JP4476884B2 - Titanium alloy with excellent formability and method for producing the same - Google Patents

Description

  The present invention relates to a titanium alloy used for a member requiring high formability, such as a press heat-processed product such as a plate heat exchanger and a separator of a fuel cell, and a casing of a mobile phone, a mobile personal computer, a camera, and the like. .

  Titanium alloys are superior in specific strength because of their low density (light weight) and high strength compared to other general-purpose metals, and demand is growing mainly for aircraft materials and space development materials. In addition, in recent years, it has been widely used as materials for automobile parts, medical materials, eyeglass materials, and golf materials.

  By the way, titanium alloys can be classified into α-type, β-type, and α + β-type in which they are mixed depending on the phase of the structure formed at room temperature. α-type and α + β-type titanium alloys have an α-type composed of hexagonal crystals, have low ductility at room temperature, and are often processed warmly or hotly. On the other hand, β-type titanium alloys composed of body-centered cubic crystals have excellent ductility in the solution treatment state as compared with α-type and α + β-type titanium alloys, and can be molded at room temperature. Furthermore, since the β-type titanium alloy can be further strengthened by performing an aging heat treatment, it is expected to be applied to further uses in the future.

  Therefore, Patent Document 1 includes V: 4.0 to 10% (meaning mass%, hereinafter the same), Sn: 2.0 to 5.0%, Al: 2.0 to 4.0%, Strength and ductility after aging heat treatment including Cr: 6.0 to 10% or Cr: 5.0 to 9.0% and Fe: 0.3 to 3.5%, the balance being Ti and inevitable impurities An excellent β-type titanium alloy is disclosed.

  Other than the above, Ti-15V-3Cr-3Sn-3Al alloy, Ti-15Mo-5Zr-3Al alloy, Ti-3Al-8V-6Cr-4Mo-4Sn alloy, Ti-13V-11Cr-3Al alloy, etc. are also β-type titanium. Known as an alloy. Although these alloys can be formed at room temperature before aging heat treatment (after solution treatment) as described above, their proof stress (for example, 0.2% proof stress) is high, so that a springback phenomenon is likely to occur during processing, and more sophisticated The ductility is insufficient to perform the forming process, and the applicable shapes are limited. Therefore, it is used for members that require high formability (for example, press-molded products such as plate heat exchangers and fuel cell separators, mobile phones, mobile PCs, cameras, etc.) and new When β-titanium alloys are used in various applications, it is desirable to develop a new β-type titanium alloy with low yield strength after solution treatment, high ductility, excellent cold workability, and excellent formability. ing.

  Therefore, in Patent Document 2, V is 15 to 25%, Al is 2.5 to 5%, Sn is 0.5 to 4%, oxygen is 0.12% or less, the remainder is Ti, and unavoidable impurities. A β-type titanium alloy excellent in cold workability is disclosed.

Patent Document 3 contains Mo and / or Nb: 0.5 to 18 wt%, V: 13 to 19 wt%, Al: 0.5 to 6 wt%, Sn: 0.5 to 6 wt%, A β-type titanium alloy having the balance of Ti and inevitable impurities and having excellent cold workability and ductility is disclosed.
JP 2004-270009 A Japanese Patent No. 2669004 Japanese Patent No. 2936754

  In the β-type titanium alloys of Patent Documents 2 and 3, a relatively large amount of V must be contained in order to improve cold workability. In the β-type titanium alloy of Patent Document 3, Mo and A relatively expensive raw material such as Nb had to be included.

  Accordingly, an object of the present invention is to provide an inexpensive β-type titanium alloy having low formability after solution treatment, high ductility, and excellent formability having excellent cold workability.

  In order to solve the above-mentioned problems, the present inventor has intensively studied the action of various elements to be contained in the β-type titanium alloy. Conventionally, Sn is considered to have an action of suppressing the formation of ω phase, and as recognized in the above-mentioned Patent Documents 2 and 3, when contained in a large amount, the base material is formed by solid solution hardening after solution treatment. It was supposed to increase the strength and deformation resistance. However, it has been found that the β-type titanium alloy of the present invention can reduce the yield strength after the solution treatment and improve the ductility by containing Sn.

  Furthermore, Fe and Cr are considered to be desirably as small as possible because they significantly increase the strength (proof strength) of the substrate in the alloy after the solution treatment (for example, Patent Document 2). However, as a result of a study by the present inventors, even if a part of V is substituted with Fe and / or Cr, the formability of the alloy is not impaired, and the cost can be reduced by reducing the V content. The present inventors have found that it can be reduced and have completed the present invention.

The titanium alloy of the present invention is
V: 6 to 13% (meaning mass%, the same shall apply hereinafter),
Sn: 4-20%,
Fe: 0.3-3.0%
Cr: 0.3 to 4.5% is included, and the balance is composed of Ti and inevitable impurities, and is excellent in formability. The content of Fe and Cr is preferably in the range of 1.5 to 3.5% in terms of Fe equivalent expressed by the following formula.
Fe equivalent = [Fe] + [Cr] /1.5 (inside [] indicates the content% of the element)

  Further, it is preferable that Al is contained in an amount of 0.3 to 5.0% and / or Cu is 0.1% or more and less than 3.0%.

