JP6265037B2 - Titanium welded tube and manufacturing method thereof - Google Patents
Titanium welded tube and manufacturing method thereof Download PDFInfo
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本発明は、管内に海水あるいは工業用水などを流し、管外に低温の媒体、例えば、LNGなどを接触させ、海水などの保有熱により低温媒体を蒸発させるタイプの熱交換器に適したチタン製伝熱管に用いることのできるチタン溶接管に関するものであり、チタン溶接管の軸方向の耐力が275MPa以上であり、且つ、管内を流れる海水あるいは工業用水などが凍結しても破裂しないチタン溶接管及びその製造方法に関する。 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.
チタン溶接管の使用用途の一つとして、発電所などの復水器が挙げられる。これらの復水器(熱交換器)では、熱交換に海水を使用するため、対海水で極めて耐食性の高いチタン製伝熱管が使用されている。通常、復水器のような一般的な熱交換器に用いられるチタン製伝熱管(チタン溶接管)は、主に帯状チタンを幅方向に湾曲させ、突き合わせ端部を溶接して製造されており、溶接ままあるいは溶接後に熱処理を施して使用している。熱処理を施すものは、一般に600℃前後で熱処理をしており、主に残留応力の除去や大気加熱による酸化被膜形成に伴い水素脆化を防止し耐食性向上を狙ったものであり(特開昭54−11012号公報)、結晶粒径や機械的特性はほとんど変わらない。これらのチタン溶接管では、溶接ままでも要求特性を十分満足し、機械的特性の面でも問題は生じていない。 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.
一方、熱交換器の中には、管内に海水あるいは工業用水などを流し、管外に0℃以下の低温の媒体、例えば、LNGなどを接触させ、海水等の保有熱により低温媒体を蒸発させるタイプのものがある。この具体例として、シェルアンドチューブタイプの熱交換器が挙げられる。このような熱交換器の伝熱管としては、上記同様の対海水での耐食性に加え、溶接管内部の海水などが凍結した場合の耐久性が求められる。これは、管内部に異物等が付着した場合、付着物の存在により管内部の海水などの流れが悪くなり、流通悪化部の海水などが低温媒体により過度に冷却され、その凍結により、海水などの流れがさらに悪化し、最終的には管が閉塞してしまうことがあり、チタン溶接管は海水などの凍結による管内部での体積膨張に伴う大きな内圧を受け、溶接管及びその近傍に亀裂が生じ、破断に至ることがあるからである。即ち、チタン溶接管内で海水などが凍結し体積膨張した場合、溶接管もそれに伴い膨張するため、この膨張代を溶接管周方向の延性で吸収する必要があるが、溶接部や熱影響部は延性の乏しい針状組織であり、この部分を起点として亀裂が入り、最悪の場合、破断に至ることがある。そのため、溶接部や熱影響部の延性を改善させる手法として、強加工による歪を加えた後、焼鈍することで、溶接部や熱影響部の針状組織を再結晶させ、溶接部の延性を改善していた。 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.
特許文献1では、シーム溶接法で製造されたチタン製伝熱管を断面減少率が30%以上となるように引抜加工し、次いで600〜800℃で熱処理をすることで、凍結繰返し試験(サンプル管に水を満たし完全にシールした後、−50℃の試験槽に浸漬して管内の水を凍結させる。その後、管内の氷を氷解させた後、さらに水を満たして完全にシールし、再度氷結させる。これを繰り返し、チタン管に亀裂が生じるまで行う。)を実施しても、耐凍結回数(チタン管に亀裂が入るまでに凍結繰り返し試験を実施した回数(亀裂発生時の試験回数は含まない))が3回以上であるチタン製伝熱管製造方法が開示されている。即ち、引抜加工時に加工歪を導入し、その後の熱処理時にチタン製伝熱管全体を再結晶させることで、溶接部及び熱影響部の延性が改善し、耐凍結回数を改善している。しかしながら、特許文献1では、冷間引抜加工工程が必要となるため、製造工程が煩雑になり、製造コストも割高となる懸念がある。 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.
