JP2004360003A

JP2004360003A - Ferritic stainless steel sheet superior in press formability and fabrication quality, and manufacturing method therefor

Info

Publication number: JP2004360003A
Application number: JP2003159275A
Authority: JP
Inventors: Yasutoshi Hideshima; 保利秀嶋; Hiroki Tomimura; 宏紀冨村; Naoto Hiramatsu; 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: 2004-12-24
Anticipated expiration: 2023-06-04
Also published as: ES2357303T3; US20090165905A1; US20040244884A1; EP1484424A1; KR100595383B1; JP3886933B2; CN1572895A; KR20040104939A; CN100363523C; DE602004028780D1; EP1484424B1

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel sheet having an improved accuracy of dimension after having been press-worked, and improved secondary hole-expandability, by controlling the form of precipitates. <P>SOLUTION: This stainless steel sheet has a composition comprising, by mass%, 0.02% or less C, 0.8% or less Si, 1.5% or less Mn, 0.050% or less P, 0.01% or less S, 8.0-35.0% Cr, 0.05% or less N, 0.05-0.40% Ti and 0.10-0.50% Nb while controlling (%Ti×%N) to less than 0.005; and has precipitates with particle diameters of 0.15 μm or larger except TiN precipitated in a density of 5,000 to 50,000 pieces per square millimeter. The manufacturing method comprises hot-rolling a stainless steel slab having the above composition at a hot-rolling-finishing temperature of 800°C or lower, annealing the hot-rolled plate at 450 to 1,080°C for a soaking time of one hour or shorter, cold-rolling it into a cold-rolled steel strip while inserting an intermediate annealing at a temperature between (the recrystallization finishing temperature -100°C) and (the recrystallization finishing temperature ) for a soaking time of one minute or shorter, and then finish-annealing it at 1,080°C or lower for a soaking time of one minute or shorter. <P>COPYRIGHT: (C)2005,JPO&NCIPI

