Treatment Method Of Foil, Production Method Of Foil, And Foil

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2023-223115, filed on Dec. 28, 2023, which is expressly incorporated herein by reference in its entirety.

BACKGROUND
Technical Field

The present invention relates to a treatment method of a foil, a production method of a foil, and a foil.

Related Art

As an example of a production method of a titanium foil, a method of producing a titanium foil from an electro-deposited titanium foil obtained by an electrochemical reaction in a fused salt bath has been proposed (for example, see Japanese Patent Application Laid-Open No. 2023-125429). The production method of a titanium foil disclosed in JP 2023-125429 includes a cold rolling step of cold-rolling an electro-deposited titanium foil obtained by an electrochemical reaction in a fused salt bath and a finishing treatment step of performing washing and/or annealing. With the fused salt electrolysis, crystal grains sequentially grow on an electrode. It has been confirmed in JP 2023-125429 that, as a result of this effect, about three to four crystal grains are arranged in an electro-deposited titanium foil with a thickness of 190 μm in the thickness direction of the electro-deposited titanium foil.

However, when seeking to limit the number of crystal grains in the foil thickness direction to a desired range, it is difficult to control the number of crystal grains aligned in the thickness direction of the titanium foil by the production method of a titanium foil disclosed in JP 2023-125429 because only cold rolling and annealing are performed on the electro-deposited titanium foil after the crystal grains have been sequentially grown on the electrode.

SUMMARY

In view of such circumstances, an object of the present invention is to provide a treatment method of a foil made of a titanium alloy, the method being capable of controlling the number of crystal grains aligned in the thickness direction of the foil of a titanium alloy. Also provided are a production method of a foil made of a titanium alloy, and a foil.

The treatment method of a foil according to the present invention includes a recrystallization heating step of heating a foil made of a titanium alloy containing tantalum under vacuum or under an inert gas atmosphere to recrystallize the structure of the foil, and a foil treatment cooling step of, after the recrystallization heating step, cooling the foil at a cooling rate that is higher than or equal to a cooling rate in which the temperature of the foil decreases by −150° C. per 10 minutes within a range until the temperature of the foil reaches 600° C. under vacuum or under an inert gas atmosphere to suppress the growth of crystal grains in the structure of the foil, the growth being associated with the recrystallization.

The treatment method of a foil according to the present invention includes a recrystallization heating step of heating a foil made of a titanium alloy containing tantalum under vacuum or under an inert gas atmosphere to recrystallize the structure of the foil, and a foil treatment cooling step of, after the recrystallization heating step, cooling the foil by forced cooling in which the temperature of the foil is lowered more quickly than in a case of natural cooling, under vacuum or under an inert gas atmosphere to suppress the growth of crystal grains in the structure of the foil, the growth being associated with the recrystallization.

In the treatment method of a foil according to the present invention, the titanium alloy contains 15 at % to 27 at % of tantalum, 1 at % to 8 at % of tin, and 0.4 at % to 1.7 at % of oxygen when the entire titanium alloy is defined as 100 at %, and the remaining portion is composed of titanium and inevitable impurities.

The production method of a foil according to the present invention includes a foil forming step of processing a titanium alloy material made of a titanium alloy containing tantalum to form a foil, and a foil treatment step of treating the foil. The foil forming step includes a rolling step of rolling the titanium alloy material to form a foil, an annealing step of annealing the titanium alloy material, which has been subjected to the rolling step, at an annealing temperature for annealing, and a foil formation cooling step of cooling the titanium alloy material that has been subjected to the annealing step. The foil treatment step includes a recrystallization heating step of heating the foil under vacuum or under an inert gas atmosphere to recrystallize the structure of the foil, and a foil treatment cooling step of, after the recrystallization heating step, cooling the foil at a cooling rate that is higher than or equal to a cooling rate in which the temperature of the foil decreases by −150° C. per 10 minutes within a range until the temperature of the foil reaches 600° C. under vacuum or under an inert gas atmosphere to suppress the growth of crystal grains in the structure of the foil, the growth being associated with the recrystallization. The rolling step refers to one lastly performed in the foil forming step.

The production method of a foil according to the present invention includes a foil forming step of processing a titanium alloy material made of a titanium alloy containing tantalum to form a foil, and a foil treatment step of treating the foil, in which the foil forming step includes a rolling step of rolling the titanium alloy material to form a foil, an annealing step of annealing the titanium alloy material, which has been subjected to the rolling step, at an annealing temperature for annealing, and a foil formation cooling step of cooling the titanium alloy material that has been subjected to the annealing step, and the foil treatment step includes a recrystallization heating step of heating the foil under vacuum or under an inert gas atmosphere to recrystallize the structure of the foil, and a foil treatment cooling step of, after the recrystallization heating step, cooling the foil by forced cooling in which the temperature of the foil is lowered more quickly than in a case of natural cooling under vacuum or under an inert gas atmosphere to suppress the growth of crystal grains in the structure of the foil, the growth being associated with the recrystallization.

In the production method of a foil according to the present invention, the titanium alloy contains 15 at % to 27 at % of tantalum, 1 at % to 8 at % of tin, and 0.4 at % to 1.7 at % of oxygen when the entire titanium alloy is defined as 100 at %, and the remaining portion is composed of titanium and inevitable impurities.

In the production method of a foil according to the present invention, the foil forming step includes, prior to the rolling step, a processing step of processing the titanium alloy material at least once, and the processing step includes a second rolling step of rolling the titanium alloy material to thin the thickness of the titanium alloy material, a second annealing step of annealing the titanium alloy material, which has been subjected to the second rolling step, at an annealing temperature for annealing, and a second foil formation cooling step of cooling the titanium alloy material that has been subjected to the second annealing step. The second rolling step refers to one performed prior to the rolling step to be lastly performed. The second annealing step refers to one performed after the second rolling step in the single processing step. The second foil formation cooling step refers to one performed prior to the foil formation cooling step.

In the production method of a foil according to the present invention, the average particle diameter by an area weighted average of crystal grains contained in the foil or the number of crystal grains aligned in the thickness direction of the foil is controlled by adjusting the rolling reduction ratio in the rolling step.

In the treatment method of a foil or the production method of a foil according to the present invention, the vacuum degree under vacuum is 1×10⁻²(Pa) or less.

In the treatment method of a foil or the production method of a foil according to the present invention, the heating temperature of the foil in the recrystallization heating step is in a range of 700 to 900° C.

In the treatment method of a foil or the production method of a foil according to the present invention, the recrystallization heating step is performed in a furnace having a housing unit configured to be able to house the foil, a vacuum generation unit configured to vacuumize the inside of the housing unit, and a heating unit configured to heat the housing unit, and the foil treatment cooling step involves externally cooling the housing unit that houses the foil under an inert gas atmosphere or under vacuum to cool the foil.

In the treatment method of a foil or the production method of a foil according to the present invention, the ratio of an α phase contained in the foil after the foil treatment cooling step is 10% or less, and the ratio of a β phase is 90% or more.

In the treatment method of a foil or the production method of a foil according to the present invention, the average particle diameter by an area weighted average of crystal grains contained in the foil or the number of crystal grains aligned in the thickness direction of the foil is controlled by adjusting the heating temperature in the recrystallization heating step.