特許文献2では、酸素含有量が0.085mass%以下であって、TIG溶接法で製造したチタン溶接管について、730〜800℃の温度範囲で30分以上焼鈍することで、耐凍結繰返し試験を実施した際に耐凍結回数が3回以上であるチタン製伝熱管製造方法が提案されている。即ち、冷間引抜加工などの冷間加工を行わずに、焼鈍のみで溶接部及び熱影響部を再結晶させる方法であり、酸素濃度を0.085mass%以下とすることで溶接部及び熱影響部の延性改善代を大きくしている。しかし、特許文献2では、熱処理前に冷間加工を実施していないため、熱処理後の溶接部、熱影響部及び母材部の結晶粒径が同程度に揃っていないと考えられる。このような場合、部分的に粗大結晶粒が発生することがあり、低耐力な粗大結晶粒部は、管内部の水の凍結時に優先的に変形し、不均一変形が生じる可能性が高く、耐凍結性の劣化が生じる可能性がある。 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.
特許文献3では、β変態組織からなるフープを用いて製造したチタン溶接管、もしくは、等軸α組織からなるフープを用いて溶接した後、β変態点以上で加熱してβ変態組織としたチタン溶接管であり、凍結繰返し試験を実施しても、耐凍結回数が3回以上であるチタン溶接管とその製造方法が開示されている。即ち、凍結時に生じるフープ応力が働いた場合、フープ材の有する集合組織により円周方向に変形し難い母材部に比べ、熱影響部はランダムな結晶方位を有するため低耐力であり、この部分への応力集中をなくすために、β変態点以上まで加熱することで集合組織を消失(結晶方位をランダム化)させている。しかしながら、特許文献3では、β変態点以上まで加熱するため、大気熱処理の場合、酸化量が多くなり、大きな歩留低下を生じる。これを防止するためには不活性ガス雰囲気や真空中で熱処理を行う必要があり、大気熱処理と比べ、真空引きや不活性ガスへの置換などにより熱処理時間が長時間化し、製造コストも割高となる懸念がある。 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.
特許文献4では、シーム溶接したチタン管に2〜5.5%の絞り加工を加え、650〜800℃の温度範囲で30分以上熱処理することでチタン管を等軸結晶組織とする方法が開示されている。しかしながら、例えば、絞り加工2%の条件では、加工時に導入される歪量が不十分であり、延性の乏しい針状組織が未再結晶のまま残存すると考えられ、耐凍結回数は3回未満となるものと推測される。また、本発明者らが検討したところ、2〜5.5%の比較的歪量の少ない場合では、熱処理温度が30分以上の長時間になると、粒成長が促進され、過度な粗大粒が発生し、耐凍結回数が3回未満まで低下することを確認している。 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.
また、溶接管の規格として、JISでは0.2%耐力の規定はないが、ASTMでは0.2%耐力が275MPa以上という規定が存在しており、最近では、チタン溶接管の要求特性として、耐凍結回数(3回以上)に加え、この耐力の基準も挙がっており、0.2%耐力が275MPa以上であることも満足する必要が出てきた。しかしながら、特許文献1〜4では、チタン溶接管の耐凍結回数に主眼が置かれており、これを改善するための延性に関しては触れられているものの、耐力については記載がなく、特許文献1〜4の製造方法では、0.2%耐力が275MPaを満足することは難しい。 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.
前述のように、管内に海水あるいは工業用水などを流し、管外に低温の媒体、例えば、LNGなどを接触させ、海水等の保有熱により低温媒体を蒸発させるタイプの熱交換器に適したチタン製伝熱管の製造に関して、耐凍結回数3回以上を達成するためには、冷間引抜工程を増やしたり、不活性ガス雰囲気もしくは真空で熱処理を実施したりする必要があり、製造工程が煩雑となり、製造コストが増加する懸念がある。 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.
そこで、本発明は、管内に海水あるいは工業用水などを流し、管外に低温の媒体を接触させるタイプの熱交換器について、追加の冷間加工工程や不活性ガス雰囲気もしくは真空で熱処理を実施することなく、軸方向の0.2%耐力が275MPa以上、且つ、凍結繰返し試験にて耐凍結回数3回以上を達成可能なチタン溶接管とその製造方法を提供することを目的とする。 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.
本発明者らは、前記課題を達成すべく、鋭意実験・検討を重ねた結果、低温の媒体を蒸発させるタイプの熱交換器に適用可能なチタン溶接管において、酸素含有量が0.10〜0.20mass%の工業用純チタンからなるチタンフープ材を、チタン管製造時に、断面減少率が3〜10%となるようにサイザースタンドもしくはサイザーミルで加工歪を付与した後、700〜800℃、10〜30分で熱処理することで、溶接管全体の組織を等軸α組織とし、母材部、熱影響部、溶接部の各部位での平均粒径が30〜100μmの範囲内であり、且つ、各部位の平均結晶粒径差を20μm以下として均一にすることで、溶接管の軸方向の0.2%耐力が275MPa以上、且つ、凍結繰返し試験にて耐凍結回数3回以上を達成できることを見出した。 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.
