JP2004002960A

JP2004002960A - Austenitic stainless steel for separator of fuel cell, and manufacturing method therefor

Info

Publication number: JP2004002960A
Application number: JP2002354689A
Authority: JP
Inventors: Kazu Shiroyama; 白山　和; Takeshi Utsunomiya; 宇都宮　武志; Sadayuki Nakamura; 中村　定幸; Akira Hironaka; 弘中　明; Naoto Hiramatsu; 平松　直人; Wakahiro Harada; 原田　和加大; Akihiro Nonomura; 野々村　明廣; Toshiro Nagoshi; 名越　敏郎
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 2002-03-13
Filing date: 2002-12-06
Publication date: 2004-01-08

Abstract

<P>PROBLEM TO BE SOLVED: To provide an austenitic stainless steel which keeps low surface contact resistance and superior corrosion resistance for a long period of time, and is suitable for a separator of a fuel cell such as a solid polymer type. <P>SOLUTION: This austenitic stainless steel used for a substrate of the separator of the fuel cell has a composition comprising 16.0-40.0% Cr, 5.0-26.0% Ni, 0.2-6.0% Cu, and the balance substantially Fe, one or more elements of 0.01-0.5% N, 0.2-6.0% Mo, 0.01-1.0% Co, 0.05-1.0% Nb, 0.05-1.0% Ti, 0.01-3.0% Al, 0.01-1.0% V, 0.001-0.5% rare earth metal, and 0.001-1.0% B, as needed. The austenitic stainless steel has a Cu rich phase precipitated and dispersed in an amount of 0.2 vol.% or more, and Cu concentrated in the surface layer so as to make Cu/(Si+Mn) to be 0.5 or more, to improve the electrical conductivity of the stainless steel sheet of the substrate. The manufacturing method comprises bright annealing the steel sheet in an atmosphere at a dew point of -30°C or lower or pickling it in a mixed acid after annealing it in the air, to precipitate and disperse the Cu rich phase or concentrate Cu in the surface layer. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【産業上の利用分野】
本発明は、固体高分子型等の燃料電池に適し、表面接触抵抗が低く耐食性に優れた燃料電池セパレータ用オーステナイト系ステンレス鋼及びその製造方法に関する。
【０００２】
【従来の技術】
燃料電池には、リン酸型燃料電池，溶融炭酸塩型燃料電池，固体電解質型燃料電池，固体高分子型燃料電池等がある。なかでも、固体高分子型燃料電池は、１００℃以下の温度で動作可能であり、短時間で起動する長所を備えている。また、各部材が固体であるため、構造が簡単でメンテナンスが容易であり、振動や衝撃に曝される用途にも適用できる。更に、出力密度が高いため小型化に適し、燃料効率が高く、騒音が小さい等の長所を備えている。これらの長所から、車両搭載用動力源としての用途が検討されている。ガソリン自動車と同等の走行距離を出せる燃料電池を自動車に搭載できると、ＮＯ_ｘ，ＳＯ_ｘの発生がほとんどなく、ＣＯ_２の発生が半減する等、環境に対して非常にクリーンな動力源になる。
【０００３】
固体高分子型燃料電池は、分子中にプロトン交換基をもつ固体高分子樹脂膜がプロトン導電性電解質として機能することを利用しており、他の形式の燃料電池と同様に固体高分子膜の一側に水素等の燃料ガスを流し、他側に空気等の酸化性ガスを流す構造になっている。
具体的には、固体高分子膜１は、両側に酸化極２及び燃料極３が接合され、それぞれガスケット４を介してセパレータ５を対向させている（図１ａ）。酸化極２側のセパレータ５には空気供給口６，空気排出口７が形成され、燃料極３側のセパレータ５には水素供給口８，水素排出口９が形成されている。
【０００４】
水素ｇ及び酸素又は空気ｏの導通，均一分配のため、水素ｇ及び酸素又は空気ｏの流動方向に延びる複数の溝１０がセパレータ５に形成されている。また、発電時に発熱があるため、給水口１１から送り込んだ冷却水ｗをセパレータ５の内部に循環させた後、排水口１２から排出させる水冷機構をセパレータ５に内蔵させている。水素供給口８から燃料極３とセパレータ５との間隙に送り込まれた水素ｇは、電子を放出したプロトンとなって固体高分子膜１を透過し、酸化極２側で電子を受け、酸化極２とセパレータ５との間隙を通過する酸素又は空気ｏによって燃焼する。そこで、酸化極２と燃料極３との間に負荷をかけるとき、電力を取り出すことができる。
【０００５】
燃料電池は、１セル当りの発電量が極く僅かである。そこで、セパレータ５，５で挟まれた固体高分子膜を１単位とし、複数のセルをスタックすることによって取出し可能な電力量を大きくしている（図１ｂ）。多数のセルをスタックした構造では、セパレータ５の抵抗が発電効率に大きな影響を及ぼす。発電効率を向上させるためには、導電性が良好で接触抵抗の低いセパレータが要求され、リン酸塩型燃料電池と同様に黒鉛質のセパレータが使用されている。黒鉛質のセパレータは、黒鉛ブロックを所定形状に切り出し、切削加工によって各種の孔や溝を形成している。そのため、材料費や加工費が高く、全体として燃料電池の価格を高騰させると共に、生産性を低下させる原因になっている。しかも、材質的に脆い黒鉛でできたセパレータでは、振動や衝撃が加えられると破損するおそれが大きい。そこで、プレス加工，パンチング加工等で作製した金属板製セパレータが期待されている（特開平２０００−２３９８０６号公報，特開平２０００−２６５２４８号公報等参照）。
【０００６】
【発明が解決しようとする課題】
酸素又は空気ｏが通過する酸化極２側は、酸性度がｐＨ２〜３の酸性雰囲気にある。このような強酸性雰囲気に耐え、しかもセパレータに要求される特性を満足する金属材料は、これまでのところ実用化されていない。たとえば、強酸に耐える金属材料としてステンレス鋼等の耐酸性材料が考えられる。ステンレス鋼は表面の不動態皮膜によって優れた耐食性を呈するが、不動態皮膜はＣｒを主体としＦｅ，Ｓｉ，Ｍｎを含む比電気抵抗の高い複合皮膜であるため、電気伝導性に劣っている。比電気抵抗、換言すれば接触抵抗が高くなると、接触部分で多量のジュール熱が発生して大きな熱損失となり、燃料電池の発電効率を低下させる。他の金属板でも、接触抵抗を高くする酸化膜が表面に存在するものがほとんどである。
【０００７】
表面に酸化皮膜や不動態皮膜を形成しない金属材料としてはＡｕが知られている。Ａｕは酸性雰囲気にも耐えうるが、非常に高価な材料であるため燃料電池のセパレータ材としては実用的でない。Ｐｔは酸化皮膜や不動態皮膜が形成されにくい金属材料であり、酸性雰囲気にも耐えうるが、Ａｕと同様に非常に高価な材料であるため燃料電池のセパレータ材としては実用的でない。汎用のステンレス鋼に導電性塗料を塗布したものも用いられているが、コスト高になること，塗料の劣化による接触抵抗の増加等が懸念される。
以上のことから、燃料電池のセパレータに対してはステンレス鋼板を無垢で適用することが好ましい。
【０００８】
【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、比較的多量のＣｕを含むオーステナイト系ステンレス鋼板を基材に用い、基材表面の不動態皮膜又は表層を改質することにより、低い表面接触抵抗及び優れた耐食性を長期にわたって維持でき、固体高分子型等の燃料電池に好適なステンレス鋼製燃料電池セパレータを提供することを目的とする。
【０００９】
本発明は、Ｃｒ：１６．０〜４０．０質量％，Ｎｉ：５．０〜２６．０質量％，Ｃｕ：０．２〜６．０質量％，残部が実質的にＦｅの組成をもつオーステナイト系ステンレス鋼板を燃料電池セパレータの基材に使用している。基材・ステンレス鋼板は、Ｎ：０．０１〜０．５質量％を含むことが好ましく、更にＭｏ：０．２〜６．０質量％，Ｃｏ：０．０１〜１．０質量％，Ｎｂ：０．０５〜１．０質量％，Ｔｉ：０．０５〜１．０質量％，Ａｌ：０．０１〜３．０質量％，Ｖ：０．０１〜１．０質量％，希土類金属（ＲＥＭ）：０．００１〜０．５質量％，Ｂ：０．００１〜１．０質量の１種又は２種以上を含むことができる。
【００１０】
基材・ステンレス鋼板の電気伝導性はＣｕリッチ相の分散析出や表層へのＣｕ濃化によって改善され、接触抵抗が低減する。燃料電池用セパレータとして有効な低接触抵抗は、マトリックスにＣｕリッチ相が０．２体積％以上の割合で分散析出した金属組織や原子比Ｃｕ／（Ｓｉ＋Ｍｎ）≧０．５にＣｕ濃化した表層で達成される。Ｃｕリッチ相の分散析出及び表層へのＣｕ濃化を併用すると、接触抵抗が一層低下する。
【００１１】
Ｃｕリッチ相は、最終焼鈍までの鋼板製造過程でたとえば８００℃前後の温度で１時間以上加熱する時効処理によってマトリックスに分散析出する。時効処理条件に応じてＣｕリッチ相の析出量を０．２体積％以上に調整できる。表層にＣｕを濃化させる手段には、最終焼鈍として露点−３０℃以下の雰囲気で光輝焼鈍する方法，大気焼鈍後にフッ酸−硝酸又は硫酸−硝酸の混酸を用いて酸洗仕上げする方法等が採用される。
【００１２】
【作用】
