JP7733311B2 - β-type titanium alloy - Google Patents

Description

The present invention relates to a β-type titanium alloy with excellent workability.

Titanium has a high specific strength, which is one of the reasons it is used for weight reduction. Titanium also has excellent compatibility with the human body. Titanium alloys, in particular, have high strength and are expected to further contribute to weight reduction. For this reason, titanium and titanium alloys are used in aircraft, transportation equipment such as motorcycles and automobiles, and consumer products such as eyeglasses. However, titanium generally has an alpha phase with an hcp structure at room temperature, and due to this anisotropy, its mechanical properties are highly anisotropic. This makes it difficult to process, which can increase manufacturing costs.

In contrast, beta-type titanium alloys have a metal structure of the beta phase with a bcc structure at room temperature, and have relatively excellent workability. They have a particularly low Young's modulus of around 80 GPa, and are often used in components that require spring properties. Representative beta-type titanium alloys include Ti-15-3 (Ti-15V-3Cr-3Sn-3Al), SSAT-2041CF (Ti-20V-4Al-1Sn), and Ti-13V-11Cr-3Al. However, with the recent trend toward lighter, smaller products and more precise shapes, there is often demand for further improvements in the workability of beta-type titanium alloys, particularly their bending workability.

Patent Document 1 describes a method for producing a high-strength Ti alloy with excellent workability, which involves heating a β-type Ti alloy or near-β-type Ti alloy containing 1.0% or less Si, performing plastic working while the Si compounds in the Ti alloy are in solid solution, and suppressing recrystallization by finely precipitating the Si compounds while maintaining a temperature range above the β transformation point. Patent Document 2 also describes a β-type titanium alloy sheet containing, by mass, 14-25% V, 2.5-5% Al, 0.5-4% Sn, and 4% or less Cr, with the balance being Ti and impurities. The sheet has a hard layer on the surface, and the hardness at a depth of 10 μm from the surface is 1.2 times or more the hardness at the center of the sheet thickness, with the hard layer thickness being 2.0-10% of the sheet thickness per side. However, Patent Documents 1 and 2 do not examine bending workability.

Japanese Patent Application Publication No. 11-343548 Japanese Patent Application Laid-Open No. 2016-176115

The present invention was made in consideration of the above circumstances, and aims to provide a β-type titanium alloy with excellent workability.

In order to solve the above problems, the present invention employs the following configuration.
[1] Contains a β phase in the structure,
When the crystal orientation of the β phase is represented by Euler angles g = {φ ₁ , Φ, φ ₂ }, and the volume fraction of crystal grains having crystal orientations in the ranges of φ ₁ : 5°-30°, Φ: 0°-60°, φ ₂ : 35°-55° is X1, and the volume fraction of crystal grains having crystal orientations in the ranges of φ ₁ : 0°-90°, Φ: 50°-60°, φ ₂ : 35°-55° is X2, a β-type titanium alloy satisfies the following formula (1):
X1/X2≧0.30…(1)
[2] In mass%,
V: 12-25%,
Al: 2.0-5.0%,
Sn: 0.5-4.0%,
O: 0.05-0.20%,
Cr: 0-4.0%,
N: 0.050% or less,
C: 0.080% or less,
H: Contains 0.020 or less,
The β-type titanium alloy according to [1], having a chemical composition consisting of the balance: Ti and impurities.
[3] The β-type titanium alloy according to [1] or [2], wherein the average grain size of the β phase is 100 μm or less.

The present invention provides a β-titanium alloy with superior bending workability compared to conventional alloys.

The inventors conducted extensive research and carried out numerous experiments using a Ti-15-3 alloy as the base composition in order to develop a beta-type titanium alloy with excellent bending workability. As a result, they discovered that in beta-type titanium alloys, the texture shown in (A) below has a significant effect on workability. Note that in this specification, the following texture will be referred to as {h11}<1/h12> fiber.

