JP3733884B2 - High chromium ferritic heat resistant steel pipe and method for producing the same - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、ボイラの過熱器管、再熱器管、主蒸気管や再熱蒸気管等に用いられる耐水蒸気酸化性に優れた高クロムフェライト系耐熱鋼管とその製造方法に関する。
【０００２】
【従来の技術】
従来の省エネルギーと地球資源の有効活用に加え、地球環境保全の観点から二酸化炭素ガスの排出量削減が重要なエネルギー課題となる中、化石燃料を燃焼させる火力発電用ボイラの高効率化が世界中においてますます重要な産業政策となっている。
【０００３】
そこで、近年、ボイラの発電効率の向上のために、ボイラの蒸気温度と蒸気圧力を高めたいわゆる超超臨界圧ボイラの新設が進められている。
【０００４】
この蒸気条件の高温高圧化にともない、ボイラの過熱器管、再熱器管等の熱交換器管や、高温蒸気をタービンに送る主蒸気管、再熱蒸気管等の配管の使用温度が上昇することから、これらの鋼管に要求される性質として、高温強度と高温耐食性に加えて、管内面の耐水蒸気酸化性が重要な課題となっている。これは、管内面の水蒸気酸化スケールが剥離した場合、タービンブレードを損傷させるだけでなく、管曲がり部等に堆積した場合、管が過熱され最悪の場合は噴破事故にもつながるからである。
【０００５】
従来技術として、高温強度と高温耐食性に優れるオーステナイト系ステンレス鋼管は、フェライト系耐熱鋼管に比べ熱膨張が大きく、管内面の水蒸気酸化スケールが剥離しやすいことから様々な工夫がなされてきた。
【０００６】
鋼管にショット加工、グラインダー加工等の表面冷間加工を施した後、溶体化処理して表面細粒層をつくり、水蒸気酸化性を高める方法（特開昭５３−１１４７２２号公報）。
【０００７】
鋼管に２０％以上の冷間加工を施し、溶体化処理により鋼管全体を細粒組織にして水蒸気酸化性を高める方法（特開昭５４−１３８８１４号公報）。
【０００８】
溶体化処理後の鋼管にショットピーニング加工を施して水蒸気酸化性を高める方法（特開平６−３２２４８９号公報）。
【０００９】
これらの方法は、いずれも、前述したように、高価なＮｉを多量に含有するオーステナイト系ステンレス鋼管に関するものである。
【００１０】
一方、Ｎｉをほとんど使わない安価なフェライト系耐熱鋼管は、高温強度と高温耐食性では、オーステナイト系ステンレス鋼管に劣るものの、熱膨張が小さく、水蒸気酸化スケールの剥離がおこりにくいことから、比較的低温部の用途に対して広く使われてきた。また、主蒸気管や再熱蒸気管の配管材は、熱疲労性を重視してフェライト系耐熱鋼管がほとんどであるが、強度と耐酸化性の制約から上限使用温度は６００℃程度であった。
【００１１】
フェライト系耐熱鋼管の水蒸気酸化性能の改善方法については、従来からＣｒ量を高める方法や各種合金元素を添加する方法、および表面処理でＣｒ濃化層を生成させる方法等がある。
【００１２】
(a) ８〜１３．５％Ｃｒ鋼に、Ｓを０．００２〜０．０４％含有させて耐水蒸気酸化性を改善する方法（特開２０００−２４８３３７号公報）。
【００１３】
(b) ７．５〜１１．５％Ｃｒ鋼に、Ｓｉを０．２〜１．０％含有させて耐水蒸気酸化性を改善する方法（特開平６−１７９９５４号公報）。
【００１４】
(c) ８．０〜１３．０％Ｃｒ鋼に、適量のＴｉとＹを複合含有させて水蒸気酸化スケールと母材との界面にこれら元素の酸化物を析出させることにより水蒸気酸化スケールの密着性を高める方法（特開平１１−９２８８０号公報）。
【００１５】
(d) １．５〜３．５％Ｃｒ鋼に、適量のＣ、ＮｂおよびＴｉを複合含有させるとともに、表面処理によってＣｒ濃化層を化学的に生成させて耐水蒸気酸化性を改善する方法（特開平３−２１１２５４号公報）。
しかし、これらの技術のうち、(a) の技術は熱間加工性が劣化するために製造性が損なわれるという欠点があり、(b) 、(c) および(d) の技術は既存の高クロムフェライト系耐熱鋼管には成分規格外であるために適用できないという欠点があり、(b) の技術には脆化が促進するという欠点もある。
【００１６】
そして、なによりこれらの技術では、水蒸気酸化スケールの成長を抑制できるが、それでも実用上過酷な繰り返し起動停止に対して、水蒸気酸化スケールの剥離防止が完全ではなかった。これは、合金成分の添加だけでは、耐水蒸気酸化性の改善に欠くことができない安定なＣｒ２Ｏ３皮膜の生成量が不十分であることが原因である。
【００１７】
一方、(d) の技術のように、化学的表面処理によりＣｒ濃化層をつくるクロマイジング処理は、水蒸気酸化初期の緻密なＣｒ２Ｏ３皮膜の生成には有利であるが、クロマイジング処理自体が特殊な処理であり、合金成分の制約、高温特殊熱処理のための設備制約や処理条件制約が多いのに加え、コストが非常に高いという欠点もあった。
【００１８】
近年の発電用ボイラでは、水蒸気の高温高圧化に加え、電力需要の変動調整の観点から、負荷変動運転や、起動停止を繰り返す運転がおこなわれるようになり、従来フェライト系耐熱鋼管が水蒸気酸化スケールの剥離問題がなく使用されていた温度域でも、新たにスケールの剥離が問題化するようになった。
【００１９】
【発明が解決しようとする課題】
本発明は、上記の実状に鑑みてなされたもので、第１の課題は、高価なオーステナイト系ステンレス鋼管に匹敵する高温強度と高温耐食性をもつ既存の高クロムフェライト系耐熱鋼管を対象とし、前記の性能を阻害するＳ、Ｓｉ、Ｙ、Ｔｉ等の合金元素を添加しなくとも良好な耐水蒸気酸化性を発揮する高クロムフェライト系耐熱鋼管を提供することにある。また、第２の課題は、高価な表面処理であるクロマイジング処理をおこなわなくても、良好な耐水蒸気酸化性を発揮する高クロムフェライト系耐熱鋼管の安価な製造方法を提供することにある。
【００２０】
【課題を解決するための手段】
本発明の要旨は、下記（１）および（２）に示される耐水蒸気酸化性に優れた高クロムフェライト系耐熱鋼管、ならびに下記（３）および（４）に示される耐水蒸気酸化性に優れた高クロムフェライト系耐熱鋼管の製造方法にある。
【００２１】
（１）質量％で、Ｃ：０．０２〜０．３％、Ｓｉ：０．０２〜０．６％、Ｍｎ：０．０１〜２％、Ｃｒ：８〜１５％、さらにＭｏ：０．１〜４％およびＷ：０．１〜４％のうちのいずれか一方または両方を含み、残部がＦｅおよび不純物であるフェライト系耐熱鋼からなり、少なくとも管内面側に深さが５０μｍ以上の脱炭層を有する高クロムフェライト系耐熱鋼管。
（２）質量％で、Ｃ：０．０２〜０．３％、Ｓｉ：０．０２〜０．６％、Ｍｎ：０．０１〜２％、Ｃｒ：８〜１５％、さらにＭｏ：０．１〜４％およびＷ：０．１〜４％のうちのいずれか一方または両方を含むとともに、下記（ａ）〜（ｈ）に記載の元素および元素群のうちから選ばれる１種または２種以上の元素を含有し、残部がＦｅおよび不純物であるフェライト系耐熱鋼からなり、少なくとも管内面側に深さが５０μｍ以上の脱炭層を有する高クロムフェライト系耐熱鋼管。
（ａ）Ｎｉ：０．１〜１．５％
（ｂ）Ｃｕ：０．０５〜３％
（ｃ）Ｎ：０．００５ 〜０．２％
