JP3755163B2

JP3755163B2 - Manufacturing method of high-strength seamless steel pipe with excellent resistance to sulfide stress cracking

Info

Publication number: JP3755163B2
Application number: JP11602395A
Authority: JP
Inventors: 邦夫近藤; 隆弘櫛田; 大迫　　一
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 1995-05-15
Filing date: 1995-05-15
Publication date: 2006-03-15
Anticipated expiration: 2021-03-15
Also published as: JPH08311551A

Description

【０００１】
【産業上の利用分野】
本発明は、耐硫化物応力割れ性に優れた高強度継目無鋼管の製造方法に関する。更に詳しくは、特定した成分系の素材（ビレット）に特定条件の加工熱処理と熱処理を組み合わせて実施する、耐硫化物応力割れ性並びに強度、靱性に優れた継目無鋼管の製造方法に関するものである。
【０００２】
【従来の技術】
巨大装置を必要とする鉄鋼製品の製造では、省プロセス、省エネルギーの観点から、オンラインでの加工熱処理の適用によるプロセスの簡素化が検討されている。特に鋼板や厚板の製造ではオフラインでの焼入れ・焼戻しによる製造はかなり減少し、オンライン処理材がほとんどを占めるまでになっている。しかしながら継目無鋼管、とりわけ高強度の継目無鋼管の製造においては、高い信頼性と高品質化の観点からかなりの製品は未だに焼入れ・焼戻し処理で製造されているのが実状である。そのため製管ラインとは別に焼入れ炉と焼戻し炉を設置し、操業する必要があった。これに対して、熱間加工後の素材が保有する熱を利用して直ちに焼入れを行う、所謂直接焼入れプロセスを導入する動きがあり、それによって焼入れ炉が不要となり工業的に大きなコストダウンが得られつつある。
【０００３】
例えば、特開昭５８−２２４１１６号、特開昭６０−７５５２３号、特開平６−１７２８５９号などの各公報に、継目無鋼管の製造過程において熱間加工後直ちに強制冷却し、直接焼入れするプロセスを用いた高強度、高耐食性を有する鋼管の製造方法が提案されている。しかしながら、直接焼入れプロセスを経て製造された製品の結晶粒径は、従来の圧延後に再び加熱して焼入れする、所謂再加熱焼入れ処理によって製造された製品と比較すると粗大であり、靱性や耐硫化物応力割れ性に劣るという問題があった。
【０００４】
結晶粒を微細化する方法としては、オンライン上で冷却と再加熱を組み合わせて、オ−ステナイトからの変態とオ−ステナイトへの逆変態の、合計２回の変態を行わせることで細粒化を図る技術が提案されている。例えば、粗圧延と仕上げ圧延の中間に冷却、再加熱のプロセスを組み込んだ、特開昭５６−３６２６号公報や、最終仕上げ圧延後に冷却と再加熱を組み合わせた、特開昭５８−９１１２３号、特開昭５８−１０４１２０号、特開昭６３−１１６２１号、特開平４−３５８０２３号の各公報が開示されている。更に、特開昭５８−１１７８３２号公報のように圧延途中および圧延後の２回、冷却・再加熱することによって結晶粒を微細化する方法も提案されている。上記した各公報に提案された方法によれば、確かに直接焼入れした鋼材の結晶粒を微細化することは可能である。しかし、▲１▼いずれの提案になるものも、高い硫化物応力割れ抵抗性が必要とされる場合には結晶粒の微細化がまだまだ不充分である、▲２▼オンラインで変態が開始あるいは完了する温度域まで強制冷却し、再び逆変態が完了する温度域まで再加熱することは、エネルギーのロスや消費の点で好ましくない、▲３▼従来のオフラインでの再加熱焼入れと比較して複雑かつ建設費の高い設備を必要とする、といった問題がある。従って、上記の各公報に提案された鋼管や鋼板の製造方法は性能面やコスト面でメリットの少ないものであり、高品質の継目無鋼管を安価な設備によって生産性高く製造したいとする産業界の期待に添うものでは必ずしもなかったのである。
【０００５】
また、結晶粒の微細化や焼入れ性を向上させる観点から、未再結晶域で加工を行い、更に再結晶させるプロセスによって微細な結晶粒を得た後、直接焼入れ・焼戻しする技術も特開昭６２−１３９８１５号、特開昭６３−２２３１２５号の各公報に提案されている。特開昭６２−１３９８１５号公報の方法では熱間圧延終了から焼入れに至るまでの間におけるボロン（Ｂ）の挙動と鋼の焼入れ効果との関係から、熱間圧延後に圧延仕上げ温度に近い温度で保持してオ−ステナイト粒を再結晶させると共に固溶Ｂを確保して強度と靱性を高めている。特開昭６３−２２３１２５号公報の方法では未再結晶温度域で充分な圧延加工を行った後、均一な再結晶粒を得るために圧延終了後Ａｒ₃点以下に下げることなく急速加熱して短時間の均熱を行い、更に、直接焼入れ・焼戻ししてＪＩＳ粒度番号で８以上の均一な細粒組織となし、強度と靱性を高めている。
【０００６】
しかしながら、この両技術はいずれも比較的再結晶や結晶粒成長が容易な低炭素鋼からなる鋼板の製造技術であって、これを高耐食性油井用鋼管のような中炭素鋼からなる継目無鋼管の製造に適用したとしても同じような効果は得難いものと考えられる。これは、未再結晶温度域すなわち比較的低温度域での大圧下仕上げ加工が鋼板、とりわけ低炭素鋼板の圧延の場合には容易であっても、複雑な圧延工程を経る鋼管それも中炭素鋼管の圧延の場合には極めて困難で、鋼板のプロセスを鋼管に応用することが簡単ではないからである。具体的には、継目無鋼管の一般的な圧延方式であるプラグミル法やマンドレルミル法による圧延を、例えば、未再結晶温度域として１０００℃以下で実施した場合、▲１▼ミルの圧延能力を超える、▲２▼表面疵や欠陥が発生する、▲３▼マンドレルバーの引き抜きが著しく困難になる、といった種々の問題が生じ実用には程遠いものである。
【０００７】
鋼管の直接焼入れプロセスにおいても圧延後あるいは圧延中の再結晶化を意図した技術が特開昭６１−２３８９１７号、特開平５−２５５７４９号、特開平５−２５５７５０号、特開平５−２７１７７２号の各公報に提案されている。
【０００８】
特開昭６１−２３８９１７号公報には、特定の化学組成を有する継目無鋼管の圧延後の加熱条件を厳密に規定して９０％以上再結晶させる技術が開示されている。しかし、継目無鋼管の加工条件については全く記載がなされていないので、継目無鋼管の一般的な圧延方式であるプラグミル法やマンドレルミル法などにおいて、単にこの公報に記載の圧延後の加熱方法を適用したとしても必ずしも整細粒の組織が得られるというものでもない。
【０００９】
特開平５−２５５７４９号公報と特開平５−２５５７５０号公報には、特定の鋼成分からなる素管を圧延途中で１１００〜９００℃まで強制冷却した後、目標の外径と肉厚を有する中空素管とするために、肉厚断面減少率で１５％以上の圧延を行い、更に、その中空素管を９００〜１０００℃に再加熱して仕上げ圧延し直接焼入れする技術が提案されている。しかしこの方法では圧延途中でたとえ超微細結晶粒が得られても、再加熱によって結晶粒が成長することとなるため、最終的に得られるオ−ステナイト粒度はＡＳＴＭNo. で高々８．９番である。更に上記の方法では、仕上げ圧延の加工量（加工率）が極めて小さいため、場合によっては結晶粒が異常成長し、必ずしも整細粒の組織になるというものでもない。
【００１０】
このように圧延途中での再加熱プロセスは結晶粒の整細粒化という点からは必ずしも好ましいものではない。再加熱温度を結晶粒を粗大化させない温度域に設定することも可能であるが、再加熱後の圧延が未再結晶域での圧延となって伸長粒組織や混粒組織となる。特に、伸長粒組織には焼入れ性が大幅に低下すると共に異方性が大きくなるという問題があり、極めて良好な耐食性を要求される継目無鋼管としては使用しかねるものである。
【００１１】
特開平５−２７１７７２号公報には、特定の鋼成分を有する鋼片を粗製管した後９００〜１０００℃に再加熱して仕上げ圧延し、直接焼入れして９０％以上のマルテンサイト組織とする技術が開示されている。しかし、鋼管の加工条件については全く記載がなく、更に上記の特開平５−２５５７４９号公報および特開平５−２５５７５０号公報の方法と同様に圧延の途中で再加熱するため、この公報に記載の方法によっても必ずしも整細粒の組織が得られるというものでもない。
【００１２】
また、最終的に得られるオ−ステナイト粒度もＡＳＴＭNo. で高々７．３番と大きいものである。
【００１３】
鋼成分と圧延機の配列に工夫を凝らし、微細粒となして直接焼入れする技術が特開平６−１７２８５４号、特開平６−１７２８５８号、特開平６−１８４７１１号の各公報に提案されている。上記の各公報に記載の技術は、剪断歪成分が極めて多い傾斜圧延機を２台以上連続配置し、それを用いて中空素管を成品形状に成形するものである。この場合、各々の傾斜圧延機での圧延温度を通常の場合より低温に設定したり、１段目の圧延温度を通常の場合より低温に設定して圧延し、加工発熱によって素管を昇温させ、最終段の傾斜圧延機による加工の後、整形のための最終仕上げ圧延を行う。この最終仕上げ圧延の前、すなわち最終段の傾斜圧延機による加工の後で中空素管が再加熱される場合もある。しかしながら、上記の公報に規定されたような圧延の温度域と圧下率では傾斜圧延機によるといえども過酷な圧延となって、製管疵が多発する場合もある。更に、この方法によっても熱間仕上げ圧延における加工率（加工量）は僅かであるため得られるオ−ステナイト粒度はＡＳＴＭNo. で高々１０．７でしかない。
【００１４】
一方、鋼を直接焼入れした後、更に１回以上再加熱焼入れすることで結晶粒を微細化し、耐硫化物応力割れ性を向上させる技術が、例えば、特開平６−２２０５３６号、特開昭６０−４３４２４号、特開昭６０−５２５２０号、特開昭６０−４６３１８号、特開昭６０−８６２０８号、特開昭６０−４６３１７号、特開昭６０−８６２０９号の各公報に提案されている。
【００１５】
このうち、特開平６−２２０５３６号公報には特定化学組成を有する鋼管を直接焼入れした後、更に再加熱焼入れする方法が開示されている。しかしながら、この方法については鋼管の加工条件、とりわけ直接焼入れ前の仕上げ圧延の条件については全く明らかにされていないので、継目無鋼管の一般的な圧延方式であるプラグミル法やマンドレルミル法によって仕上げ圧延して直接焼入れすれば、その後の再加熱焼入れ処理でかえって異常粒成長が生じる場合があって、必ずしも超微細な整粒組織とはならずに耐硫化物応力割れ性が劣ったものになってしまうこともある。
【００１６】
特開昭６０−４３４２４号公報と特開昭６０−５２５２０号公報には特定の鋼成分を有する鋼材を、直接焼入れする前の熱間加工において１１００℃以下での断面圧縮率を２０％以上とし、直接焼入れ後に再加熱焼入れする方法が提案されている。しかしこれらの方法においては１１００℃以下という低めの温度域での仕上げ圧延が規定されているものの、その加工率（断面圧縮率）は実施例からも明らかなように高々４０％程度が限界である。これは従来、継目無鋼管の製管の場合にはミルの圧延能力上、▲１▼鋼材の変形抵抗の極めて小さい高温域で圧延するか、▲２▼粗圧延と仕上げ圧延の間で再加熱して温度を上げることによって中空素管の変形抵抗を下げるか、のいずれかの手段を用いないと仕上げ圧延で高い加工率を確保することができなかったためである。しかし、上記の４０％程度の仕上げ加工率で圧延するだけでは再加熱焼入れ時の初期粒となる直接焼入れ後のオ−ステナイト粒が充分には微細にならず、従って、再加熱焼入れ処理を何回も繰り返さないと所望の超微細粒となり難いという問題が残されていた。
【００１７】
特開昭６０−４６３１８号公報と特開昭６０−８６２０８号公報には特定化学組成を有する鋼材をオ−ステナイト域で第１次熱間加工した後、変態を開始させることなくオ−ステナイト域で保定または再加熱して第２次熱間加工を行い、直接焼入れ後に再加熱焼入れする方法が提案されている。しかしこの方法の場合、１次加工と２次加工の間で変態させずに保定または再加熱を行うため、再加熱焼入れ時の初期粒となる直接焼入れ後のオ−ステナイト粒が充分には微細にならず、従って、再加熱焼入れ処理を何回も繰り返さないと所望の超微細粒となり難いという問題がある。更に、加工条件、とりわけ直接焼入れ前の２次加工の条件については全く記載がなされていないので、継目無鋼管の一般的な圧延方式によって２次加工（仕上げ加工）して直接焼入れすれば、その後の繰り返しの再加熱焼入れ処理でかえって異常粒成長が生じる場合があって、必ずしも超微細な整粒組織とはならず、耐硫化物応力割れ性が劣ったものになってしまうこともある。
【００１８】
特開昭６０−４６３１７号公報と特開昭６０−８６２０９号公報には特定の鋼成分を有する鋼材を、第１次熱間加工した後一旦変態を完了させてからオ−ステナイト域に再加熱して第２次熱間加工を行い、直接焼入れ後に再加熱焼入れする方法が開示されている。この方法の場合、１次加工と２次加工の間で変態させるため再加熱焼入れ時の初期粒となる直接焼入れ後のオ−ステナイト粒は微細になるが、変態が完了する温度域まで冷却し再びオ−ステナイトへの逆変態が完了する温度域まで再加熱することは、エネルギーのロスや消費の点で好ましくなく、また大がかりな設備が必要となってコスト面で問題がある。加えて、この方法の場合にも、加工条件、とりわけ直接焼入れ前の２次加工の条件について全く言及されていないので、継目無鋼管の一般的な圧延方式によって２次加工（仕上げ加工）して直接焼入れすれば、その後の繰り返しの再加熱焼入れ処理でかえって異常粒成長が生じる場合があって、必ずしも超微細な整粒組織とはならずに耐硫化物応力割れ性が劣ったものになってしまうこともある。
