JP6665822B2 - High strength steel sheet for sour resistant line pipe, method for producing the same, and high strength steel pipe using high strength steel sheet for sour resistant line pipe - Google Patents
High strength steel sheet for sour resistant line pipe, method for producing the same, and high strength steel pipe using high strength steel sheet for sour resistant line pipe Download PDFInfo
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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。 The present invention relates to a high-strength steel plate for a sour-resistant line pipe excellent in material uniformity in a steel plate, which is suitable for use in a line pipe in the fields of construction, marine structures, shipbuilding, civil engineering, and construction industrial machinery, and its production. It is about the method. The present invention also relates to a high-strength steel pipe using the above-mentioned high-strength steel plate for a sour resistant line pipe.
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。 In general, a line pipe is manufactured by forming a steel plate manufactured 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。 Here, in addition to strength, toughness, weldability, etc., line pipes used for transporting crude oil and natural gas containing hydrogen sulfide are resistant to hydrogen-induced cracking (HIC (Hydrogen Induced Cracking)) and sulfides. So-called sour resistance such as stress corrosion cracking (SSCC (Sulfide Stress Corrosion Cracking) resistance) is required. Among them, HIC absorbs hydrogen ions due to the corrosion reaction on the surface of the steel material, penetrates into the steel as atomic hydrogen, and diffuses and accumulates around the non-metallic inclusions such as MnS and the hard second phase structure in the steel. Therefore, it becomes molecular hydrogen, and cracks occur due to its internal pressure. This has been regarded as a problem in line pipes having a relatively low strength level with respect to oil country tubular goods, and many countermeasures 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 region of welds, and has not been regarded as a problem in relatively low-hardness line pipes. . However, in recent years, it has been reported that the mining environment for crude oil and natural gas has become increasingly severe, and that SSCC also occurs in the base material of line pipes in environments 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 under a more severe corrosive environment.
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, when manufacturing a high-strength steel sheet for a line pipe, a so-called TMCP (Thermo-Mechanical Control Process) technique that combines controlled rolling and controlled cooling is applied. In order to increase the strength of steel using this TMCP technique, 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 of the steel sheet is rapidly cooled, so that the hardness of the surface layer becomes higher than that inside the steel sheet, and the hardness distribution in the sheet thickness direction varies. Therefore, there is a problem from the viewpoint of ensuring 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、圧延後の加速冷却を中断し、表面を復熱させた後に再度加速冷却を実施することによる、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-mentioned problem, for example, Patent Documents 1 and 2 disclose accelerated cooling after rolling, and perform accelerated cooling again after reheating the surface. A method for manufacturing a small steel plate is disclosed. Patent Documents 3 and 4 disclose a method of manufacturing a steel sheet for line pipes, in which the surface of a steel sheet after accelerated cooling is heated to a higher temperature from the inside by using a high-frequency induction heating device to reduce the hardness of a surface layer portion. Have been.
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 there is unevenness in the scale thickness of the steel sheet surface, the cooling rate of the steel sheet below the steel sheet also varies during cooling, and local dispersion of the cooling stop temperature in the steel sheet becomes a problem. As a result, the material of the steel sheet varies in the sheet width direction due to the unevenness of the scale thickness. On the other hand,
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ＨＩＣ性及び耐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 of the present inventors, the high-strength steel sheets obtained by the manufacturing methods described in Patent Documents 1 to 6 have room for improvement in terms of HIC resistance and SSCC resistance under a more severe corrosive environment. It turned out that there is. The following can be considered as the reason.
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 production methods described in Patent Literatures 1 and 2, if the transformation behavior differs depending on the components of the steel sheet, there may be cases where the effect of sufficient material homogenization by recuperation cannot be obtained. Further, when the structure in the surface layer of the steel sheet obtained by the production methods described in Patent Documents 1 and 2 is a multi-phase structure such as a ferrite-bainite two-phase structure, in the micro Vickers test under a low load, the indenter has any structure. The hardness value greatly varies depending on whether the test is performed by pushing the test piece.
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 Literatures 3 and 4, since the cooling rate of the surface layer in the accelerated cooling is high, the hardness of the surface layer 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
そこで本発明は、上記課題に鑑み、より厳しい腐食環境下での耐ＨＩＣ性及び耐ＳＳＣＣ性に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。 In view of the above problems, an object of the present invention is to provide a high-strength steel sheet for a sour line pipe having excellent HIC resistance and SSCC resistance under a severer corrosive environment, together with its advantageous production method. . Another object of the present invention is to propose a high-strength steel pipe using the high-strength steel plate for a sour resistant line pipe.
本発明者らは、より厳しい腐食環境下での耐ＨＩＣ性及び耐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ＨＩＣ性に加えて、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０．５ｍｍの鋼組織を、転位密度０．５×１０14〜７．０×１０14（ｍ-2）のフェラ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特性が向上することを知見した。そして、極表層部はフェライト主体の組織でありながら、鋼板内部の組織はベイナイトとすることにより、表層硬さを抑えつつ、鋼板の強度を確保できる。 The present inventors have repeated many experiments and studies on the component composition, microstructure, and manufacturing conditions of a steel material in order to secure HIC resistance and SSCC resistance under a more severe corrosive environment. As a result, in order to further improve the SSCC resistance in addition to the HIC resistance of high-strength steel pipes, it is not enough to simply reduce the surface hardness as conventionally known, and in particular, the structure of the extreme surface layer of the steel sheet More specifically, a steel structure 0.5 mm below the surface of the steel sheet is formed into a ferrite structure having a dislocation density of 0.5 × 10 14 to 7.0 × 10 14 (m −2 ), so that a coating process after pipe forming is performed. It has been found that the increase in hardness can be suppressed, and as a result, the SSCC resistance of the steel pipe is improved. Then, while the extremely surface layer portion is mainly composed of ferrite, the structure inside the steel sheet is made of bainite, so that the strength of the steel sheet can be secured while suppressing the surface layer hardness.
