JPH09228014A - Production of alpha + beta titanium alloy seamless tube excellent in fracture toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an α+β type titanium alloy seamless tube holding high strength and ductility and furthermore excellent in fracture toughness. SOLUTION: In the method for continuously producing a seamless tube composed of α+β type titanium alloy by a hot rolling stage of perforating, stretching, forming, drawing or the like, the final rolling stage is finished in the temp. range of the βtransformation point -300 deg.C to the β transformation point +100 deg.C, cooling is executed at a cooling rate of that in air cooling, and furthermore, annealing is executed in the temp. range of the β transformation point to the β transformation point +100 deg.C for 1min to 1hr. Otherwise, according to the requisitions for strength and toughness, secondary or third heat treatment is executed.

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. More specifically, it relates to a method for manufacturing a seamless tube made of an α + β type titanium alloy having excellent fracture toughness.

[0002]

2. Description of the Related Art Titanium alloys are lightweight, have high strength, and have high corrosion resistance, and are therefore most suitable for extreme environments such as geothermal development, deep-sea oil field / gas field development, etc. at large depth, high temperature, high pressure, and high corrosion. Is attracting 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 a method of manufacturing titanium alloy pipe material, there are methods such as bending and welding a plate (welded pipe), hot extrusion (seamless pipe), plug mill, etc. It is possible to use a method of producing a pipe (continuous pipe) by continuously performing rolling such as piercing, stretching, forming, and drawing. Among them, the method of continuously forming a heated solid billet into a hollow tube by a rolling process such as piercing / drawing, forming, and drawing (hereinafter referred to as a piercing / rolling method) and a hot extrusion method are Since it is possible to manufacture a seamless pipe that does not have any defects, there is an advantage that it can be stably used for a long period of time even in the above-mentioned extreme environment applications where repair and replacement of parts are extremely difficult.

Further, in order to stably use it in such an extreme environment without repairing it for a long period of time, it must have high fracture toughness in addition to sufficient strength and ductility. Generally, α +
In order to improve the fracture toughness of β-type titanium alloy thick plates, a heat treatment including heating to a β single-phase temperature above the β transformation point is performed, and the structure mainly of needle-like α phase with excellent fracture toughness Is used. With this method, although the fracture toughness is improved, the β grains are coarsened during heating to the β single phase region, and the strength and ductility are reduced. However, currently α +
In applications such as aircraft where β-type titanium alloys are used, the allowances for the reduction of strength and ductility are within the allowable range, and in these fields it has been utilized as a material with sufficient strength, ductility and high fracture toughness. .

[0004]

However, when the α + β type titanium alloy seamless pipe is used in the extreme environment of deep depth, high temperature, high pressure, and high corrosion such as geothermal development, subsea oil field / gas field development, etc. As described above, since repair and replacement of parts are almost impossible, and moreover, they are used for a period of several decades or more, and therefore, higher mechanical properties are required as compared with the conventional application. That is, the mechanical properties obtained by simply applying the above-described method performed on thick plates as a method for improving fracture toughness are not sufficient for this ultimate environmental application, and after maintaining high strength and ductility. It is also necessary to secure high fracture toughness.

In view of the above problems, the present invention has sufficient strength and ductility to withstand extreme environments such as geothermal development, subsea oil field / gas field development, etc. of large depth, high temperature, high pressure and high corrosion. Above, it aims at providing the method of manufacturing the α + β type titanium alloy seamless tube which is further excellent in fracture toughness.

[0006]

