JP4700741B2 - Manufacturing method of steel plate for thick-walled sour line pipe with excellent toughness - Google Patents

Manufacturing method of steel plate for thick-walled sour line pipe with excellent toughness Download PDF

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JP4700741B2
JP4700741B2 JP2009035752A JP2009035752A JP4700741B2 JP 4700741 B2 JP4700741 B2 JP 4700741B2 JP 2009035752 A JP2009035752 A JP 2009035752A JP 2009035752 A JP2009035752 A JP 2009035752A JP 4700741 B2 JP4700741 B2 JP 4700741B2
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JP2010189722A (en
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均 朝日
太郎 村木
充 澤村
卓也 原
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Nippon Steel Corp
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Priority to PCT/JP2010/052797 priority patent/WO2010095755A1/en
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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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • 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
    • 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
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • 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
    • C22C38/04—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
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • 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
    • C21D9/14—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes wear-resistant or pressure-resistant pipes

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
  • Metal Rolling (AREA)

Description

本発明は、靭性及び硫化水素（Ｈ2Ｓ）を含んだ環境における耐水素誘起割れ性、即ち、耐サワー性に優れた鋼板の製造方法に関するものである。 The present invention relates to a method for producing a steel sheet having excellent toughness and resistance to hydrogen-induced cracking in an environment containing hydrogen sulfide (H 2 S), that is, sour resistance.

硫化水素を含むサワーオイル、サワーガスを輸送するラインパイプに使用される鋼管や、パイプラインの付属設備などに使用される鋼板には、耐サワー性が要求される。なお、耐サワー性とは、硫化水素を含む腐食環境における耐水素誘起割れ性（ＨＩＣ性）及び耐応力割れ性（耐ＳＳＣ性）である。   Sour resistance is required for steel pipes used for sour oils containing hydrogen sulfide, line pipes for transporting sour gas, and pipes attached to the pipes. The sour resistance is hydrogen-induced crack resistance (HIC resistance) and stress crack resistance (SSC resistance) in a corrosive environment containing hydrogen sulfide.

耐サワー性は、圧延方向に延伸化したＭｎＳの生成や、クラスター状の介在物の生成によって劣化することが知られている。また、極めて厳しい腐食環境における耐サワー性を向上させるために、Ｐ、Ｓ、Ｏ、Ｎの含有量を低下させ、Ｃａを添加して、ＭｎＳの形態を制御した鋼材を制御圧延し、水冷する方法が提案されている（例えば、特許文献１）。   It is known that the sour resistance deteriorates due to the generation of MnS stretched in the rolling direction and the generation of cluster-like inclusions. Moreover, in order to improve the sour resistance in an extremely severe corrosive environment, the content of P, S, O, and N is reduced, Ca is added, and the steel material in which the form of MnS is controlled is controlled and rolled. A method has been proposed (for example, Patent Document 1).

また、パイプラインの輸送効率の向上や薄肉化によるコスト低減などの観点から、ラインパイプ用鋼板の高強度化が要求されている。このような要求に対して、例えば、Ｘ７０程度の強度を有し、金属組織が、板厚方向に均一で、微細なベイナイトである、耐サワー性に優れた鋼板を製造する方法が提案されている（例えば、特許文献２）。   Further, from the viewpoint of improving the transportation efficiency of the pipeline and reducing the cost by reducing the wall thickness, it is required to increase the strength of the steel plate for the line pipe. In response to such demands, for example, a method for producing a steel plate having a strength of about X70, a metal structure that is uniform in the thickness direction and fine bainite and excellent in sour resistance has been proposed. (For example, Patent Document 2).

更に、寒冷地にパイプラインを敷設する際には、ラインパイプ用鋼板の低温靭性を向上させることが必要になる。このような問題に対して、低温靭性と耐サワー性を向上させた高強度鋼板の製造方法が提案されている（例えば、特許文献３〜５）。   Furthermore, when laying a pipeline in a cold region, it is necessary to improve the low temperature toughness of the steel plate for line pipe. In response to such problems, methods for producing high-strength steel sheets with improved low-temperature toughness and sour resistance have been proposed (for example, Patent Documents 3 to 5).

これらは、Ｃ量の低減によって硬度の上昇を抑制し、Ｓ量の低減とＣａの添加によってＭｎＳの形態を制御し、Ａｌ量の低減によって酸化物の形態を制御し、耐サワー性と低温靭性との両立を図ったものである。   These suppress the increase in hardness by reducing the amount of C, control the form of MnS by reducing the amount of S and adding Ca, control the form of oxide by reducing the amount of Al, sour resistance and low temperature toughness It is intended to be compatible with.

特開昭６２−１１２７２２号公報JP-A-62-112722 特開昭６１−１６５２０７号公報JP-A-61-165207 特開平０３−２３６４２０号公報Japanese Patent Laid-Open No. 03-236420 特開平０５−２９５４３４号公報JP 05-295434 A 特開平０７−２４２９４４号公報Japanese Patent Application Laid-Open No. 07-242944

耐サワー特性を確保するためには、鋼片を高温に加熱し、鋳造時に析出し、成長したＮｂＣ等の粗大な析出物を溶解させる必要がある。しかし、鋼片を高温に加熱すると、結晶粒径が粗大になる。   In order to ensure the sour resistance, it is necessary to heat the steel slab to a high temperature and to precipitate coarse precipitates such as NbC that have been deposited and grown during casting. However, when the steel slab is heated to a high temperature, the crystal grain size becomes coarse.

特に、板厚が２５ｍｍ以上の厚肉の鋼板を製造する際には、再結晶域及び未再結晶域での圧下が不十分になり、靭性、特に、落重引裂試験（Drop Weight Tear Test、ＤＷＴＴ）によって評価された落重引裂特性（ＤＷＴＴ特性ともいう。）を確保することができない。   In particular, when manufacturing a thick steel plate having a thickness of 25 mm or more, the reduction in the recrystallized region and the non-recrystallized region becomes insufficient, and the toughness, in particular, the drop weight tear test (Drop Weight Tear Test, The falling weight tear property (also referred to as DWTT property) evaluated by DWTT) cannot be ensured.

本発明は、このような問題を解決するものであって、板厚が２５ｍｍ以上である鋼板の、耐サワー特性及びＤＷＴＴ特性の両立を可能とする、耐サワー性に優れたラインパイプ用鋼板の製造方法の提供を課題とするものである。   The present invention solves such a problem, and is a steel plate for line pipes excellent in sour resistance that enables both the sour resistance and DWTT characteristics of a steel sheet having a thickness of 25 mm or more. It is an object to provide a manufacturing method.

