JP3886933B2

JP3886933B2 - Ferritic stainless steel sheet excellent in press formability and secondary workability and manufacturing method thereof

Info

Publication number: JP3886933B2
Application number: JP2003159275A
Authority: JP
Inventors: 保利秀嶋; 宏紀冨村; 直人平松
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 2003-06-04
Filing date: 2003-06-04
Publication date: 2007-02-28
Anticipated expiration: 2023-06-04
Also published as: ES2357303T3; EP1484424B1; US20090165905A1; EP1484424A1; JP2004360003A; CN1572895A; US20040244884A1; CN100363523C; DE602004028780D1; KR20040104939A; KR100595383B1

Description

【０００１】
【産業上の利用分野】
本発明は、プレス加工等で所定形状に加工され、加工後の真円度不良，ネジレ等の形状不良が少なく、引き続く穴拡げ加工等の二次加工の際に優れた穴拡げ性を呈する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】
SUS430，SUS430LXに代表されるフェライト系ステンレス鋼は、良好な耐食性を有し、高価なＮｉを含まないのでオーステナイト系ステンレス鋼に比較して経済性にも優れていることから、耐久消費財を始め広範な分野で使用されている。用途展開に伴って、フェライト系ステンレス鋼板から製品形状を得るプレス加工等の加工条件が過酷になってきている。プレス加工後に穴拡げ加工等の二次加工が施される場合もある。過酷な加工条件に対応させるため、従来材に比較して加工性が格段に優れたフェライト系ステンレス鋼板が要望されている。
【０００３】
フェライト系ステンレス鋼板の成形性向上を狙って、従来から数多くの研究が報告されている。Ｔｉ，Ｎｂの複合添加で炭窒化物を析出させ、マトリックス中のＣ，Ｎ濃度を低減させる手法が代表的な成形性向上方法である。Ｔｉ，Ｎｂの複合添加に加え、Ｍｇ系介在物を利用したリジング特性の向上（特開2000−192199号公報），成形性の評価指標であるランクフォード値（ｒ値）を向上させる熱延条件との組合せ（特公平8−26436号公報）も知られている。
【０００４】
【発明が解決しようとする課題】
フェライト系ステンレス鋼板の加工性を支配する因子としては、単にランクフォード値（ｒ値）やリジング性だけでなく、製品製造過程における一次加工品の形状凍結性や二次穴拡げ性も重要な因子と考えられている。
オーステナイト系に比較するとフェライト系ステンレス鋼板の加工性は一般に劣っており、特に一次成形後の板厚減少が大きい。しかも、一次成形後の板厚減少に大きな方向性があり、フェライト系ステンレス鋼板を円筒形状にプレス加工する際、加工条件が過酷になるほど真円度等の寸法精度のズレが大きくなる。更に、一次成形後の板厚減少にバラツキがあるため、穴拡げ加工等の二次加工の際に成形性が極端に悪化する。
【０００５】
プレス成形されたフェライト系ステンレス鋼板の寸法精度（真円度，真軸度，ネジレ等）が改善され、併せて二次穴拡げ性も改善されると、過酷な加工条件からオーステナイト系ステンレス鋼の使用を余儀なくされていた個所に安価なフェライト系ステンレス鋼板を使用でき、フェライト系ステンレス鋼板の更なる用途展開が図られる。
【０００６】
【課題を解決するための手段】
本発明は、このような要望に応えるべく案出されたものであり、析出物の粒径及び形態制御により、プレス加工後の寸法精度，二次穴拡げ性が改善されたフェライト系ステンレス鋼板を提供することを目的とする。
【０００７】
本発明のフェライト系ステンレス鋼板は、その目的を達成するため、Ｃ：０.０２質量％以下，Ｓｉ：０.８質量％以下，Ｍｎ：１.５質量％以下，Ｐ：０.０５０質量％以下，Ｓ：０.０１質量％以下，Ｃｒ：８.０〜３５.０質量％，Ｎ：０.０５質量％以下，Ｔｉ：０.０５〜０.４０質量％，Ｎｂ：０.１０〜０.５０質量％を含み、残部がＦｅ及び不可避的不純物からなり、（％Ｔｉ×％Ｎ）＜０.００５を満足する組成をもち、ＴｉＮを除く粒径０.１５μｍ以上の析出物が５０００〜５００００個／ｍｍ²の割合で析出していることを特徴とする。
【０００８】
該フェライト系ステンレス鋼板は、更に必要に応じてＮｉ：０.５質量％以下，Ｍｏ：３.０質量％以下，Ｃｕ：２.０質量％以下，Ｖ：０.３質量％以下，Ｚｒ：０.３質量％以下，Ａｌ：０.３質量％以下，Ｂ：０.０１００質量％以下の１種又は２種以上を含むことができる。
所定組成のフェライト系ステンレス鋼スラブを熱延終了温度８００℃以下で熱間圧延した後、熱延鋼帯を４５０〜１０８０℃×均熱１時間以下で熱延板焼鈍し、(再結晶完了温度−１００℃）〜（再結晶完了温度)×均熱１分以下の少なくとも１回以上の中間焼鈍を伴った冷間圧延で冷延鋼帯とし、次いで１０８０℃以下×均熱１分以下で仕上げ焼鈍することにより製造される。
【０００９】
【作用及び実施の態様】
本発明者等は、プレス加工後の寸法精度（真円度，真軸度，ネジレ等）を改善したフェライト系ステンレス鋼板を得る手段について種々検討した。その結果、ＴｉＮ及び仕上げ焼鈍後の析出物の形態が円筒形状にプレス成形した際の真円度や二次穴拡げ性に多大な影響を及ぼしていることを見出した。かかる知見をベースに、Ｃ，Ｎを炭窒化物として固溶する化学量論的な割合以上でＴｉ，Ｎｂを複合添加したフェライト系ステンレス鋼を用い、加工熱処理を最適化することにより目標特性を備えたフェライト系ステンレス鋼板が得られることを解明した。析出物の形態がプレス加工性，加工後の寸法精度に及ぼす影響は次のように推察される。