本発明者等は、燃料電池用セパレータの要求特性を満足させるため、オーステナイト系ステンレス鋼の材質，表面状態について種々予備実験した。その結果、０．２質量％以上のＣｕを含むステンレス鋼板を基材に使用し、Ｃｕを主体とする第２相（以後、Ｃｕリッチ相と称する）を０．２体積％以上の割合でマトリックスに分散析出させるとき、表面接触抵抗が低下することを見出した。
Ｃｕリッチ相が分散析出しているオーステナイト系ステンレス鋼板でも他のステンレス鋼板と同様に不動態皮膜が鋼板表面に形成されるが、Ｃｕリッチ相が表面に露出した部分では、その上下にＣｒ，Ｓｉ，Ｍｎ等を含む不動態皮膜が形成されないため、表面接触抵抗が著しく低下したものと推定される。
【００１３】
オーステナイト系ステンレス鋼板の表面にＣｕリッチ相が露出していない場合でも、最表層又は不動態皮膜中のＣｕ濃度がＳｉ，Ｍｎ濃度に対して高いと、同様に表面接触抵抗が低くなる。比電気抵抗が低いＣｕの酸化物を多く含む不動態皮膜は、比電気抵抗の高いＭｎやＳｉの酸化物を多く含む不動態皮膜に比較して低い表面接触抵抗を示し、後述の実施例からも明らかなように原子比Ｃｕ／（Ｓｉ＋Ｍｎ）≧０．５で燃料電池セパレータに要求される低表面接触抵抗を満足する。
【００１４】
最終焼鈍で表層にＣｕを濃化させる場合、露点−３０℃以下の焼鈍雰囲気における光輝焼鈍が有効である。比電気抵抗の高いＭｎ等の酸化物は、焼鈍雰囲気の露点が高くなると増量し、露点が−３０℃を超える焼鈍雰囲気ではＳｉ，Ｍｎ等の酸化進行に応じて母材内部から表層へのＳｉ，Ｍｎ等の拡散が促進され、Ｃｕ濃度が低い最表層又は不動態皮膜が形成される。これに対し、露点−３０℃以下の焼鈍雰囲気では、Ｍｎ等の金属酸化物の増量を抑えられ、結果として金属ＣｕやＣｕの酸化物が最表層又は不動態皮膜に濃化される。
【００１５】
光輝焼鈍に替え大気焼鈍後の酸洗仕上げによってもＣｕリッチ相の分散析出又は表層へのＣｕ濃化が図られる。ステンレス鋼板を大気焼鈍するとＣｒ，Ｆｅ，Ｍｎ，Ｓｉ，Ｃｕ等の酸化物を含むスケールが鋼板表面に形成されるが、スケールは酸洗によって除去され、その後に不動態皮膜が生成する。このとき、焼鈍後のステンレス鋼板を電解酸洗すると、スケール剥離後の鋼板表面に存在するＣｕ又はＣｕリッチ相が電解反応で母材よりも優先的に溶出する。そのため、電解酸洗後の鋼板表面には、Ｃｕ濃度の低い不動態皮膜が形成される。これに対し、フッ酸−硝酸，硫酸−硝酸等の混酸を用いた酸洗では、Ｃｕ又はＣｕリッチ相の優先的な溶出がなく、酸洗後に生成する不動態皮膜にＣｕ濃度の低下がない。
【００１６】
Ｃｕリッチ相の分散析出，表層へのＣｕ濃化により基材・ステンレス鋼板の表面接触抵抗を低下させる条件下で、燃料電池セパレータの腐食環境に耐え得る耐食性を有するステンレス鋼の成分を検討した。
燃料電池用セパレータは、ｐＨ２〜３の酸性雰囲気に曝され、起動時に約７０℃に昇温することもある。しかも、セパレータ間に電位差がかかるので、酸性液に対する自然浸漬時の耐食性に加えて電位が貴化した場合の酸性高電位環境における耐食性も要求される。
【００１７】
自然電位環境及び酸性高電位環境における耐食性は、１６．０〜４０．０質量％に範囲にＣｒ含有量を調整することにより改善され、更に０．０１〜０．５質量％のＮ添加で向上する。自然電位環境下で耐酸性の向上に必須の合金成分であるＣｒ添加にＮ添加を併用すると、不動態皮膜表面でＮＨ_４ ^＋が生成されて電解液の腐食環境が緩和されるために耐食性が向上すると推察される。Ｎは、接触抵抗に悪影響を及ぼすことなく、Ｃｒが全面溶解を生じるほど貴な電位の過不動態域でも有効な耐食性向上作用を呈する。
【００１８】
次いで、本発明で使用するオーステナイト系ステンレス鋼の成分，含有量等を説明する。
Ｃｒ：１６．０〜４０．０質量％
一般にステンレス鋼板の耐食性を得るために必要な合金成分であるが、燃料電池セル内の環境を考慮した含有量に設定される。具体的には、酸性，７０℃の雰囲気における電気化学特性から適正Ｃｒ含有量を検討した。自然電位の酸性雰囲気における不動態化限界電流密度からするとＣｒ濃度が高いほど電流値が低く耐食性に有利である。しかし、高電位の酸性雰囲気では、過不動態域に曝されＣｒ濃度が高いほどＣｒの溶解電流が流れやすくなる。したがって、Ｃｒ濃度が高いほど、セパレータに電位差がかかった場合の耐食性に不利となる。そこで、Ｃｒ含有量を１６．０〜４０．０質量％の範囲に設定した。１６．０質量％未満のＣｒ含有量では十分な耐酸性が得られず、４０．０質量％を超える過剰量のＣｒが含まれると高電位がかかった酸性雰囲気における耐食性が低下する。Ｃｒの過剰添加は、鋼材を硬質化しセパレータへの加工性を低下させる原因ともなる。
【００１９】
Ｎｉ：５．０〜２６．０質量％
オーステナイト相の形成及び酸性雰囲気における耐食性の確保のため、５．０質量％以上のＮｉが必要である。しかし、Ｎｉの過剰添加は溶接性や加工性に悪影響を及ぼし、コスト的にも不利となるので、Ｎｉ含有量の上限を２６．０質量％に設定した。Ｎｉ添加によってオーステナイト組織とするとき、酸性雰囲気における耐食性の向上に有効なＮの固溶も促進され、セパレータ形状への成形加工も容易になる。
【００２０】
Ｎ：０．０１〜０．５質量％
必要に応じて添加される成分であり、セパレータが曝される酸性雰囲気における耐食性の向上に顕著な作用を呈する。酸性，７０℃の燃料電池セル内の環境における電気化学特性から適正Ｎ含有量を０．０１〜０．５質量％の範囲に設定した。自然電位及び高電位何れの酸性雰囲気においてもＮの添加により腐食電流値が低くなり、０．０１質量％以上のＮ添加でセパレータの耐食性が向上する。しかし、０．５質量％を超える過剰量のＮを添加すると、窒化物の生成が助長され、結果としてＮ増量に見合った耐食性の向上が図られない。Ｎの高濃度添加は、冶金学的にも困難を伴う制御を必要とし、窒素ガス，アンモニアガス等を用いてＮを溶鋼に吸収させる製造プロセスを経るため製造コスト上昇の原因となる。
【００２１】
Ｃｕ：０．２〜６．０質量％
表面接触抵抗の低下に有効な表層へのＣｕ濃化やＣｕリッチ相の分散析出を促進させるため、比較的多量のＣｕを添加する。Ｃｕ含有量は、表層へのＣｕリッチ相又はＣｕリッチ相の分散析出によりセパレータに要求される表面接触抵抗を付与する上から、０．２〜６．０質量％の範囲に設定される。Ｃｕ添加による表面接触抵抗の低減は、０．２質量％以上でみられる。しかし、過剰量のＣｕを添加すると熱間加工性や製造性に悪影響が現れるので、Ｃｕ含有量の上限を６．０質量％に規制した。
【００２２】
本発明で使用するオーステナイト系ステンレス鋼は、Ｃｒ，Ｎｉ，Ｎ，Ｃｕを含むが、燃料電池用セパレータの要求特性である耐食性，加工性等を更に高めるため、Ｍｏ，Ｃｏ，Ｎｂ，Ｔｉ，Ａｌ，Ｖ，希土類金属（ＲＥＭ），Ｂ等の合金成分を含有させることもできる。Ｍｏ，Ｃｏ，Ｎｂ，Ｔｉ，Ａｌ，Ｖ，ＲＥＭ，Ｂ等は、酸性環境における耐食性の更なる向上に有効である。ＲＥＭは、熱間加工性に有害なＳを固定する作用も呈し、ステンレス鋼板の熱間加工性を向上させる。
Ｍｏ，Ｃｏ，Ｎｂ，Ｔｉ，Ａｌ，Ｖ，ＲＥＭ，Ｂ等を添加する場合、耐食性の向上効果及び加工性，製造性に及ぼす影響を勘案し、それぞれＭｏ：０．２〜６．０質量％，Ｃｏ：０．０１〜１．０質量％，Ｎｂ：０．０５〜１．０質量％，Ｔｉ：０．０５〜１．０質量％，Ａｌ：０．０１〜３．０質量％，Ｖ：０．０１〜１．０質量％，ＲＥＭ：０．００１〜０．５質量％，Ｂ：０．００１〜１．０質量の範囲にすることが好ましい。
【００２３】
以上の合金成分の他にＣ，Ｓｉ，Ｍｎ等も含まれるが、燃料電池用セパレータの用途では、Ｃ，Ｓｉ，Ｍｎ等の含有量を低減し、加工性，製造性にとって不利な硬質化を抑制することが好ましい。表面接触抵抗を低減する上でも、Ｃ，Ｓｉ，Ｍｎ等の含有量を低く抑えることが有効である。
たとえば、鋼材を硬質化させるＣを過剰に含ませるとプレス加工が困難になるので、Ｃ含有量を好ましくは０．０８質量％以下に規制する。Ｓｉは、鋼材の硬質化及び表面接触抵抗の増加に作用するので、好ましくは１．００質量％以下の含有量に規制する。Ｍｎは、耐食性の低下，接触抵抗の増加に作用するので、好ましくは２．００質量％以下の含有量に規制する。
【００２４】
Ｃｕリッチ相：０．２体積％以上
Ｃｕリッチ相を均一微細に析出させ、表面に露出させた部分では、その上下にＣｒ，Ｓｉ，Ｍｎ等を含む不動態皮膜が形成されず、表面接触抵抗が低下する。Ｃｕリッチ相の分散析出と表面接触抵抗の低下との関係を詳細に調査・検討した結果、Ｃｕリッチ相：０．２体積％以上で燃料電池用セパレータに要求される低表面接触抵抗を示す。
Ｃｕリッチ相を分散析出させる熱処理は、ステンレス鋼のＣｕ含有量によっても異なるが、一般的には８００℃前後で１〜２４時間の時効処理を施す条件が採用される。時効処理により、微細なＣｕリッチ相が分散析出する。
【００２５】
表層へのＣｕ濃化：Ｃｕ／（Ｓｉ＋Ｍｎ）≧０．５（原子比）
不動態皮膜又は最表層のＳｉ，Ｍｎに対するＣｕの濃度比を高くすると、表面接触電気抵抗が低下する。表層へのＣｕ濃化は、不動態皮膜又は最表層における原子比Ｃｕ／（Ｓｉ＋Ｍｎ）で定量化でき、原子比Ｃｕ／（Ｓｉ＋Ｍｎ）が０．５以上になると、燃料電池用セパレータとして満足できる程度に表面接触抵抗が低下する。Ｃｕ／（Ｓｉ＋Ｍｎ）≧０．５は、乾燥雰囲気下での光輝焼鈍や大気焼鈍後の混酸酸洗で達成される。
【００２６】
光輝焼鈍：露点−３０℃以下の雰囲気で加熱
焼鈍雰囲気の露点が高くなると、比抵抗の高いＭｎの酸化物が多くなり不動態皮膜の表面接触抵抗が増加する。Ｃｕ／（Ｓｉ＋Ｍｎ）≧０．５で表層にＣｕを濃化させるため上では、後述の実施例からも明らかなように露点−３０℃以下の焼鈍雰囲気で、好ましくは９５０〜１２００℃にステンレス鋼板を焼鈍する。光輝焼鈍により、マトリックスにＣｕリッチ相が均一に分散析出し、或いは表層にＣｕが濃化して表面接触抵抗の低い表層に改質される。
【００２７】
大気焼鈍後の混酸酸洗：
光輝焼鈍に代えて大気焼鈍後の混酸酸洗によっても、不動態皮膜又は最表層にＣｕを濃化できる。大気焼鈍では、Ｃｕリッチ相の分散析出又は表層へのＣｕ濃化のため、７５０〜１２００℃の温度域にステンレス鋼板を加熱する。大気焼鈍されたステンレス鋼板の酸洗には、酸の種類や濃度に特段の制約が加わるものではないが、一般的に濃度１０体積％程度のフッ酸−硝酸，硫酸−硝酸等が使用される。混酸酸洗では、電解酸洗で生じるＣｕ又はＣｕリッチ相の優先溶解がなく、Ｃｕ濃化の低下がない不動態皮膜が酸洗後に生成する。
【００２８】
【実施例１】
表１に示した組成をもつ各種オーステナイト系ステンレス鋼を真空溶解炉で溶製し、鋳造，熱間鍛造，熱間圧延，焼鈍・酸洗，冷間圧延を経て板厚１．０ｍｍの冷延鋼帯を製造した。比較のため、同じ条件下でＳＵＳ３０４，ＳＵＳ３０４Ｎ１の冷延鋼帯を製造した。一部の冷延鋼帯については、最終焼鈍前までの工程で８００℃×２４時間のＣｕリッチ相析出処理を施した。
【００２９】

