JP5392441B1 - Steel tube for high-strength line pipe excellent in resistance to hydrogen-induced cracking, steel plate for high-strength line pipe used therefor, and production method thereof - Google Patents

Steel tube for high-strength line pipe excellent in resistance to hydrogen-induced cracking, steel plate for high-strength line pipe used therefor, and production method thereof Download PDF

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JP5392441B1
JP5392441B1 JP2013533435A JP2013533435A JP5392441B1 JP 5392441 B1 JP5392441 B1 JP 5392441B1 JP 2013533435 A JP2013533435 A JP 2013533435A JP 2013533435 A JP2013533435 A JP 2013533435A JP 5392441 B1 JP5392441 B1 JP 5392441B1
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卓也 原
泰志 藤城
太郎 村木
豪 鈴木
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Abstract

肉厚と外径の比が０．０３５以上であっても鋼管の表層での割れを防止できる耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管であって、所定の成分組成を有し、表裏両板面の最表面から５ｍｍの深さまでの表層領域の最高硬度が３００Ｈｖ以下、表裏両板面の最表面から５ｍｍの深さまでの表層領域におけるポリゴナルフェライトとアスペクト比が３以上の加工フェライトの合計分率：０．１〜２０％であることを特徴とする。   A steel pipe for a high-strength line pipe excellent in hydrogen-induced crack resistance that can prevent cracking in the surface layer of the steel pipe even if the ratio of the wall thickness to the outer diameter is 0.035 or more, and has a predetermined component composition Processing with a maximum hardness of 300 Hv or less in the surface layer region from the outermost surface of both front and back plate surfaces to a depth of 5 mm and a polygonal ferrite and aspect ratio of 3 or more in the surface layer region from the outermost surface of both front and rear plate surfaces to a depth of 5 mm The total fraction of ferrite is 0.1 to 20%.

Description

本発明は、石油、天然ガス等の輸送用ラインパイプ等の用途に最適な耐水素誘起割れ性（以下「耐ＨＩＣ性」という）に優れたＡＰＩ規格Ｘ６０〜Ｘ８０（ＴＳ＝５００〜６５０ＭＰａ）のラインパイプ用鋼管及びこれに用いるラインパイプ用鋼板に関する。   The present invention is an API standard X60 to X80 (TS = 500 to 650 MPa) excellent in hydrogen-induced cracking resistance (hereinafter referred to as “HIC resistance”) that is optimal for uses such as transportation line pipes for petroleum and natural gas. The present invention relates to a steel pipe for a line pipe and a steel plate for a line pipe used therefor.

湿潤硫化水素（Ｈ２Ｓ）ガスが存在する環境（以下「サワー環境」という）が、石油、天然ガスの掘削、生産、輸送に存在し、そこで使用される鋼管はサワー環境に曝される。石油、天然ガス等の輸送用ラインパイプがサワー環境に曝されると、水素誘起割れ（以下「ＨＩＣ」という）の発生が懸念される。これは、サワー環境においては、表面から鋼中に水素が侵入しやすいためである。An environment in which wet hydrogen sulfide (H 2 S) gas exists (hereinafter referred to as “sour environment”) exists in the drilling, production, and transportation of oil and natural gas, and steel pipes used therein are exposed to the sour environment. If transportation line pipes for petroleum, natural gas, etc. are exposed to a sour environment, there is a concern about the occurrence of hydrogen-induced cracking (hereinafter referred to as “HIC”). This is because in the sour environment, hydrogen easily enters the steel from the surface.

ＨＩＣは、特に鋼の中心偏析部に存在する延伸化したＭｎＳや集積したＴｉやＮｂの炭窒化物あるいは酸化物集積帯における酸化物系介在物などの、鋼中の欠陥の周囲に集積した水素に起因する。   HIC is hydrogen accumulated around defects in steel, such as stretched MnS present in the center segregation part of steel, accumulated Ti and Nb carbonitrides, and oxide inclusions in the oxide accumulation zone. caused by.

サワー環境では鋼中に侵入した水素が欠陥の周囲に集積してガスとなり、その圧力による応力拡大係数（ＫＩ）が水素を含有する鋼の応力拡大係数（ＫＩＨ）を超えると、ＨＩＣが発生する。さらに、鋼の中心偏析部、介在物の周辺などが硬化していると、ＨＩＣが伝播しやすくなる。そこで、従来から、サワー環境で使用されるラインパイプについては、耐ＨＩＣ性を改善するため、延伸化したＭｎＳの生成、Ｔｉ、Ｎｂの炭窒化物の集積や、酸化物の集積を抑制し、また、中心偏析の硬化相の形成を抑制するなど、種々の提案がなされている。In the sour environment, hydrogen that has penetrated into the steel accumulates around the defect and becomes gas. When the stress intensity factor (K I ) due to the pressure exceeds the stress intensity factor (K IH ) of the steel containing hydrogen, the HIC Occur. Furthermore, when the center segregation part of steel, the periphery of inclusions, and the like are hardened, the HIC easily propagates. Therefore, conventionally, for line pipes used in sour environments, in order to improve HIC resistance, the production of stretched MnS, the accumulation of Ti and Nb carbonitrides, and the accumulation of oxides are suppressed, Various proposals have been made such as suppressing the formation of a center segregation hardening phase.

特許文献１〜３には、鋼板の中心へのＭｎの偏析を抑制することによって耐ＨＩＣ性を改善する方法が、開示されている。特許文献１には、鋼中の平均Ｍｎ含有量に対する偏析部のＭｎ含有量の比を抑制した鋼板が提案されている。特許文献２及び３には、Ｍｎ偏析スポットの大きさに加えて、偏析部のＰ濃度を限定し、さらに、Ｃａを活用した高強度ラインパイプが開示されている。   Patent Documents 1 to 3 disclose a method for improving the HIC resistance by suppressing the segregation of Mn to the center of the steel sheet. Patent Document 1 proposes a steel sheet in which the ratio of the Mn content of the segregation part to the average Mn content in the steel is suppressed. Patent Documents 2 and 3 disclose a high-strength line pipe that limits the P concentration of the segregation part in addition to the size of the Mn segregation spot and further utilizes Ca.

特許文献４には、Ｍｎの偏析に加えて、Ｎｂの偏析にも着目した、耐ＨＩＣ性に優れる熱延鋼板が開示されている。特許文献５、６では、Ｔｉ、Ｎｂの炭化物、窒化物などの介在物を抑制して耐ＨＩＣ性を改善する方法が開示されている。   Patent Document 4 discloses a hot-rolled steel sheet having excellent HIC resistance, focusing on Nb segregation in addition to Mn segregation. Patent Documents 5 and 6 disclose methods for improving HIC resistance by suppressing inclusions such as carbides and nitrides of Ti and Nb.

特許文献７、８には、Ｍｎ、Ｎｂ、Ｔｉの偏析を抑制し、さらに、中心偏析部の最高硬度を３００Ｈｖ以下とすることで、ＨＩＣの発生を防止した鋼管が開示されている。   Patent Documents 7 and 8 disclose steel pipes that prevent generation of HIC by suppressing segregation of Mn, Nb, and Ti and further setting the maximum hardness of the central segregation portion to 300 Hv or less.

特開平６−２２０５７７号公報Japanese Patent Laid-Open No. 6-220577 特開平６−２５６８９４号公報JP-A-6-256894 特開平６−２７１９７４号公報JP-A-6-271974 特開２００２−３６３６８９号公報JP 2002-36389 A 特開２００６−６３３５１号公報JP 2006-63351 A 特開２００８−７８４１号公報JP 2008-7841 A 特開２０１０−２０９４６０号公報JP 2010-209460 A 特開２０１０−２０９４６１号公報JP 2010-209461 A

前述のように、従来から、Ｍｎの偏析の抑制やＣａを利用したＭｎＳの形態制御に関する開発は盛んに行われている。しかしながら、（偏析部の最大Ｍｎ含有量）／（鋼中の平均Ｍｎ含有量）や、Ｍｎ偏析スポットの大きさを制御するだけでは、ＨＩＣを完全に防止することは困難であり、そこで、より厳密に制御することが必要である。   As described above, conventionally, development relating to suppression of segregation of Mn and morphology control of MnS using Ca has been actively performed. However, it is difficult to completely prevent HIC only by controlling (maximum Mn content of segregation part) / (average Mn content in steel) and the size of Mn segregation spot. It needs to be strictly controlled.

また、Ｍｎの偏析を解消しても、Ｎｂの偏析が問題になる。Ｎｂの偏析についても、（偏析部の最大Ｎｂ含有量）／（鋼中の平均Ｎｂ含有量）の制御だけでは不十分であり、より厳密に制御する必要がある。また、Ｎｂ−Ｔｉ−Ｃ−Ｎ系の介在物の長さや、（Ｔｉ、Ｎｂ）（Ｃ、Ｎ）系介在物の面密度及び長さを制御しても、それだけでは、ＨＩＣの発生を確実に防止することは困難であった。   Even if the segregation of Mn is eliminated, the segregation of Nb becomes a problem. Also for the segregation of Nb, it is not sufficient to control (the maximum Nb content of the segregation part) / (the average Nb content in the steel), and it is necessary to control it more strictly. In addition, even if the length of Nb-Ti-C-N inclusions and the surface density and length of (Ti, Nb) (C, N) inclusions are controlled, that alone will surely generate HIC. It was difficult to prevent.

さらに、近年、ラインパイプでは、深海を横断するプロジェクトが数多くあり、鋼管が圧潰しないように、肉厚（ｔ）と外径（Ｄ）の比（ｔ／Ｄ）が極めて高い鋼管（ｔ／Ｄ≧０．０３５）が要求されている。高ｔ／Ｄ鋼管の製造においては、鋼板から鋼管へ成形する際に成形ひずみが鋼管の内外表面に多く加わり、鋼管の内外表面付近に介在物が生成される。サワー環境などでは、鋼管の内外表面付近に介在物が存在するとＨＩＣが多発する。そのため、サワー環境下で使用可能な高ｔ／Ｄ鋼管の製造は困難であった。   Furthermore, in recent years, there are many projects in the line pipe that cross the deep sea, and the ratio of the wall thickness (t) to the outer diameter (D) (t / D) is extremely high so that the steel pipe does not crush (t / D). ≧ 0.035) is required. In the production of a high t / D steel pipe, a large amount of forming strain is applied to the inner and outer surfaces of the steel pipe when forming from the steel sheet to the steel pipe, and inclusions are generated near the inner and outer surfaces of the steel pipe. In a sour environment or the like, if there are inclusions near the inner and outer surfaces of the steel pipe, HIC occurs frequently. Therefore, it has been difficult to produce a high t / D steel pipe that can be used in a sour environment.

本発明は、以上の実情に鑑みてなされたものであり、深海を横断するラインパイプ等に使用される鋼管に最適な、ｔ／Ｄが極めて高く、鋼管全体として優れた耐ＨＩＣ性を有し、かつ鋼板の表層でのＨＩＣを防止したラインパイプ用鋼管及びこれに用いるラインパイプ鋼板の提供を課題とする。   The present invention has been made in view of the above circumstances, and is most suitable for steel pipes used for line pipes traversing the deep sea, and has an extremely high t / D and excellent HIC resistance as a whole steel pipe. An object of the present invention is to provide a steel pipe for a line pipe that prevents HIC on the surface layer of the steel sheet and a line pipe steel sheet used therefor.

本発明者らは、ｔ／Ｄが高くても、鋼管の内外表面付近で優れた耐ＨＩＣ性を有し表層でのＨＩＣを防止できる耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管及びこれに用いる鋼板を得るための条件について鋭意研究を行った。   The present inventors have a steel pipe for a high-strength line pipe that has excellent HIC resistance near the inner and outer surfaces of the steel pipe even when t / D is high and has excellent resistance to hydrogen-induced cracking that can prevent HIC on the surface layer, and The earnest research was done about the conditions for obtaining the steel plate used for this.

ｔ／Ｄが高くても、鋼管の内外表面付近で優れた耐ＨＩＣ性を有し表層でのＨＩＣを防止するためには、従来のラインパイプ用鋼管のように中心偏析部の硬さを低くするだけではなく、さらに、表層領域の硬さを低くする必要がある。一般には、表層領域は冷却速度が速く、硬くなりやすい。本発明者らは、鋼板圧延後の冷却条件を最適化することによって、従来３５０Ｈｖ程度であった鋼板の表層領域の硬さを、３００Ｈｖ以下と低くすることが可能となり、その結果、高ｔ／Ｄの鋼管であっても内外表面付近での介在物からのＨＩＣ発生を抑制でき、鋼管の内外表面付近で優れた耐ＨＩＣ性を有する鋼管が得られることを見出した。本発明は上記の知見に基づきなされたものであって、その要旨は、以下のとおりである。   Even if t / D is high, in order to have excellent HIC resistance near the inner and outer surfaces of the steel pipe and to prevent HIC on the surface layer, the hardness of the central segregation part is reduced as in conventional steel pipes for line pipes. In addition, it is necessary to lower the hardness of the surface layer region. In general, the surface layer region has a high cooling rate and tends to become hard. By optimizing the cooling conditions after rolling the steel sheet, the present inventors can reduce the hardness of the surface layer region of the steel sheet, which was conventionally about 350 Hv, to 300 Hv or less, resulting in a high t / It was found that even with the steel pipe of D, HIC generation from inclusions in the vicinity of the inner and outer surfaces can be suppressed, and a steel pipe having excellent HIC resistance near the inner and outer surfaces of the steel pipe can be obtained. The present invention has been made based on the above findings, and the gist thereof is as follows.

