JP3598981B2 - Ferritic stainless steel sheet and its manufacturing method - Google Patents
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Description
【０００１】
【発明の属する技術分野】
この発明は、冷間圧延後の表面光沢の均一性に優れ、更には優れた加工性をも有する冷間圧延用フェライト系ステンレス鋼板に関し、またその製造方法にも関するものである。
なお、本発明においては、特に断らない限り「鋼板」は鋼帯をも含めた呼称とする。
【０００２】
一般に、フェライト系ステンレス鋼板は、その優れた外観，耐食性の故に各種の厨房用品，家庭用品，建材，自動車用部材等といった様々な用途で使用されている。
しかし、近年、フェライト系ステンレス鋼板に対する特性要求は一段と高度化してきており、優れた加工性や耐食性は元より、意匠性の観点から表面光沢に関する要求も年々厳しくなりつつある。特に、建築用部材あるいはエレベ−タやエスカレ−タの側板等といった外装材については、単なる「光沢がある」との要求のみに止まらず、表面光沢のムラや色調差が少ない所謂“表面光沢の均一性”といった非常に厳しい要求がなされるようになってきた。
【０００３】
【従来の技術】
現在、工業的に製造されるフェライト系ステンレス鋼板は、連続鋳造で得られた鋳片を熱間圧延して熱間圧延鋼帯となし、得られた熱間圧延鋼帯に焼鈍・酸洗を施してから冷間圧延にて所定の製品板厚まで圧延し、次いでこれに再結晶焼鈍を施し、その後、光沢を得るために調質圧延を施して製品とされるのが一般的である。
ただ、この場合における製品単重は通常６〜１５ｔｏｎ というオ−ダ−であるので、必要量の鋼板を製造するために繰り返される一連の製造工程においては例え同一ロットであっても製造条件にバラツキ（実生産で許容される範囲内のバラツキ）が生じるのを否めず、最終製品において鋼板の幅方向及び長手方向で表面光沢等に若干の差異が生じるのを回避しがたかった。
そのため、表面光沢の均一性が厳しく要求される用途においては、現在、熱間圧延鋼帯を冷間圧延する前に該熱間圧延鋼帯の表面をグラインダ−で手入れすることが常套的に行われており、生産性の面で少なからぬ障害となっていた。
【０００４】
一方、最近の研究により、ステンレス鋼板の表面性状に及ぼす新たな影響因子が存在することが明らかになってきた。それは、冷間圧延時にロ−ル表面に生成する“ロ−ルコ−ティング”と称されるものである。
即ち、ステンレス鋼板では一般に冷間圧延後の製品表面を高光沢に仕上げるべく冷間圧延においてゼンジマ−圧延機等を用い直径２００ｍｍ以下の所謂“小径ワ−クロ−ル”で圧延がなされる。このワ−クロ−ルは各ロットの圧延を行う度に交換されるが、圧延完了後のワ−クロ−ルの表面に黒褐色の付着物が生成するのが観察され、この黒褐色の付着物を“ロ−ルコ−ティング”と称している。
【０００５】
ロ−ルコ−ティングが圧延後のステンレス鋼板表面に及ぼす影響については、例えば「第４６回塑性加工連合講演会論文集，１９９５年，第１３９〜１４０頁」や「第４７回塑性加工連合講演会論文集，１９９６年，第２８１〜２８２頁」にも報告がなされている。
これらによると、ロ−ルコ−ティングはＦｅあるいはＣｒの酸化物を主体とした組成を有しており、圧延中に生成する鋼板表面の摩耗粉あるいは酸化膜の微細粒子が剥離してロ−ル表面に付着するものと推定されている。そして、ロ−ルコ−ティングが生成すると、これがワ−クロ−ルの表面研磨によって生じる“ロ−ル表面の微細な凹凸”を埋めて平滑化してしまうため、圧延材の表面光沢が向上するものと考えられている。
【０００６】
従って、冷間圧延時にロ−ルコ−ティングをワ−クロ−ル表面に均一かつ安定して生成させることができるならば、高光沢で均一な表面性状を有した圧延板を得ることができると予想される。
【０００７】
しかしながら、冷間圧延時におけるロ−ルコ−ティングの生成挙動は必ずしも明らかになっておらず、特にフェライト系ステンレス鋼板の圧延では、ワ−クロ−ル表面にロ−ルコ−ティングが生成しないで面光沢不良をもたらしたり、ワ−クロ−ル表面に生成するロ−ルコ−ティングが幅方向で不均一となって表面光沢ムラを生じるといった問題を完全に克服することができなかった。
なお、このようなロ−ルコ−ティングの生成不良に起因した表面光沢不良は、ＳＵＳ４３０に代表される一般的なフェライト系ステンレス鋼板に比べ、成形性や耐食性を向上させるために安定化元素としてＴｉ，Ｎｂ，Ａｌ，Ｖ等を添加した所謂“高純度フェライト系ステンレス鋼板”において発生する傾向が強い。
【０００８】
冷間圧延に使用するワ−クロ−ルの表面にロ−ルコ−ティングを安定生成させる手法として、例えば特開平８−１１７８０４号公報には、圧延油のみを希釈せずにワ−クロ−ル表面に供給してステンレス鋼板を予備圧延する方法が提案されている。そして、当該特開平８−１１７８０４号公報では、このようにして表面にロ−ルコ−ティング皮膜を生成させたワ−クロ−ルをステンレス鋼板の冷間圧延に供することによってヒ−トストリ−クスを抑制できるとしている。
【０００９】
また、特開平８−２１５７１２号公報を見ると、圧延油にアルキルアミン及びアルカノ−ルアミンのうち１種以上を添加することにより圧延油のｐＨを ８．０〜１０の範囲に制御すると共に、使用する圧延油のｐＨに応じて圧延ロ−ルの電位Ｅ （Ｖ） を〔−（ｐＨ×０．１）〕から〔１−（ｐＨ×０．１）〕の範囲内に制御することによって圧延ロ−ルに安定したロ−ルコ−ティングを生成させ、焼付き疵及びオイルピットを発生させることなく高速の冷間圧延を可能とするステンレス鋼の冷間圧延方法が開示されている。
【００１０】
更に、特開平８−２２５７９４号公報には、４０℃での粘度が７〜１５０ｃＳｔ の合成エステルを基油とすると共にエマルジョンの平均粒径を５μｍ未満とした水溶性圧延油を用いた冷間圧延方法が開示されており、これによりワ−クロ−ル表面にコ−ティング皮膜が早期にムラなく生成して優れた光沢のステンレス鋼板を得ることができるとしている。
【００１１】
しかしながら、前記特開平８−２１５７１２号公報や特開平８−２２５７９４号公報に開示されている手法では、現行の圧延油を排して特殊なものを用いる必要があり、場合によっては特別な圧延油循環供給装置も必要になるなど、少なからずコスト増を余儀なくされるものであった。また、上記特開平８−２１５７１２号公報に開示されている手法によると、圧延ロ−ルへ電位を付与し管理するための設備投資も必要となる。
一方、前記特開平８−１１７８０４号公報に開示されている手法ではステンレス鋼板の冷間圧延に先立ってステンレス鋼を用いた予備圧延を施す必要があり、工程増を招くので好ましい方法とは言えなかった。
【００１２】
上述のように、ワ−クロ−ル表面にロ−ルコ−ティングが生成すると、ロ−ル研磨痕の影響が抑えられ、また冷間圧延時に生じがちなオイルピットや焼付き疵等の欠陥が抑制されて製品表面光沢が著しく向上することが知られており、圧延油の工夫によるロ−ルコ−ティング生成手法も幾つか提案されてはいたが、それでもロ−ルコ−ティングの生成機構が必ずしも明らかになっているとは言いがたく、そのため冷間圧延時に簡易かつ安定にワ−クロ−ル表面にロ−ルコ−ティングを生成させて表面光沢の均一性に優れたフェライト系ステンレス鋼板を安価に提供する手段を見出せないでいるのが現状であった。
【００１３】
【発明が解決しようとする課題】
このようなことから、本発明が目的としたのは、従来の圧延設備，圧延手法に格別な変更を加えることなく冷間圧延時にロ−ルコ−ティングを均一かつ安定して生成させ得る簡易な手法を見出し、これにより“表面光沢の均一性”等に優れたフェライト系ステンレス鋼板を安価に提供することである。
【００１４】
【課題を解決するための手段】
本発明者等は、上記目的を達成すべく、特にこれまではそれほど着目されることのなかった“冷間圧延に供される母材の表面状態”に留意し、母材の表面状態が冷間圧延時のロ−ルコ−ティング生成に及ぼす影響を各種のフェライト系ステンレス鋼について鋭意研究を行った。その結果、
ａ） ワ−クロ−ル表面におけるロ−ルコ−ティング生成の有無には、冷間圧延に供される母材の表面酸化皮膜の組成が大きく影響しており、母材の酸化皮膜組成がある一定の条件を満足する場合は母材鋼の組成によらずほぼ安定してロ−
ルコ−ティングを生成させることができる，
