JP4428550B2 - Ferritic stainless steel sheet excellent in ridging resistance and deep drawability and method for producing the same - Google Patents

Description

【００１３】
以上の工程で作製された焼鈍板から試験片を採取し、ｒ値の測定、リジング判定、ＥＢＳＰ法（Electron Backscattering Pattern）による結晶方位分布測定およびプレス加工テストを行った。なお、ｒおよびΔｒはそれぞれｒ＝（ｒ_L＋２ｒ_D＋ｒ_T）／４、Δｒ＝（ｒ_L−２ｒ_D＋ｒ_T）／２である。ただし、ｒ_L、ｒ_Dおよびｒ_Tは、それぞれ圧延方向、圧延方向に対して４５°方向および圧延方向に対して９０°方向のｒ値を示す。また、耐リジング性の判定は、Ａが最も耐リジング性が良いもの、Ｄが最も耐リジング性が悪いものとし、Ａ、Ｂ、Ｃ、Ｄの４段階評価を行った。表２に耐リジング性の判定基準を示す。
【００１４】

【００１５】
ＥＢＳＰ法による結晶方位分布測定は、各鋼板に対して、板厚の１／２深さにおける圧延板について、圧延方向；５００μｍ、幅方向；８００μｍの範囲の結晶方位分布を測定し、得られたデータをもとに、隣接する結晶粒の方位のずれ１５°以上である大角粒界の割合と（｛１１１｝面からのずれが５°以内の結晶粒の面積）／（｛１００｝面からのずれが５°以内の結晶粒の面積）を解析した。
【００１６】
また、プレス加工テストは、ポンチの寸法が２００ｍｍ×２７０ｍｍ、絞り深さが１００ｍｍ、ダイ肩半径が１０ｍｍの角筒絞りを行い、成形できたものは○、割れが生じたものは×と判定した。表３にｒ、Δｒ、リジング判定、ＥＢＳＰ法による結晶分布解析およびプレス加工テストの結果に及ぼす中間圧延と仕上げ圧延の冷延圧下配分の影響を示す。
【００１７】

【００１８】
表３より、大角粒界の割合が大きいと耐リジング性が向上するよい対応関係にある。一般的に、リジングの発生は、冷延焼鈍板内に存在する結晶方位の近い結晶粒の集団（コロニー）に起因する。コロニーの起源は、凝固柱状晶が熱延焼鈍後に残存することにより形成される圧延方向に伸びた粗大な未再結晶フェライト粒が、冷延後の焼鈍時においても再結晶による結晶方位の分散が小さいため、コロニーが形成される。コロニー内部では、見かけ上は微細な結晶組織を呈しているが、結晶方位の分散が小さいためプレス成形などの塑性変形を受けるとコロニーが大きな単結晶のように変形し、それぞれのコロニーの塑性異方性の差によりリジングが発生する。
【００１９】
ここで、コロニー内部の結晶粒界は隣接する結晶の方位のずれが小さく、ずれが１５°以上の大角粒界は非常に少ない。すなわち、大角粒界の割合が小さい鋼板では、コロニーの形成が多くリジングが発生するのに対し、大角粒界の割合が大きいと、その鋼板のリジングの原因となるコロニーの形成が小さく耐リジング性が向上し、特に、大角粒界の割合が９０％以上であるとリジング発生が軽減し、リジング判定Ｂ以上の十分な耐リジング性が得られる。
【００２０】
また、深絞りに良好な面として｛１１１｝面が、ｒ値に悪影響を及ぼす面として｛１００｝面があり、板厚中心でこの良好な面と悪影響を及ぼす面積比で整理すれば、ｒ値との優劣を明確にできる。つまり、（板厚の１／２深さにおける圧延面の｛１１１｝面からのずれが５°以内の結晶粒の面積）／（板厚の１／２深さにおける圧延面の｛１００｝面からのずれが５°以内の結晶粒の面積）がｒによく対応し、特に｛１１１｝面の面積／｛１００｝面の面積が５以上においてその傾向が顕著になり、ｒが１．４以上となる。
表３において、プレス加工テストで成形できたものは、｛１１１｝面の面積／｛１００｝面の面積が５以上のものであり、一方、｛１１１｝面の面積／｛１００｝面の面積が５より小さいものは、深絞り性が不十分で、プレス途中で激しい割れが生じた。
【００２１】
しかしながら、従来方法の２回冷延焼鈍材では、｛１１１｝面の面積／｛１００｝面の面積が５．５であり、ｒも１．４０と十分に高いがプレス加工テストで割れが生じている。この鋼板では、結晶粒界のうち隣接する結晶粒の方位のずれが１５°以上である大角粒界の割合が８１％と比較的低く、耐リジング性が低くｒ値の異方性を示すΔｒが大きかったためコーナー部で割れが発生した。すなわち、｛１１１｝集合組織が発達していても大角粒界が少なくコロニーを形成していると、満足するプレス成形性が得られず、｛１１１｝面の面積／｛１００｝面の面積が５以上であり、かつ大角粒界の割合が９０%以上必要であることが明らかになった。
【００２２】
複相化焼鈍を施す製造法において深絞り性および耐リジング性が改善される理由は、次のように考えられる。
まず、フェライト系ステンレス鋼の熱延板にＡｃ₁点以上の複相化焼鈍を施すと、（フェライト＋マルテンサイト）２相組織が得られる。フェライト相はマルテンサイト相に比べ非常に軟質であるので、中間圧延により加工を加えると、フェライト相に歪みが蓄積される。これを、箱型炉で長時間焼鈍を施すことにより、マルテンサイト相は炭化物を十分に析出し、フェライト相へ再結晶すると同時に、非常に歪みが蓄積されたフェライト相の再結晶が促進される。この時に、リジングの原因となるような凝固柱状晶に由来した粗大なフェライト粒の再結晶も促進され、再結晶粒の結晶方位は大きく分散し、コロニーの形成を阻害し、大角粒界の割合を増大させる。
【００２３】
表３において、本発明製造法の複相化焼鈍後の中間圧延率の増加とともに、結晶粒界のうち隣接する結晶粒の方位のずれが１５°以上である大角粒界の割合が増加し、対リジング性も改善された結果も、このことを裏付けている。
また、箱型焼鈍後にコロニーの形成を阻害しているため、通常工程では残存しやすいｒ値を下げる｛１００｝のコロニーの形成も少なく、続く、仕上げ圧延、仕上げ焼鈍により、ｒ値に有効な｛１１１｝集合組織は発達し、｛１１１｝面の面積／｛１００｝面の面積が５以上であり、かつ、大角粒界の割合が９０％以上の鋼板を製造することができた。
【００２４】
次に、本発明のフェライト系ステンレス鋼の成分、特性、製造条件の限定理由について説明する。なお、以下の説明における％表示は、すべて質量％を示す。Ｃ : ０．１０％以下
