: 0% or more but less than 0.25%, with the balance being Ti and impurities, and a final annealing process in which the titanium material after the last cold rolling pass is annealed, wherein the rolling reduction rate per cold rolling pass in the cold rolling process is more than 30%, and the total rolling reduction rate is 60% or more.
(11) In the manufacturing method of the titanium alloy plate described in (10) above, when the cold rolling process is performed in a plurality of cold rolling passes, the cold rolling process may include an intermediate annealing process of annealing the titanium material between the plurality of cold rolling passes, and the annealing conditions of the intermediate annealing process and the final annealing process may be such that the annealing temperature is 600°C or higher and (T _β -50)°C or lower, and the annealing temperature T (°C) and the holding time t (seconds) at the annealing temperature satisfy the following formula (1):
22000≦(T+273.15)×(Log ₁₀ (t)+20)≦27000…Formula (1)
Here, _Tβ is the β transformation point (°C).
(12) A method for producing a titanium alloy plate according to yet another embodiment of the present disclosure is a method for producing a titanium alloy plate according to any one of the above (1) to (8), comprising the steps of: 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 , but less than 0.25%, and Mn: 0% or more but less than 0.25%, with the balance being Ti and impurities; and a final annealing step of annealing the titanium material after the cold cross rolling step, wherein a total rolling reduction rate in the cold cross rolling step is 60% or more, and a cross rolling ratio, which is the ratio of the rolling reduction rate in the longitudinal direction to the rolling reduction rate in the width direction, is 0.05 or more and 20.00 or less.
(13) In the manufacturing method of the titanium alloy plate described in (12) above, when the cold rolling process or the cold cross rolling process is performed with a plurality of cold rolling passes, the cold rolling process may include an intermediate annealing process of annealing the titanium material between the plurality of cold rolling passes, and the annealing conditions of the intermediate annealing process and the final annealing process may be that the annealing temperature is 600°C or higher and (T _β -50)°C or lower, and the annealing temperature T (°C) and the holding time t (seconds) at the annealing temperature satisfy the following formula (1):
22000≦(T+273.15)×(Log ₁₀ (t)+20)≦27000…Formula (1)
Here, _Tβ is the β transformation point (°C).

(14) A further aspect of the present disclosure relates to a method for manufacturing a titanium alloy coil, which is the method for manufacturing a titanium alloy coil described in (9) above, and which comprises, in mass %, Al: more than 4.0% and not more than 6.6%, Fe: 0% or more and not more than 2.3%, V: 0% or more and not more than 4.5%, Si: 0% or more and not more than 0.60%, C: 0% or more and less than 0.080%, N: 0% or more and not more than 0.050%, O: 0% or more and not more than 0.40%, Ni: 0% or more and not more than 0.15%. %, Cr: 0% or more and less than 0.25%, and Mn: 0% or more and less than 0.25%, with the balance being Ti and impurities; and a final annealing step of annealing the titanium material after the last cold rolling pass, wherein the rolling reduction rate per cold rolling pass in the cold rolling step is more than 30%, and the total rolling reduction rate is 60% or more.

As described above, the present disclosure makes it possible to provide titanium alloy plates and titanium alloy coils having high strength, as well as methods for manufacturing the same.

FIG. 1 is an example of a (0001) pole figure from the plate thickness direction (ND) of a titanium alloy plate according to an embodiment of the present disclosure. 11 is a diagram for explaining an angle formed between a direction showing a peak of integration density and the width direction. FIG. FIG. 2 is a diagram showing an example of an optical microscope photograph of the titanium alloy plate according to the embodiment. 1 is an optical microscope photograph showing an example of a band structure. FIG. 2 is a schematic diagram for explaining a method for measuring an average plate thickness.

Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the drawings. The description will be made in the following order.
1. Titanium alloy plate 2. Manufacturing method of titanium alloy plate

<1. Titanium alloy plate>
First, the titanium alloy plate according to the present embodiment will be described with reference to Figs. 1 to 5. Fig. 1 is an example of a (0001) pole figure from the plate thickness direction (ND) of the titanium alloy plate according to the present embodiment. Fig. 2 is a diagram for explaining the angle between the direction showing the peak of the integration degree and the width direction. The (0001) pole figure from the plate thickness direction (ND) in Fig. 2 is the same as Fig. 1. Fig. 3 is a diagram showing an example of an optical microscope photograph of the titanium alloy plate 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 for explaining a method for measuring the average plate thickness. The titanium alloy plate according to the present embodiment can be manufactured by a method including a cold rolling process, as will be described in detail later.

(1.1. Chemical composition)
First, the chemical components contained in the titanium alloy plate according to this embodiment will be described. The titanium alloy plate according to this embodiment contains, in 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%, and Mn: 0% or more and less than 0.25%, with the balance being Ti and impurities. In the following description of the chemical components, unless otherwise specified, the notation "%" shall represent "mass%".

Al is an α-phase stabilizing element and has high solid solution strengthening ability. When the Al content increases, the tensile strength at room temperature increases. If the Al content exceeds 4.0%, high tensile strength can be obtained. Furthermore, the hot-rolled sheet before cold rolling can maintain high cold rolling properties. The Al content is preferably 4.5% or more, more preferably 4.6% or more. On the other hand, if the Al content exceeds 6.6%, the cold rolling properties of the hot-rolled sheet before cold rolling are significantly reduced, and regions in which Al is excessively dissolved due to solidification segregation, etc. are locally generated, and Al is regularized. This region in which Al is regularized reduces the impact toughness of the titanium alloy sheet. Therefore, the Al content is 6.6% or less, preferably 6.5% or less, and more preferably 6.4% or less.

Fe is a β-phase stabilizing element. Since Fe is an element with high solid solution strengthening ability, increasing the Fe content increases the tensile strength at room temperature. In addition, since the β phase has high workability compared to the α phase, increasing the Fe content improves the workability of the titanium alloy plate and makes it possible to improve the dimensional accuracy. Since Fe is not essential for titanium alloy plates, the lower limit of its content is 0%. However, in order to obtain the desired tensile strength while maintaining the β phase with good workability at room temperature, the Fe content is preferably 0.5% or more. The Fe content is more preferably 0.7% or more. On the other hand, since Fe is an element that is very prone to solidification segregation, if Fe is contained in excess, Fe will segregate locally, and the characteristics may vary between the part where Fe is segregated and the part where it is not segregated. In addition, if Fe is contained in excess in the titanium alloy plate, the fatigue strength may decrease. Therefore, the Fe content is preferably 2.3% or less. The Fe content is more preferably 2.1% or less, and even more preferably 2.0% or less. Incidentally, Fe is less expensive than β-phase stabilizing elements such as V and Si.

The Fe that may be contained in the titanium alloy plate according to this embodiment may be replaced with V. V is a completely soluble β-phase stabilizing element and has a solid solution strengthening ability. Since V is not essential in the titanium alloy plate, the lower limit of its content is 0%. However, in order to obtain the same solid solution strengthening ability as Fe described above, the V content is preferably 2.5% or more. The V content is more preferably 3.0% or more. Although the cost increases when Fe is replaced with V, V is less likely to segregate than Fe, so the variation in characteristics due to segregation is suppressed. As a result, it becomes easier to obtain stable characteristics in the longitudinal and width directions of the titanium alloy plate. In order to suppress the variation in characteristics due to the segregation of V, the V content is preferably 4.5% or less. As described above, V is less likely to segregate than Fe, so when manufacturing a large ingot, it is preferable that V is contained in the titanium material.

Although Si is a β-phase stabilizing element, it also dissolves in the α-phase and exhibits high solid-solution strengthening ability. As mentioned above, Fe may segregate if it is contained in a titanium alloy plate at more than 2.3%, so Si may be added as necessary to increase the strength of the titanium alloy plate. In addition, Si has a tendency to segregate in the opposite direction to O described below, and is less likely to solidify and segregate than O. Therefore, by adding an appropriate amount of Si and O to the titanium alloy plate, it is expected that high fatigue strength and tensile strength can be achieved at the same time. On the other hand, if the Si content is high, an intermetallic compound of Si called silicide is formed, and the fatigue strength of the titanium alloy plate may decrease. If the Si content is 0.60% or less, the generation of coarse silicide is suppressed, and the decrease in fatigue strength is suppressed. Therefore, the Si content is preferably 0.60% or less. The Si content is more preferably 0.50% or less, and even more preferably 0.40% or less. Since Si is not essential in the titanium alloy plate, the lower limit of the Si content is 0%, but the Si content may be, for example, 0.10% or more.

If a large amount of C is contained in a titanium alloy plate, it may reduce the ductility or workability of the titanium alloy plate. Therefore, the C content is preferably less than 0.080%. Since C is not essential in titanium alloy plate, the lower limit of its content is 0%. Note that C is an unavoidable contaminant, and the actual content is usually 0.0001% or more. The C content is more preferably 0.060% or less.

Like C, when N is contained in large amounts in titanium alloy plate, it may reduce the ductility or workability of the titanium alloy plate. Therefore, the upper limit of the N content is preferably 0.050%. Since N is not essential in titanium alloy plate, the lower limit of its content is 0%. Note that N is an unavoidable contaminant, and the actual content is usually 0.0001% or more. The N content is more preferably 0.04% or less.

Like C, when O is contained in a large amount in a titanium alloy plate, it may reduce the ductility or workability of the titanium alloy plate. Therefore, the upper limit of the O content is preferably 0.40%, more preferably 0.38%, and even more preferably 0.35%. Since O is not essential in titanium alloy plate, the lower limit of its content is 0%. Note that O is an unavoidable contaminant, and the actual content is usually 0.01% or more.

