Alpha+beta titanium alloy and method for producing same

Abstract

The present invention relates to an α+β type titanium alloy according to the present invention includes: 3.0 mass% ≤ Al ≤ 5.4 mass%; 2.5 mass% ≤ V ≤ 6.0 mass%; 1.5 mass% ≤ Cr ≤ 7.0 mass%; and 0.05 mass% ≤ Fe ≤ 3.0 mass%, with the balance being Ti and unavoidable impurities, and has a β transformation temperature Tβ of 880°C or lower.

Description

TECHNICAL FIELD

[0001] The present invention relates to an α+β type titanium alloy and a method for producing a semi-finished material composed of the α+β type titanium alloy, and more particularly to an α+β type titanium alloy having both high tensile strength and excellent cold-workability and a method for producing a semi-finished material composed of the α+β type titanium alloy.BACKGROUND ART

[0002] The practical titanium alloys are broadly classified into (1) an α-type alloy composed of an α phase (low-temperature phase) that is a close-packed hexagonal crystal, (2) a β-type alloy composed of a β phase (high-temperature phase) that is a body-centered cubic crystal, and (3) an α+β type alloy having a two-phase structure of the α phase and the β phase.

[0003] Among these, the α+β type alloy is a well-balanced material excellent in strength, specific strength, workability, corrosion resistance, and the like, and has been used as a space aircraft material, an automobile material, a material for mechanical structural parts, a general demand material, a material for medical instruments, or the like. In particular, among the α+β type alloys, the Ti-6Al-4V alloy has been widely used as a general-purpose high-power titanium alloy, and occupies about 80% of the Ti alloy usage amount.

[0004] Various proposals have been made regarding such titanium alloys.

[0005] For example, Patent Literature 1 discloses a titanium alloy for a golf club face, which contains predetermined amounts of Al, Fe, N, and O, with the balance being Ti and unavoidable impurities.

[0006] Patent Literature 1 states that both high strength and high Young's modulus can be achieved by optimizing the addition amounts of Al, O, and N as solid solution-strengthening elements of the α phase and Fe as a β-stabilizing element.

[0007] Patent Literature 2 discloses a β-type titanium alloy containing predetermined amounts of Al, Fe, and Cr, and further containing predetermined amounts of V and / or Mo, with the balance being Ti and unavoidable impurities.

[0008] Patent Literature 2 states that, in a titanium alloy further containing V or Mo in addition to Al, Fe, and Cr, in the case where the total content of V and Mo is 10 mass% or less, the influence of component segregation of Fe and Cr is alleviated.

[0009] Patent Literature 3 discloses a titanium alloy containing predetermined amounts of Al and Fe, with the balance being Ti and unavoidable impurities.

[0010] Patent Literature 3 states that the titanium alloy having such a composition has a Young's modulus of 120 GPa to 160 GPa and a tensile strength of 950 MPa to 2200 MPa.

[0011] In the titanium alloy described in Patent Literature 1, predetermined amounts of O and N are added. Therefore, in the case where the O amount and / or the N amount is high, ductility of the titanium alloy may decrease. Further, Patent Literature 1 does not mention the cold-workability of the titanium alloy.

[0012] In the β-type titanium alloy described in Patent Literature 2, component segregation is likely to occur because the amount of eutectoid-type added elements is large among β-stabilizing elements. In addition, in general, the β-type titanium alloy is inferior in high-temperature strength to the α+β type titanium alloy, and in particular, the titanium alloy containing Mo has a high density to deteriorate lightness in weight.

[0013] Further, Patent Literature 3 does not mention the cold-workability of the titanium alloy. Patent Literature 1: W O2013 / 125039 Patent Literature 2: JP 2008-133531A Patent Literature 3: JP 2006-212092A SUMMARY OF INVENTION

[0014] An object of the present invention is to provide an α+β type titanium alloy having both high tensile strength and excellent cold-workability, and a method for producing the α+β type titanium alloy.

[0015] In order to solve the above-mentioned problem, an α+β type titanium alloy according to the present invention includes: 3.0 mass % ≤ Al ≤ 5.4 mass % ; 2.5 mass % ≤ V ≤ 6.0 mass % ; 1.5 mass % ≤ Cr ≤ 7.0 mass % ; 0.05 mass % ≤ Fe ≤ 3.0 mass % , and optionally at least one of: Sn ≤ 0.5 mass % ; Zr ≤ 0.5 mass % ; Mo ≤ 0.5 mass % ; Nb ≤ 0.5 mass % ; Ta ≤ 0.5 mass % ; Ni ≤ 0.15 mass % ; Mn ≤ 0.25 mass % ; B ≤ 0.2 mass % ; Si ≤ 0.3 mass % ; O ≤ 0.4 mass % ; N ≤ 0.1 mass % ; H ≤ 0.1 mass % ; and C ≤ 0.2 mass % , with the balance being Ti and unavoidable impurities, and has a β transformation temperature T β of 880°C or lower.

[0016] A method for producing a semi-finished material composed of an α+β type titanium alloy according to the present invention, includes: a preparation step of preparing a raw material composed of the α+β type titanium alloy of the present invention; a hot-working step of performing a hot-working on the material in a temperature range of 600°C or higher and lower than the β transformation temperature T β ; and a heat treatment step of performing a heat treatment on the material after the hot-working at a temperature of T β - 200°C or higher and 1200°C or lower.

[0017] When the titanium alloy in which the component is adjusted so that T β is 880°C or lower is subjected to a hot-working in a two-phase temperature range, the acicular α phase is fractured, and a fine equiaxed α phase is formed. Subsequently, when the obtained material is heat-treated under appropriate conditions, an α+β type alloy having an equiaxed structure in which the equiaxed α phase is dominant, an acicular structure in which the acicular α phase is dominant, or a mixed structure including both the equiaxed α phase and the acicular α phase can be obtained.

[0018] The α+β type alloy thus obtained has both high tensile strength and excellent cold-workability. It is considered that high tensile strength is obtained due to solid solution-strengthening by the added element. In addition, it is considered that the reason why excellent cold-workability is obtained is that the proportion of the β phase having good cold-workability is large because T β is optimized.BRIEF DESCRIPTION OF DRAWINGS

[0019] FIG. 1 includes views each showing an example of a microstructure of an α+β type titanium alloy according to the present invention ((a): acicular structure, (b): equiaxed structure, and (c): mixed structure). FIG. 2 is a diagram showing a relationship between a β transformation temperature T β and a limit of compressibility at 25°C. DESCRIPTION OF EMBODIMENTS

[0020] Hereinafter, an embodiment of the present invention will be described in detail.[1. α+β Type Titanium Alloy][1.1. Main Constituent Elements]

[0021] An α+β type titanium alloy (hereinafter, also simply referred to as "titanium alloy") according to the present invention contains the following elements, with the balance being Ti and unavoidable impurities. Types of added elements, component ranges thereof, and reasons for limitation thereof are as follows. (1) 3.0 mass % ≤ Al ≤ 5.4 mass % :

[0022] Al is an element that contributes to stabilization of the α phase and solid solution-strengthening of the α phase. In order to obtain such an effect, the content of Al needs to be 3.0 mass% or more.

[0023] On the other hand, in the case where the content of Al is excessive, Ti 3 Al, which is an embrittlement phase, precipitates, and ductility of the titanium alloy may decrease. In addition, in the case where the content of Al is excessive, solid solution-strengthening excessively proceeds, and cold-workability of the titanium alloy may decrease. Therefore, the content of Al needs to be 5.4 mass% or less. The content of Al is preferably 4.3 mass% or less. (2) 2.5 mass % ≤ V ≤ 6.0 mass % :

[0024] V is an element that contributes to stabilization of the β phase, solid solution-strengthening of the β phase, and improvement in cold-workability by formation of the β phase. In order to obtain such an effect, the content of V needs to be 2.5 mass% or more.

[0025] On the other hand, since V is an expensive element, the raw material cost increases in the case where the content of V is excessive. Therefore, the content of V needs to be 6.0 mass% or less. The content of V is preferably 4.5 mass% or less. (3) 1.5 mass % ≤ Cr ≤ 7.0 mass % :

[0026] Cr is an element that contributes to stabilization of the β phase, solid solution-strengthening of the β phase, and improvement in cold-workability by formation of the β phase. In order to obtain such effects, the content of Cr needs to be 1.5 mass% or more.

[0027] On the other hand, since Cr is a very strong β-phase stabilizing element, the two-phase structure may not be obtained in the case where the content of Cr is excessive. In addition, in the case where the content of Cr is excessive, segregation occurs during solidification, and manufacturability may deteriorate. Therefore, the content of Cr needs to be 7.0 mass% or less.

[0028] The content of Zr is more preferably 4.0 mass% or less. (4) 0.05 mass % ≤ Fe ≤ 3.0 mass % :

[0029] Fe is an element that contributes to stabilization of the β phase and solid solution-strengthening of the β phase. Fe as an alloy element and Ti containing Fe as an impurity contribute to reduction in raw material cost because they are inexpensive. In order to obtain such an effect, the content of Fe needs to be 0.05 mass% or more. The content of Fe is preferably 0.25 mass% or more.

[0030] On the other hand, since Fe is a very strong β-phase stabilizing element, the two-phase structure may not be obtained in the case where the content of Fe is excessive. In addition, in the case where the content of Fe is excessive, segregation may occur during solidification, and manufacturability may deteriorate. Therefore, the content of Fe needs to be 3.0 mass% or less. The content of Fe is more preferably 1.3 mass% or less.[1.2. Sub Constituent Element]

[0031] The α+β type titanium alloy according to the present invention may further contain one or two or more of the following elements in addition to the above-described main constituent elements. Types of added elements, component ranges thereof, and reasons for limitation thereof are as follows. (1) Sn ≤ 0.5 mass % :

[0032] Sn is an element that contributes to solid solution-strengthening of the α phase and the β phase, and can be added as necessary.

[0033] However, in the case where the content of Sn is excessive, undissolved Ti 3 Al remains even after the solid solution heat treatment, and ductility of the titanium alloy may decrease. Therefore, the content of Sn is preferably 0.5 mass% or less. The content of Sn is more preferably 0.1 mass% or less. (2) Zr ≤ 0.5 mass % :

[0034] Zr is an element that contributes to solid solution-strengthening of the α phase and the β phase, and can be added as necessary.

[0035] However, in the case where the content of Zr is excessive, undissolved Ti 3 Al remains even after the solid solution heat treatment, and ductility of the titanium alloy may decrease. Therefore, the content of Zr is preferably 0.5 mass% or less. The content of Zr is more preferably 0.1 mass% or less. (3) Mo ≤ 0.5 mass % :

[0036] Mo is an element that contributes to solid solution-strengthening of the β phase, and can be added as necessary.

[0037] However, since Mo is a very strong β-phase stabilizing element, the two-phase structure may not be obtained in the case where the content of Mo is excessive. In addition, in the case where the content of Mo is excessive, segregation may occur during solidification. Further, since Mo is expensive, the raw material cost increases in the case where the content of Mo is excessive. Therefore, the content of Mo is preferably 0.5 mass% or less. The content of Mo is more preferably 0.1 mass% or less. (4) Nb ≤ 0.5 mass % :

[0038] Nb is an element that contributes to an improvement in oxidation resistance of the titanium alloy, and can be added as necessary.

