JP3852248B2 - Manufacturing method of martensitic stainless steel with excellent stress corrosion cracking resistance - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、耐応力腐食割れ性に優れたマルテンサイト系ステンレス鋼の製造方法に関し、詳しくは、例えば石油、天然ガスの掘削や輸送等の湿潤炭酸ガスや湿潤硫化水素ガスを含む環境において、高い応力腐食割れ抵抗を有する９５ｋｓｉグレードマルテンサイト系ステンレス鋼の製造方法に関する。
【０００２】
【従来の技術】
近年生産される石油、天然ガスは、湿潤炭酸ガスや湿潤硫化水素ガスを多量に含む場合が増加しており、その掘削、輸送においては従来の炭素鋼に代り、１３Ｃｒステンレス鋼等のマルテンサイト系ステンレス鋼が用いられるようになった。しかし、従来のマルテンサイト系ステンレス鋼は、湿潤炭酸ガスに対する耐食性（以下、単に「耐食性」と呼ぶ）は優れているが、湿潤硫化水素ガスに対する耐応力腐食割れ性（以下、単に「耐応力腐食割れ性」と呼ぶ）は十分でなく、強度、靭性、耐食性を維持しつつ、耐応力腐食割れ性に優れたマルテンサイト系ステンレス鋼が望まれていた。
【０００３】
強度、靭性、耐食性に加えて、耐応力腐食割れ性の要求を満たすマルテンサイト系ステンレス鋼として、例えば、特開昭５８−１９９８５０号公報には、Ｃ：０．１６〜０．２５ｗｔ％、Ｓｉ：１．０ｗｔ％未満、Ｍｎ：１．０ｗｔ％未満、Ｃｒ：１２．５〜１３．５ｗｔ％、Ｎｉ：０．３〜３ｗｔ％、Ｍｏ：０．５〜３．５ｗｔ％、Ｃｕ：０．５〜２ｗｔ％を含み、残部がＦｅ及び不可避不純物からなるマルテンサイト系ステンレス鋼が開示され、特開昭６１−２０７５５０号公報には、Ｃ：０．０３〜０．２０ｗｔ％、Ｓｉ：１．０ｗｔ％未満、Ｍｎ：１．０ｗｔ％未満、Ｃｒ：１２．０〜１４．０ｗｔ％、Ｂ：０．００１０〜０．００６０ｗｔ％を含有し、且つ、Ｍｏ：０．５〜４．０ｗｔ％、Ｎｉ：０．５〜６．０ｗｔ％の１種又は２種を含有し、残部がＦｅ及び不可避的不純物からなるマルテンサイト系ステンレス鋼が開示されている。しかしこれらは、極微量の硫化水素ガスを含む環境では耐応力腐食割れ性を示すものの、硫化水素ガス分圧が０．０１気圧を越える環境では、応力腐食割れが生じるため、硫化水素ガスを多く含む環境では使用できないという問題があった。
【０００４】
一方、硫化水素ガス分圧が０．０１気圧を越える環境での耐応力腐食割れ性を改善したマルテンサイト系ステンレス鋼も提案されており、例えば、特開昭６０−１７４８５９号公報には、Ｃ：０．０２ｗｔ％以下、Ｓｉ：０．５０ｗｔ％以下、Ｍｎ：０．５０〜１．５０ｗｔ％、Ｓ：０．００５ｗｔ％以下、Ｃｒ：１２〜１５ｗｔ％、Ｎｉ：３．５〜６ｗｔ％、Ｍｏ：０．５〜３ｗｔ％を含有し、残部がＦｅ及び不可避不純物からなるマルテンサイト系ステンレス鋼が開示され、特開昭６２−５４０６３号公報には、Ｃ：０．００１〜０．０５ｗｔ％、Ｓｉ：１．０ｗｔ％以下、Ｍｎ：０．３〜２．０ｗｔ％、Ｃｒ：１１．０〜１５．０ｗｔ％、Ｎｉ：３．０〜６．０ｗｔ％、Ｃａ：０．０００５〜０．００５ｗｔ％、Ａｌ：０．０１〜０．１ｗｔ％、Ｏ：０．００４０ｗｔ％以下、を含み、Ｐ：０．０１ｗｔ％以下、Ｓ：０．００５ｗｔ％以下であって、更に、Ｎ：０．０１〜０．２０ｗｔ％とＭｏ：０．０５〜３．０ｗｔ％のうち１種以上を含有して、残部はＦｅ及び不可避不純物からなるマルテンサイト系ステンレス鋼が開示されている。しかし、これらの鋼も硫化水素ガスによる応力腐食割れを完全に防止できるものではない。
【０００５】
更に、強度について検討すると、上述したマルテンサイト系ステンレス鋼は、何れも高強度化を試みると靭性及び耐応力腐食割れ性が著しく劣化し、そのため、強度又は靭性と耐応力腐食割れ性とのどちらか一方を犠牲にせざるを得ないという問題もあった。そのため、例えば高強度、耐応力腐食割れ性、耐食性、高靭性が同時に要求される高深度の油井管には適用できないという難点があった。
【０００６】
また、１３％Ｃｒ鋼をベースにして５％Ｎｉおよび２％Ｍｏを添加した高耐食性マルテンサイト系ステンレス鋼は強度の焼戻し温度依存性が大きく、９５ｋｓｉグレードの材料を製造する場合には1回の焼戻しでは目標強度を得るのが困難なため、２回の焼戻しが必要であり、製造コスト面での制約も大きかった。
【０００７】
【発明が解決しようとする課題】
本発明は、上述した従来技術における問題点を解決するためになされたもので、マルテンサイト系ステンレス鋼の耐食性を維持しつつ、強度、耐応力腐食割れ性、及び靭性を同時に改善することにより、硫化水素ガスを多く含む環境でも応力腐食割れを生じることなく使用できる９５ｋｓｉグレードのマルテンサイト系ステンレス鋼を、低コストで製造できる製造方法を提供することを目的とする。ここで目的とする性能は、炭酸ガス、硫化水素ガスを含む石油、天然ガスの掘削、輸送用鋼材に要求される性能に鑑み、以下の如くとした。
【０００８】
