JP4123722B2

JP4123722B2 - High strength steel material for oil wells excellent in sulfide cracking resistance and method for producing the same

Info

Publication number: JP4123722B2
Application number: JP2000617218A
Authority: JP
Inventors: 俊治坂本; 均朝日
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Sumitomo Metal Industries Ltd
Priority date: 1999-05-06
Filing date: 2000-05-02
Publication date: 2008-07-23
Anticipated expiration: 2020-05-02
Also published as: MXPA01011029A; EP1197571A1; US20020104592A1; EP1197571A4; AR023265A1; WO2000068450A1

Description

【０００１】
【技術分野】
本発明は、油ガス井において使用される鋼管、例えばケーシング、チュービングやドリルパイプなど、硫化物環境に曝されて使用される高強度かつ硫化物割れ(Sulfide Stress Cracking、以後ＳＳＣと略す）に対する抵抗性に優れた高強度油井用鋼材に関する。
【０００２】
【背景技術】
硫化水素が含有される油ガス井において使用される鋼材にはＳＳＣ抵抗性が必要とされる。ＳＳＣの本質は水素脆化であり、鋼材の強度が高まるほど起き易くなるため、高強度かつ耐ＳＳＣ性を両立させることは困難であった。
【０００３】
このような背景の中で、耐ＳＳＣ性に優れ、かつ降伏強度を高める技術開発がなされてきている。例えば特公平６−104849号公報では、鋼成分や金属組織を最適化して耐ＳＳＣ性を確保しつつ高強度化を達成する技術が開発されているが、この技術で到達可能な強度レベルは降伏強度120ksiが限界のようである。また特開平４−66645 号公報では、CrとNiの相互作用に着目して、これらの含有量を制御することにより、高強度化してもＳＳＣ特性と靱性を確保する技術が開示されているが、この技術でも到達可能な強度レベルは降伏強度120ksiが限界のようである。
【０００４】
一方、昨今の油井管市場ニーズによれば、十分な耐ＳＳＣ性を保証し且つ降伏強度が125ksi程度を上回る新鋼種が要望されてきており、今後の需要増が見込まれている。
【０００５】
従って、従来の到達可能レベルである120ksiの降伏強度では今後のニーズ高度化への対応力は不十分であり、新たな鋼種開発が必要となってきている。
【０００６】
【発明の開示】
このような事情に鑑み、本発明の目的は、従来到達困難であった降伏強度120ksi以上の高強度を有し、かつ十分な耐ＳＳＣ性を発揮し得る鋼材を提供することにある。
【０００７】
本発明者は、前記課題の解決に向けて研究を重ねた。その結果、本発明を構成する上で必要かつ十分な知見を得るに至った。
【０００８】
鋼材強度が高くなると、ＳＳＣは粒界から生じる。このような破壊を抑制するには、先ず金属組織として均一なものでなければならない。120ksi以上の高強度と高度な耐ＳＳＣ性を得ようとすれば、金属組織としては焼戻マルテンサイトを活用する以外にないが、この組織は可及的に均一であることが必要である。
【０００９】
もし、金属組織に異なる特性を有する異相が含まれれば、この境界あるいは異相自体が破壊起点となり、十分な耐ＳＳＣ性が得られなくなる。この組織の均一性は焼入の状態でほぼ決定される。すなわち、異相が出現するかどうかは、鋼材全体において十分かつ均一な焼入マルテンサイト組織が得られるかどうかに依存する。完全マルテンサイト組織が望ましいことは言うまでもないが、厚肉材の場合や、後述の焼入性に寄与できる元素の含有量制限などを考慮して、本発明が目的とする高強度レベルにおいて必須となる条件を検討した結果を図１に示す。
【００１０】
図１は25mm肉厚の板材を 900〜930 ℃のオーステナイト域温度から水冷する焼入処理を行い、鋼材焼入端から最も離れた位置となる肉厚中心部と焼入端部直下となる板表面直下の硬さを測定し、両測定結果の比を以って焼入性を評価すると共に、肉厚中心部から採取した試験片を用いて耐ＳＳＣ性を評価したものである。
【００１１】
図１より、肉厚中心部の硬さが表面直下部の硬さの比で95％以上であれば、120ksiを超える高強度であっても十分な耐ＳＳＣ性が得られるとの知見を得た。また、25mm程度の厚みを有する鋼材を十分に焼き入れるには、Ｃ，Mn, Moの含有量から算出される指標β(2.7Ｃ＋Mn＋２Mo）の値が 2.0以上であることが必要との知見を得た。
【００１２】
異相を含まない均一金属組織を前提として、ＳＳＣ特性を支配する強度と合金元素含有量の関係を検討した結果、図２が得られた。すなわち、ＳＳＣ特性に影響を与えるMn, Ｐの含有量を工業的に達成可能な低レベルに制御した上で、Moの含有量と焼戻後の鋼材の降伏強度YSを指標としてプロットすると、極めて良く整理できる。すなわち、YS≧120ksiの高強度材において十分な耐ＳＳＣ性を確保するには、Mo含有量は少くとも 0.5％以上が必要であり、なおかつ降伏強度YSとの関係において記述される、α＝Mo（質量％）−0.15YS(ksi) が−18.9以上であることが必要であることを知見した。
【００１３】
さらに、Ｐを低レベルとし、MnとMoの影響を詳細に調べた結果、図３に示すように、両成分には相互作用があることを知見した。すなわち、Mn量が 0.3％を下回る領域では、耐ＳＳＣ性はMo量にのみ依存するが、Mn量が 0.3〜 0.5％の範囲ではMn量が多くなる分Mo量も多くする必要があり、さらにMn量が 0.5％を超えるとMo量を増加しても耐ＳＳＣ性は改善されないようになる。したがって、Moの有益性を最大限に活用するためには、Mn量は少なくとも 0.5％以下に制限するべきで、望ましくは 0.3％未満に抑えるべきである。