（ｄ）Ｖ：０．０１〜０．５％、Ｎｂ：０．０１〜０．５％およびＴｉ：０．０１〜０．５％のいずれか１種または２種以上
（ｅ）Ｃａ：０．０００１〜０．２％およびＭｇ：０．０００１〜０．２％のいずれか一方または両方
（ｆ）Ａｌ：０．２％以下
（ｇ）Ｂ：０．０００１〜０．２％
（ｈ）Ｌａ：０．０００１〜０．２％、Ｃｅ：０．０００１〜０．２％、Ｙ：０．０００１〜０．２％、Ｐｄ：０．０００１〜０．２％およびＮｄ：０．０００１〜０．２％のいずれか１種または２種以上
【００２２】
（３）質量％で、Ｃ：０．０２〜０．３％、Ｓｉ：０．０２〜０．６％、Ｍｎ：０．０１〜２％、Ｃｒ：８〜１５％、さらにＭｏ：０．１〜４％およびＷ：０．１〜４％のうちのいずれか一方または両方を含み、残部がＦｅおよび不純物であるフェライト系耐熱鋼からなる鋼管に、最終熱処理として、９８０℃以上での焼入れ、焼ならしおよび焼なましのうちのいずれかの熱処理を、酸素分圧が１ｋＰａ以上の酸化雰囲気中でおこなうことにより、少なくとも管内面側に深さが５０μｍ以上の脱炭層を形成させる高クロムフェライト系耐熱鋼管の製造方法。
（４）質量％で、Ｃ：０．０２〜０．３％、Ｓｉ：０．０２〜０．６％、Ｍｎ：０．０１〜２％、Ｃｒ：８〜１５％、さらにＭｏ：０．１〜４％およびＷ：０．１〜４％のうちのいずれか一方または両方を含むとともに、下記（ａ）〜（ｈ）に記載の元素および元素群のうちから選ばれる１種または２種以上の元素を含有し、残部がＦｅおよび不純物であるフェライト系耐熱鋼からなる鋼管に、最終熱処理として、９８０℃以上での焼入れ、焼ならしおよび焼なましのうちのいずれかの熱処理を、酸素分圧が１ｋＰａ以上の酸化雰囲気中でおこなうことにより、少なくとも管内面側に深さ５０μｍ以上の脱炭層を形成させる高クロムフェライト系耐熱鋼管の製造方法。
（ａ）Ｎｉ：０．１〜１．５％
（ｂ）Ｃｕ：０．０５〜３％
（ｃ）Ｎ：０．００５ 〜０．２％
（ｄ）Ｖ：０．０１〜０．５％、Ｎｂ：０．０１〜０．５％およびＴｉ：０．０１〜０．５％のいずれか１種または２種以上
（ｅ）Ｃａ：０．０００１〜０．２％およびＭｇ：０．０００１〜０．２％のいずれか一方または両方
（ｆ）Ａｌ：０．２％以下
（ｇ）Ｂ：０．０００１〜０．２％
（ｈ）Ｌａ：０．０００１〜０．２％、Ｃｅ：０．０００１〜０．２％、Ｙ：０．０００１〜０．２％、Ｐｄ：０．０００１〜０．２％およびＮｄ：０．０００１〜０．２％のいずれか１種または２種以上
【００２３】
本発明者は、上記の課題を達成するため、高クロムフェライト系耐熱鋼管の耐水蒸気酸化性を支配する、鋼管内表面の水蒸気酸化初期の保護性の高い緻密なＣｒ２Ｏ３皮膜の生成挙動について鋭意研究した。その結果、以下のことを知見して上記の本発明を完成させた。
【００２４】
フェライト系耐熱鋼の耐水蒸気酸化性は、Ｃｒ含有量を適正に選定するとともに、適正な条件で熱処理をおこなうことによって、高温の水蒸気に曝される鋼表面側に従来は考慮されることがなかった脱炭層を積極的に生成させると、驚くほど向上すること、また、かかる脱炭層は、既存の熱処理設備を用いて極めて容易に生成させ得ることを知見した。以下に、その詳細を説明する。
【００２５】
すなわち、フェライト系耐熱鋼管の耐水蒸気酸化性の改善には、Ｃｒ含有量の増加が有効なことは従来から知られていた。しかし、Ｃｒ含有量の上限は９％程度で、これ以上にＣｒ含有量を増加させると、焼戻しマルテンサイト単相組織からδフェライトを含む二相組織となり、強度が損なわれる。このため、規格鋼は、前述したＳＴＢＡ２６や火ＳＴＢＡ２７の９％Ｃｒ系フェライト鋼止まりであり、これらのフェライト系耐熱鋼の場合、耐水蒸気酸化に有効な保護皮膜のＣｒ２Ｏ３皮膜が安定して生成しないため、使用上限温度は６００℃程度とされていた。
【００２６】
一方、上記の火力基準に最近規格化された火ＳＵＳ４１０Ｊ３ＴＢは、δフェライトの生成を抑制するＣｕ添加のＣｒ含有量１０％以上の鋼であり、優れた強度を有する上記のＳＵＳ３４７ＨＴＢやＳＵＳ３２１ＨＴＢに代表されるオーステナイト系ステンレス鋼に匹敵する高温強度を有している（たとえば、特開平２−２３２３４５号公報）。しかし、この火ＳＵＳ４１０Ｊ３ＴＢ鋼の耐水蒸気酸化性は、Ｃｒ含有量がＳＵＳ３４７ＨＴＢやＳＵＳ３２１ＨＴＢに代表されるオーステナイト系ステンレス鋼のＣｒ含有量の約１８％よりも低いために必ずしも十分とはいえず、特に６００℃以上の蒸気条件での使用時には、著しい酸化により厚い水蒸気酸化スケールが生成してしまい、使用に制限があった。
【００２７】
さらに、化学的表面処理であるクロマイジング処理法は、前述したように、母材の合金成分の制約や高温長時間特殊処理による靱性低下やコストが非常に高いことが問題であった。
【００２８】
これに対して、本発明者が新たに知見した高クロムフェライト系耐熱鋼の耐水蒸気酸化性の改善に必要不可欠な技術的要件は次のとおりである。
【００２９】
(1) Ｃｒ含有量を８％以上にし、かつ蒸気に曝される鋼管内表面側に深さが５０μｍ以上の脱炭層を形成させれば、高温、特に６００℃以上での耐水蒸気酸化性が飛躍的に向上する。
【００３０】
(2) 深さが５０μｍ以上の脱炭層は、９８０℃以上での焼入れ、焼ならしおよび焼なましのいずれかの最終熱処理を、酸素分圧が１ｋＰａ以上の酸化雰囲気中でおこなえば生成させることができる。また、最終熱処理が焼入れまたは焼ならしの場合には、通常、マルテンサイト組織の軟化を目的として焼戻し処理が施されるが、その際の焼戻し温度が７００℃以上、ＡＣ１変態点以下であれば、脱炭層は消滅しない。
【００３１】
上記の脱炭層による耐水蒸気酸化性改善のメカニズムは以下のとおりである。すなわち、通常のフェライト系耐熱鋼の場合、鋼中に添加含有させたＣｒ量のうちの１〜２％は、Ｃと化合してＣｒ炭化物を生成する。そして、表層部に一定深さの脱炭層が存在しない場合には、鋼表層部の固溶Ｃｒ量が低く、水蒸気酸化の初期に保護性の高い緻密なＣｒ２Ｏ３皮膜が鋼表面に十分に生成しなくなる。
【００３２】
これに対し、鋼表面に深さが５０μｍ以上の脱炭層を形成させた場合には、意外にも鋼表層部の固溶Ｃｒ量が肉厚中央部の固溶Ｃｒ量よりも１〜２％増加し、Ｃｒ含有量が８％以上の高クロムフェライト系耐熱鋼であれば、水蒸気酸化の初期に保護性の高い緻密なＣｒ２Ｏ３皮膜が鋼表面に十分に生成し、耐水蒸気酸化性が大幅に向上することが判明した。
【００３３】
つまり、耐水蒸気酸化に必要なＣｒは管内面表層部の固溶Ｃｒであり、必要な管内面表層部の固溶Ｃｒ量は深さ５０μｍ以上の脱炭層を形成させるだけで得られ、鋼全体のＣｒ含有量を必ずしも増やす必要がないので従来鋼にも適用でき、しかも高価なクロマイジング処理も必要としないので、安価な耐水蒸気酸化性に優れた高クロムフェライト系耐熱鋼管が提供できることを知見した。
【００３４】
【発明の実施の形態】
以下、本発明の高クロムフェライト系耐熱鋼管とその製造方法について詳細に説明する。なお、以下において、「％」は、特に断らない限り、「質量％」を意味する。
【００３５】
まず、本発明の高クロムフェライト系耐熱鋼管について説明する。
【００３６】
《化学組成》
素材のフェライト系耐熱鋼は、以下に述べる理由から、少なくとも、下記量のＣ、Ｓｉ、ＭｎおよびＣｒの４元素を含むフェライト系耐熱鋼であることが必要である。
【００３７】
Ｃ：０．０２〜０．３％