（１）強度：９５ｋｓｉグレード（０．２％耐力が９５ｋｓｉ以上、１１０ｋｓｉ未満）、（２）靭性：−２０℃でのシャルピー・フルサイズ試験片での吸収エネルギー値（以下、「シャルピー衝撃値」と呼ぶ）が２００Ｊ以上、（３）耐食性：３０気圧のＣＯ2 環境下の１８０℃の５％ＮａＣｌ溶液中で腐食速度が０．５ｍｍ／ｙ以下、（４）耐応力腐食割れ性：０．１気圧の硫化水素ガスを飽和させた５％ＮａＣｌ溶液中で試験片に０．２％耐力の８０％の応力を負荷して、７２０時間以上破断しないこと、（５）焼戻し温度範囲：９５ｋｓｉグレードの強度が得られる焼戻し温度範囲が５０℃以上あること。
【０００９】
【課題を解決するための手段】
本発明者等は、上記課題を解決すべく鋭意研究を重ねた結果、以下の知見を得るに至った。
【００１０】
マルテンサイト系ステンレス鋼の耐食性向上にはＣｒの増加が有効である。しかし一方で、Ｃｒの増加はδ−フェライト相を生成させて、強度及び靭性を劣化さ せる。そこで、オーステナイト生成元素であるＮｉを増加して、δ−フェライト相の生成を抑制する方法があるが、Ｎｉの増加はＡC1の変態点温度を下げて焼戻し温度に制約を及ぼすとともに、焼戻し温度によっては残留オーステナイト相を形成する。数％程度の残留オーステナイト相の存在は靭性を向上させるために有効であるが、２０％を越える多量の残留オーステナイト相は著しく強度を低下させるため、その上限に制約がある。
【００１１】
Ｃの増加も強度を上げるとともに、δ−フェライト相の生成抑制に有効であるが、焼戻し時に炭化物が析出して耐食性及び耐応力腐食割れ性を劣化させるため、その含有量は適正範囲内に制御されるべきものである。ここで、Ｃを０．０３〜０．０６ｗｔ％の範囲で添加し、合わせてＳｉを０．２５〜０．５ｗｔ％の範囲で添加すると、９５ｋｓｉグレードを得るための焼戻し温度範囲が約６０℃と非常に広くなり、その製造が容易となる。
【００１２】
さらに、Ｖを適量含有させ、且つ熱処理によりＶの炭化物をこのステンレス鋼の基地に微細な析出物として分散させることにより、靭性及び耐応力腐食割れ性を劣化させることなく高強度化することができる。そして、微細なＶ炭化物を均一に分散析出させるためにも、焼戻し条件を制御することが重要である。
【００１３】
本発明は、これらの知見に基づきなされたもので前記の課題は以下の発明により解決される。本件第１発明は、Ｃ：０．０３〜０．０６ｗｔ％、Ｓｉ：０．２５〜０．５ｗｔ％、Ｍｎ：０．０５〜０．３ｗｔ％、Ｃｒ：１２〜１６ｗｔ％、Ｎｉ：３．５〜６ｗｔ％、Ｍｏ： １．５〜２．５ｗｔ％、Ｖ：０．０１〜０．０５ｗｔ％、Ｎ：０．０３ｗｔ％以下、Ａｌ：０．０１０〜０．０２１ｗｔ％、Ｃａ：０．００１〜０．００２ｗｔ％を含有し、残部鉄および不可避不純物からなる、熱間加工されたマルテンサイト系ステンレス鋼を、ＡC3〜９８０℃の範囲に加熱してオーステナイト化した後冷却し、次いで、ＡC1−３０℃〜ＡC1＋３０℃の温度範囲で焼戻しを行うことを特徴とする耐応力腐食割れ性に優れた−２０℃でのシャルピー衝撃値が２００〜２４３Ｊのマルテンサイト系ステンレス鋼の製造方法である。本件第２発明は、本件第１発明に記載のステンレス鋼が、更に、Ｎｂ：０．０１〜０．１ｗｔ％、Ｔｉ：０．０１〜０．１ｗｔ％のうち１種又は２種以上を含有することを特徴とする耐応力腐食割れ性に優れた−２０℃でのシャルピー衝撃値が２００〜２４３Ｊのマルテンサイト系ステンレス鋼の製造方法である。
【００１４】
以下に、本発明においてマルテンサイト系ステンレス鋼の化学成分組成及び熱処理条件を上述したように限定した理由を、それぞれの作用と共に説明する。
【００１５】
（１）化学成分組成
（ａ）Ｃ：Ｃは強力なオーステナイト生成元素であり、又、高強度を得るためにも欠かせない元素である。Ｃの含有量が０．０６ｗｔ％を越えると、強度が高くなり、目標強度を満足する焼戻し温度範囲が小さくなる。一方、含有量が０．０３ｗｔ％未満では強度が低くなり、目標強度を満足する焼戻し温度範囲が小さくなる。従って、Ｃ含有量は０．０３〜０．０６ｗｔ％の範囲内に限定しなければならない。
【００１６】
（ｂ）Ｓｉ：Ｓｉは脱酸剤として必要な元素であるが、強度発現元素でもあり、０．５ｗｔ％を越えて含有させると強度が高くなり、目標強度を満足する焼戻し温度範囲が小さくなる。また、０．２５ｗｔ％未満では強度が低くなり、目標強度を満足する焼戻し温度範囲が小さくなる。従って、Ｓｉ含有量は０．２５〜０．５ｗｔ％の範囲内に限定しなければならない。
【００１７】
（ｃ）Ｍｎ：Ｍｎは脱酸、脱硫剤として有効であると共に、δ−フェライト相の出現を抑えるオーステナイト生成元素である。しかし、Ｍｎは耐応力腐食割れ性に対して有害であり、０．３ｗｔ％以下にする必要がある。一方、０．０５ｗｔ％未満では脱酸が不十分となり、鋼中の介在物が増加する。従って、Ｍｎ含有量は０．０５〜０．３ｗｔ％の範囲内に限定しなければならない。
【００１８】
（ｄ）Ｃｒ：Ｃｒはマルテンサイト系ステンレス鋼を構成する基本的な元素で、しかも耐食性を発現する重要な元素であるが、含有量が１２ｗｔ％未満では十分な耐食性が得られず、一方、１６ｗｔ％を越えるとδ−フェライト相の生成量が増加して、強度及び靭性が劣化する。従って、Ｃｒ含有量は１２〜１６ｗｔ％の範囲内に限定しなければならない。