【００１４】
本発明は、以上の知見に基づいて構成したものであり、その要旨は以下の通りである。
【００１５】
（１）質量％で、Ｃ：0.10〜0.40％、Si≦0.5％、Mn≦0.5％、Ｐ≦0.015％、Ｓ≦0.0050％、Mo：1.0 〜 2.5 ％、Al：0.005〜0.1％、Ti：0.005〜0.1％かつＮ％の3.4倍以上、Nb：0.01〜0.1％、Ｎ≦0.01％、Ｂ：0.0005〜0.0050％を含有し、残部が Fe と不純物からなり、かつksiで表される降伏強度YSとMo量の関係が下記（１）式を満足し、かつＣ、Mn、Moの含有量バランスが下記（２）式を満足することを特徴とする耐硫化物割れ性に優れた高強度油井用鋼材。
α＝Mo−0.15YS≧−18.9 ・・・（１）
β＝ 2.7Ｃ＋Mn＋２Mo≧2.0 ・・・（２）
【００１６】
（２）質量％で、Ｃ：0.10〜0.40％、Si≦0.5％、Mn≦0.5％、Ｐ≦0.015％、Ｓ≦0.0050％、Mo：1.0〜2.5％、Al：0.005〜0.1％、Ti：0.005〜0.1％、かつＮ％の3.4倍以上、Nb：0.01〜0.1％、Ｎ≦0.01％、Ｂ：0.0005〜0.0050％を含有し、かつ ksiで表される降伏強度YSとMo量の関係が下記（１）式を満足し、かつＣ、Mn、Moの含有量バランスが下記（２）式を満足し、更に、Ｖ：0.01〜0.3％、Zr：0.001〜0.010％、Ca：0.001〜0.01％、Mg：0.001〜0.01％、REM：0.001〜0.01％のうち１種または２種以上を含有し、残部がFeと不純物からなることを特徴とする耐硫化物割れ性に優れた高強度油井用鋼材。
α＝Mo−0.15YS≧−18.9 ・・・（１）
β＝2.7Ｃ＋Mn＋２Mo≧2.0 ・・・（２）
【００１７】
（３）質量％で、Ｃ：0.10〜0.40％、Si≦0.5％、Mn＜0.3％、Ｐ≦0.015％、Ｓ≦0.0050％、Mo：1.0 〜 2.5 ％、Al：0.005〜0.1％、Ti：0.005〜0.1％かつＮ％の3.4倍以上、Nb：0.01〜0.1％、Ｎ≦0.01％、Ｂ：0.0005〜0.0050％を含有し、残部が Fe と不純物からなり、かつksiで表される降伏強度YSとMo量の関係が下記（１）式を満足し、かつＣ、Mn、Moの含有量バランスが下記（２）式を満足することを特徴とする耐硫化物割れ性に優れた高強度油井用鋼材。
α＝Mo−0.15YS≧−18.9 ・・・（１）
β＝ 2.7Ｃ＋Mn＋２Mo≧2.0 ・・・（２）
【００１８】
（４）質量％で、Ｃ：0.10〜0.40％、Si≦0.5％、Mn＜0.3％、Ｐ≦0.015％、Ｓ≦0.0050％、Mo：1.0〜2.5％、Al：0.005〜0.1％、Ti：0.005〜0.1％かつＮ％の3.4倍以上、Nb：0.01〜0.1％、Ｎ≦0.01％、Ｂ：0.0005〜0.0050％を含有し、かつksiで表される降伏強度YSとMo量の関係が下記（１）式を満足し、かつＣ、Mn、Moの含有量バランスが下記（２）式を満足し、更に、Ｖ：0.01〜0.3％、Zr：0.001〜0.010％、Ca：0.001〜0.01％、Mg：0.001〜0.01％、REM：0.001〜0.01％のうち１種または２種以上を含有し、残部がFeと不純物からなることを特徴とする耐硫化物割れ性に優れた高強度油井用鋼材。
α＝Mo−0.15YS≧−18.9 ・・・（１）
β＝2.7Ｃ＋Mn＋２Mo≧2.0 ・・・（２）
【００１９】
（５）質量％で、Ｃ：0.10〜0.40％、Si≦0.5％、Mn≦0.5％、Ｐ≦0.015％、Ｓ≦0.0050％、Mo ： 1.0 〜 2.5 ％、Al：0.005〜0.1％、Ti：0.005〜0.1％かつＮ％の3.4倍以上、Nb：0.01〜0.1％、Ｎ≦0.01％、Ｂ：0.0005〜0.0050％を含有し、残部が Fe と不純物からなり、かつksiで表される降伏強度YSとMo量の関係が下記（１）式を満足し、かつＣ、Mn、Moの含有量バランスが下記（２）式を満足し、120ksi以上の降伏強度を有することを特徴とする耐硫化物割れ性に優れた高強度油井用鋼材。
α＝Mo−0.15YS≧−18.9 ・・・（１）
β＝ 2.7Ｃ＋Mn＋２Mo≧2.0 ・・・（２）
【００２０】
（６）質量％で、Ｃ：0.10〜0.40％、Si≦0.5％、Mn≦0.5％、Ｐ≦0.015％、Ｓ≦0.0050％、Mo：1.0〜2.5％、Al：0.005〜0.1％、Ti：0.005〜0.1％かつＮ％の3.4倍以上、Nb：0.01〜0.1％、Ｎ≦0.01％、Ｂ：0.0005〜0.0050％を含有し、かつksiで表される降伏強度YSとMo量の関係が下記（１）式を満足し、かつＣ、Mn、Moの含有量バランスが下記（２）式を満足し、更に、Ｖ：0.01〜0.3％、Zr：0.001〜0.010％、Ca：0.001〜0.01％、Mg：0.001〜0.01％、REM：0.001〜0.01％のうち１種または２種以上を含有し、残部がFeと不純物からなり、120ksi以上の降伏強度を有することを特徴とする耐硫化物割れ性に優れた高強度油井用鋼材。
α＝Mo−0.15YS≧−18.9 ・・・（１）
β＝2.7Ｃ＋Mn＋２Mo≧2.0 ・・・（２）
【００２１】
（７）質量％で、Ｃ：0.10〜0.40％、Si≦0.5％、Mn＜0.3％、Ｐ≦0.015％、Ｓ≦0.0050％、Mo：1.0〜2.5％、Al：0.005〜0.1％、Ti：0.005〜0.1％かつＮ％の3.4倍以上、Nb：0.01〜0.1％、Ｎ≦0.01％、Ｂ：0.0005〜0.0050％を含有し、残部が Fe と不純物からなり、かつksiで表される降伏強度YSとMo量の関係が下記（１）式を満足し、かつＣ、Mn、Moの含有量バランスが下記（２）式を満足し、120ksi以上の降伏強度を有することを特徴とする耐硫化物割れ性に優れた高強度油井用鋼材。
α＝Mo−0.15YS≧−18.9 ・・・（１）
β＝ 2.7Ｃ＋Mn＋２Mo≧2.0 ・・・（２）
【００２２】