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、残部が実質的にＦｅで、（％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／ｍｍ^２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／ｍｍ^２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，Ｆｅ_２Ｎｂ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％
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／ｍｍ^２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／ｍｍ^２以上の析出量でピン止め作用がみられるが、５００００個／ｍｍ^２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／ｍｍ^２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／ｍｍ^２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／ｍｍ^２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／ｍｍ^２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 applications]
The present invention is a ferrite which is processed into a predetermined shape by press working or the like, has less roundness defect after processing, has less shape defects such as twisting, and exhibits excellent hole expandability at the time of secondary processing such as subsequent hole expanding processing. The present invention relates to a stainless steel sheet and a method for producing the same.
[0002]
[Prior art]
Ferritic stainless steels represented by SUS430 and SUS430LX have good corrosion resistance and do not contain expensive Ni, so they are more economical than austenitic stainless steels. Used in a wide range of fields. With the development of applications, working conditions such as press working for obtaining a product shape from a ferritic stainless steel sheet have become severe. After the press working, a secondary working such as a hole expanding work may be performed. In order to cope with severe processing conditions, there is a demand for a ferritic stainless steel sheet having much better workability than conventional materials.
[0003]
Many studies have hitherto been reported with the aim of improving the formability of ferritic stainless steel sheets. A typical method of improving the formability is to precipitate carbonitrides by adding Ti and Nb in a composite manner and to reduce the C and N concentrations in the matrix. In addition to the composite 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 Rankford value (r value) as an evaluation index of formability (Japanese Patent Publication No. Hei 8-26436) is also known.
[0004]
[Problems to be solved by the invention]
Factors that govern the workability of ferritic stainless steel sheets are not only the Rankford value (r-value) and ridging properties, but also the important factors such as the shape freezing property and the secondary hole expandability of the primary processed product in the product manufacturing process. It is believed that.
The workability of a ferritic stainless steel sheet is generally inferior to that of an austenitic stainless steel sheet, and the reduction in sheet thickness after primary forming is particularly large. In addition, there is a great direction in reducing the thickness of the sheet after the primary forming. When a ferritic stainless steel sheet is pressed into a cylindrical shape, the deviation in dimensional accuracy such as roundness increases as the processing conditions become more severe. Further, since there is variation in the reduction in the thickness of the sheet after the primary forming, the formability is extremely deteriorated in the secondary processing such as the hole expanding processing.
[0005]
When the dimensional accuracy (roundness, axialness, torsion, etc.) of the press-formed ferritic stainless steel sheet is improved, and the secondary hole expandability is also improved, the austenitic stainless steel sheet is subjected to severe processing conditions. Inexpensive ferritic stainless steel sheets can be used in places where they had to be used, and further applications of ferrite stainless steel sheets can be developed.
[0006]
[Means for Solving the Problems]
The present invention has been devised to meet such a demand. By controlling the grain size and morphology of precipitates, a ferritic stainless steel sheet having improved dimensional accuracy and secondary hole expandability after press working is provided. 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% by mass or less, Si: 0.8% by mass or less, Mn: 1.5% by mass or less, P: 0.050% by mass. Hereinafter, S: 0.01% by mass or less, Cr: 8.0 to 35.0% by mass, N: 0.05% by mass or less, Ti: 0.05 to 0.40% by mass, Nb: 0.10% by mass Containing 0.50% by mass, the balance being substantially Fe, having a composition satisfying (% Ti ×% N) <0.005, and having 5,000 to 50,000 precipitates having a particle size of 0.15 μm or more excluding TiN. Characterized in that they are precipitated at a rate of particles / mm ² .
[0008]
The ferritic stainless steel sheet may further contain Ni: 0.5% by mass or less, Mo: 3.0% by mass or less, Cu: 2.0% by mass or less, V: 0.3% by mass or less, and 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 contained.
After hot rolling a ferritic stainless steel slab having a predetermined composition at a hot rolling end temperature of 800 ° C. or lower, the hot rolled steel strip is annealed at 450 to 1080 ° C. × soaking for 1 hour or less. (-100 ° C) ~ (recrystallization completion temperature) × cold-rolled steel strip by cold rolling accompanied by at least one or more intermediate annealing of soaking 1 minute or less, and then finished at 1080 ° C or less × soaking 1 minute or less. It is 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, axialness, twist, etc.) after press working. As a result, it has been found that the form of TiN and the precipitate after the finish annealing has a great influence on the roundness and the secondary hole expandability when press-formed into a cylindrical shape. Based on this knowledge, the target characteristic is optimized by optimizing the thermomechanical heat treatment using ferritic stainless steel with a complex addition of Ti and Nb in a stoichiometric ratio of C and N as carbonitride as a solid solution. It was clarified that a ferrite-based stainless steel sheet with the above was obtained. The effect of the form of the precipitate on the press workability and the dimensional accuracy after the working is presumed as follows.
[0010]
Most of C and N contained in ferritic stainless steel are precipitated as carbides and nitrides by adding Ti and Nb. Most of the precipitated carbides and nitrides excluding TiN become fine precipitates in the process of hot-rolled sheet annealing → cold rolling → finish annealing. When the manufactured steel strip is recrystallized and annealed, fine precipitates do not exhibit a pinning effect, and recrystallized grains having a specific crystal orientation grow preferentially, resulting in a mixed grain structure with large anisotropy. The large anisotropy causes the strain to concentrate in a specific direction during the primary processing of the steel sheet, and reduces the press formability and the dimensional accuracy after the pressing.
[0011]