本発明は、Ｓ及びＯの含有量を厳格に制限し、Ｃａを添加して、
〔Ｃａ〕（１−１２４〔Ｏ〕）／１．２５〔Ｓ〕
で表されるＥＳＳＰ 値を高く制御して硫化物の形態を制御し、更に、Ｃ量を低く制限し、Ｎｂ及びＣの含有量に応じて、鋼片の加熱温度を制御し、更に、熱間圧延の温度と圧下比とを制御することにより、Ｎｂ炭化物等の析出物の粗大化が抑制され、結晶粒径も微細化し、優れた耐サワー性と高い靭性の両立を備えた鋼板を製造することができるとの知見に基づいてなされたものであり、その要旨は以下のとおりである。 The present invention strictly limits the contents of S and O, adds Ca,
[Ca] (1-124 [O]) / 1.25 [S]
The ESSP value represented by is controlled to be high to control the form of sulfide, and further, the amount of C is limited to be low, the heating temperature of the billet is controlled according to the contents of Nb and C, and By controlling the temperature and rolling ratio of hot rolling, the coarsening of precipitates such as Nb carbide is suppressed, the crystal grain size is also refined, and steel sheets with both excellent sour resistance and high toughness are manufactured. It was made based on the knowledge that it can be done, and the gist thereof is as follows.

（１）質量％で、
Ｃ ：０．０１〜０．０８％、
Ｓｉ：０．１〜０．５％、
Ｍｎ：１．０〜１．５％、
Ｎｂ：０．０１０〜０．０４０％、
Ｃａ：０．００１〜０．００４％、
Ｔｉ：０．００５〜０．０３０％
を含有し、
Ａｌ：０．０８％以下、
Ｐ： ０．０１５％以下、
Ｓ： ０．０００８％以下、
Ｏ： ０．００３０％以下、
Ｎ： ０．００５０％以下
に制限し、Ｃａ、Ｏ、及び、Ｓの含有量が、
［Ｃａ］（１−１２４［Ｏ］）／１．２５［Ｓ］＞３．０
を満足し、残部Ｆｅ及び不可避的不純物からなる鋼片を、１０００〜１１５０℃の範囲内の加熱温度Ｔ１と、Ｎｂ及びＣの含有量が、
Ｔ１≧−７９７０／（ｌｏｇ（［Ｎｂ］×［Ｃ］）−３．３１）−１７０
を満足するように加熱し、粗圧延を行い、更に、仕上げ温度を８００℃以上とし、９５０℃以下の圧下比を３以上として、板厚が２５ｍｍ以上になるように、仕上げ圧延を行い、冷却速度が１０〜３０℃／ｓの加速冷却を行い、２００〜５００℃で該加速冷却を停止することを特徴とする靭性に優れた厚肉耐サワーラインパイプ用鋼板の製造方法。 (1) In mass%,
C: 0.01 to 0.08%,
Si: 0.1 to 0.5%,
Mn: 1.0 to 1.5%
Nb: 0.010 to 0.040%,
Ca: 0.001 to 0.004%,
Ti: 0.005-0.030%
Containing
Al: 0.08% or less,
P: 0.015% or less,
S: 0.0008% or less,
O: 0.0030% or less,
N: The content of Ca, O, and S is limited to 0.0050% or less,
[Ca] (1-124 [O]) / 1.25 [S]> 3.0
Satisfied, the slab ing the balance Fe and unavoidable impurities, and a heating temperature T1 in the range of 1000 to 1150 ° C., the content of Nb and C,
T1 ≧ −7970 / (log ([Nb] × [C]) − 3.31) −170
The steel sheet is heated so as to satisfy the conditions, rough rolling is performed, the finishing temperature is set to 800 ° C. or higher, the rolling ratio of 950 ° C. or lower is set to 3 or higher, and the final rolling is performed so that the plate thickness is 25 mm or higher. A method for producing a steel plate for a thick-walled sour line pipe excellent in toughness, characterized by performing accelerated cooling at a rate of 10 to 30 ° C / s and stopping the accelerated cooling at 200 to 500 ° C.

（２）前記鋼片が、質量％で、
Ｃ ：０．０１〜０．０６％、
Ｎｂ：０．０１０〜０．０２５％、
Ｔｉ：０．００５〜０．０２０％
を含有することを特徴とする上記（１）に記載の靭性に優れた厚肉耐サワーラインパイプ用鋼板の製造方法。
（３）前記鋼片が、更に、質量％で、
Ｎｉ：０．５％以下、
Ｃｕ：０．５％以下、
Ｃｒ：０．５％以下、
Ｍｏ：０．３％以下
の１種又は２種以上を含有することを特徴とする上記（２）に記載の靭性に優れた厚肉耐サワーラインパイプ用鋼板の製造方法。 (2) The steel slab is mass%,
C: 0.01 to 0.06%,
Nb: 0.010 to 0.025%,
Ti: 0.005-0.020%
The manufacturing method of the steel plate for thick-walled sour line pipes which was excellent in the toughness as described in said (1) characterized by containing.
( 3 ) The steel slab is further in mass%,
Ni: 0.5% or less,
Cu: 0.5% or less,
Cr: 0.5% or less,
Mo: The manufacturing method of the steel plate for thick-walled sour line pipes which was excellent in toughness as described in said ( 2 ) characterized by containing 1 type or 2 types or less of 0.3% or less.

（４）前記鋼片が、更に、質量％で、
Ｖ：０．０６％以下
を含有することを特徴とする上記（１）〜（３）の何れかに記載の靭性に優れた厚肉耐サワーラインパイプ用鋼板の製造方法。 ( 4 ) The steel slab is further in mass%,
V: It contains 0.06% or less, The manufacturing method of the steel plate for thick-walled sour line pipes excellent in the toughness in any one of said (1) -(3) characterized by the above-mentioned.

（５）前記鋼片が、更に、質量％で、
Ｂ：０．００２０％以下
を含有することを特徴とする上記（１）〜（４）の何れかに記載の靭性に優れた厚肉耐サワーラインパイプ用鋼板の製造方法。 ( 5 ) The steel slab is further in mass%,
B: The manufacturing method of the steel plate for thick-walled sour line pipes excellent in toughness in any one of said (1)-( 4 ) characterized by containing 0.0020% or less.

（６）前記鋼片が、更に、質量％で、
Ｍｇ：０．０１％以下
を含有することを特徴とする上記（１）〜（５）の何れかに記載の靭性に優れた厚肉耐サワーラインパイプ用鋼板の製造方法。 ( 6 ) The steel slab is further in mass%,
Mg: 0.01% or less is contained, The manufacturing method of the steel plate for thick-walled sour line pipes excellent in the toughness in any one of said (1)-( 5 ) characterized by the above-mentioned.

本発明によれば、特に、板厚が２５ｍｍ以上であり、靭性、特に、ＤＷＴＴ特性及び耐サワー性に優れた厚肉ラインパイプ用鋼板を提供することが可能になり、産業上の貢献が極めて顕著である。   According to the present invention, it is possible to provide a steel plate for a thick line pipe having a plate thickness of 25 mm or more and excellent toughness, in particular, DWTT characteristics and sour resistance. It is remarkable.