The foil of the present invention is a foil made of a titanium alloy, in which the titanium alloy contains 15 at % to 27 at % of tantalum, 1 at % to 8 at % of tin, and 0.4 at % to 1.7 at % of oxygen when the entire titanium alloy is defined as 100 at %, the remaining portion is composed of titanium and inevitable impurities, the average particle diameter by an area weighted average of crystal grains contained in the foil is in a range of 8.7 μm or more and 16.5 μm or less, the ratio of an α phase contained in the foil is 10% or less, and the ratio of a β phase is 90% or more.

According to the production method of a foil in the present invention, the excellent effect of controlling the size of crystal grains of a foil can be exerted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a production method of a foil in an embodiment of the present invention.

FIG. 2 is a Ta—Ti binary system diagram.

FIG. 3 is a graph showing the result of a cold workability evaluation test for a titanium alloy (Ti-23Ta-xSn-0.5O) in which 23 at % of tantalum, x at % of tin, and 0.5 at % of oxygen are contained when the entire titanium alloy is defined as 100 at %, and the remaining portion is composed of titanium and inevitable impurities.

FIG. 4 is a flowchart illustrating a flow of a production method of a titanium alloy material in an embodiment of the present invention.

FIG. 5A is a schematic diagram of an HIP apparatus used in a production method of a titanium alloy material in an embodiment of the present invention. FIG. 5B is a schematic diagram of an apparatus for performing a vacuum arc remelting method (VAR) used in a production method of a titanium alloy material in an embodiment of the present invention.

FIGS. 6A and 6B are diagrams illustrating an example of a configuration of a cold rolling apparatus used in production of a foil in an embodiment of the present invention.

FIG. 7A is a view illustrating an example of a configuration of a furnace used in production of a foil and a placement configuration of a foil in the furnace in an embodiment of the present invention. FIG. 7B is a view illustrating a variation example of a configuration of a furnace used in production of a foil and a placement configuration of a foil in the furnace in an embodiment of the present invention. FIG. 7C is a plan view of a base material, a titanium alloy rolled material, and a weight material housed in a housing unit of FIG. 7A.

FIG. 8 is a view for explaining a parallel cut surface and an orthogonal cut surface of a foil.

FIG. 9 is a graph illustrating a temperature change of a second foil subjected to a recrystallization heating step and a foil treatment cooling step in Example 1 of the present invention.

FIG. 10A is a photograph of an orthogonal cut surface of a first foil. FIG. 10B is a photograph of an orthogonal cut surface of a second foil. FIG. 10C is a photograph of an orthogonal cut surface of a third foil.

FIG. 11A is a photograph of an orthogonal cut surface of a fourth foil. FIG. 11B is a photograph of an orthogonal cut surface of a fifth foil. FIG. 11C is a photograph of an orthogonal cut surface of a sixth foil.

FIG. 12A is a photograph of an orthogonal cut surface of a first comparative example foil. FIG. 12B is a photograph of an orthogonal cut surface of a second comparative example foil. FIG. 12C is a photograph of an orthogonal cut surface of a third comparative example foil.

FIG. 13A is a photograph of an orthogonal cut surface of a fourth comparative example foil. FIG. 13B is a photograph of an orthogonal cut surface of a fifth comparative example foil. FIG. 13C is a photograph of an orthogonal cut surface of a sixth comparative example foil.

FIG. 14A is a view for explaining a method of counting crystal grains in photographs of orthogonal cut surfaces of first to sixth comparative example foils. FIG. 14B is a bar chart illustrating the number of crystal grains of each of the first to sixth comparative example foils counted in corresponding photographs of orthogonal cut surfaces.

FIG. 15A is a graph illustrating a ratio of an α phase contained in an orthogonal cut surface at a rolling reduction ratio of 75% of each of the first to third foils. FIG. 15B is a graph illustrating a ratio of a β phase contained in an orthogonal cut surface at a rolling reduction ratio of 75% of each of the first to third foils. FIG. 15C is a graph illustrating a ratio of an α phase contained in an orthogonal cut surface at a rolling reduction ratio of 90% of each of the fourth to sixth foils. FIG. 15D is a graph illustrating a ratio of a β phase contained in an orthogonal cut surface at a rolling reduction ratio of 90% of each of the fourth to sixth foils.

FIG. 16 is a bar chart illustrating an average particle diameter of crystal grains in an orthogonal cut surface of each of the first to sixth foils.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

A production method of a foil according to the present embodiment is to produce a foil from a titanium alloy material and includes, as illustrated in FIG. 1, a foil forming step, a cutting step, and a foil treatment step. Since the foil is composed of a titanium alloy, it may be referred to as a titanium alloy foil.

First, a titanium alloy that constitutes the titanium alloy material will be described. The titanium alloy contains at least titanium (Ti) and tantalum (Ta). However, the titanium alloy may contain additional elements other than Ti and Ta. Examples of elements other than Ti and Ta include tin (Sn). Specifically, the titanium alloy contains 15 at % or more and 27 at % or less of Ta, 0 at % or more and 8 at % or less of Sn, and 0.4 at % or more and 1.7 at % or less of oxygen (O) when the entire titanium alloy is defined as 100 at %, the remaining portion is preferably composed of Ti and inevitable impurities. The content of Ti in the remaining portion is not particularly limited as long as the most abundant element in the contained elements is Ti when considered in terms of atomic ratio. Such titanium alloys not only exhibit improved mechanical properties, such as high tensile strength, low Young's modulus and moderate elastic limits, but also offer high biocompatibility. Incidentally, “at %” used herein represents atomic percentage, and in the following description, it will be used to indicate the atomic percentage of the corresponding element when the entire titanium alloy is defined as 100 atomic % (at %). Furthermore, in the following, the numerical values attached to the elemental designations Ta, Sn, and O, which represent the composition of the titanium alloy (shown in parentheses immediately after titanium alloys in the following) indicate the atomic percentages (at %) of respective elements (Ta, Sn, O) when the entire titanium alloy is taken as 100 at %.

Furthermore, although not particularly limited, titanium alloys in the present invention are broadly classified into three types: α-type titanium alloys, which take the form of a hexagonal close-packed crystal (HCP) with an α phase as the matrix; β-type titanium alloys, which take the form of a body-centered cubic crystal (BCC) with a β phase as the matrix; and α+β-type titanium alloys, which contain both an α phase (HCP) and a β phase (BCC).

Ta causes the titanium alloy in this embodiment to undergo thermoelastic martensitic transformation. Ta functions to lower the transformation temperature from the β phase to the α phase, to stabilize the β phase at room temperature, and to suppress slip deformation (plastic deformation).

The upper limit of the Ta content is set on the basis of the melting point of the titanium alloy. FIG. 2 is a binary phase diagram of Ta—Ti. As shown in FIG. 2, when the Ta content exceeds 27 at %, the melting point of the titanium alloy may reach about 2,000 K or higher, requiring a special melting furnace and consequently increasing the manufacturing cost. In addition, incomplete melting of the Ta raw material may occur, resulting in degradation of the titanium alloy quality.

The lower limit of the Ta content is set on the basis of the above-described β phase stabilization function and mechanical properties of the titanium alloy as a material for a medical device, a biomaterial, or the like. In other words, the β phase stabilization function decreases as the Ta content decreases, and thus it is difficult to maintain the β phase at normal temperature when the Ta content is less than 15 at %. Therefore, when the Ta content is less than 15 at %, it becomes difficult to achieve the mechanical properties (Young's modulus, tensile strength, and elastic deformation strain) required for materials used in a medical device, a biomaterial, and similar applications, even with the addition of Sn. Therefore, the Ta content is preferably 15 at % or more, more preferably 19 at % or more, and most preferably 22 at % or more when the entire titanium alloy is defined as 100 at %.

