JPH10306335A - Alpha plus beta titanium alloy bar and wire rod, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an alpha plus beta titanium alloy bar and wire rod which are produced by means of a refining - rolling process and have a structural form composed essentially of equiaxed α-grains and equiaxed β-grains and in which the average grain size of the equiaxed α-grains and that of the equiaxed β-grains are regulated to <=1 μm, respectively. SOLUTION: The bar and wire rod have a β-transformation point temp. (Tβ ) of 860 to 920 deg.C and also have a composition consisting of, by weight, 3-5% Al, <=0.2% O, <=0.1% N, <=0.1% C, 2-7% V, 1.0-3.5%, in total, of one or >=2 elements selected from the group consisting of Nb, Mo, and W, 0.5-3.0%, in total, of one or >=2 elements selected from the group consisting of Cr, Mn, Fe, Co, and Ni, and the balance Ti with inevitable impurities. Moreover, the bar and wire rod have a structural form composed essentially of equiaxed α-grains and euiaxed β-transformed structures, and the equiaxed α-grains and the equiaxed β-transformed structures are superfine grains having <=1 μm average grain size, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an equiaxed α-grain and an equiaxed β
The present invention relates to a titanium alloy rod or wire having a transformed structure (hereinafter referred to as “β grain”), an ultrafine grain having an average particle diameter of 1 μm or less, and excellent in hot workability, strength and ductility, and a method for producing the same.

[0002]

2. Description of the Related Art As a means for improving the balance between strength and ductility of a metal material and improving fatigue characteristics, it is a well-known metallurgical technique to refine crystal grains constituting the material. In general, even for a titanium alloy which is lightweight and has good corrosion resistance, refinement of crystal grains is effective as a means for improving the balance between strength and ductility and improving fatigue characteristics. Further, grain refinement is also effective for improving hot workability and superplasticity, and alloys showing excellent superplasticity by having fine crystal grains and a method for producing the same have already been disclosed in several patent documents or There are reports.

For example, Japanese Patent Publication No. 8-19502 (Prior Art Document 1) discloses that, by weight%, Al: 5.5 to 6.75%, V:
3.5-4.5%, O: 0.2% or less, Fe: 0.15
To 3.0%, Cr: 0.15 to 3.0%, Mo: 0.8
A titanium alloy containing 5 to 3.15% is heated at a temperature equal to or higher than (β transformation point -200 ° C.) and lower than β transformation point, and is subsequently reduced at a temperature lower than β transformation point to a reduction ratio of 3 or more. The average particle size of the α-crystal is reduced to 6 μm or less. According to Japanese Patent Publication No. Hei 8-19503 (prior art document 2), Al: 5.5 to 6.75% by weight, V: 3.5-4.5
%, O: 0.2% or less, Cr: 0.85 to 3.15%,
Similarly, a titanium alloy containing 0.85 to 3.15% of Mo is heated at a temperature lower than the β transformation point, and subsequently subjected to reduction at a temperature lower than the β transformation point to reduce the reduction ratio to 3 or more. , The average grain size of the α-crystal is 6 μm or less, and Japanese Patent Publication No. 8-23053 (Prior Document 3) discloses that Al:
3.0-5.0%, V: 2.1-3.7%, Mo: 0.
85 to 3.15%, O: 0.15% or less, and F
e, one, two or more of Ni, Co and Cr, and 0.85% ≦ Fe% + Ni% + Co%
+ 0.9 × Cr% ≦ 3.15% and 7% ≦ 2 × Fe%
+ 2 × Ni% + 2 × Co% + 1.8 × Cr% + 1.5 ×
A titanium alloy satisfying the condition of V% + Mo% ≦ 13% is heated at a temperature of (β transformation point−250 ° C.) or more and less than the β transformation point, and then 5% with respect to the heated titanium alloy material.
It is described that by performing hot working at a rolling reduction of 0% or more, the average particle size of α grains becomes 5 μm or less.

[0004] Also, Prior Art 4 (Materials Science an
d Engineering A, A203 (1995), pages L1 to L4) include Ti-6Al-4V, Ti-3Al-5V-5M.
o, Ti-6.5Al-3.6Mo-0.6Fe-2C
With respect to the three alloys of r-0.3Si, processing with low workability in an intermediate temperature range (500 ° C. or less) where recrystallization does not occur and annealing are alternately repeated to obtain a true strain (ε) of 0.9. ~
There is a report that a grain structure of 1 μm or less can be obtained by applying a total reduction of 1 (converted to a reduction ratio of 60 to 63%). It is stated that the finer the grain, the more the β-stabilizing element is.