In order to achieve the above object, the present invention has the following methods (1) to (4).
That is, (1) a seamless tube made of α + β type titanium alloy,
In a method for continuously manufacturing by hot rolling, piercing and stretching, forming, drawing, etc., the final rolling step is finished at a temperature of β transformation point −300 ° C. or higher and β transformation point + 100 ° C. or lower, and air cooling or higher is performed. It has an excellent fracture toughness, characterized in that it is annealed for 1 minute or more and 1 hour or less at a temperature of β transformation point or more and β transformation point + 100 ° C. or less.
+ Β type titanium alloy seamless tube manufacturing method. (2) After performing the annealing described in (1), cooling is performed at a cooling rate of air cooling or higher, and further, the temperature is higher than 650 ° C and the β transformation point is -150.
A method for producing an α + β-type titanium alloy seamless tube having excellent fracture toughness, which comprises heating and holding at a temperature of less than 0 ° C for 30 minutes or more and 4 hours or less. (3) After the annealing described in (1), β transformation point −15
At a temperature of 0 ° C or higher and a β transformation point of lower than -30 ° C for 30 minutes or longer,
A second heat treatment of heating and holding for 4 hours or less and cooling at a cooling rate of air cooling or more is further performed, and further heating for 30 minutes or more and 4 hours or less at a temperature higher than 650 ° C and a β transformation point of less than -150 ° C. A method for producing an α + β type titanium alloy seamless tube having excellent fracture toughness, which comprises performing a third heat treatment for holding. (4) After the annealing described in (1), β transformation point −15
At a temperature of 0 ° C or higher and a β transformation point of lower than -30 ° C for 30 minutes or longer,
A second heat treatment of heating and holding for 4 hours or less, cooling at a cooling rate of air cooling or more, and further 450 ° C. or more, 6
A third thermophysical process of heating and holding at a temperature of less than 50 ° C. for 1 hour or more and 8 hours or less is a method for producing an α + β type titanium alloy seamless tube having excellent fracture toughness.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In order to produce an α + β type titanium alloy tube excellent in all three properties of strength, ductility and fracture toughness, the present inventors have found that the hot deformation characteristics of the α + β type titanium alloy,
As a result of conducting in-depth research on various metallurgical properties such as phase transformation and recrystallization, deformation conditions significantly different from plate rolling and hot extrusion, that is, strong shear deformation,
Reforming and grain growth of α + β type titanium alloy is remarkably suppressed in a certain processing temperature range in the drilling / rolling method, which is a very special deformation condition such as high strain rate and restraint of material in the circumferential direction. I understood. Then, the present inventors apply this knowledge, and by the piercing / rolling method, which also has the advantage that a seamless tube without a welded portion can be produced, α, which is excellent in all three characteristics of strength, ductility, and fracture toughness.
The inventors have invented a method for producing a + β type titanium alloy tube.

In the method of the present invention, in a method for continuously producing a seamless tube made of α + β type titanium alloy by a hot rolling process such as piercing and drawing, forming, drawing, etc., first, a β transformation point- Beta transformation point +10 above 300 ℃
The final rolling step was completed at a temperature of 0 ° C. or lower, and cooling was performed at a cooling rate of air cooling or higher. The process conditions are that recrystallization and grain growth are remarkably suppressed, a deformed structure in which a large amount of plastic strain is accumulated is generated, and cooling is performed to room temperature without causing recrystallization or growth on the way.

When the final temperature of the final rolling step is not less than β transformation point and not more than β transformation point + 100 ° C., the β phase transforms into acicular martensite or fine acicular α + β two-phase structure during cooling. However, at this time, most of the plastic strain in which the β phase has accumulated is frozen. On the other hand, when the finish temperature of the final rolling step is lower than the β transformation point, a deformed structure consisting of α + β two phases is formed, but during cooling, the α phase is frozen to room temperature as it is, and the β phase is martensite or It transforms into a fine needle-like α + β phase. However, most of the large amount of plastic strain that both had immediately after hot working was frozen. However, in the present invention, the final rolling process end temperature is set to the β transformation point −3.
The reason why the temperature is set to 00 ° C. or higher is that the deformation resistance is high in a temperature range lower than this temperature and sufficient hot workability cannot be secured.
In addition, the final rolling process end temperature was set to β transformation point + 100 ° C or less because even if the drilling / rolling method is used again, since the temperature is in a high temperature region where diffusion is active when the final rolling process is finished at a temperature above this temperature. This is because crystals and grain growth will occur. Further, the cooling condition is set to be air cooling or higher because at a cooling rate slower than this, recrystallization or grain growth occurs during cooling.

