JP2004307918A - Al-CONTAINING HEAT-RESISTANT FERRITIC STAINLESS STEEL SHEET SUPERIOR IN WORKABILITY AND OXIDATION RESISTANCE, AND MANUFACTURING METHOD THEREFOR - Google Patents
Al-CONTAINING HEAT-RESISTANT FERRITIC STAINLESS STEEL SHEET SUPERIOR IN WORKABILITY AND OXIDATION RESISTANCE, AND MANUFACTURING METHOD THEREFOR Download PDFInfo
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【０００１】
【発明の属する技術分野】
本発明は、暖房機器、厨房機器等の燃焼機器部材及び二輪車等の排気系部材で触媒を担持されて使用される担体、ヒートチューブ等の部品に好適な、加工性、耐酸化性に優れた耐熱フェライト系ステンレス鋼板に関する。
【０００２】
【従来の技術】
ストーブのチムニー材などの暖房器具、電熱用材料又は厨房機器の燃焼機器部材として、ＳＵＨ２１（１８Ｃｒ−３Ａｌ）等のＡｌ含有フェライト系ステンレス鋼が使用されている。Ａｌ含有フェライト系ステンレス鋼の耐酸化性は、Ａｌ２Ｏ３を主体とする酸化皮膜の緻密さによって決まるため、Ａｌ含有量が高いほど優れた耐高温酸化性を示す。一方、Ａｌ含有量が高くなると加工性が劣化するため、加工性と耐酸化性を両立し得るＡｌ含有フェライト系ステンレス鋼の開発が要求されている。
【０００３】
また、Ａｌ含有フェライト系ステンレス鋼は、自動車排気ガス浄化装置にも用いられており、排気ガス中の過酷な高温酸化環境において優れた耐酸化性を有する材料の開発が進められてきた。例えば、特許文献１には、希土類元素及びＹを添加して耐酸化性を向上させた２０Ｃｒ−５Ａｌ鋼が開示されている。
【０００４】
特に最近、環境問題の高まりから四輪車だけでなく、二輪車にも触媒方式による排気ガス浄化装置の装着が進められている。二輪車においても、２０Ｃｒ−５Ａｌ鋼の箔を使用した金属担体が使用されているが、金属担体以外に、ヒートチューブ、排気管等の部品の内面に触媒を担持する方式も採用されている。
【０００５】
それらの部材には、板厚が０．６〜１．５ｍｍ程度の鋼板が使用されるが、金属担体と同じように２０Ｃｒ−５Ａｌ鋼、ＳＵＨ２１鋼等のＡｌ含有フェライト系ステンレス鋼を適用した場合、耐酸化性は優れるものの加工性、溶接性が悪いためマフラー成形時の製造コストが高くなるばかりでなく、複雑形状の部材には適用できないという問題がある。
【０００６】
このような問題に対して、特許文献２には、ＴｉをＴｉ／（Ｃ＋Ｎ）が６以下になるように添加し、深絞り加工後の靭性を向上させたＡｌ含有フェライト系ステンレス鋼が開示されている。また、特許文献３には、低Ａｌ化及びＳｉ添加により、排気ガス中での耐酸化性、溶接性及び加工性に優れた触媒担持用耐熱フェライト系ステンレス鋼が開示されている。
【０００７】
しかし、これらの方法では、加工性の指標として考慮されているのは圧延方向の伸びだけであり、燃焼機器の複雑な形状の部品を成形するには、加工性が不十分である。また、触媒担持用耐熱フェライト系ステンレス鋼としては、更なる耐酸化の向上が要求されている。
【０００８】
【特許文献１】
特開平４−１２８３３号公報
【特許文献２】
特開平４−３５４８５７号公報
【特許文献３】
特開２０００−３１６７７３号公報
【０００９】
【発明が解決しようとする課題】
本発明の目的は、燃焼機器部材又は触媒担持用部品として最適な、加工性及び耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼冷延鋼板を提供するものである。
【００１０】
【課題を解決するための手段】
本発明者は、以上の課題に鑑み、Ａｌ含有フェライト系ステンレス鋼の加工性と耐酸化性について詳細な検討を行い、成分及び製造プロセスを最適化した。本発明の要旨は、以下のとおりである。
【００１１】
（１）質量％で、
Ｃ ：０．００１〜０．０１５％、 Ｎ ：０．００２〜０．０２％
Ｃ＋Ｎ：０．００３〜０．０２％、 Ｓｉ：０．３〜０．８％、
Ｍｎ：１．０％以下、 Ｐ ：０．０４％以下、
Ｓ ：０．０２％以下、 Ｃｒ：１３〜２０％、
Ａｌ：１．５〜２．５％未満、 Ｃｕ：０．５％以下
Ｔｉ：３×（Ｃ＋Ｎ）〜２０×（Ｃ＋Ｎ）％
を含有し、残部がＦｅ及び不可避的不純物からなり、ミクロ組織において結晶粒度番号が７〜１０であり、圧延方向に対して、０°、４５°、９０°方向の伸びの最小値Ｅｌｍｉｎ［％］、及び、ｒ値の最小値ｒｍｉｎが、それぞれ、Ｅｌｍｉｎ≧２５％、及び、ｒｍｉｎ≧１．０を満足することを特徴とする加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板。
【００１２】
（２） 質量％で、
Ｍｏ：０．１〜２．５％、 Ｎｉ：０．１〜２．５％、
Ｎｂ：０．０１〜０．５％、 Ｖ ：０．０５〜０．５％、
Ｂ ：０．０００５〜０．００５％
の１種又は２種以上を含有することを特徴とする前記（１）記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板。
【００１３】
（３） 質量％で、
Ｍｇ：０．０００５〜０．００５％、Ｃａ：０．０００５〜０．００５％、
ＲＥＭ：０．００１〜０．０１％
の１種又は２種以上を含有することを特徴とする前記（１）又は（２）記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板。
【００１４】
（４） 表面に、Ａｌ量が１５％以上であり、厚さが０．０３〜０．５μｍの酸化皮膜を有することを特徴とする、前記（１）〜（３）の何れかに記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板。
【００１５】
（５） 質量％で、
Ｃ ：０．００２〜０．０２％、 Ｎ ：０．０２％以下、
Ｓｉ：０．０５〜１．０％、 Ｍｎ：３．０％以下、
Ｐ ：０．０４％以下、 Ｓ ：０．０２％以下、
Ｃｒ：１０〜２５％、 Ａｌ：１．０〜３．０％未満
Ｔｉ：３×（Ｃ＋Ｎ）〜２０×（Ｃ＋Ｎ）％
を含有し、残部Ｆｅ及び不可避的不純物からなり、表面にＡｌ量が１５％以上であり、厚さが０．０３〜０．５μｍの酸化皮膜を有することを特徴とする、加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板。
【００１６】
（６） 質量％で、
Ｃｕ：０．１〜２．５％、 Ｍｏ：０．１〜２．５％、
Ｎｉ：０．１〜２．５％、 Ｎｂ：０．０１〜０．５％、
Ｖ：０．０５〜０．５％、 Ｂ ：０．０００５〜０．００５％
の１種又は２種以上を含有することを特徴とする前記（５）記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板。
【００１７】
（７）質量％で、
Ｍｇ：０．０００５〜０．００５％、Ｃａ：０．０００５〜０．００５％、
ＲＥＭ：０．００１〜０．０１％
の１種又は２種以上を含有することを特徴とする前記（５）又は（６）記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板。
【００１８】
（８） 前記（１）〜（３）の何れかに記載の成分からなるスラブ又はインゴットを熱間圧延、冷間圧延し、９００〜１０００℃で最終焼鈍することを特徴とする、前記（１）〜（３）の何れかに記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板の製造方法。
【００１９】
（９） 前記熱間圧延後、冷間圧延前に熱延板焼鈍を８００〜１０００℃で行うことを特徴とする（８）記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板の製造方法。
【００２０】
（１０） 前記熱延板焼鈍後、圧下率が４０％以上の冷間圧延を行い、最終焼鈍を行うことを特徴とする前記（９）記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板の製造方法。
【００２１】
（１１） 前記冷間圧延後、中間焼鈍を８００〜１０００℃で行い、最終冷間圧延を行うことを特徴とする前記（８）又は（９）記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板の製造方法。
【００２２】
（１２） 前記最終冷間圧延の圧下率が４０％以上であることを特徴とする前記（１１）記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板の製造方法。
【００２３】
（１３） 前記最終焼鈍後、ふっ酸濃度が２０〜１００ｇ／ｌ、硝酸濃度が４０〜１５０ｇ／ｌ、液温が２０〜６０℃の酸洗液中で酸洗することを特徴とする前記（８）〜（１２）の何れかに記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板の製造方法。
【００２４】
（１４） 前記（４）記載のステンレス鋼板の製造方法であって、最終焼鈍、酸洗後に、アルゴン及び／又は窒素からなる０．０００１〜０．１容量％の酸素を含む雰囲気で、６００〜９００℃で、１〜６０分加熱することを特徴とする、前記（８）〜（１３）の何れかに記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板の製造方法。
【００２５】
（１５） 前記（５）〜（７）の何れかに記載の成分からなるスラブ又はインゴットを熱間圧延、熱延板焼鈍、冷間圧延、最終焼鈍、酸洗し、アルゴン及び／又は窒素からなる０．０００１〜０．１容量％の酸素を含む雰囲気で、６００〜９００℃で、１〜６０分加熱することを特徴とする、前記（５）〜（７）の何れかに記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板の製造方法。
【００２６】
（１６） 前記（５）〜（７）の何れかに記載の成分からなるスラブ又はインゴットを熱間圧延、熱延板焼鈍、冷間圧延し、最終焼鈍として、アルゴン及び／又は窒素からなる０．０００１〜０．１容量％の酸素を含む雰囲気で、８００〜１０００℃で、１〜６０分加熱することを特徴とする、前記（５）〜（７）の何れかに記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板の製造方法。
【００２７】
【発明の実施の形態】