【００３０】
製造された冷延鋼帯を光輝焼鈍仕上げし、或いは大気焼鈍後に酸洗仕上げした。光輝焼鈍では、露点が異なる焼鈍雰囲気で１１００℃に１０秒加熱し、雰囲気の露点が焼鈍材の特性に及ぼす影響を調査した。酸洗仕上げでは、５％硝酸を用いた電解酸洗，６％硝酸＋２％フッ酸の混酸酸洗を採用し、酸洗形態の相違が焼鈍材の特性に及ぼす影響を調査した。
光輝焼鈍又は酸洗仕上げされた冷延鋼帯から切り出された試験片を透過型電子顕微鏡で観察し、マトリックスに分散析出しているＣｕリッチ相の析出量（体積％）を算出した。また、グロー発光分析で分析全元素量に対するＣｕ，Ｓｉ，Ｍｎの濃度（原子％）を求め、原子比Ｃｕ／（Ｓｉ＋Ｍｎ）に従って表層のＣｕ濃度を算出した。更に、純金製の対極及び測定端子を試験片の表面に接触させ、測定端子に１００ｇの荷重を付加した条件下で表面接触抵抗を測定した。
【００３１】
表２の調査結果にみられるように、Ｃｕが０．２質量％未満のＳＵＳ３０４，ＳＵＳ３０４Ｎ１では、Ｃｕリッチ相の析出量が０．２体積％に達せず、或いは表層のＣｕ濃化がＣｕ／（Ｓｉ＋Ｍｎ）≧０．５に至らず、高い表面接触抵抗が示された。０．２質量％以上のＣｕを添加したＡ１鋼でも、露点が−３０℃より高い雰囲気で光輝焼鈍すると、表層のＣｕ濃化がＣｕ／（Ｓｉ＋Ｍｎ）≧０．５に至らず、表面接触抵抗が依然として高い値であった。
これに対し、Ｃｕを０．２質量％以上含有するＡ１，Ａ２鋼を用い、Ｃｕリッチ相を０．２体積％以上析出させ（試験番号５，９）、或いはＣｕ／（Ｓｉ＋Ｍｎ）≧０．５で表層をＣｕ濃化すると（試験番号６，８）、表面接触抵抗が大幅に低下していた。Ｃｕリッチ相：０．２体積％以上，Ｃｕ／（Ｓｉ＋Ｍｎ）≧０．５の双方を満足する試験番号７，１０は、表面接触抵抗が一層低下していた。
【００３２】

