JP6265037B2 - Titanium welded tube and manufacturing method thereof - Google Patents

Description

  The present invention is made of titanium suitable for a heat exchanger in which seawater or industrial water is allowed to flow inside a pipe, a low-temperature medium such as LNG is brought into contact with the outside of the pipe, and the low-temperature medium is evaporated by retained heat of seawater or the like. The present invention relates to a titanium welded tube that can be used for a heat transfer tube, and has a titanium welded tube whose axial proof stress is 275 MPa or more and that does not burst even when seawater or industrial water flowing in the tube is frozen, and It relates to the manufacturing method.

  One use of titanium welded pipes is in condensers such as power plants. Since these condensers (heat exchangers) use seawater for heat exchange, titanium heat transfer tubes with extremely high corrosion resistance against seawater are used. Usually, titanium heat transfer tubes (titanium welded tubes) used in general heat exchangers such as condensers are manufactured mainly by bending strip-shaped titanium in the width direction and welding the butt ends. The heat treatment is performed as it is or after welding. In general, heat treatment is performed at around 600 ° C., and mainly aimed at improving the corrosion resistance by preventing hydrogen embrittlement due to the removal of residual stress and the formation of an oxide film by heating in the atmosphere (Japanese Patent Laid-Open No. Sho). No. 54-11012), the crystal grain size and mechanical properties are almost the same. In these titanium welded pipes, the required characteristics are sufficiently satisfied even in the as-welded state, and there are no problems in terms of mechanical characteristics.

  On the other hand, in the heat exchanger, seawater or industrial water is allowed to flow inside the pipe, and a low-temperature medium of 0 ° C. or lower, for example, LNG is brought into contact with the outside of the pipe to evaporate the low-temperature medium by the retained heat of seawater or the like. There are types. A specific example of this is a shell and tube type heat exchanger. As a heat transfer tube of such a heat exchanger, in addition to the corrosion resistance against seawater similar to the above, durability when seawater or the like inside the welded tube is frozen is required. This is because when foreign matter or the like adheres to the inside of the pipe, the flow of seawater inside the pipe deteriorates due to the presence of the adhering matter, and the seawater etc. of the circulation deterioration part is excessively cooled by a low-temperature medium. In some cases, the flow of water deteriorates further, and the tube may eventually close. The titanium welded tube receives a large internal pressure due to volume expansion inside the tube due to freezing of seawater or the like, and cracks occur in the welded tube and its vicinity. This is because it may cause breakage. That is, when seawater or the like freezes in the titanium welded pipe and expands in volume, the welded pipe also expands accordingly, so it is necessary to absorb this expansion allowance with the ductility in the circumferential direction of the welded pipe. It is a needle-like structure with poor ductility, and cracks start from this part, and in the worst case, it may break. Therefore, as a technique to improve the ductility of the welded part and heat-affected zone, after adding strain due to strong processing, annealing is performed to recrystallize the needle-like structure of the welded part and heat-affected zone, thereby improving the ductility of the welded part. It was improving.

  In Patent Document 1, a titanium heat transfer tube manufactured by a seam welding method is drawn so that the cross-section reduction rate is 30% or more, and then heat treated at 600 to 800 ° C. After filling the tube with water and completely sealing it, immerse it in a test bath at −50 ° C. to freeze the water in the tube, then defrost the ice in the tube, fill with water, seal completely, and freeze again. This is repeated until the cracks occur in the titanium tube. Even if the process is repeated, the number of freezing cycles (the number of repeated freezing tests before the titanium tube cracks) No)) is disclosed a method for producing a titanium heat transfer tube three or more times. That is, by introducing a working strain at the time of drawing and recrystallizing the entire titanium heat transfer tube at the time of the subsequent heat treatment, the ductility of the welded part and the heat-affected part is improved, and the number of freezing resistances is improved. However, in patent document 1, since a cold drawing process is required, there is a concern that the manufacturing process becomes complicated and the manufacturing cost becomes high.