ｂ） ロ−ルコ−ティングの生成に好ましいこのような母材の表面状態は、熱間圧延鋼帯の酸洗工程で使用する酸液の種類を極く一般的なものの中から選択すると共に、その濃度，温度，浸漬時間を工夫するだけで得ることができ、適正条件の酸洗を施した後は、通常の酸洗工程で一般的に行われる酸洗槽間でのブラシ研削や酸洗工程後のコイルグラインダ−を用いての表面研削等といった鋼帯表面の機械的研削を行わずに冷間圧延に供することが、均一なロ−ルコ−ティ
ングを生成させる上での重要な要件となる，
という新しい知見を得ることができた。
【００１５】
即ち、フェライト系ステンレス鋼板の冷間圧延時におけるロ−ルコ−ティング生成の有無は、当該フェライト系ステンレス鋼板の鋼組成によらず、冷間圧延に供される母材の表面酸化皮膜中のＣｒ，Ｆｅの組成比で整理すると比較的良く相関することが分かった。そして、表面酸化皮膜のうちのある酸素（Ｏ）濃度部位における「Ｃｒ／Ｆｅ比」がある特定の値以上となる領域において非常に安定してロ−ルコ−ティングが生成することを確認した。
これは、母材の表面酸化皮膜がＣｒ組成比の高いＣｒリッチな皮膜組成になると、冷間圧延時において摩耗粉が生成しやすくなったり、酸化皮膜の微細粒子がワ−クロ−ルに付着しやすくなったりするためではないかと考えられた。
【００１６】
また、このようなロ−ルコ−ティングの生成に有利な母材酸化皮膜組成を得るためには、熱間圧延フェライト系ステンレス鋼帯を酸洗する工程で、酸洗液として“硝酸”あるいは“弗硝酸のような硝酸混合液”を用い、このような酸液中で熱間圧延鋼帯の表面を一定時間以上処理すれば良いことも明らかとなった。
ここで、硝酸あるいは硝酸混合液は表面酸化皮膜の成長を促進させ、かつ皮膜中のＣｒ組成比を上げる働きがあると考えられる。
【００１７】
ただ、このような酸洗を行う場合、酸洗槽間（酸洗は複数の酸洗槽によって行われるのが通常である）に設置したブラシで鋼帯の表面研削を行ったり、酸洗後にコイルグラインダ−等で鋼帯表面を機械的に研削したりすると、酸洗によって生成したＣｒ比の高い酸化皮膜が失われてロ−ルコ−ティングの生成に悪影響を及ぼすので、これら機械的研削は厳に慎まなければならない。酸洗槽間ブラシ研削や酸洗後の機械的研削を実施しないことにより、硝酸あるいは硝酸混合液で酸洗処理された熱間圧延フェライト系ステンレス鋼帯はＣｒ比の高い酸化皮膜が生成した状態のまま冷間圧延に供されることとなって、ワ−クロ−ル表面にロ−ルコ−ティングを容易かつ安定に生成させることができる。
【００１８】
本発明は、上記知見事項等に基づいてなされたものであり、次の冷間圧延用フェライト系ステンレス鋼板並びにその製造方法を提供するものである。
▲１▼ “皮膜表層から酸素濃度が３０原子％に減少するに至るまでの範囲における皮膜中のＣｒ，Ｆｅ組成比（Ｃｒ／Ｆｅ原子比）の最大値が０．２８以上である酸化皮膜”を表面に有して成ることを特徴とする、冷間圧延後の表面光沢の均一性に優れる冷間圧延用フェライト系ステンレス鋼板。
▲２▼ 母材が、質量％にて、Ｃ：０．１０％以下，Ｓｉ：１．５ ％以下，Ｍｎ：１．５％以下，Ｐ：０．０４０ ％以下，Ｓ：０．０３０ ％以下，Ｎ：０．０５０ ％以下，Ｃｒ：１４．０〜２５．０％を含み、残部がＦｅ及び不可避的不純物である組成を有して成る、前記▲１▼記載の冷間圧延用フェライト系ステンレス鋼板。
▲３▼ 母材が、質量％にて、Ｃ：０．１０％以下，Ｓｉ：１．５ ％以下，Ｍｎ：１．５％以下，Ｐ：０．０４０ ％以下，Ｓ：０．０３０ ％以下，Ｎ：０．０５０ ％以下，Ｃｒ：１４．０〜２５．０％を含むと共に、更にＣｕ：１．５ ％以下，Ｎｂ：０．１ 〜１．０ ％，Ｔｉ：０．０３〜０．１５％，Ａｌ：０．２ ％以下及びＶ：１．０ ％以下のうちの１種又は２種以上を含有し、残部がＦｅ及び不可避的不純物である組成を有して成る、前記▲１▼記載の冷間圧延用フェライト系ステンレス鋼板。
▲４▼ 熱間圧延鋼板を酸洗処理するに際して、酸洗槽のうちの少なくとも１つの槽を硝酸あるいは硝酸混合液による酸洗槽となし、少なくともこの酸洗槽にて熱間圧延鋼板を濃度２０〜２００ｇ／Ｌ（リットル） ，温度３０〜７０℃の硝酸あるいは硝酸混合液から成る酸洗液に１５秒以上浸漬すると共に、この硝酸あるいは硝酸混合液への浸漬後は機械的表面研削を行うことなく処理製品とすることを特徴とする、前記▲１▼乃至▲３▼の何れかに記載の冷間圧延用フェライト系ステンレス鋼板の製造方法。
【００１９】
なお、 本発明では、鋼の化学成分割合を表す％は断りがない限り質量％とし、 また表面酸化皮膜中の原子比率を表す％は断りがない限り原子％とする。
以下、本発明において冷間圧延用フェライト系ステンレス鋼板の表面酸化皮膜組成，母材の化学組成，鋼板の製造条件を前記の如くに限定した理由を、必要により本発明の実施の形態をも紹介しながら説明する。
【００２０】
【発明の実施の形態】
［Ａ］ 表面酸化皮膜組成の限定理由
先にも述べたように、一般にフェライト系ステンレス鋼板は連続鋳造鋳片を熱間圧延して熱間圧延鋼帯となし、得られた熱間圧延鋼帯に焼鈍・酸洗を施してから冷間圧延にて所定の製品板厚まで圧延し、更に再結晶焼鈍を施してから光沢を得るための調質圧延を行って製品とされている。
本発明では、冷間圧延に供される酸洗後の熱間圧延フェライト系ステンレス鋼板を、“皮膜表層から酸素濃度が３０原子％に減少するに至るまでの範囲における皮膜中のＣｒ，Ｆｅ組成比（Ｃｒ／Ｆｅ原子比）の最大値が０．２８以上である酸化皮膜”を表面に有して成る構成とする。
これは、冷間圧延時におけるワ−クロ−ル表面へのロ−ルコ−ティングの生成に関し、表面酸化皮膜中のＣｒ／Ｆｅ値が高いものの方が（Ｃｒリッチな皮膜組成の方が）有利となるためである。
【００２１】
酸化皮膜中の原子比は、ＥＳＣＡ （Ｅｌｅｃｔｒｏｎ Ｓｐｅｃｔｒｏｓｃｏｐｙ ｆｏｒ Ｃｈｅｍｉｃａｌ Ａｎａｌｙｓｉｓ）やＸＰＳ （Ｘ−ｒａｙ Ｐｈｏｔｏｅｌｅｃｔｒｏｎ Ｓｐｅｃｔｒｏｓｃｏｐｙ）のような物理分析機器を用いて鋼板表面を深さ方向にスパッタリングし、各原子のＸ線強度から原子比率を求めることにより容易に測定することができる。
酸化皮膜中の原子比を確認する手法は特に規定されるものではないが、具体的な測定手順例を以下に示す。
１） 酸洗後の熱間圧延鋼板から「板厚×５ｍｍ角」の大きさに測定サンプルを切り出す。ここで、鋼板幅方向における測定サンプルの切り出し箇所は任意で良いが、鋼板の代表的な情報を得るためには幅中央部から切り出すのが好ましい。
２） 酸化皮膜中のＦｅ，Ｃｒ，Ｏの原子比率を測定するため、ＥＳＣＡにて表面酸化皮膜の分析を行う。スパッタリングガスは例えばＡｒガスを用いることができる。また、スパッタリングは、例えば３秒ピッチで合計６０秒まで行うことによって目的を達成することができる。各々のスパッタリング深さにおいてＭｇＫαを表面に照射すると、各原子において固有Ｘ線が発生し、その発生したＸ線のカウント数から原子％で組成比を求めることができる。
図１，図２及び図３に、焼鈍に続いて酸洗処理を行った後の熱間圧延フェライト系ステンレス鋼板から測定サンプルを切り出し、その酸化皮膜中Ｆｅ， Ｃｒ，Ｏについて各スパッタリング深さでの原子％の組成比を求め、深さ方向における原子比の変化状態を調べた結果例を示す。なお、図１は後述する実施例の試験番号２の例、図２は試験番号８の例、図３は試験番号１２の例（比較例） である。
３） 上記のように測定した各深さでのＦｅ，Ｃｒ，Ｏの原子比率からＣｒ／Ｆｅ比を算出する。
【００２２】
なお、スパッタリングは少なくとも“酸化皮膜表層から酸素（Ｏ）濃度が３０原子％に減少するに至るまでの範囲”について行えば良い。なぜなら、ワ−クロ−ルへのロ−ルコ−ティングの生成に影響を及ぼすのは酸化皮膜層のこの範囲の部位だからである。
一般的なＡｒガスを用いたスパッタリングであれば、最低６０秒までスパッタを行えば上記皮膜層範囲の組成比は十分に測定が可能である。
【００２３】
ところで、単に酸化皮膜層の深さ方向に原子比率を求めるだけではロ−ルコ−ティング生成に有利な皮膜組成は明らかでないが、図４で示すように、Ｃｒ／Ｆｅ原子比をＯ濃度（原子％）との関係で整理すると、ロ−ルコ−ティングの生成に有利な皮膜組成を明らかに把握することができる。
即ち、ＥＳＣＡを用いてＦｅ，Ｃｒ，Ｏの深さ方向の原子比を求めると「ワ−クロ−ルにロ−ルコ−ティングが生成するもの」と「ワ−クロ−ルにロ−ルコ−ティングが生成しないもの」とでその表面酸化皮膜に明確な差異は認められないが、Ｏ濃度とＣｒ／Ｆｅ原子比で測定結果を整理し、これと冷間圧延の結果とを照らし合わせると、ワ−クロ−ルにロ−ルコ−ティングの生成しやすい皮膜組成が明らかに浮かび上がる。
【００２４】