【００３３】
【実施例２】
表１の鋼種Ａ１，Ａ２〜Ａ４，Ａ７に実施例１と同じＣｕリッチ相析出処理を施し、電気化学試験で燃料電池セパレータ環境下での耐食性を調査した。
電気化学試験では、供試材を樹脂に埋め込んで被測定面１０ｍｍ×１０ｍｍの試験片を用意した。ｐＨ２に調整した硫酸水溶液（試験液）を７０℃に保持し、走査速度２０ｍＶ／分で自然電位から１３００ｍＶ，ＳＣＥまでのアノード分極を測定した。燃料電池セパレータが曝される雰囲気には、電位が付加されない自然電位環境，電位が付加される高電位環境がある。自然電位環境における耐食性はアノード分極の不動態化電流密度の低さ、高電位環境における耐食性は１０００ｍＶ，ＳＣＥ時の腐食電流の低さから評価できる。
図２に示すアノード分極曲線の測定結果から、Ｃｒ：１５．２質量％の鋼Ａ１は、自然電位側の不動態化限界電流密度が他の供試材よりも高い値を示し、耐食性に不足していることが判る。Ｃｒ：４０．９質量％のＡ７は、不動態化限界電流密度が低いものの、高電位側での電流値が他の供試材に比較して極めて高く、セパレータに電位が印加された状態で耐食性に劣ることが示されている。鋼Ａ２〜Ａ４は、不動態化限界電流密度，高電位側の電流値共に低く、セパレータ環境で十分な耐食性を呈することが理解できる。
【００３４】
【実施例３】
表３に示した組成をもつ各種オーステナイト系ステンレス鋼から実施例１と同様に板厚１．０ｍｍの冷延鋼帯を製造した。一部の冷延鋼帯については、最終焼鈍前までの工程で８００℃×２４時間のＣｕリッチ相析出処理を施した。
【００３５】

【００３６】
製造された冷延鋼帯に実施例１と同じ光輝焼鈍又は大気焼鈍→酸洗を施した後、実施例１と同じ条件下でＣｕリッチ相の析出量（体積％），原子比Ｃｕ／（Ｓｉ＋Ｍｎ），表面接触抵抗を調査した。
表４の調査結果にみられるように、Ｃｕを０．２質量％以上含有するＡ１１，Ａ１２鋼を用い、Ｃｕリッチ相を０．２体積％以上析出させ（試験番号１，５）、或いはＣｕ／（Ｓｉ＋Ｍｎ）≧０．５で表層をＣｕ濃化すると（試験番号２，４）、表面接触抵抗が大幅に低下していた。Ｃｕリッチ相：０．２体積％以上，Ｃｕ／（Ｓｉ＋Ｍｎ）≧０．５の双方を満足する試験番号３，６は、表面接触抵抗が一層低下していた。この結果から、耐食性向上のためにＮｉを添加した場合でも、Ｃｕリッチ相の析出又は不動態皮膜へのＣｕ濃化を図ることにより、表面接触抵抗を低下できることが確認された。
【００３７】

【００３８】
【実施例４】
表３の鋼種Ａ１１〜Ａ２６，比較鋼に実施例１と同じＣｕリッチ相析出処理を施したものを供試材とし、実施例２と同じ電気化学試験で燃料電池セパレータ環境における耐食性を調査した。
硫酸液中でのアノード分極曲線から求めた不動態電極密度及び１０００ｍＶ，ＳＣＥ時の腐食電流の測定結果を表５に示す。
【００３９】
本発明に従った試験Ｎｏ．１〜１６は、不動態化電流密度が１０μＡ／ｃｍ^２以下であることから、自然電位環境で耐食性に優れていることが判る。高電位環境下の腐食電流密度も４００μＡ／ｃｍ^２前後であり、Ｃｒ含有量が４０．０質量％を超えるＡ２８鋼と比較すると小さかった。高電位環境下でＡ２８鋼が高い腐食電流密度を示すことは、Ｃｒ含有量が高くＣｒの過不動態溶解に起因しているものと推察される。
【００４０】
更に、Ｎ含有量が高く、Ｍｏ，Ｃｏ，Ｎｂ，Ｔｉ，Ａｌ，Ｖ，ＲＥＭ，Ｂの少なくとも１種を添加したＡ１４〜Ａ２１，Ａ２３，Ａ２６鋼は、自然電位環境，高電位環境の何れにおいても不動態電流密度が小さく、一層優れた耐食性を呈した。
Ｃｒが不足するＡ２７鋼は、高電位環境下の腐食電流密度は本発明鋼なみであるものの不動態化電流密度が高く、セパレータ環境では十分な耐食性を有しているとはいえない。
【００４１】