Further, Reference Document 5 (Journal of the Japan Institute of Metals, Vol. 56, No. 11 (1992), 1351-2359)
Page) describes that, for the Ti-6Al-4V alloy, there is a limit to the fine crystal grain size that can be achieved by the normal ingot-rolling process, so other methods, namely, hydrogen storage annealing in a hydrogen atmosphere, hydrogen absorption, It is reported that a Ti alloy having a fine grain structure with an average grain size of about 1 μm can be obtained by a process including a carbide precipitation annealing, hot working, and dehydrogenation / recrystallization annealing in a vacuum.

[0006]

The fine grain size achievable in a normal ingot-rolling process depends on the alloy component,
There are limitations depending on the manufacturing process up to the final product.
Although the above-mentioned prior art documents 1 to 3 obtain a Ti alloy by a normal ingot-rolling process, the average grain size of α crystals is specified to be 5 to 6 μm or less, but the achievable fine grain size is described. There is no. It is not possible to obtain an ultrafine grain structure in which each of the α grains and the β grains has a crystal grain size of 1 μm or less based only on the knowledge disclosed in the prior art. Also in the examples, all the obtained α particle sizes are 1.9 μm or more.

[0007] Further, when defining the microstructure of α grains and β grains, it is not sufficient to simply define the α grain size. This is because, when the amount of α grains itself is small, such as immediately below the β transformation point, the size of β grains may become coarse while only the size of α grains may become small. There is no mention to mention.

[0008] Further, in the alloy having the composition shown in the prior art document 4, processing with low workability in an intermediate temperature range (500 ° C or less) where recrystallization does not occur and annealing are alternately repeated to obtain a true strain (ε). 0.9 to 1 (converted to rolling reduction, 60 to
Unless a complicated manufacturing process is applied, that is, a total reduction of 63%), a titanium alloy consisting of fine grains of 1 μm or less cannot be obtained, and the disadvantage from an economic viewpoint cannot be denied.

[0009] In addition, the alloy having the composition shown in the prior art document 5 requires a complicated manufacturing process of hydrogen storage annealing in a hydrogen atmosphere, hydride precipitation annealing, hot working, dehydrogenation and recrystallization annealing in vacuum. Otherwise, a titanium alloy consisting of fine particles of 1 μm or less cannot be obtained, and the disadvantage from an economic viewpoint cannot be denied.

It is an object of the present invention to provide a method for producing an ultrafine-grained titanium alloy having an average particle size of 1 μm or less by a usual ingot-rolling process which is advantageous from an economic viewpoint, and an object of the present invention. Things.

[0011]

In order to solve the above-mentioned object, the present inventors have studied the alloy components and the alloy components such as the β transformation point temperature (T _β ) and the manufacturing conditions such as the rolling and heat treatment conditions. By combining them, we find conditions that can achieve the purpose,
The present invention has been completed.

That is, the present invention provides (1) β transformation temperature (T
_β ) is 860 ° C. or more and 920 ° C. or less, and A
l: 3 to 5%, O: 0.2% or less, N and C: each 0.1% or less, V: 2 to 7%, further Nb, Mo, W
One or more of the group consisting of
0-3.5% by weight, and Cr, Mn, Fe, Co, Ni
One or more of the group consisting of
5 to 3.0% by weight, the balance being composed of Ti and unavoidable impurities, and having a structural form basically consisting of equiaxed α grains and an equiaxed β transformation structure. (Α + β) type titanium alloy rod or wire having an average particle diameter of 1 μm or less, (2) β transformation point temperature (T _β ) of 860 ° C. or more and 920 ° C. or less, and Al: 3 to 5% by weight. %, O:
0.2% or less, N and C: each 0.1% or less, V: 2
-7%, one or more of the group consisting of Nb, Mo, W, 1.0-3.5% by weight in total, and one of the group consisting of Cr, Mn, Fe, Co, Ni. comprises 0.5 to 3.0 wt% in total species or two or more, the remainder, to the alloy material consisting of Ti and inevitable _{impurities, (T β -250) (℃} ) above, (T _beta -25)
Start processing after heating to (° C) or less, (T _β- 100)
(° C) or less, the hot working step with a total cross-sectional reduction rate of 95% or more, followed by ( _Tβ- 100)
(° C.) or less and P = (T + 273) (20 + log
t) / 1000 (T: heat treatment temperature (° C.), t: heat treatment time (hour))
Performing a process including a heat treatment under conditions satisfying 0.0, and having a structure morphology consisting essentially of equiaxed α grains and an equiaxed β transformation structure, A method for producing an (α + β) type titanium alloy rod or wire, wherein the average particle size is 1 μm or less.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a Ti alloy according to the present invention
The β transformation point temperature (T _β ) needs to be 860 ° C or more and 920 ° C or less. If the β transformation point (T _β ) is lower than 860 ° C., the average particle size of the β phase cannot be suppressed to 1 μm or less. If the β transformation point temperature (T _β ) is higher than 920 ° C., crystal grain growth in heat treatment for obtaining an equiaxed structure cannot be suppressed, and the average grain size of each of the equiaxed α grains and the equiaxed β grains is 1 μm. The following ultrafine grain structure cannot be obtained. Therefore, the β transformation point temperature (T _β ) is 860 ° C. or more and 920 ° C.
Must be:

