JP2004217996A - Ferritic stainless steel sheet superior in formability, and manufacturing method therefor - Google Patents
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Abstract
Description
【０００１】
【発明の属する技術分野】
本発明は、成形性に優れたフェライト系ステンレス鋼板及びその製造方法に関する。
【０００２】
【従来の技術】
フェライト系ステンレス鋼は耐食性に優れており、多くの用途に用いられている。しかし、オーステナイト系ステンレス鋼の代表鋼種であるＳＵＳ３０４に比べて成形性が劣るため、成形時に割れが生じると言う問題が生じて用途が限定される場合があった。
オーステナイト系ステンレス鋼は、張り出し性の指標であるｎ値が高く、張り出しで成形を可能にしている。これに対し、フェライト系ステンレス鋼の場合にはｎ値は低く、代わりに深絞り性で成形を担っている。
【０００３】
最近では、例えば特許文献１に記載のように、熱延条件等の工夫により深絞り性の指標であるｒ値を向上させる手法が検討され、平均ｒ値で２を超える鋼種が見られるようになったが、実際の成形性はＳＵＳ３０４に比べて劣っており、フェライト系ステンレス鋼は使いづらいと考えられている。
【０００４】
深絞り性、張り出し性の両者を高める手法としては、特許文献２や特許文献３のように成分を規定する方法や、特許文献４に開示されているように、熱延時に摩擦係数、圧下率、歪速度を規定する方法が知られている。しかし、いずれの方法においても成形時の割れが完全に解消することはなく、フェライト系ステンレス鋼における加工性の問題は必ずしも解決されたとはいえない。
【０００５】
【特許文献１】
特開平１０−３３０８８７号公報
【特許文献２】
特開昭５７−１９８２４８号公報
【特許文献３】
特開昭５８−６１２５８号公報
【特許文献４】
特開平７−３１０１２２号公報
【０００６】
【発明が解決しようとする課題】
本発明は、成分及び製造方法を規定することで、成形性の極めて高いフェライト系ステンレス鋼板及びその製造方法を提供することを目的としたものである。
【０００７】
【課題を解決するための手段】
本発明者らが実際の成形である円筒絞り、角筒絞り等のＦＥＭ（有限要素法）による解析を行ったところ、実際の成形には深絞り、張り出しのどちらか一方ではなく、両者の特性を、しかも全方向の平均値で向上させることが必要であることが判明した。
このときに必要な材料特性値としては、深絞り性の指標であるｒ値（板幅／板厚の歪比）の平均値Ｒ、張り出し性の指標であるｎ値（加工硬化指数）の平均値Ｎであった。なお、平均値として、圧延方向に垂直（直角）方向（Ｃ方向）、平行方向（Ｌ方向）、４５°方向（Ｄ方向）を代表して測定し、（Ｌ方向の値＋２×（Ｄ方向の値）＋Ｃ方向の値）／４を用いた。
【０００８】
本発明者らの行った実験結果において、フェライト系ステンレス鋼においても成分及び製造方法を限定することにより、Ｒ値とＮ値を向上させれば成形割れが解消し、ＳＵＳ３０４と同等以上の成形性を示す場合が認められた。成分では特にＳｉとＰ量、組織としては最終製品板の結晶粒度番号がＲ値及びＮ値向上に重要な要素であり、製造方法では冷間圧延前の平均結晶粒径が成形性に大きな影響を及ぼすことを見出した。さらに、本発明のステンレス鋼を得るには、冷間圧延の圧延率（冷延率という）を限定し、冷間圧延前後の焼鈍温度を限定することが極めて重要であることを見出した。
【０００９】
本発明は上記知見に基づいて、十分な成形性を得るためにＲ値及びＮ値が満たすべき範囲を限定するに至って完成されたものであり、その要旨とするところは以下の通りである。
（１）圧延方向、圧延に垂直方向、４５°方向の３方向に１５％の引張塑性変形を与えたときのｒ値（板幅／板厚の歪比）の平均値として下記（１），（２）式にて定義されるＲと、圧延方向、圧延に垂直方向、４５°方向の３方向に歪みを与えたときの歪み１０〜１５％におけるｎ値（加工硬化指数）の平均値として下記（３），（４）式にて定義されるＮが、Ｒ＋１０Ｎ≧３．５の関係を満足することを特徴とする成形性に優れたフェライト系ステンレス鋼板。
ｒ＝ｌｎ（Ｗ／Ｗ０ ）／ｌｎ（ｔ／ｔ０ ） ‥‥（１）
（Ｗとｔは１５％塑性変形後、Ｗ０ とｔ０ は変形前の、それぞれ試験片の幅と厚み）
Ｒ＝（ｒＬ ＋２×ｒＤ ＋ｒＣ ）／４ ‥‥（２）
（ｒＬ は圧延方向、ｒＤ は４５°方向、ｒＣ は垂直方向の、それぞれの方向について上記（１）式に従って求めたｒ値）
ｎ＝ｌｎ（σ１５／σ１０）／ｌｎ（ε１５／ε１０） ‥‥（３）
（σ１５，ε１５は１５％引張後、σ１０，ε１０は１０％引張後の、それぞれ真応力、真歪）
Ｎ＝（ｎＬ ＋２×ｎＤ ＋ｎＣ ）／４ ‥‥（４）
（ｎＬ は圧延方向、ｎＤ は４５°方向、ｎＣ は垂直方向の、それぞれの方向について上記（３）式に従って求めたｒ値）
【００１０】
（２）ｍａｓｓ％で、
Ｃ ：０．０００５〜０．０３％、Ｓｉ：０．０１〜０．５％、
Ｍｎ：０．０１〜１％、 Ｐ ：０．０４％未満、
Ｓ ：０．０００１〜０．０１％、Ｃｒ：１０〜２５％、
Ａｌ：０．００５〜０．１％、 Ｎ ：０．０００５〜０．０３％、
Ｓｉ＋１０Ｐ≦０．５
を含有し、Ｔｉ，Ｎｂ，Ｚｒの１種または２種以上を合計で、８×（Ｃ％＋Ｎ％）〜０．８％をさらに含有し、残部鉄および不可避不純物からなることを特徴とする前記（１）に記載の成形性に優れたフェライト系ステンレス鋼板。
（３）ｍａｓｓ％で、Ｍｇ：０．０００１〜０．０１％をさらに含有することを特徴とする前記（２）に記載の成形性に優れたフェライト系ステンレス鋼板。
（４）ｍａｓｓ％で、Ｂ：０．０００５〜０．００５％をさらに含有することを特徴とする前記（２）または（３）に記載の成形性に優れたフェライト系ステンレス鋼板。
（５）ｍａｓｓ％で、
Ｍｏ：０．１〜２％、 Ｎｉ：０．１〜２％、
Ｃｕ：０．１〜２％
の１種または２種以上をさらに含有することを特徴とする前記（２）乃至（４）のいずれか１項に記載の成形性に優れたフェライト系ステンレス鋼板。
（６）ｍａｓｓ％で、
Ｃｏ：０．１〜３％、 Ｖ ：０．１〜３％
の１種または２種をさらに含有することを特徴とする前記（２）乃至（５）のいずれか１項に記載の成形性に優れたフェライト系ステンレス鋼板。
（７）結晶粒度番号が６〜８の範囲であることを特徴とする前記（１）乃至（６）のいずれか１項に記載の成形性に優れたフェライト系ステンレス鋼板。
【００１１】
（８）前記（１）乃至（７）のいずれか１項に記載のフェライト系ステンレス鋼板を製造するに際し、冷間圧延前の平均結晶粒径ｄｘを８０〜４００μｍに調整し、最終製品板の平均粒径ｄｚが下記（５）式を満足することを特徴とする成形性に優れたフェライト系ステンレス鋼板の製造方法。
ｄｘ／ｄｚ≦３ ‥‥（５）
（９）前記（２）乃至（６）のいずれか１項に記載の成分からなるフェライト系ステンレス鋼の鋳片を加熱し、総圧延率が９５％以上、仕上げ温度が９００℃以下の熱間圧延を施し、熱延板焼鈍または中間焼鈍を下記（６）式で規定される温度Ｔ１［℃］で実施し、圧延率５０〜９０％の冷間圧延を行った後、下記（７）式で規定される温度Ｔ２［℃］で最終焼鈍を実施することを特徴とする前記（８）に記載の成形性に優れたフェライト系ステンレス鋼板の製造方法。
８００≦Ｔ１≦−３０−５４５７／［ｌｏｇ｛（Ｔｉ＋Ｎｂ＋Ｚｒ）×Ｃ｝−２．６］ ‥‥（６）
Ｔ１＋５≦Ｔ２≦Ｔ１＋６０ ‥‥（７）
ここで、Ｔｉ［ｍａｓｓ％］，Ｎｂ［ｍａｓｓ％］，Ｚｒ［ｍａｓｓ％］及びＣ［ｍａｓｓ％］は、それぞれＴｉ，Ｎｂ，Ｚｒ及びＣ含有量である。
【００１２】
【発明の実施の形態】
以下、本発明について詳細に説明する。
本発明のフェライト系ステンレス鋼は、平均ｒ値Ｒと平均ｎ値ＮがＲ＋１０Ｎ≧３．５の関係を満足することを特徴とする。上限は規定しないが、５．５を超えることは、現状の技術では困難である。平均ｒ値Ｒとは、板面上で圧延方向、圧延に垂直方向、４５°方向の３方向の板状引張試験片を作製し、１５％の引張塑性変形を与えたときに発生する板幅及び板厚の歪比（ｒ値）の平均値であり、ｒ値は下記（１）式で、平均ｒ値Ｒは下記（２）式で表される。
ｒ＝ｌｎ（Ｗ／Ｗ０ ）／ｌｎ（ｔ／ｔ０ ） ‥‥（１）
（Ｗとｔは変形後、Ｗ０ とｔ０ は変形前の、それぞれ試験片の幅と厚みを表す）
Ｒ＝（ｒＬ ＋２×ｒＤ ＋ｒＣ ）／４ ‥‥（２）
（ｒＬ は圧延方向、ｒＤ は４５°方向、ｒＣ は垂直方向のｒ値）
【００１３】
また、平均ｎ値Ｎとは、板面上で圧延方向、圧延に垂直方向、４５°方向の３方向の板状引張試験片を作製し、引張試験を行ったときの１０％〜１５％における加工硬化指数ｎ値の平均値であり、ｎ値は下記（３）式で、平均ｎ値Ｎは下記（４）式で表される。
ｎ＝ｌｎ（σ１５／σ１０）／ｌｎ（ε１５／ε１０） ‥‥（３）
（σ１５，ε１５は１５％引張後、σ１０，ε１０は１０％引張後の、それぞれ真応力、真歪を表す）