本発明者は、耐酸化性が良好なＡｌ含有フェライト系ステンレス鋼である１８Ｃｒ−３Ａｌ−０．２５Ｓｉ鋼（ＳＵＨ２１鋼相当）の加工性について検討した。その結果、最終焼鈍で十分に粒成長していないことが加工性が低下した原因であることが判明した。したがって、最終焼鈍温度を高温で行うことにより加工性が向上すると考えられる。
【００２８】
しかし、Ａｌ含有フェライト系ステンレス鋼の最終焼鈍を高温で行うと、Ａｌ２Ｏ３を含有する酸化皮膜が生成し、酸洗工程でのスケール除去が困難になるという問題を生じる。
【００２９】
このような問題を解決するために、Ｃ、Ｎを低減し、Ｔｉを添加して高純化し、Ａｌ量の低減とＳｉ添加量の最適化を組み合わせることにより、耐酸化性を損なうことなく、再結晶温度を低下させて加工性を向上させたＡｌ含有耐熱フェライト系ステンレス鋼の開発を指向した。
【００３０】
本発明者は、高純化したＡｌ含有フェライト系ステンレス鋼の加工性に及ぼすＳｉの影響について検討を行った。
【００３１】
Ｃ、Ｎを低減し、Ｔｉを添加した１８Ｃｒ−２Ａｌ系のフェライト系ステンレス鋼に、０．１〜１．５％のＳｉを添加し、圧延方向に対して、０°の方向（Ｌ方向という）、４５°の方向（Ｄ方向という）、９０°の方向（Ｃ方向という）を長手とする引張試験片をＪＩＳ Ｚ ２２０１に準拠して採取し、引張試験をＪＩＳ Ｚ ２２４１に準拠して行い、またランクフォード値（ｒ値という）をＪＩＳ Ｚ ２２５４に準拠して測定した。
【００３２】
Ｓｉ量に対して、Ｌ方向、Ｄ方向、Ｃ方向の破断伸びの最小値をＥｌｍｉｎを図１に、ｒ値の最小値をｒｍｉｎを図２に示す。図１及び図２に示したように、Ｓｉ量が０．３〜０．８％の範囲において、Ｅｌｍｉｎが２５％以上、ｒｍｉｎが１．０以上になり、特に、ｒｍｉｎがＳｉの添加により向上し、０．５％超を添加すると緩やかに減少する傾向があることを見出した。
【００３３】
また、Ｓｉ量の最適化により再結晶温度も低下し、焼鈍温度を低くすることができた。これにより酸洗性も大きく改善され、酸化スケールが問題なく除去できるようになった。これらの成分及び製造方法の組み合わせにより、優れた加工性と耐酸化性を両立できるＡｌ含有耐熱フェライト系ステンレス鋼の開発に成功した。
【００３４】
更に、本発明者は耐酸化性を発現する表面皮膜に着目し、合金成分中のＡｌ量を増加させずに表面皮膜を改質し、耐酸化性を向上させる方法について検討を行った。Ａｌ含有量が３％未満のフェライト系ステンレス鋼板では、排ガス中のような厳しい酸化雰囲気で熱処理を行うと、ＡｌだけでなくＦｅ、Ｃｒ等が酸化されて、皮膜中のＡｌ量が低くなり、耐酸化が低下する。
【００３５】
本発明者は、Ｆｅ、Ｃｒの酸化を抑制するため、熱処理雰囲気中の酸素含有量に着目し、詳細な検討を行った。その結果、Ａｌ含有フェライト系ステンレス鋼を微量の酸素を含む雰囲気で熱処理すると、耐酸化性が改善されることがわかった。
【００３６】
この耐酸化性に優れた鋼の表面に生成した酸化皮膜をグロー発光分光法（ＧＤＳ）で解析した。その結果、皮膜中のＡｌ量が、Ａｌ２Ｏ３の化学量論的組成よりも低くても、耐酸化性向上に有効であることを見出した。
【００３７】
この知見を基に、耐酸化性と加工性を両立する合金成分及び酸化皮膜を形成する熱処理条件を検討し、本発明を完成させるに至った。
【００３８】
以下、本発明について詳細に説明する。
【００３９】
Ｃは、鋼中に含まれる不可避的不純物であり、過剰に含有すると加工性、溶接性が低下し、高温加熱時のＡｌ２Ｏ３皮膜の形成が不安定になる。そのためＣ量は低いほど好ましいが、Ｃ量を過度に低減するには精錬のコストが増大する。
【００４０】
本発明のＡｌ含有耐熱フェライト系ステンレス鋼を燃焼機器部材に用いる場合には、Ｃ量を０．００１〜０．０１５％の範囲とした。また、触媒担持部材に用いる場合は、Ｃ量を０．００２〜０．０２％の範囲とした。
【００４１】
Ｎは、Ｃと同様、鋼中に含まれる不可避的不純物であり、含有量が低いほど好ましいが、過度に低減するには精錬のコストが増大する。
【００４２】
Ｎは、含有量が０．０２％を超えると加工性、溶接性が低下し、高温加熱時のＡｌ２Ｏ３皮膜の形成が不安定になる。そのため、Ｎ量の上限を０．０２％以下とした。
【００４３】
本発明のＡｌ含有耐熱フェライト系ステンレス鋼を燃焼機器部材に用いる場合には、Ｎ量の下限を０．００２％以上とすることが必要であり、触媒担持部材に用いる場合も、下限を０．００２％以上とすることが好ましい。
【００４４】
本発明のＡｌ含有耐熱フェライト系ステンレス鋼を燃焼機器部材に用いる場合には、加工性の低下を防止するために、ＣとＮの合計量を０．０２％以下に制限することが必要である。Ｃ＋Ｎの下限は、精錬コストを考慮し、Ｃの下限とＮの下限の合計である０．００３％以上とした。
【００４５】
Ｓｉは、本発明のＡｌ含有耐熱フェライト系ステンレス鋼を燃焼機器材に適用する際には、極めて重要な元素である。Ｓｉは、高温酸化雰囲気において、保護性の高いＡｌ２Ｏ３皮膜の形成を促進し、耐酸化性の向上に寄与する。一方、過剰に含有すると、加工性及び溶接性が著しく低下する。
【００４６】
耐酸化性を向上させ、かつ良好な加工性を得るには、０．３〜０．８％の添加が好ましく、０．４〜０．６％の添加が最適である。
【００４７】
触媒担持部材に用いる際にもＳｉは有用な元素であり、Ａｌの代替として添加することできる。Ｓｉの添加により、エンジン排気ガス環境下において、触媒機能の維持に必要なＡｌ含有量を低減することが可能になる。
【００４８】
このような効果を発現するには、Ｓｉを０．０５％以上添加することが必要である。一方、１．０％超のＳｉの添加により伸びが低下する。従って、触媒担持部材に用いる際には、Ｓｉの添加量を０．０５〜１．０％の範囲とした。
【００４９】
Ｍｎは、加工性を劣化させる元素であり、また過剰な添加により良好な耐酸化性を有する酸化皮膜の形成を阻害する。本発明のＡｌ含有耐熱フェライト系ステンレス鋼を燃焼機器部材に用いる場合は、上限を１．０％とすることが必要であり、好ましい上限は０．５％である。
【００５０】
触媒担持用材に用いる場合は、耐酸化性の劣化を防止するために、３．０％を上限とした。一方、Ｍｎ含有量の下限は低いほど好ましいが、精錬上のコストを考慮すると好ましい下限は０．０１％であり、更に好ましい下限は０．１％である。
【００５１】
Ｐは、鋼中に不可避的に含まれる不純物であり、０．０４％を超えて含有すると加工性が低下するために０．０４％を上限とした。Ｐ量を０．０１％未満にするには精錬のコストが増大するため、０．０１％を下限とすることが好ましい。
【００５２】
Ｓは、鋼中に不可避的に含まれる不純物であり、０．０２％を超えて含有すると耐酸化性が良好な表面の酸化皮膜の形成を著しく阻害するため、０．０２％を上限とした。Ｓ量の下限は低いほど好ましいが、０．０００１％未満にするには精錬上のコストが増大する。精錬コストを考慮すると、更に好ましいＳ量の下限は、０．００５％である。
【００５３】
Ｃｒは、耐食性を向上させ、耐酸化性の良好な表面の酸化皮膜の密着性を向上させるために重要な元素であるが、過剰な添加により溶接性、加工性が著しく低下する。
【００５４】
本発明のＡｌ含有耐熱フェライト系ステンレス鋼を燃焼機器部材に用いる場合は、Ｃｒ量を１３〜２０％の範囲とすることが必要である。また、触媒担持部材に用いる場合は、Ｃｒ量を１０〜２５％の範囲とした。
【００５５】
Ａｌは、耐酸化性に優れた、Ａｌ２Ｏ３を含む酸化皮膜を形成させる本発明の必須元素である。しかし、過剰の添加により、加工性、溶接性、酸洗性を損なう。本発明のＡｌ含有耐熱フェライト系ステンレス鋼を燃焼機器部材に用いる場合は、Ａｌ量を１．５〜２．５％未満とすることが必要である。
【００５６】
触媒担持部材に用いる場合は、Ａｌ量が１．０％未満ではＳｉを添加しても酸化皮膜の生成が不十分であるため、下限を１．０％とした。また、Ａｌ量の上限は３．０％未満とした。
【００５７】
Ｔｉは、加工性及び耐酸化性に有害なＣ、Ｎを炭窒化物として固定し、酸化皮膜の密着性も改善し、更に溶接部の加工性、耐食性を確保するためにも重要な元素である。この効果は、Ｔｉ添加量が（Ｃ＋Ｎ）の含有量の３倍未満では不十分である。一方、（Ｃ＋Ｎ）の含有量の２０倍超のＴｉを添加すると、固溶Ｔｉが増加して加工性を劣化させる。
【００５８】
したがって、Ｔｉ添加量は、（Ｃ＋Ｎ）の含有量の３〜２０倍とすることが必要である。また、好ましいＴｉ添加量の範囲は、（Ｃ＋Ｎ）の８〜１５倍である。
【００５９】
Ｃｕは耐食性を向上させる元素であるが、加工性を低下させる作用が著しいため、本発明のＡｌ含有耐熱フェライト系ステンレス鋼を燃焼機器部材に適用するには、上限を０．５％に制限する必要がある。また、下限は低い方が好ましいが、０．１％未満とするには精錬のコストが増大するため、０．１％以上とすることが好ましい。
【００６０】
本発明のＡｌ含有耐熱フェライト系ステンレス鋼を触媒担持部材に適用する際には、耐食性を向上させるため０．１％以上のＣｕを添加してもよい。一方、Ｃｕを２．５％を超えて添加すると溶接性、加工性が低下するため、上限を２．５％とすることが好ましい。
【００６１】
更に、必要に応じて、Ｍｏ、Ｎｉ、Ｎｂ、Ｖ、Ｂ、Ｍｇ、Ｃａ、ＲＥＭの１種又は２種以上を添加してもよい。
【００６２】
Ｍｏは、０．１％以上の添加により、高温強度が向上するが、２．５％を超えて添加すると加工性、溶接性が低下する。そのため、Ｍｏの添加量は、０．１〜２．５％の範囲とすることが好ましい。
【００６３】
Ｎｉは、０．１％以上の添加により加工性を改善するが、２．５％を超えて添加すると溶接部にマルテンサイト相が生成し、溶接部加工性が低下する。そのため、Ｎｉの添加量は、０．１〜２．５％の範囲とすることが好ましい。
【００６４】
Ｎｂ、Ｖは、Ｔｉと同様にＣ、Ｎを固定する効果を有し、Ｎｂは０．０１％以上、Ｖは０．０５％以上の添加により、加工性、溶接部特性を改善するが、Ｎｂ、Ｖを０．５％超添加すると加工性、溶接性を劣化させる。そのため、Ｎｂの添加量を０．０１〜０．５％、Ｖの添加量を０．０５〜０．５％の範囲とすることが好ましい。
【００６５】
Ｂは、０．０００５％以上の添加により、成形時の二次加工割れを防止する効果があるが、０．００５％超を添加すると溶接性が低下する。そのため、Ｂの添加量は、０．０００５〜０．００５％の範囲とすることが好ましい。
【００６６】
Ｍｇ、Ｃａ、ＲＥＭは、表面の酸化皮膜の密着性を改善し、溶接部の組織を微細化して溶接部の加工性を向上する元素であるが、過剰に添加すると鋼板製造時に疵を生じ易くなり、また、溶接性を損なう。
【００６７】
そのため、Ｍｇ量を０．０００５〜０．００５％、Ｃａ量を０．０００５〜０．００５％、ＲＥＭ量を０．００１〜０．０１％の範囲とすることが好ましい。
【００６８】
本発明のＡｌ含有耐熱フェライト系ステンレス鋼を燃焼機器部材に用いる場合、加工性を向上させるために、ミクロ組織、伸び、ｒ値を規定することが必要である。
