JP3910419B2 - Ferritic stainless steel for alcohol fuel reformers - Google Patents
Ferritic stainless steel for alcohol fuel reformers Download PDFInfo
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- JP3910419B2 JP3910419B2 JP2001357424A JP2001357424A JP3910419B2 JP 3910419 B2 JP3910419 B2 JP 3910419B2 JP 2001357424 A JP2001357424 A JP 2001357424A JP 2001357424 A JP2001357424 A JP 2001357424A JP 3910419 B2 JP3910419 B2 JP 3910419B2
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
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Description
【０００１】
【産業上の利用分野】
本発明は、アルコール系燃料から水素を製造する改質器に適したフェライト系ステンレス鋼に関する。
【０００２】
【従来の技術】
燃料電池は、水の電気分解と逆の反応経過を経て電力を発生する装置であり、環境に有害な排ガスを発生しないことから自動車等の動力源として有望視されている。また、小規模なオンサイト発電装置としても一部ですでに実用化されている。
燃料電池の代表的な燃料として水素が使用されている。水素は、都市ガス（ＬＮＧ），石油液化ガス（ＬＰＧ），ナフサ，メタノール等の炭化水素系燃料を触媒の存在下で改質反応させることにより製造される。なかでも、メタノール，ジメチルエーテル等のアルコール系燃料の改質は、反応温度が低いため起動に要する時間が比較的短い点が長所である。生成した水素は、Ｓが含まれていないことからも燃料電池用燃料に適している。
【０００３】
【発明が解決しようとする課題】
メタノールの改質には、水蒸気酸化改質方式，部分酸化改質方式及び両者を組み合わせた併用改質方式（オートサーマル方式）がある。水蒸気改質方式では改質器の温度が３００℃程度であるため、改質器の構造材として通常のステンレス鋼を使用しても損傷は軽微であるが、改質に必要な熱量を外部から供給する必要がある。そのため、起動時は部分酸化改質やオートサーマル方式で改質し、改質器が昇温した段階で水蒸気酸化改質に切り替えるシステムが検討されている。
【０００４】
部分酸化改質やオートサーマル方式では、改質器が６００℃程度の高温に上昇することもある。しかも、多量の水蒸気の他にＣＯ，ＣＯ2等を含む酸化性雰囲気で改質器が稼動されるため、水蒸気酸化や赤スケールが発生しやすい。したがって、改質器用材料には、当該温度域で加熱・冷却されても初期の機能を損なうことがない優れた耐高温酸化性，耐赤スケール性が要求される。
更には、改質器を搭載した自動車を想定すると、自動車の走行に応じて改質器の起動停止が繰り返され、常温〜高温の幅広い温度域で改質器の構造材が加熱・冷却される。加熱・冷却によって熱歪が蓄積されると、材料破断に至る虞もある。この点，熱疲労特性に優れていることも、改質器に要求される特性の一つである。
【０００５】
【課題を解決するための手段】
本発明は、このような問題を解消すべく案出されたものであり、高温水蒸気雰囲気下で起動・停止を繰り返しても水蒸気酸化や赤スケールが発生せず、長期間にわたって安定条件下の改質反応が継続されるアルコール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。
【０００６】
本発明のアルコール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：８〜２５質量％，Ｃ：０.０３質量％以下，Ｎ：０.０３質量％以下，Ｓｉ：０.１０〜２.５質量％，Ｍｎ：１.５質量％以下，Ｓ：０.００８質量％以下，Ａｌ：０ . ０１〜４ . ０質量％を、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、Ａ＝Ｃｒ＋５（Ｓｉ＋Ａｌ）と定義されるＡ値が１３〜６０の範囲に調整された組成を有していることを特徴とする。
【０００７】
このフェライト系ステンレス鋼は、更にＹ，ＲＥＭ（希土類元素），Ｃａの１種又は２種以上を合計で０.００１〜０.１質量％含むことができる。
【０００８】
【作用】
代表的なフェライト系ステンレス鋼であるＳＵＳ４３０は、通常雰囲気では優れた耐熱・耐食性を呈するが、多量の水蒸気及びＣＯ2を含む改質器の高温雰囲気に曝されると水蒸気酸化，赤スケール化が容易に進行し、構造材としての機能が損なわれる。ＳＵＳ４３０は、加熱・冷却の繰返しに起因する熱疲労に対しても十分な耐性を備えていない。そこで、本発明者等は、水蒸気酸化，赤スケール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することによって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。
【０００９】
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が、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る。また、赤スケールは、Ｃｒ系酸化物よりもＦｅ系酸化物の方が優先的に生成することにより生じる。水蒸気酸化や赤スケールの発生によって酸化スケールが剥離すると、改質器構造材としての機能が損なわれ、改質反応が不安定になる。