Ｃは、γｍａｘを大きくする作用を呈する合金元素であり、複相化焼鈍後に得られる（フェライト＋マルテンサイト）２相組織において、十分な量のマルテンサイトとその硬さを得るために有効に作用する。しかし、冷延焼鈍後の強度を上昇させる成分であり、過剰に含まれると延性を低下させることから、Ｃ含有量の上限は０．１０％とした。
【００２５】
Ｓｉ：１．５％以下
Ｓｉは、製鋼時に脱酸剤として添加される合金成分であるが、固溶強化能が高く、過剰に含有すると材質を硬化し延性を低下させることから、その含有量の上限は１．５％に設定した。
Ｍｎ：１．０％以下
Ｍｎはオーステナイト形成元素であり、γｍａｘの制御に利用できる。また、固溶強化能が小さいため、材質を硬化する影響も少ない。しかし、過剰に含有されると溶接時にＭｎ系ヒュームが生成する等、悪影響を与えるため、その含有量の上限は１．０％に設定した。
【００２６】
Ｐ：０．００５％以下
Ｐは、その含有量に応じて熱間加工性を低下させるので、その含有量上限は０．００５％とした。
Ｓ：０．０１５％以下
Ｓは、その含有量が多くなると結晶粒界に偏析して結晶粒界を脆化させるので、その含有量上限は０．０１５％に設定した。
Ｎｉ：２．０％以下
ＮｉはＭｎと同様にオーステナイト形成元素であり、γｍａｘ制御に利用できる。しかし、Ｎｉの過剰な含有は、コストの上昇を招くことにもなるから、その含有量の上限は２．０％とした。
【００２７】
Ｃｒ：１０．０〜２０．０％
ステンレス鋼としての耐食性を得るには、Ｃｒを１０．０％以上含有させることが必要である。しかし、Ｃｒ量が高くなると、靭性や加工性の低下を招くため、その含有量は２０．０％以下にする。
Ａｌ：０．１０％以下
Ａｌは製鋼時に脱酸剤として添加される合金成分であるが、過剰量の添加は非金属介在物を増加させ、靭性低下や表面欠陥の原因になるため、その含有量の上限は０．１０％とする。
【００２８】
Ｎ：０．０５％以下
ＮもＣと同様にγｍａｘを大きくする作用を呈する元素であり、複相化焼鈍時に生成するオーステナイトを増大させ、微細組織化に有効に働き、耐リジング性を向上させる。しかし、冷延焼鈍後の強度を上昇させる成分であり、過剰に含まれると延性を低下させることから、その含有量の上限は０．０５％とした。
Ｔｉ：０．０５％以下
ＴｉはＣ、Ｎを固定し、加工性を向上させる元素であるが、Ｔｉを添加すると、鋼材コストの増大を招くことから、その含有量の上限は０．０５％とした。
【００２９】
Ｎｂ：０．０５％以下
ＮｂはＣ、Ｎを固定し、加工性を向上させる元素であるが、Ｎｂを添加すると、鋼材コストの増大を招くことから、その含有量の上限は０．０５％とした。
Ｍｏ：１．０％以下
Ｍｏは、必要に応じて添加される元素であり、耐食性の改善に寄与する。しかし、Ｍｏが過剰に添加されると、熱間加工性が低下するため、その含有量の上限は１．０％とする。
【００３０】
Ｃｕ：１．０％以下
Ｃｕは、溶製時にスクラップ等から混入する成分であり、その含有量が多くなると熱間加工性や耐食性に悪影響を及ぼすので、上限は１．０％とする。
Ｂ：０．００１０〜０．０１００％
Ｂは、必要に応じて添加する元素であり、熱延板の変態相を均一分散化し、耐リジング性を向上させる作用を呈する。０．００１０％以上の含有量でＢ添加の効果が顕著になるが、０．０１００％を越える過剰な添加は、熱間加工性や溶接性の低下を招く。
【００３１】
γｍａｘ：２０以上８０未満
本発明では、熱延板をＡｃ₁点以上のフェライト＋オーステナイトの２相域に加熱する焼鈍を連続焼鈍炉によって行い、フェライト＋マルテンサイトの２相組織を得る複相化焼鈍を有効利用することに特徴がある。したがって、本発明においては、γｍａｘの規定は重要な事項になる。すなわち、γｍａｘが２０未満であると、複相化焼鈍で得られるマルテンサイト量が少なく、中間圧延工程でフェライト相に十分な歪みの蓄積がなされず、フェライトバンドの再結晶が促進されないために、ｒ値および耐リジング性の改善が得られない。一方、γｍａｘを高めるためには、Ｃ、Ｎ、ＭｎおよびＮｉ等のオーステナイト形成元素の含有量を多くする必要があるが、これらは、鋼材の硬質化やコストの上昇を招くためγｍａｘは８０未満にする必要がある。
【００３２】
複相化焼鈍温度：Ａｃ ₁ 点〜１１００℃
高温でフェライト＋オーステナイトの２相組織を得て、複相化焼鈍後に、フェライト＋マルテンサイトの２相組織を得るためには、少なくともＡｃ₁点以上の温度で焼鈍を行うことが必須となる。しかし、１１００℃を越える温度で焼鈍を行うと、結晶粒径の粗大化、さらには、高温強度の低下による鋼板の切断の危険性が高まる。したがって、複相化焼鈍の温度は、Ａｃ₁点以上１１００℃以下の範囲とする。
【００３３】
中間冷延圧延率：１０〜５０％
複相化焼鈍後の中間圧延にて、フェライト相に歪みを蓄積させるのであるが、１０％より小さい圧延率では、その効果が得られない。また、５０％を越える中間圧延を施すと、仕上げ圧延の圧延率が小さくなり、ｒ値に有効な集合組織の発達が得られないため、中間圧延の圧延率は１０％以上５０％以下とする。
また、中間焼鈍は、マルテンサイトを炭化物とフェライトに再結晶させるために長時間焼鈍が必要となり、仮に、連続焼鈍炉で製造しようとすると、非常に効率が悪く、経済的でないため、箱型焼鈍炉による長時間焼鈍とする。
【００３４】
大角粒界の割合：９０％以上、圧延面からの小さいずれの結晶面の比：５以上
フェライト系ステンレス鋼板のプレス加工において、その加工性を示す因子として、平均的なｒ値を示すｒが用いられてきた。しかしながら、実際のプレス加工では、ｒ値が高くともΔｒが大きく異方性が大きい材料や、リジングの発生が著しい材料では、加工できない場合が多い。実際のプレス加工において好ましい材料は、異方性が小さく、深絞り性と耐リジング性を兼ね備えた材料である。そこで、図１に示した事前評価結果から、これらの特性を満足させるためには、結晶粒界のうち隣接する結晶粒の方位のずれが１５°以上である大角粒界の割合が９０％以上であり、（板厚の１／２深さにおける圧延面の｛１１１｝面からのずれが５°以内の結晶粒の面積)／（板厚の１／２深さにおける圧延面の｛１００｝面からのずれが５°以内の結晶粒の面積）を５以上に規定したものである。
【００３５】
【実施例】
表１中にＢ、Ｃ、Ｄ、Ｅに示す化学組成の鋼を溶製し、スラブとした後、熱間圧延機にて板厚４．０ｍｍの熱延板とした。この熱延板を、複相化焼鈍を施す製造方法として、表４に示す複相化焼鈍温度、中間圧延率を適用して、熱延板→複相化焼鈍→中間圧延→中間焼鈍→仕上げ圧延→仕上げ焼鈍を行い、板厚０．８ｍｍの焼鈍板とした。また、前述の熱延板を従来の製造方法、すなわち、熱延板→熱延板焼鈍（８３０℃×均熱９ｈｒ）→仕上げ冷延→仕上げ焼鈍（１回冷延焼鈍の製造方法）および熱延板→熱延板焼鈍（８３０℃×均熱９ｈｒ）→中間冷延→中間焼鈍→仕上げ冷延→仕上げ焼鈍（２回冷延焼鈍の製造方法）で作製した。
