JPWO2010104165A1 - HIC thick steel plate and UOE steel pipe - Google Patents

HIC thick steel plate and UOE steel pipe Download PDF

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JPWO2010104165A1
JPWO2010104165A1 JP2011503869A JP2011503869A JPWO2010104165A1 JP WO2010104165 A1 JPWO2010104165 A1 JP WO2010104165A1 JP 2011503869 A JP2011503869 A JP 2011503869A JP 2011503869 A JP2011503869 A JP 2011503869A JP WO2010104165 A1 JPWO2010104165 A1 JP WO2010104165A1
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伸彰 高橋
伸彰 高橋
山本　昭夫
昭夫 山本
出 湊
出 湊
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Sumitomo Metal Industries Ltd
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
    • B21C37/06—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of tubes or metal hoses; Combined procedures for making tubes, e.g. for making multi-wall tubes
    • B21C37/08—Making tubes with welded or soldered seams
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium

Abstract

Ｘ６０グレード以上の高強度で耐ＨＩＣ特性に優れた厚鋼板およびＵＯＥ鋼管は、質量％で、Ｃ：０.０２〜０.０７％、Ｓｉ：０.０５〜０.５０％、Ｍｎ：１.１〜１.６％、Ｐ：０.０１５％以下、Ｓ：０.００２％以下、Ｎｂ：０.００５〜０.０６０％、Ｔｉ：０.００５〜０.０３０％、Ａｌ：０.００５〜０.０６％、Ｃａ：０.０００５〜０.００６０％、Ｎ：０.００１５〜０.００７％、Ｃｕ、Ｎｉ、ＣｒおよびＭｏの少なくとも１種を合計で０.１％超、１.５％未満、残部Ｆｅおよび不純物からなる化学組成を有し、かつＮｂとＴｉの偏析度がそれぞれ２.０以下、Ｎｂ偏析度／Ｍｎ偏析度の比およびＴｉ偏析度／Ｍｎ偏析度の比がいずれも１.０以上、１.５以下である。Thick steel plates and UOE steel pipes having high strength and excellent HIC resistance of X60 grade or higher are C: 0.02 to 0.07%, Si: 0.05 to 0.50%, Mn: 1. 1 to 1.6%, P: 0.015% or less, S: 0.002% or less, Nb: 0.005 to 0.060%, Ti: 0.005 to 0.030%, Al: 0.005 ~ 0.06%, Ca: 0.0005-0.0060%, N: 0.0015-0.007%, and at least one of Cu, Ni, Cr and Mo in total exceeds 0.1%. It has a chemical composition consisting of less than 5%, the balance Fe and impurities, and the segregation degree of Nb and Ti is 2.0 or less, the ratio of Nb segregation degree / Mn segregation degree and the ratio of Ti segregation degree / Mn segregation degree are Both are 1.0 or more and 1.5 or less.

Description

本発明は、厚鋼板およびＵＯＥ鋼管に関し、より具体的には、ラインパイプ、海洋構造物、圧力容器等の用途に使用するのに適した、耐水素誘起割れ特性（耐ＨＩＣ特性）に優れた、Ｘ６０グレード以上の高強度の厚鋼板と、この厚鋼板を素材とするＵＯＥ鋼管とに関する。本発明において「厚鋼板」とは板厚６.０ｍｍ以上の鋼板を意味する。   The present invention relates to a thick steel plate and a UOE steel pipe, and more specifically, excellent in hydrogen-induced crack resistance (HIC resistance) suitable for use in applications such as line pipes, offshore structures, and pressure vessels. The present invention relates to a high-strength thick steel plate of X60 grade or higher and a UOE steel pipe made of this thick steel plate. In the present invention, “thick steel plate” means a steel plate having a thickness of 6.0 mm or more.

原油や天然ガスの輸送に使用されるラインパイプには、強度、靱性、溶接性等の一般的な特性の他に、硫化水素を含む腐食環境下で使用される可能性があることから、水素誘起割れ（Hydrogen Induced Cracking) (以下「ＨＩＣ」と略記する）に対する耐性が重要な特性として要求される。   Line pipes used for the transportation of crude oil and natural gas, in addition to general properties such as strength, toughness, and weldability, may be used in corrosive environments containing hydrogen sulfide. Resistance to induced cracking (hereinafter abbreviated as “HIC”) is required as an important characteristic.

過去に実際にラインパイプや油井管等でＨＩＣに起因した油漏れや破壊、爆発事故が発生したことから、ＨＩＣに関しては従来から多くの研究が行われている。その結果、ＨＩＣの発生機構に関して、腐食反応により発生した水素イオンが鋼表面に吸着し、原子状水素として鋼内部に拡散して鋼材中のＭｎＳや酸化物系介在物の周囲に集積し、分子化してガスとなり、その内圧によって割れを生ずることが明らかにされている。このため、ＨＩＣの防止策として、以下に大別される手段が開示されている。   In the past, oil leakage, destruction, and explosion accidents caused by HIC have actually occurred in line pipes, oil well pipes, etc., and many studies have been conducted on HIC. As a result, with regard to the mechanism of HIC generation, hydrogen ions generated by the corrosion reaction are adsorbed on the steel surface, diffused as atomic hydrogen inside the steel and accumulated around MnS and oxide inclusions in the steel. It has been clarified that the gas becomes gas and cracks occur due to its internal pressure. For this reason, means broadly classified as HIC prevention measures are disclosed below.

（１）特許文献１には、鋼材中にＭｎＳが存在すると、これを起点として割れが発生し、ＭｎＳが圧延時に長く伸展すると割れ感受性が増大するという知見に基づいて、鋼中のＳ含有量を低下させ、かつ鋼中にＣａやＲＥＭを添加することによって、鋼中のＳを微細な球状化したＣａＳやＲＥＭ硫化物とすることが記載されている。   (1) In Patent Document 1, when MnS is present in steel, cracks are generated from this, and based on the knowledge that when MnS extends for a long time during rolling, the cracking sensitivity increases, the S content in the steel It is described that S in steel is made into a fine spheroidized CaS or REM sulfide by reducing Ca and REM in steel.

（２）特許文献２には、鋳片の中心偏析部に相当する部位では、Ｃ、Ｍｎ、Ｐ等が偏析することによりマルテンサイトやベイナイトなどの硬い組織が形成され、これが割れの伝播経路となることを考慮して、鋼中のＣ、Ｍｎ、Ｐ等の濃度を低減すること、拡散によって偏析を軽減するための均熱処理を行うこと、さらには圧延後の冷却速度を高めることによって、硬い組織の生成を防止することが開示されている。   (2) In Patent Document 2, a hard structure such as martensite or bainite is formed by segregation of C, Mn, P, etc. in a portion corresponding to the center segregation portion of the slab, and this is a propagation path of cracks. In consideration of becoming, it is hard by reducing the concentration of C, Mn, P, etc. in the steel, performing soaking treatment to reduce segregation by diffusion, and further increasing the cooling rate after rolling. Preventing tissue formation is disclosed.

また、特許文献３、４には、連続鋳造の未凝固溶鋼が残る段階で鋳片を一旦バルジングさせた後で圧下することによって、中心偏析そのものを解消することが開示されている。
（３）特許文献５〜７には、最近求められるようになってきた鋼材の強度スペックの向上に伴って、上記の中心偏析部やＭｎＳ生成に対する単独対策では不十分な場合が多くなってきたことから、鋼中にＣｕやＮｉを添加することにより、鋼材表面に保護被膜を形成して、鋼中への水素の侵入を抑制するとともに、ＣｒやＭｏ等の添加や圧延時の加工熱処理（ＴＭＣＰ，thermomechanical control process）を併用することが開示されている。Patent Documents 3 and 4 disclose that the center segregation itself is eliminated by rolling the slab once after bulging the slab at the stage where the continuously cast unsolidified molten steel remains.
(3) In Patent Documents 5 to 7, with the improvement of the strength specifications of steel materials that have recently been required, the single countermeasures against the above-mentioned center segregation part and MnS generation have become insufficient. Therefore, by adding Cu or Ni to the steel, a protective film is formed on the surface of the steel material to suppress the intrusion of hydrogen into the steel, and the addition of Cr, Mo, etc. or the heat treatment during rolling ( It is disclosed that TMCP (thermomechanical control process) is used in combination.

