JPH09228013A - Method for manufacturing seamless tube made of α + β type titanium alloy - Google Patents

Abstract

(57) 【Abstract】 PROBLEM TO BE SOLVED: To provide strength, ductility,
Provided is a method for producing a seamless tube made of an α + β type titanium alloy, which is excellent in mechanical properties such as fatigue strength and fracture toughness. SOLUTION: In a method for continuously producing a seamless tube made of an α + β type titanium alloy by a hot rolling process such as piercing and drawing, forming, drawing, etc., a β transformation point +
The final rolling process is terminated at a temperature of β transformation point + 250 ° C. or lower at a temperature higher than 50 ° C., cooled at a cooling rate of air cooling or higher, and β
Annealing is performed in a specific temperature range below the transformation point, or two-step heat treatment is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a seamless tube made of α + β type titanium alloy.

[0002]

2. Description of the Related Art Titanium alloys are lightweight, have high strength, and have high corrosion resistance, and thus have been most suitable for extreme environments such as geothermal development, undersea oil field / gas field development, etc., in extreme depth, high temperature, high pressure, and high corrosion. Has attracted attention as a material. Among them, α + β type titanium alloys, which are widely used in aircraft applications and have a proven track record, and α + β type titanium alloys with high corrosion resistance, in which a small amount of Pd and Ru are added to improve corrosion resistance, are promising as excellent materials for extreme environments. Is being watched. In the above applications, the pipe is the main product shape, but as the method for producing the titanium alloy pipe material, the method of bending and welding the plate (welded pipe), the method by hot extrusion (seamless pipe), the plug mill It is possible to use a method of producing a pipe (continuous pipe) by continuously performing rolling such as piercing, stretching, forming, and drawing.

[0003] Of these, punching a heated solid billet,
The method of continuously forming a hollow tube by a rolling process such as drawing, forming, drawing, etc. can make a seamless pipe without a welded part, which is likely to deteriorate the characteristics, so it is extremely difficult to repair or replace parts. Even in the above extreme environment applications, in addition to being able to be used stably for a long period of time, this method has a high material yield,
This is a particularly advantageous method for a titanium alloy whose material itself is expensive because of its high production efficiency.

[0004]

However, in the above method, α
When manufacturing a seamless tube made of + β type titanium alloy, there are the following problems. That is, in the seamless pipe manufactured by piercing and rolling, unlike the case of a plate, the movement of the material during hot working is limited to the length direction, and therefore the material anisotropy is likely to occur. In particular, α + β, which contains the α-phase with the most dense hexagonal structure with poor symmetry as the main phase
Type titanium alloys have stronger anisotropy than cubic materials with high symmetry, such as steel and aluminum alloys. As a method to eliminate this material anisotropy,
By heating and holding at a temperature above the β transformation point, all materials are transformed into isotropic cubic β phase, and then α phase with random orientation is precipitated during cooling without orientation integration in a specific orientation. Or a method of stabilizing annealing in the α + β two-phase temperature range. However, in these methods, while heating and holding above the β transformation point,
There is a problem that the β crystal grains become coarse and the ductility becomes extremely poor.

In view of the above problems, the present invention has a seamless tube made of α + β type titanium alloy which has no material anisotropy and is excellent in mechanical properties such as strength, ductility, fatigue strength, fracture toughness and the like. It aims at providing the method of manufacturing.

[0006]