即ち、
(1)酸素含有量が0.10〜0.20mass%の工業用純チタンからなるチタン溶接管において、等軸α組織であり、母材部、熱影響部、溶接部の各部位での平均粒径が30〜100μmの範囲内であり、且つ、各部位の平均結晶粒径差が20μm以下であることを特徴とするチタン溶接管。
(2)チタン管製造時に、断面減少率が3〜10%となるようにサイザースタンドもしくはサイザーミルで加工した後、700〜800℃、10〜30分で熱処理することを特徴とした(1)に記載のチタン溶接管の製造方法。
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.
本発明は、管内に海水あるいは工業用水などを流し、管外に低温の媒体、例えば、LNGなどを接触させ、海水などの保有熱により低温媒体を蒸発させるタイプの熱交換器に適したチタン製伝熱管の製造に関して、引抜加工工程などの冷間加工工程を新たに追加することなく、熱処理工程も不活性ガス雰囲気中や真空中ではなく、短時間の大気熱処理のみで軸方向の0.2%耐力が275MPa以上、且つ、凍結繰返し試験にて耐凍結回数3回以上を達成可能な溶接管及びその製造方法に関するものであり、冷間加工工程の省略、製造工程の簡素化、熱処理時間の短時間化、大気熱処理化などによる大幅な製造コスト低減が可能であり、産業上の効果は計り知れない。 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.
以下、本発明について詳しく説明する。 The present invention will be described in detail below.
[酸素含有量]
本発明では、溶接管の酸素含有量が0.10〜0.20mass%の工業用純チタンからなることを特徴としている。前述したように、低温の媒体を蒸発させるタイプの熱交換器では、軸方向の0.2%耐力が275MPa以上、且つ、凍結繰返し試験にて耐凍結回数3回以上という特性が求められている。後述するが、凍結繰返し試験において耐凍結回数を3回以上とするためには、溶接管全体を均一な等軸の結晶粒とする必要があり、熱処理時にある程度結晶粒を成長させ、均一化させる必要があり、個々の結晶粒が30〜100μmまで粒成長するため、0.2%耐力は低下する傾向にある。したがって、固溶元素である酸素を0.10〜0.20mass%添加することで、0.2%耐力が275MPa以上となるようにしている。酸素量が0.10mass%未満であれば、0.2%耐力が275MPaに達しない可能性があり、逆に、0.20mass%より多く酸素を添加すると、延性が低下し、耐凍結回数が劣化する場合がある。
[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.
[断面組織と結晶粒径]
本発明では、チタン溶接管の断面組織が等軸α組織であるとしている。チタン溶接管を溶接ままで観察すると、溶接部や熱影響部は針状組織であり、この針状組織の部分は低延性のため、凍結繰返し試験を実施すると、この部分で亀裂が発生し、早期に破断に至る。したがって、低延性の針状組織部を熱処理により再結晶させ、等軸α粒として、延性の向上を図っている。この熱処理の際、低延性の針状組織の部分が残存すると、耐凍結回数が劣化するため、この部位を完全になくし、等軸α粒とすることが重要である。
[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.
また、等軸α粒になっていても、部分的に粗大結晶粒が存在すると、凍結繰返し試験時に低耐力の粗大結晶粒部に起因し不均一変形が生じる場合があり、溶接管全体を適正で均一な大きさの結晶粒径とすることで、耐凍結回数3回以上が達成できる。母材部、熱影響部、溶接部の各部位の結晶粒径は、本発明に記載の熱処理条件で熱処理すると、その部位の中ではほぼ均一になるので、母材部、熱影響部、溶接部の結晶粒径が均一であれば良い。また、具体的な結晶粒に関しては、母材部、熱影響部、溶接部の各部位での平均粒径が30〜100μmの範囲内であり、且つ、各部位の平均結晶粒径差が20μm以下であることと規定している。本発明の製造方法で得られる結晶粒径は概ね30μm以上であるため、結晶粒径の下限を30μmとしており、結晶粒径が100μmより大きくなると延性が低下し、耐凍結回数が劣化する場合があるため、結晶粒径の上限を100μmとした。母材部、熱影響部、溶接部のそれぞれの部位での平均粒径が、30〜100μmの範囲内であり、且つ、各部位の平均粒径差が20μm以下であれば、凍結繰返し試験時に、溶接管が均一に変形し、耐凍結回数3回以上が達成できる。このように、チタン管全体を均一な結晶粒径とすることによって、不均一変形をなくすことで、酸素添加量を0.10〜0.20mass%に高めても耐凍結回数3回以上の達成が可能としている。 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.