Considering the above, the Ta content is preferably 15 to 27 at %, more preferably 19 to 25 at %, and most preferably 22 to 24 at %, when the entire titanium alloy is defined as 100 at %.

Sn has an α phase stabilization function of increasing the transformation temperature to stabilize the α phase. Furthermore, Sn has a function of suppressing ω phase precipitation, which is a causal factor in increasing the Young's modulus, and enhancing the superelastic effect of the titanium alloy.

The upper limit of the Sn content is set on the basis of the workability (cold workability) of the titanium alloy. FIG. 3 is a graph showing the result of a cold workability evaluation test for a titanium alloy (Ti-23Ta-xSn-0.5O) having a Ta content of 23 at % when the entire titanium alloy is defined as 100 at %. Note that x is the Sn content (at %) when the entire titanium alloy is defined as 100 at %. In this cold workability evaluation test, first a plurality of test pieces (thickness: 1 mm, without heat treatment) were prepared while the Sn content x was changed to 0 at %, 1.5 at %, 3 at %, 6 at %, and 9 at % when the entire titanium alloy is defined as 100 at %. Then, these test pieces were cold-rolled to have a thickness of 0.1 mm (processing rate: 86%), and the number of cracks having a length equal to or greater than 1 mm in each test piece after cold rolling was counted. Note that the counting of the cracks was performed within a 140 mm span along the rolling direction of each test piece. Incidentally, the processing rate refers to the percentage (%) obtained by dividing the difference between the cross-sectional area of the material before and after cold plastic processing such as wire drawing or rolling by the cross-sectional area of the material before processing.

As shown in FIG. 3, as a result of this evaluation test, when the Sn content is 9 at %, generation of cracks having a length equal to or greater than 1 mm rapidly increased, that is, it was confirmed that the workability deteriorates rapidly. In addition, the same tendency was present even when the Ta content differed. Therefore, in order to obtain good workability, the Sn content is preferably 8 at % or less, more preferably 6 at % or less when the entire titanium alloy is defined as 100 at %.

The lower limit of the Sn content is not particularly limited, but may be 0 at % as described above. Such an Sn content is acceptable because even without addition of Sn, a titanium alloy having required mechanical properties (Young's modulus, tensile strength, and elastic deformation strain) can be obtained as long as the Ta content is 15 at % or more.

However, it is preferable to add Sn to the titanium alloy in order to further improve mechanical properties. For example, from the viewpoint of the superelasticity effect of the titanium alloy, Sn has a function of suppressing precipitation of an ω phase to prevent an increase of a Young's modulus and enhancing the superelasticity effect of the titanium alloy. This superelasticity flexibly accommodates unintended deformation. Therefore, it is preferable to add Sn to the titanium alloy. For sufficiently exerting the above-described ω phase suppression function, the Sn content when the entire titanium alloy is defined as 100 at % is preferably 1 at % or more.

Therefore, the Sn content is preferably 1 to 8 at % and more preferably 2 to 6 at % when the entire titanium alloy is defined as 100 at %.

Further, the above-described titanium alloy containing Sn has an extremely low elution amount of metal ions of Ti, Ta, and Sn as constituent elements, exhibits excellent corrosion resistance, low cytotoxicity, and high biocompatibility, is a non-magnetic body that is hardly magnetized by an external magnetic field and is extremely unlikely to adversely influence medical equipment (such as MRI) that are negatively affected by magnetism, is highly elastic and appropriately stiff, and has high processability. That is, the above-described titanium alloy containing Sn is lower in cytotoxicity than known titanium alloys and has excellent magnetic properties, corrosion resistance, mechanical properties, and processability.

Oxygen (O) has an α phase stabilization function of increasing a transformation temperature and stabilizing an α phase. Furthermore, the α phase stabilizing function of O is stronger than that of Sn. In addition, O has a function of restraining the deformation of crystals and preventing expression of shape memory ability and softening.

The O content is preferably 0.4 to 1.7 at % and more preferably 0.6 to 1.0 at % when the entire titanium alloy is defined as 100 at %. The O content can be changed by changing the particle size of at least one of Ti powder and Ta powder. Note that although it is possible to change the O content by changing the particle size of Sn powder, the O content in the titanium alloy is hardly influenced even when the particle size of Sn powder is changed, because the content of Sn powder is small in the present embodiment.

Next, with reference to FIGS. 4 and 5, a method for producing a titanium alloy material according to an embodiment of the present invention will be described below.

First, as shown in FIG. 4, a Ti powder containing titanium (Ti) as a main component, a Ta powder containing tantalum (Ta) as a main component, and an Sn powder containing tin (Sn) as a main component are prepared, and a mixing step of mixing them at a predetermined mixing ratio is performed. In the present embodiment, the Ti powder, Ta powder, and Sn powder are each sieved by a sieve having a mesh with any size of 325 (mesh: mesh/inch) or less, and are assumed to have a particle size (granularity) that can pass through the sieve. The above-mentioned Ti powder preferably contains 90% or more of Ti, more preferably 95% or more of Ti, and even more preferably 99% or more of Ti. Here, the remaining components of the Ti powder include components other than Ti. Ti in the above-mentioned Ti powder may contain pure Ti or may contain Ti having an oxide film. The above-mentioned Ta powder preferably contains 90% or more of Ta, more preferably 95% or more of Ta, and even more preferably 99% or more of Ta. Here, the remaining components of the Ta powder include components other than Ta. Ta in the above-mentioned Ta powder may contain pure Ta or may contain Ta having an oxide film. The above-mentioned Sn powder preferably contains 90% or more of tin (Sn), more preferably 95% or more of tin (Sn), and even more preferably 99% or more of Sn. Here, the remaining components of the Sn powder include components other than Sn. Sn in the above-mentioned Sn powder may contain pure Sn or may contain Sn having an oxide film.

With the same weight, a powder having a smaller particle size will have more particles constituting the powder than a powder having a larger particle size. As a result, when compared at the same weight, the smaller particle size powder has a larger surface area that reacts with oxygen than the larger particle size powder. The Ti powder and Ta powder are stabilized in the atmosphere by an oxide film. Consequently, for the same weight, the O content in the final product titanium alloy increases when Ti powder or Ta powder with a smaller particle size is used, compared to when powders with larger particle sizes are used. Therefore, the particle size of the Ti powder and that of the Ta powder affect the O content in the titanium alloy, and the O content in the titanium alloy can be adjusted by appropriately selecting the particle size of each powder. For this reason, to provide multiple titanium alloys with different oxygen contents, the particle size of at least one, the Ti powder or the Ta powder should be adjusted. When the desired oxygen content cannot be achieved by only reducing the particle size of the Ti powder and/or the Ta powder, titania powder, which contains oxygen, may be added to the mixed powder.