[0039] However, in the case where the content of Nb is excessive, the specific gravity of the titanium alloy may increase, and the specific strength may decrease. Therefore, the content of Nb is preferably 0.5 mass% or less. The content of Nb is more preferably 0.1 mass% or less. (5) Ta ≤ 0.5 mass % :

[0040] Ta is an element that contributes to solid solution-strengthening of the β phase, and can be added as necessary.

[0041] However, in the case where the content of Ta is excessive, the specific gravity of the titanium alloy may increase, and the specific strength may decrease. Therefore, the content of Ta is preferably 0.5 mass% or less. The content of Ta is more preferably 0.1 mass% or less. (6) Ni ≤ 0.15 mass % :

[0042] Ni is an element that contributes to solid solution-strengthening of the β phase, and can be added as necessary.

[0043] However, since Ni is a very strong β-phase stabilizing element, the two-phase structure may not be obtained in the case where the content of Ni is excessive. In addition, in the case where the content of Ni is excessive, segregation may occur during solidification. Therefore, the content of Ni is preferably 0.15 mass% or less. (7) Mn ≤ 0.25 mass % :

[0044] Mn is an element that contributes to solid solution-strengthening of the β phase, and can be added as necessary.

[0045] However, since Mn is a very strong β-phase stabilizing element, the two-phase structure may not be obtained in the case where the content of Mn is excessive. In addition, in the case where the content of Mn is excessive, segregation may occur during solidification. Therefore, the content of Mn is preferably 0.25 mass% or less. (8) B ≤ 0.2 mass % :

[0046] B is an element that forms a boride with Ti and contributes to refinement of crystal grains, and can be added as necessary.

[0047] However, in the case where the content of B is excessive, the boride becomes coarse, and the boride becomes a starting point of fracture, so that hot-workability and cold-workability may be significantly impaired. Therefore, the content of B is preferably 0.2 mass% or less. The content of B is more preferably 0.05 mass% or less. (9) Sn + Zr + Mo + Nb + Ta + Ni + Mn + B ≤ 2.0 mass % :

[0048] Even when the content of each of Sn, Zr, Mo, Nb, Ta, Ni, Mn, and B is within the above-described range, ductility of the titanium alloy may decrease, fatigue strength may decrease, specific strength may decrease, or segregation may occur during solidification, in the case where the total content of these elements is excessive. Therefore, the total content of these elements is preferably 2.0 mass% or less.[1.3. Unavoidable Impurities]

[0049] The "unavoidable impurities" are components mixed due to various factors such as a raw material and a production process when the α+β type titanium alloy according to the present invention is industrially produced, and the content thereof is within a range that does not adversely affect the properties of the α+β type titanium alloy according to the present invention. Specific examples of the unavoidable impurities include the followings. (1) Si ≤ 0.3 mass % :

[0050] In the case where the content of Si is excessive, ductility, cold-workability, or hot-workability of the titanium alloy may deteriorate. Therefore, the content of Si is preferably 0.3 mass% or less.

[0051] The smaller the content of Si, the better. However, an extreme reduction in the content of Si may cause an increase in manufacturing cost. Considering the manufacturing cost, the content of Si is preferably 0.0005 mass% or more. (2) O ≤ 0.4 mass % :

[0052] O is an element that contributes to solid solution-strengthening of the α phase. However, in the case where the content of O is excessive, ductility of the titanium alloy may deteriorate. Therefore, the content of O is preferably 0.4 mass% or less.

[0053] The smaller the content of O, the better. However, an extreme reduction in the content of O may cause an increase in manufacturing cost. Considering the manufacturing cost, the content of O is preferably 0.005 mass% or more. (3) N ≤ 0.1 mass % :

[0054] N is an element that contributes to solid solution-strengthening of the α phase. However, in the case where the content of N is excessive, ductility of the titanium alloy may deteriorate. Therefore, the content of N is preferably 0.1 mass% or less.

[0055] The smaller the content of N, the better. However, an extreme reduction in the content of N may cause an increase in manufacturing cost. Considering the manufacturing cost, the content of N is preferably 0.0005 mass% or more. (4) H ≤ 0.1 mass % :

[0056] In the case where the content of H is excessive, strength and ductility of the titanium alloy may deteriorate. Therefore, the content of H is preferably 0.1 mass% or less.

[0057] The smaller the content of H, the better. However, an extreme reduction in the content of H may cause an increase in manufacturing cost. Considering the manufacturing cost, the content of H is preferably 0.0005 mass% or more. (5) C ≤ 0.2 mass % :

[0058] In the case where the content of C is excessive, ductility, cold-workability, or hot-workability of the titanium alloy may deteriorate. Therefore, the content of C is preferably 0.2 mass% or less.

[0059] The smaller the content of C, the better. However, an extreme reduction in the content of C may cause an increase in manufacturing cost. Considering the manufacturing cost, the content of C is preferably 0.0005 mass% or more. (6) Total Content of Impurities ≤ 1.0 mass % :

[0060] Examples of the unavoidable impurities other than the above-described elements include P, S, Pd, Hf, and REM. The content of each of these elements is preferably 0.05 mass% or less.

[0061] In addition, even when the content of each of the unavoidable impurities is equal to or less than the above-described upper limit, ductility, cold-workability, or hot-workability of the titanium alloy may deteriorate in the case where the total content of the unavoidable impurities is excessive. Therefore, the total content of the unavoidable impurities is preferably 1.0 mass% or less. The total content is more preferably 0.5 mass% or less.[1.4. β Transformation Temperature]

[0062] The "β transformation temperature T β " refers to a lower limit temperature at which the titanium alloy becomes a β single phase, and can be measured by differential thermal analysis (DTA).

[0063] It was found that T β of the titanium alloy according to the present invention correlates with cold-workability (limit of compressibility at 25°C). As T β decreases, the cold-workability increases. It is considered that this is because the proportion of the β phase having excellent cold-workability increases as T β decreases. In order to obtain such an effect, T β needs to be 880°C or lower. T β is preferably 870°C or lower, more preferably 860°C or lower, and further preferably 850°C or lower.[1.5. Structure]

[0064] FIG. 1 shows examples of a microstructure of the α+β type titanium alloy according to the present invention. The titanium alloy according to the present invention has a two-phase structure of α+β at room temperature. In addition, the titanium alloy according to the present invention exhibits an acicular structure ((a) of FIG. 1), an equiaxed structure ((b) of FIG. 1), or a mixed structure ((c) of FIG. 1) depending on processing and heat treatment conditions.

[0065] The "acicular structure" refers to a structure in which an acicular α phase is precipitated in a matrix composed of the β phase (β phase + acicular α phase). The acicular structure can be obtained by heat-treating the titanium alloy according to the present invention at a temperature equal to or higher than T β , cooling the heat-treated titanium alloy, and precipitating the acicular α phase in association with martensitic transformation during the cooling. The titanium alloy having the acicular structure is characterized by having slightly low low-temperature strength, low ductility, and low fatigue properties, but having extremely high high-temperature strength, extremely high fracture toughness, and extremely high creep properties.

[0066] The "equiaxed structure" refers to a structure in which the equiaxed α phase is mainly present, and the β phase is present in a gap between the relatively large equiaxed α phases. The equiaxed structure can be obtained by heat-treating the titanium alloy according to the present invention at a temperature lower than T β , then heat-treating the titanium alloy at a temperature lower than the martensitic transformation-starting point (Ms point) to divide or recrystallize the acicular structure, to form the equiaxed α phase. A sufficient strain is introduced into the acicular α phase to divide or recrystallize the acicular α phase, to thereby form an equiaxed structure. The titanium alloy having the equiaxed structure is characterized by having slightly low fracture toughness, low high-temperature strength, and low creep properties, but having high ductility, extremely high low-temperature strength, and extremely high fatigue properties.

[0067] The "mixed structure" refers to a structure in which the equiaxed α phase and the β phase in which the acicular α phase is precipitated (β phase + acicular α phase) are mixed. The mixed structure can be obtained by hot-working the titanium alloy according to the present invention at a temperature lower than T β , and then performing a heat treatment in an α+β two-phase temperature range equal to or higher than the Ms point to make the α phase equiaxed, and additionally precipitating the acicular α phase along with martensitic transformation of the β phase. Finally, a structure in which the equiaxed α phase and the β phase + acicular α phase regions are mixed is obtained. The titanium alloy having the mixed structure has slightly low ductility, but has high low-temperature strength, high high-temperature strength, high fracture toughness, high creep properties, and high fatigue properties, and is a well-balanced material.

[0068] [1.6. Properties][1.6.1. Tensile Strength]

[0069] The "tensile strength" refers to a value obtained by performing a tensile test by using a No. 14A test piece having a parallel portion diameter of 6 mm in accordance with JIS Z2241 (2011).

[0070] In the titanium alloy according to the present invention, the tensile strength at 25°C can be 950 MPa or more in the case where components and / or structures are optimized.[1.6.2. Elongation]

[0071] The "elongation" refers to an elongation at rupture obtained by performing a tensile test by using a No. 14A test piece having a parallel portion diameter of 6 mm in accordance with JIS Z2241 (2011).

[0072] In the titanium alloy according to the present invention, the elongation at 25°C can be 15% or more in the case where components and / or structures are optimized. In the case where the component and / or structure are further optimized, the elongation at 25°C can be 20% or more.[1.6.3. Limit of Compressibility]

[0073] "Compressibility (%)" refers to a value represented by the following formula. Compressibility = H 1 − H 2 × 100 / H 1

[0074] Here, H 1 represents the height of the test piece before the compression test, and H 2 represents the height of the test piece after the compression test.

[0075] The "limit of compressibility" refers to the maximum compressibility at which no crack is observed in the test piece after an end face restraint compression test.

[0076] The "end face restraint compression test" refers to a test in which pressure-resistant plates are disposed above and below a test piece, and the test piece is compressed while the spread of upper and lower end faces of the test piece is restrained. The end face restraint compression test can eliminate the influence of sliding (friction) between the pressure-resistant plate and the test piece during pressurization, and therefore, the end face restraint compression test is excellent as a method for evaluating deformability and is suitable as a method for evaluating cold-workability.

[0077] The test piece had a shape of 15 mm-diameter × 22.5 mm-height, and the compression speed was 35 mm / s. Further, the test was performed at least three times under one condition, and the maximum compressibility at which no crack was observed in the compressed test piece was defined as the limit of compressibility.

[0078] In the titanium alloy according to the present invention, the limit of compressibility at 25°C can be 35% or more in the case where the components and / or structures are optimized. In the case where the components and / or structures are further optimized, the limit of compressibility at 25°C can be 40% or more, 45% or more, or 50% or more.[1.7. Application]

[0079] The present invention relates to a material composed of an α+β type titanium alloy that can be used as a raw material for manufacturing a semi-finished material, the semi-finished material manufactured from the material of the present invention, and various titanium products made from the semi-finished material.

[0080] The α+β type titanium alloy according to the present invention can be applied to various applications.