Ｃは、ボイラ用フェライト系耐熱鋼の高温引張強さ、高温クリープ破断強さを確保するうえで必要な元素であり、０．０２％以上の含有量が必要である。しかし、０．３％を超えて含有させると、Ｃｒ系炭化物が増え、耐水蒸気酸化性に有効な固溶Ｃｒ量が確保できなくなるだけでなく、溶接性が低下する。よって、Ｃ含有量は０．０２〜０．３％とした。好ましい範囲は０．０４〜０．２％、より好ましい範囲は０．０６〜０．１５％である。
【００３８】
Ｓｉ：０．０２〜０．６％
Ｓｉは、脱酸剤として添加され、また耐水蒸気酸化性を高めるためにも必要な元素であり、０．０２％以上の含有量が必要であるが、０．６％を超えて含有させると熱間加工性が悪くなる。よって、Ｓｉ含有量は０．０２〜０．６％とした。好ましい範囲は０．０５〜０．４％、より好ましい範囲は０．１〜０．３％である。
【００３９】
Ｍｎ：０．０１〜２％
Ｍｎは、Ｓと結合してＭｎＳを形成し、熱間加工性を改善する作用があるが、０．０１％未満の含有量では前記の効果が得られない。逆に、２％を超えて含有させると、硬く脆くなり、かえって熱間加工性を損なうだけでなく、溶接性をも損なわれる。よって、Ｍｎ含有量は０．０１〜２％とした。好ましい範囲は０．１〜１．０％である。
【００４０】
Ｃｒ：８〜１５％
Ｃｒは、耐水蒸気酸化性を確保するのに最も重要な元素であり、その含有量が８％未満では安定なＣｒ２Ｏ３皮膜が生成せず、所望の耐水蒸気酸化性が確保できない。一方、１５％を超えて含有させると、δフェライトが多くなり、σ相が析出する等して靱性や加工性が低下するだけでなく、高温強度および溶接性をも低下する。よって、Ｃｒ含有量は８〜１５％とした。好ましい範囲は９〜１３％、より好ましい範囲は１０．５〜１２．５％である。
【００４１】
Ｍｏ、Ｗ：いずれも０．１〜４％
ＭｏおよびＷには、高温強度を高める作用があり、いずれか一方または両方を含有させるが、いずれの元素も０．１％未満の含有量では前記の効果が得られない。一方、いずれの元素も４％を超えて含有させると、熱間加工性、溶接性が損なわれるだけでなく、高温での組織を不安定にして高温強度も損なわれる。このため、ＭｏおよびＷの含有量はいずれも０．１〜４％とした。
【００４２】
《脱炭層の深さ》
本発明の高クロムフェライト系耐熱鋼管は、少なくとも、管内面側に深さが５０μｍ以上の脱炭層を有する必要がある。これは、鋼管が上記の化学組成を満たす場合でも、高温の水蒸気に曝される管内面側に深さ５０μｍ以上の脱炭層が存在しないと、水蒸気酸化の初期に鋼表面にＣｒ２Ｏ３皮膜が生成せず、耐水蒸気酸化性が向上しないためである。このことは、後述する実施例の結果からも明らかである。
【００４３】
なお、深さ５０μｍ以上の脱炭層は、通常、高温の水蒸気に曝されることがない管外面側には必ずしも形成させる必要はない。
【００４４】
また、管内面側の脱炭層の深さは、耐水蒸気酸化性を向上させる観点のみからはいくら深くてもよいので上限は規定しない。しかし、脱炭層があまり深いと鋼管全体の耐圧強度が損なわれので、その深さは、鋼管の肉厚にもよるが、肉厚が３〜１２ｍｍ程度の薄肉鋼管の場合、深くても０．２ｍｍ程度とするのがよい。
【００４５】
さらに、本発明にいう脱炭層とは、ビッカース硬さが肉厚中心部のビッカース硬さよりも２０以上低い領域をいう。
【００４６】
本発明の高クロムフェライト系耐熱鋼管は、以上に説明した条件を満たしさえすれば、良好な耐水蒸気酸化性を発揮する。しかし、その素材鋼は、上記Ｃ、Ｓｉ、Ｍｎ、Ｃｒ、並びにＭｏまたは／およびＷの５または６元素以外に、必要に応じて、以下に示す元素のうちから選ばれる１種以上の元素を含むものであってもよく、この場合においても耐水蒸気酸化性は何ら損なわれない。
【００４７】
Ｎｉ：
Ｎｉには、靱性を改善する作用があり、その効果は不純物量レベルでも得られるが、０．１％以上の含有量で顕著になる。しかし、１．５％を超えて含有させると、クリープ強度の低下を招く。このため、添加含有させる場合のＮｉ含有量は０．１〜１．５％とするのがよい。
【００４８】
Ｃｕ：
Ｃｕには、高温強度および組織を安定にする作用があり、その効果は不純物量レベルでも得られるが、０．０５％以上の含有量で顕著になる。しかし、３％を超えて含有させると、熱間加工性および延性が損なわれる。このため、添加含有させる場合のＣｕ含有量は０．０５〜３％とするのがよい。
【００４９】
Ｎ：
Ｎは、析出強化元素で、窒化物および炭窒化物を形成してクリープ強度および靭性を向上させる作用があり、その効果は不純物量レベルでも得られるが、０．００５％以上の含有量で顕著になる。しかし、０．２％を超えて含有させると、鋼中にブローホールが多発したり、溶接欠陥の原因となる。このため、添加含有させる場合のＮ含有量は０．００５〜０．２％とするのがよい。
【００５０】
Ｖ、Ｎｂ、Ｔｉ：
これらの元素には、いずれもＣ、Ｎと結合して炭窒化物を生成し、析出強化によって高温強度および靭性を向上させる作用があり、その効果は不純物量レベルでも得られるが、いずれの元素も０．０１％の含有量で顕著になる。しかし、いずれの元素も０．５％を超えて含有させると、熱間加工性が損なわれる。このため、いずれか１種または２種以上を添加含有させる場合のこれら元素の含有量はいずれも０．０１〜０．５％とするのがよい。
【００５１】
Ｃａ、Ｍｇ：
これらの元素には、いずれも熱間加工性を向上させる作用があり、その効果は不純物量レベルでも得られるが、０．０００１％以上の含有量で顕著になる。しかし、０．２％を超えて含有させると、かえって熱間加工性が低下する。このため、いずれか一方または両方を添加含有させる場合のこれら元素の含有量はいずれも０．０００１〜０．２％とするのがよい。
【００５２】
Ａｌ：
Ａｌは、通常、脱酸剤として添加される元素であるが、その含有量が０．２％を超えると、高温クリープ強度が損なわれる。このため、添加含有させる場合のＡｌ含有量は０．２％以下とするのがよい。なお、十分な脱酸効果を得る観点からは０．００１％以上とするのがよい。
【００５３】
Ｂ：
Ｂには、炭化物を微細分散させ、高温長時間クリープ強度を向上させる作用があり、その効果は不純物量レベルでも得られるが、０．０００１％以上の含有量で顕著になる。しかし、０．２％を超えて含有させると、溶接性および熱間加工性が損なわれる。このため、添加含有させる場合のＢ含有量は０．０００１〜０．２％とするのがよい。
【００５４】
Ｌａ、Ｃｅ、Ｙ、Ｐｄ、Ｎｄ：
これらの元素には、いずれも鋼中に含まれる不純物であるＰ、ＳおよびＯ（酸素）と結合して靭性、熱間加工性および強度を向上させる作用があり、その効果は不純物量レベルでも得られるが、いずれもの元素も０．０００１％以上の含有量で顕著になる。しかし、０．２％を超えて含有させると、かえって靭性および強度が低下する。このため、いずれか１種または２種以上を添加含有させる場合のこれら元素の含有量はいずれも０．０００１〜０．２％とするのがよい。
【００５５】
次に、本発明の製造方法について説明する。
【００５６】
本発明の高クロムフェライト系耐熱鋼管の製造方法では、前述したように、最終熱処理として、９８０℃以上での焼入れ、焼ならしおよび焼なましのいずれかの熱処理を、酸素分圧が１ｋＰａ以上の酸化雰囲気中でおこなう。その理由は、９８０℃未満の温度で熱処理したのでは均質で安定な脱炭層が形成しない場合があるからであり、酸化雰囲気中の酸素分圧が１ｋＰａ未満では脱炭反応が十分に生じないためである。