ＪＰ５４−１１０１１９Ａ１JP54-10119A1 ＪＰ６１−６０８６６Ａ１JP61-60866A1 ＪＰ０９−５７４１０Ａ１JP09-57410A1 ＪＰ２００２−１０５６０４Ａ１JP2002-105604A1 ＪＰ０６−２２０５７７Ａ１JP06-220577A1 ＪＰ０９−２０９０３７Ａ１JP09-209037A1 ＪＰ２００３−２２６９２２Ａ１JP2003-226922A1

本発明者らは、主にラインパイプ等に使用されるＸ６０グレード（ＹＳ：６０ｋｓｉ、ＹＰは７０ｋｓｉ程度）以上の高強度の厚鋼板においては、上述した（１）〜（３）に説明した従来の対策では防止できないＨＩＣ発生機構があることを新たに究明した。   In the high-strength thick steel plate of X60 grade (YS: 60 ksi, YP is about 70 ksi) or higher mainly used for line pipes and the like, the conventional methods described in (1) to (3) above are used. We have newly investigated that there is an HIC generation mechanism that cannot be prevented by this measure.

本発明の目的は、Ｘ６０グレード以上の強度レベルの厚鋼板、特に著しい偏析が起こりやすい板厚１０ｍｍ以上の厚鋼板において、これまで知られているＨＩＣの防止策では防ぐことができないＨＩＣを防止することができる、耐ＨＩＣ特性に優れた厚鋼板（以下、耐ＨＩＣ厚鋼板ともいう）およびＵＯＥ鋼管を提供することである。   The object of the present invention is to prevent HIC that cannot be prevented by the known HIC prevention measures in a steel plate having a strength level of X60 grade or higher, particularly a steel plate having a thickness of 10 mm or more that is likely to cause significant segregation. It is to provide a thick steel plate (hereinafter also referred to as a HIC thick steel plate) and a UOE steel pipe excellent in HIC resistance.

本発明は、耐ＨＩＣ特性を阻害する主原因として従来から考えられていたＭｎＳ、Ｃ、Ｍｎ、Ｐ以外に、Ｎｂの炭窒化物がＨＩＣの発生起点になるという重要な知見に基づくものである。   The present invention is based on an important finding that Nb carbonitride is the starting point of HIC in addition to MnS, C, Mn, and P, which have been conventionally considered as the main cause of hindering HIC resistance. .

本発明は、質量％で、Ｃ：０.０２〜０.０７％、Ｓｉ：０.０５〜０.５０％、Ｍｎ：１.１〜１.６％、Ｐ：０.０１５％以下、Ｓ：０.００２％以下、Ｎｂ：０.００５〜０.０６０％、Ｔｉ：０.００５〜０.０３０％、Ａｌ：０.００５〜０.０６％、Ｃａ：０.０００５〜０.００６０％、Ｎ：０.００１５〜０.００７％、式(１)を満足する量のＣｕ、Ｎｉ、ＣｒおよびＭｏから選ばれた少なくとも１種、Ｖ：０〜０.１０％、残部Ｆｅおよび不純物からなる化学組成を有し、かつＮｂ偏析度が２.０以下、Ｍｎ偏析度に対するＮｂ偏析度の比α（α＝Ｎｂ偏析度／Ｍｎ偏析度）が１.０以上、１.５以下、Ｔｉ偏析度が２.０以下、Ｍｎ偏析度に対するＴｉ偏析度の比β（β＝Ｔｉ偏析度／Ｍｎ偏析度）が１.０以上、１.５以下であることを特徴する、耐ＨＩＣ特性に優れた厚鋼板である：
式(１)：０.１％＜（Ｃｕ＋Ｎｉ＋Ｃｒ＋Ｍｏ）＜１.５％
上記式中、元素記号は各元素の含有量（質量％）を意味する。In the present invention, by mass%, C: 0.02 to 0.07%, Si: 0.05 to 0.50%, Mn: 1.1 to 1.6%, P: 0.015% or less, S : 0.002% or less, Nb: 0.005 to 0.060%, Ti: 0.005 to 0.030%, Al: 0.005 to 0.06%, Ca: 0.0005 to 0.0006% , N: 0.0015 to 0.007%, at least one selected from Cu, Ni, Cr and Mo in an amount satisfying the formula (1), V: 0 to 0.10%, balance Fe and impurities Nb segregation degree is 2.0 or less, ratio α of Nb segregation degree to Mn segregation degree (α = Nb segregation degree / Mn segregation degree) is 1.0 or more and 1.5 or less, Ti HIC resistance characterized by having a segregation degree of 2.0 or less and a ratio of Ti segregation degree to Mn segregation degree β (β = Ti segregation degree / Mn segregation degree) of 1.0 or more and 1.5 or less. There is an excellent thick steel plate to sex:
Formula (1): 0.1% <(Cu + Ni + Cr + Mo) <1.5%
In said formula, an element symbol means content (mass%) of each element.

本発明において、「Ｎｂ偏析度」、「Ｍｎ偏析度」および「Ｔｉ偏析度」は、鋼板の板圧延方向に平行に切断した断面（板表面に垂直な断面）において、板厚中心から上下に５０点以上、好ましくは１００点又はそれ以上の地点で、Ｎｂ、ＭｎおよびＴｉ濃度の測定を行い、Ｎｂ、ＭｎおよびＴｉのそれぞれについて、全測定値の算術平均値である平均濃度により全測定値中の最大測定値である最高濃度を除することにより算出される値を意味する。即ち、Ｎｂ偏析度は最高Ｎｂ濃度／平均Ｎｂ濃度であり、同様にＭｎ偏析度は最高Ｍｎ濃度／平均Ｍｎ濃度であり、Ｔｉ偏析度は最高Ｔｉ濃度／平均Ｔｉ濃度である。   In the present invention, the “Nb segregation degree”, “Mn segregation degree”, and “Ti segregation degree” are measured vertically from the center of the plate thickness in a cross section (cross section perpendicular to the plate surface) cut in parallel to the plate rolling direction of the steel plate. Nb, Mn and Ti concentrations are measured at 50 points or more, preferably 100 points or more, and for each of Nb, Mn and Ti, all measured values are obtained by the average concentration which is the arithmetic average value of all measured values. It means a value calculated by dividing the maximum density which is the maximum measured value. That is, the Nb segregation degree is the highest Nb concentration / average Nb concentration, the Mn segregation degree is the highest Mn concentration / average Mn concentration, and the Ti segregation degree is the highest Ti concentration / average Ti concentration.

５０点以上、好ましくは１００点又はそれ以上の測定点は、板厚方向において全板厚の２０％以上の長さ範囲（即ち、板厚中心から上下にそれぞれ全板厚の１０％以上の長さ範囲）において、板厚中心から上下方向にほぼ均等な増分で変化させた位置とすることが好ましい。例えば、板厚中心から上下に１００μｍ（＝０.１ｍｍ）間隔でそれぞれ２５点以上、好ましくは５０点又はそれ以上の地点でＮｂ、Ｍｎ、Ｔｉの各濃度の測定を行う。いずれの場合も、得られた５０点以上での測定値から、Ｎｂ、Ｍｎ、Ｔｉの平均濃度と最大濃度を求めることにより、各元素の偏析値を算出できる。後述するように、本発明の実施例では、板厚中心から上下に０.１ｍｍ間隔で６０点ずつ（合計測定長さ１２ｍｍ）の測定により偏析度を求めた。   50 points or more, preferably 100 points or more, is a length range of 20% or more of the total plate thickness in the plate thickness direction (that is, a length of 10% or more of the total plate thickness above and below the center of the plate thickness). In the range), it is preferable that the position is changed from the center of the plate thickness in the vertical direction in almost equal increments. For example, the concentration of Nb, Mn, and Ti is measured at 25 points or more, preferably 50 points or more at intervals of 100 μm (= 0.1 mm) vertically from the center of the plate thickness. In any case, the segregation value of each element can be calculated by obtaining the average concentration and the maximum concentration of Nb, Mn, and Ti from the measured values obtained at 50 points or more. As will be described later, in the examples of the present invention, the degree of segregation was determined by measuring 60 points (total measurement length: 12 mm) at intervals of 0.1 mm from the center of the plate thickness.

「厚鋼板」とは、前述したように板厚６.０ｍｍ以上の鋼板を意味する。好ましい板厚は特に偏析が起こりやすい１０ｍｍ以上である。板厚の上限は特に規定されないが、本発明によれば板厚４０ｍｍまでの耐ＨＩＣ厚鋼板を実現できる。   The “thick steel plate” means a steel plate having a thickness of 6.0 mm or more as described above. A preferable plate thickness is 10 mm or more where segregation is particularly likely to occur. The upper limit of the plate thickness is not particularly defined, but according to the present invention, a HIC thick steel plate having a plate thickness of up to 40 mm can be realized.

別の観点からは、本発明は、上述した耐ＨＩＣ厚鋼板を素材とすることを特徴とするＵＯＥ鋼管である。
ＵＯＥ鋼管は、厚鋼板をＵ型、次にＯ型にプレス成形し、板の突き合わせ部をサブマージアーク溶接した後、内側からエキスパンダーで拡管して所定寸法に仕上げることにより製造される。From another point of view, the present invention is a UOE steel pipe characterized by using the above-mentioned HIC thick steel plate as a material.
The UOE steel pipe is manufactured by press-forming a thick steel plate into a U shape and then an O shape, and submerging arc welding the butt portion of the plate, and then expanding the tube with an expander from the inside and finishing it to a predetermined size.

