JP2009138218A - Titanium alloy member and method for producing titanium alloy member - Google Patents

Abstract

A titanium alloy member that is made of a titanium alloy material that has a low amount of dissolved oxygen and that does not easily increase deformation resistance even after repeated strong processing, and a manufacturing method for obtaining a high strength titanium alloy member. Provide a method.
For example, an area ratio of an α phase comprising a titanium alloy containing rare earth elements such as Y and La of 0.4 mass% or more, preferably 1.0 mass% or less and having a β phase or a martensite phase as a main phase. The titanium alloy material having a metal structure of 10% or less and containing an oxide of the rare earth element is subjected to a solution treatment, and then subjected to cold plastic working with a cumulative strain rate exceeding 100%.
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Description

  The present invention relates to a titanium alloy member made of a β-type titanium alloy material that is excellent in strong workability, can withstand cold plastic working with a high strain rate, and can improve strength by refining crystal grains. Further, the present invention relates to a method for producing a member made of such a titanium alloy material, and a method for producing a titanium alloy member suitable for obtaining a high-strength member.

Titanium alloys are excellent in specific strength and corrosion resistance, and are widely used in fields such as aviation, military, space, deep sea exploration, and chemical plants.
As such titanium alloys, general-purpose alloys classified into α-type, α + β-type, and β-type are known because of their structures.

  Among these, β-type titanium alloys are known to have component systems such as Ti-15V-3Cr-3Sn-3Al, Ti-22V-4Al, etc., and have a characteristic that they are supple, that is, have a low Young's modulus. Various β-type titanium-based alloys have been proposed that are less than 80 GPa.

These alloys are expected to be applied to, for example, biocompatible products (for example, artificial bones), jewelry (for example, glasses frames), sports equipment (for example, golf clubs), springs, etc. (for example, (See Patent Documents 1 to 4).
JP 2005-113227 A JP 2002-180168 A JP 2004-353039 A Japanese Patent No. 3375083

Further, as a titanium alloy other than the α-type, α + β-type, and β-type titanium alloys, there is a titanium alloy having a martensite structure. When this titanium alloy is subjected to solution treatment in a β-phase stable temperature range by appropriately controlling the amount of β-stabilizing elements such as V (vanadium), Mo (molybdenum), and Nb (niobium), α ′ and α It is characterized by the appearance of a martensite structure.
Specifically, alloy systems such as Ti—V—Al, Ti—V—Al—Fe, Ti—Nb—Sn, Ti—Nb—Al, and Ti—Mo—Sn are known (Patent Document 5 and 6).
Japanese Patent No. 3512253 Japanese Patent Publication No.59-35978

  Since these titanium alloys also have a lower Young's modulus than pure titanium, they are expected to be applied to the biocompatible products, accessories, sports equipment, springs, and the like. Ti-V alloys are easy to melt because V has a low melting point with respect to Nb and Mo, and cost is as low as general-purpose alloys such as Ti-15V-3Cr-3Sn-3Al. Therefore, application to various uses other than medical uses is expected.

On the other hand, when the alloy is required to have strength, general strength improvement methods include dispersion strengthening, solid solution strengthening, processing (structure) strengthening, and fine strengthening.
Dispersion strengthening disperses hard particles or a hard phase of metal. In a β-type titanium alloy, the strength can be significantly improved by precipitating a hard α phase by aging heat treatment. Solid solution strengthening involves adding an element that enters between lattices of metal atoms, and β-type titanium alloys use Cr, Al, Fe, Sn, etc. as additive elements. The work strengthening is a process in which dislocations, which are defects of metal crystals, are introduced at a high density and strengthened by strong work, and there are methods such as shot peening and cold rolling.

