β-type titanium alloy wire

Abstract

To provide a β-type titanium alloy wire that has excellent cold processability and high strength.SOLUTION: A β-type titanium alloy wire has a fraction of α phase in the metal structure of less than 1%, and the remainder is β phase, and in which the average crystal grain size of the β phase is 30 μm or less, and, for the total number of crystal grains whose Grain Orientation Spread value (GOS value) is measured by electron backscatter diffraction method, the ratio of the number of crystal grains having a GOS value of 2% or less is 70% or more, and the ratio of the number of crystal grains having a GOS value of 5% or more is less than 10%.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a β-type titanium alloy wire.

Background Art

[0002] Titanium and titanium alloys are lightweight, have excellent specific strength, and also have excellent corrosion resistance. Their wires are suitably used for aircraft materials, automotive part materials, medical device materials, and the like.

[0003] Among titanium alloys, β-type titanium alloys are excellent in cold workability, as disclosed in, for example, Patent Document 1. Patent Document 1 also discloses that β-type titanium alloys can be strengthened by performing aging treatment to precipitate the α-phase from the metastable β-phase.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, as a result of investigations by the present inventors, it has been found that there is room for improvement in the cold workability of β-type titanium alloys, and there is also room for improvement in the strength of β-type titanium alloys.

[0006] The present invention has been made in view of such problems, and an object thereof is to provide a β-type titanium alloy wire having excellent cold workability and high strength.

Means for Solving the Problems

[0007] As a result of various investigations, the present inventors have found that the above object can be achieved by a titanium alloy wire having the following configuration.

[0008] A β-type titanium alloy wire according to one aspect of the present invention has a metallic structure in which the fraction of the α phase is less than 1%, and the remainder is the β phase. The average crystal grain size of the β phase is 30 μm or less. The Grain Orientation Spread (GOS) value was measured by electron backscatter diffraction for the total number of crystal grains, and the GOS value was 2 ° The proportion of the number of crystal grains is as follows: 79.1 % or more, and GOS value is 5 ° The proportion of the number of crystal grains that are above 5.3% or less dea the law of nature, In mass%, V:10~25%、 Cr:0.5~5%、 Sn: 0.5~5%, and Al:0.5~5%、 It contains, with the remainder being titanium and unavoidable impurities. ru. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a β-type titanium alloy wire that has excellent cold workability and high strength. [Modes for carrying out the invention]

[0010] The inventors of this invention have conducted a detailed study on the cold workability and strength of β-type titanium alloys, focusing on the microstructure of β-type titanium alloy wires.

[0011] As a result, we discovered that by making the β-type titanium alloy wire substantially a single β phase, making the grain size fine, and allowing sufficient recrystallization of the grains, good cold workability and good strength can be achieved, thus completing the present invention.

[0012] The following describes a β-type titanium alloy wire relating to one embodiment of the present invention.

[0013] The β-type titanium alloy wire according to this embodiment has an α-phase fraction in the metal structure of less than 1%, the balance being the β-phase, the average crystal grain size of the β-phase being 30 μm or less, and the Grain Orientation Spread value (GOS value) measured by the electron backscatter diffraction method being 2 ° or less, and the proportion of the number of crystal grains being 70% or more, and the proportion of the number of crystal grains with a GOS value of 5 ° or more being less than 10%. The β-type titanium alloy wire according to this embodiment has excellent cold workability and high strength. Further, the β-type titanium alloy wire according to this embodiment can be used for aircraft materials, automobile part materials, medical device materials, and the like.

[0014] 〈β-Type Titanium Alloy〉 As the β-type titanium alloy used for the β-type titanium alloy wire according to this embodiment, a general β-type titanium alloy can be used. For example, a Ti-15V-3Cr-3Sn-3Al alloy, a Ti-22V-4Al alloy, a Ti-15Mo-5Zr-3Al alloy, or the like can be used.

[0015] Further, the β-type titanium alloy wire according to this embodiment preferably contains, as a chemical composition in mass%, V: 10 to 25%, Cr: 0.5 to 5%, Sn: 0.5 to 5%, and Al: 0.5 to 5%, with the balance being Ti and inevitable impurities. Hereinafter, this chemical composition is also referred to as "Composition A". Further, hereinafter, "mass%" for the chemical composition is simply described as "%".