[0081] Examples of titanium products to which the α+β type titanium alloy according to the present invention can be applied include (1) various sports and leisure goods such as golf club heads and bicycle gears, (2) consumer products such as eyeglass frames, and (3) various parts requiring light weight and high strength, such as various bolts, valve retainers, and seawater-resistant shafts.[2. Method for Producing α+β type Titanium Alloy]

[0082] A method for producing the α+β type titanium alloy according to the present invention includes: a preparation step of preparing a material composed of the α+β type titanium alloy according to the present invention; a hot-working step of performing a hot-working on the material in a temperature range of 600°C or higher and lower than the β transformation temperature T β ; and a heat treatment step of performing a heat treatment on the material after the hot-working at a temperature of T β - 200°C or higher and 1200°C or lower.

[0083] The method for producing the α+β type titanium alloy according to the present invention may further include an aging treatment step of subjecting the material after the heat treatment to an aging treatment at a temperature of 400°C or higher and 700°C or lower.[2.1. Preparation Step]

[0084] First, a material composed of the α+β type titanium alloy according to the present invention is prepared.

[0085] Specifically, the preparation of the raw material is preferably performed by (a) melting and casting raw materials blended so as to have a predetermined composition, to prepare an alloy ingot, (b) subjecting the molten and cast alloy ingot to a pre-hot-working as necessary, and (c) subjecting the material subjected to the pre-hot-working to a preheating treatment as necessary.[2.1.1. Melting and Casting]

[0086] First, the raw materials blended so as to have a predetermined composition are melted and cast to obtain an alloy ingot. A method for melting and casting the raw materials is not particularly limited, and an optimum method can be selected according to a purpose thereof.

[0087] Examples of the method for melting and casting the raw materials include a cold-crucible induction-melting (CCIM) method.[2.1.2. Pre-Hot-Working]

[0088] In the case where the alloy ingot produced according to the CCIM method or the like has a size equal to or smaller than the size suitable for the next step (hot-working step), the alloy ingot can be used as it is as a material for the next step.

[0089] On the other hand, in the case where the produced alloy ingot exceeds the size suitable for the next step, it is preferable to perform a pre-hot-working of the material.

[0090] The pre-hot-working is preferably started from the β single-phase temperature range. The cast structure of the alloy ingot can be divided by performing the pre-hot-working. Further, the deformation resistance is relatively small in the β single phase temperature range, and therefore, the division of the cast structure is facilitated. A pre-hot-working method is not particularly limited, and an optimal method can be selected according to the purpose. Examples of the pre-hot-working method include a hot-forging and a hot-rolling.

[0091] When the temperature of the material decreases to the α+β two-phase temperature range during the pre-hot-working, the pre-hot-working may be ended, or the pre-hot-working may be continued as it is. When the pre-hot-working is continued even after the temperature of the material decreases to the α+β two-phase temperature range, the structure may be further refined. In order to refine the alloy structure, the termination temperature of the pre-hot-working is preferably in a relatively high temperature range in the α+β two-phase temperature range. Specifically, the termination temperature of the pre-hot-working is preferably T β - 100°C or higher and lower than T β .[2.1.3. Preheating Treatment]

[0092] In the case where the pre-hot-working is performed, it is preferable to further perform a preheating treatment on the material subjected to the pre-hot-working. The preheating treatment is preferably performed in a temperature range in which the alloy structure can be homogenized, the coarsening of crystal grains can be prevented or inhibited, and the effect generated by the pre-hot-working can be maintained. The preheating treatment is preferably held in a relatively low temperature range in the β single phase temperature range. Specifically, the temperature of the preheating treatment is preferably T β or higher and T β + 80°C or lower.[2.2. Hot-Working Step]

[0093] After performing the pre-hot-working and preheating treatment of the material as necessary, the material is subjected to a hot-working in a temperature range of 600°C or higher and lower than the β transformation temperature T β .

[0094] The hot-working is performed to refine the alloy structure and adjust the form of the equiaxed α phase. Therefore, the hot-working needs to be performed in the α+β two-phase temperature range from the start to the end of the hot-working. In the case where the hot-working temperature is equal to or higher than T β , only the acicular structure is obtained even if the heat treatment is performed thereafter, and the structure control is difficult. Therefore, the hot-working temperature needs to be lower than T β .

[0095] On the other hand, in the case where the hot-working temperature is too low, the deformation resistance may excessively increase. Therefore, the hot-working temperature needs to be 600°C or higher. In order to reduce the deformation resistance, the hot-working temperature is preferably in a relatively high temperature range in the α+β two-phase temperature range.

[0096] In the maximum strain direction among principal strains in three directions generated during the hot-working, the length before the start of the hot-working is defined as "L 1 ", and the length after the end of the hot-working is defined as "L 2 ".

[0097] In this case, the "total strain ratio" refers to L 1 / L 2 in the case of L 1 > L 2 , and refers to L 2 / L 1 in the case of L 1 < L 2 .

[0098] In order to refine the alloy structure, the total strain ratio during the hot-working is preferably 3 or more. A higher total strain ratio is better.

[0099] A hot-working method is not particularly limited, and an optimal method can be selected according to the purpose. Examples of the hot-working method include a hot-forging and a hot-rolling.

[0100] After the end of the hot-working, the material may be once cooled to room temperature, or may be subjected to the next step without cooling the material.[2.3. Heat Treatment Step and Aging Treatment Step]

[0101] Next, the material after the hot-working is subjected to a heat treatment at a temperature of T β - 200°C or higher and 1200°C or lower (heat treatment step).

[0102] If necessary, the material after the heat treatment may be subjected to an aging treatment at a temperature of 400°C or higher and 700°C or lower (aging treatment step).

[0103] As the conditions of the heat treatment and the aging treatment, optimum conditions can be selected according to the purpose. When the conditions of the heat treatment and the conditions of the aging treatment are optimized, titanium alloys having different structures and / or properties can be obtained. Specific conditions for the heat treatment and the aging treatment are as follows.[2.3.1. Specific Example 1]

[0104] The heat treatment step may include a step of performing a heat treatment on the material after the hot-working at a temperature of T β - 200°C or higher and lower than T β - 50°C.

[0105] As described above, in general, the equiaxed structure can be obtained by performing the heat treatment at a temperature lower than the Ms point after hot-working. The Ms point of the α+β type titanium alloy according to the present invention is about T β - 50°C. Therefore, the heat treatment will be described by using T β .

[0106] In the case where the heat treatment temperature is T β - 50°C or higher, the acicular α phase may be precipitated, and the equiaxed structure throughout the entire surface may not be obtained. On the other hand, in the case where the heat treatment temperature is lower than T β - 200°C, it is difficult to homogenize the structure within a practical heat treatment time. In contrast, in the case where the material after hot-working is heat-treated at a temperature of T β - 200°C or higher and lower than T β - 50°C, an equiaxed structure in which the equiaxed α phase is dominant can be obtained.

[0107] A titanium alloy having the equiaxed structure is excellent in fatigue strength. In the case where the production conditions are optimized, a titanium alloy having the equiaxed structure as well as having a tensile strength at 25°C of 950 MPa or more, an elongation at 25°C of 15% or more, and a limit of compressibility at 25°C of 35% or more can be obtained.[2.3.2. Specific Example 2]

[0108] The heat treatment step may include a step of performing a heat treatment on the material after the hot-working at a temperature of T β or higher and 1200°C or lower.

[0109] Cooling after the end of the heat treatment may be air cooling or furnace cooling. In order to improve strength, toughness and ductility of the titanium alloy, it is preferable to perform cooling at a cooling rate higher than that of air cooling.

[0110] In the case where the heat treatment temperature is lower than T β , the equiaxed α phase may remain, and an acicular structure throughout the entire surface may not be obtained. On the other hand, even if the heat treatment temperature is increased more than necessary, there is no difference in effect, and there is no practical benefit. In contrast, in the case where the material after hot-working is heat-treated at a temperature of T β or higher and 1200°C or lower, an acicular structure in which the acicular α phase is dominant can be obtained.

[0111] A titanium alloy having the acicular structure is excellent in high-temperature strength and creep properties. In addition, in the case where the production conditions are optimized, a titanium alloy having the acicular structure as well as having a tensile strength at 25°C of 950 MPa or more, an elongation at 25°C of 15% or more, and / or a limit of compressibility at 25°C of 35% or more can be obtained.[2.3.3. Specific Example 3]

[0112] In the case where the heat treatment step includes a step of performing a heat treatment on the material after the hot-working at a temperature of T β or higher and 1200°C or lower, the method may further include an aging treatment step of subjecting the material after the heat treatment to an aging treatment at a temperature of 400°C or higher and 700°C or lower after the heat treatment step.

[0113] The aging treatment may be continuously performed after the end of the heat treatment without cooling the material to room temperature, or may be performed after cooling the material to room temperature.

[0114] In the case where the material is cooled to room temperature after the heat treatment, the cooling after the end of the heat treatment may be air cooling or furnace cooling. In order to improve strength, toughness and ductility of the titanium alloy, it is preferable to perform cooling at a cooling rate higher than that of air cooling.

[0115] In the case where the aging treatment temperature exceeds 700°C, coarsening of precipitates and precipitation of a ω phase, which is an embrittlement phase, may occur. On the other hand, in the case where the aging treatment temperature is lower than 400°C, it is difficult to precipitate precipitates by aging within a practical time, and the ω phase may precipitate. In contrast, in the case where the heat treatment is performed at a temperature of T β or higher and then the aging treatment is further performed at a temperature of 400°C or higher and 700°C or lower, not only the acicular structure in which the acicular α phase is dominant can be obtained, but also fine precipitates that improve strength properties can be precipitated by aging.

[0116] The titanium alloy containing the acicular structure and the aged precipitates is excellent not only in high-temperature strength and creep properties but also in strength properties. In addition, in the case where the production conditions are optimized, a titanium alloy having the acicular structure as well as having a tensile strength at 25°C of 950 MPa or more, an elongation at 25°C of 15% or more, and / or a limit of compressibility at 25°C of 35% or more can be obtained.[2.3.4. Specific Example 4]

[0117] The heat treatment step may include a step of performing a heat treatment on the material after the hot-working at a temperature of T β - 50°C or more and less than T β .

[0118] Cooling after the end of the heat treatment may be air cooling or furnace cooling. In order to improve strength, toughness and ductility of the titanium alloy, it is preferable to perform cooling at a cooling rate higher than that of air cooling.

[0119] In the case where the heat treatment temperature is lower than T β - 50°C and in the case where the heat treatment temperature is higher than T β , a mixed structure is not obtained. In contrast, in the case where the material after hot-working is heat-treated at a temperature of T β - 50°C or higher and lower than T β , a mixed structure containing both the equiaxed α phase and the acicular α phase can be obtained.

[0120] The titanium alloy having the mixed structure can achieve both excellent high-temperature properties and high fatigue strength. In addition, in the case where the production conditions are optimized, a titanium alloy having the mixed structure as well as having a tensile strength at 25°C of 950 MPa or more, an elongation at 25°C of 15% or more, and / or a limit of compressibility at 25°C of 35% or more can be obtained.[2.3.5. Specific Example 5]

[0121] In the case where the heat treatment step includes a step of performing a heat treatment on the material after the hot-working at a temperature of T β - 50°C or higher and lower than T β , the method may further include an aging treatment step of subjecting the material after the heat treatment to an aging treatment at a temperature of 400°C or higher and 700°C or lower after the heat treatment step.

[0122] The aging treatment may be continuously performed after the end of the heat treatment without cooling the material to room temperature, or may be performed after cooling the material to room temperature.

