JP7780703B2 - Titanium alloy materials and parts - Google Patents

Description

The present invention relates to titanium alloy materials and parts.

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 must have a high specific strength and be easy to reduce in weight. Lightweight parts are also required for a variety of machinery other than transportation vehicles, including the drive units and rotating bodies of precision equipment. Many of these parts are cut into 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 they must also be easy to cut.

Titanium alloy materials have a high specific strength and are lightweight. In particular, α+β titanium alloy materials such as those described in Patent Document 1 have high strength among titanium alloy materials and are suitable as materials for the above-mentioned parts.

International Publication No. 2013/094647

However, α+β titanium alloy materials have the problem that chips produced during cutting can become tangled or clog the tool, making them difficult to cut. The object of the present invention is to provide a titanium alloy material with good machinability and a part made from it.

The present invention was made to solve the above problems, and is summarized as follows: The titanium alloy material and parts using the same.

(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.

(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

(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 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 the above (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 (1) to (4) above, in which the curl diameter of the discharged chips is 5 mm or less.

(6) The titanium alloy material described in (5) above, wherein the length of the chips is 10 mm or less.

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

(8) A part made using the titanium alloy material described in any one of (1) to (7) above.

The present invention makes it possible to obtain titanium alloy materials with excellent machinability.

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. FIG. 2 is a photograph of chips discharged when cutting the conventional example, invention 1, and invention 2. FIG. 3 is a diagram schematically showing the hot working method in the first hot working.

The inventors conducted research into the machinability of titanium alloy materials and obtained the following findings (a) to (c):

(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 from α+β titanium alloy materials tend to be irregularly shaped and long. Chip shapes like this tend to easily become tangled around the cutting tool or tangle with each other. This is thought to result in reduced machinability.

(b) The reason why chips from α+β titanium alloys are long and irregularly shaped is that, in addition to being strong, α+β titanium alloys also have low thermal conductivity and highly reactive physical properties. These physical 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 demonstrated that controlling the crystal orientation of the α phase is effective.

(c) By controlling the crystal orientation of the α phase, the structure is one in which 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.

One embodiment of the present invention was developed based on the above findings. Each requirement of this embodiment will be explained in detail below.

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

Al: 4.50% or more and 6.75% or less Al (aluminum) is an element that dissolves in the α phase to strengthen the α phase. In α+β type titanium alloy materials, to obtain this effect, 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.

C: 0.100% or less Carbon (carbon) 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.

N: 0.050% or less N (nitrogen) dissolves in the α phase to strengthen it. However, excessive N content may result in excessive nitride formation, resulting in a decrease in 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-mentioned effects are to be obtained, the N content is preferably set to 0.001% or more, and more preferably set to 0.002% or more.

H: 0.016% or less H (hydrogen) is an impurity element contained in titanium alloy materials. Excessive H content may 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. Note that excessive reduction of H increases manufacturing costs, so the H content is preferably set to 0.001% or more.

O: 0.35% or less O (oxygen) dissolves in the α phase to strengthen the α phase. However, excessive O content may 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 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.

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.

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 contained 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. Therefore, 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, in order to obtain the above-mentioned effects, the V content is preferably set to 0.10% or more.

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 may increase, resulting in a decrease in strength. Furthermore, the β-phase ratio may increase even in the high temperature range, making it impossible to obtain the above-mentioned effects. 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.

Fe: 0% or more and 2.50% or less Fe (iron) 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 cause Fe segregation, which may result in a decrease in manufacturability and toughness/ductility. Furthermore, the β phase ratio may increase, making it impossible to achieve the above-mentioned effects. 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 achieve the above-mentioned effects, the Fe content is preferably set to 0.05% or more, and more preferably 0.10% or more.

Cr: 0% or more and less than 0.25% Cr (chromium) has the effect of stabilizing the β phase and improving the strength of the titanium alloy material. 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, in order to obtain the above-mentioned effects, the Cr content is preferably set to 0.03% or more.

Ni: 0% or more and less than 0.15% Ni (nickel) has the effect of stabilizing the β phase and improving the strength of the titanium alloy material. Therefore, it may be contained as needed. However, if Ni is contained in excess, Ni segregation 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 Ni content is set to less than 0.15%. The Ni content is preferably set to 0.10% or less. On the other hand, in order to obtain the above-mentioned effects, the Ni content is preferably set to 0.03% or more.