In order to achieve the above object, the present invention has the following method. That is,
(1) A seamless tube made of α + β type titanium alloy,
In the method of continuously manufacturing by hot rolling process such as piercing and stretching, forming, drawing, etc., β transformation point + 50 ° C
The final rolling step is completed in the temperature range of super β transformation point + 250 ° C or less, cooling is performed at a cooling rate of air cooling or more, and annealing is performed at a temperature of 700 ° C or more and β transformation point of less than -100 ° C for 30 minutes or more. A method for producing a seamless tube made of α + β type titanium alloy, which is characterized. And (2) a solid billet made of α + β type titanium alloy, with a β transformation point or more and a β transformation point +
It is characterized in that it is heated to a temperature of 300 ° C. or lower, perforated, cooled to a temperature below the β transformation point in the middle of a rolling process such as stretching, forming, drawing, etc., and the final rolling process is completed. 1) A method for producing a seamless tube made of the α + β type titanium alloy described in 1). And (3) In the method according to (1) or (2), instead of the annealing, heating and holding is performed for 30 minutes or more in a temperature range of β transformation point −100 ° C. or more and less than β transformation point, and cooling is performed by air cooling or more. Cool at a speed of 4 more
A method for producing a seamless tube made of α + β type titanium alloy, characterized by heating and holding at 50 to 650 ° C. for 1 hour or more and 8 hours or less, wherein (4) (1) or (2) Instead of the annealing, β transformation point −10
Heating and holding in a temperature range of 0 ° C or higher and lower than β transformation point for 30 minutes or longer, cooling at a cooling rate of air cooling or higher, and further heating at a temperature higher than 650 ° C and β transformation point of less than -100 ° C for 30 minutes or longer and 4 hours or shorter. A method for producing a seamless tube made of α + β type titanium alloy, which is characterized by holding.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The cause of the material anisotropy of α + β type titanium alloy is due to the formation of two textures, a work texture of α phase and a transformation texture from β phase. The former texture texture of α phase is inevitably generated when hot working is performed at a temperature of α + β two-phase region below the β transformation point where the α phase exists, but the material flow is limited in the length direction. This is especially noticeable in seamless tubes. As described in the section of the prior art, this work texture is resolved by annealing at a temperature above the β transformation point after hot working, but β grains become coarse during annealing, and ductility becomes extremely poor. The problem arises. Further, even if it is annealed in the two-phase temperature region below the β transformation point, this work texture cannot be resolved. The anisotropy remains.

On the other hand, the transformation texture from the β phase is subjected to hot working in a two-phase temperature region where there are many β phases immediately below the β transformation point or at a temperature above the β transformation point, and the plastic strain is used as the nucleus during cooling. To α
It is a texture that occurs when a phase is precipitated. This transformation texture is also resolved by heating and holding at a temperature above the β transformation point after hot working, but β grains become coarse during heat treatment,
The problem that ductility becomes extremely poor arises. Also, β
Even if annealed in the two-phase temperature range below the transformation point, the transformation texture does not disappear and the anisotropy remains.

On the basis of the above-mentioned texture generation principle, the present inventors have proposed a method for producing a seamless tube made of α + β type titanium alloy which does not develop the above two textures and is excellent in various characteristics including ductility. As a result of earnest research and development, the present invention has been completed.

In the method (1) of the present invention, in the method for continuously producing a seamless tube made of α + β type titanium alloy by hot rolling steps such as piercing and drawing, forming and drawing, first, β transformation point + 50 ° C above β transformation point +2
The final rolling process was completed in the temperature range of 50 ° C. or lower, and cooling was performed at a cooling rate of air cooling or higher. The reason for the limitation is
It is to prevent the formation of both the textured texture of the α phase and the transformation texture from the β phase, and also to suppress the coarsening of the β grains, which causes a decrease in ductility.

In order to prevent the formation of the α-phase working texture, the final working may be completed at the β transformation point or higher.
At the temperature just above the transformation point, the plastic strain is not sufficiently released during the cooling to the β transformation point after the end of processing, and therefore the transformation texture of the α phase is not maintained during the cooling at the temperature below the β transformation point. Will occur. However, if the final rolling is completed at a temperature higher than the β transformation point + 50 ° C, the plastic strain is released during cooling to the β transformation point, and then the α phase that precipitates during cooling at a temperature below the β transformation point. Has no texture and isotropic material is obtained. Further, the reason for deciding the final rolling at a temperature of β transformation point + 250 ° C. or lower is that if the working is finished at a temperature higher than this, not only the plastic strain is released during cooling to the β transformation point, This is because coarsening of β grains occurs and high ductility cannot be secured.

Further, the cooling rate after the end of the final rolling is regulated to be equal to or higher than the air cooling, because at a cooling rate lower than this, not only the plastic strain is released during the cooling to the β transformation point but also the β grains become coarse. The reason is that high ductility cannot be ensured due to the aging.