【００６９】
ミクロ組織は、十分に再結晶していることが必要であり、結晶粒度番号を７〜１０の範囲とした。これは、結晶粒度番号が、７未満であると結晶粒が大きすぎるため、加工時に肌荒れが生じ易く、１０を超えると、必要な加工性が得られないためである。結晶粒度番号は、ＪＩＳ Ｇ ０５５２に準拠して測定すればよい。
【００７０】
伸びとｒ値については、Ｌ方向、Ｄ方向、Ｃ方向で、ＪＩＳ Ｚ ２２４１に準拠して測定した破断伸びとＪＩＳ Ｚ ２２５４に準拠して測定したｒ値の最小値が加工性を決める重要な指標である。
【００７１】
伸びの最小値Ｅｌｍｉｎが２５％以上であり、ｒ値の最小値ｒｍｉｎが１．０以上であると、十分な加工性を確保することができる。Ｅｌｍｉｎ、ｒｍｉｎは高いほどよいが、Ｅｌｍｉｎを４０％超、ｒｍｉｎを２．５超とするためには、製造コストが著しく増大する。
【００７２】
本発明のＡｌ含有耐熱フェライト系ステンレス鋼を触媒担持部材として用いるには、耐酸化性に優れた表面皮膜を有することが必要である。
【００７３】
酸化皮膜中のＡｌ量が１５％未満であると、排ガス中等の酸化雰囲気でＦｅ、Ｃｒの拡散を阻止できず、十分な耐酸化性を有する酸化皮膜を維持することが難しくなり、耐酸化性がやや劣化する。そのため、酸化皮膜中のＡｌ量が１５％以上であることが好ましい。
【００７４】
また、酸化皮膜中のＡｌ量が５０％超になると皮膜が剥離し易くなるため、上限を５０％とすることが好ましい。
【００７５】
酸化皮膜の厚みは、０．０３μｍ未満では耐酸化性が不十分である。一方、酸化皮膜の厚みが０．５μｍを超えると効果が飽和するだけでなく、酸化皮膜の剥離等が生じ易くなる。そのため、酸化皮膜の厚みを０．０３〜０．５μｍの範囲とすることが必要である。
【００７６】
このような表面皮膜が形成された場合、Ａｌ量が１％程度であっても、耐酸化性は著しく向上する。そのため、Ａｌ量を１．５〜２．５％未満含有する燃焼機器部材として用いる場合にも、Ａｌ量が１５％以上である酸化皮膜を表面に形成させることが好ましい。
【００７７】
表面皮膜のＡｌ量、厚みはグロー発光分光法（ＧＤＳという）によって測定することができる。これは、ＧＤＳによって得られた時間とＦｅ、Ａｌ、酸素等の発光強度を示すプロファイル（ＧＤＳプロファイルという）から、Ａｌ量と表面皮膜厚さを算出する方法である。
【００７８】
本発明のＡｌ含有ステンレス鋼の製造方法は、溶解、鋳造、熱間圧延するものであり、その後は、熱延板焼鈍、酸洗、冷間圧延、最終焼鈍又は冷間圧延、中間焼鈍、酸洗、最終冷間圧延、最終焼鈍を行い、酸洗するものである。
【００７９】
更に、微量の酸素を含むアルゴン雰囲気、窒素雰囲気又はアルゴンと窒素の混合雰囲気で熱処理を行ってもよい。
【００８０】
燃焼機器部材に用いるＡｌ含有ステンレス鋼の製造方法において、加工性を向上させ、酸洗性を確保するために、最終焼鈍は極めて重要である。最終焼鈍の温度が、９００℃未満では、十分な再結晶が行われず、必要な加工性が発現しない。
【００８１】
また、１０００℃を超えると、酸化スケールが強固になるため、酸洗工程でのスケールの除去が困難となり好ましくない。従って、最終焼鈍温度は、９００〜１０００℃の範囲とすることが必要である。
【００８２】
加工性を更に向上させるには、熱間圧延後の製造工程において、熱延板焼鈍、酸洗、冷間圧延、最終焼鈍又は冷間圧延、中間焼鈍、最終冷間圧延、最終焼鈍のように、冷間圧延の前後に焼鈍を行うことが好ましい。熱延板焼鈍と中間焼鈍の両方を行ってもよい。
【００８３】
熱延板焼鈍及び中間焼鈍の焼鈍温度は、８００〜１０００℃の範囲とすることが好ましい。これは、焼鈍温度が８００℃未満では、必要な加工性が得られ難く、１０００℃を超えると、結晶粒が大きくなりすぎ、加工時に肌荒れを生じ易くなるためである。
【００８４】
冷間圧延は、中間焼鈍を行わない場合は総冷延圧下率を、中間焼鈍を行う場合は最終冷間圧延の冷延圧下率（最終冷延圧下率という）を４０％以上とすることが好ましい。
【００８５】
これは、総冷延圧下率又は最終冷延圧下率が４０％未満では、歪の導入が不十分で、不均一であり、最終焼鈍による再結晶が遅れて加工性が低下する可能性があるためである。特に好ましい条件は、総冷延圧下率が６０％以上、最終冷延圧下率が５０％以上である。
【００８６】
総冷延圧下率又は最終冷延圧下率が９５％を超えると、その効果が飽和するだけでなく、圧延ロールへの負荷が著しく大きくなる。従って、総冷延圧下率又は最終冷延圧下率の上限を９５％とすることが好ましい。
【００８７】
なお、総冷延圧下率は、熱延板の板厚と最終冷間圧延後の冷延板（最終冷延板という）の板厚の差を熱延板の板厚で除した値を百分率で表したものである。熱延板の板厚は、熱延板焼鈍後に測定しても良く、最終冷延板の板厚は最終焼鈍後又は酸洗後に測定してもよい。
【００８８】
また、最終冷延圧下率は、中間焼鈍前又は中間焼鈍後の冷延板（中間冷延板）の板厚と最終冷延板の板厚の差を中間冷延板の板厚で除した値を百分率で表したものである。
【００８９】
最終焼鈍後の酸洗は、ソルト、硝酸電解槽、硝弗酸槽から構成されるが、硝弗酸槽において、ふっ酸濃度が２０〜１００ｇ／ｌ、硝酸濃度が４０〜１５０ｇ／ｌ、液温が２０〜６０℃であることが好ましい。
【００９０】
これは、ふっ酸濃度が２０ｇ／ｌ未満、硝酸濃度が４０ｇ／ｌ未満では、酸化スケール除去が不十分あるため、スケール残りによる発色等が起こり、ふっ酸濃度が１００ｇ／ｌ超、硝酸濃度が１５０ｇ／ｌ超では、酸洗過多で母材の肌荒れが目立ち、酸洗液の液温が２０℃未満であると、酸洗が不十分となりスケール残りが発生し易く、６０℃を超えると、酸洗過多で肌荒れが目立つためである。
【００９１】
触媒担持部材に用いる本発明のＡｌ含有耐熱フェライト系ステンレス鋼の製造において、最終焼鈍、酸洗後、耐酸化性に優れた表面皮膜を形成するために、微量の酸素を含むアルゴンガス、窒素ガス、アルゴンと窒素の混合ガスの何れかの雰囲気で熱処理を行うことが必要である。
【００９２】
この雰囲気中の酸素含有量は、０．０００１〜０．１容量％であることが好ましい。これは、酸素含有量が、０．０００１容量％以下では、十分に酸化皮膜が成長できず、０．１容量％を超えると酸化皮膜中のＡｌ量が減少し、耐酸化性が低下するためである。
【００９３】
また、Ｈ２ガスを含むような還元性雰囲気では熱処理中に酸化皮膜が形成されないため好ましくない。真空中に微量の酸素を導入した雰囲気でもよいが、真空チャンバー等の設備が必要である。
【００９４】
微量の酸素を含むアルゴン及び／又は窒素からなる雰囲気は、あらかじめ酸素含有量を調節したアルゴンガスボンベ、窒素ガスボンベ、アルゴンと窒素の混合ガスボンベのガスを使用しても良く、酸素ガスとアルゴンガス及び／又は窒素ガスをガス混合機等を用いて、酸素濃度を酸素濃度計によって測定しながら、それぞれのガスの流量を調整し、混合してもよい。
【００９５】
この微量の酸素を含有するアルゴン及び／又は窒素からなる雰囲気での熱処理は、６００〜９００℃で行うことが必要である。これは、熱処理温度が６００℃未満であると良好な酸化皮膜が形成されず、９００℃を超えると、鋼板の結晶粒径が粗大になり、加工性等の性質がやや劣化するためである。
【００９６】
また、熱処理時間は１分未満では熱処理の効果にばらつきが生じ、６０分超では、鋼板の結晶粒径が粗大化し、加工性等の性質がやや劣化するため、１〜６０分とすることが好ましい。なお、加工性の観点から好ましい熱処理時間の上限は、３０分以下である。
【００９７】
また、研磨仕上げの後に熱処理を行った場合、より皮膜中のＡｌ量が高い表面皮膜が形成されるため好ましい。更に、このＡｌ含有鋼板をマフラー等の部品に組み立ててから本願発明の熱処理を行ってもよい。
【００９８】
この表面の酸化皮膜を形成させる熱処理工程の替わりに、最終焼鈍を０．０００１〜０．１容量％の酸素を含むアルゴン雰囲気、窒素雰囲気又はアルゴンと窒素の混合雰囲気で行い、その後の酸洗を省略しても良い。この際には、温度範囲は９００〜１０００℃とすることが必要である。
【００９９】
なお、本発明のＡｌ含有耐熱フェライト系ステンレス鋼を燃焼機器部材として用いる場合にも、微量の酸素を含むアルゴン雰囲気、窒素雰囲気又はアルゴンと窒素の混合雰囲気で熱処理を行うことが好ましい。熱処理の条件は、触媒担持部材に適用する際の条件と同様でよい。
【０１００】
【実施例】
更に、実施例により本発明を詳細に説明する。
【０１０１】
（実施例１）
表１に示す化学成分を有する５０ｋｇ鋼塊を溶製し、熱延を行って板厚４ｍｍの熱延板とした。
【０１０２】
【表１】
その後、冷間圧延により、２ｍｍ厚の鋼板とし、９００℃で１分保持する中間焼鈍を行って、酸洗した。その後、更に最終冷間圧延により１ｍｍ厚の鋼板とし、９５０℃で１分保持する最終焼鈍を行った後、酸洗した。
【０１０４】
中間焼鈍後、最終焼鈍後の酸洗は、ソルト、電解硝酸、硝弗酸の順で行い、硝弗酸の硝酸とふっ酸の液組成は、ふっ酸５０ｇ／ｌ、硝酸８０ｇ／ｌとし、液温は５０℃とした。
【０１０５】
これらの鋼板から、組織観察試験片、酸化試験片及び引張試験片を作製した。組織観察試験片は、板幅方向中央部の圧延方向断面位置から採取した。酸化試験片は、２０ｍｍ角の小片を採取し、表面を機械研磨し、＃４００仕上げとした。引張試験片は、ＪＩＳ Ｚ ２２０１の１３Ｂ号試験片とし、Ｌ方向、Ｄ方向及びＣ方向を長手として作製した。
【０１０６】
組織観察試験片を鏡面研磨後、エッチングし、１００倍に拡大して観察を行い、粒度番号をＪＩＳ Ｇ ０５５２に準拠して求めた。酸化試験は、大気中に８００℃、１０００℃で２００時間保持する連続酸化試験を行い、酸化増量と剥離スケールの有無で評価した。
【０１０７】
酸化増量は、以下のようにして測定した。まず試験前に重量測定を行った試験片を、試験温度に保持した炉に挿入し、試験時間経過後、炉から取り出し、素早く、ふた付の金属容器に収納し空冷する。ふた付の金属容器の重量も予め測定しておく。空冷後、試験片の入ったふた付きの金属容器の重量を測定する。
【０１０８】
このとき、空冷中に剥離したスケールは、金属容器中に残っている。酸化増量は、試験片の入ったふた付きの金属容器の重量から、試験前の試験片重量とふた付きの金属容器の重量を減じた値を試料の表面積で除して評価した。ここで、重量の単位はｍｇ、表面積の単位はｃｍ２であり、酸化増量の単位はｍｇ／ｃｍ２である。
【０１０９】
スケール剥離の有無は以下のようにして評価した。試験片をふた付きの金属容器から取り出して、外観を目視し、スケール剥離が見られるものはスケール剥離ありと評価した。スケール剥離が見られないものは、金属容器から取り出した試験後の試験片の重量測定を行った。
【０１１０】
