JP4289714B2

JP4289714B2 - Manufacturing method of medium and high carbon cold rolled steel sheet

Info

Publication number: JP4289714B2
Application number: JP08759099A
Authority: JP
Inventors: 雅人鈴木; 直人大久保; 昭史平松
Original assignee: Nippon Steel Nisshin Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 1999-03-30
Filing date: 1999-03-30
Publication date: 2009-07-01
Anticipated expiration: 2019-03-30
Also published as: JP2000282146A

Description

【０００１】
【発明の属する技術分野】
本発明は、局部延性に優れた中・高炭素冷延鋼板の製造方法に関するものであり、特に安価で製造性を向上させた製造方法に関するものである。
【０００２】
【従来の技術】
鋼中のＣ含有量が概ね0.40〜1.35質量％の、いわゆる中・高炭素冷延鋼板は、焼入れ強化が可能であるとともに、焼入れ前の焼鈍状態ではある程度の加工性も有しているため、自動車部品をはじめ各種機械部品や軸受け部品の素材として広く使用されている。部品の製造にあたっては、一般的には打抜加工や曲げ成形が施され、さらに比較的軽度な絞り加工，伸びフランジ成形が施されることもある。また、部品形状が複雑な場合は、二ないし三部品を溶接して製造される場合も多い。そしてこれらの加工部品は熱処理を経て各種用途の部品に仕上げられていく。
【０００３】
このように、素材としての中・高炭素冷延鋼板には種々の加工が施されるため、一般に炭化物の球状化処理が施される。0.40〜1.35質量％のＣを含有する中・高炭素鋼の熱延鋼板は硬く、加工性が悪いため、一次焼鈍を施して軟質化した後に冷間圧延を行い、仕上焼鈍が施される。また、冷間圧延の際、加工硬化により所望の板厚まで圧下できない場合には、一旦冷延を中止して焼鈍を施した後に、再び冷間圧延を行い、仕上焼鈍が施される。所望の板厚が薄くなると、トータルの圧下率を大きくするため、焼鈍と冷間圧延を繰り返すことになり、製造工程が煩雑になるとともに、製造コストの増加をもたらす。
【０００４】
こうした中、特開平5−171288号公報，特開平7−17968号公報には、0.80〜1.30質量％Ｃ鋼において熱間圧延条件や焼鈍方法を工夫することによって鋼板を軟質化し、一次焼鈍後の冷間圧延率を向上させる技術が紹介されている。これらは熱間圧延に際し、仕上圧延後の冷却速度を適切に制御する必要があり製造性に劣る。
【０００５】
また、特開平10−60540号公報には、0.30〜1.30質量％Ｃ鋼の熱延鋼板施す焼鈍方法を工夫することにより、鋼板を軟質化して、一次焼鈍後の冷間圧延率を向上させる技術が示されている。しかし、特開平5−171288号公報，特開平7−17968号公報,および特開平10−60540号公報は単に軟質化することによる圧下率の増大であり、冷間圧延時の耳切れ等による破断を防止するのに有効な方法は示されていない。
【０００６】
【発明が解決しようとする課題】
そこで本発明は、一次焼鈍後の鋼板において、高い圧下率まで破断することなく冷間圧延することを可能として、焼鈍と冷間圧延の繰り返し回数や冷間圧延のパス回数を低減し、安価で製造性に優れた中・高炭素冷延鋼板の製造方法を提供することを目的とする。