The tube produced through the hot working was then annealed at a temperature of 700 ° C. or higher and a β transformation point of less than −100 ° C. for 30 minutes or longer. This treatment is the release of strain due to heat shrinkage and the like caused by cooling after air cooling after the final rolling, and converts the unstable nonequilibrium structure into a sufficient stable structure, and has excellent mechanical properties, especially This is a process for ensuring high ductility. In particular, the reason why the annealing temperature is 700 ° C. or higher is that, at a temperature lower than this temperature, the release of strain and the stabilization of the structure are insufficient and sufficient ductility cannot be obtained,
Further, the annealing temperature is set to be less than the β transformation point −100 ° C., because if it is annealed at a temperature higher than this, thermal strain is generated again during cooling, or an unstable structure is generated, and ductility is lowered. Further, the reason why the annealing time is set to 30 minutes or longer is that the release of strain and the stabilization of the structure are not achieved in the time shorter than this,
This is because sufficient ductility cannot be obtained. The upper limit of the annealing time is not specified because the characteristics do not change so much even if annealing is performed for some time within the temperature range specified in the present invention. However, performing annealing for a too long time is wasteful in terms of energy, so it is desirable to perform annealing for an efficient time as appropriate according to the size and thickness of the tube. However, even in this case, as mentioned above,
A time of 30 minutes or more is necessary.

In the method (2) of the present invention, α
A solid billet made of + β-type titanium alloy is heated to a temperature of β-transformation point + 300 ° C or less above the β-transformation point to start perforation, and during the rolling process such as stretching, forming, drawing, etc., below the β-transformation point. Once cooled to the above temperature, the material temperature is raised by utilizing the heat generated during the hot working, and the final rolling process is completed at the temperature of β transformation point + 50 ° C or more and β transformation point + 250 ° C or less. By doing so, in addition to being easy to manufacture and extremely convenient, the β-grains are made smaller and more excellent mechanical properties can be obtained.

The reason why the solid billet is heated to the β transformation point or higher and the piercing is started is in the β single phase region where the hot deformation resistance is low (above the β transformation point), and most of the total working amount is This is to finish many of the piercing and stretching rolling processes,
Further, the heating temperature of the billet is set to β transformation point + 300 ° C or lower because the production scale is high at a temperature higher than the β transformation point because the production of oxide scale is severe, the yield is lowered, and surface flaws are generated. Because it is not practical.

In the middle of a series of hot working steps, cooling to the α + β two-phase temperature region below the β transformation point is determined by α
This is because by processing the + β two-phase mixed structure, the nuclei of β-grain formation are introduced when heated to a temperature above the β transformation point + 50 ° C., and the processing step in this two-phase temperature range is performed. By doing so, it is possible to obtain a β phase having a smaller particle size,
In particular, the ductility is improved. Where α + β type titanium alloy α
Although the deformation resistance in the + β two-phase temperature range is extremely large and it is difficult to work, most of the total amount of processing required to manufacture the pipe has already been completed by processing in the β range, so in the α + β two-phase temperature range The processing is slight and does not cause any trouble in manufacturing. In addition, the processing heat can be utilized with a small amount of processing by utilizing the characteristics peculiar to titanium alloys, which have low thermal conductivity. The process after the final rolling process is the same as in the present invention 1.
The same as above and as already described.

In the method (3) of the present invention, after the hot working of the method (1) or (2) of the present invention, the β transformation point is -100 ° C. or higher in place of the β annealing point instead of the annealing. The temperature is kept below 30 minutes or more, cooled at a cooling rate of air cooling or more, and further kept at 450 to 650 ° C. for 1 hour to 8 hours. This is an effective process when strength and fatigue strength are particularly important. The structural changes in this heat treatment process are as follows. First, the volume fraction of the α phase is reduced by heating and holding at a temperature of β transformation point −100 ° C. or higher and lower than the β transformation point for 30 minutes or more to form fine α grains (hereinafter referred to as primary α phase). . Next, by cooling at a cooling rate higher than air cooling, the β phase is converted into fine needle-shaped α
Convert to a phase structure or martensite structure. Furthermore, this fine acicular α-phase structure or martensite structure
It is heated and held at 450 ° C to 650 ° C for 1 hour or more and 8 hours or less, and converted into a fine and stable acicular α phase (hereinafter, secondary α phase). At this time, along with the stabilization of the structure, the strain generated during the cooling at the cooling rate higher than the air cooling is removed. Through the above series of steps, the refinement of the structure is achieved, and the strength and fatigue strength are improved.