Ni is an element that improves tensile strength and workability, similar to Fe or V. However, if the Ni content is 0.15% or more, the equilibrium phase intermetallic compound Ti ₂ Ni is generated, and the fatigue strength and room temperature ductility of the titanium alloy plate may deteriorate. Therefore, the Ni content is preferably less than 0.15%. The Ni content is more preferably 0.14% or less, 0.12% or less, and even more preferably 0.11% or less. Since Ni is not essential in the titanium alloy plate, the 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, similar to Fe or V. However, if the Cr content is 0.25% or more, the equilibrium phase intermetallic compound _TiCr2 is generated, and the fatigue strength and room temperature ductility of the titanium alloy plate may deteriorate. Therefore, the Cr content is preferably less than 0.25%. The Cr content is more preferably 0.24% or less, 0.21% or less. Since Cr is not essential in the titanium alloy plate, the lower limit of the content is 0%, but the Cr content may be, for example, 0.01% or more.

Mn, like Fe or V, is an element that improves tensile strength and workability. However, if the Mn content is 0.25% or more, the equilibrium phase intermetallic compound TiMn is generated, and the fatigue strength and room temperature ductility of the titanium alloy plate may deteriorate. Therefore, the Mn content is preferably less than 0.25%. The Mn content is more preferably 0.24% or less, and even more preferably 0.20% or less. Since Mn is not essential in titanium alloy plate, the lower limit of its content is 0%, but the Mn content may be, for example, 0.01% or more.

Taking into account the effects of the chemical components described above, the titanium alloy plate of this embodiment preferably contains, as optional elements, either Fe: 0.5-2.3% or V: 2.5-4.5%, Si: 0-0.60%, and further contains C: less than 0.080%, N: 0.050% or less, and O: 0.40% or less.

In addition, taking into consideration the effects of the chemical components described above, when the titanium alloy plate according to this embodiment contains either Fe: 0.5-2.3% or V: 2.5-4.5%, it is preferable that the titanium alloy plate contains, in place of a portion of the Fe or V, one or more elements selected from the group consisting of Ni: less than 0.15%, Cr: less than 0.25%, and Mn: less than 0.25%.

In the case where the titanium alloy plate according to the present embodiment contains Fe, when it contains one or more selected from the group consisting of Ni: less than 0.15%, Cr: less than 0.25%, and Mn: less than 0.25%, it is preferable that the total amount of Fe, Ni, Cr, and Mn is 0.5% or more and 2.3% or less. When the total amount of Fe, Ni, Cr, and Mn is 0.5% or more, high tensile strength is obtained. In addition, when the total amount of Fe, Ni, Cr, and Mn is 0.5% or more, the β phase with good workability at room temperature is maintained, improving the workability of the titanium alloy plate, and it is possible to improve the dimensional accuracy. In addition, when the total amount of Fe, Ni, Cr, and Mn is 2.3% or less, the segregation of these elements is suppressed, and it is possible to suppress the variation in properties in the titanium alloy plate.

In addition, when the titanium alloy plate according to the present embodiment contains V, and contains one or more selected from the group consisting of Ni: less than 0.15%, Cr: less than 0.25%, and Mn: less than 0.25%, the total amount of V, Ni, Cr, and Mn is preferably 2.5% or more and 4.5% or less. When the total amount of V, Ni, Cr, and Mn is 2.5% or more, high tensile strength is obtained. In addition, when the total amount of V, Ni, Cr, and Mn is 2.5% or more, the β phase with good workability at room temperature is maintained, improving the workability of the titanium alloy plate, and it is possible to improve the dimensional accuracy. In addition, when the total amount of Fe, Ni, Cr, and Mn is 4.5% or less, the segregation of these elements is suppressed, and it is possible to suppress the variation in properties in the titanium alloy plate.

The remainder of the chemical composition of the titanium alloy plate according to this embodiment may be Ti and impurities. Examples of impurities include H, Cl, Na, Mg, Ca, and B mixed in during the refining process, and Zr, Sn, Mo, Nb, Ta, and Cu mixed in from scrap. The total amount of impurities is 0.5% or less, which is not a problem. The H content is 150 ppm or less. There is a concern that B may become coarse precipitates in the ingot. Therefore, even if it is contained as an impurity, it is preferable to suppress the B content as much as possible. In the titanium alloy plate according to this embodiment, the B content is preferably 0.01% or less.

In addition, when the titanium alloy plate of this embodiment contains 0.5 to 2.3% Fe, the amount of V contained in the titanium alloy plate may be considered to be an impurity, and when the titanium alloy plate of this embodiment contains 2.5 to 4.5% V, the amount of Fe contained in the titanium alloy plate may be considered to be an impurity.

In addition, it goes without saying that the titanium alloy plate according to this embodiment may contain various elements in place of Ti, so long as it has high strength and excellent dimensional accuracy. Similarly, the elements exemplified as impurities may be contained in amounts greater than or equal to the amount considered to be an impurity, so long as the titanium alloy plate has high strength and excellent dimensional accuracy.

As explained above, the titanium alloy plate according to this embodiment may have the above chemical composition. More specifically, the chemical composition of the titanium alloy plate according to this embodiment may be, for example, Ti-6Al-4V, Ti-6Al-4V ELI, or Ti-5Al-1Fe.

(1.2. Texture and Microstructure)
Next, the texture and microstructure of the titanium alloy plate according to this embodiment will be described.

[Texture]
The titanium alloy plate according to the present embodiment has a texture in which the angle between the direction showing the peak of the degree of accumulation calculated by texture analysis in the case of the inverse pole figure using the spherical harmonic function method of the electron backscattering diffraction (EBSD) method in the (0001) pole figure from the plate thickness direction and the plate thickness direction is 65° or less when the expansion index is 16 and the Gaussian half-width is 5°. When titanium alloy is generally hot-rolled in one direction at high speed at the temperature of the β region or the α+β high temperature region with a high β phase ratio, during the phase transformation from β phase to α phase, a texture (T-texture) is formed in which the c-axis of the hexagonal close-packed structure (hcp) is oriented in the width direction perpendicular to the longitudinal direction on the rolled surface according to the variant selection rule. In the texture in which the c-axis of hcp is oriented in the width direction, large anisotropy occurs in the tensile properties in the width direction and the longitudinal direction. If there is a large anisotropy in the tensile properties in the width direction and the length direction, it may cause problems during processing.The direction showing the peak of the degree of accumulation calculated by the texture analysis (development index=16, Gaussian half-width=5°) of the inverse pole figure using the spherical harmonic function method of EBSD method corresponds to the direction in which the c-axis of hcp is most oriented.In the titanium alloy plate according to this embodiment, in the (0001) pole figure from the plate thickness direction, the angle between the direction in which the c-axis of hcp is most oriented (the direction showing the peak of accumulation) and the plate thickness direction is 65° or less, so that the anisotropy can be reduced, high workability can be ensured, and dimensional accuracy can be improved.In the (0001) pole figure from the plate thickness direction, the angle between the direction in which the c-axis of hcp is most oriented and the plate thickness direction is preferably 60° or less, more preferably 55° or less, and even more preferably 35° or less. The lower limit of the angle between the direction in which the c-axis of hcp is most oriented and the plate thickness direction is not particularly limited, but is 0 ° or more.When titanium alloy plate is manufactured by unidirectional rolling, the lower limit of the angle between the direction in which the c-axis of hcp is most oriented and the plate thickness direction is 20 ° or more.

In addition, when cold rolling is performed in one direction, the angle between the direction showing the peak of the concentration and the plate thickness direction may result in a texture (Split-TD type texture) in which the c-axis of the hcp axis is inclined toward the width direction (TD). The Split-TD type texture has excellent formability, especially excellent bendability. Therefore, the angle between the direction showing the peak of the concentration and the plate thickness direction is preferably 20° or more and 65° or less, which is a Split-TD type texture.

The (0001) pole figure can be obtained by chemically polishing the observation surface of a titanium alloy plate sample and analyzing the crystal orientation using EBSD. Specifically, the titanium alloy plate is cut in the thickness direction along the longitudinal direction at the center position in the width direction (TD), and a cross section (L cross section) is chemically polished, and the (total plate thickness) × 2 mm area of the cross section is subjected to crystal orientation analysis by the EBSD method at about 2 to 10 points at intervals of 1 to 2 μm, so that the (0001) pole figure can be drawn. The peak position of the concentration of a specific orientation in the (0001) pole figure is calculated by texture analysis of the inverse pole figure using the spherical harmonic function method using the data of OIM Analysis ^TM software (Ver. 8.1.0) made by TSL Solutions. In this case, the position with the highest contour line is the peak position of the concentration, and the value with the highest concentration among the peak positions is the maximum concentration. The degree of accumulation of a specific orientation in the (0001) pole figure indicates how many times the frequency of crystal grains with that orientation is compared to a structure with a completely random orientation distribution (degree of accumulation 1). In the above, the L-section at the center position in the width direction is used as the observation surface, but since the crystal orientation of the titanium alloy plate is uniformly distributed in the width direction, the L-section at any position in the plate width may be used as the observation surface.

