JP7396551B1 - High-strength steel plate for sour-resistant line pipe and its manufacturing method, and high-strength steel pipe using high-strength steel plate for sour-resistant line pipe - Google Patents
High-strength steel plate for sour-resistant line pipe and its manufacturing method, and high-strength steel pipe using high-strength steel plate for sour-resistant line pipe Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 187
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- 229910000734 martensite Inorganic materials 0.000 claims abstract description 15
- 230000007704 transition Effects 0.000 claims abstract description 12
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 8
- 239000012535 impurity Substances 0.000 claims abstract description 8
- 229910052804 chromium Inorganic materials 0.000 claims abstract description 7
- 238000009863 impact test Methods 0.000 claims abstract description 7
- 229910052748 manganese Inorganic materials 0.000 claims abstract description 7
- 229910052758 niobium Inorganic materials 0.000 claims abstract description 6
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 6
- 229910052698 phosphorus Inorganic materials 0.000 claims abstract description 6
- 229910052742 iron Inorganic materials 0.000 claims abstract description 5
- 229910052717 sulfur Inorganic materials 0.000 claims abstract description 5
- 238000001816 cooling Methods 0.000 claims description 72
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- 238000001953 recrystallisation Methods 0.000 claims description 31
- 239000000203 mixture Substances 0.000 claims description 21
- 238000000034 method Methods 0.000 claims description 16
- 229910052802 copper Inorganic materials 0.000 claims description 7
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- RWSOTUBLDIXVET-UHFFFAOYSA-N Dihydrogen sulfide Chemical compound S RWSOTUBLDIXVET-UHFFFAOYSA-N 0.000 description 11
- 229910000037 hydrogen sulfide Inorganic materials 0.000 description 11
- 239000000243 solution Substances 0.000 description 11
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- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 8
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- 239000000463 material Substances 0.000 description 4
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- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 3
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 3
- 238000004364 calculation method Methods 0.000 description 3
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- YZCKVEUIGOORGS-UHFFFAOYSA-N Hydrogen atom Chemical compound [H] YZCKVEUIGOORGS-UHFFFAOYSA-N 0.000 description 1
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Abstract
本発明は、耐ＨＩＣ性、耐ＳＳＣＣ性のみならず、低温靱性にも優れた耐サワーラインパイプ用高強度鋼板を提供する。本発明の耐サワーラインパイプ用高強度鋼板は、質量％で、Ｃ：０．０３０～０．０６０％、Ｓｉ：０．０１～０．５０％、Ｍｎ：０．８０～１．８０％、Ｐ：０．０１５％以下、Ｓ：０．００１５％以下、Ａｌ：０．０１０～０．０８０％、Ｃｒ：０．０５～０．５０％、Ｎｂ：０．００５～０．０８０％、Ｎ：０．００１０～０．００８０％、及びＣａ：０．０００５～０．００５０％を含有し、残部がＦｅ及び不可避的不純物からなる成分組成を有し、鋼板表面下０．２５ｍｍにおける鋼組織がグラニュラーベイナイト及び焼戻し島状マルテンサイトからなり、板厚中央における最大結晶粒径が８０μｍ以下、平均結晶粒径が２０μｍ以下であり、シャルピー衝撃試験における脆性－延性遷移温度が－１００℃以下であり、引張強さが５３５ＭＰａ以上である。The present invention provides a high-strength steel plate for sour line pipes that is excellent not only in HIC resistance and SSCC resistance but also in low-temperature toughness. The high-strength steel plate for sour-resistant line pipes of the present invention has, in mass %, C: 0.030 to 0.060%, Si: 0.01 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.080%, Cr: 0.05 to 0.50%, Nb: 0.005 to 0.080%, N : 0.0010 to 0.0080%, and Ca: 0.0005 to 0.0050%, with the balance consisting of Fe and unavoidable impurities, and the steel structure at 0.25 mm below the surface of the steel sheet is It consists of granular bainite and tempered island martensite, the maximum grain size at the center of the plate thickness is 80 μm or less, the average grain size is 20 μm or less, and the brittle-ductile transition temperature in the Charpy impact test is -100 ° C. or less, The tensile strength is 535 MPa or more.
Description
本発明は、原油や天然ガスの輸送に用いられるラインパイプに供して好適な、鋼板内の材質均一性に優れた耐サワーラインパイプ用高強度鋼板及びその製造方法に関するものである。また、本発明は、上記の耐サワーラインパイプ用高強度鋼板を用いた高強度鋼管に関するものである。 TECHNICAL FIELD The present invention relates to a high-strength steel plate for sour-resistant line pipes with excellent material uniformity within the steel plate, which is suitable for use in line pipes used for transporting crude oil and natural gas, and a method for manufacturing the same. The present invention also relates to a high-strength steel pipe using the above-described high-strength steel plate for sour-resistant line pipes.
一般に、ラインパイプは、厚板ミルや熱延ミルによって製造された鋼板を、ＵＯＥ成形、プレスベンド成形及びロール成形等によって、鋼管に成形することで製造される。 Generally, a line pipe is manufactured by forming a steel plate manufactured by a plate mill or a hot rolling mill into a steel pipe by UOE forming, press bend forming, roll forming, or the like.
ここに、硫化水素を含む原油や天然ガスの輸送に用いられるラインパイプは、強度、靭性、溶接性などの他に、耐水素誘起割れ性（耐ＨＩＣ（Hydrogen Induced Cracking）性）や耐硫化物応力腐食割れ性（耐ＳＳＣＣ（Sulfide Stress Corrosion Cracking）性）といった、いわゆる耐サワー性が必要とされる。中でもＨＩＣは、腐食反応による水素イオンが鋼表面に吸着し、原子状の水素として鋼内部に侵入し、鋼中のＭｎＳなどの非金属介在物や硬い第２相組織のまわりに拡散・集積して、分子状の水素となり、その内圧により割れを生ずるものである。このＨＩＣは、油井管に対して比較的強度レベルの低いラインパイプにおいて問題とされ、多くの対策技術が開示されてきた。一方、ＳＳＣＣに関しては、鋼管内面表層部の硬さをコントロールして、より厳しい腐食環境下での耐ＳＳＣＣ性を向上させることの重要性が指摘されている。これら耐サワー性に加えて、近年、原油や天然ガスの採掘環境がますます厳しさを増していることから、優れた低温靱性の要求が高まっている。 Line pipes used to transport crude oil and natural gas containing hydrogen sulfide are not only strong, tough, and weldable, but also have hydrogen-induced cracking resistance (HIC (Hydrogen Induced Cracking) resistance) and sulfide resistance. So-called sour resistance such as stress corrosion cracking resistance (SSCC (Sulfide Stress Corrosion Cracking) resistance) is required. Among these, HIC occurs when hydrogen ions from a corrosion reaction are adsorbed on the steel surface, penetrate into the steel as atomic hydrogen, and diffuse and accumulate around nonmetallic inclusions such as MnS and hard second phase structures in the steel. As a result, it becomes molecular hydrogen, which cracks due to its internal pressure. This HIC is considered a problem in line pipes whose strength level is relatively low compared to oil country tubular goods, and many countermeasure techniques have been disclosed. On the other hand, regarding SSCC, it has been pointed out that it is important to control the hardness of the inner surface layer of steel pipes to improve SSCC resistance under more severe corrosive environments. In addition to such sour resistance, as the mining environment for crude oil and natural gas has become increasingly harsh in recent years, there has been an increasing demand for excellent low-temperature toughness.
通常、ラインパイプ用高強度鋼板の製造に際しては、制御圧延と制御冷却を組み合わせた、いわゆるＴＭＣＰ（Thermo-Mechanical Control Process）技術が適用されている。このＴＭＣＰ技術を用いて鋼板の耐サワー性を確保するには、制御冷却時の冷却開始温度を高くし、かつ冷却速度を遅くすることが有効である。しかしながら、冷却開始温度を高くした場合、未再結晶温度域での圧延が不十分になるため、低温靱性の向上に有効である結晶粒の微細化に限度があり、優れた低温靱性を確保できなかった。 Normally, when manufacturing high-strength steel plates for line pipes, a so-called TMCP (Thermo-Mechanical Control Process) technology, which combines controlled rolling and controlled cooling, is applied. In order to ensure the sour resistance of a steel plate using this TMCP technology, it is effective to raise the cooling start temperature during controlled cooling and to slow the cooling rate. However, if the cooling start temperature is set high, rolling in the non-recrystallization temperature range becomes insufficient, which limits the grain refinement that is effective in improving low-temperature toughness, making it impossible to ensure excellent low-temperature toughness. There wasn't.
上記の問題を解決するために、例えば特許文献１には、仕上げ圧延完了温度を７００℃以上とし、平均結晶粒径を１５．０μｍ以下にする技術が提案されている。また、特許文献２には、粗圧延後仕上げ圧延開始までの保持時間を３００秒以内とし、結晶粒の成長を抑制する技術が提案されている。 In order to solve the above problems, for example, Patent Document 1 proposes a technique in which the finish rolling completion temperature is set at 700° C. or higher and the average crystal grain size is set at 15.0 μm or lower. Further, Patent Document 2 proposes a technique for suppressing the growth of crystal grains by setting the holding time after rough rolling to the start of finish rolling to within 300 seconds.
特許文献１及び特許文献２に記載の技術によって、低温靱性の向上が可能であるが、両文献に記載の技術は金属組織にフェライトが含まれているため、より厳しい腐食環境下である硫化水素分圧の高い環境における耐サワー性までは確保することが難しい。 The technologies described in Patent Document 1 and Patent Document 2 can improve low-temperature toughness, but since the technologies described in both documents contain ferrite in the metal structure, they cannot be used in hydrogen sulfide, which is a more severe corrosive environment. It is difficult to ensure sour resistance in environments with high partial pressure.
特に、寒冷地に使用されるパイプラインには優れた低温靱性が必要である。しかしながら、フェライトの生成を防ぐためには、Ａｒ３点以上の温度で冷却を開始する必要があり、圧延時の累積歪が小さくなるため、結晶粒径を微細にするのが困難であり、従来は優れた低温靱性が得られていない。In particular, pipelines used in cold regions require excellent low-temperature toughness. However, in order to prevent the formation of ferrite, it is necessary to start cooling at a temperature of Ar 3 or higher, which reduces the cumulative strain during rolling, making it difficult to make the grain size fine. Excellent low-temperature toughness has not been obtained.
そこで、本発明は、上記課題に鑑み、耐ＨＩＣ性、耐ＳＳＣＣ性のみならず、低温靱性にも優れた耐サワーラインパイプ用高強度鋼板を、その有利な製造方法と共に提供することを目的とする。また、本発明は、上記耐サワーラインパイプ用高強度鋼板を用いた高強度鋼管を提供することを目的とする。 Therefore, in view of the above problems, the present invention aims to provide a high-strength steel plate for sour line pipes that is excellent not only in HIC resistance and SSCC resistance but also in low-temperature toughness, together with an advantageous manufacturing method thereof. do. Another object of the present invention is to provide a high-strength steel pipe using the above-mentioned high-strength steel plate for sour-resistant line pipes.
