α+β type titanium alloy and method for manufacturing the same

Abstract

To provide an α+β type titanium alloy that achieves both high tensile strength and excellent cold workability, and a method for manufacturing the same. [Solution] The α+β type titanium alloy contains 3.0≦Al≦5.4 mass%, 2.5≦V≦6.0 mass%, 1.5≦Cr≦7.0 mass%, and 0.05≦Fe≦3.0 mass%, with the remainder being Ti and unavoidable impurities, and the β transformation temperature T β The temperature is 880°C or lower. Such an α+β type titanium alloy is prepared by preparing a material made of an α+β type titanium alloy and heating the material to a temperature of 600°C or higher, and the β transformation temperature T β Hot working is performed in a temperature range below T, and the material after hot working is T β It is obtained by heat treatment at a temperature between -200°C and 1200°C.

Description

[Technical Field]

[0001] The present invention relates to an α+β type titanium alloy and a method for producing the same, and more particularly, to an α+β type titanium alloy that achieves both high tensile strength and excellent cold workability and a method for producing the same. [Background Art]

[0002] Practical titanium alloys are (1) α-type alloys composed of a close-packed hexagonal α-phase (low-temperature phase), (2) β-type alloys composed of a body-centered cubic β-phase (high-temperature phase), (3) α+β-type alloys having a mixed structure of α-phase and β-phase, broadly classified into the above three categories.

[0003] Among these, α+β-type alloys are well-balanced materials excellent in strength, specific strength, workability, corrosion resistance and other properties, and are used as aerospace materials, automotive materials, materials for mechanical structural components, materials for general civilian use, materials for medical instruments and the like. In particular, among α+β-type alloys, Ti-6Al-4V alloy is widely used as a general-purpose high-strength titanium alloy, accounting for approximately 80% of the total usage amount of titanium alloys.

[0004] Various proposals have been made conventionally for such titanium alloys. For example, Patent Document 1 discloses a titanium alloy for golf club faces, which contains predetermined amounts of Al, Fe, N and O, with the balance being Ti and unavoidable impurities. This document describes that when the addition amounts of Al, O and N, which are α-phase solid solution strengthening elements, and Fe, which is a β-stabilizing element, are optimized, both high strength and high Young's modulus can be achieved.

[0005] Patent Document 2 discloses a β-type titanium alloy which contains predetermined amounts of Al, Fe and Cr, further contains a predetermined amount of V and / or Mo, with the balance being Ti and unavoidable impurities. The same document describes that in a titanium alloy further containing V or Mo in addition to Al, Fe and Cr, when the total content of V and Mo is 10 mass% or less, the influence of component segregation of Fe and Cr is mitigated.

[0006] Patent Document 3 discloses a titanium alloy containing predetermined amounts of Al and Fe, with the balance being Ti and unavoidable impurities. The same document describes that the titanium alloy having such a composition has a Young's modulus of 120 to 160 GPa and a tensile strength of 950 to 2200 MPa.

[0007] The titanium alloy described in Patent Document 1 has predetermined amounts of O and N added thereto. Therefore, when the O content and / or N content is high, there is a risk that the ductility of the titanium alloy decreases. In addition, Patent Document 1 does not mention the cold workability of the titanium alloy. The β-type titanium alloy described in Patent Document 2 is prone to component segregation because it has a large content of eutectoid additive elements among β-stabilizing elements. In addition, generally, β-type titanium alloys are inferior to α+β-type titanium alloys in high-temperature strength; in particular, titanium alloys containing Mo have high density, which impairs lightweight properties. Furthermore, Patent Document 3 does not mention the cold workability of the titanium alloy.

Prior Art Literature

Patent Literature

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0009] The problem that this invention aims to solve is to provide an α+β type titanium alloy that achieves both high tensile strength and excellent cold workability, as well as a method for manufacturing the same. [Means for solving the problem]

[0010] To solve the above problems, the α+β type titanium alloy according to the present invention is 3.0 ≤ Al ≤ 5.4 mass%, 2.5 ≤ V ≤ 6.0 mass%, 1.5 ≤ Cr ≤ 7.0 mass%, and, 0.05 ≤ Fe ≤ 3.0 mass% It contains, with the remainder consisting of Ti and unavoidable impurities. β transformation temperature T β The temperature is below 880°C.

[0011] The method for producing an α+β type titanium alloy according to the present invention is: A material preparation step for preparing a material made of an α+β type titanium alloy according to the present invention, The aforementioned material is subjected to a β-transformation temperature of 600°C or higher T β A hot working process that performs hot working in a temperature range below a certain temperature, The material after the aforementioned hot working process is T β A heat treatment process in which heat treatment is performed at a temperature of -200℃ or higher and 1200℃ or lower. It is equipped with. [Effects of the Invention]