By generating a precipitate having a particle size of a certain size or more, a pinning effect at the time of recrystallization can be expected. The pinning action suppresses preferential growth and coarsening of crystal grains having a specific crystal orientation, and improves anisotropy and, consequently, dimensional accuracy after press working. The effect of the pinning action to improve the press formability and the dimensional accuracy after the press working is, as can be seen in the examples described later, from 5,000 to 50,000 precipitates having a particle size of 0.15 μm or more excluding TiN per mm ^2. Is remarkable when precipitated at the ratio of
[0012]
However, among the precipitates, TiN is not preferable for press workability and dimensional accuracy after press work. Actually, when a steel sheet having (% Ti ×% N) exceeding 0.005 is pressed in multiple stages, cracks are generated, but coarse TiN precipitated in the form of dice is observed at the starting point of the cracks. The observation results indicate that stress is concentrated on the top of the die-shaped TiN during multi-stage pressing, and that accumulation of strain and generation of minute cracks induce cracks. Accumulation of strain and generation of minute cracks in the vicinity of TiN also deteriorate secondary hole expandability.
[0013]
Next, the components and contents of the ferritic stainless steel used in the present invention will be described.
C: 0 . Not more than 02% by mass The recrystallized ferrite is randomized at the time of final annealing by forming carbides. However, since it is a component that increases the strength, an excessive amount of C decreases workability. Since corrosion resistance is also reduced 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 the secondary hole expandability, a C content of 0.015% by mass or less is preferable. However, an excessive reduction of the C content requires a long time of refining, which causes an increase in steel material cost. In order to make the effect of carbides contributing to randomization of the recrystallized ferrite effective at the time of final annealing, the C content of 0.001% by mass or more is preferable.
[0014]
Si: 0 . 8% by mass or less An alloy component added as a deoxidizing agent during steelmaking, but has a high solid solution hardening ability, and if an excessive amount of Si exceeding 0.8% by mass is contained, material hardening and ductility decrease. cause. In order to further improve the high ductility and the secondary hole expandability, it is preferable to set the upper limit of the Si content to 0.5% by mass.
Mn: 1 . 5% by mass or less The effect of hardening the material is small due to its small solid solution strengthening ability, but if an excessive amount of Mn exceeding 1.5% by mass is included, Mn-based fumes are generated during melting, Manufacturability deteriorates.
[0015]
P: 0 . 050% by mass or less A component harmful to hot workability, and the upper limit of the P content was set to 0.050% by mass from the viewpoint of workability.
S: 0 . 01% by mass or less A harmful component that segregates at the crystal grain boundaries and embrittles the crystal grain boundaries, but by controlling the S content to 0.01% by mass or less, adverse effects due to S can be suppressed. .
[0016]
Cr: 8 . 0-35 . 0% by 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 with an increase in the amount of Cr, the upper limit of the Cr content was set to 35.0% by mass. For further improvement in high ductility and secondary hole expandability, a Cr content of 20.0% by mass or less is preferable.
N: 0 . 05 mass% or less The recrystallization ferrite is randomized at the time of final annealing as a nitride, but since it is a component for increasing strength, an excessive amount of N decreases ductility. It is preferable to reduce the content, and 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 the secondary hole expandability, the N content is preferably 0.02% by mass or less. However, an excessive reduction of the N content requires a long time of refining, which causes an increase in steel material cost. Randomization of recrystallized ferrite becomes remarkable when N content is 0.001% by mass or more.
[0017]
Ti: 0 . 05 to 0 . 40% by mass
It is an alloy component that fixes C and N and is effective in improving workability and corrosion resistance. The effect of adding Ti is exhibited at 0.05% by mass or more. However, excessive addition of Ti exceeding 0.40% by mass is not preferable because it increases the cost of steel materials and generates surface defects due to Ti-based inclusions.
Nb: 0 . 10-0 . 50% by mass
Like Ti, it is an alloy component that fixes C and N and improves workability. The Nb-based precipitates having a particle size of 0.15 μm or more, excluding TiN, which are important in the present invention, are expected to be carbides and Fe ₂ Nb, and are essential components for the precipitation of the 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 preferred because precipitates having a particle size of 0.15 μm or more are unnecessarily precipitated and the recrystallization temperature is increased.
[0018]
Ni: 0 . 5% by mass or less An alloy component added as necessary, and is effective for improving the toughness and corrosion resistance of a hot-rolled sheet. However, since the addition of Ni causes an increase in raw material cost and hardening, the upper limit of the Ni content is set to 0.5% by mass.
Mo: 3 . 0% by mass or less Alloy component added as necessary, which contributes to improvement of corrosion resistance. However, excessive addition of Mo lowers the hot workability, so when Mo is added, the upper limit is regulated to 3.0% by mass.
[0019]
Cu: 2 . 0% by mass or less An alloy component added as necessary, and is easily mixed in from a scrap or the like during melting. If an excessive amount of Cu is contained, it causes embrittlement and lowers hot workability. Therefore, the upper limit of the Cu content is set to 2.0% by mass or less.
V: 0 . 3% by mass or less, Zr: 0 . 3% by mass or less All are alloy components added as necessary, V is precipitated as solid carbide C as a carbide to improve the workability, and Zr is the workability by supplementing oxygen in the steel. ， Improve toughness. However, if added excessively, the productivity decreases, so the upper limit of the content of both V and Zr is set to 0.3% by mass.
[0020]
Al: 0 . 3% by mass or less An alloy component that is added as needed, and is added as a deoxidizer during steelmaking. However, excessive addition of Al causes an increase in nonmetallic inclusions, which causes a decrease in toughness and surface defects. Therefore, the upper limit of the Al content is set to 0.3% by mass.
B: 0 . 0100% by mass
It is an alloy component added as needed, and has an effect of fixing N and improving corrosion resistance and workability. The effect of adding B is seen at a B content of 0.0010% by mass or more. However, when an excessive amount of B exceeding 0.0100% by mass is excessively added, hot workability and weldability are reduced.