本発明者らは、耐サワー性のみならず、さらに低温靱性を確保するべく、鋼板の成分組成、ミクロ組織、及び製造条件について、数多くの実験と検討をくり返した。その結果、高強度鋼板の低温靱性をさらに向上させるためには、板厚中央における最大結晶粒径を８０μｍ以下、平均結晶粒径を２０μｍ以下にする必要があることを知見した。さらに、このような鋼組織を実現するためには、再結晶温度域における熱間圧延条件を厳密にコントロールする必要があり、その条件を見出すことに成功した。本発明は、これら知見に基づいてなされたものである。 The present inventors repeatedly conducted numerous experiments and studies regarding the composition, microstructure, and manufacturing conditions of the steel sheet in order to ensure not only sour resistance but also low-temperature toughness. As a result, it was found that in order to further improve the low-temperature toughness of a high-strength steel plate, it is necessary to make the maximum grain size at the center of the plate thickness 80 μm or less and the average grain size 20 μm or less. Furthermore, in order to achieve such a steel structure, it is necessary to strictly control the hot rolling conditions in the recrystallization temperature range, and we succeeded in finding these conditions. The present invention has been made based on these findings.
すなわち、本発明の要旨構成は次のとおりである。
［１］質量％で、Ｃ：０．０３０～０．０６０％、Ｓｉ：０．０１～０．５０％、Ｍｎ：０．８０～１．８０％、Ｐ：０．０１５％以下、Ｓ：０．００１５％以下、Ａｌ：０．０１０～０．０８０％、Ｃｒ：０．０５～０．５０％、Ｎｂ：０．００５～０．０８０％、Ｎ：０．００１０～０．００８０％、及びＣａ：０．０００５～０．００５０％を含有し、残部がＦｅ及び不可避的不純物からなる成分組成を有し、
鋼板表面下０．２５ｍｍにおける鋼組織がグラニュラーベイナイト及び焼戻し島状マルテンサイトからなり、
板厚中央における最大結晶粒径が８０μｍ以下、平均結晶粒径が２０μｍ以下であり、
シャルピー衝撃試験における脆性－延性遷移温度が－１００℃以下であり、
引張強さが５３５ＭＰａ以上である
ことを特徴とする耐サワーラインパイプ用高強度鋼板。That is, the gist of the present invention is as follows.
[1] In mass%, C: 0.030 to 0.060%, Si: 0.01 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.080%, Cr: 0.05 to 0.50%, Nb: 0.005 to 0.080%, N: 0.0010 to 0.0080%, and Ca: 0.0005 to 0.0050%, with the remainder consisting of Fe and inevitable impurities,
The steel structure at 0.25 mm below the surface of the steel plate consists of granular bainite and tempered island martensite,
The maximum crystal grain size at the center of the plate thickness is 80 μm or less, the average crystal grain size is 20 μm or less,
The brittle-ductile transition temperature in the Charpy impact test is -100°C or less,
A high-strength steel plate for sour line pipes having a tensile strength of 535 MPa or more.
［２］前記成分組成が、さらに、質量％で、Ｃｕ：０．３０％以下、Ｎｉ：０．１０％以下、及びＭｏ：０．５０％以下からなる群から選択される１種以上を含有する、前記［１］に記載の耐サワーラインパイプ用高強度鋼板。 [2] The component composition further contains, in mass%, one or more selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less. The high-strength steel plate for sour-resistant line pipes according to [1] above.
［３］前記成分組成が、さらに、質量％で、Ｖ：０．００５～０．１％、Ｔｉ：０．００５～０．１％、Ｚｒ：０．０００５～０．０２％、Ｍｇ：０．０００５～０．０２％、及びＲＥＭ：０．０００５～０．０２％からなる群から選択される１種以上を含有する、前記［１］又は［２］に記載の耐サワーラインパイプ用高強度鋼板。 [3] The component composition further includes, in mass%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0 .0005 to 0.02%, and REM: 0.0005 to 0.02%, and REM: 0.0005 to 0.02%. Strength steel plate.
［４］質量％で、Ｃ：０．０３０～０．０６０％、Ｓｉ：０．０１～０．５０％、Ｍｎ：０．８０～１．８０％、Ｐ：０．０１５％以下、Ｓ：０．００１５％以下、Ａｌ：０．０１０～０．０８０％、Ｃｒ：０．０５～０．５０％、Ｎｂ：０．００５～０．０８０％、Ｎ：０．００１０～０．００８０％、及びＣａ：０．０００５～０．００５０％を含有し、残部がＦｅ及び不可避的不純物からなる成分組成を有する鋼片を１０００～１２５０℃の温度に加熱し、
その後、前記鋼片に、再結晶温度域における総圧下率：３５％以上５５％以下、再結晶温度域における最終圧延パスの圧下率：１０％以上を満足する熱間圧延を施して鋼板とし、
その後、前記鋼板に対して、
冷却開始時の鋼板表面温度：Ａｒ３点以上、
鋼板表面下０．２５ｍｍにおける鋼板温度で７５０℃から５５０℃までの平均冷却速度：１５～３５℃／ｓ、
板厚中央における鋼板温度で７５０℃から５５０℃までの平均冷却速度：１５℃／ｓ以上、
鋼板表面下０．２５ｍｍ及び板厚中央における鋼板温度で冷却停止温度：３５０～５５０℃
の条件で制御冷却を行う
ことを特徴とする耐サワーラインパイプ用高強度鋼板の製造方法。[4] In mass%, C: 0.030 to 0.060%, Si: 0.01 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010 to 0.080%, Cr: 0.05 to 0.50%, Nb: 0.005 to 0.080%, N: 0.0010 to 0.0080%, and Ca: 0.0005 to 0.0050%, with the balance consisting of Fe and inevitable impurities, heated to a temperature of 1000 to 1250 ° C.,
Thereafter, the steel slab is subjected to hot rolling that satisfies the total rolling reduction in the recrystallization temperature range: 35% or more and 55% or less, and the rolling reduction in the final rolling pass in the recrystallization temperature range: 10% or more, to obtain a steel plate,
Then, for the steel plate,
Steel plate surface temperature at the start of cooling: Ar 3 points or more,
Average cooling rate from 750°C to 550°C at steel plate temperature 0.25mm below the steel plate surface: 15 to 35°C/s,
Average cooling rate from 750°C to 550°C at steel plate temperature at center of plate thickness: 15°C/s or more,
Cooling stop temperature at 0.25 mm below the surface of the steel plate and at the center of the plate thickness: 350 to 550°C
A method for manufacturing a high-strength steel plate for sour-resistant line pipes, characterized by performing controlled cooling under the following conditions.
［５］前記成分組成が、さらに、質量％で、Ｃｕ：０．３０％以下、Ｎｉ：０．１０％以下、及びＭｏ：０．５０％以下からなる群から選択される１種以上を含有する、前記［４］に記載の耐サワーラインパイプ用高強度鋼板の製造方法。 [5] The component composition further contains, in mass%, one or more selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less. The method for producing a high-strength steel plate for sour-resistant line pipes according to [4] above.
［６］前記成分組成が、さらに、質量％で、Ｖ：０．００５～０．１％、Ｔｉ：０．００５～０．１％、Ｚｒ：０．０００５～０．０２％、Ｍｇ：０．０００５～０．０２％、及びＲＥＭ：０．０００５～０．０２％からなる群から選択される１種以上を含有する、前記［４］又は［５］に記載の耐サワーラインパイプ用高強度鋼板の製造方法。 [6] The component composition further includes, in mass%, V: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0 .0005 to 0.02%, and REM: 0.0005 to 0.02%, and REM: 0.0005 to 0.02%. Method of manufacturing strength steel plate.
［７］前記［１］又は［２］に記載の耐サワーラインパイプ用高強度鋼板を用いた高強度鋼管。 [7] A high-strength steel pipe using the high-strength steel plate for sour-resistant line pipes according to [1] or [2] above.
［８］前記［３］に記載の耐サワーラインパイプ用高強度鋼板を用いた高強度鋼管。 [8] A high-strength steel pipe using the high-strength steel plate for sour-resistant line pipes according to [3] above.
本発明の耐サワーラインパイプ用高強度鋼板及び該耐サワーラインパイプ用高強度鋼板を用いた高強度鋼管は、耐ＨＩＣ性、耐ＳＳＣＣ性のみならず、低温靱性にも優れる。また、本発明の耐サワーラインパイプ用高強度鋼板の製造方法によれば、耐ＨＩＣ性、耐ＳＳＣＣ性のみならず、低温靱性にも優れた耐サワーラインパイプ用高強度鋼板を製造することができる。 The high-strength steel plate for sour-resistant line pipes of the present invention and the high-strength steel pipes using the high-strength steel plates for sour-resistant line pipes of the present invention are excellent not only in HIC resistance and SSCC resistance but also in low-temperature toughness. Furthermore, according to the method for producing a high-strength steel plate for sour-resistant line pipes of the present invention, it is possible to produce high-strength steel plates for sour-resistant line pipes that have excellent not only HIC resistance and SSCC resistance but also low-temperature toughness. can.
以下、本発明の耐サワーラインパイプ用高強度鋼板について、具体的に説明する。 Hereinafter, the high-strength steel plate for sour-resistant line pipes of the present invention will be specifically explained.
［成分組成］
まず、本発明による高強度鋼板の成分組成とその限定理由について説明する。以下の説明において、「％」で示す単位は、特に断らない限り全て「質量％」である。[Component composition]
First, the composition of the high-strength steel sheet according to the present invention and the reason for its limitation will be explained. In the following description, all units indicated as "%" are "% by mass" unless otherwise specified.
Ｃ：０．０３０～０．０６０％
Ｃは、強度の向上に有効に寄与するが、Ｃ含有量が０．０３０％未満では十分な強度が確保できないので、Ｃ含有量は０．０３０％以上とし、好ましくは０．０３５％以上とする。他方で、Ｃ含有量が０．０６０％を超えると、低温靭性が劣化する。また、加速冷却時に表層部や中心偏析部の硬さが上昇するため、耐ＳＳＣＣ性及び耐ＨＩＣ性が劣化する。このため、Ｃ含有量は０．０６０％以下とし、好ましくは０．０５０％以下とする。C: 0.030-0.060%
C effectively contributes to improving strength, but if the C content is less than 0.030%, sufficient strength cannot be ensured. Therefore, the C content should be 0.030% or more, preferably 0.035% or more. do. On the other hand, when the C content exceeds 0.060%, low temperature toughness deteriorates. Further, during accelerated cooling, the hardness of the surface layer portion and the central segregation portion increases, resulting in deterioration of SSCC resistance and HIC resistance. Therefore, the C content is set to 0.060% or less, preferably 0.050% or less.
Ｓｉ：０．０１～０．５０％
Ｓｉは、脱酸のために添加するが、Ｓｉ含有量が０．０１％未満では脱酸効果が十分でないので、Ｓｉ含有量は０．０１％以上とし、好ましくは０．０５％以上とする。他方で、Ｓｉ含有量が０．５０％を超えると、鋼の非熱的応力が上昇し、低温靭性が劣化するため、Ｓｉ含有量は０．５０％以下とし、好ましくは０．４５％以下とする。Si: 0.01~0.50%
Si is added for deoxidation, but if the Si content is less than 0.01%, the deoxidation effect will not be sufficient, so the Si content should be 0.01% or more, preferably 0.05% or more. . On the other hand, if the Si content exceeds 0.50%, the non-thermal stress of the steel will increase and the low temperature toughness will deteriorate, so the Si content should be 0.50% or less, preferably 0.45% or less. shall be.
