Titanium alloy materials and parts

Abstract

To provide a titanium alloy material and a titanium alloy component excellent in machinability.SOLUTION: A titanium alloy material has a chemical composition satisfying a specified chemical composition and [-4.0≤Mo+0.67V+0.28Nb+2.9Fe+1.6Cr+1.1Ni+1.6Mn-Al≤2.0]. In the titanium alloy material, an area ratio of a (0001) plane of an α-phase is 90% or higher in a range of over 65° and 90°or less, an area ratio of a (10-10) plane of the α-phase is 20% or higher in a range of over 55°and 65°or less, and an area ratio of a (11-20) plane of the α-phase is 20% or higher in a range of over 25°and 35°or less, with respect to a longer direction.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to titanium alloy materials and parts. [Background technology]

[0002] Transportation vehicles such as motorcycles, automobiles, and airplanes use a variety of parts, including bolts and nuts. To improve performance and reduce environmental impact, efforts are being made to reduce the weight of these parts, and the materials used to make these parts are required to have high specific strength and be easy to reduce weight. Lightweight parts are also required for a variety of machinery other than transportation vehicles, including the drive units and rotating bodies of precision instruments. Many of these parts are cut to their desired shape. Therefore, the materials used to make these parts are often machined with high precision, including for the purpose of reducing weight, and are therefore required to be easy to cut.

[0003] Titanium alloy materials have a high specific strength and are lightweight. In particular, the α+β type titanium alloy material described in Patent Document 1 has high strength among titanium alloy materials and is suitable as a material for the above-mentioned parts. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2013 / 094647 Summary of the Invention [Problem to be solved by the invention]

[0005] On the other hand, α+β titanium alloy materials have the problem that cutting is difficult because the chips generated during cutting tend to get tangled or clog the tool. The purpose of the present invention is to provide a titanium alloy material with good machinability and a part using the same. [Means for solving the problem]

[0006] The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is the following titanium alloy material and a part using the same.

[0007] (1) Chemical composition, in mass%, Al: 4.50% or more and 6.75% or less, C: 0.100% or less, N: 0.050% or less, H: 0.016% or less, O: 0.35% or less, Mo: 0% or more and 5.5% or less, V: 0% or more and 4.50% or less, Nb: 0% or more and 3.0% or less, Fe: 0% or more and 2.50% or less, Cr: 0% or more and less than 0.25% Ni: 0% or more and less than 0.15% Mn: 0% or more and less than 0.25% Si: 0% or more and 0.50% or less, The balance is Ti and impurities. The following formula (i) is satisfied: In a cross section perpendicular to the longitudinal direction, The area ratio of the region in which the (0001) plane of the α phase forms an angle of more than 65° and not more than 90° with respect to the longitudinal direction is 90% or more, The area ratio of the region in which the (10-10) plane of the α phase forms an angle of more than 55° and not more than 65° with respect to the longitudinal direction is 20% or more, A titanium alloy material, wherein the area ratio of a region in which the (11-20) plane of the α phase forms an angle of more than 25° and not more than 35° with respect to the longitudinal direction is 20% or more. -4.0≦Mo+0.67V+0.28Nb+2.9Fe+1.6Cr+1.1Ni+1.6Mn-Al≦2.0 (i) However, each element symbol in the above formula represents the content (mass%) of each element contained in the titanium alloy material, and if the element is not contained, it is set to zero.

[0008] (2) The chemical composition is in mass%: Al: 4.50% or more and 6.40% or less, Fe: 0.50% or more and 2.10% or less, The titanium alloy material according to (1) above, containing:

[0009] (3) In a cross section perpendicular to the longitudinal direction, The titanium alloy material according to (1) or (2) above, wherein the area ratio of the region in which the (0001) plane of the α phase makes an angle of 0° or more and 15° or less with respect to the longitudinal direction is 1% or less.

[0010] (4) The titanium alloy material according to any one of the above (1) to (3), wherein an absolute value |ΔHV| of the difference between the Vickers hardness in a cross section perpendicular to the longitudinal direction and the Vickers hardness in a cross section parallel to the longitudinal direction satisfies the following formula (ii): 10≦|ΔHV|≦30 (ii) However, |ΔHV| in the above formula is defined as follows. |ΔHV|: When the test force is 1 kgf and hardness measurements are performed at five points on each of the cross sections perpendicular to the longitudinal direction and the cross sections parallel to the longitudinal direction, the absolute value of the difference between the average Vickers hardness on the cross section perpendicular to the longitudinal direction and the average Vickers hardness on the cross section parallel to the longitudinal direction.

[0011] (5) A titanium alloy material according to any one of (1) to (4) above, wherein the curl diameter of discharged chips is 5 mm or less.

[0012] (6) The titanium alloy material according to (5) above, wherein the length of the chips is 10 mm or less.