Next, the reasons for limiting the conditions relating to the alloy components will be described. As described above, the Ti according to the present invention
The alloy has a β transformation point temperature (T _β ) of 860 ° C or more and 920 ° C
The alloying elements described below must be within the respective limited ranges and have a β transformation point temperature (T _β ) of 86.
It is added so as to satisfy the condition of 0 ° C. or more and 920 ° C. or less.

Al is an element that stabilizes the α phase in the α / β phase equilibrium and raises the β transformation point temperature (T _β ). If it is less than 3%, sufficient strength cannot be obtained, and
If it exceeds 5%, the fatigue life strength at room temperature decreases,
It is not preferable in practical use. Therefore, it is restricted to 3 to 5%.

O, N, and C are also elements that stabilize the α phase in the α / β phase equilibrium and increase the β transformation point temperature (T _β ). However, the content is O: 0.2%, N: 0.1%,
C: If it is more than 0.1%, the rolling processability is reduced, and not only is it difficult to obtain a sound product without cracks, but also the ductility and toughness of the product are significantly reduced. Therefore,
O: 0.2%, N: 0.1%, C: 0.1%, respectively
It is regulated as follows.

V is an element that stabilizes the β phase in the α / β phase equilibrium and lowers the β transformation point temperature (T _β ). The diffusion coefficient in the alloy is not as small as Nb, Mo, and W, but Cr, Mn, Fe, Co, N
Unlike i, there is an effect of increasing strength without forming an intermetallic compound that impairs ductility and toughness with Ti. If the amount is less than 2%, the effect of increasing the strength is not clear, and if it is more than 7%, the β phase is too stabilized, and the β particle size cannot be suppressed to 1 μm or less. Therefore, the amount of V is limited to 2 to 7%.

Nb, Mo, and W are also α
Is an element that stabilizes the β phase in the / β phase equilibrium.
The transformation point temperature ( _Tβ ) is decreased. Further, since these elements have a very small diffusion coefficient in the alloy, they have the effect of delaying the crystal grain growth, and the average of the equiaxed α grains and the equiaxed β grains, which is the target of the present invention, is It is an indispensable element for achieving an ultrafine grain structure having a grain size of 1 μm or less. If the total amount of these elements is less than 1%, the effect of delaying the crystal grain growth cannot be sufficiently obtained, and if the total amount exceeds 3.5%, the crystal grain growth is delayed. The effect of saturating the Ti alloy is saturated, and on the other hand, not only does the great feature of the Ti alloy that it has a low specific gravity is impaired, but also all are expensive, so there are great economical disadvantages. Therefore, the added amount of Nb, Mo, and W is limited to 1% by weight or more and 3.5% by weight or less in total of one or more kinds.

Cr, Mn, Fe, Co, and Ni are also elements that stabilize the β phase in the α / β phase equilibrium, and lower the β transformation point temperature (T _β ). In particular, Mn,
Fe, Co, and Ni have a large diffusion coefficient in the alloy and are disadvantageous from the viewpoint of suppressing crystal grain growth.
The combined addition with b, Mo, and W has the effect of suppressing crystal grain growth and reducing high-temperature deformation stress, facilitating hot working and superplastic forming. Further, the strength of the material at room temperature Contribute to the rise. The total amount added is 0.
If it is less than 5% by weight, the desired effect is not clear, and if it is more than 3% by weight in total, formation of an intermetallic compound with Ti becomes remarkable, and ductility and toughness are impaired. Therefore, when added, the total content is limited to 0.5 to 3.0% by weight.