【００３６】
上記方法により得られた鋼板を供試材として、下記の方法でｒ、Δｒおよび耐リジング性を測定した。
ｒ値：ＪＩＳ１３Ｂ号試験片を用い１５％の引張歪みを与えた後、ｒ_L、ｒ_Dおよびｒ_Tを求めた。ｒ_L、ｒ_Dおよびｒ_Tは、それぞれ圧延方向、圧延方向に対して４５°方向および圧延方向に対して９０°方向のｒ値を示す。上記の方法で求めたｒ値から、ｒおよびΔｒはｒ＝（ｒ_L＋２ｒ_D＋ｒ_T）／４、Δｒ＝（ｒ_L−２ｒ_D＋ｒ_T）／２により求めた。
【００３７】
耐リジング性：圧延方向から採取したＪＩＳ５号試験片に２０％の引張歪みを与えた後、耐リジング性の判定を行った。耐リジング性の判定は、Ａが最も耐リジング性が良いもの、Ｄが最も耐リジング性が悪いものとし、Ａ、Ｂ、Ｃ、Ｄの４段階評価を行った。
【００３８】
プレス加工テスト：ポンチの寸法が２００ｍｍ×２７０ｍｍ、絞り深さが１００ｍｍ、ダイ肩半径が１０ｍｍの角筒絞りを行い、成形できたものは○、割れが生じたものは×と判定した。
結晶方位分布測定：ＥＢＳＰ法により板厚１／２深さにおける圧延面について、圧延方向；５００μｍ、幅方向；８００μｍの範囲にわたって測定した。
【００３９】
上記した各化学組成の鋼の各製造方法および各製造条件で得られた特性値を表４に合わせて示す。
また、各鋼板の結晶粒界のうち、隣接する結晶粒の方向のずれが１５°以上である大角粒界の割合および（｛１１１｝面からのずれが５°以内の結晶粒の面積）／（｛１００｝面からのずれが５°以内の結晶粒の面積）の値とプレス加工テスト結果の関係を図１に示す。
本発明の請求項に記載された要件を満たすものはすべてプレス加工テストにおいても、割れ等の不具合は生じず、良好であった。しかしながら、複相化焼鈍温度がＡｃ₁点温度より低いもの、中間圧延率が１０％に満たないものでは、プレス加工時に割れが発生した。
【００４０】

【００４１】
【発明の効果】
以上に説明したように、本発明によれば、製鋼コストの増大や熱延鋼帯の生産性低下を招くことなく、シンク、各種器物およびコンロ用バーナー等の家庭用機器の部品、燃料タンク、給油管、モーターケース、カバーおよびフランジ等の産業用機器の部品において、主にプレス加工に供される深絞り性および耐リジング性に優れたフェライト系ステンレス鋼板が製造できる。
【図面の簡単な説明】
【図１】 各鋼板の結晶粒界のうち、隣接する結晶粒の方向のずれが１５°以上である大角粒界の割合および（｛１１１｝面からのずれが５°以内の結晶粒の面積）／（｛１００｝面からのずれが５°以内の結晶粒の面積）の値とプレス加工性の関係を示す図（実施例、比較例、従来例を含む）。[0001]
[Industrial application fields]
INDUSTRIAL APPLICABILITY The present invention is mainly used for press processing in parts of household equipment such as sinks, various items and stove burners, and parts of industrial equipment such as fuel tanks, oil supply pipes, motor cases, covers and flanges. The present invention relates to a ferritic stainless steel plate excellent in ridging resistance and deep drawing workability and a method for producing the steel plate. In addition, the steel plate in this invention shall contain a steel plate and a steel strip.
[0002]
[Prior art]
Ferritic stainless steel represented by SUS430 has good corrosion resistance, does not contain expensive Ni, and has economic advantages compared to austenitic stainless steel. Widely used. However, in recent years, in the press forming of stainless steel, more severe processing is often performed, and a ferritic stainless steel plate having further excellent workability is demanded.
[0003]
The workability of a ferritic stainless steel sheet is generally inferior to that of an austenitic stainless steel, and unique wrinkle-like surface irregularities called ridging are produced during press forming. Therefore, in ferritic stainless steel, if the press workability and ridging resistance are improved, the cheaper ferritic ferritic material, which has been difficult to apply in the past, has been used where austenitic stainless steel has been used due to severe workability. Stainless steel can be used.
[0004]