【００４２】
【発明の効果】
以上に説明したように、Ｃｒ，Ｎｉ，Ｃｕの添加量を規制し、場合によってはＮを添加し、Ｃｕリッチ相をマトリックスに分散析出させ、或いは表層にＣｕを濃化させると、オーステナイト系ステンレス鋼本来の優れた耐食性を維持しながら、長期にわたって表面接触抵抗が低位に安定維持された表面状態に改質される。耐食性は、Ｍｏ，Ｃｏ，Ｎｂ，Ｔｉ，Ａｌ，Ｖ，ＲＥＭ，Ｂ等の添加によって一層向上する。このようにして低表面接触抵抗と耐食性を両立させたオーステナイト系ステンレス鋼板は、過酷な腐食環境に曝される燃料電池用セパレータの基材として好適な材料であり、多数の燃料電池モジュールをスタックした状態でもジュール発熱が少ないため発電効率のよい燃料電池が構築される。
【図面の簡単な説明】
【図１】固体高分子膜を電解質として使用した燃料電池の内部構造を説明する断面図（ａ）及び分解斜視図（ｂ）
【図２】実施例２で用いたステンレス鋼板のアノード分極極線を示すグラフ[0001]
[Industrial applications]
The present invention relates to an austenitic stainless steel for a fuel cell separator which is suitable for a polymer electrolyte fuel cell or the like and has low surface contact resistance and excellent corrosion resistance, and a method for producing the same.
[0002]
[Prior art]
The fuel cell includes a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid electrolyte fuel cell, a polymer electrolyte fuel cell, and the like. In particular, the polymer electrolyte fuel cell can operate at a temperature of 100 ° C. or less and has an advantage of being started in a short time. In addition, since each member is solid, the structure is simple, maintenance is easy, and it can be applied to applications exposed to vibration and impact. Furthermore, it has advantages such as high output density, suitable for miniaturization, high fuel efficiency, and low noise. From these advantages, use as a vehicle-mounted power source is being studied. When it equipped with a fuel cell put out a travel distance equivalent to gasoline vehicles in a vehicle, NO _x, almost no generation of SO _x, etc. generation of CO ₂ is reduced by half, becomes very clean power source to the environment .
[0003]
Solid polymer fuel cells utilize the fact that solid polymer resin membranes with proton exchange groups in their molecules function as proton conductive electrolytes. The structure is such that a fuel gas such as hydrogen flows on one side and an oxidizing gas such as air flows on the other side.
Specifically, the solid polymer membrane 1 has an oxidizing electrode 2 and a fuel electrode 3 joined on both sides, and the separator 5 is opposed to each other via a gasket 4 (FIG. 1a). An air supply port 6 and an air discharge port 7 are formed in the separator 5 on the oxidation electrode 2 side, and a hydrogen supply port 8 and a hydrogen discharge port 9 are formed in the separator 5 on the fuel electrode 3 side.
[0004]
For conduction and uniform distribution of hydrogen g and oxygen or air o, a plurality of grooves 10 extending in the flow direction of hydrogen g and oxygen or air o are formed in separator 5. Further, since heat is generated at the time of power generation, the separator 5 has a built-in water cooling mechanism for circulating the cooling water w sent from the water supply port 11 and then discharging the cooling water w from the drain port 12. Hydrogen g sent from the hydrogen supply port 8 into the gap between the fuel electrode 3 and the separator 5 becomes protons that have emitted electrons, passes through the solid polymer membrane 1, receives electrons on the oxidation electrode 2 side, and The fuel is burned by oxygen or air o passing through the gap between the separator 2 and the separator 5. Therefore, when a load is applied between the oxidation electrode 2 and the fuel electrode 3, electric power can be taken out.
[0005]
Fuel cells generate very little power per cell. Therefore, the solid polymer film sandwiched between the