本発明者らは、Ｃａ、Ｓ、及び、Ｏの含有量から、
〔Ｃａ〕（１−１２４〔Ｏ〕）／１．２５〔Ｓ〕
によって求められるＥＳＳＰ値を制御した、板厚が２５ｍｍ以上の鋼板を製造し、耐サワー性及び靭性を評価した。 From the contents of Ca, S and O, the inventors
[Ca] (1-124 [O]) / 1.25 [S]
A steel plate having a plate thickness of 25 mm or more, in which the ESSP value obtained by the above method was controlled, was manufactured, and sour resistance and toughness were evaluated.

耐サワー性は、ＮＡＣＥ（National Association of Corrosion and Engineer）のＴＭ０２８４に準拠した試験を行い、ＨＩＣ（水素誘起割れ）の発生の有無によって評価した。ＨＩＣ破面率が５％程度以下であれば、耐サワー特性が良好であるとされる。靭性は、−４０℃でＤＷＴＴ試験を行い、延性破面率を求め、８５％を良否の判定基準とした。   The sour resistance was evaluated based on the presence or absence of occurrence of HIC (hydrogen induced cracking) by performing a test based on TM0284 of NACE (National Association of Corrosion and Engineer). If the HIC fracture surface ratio is about 5% or less, the sour resistance is considered good. As for toughness, a DWTT test was performed at −40 ° C. to determine the ductile fracture surface ratio, and 85% was used as a criterion for acceptability.

ＮＡＣＥ試験は、５％ＮａＣｌ溶液＋０．５％酢酸、ｐＨ２．７の溶液中に硫化水素ガスを飽和させて、９６時間後に割れが生成するかどうかを調査する試験方法である。   The NACE test is a test method in which hydrogen sulfide gas is saturated in a solution of 5% NaCl solution + 0.5% acetic acid, pH 2.7 to investigate whether cracks are generated after 96 hours.

ＨＩＣが発生した試料の組織及び析出物を調査した結果、耐サワー性が劣化した鋼板には、粗大なＮｂＣが析出していることが解った。次に、ＤＷＴＴ特性が低下した鋼板では、結晶粒径が粗大化していることが解った。   As a result of investigating the structure and precipitates of the sample in which HIC was generated, it was found that coarse NbC was precipitated on the steel sheet having deteriorated sour resistance. Next, it was found that the crystal grain size was coarsened in the steel sheet having a lowered DWTT characteristic.

更に、鋼板の析出状態及び粒径と、製造条件の関係を整理した結果、粗大なＮｂＣが析出している鋼板は加熱温度が低く、結晶粒が粗大化した鋼板は加熱温度が高いことが解った。また、優れた耐サワー性及び靭性を有する鋼板は、粒径の粗大化を抑制するために加熱温度をやや低下させ、かつ、ＮｂＣが固溶するように、Ｃ量及びＮｂ量を適正な範囲に制御したものであった。   Furthermore, as a result of organizing the relationship between the precipitation state and grain size of the steel sheet and the production conditions, it is understood that the steel sheet on which coarse NbC is precipitated has a low heating temperature, and the steel sheet with coarse crystal grains has a high heating temperature. It was. In addition, a steel sheet having excellent sour resistance and toughness has an appropriate range for the amount of C and the amount of Nb so that the heating temperature is lowered slightly to suppress the coarsening of the grain size, and NbC is dissolved. Was controlled.

本発明者らは、８００〜９５０℃の圧下比を３とし、２５ｍｍ以上の鋼板を製造し、加熱温度とＤＷＴＴの関係を調査した。その結果、加熱温度が１１５０℃を超えると、結晶粒径が粗大化し、ＤＷＴＴが低下することが解った。一方、加熱温度が１０００℃未満になると、粗大なＮｂＣによって、ＤＷＴＴが低下することが解った。   The inventors set the rolling ratio of 800 to 950 ° C. to 3 and manufactured a steel plate of 25 mm or more, and investigated the relationship between the heating temperature and DWTT. As a result, it was found that when the heating temperature exceeds 1150 ° C., the crystal grain size becomes coarse and the DWTT decreases. On the other hand, it was found that when the heating temperature was less than 1000 ° C., DWTT was lowered by coarse NbC.

次に、本発明者らは、加熱温度を１０００〜１１５０℃の範囲内とし、Ｎｂ及びＣが鋼中に固溶する、適正なＮｂ及びＣの含有量と、加熱温度の関係について検討を行った。その結果、Ｎｂ及びＣが鋼中に固溶するか、ＮｂＣとして鋼中に析出するかは、溶解度積に影響されるので、ｌｏｇ（［Ｎｂ］×［Ｃ］）の数値が重要になることが解った。   Next, the inventors set the heating temperature within the range of 1000 to 1150 ° C., and examined the relationship between the heating temperature and the proper Nb and C content in which Nb and C are dissolved in the steel. It was. As a result, whether Nb and C are dissolved in steel or precipitated in steel as NbC is affected by the solubility product, so that the value of log ([Nb] × [C]) is important. I understand.

本発明者らは、更に、検討を行い、加熱温度Ｔ１を１０００〜１１５０℃の範囲内とし、かつ、加熱温度Ｔ１と、Ｎｂ及びＣの含有量が、
Ｔ１≧−７９７０／（ｌｏｇ（［Ｎｂ］×［Ｃ］）−３．３１）−１７０
を満足するように、鋼片を加熱することが、耐サワー性と靭性との両立に極めて重要であることを見いだした。 The present inventors have further studied and set the heating temperature T1 within the range of 1000 to 1150 ° C., and the heating temperature T1 and the contents of Nb and C are
T1 ≧ −7970 / (log ([Nb] × [C]) − 3.31) −170
It was found that heating the slab so as to satisfy the requirements is extremely important for achieving both sour resistance and toughness.

このような条件を満たす加熱温度Ｔ１は、平衡状態で、ＮｂＣが溶解する温度に相当している。したがって、上記の関係を満足すると、鋼片に析出したＮｂＣの溶解が促進され、粗大なＮｂＣが残存せず、ＨＩＣの発生を抑制することができると考えられる。   The heating temperature T1 that satisfies such conditions corresponds to a temperature at which NbC dissolves in an equilibrium state. Therefore, if the above relationship is satisfied, it is considered that dissolution of NbC precipitated on the steel slab is promoted, coarse NbC does not remain, and generation of HIC can be suppressed.

以下、本発明について、詳細に説明する。なお、以下、％は、質量％を意味する。   Hereinafter, the present invention will be described in detail. Hereinafter, “%” means mass%.

Ｃ：０．０１〜０．０８％
Ｃ：Ｃは、鋼の強度を向上させる元素であり、その有効な量として、０．０１％以上の添加が必要である。一方、Ｃ量が０．０８％を超えると、炭化物の生成が促進されて、耐ＨＩＣ性が損なわれるので、上限を０．０８％とする。また、ＨＩＣ性、溶接性、靱性等の低下を抑制するには、Ｃ量は、０．０６％以下が好ましい。 C: 0.01 to 0.08%
C: C is an element that improves the strength of steel, and as an effective amount thereof, addition of 0.01% or more is necessary. On the other hand, if the amount of C exceeds 0.08%, the formation of carbides is promoted and the HIC resistance is impaired, so the upper limit is made 0.08%. Moreover, in order to suppress a decrease in HIC properties, weldability, toughness, etc., the C content is preferably 0.06% or less.