【００１９】
ところで、耐硫化物応力割れ性を向上するには、▲１▼素材の化学組成を特定する方法、▲２▼同じく組織を特定する方法、▲３▼熱処理技術による方法並びに▲４▼前記のものを組み合わせる方法がある。その中で成分限定に関するものとして、特開昭６２−２５３７２０号公報にはＳｉ、Ｍｎ、ＰおよびＭｏ量と降伏応力を規定する方法が、特開昭６３−２７４７１７号公報には高Ｃ鋼を選定する方法が、特開昭６２−１４９８１３号公報や特開昭６３−２３８２４２号公報にはＺｒを添加する方法がそれぞれ提案されている。また組織制御に関しては、主として焼戻しマルテンサイトからなる組織が耐硫化物応力割れ性に優れ、更に、細粒組織ほど望ましいことは周知の事実であるが、特開昭６３−９３８２２号公報にはベイナイト組織とする方法が、特開昭６２−３０８４９号公報には伸展粒とする方法が開示されている。更に、特開昭５４−１１７３１１号公報や特開昭６１−９５１９号公報には細粒組織とするための熱処理技術として、誘導加熱などによる急速加熱を適用する方法が提案されている。上記の方法には耐硫化物応力割れ性向上の効果が認められて継目無鋼管、とりわけ油井用鋼管の品質は向上するものの、従来の再加熱焼入れ・焼戻し処理を用いた技術であり、直接焼入れすることで高品質の継目無鋼管を生産性高く、それも安価な設備によって製造したいとする産業界の要請に応えるものではない。
【００２０】
【発明が解決しようとする課題】
高強度高耐食性の継目無鋼管は一般に、傾斜圧延方式によりビレットをピアサーにて穿孔し、プラグミルまたはマンドレルミルで延伸した後、サイザーまたはレデューサーなどで仕上げ加工を施し、その後焼入れ・焼戻しの調質処理を行って製造される。この焼入れ・焼戻し処理は前述したようにプロセスの合理化の観点から製管ライン上で実施しようとする動きがあり、直接焼入れプロセスはその代表的なものである。しかし、従来提案された継目無鋼管の直接焼入れ処理は前記したような幾多の問題を抱えたものであった。
【００２１】
本発明は、かかる現状に鑑みてなされたもので、鋼（素材）成分、圧延と直接焼入れ条件並びに熱処理条件を特定することによって、大きな硫化物応力割れ抵抗性と高強度および高靱性を有する継目無鋼管の製造方法を提供することを課題とする。更に、製管オフラインでの熱処理条件を特定することによって、より一層の耐硫化物応力割れ性、高強度と高靱性を有する継目無鋼管の製造方法を提供することを、もう１つの課題とする。
【００２２】
【課題を解決するための手段】
本発明者らは、上記の課題を達成するために実験・研究を重ねた結果、直接焼入れプロセスであっても成分系を特定した上で、穿孔、延伸および仕上げ圧延の条件を規定して適正な加工熱処理を施せば整粒の微細結晶粒が得られ、更に、焼戻し軟化抵抗が増大することによって、通常の再加熱焼入れ・焼戻し材に比べて著しい性能の向上が図れることを見いだした。加えて、直接焼入れ後の結晶粒径を微細にすると共に、微細な析出物を多量に分散させて粒界の移動を阻止することで異常粒成長を防止すれば、直接焼入れ後の再加熱焼入れによって整粒の超微細結晶粒が得られることを見いだした。これをまとめると下記（ａ）〜（ｋ）となる。
【００２３】
（ａ）ＮｂとＴｉを複合添加した中炭素鋼系のビレットを継目無鋼管に仕上げ圧延するに際して、仕上がり温度が８００〜１０５０℃の条件にて断面圧縮率にして４０％以上の大きな加工歪を与え、更に、仕上げ圧延後に８５０〜１１００℃の温度域の温度Ｔで時間ｔの再加熱を行い、下記ｆｎ２の値を２３５００〜２６０００となせば微細な再結晶粒が得られる。
【００２４】
ｆｎ２＝（Ｔ＋２７３）（２１＋ｌｏｇｔ）、
但し、Ｔ（℃）、ｔ（ｈ）である。
【００２５】
（ｂ）上記（ａ）の再結晶組織を直接焼入れすれば、直接焼入れのままでもオ−ステナイト粒は圧延後に再加熱焼入れした場合と同程度に微細な整粒となる。
【００２６】
（ｃ）穿孔後の延伸加工で付与された加工歪が回復する前に仕上げ圧延を行えば、上記（ａ）の仕上げ圧延は、延伸加工と仕上げ加工（仕上げ圧延）の両加工を一体化したものとできる。
【００２７】
（ｄ）穿孔後の延伸加工で付与された加工歪が回復する前に仕上げ圧延を行うためには、例えば、従来独立して配置されていた延伸圧延機としてのマンドレルミルと仕上げ圧延機としてのサイザーを、一体型の連続配置とすれば良い。
【００２８】
（ｅ）穿孔工程にコーン型のロールを有する交叉穿孔機用いて、５〜３５度の交叉角（ロールの軸線がパスラインの水平面または垂直面に対してなす角、図１参照）で穿孔すれば拡管薄肉穿孔が可能なため、得られる中空素管の肉厚は通常のバレル型ロールの穿孔機を用いて圧延した場合よりも薄くすることができ、従って、次の延伸加工と仕上げ加工の両加工を一体化した前記（ｃ）の仕上げ圧延での強加工が容易となる。従って、中空素管の変形抵抗を下げるための再加熱処理を特に施さずとも、１０５０℃以下といった比較的低温域の仕上がり温度で４０％以上の強加工が可能である。
【００２９】
（ｆ）仕上げ圧延後に上記（ａ）に示した条件で加熱保持を行えば、多量のＮｂとＴｉの微細炭窒化物が析出し、且つ、適正量の固溶したＮｂとＴｉなどを含む再結晶粒が得られる。
【００３０】
（ｇ）仕上げ圧延後の再加熱処理でＮｂとＴｉの炭窒化物が凝集粗大化したり、固溶しているＮｂとＴｉなどの大部分が炭化物や炭窒化物として析出してしまうと再加熱焼入れしても整粒の超微細粒とはならず、また焼戻し軟化抵抗の増大効果も得難い。しかし、上記（ｆ）の再結晶粒を再加熱焼入れすれば粒界の移動が阻止されて異常粒成長の防止がなされ整粒の超微細粒となると共に、再加熱焼入れ処理で析出する粗大な未固溶炭窒化物が減少するので、耐硫化物応力割れ特性が著しく向上する。
【００３１】
（ｈ）固溶したＮｂとＴｉは焼戻し時に微細な炭窒化物として析出して焼戻し軟化抵抗を大幅に増大させる。この焼戻し軟化抵抗の増大により高温での焼戻しが可能となるので、同じ強度であっても内部歪が緩和され、更に、炭化物が球状化するので耐硫化物応力割れ特性が一段と向上する。
【００３２】
（ｉ）加工と再加熱による再結晶化を熱間加工の途中で生じさせた場合には、最終加工後にもう一度再結晶させる必要があって、この場合には比較的高温域での再加熱が必要となるので再結晶粒の微細化効果は小さいものである。これに対して、上記（ａ）の直接焼入れの直前における再加熱の場合には、再結晶による微細化効果が最も大きくなり、加えて、直接焼入れ時の焼入れ温度の確保も容易であるし異方性の発生も防止できる。
【００３３】
（ｊ）仕上げ圧延後、換言すれば直接焼入れの直前に再加熱するプロセスの場合には、粗加工と最終加工（仕上げ加工）の間で再加熱するプロセスの場合とは異なって、設備と運転のコストも小さく抑えることができる。
【００３４】
（ｋ）直接焼入れの直前に再加熱処理して再結晶粒の微細化を図ると共に、適正量のＮｂとＴｉなどの固溶元素および、多量のＮｂとＴｉの微細炭窒化物を含む継目無鋼管を再加熱焼入れまたは２回の再加熱焼入れすれば、圧延後に通常の再加熱焼入れを繰り返した場合よりも一層の細粒化が可能である。特に、再加熱時の加熱保持条件が前記（ａ）の条件を満たせば、繰り返しの再加熱焼入れや焼戻し処理を行ってもＮｂとＴｉの炭窒化物は粗大化せず、再加熱焼入れ時の結晶粒の粗大化と異常粒成長が防止でき、更に、焼戻し軟化抵抗増大効果が持続されるので、通常の再加熱焼入れ処理を繰り返しても得られないほどの優れた靱性と大きな硫化物応力割れ抵抗性が得られる。
【００３５】
本発明者らは、より一層大きな硫化物応力割れ抵抗性を得るために、更なる実験・研究を重ねた結果、次の新しい知見も得た。
【００３６】
（ｌ）鋼中に不純物として含まれるＰとＳが耐硫化物応力割れ性を劣化させる原因となることは知られているが、直接焼入れの直前に再加熱して再結晶させるプロセスの場合には、ＰとＳの含有量を重量％でそれぞれ０．００５％以下と０．０００７％以下に規制すれば特に大きな硫化物応力割れ抵抗性が得られる。
【００３７】
上記の理由については完全に解明するところまで至っていないが、次のようなことが考えられる。
【００３８】
▲１▼圧延後に再加熱して焼入れるところの従来の再加熱焼入れ処理における焼入れ温度では、Ｐの含有量を０．００５重量％以下まで低減した場合でも、Ｐの偏析発生限界量が極めて小さいため、粒界偏析が残存する。一方、仕上げ圧延に続いて再加熱し直接焼入れする場合には、Ｐの固溶量が増大してＰの偏析発生限界量が０．００５重量％以上となるため偏析が殆ど解消される。
【００３９】
▲２▼従来の再加熱焼入れ処理における焼入れ温度では、Ｓの含有量を０．０００７重量％以下まで低減した場合でも固溶しきれないＭｎＳが介在物として残存する。これに対して、仕上げ圧延に続いて再加熱し直接焼入れする場合には、Ｓの固溶量が増大し０．０００７重量％以下のＳ量であればすべて固溶するので介在物が著しく減少する。
【００４０】
▲３▼極低Ｐと極低Ｓの効果は互いに独立してもたらされるので、極低Ｐと極低Ｓのいずれかを満足すれば耐硫化物応力割れ性の向上が見られ、更に、両方を満たせばより一層の耐硫化物応力割れ性の向上が達成される。
【００４１】
上記知見に基づく本発明は、下記（１）〜（５）の耐硫化物応力割れ性に優れた高強度継目無鋼管の製造方法を要旨とする。
【００４２】
（１）重量％で、Ｃ：０．２０％超〜０．５０％、Ｓｉ：０．１〜１．５％、Ｍｎ：０．１〜１．５％、Ｃｒ：０．１〜１．５％、Ｍｏ：０．１〜１．５％、Ｎｂ：０．００５〜０．５０％、Ｔｉ：０．００５〜０．５０％、Ｂ：０．０００１〜０．０１％、Ａｌ：０．００５〜０．５０％を含有し、残部はＦｅおよび不可避不純物からなり、不純物中のＮｉは０．１％以下、Ｐは０．０５％以下、Ｓは０．０１％以下、Ｎは０．０１％以下およびＯは０．０１％以下で、且つ、ｆｎ１１＞０である成分組成のビレットを、熱間で穿孔し、圧延して継目無鋼管を製造するに際し、穿孔に続いて、断面圧縮率にして４０％以上の仕上げ圧延を仕上がり温度８００〜１０５０℃で行い、その後８５０〜１１００℃の温度域の温度Ｔで時間ｔの再加熱を行って下記ｆｎ２の値を２３５００〜２６０００となしてから直ちに直接焼入れを行い、次いでＡｃ₁点以下の温度で焼戻しすることを特徴とする耐硫化物応力割れ性に優れた高強度継目無鋼管の製造方法。
但し、
ｆｎ１１＝Ｔｉ（％）−（ 48 ／ 14 ）｛Ｎ（％）｝、
ｆｎ２＝（Ｔ＋２７３）（２１＋ｌｏｇｔ）、
である。
【００４３】
（２）重量％で、Ｃ：０．２０％超〜０．５０％、Ｓｉ：０．１〜１．５％、Ｍｎ：０．１〜１．５％、Ｃｒ：０．１〜１．５％、Ｍｏ：０．１〜１．５％、Ｎｂ：０．００５〜０．５０％、Ｔｉ：０．００５〜０．５０％、Ｂ：０．０００１〜０．０１％、Ａｌ：０．００５〜０．５０％を含有し、さらに、Ｖ：０．５％以下、Ｚｒ：０．５％以下およびＣａ：０．０１％以下のうちの１種または２種以上を含有し、残部はＦｅおよび不可避不純物からなり、不純物中のＮｉは０．１％以下、Ｐは０．０５％以下、Ｓは０．０１％以下、Ｎは０．０１％以下およびＯは０．０１％以下で、且つ、ｆｎ１２＞０である成分組成のビレットを、熱間で穿孔し、圧延して継目無鋼管を製造するに際し、穿孔に続いて、断面圧縮率にして４０％以上の仕上げ圧延を仕上がり温度８００〜１０５０℃で行い、その後８５０〜１１００℃の温度域の温度Ｔで時間ｔの再加熱を行って下記ｆｎ２の値を２３５００〜２６０００となしてから直ちに直接焼入れを行い、次いでＡｃ ₁ 点以下の温度で焼戻しすることを特徴とする耐硫化物応力割れ性に優れた高強度継目無鋼管の製造方法。
但し、
ｆｎ１２＝Ｔｉ（％）−（ 48 ／ 14 ）｛Ｎ（％）−（ 14 ／ 91 ）Ｚｒ（％）｝、
ｆｎ２＝（Ｔ＋２７３）（２１＋ｌｏｇｔ）、
なお、Ｔ（℃）、ｔ（ｈ）である。
【００４４】
（３）熱間穿孔を、交叉穿孔機を用いて交叉角５〜３５度で行うことを特徴とする上記（１）または（２）に記載の耐硫化物応力割れ性に優れた高強度継目無鋼管の製造方法。
【００４５】
（４）ビレットの不純物中のＰが０．００５％以下もしくはＳが０．０００７％以下、またはＰが０．００５％以下で、且つＳが０．０００７％以下であることを特徴とする上記（１）から（３）までのいずれかに記載の耐硫化物応力割れ性に優れた高強度継目無鋼管の製造方法。
【００４６】
（５）直接焼入れと焼戻しの間で、１回または２回のＡｃ₃点〜［Ａｃ₃点＋１００℃］の温度域に加熱した後の再加熱焼入れを行うことを特徴とする上記（１）から（４）までのいずれかに記載の耐硫化物応力割れ性に優れた高強度継目無鋼管の製造方法。
以下、前記Ｚｒの項を含まないｆｎ１１及びＺｒの項を含むｆｎ１２をまとめて、ｆｎ１といい、ビレットがＺｒを含まない場合にも「ｆｎ１＝Ｔｉ（％）−（ 48 ／ 14 ）｛Ｎ（％）−（ 14 ／ 91 ）Ｚｒ（％）｝」で表すこととする。
【００４７】
【作用】
以下、本発明の各要件についてその作用効果と共に詳しく説明する。なお成分含有量の「％」は「重量％」を意味する。
【００４８】
（Ａ）ビレットの化学組成
Ｃ：
Ｃは鋼の焼入れ性を高めて強度を向上するために必要な元素であるが、その含有量が０．２０％以下では添加効果が乏しく高強度が得られない。一方、０．５０％を超えて含有すると焼き割れや遅れ破壊が起こり易くなって継目無鋼管の製造が困難となる。従って、Ｃの含有量を０．２０％超〜０．５０％とした。
【００４９】
Ｓｉ：