Ｍｎ：０．８０～１．８０％
Ｍｎは、強度の向上に有効に寄与するが、Ｍｎ含有量が０．８０％未満ではその効果が十分には発現しない。このため、Ｍｎ含有量は０．８０％以上とし、好ましくは１．００％以上とする。他方で、Ｍｎ含有量が１．８０％を超えると、加速冷却時に表層部や中心偏析部の硬さが上昇するため、耐ＳＳＣＣ性及び耐ＨＩＣ性が劣化する。また、溶接性も劣化する。このため、Ｍｎ含有量は１．８０％以下とし、好ましくは１．７０％以下とする。Mn: 0.80-1.80%
Although Mn effectively contributes to improving strength, the effect is not fully expressed when the Mn content is less than 0.80%. Therefore, the Mn content is set to 0.80% or more, preferably 1.00% or more. On the other hand, if the Mn content exceeds 1.80%, the hardness of the surface layer portion and center segregation portion increases during accelerated cooling, resulting in deterioration of SSCC resistance and HIC resistance. Moreover, weldability also deteriorates. Therefore, the Mn content is set to 1.80% or less, preferably 1.70% or less.
Ｐ：０．０１５％以下
Ｐは、不可避的不純物元素であり、低温靭性を劣化させるとともに、表層部や中心偏析部の硬さを上昇させることで、耐ＳＳＣＣ性及び耐ＨＩＣ性を劣化させる。Ｐ含有量が０．０１５％を超えると、その傾向が顕著となるため、Ｐ含有量は０．０１５％以下とし、好ましくは０．００８％以下とする。なお、Ｐ含有量は低いほどよいが、精錬コストの観点からは、Ｐ含有量は０．００１％以上とすることが好ましい。P: 0.015% or less P is an unavoidable impurity element that deteriorates low-temperature toughness and increases the hardness of the surface layer and center segregation area, thereby degrading SSCC resistance and HIC resistance. If the P content exceeds 0.015%, this tendency becomes noticeable, so the P content is set to 0.015% or less, preferably 0.008% or less. Note that the lower the P content, the better; however, from the viewpoint of refining cost, the P content is preferably 0.001% or more.
Ｓ：０．００１５％以下
Ｓは、不可避的不純物元素であり、鋼中においてはＭｎＳ介在物となり耐ＨＩＣ性を劣化させるため、少ないことが好ましい。この観点から、Ｓ含有量は０．００１５％以下とし、好ましくは０．００１０％以下とする。なお、Ｓ含有量は低いほどよいが、精錬コストの観点から、Ｓ含有量は０．０００２％以上とすることが好ましい。S: 0.0015% or less S is an unavoidable impurity element and becomes MnS inclusions in steel and deteriorates HIC resistance, so it is preferably small. From this point of view, the S content is set to 0.0015% or less, preferably 0.0010% or less. Note that the lower the S content, the better; however, from the viewpoint of refining cost, the S content is preferably 0.0002% or more.
Ａｌ：０.０１０～０.０８０％
Ａｌは、脱酸剤として添加するが、Ａｌ含有量が０.０１０％未満では、その効果が十分には発現しない。このため、Ａｌ含有量は０．０１０％以上とし、好ましくは０．０１５％以上とする。他方で、Ａｌ含有量が０.０８０％を超えると、鋼の非熱的応力が上昇し、低温靱性が劣化する。このため、Ａｌ含有量は０.０８０％以下とし、好ましくは、０．０７０％以下とする。Al: 0.010-0.080%
Al is added as a deoxidizing agent, but if the Al content is less than 0.010%, its effect will not be fully expressed. Therefore, the Al content is set to 0.010% or more, preferably 0.015% or more. On the other hand, if the Al content exceeds 0.080%, the non-thermal stress of the steel will increase and the low temperature toughness will deteriorate. Therefore, the Al content should be 0.080% or less, preferably 0.070% or less.
Ｃｒ：０.０５～０．５０％
Ｃｒは、Ｍｎと同様、低Ｃ含有量でも十分な強度を得るために有効な元素であり、この効果を得るには、Ｃｒ含有量を０．０５％以上とする必要がある。しかし、Ｃｒ含有量が多すぎると、焼入れ性が過剰になるため、加速冷却時に表層部や中心偏析部の硬さが上昇し、耐ＳＳＣＣ性及び耐ＨＩＣ性が劣化する。また、溶接性も劣化する。このため、Ｃｒ含有量は０．５０％以下とし、好ましくは０．４５％以下とする。Cr: 0.05~0.50%
Cr, like Mn, is an effective element for obtaining sufficient strength even with a low C content, and to obtain this effect, the Cr content must be 0.05% or more. However, if the Cr content is too high, the hardenability becomes excessive, so that the hardness of the surface layer part and the central segregation part increases during accelerated cooling, and the SSCC resistance and HIC resistance deteriorate. Moreover, weldability also deteriorates. Therefore, the Cr content is set to 0.50% or less, preferably 0.45% or less.
Ｎｂ：０．００５～０．０８０％
Ｎｂは、固溶Ｎｂとして存在すると制御圧延時の未再結晶温度域を拡大し、低温靭性の向上に寄与するが、Ｎｂ含有量が０．００５％未満では、その効果が十分には発現しない。このため、Ｎｂ含有量は０．００５％以上とし、好ましくは０．０１０％以上とする。他方で、Ｎｂ含有量が０．０８０％を超えると、凝固時に粗大な炭化物を晶出するため、耐ＨＩＣ性が劣化する。このため、Ｎｂ含有量は０．０８０％以下とし、好ましくは０．０６０％以下とする。Nb: 0.005-0.080%
When Nb exists as solid solution Nb, it expands the non-recrystallization temperature range during controlled rolling and contributes to improving low-temperature toughness, but this effect is not fully expressed when the Nb content is less than 0.005%. . Therefore, the Nb content is set to 0.005% or more, preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.080%, coarse carbides are crystallized during solidification, resulting in poor HIC resistance. Therefore, the Nb content is set to 0.080% or less, preferably 0.060% or less.
Ｎ：０．００１０～０．００８０％
Ｎは、強度の向上に有効に寄与するが、Ｎ含有量が０．００１０％未満では、十分な強度が確保できない。このため、Ｎ含有量は０．００１０％以上とし、好ましくは０．００１５％以上とする。他方で、Ｎ含有量が０．００８０％を超えると、加速冷却時に表層部や中心偏析部の硬さが上昇するため、耐ＳＳＣＣ性及び耐ＨＩＣ性が劣化する。また、低温靭性も劣化する。このため、Ｎ含有量は０．００８０％以下とし、好ましくは０．００７０％以下とする。N: 0.0010-0.0080%
Although N effectively contributes to improving strength, if the N content is less than 0.0010%, sufficient strength cannot be ensured. Therefore, the N content is set to 0.0010% or more, preferably 0.0015% or more. On the other hand, if the N content exceeds 0.0080%, the hardness of the surface layer portion and center segregation portion increases during accelerated cooling, resulting in deterioration of SSCC resistance and HIC resistance. In addition, low temperature toughness also deteriorates. Therefore, the N content is set to 0.0080% or less, preferably 0.0070% or less.
Ｃａ：０．０００５～０．００５０％
Ｃａは、硫化物系介在物の形態制御による耐ＨＩＣ性向上に有効な元素であるが、Ｃａ含有量が０．０００５％未満では、その添加効果が十分でない。このため、Ｃａ含有量は０．０００５％以上とし、好ましくは０．０００８％以上とする。他方で、Ｃａ含有量が０．００５０％を超えた場合、上述の効果が飽和するだけでなく、鋼の清浄度が低下することにより耐ＨＩＣ性が劣化する。このため、Ｃａ含有量は０．００５０％以下とし、好ましくは０．００４５％以下とする。Ca: 0.0005-0.0050%
Ca is an element effective in improving HIC resistance by controlling the form of sulfide-based inclusions, but if the Ca content is less than 0.0005%, the effect of its addition is not sufficient. Therefore, the Ca content is set to 0.0005% or more, preferably 0.0008% or more. On the other hand, when the Ca content exceeds 0.0050%, not only the above-mentioned effects are saturated, but also the HIC resistance deteriorates due to a decrease in the cleanliness of the steel. Therefore, the Ca content should be 0.0050% or less, preferably 0.0045% or less.
以上、本発明における成分組成の基本成分について説明したが、本発明において、成分組成は、鋼板の強度や靱性の一層の改善のために、Ｃｕ、Ｎｉ、及びＭｏからなる群から選択される１種以上を、以下の範囲で任意に含有させることができる。 The basic components of the composition in the present invention have been explained above, but in the present invention, the composition is selected from the group consisting of Cu, Ni, and Mo in order to further improve the strength and toughness of the steel sheet. More than one species can be optionally contained within the following range.
Ｃｕ：０．３０％以下
Ｃｕは、低温靭性の改善と強度の上昇に有効な元素であり、この効果を得るには、Ｃｕ含有量は０．０５％以上とすることが好ましい。しかし、Ｃｕ含有量が０．３０％を超えた場合、１ｂａｒ未満の硫化水素分圧の低い環境において、フィッシャーと呼ばれる微細割れが生成しやすくなるため、耐ＳＳＣＣ性が劣化する。このため、Ｃｕを含有する場合、Ｃｕ含有量は０．３０％以下とし、好ましくは０．２５％以下とする。Cu: 0.30% or less Cu is an element effective in improving low-temperature toughness and increasing strength, and to obtain this effect, the Cu content is preferably 0.05% or more. However, if the Cu content exceeds 0.30%, fine cracks called Fischer tend to occur in an environment with a low hydrogen sulfide partial pressure of less than 1 bar, resulting in a deterioration in SSCC resistance. Therefore, when Cu is contained, the Cu content is 0.30% or less, preferably 0.25% or less.
Ｎｉ：０．１０％以下
Ｎｉは、低温靭性の改善と強度の上昇に有効な元素であり、この効果を得るには、Ｎｉ含有量は０．０１％以上とすることが好ましい。しかし、Ｎｉ含有量が０．１０％を超えた場合、１ｂａｒ未満の硫化水素分圧の低い環境において、フィッシャーと呼ばれる微細割れが生成しやすくなるため、耐ＳＳＣＣ性が劣化する。このため、Ｎｉを含有する場合、Ｎｉ含有量は０．１０％以下とし、好ましくは０．０２％以下とする。Ni: 0.10% or less Ni is an element effective in improving low-temperature toughness and increasing strength, and to obtain this effect, the Ni content is preferably 0.01% or more. However, if the Ni content exceeds 0.10%, fine cracks called Fischer tend to occur in an environment with a low hydrogen sulfide partial pressure of less than 1 bar, resulting in a deterioration in SSCC resistance. Therefore, when Ni is contained, the Ni content should be 0.10% or less, preferably 0.02% or less.