本発明に係る耐ＨＩＣ厚鋼板およびＵＯＥ鋼管では、Ｘ６０グレード（ＹＰ７０ｋｓｉ程度）以上の高強度が得られる。また、Ｍｎ、Ｃ、Ｐ、Ｓ等の従来から提案されているＨＩＣの発生原因となる元素が制御されていることに加えて、ＮｂやＴｉの炭窒化物に起因したＨＩＣの発生を防止できるため、高強度厚鋼板にけるＨＩＣの発生を確実に防止できる。   In the HIC thick steel plate and UOE steel pipe according to the present invention, a high strength of X60 grade (about YP70 ksi) or more can be obtained. Moreover, in addition to controlling the elements that cause the generation of HIC, such as Mn, C, P, and S, which are conventionally proposed, the generation of HIC due to Nb and Ti carbonitrides can be prevented. Therefore, generation of HIC in the high-strength thick steel plate can be reliably prevented.

本発明に係る耐ＨＩＣ特性に優れた厚鋼板は、海洋構造物や圧力容器などの構造物に、あるいはＵＯＥ鋼管の製造に使用することができる。本発明に係る耐ＨＩＣ特性に優れたＵＯＥ鋼管は、特にラインパイプに好適であるが、海洋構造物においても使用可能である。ＨＩＣ発生が確実に防止できることから、製品の信頼性が著しく高まる。   The thick steel plate having excellent HIC resistance according to the present invention can be used for structures such as offshore structures and pressure vessels, or for manufacturing UOE steel pipes. The UOE steel pipe excellent in HIC resistance according to the present invention is particularly suitable for a line pipe, but can also be used in an offshore structure. Since the generation of HIC can be surely prevented, the reliability of the product is remarkably increased.

レーザーＩＣＰ法によるＭｎ、Ｐ、Ｓ、Ｎｂ、Ｃの偏析度の調査結果を示すグラフである。It is a graph which shows the investigation result of the segregation degree of Mn, P, S, Nb, C by the laser ICP method. 図２(ａ)はＭｎ濃度とＮｂ濃度との関係を示すグラフであり、図２(ｂ)はＭｎ濃度とＴｉ濃度との関係を示すグラフである。FIG. 2A is a graph showing the relationship between Mn concentration and Nb concentration, and FIG. 2B is a graph showing the relationship between Mn concentration and Ti concentration.

本発明に係る厚鋼板の化学組成は次に説明する通りである。なお、以下の説明において、鋼の化学組成に関する％はすべて質量％を意味する。
［Ｃ：０.０２％以上、０.０７％以下］
一般に、Ｃは、鋼の強度に大きな影響を及ぼす元素として知られる。Ｃ含有量が０.０２％未満ではラインパイプ等の用途に対して所定の強度を得ることが困難となる。一方、Ｃ含有量が０.０７％を超えると、上述したように連続鋳造時には鋳片の厚み中心部にマクロ偏析部を形成し、ＨＩＣの発生原因となる。そのため、Ｃ含有量は０.０２％以上、０.０７％以下とする。上記観点からＣ含有量の下限は０.０３％であることが望ましく、その上限は０.０６％であることが望ましい。The chemical composition of the thick steel plate according to the present invention is as described below. In the following description, all percentages relating to the chemical composition of steel mean mass%.
[C: 0.02% or more, 0.07% or less]
In general, C is known as an element that greatly affects the strength of steel. If the C content is less than 0.02%, it is difficult to obtain a predetermined strength for applications such as line pipes. On the other hand, if the C content exceeds 0.07%, as described above, a macro-segregation portion is formed at the thickness center portion of the slab during continuous casting, which causes generation of HIC. Therefore, the C content is set to 0.02% or more and 0.07% or less. From the above viewpoint, the lower limit of the C content is preferably 0.03%, and the upper limit is preferably 0.06%.

［Ｓｉ：０.０５％以上、０.５０％以下］
Ｓｉは、０.０５％以上含有させた場合に、一般に鋼の製造プロセスでは脱酸元素として作用し、鋼中の酸素濃度を低減するのに有効である。また、Ｓｉは鋼を強化する効果もある。しかし、Ｓｉ含有量が０.５０％を超えると島状マルテンサイトの生成を引き起し、溶接時にＨＡＺ（溶接熱影響部）の靱性を悪化させる。また、ＳｉはＴｉとの間に強い相互作用を有することから、構成元素でないにもかかわらずＴｉＮの生成に影響する。本発明においてＨＩＣの発生起点として注目するＮｂ炭窒化物は、ＴｉＮを核として析出する可能性が高く、Ｓｉ濃度が高くなりすぎると、ＨＩＣ特性の劣化を引き起こす恐れがある。したがって、Ｓｉ含有量は０.０５％以上、０.５０％以下とする。好ましいＳｉ含有量は０.０５％以上、０.３％未満である。[Si: 0.05% or more, 0.50% or less]
When Si is contained in an amount of 0.05% or more, it generally acts as a deoxidizing element in the steel manufacturing process and is effective in reducing the oxygen concentration in the steel. Si also has the effect of strengthening steel. However, if the Si content exceeds 0.50%, island-shaped martensite is generated, and the toughness of the HAZ (welding heat affected zone) is deteriorated during welding. Further, since Si has a strong interaction with Ti, it affects the generation of TiN even though it is not a constituent element. In the present invention, Nb carbonitrides that are noted as the starting point of HIC generation are highly likely to precipitate with TiN as nuclei, and if the Si concentration becomes too high, there is a risk of deteriorating HIC characteristics. Therefore, the Si content is set to 0.05% or more and 0.50% or less. A preferable Si content is 0.05% or more and less than 0.3%.

［Ｍｎ：１.１％以上、１.６％以下］
Ｍｎは，一般に鋼材の強度に大きな影響を与える元素であり、Ｍｎ含有量が１.１％未満では十分な強度を得ることが困難である。Ｍｎ含有量が１.６％を超えると、上述したようにＭｎが中心偏析部で濃化して、耐ＨＩＣ特性を劣化させる。このため、Ｍｎ含有量は１.１％以上、１.６％以下とする。中心偏析部での耐ＨＩＣ特性を確実に確保するために、Ｍｎ含有量は好ましくは１.１％以上、１.５％未満である。[Mn: 1.1% or more and 1.6% or less]
Mn is an element that generally has a great influence on the strength of a steel material. If the Mn content is less than 1.1%, it is difficult to obtain sufficient strength. If the Mn content exceeds 1.6%, Mn concentrates at the center segregation part as described above, and the HIC resistance is deteriorated. For this reason, Mn content shall be 1.1% or more and 1.6% or less. In order to ensure the HIC resistance at the center segregation part, the Mn content is preferably 1.1% or more and less than 1.5%.

［Ｐ：０.０１５％以下］
Ｐは、鋼中に不可避的に含有される不純物の一つであり、できるだけ低い方が好ましい。Ｐは、凝固時の固液界面における分配係数が小さく、著しく偏析する傾向があるので、上述したように中心偏析部で濃化して、耐ＨＩＣ特性を劣化させる。そのため、Ｐ含有量の上限を０.０１５％とする。Ｐの中心偏析部における耐ＨＩＣ特性の劣化をより確実に防止するためには、Ｐ含有量を０.００８％未満とすることが好ましい。Ｐ含有量の下限は規定されないが、極端なＰ含有量の低減には相応のコスト上昇を伴うので、Ｐ含有量は０.００４％以上とすることが望ましい。[P: 0.015% or less]
P is one of impurities inevitably contained in the steel, and is preferably as low as possible. P has a small distribution coefficient at the solid-liquid interface during solidification and has a tendency to remarkably segregate. Therefore, as described above, P is concentrated at the central segregation portion to deteriorate the HIC resistance. Therefore, the upper limit of the P content is set to 0.015%. In order to more reliably prevent the deterioration of the HIC resistance in the central segregation portion of P, the P content is preferably less than 0.008%. Although the lower limit of the P content is not specified, the extreme reduction of the P content is accompanied by a corresponding increase in cost. Therefore, the P content is preferably 0.004% or more.