Ｓｉは鋼の脱酸に必要であり、焼戻し軟化抵抗を高めて耐硫化物応力割れ性を向上するのに有効な元素であるが、過剰に含有させると鋼を脆化する作用を有する。脱酸と耐硫化物応力割れ性向上の目的からは０．１％以上含有させることが必要であるが、１．５％を超えると靱性と耐硫化物応力割れ性がかえって低下するので、その含有量を０．１〜１．５％とした。
【００５０】
Ｍｎ：
Ｍｎは鋼の脱酸と脱硫のために添加する。しかし、その含有量が０．１％未満では添加効果に乏しく、一方、１．５％を超えて含有すると鋼の靱性と耐硫化物応力割れ性が低下することになる。従って、Ｍｎの含有量を０．１〜１．５％とした。
【００５１】
Ｃｒ：
Ｃｒは鋼の焼入れ性を確保し、強度を向上するとともに耐硫化物応力割れ性を向上する元素である。しかし、その含有量が０．１％未満では充分な添加効果が得られず、１．５％を超えると靱性と耐硫化物応力割れ性がかえって低下することとなるので、その含有量を０．１〜１．５％とした。なお、Ｃｒ含有量は０．３〜１．０％とすることがより好ましい。
【００５２】
Ｍｏ：
Ｍｏは鋼の焼入れ性を高めて高強度を確保すると共に耐硫化物応力割れ性を向上するのに有効な元素である。しかし、その含有量が０．１％未満では添加効果に乏しく、一方、１．５％を超えて含有すると前記効果が飽和するだけでなく、偏析することによって逆に耐硫化物応力割れ性を劣化することとなるので、その含有量を０．１〜１．５％とした。なお、Ｍｏのより好ましい含有量は０．３〜０．８％である。
【００５３】
Ｎｂ：
Ｎｂは仕上げ圧延後の再加熱処理で微細な炭窒化物として析出して結晶粒の粗大化並びに再加熱焼入れ時の異常粒成長を防止する作用を有する。加えて、固溶Ｎｂは直接焼入れ後の焼戻し時に炭窒化物として微細に析出し焼戻し軟化抵抗を増大して耐硫化物応力割れ性を向上する効果がある。しかし、その含有量が０．００５％未満では添加効果に乏しく、０．５０％を超えると鋼の靱性が劣化するので、Ｎｂの含有量を０．００５〜０．５０％とした。なお、Ｎｂ含有量は０．０１〜０．１０％とすることがより好ましい。
【００５４】
Ｔｉ：
Ｔｉは鋼中の不純物であるＮを固定して、焼入れ時にＢを鋼中に固溶状態で存在させて鋼の焼入れ性を向上する作用がある。また、仕上げ圧延後の再加熱処理で微細な炭窒化物として析出して、結晶粒の粗大化並びに再加熱焼入れ時の異常粒成長を防止する効果がある。更に、固溶Ｔｉは直接焼入れ後の焼戻し時に微細な炭化物として析出し焼戻し軟化抵抗を増大する作用をも有する。しかし、その含有量が０．００５％未満では添加効果が小さく、一方、０．５０％を超えて含有すると鋼の靱性劣化を招くこととなる。従って、Ｔｉの含有量を０．００５〜０．５０％とした。なお、Ｔｉのより好ましい含有量は０．０１〜０．１０％である。
【００５５】
Ｂ：
Ｂは微量の添加で鋼の焼入れ性を向上し、特に厚肉材の耐硫化物応力割れ性を改善する元素である。しかし、その含有量が０．０００１％未満では所望の効果が得られず、一方、０．０１％を超えて含有すると、鋼の靱性と耐硫化物応力割れ性が劣化する。従って、Ｂの含有量を０．０００１〜０．０１％とした。
【００５６】
Ａｌ：
Ａｌは鋼の脱酸に有効な元素である。しかし、その含有量が０．００５％未満では所望の効果が得られず、０．５％を超えると介在物が多くなって鋼の靱性が劣化すると共に、継目無鋼管のネジ切り部に欠陥が発生し易くなるので、その含有量を０．００５〜０．５％とした。
【００５７】
Ｖ：
Ｖの添加は任意である。添加すれば焼戻し時に微細な炭化物として析出して耐硫化物応力割れ性を向上する効果がある。特に、Ｎｂとの複合添加で鋼に一層大きな硫化物応力割れ抵抗性を付与する作用がある。こうした効果を確実に得るには、Ｖは０．０５％以上の含有量とすることが好ましい。しかし、その含有量が０．５％を超えると鋼の靭性が劣化することとなる。従って、Ｖの含有量を０．５％以下とした。
【００５８】
Ｚｒ：
Ｚｒの添加は任意である。添加すれば引張り試験における鋼の降伏点伸びを増加する作用があり、その結果として耐硫化物応力割れ性を向上する効果を有する。この効果を確実に得るには、Ｚｒは０．０１％以上の含有量とすることが好ましい。高価な元素であると共に、０．５％を超えて含有させると介在物が多くなって鋼の靭性を劣化するので、Ｚｒの含有量の上限を０．５％とした。
【００５９】
Ｃａ：
Ｃａの添加は任意である。添加すれば鋼中のＳと反応して硫化物を形成することによって介在物の形状を改善するので、鋼の耐硫化物応力割れ性が向上する効果がある。しかし、Ｓの含有量によってその効果の度合いが異なり、また脱酸が充分なされた場合でないとかえって耐硫化物応力割れ性が劣化することもあるので、適宜添加するかしないかを選択しても良い。充分な脱酸がなされた鋼において、前記効果を確実に得るには、Ｃａは０．００１％以上の含有量とすることが好ましい。しかし、その含有量が０．０１％を超えると鋼の靭性と耐硫化物応力割れ性が劣化し、更に、継目無鋼管の表面に欠陥が生じる。従って、Ｃａの含有量を０．０１％以下とした。
なお、上記のＶ、Ｚｒ及びＢのうちのいずれか１種のみ、または２種以上の複合で含有することができる。
【００６０】
不純物元素Ｎｉ、Ｐ、Ｓ、ＮおよびＯ（酸素）はその含有量を次のとおり制限する。
【００６１】
Ｎｉ：
Ｎｉは鋼の耐硫化物応力割れ性を劣化させ、特にその含有量が０．１％を超えると耐硫化物応力割れ性の劣化が著しくなる。従って、不純物元素としてのＮｉの含有量を０．１％以下とした。
【００６２】
Ｐ：
Ｐは粒界に偏析して鋼の靱性と耐硫化物応力割れ性を劣化させ、特にその含有量が０．０５％を超えると靱性と耐硫化物応力割れ性の劣化が著しくなる。従って、不純物元素としてのＰの含有量の上限を０．０５％とした。
【００６３】
Ｓ：
Ｓは粗大な介在物を生成して鋼の靱性と耐硫化物応力割れ性を劣化させる。特にその含有量が０．０１％を超えると靱性と耐硫化物応力割れ性の劣化が著しくなるので、不純物元素としてのＳの含有量の上限を０．０１％とした。
【００６４】
ところで、直接焼入れの直前に再結晶させるために再加熱する本発明の製造方法の場合には、後の実施例にも示すように、不純物元素としてのＰおよび／またはＳの含有量の上限を更に低く規制すれば特に大きな硫化物応力割れ抵抗性が得られる。すなわち、不純物元素としてのＰの含有量の上限を０．００５％とすれば大きな硫化物応力割れ抵抗性が得られる。Ｐの含有量が０．００２％以下であれば一層効果が大きい。また、不純物元素としてのＳの含有量の上限を０．０００７％としても大きな硫化物応力割れ抵抗性が得られる。Ｓの含有量が０．０００３％以下であれば一層効果が大きい。
【００６５】
なお、極低Ｐと極低Ｓの効果は互いに独立して発揮されるので、不純物元素としてのＰの含有量を０．００５％以下とし、且つ、Ｓの含有量を０．０００７％以下とすればより一層の耐硫化物応力割れ性の向上が達成される。更に、不純物元素としてのＰの含有量を０．００２％以下とし、且つ、Ｓの含有量を０．０００３％以下とすれば鋼の硫化物応力割れ抵抗性は極めて大きなものとなる。
【００６６】
Ｎ：
ＮはＢの焼入れ性向上効果を妨げ、また鋼の靱性と耐硫化物応力割れ性を劣化させ、特にその含有量が０．０１％を超えると靱性と耐硫化物応力割れ性の劣化が著しくなる。従って、不純物元素としてのＮの含有量の上限を０．０１％とした。
【００６７】
Ｏ：
Ｏは鋼の靱性と耐硫化物応力割れ性を劣化させる。特にその含有量が０．０１％を超えると靱性と耐硫化物応力割れ性の劣化が著しくなるので、不純物元素としてのＯの含有量の上限を０．０１％とした。
【００６８】
ｆｎ１＞０：
ＴｉとＺｒはＮとの親和力が極めて大きい。従って、固溶Ｔｉを確保し直接焼入れ後の焼戻し時に微細炭化物として析出させて焼戻し軟化抵抗を増大するためにＴｉ（％）＞（48／14）｛Ｎ（％）−（14／91）Ｚｒ（％）｝であること、すなわち前記したｆｎ１が０より大なる制限を設ける必要がある。
【００６９】
（Ｂ）ビレットの加工
（Ｂ−１）ビレットの加熱：
ビレットの加熱温度は、穿孔機で熱間穿孔できる温度であれば良い。その最適温度は材質によって異なり、高温延性と高温強度を考慮して適宜決めるが、通常、１１００〜１３００℃の間に加熱する。ところで、高能率のビレット加熱を図るためにビレット長さは所定長さの整数倍の長尺状態とし、加熱炉の後段（穿孔機の前段）に設置した切断機により所定長さに切断してから穿孔することが好ましい。なお、加熱炉に挿入するビレットは分塊圧延したもの、丸型鋳型に連続鋳造して作製したものなどどのようなものでも良い。またエネルギー節減のため、ビレットは分塊圧延や連続鋳造の後、完全に室温まで冷却する前に加熱炉に装入しても良い。
【００７０】
（Ｂ−２）穿孔：
中実のビレットに熱間で貫通穴を開け中空素管（ホローシェル）を製造する工程である。この穿孔工程において、拡管薄肉穿孔を可能とし、得られる中空素管の肉厚を通常のバレル型ロールの穿孔機を用いて圧延した場合よりも薄くして、次の延伸加工と仕上げ加工の両加工を合算した仕上げ圧延での強加工を容易となすためにはコーン型ロールの交叉穿孔機を用いれば良い。この場合、交叉穿孔機の交叉角は５〜３５度とするのが良い。これは交叉角が５度未満では所望の薄肉のホローシェルが得難く、後段の加工で高加工することが難しくなるからである。交叉角は、ホローシェルのボトム端が穿孔機から抜けなくなるところの所謂「尻詰まり」の発生および圧延が不安定となることを避けるために３５度以下とすることが望ましい。なお、ビレットの温度が低下すると穿孔時に疵が発生し易くなるので、穿孔直前の前段位置に例えば誘導加熱装置などの補助加熱装置を設置して、昇温加熱してから穿孔しても良い。
【００７１】
（Ｂ−３）延伸加工と仕上げ加工：
穿孔機で穿孔された中空素管を延伸および寸法調整して所望の形状・寸法の継目無鋼管を製造する工程であって、穿孔機での加工に比べると比較的低温域での加工となるため、加工熱処理を考えた場合は有効な加工を付与する重要な工程となる。特に、穿孔後の延伸加工で付与した加工歪が回復する前に仕上げ加工（仕上げ圧延）を行えば、この両者の加工率を合算したものが所謂「仕上げ圧延（仕上げ加工）」の加工率となるため、途中で中空素管を再加熱することなく、大きな仕上げ圧延加工率を付与することができる。
【００７２】
本発明においては、断面圧縮率にして４０％以上の仕上げ圧延を仕上がり温度８００〜１０５０℃で行う必要がある。断面圧縮率で４０％未満の加工率の場合には再結晶がスムーズに進行せず、微細化効果が得られないことに加えて、時として結晶粒が異常成長することがあるからである。仕上げ圧延加工率の上限は製管対象の材質やミルの能力によって異なるため特に規定しなくても良いが、加工率が大きすぎると疵が発生しやすくなるので８０％程度を上限とすることが好ましい。圧延の仕上がり温度が１０５０℃を超えると結晶粒が粗大化して所望の細粒組織が得られないため仕上がり温度の上限を１０５０℃とした。圧延の仕上がり温度は低温であるほど再結晶粒が微細になる傾向があるが、低すぎると鋼材（中空素管）の変形抵抗が大きくなって断面圧縮率４０％以上の強加工を施すことが困難となるし、また、この仕上げ圧延後に所望のミクロ組織を得るために行う再加熱のためのエネルギー消費が大きくなるので、圧延仕上がり温度の下限を８００℃とした。
【００７３】
ところで、穿孔後の延伸加工で付与された加工歪が回復する前に仕上げ圧延を行うためには、既に述べたように、例えば、従来独立して配置されていた延伸圧延機と仕上げ圧延機を、一体型の連続配置とすれば良い。こうすることによって、製造範囲のすべてのサイズに対して仕上げ圧延加工率４０％以上を安定して確保することができる。
【００７４】
上記一体型の連続配置される延伸圧延機と仕上げ圧延機に関しては、例えば、延伸圧延機としてのマンドレルミルは、内面規制工具であるマンドレルバーの後端を拘束すると共に、延伸圧延終了後にマンドレルバーをミル入側に孔型ロール列中を通して引き戻して循環使用できるマンドレルバー拘束手段を有するタイプのものであれば良い。なかでも、前記マンドレルバー拘束手段が中空素管の延伸圧延中に管の圧延移動速度とは独立した速度でマンドレルバーの移動速度を制御可能な機能を備えるマンドレルミルを用いることが好ましい。また仕上げ圧延機としてのサイザーは、内面規制工具を有しないものであれば良いが、特に、マンドレルミルで圧延された管内のマンドレルバーから管を引き出し分離する機能を備えた所謂エキストラクティングサイザーを用いることが望ましい。
【００７５】
（Ｃ）再加熱処理
仕上げ圧延と直接焼入れ処理との間で再結晶処理としての再加熱処理を行うことが本発明の大きな特徴である。これによって、加工と加熱との組み合わせで再結晶が誘起され結晶粒の微細化が可能となる。従来の、圧延途中で粗加工と仕上げ加工の間で再加熱する場合と異なって、本発明の方法では再加熱後に加工する必要がないので再加熱温度を再結晶が進行する最低の温度に設定することができる。このため１回の再加熱だけで微細再結晶粒が得られる。
【００７６】
再加熱温度Ｔ（℃）としては、本発明の対象とする中炭素系の低合金鋼の場合、８５０℃未満の温度では再結晶化するのに長時間を要して製管効率が極めて低下する。一方、１１００℃を超える温度では結晶粒が大きく成長して粗粒化が甚だしい。従って、再加熱温度Ｔは８５０〜１１００℃の温度域の温度とした。
【００７７】
再加熱時間ｔ（ｈ）は前記したｆｎ２の値が２３５００〜２６０００となるようにする必要がある。ｆｎ２の値が２３５００未満の場合には、再結晶が完全には終了しない。ｆｎ２の値が２６０００を超えると、ＮｂとＴｉの炭窒化物が凝集粗大化したりＮｂとＴｉなどの大部分が炭窒化物として析出してしまうので、直接焼入れ後の焼戻し時に焼戻し軟化抵抗増大効果が得られなくなり、また、再加熱焼入れしても整粒の超微細粒とはならない。このため、所望の耐硫化物応力割れ性向上効果が得られない。