【００１０】
フェライト系ステンレス鋼に含まれるＣ，Ｎは、Ｔｉ，Ｎｂの添加によってほとんどが炭化物，窒化物として析出する。析出した炭化物，ＴｉＮを除く窒化物は、熱延板焼鈍→冷間圧延→仕上げ焼鈍の過程で大半が微細な析出物になる。製造した鋼帯を再結晶焼鈍する際、微細析出物がピン止め作用を発現せず、特定の結晶方位をもつ再結晶粒が優先成長する結果、異方性の大きな混粒組織となる。大きな異方性は、鋼板の一次加工時に特定方向に歪みを集中させる原因であり、プレス成形性，プレス加工後の寸法精度を低下させる。
【００１１】
粒径がある程度以上の析出物を生成することにより、再結晶時のピン止め作用を期待できる。ピン止め作用は、特定の結晶方位をもつ結晶粒の優先成長や粗大化を抑制し、異方性，ひいてはプレス加工後の寸法精度を改善する。ピン止め作用がプレス成形性、プレス加工後の寸法精度を向上させる効果は、後述の実施例にもみられるように、ＴｉＮを除く粒径０.１５μｍ以上の析出物を５０００〜５００００個／ｍｍ²の割合で析出させるとき顕著となる。
【００１２】
しかし、析出物のなかでも、ＴｉＮはプレス加工性，プレス加工後の寸法精度にとって好ましくない。実際、(％Ｔｉ×％Ｎ)が０.００５を超える鋼板を多段プレスすると割れが発生するが、サイコロ状に析出した粗大なＴｉＮが割れの起点に観察される。当該観察結果は、多段プレス時にサイコロ状ＴｉＮの頂点に応力が集中し、歪みの蓄積，微小クラックの発生が割れを誘発することを意味している。ＴｉＮ周辺における歪みの蓄積や微小クラックの発生は、二次穴拡げ性を悪化させることにもなる。
【００１３】
次いで、本発明で使用するフェライト系ステンレス鋼の成分，含有量等を説明する。
Ｃ：０ . ０２質量％以下
炭化物となって最終焼鈍時に再結晶フェライトをランダム化させるが、強度を上昇させる成分であるため過剰量のＣ含有は加工性を低下させる。炭化物析出に起因して耐食性も低下するので、可能な限りＣ含有量を低減することが望ましい。加工性，耐食性を考慮して、Ｃ含有量の上限を０.０２質量％に設定した。高い延性を確保し、二次穴拡げ性を更に改善する上では、０.０１５質量％以下のＣ含有量が好ましい。しかし、必要以上のＣ含有量低減は長時間の精錬を必要とし、鋼材コストを上昇させる原因になる。また、最終焼鈍時に再結晶フェライトのランダム化に寄与する炭化物の作用を効果的にする上で、０.００１質量％以上のＣ含有が好ましい。
【００１４】
Ｓｉ：０ . ８質量％以下
製鋼時に脱酸材として添加される合金成分であるが、固溶硬化能が高く、０.８質量％を超える過剰量のＳｉが含まれると材質硬化，延性低下を引き起こす。高延性，二次穴拡げ性の更なる改善には、Ｓｉ含有量の上限を０.５質量％に設定することが好ましい。
Ｍｎ：１ . ５質量％以下
小さな固溶強化能のため材質を硬化させる影響は少ないが、１.５質量％を超える過剰量のＭｎが含まれると、溶製時にＭｎ系ヒュームが発生し、製造性が劣化する。
【００１５】
Ｐ：０ . ０５０質量％以下
熱間加工性に有害な成分であり、加工性の観点からＰ含有量の上限を０.０５０質量％に設定した。
Ｓ：０ . ０１質量％以下
結晶粒界に偏析して結晶粒界を脆化する有害成分であるが、Ｓ含有量を０.０１質量％以下に規制することによりＳ起因の悪影響を抑制できる。
【００１６】
Ｃｒ：８ . ０〜３５ . ０質量％
ステンレス鋼に要求される耐食性を得るため、少なくとも８.０質量％のＣｒが必要である。しかし、Ｃｒの増量に伴って靭性，加工性が低下するので、Ｃｒ含有量の上限を３５.０質量％に設定した。高延性，二次穴拡げ性の更なる改善には、２０.０質量％以下のＣｒ含有量が好ましい。
Ｎ：０ . ０５質量％以下
窒化物となって最終焼鈍時に再結晶フェライトをランダム化させるが、強度上昇成分であるため過剰量のＮ含有は延性を低下させることになるので、可能な限りＮ含有量を低減することが好ましく、本発明では延性確保の観点からＮ含有量の上限を０.０５質量％に設定した。高延性，二次穴拡げ性を更に改善させる上では、０.０２質量％以下のＮ含有量が好ましい。しかし、必要以上のＮ含有量低減は長時間の精錬を必要とし、鋼材コストを上げる原因になる。再結晶フェライトのランダム化は、０.００１質量％以上のＮ含有で顕著になる。
【００１７】
Ｔｉ：０ . ０５〜０ . ４０質量％
Ｃ，Ｎを固定し、加工性，耐食性の向上に有効な合金成分であり、０.０５質量％以上でＴｉ添加の効果が発揮される。しかし、０.４０質量％を超える過剰量のＴｉ添加は、鋼材コストを上昇させ、Ｔｉ系介在物に起因する表面欠陥を発生させることから好ましくない。
Ｎｂ：０ . １０〜０ . ５０質量％
Ｔｉと同様にＣ，Ｎを固定し、加工性を向上させる合金成分である。本発明で重要なＴｉＮを除く粒径０.１５μｍ以上のＮｂ系析出物は炭化物，Ｆｅ₂Ｎｂであることが予想され、粒径０.１５μｍ以上の析出物の析出に必須の成分である。Ｎｂの添加効果は、０.１０質量％以上のＮｂ含有量で顕著になる。しかし、０.５０質量％を超えるＮｂの過剰添加は、粒径０.１５μｍ以上の析出物を必要以上に析出させ、再結晶温度を上げることにもなるので好ましくない。
【００１８】
Ｎｉ：０ . ５質量％以下
必要に応じて添加される合金成分であり、熱延板の靭性改善，耐食性の向上に有効である。しかし、Ｎｉの添加は原料コストの上昇や硬質化を招くため、Ｎｉ含有量の上限を０.５質量％に設定した。
Ｍｏ：３ . ０質量％以下
必要に応じて添加される合金成分であり、耐食性の改善に寄与する。しかし、Ｍｏの過剰添加は熱間加工性を低下させるので、Ｍｏを添加する場合には上限を３.０質量％に規制する。
【００１９】
Ｃｕ：２ . ０質量％以下
必要に応じて添加される合金成分であり、溶製時にスクラップ等から混入しやすい。過剰量のＣｕが含まれると脆化，熱間加工性低下の原因となるので、Ｃｕ含有量の上限を２.０質量％以下に設定する。