【０００７】
【課題を解決するための手段】
上記目的は、請求項１の発明、すなわち、Ｃ：0.40〜1.35質量％を含有する鋼の熱延鋼板に下記(a)の三段階焼鈍を施し、下記(b)で定義される炭化物粒径(Ｄ)と含有Ｃ量([Ｃ％])の関係が下記(c)の式を満足するように炭化物を球状化した後、冷延率90％以下の冷間圧延を行い、さらにＡc₁点未満の温度で均熱する焼鈍を施すか、又は下記(a)の三段階焼鈍を施すことを特徴とする中・高炭素冷延鋼板の製造方法によって達成できる。
【０００８】
(a)(Ａc₁−50)℃〜Ａc₁未満の温度範囲で0.5時間以上保持する１段目の加熱を行った後、Ａc₁〜（Ａc₁＋100)℃の温度範囲で0.5〜20時間保持する２段目の加熱、および（Ａr₁−80）℃〜Ａr₁の温度範囲で2〜60時間保持する３段目の加熱を連続して行い、かつ、２段目の保持温度から３段目の保持温度への冷却速度を5〜30 ℃／ｈとする。
(b)平均炭化物粒径：鋼板断面の金属組織観察において、観察視野内の個々の炭化物について測定した円相当径を全測定炭化物について平均した値をいう。ただし、観察視野は炭化物総数が300個以上となる領域とする。
(c)Ｅ値＝(22＋35×Ｄ)−15×[Ｃ％]≧20
ここで、Ａc₁は昇温過程における鋼のＡ₁変態点（℃）、Ａr₁は降温過程におけるＡ₁変態点（℃）を意味する。
【０００９】
請求項２の発明は、上記発明において対象とする鋼を、質量％において、Ｃ：0.40〜1.35％，Ｓｉ：0〜0.40％（無添加を含む），Ｍｎ：0〜1.0％（無添加を含む）を含有し、Ｐを0.03％以下，Ｓを0.01％以下，Ｔ.Ａｌを0.1％以下の含有量に制限し、残部がＦｅおよび不可避的不純物からなる鋼としたものである。
【００１０】
請求項３の発明は、同様に、Ｃ：0.40〜1.35％，Ｓｉ：0〜0.40％（無添加を含む），Ｍｎ：0〜1.0％（無添加を含む），Ｃｒ：0〜1.6％（無添加を含む），Ｍｏ：0〜0.3％（無添加を含む），Ｃｕ：0〜0.3％（無添加を含む），Ｎｉ：0〜2.0％（無添加を含む）を含有し、Ｐを0.03％以下，Ｓを0.01％以下，Ｔ.Ａｌを0.1％以下の含有量に制限し、残部がＦｅおよび不可避的不純物からなる鋼としたものである。ここで、Ｓｉ，Ｃｒ，Ｍｏ，Ｃｕ，Ｎｉの下限の0％はその元素が無添加であることを意味する。例えば請求項３で対象とする鋼の一例としては、これらの元素のうちＳｉとＣｒとＭｏだけを規定範囲内で添加し他のＣｕ，Ｎｉは添加しない鋼などが挙げられる。
【００１１】
【発明の実施の形態】
本発明者らは、一般的な中・高炭素冷延鋼板の製造方法、中でも一次焼鈍後の冷間圧延における限界の圧下率を安定的に向上させる方法について詳細に検討してきた。その結果、一次焼鈍により単に軟質化するだけでは、高圧下の冷間圧延において耳切れ等による破断を回避できない場合があることを知見した。さらに詳細に調査した結果、Ａ₁変態点を挟んだ特定条件の三段階焼鈍を一次焼鈍に採用し、一次焼鈍後の鋼板において、含有Ｃ量と炭化物粒径がある関係を満たした場合に、限界の冷間圧延率が向上し冷間圧延時の鋼板端部からの破断を安定して回避できることを見出した。
また、一次焼鈍後の鋼板は軟質でもあるため、所望の圧下率にまで冷間圧延を施すのに必要なパス回数（冷間圧延機を通過させる回数）を従来よりも低減させることも可能になる。
【００１２】
以下、本発明を特定するための事項について説明する。