【００７８】
ところで、この仕上げ圧延と直接焼入れ処理との間での再加熱処理、換言すれば直接焼入れの直前での再加熱処理を行うことで、直接焼入れ時の焼入れ温度が容易に確保でき、また、継目無鋼管の長手方向（圧延方向）およびロット間の均熱性が保証されて性能ばらつきが大幅に小さくなるという副次的効果も得られるのである。
【００７９】
なお、「８５０〜１１００℃の温度域の温度Ｔ（℃）で時間ｔ（ｈ）の再加熱を行ってｆｎ１の値を２３５００〜２６０００となす」というのは、文字どおり「８５０℃から１１００℃の間にある任意の温度Ｔに時間ｔ保持してｆｎ１の値を２３５００〜２６０００となす」ようにしても良いし、「８５０℃から１１００℃の間にある任意の温度Ｔ1 、Ｔ2 、Ｔ3 、・・でそれぞれ時間ｔ1 、ｔ2 、ｔ3 、・・保持したとすれば、温度Ｔ2 、Ｔ3 、・・での保持時間ｔ2 、ｔ3 、・・を温度Ｔ1 における保持時間ｔ21、ｔ31、・・に換算し、温度Ｔ1 で時間（ｔ1 ＋ｔ21＋ｔ31＋・・）保持してｆｎ１の値を２３５００〜２６０００となす」ようにしても良い、という意味である。
【００８０】
（Ｄ）直接焼入れ
直接焼入れ処理はオ−ステナイト状態から行う必要があるため、焼入れ温度はＡr₃点以上を確保せねばならない。しかし、本発明の方法においては仕上げ圧延した継目無鋼管を直接焼入れの直前に８５０〜１１００℃に再加熱するため、再加熱処理後直ちに直接焼入れすればＡr₃点以上の焼入れ温度は充分確保できる。
【００８１】
直接焼入れ時の冷却速度は特に限定されるものではなく、素材鋼の成分組成から判断して、継目無鋼管の肉厚全面にわたって所望の低温変態組織が得られるようなものとすれば良い。一般には空冷より速い速度である。
【００８２】
なお、本発明の対象鋼を前記の穿孔、延伸・仕上げ加工し、次いで前記の再加熱処理した後直ちに直接焼入れすれば、直接焼入れのままでもオ−ステナイト粒は圧延後に再加熱して焼入れした場合と同程度に微細な整粒となる。
【００８３】
（Ｅ）焼戻し
焼入れ処理によって得た整細粒の低温変態組織を、Ａc₁点以下の温度で焼戻し処理すると、継目無鋼管に所望の特性（強度、靱性、硫化物応力割れ抵抗性）を付与できる。すなわち、Ａc₁点以下の温度で焼戻すことによってはじめて、所望の耐硫化物応力割れ性に優れた高強度継目無鋼管を得ることができる。
【００８４】
焼戻し処理は製品の性能を決定するものであり、充分な均熱性を必要とし、温度ばらつきは±１０℃以下、好ましくは±５℃以下とすることで、強度（引張り強度、降伏強度）の変動を±５ｋｇｆ／ｍｍ²以下に抑えることができる。
【００８５】
なお、焼戻し温度に格別な下限を設ける必要はないが、高温での焼戻し処理が焼入れによって得た低温変態生成物の内部歪や内部応力を緩和・除去し、且つ、炭化物を球状化して継目無鋼管の性能向上をもたらすことからみて、５５０℃以上の温度で焼き戻すことが望ましい。
【００８６】
（Ｆ）直接焼入れと焼戻しの間で行う再加熱焼入れと中間焼戻し
本発明の方法によれば、加工と再結晶により直接焼入れ直前の組織を整細粒となせるので、直接焼入れ後に焼戻し処理するだけで良好な耐硫化物応力割れ性と優れた靱性を有する高強度継目無鋼管の製造が可能である。このため、本発明の方法を適用すれば、たとえ直接焼入れ後に焼戻し処理するだけで製造されたとしても、高強度継目無鋼管は耐硫化物応力割れ性に優れるので、この状態のままでも過酷な環境での使用に充分耐え得る。しかし場合によっては、もう一段上位の靱性と硫化物応力割れ抵抗性を要求されることもあり、この時には更に超微細な整粒組織にする必要がある。
【００８７】
前記の超微細な整粒組織は、直接焼入れ後に焼戻しとの間で、１回または２回のＡｃ₃点〜［Ａｃ₃点＋１００℃］の温度域に加熱した後の再加熱焼入れを行うことで達成できる。
【００８８】
すなわち、本発明の方法によって直接焼入れした本発明の対象鋼には、多量のＮｂとＴｉの微細炭窒化物が析出しており、且つ、適正量のＮｂとＴｉなどの固溶元素が含まれている。従って、これを再加熱焼入れすると粒界移動の阻止がなされて異常粒成長が防止されるばかりか、整粒の超微細粒が得られる。この再加熱焼入れ処理を繰り返すことで結晶粒は更に微細になり、これによって靱性と耐硫化物応力割れ性が一層向上する。しかし、本発明によれば前記したように、直接焼入れ後に焼戻し処理するだけでも良好な耐硫化物応力割れ性と優れた靱性を有する高強度継目無鋼管の製造が可能であるため、上記再加熱焼入れ処理を３回以上繰り返しても、コストアップに見合うだけの特性向上は図り難い。この故に本発明の方法においては直接焼入れと焼戻しの間で、１回または２回の再加熱焼入れの実施を規定する。
【００８９】
再加熱焼入れのための加熱温度をＡｃ₃点から［Ａｃ₃点＋１００℃］の温度域に限定する理由は、Ａｃ₃点未満の加熱温度では完全オ−ステナイト状態とならず、また、［Ａｃ₃点＋１００℃］を超える加熱温度では結晶粒が粗大化して継目無鋼管に所望の特性を付与できないからである。この再加熱焼入れのための加熱速度は特に限定されるものではないが、大きい方が好ましい。従って、電磁誘導加熱などの急速加熱手段を用いても良い。また再加熱焼入れ時の冷却速度も特に限定されるものではなく、素材鋼の成分組成から判断して、継目無鋼管の肉厚全面にわたって所望の低温変態組織が得られるようなものとすれば良い。一般には空冷より速い速度である。
【００９０】
なお、再加熱焼入れを２回行う場合には、２回目の再加熱焼入れの加熱温度を１回目の再加熱焼入れの加熱温度に比べて低くすることが、靱性と耐硫化物応力割れ性の向上にとってより好ましい。
【００９１】
ところで遅れ破壊を防止する意味合いから、直接焼入れおよび再加熱焼入れの各焼入れ処理の間で中間焼戻し処理を適宜実施しても最終焼戻し後の継目無鋼管に特性の変化は生じない。従って、各焼入れ処理の後、次の焼入れ工程までの時間が５時間を超える場合には、効果的に遅れ破壊を防止するために中間焼戻し処理を実施するのが好ましい。
【００９２】
この各焼入れ処理の間で行う中間焼戻し温度の上限は継目無鋼管に所望の特性（強度、靱性、硫化物応力割れ抵抗性）を付与するためにＡc₁点以下の温度とする必要がある。次の再加熱焼入れ処理で超微細な整粒組織を確実に得るためにはこの中間での焼戻しにおける温度は７００℃以下とすることが好ましい。また、この中間焼戻しはそれによって遅れ破壊の防止が可能な温度、例えば５００℃以上の温度で行えば充分である。
【００９３】
なお、本発明の方法は、直接焼入れをＤＱ、再加熱焼入れをＲＱ、焼戻しをＴＥ、遅れ破壊防止のための中間焼戻しをＭＴＥと表記した時、次の７つのプロセスのことをいう。
【００９４】
▲１▼ＤＱ−ＴＥ
▲２▼ＤＱ−ＲＱ−ＴＥ
▲３▼ＤＱ−ＭＴＥ−ＲＱ−ＴＥ
▲４▼ＤＱ−ＲＱ−ＲＱ−ＴＥ
▲５▼ＤＱ−ＲＱ−ＭＴＥ−ＲＱ−ＴＥ
▲６▼ＤＱ−ＭＴＥ−ＲＱ−ＲＱ−ＴＥ
▲７▼ＤＱ−ＭＴＥ−ＲＱ−ＭＴＥ−ＲＱ−ＴＥ
【００９５】
【実施例】
表１、２に示す化学組成を有する鋼を通常の方法によって溶製した。表１における鋼Ａ〜Ｌは本発明の対象鋼（以下、本発明鋼という）、表２における鋼Ｍ〜Ｔは成分のいずれかが本発明で規定する含有量の範囲から外れた比較鋼である。
【００９６】
次いで、これらの本発明鋼および比較鋼を通常の方法によって直径２２５ｍｍのビレットとなし、表３〜１０に示す条件で製管、再加熱処理、直接焼入れ、再加熱焼入れ、中間焼戻しおよび焼戻しを行った。なお、予備実験に基づき、鋼種と熱処理条件によって焼戻し温度を変化させて製品強度を米国石油協会（ＡＰＩ）規格の１１０グレード（降伏強度：７７〜８８ｋｇｆ／ｍｍ²）、１２５グレード（降伏強度：８８〜９８ｋｇｆ／ｍｍ²）、１４０グレード（降伏強度：９８〜１０９ｋｇｆ／ｍｍ²）の３段階に変化させた。
【００９７】
こうして得た製品（継目無鋼管）の強度、靱性、オ−ステナイト結晶粒度、異常粒成長の発生の有無および耐硫化物応力割れ性を調査した。なお、靱性はシャルピー衝撃試験における破面遷移温度で評価した。異常粒成長についてはＡＳＴＭ（Ｅ１１２）に準じた切片法によって、１０００μｍの距離をリニアアナライザでスキャンし、粒界との交点の数から平均結晶粒切断切片長さを求め、更に、任意の位置の２００倍のミクロ組織写真（７ｃｍ×１０ｃｍ）５視野から求めた最大結晶粒切断切片長さとの比が３倍未満の場合に異常粒成長「無」、３倍以上の場合に異常粒成長「有」と判定した。また、耐硫化物応力割れ性はＮＡＣＥ−ＴＭ０１−７７−ＭＥＴＨＯＤ−Ｂにて規定されるＳｃ値で評価した。
【００９８】
試験結果を表１１〜１８に示す。
【００９９】
一般に、高強度になればなるほど耐硫化物応力割れ性と靱性は低下する傾向があるので、以下、強度グレード別に性能を比較する。
【０１００】
本発明例である試験番号１〜１２は、交叉角０度の穿孔機を用いて穿孔し、その後仕上げ圧延と再加熱処理を行い、更に、直接焼入れ−焼戻しを行って１１０グレードに調整したものである。同等の強度レベルである従来例の試験番号９３と比較すると、結晶粒径が微細であり、靱性と耐硫化物応力割れ性が向上している。また、本発明例である試験番号２１〜３２は交叉角５〜２９度の交叉穿孔機により穿孔し、その後仕上げ圧延と再加熱処理を行い、更に、直接焼入れ−焼戻しを行って１１０グレードに調整したものである。交叉角０度の穿孔（交叉角のない穿孔）を行った試験番号１〜１２よりも交叉穿孔機を用いることにより仕上げ圧延での強加工が容易になって靱性と耐硫化物応力割れ性の一層の向上が認められる。
【０１０１】
本発明例である試験番号１３〜１６は、交叉角０度の穿孔機を用いて穿孔を行い、直接焼入れ後、更に、中間焼戻し、再加熱焼入れおよび焼戻しを実施するプロセスで１２５グレードに調整したものである。同等強度グレードの従来例である試験番号９４と比較すると結晶粒径が微細で、靱性と耐硫化物応力割れ性が向上している。本発明例の試験番号３３〜４８は交叉角５〜２９度の交叉穿孔機により穿孔して、同様に１２５グレードに調整したものである。交叉角０度の穿孔（交叉角のない穿孔）を行った試験番号１３〜１６よりも交叉穿孔機を用いることにより仕上げ圧延での強加工が容易になって靱性、耐硫化物応力割れ性のより大きな向上が生じている。
【０１０２】
本発明例の試験番号１７〜２０は、交叉角０度の穿孔機を用いて穿孔し、直接焼入れ後、更に、２回ずつの再加熱焼入れと中間焼戻しを実施し、焼戻しによって１４０グレードに調整したものである。同等の強度レベルを示す従来例の試験番号９５と比較すると結晶粒径が極めて小さく、従って、靱性と耐硫化物応力割れ性において従来例を大きく上回る性能が得られている。また、本発明例である試験番号４８〜７２は交叉角５〜３２度の交叉穿孔機により穿孔して、１４０グレードに調整したものである。交叉角０度の穿孔（交叉角のない穿孔）を行った試験番号１７〜２０よりも交叉穿孔機を用いたことにより仕上げ圧延での強加工が容易になり、靱性と耐硫化物応力割れ性の一層の向上が認められる。
【０１０３】
比較例としての試験番号７３と７４は、仕上げ圧延後再加熱処理してから直接焼入れ−焼戻しを行って１１０グレードに調整したものであるが、仕上げ圧延の加工度（加工率）、圧延仕上がり温度といった加工熱処理条件のいずれかが本発明で規定する範囲から外れるため、結晶粒微細化効果が生じず、従って、従来例の試験番号９３と比較しても靱性と耐硫化物応力割れ性が劣っている。また、試験番号７５と７６では焼入れ温度が確保できないので完全な焼入れ組織が得られない。従って、１１０グレードを得るための焼戻し温度が低くなってしまうので結晶粒は微細であるが、靱性と耐硫化物応力割れ性が低い。
【０１０４】
一方、強度レベルで１２５グレードまたは１４０グレードを示す比較例の試験番号７７〜８４も同様に、仕上げ圧延の加工度（加工率）、圧延仕上がり温度、再加熱処理といった加工熱処理条件が本発明で規定する範囲を外れるため、結晶粒微細化効果が小さく、その後に再加熱焼入れ−焼戻しの所謂「調質処理」を行っても、前履歴の影響を受けて最終の結晶粒は粗大となる。従って、特に、本発明例と比べて靱性と耐硫化物応力割れ性は劣っている。本発明例にあっては異常粒成長が生じないが、比較例の試験番号７４、７７、７８、８１および８２では異常粒成長が観察され、靱性と耐硫化物応力割れ性が劣化している。つまり、直接焼入れ後に更に調質処理を行う場合には、微細なＮｂやＴｉの炭窒化物を分散させておく必要があることが分かる。
【０１０５】
比較例である試験番号８５〜９２は、本発明で規定した製造条件は満たしているものの、化学組成が本発明で規定した条件から外れた比較鋼を用いたものである。すなわち、Ｃｒ、Ｍｏ、Ｎｂ、Ｔｉ、Ｐ、Ｓ、Ｎおよびｆｎ１のいずれかが本発明の規定条件外にあるため、本発明例の同等強度レベルのものと比較して、靱性と耐硫化物応力割れ性が劣っている。
【０１０６】
Ｐ、Ｓの含有量を減らすことが耐硫化物応力割れ性を向上させることは知られているが、本発明の直接焼入れプロセスを用いることによって、従来の再加熱焼入れ焼戻し処理に比べ、耐硫化物応力割れ性の向上効果が大きく現れる。すなわち、従来の再加熱焼入れ焼戻し処理の場合、試験番号９６、９７と試験番号９８、９９との比較から分かるように、Ｐおよび／またはＳの含有量減少による耐硫化物応力割れ性向上効果はそれほど大きく現れない。これに対して本発明の直接焼入れプロセスでは、同じ本発明例ではあっても、試験番号２１〜２６と試験番号２７〜３２との比較から、同等強度レベルでありながらＰおよび／またはＳの含有量を減少させると耐硫化物応力割れ性向上に対する効果が極めて大きく現れることが明らかである。
【０１０７】
【表１】

