JP3886785B2

JP3886785B2 - Ferritic stainless steel for petroleum fuel reformers

Info

Publication number: JP3886785B2
Application number: JP2001357420A
Authority: JP
Inventors: 学奥; 芳明堀; 幸寛川畑; 敏郎名越
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 2001-11-22
Filing date: 2001-11-22
Publication date: 2007-02-28
Anticipated expiration: 2021-11-22
Also published as: JP2003160840A

Description

【０００１】
【産業上の利用分野】
本発明は、ガソリン，ナフサ，灯油，ＬＰＧ等の石油系燃料を水素に改質する際に使用される改質器の要求特性を満足するフェライト系ステンレス鋼に関する。
【０００２】
【従来の技術】
各種化学工業分野における基礎原料，燃料電池用燃料，熱処理雰囲気用等、広範な用途に使用される水素は、石油，アルコール等の燃料を分解することにより製造している。たとえば、石油精製プラントでは、大型で連続運転される水素発生装置が使用されている。従来の水素発生装置は、ナフサや天然ガスを原料とし、水蒸気改質反応によって水素を製造している。
最近では、燃料電池用水素を得るために、各種改質器の開発が急ピッチで進められている。燃料電池用改質器としては、複数の反応管を容器に収容した多管式，大径の反応管をもつ単管式等が知られている。
【０００３】
たとえば、単管式改質器では、内壁１ａ，外壁１ｂをもつ二重管からなる反応管１に触媒を充填し、適宜の仕切りによって第１触媒層２ａ，第２触媒層２ｂを形成している（図１）。第１触媒層２ａと第２触媒層２ｂとの間に内側流路３ａ，外側流路３ｂをもつ改質ガス取出し管３を配置し、第１触媒層２ａを内側流路３ａに，第２触媒層２ｂを外側流路３ｂに連通させる。反応管１は全体がハウジング４で取り囲まれ、ハウジング４の一側壁に原料ガス供給管５が設けられ、外側流路３ｂに連通する合流管３ｃが他側壁から系外に延びている。ハウジング４の底部には、バーナ燃料ｆ，燃焼空気ｏが供給され、反応管１の内壁１ａで区画される内部空洞にフレームＦを送り込むバーナ６が設けられている。
【０００４】
原料ガスRGは、改質用水蒸気と共に反応管１内に送り込まれ、第１触媒層２ａ→内側流路３ａ→第２触媒層２ｂ→外側流路３ｂ→合流管３ｃの経路で流れる。フレームＦで内側から加熱されている第１触媒層２ａ，第２触媒層２ｂを原料ガスRGが通過する際に、改質反応（たとえば、Ｃ₃Ｈ₈＋３Ｈ₂Ｏ＝３ＣＯ＋７Ｈ₂），シフト反応（ＣＯ＋Ｈ₂Ｏ＝ＣＯ₂＋Ｈ₂）等により水素が生成する。水素は、改質取出し管から改質ガスとして直接回収され、或いはＰｄ−Ａｇ，Ｔａ等の水素透過膜を用いた選択透過法で改質ガスPGから分離回収される。
【０００５】
【発明が解決しようとする課題】
石油精製プラントの水素発生装置は、高温で連続運転されることから優れた高温クリープ強度が要求される。また、ＣＯ₂，ＳＯ₂等を含む水蒸気雰囲気に曝される。そのため、ＨＫ４０（２５Ｃｒ−２０Ｎｉ−０.４Ｃ）を始めとする耐熱合金製の遠心鋳造管が水素発生装置の構造材に使用されている。
他方、石油系燃料から水素を回収する燃料電池用改質器では、都市ガス，アルコール系燃料に比較して改質温度が８００℃以上の高温になる。しかも、水蒸気、ＣＯ₂，ＳＯ₂等を含む酸化性の雰囲気に曝され、水素の需要に応じて加熱・冷却も頻繁に繰り返される。このような過酷な環境下で十分な耐久性を呈する実用的な材料は、これまでのところ報告されていない。
【０００６】
【課題を解決するための手段】