Ｓｉ：０．１〜０．５％
Ｓｉ：Ｓｉは、脱酸元素であり、０．１％以上の添加が必要である。一方、Ｓｉ量が０．５％を超えると、溶接熱影響部（ＨＡＺ）の靱性が低下するので、上限を０．５％とする。好ましい範囲は、０．１５〜０．３５％である。 Si: 0.1 to 0.5%
Si: Si is a deoxidizing element and needs to be added in an amount of 0.1% or more. On the other hand, if the Si content exceeds 0.5%, the toughness of the weld heat affected zone (HAZ) decreases, so the upper limit is made 0.5%. A preferable range is 0.15 to 0.35%.

Ｍｎ：１．０〜１．５％
Ｍｎ：Ｍｎは、強度及び靱性を向上させる元素であり、１．０％以上の添加が必要である。一方、Ｍｎは、ＭｎＳを生成して、耐サワー性を劣化させる元素であるので、ＨＩＣを抑制するには、上限を１．５％とすることが必要である。好ましい範囲は、１．１〜１．４％である。 Mn: 1.0 to 1.5%
Mn: Mn is an element that improves strength and toughness, and it is necessary to add 1.0% or more. On the other hand, Mn is an element that generates MnS and degrades sour resistance. Therefore, to suppress HIC, the upper limit is required to be 1.5%. A preferable range is 1.1 to 1.4%.

Ｎｂ：０．０１０〜０．０４０％
Ｎｂ：Ｎｂは、未再結晶温度域を拡大して結晶粒径を微細化し、炭化物、窒化物を形成し、強度の向上に寄与する元素であり、０．０１０％以上の添加が必要である。一方、本発明では、粗大な炭化物の生成を防止することが極めて重要であり、上限を０．０４０％にすることが必要である。好ましい範囲は、０．０１１〜０．０２５％、更に好ましい範囲は、０．０１２〜０．０２０％である。 Nb: 0.010 to 0.040%
Nb: Nb is an element that expands the non-recrystallization temperature range, refines the crystal grain size, forms carbides and nitrides, and contributes to the improvement of strength. . On the other hand, in the present invention, it is extremely important to prevent the formation of coarse carbides, and the upper limit must be 0.040%. A preferred range is 0.011 to 0.025%, and a more preferred range is 0.012 to 0.020%.

Ｃａ：０．００１〜０．００４％
Ｃａ：Ｃａは、硫化物ＣａＳを生成し、圧延方向に伸長するＭｎＳの生成を抑制し、耐ＨＩＣ性の改善に顕著に寄与する元素である。Ｃａの添加量が０．００１％未満では、効果が得られないため、下限値を０．００１％とする。一方、Ｃａの添加量が０．００４％を超えると、酸化物が集積し、耐ＨＩＣ性が損なわれるので、上限を０．００４％とする。好ましい範囲は、０．００２５〜０．００３５％である。 Ca: 0.001 to 0.004%
Ca: Ca is an element that produces sulfide CaS, suppresses the production of MnS extending in the rolling direction, and contributes significantly to the improvement of HIC resistance. If the addition amount of Ca is less than 0.001%, the effect cannot be obtained, so the lower limit is set to 0.001%. On the other hand, if the amount of Ca exceeds 0.004%, oxides accumulate and the HIC resistance is impaired, so the upper limit is made 0.004%. A preferable range is 0.0025 to 0.0035%.

Ｔｉ：０．００５〜０．０３０％
Ｔｉ：Ｔｉは、脱酸剤や窒化物形成元素として結晶粒の細粒化に利用される元素であり、０．００５％以上を添加する必要がある。一方、Ｔｉを過剰に添加すると、粗大な窒化物の形成によって、靱性が低下するので、上限を０．０３０％とする。好ましい範囲は、０．０１０〜０．０２０％である。 Ti: 0.005-0.030%
Ti: Ti is an element used for refining crystal grains as a deoxidizer or nitride-forming element, and it is necessary to add 0.005% or more. On the other hand, if Ti is added excessively, the toughness decreases due to the formation of coarse nitrides, so the upper limit is made 0.030%. A preferable range is 0.010 to 0.020%.

Ａｌ：０．０８％以下
Ａｌ：Ａｌは、脱酸元素であるが、添加量が０．０８％を超えると、Ａｌ酸化物の集積クラスターが生成し、耐サワー性が損なわれるので、０．０８％以下に制限する。また、靭性が要求される場合には、Ａｌ量の上限を０．０３％にすることが好ましい。更に好ましいＡｌの上限は、０．０１％である。Ａｌ量の下限は特に限定しないが、溶鋼中の酸素量を低減させるためには、Ａｌを０．０００５％以上添加することが好ましい。 Al: 0.08% or less Al: Al is a deoxidizing element. However, if the addition amount exceeds 0.08%, an accumulation cluster of Al oxide is generated and sour resistance is impaired. Limit to 08% or less. Moreover, when toughness is requested | required, it is preferable to make the upper limit of Al amount into 0.03%. A more preferable upper limit of Al is 0.01%. The lower limit of the amount of Al is not particularly limited, but it is preferable to add 0.0005% or more of Al in order to reduce the amount of oxygen in the molten steel.

Ｐ：０．０１５％以下
Ｐ：Ｐは、不純物であり、含有量が０．０１５％を超えると、耐ＨＩＣ性を損なう。したがって、Ｐの含有量の上限を０．０１５％とする。 P: 0.015% or less P: P is an impurity. If the content exceeds 0.015%, the HIC resistance is impaired. Therefore, the upper limit of the P content is 0.015%.

Ｓ：０．０００８％以下
Ｓ：Ｓは、熱間圧延時に圧延方向に延伸するＭｎＳを生成して、耐ＨＩＣ性を低下させる元素である。したがって、本発明では、Ｓ量を低減することが必要であり、上限を０．０００８％に制限する。Ｓ量は、少ないほど好ましいが、０．０００１％未満にすることは困難である。製造コストの観点からも、０．０００１％以上にすることが好ましい。 S: 0.0008% or less S: S is an element that reduces the HIC resistance by generating MnS that extends in the rolling direction during hot rolling. Therefore, in the present invention, it is necessary to reduce the amount of S, and the upper limit is limited to 0.0008%. The smaller the amount of S, the better, but it is difficult to make it less than 0.0001%. Also from the viewpoint of manufacturing cost, it is preferably 0.0001% or more.