試験片の入ったふた付きの金属容器の重量から、試験後の試験片重量及び予め測定したふた付きの金属容器の重量を減じた値を表面積で除したスケール剥離量が、０．０１ｍｇ／ｃｍ２未満である場合はスケール剥離なし、０．０１ｍｇ／ｃｍ２以上の場合をスケール剥離ありと評価した。
【０１１１】
引張試験は、ＪＩＳ Ｚ ２２４１に準拠して行い、ｒ値の測定は、ＪＩＳ Ｚ ２２５４に準拠して行った。その結果から、破断伸び、ｒ値のＬ方向、Ｄ方向、Ｃ方向の最小値Ｅｌｍｉｎ、ｒｍｉｎを評価した。更に、加工性を評価するために、円筒絞り試験を行った。
【０１１２】
試験片を直径１００ｍｍの円形ブランクとし、ポンチの直径は５０ｍｍ、肩Ｒは５ｍｍ、ダイスの直径は５１．６ｍｍ、肩Ｒは５とし、潤滑剤としてジョンソンワックスを用いた。試験は１０回行い、絞り抜けた回数で加工性を評価した。
【０１１３】
試験の結果を表２に示す。Ｎｏ．１〜４は、本発明の請求項１の要件を満足し、Ｅｌｍｉｎが２５％、ｒｍｉｎが１．０以上であり、これらの鋼は、円筒絞り試験で全て成型できた。更に、耐酸化性についても、酸化増量が極めて少なく、概観の目視において異常酸化や剥離も見られず、良好であった。これに対し、Ｓｉが本発明の範囲よりも少ないＮｏ．５はｒｍｉｎが低く、円筒絞り試験で破断したものがあり、酸化試験においてスケール剥離が見られた。また、本発明の範囲よりもＳｉ量の多いＮｏ．６、７は、Ｅｌｍｉｎ及びｒｍｉｎが低く、円筒絞り試験で破断が生じた。
【０１１４】
【表２】
（実施例２）
表３に示す化学成分を有する鋼塊を溶製し、熱間圧延により３ｍｍの熱延板とし、９００℃で、１分保持する熱延板焼鈍を行い、酸洗した。
【０１１６】
【表３】
その後、冷間圧延により１ｍｍ厚の鋼板として、９５０℃で１分保持する最終焼鈍を行い、実施例１と同じ条件で酸洗を行って鋼板を製造した。酸洗後、目視によりスケール残りの有無を評価した。
【０１１８】
これらの鋼板から、実施例１と同様に、組織観察試験片、酸化試験片及び引張試験片を作製し、粒度番号、酸化試験及び引張試験、ｒ値の測定を行った。
【０１１９】
試験の結果を表４に示すが、Ｎｏ．８〜２２は、成分、粒度番号、Ｅｌｍｉｎ、ｒｍｉｎが本発明の範囲であり、加工性及び耐酸化性に優れている。
【０１２０】
特に、Ｎｏ．９は、１８Ｃｒ−２Ａｌ−０．５Ｓｉ鋼であり、Ｅｌｍｉｎは２９％、ｒｍｉｎは１．１と優れた加工性を示し、耐酸化性もＡｌが２％と低いがＳｉの効果によって、３％のＡｌを含有するＮｏ．２３と同等以上の耐酸化性を示す。
【０１２１】
なお、Ｎｏ．２３は、１８Ｃｒ−３Ａｌ−０．２５Ｓｉ鋼でＳＵＨ２１相当鋼であり、Ａｌが本発明の範囲よりも高いため、加工性が不十分である。
【０１２２】
また、Ａｌ量が本発明の範囲よりも低いＮｏ．２４、２５は、１０００℃での酸化試験でスケール剥離が見られ、耐酸化性が不十分あり、Ａｌが高いＮｏ．２６は、加工性が不十分であり、かつスケール残りが発生していた。
【０１２３】
【表４】
（実施例３）
表１のＣ鋼を実施例１と同様の条件で溶解、鋳造し、熱延を行って３ｍｍの熱延板とした。その後、表５に示す製造条件で鋼板を製造した。
【０１２５】
【表５】
これらの鋼板から、実施例１と同様に、組織観察試験片、酸化試験片及び引張試験片を作製し、粒度番号の測定、酸化試験及び引張試験を行った。結果を表６に示す。
【０１２７】
【表６】
酸化試験の結果は、表２のＮｏ．３とほぼ同様であり、記載を省略した。表６に示す本発明例は、Ｅｌｍｉｎが２５％以上、ｒｍｉｎが１．０以上を満足し、優れた加工性を示している。また、スケール残りや肌荒れ等もなく、優れた鋼板であることを示している。
【０１２９】
Ｎｏ．２７〜４０は、熱間圧延後、熱延板焼鈍、冷間圧延し、最終焼鈍を施したものであり、Ｎｏ．４１〜５４は、熱間圧延、冷間圧延、中間焼鈍、最終冷間圧延、最終焼鈍を施したものであり、Ｎｏ．５５は、熱延板焼鈍と中間焼鈍の両方を施したものである。
【０１３０】
Ｎｏ．２９〜４０、４３〜５５は、最終焼鈍を請求項８に係る本発明の範囲内で実施したものであり、結晶粒度、Ｅｌｍｉｎ、ｒｍｉｎが請求項１に係る発明の範囲内であり、加工性が良好である。
【０１３１】
一方、Ｎｏ．２７、４１は、最終焼鈍温度が低いため、粒成長が充分でなく、加工性に劣る。Ｎｏ．２８、４２は、最終焼鈍温度が高いため、結晶粒径が粗大化し、また、酸洗後にスケール残りが発生した。
【０１３２】
また、Ｎｏ．３４、４８は、酸洗液中の、ふっ酸と硝酸が少ないため、スケール残りが発生し、Ｎｏ．３５、４９は、ふっ酸と硝酸が多すぎるため、酸洗後に鋼板に肌荒れが生じた。
【０１３３】
更に、Ｎｏ．３６、５０は、酸洗液の液温が低すぎるため、スケール残りが発生し、Ｎｏ．３７、５１は液温が高すぎて、酸洗後に鋼板に肌荒れが生じた。
【０１３４】
（実施例４）
表７に示す化学成分を有する鋼塊を溶製し、熱延、冷延、焼鈍、酸洗し、厚さ１ｍｍの鋼板を製造した。
【０１３５】
【表７】
【表８】
これらの鋼板から小片を採取し、表８に示す雰囲気及び加熱条件で熱処理を行った。熱処理の雰囲気は、予め濃度調節した酸素―アルゴン混合ガス又は酸素−窒素混合ガスをボンベから供給することによって制御した。
【０１３８】
熱処理後の鋼板から試験片を採取し、表面の酸化皮膜をＧＤＳにより、Ｆｅ、Ｃｒ、Ａｌ、Ｓｉ、Ｍｎ、Ｃ、Ｎ、Ｔｉ、Ｏ及び添加した選択元素の深さ方向分析を行った。
【０１３９】
ＧＤＳによって得られた時間と各元素の発光強度を示すＧＤＳプロファイルから、Ａｌ量と表面皮膜厚さを測定した。結果の一例として、Ａｌ、Ｏ、Ｆｅ、ＣｒのＧＤＳプロファイルを図３に示す。
【０１４０】
母材に含まれる各元素の濃度の分析値を発光強度で除して、各元素の感度係数を求めた。感度係数の算出には、例えば、図３において、２４０ｓにおける発光強度を用いる。
【０１４１】
これは、２４０ｓのＧＤＳ分析を行うと、Ａｌ、Ｏ、Ｆｅ、Ｃｒの発光強度が一定になることから、母材に含まれる各元素の発光強度を測定していると考えられるためである。
【０１４２】
また、２４０ｓのＧＤＳ分析を行った後の試料の表面の測定部分に対応する位置の凹部の深さ（放電痕深さという）を、触針式の粗度計を用いて測定した。この放電痕深さを測定時間、即ち２４０ｓで除したものを、スパッタ速度とした。このスパッタ速度と図３に例示したＧＤＳプロファイルにより皮膜厚さを求めた。
【０１４３】
皮膜厚さの算出には、ＯのＧＤＳプロファイル（Ｏプロファイルという）を用いた。Ｏプロファイルにおいて、スパッタ開始直後の初期放電によるピークを無視して、その後のプラトー部分を酸素のピーク値とし、スパッタ開始から酸素のピーク半値までのスパッタ時間を求めた。
【０１４４】
このスパッタ時間にスパッタ速度を乗じて、皮膜厚さを決定した。なお、熱処理を行っていない試料の表面の酸化皮膜は数ｎｍ以下であり、ＧＤＳでは測定不可能であった。
【０１４５】
酸洗後の鋼板及び表８に示した熱処理後の鋼板からＪＩＳ Ｚ ２２０１に準拠して１３Ｂ号試験片を採取し、引張試験をＪＩＳ Ｚ ２２４１に準拠して行った。さらに、９０ｍｍ幅の試験片中央部に、溶接電流２００Ａ、溶接速度２ｍ／ｍｉｎで、フィラーワイヤを用いずにＴＩＧ溶接を行った。
【０１４６】
この溶接線上を試験片の中央とし、ＪＩＳ Ｚ ２２４７に準拠して、エリクセン試験機で張出加工を行った。張出加工は１０回繰り返し、平均エリクセン値と５ｍｍ以下での割れが発生した回数で溶接部加工性を評価した。
【０１４７】
また、２０ｍｍ角の酸化試験片を採取し、表面に活性アルミナを主体とする触媒層を担持し、エンジン排気ガス雰囲気で加熱試験を実施した。
【０１４８】
加熱試験は、７００℃の電気炉中に残存酸素量（約１％）、露点（約４０℃）を調節したエンジン排気ガスを導入し、その中で５ｈ保持した後、炉から取り出し、外観上の変色及び触媒層中へのＦｅ、Ｃｒ元素の拡散有無を調査し、外観上の変色が認められるまで繰り返した。
【０１４９】
この加熱試験において、外観上の変色が認められるまでの繰り返し数に５ｈを乗じたものを排ガス７００℃寿命とし、５０時間以上を○、４５時間以下を×として、合否を判定した。
【０１５０】
熱処理後の鋼の表面の酸化皮膜厚さ、Ａｌ量、引張試験結果、成形性試験結果、加熱試験結果を表９に、熱処理前の鋼の引張試験結果、成形性試験結果、加熱試験結果を表１０に示す。
【０１５１】
【表９】
【表１０】
表９に示したように、微量の酸素を含む雰囲気で熱処理され、耐酸化性の高い表面皮膜を持つ請求項５〜７に係る発明鋼は、表１０に示した比較例に比べて、排ガス７００℃寿命が長い。
【０１５４】
一方、ＡＲ鋼のようにＡｌが１％しか含有されていない鋼は、酸素量を本発明の範囲とした雰囲気での熱処理によっても、耐酸化性は合格レベルに達しない。また、ＡＳ、ＡＴ鋼のように、Ａｌ量が３％以上含まれていて耐酸化性が充分である鋼では、酸素量を本発明の範囲とした雰囲気での熱処理による効果は小さく、耐酸化性はほとんど向上しない。
【０１５５】
加工性、溶接性に関しては、酸素量を本発明の範囲とした雰囲気での熱処理による劣化は見られず、引張試験時の伸び、溶接部の加工性は、若干向上している。
【０１５６】
表９に示した請求項５〜７に係る本発明の鋼と、同等の耐酸化性を有する表１０に示した比較例、例えば、Ｎｏ．５６とＮｏ．１００を比較すると、Ｎｏ．５６の加工性、溶接性は、Ｎｏ．１００よりも優れている。
【０１５７】
【発明の効果】
本発明により、加工性と耐酸化性に優れ、暖房機器、厨房機器等の燃焼機器部材又は二輪車等のマフラー等の触媒担持部材に好適な、Ａｌ含有耐熱フェライト系ステンレス鋼を提供することができ、産業上の貢献が極めて高い。
【図面の簡単な説明】
【図１】Ｅｌｍｉｎに及ぼすＳｉ量の影響を示した図である。
【図２】ｒｍｉｎに及ぼすＳｉ量の影響を示した図である。
【図３】グロー発光分光法による皮膜分析結果の一例を示した図である。
【符号の説明】
ａ…Ａｌプロファイル
ｂ…Ａｌ量
ｃ…Ｏプロファイル
ｄ…皮膜厚さ
ｅ…Ｃｒプロファイル
ｆ…Ｆｅプロファイル[0001]
BACKGROUND OF THE INVENTION
The present invention is excellent in workability and oxidation resistance, suitable for parts such as a heat tube and a carrier that is used by supporting a catalyst in combustion equipment members such as heating equipment and kitchen equipment and exhaust system members such as motorcycles. The present invention relates to a heat resistant ferritic stainless steel sheet.