Here, the first heating and holding is performed by the β transformation point -1.
The reason why the temperature is kept at a temperature of 00 ° C. or higher and lower than the β transformation point for 30 minutes or longer and cooled at a cooling rate of air cooling or higher is as follows. That is, the primary α phase is not sufficiently miniaturized unless it is heated to a temperature of β transformation point −100 ° C. or higher,
This is because even if the subsequent heat treatment is performed, the increase in strength and fatigue strength is slight, and if heating is performed above the β transformation point, the β phase becomes coarse and ductility decreases. If it is not heated and held for more than a minute, the primary α phase is not sufficiently miniaturized.Furthermore, unless it is cooled at a cooling rate of air cooling or higher, the primary α phase that has been miniaturized will grow again and become coarse during cooling, This is because improvement in strength and fatigue strength becomes insufficient. Here, the upper limit of the heating and holding time is not set because the characteristics do not change so much even if annealing is performed for some time within the temperature range specified in the present invention. However, performing annealing for a too long time is wasteful in terms of energy, so it is desirable to perform annealing for an efficient time as appropriate according to the size and thickness of the tube. However, in this case, as described above, a time of 30 minutes or more is necessary.

The heat retention in the following low temperature heat treatment is
The reason why the temperature is 450 to 650 ° C. for 1 hour or more and 8 hours or less is
For the following reasons. That is, in the case of heating and holding at a temperature of 450 ° C. or lower, or for a time of less than 1 hour, stabilization of tissue and release of strain are insufficient, while heating and holding at a temperature of higher than 650 ° C. or a time of more than 8 hours, This is because the secondary α phase is coarsened and the strength and fatigue strength are not sufficiently improved.

Next, when the fracture toughness is more important than the strength and the fatigue strength, the method (1) or (2) of the present invention is used.
In place of the annealing, as described in the method (4) of the present invention, after the hot working is finished, heating is performed at a temperature of β transformation point −100 ° C. or higher and lower than the β transformation point for 30 minutes or more, and air cooling is performed. Cooling is performed at the above cooling rate, and heating and holding is performed at a temperature higher than 650 ° C. and a β transformation point lower than −100 ° C. for 30 minutes or more and 4 hours or less. Here, the first heat treatment, β transformation point −100
Heating and holding at a temperature below the β transformation point for 30 minutes or more
The purpose of the step of cooling at a cooling rate of air cooling or higher is the same as in the case of the method (3) of the present invention, and is as described above.

However, after this step, the strength is maintained by heating at a temperature higher than 650 ° C., which is higher than that of the method (3) of the present invention, at a β transformation point of less than −100 ° C. for 30 minutes or more and 4 hours or less. Although the fatigue strength is not so high as in the case of the method (3) of the present invention, it becomes possible to obtain the material characteristics excellent in fracture toughness. This is because crack propagation is suppressed by increasing the size of the secondary α phase.
Here, when the heating and holding temperature exceeds 650 ° C., the β transformation point is −100.
The temperature below ℃ is that, at a temperature of 650 ° C. or lower, a long holding time of more than 8 hours is required to coarsen the size of the secondary α phase to the extent that the crack propagation suppressing effect appears. At a temperature above the β transformation point of −100 ° C., as the volume fraction of the α phase decreases, most of the secondary α phase that has been generated has disappeared, and is almost the same as when annealed. This is because only the mechanical properties of Further, the heating and holding time is set to 30 minutes to 4 hours,
This is because sufficient coarsening of the secondary α phase cannot be achieved by heating and holding for less than 30 minutes, and when held for more than 4 hours,
This is because the primary α phase erodes the secondary α phase to start coarsening, and only mechanical properties comparable to those of the annealed material can be obtained.