[0012] T β When a titanium alloy whose composition has been adjusted so that its temperature is below 880°C is hot-worked in the two-phase temperature range, the needle-like α phase is destroyed, and a fine equiaxed α phase is formed. Subsequently, when the obtained material is heat-treated under appropriate conditions, an α+β type alloy is obtained having an equiaxed structure dominated by the equiaxed α phase, an acicular structure dominated by the needle-like α phase, or a mixed structure containing both the equiaxed α phase and the needle-like α phase. The α+β type alloy obtained in this way possesses both high tensile strength and excellent cold workability. The high tensile strength is thought to be due to solid solution strengthening by the added elements. Furthermore, the excellent cold workability is due to Tβ This is thought to be because the process is optimized, resulting in a higher proportion of the β phase, which has good cold workability. [Brief explanation of the drawing]

[0013] [Figure 1] This figure shows an example of the microstructure of an α+β type titanium alloy according to the present invention. [Figure 2] This figure shows the relationship between the β-transformation temperature Tβ and the critical compressibility at 25°C. [Modes for carrying out the invention]

[0014] One embodiment of the present invention will be described in detail below. [1. α+β type titanium alloy] [1.1. Main constituent elements] The α+β type titanium alloy according to the present invention (hereinafter also simply referred to as "titanium alloy") contains the following elements, with the remainder being Ti and unavoidable impurities. The types of added elements, their component ranges, and the reasons for their limitations are as follows.

[0015] (1) 3.0 ≤ Al ≤ 5.4 mass%: Al is an element that contributes to the stabilization of the α-phase and the solid solution strengthening of the α-phase. To obtain these effects, the amount of Al must be 3.0 mass% or more. On the other hand, if the amount of Al is excessive, the brittle phase Ti3Al may precipitate, which can reduce the ductility of the titanium alloy. Also, if the amount of Al is excessive, solid solution strengthening may proceed excessively, which can reduce the cold workability of the titanium alloy. Therefore, the amount of Al needs to be 5.4 mass% or less. Preferably, the amount of Al is 4.3 mass% or less.

[0016] (2) 2.5 ≤ V ≤ 6.0 mass%: V is an element that contributes to the stabilization of the β phase, solid solution strengthening of the β phase, and improvement of cold workability through the formation of the β phase. To obtain these effects, the amount of V must be 2.5 mass% or more. On the other hand, since V is an expensive element, an excess of V increases the cost of raw materials. Therefore, the amount of V needs to be 6.0 mass% or less. Preferably, the amount of V is 4.5 mass% or less.

[0017] (3) 1.5 ≤ Cr ≤ 7.0 mass%: Cr is an element that contributes to the stabilization of the β phase, solid solution strengthening of the β phase, and improvement of cold workability through the formation of the β phase. To obtain these effects, the amount of Cr must be 1.5 mass% or more. On the other hand, since Cr is a very strong β-phase stabilizing element, if the amount of Cr is excessive, a two-phase structure may not be obtained. Also, if the amount of Cr is excessive, segregation may occur during solidification, and the manufacturability may deteriorate. Therefore, the amount of Cr needs to be 7.0 mass% or less. More preferably, the amount of Cr is 4.0 mass% or less.

[0018] (4) 0.05 ≤ Fe ≤ 3.0 mass %: Fe is an element that contributes to the stabilization and solid solution strengthening of the β phase. Furthermore, since Fe as an alloying element and Ti containing Fe as an impurity are inexpensive, they contribute to reducing raw material costs. To obtain these effects, the amount of Fe must be 0.05 mass% or more. Preferably, the amount of Fe is 0.25 mass% or more. On the other hand, since Fe is a very strong β-phase stabilizing element, if the amount of Fe is excessive, a two-phase structure may not be obtained. Also, if the amount of Fe is excessive, segregation may occur during solidification, and the manufacturability may deteriorate. Therefore, the amount of Fe needs to be 3.0 mass% or less. More preferably, the amount of Fe is 1.3 mass% or less.

[0019] [1.2. Sub-constituent elements] The α+β type titanium alloy according to the present invention may further contain one or more elements in addition to the main constituent elements described above. The types of additive elements, their component ranges, and the reasons for their limitations are as follows.

[0020] (1) Sn ≤ 0.5 mass%: Sn is an element that contributes to solid solution strengthening of the α and β phases and can be added as needed. However, if the amount of Sn is excessive, undissolved Ti3Al may remain even after solution heat treatment, which can reduce the ductility of the titanium alloy. Therefore, the amount of Sn is preferably 0.5 mass% or less. More preferably, the amount of Sn is 0.1 mass% or less.

[0021] (2) Zr ≤ 0.5 mass %: Zr is an element that contributes to solid solution strengthening of the α and β phases and can be added as needed. However, if the amount of Zr is excessive, undissolved Ti3Al may remain even after solution heat treatment, which can reduce the ductility of the titanium alloy. Therefore, the amount of Zr is preferably 0.5 mass% or less. More preferably, the amount of Zr is 0.1 mass% or less.

[0022] (3) Mo ≤ 0.5 mass %: Mo is an element that contributes to solid solution strengthening of the β phase and can be added as needed. However, since Mo is a very strong β-phase stabilizing element, if the amount of Mo is excessive, a two-phase structure may not be obtained. Also, if the amount of Mo is excessive, segregation may occur during solidification. Furthermore, since Mo is expensive, if the amount of Mo is excessive, the raw material cost will increase. Therefore, the amount of Mo is preferably 0.5 mass% or less. More preferably, the amount of Mo is 0.1 mass% or less.