（１）母材となる鋼板が質量％で、Ｃ：０．０２〜０．０８％、Ｓｉ：０．０１〜０．５％、Ｍｎ：１．２〜１．８％、Ｎｂ：０．００１〜０．１０％、Ｃａ：０．０００５〜０．００５０％、Ｎ：０．００１０〜０．００６０％、Ｏ：０．０００１〜０．００３５％を含有し、かつ、Ｐ：０．０１％以下、Ｓ：０．００２０％以下、Ａｌ：０．０３０％以下、Ｔｉ：０．０３０％以下に制限し、Ｓ、Ｃａの含有量が、Ｓ／Ｃａ＜０．５を満足し、残部がＦｅ及び不可避的不純物であり、最大Ｍｎ偏析度：２．０以下、Ｎｂ偏析度：４．０以下、Ｔｉ偏析度：４．０以下、中心偏析部の未圧着部の長さ：０．１ｍｍ以下、中心偏析部の最高硬度：３００Ｈｖ以下、表裏両板面の最表面から５ｍｍの深さまでの表層領域の最高硬度：３００Ｈｖ以下、表裏両板面の最表面から５ｍｍの深さまでの表層領域におけるポリゴナルフェライトとアスペクト比が３以上の加工フェライトの合計分率：０．１〜２０％であり、鋼板の厚さｔ［ｍｍ］と造管後の鋼管の外径Ｄ［ｍｍ］が、ｔ≧２５、ｔ／Ｄ≧０．０３５を満たすことを特徴とする耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管。   (1) The steel plate used as a base material is mass%, C: 0.02-0.08%, Si: 0.01-0.5%, Mn: 1.2-1.8%, Nb: 0.0. 001 to 0.10%, Ca: 0.0005 to 0.0050%, N: 0.0010 to 0.0060%, O: 0.0001 to 0.0035%, and P: 0.01 %, S: 0.0020% or less, Al: 0.030% or less, Ti: 0.030% or less, S and Ca contents satisfy S / Ca <0.5, the balance Fe and unavoidable impurities, maximum Mn segregation degree: 2.0 or less, Nb segregation degree: 4.0 or less, Ti segregation degree: 4.0 or less, length of unbonded part of center segregation part: 0.0. 1 mm or less, the maximum hardness of the center segregation part: 300 Hv or less, the maximum hardness of the surface layer region from the outermost surfaces of the front and back plate surfaces to a depth of 5 mm: 300 Hv Below, the total fraction of the polygonal ferrite and the processed ferrite having an aspect ratio of 3 or more in the surface layer region from the outermost surface of the front and back plate surfaces to a depth of 5 mm is 0.1 to 20%, and the thickness t [ mm] and the outer diameter D [mm] of the steel pipe after pipe forming satisfy t ≧ 25 and t / D ≧ 0.035. A steel pipe for high-strength line pipe excellent in resistance to hydrogen-induced cracking.

（２）前記鋼板が、質量％で、さらに、Ｎｉ：０．０１〜２．０％、Ｃｕ：０．０１〜１．０％、Ｃｒ：０．０１〜１．０％、Ｍｏ：０．０１〜０．６０％、Ｗ：０．０１〜１．０％、Ｖ：０．０１〜０．１０％、Ｚｒ：０．０００１〜０．０５０％、Ｔａ：０．０００１〜０．０５０％、Ｂ：０．０００１〜０．００２０％、ＲＥＭ：０．０００１〜０．０１％、Ｍｇ：０．０００１〜０．０１％、Ｙ：０．０００１〜０．００５％、Ｈｆ：０．０００１〜０．００５％、及びＲｅ：０．０００１〜０．００５％のうち１種又は２種以上を含有することを特徴とする前記（１）の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管。   (2) The said steel plate is the mass%, and also Ni: 0.01-2.0%, Cu: 0.01-1.0%, Cr: 0.01-1.0%, Mo: 0.00. 01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, Ta: 0.0001 to 0.050% , B: 0.0001 to 0.0020%, REM: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, Y: 0.0001 to 0.005%, Hf: 0.0001 The high-strength line pipe excellent in resistance to hydrogen-induced cracking as described in (1) above, comprising 0.005% to 0.005% and Re: 0.0001 to 0.005% Steel pipe.

（３）表裏両板面の最表面から５ｍｍの深さまでの表層領域においてアスペクト比が３以上の加工フェライトが存在しないことを特徴とする前記（１）又は（２）の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管。   (3) In the hydrogen-induced crack resistance of (1) or (2) above, there is no processed ferrite having an aspect ratio of 3 or more in the surface layer region from the outermost surface of both front and back plate surfaces to a depth of 5 mm. Excellent steel pipe for high strength line pipe.

（４）前記（１）又は（２）の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管に用いる耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板。   (4) A steel sheet for high-strength line pipe excellent in hydrogen-induced crack resistance used in the steel pipe for high-strength line pipe excellent in hydrogen-induced crack resistance of (1) or (2).

（５）前記（３）の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管に用いる耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板。   (5) A steel sheet for high-strength line pipe excellent in hydrogen-induced crack resistance used in the steel pipe for high-strength line pipe excellent in hydrogen-induced crack resistance of (3) above.

（６）質量％で、Ｃ：０．０２〜０．０８％、Ｓｉ：０．０１〜０．５％、Ｍｎ：１．２〜１．８％、Ｎｂ：０．００１〜０．１０％、Ｃａ：０．０００５〜０．００５０％、Ｎ：０．００１０〜０．００６０％、Ｏ：０．０００１〜０．００３５％を含有し、かつ、Ｐ：０．０１％以下、Ｓ：０．００２０％以下、Ａｌ：０．０３０％以下、Ｔｉ：０．０３０％以下に制限し、Ｓ、Ｃａの含有量が、Ｓ／Ｃａ＜０．５を満足し、２次精錬後の水素の含有量が２．５ｐｐｍ以下であり、残部がＦｅ及び不可避的不純物である溶鋼を溶製する工程と、前記溶鋼を連続鋳造により鋼片とする工程と、前記鋼片を１０００℃以上に加熱する工程と、加熱した鋼片を、再結晶温度域での圧下比を２以上、未再結晶温度域での圧下比を３以上の熱間圧延し、鋼板を得る工程と、前記鋼板を、７５０℃以上の温度から４００〜６００℃になるまで冷却する冷却工程を含み、前記冷却工程は、前記鋼板の温度を上昇させる復熱処理を２回以上含み、前記復熱処理において、１回目の復熱処理の開始温度が３００℃以上であり、かつ、すべての復熱処理の終了温度が７５０℃未満であることを特徴とする耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板の製造方法。   (6) By mass%, C: 0.02-0.08%, Si: 0.01-0.5%, Mn: 1.2-1.8%, Nb: 0.001-0.10% , Ca: 0.0005 to 0.0050%, N: 0.0010 to 0.0060%, O: 0.0001 to 0.0035%, P: 0.01% or less, S: 0 0020% or less, Al: 0.030% or less, Ti: 0.030% or less, S and Ca contents satisfy S / Ca <0.5, and hydrogen after secondary refining A step of melting molten steel having a content of 2.5 ppm or less and the balance being Fe and inevitable impurities, a step of making the molten steel into a slab by continuous casting, and heating the slab to 1000 ° C. or more Hot rolling the process and the heated steel slab with a reduction ratio in the recrystallization temperature range of 2 or more and a reduction ratio in the non-recrystallization temperature range of 3 or more A step of obtaining a steel plate and a cooling step of cooling the steel plate from a temperature of 750 ° C. or higher to 400 to 600 ° C., wherein the cooling step includes a reheat treatment for increasing the temperature of the steel plate twice or more. In the reheat treatment, the first reheat treatment start temperature is 300 ° C. or higher, and the end temperatures of all reheat treatment are less than 750 ° C. Manufacturing method of steel plate for strength line pipe.

（７）前記溶鋼が、質量％で、さらに、Ｎｉ：０．０１〜２．０％、Ｃｕ：０．０１〜１．０％、Ｃｒ：０．０１〜１．０％、Ｍｏ：０．０１〜０．６０％、Ｗ：０．０１〜１．０％、Ｖ：０．０１〜０．１０％、Ｚｒ：０．０００１〜０．０５０％、Ｔａ：０．０００１〜０．０５０％、Ｂ：０．０００１〜０．００２０％、ＲＥＭ：０．０００１〜０．０１％、Ｍｇ：０．０００１〜０．０１％、Ｙ：０．０００１〜０．００５％、Ｈｆ：０．０００１〜０．００５％、及びＲｅ：０．０００１〜０．００５％のうち１種又は２種以上を含有することを特徴とする前記（６）の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板の製造方法。   (7) The molten steel is in mass%, and Ni: 0.01 to 2.0%, Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.00. 01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, Ta: 0.0001 to 0.050% , B: 0.0001 to 0.0020%, REM: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, Y: 0.0001 to 0.005%, Hf: 0.0001 The high-strength line pipe excellent in resistance to hydrogen-induced cracking as described in (6) above, containing 0.005% to 0.005% and Re: 0.0001 to 0.005% Steel plate manufacturing method.

（８）前記（６）又は（７）の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板の製造方法により得られた鋼板から鋼管を製造する方法であって、前記鋼板を管状に成形する工程と、突き合わせ部を溶接する工程を含み、鋼板の厚さｔ［ｍｍ］と造管後の鋼管の外径Ｄ［ｍｍ］が、ｔ≧２５、ｔ／Ｄ≧０．０３５を満たすことを特徴とする耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管の製造方法。   (8) A method of manufacturing a steel pipe from a steel plate obtained by the method for manufacturing a steel plate for high-strength line pipe excellent in hydrogen-induced crack resistance of (6) or (7), wherein the steel plate is formed into a tubular shape. And a step of welding the butt portion, and the thickness t [mm] of the steel sheet and the outer diameter D [mm] of the steel pipe after pipe forming satisfy t ≧ 25 and t / D ≧ 0.035. A method for producing a steel pipe for high-strength line pipes, which has excellent resistance to hydrogen-induced cracking.

本発明の高強度ラインパイプ用鋼管及び高強度ラインパイプ用鋼板は、Ｍｎ、Ｎｂ、Ｔｉの偏析が少なく、かつ中心偏析部の未圧着部の長さ及び最高硬さが抑えられ、しかも表層領域の硬さも抑えられている。その結果、耐ＨＩＣ性が確実かつ十分に優れており、サワー環境で使用されるラインパイプの素材として極めて優れている。   The steel pipe for high-strength linepipe and steel plate for high-strength linepipe of the present invention has less segregation of Mn, Nb, Ti, the length of the non-crimped part of the center segregation part and the maximum hardness, and the surface layer region The hardness of is also suppressed. As a result, the HIC resistance is surely and sufficiently excellent, and it is extremely excellent as a material for a line pipe used in a sour environment.

ＳとＣａの含有量の比Ｓ／ＣａとＨＩＣ試験におけるＨＩＣの長さ率（ＣＬＲ）との関係を示す図である。It is a figure which shows the relationship between ratio S / Ca of content of S and Ca, and the length ratio (CLR) of HIC in a HIC test. ポリゴナルフェライトと加工フェライトの合計の面積率とＨＩＣ試験におけるＨＩＣの面積率との関係を示す図である。It is a figure which shows the relationship between the total area ratio of polygonal ferrite and processed ferrite, and the area ratio of HIC in a HIC test. 本発明の製造方法における鋼板の冷却パターンの一例を示す図である。It is a figure which shows an example of the cooling pattern of the steel plate in the manufacturing method of this invention. 本発明の製造方法における鋼板の冷却パターンの他の例を示す図である。It is a figure which shows the other example of the cooling pattern of the steel plate in the manufacturing method of this invention. 従来の製造方法における鋼板の冷却パターンの一例を示す図である。It is a figure which shows an example of the cooling pattern of the steel plate in the conventional manufacturing method. 本発明のラインパイプ用鋼管の表層組織のＳＥＭ像である。It is a SEM image of the surface layer structure of the steel pipe for line pipes of the present invention. 本発明のラインパイプ用鋼管の表層組織の高度分布を示す図である。It is a figure which shows the altitude distribution of the surface layer structure of the steel pipe for line pipes of this invention. 従来のラインパイプ用鋼管の表層組織のＳＥＭ像である。It is a SEM image of the surface layer structure of the conventional steel pipe for line pipes. 従来のラインパイプ用鋼管の表層組織の硬度分布を示す図である。It is a figure which shows the hardness distribution of the surface layer structure | tissue of the conventional steel pipe for line pipes.

以下、本発明について詳細に説明する。   Hereinafter, the present invention will be described in detail.

本発明のラインパイプ用鋼管は、鋼板の厚さｔ［ｍｍ］と造管後の鋼管の外径Ｄ［ｍｍ］が、ｔ≧２５、ｔ／Ｄ≧０．０３５を満たすものであり、Ｍｎ、Ｎｂ、Ｔｉの偏析度、中心偏析部の未圧着部の長さ及び最高硬度、表裏両板面の最表面から５ｍｍの深さまでの表層領域の最高硬度及び組織を適切に規定したものである。   The steel pipe for line pipe of the present invention is such that the thickness t [mm] of the steel sheet and the outer diameter D [mm] of the steel pipe after pipe formation satisfy t ≧ 25 and t / D ≧ 0.035. , Nb, Ti segregation degree, length and maximum hardness of the non-crimped portion of the center segregation portion, maximum hardness and structure of the surface layer region from the outermost surface of both the front and back plate surfaces to a depth of 5 mm are appropriately defined. .