Ｍｏ：０．５０％以下
Ｍｏは、低温靭性の改善と強度の上昇に有効な元素であり、この効果を得るには、Ｍｏ含有量は０．０１％以上とすることが好ましく、０．１０％以上とすることがより好ましい。他方で、Ｍｏ含有量が多すぎると、１ｂａｒ未満の硫化水素分圧の低い環境において、フィッシャーと呼ばれる微細割れが生成しやすくなるため、耐ＳＳＣＣ性が劣化する。また、溶接性も劣化する。このため、Ｍｏを含有する場合、Ｍｏ含有量は０．５０％以下とし、好ましくは０．４０％以下とする。Mo: 0.50% or less Mo is an element effective in improving low-temperature toughness and increasing strength. To obtain this effect, the Mo content is preferably 0.01% or more, and 0.10% or less. % or more is more preferable. On the other hand, if the Mo content is too high, fine cracks called Fischer tend to occur in an environment with a low partial pressure of hydrogen sulfide of less than 1 bar, resulting in poor SSCC resistance. Moreover, weldability also deteriorates. Therefore, when Mo is contained, the Mo content should be 0.50% or less, preferably 0.40% or less.
本発明における成分組成は、さらに、Ｖ、Ｔｉ、Ｚｒ、Ｍｇ、及びＲＥＭからなる群から選択される１種以上を、以下の範囲で任意に含有させることもできる。 The component composition in the present invention may further optionally contain one or more selected from the group consisting of V, Ti, Zr, Mg, and REM within the following range.
Ｖ：０．００５～０．１％、Ｔｉ：０．００５～０．１％、Ｚｒ：０．０００５～０．０２％、Ｍｇ：０．０００５～０．０２％、及びＲＥＭ：０．０００５～０．０２％からなる群から選択される１種以上 V: 0.005-0.1%, Ti: 0.005-0.1%, Zr: 0.0005-0.02%, Mg: 0.0005-0.02%, and REM: 0.0005 One or more species selected from the group consisting of ~0.02%
Ｖ及びＴｉはいずれも、鋼板の強度及び低温靭性を高めるために任意に含有することができる元素である。各元素とも、含有量が０．００５％未満では、その効果が十分には発現しない。このため、これらの元素を含有する場合、含有量はそれぞれ０．００５％以上とすることが好ましい。他方で、これらの元素の含有量が０．１％を超えると、溶接部の靭性が劣化する。このため、これらの元素を含有する場合、含有量はそれぞれ０．１％以下とするのが好ましい。 Both V and Ti are elements that can be optionally included in order to improve the strength and low-temperature toughness of the steel sheet. If the content of each element is less than 0.005%, its effect will not be fully expressed. Therefore, when these elements are contained, the content is preferably 0.005% or more. On the other hand, if the content of these elements exceeds 0.1%, the toughness of the weld will deteriorate. Therefore, when these elements are contained, the content is preferably 0.1% or less.
Ｚｒ、Ｍｇ及びＲＥＭは、結晶粒微細化を通じて低温靭性を高めたり、介在物性状のコントロールを通して耐割れ性を高めたりするために任意に含有することができる元素である。各元素とも、含有量が０．０００５％未満では、その効果が十分には発現しない。このため、これらの元素を含有する場合、含有量はそれぞれ０．０００５％以上とすることが好ましい。他方で、これらの元素の含有量が０．０２％を超えると、その効果が飽和するので、これらの元素を含有する場合、含有量はそれぞれ０．０２％以下とするのが好ましい。 Zr, Mg, and REM are elements that can be optionally included in order to improve low-temperature toughness through grain refinement or to improve cracking resistance through control of inclusion properties. If the content of each element is less than 0.0005%, its effect will not be fully expressed. Therefore, when these elements are contained, the content is preferably 0.0005% or more. On the other hand, if the content of these elements exceeds 0.02%, the effect will be saturated, so when these elements are contained, it is preferable that the content is 0.02% or less.
本発明は、耐サワーラインパイプ用高強度鋼板を用いた高強度鋼管の低温靱性を改善するための技術を開示するものであるが、耐サワー性能として、いうまでもなく、耐ＨＩＣ性を満足することが必要であり、例えば、下記（１）式によって求められるＣＰ値を、１．００以下とすることが好ましい。なお、含有しない元素は０を代入すれば良い。
ＣＰ＝４．４６［％Ｃ］＋２．３７［％Ｍｎ］／６＋（１．７４［％Ｃｕ］＋１．７［％Ｎｉ］）／１５＋（１．１８［％Ｃｒ］＋１．９５［％Ｍｏ］＋１．７４［％Ｖ］）／５＋２２．３６［％Ｐ］ ・・・（１）
ただし、[％Ｘ]はＸ元素の鋼中含有量（質量％）を示す。The present invention discloses a technology for improving the low-temperature toughness of high-strength steel pipes using high-strength steel plates for sour-resistant line pipes. For example, it is preferable that the CP value determined by the following equation (1) be 1.00 or less. Note that 0 may be substituted for elements that are not contained.
CP=4.46[%C]+2.37[%Mn]/6+(1.74[%Cu]+1.7[%Ni])/15+(1.18[%Cr]+1.95[%Mo ]+1.74[%V])/5+22.36[%P]...(1)
However, [%X] indicates the content of element X in the steel (% by mass).
ここに、上記ＣＰ値は、各合金元素の含有量から中心偏析部の材質を推定するために考案された式であり、上掲（１）式のＣＰ値が高いほど中心偏析部の成分濃度が高くなり、中心偏析部の硬さが上昇する。従って、上記の（１）式において求められるＣＰ値を１．００以下とすることで、耐ＨＩＣ性を向上させることが可能となる。また、ＣＰ値が低いほど中心偏析部の硬さが低くなるため、さらに高い耐ＨＩＣ性が求められる場合は、その上限を０．９５とすれば良い。ＣＰの下限は特に限定されないが、ＣＰ値は０．７０以上であり得る。 Here, the above CP value is a formula devised to estimate the material of the central segregation part from the content of each alloying element, and the higher the CP value of the above formula (1), the higher the component concentration of the central segregation part. becomes higher, and the hardness of the center segregation area increases. Therefore, by setting the CP value determined by the above equation (1) to 1.00 or less, it is possible to improve the HIC resistance. Further, the lower the CP value, the lower the hardness of the center segregation part, so if even higher HIC resistance is required, the upper limit may be set to 0.95. Although the lower limit of CP is not particularly limited, the CP value may be 0.70 or more.
なお、上記した元素以外の残部は、Ｆｅ及び不可避的不純物からなる。ただし、本発明の作用効果を害しない限り、他の微量元素の含有を妨げない。例えば、Ｏは鋼中に不可避的に含まれる元素であるが、その含有量が０．００５０％以下、好ましくは０．００４０％以下であれば、本発明においては許容される。 Note that the remainder other than the above-mentioned elements consists of Fe and inevitable impurities. However, other trace elements may be included as long as they do not impair the effects of the present invention. For example, O is an element that is unavoidably contained in steel, but it is permissible in the present invention as long as its content is 0.0050% or less, preferably 0.0040% or less.
［鋼板表面下０．２５ｍｍの組織］
次に、本発明の耐サワーラインパイプ用高強度鋼板の鋼組織について説明する。耐ＳＳＣＣ性を向上させるとともに引張強さが５３５ＭＰａ以上の高強度化を図るために、鋼板表面下０．２５ｍｍの鋼組織を、グラニュラーベイナイト及び焼戻し島状マルテンサイトからなる組織とする。鋼組織中に、島状マルテンサイトが混在すると、低温靭性の劣化、耐ＳＳＣＣ性の劣化が生じるが、冷却停止温度が十分に高い場合には、冷却停止後に焼き戻された焼戻し島状マルテンサイト（ＴＭＡ）となるため、低温靭性及び耐ＳＳＣＣ性の劣化を防ぐことができる。また、グラニュラーベイナイト及び焼戻し島状マルテンサイトは、他のベイナイトや他のマルテンサイトと比較し、硬度が低いため、耐ＳＳＣＣ性を向上させることができる。[Structure 0.25mm below the steel plate surface]
Next, the steel structure of the high-strength steel plate for sour-resistant line pipes of the present invention will be explained. In order to improve the SSCC resistance and achieve a high tensile strength of 535 MPa or more, the steel structure 0.25 mm below the surface of the steel sheet is made of granular bainite and tempered island martensite. If island-like martensite coexists in the steel structure, it will cause deterioration of low-temperature toughness and SSCC resistance, but if the cooling stop temperature is high enough, the tempered island-like martensite that has been tempered after cooling stop will occur. (TMA), it is possible to prevent deterioration of low temperature toughness and SSCC resistance. Moreover, since granular bainite and tempered island-like martensite have lower hardness than other bainites and other martensite, SSCC resistance can be improved.
ここで、本発明における鋼板表面下０．２５ｍｍの鋼組織は、グラニュラーベイナイトが主体であることが好ましい。具体的には、グラニュラーベイナイトが面積率で９０％以上、焼戻し島状マルテンサイトが面積率で１０％以下であることが好ましい。他方で、焼戻し島状マルテンサイトの存在も必須であるため、グラニュラーベイナイトが面積率で９９％以下、焼戻し島状マルテンサイトが面積率で１％以上であることが好ましい。 Here, it is preferable that the steel structure 0.25 mm below the surface of the steel sheet in the present invention is mainly composed of granular bainite. Specifically, it is preferable that the area ratio of granular bainite is 90% or more, and the area ratio of tempered island martensite is 10% or less. On the other hand, since the presence of tempered island martensite is also essential, it is preferable that the area ratio of granular bainite is 99% or less, and the area ratio of tempered island martensite is 1% or more.
なお、本発明の高強度鋼板においては、鋼板表面下０．２５ｍｍの鋼組織が上記の条件を満足すれば、鋼板表面から深さ０．２５ｍｍまでの範囲の極表層部も同等の鋼組織を有し、その結果、上記耐ＳＳＣＣ性向上の効果が得られる。 In addition, in the high-strength steel plate of the present invention, if the steel structure 0.25 mm below the steel plate surface satisfies the above conditions, the extreme surface layer within the range from the steel plate surface to a depth of 0.25 mm also has the same steel structure. As a result, the above-mentioned effect of improving SSCC resistance can be obtained.
また、耐ＳＳＣＣ性の向上及び高強度化の効果をより十分に得るためには、表層部以外の部位も含め鋼板の全体の鋼組織が、上記の条件を満足することが好ましい。具体的には、「表層部以外の部位」を代表して、板厚中央での組織が上記の条件を満足していればよい。 Further, in order to more fully obtain the effects of improving SSCC resistance and increasing strength, it is preferable that the entire steel structure of the steel plate, including areas other than the surface layer portion, satisfies the above conditions. Specifically, it is sufficient that the structure at the center of the plate thickness satisfies the above conditions, representing "parts other than the surface layer".