[0013] (7) The titanium alloy material according to any one of (1) to (6) above, which is a rod, wire or plate material.

[0014] (8) A part made of the titanium alloy material according to any one of (1) to (7) above. [Effects of the Invention]

[0015] According to the present invention, a titanium alloy material having good machinability can be obtained. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram showing the distribution of the crystal orientation of the α phase in a cross section (T cross section) perpendicular to the longitudinal direction of each titanium alloy rod and wire. [Figure 2] FIG. 2 is a photograph of chips discharged when cutting the conventional example, invention 1, and invention 2. [Figure 3] FIG. 3 is a diagram schematically showing the hot working method in the first hot working. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present inventors have investigated the machinability of titanium alloy materials and have obtained the following findings (a) to (c).

[0018] (a) Generally, it is desirable that the chips discharged during cutting have a regular cylindrical spiral shape with a predetermined repeating curl diameter and are short in length. On the other hand, chips of α+β type titanium alloy materials tend to be irregular in shape and long in length. Such chip shapes tend to easily get tangled around the cutting tool or tangle with each other. This is thought to result in a decrease in machinability.

[0019] (b) The reason why chips from α+β titanium alloys are irregularly shaped and long is that α+β titanium alloys have high strength, low thermal conductivity, and high reactivity. These properties are partly due to the fact that the α-phase of titanium has a highly anisotropic hcp structure. For this reason, the inventors have found that controlling the crystal orientation of the α-phase is effective.

[0020] (c) By controlling the crystal orientation of the α phase, the orientation is concentrated in a narrow range compared to conventional α+β titanium alloy materials. This allows chips to be adjusted to a more desirable shape and length, improving machinability.

[0021] An embodiment of the present invention has been made based on the above findings. Each requirement of this embodiment will be described in detail below.

[0022] 1. Chemical composition of titanium alloy materials The reasons for limiting each element are as follows. In the following description, "%" for the content means "% by mass." The contents of C, N, H, O, etc. may be measured by, for example, inert gas fusion infrared absorption spectrometry, inert gas fusion thermal conductivity spectrometry, high-frequency combustion infrared absorption spectrometry, or inductively coupled plasma (ICP) atomic emission spectrometry.

[0023] Al: 4.50% or more and 6.75% or less Al (aluminum) is an element that dissolves in the α phase to strengthen it. In order to achieve this effect in α+β titanium alloy materials, the Al content is set to 4.50% or more. The Al content is preferably set to 4.80% or more, and more preferably set to 4.85% or more. However, excessive Al content reduces workability, such as hot workability. For this reason, the Al content is set to 6.75% or less. The Al content is preferably set to 6.40% or less, and more preferably set to 5.40% or less.

[0024] C: 0.100% or less Carbon (C) dissolves in the α phase to strengthen it. However, excessive C content can lead to an excessive increase in carbides, which can reduce toughness and ductility. Therefore, the C content is set to 0.100% or less. The C content is preferably set to 0.080% or less, and more preferably set to 0.060% or less. On the other hand, α+β type titanium alloy materials do not need to contain C, but because excessive reduction of C increases the manufacturing cost of raw materials and the like, and because the above-mentioned effects can be obtained, the C content is preferably set to 0.001% or more, and more preferably set to 0.002% or more.

[0025] N: 0.050% or less N (nitrogen) dissolves in the α phase to strengthen it. However, excessive N content can result in excessive nitride formation, which can reduce toughness and ductility. Therefore, the N content is set to 0.050% or less. The N content is preferably set to 0.030% or less. On the other hand, α+β type titanium alloy materials do not need to contain N, but considering that excessive reduction of N increases the manufacturing costs of raw materials and the like and that the above effects can be obtained, the N content is preferably set to 0.001% or more, and more preferably 0.002% or more.

[0026] H:0.016% or less H (hydrogen) is an impurity element contained in titanium alloy materials. Excessive H content can reduce toughness and ductility. Therefore, the H content is set to 0.016% or less. The H content is preferably set to 0.012% or less. However, excessive reduction of H increases manufacturing costs, so the H content is preferably set to 0.001% or more.

[0027] O: 0.35% or less O (oxygen) dissolves in the α phase to strengthen it. However, excessive O content can reduce toughness and ductility. Therefore, the O content is set to 0.35% or less. The O content is preferably set to 0.25% or less. On the other hand, α+β type titanium alloy materials do not need to contain O, but excessive reduction of O content increases the manufacturing cost of raw materials, etc., and in order to obtain the above-mentioned effects, the O content is preferably set to 0.050% or more, and more preferably set to 0.100% or more.