Further, with respect to Sn and Zr, which do not significantly affect the β transformation point temperature, if the β transformation point temperature is within the limitation range of the present invention, and particularly within a range that does not adversely affect the mechanical properties, It can be added.

Further, Si and B form a fine compound phase with Ti, and are expected to have an effect of suppressing the growth of crystal grains. However, if the added amount is large, the mechanical properties, particularly ductility and toughness, are reduced. Spoil. Regarding these elements, the β transformation point may be added as long as the β transformation point temperature is within the limitation range of the present invention, and as long as the mechanical properties are not adversely affected.

As described above, by combining the limited alloy components and the manufacturing conditions described below, the structure morphology consisting essentially of equiaxed α grains and equiaxed β grains, which the present invention aims to obtain, is obtained. Then, a titanium alloy rod having an ultrafine grain structure in which the average particle diameter of each of the equiaxed α grains and the equiaxed β grains is 1 μm or less can be obtained.

The rolling heating temperature is the above β transformation point temperature (T
_β ), (T _β -250) (° C.) or more, (T _β −
25) The temperature must be within (° C.) or lower. With respect to the alloy, the heating temperature is (T _beta -250) below (° C.), the rolling load becomes high, and the like cracks in the material tends to occur, the actual production, which is disadvantageous. Also, (T _β −2
5) If the temperature is higher than (° C.), the phase ratio of the β phase at that temperature is high, and as a structure after rolling, a structure composed of fine α grains and β grains elongated in the rolling direction cannot be obtained. As a result, in the structure obtained by heat treatment, the β diameter is particularly 1 μm.
It cannot be suppressed below. Therefore, rolling heating _{temperature, (T β -250) (℃} ) above, (T _beta -25)
(° C.) Limit to the following temperature range.

Hot working must be completed at (T _β -100) (° C.) or less. With respect to the alloy, the end temperature of the hot working is higher than _{(T β -100) (℃)} , its immediate recrystallization at temperatures cause grain growth, the average respectively equiaxed α grains and equiaxed beta grain An ultrafine grain structure having a grain size of 1 μm or less cannot be obtained.

Further, the hot working needs to be a strong working with a reduction ratio of total cross section of 95% or more. If the total cross-sectional reduction rate is less than 95%, the accumulation of strain for recrystallization to form an equiaxed structure is insufficient, and the heat treatment after processing does not progress the equiaxing, or Even if the formation proceeds, it is not possible to obtain an ultrafine grain structure in which the average grain size of each of the equiaxed α grains and the equiaxed β grains is 1 μm or less.

In the case of a plate material, the structure particularly in a cross section parallel to the plate surface tends to become flat and coarse, and even after the heat treatment in the next step, the average particle size of each of the equiaxed α grains and the equiaxed β grains is reduced. It is difficult to obtain an ultrafine grain structure of 1 μm or less.
The form after rolling is basically a rod or a wire.

Subsequently, it is not more than (T _β -100) (° C.) and P = (T + 273) (20 + log t) / 100
0 (T: heat treatment temperature (° C.), t: heat treatment time (hour))
Is performed under the condition that the parameter P represented by the following expression satisfies 19.2 to 20.0. In _(T β -100) (℃) higher than the heat treatment temperature,
With an ordinary industrial heat treatment time, it is not possible to obtain an ultrafine grain structure in which each of the equiaxed α grains and the equiaxed β grains has an average grain size of 1 μm or less. Also, P = (T + 273) (20 + log
Under the condition that the parameter P expressed by t) / 1000 is smaller than 19.2, the equiaxing of the structure does not proceed sufficiently, especially when the particle size is measured by the line segment method in the direction parallel to the rolling. , Β particle size cannot be less than 1 μm. Also,
Under the condition where the parameter P is larger than 20.0, crystal grain growth cannot be suppressed, and it is not possible to obtain an ultrafine grain structure in which the average grain size of each of the equiaxed α grains and the equiaxed β grains is 1 μm or less.
For example, when the heat treatment temperature is 700 ° C., the heat treatment time is in the range of about 35 minutes to 3 hours and 30 minutes. If the heat treatment time is shorter than this range, the equiaxing does not proceed sufficiently. However, if the time is too long, the crystal grain growth proceeds, and it is not possible to obtain an ultrafine grain structure in which the average grain diameter of each of the equiaxed α grains and the equiaxed β grains is 1 μm or less.