separators

5 and 5 is set as one unit, and the amount of power that can be taken out by stacking a plurality of cells is increased (FIG. 1B). In a structure in which many cells are stacked, the resistance of the separator 5 has a large effect on the power generation efficiency. In order to improve the power generation efficiency, a separator having good conductivity and low contact resistance is required, and a graphite separator is used similarly to the phosphate type fuel cell. The graphite separator cuts a graphite block into a predetermined shape and forms various holes and grooves by cutting. For this reason, material costs and processing costs are high, which increases the price of the fuel cell as a whole and causes a decrease in productivity. In addition, a separator made of graphite, which is brittle in material, is likely to be damaged when subjected to vibration or impact. Therefore, a metal plate separator manufactured by pressing, punching, or the like is expected (see JP-A-2000-239806, JP-A-2000-265248, and the like).
[0006]
[Problems to be solved by the invention]
The oxidation electrode 2 side through which oxygen or air o passes is in an acidic atmosphere having an acidity of pH 2 to 3. A metal material that withstands such a strongly acidic atmosphere and satisfies the characteristics required for the separator has not been put to practical use so far. For example, an acid-resistant material such as stainless steel is considered as a metal material that can withstand a strong acid. Although stainless steel exhibits excellent corrosion resistance due to the passivation film on the surface, the passivation film is a composite film mainly composed of Cr and containing Fe, Si, and Mn and having a high specific electrical resistance, and therefore has poor electrical conductivity. When the specific electrical resistance, in other words, the contact resistance increases, a large amount of Joule heat is generated at the contact portion, resulting in a large heat loss, and lowering the power generation efficiency of the fuel cell. Most other metal plates have an oxide film on the surface to increase the contact resistance.
[0007]
Au is known as a metal material that does not form an oxide film or a passive film on the surface. Although Au can withstand an acidic atmosphere, it is not practical as a fuel cell separator material because it is a very expensive material. Pt is a metal material on which an oxide film or a passivation film is unlikely to be formed, and can withstand an acidic atmosphere. However, Pt is a very expensive material like Au and is not practical as a fuel cell separator material. A general-purpose stainless steel coated with a conductive paint is also used. However, there is a concern about an increase in cost and an increase in contact resistance due to deterioration of the paint.
From the above, it is preferable to use a stainless steel plate as a solid for the separator of the fuel cell.
[0008]
[Means for Solving the Problems]
The present invention has been devised to solve such a problem, and uses an austenitic stainless steel sheet containing a relatively large amount of Cu as a base material and modifies a passive film or a surface layer on the base material surface. Accordingly, it is an object of the present invention to provide a stainless steel fuel cell separator that can maintain low surface contact resistance and excellent corrosion resistance for a long period of time and is suitable for a fuel cell such as a polymer electrolyte fuel cell.
[0009]
The present invention has a composition of Cr: 16.0 to 40.0% by mass, Ni: 5.0 to 26.0% by mass, Cu: 0.2 to 6.0% by mass, and the balance being substantially Fe. An austenitic stainless steel plate is used as the base material of the fuel cell separator. The base material / stainless steel sheet preferably contains N: 0.01 to 0.5% by mass, further Mo: 0.2 to 6.0% by mass, Co: 0.01 to 1.0% by mass, Nb : 0.05 to 1.0% by mass, Ti: 0.05 to 1.0% by mass, Al: 0.01 to 3.0% by mass, V: 0.01 to 1.0% by mass, rare earth metal ( REM): 0.001 to 0.5% by mass, B: 0.001 to 1.0% by mass.
[0010]
The electrical conductivity of the substrate / stainless steel sheet is improved by the dispersion precipitation of the Cu-rich phase and the concentration of Cu on the surface layer, and the contact resistance is reduced. The low contact resistance effective as a fuel cell separator is a metal structure in which a Cu-rich phase is dispersed and precipitated in a matrix at a rate of 0.2% by volume or more, and a Cu-enriched surface layer having an atomic ratio of Cu / (Si + Mn) ≧ 0.5. Is achieved in. When the dispersion precipitation of the Cu-rich phase and the concentration of Cu on the surface layer are used in combination, the contact resistance is further reduced.
[0011]
The Cu-rich phase is dispersed and precipitated in the matrix by an aging treatment of heating at a temperature of, for example, about 800 ° C. for 1 hour or more in a steel sheet manufacturing process until final annealing. The precipitation amount of the Cu-rich phase can be adjusted to 0.2% by volume or more according to the aging treatment conditions. Means for enriching Cu in the surface layer include a method of performing a bright annealing in an atmosphere having a dew point of −30 ° C. or less as a final annealing, and a method of performing a pickling finish using a mixed acid of hydrofluoric acid-nitric acid or sulfuric acid-nitric acid after air annealing. Adopted.
[0012]
[Action]
The present inventors conducted various preliminary experiments on the material and surface condition of austenitic stainless steel in order to satisfy the required characteristics of a fuel cell separator. As a result, a stainless steel sheet containing 0.2% by mass or more of Cu is used as a base material, and a second phase mainly composed of Cu (hereinafter, referred to as a Cu-rich phase) is formed in a matrix of 0.2% by volume or more. It was found that the surface contact resistance was reduced when the particles were dispersed and precipitated.
An austenitic stainless steel sheet in which a Cu-rich phase is dispersed and deposited has a passivation film formed on the steel sheet surface in the same manner as other stainless steel sheets. However, in a portion where the Cu-rich phase is exposed on the surface, Cr, Si It is presumed that the passivation film containing Mn, Mn, etc. was not formed, so that the surface contact resistance was significantly reduced.
[0013]
Even when the Cu-rich phase is not exposed on the surface of the austenitic stainless steel sheet, if the Cu concentration in the outermost layer or the passivation film is higher than the Si and Mn concentrations, the surface contact resistance similarly decreases. The passive film containing a large amount of Cu oxide having a low specific electrical resistance shows a low surface contact resistance as compared with the passive film containing a large amount of Mn or Si oxide having a high specific electric resistance. As is clear, the atomic ratio Cu / (Si + Mn) ≧ 0.5 satisfies the low surface contact resistance required for the fuel cell separator.
[0014]
When Cu is enriched in the surface layer by final annealing, bright annealing in an annealing atmosphere having a dew point of −30 ° C. or less is effective. Oxides such as Mn having a high specific resistance increase as the dew point in the annealing atmosphere increases, and in an annealing atmosphere in which the dew point exceeds −30 ° C., the amount of Si from the inside of the base material to the surface layer depends on the progress of oxidation of Si, Mn, etc. , Mn and the like are promoted, and the outermost layer or the passive film having a low Cu concentration is formed. On the other hand, in an annealing atmosphere having a dew point of −30 ° C. or lower, an increase in the amount of metal oxides such as Mn can be suppressed, and as a result, metal Cu or an oxide of Cu is concentrated in the outermost layer or the passive film.
[0015]
Dispersion and precipitation of a Cu-rich phase or concentration of Cu on the surface layer can be achieved by pickling after air annealing instead of bright annealing. When a stainless steel plate is annealed in the air, a scale containing oxides such as Cr, Fe, Mn, Si, and Cu is formed on the surface of the steel plate, but the scale is removed by pickling, and a passivation film is formed thereafter. At this time, when the anodized stainless steel sheet is electrolytically pickled, Cu or a Cu-rich phase present on the steel sheet surface after the scale exfoliation is preferentially eluted from the base material by the electrolytic reaction. Therefore, a passive film having a low Cu concentration is formed on the surface of the steel sheet after the electrolytic pickling. On the other hand, in pickling using a mixed acid such as hydrofluoric acid-nitric acid or sulfuric acid-nitric acid, there is no preferential elution of Cu or a Cu-rich phase, and there is no decrease in the Cu concentration in the passive film formed after the pickling. .
[0016]
Under the condition that the surface contact resistance of the base material / stainless steel plate is reduced by the dispersion precipitation of the Cu-rich phase and the concentration of Cu on the surface layer, the components of the stainless steel having corrosion resistance that can withstand the corrosive environment of the fuel cell separator were examined.
The fuel cell separator is exposed to an acidic atmosphere having a pH of 2 to 3, and may be heated to about 70 ° C. at startup. In addition, since a potential difference is applied between the separators, corrosion resistance in an acidic high-potential environment when the potential is noble is required in addition to corrosion resistance at the time of spontaneous immersion in an acidic liquid.
[0017]
Corrosion resistance in a self-potential environment and an acidic high-potential environment is improved by adjusting the Cr content to a range of 16.0 to 40.0% by mass, and further improved by adding 0.01 to 0.5% by mass of N. I do. When N is used in combination with Cr, which is an alloy component essential for improving acid resistance in a self-potential environment, NH ₄ ⁺ is generated on the surface of the passive film and the corrosion environment of the electrolytic solution is reduced, so that corrosion resistance is reduced. It is presumed to improve. N exerts an effective corrosion resistance improving effect even in an overpassive region of a noble potential so that Cr is completely dissolved without adversely affecting the contact resistance.
[0018]
Next, the components and contents of the austenitic stainless steel used in the present invention will be described.
Cr: 16.0 to 40.0 mass%
Generally, it is an alloy component necessary for obtaining the corrosion resistance of a stainless steel plate, but is set to a content in consideration of the environment in the fuel cell. Specifically, the appropriate Cr content was examined from the electrochemical characteristics in an acidic, 70 ° C. atmosphere. In view of the passivation limit current density in an acidic atmosphere at a natural potential, the higher the Cr concentration, the lower the current value, which is advantageous for corrosion resistance. However, in a high-potential acidic atmosphere, the higher the concentration of Cr exposed to the overpassive region, the more easily the Cr dissolution current flows. Therefore, the higher the Cr concentration, the more disadvantageous is the corrosion resistance when a potential difference is applied to the separator. Therefore, the Cr content was set in the range of 16.0 to 40.0% by mass. If the Cr content is less than 16.0% by mass, sufficient acid resistance cannot be obtained. If the Cr content exceeds 40.0% by mass, the corrosion resistance in an acidic atmosphere subjected to a high potential decreases. Excessive addition of Cr causes the steel material to harden and causes the workability of the separator to deteriorate.
[0019]
Ni: 5.0 to 26.0 mass%
In order to form an austenite phase and ensure corrosion resistance in an acidic atmosphere, Ni of 5.0 mass% or more is necessary. However, excessive addition of Ni adversely affects weldability and workability and is disadvantageous in terms of cost, so the upper limit of the Ni content was set to 26.0% by mass. When the austenitic structure is formed by adding Ni, solid solution of N effective for improving corrosion resistance in an acidic atmosphere is promoted, and forming into a separator shape is facilitated.
[0020]
N: 0.01 to 0.5% by mass
It is a component that is added as needed, and has a remarkable effect on improving corrosion resistance in an acidic atmosphere to which the separator is exposed. The proper N content was set in the range of 0.01 to 0.5% by mass from the electrochemical characteristics in the environment of the acidic, 70 ° C. fuel cell. The corrosion current value is reduced by the addition of N in the acidic atmosphere at both the natural potential and the high potential, and the corrosion resistance of the separator is improved by the addition of 0.01% by mass or more of N. However, when an excessive amount of N exceeding 0.5% by mass is added, the formation of nitrides is promoted, and as a result, improvement in corrosion resistance corresponding to the increase in N cannot be achieved. The addition of N at a high concentration requires difficult control in metallurgy, and causes a rise in manufacturing cost because of a manufacturing process of absorbing N into molten steel using nitrogen gas, ammonia gas, or the like.
[0021]
Cu: 0.2 to 6.0% by mass
A relatively large amount of Cu is added in order to promote the concentration of Cu on the surface layer effective for lowering the surface contact resistance and the dispersion and precipitation of the Cu-rich phase. The Cu content is set in the range of 0.2 to 6.0% by mass from the viewpoint of imparting the required surface contact resistance to the separator by the Cu-rich phase or the dispersed precipitation of the Cu-rich phase on the surface layer. The reduction in surface contact resistance due to the addition of Cu is seen at 0.2% by mass or more. However, the addition of an excessive amount of Cu adversely affects hot workability and manufacturability, so the upper limit of the Cu content was restricted to 6.0% by mass.
[0022]
The austenitic stainless steel used in the present invention contains Cr, Ni, N, and Cu. However, Mo, Co, Nb, Ti, and Al are used in order to further enhance the corrosion resistance, workability, and the like required characteristics of the fuel cell separator. , V, rare earth metals (REM), B, and other alloy components. Mo, Co, Nb, Ti, Al, V, REM, B, etc. are effective for further improving the corrosion resistance in an acidic environment. REM also has the effect of fixing S harmful to hot workability, and improves the hot workability of a stainless steel plate.
When adding Mo, Co, Nb, Ti, Al, V, REM, B, etc., Mo: 0.2 to 6.0% by mass in consideration of the effect of improving corrosion resistance and the effect on workability and manufacturability. , Co: 0.01 to 1.0% by mass, Nb: 0.05 to 1.0% by mass, Ti: 0.05 to 1.0% by mass, Al: 0.01 to 3.0% by mass, V : 0.01 to 1.0% by mass, REM: 0.001 to 0.5% by mass, B: 0.001 to 1.0% by mass.
[0023]
In addition to the above alloy components, C, Si, Mn, etc. are also contained. However, in the use of a fuel cell separator, the content of C, Si, Mn, etc. is reduced, and hardening which is disadvantageous to workability and manufacturability is performed. Preferably, it is suppressed. In order to reduce the surface contact resistance, it is effective to keep the contents of C, Si, Mn, and the like low.
For example, if excessively containing C for hardening the steel material, it becomes difficult to press work. Therefore, the C content is preferably regulated to 0.08% by mass or less. Since Si acts to harden the steel material and increase the surface contact resistance, the content is preferably limited to 1.00% by mass or less. Since Mn acts on a decrease in corrosion resistance and an increase in contact resistance, the content is preferably restricted to 2.00% by mass or less.
[0024]
Cu-rich phase: 0.2 volume% or more Cu-rich phase is deposited uniformly and finely, and a passivation film containing Cr, Si, Mn, etc. is not formed above and below the portion exposed on the surface, and the surface contact resistance is reduced. Decreases. As a result of detailed investigation and study of the relationship between the dispersed precipitation of the Cu-rich phase and the reduction of the surface contact resistance, a low surface contact resistance required for a fuel cell separator is shown at a Cu-rich phase of 0.2% by volume or more.
The heat treatment for dispersing and precipitating the Cu-rich phase varies depending on the Cu content of the stainless steel, but generally a condition of performing aging treatment at about 800 ° C. for 1 to 24 hours is employed. By the aging treatment, a fine Cu-rich phase is dispersed and precipitated.
[0025]
Cu concentration on surface layer: Cu / (Si + Mn) ≧ 0.5 (atomic ratio)
When the concentration ratio of Cu to Si or Mn in the passivation film or the outermost layer is increased, the surface contact electric resistance decreases. Cu concentration on the surface layer can be quantified by the atomic ratio Cu / (Si + Mn) in the passivation film or the outermost layer, and when the atomic ratio Cu / (Si + Mn) becomes 0.5 or more, a satisfactory degree as a fuel cell separator. The surface contact resistance decreases. Cu / (Si + Mn) ≧ 0.5 is achieved by bright annealing in a dry atmosphere or mixed pickling after air annealing.
[0026]
Bright annealing: When the dew point of the heating annealing atmosphere is increased in an atmosphere having a dew point of −30 ° C. or lower, the oxide of Mn having a high specific resistance increases and the surface contact resistance of the passive film increases. In order to enrich Cu in the surface layer when Cu / (Si + Mn) ≧ 0.5, the stainless steel sheet is preferably heated to 950 to 1200 ° C. in an annealing atmosphere having a dew point of −30 ° C. or less, as is clear from the examples described later. Annealing. By bright annealing, a Cu-rich phase is uniformly dispersed and precipitated in the matrix, or Cu is concentrated in the surface layer to be reformed into a surface layer having low surface contact resistance.
[0027]
Mixed acid pickling after air annealing:
Instead of bright annealing, mixed acid pickling after air annealing can also enrich Cu in the passive film or the outermost layer. In the air annealing, the stainless steel plate is heated to a temperature range of 750 to 1200 ° C. for dispersion precipitation of the Cu-rich phase or Cu concentration on the surface layer. The pickling of the stainless steel sheet annealed in the air is not subject to any particular restrictions on the type and concentration of the acid, but generally, hydrofluoric acid-nitric acid, sulfuric acid-nitric acid or the like having a concentration of about 10% by volume is used. . In the mixed acid pickling, a passive film that does not have a preferential dissolution of Cu or a Cu-rich phase generated by electrolytic pickling and has no decrease in Cu concentration is formed after the pickling.
[0028]
Embodiment 1
Various austenitic stainless steels having the compositions shown in Table 1 were melted in a vacuum melting furnace and subjected to casting, hot forging, hot rolling, annealing / pickling, and cold rolling, and then cold-rolled to a thickness of 1.0 mm. Steel strip was manufactured. For comparison, SUS304 and SUS304N1 cold-rolled steel strips were manufactured under the same conditions. Some of the cold rolled steel strips were subjected to a Cu-rich phase precipitation treatment at 800 ° C. for 24 hours in a process before final annealing.
[0029]