[0028] Mo: 0% or more and 5.5% or less Mo (molybdenum) has the effect of stabilizing the β phase and improving hot workability. Therefore, it may be added as needed. However, excessive Mo content may result in reduced manufacturability and toughness / ductility due to Mo segregation. Furthermore, the β phase ratio may increase even in high temperature regions, making it impossible to achieve the above-mentioned effects. For this reason, the Mo content is set to 5.5% or less. The Mo content is preferably set to 3.0% or less. On the other hand, to achieve the above-mentioned effects, the Mo content is preferably set to 0.9% or more, and more preferably 1.0% or more.

[0029] V: 0% or more and 4.50% or less V (vanadium) has the effect of stabilizing the β phase and improving hot workability. Therefore, it may be added as needed. However, excessive V content may increase the β phase ratio and reduce strength. Furthermore, the β phase ratio may increase even in the high temperature range, making it impossible to obtain the above-mentioned effects and even increasing raw material costs. For this reason, the V content is set to 4.50% or less. The V content is preferably set to 4.20% or less. On the other hand, to obtain the above-mentioned effects, the V content is preferably set to 0.10% or more.

[0030] Nb: 0% or more and 3.0% or less Nb (niobium) has the effect of improving oxidation resistance. Therefore, it may be contained as necessary. However, if an excessive amount of Nb is contained, the β-phase ratio increases, which may reduce strength. Furthermore, the β-phase ratio increases even in the high temperature range, which may prevent the above-mentioned effects from being obtained. Therefore, the Nb content is set to 3.0% or less. The Nb content is preferably set to 1.0% or less. On the other hand, in order to obtain the above-mentioned effects, the Nb content is preferably set to 0.1% or more, and more preferably set to 0.2% or more.

[0031] Fe: 0% or more and 2.50% or less Iron (Fe) also has the effect of stabilizing the β phase and improving hot workability. Therefore, it may be added as needed. However, excessive Fe content may lead to Fe segregation, which may reduce manufacturability and toughness / ductility. Furthermore, the β phase ratio may increase, which may prevent the above-mentioned effects from being achieved. Therefore, the Fe content is set to 2.50% or less. The Fe content is preferably set to 1.50% or less. On the other hand, to obtain the above-mentioned effects, the Fe content is preferably set to 0.05% or more, and more preferably 0.10% or more.

[0032] Cr: 0% or more and less than 0.25% Cr (chromium) has the effect of stabilizing the β phase and improving the strength of titanium alloy materials. Therefore, it may be contained as needed. However, if Cr is contained in excess, the segregation of Cr may reduce manufacturability and toughness / ductility. Furthermore, the β phase ratio may increase, making it impossible to obtain the above-mentioned effects. For this reason, the Cr content is set to less than 0.25%. The Cr content is preferably set to 0.10% or less. On the other hand, to obtain the above-mentioned effects, the Cr content is preferably set to 0.03% or more.

[0033] Ni: 0% or more and less than 0.15% Ni (nickel) has the effect of stabilizing the β phase and improving the strength of titanium alloy materials. Therefore, it may be contained as needed. However, if excessive Ni is contained, Ni segregation may decrease manufacturability and toughness / ductility. Furthermore, the β phase ratio increases, which may prevent the above-mentioned effects from being obtained. For this reason, the Ni content is set to less than 0.15%. The Ni content is preferably set to 0.10% or less. On the other hand, to obtain the above-mentioned effects, the Ni content is preferably set to 0.03% or more.

[0034] Mn: 0% or more and less than 0.25% Mn (manganese) has the effect of stabilizing the β phase and improving the strength of titanium alloy materials. Therefore, it may be contained as needed. However, if Mn is contained in excess, the segregation and evaporation of Mn may cause a decrease in manufacturability and toughness / ductility. Furthermore, the β phase ratio may increase, making it impossible to obtain the above-mentioned effects. For this reason, the Mn content is set to less than 0.25%. The Mn content is preferably set to 0.15% or less. On the other hand, to obtain the above-mentioned effects, the Mn content is preferably set to 0.03% or more.

[0035] Si: 0% or more and 0.50% or less Silicon (Si) has the effect of improving the heat resistance and increasing the strength of titanium alloy materials. However, excessive Si content can easily cause segregation, leading to the precipitation of a large amount of silicide, which can reduce toughness and ductility. For this reason, the Si content is set to 0.50% or less. The Si content is preferably set to 0.35% or less. On the other hand, to obtain the above effects, the Si content is preferably set to 0.04% or more, and more preferably set to 0.07% or more.

[0036] The contents of Mo, V, Nb, Fe, Cr, Ni, Mn, and Al must satisfy the following formula (i): -4.0≦Mo+0.67V+0.28Nb+2.9Fe+1.6Cr+1.1Ni+1.6Mn-Al≦2.0 (i) However, each element symbol in the above formula represents the content (mass%) of each element contained in the titanium alloy material, and if the element is not contained, it is set to zero.