[0016] The actions and numerical ranges of the respective alloy elements constituting Composition A will be described.

[0017] (a) V: 10 to 25% V is an element that stabilizes the β phase by solid dissolving in Ti, which constitutes the base material of the titanium alloy. To achieve this effect, a V content of 10% or more is preferable. If the V content exceeds 25%, the titanium alloy becomes a single β phase, but its age hardening properties are poor, and the time required for aging treatment becomes longer. In addition, excessive V content increases the specific gravity of the titanium alloy and also increases raw material costs, making it uneconomical. Therefore, a V content of 25% or less is preferable.

[0018] (b)Cr: 0.5~5% Cr is an element that enhances the age-hardening ability of β-type titanium alloys. To achieve this effect, a Cr content of 0.5% or more is preferable, and 1% or more is more preferable. If the Cr content is excessive, the β-stabilization becomes excessively high, reducing the age-hardening ability and lowering the strength after aging treatment. Therefore, a Cr content of 5% or less is preferable.

[0019] (c) Sn: 0.5~5% Sn (Titanium) is an element that promotes the precipitation of the α phase (hereinafter referred to as "aging precipitation") by aging treatment of β-type titanium alloys, stabilizes the precipitated α phase, and suppresses the formation of the ω phase. Therefore, Sn has the effect of widening the appropriate temperature range for aging treatment and increasing the strength of β-type titanium alloy wire after aging treatment. To achieve these effects, a Sn content of 0.5% or more is preferable, and 1% or more is more preferable. If the Sn content is excessive, the hardness of the β-type titanium alloy wire will become excessively high, whether or not aging treatment is performed. Therefore, a Sn content of 5% or less is preferable.

[0020] (d) Al: 0.5~5% Al is an element that strengthens the α-phase that precipitates during aging, thereby increasing the strength of the β-type titanium alloy after aging treatment. To achieve this effect, an Al content of 0.5% or more is preferable, and 1% or more is more preferable. If the Al content is excessive, the strength of the β-type titanium alloy after solution heat treatment will increase, degrading its workability. Therefore, an Al content of 5% or less is preferable.

[0021] (e) Other elements Composition A may contain elements other than those listed in (a) to (d) above.

[0022] Mo is an element that has a similar effect to V. Therefore, Mo may be included instead of V, or both V and Mo may be included. When Mo is included instead of V, the Mo content is preferably 10-25%, and when both V and Mo are included, the total content of V and Mo is preferably 10-25%.

[0023] Furthermore, elements such as Nb, Ta, Mn, Co, and Ni also have the effect of stabilizing the β phase, so they may be included in appropriate amounts (0.5-5% in total).

[0024] (f) Remainder In composition A, in addition to the alloy components (a) to (e) above, the remainder consists of Ti and unavoidable impurities. Unavoidable impurities include N, C, H, Fe, and O. The content of these unavoidable impurities is 0.05% or less for N and H, 0.10% or less for C, 1.00% or less for Fe, and 0.25% or less for O. This makes it possible to suppress the inhibition of obtaining the effects of the β-type titanium alloy wire according to this embodiment.

[0025] <Metal structure> (Fraction of the α-phase) In the β-type titanium alloy wire according to this embodiment, the fraction of the α phase in the metal structure (hereinafter also referred to as "α fraction") is less than 1%, with the remainder being the β phase. In other words, the metal structure of the β-type titanium alloy wire according to this embodiment is substantially a single β phase. This improves the cold workability of the β-type titanium alloy wire. In β-type titanium alloys, the α phase precipitates from the grain boundaries of the β phase, and the α phase with a hexagonal close-packed structure is less deformable than the β phase with a body-centered cubic structure. Therefore, if the α fraction is 1% or more, the cold workability is poor, and cracks may occur when cold working is performed. The α fraction can be determined by quantifying the respective intensity peaks of the β phase and α phase obtained by X-ray diffraction of the β-type titanium alloy wire, and dividing the quantified amount of α phase by the sum of the amounts of β phase and α phase.