Fine strengthening is a method of refining metal crystal grains, and it is known that the so-called Hall-Petch law is established in which metal strength, that is, yield stress and tensile strength, are inversely proportional to the square root of the crystal grain size.
As a refinement method, a strain, that is, a dislocation structure is given in the metal by rolling, forging, etc., and then a static recrystallization method in which recrystallization is caused by heat treatment using the core, or a high speed and high strain is given. There is a dynamic recrystallization method that causes recrystallization during processing.

However, since dispersion strengthening, solid solution strengthening, and work strengthening all lose the flexibility of the alloy, these methods are not suitable as a strengthening method when the flexibility of the β-type titanium alloy is required. Therefore, among the above-described strength improving methods, the fine strengthening is the strengthening method that does not lose the most suppleness.
Note that refinement of the structure can be expected not only to improve static strength but also to improve toughness such as impact resistance.

  On the other hand, in recent years, refinement technology by strong processing has progressed mainly on steel materials, and it has become clear that metal crystal grains can be effectively miniaturized by performing repeated strong processing from multiple directions. This can effectively accumulate the strain required for recrystallization by processing from multiple directions, and also gives a high cumulative strain that is difficult only by rolling from one direction due to the problem of cracking. It is said that it can be done.

  However, the β-type titanium alloy tends to cause dynamic recovery of the dislocation structure, and it has been common to finely disperse the hard α-phase described above and use it as a nucleus for recrystallization. When the hard phase cannot be used to maintain flexibility, strict refinement processing conditions are required for refinement, such as performing strong processing with a strain rate exceeding 100% cold.

β-type titanium alloy is basically a material with excellent cold malleability, but if such high strength is required, strong processing must be repeated, and the material hardens and deforms by repetition. When the resistance increases, there is a concern that cracking may occur during processing. On the other hand, if the number of times of strong processing is suppressed so as not to break, recrystallization becomes insufficient and a mixed grain structure with coarse unrecrystallized grains may result, resulting in variations in mechanical properties.
Therefore, when carrying out repeated strong processing, the material is required to have a characteristic that deformation resistance is unlikely to increase even when repeated.

  As a measure for that, it is conceivable to reduce the amount of oxygen in the titanium alloy. However, since titanium is very active and difficult to be reduced, there is a problem in that it is expensive to reduce the oxygen content.

The present invention has been made by paying attention to the above-mentioned problems relating to strength improvement by fine strengthening of conventional β-type titanium alloys.
The purpose is to reduce the oxygen content (the amount of dissolved oxygen) without cost, and it is made of a titanium alloy material that is difficult to increase in deformation resistance even after repeated strong processing. An object of the present invention is to provide a titanium alloy member that can be made into a material. Moreover, it is providing the manufacturing method which can improve the intensity | strength of such a titanium alloy member.

  In order to achieve the above-mentioned object, the present inventors have made extensive studies on alloy components and deoxidation methods. As a result, for example, by adding a predetermined amount of rare earth elements such as Y (yttrium) or La (lanthanum) to the titanium alloy, these elements function effectively as a deoxidizer, and the above problems can be solved. The invention has been completed.

  That is, the present invention is based on the above knowledge, and the titanium alloy member of the present invention is composed of a titanium alloy containing 0.4% by mass or more of a rare earth element, and mainly comprises a β phase or a martensite phase. It has a metal structure in which the area ratio of the α phase is 10% or less as a phase, and contains the rare earth element oxide.

  Further, in the method for producing a titanium alloy member of the present invention, the titanium alloy member comprises a titanium alloy containing 0.4% by mass or more of a rare earth element, and the area ratio of the α phase is 10% or less with the β phase or martensite phase as the main phase. A titanium alloy material having a metal structure and containing an oxide of the rare earth element is subjected to solution treatment, and then subjected to cold plastic working with a cumulative strain rate exceeding 100%.

  According to the present invention, since it is made of a titanium alloy containing a predetermined amount of rare earth element and contains an oxide of rare earth element, for example, cold plastic working is performed such that the cumulative strain rate exceeds 100%. Thus, a high-strength titanium alloy member can be obtained.