はじめに、本発明の鋼管及び鋼板に使用される鋼母材の成分組成の限定理由について説明する。以下、「％」は、すべて質量％を表すものとする。   First, the reasons for limiting the component composition of the steel base material used for the steel pipe and steel plate of the present invention will be described. Hereinafter, “%” represents all mass%.

〔Ｃ：０．０２〜０．０８％〕
Ｃは、鋼の強度を向上させる元素であり、０．０２％以上の添加が必要である。Ｃ量が０．０８％を超えると、炭化物の生成が促進されて耐ＨＩＣ性を損なう。より優れた耐ＨＩＣ性を確保し、溶接性、靱性の低下を抑制するためには、Ｃ量を０．０６％以下とすることが好ましい。[C: 0.02 to 0.08%]
C is an element that improves the strength of the steel and needs to be added in an amount of 0.02% or more. If the amount of C exceeds 0.08%, the formation of carbides is promoted and the HIC resistance is impaired. In order to secure more excellent HIC resistance and suppress deterioration of weldability and toughness, the C content is preferably 0.06% or less.

〔Ｓｉ：０．０１〜０．５％〕
Ｓｉは脱酸元素であり、０．０１％以上の添加が必要である。Ｓｉ量が０．５％を超えると、溶接熱影響部（ＨＡＺ）の靱性が低下する。[Si: 0.01 to 0.5%]
Si is a deoxidizing element, and addition of 0.01% or more is necessary. When the amount of Si exceeds 0.5%, the toughness of the weld heat affected zone (HAZ) decreases.

〔Ｍｎ：０．８〜１．８％〕
Ｍｎは、強度及び靱性を向上させる元素であり、０．８％以上の添加が必要である。Ｍｎ量が、１．８％を超えると、耐ＨＩＣ性が低下する。ＨＩＣをさらに抑制するには、Ｍｎ量を１．６％以下とすることが好ましい。[Mn: 0.8 to 1.8%]
Mn is an element that improves strength and toughness, and needs to be added in an amount of 0.8% or more. When the amount of Mn exceeds 1.8%, the HIC resistance decreases. In order to further suppress HIC, the Mn content is preferably 1.6% or less.

〔Ｎｂ：０．００１〜０．１０％〕
Ｎｂは、炭化物、窒化物を形成し、圧延まま鋼板の細粒化を促進し、強度の向上に寄与する元素である。その効果を得るためには、０．０００１％以上のＮｂを添加することが必要である。Ｎｂを過剰に添加すると、最大Ｎｂ偏析度が増加し、Ｎｂの炭窒化物の集積を招いて、耐ＨＩＣ性が低下するので、Ｎｂ量の上限は０．１０％とする。耐ＨＩＣ性をより重視する場合、Ｎｂ量は０．０５％以下にすることが好ましい。[Nb: 0.001 to 0.10%]
Nb is an element that forms carbides and nitrides, promotes the refinement of the steel sheet as it is rolled, and contributes to the improvement of strength. In order to obtain the effect, it is necessary to add 0.0001% or more of Nb. If Nb is added excessively, the maximum degree of segregation of Nb is increased, the accumulation of Nb carbonitrides is caused, and the HIC resistance is lowered. Therefore, the upper limit of the Nb amount is 0.10%. When HIC resistance is more important, the Nb content is preferably 0.05% or less.

〔Ｃａ：０．０００５〜０．００５０％〕
Ｃａは、硫化物ＣａＳを生成し、圧延方向に伸長するＭｎＳの生成を抑制し、耐ＨＩＣ性の改善に顕著に寄与する元素である。Ｃａの添加量が０．０００５％未満では、効果が得られない。Ｃａの添加量が０．００５０％を超えると、酸化物が集積し、耐ＨＩＣ性を損なう。[Ca: 0.0005 to 0.0050%]
Ca is an element that produces sulfide CaS, suppresses the production of MnS extending in the rolling direction, and contributes significantly to the improvement of HIC resistance. If the addition amount of Ca is less than 0.0005%, the effect cannot be obtained. When the addition amount of Ca exceeds 0.0050%, oxides accumulate and the HIC resistance is impaired.

〔Ｎ：０．００１０〜０．００６０％〕
Ｎは、ＴｉＮ、ＮｂＮなどの窒化物を形成する元素である。窒化物を利用して加熱時のオーステナイト粒径を微細にするためには、Ｎ量の添加量を０．００１０％以上とすることが必要である。Ｎの含有量が０．００６０％を超えると、ＴｉとＮｂの炭窒化物が集積しやすくなり、耐ＨＩＣ性を損なう。靭性が要求される場合には、ＴｉＮの粗大化を抑制するため、Ｎ量を０．００３５％以下にすることが好ましい。[N: 0.0010 to 0.0060%]
N is an element that forms nitrides such as TiN and NbN. In order to make the austenite grain size at the time of heating fine using nitride, it is necessary to make the amount of N added 0.0010% or more. When the N content exceeds 0.0060%, Ti and Nb carbonitrides are easily accumulated, and the HIC resistance is impaired. When toughness is required, the N content is preferably 0.0035% or less in order to suppress TiN coarsening.

〔Ｏ：０．０００１〜０．００３５％〕
Ｏは不純物であり、酸化物の集積を抑制して、耐ＨＩＣ性を向上させるために、０．００３５％以下に制限する。酸化物の生成を抑制して、母材及びＨＡＺ靭性を向上させるためには、Ｏ量を０．００３５％以下とすることが好ましく、０．００２０％以下がより好ましい。Ｏ量は少ないほど好ましいが、０．０００１％未満とするためには、精錬時間が長時間になってコストが高くなるので、下限を０．０００１％とする。[O: 0.0001 to 0.0035%]
O is an impurity and is limited to 0.0035% or less in order to suppress the accumulation of oxides and improve the HIC resistance. In order to suppress the formation of oxides and improve the base material and the HAZ toughness, the O content is preferably 0.0035% or less, and more preferably 0.0020% or less. The smaller the amount of O, the better. However, in order to make it less than 0.0001%, the refining time becomes long and the cost becomes high, so the lower limit is made 0.0001%.

〔Ｐ：０．０１％以下〕
Ｐは不純物であり、含有量が０．０１％を超えると、耐ＨＩＣ性を損ない、また、ＨＡＺの靱性が低下する。したがって、Ｐの含有量を０．０１％以下に制限する。[P: 0.01% or less]
P is an impurity. If the content exceeds 0.01%, the HIC resistance is impaired, and the toughness of the HAZ decreases. Therefore, the P content is limited to 0.01% or less.

〔Ｓ：０．０２０％以下〕
Ｓは、熱間圧延時に圧延方向に延伸するＭｎＳを生成して、耐ＨＩＣ性を低下させる元素である。本発明では、Ｓ量を０．００２０％以下に低減することが必要である。耐ＨＩＣ特性を向上させるためには、Ｓ量を０．００１０％以下とすることが好ましい。Ｓ量は、少ないほど好ましいが、０．０００１％未満にすることは困難であり、製造コストの観点から、下限を０．０００１％以上にすることが好ましい。[S: 0.020% or less]
S is an element that reduces the HIC resistance by generating MnS that extends in the rolling direction during hot rolling. In the present invention, it is necessary to reduce the amount of S to 0.0020% or less. In order to improve the HIC resistance, the S content is preferably 0.0010% or less. The smaller the amount of S, the better. However, it is difficult to make it less than 0.0001%, and the lower limit is preferably made 0.0001% or more from the viewpoint of manufacturing cost.

〔Ａｌ：０．０３０％以下〕
Ａｌは脱酸元素であるが、添加量が０．０３０％を超えるとＡｌ酸化物の集積クラスターが生成される。特に良好な靭性が要求される場合には、Ａｌ量を０．０１７％以下にすることが好ましい。Ａｌ量の下限値は特に限定しないが、溶鋼中の酸素量を低減させるためには、Ａｌを０．０００５％以上添加することが好ましい。[Al: 0.030% or less]
Al is a deoxidizing element, but when the addition amount exceeds 0.030%, an integrated cluster of Al oxides is generated. When particularly good toughness is required, the Al content is preferably 0.017% or less. Although the lower limit of the amount of Al is not particularly limited, it is preferable to add Al in an amount of 0.0005% or more in order to reduce the amount of oxygen in the molten steel.

〔Ｔｉ： ０．０３０％以下〕
Ｔｉは、通常、脱酸剤や窒化物形成元素として結晶粒の細粒化に利用される元素であるが、炭窒化物の形成によって耐ＨＩＣ性や靱性を低下させる元素である。したがって、Ｔｉの含有量は、０．０３０％以下に制限する。[Ti: 0.030% or less]
Ti is an element usually used as a deoxidizer or nitride-forming element for refining crystal grains, but is an element that reduces HIC resistance and toughness by forming carbonitride. Therefore, the Ti content is limited to 0.030% or less.

〔Ｓ／Ｃａ＜０．５〕
本発明では、Ｃａを添加して、ＣａＳを形成させることにより、Ｓの固定を図りＭｎＳの生成を抑制する。優れた耐ＨＩＣ性を得るためには、さらに、Ｓ／Ｃａの比を適切に規制することが必要である。図１に０．０４％Ｃ−１．２５％Ｍｎ鋼のＨＩＣ試験におけるＣＬＲ(ＨＩＣの長さ率)とＳ／Ｃａの関係を示す。図１に示すように、Ｓ／Ｃａの比が０．５以上になると、ＨＩＣが発生する。これは、Ｓ／Ｃａの比が０．５以上になると、ＭｎＳが生成されて、圧延時に延伸化したＭｎＳが形成され、その結果、耐ＨＩＣ性が劣化するためである。したがって、Ｓ／Ｃａの比は０．５未満とする必要がある。[S / Ca <0.5]
In the present invention, Ca is added to form CaS, thereby fixing S and suppressing generation of MnS. In order to obtain excellent HIC resistance, it is further necessary to appropriately regulate the S / Ca ratio. FIG. 1 shows the relationship between CLR (HIC length ratio) and S / Ca in the HIC test of 0.04% C-1.25% Mn steel. As shown in FIG. 1, when the S / Ca ratio is 0.5 or more, HIC occurs. This is because when the S / Ca ratio is 0.5 or more, MnS is generated and MnS stretched during rolling is formed, and as a result, the HIC resistance deteriorates. Therefore, the S / Ca ratio needs to be less than 0.5.

その他、本発明のラインパイプ用鋼管、ラインパイプ用鋼板には、必要に応じ、強度及び靱性を改善する元素として、Ｎｉ、Ｃｕ、Ｃｒ、Ｍｏ、Ｗ、Ｖ、Ｚｒ、Ｔａ、及びＢのうちから選ばれた１種又は２種以上の元素が添加してもよい。これらの任意の添加元素の添加量の限定理由は次のとおりである。   In addition, the steel pipe for line pipes and the steel sheet for line pipes of the present invention include Ni, Cu, Cr, Mo, W, V, Zr, Ta, and B as elements that improve strength and toughness as necessary. One or two or more elements selected from may be added. The reason for limiting the addition amount of these optional additive elements is as follows.

〔Ｎｉ：０．０１〜２．０％〕
Ｎｉは、靱性及び強度の改善に有効な元素であり、その効果を得るためには０．０１％以上の添加が必要である。Ｎｉの添加量が２．０％を超えると、耐ＨＩＣ性及び溶接性が低下する。[Ni: 0.01 to 2.0%]
Ni is an element effective for improving toughness and strength, and in order to obtain the effect, addition of 0.01% or more is necessary. When the added amount of Ni exceeds 2.0%, the HIC resistance and weldability deteriorate.

〔Ｃｕ：０．０１〜１．０％〕
Ｃｕは、靱性を低下させずに強度の上昇に有効な元素であり、その効果を得るためには０．０１％以上の添加が必要である。Ｃｕの添加量が１．０％を超えると、鋼片加熱時や溶接時に割れが生じやすくなる。[Cu: 0.01 to 1.0%]
Cu is an element effective for increasing strength without reducing toughness, and in order to obtain the effect, addition of 0.01% or more is necessary. If the added amount of Cu exceeds 1.0%, cracks are likely to occur during heating of the steel slab or during welding.

〔Ｃｒ：０．０１〜１．０％〕
Ｃｒは、析出強化による鋼の強度を向上させる元素であり、その効果を得るためには０．０１％以上の添加が必要である。Ｃｒの添加量が１．０％を超えると、焼入れ性が上昇して、ベイナイト組織が生じ、その結果、耐ＨＩＣ性、靱性が低下する。[Cr: 0.01 to 1.0%]
Cr is an element that improves the strength of steel by precipitation strengthening, and in order to obtain the effect, addition of 0.01% or more is necessary. When the added amount of Cr exceeds 1.0%, the hardenability is increased and a bainite structure is formed. As a result, the HIC resistance and the toughness are lowered.