６００℃程度の高温雰囲気における耐酸化性はＣｒ：８質量％以上のＣｒ添加で確保されるが、多量の水蒸気，ＣＯ2を含む改質器の雰囲気では水蒸気酸化が進行し、赤スケールが発生する。本発明においては、所定量のＳｉを添加することによって水蒸気酸化や赤スケールを防止している。
【００１０】
水蒸気酸化や赤スケールがＳｉ添加で抑制される理由は定かではないが、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、結果として酸化皮膜が強化され、Ｈ2Ｏ，ＣＯ2等の酸化性成分が酸化皮膜を透過して下地鋼に達することが抑止されるものと推察される。酸化皮膜の強化は、下地鋼からのＦｅ拡散を抑え、赤スケールの発生防止にも有効である。
【００１１】
Ｙ，ＲＥＭ，Ｃａ，Ａｌの添加も耐水蒸気酸化性，耐赤スケール性の改善に有効である。Ｙ，ＲＥＭ，Ｃａ，Ａｌ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酸化性成分の透過を抑制するものと推察される。
Ｎｂ，Ｔｉ，Ｍｏ，Ｃｕは、ステンレス鋼の高温強度、ひいては熱疲労特性を改善する。そのため、改質器の稼動・停止に伴って常温〜６００℃の広範囲で加熱・冷却が繰り返される環境下でも、熱応力に十分耐え、材料破断を抑制する効果が大きい。
【００１２】
以上の観点から、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：８〜２５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。６００℃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、２５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，低温靭性を低下させることにもなる。
【００１３】
Ｃ，Ｎ：０.０３質量％以下
本発明の成分系では、Ｃｒ系炭窒化物となって耐水蒸気酸化性，耐赤スケール性に有効なＣｒ量を消費し、高温特性に有害なＣｒ欠乏層を生成する。このような欠陥発生は、Ｃ，Ｎ含有量をそれぞれ０.０３質量％以下に規制することによって防止できる。Ｃ，Ｎ低減に起因する鋼材の軟質化は、Ｓｉ，Ｎｂ，Ｔｉ，Ｍｏ等の添加によって抑制できる。なかでも、Ｎｂ，Ｔｉは、炭窒化物の形成により固溶Ｃ，Ｎ量を減少させるため、Ｃ，Ｎ含有量の上限に関する制約も緩和する。
【００１４】
Ｓｉ：０.８〜２.５質量％，Ａｌ：４.０質量％以下
Ｃｒ系酸化物の内層に酸化物層を生成することにより酸化皮膜を強化する作用を呈し、酸化皮膜の環境遮断機能を改善する上で有効な合金成分である。０.８質量％以上のＳｉ又はＡｌ添加によって耐水蒸気酸化性の改善効果が顕著になる。また、Ａ＝Ｃｒ＋５（Ｓｉ＋Ａｌ）と定義されるＡ値を１３以上に調整するとき、酸化皮膜中のＣｒ系酸化物，Ｓｉ系酸化物，Ａｌ系酸化物が多くなり、赤スケールの発生原因であるＦｅ系酸化物の濃度が低下する。しかし、２.５質量％を超えるＳｉ添加，４.０質量％を超えるＡｌ添加や６０を超えるＡ値では、加工性，低温靭性が劣化しやすくなる。Ａ値は、溶接性を考慮するとき上限を３０以下（好ましくは、２０以下）に設定する。
【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に有効な合金成分であるが、１.５質量％を超える過剰量のＭｎ添加は熱間加工性を低下させ，鋼材を硬質化して加工性にも悪影響を及ぼす。
Ｓ：０.００８質量％以下
鋼材の熱間加工性を低下させ、溶接高温割れ感受性を高める有害成分である。そのため、Ｓ含有量は低いほど好ましく、本成分系ではＳ含有量の上限を０.００８質量％に設定した。
【００１６】
Ｙ，ＲＥＭ，Ｃａの１種又は２種以上：合計で０.００１〜０.１質量％
何れも必要に応じて添加される合金成分であり、酸化皮膜中のＣｒ系酸化物に固溶し、酸化皮膜を強化する作用を呈する。このような効果は、Ｙ，ＲＥＭ（希土類元素），Ｃａの１種又は２種以上を合計で０.００１質量％以上添加するとき顕著になる。しかし、Ｙ，ＲＥＭ（希土類元素），Ｃａの合計添加量が０.１質量％を超えると、鋼材が過度に硬質化すると共に、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。
【００１７】
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は析出硬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、熱疲労特性を改善する。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。
【００１８】
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％以下に規制されるが、高レベルの加工性や溶接性を確保するためにＰ，Ｏを更に厳格に規制する場合もある。
【００１９】
【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、外径３０ｍｍの丸棒を製造した。
【００２０】
得られた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、アルコール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雰囲気を使用した。当該雰囲気中で試験片を６００℃に昇温して２５分保持する加熱と室温まで降温して５分保持する冷却を１サイクルとし、加熱・冷却を５００回繰り返した後で試験片の重量を測定した。
【００２２】
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を超える重量変化があったものを×として耐水蒸気酸化性を評価した。酸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。また、加熱・冷却後の試験片表面を観察し、赤スケール発生の有無を調査した。