[0037] If the value in equation (i) is less than -4.0, the amount of β phase will be excessively small, which may prevent the above-mentioned effects from being obtained, making it difficult to improve machinability. Therefore, the value in equation (i) is set to -4.0 or more. The value in equation (i) is preferably set to -3.5 or more, and more preferably set to -3.0 or more. On the other hand, if the value in equation (i) exceeds 2.0, the amount of β phase will be excessively large, making it difficult to improve machinability by controlling the crystal orientation of the α phase alone. Therefore, the value in equation (i) is set to 2.0 or less. The value in equation (i) is preferably set to 1.5 or less, and more preferably set to 1.0 or less. Note that when the chemical composition of a titanium alloy material satisfies equation (i), it can be said to be substantially an α+β type alloy.

[0038] In the chemical composition of this embodiment, the balance is Ti and impurities. Here, "impurities" refers to components that are mixed in due to various factors in raw materials such as titanium sponge and manufacturing processes when industrially producing titanium alloy materials, and are acceptable within a range that does not adversely affect this embodiment. Metal elements other than those listed above that are likely to be mixed in as impurities include Sn, Zr, and Cu. These may be mixed in from raw materials, particularly when raw materials such as scrap or low-grade titanium sponge are used. These elements are desirably controlled due to concerns that their content may become high. It is preferable to keep the total content of these metal elements below 0.3%, and each element individually below 0.1%.

[0039] 2. Crystal orientation and hardness 2-1. Examination of crystal orientation and hardness The following study was conducted on the machinability of the titanium alloy material of this embodiment. Titanium alloy (α+β type titanium alloy) rods and wires with similar chemical compositions were prepared as shown in Table 1. The orientation distribution of the α phase crystal orientation was investigated when observing the cross section perpendicular to the longitudinal direction of these titanium alloy rods and wires (hereinafter simply referred to as "T cross section").

[0040] [Table 1]

[0041] Figure 1 shows the distribution of the α-phase crystal orientation in each titanium alloy rod and wire. It can be seen that, compared to the conventional material, the α-phase orientation in Inventions 1 and 2 is concentrated in a narrow, fixed range at all extremes of the (0001), (10-10), and (11-20) planes. On the other hand, in the conventional material, for example, in the (10-10) and (11-20) planes, the α-phase orientation is not concentrated in a specific angular range, but is dispersed approximately evenly across the entire angular range.

[0042] Next, Figure 2 shows the chips obtained when a cutting test was conducted on the conventional example, invention 1, and invention 2, with a cutting rate of 0.5 mm, a peripheral speed of 1000 mm / min, and a feed rate of 150 mm / min. The chips from the conventional material were unevenly curled or tangled in some places, exhibiting a shape known as a ribbon. On the other hand, the chips from invention 1 had a more regular curl than the chips from the conventional material. The chips from invention 2 had a more regular curl than invention 1, and were shorter than invention 1. This shows that invention 1 has better machinability than the conventional example, and invention 2 has better machinability than invention 1.

[0043] Furthermore, the absolute value |ΔHV| of the difference between the Vickers hardness of a cross section perpendicular to the longitudinal direction and the Vickers hardness of a cross section parallel to the longitudinal direction of these titanium alloy rods and wires was calculated. The conventional example and present invention 1 had a |ΔHV| of 7, while the present invention 2 had a |ΔHV| of 20.

[0044] 2-2.Crystal orientation In light of the above, in the titanium alloy material of this embodiment, the orientation distribution of the crystal orientation of the α phase is controlled within the following ranges with respect to the longitudinal direction. Specifically, in the T cross section, the area ratio of the region where the (0001) plane of the α phase forms an angle of more than 65° and not more than 90° with respect to the longitudinal direction is set to 90% or more. Also, in the T cross section, the area ratio of the region where the (10-10) plane of the α phase forms an angle of more than 55° and not more than 65° with respect to the longitudinal direction is set to 20% or more. Similarly, the area ratio of the region where the (11-20) plane of the α phase forms an angle of more than 25° and not more than 35° with respect to the longitudinal direction is set to 20% or more.

[0045] 2-2-1.(0001) plane In the T-section, if the area ratio of the region where the (0001) plane of the α phase forms an angle of more than 65° and not more than 90° with respect to the longitudinal direction is less than 90%, a sufficient improvement in machinability cannot be obtained. Therefore, the area ratio of the region where the (0001) plane of the α phase forms an angle of more than 65° and not more than 90° is set to 90% or more, and preferably 93% or more.

[0046] In addition, in the T-section, the area ratio of the region where the (0001) plane of the α phase forms an angle of 0° to 15° with respect to the longitudinal direction is preferably 1% or less. It is desirable that the (0001) plane of the α phase be at an angle of more than 65° to 90° with respect to the longitudinal direction, but the angle of 0 to 15° has a significantly different orientation from this angle range of more than 65° to 90°. Therefore, if this area ratio exceeds 1%, the chip length tends to be slightly longer, although it will be 10 mm or less.