本発明は、従来のフェライト系ステンレス鋼をベースとし、高温水蒸気雰囲気に曝される石油系燃料改質器の環境を考慮して鋼組成に種々の検討を加えることにより完成されたものであり、加熱初期の酸化皮膜を強化すると共に、Ｎ，Ｍｏ，Ｎｂ等の添加によって中温〜高温域での高温強度を改善し、改質器の要求特性を満足する石油系燃料改質器用フェライト系ステンレス鋼を提供することを目的とする。
【０００７】
本発明の石油系燃料改質器用フェライト系ステンレス鋼は、その目的を達成するため、Ｃｒ：８〜３５質量％，Ｃ：０.０３質量％以下，Ｎ：０.０３質量％以下，Ｍｎ：１.５質量％以下，Ｓ：０.００８質量％以下，Ｓｉ：０.８〜２.５質量％及び／又はＡｌ：０.６〜６.０質量％を含み、更にＮｂ：０ . ０５〜０ . ８０質量％，Ｔｉ：０ . ０３〜０ . ５０質量％，Ｍｏ：０ . １〜４ . ０質量％，Ｃｕ：０ . １〜４ . ０質量％の１種又は２種以上を含み、残部がＦｅ及び不可避的不純物からなり、Ｓｉ及びＡｌの合計量が１.５質量％以上に調整された組成を有していることを特徴とする。
【０００８】
このフェライト系ステンレス鋼は、更にＹ：０.００１〜０.１質量％，ＲＥＭ（希土類元素）：０.００１〜０.１質量％，Ｃａ：０.００１〜０.０１質量％の１種又は２種以上を含むことができる。
【０００９】
【作用】
ＳＵＳ４３０やＳＵＨ４０９Ｌに代表されるフェライト系ステンレス鋼は、オーステナイト系ステンレス鋼に比較すると熱疲労特性に優れているものの、多量の水蒸気を含む改質器の高温雰囲気に曝されると、水蒸気酸化が容易に進行する。また、加熱・冷却が頻繁に繰り返される改質器にあっては、より一層の優れた熱疲労特性が要求される。そこで、本発明者等は、水蒸気酸化及び熱疲労の発生メカニズムを材質面から検討し、ＳＵＳ４３０をベースとして種々の合金成分を添加し、添加合金成分が水蒸気酸化及び熱疲労に及ぼす影響を調査した。
【００１０】
高温雰囲気における水蒸気酸化は大気酸化よりも損傷が大きい。水蒸気酸化機構は必ずしも明らかでない。水蒸気酸化は水蒸気が酸素及び水素に解離して酸化反応を促進させ、水蒸気が鋼素地に直接到達して酸化を促進させること等によって生じる現象であり、結果としてスケール剥離に由来する配管系統の目詰りや鋼素地の減肉に起因する変形，穴開き等のトラブルが発生する。
【００１１】
この水蒸気酸化は、ステンレス鋼表面に生成するＣｒ系酸化物を主体とする酸化皮膜を安定化することによって抑制できる。加熱によりステンレス鋼表面に生成する酸化皮膜は、ステンレス鋼に耐酸化性を付与するものであり、８００℃程度の高温雰囲気にあっては８質量％以上のＣｒ含有量で耐酸化性の向上が顕著となる。しかし、鋼素地が高温水蒸気雰囲気に曝されると、酸化皮膜中に生成するＣｒ系酸化物が少量に留まり、Ｃｒ−Ｍｎ−Ｆｅ系のスピネル構造をもつ酸化物が多量に生成するため、酸化皮膜がポーラスになる。その結果、酸化皮膜を透過して下地鋼に到達する水蒸気，ＣＯ₂，ＳＯ₂等の腐食性成分が多くなり、下地鋼の水蒸気酸化や硫化が進行する。
【００１２】
そこで、Ｓｉ，Ａｌ添加によってＣｒ系酸化物を安定化させることにより、耐水蒸気酸化性，耐硫化性を改善する。Ｓｉ，Ａｌは、Ｃｒ系酸化物の内層にＳｉ，Ａｌの酸化物を形成し，酸化皮膜を強化する。また、Ｓｉ添加によって鋼中のＣｒ拡散が促進され、Ｃｒ系酸化物が生成しやすくなり、結果としてＣｒ系酸化物が安定した酸化皮膜によって腐食性成分の透過が抑制されることに起因するものと推察される。
Ｙ，ＲＥＭ，Ｃａの添加も耐水蒸気酸化性，耐硫化性の改善に有効である。Ｙ，ＲＥＭ，Ｃａは、酸化皮膜のＣｒ系酸化物に固溶し、酸化皮膜を強化することによって腐食性成分の透過を抑制するものと推察される。
【００１３】
石油系燃料改質器は、稼動，非稼動に応じて常温から９００℃前後の高温に至る温度域で加熱・冷却される。そのため、加熱・冷却の繰返しによって蓄積される熱疲労も大きくなる。この点、石油精製プラントの大型水素発生装置は高温で連続運転されることから、高温クリープ特性に優れた材料の使用によって問題が解決されるが、加熱・冷却が繰り返される改質器には同様な手段を適用できない。