Ｎ＝（ｎＬ ＋２×ｎＤ ＋ｎＣ ）／４ ‥‥（４）
（ｎＬ は圧延方向、ｎＤ は４５°方向、ｎＣ は垂直方向のｎ値）
【００１４】
なお、鋼板のｒ値及びｎ値は、それぞれＪＩＳ Ｚ ２２５４及びＪＩＳ Ｚ２２５３に準拠して測定すれば良い。試験片はＪＩＳ Ｚ ２２０１の５号または１３Ｂ号試験片を用いれば良く、評点距離は、２０，２５，５０ｍｍのいずれでも良い。またｒ値を求める際の、１５％塑性変形とは公称歪であり、評点距離の変位から求めることができる。ｎ値はＪＩＳ Ｚ ２２４１に準拠して引張試験を行い、得られた公称応力−公称歪曲線から求めれば良い。すなわち、公称応力−公称歪曲線から、公称歪１０％における真応力及び真歪、それぞれσ１０及びε１０を求め、さらに公称歪１５％における真応力及び真歪、それぞれσ１５及びε１５を求め、（３）式により計算することができる。試験片はＪＩＳ Ｚ ２２０１の５号または１３Ｂ号試験片を用いれば良い。
【００１５】
従来の方法では、平均のｒ値や圧延と平行方向のｎ値等が注目されていたが、実際の成形では、ｒ値とｎ値の両者が重要であることが本発明者らの実験によって明らかになった。しかも、従来ほとんど注目されていなかった平均ｎ値を求め、さらに平均ｒ値との組合わせによって成形性の指標を確立した。
上記のような関係を満足するｎ値、ｒ値を達成するには、Ｓｉ，Ｐを主とした成分規定に加えて製品板の結晶粒径を規定する必要がある。さらに本発明製品を製造するに際しては、冷間圧延前の組織を規定する必要がある。すなわち、冷間圧延前の平均結晶粒径ｄｘを８０〜４００μｍに調整することが重要である。さらに、最終製品板の平均粒径ｄｚとｄｘが、ｄｘ／ｄｚ≦３を満たすことが必要である。
これには、冷延率を５０〜９０％とし、冷間圧延前の焼鈍温度を下記（６）式で規定される温度Ｔ１［℃］とし、冷間圧延後の焼鈍温度を下記（７）式で規定される温度Ｔ２［℃］する必要がある。
８００≦Ｔ１≦−３０−５４５７／［ｌｏｇ｛（Ｔｉ＋Ｎｂ＋Ｚｒ）×Ｃ｝−２．６］ ‥‥（６）
Ｔ１＋５≦Ｔ２≦Ｔ１＋６０ ‥‥（７）
ここで、Ｔｉ［ｍａｓｓ％］，Ｎｂ［ｍａｓｓ％］，Ｚｒ［ｍａｓｓ％］及びＣ［ｍａｓｓ％］は、それぞれＴｉ，Ｎｂ，Ｚｒ及びＣ含有量である。
Ｔ１の上限は、Ｔｉ，Ｎｂ，Ｚｒ及びＣ含有量によって決まるが、この理由は、Ｔｉ，Ｎｂ及びＺｒの炭化物の析出量が低減する温度以下で焼鈍を行うことにより析出量を確保し、再結晶粒の成長を抑制して、微細な等軸の再結晶粒を得るためである。なお、（６）式において、Ｎ含有量を考慮しないのは、Ｔｉ，Ｎｂ及びＺｒの窒化物が炭化物よりも安定であり、焼鈍温度では固溶しないためである。
また、Ｔ２は、Ｔ１より５〜６０℃高い範囲で行う必要があるが、これは、ｄｘ／ｄｚ≦３を満足し、かつ粒径の粗大化や混粒組織の生成を防止するためである。なお、従来、冷間圧延後の焼鈍は、冷間圧延前の焼鈍に比べて低い温度で実施されていたが、これは、最終焼鈍の方が初期粒径が微細であり、導入されている歪量が多く、再結晶し易いためである。
【００１６】
本発明の成分の限定に関して以下に説明する。なお、下記の説明における％はｍａｓｓ％を示す。
Ｃ，Ｎは、多量に添加すると成形性が低下するため、それぞれ上限は０．０３％とした。一方、下限は精錬段階でのコストを考慮した場合には０．０００５％である。厳しい加工用途等に用いる場合には、Ｃ，Ｎ共に０．０００５〜０．０１５％とすることが好ましい。
【００１７】
Ｓｉは、脱酸元素として必要であるが、多量の添加により成形性が低下する。したがって上限は０．５％とした。下限は脱酸効果を得るために０．０１％とした。
【００１８】
Ｍｎは、多量の添加により強度を上昇させ、成形性を低下させるため、上限を１％とした。下限はコストの観点より０．０１％とした。
【００１９】
Ｐは、成形性の点からは低い方が好ましく、０．０４％未満とする必要がある。低いほど延性は向上するので、０．０２％以下にすることが好ましい。下限は原料コストの点から０．００５％程度が望ましい。
【００２０】
Ｓは、多量に添加すると耐食性、加工性が低下するため、上限は０．０１％とした。下限は近年の脱硫技術で極低化が可能なため、０．０００１％とした。
【００２１】
Ｃｒは、１０％未満ではステンレス鋼としての耐食性が不十分であり、２５％を超えると靱性が低下する場合があるため、１０〜２５％の範囲とした。また、Ｃｒ量が多くなると加工性が低下するため、加工用途としては１０〜１９％が好ましい。
【００２２】
Ｔｉ，Ｎｂ，Ｚｒは、ＣやＮを固定してｒ値及びｎ値を向上させる元素である。ＣやＮを固定するだけの量が添加されていればよく、１種または２種以上の組合わせの合計で下限を８×（Ｃ％＋Ｎ％）とした。また、多量に添加すると強度が増加して成形性の低下を招くため、上限を０．８％とした。成形性に加えて製造性等の観点から、最も好ましい範囲は０．０８〜０．３％である。
【００２３】
Ａｌは、Ｎ等を固定して加工性を向上する元素であるが、多量の添加はコストの増加をもたらすため、上限は０．１％とした。一方、Ａｌ量の低減により脱酸が困難になるため、下限は０．００５％とした。
【００２４】
Ｍｇは、成形性を向上する効果がある。その効果を発揮するのは０．０００１％以上でありこれを下限とした。また多量に添加してもその効果は飽和するため、０．０１０％を上限とした。本発明におけるＭｇ量とは、精錬時に添加され、鋼板製造後に鋼中に残存している量を示しており、不純物として精錬時にスラグ等から混入するものではない。好ましい範囲は０．０００２〜０．００３０％である。
【００２５】
Ｂは、成形性、特に二次加工性を向上する元素であり、必要に応じて０．０００５〜０．００５％の範囲で添加される。
【００２６】
Ｍｏ，Ｎｉ，Ｃｕは、耐食性を向上する元素であり、耐食性が問題となる用途では、必要に応じてＭｏ，Ｎｉ，Ｃｕのうち一種又は二種以上を組み合わせて添加する。それぞれ０．１％以上添加することにより効果が現れる。しかし、加工性を考慮すると上限はいずれも２％である。
【００２７】
Ｃｏ，Ｖは、成形性、特に張り出し性を向上させる効果を持つため、必要に応じて一種又は二種を組み合わせて添加する。それぞれ０．１％以上添加することにより効果が現れる。しかし、３％を超えると強度上昇による成形性低下が顕著になるので、３％を上限とした。
【００２８】
上記の成分規定に加えて、ＳｉとＰの量を制限することが重要である。Ｓｉ＋１０Ｐ≦０．５とすることでｎ値とｒ値の両者が向上し、結果的に極めて高い成形性が得られる。Ｓｉ＋１０Ｐは低いほど成形性が良好となり、厳しい成形が施される箇所にはＳｉ＋１０Ｐ≦０．３とすることが好ましい。下限は規定しないが、Ｓｉ及びＰの量の下限から、０．０６未満とすることは難しい。
【００２９】
本発明において、最終製品板の結晶粒度番号は６〜８の範囲にすることが好ましい。これは６未満であると粒が粗大なために成形時に肌荒れが生じやすく、８超の場合には成形性が低下するためである。結晶粒度番号はＪＩＳ Ｇ ０５２２に準ずる方法で測定すればよい。
【００３０】
本発明鋼のような鋼板を製造するには、冷間圧延前の平均結晶粒径ｄｘを８０〜４００μｍに調整することが非常に有効である。４００μｍを超える場合には成形性、特に深絞り性が低下する。冷間圧延前の結晶粒径は細かいほど最終製品板の成形性は向上するが、８０μｍ未満にするには熱延時の製造性が低下するため、８０μｍを下限とした。なお、ステンレス鋼の製造工程では数回の冷間圧延を行う場合があるが、本発明において冷間圧延前とは最終冷間圧延の前の状態を示している。すなわち、冷間圧延の前の状態とは、中間焼鈍を行う場合は、最後の中間焼鈍後の状態であり、中間焼鈍を行わない場合は、熱延板焼鈍後の状態である。結晶粒径の測定は、ＪＩＳ Ｇ ０５２２に準じて測定すれば良い。
【００３１】
冷間圧延前の平均結晶粒径ｄｘと最終製品板（焼鈍後もしくは焼鈍しない場合は冷間圧延後）の平均結晶粒径ｄｚの間には、ｄｘ／ｄｚ≦３なる関係が成り立つことが、成形性の観点から好ましい。ｄｘ／ｄｚの下限は規定しないが、１未満では成形性が低下するため、１以上とすることが好ましい。
【００３２】
本発明のフェライト系ステンレス鋼の製造方法は、溶解、連続鋳造、熱間圧延、冷間圧延及び焼鈍からなる。焼鈍は、冷間圧延後の最終焼鈍を必須とし、さらに熱間圧延後の熱延板焼鈍及び／または冷間圧延途中の中間焼鈍を行う。中間焼鈍は複数回行っても良い。最終焼鈍後にスキンパス圧延を行っても良い。また、製品の表面は２Ｂ，ＢＡ，２Ｄ，研磨肌等いずれの表面でも良い。
【００３３】
熱間圧延における総圧延率：本発明において、熱間圧延は圧延集合組織を発達させて良好な成形性を得る目的がある。総圧下率が９５％未満では、十分なＲ値が得られないため、これを下限とした。ここで総圧下率とは、熱間圧延前の鋳片の板厚と熱間圧延後の熱延板の板厚の差を、鋳片の板厚で除した値の百分率である。上限は特には設けないが、現状設備の能力を考慮すると９９．５％が限界と考えられる。成形性及び製造性の観点から好ましい範囲は９７〜９８．５％である。