Hereinafter, a β-type titanium alloy according to an embodiment of the present invention will be described.
The β-type titanium alloy of this embodiment contains a β-phase in its structure, and when the crystal orientation of the β-phase is represented by Euler angles g = { _φ1 , Φ, _φ2 }, the volume fraction of crystal grains having crystal orientations in the ranges of _φ1 : 5° to 30°, Φ: 0° to 60°, _φ2 : ₃₅ ° to 55° is X1, and the volume fraction of crystal grains having crystal orientations in the ranges of _φ1 : 0° to 90°, Φ: 50° to 60°, φ2: 35° to 55° is X2, and the β-type titanium alloy satisfies the following formula (1):
X1/X2≧0.30…(1)

Furthermore, in the β-type titanium alloy of this embodiment, it is preferable that the average crystal grain size of the β phase be 100 μm or less.

First, the texture of the β-titanium alloy of this embodiment will be described. When a β-titanium alloy is cold-rolled into a plate, the texture formed when processed with fewer rolling passes forms a texture with a _φ1 = 20° structure and {h11}<1/h12> fiber developed, compared to the texture formed when the same plate thickness is rolled with more rolling passes. β-titanium alloys with such a texture are characterized by small r-values and M-values. Generally, the lower the r-value and M-value, the better the bending workability, so the above-mentioned texture exhibits excellent bending workability. The r-value is the Lankford value, and the M-value is the Taylor factor.

In this embodiment, when the crystal orientation of the β phase is expressed by the Euler angle g = {φ ₁ , Φ, φ ₂ }, {h11}<1/h12> fiber is defined as the volume fraction of crystal grains having crystal orientations in the ranges of φ ₁ : 5° to 30°, Φ: 0° to 60°, and φ ₂ : 35° to 55°, and the volume fraction of these crystal grains is designated as X1.

On the other hand, when a β-type titanium alloy is cold-rolled, γ-fiber develops instead of {h11}<1/h12>fiber. The development of this γ-fiber increases the r-value and M-value and deteriorates bending workability. Therefore, in this embodiment, γ-fiber is defined as the volume fraction of crystal grains having crystal orientations in the ranges of _φ1 : 0° to 90°, Φ: 50° to 60°, and _φ2 : 35° to 55°, and this volume fraction of crystal grains is designated X2.

Then, it was discovered that good bending workability at room temperature can be achieved when the ratio of X1 to X2, X1/X2, satisfies the following formula (1):

X1/X2≧0.30…(1)

X1/X2 is preferably 0.35 or greater, and more preferably 0.40 or greater. Furthermore, the greater the bending workability, the better, but the upper limit is thought to be around 10.0. The upper limit in actual results was 8.0.

The following explains the crystal orientations that represent the texture. To express the crystal orientations in the texture in three dimensions, we use the notation method using Euler angles. The notation method using Euler angles (Bunge's notation method) considers three coordinate axes, RD, TD, and ND, that are mutually orthogonal to each other as the sample coordinate system (coordinate system of the plate material).

The crystal orientation distribution of a polycrystalline body is expressed by a function f(φ ₁ , Φ, φ ₂ ) using Euler angles (φ ₁ , Φ, φ ₂ ), and this function is called the crystal orientation distribution function (ODF).

The crystal orientation distribution function (ODF) can be determined by the electron backscattered diffraction (EBSD) method. The EBSD pattern is measured and analyzed by the electron backscattered diffraction (EBSD) method while scanning the inspection surface with an electron beam using a scanning electron microscope (SEM). The Euler angles (φ ₁ , Φ, φ ₂ ) of the crystal orientation at each measurement point are obtained by converting the angle relative to the plate surface through computer calculations. The ODF (f(φ ₁ , Φ, φ ₂ )) can be calculated based on the data of the measurement points within the measured field of view.

Because the β phase is uniformly distributed throughout the structure of a β-type titanium alloy, the crystal orientation distribution function can be measured at any cross section of the β-type titanium alloy. For example, a surface perpendicular to the width direction of a β-type titanium alloy plate at half the plate width (hereinafter also referred to as the "L cross section") is polished to create a measurement surface. An electron beam is scanned with a scanning electron microscope (SEM) at a 10.0 μm pitch (10.0 μm step) over a field of view of the entire plate thickness x 10 mm on this surface, and the EBSD pattern is measured and analyzed using the electron backscatter diffraction (EBSD) method. The EBSD pattern is then converted to an angle relative to the plate surface by computer calculation, yielding the Euler angles of the titanium β-phase crystals at each measurement point.