[0047] 2-2-2.(10-10) side Furthermore, if the area ratio of the region in the T cross section where the (10-10) plane of the α phase forms an angle of more than 55° and not more than 65° with respect to the longitudinal direction is less than 20%, sufficient improvement in machinability cannot be obtained. Therefore, the area ratio of the region in the T cross section where the (10-10) plane of the α phase forms an angle of more than 55° and not more than 65° with respect to the longitudinal direction is set to 20% or more, and preferably 30% or more. There is no particular restriction on the upper limit of this area ratio, but due to the symmetry of the crystal, the upper limit is usually considered to be around 45%.

[0048] 2-2-3.(11-20) side Furthermore, if the area ratio of the region in the T cross section where the (11-20) plane of the α phase forms an angle of more than 25° and not more than 35° with respect to the longitudinal direction is less than 20%, sufficient improvement in machinability cannot be obtained. Therefore, the area ratio of the region in the T cross section where the (11-20) plane of the α phase forms an angle of more than 25° and not more than 35° with respect to the longitudinal direction is set to 20% or more, and preferably 30% or more. There is no particular restriction on the upper limit of this area ratio, but due to the symmetry of the crystal, the upper limit is usually considered to be around 45%.

[0049] 2-2-4. Measuring the area ratio of each surface Here, the area ratio of each plane of the α phase mentioned above refers to the density ratio calculated at the pole of each plane, (0001), (10-10), and (11-20), when observing a T-section, from the longitudinal direction set at 0° to 90°, which is the perpendicular direction, at 5° intervals. Note that, due to the symmetry of the crystal, it is sufficient to measure within the above-mentioned range of 0 to 90°, and the density ratio of each plane is normalized so that it is 100% from 0 to 90°. In other words, the area ratio of each plane is an index showing the orientation distribution of the crystal orientation within a specified angle range.

[0050] The area ratio can be measured using EBSD. The measurement sample should have a T-section that has been polished to a mirror finish by electrolytic polishing or colloidal silica polishing. When measuring with EBSD, an area of ​​200 μm × 250 μm is observed in 2 to 5 fields of view at a step interval of 0.3 to 0.5 μm.

[0051] As described above, for each hcp plane (each orientation) in each field of view, the angle (0 to 90° divided in 5° increments) between the plane and the longitudinal direction of the titanium alloy material and the distribution of area fraction were created. The average value of all measurement points within a given angle range between the plane and the longitudinal direction of the titanium alloy material was calculated. Since the hcp prism planes (10-10) and (11-20) exist in three directions, the area fractions calculated using the analysis software were further divided by the sum to normalize (total 1). Based on the EBSD measurement results, the area fractions in each direction could be calculated using analysis software (TSL Solutions OIM Analysis™ software (Ver. 8.1.0)).

[0052] 2-3.Hardness As described above, to further improve machinability, it is desirable to increase the absolute value |ΔHV| of the difference between the Vickers hardness in a cross section perpendicular to the longitudinal direction and the Vickers hardness in a cross section parallel to the longitudinal direction. Specifically, it is preferable that the absolute value |ΔHV| of the difference between the Vickers hardness in a cross section perpendicular to the longitudinal direction and the Vickers hardness in a cross section parallel to the longitudinal direction satisfies the following formula (ii):

[0053] 10≦|ΔHV|≦30 (ii) However, |ΔHV| in the above formula is defined as follows. |ΔHV|: The absolute value of the difference between the average Vickers hardness value in the cross section perpendicular to the longitudinal direction and the average Vickers hardness value in the cross section parallel to the longitudinal direction when the test force is 1 kgf and hardness measurements are taken at five points on each of the cross sections perpendicular to the longitudinal direction.

[0054] If |ΔHV| is less than 10, the effect of improving machinability will be limited. In other words, the effect of stabilizing the deformation of the chips will be insufficient, and the deformation direction of the chips will fluctuate or they will not be broken, which will lead to the chip shape of invention 1. For this reason, |ΔHV| is preferably 10 or more, and more preferably 15 or more. On the other hand, controlling |ΔHV| so that it exceeds 30 is not practical due to the characteristics of α+β type titanium alloy materials. For this reason, |ΔHV| is preferably 30 or less, and more preferably 25 or less.

[0055] In the hardness test, the cross section perpendicular to the longitudinal direction and the cross section parallel to the longitudinal direction are used as measurement surfaces, and measurements are taken at five points using a Vickers hardness tester with a test force of 1 kgf. |ΔHV| can be calculated from the difference in the average hardness values ​​obtained for each cross section.

[0056] Furthermore, in order to satisfy formula (ii) and obtain even better machinability, the chemical composition preferably contains Al: 4.50% to 6.40% and Fe: 0.50% to 2.10%. V is an optional element and may not be included. The composition ranges of the other elements may be within the ranges mentioned above.