【０１０８】
【表２】

【０１０９】
【表３】

【０１１０】
【表４】

【０１１１】
【表５】

【０１１２】
【表６】

【０１１３】
【表７】

【０１１４】
【表８】

【０１１５】
【表９】

【０１１６】
【表１０】

【０１１７】
【表１１】

【０１１８】
【表１２】

【０１１９】
【表１３】

【０１２０】
【表１４】

【０１２１】
【表１５】

【０１２２】
【表１６】

【０１２３】
【表１７】

【０１２４】
【表１８】

【０１２５】
【発明の効果】
以上説明したように、本発明によれば、従来の再加熱焼入れ焼戻しのプロセスよりも省プロセスでありながら、これと同等以上の性能を有する継目無鋼管が得られる。更に、直接焼入れ後に再加熱焼入れ焼戻しの所謂調質処理を実施することで、従来は充分な性能が得られなかった高強度の継目無鋼管においても良好な靱性と耐硫化物応力割れ性が得られる。これによって、油井の開発コストの引き下げ、更に従来開発が困難であった深度の大きい油井の開発が可能になるなど、今後のエネルギーの安定供給に対する寄与は計り知れず、産業上の効果は極めて大きい。
【図面の簡単な説明】
【図１】交叉角を説明する図で、（ａ）は平面図、（ｂ）は側面図である。[0001]
[Industrial application fields]
The present invention relates to a method for producing a high-strength seamless steel pipe excellent in resistance to sulfide stress cracking. More specifically, the present invention relates to a method for producing a seamless steel pipe excellent in sulfide stress cracking resistance, strength, and toughness, which is implemented by combining a specified component material (billet) with a heat treatment and a heat treatment under specific conditions. .
[0002]
[Prior art]
In the manufacture of steel products that require huge equipment, from the viewpoint of process and energy saving, simplification of the process by applying online heat treatment is being considered. Especially in the production of steel plates and thick plates, the production by off-line quenching and tempering has decreased considerably, and online processing materials have become the majority. However, in the production of seamless steel pipes, in particular, high-strength seamless steel pipes, from the viewpoint of high reliability and high quality, a considerable number of products are still produced by quenching and tempering treatment. Therefore, it was necessary to install and operate a quenching furnace and a tempering furnace separately from the pipe making line. On the other hand, there is a movement to introduce a so-called direct quenching process in which quenching is performed immediately using the heat held by the material after hot working, thereby eliminating the need for a quenching furnace and resulting in a significant industrial cost reduction. It is being
[0003]
For example, JP-A-58-224116, JP-A-60-75523, JP-A-6-172859 and the like disclose a process of forced cooling immediately after hot working and direct quenching in the production process of seamless steel pipes. A method of manufacturing a steel pipe having a high strength and a high corrosion resistance has been proposed. However, the crystal grain size of the product manufactured through the direct quenching process is coarse compared to the product manufactured by the so-called reheat quenching process, in which the product is reheated and quenched after conventional rolling. There was a problem of poor stress cracking.
[0004]
As a method of refining crystal grains, online cooling and reheating are combined, and the transformation from austenite and the reverse transformation to austenite are performed twice in total to make the grains finer. Techniques for achieving this have been proposed. For example, a cooling and reheating process is incorporated between rough rolling and finish rolling, Japanese Patent Laid-Open No. 56-3626, or a combination of cooling and reheating after final finish rolling, Japanese Patent Laid-Open No. 58-91123, JP-A-58-104120, JP-A-63-11621, and JP-A-4-358023 are disclosed. Furthermore, a method for refining crystal grains by cooling and reheating twice during and after rolling as in Japanese Patent Application Laid-Open No. 58-117832 has been proposed. According to the methods proposed in the above-mentioned publications, it is possible to refine the crystal grains of the steel material that has been directly quenched. However, (1) none of the proposed proposals have sufficient crystal grain refinement when high sulfide stress cracking resistance is required. (2) On-line transformation started or completed It is not preferable in terms of energy loss and consumption to forcibly cool to the temperature range to be reheated to the temperature range where reverse transformation is completed again. (3) Compared with conventional offline reheating and quenching In addition, there is a problem that equipment with high construction costs is required. Therefore, the steel pipe and steel plate manufacturing methods proposed in each of the above publications have little merit in terms of performance and cost, and the industry wants to manufacture high quality seamless steel pipes with low cost equipment with high productivity. It did not always meet the expectations.
[0005]
In addition, from the viewpoint of improving crystal grain refinement and hardenability, there is also a technique in which fine crystal grains are obtained by a process of processing in an unrecrystallized region and further recrystallizing, and then directly quenching and tempering. 62-139815 and JP-A-63-223125. In the method of Japanese Patent Application Laid-Open No. 62-139815, from the relationship between the behavior of boron (B) from the end of hot rolling to quenching and the quenching effect of steel, the temperature is close to the rolling finish temperature after hot rolling. The austenite grains are retained and recrystallized, and the solid solution B is secured to increase the strength and toughness. In the method disclosed in Japanese Patent Laid-Open No. 63-223125, after sufficient rolling in the non-recrystallization temperature range, Ar is obtained after completion of rolling in order to obtain uniform recrystallized grains._ThreeIt is rapidly heated without lowering below the point and soaked for a short time, and further directly hardened and tempered to form a uniform fine grain structure with a JIS grain size number of 8 or more, increasing strength and toughness.
[0006]
However, both of these technologies are technologies for producing steel plates made of low carbon steel, which are relatively easy to recrystallize and grow crystal grains. These technologies are seamless steel pipes made of medium carbon steel such as steel tubes for high corrosion resistance oil wells. Even if it is applied to the production of the same, it is considered that the same effect is difficult to obtain. This is because steel pipes that have undergone a complex rolling process, even if rolling at a low recrystallization temperature range, that is, a relatively low temperature range, is easy when rolling a steel plate, particularly a low carbon steel plate. This is because it is extremely difficult in the case of rolling steel pipes, and it is not easy to apply the steel plate process to steel pipes. Specifically, when rolling by a plug mill method or a mandrel mill method, which is a general rolling method for seamless steel pipes, is performed at, for example, 1000 ° C. or less as the non-recrystallization temperature range, (1) the rolling capacity of the mill is increased. However, (2) surface flaws and defects are generated, and (3) the mandrel bar is extremely difficult to pull out, which is far from practical use.
[0007]
Techniques intended for recrystallization after rolling or during rolling are also disclosed in JP-A-61-238917, JP-A-5-255749, JP-A-5-255750, and JP-A-5-271772. Proposed in each publication.
[0008]
Japanese Patent Application Laid-Open No. 61-238917 discloses a technique for recrystallizing 90% or more by strictly defining heating conditions after rolling a seamless steel pipe having a specific chemical composition. However, since there is no description about the processing conditions of the seamless steel pipe, the heating method after rolling described in this publication is simply used in the plug mill method and the mandrel mill method which are general rolling methods of seamless steel pipes. Even if it is applied, it does not necessarily mean that a fine grained structure can be obtained.
[0009]
In JP-A-5-255549 and JP-A-5-255750, a hollow tube having a target outer diameter and thickness is obtained after forcibly cooling an element tube made of a specific steel component to 1100 to 900 ° C. during rolling. In order to obtain a raw pipe, a technique has been proposed in which rolling is performed at a thickness reduction ratio of 15% or more, and the hollow raw pipe is reheated to 900 to 1000 ° C., finish-rolled, and directly quenched. However, in this method, even if ultrafine crystal grains are obtained during rolling, crystal grains grow by reheating, so the austenite grain size finally obtained is ASTM No. at most 8.9. is there. Furthermore, in the above method, since the processing amount (processing rate) of finish rolling is extremely small, in some cases, the crystal grains grow abnormally and do not necessarily have a fine grain structure.
[0010]
Thus, the reheating process in the middle of rolling is not necessarily preferable from the viewpoint of grain refinement. Although it is possible to set the reheating temperature to a temperature range in which the crystal grains are not coarsened, the rolling after the reheating becomes rolling in the non-recrystallized area and becomes an elongated grain structure or a mixed grain structure. In particular, the elongated grain structure has a problem that the hardenability is significantly lowered and the anisotropy is increased, and it cannot be used as a seamless steel pipe that requires extremely good corrosion resistance.
[0011]
Japanese Patent Laid-Open No. 5-271773 discloses a technique in which a steel slab having a specific steel component is made into a rough tube, then reheated to 900 to 1000 ° C., finish-rolled, and directly quenched to obtain a martensite structure of 90% or more. Is disclosed. However, there is no description at all about the processing conditions of the steel pipe, and since reheating is performed in the middle of rolling in the same manner as in the methods of JP-A-5-25549 and JP-A-5-255750 described above, Even a method does not necessarily provide a fine grained structure.
[0012]
Also, the austenite grain size finally obtained is ASTM No., which is at most 7.3.
[0013]
Japanese Patent Laid-Open Nos. 6-172854, 6-172858, and 6-184711 have proposed techniques for elaborating the arrangement of steel components and rolling mills to form fine grains and directly quenching. . The technology described in each of the above publications continuously arranges two or more inclined rolling mills having an extremely large shear strain component, and uses them to form a hollow shell into a product shape. In this case, the rolling temperature in each inclined rolling mill is set to a lower temperature than usual, or the first stage rolling temperature is set to a lower temperature than usual, and the raw tube is heated by processing heat generation. After the processing by the final stage of the inclined rolling mill, final finishing rolling for shaping is performed. In some cases, the hollow shell is reheated before the final finish rolling, that is, after the processing by the final-stage inclined rolling mill. However, in the rolling temperature range and rolling reduction as defined in the above-mentioned publication, even with an inclined rolling mill, severe rolling occurs and pipe making flaws may occur frequently. Further, since the processing rate (processing amount) in the hot finish rolling is small even by this method, the austenite grain size obtained is only ASTM No. 10.7 at most.
[0014]
On the other hand, a technique for refining the crystal grains by directly re-quenching the steel after directly quenching the steel to improve the resistance to sulfide stress cracking is disclosed in, for example, JP-A-6-220536 and JP-A-60. -43424, JP-A-60-52520, JP-A-60-46318, JP-A-60-86208, JP-A-60-46317, and JP-A-60-86209. Yes.
[0015]
Among these, JP-A-6-220536 discloses a method in which a steel pipe having a specific chemical composition is directly quenched and then reheated and quenched. However, since this method has not been clarified at all about the processing conditions of the steel pipe, especially the finish rolling conditions before direct quenching, the finish rolling is performed by the plug mill method and the mandrel mill method, which are general rolling methods of seamless steel pipes. However, if it is directly quenched, abnormal grain growth may occur in the subsequent reheating quenching process, and it is not necessarily an ultrafine grained structure, and the resistance to sulfide stress cracking is inferior. Sometimes it ends up.
[0016]
In JP-A-60-43424 and JP-A-60-52520, a steel material having a specific steel component is subjected to hot working before direct quenching, and the cross-sectional compression ratio at 1100 ° C. or less is set to 20% or more. A method of reheating and quenching after direct quenching has been proposed. However, in these methods, although finish rolling in a lower temperature range of 1100 ° C. or less is specified, the processing rate (compression ratio of the cross section) is limited to about 40% at most, as is apparent from the examples. . Conventionally, in the case of seamless steel pipe production, (1) rolling in a high temperature range where the deformation resistance of the steel material is extremely small, or (2) reheating between rough rolling and finish rolling, in terms of mill rolling capacity. This is because a high working rate cannot be ensured by finish rolling unless any means of increasing the temperature to lower the deformation resistance of the hollow shell is used. However, the austenite grains after direct quenching, which are the initial grains at the time of reheating and quenching, are not sufficiently fined only by rolling at the finishing rate of about 40%, and therefore, what is necessary for reheating and quenching treatment? The problem remains that it would be difficult to obtain the desired ultrafine particles unless repeated.
[0017]
JP-A-60-46318 and JP-A-60-86208 disclose austenite region without first initiating transformation after first hot working a steel material having a specific chemical composition in the austenite region. A method is proposed in which secondary hot working is carried out by holding or reheating, followed by reheating and quenching after direct quenching. However, in this method, holding or reheating is performed without transformation between the primary processing and the secondary processing, so that the austenite grains after direct quenching that are initial grains at the time of reheating quenching are sufficiently fine. Therefore, there is a problem that it is difficult to obtain desired ultrafine grains unless the reheating and quenching process is repeated many times. Furthermore, since there is no description about the processing conditions, particularly the secondary processing conditions before direct quenching, if the secondary processing (finishing processing) is performed directly by the general rolling method of seamless steel pipe, In some cases, the repeated reheating and quenching process may cause abnormal grain growth, which does not necessarily result in an ultrafine grain size structure and may have poor resistance to sulfide stress cracking.
[0018]
In JP-A-60-46317 and JP-A-60-86209, a steel material having a specific steel component is subjected to primary hot working and then once transformed, and then reheated to the austenite region. Thus, a method of performing secondary hot working and reheating and quenching after direct quenching is disclosed. In this method, the austenite grains after direct quenching, which becomes the initial grains at the time of reheating and quenching, become fine because the transformation is performed between the primary processing and the secondary processing, but it is cooled to a temperature range where transformation is completed. Reheating to a temperature range in which reverse transformation to austenite is completed is not preferable in terms of energy loss and consumption, and requires large-scale equipment, which is problematic in terms of cost. In addition, in this method as well, there is no mention of processing conditions, particularly secondary processing conditions before direct quenching, so secondary processing (finish processing) is performed by a general rolling method of seamless steel pipes. If the direct quenching is performed, abnormal grain growth may occur in the subsequent repeated reheating and quenching process, and the resistance to sulfide stress cracking is inferior without necessarily forming an ultrafine grain size structure. Sometimes it ends up.
[0019]
By the way, in order to improve the resistance to sulfide stress cracking, (1) the method of specifying the chemical composition of the material, (2) the method of specifying the structure, (3) the method of heat treatment technique and (4) the above-mentioned There is a way to combine. Among them, regarding the component limitation, JP-A-62-253720 discloses a method for defining the amounts of Si, Mn, P and Mo and yield stress, and JP-A-63-274717 discloses a high-C steel. As a selection method, JP-A-62-149813 and JP-A-63-238242 each propose a method of adding Zr. Regarding structure control, it is a well-known fact that a structure mainly composed of tempered martensite is excellent in resistance to sulfide stress cracking, and that a finer grain structure is more desirable. However, JP-A-63-93822 discloses bainite. JP-A-62-230849 discloses a method of forming a stretched grain. Furthermore, Japanese Patent Application Laid-Open Nos. 54-117311 and 61-9519 propose a method of applying rapid heating such as induction heating as a heat treatment technique for forming a fine grain structure. Although the above-mentioned method has an effect of improving the resistance to sulfide stress cracking and improves the quality of seamless steel pipes, especially oil well steel pipes, it is a technology that uses conventional reheating quenching and tempering treatment, and is directly quenched. By doing so, high quality seamless steel pipes are highly productive, and that does not meet the demands of the industry that wants to manufacture with inexpensive equipment.
[0020]
[Problems to be solved by the invention]
High-strength and corrosion-resistant seamless steel pipes are generally pierced with a piercer by a tilt rolling method, stretched with a plug mill or mandrel mill, finished with a sizer or reducer, and then tempered by quenching and tempering. Manufactured. This quenching and tempering treatment has a movement to be carried out on the pipe making line from the viewpoint of rationalization of the process as described above, and the direct quenching process is a typical one. However, the conventionally proposed direct quenching of seamless steel pipes has many problems as described above.
[0021]
The present invention has been made in view of the present situation, and by specifying the steel (material) component, rolling and direct quenching conditions, and heat treatment conditions, a seam having high sulfide stress cracking resistance and high strength and high toughness. It aims at providing the manufacturing method of a steel-free pipe. Furthermore, it is another object to provide a method for producing a seamless steel pipe having higher resistance to sulfide stress cracking, high strength and high toughness by specifying the heat treatment conditions in the pipe making offline. .
[0022]
[Means for Solving the Problems]
As a result of repeated experiments and researches to achieve the above-mentioned problems, the present inventors specified the component system even in the direct quenching process, and specified the conditions for piercing, drawing, and finish rolling to be appropriate. It was found that fine crystal grains with uniform grain size can be obtained by subjecting to various heat treatments, and further, the temper softening resistance is increased, so that significant performance improvement can be achieved as compared with ordinary reheat quenching / tempering materials. In addition, if the grain size after direct quenching is made fine and abnormal grain growth is prevented by dispersing a large amount of fine precipitates and preventing the movement of grain boundaries, reheating quenching after direct quenching It has been found that an ultrafine crystal grain of uniform size can be obtained. These are summarized as (a) to (k) below.
[0023]
(A) When a medium carbon steel billet with a composite addition of Nb and Ti is finish-rolled into a seamless steel pipe, a large processing strain of 40% or more in terms of the cross-sectional compression ratio is obtained at a finish temperature of 800 to 1050 ° C. Further, after the finish rolling, if reheating is performed for a time t at a temperature T in the temperature range of 850 to 1100 ° C., and the value of fn2 is 23500 to 26000, fine recrystallized grains can be obtained.
[0024]
fn2 = (T + 273) (21 + logt),
However, they are T (° C.) and t (h).
[0025]
(B) If the recrystallized structure of the above (a) is directly quenched, the austenite grains become as fine as those when reheated and quenched after rolling.
[0026]
(C) If finish rolling is performed before the work strain imparted by the stretching process after drilling is recovered, the finish rolling in (a) above integrates both the stretching process and the finishing process (finish rolling). I can do it.
[0027]
(D) In order to perform finish rolling before the working strain applied in the stretching process after piercing is recovered, for example, a mandrel mill as a stretching mill and a finishing mill that have been conventionally arranged independently The sizer may be an integrated continuous arrangement.
[0028]
(E) Using a cross-piercing machine having a cone-shaped roll in the drilling process, drilling is performed at a crossing angle of 5 to 35 degrees (angle formed by the roll axis with respect to the horizontal or vertical plane of the pass line, see FIG. 1). Therefore, it is possible to reduce the wall thickness of the hollow shell obtained compared with the case of rolling using a normal barrel-type roll drilling machine. Strong processing in the finish rolling of (c), in which both processes are integrated, becomes easy. Therefore, strong processing of 40% or more is possible at a finish temperature in a relatively low temperature range of 1050 ° C. or less without particularly performing reheating treatment for reducing the deformation resistance of the hollow shell.