つまり、“皮膜表層から酸素濃度が３０原子％に減少するに至るまでの範囲における酸化皮膜中Ｃｒ／Ｆｅ原子比の最大値が０．２８に満たないと、母材表面の活性度が低下して冷間圧延時にロ−ルコ−ティングの生成に有効な摩耗粉や酸化皮膜の微細粒子が生成しにくくなり、ロ−ルコ−ティングの生成がロ−ルの幅方向で不均一となったりロ−ルコ−ティングが全く生成しなくなったりするために好ましくない。
【００２５】
なお、冷間圧延時のワ−クロ−ル表面に安定してロ−ルコ−ティングを生成させるためには、表層からＯ濃度が３０原子％となる範囲の表面酸化皮膜中Ｃｒ／Ｆｅ比の最大値は０．３０以上であることが好ましい。
【００２６】
［Ｂ］ 鋼板化学組成の限定理由
冷間圧延によるフェライト系ステンレス鋼冷延板の製造性や冷間圧延後のフェライト系ステンレス鋼冷延板の加工性，耐食性の観点からは、本発明に係る冷間圧延用フェライト系ステンレス鋼板は前記▲２▼項又は▲３▼項で示した化学組成とするのが良い。
鋼板の化学組成をこのように限定する理由は次の通りである。
【００２７】
Ｃ：
Ｃは鋼を硬質化させて加工性を低下させるほか、耐食性を劣化させる元素でもあるので、その含有量をできるだけ少なくする方が良い。そして、優れた加工性を確保するためにはＣ含有量を０．１０％以下にまで低減するのが好ましい。
【００２８】
Ｓｉ：
Ｓｉは鋼の脱酸剤として有効な成分であり、また鋼の耐酸化性を向上させる作用を有している。しかし、その含有量が １．５％を超えると含有量の増加と共に鋼の硬質化が顕著となって加工性が劣化することから、Ｓｉ含有量は １．５％以下とするのが良い。
Ｍｎ：
Ｍｎにも鋼の脱酸作用があり、また高温でのスケ−ル剥離を抑制する元素でもあるので １．５％以下の範囲で含有させるのが良い。なお、Ｍｎ含有量が １．５％を超えると発錆や孔食の起点となって耐食性が低下するだけでなく、鋼のコストが高くなり経済面で不利となる。
【００２９】
Ｐ：
Ｐは鋼の耐食性，靭性を低下させる不純物元素であり、その含有量をできるだけ低くする方が望ましい。そして、Ｐ含有量が０．０４％を超えると鋼の加工性劣化が顕著化することから、その含有量を０．０４％以下とするのが良い。
Ｓ：
Ｓは鋼の発錆や孔食の起点となり耐食性を劣化させる不純物元素であり、やはりその含有量をできるだけ低くする方が好ましい。そして、Ｓ含有量が０．０３％を超えると耐食性の劣化が顕著化することから、その含有量の上限を０．０３％とするのが良い。
【００３０】
Ｎ：
ＮはＣと同様に鋼の強度を上昇させる元素であるため、その含有量をできるだけ低減することが望まれる。そして、鋼の加工性劣化，耐食性劣化を顕著化させないという観点からＮ含有量の上限は ０．０５０％とするのが良い。
【００３１】
Ｃｒ：
Ｃｒは、鋼に表面酸化皮膜を形成させて耐食性を向上させるための主要成分である。ロ−ルコ−ティングの生成に有効なＣｒリッチな表面酸化皮膜を得るためにはＣｒを１４％以上含有させることが望まれるが、２５％を超えて含有させると製造性が劣化し、コスト上昇を招く。
【００３２】
Ｃｕ：
Ｃｕには鋼の耐食性や加工性を向上させる効果があるため、必要により含有させるのが好ましい元素である。しかしながら、 １．５％を超えてＣｕを含有させても、前記効果が飽和するだけでなく熱間加工性の劣化を招く。
【００３３】
Ｔｉ：
ＴｉにはＣ，Ｎを析出物として固定し鋼の耐食性や加工性を向上させる効果があるため、必要により含有させるのが好ましい元素であり、それらの効果を安定して得るためにはその含有量を０．０３％以上とするのが望ましい。しかし、０．１５％を超えてＴｉを含有させるとＴｉ系の大型介在物が起因となって表面品質を劣化させるおそれがあるため好ましくない。
【００３４】
Ｎｂ：
ＮｂにもＴｉと同様にＣ，Ｎを析出物として固定し鋼の耐食性や加工性を向上させる効果があるため、必要により含有させるのが好ましい元素である。そして、これらの効果が顕著化するのはＮｂ含有量が ０．１％からであるが、 １．０％を超えて含有させると鋼が硬質化するので望ましくない。
【００３５】
Ａｌ：
Ａｌは固溶Ｎを低減し降伏点を下げて加工性を改善する効果と鋼の靭性を改善する効果を発揮する元素であるので、必要により含有させるのが好ましい元素である。しかし、Ａｌ含有量が ０．２％を超えると固溶Ａｌが靭性を低下させ、製造性が劣化するという弊害が現れる。
【００３６】
Ｖ：
ＶはＣを析出物として固定し鋼の加工性を向上させる効果のほか、鋼の焼入れ性を向上させる効果を発揮する元素であるので、必要により含有させるのが好ましい元素である。しかしながら、Ｖ含有量が １．０％を超えると鋼の強度が上昇しすぎるので好ましくない。
【００３７】
［Ｃ］ 酸洗条件の限定理由
冷間圧延に供するフェライト系ステンレス鋼板の表面に“皮膜表層から酸素濃度が３０原子％に減少するに至るまでの範囲における皮膜中のＣｒ／Ｆｅ比の最大値が０．２８以上である酸化皮膜”を安定かつ均一に生成させるためには、熱間圧延後の鋼帯の酸洗処理工程において、通常は複数の酸洗槽のうちの少なくとも１槽で硝酸あるいは硝酸混合液での酸洗を行う必要がある（勿論、 酸洗槽が１槽のみの場合はその酸洗槽で硝酸あるいは硝酸混合液を用いた酸洗を行う必要がある）。
【００３８】
ここで、「硝酸混合液」とは、脱スケ−ルを促進させる目的で硝酸にその他の酸洗液を混合したものを言う。
このような硝酸混合液としては硝酸に弗酸あるいは塩酸を加えた弗硝酸，塩硝酸が挙げられ、弗酸あるいは塩酸のそれぞれ好ましい濃度としては弗酸の場合は５〜１５０ｇ／Ｌ ，塩酸の場合は１０〜２００ｇ／Ｌ である。
【００３９】
さて、硝酸は、その他の酸洗液に比較するとステンレス鋼の表面酸化皮膜の成長を促進させ、かつＣｒリッチな皮膜を形成させる働きがある。その作用により、フェライト系ステンレス鋼熱間圧延板に硝酸あるいは硝酸混合液での酸洗を行うと冷間圧延時のワ−クロ−ルにロ−ルコ−ティングの生成に有利な組成の表面酸化皮膜が安定生成するものと考えられる。
【００４０】
また、複数ある酸洗槽のうち、硝酸あるいは硝酸混合液による酸洗を行う酸洗槽以外の槽で用いられる酸洗液としては、一般的に使用される硫酸，塩酸等が挙げられる。
本発明においては、酸洗槽の少なくとも１槽に硝酸あるいは硝酸混合液を収容し、熱間圧延後のフェライト系ステンレス鋼板を少なくともこの酸洗槽に通して酸洗を行う限りにおいては、その他の酸洗槽における酸洗は上記の硫酸，塩酸等といった任意の酸洗液を適宜用いることができる。
【００４１】
なお、本発明において、酸洗液として用いられる硝酸（硝酸混合液の場合も含む）の濃度，温度、並びにそれへの被処理鋼板の浸漬時間を、それぞれ「２０〜２００ｇ／Ｌ 」，「３０〜７０℃」並びに「１５秒以上」と定めたのは、次の理由による。
【００４２】
酸洗液の硝酸濃度：
酸洗に用いられる硝酸濃度（硝酸混合液中の硝酸濃度も含む）が２０ｇ／Ｌ に満たないと、硝酸浸漬による酸化皮膜形成の効果が十分得られず、フェライト系ステンレス鋼板母材表面に形成される酸化皮膜が“安定して冷間圧延時のワ−クロ−ルにロ−ルコ−ティングが生成する皮膜組成”とはならない。一方、酸洗液の硝酸濃度を２００ｇ／Ｌ を超える高濃度としても酸化皮膜形成の効果が飽和するばかりか、酸洗液のコストが嵩むこととなる。従って、酸洗液の硝酸濃度は２０〜２００ｇ／Ｌ と定めた。
【００４３】
酸洗液の温度：
酸洗液として用いる硝酸あるいは硝酸混合液の温度が３０℃に満たないと、フェライト系ステンレス鋼板母材表面の反応が不十分となり、形成される酸化皮膜が“安定して冷間圧延時のワ−クロ−ルにロ−ルコ−ティングが生成する皮膜組成”とはならない。一方、酸洗液として用いる硝酸あるいは硝酸混合液の温度が７０℃を超えている場合には、鋼板の浸漬によって有害ガスであるＮＯｘ の発生が著しくなり、そのためＮＯｘ 除去装置等の新たな設備投資が必要となるので好ましくない。従って、酸洗液として用いる硝酸あるいは硝酸混合液の温度は３０〜７０℃と定めた。
【００４４】
浸漬時間：
酸洗液として用いる硝酸あるいは硝酸混合液への浸漬時間が１５秒に満たないと、やはりフェライト系ステンレス鋼板母材表面に形成される酸化皮膜が“安定して冷間圧延時のワ−クロ−ルにロ−ルコ−ティングが生成する皮膜組成”とはならない。従って、フェライト系ステンレス鋼板を酸洗液として用いる硝酸あるいは硝酸混合液へ浸漬する時間は１５秒以上と定めた。
【００４５】
ところで、本発明においては、酸洗液として用いる硝酸あるいは硝酸混合液への浸漬後は、酸洗時に脱スケ−ルを補助する目的で一般に行われている酸洗槽間での鋼板表面のブラシ研削や酸洗後に鋼板表面の欠陥を除去するためあるいは表面の均一化を確保するために行われているコイルグラインダ−による手入れ等は省略しなければならない。
