JP6825749B2 - High-strength steel sheet for sour-resistant pipe and its manufacturing method, and high-strength steel pipe using high-strength steel sheet for sour-resistant pipe - Google Patents
High-strength steel sheet for sour-resistant pipe and its manufacturing method, and high-strength steel pipe using high-strength steel sheet for sour-resistant pipe Download PDFInfo
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- 229910052726 zirconium Inorganic materials 0.000 claims description 7
- 229910052799 carbon Inorganic materials 0.000 claims description 6
- 229910052748 manganese Inorganic materials 0.000 claims description 6
- 229910052750 molybdenum Inorganic materials 0.000 claims description 6
- 229910052720 vanadium Inorganic materials 0.000 claims description 6
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- RWSOTUBLDIXVET-UHFFFAOYSA-N Dihydrogen sulfide Chemical compound S RWSOTUBLDIXVET-UHFFFAOYSA-N 0.000 description 22
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Description
本発明は、建築、海洋構造物、造船、土木、建設産業用機械の分野のラインパイプに使用して好適な、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およ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。 INDUSTRIAL APPLICABILITY The present invention is a high-strength steel plate for sour line pipes having excellent material uniformity in steel plates, which is suitable for use in line pipes in the fields of construction, marine structures, shipbuilding, civil engineering, and machinery for the construction industry, and production thereof. It's about the method. The present invention also relates to a high-strength steel pipe using the above-mentioned high-strength steel plate for sour-resistant 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、プレスベンド成形およ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 produced by a thick 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材表面に吸着し、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、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、一般的に油井用高強度継目無鋼管や、溶接部の高硬度域で発生することが知られており、比較的硬さが低いラインパイプではあまり問題視されてこなかった。ところが近年、原油や天然ガスの採掘環境がますます厳しさを増し、硫化水素分圧の高い、あるいはpHが低い環境において、ラインパイプの母材部においてもSSCCが生じることが報告されており、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。また、比較的、硫化水素分圧の低い環境においては、フィッシャーと呼ばれる微細割れが発生する場合があり、SSCCが生じる恐れがある。 Here, line pipes used for transporting crude oil containing hydrogen sulfide and natural gas have strength, toughness, weldability, etc., as well as 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 them, in HIC, hydrogen ions due to the corrosion reaction are adsorbed on the surface of the steel material, penetrate into the steel as atomic hydrogen, and diffuse and accumulate around non-metal inclusions such as MnS in the steel and the hard phase 2 structure. As a result, it becomes molecular hydrogen and cracks occur due to its internal pressure, which has been a problem in line pipes with a relatively low strength level with respect to oil well pipes, and many countermeasure technologies have been disclosed. On the other hand, SSCC is generally known to occur in high-strength seamless steel pipes for oil wells and in the high hardness range of welds, and has not been regarded as a problem for line pipes with relatively low hardness. .. However, in recent years, the mining environment for crude oil and natural gas has become more and more severe, and it has been reported that SSCC occurs even in the base metal part of line pipes in an environment where the partial pressure of hydrogen sulfide is high or the pH is low. It has been pointed out that it is important to control the hardness of the inner surface layer of the steel pipe to improve the SSCC resistance in a more severe corrosive environment. Further, in an environment where the partial pressure of hydrogen sulfide is relatively low, fine cracks called fisher may occur, and SSCC may occur.
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技術を用いて鋼材の高強度化を行うには、制御冷却時の冷却速度を大きくすることが有効である。しかしながら、高冷却速度で制御冷却した場合、鋼板表層部が急冷されるため、鋼板内部に比べて表層部の硬さが高くなり、板厚方向の硬さ分布にばらつきが生じる。従って、鋼板内の材質均一性を確保する観点で問題となる。 Usually, in the production of high-strength steel sheets for line pipes, so-called TMCP (Thermo-Mechanical Control Process) technology, which combines controlled rolling and controlled cooling, is applied. In order to increase the strength of the steel material using this TMCP technology, it is effective to increase the cooling rate during controlled cooling. However, when controlled cooling is performed at a high cooling rate, the surface layer portion of the steel sheet is rapidly cooled, so that the hardness of the surface layer portion is higher than that inside the steel sheet, and the hardness distribution in the plate thickness direction varies. Therefore, there is a problem from the viewpoint of ensuring the material uniformity in the steel sheet.
上記の問題を解決するために、例えば特許文献1,2には、圧延後、表層部がベイナイト変態を完了する前に表面を復熱させる高冷却速度の制御冷却を行うことによる、板厚方向の材質差が小さい鋼板の製造方法が開示されている。また、特許文献3,4には、高周波誘導加熱装置を用いて、加速冷却後の鋼板表面を内部より高温に加熱して表層部の硬さを低減した、ラインパイプ用鋼板の製造方法が開示されている。 In order to solve the above problems, for example, in Patent Documents 1 and 2, in the plate thickness direction, for example, after rolling, controlled cooling at a high cooling rate is performed to reheat the surface of the surface layer portion before completing the bainite transformation. A method for manufacturing a steel sheet having a small material difference is disclosed. Further, Patent Documents 3 and 4 disclose a method for manufacturing a steel sheet for a line pipe, which uses a high-frequency induction heating device to heat the surface of the steel sheet after accelerated cooling to a higher temperature than the inside to reduce the hardness of the surface layer portion. Has been done.
他方、鋼板表面のスケール厚さにむらがあった場合、冷却時にその下部の鋼板の冷却速度にもばらつきが生じ、鋼板内の局所的な冷却停止温度のばらつきが問題となる。その結果、スケール厚さのむらによって板幅方向に鋼板材質のばらつきが生じることになる。これに対し、特許文献5,6には、冷却直前にデスケーリングを行うことにより、スケール厚さむらに起因した冷却むらを低減して、鋼板形状を改善する方法が開示されている。 On the other hand, if the scale thickness on the surface of the steel sheet is uneven, the cooling rate of the steel sheet below the steel sheet also varies during cooling, and the local cooling stop temperature in the steel sheet becomes a problem. As a result, the steel plate material varies in the plate width direction due to the unevenness of the scale thickness. On the other hand, Patent Documents 5 and 6 disclose a method of reducing the cooling unevenness caused by the scale thickness unevenness and improving the steel sheet shape by performing descaling immediately before cooling.
しかしながら、本発明者らの検討によると、上記特許文献1〜6に記載の製造方法で得られる高強度鋼板では、より厳しい腐食環境下での耐SSCC性という観点で改善の余地があることが判明した。その理由としては、以下のようなものが考えられる。 However, according to the study by the present inventors, there is room for improvement in the high-strength steel sheet obtained by the manufacturing methods described in Patent Documents 1 to 6 from the viewpoint of SSCC resistance under a harsher corrosion environment. found. The possible reasons for this are as follows.
特許文献1,2に記載の製造方法では、鋼板の成分により変態挙動が異なると、復熱による十分な材質均質化の効果が得られない場合がある。また、特許文献1,2に記載の製造方法により得られる鋼板の表層における組織がフェライト‐ベイナイト2相組織のような複相組織の場合、低荷重のマイクロビッカース試験においては、圧子がいずれの組織を押し込んで試験するかによって硬さの値のばらつきが大きく生じる。 In the manufacturing methods described in Patent Documents 1 and 2, if the transformation behavior differs depending on the composition of the steel sheet, the effect of sufficient material homogenization by reheating may not be obtained. Further, when the structure of the surface layer of the steel plate obtained by the production methods described in Patent Documents 1 and 2 is a double-phase structure such as a ferrite-bainite two-phase structure, the indenter has any structure in the low-load micro-Vickers test. There is a large variation in the hardness value depending on whether the test is performed by pushing in.
特許文献3,4に記載の製造方法は、加速冷却における表層部の冷却速度が大きいため、鋼板表面の加熱だけでは表層部の硬さを十分に低減できない場合がある。 In the manufacturing methods described in Patent Documents 3 and 4, since the cooling rate of the surface layer portion in accelerated cooling is high, the hardness of the surface layer portion may not be sufficiently reduced only by heating the surface of the steel sheet.
他方、特許文献5,6に記載の方法では、デスケーリングにより、熱間矯正時のスケールの押し込み疵による表面性状不良の低減や、鋼板の冷却停止温度のばらつきを低減して鋼板形状を改善しているが、均一な材質を得るための冷却条件に関しては何ら配慮がなされていない。これは、鋼板表面の冷却速度がばらつくと、鋼板の硬さにばらつきが生じるからである。すなわち、冷却速度が遅いと、鋼板表面が冷却する際に、鋼板表面と冷却水の間に気泡の膜が発生する"膜沸騰"と、気泡が膜を形成する前に冷却水によって表面から分離される"核沸騰"とが同時に発生し、鋼板表面の冷却速度にばらつきが生じる。その結果、鋼板表面の硬さにばらつきを生じることになる。特許文献5,6に記載の技術ではこの点が考慮されていない。 On the other hand, in the methods described in Patent Documents 5 and 6, descaling reduces surface texture defects due to scale indentation defects during hot straightening and reduces variations in the cooling stop temperature of the steel sheet to improve the shape of the steel sheet. However, no consideration has been given to the cooling conditions for obtaining a uniform material. This is because if the cooling rate on the surface of the steel sheet varies, the hardness of the steel sheet varies. That is, when the cooling rate is slow, when the surface of the steel plate is cooled, a film of bubbles is generated between the surface of the steel sheet and the cooling water, and the bubbles are separated from the surface by the cooling water before forming the film. "Nucleate boiling" occurs at the same time, causing variations in the cooling rate of the steel plate surface. As a result, the hardness of the surface of the steel sheet varies. This point is not taken into consideration in the techniques described in Patent Documents 5 and 6.
