JP2011153345A - beta TYPE TITANIUM ALLOY HAVING EXCELLENT FATIGUE STRENGTH - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a &beta; type titanium alloy which can obtain high fatigue strength even in a high load stress of &ge;900 MPa. <P>SOLUTION: The &beta; type titanium alloy contains, by mass, 4.0 to 10.0% V, &ge;2.0% Sn and &ge;2.0% Al. In the metallic structure, &alpha; phases 1 with a width of &ge;0.05 &mu;m has a shape settled between optional two straight lines parallel at an intervals of 0.3 &mu;m, also, the length of the &alpha; phases 1 is &le;5 &mu;m, and further, the maximum grain size of &beta; phases is &le;50 &mu;m. Further, T1 defined by expression T1=937+147.7[O]+20.4[Al]+161.8[C]-19.8[Fe]-10.3[Mo]-8.4[Nb]-13.1[V]-17.0[Cr]-0.2[Sn]-8.5[Cu] is &gt;700 to &lt;800. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a β-type titanium alloy having excellent fatigue strength.

  Titanium alloys have been widely used mainly in the aircraft industry because they have various processing characteristics such as weldability, superplasticity, and diffusion bonding properties in addition to light weight, high strength, and high corrosion resistance. In order to further utilize these characteristics, in recent years it has come to be used for sports equipment such as golf equipment, such as consumer products such as automobile parts, civil engineering materials, various tools, Application to deep seas and energy development applications is also expanding.

  Among titanium alloys, β-type titanium alloys such as Ti-15V-3Cr-3Sn-3Al alloy, Ti-15Mo-5Zr-3Al alloy, Ti-13V-11Cr-3Al alloy have high strength by aging treatment. Therefore, the demand for materials for golf club heads, bicycle gears, springs, fishing gear, aircraft members and the like is increasing.

  Under such circumstances, various β-type titanium alloys have been proposed in recent years. Patent Document 1 discloses a β-type titanium alloy wire having high strength and high toughness by defining its component composition and a method for producing the same. Patent Document 2 discloses that the average particle diameter of α phase is 0.1 to 10 μm in advance. The β-type titanium alloy having an average particle size of β-phase of 0.1 to 10 μm is applied at a temperature range of “β transformation point −450 ° C.” or more and less than “β transformation point −100 ° C.” for 15 minutes to 100 hours. A production method of a β-type titanium alloy subjected to heat treatment and a titanium alloy produced by the production method have been proposed.

  In addition, as disclosed in Patent Document 3, the present applicant is also able to obtain a high strength / high ductility β-type titanium that can obtain an excellent balance between strength and ductility even when the content of expensive alloy elements V and Mo is reduced as much as possible. We are proposing alloys.

  As described above, these β-type titanium alloys have excellent strength properties and are widely used as materials for golf club heads, bicycle gears, springs, fishing gear, aircraft members, and the like. However, it is often used as a member that receives a repeated stress, and a further increase in fatigue strength is required, such as a longer life under high load repeated stress and a reduction in variation in fatigue life.

Japanese Patent Laid-Open No. 11-61297 JP 2004-156064 A JP 2004-270009 A

  This invention is made | formed in view of the said conventional situation, and makes it a subject to provide the beta type titanium alloy from which high fatigue strength is obtained under the high load stress of 900 Mpa or more.

The invention according to claim 1 is a β-type titanium alloy containing, in mass%, V of 4.0 to 10.0%, Sn of 2.0% or more, and Al of 2.0% or more, The width: width: the α phase of 0.05 μm or more is in a shape that fits between any two straight lines parallel to each other with a spacing of 0.3 μm, and the length is 5 μm or less. The β-type titanium alloy having excellent fatigue strength, wherein the maximum particle size of the β phase is 50 μm or less, and T1 defined by the following formula is more than 700 and less than 800.
T1 = 937 + 147.7 [O] +20.4 [Al] +161.8 [C] -19.8 [Fe] -10.3 [Mo] -8.4 [Nb] -13.1 [V] -17 0.0 [Cr] -0.2 [Sn] -8.5 [Cu]
However, in the above formula, [] indicates the content (% by mass) of each element.