Further, after the heat treatment, aging treatment may be performed for the purpose of strengthening by precipitation strengthening, etc., but the structure is obtained by the heat treatment in the previous stage, that is, basically, equiaxed α grains and equiaxed α grains are formed. Heat treatment such as aging treatment may be performed as long as the average particle size of the equiaxed α grains and the equiaxed β grains does not deviate from the ultrafine grain structure of 1 μm or less.

As described above, by combining the limited alloy components and the production conditions, the microstructure basically has a structure of equiaxed α grains and equiaxed β grains, and the average grain size of the equiaxed α grains and the equiaxed β grains respectively. Consisting of an ultrafine grain structure with a diameter of 1 μm or less (α + β)
A shaped titanium alloy bar can be obtained.

[0030]

【Example】

(Example 1) Alloys having the compositions shown in Table 1 were produced by arc melting. After hot forging, it was subjected to hot rolling. The β transformation temperature (T _β ) is obtained by continuously measuring the electric resistance from room temperature to 1000 ° C. on a sample taken from a hot forged material, and is obtained by bending a temperature-electric resistance curve, and Evaluation was made by microstructure observation.

From the forged material, 50 mmφ round bars were sampled, and as shown in Table 2, the heating temperature was set at (T _β -110) ° C. as a guide. After heating, the plate is subjected to caliber rolling at a cross-sectional reduction rate of 5 to 20% per pass.
The finishing temperature after finishing (total cross-sectional reduction rate: 96%) is as shown in Table 2. The alloy number B8
No. to B10 had poor rolling workability and caused many cracks during rolling.

Subsequently, a heat treatment was carried out at 700 ° C. for 1 hour and air-cooled. Parameter P corresponding to this heat treatment condition
Is 19.46. The microstructure was observed for a cross section parallel to the rolling longitudinal direction, and the crystal grain size was measured by a line segment method in a direction parallel to the rolling direction and in a direction perpendicular to the rolling direction. The ASTM tensile test piece (parallel diameter:
6.25 mmφ, gauge length: 25 mm) were taken, and the tensile properties at room temperature were evaluated. The initial strain rate in the tensile test is approximately 1 × 10 ⁻³ s ⁻¹ . Also, 8
A cylindrical compression test specimen having a diameter of 12 mm and a height of 12 mm was collected,
A compression test was performed at 00 ° C. at a strain rate of 1 × 10 ⁻³ s ⁻¹ to determine the compression deformation resistance. In addition, alloy numbers B8 to B1
With respect to 0, since a sufficient amount of a sound part without cracks was not obtained, it was not subjected to the tensile and compression tests.

Table 3 shows the measurement results of the crystal grain size. In the alloy numbers A1 to A11 of the present invention, in the cross-sectional structure parallel to the rolling, α grains and β grains measured by a line segment method in a direction parallel to the rolling and in a direction perpendicular to the rolling are all included. 1 μm or less, and it can be seen that α-particles and β-particles have an equiaxed structure with a particle diameter of 1 μm or less.

On the other hand, the comparative examples (B1 to B7, B1
In (1, B12), any of the measured values of α grains and β grains measured by the line segment method in the direction parallel to the rolling and the direction perpendicular to the rolling is a value larger than 1 μm.
For example, in alloy number B3 in which the amount of V added is large and T _β is lower than 860 ° C., the α grains are 1 μm or less in a direction parallel to rolling and in a direction perpendicular to rolling, but β The measured value of the grain is 2.38 μm and 1 μm
Greater than. The measured value of β grains in the direction perpendicular to the rolling is 0.90 μm, which indicates that the β grains are not sufficiently equiaxed.

Further, for example, in the alloy number B5 of the group consisting of Nb, Mo and W with a small amount of element added, α
Being unable to suppress any grain growth of β grains, α grains, β grains
The measured value of each grain is a value larger than 1 μm.