[0026] (Average grain size) In the β-type titanium alloy wire according to this embodiment, the average grain size of the β phase (hereinafter also simply referred to as "average grain size") is 30 μm or less. This improves the cold workability of the β-type titanium alloy wire and allows for sufficiently high strength. If the average grain size is greater than 30 μm and the grains are coarse, the cold workability is poor and sufficient strength cannot be obtained. The average grain size is preferably 25 μm or less. There is no particular lower limit for the average grain size, but it is preferably 5 μm or more. The average grain size can be determined by converting the average area of ​​the grains obtained by defining the area where the crystal orientation difference between adjacent measurement points is 15° or more as a grain boundary in a scanning electron microscope image taken using electron backscatter diffraction (EBSD) to the average grain size.

[0027] (GOS value) In the β-type titanium alloy wire according to this embodiment, the Grain Orientation Spread (GOS) value (GOS value) was measured by the EBSD method for the total number of crystal grains, and the GOS value was 2 ° The proportion of crystal grains that meet the following criteria is 70% or more, and the GOS value is 5 °The percentage of crystal grains that meet the above criteria is less than 10%.

[0028] The GOS value is the average orientation difference for each crystal grain, measured by the EBSD method using a scanning electron microscope. It is the average of the orientation differences between a single point within a crystal grain and all the measurement points that make up that grain. A smaller GOS value for a crystal grain indicates a smaller orientation difference within that grain and that recrystallization is more advanced. ° In this embodiment, "the following" means that recrystallization has progressed sufficiently in the crystal and is substantially completed. For a given crystal grain, "the GOS value is 5 ° In this embodiment, "the above" means that the recrystallization process in the crystal is insufficient and has not been completed.

[0029] The total number of crystal grains for which the GOS value was measured was 2 ° The proportion of crystal grains that are below 70% and the GOS value is 5 ° If the proportion of crystal grains that are above 10% is less than 10%, the cold workability of β-type titanium alloy wire can be improved. ° The percentage of crystal grains that are below 70%, or the GOS value is 5 ° If the proportion of crystal grains that are above this level is 10% or more, the cold workability is poor. GOS value is 2 ° The proportion of crystal grains that are below 90% is preferable. Also, the GOS value is 5 ° The percentage of crystal grains that meet the above criteria is preferably less than 1%.

[0030] When measuring GOS values, in order to understand the overall trend of the β-type titanium alloy wire being measured, the scanning electron microscope should be positioned so that 100 or more crystal grains, preferably 500 or more, are included in the field of view, and the GOS values ​​should be measured for all crystal grains within the field of view.

[0031] <Tensile strength> The β-type titanium alloy wire according to this embodiment has a tensile strength of 800 N / mm². 2Preferably, the tensile strength is 800 N / mm². 2 By doing so, the β-type titanium alloy wire can be made to have sufficiently high strength. The tensile strength of the β-type titanium alloy wire according to this embodiment is measured by a tensile test using a test piece with a diameter of 5.25 mm and a test piece length of 350 mm, with a gauge length of 200 mm and a tensile speed of 20 mm / min. This tensile test is performed for a number of tests N=3, and the average value is taken as the tensile strength of the β-type titanium alloy wire. Since the tensile strength of titanium alloy is greatly affected by the tensile speed, the tensile speed is fixed at 20 mm / min.

[0032] <Shape, dimensions> The β-type titanium alloy wire according to this embodiment does not have a specific dimension, but for example, its diameter can be 1 to 10 mm. The cross-sectional shape is not limited to a circle, but can be elliptical, square, or the like.

[0033] <Manufacturing method> The β-type titanium alloy wire according to this embodiment can be manufactured, for example, by cold working a raw wire material of β-type titanium alloy with a predetermined reduction ratio, holding the pre-worked wire obtained by cold working at a temperature above the β-transformation temperature for a predetermined time, and then water-cooling it.

[0034] More specifically, a raw wire of a β-type titanium alloy of a desired composition is cold-worked with a predetermined reduction ratio to obtain a pre-worked wire of the desired dimensions. This allows for the introduction of strain and lattice defects into the microstructure of the pre-worked wire.