また、特許文献1〜6では、硫化水素分圧が比較的低い環境におけるフィッシャーのような微細割れを回避する条件は明確でなかった。 Further, in Patent Documents 1 to 6, the conditions for avoiding fine cracks such as Fisher in an environment where the partial pressure of hydrogen sulfide is relatively low are not clear.
そこで本発明は、上記課題に鑑み、耐HIC性のみならず、より厳しい腐食環境下での耐SSCC性および1bar未満の硫化水素分圧の低い環境における耐SSCC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案することを目的とする。 Therefore, in view of the above problems, the present invention is a sour line pipe excellent not only in HIC resistance but also in SSCC resistance in a harsher corrosion environment and SSCC resistance in an environment with a low partial pressure of hydrogen sulfide of less than 1 bar. It is an object of the present invention to provide a high-strength steel plate for use together with its advantageous manufacturing method. Another object of the present invention is to propose a high-strength steel pipe using the above-mentioned high-strength steel plate for sour-resistant pipes.
本発明者らは、より厳しい腐食環境下での耐SSCC性を確保するべく、鋼材の成分組成、ミクロ組織および製造条件について、数多くの実験と検討を繰り返した。その結果、高強度鋼管の耐SSCC性をさらに向上させるためには、従来知見どおり単に表層硬さを抑えることだけでは不十分であり、特に鋼板の極表層部の組織、具体的には鋼板表面下0.25mmの鋼組織を、転位密度1.0×1014〜7.0×1014(m−2)のベイナイト組織とすることで、造管後のコーティング過程において硬さの上昇代を抑えることができ、結果として鋼管の耐SSCC性が向上することを知見した。さらに、このような鋼組織を実現するためには、鋼板表面下0.25mmにおける冷却速度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。また、1bar超えの硫化水素分圧の高い環境では、Mo添加が初期き裂発生抑制に有効であること、1bar未満の硫化水素分圧の低い環境ではNi添加を抑制することがフィッシャーのような微細割れを回避するのに有効であることを見出した。本発明は、この知見をもとになされたものである。The present inventors have repeated numerous experiments and studies on the composition, microstructure and manufacturing conditions of steel materials in order to ensure SSCC resistance in a more severe corrosive environment. As a result, in order to further improve the SSCC resistance of high-strength steel pipes, it is not sufficient to simply suppress the surface hardness as previously found, and in particular, the structure of the polar surface layer of the steel sheet, specifically the surface of the steel sheet. By forming the steel structure of the lower 0.25 mm into a bainite structure with a dislocation density of 1.0 × 10 14 to 7.0 × 10 14 (m- 2 ), the increase in hardness can be increased in the coating process after pipe formation. It was found that it can be suppressed and as a result, the SSCC resistance of the steel pipe is improved. Furthermore, in order to realize such a steel structure, it is necessary to strictly control the cooling rate at 0.25 mm below the surface of the steel sheet, and we have succeeded in finding the condition. Further, in an environment where the partial pressure of hydrogen sulfide exceeds 1 bar, the addition of Mo is effective in suppressing the generation of initial cracks, and in an environment where the partial pressure of hydrogen sulfide is less than 1 bar, suppressing the addition of Ni is like Fisher. It was found to be effective in avoiding microcracks. The present invention is based on this finding.
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:0.02〜0.08%、Si:0.01〜0.50%、Mn:0.50〜1.80%、P:0.001〜0.015%、S:0.0002〜0.0015%、Al:0.01〜0.08%、Mo:0.01〜0.50%およびCa:0.0005〜0.005%を含有し、さらに、Nb:0.005〜0.1%およびTi:0.005〜0.1%から選ばれる1種以上を含有し、残部がFeおよ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、転位密度1.0×1014〜7.0×1014(m−2)のベイナイト組織であり、
鋼板表面下0.25mmにおけるビッカース硬さのばらつきが、標準偏差をσとしたときに3σで30HV以下であり、
板厚方向のビッカース硬さのばらつきが、標準偏差をσとしたときに3σで30HV以下であり、
520MPa以上の引張強さを有する
ことを特徴とする耐サワーラインパイプ用高強度鋼板。That is, the gist structure of the present invention is as follows.
[1] In terms of mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%. , S: 0.0002 to 0.0015%, Al: 0.01 to 0.08%, Mo: 0.01 to 0.50% and Ca: 0.0005 to 0.005%, and further It contains one or more selected from Nb: 0.005 to 0.1% and Ti: 0.005 to 0.1%, and has a component composition in which the balance consists of Fe and unavoidable impurities.
The steel structure at 0.25 mm below the surface of the steel sheet is a bainite structure with a dislocation density of 1.0 × 10 14 to 7.0 × 10 14 (m- 2 ).
The variation in Vickers hardness at 0.25 mm below the surface of the steel sheet is 30 HV or less at 3σ when the standard deviation is σ.
The variation in Vickers hardness in the plate thickness direction is 30 HV or less at 3σ when the standard deviation is σ.
A high-strength steel sheet for sour line pipes, which has a tensile strength of 520 MPa or more.
[2]前記成分組成が、さらに、質量%で、Cu:0.50%以下、Ni:0.10%以下およびCr:0.50%以下のうちから選んだ1種以上を含有する、上記[1]に記載の耐サワーラインパイプ用高強度鋼板。 [2] The above-mentioned component composition further contains at least one selected from Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less in mass%. The high-strength steel plate for sour-resistant pipes according to [1].
[3]前記成分組成が、さらに、質量%で、V:0.005〜0.1%、Zr:0.0005〜0.02%、Mg:0.0005〜0.02%およびREM:0.0005〜0.02%のうちから選んだ1種以上を含有する、上記[1]または[2]に記載の耐サワーラインパイプ用高強度鋼板。 [3] Further, the component composition is, in mass%, V: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02% and REM: 0. The high-strength steel sheet for sour line pipe according to the above [1] or [2], which contains at least one selected from 0005 to 0.02%.
[4]質量%で、C:0.02〜0.08%、Si:0.01〜0.50%、Mn:0.50〜1.80%、P:0.001〜0.015%、S:0.0002〜0.0015%、Al:0.01〜0.08%、Mo:0.01〜0.50%およびCa:0.0005〜0.005%を含有し、さらに、Nb:0.005〜0.1%およびTi:0.005〜0.1%から選ばれる1種以上を含有し、残部がFeおよび不可避的不純物の成分組成を有する鋼片を、1000〜1300℃の温度に加熱したのち、熱間圧延して鋼板とし、
その後前記鋼板に対して、
冷却開始時の鋼板表面温度:(Ar3−10℃)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:50℃/s以下、
鋼板平均温度で750℃から550℃までの平均冷却速度:15℃/s以上、
鋼板表面下0.25mmにおける鋼板温度で550℃から冷却停止時の温度まで平均冷却速度:150℃/s以上、および
鋼板平均温度で冷却停止温度:250〜550℃
の条件で制御冷却を行い、
その後、前記鋼板に対して、鋼板平均温度:前記冷却停止温度超え、かつ450〜600℃の条件で再加熱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 terms of mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%. , S: 0.0002 to 0.0015%, Al: 0.01 to 0.08%, Mo: 0.01 to 0.50% and Ca: 0.0005 to 0.005%, and further 1000 to 1300 steel pieces containing one or more selected from Nb: 0.005 to 0.1% and Ti: 0.005 to 0.1%, and the balance having a component composition of Fe and unavoidable impurities. After heating to a temperature of ℃, it is hot-rolled to make a steel sheet.
After that, with respect to the steel sheet
Cooling at the start of the steel sheet surface temperature: (Ar 3 -10 ℃) or higher,
Average cooling rate from 750 ° C to 550 ° C at steel plate temperature 0.25 mm below the surface of the steel plate: 50 ° C / s or less,
Average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature: 15 ° C / s or more,
Average cooling rate: 150 ° C / s or more from 550 ° C to the temperature at which cooling is stopped at a steel plate temperature of 0.25 mm below the surface of the steel sheet, and cooling stop temperature: 250 to 550 ° C at the average temperature of the steel sheet.
Control cooling under the conditions of
After that, the steel sheet is reheated under the conditions of the average temperature of the steel sheet: the cooling stop temperature and 450 to 600 ° C., which is a method for producing a high-strength steel sheet for sour line pipe.
[5]前記成分組成が、さらに、質量%で、Cu:0.50%以下、Ni:0.10%以下およびCr:0.50%以下のうちから選んだ1種以上を含有する、上記[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 above-mentioned component composition further contains at least one selected from Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less in mass%. The method for manufacturing a high-strength steel plate for sour-resistant pipes according to [4].
[6]前記成分組成が、さらに、質量%で、V:0.005〜0.1%、Zr:0.0005〜0.02%、Mg:0.0005〜0.02%およびREM:0.0005〜0.02%のうちから選んだ1種以上を含有する、上記[4]または[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] Further, the component composition is, in mass%, V: 0.005 to 0.1%, Zr: 0.0005 to 0.02%, Mg: 0.0005 to 0.02% and REM: 0. The method for producing a high-strength steel sheet for sour line pipes according to the above [4] or [5], which contains at least one selected from 0005 to 0.02%.
[7]上記[1]〜[3]のいずれか一項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 pipe according to any one of the above [1] to [3].
本発明の耐サワーラインパイプ用高強度鋼板およ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性のみならず、より厳しい腐食環境下での耐SSCC性および1bar未満の硫化水素分圧の低い環境における耐SSCC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性のみならず、より厳しい腐食環境下での耐SSCC性および1bar未満の硫化水素分圧の低い環境における耐SSCC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 pipe for sour line pipes of the present invention and the high-strength steel pipe using the high-strength steel plate for sour line pipes have not only HIC resistance but also SSCC resistance under more severe corrosion environment and less than 1 bar. It also has excellent SSCC resistance in an environment with low hydrogen sulfide partial pressure. Further, according to the method for producing a high-strength steel plate for sour line pipes of the present invention, not only HIC resistance but also SSCC resistance in a harsher corrosion environment and resistance in an environment with a low hydrogen sulfide partial pressure of less than 1 bar are obtained. It is possible to manufacture a high-strength steel plate for sour line pipes having excellent SSCC properties.