In the present invention, the α + β-type titanium alloy means that the main phase is two phases of α + β at room temperature in the equilibrium state, and when quenched from a single-phase temperature range above the β transformation point, all or part of Martensitic transformation type alloy, Ti-6Al-4V, Ti-6Al-6V-2S
n, Ti-6Al-2Sn-4Zr-6Mo, Ti-6
Al-1.7Fe-0.2Si, Ti-5.5Al-1
Fe-0.15 wt% oxygen-0.05 wt% nitrogen, Ti
-5Al-2.5Fe, Ti-1.5Fe-0.5 wt% oxygen-0.04 wt% nitrogen, etc. correspond to this. Also, Ti-6Al-4V-0.2% Pd, such as Pd and Ru.
Alloys with improved corrosion resistance by adding platinum group elements such as Ti-6Al-4 with reduced amount of interstitial impurity elements
V-ELI (Extra-Low Interstitials) and the like also belong to α + β type titanium alloys. In the equilibrium state, these α + β type titanium alloys have FeTi phase, ω phase, silicide, and Ti−.
Al-based ordered phase, Ti-O-based ordered phase, Ti-N-based ordered phase,
Some of them contain intermetallic compound phases, etc., but basically, in the temperature range below the β transformation point, the two phases α + β are basically used, and above the β transformation point, the volume fraction of the α phase is zero. At the following temperatures, the proportion of α phase increases with decreasing temperature,
At room temperature, depending on the alloy type, it is approximately 75% to 95%.
% Α phase and the balance β phase.

[0023]

The present invention will be described in more detail with reference to the following examples. (Test 1) Ti-6Al-4V by vacuum arc melting
Was melted and subjected to hot forging to manufacture a solid billet having a square cross section of 210 mm × 210 mm. The β transformation point of this Ti alloy is 1000 ° C. This billet was heated to the "billet heating temperature" shown in Table 1, and continuously hot-worked into a tube having an outer diameter of 160 mm and a thickness of 18 mm through each step of perforation, stretching in three stages, and regular shape. . During this series of hot working processes, the temperature when the outer surface temperature of the material is the lowest, that is, the minimum temperature in the process from the time the billet is carried out of the heating furnace until the end of the standard process, which is the final rolling process, is ,
In Table 1, in the column of "Minimum temperature during hot working process", in Table 1 in the column of "Final rolling process end temperature" the material temperature immediately after the end of the standard process, and in the cooling conditions after the end of the standard process. Is shown in Table 1 in the column "Cooling after the final rolling step". The tube after hot working was heated and held at the temperature and time described in the column "Heat treatment" in Table 1 and cooled, and thereafter, the test piece (distance between scores 25 mm, diameter 6 0.25 mm) was sampled and a tensile test was conducted. The test results are also shown in Table 1.

In Table 1, test numbers 1 to 3 are conventional examples. Test No. 1 is a case where the final rolling process end temperature is below the β transformation point and annealing is also performed in the α + β two-phase region below the β transformation point. In this method, there is a working texture and working strain of the α phase. Due to the transformation texture from the β phase, the difference in tensile strength between the longitudinal direction and the circumferential direction has expanded to about 100 MPa, and the elongation is also good in the longitudinal direction, but 10 in the radial direction.
Only low values of less than% have been obtained. Test No. 2 is a case where the annealing is performed at a temperature of β transformation point or higher, and the characteristic difference between the length direction and the circumferential direction is eliminated, but the β grains are coarsened and the elongation value becomes extremely small in both directions. There is. Further, Test No. 3 was a case where the sample was heated and held in the β single-phase region, water-cooled, and then reheated and held in the α + β two-phase temperature region, and the β-grains were coarsened, so that the elongation value was insufficient.

In contrast to the above conventional example, the method (1) of the present invention
Test Nos. 4, 5, 7, 10, 12, 15 which are examples of
In both cases, there is no characteristic difference in the length direction and the circumferential direction. 100
A high tensile strength of 0 MPa or more and a high elongation value of 10% or more are obtained in both directions. On the other hand, in the test numbers 6, 8, 9, 11, 13, and 14 of the comparative example, in the test number 6, the anisotropy, that is, the characteristic difference between the length direction and the circumferential direction strongly appears, and the test number Numbers 8, 9, 11, 13,
No. 14 had no anisotropy, but all had low elongation values of less than 10%. The reason for this is as follows. That is, in Test No. 6, since the end temperature of the final rolling step was lower than the temperature at the β-transformation point + 50 ° C. or higher specified in the present invention, a transformation texture was generated during cooling and was eliminated even in the subsequent annealing. No, strong anisotropy occurred. Exam number 8
Indicates that the final rolling step end temperature was higher than the temperature specified in the method (1) of the present invention, so the β grains coarsened during cooling, and test number 9 indicates that the cooling after the final rolling step was the method of the present invention. Since the furnace cooling was slower than the cooling condition specified in (1), β grains were also coarsened during cooling, and the elongation values were low in both test numbers 8 and 9. Test number 1
No. 1 was because the annealing temperature was lower than the temperature specified by the method (1) of the present invention, strain release and structure stabilization were insufficient, and high ductility was not obtained. Since the temperature was higher than the temperature specified in the method (1) of the present invention, thermal strain was generated again during cooling, an unstable structure was generated, and ductility was lowered. Furthermore, in Test No. 14, since the annealing time was shorter than the annealing time specified in the method (1) of the present invention, the release of strain and the stabilization of the structure were not achieved, and sufficient ductility was not obtained.