  In Patent Document 2, an oxygen content is 0.085 mass% or less, and a titanium welded tube manufactured by the TIG welding method is annealed for 30 minutes or more in a temperature range of 730 to 800 ° C. A titanium heat transfer tube manufacturing method has been proposed in which the number of times of freezing is three or more when implemented. That is, it is a method of recrystallizing the welded part and the heat-affected zone only by annealing without performing cold working such as cold drawing, etc., and by setting the oxygen concentration to 0.085 mass% or less, the welded part and the thermal influence The cost for improving the ductility of the department has been increased. However, in Patent Document 2, since cold working is not performed before the heat treatment, it is considered that the crystal grain sizes of the welded portion, the heat-affected portion, and the base material portion after the heat treatment are not uniform. In such a case, coarse crystal grains may be partially generated, and the low yield strength coarse crystal grain part is preferentially deformed when the water inside the tube is frozen, and there is a high possibility that non-uniform deformation occurs. Degradation of freezing resistance may occur.

  In Patent Document 3, titanium welded pipe manufactured using a hoop composed of a β-transformed structure, or titanium welded using a hoop composed of an equiaxed α-structure and then heated above the β-transformation point to form a β-transformed structure. A titanium welded pipe that is a welded pipe and has a number of freeze resistances of 3 or more even when the freeze repeated test is performed, and a method for manufacturing the same are disclosed. That is, when the hoop stress generated during freezing works, the heat-affected zone has a random crystal orientation compared to the base material that is difficult to deform in the circumferential direction due to the texture of the hoop material, and this part has low proof stress. In order to eliminate the stress concentration on the surface, the texture is eliminated (crystal orientation is randomized) by heating to the β transformation point or higher. However, in patent document 3, since it heats more than (beta) transformation point, in the case of atmospheric heat processing, the amount of oxidations will increase and a big yield fall will arise. In order to prevent this, it is necessary to perform heat treatment in an inert gas atmosphere or vacuum. Compared to atmospheric heat treatment, the heat treatment time is prolonged due to vacuuming or replacement with inert gas, and the manufacturing cost is also high. There are concerns.

  Patent Document 4 discloses a method in which a titanium tube is made into an equiaxed crystal structure by adding 2 to 5.5% drawing to a seam-welded titanium tube and heat-treating it at a temperature range of 650 to 800 ° C. for 30 minutes or more. Has been. However, for example, under the condition of 2% drawing, the amount of strain introduced at the time of processing is insufficient, and it is considered that a needle-like structure having poor ductility remains unrecrystallized, and the number of freeze resistance is less than 3 times. Presumed to be. In addition, as a result of studies by the present inventors, when the amount of strain is relatively small, 2 to 5.5%, when the heat treatment temperature is a long time of 30 minutes or more, grain growth is promoted and excessive coarse grains are formed. It has been confirmed that the number of freeze-resistant times drops to less than 3 times.

  In addition, as a standard for welded pipes, JIS does not provide a 0.2% proof stress, but ASTM provides a 0.2% proof stress of 275 MPa or more. Recently, as a required characteristic of a titanium welded pipe, In addition to the number of times of freezing (3 times or more), the standard of the yield strength is listed, and it has become necessary to satisfy that the 0.2% yield strength is 275 MPa or more. However, in Patent Documents 1 to 4, the main focus is on the number of times of freezing of titanium welded pipes, and although the ductility for improving this is mentioned, there is no description about the proof stress, and Patent Documents 1 to In the production method 4, it is difficult for the 0.2% proof stress to satisfy 275 MPa.

JP-A 61-186461 Japanese Unexamined Patent Publication No. 63-192852 JP-A-1-188655 JP-A-2-190458

  As described above, titanium suitable for a heat exchanger in which seawater or industrial water is allowed to flow inside a pipe, a low-temperature medium such as LNG is brought into contact with the outside of the pipe, and the low-temperature medium is evaporated by the retained heat of seawater or the like. Regarding the production of heat transfer tubes, it is necessary to increase the number of cold drawing processes or to perform heat treatment in an inert gas atmosphere or vacuum in order to achieve more than 3 times of freezing resistance. There is a concern that the manufacturing cost will increase.

  Therefore, the present invention performs heat treatment in an additional cold working step or in an inert gas atmosphere or vacuum on a heat exchanger of a type in which seawater or industrial water is allowed to flow inside the pipe and a low-temperature medium is contacted outside the pipe. Therefore, an object of the present invention is to provide a titanium welded tube that has a 0.2% proof stress in the axial direction of 275 MPa or more and that can achieve a freeze resistance of 3 times or more in a repeated freeze test and a method for manufacturing the same.