以下、本開示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 sheet for sour-resistant pipes of the present disclosure will be specifically described.
[成分組成]
まず、本開示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%で示す単位は全て質量%である。[Ingredient composition]
First, the component composition of the high-strength steel sheet according to the present disclosure and the reason for its limitation will be described. In the following description, all units indicated by% are mass%.
C:0.02〜0.08%
Cは、強度の向上に有効に寄与するが、含有量が0.02%未満では十分な強度が確保できず、一方0.08%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性および耐HIC性が劣化する。また、靭性も劣化する。このため、C量は0.02〜0.08%の範囲に限定する。C: 0.02 to 0.08%
C effectively contributes to the improvement of strength, but if the content is less than 0.02%, sufficient strength cannot be secured, while if it exceeds 0.08%, the hardness of the surface layer portion and the central segregation portion is hard during accelerated cooling. The SSCC resistance and the HIC resistance are deteriorated due to the increase in the resistance. In addition, toughness also deteriorates. Therefore, the amount of C is limited to the range of 0.02 to 0.08%.
Si:0.01〜0.50%
Siは、脱酸のため添加するが、含有量が0.01%未満では脱酸効果が十分でなく、一方0.50%を超えると靭性や溶接性を劣化させるため、Si量は0.01〜0.50%の範囲に限定する。Si: 0.01 to 0.50%
Si is added for deoxidation, but if the content is less than 0.01%, the deoxidizing effect is not sufficient, while if it exceeds 0.50%, the toughness and weldability deteriorate, so the amount of Si is 0. Limited to the range of 01 to 0.50%.
Mn:0.50〜1.80%
Mnは、強度、靭性の向上に有効に寄与するが、含有量が0.50%未満ではその添加効果に乏しく、一方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性および耐HIC性が劣化する。また、溶接性も劣化する。このため、Mn量は0.50〜1.80%の範囲に限定する。Mn: 0.50 to 1.80%
Mn effectively contributes to the improvement of strength and toughness, but its addition effect is poor when the content is less than 0.50%, while when it exceeds 1.80%, the hardness of the surface layer portion and the central segregation portion during accelerated cooling. As a result, SSCC resistance and HIC resistance deteriorate. Weldability also deteriorates. Therefore, the amount of Mn is limited to the range of 0.50 to 1.80%.
P:0.001〜0.015%
Pは、不可避不純物元素であり、溶接性を劣化させるとともに、中心偏析部の硬さを上昇させることで耐HIC性を劣化させる。0.015%を超えるとその傾向が顕著となるため、上限を0.015%に規定する。好ましくは0.008%以下である。含有量は低いほどよいが、精錬コストの観点から0.001%以上とする。P: 0.001 to 0.015%
P is an unavoidable impurity element, which deteriorates weldability and also deteriorates HIC resistance by increasing the hardness of the central segregated portion. If it exceeds 0.015%, the tendency becomes remarkable, so the upper limit is set to 0.015%. It is preferably 0.008% or less. The lower the content, the better, but from the viewpoint of refining cost, it should be 0.001% or more.
S:0.0002〜0.0015%
Sは、不可避不純物元素であり、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が、0.0015%までは許容される。含有量は低いほどよいが、精錬コストの観点から0.0002%以上とする。S: 0.0002 to 0.0015%
S is an unavoidable impurity element and is preferably a small amount because it becomes an MnS inclusion in steel and deteriorates HIC resistance, but up to 0.0015% is allowed. The lower the content, the better, but from the viewpoint of refining cost, it should be 0.0002% or more.
Al:0.01〜0.08%
Alは、脱酸剤として添加するが、0.01%未満では添加効果がなく、一方、0.08%を超えると鋼の清浄度が低下し、靱性が劣化するため、Al量は0.01〜0.08%の範囲に限定する。Al: 0.01 to 0.08%
Al is added as an antacid, but if it is less than 0.01%, there is no effect of addition, while if it exceeds 0.08%, the cleanliness of the steel is lowered and the toughness is deteriorated, so that the amount of Al is 0. Limited to the range of 01 to 0.08%.
Mo:0.01〜0.50%
Moは、靭性の改善と強度の上昇に有効な元素であり、硫化水素分圧によらず耐SSCC性の向上に有効な元素である。この効果を得るには0.01%以上を含有することが必要であり、0.10%以上を含有することが好ましい。一方で、含有量が多すぎると、焼入れ性が過剰になるため、後述する転位密度が高くなり、耐SSCC性が劣化する。また、溶接性も劣化する。このため、Mo量は0.50%以下とし、好ましくは0.40%以下とする。Mo: 0.01-0.50%
Mo is an element effective for improving toughness and increasing strength, and is an element effective for improving SSCC resistance regardless of the partial pressure of hydrogen sulfide. In order to obtain this effect, it is necessary to contain 0.01% or more, and preferably 0.10% or more. On the other hand, if the content is too large, the hardenability becomes excessive, so that the dislocation density described later becomes high and the SSCC resistance deteriorates. Weldability also deteriorates. Therefore, the amount of Mo is 0.50% or less, preferably 0.40% or less.
Ca:0.0005〜0.005%
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、0.0005%未満ではその添加効果が十分でない。一方、0.005%を超えた場合、効果が飽和するだけでなく、鋼の清浄度の低下により耐HIC性を劣化させるので、Ca量は0.0005〜0.005%の範囲に限定する。Ca: 0.0005 to 0.005%
Ca is an element effective for improving HIC resistance by controlling the morphology of sulfide-based inclusions, but its addition effect is not sufficient if it is less than 0.0005%. On the other hand, if it exceeds 0.005%, not only the effect is saturated but also the HIC resistance is deteriorated due to the decrease in the cleanliness of the steel, so the Ca amount is limited to the range of 0.0005 to 0.005%. ..
Nb:0.005〜0.1%およびTi:0.005〜0.1%のうちから選んだ1種以上
NbおよびTiはいずれも、鋼板の強度および靭性を高めるために有効な元素である。各元素とも、含有量が0.005%未満ではその添加効果に乏しく、一方0.1%を超えると溶接部の靭性が劣化する。よって、NbおよびTiの少なくとも1種を、各々0.005〜0.1%の範囲で添加するものとする。One or more selected from Nb: 0.005 to 0.1% and Ti: 0.005 to 0.1% Nb and Ti are both effective elements for increasing the strength and toughness of the steel sheet. .. If the content of each element is less than 0.005%, the effect of adding the element is poor, while if it exceeds 0.1%, the toughness of the welded portion deteriorates. Therefore, at least one of Nb and Ti shall be added in the range of 0.005 to 0.1%, respectively.
以上、本開示の基本成分について説明したが、本開示の成分組成は、鋼板の強度や靱性の一層の改善のために、Cu,NiおよびCrのうちから選んだ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 present disclosure have been described above, but the component composition of the present disclosure includes one or more selected from Cu, Ni and Cr in the following range in order to further improve the strength and toughness of the steel sheet. Can be arbitrarily contained in.
Cu:0.50%以下
Cuは、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0.05%以上を含有することが好ましいが、含有量が多すぎると溶接性が劣化するため、Cuを添加する場合は0.50%を上限とする。Cu: 0.50% or less Cu is an element effective for improving toughness and increasing strength, and it is preferable to contain 0.05% or more in order to obtain this effect, but if the content is too large, welding is performed. Since the property deteriorates, the upper limit is 0.50% when Cu is added.
Ni:0.10%以下
Niは、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0.01%以上を含有することが好ましいが、0.10%を超えて添加すると、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を生成しやすくするため、Niを添加する場合は0.10%を上限とする。好ましくは、0.02%以下とする。Ni: 0.10% or less Ni is an element effective for improving toughness and increasing strength, and it is preferable to contain 0.01% or more in order to obtain this effect, but it exceeds 0.10%. When Ni is added, the upper limit is 0.10% when Ni is added in order to facilitate the formation of fine cracks called Fisher in an environment where the partial pressure of hydrogen sulfide is less than 1 bar. Preferably, it is 0.02% or less.
Cr:0.50%以下
Crは、Mnと同様、低C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0.05%以上を含有することが好ましいが、含有量が多すぎると、焼入れ性が過剰になるため、後述する転位密度が高くなり、耐SSCC性が劣化する。また、溶接性も劣化する。このため、Crを添加する場合は0.50%を上限とする。Cr: 0.50% or less Cr is an element effective for obtaining sufficient strength even at low C like Mn, and it is preferable to contain 0.05% or more in order to obtain this effect. If the amount is too large, the hardenability becomes excessive, so that the dislocation density described later becomes high and the SSCC resistance deteriorates. Weldability also deteriorates. Therefore, when Cr is added, the upper limit is 0.50%.
本開示の成分組成は、さらに、V,Zr,MgおよびREMのうちから選んだ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 of the present disclosure may further optionally contain one or more selected from V, Zr, Mg and REM within the following range.
