JP3941211B2 - Manufacturing method of steel plate for high-strength line pipe with excellent HIC resistance - Google Patents

Manufacturing method of steel plate for high-strength line pipe with excellent HIC resistance Download PDF

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JP3941211B2
JP3941211B2 JP08354198A JP8354198A JP3941211B2 JP 3941211 B2 JP3941211 B2 JP 3941211B2 JP 08354198 A JP08354198 A JP 08354198A JP 8354198 A JP8354198 A JP 8354198A JP 3941211 B2 JP3941211 B2 JP 3941211B2
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steel
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steel plate
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strength
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JPH11279639A (en
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茂 遠藤
信行 石川
稔 諏訪
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JFE Steel Corp
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JFE Steel Corp
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Description

【0001】
【発明の属する技術分野】
本発明は、厚板ミルや熱延ミルにて製造され、UOE成形、プレスべンド成形、ロール成形などにより管状に成形され、サブマージドアーク溶接や電縫溶接などにより溶接接合されて、原油や天然ガスを輸送するためのラインパイプとして利用される鋼板の製造方法に係り、耐水素誘起割れ性に優れた、強度レべルがAPI規格X65グレード以上のラインパイプの素材として使用される鋼板の製造方法に関する。
【0002】
【従来の技術】
一般に、硫化水素を含む原油や天然ガスの輸送に用いられるラインパイプには、強度・靭性・溶接性などパイプラインとして必要な特性の他に、耐水素誘起割れ性(耐HIC性)や耐応力腐食割れ性(耐SSCC性)などのいわゆる耐サワー性能が要求される。ここでHICは、腐食反応により生成した水素イオンが鋼表面に吸着し、原子状の水素として鋼内部に侵入、鋼中のMnSなどの非金属介在物や硬い第2相組織のまわりに拡散・集積し、その内圧により割れを生ずるものとされている。このため、HICの発生を防ぐために以下の方法がこれまでに考案されている。
(1)鋼中のS含有量を下げるとともに、CaやREMなどを適量添加することにより、長く伸展したMnSの生成を抑制し、応力集中の小さい微細に分散した球状の介在物に形態を変えて割れの発生・伝播を抑制する(例えば、特開昭54−110119号公報)。
(2)中央偏析部での割れについては、起点となりうる島状マルテンサイトの生成を抑制するとともに、割れの伝播経路となりやすいマルテンサイトやべイナイトなどの硬化組織の生成を抑制するために、鋼中のC、Mn、Pなど偏析傾向の高い元素の含有量を低減したり、圧延前のスラブ加熱段階で合金元素の偏析を解消するための均熱処理を施す、あるいは圧延後の冷却時の変態途中でのCの拡散による硬化組織の生成を防ぐために加速冷却を施す(例えば、特開昭61−60866号公報、特開昭61−165207号公報など)。
【0003】
(3)焼入れ・焼戻しなどの熱処理を施したり、圧延仕上温度をオーステナイトの再結晶温度以上とするなど、割れ感受性の低いミクロ組織を得る(特開昭54−12782号公報、特開昭62−7819号公報、特開平6−73450号公報)。
(4)鋼中へのCuの添加により、表面に保護膜を形成して、鋼中への水素侵入を抑制する(特開昭52−111815号公報)。
【0004】
これらの方法を採用することにより耐HIC性は向上し、耐サワー性を必要とするラインパイプもAPI規格X65グレードまで大量生産されるようになった。
【0005】
しかしながら、近年になって輸送効率の増大や敷設費用低減のために、より高強度の鋼管に対する要求が高まり、サワー環境で使用されるラインパイプにもX80グレードまでの高強度化が要求される可能性がでてきた。しかしながら、HICは強度の上昇とともに発生しやすくなるため、上記(1)〜(4)の方法では完全にHICの発生を抑制することができなくなってきた。
このような高強度材になると、上記(1)の形態制御を行った介在物からも割れが発生するようになり、(2)の中央偏析対策を施した中心部以外の部分で割れが発生するようになる。また(3)の焼入れ焼戻し処理や再結晶温度域仕上による組織制御はラインパイプの大量生産にはコスト・能率の面から不適当であるし、充分な低温靭性も得にくい。さらに(4)のCu被膜の効果も、pHの低い環境ではその効果が期待できず、実際にpHが約3の硫化水素を飽和させた5%NaCl+0.5%CH3 COOH水溶液(通称NACE溶液)では、被膜の効果が得られていない。