熱間圧延における仕上げ温度：仕上げ温度は、９００℃を超えると熱間圧延の加工歪みが回復して、熱延後の巻取り工程あるいは焼鈍工程での再結晶を生じ難くなる。したがって、仕上げ圧延温度の上限を９００℃以下とすることが必要である。一方、仕上げ温度の下限は、７００℃よりも低いと圧延荷重が増加し、熱延板に割れや表面疵を生じ易いため７００℃以上を下限とすることが好ましい。
【００３４】
熱延板焼鈍温度または中間焼鈍温度：熱延板焼鈍または中間焼鈍の温度は下記（６）式で規定される温度Ｔ１［℃］とする。
８００≦Ｔ１≦−３０−５４５７／［ｌｏｇ｛（Ｔｉ＋Ｎｂ＋Ｚｒ）×Ｃ｝−２．６］ ‥‥（６）
ここで、Ｔｉ［ｍａｓｓ％］，Ｎｂ［ｍａｓｓ％］，Ｚｒ［ｍａｓｓ％］及びＣ［ｍａｓｓ％］は、それぞれＴｉ，Ｎｂ，Ｚｒ及びＣ含有量である。
これは、焼鈍温度が８００℃未満であると未再結晶が残存し、結晶粒が粗大になり、十分な加工性が得られないためである。また（６）式右辺より高い場合には、再結晶するものの結晶粒径が粗大になりすぎて十分な加工性が得られないため、これを上限とした。この式中にＴｉ，Ｎｂ，Ｚｒ及びＣの量が含まれている理由は、焼鈍をＴｉ，Ｎｂ及びＺｒの炭化物が溶解する温度以下で行い、析出量を確保するためである。これにより再結晶粒の成長を抑制して、微細かつ等軸な再結晶粒が得られる。なお、複数回の中間焼鈍を行う場合は、少なくとも最後の中間焼鈍の温度が（６）式を満たす必要がある。また、Ｔｉ，Ｎｂ及びＺｒの窒化物は、炭化物よりも安定であり、焼鈍温度では溶解しないため、（６）式においてＮ含有量を考慮する必要はない。
【００３５】
冷延率：本発明では、最終冷間圧延における冷延率を規定する。なお、冷延率は、最終冷延前の板厚と最終冷延後の板厚の差を最終冷延前の板厚で除した値の百分率である。
ここで、最終冷間圧延とは、中間焼鈍を行う場合は、中間焼鈍後、最終焼鈍前の冷間圧延であり、冷延率は中間焼鈍前または中間焼鈍後の板厚から最終冷延後または最終焼鈍後の板厚を減じて中間焼鈍前または中間焼鈍後の板厚で除した値の百分率である。なお、複数回の中間焼鈍を行う場合は、最後の中間焼鈍後、最終焼鈍前の冷間圧延が最終冷間圧延である。また、中間焼鈍を行わない場合は、熱間圧延後、最終焼鈍前の冷間圧延が最終冷間圧延であり、冷延率は熱間圧延後または熱延板焼鈍後の板厚から最終冷延後または最終焼鈍後の板厚を減じて熱間圧延後または熱延板焼鈍後の板厚で除した値の百分率である。
冷延率は、５０〜９０％とする必要がある。これは、冷延率が５０％未満であると十分な圧延集合組織が発達せず、加工性が劣り、また９０％以上の圧延は加工性の向上代が飽和するばかりか、圧延負荷が増大するためである。
最終焼鈍温度：最終冷間圧延後には下記（７）式で規定される温度Ｔ２［℃］で最終焼鈍を実施する。
Ｔ１＋５≦Ｔ２≦Ｔ１＋６０ ‥‥（７）
【００３６】
本発明においては、最終焼鈍温度は冷間圧延前の焼鈍温度Ｔ１より５〜６０℃高い範囲で行う必要がある。これは、Ｔ１＋５℃未満であると再結晶しても、前述のｄｘ／ｄｚ≦３なる関係が成り立たず、良好な成形性が得られないためである。またＴ１＋６０℃を超える温度では、再結晶粒径が粗大になり、また、混粒組織となって、製品板加工時の肌荒れが生じたり、成形性が劣化したりするためである。
また最終焼鈍後に、形状矯正や降伏点伸び防止を目的としてスキンパス圧延を行っても構わない。実施する場合、０．３〜１．０％の伸び率とすることが好ましい。
【００３７】
【実施例】
以下に本発明の実施例を示す。
［実施例１］
表１に示すフェライト系ステンレス鋼を溶製し、熱延後、冷延、焼鈍等により０．８ｍｍの鋼板を作成した。熱間圧延の条件は、総圧延率が９５〜９９．５％、仕上げ温度が７００〜９００℃とした。冷間圧延後、中間焼鈍を行った。中間焼鈍は、下限を８００℃以上とし、上限を表１の（６）式の右辺の計算値の欄に示した温度とし、その範囲内で行った。その後、圧延率５０〜９０％の冷間圧延を行い、中間焼鈍よりも５〜６０℃高い温度で最終焼鈍を行った。
【００３８】
その後、角筒深絞り成形及び円筒深絞り成形試験を行った。なお、角筒深絞り成形試験は、角筒ポンチ及び角型ダイスを用いて深絞り試験を行うものであり、張り出し成形性を評価するために実施した試験である。角筒深絞り成形試験及び円筒深絞り成形試験の共通した条件として、成形高さは４０ｍｍ、潤滑はグリース状の潤滑油及びテフロン（登録商標）シート張り付けとした。素板サイズ及び形状は、鋼種毎に適宜調整した。角筒深絞り試験のポンチは４０ｍｍ角、ダイスは板厚に応じて４２ｍｍ角、ポンチ及びダイスの肩Ｒ並びにコーナーＲは全て５Ｒのものを使用した。円筒深絞り試験のポンチは直径５０ｍｍ、ダイスは板厚に応じて直径５２ｍｍ、ポンチ及びダイスの肩Ｒは共に５のものを使用した。
【００３９】
以下の角筒深絞り成形及び円筒深絞り成形試験において、成形高さ４０ｍｍまでに破断を生じたものを×とした。成形高さ４０ｍｍ成形できたものについては目視による判定を行った。まず、縦壁部、ポンチ肩部、ポンチ肩部近傍のいずれか１以上に、筋状に板厚が薄くなっている局部くびれが見られたものを、□とした。この局部くびれを減肉という。また、表面に凹凸を生じたものを△とした。この凹凸は筋状ではなく、ディンプル状に観察されるものであり、これを肌荒れという。なお、減肉はｒ値、延性の低下により生じ、肌荒れは粒径が大きいと発生するものであり、△と□は、どちらが成形性が劣るとはいえないが、いずれも成形性の観点から好ましくない。さらに、減肉及び肌荒れが見られないものを○とした。
【００４０】
鋼板のｒ値及びｎ値は、Ｌ，Ｄ，Ｃ方向について、それぞれＪＩＳ Ｚ ２２５４及びＪＩＳ Ｚ ２２５３に準拠して測定した。試験片はＪＩＳ Ｚ ２２０１の５号試験片を用いて評点距離を２５ｍｍとした。
ｒ値は、付加ひずみ量を１５％として、変形前後の試験片の幅と厚みから（１）式によって求めた。
ｒ＝ｌｎ（Ｗ／Ｗ０ ）／ｌｎ（ｔ／ｔ０ ） ‥‥（１）
ここで、Ｗとｔは１５％塑性変形後、Ｗ０ とｔ０ は変形前の、それぞれ試験片の幅と厚みであり、１５％塑性変形とは評点距離の変位から求めた公称歪である。
さらに、（２）式によりｒ値の平均値Ｒを求めた。
Ｒ＝（ｒＬ ＋２×ｒＤ ＋ｒＣ ）／４ ‥‥（２）
ここで、ｒＬ は圧延方向、ｒＤ は４５°方向、ｒＣ は垂直方向の、それぞれの方向について（１）式に従って求めたｒ値である。
【００４１】
ｎ値はＪＩＳ Ｚ ２２４１に準拠して引張試験を行い、得られた公称応力−公称歪曲線から求めた。すなわち、公称応力−公称歪曲線から、公称歪１０％における真応力及び真歪、それぞれσ１０及びε１０を求め、さらに公称歪１５％における真応力及び真歪、それぞれσ１５及びε１５を求め、（３）式により計算した。
ｎ＝ｌｎ（σ１５／σ１０）／ｌｎ（ε１５／ε１０） ‥‥（３）
さらに、（４）式により、ｎ値の平均値Ｎを求めた。
Ｎ＝（ｎＬ ＋２×ｎＤ ＋ｎＣ ）／４ ‥‥（４）
ここで、ｎＬ は圧延方向、ｎＤ は４５°方向、ｎＣ は垂直方向の、それぞれの方向について（３）式に従って求めたｎ値である。
【００４２】
各種評価結果を表２に示す。表２から明らかなように、本発明鋼は比較鋼に比べて成形性が優れている。
【００４３】
【表１】
【表２】
［実施例２］
表１−Ｃ，Ｋ及びＭのフェライト系ステンレス鋼製品板について製造条件を変えて結晶粒度番号が異なる数種のサンプルを作製した。本発明鋼及び鋼種Ｍは、以下の条件により製造条件した。熱間圧延の条件は、総圧延率が９５〜９９．５％、仕上げ温度が７００〜９００℃とし、冷間圧延後、中間焼鈍を行った。中間焼鈍は、下限を８００℃以上とし、上限を表１の（６）式の右辺の計算値の欄に示した温度とし、その範囲内で行った。その後、圧延率５０〜９０％の冷間圧延を行い、中間焼鈍よりも５〜６０℃高い温度で最終焼鈍を行った。比較鋼は、冷間圧延までは、本発明鋼及び鋼種Ｍと同様の製造条件としたが、最終焼鈍温度を中間焼鈍よりも５℃未満高い温度または６０℃超高い温度で行った。
【００４６】
その後角筒深絞り成形及び円筒深絞り成形試験を行った。成形条件は実施例１と同じである。各種評価結果を表３に示す。
表３から明らかなように、本発明鋼は成形可能で成形性が優れているが、比較鋼は成形不可であったり、あるいは肌荒れが生じたり、一部の減肉が激しかったりしている。なお、成形性が△のものは、最終焼鈍温度が本発明の範囲よりも高く、成形性が□のものは、最終焼鈍温度が本発明の範囲よりも低かった比較例である。
【００４７】
【表３】
［実施例３］
表１−Ｃ及びＫのフェライト系ステンレス鋼製品板について凝固、熱延及び焼鈍条件を変えて冷間圧延前の結晶粒径を調整したサンプルを作製し、冷間圧延、焼鈍後に角筒深絞り成形及び円筒深絞り成形試験を行った。製造条件を表４に示す。成形条件は、成型高さを１１０ｍｍにし、それ以外の条件は実施例１と同様にした。
製造条件及び各種評価結果を表５に示す。表５から明らかなように、本発明法によって製造された鋼は成形可能であるが、比較法によって製造された鋼は成形不可であったり、あるいは肌荒れが生じたり、一部の減肉が激しかったりしている。