〔Ｍｏ：０．０１〜０．６０％〕
Ｍｏは、焼入れ性を向上させると同時に、炭窒化物を形成し強度を改善する元素であり、その効果を得るためには、０．０１％以上の添加が必要である。Ｍｏの添加量が０．６０％を超えると、コストが上昇する。鋼の強度が過度に上昇すると、耐ＨＩＣ性及び靱性が低下することがあるので、好ましいＭｏの添加量は０．２０％以下である。[Mo: 0.01-0.60%]
Mo is an element that improves hardenability and at the same time forms carbonitrides and improves strength. In order to obtain the effect, addition of 0.01% or more is necessary. If the amount of Mo exceeds 0.60%, the cost increases. If the strength of the steel is excessively increased, the HIC resistance and toughness may be lowered. Therefore, the preferable addition amount of Mo is 0.20% or less.

〔Ｗ：０．０１〜１．０％〕
Ｗは、強度の向上に有効な元素であり、その効果を得るためには０．０１％以上の添加が必要である。Ｗの添加量が１．０％を超えると、靱性が低下することがある。[W: 0.01 to 1.0%]
W is an element effective for improving the strength, and in order to obtain the effect, addition of 0.01% or more is necessary. If the addition amount of W exceeds 1.0%, the toughness may be lowered.

〔Ｚｒ：０．０００１〜０．０５０％〕
Ｚｒは、Ｖと同様に炭化物、窒化物を形成して強度の向上に寄与する元素であり、その効果を得るためには、０．０００１％以上を添加することが必要である。Ｚｒの添加量が０．０５０％を超えると、靱性が低下することがある。[Zr: 0.0001 to 0.050%]
Zr is an element that contributes to the improvement of strength by forming carbides and nitrides like V, and in order to obtain the effect, it is necessary to add 0.0001% or more. If the amount of Zr added exceeds 0.050%, the toughness may decrease.

〔Ｔａ：０．０００１〜０．０５０％〕
Ｔａも、Ｖと同様に炭化物、窒化物を形成し強度の向上に寄与する元素であり、その効果を得るためには、０．０００１％以上を添加する必要がある。Ｔａの添加量が０．０５０％を超えると、靱性が低下することがある。[Ta: 0.0001 to 0.050%]
Ta, like V, is an element that forms carbides and nitrides and contributes to the improvement of strength. In order to obtain the effect, it is necessary to add 0.0001% or more. If the amount of Ta exceeds 0.050%, the toughness may decrease.

〔Ｂ：０．０００１〜０．００２０％〕
Ｂは、鋼の粒界に偏析して焼入れ性の向上に著しく寄与する元素である。この効果を得るには、０．０００１％以上のＢの添加が必要である。Ｂは、ＢＮを生成し、固溶Ｎを低下させて、溶接熱影響部の靱性の向上にも寄与する元素であるため、０．０００５％以上の添加が好ましい。Ｂの添加量が０．００２０％を超えると、粒界への偏析が過剰になり、靱性が低下することがある。[B: 0.0001 to 0.0020%]
B is an element that segregates at the grain boundaries of steel and contributes significantly to improving the hardenability. In order to obtain this effect, 0.0001% or more of B must be added. B is an element that generates BN, lowers the solid solution N, and contributes to the improvement of the toughness of the weld heat affected zone, so addition of 0.0005% or more is preferable. When the addition amount of B exceeds 0.0020%, segregation to the grain boundary becomes excessive, and the toughness may be lowered.

さらに、本発明のラインパイプ用鋼管、ラインパイプ用鋼板には、酸化物や硫化物などの介在物を制御するために、必要に応じて、ＲＥＭ（希土類元素）、Ｍｇ、Ｙ、Ｈｆ、及びＲｅのうちから選ばれた１種又は２種以上を添加してもよい。これらの任意の添加元素の添加量の限定理由は次のとおりである。   Furthermore, in order to control inclusions such as oxides and sulfides, the steel pipe for line pipes and the steel sheet for line pipes of the present invention include REM (rare earth element), Mg, Y, Hf, and One or more selected from Re may be added. The reason for limiting the addition amount of these optional additive elements is as follows.

〔ＲＥＭ（希土類元素）：０．０００１〜０．０１％〕
ＲＥＭは、脱酸剤及び脱硫剤として添加される元素であり、その効果を得るためには０．０００１％以上の添加が必要である。ＲＥＭの添加量が０．０１０％を超えると、粗大な酸化物を生じて、耐ＨＩＣ性や、母材及びＨＡＺの靱性が低下することがある。[REM (rare earth element): 0.0001 to 0.01%]
REM is an element added as a deoxidizing agent and a desulfurizing agent, and 0.0001% or more must be added to obtain the effect. When the amount of REM added exceeds 0.010%, coarse oxides are generated, and the HIC resistance, the toughness of the base material and HAZ may be lowered.

〔Ｍｇ：０．０００１〜０．０１％〕
Ｍｇは、脱酸剤及び脱硫剤として添加される元素であり、特に、微細な酸化物を生じて、ＨＡＺ靭性の向上にも寄与する。この効果を得るには、０．０００１％以上のＭｇを添加することが必要である。Ｍｇの添加量が０．０１０％を超えると、酸化物が凝集、粗大化しやすくなり、耐ＨＩＣ性や、母材及びＨＡＺの靱性が低下することがある。[Mg: 0.0001 to 0.01%]
Mg is an element added as a deoxidizing agent and a desulfurizing agent. In particular, a fine oxide is generated and contributes to improvement of HAZ toughness. In order to obtain this effect, it is necessary to add 0.0001% or more of Mg. If the added amount of Mg exceeds 0.010%, the oxide tends to aggregate and coarsen, and the HIC resistance and the toughness of the base material and HAZ may be lowered.

〔Ｙ：０．０００１〜０．００５％〕
Ｙは、Ｃａと同様、硫化物を生成し、圧延方向に伸長したＭｎＳの生成を抑制し、耐ＨＩＣ性の向上に寄与する元素である。このような効果を得るには、Ｙを０．０００１％以上添加することが必要である。Ｙの添加量が０．００５％を超えると、酸化物が増加して、凝集、粗大化し、耐ＨＩＣ性を損なう。[Y: 0.0001 to 0.005%]
Y, like Ca, is an element that generates sulfides, suppresses the generation of MnS elongated in the rolling direction, and contributes to improvement in HIC resistance. In order to obtain such an effect, it is necessary to add Y in an amount of 0.0001% or more. When the added amount of Y exceeds 0.005%, the oxides increase, aggregate and coarsen, and the HIC resistance is impaired.

〔Ｈｆ：０．０００１〜０．００５％〕
Ｈｆも、Ｃａと同様、硫化物を生成し、圧延方向に伸長したＭｎＳの生成を抑制し、耐ＨＩＣ性の向上に寄与する元素である。このような効果を得るには、Ｈｆを０．０００１％以上添加することが好ましい。一方、Ｈｆの量が０．００５％を超えると、酸化物が増加して、凝集、粗大化し、耐ＨＩＣ性を損なう。[Hf: 0.0001 to 0.005%]
Hf, like Ca, is an element that generates sulfides and suppresses the generation of MnS elongated in the rolling direction and contributes to improvement in HIC resistance. In order to obtain such an effect, it is preferable to add 0.0001% or more of Hf. On the other hand, if the amount of Hf exceeds 0.005%, the oxides increase, aggregate and coarsen, and the HIC resistance is impaired.

〔Ｒｅ：０．０００１〜０．００５％〕
Ｒｅも、Ｃａと同様、硫化物を生成し、圧延方向に伸長したＭｎＳの生成を抑制し、耐ＨＩＣ性の向上に寄与する元素である。このような効果を得るには、Ｒｅを０．０００１％以上添加する必要がある。Ｒｅの添加量が０．００５％を超えると、酸化物が増加して、凝集、粗大化し、耐ＨＩＣ性を損なう。[Re: 0.0001 to 0.005%]
Re, like Ca, is an element that generates sulfides and suppresses the generation of MnS elongated in the rolling direction and contributes to improvement in HIC resistance. In order to obtain such an effect, it is necessary to add 0.0001% or more of Re. When the added amount of Re exceeds 0.005%, the oxide increases, aggregates and coarsens, and the HIC resistance is impaired.

以上の各元素の残部は、Ｆｅ及び不可避的不純物である。なお、前述のＮｉ、Ｃｕ、Ｃｒ、Ｍｏ、Ｗ、Ｖ、Ｚｒ、Ｔａ、及びＢについては、いずれも前記の下限値未満の微量を不純物として含有することは許容される。また、ＲＥＭ、Ｍｇ、Ｙ、Ｈｆ、及びＲｅについても、それぞれの下限値未満の極微量を不純物として含有することは許容される。   The balance of each of the above elements is Fe and inevitable impurities. It should be noted that the above-described Ni, Cu, Cr, Mo, W, V, Zr, Ta, and B are all allowed to contain a trace amount less than the lower limit as an impurity. Also, REM, Mg, Y, Hf, and Re are allowed to contain trace amounts less than the respective lower limit values as impurities.

次に、本発明のラインパイプ用鋼管、及び鋼板の組織について説明する。   Next, the structure of the steel pipe for line pipes and the steel sheet of the present invention will be described.

〔最大Ｍｎ偏析度：２．０以下、Ｎｂ偏析度：４．０以下、Ｔｉ偏析度：４．０以下〕
ＨＩＣは、鋼の中心偏析部に存在する延伸化したＭｎＳや、集積したＴｉやＮｂの炭窒化物などの周囲に集積した水素に起因する。[Maximum Mn segregation degree: 2.0 or less, Nb segregation degree: 4.0 or less, Ti segregation degree: 4.0 or less]
The HIC is caused by hydrogen accumulated around the stretched MnS present in the center segregation portion of the steel, accumulated Ti, Nb carbonitride and the like.

延伸化した粗大なＭｎＳを抑制するためには、鋼板及び鋼管の最大Ｍｎ偏析度を２．０以下にすることが必要である。さらに、集積したＴｉ、Ｎｂの炭窒化物の抑制によって、ラインパイプ用鋼管及びラインパイプ用鋼板のＨＩＣの発生を顕著に防止できる。   In order to suppress the stretched coarse MnS, the maximum Mn segregation degree of the steel plate and the steel pipe needs to be 2.0 or less. Furthermore, by suppressing the accumulated Ti and Nb carbonitrides, the occurrence of HIC in the steel pipe for line pipe and the steel sheet for line pipe can be remarkably prevented.

Ｔｉ、Ｎｂの炭窒化物の集積を抑制するには、Ｎ量を０．００５０％以下、Ｃ量を０．０６％以下とし、かつ、ＮｂとＴｉの最大偏析度をそれぞれ４．０以下にすればよい。   To suppress the accumulation of carbonitrides of Ti and Nb, the N content is 0.0050% or less, the C content is 0.06% or less, and the maximum segregation degree of Nb and Ti is 4.0 or less, respectively. do it.

最大Ｍｎ偏析度とは、鋼板の板厚方向のＭｎ濃度分布、鋼管の管壁の肉厚方向のＭｎ濃度分布における、平均のＭｎ量に対する中心偏析部の最大のＭｎ量である。同様に、Ｎｂ偏析度とＴｉ偏析度は、鋼板の板厚方向のＮｂ、Ｔｉの濃度分布、鋼管の管壁の肉厚方向のＮｂ、Ｔｉの濃度分布における、平均のＮｂ量（Ｔｉ量）に対する中心偏析部の平均化した最大のＮｂ量（Ｔｉ量）である。   The maximum Mn segregation degree is the maximum Mn amount of the central segregation portion with respect to the average Mn amount in the Mn concentration distribution in the plate thickness direction of the steel sheet and the Mn concentration distribution in the thickness direction of the tube wall of the steel pipe. Similarly, the Nb segregation degree and the Ti segregation degree are the average Nb amount (Ti amount) in the concentration distribution of Nb and Ti in the plate thickness direction of the steel sheet and the concentration distribution of Nb and Ti in the thickness direction of the tube wall of the steel pipe. It is the maximum Nb amount (Ti amount) averaged at the center segregation part.

最大Ｍｎ偏析度は、ＥＰＭＡ（Ｅｌｅｃｔｒｏｎ Ｐｒｏｂｅ Ｍｉｃｒｏ Ａｎａｌｙｚｅｒ）、又はＥＰＭＡによる測定結果を画像処理することができるＣＭＡ（Ｃｏｍｐｕｔｅｒ Ａｉｄｅｄ Ｍｉｃｒｏ Ａｎａｌｙｚｅｒ）によって鋼板及び鋼管のＭｎ濃度分布を測定することにより求められる。測定対象は、ＨＩＣ試験片（２０ｍｍ幅×２０ｍｍ厚×１００ｍｍ長さ）とし、そのＨＩＣ試験片の２０ｍｍ幅（試験片幅）×２０ｍｍ厚（ＨＩＣ）試験片厚)を測定領域とする。Ｎｂ偏析度及びＴｉ偏析度についても同様に、同じ領域を、ＥＰＭＡ又はＣＭＡによって測定して、それぞれＮｂ濃度分布及びＴｉ濃度分布を測定すればよい。ＥＰＭＡ（又はＣＭＡ）のプローブ径は２μｍとする。   The maximum Mn segregation degree is obtained by measuring the Mn concentration distribution of the steel sheet and the steel pipe by EPMA (Electron Probe Micro Analyzer) or CMA (Computer Aided Micro Analyzer) capable of image processing the measurement result by EPMA. The measurement object is an HIC test piece (20 mm width × 20 mm thickness × 100 mm length), and the measurement area is 20 mm width (test piece width) × 20 mm thickness (HIC test piece thickness) of the HIC test piece. Similarly, for the Nb segregation degree and the Ti segregation degree, the same region may be measured by EPMA or CMA, and the Nb concentration distribution and the Ti concentration distribution may be measured, respectively. The probe diameter of EPMA (or CMA) is 2 μm.