熱疲労試験では、自由熱膨張に相当する歪量を付与（拘束率１００％）しながら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１０００サイクル以上繰り返しても破損しなかった試験片を○，１０００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。
【００２３】
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、アルコール系燃料改質器材料としての要求特性を十分に満足していた。また、試験片の表面に赤スケールが観察されなかった。
他方、鋼種番号６，７のフェライト系ステンレス鋼は、高温保持した後での重量変化が多く、水蒸気酸化，赤スケール発生が進行していた。水蒸気酸化，赤スケール発生は、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。更には、熱疲労特性も不足していた。
【００２４】
【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、Ｓｉ，Ａｌ添加でＣｒ系酸化物を安定化させ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劣化せずに高い環境遮断機能を発現するため、水蒸気酸化や赤スケール発生が抑えられる。Ｓｉ添加は、鋼材の高温強度、ひいては熱疲労特性の改善にも有効である。そのため、多量のＨ2Ｏ，ＣＯ2，ＣＯを含む高温雰囲気に曝され、しかも高温〜常温の広い温度域にわたって加熱・冷却が繰り返されるアルコール系燃料改質器に好適な材料として使用される。[0001]
[Industrial application fields]
The present invention relates to a ferritic stainless steel suitable for a reformer that produces hydrogen from alcohol fuel.
[0002]
[Prior art]
A fuel cell is a device that generates electric power through a reaction process opposite to that of water electrolysis, and is considered promising as a power source for automobiles and the like because it does not generate exhaust gas harmful to the environment. Some small on-site power generation devices have already been put into practical use.
Hydrogen is used as a typical fuel for fuel cells. Hydrogen is produced by a reforming reaction of hydrocarbon fuels such as city gas (LNG), petroleum liquefied gas (LPG), naphtha and methanol in the presence of a catalyst. Among these, reforming of alcohol fuels such as methanol and dimethyl ether has an advantage in that the time required for starting is relatively short because the reaction temperature is low. The generated hydrogen is suitable for fuel cell fuel because it does not contain S.
[0003]
[Problems to be solved by the invention]
For reforming methanol, there are a steam oxidation reforming method, a partial oxidation reforming method, and a combined reforming method (autothermal method) combining both. In the steam reforming method, the temperature of the reformer is about 300 ° C., so even if ordinary stainless steel is used as the structural material of the reformer, the damage is slight, but the amount of heat required for reforming is externally applied. It is necessary to supply. Therefore, a system is being studied in which reforming is performed by partial oxidation reforming or autothermal method at startup, and switching to steam oxidation reforming when the temperature of the reformer rises.