（８）質量％で、Ｃ：0.10〜0.40％、Si≦0.5％、Mn＜0.3％、Ｐ≦0.015％、Ｓ≦0.0050％、Mo：1.0〜2.5％、Al：0.005〜0.1％、Ti：0.005〜0.1％かつＮ％の3.4倍以上、Nb：0.01〜0.1％、Ｎ≦0.01％、Ｂ：0.0005〜0.0050％を含有し、かつksiで表される降伏強度YSとMo量の関係が下記（１）式を満足し、かつＣ、Mn、Moの含有量バランスが下記（２）式を満足し、更に、Ｖ：0.01〜0.3％、Zr：0.001〜0.010％、Ca：0.001〜0.01％、Mg：0.001〜0.01％、REM：0.001〜0.01％のうち１種または２種以上を含有し、残部がFeと不純物からなり、120ksi以上の降伏強度を有することを特徴とする耐硫化物割れ性に優れた高強度油井用鋼材。
α＝Mo−0.15YS≧−18.9 ・・・（１）
β＝2.7Ｃ＋Mn＋２Mo≧2.0 ・・・（２）
【００２３】
（９）請求項１〜８のいずれかの項に記載の成分を含有し、残部が Fe と不純物からなり、かつＣ、Mn、Moの含有量バランスが下記（２）式を満足した鋼を、熱間加工した後、Ac₃点＋20℃以上かつ1000℃以下の温度域に加熱してオーステナイト化した後、焼入処理を施し、この冷却表面から最も離れた鋼材位置における焼入ままの硬さが、冷却表面の硬さの95％以上となる金属組織とし、続いて620〜720℃の温度で焼き戻すことを特徴とする耐硫化物割れ特性に優れた高強度油井用鋼材の製造方法。
β＝2.7Ｃ＋Mn＋２Mo≧2.0 ・・・（２）
【００２４】
（10）NACE TMO177-A の定荷重式硫化物割れ試験によって求められる割れ発生限界応力が降伏強度の80％以上であることを特徴とする上記（１）〜（８）のいずれかに記載の耐硫化物割れ性に優れた高強度油井用鋼材。
【００２５】
【発明を実施するための最良の形態】
以下に本発明を詳細に説明する。
【００２６】
まず本発明の合金成分限定理由を述べる。成分の含有量は質量％である。
【００２７】
Ｃ：Ｃは目的の高強度と耐硫化物割れ性を同時に確保するために必須の元素である。これら強度と耐硫化物割れ性は、焼入性に依存しており、0.10％未満の含有量では、不完全焼入となるため強度は低下し、仮に焼戻条件を調節して所要強度を得たとしても十分な耐硫化物割れ性が得られない。一方、0.40％を超えて含有させても、耐硫化物割れ性は飽和すると共に焼き割れや置き割れの感受性が高くなる。このため、適正範囲を0.10〜0.40％とした。
【００２８】
Si：Siは製鋼工程における脱酸剤が残存したものであるが、 0.5％を超えて含有すると鋼が脆化し、耐硫化物割れ性も劣化するため、上限を 0.5％とした。
【００２９】
Mn：Mnは耐硫化物割れ性に有害な元素であり添加すべきでないが、焼入性を向上させる作用も有しており、焼入性を向上させるＣやMoの含有量が少なく焼入能が不十分な場合には、 0.5％を上限として含有させても良い。しかしながら 0.5％を超えて含有させると、完全焼入されても満足すべき耐硫化物割れ性は得られないので、 0.5％を上限とした。なお、Mnの好ましい含有量は 0.3％未満である。
【００３０】
Ｐ：Ｐは粒界に偏析して耐硫化物割れ性を劣化させる不純物元素であり、可及的低レベルに抑制すべきである。コストも加味した現状精錬技術で安定的に工業生産可能な許容レベルとして、 0.015％を上限とした。
【００３１】
Ｓ：Ｓも粒界に偏析して耐硫化物割れ性を劣化させる不純物元素である。本発明では、Ｓを固定するMnを含有させないのが基本であるため、可及的低レベルに抑制すべきであるが、0.0050％未満の範囲では著しい耐ＳＳＣ性の劣化が見られないことから、0.0050％を含有量の上限とした。
【００３２】
Mo：Moは本発明における必須元素の１つで、耐ＳＳＣ性に有害なＰの粒界偏析を抑制する元素であると共に、焼戻軟化抵抗を高めるので高強度を得るには格好の元素である。図２で示すように、YS≧120ksiの高強度領域で十分な耐ＳＳＣ性を確保するには少くとも 0.5％以上の含有が必要であり、YSが高いほどMo含有量も多く含有させる必要があり、好ましい範囲は 1.0％以上である。しかしながら、多量に含有させてもその効果は飽和すると共に、強度調整自由度が狭縮化するため、 2.5％を上限とする。
【００３３】
Al：Alは製鋼工程で鋼を十分に脱酸するために必要であり、 0.005％以上を含有させる。しかし多量に含有させるとアルミナ系介在物量が増えて、ＳＳＣ感受性が高まる危険性があるため、 0.1％を上限とした。
【００３４】
Ti：Tiは、後述のＢの焼入性を十分に奏効させるために含有させる。すなわち、BNの析出を防止するために、予めＮを TiNとして固定する必要があり、そのために 0.005％以上かつＮ含有量の 3.4倍量以上を含有させる。しかしながら、多量の含有は粗大な TiNの析出を助長してＳＳＣ感受性を高めるので、 0.1％を上限とした。
【００３５】
Nb：Nbは、その細粒化効果を通してＰの粒界偏析を軽減するため耐ＳＳＣ性改善に有効な元素であり、0.01％以上を含有させる。しかし、多量に含有させても細粒化効果は飽和し、むしろ炭化物粗大化による粒界強度低下によって耐ＳＳＣ性が低下するため、上限を 0.1％とした。
【００３６】
Ｎ：ＮはＢの焼入性を阻害する不純物元素であり、可及的低レベルに抑制すべきである。コストも加味した現状精錬技術で安定的に工業生産可能な許容レベルとして0.01％を上限とした。
【００３７】
Ｂ：Ｂは著しく焼入性を改善する元素であり、本発明における焼入性確保に必須の元素である。0.0005％未満の含有では、十分な焼入性が確保できないため、これを下限含有量とした。また、0.0050％を超えて含有させても焼入性改善効果は飽和しており、むしろ炭硼化物の析出が顕著になって耐ＳＳＣ性が劣化するため、上限含有量を0.0050％とした。
【００３８】
α＝Mo−0.15YS：図２に示すように、降伏強度YS(ksi) およびMo含有量（質量％）の関数として算出されるαの値が−18.9以上になると、優れた耐ＳＳＣ性が得られ、−18.9を下回ると、個々の成分が前記条件を満たしても満足すべき耐ＳＳＣ性が得られない。このことから、前記成分条件に加えてα≧−18.9を本発明の必須要件とした。
【００３９】