In addition to the above-mentioned components, Ca, Mg, Co, REM and the like may be mixed in from the scrap as a raw material during melting. These components do not adversely affect the roundness at the time of drawing and the dimensional accuracy after pressing, unless they are contained in a particularly large amount.
[0021]
( % Ti ×% N ) <0 . 005
With the increase of (% Ti ×% N), coarse TiN is generated or TiN forms a cluster. Coarse TiN or cluster-like TiN accumulates strain during primary processing to generate minute cracks, and as a result, promotes cracking early in drawing. The adverse effect caused by coarse TiN or cluster-like TiN is suppressed by regulating (% Ti ×% N) to less than 0.005, as will be confirmed in Examples described later.
[0022]
The particle size except TiN 0. Precipitates of 15 μm or more: 5000 to 50,000 / mm ²
When carbides and nitrides having a grain size of 0.15 μm or more are precipitated, preferential growth of crystal grains having a specific crystal orientation and coarsening of the crystal grains are suppressed by the pinning action of the precipitates, and anisotropy and, consequently, cylinders The roundness after drawing and the dimensional accuracy after pressing are improved.
Precipitates are carbides, nitrides, Laves phases of Ti and Nb other than TiN, and mixtures thereof. However, the TiN precipitated in the form of dice causes stress concentration at the apex of the precipitate particles during processing, causing the accumulation of strain and the generation of minute cracks, and tends to be the starting point of cracking. Precipitates effective for dimensional accuracy were excluded.
[0023]
When precipitates having a particle size of 0.15 μm or more, excluding TiN, are deposited and distributed at a rate of 5,000 to 50,000 / mm ² , as will be confirmed in Examples described later, pressing is performed by the pinning action of the precipitates. Formability and dimensional accuracy after pressing are improved.
Precipitates having a particle size of 0.15 μm or more are effective for press formability and dimensional accuracy after press working, and the larger the particle size, the greater the improving effect. However, coarse precipitates exceeding 1.0 μm, depending on the shape of the precipitates, accumulate strain around the precipitates during press working, generate minute cracks and reduce shape freezing during forming. Not preferred. A pinning effect is observed at a deposition amount of 5,000 particles / mm ² or more, but at a deposition amount of more than 50,000 particles / mm ² , the ductility is rather lowered and the cylindrical drawability is deteriorated. An excessive amount of precipitation is not preferable in that it raises the recrystallization temperature of the steel material and makes recrystallization annealing difficult.
[0024]
Next, the manufacturing conditions will be described. The control of the morphology of the precipitates is possible only by setting the manufacturing conditions.
Hot rolling end temperature: 800 ° C or less The finishing hot rolling temperature is lowered in order to generate a large number of core sites for precipitates having a particle size of 0.15 µm or more in the final annealing material. The nucleus sites generated after hot rolling are ferrite grain boundaries and internal strain, and a hot rolling end temperature of 800 ° C. or less is required to incorporate a large amount of nucleus sites.
[0025]
Hot-rolled sheet annealing: 450 to 1080 ° C x soaking 1 hour or less By hot-rolled sheet annealing, the precipitates are tempered to a target precipitation state, and precipitates having a grain size of 0.15 µm or more after final annealing. can get. When the hot-rolled sheet annealing temperature is lower than 450 ° C., almost no precipitates are formed. On the other hand, when the temperature exceeds 1080 ° C., the precipitates other than TiN form a solid solution 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 appropriately controlled, and the annealing time is set to 1 hour or less in order to suppress the coarse growth of the precipitates.
[0026]
Intermediate annealing: ( recrystallization completion temperature-100 ° C ) - ( recrystallization completion temperature ) × soaking 1 minute or less In order to suppress re-solid solution of precipitates obtained by hot-rolled sheet annealing, relatively Intermediate annealing at low temperature. In order to remove the strain introduced by the cold rolling and to soften, it is preferable to set the intermediate annealing temperature immediately below the recrystallization completion temperature. Within the temperature range of (1), although there are some regions of the rolled structure that have not been recrystallized, both suppression of re-solid solution of precipitates and softening are achieved. The annealing time is set to a short time of 1 minute or less, assuming a normal continuous annealing line, in order to suppress the solid solution of the precipitate again.
[0027]
Finish annealing: 1080 ° C. or less × soaking 1 minute or less Although the rolling structure is eliminated by the finish annealing, if the annealing temperature is too high, the mass production productivity is lowered, and of course, the precipitates are re-dissolved to form crystals. The upper limit of the annealing temperature is set to 1050 ° C., because the coarsening of the grains causes a decrease in toughness. 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 effect of TiN, which is likely to precipitate on ferritic stainless steel, and the effect of the precipitation form on dimensional accuracy after press working and secondary hole expandability were investigated under the following conditions.
By laboratory dissolution, C: 0.007% by mass, Si: 0.40% by mass, Mn: 0.25% by mass, P: 0.030% by mass, S: 0.0005% by mass, Cu: 0 0.055% 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%, N: 0. Nine types of slabs changed in the range of 005 to 0.035% were melted.
Table 1 also 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. Steel type No. Each of the hot-rolled sheets 1 to 7 was annealed at 800 ° C. for 60 seconds, cold-rolled to a thickness of 2 mm after pickling. Further, a cold-rolled sheet having a sheet thickness of 0.5 mm was manufactured by cold rolling accompanied by intermediate annealing of (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.
Steel type No. The hot-rolled sheets 8 and 9 were pickled after hot-rolled sheet annealing and cold-rolled to a sheet thickness of 2 mm. Furthermore, after performing intermediate annealing and cold-rolling to a sheet thickness of 0.5 mm, finish annealing was performed to obtain a cold-rolled annealed sheet having a sheet thickness of 0.5 mm. Steel type No. Table 2 shows the conditions of hot rolled sheet annealing, intermediate annealing, and finish annealing for Nos. 8 and 9.
[0031]