【0006】
このような課題に対応すべく最近、耐サワー性を有するX80グレードのラインパイプ用鋼板の製造方法がいくつか開示されている。その骨子は、低S・Ca添加により介在物の形態制御を行いつつ、低C、低Mnとして中央偏析を抑制し、それに伴う強度低下をCr添加(特開平5−9575号公報)、Cr−Mo添加(特開平5−271766号公報、特開平7−109519号公報)、Ni‐Cr−Mo添加(特開平7−173536号公報)と圧延後の加速冷却で補うというものである。
【0007】
【発明が解決しようとする課題】
しかしながら、上記したX80の製造方法に関係する製造技術(特開平5−9575号公報、特開平5−271766号公報、特開平7−109519号公報、特開平7−173536号公報)は、いずれも中央偏析部のHIC発生防止方法であって、中央偏析部以外の部分で発生するHICの防止については、具体的な割れ対策とはなっていない。すなわち、サワー環境で使用される鋼管の強度水準が上昇すると、素材である鋼板の介在物の形態制御と中央偏析部の組織制御を行なっても、HICが発生しやすくなり、特に加速冷却を施した材料では表面近くの硬さが上昇し、HICが発生しやすくなる。このような表面近くのHICの発生防止が大きな課題となる。
【0008】
本発明の目的は、API規格X65に加えてX70,X80といった高強度材で、耐HIC性に優れた鋼材を経済的に安定して製造する方法を提供することにある。
【0009】
【課題を解決するための手段】
前記課題を解決し目的を達成するために、本発明は以下に示す手段を用いている。
(1)本発明の製造方法は、質量%で、C:0.03〜0.08%と、Si:0.05〜0.5%と、Mn:1〜1.8%と、P:0.01%以下と、S:0.002%以下と、Nb:0.005〜0.05%と、Ti:0.005〜0.02%と、Al:0.01〜0.07%と、Ca:0.0005〜0.004%とを含有し、かつ炭素当量:Ceq≧0.28%であり、残部がFe及び不可避的不純物からなり、さらに取鍋精錬時のスラグのトータルFe+MnOが0.5〜3%を満足する鋼板を製造する方法において、
該鋼を1000〜1200℃に加熱して熱間圧延する工程と、
熱間圧延した後の鋼板を、鋼板表面温度で500℃以下となるまで加速冷却した後、一旦冷却を中断し、鋼板表面温度が500℃以上になるまで復熱させる工程と、
鋼板表面温度が500℃以上になるまで復熱させた鋼板を、再び復熱時の鋼板表面温度以下で600℃以下の鋼板表面温度まで、3〜50℃/秒の鋼板平均冷却速度で加速冷却する工程と、
を備えたことを特徴とする、耐HIC性に優れた高強度ラインパイプ用鋼板の製造方法である。
【0010】
但し、炭素当量:Ceq=C%+Mn%/6+(Cu%+Ni%)/15+(Cr%+Mo%+V%)/5
(2)本発明の製造方法は、鋼成分として、質量%でさらに、Cu:0.5%以下、Ni:0.5%以下、Cr:0.5%以下、Mo:0.5%以下、及びV:0.1%以下の群から選択された1種または2種以上を含有することを特徴とする、請求項1に記載の耐HIC性に優れた高強度ラインパイプ用鋼板の製造方法である。
【0011】
【発明の実施の形態】
本発明者らは、上記の課題を解決すべく、添加元素と熱処理条件を変化させて種々の成分系・ミクロ組織を有する母材を作成し、耐HIC性と強度、靭性とを調べた。
【0012】
その結果、添加元素量ならびに式(1)で示される炭素当量、取鍋スラグのトータルFe+MnOとを規定した鋼に間欠冷却型の加速冷却を行うことにより表層部の硬さの上昇を抑え、高強度と靭性、良好な耐HIC性能が得られることが分かった。
【0013】
炭素当量:Ceq=C%+Mn%/6+(Cu%+Ni%)/15+(Cr%+Mo%+V%)/5 …(1)
以上の知見に基づき、本発明者らは、特定量の化学成分を有し、取鍋スラグのトータルFe+MnOを規定した鋼の熱間圧延条件、冷却中断、復熱工程を含む加速冷却条件を一定範囲内に制御するようにして、耐HIC性に優れた高強度ラインパイプ用鋼の製造方法を見出し、本発明を完成させた。
すなわち、本発明は、鋼組成及び製造条件を下記範囲に限定することにより、耐HIC性に優れたAPI規格X80グレードのラインパイプ用鋼板を安価にかつ安定して製造する方法を提供することができる。
【0014】
以下に、本発明の成分添加理由、成分限定理由、及び製造条件の限定理由について説明する。
(1)成分組成範囲
C:0.03〜0.08%
Cは鋼の強化元素として必要でありX65からX80の所定の強度を確保するためには0.03%以上の含有が必要である。一方、0.08%を超える過剰なCの含有は鋼板の靭性と耐HIC性の劣化を招くので0.08%以下とする必要があり、溶接性や耐硫化物応力腐食割れ性の観点からもC量の低減が望ましいため、上限は0.08%である。
Si:0.05〜0.5%
Siは脱酸のために添加され、0.05%未満では充分な脱酸効果が得られず、一方0.5%を越えると靭性や溶接性の劣化を引き起こすため、0.05〜0.5%である。
【0015】
Mn:1〜1.8%
Mnは鋼の強度および靭性の向上に有効な鋼の基本元素として添加されるが、1%未満ではその効果が小さく、また1.8%を越えると溶接性と耐HIC性が著しく劣化するため、1〜1.8%である。
【0016】
P:0.01%以下
本発明鋼の場合、Pは溶接性と耐HIC性とを劣化させる不純物元素であり極力低減することが望ましいが、過度の脱Pはコスト上昇を招くため上限は0.01%である。
S:0.002%以下