  As a result of intensive experiments and examinations to achieve the above-mentioned problems, the present inventors have found that a titanium welded tube applicable to a heat exchanger of a type that evaporates a low-temperature medium has an oxygen content of 0.10 to 0.10. A titanium hoop material made of pure titanium for industrial use of 0.20 mass% is subjected to processing strain with a sizer stand or a sizer mill so that the cross-section reduction rate is 3 to 10% at the time of manufacturing a titanium tube, and then 700 to 800 ° C., By heat-treating for 10 to 30 minutes, the entire structure of the welded pipe is made into an equiaxed α structure, and the average particle diameter in each part of the base material part, the heat-affected part, and the welded part is in the range of 30 to 100 μm. In addition, by making the average crystal grain size difference of each part equal to 20 μm or less, the 0.2% proof stress in the axial direction of the welded pipe is 275 MPa or more, and the number of times of freezing is 3 times or more in the repeated freeze test. Find what you can It was.

That is,
(1) In a titanium welded pipe made of industrial pure titanium having an oxygen content of 0.10 to 0.20 mass%, it is an equiaxed α structure, and is an average at each part of the base material portion, the heat affected zone, and the weld zone. A titanium welded tube characterized in that the particle size is in the range of 30 to 100 µm, and the average crystal particle size difference at each site is 20 µm or less.
(2) (1) characterized in that at the time of manufacturing the titanium tube, after being processed with a sizer stand or a sizer mill so that the cross-sectional reduction rate is 3 to 10%, heat treatment is performed at 700 to 800 ° C. for 10 to 30 minutes The manufacturing method of the titanium welding pipe of description.

  The present invention is made of titanium suitable for a heat exchanger in which seawater or industrial water is allowed to flow inside a pipe, a low-temperature medium such as LNG is brought into contact with the outside of the pipe, and the low-temperature medium is evaporated by retained heat of seawater or the like. Regarding the manufacture of heat transfer tubes, there is no need to add a cold working process such as a drawing process, and the heat treatment process is not performed in an inert gas atmosphere or in a vacuum, but only in a short period of atmospheric heat treatment, 0.2 in the axial direction. The present invention relates to a welded pipe that can achieve a yield strength of 275 MPa or more and a freeze resistance of 3 times or more in a repeated freezing test and a manufacturing method thereof, omitting the cold working process, simplifying the manufacturing process, and reducing the heat treatment time. The manufacturing cost can be greatly reduced by shortening the time and heat treatment in the atmosphere, and the industrial effects are immeasurable.

  The present invention will be described in detail below.

[Oxygen content]
The present invention is characterized in that the oxygen content of the welded pipe is made of industrial pure titanium having a 0.10 to 0.20 mass%. As described above, the heat exchanger of the type that evaporates a low-temperature medium is required to have the characteristics that the 0.2% proof stress in the axial direction is 275 MPa or more, and the number of times of freezing is three or more in the repeated freeze test. . As will be described later, in order to increase the number of freezing resistances to 3 times or more in the freezing repeated test, it is necessary to make the entire welded tube uniform crystal grains with uniform axes. It is necessary, and since individual crystal grains grow to 30 to 100 μm, the 0.2% yield strength tends to decrease. Therefore, by adding 0.10 to 0.20 mass% of oxygen which is a solid solution element, the 0.2% proof stress is made to be 275 MPa or more. If the amount of oxygen is less than 0.10 mass%, the 0.2% yield strength may not reach 275 MPa. Conversely, if oxygen is added in an amount of more than 0.20 mass%, the ductility decreases and the number of times of freezing is reduced. May deteriorate.

[Cross sectional structure and crystal grain size]
In the present invention, the cross-sectional structure of the titanium welded tube is an equiaxed α structure. When the titanium welded tube is observed as welded, the welded part and the heat-affected zone have a needle-like structure, and the portion of this needle-like structure has low ductility. Breaks early. Therefore, the low ductility acicular structure is recrystallized by heat treatment to obtain equiaxed α grains to improve ductility. During this heat treatment, if the portion of the low ductility needle-like structure remains, the number of freezing resistance deteriorates. Therefore, it is important to eliminate this part completely and to form equiaxed α grains.