  The invention according to claim 2 further includes, in mass%, 5.0 to 9.0% of Cr, 0.3 to 3.5% of Fe, 2.0 to 5.0% of Sn, and 2 of Al. The β-type titanium alloy having excellent fatigue strength according to claim 1, comprising 0.0 to 4.5%, the balance being Ti and inevitable impurities.

The invention according to claim 3 further includes 0.5 mass% or less of Si, and T1 defined by the following formula is more than 700 and less than 800, and the fatigue strength according to claim 2 It is an excellent β-type titanium alloy.
T1 = 937 + 147.7 [O] +20.4 [Al] +161.8 [C] -19.8 [Fe] -10.3 [Mo] -8.4 [Nb] -13.1 [V] -17 0.0 [Cr] -0.2 [Sn] -8.5 [Cu] -12 [Si]
However, in the above formula, [] indicates the content (% by mass) of each element.

  According to the β-type titanium alloy excellent in fatigue strength of the present invention, high fatigue strength can be obtained even under a high load stress of 900 MPa or more.

It is a schematic diagram which shows the metal structure of beta type titanium of one Embodiment of this invention. It is a schematic diagram which shows the largest (alpha) phase which satisfies the requirements of this invention. It is process drawing which shows the manufacturing method of beta type titanium of one Embodiment of this invention with temperature conditions. The state of the metal structure in each manufacturing process of β-type titanium of the embodiment is shown, (a) is a schematic diagram in each manufacturing process when the recovery heat treatment is not performed, (b) is the recovery heat treatment It is a schematic diagram in each manufacturing process when implemented. It is a microscope picture which shows the metal structure of the beta type titanium alloy which satisfies the requirements of the present invention. It is a microscope picture which shows the metal structure of the beta type titanium alloy which does not satisfy the requirements of the present invention. It is a front view of the test piece which shows the shape and dimension of the test piece used for the Ono-type rotary bending fatigue test of the Example.

  In order to develop a β-type titanium alloy excellent in fatigue strength capable of obtaining high fatigue strength even under a high load stress of 900 MPa or more, the present inventors have advanced earnestly, experiments, and research. As a result, the content of alloying elements added to titanium, particularly V, Sn, and Al, is specified, the shape of the α phase existing as a metal structure is made appropriate, and the largest particles of β phase in the metal structure By defining the diameter, it was found that high fatigue strength was obtained even under a high load stress of 900 MPa or more, and the present invention was completed.

  Hereinafter, the present invention will be described in detail based on embodiments.

  In the present invention, the component composition of the β-type titanium alloy, the form of the α phase in the metal structure, and the maximum particle size of the β phase in the metal structure, first, the type of alloy element to be added to titanium, The reason for determining the content of each alloy element will be described in detail for each element. In addition, all% described in this specification shows the mass%.

V: 4.0 to 10.0%
V is an extremely effective element for providing an excellent strength-ductility balance as a titanium alloy. However, V is also an expensive element, and it is necessary to define an upper limit in order to reduce costs. Therefore, in the present invention, the upper limit is defined as 10.0%. A more preferable upper limit is 8.0% from the viewpoint of cost reduction. In addition, the ductility improving effect after the aging treatment obtained by adding V is extremely important in the present invention, and in order to effectively exhibit the effect, it is necessary to contain at least 4.0%. A more preferred lower limit is 5.0%.

Cr: 5.0-9.0%
Cr is an effective element for suppressing age hardening and suppressing an excessive increase in strength to avoid shortage of ductility, but in order to obtain the effect, 5.0% or more needs to be contained. The lower limit of the preferable content is 6.0%. As the Cr content is increased, age hardening is delayed. As a matter of course, age hardening proceeds as the aging time is increased. However, extremely increasing the aging time in actual production is not suitable for the actual situation because it reduces productivity. The normal aging time applied to β-type titanium alloys applied to aircraft members and the like is about 8 hours, and the upper limit for securing sufficient strength at this aging time is 9.0% content it is conceivable that. The upper limit of the preferable content is 8.0%.