[0032]
[Investigation of precipitation rate and precipitation form of precipitates]
A test piece was cut out from each cold-rolled annealed plate, and after 10% acetylacetone-1% tetramethylammonium chloride-methyl alcohol was used as an electrolytic solution for non-aqueous solvent-based constant potential electrolytic etching, the form of the precipitate was determined by a scanning electron microscope. Was observed. A cross section of the plate thickness parallel to the rolling direction was taken as an object to be observed, and precipitates were observed in arbitrary 50 visual fields, and the maximum length of each precipitate was measured as the particle size of the precipitate.
[0033]
[Survey of dimensional accuracy after press working]
The blank cut out from each cold-rolled annealed plate is multi-stage pressed to form a cylindrical shape as shown in FIG. 1, and the maximum radius and the minimum radius of the cylindrical portion C at a position 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 roundness.
[0034]
[Survey 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 processed product having a height of 10 mm is produced from each cold-rolled annealed plate by a flange-fixed beading process. A blank having a diameter of 92 mm was cut out. A punched hole having a diameter of 10 mm was formed at 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 punching was performed with a flat-head punch. The hole was pushed open until cracks occurred at the hole edge. The diameter of the hole at the time of occurrence of cracks was measured, and the secondary hole expansion ratio was calculated as follows: secondary hole expansion ratio = [(diameter of punched hole after test−diameter of punched hole before test) / diameter of punched hole before test] × 100 (%). The rate was calculated.
[0035]
As can be seen from the survey results in Table 3, when (% Ti ×% N) exceeds 0.005, cracks occur in the middle of the multi-stage drawing, and any steel type having a small Nb content of 0.02 mass% Even after the production process, the roundness was poor. As a result of observing the type of steel in which cracks occurred and the type of steel having 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 in all types of steel. On the other hand, in the case of Nb: 0.3% by mass, when the number of precipitates having a particle size of 0.15 μm or more excluding TiN becomes a certain number or more by combination with the thermomechanical treatment conditions, the roundness is improved. On the other hand, when the precipitates were excessively generated, the roundness tended to decrease.
[0036]
The secondary hole expandability was also extremely poor in the steel type of (% Ti ×% N) ≧ 0.005, and a low secondary hole expansion ratio was shown 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 having a particle size of 0.15 μm or more excluding TiN reaches a certain number or more, the secondary hole expandability is improved, but the precipitates are excessively increased. When it is generated, the secondary hole expandability tends to decrease.
The above results show that the dimensional accuracy after press working and the secondary hole expandability depend on the form of the precipitate having a particle size of 0.15 μm or more excluding TiN. By optimizing the conditions for thermomechanical treatment and adjusting the amount of precipitates having a particle size of 0.15 μm or more excluding TiN to be in the range of 5,000 to 50,000 particles / mm ² , excellent dimensional accuracy after press working can be achieved. It is confirmed that the next hole expandability is obtained.
[0037]