[0023] (4) Nb ≤ 0.5 mass %: Nb is an element that contributes to improving the oxidation resistance of titanium alloys and can be added as needed. However, if the amount of Nb is excessive, the specific gravity of the titanium alloy may increase, and the specific strength may decrease. Therefore, the amount of Nb is preferably 0.5 mass% or less. More preferably, the amount of Nb is 0.1 mass% or less.

[0024] (5) Ta ≤ 0.5 mass %: Ta is an element that contributes to solid solution strengthening of the β phase and can be added as needed. However, if the amount of Ta is excessive, the specific gravity of the titanium alloy may increase, and the specific strength may decrease. Therefore, the amount of Ta is preferably 0.5 mass% or less. More preferably, the amount of Ta is 0.1 mass% or less.

[0025] (6) Ni ≤ 0.15 mass%: Ni is an element that contributes to solid solution strengthening of the β phase and can be added as needed. However, since Ni is a very strong β-phase stabilizing element, an excess of Ni may prevent the acquisition of a two-phase structure. Furthermore, an excess of Ni may cause segregation during solidification. Therefore, a Ni content of 0.15 mass% or less is preferable.

[0026] (7) Mn ≤ 0.25 mass %: Mn is an element that contributes to solid solution strengthening of the β phase and can be added as needed. However, since Mn is a very strong β-phase stabilizing element, if the amount of Mn is excessive, a two-phase structure may not be obtained. Also, if the amount of Mn is excessive, segregation may occur during solidification. Therefore, the amount of Mn is preferably 0.25 mass% or less.

[0027] (8) B ≤ 0.2 mass %: B is an element that forms borides with Ti and contributes to grain refinement, and can be added as needed. However, if the amount of B is excessive, the borides become coarse, and these borides can act as the starting point for fracture, which can significantly impair both hot and cold workability. Therefore, the amount of B is preferably 0.2 mass% or less. More preferably, the amount of B is 0.05 mass% or less.

[0028] (9) Sn + Zr + Mo + Nb + Ta + Ni + Mn + B ≤ 2.0 mass%: Even if the content of Sn, Zr, Mo, Nb, Ta, Ni, Mn, and B is within the ranges described above, if the total amount of these elements becomes excessive, the ductility of the titanium alloy will decrease, fatigue strength will decrease, specific strength will decrease, or segregation may occur during solidification. Therefore, the total amount of these elements is preferably 2.0 mass% or less.

[0029] [1.3. Inevitable Impurities] "Unavoidable impurities" refer to components that are introduced into the α+β type titanium alloy according to the present invention due to various factors such as raw materials and manufacturing processes during industrial production, and whose content is within a range that does not adversely affect the properties of the α+β type titanium alloy according to the present invention. Specifically, unavoidable impurities include the following:

[0030] (1) Si ≤ 0.3 mass %: Excessive Si content can degrade the ductility, cold workability, or hot workability of titanium alloys. Therefore, a Si content of 0.3 mass% or less is preferable. A lower Si content is generally better. However, extreme reductions in Si content can increase manufacturing costs. Considering manufacturing costs, a Si content of 0.0005 mass% or higher is preferable.

[0031] (2) O ≤ 0.4 mass %: Oxygen (O) is an element that contributes to the solid solution strengthening of the α phase. However, if the amount of O is excessive, the ductility of the titanium alloy may deteriorate. Therefore, the amount of O is preferably 0.4 mass% or less. A lower oxygen content is preferable. However, extreme reductions in oxygen content can increase manufacturing costs. Considering manufacturing costs, an oxygen content of 0.005 mass% or higher is preferable.

[0032] (3) N ≤ 0.1 mass %: N is an element that contributes to solid solution strengthening of the α phase. However, if the amount of N is excessive, the ductility of the titanium alloy may deteriorate. Therefore, the amount of N is preferably 0.1 mass% or less. A lower nitrogen content is preferable. However, extreme reductions in nitrogen content can increase manufacturing costs. Considering manufacturing costs, an nitrogen content of 0.0005 mass% or higher is preferable.

[0033] (4) H ≤ 0.1 mass %: If the amount of hydrogen (H) is excessive, the strength and ductility of the titanium alloy may deteriorate. Therefore, the amount of hydrogen is preferably 0.1 mass% or less. A lower H content is preferable. However, extreme reductions in H content can increase manufacturing costs. Considering manufacturing costs, an H content of 0.0005 mass% or higher is preferable.

[0034] (5) C ≤ 0.2 mass %: If the carbon content is excessive, the ductility, cold workability, or hot workability of the titanium alloy may deteriorate. Therefore, the carbon content is preferably 0.2 mass% or less. A lower carbon content is preferable. However, extreme reductions in carbon content can increase manufacturing costs. Considering manufacturing costs, a carbon content of 0.0005 mass% or higher is preferable.