[0029]
(F) If heat holding is performed after the finish rolling under the conditions shown in (a) above, a large amount of fine carbonitrides of Nb and Ti precipitate, and an appropriate amount of solid solution containing Nb and Ti, etc. Crystal grains are obtained.
[0030]
(G) Reheating when Nb and Ti carbonitrides are agglomerated and coarsened by reheating treatment after finish rolling, or most of the solid solution such as Nb and Ti is precipitated as carbides and carbonitrides. Even if quenched, it does not become an ultrafine grain of sized particles, and it is difficult to obtain an effect of increasing the temper softening resistance. However, if the recrystallized grains of (f) above are reheated and quenched, the movement of grain boundaries is prevented, abnormal grain growth is prevented, and ultrafine grains are sized. Since undissolved carbonitrides are reduced, the resistance to sulfide stress cracking is significantly improved.
[0031]
(H) The dissolved Nb and Ti precipitate as fine carbonitrides during tempering and greatly increase the temper softening resistance. Since this temper softening resistance increase enables tempering at a high temperature, the internal strain is alleviated even at the same strength, and further, the carbide is spheroidized, so that the sulfide stress cracking resistance is further improved.
[0032]
(I) When recrystallization by processing and reheating occurs in the middle of hot processing, it is necessary to recrystallize again after the final processing. In this case, reheating in a relatively high temperature range is required. Since it is necessary, the effect of recrystallizing grains is small. On the other hand, in the case of reheating immediately before the direct quenching of the above (a), the refining effect by recrystallization becomes the largest, and in addition, it is easy to ensure the quenching temperature during the direct quenching. It is also possible to prevent the occurrence of directionality.
[0033]
(J) In the case of the process of reheating after finish rolling, in other words, directly before quenching, the equipment and operation are different from the process of reheating between roughing and final processing (finishing). The cost can be kept small.
[0034]
(K) A reheat treatment is performed immediately before direct quenching to refine the recrystallized grains, and a seamless solution containing an appropriate amount of solid solution elements such as Nb and Ti and a large amount of fine carbonitrides of Nb and Ti. If the steel pipe is reheat-quenched or reheat-quenched twice, further refinement can be achieved as compared with the case where normal reheat quenching is repeated after rolling. In particular, if the heating and holding conditions during reheating satisfy the above condition (a), Nb and Ti carbonitrides do not coarsen even after repeated reheating quenching and tempering treatment, and during reheating and quenching. Can prevent grain coarsening and abnormal grain growth, and further maintains the effect of increasing resistance to temper softening. Excellent toughness and large sulfide stress cracking that cannot be obtained even after repeated reheating and quenching treatment. Resistance is obtained.
[0035]
The inventors of the present invention have obtained the following new findings as a result of repeated experiments and research in order to obtain even greater resistance to sulfide stress cracking.
[0036]
(L) It is known that P and S contained as impurities in steel cause deterioration of the resistance to sulfide stress cracking, but in the case of a process of reheating and recrystallization immediately before direct quenching. In particular, if the P and S contents are regulated to 0.005% or less and 0.0007% or less by weight%, particularly high sulfide stress cracking resistance can be obtained.
[0037]
Although the reason for the above has not been fully elucidated, the following can be considered.
[0038]
(1) At the quenching temperature in the conventional reheating quenching process where reheating is performed after rolling, even when the P content is reduced to 0.005% by weight or less, the segregation generation limit amount of P is extremely small. Therefore, grain boundary segregation remains. On the other hand, in the case of reheating and direct quenching subsequent to finish rolling, the amount of dissolved P is increased and the segregation limit amount of P is 0.005% by weight or more, so that segregation is almost eliminated.
[0039]
(2) At the quenching temperature in the conventional reheating quenching process, even when the S content is reduced to 0.0007% by weight or less, MnS that cannot be completely dissolved remains as inclusions. On the other hand, when reheating and directly quenching after finish rolling, the solid solution amount of S increases, and if the amount of S is 0.0007% by weight or less, all of the solid solution is dissolved, so the inclusions are significantly reduced. To do.
[0040]
(3) Since the effects of extremely low P and extremely low S are brought about independently of each other, if either the extremely low P or the extremely low S is satisfied, the resistance to sulfide stress cracking can be improved. If this is satisfied, further improvement in resistance to sulfide stress cracking can be achieved.
[0041]
  The present invention based on the above findings includes the following (1) to(5)The summary is a method for producing a high-strength seamless steel pipe excellent in resistance to sulfide stress cracking.
[0042]
  (1) By weight%, C: more than 0.20% to 0.50%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.5%, Cr: 0.1 to 1. 5%, Mo: 0.1 to 1.5%, Nb: 0.005 to 0.50%, Ti: 0.005 to 0.50%, B: 0.0001 to 0.01%, Al: 0 0.005-0.50%And the balance consists of Fe and inevitable impurities, Ni in the impurities is 0.1% or less, P is 0.05% or less, S is 0.01% or less, N is 0.01% or less, and O is 0 .01% or less, andfn11When producing a seamless steel pipe by hot-drilling a billet having a component composition of> 0 and rolling it, a finish rolling with a cross-sectional compression ratio of 40% or more is finished after the drilling. After that, after reheating for a time t at a temperature T in the temperature range of 850 to 1100 ° C. to make the value of fn2 below 23500 to 26000, direct quenching is performed immediately, and then Ac₁A method for producing a high-strength seamless steel pipe excellent in sulfide stress cracking resistance, characterized by tempering at a temperature below the point.
However,
  fn11 = Ti (%) − ( 48 / 14 ) {N (%)},
  fn2 = (T + 273) (21 + log t),
It is.
[0043]
  (2) By weight, C: more than 0.20% to 0.50%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.5%, Cr: 0.1 to 1. 5%, Mo: 0.1 to 1.5%, Nb: 0.005 to 0.50%, Ti: 0.005 to 0.50%, B: 0.0001 to 0.01%, Al: 0 0.005 to 0.50%, further containing one or more of V: 0.5% or less, Zr: 0.5% or less, and Ca: 0.01% or less, and the balance Consists of Fe and inevitable impurities, Ni in impurities is 0.1% or less, P is 0.05% or less, S is 0.01% or less, N is 0.01% or less, and O is 0.01% or less In addition, when a billet having a component composition of fn12> 0 is hot drilled and rolled to produce a seamless steel pipe, the cross-sectional compression ratio is 40% or more following the drilling. Finish rolling is performed at a finished temperature of 800 to 1050 ° C., and then reheating for a time t at a temperature T in the temperature range of 850 to 1100 ° C., and the value of fn2 is 23500 to 26000, and then directly quenching is performed. Then Ac ₁ A method for producing a high-strength seamless steel pipe excellent in sulfide stress cracking resistance, characterized by tempering at a temperature below the point.
However,
  fn12 = Ti (%) − ( 48 / 14 ) {N (%)-( 14 / 91 ) Zr (%)},
  fn2 = (T + 273) (21 + log t),
Note that T (° C.) and t (h).
[0044]
  (3)The hot drilling is performed at a crossing angle of 5 to 35 degrees using a cross drilling machine (1)Or (2)The manufacturing method of the high intensity | strength seamless steel pipe excellent in the sulfide stress cracking resistance of description to description.
[0045]
  (4) BilletP in the impurity is 0.005% or less or S is 0.0007% or less, or P is 0.005% or less and S is 0.0007% or less (1)To (3)The manufacturing method of the high intensity | strength seamless steel pipe excellent in the sulfide stress cracking resistance in any one of these.
[0046]
  (5)1 or 2 Ac between direct quenching and tempering_ThreePoint ~ [Ac_ThreeFrom the above (1), characterized by performing reheating and quenching after heating to a temperature range of point + 100 ° C](4)The manufacturing method of the high intensity | strength seamless steel pipe excellent in the sulfide stress cracking resistance in any one of to.
  Hereinafter, fn11 not including the Zr term and fn12 including the Zr term are collectively referred to as fn1, and even when the billet does not include Zr, “fn1 = Ti (%) − ( 48 / 14 ) {N (%)-( 14 / 91 ) Zr (%)} ”.
[0047]
[Action]
Hereafter, each requirement of this invention is demonstrated in detail with the effect. “%” Of the component content means “% by weight”.
[0048]
(A) Billet chemical composition
C:
C is an element necessary for improving the hardenability of the steel and improving the strength. However, if its content is 0.20% or less, the effect of addition is poor and high strength cannot be obtained. On the other hand, when it contains exceeding 0.50%, a fire crack and a delayed fracture will occur easily and manufacture of a seamless steel pipe will become difficult. Therefore, the C content is set to more than 0.20% to 0.50%.
[0049]
Si:
Si is necessary for deoxidation of steel and is an effective element for improving the temper softening resistance and improving the resistance to sulfide stress cracking. However, when contained in excess, it has the effect of embrittlement of the steel. For the purpose of deoxidation and improvement of resistance to sulfide stress cracking, it is necessary to contain 0.1% or more. However, if it exceeds 1.5%, the toughness and resistance to sulfide stress cracking will be reduced. The content was set to 0.1 to 1.5%.
[0050]
Mn:
Mn is added for deoxidation and desulfurization of steel. However, if the content is less than 0.1%, the effect of addition is poor. On the other hand, if the content exceeds 1.5%, the toughness and sulfide stress cracking resistance of the steel are lowered. Therefore, the Mn content is set to 0.1 to 1.5%.
[0051]
Cr:
Cr is an element that ensures the hardenability of the steel, improves the strength, and improves the resistance to sulfide stress cracking. However, if the content is less than 0.1%, a sufficient addition effect cannot be obtained, and if it exceeds 1.5%, the toughness and the resistance to sulfide stress cracking are reduced, so the content is reduced to 0. 0.1 to 1.5%. The Cr content is more preferably 0.3 to 1.0%.
[0052]
Mo:
Mo is an element effective for enhancing the hardenability of steel to ensure high strength and improving the resistance to sulfide stress cracking. However, if the content is less than 0.1%, the effect of addition is poor. On the other hand, if the content exceeds 1.5%, not only the effect is saturated, but also the resistance to sulfide stress cracking is caused by segregation. Since it will deteriorate, the content was made 0.1 to 1.5%. In addition, more preferable content of Mo is 0.3 to 0.8%.
[0053]
Nb:
Nb precipitates as fine carbonitride by reheating treatment after finish rolling, and has the effect of preventing coarsening of crystal grains and abnormal grain growth during reheating and quenching. In addition, solid solution Nb has the effect of improving the resistance to sulfide stress cracking by increasing the temper softening resistance by finely depositing as carbonitride during tempering after direct quenching. However, if the content is less than 0.005%, the effect of addition is poor, and if it exceeds 0.50%, the toughness of the steel deteriorates, so the Nb content was made 0.005 to 0.50%. The Nb content is more preferably 0.01 to 0.10%.
[0054]
Ti:
Ti fixes N, which is an impurity in the steel, and has the effect of improving the hardenability of the steel by making B exist in the solid solution in the steel during quenching. Moreover, it has the effect of preventing coarse grain growth and abnormal grain growth during reheating and quenching by precipitating as fine carbonitride by reheating treatment after finish rolling. Further, solute Ti precipitates as fine carbides during tempering after direct quenching and has the effect of increasing the temper softening resistance. However, if the content is less than 0.005%, the effect of addition is small. On the other hand, if the content exceeds 0.50%, the toughness of the steel is deteriorated. Therefore, the Ti content is set to 0.005 to 0.50%. In addition, more preferable content of Ti is 0.01 to 0.10%.
[0055]
B:
B is an element that improves the hardenability of steel by adding a small amount, and particularly improves the resistance to sulfide stress cracking of thick materials. However, if the content is less than 0.0001%, the desired effect cannot be obtained. On the other hand, if the content exceeds 0.01%, the toughness and resistance to sulfide stress cracking of steel deteriorate. Therefore, the B content is set to 0.0001 to 0.01%.
[0056]
Al:
Al is an element effective for deoxidation of steel. However, if the content is less than 0.005%, the desired effect cannot be obtained, and if it exceeds 0.5%, inclusions increase and the toughness of the steel deteriorates, and the threaded portion of the seamless steel pipe is defective. Therefore, the content is set to 0.005 to 0.5%.
[0057]
  V:
  VThe addition of is optional.If added, it has the effect of precipitating as fine carbides during tempering and improving the resistance to sulfide stress cracking. In particular, the combined addition with Nb has the effect of imparting greater sulfide stress cracking resistance to the steel. In order to obtain such an effect reliably, it is preferable that V is 0.05% or more. However, if its content exceeds 0.5%, the toughness of the steel will deteriorate. Therefore, the V content is 0.5% or less.
[0058]
  Zr:
  ZrThe addition of is optional. If added, it has the effect of increasing the yield point elongation of the steel in the tensile test, and as a result, it has the effect of improving the resistance to sulfide stress cracking. In order to reliably obtain this effect, the content of Zr is preferably 0.01% or more. In addition to being an expensive element, if the content exceeds 0.5%, inclusions increase and the toughness of the steel deteriorates, so the upper limit of the Zr content was set to 0.5%.
[0059]
  Ca:
  CaThe addition of is optional. If added, it reacts with S in the steel to form sulfides, thereby improving the shape of the inclusions, so that there is an effect of improving the resistance to sulfide stress cracking of the steel. However, the degree of the effect varies depending on the content of S, and the resistance to sulfide stress cracking may be deteriorated unless deoxidation is sufficiently performed. good. In steel that has been sufficiently deoxidized, Ca is preferably contained in an amount of 0.001% or more in order to reliably obtain the above-described effects. However, if its content exceeds 0.01%, the toughness of the steel and the resistance to sulfide stress cracking deteriorate, and further, defects occur on the surface of the seamless steel pipe. Therefore, the Ca content is set to 0.01% or less.
  In addition, it can contain only 1 type in said V, Zr, and B, or 2 or more types of composite.
[0060]
Impurity elements Ni, P, S, N and O (oxygen) limit their contents as follows.
[0061]
Ni:
Ni deteriorates the resistance to sulfide stress cracking of steel, and particularly when its content exceeds 0.1%, the resistance to sulfide stress cracking deteriorates remarkably. Therefore, the content of Ni as an impurity element is set to 0.1% or less.
[0062]
P:
P segregates at the grain boundaries and degrades the toughness and sulfide stress cracking resistance of the steel. In particular, if its content exceeds 0.05%, the toughness and sulfide stress cracking resistance deteriorate significantly. Therefore, the upper limit of the content of P as the impurity element is set to 0.05%.
[0063]
S:
S produces coarse inclusions and degrades the toughness and sulfide stress cracking resistance of the steel. In particular, when the content exceeds 0.01%, the toughness and sulfide stress cracking resistance deteriorate significantly, so the upper limit of the content of S as an impurity element was set to 0.01%.
[0064]
By the way, in the case of the production method of the present invention in which reheating is performed for recrystallization immediately before direct quenching, the upper limit of the content of P and / or S as an impurity element is set as shown in the following examples. If it is further regulated, a particularly large sulfide stress cracking resistance can be obtained. That is, if the upper limit of the content of P as an impurity element is 0.005%, a large sulfide stress cracking resistance can be obtained. If the P content is 0.002% or less, the effect is even greater. Moreover, even if the upper limit of the content of S as an impurity element is 0.0007%, a large sulfide stress cracking resistance can be obtained. If the S content is 0.0003% or less, the effect is even greater.
[0065]
Since the effects of extremely low P and extremely low S are exhibited independently of each other, the content of P as an impurity element is 0.005% or less, and the content of S is 0.0007% or less. By doing so, further improvement in resistance to sulfide stress cracking can be achieved. Furthermore, if the content of P as an impurity element is 0.002% or less and the content of S is 0.0003% or less, the sulfide stress cracking resistance of steel becomes extremely large.
[0066]
N:
N hinders the effect of improving the hardenability of B, and also deteriorates the toughness and sulfide stress cracking resistance of steel. Especially when its content exceeds 0.01%, the deterioration of toughness and sulfide stress cracking resistance is remarkable. Become. Therefore, the upper limit of the content of N as an impurity element is set to 0.01%.
[0067]
O:
O deteriorates the toughness and sulfide stress cracking resistance of steel. In particular, if the content exceeds 0.01%, the toughness and sulfide stress cracking resistance deteriorate significantly, so the upper limit of the content of O as an impurity element was set to 0.01%.
[0068]
fn1> 0:
Ti and Zr have an extremely high affinity for N. Therefore, Ti (%)> (48/14) {N (%)-(14/91) Zr in order to increase the temper softening resistance by securing solid solution Ti and precipitating as fine carbides during tempering after direct quenching. (%)}, That is, it is necessary to provide a restriction that fn1 is greater than zero.
[0069]
(B) Billet processing
(B-1) Heating of billet:
The heating temperature of the billet may be any temperature that allows hot drilling with a punch. The optimum temperature varies depending on the material, and is appropriately determined in consideration of high temperature ductility and high temperature strength, but is usually heated between 1100 and 1300 ° C. By the way, in order to heat the billet with high efficiency, the billet length is set to be an integral multiple of the predetermined length, and the billet is cut to a predetermined length by a cutting machine installed at the rear stage of the heating furnace (the front stage of the drilling machine). It is preferable to perforate. The billet to be inserted into the heating furnace may be any one such as a piece-rolled one or a piece produced by continuous casting in a round mold. In order to save energy, the billet may be charged into a heating furnace after being subjected to split rolling or continuous casting and before being completely cooled to room temperature.
[0070]
(B-2) Perforation:
In this process, a hollow billet (hollow shell) is manufactured by opening a through-hole in a solid billet hot. In this drilling process, expanded thin-walled drilling is possible, and the thickness of the resulting hollow shell is made thinner than when rolled using a normal barrel-type roll drilling machine, and both the next stretching and finishing processes are performed. In order to facilitate the strong processing in finish rolling with the total processing, a cone-type roll cross punching machine may be used. In this case, the crossing angle of the cross punching machine is preferably 5 to 35 degrees. This is because if the crossing angle is less than 5 degrees, it is difficult to obtain a desired thin hollow shell, and it is difficult to perform high processing in subsequent processing. The crossing angle is desirably set to 35 degrees or less in order to avoid the occurrence of so-called “bottom clogging” where the bottom end of the hollow shell cannot be removed from the drilling machine and the rolling becoming unstable. Note that if the temperature of the billet is lowered, wrinkles are likely to occur during drilling. Therefore, an auxiliary heating device such as an induction heating device may be installed at a previous stage immediately before drilling, and the drilling may be performed after heating.
[0071]
(B-3) Stretching and finishing:
A process of producing a seamless steel pipe having a desired shape and size by stretching and adjusting the dimensions of a hollow hollow pipe drilled by a drilling machine, which is processed in a relatively low temperature region as compared with processing by a drilling machine. For this reason, it is an important process for giving effective processing when considering the heat treatment. In particular, if the finishing process (finish rolling) is performed before the processing strain imparted by the stretching process after the drilling is recovered, the sum of the processing rates of both is the so-called "finish rolling (finishing process)" processing rate. Therefore, a large finish rolling ratio can be imparted without reheating the hollow shell in the middle.