Ｖ：０ . ３質量％以下，Ｚｒ：０ . ３質量％以下
何れも必要に応じて添加される合金成分であり、Ｖは固溶Ｃを炭化物として析出させ加工性を向上させ、Ｚｒは鋼中の酸素を補足して加工性，靭性を向上させる。しかし、過剰添加すると製造性が低下するので、Ｖ，Ｚｒ共に含有量の上限を０.３質量％に設定する。
【００２０】
Ａｌ：０ . ３質量％以下
必要に応じて添加される合金成分であり、製鋼時に脱酸材として添加される。しかし、Ａｌの過剰添加は非金属介在物の増加を招き、靭性低下，表面欠陥の原因となるため、Ａｌ含有量の上限を０.３質量％に設定する。
Ｂ：０ . ０１００質量％以下
必要に応じて添加される合金成分であり、Ｎを固定し、耐食性，加工性を改善する作用を呈する。Ｂの添加効果は、Ｂ含有量０.００１０質量％以上でみられる。しかし、０.０１００質量％を超える過剰量のＢを過剰に添加すると、熱間加工性，溶接性が低下する。
以上に挙げた成分以外にＣａ，Ｍｇ，Ｃｏ，REM等が溶製中に原料であるスクラップから混入することもある。これらの成分は、格別多量に含まれる場合を除き、絞り加工時の真円度やプレス加工後の寸法精度に悪影響を及ぼさない。
【００２１】
( ％Ｔｉ×％Ｎ ) ＜０ . ００５
(％Ｔｉ×％Ｎ)の増加に伴って粗大なＴｉＮが生成し、或いはＴｉＮがクラスターを形成する。粗大なＴｉＮやクラスター状ＴｉＮは、一次加工時に歪みを蓄積して微小クラックを発生させ、結果として絞り加工の早期に割れ発生を促進させる。粗大なＴｉＮやクラスター状ＴｉＮに起因する悪影響は、後述の実施例でも確認されるように、(％Ｔｉ×％Ｎ)を０.００５未満に規制することにより抑制される。
【００２２】
ＴｉＮを除く粒径０ . １５μｍ以上の析出物：５０００〜５００００個／ｍｍ ²
粒径０.１５μｍ以上の炭化物，窒化物を析出させるとき、析出物のピン止め作用によって特定の結晶方位をもつ結晶粒の優先成長や結晶粒の粗大化が抑制され、異方性，ひいては円筒絞り後の真円度性やプレス加工後の寸法精度が改善される。
析出物は、ＴｉＮを除くＴｉ，Ｎｂの炭化物，窒化物，ラーベス相及びこれらの混合物である。しかし、サイコロ状に析出するＴｉＮは、加工時に析出物粒子の頂点で応力集中を生じさせ、歪みの蓄積，微小クラックの発生を引き起こして割れの起点になりやすいので、プレス成形性，プレス加工後の寸法精度に有効な析出物から除外した。
【００２３】
粒径０.１５μｍ以上の析出物のＴｉＮを除く析出物を５０００〜５００００個／ｍｍ²の割合で析出分布させるとき、後述の実施例で確認されるように、析出物のピン止め作用によってプレス成形性，プレス加工後の寸法精度が改善される。
析出物は、粒径０.１５μｍ以上でプレス成形性，プレス加工後の寸法精度に有効であり、粒径が大きくなるほど改善作用も大きくなる。しかし、１.０μｍを超える粗大析出物は、析出物の形状にもよるがプレス加工時に析出物周辺に歪みを蓄積し、微小クラックを発生させて成形加工時の形状凍結性を低下させることから好ましくない。５０００個／ｍｍ²以上の析出量でピン止め作用がみられるが、５００００個／ｍｍ²を超える析出量では却って延性が低下し、円筒絞り性が悪化する。過剰な析出量は鋼材の再結晶温度を上昇させ、再結晶焼鈍を困難にする点でも好ましくない。
【００２４】
次に、製造条件について説明するが、析出物の形態制御は当該製造条件の設定により始めて可能になる。
熱延終了温度：８００℃以下
粒径０.１５μｍ以上の析出物用の核サイトを最終焼鈍材に多量生成させるため、仕上げ熱延温度を低温化している。熱延後に生成する核サイトはフェライト結晶粒界や内部歪みであり、多量の核サイトを内蔵させるために８００℃以下の熱延終了温度が必要である。
【００２５】
熱延板焼鈍：４５０〜１０８０℃×均熱１時間以下
熱延板焼鈍により、析出物を目標の析出状態に調質され、最終焼鈍後に粒径０.１５μｍ以上の析出物が得られる。熱延板焼鈍温度が４５０℃未満では析出物がほとんど生成せず、逆に１０８０℃を超えるとＴｉＮを除く析出物がマトリックスに固溶する。そのため、４５０〜１０８０℃の温度域に熱延板焼鈍温度を設定し、析出物の個数を適正管理し、析出物の粗大成長を抑制するため均熱１時間以下に焼鈍時間を設定する。
【００２６】
中間焼鈍： ( 再結晶完了温度−１００℃ ) 〜 ( 再結晶完了温度 ) ×均熱１分以下
熱延板焼鈍で得られた析出物の再固溶を抑制するため、比較的低温域で中間焼鈍する。冷間圧延で導入された歪みを除去して軟質化させる上で、中間焼鈍温度を再結晶完了温度直下に設定することが好ましいが、(再結晶完了温度−１００℃)〜(再結晶完了温度)の温度範囲であれば、再結晶されていない圧延組織領域が若干存在するものの、析出物の再固溶抑制，軟質化の双方が達成される。焼鈍時間は、析出物の再固溶を抑制するため、通常の連続焼鈍ラインを想定して１分以下の短時間に設定する。
【００２７】
仕上げ焼鈍：１０８０℃以下×均熱１分以下
仕上げ焼鈍で圧延組織が解消されるが、高すぎる焼鈍温度では量産製造性が低下することは勿論、析出物が再固溶し結晶粒を粗大化して靭性低下の原因となるので、焼鈍温度の上限を１０５０℃に設定する。焼鈍時間は、連続焼鈍ラインを想定して１分以下の短時間に設定される。
【００２８】
【実施例１：基礎実験】
フェライト系ステンレス鋼に析出しやすいＴｉＮの影響及びプレス加工後の寸法精度，二次穴拡げ性に析出形態が及ぼす影響を次の条件下で調査した。
実験室的な溶解により、Ｃ：０.００７質量％，Ｓｉ：０.４０質量％，Ｍｎ：０.２５質量％，Ｐ：０.０３０質量％，Ｓ：０.０００５質量％，Ｃｕ：０.０５質量％，Ｃｒ：１６.５０質量％，Ａｌ：０.０４質量％を含み、Ｎｂを０.０２〜０.３０質量％，Ｔｉを０.０５〜０．３０％，Ｎを０.００５〜０.０３５％の範囲で変化させた９種の鋳片を溶製した。
各鋳片のＮｂ，Ｔｉ，Ｎ含有量と共に(％Ｔｉ×％Ｎ)，再結晶終了温度を表１に併せ示す。
【００２９】