V:0.005〜0.1%、Zr:0.0005〜0.02%、Mg:0.0005〜0.02%およびREM:0.0005〜0.02%のうちから選んだ1種以上
Vは、鋼板の強度および靭性を高めるために任意に添加することができる元素である。含有量が0.005%未満ではその添加効果に乏しく、一方0.1%を超えると溶接部の靭性が劣化するので、添加する場合は0.005〜0.1%の範囲とするのが好ましい。Zr,MgおよびREMは、結晶粒微細化を通じて靭性を高めたり、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添加することができる元素である。これらの元素は、いずれも、含有量が0.0005%未満ではその添加効果に乏しく、一方0.02%を超えるとその効果が飽和するので、添加する場合はいずれも0.0005〜0.02%の範囲とするのが好ましい。One selected from V: 0.005-0.1%, Zr: 0.0005-0.02%, Mg: 0.0005-0.02% and REM: 0.0005-0.02% As described above, V is an element that can be arbitrarily added in order to increase the strength and toughness of the steel sheet. If the content is less than 0.005%, the effect of addition is poor, while if it exceeds 0.1%, the toughness of the weld deteriorates. Therefore, when adding, the range should be 0.005 to 0.1%. preferable. Zr, Mg and REM are elements that can be arbitrarily added to increase toughness through grain refinement and to increase crack resistance through control of inclusion properties. When the content of each of these elements is less than 0.0005%, the effect of adding them is poor, while when the content exceeds 0.02%, the effect is saturated. Therefore, when they are added, they are all 0.0005-0. It is preferably in the range of 02%.
本開示は、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耐SSCC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性を同時に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、添加しない元素は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。This disclosure discloses a technique for improving the SSCC resistance of a high-strength steel pipe using a high-strength steel plate for a sour line pipe, but it goes without saying that the sour resistance is HIC resistance. At the same time, it is necessary to be satisfied, and for example, the CP value obtained by the following equation (1) is preferably 1.00 or less. In addition, 0 may be substituted for the element which is not added.
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 (mass%) of the X element in the steel.
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、HIC試験での割れ発生を抑制す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。 Here, the CP value is an equation devised to estimate the material of the central segregation portion from the content of each alloy element, and the higher the CP value of the above equation (1), the higher the component concentration of the central segregation portion. Increases, and the hardness of the central segregated portion increases. Therefore, by setting the CP value obtained in the above equation (1) to 1.00 or less, it is possible to suppress the occurrence of cracks in the HIC test. Further, the lower the CP value, the lower the hardness of the central segregated portion. Therefore, when higher HIC resistance is required, the upper limit thereof may be 0.95.
なお、上記した元素以外の残部は、Feおよ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、N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0.007%以下、好ましくは0.006%以下であれば、本発明においては許容される。 The balance other than the above-mentioned elements consists of Fe and unavoidable impurities. However, the inclusion of other trace elements is not hindered as long as the effects of the present invention are not impaired. For example, N is an element unavoidably contained in steel, but if the content thereof is 0.007% or less, preferably 0.006% or less, it is acceptable in the present invention.
[鋼板の組織]
次に、本開示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。引張強さが520MPa以上の高強度化を図るために、鋼組織は、ベイナイト組織とする必要がある。特に、表層部は、マルテンサイトや島状マルテンサイト(MA)等の硬質相が生成した場合、表層硬さが上昇し、鋼板内の硬さのばらつきが増大して材質均一性が阻害される。表層硬さの上昇を抑制するために、表層部の鋼組織についてはベイナイト組織とする。表層部以外の部位もベイナイト組織であり、当該部位を代表して板厚中央での組織がベイナイト組織であればよい。ここで、ベイナイト組織は、変態強化に寄与する加速冷却時あるいは加速冷却後に変態するベイニティックフェライトまたはグラニュラーフェライトと称される組織を含むものとする。ベイナイト組織中に、フェライトやマルテンサイト、パーライト、島状マルテンサイト、残留オーステナイトなどの異種組織が混在すると、強度の低下や靭性の劣化、表層硬さの上昇などが生じるため、ベイナイト相以外の組織分率は少ない程良い。ただし、ベイナイト相以外の組織の体積分率が十分に低い場合には、それらの影響が無視できるので、ある程度の量であれば許容される。具体的に、本開示では、ベイナイト以外の鋼組織(フェライト、マルテンサイト、パーライト、島状マルテンサイト、残留オーステナイト等)の合計が体積分率で5%未満であれば、大きな影響がないので許容されるものとする。[Structure of steel plate]
Next, the steel structure of the high-strength steel sheet for sour-resistant pipes of the present disclosure will be described. The steel structure needs to have a bainite structure in order to increase the tensile strength to 520 MPa or more. In particular, when a hard phase such as martensite or island-shaped martensite (MA) is generated in the surface layer portion, the surface layer hardness increases, the variation in hardness in the steel sheet increases, and the material uniformity is impaired. .. In order to suppress the increase in surface hardness, the steel structure of the surface layer shall be a bainite structure. A portion other than the surface layer portion is also a bainite structure, and the structure at the center of the plate thickness may be a bainite structure on behalf of the portion. Here, the bainite structure includes a structure called bainitic ferrite or granular ferrite that transforms during accelerated cooling or after accelerated cooling, which contributes to strengthening the transformation. When heterogeneous structures such as ferrite, martensite, pearlite, island-like martensite, and retained austenite are mixed in the bainite structure, the strength decreases, the toughness deteriorates, and the surface hardness increases. Therefore, structures other than the bainite phase The smaller the fraction, the better. However, if the volume fraction of the tissue other than the bainite phase is sufficiently low, those effects can be ignored, and a certain amount is acceptable. Specifically, in the present disclosure, if the total volume fraction of steel structures other than bainite (ferrite, martensite, pearlite, island-like martensite, retained austenite, etc.) is less than 5%, there is no significant effect and is acceptable. It shall be done.
また、ベイナイト組織にも冷却速度に応じた種々の形態があるが、本開示においては、鋼板の極表層部の組織、具体的には鋼板表面下0.25mmの鋼組織を、転位密度1.0×1014〜7.0×1014(m−2)のベイナイト組織とすることが肝要である。造管後のコーティング過程において転位密度が減少するため、鋼板表面下0.25mmの転位密度が7.0×1014(m−2)以下であれば、時効硬化による硬さの上昇代を最小限に抑えることができる。逆に、鋼板表面下0.25mmの転位密度が7.0×1014(m−2)を超えると、造管後のコーティング過程において転位密度が減少せず、時効硬化で硬度が大きく上昇して耐SSCC性を劣化させる。造管後に良好な耐SSCC性を得るために好ましい転位密度の範囲は6.0×1014(m−2)以下である。一方、鋼板表面下0.25mmの転位密度が1.0×1014(m−2)未満では鋼板として強度を維持できなくなる。X65グレードの強度を確保するため、2.0×1014(m−2)以上の転位密度を有することが好ましい。なお、本開示の高強度鋼板においては、鋼板表面下0.25mmの鋼組織における転位密度が上記範囲であれば、鋼板表面から深さ0.25mmの範囲の極表層部も同等の転位密度を有し、その結果、上記耐SSCC性向上の効果が得られるものである。Further, the bainite structure also has various forms depending on the cooling rate. In the present disclosure, the structure of the polar surface layer of the steel sheet, specifically, the steel structure 0.25 mm below the surface of the steel sheet is defined as the dislocation density 1. It is important to have a bainite structure of 0 × 10 14 to 7.0 × 10 14 (m- 2 ). Since the dislocation density decreases in the coating process after pipe formation, if the dislocation density of 0.25 mm below the surface of the steel sheet is 7.0 × 10 14 (m- 2 ) or less, the increase in hardness due to age hardening is minimized. It can be suppressed to the limit. On the contrary, when the dislocation density of 0.25 mm below the surface of the steel sheet exceeds 7.0 × 10 14 (m- 2 ), the dislocation density does not decrease in the coating process after pipe formation, and the hardness increases significantly by age hardening. It deteriorates SSCC resistance. The preferred range of dislocation densities for obtaining good SSCC resistance after pipe formation is 6.0 × 10 14 (m- 2 ) or less. On the other hand, if the dislocation density of 0.25 mm below the surface of the steel sheet is less than 1.0 × 10 14 (m- 2 ), the strength of the steel sheet cannot be maintained. In order to secure the strength of X65 grade, it is preferable to have a dislocation density of 2.0 × 10 14 (m- 2 ) or more. In the high-strength steel sheet of the present disclosure, if the dislocation density in the steel structure 0.25 mm below the surface of the steel sheet is within the above range, the dislocation density is the same for the polar surface layer portion in the range of 0.25 mm deep from the surface of the steel sheet. As a result, the effect of improving the SSCC resistance can be obtained.
なお、鋼板表面下0.25mmでの転位密度を7.0×1014(m−2)以下とすると、表面下0.25mmでのHV0.1が230以下となる。鋼管の耐SSCC性を確保する観点から、鋼板の表層硬さを抑制することが重要であるが、鋼板の表面下0.25mmでのHV0.1を230以下にすることで、造管後250℃で1時間のコーティング熱処理過程を経たのちの、表面下0.25mmでのHV0.1を260以下に抑えることができ、耐SSCC性を確保することができる。If the dislocation density at 0.25 mm below the surface of the steel sheet is 7.0 × 10 14 (m- 2 ) or less, the HV 0.1 at 0.25 mm below the surface is 230 or less. From the viewpoint of ensuring the SSCC resistance of the steel pipe, it is important to suppress the surface hardness of the steel sheet. However, by setting the HV 0.1 at 0.25 mm below the surface of the steel sheet to 230 or less, 250 after pipe formation. After undergoing a coating heat treatment process at ° C. for 1 hour, HV0.1 at 0.25 mm below the surface can be suppressed to 260 or less, and SSCC resistance can be ensured.