本発明では、Ｃ：0.40〜1.35質量％を含有する鋼を対象とする。Ｃは炭素鋼においては最も基本となる合金元素であり、その含有量によって最終的な製品における焼入れ硬さおよび炭化物量が大きく変動する。また、製造段階における鋼板の加工性すなわち冷間圧延時の圧下限界に及ぼす影響も大きい。Ｃ含有量が0.40質量％以下の鋼は、比較的加工性が良く冷間圧延の際に圧下限界が問題となることはない。一方、Ｃ含有量が1.35質量％を超える鋼においても、本発明を適用することによる圧下率増大の効果は得られるが、熱間圧延後の靱性が低下して鋼帯の製造性・取扱い性が悪くなるとともに、焼鈍後にも十分に軟質化しないため、冷間圧延において90％といった高い圧下率が付与できない。したがって、本発明では、Ｃ含有量が0.40〜1.35質量％の範囲の鋼を対象とする。
【００１３】
本発明ではＣ以外の添加元素および不純物元素の含有量を特別に限定していない一般的な市販鋼に対して、製造工程の省略・簡略化が可能であるが、最終的な製品の材質を向上させるために種々の元素の含有量を限定した鋼に対しても適用できる。
Ｓは、ＭｎＳ系介在物を形成する元素である。この介在物の量が多くなると製品の靭性劣化につながるため、鋼中のＳ含有量はできるだけ低減することが望ましい。このため、Ｓ含有量を0.01質量％以下に低減した鋼を使用することが望ましい。
【００１４】
Ｐは、延性や靱性を劣化させるので、0.03質量％以下の含有量とすることが望ましい。Ａｌは溶鋼の脱酸剤として添加されるが、鋼中のＴ.Ａｌ量が0.1質量％を超えると鋼の清浄度が損なわれて鋼板に表面疵が発生しやすくなるので、Ｔ.Ａｌ含有量は0.1質量％以下とすることが望ましい。
【００１５】
Ｓｉは、局部延性に対して影響の大きい元素の１つである。Ｓｉを過剰に添加すると固溶強化作用によりフェライトが硬化し、加工性劣化の原因となる。またＳｉ含有量が増加すると製造過程で鋼板表面にスケール疵が発生する傾向を示し、表面品質の低下を招く。そこでＳｉを添加するに際しては0.40質量％以下の含有量となるようにする。
Ｍｎは、鋼板の耐摩耗性向上に有効な添加元素である。1.0質量％を超えて多量に含有させるとフェライトが硬化し、加工性の劣化を招く。そこで、Ｍｎは1.0質量％以下の範囲で含有させることが望ましい。
【００１６】
また本発明では必要に応じてＣｒ，Ｍｏ，Ｃｕ，Ｎｉ等の元素を添加して諸特性の改善を図ることができる。
Ｃｒは、焼入れ性を改善するとともに焼戻し軟化抵抗を大きくする元素である。しかし、1.6質量％を超える多量のＣｒが含まれると三段階焼鈍を施して炭化物を球状化しても、軟質化しにくく加工性が劣化するようになる。したがってＣｒを添加する場合は1.6質量％以下の範囲とする。
Ｍｏは、少量の添加でＣｒと同様に焼入れ性・焼戻し軟化抵抗の改善に寄与する。しかし、0.3質量％を超える多量のＭｏが含まれると３段階焼鈍を施しても軟質化しにくく加工性が劣化するようになる。したがってＭｏを添加する場合は0.3質量％以下の範囲とする。
【００１７】
Ｃｕは、熱延中に生成する酸化スケールの剥離性を向上させるので、鋼板の表面性状の改善に有効である。しかし、0.3質量％以上含有させると溶融金属脆化により鋼板表面に微細なクラックが生じやすくなるので、Ｃｕは0.3質量％以下の範囲で添加できる。Ｃｕ含有量の好ましい範囲は0.10〜0.15質量％である。
Ｎｉは、焼入れ性を改善するとともに低温脆性を防止する合金成分である。またＮｉは、Ｃｕ添加によって問題となる溶融金属脆化の悪影響を打ち消す作用を示すので、特にＣｕを約0.2％以上添加する場合にはＣｕ添加量と同程度のＮｉを添加することが極めて効果的である。しかし、2.0質量％を超える多量のＮｉが含まれると３段階焼鈍を施しても軟質化しにくく焼入れ前のプレス成形性や加工性が劣化するようになる。したがってＮｉを添加する場合は2.0質量％以下の範囲とする。