β＝ 2.7Ｃ＋Mn＋２Mo：前記成分条件に加えて、十分な焼入性を確保するために、Ｃ，Mn, Moといった焼入性に寄与する合金元素の含有量（質量％）から算出される指標βを、図１に示すように 2.0以上とした。なお、上限はＣ，Mn, Moの上限含有量から6.11と算出される。
【００４０】
本発明では、上記元素に加えて、必要に応じて以下のうち１種または２種以上を選択的に含有させる。
【００４２】
Ｖ：Ｖは焼戻軟化抵抗を高める作用があり、0.01％以上の含有で高強度化に有利であるが、多量に含有させると耐ＳＳＣ性が劣化するため、 0.3％を上限とした。
【００４３】
Zr：ZrはＰの粒界偏析を抑制する作用がある。そのためには 0.001％以上の含有が必要であるが、高価な元素であると共に多量に含有させると酸化物が増えてＳＳＣ感受性が高まる危険性があるため、0.010％を上限とした。
【００４４】
Ca, Mg, REM ：これらの元素は介在物の形状を球状化して応力集中を軽減すると共に、Ｓを固定してＳの粒界偏析を軽減する作用を有する。いずれも、 0.001％未満の含有では十分な効果が得られず、多すぎると酸化物が増えてＳＳＣ感受性を高める危険性があるため、 0.010％を上限とした。
【００４５】
本発明では、前記の合金組成を有する鋼を、転炉、電気炉などで溶製して鋳造し、通常の熱間圧延方法によって、管、板、棒などの所望の形状に造形した後、焼入、焼戻の熱処理を施して所望の強度に調質する。
【００４６】
本発明における熱処理条件について以下に説明する。
【００４７】
焼入のためのオーステナイト化温度は、 Ac_３点＋20℃以上かつ1000℃以下とする。 Ac_３点＋20℃未満では鋼材のオーステナイト化が不十分で均一マルテンサイト組織を得るのが困難である。一方、1000℃を上回ると粒成長が顕著となり粒界面積が減少するため、Ｐなどの偏析元素による耐ＳＳＣ性低減作用が発現されてしまう。したがって、適正なオーステナイト化温度条件として Ac_３＋20℃〜1000℃の範囲とした。なお、本発明の鋼材における代表的 Ac_３点は 830℃である。
【００４８】
さらに前述の如く、満足すべき耐ＳＳＣ性を得るには、このオーステナイト化温度から焼入れられた鋼材は組織均一性を有する必要があり、焼入端から最も離れた部位の硬さが焼入端直下の硬さとの比において95％以上の値を確保することが必要である。本発明の成分組成では、25mm程度の厚肉材においても所定の焼入性を確保できる。
【００４９】
焼戻の温度条件としては 620〜720 ℃を適正範囲とした。 620℃未満の低温条件ではYSが高くなり過ぎて前記αが低くなってしまうため、満足すべき耐ＳＳＣ性が得られない。一方、 720℃を超えると、２相領域に突入して組織均一性が崩れることによりＳＳＣ感受性が高まる危険性がある。このため、適正な焼戻温度条件として 620〜720 ℃を設定した。
【００５０】
上記の温度範囲において、例えば焼入れ温度を制御する等により、YSの値を所定の範囲とすることができ、α＝Mo−0.15YS≧−18.9を満足させ得る。
【００５１】
以上のように、金属組織および成分を適正化することにより、従来到達困難であったYS≧120ksiの高強度耐サワー鋼材が得られるが、YSが120ksi未満の強度領域に適用した場合、やはり耐ＳＳＣ性は改善される。図４は、本発明で規定する金属組織と成分の条件を満たし、YSを 117〜120ksiに調質した鋼材に対し、NACE TMO177-A の定荷重ＳＳＣ試験で付加応力を変化させて破断限界応力（σth）を求めた結果を示す。これより、本発明の構成因子はYS≧120ksiの領域のみならず、YSが120ksi未満の鋼材に対してもσthの改善という形で耐ＳＳＣ性を向上させていると言える。
【００５２】
【実施例】
表１に示す化学組成の鋼を真空溶解炉にて溶製し、得られた鋼塊を熱間圧延に供して肉厚25mmの板とした。この板を 900〜1025℃に加熱しオーステナイト化した後、水冷する焼入処理を行い、焼入ままで肉厚中心部と表面直下部の硬さを測定し、両者の比を求めて焼入性を評価すると共に、引き続いて 630〜730 ℃の温度で焼戻処理を行い、この板材の肉厚中心部から丸棒引張試験片を採取して引張試験を実施した。
【００５３】
また、併せて肉厚中心部より NACE-TM0177-Aに規定される平行部長さ25mm、直径 6.2mmの丸棒形状の硫化物割れ試験片を採取し、 0.5％酢酸＋５％NaClを含有し H_２Sを１気圧の分圧で飽和させた25℃の腐食溶液中において、降伏強度の80％の一定応力を付加したまま、 720時間の試験を実施した。必要に応じて、80％YSの付加応力で破断したものについては５％YSずつ付加応力を減じ、80％YSの付加応力で破断しなかったものについては５％YSずつ付加応力を増して追加試験を行い割れ発生限界付加応力（σth）を求めた。
【００５４】
焼入性は、肉厚中心部硬さが表面直下硬さの95％以上であれば良好と評価した。耐ＳＳＣ性は 720時間の試験時間中、破断しなかったものを良好と評価した。試験結果を表２および表３に示す。
【００５５】
表２における No.4 〜 10 、 No.101 、 No.102 および No.104は本発明の範囲に含まれる試験結果であるが、YS≧120ksiの高強度と、優れた耐ＳＳＣ性を同時に満足している。
【００５６】
一方、比較例の No.18から30およびNo.105〜107 は成分が本発明の範囲を外れるため、良好な耐ＳＳＣ性が得られていない。また、成分は本発明の範囲内であるにもかかわらず、比較例のNo.14 はαが本発明の範囲を外れており、比較例の No.15 では、それぞれ焼戻し温度、オーステナイト化温度が本発明の範囲を外れているため、十分な耐ＳＳＣ性が得られていない。
【００５７】
また、表３におけるNo.108〜109 は、YSが120ksi未満であるが、同等YSの比較例 No.113, 114に比べてσthが高く耐ＳＳＣ性が改善されている。本発明の No.110, 111はYSが120ksiを超えており、σthもYSの85％という高い値となっている。一方、比較例 No.112 はYSが低いにもかかわらずσthは YSの85％に止まっている。また、No.115では耐ＳＳＣ性は不十分なものとなっている。
【００５８】
【表１】