For example, Ti powder or titania powder, Ta powder, and Sn powder are uniformly mixed in a mixing ratio that, when the entire titanium alloy is defined as 100 at %, the mixture contains 15 to 27 at % of Ta, 1 to 8 at % of Sn, 0.4 to 1.7 at % of O, and the remaining portion is composed of titanium (Ti) and inevitable impurities. When the above is converted into percentage by weight (wt %) and the entire titanium alloy is defined as 100 wt %, the amount of Ta is 40 to 56 wt %, the amount of Sn is 2 to 10 wt %, the amount of 0 is 0.1 to 0.3 wt %, and the remaining portion is composed of Ti and inevitable impurities. For example, when the titanium alloy is Ti-23.4Ta-3.4Sn-xO titanium alloy and the entire titanium alloy is defined as 100 wt %, the powders are mixed so that the amount of Ta powder is 52 wt %, the amount of Sn powder is 5 wt %, and the remaining portion is Ti powder or titania powder. Then, the O content is adjusted by adjusting the particle size of Ti powder or Ta powder having an oxide film on their surface, or by using a titania powder. In the present embodiment, since the content of Sn powder is small, it is presumed that the effect of Sn powder on the O content is small.

Next, as shown in FIG. 4, the mixed powder of Ti powder or titania powder, Ta powder, and Sn powder (hereinafter, simply referred to as mixed powder) that was uniformly mixed in the mixing step is subjected to a solidifying step in which the mixed powder is solidified by solid phase diffusion bonding under vacuum. By performing the solidifying step, the mixed powder becomes a solidified body. Since the mixed powder undergoes solid phase diffusion bonding in a state of being disposed under vacuum, it is possible to restrict an unexpected entry of oxygen (O) into the solidified body from the outside air. Note that the solidified body in the present embodiment refers to a single mass formed by a mixed powder that has undergone solid phase diffusion bonding. In addition, in the solidifying step, for example, any of a normal heat treatment (for example, a sintering treatment by heating), a pressurization heat treatment that simultaneously performs pressurization and heating, or the like is adopted. As the pressurization heat treatment, for example, any of a hot isostatic pressing method (HIP: Hot Isostatic Pressing), a spark isostatic pressing method (SIP: Spark Isostatic Pressing), or a spark plasma sintering method (SPS: Spark Plasma Sintering) is used, and the mixed powder is solidified by any of these methods. Incidentally, HIP and SIP are performed under isotropic (hydrostatic) pressurization, and SPS is performed under directional pressurization (axial pressurization) because a press machine and a metal mold are used. The pressure under vacuum where the mixed powder is disposed in the present embodiment is, for example, preferably 1.0×10⁻¹(Pa) or lower, more preferably within a range of 1.0×10⁻²to 1.0×10⁻⁵(Pa), further preferably within a range of 1.0×10⁻²to 1.0×10⁻³(Pa).

Here, referring to FIG. 5A, a treatment by a hot isostatic pressing method in the solidifying step (hereinafter referred to as an HIP treatment) will be described by way of example. First, the mixed powder 5 described above is filled in an HIP container 2 while being pressurized. The HIP container 2 is constituted by, for example, a cylindrical container in which one end is open and the other end is closed, and a cover. The mixed powder 5 is filled in the cylindrical container while being compressed, and the cylindrical container is placed in a vacuum chamber of an electron beam apparatus (not shown). Then, the pressure in the vacuum chamber is adjusted in a vacuum state to fall within a range of, for example, 1.0×10⁻²to 1.0×10⁻³(Pa), and electron-beam welding is performed to weld the cover on the opening of the HIP container 2 for sealing of the HIP container 2. Thus, the mixed powder is placed under vacuum in the HIP container 2.

The material of the HIP container 2 is preferably a material other than Ta. Ta is very expensive, and it is not practical for use in mass production. For this reason, for example, a material containing Ti or iron as a main component is preferably used as the material of the HIP container 2, and a material containing iron as a main component is more preferable. Note that, in general HIP processes, an HIP container 2 is typically made of the same material as the material having the highest melting point among the materials in the mixed powder 5 (here Ta).

Then, the HIP container 2 is placed inside a heat insulating portion 3A of an HIP furnace 3 of the HIP device 1. Note that the HIP device 1 is configured so that the inner area of the heat insulating portion 3A of the HIP furnace 3 can be brought into a high-temperature and high-pressure atmosphere by a substantially inert gas such as argon and by heating with a heater 4. The gas is supplied to the inside of the HIP furnace 3 from the outside through a gas introduction passage 3B of the HIP furnace 3. When a high temperature and a high pressure are applied to the HIP container 2 for a predetermined period of time, the mixed powder 5 is pressurized and heated via the HIP container 2. As a result, the mixed powder 5 becomes a solidified body by solid phase diffusion bonding. Since the mixed powder 5 is hermetically sealed in the HIP container 2 in a vacuum state, even if the mixed powder 5 is pressurized and heated via the HIP container 2, it is possible to restrict an unexpected entry of oxygen (O) into the solidified body from the outside air. Note that, immediately after the HIP treatment, the HIP container 2 and the solidified body are firmly bonded to each other. Therefore, in order to separate the HIP container 2 and the solidified body, the HIP container 2 and a layer in which the solidified body is fused with the HIP container 2 are cut by a working machine. As a result, only the solidified body remains. Finally, a solidified body having a cylindrical shape is formed.

During the HIP treatment, the temperature inside the HIP furnace 3 of the HIP device is set to, for example, 1000° C. and the pressure inside the HIP furnace 3 of the HIP device is set to 98 MPa. The HIP container 2 is then placed under these conditions for a predetermined period of time to form a solidified body. The internal temperature of the HIP furnace 3 may be any temperature as long as the HIP container 2 is not damaged or melted. The internal temperature of the HIP furnace 3 is preferably, for example, 700° C. to 1600° C., more preferably 900° C. to 1400° C., and still more preferably 1000° C. to 1200° C. The internal pressure of the HIP furnace 3 is preferably 50 to 200 (MPa), 70 to 180 (MPa), and 90 to 120 (MPa).

Next, as shown in FIG. 4, a melting step of melting the solidified body of the mixed powder, which has been solidified in the solidifying step, is performed. The solidified body is melted in the melting step to become an ingot of the titanium alloy. As described above, the solidified body has the respective components that are not uniformly dispersed. However, the respective components are melted (dissolved) and uniformly dispersed by performing the melting step, and thus it is possible to obtain an ingot of the titanium alloy in which the respective components are uniformly dispersed. In the melting step, to simultaneously melt and uniformly diffuse Ta, Sn and Ti having different melting points, it is preferable to melt the solidified body at a temperature at which all of Ta, Sn, and Ti can be melted at the same time. In the melting step, for example, any one of a vacuum arc remelting method (VAR: Vacuum Arc Remelting), an electroslag remelting method (ESR: ElectroSlag Remelting), a vacuum induction melting method (VIM: Vacuum Induction Melting), a cold crucible induction melting method (CCIM: Cold Crucible Induction Melting), a plasma arc melting method (PAM: Plasma Arc Melting), and an electron beam melting method (EBM: Electron Beam Melting) is used to melt the solidified body.

Here, referring to FIG. 5B, a case where a vacuum arc remelting method (VAR) is adopted in the melting step will be described. First, a cylindrical solidified body provided in the solidifying step is connected to a rod 9 suspended in an arc melting furnace 8 as a consumable electrode 6. As a result, the consumable electrode 6 is suspended and supported by the rod 9 in a state in which the molten metal pool 10 is positioned directly below the arc melting furnace 8. In this state, when a current is allowed to flow through the consumable electrode 6 via the rod 9, an arc discharge occurs between the consumable electrode 6 and the molten metal pool 10. Due to the high heat generated by the arc discharge, the consumable electrode 6 is heated and melted, accumulating directly below and becoming an ingot 11 of the titanium alloy.