［板厚中央における最大結晶粒径及び平均結晶粒径］
本発明においては、粗大な結晶粒の形成を抑制することが肝要である。すなわち、最大結晶粒径又は平均結晶粒径が大きいと、低温靭性は劣化する。特に、板厚中央における最大結晶粒径が８０μｍ超であると、粗大な結晶粒が破壊の起点になりやすいので、低温靭性は著しく劣化する。平均結晶粒径が２０μｍ超でも、低温靭性は劣化する。よって、板厚中央における最大結晶粒径を８０μｍ以下、平均結晶粒径を２０μｍ以下とする必要がある。板厚中央における最大結晶粒径及び平均結晶粒径は、小さいほど好ましいため、その下限は特に限定されないが、本発明において、板厚中央における最大結晶粒径は５０μｍ以上であり得、平均結晶粒径は１０μｍ以上であり得る。なお、「結晶粒径」は、板厚中央における１ｍｍ×１ｍｍの範囲における結晶粒の粒径を測定し、そのうちの最大値を「最大結晶粒径」として採用し、平均値を「平均結晶粒径」として採用するものとし、結晶粒径の定義は円相当径とした。ここで、板厚中央とは、全厚の１／２位置を指す。[Maximum grain size and average grain size at the center of plate thickness]
In the present invention, it is important to suppress the formation of coarse crystal grains. That is, when the maximum grain size or the average grain size is large, the low temperature toughness deteriorates. In particular, if the maximum crystal grain size at the center of the plate thickness is more than 80 μm, the coarse crystal grains tend to become the starting point of fracture, and the low-temperature toughness is significantly deteriorated. Even if the average grain size exceeds 20 μm, low-temperature toughness deteriorates. Therefore, the maximum crystal grain size at the center of the plate thickness must be 80 μm or less, and the average crystal grain size must be 20 μm or less. Since the smaller the maximum crystal grain size and the average crystal grain size at the center of the plate thickness, the lower limit is not particularly limited, but in the present invention, the maximum crystal grain size at the center of the plate thickness may be 50 μm or more, and the average crystal grain The diameter can be 10 μm or more. In addition, "crystal grain size" measures the grain size of crystal grains in a range of 1 mm x 1 mm at the center of the plate thickness, adopts the maximum value as "maximum grain size", and calculates the average value as "average grain size". The grain size was defined as the equivalent circle diameter. Here, the center of the plate thickness refers to the 1/2 position of the total thickness.
［脆性－延性遷移温度］
本発明の高強度鋼板は、シャルピー衝撃試験における脆性－延性遷移温度が－１００℃以下であるものとする。これにより、優れた低温靭性を確保することができる。脆性－延性遷移温度の下限は特に限定されないが、本発明において、脆性－延性遷移温度は－１２０℃以上であり得る。[Brittle-ductile transition temperature]
The high-strength steel plate of the present invention has a brittle-ductile transition temperature of -100°C or lower in a Charpy impact test. Thereby, excellent low-temperature toughness can be ensured. The lower limit of the brittle-ductile transition temperature is not particularly limited, but in the present invention, the brittle-ductile transition temperature may be -120°C or higher.
［引張強さ］
本発明の高強度鋼板は、ＡＰＩ ５ＬのＸ６５グレード以上の強度を有する鋼管用の鋼板であることから、５３５ＭＰａ以上の引張強さを有するものとする。引張強さの上限は特に限定されないが、本発明の高強度鋼板の引張強さは６００ＭＰａ以下であり得る。[Tensile strength]
Since the high-strength steel plate of the present invention is a steel plate for steel pipes having a strength of API 5L X65 grade or higher, it has a tensile strength of 535 MPa or higher. Although the upper limit of the tensile strength is not particularly limited, the tensile strength of the high-strength steel plate of the present invention may be 600 MPa or less.
［鋼板の厚さ］
本発明の高強度鋼板は、特に板厚は限定されないが、１２～３９ｍｍの厚さを有するものが上述の用途に適している。[Thickness of steel plate]
The high-strength steel plate of the present invention is not particularly limited in thickness, but those having a thickness of 12 to 39 mm are suitable for the above-mentioned uses.
［耐サワーラインパイプ用高強度鋼板の製造方法］
以下、上記耐サワーラインパイプ用高強度鋼板を製造するための製造方法及び製造条件について、具体的に説明する。本発明の製造方法は、上記成分組成を有する鋼片（例えば、スラブ）を加熱したのち、熱間圧延して鋼板とし、その後当該鋼板に対して所定条件下での制御冷却を行う。[Method for manufacturing high-strength steel plate for sour-resistant line pipes]
Hereinafter, the manufacturing method and manufacturing conditions for manufacturing the above-mentioned high-strength steel plate for sour-resistant line pipes will be specifically explained. In the manufacturing method of the present invention, a steel piece (for example, a slab) having the above-mentioned composition is heated, then hot rolled into a steel plate, and then the steel plate is subjected to controlled cooling under predetermined conditions.
〔鋼片加熱温度〕
鋼片加熱温度：１０００～１２５０℃
鋼片加熱温度が１０００℃未満では、炭化物の固溶が不十分となり、固溶強化量が少なくなるため、必要な強度が得られない。このため、鋼片加熱温度は１０００℃以上とし、好ましくは１０３０℃以上とする。他方で、鋼片加熱温度が１２５０℃を超えると、結晶粒が極端に粗大化し、靭性が劣化する。このため、鋼片加熱温度は１２５０℃以下とし、好ましくは１２００℃以下とする。なお、この温度は加熱炉の炉内温度であり、鋼片は中心部までこの温度に加熱されるものとする。[Silver heating temperature]
Steel billet heating temperature: 1000-1250℃
If the heating temperature of the steel billet is less than 1000° C., the solid solution of carbides becomes insufficient and the amount of solid solution strengthening decreases, making it impossible to obtain the necessary strength. For this reason, the steel billet heating temperature is set to 1000°C or higher, preferably 1030°C or higher. On the other hand, if the steel billet heating temperature exceeds 1250°C, the crystal grains become extremely coarse and the toughness deteriorates. For this reason, the steel billet heating temperature is set to 1250°C or lower, preferably 1200°C or lower. Note that this temperature is the temperature inside the heating furnace, and it is assumed that the steel slab is heated to this temperature up to the center.
〔熱間圧延条件〕
再結晶温度域における総圧下率：３５％以上５５％以下
最大結晶粒を微細にするためには、再結晶温度域における熱間圧延で、結晶粒の再結晶を促進し、粗大粒の形成を抑制する必要がある。再結晶温度域における総圧下率が３５％未満の場合、再結晶が不十分であるため、粗大粒が残存し、低温靭性が劣化する。よって、再結晶温度域における総圧下率は３５％以上とし、好ましくは３８％以上とする。他方で、再結晶温度域における総圧下率が５５％を超えると、最大結晶粒の粗大化は抑制できるが、未再結晶温度域での圧下が不足するため、平均結晶粒径を２０μｍ以下にできず、低温靭性が劣化する。よって、再結晶温度域における総圧下率は５５％以下とし、好ましくは５２％以下とする。ここで、「再結晶温度域」とは、以下の式から求められるＴｎｒ以上の温度域を意味する。なお、鋼板の表面温度は放射温度計等で測定することができ、各板厚位置の鋼板温度に変換することができる。
Ｔｎｒ（℃）＝１７４×ｌｏｇ｛[％Ｎｂ]×（[％Ｃ]＋１２／１４[％Ｎ]）｝＋１４４４
ただし、[％Ｘ]はＸ元素の鋼中含有量（質量％）を示す。[Hot rolling conditions]
Total rolling reduction ratio in the recrystallization temperature range: 35% or more and 55% or less In order to make the maximum crystal grains fine, hot rolling in the recrystallization temperature range promotes recrystallization of the crystal grains and prevents the formation of coarse grains. need to be suppressed. When the total rolling reduction in the recrystallization temperature range is less than 35%, recrystallization is insufficient, so coarse grains remain and low-temperature toughness deteriorates. Therefore, the total reduction rate in the recrystallization temperature range is set to 35% or more, preferably 38% or more. On the other hand, if the total reduction ratio in the recrystallization temperature range exceeds 55%, coarsening of the maximum crystal grains can be suppressed, but the reduction in the non-recrystallization temperature range is insufficient, so the average crystal grain size cannot be reduced to 20 μm or less. This results in poor low-temperature toughness. Therefore, the total reduction rate in the recrystallization temperature range is 55% or less, preferably 52% or less. Here, the "recrystallization temperature range" means a temperature range equal to or higher than Tnr determined from the following equation. Note that the surface temperature of the steel plate can be measured with a radiation thermometer or the like, and can be converted into the steel plate temperature at each plate thickness position.
Tnr (℃) = 174 x log {[%Nb] x ([%C] + 12/14[%N])} + 1444
However, [%X] indicates the content of element X in the steel (% by mass).
再結晶温度域における最終圧延パスの圧下率：１０％以上
再結晶温度域における総圧下率を３５％以上５５％以下にすることに加えて、再結晶温度域における最終圧延パスの圧下率を十分に確保し、再結晶を十分に促進させることで、粗大粒が存在しない均一粒の状態で未再結晶温度域での圧延を開始する必要がある。再結晶温度域における最終圧延パスの圧下率が１０％未満の場合、再結晶が不十分であるため、粗圧延後仕上げ圧延開始までの保持時間の間に粗大粒に成長するため、低温靭性が劣化する。よって、再結晶温度域における最終圧延パスの圧下率は１０％以上とし、好ましくは１１％以上とする。再結晶温度域における最終圧延パスの圧下率の上限は特に限定されず、高いほど好ましいが、当該圧下率は２０％以下であり得る。Reduction ratio of the final rolling pass in the recrystallization temperature range: 10% or more In addition to setting the total reduction ratio of the final rolling pass in the recrystallization temperature range to 35% or more and 55% or less, the reduction ratio of the final rolling pass in the recrystallization temperature range should be set sufficiently. It is necessary to start rolling in the non-recrystallization temperature range in a state of uniform grains without coarse grains by ensuring that the grains are uniform and recrystallization is sufficiently promoted. If the rolling reduction ratio of the final rolling pass in the recrystallization temperature range is less than 10%, recrystallization is insufficient and coarse grains grow during the holding time after rough rolling until the start of finish rolling, resulting in poor low-temperature toughness. to degrade. Therefore, the reduction ratio of the final rolling pass in the recrystallization temperature range is set to 10% or more, preferably 11% or more. The upper limit of the rolling reduction in the final rolling pass in the recrystallization temperature range is not particularly limited, and is preferably as high as possible, but the rolling reduction may be 20% or less.
再結晶温度域における圧延の完了後に行われる未再結晶温度域における圧延は、低温で圧延した方が、歪が多く導入されるため、結晶粒の微細化に有効である。このため、後述する冷却処理の冷却開始温度を遵守できる範囲内において、できるだけ低温で圧延するのが好ましい。 Rolling in the non-recrystallization temperature range, which is performed after completion of rolling in the recrystallization temperature range, is more effective in refining crystal grains because rolling at a lower temperature introduces more strain. For this reason, it is preferable to roll at as low a temperature as possible within a range that can comply with the cooling start temperature of the cooling treatment described later.
熱間圧延工程において、高い低温靱性を得るには、圧延終了温度は低いほどよい。その反面、硫化水素分圧が高い環境下においても耐サワー性を確保する観点からは、後述する制御冷却の冷却開始温度を鋼板表面温度でＡｒ３点以上とする必要があることを踏まえて、圧延終了温度を設定する必要がある。ここで、Ａｒ３点とは、冷却中におけるフェライト変態開始温度を意味し、例えば、鋼の成分から以下の式で求めることができる。なお、鋼板の表面温度は放射温度計等で測定することができる。
Ａｒ３点（℃）＝９１０－３１０[％Ｃ]－８０[％Ｍｎ]－２０[％Ｃｕ]－１５[％Ｃｒ]－５５[％Ｎｉ]－８０[％Ｍｏ]
ただし、[％Ｘ]はＸ元素の鋼中含有量（質量％）を示す。In the hot rolling process, in order to obtain high low-temperature toughness, the lower the rolling end temperature, the better. On the other hand, from the perspective of ensuring sour resistance even in an environment with high hydrogen sulfide partial pressure, it is necessary to set the cooling start temperature of controlled cooling described below to Ar 3 points or higher at the steel plate surface temperature. It is necessary to set the rolling end temperature. Here, the Ar 3 point means the temperature at which ferrite transformation starts during cooling, and can be determined, for example, from the composition of the steel using the following formula. Note that the surface temperature of the steel plate can be measured with a radiation thermometer or the like.