  In addition, even if the grains are equiaxed α grains, if there are some coarse grains, non-uniform deformation may occur due to low yield strength coarse grains during the repeated freeze test. By setting the crystal grain size to a uniform size, it is possible to achieve a freeze resistance of 3 times or more. The crystal grain size of each part of the base metal part, the heat affected part, and the welded part is almost uniform in the part when heat treatment is performed under the heat treatment conditions described in the present invention. It is sufficient that the crystal grain size of the part is uniform. In addition, regarding specific crystal grains, the average grain size in each part of the base material part, the heat affected zone, and the welded part is in the range of 30 to 100 μm, and the average crystal grain size difference in each part is 20 μm. It stipulates that Since the crystal grain size obtained by the production method of the present invention is approximately 30 μm or more, the lower limit of the crystal grain size is set to 30 μm. When the crystal grain size is larger than 100 μm, the ductility is lowered and the number of freezing resistance may be deteriorated. For this reason, the upper limit of the crystal grain size was set to 100 μm. If the average particle size in each part of the base material part, the heat affected part and the welded part is in the range of 30 to 100 μm, and the difference in average particle diameter in each part is 20 μm or less, during the freezing repeated test The welded pipe is uniformly deformed, and the number of freezing times can be achieved three times or more. In this way, by making the entire titanium tube have a uniform crystal grain size, non-uniform deformation is eliminated, and even if the oxygen addition amount is increased to 0.10 to 0.20 mass%, the number of times of freezing is achieved three times or more. Is possible.

[Processing and heat treatment]
The present invention is characterized in that it is processed by a sizer stand or a sizer mill so that the cross-section reduction rate is 3 to 10% when the titanium tube is manufactured. In the production of welded pipes for normal use, the cross-section reduction rate at the sizer stand is about 1%, and it is used as an application for adjusting the outer shape at the end of the line. An object of the present invention is to use this sizer stand to impart processing strain necessary for recrystallization for obtaining an equiaxed α structure at the welded portion and the heat-affected zone. It was found that the amount of strain for making the crystal grain size of the base metal part, the heat-affected zone, and the weld zone uniform should generally be 3% or more of the cross-sectional reduction rate of the titanium tube. This is because when the cross-section reduction rate is less than 3%, the amount of processing strain introduced is small, the entire welded tube does not become completely equiaxed crystal grains, a part of the needle-like structure remains, and the number of times of freezing is three times. It is because there is a possibility that it is less than. In the sense of promoting recrystallization, it is important to impart sufficient processing strain, and it is desirable to perform processing at a cross-section reduction rate of 6 to 10%. Furthermore, the upper limit of the range that can be processed by the sizer stand is 10%. However, even if the cross-section reduction rate exceeds 10%, it works in the direction of promoting recrystallization, so it is considered that the same effect as the present invention is obtained. Processing with this sizer may be performed with either a sizer stand or a sizer mill.

  Moreover, about the heat processing after a process, it is supposed to heat-process at 700-800 degreeC and 10 to 30 minutes. When the heat treatment temperature is less than 700 ° C., the crystal grain size may be less than 30 μm, and the average grain size difference in each part of the base material portion, the heat affected zone and the weld zone is greater than 20 μm, and the number of times of freezing is 3 Times may not be achieved. At a temperature higher than 800 ° C., the crystal grain size becomes larger than 100 μm, and excessive coarsening of the crystal grain size may deteriorate the number of freezing resistance, so the temperature is set to 700 to 800 ° C. With respect to the heat treatment temperature, if it is less than 10 minutes, recrystallization is insufficient, and there are rare cases where a needle-like structure remains in the cross-sectional structure, or recrystallization is achieved and the entire structure becomes equiaxed α grains. Even in this case, the average crystal grain size difference in each part of the base material part, the heat affected zone and the weld zone may exceed 20 μm, and the heat treatment time is 10 minutes in order to sufficiently grow the grains and make the crystal grain size uniform. That's it. Further, in the heat treatment at high temperature, if the heat treatment time is prolonged, excessive crystal grain coarsening may occur and productivity may be deteriorated. Therefore, the upper limit of the heat treatment time is set to 30 minutes. Preferably, it is 15 minutes or more and 28 minutes or less. The heat treatment may be performed in any atmosphere of atmospheric heat treatment, vacuum heat treatment, and inert gas atmosphere, but in the embodiment, the heat treatment is performed by air heat treatment from the viewpoint of cost.