【００５７】
最終熱処理の温度は、９８０℃以上であればいくら高くてもよく、上限は特に規定しない。しかし、熱処理温度を高くすればするほど、酸化スケールが厚くなって材料損失が増加するだけでなく、表面の凹凸が著しくなって製品としての見栄えが損なわれ、コストのかかる表面研磨や表面研削を余儀なくされるようになるので、熱処理温度の上限は１２００℃、好ましくは１１５０℃とするのがよい。
【００５８】
最終熱処理での保持時間は、特に規定しないが、１分以上保持すれば十分である。しかし、あまり長く保持すると、加熱温度の場合と同様に、酸化スケールが厚くなって材料損失が増加するだけでなく、表面の凹凸が著しくなる。このため、保持時間は、長くても、肉厚２５ｍｍ当たり３時間以内とするのが望ましい。
【００５９】
加熱雰囲気は、酸素分圧が１ｋＰａ以上であればよく、その上限は特に規定する必要はないが、過度な酸素分圧の上昇は費用が嵩むので、加熱雰囲気は酸素分圧が２１ｋＰａ以下の大気雰囲気とするのが望ましい。
【００６０】
上記条件による最終熱処理が焼入れまたは焼ならしの場合には、前述したように、通常、マルテンサイト組織の軟化を目的として焼戻し処理がおこなわれる。しかし、７００℃未満で焼戻したのでは、軟化が不十分で、高温クリープ強度が低下する。このため、最終熱処理が焼入れまたは焼ならしの場合におこなう焼戻し処理は、７００℃以上、ＡＣ１変態点以下、好ましくは７５０℃以上、ＡＣ１変態点以下でおこなうのが望ましい。また、その処理は、最終熱処理の焼入れまたは焼ならしと同じ雰囲気中でおこなってよい。
【００６１】
上記条件による最終熱処理は、鋼管の成形加工中におこなってもよく、熱処理時に生成する表面の酸化スケール層は酸洗等により除去しても除去しなくてもよい。しかし、酸化スケール層には、水蒸気酸化の初期にＣｒ２Ｏ３皮膜の生成を促進させる作用があるので、除去しない方が望ましい。
【００６２】
なお、本発明が対象とするＣｒ含有量が８％以上の高クロムフェライト系耐熱鋼の酸化スケールによる材料損失は軽微であり、また表面の凸凹についても本発明対象鋼では実用上の問題にはならないレベルである。
【００６３】
【実施例】
表１に示す化学組成を有する８種類の鋼を、容量が５０ｋｇの真空溶解炉を用いて溶製し、得られた鋼塊を熱間鍛造して板厚１０ｍｍの鋼板を製作した。
【００６４】
なお、代符Ａ〜Ｇの鋼は本発明で規定する化学組成を満たす鋼であり、代符Ａ鋼はＪＩＳに規定されているＳＴＢＡ２６、代符Ｂ鋼は火力基準に規定されている火ＳＴＢＡ２８、代符Ｃ鋼は同じく火力基準に規定されている火ＳＵＳ４１０Ｊ３、代符Ｄ鋼はＤＩＮ規格（ＤＩＮ１７１７５）に規定させているＸ２０ＣｒＭｏＶ１２１の既存鋼である。また、代符Ｈ鋼はＣｒ含有量が本発明で規定する範囲を外れる比較鋼である。
【００６５】
得られた各代符鋼からなる鋼板は、表２に示す種々の条件で熱処理して脱炭層の深さが異なる鋼板に作り分けし、熱処理により生成した酸化スケール層を除去した後、下記の水蒸気酸化試験に供した。
【００６６】
なお、脱炭層の深さは、酸化スケールの除去後、鋼板の肉厚方向のビッカース硬さを測定し、前述したように、その硬さが肉厚中心部のビッカース硬さよりも２０以上低い鋼板表面から最も離れた位置までの距離をその深さとして求めた。
【００６７】
水蒸気酸化試験は、試験片を６５０℃の水蒸気に１０００時間曝しておこない、試験後の試験片断面を顕微鏡観察して水蒸気酸化スケール層の厚さを１０点測定し、その平均値を各鋼板の水蒸気酸化スケール層の厚さとした。
【００６８】
結果は表２に示すとおりであり、Ｃ、Ｓｉ、Ｍｎ、Ｃｒ並びにＭｏまたは／およびＷの含有量と最終熱処理条件が本発明で規定する範囲内であり、脱炭層の深さが５０μｍ以上の鋼板（試番１、４、６、７、１０、１２、１４および１６）の水蒸気酸化スケールの厚さは、母材のＣｒ含有量によって異なるが、９％Ｃｒ系の鋼（試番１、４および１２）では３２〜３７μｍ、１１〜１４％Ｃｒ系の鋼（試番６、７、１０、１４および１６）では１８〜３６μｍと薄い。
【００６９】
これに対し、Ｃ、Ｓｉ、Ｍｎ、Ｃｒ並びにＭｏまたは／およびＷの含有量は本発明で規定する範囲内であるが、最終熱処理条件が本発明で規定する範囲を外れていて脱炭層の深さが５０μｍ未満の鋼板（試番５および８）の水蒸気酸化スケールの厚さは、母材のＣｒ含有量によって異なるが、９％Ｃｒ系の鋼（試番５）では７０μｍ、１１〜１４％Ｃｒ系の鋼（試番８）では５３μｍと厚い。
【００７０】
また、Ｃ、Ｓｉ、Ｍｎ、Ｃｒ並びにＭｏまたは／およびＷの含有量は本発明で規定する範囲内であるが、最終熱処理を従来の光輝炉でおこなった脱炭層の深さが０μｍの鋼板（試番３、５、９、１１、１３、１５および１７）の水蒸気酸化スケールの厚さは、母材のＣｒ含有量によって異なるが、９％Ｃｒ系の鋼（試番３、５および１３）では７８〜９５μｍ、１１〜１４％Ｃｒ系の鋼（試番９、１１、１５および１７）では４３〜５８μｍと厚い。
【００７１】
さらに、最終熱処理条件は本発明で規定する範囲内で、脱炭層の深さも５０μｍ以上ではあるが、Ｃｒ含有量が本発明で規定する範囲を外れる鋼板（試番１８）の水蒸気酸化スケールの厚さは１１２μｍと厚く、最終熱処理が従来の光輝炉でおこなわれていて脱炭層の深さが０μｍの鋼板（試番１９）の水蒸気酸化スケールの厚さとほとんど変わらない。
【００７２】
次に、一部の鋼板、具体的には、試番６の鋼板、試番９の鋼板および試番１９の鋼板について、水蒸気酸化試験後の断面スケール構造をＳＥＭのＥＤＸ分析により観察した。その結果、試番６の鋼板の水蒸気酸化スケールと母材との界面には極めて薄いＣｒ２Ｏ３皮膜と考えられるＣｒ濃度の高い層が確認されたが、試番９の鋼板および試番１９の鋼板にはこのＣｒの濃化層は認められなかった。このことは、深さ５０μｍ以上の脱炭層を有する場合においてのみ、水蒸気酸化スケールの生成初期に緻密なＣｒ２Ｏ３と考えられる保護性の高い皮膜が生成し、以後の水蒸気酸化スケール成長が抑制されていることを意味している。
【００７３】
【表１】
【表２】
【発明の効果】
本発明の高クロムフェライト系耐熱鋼管は、耐水蒸気酸化性に優れるので、事業用や産業用の発電ボイラの過熱器管、再熱器管、主蒸気管や再熱蒸気管等に用いた場合、水蒸気酸化スケールの剥離問題が抑制されるため、ボイラの稼働率が向上する。また、本発明の方法によれば、特別な熱処理設備を用いる必要がないので、耐水蒸気酸化性に優れる高クロムフェライト系耐熱鋼管を高能率に製造でき、製造コストの低減が図れる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high chromium ferritic heat resistant steel pipe excellent in steam oxidation resistance used for a superheater pipe, a reheater pipe, a main steam pipe, a reheat steam pipe and the like of a boiler, and a method for producing the same.