By the way, it is known that the press formability of ferritic stainless steel depends on the r value. As an index indicating the r value, r indicating an average r value is used. Many techniques for improving the r value have been tried so far. For example, Japanese Patent Application Laid-Open No. 53-48018 proposes a technique for improving the r value by reducing C and N as much as possible and adding one or both of Ti and Nb. However, this technique takes time for refining in order to reduce C and N, and the cost of raw materials increases due to the addition of expensive Ti and Nb, which increases the cost of steelmaking.
[0005]
In order to improve the ridging property, a technique using so-called delay rolling in which a material is temporarily kept waiting during rolling to increase the time between passes is proposed in, for example, Japanese Patent Laid-Open No. 62-199721. . However, since the above technique is a method of giving a large strain in a low temperature region, it causes a biting failure and a shape failure, and also increases a rolling time, resulting in a decrease in productivity. Therefore, this is not an economical method.
[0006]
[Problems to be solved by the invention]
The present invention has been devised to solve such a problem, and has the deep drawability required for press working without increasing the steelmaking cost and reducing the productivity of the hot-rolled steel strip. And, it is an object to propose a ferritic stainless steel having satisfactory ridging resistance and a work heat treatment technique after hot rolling that can exhibit such characteristics.
[0007]
[Means for Solving the Problems]
In order to achieve the object, the ferritic stainless steel excellent in ridging resistance and deep drawing workability of the present invention is C: 0.10% or less, Si: 1.5% or less, Mn: 1 0.0% or less, P: 0.050% or less, S: 0.015% or less, Ni: 2.0% or less, Cr: 10.0 to 20.0%, Al: 0.10% or less, N: If necessary, Ti: 0.05% or less, Nb: 0.05% or less, Mo: 1.0% or less, Cu: 1.0% or less, B: 0.00% or less. 1 type or 2 types or more of 0010-0.0100%, the balance consists of Fe and inevitable impurities, and γmax defined by the formula (5) is 20 or more and less than 80, The deviation of the orientation of adjacent crystal grains in the grain boundary of the rolled surface at 1/2 depth is 15 ° or more. The ratio of a certain large-angle grain boundary is 90% or more, and further has the formula (6).
[0008]
Formula (5)
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-50Nb-52Al + 470N + 189 formula (6)
X / Y ≧ 5
Where X is the area of the crystal grains within 5 ° of the rolled surface at a depth of 1/2 of the plate thickness, and Y is {100 of the rolled surface at the depth of 1/2 of the plate thickness. } The area of the crystal grain within 5 ° from the plane.
