JP2010189735A - Titanium alloy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a titanium alloy whose work hardening is suppressed so as to obtain stable, high workability while maintaining its high strength. <P>SOLUTION: The titanium alloy has a chemical composition at least comprising Nb and O among Nb, Zr, Ta and O, and in which Nb equivalent defined by formula (1): Nb equivalent(at%)=Nb content(at%)+0.875×Zr content(at%)+0.75×Ta content(at%)+4×O content(at%) is 32 to 39 at%; wherein, the O content is 0.5 to 1.5 at%, the Zr content is ≤4 at%, and the balance Ti with inevitable impurities; wherein, the hardness of the solution-treated material is ≥200 Hv, the hardness of the 95% cold-worked material is ≤300 Hv, and a work hardening rate defined by formula (2): the work hardening rate (%)=äthe hardness (Hv) of the 95% cold-worked material-the hardness (Hv) of the solution-treated material}/äthe hardness (Hv) of the solution-treated material}×100 is ≤20%. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a titanium alloy, in particular, a β-type titanium alloy having a low work hardening rate and excellent workability.

  Titanium alloys are excellent in specific strength and corrosion resistance, and are therefore widely used in special fields such as aviation, space, deep sea, and chemical plants. In particular, β-type titanium alloys are characterized by high corrosion resistance and low elastic modulus, and are expected to be applied to fastening members such as screws used in corrosive environments.

  β-type titanium alloys are superior in workability compared to α-type and α + β-type titanium alloys, but depending on the alloy composition, ω-phase may be generated during quenching or processing, resulting in embrittlement or α ”martensite transformation. In many cases, remarkable work hardening occurs and workability deteriorates.

  In Patent Document 1, the Mo equivalent defined by the linear form of each content of Mo, V, W, Nb, Ta, Fe, Cr, Ni, Co, Cu, and Al is in the range of 3 to 11% by mass. Contains one or more of these alloy elements, contains 0.3 to 3% by mass of interstitial solid solution elements consisting of one or more of O, N or C, and Al content is 1.8% by mass or less A titanium alloy having excellent workability and having a low Young's modulus, which is characterized by being a β single phase at least at room temperature, is disclosed.

  Patent Document 2 includes, by mass ratio, Mo: 13.5 to 16.9%, Nb: 0.5 to 5.5%, Zr: 0.5 to 5.0%, and O: 0.20 or less. A biomedical β-type titanium alloy is disclosed, characterized in that it contains Ti and the inevitable impurities, and the β transformation point is in the range of 700 to 750 ° C.

  However, in order to ensure high workability stably while maintaining high strength, an alloy design from the viewpoint of suppressing work hardening has been further required.

JP 2004-162171 A Japanese Patent Laid-Open No. 2003-73761

  An object of this invention is to provide the titanium alloy which suppressed work hardening so that high workability may be obtained stably, maintaining high intensity | strength.

In order to achieve the above object, according to the present invention, at least Nb and O among Nb, Zr, Ta, and O are contained, and the following formula (1):
Nb equivalent (at%) = Nb content (at%) + 0.875 × Zr content (at%) + 0.75 × Ta content (at%) + 4 × O content (at%) (1) )
The Nb equivalent defined by the formula is 32 to 39 at%, the O content is 0.5 to 1.5 at%, the Zr content is 4 at% or less, and the balance has a chemical composition consisting of Ti and inevitable impurities. And
The hardness of the solution treatment material is 200 Hv or more, the hardness of the 95% cold-worked material is 300 Hv or less, and the following formula (2):
Work hardening rate (%) = {95% hardness of cold worked material (Hv) −hardness of solution treated material (Hv)} / {hardness of solution treated material (Hv)} × 100. (2)
A titanium alloy characterized in that the work hardening rate defined by is 20% or less is provided.

  According to the present invention, the Nb equivalent, O content, and Zr content are limited within the specified ranges, and the hardness and work hardening rate are specified to ensure high workability while maintaining high strength. Can do.

  In the present invention, the martensitic transformation start temperature Ms point is maintained at a temperature sufficiently lower than room temperature in order to improve cold workability. Here, when this inventor defined Nb equivalent by Formula (1), it discovered newly that Ms point could be represented by Formula (1A) by Nb equivalent.

Nb equivalent (at%) = Nb content (at%) + 0.875 × Zr content (at%) + 0.75 × Ta content (at%) + 4 × O content (at%) (1) )
Ms point (K) = 1380−43 × Nb equivalent (at%) (1A)
The Nb equivalent defined by the formula (1) is when the other elements in the formula (1) are increased by 1 at% in place of Nb with respect to the decrease amount of the Ms point when the Nb is increased by 1 at%. The ratio of the amount of decrease in the Ms point is expressed in a linear form as a coefficient of each element.

  In the present invention, the reason for limiting the chemical composition of the titanium alloy is as follows.

[Nb equivalent is 32 to 39 at%]
When the Nb equivalent is less than 32 at%, the work hardening rate exceeds 20%. By setting the Nb equivalent to 32 at% or more, martensitic transformation can be completely suppressed, and the work hardening rate can be easily reduced to 20% or less. On the other hand, if the Nb equivalent exceeds 39 at%, the melting point becomes too high, which is disadvantageous in production, and the material cost also increases. Therefore, the Nb equivalent is limited to 32 to 39 at%.