Ar 3 points (°C) = 910-310[%C]-80[%Mn]-20[%Cu]-15[%Cr]-55[%Ni]-80[%Mo]
However, [%X] indicates the content of element X in the steel (% by mass).
〔制御冷却条件〕
冷却開始温度：鋼板表面温度でＡｒ３点以上
冷却開始時の鋼板表面温度がＡｒ３点未満の場合、制御冷却前にフェライトが生成して、強度低下が大きくなると共に耐サワー性が劣化する。このため、冷却開始時の鋼板表面温度はＡｒ３点以上とする。なお、冷却開始時の鋼板表面温度は、圧延終了温度以下となる。冷却開始時の鋼板表面温度の上限は特に限定されないが、当該温度は９００℃以下であり得る。[Controlled cooling conditions]
Cooling start temperature: Steel plate surface temperature of 3 Ar points or more If the steel plate surface temperature at the start of cooling is less than 3 Ar points, ferrite is generated before controlled cooling, resulting in a large decrease in strength and deterioration of sour resistance. For this reason, the steel plate surface temperature at the start of cooling is set to Ar 3 points or higher. Note that the steel sheet surface temperature at the start of cooling is equal to or lower than the rolling end temperature. Although the upper limit of the steel sheet surface temperature at the start of cooling is not particularly limited, the temperature may be 900° C. or lower.
優れた耐ＳＳＣＣ性を得つつ、高強度化を図るためには、鋼板表面下０．２５ｍｍ及び板厚中央における冷却速度を制御する必要がある。 In order to achieve high strength while obtaining excellent SSCC resistance, it is necessary to control the cooling rate 0.25 mm below the surface of the steel sheet and at the center of the sheet thickness.
鋼板表面下０．２５ｍｍにおける鋼板温度で７５０℃から５５０℃までの平均冷却速度：１５～３５℃／ｓ
鋼板表面下０．２５ｍｍにおける鋼板温度で７５０℃から５５０℃までの平均冷却速度を極力遅くし、グラニュラーベイナイトを造り込むことが重要である。７５０℃から５５０℃までの温度域がベイナイト変態において重要な温度域となるので、この温度域における冷却速度を制御することが重要になる。すなわち、平均冷却速度が３５℃／ｓ超えでは、ラスベイナイトが生成し、造管後の耐ＳＳＣＣ性が劣化する。そのため、当該平均冷却速度は３５℃／ｓ以下とし、好ましくは３０℃／ｓ以下とする。他方で、当該平均冷却速度が１５℃／ｓ未満では、フェライトやパーライトが生成して強度不足となるため、これを防ぐ観点から、当該平均冷却速度は１５℃／ｓ以上とする。Average cooling rate from 750℃ to 550℃ at steel plate temperature 0.25mm below the steel plate surface: 15-35℃/s
It is important to make the average cooling rate from 750° C. to 550° C. at a temperature of 0.25 mm below the surface of the steel sheet as slow as possible, and to build in granular bainite. Since the temperature range from 750°C to 550°C is an important temperature range for bainite transformation, it is important to control the cooling rate in this temperature range. That is, when the average cooling rate exceeds 35° C./s, lath bainite is generated and the SSCC resistance after pipe formation is deteriorated. Therefore, the average cooling rate is 35° C./s or less, preferably 30° C./s or less. On the other hand, if the average cooling rate is less than 15° C./s, ferrite and pearlite will be produced and the strength will be insufficient. To prevent this, the average cooling rate is set to 15° C./s or more.
なお、鋼板表面下０．２５ｍｍにおける鋼板温度で５５０℃以下の冷却については、冷却速度が遅い場合、安定した核沸騰状態での冷却にならず、鋼板の極表層部で組織がばらつくおそれがある。このため、鋼板表面下０．２５ｍｍにおける鋼板温度で５５０℃から冷却停止温度までの平均冷却速度は１５０℃／ｓ以上が好ましい。組織がばらつくおそれがあるため、当該平均冷却速度は２５０℃／ｓ以下が好ましい。 In addition, when cooling the steel plate at a temperature of 550°C or less at a temperature 0.25 mm below the surface of the steel plate, if the cooling rate is slow, cooling may not occur in a stable nucleate boiling state, and the structure may vary in the extreme surface layer of the steel plate. . For this reason, the average cooling rate from 550° C. to the cooling stop temperature at the steel sheet temperature 0.25 mm below the surface of the steel sheet is preferably 150° C./s or more. Since there is a possibility that the structure may vary, the average cooling rate is preferably 250° C./s or less.
板厚中央における鋼板温度で７５０℃から５５０℃までの平均冷却速度：１５℃／ｓ以上
板厚中央における鋼板温度で７５０℃から５５０℃までの平均冷却速度が１５℃／ｓ未満では、フェライトが生成し、強度低下や耐ＨＩＣ性の劣化が生じる。このため、当該平均冷却速度は１５℃／ｓ以上とする。低温靭性のばらつき抑制の観点からは、当該平均冷却速度は１７℃／ｓ以上とすることが好ましい。当該冷却速度の上限は特に限定されないが、ラスベイナイトが生成しないように、当該平均冷却速度は３５℃／ｓ以下とすることが好ましい。なお、板厚中央における鋼板温度で５５０℃以下の冷却については、特に限定されないが、低温靭性のばらつき抑制の観点から、平均冷却速度は１５℃／ｓ以上３５℃／ｓ以下とすることが好ましい。Average cooling rate from 750°C to 550°C at the steel plate temperature at the center of the plate thickness: 15°C/s or more If the average cooling rate from 750°C to 550°C at the steel plate temperature at the center of the plate thickness is less than 15°C/s, ferrite This results in a decrease in strength and deterioration in HIC resistance. Therefore, the average cooling rate is set to 15° C./s or more. From the viewpoint of suppressing variations in low-temperature toughness, the average cooling rate is preferably 17° C./s or more. Although the upper limit of the cooling rate is not particularly limited, it is preferable that the average cooling rate is 35° C./s or less so that lath bainite is not generated. Note that cooling to a steel plate temperature of 550 °C or less at the center of the plate thickness is not particularly limited, but from the viewpoint of suppressing variations in low-temperature toughness, it is preferable that the average cooling rate is 15 °C / s or more and 35 °C / s or less. .
なお、鋼板表面下０．２５ｍｍ及び板厚中央における鋼板温度は、物理的に直接測定することはできないが、放射温度計にて測定された冷却開始時の表面温度と目標の冷却停止時の表面温度をもとに、例えばプロセスコンピューターを用いて差分計算により板厚断面内の温度分布を計算し、その結果からリアルタイムに求めることができる。当該温度分布における鋼板表面下０．２５ｍｍでの温度を本明細書における「鋼板表面下０．２５ｍｍにおける鋼板温度」とし、当該温度分布における板厚中央の温度を本明細書における「板厚中央における鋼板温度」とする。 Note that the steel plate temperature 0.25 mm below the steel plate surface and at the center of the plate thickness cannot be physically measured directly, but the surface temperature at the start of cooling measured with a radiation thermometer and the target surface temperature at the end of cooling. Based on the temperature, the temperature distribution within the thickness section of the plate can be calculated by differential calculation using, for example, a process computer, and the temperature distribution can be determined in real time from the results. The temperature at 0.25 mm below the surface of the steel plate in this temperature distribution is referred to as the "steel plate temperature at 0.25 mm below the surface of the steel plate" in this specification, and the temperature at the center of the plate thickness in the temperature distribution is referred to as "the temperature at the center of the plate thickness" in this specification. "Steel plate temperature".
冷却停止温度：鋼板表面下０．２５ｍｍ及び板厚中央における鋼板温度で３５０～５５０℃
冷却停止温度が５５０℃を超えると、ベイナイト変態が不完全になり、十分な強度が得られない。このため、冷却停止温度は５５０℃以下とする。他方で、冷却停止温度が３５０℃未満では、島状マルテンサイトが十分に焼き戻されないため、低温靭性が劣化する。さらに、冷却停止温度が２５０℃未満では、耐ＳＳＣＣ性も劣化する。このため、冷却停止温度は３５０℃以上とし、好ましくは４００℃以上とする。Cooling stop temperature: 350 to 550°C at the steel plate temperature 0.25 mm below the steel plate surface and at the center of the plate thickness
If the cooling stop temperature exceeds 550°C, bainite transformation will be incomplete and sufficient strength will not be obtained. Therefore, the cooling stop temperature is set to 550°C or less. On the other hand, if the cooling stop temperature is less than 350°C, the island-like martensite is not sufficiently tempered, resulting in poor low-temperature toughness. Furthermore, if the cooling stop temperature is less than 250° C., SSCC resistance also deteriorates. Therefore, the cooling stop temperature is set to 350°C or higher, preferably 400°C or higher.
［高強度鋼管］
本発明の高強度鋼板を、プレスベンド成形、ロール成形、ＵＯＥ成形等で管状に成形した後、突き合わせ部を溶接することにより、原油や天然ガスの輸送に好適な鋼板内の材質均一性に優れた耐サワーラインパイプ用高強度鋼管（ＵＯＥ鋼管、電縫鋼管、スパイラル鋼管等）を製造することができる。また、本発明の高強度鋼板を鋼管に用いることにより、溶接部に高硬度域が存在しても、耐ＳＳＣＣ性に優れる鋼管を製造することができる。[High strength steel pipe]
By forming the high-strength steel plate of the present invention into a tubular shape by press bending, roll forming, UOE forming, etc., and then welding the butted parts, the steel plate has excellent material uniformity, which is suitable for transporting crude oil and natural gas. High-strength steel pipes for sour-resistant line pipes (UOE steel pipes, ERW steel pipes, spiral steel pipes, etc.) can be manufactured. Moreover, by using the high-strength steel plate of the present invention in a steel pipe, a steel pipe with excellent SSCC resistance can be manufactured even if a high hardness region exists in the welded portion.
例えば、ＵＯＥ鋼管は、鋼板の端部を開先加工し、Ｃプレス、Ｕプレス、Ｏプレスで鋼管形状に成形した後、内面溶接及び外面溶接で突き合わせ部をシーム溶接し、さらに必要に応じて拡管工程を経て製造される。また、溶接方法は十分な継手強度と継手靭性が得られる方法であれば、いずれの方法でも良いが、優れた溶接品質と製造能率の観点から、サブマージアーク溶接を用いることが好ましい。また、プレスベンド成形により管状に成形した後、突き合せ部をシーム溶接した鋼管に対しても、拡管を実施することができる。 For example, UOE steel pipes are made by groove-processing the ends of a steel plate, forming them into a steel pipe shape using a C press, a U press, or an O press, and then seam welding the butt portions using inner and outer welding, and then welding the butt parts as necessary. Manufactured through a tube expansion process. Furthermore, any welding method may be used as long as sufficient joint strength and joint toughness can be obtained, but from the viewpoint of excellent welding quality and manufacturing efficiency, it is preferable to use submerged arc welding. Moreover, expansion can also be performed on a steel pipe that has been formed into a tubular shape by press bending and then seam-welded at the butt portions.