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 added as needed. However, excessive Mn content may result in reduced manufacturability and toughness/ductility due to segregation and evaporation of Mn. Furthermore, the β phase ratio may increase, making it impossible to achieve 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 achieve the above-mentioned effects, the Mn content is preferably set to 0.03% or more.

Si: 0% or more and 0.50% or less Si (silicon) 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, in order 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.

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.

If the side value in formula (i) is less than -4.0, the amount of β phase will be excessively small, which may prevent the above-mentioned effects from being achieved and make it difficult to improve machinability. For this reason, the side value in formula (i) is set to -4.0 or greater. The side value in formula (i) is preferably set to -3.5 or greater, and more preferably set to -3.0 or greater. On the other hand, if the side value in formula (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. For this reason, the side value in formula (i) is set to 2.0 or less. The side value in formula (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 formula (i), it can be said to be essentially an α+β type alloy.

In the chemical composition of this embodiment, the balance is Ti and impurities. Here, "impurities" refers to components that are mixed in during the industrial production of titanium alloy material due to various factors in the raw materials, such as titanium sponge, and the manufacturing process, 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 should be controlled as there is a risk of their content increasing. It is preferable to keep the total content of these metal elements to less than 0.3%, and each element individually to 0.1% or less.

2. Crystal orientation and hardness 2-1. Investigation of crystal orientation and hardness The following investigation was carried out regarding the machinability of the titanium alloy material of this embodiment. Titanium alloy wire rods (α+β titanium alloy) 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 wire rods (hereinafter simply referred to as "T cross section").

Figure 1 shows the distribution of the α-phase crystal orientation for each titanium alloy wire rod. It can be seen that, compared to the conventional material, Inventions 1 and 2 have α-phase orientation concentrated in a narrow, fixed range at all extremes of the (0001), (10-10), and (11-20) planes. On the other hand, with 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.

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 per run, a peripheral speed of 1000 mm/min, and a feed rate of 150 mm/min. The chips from the conventional material were unevenly curled and tangled in some places, giving them 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, like invention 1, had a more regular curl, and were shorter in length than invention 1. This shows that invention 1 has superior machinability to the conventional example, and invention 2 has superior machinability to invention 1.

Furthermore, when 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 was calculated for these titanium alloy rods and wires, the |ΔHV| was 7 for the conventional example and invention 1, while the |ΔHV| for invention 2 was 20.

2-2. Crystal Orientation In light of the above, in the titanium alloy material of this embodiment, the orientation distribution of the α-phase crystal orientation is controlled within the following ranges, with the longitudinal direction as the reference. 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.

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 effect of improving 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.

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° or more and 15° or less with respect to the longitudinal direction is preferably 1% or less. It is desirable for the (0001) plane of the α phase to be at an angle of more than 65° and less than 90° with respect to the longitudinal direction, but compared to this angle range of more than 65° and less than 90°, the angle between 0 and 15° has a significantly different orientation. For this reason, if this area ratio exceeds 1%, the chip length tends to be slightly longer, although it will be 10 mm or less.

2-2-2. (10-10) Plane 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 achieved. For this reason, 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. The upper limit of this area ratio is not particularly limited, but due to the symmetry of the crystal, the upper limit is usually considered to be around 45%.

2-2-3. (11-20) Plane 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 achieved. For this reason, 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. The upper limit of this area ratio is not particularly limited, but due to the symmetry of the crystal, the upper limit is usually considered to be around 45%.

2-2-4. Measurement of the Area Ratio of Each Plane 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, with the longitudinal direction set to 0° and the perpendicular direction to 90°, 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 range of angles.

The above 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 x 250 μm is observed in 2 to 5 fields of view, with a step interval of 0.3 to 0.5 μm.

As described above, for each hcp plane (each orientation) in each field of view, a distribution of the angle (0 to 90° divided in 5° increments) and area ratio with respect to the longitudinal direction of the titanium alloy material is created. The average value of all measurement points within a specified range of angles with respect to the longitudinal direction of the titanium alloy material is calculated. Since the hcp cylindrical planes (10-10) and (11-20) exist in three directions, the area ratios calculated using the analysis software are further divided by the total and normalized (to a total of 1). Based on the EBSD measurement results, the area ratios in each direction can be calculated using analysis software (OIM Analysis™ software (Ver. 8.1.0) manufactured by TSL Solutions, Inc.).