熱疲労特性の改善には、ステンレス鋼の高温強度を高めることが有効な手段である。本発明では、Ｎｂ，Ｔｉ，Ｍｏ，Ｃｕの１種又は２種以上を添加することによって高温強度，ひいては熱疲労特性を改善している。Ｍｏ，Ｃｕは固溶強化、Ｎｂ，Ｔｉは固溶強化や析出強化によって熱疲労特性を改善する。
【００１４】
以上の観点から、石油系燃料改質器に使用されるフェライト系ステンレス鋼の成分・組成を次のように定めた。
Ｃｒ：８〜３５質量％
ステンレス鋼に必要な耐食性，耐酸化性を付与する上で必要な合金成分である。８００℃前後における高温耐酸化性を確保するためには、８質量％以上のＣｒが必要である。しかし、３５質量％を超える過剰量のＣｒが含まれると、フェライト系ステンレス鋼の加工性，低温靭性が低下する。
Ｃ，Ｎ：０.０３質量％以下
高温強度，特にクリープ特性を改善する成分であるが、フェライト系ステンレス鋼に過剰添加すると加工性，低温靭性を著しく低下させる。また、ＴｉやＮｂとの反応によって炭窒化物を生成しやすく、高温強度の改善に有効な固溶Ｔｉや固溶Ｎｂを減少させる。したがって、本成分系ではＣ，Ｎ含有量は少ないほど好ましく、共に上限を０.０３質量％に設定した。
【００１５】
Ｍｎ：１.５質量％以下
フェライト系ステンレス鋼の耐スケール剥離性を向上させる成分であるが、１.５質量％を超える過剰量のＭｎが含まれると鋼材が硬質化し、加工性，低温靭性が低下する。高レベルの加工性，低温靭性を確保する上では、Ｍｎ含有量の上限を０.５質量％にすることが好ましい。
Ｓ：０.００８質量％以下
熱間加工性，耐溶接高温割れ性に悪影響を及ぼす成分であり、異常酸化の起点にもなる。そのため、Ｓ含有量は可能な限り低くすることが好ましく、上限を０.００８質量％に設定した。
【００１６】
Ｓｉ：０.８〜２.５質量％
Ｃｒ系酸化物の安定化に有効な合金成分であり、０.８質量％以上の含有量でＳｉの添加効果が顕著になる。しかし、２.５質量％を超える過剰量のＳｉが含まれると、加工性，特に延性を著しく低下させ、低温靭性も低下する。また、鋼表面に疵が生成しやすくなり、製造性も低下する。
Ａｌ：０.６〜６.０質量％
Ｓｉと同様にＣｒ系酸化物の安定化に有効な合金成分であり、０.６質量％以上の含有量でＡｌの添加効果が顕著になる。しかし、６.０質量％を超える過剰量のＡｌが含まれると、加工性，低温靭性が著しく低下する。
Ａｌ及びＳｉの過剰添加に起因する欠陥を発生させることなく酸化皮膜を強化する上では、Ｓｉ，Ａｌの合計添加量を１.５質量％以上に設定することが重要である。合計添加量が１.５質量％に満たないと、Ｃｒ系酸化物を安定させるためにＳｉ，Ａｌの何れか一方を多量に添加する必要が生じ、加工性，低温靭性低下の原因になりやすい。
【００１７】
Ｙ：０.００１〜０.１質量％，
ＲＥＭ（希土類元素）：０.００１〜０.１質量％，
Ｃａ：０.００１〜０.０１質量％
何れも必要に応じて添加される合金成分であり、酸化皮膜中に固溶し、酸化皮膜を強化する作用を呈する。このような効果は、Ｙ：０.００１質量％以上，ＲＥＭ：０.００１質量％以上，Ｃａ：０.００１質量％以上で顕著になる。しかし、０.１質量％を超える過剰量のＹ，０.１質量％を超える過剰量のＲＥＭ，０.０１質量％を超える過剰量のＣａを添加すると、鋼材が過度に硬質化するばかりでなく、製造時に表面疵が生じやすくなり製造コストの上昇を招く。
【００１８】
Ｎｂ：０.０５〜０.８０質量％，Ｔｉ：０.０３〜０.５０質量％
Ｍｏ：０.１〜４.０質量％，Ｃｕ：０.１〜４.０質量％
何れも必要に応じて添加される合金成分であり、Ｍｏ，Ｃｕは固溶強化、Ｎｂ，Ｔｉは析出強化によってフェライト系ステンレス鋼の高温強度を更に向上させる。それぞれＭｏ：０.１質量％以上，Ｃｕ：０.１質量％以上，Ｎｂ：０.０５質量％以上，Ｔｉ：０.０３質量％以上で添加効果が顕著になる。しかし、過剰量のＣｕが含まれると熱間加工性が低下し、過剰量のＭｏ，Ｎｂ，Ｔｉが含まれると鋼材が過度に硬質化するので、それぞれの上限をＭｏ：４.０質量％，Ｃｕ：４.０質量％，Ｎｂ：０.８０質量％，Ｔｉ：０.５０質量％に設定した。
【００１９】