  Hereinafter, the present invention will be described in more detail with reference to examples. In Tables 1 and 2, numerical values outside the scope of the present invention are underlined.

  In Examples and Comparative Examples shown in Table 1, titanium welded pipes were manufactured using 1.2 mm thick hoop materials made of pure industrial titanium having different oxygen contents. At this time, pipe making was performed using a sizer stand at the end of the titanium welded pipe production line so that the cross-sectional reduction rate was 1 to 10% from the outer diameter before passing through the sizer stand. In addition, the cross-sectional structure | tissue of the hoop material employ | adopted the thing with an average particle diameter of 20-30 micrometers which consists of an equiaxed alpha structure | tissue, and it piped so that the outer diameter of the welded pipe after pipe forming might be set to (phi) 19mm. After forming the welded pipe, the welded pipe was cut to about 500 mm and subjected to atmospheric heat treatment at a temperature of 650 to 850 ° C. for 5 to 60 minutes. After the atmospheric heat treatment, cross-sectional structure observation (structure morphology and crystal grain size of each part (base material part, heat-affected part, welded part)), tensile test, and freezing repeated test were performed. After these tests, the 0.2% proof stress and the number of freeze resistances were evaluated, and those having a 0.2% proof stress of 275 MPa or more and a freeze resistance number of 3 or more were regarded as acceptable. Cross-sectional structure observation was carried out at a magnification of 100 times, and the average crystal grain size was calculated by a cutting method so that the number of crystal grains measured at the base material, the heat-affected zone and the weld zone was 200 or more, respectively. . In addition, since the melted portion and the heat affected zone have a smaller area than the base material portion, several visual fields were observed in the longitudinal direction. The tensile test is based on the JIS standard and a tubular tensile test is performed. A JIS No. 11 test piece is used as the tensile test piece. In addition, the freeze repeated test piece uses a welded tube with a length of 350 mm cut out and welded with a titanium disc of about 10 mm thickness and φ19 mm at the end of the tube. One end of the tube is filled with water inside the tube. Therefore, the M8 bolt hole was processed. At the time of the test, M8 bolts were inserted into the bolt holes and the water inside was sealed to carry out the test (rubber packing and washers were also used for sealing). In the cycle of the freezing repeated test, the titanium welded tube filled with water and sealed as described above was immersed in a -50 ° C test bath for 5 minutes to freeze the water in the tube, and then the titanium welded tube was heated to about 90 ° C. Soak the ice in the tube. Thereafter, if there is no crack in the welded tube, the welded tube is again filled with water, completely sealed, and immersed again in a −50 ° C. test bath. This is repeated until the titanium tube is cracked, and the number of tests in which the titanium tube is maintained without cracking for 5 minutes in the test bath at −50 ° C. until the titanium tube is cracked is recorded. did. In addition, what mixed alcohol and liquid nitrogen was used for the test tank of -50 degreeC.

  No. 1 to No. 29 Examples and Comparative Examples are cases in which a pure titanium hoop material having an oxygen content of 0.15 mass% was used.

  No. A comparative example 1 is an example in which a welded pipe is manufactured under normal pipe-making conditions, and the cross-sectional reduction rate of the sizer stand is 1%, and no strong processing is performed and no heat treatment is performed. Since the heat-affected zone and the weld zone remained welded, they exhibited a needle-like structure, and the 0.2% proof stress was 275 MPa or more, but the number of freezing proofs was zero.

  No. In Comparative Example 2, the size reduction ratio of the sizer stand is 1%, a strong pipe is not carried out, and a welded pipe manufactured under normal pipe making conditions is heat-treated at 800 ° C. for 10 minutes. In the cross-sectional structure after heat treatment, the base metal part and the heat-affected zone exhibited an equiaxed α structure, but the welded part was not sufficiently recrystallized because it was not subjected to strong processing with a sizer stand. The number of freezing resistance was not as high as 3 times.

