JP5884201B2 - Hot-rolled steel sheet for high-strength line pipe with a tensile strength of 540 MPa or more - Google Patents

Hot-rolled steel sheet for high-strength line pipe with a tensile strength of 540 MPa or more Download PDF

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JP5884201B2
JP5884201B2 JP2014558502A JP2014558502A JP5884201B2 JP 5884201 B2 JP5884201 B2 JP 5884201B2 JP 2014558502 A JP2014558502 A JP 2014558502A JP 2014558502 A JP2014558502 A JP 2014558502A JP 5884201 B2 JP5884201 B2 JP 5884201B2
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聡太 後藤
聡太 後藤
俊介 豊田
俊介 豊田
岡部　能知
能知 岡部
雪彦 岡崎
雪彦 岡崎
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Description

本発明は耐水素誘起割れ性(hydrogen induced cracking resistance)（以下耐ＨＩＣ性と呼ぶ）を有し、ＡＰＩ（American Petroleum Institute）規格 Ｘ７０以上の強度を有する、原油または、天然ガスといったエネルギー資源を輸送するためのラインパイプ用電縫鋼管素材として用いるのに好適な熱延鋼板およびその製造方法に関する。   The present invention transports energy resources such as crude oil or natural gas having hydrogen induced cracking resistance (hereinafter referred to as HIC resistance) and strength of API (American Petroleum Institute) standard X70 or higher. The present invention relates to a hot-rolled steel sheet suitable for use as an electric-welded steel pipe material for line pipes and a method for producing the same.

従来、ラインパイプには、輸送効率の観点から、大径で厚肉な鋼管が製造可能なＵＯＥ鋼管が主として使用されてきたが、最近では、ＵＯＥ鋼管に代わり生産性が高く、より安価なコイル形状の熱延鋼板（熱延鋼帯）を素材とした高強度電縫鋼管（high strength electric resistance welded steel pipe）の普及が進んでいる。コスト面以外にも、電縫鋼管は肉厚（wall thickness）の偏差（deviation）や真円度（roundness）がＵＯＥ鋼管に比べ優れるといった利点がある。一方で、電縫鋼管の造管方法（pipe production method）は冷間ロール成形（cold roll forming）であり、鋼管を製造する際に付与される塑性ひずみ(plastic strain)がＵＯＥ鋼管と比べて格段に大きいという特徴がある。   Conventionally, UOE steel pipes that can produce large-diameter and thick-walled steel pipes have been mainly used as line pipes from the viewpoint of transportation efficiency. Recently, however, UOE steel pipes have been replaced with higher productivity and less expensive coils. High strength electric resistance welded steel pipe made of hot-rolled steel sheet (hot-rolled steel strip) in shape is spreading. In addition to cost, ERW steel pipes have the advantage that wall thickness deviation and roundness are superior to UOE steel pipes. On the other hand, the pipe production method of ERW steel pipe is cold roll forming, and the plastic strain applied when manufacturing steel pipe is much higher than that of UOE steel pipe. There is a feature that is large.

近年の原油および、天然ガス開発は、エネルギー需要の増加と採掘技術の進歩により油田および、ガス田の極地化や、高深度化が進んでいる。こういった場所で用いられるラインパイプには強度、靭性および溶接性に加えて、耐ＨＩＣ性や耐硫化物応力腐食割れ（sulfate stress corrosion cracking resistance）（ＳＳＣ）といったいわゆる耐サワー特性(sour resistance)が求められる。敷設された後に応力が負荷されないラインパイプでは、特に耐ＨＩＣ性が重要となる。   In recent years, the development of crude oil and natural gas has become more polar and deeper in oil and gas fields due to increased energy demand and advances in mining technology. In addition to strength, toughness and weldability, line pipes used in these locations have so-called sour resistance such as HIC resistance and sulfate stress corrosion cracking resistance (SSC). Is required. For line pipes that are not stressed after being laid, HIC resistance is particularly important.

ＨＩＣは腐食反応により生成した水素イオンが鋼表面で水素原子となり鋼中に侵入して、ＭｎＳなどの介在物、ＮｂＣなどの粗大な炭化物や硬質第二相(second hard phase)のまわりに集積することで内圧を生じさせ、最終的に鋼材に割れを発生させるものである。また、鋼材に塑性ひずみが付与された場合、前記介在物、炭化物および硬質第二相周辺には多数の転位(dislocation)が導入されることによって、より水素原子が集積しやすくなるため、ＨＩＣが助長される。   In HIC, hydrogen ions generated by the corrosion reaction become hydrogen atoms on the steel surface and penetrate into the steel and accumulate around inclusions such as MnS, coarse carbides such as NbC, and the second hard phase. In this way, an internal pressure is generated, and a crack is finally generated in the steel material. In addition, when plastic strain is applied to the steel material, a large number of dislocations are introduced around the inclusions, carbides, and hard second phase, which facilitates the accumulation of hydrogen atoms. Be encouraged.

上記したＨＩＣの問題を解決するために、従来から種々の解決策が提案されている。   In order to solve the above HIC problem, various solutions have been proposed.

特許文献１では、Ｓ、Ｏ（酸素）およびＮのそれぞれと結合して介在物を形成する元素の含有量の合計を０．０１％以下として、あるいは介在物の最大径を５μｍ以下に制御して、ＨＩＣの起点となる介在物を無害化し、さらに中心偏析部（center segregation part）の硬度をＨｖ３３０以下とすることで耐ＨＩＣを向上させる方法が開示されている。   In Patent Document 1, the total content of elements that combine with each of S, O (oxygen) and N to form inclusions is controlled to 0.01% or less, or the maximum diameter of inclusions is controlled to 5 μm or less. Thus, there is disclosed a method for improving the HIC resistance by detoxifying the inclusion that becomes the starting point of the HIC and further setting the hardness of the center segregation part to Hv 330 or less.

特許文献２では、ＨＩＣの起点となるＴｉＮの大きさを小さくすることでＨＩＣ面積率(area ratio of HIC)を小さくする方法が開示されている。具体的にはＡｌとＣａの添加量を調整し、ＣａＯ／Ａｌ２Ｏ３の重量比を１．２〜１．５とすることで溶鋼中のＡｌ−Ｃａ系硫化物を微細化し、それを核として生成するＡｌ−Ｔｉ−Ｃａ系複合介在物を３０μｍ以下とする。Patent Document 2 discloses a method of reducing the HIC area ratio (area ratio of HIC) by reducing the size of TiN that is the starting point of HIC. Specifically, the amount of Al and Ca added is adjusted, and the weight ratio of CaO / Al 2 O 3 is set to 1.2 to 1.5 to refine the Al—Ca sulfide in the molten steel. The Al—Ti—Ca-based composite inclusions generated as nuclei are 30 μm or less.