[0004]
In the partial oxidation reforming or autothermal system, the reformer may rise to a high temperature of about 600 ° C. Moreover, since the reformer is operated in an oxidizing atmosphere containing CO, CO 2 and the like in addition to a large amount of steam, steam oxidation and red scale are likely to occur. Therefore, the reformer material is required to have excellent high temperature oxidation resistance and red scale resistance that do not impair the initial function even when heated and cooled in the temperature range.
Furthermore, assuming an automobile equipped with a reformer, the reformer is repeatedly started and stopped as the automobile travels, and the structural material of the reformer is heated and cooled in a wide temperature range from room temperature to high temperature. . If thermal strain accumulates due to heating and cooling, there is a risk of material breakage. In this respect, excellent thermal fatigue characteristics are also required for the reformer.
[0005]
[Means for Solving the Problems]
The present invention has been devised to solve such a problem, and even if it is repeatedly started and stopped in a high-temperature steam atmosphere, steam oxidation and red scale do not occur, and the stable condition is improved over a long period of time. An object of the present invention is to provide a ferritic stainless steel for an alcohol fuel reformer in which a quality reaction is continued.
[0006]
In order to achieve the object, the ferritic stainless steel for alcohol-based fuel reformer of the present invention has Cr: 8 to 25% by mass, C: 0.03% by mass or less, N: 0.03% by mass or less, Si: 0.10 to 2.5 wt%, Mn: 1.5 wt% or less, S: 0.008 mass% or less, Al:.. 0 01 to 4 0% by weight, further Nb:. 0 05~0. 80 wt%, Ti:.. 0 03~0 50 wt%, Mo:.. 0 1 to 4 0% by mass, Cu:.. 0 1 to 4 wherein one or more of 0 mass%, the balance Is composed of Fe and inevitable impurities, and has a composition in which an A value defined as A = Cr + 5 (Si + Al) is adjusted to a range of 13 to 60.
[0007]
The ferritic stainless steel may further comprise 0.001 to 0.1 wt% Y, REM (rare earth element), one of Ca or two or more in total.
[0008]
[Action]
SUS430, which is a typical ferritic stainless steel, exhibits excellent heat resistance and corrosion resistance in a normal atmosphere, but when exposed to a high temperature atmosphere of a reformer containing a large amount of steam and CO 2 , steam oxidation and red scale conversion can be achieved. It progresses easily and the function as a structural material is impaired. SUS430 does not have sufficient resistance against thermal fatigue caused by repeated heating and cooling. Therefore, the present inventors have studied the generation mechanism of steam oxidation, red scale, and thermal fatigue from the viewpoint of material, and by adding various alloy components based on SUS430, the added alloy component affects the steam oxidation and thermal fatigue. The impact was investigated.
[0009]
Steam oxidation in a high temperature atmosphere is more damaging than atmospheric oxidation. Although the steam oxidation mechanism is not necessarily clear, it is a phenomenon caused by water vapor dissociating into oxygen and hydrogen to promote an oxidation reaction, and water vapor directly reaches the steel substrate to promote oxidation. Further, the red scale is generated when the Fe-based oxide is preferentially generated over the Cr-based oxide. When the oxide scale is peeled off due to the generation of steam oxidation or red scale, the function as the reformer structure material is impaired, and the reforming reaction becomes unstable.
Oxidation resistance in a high temperature atmosphere of about 600 ° C is ensured by the addition of Cr: 8 mass% or more, but steam oxidation proceeds in a reformer atmosphere containing a large amount of steam and CO 2 , and red scale is generated. To do. In the present invention, steam oxidation and red scale are prevented by adding a predetermined amount of Si.
[0010]
The reason why steam oxidation and red scale are suppressed by the addition of Si is not clear, but the addition of Si promotes Cr diffusion in the steel, easily forms Cr-based oxides, and as a result, the oxide film is strengthened, resulting in H 2. It is presumed that oxidizing components such as O and CO 2 are prevented from passing through the oxide film and reaching the base steel. The strengthening of the oxide film suppresses Fe diffusion from the base steel and is effective in preventing the occurrence of red scale.