[0002]
[Prior art]
In addition to conventional energy saving and effective use of earth resources, reducing CO2 gas emissions is an important energy issue from the viewpoint of global environmental conservation. As a result, high-efficiency boilers for thermal power generation that burn fossil fuels Has become an increasingly important industrial policy.
[0003]
Therefore, in recent years, in order to improve the power generation efficiency of the boiler, a so-called super supercritical pressure boiler in which the steam temperature and the steam pressure of the boiler are increased is being promoted.
[0004]
As the steam conditions increase in temperature and pressure, the operating temperature of heat exchanger tubes such as boiler superheater tubes and reheater tubes, and piping such as main steam tubes and reheat steam tubes that send high-temperature steam to the turbine increases. Therefore, as properties required for these steel pipes, in addition to high temperature strength and high temperature corrosion resistance, steam oxidation resistance on the inner surface of the pipe is an important issue. This is because when the steam oxidation scale on the inner surface of the pipe is peeled off, not only the turbine blade is damaged, but also when it is deposited on a bent portion of the pipe, the pipe is overheated, and in the worst case, it leads to a blowout accident.
[0005]
As a prior art, an austenitic stainless steel pipe excellent in high-temperature strength and high-temperature corrosion resistance has a large thermal expansion compared to a ferritic heat-resistant steel pipe, and various ideas have been made because the steam oxidation scale on the inner surface of the pipe is easy to peel off.
[0006]
A method of improving the water vapor oxidation property by subjecting a steel pipe to surface cold working such as shot processing and grinder processing, followed by solution treatment to form a surface fine-grained layer (Japanese Patent Laid-Open No. 53-114722).
[0007]
A method in which the steel pipe is subjected to cold working of 20% or more, and the entire steel pipe is made into a fine-grained structure by solution treatment to improve steam oxidation (Japanese Patent Laid-Open No. 54-138814).
[0008]
A method of improving the steam oxidation property by subjecting a steel pipe after solution treatment to shot peening (Japanese Patent Laid-Open No. 6-322489).
[0009]
These methods all relate to an austenitic stainless steel pipe containing a large amount of expensive Ni as described above.
[0010]
On the other hand, inexpensive ferritic heat-resistant steel pipes that use almost no Ni are inferior to austenitic stainless steel pipes in terms of high-temperature strength and high-temperature corrosion resistance, but have low thermal expansion and are difficult to peel off the steam oxide scale. Have been widely used for Moreover, most of the piping materials for main steam pipes and reheat steam pipes are ferritic heat-resistant steel pipes with emphasis on thermal fatigue, but the upper limit operating temperature was about 600 ° C due to strength and oxidation resistance constraints. .
[0011]
Conventional methods for improving the steam oxidation performance of ferritic heat resistant steel pipes include a method for increasing the Cr content, a method for adding various alloy elements, and a method for generating a Cr concentrated layer by surface treatment.
[0012]
(a) A method of improving the steam oxidation resistance by adding 0.002 to 0.04% of S to 8 to 13.5% Cr steel (Japanese Patent Laid-Open No. 2000-248337).
[0013]
(b) A method of improving the steam oxidation resistance by adding 0.2 to 1.0% of Si to 7.5 to 11.5% Cr steel (Japanese Patent Laid-Open No. Hei 6-179954).
[0014]
(c) Adhesion of the steam oxide scale by adding an appropriate amount of Ti and Y to 8.0 to 13.0% Cr steel and precipitating oxides of these elements at the interface between the steam oxide scale and the base material A method for improving the property (Japanese Patent Laid-Open No. 11-92880).
[0015]
(d) A method of improving the steam oxidation resistance by adding a proper amount of C, Nb and Ti to 1.5-3.5% Cr steel in combination and chemically generating a Cr concentrated layer by surface treatment (JP-A-3-21254).
However, among these technologies, the technology (a) has the disadvantage that the manufacturability is impaired because the hot workability deteriorates, and the technologies (b), (c), and (d) The chromium ferritic heat resistant steel pipe has a defect that it cannot be applied because it is out of the component standard, and the technique (b) has a defect that embrittlement is accelerated.
[0016]
Above all, with these technologies, the growth of the steam oxide scale can be suppressed. However, the steam oxide scale is not completely prevented from peeling against repeated severe start and stop in practical use. This is because stable Cr is indispensable for improving steam oxidation resistance only by adding alloy components.2O3The reason is that the amount of film produced is insufficient.
[0017]
On the other hand, as in the technique of (d), the chromizing treatment for forming a Cr concentrated layer by chemical surface treatment is a dense Cr in the initial stage of steam oxidation.2O3Although it is advantageous for the formation of the film, the chromizing treatment itself is a special treatment, and there are many restrictions on the alloy components, equipment restrictions for high-temperature special heat treatment, and treatment condition restrictions, and the cost is very high. There were also drawbacks.
[0018]
In recent boilers for power generation, in addition to increasing the temperature and pressure of steam, in addition to adjusting fluctuations in power demand, load fluctuation operation and operation that repeatedly starts and stops are performed. Even in the temperature range where there was no problem of peeling, scale peeling became a new problem.
[0019]
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and a first problem is directed to an existing high chromium ferritic heat resistant steel pipe having high temperature strength and high temperature corrosion resistance comparable to an expensive austenitic stainless steel pipe. An object of the present invention is to provide a high chromium ferritic heat resistant steel pipe that exhibits good resistance to steam oxidation without adding an alloying element such as S, Si, Y, Ti, etc. A second problem is to provide an inexpensive method for producing a high chromium ferritic heat resistant steel pipe that exhibits good steam oxidation resistance without performing chromizing treatment, which is an expensive surface treatment.
[0020]
[Means for Solving the Problems]
The gist of the present invention is the following (1).And shown in (2)High chromium ferritic heat resistant steel pipe with excellent steam oxidation resistanceAndfollowingShown in (3) and (4)It exists in the manufacturing method of the high chromium ferritic heat-resistant steel pipe excellent in steam oxidation resistance.
[0021]
(1) By mass%, C: 0.02 to 0.3%, Si: 0.02 to 0.6%, Mn: 0.01 to 2%, Cr: 8 to 15%, and Mo: 0.0. Including one or both of 1-4% and W: 0.1-4%The balance is Fe and impuritiesA high chromium ferritic heat resistant steel pipe made of ferritic heat resistant steel and having a decarburized layer with a depth of 50 μm or more at least on the inner surface side of the pipe.
(2) By mass%, C: 0.02 to 0.3%, Si: 0.02 to 0.6%, Mn: 0.01 to 2%, Cr: 8 to 15%, and Mo: 0.0. One or two selected from the elements and element groups described in (a) to (h) below, including any one or both of 1 to 4% and W: 0.1 to 4% A high-chromium ferritic heat-resistant steel pipe comprising a ferritic heat-resistant steel containing the above elements, the balance being Fe and impurities, and having a decarburized layer having a depth of at least 50 μm on the inner surface side of the pipe.
(A) Ni: 0.1 to 1.5%
(B) Cu: 0.05-3%
(C) N: 0.005 ~ 0.2%
(D) V: 0.01 to 0.5%, Nb: 0.01 to 0.5% and Ti: 0.01 to 0.5%, one or more
(E) One or both of Ca: 0.0001 to 0.2% and Mg: 0.0001 to 0.2%
(F) Al: 0.2% or less
(G) B: 0.0001 to 0.2%
(H) La: 0.0001 to 0.2%, Ce: 0.0001 to 0.2%, Y: 0.0001 to 0.2%, Pd: 0.0001 to 0.2%, and Nd: 0 Any one or more of 0.0001-0.2%
[0022]
(3)In mass%, C: 0.02-0.3%, Si: 0.02-0.6%, Mn: 0.01-2%, Cr: 8-15%, and Mo: 0.1-4 % And W: including one or both of 0.1 to 4%The balance is Fe and impuritiesBy performing any one of quenching, normalizing and annealing at 980 ° C. or higher in an oxidizing atmosphere having an oxygen partial pressure of 1 kPa or higher as a final heat treatment on a steel pipe made of ferritic heat resistant steel And a method for producing a high chromium ferritic heat-resistant steel pipe, wherein a decarburized layer having a depth of 50 μm or more is formed at least on the inner surface side of the pipe.