［Ｓ：０.００２％以下］
Ｓも鋼中に不可避的に含有される不純物の一つであり、できるだけ低い方が好ましい。Ｐと同様に、Ｓも凝固時の固液界面における分配係数が小さく、著しく偏析する傾向がある元素である。その上、上述したように、偏析部ではＭｎＳを生成してＨＩＣの発生起点となる。このため、Ｓ含有量の上限を０.００２％とする。高強度鋼のように、より要求レベルの厳しい条件で安定して耐ＨＩＣ特性を得るためには、Ｓ含有量の上限を０.００１％とすることが好ましい。Ｓ含有量の下限は規定されないが、極端なＳ含有量の低減には相応のコスト上昇を伴うので、Ｓ含有量は０.０００３％以上とすることが望ましい。[S: 0.002% or less]
S is one of the impurities inevitably contained in the steel and is preferably as low as possible. Like P, S is an element that has a small distribution coefficient at the solid-liquid interface during solidification and has a tendency to remarkably segregate. In addition, as described above, the segregation part generates MnS and becomes the starting point of HIC generation. For this reason, the upper limit of the S content is set to 0.002%. In order to obtain stable HIC resistance under more severe requirements such as high strength steel, the upper limit of the S content is preferably 0.001%. Although the lower limit of the S content is not specified, an extreme reduction in the S content is accompanied by a corresponding increase in cost, so the S content is preferably 0.0003% or more.

［Ｎｂ：０.００５％以上、０.０６０％以下］
Ｎｂは、鋼中で炭窒化物を形成して、鋼の強度を高めるとともに靱性の向上にも有効な元素である。そのため、Ｎｂを０.００５％以上含有させる。特にＴＭＣＰにおいては、固溶及び析出を制御することにより厚鋼板のミクロ組織を制御するためにＮｂが用いられる。この効果を得るためにも、Ｎｂを０.００５％以上含有させる。しかし、Ｎｂ含有量が０.０６０％を超えると、加熱時にも固溶せず、組織制御ができなくなる。同時にＮｂ量の増加は、Ｎｂ炭窒化物量の増加を意味し、耐ＨＩＣ特性の低下を引き起こす可能性がある。このためＮｂ含有量は０.００５％以上、０.０６０％以下とする。高強度鋼のように、より要求レベルの厳しい条件で安定して耐ＨＩＣ特性を確保するためには、Ｎｂ含有量を０.０１０％以上、０.０４０％以下とすることが好ましい。[Nb: 0.005% or more, 0.060% or less]
Nb is an element that forms carbonitrides in steel to increase the strength of the steel and is effective in improving toughness. Therefore, Nb is contained 0.005% or more. In particular, in TMCP, Nb is used to control the microstructure of the thick steel plate by controlling solid solution and precipitation. In order to obtain this effect, 0.005% or more of Nb is contained. However, if the Nb content exceeds 0.060%, it does not dissolve even during heating, and the structure cannot be controlled. At the same time, an increase in the amount of Nb means an increase in the amount of Nb carbonitride, which may cause a decrease in HIC resistance. For this reason, Nb content shall be 0.005% or more and 0.060% or less. In order to ensure stable HIC resistance under more severe requirements, such as high-strength steel, the Nb content is preferably 0.010% or more and 0.040% or less.

［Ｔｉ：０.００５％以上、０.０３０％以下］
Ｔｉは、鋼の強度を向上させるとともに、鋼中のＮをＴｉＮとして固定し、ＮｂＮやＡｌＮの析出量を減少させることから、連続鋳造の鋳片の曲げ・矯正時のオーステナイト粒界へのＮｂＮやＡｌＮの動的析出に起因した鋳片表面割れを防止する効果もある。このような効果を得るために、Ｔｉを０.００５％以上含有させる。しかし、Ｔｉ含有量が０.０３０％を超えると、Ｔｉ炭化物が多数生成し、ＨＡＺ靱性を低下させるとともに、粗大なＴｉＮが生成する原因となる。また、上述したようにＮｂ炭窒化物はＴｉＮを核として析出する可能性が高いため、粗大なＴｉＮの存在は耐ＨＩＣ特性の低下を引き起こす。このため、Ｔｉ含有量は０.００５％以上、０.０３０％以下とする。好ましいＴｉ含有量は０.０１０％以上、０.０２５％以下である。[Ti: 0.005% or more, 0.030% or less]
Ti improves the strength of the steel, fixes N in the steel as TiN, and reduces the precipitation amount of NbN and AlN. There is also an effect of preventing slab surface cracks caused by dynamic precipitation of AlN. In order to obtain such an effect, 0.005% or more of Ti is contained. However, if the Ti content exceeds 0.030%, a large number of Ti carbides are generated, which reduces HAZ toughness and causes coarse TiN to be generated. Further, as described above, since Nb carbonitride is highly likely to precipitate with TiN as a nucleus, the presence of coarse TiN causes a decrease in HIC resistance. For this reason, Ti content shall be 0.005% or more and 0.030% or less. A preferable Ti content is 0.010% or more and 0.025% or less.

［Ａｌ：０.００５％以上、０.０６％以下］
Ａｌも脱酸元素として鋼中の酸素濃度を低減するために有効な元素の一つである。脱酸のために必要となるＡｌ含有量は０.００５％以上となる。Ａｌ含有量がこれを下回ると、脱硫も不十分になる上、Ｃａ添加の歩留まりが悪化し、その効果も充分得られなくなり、鋼中の硫化物やＳの偏析に起因してＨＩＣが発生する。しかし、脱酸に伴い生成するアルミナはＨＩＣの原因となることがあるので、Ａｌ含有量は０.０６％以下とする。同じ理由で、Ａｌ含有量は０.０４％以下とすることが好ましい。[Al: 0.005% or more, 0.06% or less]
Al is one of the elements effective for reducing the oxygen concentration in steel as a deoxidizing element. The Al content necessary for deoxidation is 0.005% or more. If the Al content is less than this, desulfurization will be insufficient, the yield of Ca addition will deteriorate, and the effect will not be sufficiently obtained, and HIC will occur due to the segregation of sulfides and S in the steel. . However, since the alumina produced by deoxidation may cause HIC, the Al content is set to 0.06% or less. For the same reason, the Al content is preferably 0.04% or less.

［Ｃａ：０.０００５％以上、０.００６０％以下］
耐ＨＩＣ鋼では、ＣａはＳ濃度を低減させ、ＭｎＳの生成を防止すると共に、硫化物の形態制御のために半ば必須の添加元素である。このため、Ｃａを０.０００５％以上含有させる。しかし、Ｃａ含有量が０.００６０％を超えると、その効果は飽和し、製造コストの増加を招く。このため、Ｃａ含有量は０.０００５％以上、０.００６０％以下とする。[Ca: 0.0005% or more, 0.0006% or less]
In HIC-resistant steel, Ca is a semi-essential additive element for reducing the S concentration, preventing the formation of MnS, and controlling the form of sulfide. For this reason, 0.0005% or more of Ca is contained. However, when the Ca content exceeds 0.0006%, the effect is saturated and the manufacturing cost increases. For this reason, Ca content shall be 0.0005% or more and 0.0006% or less.

［Ｎ：０.００１５％以上、０.００７％以下］
Ｎは、鋼を転炉などの大気雰囲気で溶製する場合には鋼中に不可避的に侵入する元素である。Ｎは本発明で着目する粗大なＮｂ炭窒化物の構成元素である。Ｎｂ炭窒化物は、直接Ｎと優先的に結びつくことはないが、晶出したＴｉＮを核として析出することが知られている。Ｎは鋼材中でＡｌやＴｉなどと窒化物を形成する元素であり、これらの窒化物は熱間加工の過程でピン留め粒子として結晶粒を微細化する効果を有することから、鋼材の機械特性に影響を与えるとともに、ミクロ組織形成にも影響を与える。このため、Ｎを０.００１５％以上の濃度とする必要がある。一方で、前述のようにこれらの窒化物が連続鋳造時にオーステナイト粒界に動的析出することにより、鋳片表面割れの原因となることから、Ｎの上限は０.００７％となる。[N: 0.0015% or more, 0.007% or less]
N is an element that inevitably enters the steel when the steel is melted in an air atmosphere such as a converter. N is a constituent element of coarse Nb carbonitride which is focused in the present invention. Nb carbonitrides are not directly associated with N preferentially, but are known to precipitate with crystallized TiN as nuclei. N is an element that forms nitrides with Al, Ti, etc. in steel materials, and these nitrides have the effect of refining crystal grains as pinned particles during the hot working process. Affects the formation of microstructures. For this reason, N needs to be a concentration of 0.0015% or more. On the other hand, as described above, these nitrides are dynamically precipitated at the austenite grain boundaries during continuous casting, causing cracks on the slab surface, so the upper limit of N is 0.007%.

［０.１％＜（Ｃｕ＋Ｎｉ＋Ｃｒ＋Ｍｏ）＜１.５％］
(１)式に関して述べたように、上記各元素記号はその元素の含有量（質量％）を意味する。[0.1% <(Cu + Ni + Cr + Mo) <1.5%]
As described with respect to the formula (1), each element symbol means the content (% by mass) of the element.