また、本開示の高強度鋼板では、鋼板表面下0.25mmにおけるビッカース硬さのばらつきが、標準偏差をσとしたときに3σで30HV以下であることも重要である。鋼板表面下0.25mmにおけるビッカース硬さを測定した際の3σが30HV超えの場合、鋼板の極表層における硬さばらつき、すなわち、極表層に局所的な高硬度部位が存在することにより、当該部位を起点とした耐SSCC性の劣化が生じるからである。なお、標準偏差σを求める際、100点以上、ビッカース硬さを測定しておくことが好ましい。 Further, in the high-strength steel sheet of the present disclosure, it is also important that the variation in Vickers hardness at 0.25 mm below the surface of the steel sheet is 30 HV or less at 3σ when the standard deviation is σ. When 3σ when measuring the Vickers hardness at 0.25 mm below the surface of the steel sheet exceeds 30 HV, the hardness variation in the polar surface layer of the steel sheet, that is, the presence of a local high hardness portion on the polar surface layer causes the portion. This is because the SSCC resistance deteriorates starting from. When determining the standard deviation σ, it is preferable to measure the Vickers hardness at 100 points or more.
また、同様の考え方で、本開示の高強度鋼板では、板厚方向のビッカース硬さのばらつきが、標準偏差をσとしたときに3σで30HV以下であることも重要である。 Further, in the same way of thinking, in the high-strength steel sheet of the present disclosure, it is also important that the variation of Vickers hardness in the plate thickness direction is 30 HV or less at 3σ when the standard deviation is σ.
本開示の高強度鋼板は、API 5LのX60グレード以上の強度を有する鋼管用の鋼板であるので、520MPa以上の引張強さを有するものとする。 Since the high-strength steel sheet of the present disclosure is a steel sheet for steel pipes having a strength of X60 grade or higher of API 5L, it is assumed to have a tensile strength of 520 MPa or higher.
[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および製造条件について、具体的に説明する。本開示の製造方法は、上記成分組成を有する鋼片の加熱したのち、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い、その後鋼板を再加熱する。[Production method]
Hereinafter, the manufacturing method and manufacturing conditions for manufacturing the high-strength steel sheet for sour-resistant pipes will be specifically described. In the manufacturing method of the present disclosure, a steel piece having the above-mentioned composition is heated, then hot-rolled to obtain a steel sheet, and then the steel sheet is controlledly cooled under predetermined conditions, and then the steel sheet is reheated.
〔スラブ加熱温度〕
スラブ加熱温度:1000〜1300℃
スラブ加熱温度が1000℃未満では、炭化物の固溶が不十分で必要な強度が得られず、一方1300℃を超えると靭性が劣化するため、スラブ加熱温度は1000〜1300℃とする。なお、この温度は加熱炉の炉内温度であり、スラブは中心部までこの温度に加熱されるものとする。[Slab heating temperature]
Slab heating temperature: 1000-1300 ° C
If the slab heating temperature is less than 1000 ° C., the solid solution of the carbide is insufficient and the required strength cannot be obtained. On the other hand, if the slab heating temperature exceeds 1300 ° C., the toughness deteriorates. Therefore, the slab heating temperature is set to 1000 to 1300 ° C. It should be noted that this temperature is the temperature inside the heating furnace, and the slab is heated to this temperature up to the center.
〔圧延終了温度〕
熱間圧延工程において、高い母材靱性を得るには、圧延終了温度は低いほどよいが、その反面、圧延能率が低下するため、鋼板表面温度における圧延終了温度は、必要な母材靱性と圧延能率を勘案して設定する必要がある。強度および耐HIC性を向上させる観点からは、圧延終了温度を、鋼板表面温度でAr3変態点以上とすることが好ましい。ここで、Ar3変態点とは、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。また、高い母材靱性を得るためにはオーステナイト未再結晶温度域に相当する950℃以下の温度域での圧下率を60%以上とすることが望ましい。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(℃)=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。[Rolling end temperature]
In the hot rolling process, in order to obtain high base metal toughness, the lower the rolling end temperature, the better, but on the other hand, the rolling efficiency decreases, so the rolling end temperature at the steel sheet surface temperature is the required base material toughness and rolling. It is necessary to set in consideration of efficiency. From the viewpoint of improving the strength and HIC resistance, it is preferable that the rolling end temperature is the Ar 3 transformation point or higher at the steel sheet surface temperature. Here, the Ar 3 transformation point means the ferrite transformation start temperature during cooling, and can be obtained from the components of steel, for example, by the following equation. Further, in order to obtain high base metal toughness, it is desirable that the reduction rate in the temperature range of 950 ° C. or lower, which corresponds to the austenite unrecrystallized temperature range, be 60% or more. The surface temperature of the steel sheet can be measured with a radiation thermometer or the like.
Ar 3 (° C.) = 910-310 [% C] -80 [% Mn] -20 [% Cu] -15 [% Cr] -55 [% Ni] -80 [% Mo]
However, [% X] indicates the content (mass%) of the X element in the steel.
〔制御冷却の冷却開始温度〕
冷却開始温度:鋼板表面温度で(Ar3−10℃)以上
冷却開始時の鋼板表面温度が低いと、制御冷却前のフェライト生成量が多くなり、特にAr3変態点からの温度降下量が10℃を超えると体積分率で5%を超えるフェライトが生成して、強度低下が大きくなると共に耐HIC性が劣化するため、冷却開始時の鋼板表面温度は(Ar3−10℃)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。[Cooling start temperature for controlled cooling]
Cooling start temperature: the steel plate surface temperature (Ar 3 -10 ° C.) or higher cooling starting steel sheet surface temperature is low, the number of ferrite amount before controlled cooling, in particular the amount of temperature drop of from Ar 3 transformation point is 10 ° C. the generated ferrite exceeding 5% by volume fraction exceeds, the strength decreases to HIC resistance is deteriorated with increases, the steel sheet surface temperature of the cooling at the start and (Ar 3 -10 ℃) or higher .. The surface temperature of the steel sheet at the start of cooling is equal to or lower than the rolling end temperature.
〔制御冷却の冷却速度〕
高強度化を図りつつ、鋼板内の硬さのばらつきを低減し、材質均一性を向上させるためには、表層部の冷却速度と鋼板内の平均冷却速度を制御することが重要である。特に、鋼板表面下0.25mmにおける転位密度と3σを既述の範囲とするためには、鋼板表面下0.25mmにおける冷却速度を制御する必要がある。[Cooling rate of controlled cooling]
It is important to control the cooling rate of the surface layer and the average cooling rate in the steel sheet in order to reduce the variation in hardness in the steel sheet and improve the material uniformity while increasing the strength. In particular, in order to set the dislocation density and 3σ at 0.25 mm below the surface of the steel sheet within the above-mentioned ranges, it is necessary to control the cooling rate at 0.25 mm below the surface of the steel sheet.
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:50℃/s以下
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が50℃/sを超えると、鋼板表面下0.25mmにおける転位密度7.0×1014(m−2)超えとなってしまう。その結果、鋼板表面下0.25mmのHV0.1が230を超え、造管後のコーティング過程を経たのち、表面下0.25mmでのHV0.1が260を超え、鋼管の耐SSCC性が劣化する。そのため、当該平均冷却速度は50℃/s以下とする。好ましくは45℃/s以下、より好ましくは40℃/s以下である。当該平均冷却速度の下限は特に限定されないが、冷却速度が過度に小さくなると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、20℃/s以上とすることが好ましい。Average cooling rate from 750 ° C to 550 ° C at 0.25 mm below the surface of the steel sheet: 50 ° C / s or less Average cooling rate from 750 ° C to 550 ° C at 0.25 mm below the surface of the steel sheet is 50 ° C / If it exceeds s, the dislocation density at 0.25 mm below the surface of the steel sheet exceeds 7.0 × 10 14 (m- 2 ). As a result, the HV0.1 of 0.25 mm below the surface of the steel sheet exceeds 230, and after the coating process after pipe making, the HV0.1 of 0.25 mm below the surface exceeds 260, and the SSCC resistance of the steel pipe deteriorates. To do. Therefore, the average cooling rate is set to 50 ° C./s or less. It is preferably 45 ° C./s or less, more preferably 40 ° C./s or less. The lower limit of the average cooling rate is not particularly limited, but if the cooling rate becomes excessively small, ferrite and pearlite are generated and the strength becomes insufficient. Therefore, from the viewpoint of preventing this, it is preferably 20 ° C./s or more.
鋼板平均温度で750℃から550℃までの平均冷却速度:15℃/s以上
鋼板平均温度で750℃から550℃までの平均冷却速度が15℃/s未満では、ベイナイト組織が得られずに強度低下や耐HIC性の劣化が生じる。このため、鋼板平均温度での冷却速度は15℃/s以上とする。鋼板強度と硬さのばらつきの観点からは、鋼板平均の冷却速度は20℃/s以上とすることが好ましい。当該平均冷却速度の上限は特に限定されないが、低温変態生成物が過剰に生成しないように、80℃/s以下とすることが好ましい。Average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature: 15 ° C / s or more If the average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature is less than 15 ° C / s, bainite structure cannot be obtained and strength Deterioration and deterioration of HIC resistance occur. Therefore, the cooling rate at the average temperature of the steel sheet is set to 15 ° C./s or more. From the viewpoint of variations in steel sheet strength and hardness, the average cooling rate of the steel sheet is preferably 20 ° C./s or more. The upper limit of the average cooling rate is not particularly limited, but it is preferably 80 ° C./s or less so that the low temperature transformation product is not excessively produced.