[0123] In the case where the material is cooled to room temperature after the heat treatment, the cooling after the end of the heat treatment may be air cooling or furnace cooling. In order to improve strength, toughness and ductility of the titanium alloy, it is preferable to perform cooling at a cooling rate higher than that of air cooling.

[0124] In the case where the aging treatment temperature exceeds 700°C, coarsening of precipitates and precipitation of the ω phase, which is an embrittlement phase, may occur. On the other hand, in the case where the aging treatment temperature is lower than 400°C, it is difficult to precipitate precipitates by aging within a practical time, and the ω phase may precipitate. In contrast, in the case where the heat treatment is performed at a temperature of T β - 50°C or higher and lower than T β and then the aging treatment is further performed at a temperature of 400°C or higher and 700°C or lower, not only the mixed structure including both the equiaxed α phase and the acicular α phase can be obtained, but also fine precipitates that improve strength properties can be precipitated by aging.

[0125] The titanium alloy containing the mixed structure and the aged precipitates can achieve not only excellent high-temperature properties and high fatigue strength but also excellent strength properties. In addition, in the case where the production conditions are optimized, a titanium alloy having the mixed structure as well as having a tensile strength at 25°C of 950 MPa or more, an elongation at 25°C of 15% or more, and / or a limit of compressibility at 25°C of 35% or more can be obtained.[3. Effects]

[0126] When the titanium alloy in which the component is adjusted so that T β is 880°C or lower is subjected to a hot-working in a two-phase temperature range, the acicular α phase is fractured, and a fine equiaxed α phase is formed. Subsequently, when the obtained material is heat-treated under appropriate conditions, the α+β type alloy having the equiaxed structure in which the equiaxed α phase is dominant, the acicular structure in which the acicular α phase is dominant, or the mixed structure including both the equiaxed α phase and the acicular α phase can be obtained.

[0127] The α+β type alloy thus obtained has both high tensile strength and excellent cold-workability. It is considered that high tensile strength is obtained due to solid solution-strengthening by the added element. In addition, it is considered that the reason why excellent cold-workability is obtained is that the proportion of the β phase having good cold-workability is large because T β is optimized.EXAMPLES(Examples 1 to 8 and Comparative Examples 1 to 9)[1. Preparation of Samples]

[0128] Raw materials blended so as to have a predetermined composition were melted by the CCIM method to obtain an ingot of 115 mm-diameter × 100 mm-height. This ingot was heated to a temperature of T β or higher and hot-forged (pre-forged) into a 40 mm cube. Further, the pre-forged material was heated in the α+β two-phase temperature range and hot-forged (finish forged) to a diameter of 20 mm. After forging, a heat treatment was performed in a range of T β - 200°C or higher and lower than T β - 50°C.[2. Test Method][2.1. Tensile Test]

[0129] A tensile test was performed at 25°C in accordance with JIS Z2241 (2011) to measure tensile strength and elongation at rupture. A No. 14A test piece having a parallel portion diameter of 6 mm was used as the test piece.[2.2. Compression Test]

[0130] An end face restraint compression test was performed at 25°C to measure limit of compressibility. A cylindrical test piece of 15 mm-diameter × 22.5 mm-height was used as the test piece. In addition, in order to ensure accuracy, the end face restraint compression test was performed three times under one condition. The maximum compressibility at which no crack occurred in all three times was defined as the limit of compressibility.[3. Results]

[0131] The results are shown in Table 1. Table 1 also shows the components of each sample. FIG. 2 shows a relationship between the β transformation temperature T β and the limit of compressibility at 25°C. The followings can be found from Table 1 and FIG. 2. (1) The tensile strength was low in Comparative Example 1. The reason is considered to be that Cr was not contained. Further, the elongation was low in Comparative Example 1. The reason is considered to be that the content of Al was excessive and Cr was not contained. Further, the limit of compressibility was low in Comparative Example 1. The reason is considered to be that T β exceeded 880°C. (2) In Comparative Example 2, the tensile strength and elongation were low. The reason is considered to be that V and Cr were not contained. Further, the limit of compressibility was low in Comparative Example 2. The reason is considered to be that T β exceeded 880°C. (3) The limit of compressibility was low in Comparative Example 3. The reason is considered to be that T β exceeded 880°C. (4) The elongation was low in Comparative Example 4. The reason is considered to be that the content of Fe was slightly large (exceeded 1.5 mass%) and the content of Cr was small. Further, the limit of compressibility was low in Comparative Example 4. The reason is considered to be that T β exceeded 880°C. (5) The tensile strength was low in Comparative Example 5. The reason is considered to be that V was not contained. Further, the limit of compressibility was low in Comparative Example 5. The reason is considered to be that T β exceeded 880°C. (6) The elongation was low in Comparative Example 6. The reason is considered to be that the content of V was small. Further, the limit of compressibility was low in Comparative Example 6. The reason is considered to be that T β exceeded 880°C. (7) The elongation was low in Comparative Example 7. The reason is considered to be that the content of Fe was slightly large (exceeded 1.5 mass%) and the content of Cr was small. Further, the limit of compressibility was low in Comparative Example 7. The reason is considered to be that T β exceeded 880°C. (8) The limit of compressibility was low in Comparative Example 8. The reason is considered to be that T β exceeded 880°C. (9) The tensile strength and elongation were low in Comparative Example 9. The reason is considered to be that the content of V was small. Further, the limit of compressibility was low in Comparative Example 9. The reason is considered to be that T β exceeded 880°C. (10) In Examples 1 to 8, the tensile strength was 950 MPa or more, the elongation was 15% or more, and the limit of compressibility was 35% or more. (11) In Examples 1 and 6, the elongation was 20% or more, and the limit of compressibility was 50% or more. The reason is considered to be that T β was 840°C or lower. (12) A correlation was observed between T β and the limit of compressibility. Refer to FIG. 2. It was found that it is effective to set T β to 880°C or lower in order to control the limit of compressibility to 35% or more. Table 1 Composition (mass%)TiAlVCrFeONHCSiOthersEx. 1Bal.4.13.23.00.900.190.010.010.010.03Ex. 2Bal.4.53.13.00.970.210.01< 0.010.010.03Ex. 3Bal.4.53.13.00.070.190.010.010.010.03Ex. 4Bal.4.02.93.02.100.140.010.010.010.03Ex. 5Bal.3.92.96.00.110.110.010.010.010.02Ex. 6Bal.4.13.23.00.900.190.010.010.010.030.03Nb0.07Mo0.03TaEx. 7Bal.4.13.23.00.900.190.010.010.010.030.02Sn0.04Zr0.05Mn0.07NiEx. 8Bal.4.13.23.00.900.190.010.010.010.030.01BComp. Ex. 1Bal.6.44.30.00.190.150.010.010.010.06-Comp. Ex. 2Bal.5.00.00.02.500.140.010.010.010.05Comp. Ex. 3Bal.4.53.11.00.990.240.010.010.010.03Comp. Ex. 4Bal.4.82.91.01.800.100.010.01<0.010.03-Comp. Ex. 5Bal.5.00.01.11.900.100.010.010.010.05-Comp. Ex. 6Bal.4.92.01.51.800.100.010.01<0.010.04-Comp. Ex. 7Bal.4.93.50.71.800.100.010.01<0.010.05-Comp. Ex. 8Bal.6.064.141.550.250.130.010.010.010.06-Comp. Ex. 9Bal.3.982.232.020.190.060.010.010.010.05- Table 1 (Continued) T β (°C)StructureTensile strengthElongation at ruptureLimit of compressibilityEx. 1832α+βAAAAAAEx. 2847α+βAAAAEx. 3866α+βAAAEx. 4878α+βAAAEx. 5861α+βAAAEx. 6832α+βAAAAAAEx. 7832α+βAAAEx. 8832α+βAAAAComp. Ex. 1995α+βBBBComp. Ex. 2950α+βBBBComp. Ex. 3889α+βAABComp. Ex. 4924α+βABBComp. Ex. 5970α+βBABComp. Ex. 6928α+βABBComp. Ex. 7920α+βABBComp. Ex. 8940α+βAABComp. Ex. 9890α+βBBB Tensile strength at 25°C was evaluated as: A: 950 MPa or more, and B: less than 950 MPa. Elongation at rupture at 25°C was evaluated as: AA: 20% or more, A: 15% or more, and B: less than 15%. Maximum limit of compressibility at 25°C was evaluated as: AAA: 50% or more, AA: 45% or more, A: 35% or more, and B: less than 35%.

[0132] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

[0133] This application is based on Japanese Patent Application No. 2025-034193 filed on March 5, 2025, the contents of which are incorporated herein by reference.INDUSTRIAL APPLICABILITY

[0134] The α+β titanium alloy according to the present invention can be used for various structural parts, corrosion resistant parts, and the like used in a golf club head, a chemical industrial device, electrical equipment, space equipment, an aircraft, a ship, a vehicle, a medical device, a condenser, a heat exchanger, a seawater desalination device, and the like.

Examples

examples 1 to 8

(Examples 1 to 8 and Comparative Examples 1 to 9)

[1. Preparation of Samples]

[0128]Raw materials blended so as to have a predetermined composition were melted by the CCIM method to obtain an ingot of 115 mm-diameter × 100 mm-height. This ingot was heated to a temperature of T β or higher and hot-forged (pre-forged) into a 40 mm cube. Further, the pre-forged material was heated in the α+β two-phase temperature range and hot-forged (finish forged) to a diameter of 20 mm. After forging, a heat treatment was performed in a range of T β - 200°C or higher and lower than T β - 50°C.

[2. Test Method]

[2.1. Tensile Test]

[0129]A tensile test was performed at 25°C in accordance with JIS Z2241 (2011) to measure tensile strength and elongation at rupture. A No. 14A test piece having a parallel portion diameter of 6 mm was used as the test piece.

[2.2. Compression Test]

[0130]An end face restraint compression test was performed at 25°C to measure limit of compressibility. A cylindrical test piece o...

TECHNICAL FIELD

[0001] The present invention relates to an α+β type titanium alloy and a method for producing a semi-finished material composed of the α+β type titanium alloy, and more particularly to an α+β type titanium alloy having both high tensile strength and excellent cold-workability and a method for producing a semi-finished material composed of the α+β type titanium alloy.BACKGROUND ART

[0002] The practical titanium alloys are broadly classified into (1) an α-type alloy composed of an α phase (low-temperature phase) that is a close-packed hexagonal crystal, (2) a β-type alloy composed of a β phase (high-temperature phase) that is a body-centered cubic crystal, and (3) an α+β type alloy having a two-phase structure of the α phase and the β phase.

[0003] Among these, the α+β type alloy is a well-balanced material excellent in strength, specific strength, workability, corrosion resistance, and the like, and has been used as a space aircraft material, an automobile material, a material for mechanical structural parts, a general demand material, a material for medical instruments, or the like. In particular, among the α+β type alloys, the Ti-6Al-4V alloy has been widely used as a general-purpose high-power titanium alloy, and occupies about 80% of the Ti alloy usage amount.

[0004] Various proposals have been made regarding such titanium alloys.

[0005] For example, Patent Literature 1 discloses a titanium alloy for a golf club face, which contains predetermined amounts of Al, Fe, N, and O, with the balance being Ti and unavoidable impurities.