さらに、このような鋼組織を実現するためには、制御冷却の直前に所定条件でデスケーリングを行い、しかもその後の制御冷却では鋼板表面下０．５ｍｍ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。本発明は、この知見をもとになされたものである。なお、特許文献５，６などの従来のデスケーリング技術は、スケールを薄くすることによってその後の冷却むらを減らすことを目的としたものにすぎない。本発明では、デスケーリングを所定条件で行うことと、制御冷却を所定条件で行うこととによって、上記のような極表層で転位密度を抑えたフェライトを形成するものである。
Furthermore, in order to realize such a steel structure, it is necessary to perform descaling under predetermined conditions immediately before controlled cooling, and to precisely control the cooling rate at 0.5 mm below the steel sheet surface in the controlled cooling thereafter. Succeeded in finding the conditions. The present invention has been made based on this finding. Note that the conventional descaling techniques such as
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％およびＣａ：０．０００５〜０．００５％を含有し、以下の式（１）によって求められるＣＰ値が１．００以下となり、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０．５ｍｍにおける鋼組織が、転位密度０．５×１０14〜７．０×１０14（ｍ-2）のフェライト組織であり、
板厚中央における鋼組織がベイナイト組織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。
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。
That is, the gist configuration of the present invention is as follows.
[1] In 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%, and Ca: 0.0005 to 0.005%, and the CP value obtained by the following formula (1) is 1 .00 or less, and the balance has a component composition consisting of Fe and unavoidable impurities,
The steel structure at 0.5 mm below the surface of the steel sheet is a ferrite structure having a dislocation density of 0.5 × 10 14 to 7.0 × 10 14 (m −2 ),
A high-strength steel plate for a sour-resistant line pipe, wherein the steel structure at the center of the plate thickness is a bainite structure.
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)
Here, [% X] indicates the content (% by mass) of X element in steel.
［２］前記成分組成が、さらに、質量％で、Ｃｕ：０．５０％以下、Ｎｉ：０．５０％以下、Ｃｒ：０．５０％以下およびＭｏ：０．５０％以下のうちから選んだ１種又は２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 component composition is further selected from among Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Mo: 0.50% or less by mass%. The high-strength steel sheet for a sour-resistant line pipe according to the above [1], comprising one or more kinds.
［３］前記成分組成が、さらに、質量％で、Ｎｂ：０．００５〜０．１％、Ｖ：０．００５〜０．１％およびＴｉ：０．００５〜０．１％のうちから選んだ１種又は２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] The component composition is further selected from Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1% by mass%. The high-strength steel sheet for sour resistant line pipe according to the above [1] or [2], comprising one or more kinds.
［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％およびＣａ：０．０００５〜０．００５％を含有し、以下の式（１）によって求められるＣＰ値が１．００以下となり、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、（Ａｒ3＋２０℃）以下の鋼板表面温度で、鋼板表面での噴射流の衝突圧が１ＭＰａ以上のデスケーリングを行い、
その後前記鋼板に対して、
冷却開始時の鋼板表面温度：（Ａｒ3−１００℃）以上（Ａｒ3−１０℃）以下、
鋼板表面下０．５ｍｍにおける鋼板温度で７５０℃から５５０℃までの平均冷却速度：８０℃／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、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。
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。
[4] In 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%, and Ca: 0.0005 to 0.005%, and the CP value obtained by the following formula (1) is 1 1.00 or less, the remainder is a steel slab having a component composition of Fe and unavoidable impurities, after heating to a temperature of 1000 ~ 1300 ℃, hot-rolled into a steel sheet,
Thereafter, the steel sheet is subjected to descaling at a steel sheet surface temperature of (Ar 3 + 20 ° C.) or less and a collision pressure of a jet flow on the steel sheet surface of 1 MPa or more,
Then, for the steel plate,
Steel sheet surface temperature at the start of cooling: (Ar 3 -100 ° C) or more and (Ar 3 -10 ° C) or less,
Average cooling rate from 750 ° C. to 550 ° C. at a steel sheet temperature 0.5 mm below the steel sheet surface: 80 ° C./s or less,
Average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature: 15 ° C / s or more, and cooling stop temperature at average steel sheet temperature: 250 to 550 ° C
A method for producing a high-strength steel plate for a sour-resistant line pipe, characterized in that controlled cooling is performed under the following conditions.
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)
Here, [% X] indicates the content (% by mass) of X element in steel.
［５］前記成分組成が、さらに、質量％で、Ｃｕ：０．５０％以下、Ｎｉ：０．５０％以下、Ｃｒ：０．５０％以下およびＭｏ：０．５０％以下のうちから選んだ１種又は２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 component composition is further selected from Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Mo: 0.50% or less by mass%. The method for producing a high-strength steel sheet for a sour resistant line pipe according to the above [4], comprising one or more kinds.
［６］前記成分組成が、さらに、質量％で、Ｎｂ：０．００５〜０．１％、Ｖ：０．００５〜０．１％およびＴｉ：０．００５〜０．１％のうちから選んだ１種又は２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] The above-mentioned component composition is further selected from Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1% by mass%. The method for producing a high-strength steel sheet for a sour-resistant line pipe according to the above [4] or [5], comprising one or more kinds.
［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 line 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、より厳しい腐食環境下での耐ＨＩＣ性及び耐ＳＳＣＣ性に優れる。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、より厳しい腐食環境下での耐ＨＩＣ性及び耐ＳＳＣＣ性に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 sheet for a sour line pipe and the high-strength steel pipe using the high-strength steel sheet for a sour line pipe according to the present invention are excellent in HIC resistance and SSCC resistance under severer corrosive environment. Further, according to the method for producing a high-strength steel plate for a sour line pipe according to the present invention, it is possible to produce a high-strength steel plate for a sour line pipe excellent in HIC resistance and SSCC resistance under a more severe corrosive environment. it can.