Next, in the method (1) of the present invention, annealing is carried out by heating and holding at a temperature not lower than the β transformation point and not higher than the β transformation point + 100 ° C. for 1 minute or more and 1 hour or less. The process condition is to heat to a β single phase region above the β transformation point to generate a needle-shaped α phase having excellent fracture toughness during cooling.
In products such as ordinary thick plates, the β phase undergoes grain growth during this process, so the fracture toughness is improved, but the strength and ductility are reduced. However, in the case of the piercing / rolling method, if the final rolling process end temperature and the cooling conditions after hot working are within the ranges specified in the present invention, recrystallization or grain growth does not occur at this stage, and a large amount of plasticity Since strain is accumulated, even if it is heated above the β transformation point, strain is released preferentially and then grain growth occurs, so the β grain size is smaller than that of products such as thick plates. It is possible to improve fracture toughness without significantly reducing strength and ductility. However, when the annealing temperature exceeds the β transformation point + 100 ° C., diffusion is activated, strain is released within a short time, β grain growth occurs subsequently, and strength and ductility decrease. The heating hold time is
If the heating and holding is not performed for 1 minute or more, the release of strain is insufficient, the toughness is insufficiently improved, and the ductility is reduced. When heated and held for more than 1 hour, β-grain growth starts following the release of strain, and the strength and ductility decrease.

In the methods (2) to (4) of the present invention, the annealing described in the method (1) of the present invention is followed by the second and third heat treatments. These heat treatments are for further improving the strong fracture toughness. That is,
In the method (2) of the present invention, after performing the annealing described in the method (1) of the present invention, cooling is performed at a cooling rate other than air cooling, and further, 6
It was decided to heat and hold at a temperature of more than 50 ° C. to β transformation point of less than −150 ° C. for 30 minutes to 4 hours. This step is a heat treatment effective to enhance both toughness and strength. That is, by performing the cooling in the annealing step described in the method (1) of the present invention by air cooling or more, the β phase having a fine grain size is converted into fine martensite or fine acicular α +.
Converted to β structure, and more than 650 ℃ or β transformation point-
By heating and holding at a temperature of less than 150 ° C. for 30 minutes or more to 4 hours or less, a part is grown to a relatively coarse α phase and the crack propagation resistance is increased, while the rest is a fine needle-like α phase. , To increase the strength.

If the cooling after annealing is not performed by air cooling or more, the entire material is transformed into a coarse acicular α-phase structure, so that the increase in strength is not achieved and the method (1) of the present invention is used. Only the same characteristics can be obtained. Further, the heating and holding temperature and time of the next second heat treatment should be 30 minutes or more at a temperature higher than 650 ° C. or lower than β transformation point −150 ° C. The reason for the limitation is that at 650 ° C. or less, the proportion of the fine acicular α-phase structure becomes large and the strength is remarkably increased, but the toughness is insufficiently improved, and the β transformation point of −150 ° C. When it exceeds the above range, the amount of fine acicular α-phase is reduced, so that high strength cannot be obtained, and only properties comparable to those of the method (1) of the present invention can be obtained. Further, if not held for 30 minutes or more, the structure is not stabilized and the fracture toughness is insufficiently improved. This heat treatment time does not significantly change the characteristics even if it exceeds 4 hours, but since the desired characteristics have already been obtained in 4 hours, it is energyless to continue the heat treatment any longer. In method (2), the upper limit was 4 hours.

Next, in the method (3) of the present invention, after the annealing described in the method (1) of the present invention, the β transformation point is -150 ° C.
Or more or β transformation point at a temperature below -30 ° C for 30 minutes or more
A second heat treatment is performed, in which the material is kept heated for 4 hours or less and cooled at a cooling rate of air cooling or more, and further heated to a temperature higher than 650 ° C or lower than the β transformation point -150 ° C for 30 minutes to 4 hours. It was decided to perform the third heat treatment for holding.
This heat treatment is an effective heat treatment for obtaining higher toughness than the method (2) of the present invention.