[0006] Patent Literature 1 states that both high strength and high Young's modulus can be achieved by optimizing the addition amounts of Al, O, and N as solid solution-strengthening elements of the α phase and Fe as a β-stabilizing element.

[0007] Patent Literature 2 discloses a β-type titanium alloy containing predetermined amounts of Al, Fe, and Cr, and further containing predetermined amounts of V and / or Mo, with the balance being Ti and unavoidable impurities.

[0008] Patent Literature 2 states that, in a titanium alloy further containing V or Mo in addition to Al, Fe, and Cr, in the case where the total content of V and Mo is 10 mass% or less, the influence of component segregation of Fe and Cr is alleviated.

[0009] Patent Literature 3 discloses a titanium alloy containing predetermined amounts of Al and Fe, with the balance being Ti and unavoidable impurities.

[0010] Patent Literature 3 states that the titanium alloy having such a composition has a Young's modulus of 120 GPa to 160 GPa and a tensile strength of 950 MPa to 2200 MPa.

[0011] In the titanium alloy described in Patent Literature 1, predetermined amounts of O and N are added. Therefore, in the case where the O amount and / or the N amount is high, ductility of the titanium alloy may decrease. Further, Patent Literature 1 does not mention the cold-workability of the titanium alloy.

[0012] In the β-type titanium alloy described in Patent Literature 2, component segregation is likely to occur because the amount of eutectoid-type added elements is large among β-stabilizing elements. In addition, in general, the β-type titanium alloy is inferior in high-temperature strength to the α+β type titanium alloy, and in particular, the titanium alloy containing Mo has a high density to deteriorate lightness in weight.

[0013] Further, Patent Literature 3 does not mention the cold-workability of the titanium alloy. Patent Literature 1: W O2013 / 125039 Patent Literature 2: JP 2008-133531A Patent Literature 3: JP 2006-212092A SUMMARY OF INVENTION

[0014] An object of the present invention is to provide an α+β type titanium alloy having both high tensile strength and excellent cold-workability, and a method for producing the α+β type titanium alloy.

[0015] In order to solve the above-mentioned problem, an α+β type titanium alloy according to the present invention includes: 3.0 mass % ≤ Al ≤ 5.4 mass % ; 2.5 mass % ≤ V ≤ 6.0 mass % ; 1.5 mass % ≤ Cr ≤ 7.0 mass % ; 0.05 mass % ≤ Fe ≤ 3.0 mass % , and optionally at least one of: Sn ≤ 0.5 mass % ; Zr ≤ 0.5 mass % ; Mo ≤ 0.5 mass % ; Nb ≤ 0.5 mass % ; Ta ≤ 0.5 mass % ; Ni ≤ 0.15 mass % ; Mn ≤ 0.25 mass % ; B ≤ 0.2 mass % ; Si ≤ 0.3 mass % ; O ≤ 0.4 mass % ; N ≤ 0.1 mass % ; H ≤ 0.1 mass % ; and C ≤ 0.2 mass % , with the balance being Ti and unavoidable impurities, and has a β transformation temperature T β of 880°C or lower.

[0016] A method for producing a semi-finished material composed of an α+β type titanium alloy according to the present invention, includes: a preparation step of preparing a raw material composed of the α+β type titanium alloy of the present invention; a hot-working step of performing a hot-working on the material in a temperature range of 600°C or higher and lower than the β transformation temperature T β ; and a heat treatment step of performing a heat treatment on the material after the hot-working at a temperature of T β - 200°C or higher and 1200°C or lower.

[0017] When the titanium alloy in which the component is adjusted so that T β is 880°C or lower is subjected to a hot-working in a two-phase temperature range, the acicular α phase is fractured, and a fine equiaxed α phase is formed. Subsequently, when the obtained material is heat-treated under appropriate conditions, an α+β type alloy having an equiaxed structure in which the equiaxed α phase is dominant, an acicular structure in which the acicular α phase is dominant, or a mixed structure including both the equiaxed α phase and the acicular α phase can be obtained.

[0018] The α+β type alloy thus obtained has both high tensile strength and excellent cold-workability. It is considered that high tensile strength is obtained due to solid solution-strengthening by the added element. In addition, it is considered that the reason why excellent cold-workability is obtained is that the proportion of the β phase having good cold-workability is large because T β is optimized.BRIEF DESCRIPTION OF DRAWINGS

[0019] FIG. 1 includes views each showing an example of a microstructure of an α+β type titanium alloy according to the present invention ((a): acicular structure, (b): equiaxed structure, and (c): mixed structure). FIG. 2 is a diagram showing a relationship between a β transformation temperature T β and a limit of compressibility at 25°C. DESCRIPTION OF EMBODIMENTS

[0020] Hereinafter, an embodiment of the present invention will be described in detail.[1. α+β Type Titanium Alloy][1.1. Main Constituent Elements]

[0021] An α+β type titanium alloy (hereinafter, also simply referred to as "titanium alloy") according to the present invention contains the following elements, with the balance being Ti and unavoidable impurities. Types of added elements, component ranges thereof, and reasons for limitation thereof are as follows. (1) 3.0 mass % ≤ Al ≤ 5.4 mass % :

[0022] Al is an element that contributes to stabilization of the α phase and solid solution-strengthening of the α phase. In order to obtain such an effect, the content of Al needs to be 3.0 mass% or more.

[0023] On the other hand, in the case where the content of Al is excessive, Ti 3 Al, which is an embrittlement phase, precipitates, and ductility of the titanium alloy may decrease. In addition, in the case where the content of Al is excessive, solid solution-strengthening excessively proceeds, and cold-workability of the titanium alloy may decrease. Therefore, the content of Al needs to be 5.4 mass% or less. The content of Al is preferably 4.3 mass% or less. (2) 2.5 mass % ≤ V ≤ 6.0 mass % :

[0024] V is an element that contributes to stabilization of the β phase, solid solution-strengthening of the β phase, and improvement in cold-workability by formation of the β phase. In order to obtain such an effect, the content of V needs to be 2.5 mass% or more.

[0025] On the other hand, since V is an expensive element, the raw material cost increases in the case where the content of V is excessive. Therefore, the content of V needs to be 6.0 mass% or less. The content of V is preferably 4.5 mass% or less. (3) 1.5 mass % ≤ Cr ≤ 7.0 mass % :

[0026] Cr is an element that contributes to stabilization of the β phase, solid solution-strengthening of the β phase, and improvement in cold-workability by formation of the β phase. In order to obtain such effects, the content of Cr needs to be 1.5 mass% or more.

[0027] On the other hand, since Cr is a very strong β-phase stabilizing element, the two-phase structure may not be obtained in the case where the content of Cr is excessive. In addition, in the case where the content of Cr is excessive, segregation occurs during solidification, and manufacturability may deteriorate. Therefore, the content of Cr needs to be 7.0 mass% or less.

[0028] The content of Zr is more preferably 4.0 mass% or less. (4) 0.05 mass % ≤ Fe ≤ 3.0 mass % :

[0029] Fe is an element that contributes to stabilization of the β phase and solid solution-strengthening of the β phase. Fe as an alloy element and Ti containing Fe as an impurity contribute to reduction in raw material cost because they are inexpensive. In order to obtain such an effect, the content of Fe needs to be 0.05 mass% or more. The content of Fe is preferably 0.25 mass% or more.

[0030] On the other hand, since Fe is a very strong β-phase stabilizing element, the two-phase structure may not be obtained in the case where the content of Fe is excessive. In addition, in the case where the content of Fe is excessive, segregation may occur during solidification, and manufacturability may deteriorate. Therefore, the content of Fe needs to be 3.0 mass% or less. The content of Fe is more preferably 1.3 mass% or less.[1.2. Sub Constituent Element]

[0031] The α+β type titanium alloy according to the present invention may further contain one or two or more of the following elements in addition to the above-described main constituent elements. Types of added elements, component ranges thereof, and reasons for limitation thereof are as follows. (1) Sn ≤ 0.5 mass % :

[0032] Sn is an element that contributes to solid solution-strengthening of the α phase and the β phase, and can be added as necessary.

[0033] However, in the case where the content of Sn is excessive, undissolved Ti 3 Al remains even after the solid solution heat treatment, and ductility of the titanium alloy may decrease. Therefore, the content of Sn is preferably 0.5 mass% or less. The content of Sn is more preferably 0.1 mass% or less. (2) Zr ≤ 0.5 mass % :

[0034] Zr is an element that contributes to solid solution-strengthening of the α phase and the β phase, and can be added as necessary.

[0035] However, in the case where the content of Zr is excessive, undissolved Ti 3 Al remains even after the solid solution heat treatment, and ductility of the titanium alloy may decrease. Therefore, the content of Zr is preferably 0.5 mass% or less. The content of Zr is more preferably 0.1 mass% or less. (3) Mo ≤ 0.5 mass % :

[0036] Mo is an element that contributes to solid solution-strengthening of the β phase, and can be added as necessary.

[0037] However, since Mo is a very strong β-phase stabilizing element, the two-phase structure may not be obtained in the case where the content of Mo is excessive. In addition, in the case where the content of Mo is excessive, segregation may occur during solidification. Further, since Mo is expensive, the raw material cost increases in the case where the content of Mo is excessive. Therefore, the content of Mo is preferably 0.5 mass% or less. The content of Mo is more preferably 0.1 mass% or less. (4) Nb ≤ 0.5 mass % :

[0038] Nb is an element that contributes to an improvement in oxidation resistance of the titanium alloy, and can be added as necessary.

[0039] However, in the case where the content of Nb is excessive, the specific gravity of the titanium alloy may increase, and the specific strength may decrease. Therefore, the content of Nb is preferably 0.5 mass% or less. The content of Nb is more preferably 0.1 mass% or less. (5) Ta ≤ 0.5 mass % :

[0040] Ta is an element that contributes to solid solution-strengthening of the β phase, and can be added as necessary.

[0041] However, in the case where the content of Ta is excessive, the specific gravity of the titanium alloy may increase, and the specific strength may decrease. Therefore, the content of Ta is preferably 0.5 mass% or less. The content of Ta is more preferably 0.1 mass% or less. (6) Ni ≤ 0.15 mass % :

[0042] Ni is an element that contributes to solid solution-strengthening of the β phase, and can be added as necessary.

[0043] However, since Ni is a very strong β-phase stabilizing element, the two-phase structure may not be obtained in the case where the content of Ni is excessive. In addition, in the case where the content of Ni is excessive, segregation may occur during solidification. Therefore, the content of Ni is preferably 0.15 mass% or less. (7) Mn ≤ 0.25 mass % :

[0044] Mn is an element that contributes to solid solution-strengthening of the β phase, and can be added as necessary.

[0045] However, since Mn is a very strong β-phase stabilizing element, the two-phase structure may not be obtained in the case where the content of Mn is excessive. In addition, in the case where the content of Mn is excessive, segregation may occur during solidification. Therefore, the content of Mn is preferably 0.25 mass% or less. (8) B ≤ 0.2 mass % :

[0046] B is an element that forms a boride with Ti and contributes to refinement of crystal grains, and can be added as necessary.