Ｏ：０．００３０％以下
Ｏ：Ｏは、不純物であり、酸化物の集積を抑制して、耐ＨＩＣ性を向上させるためには、上限を０．００３０％に制限することが必要である。酸化物の生成を抑制して、靭性を向上させるためには、Ｏ量を０．００２０％以下とすることが好ましい。 O: 0.0030% or less O: O is an impurity, and it is necessary to limit the upper limit to 0.0030% in order to suppress the accumulation of oxides and improve the HIC resistance. In order to suppress the formation of oxides and improve toughness, the O content is preferably 0.0020% or less.

Ｎ：０．００５０％以下
Ｎ：Ｎは、不純物であり、Ｎの含有量が０．００５０％を超えると、ＴｉとＮｂの炭窒化物が集積し易くなり、耐ＨＩＣ性を損なう。したがって、Ｎ量の上限を０．００５０％とする。なお、靭性などが要求される場合には、ＴｉＮの粗大化を抑制するため、Ｎ量の上限を０．００３５％にすることが好ましい。ＴｉＮ、ＮｂＮなどの窒化物を利用し、加熱時のオーステナイト粒径の微細化を図る場合は、０．００１０％以上のＮを含有させることが好ましい。 N: 0.0050% or less N: N is an impurity. When the content of N exceeds 0.0050%, carbonitrides of Ti and Nb are likely to accumulate, and the HIC resistance is impaired. Therefore, the upper limit of the N amount is set to 0.0050%. In addition, when toughness etc. are requested | required, in order to suppress the coarsening of TiN, it is preferable to make the upper limit of N amount 0.0035%. When a nitride such as TiN or NbN is used to reduce the austenite grain size during heating, it is preferable to contain 0.0010% or more of N.

［Ｃａ］（１−１２４［Ｏ］）／１．２５［Ｓ］＞３．０
本発明では、［Ｃａ］（１−１２４［Ｏ］）／１．２５［Ｓ］、即ち、ＥＳＳＰ値を大きくすることが必要である。ＥＳＳＰ値は、Ｃａが酸化物を形成することを考慮し、ＣａＳを生成させるために必要な、Ｓ量に対するＣａ量の比である。Ｃａを添加して、ＣａＳを形成させてＳを固定するためには、ＥＳＳＰ値を、３．０超とすることが必要である。 [Ca] (1-124 [O]) / 1.25 [S]> 3.0
In the present invention, [Ca] (1-124 [O]) / 1.25 [S], that is, it is necessary to increase the ESSP value. The ESSP value is a ratio of the Ca amount to the S amount necessary for generating CaS considering that Ca forms an oxide. In order to fix Ca by adding Ca to form CaS, the ESSP value needs to be more than 3.0.

なお、Ｓ量が０になると、ＥＳＳＰは無限大になるが、この場合、ＭｎＳの生成は有り得ない。したがって、Ｃａ量が上述の範囲内であれば、ＥＳＳＰ値の上限を規定する必要はない。   Note that when the S amount becomes 0, ESSP becomes infinite, but in this case, generation of MnS is impossible. Therefore, if the Ca content is within the above range, there is no need to define the upper limit of the ESSP value.

なお、本発明においては、強度及び靱性を改善する元素として、Ｎｉ、Ｃｕ、Ｃｒ、Ｍｏ、Ｖ、Ｂのうち、１種又は２種以上の元素を添加することが好ましい。   In the present invention, it is preferable to add one or more elements among Ni, Cu, Cr, Mo, V, and B as elements for improving strength and toughness.

Ｎｉ：０．５％以下
Ｎｉ：Ｎｉは、靱性及び強度の改善に有効な元素であり、耐食性の向上にも寄与するため、０．０１％以上の添加が好ましい。一方、Ｎｉは高価な元素であるので、製造コストを削減するために、上限を、０．５％に制限することが好ましい。 Ni: 0.5% or less Ni: Ni is an element effective for improving toughness and strength, and contributes to improvement of corrosion resistance. Therefore, addition of 0.01% or more is preferable. On the other hand, since Ni is an expensive element, the upper limit is preferably limited to 0.5% in order to reduce manufacturing costs.

Ｃｕ：０．５％以下
Ｃｕ：Ｃｕは、強度の上昇に有効な元素であり、耐食性の向上にも寄与するので、０．０１％以上の添加が好ましい。一方、Ｃｕも高価な元素であるので、製造コストを削減するために、上限を、０．５％に制限することが好ましい。 Cu: 0.5% or less Cu: Cu is an element effective for increasing the strength and contributes to the improvement of corrosion resistance, so 0.01% or more is preferably added. On the other hand, since Cu is also an expensive element, it is preferable to limit the upper limit to 0.5% in order to reduce manufacturing costs.

Ｃｒ：０．５％以下
Ｃｒ：Ｃｒは、強度の上昇に有効な元素であり、０．０１％以上の添加が好ましい。一方、多量に添加すると、焼入れ性が高くなり、靱性が低下することがあるので、上限を、０．５％とすることが好ましい。 Cr: 0.5% or less Cr: Cr is an element effective for increasing the strength, and is preferably added in an amount of 0.01% or more. On the other hand, if added in a large amount, the hardenability becomes high and the toughness may be lowered, so the upper limit is preferably made 0.5%.

Ｍｏ：０．３％以下
Ｍｏ：Ｍｏは、焼入れ性を向上させると同時に、炭窒化物を形成し強度を改善する元素であり、その添加効果を得るためには、０．０１％以上の添加が好ましい。一方、Ｍｏは高価な元素であるので、製造コストを削減するために、上限を、０．３％にすることが好ましい。鋼の強度が上昇すると、ＨＩＣ性及び靱性が低下することがあるので、好ましい上限は、０．２％である。 Mo: 0.3% or less Mo: Mo is an element that improves hardenability and at the same time forms carbonitride to improve strength. To obtain the effect of addition, 0.01% or more is added. Is preferred. On the other hand, since Mo is an expensive element, the upper limit is preferably set to 0.3% in order to reduce manufacturing costs. When the strength of the steel increases, the HIC property and toughness may decrease, so the preferable upper limit is 0.2%.

Ｖ：０．０６％以下
Ｖ：Ｖは、炭化物、窒化物を形成し、強度の向上に寄与する元素であり、添加効果を得るために、０．０１％以上の添加が好ましい。一方、０．０６％を超えるＶを添加すると、靱性の低下を招くことがあるので、上限を、０．０６％とすることが好ましい。 V: 0.06% or less V: V is an element that forms carbides and nitrides and contributes to the improvement of strength, and in order to obtain the effect of addition, addition of 0.01% or more is preferable. On the other hand, if V exceeding 0.06% is added, the toughness may be lowered, so the upper limit is preferably made 0.06%.

Ｂ：０．００２０％以下
Ｂ：Ｂは、鋼の粒界に偏析して焼入れ性の向上に著しく寄与する元素である。この添加効果を得るには、０．０００１％以上の添加が好ましい。一方。Ｂを過剰に添加すると、粒界への偏析が過剰になり、靱性の低下を招くことがあるので、上限を、０．００２０％とすることが好ましい。 B: 0.0020% or less B: B is an element that segregates at the grain boundaries of steel and contributes significantly to improving the hardenability. In order to obtain this addition effect, addition of 0.0001% or more is preferable. on the other hand. When B is added excessively, segregation to the grain boundary becomes excessive and the toughness may be lowered. Therefore, the upper limit is preferably made 0.0020%.