[0072]
In the present invention, it is necessary to perform finish rolling with a cross-sectional compression ratio of 40% or more at a finishing temperature of 800 to 1050 ° C. This is because in the case of a processing rate of less than 40% in terms of the cross-sectional compression rate, recrystallization does not proceed smoothly and a fine effect cannot be obtained, and sometimes crystal grains sometimes grow abnormally. The upper limit of the finish rolling processing rate does not need to be specified because it varies depending on the material to be pipe-made and the capability of the mill. However, if the processing rate is too large, wrinkles are likely to occur, so the upper limit is about 80%. preferable. When the rolling finish temperature exceeds 1050 ° C., the crystal grains become coarse and a desired fine grain structure cannot be obtained, so the upper limit of the finish temperature was set to 1050 ° C. The lower the finishing temperature of the rolling, the finer the recrystallized grains tend to be. However, if the temperature is too low, the deformation resistance of the steel material (hollow blank) increases, and strong processing with a cross-section compression ratio of 40% or more may be applied. It becomes difficult, and energy consumption for reheating performed to obtain a desired microstructure after the finish rolling increases, so the lower limit of the rolling finish temperature is set to 800 ° C.
[0073]
  By the way, in order to perform finish rolling before the processing strain applied in the stretching process after piercing is recovered, as already described,For example,What is necessary is just to let the extending | stretching rolling mill and finish rolling mill which were conventionally arrange | positioned independently be an integral continuous arrangement | positioning. By doing so, a finish rolling ratio of 40% or more can be stably secured for all sizes in the manufacturing range.
[0074]
With respect to the above-described one-piece continuously-drawn rolling mill and finish rolling mill, for example, a mandrel mill as a stretching mill constrains the rear end of a mandrel bar, which is an inner surface regulating tool, and after completion of the stretching rolling, As long as it is a type having mandrel bar restraining means that can be circulated and returned to the mill entry side through the hole-type roll train. In particular, it is preferable to use a mandrel mill having a function that allows the mandrel bar restraining means to control the moving speed of the mandrel bar at a speed independent of the rolling speed of the pipe during the drawing and rolling of the hollow shell. The sizer as a finishing mill may be any one that does not have an inner surface regulating tool, and in particular, a so-called extracting sizer having a function of pulling and separating a pipe from a mandrel bar in a pipe rolled by a mandrel mill. It is desirable to use it.
[0075]
(C) Reheating treatment
A major feature of the present invention is that a reheating treatment as a recrystallization treatment is performed between the finish rolling and the direct quenching treatment. As a result, recrystallization is induced by a combination of processing and heating, and crystal grains can be made finer. Unlike the conventional case where reheating is performed between roughing and finishing during rolling, the method of the present invention does not require processing after reheating, so the reheating temperature is set to the lowest temperature at which recrystallization proceeds. can do. For this reason, fine recrystallized grains can be obtained by only one reheating.
[0076]
As the reheating temperature T (° C.), in the case of the medium carbon low alloy steel which is the object of the present invention, it takes a long time to recrystallize at a temperature lower than 850 ° C. To do. On the other hand, at temperatures exceeding 1100 ° C., the crystal grains grow greatly and coarsening is significant. Accordingly, the reheating temperature T is set to a temperature range of 850 to 1100 ° C.
[0077]
The reheating time t (h) needs to be such that the value of fn2 is 23500 to 26000. When the value of fn2 is less than 23500, recrystallization does not end completely. When the value of fn2 exceeds 26000, Nb and Ti carbonitrides are agglomerated and coarsened, and most of Nb and Ti and the like are precipitated as carbonitrides, so that the effect of increasing the temper softening resistance during tempering after direct quenching. Can not be obtained, and even when re-heated and hardened, it does not become a sized ultra-fine grain. For this reason, the desired effect of improving the resistance to sulfide stress cracking cannot be obtained.
[0078]
By the way, the reheating process between the finish rolling and the direct quenching process, in other words, the reheating process immediately before the direct quenching, the quenching temperature during the direct quenching can be easily secured, and the seam A secondary effect is also obtained in that the uniformity of performance is greatly reduced by guaranteeing the heat uniformity between the longitudinal direction (rolling direction) of the steelless pipe and the lot.
[0079]
In addition, “reheating for a time t (h) at a temperature T (° C.) in the temperature range of 850 to 1100 ° C. to change the value of fn1 to 23500 to 26000” literally means “from 850 ° C. to 1100 ° C. It is also possible to hold the time t at an arbitrary temperature T between them and set the value of fn1 to 23500-26000 ”or“ any temperature T1, T2, T3,... Between 850 ° C. and 1100 ° C. If, at t1, t2, t3, and so on, the holding times t2, t3, ... at temperatures T2, T3, ... are converted to holding times t21, t31, ... at temperature T1, respectively. , The temperature (T1 + t21 + t31 + ..) is maintained at the temperature T1, and the value of fn1 is set to 23500 to 26000 ”.
[0080]
(D) Direct quenching
Since the direct quenching process must be performed from the austenite state, the quenching temperature is Ar._ThreeYou have to secure more points. However, in the method of the present invention, the finish-rolled seamless steel pipe is reheated to 850 to 1100 ° C. immediately before direct quenching._ThreeA sufficient quenching temperature above the point can be secured.
[0081]
The cooling rate at the time of direct quenching is not particularly limited, and may be such that a desired low-temperature transformation structure can be obtained over the entire thickness of the seamless steel pipe as judged from the component composition of the raw steel. Generally, it is faster than air cooling.
[0082]
In addition, if the target steel of the present invention is directly tempered immediately after the above-described perforation, stretching and finishing, and then the reheating treatment, the austenite grains are reheated and quenched after rolling even in the direct quenching. It becomes as fine as possible.
[0083]
(E) Tempering
The low-temperature transformation structure of the fine grain obtained by quenching treatment is expressed as Ac₁When the tempering treatment is performed at a temperature below the point, desired characteristics (strength, toughness, sulfide stress cracking resistance) can be imparted to the seamless steel pipe. That is, Ac₁Only after tempering at a temperature below the point can a high-strength seamless steel pipe excellent in desired sulfide stress cracking resistance can be obtained.
[0084]
The tempering process determines the performance of the product, requires sufficient soaking, and the temperature variation is ± 10 ° C or less, preferably ± 5 ° C or less, so that the strength (tensile strength, yield strength) changes. ± 5 kgf / mm²The following can be suppressed.
[0085]
Although it is not necessary to set a special lower limit to the tempering temperature, the tempering treatment at a high temperature alleviates and removes internal strain and internal stress of the low-temperature transformation product obtained by quenching, and spheroidizes the carbide to make it seamless. In view of improving the performance of the steel pipe, it is desirable to temper at a temperature of 550 ° C. or higher.
[0086]
(F) Reheating quenching and intermediate tempering performed between direct quenching and tempering
According to the method of the present invention, the structure immediately before quenching can be made into fine grain by processing and recrystallization, so that it has high sulfide stress cracking resistance and excellent toughness just by tempering after direct quenching. High-strength seamless steel pipes can be manufactured. For this reason, if the method of the present invention is applied, the high-strength seamless steel pipe is excellent in sulfide stress cracking resistance even if it is manufactured by tempering directly after quenching. Can withstand use in the environment. However, in some cases, higher toughness and sulfide stress cracking resistance may be required, and at this time, it is necessary to make a finer sized structure.
[0087]
The ultra-fine sized structure is obtained once or twice between direct tempering and tempering._ThreePoint ~ [Ac_ThreeIt can be achieved by performing reheating and quenching after heating to a temperature range of [point + 100 ° C.].
[0088]
That is, in the target steel of the present invention directly quenched by the method of the present invention, a large amount of fine carbonitrides of Nb and Ti are precipitated, and appropriate amounts of solid solution elements such as Nb and Ti are included. ing. Therefore, when this is re-heated and quenched, grain boundary movement is prevented and abnormal grain growth is prevented, and sized ultrafine grains are obtained. By repeating this reheating and quenching treatment, the crystal grains become finer, thereby further improving toughness and resistance to sulfide stress cracking. However, according to the present invention, as described above, it is possible to produce a high-strength seamless steel pipe having good sulfide stress cracking resistance and excellent toughness just by tempering after direct quenching. Even if the quenching process is repeated three times or more, it is difficult to improve the characteristics to meet the cost increase. Therefore, in the method of the present invention, one or two reheating quenching operations are defined between direct quenching and tempering.
[0089]
The heating temperature for reheating and quenching is Ac._ThreeFrom the point [Ac_ThreeThe reason for limiting the temperature range to the point + 100 ° C. is Ac_ThreeWhen the heating temperature is lower than the point, the complete austenite state is not obtained, and [Ac_ThreeThis is because, when the heating temperature exceeds the point + 100 ° C., the crystal grains become coarse and the desired characteristics cannot be imparted to the seamless steel pipe. The heating rate for this reheating and quenching is not particularly limited, but a larger one is preferable. Accordingly, rapid heating means such as electromagnetic induction heating may be used. Also, the cooling rate at the time of reheating and quenching is not particularly limited, and it may be such that a desired low-temperature transformation structure can be obtained over the entire thickness of the seamless steel pipe as judged from the composition of the raw steel. . Generally, it is faster than air cooling.
[0090]
When reheating and quenching is performed twice, the toughness and resistance to sulfide stress cracking can be improved by lowering the heating temperature of the second reheating and quenching compared to the heating temperature of the first reheating and quenching. More preferred.
[0091]
By the way, from the meaning of preventing delayed fracture, even if an intermediate tempering process is appropriately performed between the quenching processes of direct quenching and reheating quenching, the characteristics of the seamless steel pipe after final tempering do not change. Therefore, after each quenching process, when the time until the next quenching process exceeds 5 hours, it is preferable to carry out an intermediate tempering process in order to effectively prevent delayed fracture.
[0092]
The upper limit of the intermediate tempering temperature performed between the quenching treatments is required to give the seamless steel pipe the desired characteristics (strength, toughness, sulfide stress cracking resistance).₁The temperature must be below the point. In order to surely obtain an ultrafine sized structure by the subsequent reheating and quenching treatment, the temperature in this intermediate tempering is preferably 700 ° C. or lower. Further, it is sufficient that the intermediate tempering is performed at a temperature at which delayed fracture can be prevented, for example, at a temperature of 500 ° C. or more.
[0093]
The method of the present invention refers to the following seven processes, when direct quenching is represented as DQ, reheat quenching is represented as RQ, tempering is represented as TE, and intermediate tempering for preventing delayed fracture is represented as MTE.
[0094]
(1) DQ-TE
(2) DQ-RQ-TE
(3) DQ-MTE-RQ-TE
(4) DQ-RQ-RQ-TE
(5) DQ-RQ-MTE-RQ-TE
(6) DQ-MTE-RQ-RQ-TE
(7) DQ-MTE-RQ-MTE-RQ-TE
[0095]
【Example】
Steels having chemical compositions shown in Tables 1 and 2 were melted by an ordinary method. Steels A to L in Table 1 are target steels of the present invention (hereinafter referred to as steels of the present invention), and Steels M to T in Table 2 are comparative steels whose components are out of the content range defined in the present invention. is there.
[0096]
Then, these steels of the present invention and comparative steels were made into billets having a diameter of 225 mm by the usual method, and pipe making, reheating treatment, direct quenching, reheating quenching, intermediate tempering and tempering were performed under the conditions shown in Tables 3-10. It was. Based on preliminary experiments, the tempering temperature was changed depending on the steel type and heat treatment conditions, and the product strength was changed to 110 grade (yield strength: 77 to 88 kgf / mm) of the American Petroleum Institute (API) standard.²), 125 grade (yield strength: 88-98 kgf / mm)²), 140 grade (yield strength: 98-109 kgf / mm)²3).
[0097]
The product (seamless steel pipe) thus obtained was examined for strength, toughness, austenite grain size, occurrence of abnormal grain growth, and resistance to sulfide stress cracking. The toughness was evaluated by the fracture surface transition temperature in the Charpy impact test. For abnormal grain growth, a distance of 1000 μm is scanned with a linear analyzer according to the section method according to ASTM (E112), and the average grain cut section length is obtained from the number of intersections with the grain boundary. 200 times microstructure photograph (7 cm × 10 cm) Abnormal grain growth is “No” when the ratio to the maximum grain cut section length obtained from 5 fields of view is less than 3 times, and “Abnormal grain growth” is present when the ratio is 3 times or more Was determined. Further, the resistance to sulfide stress cracking was evaluated by the Sc value defined by NACE-TM01-77-METHOD-B.
[0098]
Test results are shown in Tables 11-18.
[0099]
In general, the higher the strength, the lower the resistance to sulfide stress cracking and toughness. Therefore, the performance will be compared for each strength grade.
[0100]
Test Nos. 1 to 12, which are examples of the present invention, were drilled using a drilling machine with a crossing angle of 0 °, then subjected to finish rolling and reheating treatment, and further subjected to direct quenching and tempering to adjust to 110 grade. It is. Compared to test number 93 of the conventional example having the same strength level, the crystal grain size is fine, and the toughness and sulfide stress cracking resistance are improved. In addition, test numbers 21 to 32 which are examples of the present invention are drilled by a cross punching machine having a crossing angle of 5 to 29 degrees, and then subjected to finish rolling and reheating treatment, and further subjected to direct quenching and tempering to be adjusted to 110 grade. It is what. By using a cross-drilling machine rather than Test Nos. 1 to 12 with a 0-degree cross-drilling (drilling without a crossing angle), it becomes easier to perform strong work in finish rolling and toughness and sulfide stress cracking resistance. Further improvement is observed.
[0101]
Test Nos. 13 to 16, which are examples of the present invention, were drilled using a drilling machine having a crossing angle of 0 °, and after direct quenching, the grades were adjusted to 125 grades by a process of intermediate tempering, reheating quenching and tempering. Is. Compared with test number 94 which is a conventional example of equivalent strength grade, the crystal grain size is fine, and the toughness and sulfide stress cracking resistance are improved. Test Nos. 33 to 48 of the present invention were drilled with a cross drilling machine having a crossing angle of 5 to 29 degrees and similarly adjusted to 125 grades. By using a cross drilling machine rather than test Nos. 13 to 16 where drilling was performed at a crossing angle of 0 degrees (drilling without a crossing angle), strong processing in finish rolling was facilitated, and toughness and sulfide stress cracking resistance were improved. Greater improvements are occurring.
[0102]
Test Nos. 17 to 20 of the present invention were drilled using a drilling machine with a crossing angle of 0 °, and after direct quenching, reheating quenching and intermediate tempering were performed twice and adjusted to 140 grades by tempering. It is a thing. Compared with the test number 95 of the conventional example showing an equivalent strength level, the crystal grain size is extremely small, and therefore, the performance far exceeding the conventional example is obtained in toughness and resistance to sulfide stress cracking. Test numbers 48 to 72, which are examples of the present invention, are drilled with a cross drilling machine having a crossing angle of 5 to 32 degrees and adjusted to 140 grades. By using a cross-drilling machine rather than test Nos. 17 to 20 in which a 0-degree crossing (drilling without a crossing angle) was performed, strong processing in finish rolling became easier, and toughness and resistance to sulfide stress cracking were achieved. A further improvement is recognized.
[0103]
Test numbers 73 and 74 as comparative examples were prepared by reheating after finish rolling and then directly quenching and tempering to adjust to 110 grade, but the finish rolling degree (working rate) and rolling finish temperature. Therefore, the grain refinement effect does not occur, and therefore the toughness and the resistance to sulfide stress cracking are inferior to those of the conventional test number 93. ing. Further, in the test numbers 75 and 76, since the quenching temperature cannot be secured, a completely quenched structure cannot be obtained. Accordingly, since the tempering temperature for obtaining the 110 grade is lowered, the crystal grains are fine, but the toughness and the resistance to sulfide stress cracking are low.
[0104]
On the other hand, the test numbers 77 to 84 of the comparative examples showing the 125th grade or the 140th grade at the strength level similarly define the heat treatment conditions such as the finish rolling degree (working rate), the finished rolling temperature, and the reheating treatment in the present invention. Therefore, even if the so-called “tempering treatment” of reheating quenching-tempering is performed thereafter, the final crystal grain becomes coarse due to the influence of the previous history. Therefore, in particular, the toughness and the resistance to sulfide stress cracking are inferior compared with the examples of the present invention. In the example of the present invention, abnormal grain growth does not occur, but in the comparative test numbers 74, 77, 78, 81 and 82, abnormal grain growth is observed, and the toughness and the resistance to sulfide stress cracking are deteriorated. . That is, it is understood that fine Nb and Ti carbonitrides need to be dispersed when further tempering is performed after direct quenching.
[0105]
Test numbers 85 to 92, which are comparative examples, use comparative steels whose chemical composition deviates from the conditions specified in the present invention, although the manufacturing conditions specified in the present invention are satisfied. That is, since any one of Cr, Mo, Nb, Ti, P, S, N and fn1 is outside the specified conditions of the present invention, toughness and sulfide resistance are compared with those of the equivalent strength level of the examples of the present invention. Stress cracking is inferior.
[0106]
It is known that reducing the content of P and S improves the resistance to sulfide stress cracking, but by using the direct quenching process of the present invention, compared to the conventional reheat quenching and tempering treatment, the sulfuration resistance is improved. The effect of improving the physical stress cracking property appears greatly. That is, in the case of the conventional reheating quenching and tempering treatment, as can be seen from the comparison between the test numbers 96 and 97 and the test numbers 98 and 99, the effect of improving the resistance to sulfide stress cracking due to the decrease in the content of P and / or S is It doesn't appear so big. On the other hand, in the direct quenching process of the present invention, even if it is the same example of the present invention, from the comparison of test numbers 21 to 26 and test numbers 27 to 32, the inclusion of P and / or S while being at the same strength level It is clear that when the amount is decreased, the effect of improving the resistance to sulfide stress cracking is very significant.
[0107]
[Table 1]