(4) By mass%, C: 0.02 to 0.3%, Si: 0.02 to 0.6%, Mn: 0.01 to 2%, Cr: 8 to 15%, and Mo: 0.00. One or two selected from the elements and element groups described in the following (a) to (h), including any one or both of 1 to 4% and W: 0.1 to 4% To the steel pipe made of ferritic heat-resistant steel containing the above elements and the balance being Fe and impurities, as a final heat treatment, any one of quenching, normalizing, and annealing at 980 ° C. or higher is performed. A method for producing a high chromium ferritic heat-resistant steel pipe, wherein a decarburized layer having a depth of 50 μm or more is formed at least on the inner surface of the pipe by performing in an oxidizing atmosphere having an oxygen partial pressure of 1 kPa or more.
(A) Ni: 0.1 to 1.5%
(B) Cu: 0.05-3%
(C) N: 0.005 ~ 0.2%
(D) V: 0.01 to 0.5%, Nb: 0.01 to 0.5% and Ti: 0.01 to 0.5%, one or more
(E) One or both of Ca: 0.0001 to 0.2% and Mg: 0.0001 to 0.2%
(F) Al: 0.2% or less
(G) B: 0.0001 to 0.2%
(H) La: 0.0001 to 0.2%, Ce: 0.0001 to 0.2%, Y: 0.0001 to 0.2%, Pd: 0.0001 to 0.2%, and Nd: 0 Any one or more of 0.0001-0.2%
[0023]
In order to achieve the above-mentioned object, the present inventor controls dense oxidation of high-chromium ferritic heat-resisting steel pipe with high protection property at the initial stage of steam oxidation on the inner surface of the steel pipe.2O3The formation behavior of the film was studied earnestly. As a result, the present invention was completed by finding out the following.
[0024]
The steam oxidation resistance of ferritic heat-resisting steel is selected by appropriately selecting the Cr content and performing heat treatment under appropriate conditions.WhenBy actively generating a decarburized layer that has not been considered in the past on the steel surface exposed to high-temperature water vapor, it is surprisingly improved. It has been found that it can be generated very easily by using. The details will be described below.
[0025]
That is, it has been conventionally known that an increase in Cr content is effective for improving the steam oxidation resistance of ferritic heat resistant steel pipes. However, the upper limit of the Cr content is about 9%, and if the Cr content is further increased, the tempered martensite single phase structure becomes a two-phase structure containing δ ferrite and the strength is impaired. For this reason, the standard steel is the 9% Cr ferritic steel stop of STBA26 and Fire STBA27 described above, and in the case of these ferritic heat resistant steels, Cr is a protective film effective for steam oxidation resistance.2O3Since the film was not stably formed, the upper limit temperature for use was set to about 600 ° C.
[0026]
On the other hand, fire SUS410J3TB, which has recently been standardized to the above-mentioned thermal power standard, is a steel with a Cr content of 10% or more added with Cu that suppresses the formation of δ ferrite, and is represented by the above-mentioned SUS347HTB and SUS321HTB having excellent strength. Has a high temperature strength comparable to that of austenitic stainless steel (for example, JP-A-2-232345). However, the steam oxidation resistance of this fire SUS410J3TB steel is not necessarily sufficient because the Cr content is lower than about 18% of the Cr content of austenitic stainless steel represented by SUS347HTB and SUS321HTB. When used under steam conditions at or above ° C., a significant steam oxidation scale was generated due to significant oxidation, limiting the use.
[0027]
Furthermore, as described above, the chromizing treatment method, which is a chemical surface treatment, has a problem in that the alloy components of the base material are restricted, the toughness is reduced due to the high temperature and long time special treatment, and the cost is very high.
[0028]
On the other hand, technical requirements indispensable for improving the steam oxidation resistance of the high chromium ferritic heat resistant steel newly discovered by the present inventors are as follows.
[0029]
(1) If a decarburized layer with a depth of 50 μm or more is formed on the inner surface side of the steel pipe exposed to steam with a Cr content of 8% or more, steam oxidation resistance at high temperatures, particularly 600 ° C. or more, is achieved. Improve dramatically.
[0030]
(2) A decarburized layer with a depth of 50 μm or more is generated if the final heat treatment at 980 ° C. or higher is performed in an oxidizing atmosphere with an oxygen partial pressure of 1 kPa or higher. be able to. Further, when the final heat treatment is quenching or normalizing, a tempering process is usually performed for the purpose of softening the martensite structure.C1If it is below the transformation point, the decarburized layer will not disappear.
[0031]
The mechanism for improving the steam oxidation resistance by the decarburized layer is as follows. That is, in the case of ordinary ferritic heat resistant steel, 1 to 2% of the amount of Cr added and contained in the steel combines with C to produce Cr carbide. And when a decarburized layer of a certain depth does not exist in the surface layer part, the amount of solid solution Cr in the steel surface layer part is low, and dense Cr with high protection at the initial stage of steam oxidation2O3The film is not sufficiently formed on the steel surface.
[0032]
In contrast, when a decarburized layer having a depth of 50 μm or more is formed on the steel surface, the amount of solid solution Cr in the steel surface layer is surprisingly 1 to 2% higher than the amount of solid solution Cr in the center of the wall. If the high chromium ferritic heat-resisting steel with a Cr content of 8% or more, the dense Cr with high protective properties at the initial stage of steam oxidation2O3It was found that the film was sufficiently formed on the steel surface and the steam oxidation resistance was greatly improved.
[0033]
In other words, Cr required for steam oxidation resistance is the solid solution Cr of the tube inner surface layer portion, and the necessary amount of the solid solution Cr of the tube inner surface layer portion can be obtained simply by forming a decarburized layer having a depth of 50 μm or more. Since it is not always necessary to increase the Cr content of steel, it can be applied to conventional steel, and it does not require expensive chromizing treatment, so it can be found that an inexpensive high chromium ferritic heat resistant steel pipe with excellent steam oxidation resistance can be provided. did.
[0034]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the high chromium ferritic heat resistant steel pipe of the present invention and the manufacturing method thereof will be described in detail. In the following, “%” means “mass%” unless otherwise specified.
[0035]
First, the high chromium ferritic heat resistant steel pipe of the present invention will be described.
[0036]
<Chemical composition>
The ferritic heat resistant steel as a raw material needs to be a ferritic heat resistant steel containing at least the following amounts of four elements of C, Si, Mn, and Cr for the following reasons.
[0037]
C: 0.02-0.3%
C is an element necessary for securing high-temperature tensile strength and high-temperature creep rupture strength of ferritic heat-resistant steel for boilers, and a content of 0.02% or more is necessary. However, if the content exceeds 0.3%, Cr-based carbides increase, and it becomes impossible not only to secure a solid solution Cr amount effective for steam oxidation resistance, but also to deteriorate weldability. Therefore, the C content is set to 0.02 to 0.3%. A preferable range is 0.04 to 0.2%, and a more preferable range is 0.06 to 0.15%.
[0038]
Si: 0.02 to 0.6%
Si is an element that is added as a deoxidizer and is also necessary for improving the steam oxidation resistance, and a content of 0.02% or more is necessary. Hot workability deteriorates. Therefore, the Si content is set to 0.02 to 0.6%. A preferable range is 0.05 to 0.4%, and a more preferable range is 0.1 to 0.3%.
[0039]
Mn: 0.01-2%
Mn combines with S to form MnS to improve hot workability, but the above effect cannot be obtained with a content of less than 0.01%. On the other hand, if it exceeds 2%, it becomes hard and brittle, which not only impairs hot workability but also deteriorates weldability. Therefore, the Mn content is set to 0.01 to 2%. A preferable range is 0.1 to 1.0%.
[0040]
Cr: 8-15%
Cr is the most important element for ensuring steam oxidation resistance, and if its content is less than 8%, it is stable Cr.2O3A film is not formed, and the desired steam oxidation resistance cannot be ensured. On the other hand, if the content exceeds 15%, the amount of δ ferrite increases, and not only the toughness and workability decrease due to precipitation of the σ phase, but also the high temperature strength and weldability decrease. Therefore, the Cr content is 8-15%. A preferable range is 9 to 13%, and a more preferable range is 10.5 to 12.5%.