  Since the titanium alloy has an Fe content of 0.3 to 3.0%, sponge titanium containing 0.2 to 2% of Fe (hereinafter also referred to as lower sponge titanium), and other additive elements, Can be manufactured by melting.

  The titanium alloy of the present invention is excellent in formability because it has high ductility after solution treatment, low proof stress, and excellent cold workability. Furthermore, since the titanium alloy of the present invention can be manufactured using lower sponge titanium, the manufacturing cost can be reduced.

  The action of the alloy element in the β-type titanium alloy of the present invention will be described.

  V is an all-solid-solution type β-phase stabilizing element, and if it is less, ductility after solution treatment is significantly reduced, and cold workability is also deteriorated. Therefore, it is necessary to contain 6% or more (preferably 7% or more, more preferably 8% or more). However, even if contained excessively, the ductility is not improved, and the raw material cost is increased. Therefore, the upper limit of the content of V needs to be suppressed to 13% (preferably 12.5%, more preferably 12%).

Sn is a neutral element, and the titanium alloy of the present invention has an effect of improving ductility after solution treatment. Therefore, it is necessary to contain 4% or more (preferably more than 5%, more preferably more than 6%). On the other hand, Sn is an element with a higher density than Ti, so if it is contained in a large amount, the low density characteristics inherent in Ti will be impaired, and even if it is excessively contained, no improvement in ductility can be expected. In this case, Ti ₃ Sn precipitates and the ductility is lowered, which in turn increases the raw material cost. Therefore, the upper limit of the Sn content needs to be suppressed to 20% (preferably 15%, more preferably 12%).

  Fe is a eutectoid β-phase stabilizing element. If it is too small, stress-induced martensitic transformation occurs due to cold working, and an unstable β-phase is formed in the titanium alloy, which easily causes cracking during cold working. Become. Therefore, the Fe content needs to be 0.3% or more (preferably 0.5% or more, more preferably 1.0% or more). However, when Fe is contained in a large amount, the yield strength becomes too high due to solid solution hardening, and cold workability is impaired. Therefore, the upper limit of the Fe content needs to be suppressed to 3.0% (preferably 2.5%, more preferably 2.0%).

  Like Fe, Cr is a eutectoid β-phase stabilizing element, and cold workability is insufficient with a small amount, so 0.3% or more (preferably 0.5% or more, more preferably 1). (0.0% or more). However, if excessively contained, the yield strength after the solution treatment is increased, and the cold workability is impaired. Therefore, the upper limit of the Cr content needs to be suppressed to 4.5% (preferably 4.0%, more preferably 3.5%).

  Further, the contents of Fe and Cr may be adjusted independently as described above, but the solution treatment can be performed by adjusting together with the Fe equivalents defined by the contents of Fe and Cr. Later yield strength can be reduced, ductility can be improved, and cold workability can also be improved. The Fe equivalent value is desirably 1.5% or more (preferably 2.0% or more) and 3.5% or less (preferably 3.0% or less).

  The titanium alloy of the present invention can contain Al, Cu, N, C, Si, etc. in addition to the above main components depending on the purpose. Among these, Al and Cu are preferably contained from the viewpoint of improving ductility after the solution treatment.

  Al, like Sn, can suppress the formation of the ω phase, and its effect is higher than that of Sn, and is a relatively inexpensive and low-density (light) element. Therefore, the ductility after the solution treatment can be improved by adding 0.3% or more (preferably 0.5% or more, more preferably 1.0% or more) to the titanium alloy. However, when Al is contained excessively, the yield strength after the solution treatment becomes too high, the ductility is lowered, and the cold workability is also impaired. Therefore, the upper limit of the Al content is preferably suppressed to 5.0% (more preferably 4.5%, particularly 4.0%).

  By containing Cu in an amount of 0.1% or more (preferably 0.2% or more, more preferably 0.3% or more), the ductility after the solution treatment can be improved and the yield strength can be reduced. However, since excessive cold workability is impaired, the addition amount of Cu is preferably suppressed to less than 3.0% (more preferably less than 2.5%).

  The β-type titanium alloy of the present invention has high ductility after solution treatment, low yield strength, excellent cold workability, and excellent formability. Therefore, alloy products using the β-titanium alloy can be used in applications that require high formability and cold workability regardless of the shape (for example, plate shape, rod shape, wire shape, tubular shape, etc.). .

  The production of the β-type titanium alloy is not particularly limited, but it is preferable to use sponge titanium as a main raw material, and the sponge titanium together with other additive elements such as arc melting method, electron beam melting method, plasma arc melting method (preferably Can be manufactured by melting by an arc melting method. Usually, Fe needs to be set to a small value because the hardness of the titanium alloy after the solution treatment is increased, and therefore, high purity sponge titanium with a Fe content of about 0.05% or less is used. Manufactured. On the other hand, in the titanium alloy of the present invention, the action is positively utilized by increasing the content of Fe. Therefore, it is necessary to use lower sponge titanium containing 0.2 to 2% of Fe. Manufacturing costs can be reduced.