[0035] (6) Total amount of impurities ≤ 1.0 mass%: Other unavoidable impurities besides the elements mentioned above include, for example, P, S, Pd, Hf, and REM. The content of each of these elements is preferably 0.05 mass% or less. Furthermore, even if the content of each unavoidable impurity is below the upper limit mentioned above, if the total amount of unavoidable impurities becomes excessive, the ductility, cold workability, or hot workability of the titanium alloy may deteriorate. Therefore, the total amount of unavoidable impurities is preferably 1.0 mass% or less. More preferably, the total amount is 0.5 mass% or less.

[0036] [1.4. β-transformation temperature] "β transformation temperature T β "Beta" refers to the lower limit temperature at which a titanium alloy becomes a single β phase, and can be measured by suggestive thermal analysis (DTA). The titanium alloy T according to the present invention βwas confirmed to have a correlation with cold workability (critical compression ratio at 25°C). T β The lower this is, the higher the cold workability. This is because T β The lower this is, the higher the proportion of the β phase that excels in cold workability, which is considered to be the reason. To obtain such an effect, T β needs to be 880°C or lower. T β is preferably 870°C or lower, 860°C or lower, or 850°C or lower.

[0037] [1.5. Structure] Fig. 1 shows an example of the microstructure of an α+β type titanium alloy according to the present invention. The titanium alloy according to the present invention exhibits an α+β dual-phase structure at room temperature. Further, the titanium alloy according to the present invention exhibits an acicular structure, a mixed structure, or an equiaxed structure depending on processing and heat treatment conditions.

[0038] The "acicular structure" refers to a structure (β phase + acicular α phase) in which acicular α phase is precipitated in a matrix composed of the β phase. The acicular structure is obtained by subjecting the titanium alloy according to the present invention to T β heat treatment at a temperature equal to or higher than that, followed by cooling, and precipitating the acicular α phase along with martensitic transformation during cooling. A titanium alloy having an acicular structure is characterized in that its low-temperature strength is slightly low, and its ductility and fatigue properties are low, but its high-temperature strength, fracture toughness, and creep properties are extremely high.

[0039] The "equiaxed structure" refers to a structure mainly composed of equiaxed α phase, with the β phase present in the gaps between relatively large equiaxed α phase grains. The equiaxed structure is obtained by subjecting the titanium alloy according to the present invention to T β heat treatment at a temperature lower than that, followed by heat treatment at a temperature lower than the martensitic transformation start point (Ms point), to fragment or recrystallize the acicular structure and make the α phase equiaxed. An equiaxed structure is obtained by introducing sufficient strain into the acicular α phase and fragmenting or recrystallizing the acicular α phase. A titanium alloy having an equiaxed structure is characterized in that its fracture toughness is slightly low, and its high-temperature strength and creep properties are low, but its ductility is high, and its low-temperature strength and fatigue properties are extremely high.

[0040] "Mixed structure" refers to a structure in which equiaxed α-phase and β-phase (β-phase + acicular α-phase) with needle-shaped α-phase precipitates are mixed. Mixed structure is used in the titanium alloy according to the present invention. β This material is obtained by hot working at a temperature below Ms, followed by heat treatment in the α+β two-phase temperature region above the Ms point, which causes the α phase to become equiaxed, and in addition, causes the precipitation of needle-shaped α phase due to the martensitic transformation of the β phase. The final structure is a mixture of equiaxed α phase and β phase + needle-shaped α phase regions. Titanium alloys with this mixed structure have slightly lower ductility, but high low-temperature strength, high-temperature strength, fracture toughness, creep properties, and fatigue properties, making them well-balanced materials.

[0041] [1.6. Characteristics] [1.6.1. Tensile Strength] "Tensile strength" refers to the value obtained by conducting a tensile test using a No. 14A test specimen with a parallel section diameter of 6 mm, in accordance with JIS Z2241:2011. In the titanium alloy according to the present invention, optimizing the composition and / or structure results in a tensile strength of 950 MPa or more at 25°C.

[0042] [1.6.2. Growth] "Elongation" refers to the elongation at break obtained by performing a tensile test using a No. 14A test specimen with a parallel section diameter of 6 mm, in accordance with JIS Z2241:2011. In the titanium alloy according to the present invention, optimizing the composition and / or structure results in an elongation of 15% or more at 25°C. Further optimization of the composition and / or structure results in an elongation of 20% or more at 25°C.

[0043] [1.6.3. Maximum Compression Ratio] "Compression ratio (%)" refers to the value expressed by the following formula. Compression ratio = (H1 - H2) × 100 / H1 however, H1 is the height of the specimen before the compression test. H2 is the height of the specimen after the compression test.

[0044] "Critical compressibility" refers to the maximum compressibility at which no cracks are observed in the test specimen after an end-face restrained compression test. An "end-face restrained compression test" is a test in which pressure plates are placed above and below the test specimen, and the specimen is compressed while restraining the spreading of the upper and lower end faces of the specimen. Because the end-face restrained compression test can eliminate the effect of slippage (friction) between the pressure plates and the test specimen during pressurization, it is an excellent method for evaluating deformability and is suitable for evaluating cold workability. The test specimen shape was φ15 × 22.5 mm, and the reduction speed was 35 mm / s. Furthermore, at least three tests were performed for each condition, and the maximum compression ratio at which no cracks were observed in the compressed specimen was defined as the limit compression ratio.