The mechanism by which higher toughness is obtained is as follows. First, after annealing according to the method (1) of the present invention, heating is performed in a high temperature α + β two-phase temperature range of β transformation point −150 ° C. or higher to β transformation point −30 ° C. or higher for 30 minutes to 4 hours. By the second heat treatment to hold, the coarse α phase is β
The organization exists in the phase. The coarse α phase has a particularly strong effect of preventing crack propagation and further improves toughness. Next, the β phase is transformed into martensite or fine acicular α phase by cooling at a cooling rate of air cooling or higher. Further, as a third heat treatment, heating and holding is performed at a temperature higher than 650 ° C. or lower than β transformation point −150 ° C. for 30 minutes to 4 hours,
A part is converted into a relatively coarse α phase, and the propagation resistance of cracks is further increased, while the balance is made into a fine needle-like α phase to increase the strength, and the latter step is the method of the present invention ( It is similar to the mechanism in 2).

Here, the temperature range and time of the second heat treatment are set to a β transformation point of −150 ° C. or higher to a β transformation point of −30 ° C.
The reason why it is limited to less than 30 minutes to 4 hours is as follows. That is, at a temperature above the β transformation point −30 ° C.,
Since the α phase volume fraction is low, it is difficult to generate a coarse α phase effective for improving toughness, and at a temperature below the β transformation point −150 ° C.
The coarsening of the phase is insufficient, and the coarsening of the α phase is insufficient unless it is held for 30 minutes or more, and the coarsening of the α phase is sufficiently achieved even if it is held for more than 4 hours. However, holding any more is energetically useless. In the subsequent process, cooling with air cooling or higher is performed. If the cooling rate is slower than air cooling, marthesite or fine acicular α phase is not generated during cooling, and the effect of this heat treatment becomes insufficient. Because. Further, the third heat treatment is performed by heating and holding at a temperature higher than 650 ° C. to a β transformation point of lower than −150 ° C. for a time of 30 minutes or more and 4 hours or less. First, the mechanism in the method (2) of the present invention. As described in the explanation of.

According to the method (4) of the present invention, in the production method described in the method (3) of the present invention, after the second heat treatment, at a temperature of 450 ° C. to less than 650 ° C. for 1 hour to 8 hours. The third heat treatment of heating and holding for the following time was performed. This heat treatment is a case where strength is particularly emphasized, and although the toughness is somewhat lowered as compared with the method (3) of the present invention, the strength is increased. Up to the second heat treatment step, it is exactly the same as the method (3) of the present invention, but the final third heat treatment is held by heating at a temperature of 450 ° C. or higher and lower than 650 ° C. for 1 hour to 8 hours. By doing so, more fine α phase is generated than in the case of the method (3) of the present invention,
This is intended to improve strength.

The condition of the third heat treatment is set to 450 ° C. or more and less than 650 ° C. for 1 hour to 8 hours inclusive because the α phase is too fine and the toughness is lowered at less than 450 ° C. At 650 ° C. or higher, a part of the α phase becomes relatively coarse, and the toughness described in the method (3) of the present invention is achieved, but the strength is not significantly improved. In addition, the tissue will not be sufficiently stabilized unless heating and holding for 1 hour or more,
The toughness deteriorates, the structure has already been formed in a heating and holding time of less than 8 hours, there is no characteristic change, and further heat treatment is energetically useless.

In the present invention, the α + β-type titanium alloy means that when it is quenched from a single-phase temperature range above the β transformation point, the two phases of αβ are the main phases at room temperature in the equilibrium state, and some or all. Is a type of alloy that undergoes martensitic transformation,
Ti-6Al-4V, Ti-6Al-6V-2Sn, T
i-6Al-2Sn-4Zr-6Mo, Ti-6Al-
1.7Fe-0.2Si, Ti-5.5Al-1Fe-
0.15 wt% oxygen-0.05 wt% nitrogen, Ti-5A
1-2.5Fe, Ti-1.5Fe-0.5 wt% oxygen-0.04 wt% nitrogen, etc. correspond to this. Also, T
i-6Al-4V-0.2% Pd and other alloys in which platinum group elements such as Pd and Ru are further added to improve corrosion resistance, and Ti-6Al-4V- in which the amount of interstitial impurity elements is reduced.
ELI (Extra-Low Interstitials) etc. 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-A.
There are those containing an 1-system ordered phase, a Ti-O system ordered phase, a Ti-N system ordered phase, an intermetallic compound phase, etc., but in the temperature range below the β transformation point, essentially two phases of α + β are basically used. The volume fraction of the α phase is zero above the β transformation point, and the percentage of the α phase increases as the temperature decreases at temperatures below that. At room temperature, it varies depending on the alloy type, but is generally 75% to 95%. It is composed of the α phase and the balance β phase.