The ODF of the β phase is calculated using the data measured under the above conditions using TSL Solutions' OIM Analysis software (version 8.1.0). Texture analysis using the spherical harmonics method of electron backscatter diffraction (EBSD) is used to calculate the ODF (expansion index = 16, Gaussian half-width = 5°). The symmetry of the rolling deformation is taken into account, and the calculation is performed so that the result is symmetrical with respect to the thickness direction, rolling direction, and width direction.

The present invention is directed to β-type titanium alloys. That is, titanium alloys in which the proportion of β phase measured by the electron backscatter diffraction method is greater than 50% by area. The proportion of β phase is preferably 70% or more by area, and more preferably 90% or more by area.

In β-type titanium alloys, formability improves when the average crystal grain size of the β phase is coarse. However, when β-type titanium alloys are made into thin plate materials, if the average crystal grain size is too coarse, the number of crystal grains relative to the plate thickness decreases, which in turn reduces plastic deformability and bending workability. An average crystal grain size of 100 μm or less ensures a sufficient number of crystal grains relative to the plate thickness and does not reduce workability, so the average crystal grain size of the β phase is preferably 100 μm or less. It is even more preferably 50 μm or less. On the other hand, when the β phase has fine grains, properties such as fatigue resistance improve, but the strength increases, resulting in reduced ductility. An average crystal grain size of 10 μm or more does not reduce ductility, so an average crystal grain size of 10 μm or more is preferred.

The grain size of the β phase is measured under the same conditions as when measuring the volume fraction of grains with a specified crystal orientation, and the average grain size of 100 or more grains is calculated. The grain size of the grains is the equivalent circle diameter.

The β-titanium alloy of this embodiment needs only to contain a β phase and satisfy the ratio X1/X2, the volume fraction ratio of crystal grains having the specified crystal orientation described above, of 0.30 or greater. There is no need to limit the chemical composition of the titanium alloy, but examples of chemical compositions applicable to the β-titanium alloy of this embodiment are listed below.

In other words, the β-type titanium alloy of this embodiment may have a chemical composition containing, in mass%, V: 12-25%, Al: 2.0-5.0%, Sn: 0.5-4.0%, O: 0.05-0.20%, Cr: 0-4.0%, N: 0.050% or less, C: 0.080% or less, H: 0.020% or less, with the balance being Ti and impurities. In the following description, "mass%" will be abbreviated as "%".

[V content]
V is an important element for stabilizing the β phase and achieving a single β phase structure at room temperature before aging. In addition, because it is resistant to solidification segregation during melting, it can be easily incorporated in relatively large amounts. On the other hand, if the V content is low, even rapid cooling from a high-temperature β single-phase region, such as water cooling, may result in martensitic transformation without the β phase remaining, thereby degrading workability at room temperature. Therefore, the V content is preferably 12% or more, and more preferably 13% or more. On the other hand, if the V content exceeds 25%, the cold workability of the alloy may be significantly degraded due to a decrease in ductility caused by solid solution strengthening. Furthermore, the stability of the β phase becomes too high, making it difficult for the α phase to precipitate, which may lengthen the time required for aging and further prevent sufficient strengthening from being achieved by aging. Therefore, the V content should be 25% or less, preferably 23% or less, and more preferably 22% or less.

[Al content]
β-type titanium alloys are strengthened by aging treatment in the final product, and it is preferable to include Al, an α-phase stabilizing element, to obtain a sufficient increase in strength during this process. Furthermore, Al also has the effect of suppressing the precipitation of brittle ω-phase during aging treatment and promoting the precipitation of α-phase. If the Al content is less than 2.0%, the effects of increasing strength and suppressing ω-phase are not sufficiently obtained. On the other hand, if the Al content exceeds 5.0%, the alloy becomes too hard in the β-phase single phase state, reducing cold workability. Therefore, the Al content is set to 2.0-5.0%, preferably 4.0% or less.