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 plate for a sour-resistant line pipe 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。
[Component composition]
First, the component composition of the high-strength steel sheet according to the present disclosure and the reason for the 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の硬さが上昇するため、耐ＨＩＣ特性および耐ＳＳＣＣ特性が劣化する。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 the 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 increases during accelerated cooling. , HIC resistance and SSCC resistance deteriorate. Further, the toughness also deteriorates. For this reason, the C content 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, and if it exceeds 0.50%, toughness and weldability are deteriorated. It is 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中心偏析部の硬さが上昇するため、耐ＨＩＣ特性が劣化する。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 if the content is less than 0.50%, the effect of its addition is poor. On the other hand, if it exceeds 1.80%, the hardness of the central segregation part increases during accelerated cooling. Therefore, the HIC resistance deteriorates. In addition, weldability also deteriorates. For this reason, 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特性を劣化させる。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, and degrades the weldability and degrades the HIC resistance by increasing the hardness of the central segregation part. If it exceeds 0.015%, the tendency becomes remarkable, so the upper limit is set to 0.015%. Preferably it is 0.008% or less. The lower the content, the better, but from the viewpoint of refining cost, the content is set to 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特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: 0.0002-0.0015%
S is an unavoidable impurity element and is preferably small in steel because it becomes MnS inclusions and degrades the HIC resistance, but is allowed up to 0.0015%. The lower the content, the better, but from the viewpoint of refining cost, the content is set to 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 a deoxidizing agent, but if it is less than 0.01%, there is no effect, whereas if it exceeds 0.08%, the cleanliness of the steel decreases and the toughness deteriorates. It is limited to the range of 01 to 0.08%.
Ｃａ：０．０００５〜０．００５％
Ｃａは、硫化物系介在物の形態制御による耐ＨＩＣ特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特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 the HIC resistance by controlling the form of the sulfide-based inclusions, but if it is less than 0.0005%, the effect of its addition is not sufficient. On the other hand, when the content exceeds 0.005%, not only the effect is saturated, but also the HIC resistance is deteriorated due to a decrease in the cleanliness of the steel. Therefore, the Ca content is limited to the range of 0.0005 to 0.005%. .
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，ＣｒおよびＭｏのうちから選んだ１種又は２種以上を、以下の範囲で任意に含有させることができる。 Although the basic components of the present disclosure have been described above, the component composition of the present disclosure may be one or more selected from Cu, Ni, Cr, and Mo in order to further improve the strength and toughness of the steel sheet. Can be arbitrarily contained in the following range.
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. To obtain this effect, it is preferable to contain 0.05% or more. Therefore, when Cu is added, the upper limit is 0.50%.
Ｎｉ：０．５０％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０．０５％以上を含有することが好ましいが、含有量が多すぎると経済的に不利なだけでなく、溶接熱影響部の靱性が劣化するため、Ｎｉを添加する場合は０．５０％を上限とする。
Ni: 0.50% or less Ni is an element effective for improving toughness and increasing strength. To obtain this effect, it is preferable to contain 0.05% or more. Not only is it disadvantageous, but also the toughness of the heat affected zone deteriorates, so when Ni is added, the upper limit is 0.50%.
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溶接性が劣化するため、Ｃｒを添加する場合は０．５０％を上限とする。
Cr: 0.50% or less Cr, like Mn, is an element effective for obtaining sufficient strength even at low C, and it is preferable to contain 0.05% or more in order to obtain this effect. If the amount is too large, the weldability deteriorates. Therefore, when Cr is added, the upper limit is 0.50%.
Ｍｏ：０．５０％以下
Ｍｏ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、Ｍｏを添加する場合は０．５０％を上限とする。
Mo: 0.50% or less Mo is an element effective for improving toughness and increasing strength. To obtain this effect, it is preferable to contain 0.05% or more. Therefore, when Mo is added, the upper limit is 0.50%.
本開示の成分組成は、さらに、Ｎｂ，ＶおよびＴｉのうちから選んだ１種又は２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 contain one or more selected from Nb, V, and Ti within the following range.
Ｎｂ：０．００５〜０．１％、Ｖ：０．００５〜０．１％およびＴｉ：０．００５〜０．１％のうちから選んだ１種又は２種以上
Ｎｂ，Ｖ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任意に添加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、添加する場合はいずれも０．００５〜０．１％の範囲とするのが好ましい。
Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.1%, at least one selected from the group consisting of Nb, V, and Ti. Is also an element that can be arbitrarily added to enhance the strength and toughness of the steel sheet. If the content of each element is less than 0.005%, the effect of the addition is poor, while if the content exceeds 0.1%, the toughness of the welded portion is deteriorated. It is preferable to set it in the range.
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性能を同時に満足することが必要であるので、下記（１）式によって求められるＣＰ値を、１．００以下とする。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。
The present disclosure discloses a technique for improving the SSCC resistance of a high-strength steel pipe using a high-strength steel plate for a sour resistant line pipe. Since it is necessary to satisfy at the same time, the CP value obtained by the following equation (1) is set to 1.00 or less. Note that 0 may be substituted for an element that 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)
Here, [% X] indicates the content (% by mass) of X element in 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特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 a formula devised for estimating the material of the central segregation portion from the content of each alloy element. The higher the CP value of the above formula (1), the higher the component concentration of the central segregation portion. And the hardness of the center segregation part increases. Therefore, by setting the CP value obtained in the above equation (1) to 1.00 or less, it becomes possible to suppress the occurrence of cracks in the HIC test. In addition, the lower the CP value, the lower the hardness of the center segregation portion. Therefore, if higher HIC resistance is required, the upper limit may be set to 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。 The balance other than the above-mentioned elements is composed of Fe and inevitable impurities. However, the content of other trace elements is not prevented as long as the function and effect of the present invention are not impaired.