Ｍｇ：０．０１％以下
Ｍｇ：Ｍｇは、脱酸剤及び脱硫剤として作用する元素であり、特に、微細な酸化物を生じて、粒径の粗大化を抑制するので、靭性の向上に有効である。この添加効果を得るには、０．０００１％以上の添加が好ましい。一方、Ｍｇを０．０１％超添加すると、酸化物が凝集、粗大化し易くなり、ＨＩＣ性や靱性を低下させることがある。したがって、Ｍｇ量の上限を、０．０１％とすることが好ましい。 Mg: 0.01% or less Mg: Mg is an element that acts as a deoxidizing agent and a desulfurizing agent. In particular, it produces fine oxides and suppresses coarsening of the particle size, so it is effective in improving toughness. It is. In order to obtain this addition effect, addition of 0.0001% or more is preferable. On the other hand, if Mg is added in an amount of more than 0.01%, the oxide tends to aggregate and coarsen, and the HIC property and toughness may be lowered. Therefore, it is preferable that the upper limit of the Mg amount is 0.01%.

上記の成分を含有する鋼は、製鋼工程で溶製後、連続鋳造により鋼片とし、鋼片を加熱し、粗圧延と仕上げ圧延からなる厚板圧延を施して、鋼板とされる。本発明では、鋼片の加熱温度、仕上げ圧延の温度及び圧下比、圧延後の加速冷却の条件が重要である。   The steel containing the above components is made into a steel plate by melting it in the steel making process, making it a steel slab by continuous casting, heating the steel slab, and performing thick plate rolling consisting of rough rolling and finish rolling. In the present invention, the steel slab heating temperature, the finish rolling temperature and rolling reduction ratio, and the accelerated cooling conditions after rolling are important.

加熱温度Ｔ１：１０００〜１１５０℃
まず、本発明では、鋼片の加熱温度の範囲が重要である。加熱温度が１０００℃未満であると、鋼片に析出したＮｂＣが固溶せず、粗大なＮｂＣが鋼板に残存し、耐サワー性が低下する。 Heating temperature T1: 1000-1150 ° C
First, in the present invention, the range of the heating temperature of the steel slab is important. When the heating temperature is less than 1000 ° C., NbC deposited on the steel slab does not dissolve, and coarse NbC remains in the steel sheet, sour resistance decreases.

一方、加熱温度が１１５０℃を超えると、鋼板の結晶粒径が粗大になり、２５ｍｍを超える厚さの鋼板では粗圧延域での再結晶が不十分になり、最終的な鋼板の結晶粒度は加熱時の結晶粒度の影響を強く受ける。特に、ＤＷＴＴ特性は、結晶粒度の影響を受け易いため、結晶粒径が大きくなると、ＤＷＴＴ特性が低下する。したがって、鋼片の加熱温度Ｔ１は、１０００〜１１５０℃の範囲内とする。   On the other hand, when the heating temperature exceeds 1150 ° C., the crystal grain size of the steel sheet becomes coarse, and in the steel sheet having a thickness of more than 25 mm, recrystallization in the rough rolling region becomes insufficient. It is strongly affected by the grain size during heating. In particular, since the DWTT characteristic is easily affected by the crystal grain size, the DWTT characteristic decreases as the crystal grain size increases. Therefore, the heating temperature T1 of the steel slab is in the range of 1000 to 1150 ° C.

Ｔ１≧−７９７０／（ｌｏｇ（［Ｎｂ］×［Ｃ］）−３．３１）−１７０
更に、鋼片に析出したＮｂＣを固溶させるためには、鋼片の加熱温度Ｔ１と、Ｎｂ及びＣの含有量の関係を適正に制御することが重要である。これは、上述のように、ＮｂＣの溶解、析出は、溶解度積に影響されるので、ｌｏｇ（［Ｎｂ］×［Ｃ］）の数値が重要になるからである。 T1 ≧ −7970 / (log ([Nb] × [C]) − 3.31) −170
Furthermore, in order to dissolve NbC precipitated on the steel slab, it is important to appropriately control the relationship between the heating temperature T1 of the steel slab and the contents of Nb and C. This is because the numerical value of log ([Nb] × [C]) becomes important because dissolution and precipitation of NbC are affected by the solubility product as described above.

本発明では、加熱温度を１０００〜１１５０℃の範囲内とし、更に、鋼片の加熱温度Ｔ１と、Ｎｂ及びＣの含有量が、
Ｔ１≧−７９７０／（ｌｏｇ（［Ｎｂ］×［Ｃ］）−３．３１）−１７０
を満足することが必要である。これにより、鋼板に粗大なＮｂＣが残存せず、耐サワー性を向上させることができる。 In the present invention, the heating temperature is in the range of 1000 to 1150 ° C., and the steel slab heating temperature T1 and the contents of Nb and C are
T1 ≧ −7970 / (log ([Nb] × [C]) − 3.31) −170
It is necessary to satisfy Thereby, coarse NbC does not remain on the steel sheet, and sour resistance can be improved.

鋼片を加熱した後、粗圧延を行い、更に、仕上げ圧延を行う。   After heating the steel slab, rough rolling is performed, and then finish rolling is performed.

仕上げ温度：８００℃以上
熱間圧延の仕上げ温度は、組織を均質にするために、８００℃以上とする。これは、成分組成よっては、８００℃未満でフェライトが生成して、圧延後の鋼板の組織が層状になるためである。組織が層状になると、耐サワー性が低下することがある。また、仕上げ圧延の条件によっては、鋼板に加工フェライトが残存し、靭性が損なわれることがある。 Finishing temperature: 800 ° C. or higher The hot rolling finishing temperature is 800 ° C. or higher in order to make the structure homogeneous. This is because, depending on the component composition, ferrite is generated at a temperature lower than 800 ° C., and the structure of the steel sheet after rolling becomes layered. When the structure is stratified, the sour resistance may decrease. Moreover, depending on the conditions of finish rolling, the processed ferrite may remain on the steel sheet and the toughness may be impaired.

９５０℃以下の圧下比：３以上
仕上げ圧延では、結晶粒径を微細化するために、圧延温度と圧下比を制御することが必要である。特に、低温での圧下比を大きくして仕上げ圧延を行うことにより、鋼板の組織を微細にすることができる。圧延温度が９５０℃を超えている場合、再結晶が生じるので、９５０℃以下での圧下比が重要である。 Rolling ratio of 950 ° C. or lower: 3 or higher In finish rolling, it is necessary to control the rolling temperature and the rolling ratio in order to refine the crystal grain size. In particular, the structure of the steel sheet can be made fine by performing finish rolling while increasing the reduction ratio at a low temperature. Since recrystallization occurs when the rolling temperature exceeds 950 ° C., the reduction ratio at 950 ° C. or lower is important.