[Sn content]
Although Sn is a neutral element, it has the same effect as Al described above. Therefore, by substituting Sn for Al, strengthening can be achieved without impairing cold workability. If the Sn content is less than 0.5%, the effect of substituting Al is small, resulting in poor strengthening. On the other hand, if the Sn content exceeds 4.0%, the material becomes too hard in the single β phase state, reducing cold workability. Therefore, the Sn content is set to 0.5 to 4.0%.

[Cr content]
Cr is a eutectoid reaction element and has a higher β-stabilizing ability than V, so even a small amount of Cr is effective in stabilizing the β phase and ensuring cold workability. On the other hand, Cr is an element that easily segregates during melting, and adding a large amount of Cr increases the strength variation within the ingot. In addition, the β phase is too stable, making it difficult for the α phase to precipitate, which lengthens the time required for aging treatment and prevents sufficient strengthening through aging treatment. Therefore, the above problems are unlikely to occur if the Cr content is up to 4.0%, so this may be the upper limit. Preferably, it is 3.5% or less. Cr does not need to be added. Therefore, the lower limit of the Cr content is 0% or more, preferably more than 0%.

[O content]
O is an element with high solid solution strengthening ability. If the O content is less than 0.05%, the strength is insufficient. Therefore, the O content is 0.05% or more, preferably 0.06% or more. On the other hand, if the O content exceeds 0.20%, the strength becomes too high, significantly reducing the formability. Therefore, the O content is 0.20% or less, preferably 0.18% or less.

[N content, C content, H content]
If the N content, C content, and H content are each excessive, the ductility of the β-type titanium alloy will decrease. Therefore, the N content is set to 0.050% or less, the C content to 0.080% or less, and the H content to 0.020% or less. The C content is preferably less than 0.050%. The contents of these elements may be 0%, but setting the N content to less than 0.0001%, the C content to less than 0.0001%, and the H content to less than 0.00001% will significantly increase smelting costs. Therefore, the N content may be set to 0.0001% or more, the C content to 0.0001% or more, and the H content to 0.00001% or more. The N content may be set to 0.001% or more, the C content to 0.001% or more, and the H content to 0.001% or more.

The balance is Ti and impurities.

Furthermore, when the β-titanium alloy of this embodiment is made into a plate material, the plate thickness is preferably 4 mm or less. The form of the β-titanium alloy of this embodiment is not limited to a plate material, and it may also be a rod material or a tube material.

Next, a method for producing the β-type titanium alloy of this embodiment will be described.
First, a titanium alloy slab is produced. The method for producing the titanium alloy slab is not particularly limited, but it can be produced by, for example, vacuum arc melting (VAR) or electron beam melting (EBR). The surface of the obtained titanium alloy slab may be cut if necessary.

Next, in the hot rolling process, the titanium alloy slab is heated to 800°C to 1100°C and then hot rolled. Hot rolling is preferably performed under conditions that result in a total rolling reduction of 80% or more, and preferably 90% or more. By setting the total rolling reduction to 80% or more, the strain necessary for recrystallization during annealing can be imparted.

After the hot rolling process, the titanium material may or may not be annealed at a soaking temperature of 600°C to 950°C for a soaking time of 1 minute to 30 minutes (60 seconds to 1800 seconds). Cooling after annealing can be achieved by natural cooling. Annealing may be performed in an air atmosphere, an inert atmosphere, or a vacuum atmosphere. When hot-rolled sheet annealing is performed, the unrecrystallized grains in the hot-rolled sheet can be precipitated as fine recrystallized grains, resulting in more uniform and finer crystals in the metal structure of the final titanium alloy sheet.

After annealing, the titanium material may be subjected to pickling or cutting to remove oxide scale, etc., and further cleaning treatment, if necessary.