  Hereinafter, the titanium alloy member of the present invention and the method for producing the titanium alloy member will be described in more detail together with reasons for limiting various numerical values. In the present specification, “%” represents mass percentage unless otherwise specified.

  As described above, the titanium alloy member of the present invention is made of a titanium alloy containing 0.4% or more of a rare earth element, and has a β phase or martensite phase as a main phase and an α phase area ratio of 10% or less. Has an organization. And since rare earth element oxides are contained, the amount of dissolved oxygen is small, and it can withstand repeated strong processing. By strengthening the structure based on such strong processing, high strength titanium alloy members It can be.

In other words, even if the same alloy component is used, if the average oxygen content (total oxygen content) in the alloy is small, the hardening due to repeated strong processing saturates at a relatively low level, whereas the average oxygen content When the amount is large, curing proceeds, the deformation resistance increases remarkably, and cracking occurs.
On the other hand, even when the average oxygen content is high, the oxygen content (solid oxygen content) in the β phase or martensite phase constituting the alloy is obtained by forming and growing a rare earth oxide by adding an appropriate amount of rare earth elements. Is reduced, the deformation resistance becomes the same as that of an alloy having a low average oxygen content. Therefore, this makes it possible to obtain an alloy having excellent repeated workability at low cost.

In the titanium alloy member of the present invention, the β phase or martensite phase is the main phase of the metal structure, and the rare earth element oxide is finely dispersed in the metal structure.
Here, when other metal structures other than this are used as the main phase, they are not supple and relatively high in strength, so that it is not possible to fully utilize the advantages of reducing the amount of dissolved oxygen due to the formation of rare earth oxides. .
Even in the case of a β-type alloy, if a large amount of α-phase is precipitated, the strength is greatly improved, but at the same time, the flexibility is remarkably impaired, so that the area ratio is required to be 10% or less.

The components of the titanium alloy material used in the present invention are those that exhibit the above-described metal structure, that is, a component system in which a β-phase or martensite phase is the main phase and a metal structure having an α-phase area ratio of 10% or less is obtained. As long as it is, there is no particular limitation.
Specific component systems include Ti-15V-3Cr-3Sn-3Al, Ti-22V-4Al, Ti-15V-2Al, Ti-10V-1.5Al-4.5Fe, Ti-35Nb-4Sn, In addition to Ti-38Nb-1.4Al, Ti-15Mo-3Al, Ti-15V, Ti-15Mo-5Zr-3Al, Ti-6Al-4V-10Cr-1.3C, Ti-15V-6Cr-4Al, Ti- 20V-3.5Al-1Sn etc. can be mentioned.

Further, the titanium alloy member according to the present invention contains 0.4% or more, preferably 1.0% or less of rare earth elements.
That is, if the addition amount of the rare earth element is less than 0.4%, the effect of sufficiently reducing the amount of dissolved oxygen cannot be obtained. If the addition amount exceeds 1.0%, the effect is saturated. However, the effect is not improved, and it becomes uneconomical.

Furthermore, in the titanium alloy member according to the present invention, it is desirable that the roughly elliptical rare earth element oxide is dispersed with a minor axis radius of 0.1 to 5 μm.
In the inclusion for the purpose of improving machinability, it is preferable that the inclusion is finely and uniformly dispersed. On the other hand, since the purpose of the present invention is to reduce the amount of dissolved oxygen accompanying oxide growth, if the minor axis radius is less than 0.1 μm, a sufficient amount of dissolved oxygen can be reduced. On the other hand, if the thickness exceeds 5 μm, adverse effects such as a decrease in fatigue strength may occur.

Furthermore, the usefulness of the titanium alloy member of the present invention is particularly remarkable when an alloy having a high average oxygen content is used as a material.
For example, even a commercially available alloy having an average oxygen content of 0.15% or more, the amount of dissolved oxygen can be greatly reduced. It is possible to obtain excellent characteristics that hardly cause cracks. It should be noted that even if a material with a reduced amount of oxygen is used through thorough reduction treatment, the increase in deformation resistance due to repeated processing cannot be suppressed, but in that case, the cost of the material increases significantly. Will end up.