[0108]
[Table 2]

[0109]
[Table 3]

[0110]
[Table 4]

[0111]
[Table 5]

[0112]
[Table 6]

[0113]
[Table 7]

[0114]
[Table 8]

[0115]
[Table 9]

[0116]
[Table 10]

[0117]
[Table 11]

[0118]
[Table 12]

[0119]
[Table 13]

[0120]
[Table 14]

[0121]
[Table 15]

[0122]
[Table 16]

[0123]
[Table 17]

[0124]
[Table 18]

[0125]
【The invention's effect】
As described above, according to the present invention, a seamless steel pipe having a performance equivalent to or higher than that of the conventional reheating quenching and tempering process can be obtained. Furthermore, by performing so-called tempering treatment of reheating quenching and tempering after direct quenching, good toughness and sulfide stress cracking resistance can be obtained even in high-strength seamless steel pipes that have not been able to obtain sufficient performance. It is done. This will reduce the development cost of oil wells and enable the development of deep wells that were difficult to develop in the past, making a significant contribution to the stable supply of energy in the future. .
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram for explaining a crossing angle, where (a) is a plan view and (b) is a side view.

Claims

% By weight: C: more than 0.20% to 0.50%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.5%, Cr: 0.1 to 1.5%, Mo: 0.1-1.5%, Nb: 0.005-0.50%, Ti: 0.005-0.50%, B: 0.0001-0.01%, Al: 0.005- Containing 0.50 % , the balance is composed of Fe and inevitable impurities, Ni in the impurities is 0.1% or less, P is 0.05% or less, S is 0.01% or less, N is 0.01% When a billet having a composition of not more than 0.01% and less than O% and fn11 > 0 is hot-drilled and rolled to produce a seamless steel pipe, the cross-sectional compressibility is set after the drilling. 40% or more finish rolling is performed at a finished temperature of 800 to 1050 ° C., and then reheated for a time t at a temperature T in the temperature range of 850 to 1100 ° C. What the following values fn2 performed immediately direct quenching from the form and 23500-26000, then a high strength seamless steel pipe having excellent sulfide stress cracking resistance, characterized by tempering at temperatures below Ac ₁ point Production method.
here,
fn11 = Ti (%)-(48/14) {N (%)} ,
fn2 = (T + 273) (21 + log t),
Note that T (° C.) and t (h).

% By weight: C: more than 0.20% to 0.50%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.5%, Cr: 0.1 to 1.5%, Mo: 0.1-1.5%, Nb: 0.005-0.50%, Ti: 0.005-0.50%, B: 0.0001-0.01%, Al: 0.005- Containing 0.50%, further containing one or more of V: 0.5% or less, Zr: 0.5% or less and Ca: 0.01% or less, with the balance being Fe and Consisting of inevitable impurities, Ni in the impurities is 0.1% or less, P is 0.05% or less, S is 0.01% or less, N is 0.01% or less and O is 0.01% or less, and When producing a seamless steel pipe by hot-drilling and rolling a billet having a component composition of fn12> 0, the cross-sectional compression ratio is 40% or more after the drilling. Rolling is performed at a finished temperature of 800 to 1050 ° C., followed by reheating for a time t at a temperature T in the temperature range of 850 to 1100 ° C., and immediately quenching immediately after setting the value of fn2 to 23500 to 26000, Ac ₁₁ A method for producing a high-strength seamless steel pipe excellent in sulfide stress cracking resistance, characterized by tempering at a temperature below the point.
here,
fn12 = Ti (%) − ( 48 / 14 ) {N (%)-( 14 / 91 ) Zr (%)},
fn2 = (T + 273) (21 + log t),
Note that T (° C.) and t (h).

The method for producing a high-strength seamless steel pipe excellent in sulfide stress cracking resistance according to claim 1 or 2 , wherein hot drilling is performed at a crossing angle of 5 to 35 degrees using a cross drilling machine.

P is 0.0007% 0.005% or less, or S in the impurities of the billet below, or P is 0.005% or less, and claims 1, S is equal to or less than 0.0007% A method for producing a high-strength seamless steel pipe excellent in sulfide stress cracking resistance according to any one of items 3 to 3 .

Directly between the quenching and tempering, the one or two Ac ₃ point - that reheating quenching after heating to a temperature range of [Ac ₃ point + 100 ° C.] claim 1, wherein up to 4 The manufacturing method of the high intensity | strength seamless steel pipe excellent in the sulfide stress cracking resistance in any one.