  In addition, the obtained ingot may be subjected to aging heat treatment (for example, air cooling or water cooling after treatment at 450 to 600 ° C. for 4 to 12 hours) after cold working / molding to improve the strength. .

  A raw material containing the elements shown in Table 1 and the balance being Ti and inevitable impurities was melted by an arc melting method to obtain an ingot of titanium alloy (diameter 50 mm × 15 mm: weight about 120 g). The obtained ingot was hot-rolled at 1200 ° C. until the thickness reached 10 mm, and then hot-rolled again at 1000 ° C., and then the scale formed on the surface was removed to make the thickness 4 mm. Thereafter, cold working is performed until the plate thickness reaches 1.2 mm, and a solution treatment is performed by holding in an atmospheric furnace at 850 ° C. for 10 minutes. After air cooling, the scale formed on the surface is removed to remove the thickness. Was prepared to 1.0 mm to prepare a specimen. A part of the end portion of the obtained test material was cut out, component analysis was performed, and the obtained results are shown in Table 1.

(Evaluation of cold workability)
A 20 mm wide × 100 mm long plate piece was cut out from the test material, and cold-worked again (cold working rate 80%) until the thickness of the cut-out plate piece reached 0.2 mm. Was visually confirmed, the cold workability was evaluated according to the following criteria, and the results are shown in Table 1.
○: Ear cracks were not observed. Δ: Ear cracks less than 1 mm were observed. ×: Ear cracks greater than 1 mm were observed.

(Test method for breaking elongation and 0.2% proof stress)
A test piece having a gauge length of 25.0 mm and a thickness of 1.0 mm was cut out from the test material, the elongation at break and 0.2% proof stress were measured according to the test method of JIS Z 2241. The results are shown in Table 1.

  Test No. Nos. 1 to 18 are titanium alloys that satisfy the requirements defined in the present invention. 19 to 28 are comparative examples lacking any of the requirements defined in the present invention.

  In the titanium alloy (Test No. 19) in which V is greater than the specified amount of the present invention, no cracks are observed, but the proof stress (0.2% proof stress) is 750 MPa or higher, which is higher than that of the titanium alloy of the present invention. (Elongation at break) was also about 19.0%. On the contrary, in the case where the content of V is small, the yield strength is higher than that of the titanium alloy of the present invention, the ductility is low, and the ear cracks are generated (Test No. 20).

  About Sn, yield strength fell and ductility also increased as content increased (test No. 2 and 4-6). However, alloys with Sn other than the specified amount of the present invention (test Nos. 21 and 28) had high yield strength, low ductility, and ear cracks.

  Regarding the content of Fe and Cr, those outside the range specified in the present invention had high proof stress and low ductility (Test Nos. 22 and 23 for Fe and Test Nos. 24 and 25 for Cr). In particular, ear cracks occurred (test Nos. 22 to 24) except for an alloy containing excessive Cr (test No. 25).

  Further, even in alloys (test Nos. 8 and 11) in which the Fe and Cr contents satisfy the above requirements and the Fe equivalent is 3.5% or more, the proof stress is low compared to all the comparative examples, and the cold workability and ductility are low. Shows the same or better characteristics, but by adjusting the Fe equivalent within the range of 1.5 to 3.5%, the proof stress can be reduced and the ductility can be improved. (Test Nos. 1 to 7, 9, 10, 12 to 18).

  Moreover, by including Al in addition to the basic element, the yield strength could be reduced and the ductility could be improved (Test Nos. 12 to 15). However, when it contained excessively, ductility was impaired, proof stress improved, and the ear crack had arisen (test No. 26).

  Also, Cu can reduce the yield strength and improve the ductility (Test Nos. 16 and 17). However, if it is excessively contained, the ductility is impaired, the yield strength is improved, and the ear cracks are generated. (Test No. 27).

  Furthermore, by containing Al and Cu at the same time, the yield strength could be reduced and the ductility could be improved as compared with the case of containing either of them alone (Test No. 18).

Claims (5)

V: 6 to 13% (meaning mass%, the same shall apply hereinafter),
Sn: 4-20%,
Fe: 0.3-3.0%
Cr: A titanium alloy having excellent formability characterized by containing 0.3 to 4.5% and the balance being made of Ti and inevitable impurities. The titanium alloy according to claim 1, wherein the content of Fe and Cr is in the range of 1.5 to 3.5% in terms of Fe equivalent expressed by the following formula.
Fe equivalent = [Fe] + [Cr] /1.5 (inside [] represents the content mass% of the element).   The titanium alloy according to claim 1 or 2, further comprising Al: 0.3 to 5.0%.   Furthermore, Cu: 0.1% or more and less than 3.0%, The titanium alloy of any one of Claims 1-3. The titanium alloy according to any one of claims 1 to 4,
A method for producing a titanium alloy, which comprises producing sponge titanium containing 0.2 to 2% of Fe and other additive elements.