【００１９】
（ｅ）Ｎｉ：Ｎｉは耐食性を向上させると共に、オーステナイトの生成に極めて有効な元素であるが、３．５ｗｔ％未満ではその効果が少なく、一方、含有量が増加するとＡC1変態点を下げて、焼戻し温度に制約を及ぼすため、６ｗｔ％以下にする必要がある。従って、Ｎｉ含有量は３．５〜６ｗｔ％の範囲に限定しなければならない。
【００２０】
（ｆ）Ｍｏ：Ｍｏは特に耐応力腐食割れ性及び耐食性に有効な元素であるが、１．５ｗｔ％未満ではその効果が少なく、一方、２．５ｗｔ％を越えると過剰なδ−フェライトを出現させる。従って、Ｍｏ含有量は１．５〜２．５ｗｔ％の範囲に限定しなければならない。
【００２１】
（ｇ）Ｖ：Ｖは強力な炭化物生成元素であり、微細な炭化物を析出させて結晶粒を微細化し、耐応力腐食割れ性を向上させる。又、微細な炭化物の析出は強度向上にも寄与する。しかし、フェライト生成元素でもあり、δ−フェライト相を増加させる。含有量が０．０１ｗｔ％未満では耐応力腐食割れ性の向上効果が現れず、一方、０．０５ｗｔ％を越えると、その効果は飽和すると共にδ−フェライト相が増加する。従って、Ｖ含有量は０．０１〜０．０５ｗｔ％の範囲に限定しなければならない。
【００２２】
（ｈ）Ｎ：Ｎは耐食性向上に有害な元素であるが、オーステナイト生成元素でもある。０．０３ｗｔ％を越えて含有させると、焼戻し時に窒化物となって析出し、耐食性、耐応力腐食割れ性、及び靭性を劣化させる。従って、Ｎ含有量は０．０３ｗｔ％以下に限定しなければならない。
【００２３】
（ｉ）付加成分としてのＮｂ及びＴｉ：Ｎｂ及びＴｉは強力な炭化物生成元素であり、微細な炭化物を析出させて結晶粒を微細化し、耐応力腐食割れ性を向上させる。しかし、共にフェライト生成元素でもあり、δ−フェライト相を増加させる。Ｎｂ及びＴｉの含有量が０．０１ｗｔ％未満では耐応力腐食割れ性の向上効果が現れず、一方、０．１ｗｔ％を越えると、その効果は飽和すると共にδ−フェライト相が増加する。従って、Ｎｂ含有量及びＴｉ含有量は、共に０．０１〜０．１ｗｔ％の範囲に限定することが好ましい。
【００２４】
（２）熱処理条件
（ａ）オーステナイト化温度：加熱温度がＡC3温度より低いと、組織全体が均一にはオーステナイト化されないため、均質な焼入れマルテンサイト組織が得られない。この段階で均質なマルテンサイト組織が得られていないと、これ以降の熱処理によっても焼戻し効果が十分に得られないばかりか、最終製品の特性も安定しない。一方、加熱温度が９８０℃を越えると、結晶粒が粗大化して十分な強度が得られないばかりでなく、靭性が劣化する。従って、オーステナイト化温度はＡC3〜９８０℃の範囲に限定しなければならない。
【００２５】
（ｂ）焼戻し温度：焼戻し処理は、Ｖの微細な炭化物を均一に分散析出させて高強度化させるとともに、靭性及び耐応力腐食割れ性を向上させるために必要である。従来の１３％Ｃｒ−５％Ｎｉ−２％Ｍｏ系のマルテンサイト系ステンレス鋼は、ＡC1点近傍で焼戻しを行うと強度が９５ｋｓｉグレードの規格を下回る可能性が高いため、コスト高となる２回の焼戻しを行うしか製造方法がなかった。しかし、本発明鋼のようにＣとＳｉの含有量を規定して複合添加すれば、ＡC1点の前後３０℃の範囲で焼戻しを行い、９５ｋｓｉグレードの材料を得ることが容易となる。焼戻し温度がＡC1−３０℃より低いと強度が十分に下らず、また、焼戻し温度がＡC1＋３０℃より高いと冷却時に新たにマルテンサイト相が生成して耐応力腐食割れ性が劣化するので、焼戻し温度はＡC1−３０℃〜ＡC1＋３０℃を満足する範囲に限定しなければならない。
【００２６】
【発明の実施の形態】
転炉、電気炉、及び、炉外精錬炉等により上記化学成分組成に溶製された溶鋼を普通造塊法又は連続鋳造法により鋳片にする。それを、熱間加工により鋼板又は継目無鋼管に製造した後、ＡC3〜９８０℃の範囲に加熱してオーステナイト化した後冷却し、次いでＡC1−３０℃〜ＡC1＋３０℃の範囲に焼戻し処理を行う。尚、焼戻し温度を決めるＡC1温度及びＡC3温度は、溶製されたマルテンサイト系ステンレス鋼の化学成分組成から予め定めておくこととする。本発明鋼の成分範囲では、ＡC1温度は６００℃〜６４０℃、ＡC3温度は７７０℃〜８１０℃となる。２回の焼戻しは不要である。
【００２７】
上記溶製の際に、不可避不純物として硫黄（Ｓ）及び燐（Ｐ）が残留する。これらは何れも鋼の熱間加工性及び耐応力腐食割れ性を劣化させる元素であり、少ない程好ましい。しかし、本発明者らの経験では、Ｓは０．０１ｗｔ％以下、Ｐは０．０４ｗｔ％以下であれば、本発明の目的とする耐応力腐食割れ性を確保できると共に、熱間圧延鋼板及び継目無鋼管の製造に支障を来すことがないので、この程度まで低減すれば十分である。
【００２８】