【００４９】
【表４】
【表５】
【発明の効果】
上記のように、本発明は成形性に優れたフェライト系ステンレス鋼板を提供でき、その産業上の価値は極めて高い。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a ferritic stainless steel sheet excellent in formability and a method for producing the same.
[0002]
[Prior art]
Ferritic stainless steel has excellent corrosion resistance and is used in many applications. However, since the formability is inferior to SUS304, which is a representative type of austenitic stainless steel, there is a problem that cracks occur at the time of forming, and the use thereof is sometimes limited.
Austenitic stainless steel has a high n value, which is an index of overhanging property, and enables forming by overhanging. On the other hand, in the case of ferritic stainless steel, the n value is low, and instead, it is formed by deep drawing.
[0003]
Recently, for example, as described in Patent Literature 1, a method of improving the r value as an index of deep drawability by devising hot rolling conditions and the like has been studied. However, the actual formability is inferior to SUS304, and it is considered that ferritic stainless steel is difficult to use.
[0004]
As a technique for improving both the deep drawability and the overhang property, a method of defining the components as in Patent Document 2 and Patent Document 3, and a friction coefficient and reduction ratio during hot rolling as disclosed in Patent Document 4 A method for defining the strain rate is known. However, none of these methods completely eliminates cracking during molding, and it cannot be said that the problem of workability in ferritic stainless steel has always been solved.
[0005]
[Patent Document 1]
JP-A-10-330887 [Patent Document 2]
JP-A-57-198248 [Patent Document 3]
JP-A-58-61258 [Patent Document 4]
JP-A-7-310122
[Problems to be solved by the invention]
An object of the present invention is to provide a ferritic stainless steel sheet having extremely high formability and a method for producing the same by specifying the components and the production method.
[0007]
[Means for Solving the Problems]
When the present inventors performed analysis by FEM (finite element method) of cylindrical drawing, square tube drawing, etc., which were actual moldings, the actual molding did not involve either deep drawing or overhanging, but the characteristics of both. It has been found that it is necessary to improve the average value in all directions.
The material characteristic values required at this time include an average value R of an r value (strain ratio of sheet width / sheet thickness) as an index of deep drawability, and an average value of an n value (work hardening index) as an index of overhang property. Value N. In addition, as an average value, measurement was performed on behalf of a direction perpendicular to the rolling direction (right angle) (C direction), a parallel direction (L direction), and a 45 ° direction (D direction). Value) + value in the C direction) / 4.
[0008]
According to the results of experiments conducted by the present inventors, by limiting the components and the production method even for ferritic stainless steel, if the R value and the N value are improved, forming cracks are eliminated and the formability is equal to or higher than SUS304. Was observed. In terms of components, particularly the amounts of Si and P, and the grain size number of the final product sheet is an important factor for improving the R value and the N value of the structure, and in the production method, the average crystal grain size before cold rolling has a great effect on the formability Was found. Furthermore, in order to obtain the stainless steel of the present invention, it has been found that it is extremely important to limit the rolling reduction of cold rolling (referred to as cold rolling reduction) and to limit the annealing temperature before and after cold rolling.