【００３０】
各鋳片を最終仕上げ温度７５０℃で熱間圧延し、板厚４ｍｍの熱延板を得た。
鋼種Ｎo.１〜７の熱延板を８００℃×６０秒で熱延板焼鈍し、酸洗後に板厚２ｍｍまで冷間圧延した。更に、(再結晶完了温度−５０℃)×６０秒の中間焼鈍を伴った冷間圧延により板厚０.５ｍｍの冷延板を製造した。冷延板を１０００℃×６０秒で仕上げ焼鈍し、板厚０.５ｍｍの冷延焼鈍板を得た。
鋼種Ｎo.８，９の熱延板については、熱延板焼鈍後、酸洗し、板厚２ｍｍまで冷間圧延した。更に中間焼鈍を施し、板厚０.５ｍｍまで冷間圧延した後、仕上げ焼鈍して板厚０.５ｍｍの冷延焼鈍板を得た。鋼種Ｎo.８，９の熱延板焼鈍，中間焼鈍，仕上げ焼鈍の条件を表２に示す。
【００３１】

【００３２】
〔析出物の析出割合，析出形態の調査〕
各冷延焼鈍板から試験片を切り出し、１０％アセチルアセトン−１％テトラメチルアンモニウムクロライド−メチルアルコールを電解液に用いて非水溶媒系定電位電解エッチングした後、走査型電子顕微鏡により析出物の形態を観察した。
圧延方向に平行な板厚断面を観察対象に採り、任意の５０視野で析出物を観察し、個々の析出物について最大長さを当該析出物の粒径として測定した。
【００３３】
〔プレス加工後の寸法精度の調査〕
各冷延焼鈍板から切り出されたブランクを多段プレスして図１の円筒形状に成形し、フランジＦから５ｍｍの位置にある円筒部Ｃの最大半径，最小半径をレーザ変位計で測定し、(最大直径−最小直径)／(最小直径)の比率を算出した。該比率を真円度としてプレス加工後の寸法精度を評価した。
【００３４】
〔二次穴拡げ性の調査〕
直径１０３ｍｍ，肩半径１０ｍｍのパンチ及び直径１０５ｍｍ，肩半径８ｍｍのダイスを用い、フランジをビードで固定した張出し加工によって各冷延焼鈍板から張出し高さ１０ｍｍの加工品を作製し、加工品底部から直径９２ｍｍのブランクを切り出した。ブランクの中央に直径１０ｍｍの打抜き穴をクリアランス１０％で開けた後、二次穴拡げ試験に供した。
二次穴拡げ試験では、直径４０ｍｍ，肩半径３ｍｍのパンチ及び直径４２ｍｍ，肩半径３ｍｍのダイスを用い、打抜き穴のバリ方向をダイス側に設定し、フランジをビードで固定し、平頭パンチにより打抜き穴を穴縁に割れが発生するまで押し広げた。割れ発生時の穴直径を測定し、二次穴拡げ率＝[(試験後の打抜き穴直径−試験前の打抜き穴直径)／試験前の打抜き穴直径]×１００(％)として二次穴拡げ率を算出した。
【００３５】
表３の調査結果にみられるように、(％Ｔｉ×％Ｎ)が０.００５を超えると多段絞りの途中で割れが発生し、Ｎｂ含有量が０.０２質量％と少ない鋼種は何れの製造工程を経ても真円度に劣っていた。割れが発生した鋼種や真円度の劣る鋼種を観察した結果、何れの鋼種でもＴｉＮを除く粒径０.１５μｍ以上の析出物の個数が非常に少ないことが判った。他方、Ｎｂ：０.３質量％の鋼種では、加工熱処理条件との組合せによってＴｉＮを除く粒径０.１５μｍ以上の析出物がある程度以上の個数になると、真円度の向上がみられたが、析出物を過剰に生成させると却って真円度が低下する傾向にあった。
【００３６】
二次穴拡げ性も、(％Ｔｉ×％Ｎ)≧０.００５の鋼種では非常に劣っており、Ｎｂ：０.０２質量％の鋼種でも低い二次穴拡げ率が示された。他方、Ｎｂ：０.３質量％の鋼種では、ＴｉＮを除く粒径０.１５μｍ以上の析出物がある程度以上の個数になると二次穴拡げ性の改善がみられたが、析出物を過剰に生成させると却って二次穴拡げ性が低下する傾向にあった。
以上の結果は、ＴｉＮを除く粒径０.１５μｍ以上の析出物の形態にプレス加工後の寸法精度，二次穴拡げ性が依存していることを示す。そして、加工熱処理条件を最適化してＴｉＮを除く粒径０.１５μｍ以上の析出物の析出量を５０００〜５００００個／ｍｍ²の範囲に調整することにより、優れたプレス加工後の寸法精度，二次穴拡げ性が得られることが確認される。
【００３７】

【００３８】
【実施例２】
表４に示す組成のステンレス鋼を真空溶解炉で溶製し、得られた鋳片を板厚４.０ｍｍに熱間圧延した。各熱延板を熱延板焼鈍後、酸洗し、板厚２ｍｍまで冷間圧延した。更に中間焼鈍を施し、板厚０.５ｍｍまで冷間圧延した後、仕上げ焼鈍し、板厚０.５ｍｍの冷延焼鈍板を得た。仕上げ焼鈍温度，熱延板焼鈍，中間焼鈍，仕上げ焼鈍の熱処理条件を表５に示す。表４中、鋼種Ａ〜Ｈが本発明で規定した組成条件を満足し、鋼種Ｉ〜Ｌが本発明で規定した範囲を外れる。
【００３９】

【００４０】

【００４１】
各冷延焼鈍板について、実施例１と同様に析出物の形態，析出量及びプレス加工後の寸法精度，二次穴拡げ性を調査した。
表６の調査結果にみられるように、ＴｉＮを除く粒径０.１５μｍ以上の析出物が５０００〜５００００個／ｍｍ²の割合で分散しているフェライト系ステンレス鋼板をプレス成形したとき、２.５％以下と真円度が極めて良好な加工品が得られた。
【００４２】
他方、組成的には本発明で既定した条件を満足するものの製造条件が不適切な比較材（試験Ｎos.Ａ２，Ｂ２，Ｃ２，Ｄ２）は、ＴｉＮを除く粒径０.１５μｍ以上の析出物の析出割合が５０００〜５００００個／ｍｍ²の範囲を外れ、プレス加工後の寸法精度，二次穴拡げ性に劣っていた。過剰のＣを含む試験Ｎo.１は材料自体が硬質であり、過剰のＮｂを含む試験Ｎo.Ｋは材料自体の強度が高すぎるため、何れの鋼板も所定形状の成形品に加工する前に割れが発生した。(％Ｔｉ×％Ｎ)が０.００５を超える試験Ｎo.Ｌは、所定形状の成形品に加工する前に粗大なＴｉＮを起点とする割れが発生した。Ｎｂが不足する試験Ｎo.Ｊは、プレス加工後の真円度に劣っていた。
以上の対比結果は、ＴｉＮを除く粒径０.１５μｍ以上の析出物の形態制御によって、プレス加工後の寸法精度，二次穴拡げ性に優れたフェライト系ステンレス鋼板となることを示している。
【００４３】