このように、Ｃｒの増加による金属組織の制約を考慮しつつ、１３％Ｃｒ−５％Ｎｉ−２％Ｍｏ系のマルテンサイト系ステンレス鋼にＣおよびＳｉの成分範囲を規定して複合添加するとともにＶを一定量含有させ、且つ熱処理条件を一定範囲内に調整して、Ｖの炭化物を粒内に均一に分散析出させてマルテンサイト系ステンレス鋼を製造することで、従来のマルテンサイト系ステンレス鋼では実現し得なかった高靭性で耐応力腐食割れ性に優れた９５ｋｓｉグレードマルテンサイト系ステンレス鋼を低コストで製造することが可能となる。
【００２９】
【実施例】
１０種類の化学成分組成のマルテンサイト系ステンレス鋼を真空溶解炉により溶製し、鋳片とした後、この鋳片を熱間圧延にて厚み１２ｍｍの鋼板とした。表１に、これら１０種類の供試鋼の化学成分組成を示す。表１に示すように、鋼Ｎｏ．１〜５の化学成分組成は本発明の範囲内であるのに対し、鋼Ｎｏ．６はＣとＳｉが、鋼Ｎｏ．７はＣとＭｏとＶが、鋼Ｎｏ．８はＣとＳｉとＣｒとＭｏが、鋼Ｎｏ．９はＣとＳｉとＮｉとＮが、又、鋼Ｎｏ．１０はＣとＳｉとＶが、それぞれ本発明の範囲を外れている。
【００３０】
【表１】
その後、これらの鋼板を加熱してオーステナイト化した後空冷し、次いで、３０分間の焼戻し処理を行った後、機械的性質、耐食性、及び耐応力腐食割れ性を調査した。その際、鋼の化学成分組成と熱処理条件との組み合せを変更して、合計１６水準の試験を行った。オーステナイト化温度は全ての試験において本発明の範囲内とし、又、焼戻し温度も９５ｋｓｉグレードの材料が得られる範囲内とした。
【００３２】
表２に１６水準の各試験における供試鋼のＡC1温度、ＡC3温度、９５ｋｓｉグレードを満足する焼戻し温度範囲、熱処理温度（オーステナイト化温度Ｑ／焼戻し温度Ｔ）、及び調査結果を示す。機械的性質、耐食性、及び耐応力腐食割れ性の調査は、前述した条件下で実施した。
【００３３】
【表２】
表２に示すように、本発明の範囲内の化学成分組成の鋼を、本発明の範囲内の熱処理条件で処理することにより、０．２％耐力及びシャルピー衝撃値は目標値を上回り、また腐食速度も目標値を達成すると共に応力腐食割れ（ＳＳＣ）も発生せず、耐食性及び耐応力腐食割れ性も目標値を達成した。
【００３５】
【発明の効果】
本発明では、化学成分組成及び熱処理条件を特定することにより、高靭性及び高強度を維持しつつ、炭酸ガス腐食に対する耐食性はもとより、硫化水素ガスを多量に含む環境での耐応力腐食割れ性に極めて優れた９５ｋｓｉグレードマルテンサイト系ステンレス鋼を、２回の焼戻しをすることなく１回の焼戻しで安定して安価に製造することが可能となる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing martensitic stainless steel having excellent stress corrosion cracking resistance, and in particular, in an environment containing wet carbon dioxide gas or wet hydrogen sulfide gas, such as oil and natural gas drilling and transportation. The present invention relates to a method for producing 95 ksi grade martensitic stainless steel having stress corrosion cracking resistance.
[0002]
[Prior art]
Oil and natural gas produced in recent years have increased in cases where they contain a large amount of wet carbon dioxide or wet hydrogen sulfide gas, and in their excavation and transportation, martensitic systems such as 13Cr stainless steel are used instead of conventional carbon steel. Stainless steel has been used. However, conventional martensitic stainless steel has excellent corrosion resistance against wet carbon dioxide gas (hereinafter simply referred to as “corrosion resistance”), but stress corrosion cracking resistance against wet hydrogen sulfide gas (hereinafter simply referred to as “stress corrosion resistance”). Martensitic stainless steel having excellent stress corrosion cracking resistance while maintaining strength, toughness, and corrosion resistance has been desired.