Fe: 0.3-3.5%
Fe is a eutectoid β-stabilizing element like Cr, and when added together with Cr, the strength-ductility balance can be further increased. Such an effect can be effectively expressed by adding 0.3% or more of Fe. However, when the Fe content exceeds 3.5%, the aging is delayed as in the case of Cr and high strength is obtained. It becomes an obstacle to crystallization. Therefore, the Fe content is preferably 3.5% or less. A more preferred upper limit is 3.0%.

Sn: 2.0-5.0%
Sn is an element necessary for obtaining high ductility by aging treatment after cold working, and when its content is less than 2.0%, an ωδ phase appears in the titanium alloy, and aging after cold working. Causes ductile deterioration in processing. Therefore, it is desirable to contain 2.0% or more, preferably 2.5% or more. However, Sn has a higher density than Ti and is expensive for high-purity products. Therefore, for the purpose of cost reduction, it is necessary to reduce the content as much as possible. Therefore, it is desirable that the content is at most 5.0% or less, preferably 4.5% or less.

Al: 2.0 to 4.5%
Al is an element necessary for increasing the strength after the aging treatment, and in order to exert the effect, it is desirable to contain 2.0% or more, preferably 2.5% or more. However, if the content is too large, the ductility is lowered and cold workability is impaired. Therefore, it is necessary to suppress the content to 4.5% or less, preferably 4.0% or less at most. is there.

  The upper and lower limits of the content of the alloying element added to titanium have been described above. In the β-type titanium alloy of the present invention, V is 4.0 to 10.0%, Sn is 2.0% or more, and Al is 2%. It is essential to contain 0.0% or more. When manufacturing a wire, the temperature rise especially at the time of rolling becomes a problem, and the temperature rise at the time of rolling invites coarsening of the particle size of β phase. Furthermore, the high rolling load also impairs economic efficiency. Conventionally, typical β-stabilizing elements such as V and Mo have been often added, but these are expensive elements, and when added in large quantities, deformation resistance during rolling increases. Therefore, the above-mentioned problem is promoted. Therefore, it is conceivable to add an inexpensive β-stabilizing element in place of V and Mo instead of V and Mo such as Fe, Cr, Nb, Cu, and Si, but it is difficult to completely replace them, and the structure ( In terms of the stability of the phase) and the effect of improving the strength after aging, it does not reach the addition of V and Mo. As a result of the study by the present inventors, the above problem can be solved by keeping the V content lower than before (specifically, 4.0 to 10.0%) and adding appropriate amounts of Sn and Al. It was found. As described above, Sn is an element effective for stabilizing the β phase, and Al is an element effective for increasing the strength after aging. By adding 2.0% or more of each of these, This makes it possible to reduce the amount of V to be added to 4.0 to 10.0% as compared with the prior art. In the present invention, it is possible to contain Mo, but it is not always necessary to add it as shown in the examples below. In the case where Mo is added, considering the above-mentioned problems, it is set to 10.0% or less, preferably 8.0% or less, more preferably 7.0% or less.

  Further, Fe, Cr, Nb, Cu, and Si can be included as β-stabilizing elements because they are less expensive than V and Mo. Among them, Si is particularly inexpensive, and it is effective to contain it in a β-type titanium alloy as a β-stabilizing element. However, if a large amount of Si is added, there is a possibility that silicide is generated and embrittlement is caused. Therefore, when adding Si, it is 0.5% at most.

  The reason for limiting the component range of the alloy element added to the β-type titanium alloy of the present invention is as described above, and the balance is Ti and inevitable impurities.

  In addition, the β-type titanium alloy of the present invention is required to satisfy the requirement of 700 <T1 <800.

T1 shown here represents the content (mass%) of each alloy element to be added to the β-type titanium alloy by [].
T1 = 937 + 147.7 [O] +20.4 [Al] +161.8 [C] -19.8 [Fe] -10.3 [Mo] -8.4 [Nb] -13.1 [V] -17 0.0 [Cr] -0.2 [Sn] -8.5 [Cu]
Can be expressed as This equation is an equation for obtaining the β transformation temperature (β transformation point) in consideration of the influence of each additive element.