最大Ｍｎ偏析度は、ＥＰＭＡを用い、５０μｍのビーム径で、２０ｍｍ幅（ＨＩＣ試験片幅）×２０ｍｍ厚（ＨＩＣ試験片厚）の測定領域におけるＭｎの濃度を、板厚方向及び板幅方向に等間隔で測定して、Ｍｎの濃度分布を測定し、測定したＭｎ濃度分布における平均値を平均Ｍｎ濃度とする。次に、最もＭｎ量が濃化していた個所を含む１ｍｍ（幅）×１ｍｍ（厚み）の領域について、ビーム径を２μｍに変更し、板厚方向及び板幅方向に等間隔で５０点×５０点のＭｎ濃度を測定し、分布から最大Ｍｎ濃度を求める。そして、２μｍのビーム径で得られた最大Ｍｎ濃度と、５０μｍのビーム径で得られた平均Ｍｎ濃度の比を、最大Ｍｎ偏析度と定義する。   For the maximum Mn segregation degree, the concentration of Mn in the measurement area of 20 mm width (HIC test piece width) × 20 mm thickness (HIC test piece thickness) is measured in the plate thickness direction and plate width direction using EPMA with a beam diameter of 50 μm. The Mn concentration distribution is measured at regular intervals, and the average value in the measured Mn concentration distribution is defined as the average Mn concentration. Next, for the region of 1 mm (width) × 1 mm (thickness) including the portion where the amount of Mn was most concentrated, the beam diameter was changed to 2 μm, and 50 points × 50 at equal intervals in the plate thickness direction and the plate width direction. The Mn concentration at the point is measured, and the maximum Mn concentration is obtained from the distribution. The ratio of the maximum Mn concentration obtained with a beam diameter of 2 μm and the average Mn concentration obtained with a beam diameter of 50 μm is defined as the maximum Mn segregation degree.

Ｎｂ偏析度、Ｔｉ偏析度も、同様にＥＰＭＡを用い、５０μｍのビーム径で２０ｍｍ幅（ＨＩＣ試験片幅）×２０ｍｍ厚（ＨＩＣ試験片厚）の測定領域におけるＮｂ、Ｔｉの板厚方向の濃度分布を測定した後、最もＮｂ、Ｔｉ量が濃化していた場所を、さらに２μｍのビーム径で１ｍｍ（幅）×１ｍｍ（厚み）の領域のＮｂ、Ｔｉの濃度を測定することにより求める。   Similarly, the Nb segregation degree and the Ti segregation degree use EPMA, and the concentration of Nb and Ti in the thickness direction in a measurement region of 20 mm width (HIC test piece width) × 20 mm thickness (HIC test piece thickness) with a beam diameter of 50 μm. After the distribution is measured, the place where the amount of Nb and Ti is most concentrated is obtained by measuring the concentration of Nb and Ti in a region of 1 mm (width) × 1 mm (thickness) with a beam diameter of 2 μm.

Ｎｂ、Ｔｉの炭窒化物などの介在物が存在すると、偏析度が見かけ上大きくなることがあるが、介在物が存在する場合には、Ｎｂ、Ｔｉの濃度分布で急激にピークが立ち上がる領域として判別することができるから、その領域での測定値は除いて各偏析度を求める。   When inclusions such as Nb and Ti carbonitrides exist, the degree of segregation may appear to be large. However, when inclusions exist, the region where the peaks suddenly rise in the concentration distribution of Nb and Ti. Since it can be discriminated, each segregation degree is obtained by excluding the measured value in that region.

次に、最大Ｍｎ偏析度、Ｎｂ偏析度及びＴｉ偏析度を抑制するための具体的方法を説明する。   Next, a specific method for suppressing the maximum Mn segregation degree, the Nb segregation degree, and the Ti segregation degree will be described.

Ｍｎ、Ｎｂ及びＴｉの偏析を抑制するには、連続鋳造における最終凝固時の軽圧下（英語 ｓｏｆｔ ｒｅｄｕｃｔｉｏｎ）が最適である。最終凝固時の軽圧下は、鋳造の冷却の不均一に起因する、凝固部と未凝固部との混在を解消するために施すものである。これにより、凝固収縮に伴う空隙をなくして、未凝固部の溶鋼流動を抑えて、鋼片を均一に凝固させることができる。   In order to suppress segregation of Mn, Nb, and Ti, light reduction (English soft reduction) at the time of final solidification in continuous casting is optimal. The light reduction at the time of final solidification is performed in order to eliminate mixing of the solidified portion and the unsolidified portion due to non-uniform cooling of the casting. Thereby, the space | gap accompanying a solidification shrinkage can be eliminated, the molten steel flow of an unsolidified part can be suppressed, and a steel piece can be solidified uniformly.

また、幅方向に不均一な凝固が生じた後で軽圧下を加えると、凝固部の変形抵抗が大きいことに起因して、未凝固部の溶鋼流動を抑えることができなくなる。したがって、このようなＷ型の凝固を生じさせないようにするためには、鋳片の最終凝固位置における中心固相率の幅方向の分布に応じて圧下量を制御しながら軽圧下することが好ましい。これにより、幅方向でも中心偏析が抑制され、最大Ｍｎ偏析度、Ｎｂ偏析度、Ｔｉ偏析度をさらに小さくすることができる。   Moreover, if light reduction is applied after uneven solidification has occurred in the width direction, the molten steel flow in the unsolidified portion cannot be suppressed due to the large deformation resistance of the solidified portion. Therefore, in order to prevent such W-type solidification from occurring, it is preferable to lightly reduce the amount of reduction while controlling the amount of reduction according to the distribution in the width direction of the central solid fraction at the final solidification position of the slab. . Thereby, center segregation is suppressed also in the width direction, and the maximum Mn segregation degree, Nb segregation degree, and Ti segregation degree can be further reduced.

〔中心偏析部の未圧着部の長さ：０．１ｍｍ以下〕
鋼板の中心偏析部に、長さ０．１ｍｍ以上の未圧着部があると、ＨＩＣの起点となり、耐ＨＩＣ性が劣化する。未圧着部とは、凝固時に鋼片に生じた空隙が、熱間圧延によって圧着されず、鋼板に残存したものである。未圧着部の長さは、超音波等の非破壊検査で測定できる。[Length of unseparated portion of center segregation portion: 0.1 mm or less]
If there is an uncompressed portion having a length of 0.1 mm or more in the center segregation portion of the steel plate, it becomes a starting point of HIC and the HIC resistance is deteriorated. The uncrimped portion is a portion in which a void generated in a steel piece during solidification is not crimped by hot rolling and remains on the steel plate. The length of the non-bonded portion can be measured by nondestructive inspection such as ultrasonic waves.

中心偏析部に未圧着部が残存する原因は、主として、熱間圧延前に鋼片に含まれる水素である。鋼を転炉及び２次精錬によって溶製した後連続鋳造する際に、鋼は凝固し、冷却されて収縮するので、特に鋼片の中心部には空隙が生じやすい。この空隙が負圧の場合、鋼片に含まれる水素量が多いと、水素ガスが空隙中に侵入する。２次精錬によって溶製した際に鋼に含有される水素は、ほとんどそのまま、連続鋳造後の鋼片中の空隙内に残存する。   The cause of the unbonded portion remaining in the center segregation portion is mainly hydrogen contained in the steel slab before hot rolling. When the steel is continuously cast after being melted by a converter and secondary refining, the steel is solidified, cooled, and contracted, so that a void is likely to be generated particularly at the center of the steel slab. When this void has a negative pressure, if the amount of hydrogen contained in the steel slab is large, hydrogen gas enters the void. The hydrogen contained in the steel when melted by secondary refining remains almost intact in the voids in the steel slab after continuous casting.

連続鋳造後の熱間圧延のための加熱時には、鋼片の組織が、面心立方晶で固溶できる水素量の多いオーステナイトであるので、水素が鋼片の外に放散されない。鋼片を加熱して熱間圧延により圧下を加えれば、鋼片に内部の空隙は小さくなるが、空隙に含まれる水素ガスの圧力は、空隙のサイズに反比例して高くなる。そのため、熱間圧延によっても空隙を圧着させることができず、鋼板内部、特に中心偏析部に未圧着部が残存する。   At the time of heating for hot rolling after continuous casting, the structure of the steel slab is austenite having a large amount of hydrogen that can be dissolved in face-centered cubic crystals, so that hydrogen is not diffused out of the steel slab. If the steel slab is heated and subjected to reduction by hot rolling, the internal voids in the steel slab become smaller, but the pressure of hydrogen gas contained in the voids increases in inverse proportion to the size of the voids. For this reason, the gap cannot be crimped even by hot rolling, and an uncompressed portion remains in the steel sheet, particularly in the central segregation portion.

本発明者らが鋼中の水素量と未圧着部の長さとの関係を詳細に調査した結果、鋼中の水素量を２．５ｐｐｍ以下に抑えれば、鋼板の中心偏析部に残存する未圧着部の長さが０．１ｍｍ以下になることが分かった。ここでいう鋼中の水素量は、２次精錬後に採取した溶鋼を燃焼法で測定したものである。   As a result of detailed investigations on the relationship between the amount of hydrogen in steel and the length of the uncompressed portion, the present inventors have found that if the amount of hydrogen in the steel is suppressed to 2.5 ppm or less, the amount remaining in the center segregation portion of the steel plate It was found that the length of the crimping part was 0.1 mm or less. The amount of hydrogen in the steel here is a value obtained by measuring molten steel collected after secondary refining by a combustion method.

２次精錬における水素量を低減するには、２次精錬を行う際の雰囲気中の水素分圧を低下させればよい。たとえば、雰囲気中に不活性ガスや、窒素などを吹き込むことで、水素分圧を低下させることができる。   In order to reduce the amount of hydrogen in secondary refining, the hydrogen partial pressure in the atmosphere during secondary refining may be reduced. For example, the hydrogen partial pressure can be reduced by blowing inert gas, nitrogen, or the like into the atmosphere.

なお、熱間圧延後に鋼板に残留する水素量は、鋼板が冷却され金属組織がオーステナイトからフェライト、ベイナイト、マルテンサイト、パーライトなどに変態すれば水素が外部に放散するので、２次精錬後の水素量に比べると低下する。   The amount of hydrogen remaining in the steel sheet after hot rolling is such that hydrogen is dissipated to the outside if the steel sheet is cooled and the metal structure is transformed from austenite to ferrite, bainite, martensite, pearlite, etc. Reduced compared to amount.

〔中心偏析部の最高硬度：３００Ｈｖ以下〕
耐ＨＩＣ性改善には、中心偏析部の最高硬さを３００Ｈｖ以下とすることが有効である。中心偏析部最高硬さの上限を３００Ｈｖ以下とすることによって、確実にＨＩＣの発生を防止することができる。中心偏析部の最高硬さは、３％硝酸＋９７％ナイタール溶液で腐食した後、ＪＩＳ Ｚ ２２４４に準拠し、２５ｇの荷重でビッカース硬さ試験を行って、測定する。中心偏析部とは、ＥＰＭＡやＣＭＡによって測定したＭｎの濃度が最大になる部位である。[Maximum hardness of center segregation part: 300 Hv or less]
In order to improve the HIC resistance, it is effective to set the maximum hardness of the center segregation part to 300 Hv or less. By setting the upper limit of the center segregation portion maximum hardness to 300 Hv or less, generation of HIC can be reliably prevented. The maximum hardness of the center segregation part is measured by corroding with 3% nitric acid + 97% nital solution and then performing a Vickers hardness test with a load of 25 g in accordance with JIS Z 2244. The center segregation part is a part where the concentration of Mn measured by EPMA or CMA is maximized.

〔表裏両板面の最表面から５ｍｍの深さまでの表層領域の最高硬度：３００Ｈｖ以下〕
鋼板における表裏の板面（圧延面）の最表面から板の厚み方向（深さ方向）へ５ｍｍまでの領域（表層領域）、及び鋼管における内外の最表面から管壁の厚み方向（深さ方向）へ５ｍｍまでの領域（表層領域）の最高硬さを、いずれも３００Ｈｖ以下とすると、耐ＨＩＣ性を確実に高めることができる。すなわち、最表面から５ｍｍまでの表層領域の最高硬さの上限を３００Ｈｖ以下とすることによって、ｔ／Ｄが０．０３５以上と高いラインパイプ用鋼管においても、確実に介在物やブリスターなどの表層に起因するＨＩＣの発生を防止することができる。表層領域の最高硬さは、最表面から５ｍｍの深さの位置まで、深さ方向に所定間隔（例えば０．１ｍｍ間隔）ごとにビッカース硬さ試験を行い、そのうちの最も高い値を最高硬さとする。具体的には、０．１３％硝酸＋９７％ナイタール溶液で腐食した後、ＪＩＳ Ｚ ２２４４に準拠し、２５ｇの荷重で、最表面から５ｍｍの深さの位置まで、０．１ｍｍ間隔の５０点×５０点のビッカース硬さ試験を行って、調べる。[Maximum hardness of the surface layer region from the outermost surface of both front and back plate surfaces to a depth of 5 mm: 300 Hv or less]
A region (surface region) from the outermost surface of the front and back plates (rolled surface) in the steel plate to the thickness direction (depth direction) of the plate up to 5 mm, and the thickness direction (depth direction) from the inner and outer outer surfaces of the steel pipe ) If the maximum hardness of the region (surface layer region) up to 5 mm is 300 Hv or less, the HIC resistance can be reliably improved. That is, by setting the upper limit of the maximum hardness of the surface layer region from the outermost surface to 5 mm to 300 Hv or less, even in steel pipes for line pipes with a high t / D of 0.035 or more, the surface layer of inclusions, blisters, etc. It is possible to prevent the occurrence of HIC due to. The maximum hardness of the surface layer region is determined by performing a Vickers hardness test at a predetermined interval (for example, 0.1 mm interval) in the depth direction from the outermost surface to a depth of 5 mm. To do. Specifically, after corroding with 0.13% nitric acid + 97% nital solution, according to JIS Z 2244, with a load of 25 g, 50 points at intervals of 0.1 mm from the outermost surface to a depth of 5 mm × A 50-point Vickers hardness test is performed and examined.