[0038]
Embodiment 2
Stainless steel having the composition shown in Table 4 was melted in a vacuum melting furnace, and the obtained slab was hot-rolled to a thickness of 4.0 mm. Each hot-rolled sheet was annealed, pickled, and cold-rolled to a thickness of 2 mm. Furthermore, after performing intermediate annealing and cold-rolling to a sheet thickness of 0.5 mm, finish annealing was performed to obtain a cold-rolled annealed sheet having a sheet thickness of 0.5 mm. Table 5 shows the heat treatment conditions for the finish annealing temperature, hot-rolled sheet annealing, intermediate annealing, and finish annealing. In Table 4, steel types A to H satisfy the composition conditions specified in the present invention, and steel types I to L are out of the range specified in the present invention.
[0039]

[0040]

[0041]
With respect to each cold-rolled annealed sheet, the form of the precipitate, the amount of precipitation, the dimensional accuracy after press working, and the expandability of the secondary hole were investigated in the same manner as in Example 1.
As can be seen from the survey 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 5,000 to 50,000 / mm ² is press-formed. A processed product having an extremely good roundness of 5% or less was obtained.
[0042]
On the other hand, a comparative material (test Nos. A2, B2, C2, and D2) which satisfies the conditions specified in the present invention but has inappropriate production conditions is a precipitate having a particle size of 0.15 μm or more excluding TiN. Was out of the range of 5,000 to 50,000 / mm ² , and the dimensional accuracy after press working and the secondary hole expandability were poor. Test No. containing excess C In Test No. 1, the material itself was hard and contained excessive Nb. As for K, since the strength of the material itself was too high, cracks occurred before any steel sheet was processed into a molded article having a predetermined shape. Test No. (% Ti ×% N) exceeding 0.005. As for L, a crack originated from coarse TiN occurred before being processed into a molded product having a predetermined shape. Test No. for which Nb is insufficient J was inferior in roundness after press working.
The above comparison results show that by controlling the morphology of precipitates having a particle size of 0.15 μm or more excluding TiN, a ferritic stainless steel sheet having excellent dimensional accuracy and secondary hole expandability after press working is obtained.
[0043]

[0044]
【The invention's effect】
As described above, in a ferritic stainless steel having a specified composition and composition, when a precipitate having a particle size of 0.15 μm or more excluding TiN is dispersed in a matrix at a rate of 5,000 to 50,000 particles / mm ² , pressing is performed. A ferritic stainless steel sheet with excellent dimensional accuracy and secondary hole expandability after processing can be obtained. Precipitate morphology and distribution control are achieved by proper 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 processing and hole expandability during secondary processing is used for IT-related parts such as encapsulants for organic EL elements with strict dimensional accuracy, various precision press products, sinks, Household appliances and parts such as fixtures and stove burners, fuel tanks and filler pipes, motor cases, covers, sensor caps, injector pipes, thermostat valves, bearing seal materials, parts of industrial equipment such as flanges, building components It is used as a substitute for expensive austenitic materials in a wide range of fields.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining the calculation of the roundness of a cylindrical workpiece formed by a multi-stage press.

Claims

C: 0.02% by mass or less, Si: 0.8% by mass or less, Mn: 1.5% by mass or less, P: 0.050% by mass or less, S: 0.01% by mass or less, Cr: 8.0 35.0% by mass, N: 0.05% by mass or less, Ti: 0.05 to 0.40% by mass, Nb: 0.10 to 0.50% by mass, and the balance is substantially Fe. (% Ti ×% N) <0.005, and precipitates having a particle size of 0.15 μm or more excluding TiN are precipitated at a rate of 5,000 to 50,000 / mm ^2. Ferritic stainless steel sheet with excellent press formability and secondary workability.

Further, Ni: 0.5% by mass or less, Mo: 3.0% by mass or less, Cu: 2.0% by mass or less, V: 0.3% by mass or less, Zr: 0.3% by mass or less, Al: 0 2. The ferritic stainless steel sheet according to claim 1, which contains one or more of 0.3% by mass or less and B: 0.0100% by mass or less.

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