Figure 1 shows an example of a (0001) pole figure from the plate thickness direction (ND) of the titanium alloy plate according to this embodiment. In Figure 1, the poles of each detected crystal orientation are accumulated according to the inclination to the final rolling direction (RD) and the final rolling width direction (TD), and the accumulation contour is drawn on the (0001) pole figure. The part where the contour line is highest in the figure is the peak P1 of the crystal grain. Therefore, in this embodiment, the angle between the direction showing the peak P1 of the crystal grain and the ND is 65° or less. Usually, the maximum accumulation degree is the accumulation degree of the peak P1 of the crystal grain.

In addition, in the titanium alloy plate according to this embodiment, in the (0001) pole figure from the plate thickness direction, the angle between the direction showing the peak of the accumulation degree calculated by texture analysis when the expansion index is 16 and the Gaussian half-width is 5° for the inverse pole figure using the spherical harmonic function method of the electron backscattering diffraction method and the width direction may be 10° or less. The angle between the direction showing the peak of the accumulation degree and the width direction is the angle θ2 between the direction from the center of the (0001) pole figure from the plate thickness direction to the position showing the peak of the accumulation degree and the width direction (TD) as shown in FIG. The angle is preferably 5° or less, more preferably 3° or less, from the viewpoint of manufacturing and the observation method of the structure.
In addition, in the (0001) pole figure of the titanium alloy plate according to this embodiment, in the inverse pole figure using the spherical harmonic function method of the electron backscattering diffraction method, the angle between the direction showing the peak of the degree of accumulation calculated by texture analysis when the expansion index is 16 and the Gaussian half-width is 5° and the plate thickness direction may be 35° or less.

[Microstructure]
The titanium alloy plate according to the present embodiment has an area ratio of α phase of 80% or more. The titanium alloy plate according to the present embodiment contains a large amount of α stabilizing elements to increase strength. Therefore, if the amount of β stabilizing elements added is further increased, it becomes impossible to manufacture the titanium alloy plate by cold rolling due to excessive strength. Therefore, the titanium alloy plate according to the present embodiment has an area ratio of α phase of 80% or more. The area ratio of α phase may be, for example, 82% or more. The upper limit of the area ratio of α phase is not particularly limited, and the area ratio of α phase may be, for example, 100% or less or 98% or less. The structure of the titanium alloy plate according to the present embodiment is composed of an α phase and a remaining structure, and the remaining structure includes a β phase, TiFe, Ti ₃ Al, and silicide.

In the titanium alloy plate according to the present embodiment, the area ratio of the α phase having an equivalent circle diameter of 1 μm or more is more than 53%. If the area ratio of 1 μm or less is high, the ductility at room temperature may be poor, so the area ratio of the α phase having an equivalent circle diameter of 1 μm or more is more than 53%. The area ratio of the α phase having an equivalent circle diameter of 1 μm or more may be 55% or more, or may be 60% or more. The upper limit of the area ratio of the α phase having an equivalent circle diameter of 1 μm or more is not particularly limited, and the area ratio of the α phase having an equivalent circle diameter of 1 μm or more may be, for example, 98% or less. The microstructure of the titanium alloy plate according to the present embodiment is, for example, as shown in FIG. 3. There is no particular limit to the upper limit of the equivalent circle diameter of the α phase, and the equivalent circle diameter of the α phase is, for example, 20 μm or less.

The area ratio of the α phase and the area ratio of the α phase with a circle equivalent diameter of 1 μm or more are measured by the following method. A cross section (L cross section) of a titanium alloy plate cut in the thickness direction along the longitudinal direction at the center position in the width direction (TD) is chemically polished, and a crystal orientation analysis is performed by the EBSD method on an area of (total plate thickness) x 200 μm of the cross section, for about 2 to 5 fields of view in steps of 1 to 5 μm. The α phase is identified by this EBSD crystal orientation analysis. The area ratio of the α phase is the area ratio of the α phase. In addition, the circle equivalent diameter (area A = π x (grain diameter D / 2)2) of the α phase observed in the above field of view is calculated, and the total area of the α phase with a circle equivalent diameter of 1 μm or more with respect to the area of the above region is the area ratio of the α phase with a circle equivalent diameter of 1 μm or more. The crystal grains of the α phase with a circle equivalent diameter of 1 μm or more are to include the band structure described later. In the above, the area ratio of the α phase and the area ratio of the α phase having a circle equivalent diameter of 1 μm or more are measured based on an L cross section at the center position in the width direction. However, since the α phase is uniformly distributed in the width direction, the area ratio of the α phase and the area ratio of the α phase having a circle equivalent diameter of 1 μm or more may also be measured based on an L cross section at any position in the sheet width.

The titanium alloy plate according to this embodiment has a microstructure consisting of an equiaxed structure with an aspect ratio of 3.0 or less and a band structure with an aspect ratio of more than 3.0 that is elongated in the longitudinal direction, and it is preferable that the average crystal grain size of the equiaxed structure is 0.1 μm or more and 20.0 μm or less, and the area ratio of the band structure to the area of the microstructure is 10.0% or less. Each structure is described below.

When titanium alloys are hot-rolled at temperatures in the α+β or β regions, a structure called a "band structure" may be formed as shown in FIG. 4. The band structure referred to here is, for example, a structure elongated in the longitudinal direction as shown in FIG. 4. Specifically, it refers to crystal grains whose aspect ratio, expressed by the long axis/short axis of the crystal grain, exceeds 3.0. The titanium alloy plate according to this embodiment may have a band structure elongated in the longitudinal direction. If a band structure is formed, it may cause anisotropy in strength or defects during forming. Therefore, it is better to have as little band structure 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. On the other hand, since it is better to have no band structure, the lower limit is 0%.

The aspect ratio and the area ratio of the band structure can be calculated as follows. A cross section (L cross section) of a titanium alloy plate cut in the thickness direction along the longitudinal direction at the center of the width direction (TD) is chemically polished, and an area of (total plate thickness) x 200 μm of the cross section is subjected to crystal orientation analysis by the EBSD method for about 2 to 5 fields of view in steps of 1 to 5 μm. From the results of this EBSD crystal orientation analysis, the aspect ratio is calculated for each crystal grain. Then, the area ratio of crystal grains with an aspect of more than 3.0 is calculated. Note that in the above, the aspect ratio and the area ratio of the band structure are calculated based on the L cross section at the center of the width direction, but since the band structure is uniformly distributed in the width direction, the aspect ratio and the area ratio of the band structure may be calculated based on the L cross section at any plate width position.

The remaining part of the microstructure other than the band structure is preferably an equiaxed structure formed by recrystallization. The titanium alloy plate preferably has an equiaxed structure from the viewpoint of formability, and in particular, the titanium alloy plate is preferably fine-grained because it may be formed by utilizing superplastic properties. From the viewpoint of formability and superplasticity, the average grain size of the equiaxed structure is preferably 20.0 μm or less. The average grain size of the equiaxed structure is more preferably 15.0 μm or less, even more preferably 10.0 μm or less, and even more preferably 8.0 μm or less. On the other hand, if the average grain size of the equiaxed structure is less than 0.5 μm, the strength may become too large due to the grain refinement effect, and the ductility may be significantly reduced. As a result, the workability may be reduced, especially in the cold (room temperature). Therefore, the average grain size of the equiaxed structure is preferably 0.5 μm or more. The average grain size of the equiaxed structure is more preferably 1.0 μm or more.
In addition, the equiaxed structure and band structure are more than 80% α phase, and β phase exists between the α phases.

The presence or absence of recrystallization can be determined by measuring the aspect ratio of the crystal grain (ratio of long axis to short axis). If the aspect ratio is 3.0 or less, the crystal grain can be determined to be a recrystallized grain. The lower limit of the aspect ratio of an equiaxed structure is 1.0.

The average grain size of the equiaxed structure can be calculated as follows: The circle-equivalent grain size (area A = π × (grain size D/2) ² ) is calculated from the grain area measured by EBSD for the equiaxed structure, and the number-based average value is defined as the average grain size of the equiaxed structure.

(1.3. 0.2% proof stress)
The smaller of the 0.2% proof stress in the longitudinal direction or the 0.2% proof stress in the width direction at 25 ° C. of the titanium alloy plate according to this embodiment is preferably 700 MPa or more. Hereinafter, the smaller of the 0.2% proof stress in the longitudinal direction or the 0.2% proof stress in the width direction is simply referred to as 0.2% proof stress. In the aircraft field, etc., a tensile strength close to the tensile strength at 25 ° C. of Ti-6Al-4V, which is a general-purpose α + β type titanium alloy, is often required. If the 0.2% proof stress at 25 ° C. of the titanium alloy plate is 700 MPa or more, it can be used in applications requiring high strength. The 0.2% proof stress at 25 ° C. of the titanium alloy plate is more preferably 730 MPa or more. On the other hand, if the strength is too high, the strength of the hot-rolled plate before cold rolling is also high, making it difficult to cold-roll the hot-rolled plate, and the number of cold rolling passes may increase, resulting in increased costs. Also, if the strength is too high, the notch sensitivity will be high and the plate may break. Therefore, the 0.2% proof stress of the titanium alloy plate at 25 ° C is preferably 1200 MPa or less. The 0.2% proof stress of the titanium alloy plate at 25 ° C is more preferably 1150 MPa or less. Furthermore, if the 0.2% proof stress of the titanium alloy plate at 25 ° C is 1000 MPa or less, the cracking during rolling is further suppressed, so the 0.2% proof stress of the titanium alloy plate at 25 ° C is even more preferably 1100 MPa or less. The 0.2% proof stress can be measured by a method conforming to JIS Z2241:2011. That is, the 0.2% proof stress in the longitudinal direction and the 0.2% proof stress in the transverse direction can be measured by a method conforming to JIS Z2241:2011. The longitudinal direction referred to 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.