Caを添加してMnSからCaS系の介在物に形態制御を行ったとしても、X80グレードの高強度材の場合には微細に分散したCaS系介在物も割れの起点となり得るために、S含有量を0.002%以下に低減する必要がある。
【0017】
Nb:0.005〜0.05%
Nbは圧延時や焼入れ時の粒成長を抑制することによりミクロ組織を微細化し、ラインパイプとして充分な靭性を付与するために必要な成分である。0.005%以上でその効果が顕著であり、0.05%を超えるとその効果がほぼ飽和して溶接熱影響部の靭性を劣化させるため、0.005〜0.05%である。
【0018】
Ti:0.005〜0.02%
TiはTiNを形成してスラブ加熱時と焼入れ時の粒成長を抑制し、結果としてミクロ組織の微細化をもたらして靭性を改善する効果があるが、その効果は0.005%以上で現われ、0.02%を越えると逆に靭性の劣化を引き起こすため、0.005〜0.02%である。
Al:0.01〜0.07%
Alは脱酸剤として添加され、0.01%以上でその効果が顕著であり、0.07%を超えると清浄度が低下して耐HIC性の劣化を引き起こすため、0.01〜0.07%である。
【0019】
Ca:0.0005〜0.004%
Caは硫化物系介在物の形態制御に不可欠な元素であり、0.0005%以上でその効果が現われ、0.004%を超えると効果が飽和し、逆に清浄度を低下させて耐HIC性を劣化させるため、0.0005〜0.004%である。
炭素当量:Ceq≧0.28%,但し、炭素当量:Ceq=C%+Mn%/6+(Cu%+Ni%)/15+(Cr%+Mo%+V%)/5
炭素当量CeqはX65からX80としての充分な強度を得るために0.28%以上が必要であるので、その下限は0.28%である。その上限は特に限定しない。なおCeqは次式で示される。
【0020】
Ceq=C%+Mn%/6+(Cu%+Ni%)/15+(Cr%+Mo%+V%)/5
トータルFe+MnO:0.5〜3%
取鍋精錬時のスラグのトータルFe+MnOが3%を超えると鋼板表面近傍でのHICが発生するので上限は3%である。また、0.5%を下回るようにトータルFe+MnOを制御することは経済性を阻害する。
本発明では上記の成分以外に、必要に応じて以下の選択成分群から選択された1種または2種以上を含有してもよい。
【0021】
(選択成分群)
Cu:0.5%以下
Cuは靭性の改善と強度の上昇に有効な元素の1つであるが,0.5%を超えるCuの含有は溶接性を阻害するため、添加する場合には0.5%以下に限定されなければならない。
【0022】
Ni:0.5%以下
Niは靭性の改善と強度の上昇に有効な元素の1つであるが、0.5%を超えると効果が飽和して応力腐食割れが発生しやすくなるため、添加する場合は0.5%以下である。
【0023】
Mo:0.5%以下
Moは靭性の改善と強度の上昇に有効な元素の1つであるが、0.5%を超えると効果が飽和し、溶接性や耐HIC性を阻害するため、添加する場合は0.5%以下である。
Cr:0.5%以下
CrはMnとともに低CでもX80グレードとして充分な強度を得るために有効な元素であるが、0.5%を超えて添加すると溶接性に悪影響を与えるため、上限は0.5%である。
V:0.1%以下
適量のVの添加は靭性・溶接性や耐サワー性を劣化させずに強度を高めるため、Crとともに低CでもX80グレードとして充分な強度を得るために有効な任意添加元素であるが、0.1%を越えると溶接性を著しく損なうため0.1%以下である。
上記の成分組成範囲に調整することにより、良好な耐HIC性に加えて、良好な靭性も有するX65,X70,X80グレードといった高強度鋼板を得ることが可能となる。
【0024】
このような特性の鋼板は以下の製造方法により製造することができる。
(2)鋼板製造工程
(製造方法)
上記の成分組成範囲に調整した鋼を溶製し、連続鋳造で得られた鋼スラブを1000〜1200℃に加熱して熱間圧延し、鋼板表面温度で500℃以下となるまで加速冷却した後、一旦冷却を中断し、鋼板表面温度が500℃以上になるまで復熱させる。次いで、再び600℃以下の鋼板表面温度まで、3〜50℃/秒の平均冷却速度で加速冷却する。
【0025】
a.スラブ加熱温度
スラブ加熱温度が1000℃を下回ると充分な強度が得られない。またスラブ加熱温度が1200℃を超えると良好な靭性が得られない。従って、スラブ加熱温度は1000〜1200℃である。また、熱間圧延終了温度はAr3 変態温度以上であることが望ましい。
b.加速冷却開始温度
加速冷却開始温度が低いと復熱に時間を要する、また、耐HIC性も劣化するので750℃+300/t以上が望ましい。ここでtは鋼板板厚(mm)。
【0026】
c.加速冷却中断表面温度
加速冷却中断時の表面温度が500℃を上回ると、表面近傍での変態が充分に進行しておらず、復熱後の急冷時にべイナイトなどに変態し硬化してしまう。従って加速冷却中断時の表面温度は500℃以下である。
d.表面復熱温度
表面復熱温度が500℃未満では、500℃以下まで冷却した時に変態した表層部分の硬さが低下せずHICの発生原因となるので表面復熱温度は500℃以上である。
【0027】
e.加速冷却停止温度
加速冷却停止温度が表面温度で600℃を上回ると充分な強度が得られない場合がある。従って、加速冷却停止温度は600℃以下である。
f.冷却速度
鋼板の平均冷却速度が3℃/秒未満になると充分な強度が得られない場合がある。また、50℃/秒を超えると強度が上昇し耐HIC性の劣化をまねく。従って、冷却速度は3〜50℃/秒である。
【0028】
上記条件を満たすかぎりその他の鋼板の圧延条件は特に規定しない。
また鋼管の成形方法も冷間であるかぎり特に規定しない。
以下に本発明の実施例を挙げ、本発明の効果を立証する。
【0029】
【実施例】