耐ＨＩＣ鋼では、ＭｎＳ発生およびＣ偏析の低減の目的で、Ｃ、Ｍｎの含有量の上限が定められる。このため、強度を確保するために、Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏから選ばれた少なくとも１種の合金元素を含有させる。強度の向上効果を確実に得るためには、これら合金元素の含有量の総量は０.１％よりも多いことが有効である。しかし、これら合金元素の含有量が多過ぎると、焼き入れ性の上昇を伴い、強度上昇とともに一部組織の硬化を引き起こし、これにより耐ＨＩＣ性が劣化する。そこで、本発明では、これら合金元素の含有量の総量は１.５％未満とする。   In HIC-resistant steel, the upper limit of the C and Mn contents is determined for the purpose of reducing MnS generation and C segregation. For this reason, in order to ensure strength, at least one alloy element selected from Cu, Ni, Cr, and Mo is contained. In order to surely obtain the effect of improving the strength, it is effective that the total content of these alloy elements is more than 0.1%. However, if the content of these alloy elements is too large, the hardenability is increased, and the strength is increased and part of the structure is hardened. As a result, the HIC resistance is deteriorated. Therefore, in the present invention, the total content of these alloy elements is less than 1.5%.

これらの合金元素それぞれに含有量についても説明する。これらの説明からわかるように、これら４種の元素のすべてを含有させることが好ましいが、目標とする強度レベルによっては１種〜３種の添加でもよい。しかし、その場合でも、それらの合計量が０.１％超、１.５％未満となるようにする。   The content of each of these alloy elements will also be described. As can be seen from these explanations, it is preferable to contain all of these four elements, but depending on the target strength level, one to three additions may be added. However, even in that case, the total amount thereof is made to be more than 0.1% and less than 1.5%.

Ｃｕは、０.１％以上含有することにより鋼の焼き入れ性を向上させる。一方、Ｃｕ含有量が０.５％を超えると、鋼の熱間加工性や被削性が低下し、また連続鋳造時には、カッパー割れと称する表面割れを誘発する。したがって、Ｃｕ含有量は０.１％以上、０.５％以下であることが望ましい。Ｃｕを０.２％以上含有する場合には、カッパー割れを防止するために、Ｃｕの（１／３）以上の量のＮｉを併せて含有させることが望ましい。   Cu improves the hardenability of steel by containing 0.1% or more. On the other hand, if the Cu content exceeds 0.5%, the hot workability and machinability of the steel deteriorate, and surface cracks called copper cracks are induced during continuous casting. Therefore, the Cu content is desirably 0.1% or more and 0.5% or less. When Cu is contained in an amount of 0.2% or more, it is desirable to contain Ni in an amount of (1/3) or more of Cu in order to prevent copper cracking.

Ｎｉには、固溶強化によって鋼の強度を向上させるとともに、靱性を改善する効果もある。これらの効果はＮｉ含有量が０.１％以上で得られるが、１.０％を超えてＮｉを含有させてもその効果は頭打ちとなり、むしろ溶接性が悪化する。このため、Ｎｉ含有量は０.１％以上、１.０％以下であることが望ましい。   Ni has the effect of improving the toughness as well as improving the strength of the steel by solid solution strengthening. These effects are obtained when the Ni content is 0.1% or more. However, even if Ni is contained in excess of 1.0%, the effect reaches a peak, and the weldability is rather deteriorated. For this reason, the Ni content is desirably 0.1% or more and 1.0% or less.

Ｃｒは鋼の強度、靱性を高めるので、特に高強度が要求される鋼においてＣｒ添加が有効である。Ｃｒは少量含有させるだけで強度上昇の大幅に寄与することは、炭素当量Ｃｅｑ（＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ）／５＋（Ｃｕ＋Ｎｉ）／１５）からもわかる。この効果は０.０５％以上のＣｒ含有量で得られる。一方、０.５％を超える量のＣｒを含有させると溶接割れが発生する。そこで、Ｃｒ含有量は０.０５％以上、０.５％以下であることが好ましい。   Since Cr increases the strength and toughness of steel, the addition of Cr is effective particularly in steels that require high strength. It can be seen from the carbon equivalent Ceq (= C + Mn / 6 + (Cr + Mo) / 5 + (Cu + Ni) / 15) that Cr contributes significantly by increasing the amount of Cr. This effect is obtained with a Cr content of 0.05% or more. On the other hand, if an amount of Cr exceeding 0.5% is contained, a weld crack occurs. Therefore, the Cr content is preferably 0.05% or more and 0.5% or less.

Ｍｏは、鋼の焼き入れ性を向上させ、強度上昇に寄与する。また、Ｍｏはミクロ偏析し難い元素であり、中心偏析に起因するＨＩＣの発生を抑制する効果がある。これらのＭｏの効果は０.０２％以上のＭｏ含有量で得られる。しかし、Ｍｏはコスト増加につながる高価な元素であるばかりか、０.５％を超えてＭｏを含有させると、ベイナイト相やマルテンサイト相などの硬化相が生成して、ＨＩＣをむしろ悪化させる。そのため、Ｍｏ含有量は、０.０２％以上、０.５％以下とすることが好ましい。耐ＨＩＣ特性に及ぼす影響は、上述した他の３元素と比較して大きいため、Ｍｏ含有量はより好ましくは０.３％以下である。   Mo improves the hardenability of steel and contributes to an increase in strength. Mo is an element that is difficult to segregate microscopically and has an effect of suppressing the generation of HIC due to central segregation. These effects of Mo are obtained with a Mo content of 0.02% or more. However, Mo is not only an expensive element that leads to an increase in cost, but if Mo is contained in an amount exceeding 0.5%, a hardened phase such as a bainite phase or a martensite phase is generated, and the HIC is rather deteriorated. Therefore, the Mo content is preferably 0.02% or more and 0.5% or less. Since the influence on the HIC resistance is larger than that of the other three elements described above, the Mo content is more preferably 0.3% or less.

次に、任意元素について説明する。
［Ｖ：０.１０％以下］
Ｖは、鋼中でフェライト中への固溶並びに炭窒化物を形成して鋼の強度を高めるために有効であるので、特に高強度が要求される場合にはＶを含有させてもよい。しかし、Ｖ含有量が０.１％を超えると、ＨＡＺ靱性に悪影響を与える。このため、Ｖを含有させる場合にはその含有量は０.１０％以下とする。なお、Ｖによる上記効果を確実に得るには、Ｖ含有量は０.０１％であることが望ましい。Next, arbitrary elements will be described.
[V: 0.10% or less]
V is effective for increasing the strength of steel by forming a solid solution and carbonitride in ferrite in steel, and therefore V may be contained particularly when high strength is required. However, if the V content exceeds 0.1%, the HAZ toughness is adversely affected. For this reason, when it contains V, the content is made into 0.10% or less. In addition, in order to acquire the said effect by V reliably, it is desirable that V content is 0.01%.

上記以外は、Ｆｅおよび不純物である。
［Ｎｂ偏析度：２.０以下；Ｎｂ偏析度／Ｍｎ偏析度（α）：１.０以上、１.５以下］
［Ｔｉ偏析度：２.０以下；Ｔｉ偏析度／Ｍｎ偏析度（β）：１.０以上、１.５以下］
「Ｍｎ偏析度」、「Ｎｂ偏析度」および「Ｔｉ偏析度」は、板厚中央部での偏析度であり、前述した通りに求められる。Other than the above are Fe and impurities.
[Nb segregation degree: 2.0 or less; Nb segregation degree / Mn segregation degree (α): 1.0 or more, 1.5 or less]
[Ti segregation degree: 2.0 or less; Ti segregation degree / Mn segregation degree (β): 1.0 or more, 1.5 or less]
“Mn segregation degree”, “Nb segregation degree”, and “Ti segregation degree” are segregation degrees at the center of the plate thickness, and are determined as described above.

本発明者らは、Ｘ６０グレード（ＹＰ７０ｋｓｉ程度）以上のラインパイプ等に使用される厚鋼板において、耐ＨＩＣ特性を阻害する主な原因として従来から考えられていたＭｎＳやＣ、Ｍｎ、Ｐを低減しても、ＨＩＣが発生する場合があることを知見した。この理由は、従来のＭｎＳやマクロ偏析等のＨＩＣの対策を行っても、鋼中に残存するＮｂやＴｉの炭窒化物が基点となって割れが発生することがあるためと考えられた。   The present inventors have reduced MnS, C, Mn, and P, which have been conventionally considered as the main cause of hindering HIC resistance in thick steel plates used for X60 grade (about YP70 ksi) line pipes and the like. Even so, it has been found that HIC may occur. The reason for this is thought to be that cracks may occur due to the Nb or Ti carbonitrides remaining in the steel as the starting point even when conventional HIC measures such as MnS and macrosegregation are taken.