[O content is 0.5-1.5at%]
Even if the Nb equivalent is in the specified range of 32-39 at%, if O is less than 0.5 at%, the work hardening rate exceeds 20%. When O exceeds 1.5 at%, solution hardness will exceed 300 Hv and ductility will fall. Accordingly, the O content is limited to 0.5 to 1.5 at%.

[Zr content is less than 4at%]
Even if the Nb equivalent is 32 to 39 at% and the O content is 0.5 to 1.5 at%, if the Zr content exceeds 4 at%, the hardness exceeds 300 Hv during processing, and processing becomes difficult. .

  Moreover, the reason which limited hardness and work hardening rate is as follows.

[Hardness: 200Hv or more in solution state, 300Hv or less in 95% processing state]
When the hardness in the solution state is less than 200 Hv, the strength is too low, and rather supports various processes such as rolling. On the other hand, if the hardness in the 95% processed state exceeds 300 Hv, the ductility is insufficient. Therefore, the hardness is 200 Hv or more in the solution state and 300 Hv or less in the 95% processed state.

[Work hardening rate: 20% or less]
In the present invention, the work hardening rate is defined by the following formula (2).

Work hardening rate (%) = {95% hardness of cold rolled material (Hv) −hardness of solution treated material (Hv)} / {hardness of solution treated material (Hv)} × 100
When the work hardening rate exceeds 20%, high workability can be secured stably. Therefore, the work hardening rate in the 95% processed state with respect to the solution state is 20% or less.

  Titanium alloys having various chemical compositions shown in Table 1 were prepared. Alloy numbers 1 to 8 are examples of the present invention, and alloy numbers 9 to 18 are comparative examples. The production procedure and conditions were as follows.

<Ingot production>
Sponge Ti (purity 99.99%), massive Nb (purity 99.9%), massive Ta (purity 99.9%), massive Zr (purity 99.9%), TiO ₂ powder are used as alloy raw materials, A button-shaped alloy ingot was produced by a consumable tungsten electrode type Ar arc melting method. In order to obtain a homogeneous alloy ingot, the operation of turning the once solidified ingot upside down and re-dissolving was repeated for a total of six times of melting.

<Homogenization treatment (also serves as solution treatment)>
In order to further reduce segregation, homogenization was performed under the following conditions. This treatment also serves as a solution treatment.

The ingot was vacuum sealed in a quartz tube, held at 1273 K for 7.2 ks in an atmospheric furnace, and then air-cooled to room temperature. The ingot was taken out from the quartz tube, and the oxide film formed on the ingot surface was removed by etching. Etching was performed in a 343K water bath using a mixed solution of H ₂ O: HNO ₃ : HF = 5: 4: 1 as an etchant.

<Cold processing>
The ingot was subjected to cold working with a working rate of 95% by cold rolling using a four-stage roll to form a thin plate having a thickness of about 0.5 mm.

  The Vickers hardness of the ingot before cold rolling and the thin plate after cold rolling was measured. The results are shown in Table 1.

  In Alloy Nos. 1 to 8 of the present invention example, the hardness (solution treated material, 95% cold worked material) and work hardening rate all satisfy the specified range of the present invention.

  On the other hand, since alloy numbers 9 to 18 of the comparative examples have at least a part of the chemical composition outside the specified range of the present invention, at least one of hardness and work hardening rate satisfies the specified range of the present invention. Absent.

  That is, in alloy numbers 9 to 10 and 13 to 15 of the comparative examples, the Nb equivalent is less than the specified lower limit value 32 at% of the present invention, so the work hardening rate exceeds the specified upper limit value 20% of the present invention.

  In Alloy Nos. 11 and 12 of Comparative Examples, the Nb equivalent is within the specified range of the present invention, but the O content is less than the specified lower limit of 0.5 at% of the present invention, so the work hardening rate is specified by the present invention. The upper limit is over 20%. On the other hand, in the alloy numbers 16 and 17 of the comparative examples, the Nb equivalent is within the specified range of the present invention, but the O content exceeds the specified upper limit of 1.5 at% of the present invention. Is over Hv300.

  In Alloy No. 18 of the comparative example, the Nb equivalent is within the specified range of the present invention, and the O content is also within the specified range of the present invention, but the Zr content exceeds the specified upper limit of 4 at% of the present invention. The hardness of the 95% cold-worked material exceeds 300 Hv.

  ADVANTAGE OF THE INVENTION According to this invention, the titanium alloy which suppressed work hardening is provided so that high workability can be obtained stably, maintaining high intensity | strength.

Claims (1)

It contains at least Nb and O among Nb, Zr, Ta, and O, and the following formula (1):
Nb equivalent (at%) = Nb content (at%) + 0.875 × Zr content (at%) + 0.75 × Ta content (at%) + 4 × O content (at%) (1) )
The Nb equivalent defined by the formula is 32 to 39 at%, the O content is 0.5 to 1.5 at%, the Zr content is 4 at% or less, and the balance has a chemical composition consisting of Ti and inevitable impurities. And
The hardness of the solution treatment material is 200 Hv or more, the hardness of the 95% cold-worked material is 300 Hv or less, and the following formula (2):
Work hardening rate (%) = {95% hardness of cold worked material (Hv) −hardness of solution treated material (Hv)} / {hardness of solution treated material (Hv)} × 100. (2)
A titanium alloy characterized in that the work hardening rate defined by is 20% or less.