その他の成分について本発明では特に規定するものではないが、一般的な不純物元素でありＰ，Ｏ，Ｎｉ等は可能な限り低減することが好ましい。通常はＰ：０.０４質量％以下，Ｏ：０.０２質量％以下，Ｎｉ：０.６質量％以下に規制されるが、高レベルの加工性や溶接性を確保する場合にはＰ，Ｏ，Ｎｉを更に厳しく規制する。また、耐熱性の改善に有効なＷ，Ｔａ，Ｖ，Ｚｒや熱間加工性の改善に有効なＢ，Ｍｇ，Ｃｏ等の元素も必要に応じて添加できる。
【００２０】
【実施例】
表１の成分・組成をもつ各種フェライト系ステンレス鋼を３０ｋｇ真空溶解炉で溶製し、インゴットに鋳造した。インゴットを粗圧延した後、熱延，焼鈍酸洗，冷延，仕上げ焼鈍を経て板厚２.０ｍｍの冷延焼鈍材を製造した。
また、別のインゴットを熱間鍛造，焼鈍して外径３０ｍｍの丸棒を製造した。
【００２１】

【００２２】
各フェライト系ステンレス鋼から試験片を切り出し、冷延焼鈍板を高温水蒸気酸化試験に、焼鈍丸棒を熱疲労試験に供した。
高温水蒸気酸化試験では、石油系燃料改質器が曝される雰囲気を想定し、５０体積％Ｈ₂Ｏ＋２０体積％ＣＯ₂及び５０体積％Ｈ₂Ｏ＋１０ｐｐｍＳＯ₂の２種類の雰囲気を用意した。当該雰囲気中で試験片を９００℃に２５分保持する加熱及び室温まで降温して５分保持する冷却を１サイクルとする加熱・冷却を５００回繰り返した後、試験片の重量を測定した。測定結果を試験前の重量と比較し、重量変化が２.０ｍｇ／ｃｍ²以下を○，２.０ｍｇ／ｃｍ²を超える重量増加があったものを×として耐水蒸気酸化性を評価した。酸化，硫化が生じていないものほど、酸化皮膜の環境遮断機能が強く、耐水蒸気酸化性に優れているといえる。
【００２３】
熱疲労試験では、自由熱膨張に対し５０％の歪量を付加するように制御して２００〜９００℃の温度域で試験片を繰返し加熱・冷却した。初期の最大引張り応力が３／４まで低下したときの繰返し数を破損繰返し数と定義し、加熱・冷却を５００サイクル以上繰り返しても破損しなかった試験片を○，５００サイクル未満の加熱・冷却で破損繰返し数に達した試験片を×として熱疲労特性を評価した。
表２の試験結果にみられるように、本発明に従った鋼種番号１〜５のフェライト系ステンレス鋼は、何れも耐水蒸気酸化性，熱疲労特性に優れており、改質器材料としての要求特性を十分に満足していた。
【００２４】
他方、鋼種番号６〜８のフェライト系ステンレス鋼は、高温保持した後で試験片表面に酸化スケールの亀裂等、多数の損傷が発生しており、耐水蒸気酸化性に劣っていた。損傷の発生は、Ｓｉ、Ａｌ含有量が不足するために酸化皮膜のＣｒ系酸化物が不安定で、高温保持中に酸化皮膜を透過したＨ₂Ｏ，ＣＯ₂，ＳＯ₂等が下地鋼をアタックしたことによるものと推察される。熱疲労特性にも劣っていた。熱疲労特性は０．３１質量％のＮｂを添加した鋼種番号８で改善がみられたが、耐水蒸気酸化性は依然として不十分であった。
【００２５】

【００２６】
【発明の効果】
以上に説明したように、本発明のフェライト系ステンレス鋼は、Ｃｒ系酸化物が安定化した酸化皮膜が表面に形成され、高温雰囲気に長時間曝された状態でも酸化皮膜が優れた環境遮断機能を呈し、高温水蒸気雰囲気下での酸化や硫化が防止される。また、組織強化により優れた熱疲労特性が維持される。そのため、過酷な高温水蒸気雰囲気下で稼動され、高温〜常温の広い温度域にわたって加熱・冷却が繰り返される石油系燃料改質器に好適な材料として使用される。
【図面の簡単な説明】
【図１】単管式石油系燃料改質器の内部構造を示す概略図[0001]
[Industrial application fields]
The present invention relates to a ferritic stainless steel that satisfies the required characteristics of a reformer used when reforming petroleum-based fuels such as gasoline, naphtha, kerosene, and LPG to hydrogen.