[0009]
Further, in order to achieve the object, the production method of the present invention, in producing the ferritic stainless steel, a steel slab composed of the components described in each section, hot-rolled steel strip through hot rolling When producing a cold-rolled steel sheet or steel strip by combining cold rolling and annealing, the hot-rolled steel strip is heated to a ferrite-austenite two-phase region of Ac ₁ point or higher and 1100 ° C or lower. Is performed by a continuous annealing furnace, followed by further intermediate cold rolling at a rolling rate of 10% or more and 50% or less, followed by annealing in a box furnace, followed by finish cold rolling and recrystallization annealing. To do.
[0010]
[Embodiments of the present invention]
In order to solve the above-mentioned problems, the present inventors have developed an alloy composition and metal for improving the deep drawability and ridging resistance of a ferritic stainless steel sheet without increasing the steelmaking cost and inviting the productivity of a hot-rolled steel strip. We have studied in detail the microstructure and the heat treatment after hot rolling. In addition, the selected materials were subjected to press working, and the characteristics required for the press working were investigated. As a result, hot-rolled sheet annealing was subjected to multi-phase annealing at Ac ₁ point or higher, and the obtained (ferrite + martensite) two-phase structure was processed by cold rolling, and then annealed in a box furnace. It has been found that the ferritic stainless steel sheet that has undergone finish cold rolling and recrystallization annealing has markedly improved ridging resistance and deep drawability. In addition, the characteristics required for press working were clarified. The present invention has been completed based on this finding. Hereinafter, embodiments of the invention will be described based on experimental results.
[0011]
Using a hot-rolled sheet with a thickness of 4 mm and having a chemical composition shown in steel type A in Table 1, in a laboratory, hot-rolled sheet → double phase annealing (1050 ° C. × soaking 0 sec) → intermediate cold rolling → Intermediate annealing (830 ° C. × soaking 9 hr) → finish cold rolling → finish annealing was performed to obtain an annealed plate having a thickness of 0.8 mm. Here, the cold rolling reduction distribution of intermediate rolling and finish rolling was changed. Moreover, the above-mentioned hot-rolled sheet is manufactured by a conventional manufacturing method, that is, hot-rolled sheet → hot-rolled sheet annealing (830 ° C. × 9 hr) → finished cold-rolled → finished annealing (one-time cold-rolled annealing) and hot-rolled sheet → hot-rolled. It was produced by sheet annealing (830 ° C. × 9 hr) → intermediate cold rolling → intermediate annealing → finish cold rolling → finish annealing (twice cold rolling annealing).
[0012]

[0013]
Test pieces were collected from the annealed plates produced in the above steps, and subjected to r value measurement, ridging determination, crystal orientation distribution measurement by EBSP method (Electron Backscattering Pattern), and press working test. Note that r and Δr are r = (r _L + 2r _D + r _T ) / 4 and Δr = (r _L −2r _D + r _T ) / 2, respectively. However, r _L , r _D and r _T indicate r values in the rolling direction, the 45 ° direction with respect to the rolling direction, and the 90 ° direction with respect to the rolling direction, respectively. In addition, the determination of ridging resistance was evaluated by a four-step evaluation of A, B, C, and D, with A being the best ridging resistance and D being the worst ridging resistance. Table 2 shows criteria for determining ridging resistance.
[0014]

[0015]
The crystal orientation distribution measurement by the EBSP method was obtained by measuring the crystal orientation distribution in the rolling direction: 500 μm, the width direction: 800 μm for each steel plate at the half depth of the plate thickness. Based on the data, the ratio of large-angle grain boundaries whose deviation of orientation of adjacent crystal grains is 15 ° or more and (area of crystal grains within 5 ° of deviation from {111} plane) / (from {100} plane The area of crystal grains with a deviation within 5 ° was analyzed.
[0016]
In the press work test, a rectangular tube drawing with a punch size of 200 mm × 270 mm, a drawing depth of 100 mm, and a die shoulder radius of 10 mm was performed. . Table 3 shows the influence of cold rolling reduction distribution of intermediate rolling and finish rolling on the results of r, Δr, ridging judgment, crystal distribution analysis by EBSP method, and press working test.
[0017]

[0018]
From Table 3, when the ratio of the large-angle grain boundary is large, the ridging resistance is improved. Generally, ridging is caused by a group (colony) of crystal grains having a close crystal orientation existing in a cold-rolled annealed plate. The origin of the colony is that the coarse non-recrystallized ferrite grains extending in the rolling direction formed by the solidified columnar crystals remaining after hot rolling annealing are dispersed in the crystal orientation by recrystallization even during annealing after cold rolling. Since it is small, colonies are formed. Inside the colony, it appears to have a fine crystal structure, but since the dispersion of crystal orientation is small, the colony deforms like a large single crystal when subjected to plastic deformation such as press molding, and the plasticity of each colony is different. Ridging occurs due to the difference in directionality.
[0019]
Here, the crystal grain boundary inside the colony has a small deviation in the orientation of adjacent crystals, and there are very few large-angle grain boundaries with a deviation of 15 ° or more. That is, in a steel plate with a small proportion of large angle grain boundaries, lysing occurs with a large amount of colonies, whereas when the proportion of large angle grain boundaries is large, the formation of colonies that cause ridging of the steel plate is small and ridging resistance In particular, when the ratio of the large-angle grain boundaries is 90% or more, the generation of ridging is reduced and sufficient ridging resistance higher than the ridging judgment B is obtained.