[0009]
The present invention has been completed based on the above findings by limiting the range that the R value and the N value must satisfy in order to obtain sufficient formability, and the gist thereof is as follows.
(1) As an average value of r values (strain ratio of sheet width / sheet thickness) when 15% tensile plastic deformation is applied in three directions of a rolling direction, a direction perpendicular to rolling, and a direction of 45 °, the following (1), R defined by the formula (2) and the average value of the n value (work hardening index) at a strain of 10 to 15% when strain is applied in three directions of a rolling direction, a direction perpendicular to rolling, and a 45 ° direction. A ferritic stainless steel sheet excellent in formability, wherein N defined by the following formulas (3) and (4) satisfies the relationship of R + 10N ≧ 3.5.
r = ln (W / W0) / ln (t / t0) ‥‥ (1)
(W and t are the width and thickness of the test piece after 15% plastic deformation and W0 and t0 are before deformation, respectively)
R = (rL + 2 × rD + rC) / 4 ‥‥ (2)
(RL is a rolling direction, rD is a 45 ° direction, and rC is a vertical direction. R values obtained in accordance with the above equation (1) for each direction)
n = ln (σ15 / σ10) / ln (ε15 / ε10) ‥‥ (3)
(Σ15 and ε15 are true stress and true strain after 15% tension, and σ10 and ε10 are true stress and true strain after 10% tension, respectively)
N = (nL + 2 × nD + nC) / 4 ‥‥ (4)
(NL is the rolling direction, nD is the 45 ° direction, and nC is the vertical direction, the r value obtained according to the above formula (3) for each direction)
[0010]
(2) mass%
C: 0.0005 to 0.03%, Si: 0.01 to 0.5%,
Mn: 0.01 to 1%, P: less than 0.04%,
S: 0.0001 to 0.01%, Cr: 10 to 25%,
Al: 0.005 to 0.1%, N: 0.0005 to 0.03%,
Si + 10P ≦ 0.5
And one or more of Ti, Nb and Zr in total of 8 × (C% + N%) to 0.8%, with the balance being iron and unavoidable impurities. The ferritic stainless steel sheet excellent in formability according to the above (1).
(3) The ferritic stainless steel sheet excellent in formability according to the above (2), further comprising Mg: 0.0001 to 0.01% by mass%.
(4) The ferritic stainless steel sheet excellent in formability according to the above (2) or (3), further comprising B: 0.0005 to 0.005% by mass%.
(5) mass%,
Mo: 0.1 to 2%, Ni: 0.1 to 2%,
Cu: 0.1 to 2%
The ferritic stainless steel sheet excellent in formability according to any one of the above (2) to (4), further comprising one or more of the following.
(6) In mass%,
Co: 0.1-3%, V: 0.1-3%
The ferritic stainless steel sheet excellent in formability according to any one of the above (2) to (5), further comprising one or two of the following.
(7) The ferritic stainless steel sheet excellent in formability according to any one of (1) to (6), wherein the crystal grain size number is in the range of 6 to 8.
[0011]
(8) In producing the ferritic stainless steel sheet according to any one of the above (1) to (7), the average crystal grain size dx before cold rolling is adjusted to 80 to 400 μm, and the final product sheet A method for producing a ferritic stainless steel sheet having excellent formability, wherein the average particle diameter dz satisfies the following expression (5).
dx / dz ≦ 3 ‥‥ (5)
(9) A slab of ferritic stainless steel comprising the component according to any one of the above (2) to (6) is heated to a hot rolling having a total rolling reduction of 95% or more and a finishing temperature of 900 ° C or less. Rolling is performed, hot-rolled sheet annealing or intermediate annealing is performed at a temperature T1 [° C.] defined by the following equation (6), and after cold rolling at a rolling reduction of 50 to 90%, the following equation (7) is applied. The method for producing a ferritic stainless steel sheet having excellent formability according to the above (8), wherein the final annealing is performed at a temperature T2 [° C.] specified in (1).
800 ≦ T1 ≦ −30−5457 / [log {(Ti + Nb + Zr) × C} −2.6] {(6)
T1 + 5 ≦ T2 ≦ T1 + 60 (7)
Here, Ti [mass%], Nb [mass%], Zr [mass%] and C [mass%] are the contents of Ti, Nb, Zr and C, respectively.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
The ferritic stainless steel of the present invention is characterized in that the average r value R and the average n value N satisfy the relationship of R + 10N ≧ 3.5. The upper limit is not specified, but exceeding 5.5 is difficult with the current technology. The average r-value R is the width of a sheet generated when a plate-shaped tensile test specimen in three directions of a rolling direction, a direction perpendicular to rolling, and a 45 ° direction is prepared on a sheet surface and subjected to 15% tensile plastic deformation. And the average value of the strain ratio (r value) of the plate thickness, the r value is represented by the following equation (1), and the average r value R is represented by the following equation (2).
r = ln (W / W0) / ln (t / t0) ‥‥ (1)
(W and t represent the width and thickness of the test piece after deformation and W0 and t0 respectively before deformation)
R = (rL + 2 × rD + rC) / 4 ‥‥ (2)
(RL is the rolling direction, rD is the 45 ° direction, and rC is the r value in the vertical direction)
[0013]
The average n value N refers to a rolling direction on a sheet surface, a direction perpendicular to rolling, and a plate-like tensile test specimen in three directions of 45 ° directions, and 10% to 15% when a tensile test is performed. The work hardening index is an average value of the n value, the n value is represented by the following formula (3), and the average n value N is represented by the following formula (4).
n = ln (σ15 / σ10) / ln (ε15 / ε10) ‥‥ (3)
(Σ15 and ε15 represent true stress and true strain after 15% tension, and σ10 and ε10 represent true stress and strain after 10% tension, respectively)
N = (nL + 2 × nD + nC) / 4 ‥‥ (4)
(NL is the rolling direction, nD is the 45 ° direction, and nC is the vertical n value)
[0014]
The r value and the n value of the steel plate may be measured according to JIS Z 2254 and JIS Z 2253, respectively. The test piece may be a JIS Z 2201 No. 5 or No. 13B test piece, and the evaluation distance may be any of 20, 25, and 50 mm. The 15% plastic deformation at the time of obtaining the r value is a nominal strain and can be obtained from the displacement of the rating distance. The n value may be determined from a nominal stress-nominal strain curve obtained by performing a tensile test in accordance with JIS Z 2241. That is, from the nominal stress-nominal strain curve, the true stress and true strain at a nominal strain of 10%, σ10 and ε10, respectively, are further determined. Further, the true stress and true strain at a nominal strain of 15%, σ15 and ε15, respectively, are determined. It can be calculated by the formula. The test piece may be a JIS Z 2201 No. 5 or 13B test piece.
[0015]
In the conventional method, attention was paid to the average r value and the n value in the direction parallel to the rolling. However, in the actual forming, it was confirmed by experiments by the present inventors that both the r value and the n value are important. It was revealed. In addition, an average n value, which has been hardly noticed in the past, was determined, and an index of formability was established by combining the average n value with the average r value.
In order to achieve the n value and the r value that satisfy the above relationship, it is necessary to define the crystal grain size of the product sheet in addition to the component definition mainly of Si and P. Further, when manufacturing the product of the present invention, it is necessary to define the structure before cold rolling. That is, it is important to adjust the average crystal grain size dx before cold rolling to 80 to 400 μm. Furthermore, it is necessary that the average particle diameters dz and dx of the final product plate satisfy dx / dz ≦ 3.
For this, the cold-rolling rate is set to 50 to 90%, the annealing temperature before cold rolling is set to a temperature T1 [° C.] specified by the following formula (6), and the annealing temperature after cold rolling is set to the following (7). It is necessary to keep the temperature T2 [° C.] defined by the equation.
800 ≦ T1 ≦ −30−5457 / [log {(Ti + Nb + Zr) × C} −2.6] {(6)
T1 + 5 ≦ T2 ≦ T1 + 60 (7)
Here, Ti [mass%], Nb [mass%], Zr [mass%] and C [mass%] are the contents of Ti, Nb, Zr and C, respectively.
The upper limit of T1 is determined by the contents of Ti, Nb, Zr and C. The reason for this is that the amount of precipitation is secured by annealing at a temperature lower than the temperature at which the amount of precipitation of carbides of Ti, Nb and Zr decreases. This is to suppress the growth of crystal grains and obtain fine equiaxed recrystallized grains. The reason why the N content is not considered in the equation (6) is that nitrides of Ti, Nb and Zr are more stable than carbides and do not form a solid solution at the annealing temperature.
Further, T2 needs to be performed in a range higher than T1 by 5 to 60 ° C., in order to satisfy dx / dz ≦ 3 and to prevent coarsening of the particle size and generation of a mixed grain structure. . Note that, conventionally, annealing after cold rolling was performed at a lower temperature than annealing before cold rolling, but this is because the final annealing has a finer initial grain size and is introduced. This is because the amount of strain is large and recrystallization is easy.