[加工及び熱処理]
本発明では、チタン管製造時に断面減少率が3〜10%となるようにサイザースタンドもしくはサイザーミルで加工することを特徴としている。通常用途の溶接管の製造においては、サイザースタンドでの断面減少率は1%程度であり、ライン終盤で外形形状を整える用途として使われている。本発明では、このサイザースタンドを活用し、溶接部及び熱影響部で等軸α組織を得るための再結晶に必要な加工歪みを付与することを目的としている。母材部、熱影響部、溶接部の結晶粒径を均一にするための歪量は、概ね、チタン管の断面減少率を3%以上にすれば良いと分かった。これは、断面減少率が3%未満では、加工歪の導入量が少なく、溶接管全体が完全な等軸結晶粒にならず、一部で針状組織が残存し、耐凍結回数が3回に満たない可能性があるからである。再結晶を促進させるという意味では、十分に加工歪を付与することが重要であり、断面減少率6〜10%で加工することが望ましい。さらに、サイザースタンドでの加工可能な範囲として、上限を10%とした。しかしながら、断面減少率10%超であっても、再結晶を促進する方向に働くので、本発明と同等の効果が得られると考えられる。このサイザーでの加工は、サイザースタンドで実施しても、サイザーミルで実施しても、どちらでも構わない。
[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.
また、加工後の熱処理について、700〜800℃、10〜30分で熱処理することとしている。熱処理温度が700℃未満では、結晶粒径が30μm未満となる場合があり、且つ、母材部、熱影響部、溶接部の各部位の平均粒径差が20μmより大きくなり、耐凍結回数3回を達成できない場合がある。800℃より高い温度では、結晶粒径が100μmより大きくなり、過度な結晶粒径の粗大化が耐凍結回数を劣化させる場合があるため、700〜800℃とした。熱処理温度については、10分未満では、再結晶が不十分であり、断面組織に針状組織が残存する場合が稀に存在し、あるいは、再結晶は達成され組織全体が等軸α粒になっていても、母材部、熱影響部、溶接部の各部位の平均結晶粒径差が20μmを超える場合があり、十分に粒成長させ結晶粒径を揃えるためにも、熱処理時間は10分以上としている。また、高温での熱処理では熱処理時間を長時間化すると、過度な結晶粒粗大化が生じることや、生産性の悪化も懸念されるため、熱処理時間の上限を30分とした。好ましくは、15分以上28分以下である。熱処理は、大気熱処理、真空熱処理、不活性ガス雰囲気のいずれの雰囲気で行っても良いが、実施例ではコストの観点から大気熱処理にて熱処理を行っている。 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.
以下、実施例により本発明をさらに詳細に説明する。表1、2において、本発明範囲から外れる数値にアンダーラインを付している。 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.
表1に示す実施例及び比較例において、酸素含有量の異なる工業用純チタンからなる1.2mm厚のフープ材を用いて、チタン溶接管を製造した。この際、チタン溶接管製造ライン終盤にあるサイザースタンドを用いて、サイザースタンド通過前の外径から断面減少率が1〜10%となるように造管を行った。なお、フープ材の断面組織は、等軸α組織からなる平均粒径20〜30μmのものを採用しており、造管後の溶接管の外径がφ19mmとなるように造管を行った。溶接管造管後、溶接管を500mm程度に切断し、650〜850℃の温度で、5〜60分間、大気熱処理を行った。大気熱処理後、断面組織観察(組織形態及び各部位(母材部、熱影響部、溶接部)の結晶粒径)、引張試験、凍結繰返し試験を行った。これら試験後に、0.2%耐力と耐凍結回数を評価し、0.2%耐力が275MPa以上、耐凍結回数が3回以上のものを合格とした。断面組織観察は、100倍の倍率で実施しており、母材、熱影響部、溶接部でそれぞれ測定した結晶粒の数が200個以上となるように切断法により平均結晶粒径を算出した。また、溶融部、熱影響部は、母材部に比べ、面積が少ないので、長手方向に数視野観察した。引張試験は、JIS規格に準拠し、管状引張試験を行っており、引張試験片はJIS 11号の試験片を用いた。また、凍結繰返し試験片は、350mm長さの溶接管を切り出し、管端に約10mm厚でφ19mmのチタン円盤を溶接したものを使用しており、片方の管端には管内部に水を充填させるため、M8のボルト穴を加工した。試験時には、ボルト穴にM8ボルトを差し込み、内部の水を密封して試験を実施した(密封のためゴムパッキンやワッシャーなども使用)。凍結繰返し試験のサイクルは、前記要領で水を充填させシールしたチタン溶接管を−50℃の試験槽に5分間浸漬して管内の水を凍結させ、その後、チタン溶接管を90℃程度のお湯に浸漬させ管内の氷を氷解させる。その後、溶接管に亀裂がなければ、再度、溶接管内に水を充填して完全にシールし、再度−50℃の試験槽に浸漬させる。これを繰り返し、チタン管に亀裂が生じるまで行い、チタン管に亀裂が入るまでに、−50℃の試験槽で5分間チタン管が割れずに維持された試験回数を記録し、耐凍結回数とした。なお、−50℃の試験槽は、アルコールと液体窒素を混合したものを使用した。 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からNo.29の実施例及び比較例は、酸素量が0.15mass%の純チタンフープ材を用いた場合である。 