そこで、厚鋼板の偏析度とＨＩＣ試験とを行ってその相関関係を調査することによって、Ｃ、Ｍｎ、Ｐ、Ｓに加えて、Ｎｂ、Ｔｉに関しても厚鋼板における偏析度によりＨＩＣの発生原因となることを知見した。つまり、従来の知見に加えて、新たにＮｂとＴｉの偏析をも制御することにより、Ｘ６０グレード以上のラインパイプ等に使用される厚鋼板の耐ＨＩＣ特性を向上させることができる。   Therefore, by investigating the correlation between the segregation degree of the thick steel plate and the HIC test, in addition to C, Mn, P, and S, Nb and Ti also caused the occurrence of HIC due to the segregation degree in the thick steel plate. I found out that That is, in addition to the conventional knowledge, by newly controlling the segregation of Nb and Ti, it is possible to improve the HIC resistance of thick steel plates used for X60 grade line pipes and the like.

本発明者らは、製造された厚鋼板から採取した試験片についてＨＩＣ試験を行った。割れは全て板厚中央部において発生していた。このことから、中心偏析部でＨＩＣが発生することがわかる。割れが発生した試験片について割れ発生部の詳細な調査を行った。割れの起点となった介在物について、ＳＥＭ／ＥＤＳにより分析したところ、濃度は種々変化する場合があるものの、主にＮｂを含有し、Ｎｂ（Ｃ,Ｎ）と表記される炭窒化物（１０体積％以下のＴｉを含む、本明細書では「Ｎｂ炭窒化物」と称する）であることが判明した。   The present inventors performed an HIC test on a test piece collected from the manufactured thick steel plate. All cracks occurred at the center of the plate thickness. From this, it can be seen that HIC occurs at the central segregation part. A detailed investigation of the crack occurrence portion was performed on the test piece where the crack occurred. The inclusions that became the starting point of cracking were analyzed by SEM / EDS, and although the concentration may vary in various ways, the carbonitrides (Nb (C, N)) that mainly contain Nb (10 This was found to be “Nb carbonitride” in the present specification, containing up to vol% Ti.

より広範囲の組成で中心偏析部の調査をすることにより、各元素の濃化程度（偏析度）を把握することができる。偏析度調査は、ＥＰＭＡ（電子線マイクロアナライザー、Electron Probe Micro Analyzer）法、レーザーＩＣＰ（レーザーアブレーション誘導結合プラズマ、Laser Ablation Inductively Coupled Plasma）法、又は化学分析法を用いることにより実施できる。   By investigating the center segregation part with a wider composition, it is possible to grasp the degree of concentration (segregation degree) of each element. The segregation degree investigation can be performed by using an EPMA (Electron Probe Micro Analyzer) method, a laser ICP (Laser Ablation Inductively Coupled Plasma) method, or a chemical analysis method.

本発明者らは、圧延された厚鋼板より、板厚中央部の各元素の偏析度を調査し、ＨＩＣ発生有無との相関を調べた。
各元素の偏析度は、レーザーＩＣＰ法により調査した。使用した装置は島津製作所製のレーザーＩＣＰ分析装置であった。レーザーＩＣＰ法は、サンプルの切断面に対しレーザービームを照射し、発生した蒸気をキャリアガスによって搬送し、誘導プラズマ中で発光させ、この光の波長と光度を分析して、各元素の定量分析を行う方法であり、発光分析の１種である。サンプルを１方向に移動させてレーザービームを照射し、分析を繰り返すことによって、各元素について一定長さにおける濃度変化を調べることができる。The present inventors investigated the degree of segregation of each element at the center of the plate thickness from the rolled steel plate, and investigated the correlation with the presence or absence of HIC.
The degree of segregation of each element was investigated by the laser ICP method. The apparatus used was a laser ICP analyzer manufactured by Shimadzu Corporation. The laser ICP method irradiates a cut surface of a sample with a laser beam, conveys the generated vapor with a carrier gas, emits light in induction plasma, analyzes the wavelength and intensity of this light, and quantitatively analyzes each element. This is a type of emission analysis. By moving the sample in one direction, irradiating a laser beam, and repeating the analysis, it is possible to examine the concentration change at a certain length for each element.

レーザーＩＣＰ装置では、１００μｍ（＝０.１ｍｍ）毎の測定（１００μｍずつサンプルを移動させた測定）が可能であるので、１０ｍｍの長さにおいて１００点の測定結果を得ることができる。本発明では、厚鋼板を板圧延方向に平行に板面に垂直に切断した断面サンプルを用いて、切断面の板厚中心から上下に６ｍｍずつ合計１２ｍｍ長さにおいて１２０点での各元素の濃度の測定値を得る。それによりマクロ偏析を十分に評価することができる。各元素の偏析度は、得られた１２０点での測定値の算術平均値を厚鋼板におけるその元素の平均濃度とし、その平均濃度で測定値の中の最大値（最高濃度）を除することにより求められる。   In the laser ICP apparatus, measurement every 100 μm (= 0.1 mm) (measurement by moving the sample by 100 μm) is possible, so that 100 points of measurement results can be obtained in a length of 10 mm. In the present invention, using a cross-sectional sample obtained by cutting a thick steel plate parallel to the plate rolling direction and perpendicular to the plate surface, the concentration of each element at 120 points in a total length of 12 mm, 6 mm above and below the plate thickness center of the cut surface. Get the measured value. Thereby, macrosegregation can be fully evaluated. For the segregation degree of each element, the arithmetic average value of the measured values obtained at 120 points is regarded as the average concentration of the element in the thick steel plate, and the maximum value (maximum concentration) among the measured values is divided by the average concentration. Is required.

各元素の偏析度は、レーザーＩＣＰ法により得られた１２０点の測定値（発光ピーク強度）から各元素の濃度（質量％）を実際に求めて、最高濃度／平均濃度を算出することにより求めることもできる。しかし、この濃度と測定で得られた各元素に帰属する発光ピーク強度とは正比例関係にあるので、各元素について最高ピーク強度／平均ピーク強度の比として偏析度を簡便に求めることができる。   The segregation degree of each element is obtained by actually obtaining the concentration (mass%) of each element from the measured values (emission peak intensity) at 120 points obtained by the laser ICP method, and calculating the maximum concentration / average concentration. You can also. However, since this concentration and the luminescence peak intensity attributed to each element obtained by measurement are in a directly proportional relationship, the segregation degree can be easily obtained as the ratio of maximum peak intensity / average peak intensity for each element.

レーザービーム径は約１ｍｍ程度あるため、１ｍｍ以下の介在物は、全て測定データの中に吸収されることとなる。介在物の大きさは通常は数μｍ程度、大きいとされるものでも数十μｍ程度であるので、本測定により十分に介在物を含んだ偏析度調査を行うことができる。   Since the laser beam diameter is about 1 mm, all inclusions of 1 mm or less are absorbed in the measurement data. Since the size of the inclusion is usually about several μm, and even a large one is about several tens of μm, the measurement of the degree of segregation sufficiently including the inclusion can be performed by this measurement.

図１は、ＨＩＣ発生率が８.８％である厚鋼板について、レーザーＩＣＰ法により求めたＭｎ、Ｐ、Ｓ、Ｎｂ、Ｃの偏析度の結果を示すグラフである。図１のグラフにおける横軸は板厚方向の測定点位置（板厚中心が６０＜単位は０.１ｍｍ＞、測定長さは板厚中心から上下方向に各６ｍｍ、測定点は合計１２０点）であり、縦軸は各元素の測定結果（発光ピーク強度、任意単位）である。縦軸は、元素によりレベル差が生じるが、この測定方法により平均組成および偏析度を算出することができる。   FIG. 1 is a graph showing the results of segregation degrees of Mn, P, S, Nb, and C obtained by a laser ICP method for a thick steel plate having an HIC generation rate of 8.8%. The horizontal axis in the graph of FIG. 1 is the measurement point position in the plate thickness direction (the plate thickness center is 60 <unit is 0.1 mm>, the measurement length is 6 mm each in the vertical direction from the plate thickness center, and the measurement points are 120 points in total) The vertical axis represents the measurement result (luminescence peak intensity, arbitrary unit) of each element. The level of the vertical axis varies depending on the element, but the average composition and the degree of segregation can be calculated by this measurement method.

上述したように、レーザーＩＣＰ法以外の方法も使用可能である。例えば化学分析等を用いて対象部位の元素を測定することによって偏析のない部分との分析結果との比較により偏析度を求めることも可能である。ＥＰＭＡ法や、レーザーＩＣＰ法において定量を発光分析ではなく質量分析により行うレーザー発光質量分析法（Laser Ablation Inductive Coupled Plasma Mass Spectroscopy、ＬＡ−ＩＣＰ−ＭＳ法）により偏析度を求めることもできる。   As described above, methods other than the laser ICP method can also be used. For example, the degree of segregation can be determined by measuring the element at the target site using chemical analysis or the like and comparing the result with the analysis result of the part without segregation. The degree of segregation can also be determined by laser emission mass spectrometry (Laser Ablation Inductive Coupled Plasma Mass Spectroscopy, LA-ICP-MS method) in which quantitative determination is performed by mass spectrometry instead of emission analysis in the EPMA method or laser ICP method.