これは、鋼板表面を機械的に研削すると、折角硝酸あるいは硝酸混合液による酸洗で得られた表面の酸化皮膜の組成が変化してしまい、冷間圧延時にロ−ルコ−ティングが生成しにくくなるためである。
また、ブラシやコイルグラインダ−の研削目が製品表面にスジ状の欠陥として残存する可能性もあり、この点からも酸洗槽間でのブラシ研削や酸洗後のコイルグラインダ−による手入れは避けるべきである。
【００４６】
しかしながら、表面欠陥を除去するためにコイルグラインダ−による表面研削が回避できない場合は、コイルグラインダ−にて鋼板表面を研削した後に、更に本発明に係る酸洗処理（特定硝酸濃度，温度の硝酸あるいは硝酸混合液に特定時間以上浸漬して酸洗する処理）を行えば良い。このような再度の酸洗処理を行うことにより、ロ−ルコ−ティング生成に有利な表面酸化皮膜を回復させることができる。
【００４７】
続いて、本発明を実施例により更に具体的に説明する。
【実施例】
〔実施例１〕
表１に示す鋼組成を有するフェライト系ステンレス鋼を８０ｔｏｎ の電気炉で溶製し、連続鋳造法にて鋳片とし、１２００℃×３時間の加熱を行い、熱間厚により厚さ ３．０ｍｍ，幅１０００ｍｍの熱間圧延鋼帯とした。
【００４８】
【表１】
この熱間圧延鋼帯にバッチ型焼鈍炉にて８５０℃で１４時間の焼鈍を施し、次いで連続焼鈍・酸洗ラインにて焼鈍することなくショットブラストと酸洗により酸化スケ−ルの除去を行った。
酸洗ラインの酸洗槽は３槽からなり、第１槽及び第２槽は２０％Ｈ２ ＳＯ４ ，８０℃の同一条件とし、第３槽の酸洗条件を表２に示す通りに変化させた。
【００５０】
【表２】
このようにして得られた各フェライト系ステンレス鋼帯の表面酸化皮膜の組成を確認するために、各鋼帯の長手方向の中央部から切り板片を採取し、更にその幅中央の位置からＥＳＣＡに供試するサンプルを採取した。サンプルの寸法は厚さ ３．０ｍｍ×幅，長さとも５ｍｍである。
これらのサンプルについて、ＥＳＣＡにてＡｒガスで３秒ピッチで合計６０秒のスパッタリングを行い、各ピッチでのＦｅ，Ｃｒ，Ｏの原子％での組成比を求めた。そして、各深さでのＦｅ，Ｃｒの原子％からＣｒ／Ｆｅの最大値を求めた。
【００５２】
また、上記の酸洗処理を施したステンレス鋼帯を冷間圧延した際におけるロ−ルコ−ティングの生成状況を確認すべく、各鋼帯を冷間圧延に供試した。
冷間圧延は、２０段のゼンジマ−圧延機を用い、ワ−クロ−ル径９０ｍｍ，圧延速度２００ｍｐｍ の条件で母材厚さ ３．０ｍｍから製品厚さ ０．６ｍｍまで圧延した。
圧延ワ−クロ−ルは、各ステンレス鋼帯の圧延が完了する度に新ロ−ルと交換し、圧延完了後のワ−クロ−ル交換時にロ−ルコ−ティングの生成状況を目視にて確認した。
【００５３】
冷延圧延により製品厚まで圧延された各鋼帯は、その後連続焼鈍酸洗ラインにて再結晶焼鈍を施した後、酸洗処理にてスケ−ルを除去し、その後調質圧延を施すことにより冷延製品となした。
このようにして製造されたステンレス冷延鋼帯の表面光沢及び均一性を評価するために、鋼帯の幅方向及び長手方向の外観を目視で観察すると共に、先後端より切り板サンプルを採取し圧延９０°方向の光沢度の測定を行った。
これらの結果を表３に示す。
【００５４】
【表３】
表３に示される結果からも明らかなように、本発明に係る熱間圧延ステンレス鋼帯は表面酸化皮膜中のＣｒ／Ｆｅ組成比の最大値が０．２８以上となりロ−ルコ−ティングが生成しやすい皮膜構造となっている。そのため、冷間圧延においてロ−ルコ−ティングが均一に生成し、高光沢で均一性に優れる冷延製品表面を得ることができる。
また、光沢度測定結果を見ても、先端部及び後端部とも安定して光沢度５００以上の高光沢の表面となっており、先後端の光沢度の差も１５以下と非常に小さくて表面光沢の均一性にも優れていることが分かる。
【００５６】
これに対して、酸化皮膜中のＣｒ／Ｆｅ組成比の最大値が０．２８に満たない試験番号１１〜１７は、冷間圧延時にロ−ルコ−ティングが生成しなかったり、生成しても幅方向で不均一となり、そのため製品表面で光沢が不均一となっている。
光沢度で比較しても、光沢度５００以上の高い光沢を安定して得ることができず、先後端での光沢度の差も２０以上で本発明例よりも表面の均一性に劣るものである。
【００５７】
また、酸洗液温の高い試験番号１８及び酸洗液の硝酸濃度が高い試験番号１９は、共に冷延製品表面の光沢度，均一性に優れるものの、酸洗時のＮＯｘ 発生量が多くて作業環境を著しく汚染したり酸の原単位が上昇してコスト高を招くため、これらの観点から好ましいとは言えなかった。
【００５８】
〔実施例２〕
表４に示す化学組成のフェライト系ステンレス鋼を、実施例１と同じ方法で熱間圧延鋼帯とした。
【００５９】
【表４】
次に、各熱間圧延鋼帯について、バッチ型焼鈍炉あるいは連続焼鈍酸洗ラインにて表５に示す熱処理を行った後、表６に示す酸洗条件で酸洗処理を行った。
【００６１】
【表５】
【表６】
このようにして得られたフェライト系ステンレス鋼帯について、実施例１と同じ手法にて表面酸化皮膜の組成比を調査し、冷間圧延時のワ−クロ−ルコ−ティングの生成状況及び冷延製品の表面評価を行った。
【００６４】
また、得られた冷延製品の加工性を評価するため、各フェライト系ステンレス冷延鋼帯の圧延直角方向からそれぞれＪＩＳ Ｚ ２２０１に規定されるＪＩＳ１３Ｂ号試験片を採取し、ＪＩＳ Ｚ ２２４１に規定される方法で常温の引張試験を行ってその伸びを測定した。
【００６５】
更に、冷延製品の耐食性を評価するため、ＪＩＳ Ｚ ２３７１に規定される塩水噴霧試験を連続７日間行い、７日後の試験片表面を光学顕微鏡の５０倍視野にて観察して発銹の有無を評価した。
【００６６】
加えて、各製造工程毎に鋼帯の外観観察を行い、鋼帯の破断，幅部の割れ（以下“耳割れ”と称する）の発生の有無を観察し、製造性の評価とした。
これらの結果を表７に示す。
【００６７】
【表７】
表７に示される結果からも明らかな通り、本発明に係るフェライト系ステンレス鋼帯は、冷延圧延時にロ−ルコ−ティングが安定して生成し高光沢で均一性に優れる冷延製品表面を得ることができ、また製造性も良好であるほか、冷延製品とした際の加工性，耐食性にも優れていることが分かる。
【００６９】
これに対して、鋼板の化学組成が請求項２及び３に係る発明の規定範囲から外れている試験番号３３〜４２では、高光沢かつ均一性に優れる表面性状の冷延製品を得ることができるものの、製造性並びに冷延製品の加工性，耐食性を同時に良好な範囲で満足することができない。
また、酸洗条件が不適切で所要の表面酸化皮膜を確保できない試験番号４３，４４は、冷間圧延時にロ−ルコ−ティングが生成しなかったり、生成しても幅方向で不均一となり、そのため製品表面で光沢が不均一となっている。
【００７０】
【発明の効果】
以上に説明した如く、本発明によれば、酸洗条件の簡易な調整によるだけで冷間圧延時のワ−クロ−ルにロ−ルコ−ティングを均一に生成させて高光沢で光沢の均一性に優れた冷延製品を安定製造できるようにした冷間圧延用フェライト系ステンレス鋼板を提供することができ、また鋼板組成の調整をも加味することにより、高光沢でかつ光沢の均一性に優れるだけではなく、同時に優れた加工性，耐食性をも兼備した冷延製品が得られる冷間圧延用フェライト系ステンレス鋼板の提供も可能にするなど、産業上有用な効果がもたらされる。
【図面の簡単な説明】
【図１】本発明に係るフェライト系ステンレス鋼板（実施例１の試験番号２） の表面のＥＳＣＡ分析結果を示したグラフである。
【図２】本発明に係るフェライト系ステンレス鋼板（実施例１の試験番号８） の表面のＥＳＣＡ分析結果を示したグラフである。
【図３】比較例に係るフェライト系ステンレス鋼板（実施例１の試験番号１２）の表面のＥＳＣＡ分析結果を示したグラフである。
【図４】図１乃至３の結果をＣｒ／Ｆｅ原子比とＯ濃度（原子％）との関係で整理したグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a ferritic stainless steel sheet for cold rolling which has excellent uniformity of surface gloss after cold rolling and also has excellent workability, and also relates to a method for producing the same.