Incidentally, the ingot 11 of the titanium alloy is connected to the rod 9 as the consumable electrode 6, and then, installed in the arc melting furnace 8 as described above. A current may be allowed to flow therethrough again to melt the ingot 11. This process may be repeatedly performed multiple times because the reliability of the uniformity of the respective components can be increased. As a result, the titanium alloy is formed.

Next, as shown in FIG. 4, cold working is performed on the ingot of the titanium alloy formed in the melting step. A titanium alloy material is formed by cold working of the ingot of the titanium alloy.

Next, as shown in FIG. 4, a heat treatment step of heat treating the titanium alloy material provided in the cold working step is performed. The heat treatment temperature is preferably, for example, 600° C. to 1000° C., and more preferably 700° C. to 900° C. Note that the heat treatment step may be omitted.

Next, as shown in FIG. 4, an aging step is performed on the titanium alloy material that has been subjected to the heat treatment in the heat treatment step, or on the titanium alloy material that has not been subjected to the heat treatment step after the cold working step. The aging temperature is preferably 200° C. to 550° C., and more preferably 300° C. to 500° C. When the titanium alloy material in the present embodiment is subjected to the aging treatment for a predetermined period of time, equiaxed a phase or the like is precipitated on the titanium alloy material. The α phase is not limited to the equiaxed structure and may include other forms.

Next, the foil forming step will be described. The foil forming step has a processing step performed at least once.

In the processing step, the titanium alloy material produced as described above is rolled to be processed into a titanium alloy plate material (hereinafter, referred to as a titanium alloy rolled material) having a further thinner thickness. Note that in an initial processing step, a titanium alloy material formed in a plate shape (hereinafter, referred to as a titanium alloy plate material) is rolled. The initial thickness of the titanium alloy plate material is, for example, 5 mm, but is not limited thereto and may be another thickness.

The processing step may be performed once or may be repeated multiple times. The number of processing steps varies depending on the thickness of the titanium alloy plate material, the thickness of the final foil, resistance characteristics of the titanium alloy plate material, and the like. Specifically, the processing step has, as illustrated in FIG. 1, a rolling step, an annealing step, and a foil formation cooling step.

In an initial rolling step, a titanium alloy plate material is rolled to be processed into a titanium alloy rolled material having a further thinner thickness. In the second and subsequent rolling steps, the titanium alloy rolled material that has been subjected to the first processing step is rolled to further thin the plate thickness of the titanium alloy rolled material. Specifically, in each rolling step, a titanium alloy plate material 12 is rolled at a preset rolling reduction ratio by performing rolling processing once or multiple times. In the rolling step according to the present embodiment, for example, a cold rolling apparatus 20 having a pair of rolling rollers 21 and 22 as illustrated in FIG. 6A is used.

The pair of rolling rollers 21 and 22 are faced each other and configured that a distance D of a space 23 between the pair of rolling rollers 21 and 22 are changeable. The cold rolling apparatus 20 is configured such that the pair of rolling rollers 21 and 22 can rotate. When the titanium alloy plate material 12 is inserted into the space 23 between the pair of rolling rollers 21 and 22 from one side of the cold rolling apparatus 20, the titanium alloy plate material 12 moves to pass through the space 23 by rotations of the pair of rolling rollers 21 and 22 and is transferred to the other side of the cold rolling apparatus 20, as illustrated in FIG. 6B. At this time, a region where the titanium alloy plate material 12 passes through the space 23 is rolled by the pair of rolling rollers 21 and 22 to have a thickness corresponding to the distance D of the space 23 between the pair of rolling rollers 21 and 22 and be further thinned.

The titanium alloy plate material 12 is inserted between the pair of rolling rollers 21 and 22 once in one rolling step or multiple times while varying the distance D of the space 23, so as to be processed into a titanium alloy rolled material rolled at a desired rolling reduction ratio. The rolling step prior to the last rolling step may be referred to as a second rolling step in distinction from the last rolling step.

In the annealing step, the titanium alloy rolled material, which has been rolled in the rolling step, is heated at an annealing temperature for performing an annealing treatment. Specifically, the annealing step is performed on a titanium alloy rolled material that has been rolled, in one rolling step, only an amount that corresponds to a preset rolling reduction ratio. The annealing treatment is performed while the titanium alloy rolled material is placed in a furnace (not illustrated). Further, the annealing treatment may be performed in the atmosphere or under vacuum. The annealing treatment eliminates strain and residual stress of the titanium alloy rolled material. The annealing temperature is preferably in a range of 650 to 900° C., more preferably in a range of 680 to 800° C., and further preferably in a range of 700 to 750° C., but is not limited thereto and may be another temperature range. The annealing step to be combined with the second rolling step in the processing step may be referred to as a second annealing step.

In the foil formation cooling step, the titanium alloy rolled material subjected to the annealing step is cooled by being left to stand for a prescribed time. Specifically, in the foil formation cooling step, for example, the heating action of the furnace housing the titanium alloy rolled material is terminated, and the titanium alloy rolled material is left to stand (furnace cooling). The leaving time for cooling is, for example, in a range of 15 to 24 hours at room temperature (for example, in a range of 10° C. to 30° C.) of the furnace location. Note that in the foil formation cooling step, the titanium alloy rolled material may be rapidly cooled by forced cooling in which the temperature of the titanium alloy rolled material is lowered quicker than in a case of furnace cooling (natural cooling).

As described above, the rolling step, the annealing step, and the foil formation cooling step are also sequentially performed in second and subsequent processing steps. However, the rolling reduction ratio may differ or may be the same among the processing steps.

The titanium alloy plate material becomes a foil through one processing step or multiple processing steps. The thickness of the foil is, for example, 0.03 mm, but is not limited thereto and preferably any of 0.01 mm or more and 0.1 mm or less, more preferably any of 0.01 mm or more and 0.07 mm or less, and further preferably any of 0.02 mm or more and 0.05 mm or less.

In the cutting step, the foil is cut to a predetermined size. Here, the foil may be cut to a size that facilitates the treatment in the foil treatment step or may be cut to a size for shipping. In the former case, a cutting step of cutting the foil to a size for shipping is separately added after the foil treatment step.

The foil treatment step is performed on the foil formed by the foil forming step. In that context, it can be said that the foil treatment step is a treatment method of a foil. Specifically, the foil treatment step has, as illustrated in FIG. 1, a recrystallization heating step and a foil treatment cooling step.

In the recrystallization heating step, the foil is heated to recrystallize the structure of the foil. In that context, it can be said that in the recrystallization heating step, a heat treatment for recrystallization (recrystallization heat treatment) is performed on the foil. The recrystallization heating step is preferably performed under an inert gas atmosphere or under vacuum. Examples of the inert gas include argon, neon, and helium. Here, the heating temperature (hereinafter, the recrystallization heating temperature) is preferably in a range of 760 to 900° C., more preferably in a range of 750 to 850° C., and further preferably in a range of 780 to 820° C., but is not limited thereto and may be another temperature as long as it is a temperature at which the structure of the foil can be recrystallized.

Multiple processing strains (residual strains by processing) occur in the foil that has undergone the foil forming step (processing step). When such a foil is heated at a recrystallization heating temperature, the processing strains serves as nuclei of crystal grains, so that new crystal grains gradually grow. The new crystal grains grow even larger when the heating time is lengthened. Therefore, the heating time (hereinafter, the recrystallization heating time) in the recrystallization heating step is preferably in a range of 30 to 100 minutes, more preferably in a range of 30 to 80 minutes, and further preferably in a range of 30 to 60 minutes, but is not limited thereto and may be another range.