図１のグラフの例では、Ｎｂ偏析度＝２.１、Ｔｉ偏析度＝１.８、Ｍｎ偏析度＝１.３であることが判るので、Ｎｂ偏析度／Ｍｎ偏析度(α)＝１.７、Ｔｉ偏析度／Ｍｎ偏析度(β)＝１.４と算出することができる。   In the example of the graph of FIG. 1, it can be seen that Nb segregation degree = 2.1, Ti segregation degree = 1.8, and Mn segregation degree = 1.3, so that Nb segregation degree / Mn segregation degree (α) = 1. 0.7, Ti segregation degree / Mn segregation degree (β) = 1.4.

図２(ａ)は、図１のデータを用いて、横軸をＭｎ濃度、縦軸をＮｂ濃度としたグラフであり、図２(ｂ)は、横軸をＭｎ濃度、縦軸をＴｉ濃度としたグラフである。なお、各元素の濃度は実際には発光ピーク強度を用いている。これらのグラフを比較することにより、Ｍｎ偏析度とＮｂ偏析度またはＴｉ偏析度とを比較することができる。   FIG. 2A is a graph using the data of FIG. 1 with the horizontal axis representing the Mn concentration and the vertical axis representing the Nb concentration. FIG. 2B illustrates the horizontal axis representing the Mn concentration and the vertical axis representing the Ti concentration. It is a graph. The concentration of each element actually uses the emission peak intensity. By comparing these graphs, the degree of Mn segregation and the degree of Nb segregation or Ti segregation can be compared.

図２(ａ)および図２(ｂ)に示すグラフから、Ｍｎ偏析度がスラブにおける各元素の偏析を代表しており、スラブ偏析度（Ｍｎ偏析度）が悪化すれば、Ｎｂ偏析度やＴｉ偏析度も同様に悪化することがわかる。さらに、Ｍｎ偏析度がある臨界点を超えると、これらの図に丸で囲んで示すようにＮｂやＴｉが一気に介在物として析出する傾向があることも判明した。   From the graphs shown in FIG. 2A and FIG. 2B, the Mn segregation degree represents the segregation of each element in the slab, and if the slab segregation degree (Mn segregation degree) deteriorates, the Nb segregation degree and Ti It can be seen that the degree of segregation deteriorates as well. Furthermore, it has also been found that when the Mn segregation degree exceeds a certain critical point, Nb and Ti tend to precipitate as inclusions all at once as shown in circles in these figures.

本発明者らは、大量に発生した介在物がＨＩＣの原因になり得ると考え、ＨＩＣの発生有無とＮｂ偏析度／Ｍｎ偏析度（α）およびＴｉ偏析度／Ｍｎ偏析度（β）との関係を調査した。その結果、Ｎｂ偏析度が２.０以下で、Ｎｂ偏析度／Ｍｎ偏析度の比であるαが１.０以上、１.５以下であり、かつＴｉ偏析度が２.０以下で、Ｔｉ偏析度／Ｍｎ偏析度の比であるβが１.０以上、１.５以下であると、ＨＩＣ発生を著しく抑制することができることが判明した。   The present inventors consider that inclusions generated in a large amount can cause HIC, and the presence / absence of HIC, Nb segregation degree / Mn segregation degree (α), and Ti segregation degree / Mn segregation degree (β). The relationship was investigated. As a result, the Nb segregation degree is 2.0 or less, the ratio of Nb segregation degree / Mn segregation degree α is 1.0 or more and 1.5 or less, and the Ti segregation degree is 2.0 or less. It has been found that the occurrence of HIC can be remarkably suppressed when β, which is the ratio of segregation degree / Mn segregation degree, is 1.0 or more and 1.5 or less.

本発明に従ったＨＩＣ発生有無の判定基準は、従来から知られている判定基準のみでは判定できない、Ｘ６０グレード以上の高強度鋼の耐ＨＩＣ特性に関して特に有効である。従来提案されているＣ、Ｍｎ、Ｐ、Ｓの低減とともに、本発明に従ったＮｂ、Ｔｉの偏析度の判定基準を併用すれば、ＨＩＣ発生を効果的に防止できることは言うまでもない。   The determination criterion for the presence or absence of HIC occurrence according to the present invention is particularly effective with respect to the HIC resistance of high strength steel of X60 grade or higher, which cannot be determined only by a conventionally known determination criterion. It goes without saying that the generation of HIC can be effectively prevented by using the criteria for segregation degree of Nb and Ti according to the present invention together with the conventionally proposed reduction of C, Mn, P and S.

厚鋼板の化学組成が同じでも、製造条件が異なると偏析の状況は異なるので、上述した偏析度の要件（Ｎｂ、Ｔｉの偏析度およびα、βの値）を満たしたり、満たさなかったりすることがある。従って、化学組成に加えて、製鋼および圧延の条件を、偏析が起こりにくいように設定することによって、本発明に係る厚鋼板を製造することができる。偏析低減に有効な操業条件について、以下に簡単に説明する。   Even if the chemical composition of the steel plate is the same, the segregation situation differs if the manufacturing conditions are different. There is. Therefore, in addition to the chemical composition, the thick steel plate according to the present invention can be manufactured by setting the steelmaking and rolling conditions so that segregation does not easily occur. The operating conditions effective for reducing segregation are briefly described below.

製鋼では、ＩＲ（インジェクション・リファイニング）の適用およびＣａを添加することによる酸化物の形態制御が偏析の低減に有効である。
次工程である連続鋳造（ＣＣ）では、鋳片の中心部が凝固する際に鋳片の凝固収縮量に相当する程度、あるいはやや上回る程度に鋳片に勾配を設けることで、板厚方向の偏析の低減を図ることできる。一方、鋳込み幅方向と鋳込み長さ方向で顕著な凝固不均一が発生しないように、適正な水冷条件の採用および鋳込み速度の選択も偏析低減に有効である。In steelmaking, application of IR (injection refining) and control of oxide morphology by adding Ca are effective in reducing segregation.
In the next process, continuous casting (CC), when the center part of the slab solidifies, the slab is provided with a gradient to the extent corresponding to or slightly above the solidification shrinkage of the slab. Segregation can be reduced. On the other hand, the use of appropriate water-cooling conditions and the selection of the casting speed are also effective in reducing segregation so that significant solidification non-uniformity does not occur in the casting width direction and the casting length direction.

鋳片から厚板への熱間圧延（これはＣＣに続けて行う直送圧延であってもよい）では、先ず１１００℃以上、１２００℃以下での加熱を実施することが有効である。これにより、鋳片段階で晶出したＮｂを固溶させて、Ｎｂ炭窒化物となることを防ぐことができる。Ｎｂの固溶のために、Ｎｂ量に伴って加熱温度や時間を調整することが有効である。Ｎｂ含有量が多い場合には、加熱温度および／または時間を増大させる。   In hot rolling from a slab to a thick plate (this may be direct rolling performed after CC), it is effective to first perform heating at 1100 ° C. or more and 1200 ° C. or less. Thereby, it is possible to prevent Nb crystallized in the slab stage from becoming a solid solution and becoming Nb carbonitride. For solid solution of Nb, it is effective to adjust the heating temperature and time according to the amount of Nb. When the Nb content is high, the heating temperature and / or time is increased.

圧延後、Ａｒ３点以上で水冷を開始する。水冷開始温度をこのように制限するのは、水冷をＡｒ３点以下の温度から開始すると、水冷前にフェライトが生成して炭素の排出が始まっているため、水冷により多量に炭素を含む硬化組織が形成され、ＨＩＣ特性が劣化するからである。After rolling, water cooling is started at Ar 3 points or more. The reason for limiting the water cooling start temperature in this way is that when water cooling is started from a temperature of 3 points or less of Ar, ferrite is generated before water cooling and carbon discharge starts. Therefore, a hardened structure containing a large amount of carbon by water cooling. This is because the HIC characteristics deteriorate.

水冷が有効であるのは、ＣやＰ等の元素の拡散をできるだけ低減し、ＣがＮｂと結びつくことを防止できるからである。温度の低下とともに各元素の拡散速度は低下するが、圧延後に水冷せずに放冷すると、より高温にさらされる時間が長く、元素の拡散を助長する可能性がある。拡散した元素は、粒界や介在物に偏析する。   The reason why water cooling is effective is that diffusion of elements such as C and P can be reduced as much as possible, and C can be prevented from being combined with Nb. Although the diffusion rate of each element decreases with a decrease in temperature, if it is allowed to cool without water cooling after rolling, it may take a longer time to be exposed to a higher temperature, which may promote element diffusion. The diffused elements segregate at the grain boundaries and inclusions.