表1にその化学成分を示した鋼(A〜K:本発明鋼、L〜R:比較鋼)を表2に示した圧延加速冷却条件で熱間圧延した(A−4,7,B−1〜K−1:本発明鋼板、A−1〜3,5,6,8,9,L−1,2,M−1〜R−1:比較鋼板)。
鋼板の機械的性質(降伏強さ、引張強さ、靭性)、耐HIC性、及び溶接性を表2に示す。HIC試験はpHが約3の硫化水素を飽和させた5%NaCl+0.5%CH3 COOH水溶液(通称NACE溶液)中で行い、割れ長さ率(CLR)が15%以下で耐HIC性は良好と判断した。靭性はシャルピー衝撃試験での破面遷移温度が−60℃以下の場合良好とした。強度は降伏強さが448MPa以上で良好と判断した。また、溶接性は実鋼管のシーム溶接に相当するサブマージアーク溶接を行ない、溶接高温割れ、低温割れの有無を溶接部の断面観察により調査した。溶接部に割れの発生の無い場合を、溶接性は良好と判断した。
本発明の鋼に本発明の圧延加速冷却処理を行った本発明鋼板A−4,A−7,B−1,C−1,D−1,E−1,F−1,G−1,H−1,I−1,J−1,K−1ではいずれも充分な強度と良好な耐HIC性能が得られた。一方、本発明の鋼を用いても本発明の圧延加速冷却を行わない比較鋼板A−1,A−2,A−3,A−5,A−6,A−8,A−9では充分な性能が得られていない。また、本発明でない鋼に本発明の圧延加速冷却を行った比較鋼板L−1,Q−1,R−1あるいは、本発明でない鋼に本発明でない圧延加速冷却を行った比較鋼板L−2,M−1,N−1,O−1,P−1では充分な性能が得られていない。
【0030】
【表1】
【0031】
【表2】
【0032】
【発明の効果】
以上説明したように、本発明によれば鋼組成及び製造条件を特定することにより、耐HIC性に優れたAPI規格X80グレードのラインパイプ用鋼板を安価にかつ安定して製造することが可能となった。
[0001]
BACKGROUND OF THE INVENTION
The present invention is manufactured by a thick plate mill or a hot rolling mill, formed into a tubular shape by UOE forming, press bend forming, roll forming, etc., welded and joined by submerged arc welding, electric seam welding, etc. It relates to a method of manufacturing steel sheets used as line pipes for transporting natural gas, and has excellent resistance to hydrogen-induced cracking and is used as a material for line pipes with a strength level of API standard X65 grade or higher. It relates to a manufacturing method.
[0002]
[Prior art]
In general, line pipes used for the transportation of crude oil and natural gas containing hydrogen sulfide, in addition to the properties required for pipelines such as strength, toughness and weldability, are also resistant to hydrogen-induced cracking resistance (HIC resistance) and stress resistance. So-called sour resistance such as corrosion cracking resistance (SSCC resistance) is required. Here, in the HIC, hydrogen ions generated by the corrosion reaction are adsorbed on the steel surface, penetrate into the steel as atomic hydrogen, diffuse around non-metallic inclusions such as MnS in the steel and hard second phase structure. It accumulates and cracks are caused by the internal pressure. For this reason, the following methods have been devised so far to prevent the occurrence of HIC.
(1) By reducing the S content in the steel and adding an appropriate amount of Ca, REM, etc., the formation of long extended MnS is suppressed, and the form is changed to finely dispersed spherical inclusions with low stress concentration. This suppresses the generation and propagation of cracks (for example, JP-A No. 54-110119).