Further, as described above, the components of the titanium alloy member of the present invention are not particularly limited except that an appropriate amount of rare earth element is added.
In other words, in all alloys having a metal structure with a β-phase or martensite phase as the main phase and an α-phase area ratio of 10% or less, the deformation resistance is remarkably increased with respect to repeated strong working while maintaining its flexibility. Can be prevented, and the strength can be improved by miniaturization.

  And, the titanium alloy member of the present invention exhibits its performance especially by repeated strong processing, especially when a recrystallized structure is obtained by strong processing from multiple directions. There is also an advantage that the same mechanical characteristics can be obtained. However, the processing method is not particularly limited, and has an advantage of low deformation resistance even in one strong processing.

In the method for producing a titanium alloy member of the present invention, it is composed of a titanium alloy containing 0.4% by mass or more of a rare earth element, the area ratio of the α phase is 10% or less, and the β phase or the martensite phase is the main phase. A titanium alloy material having a metal structure and containing the rare earth element oxide is used. And after performing the solution treatment to the said raw material, it is trying to perform the cold plastic processing in which an accumulation distortion rate exceeds 100%, and, thereby, a high strength titanium alloy member can be obtained.
Here, the cumulative distortion rate is a cumulative value of the distortion rate repeatedly applied in a predetermined direction of the sample. For example, if 10% strain application is repeated 10 times, the cumulative distortion rate becomes 100%. The reason for repeatedly applying the predetermined cumulative strain amount is that the higher the strain rate, the easier the recrystallization and the higher the strength by refining the crystal grains, but the higher the strain at one time, the higher the deformation resistance. This is because cracks are more likely to occur. The more important reason is that it is necessary to apply strain from multiple directions in order to obtain fine crystal grains isotropically, but if the strain applied from one direction is too large, that direction This is because it becomes difficult to apply the strain from the other direction.

The larger the accumulated strain to be applied, the easier it is to refine the obtained crystal grains, while the so-called Hall-Petch law is known that the strength is proportional to the -1/2 power of the crystal grain size. If possible, the strength can be improved. For example, in order to make the strength of a β single-phase alloy used with a grain size of approximately several hundred μm about the same as that of 6Al-4V titanium, the average crystal grain should be about 1/10 so that the strength is 2 to 3 times. However, in order to achieve this only by cold or warm rolling, strong rolling of 90% or more is necessary, so that the material is easily broken and it is difficult to obtain an isotropic structure.
In the present invention, since the ductility decrease due to repetition is small, it is possible to repeatedly press the accumulated strain exceeding 100%, and it is possible to obtain an isotropically high-strength titanium alloy member that has been difficult to obtain conventionally. If the cumulative strain is 100% or less, crystal grain refinement that leads to a significant improvement in strength cannot be achieved, but it should be more than that, but if the crystal grain is 1 μm or less, the ductility is impaired, so the cumulative strain is 700% or less. It is more preferable.

Moreover, in the said manufacturing method, it is desirable to hold | maintain more than a recrystallization temperature after cold plastic working.
Although the recrystallization temperature varies depending on the component, in the case of β-type titanium alloy, new crystal grains can be generated and grown by maintaining the temperature at approximately 600 ° C. to 800 ° C. The higher the processing rate, the finer the crystal grain size becomes. Easy to make.

  EXAMPLES Hereinafter, although this invention is demonstrated further in detail based on an Example, this invention is not limited to these Examples.

As a β-type titanium alloy, a Ti-22V-3Al alloy was used as a base, and Y was selected as a rare earth element. In this alloy, the β phase becomes a stable phase at room temperature, and martensite is generated during strong processing.
About 99.9% pure Ti, V, Al, Y pure metals and titanium oxide were used, and the average oxygen content and Y content were changed by arc melting in an argon atmosphere as shown in Table 1. 15 types of 90 g ingots (Examples 1 to 7, Comparative Examples 1 to 8) were melted.