[0002]
[Prior art]
Al-containing ferritic stainless steel such as SUH21 (18Cr-3Al) is used as a heating appliance such as a chimney material for a stove, a material for electric heating, or a combustion appliance member of a kitchen appliance. The oxidation resistance of Al-containing ferritic stainless steel is Al 2 O 3 Therefore, the higher the Al content, the better the high temperature oxidation resistance. On the other hand, since the workability deteriorates as the Al content increases, the development of an Al-containing ferritic stainless steel that can achieve both workability and oxidation resistance is required.
[0003]
Further, Al-containing ferritic stainless steel is also used in automobile exhaust gas purification devices, and development of materials having excellent oxidation resistance in a severe high-temperature oxidation environment in exhaust gas has been advanced. For example,
[0004]
Recently, in particular, due to increasing environmental problems, not only automobiles but also motorcycles are being equipped with catalytic exhaust gas purification devices. In motorcycles, a metal carrier using a 20Cr-5Al steel foil is used. In addition to the metal carrier, a system in which a catalyst is supported on the inner surface of a part such as a heat tube or an exhaust pipe is also employed.
[0005]
For those members, steel plates with a thickness of about 0.6 to 1.5 mm are used, but when Al-containing ferritic stainless steels such as 20Cr-5Al steel and SUH21 steel are applied in the same manner as the metal carrier. However, although the oxidation resistance is excellent, the workability and weldability are poor, so that not only the manufacturing cost at the time of muffler molding increases, but there is a problem that it cannot be applied to a member having a complicated shape.
[0006]
For such problems,
[0007]
However, in these methods, only the elongation in the rolling direction is considered as an index of workability, and the workability is insufficient to form a complex-shaped part of a combustion device. Further, as a heat-resistant ferritic stainless steel for supporting a catalyst, further improvement in oxidation resistance is required.
[0008]
[Patent Document 1]
JP-A-4-12833
[Patent Document 2]
JP-A-4-354857
[Patent Document 3]
JP 2000-316773 A
[0009]
[Problems to be solved by the invention]
An object of the present invention is to provide an Al-containing heat-resistant ferritic stainless steel cold-rolled steel sheet that is optimal as a combustion equipment member or a catalyst-supporting part and excellent in workability and oxidation resistance.
[0010]
[Means for Solving the Problems]
In view of the above problems, the present inventor has made a detailed study on the workability and oxidation resistance of Al-containing ferritic stainless steel and optimized the components and the manufacturing process. The gist of the present invention is as follows.
[0011]
(1) In mass%,
C: 0.001-0.015%, N: 0.002-0.02%
C + N: 0.003 to 0.02%, Si: 0.3 to 0.8%,
Mn: 1.0% or less, P: 0.04% or less,
S: 0.02% or less, Cr: 13-20%,
Al: Less than 1.5 to 2.5%, Cu: 0.5% or less
Ti: 3 × (C + N) to 20 × (C + N)%
The balance is Fe and inevitable impurities, the grain size number is 7 to 10 in the microstructure, and the minimum value El of elongation in the 0 °, 45 °, and 90 ° directions with respect to the rolling direction min [%] And the minimum value r of the r value min Are El min ≧ 25% and r min An Al-containing heat-resistant ferritic stainless steel sheet excellent in workability and oxidation resistance, characterized by satisfying ≧ 1.0.
[0012]
(2) By mass%
Mo: 0.1-2.5%, Ni: 0.1-2.5%,
Nb: 0.01 to 0.5%, V: 0.05 to 0.5%,
B: 0.0005 to 0.005%
The Al-containing heat-resistant ferritic stainless steel sheet having excellent workability and oxidation resistance as described in (1) above, comprising one or more of the above.
[0013]
(3) In mass%,
Mg: 0.0005 to 0.005%, Ca: 0.0005 to 0.005%,
REM: 0.001 to 0.01%
The Al-containing heat-resistant ferritic stainless steel sheet having excellent workability and oxidation resistance as described in (1) or (2) above, comprising one or more of the above.
[0014]
(4) The surface is provided with an oxide film having an Al amount of 15% or more and a thickness of 0.03 to 0.5 μm on the surface, according to any one of (1) to (3), Al-containing heat-resistant ferritic stainless steel sheet with excellent workability and oxidation resistance.
[0015]
(5) By mass%
C: 0.002 to 0.02%, N: 0.02% or less,
Si: 0.05-1.0%, Mn: 3.0% or less,
P: 0.04% or less, S: 0.02% or less,
Cr: 10 to 25%, Al: 1.0 to less than 3.0%
Ti: 3 × (C + N) to 20 × (C + N)%
Workability and oxidation resistance, characterized by comprising a balance Fe and inevitable impurities, having an Al amount of 15% or more on the surface, and having an oxide film with a thickness of 0.03 to 0.5 μm Al-containing heat-resistant ferritic stainless steel sheet with excellent properties.
[0016]
(6) In mass%,
Cu: 0.1 to 2.5%, Mo: 0.1 to 2.5%,
Ni: 0.1 to 2.5%, Nb: 0.01 to 0.5%,
V: 0.05-0.5%, B: 0.0005-0.005%
The Al-containing heat-resistant ferritic stainless steel sheet having excellent workability and oxidation resistance as described in (5) above, comprising one or more of the above.
[0017]
(7) By mass%
Mg: 0.0005 to 0.005%, Ca: 0.0005 to 0.005%,
REM: 0.001 to 0.01%
The Al-containing heat-resistant ferritic stainless steel sheet having excellent workability and oxidation resistance as described in (5) or (6) above, comprising one or more of the above.
[0018]
(8) The slab or ingot comprising the component according to any one of (1) to (3) is hot-rolled and cold-rolled and finally annealed at 900 to 1000 ° C. ) To (3) A method for producing an Al-containing heat-resistant ferritic stainless steel sheet having excellent workability and oxidation resistance.
[0019]
(9) The Al-containing heat-resistant ferritic stainless steel having excellent workability and oxidation resistance according to (8), wherein hot-rolled sheet annealing is performed at 800 to 1000 ° C. after the hot rolling and before cold rolling. A method of manufacturing a steel sheet.
[0020]
(10) After the hot-rolled sheet annealing, cold rolling with a rolling reduction of 40% or more is performed, and final annealing is performed. The Al-containing heat-resistant excellent workability and oxidation resistance according to (9) above Manufacturing method of ferritic stainless steel sheet.
[0021]
(11) Al excellent in workability and oxidation resistance according to (8) or (9), wherein intermediate annealing is performed at 800 to 1000 ° C. after the cold rolling, and final cold rolling is performed. Method for producing heat-resistant ferritic stainless steel sheet.
[0022]
(12) The method for producing an Al-containing heat-resistant ferritic stainless steel sheet having excellent workability and oxidation resistance according to (11), wherein the rolling reduction of the final cold rolling is 40% or more.
[0023]
(13) After the final annealing, pickling in a pickling solution having a hydrofluoric acid concentration of 20 to 100 g / l, a nitric acid concentration of 40 to 150 g / l, and a liquid temperature of 20 to 60 ° C. A method for producing an Al-containing heat-resistant ferritic stainless steel sheet having excellent workability and oxidation resistance according to any one of 8) to (12).
[0024]
(14) The method for producing a stainless steel plate according to (4), wherein after annealing and pickling, in an atmosphere containing 0.0001 to 0.1% by volume of oxygen composed of argon and / or nitrogen, 600 to The method for producing an Al-containing heat-resistant ferritic stainless steel sheet excellent in workability and oxidation resistance according to any one of (8) to (13), wherein heating is performed at 900 ° C. for 1 to 60 minutes.
[0025]
(15) From the argon and / or nitrogen, the slab or ingot comprising the component according to any one of (5) to (7) is hot-rolled, hot-rolled sheet annealed, cold-rolled, final annealed, pickled. The processing according to any one of (5) to (7) above, wherein heating is performed at 600 to 900 ° C. for 1 to 60 minutes in an atmosphere containing 0.0001 to 0.1% by volume of oxygen. Of Al-containing heat-resistant ferritic stainless steel sheet with excellent heat resistance and oxidation resistance.
[0026]
(16) A slab or ingot comprising the component according to any one of (5) to (7) above is hot-rolled, hot-rolled sheet annealed, cold-rolled, and finally annealed as 0 consisting of argon and / or nitrogen. The workability as set forth in any one of (5) to (7) above, which is heated at 800 to 1000 ° C. for 1 to 60 minutes in an atmosphere containing oxygen of 0.0001 to 0.1% by volume, A method for producing an Al-containing heat-resistant ferritic stainless steel sheet having excellent oxidation resistance.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
The inventor examined the workability of 18Cr-3Al-0.25Si steel (equivalent to SUH21 steel), which is an Al-containing ferritic stainless steel with good oxidation resistance. As a result, it was found that the lack of sufficient grain growth in the final annealing was the cause of the decrease in workability. Therefore, it is considered that workability is improved by performing the final annealing temperature at a high temperature.
[0028]
However, when the final annealing of Al-containing ferritic stainless steel is performed at a high temperature, Al 2 O 3 This causes a problem that an oxide film containing s is formed and it becomes difficult to remove the scale in the pickling process.
[0029]
In order to solve such a problem, C and N are reduced, Ti is added to improve purity, and the combination of reduction of Al amount and optimization of Si addition amount is combined without impairing oxidation resistance. The aim was to develop an Al-containing heat-resistant ferritic stainless steel that was improved in workability by lowering the recrystallization temperature.
[0030]
The present inventor examined the influence of Si on the workability of highly purified Al-containing ferritic stainless steel.
[0031]
0.1-1.5% Si is added to 18Cr-2Al ferritic stainless steel with C and N added and Ti added, and the direction of 0 ° (referred to as L direction) with respect to the rolling direction. ), A tensile test piece having a direction of 45 ° (referred to as D direction) and a direction of 90 ° (referred to as C direction) as a longitudinal direction was collected in accordance with JIS Z 2201, and a tensile test was performed in accordance with JIS Z 2241. In addition, the Rankford value (referred to as r value) was measured in accordance with JIS Z 2254.
[0032]
The minimum value of the elongation at break in the L direction, D direction, and C direction with respect to the Si amount is El. min FIG. 1 shows the minimum value of r as r min Is shown in FIG. As shown in FIGS. 1 and 2, when the Si content is in the range of 0.3 to 0.8%, El min Is 25% or more, r min Becomes 1.0 or more, especially r min Was found to be improved by the addition of Si, and when it added over 0.5%, it was found that there was a tendency to decrease gradually.
[0033]
Moreover, the recrystallization temperature was lowered by the optimization of the Si amount, and the annealing temperature could be lowered. As a result, the pickling property is greatly improved, and the oxide scale can be removed without any problem. By combining these components and manufacturing methods, we have succeeded in developing an Al-containing heat-resistant ferritic stainless steel that can achieve both excellent workability and oxidation resistance.
[0034]
Further, the inventor has focused on a surface film that exhibits oxidation resistance, and has studied a method for improving the oxidation resistance by modifying the surface film without increasing the amount of Al in the alloy components. In a ferritic stainless steel sheet with an Al content of less than 3%, when heat treatment is performed in a harsh oxidizing atmosphere such as in exhaust gas, not only Al but Fe, Cr, etc. are oxidized, and the amount of Al in the film is reduced. Reduces oxidation resistance.
[0035]
In order to suppress the oxidation of Fe and Cr, the present inventor has focused on the oxygen content in the heat treatment atmosphere and conducted detailed studies. As a result, it was found that when the Al-containing ferritic stainless steel was heat-treated in an atmosphere containing a small amount of oxygen, the oxidation resistance was improved.