[0057] 3. Target characteristics In the titanium alloy material of this embodiment, it is preferable that the discharged chips satisfy the following range when a predetermined cutting process is performed, as an index of machinability. Specifically, when a cutting test is performed under the same conditions as those shown in Figure 2, i.e., under conditions of 0.5 mm cutout amount, 1000 mm / min peripheral speed, and 150 mm / min feed, it is preferable that the curl diameter of the discharged chips is 5 mm or less, as in invention 2. Furthermore, as an index of good machinability, in addition to the above curl diameter, it is preferable that the length of the chips is 10 mm or less.

[0058] 4. Shape of titanium alloy material As described above, the titanium alloy material of this embodiment may be in the form of, for example, a rod, wire, plate, etc. Note that the specific dimensions of the shape are not important.

[0059] 5. Parts The part of this embodiment may be manufactured using the titanium alloy material of this embodiment described above.

[0060] 6. Manufacturing method A preferred method for producing the titanium alloy material according to this embodiment will now be described. The titanium alloy material according to this embodiment can be stably produced by the method described below.

[0061] A hot-worked material such as a cast piece, slab, bloom, or billet of a titanium alloy having a chemical composition within the above-mentioned range is manufactured. The manufacturing method of the hot-worked material is not particularly limited and may be a conventional method. Next, the obtained hot-worked material is subjected to a first hot working (hereinafter simply referred to as "first hot working") and a second hot working (hereinafter simply referred to as "second hot working"). These two hot working steps will be explained below.

[0062] 6-1. First hot processing process The above-mentioned hot-worked material is subjected to a first hot working process. In the first hot working process, it is necessary to develop the crystal orientation of the β phase (bcc structure) that transforms into the α phase (hcp structure) in the longitudinal direction of the titanium alloy material during cooling. This develops a so-called fiber texture. This allows a titanium alloy material with the above-mentioned crystal orientation to be obtained. Specifically, the titanium alloy material is hot-worked in a temperature range where the β phase fraction is 20% or more. That is, the hot working is performed so that the β phase fraction at the lowest temperature during hot working is 20% or more. This is because if the hot working is performed in a temperature range where the β phase fraction is less than 20%, it will be impossible to obtain an α phase with the desired crystal orientation in the subsequent process. The β phase fraction is the minimum β phase fraction that can be calculated using the method described below.

[0063] In the first hot working, hot working is performed with a rectangular cross section as shown in Fig. 3. In this hot working, reduction may be applied from all four peripheral faces. The distribution of reduction may be adjusted as appropriate, but it is preferable that the total area reduction rate described below is satisfied.

[0064] Hot working a rectangular cross section allows for more uniform deformation all the way to the inside. Furthermore, by pressing down the overhangs that occur on surfaces other than the pressed surface, the same deformation can be achieved on the surface layers on all four sides. In contrast, when pressing a circular (elliptical) cross section, the overhang area is narrow, making it difficult to achieve uniform deformation on the inside and surface layers.

[0065] In the first hot processing, it is not necessary to process the cross section into a rectangular shape in all hot processing steps; it is sufficient to include a step involving hot working from a rectangular cross section to a rectangular cross section. This is because it is possible to obtain the α phase with the desired angle range in each plane. For example, a material for hot processing with a circular or rectangular cross section may be hot worked to a rectangular cross section, and then further hot worked while the cross section remains rectangular. Alternatively, a material for hot processing with a circular or rectangular cross section may be hot worked to a rectangular cross section, and then further hot worked while the cross section remains rectangular, and then hot worked to a circular or rectangular cross section. The total area reduction rate when hot working from a rectangular cross section to a rectangular cross section is preferably 50% or more, and more preferably 60% or more.

[0066] The rectangular cross section described above only needs to have four corners, and may have R, chamfers, etc.

[0067] 6-2.Second hot working process In the second hot working, the desired crystal orientation is developed by applying a large degree of working (heavy working) while significantly changing the phase ratios of α and β. Even if the temperature drops during working and the β phase transforms to α, it is desirable to actively generate heat during working to cause the α phase to transform to β phase and continue working.

[0068] By performing such second hot working, the structure formed during the first hot working can be developed into a structure having a desired crystal orientation. In order to perform the above-mentioned structure control during the second hot working, the titanium alloy material is heated to a temperature range in which the β-phase fraction is 30 to 60%, and hot working is performed. During hot working, the temperature of the titanium alloy material gradually decreases, but even if the temperature decreases, it is preferable to perform hot working while maintaining a temperature in which the β-phase fraction is 7% or more. During hot working, it is more preferable to maintain a temperature range in which the β-phase fraction is at least 10% or more.