(2) Regarding cracks at the central segregation part, in order to suppress the formation of island-like martensite that can be the starting point, and to suppress the formation of hardened structures such as martensite and bainite that are likely to be propagation paths of cracks, Reduce the content of elements with a high segregation tendency, such as C, Mn, P, etc., perform soaking treatment to eliminate segregation of alloy elements in the slab heating stage before rolling, or transformation during cooling after rolling Accelerated cooling is performed in order to prevent formation of a hardened structure due to C diffusion in the middle (for example, Japanese Patent Laid-Open Nos. 61-60866 and 61-165207).
[0003]
(3) A microstructure with low cracking sensitivity is obtained, for example, by performing heat treatment such as quenching and tempering or setting the rolling finishing temperature to be higher than the recrystallization temperature of austenite (Japanese Patent Laid-Open Nos. 54-12882 and 62- 7819, JP-A-6-73450).
(4) By adding Cu to the steel, a protective film is formed on the surface to suppress hydrogen intrusion into the steel (Japanese Patent Laid-Open No. 52-111815).
[0004]
By adopting these methods, HIC resistance has been improved, and line pipes that require sour resistance have been mass-produced up to API standard X65 grade.
[0005]
However, in recent years, the demand for higher-strength steel pipes has increased in order to increase transportation efficiency and reduce laying costs, and line pipes used in sour environments may be required to have high strength up to X80 grade. Sex came out. However, since HIC is likely to be generated with an increase in strength, the methods (1) to (4) cannot completely suppress the generation of HIC.
When such a high-strength material is used, cracks also occur from inclusions that have been subjected to form control in (1) above, and cracks occur in parts other than the central part that has been subjected to the central segregation countermeasure in (2). To come. Moreover, the structure control by quenching and tempering treatment (3) and recrystallization temperature range finishing is not suitable for mass production of line pipes from the viewpoint of cost and efficiency, and sufficient low temperature toughness is difficult to obtain. Furthermore, the effect of the Cu coating of (4) cannot be expected in an environment having a low pH, and a 5% NaCl + 0.5% CH 3 COOH aqueous solution (commonly known as NACE solution) in which hydrogen sulfide having a pH of about 3 is actually saturated. ), The film effect is not obtained.
[0006]
Recently, several methods for producing steel plates for X80 grade line pipes having sour resistance have been disclosed to cope with such problems. The essence is that the inclusions are controlled by addition of low S and Ca, and the central segregation is suppressed as low C and low Mn, and the resulting strength reduction is Cr addition (JP-A-5-9575), Cr- This is supplemented by Mo addition (Japanese Patent Laid-Open Nos. 5-271766 and 7-109519), Ni-Cr-Mo addition (Japanese Patent Laid-Open No. 7-173536) and accelerated cooling after rolling.
[0007]
[Problems to be solved by the invention]
However, any of the manufacturing techniques (JP-A-5-9575, JP-A-5-271766, JP-A-7-109519, JP-A-7-173536) related to the above-described X80 manufacturing method are all used. It is a method for preventing the occurrence of HIC in the central segregation part, and the prevention of HIC generated in parts other than the central segregation part is not a specific countermeasure against cracking. That is, when the strength level of the steel pipe used in the sour environment increases, HIC tends to occur even if the form control of the inclusions in the steel plate as the material and the structure control of the central segregation part are performed, and particularly accelerated cooling is applied. With the material, the hardness near the surface increases, and HIC tends to occur. Preventing the occurrence of HIC near the surface is a major issue.
[0008]
An object of the present invention is to provide a method for economically and stably producing a steel material excellent in HIC resistance using high strength materials such as X70 and X80 in addition to API standard X65.
[0009]
[Means for Solving the Problems]
In order to solve the above problems and achieve the object, the present invention uses the following means.
(1) The manufacturing method of this invention is mass %, C: 0.03-0.08%, Si: 0.05-0.5%, Mn: 1-1.8%, P: 0.01% or less, S: 0.002% or less, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.02%, Al: 0.01 to 0.07% And Ca: 0.0005-0.004%, carbon equivalent: Ceq ≧ 0.28%, the balance is made of Fe and inevitable impurities, and the total Fe + MnO of slag during ladle refining In the method of manufacturing a steel sheet satisfying 0.5 to 3%,
Heating the steel to 1000-1200 ° C. and hot rolling;
The steel sheet after hot rolling is accelerated and cooled until the steel sheet surface temperature reaches 500 ° C. or lower, and then the cooling is temporarily interrupted and reheated until the steel sheet surface temperature reaches 500 ° C. or higher.
The steel plate reheated until the steel plate surface temperature reaches 500 ° C. or higher is accelerated and cooled again at a steel plate average cooling rate of 3 to 50 ° C./sec. And a process of
It is a manufacturing method of the steel plate for high strength line pipes excellent in HIC resistance characterized by having provided.