[0045] In the titanium alloy according to the present invention, optimizing the composition and / or microstructure results in a critical compressibility of 35% or more at 25°C. Further optimization of the composition and / or microstructure results in a critical compressibility of 40% or more, 45% or more, or 50% or more at 25°C.

[0046] [1.7. Usage] The α+β type titanium alloy according to the present invention can be applied to a variety of uses. Examples of titanium products to which the α+β type titanium alloy according to the present invention is applied include: (1) Various sports and leisure goods such as golf club heads and bicycle gears, (2) Consumer goods such as eyeglass frames, (3) Various bolts, valve retainers, seawater-resistant shafts, and other parts that require light weight and high strength. These are some examples.

[0047] [2. Method for manufacturing α+β type titanium alloy] The method for producing an α+β type titanium alloy according to the present invention is: A preparation step for preparing a material made of an α+β type titanium alloy according to the present invention, The aforementioned material is subjected to a β-transformation temperature of 600°C or higher T β A hot working process that performs hot working in a temperature range below a certain temperature, The material after the aforementioned hot working process is Tβ A heat treatment process in which heat treatment is performed at a temperature of -200℃ or higher and 1200℃ or lower. It is equipped with. The method for producing an α+β type titanium alloy according to the present invention may further include an aging treatment step in which the material after heat treatment is subjected to aging treatment at a temperature of 400°C to 700°C.

[0048] [2.1. Preparation process] First, a material made of the α+β type titanium alloy according to the present invention is prepared. The preparation of the materials is as follows: (a) The raw materials, which have been blended to have a predetermined composition, are melted and cast, (b) If necessary, preheat working of the melted and cast alloy ingot, (c) If necessary, preheat the preheated material. It is preferable to do so.

[0049] [2.1.1. Melting and Casting] First, the raw materials, which have been blended to achieve the desired composition, are melted and cast. The method of melting and casting the raw materials is not particularly limited, and the most suitable method can be selected depending on the purpose. One example of a method for melting and casting raw materials is the cold crucible induction melting (CCIM) method.

[0050] [2.1.2. Preheating and working] When an alloy ingot produced by the CCIM method or similar is smaller than or equal to the size suitable for the next process (hot working process), it can be used directly as the material for the next process. On the other hand, if the manufactured alloy ingot exceeds a size suitable for the next process, it is preferable to preheat and work the material.

[0051] Preheating is preferably started in the β single-phase temperature range. Preheating allows for the fragmentation of the cast structure of the alloy ingot. Furthermore, since deformation resistance is relatively low in the β single-phase temperature range, fragmentation of the cast structure is facilitated. The method of preheating is not particularly limited, and the most suitable method can be selected according to the purpose. Examples of preheating methods include hot forging and hot rolling.

[0052] If the material temperature drops to the α+β two-phase temperature range during preheating, the preheating may be terminated, or it may be continued. If preheating is continued after the material temperature drops to the α+β two-phase temperature range, the microstructure may be further refined. In order to refine the alloy microstructure, the termination temperature of preheating is preferably in a relatively high temperature range within the α+β two-phase temperature range. Specifically, the termination temperature of preheating is T β -100℃ or moreT β Less than is preferable.

[0053] [2.1.3. Preheating Treatment] When preheating is performed, it is preferable to further preheat the preheated material. The preheating treatment should preferably be performed in a temperature range that can homogenize the alloy structure, suppress grain coarsening, and maintain the effects produced by the preheating. The preheating treatment should preferably be maintained in a relatively low temperature range within the β single-phase temperature range. Specifically, the temperature of the preheating treatment should be T β More than T β A temperature of +80°C or lower is preferable.

[0054] [2.2. Hot Working Process] After preheating and processing the material as necessary, the material is heated to 600°C or higher, and then to the β transformation temperature T β Hot working is performed in a temperature range below a certain temperature.

[0055] Hot working is performed to refine the alloy structure and adjust the morphology of the equiaxed α phase. Therefore, hot working must be performed within the α+β two-phase temperature range from start to finish. The hot working temperature is T β If the temperature exceeds this level, even if heat treatment is performed afterward, only a needle-like structure will be obtained, making it difficult to control the structure. Therefore, the hot working temperature should be T β It must be less than [a certain value]. On the other hand, if the hot working temperature is too low, the deformation resistance may increase excessively. Therefore, the hot working temperature needs to be 600°C or higher. To minimize deformation resistance, the hot working temperature is preferably in a relatively high temperature range within the α+β two-phase temperature region.

[0056] During hot working, the length of the material in the direction of maximum strain before the start of hot working is defined as "L1," and the length after the end of hot working is defined as "L2." In this case, "total molding ratio" means: When L1 > L2, it is called L1 / L2. L1 <L2であるときにはL2 / L1をいう。 To refine the alloy structure, a total forming ratio of 3 or higher during hot working is preferable. A higher total forming ratio is better.