［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、板厚中心における鋼組織は、ベイナイト組織とする必要がある。一方、表層硬さの上昇を抑制するために、表層部０．５ｍｍの鋼組織についてはフェライト組織とする。表層部のフェライト層が厚すぎると、フェライト変態に伴うＣ拡散によりフェライト相直下の硬さが上昇する恐れがあるため、フェライト層は１．０ｍｍ以下が好ましい。フェライト組織中に、ベイナ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、ミクロ硬さ評価時、個別の層を評価することにより表層硬さのばらつきが大きくなるため、フェライト相以外の組織分率は少ない程良い。ただし、フェラ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、フェライト以外の鋼組織（ベイナイト、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。
[Steel structure]
Next, the steel structure of the high-strength steel plate for a sour-resistant line pipe of the present disclosure will be described. In order to achieve a high tensile strength of 520 MPa or more, the steel structure at the center of the sheet thickness needs to have a bainite structure. On the other hand, in order to suppress an increase in surface hardness, the steel structure having a surface layer portion of 0.5 mm has a ferrite structure. If the ferrite layer in the surface portion is too thick, the hardness immediately below the ferrite phase may increase due to C diffusion accompanying the ferrite transformation, and therefore the ferrite layer is preferably 1.0 mm or less. When heterogeneous structures such as bainite, martensite, pearlite, island-like martensite, and retained austenite are mixed in the ferrite structure, the evaluation of the individual layers at the time of microhardness evaluation results in a large variation in surface layer hardness. The smaller the structural fraction other than the ferrite phase, the better. However, when the volume fraction of the structure other than the ferrite phase is sufficiently low, their influence is negligible, so that a certain amount is permissible. Specifically, in the present disclosure, if the total of the steel structures other than ferrite (bainite, martensite, pearlite, island-like martensite, retained austenite, etc.) is less than 5% by volume, there is no significant effect. Shall be accepted.
また、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０．５ｍｍの鋼組織を、転位密度０．５×１０14〜７．０×１０14（ｍ-2）のフェラ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０．５ｍｍ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０．５ｍｍ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０．５ｍｍの転位密度が０．５×１０14（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０．５ｍｍ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０．５ｍｍ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, in the present disclosure, the structure of the extreme surface layer portion of the steel sheet, specifically, the steel structure of 0.5 mm below the surface of the steel sheet is changed to a dislocation density of 0.5 × 10 14 to 7.0 × 10 14 (m −2 ). It is important to have a ferrite structure of Since the dislocation density decreases during the coating process after pipe forming, if the dislocation density at 0.5 mm below the steel sheet surface is 7.0 × 10 14 (m −2 ) or less, the increase in hardness due to age hardening is minimized. Can be minimized. Conversely, when the dislocation density at 0.5 mm below the steel sheet surface exceeds 7.0 × 10 14 (m −2 ), the dislocation density does not decrease in the coating process after pipe forming, and the hardness increases significantly by age hardening. Degrades SSCC resistance. The preferred range of dislocation density for obtaining good SSCC resistance after pipe forming is 6.0 × 10 14 (m −2 ) or less. On the other hand, if the dislocation density at 0.5 mm below the steel sheet surface is less than 0.5 × 10 14 (m −2 ), the steel sheet cannot maintain its strength. In order to secure the strength of the X65 grade, it is preferable to have a dislocation density of 2.0 × 10 14 (m −2 ) or more. Note that, in the high-strength steel sheet of the present disclosure, if the dislocation density in the steel structure of 0.5 mm below the steel sheet surface is in the above range, the same superfluous layer part in the range of 0.5 mm in depth from the steel sheet surface has the same dislocation density. As a result, the effect of improving the SSCC resistance can be obtained.
なお、鋼板表面下０．５ｍｍでの転位密度を７．０×１０14（ｍ-2）以下とすると、表面下０．５ｍｍ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０．５ｍｍでのＨＶ０．１を２３０以下にすることで、造管後コーティング過程を経たのちの、表面下０．５ｍｍ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 0.5 mm below the surface of the steel sheet is 7.0 × 10 14 (m −2 ) or less, the HV 0.1 at 0.5 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 0.5 mm below the surface of the steel sheet to 230 or less, the coating after pipe formation is reduced. HV0.1 at 0.5 mm below the surface after the process can be suppressed to 260 or less, and SSCC resistance can be secured.
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 plate of the present disclosure is a steel plate for a steel pipe having a strength of X5 grade of API 5L or more, it has a tensile strength of 520 MPa or more.
［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、その後鋼板に対して所定条件下でのデスケーリングを行い、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。
[Production method]
Hereinafter, a manufacturing method and manufacturing conditions for manufacturing the high-strength steel plate for a sour resistant line pipe will be specifically described. The manufacturing method of the present disclosure, after heating a slab having the above-described composition, hot-rolled to a steel sheet, and then descaling the steel sheet under predetermined conditions, and then performing predetermined conditions on the steel sheet. Perform controlled cooling below.
〔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 required solidity cannot be obtained due to insufficient solid solution of carbides, while if it exceeds 1300 ° C., the toughness deteriorates. 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性能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 step, in order to obtain high base material toughness, the lower the rolling end temperature, the better. However, on the other hand, since the rolling efficiency is reduced, the rolling end temperature at the steel sheet surface temperature is set to the required base material toughness and rolling. It is necessary to set in consideration of efficiency. From the viewpoint of improving the strength and the HIC resistance, it is preferable that the rolling end temperature be equal to or higher than the Ar 3 transformation point at the steel sheet surface temperature. Here, the Ar 3 transformation point means a ferrite transformation start temperature during cooling, and can be determined, for example, from the steel composition by the following equation. Further, in order to obtain a high base material toughness, it is desirable that the rolling reduction in a temperature range of 950 ° C. or less corresponding to the austenite non-recrystallization temperature range is 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]
Here, [% X] indicates the content (% by mass) of X element in steel.