また特許文献３では、板厚方向の中央部から板厚方向へ向けて板厚の５％の距離にある領域におけるＮｂ濃度を０．０６０％以下とすると共にＴｉ濃度を０．０２５％以下に抑えることで、ＨＩＣ起点となるＮｂおよびＴｉの炭窒化物を生成し難くする方法が開示されている。   Moreover, in patent document 3, Nb density | concentration in the area | region which is 5% of board thickness toward the board thickness direction from the center part of board thickness direction shall be 0.060% or less, and Ti density shall be 0.025% or less. Disclosed is a method for making it difficult to produce Nb and Ti carbonitrides serving as HIC starting points.

特許文献４では、鋼に添加するＭｎ量を低減し中心偏析を軽減することで耐ＨＩＣ性を高め、比較的中心偏析し難いＣｒおよびＭｏを活用することで耐ＨＩＣ性に優れた高強度ラインパイプを製造する方法が開示されている。   In Patent Document 4, a high-strength line excellent in HIC resistance by using Cr and Mo, which increases HIC resistance by reducing the amount of Mn added to steel and reduces central segregation, and is relatively difficult to segregate center. A method of manufacturing a pipe is disclosed.

特開２００６−６３３５１号公報JP 2006-63351 A 特許第４３６３４０３号公報（国際公開WO2005/075694号公報）Japanese Patent No. 4363403 (International Publication WO2005 / 075694) 特開２０１１−６３８４０号公報JP 2011-63840 A 特許第２６４７３０２号公報（特開平５−２７１７６６号公報）Japanese Patent No. 2647302 (JP-A-5-271766)

しかしながら、特許文献１〜３に開示された方法によりＨＩＣ起点をある程度の無害化することは可能であるが、特にＡＰＩ Ｘ７０以上の高強度鋼板においては、ＨＩＣへの感受性（sensitivity）が増加するため、介在物量と大きさを制御しただけでは十分な効果は得られない。   However, although it is possible to make the HIC origin harmless to some extent by the methods disclosed in Patent Documents 1 to 3, especially in high-strength steel sheets of API X70 or higher, the sensitivity to HIC increases (sensitivity). However, a sufficient effect cannot be obtained only by controlling the amount and size of inclusions.

また、特許文献４に開示されたＣｒおよびＭｏを活用する方法を用いて、ＣｒおよびＭｏを過剰に添加すると中心偏析部でのマルテンサイト生成が助長され、耐ＨＩＣ性が低下するという問題がある。   Further, when Cr and Mo are added excessively using the method of utilizing Cr and Mo disclosed in Patent Document 4, martensite formation is promoted in the central segregation part, and there is a problem that HIC resistance is lowered. .

本発明は、上記課題を鑑みてなされたもので、ＡＰＩ Ｘ７０以上の高強度電縫管ラインパイプの素材として好適な、耐ＨＩＣ性に優れた高強度ラインパイプ用熱延鋼板を提供することを目的とする。   The present invention has been made in view of the above problems, and provides a hot-rolled steel sheet for high-strength line pipes excellent in HIC resistance, suitable as a material for high-strength ERW line pipes of API X70 or higher. Objective.

ここで耐ＨＩＣ性に優れるとは、ＮＡＣＥ溶液（ＮＡＣＥ ＴＭ−０２８４ ｓｏｌｕｔｉｏｎＡ ：５％ＮａＣｌ＋０．５％ＣＨ３ＯＯＨ、１気圧飽和Ｈ２Ｓ、ｐＨ＝３．０〜４．０）中に９６ｈｒ浸漬した後の割れ長さ率（ＣＬＲ）が１５％以下であることをいう。Here, excellent HIC resistance means that it is immersed for 96 hours in a NACE solution (NACE TM-0284 solution A: 5% NaCl + 0.5% CH 3 OOH, 1 atm saturated H 2 S, pH = 3.0 to 4.0). This means that the crack length ratio (CLR) after being 15% or less.

本発明は、ＨＩＣ感受性が高まるＴＳ：５４０ＭＰａ以上の高強度ラインパイプ用熱延鋼板において、鋼組成の調整により中心偏析部の焼入性の向上を通じて中心偏析部の組織を細粒化し、ＨＩＣ伝播抵抗（propagation resistance of HIC）を高めることによって、ＨＩＣの発生起点となる介在物が多少存在する場合においても、割れ長さ率ＣＬＲを小さく抑制するという考えに基づき完成したものである。すなわち、本発明の要旨は以下の通りである。   In the present invention, the hot-rolled steel sheet for high strength line pipes with TS: 540 MPa or higher with increased HIC sensitivity is used to refine the structure of the center segregation part by improving the hardenability of the center segregation part by adjusting the steel composition and to propagate the HIC. By increasing the resistance (propagation resistance of HIC), the present invention has been completed based on the idea of suppressing the crack length ratio CLR to be small even when there are some inclusions that are the starting point of HIC. That is, the gist of the present invention is as follows.

［１］成分組成が、質量％で、Ｃ：０．０２〜０．０６％、Ｓｉ：０．０５〜０．２５％、Ｍｎ：０．６０〜１．１０％、Ｐ：０．００８％以下、Ｓ：０．００１０％以下、Ｎｂ：０．０２０〜０．０６０％、Ｔｉ：０．００１〜０．０２０％、Ａｌ：０．０１〜０．０８％、Ｃａ：０．０００５〜０．００５０％を含有し、さらに、Ｃｕ：０．５０％以下、Ｎｉ：０．５０％以下、Ｃｒ：０．５０％以下、Ｍｏ：０．０５％以下、Ｖ：０．１０％以下の中から選ばれる１種以上を含有し、残部Ｆｅおよび不可避的不純物からなり、下記式（１）を満たすことを特徴とする引張強さ５４０ＭＰａ以上の耐ＨＩＣ性に優れた高強度ラインパイプ用熱延鋼板。   [1] Component composition is mass%, C: 0.02 to 0.06%, Si: 0.05 to 0.25%, Mn: 0.60 to 1.10%, P: 0.008% Hereinafter, S: 0.0010% or less, Nb: 0.020 to 0.060%, Ti: 0.001 to 0.020%, Al: 0.01 to 0.08%, Ca: 0.0005 to 0 .0050%, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.05% or less, V: 0.10% or less Hot-rolling for high-strength line pipe excellent in HIC resistance having a tensile strength of 540 MPa or more, comprising at least one selected from the group consisting of the remaining Fe and inevitable impurities and satisfying the following formula (1) steel sheet.