In the present invention, the term “steel sheet” is also used to include a steel strip unless otherwise specified.
[0002]
In general, ferritic stainless steel sheets are used in various applications such as various kitchen articles, household articles, building materials, automobile members, etc. because of their excellent appearance and corrosion resistance.
However, in recent years, the requirements for characteristics of ferritic stainless steel sheets have been further enhanced, and the requirements regarding surface gloss have been stricter year by year from the viewpoint of designability, as well as excellent workability and corrosion resistance. In particular, exterior materials such as architectural members or side plates of elevators and escalators are not limited only to the requirement of “glossy”, and so-called “surface glossiness” with little unevenness of surface gloss and color difference. Extremely stringent requirements such as "uniformity" have come to be made.
[0003]
[Prior art]
Currently, industrially manufactured ferritic stainless steel sheets are hot rolled into slabs obtained by continuous casting to form hot rolled steel strips, and the resulting hot rolled steel strips are subjected to annealing and pickling. Generally, the product is rolled to a predetermined product thickness by cold rolling, then subjected to recrystallization annealing, and then subjected to temper rolling in order to obtain luster, thereby obtaining a product.
However, since the unit weight of the product in this case is usually on the order of 6 to 15 tons, even in the same series of production steps repeated to produce the required amount of steel sheet, the production conditions vary. (Variation within the range allowed in actual production) was unavoidable, and it was difficult to avoid a slight difference in the surface gloss and the like in the width direction and the longitudinal direction of the steel sheet in the final product.
Therefore, in applications where the uniformity of surface gloss is strictly required, at present, it is customary to grind the surface of the hot-rolled steel strip with a grinder before cold-rolling the hot-rolled steel strip. This has been a considerable obstacle to productivity.
[0004]
On the other hand, recent studies have revealed that there are new influencing factors on the surface properties of stainless steel sheets. It is called "roll coating" generated on the roll surface during cold rolling.
That is, a stainless steel sheet is generally rolled in a so-called "small diameter wheel" having a diameter of 200 mm or less in a cold roll using a Sendzimir rolling mill or the like in order to finish the product surface after cold rolling with high gloss. This work roll is replaced every time rolling of each lot is performed. However, it is observed that black brown deposits are formed on the surface of the work roll after the completion of rolling, and the black brown deposits are removed. This is called "roll coating".
[0005]
Regarding the effect of roll coating on the surface of a stainless steel plate after rolling, see, for example, "The 46th Joint Lecture on Plastic Working, 1995, pp. 139-140" and "47th Joint Lecture on Plastic Working". Thesis, 1996, pp. 281-282. "
According to these, roll coating has a composition mainly composed of an oxide of Fe or Cr, and abrasion powder or fine particles of an oxide film formed on the surface of a steel sheet generated during rolling are exfoliated and rolls are formed. It is presumed to adhere to the surface. When roll coating is generated, the roll coating fills in "fine irregularities on the roll surface" caused by the surface polishing of the work roll and smoothes the roll, thereby improving the surface gloss of the rolled material. It is believed that.
[0006]
Therefore, if roll coating can be uniformly and stably generated on the surface of the roll at the time of cold rolling, a rolled plate having high gloss and uniform surface properties can be obtained. is expected.
[0007]
However, the behavior of the formation of roll coating during cold rolling is not always clear. In particular, when rolling a ferritic stainless steel sheet, no roll coating is formed on the surface of the work roll and the surface is not rolled. It has not been possible to completely overcome problems such as poor gloss and roll coating generated on the surface of the work roll, which is not uniform in the width direction, resulting in uneven surface gloss.
The poor surface gloss due to such poor roll coating generation is caused by Ti as a stabilizing element in order to improve formability and corrosion resistance as compared with a general ferritic stainless steel sheet represented by SUS430. , Nb, Al, V, etc., have a strong tendency to occur in a so-called “high-purity ferritic stainless steel sheet”.
[0008]
As a method for stably producing roll coating on the surface of a work roll used for cold rolling, for example, Japanese Patent Application Laid-Open No. HEI 8-117804 discloses a method of diluting only a rolling oil without diluting the rolling oil. A method of pre-rolling a stainless steel sheet by supplying it to the surface has been proposed. In Japanese Unexamined Patent Publication No. HEI 8-117804, the heat-stroke is obtained by subjecting the roll having the roll coating film formed on the surface thereof to cold rolling of a stainless steel plate. It can be suppressed.
[0009]
According to JP-A-8-215712, the pH of the rolling oil is controlled to be in a range of 8.0 to 10 by adding at least one of an alkylamine and an alkanolamine to the rolling oil. Rolling is performed by controlling the potential E (V) of the rolling roll within the range of [-(pH × 0.1)] to [1- (pH × 0.1)] according to the pH of the rolling oil to be rolled. There is disclosed a cold rolling method for stainless steel, which produces a stable roll coating on a roll and enables high-speed cold rolling without generating seizure flaws and oil pits.
[0010]
Further, JP-A-8-225794 discloses cold rolling using a water-soluble rolling oil having a base oil of a synthetic ester having a viscosity of 7 to 150 cSt at 40 ° C. and having an average particle diameter of less than 5 μm. A method is disclosed, whereby a coating film is formed on the surface of a work roll at an early stage without unevenness, whereby a stainless steel plate having excellent gloss can be obtained.
[0011]
However, in the method disclosed in the above-mentioned JP-A-8-215712 and JP-A-8-225794, it is necessary to discharge the existing rolling oil and use a special one. For example, a circulating supply device was required, which necessitated a considerable increase in cost. Further, according to the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. Hei 8-215712, capital investment for applying and managing a potential to the rolling roll is also required.
On the other hand, in the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 8-117804, it is necessary to perform preliminary rolling using stainless steel prior to cold rolling of a stainless steel plate, which leads to an increase in the number of steps, which is not a preferable method. Was.
[0012]
As described above, when roll coating is formed on the work roll surface, the effect of roll polishing marks is suppressed, and defects such as oil pits and seizures that tend to occur during cold rolling are reduced. It is known that the roll coating is suppressed and the surface gloss of the product is remarkably improved, and some roll coating generation methods by devising rolling oil have been proposed. However, the roll coating generation mechanism is not always required. It is hard to say that it has been clarified. For this reason, a roll coating is easily and stably generated on the surface of the roll at the time of cold rolling, and a ferritic stainless steel sheet having excellent surface gloss uniformity is inexpensive. At present, they have not been able to find a means to provide such services.
[0013]
[Problems to be solved by the invention]
In view of the above, an object of the present invention is to provide a simple rolling mill capable of uniformly and stably producing a roll coating at the time of cold rolling without making any special changes to conventional rolling equipment and rolling techniques. An object of the present invention is to find a method and thereby provide a ferritic stainless steel sheet excellent in “uniformity of surface gloss” and the like at low cost.