【００５９】
【表２】

【００６０】
【表３】

【００６１】
【産業上の利用の可能性】
以上のように本発明によれば、降伏強度120ksi以上の高強度と優れた耐ＳＳＣ性を両立させた油井用鋼材が得られる。
【図面の簡単な説明】
【図１】図１は、焼入ままの状態における表層直下部硬さに対する肉厚中心部硬さの比を鋼成分の焼入性指標βで整理すると共に、焼戻し後の鋼材の耐ＳＳＣ性とβの関係を示したものである。
【図２】図２は、焼入性が良好な鋼を対象として、耐ＳＳＣ性をMo含有量とYSの関係で示したものである。
【図３】図３は、YSが同等の鋼を対象として耐ＳＳＣ性をMnとMoの含有量に対して示した図である。
【図４】図４は、破断限界応力（σth）とYSの比を耐ＳＳＣ性の指標としてYSが変化した場合の耐ＳＳＣ性を示した図である。[0001]
【Technical field】
The present invention relates to steel pipes used in oil and gas wells, such as casings, tubing and drill pipes, which are used for high strength and resistance to sulfide cracking (Sulfide Stress Cracking, hereinafter abbreviated as SSC). The present invention relates to a high-strength steel material for oil wells with excellent properties.
[0002]
[Background]
Steel materials used in oil and gas wells containing hydrogen sulfide are required to have SSC resistance. The essence of SSC is hydrogen embrittlement, and it becomes easier to occur as the strength of the steel increases, so it was difficult to achieve both high strength and SSC resistance.
[0003]
Against this background, technical development has been made that is excellent in SSC resistance and increases the yield strength. For example, in Japanese Patent Publication No. 6-104849, a technology has been developed that achieves high strength while ensuring SSC resistance by optimizing the steel composition and metal structure. Strength 120ksi seems to be the limit. Japanese Patent Application Laid-Open No. 4-66645 discloses a technique for securing SSC characteristics and toughness even when the strength is increased by controlling the content of these elements by paying attention to the interaction between Cr and Ni. The strength level that can be reached with this technology seems to be limited to the yield strength of 120 ksi.
[0004]
On the other hand, according to recent needs for oil well pipe market, there is a demand for a new steel grade that guarantees sufficient SSC resistance and has a yield strength exceeding about 125 ksi, and future demand is expected to increase.
[0005]
Therefore, the yield strength of 120 ksi, which is a conventional reachable level, is insufficient to cope with future needs, and new steel types need to be developed.
[0006]
DISCLOSURE OF THE INVENTION
In view of such circumstances, an object of the present invention is to provide a steel material having a high strength of yield strength of 120 ksi or more, which has been difficult to achieve in the past, and capable of exhibiting sufficient SSC resistance.
[0007]
The present inventor has made researches to solve the above problems. As a result, the inventors have obtained knowledge necessary and sufficient for configuring the present invention.
[0008]
As the steel strength increases, SSC arises from the grain boundaries. In order to suppress such destruction, first, the metal structure must be uniform. In order to obtain a high strength of 120 ksi or higher and high SSC resistance, there is no use other than using tempered martensite as the metal structure, but this structure must be as uniform as possible.
[0009]
If the metal structure includes a different phase having different characteristics, this boundary or the different phase itself becomes a fracture starting point, and sufficient SSC resistance cannot be obtained. The uniformity of the structure is almost determined by the quenching state. That is, whether a heterogeneous phase appears depends on whether a sufficient and uniform quenched martensite structure can be obtained in the entire steel material. Needless to say, a complete martensite structure is desirable, but in the case of a thick material or in consideration of restrictions on the content of elements that can contribute to the hardenability described later, it is essential at the high strength level intended by the present invention. The results of studying the conditions are shown in FIG.
[0010]
Fig. 1 shows a plate with a thickness of 25mm thick, which is water-cooled from an austenite temperature of 900 to 930 ° C and is located farthest from the steel quenching end and directly below the quenching end. The hardness just under the surface is measured, and the hardenability is evaluated by the ratio of both measurement results, and the SSC resistance is evaluated by using a test piece taken from the center of the thickness.
[0011]
From FIG. 1, the knowledge that sufficient SSC resistance can be obtained even at high strength exceeding 120 ksi if the hardness at the center of the wall thickness is 95% or more in terms of the ratio of the hardness directly below the surface. It was. Moreover, in order to fully quench a steel material having a thickness of about 25 mm, the knowledge that the value of the index β (2.7C + Mn + 2Mo) calculated from the contents of C, Mn, and Mo is required to be 2.0 or more was obtained. It was.
[0012]
As a result of studying the relationship between the strength governing the SSC characteristics and the alloy element content on the premise of a uniform metal structure that does not include a heterogeneous phase, FIG. 2 was obtained. That is, when the contents of Mn and P that affect the SSC characteristics are controlled to a low level that can be achieved industrially, the Mo content and the yield strength YS of the steel after tempering are plotted as indices. Can be organized well. That is, in order to ensure sufficient SSC resistance in a high-strength material with YS ≧ 120 ksi, the Mo content needs to be at least 0.5% and is described in relation to the yield strength YS, α = Mo It was found that (% by mass) −0.15YS (ksi) needs to be −18.9 or more.
[0013]
Furthermore, as a result of investigating the influence of Mn and Mo in detail by setting P to a low level, it was found that both components interact as shown in FIG . That is, in the region where the Mn amount is less than 0.3%, the SSC resistance depends only on the Mo amount, but when the Mn amount is in the range of 0.3 to 0.5%, it is necessary to increase the Mo amount as the Mn amount increases. If the Mn content exceeds 0.5%, the SSC resistance will not be improved even if the Mo content is increased. Therefore, to maximize the benefits of Mo, the Mn content should be limited to at least 0.5% and preferably less than 0.3%.
[0014]
The present invention is configured based on the above knowledge, and the gist thereof is as follows.
[0015]
(1) By mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn ≦ 0.5%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5 % , Al: 0.005 to 0.1%, Ti: Yield strength expressed as ksi containing 0.005 to 0.1% and 3.4% or more of N%, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, the balance being Fe and impurities The relationship between YS and Mo content satisfies the following formula (1), and the C, Mn, Mo content balance satisfies the following formula (2). Steel material for oil wells.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)
[0016]
(2) By mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn ≦ 0.5%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: The relationship between yield strength YS and Mo content is 0.005 to 0.1% and more than 3.4% of N%, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, and expressed by ksi. It satisfies the following equation (1), and C, Mn, the content balance of Mo satisfies the following expression (2), a further, V: 0.01~0.3%, Zr: 0.001~0.010%, Ca: 0.001~ 0.01%, Mg: 0.001 to 0.01%, REM: One or more of 0.001 to 0.01%, containing the balance of Fe and impurities, high strength with excellent resistance to sulfide cracking Steel material for oil wells.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)
[0017]
(3) By mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn <0.3%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5 % , Al: 0.005 to 0.1%, Ti: Yield strength expressed as ksi containing 0.005 to 0.1% and 3.4% or more of N%, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, the balance being Fe and impurities The relationship between YS and Mo content satisfies the following formula (1), and the C, Mn, Mo content balance satisfies the following formula (2). Steel material for oil wells.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)
[0018]