表１に示す成分組成からなる鋼（鋼種Ａ～ＡＥ）を、連続鋳造法によりスラブとし、表２に示す条件で加熱、熱間圧延、及び制御冷却を実施して、鋼板を得た。その後、鋼板の端部を開先加工し、Ｃプレス、Ｕプレス、Ｏプレスで鋼管形状に成形した後、突き合わせ部を内面側及び外面側からサブマージアーク溶接でシーム溶接し、拡管工程を経て鋼管にした。 Steel (steel types A to AE) having the composition shown in Table 1 was made into a slab by a continuous casting method, and heated, hot rolled, and controlled cooling were performed under the conditions shown in Table 2 to obtain a steel plate. After that, the ends of the steel plate are grooved and formed into a steel pipe shape using a C press, a U press, and an O press.The butted parts are seam welded by submerged arc welding from the inner and outer sides, and the steel pipe is expanded through a pipe expansion process. I made it.
［組織の特定及び面積率の算出並びに最大結晶粒径及び平均結晶粒径の算出］
上記に従って得られた鋼板の板長中央部かつ板幅中央部より金属組織観察用サンプルを採取した。このサンプルの板幅方向に垂直な断面を鏡面研磨したあと、コロイダルシリカでエッチングを行ってから、鋼板表面下０．２５ｍｍ及び板厚中央（板全厚の１／２位置）の位置でそれぞれ１ｍｍ×１ｍｍの視野でＥＢＳＤ（Electron Backscatter Diffraction）法にて結晶データを収集した（測定ステップ：０．８μｍ）。データ収集後、ＯＩＭ－Ａｎａｌｙｓｉｓ及び画像処理ソフト（ＩｍａｇｅＪ）を用いて、鋼板表面下０．２５ｍｍでの組織及び板厚中央の位置での組織を特定し、各相の面積率を算出するとともに、板厚中央の最大結晶粒径及び平均結晶粒径を算出した。なお、結晶粒径の定義は円相当径とした。これら測定の結果を表３に示す。[Identification of structure, calculation of area ratio, and calculation of maximum crystal grain size and average crystal grain size]
A sample for metallographic observation was taken from the center of the plate length and the center of the plate width of the steel plate obtained as described above. After mirror-polishing the cross section perpendicular to the plate width direction of this sample, etching with colloidal silica, etching the sample at 0.25 mm below the surface of the steel plate and 1 mm at the center of the plate thickness (1/2 position of the total plate thickness). Crystal data was collected using the EBSD (Electron Backscatter Diffraction) method in a field of view of ×1 mm (measurement step: 0.8 μm). After collecting the data, use OIM-Analysis and image processing software (ImageJ) to identify the structure at 0.25 mm below the surface of the steel sheet and the structure at the center of the sheet thickness, and calculate the area ratio of each phase. The maximum crystal grain size and average crystal grain size at the center of the plate thickness were calculated. Note that the grain size was defined as the equivalent circle diameter. The results of these measurements are shown in Table 3.
［脆性－延性遷移温度の導出］
シャルピー衝撃試験片を、板厚１／２位置から、試験片長手方向が板幅方向と一致するように採取し、シャルピー衝撃試験に供し、「鉄鋼のシャルピー吸収エネルギー遷移曲線の新しい数式表示法と破壊靭性評価（日本材料強度学会誌 １７ １－１３、１９８２）」に記載の方法に基づいて、脆性－延性遷移温度を導出した。その結果を表３に示す。[Derivation of brittle-ductile transition temperature]
Charpy impact test specimens were taken from the 1/2 position of the plate thickness so that the longitudinal direction of the specimen coincided with the width direction of the plate, and subjected to the Charpy impact test. The brittle-ductile transition temperature was derived based on the method described in "Evaluation of Fracture Toughness (Journal of the Japan Society for Strength of Materials 17 1-13, 1982)". The results are shown in Table 3.
［引張強さ及び降伏強さの測定］
全厚引張試験片を、試験片長手方向が板幅方向と一致するように採取し、引張試験を行い、引張強さ及び降伏強さを測定した。その結果を表３に示す。[Measurement of tensile strength and yield strength]
A full-thickness tensile test piece was taken so that the longitudinal direction of the test piece coincided with the width direction of the plate, a tensile test was conducted, and the tensile strength and yield strength were measured. The results are shown in Table 3.
［耐ＳＳＣＣ性の評価］
図１に示すように、得られた鋼管から切り出した試験片（クーポン； coupon）をフラットニングした後、５×１５×１１５ｍｍのＳＳＣＣ試験片を鋼管内面より採取した。このとき、溶接部を含まない母材だけの試験片のほかに、溶接部と母材の両方を含む試験片を採取した。被検面である内面は、最表層の状態を残すために黒皮付きのままとした。すなわち、鋼板表面下０．２５ｍｍは試験片に含まれている。かくして採取したＳＳＣＣ試験片に、各鋼管の実際の降伏強度（０．５％ＹＳ）の９０％の応力を負荷し、ＮＡＣＥ規格 ＴＭ０１７７ Ｓｏｌｕｔｉｏｎ Ａ溶液を用い、硫化水素分圧：１ｂａｒにて、ＥＦＣ１６規格に準拠して４点曲げＳＳＣＣ試験を行った。
また、同様にＮＡＣＥ規格 ＴＭ０１７７ Ｓｏｌｕｔｉｏｎ Ｂ溶液を用い、硫化水素分圧：０．１ｂａｒ＋二酸化炭素分圧：０．９ｂａｒにて、ＥＦＣ１６規格に準拠して４点曲げＳＳＣＣ試験を行った。
さらに、ＮＡＣＥ規格 ＴＭ０１７７ Ｓｏｌｕｔｉｏｎ Ａ溶液を用い、硫化水素分圧：１６ｂａｒ＋二酸化炭素分圧：５ｂａｒにて、ＥＦＣ１６規格に準拠して４点曲げＳＳＣＣ試験を行った。
試験片を溶液に７２０時間浸漬した後、溶接部を含まない母材だけの試験片と、溶接部と母材の両方を含む試験片との両方において、割れが認められない場合を耐ＳＳＣＣ性が良好と判断して○、また少なくとも一方の試験片において割れが発生した場合を耐ＳＳＣＣ性が不良と判断して×とした。評価結果を表３に示す。[Evaluation of SSCC resistance]
As shown in FIG. 1, a test piece (coupon) cut out from the obtained steel pipe was flattened, and then a 5 x 15 x 115 mm SSCC test piece was taken from the inner surface of the steel pipe. At this time, in addition to a test piece containing only the base metal without the welded part, a test piece containing both the welded part and the base metal was collected. The inner surface, which is the surface to be tested, was left with a black crust to preserve the outermost layer. That is, 0.25 mm below the surface of the steel plate was included in the test piece. A stress of 90% of the actual yield strength (0.5% YS) of each steel pipe was applied to the SSCC specimen thus collected, and EFC16 was applied using NACE standard TM0177 Solution A solution at hydrogen sulfide partial pressure: 1 bar. A four-point bending SSCC test was conducted in accordance with the standard.
Similarly, a four-point bending SSCC test was conducted using NACE standard TM0177 Solution B solution at hydrogen sulfide partial pressure: 0.1 bar + carbon dioxide partial pressure: 0.9 bar in accordance with EFC16 standard.
Furthermore, a four-point bending SSCC test was conducted using NACE standard TM0177 Solution A solution at hydrogen sulfide partial pressure: 16 bar + carbon dioxide partial pressure: 5 bar in accordance with EFC16 standard.
After immersing the test piece in the solution for 720 hours, SSCC resistance is determined when no cracks are observed in both the test piece with only the base metal without the weld and the test piece with both the weld and the base metal. The test piece was judged as good and marked as ○, and the case where cracking occurred in at least one of the test pieces was judged as poor in SSCC resistance and marked as poor. The evaluation results are shown in Table 3.
［耐ＨＩＣ性の評価］
耐ＨＩＣ性は、ＮＡＣＥ規格 ＴＭ０１７７ Ｓｏｌｕｔｉｏｎ Ａ溶液を用い、硫化水素分圧：１ｂａｒにて、９６時間浸漬のＨＩＣ試験により調べた。また、ＮＡＣＥ規格 ＴＭ０１７７ Ｓｏｌｕｔｉｏｎ Ｂ溶液を用い、硫化水素分圧：０．１ｂａｒ＋二酸化炭素分圧：０．９ｂａｒにて、９６時間浸漬のＨＩＣ試験により調べた。耐ＨＩＣ性は、ＨＩＣ試験で割れ面積率（ＣＡＲ）が５％以下となった場合を耐ＨＩＣ性が良好と判断して〇、５％を超えた場合を耐ＨＩＣ性が不良と判断して×とした。評価結果を表３に示す。[Evaluation of HIC resistance]
The HIC resistance was examined by a 96-hour immersion HIC test at a hydrogen sulfide partial pressure of 1 bar using NACE standard TM0177 Solution A solution. Further, using NACE standard TM0177 Solution B solution, it was investigated by a HIC test of 96-hour immersion at hydrogen sulfide partial pressure: 0.1 bar + carbon dioxide partial pressure: 0.9 bar. HIC resistance is determined to be good if the crack area ratio (CAR) is 5% or less in the HIC test, and poor HIC resistance if it exceeds 5%. It was set as ×. The evaluation results are shown in Table 3.
本発明の目標範囲は、耐サワーラインパイプ用高強度鋼板として脆性－延性遷移温度：－１００℃以下、引張強さ：５３５ＭＰａ以上、鋼板表面下０．２５ｍｍ位置における鋼組織はグラニュラーベイナイト及び焼戻し島状マルテンサイトからなり、板厚中央における最大結晶粒径が８０μｍ以下、平均結晶粒径が２０μｍ以下であり、ＳＳＣＣ試験で割れが認められないこと、ＨＩＣ試験で割れ面積率（ＣＡＲ）が５％以下であることとした。 The target range of the present invention is to have a brittle-ductile transition temperature of -100°C or less, a tensile strength of 535 MPa or more, and a steel structure of granular bainite and tempered islands at a position 0.25 mm below the surface of the steel plate as a high-strength steel plate for sour-resistant line pipes. The maximum crystal grain size at the center of the plate thickness is 80 μm or less, the average crystal grain size is 20 μm or less, no cracks are observed in the SSCC test, and the crack area ratio (CAR) is 5% in the HIC test. It was decided to be as follows.