[0030]
The produced cold-rolled steel strip was subjected to bright annealing finish or pickling finish after air annealing. In bright annealing, heating was performed at 1100 ° C. for 10 seconds in annealing atmospheres having different dew points, and the influence of the dew point of the atmosphere on the properties of the annealed material was investigated. In the pickling finishing, electrolytic pickling using 5% nitric acid and mixed pickling of 6% nitric acid + 2% hydrofluoric acid were adopted, and the influence of the difference in pickling form on the properties of the annealed material was investigated.
A specimen cut out from the bright-annealed or pickled-finished cold-rolled steel strip was observed with a transmission electron microscope, and the amount (% by volume) of the Cu-rich phase dispersed and precipitated in the matrix was calculated. Further, the concentration (atomic%) of Cu, Si, and Mn with respect to the total amount of the analyzed elements was determined by glow emission analysis, and the Cu concentration of the surface layer was calculated according to the atomic ratio Cu / (Si + Mn). Further, the counter electrode and the measuring terminal made of pure gold were brought into contact with the surface of the test piece, and the surface contact resistance was measured under the condition that a load of 100 g was applied to the measuring terminal.
[0031]
As can be seen from the survey results in Table 2, in SUS304 and SUS304N1 in which Cu is less than 0.2% by mass, the precipitation amount of the Cu-rich phase does not reach 0.2% by volume, or the Cu concentration in the surface layer is Cu / (Si + Mn) ≧ 0.5 was not achieved, indicating a high surface contact resistance. Even in A1 steel to which 0.2% by mass or more of Cu is added, when bright annealing is performed in an atmosphere having a dew point higher than −30 ° C., the concentration of Cu in the surface layer does not reach Cu / (Si + Mn) ≧ 0.5, and the surface contact resistance increases. Was still high.
On the other hand, using A1, A2 steel containing 0.2% by mass or more of Cu, a Cu-rich phase is precipitated by 0.2% by volume or more (test numbers 5 and 9), or Cu / (Si + Mn) ≧ 0. When the surface layer was concentrated with Cu in Test No. 5 (Test Nos. 6 and 8), the surface contact resistance was significantly reduced. Test Nos. 7 and 10 satisfying both Cu-rich phase: 0.2% by volume or more and Cu / (Si + Mn) ≧ 0.5, the surface contact resistance was further reduced.
[0032]