2-3. Hardness As described above, in order 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):

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 performed at five points on each of the cross sections perpendicular to the longitudinal direction.

If |ΔHV| is less than 10, the effect of improving machinability will be limited. In other words, the effect of stabilizing chip deformation will be insufficient, and the chip deformation direction may fluctuate or the chip may not be broken, resulting in the chip shape described in 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 realistic 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.

In hardness testing, the measurement surfaces are a cross section perpendicular to the longitudinal direction and a cross section parallel to the longitudinal direction, 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.

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

3. Targeted Properties In the titanium alloy material of this embodiment, when a predetermined cutting process is performed, it is preferable that the discharged chips satisfy the following range 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 per cut, 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 curl diameter, it is preferable that the length of the chips is 10 mm or less.

4. Shape of Titanium Alloy Material As described above, the shape of the titanium alloy material of this embodiment may be, for example, a rod or wire, a plate, etc. Note that the specific dimensions of the shape are not important.

5. Components Components of this embodiment may be manufactured using the titanium alloy material of this embodiment described above.

6. Manufacturing Method A preferred manufacturing method for the titanium alloy material according to this embodiment will be described. The titanium alloy material according to this embodiment can be stably manufactured by the method described below.

Hot-worked materials such as cast pieces, slabs, blooms, and billets of titanium alloys having chemical compositions within the above-mentioned ranges are manufactured. There are no particular limitations on the method for manufacturing the hot-worked materials; conventional methods can be used. The resulting hot-worked materials are then subjected to a first hot working process (hereinafter simply referred to as "first hot working") and a second hot working process (hereinafter simply referred to as "second hot working"). These two hot working steps are described below.

6-1. First Hot Working Step 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 for the production of a titanium alloy material with the above-mentioned crystal orientation. Specifically, hot working is performed in a temperature range where the β phase fraction of the titanium alloy material is 20% or more. That is, 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 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 subsequent processes. The β phase fraction refers to the minimum β phase fraction that can be calculated using the method described below.

In the first hot working step, hot working is performed on a rectangular cross section, as shown in Figure 3. During this hot working step, reduction is applied from all four peripheral faces. The distribution of reductions can be adjusted as appropriate, but it is preferable to satisfy the total area reduction rate described below.

Hot working a rectangular cross section allows for more uniform deformation all the way to the interior. Furthermore, by pressing down the overhangs that occur on surfaces other than the pressed surface, it is possible to achieve the same deformation on the surface 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 interior and surface.

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 α-phase with the desired angle range on each surface. 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 still in the rectangular cross section. 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 still in the rectangular cross section, and then hot worked to a circular or rectangular cross section. The total area reduction when hot working from a rectangular cross section to a rectangular cross section is preferably 50% or more, and more preferably 60% or more.

The rectangular cross section mentioned above only needs to have four corners, and may also have R, chamfers, etc.

6-2. Second hot working process In the second hot working process, 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 a transformation from β to α occurs, it is desirable to actively generate heat during working to cause the transformation from α to β and continue working.

By performing this second hot working, the structure formed during the first hot working can be developed into a structure with the desired crystal orientation. In order to achieve the above-mentioned structure control during the second hot working, the material is heated to a temperature range where the β-phase fraction is 30-60%, 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 maintain a temperature where the β-phase fraction is 7% or more and perform hot working. During hot working, it is more preferable to maintain a temperature range where the β-phase fraction is at least 10%.

Because 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 suppresses temperature drop and makes it easier to 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 higher. This allows for active utilization of processing heat. If the strain rate during processing is less than 10/s, sufficient processing heat will not be generated, resulting in a large temperature drop during hot working, making it impossible to achieve the desired crystal orientation. Therefore, the strain rate during processing is preferably 10/s or higher, and more preferably 15/s or higher.

Note that if the strain rate during processing exceeds 50/s, excessive processing heat will be generated, making it impossible to obtain the desired structure. For this reason, the strain rate during processing should be 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° to 15° with respect to the longitudinal direction to 1% or less.