９５０℃以下での圧下比が、３未満であると、粒径が微細化せず、均質な組織を得ることが困難になり、靭性及び耐サワー性が低下する。したがって、９５０℃以下、仕上げ圧延が終了するまでの圧下比を３以上とする。９５０℃以下から仕上げ圧延が終了するまでの圧下比は、９５０℃における板厚に対する圧延後の板厚の比である。   When the rolling ratio at 950 ° C. or less is less than 3, the particle size is not refined, making it difficult to obtain a homogeneous structure, and toughness and sour resistance are reduced. Therefore, the reduction ratio until the finish rolling is finished at 950 ° C. or less is 3 or more. The reduction ratio from 950 ° C. or lower to the end of finish rolling is the ratio of the plate thickness after rolling to the plate thickness at 950 ° C.

板厚：２５ｍｍ以上
本発明は、板厚が２５ｍｍ以上の鋼板の組織を、微細かつ均質にするものである。即ち、板厚が２５ｍｍ以上になると、仕上げ圧延の圧下比を確保することが困難になり、組織を微細化するためには、加熱温度を低くする必要が生じる。したがって、本発明の製造方法では、仕上げ圧延後の板厚を２５ｍｍ以上とする。 Plate thickness : 25 mm or more In the present invention, the structure of a steel plate having a plate thickness of 25 mm or more is made fine and uniform. That is, when the plate thickness is 25 mm or more, it becomes difficult to secure the reduction ratio of finish rolling, and it is necessary to lower the heating temperature in order to refine the structure. Therefore, in the manufacturing method of the present invention, the plate thickness after finish rolling is set to 25 mm or more.

仕上げ圧延後には、加速冷却を行う。加速冷却は、仕上げ圧延後、直ちに行うが、製造ラインの構成上、若干、温度が低下してもよい。なお、加速冷却の開始温度は、ポリゴナルフェライトの生成を抑制し、フェライトと低温変態相（ベイナイトやマルテンサイト）の層状組織となることを防止するため、７００℃以上とすることが好ましい。   After finish rolling, accelerated cooling is performed. Although accelerated cooling is performed immediately after finish rolling, the temperature may be slightly lowered due to the configuration of the production line. The accelerated cooling start temperature is preferably 700 ° C. or higher in order to suppress the formation of polygonal ferrite and prevent the formation of a layered structure of ferrite and a low-temperature transformation phase (bainite or martensite).

加速冷却の冷却速度：１０〜３０℃／ｓ
加速冷却は、鋼板の組織を微細なアシキュラーフェライトやベイニティックフェライトにするために行うものである。ポリゴナルフェライト変態を抑制し、パーライトの生成を防止するには、冷却速度を１０℃／ｓ以上にすることが必要である。 Accelerated cooling rate: 10-30 ° C / s
Accelerated cooling is performed in order to make the structure of the steel sheet into fine acicular ferrite or bainitic ferrite. In order to suppress polygonal ferrite transformation and prevent the formation of pearlite, the cooling rate needs to be 10 ° C./s or more.

一方、加速冷却の冷却速度が３０℃／ｓを超えると、マルテンサイトが過剰に生成して、硬度が不均一になり、耐サワー性及び靭性が低下する。したがって、加速冷却の冷却速度は１０〜３０℃／ｓとする。なお、冷却速度は鋼板の板厚中心での冷却速度である。   On the other hand, when the cooling rate of accelerated cooling exceeds 30 ° C./s, martensite is excessively generated, the hardness becomes nonuniform, and sour resistance and toughness are lowered. Therefore, the cooling rate of accelerated cooling is 10 to 30 ° C./s. The cooling rate is the cooling rate at the center of the thickness of the steel plate.

加速冷却の停止温度：２００〜５００℃
加速冷却の停止温度は、マルテンサイトの生成を抑制するために、２００〜５００℃の範囲内とする。ポリゴナルフェライト変態を抑制し、パーライトの生成を防止するには、加速冷却の停止温度を５００℃以下にすることが必要である。 Accelerated cooling stop temperature: 200-500 ° C
The accelerated cooling stop temperature is set to be in the range of 200 to 500 ° C. in order to suppress the formation of martensite. In order to suppress the polygonal ferrite transformation and prevent the formation of pearlite, it is necessary to set the stop temperature of accelerated cooling to 500 ° C. or lower.

一方、加速冷却の停止速度が２００℃以下になると、マルテンサイトが過剰に生成して、硬度が不均一になり、耐サワー性及び靭性が低下する。   On the other hand, when the accelerated cooling stop speed is 200 ° C. or less, martensite is excessively generated, the hardness becomes non-uniform, and sour resistance and toughness deteriorate.

表１に示す化学成分を有する鋼を転炉、二次精錬で溶製し、連続鋳造で２５０ｍｍ厚の鋼片を製造した。得られた鋼片を表２に示す条件で熱間圧延を行い、鋼板とした。製造後の鋼板のＨＩＣ性をＮＡＣＥ試験によって評価した。   Steel having chemical components shown in Table 1 was melted by a converter and secondary refining, and a steel piece having a thickness of 250 mm was produced by continuous casting. The obtained steel slab was hot-rolled under the conditions shown in Table 2 to obtain a steel plate. The HIC property of the steel sheet after production was evaluated by a NACE test.

ＮＡＣＥ試験の条件は、５％ＮａＣｌ溶液＋０．５％酢酸、ｐＨ２．７の溶液中に硫化水素ガスを飽和させて、浸漬時間を９６時間とし、割れの有無を観察し、ＨＩＣ破面率（ＣＡＲ）を測定した。   The NACE test was performed by saturating hydrogen sulfide gas in a solution of 5% NaCl solution + 0.5% acetic acid, pH 2.7, dipping time of 96 hours, observing the presence of cracks, CAR) was measured.

靭性は、ＤＷＴＴ試験で評価した。鋼板から、ＡＰＩ、５Ｌ３、ＡＳＴＭ、Ｅ４３６に準拠して、板幅方向を長手方向とし、ノッチを板厚方向と平行にして設けたプレスノッチ試験片を作製した。ＤＷＴＴは−３５℃で行い、延性破面率（ＳＡ）を求めた。   Toughness was evaluated by the DWTT test. A press notch test piece was prepared from the steel plate in accordance with API, 5L3, ASTM, E436, with the plate width direction as the longitudinal direction and the notch parallel to the plate thickness direction. DWTT was performed at −35 ° C. to determine the ductile fracture surface ratio (SA).

結果を表２に示す。Ｎｏ．１〜８は、鋼板の成分及び製造条件が本発明の範囲内であり、ＣＡＲが５％以下となり、良好な耐サワー性を有している。一方、Ｎｏ．１０は、ＥＳＳＰ値が本発明の範囲よりも低く、耐サワー性が低下した例である。 The results are shown in Table 2. No. In Nos. 1 to 8 , the components and production conditions of the steel sheet are within the scope of the present invention, and the CAR is 5% or less and has good sour resistance. On the other hand, no. 10 is an example in which the ESSP value is lower than the range of the present invention and sour resistance is lowered.