[0047] However, in the case where the content of B is excessive, the boride becomes coarse, and the boride becomes a starting point of fracture, so that hot-workability and cold-workability may be significantly impaired. Therefore, the content of B is preferably 0.2 mass% or less. The content of B is more preferably 0.05 mass% or less. (9) Sn + Zr + Mo + Nb + Ta + Ni + Mn + B ≤ 2.0 mass % :

[0048] Even when the content of each of Sn, Zr, Mo, Nb, Ta, Ni, Mn, and B is within the above-described range, ductility of the titanium alloy may decrease, fatigue strength may decrease, specific strength may decrease, or segregation may occur during solidification, in the case where the total content of these elements is excessive. Therefore, the total content of these elements is preferably 2.0 mass% or less.[1.3. Unavoidable Impurities]

[0049] The "unavoidable impurities" are components mixed due to various factors such as a raw material and a production process when the α+β type titanium alloy according to the present invention is industrially produced, and the content thereof is within a range that does not adversely affect the properties of the α+β type titanium alloy according to the present invention. Specific examples of the unavoidable impurities include the followings. (1) Si ≤ 0.3 mass % :

[0050] In the case where the content of Si is excessive, ductility, cold-workability, or hot-workability of the titanium alloy may deteriorate. Therefore, the content of Si is preferably 0.3 mass% or less.

[0051] The smaller the content of Si, the better. However, an extreme reduction in the content of Si may cause an increase in manufacturing cost. Considering the manufacturing cost, the content of Si is preferably 0.0005 mass% or more. (2) O ≤ 0.4 mass % :

[0052] O is an element that contributes to solid solution-strengthening of the α phase. However, in the case where the content of O is excessive, ductility of the titanium alloy may deteriorate. Therefore, the content of O is preferably 0.4 mass% or less.

[0053] The smaller the content of O, the better. However, an extreme reduction in the content of O may cause an increase in manufacturing cost. Considering the manufacturing cost, the content of O is preferably 0.005 mass% or more. (3) N ≤ 0.1 mass % :

[0054] N is an element that contributes to solid solution-strengthening of the α phase. However, in the case where the content of N is excessive, ductility of the titanium alloy may deteriorate. Therefore, the content of N is preferably 0.1 mass% or less.

[0055] The smaller the content of N, the better. However, an extreme reduction in the content of N may cause an increase in manufacturing cost. Considering the manufacturing cost, the content of N is preferably 0.0005 mass% or more. (4) H ≤ 0.1 mass % :

[0056] In the case where the content of H is excessive, strength and ductility of the titanium alloy may deteriorate. Therefore, the content of H is preferably 0.1 mass% or less.

[0057] The smaller the content of H, the better. However, an extreme reduction in the content of H may cause an increase in manufacturing cost. Considering the manufacturing cost, the content of H is preferably 0.0005 mass% or more. (5) C ≤ 0.2 mass % :

[0058] In the case where the content of C is excessive, ductility, cold-workability, or hot-workability of the titanium alloy may deteriorate. Therefore, the content of C is preferably 0.2 mass% or less.

[0059] The smaller the content of C, the better. However, an extreme reduction in the content of C may cause an increase in manufacturing cost. Considering the manufacturing cost, the content of C is preferably 0.0005 mass% or more. (6) Total Content of Impurities ≤ 1.0 mass % :

[0060] Examples of the unavoidable impurities other than the above-described elements include P, S, Pd, Hf, and REM. The content of each of these elements is preferably 0.05 mass% or less.

[0061] In addition, even when the content of each of the unavoidable impurities is equal to or less than the above-described upper limit, ductility, cold-workability, or hot-workability of the titanium alloy may deteriorate in the case where the total content of the unavoidable impurities is excessive. Therefore, the total content of the unavoidable impurities is preferably 1.0 mass% or less. The total content is more preferably 0.5 mass% or less.[1.4. β Transformation Temperature]

[0062] The "β transformation temperature T β " refers to a lower limit temperature at which the titanium alloy becomes a β single phase, and can be measured by differential thermal analysis (DTA).

[0063] It was found that T β of the titanium alloy according to the present invention correlates with cold-workability (limit of compressibility at 25°C). As T β decreases, the cold-workability increases. It is considered that this is because the proportion of the β phase having excellent cold-workability increases as T β decreases. In order to obtain such an effect, T β needs to be 880°C or lower. T β is preferably 870°C or lower, more preferably 860°C or lower, and further preferably 850°C or lower.[1.5. Structure]

[0064] FIG. 1 shows examples of a microstructure of the α+β type titanium alloy according to the present invention. The titanium alloy according to the present invention has a two-phase structure of α+β at room temperature. In addition, the titanium alloy according to the present invention exhibits an acicular structure ((a) of FIG. 1), an equiaxed structure ((b) of FIG. 1), or a mixed structure ((c) of FIG. 1) depending on processing and heat treatment conditions.

[0065] The "acicular structure" refers to a structure in which an acicular α phase is precipitated in a matrix composed of the β phase (β phase + acicular α phase). The acicular structure can be obtained by heat-treating the titanium alloy according to the present invention at a temperature equal to or higher than T β , cooling the heat-treated titanium alloy, and precipitating the acicular α phase in association with martensitic transformation during the cooling. The titanium alloy having the acicular structure is characterized by having slightly low low-temperature strength, low ductility, and low fatigue properties, but having extremely high high-temperature strength, extremely high fracture toughness, and extremely high creep properties.

[0066] The "equiaxed structure" refers to a structure in which the equiaxed α phase is mainly present, and the β phase is present in a gap between the relatively large equiaxed α phases. The equiaxed structure can be obtained by heat-treating the titanium alloy according to the present invention at a temperature lower than T β , then heat-treating the titanium alloy at a temperature lower than the martensitic transformation-starting point (Ms point) to divide or recrystallize the acicular structure, to form the equiaxed α phase. A sufficient strain is introduced into the acicular α phase to divide or recrystallize the acicular α phase, to thereby form an equiaxed structure. The titanium alloy having the equiaxed structure is characterized by having slightly low fracture toughness, low high-temperature strength, and low creep properties, but having high ductility, extremely high low-temperature strength, and extremely high fatigue properties.

[0067] The "mixed structure" refers to a structure in which the equiaxed α phase and the β phase in which the acicular α phase is precipitated (β phase + acicular α phase) are mixed. The mixed structure can be obtained by hot-working the titanium alloy according to the present invention at a temperature lower than T β , and then performing a heat treatment in an α+β two-phase temperature range equal to or higher than the Ms point to make the α phase equiaxed, and additionally precipitating the acicular α phase along with martensitic transformation of the β phase. Finally, a structure in which the equiaxed α phase and the β phase + acicular α phase regions are mixed is obtained. The titanium alloy having the mixed structure has slightly low ductility, but has high low-temperature strength, high high-temperature strength, high fracture toughness, high creep properties, and high fatigue properties, and is a well-balanced material.

[0068] [1.6. Properties][1.6.1. Tensile Strength]

[0069] The "tensile strength" refers to a value obtained by performing a tensile test by using a No. 14A test piece having a parallel portion diameter of 6 mm in accordance with JIS Z2241 (2011).

[0070] In the titanium alloy according to the present invention, the tensile strength at 25°C can be 950 MPa or more in the case where components and / or structures are optimized.[1.6.2. Elongation]

[0071] The "elongation" refers to an elongation at rupture obtained by performing a tensile test by using a No. 14A test piece having a parallel portion diameter of 6 mm in accordance with JIS Z2241 (2011).

[0072] In the titanium alloy according to the present invention, the elongation at 25°C can be 15% or more in the case where components and / or structures are optimized. In the case where the component and / or structure are further optimized, the elongation at 25°C can be 20% or more.[1.6.3. Limit of Compressibility]

[0073] "Compressibility (%)" refers to a value represented by the following formula. Compressibility = H 1 − H 2 × 100 / H 1

[0074] Here, H 1 represents the height of the test piece before the compression test, and H 2 represents the height of the test piece after the compression test.

[0075] The "limit of compressibility" refers to the maximum compressibility at which no crack is observed in the test piece after an end face restraint compression test.

[0076] The "end face restraint compression test" refers to a test in which pressure-resistant plates are disposed above and below a test piece, and the test piece is compressed while the spread of upper and lower end faces of the test piece is restrained. The end face restraint compression test can eliminate the influence of sliding (friction) between the pressure-resistant plate and the test piece during pressurization, and therefore, the end face restraint compression test is excellent as a method for evaluating deformability and is suitable as a method for evaluating cold-workability.

[0077] The test piece had a shape of 15 mm-diameter × 22.5 mm-height, and the compression speed was 35 mm / s. Further, the test was performed at least three times under one condition, and the maximum compressibility at which no crack was observed in the compressed test piece was defined as the limit of compressibility.

[0078] In the titanium alloy according to the present invention, the limit of compressibility at 25°C can be 35% or more in the case where the components and / or structures are optimized. In the case where the components and / or structures are further optimized, the limit of compressibility at 25°C can be 40% or more, 45% or more, or 50% or more.[1.7. Application]

[0079] The present invention relates to a material composed of an α+β type titanium alloy that can be used as a raw material for manufacturing a semi-finished material, the semi-finished material manufactured from the material of the present invention, and various titanium products made from the semi-finished material.

[0080] The α+β type titanium alloy according to the present invention can be applied to various applications.

[0081] Examples of titanium products to which the α+β type titanium alloy according to the present invention can be applied include (1) various sports and leisure goods such as golf club heads and bicycle gears, (2) consumer products such as eyeglass frames, and (3) various parts requiring light weight and high strength, such as various bolts, valve retainers, and seawater-resistant shafts.[2. Method for Producing α+β type Titanium Alloy]

[0082] A method for producing the α+β type titanium alloy according to the present invention includes: a preparation step of preparing a material composed of the α+β type titanium alloy according to the present invention; a hot-working step of performing a hot-working on the material in a temperature range of 600°C or higher and lower than the β transformation temperature T β ; and a heat treatment step of performing a heat treatment on the material after the hot-working at a temperature of T β - 200°C or higher and 1200°C or lower.

[0083] The method for producing the α+β type titanium alloy according to the present invention may further include an aging treatment step of subjecting the material after the heat treatment to an aging treatment at a temperature of 400°C or higher and 700°C or lower.[2.1. Preparation Step]

[0084] First, a material composed of the α+β type titanium alloy according to the present invention is prepared.

[0085] Specifically, the preparation of the raw material is preferably performed by (a) melting and casting raw materials blended so as to have a predetermined composition, to prepare an alloy ingot, (b) subjecting the molten and cast alloy ingot to a pre-hot-working as necessary, and (c) subjecting the material subjected to the pre-hot-working to a preheating treatment as necessary.[2.1.1. Melting and Casting]

[0086] First, the raw materials blended so as to have a predetermined composition are melted and cast to obtain an alloy ingot. A method for melting and casting the raw materials is not particularly limited, and an optimum method can be selected according to a purpose thereof.

[0087] Examples of the method for melting and casting the raw materials include a cold-crucible induction-melting (CCIM) method.[2.1.2. Pre-Hot-Working]

[0088] In the case where the alloy ingot produced according to the CCIM method or the like has a size equal to or smaller than the size suitable for the next step (hot-working step), the alloy ingot can be used as it is as a material for the next step.

[0089] On the other hand, in the case where the produced alloy ingot exceeds the size suitable for the next step, it is preferable to perform a pre-hot-working of the material.