  No. The comparative example 3 is an example in which processing with a cross-section reduction rate of 10% was performed with a sizer stand, but no subsequent heat treatment was performed. No. As in Comparative Example 1, since heat treatment was not performed, the cross-sectional structure of the welded tube was a needle-like structure at the heat-affected zone and the weld zone, and the number of freeze resistances was zero.

  No. 4-No. The comparative example of No. 7 is a case where processing with a cross-section reduction rate of 2% was performed with a sizer stand. Although heat treatment was performed at a temperature of 700 to 800 ° C. for 10 to 30 minutes, the 0.2% proof stress satisfied 275 MPa under any condition, but the number of freezing proofs was less than three. No. 4-No. In the comparative example of 6, the low ductility needle-like structure remained, so it is considered that the number of freeze-resistant times was low. No. In Comparative Example 7, the needle-like structure did not remain and was an equiaxed α structure. However, the difference in crystal grain size between the base material part, the heat-affected part, and the welded part was not equal to or less than 20 μm, and the It is estimated that the number of freezes was low.

  No. 8-No. Twelve examples and comparative examples are cases in which the cross-sectional reduction rate at the sizer stand is set to 3%, which is larger than usual.

  No. In Comparative Example 8, heat treatment was performed at 650 ° C. for 10 minutes, but recrystallization was insufficient, a needle-like structure remained in the welded portion, and the number of freezing resistances did not reach 3 times. .

  No. 9-No. In Example 11, the heat treatment temperature was changed from 700 ° C. to 800 ° C. within the scope of the present invention, but the heat treatment time was 10 minutes. In any of the Examples, the average particle size difference at each part was as small as 20 μm or less, the 0.2% proof stress was 275 MPa or more, and the freeze resistance was 3 times or more.

  No. The comparative example of 12 is a case where the heat treatment temperature is further increased and the heat treatment is performed at 850 ° C. for 10 minutes. By increasing the heat treatment temperature up to 850 ° C., the crystal grain size is larger than 100 μm, and although the difference in crystal grain size in the entire welded pipe is small, the number of times of freezing is No. 9-No. Compared to the ten examples, it was lower and did not satisfy the criteria. Excessive coarsening of crystal grains produced low-ductile coarse grains, resulting in non-uniform deformation and a decrease in the number of freeze resistances.

  No. 13-No. The 15th embodiment is a case where the cross-section reduction rate at the sizer stand is further increased to 5%. Similarly, the heat treatment temperature was changed in the range of 700 to 800 ° C. and held for 10 minutes. In each case, the average particle size difference at each part was as small as 20 μm or less, the 0.2% proof stress was 275 MPa or more, and The number of times of freezing was more than 3 times.

  No. 16-No. 29 examples and comparative examples are cases where the cross-section reduction rate at the sizer stand is increased to 6% or 10%.

  No. 16-No. A comparative example of 17 is a case where the heat treatment after processing with a sizer stand is performed at 700 to 800 ° C. for 5 minutes, but the 0.2% proof stress was 275 MPa or more, but the number of times of freezing was three times. Was less than. No. In Comparative Example 16, the retention time was short, and the crystal was not sufficiently recrystallized, and a low ductility needle-like structure remained. No. In Comparative Example 17, although an equiaxed α structure was obtained by recrystallization, there was a difference in the crystal grain size in the entire welded tube, which was thought to be due to insufficient retention time and insufficient grain growth. Therefore, it seems that the number of freeze-resistant times was low.

  No. 18-No. 22 Examples and Comparative Examples are cases where the heat treatment was changed in the range of 650 to 850 ° C. and held for 10 minutes.

  No. Comparative Example 18 was an example of heat treatment at 650 ° C. for 10 minutes. Sufficient recrystallization did not occur because the holding temperature was low, and a needle-like structure remained in the heat-affected zone and the weld zone. . Since the low ductility needle-like structure remained, it is considered that the number of freezing resistance was less than 3.

  No. 19-No. In Example 21, heat treatment conditions were maintained at 700 to 800 ° C. for 10 minutes. Since the average particle size difference at each site was 20 μm or less, the 0.2% proof stress was 275 MPa or more, and the freeze resistance was 3 times or more.

  No. The comparative example of 22 is a case where the heat treatment temperature is increased to 850 ° C. for 10 minutes. Crystal grains larger than 100 μm are partially generated, and deterioration of the number of freezing resistance considered to be due to the presence of such coarse crystal grain portions is observed. 19-No. The number of times of freezing resistance was lower than that of Example 21, and the standard was not satisfied.