(1.4. Anisotropy)
The titanium alloy plate according to the present embodiment preferably has a yield strength ratio σT/σL, which is the ratio of the 0.2% yield strength σT in the width direction at 25 ° C. to the 0.2% yield strength σL in the longitudinal direction at 25 ° C., of 0.85 or more and 1.18 or less. As described above, α+β-type titanium has a hcp phase (α phase), so it shows high anisotropy in the hcp direction. As described above, when T-texture is formed, anisotropy becomes large, so it is sometimes desired to make anisotropy as small as possible, especially in the aircraft field. Therefore, the yield strength ratio σT/σL is better as close to 1.00 as possible, but if the yield strength ratio σT/σL is 1.18 or less, better formability can be obtained. The yield strength ratio σT/σL is more preferably 1.16 or less, even more preferably 1.15 or less, and even more preferably 1.14 or less. In the case of cold cross rolling, which performs cold rolling in the longitudinal direction and the width direction, the yield strength ratio σT/σL can be 0.85 or more and 1.10 or less. The yield strength ratio σT/σL of the titanium alloy plate manufactured by cold cross rolling is preferably 0.90 or more, more preferably 0.95 or more. In addition, the yield strength ratio σT/σL of the titanium alloy plate manufactured by cold cross rolling is preferably 1.05 or less. In the case of unidirectional cold rolling in the longitudinal direction, it is difficult to make the yield strength ratio σT/σL less than 1.05, and it can be 1.05 or more. In addition, since it is possible to manufacture a titanium alloy plate with a yield strength ratio σT/σL of more than 1.18 by unidirectional cold rolling, σT/σL may be more than 1.18.

(1.5. Average plate thickness)
The average thickness of the titanium alloy plate according to the present embodiment is 2.5 mm or less. For example, the average thickness of the titanium alloy plate can be 2.5 mm or less by using a titanium material containing the above chemical components according to the manufacturing method of the titanium alloy plate described later. Since the titanium material having an Al content of more than 4.0% and not more than 6.6% has a large deformation resistance, the allowable load of the rolling machine may be exceeded when manufacturing a thin plate with a general rolling machine. Therefore, it is difficult to manufacture a titanium alloy plate containing the above chemical components and having a thickness of 2.5 mm or less. In addition, when hot rolling is performed without using pack rolling, the temperature drops rapidly as the plate thickness becomes thinner, and the deformation resistance increases. As a result, when hot rolling a high-strength material, the allowable load of the rolling machine may be exceeded, making it difficult to make the average thickness 2.5 mm or less. On the other hand, although there is no particular limit on the lower limit of the average thickness of the titanium alloy plate according to the present embodiment, in a titanium alloy having the above strength, the average thickness is often 0.1 mm or more in reality. Therefore, the average thickness of the titanium alloy plate according to the present embodiment is preferably 0.1 mm or more. The thickness of the titanium alloy plate according to this embodiment is preferably 2.0 mm or less, more preferably 1.5 mm or less. The average thickness of the titanium alloy plate according to this embodiment is more preferably 0.2 mm or more.

Here, the method for measuring the average plate thickness will be described with reference to Figure 5. The plate thickness at each position is measured at 5 or more positions at intervals of 1 m or more in the longitudinal direction using an X-ray, micrometer or caliper, including the center position in the width direction (TD) and positions at a distance of 1/4 of the plate width from each end in the width direction, and the average of the measured plate thicknesses is defined as the average plate thickness.

(1.6. Plate thickness dimensional accuracy)
The thickness dimensional accuracy of the titanium alloy plate according to this embodiment (hereinafter, the thickness dimensional accuracy may be simply referred to as thickness dimensional accuracy) is preferably 5.0% or less with respect to the average plate thickness. In pack rolling, a titanium alloy thin plate is manufactured by hot rolling a titanium material that is laminated in multiple layers and wrapped in steel material, but since the deformation resistance of the laminated titanium material varies greatly depending on the temperature distribution, it is difficult to manufacture a thin plate with a uniform thickness. However, since the titanium alloy plate according to this embodiment is manufactured through cold rolling as described later, it is a titanium alloy thin plate with excellent thickness dimensional accuracy. The dimensional accuracy of the titanium alloy plate according to this embodiment is more preferably 4.0% or less with respect to the average plate thickness, and even more preferably 2.0% or less with respect to the average plate thickness.

The thickness dimensional accuracy is measured by the following method. The thickness of each position is measured at 5 or more positions at intervals of 1 m or more in the longitudinal direction, including the center position in the width direction (TD) and positions at a distance of 1/4 of the width from each end in the width direction, using X-rays, a micrometer, or a vernier caliper. Using the actually measured thickness d and the above average thickness dave, the maximum value of a' calculated by the following formula (101) is taken as the thickness dimensional accuracy a.
a'=(d-dave)/dave×100...Formula (101)

The above describes the titanium alloy plate according to this embodiment. The titanium alloy plate according to this embodiment has high strength because it has the above-mentioned chemical components and metal structure. The titanium alloy plate according to this embodiment described above may be manufactured by any method, but can also be manufactured, for example, by the manufacturing method of the titanium alloy plate according to this embodiment described below.

<2. Manufacturing method of titanium alloy plate>
The manufacturing method of the titanium alloy plate according to the present embodiment includes a slab manufacturing process for manufacturing a titanium alloy slab, a hot rolling process for hot rolling the titanium alloy slab, a cold rolling process for cold rolling the titanium material after the hot rolling process, and a temper rolling/tensile straightening process for temper rolling or tensile straightening the titanium material after the cold rolling process, as necessary. Each step of the manufacturing method of the titanium alloy plate according to the present embodiment will be described below. In the cold rolling process, one-way cold rolling is performed in which the titanium material after the hot rolling process is subjected to one or more cold rolling passes only in the longitudinal direction, or cold cross rolling is performed in which the titanium material is subjected to cold rolling passes in the longitudinal direction and the width direction. In the following, as the first manufacturing method, a case in which one-way cold rolling is performed on the titanium material after the hot rolling process in the cold rolling process will be described, and as the second manufacturing method, a case in which cold cross rolling is performed on the titanium material after the hot rolling process will be described.

[First manufacturing method]
(2.1. Slab manufacturing process)
In the slab manufacturing process, a titanium alloy slab is manufactured. The manufacturing method of the titanium alloy slab is not particularly limited, and for example, it can be manufactured by the following procedure. First, an ingot is produced from sponge titanium by various melting methods such as a vacuum arc melting method, an electron beam melting method, or a hearth melting method such as a plasma melting method. Next, the obtained ingot is hot forged at a temperature in the α-phase high temperature region, the α + β two-phase region, or the β-phase single-phase region to obtain a titanium alloy slab. Note that the titanium alloy slab may be subjected to pretreatment such as cleaning treatment and cutting as necessary. In addition, when the titanium alloy slab is made into a rectangular shape that can be hot rolled by the hearth melting method, it may be subjected to hot rolling without hot forging or the like. The produced titanium alloy slab contains, by mass%, Al: more than 4.0% and not more than 6.6%, Fe: 0% or more and not more than 2.3%, V: 0% or more and not more than 4.5%, Si: 0% or more and not more than 0.60%, C: 0% or more and not more than 0.080%, N: 0% or more and not more than 0.050%, O: 0% or more and not more than 0.40%, Ni: 0% or more and not more than 0.15%, Cr: 0% or more and not more than 0.25%, and Mn: 0% or more and not more than 0.25%.

(2.2. Hot rolling process)
In the hot rolling process, the titanium alloy slab is heated and then hot rolled. For example, the titanium alloy slab may be heated to a temperature range equal to or higher than the β transformation point T _β ° C., and then rolled so that the total reduction is 80% or more. However, if hot rolling is started from a temperature range equal to or lower than the temperature range of the α + β phase, the titanium alloy slab may crack, or the above-mentioned metal structure may not be obtained even if the titanium alloy slab does not crack. Therefore, in this process, hot rolling is started from the temperature range of the β phase. In addition, the finishing temperature, which is the temperature immediately after hot rolling, is set to the temperature range of the α + β phase, and varies depending on the composition of the titanium alloy slab, but is, for example, (T _β -250) ° C. or higher and (T _β -50) ° C. or lower. In addition, hot rolling may be performed so that the reduction is the above reduction in one hot rolling, or hot rolling may be performed so that the above reduction is achieved by multiple hot rolling. The titanium material after this hot rolling process contains, by mass%, Al: more than 4.0% and not more than 6.6%, Fe: 0% or more and not more than 2.3%, V: 0% or more and not more than 4.5%, Si: 0% or more and not more than 0.60%, C: 0% or more and not more than 0.080%, N: 0% or more and not more than 0.050%, O: 0% or more and not more than 0.40%, Ni: 0% or more and not more than 0.15%, Cr: 0% or more and not more than 0.25%, and Mn: 0% or more and not more than 0.25%.

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

In the hot rolling process, the titanium alloy slab can be continuously hot rolled using known continuous hot rolling equipment. When continuous hot rolling equipment is used, the titanium alloy slab is hot rolled and then wound by a winder to become a titanium alloy hot rolled coil. Therefore, the titanium material after the hot rolling process includes plate-shaped titanium material and coil-shaped titanium material that is longer than the plate-shaped titanium material.