【００４４】
【発明の効果】
以上に説明したように、成分・組成が特定されたフェライト系ステンレス鋼において、ＴｉＮを除く粒径０.１５μｍ以上の析出物を５０００〜５００００個／ｍｍ²の割合でマトリックスに分散させるとき、プレス加工後の寸法精度，二次穴拡げ性に優れたフェライト系ステンレス鋼板が得られる。析出物の形態，分布制御は、熱延終了温度，熱延板焼鈍，中間焼鈍，仕上げ焼鈍の適正管理により達成される。プレス加工後の寸法精度，二次加工時の穴拡げ性が改善されたフェライト系ステンレス鋼板は、寸法精度が厳しい有機ＥＬ素子用封止材等のＩＴ関連部品や各種精密プレス製品，シンク，各種器物，コンロ用バーナ等の家庭用機器・部品，燃料用のタンクや給油管，モータケース，カバー，センサーキャップ，インジェクタ管，サーモスタットバルブ，ベアリングシール材，フランジ等の産業用機器の部品，建築部材等、広範な分野で高価なオーステナイト系に代わる材料として使用される。
【図面の簡単な説明】
【図１】多段プレスで成形した円筒状加工品の真円度を求めることを説明する図[0001]
[Industrial application fields]
The present invention is a ferrite that is processed into a predetermined shape by press processing or the like, has few roundness defects after processing, shape defects such as torsion, etc., and exhibits excellent hole expandability in subsequent secondary processing such as hole expansion processing. The present invention relates to a stainless steel plate and a method for producing the same.
[0002]
[Prior art]
Ferritic stainless steel represented by SUS430 and SUS430LX has good corrosion resistance and does not contain expensive Ni, so it is more economical than austenitic stainless steel. Used in a wide range of fields. With the development of applications, processing conditions such as press processing for obtaining a product shape from a ferritic stainless steel sheet have become severe. There is a case where secondary processing such as hole expansion processing is performed after the press processing. In order to cope with harsh processing conditions, a ferritic stainless steel sheet is required that has significantly better workability than conventional materials.
[0003]
Many studies have been reported to improve the formability of ferritic stainless steel sheets. A typical formability improving method is a method of precipitating carbonitride by combined addition of Ti and Nb to reduce the C and N concentration in the matrix. In addition to the combined addition of Ti and Nb, improvement of ridging characteristics using Mg-based inclusions (Japanese Patent Laid-Open No. 2000-192199), hot rolling conditions for improving the Rankford value (r value), which is an evaluation index of formability A combination with (Japanese Patent Publication No. 8-26436) is also known.
[0004]
[Problems to be solved by the invention]
Factors governing the workability of ferritic stainless steel sheets are not only the Lankford value (r value) and ridging properties, but also the shape freezing properties and secondary hole expandability of the primary processed product in the product manufacturing process. It is believed that.
Compared with austenite, the ferritic stainless steel sheet is generally inferior in workability, and the thickness reduction after primary forming is particularly large. In addition, there is a great direction in reducing the plate thickness after the primary forming, and when the ferritic stainless steel plate is pressed into a cylindrical shape, the deviation of dimensional accuracy such as roundness increases as the processing conditions become severe. Furthermore, since the thickness reduction after the primary forming varies, the formability is extremely deteriorated during secondary processing such as hole expansion processing.
[0005]
If the dimensional accuracy (roundness, uniaxiality, torsion, etc.) of the press-formed ferritic stainless steel sheet is improved and the secondary hole expansibility is also improved, the austenitic stainless steel can Inexpensive ferritic stainless steel sheets can be used in places where they were forced to use, and further applications of ferritic stainless steel sheets can be developed.
[0006]
[Means for Solving the Problems]
The present invention has been devised to meet such demands. A ferritic stainless steel sheet having improved dimensional accuracy and secondary hole expansibility after press working by controlling the particle size and form of precipitates. The purpose is to provide.
[0007]
In order to achieve the object, the ferritic stainless steel sheet of the present invention has C: 0.02 mass% or less, Si: 0.8 mass% or less, Mn: 1.5 mass% or less, P: 0.050 mass%. Hereinafter, S: 0.01 mass% or less, Cr: 8.0-35.0 mass%, N: 0.05 mass% or less, Ti: 0.05-0.40 mass%, Nb: 0.10 A precipitate containing 0.50% by mass, the balance being Fe and inevitable impurities , having a composition satisfying (% Ti ×% N) <0.005, and having a particle size of 0.15 μm or more excluding TiN It is characterized by being deposited at a rate of ˜50000 pieces / mm ² .
[0008]
If necessary, the ferritic stainless steel sheet may further comprise Ni: 0.5 mass% or less, Mo: 3.0 mass% or less, Cu: 2.0 mass% or less, V: 0.3 mass% or less, Zr: One or more of 0.3% by mass or less, Al: 0.3% by mass or less, and B: 0.0100% by mass or less can be included.
After hot rolling a ferritic stainless steel slab having a predetermined composition at a hot rolling finish temperature of 800 ° C. or lower, the hot rolled steel strip is annealed at 450 to 1080 ° C. × soaking for 1 hour or shorter (recrystallization completion temperature). -100 ° C) to (recrystallization completion temperature) x cold rolling with at least one intermediate annealing of 1 minute or less soaking into a cold-rolled steel strip, then finishing at 1080 ° C or less x soaking 1 minute or less Manufactured by annealing.
[0009]
[Action and embodiment]
The present inventors have studied various means for obtaining a ferritic stainless steel sheet with improved dimensional accuracy (roundness, uniaxiality, torsion, etc.) after press working. As a result, it has been found that the form of the precipitates after TiN and finish annealing has a great influence on the roundness and the secondary hole expansibility when pressed into a cylindrical shape. Based on this knowledge, ferritic stainless steel with a combined addition of Ti and Nb in a stoichiometric proportion or higher where C and N are dissolved as carbonitrides is used, and the target characteristics are achieved by optimizing the thermomechanical treatment. It was clarified that the provided ferritic stainless steel sheet was obtained. The influence of the form of precipitates on press workability and post-working dimensional accuracy is presumed as follows.
[0010]
Most of C and N contained in ferritic stainless steel is precipitated as carbides and nitrides by the addition of Ti and Nb. Most of the precipitated carbides and nitrides except TiN become fine precipitates in the process of hot-rolled sheet annealing → cold rolling → finish annealing. When the manufactured steel strip is subjected to recrystallization annealing, fine precipitates do not exhibit a pinning action, and recrystallized grains having a specific crystal orientation preferentially grow, resulting in a mixed grain structure with large anisotropy. The large anisotropy is a cause of concentration of strain in a specific direction during the primary processing of the steel sheet, and reduces press formability and dimensional accuracy after press processing.
[0011]
By generating a precipitate having a particle size of a certain level or more, a pinning action during recrystallization can be expected. The pinning action suppresses the preferential growth and coarsening of crystal grains having a specific crystal orientation, and improves the anisotropy and thus the dimensional accuracy after press working. Pinning action press formability, the effect of improving the dimensional accuracy after the press working, as also seen in the examples below, 5,000 to 50,000 pieces of grain size 0.15μm or more precipitates except TiN / mm ² It becomes remarkable when it precipitates in the ratio.
[0012]
However, among the precipitates, TiN is not preferable for press workability and dimensional accuracy after press work. Actually, cracking occurs when a steel plate having (% Ti ×% N) exceeding 0.005 is multistage pressed, but coarse TiN deposited in a dice shape is observed at the starting point of the cracking. This observation result means that stress concentrates on the apex of the dice-like TiN during multi-stage pressing, and accumulation of strain and generation of microcracks induce cracking. Accumulation of strain around the TiN and the occurrence of microcracks also deteriorate secondary hole expansibility.
[0013]
Next, components, contents and the like of the ferritic stainless steel used in the present invention will be described.
C:. 0 02 Although mass% or less become carbides to randomize recrystallized ferrite during final annealing, C content of excess for a component to increase the strength decreases the workability. Since corrosion resistance also decreases due to carbide precipitation, it is desirable to reduce the C content as much as possible. In consideration of workability and corrosion resistance, the upper limit of the C content was set to 0.02% by mass. In order to secure high ductility and further improve secondary hole expansibility, a C content of 0.015% by mass or less is preferable. However, reducing the C content more than necessary requires refining for a long time, which increases the cost of steel. Moreover, in order to make the effect | action of the carbide | carbonized_material which contributes to randomization of recrystallized ferrite at the time of final annealing effective, 0.001 mass% or more of C containing is preferable.
[0014]
Si:. 0 but when 8 wt% or less steel is an alloy component added as a deoxidizing material, a solid solution hardening ability is high, excessive addition of Si more than 0.8 mass% Material Causes hardening and ductility degradation. For further improvement of high ductility and secondary hole expansibility, it is preferable to set the upper limit of the Si content to 0.5 mass%.
Mn:. 1 when it 5 Effect of curing the material for mass% or less small solid solution strengthening ability is small, contains an excess amount of Mn in excess of 1.5 mass%, Mn system during melting Hume Occurs and the manufacturability deteriorates.
[0015]
P:. 0 a harmful component 050 mass% or less hot workability, an upper limit of P content in the 0.050 wt% in view of workability.
S:. 0 01 wt% or less but segregated in the grain boundary is a harmful components that embrittle the grain boundaries, the S caused by regulating the S content below 0.01 wt% Adverse effects can be suppressed.