Specifically, in the recrystallization heating step, a foil 13 is housed in a housing unit 31 of a furnace 30 as illustrated in FIG. 7A. The housing unit 31 has an inlet and outlet port 31A. The inlet and outlet port 31A is opened and closed by an opening and closing unit 31B. When the opening and closing unit 31B is closed, the state of the housing unit 31 becomes a sealed state. At this time, the foil 13 is placed on a base material 32 in the housing unit 31. Further, a weight material 33 is put on the foil 13. As a result, the foil 13 is sandwiched between the base material 32 and the weight material 33. The base material 32 and the weight material 33 are preferably formed in a plate shape. Note that one or a plurality of the foils 13 and one weight material 33 may be alternately stacked as illustrated in FIG. 7A, or a plurality of the foils 13 may be stacked on the base material 32, and the weight material 33 may be put on the foil 13 of the top layer as illustrated in FIG. 7B. Alternatively, the base material 32 may be omitted, and the foil 13 may be directly placed on the bottom surface of the housing unit 31. In this case, the foil 13 is sandwiched between the bottom surface of the housing unit 31 and the weight material 33.

Then, when the recrystallization heating step is performed under vacuum, the inside of the housing unit 31 is vacuumized by a vacuum generation unit 35. An example of the vacuum generation unit 35 is a pump to draw the air inside the housing unit 31. According to the present embodiment, the pressure (vacuum degree) under vacuum inside the housing unit 31 is, for example, preferably 1.0 (Pa) or less, more preferably 1.0×10⁻¹or less, and further preferably 1.0×10⁻²or less. Such the pressure (vacuum degree) under vacuum serves to suppress oxidation of the foil 13.

The inside of the housing unit 31 under vacuum is heated by a heating unit 34 at a recrystallization heating temperature and for a recrystallization heating time selected from the above-described ranges of the recrystallization heating temperature and the recrystallization heating time, respectively. Accordingly, the foil 13 is heated under vacuum. The foil 13 is sandwiched between the base material 32 and the weight material 33, to thereby be corrected to a straight posture. As a result, heat induced warping of the foil 13 is prevented.

As illustrated in FIG. 7C, it is preferable that the base material 32 have a shape and size that can be in contact with the entire region of a planar surface on one side of the foil 13, and it is preferable that the weight material 33 have a shape and size that can be in contact with the entire region of a planar surface on the other side of the foil 13, in order to reliably prevent the foil 13 from warping. The materials of the base material 32 and the weight material 33 are preferably a material that does not bond to the foil 13 when heated at the recrystallization heating temperature. An example of such a material is molybdenum (Mo).

In the foil treatment cooling step, the foil is cooled after the recrystallization heating step. The foil treatment cooling step is preferably performed under an inert gas atmosphere or under vacuum. The cooling in the foil treatment cooling step is forced cooling in which the temperature of the foil is lowered quicker than in a case of furnace cooling (natural cooling). The forced cooling is preferably cooling at a cooling rate that is higher than or equal to a cooling rate in which the temperature of the foil decreases by −150° C. per 10 minutes within a range until the temperature of the foil reaches 600° C. A specific aspect of the forced cooling is, for example, an aspect of forced-cooling the foil by air from an air blower, but the aspect of the forced cooling is not limited thereto and may be an aspect of forced-cooling the foil in a refrigerator or a freezer in which the temperature is lower than room temperature.

When the foil is forced-cooled by blown air, air is blown, as illustrated in FIGS. 7A and 7B, by an air blower 40 from the outside of the housing unit 31 to the housing unit 31 housing the foil under an inert gas atmosphere or under vacuum to forced-cool the foil in the foil treatment cooling step. When the foil is forced-cooled in a refrigerator or a freezer, for example, the housing unit 31 is transferred to a refrigerator or a freezer by a transfer mechanism (not illustrated) that transfers the housing unit 31 to a refrigerator or a freezer, and the foil is forced-cooled in the refrigerator or the freezer in a state of being housed in the housing unit 31. The forced cooling by blown air or by a refrigerator or a freezer suppresses the growth of crystal grains in the structure of the foil being associated with recrystallization. As a result, growth of crystal grains in the structure of the foil stops midway, and a plurality of crystal grains having an appropriate size can be provided in the structure of the foil.

Note that when the foil treatment cooling step is not performed, and the foil is furnace-cooled (natural-cooled) by terminating the heating action of the furnace and leaving the foil to stand, growth of crystal grains does not stop midway, and the size per one crystal grain increases. Therefore, the number of crystal grains contained in the foil decreases in the thickness direction of the foil. In a case of cooling the foil by furnace cooling (natural cooling), for example, the number of crystal grains contained in the foil in the thickness direction of the foil is about 1 to 3 when the thickness of the foil according to the present embodiment is 0.03 mm.

On the other hand, when the foil treatment cooling step is performed instead of furnace cooling (natural cooling), growth of crystal grains is suppressed, so that the size per one crystal grain is smaller than in a case of furnace cooling (natural cooling). Therefore, the number of crystal grains contained in the foil increases in the thickness direction of the foil. In a case of cooling the foil in the foil treatment cooling step, for example, the number of crystal grains contained in the foil can be 5 or more in the thickness direction of the foil, when the thickness of the foil according to the present embodiment is 0.03 mm.

As described above, crystal grains of the foil grow in the recrystallization heating step, but the growth of crystal grains can be suppressed by forced cooling in the foil treatment cooling step. Therefore, according to the foil treatment step (treatment method of foil), the number of crystal grains aligned in the thickness direction of the foil of the titanium alloy can be controlled.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

In order to verify the foil of the present invention, the inventors of the present application prepared a first foil to a sixth foil corresponding to the foil of the present invention and a first comparative example foil to a sixth comparative example foil as a comparative example as follows.

First, in order to prepare a first foil to a third foil, three titanium alloy plate materials with a thickness of 5 mm were subjected to a foil treatment step after undergoing a processing step five times. Specifically, in the rolling step of the first processing step, using a cold rolling apparatus 20 as shown in FIGS. 6A and 6B, three titanium alloy plate materials were subjected to rolling with a rolling reduction ratio of 60%, so that three titanium alloy rolled material with a thickness of 2 mm is formed. The titanium alloy plate material was made of a titanium alloy having a Ti-23Ta-3Sn-0.6O composition.

Then, in the annealing step, the three titanium alloy rolled materials with a thickness of 2 mm were placed in a furnace and heated at an annealing temperature of 700° C. for 30 minutes. In the foil formation cooling step, the heating operation in the furnace containing the three titanium alloy rolled materials with a thickness of 2 mm was stopped, and the three titanium alloy rolled materials with a thickness of 2 mm were left for 24 hours in the furnace.

In the rolling step of the second processing step, using the cold rolling apparatus 20, the three titanium alloy rolled materials with a thickness of 2 mm were subjected to rolling with a rolling reduction ratio of 60%, so that three titanium alloy rolled materials with a thickness of 0.8 mm is formed. The annealing step and the foil formation cooling step for the three titanium alloy rolled materials with a thickness of 0.8 mm were the same as those in the first processing step.

In the rolling step of the third processing step, using the cold rolling apparatus 20, the three titanium alloy rolled materials with a thickness of 0.8 mm were subjected to rolling with a rolling reduction ratio of 60%, so that three titanium alloy rolled materials with a thickness of 0.32 mm is formed. The annealing step and the foil formation cooling step for the three titanium alloy rolled materials with a thickness of 0.32 mm were the same as those in the first processing step.