[0035] Cold working to obtain pre-processed wire is preferably performed with a reduction ratio of 10-80% from the raw wire to the pre-processed wire. If the reduction ratio is less than 10%, sufficient strain and lattice defects cannot be introduced into the metal structure of the pre-processed wire, and the average grain size after recrystallization by solution heat treatment may not be able to be 30 μm or less. If the reduction ratio exceeds 80%, cracks and fractures may occur in the pre-processed wire. A reduction ratio of 50% or more is more preferable.

[0036] Next, a solution heat treatment is performed on the pre-processed wire, which involves holding it at a temperature above the β-transformation temperature for a predetermined time and then water-cooling, to obtain a β-type titanium alloy wire according to this embodiment.

[0037] In this embodiment, solution heat treatment is a heat treatment aimed at maintaining the pre-processed wire above its β transformation temperature to form a β single phase, and then rapidly cooling it to maintain the β single phase even at room temperature. In this embodiment, if the α fraction is less than 1%, the α phase may precipitate during the cooling of the solution heat treatment.

[0038] Furthermore, in this embodiment, the solution heat treatment also aims to promote the recrystallization of the metal structure of the pre-processed wire. By promoting the recrystallization of the metal structure of the pre-processed wire in which strain and lattice defects have been introduced, the average grain size of the β phase after solution heat treatment can be reduced to 30 μm or less. In addition, the GOS value is measured for the total number of crystal grains after solution heat treatment. ° The proportion of crystal grains that are below 70% and the GOS value is 5 ° The proportion of crystal grains that meet the above criteria can be less than 10%.

[0039] The solution heat treatment conditions vary depending on the chemical composition of the pre-processed wire. For example, in the case of composition A, it is preferable to set the holding temperature to 720-820°C, which is above the β-transformation temperature, and the holding time to 350-370 seconds. A holding temperature of 800°C or lower is more preferable. A holding time of 355 seconds or more is more preferable, and 365 seconds or less is more preferable.

[0040] The β-type titanium alloy wire according to this embodiment, obtained by the above manufacturing method, may be subjected to aging treatment after cold working. The β-type titanium alloy wire according to this embodiment has good cold workability, so it can be easily processed into a desired shape, and it has sufficient strength even without aging treatment. However, by performing aging treatment after cold working the β-type titanium alloy wire, the α phase is precipitated from the grain boundaries of the β phase, which can further improve the strength. The conditions for aging treatment are preferably such that the holding temperature of the β-type titanium alloy wire is 350 to 650°C and the holding time is 1 to 10 hours.

[0041] As described above, this specification discloses various aspects of technology, the main technologies among them are summarized below.

[0042] As described above, the β-type titanium alloy wire according to one aspect of the present invention has a metallic structure in which the fraction of α-phase is less than 1%, the remainder being β-phase, the average grain size of the β-phase is 30 μm or less, and the Grain Orientation Spread (GOS) value measured by electron beam backscatter diffraction is 2 for the total number of crystal grains. ° The proportion of crystal grains that meet the following criteria is 70% or more, and the GOS value is 5 ° The percentage of crystal grains that meet the above criteria is less than 10%.

[0043] This configuration makes it possible to obtain a β-type titanium alloy wire that has excellent cold workability and high strength.

[0044] The β-type titanium alloy wire with the above configuration may contain, by mass%, V: 10-25%, Cr: 0.5-5%, Sn: 0.5-5%, and Al: 0.5-5%, with the remainder being titanium and unavoidable impurities.

[0045] This configuration makes it possible to obtain β-type titanium alloy wire that has superior cold workability and high strength.

[0046] The β-type titanium alloy wire with the above configuration has a tensile strength of 800 N / mm². 2 That's fine too.

[0047] This configuration makes it possible to obtain β-type titanium alloy wire that has superior cold workability and high strength. [Examples]

[0048] <sample> The samples used were 5.25 mm diameter β-type titanium alloy wires having the chemical compositions of alloy types a and b shown in Table 1. Alloy types a and b satisfy composition A described above.