[0011]
Addition of Y, REM, Ca, and Al is also effective in improving steam oxidation resistance and red scale resistance. It is inferred that Y, REM, Ca, and Al are dissolved in the Cr-based oxide of the oxide film and suppress the permeation of the oxidizing component by strengthening the oxide film.
Nb, Ti, Mo, and Cu improve the high-temperature strength of stainless steel and thus the thermal fatigue characteristics. Therefore, even in an environment where heating / cooling is repeated in a wide range from room temperature to 600 ° C. along with the operation / stop of the reformer, the effect of sufficiently resisting thermal stress and suppressing material breakage is great.
[0012]
From the above viewpoint, the composition and composition of ferritic stainless steel used in the reformer were determined as follows.
Cr: 8 to 25% 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 600 ° C., 8 mass% or more of Cr is required. However, when an excessive amount of Cr exceeding 25% by mass is contained, the workability and low temperature toughness of the ferritic stainless steel are also lowered.
[0013]
C, N: 0.03 mass% or less In the component system of the present invention, Cr-based carbonitrides are consumed, consuming an amount of Cr effective for steam oxidation resistance and red scale resistance, and Cr deficiency harmful to high temperature characteristics Create a layer. Such defect generation can be prevented by regulating the C and N contents to 0.03 mass% or less. Softening of the steel material due to C and N reduction can be suppressed by adding Si, Nb, Ti, Mo, or the like. Especially, since Nb and Ti reduce the amount of solid solution C and N by formation of carbonitride, the restrictions regarding the upper limit of C and N content are eased.
[0014]
Si: 0.8 to 2.5% by mass, Al: 4.0% by mass or less The effect of strengthening the oxide film by forming an oxide layer in the inner layer of the Cr-based oxide, and the environmental barrier function of the oxide film It is an effective alloy component in improving Addition of 0.8 mass% or more of Si or Al makes the steam oxidation resistance improving effect remarkable. In addition, when the A value defined as A = Cr + 5 (Si + Al) is adjusted to 13 or more, the amount of Cr-based oxide, Si-based oxide, and Al-based oxide in the oxide film increases, which causes red scale. The density | concentration of a certain Fe type oxide falls. However, when Si is added in excess of 2.5 mass%, Al is added in excess of 4.0 mass%, or the A value exceeds 60, workability and low temperature toughness tend to deteriorate. The upper limit of the A value is set to 30 or less (preferably 20 or less) when considering weldability.
[0015]
Mn: 1.5% by mass or less An alloy component effective for the scale peel resistance of ferritic stainless steel, but excessive addition of Mn exceeding 1.5% by mass reduces hot workability and hardens the steel. Will adversely affect processability.
S: 0.008% by mass or less S is a harmful component that reduces the hot workability of the steel material and increases the sensitivity to welding hot cracking. Therefore, the lower the S content, the better. In this component system, the upper limit of the S content is set to 0.008% by mass.
[0016]
One or more of Y, REM and Ca: 0.001 to 0.1% by mass in total
Any of these is an alloy component that is added as necessary, and exhibits an action of strengthening the oxide film by being dissolved in the Cr-based oxide in the oxide film. Such an effect becomes conspicuous when one or more of Y, REM (rare earth element), and Ca are added in an amount of 0.001% by mass or more in total. However, if the total amount of Y, REM (rare earth element) and Ca exceeds 0.1% by mass, the steel material becomes excessively hard, and surface flaws are liable to occur during production, leading to an increase in production cost.
[0017]
Nb: 0.05 to 0.80% by mass, Ti: 0.03 to 0.50% by mass,
Mo: 0.1-4.0 mass%, Cu: 0.1-4.0 mass%
Both are alloy components added as necessary. Mo and Cu further enhance the high temperature strength of ferritic stainless steel by solid solution strengthening and Nb and Ti by precipitation hardening to improve thermal fatigue characteristics. 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.