[0010]
However, carbon equivalent: Ceq = C% + Mn% / 6 + (Cu% + Ni%) / 15+ (Cr% + Mo% + V%) / 5
(2) The production method of the present invention further includes, as a steel component, in mass %, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less. And V: containing one or more selected from the group of 0.1% or less, production of a steel plate for high-strength line pipe excellent in HIC resistance according to claim 1 Is the method.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
In order to solve the above-mentioned problems, the present inventors made base materials having various component systems and microstructures by changing additive elements and heat treatment conditions, and examined HIC resistance, strength, and toughness.
[0012]
As a result, an increase in the hardness of the surface layer portion is suppressed by performing intermittent cooling type accelerated cooling on the steel in which the amount of added element and the carbon equivalent represented by the formula (1) and the total Fe + MnO of the ladle slag are defined. It was found that strength and toughness and good HIC resistance were obtained.
[0013]
Carbon equivalent: Ceq = C% + Mn% / 6 + (Cu% + Ni%) / 15+ (Cr% + Mo% + V%) / 5 (1)
Based on the above knowledge, the present inventors have fixed a constant accelerated cooling condition including a hot rolling condition, a cooling interruption, and a recuperation process of steel having a specific amount of chemical components and defining the total Fe + MnO of ladle slag. The present invention was completed by finding a method for producing steel for high-strength line pipe excellent in HIC resistance by controlling within the range.
That is, the present invention provides a method for stably and inexpensively manufacturing a steel plate for API pipes of API standard X80 grade having excellent HIC resistance by limiting the steel composition and manufacturing conditions to the following ranges. it can.
[0014]
Below, the reason for component addition of the present invention, the reason for component limitation, and the reason for limitation of production conditions will be described.
(1) Component composition range C: 0.03 to 0.08%
C is necessary as a strengthening element of steel, and in order to ensure a predetermined strength of X65 to X80, it is necessary to contain 0.03% or more. On the other hand, the excessive C content exceeding 0.08% causes deterioration of the toughness and HIC resistance of the steel sheet, so it is necessary to be 0.08% or less, from the viewpoint of weldability and resistance to sulfide stress corrosion cracking. However, since it is desirable to reduce the amount of C, the upper limit is 0.08%.
Si: 0.05-0.5%
Si is added for deoxidation. If it is less than 0.05%, a sufficient deoxidation effect cannot be obtained. On the other hand, if it exceeds 0.5%, deterioration of toughness and weldability is caused. 5%.
[0015]
Mn: 1 to 1.8%
Mn is added as a basic element of steel effective in improving the strength and toughness of steel. However, if it is less than 1%, its effect is small, and if it exceeds 1.8%, weldability and HIC resistance are remarkably deteriorated. 1 to 1.8%.
[0016]
P: 0.01% or less In the case of the steel of the present invention, P is an impurity element that deteriorates weldability and HIC resistance, and it is desirable to reduce it as much as possible. 0.01%.
S: Even when 0.002% or less of Ca is added to control the morphology of MnS to CaS inclusions, in the case of X80 grade high strength materials, finely dispersed CaS inclusions are also the origin of cracking. Therefore, it is necessary to reduce the S content to 0.002% or less.
[0017]
Nb: 0.005 to 0.05%
Nb is a component necessary to refine the microstructure by suppressing grain growth during rolling or quenching and to impart sufficient toughness as a line pipe. The effect is significant at 0.005% or more, and when it exceeds 0.05%, the effect is almost saturated and the toughness of the weld heat affected zone is deteriorated, so 0.005 to 0.05%.
[0018]
Ti: 0.005-0.02%
Ti forms TiN and suppresses grain growth during slab heating and quenching, and as a result, it has the effect of improving the toughness by reducing the microstructure, but the effect appears at 0.005% or more, On the other hand, if it exceeds 0.02%, the toughness is deteriorated, so 0.005 to 0.02%.
Al: 0.01 to 0.07%
Al is added as a deoxidizing agent, and the effect is remarkable at 0.01% or more. When it exceeds 0.07%, the cleanliness is lowered and the HIC resistance is deteriorated. 07%.
[0019]
Ca: 0.0005 to 0.004%
Ca is an element indispensable for controlling the form of sulfide inclusions, and its effect appears at 0.0005% or more, and when it exceeds 0.004%, the effect is saturated. In order to deteriorate the property, the content is 0.0005 to 0.004%.
Carbon equivalent: Ceq ≧ 0.28%, where carbon equivalent: Ceq = C% + Mn% / 6 + (Cu% + Ni%) / 15+ (Cr% + Mo% + V%) / 5
The carbon equivalent Ceq needs to be 0.28% or more in order to obtain a sufficient strength as X65 to X80, so the lower limit is 0.28%. The upper limit is not particularly limited. Ceq is expressed by the following equation.