[0020]
Also, the {111} plane is a good surface for deep drawing and the {100} plane is a surface that adversely affects the r value. The superiority and inferiority of the value can be clearly defined That is, (area of crystal grains within 5 ° of deviation from {111} plane of rolled surface at 1/2 depth of plate thickness) / ({100} surface of rolled surface at 1/2 depth of plate thickness) (The area of the crystal grains with a deviation from 5 ° within 5 °) corresponds well to r, particularly when the area of the {111} plane / the area of the {100} plane is 5 or more, and this tendency becomes remarkable. That's it.
In Table 3, what can be formed by the press working test is that the {111} plane area / {100} plane area is 5 or more, while the {111} plane area / {100} plane area. When the diameter was less than 5, the deep drawability was insufficient, and severe cracking occurred during pressing.
[0021]
However, in the conventional cold-rolled annealing material, the {111} plane area / {100} plane area is 5.5 and r is sufficiently high at 1.40, but cracking occurs in the press work test. ing. In this steel sheet, the proportion of large-angle grain boundaries in which the deviation in orientation of adjacent crystal grains is 15 ° or more of the crystal grain boundaries is relatively low at 81%, ridging resistance is low, and Δr indicating r-value anisotropy. The crack was generated at the corner because of the large. That is, even if the {111} texture is developed, if there are few large-angle grain boundaries and colonies are formed, satisfactory press formability cannot be obtained, and the area of {111} plane / area of {100} plane is It was revealed that the ratio of 5 or more and the ratio of large-angle grain boundaries required 90% or more.
[0022]
The reason why the deep drawability and ridging resistance are improved in the production method in which the multiphase annealing is performed is considered as follows.
First, when a hot-rolled sheet of ferritic stainless steel is subjected to a multiphase annealing at Ac ₁ point or more, a (ferrite + martensite) two-phase structure is obtained. Since the ferrite phase is much softer than the martensite phase, strain is accumulated in the ferrite phase when processing is performed by intermediate rolling. By annealing this in a box furnace for a long time, the martensite phase sufficiently precipitates carbides and recrystallizes into the ferrite phase, and at the same time, recrystallization of the ferrite phase with very accumulated strain is promoted. . At this time, recrystallization of coarse ferrite grains derived from solidified columnar crystals that cause ridging is also promoted, the crystal orientation of the recrystallized grains is greatly dispersed, the formation of colonies is inhibited, and the ratio of large-angle grain boundaries Increase.
[0023]
In Table 3, along with the increase in the intermediate rolling rate after the multiphase annealing of the production method of the present invention, the proportion of large-angle grain boundaries in which the deviation of the orientation of adjacent crystal grains among the grain boundaries is 15 ° or more increases. The results of improved ridging properties confirm this.
In addition, since the formation of colonies is inhibited after box-type annealing, the formation of {100} colonies, which lowers the r value that tends to remain in the normal process, is small, and is effective for the r value by the subsequent finish rolling and finish annealing. The {111} texture was developed, and a steel sheet having an area of {111} face / area of {100} face of 5 or more and a large-angle grain boundary ratio of 90% or more could be produced.
[0024]
Next, the reasons for limiting the components, characteristics, and production conditions of the ferritic stainless steel of the present invention will be described. In addition, the% display in the following description shows mass% altogether. C : 0.10% or less C is an alloy element exhibiting an effect of increasing γmax, and a sufficient amount of martensite and its hardness in a two-phase structure (ferrite + martensite) obtained after double phase annealing. It works effectively to obtain. However, it is a component that increases the strength after cold rolling annealing, and if included excessively, the ductility is reduced, so the upper limit of the C content was 0.10%.
[0025]
Si: 1.5% or less Si is an alloy component added as a deoxidizer during steelmaking, but has a high solid solution strengthening ability, and if contained excessively, the material is hardened and the ductility is lowered. The upper limit was set at 1.5%.
Mn: 1.0% or less Mn is an austenite forming element and can be used to control γmax. Moreover, since the solid solution strengthening ability is small, the influence of curing the material is small. However, if it is contained excessively, it causes adverse effects such as generation of Mn-based fumes during welding, so the upper limit of its content was set to 1.0%.
[0026]
P: 0.005% or less P lowers the hot workability in accordance with its content, so the upper limit of its content was made 0.005%.
S: 0.015% or less S is segregated at the crystal grain boundaries and embrittles the crystal grain boundaries as the content increases. Therefore, the upper limit of the content is set to 0.015%.
Ni: 2.0% or less Ni, like Mn, is an austenite-forming element and can be used for γmax control. However, excessive content of Ni causes an increase in cost, so the upper limit of the content was set to 2.0%.
[0027]
Cr: 10.0-20.0%
In order to obtain the corrosion resistance as stainless steel, it is necessary to contain 10.0% or more of Cr. However, when the amount of Cr increases, the toughness and workability are reduced, so the content is made 20.0% or less.
Al: 0.10% or less Al is an alloy component that is added as a deoxidizer during steelmaking, but excessive addition increases nonmetallic inclusions and causes toughness reduction and surface defects. The upper limit of the amount is 0.10%.
[0028]
N: 0.05% or less N is also an element exhibiting an effect of increasing γmax in the same manner as C. It increases austenite generated during double phase annealing, works effectively for fine structure, and improves ridging resistance. . However, it is a component that increases the strength after cold rolling annealing, and if it is excessively contained, it lowers the ductility, so the upper limit of its content was made 0.05%.
Ti: 0.05% or less Ti is an element that fixes C and N and improves workability. However, adding Ti causes an increase in steel material cost, so the upper limit of its content is 0.05%. It was.