[0090] The pre-hot-working is preferably started from the β single-phase temperature range. The cast structure of the alloy ingot can be divided by performing the pre-hot-working. Further, the deformation resistance is relatively small in the β single phase temperature range, and therefore, the division of the cast structure is facilitated. A pre-hot-working method is not particularly limited, and an optimal method can be selected according to the purpose. Examples of the pre-hot-working method include a hot-forging and a hot-rolling.

[0091] When the temperature of the material decreases to the α+β two-phase temperature range during the pre-hot-working, the pre-hot-working may be ended, or the pre-hot-working may be continued as it is. When the pre-hot-working is continued even after the temperature of the material decreases to the α+β two-phase temperature range, the structure may be further refined. In order to refine the alloy structure, the termination temperature of the pre-hot-working is preferably in a relatively high temperature range in the α+β two-phase temperature range. Specifically, the termination temperature of the pre-hot-working is preferably T β - 100°C or higher and lower than T β .[2.1.3. Preheating Treatment]

[0092] In the case where the pre-hot-working is performed, it is preferable to further perform a preheating treatment on the material subjected to the pre-hot-working. The preheating treatment is preferably performed in a temperature range in which the alloy structure can be homogenized, the coarsening of crystal grains can be prevented or inhibited, and the effect generated by the pre-hot-working can be maintained. The preheating treatment is preferably held in a relatively low temperature range in the β single phase temperature range. Specifically, the temperature of the preheating treatment is preferably T β or higher and T β + 80°C or lower.[2.2. Hot-Working Step]

[0093] After performing the pre-hot-working and preheating treatment of the material as necessary, the material is subjected to a hot-working in a temperature range of 600°C or higher and lower than the β transformation temperature T β .

[0094] The hot-working is performed to refine the alloy structure and adjust the form of the equiaxed α phase. Therefore, the hot-working needs to be performed in the α+β two-phase temperature range from the start to the end of the hot-working. In the case where the hot-working temperature is equal to or higher than T β , only the acicular structure is obtained even if the heat treatment is performed thereafter, and the structure control is difficult. Therefore, the hot-working temperature needs to be lower than T β .

[0095] On the other hand, in the case where the hot-working temperature is too low, the deformation resistance may excessively increase. Therefore, the hot-working temperature needs to be 600°C or higher. In order to reduce the deformation resistance, the hot-working temperature is preferably in a relatively high temperature range in the α+β two-phase temperature range.

[0096] In the maximum strain direction among principal strains in three directions generated during the hot-working, the length before the start of the hot-working is defined as "L 1 ", and the length after the end of the hot-working is defined as "L 2 ".

[0097] In this case, the "total strain ratio" refers to L 1 / L 2 in the case of L 1 > L 2 , and refers to L 2 / L 1 in the case of L 1 < L 2 .

[0098] In order to refine the alloy structure, the total strain ratio during the hot-working is preferably 3 or more. A higher total strain ratio is better.

[0099] A hot-working method is not particularly limited, and an optimal method can be selected according to the purpose. Examples of the hot-working method include a hot-forging and a hot-rolling.

[0100] After the end of the hot-working, the material may be once cooled to room temperature, or may be subjected to the next step without cooling the material.[2.3. Heat Treatment Step and Aging Treatment Step]

[0101] Next, the material after the hot-working is subjected to a heat treatment at a temperature of T β - 200°C or higher and 1200°C or lower (heat treatment step).

[0102] If necessary, the material after the heat treatment may be subjected to an aging treatment at a temperature of 400°C or higher and 700°C or lower (aging treatment step).

[0103] As the conditions of the heat treatment and the aging treatment, optimum conditions can be selected according to the purpose. When the conditions of the heat treatment and the conditions of the aging treatment are optimized, titanium alloys having different structures and / or properties can be obtained. Specific conditions for the heat treatment and the aging treatment are as follows.[2.3.1. Specific Example 1]

[0104] The heat treatment step may include a step of performing a heat treatment on the material after the hot-working at a temperature of T β - 200°C or higher and lower than T β - 50°C.

[0105] As described above, in general, the equiaxed structure can be obtained by performing the heat treatment at a temperature lower than the Ms point after hot-working. The Ms point of the α+β type titanium alloy according to the present invention is about T β - 50°C. Therefore, the heat treatment will be described by using T β .

[0106] In the case where the heat treatment temperature is T β - 50°C or higher, the acicular α phase may be precipitated, and the equiaxed structure throughout the entire surface may not be obtained. On the other hand, in the case where the heat treatment temperature is lower than T β - 200°C, it is difficult to homogenize the structure within a practical heat treatment time. In contrast, in the case where the material after hot-working is heat-treated at a temperature of T β - 200°C or higher and lower than T β - 50°C, an equiaxed structure in which the equiaxed α phase is dominant can be obtained.

[0107] A titanium alloy having the equiaxed structure is excellent in fatigue strength. In the case where the production conditions are optimized, a titanium alloy having the equiaxed structure as well as having a tensile strength at 25°C of 950 MPa or more, an elongation at 25°C of 15% or more, and a limit of compressibility at 25°C of 35% or more can be obtained.[2.3.2. Specific Example 2]

[0108] The heat treatment step may include a step of performing a heat treatment on the material after the hot-working at a temperature of T β or higher and 1200°C or lower.

[0109] Cooling after the end of the heat treatment may be air cooling or furnace cooling. In order to improve strength, toughness and ductility of the titanium alloy, it is preferable to perform cooling at a cooling rate higher than that of air cooling.

[0110] In the case where the heat treatment temperature is lower than T β , the equiaxed α phase may remain, and an acicular structure throughout the entire surface may not be obtained. On the other hand, even if the heat treatment temperature is increased more than necessary, there is no difference in effect, and there is no practical benefit. In contrast, in the case where the material after hot-working is heat-treated at a temperature of T β or higher and 1200°C or lower, an acicular structure in which the acicular α phase is dominant can be obtained.

[0111] A titanium alloy having the acicular structure is excellent in high-temperature strength and creep properties. In addition, in the case where the production conditions are optimized, a titanium alloy having the acicular structure as well as having a tensile strength at 25°C of 950 MPa or more, an elongation at 25°C of 15% or more, and / or a limit of compressibility at 25°C of 35% or more can be obtained.[2.3.3. Specific Example 3]

[0112] In the case where the heat treatment step includes a step of performing a heat treatment on the material after the hot-working at a temperature of T β or higher and 1200°C or lower, the method may further include an aging treatment step of subjecting the material after the heat treatment to an aging treatment at a temperature of 400°C or higher and 700°C or lower after the heat treatment step.

[0113] The aging treatment may be continuously performed after the end of the heat treatment without cooling the material to room temperature, or may be performed after cooling the material to room temperature.

[0114] In the case where the material is cooled to room temperature after the heat treatment, the cooling after the end of the heat treatment may be air cooling or furnace cooling. In order to improve strength, toughness and ductility of the titanium alloy, it is preferable to perform cooling at a cooling rate higher than that of air cooling.

[0115] In the case where the aging treatment temperature exceeds 700°C, coarsening of precipitates and precipitation of a ω phase, which is an embrittlement phase, may occur. On the other hand, in the case where the aging treatment temperature is lower than 400°C, it is difficult to precipitate precipitates by aging within a practical time, and the ω phase may precipitate. In contrast, in the case where the heat treatment is performed at a temperature of T β or higher and then the aging treatment is further performed at a temperature of 400°C or higher and 700°C or lower, not only the acicular structure in which the acicular α phase is dominant can be obtained, but also fine precipitates that improve strength properties can be precipitated by aging.

[0116] The titanium alloy containing the acicular structure and the aged precipitates is excellent not only in high-temperature strength and creep properties but also in strength properties. In addition, in the case where the production conditions are optimized, a titanium alloy having the acicular structure as well as having a tensile strength at 25°C of 950 MPa or more, an elongation at 25°C of 15% or more, and / or a limit of compressibility at 25°C of 35% or more can be obtained.[2.3.4. Specific Example 4]

[0117] The heat treatment step may include a step of performing a heat treatment on the material after the hot-working at a temperature of T β - 50°C or more and less than T β .

[0118] Cooling after the end of the heat treatment may be air cooling or furnace cooling. In order to improve strength, toughness and ductility of the titanium alloy, it is preferable to perform cooling at a cooling rate higher than that of air cooling.

[0119] In the case where the heat treatment temperature is lower than T β - 50°C and in the case where the heat treatment temperature is higher than T β , a mixed structure is not obtained. In contrast, in the case where the material after hot-working is heat-treated at a temperature of T β - 50°C or higher and lower than T β , a mixed structure containing both the equiaxed α phase and the acicular α phase can be obtained.

[0120] The titanium alloy having the mixed structure can achieve both excellent high-temperature properties and high fatigue strength. In addition, in the case where the production conditions are optimized, a titanium alloy having the mixed structure as well as having a tensile strength at 25°C of 950 MPa or more, an elongation at 25°C of 15% or more, and / or a limit of compressibility at 25°C of 35% or more can be obtained.[2.3.5. Specific Example 5]

[0121] In the case where the heat treatment step includes a step of performing a heat treatment on the material after the hot-working at a temperature of T β - 50°C or higher and lower than T β , the method may further include an aging treatment step of subjecting the material after the heat treatment to an aging treatment at a temperature of 400°C or higher and 700°C or lower after the heat treatment step.

[0122] The aging treatment may be continuously performed after the end of the heat treatment without cooling the material to room temperature, or may be performed after cooling the material to room temperature.

[0123] In the case where the material is cooled to room temperature after the heat treatment, the cooling after the end of the heat treatment may be air cooling or furnace cooling. In order to improve strength, toughness and ductility of the titanium alloy, it is preferable to perform cooling at a cooling rate higher than that of air cooling.

[0124] In the case where the aging treatment temperature exceeds 700°C, coarsening of precipitates and precipitation of the ω phase, which is an embrittlement phase, may occur. On the other hand, in the case where the aging treatment temperature is lower than 400°C, it is difficult to precipitate precipitates by aging within a practical time, and the ω phase may precipitate. In contrast, in the case where the heat treatment is performed at a temperature of T β - 50°C or higher and lower than T β and then the aging treatment is further performed at a temperature of 400°C or higher and 700°C or lower, not only the mixed structure including both the equiaxed α phase and the acicular α phase can be obtained, but also fine precipitates that improve strength properties can be precipitated by aging.

[0125] The titanium alloy containing the mixed structure and the aged precipitates can achieve not only excellent high-temperature properties and high fatigue strength but also excellent strength properties. In addition, in the case where the production conditions are optimized, a titanium alloy having the mixed structure as well as having a tensile strength at 25°C of 950 MPa or more, an elongation at 25°C of 15% or more, and / or a limit of compressibility at 25°C of 35% or more can be obtained.[3. Effects]

[0126] When the titanium alloy in which the component is adjusted so that T β is 880°C or lower is subjected to a hot-working in a two-phase temperature range, the acicular α phase is fractured, and a fine equiaxed α phase is formed. Subsequently, when the obtained material is heat-treated under appropriate conditions, the α+β type alloy having the equiaxed structure in which the equiaxed α phase is dominant, the acicular structure in which the acicular α phase is dominant, or the mixed structure including both the equiaxed α phase and the acicular α phase can be obtained.