[0019]

The present invention will be described in more detail with reference to the following examples. [Test 1] Ti-6Al-4V having a β transformation point of 990 ° C was melted by vacuum arc melting, and 120 mm by hot forging.
After making a thick slab and a billet having a diameter of 170 mm, a thick plate having a thickness of 13 mm was manufactured by plate rolling, or a tube having an outer diameter of 165 mm and a thickness of 25 mm was manufactured by hot extrusion. At this time, the temperature immediately after the end of hot working is 900 for thick plates.
C., 1050.degree. C. for tubes.

After the hot working, all the materials were air-cooled and the heat treatment shown in Table 1 was performed. From this material, tensile test pieces (distance between scores 25 mm, diameter 6.25 mm) and fracture toughness test pieces (fatigue pre-crack introduced at the tip of the mechanical notch, thickness 1
2.7 mm) was cut out and subjected to a tensile test and a fracture toughness test to determine the tensile strength, elongation and K _IC . For thick plates, tensile test specimens are taken from two directions so that they are parallel and perpendicular to the final rolling direction, and fracture toughness test specimens are such that the notches are parallel and perpendicular to the final rolling direction. Two
Collected from the direction. In the case of a pipe, the tensile test piece is taken from two directions, the length direction and the circumferential direction, and the fracture toughness test piece is made so that the notch is parallel to the length direction and the circumferential direction.
Collected from the direction. The test results shown in Table 1 are all average values in two directions.

All of the examples shown in Table 1 are reference examples, and Test No. 1 is for a thick plate that has been annealed after hot working in the α + β two-phase temperature region below the β transformation point. In this case, the tensile strength and elongation are high, but the K _IC is low. Test Nos. 2 and 3 are cases in which heat treatment including heating to the β transformation point or higher is performed to enhance toughness, and K _IC is certainly high, but during heating to the β single phase region. β grains are coarsened, and strength and ductility are reduced. Test numbers 4 and 5
The same applies to the case of the hot extruded pipe as shown in Fig. 9, where K _IC is high, but tensile strength and elongation are low.

[0022]

[Table 1]

[Test 2] Ti-6Al-4V having a β transformation point of 990 ° C. was melted by vacuum arc melting, and a solid billet of 210 mm × 210 mm square section was manufactured by hot forging. This billet is continuously drilled, drawn in three steps, and subjected to a standard process to obtain an outer diameter of 160 mm and a thickness of 1
Hot worked into 8 mm seamless tubes. The material temperature immediately after the end of the final rolling process is shown in the column "Temperature of the final rolling process" in Table 2, and the cooling conditions after the end of the rolling process are shown in the column "Cooling after the final rolling process" in Table 2. . The tube after hot working is heated and held and cooled under the temperature, time and cooling conditions described in the column "heat treatment" in Table 2, and tensile test pieces are taken in parallel with the length direction and the circumferential direction of the tube. A tensile test was conducted. Also,
A fracture toughness test piece was cut out so that the notch was parallel to the length direction and the circumferential direction of the pipe, and the fracture toughness test was performed.
The test pieces are the same as those used in Test 1. The test results are shown in Table 2, and each value is an average value of test pieces taken from two directions.

In Table 2, test numbers 6, 8, 9, 1
0, 13, 16 are examples of the method (1) of the present invention,
All show a high tensile strength of 950 MPa or more, a high elongation of 10% or more, and a high K _IC of 80 MPa · m ^1/2 or more, and all three properties of strength, ductility, and toughness are high. It was

On the other hand, among the comparative examples shown in Table 2,
Test numbers 7, 12, 15, and 17 did not have high strength and ductility, test number 14 did not have high ductility and toughness, and test number 18 did not have high toughness. Further, in Test No. 11, deep flaws occurred during hot working, and it was not possible to even collect a test piece. The above is because any one of the final rolling process end temperature, the cooling rate after the final rolling process, the annealing temperature, and the annealing time is outside the range specified in the method (1) of the present invention.