(4) By mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn <0.3%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: The relationship between yield strength YS and Mo content is 0.005 to 0.1% and 3.4% or more of N%, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, and expressed in ksi. (1) satisfies the formula, and C, Mn, the content balance of Mo satisfies the following expression (2), a further, V: 0.01~0.3%, Zr: 0.001~0.010%, Ca: 0.001~0.01 %, Mg: 0.001 to 0.01%, REM: 0.001 to 0.01% of one or more, and the remainder is composed of Fe and impurities. High strength oil well with excellent resistance to sulfide cracking Steel material.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)
[0019]
(5) By mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn ≦ 0.5%, P ≦ 0.015%, S ≦ 0.0050%, Mo : 1.0 to 2.5 % , Al: 0.005 to 0.1%, Ti: Yield strength expressed as ksi containing 0.005 to 0.1% and 3.4% or more of N%, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, the balance being Fe and impurities The relationship between YS and Mo content satisfies the following formula (1), and the C, Mn, Mo content balance satisfies the following formula (2), and has a yield strength of 120 ksi or more. High-strength oil well steel with excellent cracking properties.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)
[0020]
(6) By mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn ≦ 0.5%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: The relationship between yield strength YS and Mo content is 0.005 to 0.1% and 3.4% or more of N%, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, and expressed in ksi. (1) satisfies the formula, and C, Mn, the content balance of Mo satisfies the following expression (2), a further, V: 0.01~0.3%, Zr: 0.001~0.010%, Ca: 0.001~0.01 %, Mg: 0.001 to 0.01%, REM: 0.001 to 0.01%, one or more of them, the balance being Fe and impurities, with a yield strength of 120 ksi or more High strength steel for oil wells with excellent cracking properties.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)
[0021]
(7) By mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn <0.3%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: Yield strength expressed as ksi containing 0.005 to 0.1% and 3.4% or more of N%, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, the balance being Fe and impurities The relationship between YS and Mo content satisfies the following formula (1), and the C, Mn, Mo content balance satisfies the following formula (2), and has a yield strength of 120 ksi or more. High-strength oil well steel with excellent cracking properties.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)
[0022]
(8) By mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn <0.3%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: The relationship between yield strength YS and Mo content is 0.005 to 0.1% and 3.4% or more of N%, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, and expressed in ksi. (1) satisfies the formula, and C, Mn, the content balance of Mo satisfies the following expression (2), a further, V: 0.01~0.3%, Zr: 0.001~0.010%, Ca: 0.001~0.01 %, Mg: 0.001 to 0.01%, REM: 0.001 to 0.01%, one or more of them, the balance being Fe and impurities, with a yield strength of 120 ksi or more High strength steel for oil wells with excellent cracking properties.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)
[0023]
(9) A steel containing the component according to any one of claims 1 to 8, the balance being Fe and impurities, and the balance of content of C, Mn, and Mo satisfying the following formula (2): After hot working, the steel is heated to a temperature range of Ac ₃ point + 20 ° C or higher and 1000 ° C or lower to austenite, then hardened, and hardened as-hardened at the position of the steel farthest from the cooling surface. Is a metal structure that is 95% or more of the hardness of the cooling surface, and subsequently tempered at a temperature of 620 to 720 ° C. .
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)
[0024]
(10) NACE TMO177-A the crack occurrence limit stress determined by the constant-loaded sulfide cracking test is characterized in that at least 80% of the yield strength of (1) to according to any one of (8) High-strength oil well steel with excellent sulfide cracking resistance.
[0025]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is described in detail below.
[0026]
First, the reasons for limiting the alloy components of the present invention will be described. The content of the component is mass% .
[0027]
C: C is an essential element for ensuring the desired high strength and resistance to sulfide cracking at the same time. These strengths and sulfide cracking resistance depend on hardenability. If the content is less than 0.10%, incomplete quenching results in lower strength. Temporarily adjusting the tempering conditions will reduce the required strength. Even if it is obtained, sufficient sulfide cracking resistance cannot be obtained. On the other hand, even if the content exceeds 0.40%, the resistance to sulfide cracking is saturated and the sensitivity to burning cracks and set cracks is increased. For this reason, the appropriate range was made 0.10 to 0.40%.
[0028]
Si: Si remains the deoxidizer in the steelmaking process, but if contained over 0.5%, the steel becomes brittle and the resistance to sulfide cracking deteriorates, so the upper limit was made 0.5%.
[0029]
Mn: Mn is an element harmful to sulfide cracking resistance and should not be added, but it also has the effect of improving hardenability and quenching with low C and Mo contents to improve hardenability. If the performance is insufficient, 0.5% may be included as the upper limit. However, if it exceeds 0.5%, satisfactory sulfide cracking resistance cannot be obtained even if it is completely quenched, so 0.5% was made the upper limit. A preferable content of Mn is less than 0.3%.
[0030]
P: P is an impurity element that segregates at the grain boundaries and degrades the resistance to sulfide cracking, and should be suppressed to the lowest possible level. The upper limit of 0.015% is the acceptable level that enables stable industrial production with the current refining technology, which also includes cost.
[0031]
S: S is also an impurity element that segregates at grain boundaries and degrades the resistance to sulfide cracking. In the present invention, since it is fundamental not to contain Mn for fixing S, it should be suppressed to the lowest possible level. However, in the range of less than 0.0050%, there is no significant deterioration in SSC resistance. 0.0050% was made the upper limit of the content.
[0032]
Mo: Mo is one of the essential elements in the present invention, and is an element that suppresses the grain boundary segregation of P, which is harmful to SSC resistance, and is a good element to obtain high strength because it increases resistance to temper softening. is there. As shown in Fig. 2, it is necessary to contain at least 0.5% or more in order to ensure sufficient SSC resistance in the high strength region of YS ≥ 120 ksi, and the higher the YS, the more Mo content must be contained. The preferred range is 1.0% or more. However, even if contained in a large amount, the effect is saturated and the degree of freedom in adjusting the strength is narrowed, so the upper limit is set to 2.5%.
[0033]
Al: Al is necessary for fully deoxidizing steel in the steel making process, and contains 0.005% or more. However, if it is contained in a large amount, there is a risk that the amount of alumina inclusions increases and the SSC sensitivity increases, so 0.1% was made the upper limit.
[0034]
Ti: Ti is contained in order to sufficiently achieve the hardenability of B described later. That is, in order to prevent the precipitation of BN, it is necessary to fix N as TiN in advance, so that 0.005% or more and 3.4 times or more of the N content are contained. However, a large content promotes coarse TiN precipitation and enhances SSC sensitivity, so the upper limit was made 0.1%.
[0035]
Nb: Nb is an element effective for improving the SSC resistance in order to reduce grain boundary segregation of P through its fine graining effect, and is contained in an amount of 0.01% or more. However, even if it is contained in a large amount, the effect of refining is saturated, and rather the SSC resistance is lowered due to the decrease in grain boundary strength due to coarsening of carbides, so the upper limit was made 0.1%.
[0036]
N: N is an impurity element that impairs the hardenability of B, and should be suppressed to the lowest possible level. The upper limit of 0.01% is the acceptable level that enables stable industrial production with the current refining technology, which also includes costs.