The titanium material after the hot rolling process may be annealed by a known method, pickled or cut to remove oxide scale, or cleaned, if necessary. For example, the titanium material after the hot rolling process is annealed at a temperature of 650°C to 800°C for 20 minutes to 90 minutes. This allows the unrecrystallized grains of the hot rolled plate to precipitate as fine recrystallized grains, making the crystals in the metal structure of the finally obtained titanium alloy plate more uniform and fine. The annealing may be performed in any of an air atmosphere, an inert atmosphere, and a vacuum atmosphere.

In addition, in the above-mentioned manufacturing method of titanium alloy plate, the titanium material after the hot rolling process corresponds to the titanium material disclosed herein.

(2.3. Cold rolling process)
In this process, the titanium material after the hot rolling process is subjected to one or more cold rolling passes in its longitudinal direction. 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. This cold rolling process makes the c-axis of hcp approach ND. However, if the rolling rate per cold rolling pass and the total rolling rate are too small, the crystal orientation hardly changes, and the angle between the direction showing the peak of the concentration and the plate thickness direction does not become 65° or less. In this case, the anisotropy of the titanium alloy plate is not improved. In addition, although the above-mentioned band structure is formed by hot rolling, if the cold rolling rate per cold rolling pass and the total cold rolling rate in the cold rolling after hot rolling are small, the band structure remains in the titanium material without being destroyed. Therefore, the rolling ratio per cold rolling pass in the cold rolling step is more than 30%, and the total rolling ratio is 60% or more. The total rolling ratio is preferably 70% or more.

Note that one cold rolling pass here refers to cold rolling that is performed continuously. Specifically, the cold rolling pass refers to cold rolling from after the hot rolling process until the titanium material reaches the final product thickness, or from after the hot rolling process until before the temper rolling process, if the temper rolling process described below is performed after the hot rolling process. However, if intermediate annealing is performed in the cold rolling process, the cold rolling from the hot rolling process until the intermediate annealing process, and the cold rolling from the intermediate annealing process until the titanium material reaches the final product thickness, or before the temper rolling process, are each called a cold rolling pass. In addition, if intermediate annealing is performed multiple times, the cold rolling from the previous intermediate annealing process to the subsequent intermediate annealing process is also called a cold rolling pass. Note that the rolling rate by each cold rolling mill may be any rate as long as the rolling rate per run is more than 30%.

In this cold rolling process, manufacturing costs can be reduced by rolling titanium material into long hot-rolled sheets or long hot-rolled coils in the rolling direction.

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

[Intermediate annealing process]
In the cold rolling process, when a plurality of cold rolling passes are performed, it is preferable to have an intermediate annealing step in which the titanium material is annealed between the plurality of cold rolling passes. In the intermediate annealing step, it is preferable to anneal the intermediate material in the cold rolling process so that the annealing temperature T is 600° C. or higher (T _β -50)° C. or lower, and the annealing temperature T (° C.) and the holding time t (seconds) at the annealing temperature T satisfy the following formula (102). In the following formula (102), (T+273.15)×(Log ₁₀ (t)+20) is the Larson-Miller parameter.
22000≦(T+273.15)×(Log ₁₀ (t)+20)≦27000…Equation (102)
Here, _Tβ is the β transformation point (°C).

[Final annealing process]
The final annealing process is a process of annealing the titanium material after the last cold rolling pass. The annealing conditions in the final annealing process are not particularly limited, but in order to improve the formability of the titanium alloy plate, it is preferable that the annealing temperature T is 600°C or higher and (T _β -50)°C or lower, and the annealing temperature T (°C) and the holding time t (seconds) at the annealing temperature T satisfy the above formula (102).

By carrying out the intermediate annealing step and the final annealing step under the above conditions, the uncrystallized grains are recrystallized and the c-axis of the α phase approaches the ND direction. This makes it possible to reduce the anisotropy of the titanium alloy sheet. In addition, the recrystallization causes an excess amount of band structure in the microstructure to disappear. On the other hand, if the annealing temperature is equal to or higher than the β transformation point T _β , a phase transformation from the β phase to the α phase occurs, and the α phase thus produced becomes an acicular structure. In addition, even if the annealing temperature is just below the β transformation point, a bimodal structure in which an equiaxed structure and an acicular structure are mixed is formed. The acicular structure and the bimodal structure may cause internal cracks and edge cracks during cold rolling. Furthermore, the acicular structure or the bimodal structure often becomes a coarse grain, making it difficult to exhibit superplastic properties. In the intermediate annealing step and the final annealing step, the annealing temperature T is 600°C or more and (T _β -50)°C or less, and the annealing temperature T and the annealing time t are determined so that they satisfy the above formula (102), so that the c-axis of the α phase approaches the ND direction by recrystallization, and the anisotropy of the titanium alloy sheet is further reduced, and the band structure in the microstructure can be further reduced. Furthermore, in the intermediate annealing step and the final annealing step, the annealing temperature T is 600°C or more and (T _β -50)°C or less, and the annealing temperature T and the annealing time t are determined so that they satisfy the above formula (102), so that the fine equiaxed structure is increased, internal cracks and edge cracks during cold rolling are suppressed, and superplastic properties are easily developed.

(2.4. Temper rolling and tensile straightening process)
The titanium alloy plate is manufactured through the above-mentioned cold rolling process, and the titanium alloy plate after the cold rolling process is preferably subjected to temper rolling to adjust mechanical properties or tensile straightening to straighten the shape, as necessary. The rolling reduction in temper rolling is preferably 10% or less, and the elongation in tensile straightening is preferably 5% or less. Note that temper rolling and tensile straightening do not have to be performed if they are not necessary.

According to the first manufacturing method, in the cold rolling process in which the hot-rolled plate manufactured using the above titanium alloy plate material is cold-rolled once or more in the longitudinal direction, the rolling reduction rate per cold rolling is more than 30% and the total rolling reduction rate is 60% or more, so that a titanium alloy plate is obtained in which, in the (0001) pole figure from the plate thickness direction, the angle between the direction showing the peak of the accumulation calculated by texture analysis when the expansion index is 16 and the Gaussian half-width is 5° is 65° or less in the inverse pole figure using the spherical harmonic function method of the EBSD method. In addition, according to the first manufacturing method, the average plate thickness of the titanium alloy plate can be set to 2.5 mm or less, and the dimensional accuracy of the plate thickness can be set to 5.0% or less with respect to the average plate thickness.

In addition, according to the first manufacturing method, the metal structure of the titanium alloy plate has a microstructure consisting of an equiaxed structure with an aspect ratio of 3.0 or less and a band structure with an aspect ratio of more than 3.0 and extending in the longitudinal direction, the average crystal grain size of the equiaxed structure is 0.1 μm or more and 20.0 μm or less, and the area ratio of the band structure to the area of the microstructure is 10.0% or less. This further reduces the anisotropy of the titanium alloy plate.
Furthermore, according to the first manufacturing method, 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.

In addition, according to the first manufacturing method, cold rolling refines the crystal grains, making it easier to develop superplastic properties, and the titanium alloy plate has excellent workability in thin plate forming.

According to the manufacturing method of the titanium alloy plate according to the present embodiment, since it includes a unidirectional cold rolling process, it is possible to manufacture a long titanium alloy plate and a titanium alloy coil. Therefore, the above manufacturing method can also be said to be a manufacturing method of a titanium alloy coil. Therefore, it goes without saying that the titanium alloy coil manufactured by the above manufacturing method has the same characteristics as the titanium alloy plate of the present disclosure. Specifically, the titanium alloy coil of the present disclosure contains, by mass%, Al: more than 4.0% and not more than 6.6%, Fe: 0% or more and not more than 2.3%, V: 0% or more and not more than 4.5%, Si: 0% or more and not more than 0.60%, C: 0% or more and not more than 0.080%, N: 0% or more and not more than 0.050%, O: 0% or more and not more than 0.40%, Ni: 0% or more and not more than 0.15%, Cr: 0% or more and not more than 0.25%, and Mn: 0% or more and not more than 0.25%, with the balance being Ti and and impurities, the area ratio of the α phase is 80% or more, the area ratio of the α phase having a circle equivalent diameter of 1 μm or more is more than 53%, in a (0001) pole figure from the plate thickness direction, the inverse pole figure using the spherical harmonic function method of electron backscatter diffraction has an expansion index of 16, and the angle between the direction showing the peak of the accumulation degree calculated by texture analysis when the Gaussian half width is 5° and the plate thickness direction is 65° or less, and the average plate thickness is 2.5 mm or less.

In addition, when manufacturing a titanium alloy coil, the above-mentioned "longitudinal direction" corresponds to the longitudinal direction of the titanium alloy coil, and the "width direction" corresponds to the direction perpendicular to the longitudinal direction of the rolled surface of the titanium alloy coil.
So far, the first manufacturing method has been described.

[Second manufacturing method]
Next, the second manufacturing method will be described. The second manufacturing method is different from the first manufacturing method in the cold rolling step, but the other steps are the same as those of the first manufacturing method. Therefore, here, the cold rolling step will be described in detail, and the description of the other steps will be omitted.

The cold rolling process in the second manufacturing method is a cold cross rolling process in which cold rolling passes are performed in the longitudinal and width directions on the titanium material after the hot rolling process.