When Si is added as a β-stabilizing element, T1 represents the content (mass%) of each alloy element to be added to the β-type titanium alloy by [].
T1 = 937 + 147.7 [O] +20.4 [Al] +161.8 [C] -19.8 [Fe] -10.3 [Mo] -8.4 [Nb] -13.1 [V] -17 0.0 [Cr] -0.2 [Sn] -8.5 [Cu] -12 [Si]
Can be expressed as

  Regardless of the value calculated by either formula, when this T1 is 700 or less, the β phase is too thermodynamically stabilized, so that the formation of an aging α phase is suppressed and the aging time becomes very long. For this reason, productivity is lowered and it cannot be industrially realized. On the other hand, when T1 is 800 or more, an α phase is generated during cooling after rolling or annealing, and a coarse linear α phase is generated to reduce fatigue strength. Therefore, 700 <T1 <800 is defined as a requirement of the present invention. A preferable lower limit of T1 is 730, and a more preferable lower limit of T1 is 750, and a preferable upper limit of T1 is 780, and a more preferable lower limit of T1 is 770.

  Next, the metal structure of the β-type titanium alloy of the present invention will be described. The metal structure of the β-type titanium alloy of the present invention has a shape as shown in FIG. 2 is a substantially spherical β phase (parent phase), and 1 (1a, 1b) is a linear α phase. 1 a is a grain boundary α phase existing in the β grain boundary (between the β phases), and 1 b is an intragranular α phase existing in the β phase 2. In the present invention, among these α phases 1, the form (shape and size) of α phase 1 having a width of 0.05 μm or more and the maximum particle size of β phase 2 are required for the invention. For reference, FIG. 5 shows a micrograph of the metal structure of a β-type titanium alloy that satisfies the requirements of the present invention, and FIG. 6 shows a photomicrograph of the metal structure of a β-type titanium alloy that does not satisfy the requirements of the present invention.

  FIG. 2 shows the maximum α phase 1 that satisfies the requirements of the present invention. The α phase 1 has a width of 0.05 μm or more and is a linear shape that fits between any two straight lines spaced apart by 0.3 μm and has a maximum length of 5 μm. It is.

  In β-type titanium alloys, the fact that the α phase 1 with a certain width or more is continuously generated in the metal structure in a straight line, cracks occur at an early stage, leading to a significant decrease in fatigue life. The inventors have found. On the other hand, it was also confirmed that even if the α phase 1 is continuous, if the α phase 1 is curved, it is difficult to become a crack occurrence site. Therefore, the present inventors consider that it is important to reduce the linear α-phase 1 in order to exert a fatigue life superior to that of a conventional β-type titanium alloy. As a result, regarding the α phase 1, it has been found that by satisfying the above requirements, the β-type titanium alloy exhibits a high fatigue life and a high fatigue strength can be obtained.

  When the width of the α phase 1 formed in the metal structure is less than 0.05 μm, sufficient aging is not achieved and the strength is insufficient or the ductility is lowered and the fatigue life is reduced. Therefore, the α phase 1 having a width of 0.05 μm or more must be present.

  In the present invention, it is also a requirement that the maximum particle size of β phase 2 be 50 μm or less. The α phase 1 generated by aging is preferentially precipitated on the β grain boundaries. At this time, if the β grain boundary is linear, a continuous α phase 1 is naturally generated, and a crack is generated. Therefore, when there is a long linear β grain boundary, the α phase 1 having a width of 0.05 μm or more does not have a shape that fits between any two straight lines spaced apart by 0.3 μm, In addition, there is a high possibility that the length will not be 5 μm or less. In particular, when non-recrystallized grains extending in the rolling direction are present in the microstructure, a linear continuous α phase 1 is easily generated at the β grain boundary, which greatly affects the fatigue life. Since fatigue cracks are mostly generated and propagated in the most fragile locations in the metal structure, it is important to define the maximum particle size, not the average particle size of β phase 2, and in the present invention The maximum particle size of the β phase was defined to be 50 μm or less.