In the rolling step of the fourth processing step, using the cold rolling apparatus 20, three titanium alloy rolled materials with a thickness of 0.32 mm were subjected to rolling with a rolling reduction ratio of 62.5%, so that three titanium alloy rolled materials with a thickness of 0.12 mm is formed. The annealing step and the foil formation cooling step for the three titanium alloy rolled materials with a thickness of 0.12 mm were the same as those in the first processing step.

In the rolling step of the fifth processing step, using the cold rolling apparatus 20, the three titanium alloy rolled materials with a thickness of 0.12 mm were subjected to rolling with a rolling reduction ratio (final rolling reduction ratio) of 75%, so that three foils that were the titanium alloy rolled materials with a thickness of 0.03 mm were formed. The annealing step and the foil formation cooling step for the foils were the same as those in the first processing step. The three foils correspond to a first foil, a second foil, and a third foil, respectively.

In the foil treatment step, the recrystallization heating temperature for the first foil was set to 780° C., the recrystallization heating temperature for the second foil was set to 800° C., and the recrystallization heating temperature for the third foil was set to 820° C., and the recrystallization heating time was set to 30 minutes. Based on the above settings, heat treatment (recrystallization heating treatment) was performed on the first foil to the third foil in a furnace under vacuum (1.0×10⁻¹(Pa)). After that, blowing was performed on a housing unit 31 of the furnace 30 using an air blower for 5 hours at 6.8 (m³/min) (foil treatment cooling step). As a result, the first foil to the third foil were forced-cooled, so that the temperature in the housing unit 31 of the furnace was 100° C. or lower. The temperature change of the second foil having been subjected to the recrystallization heating step and the foil treatment cooling step is shown in FIG. 9. In this blowing, the first foil to the third foil were cooled at a cooling rate in which the temperatures of the first foil to the third foil decrease by −300° C. per 10 minutes within a range until the temperatures of the first foil to the third foil reach 600° C. Thus, the first foil to the third foil were completed.

Similarly, in order to prepare the fourth foil to the sixth foil, three titanium alloy plate materials with a thickness of 5 mm were subjected to a foil treatment step after undergoing a processing step four times. The processing steps up to the third processing step were the same as those for the first foil to the third foil. In the rolling step of the fourth processing step, using the cold rolling apparatus 20, three titanium alloy rolled materials with a thickness of 0.32 mm were subjected to rolling with a final rolling reduction ratio of about 90%, so that three foils that were titanium alloy rolled materials with a thickness of 0.03 mm were formed. The annealing step and the foil formation cooling step for the foils were the same as those for the first foil to the third foil. The three foils corresponds to a fourth foil, a fifth foil, and a sixth foil, respectively.

In the foil treatment step, the recrystallization heating temperature for the fourth foil was set to 780° C., the recrystallization heating temperature for the fifth foil was set to 800° C., and the recrystallization heating temperature for the sixth foil was set to 820° C., and the recrystallization heating time was set to 30 minutes. Based on the above settings, heat treatment (recrystallization heating treatment) was performed on the fourth foil to the sixth foil in a furnace under vacuum (1.0×10⁻¹(Pa)). After that, blowing was performed on the housing unit 31 of the furnace 30 using an air blower for 5 hours at 6.8 (m³/min). As a result, the fourth foil to the sixth foil were forced-cooled, so that the temperature in the housing unit 31 of the furnace was 100° C. or lower. As described above, the fourth foil to the sixth foil were completed. In this blowing, as in the case of the first foil to the third foil, the fourth foil to the sixth foil were cooled at a cooling rate in which the temperatures of the fourth foil to the sixth foil decrease by −300° C. per 10 minutes within a range until the temperatures of the fourth foil to the six foil reach 600° C.

Next, in order to prepare the first comparative example foil to the sixth comparative example foil, six titanium alloy plate materials with a thickness of 5 mm were subjected to a foil treatment step after undergoing a processing step six times. Specifically, the contents of the processing steps up to the fourth processing step were the same as those for the first foil to the third foil. In the fifth processing step, six titanium alloy rolled materials were subjected to rolling with a rolling reduction ratio of 58.3%, so that six titanium alloy rolled materials with a thickness of 0.05 mm is formed. In the sixth processing step, the six titanium alloy rolled materials were subjected to rolling with a final rolling reduction ratio of 40%, so that six foils that were titanium alloy rolled material with a thickness of 0.03 mm were formed. The annealing step and the foil formation cooling step in the fifth and sixth processing steps were the same as those for the first to third foils. The six foils correspond to the first comparative example foil to the sixth comparative example foil, respectively.

In the foil treatment step, the recrystallization heating temperature for the first comparative example foil was set to 780° C. and the recrystallization heating time therefor was set to 30 minutes. The recrystallization heating temperature for the second comparative example foil was set to 800° C. and the recrystallization heating time therefor was set to 30 minutes. The recrystallization heating temperature for the third comparative example foil was set to 840° C. and the recrystallization heating time therefor was set to 30 minutes. The recrystallization heating temperature for the fourth comparative example foil was set to 780° C. and the recrystallization heating time therefor was set to 60 minutes. The recrystallization heating temperature for the fifth comparative example foil was set to 800° C. and the recrystallization heating time therefor was set to 60 minutes. The recrystallization heating temperature for the sixth comparative example foil was set to 840° C. and the recrystallization heating time therefor was set to 60 minutes. Based on the above settings, heat treatment (recrystallization heating treatment) was performed on the first comparative example foil to the sixth comparative example foil under vacuum (1.0×10⁻¹(Pa)). After that, the heating operation of the furnace was terminated, and the first comparative example foil to the sixth comparative example foil were left to be furnace-cooled (natural cooling). As a result, the temperature in the housing unit 31 was reduced to 100° C. or less. The furnace cooling (natural cooling) was performed at a cooling rate in which the temperatures of the first comparative example foil to the sixth comparative example foil decrease by −100° C. per 10 minutes within a range until the temperatures of the first comparative example foil to the sixth comparative example foil reach 600° C.

Here, when the cold rolling apparatus 20 is used in the rolling step, as illustrated in FIGS. 6A and 6B, a length direction of the foil along a transfer direction E (see FIGS. 4(A) and 4(B)) of the titanium alloy material (titanium alloy rolled material) or the foil is defined as a passing-side length direction A, and a direction orthogonal to both the passing-side length direction A and a thickness direction C of the foil is defined as an orthogonal direction B. Orthogonal cut surfaces V (see FIG. 8) obtained by cutting the first to sixth foils along the orthogonal direction B were observed and photographed by an electron microscope (FE-SEM JSM7800F-Prime manufactured by JEOL Ltd.) at an observation magnification of ×4000. The obtained photographs are shown in FIG. 10 and FIG. 11.

Further, orthogonal cut surfaces V (see FIG. 8) obtained by cutting the first to sixth comparative example foils along the orthogonal direction B were observed and photographed by an electron microscope (FE-SEM JSM7800F-Prime manufactured by JEOL Ltd.) at an observation magnification of ×4000. The obtained photographs are shown in FIG. 12 and FIG. 13.