【００１８】
本発明は、中・高炭素冷延鋼板の製造において、一次焼鈍にＡ₁変態点を挟んだ三段階の温度範囲で順次加熱する「三段階焼鈍」を施して炭化物を球状化し、Ｃ含有量([Ｃ％])と炭化物粒径(Ｄ)との関係がＥ値＝(22＋35×Ｄ)−15×[Ｃ％]≧20を満足させることにより、冷延工程の省略または簡略化する点に大きな特徴がある。
【００１９】
一般的に、鋼をＡc₁点以上の温度に加熱すると炭化物のうち微細なものはオーステナイト中に溶解し、その後Ａr₁点以下の温度に冷却すると再び炭化物として析出する。その際、Ａc₁点以上の温度域で未溶解炭化物をある程度多く残存させることができた場合には、冷却速度を遅くすると、オーステナイト中に固溶したＣはパーライトを生成せずに未溶解炭化物を核として析出するので、焼鈍後の炭化物は球状化する。またこの場合、Ａc₁点以上の加熱によって炭化物の数は焼鈍前より減少し、しかも冷却速度が遅いときは冷却時に新たに核生成しないので、結果的に焼鈍後の炭化物数は焼鈍前より減少する。トータル炭素量は一定だから、炭化物数の減少は炭化物平均粒径の増大を意味する。
【００２０】
しかしながら、Ａc₁点以上の温度域において、平衡的には炭化物のすべてまたは一部が溶解する領域である。このため、通常はＡc₁点以上の温度域に加熱すると未溶解炭化物の個数は少なくなり、その後Ａr₁点以下の温度への冷却過程で、オーステナイト中に固溶したＣはラメラ間隔の広い再生パーライトとして析出する。その結果、炭化物の球状化せず、加工性の良い鋼板は得られない。
【００２１】
そこで、本発明者らは検討を重ねた結果、鋼板をＡc₁点以上へ加熱する前に、予めＡc₁点未満の特定温度域で一定時間以上加熱する処理を行えば、Ａc₁点以上の温度域において未溶解炭化物を適切量残存させることが可能であることを知見した。加えて、Ａr₁点以下への冷却後に特定温度域で特定時間保持することによって、炭化物を球状化および粗大化させることが可能になることもわかった。このような知見に基づいて規定した本発明の三段階焼鈍の条件について次に説明する。
【００２２】
〔Ａc₁点未満での加熱保持〕
１段目の加熱の目的は、Ａc₁点未満の温度に鋼板を保持し、熱間圧延で生成したパーライトを分断して、炭化物（セメンタイト）の球状化を図ることである。分断された炭化物は比較的細かいものの、球状化の進行より炭化物単位体積当たりの表面積が減少するので、結果的に２段目のＡc₁点以上の加熱時に、炭化物／オーステナイト界面面積の減少効果で炭化物の固溶を遅らせることができる。熱延パーライトの分断・球状化反応促進のためにはＡc₁点未満の範囲でなるべく高温が望ましい。Ａc₁−50℃より低温では球状化が十分に進まない。一方、Ａc₁点以上になると界面面積の大きい熱延パーライトは容易にオーステナイトに固溶してしまうので目的が達成できない。したがって１段目の加熱温度はＡc₁−50℃〜Ａc₁未満の温度範囲とした。また、その温度範囲での保持時間が0.5時間未満では球状化が十分に図れないので、１段目の加熱保持時間は0.5時間以上とした。保持時間の上限は特に規定する必要はないが、工業的な実施を考慮したとき20時間以内とすることが望ましい。
【００２３】
なお、この１段目の加熱を行った後は、そのまま昇温して２段目の加熱を実施してもよいし、一旦常温まで冷却したのち改めて昇温して２段目の加熱に供してもよい。設備の都合等により１回の加熱で0.5時間以上の保持時間を確保できないときは、この１段目の加熱を複数回に分けて行ってもよい。その場合は上記温度範囲内での保持時間がトータル0.5時間以上となるようにする。
【００２４】
〔Ａc₁点以上での加熱保持〕