鋼板表面下0.25mmにおける鋼板温度で550℃から冷却停止時の温度まで平均冷却速度:150℃/s以上
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、安定した核沸騰状態での冷却が必要であり、水量密度の上昇が不可欠である。鋼板表面下0.25mmにおける鋼板温度で550℃から冷却停止時の温度まで平均冷却速度が150℃/s未満の場合、核沸騰状態での冷却にならず、鋼板の極表層部で硬さばらつきが生じ、鋼板表面下0.25mmにおける3σが30HVを超えてしまい、その結果耐SSCC性が劣化する。そのため、当該平均冷却速度は150℃/s以上とする。好ましくは170℃/s以上である。当該平均冷却速度の上限は特に限定されないが、設備上の制約から、250℃/s以下とすることが好ましい。Average cooling rate from 550 ° C to the temperature at which cooling is stopped at a steel plate temperature of 0.25 mm below the surface of the steel plate: 150 ° C / s or more Stable nuclear boiling for cooling at a steel plate temperature of 0.25 mm below the surface of the steel plate of 550 ° C or less Cooling in the state is required, and an increase in water density is essential. If the average cooling rate is less than 150 ° C / s from 550 ° C to the temperature at which cooling is stopped at a steel plate temperature of 0.25 mm below the surface of the steel sheet, cooling does not occur in the nucleate boiling state, and the hardness varies at the extreme surface layer of the steel sheet. , And 3σ at 0.25 mm below the surface of the steel sheet exceeds 30 HV, resulting in deterioration of SSCC resistance. Therefore, the average cooling rate is set to 150 ° C./s or higher. It is preferably 170 ° C./s or higher. The upper limit of the average cooling rate is not particularly limited, but is preferably 250 ° C./s or less due to equipment restrictions.
なお、鋼板表面下0.25mmおよび鋼板平均温度は、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方向の温度の平均値を本明細書における「鋼板平均温度」とする。 Although the 0.25 mm below the surface of the steel plate and the average temperature of the steel plate cannot be physically measured directly, the surface temperature at the start of cooling and the surface temperature at the target cooling stop measured by a radiation thermometer are also included. In addition, for example, the temperature distribution in the plate thickness cross section can be obtained in real time by difference calculation using a process computer. The temperature at 0.25 mm below the surface of the steel sheet in the temperature distribution is defined as "the temperature of the steel sheet at 0.25 mm below the surface of the steel sheet" in the present specification, and the average value of the temperatures in the thickness direction in the temperature distribution is the "steel plate" in the present specification. "Average temperature".
〔冷却停止温度〕
冷却停止温度:鋼板平均温度で250〜550℃
圧延終了後、制御冷却でベイナイト変態の温度域である250〜550℃まで急冷することにより、ベイナイト相を生成させる。冷却停止温度が550℃を超えると、ベイナイト変態が不完全であり、十分な強度が得られない。また、冷却停止温度が250℃未満では、表層部の硬さ上昇が著しくなり、鋼板表面下0.25mmでの転位密度7.0×1014(m−2)超えとなるため、耐SSCC性が劣化する。また、中心偏析部の硬さも高くなり、耐HIC性も劣化する。そこで、鋼板内の材質均一性の劣化を抑制するため、制御冷却の冷却停止温度は鋼板平均温度で250〜550℃とする。[Cooling stop temperature]
Cooling stop temperature: 250 to 550 ° C at average steel sheet temperature
After the rolling is completed, the bainite phase is formed by quenching to 250 to 550 ° C., which is the temperature range of bainite transformation, by controlled cooling. If the cooling stop temperature exceeds 550 ° C., the bainite transformation is incomplete and sufficient strength cannot be obtained. Further, when the cooling stop temperature is less than 250 ° C., the hardness of the surface layer portion increases remarkably, and the dislocation density exceeds 7.0 × 10 14 (m- 2 ) at 0.25 mm below the surface of the steel sheet. Therefore, SSCC resistance Deteriorates. In addition, the hardness of the central segregation portion becomes high, and the HIC resistance also deteriorates. Therefore, in order to suppress the deterioration of the material uniformity in the steel sheet, the cooling stop temperature of the control cooling is set to 250 to 550 ° C. as the average temperature of the steel sheet.
〔再加熱条件〕
再加熱温度:鋼板平均温度で、冷却停止温度超え、かつ450〜600℃
本開示では、圧延終了後、制御冷却でベイナイト変態の温度域である250〜550℃まで急冷した後、鋼板をオンライン再加熱に供する。当該再加熱によって、鋼板を冷却停止温度よりも高い温度にしてベイナイト相を焼戻し軟化させることで、耐SSCC性を向上することができる。ただし、再加熱温度が450℃未満では表層軟化効果が不十分であり、再加熱温度が600℃を超えると強度低下およびDWTT(Drop Weight Tear Test:落重引裂試験)特性の劣化を招くため、再加熱温度は450〜600℃とする。[Reheating conditions]
Reheating temperature: The average temperature of the steel sheet, which exceeds the cooling stop temperature and is 450 to 600 ° C.
In the present disclosure, after the rolling is completed, the steel sheet is subjected to online reheating after being rapidly cooled to 250 to 550 ° C., which is a temperature range of bainite transformation by controlled cooling. By the reheating, the temperature of the steel sheet is set higher than the cooling stop temperature to temper and soften the bainite phase, so that the SSCC resistance can be improved. However, if the reheating temperature is less than 450 ° C, the surface softening effect is insufficient, and if the reheating temperature exceeds 600 ° C, the strength decreases and the DWTT (Drop Weight Tear Test) characteristics deteriorate. The reheating temperature is 450 to 600 ° C.
本開示においては、製造効率や熱処理に要する燃料コストを削減する観点から、制御冷却の停止後、直ちに再加熱を実施することが好ましい。ここで、制御冷却停止後、直ちに再加熱を実施するとは、制御冷却停止後、120秒以内に再加熱することを言う。 In the present disclosure, from the viewpoint of reducing the production efficiency and the fuel cost required for the heat treatment, it is preferable to carry out reheating immediately after the control cooling is stopped. Here, to perform reheating immediately after the control cooling is stopped means to reheat within 120 seconds after the control cooling is stopped.
表層軟化のためには、再加熱の際に、再加熱開始温度、すなわち、冷却停止温度より50℃以上昇温することが望ましい。また、再加熱後の冷却は、基本的に、空冷とすることが好ましい。 For surface softening, it is desirable to raise the temperature by 50 ° C. or higher from the reheating start temperature, that is, the cooling stop temperature at the time of reheating. Further, the cooling after reheating is basically preferably air cooling.
[高強度鋼管]
本開示の高強度鋼板を、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。[High-strength steel pipe]
The high-strength steel sheet of the present disclosure is formed into a tubular shape by press bend forming, roll forming, UOE forming, etc., and then the butt portion is welded to provide excellent material uniformity in the steel sheet suitable for transporting crude oil and natural gas. High-strength steel pipes for sour-resistant pipes (UOE steel pipes, welded steel pipes, spiral steel pipes, etc.) can be manufactured.
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、内面溶接およ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。 For example, in a UOE steel pipe, the end portion of a steel sheet is grooved, formed into a steel pipe shape by C press, U press, and O press, and then the butt portion is seam welded by inner surface welding and outer surface welding, and further, if necessary. Manufactured through a pipe expansion process. The welding method may be any method as long as sufficient joint strength and joint toughness can be obtained, but from the viewpoint of excellent welding quality and manufacturing efficiency, submerged arc welding is preferably used.
表1に示す成分組成になる鋼(鋼種A〜M)を、連続鋳造法によりスラブとし、表2に示す温度に加熱したのち、表2に示す圧延終了温度および圧下率の熱間圧延をして、表2に示す板厚の鋼板とした。その後、鋼板に対して、表2に示す条件下で水冷型の制御冷却装置を用いて制御冷却を行った。その後直ちに、オンライン誘導加熱装置を用いて、鋼板平均温度が表2の「再加熱温度」となるように、鋼板を再加熱した。 Steels (steel grades A to M) having the composition shown in Table 1 are made into slabs by a continuous casting method, heated to the temperatures shown in Table 2, and then hot-rolled at the rolling end temperature and rolling reduction ratio shown in Table 2. The steel plate with the thickness shown in Table 2 was used. Then, the steel sheet was controlled-cooled using a water-cooled control cooling device under the conditions shown in Table 2. Immediately thereafter, the steel sheet was reheated using an online induction heating device so that the average temperature of the steel sheet became the “reheating temperature” in Table 2.
[組織の特定]
得られた鋼板のミクロ組織を、光学顕微鏡および走査型電子顕微鏡により観察した。鋼板表面下0.25mmの位置での組織と、板厚中央での組織を、表2に示す。[Organization identification]
The microstructure of the obtained steel sheet was observed with an optical microscope and a scanning electron microscope. Table 2 shows the structure at a position 0.25 mm below the surface of the steel sheet and the structure at the center of the sheet thickness.
[引張強度の測定]
圧延方向に直角な方向の全厚試験片を引張試験片として引張試験を行い、引張強度を測定した。結果を表2に示す。[Measurement of tensile strength]
A tensile test was performed using a full-thickness test piece in the direction perpendicular to the rolling direction as a tensile test piece, and the tensile strength was measured. The results are shown in Table 2.