[0041]
Mo, W: Both 0.1 to 4%
Mo and W have the effect of increasing the high-temperature strength and contain either one or both, but the above effects cannot be obtained when the content of any element is less than 0.1%. On the other hand, if any element is contained in excess of 4%, not only hot workability and weldability are impaired, but also the structure at high temperature becomes unstable and the high temperature strength is also impaired. For this reason, the contents of Mo and W are both set to 0.1 to 4%.
[0042]
《Decarburized layer depth》
The high chromium ferritic heat resistant steel pipe of the present invention needs to have at least a decarburized layer having a depth of 50 μm or more on the inner surface side of the pipe. This is because even when the steel pipe satisfies the above chemical composition, if there is no decarburized layer having a depth of 50 μm or more on the inner surface side of the pipe exposed to high-temperature steam, Cr is formed on the steel surface at the initial stage of steam oxidation.2O3This is because no film is formed and the steam oxidation resistance is not improved. This is also clear from the results of Examples described later.
[0043]
It should be noted that the decarburized layer having a depth of 50 μm or more is not necessarily formed on the outer surface side of the tube that is not exposed to high-temperature steam.
[0044]
Further, the depth of the decarburized layer on the inner surface side of the pipe may be as deep as possible only from the viewpoint of improving the steam oxidation resistance, so no upper limit is defined. However, if the decarburized layer is too deep, the pressure resistance of the entire steel pipe is impaired, and the depth depends on the thickness of the steel pipe, but in the case of a thin steel pipe having a thickness of about 3 to 12 mm, the depth is 0. It is good to be about 2 mm.
[0045]
Furthermore, the decarburized layer referred to in the present invention is a Vickers hardness.TheIs a region that is 20 or more times lower than the Vickers hardness at the center of the wall thickness.
[0046]
The high chromium ferritic heat resistant steel pipe of the present invention exhibits good steam oxidation resistance as long as the above-described conditions are satisfied. However, in addition to the above-mentioned C, Si, Mn, Cr, and 5 or 6 elements of Mo or / and W, the material steel contains one or more elements selected from the following elements as necessary. In this case, the steam oxidation resistance is not impaired at all.
[0047]
Ni:
Ni has an effect of improving toughness, and the effect can be obtained even at the impurity level, but becomes remarkable at a content of 0.1% or more. However, if the content exceeds 1.5%, the creep strength is reduced. For this reason, the Ni content in the case of addition is preferably 0.1 to 1.5%.
[0048]
Cu:
Cu has the effect of stabilizing the high-temperature strength and the structure, and the effect can be obtained even at the impurity level, but becomes remarkable at a content of 0.05% or more. However, if it exceeds 3%, hot workability and ductility are impaired. For this reason, the Cu content when added and contained is preferably 0.05 to 3%.
[0049]
N:
N is a precipitation strengthening element, and has the effect of improving the creep strength and toughness by forming nitrides and carbonitrides, and the effect can be obtained even at the impurity level, but is remarkable at a content of 0.005% or more. become. However, if the content exceeds 0.2%, blowholes are frequently generated in the steel or welding defects are caused. For this reason, the N content in the case of addition is preferably 0.005 to 0.2%.
[0050]
V, Nb, Ti:
All of these elements combine with C and N to produce carbonitrides, and have the effect of improving high temperature strength and toughness by precipitation strengthening. The effect can be obtained even at the impurity level. Is also noticeable at a content of 0.01%. However, if any element exceeds 0.5%, hot workability is impaired. For this reason, the content of these elements in the case where any one or two or more of them are added and contained is preferably 0.01 to 0.5%.
[0051]
Ca, Mg:
All of these elements have an effect of improving hot workability, and the effect can be obtained even at the impurity level, but becomes remarkable at a content of 0.0001% or more. However, if the content exceeds 0.2%, the hot workability is rather lowered. For this reason, when adding any one or both of them, the content of these elements is preferably 0.0001 to 0.2%.
[0052]
Al:
Al is an element usually added as a deoxidizer, but if its content exceeds 0.2%, the high temperature creep strength is impaired. For this reason, when it is added and contained, the Al content is preferably 0.2% or less. From the viewpoint of obtaining a sufficient deoxidizing effect, the content is preferably 0.001% or more.
[0053]
B:
B has the effect of finely dispersing carbides and improving the high-temperature and long-term creep strength. The effect can be obtained even at the impurity level, but becomes remarkable at a content of 0.0001% or more. However, if the content exceeds 0.2%, weldability and hot workability are impaired. For this reason, the B content when added and contained is preferably 0.0001 to 0.2%.
[0054]
La, Ce, Y, Pd, Nd:
These elements all have the effect of improving the toughness, hot workability and strength by combining with P, S and O (oxygen), which are impurities contained in the steel, and the effect is even at the impurity level. Although it is obtained, any element becomes significant at a content of 0.0001% or more. However, if the content exceeds 0.2%, the toughness and strength are rather lowered. For this reason, the content of these elements when any one or two or more of them are added and contained is preferably 0.0001 to 0.2%.
[0055]
Next, the manufacturing method of this invention is demonstrated.
[0056]
In the method for producing a high chromium ferritic heat-resistant steel pipe of the present invention, as described above, any one of quenching, normalizing, and annealing at 980 ° C. or higher is used as the final heat treatment, and the oxygen partial pressure is 1 kPa or higher. In an oxidizing atmosphere. The reason is that a heat treatment at a temperature lower than 980 ° C. may not form a homogeneous and stable decarburized layer, and if the oxygen partial pressure in the oxidizing atmosphere is less than 1 kPa, the decarburization reaction does not occur sufficiently. It is.
[0057]
The final heat treatment temperature may be as high as 980 ° C. or higher, and the upper limit is not particularly defined. However, the higher the heat treatment temperature, the thicker the oxide scale and the greater the material loss, as well as the concavity and convexity of the surface, which impairs the appearance of the product, resulting in costly surface polishing and surface grinding. Therefore, the upper limit of the heat treatment temperature is 1200 ° C., preferably 1150 ° C.
[0058]
The holding time in the final heat treatment is not particularly defined, but it is sufficient to hold it for 1 minute or more. However, if held for too long, the oxide scale becomes thick and the material loss increases as in the case of the heating temperature, and the surface irregularities become remarkable. For this reason, it is desirable that the holding time is 3 hours or less per 25 mm thickness at the longest.
[0059]
The heating atmosphere only needs to have an oxygen partial pressure of 1 kPa or higher, and the upper limit thereof need not be specified. However, an excessive increase in the oxygen partial pressure is expensive, so the heating atmosphere is an atmosphere with an oxygen partial pressure of 21 kPa or less. An atmosphere is desirable.
[0060]
When the final heat treatment under the above conditions is quenching or normalizing, as described above, the tempering treatment is usually performed for the purpose of softening the martensite structure. However, when tempering is performed at a temperature lower than 700 ° C., the softening is insufficient and the high temperature creep strength is lowered. For this reason, the tempering performed when the final heat treatment is quenching or normalization is performed at 700 ° C. or higher.C1A transformation point or lower, preferably 750 ° C. or higher, AC1It is desirable to do it below the transformation point. In addition, the treatment may be performed in the same atmosphere as quenching or normalization of the final heat treatment.
[0061]
The final heat treatment under the above conditions may be performed during the forming process of the steel pipe, and the surface oxide scale layer formed during the heat treatment may be removed by pickling or the like. However, the oxide scale layer contains Cr at the initial stage of steam oxidation.2O3Since it has an effect of promoting the formation of a film, it is desirable not to remove it.
[0062]
It should be noted that the material loss due to the oxide scale of the high chromium ferritic heat-resistant steel having a Cr content of 8% or more, which is the subject of the present invention, is minor, and the surface irregularities are also practical problems in the subject steel. It is a level that should not be.
[0063]
【Example】
Eight types of steel having the chemical composition shown in Table 1 were melted using a vacuum melting furnace having a capacity of 50 kg, and the obtained steel ingot was hot forged to produce a steel plate having a thickness of 10 mm.
[0064]
The steels of the symbols A to G satisfy the chemical composition defined in the present invention, the symbol A steel is STBA26 defined in JIS, and the symbol B steel is fire STBA28 defined in the thermal power standard. The steel C is the existing steel of X20CrMoV121 specified in the DIN standard (DIN 17175). The steel H is a comparative steel whose Cr content is outside the range defined in the present invention.
[0065]
The obtained steel plates made of each steel plate were heat-treated under various conditions shown in Table 2 to make steel plates with different depths of decarburized layers, and after removing the oxide scale layer generated by the heat treatment, Subjected to a steam oxidation test.