[0057] There are no particular limitations on the method of hot working; the most suitable method can be selected depending on the purpose. Examples of hot working methods include hot forging and hot rolling. After the hot working is complete, the material may be cooled to room temperature, or it may be used for the next process without cooling.

[0058] [2.3. Heat treatment process, aging treatment process] Next, the material after the hot working process is T β Heat treatment is performed at a temperature between -200°C and 1200°C (heat treatment process). Furthermore, if necessary, the heat-treated material may be subjected to aging treatment at a temperature of 400°C to 700°C (aging treatment step).

[0059] The conditions for heat treatment and aging treatment can be selected to be optimal according to the purpose. Optimizing the heat treatment and aging treatment conditions can yield titanium alloys with different microstructures and / or properties. Specific examples of heat treatment and aging treatment conditions include the following:

[0060] [2.3.1. Specific Example 1] The heat treatment step involves T β -200℃ or moreT β The process may include a step of heat treatment at a temperature below -50°C. As described above, generally, an equiaxed structure can be obtained by heat treatment below the Ms point after hot working. The Ms point of the α+β type titanium alloy according to the present invention is approximately T β It will become -50℃. Therefore, T β We will use this to explain heat treatment.

[0061] The heat treatment temperature is T β At temperatures above -50°C, needle-shaped α-phase may precipitate, preventing the acquisition of a uniformly equiaxed structure. On the other hand, if the heat treatment temperature is T β Below -200℃, it becomes difficult to homogenize the structure within a practical heat treatment time. In contrast, the material after hot working is T β -200℃ or moreT β When heat-treated at temperatures below -50°C, an equiaxed structure dominated by the equiaxed α phase is obtained. Titanium alloys with an equiaxed structure exhibit excellent fatigue strength. Furthermore, by optimizing the manufacturing conditions, it is possible to obtain titanium alloys with an equiaxed structure, a tensile strength of 950 MPa or more at 25°C, an elongation of 15% or more at 25°C, and a critical compressibility of 35% or more at 25°C.

[0062] [2.3.2. Specific Example 2] The heat treatment step involves T β The process may also include a step of heat treatment at a temperature of 1200°C or lower. Cooling after heat treatment can be done by air cooling or furnace cooling. To improve the strength and ductility of the titanium alloy, it is preferable to cool it at a rate faster than air cooling.

[0063] The heat treatment temperature is T β If the temperature is below a certain level, equiaxed α-phase may remain, and a uniform needle-like structure cannot be obtained. On the other hand, raising the heat treatment temperature more than necessary does not make a difference in effect and is not practical. In contrast, the material after hot working is T β When heat treatment is performed at temperatures below 1200°C, a needle-like structure dominated by the needle-shaped α phase is obtained. Titanium alloys with a needle-like structure exhibit excellent high-temperature strength and creep properties. Furthermore, by optimizing the manufacturing conditions, titanium alloys with a needle-like structure, a tensile strength of 950 MPa or more at 25°C, an elongation of 15% or more at 25°C, and / or a critical compressibility of 35% or more at 25°C can be obtained.

[0064] [2.3.3. Specific Example 3] The heat treatment step involves the material after the hot working process T β If the process includes heat treatment at a temperature of 1200°C or lower, The method may further include an aging treatment step after the heat treatment step, in which the heat-treated material is subjected to aging treatment at a temperature of 400°C to 700°C.

[0065] The aging treatment may be performed immediately after the heat treatment is completed without cooling the material to room temperature, or it may be performed after the material has been cooled to room temperature. When cooling the material to room temperature after heat treatment, cooling after the heat treatment can be done by air cooling or furnace cooling. To improve the strength and ductility of titanium alloys, it is preferable to cool them at a rate faster than air cooling.

[0066] If the aging treatment temperature exceeds 700°C, there is a risk of coarsening of the precipitate and precipitation of the brittle ω phase. On the other hand, if the aging treatment temperature is below 400°C, it becomes difficult to age the precipitate within a practical time, and there is a risk of precipitation of the ω phase. In contrast, T β After heat treatment at the above temperatures, further aging treatment at a temperature between 400°C and 700°C not only yields a needle-like structure dominated by the needle-shaped α phase, but also allows for the aging of fine precipitates that improve strength properties.

[0067] Titanium alloys containing needle-like structures and age precipitates exhibit not only excellent high-temperature strength and creep properties, but also superior strength properties. Furthermore, by optimizing the manufacturing conditions, titanium alloys with needle-like structures, a tensile strength of 950 MPa or more at 25°C, an elongation of 15% or more at 25°C, and / or a critical compressibility of 35% or more at 25°C can be obtained.

[0068] [2.3.4. Specific Example 4] The heat treatment step involves T β -50℃ or moreT β The process may also include a step of heat treatment at a temperature below a certain level. Cooling after heat treatment can be done by air cooling or furnace cooling. To improve the strength and ductility of the titanium alloy, it is preferable to cool it at a rate faster than air cooling.