  Next, the manufacturing method of (beta) type titanium of this invention is demonstrated. There are considered to be many methods for producing the β-type titanium of the present invention, and the member to be produced includes a wire, a plate, a forged material, etc. Here, an example in the case of producing a wire will be described.

  FIG. 3 shows the manufacturing method together with temperature conditions, and the broken line indicates the β transformation point. As shown in FIG. 3, for example, the β-type titanium of the present invention includes: “A. Hot rolling → (B. Cold wire drawing →) (C. Recrystallization treatment →) D. Cold wire drawing → (E It can be manufactured through a process of “.Recovery heat treatment →) F. Aging treatment”.

  First, A. Although the base material is hot-rolled, the finishing temperature is set to the β transformation point (T1) or higher. C. after hot rolling and cooling. Cold-drawing is performed with a reduction in area of 20 to 50%. Finally, F.A. If an aging treatment is carried out, a β-type titanium alloy can be produced. This aging treatment is carried out at a temperature range of (β transformation point−300 ° C.) to (β transformation point−150 ° C.) for 2 hours, Implement by air cooling (including forced air cooling) or water cooling.

  D. Before cold drawing, C.I. Recrystallization treatment may be performed. In performing the recrystallization treatment, B.B. It is recommended to perform cold drawing. B. When performing cold drawing, it is necessary to carry out with a reduction in area of 20% or more. When cold drawing is performed with a reduction in area of less than 20%, the growth of β crystal grains is rather promoted by the influence of the subsequent heat treatment and the maximum particle size of β phase 2 exceeds 50 μm. There is a possibility. In addition, it is recommended that the recrystallization treatment at this time be air-cooled or water-cooled after being held for about 30 seconds to 3 hours in a temperature range of β transformation point to (β transformation point + 50 ° C.).

  In addition, if necessary, D.I. After cold drawing, E.I. Recovery heat treatment may be performed. The recovery heat treatment is performed by setting the holding time in the temperature range from (β transformation point−100 ° C.) to β transformation point to 2 hours or less and then performing air cooling.

  Although the β-type titanium of the present invention can be manufactured as described above, the difference in the metal structure of the β-type titanium when the recovery heat treatment is performed and when it is not performed will be described with reference to FIG. .

  4A shows the case where the recovery heat treatment is not performed, and FIG. 4B shows the case where the recovery heat treatment is performed. D. A linear slip band 3 is formed in the β phase 2 after cold drawing. By carrying out the recovery heat treatment, the slip band 3 can be made wavy. The length of the α phase after the aging treatment can be shortened. Although this recovery heat treatment has been described as not necessarily required in the description of the manufacturing conditions, it can be considered effective to make the α phase 1 in a desired form.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.

  In this example, first, a billet of 60 mm square was produced and hot rolled to obtain a wire diameter of 16 to 21.5 mmφ. Thereafter, the wire diameter is finally obtained through steps of recrystallization treatment (retention time is 20 minutes), cold drawing, recovery heat treatment (retention time is 40 seconds), and aging treatment (retention time is 2 hours, forced air cooling): Wires having a diameter of 6 to 12.6 mm were prepared. Table 1 shows the component composition of each wire made of the produced β-type titanium alloy.

  Observation and measurement of the metal structure of each wire rod made of the produced β-type titanium alloy and measurement and evaluation of fatigue life were performed as follows.

  The form of the metal structure of the β-type titanium alloy was observed by the following method. As for the α phase morphology, a field emission scanning microscope (FE-SEM) (manufactured by JEOL Ltd., JSM5410) is used to observe the α phase by azimuth analysis by electron back-scattering analysis image method (EBSD). The form was confirmed by SEM image. On the other hand, with respect to the β phase, after mirror processing is performed on a cross section parallel to the rolling direction of the wire, the processed surface is etched with nitric hydrofluoric acid, and is observed at a magnification of 100 times using an optical microscope. The maximum diameter (major axis) of the β grains was taken as the maximum particle diameter.