[0002]
[Prior art]
Hydrogen used in a wide range of applications such as basic raw materials, fuel for fuel cells and heat treatment atmosphere in various chemical industries is produced by decomposing fuels such as petroleum and alcohol. For example, a large and continuous hydrogen generator is used in an oil refinery plant. A conventional hydrogen generator uses naphtha or natural gas as a raw material to produce hydrogen by a steam reforming reaction.
Recently, various reformers have been developed at a rapid pace in order to obtain hydrogen for fuel cells. As a reformer for a fuel cell, a multi-tube type in which a plurality of reaction tubes are accommodated in a container, a single-tube type having a large-diameter reaction tube, and the like are known.
[0003]
For example, in a single-tube reformer, a catalyst is filled in a reaction tube 1 composed of a double tube having an inner wall 1a and an outer wall 1b, and a first catalyst layer 2a and a second catalyst layer 2b are formed by appropriate partitions. (Fig. 1). A reformed gas extraction pipe 3 having an inner flow path 3a and an outer flow path 3b is disposed between the first catalyst layer 2a and the second catalyst layer 2b, and the first catalyst layer 2a is connected to the inner flow path 3a. The catalyst layer 2b is communicated with the outer flow path 3b. The reaction tube 1 is entirely surrounded by a housing 4, a source gas supply pipe 5 is provided on one side wall of the housing 4, and a junction pipe 3c communicating with the outer flow path 3b extends from the other side wall to the outside of the system. A burner 6 is provided at the bottom of the housing 4 and is supplied with burner fuel f and combustion air o and feeds the frame F into an internal cavity defined by the inner wall 1 a of the reaction tube 1.
[0004]
The raw material gas RG is fed into the reaction tube 1 together with the reforming steam, and flows through the path of the first catalyst layer 2a → the inner flow path 3a → the second catalyst layer 2b → the outer flow path 3b → the merge pipe 3c. When the raw material gas RG passes through the first catalyst layer 2a and the second catalyst layer 2b heated from the inside in the frame F, a reforming reaction (for example, C ₃ H ₈ + 3H ₂ O = 3CO + 7H ₂ ), shift reaction Hydrogen is generated by (CO + H ₂ O═CO ₂ + H ₂ ) or the like. Hydrogen is directly recovered as a reformed gas from the reforming extraction pipe, or separated and recovered from the reformed gas PG by a selective permeation method using a hydrogen permeable membrane such as Pd—Ag, Ta.
[0005]
[Problems to be solved by the invention]
Since the hydrogen generator of an oil refinery plant is continuously operated at a high temperature, an excellent high temperature creep strength is required. Further, it is exposed to a water vapor atmosphere containing CO ₂ , SO _{2 and the} like. For this reason, centrifugal cast tubes made of heat-resistant alloys such as HK40 (25Cr-20Ni-0.4C) are used as the structural material of the hydrogen generator.
On the other hand, in a reformer for a fuel cell that recovers hydrogen from petroleum-based fuel, the reforming temperature is 800 ° C. or higher compared to city gas and alcohol-based fuel. Moreover, it is exposed to an oxidizing atmosphere containing water vapor, CO ₂ , SO _2, etc., and heating and cooling are frequently repeated according to the demand for hydrogen. A practical material that exhibits sufficient durability under such a harsh environment has not been reported so far.
[0006]
[Means for Solving the Problems]
The present invention is based on conventional ferritic stainless steel, and has been completed by adding various studies to the steel composition in consideration of the environment of a petroleum fuel reformer exposed to a high-temperature steam atmosphere. Ferritic stainless steel for petroleum fuel reformers that enhances the high temperature strength in the medium to high temperature range by adding N, Mo, Nb, etc., while satisfying the required characteristics of the reformer, while strengthening the oxide film at the initial stage of heating The purpose is to provide.