[0014]
[Means for Solving the Problems]
In order to achieve the above object, the present inventors have paid particular attention to the “surface condition of the base material subjected to cold rolling” which has not received much attention so far, and The effect on roll coating formation during cold rolling was studied extensively for various ferritic stainless steels. as a result,
a) The composition of the surface oxide film of the base material to be subjected to cold rolling greatly affects the presence or absence of roll coating on the surface of the work roll. When certain conditions are satisfied, the row is almost stable regardless of the composition of the base steel.
Can generate a coating,
b) The surface condition of such a base material that is preferable for the formation of roll coating is selected from the most common types of acid solution used in the pickling step of a hot-rolled steel strip, and It can be obtained only by devising its concentration, temperature, and immersion time. After pickling under appropriate conditions, brush grinding or pickling between pickling tanks generally performed in a normal pickling process A uniform roll coat can be obtained by subjecting the steel strip to cold rolling without mechanical grinding such as surface grinding using a coil grinder after the process.
Is an important requirement for generating
Was obtained.
[0015]
That is, the presence or absence of roll coating during cold rolling of a ferritic stainless steel sheet depends on the presence of Cr in the surface oxide film of the base material subjected to cold rolling regardless of the steel composition of the ferritic stainless steel sheet. , Fe, it was found that the correlation was relatively good. Then, it was confirmed that the roll coating was formed very stably in a region where the "Cr / Fe ratio" at a certain oxygen (O) concentration site in the surface oxide film was a specific value or more.
This is because if the surface oxide film of the base material has a Cr-rich film composition with a high Cr composition ratio, abrasion powder is likely to be generated during cold rolling, or fine particles of the oxide film adhere to the workpiece. It was thought that it might be easier to do.
[0016]
Further, in order to obtain a base metal oxide film composition which is advantageous for producing such roll coating, in the step of pickling a hot-rolled ferritic stainless steel strip, "nitric acid" or "nitric acid" is used as a pickling solution. It was also clarified that the surface of the hot-rolled steel strip should be treated in such an acid solution for a certain period of time or more using a "mixed solution of nitric acid such as hydrofluoric acid".
Here, it is considered that nitric acid or a mixed solution of nitric acid promotes the growth of the surface oxide film and has a function of increasing the Cr composition ratio in the film.
[0017]
However, when performing such pickling, the surface of the steel strip is ground with a brush installed between the pickling tanks (the pickling is usually performed by a plurality of pickling tanks), or after pickling. If the steel strip surface is mechanically ground with a coil grinder or the like, the oxide film having a high Cr ratio generated by the pickling is lost, which adversely affects the formation of roll coating. You have to be strict. A hot-rolled ferritic stainless steel strip pickled with nitric acid or a mixture of nitric acid is a state in which an oxide film with a high Cr ratio has been formed by not performing brush grinding between pickling tanks or mechanical grinding after pickling. As it is subjected to cold rolling as it is, roll coating can be easily and stably generated on the surface of the work roll.
[0018]
The present invention has been made based on the above findings and the like, and provides the following ferritic stainless steel sheet for cold rolling and a method for producing the same.
{Circle around (1)} “Oxide film having the maximum value of the Cr / Fe composition ratio (Cr / Fe atomic ratio) in the film in the range from the surface layer of the film until the oxygen concentration decreases to 30 atomic% is 0.28 or more” A ferritic stainless steel sheet for cold rolling, having excellent uniformity of surface gloss after cold rolling, characterized by having a surface thereof.
{Circle around (2)} The base material is C: 0.10% or less, Si: 1.5% or less, Mn: 1.5% or less, P: 0.040% or less, S: 0.030% by mass%. The ferrite for cold rolling according to the above item (1), comprising: N: 0.050% or less, Cr: 14.0 to 25.0%, the balance being Fe and an unavoidable impurity. Stainless steel sheet.
{Circle around (3)} In terms of mass%, C: 0.10% or less, Si: 1.5% or less, Mn: 1.5% or less, P: 0.040% or less, S: 0.030% by mass% Hereinafter, N: 0.050% or less, Cr: 14.0 to 25.0%, Cu: 1.5% or less, Nb: 0.1 to 1.0%, Ti: 0.03 to 0.15%, Al: 0.2% or less, and V: 1.0% or less, the composition containing one or more of the following, with the balance being Fe and inevitable impurities. Ferrite stainless steel sheet for cold rolling according to (1).
{Circle around (4)} When pickling a hot-rolled steel sheet, at least one of the pickling tanks is a pickling tank using nitric acid or a mixed solution of nitric acid. It is immersed in a pickling solution composed of a nitric acid or nitric acid mixture at 20 to 200 g / L (liter) at a temperature of 30 to 70 ° C. for 15 seconds or longer, and after immersion in the nitric acid or nitric acid mixture, mechanical surface grinding is performed. The method for producing a ferritic stainless steel sheet for cold rolling according to any one of the above (1) to (3), wherein the product is a processed product without any treatment.
[0019]
In the present invention,% representing the chemical composition ratio of steel is represented by mass% unless otherwise specified, and% representing the atomic ratio in the surface oxide film is represented by atomic% unless otherwise specified.
Hereinafter, the reasons why the surface oxide film composition of the ferritic stainless steel sheet for cold rolling, the chemical composition of the base metal, and the manufacturing conditions of the steel sheet are limited as described above, and the embodiments of the present invention will be introduced as necessary. It will be explained while doing so.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
[A] Reason for limiting composition of surface oxide film
As described above, in general, a ferritic stainless steel sheet is hot-rolled from a continuous cast slab to form a hot-rolled steel strip, and the obtained hot-rolled steel strip is annealed, pickled, and then cooled. The product is rolled to a predetermined product thickness by cold rolling, further subjected to recrystallization annealing, and then subjected to temper rolling to obtain luster.
In the present invention, the hot-rolled ferritic stainless steel sheet after pickling to be subjected to cold rolling is referred to as “Cr, Fe composition in the coating in a range from the surface of the coating until the oxygen concentration decreases to 30 atomic%. An oxide film having a maximum value of the ratio (Cr / Fe atomic ratio) of 0.28 or more is provided on the surface.
This is related to the formation of roll coating on the surface of the roll at the time of cold rolling, in which the Cr / Fe value in the surface oxide film is higher (the Cr-rich film composition is more advantageous). This is because
[0021]
The atomic ratio in the oxide film is determined by sputtering the steel sheet surface in the depth direction by using a physical analysis device such as ESCA (Electron Spectroscopy for Chemical Analysis) or XPS (X-ray Photoelectron Spectroscopy), and X of each atomic line. It can be easily measured by determining the atomic ratio from.
The method for confirming the atomic ratio in the oxide film is not particularly limited, but a specific example of the measurement procedure is shown below.
1) A measurement sample is cut out from a hot-rolled steel sheet after pickling to a size of “sheet thickness × 5 mm square”. Here, the cut-out point of the measurement sample in the steel sheet width direction may be arbitrary, but it is preferable to cut out from the center of the width in order to obtain representative information of the steel sheet.
2) In order to measure the atomic ratio of Fe, Cr and O in the oxide film, the surface oxide film is analyzed by ESCA. As the sputtering gas, for example, Ar gas can be used. The object can be achieved by performing the sputtering at a pitch of 3 seconds for a total of 60 seconds, for example. When the surface is irradiated with MgKα at each sputtering depth, a unique X-ray is generated in each atom, and the composition ratio can be determined in atomic% from the number of generated X-rays.
1, 2 and 3, measurement samples were cut out from a hot-rolled ferritic stainless steel sheet after performing an acid pickling treatment after annealing, and Fe, Cr, and O in the oxide film at each sputtering depth. The following shows an example of a result obtained by determining the composition ratio of the atomic percentage of, and examining the state of change of the atomic ratio in the depth direction. FIG. 1 shows an example of test number 2 in the embodiment described later, FIG. 2 shows an example of test number 8, and FIG. 3 shows an example of test number 12 (comparative example).
3) The Cr / Fe ratio is calculated from the atomic ratios of Fe, Cr, and O at each depth measured as described above.
[0022]
Note that the sputtering may be performed at least for "the range from the surface of the oxide film to the point where the oxygen (O) concentration is reduced to 30 atomic%". This is because it is the portion of the oxide film layer in this range that affects the formation of roll coating on the work roll.
In the case of general sputtering using Ar gas, the composition ratio in the range of the coating layer can be sufficiently measured by performing sputtering for at least 60 seconds.
[0023]
By the way, merely finding the atomic ratio in the depth direction of the oxide film layer does not clarify the film composition that is advantageous for roll coating generation, but as shown in FIG. %), It is possible to clearly understand the film composition advantageous for the formation of roll coating.
That is, when the atomic ratio of Fe, Cr and O in the depth direction is obtained by using ESCA, it is found that "roll coat is formed on the work roll" and "roll coat is formed on the work roll." Although no clear difference is recognized in the surface oxide film between "the one that does not generate the pitting" and the result of the measurement based on the O concentration and the Cr / Fe atomic ratio, and comparing this with the result of the cold rolling, The coating composition on which roll coating is liable to be formed clearly appears on the work roll.
[0024]
In other words, if the maximum value of the Cr / Fe atomic ratio in the oxide film in the range from the surface layer of the film to the point where the oxygen concentration is reduced to 30 atomic% is less than 0.28, the activity of the base material surface decreases. It is difficult to generate wear powder and fine particles of an oxide film effective in generating roll coating during cold rolling, and the generation of roll coating becomes non-uniform in the width direction of the roll. -It is not preferable because no coating is generated at all.