[0016]
The limitation of the components of the present invention will be described below. In the following description,% indicates mass%.
If C and N are added in large amounts, the moldability decreases, so the upper limits were each set to 0.03%. On the other hand, the lower limit is 0.0005% in consideration of the cost in the refining stage. When used for severe processing applications, it is preferable that both C and N be 0.0005 to 0.015%.
[0017]
Although Si is necessary as a deoxidizing element, the moldability is reduced by adding a large amount. Therefore, the upper limit was set to 0.5%. The lower limit was set to 0.01% in order to obtain a deoxidizing effect.
[0018]
The upper limit of Mn is set to 1% because the addition of a large amount increases the strength and lowers the moldability. The lower limit is set to 0.01% from the viewpoint of cost.
[0019]
P is preferably low from the viewpoint of moldability, and needs to be less than 0.04%. Since the ductility improves as the temperature decreases, the content is preferably set to 0.02% or less. The lower limit is preferably about 0.005% from the viewpoint of raw material cost.
[0020]
If S is added in a large amount, the corrosion resistance and workability deteriorate, so the upper limit was made 0.01%. The lower limit is set to 0.0001% because a very low level can be achieved by a recent desulfurization technique.
[0021]
If the Cr content is less than 10%, the corrosion resistance of stainless steel is insufficient, and if it exceeds 25%, the toughness may be reduced. Further, when the Cr content is large, the workability is reduced, and therefore, the use of 10 to 19% is preferable for processing applications.
[0022]
Ti, Nb, and Zr are elements that fix C and N to improve the r value and the n value. It is sufficient that an amount sufficient to fix C and N is added, and the lower limit is set to 8 × (C% + N%) in total of one or two or more kinds. Further, if added in a large amount, the strength is increased and the moldability is lowered, so the upper limit was made 0.8%. The most preferable range is 0.08 to 0.3% from the viewpoint of productivity and the like in addition to moldability.
[0023]
Al is an element that improves workability by fixing N and the like, but the addition of a large amount increases the cost, so the upper limit was set to 0.1%. On the other hand, since the deoxidation becomes difficult due to the reduction of the Al content, the lower limit was made 0.005%.
[0024]
Mg has an effect of improving formability. The effect is exhibited at 0.0001% or more, and the lower limit is set. Further, even if a large amount is added, the effect is saturated, so the upper limit was made 0.010%. The Mg content in the present invention indicates the amount added during refining and remaining in the steel after the steel sheet is manufactured, and is not mixed as impurities from slag or the like during refining. The preferred range is 0.0002 to 0.0030%.
[0025]
B is an element that improves moldability, particularly secondary workability, and is added in the range of 0.0005 to 0.005% as necessary.
[0026]
Mo, Ni, and Cu are elements that improve corrosion resistance, and in applications where corrosion resistance is a problem, Mo, Ni, or Cu may be added alone or in combination of two or more as needed. The effect is exhibited by adding 0.1% or more of each. However, considering the workability, the upper limit is 2% in each case.
[0027]
Since Co and V have the effect of improving the formability, especially the overhanging property, Co or V may be added alone or in combination of two or more as necessary. The effect is exhibited by adding 0.1% or more of each. However, if it exceeds 3%, the moldability is significantly reduced due to an increase in strength, so the upper limit is set to 3%.
[0028]
It is important to limit the amounts of Si and P in addition to the above-mentioned component definition. By setting Si + 10P ≦ 0.5, both the n value and the r value are improved, and as a result, extremely high formability is obtained. The lower the value of Si + 10P, the better the formability, and it is preferable to set Si + 10P ≦ 0.3 at a location where severe molding is performed. Although the lower limit is not specified, it is difficult to set the lower limit to less than 0.06 from the lower limits of the amounts of Si and P.
[0029]
In the present invention, the grain size number of the final product plate is preferably in the range of 6 to 8. This is because if it is less than 6, the surface is likely to be roughened at the time of molding because the grains are coarse, and if it is more than 8, the moldability deteriorates. The crystal grain size number may be measured by a method according to JIS G 0522.
[0030]
In order to manufacture a steel sheet such as the steel of the present invention, it is very effective to adjust the average crystal grain size dx before cold rolling to 80 to 400 μm. If it exceeds 400 μm, the formability, especially the deep drawability, will decrease. Although the formability of the final product sheet improves as the crystal grain size before cold rolling becomes smaller, the lower limit is set to 80 μm because the productivity at the time of hot rolling decreases to less than 80 μm. In the manufacturing process of stainless steel, cold rolling may be performed several times. In the present invention, “before cold rolling” means a state before final cold rolling. That is, the state before cold rolling is a state after the last intermediate annealing when performing intermediate annealing, and a state after hot-rolled sheet annealing when not performing intermediate annealing. The crystal grain size may be measured according to JIS G 0522.
[0031]
The relationship of dx / dz ≦ 3 holds between the average grain size dx before cold rolling and the average grain size dz of the final product sheet (after annealing or after cold rolling if not annealed), It is preferable from the viewpoint of moldability. Although the lower limit of dx / dz is not specified, if it is less than 1, the moldability is reduced, so that it is preferably at least 1.
[0032]
The method for producing a ferritic stainless steel of the present invention comprises melting, continuous casting, hot rolling, cold rolling and annealing. Annealing requires final annealing after cold rolling, and further hot-rolled sheet annealing after hot rolling and / or intermediate annealing during cold rolling. Intermediate annealing may be performed several times. After the final annealing, skin pass rolling may be performed. The surface of the product may be any surface such as 2B, BA, 2D, and polished skin.
[0033]
Total rolling reduction in hot rolling: In the present invention, the purpose of hot rolling is to develop a rolling texture and obtain good formability. If the total rolling reduction is less than 95%, a sufficient R value cannot be obtained, so this was set as the lower limit. Here, the total rolling reduction is a percentage of a value obtained by dividing the difference between the thickness of the cast slab before hot rolling and the thickness of the hot rolled sheet after hot rolling by the thickness of the slab. Although there is no particular upper limit, 99.5% is considered to be the limit in view of the current capacity of the facility. A preferable range is 97 to 98.5% from the viewpoint of moldability and manufacturability.
Finishing temperature in hot rolling: When the finishing temperature exceeds 900 ° C., the processing distortion of hot rolling recovers, and recrystallization in the winding step or annealing step after hot rolling hardly occurs. Therefore, it is necessary to set the upper limit of the finish rolling temperature to 900 ° C. or less. On the other hand, if the lower limit of the finishing temperature is lower than 700 ° C., the rolling load increases, and cracks and surface flaws are easily generated in the hot-rolled sheet.
[0034]
Hot-rolled sheet annealing temperature or intermediate annealing temperature: The temperature of hot-rolled sheet annealing or intermediate annealing is a temperature T1 [° C.] specified by the following equation (6).
800 ≦ T1 ≦ −30−5457 / [log {(Ti + Nb + Zr) × C} −2.6] {(6)
Here, Ti [mass%], Nb [mass%], Zr [mass%] and C [mass%] are the contents of Ti, Nb, Zr and C, respectively.
This is because if the annealing temperature is lower than 800 ° C., unrecrystallized crystal remains, crystal grains become coarse, and sufficient workability cannot be obtained. If the value is higher than the right side of the expression (6), recrystallization occurs, but the crystal grain size becomes too large and sufficient workability cannot be obtained. The reason why the amounts of Ti, Nb, Zr, and C are included in this equation is that annealing is performed at a temperature lower than the temperature at which carbides of Ti, Nb, and Zr are dissolved to secure the amount of precipitation. Thereby, the growth of the recrystallized grains is suppressed, and fine and equiaxed recrystallized grains are obtained. In the case where the intermediate annealing is performed a plurality of times, at least the temperature of the final intermediate annealing needs to satisfy the expression (6). Further, the nitrides of Ti, Nb and Zr are more stable than carbides and do not dissolve at the annealing temperature, so that it is not necessary to consider the N content in equation (6).
[0035]
Cold rolling rate: In the present invention, the cold rolling rate in final cold rolling is specified. The cold rolling ratio is a percentage of a value obtained by dividing a difference between a sheet thickness before final cold rolling and a sheet thickness after final cold rolling by a sheet thickness before final cold rolling.
Here, the final cold rolling, when performing the intermediate annealing, is a cold rolling after the intermediate annealing, and before the final annealing, the cold rolling rate after the final cold rolling from the sheet thickness before the intermediate annealing or after the intermediate annealing. Alternatively, it is a percentage of a value obtained by subtracting the sheet thickness after the final annealing and dividing by the sheet thickness before or after the intermediate annealing. In addition, when performing multiple times of intermediate annealing, cold rolling before final annealing after final intermediate annealing is final cold rolling. When intermediate annealing is not performed, cold rolling after hot rolling and before final annealing is final cold rolling, and the cold rolling ratio is determined based on the thickness after hot rolling or after hot rolling sheet annealing. It is the percentage of the value obtained by subtracting the sheet thickness after rolling or final annealing and dividing by the sheet thickness after hot rolling or hot rolling sheet annealing.