In all of the above tests, the minimum temperature during the hot working step was at or above the β transformation point, that is, the entire hot working step was performed in the β single phase region. Test Nos. 16, 17, and 18 show examples in which the final rolling process was completed at a temperature of β transformation point + 50 ° C. or higher due to heat generation during processing after passing through the α + β two-phase temperature range below the β transformation point. They are,
All had no anisotropy, and in addition to a high strength of 1000 MPa or more, an even higher elongation value of 14% or more was obtained. This is the effect that the β grains became finer. here,
There is a difference in characteristics between test number 16 in which the billet heating temperature is in the α + β two-phase temperature range with high deformation resistance and test numbers 17 and 18 in which the billet heating temperature is at or above the β transformation point of deformation resistance. rare. Therefore, the method (2) of the present invention in which the main processing is carried out in the β region of low deformation resistance, passes through the α + β two-phase temperature region on the way, and is finally finished at the temperature of β transformation point + 50 ° C. or higher is the manufacturability. It is also an excellent method from the viewpoint of. However, the heating temperature of the billet is β transformation point +3
In Test No. 19 in which the temperature exceeded 00 ° C., surface defects due to oxidation were severe, and the characteristics could not be evaluated.

[0027]

[Table 1]

(Test 2) Ti-
Ti-6Al-4V-ELI-0.1Pd obtained by adding 0.1% by weight of Pd to 6Al-4V-ELI was melted and hot-forged to manufacture a solid billet having a square cross section of 210 mm × 210 mm. The β transformation point of this Ti alloy is 950 ° C
Met. This billet was heated to 1150 ° C., continuously subjected to perforation, three-step drawing, and a standard process, to be hot-worked into a tube having an outer diameter of 160 mm and a thickness of 20 mm and air-cooled. During this series of hot working steps, the temperature when the outer surface temperature of the raw material is the lowest, that is, the minimum temperature in the process from the time when the billet is carried out of the heating furnace until the end of the standard process, which is the final rolling process, is , Α + β two-phase temperature range 93
At 0 ° C., the temperature was 1030 ° C. immediately after the completion of the standard process. This tube was cut into a length of about 50 cm, various heat treatments shown in Table 2 were performed, a test piece was taken in the same manner as in Test 1 in parallel with the length direction and the circumferential direction of the tube, and a tensile test was performed. It was Further, an hourglass-shaped test piece having a minimum diameter of 8 mm was sampled in parallel with the length direction and the circumferential direction, and a rotary bending fatigue test was performed to evaluate the fatigue strength in both directions. At this time, the fatigue strength was defined as the maximum strength that does not break under a repeated load of 10 ⁷ cycles. The test results are shown in Table 2.