〔デスケーリング〕
熱間圧延の終了後、制御冷却の直前に、鋼板に対してデスケーリングを行う。これは、表層部に均一にフェライトを形成させるために重要な工程の一つである。
(Descaling)
After the end of the hot rolling and immediately before the controlled cooling, the steel sheet is descaled. This is one of the important steps for uniformly forming ferrite on the surface layer.
デスケーリング開始時の鋼板表面温度：（Ａｒ3＋２０℃）以下
デスケーリング開始時の鋼板表面温度が（Ａｒ3＋２０℃）を超える場合、鋼板表面下０．５ｍｍの位置までフェライトが形成されず、表層硬さの上昇、耐ＳＳＣＣ性の劣化を招くからである。当該鋼板表面温度の下限は特に限定されないが、フェライト層の過剰形成を避ける観点から（Ａｒ3−３０℃）以上とすることが好ましい。
Steel sheet surface temperature at the start of descaling: (Ar 3 + 20 ° C.) or less If the steel sheet surface temperature at the start of descaling exceeds (Ar 3 + 20 ° C.), no ferrite is formed up to 0.5 mm below the steel sheet surface, This is because the surface hardness increases and SSCC resistance deteriorates. The lower limit of the steel sheet surface temperature is not particularly limited, but is preferably (Ar 3 -30 ° C.) or more from the viewpoint of avoiding excessive formation of the ferrite layer.
鋼板表面での噴射流の衝突圧：１ＭＰａ以上
鋼板表面での噴射流の衝突圧が１ＭＰａ未満では、デスケーリングが不十分でスケールむらが生じる場合があり、鋼板表面下０．５ｍｍにおいて均一なフェライト相の形成が困難となるため、１ＭＰａ以上とする。衝突圧の上限は特に限定されないが、設備能力の観点から１０ＭＰａ以下とすることが好ましい。
When the collision pressure of the jet flow on the surface of the steel plate is 1 MPa or more, if the collision pressure of the jet flow on the surface of the steel plate is less than 1 MPa, the descaling may be insufficient and scale unevenness may occur. Since it is difficult to form a phase, the pressure is set to 1 MPa or more. The upper limit of the collision pressure is not particularly limited, but is preferably 10 MPa or less from the viewpoint of the equipment capacity.
〔制御冷却の冷却開始温度〕
冷却開始時の鋼板表面温度：（Ａｒ3−１００℃）以上（Ａｒ3−１０℃）以下
冷却開始時の鋼板表面温度は特に重要で、制御冷却前に鋼板表層において十分にフェライト変態させておく必要がある。Ａｒ3変態点からの温度降下量が１０℃未満であると、表層組織はフェライト−ベイナイト２相組織となり、また、鋼板表面下０．５ｍｍ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。このため、冷却開始時の鋼板表面温度は、（Ａｒ3−１０℃）以下とした。好ましくは（Ａｒ3−３０℃）以下である。また、Ａｒ3変態点からの温度降下量が１００℃を超えると、板厚全域でのフェライト変態が進行し、板厚中心部ではフェライト−ベイナイト２相組織となり、強度低下が大きくなると共に耐ＨＩＣ特性が劣化する。このため、冷却開始時の鋼板表面温度は（Ａｒ3−１００℃）以上とする。さらに好ましくは（Ａｒ3−８０℃）以上である。
[Cooling start temperature for control cooling]
Steel sheet surface temperature at the start of cooling: (Ar 3 -100 ° C.) or more and (Ar 3 -10 ° C.) or less The steel sheet surface temperature at the start of cooling is particularly important, and the surface of the steel sheet is sufficiently transformed into ferrite before controlled cooling. There is a need. If the amount of temperature drop from the Ar 3 transformation point is less than 10 ° C., the surface layer structure becomes a ferrite-bainite two-phase structure, and the dislocation density at a depth of 0.5 mm below the steel sheet surface is 7.0 × 10 14 (m −2). ), The SSCC resistance deteriorates. For this reason, the steel sheet surface temperature at the start of cooling was set to (Ar 3 -10 ° C.) or less. Preferably not more than (Ar 3 -30 ℃). When the temperature drop from the Ar 3 transformation point exceeds 100 ° C., the ferrite transformation proceeds in the entire thickness of the sheet, a ferrite-bainite two-phase structure is formed in the center of the sheet thickness, the strength decrease is increased, and the HIC resistance is increased. The characteristics deteriorate. For this reason, the steel sheet surface temperature at the start of cooling is (Ar 3 -100 ° C.) or higher. It is more preferably (Ar 3 -80 ° C.) or higher.
〔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（具体的には鋼板表面下０．５ｍｍの深さ）での冷却速度を抑制しつつ、板厚中心の変態温度区間での冷却速度を確保する必要がある。
[Cooling rate of controlled cooling]
In order to reduce the variation in hardness in the steel sheet and improve the material uniformity while increasing the strength, the cooling rate at the surface layer (specifically, 0.5 mm below the steel sheet surface) is suppressed. At the same time, it is necessary to secure a cooling rate in the transformation temperature section at the center of the sheet thickness.