The total rolling ratio, including both the longitudinal rolling and the widthwise rolling in this process, is 60% or more. The final rolling direction in this process is the longitudinal direction, and the direction perpendicular to the longitudinal direction is the width direction. If the total rolling ratio is 60% or more, the c-axis of the hcp is more oriented in the ND, making it possible to produce a titanium alloy thin plate with small anisotropy. The higher the rolling ratio, the closer the c-axis of the α-phase of the titanium alloy thin plate is to the plate thickness direction and the greater the degree of integration, so there is no upper limit to the rolling ratio.

The cross rolling ratio is not particularly limited, and is, for example, 0.05 or more and 20.00 or less. The cross rolling ratio here refers to the longitudinal rolling ratio (longitudinal rolling ratio/width rolling ratio) applied from 4 mm to the target plate thickness to achieve the plate thickness. If the cross rolling ratio is 0.05 or more and 20.00 or less, the c-axis of the hcp is more oriented in the ND, making it possible to manufacture a thin plate with small anisotropy. In addition, it is possible to reduce the excessive band structure. The cross rolling ratio is more preferably 0.07 or more and 15.00 or less.

The rolling ratio per cold rolling pass is not particularly limited as long as the total rolling ratio is 60% or more. In addition, one cold rolling pass referred to here refers to cold rolling in the longitudinal direction or cold rolling in the width direction that is continuously performed on the hot rolled plate. Therefore, in this cold cross rolling process, when the hot rolled plate is subjected to cold rolling in the longitudinal direction and cold rolling in the width direction multiple times, the total number of times is the number of cold rolling passes. For example, when the hot rolled plate is subjected to one cold rolling in the longitudinal direction and one cold rolling in the width direction, the number of cold rolling passes is two. In the second manufacturing method, rolling in the longitudinal direction or rolling in the width direction may be performed multiple times. In addition, even if the plate thickness is 4 mm or less, reheating, etc. may be performed. In addition, hot rolling in the width direction may be performed every time hot rolling in the longitudinal direction is performed once or several times.
Furthermore, rolling in the width direction may be carried out at any time.

The rolling ratio per cold rolling pass is not particularly limited and can be, for example, 5% or more. The rolling ratio per cold rolling pass is preferably 10% or more, more preferably 20% or more. The rolling ratio per cold rolling pass may be 80% or less, or 75% or less.

The rolling temperature in the cold cross rolling process is preferably 500°C or less. If the rolling temperature is 500°C or less, high dimensional accuracy can be obtained, and the crystal grains are refined during rolling. The rolling temperature is more preferably 400°C or less. There is no particular lower limit for the cold rolling temperature, and the cold rolling temperature can be, for example, room temperature or higher. Here, room temperature is intended to be 0°C or higher.

According to the second manufacturing method, a titanium alloy plate is obtained in which, in the (0001) pole figure from the plate thickness direction, the angle between the direction showing the peak of the accumulation calculated by texture analysis when the expansion index is 16 and the Gaussian half-width is 5° and the plate thickness direction is 35° or less, and the ratio of the 0.2% yield strength in the width direction to the 0.2% yield strength in the longitudinal direction is 0.85 to 1.10. By performing multiple rolling passes in the longitudinal direction and multiple rolling passes in the width direction, it is possible to make the ratio of the 0.2% yield strength in the width direction to the 0.2% yield strength in the longitudinal direction close to 1.00.

Furthermore, when the titanium material contains a large amount of a β-phase stabilizing element such as V, hot rolling in one direction at high speed at a temperature in the β region or in the α+β high temperature region where the β phase ratio is high is likely to form a T-texture, and the anisotropy of the titanium alloy thin plate is likely to increase. However, according to the second manufacturing method, cold cross rolling is performed, so the formation of T-texture is suppressed even when the titanium material contains a β-phase stabilizing element such as V. As a result, it is possible to manufacture a titanium alloy plate with small anisotropy.

According to the second manufacturing method, the metal structure of the titanium alloy plate has a microstructure consisting of an equiaxed structure with an aspect ratio of 3.0 or less and a band structure with an aspect ratio of more than 3.0 and elongated in the longitudinal direction, the average crystal grain size of the equiaxed structure is 0.1 μm or more and 20.0 μm or less, and the area ratio of the band structure to the area of the microstructure is 10.0% or less. This further reduces the anisotropy of the titanium alloy plate.

Below, the embodiment of the present disclosure will be specifically explained while showing examples. Note that the examples shown below are merely examples of the present disclosure, and the present disclosure is not limited to the examples below.

Example 1
1. Manufacturing of titanium alloy plate First, a titanium alloy ingot, which is a raw material of titanium alloy plate having the chemical composition shown in Table 1, was manufactured by either vacuum arc remelting (VAR), electron beam remelting (EBR), or plasma arc melting (PAM), and then a titanium alloy slab having a thickness of 150 mm, a width of 800 mm, and a length of 5000 mm was manufactured by blooming or forging. Then, these titanium alloy slabs were subjected to hot rolling, hot rolled sheet annealing, shot blasting, and pickling to obtain a hot rolled sheet having a thickness of 4 mm. In the hot rolling, the titanium alloy slab was heated to 1050 to 1100°C so that the temperature was equal to or higher than the β transformation point _Tβ , and the hot rolling was started from that temperature, and the finishing temperature was set to 800 to 950°C so that the temperature was equal to or lower than the _β transformation point Tβ. The elements other than those listed in Table 1 were Ti and impurities.

The chemical components of the hot-rolled sheets were measured for Al, Fe, Si, Ni, Cr, Mn, and V by ICP emission spectrometry. O and N were measured by inert gas fusion, thermal conductivity, and infrared absorption using an oxygen and nitrogen simultaneous analyzer. C was measured by infrared absorption using a carbon and sulfur simultaneous analyzer. The chemical components of each of the hot-rolled sheets produced were equal to the chemical components of the titanium alloy slabs shown in Table 1. In addition, for the titanium materials A to P shown in Table 1, a titanium alloy phase diagram was obtained by the CALPHAD method using Thermo-Calc, an integrated thermodynamic calculation system from Thermo-Calc Software AB, and a specified database (TI3), and the β transformation point T _β was calculated.

Next, the obtained hot-rolled sheet was subjected to a cold rolling process under the conditions shown in Table 2. Inventive Examples 1 to 18, 30, and Comparative Example 3 in Table 2 are examples in which cold rolling with a rolling ratio of 35 to 60% per cold rolling pass and intermediate annealing under the conditions shown in Table 2 were repeated until the total rolling ratio was 70 to 94%. Inventive Example 19 is an example in which cold rolling with a rolling ratio of 35% per cold rolling pass and intermediate annealing under the conditions shown in Table 2 were repeated until the total rolling ratio was 60%. Inventive Example 20 is an example in which cold rolling was performed at a cold rolling temperature of 300°C. Inventive Example 21 is an example in which cold rolling with a rolling ratio of 40% per cold rolling pass and intermediate annealing under the conditions shown in Table 2 were repeated until the total rolling ratio was 78%. The intermediate annealing step in Example 21 is an example that does not satisfy the above formula (102). Examples 22 and 23 are examples in which cold rolling was performed with a rolling reduction of 75% and 60%, respectively, without intermediate annealing. Examples 24 to 26 are examples in which cold rolling was performed with a first cold rolling pass with a rolling reduction of 75%, followed by intermediate annealing under the conditions shown in Table 2, and then cold rolling with a second cold rolling pass with a rolling reduction of 50%, resulting in a total rolling reduction of 88%. Inventive Examples 27 to 29, the first cold rolling pass was cold rolled at a rolling ratio of 50%, followed by a first intermediate annealing under the conditions shown in Table 2, a second cold rolling pass was cold rolled at a rolling ratio of 50%, a second intermediate annealing was performed under the conditions shown in Table 2 after the second cold rolling pass, and a third cold rolling pass was cold rolled at a rolling ratio of 60% to achieve a total rolling ratio of 90%. The reference example is a hot-rolled sheet that was not subjected to a cold rolling process. Comparative Example 1 is an example in which the rolling ratio per pass is 20% and the total rolling ratio is 59%. Comparative Example 2 is an example in which the total rolling ratio is 50%. In Comparative Example 4, which used titanium material O with a high Al content, surface cracks and severe edge cracks occurred during cold rolling after hot rolling. Therefore, in Comparative Example 4, intermediate annealing and final annealing were not performed. In Table 2, "T _β " is the β transformation point, and "Larson-Miller parameter" is a value of (T + 273.15) × (Log ₁₀ (t) + 20). In Table 2, "Pattern A" indicates a cold rolling pattern in which the reduction ratio of the first cold rolling pass is 75% and the reduction ratio of the second cold rolling pass is 50%. In Table 2, "Pattern B" indicates a cold rolling pattern in which the reduction ratio of the first cold rolling pass is 50%, the reduction ratio of the second cold rolling pass is 50%, and the reduction ratio of the third cold rolling pass is 60%.

2. Evaluation The titanium alloy plates according to the invention examples, reference examples and comparative examples were evaluated for the following items.

2.1 Chemical Composition The chemical compositions of the titanium alloy sheets according to the invention examples, reference examples, and comparative examples were measured in the same manner as the method for measuring the chemical compositions of the hot-rolled sheets.