０．６０≦ＣＰ≦０．９０ ・・・・・（１）
なお、ＣＰは、ＣＰ＝４．４６×Ｃ＋２．３７×Ｍｎ／６＋（１．１８×Ｃｒ＋１．９５×Ｍｏ＋１．７４×Ｖ）／５＋（１．７４×Ｃｕ＋１．７０×Ｎｉ）／１５ から求められ、式中の元素記号は各元素の質量％を意味する。0.60 ≦ CP ≦ 0.90 (1)
CP is obtained from CP = 4.46 × C + 2.37 × Mn / 6 + (1.18 × Cr + 1.95 × Mo + 1.74 × V) / 5 + (1.74 × Cu + 1.70 × Ni) / 15 The element symbol in the formula means mass% of each element.

［２］前記成分組成に加えてさらに、下記式（２）を満たすことを特徴とする前記［１］に記載の引張強さ５４０ＭＰａ以上の耐ＨＩＣ性に優れた高強度ラインパイプ用熱延鋼板。   [2] In addition to the above component composition, the following formula (2) is further satisfied: [1] The hot rolled steel sheet for high strength line pipe excellent in HIC resistance having a tensile strength of 540 MPa or more. .

ＣＭ≦３．０５ ・・・・・（２）
ここで、ＣＭは、ＣＭ＝２．３７×Ｍｎ＋２．３４×Ｍｏ＋０．５９×Ｃｒ＋０．１７×Ｎｉ から求められ、式中の元素記号は各元素の質量％を意味する。CM ≦ 3.05 (2)
Here, CM is obtained from CM = 2.37 × Mn + 2.34 × Mo + 0.59 × Cr + 0.17 × Ni, and the element symbol in the formula means mass% of each element.

［３］前記成分組成に加えてさらに、中心偏析部の金属組織が面積分率で９５％以上のベイニティックフェライト組織(bainitic-ferrite microstructure)であり、ベイニティックフェライト組織の平均粒径が８．０μｍ以下であることを特徴とする前記［１］または［２］に記載の引張強さ５４０ＭＰａ以上の耐ＨＩＣ性に優れた高強度ラインパイプ用熱延鋼板。   [3] In addition to the component composition, the metal structure of the central segregation part is a bainitic-ferrite microstructure with an area fraction of 95% or more, and the average particle size of the bainitic ferrite structure is The hot-rolled steel sheet for high-strength line pipe excellent in HIC resistance having a tensile strength of 540 MPa or more as described in [1] or [2] above, which is 8.0 μm or less.

［４］前記［１］または［２］に記載の成分組成を有する鋼のスラブを、１１００℃〜１３００℃の温度に加熱し、粗圧延に引き続き、９３０℃以下での累積圧下率(cumulative rolling reduction ratio)が２０％以上となるように仕上圧延を行った後、板厚中心で１０〜１００℃／ｓの平均冷却速度で３８０〜６００℃まで加速冷却（accelerated cooling）を行い、その後コイルに巻取ることを特徴とする引張強さ５４０ＭＰａ以上の耐ＨＩＣ性に優れた高強度ラインパイプ用熱延鋼板の製造方法。   [4] A steel slab having the composition described in [1] or [2] is heated to a temperature of 1100 ° C. to 1300 ° C., followed by rough rolling, and a cumulative rolling ratio (cumulative rolling at 930 ° C. or lower). After finishing rolling so that the reduction ratio is 20% or more, accelerated cooling is performed to 380 to 600 ° C. at an average cooling rate of 10 to 100 ° C./s at the center of the plate thickness, and then the coil is applied to the coil. A method for producing a hot-rolled steel sheet for a high-strength line pipe excellent in HIC resistance having a tensile strength of 540 MPa or more, characterized by winding.

本発明によれば、多少の介在物が存在する場合であっても、中心偏析部の組織を微細に制御することによりＨＩＣを抑制し、ＮＡＣＥ溶液に相当する過酷環境下においても問題なく使用できるＡＰＩ Ｘ７０以上電縫鋼管ラインパイプに好適な、耐ＨＩＣ性に優れた高強度熱延鋼板を製造することができる。また本発明により製造される熱延鋼板はＡＰＩ Ｘ７０以上のスパイラル鋼管ラインパイプ（spiral steel pipe for linepipe）にも使用できる。   According to the present invention, even when some inclusions are present, HIC is suppressed by finely controlling the structure of the central segregation part, and can be used without any problem even in a harsh environment corresponding to a NACE solution. API X70 or higher A high-strength hot-rolled steel sheet excellent in HIC resistance suitable for an electric-welded steel pipe line pipe can be produced. Further, the hot-rolled steel sheet produced according to the present invention can also be used for a spiral steel pipe for linepipe of API X70 or higher.

以下に本発明の各構成要件の限定理由について説明する。   The reasons for limiting the respective constituent requirements of the present invention will be described below.

１．成分組成について
はじめに、本発明の鋼の成分組成を規定した理由を説明する。なお、成分％は、すべて質量％を意味する。1. About component composition First, the reason which prescribed | regulated the component composition of the steel of this invention is demonstrated. In addition, all component% means the mass%.

Ｃ：０．０２〜０．０６％
Ｃは鋼の高強度化に大きく寄与する元素であり、０．０２％以上の含有でその効果を発揮するが、０．０６％を超える含有はパーライト組織(pearlite microstructure)のような第二相の生成を容易にするため、耐ＨＩＣ性が悪化する。このためＣ量は０．０２〜０．０６％の範囲とする。好ましくは、０．０３〜０．０５％の範囲である。C: 0.02 to 0.06%
C is an element that greatly contributes to increasing the strength of steel, and when 0.02% or more is contained, the effect is exhibited. However, if it exceeds 0.06%, the second phase such as pearlite microstructure is used. This facilitates the generation of HIC resistance. For this reason, C amount is taken as 0.02 to 0.06% of range. Preferably, it is 0.03 to 0.05% of range.