[0007]
In order to achieve the object, the ferritic stainless steel for petroleum fuel reformer of the present invention has Cr: 8 to 35 mass%, C: 0.03 mass% or less, N: 0.03 mass% or less, Mn: 1.5 wt% or less, S: 0.008 mass% or less, Si: 0.8-2.5 mass% and / or Al: 0.6-6.0 comprises a mass%, further Nb:. 0 05 . ~ 0 80 wt%, Ti:.. 0 03 to 0 50 wt%, Mo:.. 0 1-4 0% by mass, Cu:. 0 1-4 one or more 0 mass%. And the balance is composed of Fe and inevitable impurities , and the total amount of Si and Al is adjusted to 1.5% by mass or more.
[0008]
This ferritic stainless steel is further classified as Y: 0.001 to 0.1% by mass, REM (rare earth element): 0.001 to 0.1% by mass, and Ca: 0.001 to 0.01% by mass. Or 2 or more types can be included.
[0009]
[Action]
Ferritic stainless steel represented by SUS430 and SUH409L is superior in thermal fatigue properties compared to austenitic stainless steel, but is easily oxidized by steam when exposed to a high-temperature atmosphere of a reformer containing a large amount of steam. Proceed to. Further, in a reformer in which heating / cooling is frequently repeated, further excellent thermal fatigue characteristics are required. Therefore, the present inventors examined the generation mechanism of steam oxidation and thermal fatigue from the viewpoint of material, added various alloy components based on SUS430, and investigated the effect of the added alloy components on steam oxidation and thermal fatigue. .
[0010]
Steam oxidation in a high temperature atmosphere is more damaging than atmospheric oxidation. The steam oxidation mechanism is not always clear. Steam oxidation is a phenomenon that occurs when water vapor dissociates into oxygen and hydrogen to promote the oxidation reaction, and water vapor directly reaches the steel substrate to promote oxidation. Troubles such as clogging and deformation due to thinning of the steel substrate, hole opening, etc. occur.
[0011]
This steam oxidation can be suppressed by stabilizing an oxide film mainly composed of Cr-based oxide formed on the surface of stainless steel. The oxide film formed on the stainless steel surface by heating imparts oxidation resistance to the stainless steel, and in a high temperature atmosphere of about 800 ° C., the oxidation resistance is improved with a Cr content of 8% by mass or more. Become prominent. However, when the steel substrate is exposed to a high-temperature steam atmosphere, only a small amount of Cr-based oxide is formed in the oxide film, and a large amount of oxide having a Cr-Mn-Fe-based spinel structure is generated. The film becomes porous. As a result, corrosive components such as water vapor, CO ₂ and SO ₂ that pass through the oxide film and reach the base steel increase, and steam oxidation and sulfidation of the base steel proceed.
[0012]
Therefore, by stabilizing the Cr-based oxide by adding Si and Al, the steam oxidation resistance and sulfidation resistance are improved. Si and Al form an oxide of Si and Al in the inner layer of the Cr-based oxide to strengthen the oxide film. Moreover, Cr diffusion in steel is promoted by addition of Si, and it becomes easy to produce Cr-based oxides. As a result, the transmission of corrosive components is suppressed by the stable oxide film of Cr-based oxides. It is guessed.
Addition of Y, REM, and Ca is also effective for improving steam oxidation resistance and sulfidation resistance. It is presumed that Y, REM, and Ca are dissolved in the Cr-based oxide of the oxide film and suppress the permeation of corrosive components by strengthening the oxide film.
[0013]
Petroleum-based fuel reformers are heated and cooled in a temperature range from room temperature to a high temperature of around 900 ° C. according to operation and non-operation. Therefore, thermal fatigue accumulated by repeated heating and cooling is also increased. In this respect, large-scale hydrogen generators in oil refining plants operate continuously at high temperatures, so the problem is solved by using materials with excellent high-temperature creep characteristics, but the same applies to reformers that are repeatedly heated and cooled. Cannot be applied.
Increasing the high temperature strength of stainless steel is an effective means for improving thermal fatigue properties. In the present invention, the high-temperature strength and thus the thermal fatigue characteristics are improved by adding one or more of Nb, Ti, Mo, and Cu. Mo and Cu improve the thermal fatigue characteristics by solid solution strengthening, and Nb and Ti improve by solid solution strengthening and precipitation strengthening.
[0014]
From the above viewpoint, the components and composition of ferritic stainless steel used in petroleum fuel reformers were determined as follows.