2.2. Accumulation Peak Position The observed surface of the titanium alloy plate samples according to each of the invention examples, reference examples, and comparative examples was chemically polished, and the crystal orientation was analyzed using electron backscatter diffraction to obtain a (0001) pole figure. Specifically, the L cross section was chemically polished at the center position in the width direction (TD) of each sample, and the crystal orientation analysis was performed by EBSD method on the cross section for about 2 to 10 fields of view at intervals of 1 to 2 μm in an area of (total plate thickness) × 2 mm, and a (0001) pole figure was drawn. The accumulation peak position of a specific orientation in the (0001) pole figure was calculated by texture analysis of the inverse pole figure using the spherical harmonic function method (expansion index = 16, Gaussian half-width = 5°) using the data from TSL Solutions' OIM Analysis software.

2.3. Area ratio of α phase and area ratio of α phase with circle equivalent diameter of 1 μm or more The area ratio of α phase and area ratio of α phase with circle equivalent diameter of 1 μm or more were measured by the following method. The cross section of the titanium alloy plate cut perpendicular to the width direction (TD) at the center position in the width direction (TD) was chemically polished, and the crystal orientation analysis was performed by the EBSD method on the area of (total plate thickness) × 200 μm of the cross section, for about 2 to 5 fields of view in steps of 1 to 5 μm. The area ratio of the α phase to the area of the above area was defined as the area ratio of the α phase. In addition, the circle equivalent diameter (area A = π × (grain diameter D / 2) ² ) of the α phase observed in the above field of view was calculated, and the total area of the α phase with circle equivalent diameter of 1 μm or more to the area of the above area was defined as the area ratio of the α phase with circle equivalent diameter of 1 μm or more. The crystal grains of the α phase with circle equivalent diameter of 1 μm or more were assumed to include a band structure described later.

2.4. Aspect ratio and band structure area ratio A cross section of a titanium alloy plate sample cut perpendicular to the width direction (TD) at the center position in the width direction (TD) was chemically polished, and a crystal orientation analysis was performed by EBSD method on an area of (total plate thickness) x 200 μm of the cross section, with steps of 1 to 5 μm and about 2 to 5 fields of view. From the results of this EBSD crystal orientation analysis, the aspect ratio of each crystal grain was calculated. The area ratio of crystal grains with an aspect of more than 3.0 was calculated as the band structure area ratio.

2.5. Average grain size of equiaxed structure The average grain size of the equiaxed structure was determined by calculating the circle equivalent grain size (area A = π × (grain size D / 2)2) from the grain area measured by EBSD for the equiaxed structure, and the number-based average value was defined as the average grain size of the equiaxed structure.

2.6. 0.2% Yield Strength The 0.2% yield strength at 25°C of the titanium alloy plates according to each of the invention examples, reference examples and comparative examples was measured in accordance with JIS Z 2241:2011.

2.7. Average plate thickness dave
The average thickness dave of the titanium alloy plate according to each invention example, reference example and comparative example is measured by the following method. For each titanium alloy plate manufactured, the thickness of each position is measured at 5 or more positions at intervals of 1 m or more in the longitudinal direction at the center position in the width direction and at the positions at 1/4 of the width direction from both ends of the width direction by X-ray, micrometer or caliper, and the average value of the measured thickness is taken as the average thickness dave.

2.8. Plate thickness dimensional accuracy a
The thickness dimensional accuracy a of the titanium alloy plate according to each invention example, reference example and comparative example is determined by using the plate thickness d that is actually measured by the above-mentioned method and the above-mentioned average plate thickness dave, and calculates the maximum value of a' by the following formula (101).
a'=(d-dave)/dave×100...Formula (101)

3. Results The above evaluation results are shown in Table 3. In addition, "θ" shown in Table 3 is the angle between the direction showing the peak of the degree of accumulation calculated by texture analysis when the expansion index is 16 and the Gaussian half width is 5° in the inverse pole figure using the spherical harmonic function method of the backscattered electron diffraction method in the (0001) pole figure from the plate thickness direction and the plate thickness direction. In addition, "θ2" shown in Table 3 is the angle between the direction from the center of the pole figure showing the peak of the degree of accumulation calculated by texture analysis when the expansion index is 16 and the Gaussian half width is 5° in the inverse pole figure using the spherical harmonic function method of the backscattered electron diffraction method in the (0001) pole figure from the plate thickness direction and the plate width direction.

For all of the invention examples 1 to 30, the reference example, and the comparative examples 1 to 4, the contents of Al, Fe, Si, Ni, Cr, Mn, V, O, N and C in the produced titanium alloy plates were equal to the contents of the above elements in the hot-rolled plates used.

For Examples 1 to 20, the angle θ between the direction showing the peak of the density in the (0001) pole figure and the sheet thickness direction was 65° or less, and the angle θ2 between the direction showing the peak of the density and the width direction was 0°. The average crystal grain size of the equiaxed structure was 0.1 μm or more and 20.0 μm or less, and the area ratio of the band structure was 10% or less. The area ratio of the α phase was 80% or more in all cases, and the area ratio of the α phase with a circle equivalent diameter of 1 μm 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 longitudinal 0.2% yield strength at 25°C was 700 MPa or more, and the yield strength ratio σT/σL, which is the ratio of the longitudinal 0.2% yield strength σT at 25°C to the width direction 0.2% yield strength σL at 25°C, was 1.05 to 1.18.

For Example 21, the angle θ between the direction showing the peak of the density in the (0001) pole figure and the sheet thickness direction was 49°, and the angle θ2 between the direction showing the peak of the density and the width direction was 0°. The average grain size of the equiaxed structure was 1.8 μm, and the area ratio of the band structure was 5.0%. The area ratio of the α phase was 88% or more, and the area ratio of the α phase with a circle equivalent diameter of 1 μm or more was 88%. The average sheet thickness was 0.9 mm, and the dimensional accuracy was 2.0%. The 0.2% yield strength at 25°C was 805 MPa, and the yield strength ratio σT/σL was 1.12.

For Examples 22 and 23, the angle θ between the direction showing the peak of the density in the (0001) pole figure and the sheet thickness direction was 50°, and the angle θ2 between the direction showing the peak of the density and the width direction was 0°. The average grain size of the equiaxed structure of Example 22 was 3.5 μm, and the average grain size of the equiaxed structure of Example 23 was 10.5 μm. The area ratios of the band structure were 15.0% and 20.0%, respectively. The area ratios of the α phase were 80% or more in both cases, and the area ratios of the α phase with a circle equivalent diameter of 1 μm or more were more than 53%. The average sheet thicknesses were 1.0 mm and 1.6 mm, and the dimensional accuracy was 2.0% and 2.5%. The 0.2% yield strength at 25°C was 700 MPa or more, and the yield strength ratios σT/σL were 1.11 and 1.15.

For Examples 24 to 26, the angle θ between the direction showing the peak of the density in the (0001) pole figure and the sheet thickness direction was 65° or less, and the angle θ2 between the direction showing the peak of the density and the width direction was 0°. The average grain size of the equiaxed structure was 0.1 μm or more and 20.0 μm or less, and the area ratio of the band structure was 10% or less. The area ratio of the α phase was 80% or more in all cases, and the area ratio of the α phase with a circle equivalent diameter of 1 μm or more was more than 53%. The average sheet thickness was 0.5 mm in all cases, and the dimensional accuracy was 1.0 in all cases. The 0.2% longitudinal yield strength at 25°C was 700 MPa or more, and the yield strength ratio σT/σL was 1.05 to 1.18.

For Examples 27 to 29, the angle θ between the direction showing the peak of the density in the (0001) pole figure and the sheet thickness direction was 65° or less, and the angle θ2 between the direction showing the peak of the density and the width direction was 0°. The average grain size of the equiaxed structure was 0.1 μm or more and 20.0 μm or less, and the area ratio of the band structure was 10% or less. The area ratio of the α phase was 80% or more in all cases, and the area ratio of the α phase with a circle equivalent diameter of 1 μm or more was more than 53%. The average sheet thickness was 0.4 mm in all cases, and the dimensional accuracy was 1.0% or less in all cases. The 0.2% yield strength in the longitudinal direction was 700 MPa or more, and the yield strength ratio σT/σL was 1.05 to 1.18.

For Example 30, the angle θ between the direction showing the peak of the density in the (0001) pole figure and the sheet thickness direction was 45°, and the angle θ2 between the direction showing the peak of the density and the width direction was 0°. The average crystal grain size of the equiaxed structure was 3.5 μm, and the area ratio of the band structure was 5.0%. The area ratio of the α phase was 85% or more, and the area ratio of the α phase with a circle equivalent diameter of 1 μm or more was 80%. The average sheet thickness was 1.0 mm, and the dimensional accuracy was 1.5%. The 0.2% yield strength was 800 MPa, and the yield strength ratio σT/σL was 1.14.

For the reference example, the angle θ between the direction showing the peak of the density in the (0001) pole figure and the plate thickness direction was more than 65°. Therefore, the yield strength ratio σT/σL was more than 1.18, indicating strong anisotropy.

In addition, in Comparative Example 1, the rolling rate per run was as low as 20%, and the total rolling rate was also as low as 59%. Therefore, the angle θ between the direction showing the peak of the density in the (0001) pole figure and the plate thickness direction was more than 65°. Therefore, the yield strength ratio σT/σL was more than 1.18, indicating strong anisotropy. In Comparative Example 2, the rolling rate per run was 50%, but intermediate annealing and cold rolling were not repeated, and the total rolling rate was small at 50%. Therefore, the angle θ between the direction showing the peak of the density in the (0001) pole figure and the plate thickness direction was more than 65°. Therefore, the yield strength ratio σT/σL was more than 1.18, indicating strong anisotropy. In Comparative Example 3, the Al content was low, so the 0.2% yield strength was as low as 598 MPa. In Comparative Example 4, as described above, surface cracks and severe edge cracks occurred during cold rolling.