[0037]
B: B is an element that remarkably improves hardenability, and is an essential element for ensuring hardenability in the present invention. If the content is less than 0.0005%, sufficient hardenability cannot be secured, so this was made the lower limit content. Further, even if the content exceeds 0.0050%, the effect of improving hardenability is saturated, and rather the precipitation of carbon boride becomes remarkable and the SSC resistance deteriorates, so the upper limit content was made 0.0050%.
[0038]
α = Mo−0.15YS: As shown in FIG. 2, when the value of α calculated as a function of yield strength YS (ksi) and Mo content (mass%) is −18.9 or more, excellent SSC resistance is obtained. If it is less than −18.9, satisfactory SSC resistance cannot be obtained even if each component satisfies the above conditions. Therefore, in addition to the component conditions, α ≧ −18.9 is set as an essential requirement of the present invention.
[0039]
β = 2.7C + Mn + 2Mo: In addition to the above component conditions, in order to ensure sufficient hardenability, an index β calculated from the content ( mass %) of alloy elements contributing to hardenability such as C, Mn, Mo Was 2.0 or more as shown in FIG. The upper limit is calculated as 6.11 from the upper limit contents of C, Mn, and Mo.
[0040]
In this invention, in addition to the said element, 1 type (s) or 2 or more types are selectively contained among the following as needed.
[0042]
V: V has the effect of increasing the temper softening resistance, and if contained in an amount of 0.01% or more, it is advantageous for increasing the strength. However, if contained in a large amount, the SSC resistance deteriorates, so 0.3% was made the upper limit.
[0043]
Zr: Zr has the effect of suppressing the grain boundary segregation of P. For this purpose, a content of 0.001% or more is necessary. However, since it is an expensive element and contains a large amount, there is a risk that the oxides increase and the SSC sensitivity increases, so 0.010% was made the upper limit.
[0044]
Ca, Mg, REM: These elements have the effect of reducing the stress concentration by spheroidizing the shape of the inclusions, and reducing S grain boundary segregation by fixing S. In any case, if the content is less than 0.001%, a sufficient effect cannot be obtained, and if it is too much, there is a risk of increasing the oxide and increasing the SSC sensitivity, so 0.010% was made the upper limit.
[0045]
In the present invention, the steel having the above alloy composition is melted and cast in a converter, an electric furnace, etc., and shaped into a desired shape such as a tube, a plate, or a rod by a normal hot rolling method, Heat treatment such as quenching and tempering is performed to obtain a desired strength.
[0046]
The heat treatment conditions in the present invention will be described below.
[0047]
The austenitizing temperature for quenching is Ac ₃ points + 20 ° C to 1000 ° C. If it is less than Ac ₃ point + 20 ° C., it is difficult to obtain a uniform martensite structure due to insufficient austenitization of the steel material. On the other hand, when the temperature is higher than 1000 ° C., the grain growth becomes remarkable and the grain boundary area decreases, so that the SSC resistance reducing action by the segregating element such as P is exhibited. Therefore, the range of Ac _{3 +} 20 ℃ ~1000 ℃ as an appropriate austenitizing temperature conditions. The typical Ac ₃ point in the steel material of the present invention is 830 ° C.
[0048]
Further, as described above, in order to obtain satisfactory SSC resistance, the steel material quenched from this austenitizing temperature needs to have structural uniformity, and the hardness of the part farthest from the quenching end is the quenching end. It is necessary to ensure a value of 95% or more in the ratio with the hardness directly below. With the component composition of the present invention, a predetermined hardenability can be secured even with a thick material of about 25 mm.
[0049]
The appropriate temperature range for tempering was 620-720 ° C. Under low temperature conditions of less than 620 ° C., YS becomes too high and α becomes low, so that satisfactory SSC resistance cannot be obtained. On the other hand, when it exceeds 720 ° C., there is a risk that the SSC sensitivity is increased by entering the two-phase region and breaking the tissue uniformity. For this reason, 620-720 degreeC was set as appropriate tempering temperature conditions.
[0050]
In the above temperature range, for example, by controlling the quenching temperature, the value of YS can be made a predetermined range, and α = Mo−0.15YS ≧ −18.9 can be satisfied.
[0051]
As described above, by optimizing the metal structure and components, a high strength sour-resistant steel material with YS ≧ 120 ksi, which was difficult to achieve in the past, can be obtained, but when applied to a strength region where YS is less than 120 ksi, SSC properties are improved. Fig. 4 shows the fracture limit stress of steel material that satisfies the microstructure and component conditions specified in the present invention and is tempered with YS of 117 to 120 ksi by changing the applied stress in the constant load SSC test of NACE TMO177-A. The result of obtaining (σth) is shown. From this, it can be said that the constituent factor of the present invention improves the SSC resistance in the form of improvement of σth not only in the region of YS ≧ 120 ksi but also for steel materials with YS less than 120 ksi.
[0052]
【Example】
Steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace, and the obtained steel ingot was subjected to hot rolling to obtain a plate having a thickness of 25 mm. This plate is heated to 900-125 ° C to form austenite, and then it is quenched with water, and the hardness at the center of the thickness and directly below the surface is measured while quenching, and the ratio between the two is determined and quenched. In addition to evaluating the properties, a tempering treatment was subsequently performed at a temperature of 630 to 730 ° C., and a tensile test was conducted by collecting a round bar tensile test piece from the center of the thickness of the plate.
[0053]
At the same time, a round bar-shaped sulfide crack test piece with a parallel part length of 25 mm and a diameter of 6.2 mm specified in NACE-TM0177-A is collected from the center of the thickness, and contains 0.5% acetic acid + 5% NaCl. The test was conducted for 720 hours in a corrosive solution at 25 ° C. saturated with ₂ S at a partial pressure of 1 atm, with a constant stress of 80% of the yield strength applied. If necessary, the additional stress is reduced by 5% YS for those that break at 80% YS applied stress, and the additional stress is increased by 5% YS for those that do not break at 80% YS additional stress. Tests were conducted to determine the crack initiation critical additional stress (σth).
[0054]
The hardenability was evaluated as good when the thickness center hardness was 95% or more of the hardness just below the surface. The SSC resistance was evaluated as good if it did not break during the test time of 720 hours. The test results are shown in Tables 2 and 3.
[0055]
Nos . 4 to 10 , No. 101 , No. 102, and No. 104 in Table 2 are test results included in the scope of the present invention, and simultaneously satisfy high strength of YS ≧ 120 ksi and excellent SSC resistance. is doing.
[0056]
On the other hand, Nos. 18 to 30 and Nos. 105 to 107 of the comparative examples are out of the scope of the present invention, so that good SSC resistance is not obtained. Furthermore, even though components are within the scope of the present invention, No.14 of the comparative example α are outside the scope of the present invention, in No.15 of the comparative example, respectively tempering temperature, austenitizing temperature However, since it is out of the scope of the present invention, sufficient SSC resistance is not obtained.
[0057]
Further, in Nos. 108 to 109 in Table 3, YS is less than 120 ksi, but σth is higher and SSC resistance is improved as compared with Comparative Examples No. 113 and 114 of the equivalent YS. In Nos. 110 and 111 of the present invention, YS exceeds 120 ksi, and σth is as high as 85% of YS. On the other hand, in Comparative Example No. 112, σth is only 85% of YS although YS is low. In No. 115, the SSC resistance is insufficient.
[0058]
[Table 1]