The ingot was subjected to a homogenization treatment at 1150 ° C. for 24 hours in a vacuum, followed by a solution treatment at 950 ° C. for 2 hours, and cooled in oil at room temperature.
Thereafter, for comparison, some materials (Example 4, Comparative Examples 5 and 6) were subjected to an aging treatment at 400 ° C. to precipitate an α phase.

A test piece was processed from the ingot after the heat treatment, and repeated pressing experiments were performed. The pressing method includes a repeated rolling method and an extrusion method with strong shearing. Here, as shown in FIG. 1, a so-called multi-axis forging method in which the pressing direction is changed and repeated forging is adopted.
In the present invention, as long as a cumulative strain exceeding 100% is given, the processing method is not limited, and various pressing methods can be combined.

That is, as shown in FIG. 1A, first, the test piece is formed into a shape of 7.4 mm × 7 mm × 6.7 mm and pressed in the direction of the arrow so that the longest side of 7.4 mm is 6.7 mm. (True strain 10%). As a result, as shown in FIG. 1B, the 7 mm side is expanded to 7.4 mm and the 6.7 mm side is expanded to 7 mm for the other sides. 4mm x 7mm x 6.7mm.
Thus, as shown in FIG.1 (c), it can press repeatedly by always pressing the longest side. In addition, since it becomes 1 cycle by pressing from 3 directions, the repetition number was shown by the cycle number below. That is, a cumulative distortion ratio of 30% is given in one cycle, 150% in five cycles, and 600% in 20 cycles.

At this time, a 600 kN hydraulic hand press was used for pressing, and the change in deformation resistance at room temperature was recorded as a reaction force during pressing. Thereafter, a sample having a plate thickness of 3 mm was maintained at a recrystallization temperature of 700 ° C. for 7 minutes, and then cooled in oil at room temperature.
Then, a bending test piece having a length of 12 mm, a width of 3 mm, and a thickness of 2 mm is cut out from a part of the sample by electric discharge machining, and a three-point bending test is performed at room temperature with a support span of 8 mm and a displacement speed of 0.3 mm / min. The maximum stress at this time was defined as the bending strength.

In addition, about the component analysis of each alloy raw material, the infrared rays absorption method was used for the measurement of oxygen amount, and the plasma emission analysis was used for the other analysis.
The crystal grains are observed with an optical microscope, and the state of inclusions is observed with a scanning electron microscope. The minor axis radius is measured to obtain an average value, and inclusions are identified with EDX (X-ray microanalyzer). It was.

Here, since the average (total) oxygen amount Ot (%) in the alloy by the infrared absorption method is considered to be the sum of the dissolved oxygen amount Os (%) and the amount contained as the Y oxide, the Y content Yc Is present as an oxide, the solid solution oxygen amount Os can be obtained from the following equation from the atomic weight of Y 89 and the atomic weight 16 of oxygen.
Os = Ot−Yc × (3 × 16) / (2 × 89)

Furthermore, for some samples, in order to investigate the improvement effect of malleability due to oxygen reduction, static pressing is performed using a hand press after solutionization, and the thickness is reduced by 10% with respect to the initial thickness of the sample. The presence or absence of cracks was visually confirmed every time, the thickness reduction rate at which cracks occurred was investigated, and the crack generation limit was set.
These results are summarized in Table 1.

Moreover, as a representative example of the structure | tissue after solution treatment, the scanning electron microscope observation result of the comparative example 2 and Example 6 is shown to Fig.2 (a) and (b), respectively. The elliptical particles found on the structure are recognized as oxides of Y (Y ₂ O ₃ ), as shown in FIG.
This yttrium oxide is dispersed in the grain boundaries and grains, and the amount corresponds to the Y content. Therefore, it is considered that the amount of dissolved oxygen is reduced according to the Y content. . Note that the size and shape of the yttrium oxide do not change before and after the solution treatment.