[0066]
The depth of the decarburized layer is measured by measuring the Vickers hardness in the thickness direction of the steel sheet after removing the oxide scale, and as described above, the steel sheet is 20 or more times lower than the Vickers hardness at the center of the thickness. The distance to the position farthest from the surface was determined as the depth.
[0067]
The steam oxidation test is performed by exposing the test piece to steam at 650 ° C. for 1000 hours, observing a cross section of the test piece after the test with a microscope, measuring the thickness of the steam oxidation scale layer at 10 points, and calculating the average value of each steel plate. The thickness of the steam oxidation scale layer was used.
[0068]
The results are as shown in Table 2, and the contents of C, Si, Mn, Cr and Mo or / and W and the final heat treatment conditions are within the range defined in the present invention, and the depth of the decarburized layer is 50 μm or more. The thickness of the steam oxidation scale of the steel plates (Trial Nos. 1, 4, 6, 7, 10, 12, 14 and 16) varies depending on the Cr content of the base material, but the 9% Cr steel (Trial No. 1, 4 and 12) are as thin as 18 to 36 μm in 32 to 37 μm and 11 to 14% Cr steel (trial numbers 6, 7, 10, 14 and 16).
[0069]
On the other hand, the contents of C, Si, Mn, Cr, and Mo or / and W are within the range specified by the present invention, but the final heat treatment conditions are outside the range specified by the present invention, and the depth of the decarburized layer The thickness of the steam oxidation scale of steel plates having a thickness of less than 50 μm (trial numbers 5 and 8) varies depending on the Cr content of the base material, but 70 μm and 11-14% for 9% Cr-based steel (trial number 5). Cr steel (Trial No. 8) is as thick as 53 μm.
[0070]
In addition, the content of C, Si, Mn, Cr, and Mo or / and W is within the range specified in the present invention, but the steel plate having a decarburized layer depth of 0 μm in which the final heat treatment was performed in a conventional bright furnace ( The thickness of the steam oxidation scale of trial numbers 3, 5, 9, 11, 13, 15 and 17) varies depending on the Cr content of the base material, but is 9% Cr-based steel (trial numbers 3, 5, and 13). Then, 78 to 95 μm, 11 to 14% Cr steel (trial numbers 9, 11, 15 and 17) is as thick as 43 to 58 μm.
[0071]
Further, the final heat treatment condition is within the range specified by the present invention, and the depth of the decarburized layer is 50 μm or more, but the thickness of the steam oxidation scale of the steel plate (sample 18) whose Cr content is outside the range specified by the present invention. The thickness is 112 μm, which is almost the same as the thickness of the steam oxidation scale of a steel plate (trial No. 19) in which the final heat treatment is performed in a conventional bright furnace and the depth of the decarburized layer is 0 μm.
[0072]
Next, the cross-sectional scale structure after the steam oxidation test was observed by EDX analysis of SEM for some of the steel plates, specifically, the steel plate of trial No. 6, the steel plate of trial No. 9, and the steel plate of trial No. 19. As a result, an extremely thin Cr film is formed at the interface between the steam oxidation scale of the steel plate No. 6 and the base material.2O3A layer having a high Cr concentration, which was considered to be a film, was confirmed. However, this Cr concentrated layer was not observed in the steel plate No. 9 and the steel plate No. 19. This is because only in the case of having a decarburized layer with a depth of 50 μm or more, dense Cr is formed at the initial stage of the generation of the steam oxidation scale.2O3This means that a highly protective film considered to be generated is generated, and the subsequent steam oxide scale growth is suppressed.
[0073]
[Table 1]
[Table 2]
【The invention's effect】
Since the high chromium ferritic heat-resistant steel pipe of the present invention is excellent in steam oxidation resistance, it is used for superheater pipes, reheater pipes, main steam pipes, reheat steam pipes, etc. of power boilers for business and industrial use. Since the problem of peeling of the steam oxide scale is suppressed, the operating rate of the boiler is improved. In addition, according to the method of the present invention, since it is not necessary to use a special heat treatment facility, a high chromium ferritic heat resistant steel pipe excellent in steam oxidation resistance can be manufactured with high efficiency, and the manufacturing cost can be reduced.
Claims (4)
（ａ）Ｎｉ：０．１〜１．５％
（ｂ）Ｃｕ：０．０５〜３％
（ｃ）Ｎ：０．００５ 〜０．２％
（ｄ）Ｖ：０．０１〜０．５％、Ｎｂ：０．０１〜０．５％およびＴｉ：０．０１〜０．５％のいずれか１種または２種以上
（ｅ）Ｃａ：０．０００１〜０．２％およびＭｇ：０．０００１〜０．２％のいずれか一方または両方
（ｆ）Ａｌ：０．２％以下
（ｇ）Ｂ：０．０００１〜０．２％
（ｈ）Ｌａ：０．０００１〜０．２％、Ｃｅ：０．０００１〜０．２％、Ｙ：０．０００１〜０．２％、Ｐｄ：０．０００１〜０．２％およびＮｄ：０．０００１〜０．２％のいずれか１種または２種以上In mass%, C: 0.02-0.3%, Si: 0.02-0.6%, Mn: 0.01-2%, Cr: 8-15%, and Mo: 0.1-4 % And W: either one or both of 0.1 to 4% and one or more elements selected from the elements and element groups described in (a) to (h) below A high-chromium ferritic heat-resistant steel pipe having a decarburized layer with a depth of 50 μm or more at least on the inner surface of the pipe.
(A) Ni: 0.1 to 1.5%
(B) Cu: 0.05-3%
(C) N: 0.005 to 0.2%
(D) V: 0.01 to 0.5%, Nb: 0.01 to 0.5%, and Ti: 0.01 to 0.5%, one or more (e) Ca: 0 One or both of 0.0001 to 0.2% and Mg: 0.0001 to 0.2% (f) Al: 0.2% or less (g) B: 0.0001 to 0.2%
(H) La: 0.0001 to 0.2%, Ce: 0.0001 to 0.2%, Y: 0.0001 to 0.2%, Pd: 0.0001 to 0.2%, and Nd: 0 Any one or more of .0001 to 0.2%
（ａ）Ｎｉ：０．１〜１．５％
（ｂ）Ｃｕ：０．０５〜３％
（ｃ）Ｎ：０．００５ 〜０．２％
（ｄ）Ｖ：０．０１〜０．５％、Ｎｂ：０．０１〜０．５％およびＴｉ：０．０１〜０．５％のいずれか１種または２種以上
（ｅ）Ｃａ：０．０００１〜０．２％およびＭｇ：０．０００１〜０．２％のいずれか一方または両方
（ｆ）Ａｌ：０．２％以下
（ｇ）Ｂ：０．０００１〜０．２％
（ｈ）Ｌａ：０．０００１〜０．２％、Ｃｅ：０．０００１〜０．２％、Ｙ：０．０００１〜０．２％、Ｐｄ：０．０００１〜０．２％およびＮｄ：０．０００１〜０．２％のいずれか１種または２種以上In mass%, C: 0.02-0.3%, Si: 0.02-0.6%, Mn: 0.01-2%, Cr: 8-15%, and Mo: 0.1-4 % And W: either one or both of 0.1 to 4% and one or more elements selected from the elements and element groups described in (a) to (h) below As a final heat treatment, any one of quenching, normalizing, and annealing at 980 ° C. or higher is applied to a steel pipe made of a ferritic heat resistant steel with the balance being Fe and impurities. Is carried out in an oxidizing atmosphere of 1 kPa or more, thereby forming a high chromium ferritic heat resistant steel pipe that forms a decarburized layer having a depth of 50 μm or more on at least the inner surface of the pipe.
(A) Ni: 0.1 to 1.5%
(B) Cu: 0.05-3%
(C) N: 0.005 to 0.2%
(D) V: 0.01 to 0.5%, Nb: 0.01 to 0.5%, and Ti: 0.01 to 0.5%, one or more (e) Ca: 0 One or both of 0.0001 to 0.2% and Mg: 0.0001 to 0.2% (f) Al: 0.2% or less (g) B: 0.0001 to 0.2%
(H) La: 0.0001 to 0.2%, Ce: 0.0001 to 0.2%, Y: 0.0001 to 0.2%, Pd: 0.0001 to 0.2%, and Nd: 0 Any one or more of .0001 to 0.2%
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