Next, the annealed β-type titanium alloy is cold-rolled one or more times in its longitudinal direction. The number of cold-rolling passes may be three or less. The reduction rate per pass in the cold-rolling process is 65% or more, preferably 80% or more. By increasing the reduction rate per pass to 65% or more, it is possible to develop {h11}<1/h12> fiber and increase its volume fraction X1. As a result, by increasing X1/X2 to 0.30 or more, the bending workability of the β-type titanium alloy can be improved.

When cold rolling is performed multiple times, intermediate annealing may be performed. The conditions for intermediate annealing are preferably a soaking temperature of 750°C or higher and 950°C or lower. Furthermore, the holding time t (seconds) at the soaking temperature T (°C) is preferably set to satisfy the following formula (2). Formula (2) is the Larson-Miller parameter.

22000≦(T+273.15)×(Log ₁₀ (t)+20)≦28000…(2)

Cooling after intermediate annealing can be done by natural cooling. Intermediate annealing can be performed in air, an inert atmosphere, or a vacuum atmosphere.

Final annealing may also be performed on the cold-rolled β-type titanium alloy. The conditions for final annealing are preferably a soaking temperature of 750°C or higher and 950°C or lower. The holding time t (seconds) at the soaking temperature T (°C) is preferably set to satisfy the above formula (2). Cooling after final annealing may be natural cooling. Final annealing may be performed in an air atmosphere, an inert atmosphere, or a vacuum atmosphere. Performing final annealing removes strain that occurred during cold rolling and allows the material properties to be adjusted to be suitable for bending.

Next, the present invention will be described in more detail using examples. However, the examples described below are merely examples of embodiments of the present invention and are not intended to limit the scope of the present invention.

Slabs made of β-titanium alloy were produced using the vacuum arc melting (VAR) method. Specifically, metal raw materials, the composition of which was adjusted to achieve the chemical compositions shown in Table 1, were vacuum arc melted to produce molten metal, which was then charged into a water-cooled copper mold to produce slabs made of β-titanium alloy with a thickness of 200 mm. Blank cells in Table 1 indicate that no alloying elements were intentionally added.

The slab was then heated to 950°C to 1100°C and hot-rolled. The hot-rolling conditions were such that the total rolling reduction was 80% or more. This resulted in a hot-rolled sheet made of β-type titanium alloy with a thickness of 3mm to 6mm.

The hot-rolled sheet was then annealed under the conditions shown in Table 2A. The annealing atmosphere was a vacuum. After annealing, the hot-rolled sheet was pickled to remove oxide scale.

Next, the hot-rolled annealed β-type titanium alloy sheet was cold-rolled one or more times in its longitudinal direction, and intermediate annealing was performed as necessary. Tables 2A and 2B show the cold-rolling conditions and intermediate annealing conditions. The annealing atmosphere was air.

Next, a portion of the cold-rolled β-type titanium alloy plate was subjected to final annealing under the conditions shown in Table 2B. The annealing atmosphere was a vacuum. In this way, β-type titanium alloy plates No. 1 to 21 were obtained. The thickness of the obtained titanium alloy plates was all 1.0 mm or less.

We will now explain how to evaluate the texture. To express the crystal orientation in the texture in three dimensions, we used the Euler angle notation method. The Euler angle notation method (Bunge's notation method) considers three coordinate axes, RD, TD, and ND, which are mutually orthogonal to each other, as the sample coordinate system (coordinate system for the plate material).

The crystal orientation distribution of a polycrystalline body is expressed by a function f(φ ₁ , Φ, φ ₂ ) using Euler angles (φ ₁ , Φ, φ ₂ ), and this function is called the crystal orientation distribution function (ODF).

The crystal orientation distribution function (ODF) can be determined by the electron backscattered diffraction (EBSD) method. The EBSD pattern was measured and analyzed by the electron backscattered diffraction (EBSD) method while scanning the inspection surface with an electron beam using a scanning electron microscope (SEM), and the Euler angles (φ ₁ , Φ, φ 2 ) of the crystal orientation at each measurement point were obtained by converting the angle relative to the plate surface by computer calculation. The ODF (f(φ ₁ , Φ, φ ₂ )) was calculated based on the data of the measurement points within _the measured field of view.