[ビッカース硬さの測定]
圧延方向に直角な断面について、JIS Z 2244に準拠して、鋼板表面下0.25mmの位置において100点のビッカース硬さ(HV0.1)を測定し、その平均値および標準偏差σを求めた。平均値と3σの値を、鋼板表面下0.25mmでの平均硬さ及び硬さばらつきとして、表2に示す。また、同じく圧延方向に直角な断面について、JIS Z 2244に準拠して、鋼板表面下0.25mmの位置から板厚方向に0.5mmピッチで、鋼板の反対側の表面下0.25mmの位置まで、ビッカース硬さ(HV0.1)を測定し、その標準偏差σを求めた。3σの値を、板厚方向の硬さばらつきとして、表2に示す。なお、板厚方向のビッカース硬さ測定を0.5mmピッチで続けた場合に、測定開始側の反対側の鋼板表面下1.25mm以内の位置で最初の測定の後は、測定開始側の反対側の鋼板表面下0.25mm位置のビッカース硬度を測定して測定終了とする。ここで、通常用いられるHV10に代えてHV0.1で測定したのは、HV0.1で測定することにより圧痕が小さくなるので、より表面に近い位置での硬さ情報や、よりミクロ組織に敏感な硬さ情報をすることが可能となるからである。[Measurement of Vickers hardness]
For the cross section perpendicular to the rolling direction, 100 points of Vickers hardness (HV0.1) were measured at a position 0.25 mm below the surface of the steel sheet in accordance with JIS Z 2244, and the average value and standard deviation σ were obtained. .. Table 2 shows the average value and the value of 3σ as the average hardness and the hardness variation at 0.25 mm below the surface of the steel sheet. Similarly, with respect to the cross section perpendicular to the rolling direction, in accordance with JIS Z 2244, the position is 0.25 mm below the surface of the steel sheet at a pitch of 0.5 mm in the thickness direction from the position 0.25 mm below the surface of the steel sheet. The Vickers hardness (HV0.1) was measured up to, and the standard deviation σ was determined. The value of 3σ is shown in Table 2 as the hardness variation in the plate thickness direction. When the Vickers hardness measurement in the plate thickness direction is continued at a pitch of 0.5 mm, after the first measurement at a position within 1.25 mm below the surface of the steel plate on the opposite side of the measurement start side, the opposite side of the measurement start side. The Vickers hardness at a position 0.25 mm below the surface of the steel plate on the side is measured to complete the measurement. Here, the measurement with HV0.1 instead of the normally used HV10 is because the indentation becomes smaller by the measurement with HV0.1, so that the hardness information at a position closer to the surface and more sensitive to the microstructure are obtained. This is because it is possible to provide various hardness information.
[転位密度]
平均的な硬度を有する位置からX線回折用のサンプルを採取、サンプル表面を研磨してスケールを除去し、鋼板表面下0.25mmの位置においてX線回折測定を行った。転位密度はX線回折測定の半価幅βから求める歪みから換算する手法を用いた。通常のX線回折により得られる回折強度曲線では、波長の異なるKα1線とKα2線の2つが重なっているため、Rachingerの方法により分離する。歪みの抽出には、以下に示すWilliamsson−Hall法を用いる。半価幅の広がりは結晶子のサイズDとひずみεが影響し、両因子の和として次式で計算できる。β=β1+β2=(0.9λ/(D×cosθ))+2ε×tanθとなる。さらにこの式を変形し、βcosθ/λ=0.9λ/D+2ε×sinθ/λとなる。sinθ/λに対してβcosθ/λをプロットすることにより、直線の傾きからひずみεが算出される。なお、算出に用いる回折線は(110)、(211)、および(220)とする。ひずみεから転位密度の換算はρ=14.4ε2/b2を用いた。なお、θはX線回折のθ‐2θ法より算出されるピーク角度を意味し、λはX線回折で使用するX線の波長を意味する。bはFe(α)のバーガース・ベクトルで、本実施例においては、0.25nmとした。[Dislocation density]
A sample for X-ray diffraction was taken from a position having average hardness, the surface of the sample was polished to remove scale, and X-ray diffraction measurement was performed at a position 0.25 mm below the surface of the steel sheet. The dislocation density was converted from the strain obtained from the half-value width β of the X-ray diffraction measurement. In the diffraction intensity curve obtained by ordinary X-ray diffraction, since two Kα1 lines and Kα2 lines having different wavelengths overlap, they are separated by the method of Rachinger. The Williamsson-Hall method shown below is used to extract the strain. The spread of the half-value range is affected by the crystallite size D and strain ε, and can be calculated by the following equation as the sum of both factors. β = β1 + β2 = (0.9λ / (D × cosθ)) + 2ε × tanθ. Further modifying this equation, βcosθ / λ = 0.9λ / D + 2ε × sinθ / λ. By plotting βcosθ / λ against sinθ / λ, the strain ε is calculated from the slope of a straight line. The diffraction lines used for the calculation are (110), (211), and (220). For the conversion of the dislocation density from the strain ε, ρ = 14.4 ε 2 / b 2 was used. Note that θ means the peak angle calculated by the θ-2θ method of X-ray diffraction, and λ means the wavelength of X-rays used in X-ray diffraction. b is a Burgers vector of Fe (α), which is 0.25 nm in this example.
[耐SSCC性の評価]
耐SSCC性は、これら各鋼板の一部を用いて造管して評価した。造管は、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、内面および外面の突き合わせ部をサブマージアーク溶接でシーム溶接し、拡管工程を経て製造した。図1に示すように、得られた鋼管から切り出したクーポン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、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。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規格の4点曲げSSCC試験に準拠して行った。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規格の4点曲げSSCC試験に準拠して行った。さらに、NACE規格 TM0177 Solution A溶液を用い、硫化水素分圧:2bar+二酸化炭素分圧:3barについても、EFC16規格の4点曲げSSCC試験に準拠して行った。720時間の浸漬後に、割れが認められない場合を耐SSCC性が良好と判断して○、また割れが発生した場合を不良と判断して×とした。結果を表2に示す。[Evaluation of SSCC resistance]
The SSCC resistance was evaluated by forming a pipe using a part of each of these steel sheets. Pipe making is manufactured through a pipe expansion process in which the end of the steel sheet is grooved, formed into a steel pipe shape by C press, U press, and O press, and then the butt portion of the inner and outer surfaces is seam welded by submerged arc welding. did. As shown in FIG. 1, after flattening the coupon cut out from the obtained steel pipe, a 5 × 15 × 115 mm SSCC test piece was collected from the inner surface of the steel pipe. At this time, the inner surface, which is the surface to be inspected, was left with black skin in order to retain the state of the outermost layer. A stress of 90% of the actual yield strength (0.5% YS) of each steel pipe is applied to the collected SSCC test piece, and using NACE standard TM0177 Solution A solution, hydrogen sulfide partial pressure: 1 bar, EFC 16 standard. The 4-point bending SSCC test was performed in accordance with the above. Similarly, using a NACE standard TM0177 Solution B solution, hydrogen sulfide partial pressure: 0.1 bar + carbon dioxide partial pressure: 0.9 bar was performed in accordance with the EFC16 standard 4-point bending SSCC test. Furthermore, using the NACE standard TM0177 Solution A solution, hydrogen sulfide partial pressure: 2 bar + carbon dioxide partial pressure: 3 bar was also performed in accordance with the 4-point bending SSCC test of the EFC16 standard. When no cracks were observed after 720 hours of immersion, the SSCC resistance was judged to be good, and when cracks occurred, it was judged to be defective and marked as x. The results are shown in Table 2.
[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試験で割れ長さ率(CLR)が15%以下となった場合を良好と判断して○、15%を超えた場合を×とした。結果を表2に示す。[Evaluation of HIC resistance]
The HIC resistance was examined by a 96-hour immersion HIC test at a partial pressure of hydrogen sulfide of 1 bar using a NACE standard TM0177 Solution A solution. In addition, using a NACE standard TM0177 Solution B solution, hydrogen sulfide partial pressure: 0.1 bar + carbon dioxide partial pressure: 0.9 bar was examined by a 96-hour immersion HIC test. The HIC resistance was evaluated as good when the crack length ratio (CLR) was 15% or less in the HIC test, and was evaluated as x when it exceeded 15%. The results are shown in Table 2.
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引張強度:520MPa以上、表面下0.25mm位置とt/2位置ともミクロ組織はベイナイト組織、表面下0.25mmでのHV0.1が230以下、その鋼板を用いて造管した高強度鋼管においてSSCC試験で割れが認められないこと、HIC試験で割れ長さ率(CLR)が15%以下であることとした。 The target range of the present invention is a high-strength steel sheet for sour line pipes with a tensile strength of 520 MPa or more, a bainite structure at both 0.25 mm and t / 2 positions below the surface, and HV0.1 at 0.25 mm below the surface. It was determined that no cracks were observed in the SSCC test and the crack length ratio (CLR) was 15% or less in the HIC test in the high-strength steel pipe formed by using the steel plate.
表2に示したように、No.1〜No.15は、成分組成およ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、鋼板として引張強度:520MPa以上、表面下0.25mm位置とt/2位置ともミクロ組織はベイナイト組織、表面下0.25mmでのHV0.1が230以下であり、その鋼板を用いて造管した高強度鋼管において耐SSCC性および耐HIC性も良好であった。 As shown in Table 2, No. 1-No. Reference numeral 15 denotes an example of the invention in which the component composition and the production conditions satisfy the appropriate range of the present invention. In both cases, the tensile strength of the steel sheet is 520 MPa or more, the microstructure at both the 0.25 mm and t / 2 positions below the surface is a bainite structure, and the HV 0.1 at 0.25 mm below the surface is 230 or less. The high-strength steel pipe produced had good SSCC resistance and HIC resistance.