The cold rolling reduction needs to be 50 to 90%. This is because if the cold rolling reduction is less than 50%, a sufficient rolling texture is not developed and the workability is inferior, and if the rolling is 90% or more, not only the improvement in workability is saturated, but also the rolling load increases. To do that.
Final annealing temperature: After the final cold rolling, final annealing is performed at a temperature T2 [° C.] defined by the following equation (7).
T1 + 5 ≦ T2 ≦ T1 + 60 (7)
[0036]
In the present invention, the final annealing temperature needs to be 5 to 60 ° C. higher than the annealing temperature T1 before cold rolling. This is because the relationship of dx / dz ≦ 3 does not hold even if recrystallization is performed at a temperature lower than T1 + 5 ° C., and good moldability cannot be obtained. At a temperature exceeding T1 + 60 ° C., the recrystallized grain size becomes coarse, and a mixed grain structure is formed, resulting in rough surface during processing of a product plate or deterioration of formability.
After the final annealing, skin pass rolling may be performed for the purpose of shape correction and prevention of yield point elongation. When carrying out, it is preferable to set the elongation at 0.3 to 1.0%.
[0037]
【Example】
Examples of the present invention will be described below.
[Example 1]
Ferritic stainless steels shown in Table 1 were melted and hot rolled, then cold rolled, annealed, and the like to prepare 0.8 mm steel plates. The conditions of the hot rolling were such that the total rolling ratio was 95 to 99.5% and the finishing temperature was 700 to 900 ° C. After cold rolling, intermediate annealing was performed. The intermediate annealing was performed within the range, with the lower limit being 800 ° C. or higher and the upper limit being the temperature shown in the column of the calculated value on the right side of Equation (6) in Table 1. Thereafter, cold rolling was performed at a rolling reduction of 50 to 90%, and final annealing was performed at a temperature 5 to 60 ° C. higher than that of the intermediate annealing.
[0038]
Thereafter, a square tube deep drawing and a cylindrical deep drawing test were performed. In addition, the square tube deep drawing forming test is to perform a deep drawing test using a square tube punch and a square die, and is a test conducted to evaluate the overhang formability. As common conditions for the square tube deep drawing test and the cylindrical deep drawing test, the forming height was 40 mm, and the lubrication was grease-like lubricating oil and Teflon (registered trademark) sheet pasting. The base plate size and shape were appropriately adjusted for each steel type. The punch used in the square tube deep drawing test was a 40 mm square, the die was a 42 mm square according to the plate thickness, and the shoulder R and the corner R of the punch and the die were all 5 R. In the cylindrical deep drawing test, a punch having a diameter of 50 mm, a die having a diameter of 52 mm according to the thickness of the plate, and having a shoulder R of 5 for the punch and the die were used.
[0039]
In the following square cylinder deep drawing and cylindrical deep drawing tests, those which fractured up to a molding height of 40 mm were evaluated as x. For a molded product having a molding height of 40 mm, visual judgment was performed. First, a line where the local constriction in which the thickness was reduced in a streak shape was observed in one or more of the vertical wall portion, the punch shoulder portion, and the vicinity of the punch shoulder portion was marked as □. This local constriction is called thinning. In addition, a sample having irregularities on the surface was designated as Δ. The irregularities are observed not in the form of stripes but in the form of dimples, and this is called rough skin. In addition, the wall thickness reduction is caused by a decrease in the r value and ductility, and the surface roughness is generated when the particle size is large. Both Δ and □ cannot be said to be inferior in moldability, but both are from the viewpoint of moldability. Not preferred. Further, those in which no thinning and rough skin were observed were evaluated as ○.
[0040]
The r value and the n value of the steel sheet were measured in the L, D, and C directions according to JIS Z 2254 and JIS Z 2253, respectively. As the test piece, a JIS Z 2201 No. 5 test piece was used, and the evaluation distance was 25 mm.
The r value was determined from the width and thickness of the test piece before and after the deformation, using the equation (1), with the added strain amount being 15%.
r = ln (W / W0) / ln (t / t0) ‥‥ (1)
Here, W and t are the width and thickness of the test piece after 15% plastic deformation and W0 and t0 before deformation, respectively, and the 15% plastic deformation is the nominal strain obtained from the displacement of the rating distance.
Further, the average value R of the r values was determined by the equation (2).
R = (rL + 2 × rD + rC) / 4 ‥‥ (2)
Here, rL is a rolling direction, rD is a 45 ° direction, and rC is a vertical value, which is an r value obtained in accordance with equation (1).
[0041]
The n value was determined from a nominal stress-nominal strain curve obtained by conducting a tensile test according to JIS Z 2241. That is, from the nominal stress-nominal strain curve, the true stress and true strain at a nominal strain of 10%, σ10 and ε10, respectively, are further determined. Further, the true stress and true strain at a nominal strain of 15%, σ15 and ε15, respectively, are determined. It was calculated by the formula.
n = ln (σ15 / σ10) / ln (ε15 / ε10) ‥‥ (3)
Further, the average value N of the n values was determined by the equation (4).
N = (nL + 2 × nD + nC) / 4 ‥‥ (4)
Here, nL is a rolling direction, nD is a 45 ° direction, and nC is a vertical value in each direction, which is an n value obtained according to the equation (3).
[0042]
Table 2 shows various evaluation results. As is clear from Table 2, the steel of the present invention has excellent formability as compared with the comparative steel.
[0043]
[Table 1]
[Table 2]
[Example 2]
With respect to the ferrite-based stainless steel product sheets of Table 1-C, K and M, several kinds of samples having different crystal grain size numbers were produced by changing production conditions. The steel of the present invention and the steel type M were manufactured under the following conditions. The hot rolling conditions were such that the total rolling ratio was 95 to 99.5%, the finishing temperature was 700 to 900 ° C., and after cold rolling, intermediate annealing was performed. The intermediate annealing was performed within the range, with the lower limit being 800 ° C. or higher and the upper limit being the temperature shown in the column of the calculated value on the right side of Equation (6) in Table 1. Thereafter, cold rolling was performed at a rolling reduction of 50 to 90%, and final annealing was performed at a temperature 5 to 60 ° C. higher than that of the intermediate annealing. The production conditions of the comparative steel were the same as those of the steel of the present invention and the steel type M until the cold rolling, but the final annealing temperature was lower than the intermediate annealing by 5 ° C. or higher than 60 ° C.
[0046]
Thereafter, a square tube deep drawing and a cylindrical deep drawing test were performed. The molding conditions are the same as in Example 1. Table 3 shows various evaluation results.
As is evident from Table 3, the steel of the present invention is moldable and has excellent moldability, but the comparative steel is not moldable, has a rough surface, or has a severely reduced thickness. In addition, those with the moldability of Δ are those in which the final annealing temperature is higher than the range of the present invention, and those with the formability of □ are comparative examples in which the final annealing temperature is lower than the range of the present invention.
[0047]
[Table 3]
[Example 3]
For the ferritic stainless steel product sheets of Table 1-C and K, solidification, hot rolling and annealing conditions were changed to prepare samples in which the crystal grain size before cold rolling was adjusted, and then cold rolling and deep drawing of rectangular cylinders after annealing were performed. Forming and cylindrical deep drawing tests were performed. Table 4 shows the manufacturing conditions. The molding conditions were such that the molding height was 110 mm, and the other conditions were the same as in Example 1.
Table 5 shows the manufacturing conditions and various evaluation results. As is clear from Table 5, the steel manufactured by the method of the present invention can be formed, but the steel manufactured by the comparative method cannot be formed, or the surface is roughened, and some of the wall thickness is severe. Or
[0049]
[Table 4]
[Table 5]
【The invention's effect】
As described above, the present invention can provide a ferritic stainless steel sheet excellent in formability, and its industrial value is extremely high.