[0018]
Other components are not particularly defined in the present invention, but are generally impurity elements, and it is preferable to reduce P, O, etc. as much as possible. Usually, P is controlled to 0.04 mass% or less, and O is controlled to 0.02 mass% or less, but P and O may be more strictly regulated to ensure a high level of workability and weldability. .
[0019]
【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.
[0020]
A test piece was cut out from each ferritic stainless steel obtained, and 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, an atmosphere to which the alcohol-based fuel reformer was exposed was assumed, and a 50 volume% H 2 O + 20 volume% CO 2 atmosphere was used. Heating the test piece to 600 ° C. in the atmosphere and holding it for 25 minutes and cooling it to room temperature and holding it for 5 minutes are one cycle, and after repeating heating and cooling 500 times, the weight of the test piece is measured. It was measured.
[0022]
The measurement result was compared with the weight before the test, and the steam oxidation resistance was evaluated by setting the weight change to 2.0 mg / cm 2 or less as ◯, and the weight change exceeding 2.0 mg / cm 2 as x. It can be said that the more the oxide is not produced, the stronger the environmental barrier function of the oxide film, and the better the steam oxidation resistance. Moreover, the surface of the test piece after heating / cooling was observed to investigate the occurrence of red scale.
In the thermal fatigue test, the test piece was repeatedly heated and cooled in a temperature range of 200 to 900 ° C. while applying a strain corresponding to free thermal expansion (constraint rate 100%). 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 1000 cycles or more is heated and cooled for less than 1000 cycles. The thermal fatigue property was evaluated by setting the test piece that reached the number of repetitions of damage as x.
[0023]
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 alcohol-based fuel reformer materials. The required characteristics were fully satisfied. Further, no red scale was observed on the surface of the test piece.
On the other hand, the ferritic stainless steels of steel type Nos. 6 and 7 have a large weight change after being kept at high temperature, and steam oxidation and red scale generation have progressed. Steam oxidation and red scale generation are caused by the lack of Si and Al contents, so that the Cr-based oxide of the oxide film is unstable, and H 2 O, CO 2 , SO 2, etc. that permeate the oxide film during holding at a high temperature. This is presumably due to the attack of the base steel. Furthermore, thermal fatigue characteristics were insufficient.
[0024]
【The invention's effect】
As described above, the ferritic stainless steel of the present invention has an oxide film formed on the surface by stabilizing the Cr-based oxide by adding Si and Al, and the oxide film can be exposed to a high temperature atmosphere for a long time. Since it exhibits a high environmental barrier function without deterioration, steam oxidation and red scale generation can be suppressed. Si addition is also effective in improving the high temperature strength of the steel material, and hence the thermal fatigue characteristics. Therefore, it is used as a material suitable for an alcohol-based fuel reformer that is exposed to a high-temperature atmosphere containing a large amount of H 2 O, CO 2 , and CO and that is repeatedly heated and cooled over a wide temperature range from high temperature to normal temperature.
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| WO2015064739A1 (en) | 2013-11-01 | 2015-05-07 | 新日鐵住金ステンレス株式会社 | Ferritic stainless steel for use in fuel reformer and method of manufacturing ferritic stainless steel |
| US10544490B2 (en) | 2014-07-29 | 2020-01-28 | Nippon Steel & Sumikin Stainless Steel Corporation | Ferritic stainless steel for fuel cell and method for producing the same |
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Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2015064739A1 (en) | 2013-11-01 | 2015-05-07 | 新日鐵住金ステンレス株式会社 | Ferritic stainless steel for use in fuel reformer and method of manufacturing ferritic stainless steel |
| EP3064606A1 (en) * | 2013-11-01 | 2016-09-07 | Nippon Steel & Sumikin Stainless Steel Corporation | Ferritic stainless steel for use in fuel reformer and method of manufacturing ferritic stainless steel |
| EP3064606A4 (en) * | 2013-11-01 | 2017-03-29 | Nippon Steel & Sumikin Stainless Steel Corporation | Ferritic stainless steel for use in fuel reformer and method of manufacturing ferritic stainless steel |
| US10544490B2 (en) | 2014-07-29 | 2020-01-28 | Nippon Steel & Sumikin Stainless Steel Corporation | Ferritic stainless steel for fuel cell and method for producing the same |
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