[0020]
Ceq = C% + Mn% / 6 + (Cu% + Ni%) / 15+ (Cr% + Mo% + V%) / 5
Total Fe + MnO: 0.5-3%
If the total Fe + MnO of the slag during ladle refining exceeds 3%, HIC is generated near the steel sheet surface, so the upper limit is 3%. Moreover, controlling total Fe + MnO to be less than 0.5% hinders economic efficiency.
In this invention, you may contain the 1 type (s) or 2 or more types selected from the following selected component groups as needed other than said component.
[0021]
(Selected ingredient group)
Cu: 0.5% or less Cu is one of the elements effective for improving toughness and increasing strength, but inclusion of Cu exceeding 0.5% inhibits weldability, so when added, it is 0. Must be limited to 5% or less.
[0022]
Ni: 0.5% or less Ni is one of the elements effective in improving toughness and increasing strength. However, if it exceeds 0.5%, the effect is saturated and stress corrosion cracking is likely to occur. When it is, it is 0.5% or less.
[0023]
Mo: 0.5% or less Mo is one of the elements effective for improving toughness and increasing strength. However, if it exceeds 0.5%, the effect is saturated, and weldability and HIC resistance are impaired. When adding, it is 0.5% or less.
Cr: 0.5% or less Cr is an effective element for obtaining sufficient strength as X80 grade even at low C together with Mn, but if added over 0.5%, the weldability is adversely affected, so the upper limit is 0.5%.
V: 0.1% or less Appropriate amount of V is added to increase strength without deteriorating toughness, weldability and sour resistance, and is an optional addition effective for obtaining sufficient strength as X80 grade with Cr as well as low C Although it is an element, if it exceeds 0.1%, weldability is remarkably impaired, so it is 0.1% or less.
By adjusting to the above component composition range, it becomes possible to obtain high-strength steel sheets such as X65, X70, and X80 grades having good toughness in addition to good HIC resistance.
[0024]
A steel plate having such characteristics can be manufactured by the following manufacturing method.
(2) Steel plate manufacturing process (manufacturing method)
After melting the steel adjusted to the above component composition range, heating the steel slab obtained by continuous casting to 1000 to 1200 ° C, hot rolling, and accelerated cooling until the steel sheet surface temperature becomes 500 ° C or less Then, the cooling is temporarily interrupted, and reheating is performed until the steel sheet surface temperature is 500 ° C. or higher. Next, accelerated cooling is performed again at an average cooling rate of 3 to 50 ° C./second until the steel sheet surface temperature is 600 ° C. or less.
[0025]
a. Slab heating temperature If the slab heating temperature is below 1000 ° C, sufficient strength cannot be obtained. Moreover, when the slab heating temperature exceeds 1200 ° C., good toughness cannot be obtained. Therefore, the slab heating temperature is 1000 to 1200 ° C. Further, it is desirable that the hot rolling end temperature is equal to or higher than the Ar 3 transformation temperature.
b. Accelerated cooling start temperature When the accelerated cooling start temperature is low, it takes time for recuperation, and the HIC resistance is also deteriorated, so 750 ° C. + 300 / t or more is desirable. Here, t is a steel plate thickness (mm).
[0026]
c. Accelerated cooling interrupted surface temperature When the surface temperature at the time of interrupting accelerated cooling exceeds 500 ° C., the transformation in the vicinity of the surface does not proceed sufficiently, and transforms into bainite or the like during rapid cooling after recuperation and hardens. Therefore, the surface temperature when accelerating cooling is interrupted is 500 ° C. or less.
d. Surface recuperation temperature If the surface recuperation temperature is less than 500 ° C, the hardness of the surface layer portion transformed when cooled to 500 ° C or less does not decrease and causes HIC generation, so the surface recuperation temperature is 500 ° C or more.
[0027]
e. Accelerated cooling stop temperature When the accelerated cooling stop temperature exceeds 600 ° C. at the surface temperature, sufficient strength may not be obtained. Therefore, the accelerated cooling stop temperature is 600 ° C. or lower.
f. Cooling rate If the average cooling rate of the steel sheet is less than 3 ° C / second, sufficient strength may not be obtained. On the other hand, if it exceeds 50 ° C./second, the strength increases and the HIC resistance is deteriorated. Therefore, the cooling rate is 3 to 50 ° C./second.
[0028]
As long as the above conditions are satisfied, other rolling conditions for the steel sheet are not particularly specified.
Also, the method for forming the steel pipe is not particularly limited as long as it is cold.
Examples of the present invention will be given below to prove the effects of the present invention.
[0029]
【Example】
Steels whose chemical components are shown in Table 1 (AK: invention steel, LR: comparative steel) were hot-rolled under the rolling accelerated cooling conditions shown in Table 2 (A-4, 7, B-). 1 to K-1: steel plate of the present invention, A-1 to 3, 5, 6, 8, 9, L-1, 2, M-1 to R-1: comparative steel plate).