The number of crystal grains aligned in the thickness direction of the foil in the cross sections of the first to sixth comparative example foils shown in FIG. 12 and FIG. 13 was counted. The result is shown in FIG. 14B. A method of counting the number of crystal grains is, as illustrated in FIG. 14A, drawing lines (see dotted lines) along the thickness direction C of the foil in the cross section of the foil and counting the crystal grains on the lines along the thickness direction C of the foil. Note that in FIG. 14A, the cross-sectional photograph of the second comparative example foil shown in FIG. 12(B) is used as an example.

The number of crystal grains aligned in the thickness direction of each of the first comparative example foil, the second comparative example foil, the fourth comparative example foil, and the fifth comparative example foil was 3. On the other hand, the number of crystal grains aligned in the thickness direction of each of the third comparative example foil and the sixth comparative example foil was 3 or less. As confirmed from the above, the number of crystal grains aligned in the thickness direction of the foil in the cross sections of the first to sixth comparative example foils was 3 or less. On the other hand, it could be confirmed that at least five or more crystal grains were aligned in the thickness direction of the foil in the orthogonal cut surfaces V of the first to sixth foils shown in FIG. 10 and FIG. 11.

From the above, it can be said that growth of crystal grains proceeds to lengthen the length per one crystal grain in the thickness direction C of the foil in the foil not subjected to the foil treatment cooling step, while growth of crystal grains is suppressed to suppress the size (length in the thickness direction of the foil) per one crystal grain in the thickness direction of the foil in the foil subjected to the foil treatment cooling step.

Further, in a comparison of the orthogonal cut surfaces V of the first to third foils, the second foil and the third foil contain more crystal grains that are larger than those in the first foil. Further, in a comparison of the orthogonal cut surfaces V of the fourth to sixth foils, the fifth foil and the sixth foil contain more crystal grains that are larger than those in the fourth foil. While the recrystallization heating temperature for the first foil and the fourth foil is 780° C., the recrystallization heating temperature for other foils (the second, third, fifth, and sixth foils) is 800° C. or higher. It is estimated that the difference in the recrystallization heating temperature influences the size of crystal grains. Therefore, the recrystallization heating temperature is preferably lower than 800° C. in order to generate many crystal grains in the thickness direction of the foil. Conversely, the recrystallization heating temperature is preferably 800° C. or higher in order to suppress the number of crystal grains in the thickness direction of the foil.

The ratios of an α phase and a β phase contained in the orthogonal cut surface V of each of the first to sixth foils is illustrated in the graph of FIG. 15. The ratios of an α phase and a β phase contained in the orthogonal cut surface V was derived on the basis of a scan area (28.3 μm×21.2 μm) of each of a parallel cut surface P and an orthogonal cut surface V by an EBSD analyzer (Symmetry S2 manufactured by Oxford Instruments).

As seen in the graph of FIG. 15, an α phase is 3% or less, and a β phase is 97% or more, in all of the first to sixth foils. That is, it could be confirmed that according to the production method of a foil or the treatment method of a foil according to the present examples, an α phase contained in the foil can be 10% or less, and a β phase can be 90% or more.

Further, it is understood from the graph of FIG. 15 that there is a tendency that as the recrystallization heating temperature increases, the ratio of an α phase decreases, and the ratio of a β phase increases, in both the foil having been rolled with a final rolling reduction ratio of 75% and the foil having been rolled with a final rolling reduction ratio of 90%.

The average particle diameter in the orthogonal cut surface V of each of the first to sixth foils is illustrated in the graph of FIG. 16. The average particle diameters shown in the graph of FIG. 16 are those of particles contained in the regions of the orthogonal cut surfaces V shown in FIG. 10 and FIG. 11, and were derived by an area weighted average. The area weighted average is an average value of values obtained by multiplying the ratio of the area of each crystal grain to the total area by the area value of each crystal grain. The average particle diameter by an area weighted average was derived by an EBSD analyzer (Symmetry S2 manufactured by Oxford Instruments).

As seen in the graph of FIG. 16, the average particle diameter by an area weighted average of crystal grains is 12.22 μm in the second foil, which is the largest, and is 8.84 μm in the fourth foil, which is the smallest. This demonstrated that the average particle diameter by an area weighted average of crystal grains contained in the foil of the titanium alloy is in a range of 8.8 μm or more and 12.3 μm or less. As a result, it could be confirmed that according to the production method of a foil or the treatment method of a foil in the present Examples, the average particle diameter by an area weighted average of crystal grains contained in the foil of titanium alloy can be controlled to fall within a range of 8.8 μm or more and 12.3 μm or less. Note that the range of the average particle diameter by an area weighted average of crystal grains contained in the foil of the titanium alloy can be further widened, and it can be estimated that the range can be widened to 8.7 μm or more and 16.5 μm or less. Further, the above-described range of the average particle diameter can be adjusted, for example, by adjusting the rolling reduction ratio in the rolling step and/or by adjusting the heating temperature in the recrystallization heating step.

As understood from the graph of FIG. 16, the foils having been rolled with a final rolling reduction ratio of 75% have an average particle diameter larger than that of the foils having been rolled with a final rolling reduction ratio of 90%. Accordingly, it can be estimated that the average particle diameter by an area weighted average is larger when the final rolling reduction ratio is low than that when the final rolling reduction ratio is high.

Further, it is understood that in both of the foils having been rolled with a final rolling reduction ratio of 75% and the foils having been rolled with a final rolling reduction ratio of 90%, the average particle diameter by an area weighted average is larger when the recrystallization heating temperature is 800° C. or higher than that when the recrystallization heating temperature is 780° C. This demonstrated that the average particle diameter by an area weighted average further increases when the recrystallization heating temperature is higher. This point can also be confirmed by visually observing the size of crystal grains shown in FIG. 10 and FIG. 11, as described in <Comparison of crystal grains>.

From the above results, it was confirmed that the number of crystal grains aligned in the thickness direction of the foil or the average particle diameter by an area weighted average can be controlled by adjusting the final rolling reduction ratio in the rolling step to be lastly performed, and the recrystallization heating temperature in the recrystallization heating step.

As a result of studying the orthogonal cut surfaces of the first to sixth comparative example foils, it could be confirmed that suppression of the growth of crystal grains in the structure of the foil being associated with recrystallization is insufficient at a first cooling rate (cooling rate by furnace cooling) in which the temperature of the foil decreases by −100° C. per 10 minutes within a range until the temperature of the foil reaches 600° C. Further, as a result of studying the orthogonal cut surfaces of the first to sixth foils according to the present examples, it could be confirmed that the growth of crystal grains in the structure of the foil being associated with recrystallization can be suppressed at a second cooling rate in which the temperature of the foil decreases by −300° C. per 10 minutes within a range until the temperature of the foil reaches 600° C. From the above study results, it can be estimated that even at a third cooling rate in which the temperature of the foil decreases by −150° C. per 10 minutes within a range until the temperature of the foil reaches 600° C., the growth of crystal grains in the structure of the foil being associated with recrystallization can be suppressed to suppress the size (length in the thickness direction of the foil) per one crystal grain in the thickness direction of the foil. In brief, it is considered that when the foil is cooled at a cooling rate higher than the third cooling rate in the foil treatment cooling step, the number of crystal grains in the thickness direction of the foil can be controlled.

It should be noted that the production method of a foil, the treatment method of a foil, and the foil of the present invention are not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention. Of course, all production methods of a foil, treatment methods of a foil, and foils constituted by extracting and appropriately combining respective constituent elements in the present embodiments are also encompassed by the scope of the present invention.

Treatment Method Of Foil, Production Method Of Foil, And Foil

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