[0029]
Nb: 0.05% or less Nb is an element that fixes C and N and improves workability. However, if Nb is added, the steel material cost increases, so the upper limit of its content is 0.05%. It was.
Mo: 1.0% or less Mo is an element added as necessary, and contributes to improvement of corrosion resistance. However, if Mo is added excessively, the hot workability deteriorates, so the upper limit of its content is 1.0%.
[0030]
Cu: 1.0% or less Cu is a component mixed from scrap or the like at the time of melting, and if its content increases, it adversely affects hot workability and corrosion resistance, so the upper limit is made 1.0%.
B: 0.0010 to 0.0100%
B is an element that is added as necessary, and has the effect of uniformly dispersing the transformed phase of the hot-rolled sheet and improving ridging resistance. When the content is 0.0010% or more, the effect of addition of B becomes remarkable, but excessive addition exceeding 0.0100% causes a decrease in hot workability and weldability.
[0031]
[gamma] max: 20 or more and less than 80 In the present invention, annealing is performed in a continuous annealing furnace to heat a hot-rolled sheet in a two-phase region of ferrite and austenite at Ac ₁ point or more, and a two-phase structure of ferrite and martensite is obtained. It is characterized by effectively using the obtained multiphase annealing. Therefore, in the present invention, the definition of γmax is an important matter. That is, if γmax is less than 20, the amount of martensite obtained by the multi-phase annealing is small, sufficient accumulation of strain is not made in the ferrite phase in the intermediate rolling process, and recrystallization of the ferrite band is not promoted. The r value and ridging resistance cannot be improved. On the other hand, in order to increase γmax, it is necessary to increase the content of austenite-forming elements such as C, N, Mn, and Ni. However, these cause hardness of the steel material and increase in cost, so γmax is less than 80. It is necessary to.
[0032]
Duplex annealing temperature: Ac ₁ point to 1100 ° C
In order to obtain a two-phase structure of ferrite + austenite at a high temperature and obtain a two-phase structure of ferrite + martensite after multi-phase annealing, it is essential to perform annealing at a temperature of at least Ac ₁ point or more. However, when annealing is performed at a temperature exceeding 1100 ° C., the risk of cutting the steel sheet due to the coarsening of the crystal grain size and the decrease in the high-temperature strength is increased. Therefore, the temperature of the multiphase annealing is set to a range of Ac ₁ point or more and 1100 ° C. or less.
[0033]
Intermediate cold rolling ratio: 10-50%
In the intermediate rolling after the duplex annealing, strain is accumulated in the ferrite phase, but the effect cannot be obtained at a rolling rate smaller than 10%. Further, if intermediate rolling exceeding 50% is performed, the rolling rate of finish rolling becomes small and the development of a texture effective for the r value cannot be obtained. Therefore, the rolling rate of intermediate rolling is 10% or more and 50% or less. .
Also, intermediate annealing requires long-time annealing to recrystallize martensite into carbide and ferrite, and if it is attempted to manufacture in a continuous annealing furnace, it is very inefficient and not economical. Long-term annealing with a furnace.
[0034]
Ratio of large-angle grain boundaries: 90% or more, ratio of any small crystal face from rolled surface: 5 or more In press working of ferritic stainless steel sheet, average r value as a factor indicating its workability R has been used. However, in actual pressing, even if the r value is high, it is often impossible to process a material having a large Δr and a large anisotropy or a material in which ridging is significant. A material preferable for actual pressing is a material having a small anisotropy and having both deep drawability and ridging resistance. Therefore, from the preliminary evaluation results shown in FIG. 1, in order to satisfy these characteristics, the ratio of the large-angle grain boundaries in which the deviation of the orientation of the adjacent crystal grains among the grain boundaries is 15 ° or more is 90% or more. (The area of crystal grains within 5 ° of deviation from the {111} plane of the rolled surface at ½ depth of the plate thickness) / ({100} of the rolled surface at ½ depth of the plate thickness) The area of crystal grains whose deviation from the surface is within 5 ° is defined as 5 or more.
[0035]
【Example】
In Table 1, steels having chemical compositions indicated by B, C, D, and E were melted to form slabs, and then hot rolled sheets having a thickness of 4.0 mm were obtained using a hot rolling mill. As a manufacturing method for subjecting this hot-rolled sheet to multi-phase annealing, applying the multi-phase annealing temperature and intermediate rolling rate shown in Table 4, hot-rolled sheet → multi-phase annealing → intermediate rolling → intermediate annealing → finishing Rolling → finish annealing was performed to obtain an annealing plate having a thickness of 0.8 mm. Moreover, the above-mentioned hot-rolled sheet is manufactured by a conventional manufacturing method, that is, hot-rolled sheet → hot-rolled sheet annealing (830 ° C. × soaking 9 hours) → finish cold rolling → finish annealing (manufacturing method of one cold-rolled annealing) and heat. It was produced by rolled sheet → hot-rolled sheet annealing (830 ° C. × soaking 9 hr) → intermediate cold rolling → intermediate annealing → finish cold rolling → finish annealing (manufacturing method of twice cold rolling annealing).
[0036]
Using the steel sheet obtained by the above method as a test material, r, Δr, and ridging resistance were measured by the following methods.
r value : After giving a tensile strain of 15% using a JIS No. 13B test piece, r _L , r _D and r _T were determined. r _L , r _D and r _T represent r values in the rolling direction, the 45 ° direction with respect to the rolling direction, and the 90 ° direction with respect to the rolling direction, respectively. From the r value obtained by the above method, r and Δr were obtained by r = (r _L + 2r _D + r _T ) / 4 and Δr = (r _L −2r _D + r _T ) / 2.