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.1の比較例は、通常の造管条件で溶接管を製造した例であり、サイザースタンドの断面減少率は1%であって強加工は行わず、熱処理も行っていない場合である。熱影響部や溶接部は、溶接ままのため、針状組織を呈しており、0.2%耐力は275MPa以上であるものの、耐凍結回数は0回であった。 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.2の比較例は、サイザースタンドの断面減少率は1%であって強加工を実施せず、通常の造管条件で製造した溶接管を、800℃、10分で熱処理した例である。熱処理後の断面組織において、母材部と熱影響部は等軸α組織を呈していたが、サイザースタンドでの強加工を実施しなかったため、溶接部は十分再結晶しておらず針状組織であり、耐凍結回数も3回には及ばなかった。 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.3の比較例は、サイザースタンドで断面減少率10%の加工を実施したが、その後の熱処理を実施しなかった例である。No.1の比較例と同様、熱処理を実施していないため、溶接管の断面組織は、熱影響部と溶接部で針状組織となっており、耐凍結回数は0回であった。 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.7の比較例は、サイザースタンドで断面減少率2%の加工を実施した場合である。温度700〜800℃で10〜30分保持する熱処理を実施しているが、いずれの条件でも0.2%耐力は275MPaを満足するものの、耐凍結回数は3回未満であった。No.4〜No.6の比較例では、低延性の針状組織が残存していたため、耐凍結回数が低かったと考えられる。また、No.7の比較例では、針状組織は残存しておらず、等軸α組織であったが、母材部、熱影響部、溶接部の結晶粒径差が20μm以下に揃っておらず、耐凍結回数が低かったものと推測される。 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.12の実施例及び比較例は、サイザースタンドでの断面減少率を通常よりも大きく3%とした場合である。 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.8の比較例では、650℃、10分の熱処理を実施しているが、再結晶が不十分で、溶接部に針状組織が残存しており、耐凍結回数が3回に達していなかった。 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.11の実施例は、熱処理温度を本発明範囲内で700℃〜800℃とそれぞれ変えているが、熱処理時間は10分とした例である。いずれの実施例でも各部位の平均粒径差が20μm以下と小さく、0.2%耐力は275MPa以上、且つ、耐凍結回数は3回以上を満足していた。 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.12の比較例は、さらに熱処理温度を高くし、850℃、10分で熱処理を実施した場合である。850℃まで熱処理温度を高くしたことで、結晶粒径が100μmより大きくなっており、溶接管全体での結晶粒径差は小さいものの、耐凍結回数はNo.9〜No.10の実施例と比較し、低下しており、基準を満足しなかった。過度な結晶粒の粗大化により、低延性な粗大粒が生じたため、不均一変形となり、耐凍結回数が劣化したものと考えられる。 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.15の実施例は、サイザースタンドでの断面減少率をさらに大きく5%とした場合である。同様に、熱処理温度を700〜800℃の範囲で変化させ、10分保持を行ったが、いずれも各部位の平均粒径差が20μm以下と小さく、0.2%耐力は275MPa以上、且つ、耐凍結回数は3回以上を満足していた。 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の実施例及び比較例は、サイザースタンドでの断面減少率を6%あるいは10%まで大きくした場合である。 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.17の比較例は、サイザースタンドでの加工後の熱処理時を700〜800℃、5分で行った場合であるが、0.2%耐力は275MPa以上であったが、耐凍結回数は3回未満であった。No.16の比較例では、保持時間が短いことに起因し、十分再結晶しておらず、低延性の針状組織が残存していたため、耐凍結回数が低かったと推定される。また、No.17の比較例では、再結晶により等軸α組織が得られたものの、保持時間が少なく十分粒成長しなかったことに起因すると考えられる溶接管全体での結晶粒径差が生じており、これにより、耐凍結回数が低かったと思われる。 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の実施例及び比較例は、熱処理を650〜850℃の範囲で変化させ、10分間保持した場合である。 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.18の比較例は、650℃、10分で熱処理した例であり、保持温度が低いためか、十分な再結晶が生じておらず、熱影響部、溶接部で針状組織が残存していた。