[0025]
In order to stably produce roll coating on the surface of the roll at the time of cold rolling, the Cr / Fe ratio in the surface oxide film in the range where the O concentration is 30 atomic% from the surface layer is determined. The maximum value is preferably 0.30 or more.
[0026]
[B] Reason for limiting chemical composition of steel sheet
From the viewpoint of the productivity of cold-rolled ferritic stainless steel sheet by cold rolling and the workability and corrosion resistance of the cold-rolled ferritic stainless steel sheet after cold rolling, the ferritic stainless steel sheet for cold rolling according to the present invention is: The chemical composition described in the above item (2) or (3) is preferably used.
The reasons for limiting the chemical composition of the steel sheet in this way are as follows.
[0027]
C:
C is an element that hardens steel to reduce workability and also deteriorates corrosion resistance. Therefore, it is better to reduce the content of C as much as possible. In order to ensure excellent workability, it is preferable to reduce the C content to 0.10% or less.
[0028]
Si:
Si is a component effective as a deoxidizing agent for steel, and has an effect of improving the oxidation resistance of steel. However, when the content exceeds 1.5%, the hardening of the steel becomes remarkable as the content increases and the workability deteriorates. Therefore, the Si content is preferably set to 1.5% or less.
Mn:
Since Mn also has a deoxidizing effect on steel and is an element that suppresses scale peeling at high temperatures, it is preferable to contain Mn in a range of 1.5% or less. If the Mn content exceeds 1.5%, not only does rusting and pitting occur, the corrosion resistance is lowered, but also the cost of steel increases, which is economically disadvantageous.
[0029]
P:
P is an impurity element that lowers the corrosion resistance and toughness of steel, and it is desirable that the content of P be as low as possible. If the P content exceeds 0.04%, the workability of the steel deteriorates remarkably. Therefore, the P content is preferably set to 0.04% or less.
S:
S is an impurity element which becomes a starting point of rusting and pitting of steel and deteriorates corrosion resistance, and it is preferable that the content of S is as low as possible. If the S content exceeds 0.03%, the deterioration of corrosion resistance becomes remarkable, so the upper limit of the content is preferably set to 0.03%.
[0030]
N:
Since N is an element that increases the strength of steel like C, it is desired to reduce its content as much as possible. The upper limit of the N content is preferably set to 0.050% from the viewpoint that the workability and corrosion resistance of the steel are not remarkably deteriorated.
[0031]
Cr:
Cr is a main component for improving the corrosion resistance by forming a surface oxide film on steel. In order to obtain a Cr-rich surface oxide film effective for the formation of roll coating, it is desired to contain Cr in an amount of 14% or more, but if it exceeds 25%, the productivity is deteriorated and the cost increases. Invite.
[0032]
Cu:
Since Cu has an effect of improving the corrosion resistance and workability of steel, it is a preferable element to be contained as necessary. However, even if Cu is contained in excess of 1.5%, not only the above effect is saturated, but also hot workability is deteriorated.
[0033]
Ti:
Since Ti has the effect of fixing C and N as precipitates and improving the corrosion resistance and workability of steel, it is a preferable element to contain Ti as necessary, and in order to obtain those effects stably, contains Ti. The amount is desirably 0.03% or more. However, if the content of Ti exceeds 0.15%, large Ti-based inclusions may cause deterioration of surface quality, which is not preferable.
[0034]
Nb:
Like Nb, Nb has the effect of fixing C and N as precipitates and improving the corrosion resistance and workability of steel. Therefore, it is preferable to include Nb as necessary. These effects become remarkable when the Nb content is from 0.1%. However, when the Nb content exceeds 1.0%, the steel hardens, which is not desirable.
[0035]
Al:
Al is an element that exhibits the effect of reducing the solute N and lowering the yield point to improve workability and the effect of improving the toughness of steel, and is therefore preferably contained as necessary. However, if the Al content exceeds 0.2%, the solute Al lowers the toughness, resulting in an adverse effect that the productivity is deteriorated.
[0036]
V:
V is an element that has an effect of fixing C as a precipitate to improve the workability of steel and also has an effect of improving the hardenability of steel. Therefore, it is preferable to include V as necessary. However, when the V content exceeds 1.0%, the strength of the steel is excessively increased, which is not preferable.
[0037]
[C] Reasons for limiting pickling conditions
An oxide film having a maximum Cr / Fe ratio of 0.28 or more in the range from the surface layer of the ferrite stainless steel sheet to the oxygen concentration reduced to 30 atomic% on the surface of the ferritic stainless steel sheet subjected to cold rolling. In order to stably and uniformly produce "", in the pickling treatment step of the steel strip after hot rolling, usually, at least one of a plurality of pickling tanks is pickled with nitric acid or a mixed solution of nitric acid. (Of course, when there is only one pickling tank, it is necessary to perform pickling using nitric acid or a mixed solution of nitric acid in the pickling tank).
[0038]
Here, the "mixed solution of nitric acid" refers to a mixture of nitric acid and another pickling solution for the purpose of accelerating descaling.
Examples of such a nitric acid mixture include hydrofluoric acid and hydrochloric nitric acid obtained by adding hydrofluoric acid or hydrochloric acid to nitric acid. Preferred concentrations of hydrofluoric acid or hydrochloric acid are 5 to 150 g / L for hydrofluoric acid, and Is 10 to 200 g / L.
[0039]
By the way, nitric acid has a function of promoting the growth of a surface oxide film on stainless steel and forming a Cr-rich film as compared with other pickling solutions. By the action, when the hot-rolled ferritic stainless steel sheet is pickled with nitric acid or a mixed solution of nitric acid, the surface oxidation of a composition advantageous for forming roll coating on the work roll during cold rolling. It is considered that the film is stably formed.
[0040]
Among the plurality of pickling tanks, examples of the pickling liquid used in a tank other than the pickling tank for pickling with nitric acid or a mixed solution of nitric acid include sulfuric acid and hydrochloric acid which are generally used.
In the present invention, nitric acid or a mixed solution of nitric acid is accommodated in at least one of the pickling tanks, and the hot-rolled ferritic stainless steel sheet is passed through at least the pickling tank to perform pickling. For the pickling in the pickling tank, any pickling solution such as the above-mentioned sulfuric acid, hydrochloric acid or the like can be appropriately used.
[0041]
In the present invention, the concentration and temperature of nitric acid (including the case of nitric acid mixture) used as the pickling solution, and the immersion time of the steel sheet to be treated therein are respectively set to "20 to 200 g / L" and "30". -70 ° C "and" 15 seconds or more "are determined for the following reasons.
[0042]
Nitric acid concentration of pickling solution:
If the concentration of nitric acid used for pickling (including the concentration of nitric acid in the mixed solution of nitric acid) is less than 20 g / L, the effect of forming an oxide film by immersing in nitric acid is not sufficiently obtained, and the nitric acid is formed on the surface of the base material of the ferritic stainless steel sheet. The resulting oxide film does not have a "film composition which stably forms roll coating on the roll during cold rolling". On the other hand, even if the concentration of nitric acid in the pickling solution is as high as 200 g / L, the effect of forming an oxide film is not only saturated, but also the cost of the pickling solution increases. Therefore, the concentration of nitric acid in the pickling solution was determined to be 20 to 200 g / L.
[0043]
Pickling solution temperature:
If the temperature of the nitric acid or the nitric acid mixture used as the pickling solution is lower than 30 ° C., the reaction on the surface of the base material of the ferritic stainless steel sheet becomes insufficient, and the formed oxide film is “stable during cold rolling. -Film composition in which roll coating is formed on the roll. On the other hand, when the temperature of nitric acid or a mixed solution of nitric acid used as a pickling solution is higher than 70 ° C., NOx, which is a harmful gas due to the x Generation is remarkable, and therefore NO x It is not preferable because new equipment investment such as a removing device is required. Therefore, the temperature of nitric acid or a mixed solution of nitric acid used as the pickling solution was determined to be 30 to 70 ° C.
[0044]
Immersion time:
If the immersion time in the nitric acid used as the pickling solution or the nitric acid mixed solution is less than 15 seconds, the oxide film formed on the base material of the ferritic stainless steel sheet is also "stably cold-rolled during cold rolling." Does not result in a film composition that causes roll coating on the roll. Therefore, the time for immersing the ferritic stainless steel sheet in nitric acid or a mixed solution of nitric acid used as a pickling solution is determined to be 15 seconds or more.
[0045]
By the way, in the present invention, after immersion in nitric acid or a mixed solution of nitric acid used as a pickling solution, a brush on the surface of a steel sheet between pickling tanks generally used for assisting descaling during pickling. After grinding or pickling, the maintenance by a coil grinder performed to remove defects on the surface of the steel sheet or to ensure the surface is uniform must be omitted.
This is because when the surface of the steel sheet is mechanically ground, the composition of the oxide film on the surface obtained by pickling with a nitric acid or a mixed solution of nitric acid changes, so that roll coating is not easily generated during cold rolling. It is because it becomes.