[0036]
The oxide film formed on the surface of the steel having excellent oxidation resistance was analyzed by glow emission spectroscopy (GDS). As a result, the amount of Al in the film is Al 2 O 3 It has been found that even if it is lower than the stoichiometric composition, it is effective for improving the oxidation resistance.
[0037]
Based on this knowledge, the present inventors have completed the present invention by examining the alloy components having both oxidation resistance and workability and heat treatment conditions for forming an oxide film.
[0038]
Hereinafter, the present invention will be described in detail.
[0039]
C is an unavoidable impurity contained in the steel, and if contained excessively, workability and weldability deteriorate, and Al during high temperature heating 2 O 3 The film formation becomes unstable. Therefore, the lower the amount of C, the better. However, refining costs increase to reduce the amount of C excessively.
[0040]
When the Al-containing heat-resistant ferritic stainless steel of the present invention is used as a combustion equipment member, the C content is set in the range of 0.001 to 0.015%. Moreover, when using for a catalyst carrying member, C amount was made into the range of 0.002-0.02%.
[0041]
N, like C, is an unavoidable impurity contained in steel, and the lower the content, the better. However, refining costs increase for excessive reduction.
[0042]
When N content exceeds 0.02%, workability and weldability decrease, and Al during high temperature heating 2 O 3 The film formation becomes unstable. Therefore, the upper limit of the N amount is set to 0.02% or less.
[0043]
When the Al-containing heat-resistant ferritic stainless steel of the present invention is used for a combustion equipment member, the lower limit of the N amount needs to be 0.002% or more. It is preferable to set it to 002% or more.
[0044]
When the Al-containing heat-resistant ferritic stainless steel of the present invention is used for a combustion equipment member, it is necessary to limit the total amount of C and N to 0.02% or less in order to prevent deterioration of workability. . The lower limit of C + N is set to 0.003% or more, which is the sum of the lower limit of C and the lower limit of N, in consideration of refining costs.
[0045]
Si is an extremely important element when the Al-containing heat-resistant ferritic stainless steel of the present invention is applied to combustion equipment materials. Si is a highly protective Al material in a high-temperature oxidizing atmosphere. 2 O 3 Promotes film formation and contributes to improved oxidation resistance. On the other hand, when it contains excessively, workability and weldability will fall remarkably.
[0046]
In order to improve oxidation resistance and obtain good workability, the addition of 0.3 to 0.8% is preferable, and the addition of 0.4 to 0.6% is optimal.
[0047]
Si is also a useful element when used as a catalyst support member, and can be added as a substitute for Al. The addition of Si makes it possible to reduce the Al content necessary for maintaining the catalytic function in the engine exhaust gas environment.
[0048]
In order to exhibit such effects, it is necessary to add 0.05% or more of Si. On the other hand, the elongation is reduced by adding more than 1.0% of Si. Therefore, when used as a catalyst-carrying member, the amount of Si added is in the range of 0.05 to 1.0%.
[0049]
Mn is an element that deteriorates workability, and excessive addition inhibits formation of an oxide film having good oxidation resistance. When the Al-containing heat-resistant ferritic stainless steel of the present invention is used for a combustion equipment member, the upper limit needs to be 1.0%, and the preferable upper limit is 0.5%.
[0050]
When used as a catalyst-carrying material, 3.0% was made the upper limit in order to prevent deterioration of oxidation resistance. On the other hand, the lower the lower limit of the Mn content, the better. However, considering the refining cost, the preferable lower limit is 0.01%, and the more preferable lower limit is 0.1%.
[0051]
P is an impurity inevitably contained in the steel, and if it exceeds 0.04%, the workability deteriorates, so 0.04% was made the upper limit. Refining costs increase when the P content is less than 0.01%, so 0.01% is preferable as the lower limit.
[0052]
S is an impurity inevitably contained in the steel, and if it exceeds 0.02%, the formation of an oxide film on the surface with good oxidation resistance is remarkably inhibited, so 0.02% was made the upper limit. . The lower the lower limit of the amount of S, the better. However, refining costs increase to make it less than 0.0001%. Considering the refining cost, the more preferable lower limit of the amount of S is 0.005%.
[0053]
Cr is an important element for improving the corrosion resistance and improving the adhesion of the oxide film on the surface having good oxidation resistance. However, excessive addition significantly reduces weldability and workability.
[0054]
When the Al-containing heat-resistant ferritic stainless steel of the present invention is used for a combustion equipment member, it is necessary that the Cr content be in the range of 13 to 20%. Moreover, when using for a catalyst support member, Cr amount was made into the range of 10-25%.
[0055]
Al is excellent in oxidation resistance, Al 2 O 3 It is an essential element of the present invention for forming an oxide film containing. However, workability, weldability, and pickling properties are impaired by excessive addition. When the Al-containing heat-resistant ferritic stainless steel of the present invention is used for a combustion equipment member, it is necessary that the Al content be 1.5 to less than 2.5%.
[0056]
When used for a catalyst-carrying member, if the Al content is less than 1.0%, the formation of an oxide film is insufficient even if Si is added, so the lower limit was made 1.0%. Further, the upper limit of the Al amount is less than 3.0%.
[0057]
Ti is an important element for fixing C and N, which are harmful to workability and oxidation resistance, as carbonitride, improving adhesion of oxide film, and ensuring workability and corrosion resistance of welds. is there. This effect is insufficient when the Ti content is less than 3 times the content of (C + N). On the other hand, when Ti more than 20 times the content of (C + N) is added, the solid solution Ti increases and the workability deteriorates.
[0058]
Therefore, the Ti addition amount needs to be 3 to 20 times the content of (C + N). Moreover, the range of preferable Ti addition amount is 8 to 15 times of (C + N).
[0059]
Although Cu is an element that improves corrosion resistance, it has a remarkable effect of reducing workability. Therefore, in order to apply the Al-containing heat-resistant ferritic stainless steel of the present invention to combustion equipment members, the upper limit is limited to 0.5%. There is a need. Further, the lower limit is preferred, but if it is less than 0.1%, the refining cost increases, so 0.1% or more is preferred.
[0060]
When the Al-containing heat-resistant ferritic stainless steel of the present invention is applied to a catalyst support member, 0.1% or more of Cu may be added in order to improve corrosion resistance. On the other hand, if Cu is added in excess of 2.5%, weldability and workability deteriorate, so the upper limit is preferably made 2.5%.
[0061]
Furthermore, you may add 1 type (s) or 2 or more types of Mo, Ni, Nb, V, B, Mg, Ca, and REM as needed.
[0062]
When Mo is added in an amount of 0.1% or more, the high-temperature strength is improved, but if added over 2.5%, workability and weldability are lowered. Therefore, the addition amount of Mo is preferably in the range of 0.1 to 2.5%.
[0063]
Ni improves the workability by adding 0.1% or more, but if added over 2.5%, a martensite phase is generated in the welded portion, and the welded workability is lowered. Therefore, the addition amount of Ni is preferably in the range of 0.1 to 2.5%.
[0064]
Nb and V have the effect of fixing C and N in the same manner as Ti. Nb is added by 0.01% or more, and V is added by 0.05% or more. Addition of Nb and V exceeding 0.5% degrades workability and weldability. Therefore, it is preferable that the amount of Nb added is in the range of 0.01 to 0.5% and the amount of V is in the range of 0.05 to 0.5%.
[0065]
When B is added in an amount of 0.0005% or more, there is an effect of preventing secondary processing cracks during molding, but if over 0.005% is added, the weldability decreases. Therefore, the addition amount of B is preferably in the range of 0.0005 to 0.005%.
[0066]
Mg, Ca, and REM are elements that improve the adhesion of the oxide film on the surface and refine the welded structure to improve the workability of the welded part. Moreover, weldability is impaired.
[0067]
Therefore, it is preferable that the Mg amount is 0.0005 to 0.005%, the Ca amount is 0.0005 to 0.005%, and the REM amount is 0.001 to 0.01%.
[0068]
When the Al-containing heat-resistant ferritic stainless steel of the present invention is used for a combustion equipment member, it is necessary to define the microstructure, elongation, and r value in order to improve workability.
[0069]
The microstructure must be sufficiently recrystallized, and the crystal grain size number was in the range of 7-10. This is because if the crystal grain size number is less than 7, the crystal grains are too large, so that rough skin is likely to occur during processing, and if it exceeds 10, the required processability cannot be obtained. The crystal grain size number may be measured according to JIS G 0552.
[0070]
Regarding the elongation and the r value, in the L direction, the D direction, and the C direction, the elongation at break measured according to JIS Z 2241 and the minimum value of the r value measured according to JIS Z 2254 are important for determining workability. It is an indicator.
[0071]
Minimum value of elongation El min Is 25% or more and the minimum r value r min When it is 1.0 or more, sufficient workability can be secured. El min , R min Is better, but El min Over 40%, r min In order to make the value over 2.5, the manufacturing cost is remarkably increased.
[0072]
In order to use the Al-containing heat-resistant ferritic stainless steel of the present invention as a catalyst-carrying member, it is necessary to have a surface film excellent in oxidation resistance.
[0073]
If the Al content in the oxide film is less than 15%, it is difficult to prevent the diffusion of Fe and Cr in an oxidizing atmosphere such as in exhaust gas, and it becomes difficult to maintain an oxide film having sufficient oxidation resistance. Slightly deteriorates. Therefore, the Al content in the oxide film is preferably 15% or more.
[0074]
Moreover, when the amount of Al in the oxide film exceeds 50%, the film is easily peeled off, so the upper limit is preferably 50%.
[0075]
If the thickness of the oxide film is less than 0.03 μm, the oxidation resistance is insufficient. On the other hand, when the thickness of the oxide film exceeds 0.5 μm, not only the effect is saturated, but also the oxide film is easily peeled off. Therefore, the thickness of the oxide film needs to be in the range of 0.03 to 0.5 μm.
[0076]
When such a surface film is formed, even if the Al amount is about 1%, the oxidation resistance is remarkably improved. Therefore, even when used as a combustion equipment member containing an Al amount of less than 1.5 to 2.5%, it is preferable to form an oxide film having an Al amount of 15% or more on the surface.
[0077]
The Al content and thickness of the surface film can be measured by glow emission spectroscopy (referred to as GDS). This is a method for calculating the amount of Al and the thickness of the surface film from the time obtained by GDS and a profile (referred to as GDS profile) showing the emission intensity of Fe, Al, oxygen and the like.
[0078]
The production method of the Al-containing stainless steel of the present invention is melting, casting, hot rolling, and thereafter hot-rolled sheet annealing, pickling, cold rolling, final annealing or cold rolling, intermediate annealing, acid Washing, final cold rolling, and final annealing are performed and pickling is performed.
[0079]
Further, the heat treatment may be performed in an argon atmosphere containing a small amount of oxygen, a nitrogen atmosphere, or a mixed atmosphere of argon and nitrogen.
[0080]
In the method for producing Al-containing stainless steel used for combustion equipment members, final annealing is extremely important in order to improve workability and ensure pickling properties. If the temperature of final annealing is less than 900 degreeC, sufficient recrystallization will not be performed but required workability will not be expressed.
[0081]
On the other hand, if the temperature exceeds 1000 ° C., the oxide scale becomes strong, and it is difficult to remove the scale in the pickling step, which is not preferable. Therefore, the final annealing temperature needs to be in the range of 900 to 1000 ° C.
[0082]
In order to further improve workability, in the manufacturing process after hot rolling, hot rolled sheet annealing, pickling, cold rolling, final annealing or cold rolling, intermediate annealing, final cold rolling, final annealing, etc. It is preferable to perform annealing before and after cold rolling. Both hot-rolled sheet annealing and intermediate annealing may be performed.
[0083]
The annealing temperature for hot-rolled sheet annealing and intermediate annealing is preferably in the range of 800 to 1000 ° C. This is because if the annealing temperature is less than 800 ° C., the required workability is difficult to obtain, and if it exceeds 1000 ° C., the crystal grains become too large and rough skin is likely to occur during processing.