低延性の針状組織が残存したため、耐凍結回数は3回未満であったと考えられる。 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.21の実施例は、熱処理条件を700〜800℃で10分間保持した例である。各部位の平均粒径差が20μm以下となっていたため、0.2%耐力は275MPa以上、且つ、耐凍結回数は3回以上を満足していた。 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.22の比較例は、熱処理温度を高温にし、850℃、10分とした場合である。部分的に100μmより大きい結晶粒が生じており、このような粗大結晶粒部が存在したことに起因したと思われる耐凍結回数の劣化が見られ、No.19〜No.21の実施例よりも耐凍結回数が低く、基準を満たしていなかった。 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.27の実施例及び比較例は、No.18〜No.22の実施例及び比較例よりも熱処理時の保持時間を長くし、650〜850℃、25〜30分で熱処理した場合である。 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.23の比較例は、650℃、30分で熱処理した場合であるが、No.18の比較例と同様、十分な再結晶が生じておらず、溶接部で針状組織が残存していた。ゆえに、耐凍結回数も低かったと推定される。 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.26の実施例は、熱処理温度を700〜800℃で変化させ、25〜30分保持した場合である。こちらの例でも、No.19〜No.21の実施例と同様、平均粒径差は小さく、0.2%耐力と耐凍結回数の合格基準を満足していた。 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.27の比較例は、850℃、30分とした場合である。耐凍結回数が2回と、No.24〜No.26の実施例よりも1回低下していた。断面組織観察では、溶接管全体で結晶粒の粗大化が生じており、これによる低延性か寄与したものと思われる。 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の実施例及び比較例は、さらに熱処理時間を長くし、700〜800℃、60分で熱処理した場合である。いずれの条件でも、母材部、熱影響部、溶接部の結晶粒径が粗大化しており、耐凍結回数は3回未満であった。 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の実施例及び比較例は、酸素含有量を0.08〜0.25%まで変化させたフープ材を用いて、サイザースタンドで断面減少率10%の加工を実施した例である。 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.30、No31の比較例は、酸素量が0.08mass%の素材を造管後、700℃〜800℃で10分間熱処理した場合である。各部位での平均粒径は30〜100μmの範囲であり、平均粒径差は20μm以下であるため、耐凍結回数3回以上は満足したが、酸素量が0.08mass%と少なく、0.2%耐力が275MPa以上とならなかった。 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.34の実施例は、酸素量が0.1mass%のフープ材を使用した例であり、熱処理時の保持温度を700〜800℃の範囲で変化させ、10分保持している場合である。いずれも各部位の平均粒径差が20μm以下であり、0.2%耐力は275MPa以上、且つ、耐凍結回数は3回以上を満足していた。 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.37の実施例は、酸素量が0.2mass%のフープ材を使用した例であり、同様に、熱処条件を700℃〜800℃、10分とした場合である。No.32〜No.34の実施例よりも酸素量を上昇させているため、0.2%耐力は高くなっているが、溶接管全体の平均粒径は30〜100μmの範囲で、且つ、各部位の平均粒径差が20μm以下となっており、耐凍結回数の基準も満足していた。 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.39の比較例は、酸素量が0.25mass%のフープ材を使用した例であり、それぞれ700〜800℃で、10分間熱処理した場合である。酸素量が0.25mass%と多いフープを用いたため、延性に乏しく、耐凍結回数が3回の基準を下回ったと思われる。 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.
以上の実施例では、いずれの部位でも、結晶粒径30〜100μmの範囲であり、且つ、同一溶接管内では粒径が均一に揃っていたため、不均一変形が抑制され、耐凍結回数が改善したものと考えられる。 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.
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