Cr: 8 to 35% by mass
It is an alloy component necessary for imparting the necessary corrosion resistance and oxidation resistance to stainless steel. In order to ensure high-temperature oxidation resistance at around 800 ° C., 8 mass% or more of Cr is necessary. However, if an excessive amount of Cr exceeding 35% by mass is contained, the workability and low temperature toughness of the ferritic stainless steel are lowered.
C, N: 0.03 mass% or less A component that improves high-temperature strength, particularly creep properties, but when added excessively to ferritic stainless steel, workability and low-temperature toughness are significantly reduced. In addition, carbonitrides are easily generated by reaction with Ti and Nb, and solid solution Ti and solid solution Nb effective in improving high temperature strength are reduced. Therefore, in this component system, the lower the C and N content, the better.
[0015]
Mn: 1.5% by mass or less A component that improves the scale peel resistance of ferritic stainless steel, but if an excessive amount of Mn exceeding 1.5% by mass is contained, the steel material becomes hard, and the workability and low temperature toughness are increased. Decreases. In order to ensure a high level of workability and low temperature toughness, the upper limit of the Mn content is preferably 0.5% by mass.
S: 0.008% by mass or less S is a component that adversely affects hot workability and welding hot cracking resistance, and also serves as a starting point for abnormal oxidation. Therefore, the S content is preferably as low as possible, and the upper limit is set to 0.008% by mass.
[0016]
Si: 0.8-2.5 mass%
It is an alloy component effective for stabilizing the Cr-based oxide, and the effect of adding Si becomes remarkable when the content is 0.8% by mass or more. However, when an excessive amount of Si exceeding 2.5% by mass is contained, workability, particularly ductility, is remarkably lowered, and low temperature toughness is also lowered. Moreover, it becomes easy to generate | occur | produce a flaw on the steel surface, and productivity falls.
Al: 0.6-6.0 mass%
Like Si, it is an effective alloy component for stabilizing the Cr-based oxide, and the effect of adding Al becomes remarkable when the content is 0.6% by mass or more. However, if an excessive amount of Al exceeding 6.0% by mass is contained, workability and low temperature toughness are remarkably lowered.
In order to strengthen the oxide film without generating defects due to the excessive addition of Al and Si, it is important to set the total addition amount of Si and Al to 1.5% by mass or more. If the total addition amount is less than 1.5% by mass, it is necessary to add a large amount of either Si or Al to stabilize the Cr-based oxide, which is likely to cause deterioration in workability and low temperature toughness. .
[0017]
Y: 0.001 to 0.1% by mass,
REM (rare earth element): 0.001 to 0.1% by mass,
Ca: 0.001 to 0.01% by mass
Any of these is an alloy component that is added as necessary, and exhibits an action of solid-dissolving in the oxide film and strengthening the oxide film. Such an effect becomes remarkable when Y: 0.001 mass% or more, REM: 0.001 mass% or more, and Ca: 0.001 mass% or more. However, adding an excess amount of Y exceeding 0.1% by mass, an excess amount of REM exceeding 0.1% by mass, and an excess amount of Ca exceeding 0.01% by mass only makes the steel material excessively hardened. In addition, surface flaws are likely to occur during manufacturing, leading to an increase in manufacturing cost.
[0018]
Nb: 0.05 to 0.80 mass%, Ti: 0.03 to 0.50 mass%
Mo: 0.1-4.0 mass%, Cu: 0.1-4.0 mass%
Both are alloy components added as necessary, and Mo and Cu further improve the high temperature strength of ferritic stainless steel by solid solution strengthening and Nb and Ti by precipitation strengthening. The effect of addition becomes remarkable when Mo: 0.1% by mass or more, Cu: 0.1% by mass or more, Nb: 0.05% by mass or more, and Ti: 0.03% by mass or more, respectively. However, when an excessive amount of Cu is included, hot workability is reduced, and when an excessive amount of Mo, Nb, and Ti is included, the steel material is excessively hardened. Therefore, each upper limit is set to Mo: 4.0% by mass. , Cu: 4.0% by mass, Nb: 0.80% by mass, Ti: 0.50% by mass.
[0019]
The other components are not particularly defined in the present invention, but are generally impurity elements, and it is preferable to reduce P, O, Ni and the like as much as possible. Usually, P is controlled to 0.04 mass% or less, O: 0.02 mass% or less, and Ni: 0.6 mass% or less. However, in order to ensure a high level of workability and weldability, P, O and Ni are more strictly regulated. Also, elements such as W, Ta, V, Zr effective for improving heat resistance and B, Mg, Co effective for improving hot workability can be added as required.