[0069] The heat treatment temperature is T β If the temperature is below -50℃, and T β In either of the higher cases, a mixed structure cannot be obtained. In contrast, the material after hot working is T β -50℃ or moreT β When heat treatment is performed at temperatures below a certain level, a mixed structure containing both equiaxed α-phase and acicular α-phase is obtained. Titanium alloys with a mixed structure can achieve both excellent high-temperature properties and high fatigue strength. Furthermore, by optimizing the manufacturing conditions, titanium alloys with a mixed structure, a tensile strength of 950 MPa or more at 25°C, an elongation of 15% or more at 25°C, and / or a critical compressibility of 35% or more at 25°C can be obtained.

[0070] [2.3.5. Specific Example 5] The heat treatment process involves the material after hot forging being T β -50℃ or moreT β If the process includes heat treatment at a temperature below, The method may further include an aging treatment step after the heat treatment step, in which the heat-treated material is subjected to aging treatment at a temperature of 400°C to 700°C.

[0071] The aging treatment may be performed immediately after the heat treatment is completed without cooling the material to room temperature, or it may be performed after the material has been cooled to room temperature. When cooling the material to room temperature after heat treatment, cooling after the heat treatment can be done by air cooling or furnace cooling. To improve the strength and ductility of titanium alloys, it is preferable to cool them at a rate faster than air cooling.

[0072] If the aging treatment temperature exceeds 700°C, there is a risk of coarsening of the precipitate and precipitation of the brittle ω phase. On the other hand, if the aging treatment temperature is below 400°C, it becomes difficult to age the precipitate within a practical time, and there is a risk of precipitation of the ω phase. In contrast, T β -50℃ or moreT β If heat treatment is performed at a temperature below 700°C, followed by further aging treatment at a temperature between 400°C and 700°C, a mixed structure containing both equiaxed α-phase and acicular α-phase can be obtained, and fine precipitates that improve strength properties can be precipitated by aging.

[0073] Titanium alloys containing mixed structures and age precipitates not only achieve both excellent high-temperature properties and high fatigue strength, but also possess superior strength characteristics. Furthermore, by optimizing the manufacturing conditions, titanium alloys with a mixed structure, a tensile strength of 950 MPa or more at 25°C, an elongation of 15% or more at 25°C, and / or a critical compressibility of 35% or more at 25°C can be obtained.

[0074] [3. Effect] T β When a titanium alloy whose composition has been adjusted so that its temperature is below 880°C is hot-worked in the two-phase temperature range, the needle-like α phase is destroyed, and a fine equiaxed α phase is formed. Subsequently, when the obtained material is heat-treated under appropriate conditions, an α+β type alloy is obtained having an equiaxed structure dominated by the equiaxed α phase, an acicular structure dominated by the needle-like α phase, or a mixed structure containing both the equiaxed α phase and the needle-like α phase. The α+β type alloy obtained in this way possesses both high tensile strength and excellent cold workability. The high tensile strength is thought to be due to solid solution strengthening by the added elements. Furthermore, the excellent cold workability is due to T β This is thought to be because the process is optimized, resulting in a higher proportion of the β phase, which has good cold workability. [Examples]

[0075] (Examples 1-8, Comparative Examples 1-9) [1. Sample Preparation] The raw materials, blended to achieve the predetermined composition, were melted using the CCIM method to obtain an ingot measuring φ115 × 100 mm. This ingot was then processed using the T β The material was heated to the above temperature and hot-forged (pre-forged) into a 40mm square. Furthermore, the pre-forged material was heated to the α+β two-phase temperature region and hot-forged (finish forging) into a diameter of 20mm. After forging, T β -200℃ or moreT β Heat treatment was performed in a temperature range below -50°C.

[0076] [2. Test Method] [2.1. Tensile Test] Tensile tests were conducted at 25°C in accordance with JIS Z2241:2011, and the tensile strength and elongation at break were measured. A No. 14A specimen with a parallel section diameter of 6 mm was used.

[0077] [2.2. Compression Test] End-face restrained compression tests were performed at 25°C to measure the critical compressibility. A cylindrical specimen measuring φ15 × 22.5 mm was used. To ensure accuracy, the end-face restrained compression test was performed three times under each condition. The maximum compressibility achieved when no cracks occurred in any of the three tests was defined as the critical compressibility.

[0078] [3. Results] The results are shown in Table 1. Table 1 also shows the components of each sample. Figure 2 shows the β-transformation temperature T. β The relationship between this and the critical compressibility at 25°C is shown. From Table 1 and Figure 2, the following can be seen.

[0079] (1) Comparative Example 1 has low tensile strength. This is thought to be because it does not contain Cr. Also, Comparative Example 1 has low elongation. This is thought to be because the amount of Al is excessive and it does not contain Cr. Furthermore, Comparative Example 1 has a low critical compressibility. This is because T β This is thought to be because the temperature exceeds 880°C. (2) Comparative Example 2 has low tensile strength and elongation. This is thought to be because it does not contain V and Cr. Furthermore, Comparative Example 2 has a low critical compressibility. This is because T β This is thought to be because the temperature exceeds 880°C.