Table 2 shows the mechanical properties (yield strength, tensile strength, toughness), HIC resistance, and weldability of the steel sheet. The HIC test is conducted in 5% NaCl + 0.5% CH 3 COOH aqueous solution (commonly called NACE solution) saturated with hydrogen sulfide having a pH of about 3, and the crack length ratio (CLR) is 15% or less and the HIC resistance is good. It was judged. Toughness was considered good when the fracture surface transition temperature in the Charpy impact test was −60 ° C. or lower. The strength was judged to be good when the yield strength was 448 MPa or more. In addition, the weldability was determined by performing submerged arc welding corresponding to seam welding of a real steel pipe, and examining the presence or absence of weld hot cracking and cold cracking by observing the cross section of the weld. The weldability was judged to be good when there was no crack in the weld.
Invented steel sheets A-4, A-7, B-1, C-1, D-1, E-1, F-1, G-1, With H-1, I-1, J-1, and K-1, sufficient strength and good HIC resistance were obtained. On the other hand, the comparative steel plates A-1, A-2, A-3, A-5, A-6, A-8, and A-9 that do not perform the rolling accelerated cooling of the present invention are sufficient even when the steel of the present invention is used. Performance has not been obtained. Moreover, the comparative steel plates L-1, Q-1, and R-1 that have been subjected to the rolling accelerated cooling of the present invention on the steel that is not the present invention, or the comparative steel plates L-2 that have been subjected to the rolling accelerated cooling that is not the present invention to the steel that is not the present invention , M-1, N-1, O-1, and P-1 do not provide sufficient performance.
[0030]
[Table 1]
[0031]
[Table 2]
[0032]
【The invention's effect】
As described above, according to the present invention, by specifying the steel composition and manufacturing conditions, it is possible to stably and inexpensively manufacture steel sheets for line pipes of API standard X80 grade having excellent HIC resistance. became.

Claims (2)

質量%で、C:0.03〜0.08%と、Si:0.05〜0.5%と、Mn:1〜1.8%と、P:0.01%以下と、S:0.002%以下と、Nb:0.005〜0.05%と、Ti:0.005〜0.02%と、Al:0.01〜0.07%と、Ca:0.0005〜0.004%とを含有し、かつ炭素当量:Ceq≧0.28%であり、残部がFe及び不可避的不純物からなり、さらに取鍋精錬時のスラグのトータルFe+MnOが0.5〜3%を満足する鋼板を製造する方法において、
該鋼を1000〜1200℃に加熱して熱間圧延する工程と、
熱間圧延した後の鋼板を、鋼板表面温度で500℃以下となるまで加速冷却した後、一旦冷却を中断し、鋼板表面温度が500℃以上になるまで復熱させる工程と、
鋼板表面温度が500℃以上になるまで復熱させた鋼板を、再び復熱時の鋼板表面温度以下で600℃以下の鋼板表面温度まで、3〜50℃/秒の鋼板平均冷却速度で加速冷却する工程と、
を備えたことを特徴とする、耐HIC性に優れた高強度ラインパイプ用鋼板の製造方法。
但し、炭素当量:Ceq=C%+Mn%/6+(Cu%+Ni%)/15+(Cr%+Mo%+V%)/5
In mass %, C: 0.03-0.08%, Si: 0.05-0.5%, Mn: 1-1.8%, P: 0.01% or less, S: 0 0.002% or less, Nb: 0.005-0.05%, Ti: 0.005-0.02%, Al: 0.01-0.07%, Ca: 0.0005-0. 004% and carbon equivalent: Ceq ≧ 0.28%, the balance consists of Fe and inevitable impurities, and the total Fe + MnO of slag during ladle refining satisfies 0.5 to 3% In the method of manufacturing a steel plate,
Heating the steel to 1000-1200 ° C. and hot rolling;
The steel sheet after hot rolling is accelerated and cooled until the steel sheet surface temperature reaches 500 ° C. or lower, and then the cooling is temporarily interrupted and reheated until the steel sheet surface temperature reaches 500 ° C. or higher.
The steel plate reheated until the steel plate surface temperature reaches 500 ° C. or higher is accelerated and cooled again at a steel plate average cooling rate of 3 to 50 ° C./sec. And the process of
The manufacturing method of the steel plate for high strength line pipes excellent in HIC resistance characterized by having provided.
However, carbon equivalent: Ceq = C% + Mn% / 6 + (Cu% + Ni%) / 15+ (Cr% + Mo% + V%) / 5
鋼成分として、質量%でさらに、Cu:0.5%以下、Ni:0.5%以下、Cr:0.5%以下、Mo:0.5%以下、及びV:0.1%以下の群から選択された1種または2種以上を含有することを特徴とする、請求項1に記載の耐HIC性に優れた高強度ラインパイプ用鋼板の製造方法。As a steel component, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, and V: 0.1% or less in mass % The manufacturing method of the steel plate for high strength line pipes excellent in HIC resistance of Claim 1 characterized by containing 1 type, or 2 or more types selected from the group.
JP08354198A 1998-03-30 1998-03-30 Manufacturing method of steel plate for high-strength line pipe with excellent HIC resistance Expired - Fee Related JP3941211B2 (en)

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