Ｓｉ：０．０５〜０．２５％
Ｓｉは、固溶強化と熱間圧延時のスケールオフ量(scale-off quantity)を小さくするために添加する元素であり、０．０５％以上の含有でその効果を発揮するが、０．２５％を超えると赤スケール(red scale)が過剰に成長し、熱間圧延時の冷却むら(cooling ununiformity)を生じ、外観や材質の均一性(uniformity)が悪化する。このため、Ｓｉ量は０．０５〜０．２５％の範囲とする。より好ましくは、０．１０〜０．２５％である。加えて、Ｓｉは電縫溶接時にＭｎＳｉ系の酸化物を形成し、電縫溶接部の靭性(toughness)を悪化させるので、Ｍｎ／Ｓｉ比が４．０以上１２以下となるように含有することが好ましい。Si: 0.05-0.25%
Si is an element added to reduce the scale-off quantity at the time of solid solution strengthening and hot rolling, and when it is contained in an amount of 0.05% or more, its effect is exhibited. If the percentage exceeds 50%, the red scale grows excessively, resulting in cooling ununiformity during hot rolling, and the appearance and uniformity of the material deteriorate. For this reason, Si amount is taken as 0.05 to 0.25% of range. More preferably, it is 0.10 to 0.25%. In addition, Si forms an MnSi-based oxide during ERW welding and deteriorates the toughness of the ERW weld, so that the Mn / Si ratio should be 4.0 or more and 12 or less. Is preferred.

Ｍｎ：０．６０〜１．１０％
Ｍｎは鋼組織の微細化を通じて強度、靭性に寄与する元素であり、０．６０％以上の含有でその効果を発揮する。一方でＭｎ含有量の増加は中心偏析部での微細マルテンサイト組織の形成を助長し、さらにＨＩＣの起点となるＭｎＳの生成を助長するため、その含有量は１．１０％以下に抑える必要がある。このため、Ｍｎ量は０．６０〜１．１０％の範囲とする。好ましくは、０．８０〜１．１０％の範囲である。より好ましくは、０．８０〜１．０５％である。Mn: 0.60 to 1.10%
Mn is an element that contributes to strength and toughness through refinement of the steel structure, and exhibits its effect when contained in an amount of 0.60% or more. On the other hand, the increase in the Mn content promotes the formation of a fine martensite structure in the central segregation part, and further promotes the generation of MnS as a starting point of HIC. is there. For this reason, the amount of Mn shall be 0.60 to 1.10% of range. Preferably, it is 0.80 to 1.10% of range. More preferably, it is 0.80 to 1.05%.

Ｐ：０．００８％以下
Ｐは不可避的不純物元素であり、中心偏析部の硬度を著しく上昇させ、耐ＨＩＣ性を悪化させるため、その含有量はできるだけ低いほうが好ましいが、０．００８％までは許容される。さらに、Ｐを極めて低くするためには精錬時間(refining time)の長時間化によるコスト上昇を伴うため、０．００２％以上とすることが好ましい。P: 0.008% or less P is an unavoidable impurity element, which significantly increases the hardness of the central segregation part and deteriorates the HIC resistance. Therefore, the content is preferably as low as possible, but up to 0.008% Permissible. Furthermore, in order to make P very low, it is accompanied by a cost increase due to a long refining time, so 0.002% or more is preferable.

Ｓ：０．００１０％以下
Ｓは、Ｐと同様に鋼中に不可避的に含まれる元素であり、鋼中ではＭｎＳを生成するため、その含有量はできるだけ低いほうが好ましいが、０．００１０％までは許容される。より好ましくは、０．０００６％以下である。S: 0.0010% or less S is an element inevitably contained in steel as in P, and since MnS is produced in steel, its content is preferably as low as possible, but up to 0.0010% Is acceptable. More preferably, it is 0.0006% or less.

Ｎｂ：０．０２０〜０．０６０％
Ｎｂは、熱延鋼板製造時の巻取工程(coiling process)においてＮｂ炭窒化物として微細に析出し、鋼の強度向上に寄与する元素である。また電縫溶接時にオーステナイト粒の成長を抑制し、溶接部の靭性の向上に寄与する元素である。０．０２０％以上の含有でその効果を発揮する。一方、０．０６０％を超えるとＨＩＣの起点となる粗大なＮｂ炭窒化物が生成しやすくなる。そのため、Ｎｂ量は０．０２０〜０．０６０％の範囲とする。好ましくは０．０３０〜０．０５０％の範囲である。Nb: 0.020 to 0.060%
Nb is an element that precipitates finely as Nb carbonitride in the coiling process during the production of hot-rolled steel sheets and contributes to improving the strength of the steel. In addition, it is an element that suppresses the growth of austenite grains during electric resistance welding and contributes to improvement of the toughness of the weld. The effect is exhibited when the content is 0.020% or more. On the other hand, if it exceeds 0.060%, coarse Nb carbonitrides that are the starting point of HIC are likely to be generated. Therefore, the Nb amount is set to a range of 0.020 to 0.060%. Preferably it is 0.030 to 0.050% of range.

Ｔｉ：０．００１〜０．０２０％
Ｔｉは、鋼の靭性を著しく悪化させるＮをＴｉＮとして固定し無害化するために添加する元素である。０．００１％を超える含有でその効果を発揮する。一方で、０．０２０％を超えるとＦｅのへき開面に沿って析出するＴｉ炭窒化物の量が増加し、鋼の靭性を悪化させる。そのため、Ｔｉ量は０．００１〜０．０２０％の範囲とする。好ましくは０．００５〜０．０１５％の範囲である。Ti: 0.001 to 0.020%
Ti is an element added to fix and detoxify N as TiN, which significantly deteriorates the toughness of steel. The effect is exhibited when the content exceeds 0.001%. On the other hand, if it exceeds 0.020%, the amount of Ti carbonitride that precipitates along the cleavage plane of Fe increases, and the toughness of the steel deteriorates. Therefore, the Ti amount is set to a range of 0.001 to 0.020%. Preferably it is 0.005 to 0.015% of range.

Ａｌ：０．０１〜０．０８％
Ａｌは脱酸剤として添加するが、０．０１％未満では脱酸が十分でなく、一方、０．０８％を超えると鋼中に残存する粗大なＡｌ系酸化物量が増加し、耐ＨＩＣ性と靭性を悪化させる。そのため、Ａｌ量は０．０１〜０．０８％の範囲とする。好ましくは０．０１〜０．０５％の範囲である。Al: 0.01 to 0.08%
Al is added as a deoxidizing agent, but if it is less than 0.01%, deoxidation is not sufficient. On the other hand, if it exceeds 0.08%, the amount of coarse Al-based oxide remaining in the steel increases, and HIC resistance And worsen toughness. Therefore, the Al content is set to a range of 0.01 to 0.08%. Preferably it is 0.01 to 0.05% of range.