水冷速度は５℃／ｓｅｃ以上、３０℃／ｓｅｃ以下であることが望ましい。水冷速度が５℃／ｓｅｃ未満であると拡散を助長し、水冷速度が３０℃／ｓｅｃ超であると焼きが過度に入り、硬化組織を生じるからである。   The water cooling rate is desirably 5 ° C./sec or more and 30 ° C./sec or less. This is because if the water cooling rate is less than 5 ° C./sec, diffusion is promoted, and if the water cooling rate exceeds 30 ° C./sec, baking occurs excessively and a hardened structure is generated.

本発明に係る厚鋼板を素材として製造されるＵＯＥ鋼管は、ＴｉやＮｂの粗大な炭窒化物に起因したＨＩＣを生じることがなく、腐食環境下で使用された場合の信頼性が高まる。ＵＯＥ鋼管の製造方法は当業者には周知であり、本発明においても、従来技術と同様の方法でＵＯＥ鋼管を製造することができる。   The UOE steel pipe manufactured using the thick steel plate according to the present invention does not generate HIC due to coarse carbonitrides of Ti and Nb, and the reliability when used in a corrosive environment is increased. A method for manufacturing a UOE steel pipe is well known to those skilled in the art. In the present invention, a UOE steel pipe can be manufactured by a method similar to the conventional technique.

本発明を、実施例を参照しながら、さらに具体的に説明する。
表１に示す化学組成とＡｒ３点を有する鋼種Ｎｏ.１〜１４を、厚さ３００ｍｍ、幅２３００ｍｍの垂直曲げ型連続鋳造機を用いて、０.７ｍ／ｍｉｎ以上、０.８ｍ／ｍｉｎ以下の鋳造速度で連続鋳造して鋳片（スラブ）を得た。The present invention will be described more specifically with reference to examples.
Using steel type Nos. 1 to 14 having the chemical composition and Ar 3 points shown in Table 1, 0.7 m / min or more and 0.8 m / min or less using a vertical bending type continuous casting machine having a thickness of 300 mm and a width of 2300 mm. A slab was obtained by continuous casting at a casting speed of.

得られた鋳片を、約１１００℃以上、１２００℃以下に加熱し、仕上圧延温度が約７５０℃以上、８５０℃以下となる条件で熱間圧延を行って、厚さ２５.４ｍｍ前後に仕上げた。熱間圧延後は直ちに水冷を行って、約４５０℃以上、５５０℃以下で冷却を停止し、その後は空冷した。水冷の冷却速度は１０〜３０℃／ｓｅｃであった。   The obtained slab is heated to about 1100 ° C. or more and 1200 ° C. or less, and hot-rolled under conditions where the finish rolling temperature is about 750 ° C. or more and 850 ° C. or less, and finished to a thickness of about 25.4 mm. It was. Immediately after the hot rolling, water cooling was performed, cooling was stopped at about 450 ° C. or more and 550 ° C. or less, and then air cooling was performed. The cooling rate for water cooling was 10 to 30 ° C./sec.

この厚鋼板について、前述したレーザーＩＣＰ法を用いた偏析度調査（測定点１２０点）、引張試験およびＨＩＣ試験を行った。試験結果を鋼の化学組成とともに表１に示す。
ＨＩＣ試験は、ＮＡＣＥ ＴＭ−０２−８４で規定されるＮＡＣＥ試験に供し、ＨＩＣ発生率として割れ面積率（ＣＡＲ, Crack Area Ratio）を測定した。ＣＡＲが３％以下を合格と判定し、３％を超える場合は実質的なＨＩＣ発生ありとして、不合格であると判定した。About this thick steel plate, the segregation degree investigation (measurement point 120 points) using the laser ICP method mentioned above, the tension test, and the HIC test were done. The test results are shown in Table 1 together with the chemical composition of the steel.
The HIC test was subjected to the NACE test specified by NACE TM-02-84, and the crack area ratio (CAR) was measured as the HIC generation rate. When CAR was 3% or less, it was determined to be acceptable, and when it exceeded 3%, it was determined that there was substantial HIC occurrence and that it was rejected.

Figure 2010104165
Figure 2010104165

表１からわかるように、ＮｂおよびＴｉの偏析度比αおよびβが１.５以下と小さい発明鋼の厚鋼板は、Ｘ６０グレード以上で、引張り強度が５２０ＭＰａ以上の強度を有するとともに、ＨＩＣ発生率（ＣＡＲ）が３％以下であった。従って、この厚鋼板から信頼性の高いＵＯＥ鋼管を製造できることは当業者には明らかである。   As can be seen from Table 1, the steel plate of the invention steel having a small segregation degree ratio α and β of Nb and Ti as small as 1.5 or less has a strength of X60 grade or higher, a tensile strength of 520 MPa or higher, and an HIC generation rate. (CAR) was 3% or less. Accordingly, it is obvious to those skilled in the art that a highly reliable UOE steel pipe can be manufactured from this thick steel plate.

これに対し、比較鋼の厚鋼板は、強度またはＨＩＣ発生率のいずれかが不芳であった。特に、鋼種Ｎｏ.４、１１および１２は、鋼の化学組成は本発明で規定する条件を満たしているが、偏析度に関して、Ｎｂ偏析度の値、Ｎｂ偏析度／Ｍｎ偏析度の比（α）又はＴｉ偏析度／Ｍｎ偏析度の比（β）が本発明の範囲を満たしていない例であり、いずれも強度は十分であるが、ＨＩＣ発生率が高くなった。
On the other hand, either the strength or the HIC occurrence rate was unsatisfactory for the thick steel plate of the comparative steel. In particular, steel types Nos. 4, 11 and 12 have the chemical composition of the steel satisfying the conditions specified in the present invention, but regarding the degree of segregation, the value of Nb segregation degree, the ratio of Nb segregation degree / Mn segregation degree (α ) Or Ti segregation degree / Mn segregation degree ratio (β) is an example that does not satisfy the scope of the present invention, and in all cases the strength is sufficient, but the HIC generation rate is high.

Claims (3)

質量％で、Ｃ：０.０２〜０.０７％、Ｓｉ：０.０５〜０.５０％、Ｍｎ：１.１〜１.６％、Ｐ：０.０１５％以下、Ｓ：０.００２％以下、Ｎｂ：０.００５〜０.０６０％、Ｔｉ：０.００５〜０.０３０％、Ａｌ：０.００５〜０.０６％、Ｃａ：０.０００５〜０.００６０％、Ｎ：０.００１５〜０.００７％、式(１)を満足する量のＣｕ、Ｎｉ、ＣｒおよびＭｏから選ばれた少なくとも１種、Ｖ：０〜０.１０％、残部Ｆｅおよび不純物からなる化学組成を有し、かつＮｂ偏析度が２.０以下、Ｍｎ偏析度に対するＮｂ偏析度の比α（α＝Ｎｂ偏析度／Ｍｎ偏析度）が１.０以上、１.５以下、Ｔｉ偏析度が２.０以下、Ｍｎ偏析度に対するＴｉ偏析度の比β（β＝Ｔｉ偏析度／Ｍｎ偏析度）が１.０以上、１.５以下であることを特徴する、耐ＨＩＣ特性に優れた厚鋼板。
式(１)：０.１％＜（Ｃｕ＋Ｎｉ＋Ｃｒ＋Ｍｏ）＜１.５％
式中、元素記号は各元素の含有量（質量％）を意味する。By mass%, C: 0.02 to 0.07%, Si: 0.05 to 0.50%, Mn: 1.1 to 1.6%, P: 0.015% or less, S: 0.002 %: Nb: 0.005 to 0.060%, Ti: 0.005 to 0.030%, Al: 0.005 to 0.06%, Ca: 0.0005 to 0.0006%, N: 0 .0015 to 0.007%, a chemical composition comprising at least one selected from Cu, Ni, Cr and Mo in an amount satisfying the formula (1), V: 0 to 0.10%, the balance Fe and impurities. And Nb segregation degree is 2.0 or less, ratio α of Nb segregation degree to Mn segregation degree (α = Nb segregation degree / Mn segregation degree) is 1.0 or more and 1.5 or less, and Ti segregation degree is 2 Excellent HIC resistance, characterized in that the ratio β of Ti segregation to Mn segregation is β or less (β = Ti segregation / Mn segregation) is 1.0 or more and 1.5 or less. Thick steel plate.
Formula (1): 0.1% <(Cu + Ni + Cr + Mo) <1.5%
In the formula, the element symbol means the content (% by mass) of each element. 前記化学組成が０.０１〜０.１０％のＶを含有する、請求項１に記載の厚鋼板。   The thick steel plate according to claim 1, wherein the chemical composition contains V of 0.01 to 0.10%. 請求項１または２に記載の厚鋼板を素材とすることを特徴とするＵＯＥ鋼管。   A UOE steel pipe made of the thick steel plate according to claim 1 or 2.
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