[0080] (3) Comparative Example 3 has a low limit compression ratio. This is because T β This is thought to be because the temperature exceeds 880°C. (4) Comparative Example 4 has low elongation. This is thought to be because the Fe content is slightly high (exceeding 1.5 mass%) and the Cr content is low. Furthermore, Comparative Example 4 has a low critical compressibility. This is because T β This is thought to be because the temperature exceeds 880°C.

[0081] (5) Comparative Example 5 has low tensile strength. This is thought to be because it does not contain V. Furthermore, Comparative Example 5 has low critical compressibility. This is because T β This is thought to be because the temperature exceeds 880°C. (6) Comparative Example 6 has low elongation. This is thought to be due to a small amount of V. Furthermore, Comparative Example 6 has a low critical compression ratio. This is because T β This is thought to be because the temperature exceeds 880°C.

[0082] (7) Comparative Example 7 has low elongation. This is thought to be because the Fe content is slightly high (exceeding 1.5 mass%) and the Cr content is low. Furthermore, Comparative Example 7 has a low critical compressibility. This is because T β This is thought to be because the temperature exceeds 880°C. (8) Comparative Example 8 has a low limit compression ratio. This is because T β This is thought to be because the temperature exceeds 880°C. (9) Comparative Example 9 has low tensile strength and elongation. This is thought to be due to the small amount of V. Furthermore, Comparative Example 9 has a low critical compressibility. This is because T β This is thought to be because the temperature exceeds 880°C.

[0083] (10) Examples 1 to 8 all had a tensile strength of 950 MPa or more, an elongation of 15% or more, and a critical compressibility of 35% or more. (11) Examples 1 and 6 had an elongation of 20% or more and a critical compression ratio of 50% or more. This is because T β This is thought to be because the temperature is below 840℃. (12)T β A correlation was observed between T and the critical compression ratio. See Figure 2. In order to achieve a critical compression ratio of 35% or higher, T β It was found that keeping the temperature below 880°C is effective.

[0084] [Table 1]

[0085] Although embodiments of the present invention have been described in detail above, the present invention is not limited in any way to the above embodiments, and various modifications are possible without departing from the spirit of the present invention. [Industrial applicability]

[0086] The α+β type titanium alloy according to the present invention can be used in various structural components, corrosion-resistant components, etc., used in golf club heads, chemical industrial equipment, electrical equipment, space equipment, aircraft, ships, vehicles, medical devices, condensers, heat exchangers, seawater desalination plants, and the like.

Claims

1. 3.0≦Al≦5.4mass%, 2.5≦V≦6.0mass%, 1.5 ≤ Cr ≤ 7.0 mass%, and, 0.05≦Fe≦3.0mass% It contains, with the remainder consisting of Ti and unavoidable impurities. β-transformation temperature T β The temperature is below 880°C. α+β type titanium alloy.

2. Sn≦0.5mass%, Zr≦0.5mass%, Mo≦0.5mass%, Nb≦0.5mass%, Ta≦0.5mass%, Ni≦0.15mass%, Mn ≤ 0.25 mass%, and, B≦0.2mass% It further comprises at least one element selected from the group consisting of, Sn + Zr + Mo + Nb + Ta + Ni + Mn + B ≤ 2.0 mass% The α+β type titanium alloy according to claim 1 that satisfies the requirements.

3. The tensile strength at 25°C is 950 MPa or higher. The elongation at 25°C is 15% or more, and The critical compressibility at 25°C is 35% or higher. The α+β type titanium alloy according to claim 1.

4. A preparation step for preparing a material made of the α+β type titanium alloy described in claim 1 or 2, The aforementioned material is subjected to a β transformation temperature of 600°C or higher T β A hot working process that performs hot working in a temperature range below a certain temperature, The material after the aforementioned hot working process is T β A heat treatment process in which heat treatment is performed at a temperature of -200°C or higher and 1200°C or lower. A method for manufacturing α+β type titanium alloys equipped with the necessary components.

5. The heat treatment step involves the material after hot working being T β -200℃ or moreT β A method for producing an α+β type titanium alloy according to claim 4, comprising the step of performing heat treatment at a temperature of less than -50°C.

6. The heat treatment step involves the material after hot working being T β A method for producing an α+β type titanium alloy according to claim 4, comprising the step of performing heat treatment at a temperature of 1200°C or lower.

7. A method for producing an α+β type titanium alloy according to claim 6, further comprising an aging treatment step after the heat treatment step, in which the material after heat treatment is subjected to aging treatment at a temperature of 400°C to 700°C.

8. The heat treatment step involves the material after hot working being T β -50℃ or moreT β A method for producing an α+β type titanium alloy according to claim 4, comprising the step of performing heat treatment at a temperature below a certain level.

9. A method for producing an α+β type titanium alloy according to claim 8, further comprising an aging treatment step after the heat treatment step, in which the material after heat treatment is subjected to aging treatment at a temperature of 400°C to 700°C.

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