[0037]
Ridging resistance : 20% tensile strain was applied to a JIS No. 5 specimen taken from the rolling direction, and then ridging resistance was determined. For the determination of ridging resistance, A was the one with the best ridging resistance and D was the one with the lowest ridging resistance, and four-level evaluation of A, B, C, and D was performed.
[0038]
Press working test : A rectangular tube drawing with a punch size of 200 mm x 270 mm, a drawing depth of 100 mm, and a die shoulder radius of 10 mm was performed.
Measurement of crystal orientation distribution : Measured over the range of rolling direction: 500 μm, width direction: 800 μm on a rolled surface at a plate thickness of 1/2 depth by the EBSP method.
[0039]
Table 4 shows the characteristic values obtained by the respective production methods and production conditions of the steels having the chemical compositions described above.
Further, among the crystal grain boundaries of each steel plate, the ratio of large-angle grain boundaries whose deviation in the direction of adjacent crystal grains is 15 ° or more and (the area of crystal grains whose deviation from the {111} plane is within 5 °) / FIG. 1 shows the relationship between the value of (the area of crystal grains whose deviation from the {100} plane is within 5 °) and the press work test results.
Anything that satisfies the requirements described in the claims of the present invention did not cause defects such as cracks in the press work test, and was satisfactory. However, in the case where the duplex annealing temperature is lower than the Ac ₁ point temperature and the intermediate rolling rate is less than 10%, cracks occurred during press working.
[0040]

[0041]
【The invention's effect】
As described above, according to the present invention, without incurring an increase in steelmaking costs and a decrease in the productivity of hot-rolled steel strip, parts of household equipment such as a sink, various items and a stove burner, a fuel tank, Ferritic stainless steel sheets with excellent deep drawability and ridging resistance, which are mainly used for press processing, can be manufactured for parts of industrial equipment such as oil supply pipes, motor cases, covers and flanges.
[Brief description of the drawings]
FIG. 1 shows the ratio of large-angle grain boundaries in which the deviation in the direction of adjacent crystal grains is 15 ° or more among the grain boundaries of each steel sheet, and the area of crystal grains in which deviation from the ({111} plane is within 5 ° ) / (A graph showing the relationship between the value of (the area of crystal grains within 5 ° of deviation from the {100} plane) and press workability (including examples, comparative examples, and conventional examples).

Claims (3)

C: 0.10% or less, Si: 1.5% or less, Mn: 1.0% or less, P: 0.050% or less, S: 0.015% or less, Ni: 2.0% Hereinafter, Cr: 10.0 to 20.0%, Al: 0.10% or less, N: 0.05% or less, the balance is made of Fe and inevitable impurities, and is defined by the formula (1) The ratio of large-angle grain boundaries in which γmax is 20 or more and less than 80, and the deviation of the orientation of adjacent crystal grains is 15 ° or more among the grain boundaries of the rolled surface at ½ depth of the plate thickness is A ferritic stainless steel sheet excellent in ridging resistance and deep drawability, characterized by being 90% or more and satisfying the formula (2).
Formula (1)
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-52Al + 470N + 189
Formula (2)
X / Y ≧ 5
Where X is the area of the crystal grains within 5 ° of the rolled surface at a depth of 1/2 of the plate thickness, and Y is {100 of the rolled surface at the depth of 1/2 of the plate thickness. } The area of the crystal grain within 5 ° from the plane. C: 0.10% or less, Si: 1.5% or less, Mn: 1.0% or less, P: 0.050% or less, S: 0.015% or less, Ni: 2.0% Cr: 10.0 to 20.0%, Al: 0.10% or less, N: 0.05% or less, Ti: 0.05% or less, Nb: 0.05% or less, Including one or more of Mo: 1.0% or less, Cu: 1.0% or less, B: 0.0010 to 0.0100%, the balance being Fe and inevitable impurities, and the formula ( Γmax defined in 3) is 20 or more and less than 80, and further, a large-angle grain having a deviation of the orientation of adjacent crystal grains of 15 ° or more among grain boundaries on a rolling surface having a depth of ½ of the plate thickness. Ridging resistance and deep drawability characterized in that the ratio of the boundary is 90% or more and further satisfies the formula (4) An excellent ferritic stainless steel sheet.
Formula (3)
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-50Nb-52Al + 470N + 189 Formula (4)
X / Y ≧ 5
Where X is the area of the crystal grains within 5 ° of the rolled surface at a depth of 1/2 of the plate thickness, and Y is {100 of the rolled surface at the depth of 1/2 of the plate thickness. } The area of the crystal grain within 5 ° from the plane. In producing the ferritic stainless steel according to claim 1 or 2, a steel slab comprising the components described in the respective items is formed into a hot-rolled steel strip through hot rolling, and a combination of cold rolling and annealing. When producing a cold-rolled steel sheet or a steel strip, after performing a dual-phase annealing in which a hot-rolled steel strip is heated to a two-phase region of ferrite + austenite of Ac ₁ point or more and 1100 ° C. or less by a continuous annealing furnace, It is excellent in ridging resistance and deep drawability, characterized by performing intermediate cold rolling at a rolling rate of 10% or more and 50% or less, followed by annealing in a box furnace, finish cold rolling and recrystallization annealing. A method for producing a ferritic stainless steel sheet.

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