[0003]
As martensitic stainless steel that satisfies the requirements for stress corrosion cracking resistance in addition to strength, toughness, and corrosion resistance, for example, JP-A-58-199850 discloses C: 0.16-0.25 wt%, Si : Less than 1.0 wt%, Mn: less than 1.0 wt%, Cr: 12.5-13.5 wt%, Ni: 0.3-3 wt%, Mo: 0.5-3.5 wt%, Cu: 0. A martensitic stainless steel containing 5 to 2 wt%, the balance being Fe and unavoidable impurities is disclosed. JP-A 61-207550 discloses C: 0.03 to 0.20 wt%, Si: 1. Less than 0 wt%, Mn: less than 1.0 wt%, Cr: 12.0 to 14.0 wt%, B: 0.0010 to 0.0060 wt%, and Mo: 0.5 to 4.0 wt%, Ni: 0.5 to 6.0 wt% or one type Containing seeds, martensitic stainless steel balance being Fe and inevitable impurities. However, they exhibit stress corrosion cracking resistance in an environment containing a very small amount of hydrogen sulfide gas, but stress corrosion cracking occurs in an environment where the hydrogen sulfide gas partial pressure exceeds 0.01 atm. There was a problem that it could not be used in the environment that included.
[0004]
On the other hand, martensitic stainless steel having improved stress corrosion cracking resistance in an environment where the hydrogen sulfide gas partial pressure exceeds 0.01 atm has been proposed. For example, Japanese Patent Laid-Open No. 60-174859 discloses C : 0.02 wt% or less, Si: 0.50 wt% or less, Mn: 0.50 to 1.50 wt%, S: 0.005 wt% or less, Cr: 12 to 15 wt%, Ni: 3.5 to 6 wt%, A martensitic stainless steel containing Mo: 0.5 to 3 wt%, the balance being Fe and inevitable impurities is disclosed, and Japanese Patent Application Laid-Open No. 62-54063 discloses C: 0.001 to 0.05 wt%. , Si: 1.0 wt% or less, Mn: 0.3-2.0 wt%, Cr: 11.0-15.0 wt%, Ni: 3.0-6.0 wt%, Ca: 0.0005-0. 005 wt%, Al: 0.01-0. wt%, O: 0.0040 wt% or less, P: 0.01 wt% or less, S: 0.005 wt% or less, and further, N: 0.01-0.20 wt% and Mo: 0.0. A martensitic stainless steel containing at least one of 05 to 3.0 wt%, the balance being Fe and inevitable impurities is disclosed. However, these steels cannot completely prevent stress corrosion cracking caused by hydrogen sulfide gas.