[0026]

[Table 2]

[Test 3] Ti-6Al used in Test 2
A -4V billet was hot-worked into a seamless tube having an outer diameter of 160 mm and a thickness of 20 mm by successively passing through each step of perforation, stretching in three steps, and standardization. The material temperature immediately after the final standard process is 1000, that is, the final rolling process end temperature is 1000.
In ° C, the cooling condition after the completion of the standard process, that is, the cooling rate after the final rolling process is air cooling. This seamless tube was cut into a length of 50 cm, subjected to the heat treatment described in the column "Heat treatment" in Table 3, and test pieces were taken in the same manner as in Test 2 to perform a tensile test and a fracture toughness test. The test results are as shown in Table 3, and each characteristic value is an average value of tests taken from two directions.

In Table 3, test numbers 19, 20, 2
Nos. 3 and 26 are examples of the method (2) of the present invention, and all have tensile strength of 970 MPa or more, elongation of 10% or more, and 1
The K _IC of 00 MPa · m ^1/2 is shown, and higher strength and toughness are obtained as compared with the examples of the method (1) of the present invention shown in Table 2. In contrast, test numbers 22 and 2
No. 4, although the strength was increased, the K _IC was almost the same as that of the example of the method (1) of the present invention shown in Table 2, and the fracture toughness was not improved. The test numbers 25 and 27 are
Both strength and toughness were comparable to the examples of method (1) of the invention shown in Table 2. This is because any one of the cooling rate after annealing, the heating holding temperature of the subsequent heat treatment, and the time was out of the range specified by the method (2) of the present invention. Test No. 21 had a high level of strength, ductility, and toughness, but after annealing for 4 hours 3
3 hours 3 hours despite re-heat treatment for 0 minutes
Only the same characteristics as those of Test No. 20 in which only a short heat treatment of 0 minutes was performed were obtained, which is a waste of energy.

[0029]

[Table 3]

[Test 4] Ti-6A used in Test 3
A material obtained by cutting a l-4V seamless tube into a length of 50 cm was annealed at 1030 ° C for 15 minutes, air-cooled, and then subjected to the second and third heat treatments described in the "heat treatment" column of Table 4. In the same manner as in Test 2 and Test 3, test pieces were collected and subjected to a tensile test and a fracture toughness test. The test results are shown in Table 4, and each characteristic value is an average value of test pieces taken from two directions.

In Table 4, test numbers 28, 30, 3
2, 35, 38, 39, 41 are examples of the method (3) of the present invention, and all have a tensile strength of 970 MPa or more, 1
The elongation is 0% or more and the K _IC is 120 MPa · m ^1/2 or more, and the toughness is higher than that of the example of the method (2) of the present invention shown in Table 3.

On the other hand, test numbers 29, 31, 34,
40 did not improve K _IC , and in test numbers 36 and 37, neither tensile strength nor K _IC improved from the examples of the method (2) of the present invention shown in Table 3. This is because any of the temperature, time, cooling rate of the second heat treatment, and time of the third heat treatment was outside the range specified in the method (3) of the present invention. In addition, since the K _IC of Test No. 43 was similar to that of the example of the method (2) of the present invention shown in Table 3, the toughness, which is the object of the method (3) of the present invention, was achieved. However, the K _IC is a relatively high value of 100 MPa · m ^1/2 , the elongation is 10% or more, and the tensile strength is remarkably high and exceeds 1000 MPa. The characteristics were extremely suitable for applications that require strength. This corresponds to an embodiment of method (4) of the invention.

[0033]

[Table 4]

[Test 5] Ti-6A used in Test 3
A material obtained by cutting a l-4V seamless tube into a length of 50 cm was annealed at 1030 ° C for 15 minutes, air-cooled, and then 9
A second heat treatment of heating and holding at 40 ° C. for 1 hour and then water cooling was performed, and further, a third heat treatment shown in Table 5 was performed, and a test 2 was performed.
Test pieces were sampled in the same manner as in No. 4 to perform a tensile test and a fracture toughness test. The test results are shown in Table 5, and each characteristic value is an average value of test pieces taken from two directions.