Ｃａ：０．０００５〜０．００５０％
Ｃａは硫化物系介在物の形態制御による耐ＨＩＣ性向上に有効な元素であり、０．０００５％以上の含有でその効果を発揮する。一方で、０．００５０％を超えると、効果が飽和するだけでなく、Ｃａの酸化物を多く形成し、耐ＨＩＣ性を悪化させる。そのため、Ｃａ量は０．０００５〜０．００５０％の範囲とする。好ましくは０．００１０〜０．００３０％の範囲である。Ca: 0.0005 to 0.0050%
Ca is an element effective for improving the HIC resistance by controlling the form of sulfide inclusions, and exerts its effect when contained in an amount of 0.0005% or more. On the other hand, if it exceeds 0.0050%, not only the effect is saturated, but a large amount of Ca oxide is formed, and the HIC resistance is deteriorated. Therefore, the Ca content is in the range of 0.0005 to 0.0050%. Preferably it is 0.0010 to 0.0030% of range.

本発明では、さらにＣｕ、Ｎｉ、Ｃｒ、Ｍｏ、Ｖのうちから１種以上を以下の範囲で含有させることができる。   In the present invention, one or more of Cu, Ni, Cr, Mo, and V can be further contained in the following range.

Ｃｕ：０．５０％以下
Ｃｕは焼入性向上を通じて鋼の靭性および強度向上に寄与する元素であり、同様の効果を有するＭｎやＭｏと比較して中心偏析部への濃化が少ないため、耐ＨＩＣ性を悪化させずに鋼を強化することができるので、強度グレードに応じて添加する。０．０５％以上の含有でその効果を発揮するが、０．５０％を超えるとその効果は飽和し、これ以上の含有は余計なコスト上昇を招く。そのため、Ｃｕ量は０．５０％以下とする。好ましくは、０．４０％以下である。Cu: 0.50% or less Cu is an element that contributes to improving the toughness and strength of steel through improved hardenability, and is less concentrated in the central segregation part than Mn and Mo having the same effect. Since steel can be strengthened without deteriorating HIC resistance, it is added according to the strength grade. The effect is exhibited when the content is 0.05% or more. However, when the content exceeds 0.50%, the effect is saturated, and the content exceeding this causes an extra cost increase. Therefore, the Cu content is 0.50% or less. Preferably, it is 0.40% or less.

Ｎｉ：０．５０％以下
Ｎｉは、Ｃｕと同様に焼入性向上を通じて鋼の靭性および強度向上に寄与する元素であり、同様の効果を有するＭｎやＭｏと比較して中心偏析部への濃化が少ないため、耐ＨＩＣ性を悪化させずに鋼を強化することができるので、強度グレードに応じて添加する。０．０５％以上の含有でその効果を発揮するが、０．５０％を超えて含有するとその効果は飽和し、これ以上の含有は余計なコスト上昇を招く。そのため、Ｎｉ量は０．５０％以下とする。好ましくは、０．４０％以下である。Ni: 0.50% or less Ni, like Cu, is an element that contributes to improving the toughness and strength of steel through improved hardenability. Compared with Mn and Mo having the same effect, Ni is concentrated in the central segregation part. Therefore, the steel can be strengthened without deteriorating the HIC resistance, so it is added depending on the strength grade. The effect is exhibited when the content is 0.05% or more, but when the content exceeds 0.50%, the effect is saturated, and the content exceeding this causes an extra cost increase. Therefore, the Ni content is 0.50% or less. Preferably, it is 0.40% or less.

Ｃｒ：０．５０％以下
Ｃｒは焼入性を高め、鋼の靭性および強度向上に有効に作用する元素であり、０．０５％以上の含有でその効果を発揮するが、電縫溶接時にＣｒ酸化物を形成し溶接部の靭性を著しく悪化させる。これを抑制するために、Ｃｒ量は０．５０％以下とする。好ましくは０．３０％以下である。Cr: 0.50% or less Cr is an element that enhances hardenability and effectively works to improve the toughness and strength of steel. An oxide is formed, and the toughness of the weld is significantly deteriorated. In order to suppress this, the Cr content is 0.50% or less. Preferably it is 0.30% or less.

Ｍｏ：０．５０％以下
Ｍｏは焼入性を高め、鋼の靭性および強度向上に極めて有効に作用する元素であり、０．０５％以上の含有でその効果を発揮するが、０．５０％を超えた範囲ではその効果は飽和し、これ以上の含有は余計なコスト上昇を招く。そのため、Ｍｏ量は０．５０％以下とする。好ましくは０．３０％以下である。Mo: 0.50% or less Mo is an element that enhances hardenability and works extremely effectively to improve the toughness and strength of steel. The effect is saturated in the range exceeding 1, and inclusion beyond this causes an extra cost increase. Therefore, the Mo amount is 0.50% or less. Preferably it is 0.30% or less.

Ｖ：０．１０％以下
Ｖは固溶強化（solute strengthening）および析出強化（precipitation strengthening）を通じて０．００５％以上の含有で鋼の強度向上に寄与する元素であるが、０．１０％を超えると中心偏析部の硬度が高くなり、耐ＨＩＣ性を悪化させる。そのため、Ｖ量は０．１０％以下とする。好ましくは、０．０８０％以下である。V: 0.10% or less V is an element that contributes to improving the strength of steel with a content of 0.005% or more through solute strengthening and precipitation strengthening, but exceeds 0.10% And the hardness of the center segregation part becomes high and the HIC resistance is deteriorated. Therefore, the V amount is 0.10% or less. Preferably, it is 0.080% or less.

ＣＰ：０．６０〜０．９０
本発明においては、各合金元素の含有量から求められるＣＰ値が下記式（１）を満たすものとする。CP: 0.60-0.90
In the present invention, the CP value obtained from the content of each alloy element satisfies the following formula (1).

０．６０≦ＣＰ≦０．９０ ・・・・・（１）
なお、ＣＰは、ＣＰ＝４．４６×Ｃ＋２．３７×Ｍｎ／６＋（１．１８×Ｃｒ＋１．９５×Ｍｏ＋１．７４×Ｖ）／５＋（１．７４×Ｃｕ＋１．７０×Ｎｉ）／１５ から求められ、式中の元素記号は各元素の質量％を意味する。なお添加しない元素については０とする。0.60 ≦ CP ≦ 0.90 (1)
CP is obtained from CP = 4.46 × C + 2.37 × Mn / 6 + (1.18 × Cr + 1.95 × Mo + 1.74 × V) / 5 + (1.74 × Cu + 1.70 × Ni) / 15 The element symbol in the formula means mass% of each element. The element not added is set to 0.