As is clear from the results in Table 1, in the repeated forging of Examples 1 to 7, a low reaction force was exhibited from the initial stage of pressing, and the reaction force was saturated at a low level even when pressing was repeated. confirmed. In addition, the obtained structure was homogeneous and fine equiaxed crystal grains without any cracks.
As can be seen by comparing the case of Example 7 with other examples, the effect of Y for repeatedly suppressing the reaction force is saturated at about 1%. Therefore, even if it adds more than this, even if the effect which suppresses repeated reaction force is acquired similarly, since it becomes costly, more preferable content is 0.4 to 1.0%.

In contrast, in Comparative Example 1, since the α phase was dispersed in a large amount, the reaction force was high from the initial stage of repeated pressing, and cracking occurred.
Further, in Comparative Examples 2 to 4, since the content of Y is small, the amount of dissolved oxygen is high, the repeated reaction force is greatly increased, and not only pressing becomes difficult, but also cracking or mixing of the structure is caused. As a result, it became a grain.

  Moreover, about bending strength, compared with the case of the comparative examples 7 and 8 which did not press-process, and the comparative example 2 which the crack did not generate | occur | produce, in Examples 1-7 of this invention, all were the bending excellent. Intensity was shown.

The titanium alloy member according to the present invention is excellent in repeated malleability and can be improved in strength by repeating strong processing. However, depending on the application, static plastic workability at low cost, that is, cold and warm can be obtained. It is also effective in obtaining the malleability necessary for inter-rolling and forging. For example, in applications where isotropic mechanical properties and flexibility are not required, a high degree of work hardening may be used without recrystallization.
As shown in Table 1, in Examples 4 and 7 of the present invention, it is confirmed that the crack limit is superior as compared with Comparative Examples 1 to 6.

  In addition, it is not always necessary to perform post-processing recrystallization heat treatment, and it is also possible to perform molding using so-called dynamic recrystallization in which continuous recrystallization occurs by giving high processing strain and high strain rate.

  In this example, a Ti-22V-3Al alloy was used as the base alloy and Y was used as the rare earth element. However, based on the various alloys shown above, ytterbium, erbium, selenium, etc. Needless to say, these rare earth elements may be used.

(A)-(c) is explanatory drawing which shows the point of the multi-axis forging method applied in the Example. It is an electron micrograph which shows the structure | tissue after the solution treatment of the alloy raw material concerning the comparative example (a) and Example (b) of this invention. It is a chart figure which shows the identification result by EDX of the inclusion of FIG.

Claims (6)

  It consists of a titanium alloy containing 0.4% by mass or more of a rare earth element, has a metal structure in which the β phase or martensite phase is the main phase and the area ratio of the α phase is 10% or less, and contains the oxide of the rare earth element. A titanium alloy member characterized in that   The titanium alloy member according to claim 1, wherein the rare earth element content is 0.4 to 1.0 mass%.   3. The titanium alloy member according to claim 1, wherein the rare earth element oxide has a substantially elliptical shape with a minor axis of 0.1 to 5 μm.   It consists of a titanium alloy containing 0.4% by mass or more of a rare earth element, has a metal structure in which the β phase or martensite phase is the main phase and the area ratio of the α phase is 10% or less, and contains the oxide of the rare earth element. A method for producing a titanium alloy member, comprising subjecting a titanium alloy material being subjected to solution treatment to cold plastic working with a cumulative strain rate exceeding 100%.   The method for producing a titanium alloy member according to claim 4, wherein the rare earth element content is 0.4 to 1.0 mass%.   The method for producing a titanium alloy member according to claim 4 or 5, wherein after the cold plastic working, the temperature is maintained at a recrystallization temperature or higher.

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