〔表裏両板面の最表面から５ｍｍの深さまでの表層領域におけるポリゴナルフェライトとアスペクト比が３以上の加工フェライトの合計分率：０．１〜２０％〕
耐ＨＩＣ性向上のためには、基本的には、母材の鋼組織が均一かつ微細なアシキュラーフェライト又はベイナイト組織であることが望ましい。したがって、耐ＨＩＣ性を考慮した本発明の高強度ラインパイプ用の鋼母材の組織は、基本的にはベイナイトあるいはアシキュラーフェライトが好ましい。しかしながら、厚肉のラインパイプ用鋼管は、ＤＷＴＴのような落重試験に耐え得る特性が要求される場合が多い。[Total fraction of polygonal ferrite and processed ferrite with an aspect ratio of 3 or more in the surface layer region from the outermost surface of both front and back plates to a depth of 5 mm: 0.1 to 20%]
In order to improve the HIC resistance, it is basically desirable that the steel structure of the base material is a uniform and fine acicular ferrite or bainite structure. Therefore, the structure of the steel base material for the high-strength line pipe of the present invention considering HIC resistance is basically preferably bainite or acicular ferrite. However, steel pipes for thick line pipes are often required to have characteristics that can withstand drop weight tests such as DWTT.

厚肉の鋼管の場合、従来のラインパイプ用鋼管の組織では、ＤＷＴＴのような落重試験に耐える特性は得られないが、０．１％以上のポリゴナルフェライト、加工フェライトを生成させれば、ＤＷＴＴ特性が改善される。しかしながら、表層領域のポリゴナルフェライトと加工フェライトの合計の分率が２０％を超える組織になると、耐ＨＩＣ性が急激に低下する。   In the case of a thick-walled steel pipe, the structure of a conventional steel pipe for line pipes does not have the characteristics to withstand a drop weight test such as DWTT, but if 0.1% or more of polygonal ferrite and processed ferrite are generated, DWTT characteristics are improved. However, when the total fraction of polygonal ferrite and processed ferrite in the surface layer region exceeds 20%, the HIC resistance is drastically lowered.

図２に、鋼管の表層のポリゴナルフェライトと加工フェライトの合計の面積率と、ＨＩＣ面積率との関係を示す。図中の３時、６時、９時は、試験片を採取した鋼管の周方向の位置であり、溶接部を０時とし、鋼管の底部から見た３時（９０°）、６時（１８０°）、９時（２７０°）の位置から試験片を採取し、組織を観察した。   FIG. 2 shows the relationship between the total area ratio of polygonal ferrite and processed ferrite on the surface layer of the steel pipe and the HIC area ratio. In the figure, 3 o'clock, 6 o'clock, and 9 o'clock are the positions in the circumferential direction of the steel pipe from which the test piece was taken, the welded part was taken as 0 o'clock, and 3 o'clock (90 °) and 6 o'clock as viewed from the bottom of the steel pipe ( 180 °), a specimen was taken from 9 o'clock (270 °), and the tissue was observed.

表層のポリゴナルフェライトと加工フェライトの合計の面積率が２０％を超えると、ＨＩＣ面積率は３％を大きく超える。したがって、表層領域にポリゴナルフェライトと加工フェライトが含まれる場合でも、耐ＨＩＣ性を確実に向上させるためには、ポリゴナルフェライトと加工フェライトの合計の分率を面積率で２０％以下に抑制することが望ましい。耐ＨＩＣ性の観点からは、加工フェライトは少ないほどよく、加工フェライトの分率が面積率で１０％以下であることが好ましく、存在しないことがより好ましい。   When the total area ratio of polygonal ferrite and processed ferrite in the surface layer exceeds 20%, the HIC area ratio greatly exceeds 3%. Therefore, even when polygonal ferrite and processed ferrite are included in the surface region, in order to reliably improve the HIC resistance, the total fraction of polygonal ferrite and processed ferrite is suppressed to 20% or less in terms of area ratio. It is desirable. From the viewpoint of HIC resistance, the smaller the processed ferrite, the better. The fraction of processed ferrite is preferably 10% or less in terms of area ratio, more preferably not present.

ポリゴナルフェライトと加工フェライトの分率の測定方法は、２００倍の光学顕微鏡写真を５枚撮影し、ポリゴナルフェライトと加工フェライトを抽出して、画像解析によって求めた値とする。２００倍の光学顕微鏡で観察した場合、白い領域がポリゴナルフェライト又は加工フェライトであり、アスペクト比（横長さと縦長さの比）が３未満のものをポリゴナルフェライト、アスペクト比が３以上のものを加工フェライトと定義する。   The method of measuring the fraction of polygonal ferrite and processed ferrite is to take five 200 × optical micrographs, extract the polygonal ferrite and processed ferrite, and use the values obtained by image analysis. When observed with a 200 × optical microscope, the white area is polygonal ferrite or processed ferrite, the aspect ratio (the ratio of the horizontal length to the vertical length) is less than 3, the polygonal ferrite is the one with an aspect ratio of 3 or more Defined as processed ferrite.

ここで、表層領域のポリゴナルフェライトと加工フェライトの合計分率を０．１％以上２０％以下に抑制するためには、後述する製造方法に従えばよい。すなわち、圧延終了温度及び／又は水冷開始温度を７５０℃以上にすれば、表層領域のポリゴナルフェライトと加工フェライトの分率を２０％以下にすることが可能である。ただし、圧延終了温度及び／又は水冷開始温度が７５０℃を下回ると、表層領域のポリゴナルフェライトと加工フェライトが２０％を越えて増加する傾向を示すので、圧延終了温度及び／又は水冷開始温度を７５０℃以上とすることが好ましい。また、ポリゴナルフェライトと加工フェライトの分率を１０％以下にするためには、圧延終了温度又は水冷開始温度を７７０℃以上することがさらに好ましい。   Here, in order to suppress the total fraction of polygonal ferrite and processed ferrite in the surface layer region to 0.1% or more and 20% or less, a manufacturing method described later may be followed. That is, if the rolling end temperature and / or the water cooling start temperature is set to 750 ° C. or higher, the fraction of polygonal ferrite and processed ferrite in the surface layer region can be set to 20% or lower. However, when the rolling end temperature and / or the water cooling start temperature is lower than 750 ° C., the polygonal ferrite and the processed ferrite in the surface layer region tend to increase by over 20%. It is preferable to set it as 750 degreeC or more. In order to make the fraction of polygonal ferrite and processed ferrite 10% or less, it is more preferable to set the rolling end temperature or the water cooling start temperature to 770 ° C. or higher.

上記の分率は、Ｌ断面（板厚方向と圧延方向の面）で観察したときの面積率を意味する。また、上記のポリゴナルフェライト、及び加工フェライト以外の組織、すなわち表層領域の８０％以上の面積を占める組織は、ベイナイト及び／又はアシキュラーフェライトパーライトであればよい。   The above-mentioned fraction means an area ratio when observed in the L cross section (surface in the plate thickness direction and rolling direction). Moreover, the structure other than the polygonal ferrite and the processed ferrite, that is, the structure occupying an area of 80% or more of the surface layer region may be bainite and / or acicular ferrite pearlite.

上記の表層領域より内部の側の組織は特に限定しないが、引張り強度が５００ＭＰａ以上の高強度ラインパイプ用鋼板、鋼管として、母材高強度、母材靭性、ＨＡＺ靭性、溶接性などを確保するためには、アシキュラーフェライト又はベイナイト主体の組織とすればよい。   The structure on the inner side from the surface layer region is not particularly limited, but as a steel plate and steel pipe for a high-strength line pipe with a tensile strength of 500 MPa or more, a high base metal strength, base material toughness, HAZ toughness, weldability, etc. are ensured. For this purpose, a structure mainly composed of acicular ferrite or bainite may be used.

次に本発明のラインパイプ用鋼板、ラインパイプ用鋼管を製造するための好ましい方法について説明する。   Next, the preferable method for manufacturing the steel plate for line pipes and the steel pipe for line pipes of this invention is demonstrated.

製鋼工程で、上述した成分組成を有する鋼を、２次精錬後の溶鋼中の水素量が２．５ｐｐｍ以下となるように常法により溶製後、連続鋳造により鋼片とし、鋼片を再加熱して厚板圧延を施し、鋼板とする。連続鋳造時には、上述のように、鋳片の最終凝固位置における中心固相率の幅方向の分布に応じて圧下量を制御しながら軽圧下を加えることが好ましい。   In the steelmaking process, the steel having the above-described composition is melted by a conventional method so that the hydrogen content in the molten steel after the secondary refining is 2.5 ppm or less, and then steel slab is obtained by continuous casting. Heat to thick plate rolling to obtain a steel plate. During continuous casting, as described above, it is preferable to apply light reduction while controlling the amount of reduction in accordance with the distribution in the width direction of the central solid phase ratio at the final solidification position of the slab.

連続鋳造後の鋼片の再加熱温度は１０００℃以上とし、再結晶温度域での圧下比を２以上、未再結晶域での圧下比を３以上にして厚板圧延を行えば、平均旧オーステナイト粒径を２０μｍ以下にすることができる。圧延終了後の鋼板に、水冷を、開始温度を７５０℃以上、停止温度を４００〜６００℃として施す。ここでいう水冷の停止温度とは、冷却水を停止し、その後復熱により上昇した鋼板の温度の最高温度をいう。   If the steel plate is rolled at a reheating temperature of 1000 ° C or higher after continuous casting, a reduction ratio in the recrystallization temperature range of 2 or more, and a reduction ratio in the non-recrystallization range of 3 or more, The austenite particle size can be 20 μm or less. The steel plate after rolling is subjected to water cooling at a start temperature of 750 ° C. or higher and a stop temperature of 400 to 600 ° C. The stop temperature of water cooling here refers to the maximum temperature of the steel sheet that has been raised by recuperation after stopping the cooling water.

再結晶温度域は、圧延後に再結晶が生じる温度範囲であり、本発明の鋼の成分では概ね９００℃超である。未再結晶温度域は、圧延後に再結晶及びフェライト変態が生じない温度範囲であり、本発明の鋼の成分では概ね７５０〜９００℃である。再結晶温度域での圧延を再結晶圧延又は粗圧延といい、未再結晶温度域での圧延を未再結晶圧延又は仕上げ圧延という。   The recrystallization temperature range is a temperature range where recrystallization occurs after rolling, and is generally higher than 900 ° C. for the components of the steel of the present invention. The non-recrystallization temperature range is a temperature range in which recrystallization and ferrite transformation do not occur after rolling, and is generally 750 to 900 ° C. for the steel component of the present invention. Rolling in the recrystallization temperature range is called recrystallization rolling or rough rolling, and rolling in the non-recrystallization temperature range is called non-recrystallization rolling or finish rolling.

未再結晶圧延後、７５０℃以上の温度から水冷を開始し、水冷停止温度を４００℃以上とすることにより、中心偏析部の最大硬度を３００Ｈｖ以下に抑制することができる。水冷開始温度を７５０℃未満にすると、冷却開始前にフェライトが多く生成され、フェライトからＣ（炭素）がオーステナイトへ排出され、オーステナイト相にＣが濃化される。その結果、Ｃが濃縮したオーステナイト相は、冷却過程で、多量のＣ量を含む硬質のマルテンサイトに変態する。   After the non-recrystallization rolling, the maximum hardness of the center segregation part can be suppressed to 300 Hv or less by starting water cooling from a temperature of 750 ° C. or higher and setting the water cooling stop temperature to 400 ° C. or higher. When the water cooling start temperature is less than 750 ° C., a large amount of ferrite is generated before the start of cooling, C (carbon) is discharged from the ferrite to austenite, and C is concentrated in the austenite phase. As a result, the austenite phase enriched with C is transformed into hard martensite containing a large amount of C during the cooling process.

これに対し、水冷開始温度を７５０℃以上にすると、硬質のマルテンサイトの生成を抑制できるので、中心偏析部の最大硬度を３００Ｈｖ以下に抑制することができる。また、水冷停止温度を４００℃以上にすると、変態後の硬質なマルテンサイトが一部分解するので、中心偏析部の最大硬度を３００Ｈｖ以下に抑制することができる。水冷停止温度が高すぎると鋼管の強度が低下するので、水冷停止温度は６００℃以下とする。   On the other hand, when the water cooling start temperature is set to 750 ° C. or higher, generation of hard martensite can be suppressed, so that the maximum hardness of the center segregation portion can be suppressed to 300 Hv or less. Further, when the water cooling stop temperature is set to 400 ° C. or higher, the hard martensite after transformation is partially decomposed, so that the maximum hardness of the center segregation portion can be suppressed to 300 Hv or less. If the water cooling stop temperature is too high, the strength of the steel pipe is lowered, so the water cooling stop temperature is set to 600 ° C. or less.