２段目の加熱の目的は、１段目の加熱を経た鋼板をＡc₁点以上の温度に保持し、オーステナイト化した部分において微細な炭化物を固溶・消失させるとともに比較的大きな球状炭化物を未溶解のまま残すこと、および、過共析鋼においては炭化物をオストワルド成長させることである。つまり、続く３段目の加熱で炭化物析出の核となるべき未溶解炭化物の数および分散状態を決定付ける工程である。加熱温度がＡc₁点未満ではオーステナイトが生成しない。一方、Ａc₁＋100℃の温度を超えると、１段目の加熱で炭化物が球状化されていても、その多くはオーステナイト中に固溶・消失し、未溶解炭化物の数が少なくなりすぎるか、または存在しなくなる。そうなると３段目への冷却過程で再生パーライトが生成し、炭化物の球状化が実現できない。加熱保持時間が0.5時間未満ではオーステナイト中への微細炭化物の固溶が不十分であり、20時間を超える長時間加熱ではより平衡状態に近づくため未溶解炭化物の数が減少しすぎる。したがって、２段目の加熱はＡc₁〜Ａc₁＋100℃の温度範囲で0.5〜20時間保持することとした。
【００２５】
〔降温過程で行うＡr₁点以下での加熱保持〕
３段目の加熱の目的は、１段目〜２段目の加熱を経た鋼板をＡr₁点以下の温度に保持し、２段目の温度からの冷却でオーステナイト→フェライト変態に伴ってオーステナイトから吐き出されるＣを未溶解炭化物を核として析出させるとともに、これらの炭化物をオストワルド成長させることである。つまり、炭化物の数は２段目の加熱で残存させた未溶解炭化物の数をほぼそのまま維持し、かつ炭化物の球状化率を高める工程である。保持温度がＡr₁点以下でないとオーステナイト→フェライト変態が起こらない。また、保持温度がＡr₁−80℃より低温の場合や、保持時間が２時間未満では、オストワルド成長が十分進まない。ただし、保持時間が60時間を超えてもその効果が飽和し、工業的なメリットはない。したがって、３段目の加熱はＡr₁−80℃〜Ａr₁の温度範囲で2〜60時間保持することとした。
【００２６】
〔Ａr₁点以下の保持温度への冷却速度〕
この冷却速度が速いとオーステナイトの過冷度が大きくなり、再生パーライトが生成しやすくなる。再生パーライトの生成を十分抑制するためには冷却速度を30℃／ｈ以下とする必要がある。一方、冷却速度を5℃／ｈより遅くしても再生パーライト抑制効果は飽和し、工業的メリットがない。したがって、当該冷却速度は5〜30℃／ｈに規定した。
【００２７】
以上の焼鈍によって、炭化物が球状化・粗大化するが、Ｃ含有量([Ｃ％])と炭化物粒径(Ｄ)がＥ値＝(22＋35×Ｄ)−15×[Ｃ％]≧20を満足する場合には、耳切れ等による破断を起こすことなく冷間圧延における限界の圧下率を向上することができる。
このＥ値は鋼板の局部的な変形能を評価した値であり、Ｅ値が大きいほど冷間圧延時に耳切れ等による破断トラブルが発生しにくくなる。また、Ｃ含有量が少ないほど、また炭化物粒径（Ｄ）が大きいほど冷延工程の省略または簡略化により製造性が向上することを意味している。すなわち、加工性に劣るＣ含有量の多い鋼においても、炭化物を球状化して、その粒径を大きくすることにより冷間圧延における限界の圧下率が向上する。
本発明では、三段階焼鈍を施して、Ｃ含有量と炭化物粒径の関係を適正範囲に調整することで、冷延率90％までの冷間圧延を施すことができる。
【００２８】
ここで、平均炭化物粒径は、鋼板断面の金属組織観察（例えば走査電子顕微鏡観察）において、観察視野内の個々の炭化物について測定した円相当径を全測定炭化物について平均した値を意味する。ただし、観察視野は炭化物総数が300個以上となる領域とする。
【００２９】