これに対し、No.16〜No.23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.16は、スラブ加熱温度が低いため、ミクロ組織の均質化と炭化物の固溶が不十分であり低強度であった。No.17は、冷却開始温度が低く、フェライトが析出した層状組織となったため、低強度であるとともに、造管後の耐HIC性が劣化した。No.18は、制御冷却条件が本発明範囲外で、ミクロ組織として板厚中心部でベイナイト組織が得られず、フェライト+パーライト組織となったため、低強度であるとともに、造管後の耐HIC性が劣化した。No.19は、冷却停止温度が低く、表面下0.25mmでの転位密度が高くなって、HV0.1が230を超えたため、造管後の耐SSCC性が劣っていた。また、中心偏析部の硬さも高くなったため耐HIC性も劣化した。No.20およびNo.23は、鋼板表面下0.25mmにおける鋼板温度で750℃から550℃での平均冷却速度が50℃/sを大きく超えたため、表面下0.25mmでの転位密度が高くなって、HV0.1が230を超え、造管後の耐SSCC性が劣っていた。また、No.23では表層部での耐HIC性も劣化した。No.21およびNo.22は、鋼板表面下0.25mmにおける550℃以下での平均冷却速度が150℃/sに満たないため、鋼板の不均一冷却が顕著となり、HV0.1が平均で230以下を満足したものの、硬さばらつきが大きく、局所的に硬さが高い部分を生じたため、造管後の耐SSCC性が劣っていた。No.24〜No.27は、鋼板の成分組成が本発明の範囲外であり、表面下0.25mmでの転位密度が高くなってHV0.1が230を超えたため、造管後の耐SSCC性が劣っていた。また、No.24〜No.27については、中心偏析部の硬さが増加したため、耐HIC性も劣っていた。No.28は、鋼板のNi量が過多であるため、硫化水素分圧の低い環境での耐SSCC性が劣化した。No.29は、鋼板がMoを含まないため、硫化水素分圧2Barという非常に厳しい腐食環境下では耐SSCC性が劣化した。No.30は、再加熱を実施しなかったので表層軟化の効果がなく、硫化水素分圧2Barという非常に厳しい腐食環境下では耐SSCC性が劣化する場合があった。No.31は、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が50℃/sを超えたため、硫化水素分圧2Barという非常に厳しい腐食環境下では耐SSCC性が劣化する場合があった。 On the other hand, No. 16-No. 23 is a comparative example in which the component composition is within the range of the present invention, but the production conditions are outside the range of the present invention. No. In No. 16, since the slab heating temperature was low, the homogenization of the microstructure and the solid solution of carbides were insufficient, and the strength was low. No. No. 17 had a low cooling start temperature and had a layered structure in which ferrite was precipitated, so that the strength was low and the HIC resistance after pipe formation was deteriorated. No. 18 had a low strength and HIC resistance after pipe formation because the controlled cooling conditions were outside the scope of the present invention, a bainite structure could not be obtained at the center of the plate thickness as a microstructure, and a ferrite + pearlite structure was formed. The sex has deteriorated. No. In No. 19, the cooling stop temperature was low, the dislocation density at 0.25 mm below the surface was high, and the HV 0.1 exceeded 230, so that the SSCC resistance after pipe formation was inferior. In addition, the hardness of the central segregated portion was also increased, so that the HIC resistance was also deteriorated. In No. 20 and No. 23, the average cooling rate from 750 ° C. to 550 ° C. at the steel sheet temperature 0.25 mm below the surface of the steel sheet greatly exceeded 50 ° C./s, so that the dislocation density at 0.25 mm below the surface was high. As a result, HV0.1 exceeded 230, and SSCC resistance after pipe making was inferior. In addition, No. In No. 23, the HIC resistance at the surface layer was also deteriorated. In No. 21 and No. 22, since the average cooling rate at 550 mm or less below the surface of the steel sheet is less than 150 ° C./s, uneven cooling of the steel sheet becomes remarkable, and HV0.1 is 230 on average. Although the following was satisfied, the SSCC resistance after pipe formation was inferior because the hardness variation was large and a portion having a locally high hardness was generated. In No. 24 to No. 27, the composition of the steel sheet was out of the range of the present invention, the dislocation density at 0.25 mm below the surface was high, and the HV 0.1 exceeded 230, so that the SSCC resistance after pipe formation was increased. The sex was inferior. In addition, No. 24 to No. With respect to 27, the hardness of the central segregated portion increased, so that the HIC resistance was also inferior. No. In No. 28, since the amount of Ni in the steel sheet was excessive, the SSCC resistance in an environment where the partial pressure of hydrogen sulfide was low deteriorated. No. In No. 29, since the steel sheet does not contain Mo, the SSCC resistance deteriorated in a very severe corrosion environment of hydrogen sulfide partial pressure of 2 Bar. No. Since No. 30 was not reheated, there was no effect of surface softening, and SSCC resistance may deteriorate in a very severe corrosion environment of hydrogen sulfide partial pressure of 2 Bar. No. In No. 31, the average cooling rate from 750 ° C. to 550 ° C. exceeded 50 ° C./s at the steel plate temperature 0.25 mm below the surface of the steel sheet, so that the SSCC resistance deteriorated in a very severe corrosion environment with a partial pressure of hydrogen sulfide of 2 Bar. There was a case.
本発明によれば、耐HIC性のみならず、より厳しい腐食環境下での耐SSCC性および1bar未満の硫化水素分圧の低い環境における耐SSCC性にも優れた耐サワーラインパイプ用高強度鋼板を供給することができる。よって、この鋼板を冷間成形して製造した鋼管(電縫鋼管、スパイラル鋼管、UOE鋼管等)は、耐サワー性を要する硫化水素を含む原油や天然ガスの輸送に好適に使用することができる。
According to the present invention, a high-strength steel plate for sour line pipes having excellent not only HIC resistance but also SSCC resistance in a harsher corrosion environment and SSCC resistance in an environment with a low partial pressure of hydrogen sulfide of less than 1 bar. Can be supplied. Therefore, steel pipes (electrosewn steel pipes, spiral steel pipes, UOE steel pipes, etc.) manufactured by cold forming this steel sheet can be suitably used for transporting crude oil and natural gas containing hydrogen sulfide, which requires sour resistance. ..
Claims (7)
板厚中央部(t/2位置)でのミクロ組織がベイナイト組織であり、鋼板表面下0.25mmにおける鋼組織が、転位密度1.0×1014〜7.0×1014(m-2)のベイナイト組織であり、
鋼板表面下0.25mmにおけるビッカース硬さのばらつきが、標準偏差をσとしたときに3σで30HV以下であり、
板厚方向のビッカース硬さのばらつきが、標準偏差をσとしたときに3σで30HV以下であり、
520MPa以上の引張強さを有する
ことを特徴とする耐サワーラインパイプ用高強度鋼板。 By mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%, S: It contains 0.0002 to 0.0015%, Al: 0.01 to 0.08%, Mo: 0.01 to 0.50% and Ca: 0.0005 to 0.005%, and further contains Nb: 0. It contains .005 to 0.1% , optionally Ti: 0.005 to 0.1 %, and has a component composition with the balance consisting of Fe and unavoidable impurities.
The microstructure at the center of the plate thickness (t / 2 position) is the bainite structure, and the steel structure at 0.25 mm below the surface of the steel sheet has a dislocation density of 1.0 × 10 14 to 7.0 × 10 14 (m- 2). ) Is a bainite organization
The variation in Vickers hardness at 0.25 mm below the surface of the steel sheet is 30 HV or less at 3σ when the standard deviation is σ.
The variation in Vickers hardness in the plate thickness direction is 30 HV or less at 3σ when the standard deviation is σ.
A high-strength steel sheet for sour line pipes, which has a tensile strength of 520 MPa or more.
その後前記鋼板に対して、
冷却開始時の鋼板表面温度:(Ar3−10℃)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:50℃/s以下、
鋼板平均温度で750℃から550℃までの平均冷却速度:15℃/s以上、
鋼板表面下0.25mmにおける鋼板温度で550℃から冷却停止時の温度まで平均冷却速度:150℃/s以上、および
鋼板平均温度で冷却停止温度:250〜550℃
の条件で制御冷却を行い、
その後、前記鋼板に対して、鋼板平均温度:前記冷却停止温度超え、かつ450〜600℃の条件で再加熱を行って、板厚中央部(t/2位置)でのミクロ組織がベイナイト組織であり、鋼板表面下0.25mmにおける鋼組織が、転位密度1.0×10 14 〜7.0×10 14 (m -2 )のベイナイト組織であり、鋼板表面下0.25mmにおけるビッカース硬さのばらつきが、標準偏差をσとしたときに3σで30HV以下であり、板厚方向のビッカース硬さのばらつきが、標準偏差をσとしたときに3σで30HV以下であり、520MPa以上の引張強さを有する高強度鋼板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。 By mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%, S: It contains 0.0002 to 0.0015%, Al: 0.01 to 0.08%, Mo: 0.01 to 0.50% and Ca: 0.0005 to 0.005%, and further contains Nb: 0. A steel piece containing .005 to 0.1% , optionally Ti: 0.005 to 0.1 %, and the balance having a composition of Fe and unavoidable impurities, at a temperature of 1000 to 1300 ° C. After heating, it is hot-rolled to make a steel sheet.
After that, with respect to the steel sheet
Cooling at the start of the steel sheet surface temperature: (Ar 3 -10 ℃) or higher,
Average cooling rate from 750 ° C to 550 ° C at steel plate temperature 0.25 mm below the surface of the steel plate: 50 ° C / s or less,
Average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature: 15 ° C / s or more,
Average cooling rate: 150 ° C / s or more from 550 ° C to the temperature at which cooling is stopped at a steel plate temperature of 0.25 mm below the surface of the steel sheet, and cooling stop temperature: 250 to 550 ° C at the average temperature of the steel sheet.
Control cooling under the conditions of
Then, with respect to the steel sheet, steel sheet average temperature: the more than cooling stop temperature, and I rows reheated under the conditions of 450 to 600 ° C., microstructure bainite structure at the mid-thickness portion (t / 2 position) The steel structure at 0.25 mm below the surface of the steel sheet is a bainite structure with a dislocation density of 1.0 × 10 14 to 7.0 × 10 14 (m- 2 ), and the Vickers hardness at 0.25 mm below the surface of the steel sheet. The variation of Vickers hardness in the plate thickness direction is 30 HV or less at 3σ when the standard deviation is σ, and the tensile strength is 520 MPa or more when the standard deviation is σ. A method for manufacturing a high-strength steel sheet for a sour-resistant pipe, which comprises manufacturing a high-strength steel sheet having a standard deviation.
A high-strength steel pipe using the high-strength steel plate for sour-resistant line pipe according to any one of claims 1 to 3.
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