[0016]
Cr:.. 8 0~35 0 mass%
In order to obtain the corrosion resistance required for stainless steel, at least 8.0% by mass of Cr is required. However, since the toughness and workability decrease as the amount of Cr increases, the upper limit of the Cr content is set to 35.0% by mass. For further improvement of high ductility and secondary hole expansibility, a Cr content of 20.0 mass% or less is preferable.
N:. 0 05 wt% or less but becomes nitride to randomize recrystallized ferrite during final annealing, the N content of the excess for an increase in strength component will reduce the ductility, It is preferable to reduce the N content as much as possible. In the present invention, the upper limit of the N content is set to 0.05% by mass from the viewpoint of ensuring ductility. In order to further improve the high ductility and secondary hole expansibility, an N content of 0.02% by mass or less is preferable. However, reducing the N content more than necessary requires refining for a long time, which increases the cost of steel. Randomization of recrystallized ferrite becomes remarkable when N content is 0.001% by mass or more.
[0017]
Ti:.. 0 05~0 40 wt%
It is an alloy component that fixes C and N and is effective for improving workability and corrosion resistance. The effect of addition of Ti is exhibited at 0.05% by mass or more. However, an excessive amount of Ti addition exceeding 0.40 mass% is not preferable because it increases the cost of steel materials and generates surface defects due to Ti inclusions.
Nb:.. 0 10~0 50 wt%
Like Ti, it is an alloy component that fixes C and N and improves workability. The Nb-based precipitate having a particle size of 0.15 μm or more excluding TiN, which is important in the present invention, is expected to be a carbide, Fe ₂ Nb, and is an essential component for the precipitation of precipitates having a particle size of 0.15 μm or more. The effect of adding Nb becomes remarkable when the Nb content is 0.10% by mass or more. However, excessive addition of Nb exceeding 0.50% by mass is not preferable because precipitates having a particle size of 0.15 μm or more are precipitated more than necessary and the recrystallization temperature is raised.
[0018]
Ni:. 0 is 5 wt% or less alloy components added as required, improving toughness of the hot-rolled sheet is effective in improving corrosion resistance. However, the addition of Ni causes an increase in raw material cost and hardening, so the upper limit of Ni content was set to 0.5 mass%.
Mo:. 3 0 wt% or less alloy components added as required, contributes to the improvement of corrosion resistance. However, excessive addition of Mo reduces hot workability, so when adding Mo, the upper limit is regulated to 3.0% by mass.
[0019]
Cu:. 2 0 is the mass% or less alloying element be added if necessary, easily mixed from scraps during melting. If an excessive amount of Cu is contained, it causes embrittlement and a decrease in hot workability, so the upper limit of the Cu content is set to 2.0 mass% or less.
V:. 0 3 mass% or less, Zr:. 0 a 3 wt% or less alloy component both being optionally added, V is to improve the workability to precipitate the solute C as carbides Zr supplements oxygen in steel and improves workability and toughness. However, since the productivity decreases when excessively added, the upper limit of the content of both V and Zr is set to 0.3% by mass.
[0020]
Al:. 0 a 3 wt% alloy components added as needed will be added during the steel making as a deoxidizer. However, excessive addition of Al causes an increase in non-metallic inclusions and causes toughness reduction and surface defects, so the upper limit of Al content is set to 0.3% by mass.
B:. 0 is an alloy component to be added according to 0100 wt% or less necessary, to secure the N, exhibits an effect of improving corrosion resistance, the workability. The effect of adding B is observed when the B content is 0.0010% by mass or more. However, if an excessive amount of B exceeding 0.0100% by mass is added excessively, hot workability and weldability deteriorate.
In addition to the components listed above, Ca, Mg, Co, REM, and the like may be mixed from the raw material scrap during melting. These components do not adversely affect the roundness at the time of drawing and the dimensional accuracy after press processing, except when included in a particularly large amount.
[0021]
(% Ti ×% N) < 0. 005
Coarse TiN is generated as (% Ti ×% N) increases, or TiN forms clusters. Coarse TiN or clustered TiN accumulates strain during primary processing to generate microcracks, and as a result, promotes cracking early in the drawing process. The adverse effects caused by coarse TiN and clustered TiN are suppressed by restricting (% Ti ×% N) to less than 0.005, as will be confirmed in examples described later.
[0022]
. The particle size 0 15μm or more of the precipitate with the exception of TiN: 5000~50000 pieces / mm ²
When precipitating carbides and nitrides with a grain size of 0.15 μm or more, the pinning action of the precipitates suppresses the preferential growth of crystal grains with a specific crystal orientation and the coarsening of the crystal grains. The roundness after drawing and the dimensional accuracy after pressing are improved.
Precipitates are Ti, Nb carbides, nitrides, Laves phases and mixtures thereof excluding TiN. However, TiN that precipitates in a dice shape causes stress concentration at the apex of the precipitate particles during processing, which tends to cause strain accumulation and generation of microcracks. It was excluded from precipitates effective for dimensional accuracy.
[0023]
When depositing the precipitate excluding TiN having a particle size of 0.15 μm or more at a rate of 5000 to 50000 pieces / mm ² , as shown in the examples described later, the precipitate is pressed by the pinning action. Formability and dimensional accuracy after pressing are improved.
Precipitates are effective for press formability and dimensional accuracy after pressing when the particle size is 0.15 μm or more, and the effect of improvement increases as the particle size increases. However, coarse precipitates exceeding 1.0 μm, depending on the shape of the precipitates, accumulate strain around the precipitates during press working, and generate microcracks, reducing the shape freezing property during forming. It is not preferable. A pinning action is observed at a deposition amount of 5000 pieces / mm ² or more, but at a deposition amount exceeding 50000 pieces / mm ² , the ductility is lowered and the cylindrical drawability is deteriorated. An excessive amount of precipitation is also undesirable in that it increases the recrystallization temperature of the steel material and makes recrystallization annealing difficult.
[0024]
Next, manufacturing conditions will be described. The form control of precipitates can be performed only by setting the manufacturing conditions.
Hot rolling end temperature: 800 ° C. or lower The final hot rolling temperature is lowered in order to produce a large amount of core sites for precipitates having a particle size of 0.15 μm or more in the final annealed material. The nuclear sites generated after hot rolling are ferrite grain boundaries and internal strains, and a hot rolling end temperature of 800 ° C. or lower is necessary to incorporate a large amount of nuclear sites.
[0025]
Hot-rolled sheet annealing: 450-1080 [deg.] C. x soaking for 1 hour or less The precipitates are tempered to a target precipitation state by hot-rolled sheet annealing, and precipitates with a particle size of 0.15 [ mu] m or more are obtained after final annealing. can get. When the hot-rolled sheet annealing temperature is less than 450 ° C., almost no precipitate is generated. Conversely, when it exceeds 1080 ° C., the precipitate excluding TiN is dissolved in the matrix. Therefore, the hot-rolled sheet annealing temperature is set in a temperature range of 450 to 1080 ° C., the number of precipitates is properly controlled, and the annealing time is set to 1 hour or less for soaking in order to suppress coarse growth of the precipitates.
[0026]
Intermediate annealing: ( recrystallization completion temperature−100 ° C. ) to ( recrystallization completion temperature ) × soaking for 1 minute or less <In order to suppress re-solution of precipitates obtained by hot-rolled sheet annealing, Intermediate annealing at low temperature. In order to remove the strain introduced in the cold rolling and soften it, it is preferable to set the intermediate annealing temperature directly below the recrystallization completion temperature, but (recrystallization completion temperature−100 ° C.) to (recrystallization completion temperature). In the temperature range of (2), although there are some rolled structure regions that are not recrystallized, both re-solution suppression and softening of the precipitate are achieved. In order to suppress re-dissolution of precipitates, the annealing time is set to a short time of 1 minute or less assuming a normal continuous annealing line.
[0027]
Finish annealing: 1080 ° C. or less × soaking for 1 minute or less The rolling structure is eliminated by finish annealing, but mass production productivity is lowered at an annealing temperature that is too high. Since the grains become coarse and cause toughness reduction, the upper limit of the annealing temperature is set to 1050 ° C. The annealing time is set to a short time of 1 minute or less assuming a continuous annealing line.
[0028]
[Example 1: Basic experiment]
The effects of TiN, which tends to precipitate on ferritic stainless steel, as well as the effect of precipitation on the dimensional accuracy and secondary hole expansibility after pressing were investigated under the following conditions.
By laboratory dissolution, C: 0.007 mass%, Si: 0.40 mass%, Mn: 0.25 mass%, P: 0.030 mass%, S: 0.0005 mass%, Cu: 0 0.05% by mass, Cr: 16.50% by mass, Al: 0.04% by mass, Nb 0.02 to 0.30% by mass, Ti 0.05 to 0.30%, and N 0.0%. Nine types of slabs varied from 005 to 0.035%.
Table 1 shows the recrystallization end temperature together with the Nb, Ti, and N contents of each slab (% Ti ×% N).
[0029]