  No. 23-No. No. 27 Examples and Comparative Examples 18-No. This is a case where the holding time at the time of heat treatment is made longer than that of Example 22 and Comparative Example 22 and heat treatment is performed at 650 to 850 ° C. for 25 to 30 minutes.

  No. No. 23 is a case where heat treatment is performed at 650 ° C. for 30 minutes. As in the 18 comparative examples, sufficient recrystallization did not occur, and a needle-like structure remained at the weld. Therefore, it is estimated that the number of times of freezing resistance was low.

  No. 24-No. The example of 26 is a case where heat processing temperature is changed at 700-800 degreeC, and it hold | maintains for 25-30 minutes. In this example, no. 19-No. Similar to Example 21, the average particle size difference was small, and the acceptance criteria of 0.2% proof stress and anti-freezing frequency were satisfied.

  No. The comparative example of 27 is a case where it is 850 degreeC and 30 minutes. No. of freezing is 2 times. 24-No. It was once lower than 26 examples. In the cross-sectional structure observation, the entire welded pipe has coarsened crystal grains, which may be attributed to the low ductility.

  No. 28, no. 29 Examples and Comparative Examples are cases where the heat treatment time was further increased and heat treatment was performed at 700 to 800 ° C. for 60 minutes. Under any condition, the crystal grain sizes of the base metal part, the heat-affected part, and the welded part were coarsened, and the number of freeze resistances was less than 3.

  No. 30-No. 39 Examples and Comparative Examples are examples in which processing with a cross-section reduction rate of 10% was performed with a sizer stand using a hoop material in which the oxygen content was changed from 0.08 to 0.25%.

  No. A comparative example of No. 30 and No. 31 is a case where a material having an oxygen content of 0.08 mass% is heat-treated at 700 to 800 ° C. for 10 minutes after being formed. The average particle size at each part is in the range of 30 to 100 μm, and the average particle size difference is 20 μm or less. The 2% proof stress was not 275 MPa or more.

  No. 32-No. Example 34 is an example in which a hoop material having an oxygen amount of 0.1 mass% was used, and the holding temperature during heat treatment was changed in the range of 700 to 800 ° C. and held for 10 minutes. In each case, the average particle size difference at each site was 20 μm or less, the 0.2% proof stress was 275 MPa or more, and the number of freezing resistances was 3 times or more.

  No. 35-No. Example 37 is an example in which a hoop material having an oxygen content of 0.2 mass% is used. Similarly, the heat treatment conditions are 700 ° C. to 800 ° C. for 10 minutes. No. 32-No. Since the oxygen amount is increased as compared with Example 34, the 0.2% proof stress is high, but the average particle size of the entire welded pipe is in the range of 30 to 100 μm, and the average particle size of each part The difference was 20 μm or less, and the criteria for the number of times of freezing were satisfied.

  No. 38-No. The comparative example of 39 is an example using a hoop material having an oxygen content of 0.25 mass%, and is a case where heat treatment is performed at 700 to 800 ° C. for 10 minutes. Since a hoop having a large oxygen amount of 0.25 mass% was used, the ductility was poor and the number of times of freezing resistance was considered to have fallen below the standard of 3.

  In the above examples, the crystal grain size is in the range of 30 to 100 μm in any part, and the grain sizes are uniform in the same welded pipe, so that non-uniform deformation is suppressed and the number of freeze resistances is improved. It is considered a thing.

Claims (2)

  In a titanium welded tube made of pure titanium for industrial use with an oxygen content of 0.10 to 0.20 mass%, it is an equiaxed α structure, and the average particle size at each part of the base metal part, the heat affected zone, and the weld zone is A titanium welded tube characterized in that it is within a range of 30 to 100 μm, and an average crystal grain size difference at each part is 20 μm or less.   2. The titanium according to claim 1, wherein the titanium tube is heat-treated at 700 to 800 ° C. for 10 to 30 minutes after being processed with a sizer stand or a sizer mill so that the cross-sectional reduction rate becomes 3 to 10% at the time of manufacturing the titanium tube. Manufacturing method of welded pipe.