また、Ｎｏ．１１は、Ｃ量が多く、加熱温度が低く、
Ｔ１≧−７９７０／（ｌｏｇ（［Ｎｂ］×［Ｃ］）−３．３１）−１７０
を満足しないため、耐ＨＩＣ性が低下した例である。Ｎｏ．１２は、加熱温度が低く、靭性が劣化した例である。 No. 11 has a large amount of C and a low heating temperature,
T1 ≧ −7970 / (log ([Nb] × [C]) − 3.31) −170
This is an example in which the HIC resistance is lowered. No. No. 12 is an example in which the heating temperature is low and the toughness is deteriorated.

Figure 0004700741
Figure 0004700741

Figure 0004700741
Figure 0004700741

前述したように、本発明によれば、特に、板厚が２５ｍｍ以上であり、靭性、特に、ＤＷＴＴ特性及び耐サワー性に優れた厚肉ラインパイプ用鋼板を提供することが可能になるしたがって、本発明は、産業上の貢献が極めて顕著であり、産業上の利用可能性が大きいものである。   As described above, according to the present invention, in particular, it becomes possible to provide a steel plate for a thick line pipe having a plate thickness of 25 mm or more and excellent in toughness, in particular, DWTT characteristics and sour resistance. The industrial contribution of the present invention is extremely remarkable, and the industrial applicability is great.

Claims (6)

質量％で、
Ｃ ：０．０１〜０．０８％、
Ｓｉ：０．１〜０．５％、
Ｍｎ：１．０〜１．５％、
Ｎｂ：０．０１０〜０．０４０％、
Ｃａ：０．００１〜０．００４％、
Ｔｉ：０．００５〜０．０３０％
を含有し、
Ａｌ：０．０８％以下、
Ｐ： ０．０１５％以下、
Ｓ： ０．０００８％以下、
Ｏ： ０．００３０％以下、
Ｎ： ０．００５０％以下
に制限し、Ｃａ、Ｏ、及び、Ｓの含有量が、
［Ｃａ］（１−１２４［Ｏ］）／１．２５［Ｓ］＞３．０
を満足し、残部Ｆｅ及び不可避的不純物からなる鋼片を、１０００〜１１５０℃の範囲内の加熱温度Ｔ１と、Ｎｂ及びＣの含有量とが、
Ｔ１≧−７９７０／（ｌｏｇ（［Ｎｂ］×［Ｃ］）−３．３１）−１７０
を満足するように加熱し、粗圧延を行い、更に、仕上げ温度を８００℃以上とし、９５０℃以下の圧下比を３以上として、板厚が２５ｍｍ以上になるように、仕上げ圧延を行い、冷却速度が１０〜３０℃／ｓの加速冷却を行い、２００〜５００℃で該加速冷却を停止することを特徴とする靭性に優れた厚肉耐サワーラインパイプ用鋼板の製造方法。 % By mass
C: 0.01 to 0.08%,
Si: 0.1 to 0.5%,
Mn: 1.0 to 1.5%
Nb: 0.010 to 0.040%,
Ca: 0.001 to 0.004%,
Ti: 0.005-0.030%
Containing
Al: 0.08% or less,
P: 0.015% or less,
S: 0.0008% or less,
O: 0.0030% or less,
N: The content of Ca, O, and S is limited to 0.0050% or less,
[Ca] (1-124 [O]) / 1.25 [S]> 3.0
Satisfied, the slab ing the balance Fe and unavoidable impurities, and a heating temperature T1 in the range of 1000 to 1150 ° C., and the content of Nb and C,
T1 ≧ −7970 / (log ([Nb] × [C]) − 3.31) −170
The steel sheet is heated so as to satisfy the conditions, rough rolling is performed, the finishing temperature is set to 800 ° C. or higher, the rolling ratio of 950 ° C. or lower is set to 3 or higher, and the final rolling is performed so that the plate thickness is 25 mm or higher. A method for producing a steel plate for a thick-walled sour line pipe excellent in toughness, characterized by performing accelerated cooling at a rate of 10 to 30 ° C / s and stopping the accelerated cooling at 200 to 500 ° C. 前記鋼片が、質量％で、The billet is mass%,
Ｃ ：０．０１〜０．０６％、C: 0.01 to 0.06%,
Ｎｂ：０．０１０〜０．０２５％、Nb: 0.010 to 0.025%,
Ｔｉ：０．００５〜０．０２０％Ti: 0.005-0.020%
を含有することを特徴とする請求項１に記載の靭性に優れた厚肉耐サワーラインパイプ用鋼板の製造方法。The manufacturing method of the steel plate for thick-walled sour-line pipes excellent in toughness of Claim 1 characterized by the above-mentioned. 前記鋼片が、更に、質量％で、
Ｎｉ：０．５％以下、
Ｃｕ：０．５％以下、
Ｃｒ：０．５％以下、
Ｍｏ：０．３％以下
の１種又は２種以上を含有することを特徴とする請求項２に記載の靭性に優れた厚肉耐サワーラインパイプ用鋼板の製造方法。 The steel slab is further mass%,
Ni: 0.5% or less,
Cu: 0.5% or less,
Cr: 0.5% or less,
Mo: 0.3% or less of 1 type or 2 types or more, The manufacturing method of the steel plate for thick-walled sour line pipes excellent in toughness of Claim 2 characterized by the above-mentioned. 前記鋼片が、更に、質量％で、
Ｖ：０．０６％以下
を含有することを特徴とする請求項１〜３の何れか１項に記載の靭性に優れた厚肉耐サワーラインパイプ用鋼板の製造方法。 The steel slab is further mass%,
V: It contains 0.06% or less, The manufacturing method of the steel plate for thick-walled sour line pipes excellent in toughness of any one of Claims 1-3 characterized by the above-mentioned. 前記鋼片が、更に、質量％で、
Ｂ：０．００２０％以下
を含有することを特徴とする請求項１〜４の何れか１項に記載の靭性に優れた厚肉耐サワーラインパイプ用鋼板の製造方法。 The steel slab is further mass%,
B: 0.0020% or less is contained, The manufacturing method of the steel plate for thick-walled sour line pipes excellent in toughness of any one of Claims 1-4 characterized by the above-mentioned. 前記鋼片が、更に、質量％で、
Ｍｇ：０．０１％以下
を含有することを特徴とする請求項１〜５の何れか１項に記載の靭性に優れた厚肉耐サワーラインパイプ用鋼板の製造方法。 The steel slab is further mass%,
Mg: 0.01% or less is contained, The manufacturing method of the steel plate for thick-walled sour line pipes excellent in toughness of any one of Claims 1-5 characterized by the above-mentioned.
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