ＣＰ値は中心偏析部の焼入性を示す指標であり、鋼組成をＣＰ値が０．６０以上になるように調整することで、中心偏析部において８．０μｍ以下の微細なベイニティックフェライト組織が得られる。一方、ＣＰ値が０．９０を超えると、過度に焼入性が向上し、中心偏析部硬度が上昇する。従って、ＣＰ値は０．６０〜０．９０とする。より好ましくは、０．７０〜０．９０である。   The CP value is an index indicating the hardenability of the central segregation part. By adjusting the steel composition so that the CP value is 0.60 or more, fine bainitic ferrite of 8.0 μm or less in the central segregation part. Organization is obtained. On the other hand, if the CP value exceeds 0.90, the hardenability is excessively improved and the center segregation hardness increases. Therefore, the CP value is 0.60 to 0.90. More preferably, it is 0.70-0.90.

ＣＭ：３．０５以下
中心偏析部に生成する微細マルテンサイト組織は耐ＨＩＣ性を悪化させる。微細マルテンサイト組織生成に寄与する元素はＭｎ、Ｍｏ、Ｃｒ、Ｎｉであり、これら元素の微細マルテンサイト生成量への影響度を数値化したものがＣＭ値である。中心偏析部に生成する微細マルテンサイト組織を面積分率で５％未満とするためには、下記に示されるＣＭ値が、下記式（２）を満たすものとする。CM: 3.05 or less The fine martensite structure generated in the center segregation part deteriorates the HIC resistance. Elements that contribute to the formation of fine martensite structure are Mn, Mo, Cr, and Ni. The CM value is obtained by quantifying the degree of influence of these elements on the amount of fine martensite produced. In order to make the fine martensite structure generated in the central segregation part less than 5% in area fraction, the CM value shown below satisfies the following formula (2).

ＣＭ≦３．０５ ・・・・・（２）
ここで、ＣＭは、ＣＭ＝２．３７×Ｍｎ＋２．３４×Ｍｏ＋０．５９×Ｃｒ＋０．１７×Ｎｉ から求められ、式中の元素記号は各元素の質量％を意味する。より好ましくは、２．９５以下である。CM ≦ 3.05 (2)
Here, CM is obtained from CM = 2.37 × Mn + 2.34 × Mo + 0.59 × Cr + 0.17 × Ni, and the element symbol in the formula means mass% of each element. More preferably, it is 2.95 or less.

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妨げない限り、他の微量元素の含有は制限されない。   The balance other than the above-described elements is composed of Fe and inevitable impurities. However, the content of other trace elements is not limited as long as the effects of the present invention are not hindered.

２．金属組織について
次に、本発明鋼の金属組織について説明する。2. Next, the metal structure of the steel of the present invention will be described.

本発明鋼の金属組織は、靭性に優れた組織であるベイニティックフェライト組織とする必要がある。ベイニティックフェライト組織中に、微細マルテンサイトや上部ベイナイト、パーライトなどの異種組織相が存在すると、水素のトラップサイトとなるため、耐ＨＩＣ性は低下する。このため、ベイニティックフェライト組織以外の組織分率は少なければ少ないほど良い。ただし、ベイニティックフェライト組織以外の面積分率が極めて低い場合には、その影響は無視できるほど小さいので、ある程度の量までは許容できる。具体的には、ベイニティックフェライト組織以外の鋼組織（微細マルテンサイト、上部ベイナイト、パーライトなど）の中心偏析部に占める面積分率の合計が５％以下であれば、本発明に含まれる。   The metal structure of the steel of the present invention needs to be a bainitic ferrite structure which is a structure having excellent toughness. In the bainitic ferrite structure, if a different structure phase such as fine martensite, upper bainite, or pearlite is present, it becomes a hydrogen trap site, so that the HIC resistance is lowered. For this reason, the smaller the fraction of the structure other than the bainitic ferrite structure, the better. However, when the area fraction other than the bainitic ferrite structure is extremely low, the influence is so small that it can be ignored. Specifically, it is included in the present invention if the total area fraction of the central segregation portion of the steel structure (fine martensite, upper bainite, pearlite, etc.) other than the bainitic ferrite structure is 5% or less.

ベイニティックフェライト組織の平均粒径：８．０μｍ以下
ラインパイプ用途として十分な靭性（ｖＴｒｓ≦−８０℃）を得るためには、ベイニティックフェライトの平均粒径を８．０μｍ以下とする必要がある。また、ＨＩＣき裂伝播抵抗を高めるためにも平均粒径を８．０μｍ以下とすることが望ましい。好ましくは６．０μｍ以下である。Average particle size of bainitic ferrite structure: 8.0 μm or less In order to obtain sufficient toughness (vTrs ≦ −80 ° C.) for line pipe applications, the average particle size of bainitic ferrite needs to be 8.0 μm or less. There is. Further, in order to increase the HIC crack propagation resistance, it is desirable that the average particle size is 8.0 μm or less. Preferably it is 6.0 μm or less.

３．製造条件について
次に上記鋼組織を達成するための製造条件について説明する。3. Manufacturing conditions Next, manufacturing conditions for achieving the steel structure will be described.

スラブ加熱温度は１１００℃以上１３００oＣ以下とする。１１００℃未満では、連鋳工程で鋼中に生成した炭化物を完全に固溶させるのに不十分であり、必要な強度が得られない。一方、１３００℃を超える加熱では、オーステナイト粒が著しく粗大化するために靭性が悪化する。なお、この温度は加熱炉の炉内温度であり、スラブ中心(center of slab)までこの温度に加熱されるものとする。   Slab heating temperature shall be 1100 degreeC or more and 1300 degrees C or less. If it is less than 1100 degreeC, it is inadequate to make the carbide | carbonized_material produced | generated in steel in the continuous casting process completely dissolve, and required intensity | strength cannot be obtained. On the other hand, when the temperature exceeds 1300 ° C., the austenite grains become extremely coarse and the toughness deteriorates. Note that this temperature is the furnace temperature of the heating furnace and is heated to this temperature up to the center of the slab.