[0030]
Each slab was hot-rolled at a final finishing temperature of 750 ° C. to obtain a hot-rolled sheet having a thickness of 4 mm.
Hot rolled sheets of steel types No. 1-7 were annealed at 800 ° C. for 60 seconds and cold rolled to a thickness of 2 mm after pickling. Furthermore, a cold-rolled sheet having a thickness of 0.5 mm was manufactured by cold rolling with intermediate annealing (recrystallization completion temperature−50 ° C.) × 60 seconds. The cold-rolled sheet was finish-annealed at 1000 ° C. for 60 seconds to obtain a cold-rolled annealed sheet having a thickness of 0.5 mm.
The hot rolled sheets of steel types Nos. 8 and 9 were pickled and hot rolled to a thickness of 2 mm after annealing. Further, intermediate annealing was performed, and after cold rolling to a thickness of 0.5 mm, finish annealing was performed to obtain a cold-rolled annealing plate having a thickness of 0.5 mm. Table 2 shows the conditions of hot-rolled sheet annealing, intermediate annealing, and finish annealing of steel types No. 8 and 9.
[0031]

[0032]
[Investigation of precipitation rate and form of precipitates]
A test piece was cut out from each cold-rolled annealed plate, 10% acetylacetone-1% tetramethylammonium chloride-methyl alcohol was used as an electrolyte solution, non-aqueous solvent-based constant-potential electrolytic etching, and the form of precipitates by a scanning electron microscope Was observed.
A plate thickness cross section parallel to the rolling direction was taken as an observation object, and precipitates were observed in arbitrary 50 fields of view, and the maximum length of each precipitate was measured as the particle size of the precipitate.
[0033]
[Investigation of dimensional accuracy after press working]
A blank cut from each cold-rolled annealed plate is multi-stage pressed to form the cylindrical shape of FIG. 1, and the maximum radius and the minimum radius of the cylindrical portion C located 5 mm from the flange F are measured with a laser displacement meter. The ratio of maximum diameter-minimum diameter / minimum diameter was calculated. The dimensional accuracy after press working was evaluated using the ratio as the roundness.
[0034]
[Investigation of secondary hole expandability]
Using a punch having a diameter of 103 mm and a shoulder radius of 10 mm and a die having a diameter of 105 mm and a shoulder radius of 8 mm, a workpiece having a protruding height of 10 mm is produced from each cold-rolled annealed plate by an extension process in which the flange is fixed with a bead. A blank with a diameter of 92 mm was cut out. A punched hole having a diameter of 10 mm was formed in the center of the blank with a clearance of 10%, and then subjected to a secondary hole expansion test.
In the secondary hole expansion test, a punch with a diameter of 40 mm and a shoulder radius of 3 mm and a die with a diameter of 42 mm and a shoulder radius of 3 mm were used, the burr direction of the punched hole was set to the die side, the flange was fixed with a bead, and punched with a flat head punch. The hole was expanded until cracking occurred at the hole edge. Measure the hole diameter at the time of crack occurrence, secondary hole expansion rate = [(punched hole diameter after test-punched hole diameter before test) / punched hole diameter before test] x 100 (%) The rate was calculated.
[0035]
As can be seen from the results of the investigation in Table 3, when (% Ti ×% N) exceeds 0.005, cracking occurs in the middle of multistage drawing, and Nb content is as low as 0.02 mass%. Even after the manufacturing process, the roundness was inferior. As a result of observing the steel types with cracks and inferior roundness, it was found that the number of precipitates having a particle size of 0.15 μm or more excluding TiN was very small. On the other hand, in the steel type of Nb: 0.3% by mass, roundness was improved when the number of precipitates having a particle size of 0.15 μm or more excluding TiN became a certain number or more by combination with the thermomechanical treatment conditions. On the other hand, when the precipitates are generated excessively, the roundness tends to decrease.
[0036]
The secondary hole expandability was also very inferior in the steel type of (% Ti ×% N) ≧ 0.005, and a low secondary hole expansion rate was shown even in the steel type of Nb: 0.02 mass%. On the other hand, in the steel type of Nb: 0.3% by mass, when the number of precipitates with a particle size of 0.15 μm or more excluding TiN reached a certain number, the secondary hole expansibility was improved. On the contrary, the secondary hole expansibility tends to decrease.
The above results show that dimensional accuracy after press working and secondary hole expansibility depend on the form of precipitates having a particle size of 0.15 μm or more excluding TiN. Then, by optimizing the heat treatment conditions and adjusting the amount of precipitates with a particle size of 0.15 μm or more excluding TiN to the range of 5000 to 50000 pieces / mm ² , excellent dimensional accuracy after press working, It is confirmed that the next hole expandability can be obtained.
[0037]