Test number 20, which was annealed by air cooling at 750 ° C. for 1 hour after hot working, is an example of the method (2) of the present invention, which has no anisotropy and a tensile strength of about 930 MPa and 17 %, High elongation, and rotational bending fatigue strength of about 450 MPa were obtained. On the other hand, test numbers 21, 22, 24, 27, 2 to which the method (3) of the present invention is applied
All of 8, 31, 32 have no anisotropy and 12%
1000MP while maintaining the above relatively high elongation value
A high strength of a or higher and a high rotational bending fatigue strength of 480 MPa or higher have been achieved, and the method (3) of the present invention is extremely effective for improving the strength and the fatigue strength. On the other hand, Test No. 23 is obtained by performing low temperature heat treatment after annealing in the method (1) of the present invention, and the temperature of the first heat treatment is the β transformation point -100 of the lower limit temperature defined in the method (3) of the present invention. Since the temperature was lower than 0 ° C, the primary α phase was not sufficiently refined, and even after the subsequent heat treatment, only a slight increase in strength and fatigue strength was achieved. In Test No. 25, the temperature of the first heat treatment was at or above the β transformation point as conventionally performed, so that the β phase was coarsened and the ductility was reduced. In Test No. 26, the heat retention time of the first heat treatment was less than 30 minutes specified in the method (3) of the present invention, so the refinement of the primary α phase was insufficient, and the strength and fatigue strength were insignificant. Only improved. Further, in the test number 29, the cooling rate after the first heat treatment was the method (3) of the present invention.
Since the furnace cooling was slower than the cooling condition of No. 3, the refined primary α-phase grew again during cooling and coarsened, and improvement in strength and fatigue strength became insufficient. Furthermore, in Test No. 30, the temperature of the second low temperature heat treatment was less than 450 ° C. which is the lower limit temperature defined in the method (3) of the present invention. Since the time was less than the lower limit time defined by the method (3) of the present invention, the stabilization of the structure and the release of strain were insufficient, and a sufficient ductility value could not be obtained. In Test Nos. 33 and 34, the temperature and the heat holding time of the second heat treatment performed exceeded the upper limits in the method (3) of the present invention, so that the secondary α phase was coarsened, and the strength and fatigue were increased. The strength did not improve sufficiently. The test number 33 is an example of the method (4) of the present invention, and although not shown in Table 2, the fracture toughness was excellent.

[0030]

[Table 2]

(Test 3) Ti-6Al- used in Test 2
4V-ELI-0.1Pd solid billet (β transformation point:
950 ° C.) to 1150 ° C., perforation, three-step drawing, and regular steps are continuously performed to obtain an outer diameter of 160 m.
The pipe was hot worked into a tube of m and 20 mm in thickness and air-cooled. During this series of hot working steps, the temperature when the outer surface temperature of the raw material is the lowest, that is, the minimum temperature in the process from the time when the billet is carried out of the heating furnace until the end of the standard process, which is the final rolling process, is , Β single-phase temperature range was 1000 ° C., and the temperature immediately after the completion of the routine process was 1050 ° C. This tube is about 5
Cut to a length of 0 cm and perform various heat treatments shown in Table 3,
A test piece similar to that of Test 1 was taken in parallel with the length direction and the circumferential direction of the tube, and the tensile test was performed. Further, using a fracture toughness test piece (thickness: 12.7 mm) in which fatigue pre-cracks were introduced at the tip of the mechanical notch in the lengthwise direction and the circumferential direction, the fracture toughness in both directions was evaluated by the K _IC value. The test results are shown in Table 3.

Test No. 36, which was annealed by air cooling at 750 ° C. for 1 hour after hot working, is an example of the method (1) of the present invention, has no anisotropy, and is processed in the α + β region on the way. Although not as good as Test No. 20 (Example of the method (2) of the present invention) that has been subjected, it has excellent tensile strength of about 920 MPa and high elongation of more than 14%, and excellent K _{IC of} about 94 to 95 MPa · m ^1/2. The mechanical properties obtained were obtained. On the other hand, test numbers 37, 38, 40, 4 to which the method (4) of the present invention was applied
No. 3,44 have no anisotropy and 930 MPa
While maintaining the above high tensile strength and sufficient elongation of 14% or more, a high K _IC of 110 MPa · m ^1/2 or more has been achieved, and the method (4) of the present invention is extremely effective for improving fracture toughness. Is. On the other hand, test number 39 is the method (3) of the present invention, in which the temperature of the second low temperature heat treatment is
Since it did not reach the temperature specified by the method (4) of the present invention, and in Test No. 42, the heating holding time did not reach the lower limit time specified by the method (4) of the present invention, it bears a crack propagation suppressing effect. Insufficient coarsening of the secondary α phase causes K
_IC improvement was not achieved sufficiently. Also, test number 41
Then, since the temperature of the second heat treatment exceeded the upper limit temperature of the method (4) of the present invention, the secondary α produced in the first heat treatment was
Many of the phases disappeared, and the strength, toughness, and fracture toughness were not improved from the annealed material (Test No. 36). Also,
In Test No. 45, the time of the second heat treatment performed exceeded the upper limit value in the method (4) of the present invention, so the primary α phase eroded the secondary α phase and coarsened, and the annealed material (Test No. 3
Only strength, ductility, and fracture toughness comparable to 6) were obtained.