Because the β phase is uniformly distributed throughout the structure of β-type titanium alloys, a surface perpendicular to the width direction of the β-type titanium alloy plate at half the width (hereinafter referred to as the "L-section") was polished to create the measurement surface. The EBSD pattern was measured and analyzed using the electron backscatter diffraction (EBSD) method while scanning an electron beam with a scanning electron microscope (SEM) at a 10.0 μm pitch (10.0 μm step) over a field of view of the entire plate thickness x 10 mm on this surface. The EBSD pattern was then converted into an angle relative to the plate surface using computer calculations to obtain the Euler angles of the titanium β-phase crystals at each measurement point.

The ODF of the β phase was calculated for the data measured under the above conditions using TSL Solutions' OIM Analysis software (version 8.1.0). Texture analysis using the spherical harmonics method of electron backscatter diffraction (EBSD) was used to calculate the ODF (expansion index = 16, Gaussian half-width = 5°). Taking into account the symmetry of rolling deformation, calculations were performed to ensure line symmetry in the thickness direction, rolling direction, and width direction. The results are shown in Table 3.

The bending workability was evaluated as follows. A three-point bending test was performed on the obtained titanium alloy plate (size: length in the rolling direction 60 mm, length in the rolling width direction 30 mm). The test conditions were as follows: the bending axis was set in the rolling direction, the bending radius was 0.4 mm, and the clearance CL (mm) satisfied the following formula (3) for the plate thickness A of ₁ mm, and the bending test was performed at a plunge speed of 10 min/mm.

CL=2.0×(A ₁ +1)…(3)

To evaluate workability, the surface of the bent section at a plunge angle of 80° was visually inspected and observed under an optical microscope at magnifications of 20x to 100x, with 2 to 10 fields of view being observed. The presence of cracks was marked "X" and the absence of cracks was marked "O". Furthermore, cases where there were no cracks but the surface had wrinkles or large irregularities were marked "△". Cases with cracks were deemed "failure", and cases with no cracks marked "○" or "△" were deemed "pass". The results are shown in Table 3.

As shown in Tables 1 to 3, when Nos. 5, 6, 8, 20, and 21 were cold-rolled one to three times, the cold-rolling reduction was less than 65% in at least one cold-rolling. As a result, the X1/X2 ratio was less than 0.30, and bending workability was reduced.

In addition, Nos. 16 and 19 had Larson-Miller parameters outside the specified range, which deviated from the preferred manufacturing conditions. As a result, X1/X2 was less than 0.30, resulting in poor bending workability.

On the other hand, as shown in Tables 1 to 3, for Nos. 1 to 4, 7, 9 to 15, 17, and 18, the hot-rolled sheet annealing conditions, cold rolling conditions, intermediate annealing conditions, and final annealing conditions were within the preferred ranges, so X1/X2 was 0.30 or greater, and bending workability was good. Furthermore, Nos. 1 to 4, 7, 9 to 15, 17, and 18 had a β-phase area fraction of over 50%, confirming that they were β-type alloys.

Claims (3)

Contains β phase in the structure,
When the crystal orientation of the β phase is represented by Euler angles g = {φ ₁ , Φ, φ ₂ }, and the volume fraction of crystal grains having crystal orientations in the ranges of φ ₁ : 5°-30°, Φ: 0°-60°, φ ₂ : 35°-55° is X1, and the volume fraction of crystal grains having crystal orientations in the ranges of φ ₁ : 0°-90°, Φ: 50°-60°, φ ₂ : 35°-55° is X2, a β-type titanium alloy satisfies the following formula (1):
X1/X2≧0.30…(1) In mass%,
V: 12-25%,
Al: 2.0-5.0%,
Sn: 0.5-4.0%,
O: 0.05-0.20%,
Cr: 0-4.0%,
N: 0.050% or less,
C: 0.080% or less,
H: Contains 0.020 or less,
The β-type titanium alloy according to claim 1, having a chemical composition consisting of the balance: Ti and impurities. The β-type titanium alloy according to claim 1 or 2, wherein the average grain size of the β phase is 100 μm or less.