[0038]
[Example 2]
Stainless steel having the composition shown in Table 4 was melted in a vacuum melting furnace, and the resulting slab was hot rolled to a plate thickness of 4.0 mm. Each hot-rolled sheet was subjected to hot-rolled sheet annealing, pickled, and cold-rolled to a thickness of 2 mm. Further, intermediate annealing was performed, and after cold rolling to a thickness of 0.5 mm, finish annealing was performed to obtain a cold-rolled annealing plate having a thickness of 0.5 mm. Table 5 shows the heat treatment conditions of finish annealing temperature, hot rolled sheet annealing, intermediate annealing, and finish annealing. In Table 4, steel types A to H satisfy the composition conditions defined in the present invention, and steel types I to L deviate from the ranges defined in the present invention.
[0039]

[0040]

[0041]
About each cold-rolled annealing board, it carried out similarly to Example 1, and investigated the form of the precipitate, the precipitation amount, the dimensional accuracy after press work, and the secondary hole expansibility.
As seen in the investigation results in Table 6, when a ferritic stainless steel sheet in which precipitates having a particle size of 0.15 μm or more excluding TiN are dispersed at a rate of 5000 to 50000 pieces / mm ² is pressed. A processed product with an extremely good roundness of 5% or less was obtained.
[0042]
On the other hand, a comparative material (test Nos. A2, B2, C2, D2) that satisfies the conditions defined in the present invention in terms of composition but is inadequate in manufacturing conditions is a precipitate having a particle size of 0.15 μm or more excluding TiN. The deposition rate was out of the range of 5000 to 50000 pieces / mm ² , and the dimensional accuracy after press working and secondary hole expansibility were inferior. The test No. 1 containing excess C is hard in the material itself, and the test No. K containing excess Nb is too strong in the material itself. Therefore, before processing any steel plate into a molded product of a predetermined shape, Cracking occurred. In the test No. L in which (% Ti ×% N) exceeds 0.005, cracks originating from coarse TiN occurred before being processed into a molded product having a predetermined shape. Test No. J in which Nb was insufficient was inferior in roundness after press working.
The above comparison results show that a ferritic stainless steel sheet excellent in dimensional accuracy after press working and secondary hole expandability can be obtained by controlling the form of precipitates having a particle size of 0.15 μm or more excluding TiN.
[0043]

[0044]
【The invention's effect】
As described above, in a ferritic stainless steel whose components and compositions are specified, when a precipitate having a particle size of 0.15 μm or more excluding TiN is dispersed in a matrix at a rate of 5000 to 50000 pieces / mm ² , Ferritic stainless steel sheet with excellent dimensional accuracy after processing and secondary hole expandability can be obtained. Precipitation morphology and distribution control are achieved by appropriate management of hot rolling end temperature, hot rolled sheet annealing, intermediate annealing, and finish annealing. Ferritic stainless steel sheet with improved dimensional accuracy after press working and hole expandability during secondary processing, IT related parts such as organic EL element sealing materials with strict dimensional accuracy, various precision press products, sinks, various Household equipment and parts such as containers, stove burners, fuel tanks and fuel supply pipes, motor cases, covers, sensor caps, injector pipes, thermostat valves, bearing seal materials, flanges and other industrial equipment parts, building components It is used as an alternative to expensive austenitic materials in a wide range of fields.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining the roundness of a cylindrical workpiece formed by a multi-stage press.

Claims

C: 0.02 mass% or less, Si: 0.8 mass% or less, Mn: 1.5 mass% or less, P: 0.050 mass% or less, S: 0.01 mass% or less, Cr: 8.0 -35.0% by mass, N: 0.05% by mass or less, Ti: 0.05-0.40% by mass, Nb: 0.10-0.50% by mass, the balance being Fe and inevitable impurities And having a composition satisfying (% Ti ×% N) <0.005, precipitates having a particle size of 0.15 μm or more excluding TiN are precipitated at a rate of 5000 to 50000 pieces / mm ^2. Ferritic stainless steel sheet with excellent press formability and secondary workability.

Furthermore, Ni: 0.5 mass% or less, Mo: 3.0 mass% or less, Cu: 2.0 mass% or less, V: 0.3 mass% or less, Zr: 0.3 mass% or less, Al: 0 The ferritic stainless steel sheet according to claim 1, comprising one or more of 0.3 mass% or less and B: 0.0100 mass% or less.

After hot-rolling a ferritic stainless steel slab having the composition of claim 1 or 2 at a hot rolling end temperature of 800 ° C or lower, the hot rolled steel strip is annealed at 450 to 1080 ° C x soaking for 1 hour or shorter. (Recrystallization completion temperature−100 ° C.) to (recrystallization completion temperature) × soaking at least one intermediate annealing of 1 minute or less to form a cold-rolled steel strip, then 1080 ° C. or less × A method for producing ferritic stainless steel excellent in press formability and secondary workability, characterized by finishing annealing in less than 1 minute.