  For the fatigue life of the β-type titanium alloy, the test piece shown in FIG. 7 was collected from the wire prepared by the method described above, and an Ono type rotating bending fatigue test was performed under a high load stress of 950 MPa. The frequency is 2700 to 3000 rpm. In this example, as a result of this Ono type rotating bending fatigue test, it was determined that a β-type titanium alloy having excellent fatigue strength was one having a cycle number of up to 200,000 (2.0E + 05).

  The test results are shown in Table 2.

  The numbers 2 to 4 and 9 to 14 in Table 2 are invention examples that satisfy all the requirements of the present invention. In these inventive examples, the number of fracture cycles obtained in the Ono-type rotating bending fatigue test is all over 200,000 (2.0E + 05), and it can be determined that the β-type titanium alloy has excellent fatigue strength.

  On the other hand, No. 1 is a comparative example in which the maximum particle size of the β phase and the length of the α phase do not satisfy the requirements of the present invention, and No. 5 is a cold wire drawing in which the length of the α phase does not satisfy the requirements of the present invention. A comparative example in which the area reduction ratio does not satisfy the above-described manufacturing conditions of the wire, No. 6 is a comparative example in which the length of the α phase does not satisfy the requirements of the present invention, and No. 7 is an α phase having a width of 0.05 μm or more in the metal structure No. 8 is a comparative example in which the requirements of the present invention are not satisfied and the aging treatment temperature does not satisfy the above-described manufacturing conditions of the wire, No. 8 is that the maximum particle size of the β phase and the length of the α phase satisfy the requirements of the present invention. Comparative example in which the conditions for hot rolling are not satisfied and the manufacturing conditions for the wire described above are not satisfied, No. 15 indicates that T1 does not satisfy the requirements of the present invention and the α phase having a width of 0.05 μm or more does not exist in the metal structure However, this is a comparative example that does not satisfy the requirements of the present invention.

  In these comparative examples, the number of rupture cycles obtained in the Ono-type rotating bending fatigue test is all 200,000 (2.0E + 05) or less, and it cannot be said that the β-type titanium alloy has excellent fatigue strength.

DESCRIPTION OF SYMBOLS 1 ... alpha phase 1a ... grain boundary alpha phase 1b ... intragranular alpha phase 2 ... beta phase 3 ... slip band

Claims (3)

A β-type titanium alloy containing, in mass%, V of 4.0 to 10.0%, Sn of 2.0% or more, and Al of 2.0% or more,
In the metal structure, the α phase having a width of 0.05 μm or more is a shape that fits between any two straight lines parallel to each other with a spacing of 0.3 μm, and the length is 5 μm or less. ,
The maximum particle size of the β phase is 50 μm or less,
A β-type titanium alloy having excellent fatigue strength, wherein T1 defined by the following formula is more than 700 and less than 800.
T1 = 937 + 147.7 [O] +20.4 [Al] +161.8 [C] -19.8 [Fe] -10.3 [Mo] -8.4 [Nb] -13.1 [V] -17 0.0 [Cr] -0.2 [Sn] -8.5 [Cu]
However, in the above formula, [] indicates the content (% by mass) of each element.   Furthermore, by mass%, Cr is 5.0 to 9.0%, Fe is 0.3 to 3.5%, Sn is 2.0 to 5.0%, and Al is 2.0 to 4.5%. The β-type titanium alloy having excellent fatigue strength according to claim 1, further comprising Ti and inevitable impurities. The β-type titanium alloy having excellent fatigue strength according to claim 2, further comprising 0.5% by mass or less of Si, and T1 defined by the following formula being more than 700 and less than 800.
T1 = 937 + 147.7 [O] +20.4 [Al] +161.8 [C] -19.8 [Fe] -10.3 [Mo] -8.4 [Nb] -13.1 [V] -17 0.0 [Cr] -0.2 [Sn] -8.5 [Cu] -12 [Si]
However, in the above formula, [] indicates the content (% by mass) of each element.

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