さらに、最表面から５ｍｍまでの表層領域の最高硬度を３００Ｈｖ以下に抑えるためには、水冷停止温度を４００℃から６００℃以下にする以外に、表層の冷却パターンを最適化する必要がある。具体的には、表層の冷却の際に、少なくとも２回以上の復熱を行うことにより、最表層から５ｍｍまでの最高硬度を３００Ｈｖ以下にすることができる。これは、復熱を行うことにより焼戻し効果が発揮されて、表層領域の硬度を下げることができるからである。復熱温度の下限は３００℃とし、上限温度は、７５０℃とすることが好ましい。復熱温度が３００℃未満になると、５０％以上のマルテンサイトが生じて、硬化し、表層の硬度が低下しなくなる。復熱温度が７５０℃を越えると、表層領域の硬度が下がりすぎる。   Furthermore, in order to suppress the maximum hardness of the surface layer region from the outermost surface to 5 mm to 300 Hv or less, it is necessary to optimize the cooling pattern of the surface layer in addition to setting the water cooling stop temperature to 400 ° C. to 600 ° C. or less. Specifically, when the surface layer is cooled, the maximum hardness from the outermost layer to 5 mm can be reduced to 300 Hv or less by performing recuperation at least twice. This is because the tempering effect is exhibited by recuperating and the hardness of the surface layer region can be lowered. The lower limit of the recuperation temperature is preferably 300 ° C., and the upper limit temperature is preferably 750 ° C. When the recuperation temperature is less than 300 ° C., 50% or more of martensite is generated and cured, and the hardness of the surface layer does not decrease. When the recuperation temperature exceeds 750 ° C., the hardness of the surface layer region is too low.

図３Ａ、図３Ｂに、本発明における冷却工程の冷却パターンの例を示す。グラフ中の１が復熱処理による温度変化であり、２の温度が復熱開始温度、３の温度が復熱終了温度である。グラフ中の４の温度が水冷停止温度である。このような冷却パターンは、冷却水を吐出するノズルのオン、オフの切り替えや、水量の調整により制御することができる。   3A and 3B show examples of cooling patterns in the cooling process according to the present invention. In the graph, 1 is a temperature change due to reheat treatment, 2 is a recuperation start temperature, and 3 is a recuperation end temperature. The temperature of 4 in the graph is the water cooling stop temperature. Such a cooling pattern can be controlled by switching on and off a nozzle that discharges cooling water, and adjusting the amount of water.

図３Ｃは、従来の製造方法による冷却パターンである。冷却水を停止した後は鋼板の温度が上昇するので、１回の復熱が含まれる。   FIG. 3C is a cooling pattern according to a conventional manufacturing method. Since the temperature of the steel plate rises after the cooling water is stopped, one recuperation is included.

さらに、上述した水冷開始温度を７５０℃以上にすることによって、最表面から５ｍｍまでの表層領域の組織を、ポリゴナルフェライトと加工フェライトの合計分率を２０％以下に抑制することができる。水冷開始温度が７５０℃より低温になると、５００ＭＰａ以上の鋼のγ／α変態温度を下回るので、ポリゴナルフェライトや加工フェライトが容易に生成し、ポリゴナルフェライトと加工フェライトの合計分率が２０％を超える。   Furthermore, by setting the above-described water cooling start temperature to 750 ° C. or higher, the total fraction of polygonal ferrite and processed ferrite can be suppressed to 20% or less in the structure of the surface layer region from the outermost surface to 5 mm. When the water cooling start temperature is lower than 750 ° C., it falls below the γ / α transformation temperature of steel of 500 MPa or more, so polygonal ferrite and processed ferrite are easily generated, and the total fraction of polygonal ferrite and processed ferrite is 20%. Over.

上述のようにして得られた鋼板を用いてラインパイプ用鋼管を製造するにあたっては、母材鋼板を管状に成形した後、突き合わせ部をアーク溶接し、溶接鋼管とすればよい。ここで、成形工程としては、鋼板をＣプレス、Ｕプレス、ＯプレスするＵＯＥ工程が好ましい。またアーク溶接としては、溶接金属の靭性と生産性の観点から、サブマージドアーク溶接を採用することが好ましい。アーク溶接時の入熱は特に限定しないが、通常は、２．０〜１５．０ｋＪ／ｍｍとすることが好ましい。   In manufacturing a steel pipe for a line pipe using the steel plate obtained as described above, the base steel plate is formed into a tubular shape, and then the butt portion is arc-welded to form a welded steel pipe. Here, the forming step is preferably a UOE step of C-pressing, U-pressing or O-pressing the steel plate. Moreover, as arc welding, it is preferable to employ submerged arc welding from the viewpoint of the toughness and productivity of the weld metal. Although the heat input at the time of arc welding is not specifically limited, Usually, it is preferable to set it as 2.0-15.0 kJ / mm.

以下、本発明について実施例によって具体的に説明する。なお、以下の実施例は、本発明による具体的な効果を示すためのものであって、実施例に記載された条件が本発明の技術的範囲を限定するものでないことはもちろんである。   Hereinafter, the present invention will be specifically described with reference to examples. It should be noted that the following examples are for showing specific effects according to the present invention, and of course, the conditions described in the examples do not limit the technical scope of the present invention.

表１Ａ〜表１Ｃに示す化学成分を有する鋼１〜３５を溶製し、連続鋳造により、厚みが２４０ｍｍ又は３００ｍｍである鋼片とした。また、表１Ａ〜表１Ｃには、溶鋼の水素量の分析値も示した。   Steels 1 to 35 having chemical components shown in Tables 1A to 1C were melted, and steel pieces having a thickness of 240 mm or 300 mm were obtained by continuous casting. Moreover, in Table 1A-Table 1C, the analytical value of the hydrogen content of molten steel was also shown.

連続鋳造では、最終凝固時に、圧下率約２％の軽圧下を実施した。得られた鋼片を１１００〜１２５０℃に加熱し、９００℃超の再結晶温度域で熱間圧延を行い、引き続き、７５０〜９５０℃の未再結晶温度域での熱間圧延を行った。   In continuous casting, light reduction with a reduction rate of about 2% was performed at the time of final solidification. The obtained steel slab was heated to 1100 to 1250 ° C., hot-rolled in a recrystallization temperature range exceeding 900 ° C., and subsequently hot-rolled in a non-recrystallization temperature range of 750 to 950 ° C.

熱間圧延後は、７５０℃以上から水冷を開始し、４００〜６００℃の温度で水冷を停止した。その間、１回又は２回、鋼板の温度を上昇させる復熱処理を行った。鋼板の温度は、冷却水を停止することによって上昇させた。１回目の復熱開始、終了温度、２回目の復熱開始温度を表２Ａに示す。２回目の復熱終了温度（復熱回数が１回の場合は１回目の復熱終了温度）は、冷却終了温度である。   After hot rolling, water cooling was started from 750 ° C. or higher, and water cooling was stopped at a temperature of 400 to 600 ° C. In the meantime, reheat treatment was performed to raise the temperature of the steel sheet once or twice. The temperature of the steel plate was raised by stopping the cooling water. The first recuperation start and end temperatures and the second recuperation start temperature are shown in Table 2A. The second recuperation end temperature (or the first recuperation end temperature when the number of recuperations is one) is the cooling end temperature.

得られた鋼板を、Ｃプレス、Ｕプレス、Ｏプレスによって管状に成形し、端面を仮付け溶接し、内外面から本溶接を行った後、拡管後、ラインパイプ用の鋼管とした。なお、本溶接には、サブマージドアーク溶接を適用した。   The obtained steel plate was formed into a tubular shape by a C press, U press, and O press, end surfaces were tack welded, main welding was performed from the inner and outer surfaces, and after pipe expansion, a steel pipe for a line pipe was obtained. Note that submerged arc welding was applied to the main welding.

得られた鋼板及び鋼管から引張試験片、ＨＩＣ試験片、マクロ試験片を採取し、それぞれの試験に供した。ＨＩＣ試験は、ＮＡＣＥＴＭ０２８４に準拠して行った。また、マクロ試験片を用いて、Ｍｎ、Ｎｂ、Ｔｉの偏析度をＥＰＭＡによって測定した。ＥＰＭＡによる偏析度の測定は、プローブ径５０μｍ及び２μｍで行った。   Tensile test pieces, HIC test pieces, and macro test pieces were collected from the obtained steel plates and steel pipes and used for each test. The HIC test was performed according to NACETM0284. Moreover, the segregation degree of Mn, Nb, and Ti was measured by EPMA using a macro test piece. The degree of segregation by EPMA was measured with probe diameters of 50 μm and 2 μm.

また、中心偏析部のビッカース硬度、及び鋼板、鋼管の最表面から深さ５ｍｍまでの表層領域におけるビッカース硬度をＪＩＳ Ｚ ２２４４に準拠して測定した。ビッカース硬度の測定は、荷重を２５ｇとし、ＥＰＭＡによって測定した厚み方向のＭｎ濃度の分布における、Ｍｎ濃度が最も高い部位で行った。   Further, the Vickers hardness of the central segregation part and the Vickers hardness in the surface layer region from the outermost surface of the steel plate and the steel pipe to the depth of 5 mm were measured according to JIS Z 2244. Vickers hardness was measured at a site where the load was 25 g and the Mn concentration was highest in the distribution of Mn concentration in the thickness direction measured by EPMA.

さらに、鋼管の最表面から５ｍｍの深さまでの表層領域のミクロ組織を同定するため、２００倍の光学顕微鏡写真を、Ｌ断面(板厚方向と圧延方向の面)についてそれぞれ５枚撮影して、ポリゴナルフェライトと加工フェライトの面積率（分率）を測定し、これらの合計分率を算出した。   Furthermore, in order to identify the microstructure of the surface layer region from the outermost surface of the steel pipe to a depth of 5 mm, five optical micrographs of 200 times were taken for each of the L cross sections (surfaces in the plate thickness direction and rolling direction), The area fraction (fraction) of polygonal ferrite and processed ferrite was measured, and the total fraction of these was calculated.

表２Ａ〜表２Ｃに、鋼板の板厚、最大Ｍｎ偏析度、Ｎｂ偏析度、Ｔｉ偏析度、未圧着部の長さ、中心偏析部の最高硬さ、表面領域の最高硬度、引張り強度及びＨＩＣ試験によって求められたＨＩＣの長さ率（ＣＬＲ）、表層領域におけるポリゴナルフェライトと加工フェライトの合計の分率を示す。   Tables 2A to 2C show steel sheet thickness, maximum Mn segregation degree, Nb segregation degree, Ti segregation degree, length of unbonded part, maximum hardness of central segregation part, maximum hardness of surface region, tensile strength and HIC. The length ratio (CLR) of HIC obtained by the test and the total fraction of polygonal ferrite and processed ferrite in the surface layer region are shown.

表３に、鋼管の肉厚、本溶接の入熱量、ＨＩＣ試験によって求められたＨＩＣの長さ率（ＣＬＲ）を示す。なお、鋼管における最大Ｍｎ偏析度、Ｎｂ偏析度、Ｔｉ偏析度、未圧着部の長さ、中心偏析部の最高硬さは、いずれも鋼板と同等であり、また鋼管の引張り強度は、鋼板よりも１０〜２０ＭＰａ程度大きくなっている。   Table 3 shows the thickness of the steel pipe, the heat input of the main welding, and the length ratio (CLR) of the HIC determined by the HIC test. In addition, the maximum Mn segregation degree, Nb segregation degree, Ti segregation degree, the length of the non-crimped part, and the maximum hardness of the central segregation part in the steel pipe are all equivalent to the steel sheet, and the tensile strength of the steel pipe is Is also increased by about 10 to 20 MPa.

鋼板１〜２３は本発明例である。表２Ａ〜表２Ｃに示すように、これらの鋼板の最大Ｍｎ偏析度は１．６以下、Ｎｂ偏析度は４．０以下、Ｔｉ偏析度は４．０以下であった。また、鋼板の上下面及び鋼管の内外面の最表層から５ｍｍ厚までの最高硬度が３００Ｈｖ以下、中心偏析部の最高硬さは３００Ｈｖ以下であった、さらに、ポリゴナルフェライトと加工フェライトの合計の分率も２０％以下であった。したがって、ＨＩＣ試験によるＨＩＣは発生していない。これらの鋼板１〜２３を素材とする鋼管についても、表３に示すように同様な結果が得られた。   Steel plates 1 to 23 are examples of the present invention. As shown in Tables 2A to 2C, the maximum Mn segregation degree of these steel sheets was 1.6 or less, the Nb segregation degree was 4.0 or less, and the Ti segregation degree was 4.0 or less. Further, the maximum hardness from the outermost surface of the upper and lower surfaces of the steel plate and the inner and outer surfaces of the steel pipe to 5 mm thickness was 300 Hv or less, and the maximum hardness of the central segregation part was 300 Hv or less. Furthermore, the total of polygonal ferrite and processed ferrite The fraction was also 20% or less. Therefore, no HIC is generated by the HIC test. Similar results were obtained for steel pipes made of these steel plates 1 to 23 as shown in Table 3.