[0069] Since a temperature drop during hot working is unavoidable, it is preferable to utilize processing heat to control the temperature during hot working. While it is generally desirable to suppress processing heat, the titanium alloy material of this embodiment actively utilizes processing heat because it is easier to suppress temperature drop and perform hot working in a temperature range that results in a β-phase fraction within the above-mentioned range. To generate processing heat, the strain rate is set to 10 / s or more. This allows for active utilization of processing heat. If the strain rate during processing is less than 10 / s, insufficient processing heat is generated, resulting in a large temperature drop during hot working, making it impossible to obtain the desired crystal orientation. Therefore, the strain rate during processing is preferably 10 / s or more, and more preferably 15 / s or more.

[0070] If the strain rate during processing exceeds 50 / s, excessive processing heat is generated, making it impossible to obtain the desired structure. Therefore, the strain rate during processing is set to 50 / s or less. In particular, a strain rate of 40 / s or less is preferable in order to keep the area ratio of the region where the (0001) plane of the α phase forms an angle of 0° or more and 15° or less with respect to the longitudinal direction to 1% or less.

[0071] In the second hot working step, unlike the first hot working step, the cross-sectional shape during working is not particularly limited. The cross-sectional shape may be either circular or rectangular. The total area reduction rate during working is 70% or more, preferably 80% or more. After this second hot working step, it is recommended to avoid any processing that results in an area reduction rate of more than 5%, specifically, cold rolling, wire drawing, drawing, etc., because this will disrupt the crystal orientation of the titanium alloy material.

[0072] Before and after the first and second hot working steps, surface treatment, shape correction, heat treatment, etc. may be performed as necessary. Therefore, the first and second hot working steps do not necessarily have to be performed consecutively. For example, after the first hot working step, the temperature may be lowered to room temperature or 500°C or less, and then the second hot working step may be performed, or the second hot working step may be performed in two or three steps. When heat treatment is performed, it may be performed at a temperature of β transformation point -50°C or less. The hot working refers to either hot rolling or hot forging, or a combination of both.

[0073] In the first hot working and the second hot working described above, the correspondence relationship between the β phase ratio and the temperature required for temperature control can be obtained by the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) method in addition to the chemical composition of the titanium alloy material itself, and can be confirmed, for example, by using Thermo-Calc, a comprehensive thermodynamic calculation system manufactured by Thermo-Calc Software AB, and a predetermined database (TI3). The same applies to the β transformation point.

[0074] After the first hot working step and the second hot working step, cooling is carried out as appropriate, and an α+β type titanium alloy material can be obtained.

[0075] The titanium alloy material according to the present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0076] A material for hot working having the chemical composition shown in Table 2 was manufactured. This material for hot working was subjected to first hot working and second hot working under the manufacturing conditions shown in Table 3 to obtain a titanium alloy material. The changes in cross-sectional shape during the first hot working and second hot working are as shown in Tables 4 and 5. That is, A to D in the item "Changes during working in the first hot working step" in Table 3 (Table 4) correspond to steps A to D in Table 4, respectively. Similarly, 1 to 4 in the item "Changes during working in the second hot working step" in Table 3 (Table 5) correspond to steps 1 to 4 in Table 5.

[0077] [Table 2]

[0078] [Table 3]

[0079] [Table 4]

[0080] [Table 5]

[0081] The distribution of crystal orientation and |ΔHV| of the titanium alloy material obtained through the above process were investigated by the following procedure, and a cutting test was conducted to evaluate the machinability.

[0082] (Distribution of crystal orientation) The T-section of the titanium alloy material was polished to a mirror finish by electrolytic polishing or colloidal silica polishing, and then measured using EBSD. EBSD measurements were performed over a 200 μm x 250 μm area, measuring 2 to 5 fields of view at 0.3 to 0.5 μm intervals. The angle (0 to 90°, divided in 5° increments) and area ratio distribution for each surface in each field of view relative to the longitudinal direction of the titanium alloy material were calculated.

[0083] The average value of all measurement points was calculated for the angle range formed with the longitudinal direction of each titanium alloy material. Note that since the hcp prism planes (10-10) and (11-20) exist in three directions, the area ratio calculated using the analysis software was further divided by the total for normalization. The area ratio in each direction was calculated based on the EBSD measurement results using analysis software (TSL Solutions OIM Analysis™ software (Ver. 8.1.0)).

[0084] (Hardness test) In the hardness test, the cross section perpendicular to the longitudinal direction and the cross section parallel to the longitudinal direction were used as the measurement surfaces, and measurements were taken at five points using a Vickers hardness tester with a test force of 1 kgf. |ΔHV| was calculated from the difference in the average hardness values ​​obtained for each cross section.