In Table 5, test numbers 44, 45 and 48
Is an embodiment of the method (4) of the present invention.
Tensile strength of 00 MPa or more, elongation of 10% or more, 100 M
It shows a K _IC of Pa · m ^1/2 or more, and the fracture toughness is not as high as that of the example of the method (3) of the present invention shown in Table 4, but a seam having a remarkably high strength and a relatively high toughness. No tube is obtained.

On the other hand, in Test Nos. 46 and 50, although high strength was obtained, the K _IC was only about 80 MPa · m ^1/2 , and the fracture toughness was considerably low. This is because in the test number 46, the time of the third heat treatment was shorter than the time specified in the method (4) of the present invention, and in the test number 50, the temperature of the third heat treatment was the method of the present invention. This is because the temperature was lower than the temperature specified in (4). Further, in the test number 49, the same characteristics as those of the other examples of the method (4) of the present invention were obtained, but the characteristics were the same as those of the test number 48 in which the time of the third heat treatment was shorter than this. No effect of long-term heat treatment is observed. That is, it is a waste of energy. The test number 47 is the third
Since the heat treatment temperature was higher than the temperature specified by the method (4) of the present invention and the temperature specified by the method (3) of the present invention, although it exhibited extremely high fracture toughness, the strength was not sufficiently improved. There wasn't.

[0037]

[Table 5]

[Test 6] By vacuum arc melting, Ti-
Ti-6Al-4V-0.1Pd-ELI, in which 0.1% by weight of Pd was further added to 6Al-4V-ELI, was melted and hot-forged to form a slab having a thickness of 120 mm and 10
A solid billet having a square cross section of mm × 210 mm was manufactured.
The β transformation point is 960 ° C.

The slab was made into a thick plate having a thickness of 13 mm by hot rolling, and the heat treatment shown in Table 6 was performed, and Ti-6A of Test 1 was tested.
Test pieces were sampled in the same manner as the 1-4V thick plate, and the tensile strength, elongation and K _IC were determined. At this time, the final rolling process end temperature is 900 ° C., and the cooling after the rolling is air cooling. The billet is continuously subjected to the steps of perforation, three-stage drawing, and standardization, and is hot worked into a seamless tube having an outer diameter of 160 mm and a thickness of 18 mm, and the heat treatment shown in Table 6 is performed, and the Ti of Test 2 is tested. -6
Test pieces were sampled in the same manner as the Al-4V seamless tube, and the tensile strength, elongation and K _IC were determined. At this time, the temperature at the end of the final rolling step is 1000 ° C., and the cooling after the rolling is air cooling. All test pieces were sampled from two directions as in Tests 1 to 5, and all the results in Table 6 are average values in two directions.

In Table 6, Test No. 51 is an example of a thick plate and is a reference example. In order to increase the toughness, heat treatment including heating to the β transformation point or higher is performed, and although K _IC is certainly high, β grains are coarsened while heating to the β single phase region, Strength and ductility are reduced. On the other hand, the seamless pipes of Test Nos. 52 to 55, which are examples of the present invention, have high tensile strength of 900 MPa or more, high elongation of 13% or more, and high K of 90 MPa · m ^1/2 or more. _IC has been obtained, and an excellent seamless tube is obtained with all three characteristics of strength, ductility, and toughness. In particular, the test number 53 which is an example of the method (2) of the present invention has higher strength and toughness than the test number 52 which is an example of the method (1) of the present invention. In Test No. 54, which is an example of the method (3), higher toughness is obtained. Test No. 55, which is an example of the method (4) of the present invention, has a toughness similar to that of Test No. 53, but an extremely high tensile strength of 980 MPa or more is obtained.