鋼板２４〜４３は本発明の範囲外である比較例である。鋼板２４〜３５は基本成分、又は任意の添加元素のうちいずれかの元素が、本発明の範囲外である。鋼板３６〜４３は、本発明の製造条件を満足しない。その結果、鋼板（表２Ａ〜表２Ｃ参照）、鋼管（表３参照）のいずれにおいても、ＨＩＣ試験にてＨＩＣが発生し、ＣＬＲが３％を超えた、又は０℃でのＤＷＴＴの延性破面率が８５％に満たなかった。   Steel plates 24-43 are comparative examples that are outside the scope of the present invention. In the steel plates 24 to 35, any of the basic components or any of the additional elements is outside the scope of the present invention. The steel plates 36 to 43 do not satisfy the production conditions of the present invention. As a result, in any of steel plates (see Tables 2A to 2C) and steel pipes (see Table 3), HIC was generated in the HIC test, CLR exceeded 3%, or ductile fracture of DWTT at 0 ° C. The area ratio was less than 85%.

図４Ａに、本願発明の製造方法で製造した鋼１１の最表層から５ｍｍ厚までの硬度分布、図４Ｂに鋼１１の表層の組織写真を示す。また、図５Ａに、従来法で製造した鋼４０の最表層から５ｍｍ厚までの硬度分布、図５Ｂに鋼４０の表層の組織写真を示す。   FIG. 4A shows a hardness distribution from the outermost surface layer of the steel 11 manufactured by the manufacturing method of the present invention to a thickness of 5 mm, and FIG. 4B shows a structure photograph of the surface layer of the steel 11. FIG. 5A shows a hardness distribution from the outermost layer of steel 40 manufactured by a conventional method to a thickness of 5 mm, and FIG. 5B shows a structural photograph of the surface layer of steel 40.

図４Ａに示す本発明の鋼板の硬度分布は、最大硬度が２４５Ｈｖと低いが、図５Ａに示す従来法で製造した鋼板の硬度分布は、局所的に硬度が３００Ｈｖを超える部分があり、これは、ＨＩＣの起点となり得る。図５Ｂに示す従来法で製造した鋼板は、復熱回数が１回なので、母材が十分に焼戻されず、硬い組織が生成している。   The hardness distribution of the steel sheet of the present invention shown in FIG. 4A is as low as 245 Hv in the maximum hardness, but the hardness distribution of the steel sheet manufactured by the conventional method shown in FIG. Can be the starting point of HIC. Since the steel plate manufactured by the conventional method shown in FIG. 5B has one recuperation, the base material is not tempered sufficiently, and a hard structure is generated.

１ 復熱処理
２ 復熱開始温度
３ 復熱終了温度
４ 水冷停止温度1 Recovery heat treatment 2 Recovery heat start temperature 3 Recovery heat end temperature 4 Water cooling stop temperature

Claims (8)

母材となる鋼板が質量％で、
Ｃ ：０．０２〜０．０８％、
Ｓｉ：０．０１〜０．５％、
Ｍｎ：１．２〜１．８％、
Ｎｂ：０．００１〜０．１０％、
Ｃａ：０．０００５〜０．００５０％、
Ｎ ：０．００１０〜０．００６０％、
Ｏ ：０．０００１〜０．００３５％
を含有し、かつ、
Ｐ ：０．０１％以下、
Ｓ ：０．００２０％以下、
Ａｌ：０．０３０％以下、
Ｔｉ：０．０３０％以下
に制限し、Ｓ、Ｃａの含有量が、
Ｓ／Ｃａ＜０．５
を満足し、残部がＦｅ及び不可避的不純物であり、
最大Ｍｎ偏析度：２．０以下、
Ｎｂ偏析度：４．０以下、
Ｔｉ偏析度：４．０以下、
中心偏析部の未圧着部の長さ：０．１ｍｍ以下、
中心偏析部の最高硬度：３００Ｈｖ以下、
表裏両板面の最表面から５ｍｍの深さまでの表層領域の最高硬度：３００Ｈｖ以下、
表裏両板面の最表面から５ｍｍの深さまでの表層領域におけるポリゴナルフェライトとアスペクト比が３以上の加工フェライトの合計分率：０．１〜２０％
であり、
鋼板の厚さｔ［ｍｍ］と造管後の鋼管の外径Ｄ［ｍｍ］が、
ｔ≧２５
ｔ／Ｄ≧０．０３５
を満たすことを特徴とする耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管。The base steel plate is mass%
C: 0.02 to 0.08%,
Si: 0.01 to 0.5%,
Mn: 1.2-1.8%
Nb: 0.001 to 0.10%,
Ca: 0.0005 to 0.0050%,
N: 0.0010 to 0.0060%,
O: 0.0001 to 0.0035%
Containing, and
P: 0.01% or less,
S: 0.0020% or less,
Al: 0.030% or less,
Ti: limited to 0.030% or less, S, Ca content,
S / Ca <0.5
And the balance is Fe and inevitable impurities,
Maximum Mn segregation degree: 2.0 or less,
Nb segregation degree: 4.0 or less,
Ti segregation degree: 4.0 or less,
The length of the unseparated portion of the center segregation portion: 0.1 mm or less,
Maximum hardness of center segregation part: 300 Hv or less,
Maximum hardness of the surface layer region from the outermost surface of the front and back plate surfaces to a depth of 5 mm: 300 Hv or less,
Total fraction of polygonal ferrite and processed ferrite having an aspect ratio of 3 or more in the surface layer region from the outermost surface of both front and back plate surfaces to a depth of 5 mm: 0.1 to 20%
And
The thickness t [mm] of the steel plate and the outer diameter D [mm] of the steel pipe after pipe making are as follows:
t ≧ 25
t / D ≧ 0.035
A steel pipe for high-strength line pipes with excellent resistance to hydrogen-induced cracking. 前記鋼板が、質量％で、さらに、
Ｎｉ：０．０１〜２．０％、
Ｃｕ：０．０１〜１．０％、
Ｃｒ：０．０１〜１．０％、
Ｍｏ：０．０１〜０．６０％、
Ｗ ：０．０１〜１．０％、
Ｖ ：０．０１〜０．１０％、
Ｚｒ：０．０００１〜０．０５０％、
Ｔａ：０．０００１〜０．０５０％、
Ｂ ：０．０００１〜０．００２０％、
ＲＥＭ：０．０００１〜０．０１％、
Ｍｇ：０．０００１〜０．０１％、
Ｙ ：０．０００１〜０．００５％、
Ｈｆ：０．０００１〜０．００５％、及び
Ｒｅ：０．０００１〜０．００５％
のうち１種又は２種以上を含有することを特徴とする請求項１に記載の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管。The steel sheet is in mass%, and
Ni: 0.01 to 2.0%,
Cu: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Mo: 0.01-0.60%,
W: 0.01-1.0%
V: 0.01 to 0.10%,
Zr: 0.0001 to 0.050%,
Ta: 0.0001 to 0.050%,
B: 0.0001 to 0.0020%,
REM: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%
Y: 0.0001 to 0.005%,
Hf: 0.0001 to 0.005%, and Re: 0.0001 to 0.005%
The steel pipe for high-strength line pipe excellent in resistance to hydrogen-induced cracking according to claim 1, wherein one or more of them are contained. 表裏両板面の最表面から５ｍｍの深さまでの表層領域においてアスペクト比が３以上の加工フェライトが存在しないことを特徴とする請求項１又は２に記載の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管。   3. High strength excellent in resistance to hydrogen-induced cracking according to claim 1 or 2, characterized in that there is no processed ferrite having an aspect ratio of 3 or more in the surface layer region from the outermost surface of both front and back plates to a depth of 5 mm. Steel pipe for line pipe. 請求項１又は２に記載の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管に用いる耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板。   A steel sheet for high-strength line pipe excellent in hydrogen-induced crack resistance used in the steel pipe for high-strength line pipe excellent in hydrogen-induced crack resistance according to claim 1 or 2. 請求項３に記載の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管に用いる耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板。   A steel sheet for high-strength line pipe excellent in hydrogen-induced crack resistance, which is used in the steel pipe for high-strength line pipe excellent in hydrogen-induced crack resistance according to claim 3. 質量％で、
Ｃ ：０．０２〜０．０８％、
Ｓｉ：０．０１〜０．５％、
Ｍｎ：１．２〜１．８％、
Ｎｂ：０．００１〜０．１０％、
Ｃａ：０．０００５〜０．００５０％、
Ｎ ：０．００１０〜０．００６０％、
Ｏ ：０．０００１〜０．００３５％
を含有し、かつ、
Ｐ ：０．０１％以下、
Ｓ ：０．００２０％以下、
Ａｌ：０．０３０％以下、
Ｔｉ：０．０３０％以下
に制限し、Ｓ、Ｃａの含有量が、
Ｓ／Ｃａ＜０．５
を満足し、
２次精錬後の水素の含有量が２．５ｐｐｍ以下であり、残部がＦｅ及び不可避的不純物である溶鋼を溶製する工程と、
前記溶鋼を連続鋳造により鋼片とする工程と、
前記鋼片を１０００℃以上に加熱する工程と、
加熱した鋼片を、再結晶温度域での圧下比を２以上、未再結晶温度域での圧下比を３以上の熱間圧延し、鋼板を得る工程と、
前記鋼板を、７５０℃以上の温度から４００〜６００℃になるまで冷却する冷却工程を含み、
前記冷却工程は、前記鋼板の温度を上昇させる復熱処理を２回以上含み、
前記復熱処理において、１回目の復熱処理の開始温度が３００℃以上であり、かつ、すべての復熱処理の終了温度が７５０℃未満である
ことを特徴とする耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板の製造方法。% By mass
C: 0.02 to 0.08%,
Si: 0.01 to 0.5%,
Mn: 1.2-1.8%
Nb: 0.001 to 0.10%,
Ca: 0.0005 to 0.0050%,
N: 0.0010 to 0.0060%,
O: 0.0001 to 0.0035%
Containing, and
P: 0.01% or less,
S: 0.0020% or less,
Al: 0.030% or less,
Ti: limited to 0.030% or less, S, Ca content,
S / Ca <0.5
Satisfied,
A step of melting molten steel in which the content of hydrogen after secondary refining is 2.5 ppm or less and the balance is Fe and inevitable impurities;
Making the molten steel into a steel piece by continuous casting;
Heating the steel slab to 1000 ° C. or higher;
A step of hot-rolling the heated steel slab at a reduction ratio in the recrystallization temperature range of 2 or more and a reduction ratio in the non-recrystallization temperature range of 3 or more to obtain a steel sheet;
A cooling step of cooling the steel sheet from a temperature of 750 ° C. or higher to 400 to 600 ° C.,
The cooling step includes a reheat treatment for increasing the temperature of the steel sheet twice or more,
In the reheat treatment, the first heat treatment start temperature is 300 ° C. or higher, and the end temperature of all heat treatment is less than 750 ° C. Manufacturing method of steel plate for line pipe. 前記溶鋼が、質量％で、さらに、
Ｎｉ：０．０１〜２．０％、
Ｃｕ：０．０１〜１．０％、
Ｃｒ：０．０１〜１．０％、
Ｍｏ：０．０１〜０．６０％、
Ｗ ：０．０１〜１．０％、
Ｖ ：０．０１〜０．１０％、
Ｚｒ：０．０００１〜０．０５０％、
Ｔａ：０．０００１〜０．０５０％、
Ｂ ：０．０００１〜０．００２０％、
ＲＥＭ：０．０００１〜０．０１％、
Ｍｇ：０．０００１〜０．０１％、
Ｙ ：０．０００１〜０．００５％、
Ｈｆ：０．０００１〜０．００５％、及び
Ｒｅ：０．０００１〜０．００５％
のうち１種又は２種以上を含有することを特徴とする請求項６に記載の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板の製造方法。The molten steel is in mass%, and
Ni: 0.01 to 2.0%,
Cu: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Mo: 0.01-0.60%,
W: 0.01-1.0%
V: 0.01 to 0.10%,
Zr: 0.0001 to 0.050%,
Ta: 0.0001 to 0.050%,
B: 0.0001 to 0.0020%,
REM: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%
Y: 0.0001 to 0.005%,
Hf: 0.0001 to 0.005%, and Re: 0.0001 to 0.005%
The manufacturing method of the steel plate for high intensity | strength line pipe excellent in the hydrogen-induced cracking resistance of Claim 6 characterized by including 1 type, or 2 or more types among these. 請求項６又は７に記載の耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板の製造方法により得られた鋼板から鋼管を製造する方法であって、
前記鋼板を管状に成形する工程と、
突き合わせ部を溶接する工程
を含み、
鋼板の厚さｔ［ｍｍ］と造管後の鋼管の外径Ｄ［ｍｍ］が、
ｔ≧２５
ｔ／Ｄ≧０．０３５
を満たすことを特徴とする耐水素誘起割れ性に優れた高強度ラインパイプ用鋼管の製造方法。A method for producing a steel pipe from a steel sheet obtained by the method for producing a steel sheet for high-strength line pipe excellent in hydrogen-induced crack resistance according to claim 6 or 7,
Forming the steel sheet into a tubular shape;
Including the step of welding the butt,
The thickness t [mm] of the steel plate and the outer diameter D [mm] of the steel pipe after pipe making are as follows:
t ≧ 25
t / D ≧ 0.035
The manufacturing method of the steel pipe for high-strength line pipe excellent in hydrogen-induced cracking resistance characterized by satisfying.
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