[0127] The α+β type alloy thus obtained has both high tensile strength and excellent cold-workability. It is considered that high tensile strength is obtained due to solid solution-strengthening by the added element. In addition, it is considered that the reason why excellent cold-workability is obtained is that the proportion of the β phase having good cold-workability is large because T β is optimized.EXAMPLES(Examples 1 to 8 and Comparative Examples 1 to 9)[1. Preparation of Samples]

[0128] Raw materials blended so as to have a predetermined composition were melted by the CCIM method to obtain an ingot of 115 mm-diameter × 100 mm-height. This ingot was heated to a temperature of T β or higher and hot-forged (pre-forged) into a 40 mm cube. Further, the pre-forged material was heated in the α+β two-phase temperature range and hot-forged (finish forged) to a diameter of 20 mm. After forging, a heat treatment was performed in a range of T β - 200°C or higher and lower than T β - 50°C.[2. Test Method][2.1. Tensile Test]

[0129] A tensile test was performed at 25°C in accordance with JIS Z2241 (2011) to measure tensile strength and elongation at rupture. A No. 14A test piece having a parallel portion diameter of 6 mm was used as the test piece.[2.2. Compression Test]

[0130] An end face restraint compression test was performed at 25°C to measure limit of compressibility. A cylindrical test piece of 15 mm-diameter × 22.5 mm-height was used as the test piece. In addition, in order to ensure accuracy, the end face restraint compression test was performed three times under one condition. The maximum compressibility at which no crack occurred in all three times was defined as the limit of compressibility.[3. Results]

[0131] The results are shown in Table 1. Table 1 also shows the components of each sample. FIG. 2 shows a relationship between the β transformation temperature T β and the limit of compressibility at 25°C. The followings can be found from Table 1 and FIG. 2. (1) The tensile strength was low in Comparative Example 1. The reason is considered to be that Cr was not contained. Further, the elongation was low in Comparative Example 1. The reason is considered to be that the content of Al was excessive and Cr was not contained. Further, the limit of compressibility was low in Comparative Example 1. The reason is considered to be that T β exceeded 880°C. (2) In Comparative Example 2, the tensile strength and elongation were low. The reason is considered to be that V and Cr were not contained. Further, the limit of compressibility was low in Comparative Example 2. The reason is considered to be that T β exceeded 880°C. (3) The limit of compressibility was low in Comparative Example 3. The reason is considered to be that T β exceeded 880°C. (4) The elongation was low in Comparative Example 4. The reason is considered to be that the content of Fe was slightly large (exceeded 1.5 mass%) and the content of Cr was small. Further, the limit of compressibility was low in Comparative Example 4. The reason is considered to be that T β exceeded 880°C. (5) The tensile strength was low in Comparative Example 5. The reason is considered to be that V was not contained. Further, the limit of compressibility was low in Comparative Example 5. The reason is considered to be that T β exceeded 880°C. (6) The elongation was low in Comparative Example 6. The reason is considered to be that the content of V was small. Further, the limit of compressibility was low in Comparative Example 6. The reason is considered to be that T β exceeded 880°C. (7) The elongation was low in Comparative Example 7. The reason is considered to be that the content of Fe was slightly large (exceeded 1.5 mass%) and the content of Cr was small. Further, the limit of compressibility was low in Comparative Example 7. The reason is considered to be that T β exceeded 880°C. (8) The limit of compressibility was low in Comparative Example 8. The reason is considered to be that T β exceeded 880°C. (9) The tensile strength and elongation were low in Comparative Example 9. The reason is considered to be that the content of V was small. Further, the limit of compressibility was low in Comparative Example 9. The reason is considered to be that T β exceeded 880°C. (10) In Examples 1 to 8, the tensile strength was 950 MPa or more, the elongation was 15% or more, and the limit of compressibility was 35% or more. (11) In Examples 1 and 6, the elongation was 20% or more, and the limit of compressibility was 50% or more. The reason is considered to be that T β was 840°C or lower. (12) A correlation was observed between T β and the limit of compressibility. Refer to FIG. 2. It was found that it is effective to set T β to 880°C or lower in order to control the limit of compressibility to 35% or more. Table 1 Composition (mass%)TiAlVCrFeONHCSiOthersEx. 1Bal.4.13.23.00.900.190.010.010.010.03Ex. 2Bal.4.53.13.00.970.210.01< 0.010.010.03Ex. 3Bal.4.53.13.00.070.190.010.010.010.03Ex. 4Bal.4.02.93.02.100.140.010.010.010.03Ex. 5Bal.3.92.96.00.110.110.010.010.010.02Ex. 6Bal.4.13.23.00.900.190.010.010.010.030.03Nb0.07Mo0.03TaEx. 7Bal.4.13.23.00.900.190.010.010.010.030.02Sn0.04Zr0.05Mn0.07NiEx. 8Bal.4.13.23.00.900.190.010.010.010.030.01BComp. Ex. 1Bal.6.44.30.00.190.150.010.010.010.06-Comp. Ex. 2Bal.5.00.00.02.500.140.010.010.010.05Comp. Ex. 3Bal.4.53.11.00.990.240.010.010.010.03Comp. Ex. 4Bal.4.82.91.01.800.100.010.01<0.010.03-Comp. Ex. 5Bal.5.00.01.11.900.100.010.010.010.05-Comp. Ex. 6Bal.4.92.01.51.800.100.010.01<0.010.04-Comp. Ex. 7Bal.4.93.50.71.800.100.010.01<0.010.05-Comp. Ex. 8Bal.6.064.141.550.250.130.010.010.010.06-Comp. Ex. 9Bal.3.982.232.020.190.060.010.010.010.05- Table 1 (Continued) T β (°C)StructureTensile strengthElongation at ruptureLimit of compressibilityEx. 1832α+βAAAAAAEx. 2847α+βAAAAEx. 3866α+βAAAEx. 4878α+βAAAEx. 5861α+βAAAEx. 6832α+βAAAAAAEx. 7832α+βAAAEx. 8832α+βAAAAComp. Ex. 1995α+βBBBComp. Ex. 2950α+βBBBComp. Ex. 3889α+βAABComp. Ex. 4924α+βABBComp. Ex. 5970α+βBABComp. Ex. 6928α+βABBComp. Ex. 7920α+βABBComp. Ex. 8940α+βAABComp. Ex. 9890α+βBBB Tensile strength at 25°C was evaluated as: A: 950 MPa or more, and B: less than 950 MPa. Elongation at rupture at 25°C was evaluated as: AA: 20% or more, A: 15% or more, and B: less than 15%. Maximum limit of compressibility at 25°C was evaluated as: AAA: 50% or more, AA: 45% or more, A: 35% or more, and B: less than 35%.

[0132] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

[0133] This application is based on Japanese Patent Application No. 2025-034193 filed on March 5, 2025, the contents of which are incorporated herein by reference.INDUSTRIAL APPLICABILITY

[0134] The α+β titanium alloy according to the present invention can be used for various structural parts, corrosion resistant parts, and the like used in a golf club head, a chemical industrial device, electrical equipment, space equipment, an aircraft, a ship, a vehicle, a medical device, a condenser, a heat exchanger, a seawater desalination device, and the like.

Examples

examples 1 to 8

(Examples 1 to 8 and Comparative Examples 1 to 9)

[1. Preparation of Samples]

[0128]Raw materials blended so as to have a predetermined composition were melted by the CCIM method to obtain an ingot of 115 mm-diameter × 100 mm-height. This ingot was heated to a temperature of T β or higher and hot-forged (pre-forged) into a 40 mm cube. Further, the pre-forged material was heated in the α+β two-phase temperature range and hot-forged (finish forged) to a diameter of 20 mm. After forging, a heat treatment was performed in a range of T β - 200°C or higher and lower than T β - 50°C.

[2. Test Method]

[2.1. Tensile Test]

[0129]A tensile test was performed at 25°C in accordance with JIS Z2241 (2011) to measure tensile strength and elongation at rupture. A No. 14A test piece having a parallel portion diameter of 6 mm was used as the test piece.

[2.2. Compression Test]

[0130]An end face restraint compression test was performed at 25°C to measure limit of compressibility. A cylindrical test piece o...

Claims

1. An α+β type titanium alloy, consisting of: <maths> 3.0 mass % ≤ A 1 ≤ 5.4 mass % ; 2.5 mass % ≤ V ≤ 6.0 mass % ; 1.5 mass % ≤ Cr ≤ 7.0 mass % ; 0.05 mass % ≤ Fe ≤ 3.0 mass % ; and optionally at least one of: Sn ≤ 0.5 mass % ; Zr ≤ 0.5 mass % ; Mo ≤ 0.5 mass % ; Nb ≤ 0.5 mass % ; Ta ≤ 0.5 mass % ; Ni ≤ 0.15 mass % ; Mn ≤ 0.25 mass % ; B ≤ 0.2 mass % ; Si ≤ 0.3 mass % ; O ≤ 0.4 mass % ; N ≤ 0.1 mass % ; H ≤ 0.1 mass % ; and C ≤ 0.2 mass % , with the balance being Ti and unavoidable impurities, and having a β transformation temperature Tβ of 880°C or lower.

2. The α+β type titanium alloy according to claim 1, satisfying: Sn + Zr + Mo + Nb + Ta + Ni + Mn + B ≤ 2.0 mass % .

3. The α+β type titanium alloy according to claim 1 or 2, having a tensile strength at 25°C of 950 MPa or more, an elongation at 25°C of 15% or more, and a limit of compressibility at 25°C of 35% or more.

4. A method for producing a semi-finished material composed of an α+β type titanium alloy, comprising: a preparation step of preparing a raw material composed of the α+β type titanium alloy according to any one of claims 1 to 3; a hot-working step of performing a hot-working on the material in a temperature range of 600°C or higher and lower than the β transformation temperature Tβ; and a heat treatment step of performing a heat treatment on the material after the hot-working at a temperature of Tβ - 200°C or higher and 1200°C or lower.

5. The method according to claim 4, wherein the heat treatment step includes a step of performing a heat treatment on the material after the hot-working at a temperature of Tβ - 200°C or higher and lower than Tβ - 50°C.

6. The method according to claim 4, wherein the heat treatment step includes a step of performing a heat treatment on the material after the hot-working at a temperature of Tβ or higher and 1200°C or lower.

7. The method according to claim 6, further comprising, after the heat treatment step, an aging treatment step of subjecting the material after the heat treatment to an aging treatment at a temperature of 400°C or higher and 700°C or lower.

8. The method according to claim 4, wherein the heat treatment step includes a step of performing a heat treatment on the material after the hot-working at a temperature of Tβ - 50°C or higher and lower than Tβ.

9. The method according to claim 8, further comprising, after the heat treatment step, an aging treatment step of subjecting the material after the heat treatment to an aging treatment at a temperature of 400°C or higher and 700°C or lower.

10. A use of the α+β type titanium alloy according to any one of claims 1 to 3, or as manufactured according to any one of claims 4 to 9, for a structural part or a corrosion resistant part.

11. The use according to claim 10, for sports or leisure goods or for consumer products.

12. The use according to claim 11, for golf club heads or bicycle gears.

13. The use according to claim 11, for eyeglass frames.

14. The use according to claim 10, for bolts, valve retainers, or seawater-resistant shafts.

15. The use according to claim 10, for a chemical industrial device, electrical equipment, space equipment, an aircraft, a ship, a vehicle, a medical device, a condenser, a heat exchanger, or a seawater desalination device.

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