[0041]

[Table 6]

[Test 7] Ti-
A slab with a thickness of 120 mm and a billet with a square cross section of 210 mm × 210 mm were manufactured by melting 1.5Fe-0.5 wt% oxygen-0.04 wt% nitrogen and hot forging. The β transformation point is 960 ° C. The slab is hot-rolled into a thick plate having a thickness of 13 mm, and the heat treatment described in Table 7 is performed.
Similarly to the Ti-6Al-4V thick plate of Test 1, test pieces were sampled and the tensile strength, elongation and K _IC were determined. At this time, the final rolling process end temperature is 900 ° C., and the cooling after the rolling is air cooling. The billet has an outer diameter of 160 mm and a thickness of 18 mm after being continuously subjected to the steps of perforation, three-stage drawing, and standard forming.
The seamless tube of No. 2 was hot worked and heat-treated as shown in Table 7, and a test piece was sampled in the same manner as the Ti-6Al-4V seamless tube of Test 2 to determine the tensile strength, elongation and K _IC . At this time, the temperature at the end of the final rolling step is 950 ° C., and the cooling after the rolling is air cooling. All the test pieces were sampled from two directions as in Tests 1 to 6, and the results in Table 7 were all 2
It is the average value of the other.

In Table 7, test number 56 is an example of a thick plate and is a reference example. In order to increase the toughness, heat treatment including heating to the β transformation point or higher is performed, and although the K _IC is certainly high, the β grains become coarse during heating to the β single phase region, Strength and ductility are reduced. In contrast, seamless pipe of Example a is test numbers 57 to 60 of the present invention are all 950MPa or more high tensile strength, 14% or more of the high elongation, 60 MPa · m ^1/2 or more high K _IC has been obtained, and an excellent seamless tube is obtained with all three characteristics of strength, ductility, and toughness. In particular, the test number 58 which is an example of the method (2) of the present invention has higher strength and toughness than the test number 57 which is an example of the method (1) of the present invention. In Test No. 59, which is an example of the method (3), higher toughness is obtained. Test No. 60, which is an example of the method (4) of the present invention, has a toughness similar to that of Test No. 58, but an extremely high tensile strength of 1050 MPa or more is obtained.

[0044]

[Table 7]

[0045]

As described above, by applying the present invention, geothermal development, subsea oil and gas field development,
It is possible to manufacture an α + β-type titanium alloy seamless tube which has sufficient strength and ductility capable of withstanding the extreme environment of large depth, high temperature, high pressure, and high corrosion, and has excellent fracture toughness.

Claims (4)

[Claims] 1. A β transformation point -30 in 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.
The final rolling process is completed at a temperature of 0 ° C. or higher and β transformation point + 100 ° C. or lower, cooled at an cooling rate of air cooling or higher, and further, at a temperature of β transformation point or higher and β transformation point + 100 ° C. or lower for 1 minute or longer,
A method for producing an α + β type titanium alloy seamless tube having excellent fracture toughness, characterized by performing annealing for 1 hour or less. 2. After the annealing according to claim 1, cooling is performed at a cooling rate of air cooling or more, and further, at a temperature of more than 650 ° C. and β transformation point of less than −150 ° C. for 30 minutes or more and 4 hours or less. Α + β with excellent fracture toughness characterized by being heated and held
Type titanium alloy seamless tube manufacturing method. 3. After the annealing according to claim 1, the temperature is 30 at a temperature of β transformation point of −150 ° C. or more and less than β transformation point of −30 ° C.
A second heat treatment is performed, in which the material is heated and held for a time of not less than 4 minutes and not less than 4 minutes, and cooled at a cooling rate of not less than air cooling, and further 650 ° C.
A method for producing an α + β type titanium alloy seamless tube having excellent fracture toughness, which comprises performing a third heat treatment in which the temperature is maintained at a temperature below the super-β transformation point of −150 ° C. for 30 minutes or more and 4 hours or less. 4. After the annealing as set forth in claim 1, the temperature is set to a temperature not lower than β transformation point −150 ° C. and lower than β transformation point −30 ° C.
A second heat treatment is performed, in which the material is heated and maintained for a time of not less than 4 minutes and not less than 4 minutes, and cooled at a cooling rate of not less than air cooling, and further 450 ° C
As described above, the method for producing an α + β-type titanium alloy seamless tube having excellent fracture toughness, which comprises performing the third thermodynamics of heating and holding at a temperature of less than 650 ° C. for 1 hour or more and 8 hours or less.