[0033]

[Table 3]

(Test 4) By vacuum arc melting, 1.5
Ti-1.5Fe-0.5 [O] -0.04 containing wt% Fe, 0.5 wt% oxygen, 0.04 wt% nitrogen
[N] was melted and subjected to hot forging to manufacture a solid billet having a square cross section of 210 mm × 210 mm. The β transformation point of this Ti alloy was 950 ° C. This billet is 100
The tube was heated to 0 ° C., continuously perforated, stretched in three steps, and subjected to a standard process, and then hot-worked into a tube having an outer diameter of 160 mm and a thickness of 20 mm, followed by air cooling. During this series of hot working steps, the temperature when the outer surface temperature of the raw material is the lowest, that is, the minimum temperature in the process from the time when the billet is carried out of the heating furnace until the end of the standard process, which is the final rolling process, is , Β single-phase temperature range of 1000 ° C., the temperature immediately after the completion of the standard process is 105
It was 0 ° C. This tube was cut into a length of about 50 cm, various heat treatments shown in Table 4 were performed, various test pieces were cut out in parallel with the length direction and the circumferential direction of the tube, and a tensile test similar to Test 1 and Test 2 were performed. A similar fatigue test and a fracture toughness test similar to Test 3 were performed. The test results are shown in Table 4.

Test No. 46 of the conventional example annealed above the β transformation point has no anisotropy, but the elongation is low due to the coarsening of β grains. On the other hand, the test number 47, which is an example of the method (1) of the present invention, has no anisotropy and is 950
High strength above MPa, high elongation above 14%, 500M
High rotational bending fatigue strength of Pa or more, 100 MPa · m
A high K _{IC of} over ^1/2 is obtained. Further, in Test No. 48 to which the method (3) of the present invention was applied, there was no anisotropy, and high strength exceeding 1050 MPa and high rotational bending fatigue strength exceeding 530 MPa were achieved while maintaining sufficient elongation of 11% or more. Has been done. Further, in the test number 49 to which the method (4) of the present invention is applied, there is no anisotropy, and while maintaining strength and elongation equal to or higher than those of the test number 47, 1
A high K _IC exceeding 10 MPa is obtained.

[0036]

[Table 4]

[0037]

As described above, by applying the method of the present invention, an α + β type titanium alloy having no material anisotropy and having excellent mechanical properties such as strength, ductility, fatigue strength and fracture toughness. It is possible to manufacture a seamless tube consisting of

Claims (4)

[Claims] 1. A method for continuously producing a seamless tube made of an α + β type titanium alloy by hot rolling processes such as piercing and drawing, forming, drawing and the like, in which β transformation point +
Finish the final rolling step at a temperature of more than 50 ° C β transformation point + 250 ° C or less, cool at a cooling rate of air cooling or more, and anneal at 700 ° C or more and β transformation point of less than -100 ° C for 30 minutes or more. A method for producing a seamless tube made of α + β type titanium alloy, characterized by: 2. A solid billet composed of an α + β type titanium alloy is heated to a temperature not lower than the β transformation point and not higher than the β transformation point and not higher than 300 ° C. for perforation, and β is formed during the rolling process such as stretching, forming and drawing. The method for producing a seamless tube made of an α + β type titanium alloy according to claim 1, characterized in that the final rolling step is finished by cooling to a temperature below the transformation point. 3. The manufacturing method according to claim 1, wherein, instead of the annealing, the temperature is maintained at a temperature of β transformation point −100 ° C. or higher and lower than the β transformation point for 30 minutes or more, and a cooling rate of air cooling or higher. A method for producing a seamless tube made of an α + β type titanium alloy, which is characterized by cooling at 4, and heating and holding at 450 to 650 ° C. for 1 hour or more and 8 hours or less. 4. The manufacturing method according to claim 1, wherein, instead of the annealing, the material is heated and held at a temperature of β transformation point −100 ° C. or more and less than β transformation point for 30 minutes or more, and a cooling rate of air cooling or more. Cooled at 650 ° C and further β transformation point -100
A method for producing a seamless pipe made of an α + β type titanium alloy, which comprises heating at a temperature lower than 0 ° C. for 30 minutes or more and 4 hours or less.