[0005]
Further, when examining the strength, the above-described martensitic stainless steels are significantly deteriorated in toughness and stress corrosion cracking resistance when attempting to increase the strength. There was also a problem that one had to be sacrificed. Therefore, for example, there is a problem that it cannot be applied to a deep oil well pipe that requires high strength, stress corrosion cracking resistance, corrosion resistance, and high toughness at the same time.
[0006]
In addition, high corrosion resistance martensitic stainless steel based on 13% Cr steel and containing 5% Ni and 2% Mo has a high strength tempering temperature dependency. Since it is difficult to obtain the target strength by tempering, tempering twice is necessary, and the manufacturing cost is very limited.
[0007]
[Problems to be solved by the invention]
The present invention was made in order to solve the problems in the prior art described above, while simultaneously improving the strength, stress corrosion cracking resistance, and toughness while maintaining the corrosion resistance of martensitic stainless steel. An object of the present invention is to provide a production method capable of producing a 95 ksi grade martensitic stainless steel that can be used without causing stress corrosion cracking even in an environment containing a large amount of hydrogen sulfide gas. The target performance here is as follows in view of the performance required for steel for transportation of petroleum, natural gas drilling and transportation including carbon dioxide gas and hydrogen sulfide gas.
[0008]
(1) Strength: 95 ksi grade (0.2% proof stress is 95 ksi or more and less than 110 ksi), (2) Toughness: absorbed energy value with Charpy full size test piece at −20 ° C. (hereinafter “Charpy impact value”) 200J or more) (3) Corrosion resistance: Corrosion rate of 0.5 mm / y or less in 180% C 5% NaCl solution under 30 atm CO 2 environment, (4) Stress corrosion cracking resistance: 0. The test piece is loaded with 80% stress of 0.2% proof stress in a 5% NaCl solution saturated with 1 atm of hydrogen sulfide gas, and does not break for more than 720 hours. (5) Tempering temperature range: 95 ksi grade The tempering temperature range for obtaining the strength of 50 ° C. or more.
[0009]
[Means for Solving the Problems]
As a result of intensive studies to solve the above problems, the present inventors have obtained the following knowledge.
[0010]
The improvement of corrosion resistance martensitic stainless steel increased Cr are valid. On the other hand, however, an increase in Cr produces a δ-ferrite phase, which degrades strength and toughness. Therefore, there is a method of increasing the austenite-forming element Ni to suppress the formation of the δ-ferrite phase. However, the increase in Ni lowers the transformation temperature of A C1 and restricts the tempering temperature. Depending on the case, a residual austenite phase is formed. The presence of a residual austenite phase of about several percent is effective for improving toughness, but a large amount of residual austenite phase exceeding 20% significantly reduces the strength, so that the upper limit is limited.
[0011]
Increasing C increases the strength and is effective in suppressing the formation of δ-ferrite phase, but carbides precipitate during tempering and degrade corrosion resistance and stress corrosion cracking resistance, so the content is controlled within an appropriate range. Is to be done. Here, when C is added in the range of 0.03 to 0.06 wt% and Si is added in the range of 0.25 to 0.5 wt%, the tempering temperature range for obtaining the 95 ksi grade is about 60 ° C. It becomes very wide and easy to manufacture.
[0012]
Furthermore, it is possible to increase the strength without deteriorating the toughness and stress corrosion cracking resistance by containing an appropriate amount of V and dispersing the carbide of V as fine precipitates on this stainless steel base by heat treatment. . In order to uniformly disperse and precipitate fine V carbides, it is important to control the tempering conditions.
[0013]
The present invention has been made based on these findings, and the above-mentioned problems can be solved by the following invention. This 1st invention is C: 0.03-0.06 wt%, Si: 0.25-0.5 wt%, Mn: 0.05-0.3 wt%, Cr: 12-16 wt%, Ni: 3. 5 to 6 wt%, Mo: 1.5 to 2.5 wt%, V: 0.01 to 0.05 wt%, N: 0.03 wt% or less , Al: 0.010 to 0.021 wt%, Ca: 0. containing 001～0.002Wt%, ing a balance of iron and inevitable impurities, hot-worked martensitic stainless steel, and cooled after austenite by heating to a range of a C3 ~980 ℃, then A martensitic stainless steel having excellent stress corrosion cracking resistance and a Charpy impact value at −20 ° C. of 200 to 243J, characterized by tempering in a temperature range of A C1 −30 ° C. to A C1 + 30 ° C. It is a manufacturing method. In the second invention, the stainless steel described in the first invention further contains one or more of Nb: 0.01 to 0.1 wt% and Ti: 0.01 to 0.1 wt%. This is a method for producing martensitic stainless steel having excellent stress corrosion cracking resistance and a Charpy impact value at −20 ° C. of 200 to 243J .
[0014]
The reason why the chemical component composition and the heat treatment conditions of the martensitic stainless steel are limited as described above in the present invention will be described together with the respective actions.