[0084]
In cold rolling, if the intermediate annealing is not performed, the total cold rolling reduction ratio may be 40% or more, and if the intermediate annealing is performed, the cold rolling reduction ratio of the final cold rolling (referred to as the final cold rolling reduction ratio) may be 40% or more. preferable.
[0085]
This is because when the total cold rolling reduction ratio or final cold rolling reduction ratio is less than 40%, the introduction of strain is insufficient and non-uniform, and recrystallization due to final annealing may be delayed and workability may be reduced. Because. Particularly preferable conditions are a total cold rolling reduction ratio of 60% or more and a final cold rolling reduction ratio of 50% or more.
[0086]
When the total cold rolling reduction ratio or the final cold rolling reduction ratio exceeds 95%, not only the effect is saturated, but also the load on the rolling roll is significantly increased. Therefore, the upper limit of the total cold rolling reduction rate or the final cold rolling reduction rate is preferably 95%.
[0087]
The total cold rolling reduction ratio is the percentage obtained by dividing the difference between the thickness of the hot rolled sheet and the thickness of the cold rolled sheet after the final cold rolling (called the final cold rolled sheet) by the thickness of the hot rolled sheet. It is represented by. The thickness of the hot-rolled sheet may be measured after hot-rolled sheet annealing, and the thickness of the final cold-rolled sheet may be measured after final annealing or after pickling.
[0088]
In addition, the final cold rolling reduction ratio is obtained by dividing the difference between the thickness of the cold rolled sheet before intermediate annealing or after the intermediate annealing (intermediate cold rolled sheet) and the thickness of the final cold rolled sheet by the thickness of the intermediate cold rolled sheet. The value is expressed as a percentage.
[0089]
The pickling after the final annealing is composed of a salt, a nitric acid electrolytic tank, and a nitric hydrofluoric acid tank. In the nitric hydrofluoric acid tank, the hydrofluoric acid concentration is 20 to 100 g / l, the nitric acid concentration is 40 to 150 g / l, liquid The temperature is preferably 20 to 60 ° C.
[0090]
This is because when the hydrofluoric acid concentration is less than 20 g / l and the nitric acid concentration is less than 40 g / l, oxidation scale removal is insufficient, and coloration due to the remaining scale occurs. The hydrofluoric acid concentration exceeds 100 g / l and the nitric acid concentration is low. If it exceeds 150 g / l, the surface of the base material will be rough due to excessive pickling, and if the liquid temperature of the pickling solution is less than 20 ° C., pickling becomes insufficient and scale residue tends to occur. This is because rough skin is conspicuous due to excessive pickling.
[0091]
In the production of the Al-containing heat-resistant ferritic stainless steel of the present invention used as a catalyst support member, after final annealing and pickling, an argon gas containing a small amount of oxygen and a nitrogen gas are used to form a surface film having excellent oxidation resistance. It is necessary to perform the heat treatment in any atmosphere of a mixed gas of argon and nitrogen.
[0092]
The oxygen content in the atmosphere is preferably 0.0001 to 0.1% by volume. This is because when the oxygen content is 0.0001% by volume or less, the oxide film cannot be sufficiently grown, and when it exceeds 0.1% by volume, the amount of Al in the oxide film decreases and the oxidation resistance decreases. It is.
[0093]
H 2 In a reducing atmosphere containing gas, an oxide film is not formed during heat treatment, which is not preferable. An atmosphere in which a small amount of oxygen is introduced into the vacuum may be used, but equipment such as a vacuum chamber is required.
[0094]
As an atmosphere composed of argon and / or nitrogen containing a small amount of oxygen, an argon gas cylinder, a nitrogen gas cylinder, or a mixed gas cylinder of argon and nitrogen whose oxygen content is adjusted in advance may be used. Alternatively, nitrogen gas may be mixed by adjusting the flow rate of each gas while measuring the oxygen concentration with an oxygen concentration meter using a gas mixer or the like.
[0095]
The heat treatment in an atmosphere composed of argon and / or nitrogen containing a trace amount of oxygen needs to be performed at 600 to 900 ° C. This is because if the heat treatment temperature is less than 600 ° C., a good oxide film is not formed, and if it exceeds 900 ° C., the crystal grain size of the steel sheet becomes coarse and properties such as workability are slightly deteriorated.
[0096]
In addition, if the heat treatment time is less than 1 minute, the effect of the heat treatment varies, and if it exceeds 60 minutes, the crystal grain size of the steel sheet becomes coarse and the properties such as workability are slightly deteriorated, so that the heat treatment time may be 1 to 60 minutes. preferable. In addition, the upper limit of the heat treatment time preferable from the viewpoint of workability is 30 minutes or less.
[0097]
In addition, when heat treatment is performed after the polishing finish, a surface film having a higher Al content in the film is formed, which is preferable. Furthermore, the heat treatment according to the present invention may be performed after the Al-containing steel plate is assembled into a part such as a muffler.
[0098]
Instead of the heat treatment step for forming the oxide film on the surface, the final annealing is performed in an argon atmosphere containing 0.0001 to 0.1% by volume of oxygen, a nitrogen atmosphere, or a mixed atmosphere of argon and nitrogen, and then pickling is performed. May be omitted. In this case, the temperature range needs to be 900 to 1000 ° C.
[0099]
Even when the Al-containing heat-resistant ferritic stainless steel of the present invention is used as a combustion equipment member, it is preferable to perform heat treatment in an argon atmosphere containing a trace amount of oxygen, a nitrogen atmosphere, or a mixed atmosphere of argon and nitrogen. The heat treatment conditions may be the same as those applied to the catalyst-carrying member.
[0100]
【Example】
Further, the present invention will be described in detail by way of examples.
[0101]
(Example 1)
A 50 kg steel ingot having the chemical components shown in Table 1 was melted and hot rolled to obtain a hot rolled plate having a thickness of 4 mm.
[0102]
[Table 1]
Then, it cold-rolled into a 2 mm-thick steel plate, subjected to intermediate annealing that was held at 900 ° C. for 1 minute, and pickled. Thereafter, the steel sheet was further made into a 1 mm thick steel plate by final cold rolling, subjected to final annealing that was held at 950 ° C. for 1 minute, and then pickled.
[0104]
After the intermediate annealing, the pickling after the final annealing is performed in the order of salt, electrolytic nitric acid, and nitric hydrofluoric acid. The nitric hydrofluoric acid nitric acid and hydrofluoric acid liquid composition is 50 g / l hydrofluoric acid and 80 g / l nitric acid, The liquid temperature was 50 ° C.
[0105]
From these steel plates, a structure observation specimen, an oxidation specimen, and a tensile specimen were prepared. The structure observation specimen was collected from the cross-sectional position in the rolling direction at the center in the sheet width direction. As the oxidation test piece, a 20 mm square piece was collected, and the surface was mechanically polished to give a # 400 finish. The tensile test piece was a JIS Z 2201 No. 13B test piece, and the L direction, the D direction, and the C direction were formed as the long sides.
[0106]
The tissue observation test piece was mirror-polished, etched, magnified 100 times, and observed, and the particle size number was determined in accordance with JIS G 0552. In the oxidation test, a continuous oxidation test was conducted in the atmosphere at 800 ° C. and 1000 ° C. for 200 hours, and the evaluation was made based on the increase in oxidation and the presence or absence of a peeling scale.
[0107]
The increase in oxidation was measured as follows. First, a test piece that has been weighed before the test is inserted into a furnace maintained at the test temperature, and after the test time has elapsed, it is taken out of the furnace, quickly stored in a metal container with a lid, and air-cooled. The weight of the metal container with the lid is also measured in advance. After air cooling, the weight of the metal container with the lid containing the test piece is measured.
[0108]
At this time, the scale peeled off during air cooling remains in the metal container. The increase in oxidation was evaluated by dividing the value obtained by subtracting the weight of the test specimen before the test and the weight of the metal container with the lid from the weight of the metal container with the lid containing the test piece by the surface area of the sample. Here, the unit of weight is mg and the unit of surface area is cm. 2 The unit of increase in oxidation is mg / cm. 2 It is.
[0109]
The presence or absence of scale peeling was evaluated as follows. The test piece was taken out from the metal container with a lid, the appearance was visually observed, and the case where scale peeling was observed was evaluated as having scale peeling. For the case where scale peeling was not observed, the weight of the test piece after the test taken out from the metal container was measured.
[0110]
The scale peel-off amount obtained by dividing the weight of the metal container with the test piece with the lid from the weight of the test piece after the test and the weight of the metal container with the lid measured in advance by the surface area is 0.01 mg / cm 2 If it is less than, there is no scale peeling, 0.01 mg / cm 2 The above cases were evaluated as having scale peeling.
[0111]
The tensile test was performed according to JIS Z 2241, and the r value was measured according to JIS Z 2254. From the result, elongation at break, minimum value El in L direction, D direction and C direction of r value min , R min Evaluated. Furthermore, in order to evaluate workability, a cylindrical drawing test was performed.
[0112]
The test piece was a circular blank with a diameter of 100 mm, the punch diameter was 50 mm, the shoulder R was 5 mm, the die diameter was 51.6 mm, the shoulder R was 5, and Johnson wax was used as the lubricant. The test was performed 10 times, and the workability was evaluated by the number of times of drawing.
[0113]
The test results are shown in Table 2. No. 1-4 satisfy the requirements of
[0114]
[Table 2]
(Example 2)
Steel ingots having the chemical components shown in Table 3 were melted and hot-rolled into 3 mm hot-rolled plates, annealed at 900 ° C. for 1 minute, and pickled.
[0116]
[Table 3]
Then, the final annealing which hold | maintains at 950 degreeC for 1 minute was performed as a 1 mm-thick steel plate by cold rolling, and it pickled on the same conditions as Example 1, and manufactured the steel plate. After pickling, the presence or absence of scale residue was evaluated visually.
[0118]
From these steel plates, similarly to Example 1, a structure observation test piece, an oxidation test piece, and a tensile test piece were prepared, and a particle size number, an oxidation test, a tensile test, and an r value were measured.
[0119]
The test results are shown in Table 4. 8-22 are component, particle size number, El min , R min Is within the scope of the present invention and is excellent in workability and oxidation resistance.
[0120]
In particular, no. 9 is 18Cr-2Al-0.5Si steel, El min Is 29%, r min Shows excellent workability with 1.1, and the oxidation resistance is as low as 2% Al. Oxidation resistance equal to or better than 23.
[0121]
In addition, No. No. 23 is 18Cr-3Al-0.25Si steel and is SUH21 equivalent steel, and Al is higher than the range of the present invention, so that workability is insufficient.
[0122]
In addition, the Al content is lower than the range of the present invention. Nos. 24 and 25 were scale peelings observed in an oxidation test at 1000 ° C., oxidation resistance was insufficient, and Al was high. In No. 26, the workability was insufficient and the remaining scale was generated.
[0123]
[Table 4]
Example 3
The steel C in Table 1 was melted and cast under the same conditions as in Example 1 and hot rolled to obtain a 3 mm hot rolled sheet. Then, the steel plate was manufactured on the manufacturing conditions shown in Table 5.
[0125]
[Table 5]
From these steel plates, in the same manner as in Example 1, a structure observation test piece, an oxidation test piece, and a tensile test piece were prepared, and a particle size number measurement, an oxidation test, and a tensile test were performed. The results are shown in Table 6.
[0127]
[Table 6]
The results of the oxidation test are shown in No. 2 of Table 2. This is almost the same as 3, and the description is omitted. Examples of the present invention shown in Table 6 are El min Is 25% or more, r min Satisfies 1.0 or more, indicating excellent workability. Moreover, it has shown that it is an excellent steel plate without a scale remainder, rough skin, etc.
[0129]
No. Nos. 27 to 40 are hot-rolled sheets, hot-rolled sheet annealed, cold-rolled, and subjected to final annealing. Nos. 41 to 54 are subjected to hot rolling, cold rolling, intermediate annealing, final cold rolling, and final annealing. No. 55 performs both hot-rolled sheet annealing and intermediate annealing.