[0085] (Evaluation of machinability) A cutting test was conducted to evaluate machinability. In the cutting test, commercially available carbide cutting tips and commercially available water-soluble metalworking oil (Brother Corporation's Synergy 735) were used. The cutting was performed at a feed rate of 0.5 mm per cut, a peripheral speed of approximately 1000 mm / min, and a feed rate of approximately 150 mm / min. The chip shapes were evaluated. Chips that were spirally curled and short and broken were rated A. Chips slightly longer than A were rated B. Chips that were spirally curled and untangled were rated C, and ribbon-like and tangled were rated D. A was defined as a chip curl diameter of 5 mm or less and a chip length of 10 mm or less, and B was defined as a chip length greater than 10 mm. The results are shown in Table 6.

[0086] [Table 6]

[0087] Nos. 1 to 23, which satisfy the requirements of this embodiment, exhibited good machinability. In particular, Nos. 10 to 23, which satisfy the hardness requirement, exhibited even better machinability. Note that No. 17, which does not satisfy the requirement for the area ratio of the region forming an angle of 0 to 15° with the (0001) plane of the α phase, exhibited relatively good machinability, but produced longer chips than, for example, Nos. 12 to 16, which have the same composition.

[0088] On the other hand, Nos. 24 to 31, which did not satisfy the requirements of this embodiment, had poor machinability. In particular, Nos. 24 and 27 were not hot-worked into a rectangular shape in the first hot-working step, so the desired α-phase structure was not formed, and machinability was reduced. In addition, Nos. 25 and 28 were not formed into the desired α-phase structure because the strain rate in the second hot-working step was too slow, so processing heat was not generated sufficiently, and the desired α-phase structure was not formed, and machinability was reduced. In Nos. 26 and 29, the temperature conditions in the second hot-working step were outside the preferred range, so the desired α-phase structure was not formed, and machinability was reduced. In addition, Nos. 30 and 31 had chemical compositions that did not satisfy the requirements of this embodiment, so machinability was reduced.

Claims

1. The chemical composition, in mass%, is Al: 4.50% or more and 6.75% or less, C: 0.100% or less, N: 0.050% or less, H: 0.016% or less, O: 0.35% or less, Mo: 0% or more and 5.5% or less, V: 0% or more and 4.50% or less, Nb: 0% or more and 3.0% or less, Fe: 0% or more and 2.50% or less, Cr: 0% or more and less than 0.25% Ni: 0% or more and less than 0.15% Mn: 0% or more and less than 0.25% Si: 0% or more and 0.50% or less, The balance is Ti and impurities. The following formula (i) is satisfied: In a cross section perpendicular to the longitudinal direction, The area ratio of the region in which the (0001) plane of the α phase forms an angle of more than 65° and not more than 90° with respect to the longitudinal direction is 90% or more, The area ratio of the region in which the (10-10) plane of the α phase forms an angle of more than 55° and not more than 65° with respect to the longitudinal direction is 20% or more, The area ratio of the region in which the (11-20) plane of the α phase forms an angle of more than 25° and not more than 35° with respect to the longitudinal direction is 20% or more, Titanium alloy material in the form of rod, wire or plate. -4.0≦Mo+0.67V+0.28Nb+2.9Fe+1.6Cr+1.1Ni+1.6Mn-Al≦2.0...(i) However, each element symbol in the above formula represents the content (mass%) of each element contained in the titanium alloy material, and if the element is not contained, it is set to zero.

2. The chemical composition is, in mass %, Al: 4.50% or more and 6.40% or less, Fe: 0.50% or more and 2.10% or less, The titanium alloy material according to claim 1, comprising:

3. In a cross section perpendicular to the longitudinal direction, 3. The titanium alloy material according to claim 1, wherein the area ratio of the region in which the (0001) plane of the α phase forms an angle of 0° or more and 15° or less with respect to the longitudinal direction is 1% or less.

4. The titanium alloy material according to any one of claims 1 to 3, wherein the absolute value |ΔHV| of the difference between the Vickers hardness in the cross section perpendicular to the longitudinal direction and the Vickers hardness in the cross section parallel to the longitudinal direction satisfies the following formula (ii): 10≦|ΔHV|≦30...(ii) However, |ΔHV| in the above formula is defined as follows. |ΔHV|: When the test force is 1 kgf and hardness measurements are performed at five points on each of the cross section perpendicular to the longitudinal direction and the cross section parallel to the longitudinal direction, the absolute value of the difference between the average Vickers hardness on the cross section perpendicular to the longitudinal direction and the average Vickers hardness on the cross section parallel to the longitudinal direction.

5. A titanium alloy material according to any one of claims 1 to 4, wherein when a cutting test is conducted with a cutting amount of 0.5 mm per run, a peripheral speed of 1000 mm / min, and a feed rate of 150 mm / min, the curl diameter of the discharged chips is 5 mm or less.

6. 6. The titanium alloy material according to claim 5, wherein the length of the chip is 10 mm or less.

7. A part using the titanium alloy material according to any one of claims 1 to 6.

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