[0015]
(1) Chemical component composition (a) C: C is a strong austenite-forming element and is also an indispensable element for obtaining high strength. When the C content exceeds 0.06 wt%, the strength increases and the tempering temperature range that satisfies the target strength decreases. On the other hand, when the content is less than 0.03 wt%, the strength is lowered, and the tempering temperature range satisfying the target strength is reduced. Therefore, the C content must be limited to a range of 0.03 to 0.06 wt%.
[0016]
(B) Si: Si is an element necessary as a deoxidizer, but it is also an element that develops strength. If it exceeds 0.5 wt%, the strength increases and the tempering temperature range that satisfies the target strength decreases. . On the other hand, if it is less than 0.25 wt%, the strength becomes low, and the tempering temperature range that satisfies the target strength becomes small. Therefore, the Si content must be limited to the range of 0.25 to 0.5 wt%.
[0017]
(C) Mn: Mn is an austenite generating element that is effective as a deoxidizing and desulfurizing agent and suppresses the appearance of a δ-ferrite phase. However, Mn is harmful to the stress corrosion cracking resistance and needs to be 0.3 wt% or less. On the other hand, if it is less than 0.05 wt%, deoxidation becomes insufficient, and inclusions in the steel increase. Therefore, the Mn content must be limited to the range of 0.05 to 0.3 wt%.
[0018]
(D) Cr: Cr is a basic element constituting martensitic stainless steel, and is an important element that expresses corrosion resistance. However, if the content is less than 12 wt%, sufficient corrosion resistance cannot be obtained, If it exceeds 16 wt%, the amount of δ-ferrite phase produced increases, and the strength and toughness deteriorate. Therefore, the Cr content must be limited to a range of 12 to 16 wt%.
[0019]
(E) Ni: Ni is an element that improves corrosion resistance and is extremely effective in the formation of austenite. However, if it is less than 3.5 wt%, its effect is small, whereas if the content increases, the A C1 transformation point is lowered. In order to limit the tempering temperature, it is necessary to make it 6 wt% or less. Therefore, the Ni content must be limited to the range of 3.5 to 6 wt%.
[0020]
(F) Mo: Mo is an element particularly effective for stress corrosion cracking resistance and corrosion resistance. However, if it is less than 1.5 wt%, its effect is small, while if it exceeds 2.5 wt%, excessive δ-ferrite appears. Let Therefore, the Mo content must be limited to a range of 1.5 to 2.5 wt%.
[0021]
(G) V: V is a strong carbide-forming element, which precipitates fine carbides to refine crystal grains and improves stress corrosion cracking resistance. Moreover, the precipitation of fine carbides also contributes to the strength improvement. However, it is also a ferrite-forming element and increases the δ-ferrite phase. If the content is less than 0.01 wt%, the effect of improving the stress corrosion cracking resistance does not appear. On the other hand, if the content exceeds 0.05 wt%, the effect is saturated and the δ-ferrite phase increases. Therefore, the V content must be limited to a range of 0.01 to 0.05 wt%.
[0022]
(H) N: N is an element harmful to the improvement of corrosion resistance, but is also an austenite generating element. When the content exceeds 0.03 wt%, it precipitates as nitride during tempering, and deteriorates corrosion resistance, stress corrosion cracking resistance, and toughness. Therefore, the N content must be limited to 0.03 wt% or less.
[0023]
(I) Nb and Ti as additional components: Nb and Ti are strong carbide generating elements, and precipitate fine carbides to refine crystal grains and improve stress corrosion cracking resistance. However, both are ferrite-forming elements and increase the δ-ferrite phase. When the content of Nb and Ti is less than 0.01 wt%, the effect of improving the stress corrosion cracking resistance does not appear. On the other hand, when the content exceeds 0.1 wt%, the effect is saturated and the δ-ferrite phase increases. Therefore, it is preferable that both the Nb content and the Ti content are limited to the range of 0.01 to 0.1 wt%.
[0024]
(2) Heat treatment conditions (a) Austenitizing temperature: If the heating temperature is lower than the AC3 temperature, the entire structure is not uniformly austenitized, so that a homogeneous quenched martensite structure cannot be obtained. If a homogeneous martensite structure is not obtained at this stage, not only a sufficient tempering effect can be obtained by the subsequent heat treatment, but also the properties of the final product are not stable. On the other hand, when the heating temperature exceeds 980 ° C., the crystal grains become coarse and sufficient strength cannot be obtained, and the toughness deteriorates. Therefore, the austenitizing temperature must be limited to the range of A C3 to 980 ° C.