[0059]
[Table 2]

[0060]
[Table 3]

[0061]
[Possibility of industrial use]
As described above, according to the present invention, it is possible to obtain an oil well steel material that achieves both high strength with a yield strength of 120 ksi or more and excellent SSC resistance.
[Brief description of the drawings]
FIG. 1 shows the ratio of the thickness center hardness to the hardness just below the surface layer in the as-quenched state by the hardenability index β of the steel component and the SSC resistance of the steel after tempering. Is the relationship between and β.
FIG. 2 is a graph showing SSC resistance in relation to Mo content and YS for steel with good hardenability.
FIG. 3 is a diagram showing the SSC resistance with respect to the contents of Mn and Mo for steels with equivalent YS.
FIG. 4 is a graph showing SSC resistance when YS changes using the ratio between the breaking limit stress (σth) and YS as an index of SSC resistance.

Claims

In mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn ≦ 0.5%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1 Ys and Mo containing 3.4% or more N% and N%, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, the balance being Fe and impurities, and expressed by ksi High-strength steel for oil wells with excellent resistance to sulfide cracking, characterized in that the relationship between the amounts satisfies the following formula (1) and the C, Mn, Mo content balance satisfies the following formula (2): .
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)

In mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn ≦ 0.5%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1 % And N% 3.4 times or more, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, and the relationship between the yield strength YS expressed by ksi and the Mo amount is (1) It satisfies the formula, and C, Mn, the content balance of Mo satisfies the following expression (2), a further, V: 0.01~0.3%, Zr: 0.001~0.010%, Ca: 0.001~0.01%, Mg : 0.001 to 0.01%, REM: High strength steel for oil wells with excellent resistance to sulfide cracking, characterized in that it contains one or more of 0.001 to 0.01% and the balance consists of Fe and impurities.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)

In mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn <0.3%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1 Ys and Mo containing 3.4% or more of N%, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, the balance being Fe and impurities, and expressed by ksi High-strength steel for oil wells with excellent resistance to sulfide cracking, characterized in that the relationship between the amounts satisfies the following formula (1) and the C, Mn, Mo content balance satisfies the following formula (2): .
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)

In mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn <0.3%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1 % And N% 3.4 times or more, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, and the relationship between the yield strength YS expressed by ksi and the Mo amount is (1) It satisfies the formula, and C, Mn, the content balance of Mo satisfies the following expression (2), a further, V: 0.01~0.3%, Zr: 0.001~0.010%, Ca: 0.001~0.01%, Mg : 0.001 to 0.01%, REM: High strength steel for oil wells with excellent resistance to sulfide cracking, characterized in that it contains one or more of 0.001 to 0.01% and the balance consists of Fe and impurities.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)

In mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn ≦ 0.5%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1 Ys and Mo containing 3.4% or more of N%, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, the balance being Fe and impurities, and expressed by ksi Sulfide crack resistance, characterized in that the relationship between the quantities satisfies the following formula (1) and the C, Mn, Mo content balance satisfies the following formula (2) and has a yield strength of 120 ksi or more. High strength steel material for oil wells.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)

In mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn ≦ 0.5%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1 % And N% 3.4 times or more, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, and the relationship between the yield strength YS expressed by ksi and the Mo amount is (1) It satisfies the formula, and C, Mn, the content balance of Mo satisfies the following expression (2), a further, V: 0.01~0.3%, Zr: 0.001~0.010%, Ca: 0.001~0.01%, Mg : 0.001 to 0.01%, REM: One or more of 0.001 to 0.01%, with the balance being Fe and impurities, with yield strength of 120 ksi or more Excellent high strength steel for oil wells.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)

In mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn <0.3%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1 Ys and Mo containing 3.4% or more of N%, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, the balance being Fe and impurities, and expressed by ksi Sulfide crack resistance, characterized in that the relationship between the quantities satisfies the following formula (1) and the C, Mn, Mo content balance satisfies the following formula (2) and has a yield strength of 120 ksi or more. High strength steel material for oil wells.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)

In mass%, C: 0.10 to 0.40%, Si ≦ 0.5%, Mn <0.3%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1 % And N% 3.4 times or more, Nb: 0.01 to 0.1%, N ≦ 0.01%, B: 0.0005 to 0.0050%, and the relationship between the yield strength YS expressed by ksi and the Mo amount is (1) It satisfies the formula, and C, Mn, the content balance of Mo satisfies the following expression (2), a further, V: 0.01~0.3%, Zr: 0.001~0.010%, Ca: 0.001~0.01%, Mg : 0.001 to 0.01%, REM: One or more of 0.001 to 0.01%, with the balance being Fe and impurities, with yield strength of 120 ksi or more Excellent high strength steel for oil wells.
α = Mo−0.15YS ≧ −18.9 (1)
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)

A steel containing the component according to any one of claims 1 to 8, wherein the balance is Fe and impurities, and the balance of content of C, Mn, and Mo satisfies the following formula (2): After processing, it is heated to a temperature range of Ac ₃ point + 20 ° C or more and 1000 ° C or less to austenite, and then subjected to quenching treatment, the hardness as quenched in the steel material position farthest from this cooling surface, A method for producing a high-strength oil well steel material excellent in sulfide cracking characteristics, characterized by having a metal structure that is 95% or more of the hardness of the cooling surface and subsequently tempering at a temperature of 620 to 720 ° C.
β = 2.7C + Mn + 2Mo ≧ 2.0 (2)

The sulfide resistance according to any one of claims 1 to 8, wherein a crack initiation limit stress obtained by a constant load type sulfide cracking test of NACE TMO177-A is 80% or more of a yield strength. High strength steel for oil wells with excellent cracking properties.