仕上圧延工程では、９３０℃以下の累計圧下率が２０％以上となる条件で仕上圧延を施す必要がある。累積圧下率が２０％未満であるとベイニティックフェライト組織の生成サイトが不足し、粗大組織となるため靭性が悪化する。しかし累積圧下率が８０％を超えると、その効果が飽和するばかりか、圧延機に多大な負荷をかけることになるため、累積圧下率の上限は８０％以下とすることが好ましい。   In the finish rolling process, it is necessary to perform finish rolling under the condition that the cumulative rolling reduction at 930 ° C. or less is 20% or more. When the cumulative rolling reduction is less than 20%, the site for forming the bainitic ferrite structure is insufficient, and the toughness deteriorates due to the coarse structure. However, when the cumulative rolling reduction exceeds 80%, not only the effect is saturated, but also a great load is applied to the rolling mill. Therefore, the upper limit of the cumulative rolling reduction is preferably 80% or less.

板厚中心の平均冷却速度は１０〜１００℃／ｓとする。１０℃／ｓ未満の冷却速度では、Ｃｕ、Ｎｉ、Ｃｒといった焼入性向上元素を添加したとしても、フェライトおよび/またはパーライト組織の面積分率が５％超えとなるため、１０℃／ｓ以上の冷速速度が必要となる。一方で、１００℃／ｓを超える場合はマルテンサイト組織の面分率が５％超えとなる。板厚中心の冷却速度は、事前に調査したランナウト(run-out)の冷却能力(cooling capacity)（熱伝達率(heat-transfer coefficient)）とランナウト上で放射温度計(radiation thermometer)により測定された鋼板の表面温度を用いて伝熱計算（heat-transfer calculation）を行い、板厚中心の温度履歴（temperature history）を求めることで算出した。   The average cooling rate at the center of the plate thickness is 10 to 100 ° C./s. At a cooling rate of less than 10 ° C./s, even if a hardenability improving element such as Cu, Ni, Cr is added, the area fraction of the ferrite and / or pearlite structure exceeds 5%, so that it is 10 ° C./s or more. The cold speed of is required. On the other hand, when it exceeds 100 ° C./s, the area fraction of the martensite structure exceeds 5%. The cooling rate at the center of the plate thickness is measured with a run-out cooling capacity (heat-transfer coefficient) and a radiation thermometer on the run-out. The heat-transfer calculation was performed using the surface temperature of the steel plate, and the temperature history at the center of the plate thickness was calculated.

冷却停止の温度範囲は３８０℃以上６００℃以下とする。６００℃を超えるとＮｂ炭窒化物といった析出強化粒子の粗大化により強度が低下する。さらに中心偏析部での炭素の濃化が促進され、微細マルテンサイト、上部ベイナイトおよびパーライトが生成しやすくなる。一方で、３８０℃を下回る場合は、鋼板の変形抵抗（deformation resistance）が増加し、コイル状に巻き取ることが困難となるばかりか、Ｎｂ炭窒化物といった析出強化粒子が析出しないため、強度が低下する。   The temperature range of cooling stop is 380 ° C. or more and 600 ° C. or less. If it exceeds 600 ° C., the strength decreases due to coarsening of precipitation strengthening particles such as Nb carbonitride. Furthermore, the concentration of carbon in the central segregation part is promoted, and fine martensite, upper bainite and pearlite are easily generated. On the other hand, when the temperature is lower than 380 ° C., the deformation resistance of the steel sheet is increased, and it becomes difficult to wind up in a coil shape, and precipitation strengthening particles such as Nb carbonitride do not precipitate, so that the strength is increased. descend.

表１に示す組成の鋼素材に、表２に示す熱間圧延条件および冷却条件で熱間圧延を行ないコイル状に巻取り、表２に示す板厚の熱延鋼板とした。   A steel material having the composition shown in Table 1 was hot-rolled under the hot rolling conditions and cooling conditions shown in Table 2 and wound into a coil shape to obtain a hot-rolled steel sheet having a thickness shown in Table 2.

Figure 0005884201
Figure 0005884201

Figure 0005884201
Figure 0005884201

得られた熱延鋼板から、試験片を採取し、組織観察、引張試験、シャルピー衝撃試験、硬度測定およびＨＩＣ試験を実施し、引張特性、靭性および耐ＨＩＣ性を評価した。   From the obtained hot-rolled steel sheet, a test piece was collected and subjected to structure observation, tensile test, Charpy impact test, hardness measurement and HIC test to evaluate tensile properties, toughness and HIC resistance.

得られた熱延鋼板から組織観察用試験片を採取し、圧延方向断面を研磨し、２％ナイタールに３０秒以上浸漬し、偏析線(segregation line)を現出させたのち、走査電子顕微鏡（electron scanning microscope）（倍率２０００倍）で、偏析位置を５視野以上撮影し、組織の種類、ベイニティックフェライト粒径、微細マルテンサイト、上部ベイナイトおよびパーライトといった有害第２相の面積分率を測定した。鋼組織は板厚方向１／４ｔ位置から採取した組織観察用試験片から判定した。   A specimen for microstructure observation was collected from the obtained hot rolled steel sheet, the cross section in the rolling direction was polished, immersed in 2% nital for 30 seconds or more, and a segregation line was revealed, and then a scanning electron microscope ( Using an electron scanning microscope (magnification 2000 times), segregation positions were photographed over 5 fields of view, and the area fraction of harmful second phase such as tissue type, bainitic ferrite grain size, fine martensite, upper bainite and perlite was measured. did. The steel structure was determined from a structure observation specimen taken from a 1/4 t position in the plate thickness direction.

引張試験は、得られた熱延鋼板から、圧延方向に直交する方向（Ｃ方向）が長手方向となるように、ＡＰＩ−５Ｌの規定に準拠して、室温にて引張試験を実施し、降伏応力ＹＳ（公称ひずみ０．５％での変形応力）と引張強さＴＳを求めた。   The tensile test is performed at room temperature in accordance with the provisions of API-5L so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction from the obtained hot-rolled steel sheet, and yielding is performed. Stress YS (deformation stress at a nominal strain of 0.5%) and tensile strength TS were determined.