[0025]
(B) Tempering temperature: The tempering treatment is necessary for uniformly dispersing and precipitating fine carbides of V to increase the strength and improving toughness and stress corrosion cracking resistance. Conventional 13% Cr-5% Ni- 2% Mo martensitic stainless steel, the strength is performed tempering near point A C1 because likely below the standard 95ksi grade, is costly 2 The only production method was tempering once. However, if the C and Si contents are specified and added together as in the steel of the present invention, tempering is performed in the range of 30 ° C. before and after the A C1 point, and it becomes easy to obtain a 95 ksi grade material. If the tempering temperature is lower than A C1 −30 ° C., the strength is not sufficiently lowered, and if the tempering temperature is higher than A C1 + 30 ° C., a new martensite phase is generated during cooling and the stress corrosion cracking resistance deteriorates. Therefore, the tempering temperature must be limited to a range satisfying A C1 −30 ° C. to A C1 + 30 ° C.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Molten steel melted to the above chemical composition by a converter, electric furnace, out-of-furnace refining furnace or the like is made into a slab by a normal ingot casting method or a continuous casting method. After it is manufactured into a steel plate or seamless steel pipe by hot working, it is heated to a range of A C3 to 980 ° C., cooled to austenite, and then tempered to a range of A C1 -30 ° C. to A C1 + 30 ° C. Process. The A C1 temperature and the A C3 temperature that determine the tempering temperature are determined in advance from the chemical composition of the molten martensitic stainless steel. The composition range of the steel of the present invention, A C1 temperature of 600 ℃ ~640 ℃, A C3 temperature becomes 770 ℃ ~810 ℃. Two tempers are not required.
[0027]
During the melting, sulfur (S) and phosphorus (P) remain as inevitable impurities. These are all elements that degrade the hot workability and stress corrosion cracking resistance of steel, and the smaller the better. However, in the experience of the present inventors, if S is 0.01 wt% or less and P is 0.04 wt% or less, the stress corrosion cracking resistance intended by the present invention can be secured, and a hot-rolled steel plate and Since there is no hindrance to the production of seamless steel pipes, it is sufficient to reduce to this extent.
[0028]
In this way, while considering the restriction of the metal structure due to the increase of Cr, the C and Si component ranges are defined and added to the 13% Cr-5% Ni-2% Mo martensitic stainless steel in combination. A conventional martensitic stainless steel is produced by containing a certain amount of V and adjusting the heat treatment conditions within a certain range to produce a martensitic stainless steel by uniformly dispersing and precipitating V carbide in the grains. Thus, it becomes possible to produce 95 ksi grade martensitic stainless steel having high toughness and excellent stress corrosion cracking resistance that could not be realized at low cost.
[0029]
【Example】
Ten martensitic stainless steels having chemical composition were melted in a vacuum melting furnace to form a slab, and the slab was hot rolled to form a steel plate having a thickness of 12 mm. Table 1 shows the chemical composition of these ten types of test steels. As shown in Table 1, steel no. While the chemical composition of 1-5 is within the scope of the present invention, steel no. No. 6 is C and Si, steel no. No. 7, C, Mo and V are steel No. No. 8 is C, Si, Cr and Mo. No. 9 is C, Si, Ni and N. 10, C, Si, and V are out of the scope of the present invention.
[0030]
[Table 1]
Thereafter, these steel sheets were heated to austenite and then air-cooled, followed by tempering for 30 minutes, and then the mechanical properties, corrosion resistance, and stress corrosion cracking resistance were investigated. At that time, the combination of the chemical composition of the steel and the heat treatment conditions was changed, and a total of 16 tests were conducted. The austenitizing temperature was within the range of the present invention in all tests, and the tempering temperature was also within the range where a 95 ksi grade material was obtained.
[0032]
Table 2 shows the A C1 temperature, A C3 temperature, the tempering temperature range satisfying the 95 ksi grade, the heat treatment temperature (austenitizing temperature Q / tempering temperature T), and the investigation results in each of the 16 levels. The investigation of mechanical properties, corrosion resistance, and stress corrosion cracking resistance was performed under the conditions described above.
[0033]
[Table 2]
As shown in Table 2, by treating steel having a chemical composition within the range of the present invention under the heat treatment conditions within the range of the present invention, the 0.2% proof stress and Charpy impact value exceeded the target values, and Corrosion rate achieved the target value and stress corrosion cracking (SSC) did not occur, and the corrosion resistance and stress corrosion cracking resistance achieved the target value.
[0035]
【The invention's effect】
In the present invention, by specifying the chemical composition and heat treatment conditions, while maintaining high toughness and high strength, not only corrosion resistance against carbon dioxide corrosion but also stress corrosion cracking resistance in an environment containing a large amount of hydrogen sulfide gas. An extremely excellent 95 ksi grade martensitic stainless steel can be manufactured stably and inexpensively by one tempering without tempering twice.
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| EP1498501B1 (en) * | 2002-04-12 | 2015-04-08 | Nippon Steel & Sumitomo Metal Corporation | Method for producing martensitic stainless steel |
| JP5924256B2 (en) | 2012-06-21 | 2016-05-25 | Ｊｆｅスチール株式会社 | High strength stainless steel seamless pipe for oil well with excellent corrosion resistance and manufacturing method thereof |
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| CN103469114A (en) * | 2013-08-02 | 2013-12-25 | 安徽三联泵业股份有限公司 | High-toughness stainless steel material used for water pump shell, and preparation method thereof |
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