Example 2
In the same manner as in Example 1, hot-rolled steel sheets having chemical compositions shown in A, B, C, E and M in Table 1 and a thickness of 4 mm were produced.

Next, the obtained hot-rolled sheet was subjected to a cold rolling process under the conditions shown in Table 4. In invention examples 31 to 37 in Table 2, multiple cold rolling passes were performed so that the rolling ratio per cold rolling pass was 5% or more and the total rolling ratio shown in Table 4 was achieved. In invention examples 31 to 35 in Table 4, multiple cold rolling passes at a rolling temperature of 25°C and intermediate annealing under the conditions shown in Table 2 were repeated, and cold cross rolling was performed until the total rolling ratio reached 60 to 75%. Intermediate annealing was performed at a temperature of 680 to 900°C for 60 to 28,800 seconds, and final annealing was performed at a temperature of 650 to 930°C for 120 to 28,800 seconds. The cross rolling ratio of invention examples 32 to 36 was 0.4 to 7.0. Inventive Example 36 is an example in which multiple cold rolling passes at a rolling temperature of 400 ° C and intermediate annealing under the conditions shown in Table 4 were repeated, and cold cross rolling was performed until the total rolling reduction was 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 in which multiple cold rolling passes at a rolling temperature of 25 ° C and intermediate annealing under the conditions shown in Table 4 were repeated, and cold cross rolling was performed until the total rolling reduction was 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. At this time, the rolling in the cross direction was performed while appropriately cutting according to the width of the rolling roll so as to obtain a size that can be rolled.

For the titanium alloy plates according to each invention example, the same items as in Example 1 were evaluated in the same manner as in Example 1. The evaluation results are shown in Table 5.

For Examples 31 to 37, the angle θ between the direction showing the peak of the accumulation 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 μm or more and 10.0 μm or less, and the area ratio of the band structure was 10% or less. The area ratio of the α phase was 80% or more in all cases, and the area ratio of the α phase with a circle equivalent diameter of 1 μm or more exceeded 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 0.2% yield strength at 25°C was 700 MPa or more, and the yield strength ratio σT/σL, which is the ratio of the 0.2% yield strength σT in the width direction at 25°C to the 0.2% yield strength σL in the longitudinal direction at 25°C, was 0.85 to 1.10.

Although the preferred embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to such examples. It is clear that a person with ordinary knowledge in the technical field to which the present disclosure pertains can conceive of various modified or revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present disclosure.

Claims (14)

In mass percent,
Al: more than 4.0%, less than 6.6%,
Fe: 0% or more, 2.3% or less,
V: 0% or more, 4.5% or less,
Si: 0% or more, 0.60% or less,
C: 0% or more and less than 0.080%;
N: 0% or more, 0.050% or less,
O: 0% or more, 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%;
with the remainder being Ti and impurities,
The area ratio of the α phase is 80% or more,
The area ratio of α phase having an equivalent circle diameter of 1 μm or more is more than 53%,
In a (0001) pole figure from the thickness direction, an inverse pole figure using the spherical harmonic function method of electron backscatter diffraction has an expansion index of 16, and the angle between the direction showing the peak of the accumulation degree calculated by texture analysis when the Gaussian half width is 5° and the thickness direction is 65° or less;
A titanium alloy plate having an average plate thickness of 2.5 mm or less. The microstructure is composed of an equiaxed structure having an aspect ratio of 3.0 or less and a band structure having an aspect ratio of more than 3.0 and extending in the longitudinal direction;
The average grain size of the equiaxed structure is 0.5 μm or more and 20.0 μm or less,
The titanium alloy plate according to claim 1, wherein the area ratio of the band structure to the area of the microstructure is 10.0% or less. In mass%, Fe: 0.5% or more and 2.3% or less, or V: 2.5% or more and 4.5% or less;
The titanium alloy plate according to claim 1 or 2, comprising: In place of a portion of the Fe or V,
Ni: less than 0.15%
Cr: less than 0.25%; and
The titanium alloy plate according to claim 3 , containing one or more selected from the group consisting of: Mn: less than 0.25%. A titanium alloy plate according to any one of claims 1 to 4, in which the smaller of the 0.2% yield strength in the longitudinal direction at 25°C and the 0.2% yield strength in the width direction at 25°C is 700 MPa or more and 1200 MPa or less. In a (0001) pole figure from the plate thickness direction, an inverse pole figure obtained by using the spherical harmonic function method of electron backscatter diffraction has an expansion index of 16 and a Gaussian half-width of 5°, and the angle between the direction showing the peak of the accumulation degree calculated by texture analysis and the width direction is 10° or less;
The titanium alloy plate according to any one of claims 1 to 5, wherein the ratio of the 0.2% proof stress in the width direction to the 0.2% proof stress in the longitudinal direction is 1.05 or more and 1.18 or less. In the (0001) pole figure from the thickness direction, the inverse pole figure using the spherical harmonic function method of electron backscatter diffraction has an expansion index of 16, and the angle between the thickness direction and the direction showing the peak of the accumulation degree calculated by texture analysis when the Gaussian half width is 5° is 35° or less.
The titanium alloy plate according to any one of claims 1 to 5, wherein the ratio of the 0.2% 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. A titanium alloy plate according to any one of claims 1 to 7, in which the dimensional accuracy of the plate thickness is 5.0% or less of the average plate thickness. In mass percent,
Al: more than 4.0%, less than 6.6%,
Fe: 0% or more, 2.3% or less,
V: 0% or more, 4.5% or less,
Si: 0% or more, 0.60% or less,
C: 0% or more and less than 0.080%;
N: 0% or more, 0.050% or less,
O: 0% or more, 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%;
with the remainder being Ti and impurities,
The area ratio of the α phase is 80% or more,
The area ratio of α phase having an equivalent circle diameter of 1 μm or more is more than 53%,
In a (0001) pole figure from the thickness direction, an inverse pole figure using the spherical harmonic function method of electron backscatter diffraction has an expansion index of 16, and the angle between the direction showing the peak of the accumulation degree calculated by texture analysis when the Gaussian half width is 5° and the thickness direction is 65° or less;
A titanium alloy coil having an average thickness of 2.5 mm or less. A method for producing a titanium alloy plate according to any one of claims 1 to 8,
a cold rolling process of performing one or more cold rolling passes in the longitudinal direction of a titanium material containing, 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 less than 0.60%, 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%, and Mn: 0% or more and less than 0.25%, with the balance being Ti and impurities;
and a final annealing step of annealing the titanium material after the last cold rolling pass,
A method for producing a titanium alloy plate, wherein the cold rolling step has a rolling ratio per cold rolling pass of more than 30% and a total rolling ratio of 60% or more. The cold rolling process, when performing a plurality of cold rolling passes, includes an intermediate annealing process of annealing the titanium material between the plurality of cold rolling passes, and the annealing conditions of the intermediate annealing process and the final annealing process are that the annealing temperature is 600 ° C or more and (T _β -50) ° C or less, and the annealing temperature T (° C.) and the holding time t (seconds) at the annealing temperature satisfy the following formula (1):
22000≦(T+273.15)×(Log ₁₀ (t)+20)≦27000…Formula (1)
Here, _Tβ is the β transformation point (°C). A method for producing a titanium alloy plate according to any one of claims 1 to 8,
a cold cross rolling process in which cold rolling passes are performed in the longitudinal and transverse directions of the titanium material, the titanium material containing, by mass%, Al: more than 4.0% and not more than 6.6%, Fe: 0% or more and not more than 2.3%, V: 0% or more and not more than 4.5%, Si: 0% or more and not more than 0.60%, C: 0% or more and less than 0.080%, N: 0% or more and not more than 0.050%, O: 0% or more and not more than 0.40%, 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%, with the balance being Ti and impurities;
A final annealing step of annealing the titanium material after the cold cross rolling step,
The total rolling ratio in the cold cross rolling step is 60% or more,
A method for producing a titanium alloy plate, wherein a cross rolling ratio, which is a ratio of the rolling ratio in the longitudinal direction to the rolling ratio in the width direction, is 0.05 or more and 20.00 or less. The cold cross rolling process, when performing a plurality of cold rolling passes, includes an intermediate annealing process of annealing the titanium material between the plurality of cold rolling passes, and the annealing conditions of the intermediate annealing process and the final annealing process are that the annealing temperature is 600 ° C or more and (T _β -50) ° C or less, and the annealing temperature T (° C.) and the holding time t (seconds) at the annealing temperature satisfy the following formula (1):
22000≦(T+273.15)×(Log ₁₀ (t)+20)≦27000…Formula (1)
Here, _Tβ is the β transformation point (°C). A method for producing a titanium alloy coil according to claim 9,
a cold rolling process of performing one or more cold rolling passes in the longitudinal direction of a titanium material containing, by mass%, Al: more than 4.0% and not more than 6.6%, Fe: 0% or more and not more than 2.3%, V: 0% or more and not more than 4.5%, Si: 0% or more and not more than 0.60%, C: 0% or more and less than 0.080%, N: 0% or more and not more than 0.050%, O: 0% or more and not more than 0.40%, 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%, with the balance being Ti and impurities;
and a final annealing step of annealing the titanium material after the last cold rolling pass,
A method for producing a titanium alloy coil, wherein the cold rolling step has a rolling ratio per cold rolling pass of more than 30% and a total rolling ratio of 60% or more.

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