鋼板表面下０．５ｍｍにおける鋼板温度で７５０℃から５５０℃までの平均冷却速度：８０℃／ｓ以下
鋼板表面下０．５ｍｍにおける鋼板温度で７５０℃から５５０℃までの平均冷却速度が８０℃／ｓを超えると、鋼板表面下０．５ｍｍ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０．５ｍｍ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０．５ｍｍ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８０℃／ｓ以下とする。好ましくは６０℃／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、フェライト層の過剰形成回避の観点から、１０℃／ｓ以上とすることが好ましい。
Average cooling rate from 750 ° C to 550 ° C at a steel sheet temperature of 0.5mm below the steel sheet surface: 80 ° C / s or less Average cooling rate from 750 ° C to 550 ° C at a steel sheet temperature of 0.5mm below the steel sheet surface is 80 ° C / s. If it exceeds s, the dislocation density at a depth of 0.5 mm below the steel sheet surface will exceed 7.0 × 10 14 (m −2 ). As a result, the HV0.1 at 0.5 mm below the steel plate surface exceeds 230, and after the coating process after pipe making, the HV0.1 at 0.5 mm below the surface exceeds 260, deteriorating the SSCC resistance of the steel tube. I do. Therefore, the average cooling rate is set to 80 ° C./s or less. Preferably, it is 60 ° C./s or less. The lower limit of the average cooling rate is not particularly limited, but is preferably 10 ° C./s or more from the viewpoint of avoiding excessive formation of the ferrite layer.
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、板厚中央部においてベイナイト組織が得られずに、強度低下や耐ＨＩＣ特性の劣化が生じたり、硬さのばらつきが大きくなったりする。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、マルテンサイト等の硬質相形成による硬さばらつきを回避する観点から、８０℃／ｓ以下とすることが好ましい。
Average cooling rate from 750 ° C to 550 ° C at an average temperature of steel sheet: 15 ° C / s or more If the average cooling rate from 750 ° C to 550 ° C at an average temperature of the steel sheet is less than 15 ° C / s, bainite structure is formed at the center of the sheet thickness. Otherwise, the strength is reduced, the HIC resistance is degraded, and the hardness varies greatly. 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 the strength and hardness of the steel sheet, the average cooling rate of the steel sheet is preferably set to 20 ° C./s or more. The upper limit of the average cooling rate is not particularly limited, but is preferably 80 ° C./s or less from the viewpoint of avoiding hardness variation due to formation of a hard phase such as martensite.
なお、鋼板表面下０．５ｍｍ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０．５ｍｍでの温度を本明細書における「鋼板表面下０．５ｍｍ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 0.5 mm below the steel sheet surface and the average steel sheet temperature cannot be directly measured physically, the surface temperature at the start of cooling measured by the radiation thermometer and the surface temperature at the target cooling stop are also measured. At the same time, for example, the temperature distribution in the thickness cross section can be obtained in real time by a difference calculation using a process computer. The temperature at 0.5 mm below the surface of the steel sheet in the temperature distribution is referred to as “the steel sheet temperature at 0.5 mm below the surface of the steel sheet” in this specification, and the average value of the temperature in the thickness direction in the temperature distribution is referred to as “the steel sheet in this specification. Average temperature ".
〔冷却停止温度〕
鋼板平均温度で冷却停止温度：２５０〜５５０℃
表層のフェライト変態が完了後、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、マルテンサイトや島状マルテンサイト（ＭＡ）が生成し、特に表層部の硬さ上昇が著しくなり、板厚方向の硬さのばらつきが大きくなる。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 at the average temperature of the steel sheet: 250 to 550 ° C
After the ferrite transformation of the surface layer is completed, the bainite phase is generated by rapid cooling to 250 to 550 ° C. which is the temperature range of bainite transformation by controlled cooling. If the cooling stop temperature exceeds 550 ° C., bainite transformation is incomplete and sufficient strength cannot be obtained. If the cooling stop temperature is lower than 250 ° C., martensite or island-like martensite (MA) is generated, and the hardness of the surface layer in particular increases remarkably, and the variation in hardness in the thickness direction increases. 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.
［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, and the like, and by welding the butted portions, the material uniformity in the steel sheet suitable for transporting crude oil and natural gas is excellent. It is possible to manufacture high-strength steel pipes (such as UOE steel pipes, ERW steel pipes, and spiral steel pipes) for sour resistant line pipes.
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, UOE steel pipes, beveling the end of the steel plate, forming the steel pipe shape by C-press, U-press, O-press, then seam-welding the butt portion by internal welding and external welding, and further if necessary It is manufactured through an expansion process. In addition, any welding method may be used as long as sufficient joint strength and joint toughness can be obtained. However, from the viewpoint of excellent welding quality and production efficiency, it is preferable to use submerged arc welding.
表１に示す成分組成になる鋼（鋼種Ａ〜Ｉ）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、表２に示す鋼板表面温度にて、鋼板表面での噴射流の衝突圧が２．０ＭＰａとなるように鋼板にデスケーリングを行った。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。 Steel (steel types A to I) having the composition shown in Table 1 was formed into a slab by a continuous casting method, heated to the temperature shown in Table 2, and then subjected to hot rolling at the rolling end temperature and rolling reduction shown in Table 2. Thus, a steel plate having a plate thickness shown in Table 2 was obtained. Then, at the steel sheet surface temperature shown in Table 2, the steel sheet was descaled such that the collision pressure of the jet flow on the steel sheet surface was 2.0 MPa. Thereafter, the steel sheet was subjected to controlled cooling using a water-cooled controlled cooling device under the conditions shown 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０．５ｍｍの位置での組織と、板厚中央での組織を、表２に示す。
[Identify organization]
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 0.5 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 a direction perpendicular to the rolling direction as a tensile test piece, and the tensile strength was measured. Table 2 shows the results.
［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０．５ｍｍの位置において５０点のビッカース硬さ（ＨＶ０．１）を測定し、その平均値を求めた。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]
With respect to a cross section perpendicular to the rolling direction, Vickers hardness (HV0.1) at 50 points was measured at a position 0.5 mm below the steel sheet surface according to JIS Z 2244, and the average value was determined. Here, the measurement at HV0.1 instead of the commonly used HV10 is because the indentation is reduced by measuring at HV0.1, so that the hardness information at a position closer to the surface and the sensitivity to the microstructure are more sensitive. This is because it is possible to provide information on the hardness of the object.