FIG. 1 shows the measurement results of the α grain size and the β grain size in the direction parallel to the rolling. It is an example of the present invention. T _β
Are 860 ° C or more and 920 ° C or less, alloy numbers A1 to A
In No. 11, a microstructure of 1 μm or less was obtained for both α grains and β grains. On the other hand, T _β is 860 ° C
For comparative examples that are lower or higher than 920 ° C.
Either α grains or β grains or both are larger than 1 μm. Even when T _β is in the range of 860 ° C. or more and 920 ° C. or less, alloy No. B5, in which the amount of addition of the group of Nb, Mo, and W is small, cannot suppress the grain growth,
Both β grains are larger than 1 μm. In addition, alloy numbers B8 to B10 with a large amount of added O, N, and C have a fine structure, but as described above, generate a large number of cracks during rolling, and can obtain a sound product. Did not.

Table 4 shows the results of the tensile test at room temperature and the compressive deformation resistance at 700 ° C. Alloy Nos. A1 to A11 of the present invention examples having an equiaxed submicron structure
Shows a good strength-ductility balance at room temperature, as shown in FIG. Furthermore, the deformation resistance at 700 ° C. is low due to the formation of a submicron structure. That is, in the alloy numbers A1 to A11 of the present invention, a good balance between strength and ductility at room temperature and a good balance between hot workability are obtained. It can be seen that this has been achieved.

On the other hand, the alloy number B used for the mechanical test
In 1 to B7, B11 and B12, generally ductility,
In particular, since the drawing is low and the compressive deformation resistance at 700 ° C. is high, a good balance between strength and ductility at room temperature and good hot workability have not been achieved at the same time. For example, alloy number B6, in which the amount of addition of the group consisting of Cr, Mn, Fe, Co, and Ni is small, exhibits relatively large ductility at room temperature, but has low strength at room temperature and high deformation resistance at 700 ° C. Conversely, Cr, Mn, Fe, C
Alloy No. B7 with a large addition amount of the group consisting of o and Ni
Has a relatively high strength at room temperature and a relatively low deformation resistance at 700 ° C., but a low ductility at room temperature.

(Example 2) Alloy number A2 (T _β :
(881 ° C.) after arc melting and hot forging, a 50 mmφ round bar was sampled, and as shown in Table 5, the heating temperature was changed to five levels, and the cross-sectional reduction rate per pass was 5 to 20%. 15 mmφ (total cross-section reduction rate: 91 mm)
%) And 10 mmφ (total cross-sectional reduction rate: 96%). The finishing temperature is as shown in Table 5.
The heating temperature (T _beta -25) lower than ° C., 600 ° C.
In the case of No. 1, rolling was stopped because cracks occurred in the early stage of rolling. Subsequently, a heat treatment was performed at 700 ° C. for 1 hour and air-cooled. Parameter P corresponding to this heat treatment condition
Is 19.46. The microstructure was observed for a cross section parallel to the rolling longitudinal direction, and the crystal grain size was measured by a line segment method in a direction parallel to the rolling direction and in a direction perpendicular to the rolling direction.

Table 6 shows the results of measuring the crystal grain size. Thus, the rolling heating temperature is set to 860 ° C. higher than (T _β− 25) ° C.
In alloy Nos. D1 and D2 and alloy Nos. D2 to D5 in which the cross-sectional reduction rate in rolling was smaller than 95%, the size of β grains exceeded 1 μm in all cases, and the average grain size of α grains and β grains respectively An ultrafine grain structure having a diameter of 1 μm or less was not obtained.

(Example 3) Alloy No. A2 (T _β :
(881 ° C.) after arc melting and hot forging, collecting a 50 mmφ round bar, heating to 840 ° C., and subjecting to caliber rolling at a cross-sectional reduction rate of 5 to 20% per pass, as shown in Table 7. The finish temperature was changed to 10 mmφ (total cross-section reduction rate: 96%). Subsequently, a heat treatment was performed at 700 ° C. for 1 hour and air-cooled. The value of the parameter P corresponding to this heat treatment condition is 19.46. The microstructure was observed for a cross section parallel to the rolling longitudinal direction, and the crystal grain size was measured by a line segment method in a direction parallel to the rolling direction and in a direction perpendicular to the rolling direction. Table 7 shows the measurement results. With alloy number F1, in which the finishing temperature was higher than ( _Tβ- 100) ° C., an ultrafine grain structure in which each of the α grains and the β grains had an average grain size of 1 μm or less was not obtained.