In the second hot working step, unlike the first hot working step, there are no particular restrictions on the cross-sectional shape during working. The cross-sectional shape may be either circular or rectangular. The total area reduction during working is 70% or more, and preferably 80% or more. After this second hot working step, it is recommended that any processing that results in an area reduction of more than 5%, such as cold rolling, wire drawing, or drawing, be avoided, as this would disrupt the crystal orientation of the titanium alloy material.

Before or after the first and second hot working steps, surface conditioning, shape correction, heat treatment, etc. may be performed as needed. 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 below 500°C, and then the second hot working step may be performed, or the second hot working step may be performed in two or three separate steps. If heat treatment is performed, it may be performed at a temperature below the β transformation point -50°C. The above-mentioned hot working refers to either hot rolling or hot forging, or a combination of both.

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

After the first and second hot working steps, cooling can be carried out as needed to obtain an α+β titanium alloy material.

The following examples will explain the titanium alloy material according to the present invention in more detail, but the present invention is not limited to these examples.

A hot-worked material having the chemical composition shown in Table 2 was manufactured. This hot-worked material was subjected to first and second hot working processes under the manufacturing conditions shown in Table 3 to obtain a titanium alloy material. The changes in cross-sectional shape during the first and second hot working processes are shown in Tables 4 and 5. That is, items A to D in the section on changes during processing in the first hot working step (Table 4) in Table 3 correspond to steps A to D in Table 4, respectively. Similarly, items 1 to 4 in the section on changes during processing in the second hot working step (Table 5) in Table 3 correspond to steps 1 to 4 in Table 5.

The crystal orientation distribution and |ΔHV| of the titanium alloy material obtained through the above process were investigated using the following procedure, and cutting tests were conducted to evaluate its machinability.

(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. In the EBSD measurement, a 200 μm × 250 μm area was measured in steps of 0.3 to 0.5 μm, covering 2 to 5 fields of view. For each surface in each field of view, the angle (0 to 90° divided in 5° increments) between the longitudinal direction of the titanium alloy material and the surface area was measured.

The average value of all measurement points was calculated for the angle range formed with the longitudinal direction of each titanium alloy material. Since the hcp cylindrical surfaces (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. Based on the EBSD measurement results, the area ratio in each direction was calculated using analysis software (OIM Analysis™ software (Ver. 8.1.0) manufactured by TSL Solutions Co., Ltd.).

(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 measurement surfaces, and measurements were made 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.

(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 (Synergy 735 manufactured by Brother Corporation) were used. 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, and the shape of the chips was evaluated. Chips that were spirally curled and short and broken were evaluated as A. Chips that were slightly longer than A were evaluated as B. Chips that were spirally curled and not tangled were evaluated as C, and chips that were ribbon-like and tangled were evaluated as D. Here, A represented a chip curl diameter of 5 mm or less and a chip length of 10 mm or less, and B represented a chip length of more than 10 mm. The results are shown in Table 6 below.

Nos. 1 to 23, which meet the requirements of this embodiment, exhibited good machinability. In particular, Nos. 10 to 23, which meet the hardness requirements, exhibited even better machinability. Note that No. 17, which does not meet the requirements for the area ratio of the α-phase (0001) plane's (area ratio) angle of 0 to 15 degrees, exhibited relatively good machinability, but produced longer chips than Nos. 12 to 16, which have the same composition.

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, resulting in poor machinability. Furthermore, Nos. 25 and 28 were not hot-worked to the desired α-phase structure because the strain rate in the second hot-working step was too slow, resulting in insufficient processing heat and poor machinability. Nos. 26 and 29 were hot-worked outside the preferred temperature range in the second hot-working step, so the desired α-phase structure was not formed, resulting in poor machinability. Furthermore, Nos. 30 and 31 had poor machinability because their chemical compositions did not satisfy the requirements of this embodiment.

Claims (7)

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. 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: 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. 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. The titanium alloy material according to any one of claims 1 to 4, wherein when a cutting test is carried out under the conditions of a single removal rate of 0.5 mm, a peripheral speed of 1000 mm/min, and a feed rate of 150 mm /min, the curl diameter of discharged chips is 5 mm or less. The titanium alloy material according to claim 5, wherein the length of the chips is 10 mm or less. A part made of the titanium alloy material according to any one of claims 1 to 6 .