[0033]
Embodiment 2
The steel types A1, A2 to A4, and A7 shown in Table 1 were subjected to the same Cu-rich phase precipitation treatment as in Example 1, and corrosion resistance in a fuel cell separator environment was examined by an electrochemical test.
In the electrochemical test, a test piece having a surface to be measured having a size of 10 mm × 10 mm was prepared by embedding a test material in a resin. The sulfuric acid aqueous solution (test solution) adjusted to pH 2 was maintained at 70 ° C., and the anodic polarization from the natural potential to 1,300 mV, SCE was measured at a scanning speed of 20 mV / min. The atmosphere to which the fuel cell separator is exposed includes a natural potential environment where no potential is applied and a high potential environment where potential is applied. The corrosion resistance in the self-potential environment can be evaluated from the low passivation current density of anodic polarization, and the corrosion resistance in the high-potential environment can be evaluated from 1000 mV and the low corrosion current at SCE.
From the measurement results of the anodic polarization curve shown in FIG. 2, the steel A1 with Cr: 15.2% by mass has a higher passivation limit current density on the spontaneous potential side than the other test materials, and lacks corrosion resistance. You can see that it is. Cr: 40.9% by mass of A7, although having a low passivation limit current density, has a very high current value on the high potential side as compared with the other test materials, and has a potential applied to the separator. It is shown that the corrosion resistance is poor. It can be understood that the steels A2 to A4 have low passivation limit current density and low current value on the high potential side, and exhibit sufficient corrosion resistance in a separator environment.
[0034]
Embodiment 3
Cold rolled steel strips having a thickness of 1.0 mm were produced from various austenitic stainless steels having the compositions shown in Table 3 in the same manner as in Example 1. Some of the cold rolled steel strips were subjected to a Cu-rich phase precipitation treatment at 800 ° C. for 24 hours in a process before final annealing.
[0035]

[0036]
After the produced cold-rolled steel strip was subjected to the same bright annealing or atmospheric annealing as in Example 1, and then pickled, the amount of Cu-rich phase precipitated (volume%) and the atomic ratio Cu / ( Si + Mn) and surface contact resistance were investigated.
As can be seen from the inspection results in Table 4, using A11 and A12 steels containing 0.2% by mass or more of Cu, 0.2% by volume or more of a Cu-rich phase was precipitated (test numbers 1 and 5), or When /(Si+Mn)≧0.5 and the surface layer was concentrated with Cu (Test Nos. 2 and 4), the surface contact resistance was significantly reduced. Test Nos. 3 and 6 satisfying both Cu-rich phase: 0.2% by volume or more and Cu / (Si + Mn) ≧ 0.5 had further reduced surface contact resistance. From these results, it was confirmed that even when Ni was added for improving the corrosion resistance, the surface contact resistance could be reduced by precipitating a Cu-rich phase or concentrating Cu in the passive film.
[0037]

[0038]
Embodiment 4
The steels A11 to A26 in Table 3 and the comparative steels subjected to the same Cu-rich phase precipitation treatment as in Example 1 were used as test materials, and the corrosion resistance in a fuel cell separator environment was investigated by the same electrochemical test as in Example 2.
Table 5 shows the measurement results of the passivation electrode density obtained from the anodic polarization curve in the sulfuric acid solution and the corrosion current at 1000 mV and SCE.
[0039]
Test No. 1 according to the present invention. Nos. 1 to 16 have a passivation current density of 10 μA / cm ² or less, indicating that they have excellent corrosion resistance in a self-potential environment. The corrosion current density under a high potential environment was also about 400 μA / cm ² , which was smaller than that of A28 steel having a Cr content exceeding 40.0% by mass. The fact that the A28 steel shows a high corrosion current density under a high potential environment is presumed to be due to the high Cr content and the overpassive dissolution of Cr.
[0040]
Further, A14 to A21, A23, and A26 steels having a high N content and adding at least one of Mo, Co, Nb, Ti, Al, V, REM, and B can be used in either a self-potential environment or a high-potential environment. Also, the passivation current density was small, and further excellent corrosion resistance was exhibited.
The A27 steel lacking Cr has a high passivation current density in a high potential environment, although it has a corrosion current density similar to that of the steel of the present invention, and cannot be said to have sufficient corrosion resistance in a separator environment.
[0041]

[0042]
【The invention's effect】
As described above, when the addition amounts of Cr, Ni, and Cu are regulated, and in some cases, N is added, and a Cu-rich phase is dispersed and precipitated in the matrix, or when Cu is concentrated in the surface layer, the austenitic stainless steel is used. While maintaining the excellent corrosion resistance inherent in steel, the surface state is modified to a surface state in which the surface contact resistance is stably maintained at a low level for a long time. The corrosion resistance is further improved by the addition of Mo, Co, Nb, Ti, Al, V, REM, B and the like. The austenitic stainless steel plate that has both low surface contact resistance and corrosion resistance in this way is a suitable material as a base material of a fuel cell separator exposed to a severe corrosive environment, and has stacked a large number of fuel cell modules. Even in this state, a fuel cell with high power generation efficiency is constructed because of little Joule heat generation.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view (a) and an exploded perspective view (b) illustrating an internal structure of a fuel cell using a solid polymer membrane as an electrolyte.
FIG. 2 is a graph showing the anodic polarization pole of the stainless steel sheet used in Example 2.

Claims

Cr: 16.0 to 40.0% by mass, Ni: 5.0 to 26.0% by mass, Cu: 0.2 to 6.0% by mass, the balance being substantially Fe, and a Cu-rich phase Austenitic stainless steel for a fuel cell separator, wherein the austenitic stainless steel is dispersed and precipitated in a matrix at a rate of 0.2% by volume or more.

Cr: 16.0 to 40.0% by mass, Ni: 5.0 to 26.0% by mass, Cu: 0.2 to 6.0% by mass, the balance substantially having a composition of Fe, Si, Mn An austenitic stainless steel for a fuel cell separator, wherein Cu is concentrated in the surface layer at an atomic ratio Cu / Cu / (Si + Mn) of 0.5 or more with respect to.

Cr: 16.0 to 40.0% by mass, Ni: 5.0 to 26.0% by mass, Cu: 0.2 to 6.0% by mass, the balance being substantially Fe, and a Cu-rich phase Are dispersed and precipitated in the matrix at a ratio of 0.2% by volume or more, and Cu is concentrated in the surface layer at an atomic ratio of Cu to Si and Mn of Cu / (Si + Mn) of 0.5 or more. Austenitic stainless steel for fuel cell separators.

The austenitic stainless steel for a fuel cell separator according to any one of claims 1 to 3, further comprising N: 0.01 to 0.5% by mass.

Further, Mo: 0.2 to 6.0% by mass, Co: 0.01 to 1.0% by mass, Nb: 0.05 to 1.0% by mass, Ti: 0.05 to 1.0% by mass, Al : 0.01 to 3.0% by mass, V: 0.01 to 1.0% by mass, rare earth metal (REM): 0.001 to 0.5% by mass, B: 0.001 to 1.0% by mass. The austenitic stainless steel for a fuel cell separator according to any one of claims 1 to 4, comprising one or more kinds.

A fuel cell separator comprising the austenitic stainless steel according to claim 1 as a base material.

An austenitic stainless steel sheet containing Cr: 16.0 to 40.0% by mass, Ni: 5.0 to 26.0% by mass, and Cu: 0.2 to 6.0% by mass was subjected to a dew point of −30 ° C. as final annealing. A method for producing an austenitic stainless steel for a fuel cell separator, comprising bright annealing in the following atmosphere.

After an austenitic stainless steel sheet containing Cr: 16.0 to 40.0% by mass, Ni: 5.0 to 26.0% by mass, and Cu: 0.2 to 6.0% by mass, it is hydrofluoric acid- A process for producing an austenitic stainless steel for a fuel cell separator, wherein the austenitic stainless steel for a fuel cell separator is finished by pickling using nitric acid or a mixed acid of sulfuric acid and nitric acid.