[0049] [Table 1]

[0050] <Method for preparing the sample> Samples were prepared under the conditions of No. 1 to 9 shown in Table 2. A cylindrical β-type titanium alloy raw wire with a predetermined diameter was cold-drawn to a diameter of 5.25 mm with a reduction ratio of 10 to 80% to obtain a pre-processed wire. This pre-processed wire was then heated using a solution heat treatment in an atmospheric furnace or an argon gas atmosphere furnace at a holding temperature of 680 to 820°C for a holding time of 348 to 379 seconds, and then directly or indirectly water-cooled to produce samples No. 1 to 12. Direct water cooling is the cooling of the heated pre-processed wire by directly immersing it in water. Indirect water cooling is the cooling of the heated pre-processed wire by passing it through a water-cooled pipe.

[0051] [Table 2]

[0052] The characteristic values ​​of the prepared samples No. 1 to 9, namely the average crystal grain size, α fraction, and GOS value, were measured, and the GOS value relative to the total number of crystal grains was 2. ° The following are the proportions of the number of crystal grains and the GOS value of 5 ° The proportion of crystal grains and tensile strength are shown in Table 3. Each characteristic value was measured using the method described above. GOS values ​​were measured for 100 or more crystal grains in each sample.

[0053] Samples No. 1 to 3 were comparative examples in which at least one of the crystal grain size, α fraction, and proportion of crystal grains satisfying each GOS value did not meet the requirements of the present invention. Samples No. 4 to 9 were examples of the present invention in which the crystal grain size, α fraction, and GOS value all met the requirements of the present invention.

[0054] [Table 3]

[0055] <Testing method for the sample> The obtained samples No. 1 to 9 were cut into 30 mm long pieces, and cap bolts were fabricated by cold heading of the pieces. The cap bolts were shaped according to JIS B 1176, with a nominal diameter of M6 and a length of 3 mm.

[0056] Cold heading was performed on 10 to 60 individual pieces of each sample. Those that could be processed into cap bolts without cracking were classified as "successful," while those that cracked were classified as "unsuccessful." Based on these results, the processing success rate for each sample was calculated. In Table 3, the processing success rate is indicated as follows: "◎" for over 90%, "○" for over 70%, "△" for 50-70%, and "×" for less than 50%.

[0057] <Test Results> Samples No. 4 to 9, which are embodiments of the present invention, all received a ○ or ◎ rating for processability, indicating good processability. In particular, samples No. 7 to 9, in which the proportion of crystal grains with a GOS value of 2% or less was 90% or more, received an ◎ rating for processability, indicating excellent processability. Furthermore, all samples No. 4 to 9 had a tensile strength of 800 N / mm². 2 The above indicates that it possessed good strength.

[0058] On the other hand, comparative examples No. 1 and 2 had a tensile strength of 847 N / mm². 2 Although the strength and other properties were good, the processability was poor, receiving a failing grade. This is likely due to the high alpha fraction of 3% or more.

[0059] Comparative example No. 3 had a workability rating of △, indicating poor workability, and its tensile strength was 800 N / mm². 2 It was less than 30 μm and also inferior in strength. This is because sample No. 3 had an average crystal grain size exceeding 30 μm, and the GOS value relative to the total number of crystal grains for which the GOS value was measured was 2 ° The proportion of crystal grains that were below the specified number was less than 70%, which is thought to be due to insufficient recrystallization.

Claims

1. The α-phase fraction in the metal structure is less than 1%, with the remainder being the β-phase. The average crystal grain size of the β phase is 30 μm or less. The Grain Orientation Spread (GOS) value was measured by electron beam backscatter diffraction, and the proportion of grains with a GOS value of 2° or less was 79.1% or more, and the proportion of grains with a GOS value of 5° or more was 5.3% or less. In mass percent, V: 10-25%, Cr: 0.5-5%, Sn: 0.5–5%, and Al: 0.5-5%, A β-type titanium alloy wire containing [a specific component], with the remainder being titanium and unavoidable impurities.

2. Tensile strength of 800 N / mm ２ The above is the β-type titanium alloy wire according to claim 1.

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