［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０．５ｍｍ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 collected from a position having an average hardness, the sample surface was polished to remove scale, and X-ray diffraction measurement was performed at a position of 0.5 mm below the steel sheet surface. The method of converting the dislocation density from the strain obtained from the half width β of the X-ray diffraction measurement was used. In a diffraction intensity curve obtained by ordinary X-ray diffraction, two Kα1 rays and Kα2 rays having different wavelengths are overlapped with each other, and are separated by the Rachinger method. The Williamsson-Hall method described below is used to extract the distortion. The spread of the half width is affected by the crystallite size D and the strain ε, and can be calculated by the following equation as the sum of both factors. β = β1 + β2 = (0.9λ / (D × cos θ)) + 2ε × tan θ. This equation is further transformed to βcos θ / λ = 0.9λ / D + 2ε × sin θ / λ. By plotting βcosθ / λ against sinθ / λ, strain ε is calculated from the slope of the straight line. The diffraction lines used for 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. Here, θ means the peak angle calculated by the θ-2θ method of X-ray diffraction, and λ means the wavelength of X-ray used in X-ray diffraction. b is the Burgers vector of Fe (α), which was 0.25 nm in this embodiment.
［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。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]
SSCC resistance was evaluated by forming a pipe using a part of each of these steel sheets. The pipe is manufactured by beveling the end of the steel plate, forming it into a steel pipe shape by C press, U press, and O press, then seam welding the inner and outer butt joints by submerged arc welding and then expanding the pipe. did. As shown in FIG. 1, a coupon cut out from the obtained steel pipe was flattened, and 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 blackened in order to leave the state of the outermost layer. A 90% stress of the actual yield strength (0.5% YS) of each steel pipe was applied to the sampled SSCC specimens, and NACE standard TM0177 Solution A solution was used, hydrogen sulfide partial pressure: 1 bar, and EFC16 standard. Was performed according to the 4-point bending SSCC test. When no cracks were observed after 720 hours of immersion, SSCC resistance was determined to be good, and when cracks occurred, it was determined to be poor, and x was determined. Table 2 shows the results.
［耐ＨＩＣ性の評価］
耐ＨＩＣ特性は、ＮＡＣＥ Ｓｔａｎｄａｒｄ ＴＭ−０２−８４に準じた浸漬時間９６時間のＨＩＣ試験を行い、割れが認められない場合を耐ＨＩＣ特性良好と判断して○で、割れが発生した場合を×として評価した。結果を表２に示す。
[Evaluation of HIC resistance]
The HIC resistance was evaluated by performing a HIC test for a dipping time of 96 hours in accordance with NACE Standard TM-02-84. If no cracks were observed, the HIC resistance was judged to be good. Was evaluated. Table 2 shows the results.
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０．５ｍｍ位置におけるミクロ組織はフェライト相とし、ｔ／２位置におけるミクロ組織はベイナイト組織とし、表面下０．５ｍｍ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、およびＨＩＣ試験による割れが認められないこととした。 The target range of the present invention is a high-strength steel sheet for a sour-resistant line pipe having a tensile strength of 520 MPa or more, a microstructure at a position 0.5 mm below the surface being a ferrite phase, a microstructure at a position t / 2 being a bainite structure, HV0.1 at 0.5 mm was 230 or less, and it was determined that no crack was observed in the SSCC test and no crack was observed in the HIC test in a high-strength steel pipe formed using the steel sheet.
表２に示したように、Ｎｏ．１〜Ｎｏ．９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０．５ｍｍ位置におけるミクロ組織はフェライト相、ｔ／２位置におけるミクロ組織はベイナイト組織、表面下０．５ｍｍ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 to No. 9 is an invention example in which the component composition and the production conditions satisfy the appropriate range of the present invention. In any case, the steel sheet has a tensile strength of 520 MPa or more, the microstructure at a position 0.5 mm below the surface is a ferrite phase, the microstructure at a position t / 2 is a bainite structure, and the HV0.1 at 0.5 mm below the surface is 230 or less. The SSCC resistance and the HIC resistance of the high-strength steel pipe formed using the steel sheet were also good.