[0020]
【Example】
Various ferritic stainless steels having the components and compositions shown in Table 1 were melted in a 30 kg vacuum melting furnace and cast into an ingot. After roughly rolling the ingot, a cold-rolled annealed material having a thickness of 2.0 mm was manufactured through hot rolling, annealing pickling, cold rolling, and finish annealing.
Another ingot was hot forged and annealed to produce a round bar with an outer diameter of 30 mm.
[0021]

[0022]
A test piece was cut out from each ferritic stainless steel, the cold-rolled annealed plate was subjected to a high temperature steam oxidation test, and the annealed round bar was subjected to a thermal fatigue test.
In the high-temperature steam oxidation test, _two atmospheres of 50 volume% H ₂ O + 20 volume% CO ₂ and 50 volume% H ₂ O + 10 ppm SO ₂ were prepared assuming an atmosphere to which the petroleum fuel reformer was exposed. The heating of the test piece at 900 ° C. for 25 minutes in the atmosphere and the heating / cooling with one cycle of cooling to room temperature and holding for 5 minutes were repeated 500 times, and then the weight of the test piece was measured. The measurement result was compared with the weight before the test, and the steam oxidation resistance was evaluated with a change in weight of 2.0 mg / cm ² or less as ◯ and a weight increase exceeding 2.0 mg / cm ² as x. It can be said that the one that does not oxidize or sulfidize has a stronger environmental barrier function of the oxide film and is superior in steam oxidation resistance.
[0023]
In the thermal fatigue test, the test piece was repeatedly heated and cooled in a temperature range of 200 to 900 ° C. while controlling to add a strain amount of 50% to free thermal expansion. The number of repetitions when the initial maximum tensile stress is reduced to 3/4 is defined as the number of failure repetitions. A test piece that did not break even after repeated heating and cooling for 500 cycles or more is heated and cooled for less than 500 cycles. The thermal fatigue property was evaluated by setting the test piece that reached the number of repetitions of damage as x.
As can be seen from the test results in Table 2, the ferritic stainless steels of steel types 1 to 5 according to the present invention are all excellent in steam oxidation resistance and thermal fatigue properties, and are required as reformer materials. The characteristics were fully satisfied.
[0024]
On the other hand, the ferritic stainless steels of steel type numbers 6 to 8 were inferior in steam oxidation resistance because many damages such as oxide scale cracks were generated on the surface of the test piece after being kept at a high temperature. Damage occurs because the Cr-based oxide of the oxide film is unstable due to insufficient Si and Al contents, and H ₂ O, CO ₂ , SO ₂ etc. that permeate the oxide film while maintaining high temperature This is probably due to the attack. The thermal fatigue properties were also inferior. Although the thermal fatigue characteristics were improved in steel type No. 8 to which 0.31% by mass of Nb was added, the steam oxidation resistance was still insufficient.
[0025]

[0026]
【The invention's effect】
As described above, the ferritic stainless steel of the present invention has an oxide film with a stable Cr-based oxide formed on the surface, and the oxide film has an excellent environmental barrier function even when exposed to a high temperature atmosphere for a long time. This prevents oxidation and sulfidation in a high-temperature steam atmosphere. In addition, excellent thermal fatigue characteristics are maintained by strengthening the structure. Therefore, it is used as a material suitable for a petroleum fuel reformer that is operated in a harsh high-temperature steam atmosphere and is repeatedly heated and cooled over a wide temperature range from high temperature to normal temperature.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing the internal structure of a single-pipe petroleum fuel reformer.

Claims

Cr: 8-35 mass%, C: 0.03 mass% or less, N: 0.03 mass% or less, Mn: 1.5 mass% or less, S: 0.008 mass% or less, Si: 0.8- 2.5 wt% and / or Al: includes 0.6 to 6.0 mass%, further Nb:.. 0 05~0 80 wt%, Ti:.. 0 03~0 50 wt%, Mo: 0 .. 1-4 0 wt%, Cu:.. 0 1-4 includes one or more of 0 mass%, the balance being Fe and unavoidable impurities, the total amount of Si and Al is 1.5 A ferritic stainless steel for petroleum fuel reformer characterized by having a composition adjusted to at least mass%.

Further, Y: 0.001 to 0.1% by mass, REM (rare earth element): 0.001 to 0.1% by mass, Ca: 0.001 to 0.01% by mass, or one or more. Item 2. A ferritic stainless steel for petroleum fuel reformer according to Item 1.