Claims (9)
圧延方向、圧延に垂直方向、４５°方向の３方向に歪みを与えたときの歪み１０〜１５％におけるｎ値（加工硬化指数）の平均値として下記（３），（４）式にて定義されるＮが、
Ｒ＋１０Ｎ≧３．５の関係を満足することを特徴とする成形性に優れたフェライト系ステンレス鋼板。
ｒ＝ｌｎ（Ｗ／Ｗ０ ）／ｌｎ（ｔ／ｔ０ ） ‥‥（１）
（Ｗとｔは１５％塑性変形後、Ｗ０ とｔ０ は変形前の、それぞれ試験片の幅と厚み）
Ｒ＝（ｒＬ ＋２×ｒＤ ＋ｒＣ ）／４ ‥‥（２）
（ｒＬ は圧延方向、ｒＤ は４５°方向、ｒＣ は垂直方向の、それぞれの方向について上記（１）式に従って求めたｒ値）
ｎ＝ｌｎ（σ１５／σ１０）／ｌｎ（ε１５／ε１０） ‥‥（３）
（σ１５、ε１５は１５％引張後、σ１０、ε１０は１０％引張後の、それぞれ真応力、真歪）
Ｎ＝（ｎＬ ＋２×ｎＤ ＋ｎＣ ）／４ ‥‥（４）
（ｎＬ は圧延方向、ｎＤ は４５°方向、ｎＣ は垂直方向の、それぞれの方向について上記（３）式に従って求めたｒ値）The average value of r value (strain ratio of sheet width / sheet thickness) when 15% tensile plastic deformation is applied in three directions of the rolling direction, the perpendicular direction to rolling, and the 45 ° direction is as follows (1), (2) R defined by the formula:
Defined by the following formulas (3) and (4) as the average value of the n value (work hardening index) at a strain of 10 to 15% when strain is applied in three directions of the rolling direction, the vertical direction to the rolling, and the 45 ° direction. N
A ferritic stainless steel sheet excellent in formability, characterized by satisfying a relationship of R + 10N ≧ 3.5.
r = ln (W / W0) / ln (t / t0) ‥‥ (1)
(W and t are the width and thickness of the test piece after 15% plastic deformation and W0 and t0 are before deformation, respectively)
R = (rL + 2 × rD + rC) / 4 ‥‥ (2)
(RL is a rolling direction, rD is a 45 ° direction, and rC is a vertical direction. R values obtained in accordance with the above equation (1) for each direction)
n = ln (σ15 / σ10) / ln (ε15 / ε10) ‥‥ (3)
(Σ15 and ε15 are true stress and true strain after 15% tension, and σ10 and ε10 are 10% tension after tension, respectively)
N = (nL + 2 × nD + nC) / 4 ‥‥ (4)
(NL is the rolling direction, nD is the 45 ° direction, and nC is the vertical direction, the r value obtained according to the above formula (3) for each direction)
Ｃ ：０．０００５〜０．０３％、
Ｓｉ：０．０１〜０．５％、
Ｍｎ：０．０１〜１％、
Ｐ ：０．０４％未満、
Ｓ ：０．０００１〜０．０１％、
Ｃｒ：１０〜２５％、
Ａｌ：０．００５〜０．１％、
Ｎ ：０．０００５〜０．０３％、
Ｓｉ＋１０Ｐ≦０．５を含有し、
Ｔｉ，Ｎｂ，Ｚｒの１種または２種以上を合計で８×（Ｃ％＋Ｎ％）〜０．８％をさらに含有し、
残部鉄および不可避不純物からなることを特徴とする請求項１に記載の成形性に優れたフェライト系ステンレス鋼板。mass%,
C: 0.0005 to 0.03%,
Si: 0.01-0.5%,
Mn: 0.01-1%,
P: less than 0.04%,
S: 0.0001 to 0.01%,
Cr: 10 to 25%,
Al: 0.005 to 0.1%,
N: 0.0005 to 0.03%,
Containing Si + 10P ≦ 0.5,
One or more of Ti, Nb, and Zr, further containing 8 × (C% + N%) to 0.8% in total;
The ferritic stainless steel sheet excellent in formability according to claim 1, comprising a balance of iron and unavoidable impurities.
Ｍｇ：０．０００１〜０．０１％をさらに含有することを特徴とする請求項２に記載の成形性に優れたフェライト系ステンレス鋼板。mass%,
The ferritic stainless steel sheet having excellent formability according to claim 2, further comprising Mg: 0.0001 to 0.01%.
Ｂ ：０．０００５〜０．００５％をさらに含有することを特徴とする請求項２または３に記載の成形性に優れたフェライト系ステンレス鋼板。mass%,
B: The ferritic stainless steel sheet excellent in formability according to claim 2 or 3, further containing 0.0005 to 0.005%.
Ｍｏ：０．１〜２％、
Ｎｉ：０．１〜２％、
Ｃｕ：０．１〜２％
の１種または２種以上をさらに含有することを特徴とする請求項２乃至４のいずれか１項に記載の成形性に優れたフェライト系ステンレス鋼板。mass%,
Mo: 0.1-2%,
Ni: 0.1 to 2%,
Cu: 0.1 to 2%
The ferritic stainless steel sheet excellent in formability according to any one of claims 2 to 4, further comprising one or more of the following.
Ｃｏ：０．１〜３％、
Ｖ ：０．１〜３％
の１種または２種をさらに含有することを特徴とする請求項２乃至５のいずれか１項に記載の成形性に優れたフェライト系ステンレス鋼板。mass%,
Co: 0.1-3%,
V: 0.1-3%
The ferritic stainless steel sheet excellent in formability according to any one of claims 2 to 5, further comprising one or two of the following.
ｄｘ／ｄｚ≦３ ‥‥（５）In producing the ferritic stainless steel sheet according to any one of claims 1 to 7, the average grain size dx before cold rolling is adjusted to 80 to 400 µm, and the average grain size dz of the final product sheet is as follows. A method for producing a ferritic stainless steel sheet excellent in formability, characterized by satisfying the expression (5).
dx / dz ≦ 3 ‥‥ (5)
８００≦Ｔ１≦−３０−５４５７／［ｌｏｇ｛（Ｔｉ＋Ｎｂ＋Ｚｒ）×Ｃ｝−２．６］ ‥‥（６）
Ｔ１＋５≦Ｔ２≦Ｔ１＋６０ ‥‥（７）
ここで、Ｔｉ［ｍａｓｓ％］，Ｎｂ［ｍａｓｓ％］，Ｚｒ［ｍａｓｓ％］及びＣ［ｍａｓｓ％］は、それぞれＴｉ，Ｎｂ，Ｚｒ及びＣ含有量である。A slab of ferritic stainless steel comprising the components according to any one of claims 2 to 6 is heated and subjected to hot rolling at a total rolling reduction of 95% or more and a finishing temperature of 900 ° C or less, After performing sheet annealing or intermediate annealing at a temperature T1 [° C.] specified by the following equation (6) and performing cold rolling at a rolling reduction of 50 to 90%, a temperature T2 specified by the following equation (7) is used. The method for producing a ferritic stainless steel sheet excellent in formability according to claim 8, wherein the final annealing is performed at [° C].
800 ≦ T1 ≦ −30−5457 / [log {(Ti + Nb + Zr) × C} −2.6] {(6)
T1 + 5 ≦ T2 ≦ T1 + 60 (7)
Here, Ti [mass%], Nb [mass%], Zr [mass%] and C [mass%] are the contents of Ti, Nb, Zr and C, respectively.
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Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2006124770A (en) * | 2004-10-28 | 2006-05-18 | Jfe Steel Kk | Method for manufacturing ferritic stainless steel sheet |
| JP2006328525A (en) * | 2005-01-24 | 2006-12-07 | Nippon Steel & Sumikin Stainless Steel Corp | Low carbon-low nitrogen ferritic stainless steel thin sheet having reduced plane anisotropy upon forming and having excellent ridging resistance and roughening resistance, and method for producing the same |
| JP2010043321A (en) * | 2008-08-12 | 2010-02-25 | Nippon Steel & Sumikin Stainless Steel Corp | Ferritic stainless steel sheet having reduced surface roughness due to working and method for producing the same |
| JP2020532651A (en) * | 2017-08-31 | 2020-11-12 | ポスコＰｏｓｃｏ | Ferritic stainless steel with improved heat dissipation and workability and its manufacturing method |
| WO2023089693A1 (en) | 2021-11-17 | 2023-05-25 | 日鉄ステンレス株式会社 | Ferritic stainless steel sheet |
-
2003
- 2003-01-14 JP JP2003006073A patent/JP2004217996A/en not_active Withdrawn
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2006124770A (en) * | 2004-10-28 | 2006-05-18 | Jfe Steel Kk | Method for manufacturing ferritic stainless steel sheet |
| JP4581630B2 (en) * | 2004-10-28 | 2010-11-17 | Ｊｆｅスチール株式会社 | Manufacturing method of ferritic stainless steel sheet and target temperature setting method in continuous annealing process |
| JP2006328525A (en) * | 2005-01-24 | 2006-12-07 | Nippon Steel & Sumikin Stainless Steel Corp | Low carbon-low nitrogen ferritic stainless steel thin sheet having reduced plane anisotropy upon forming and having excellent ridging resistance and roughening resistance, and method for producing the same |
| JP2010043321A (en) * | 2008-08-12 | 2010-02-25 | Nippon Steel & Sumikin Stainless Steel Corp | Ferritic stainless steel sheet having reduced surface roughness due to working and method for producing the same |
| JP2020532651A (en) * | 2017-08-31 | 2020-11-12 | ポスコＰｏｓｃｏ | Ferritic stainless steel with improved heat dissipation and workability and its manufacturing method |
| WO2023089693A1 (en) | 2021-11-17 | 2023-05-25 | 日鉄ステンレス株式会社 | Ferritic stainless steel sheet |
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