シャルピー衝撃試験は、得られた熱延鋼板の板厚中央部から、圧延方向に直交する方向（Ｃ方向）が長手方向となるようにＶノッチ試験片を採取し、ＪＩＳ Ｚ ２２４２の規定に準拠して−１４０℃〜０℃の範囲でシャルピー衝撃試験（Charpy impact test）を実施し、吸収エネルギー（absorbed energy）と脆性破面率（percent brittle fracture）を測定し、脆性破面率が５０％となる温度（破面遷移温度（fracture transition temperature））を求めた。なお、各温度での試験片は３本とし、得られた吸収エネルギーと脆性破面率の算術平均を求めた。   In the Charpy impact test, a V-notch test piece was taken from the center of the thickness of the obtained hot-rolled steel sheet so that the direction perpendicular to the rolling direction (C direction) was the longitudinal direction, and conformed to JIS Z 2242. Then, the Charpy impact test is performed in the range of -140 ° C to 0 ° C, the absorbed energy and the percent brittle fracture are measured, and the brittle fracture rate is 50% Temperature (fracture transition temperature) was obtained. In addition, the test piece in each temperature was set to three, and the arithmetic mean of the obtained absorbed energy and the brittle fracture surface ratio was calculated | required.

なお、破面遷移温度（ｖＴｒｓ）は−８０℃以下を良好（○）であるとした。   The fracture surface transition temperature (vTrs) was -80 ° C. or less as good (◯).

ＨＩＣ試験は、得られた熱延鋼板から、長手方向が鋼板の圧延方向となるように鋼板板厚×２０ｍｍ幅×１００ｍｍ長さのＨＩＣ試験片を採取し、ＮＡＣＥ ＴＭ ０２８４の規定に準拠し、Ａ溶液にて耐ＨＩＣ性を評価した。なお、試験片本数は１０本とし、電縫鋼管成形時の塑性ひずみの影響を反映すべく、予め幅方向に１０％の圧縮歪を付与した。この結果、全ての試験片において割れ長さ率（ＣＬＲ）≦１５％となったコイルを耐ＨＩＣ性が良好（○）であると判断する。いずれかの試験片でＣＬＲ＞１５％となったコイルは耐ＨＩＣ性不良（×）であると判断する。   In the HIC test, from the obtained hot-rolled steel sheet, a HIC test piece having a steel plate thickness × 20 mm width × 100 mm length is collected so that the longitudinal direction is the rolling direction of the steel plate, and conforms to the regulations of NACE TM 0284, The HIC resistance was evaluated with the solution A. Note that the number of test pieces was 10, and 10% of compressive strain was applied in the width direction in advance in order to reflect the influence of plastic strain at the time of forming the ERW steel pipe. As a result, the coil having a crack length ratio (CLR) ≦ 15% in all the test pieces is judged to have good HIC resistance (◯). A coil having CLR> 15% in any of the test pieces is judged to have poor HIC resistance (x).

得られた結果を表３に示す。   The obtained results are shown in Table 3.

Figure 0005884201
Figure 0005884201

本発明例はいずれも、ＴＳ≧５４０ＭＰａの高強度と良好な耐ＨＩＣ性を持つ鋼板となっている。一方、本発明の範囲を外れる比較例は、所望の強度、靭性を得られていないか、耐ＨＩＣ性が低下しているかして、耐ＨＩＣ性に優れた高強度電縫鋼管用の熱延鋼板として、所望の特性を確保できていない。
All of the examples of the present invention are steel plates having high strength of TS ≧ 540 MPa and good HIC resistance. On the other hand, the comparative examples that are out of the scope of the present invention show that the desired strength and toughness are not obtained, or the HIC resistance is lowered. As a steel plate, desired properties cannot be secured.

Claims (1)

成分組成が、質量％で、Ｃ：０．０２〜０．０６％、Ｓｉ：０．０５〜０．１９％、Ｍｎ：０．６０〜１．１０％、Ｐ：０．００８％以下、Ｓ：０．００１０％以下、Ｎｂ：０．０２０〜０．０６０％、Ｔｉ：０．００１〜０．０２０％、Ａｌ：０．０１〜０．０８％、Ｃａ：０．０００５〜０．００５０％を含有し、さらに、Ｃｕ：０．５０％以下、Ｎｉ：０．５０％以下、Ｃｒ：０．５０％以下、Ｍｏ：０．５０％以下、Ｖ：０．１０％以下の中から選ばれる１種以上を含有し、残部Ｆｅおよび不可避的不純物からなり、下記式（１）および式（２）を満たし、中心偏析部の金属組織が面積分率で９５％以上のベイニティックフェライト組織であり、ベイニティックフェライト組織の平均粒径が８．０μｍ以下であることを特徴とする引張強さ５４０ＭＰａ以上の高強度ラインパイプ用熱延鋼板。
０．６０≦ＣＰ≦０．９０ ・・・・・（１）
なお、ＣＰは、ＣＰ＝４．４６×Ｃ＋２．３７×Ｍｎ／６＋（１．１８×Ｃｒ＋１．９５×Ｍｏ＋１．７４×Ｖ）／５＋（１．７４×Ｃｕ＋１．７０×Ｎｉ）／１５ から求められ、式中の元素記号は各元素の質量％を意味する。
ＣＭ≦３．０５ ・・・・・（２）
ここで、ＣＭは、ＣＭ＝２．３７×Ｍｎ＋２．３４×Ｍｏ＋０．５９×Ｃｒ＋０．１７×Ｎｉから求められ、式中の元素記号は各元素の質量％を意味する。 Component composition, by mass%, C: 0.02~0.06%, Si : 0.05~ 0.19%, Mn: 0.60~1.10%, P: 0.008% or less, S : 0.0010% or less, Nb: 0.020 to 0.060%, Ti: 0.001 to 0.020%, Al: 0.01 to 0.08%, Ca: 0.0005 to 0.0050% In addition, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.10% or less contain one or more, and a balance of Fe and unavoidable impurities, the following formula (1) and (2) meets the metal structure of the center segregation area is at an area fraction of 95% or more bainitic ferrite structure The average grain size of the bainitic ferrite structure is 8.0 μm or less. Hot-rolled steel sheet for high-strength line pipes with a tensile strength of 540 MPa or more.
0.60 ≦ CP ≦ 0.90 (1)
CP is obtained from CP = 4.46 × C + 2.37 × Mn / 6 + (1.18 × Cr + 1.95 × Mo + 1.74 × V) / 5 + (1.74 × Cu + 1.70 × Ni) / 15 The element symbol in the formula means mass% of each element.
CM ≦ 3.05 (2)
Here, CM is obtained from CM = 2.37 × Mn + 2.34 × Mo + 0.59 × Cr + 0.17 × Ni, and the element symbol in the formula means mass% of each element.
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