表２及び表３に示したように、Ｎｏ．１～１０，３９～４２は、成分組成及び製造条件が本発明の適正範囲を満足する発明例である。いずれも、鋼板として脆性－延性遷移温度：－１００℃以下、引張強さ：５３５ＭＰａ以上、鋼板表面下０．２５ｍｍにおける鋼組織はグラニュラーベイナイト及び焼戻し島状マルテンサイトからなり、板厚中央における最大結晶粒径が８０μｍ以下、平均結晶粒径が２０μｍ以下であり、耐ＳＳＣＣ性及び耐ＨＩＣ性が良好であった。 As shown in Tables 2 and 3, No. Nos. 1 to 10 and 39 to 42 are invention examples whose component compositions and manufacturing conditions satisfy the appropriate range of the present invention. In both cases, the steel plate has a brittle-ductile transition temperature of -100°C or less, a tensile strength of 535 MPa or more, and the steel structure 0.25 mm below the surface of the steel plate consists of granular bainite and tempered island-like martensite, with the largest crystal grain at the center of the plate thickness. The grain size was 80 μm or less, the average crystal grain size was 20 μm or less, and the SSCC resistance and HIC resistance were good.
これに対し、Ｎｏ.１１～２５，３７～３８は、鋼板の成分組成が本発明の範囲外である。Ｎｏ.１１，１４，３７は固溶強化が十分でなく、強度が不足した。Ｎｏ.１２，１５～１６，１９，２１は、硬さが上昇したため、耐ＳＳＣＣ性及び耐ＨＩＣ性が劣っていた。Ｎｏ.１２，２１は、低温での強度上昇が大きく、低温靭性が劣化した。Ｎｏ.１６は清浄度が低下し、低温靭性が劣化した。Ｎｏ.１７，２０，２２，３８は、介在物又は炭化物が生成したため、耐ＨＩＣ性が劣っていた。Ｎｏ.１３，１８は、鋼の非熱的応力の上昇によって低温靭性が劣化した。Ｎｏ．２３～２５は、局部腐食が促進されたため、０．１ｂａｒ－Ｈ２Ｓ＋０．９ｂａｒ－ＣＯ２での耐ＳＳＣＣ性が劣化した。On the other hand, in Nos. 11 to 25 and 37 to 38, the compositions of the steel plates are outside the scope of the present invention. Nos. 11, 14, and 37 did not have sufficient solid solution strengthening, resulting in insufficient strength. Nos. 12, 15 to 16, 19, and 21 had poor SSCC resistance and HIC resistance due to increased hardness. In Nos. 12 and 21, the strength increased significantly at low temperatures and the low temperature toughness deteriorated. In No. 16, the cleanliness decreased and the low temperature toughness deteriorated. Nos. 17, 20, 22, and 38 had poor HIC resistance because inclusions or carbides were generated. In Nos. 13 and 18, the low-temperature toughness deteriorated due to the increase in non-thermal stress of the steel. No. In samples Nos. 23 to 25, the SSCC resistance at 0.1 bar-H 2 S+0.9 bar-CO 2 deteriorated due to accelerated local corrosion.
Ｎｏ．２６～３６は、成分組成は本発明の範囲内であるが、製造条件が本発明の範囲外の比較例である。Ｎｏ．２６は、スラブ加熱温度が低いため、炭化物の固溶が不十分であり低強度であった。Ｎｏ．２７は、スラブ加熱温度が高いため、結晶粒が粗大化し低温靭性が劣化した。Ｎｏ．２８は、再結晶温度域での総圧下率が不足したため、粗大粒が残存し、低温靭性が劣化した。Ｎｏ．２９は、再結晶温度域での総圧下率が過多のため、平均結晶粒径が大きく、低温靭性が劣化した。Ｎｏ．３０は、再結晶温度域での最終パスの圧下率が不足したため、粗大粒が残存し、低温靭性が劣化した。Ｎｏ．３１は、冷却開始温度が低く、表層にフェライトが一部生成したため、低強度であるとともに、高圧での耐ＳＳＣＣ性が劣化した。Ｎｏ.３２は、平均冷却速度が低く、板厚中央までフェライトが一部生成したため、低強度であるとともに、耐ＨＩＣ性及び高圧での耐ＳＳＣＣ性が劣っていた。Ｎｏ.３３は、平均冷却速度が高く、表層にラスベイナイトが一部生成したため、高圧での耐ＳＳＣＣ性が劣っていた。Ｎｏ．３４は、冷却停止温度が低く、島状マルテンサイトが十分に焼き戻されなかったため、低温靭性が劣化した。Ｎｏ．３５は、冷却停止温度がさらに低いため、低温靭性に加えて高圧での耐ＳＳＣＣ性が劣化した。Ｎｏ．３６は、冷却停止温度が高く、板厚中央までフェライトが一部生成したため、低強度であるとともに、耐ＨＩＣ性及び高圧での耐ＳＳＣＣ性が劣っていた。 No. Samples Nos. 26 to 36 are comparative examples whose component compositions are within the scope of the present invention, but whose manufacturing conditions are outside the scope of the present invention. No. In No. 26, since the slab heating temperature was low, solid solution of carbide was insufficient and the strength was low. No. In No. 27, the slab heating temperature was high, so the crystal grains became coarse and the low-temperature toughness deteriorated. No. In No. 28, the total rolling reduction in the recrystallization temperature range was insufficient, so coarse grains remained and low-temperature toughness deteriorated. No. In No. 29, the total rolling reduction in the recrystallization temperature range was too large, resulting in a large average grain size and poor low-temperature toughness. No. In No. 30, the reduction rate in the final pass in the recrystallization temperature range was insufficient, so coarse grains remained and the low-temperature toughness deteriorated. No. In No. 31, the cooling start temperature was low and some ferrite was formed on the surface layer, resulting in low strength and poor SSCC resistance at high pressure. In No. 32, the average cooling rate was low and some ferrite was formed up to the center of the plate thickness, resulting in low strength and poor HIC resistance and SSCC resistance at high pressure. In No. 33, the average cooling rate was high and some lath bainite was formed on the surface layer, so the SSCC resistance at high pressure was poor. No. In No. 34, the cooling stop temperature was low and the island-like martensite was not sufficiently tempered, resulting in poor low-temperature toughness. No. In No. 35, the cooling stop temperature was even lower, so in addition to the low-temperature toughness, the SSCC resistance at high pressure deteriorated. No. In No. 36, the cooling stop temperature was high and some ferrite was formed up to the center of the plate thickness, so the strength was low and the HIC resistance and SSCC resistance at high pressure were poor.
本発明によれば、耐ＨＩＣ性、耐ＳＳＣＣ性のみならず、低温靱性にも優れた耐サワーラインパイプ用高強度鋼板及び高強度鋼管を提供することができる。
According to the present invention, it is possible to provide a high-strength steel plate for a sour line pipe and a high-strength steel pipe that are excellent not only in HIC resistance and SSCC resistance but also in low-temperature toughness.
Claims (8)
鋼板表面下０．２５ｍｍにおける鋼組織がグラニュラーベイナイト及び焼戻し島状マルテンサイトからなり、
板厚中央における最大結晶粒径が８０μｍ以下、平均結晶粒径が２０μｍ以下であり、
シャルピー衝撃試験における脆性－延性遷移温度が－１００℃以下であり、
引張強さが５３５ＭＰａ以上である
ことを特徴とする耐サワーラインパイプ用高強度鋼板。 In mass%, C: 0.030 to 0.060%, Si: 0.01 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S: 0.0015 % or less, Al: 0.010 to 0.080%, Cr: 0.05 to 0.50%, Nb: 0.005 to 0.080%, N: 0.0010 to 0.0080%, and Ca: Contains 0.0005 to 0.0050%, with the remainder consisting of Fe and unavoidable impurities,
The steel structure at 0.25 mm below the surface of the steel plate consists of granular bainite and tempered island martensite,
The maximum crystal grain size at the center of the plate thickness is 80 μm or less, the average crystal grain size is 20 μm or less,
The brittle-ductile transition temperature in the Charpy impact test is -100°C or less,
A high-strength steel plate for sour line pipes having a tensile strength of 535 MPa or more.
含有し、残部がＦｅ及び不可避的不純物からなる成分組成を有する鋼片を１０００～１２５０℃の温度に加熱し、
その後、前記鋼片に、再結晶温度域における総圧下率：３５％以上５５％以下、再結晶温度域における最終圧延パスの圧下率：１０％以上を満足する熱間圧延を施して鋼板とし、
その後、前記鋼板に対して、
冷却開始時の鋼板表面温度：Ａｒ３点以上、
鋼板表面下０．２５ｍｍにおける鋼板温度で７５０℃から５５０℃までの平均冷却速度：１５～３５℃／ｓ、
板厚中央における鋼板温度で７５０℃から５５０℃までの平均冷却速度：１５℃／ｓ以上、
鋼板表面下０．２５ｍｍ及び板厚中央における鋼板温度で冷却停止温度：３５０～５５０℃
の条件で制御冷却を行って、鋼板表面下０．２５ｍｍにおける鋼組織がグラニュラーベイナイト及び焼戻し島状マルテンサイトからなり、板厚中央における最大結晶粒径が８０μｍ以下、平均結晶粒径が２０μｍ以下であり、シャルピー衝撃試験における脆性－延性遷移温度が－１００℃以下であり、引張強さが５３５ＭＰａ以上である耐サワーラインパイプ用高強度鋼板を製造する
ことを特徴とする耐サワーラインパイプ用高強度鋼板の製造方法。 In mass%, C: 0.030 to 0.060%, Si: 0.01 to 0.50%, Mn: 0.80 to 1.80%, P: 0.015% or less, S: 0.0015 % or less, Al: 0.010 to 0.080%, Cr: 0.05 to 0.50%, Nb: 0.005 to 0.080%, N: 0.0010 to 0.0080%, and Ca: A steel piece having a composition containing 0.0005 to 0.0050% and the remainder consisting of Fe and inevitable impurities is heated to a temperature of 1000 to 1250 ° C.
Thereafter, the steel slab is subjected to hot rolling that satisfies the total rolling reduction in the recrystallization temperature range: 35% or more and 55% or less, and the rolling reduction in the final rolling pass in the recrystallization temperature range: 10% or more, to obtain a steel plate,
Then, for the steel plate,
Steel plate surface temperature at the start of cooling: Ar 3 points or more,
Average cooling rate from 750°C to 550°C at steel plate temperature 0.25mm below the steel plate surface: 15 to 35°C/s,
Average cooling rate from 750°C to 550°C at steel plate temperature at center of plate thickness: 15°C/s or more,
Cooling stop temperature at 0.25 mm below the surface of the steel plate and at the center of the plate thickness: 350 to 550°C
By performing controlled cooling under the following conditions, the steel structure 0.25 mm below the surface of the steel plate consists of granular bainite and tempered island martensite, the maximum grain size at the center of the plate thickness is 80 μm or less, and the average grain size is 20 μm or less. To produce a high-strength steel plate for sour-resistant line pipes, which has a brittle-ductile transition temperature of -100°C or less in a Charpy impact test and a tensile strength of 535 MPa or more.
A method for manufacturing a high-strength steel plate for sour-resistant line pipes.
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| WO2021020220A1 (en) | 2019-07-31 | 2021-02-04 | Ｊｆｅスチール株式会社 | High-strength steel sheet for sour resistant line pipe, manufacturing method thereof, and high-strength steel pipe made using high-strength steel sheet for sour resistant line pipe |
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| WO2021020220A1 (en) | 2019-07-31 | 2021-02-04 | Ｊｆｅスチール株式会社 | High-strength steel sheet for sour resistant line pipe, manufacturing method thereof, and high-strength steel pipe made using high-strength steel sheet for sour resistant line pipe |
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