Also, there is a possibility that the brush or coil grinder grind marks may remain as streak-like defects on the product surface. From this point, avoid brush grinding between pickling tanks and care by a coil grinder after pickling. Should.
[0046]
However, if surface grinding by a coil grinder cannot be avoided in order to remove surface defects, the steel plate surface is ground with a coil grinder, and then the pickling treatment according to the present invention (for example, nitric acid with a specific nitric acid concentration and temperature). (A process of immersing in a nitric acid mixed solution for a specific time or more and pickling) may be performed. By performing such pickling again, a surface oxide film advantageous for roll coating can be recovered.
[0047]
Next, the present invention will be described more specifically with reference to examples.
【Example】
[Example 1]
A ferritic stainless steel having a steel composition shown in Table 1 was melted in an electric furnace of 80 tons, cast into a slab by a continuous casting method, heated at 1200 ° C. for 3 hours, and was heated to a thickness of 3.0 mm. And a hot-rolled steel strip having a width of 1000 mm.
[0048]
[Table 1]
This hot-rolled steel strip is annealed at 850 ° C. for 14 hours in a batch-type annealing furnace, and then the oxide scale is removed by shot blasting and pickling without annealing in a continuous annealing / pickling line. Was.
The pickling tank of the pickling line consists of three tanks, the first and second tanks are 20% H 2 SO 4 , 80 ° C., and the pickling conditions in the third tank were changed as shown in Table 2.
[0050]
[Table 2]
In order to confirm the composition of the surface oxide film of each of the ferritic stainless steel strips obtained in this manner, a piece of a cut plate was sampled from the center in the longitudinal direction of each steel strip, and ESCA was measured from the center of the width. A sample to be tested was taken. The dimensions of the sample are 3.0 mm thick × 5 mm wide and 5 mm long.
These samples were subjected to sputtering by Ar gas at a pitch of 3 seconds by ESCA for a total of 60 seconds, and the composition ratio of Fe, Cr, and O in atomic% at each pitch was determined. Then, the maximum value of Cr / Fe was determined from the atomic% of Fe and Cr at each depth.
[0052]
Each steel strip was subjected to a cold rolling test in order to confirm the state of roll coating generated when the pickled stainless steel strip was cold rolled.
The cold rolling was carried out from a base material thickness of 3.0 mm to a product thickness of 0.6 mm under conditions of a work roll diameter of 90 mm and a rolling speed of 200 mpm using a 20-stage Sendzimer rolling mill.
The rolling wheel is replaced with a new roll each time the stainless steel strip is completely rolled, and the roll coating is visually observed when the roll is replaced after the rolling is completed. confirmed.
[0053]
Each steel strip that has been rolled to the product thickness by cold rolling is then subjected to recrystallization annealing in a continuous annealing and pickling line, the scale is removed by pickling, and then temper rolling is performed. It became a cold rolled product.
In order to evaluate the surface gloss and uniformity of the stainless steel cold-rolled steel strip manufactured in this manner, the appearance of the steel strip in the width direction and the longitudinal direction was visually observed, and a cut plate sample was taken from the front and rear ends. The glossiness in the direction of rolling 90 ° was measured.
Table 3 shows the results.
[0054]
[Table 3]
As is clear from the results shown in Table 3, in the hot-rolled stainless steel strip according to the present invention, the maximum value of the Cr / Fe composition ratio in the surface oxide film was 0.28 or more, and roll coating was generated. It has a film structure that is easy to do. Therefore, roll coating is uniformly generated in cold rolling, and a cold rolled product surface having high gloss and excellent uniformity can be obtained.
Also, the glossiness measurement results show that both the front end and the rear end have a stable high gloss surface with a gloss of 500 or more, and the difference in gloss between the front and rear ends is very small, 15 or less. It can be seen that the uniformity of the surface gloss is also excellent.
[0056]
On the other hand, in Test Nos. 11 to 17 in which the maximum value of the Cr / Fe composition ratio in the oxide film is less than 0.28, no roll coating is generated during cold rolling, or even if roll coating is generated. It is not uniform in the width direction, so that the gloss is uneven on the product surface.
Even when compared in terms of gloss, a high gloss of 500 or more cannot be stably obtained, and the difference in gloss between the front and rear ends is 20 or more, which is inferior to the surface uniformity of the present invention. is there.
[0057]
Test No. 18 with a high pickling solution temperature and Test No. 19 with a high nitric acid concentration in the pickling solution both have excellent glossiness and uniformity on the surface of the cold-rolled product, but have NO during pickling. x Since the generation amount is large, the working environment is remarkably polluted and the basic unit of acid is increased, which leads to an increase in cost. Therefore, it is not preferable from these viewpoints.
[0058]
[Example 2]
A ferritic stainless steel having the chemical composition shown in Table 4 was used as a hot-rolled steel strip in the same manner as in Example 1.
[0059]
[Table 4]
Next, each of the hot-rolled steel strips was subjected to a heat treatment shown in Table 5 in a batch annealing furnace or a continuous annealing pickling line, and then to a pickling treatment under the pickling conditions shown in Table 6.
[0061]
[Table 5]
[Table 6]
With respect to the ferritic stainless steel strip thus obtained, the composition ratio of the surface oxide film was investigated in the same manner as in Example 1, and the state of formation of the work-coating during cold rolling and the cold rolling were performed. The surface of the product was evaluated.
[0064]
Further, in order to evaluate the workability of the obtained cold-rolled product, a JIS No. 13B test piece specified in JIS Z 2201 was sampled from a direction perpendicular to the rolling of each ferritic stainless steel cold-rolled steel strip, and specified in JIS Z 2241. The tensile test at room temperature was performed by the method described above, and the elongation was measured.
[0065]
Further, in order to evaluate the corrosion resistance of the cold rolled product, a salt spray test specified in JIS Z 2371 was continuously performed for 7 days, and after 7 days, the surface of the test piece was observed with a 50-fold visual field of an optical microscope to check for rust. Was evaluated.
[0066]
In addition, the appearance of the steel strip was observed in each manufacturing process, and the presence or absence of breakage of the steel strip and occurrence of cracks in the width portion (hereinafter referred to as “ear cracks”) was evaluated to evaluate the productivity.
Table 7 shows the results.
[0067]
[Table 7]
As is evident from the results shown in Table 7, the ferritic stainless steel strip according to the present invention has a roll-coated product surface that is stably formed at the time of cold rolling and has high gloss and excellent uniformity. It can be seen that they can be obtained, have good manufacturability, and have excellent workability and corrosion resistance when used as cold rolled products.
[0069]
On the other hand, in Test Nos. 33 to 42 in which the chemical composition of the steel sheet is out of the specified range of the inventions according to Claims 2 and 3, it is possible to obtain a cold-rolled product having high gloss and excellent surface uniformity. However, it is not possible to simultaneously satisfy the manufacturability and the workability and corrosion resistance of the cold rolled product within a favorable range.
In Test Nos. 43 and 44, in which the required surface oxide film could not be obtained due to improper pickling conditions, roll coating did not occur during cold rolling, or even if roll coating was formed, the roll coating became uneven in the width direction. Therefore, the gloss is uneven on the product surface.
[0070]
【The invention's effect】
As described above, according to the present invention, by simply adjusting the pickling conditions, roll coating can be uniformly formed on the work roll during cold rolling, thereby achieving high gloss and uniform gloss. Can provide a ferritic stainless steel sheet for cold rolling that enables stable production of cold-rolled products with excellent heat resistance.In addition, by taking into account the adjustment of the steel sheet composition, high gloss and uniformity of gloss can be achieved. Industrially useful effects are provided, such as the provision of a ferritic stainless steel sheet for cold rolling that can provide a cold-rolled product that is not only excellent but also has excellent workability and corrosion resistance.
[Brief description of the drawings]
FIG. 1 is a graph showing the results of ESCA analysis of the surface of a ferritic stainless steel sheet according to the present invention (test number 2 in Example 1).
FIG. 2 is a graph showing the results of ESCA analysis of the surface of a ferritic stainless steel sheet according to the present invention (Test No. 8 of Example 1).
FIG. 3 is a graph showing the results of ESCA analysis of the surface of a ferritic stainless steel sheet according to a comparative example (test number 12 in Example 1).
FIG. 4 is a graph in which the results of FIGS. 1 to 3 are arranged in relation to the Cr / Fe atomic ratio and the O concentration (atomic%).
Claims (4)
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| EP4119697A4 (en) * | 2020-03-12 | 2024-04-17 | Nippon Steel Stainless Steel Corp | Ferritic stainless steel and method for manufacturing same |
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| JP5684547B2 (en) * | 2010-11-26 | 2015-03-11 | 新日鐵住金ステンレス株式会社 | Ferritic stainless steel sheet for urea SCR system parts and method for producing the same |
| JP5724436B2 (en) * | 2011-02-18 | 2015-05-27 | Ｊｆｅスチール株式会社 | Stainless steel with excellent corrosion resistance |
| KR101423823B1 (en) | 2012-06-28 | 2014-07-25 | 주식회사 포스코 | Low chrome ferritic stainless steel with improved corrosion resistance and ridging property |
| CN111118510A (en) * | 2019-12-20 | 2020-05-08 | 唐山钢铁集团高强汽车板有限公司 | Manufacturing method of boron-containing pickled steel strip |
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