[0130]
No. Nos. 29 to 40 and 43 to 55 are obtained by carrying out final annealing within the scope of the present invention according to claim 8, and the grain size, El min , R min Is within the scope of the invention according to
[0131]
On the other hand, no. Since No. 27 and 41 have a low final annealing temperature, the grain growth is not sufficient and the workability is inferior. No. In Nos. 28 and 42, since the final annealing temperature was high, the crystal grain size became coarse, and scale residue occurred after pickling.
[0132]
No. Nos. 34 and 48 have scale residue due to less hydrofluoric acid and nitric acid in the pickling solution. Since 35 and 49 had too much hydrofluoric acid and nitric acid, the steel plate was roughened after pickling.
[0133]
Furthermore, no. In Nos. 36 and 50, the temperature of the pickling solution is too low, so that scale residue occurs. Nos. 37 and 51 were too hot, and the steel plate was roughened after pickling.
[0134]
Example 4
Steel ingots having chemical components shown in Table 7 were melted and hot rolled, cold rolled, annealed, and pickled to produce a steel plate having a thickness of 1 mm.
[0135]
[Table 7]
[Table 8]
Small pieces were collected from these steel plates and heat-treated under the atmosphere and heating conditions shown in Table 8. The atmosphere of the heat treatment was controlled by supplying an oxygen-argon mixed gas or oxygen-nitrogen mixed gas whose concentration was adjusted in advance from a cylinder.
[0138]
A specimen was collected from the heat-treated steel sheet, and the oxide film on the surface was analyzed by GDS in the depth direction for Fe, Cr, Al, Si, Mn, C, N, Ti, O and the added selected elements.
[0139]
From the time obtained by GDS and the GDS profile showing the emission intensity of each element, the Al content and the surface film thickness were measured. As an example of the results, GDS profiles of Al, O, Fe, and Cr are shown in FIG.
[0140]
The analysis value of the concentration of each element contained in the base material was divided by the emission intensity to obtain the sensitivity coefficient of each element. For example, the light emission intensity at 240 s in FIG. 3 is used to calculate the sensitivity coefficient.
[0141]
This is because the emission intensity of each element contained in the base material is considered to be measured because the emission intensity of Al, O, Fe, and Cr becomes constant when a 240 s GDS analysis is performed.
[0142]
Further, the depth of the concave portion (referred to as the discharge trace depth) at a position corresponding to the measurement portion on the surface of the sample after the 240 s GDS analysis was measured using a stylus type roughness meter. A value obtained by dividing the depth of the discharge trace by the measurement time, that is, 240 s was taken as the sputtering rate. The film thickness was determined from this sputtering rate and the GDS profile illustrated in FIG.
[0143]
For the calculation of the film thickness, a GDS profile of O (referred to as O profile) was used. In the O profile, the peak due to the initial discharge immediately after the start of sputtering was ignored, the subsequent plateau portion was taken as the peak value of oxygen, and the sputtering time from the start of sputtering to the half peak value of oxygen was determined.
[0144]
The film thickness was determined by multiplying the sputtering time by the sputtering rate. Note that the oxide film on the surface of the sample that was not heat-treated was several nm or less and could not be measured by GDS.
[0145]
From the steel plate after pickling and the steel plate after heat treatment shown in Table 8, No. 13B test piece was sampled according to JIS Z 2201, and a tensile test was conducted according to JIS Z 2241. Further, TIG welding was performed at the center of the 90 mm wide test piece at a welding current of 200 A and a welding speed of 2 m / min without using a filler wire.
[0146]
The weld line was set at the center of the test piece, and overhanging was performed with an Erichsen tester in accordance with JIS Z 2247. The overhanging process was repeated 10 times, and the weldability was evaluated based on the average Erichsen value and the number of occurrences of cracks of 5 mm or less.
[0147]
Further, a 20 mm square oxidation test piece was collected, a catalyst layer mainly composed of activated alumina was supported on the surface, and a heating test was performed in an engine exhaust gas atmosphere.
[0148]
In the heating test, engine exhaust gas with a residual oxygen amount (about 1%) and dew point (about 40 ° C) adjusted was introduced into an electric furnace at 700 ° C, held for 5 hours, then removed from the furnace, And the presence or absence of Fe and Cr elements in the catalyst layer were investigated, and the process was repeated until discoloration was observed.
[0149]
In this heating test, the product obtained by multiplying the number of repetitions until a discoloration on the appearance was multiplied by 5 h was defined as the exhaust gas 700 ° C. life, ○ for 50 hours or more, and x for 45 hours or less, and pass / fail was judged.
[0150]
Table 9 shows the oxide film thickness, Al content, tensile test results, formability test results, and heat test results of the steel surface after heat treatment. Table 9 shows the tensile test results, formability test results, and heat test results of the steel before heat treatment. Table 10 shows.
[0151]
[Table 9]
[Table 10]
As shown in Table 9, the invention steels according to
[0154]
On the other hand, a steel containing only 1% Al, such as AR steel, does not reach an acceptable level of oxidation resistance even by heat treatment in an atmosphere in which the amount of oxygen is within the range of the present invention. In addition, in steels such as AS and AT steels that contain 3% or more of Al and have sufficient oxidation resistance, the effect of heat treatment in an atmosphere in which the oxygen content is within the scope of the present invention is small, and oxidation resistance Sex is hardly improved.
[0155]
With respect to workability and weldability, there was no deterioration due to heat treatment in an atmosphere in which the oxygen content was within the range of the present invention, and the elongation during the tensile test and the workability of the welded portion were slightly improved.
[0156]
The steel of the present invention according to
[0157]
【The invention's effect】
According to the present invention, it is possible to provide Al-containing heat-resistant ferritic stainless steel that is excellent in workability and oxidation resistance and is suitable for a catalyst supporting member such as a combustion device member such as a heating device and a kitchen device or a muffler such as a motorcycle. , Industrial contribution is extremely high.
[Brief description of the drawings]
FIG. 1 El min It is the figure which showed the influence of the amount of Si which gives to.
FIG. 2 r min It is the figure which showed the influence of the amount of Si which gives to.
FIG. 3 is a diagram showing an example of a film analysis result by glow emission spectroscopy.
[Explanation of symbols]
a ... Al profile
b ... Al content
c ... O profile
d: Film thickness
e ... Cr profile
f ... Fe profile
Claims (16)
Ｃ ：０．００１〜０．０１５％、
Ｎ ：０．００２〜０．０２％、
Ｃ＋Ｎ：０．００３〜０．０２％、
Ｓｉ：０．３〜０．８％、
Ｍｎ：１．０％以下、
Ｐ ：０．０４％以下、
Ｓ ：０．０２％以下、
Ｃｒ：１３〜２０％、
Ａｌ：１．５〜２．５％未満、
Ｃｕ：０．５％以下
Ｔｉ：３×（Ｃ＋Ｎ）〜２０×（Ｃ＋Ｎ）％
を含有し、残部がＦｅ及び不可避的不純物からなり、ミクロ組織において結晶粒度番号が７〜１０であり、圧延方向に対して、０°、４５°、９０°方向の伸びの最小値Ｅｌｍｉｎ［％］、及び、ｒ値の最小値ｒｍｉｎが、それぞれ、Ｅｌｍｉｎ≧２５％、及び、ｒｍｉｎ≧１．０を満足することを特徴とする加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板。% By mass
C: 0.001 to 0.015%,
N: 0.002 to 0.02%,
C + N: 0.003 to 0.02%,
Si: 0.3-0.8%
Mn: 1.0% or less,
P: 0.04% or less,
S: 0.02% or less,
Cr: 13-20%,
Al: 1.5 to less than 2.5%,
Cu: 0.5% or less Ti: 3 × (C + N) to 20 × (C + N)%
The balance consists of Fe and inevitable impurities, the crystal grain size number is 7 to 10 in the microstructure, and the minimum value El min [0, 45, 90 ° elongation relative to the rolling direction El min [ %] And the minimum value r min of the r value satisfy El min ≧ 25% and r min ≧ 1.0, respectively, and Al-containing heat resistance excellent in workability and oxidation resistance Ferritic stainless steel sheet.
Ｍｏ：０．１〜２．５％、
Ｎｉ：０．１〜２．５％、
Ｎｂ：０．０１〜０．５％、
Ｖ ：０．０５〜０．５％、
Ｂ ：０．０００５〜０．００５％
の１種又は２種以上を含有することを特徴とする請求項１記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板。% By mass
Mo: 0.1 to 2.5%,
Ni: 0.1 to 2.5%,
Nb: 0.01-0.5%
V: 0.05-0.5%
B: 0.0005 to 0.005%
The Al-containing heat-resistant ferritic stainless steel sheet having excellent workability and oxidation resistance according to claim 1, comprising one or more of the following.
Ｍｇ：０．０００５〜０．００５％、
Ｃａ：０．０００５〜０．００５％、
ＲＥＭ：０．００１〜０．０１％
の１種又は２種以上を含有することを特徴とする請求項１又は２記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板。% By mass
Mg: 0.0005 to 0.005%,
Ca: 0.0005 to 0.005%,
REM: 0.001 to 0.01%
The Al-containing heat-resistant ferritic stainless steel sheet excellent in workability and oxidation resistance according to claim 1 or 2, characterized by containing at least one of the following.
Ｃ ：０．００２〜０．０２％、
Ｎ ：０．０２％以下、
Ｓｉ：０．０５〜１．０％、
Ｍｎ：３．０％以下、
Ｐ ：０．０４％以下、
Ｓ ：０．０２％以下、
Ｃｒ：１０〜２５％、
Ａｌ：１．０〜３．０％未満、
Ｔｉ：３×（Ｃ＋Ｎ）〜２０×（Ｃ＋Ｎ）％
を含有し、残部Ｆｅ及び不可避的不純物からなり、表面にＡｌ量が１５％以上であり、厚さが０．０３〜０．５μｍの酸化皮膜を有することを特徴とする、加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板。% By mass
C: 0.002 to 0.02%,
N: 0.02% or less,
Si: 0.05 to 1.0%,
Mn: 3.0% or less,
P: 0.04% or less,
S: 0.02% or less,
Cr: 10 to 25%,
Al: 1.0 to less than 3.0%,
Ti: 3 × (C + N) to 20 × (C + N)%
Workability and oxidation resistance, characterized by comprising a balance Fe and inevitable impurities, having an Al amount of 15% or more on the surface, and having an oxide film with a thickness of 0.03 to 0.5 μm Al-containing heat-resistant ferritic stainless steel sheet with excellent properties.
Ｃｕ：０．１〜２．５％、
Ｍｏ：０．１〜２．５％、
Ｎｉ：０．１〜２．５％、
Ｎｂ：０．０１〜０．５％、
Ｖ ：０．０５〜０．５％、
Ｂ ：０．０００５〜０．００５％
の１種又は２種以上を含有することを特徴とする請求項５記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板。% By mass
Cu: 0.1 to 2.5%,
Mo: 0.1 to 2.5%,
Ni: 0.1 to 2.5%,
Nb: 0.01-0.5%
V: 0.05-0.5%
B: 0.0005 to 0.005%
The Al-containing heat-resistant ferritic stainless steel sheet having excellent workability and oxidation resistance according to claim 5, comprising one or more of the following.
Ｍｇ：０．０００５〜０．００５％、
Ｃａ：０．０００５〜０．００５％、
ＲＥＭ：０．００１〜０．０１％
の１種又は２種以上を含有することを特徴とする請求項５又は６記載の加工性、耐酸化性に優れたＡｌ含有耐熱フェライト系ステンレス鋼板。% By mass
Mg: 0.0005 to 0.005%,
Ca: 0.0005 to 0.005%,
REM: 0.001 to 0.01%
The Al-containing heat-resistant ferritic stainless steel sheet excellent in workability and oxidation resistance according to claim 5 or 6, characterized by containing at least one of the following.
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