(Example 4) Alloy number A2 (T _β :
(881 ° C.) after arc melting and hot forging, collecting a 50 mmφ round bar, heating to 840 ° C., and subjecting the bar to caliber rolling at a cross-sectional reduction rate of 5 to 20% per pass, and 10 mm φ
(Total cross-section reduction rate: 96%). Finishing temperature is 6
70 ° C. After cutting the obtained round bar, it was heat-treated under the heat treatment conditions (temperature, time) shown in Table 8, and air-cooled. In the table, the value of the parameter P is also described. The microstructure was observed for a cross section parallel to the rolling longitudinal direction, and the crystal grain size was measured by a line segment method in a direction parallel to the rolling direction and in a direction perpendicular to the rolling direction.

Table 9 shows the measurement results. As can be seen from this result, the parameter P (P = (T + 273) (20
+ Log t) / 1000) for alloy numbers G1 to G4 and A2 in the range of 19.2 to 20.0, α
An ultrafine grain structure in which each of the grains and the β grains had an average grain size of 1 μm or less was obtained. On the other hand, alloy numbers H1 and H3, whose P values are smaller than 19.2, have β grains measured in a direction parallel to the rolling, although the measured values of β grains measured by a line segment method in a direction perpendicular to the rolling are small. The measured value of the grains is large, and it can be seen that the equiaxing is not sufficiently advanced. Also, if the P value is 2
For alloy numbers H2, H4, H5 greater than 0.0,
The crystal grain growth during the heat treatment was not suppressed, and an ultrafine grain structure in which the average grain size of each of the α grains and the β grains was 1 μm or less was not obtained.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

[0047]

[Table 4]

[0048]

[Table 5]

[0049]

[Table 6]

[0050]

[Table 7]

[0051]

[Table 8]

[0052]

[Table 9]

[0053]

As described above, the titanium alloy rod or wire manufactured by the predetermined manufacturing method with respect to the titanium alloy having the alloy composition according to the present invention has an average grain diameter of each of α grains and β grains of 1 μm or less. It has a certain ultra-fine grain structure. Titanium alloy rod and wire having such an ultrafine grain structure has good strength-ductility balance at room temperature and good hot workability, and as a product, or as a material for superplastic working or hot forging. Are also suitable.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between T _β and measured values of the particle diameters of α grains and β grains measured by a line segment method along a direction parallel to rolling for invention examples and comparative examples.

FIG. 2 is a diagram showing a relationship between tensile strength and ductility (drawing) for an invention example and a comparative example.

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Claims (2)

[Claims] 1. The β transformation point temperature (T _β ) is 860 ° C. or higher and 9
20 ° C. or less, Al: 3 to 5% by weight, O:
0.2% or less, N and C: each 0.1% or less, V: 2
-7%, one or more of the group consisting of Nb, Mo, W, 1.0-3.5% by weight in total, and one of the group consisting of Cr, Mn, Fe, Co, Ni. Containing 0.5 to 3.0% by weight of a kind or two or more kinds in total, consisting of a balance, Ti and unavoidable impurities, and having a structure form basically consisting of equiaxed α grains and equiaxed β transformation structure, etc. Α axis grain and equiaxed β transformation structure, each average grain size is 1μm
An (α + β) type titanium alloy rod or wire, characterized by the following. 2. The β transformation point temperature (T _β ) is at least 860 ° C.
20 ° C. or less, Al: 3 to 5% by weight, O:
0.2% or less, N and C: each 0.1% or less, V: 2
-7%, one or more of the group consisting of Nb, Mo, W, 1.0-3.5% by weight in total, and one of the group consisting of Cr, Mn, Fe, Co, Ni. comprises 0.5 to 3.0 wt% in total species or two or more, the remainder, to the alloy material consisting of Ti and inevitable _{impurities, (T β -250) (℃} ) _above, (T β -25) ( ° C.) to start the heating after processing below, _(T β -100) (ends the processing at ° C.) or less, and the processing between the total reduction of area of 95% or more thermal processes, it pulled followed by (T _beta -100 ) (° C.) or less and P =
(T + 273) (20 + log t) / 1000 (T: heat treatment temperature (° C.), t: heat treatment time (hour)) and a step including heat treatment under the condition that parameter P satisfies 19.2 to 20.0. Performed, has a structure morphology consisting essentially of equiaxed α grains and equiaxed β transformation structure, wherein the average grain size of the equiaxed α grains and the equiaxed β transformation structure is 1 μm or less. A method for producing a (α + β) type titanium alloy rod or wire.