これに対し、Ｎｏ．１０〜Ｎｏ．１９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．１０は、デスケーリングを使用しなかったため、表層組織がフェライト相のみとならず、耐ＳＳＣＣ性に劣る。Ｎｏ．１１は、デスケーリングの温度、冷却開始温度及び冷却速度が本発明範囲外で、表層の転位密度及び硬さが大きくなり、耐ＳＳＣＣ性の劣化を招いている。Ｎｏ．１２はデスケーリングの温度、及び冷却開始温度が本発明範囲外であり、表層組織がベイナイトであるため、表層の転位密度及び硬さが大きくなり、耐ＳＳＣＣ性の劣化を招いている。Ｎｏ.１３は冷却開始温度が低く、板全域でフェライトが形成され、強度不足となり、耐ＨＩＣ性が劣化した。Ｎｏ．１４はデスケーリング開始温度が高く、表層に十分フェライトが形成されなかったため、表層硬さ、転位密度が大きくなり、ＳＳＣＣ発生を招いた。Ｎｏ．１５は、デスケーリング条件は範囲内であるものの、冷却開始温度が高く、表層フェライト形成が充分でなかったため、ＳＳＣＣの発生を招いた。Ｎｏ．１６は、鋼板表面下０．５ｍｍでの冷却速度が著しく速いため、表層部にマルテンサイトが形成され、その結果表層部の硬さが上昇し、耐ＳＳＣＣ性も劣化した。Ｎｏ．１７は、Ｎｏ．１６ほど冷却速度が速くないため表層組織はフェライトであるが、本発明範囲外の冷却速度であるため、表層部硬さの上昇、耐ＳＳＣＣ性の劣化を招いた。Ｎｏ．１８およびＮｏ.１９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、低強度であった。Ｎｏ.２０〜Ｎｏ.２３は、鋼板の成分組成が本発明の範囲外であり、ＨＩＣ割れを生じた。 On the other hand, no. 10-No. 19 is a comparative example in which the component composition is within the range of the present invention, but the production conditions are out of the range of the present invention. No. In No. 10, since no descaling was used, the surface layer structure was not only a ferrite phase, and the SSCC resistance was poor. No. In No. 11, the descaling temperature, the cooling start temperature, and the cooling rate were out of the range of the present invention, and the dislocation density and hardness of the surface layer were increased, resulting in deterioration of SSCC resistance. No. In No. 12, since the descaling temperature and the cooling start temperature are out of the range of the present invention, and the surface layer structure is bainite, the dislocation density and the hardness of the surface layer are increased, and the SSCC resistance is deteriorated. In No. 13, the cooling start temperature was low, ferrite was formed throughout the plate, the strength was insufficient, and the HIC resistance was deteriorated. No. In No. 14, since the descaling start temperature was high and ferrite was not sufficiently formed on the surface layer, the surface layer hardness and dislocation density were increased, and SSCC was generated. No. In No. 15, although the descaling condition was within the range, the cooling start temperature was high and the formation of the surface ferrite was not sufficient, so that SSCC was generated. No. In No. 16, since the cooling rate at 0.5 mm below the steel sheet surface was extremely high, martensite was formed in the surface layer, and as a result, the hardness of the surface layer was increased and the SSCC resistance was also deteriorated. No. No. 17 is No. Although the cooling rate is not as fast as 16, the surface layer structure is ferrite. However, since the cooling rate is out of the range of the present invention, the hardness of the surface layer is increased and the SSCC resistance is deteriorated. No. In Nos. 18 and 19, the controlled cooling conditions were out of the range of the present invention, and no bainite structure was obtained at the center of the plate thickness as the microstructure, and the samples had low strength. In No. 20 to No. 23, the component composition of the steel sheet was out of the range of the present invention, and HIC cracking occurred.
本発明によれば、より厳しい腐食環境下での耐ＨＩＣ性及び耐ＳＳＣＣ性に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。 ADVANTAGE OF THE INVENTION According to this invention, the high strength steel plate for sour line pipes excellent in HIC resistance and SSCC resistance under severer corrosion environment can be supplied. Therefore, a steel pipe (such as an electric resistance welded steel pipe, a spiral steel pipe, or a UOE steel pipe) manufactured by cold-forming this steel sheet can be suitably used for transporting crude oil or natural gas containing hydrogen sulfide that requires sour resistance. .
Claims (7)
鋼板表面下０．５ｍｍにおける鋼組織が、転位密度０．５×１０14〜７．０×１０14（ｍ-2）のフェライト組織であり、
板厚中央における鋼組織がベイナイト組織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。
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。 In 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% and Ca: 0.0005 to 0.005%, and has a CP value of 1.00 or less determined by the following formula (1). And the balance has a component composition consisting of Fe and unavoidable impurities,
The steel structure at 0.5 mm below the surface of the steel sheet is a ferrite structure having a dislocation density of 0.5 × 10 14 to 7.0 × 10 14 (m −2 ),
A high-strength steel plate for a sour-resistant line pipe, wherein the steel structure at the center of the plate thickness is a bainite structure.
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)
Here, [% X] indicates the content (% by mass) of X element in steel.
その後前記鋼板に対して、（Ａｒ3＋２０℃）以下の鋼板表面温度で、鋼板表面での噴射流の衝突圧が１ＭＰａ以上のデスケーリングを行い、
その後前記鋼板に対して、
冷却開始時の鋼板表面温度：（Ａｒ3−１００℃）以上（Ａｒ3−１０℃）以下、
鋼板表面下０．５ｍｍにおける鋼板温度で７５０℃から５５０℃までの平均冷却速度：８０℃／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、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って、鋼板表面下０．５ｍｍにおける鋼組織が、転位密度０．５×１０ 14 〜７．０×１０ 14 （ｍ -2 ）のフェライト組織であり、板厚中央における鋼組織がベイナイト組織である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。
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。 In 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% and Ca: 0.0005 to 0.005%, and has a CP value of 1.00 or less determined by the following formula (1). And the remainder is heated to a temperature of 1000 to 1300 ° C., after the steel slab having the component composition of Fe and unavoidable impurities is hot-rolled into a steel sheet,
Thereafter, the steel sheet is subjected to descaling at a steel sheet surface temperature of (Ar 3 + 20 ° C.) or less and a collision pressure of a jet flow on the steel sheet surface of 1 MPa or more,
Then, for the steel plate,
Steel sheet surface temperature at the start of cooling: (Ar 3 -100 ° C) or more and (Ar 3 -10 ° C) or less,
Average cooling rate from 750 ° C. to 550 ° C. at a steel sheet temperature 0.5 mm below the steel sheet surface: 80 ° C./s or less,
Average cooling rate from 750 ° C to 550 ° C at average steel sheet temperature: 15 ° C / s or more, and cooling stop temperature at average steel sheet temperature: 250 to 550 ° C
What condition row control cooling at the, steel structure in the surface of the steel sheet under 0.5mm is a ferritic structure of the dislocation density 0.5 × 10 14 ~7.0 × 10 14 (m -2), mid-thickness A method for producing a high-strength steel sheet for a sour-resistant line pipe, characterized by producing a high-strength steel sheet having a bainite structure in (a) .
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)
Here, [% X] indicates the content (% by mass) of X element in steel.
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