一次焼鈍後、冷間圧延を施した鋼板のひずみや加工硬化を解消し、軟質で加工性の良い鋼板を得るための焼鈍としては、Ａc₁点未満の均熱にて焼鈍を行えばよい。ただし、さらに炭化物を粗大化する場合には、一次焼鈍と同様の条件にて三段階焼鈍を行うことが効果的である。
【００３０】
トータルの冷延率が90％を超える場合には、請求項１に記載の一次焼鈍を施し、その後の冷延・焼鈍を２回以上繰り返すことができる。この場合においても、一次焼鈍に三段階焼鈍を施すと、1回の冷間圧延工程におけるパス回数の低減や、冷延・焼鈍の工程省略ができるので製造性が向上する。
例えば、5mmの熱延板から0.32mmの冷延焼鈍材を作製する場合、従来の方法では、一次焼鈍後に圧下率60％までの冷間圧延が可能な場合においても、冷延・焼鈍を３回繰り返すことになる。これに対し、本発明の三段階焼鈍を一次焼鈍に適用することにより、冷延率90％の冷間圧延を施して板厚を0.５mmとし、焼鈍後さらに冷延率36％の冷間圧延を施して板厚を0.32mmとすることができるので、冷延・焼鈍の工程を１回省略することができる。
【００３１】
【実施例】
〔実施例〕
表１に、供試鋼板の化学成分，Ａc₁変態点，およびＡr₁変態点を示す。Ａc₁変態点およびＡr₁変態点は、直径5ｍｍ×長さ10ｍｍの供試鋼試験片を「10℃／ｈで昇温→900℃で10分間保持して完全にオーステナイト化→10℃／ｈで冷却」というヒートパターンで加熱・冷却しながら試験片の収縮・膨張を測定し、その収縮・膨張曲線の変化から求めた。
【００３２】
【表１】

【００３３】
表１の鋼について、仕上パス温度850℃，仕上熱延での全圧下率80％，仕上圧延における最終パスの圧下率10％，巻取温度600℃の条件で熱間圧延を行って厚さ5.0ｍｍの熱延鋼帯を製造し、酸洗した後、各鋼板を種々の条件で一次焼鈍した。一次焼鈍後の鋼板について、平均炭化物粒径を測定した。また、冷延率90％までの冷間圧延を施し、圧下率40％の時のパス回数、および耳切れ発生時の冷延率について調べた。
【００３４】
平均炭化物粒径は、走査電子顕微鏡により鋼板断面の一定領域内を観察し、画像処理装置（ニレコ社製、ＬＵＺＥＸ IIIＵ）を利用して、個々の炭化物の円相当径を算出し、それを全測定炭化物について平均して求めた。その際、測定炭化物総数は300〜1000個の範囲であった。
これらの試験結果を一次焼鈍条件、Ｅ値、一次焼鈍後の硬さと併せて表２に示す。
【００３５】
【表２】

【００３６】
Ｃ含有量が多いＨ鋼は、本発明の規定範囲内で焼鈍し、Ｅ値を20以上としても、硬さが硬く、圧下率55％で耳切れが発生した。Ｂ、Ｄ鋼において、本発明範囲外のＡc₁点以下の温度で一段均熱の焼鈍を施したNo.１，２は炭化物粒径が小さいため、Ｅ値が20より小さくなり、60％以下の圧下率で耳切れが発生した。Ｄ鋼において三段階焼鈍を施しても、三段目の温度が低いNo.２１や、三段目の保持時間が短いNo.２２も同様に、炭化物粒径が小さくＥ値が20より小さくなっており、圧下率55％で耳切れが発生した。
また、一段目の保持温度が低いNo.１６、二段目の温度が高いNo.１７、二段目の時間が長いNo.１８および二段目からの冷却速度が速いNo.１９は再生パーライトが生成したため、炭化物が球状化せず、本発明規定の三段階焼鈍を施したもの（No.４，９，１５）と同等の硬さにも拘らず、圧下率50％以下で耳切れが発生した。三段目の温度がＡr₁点を超えているNo.２０についても三段目からの冷却段階において、パーライトを生成したため45％で耳切れが発生した。
【００３７】
一方、Ｃ量が本発明で規定した範囲内である鋼板に、適正な条件にて三段階焼鈍を施したNo.３〜１２，１４，１５はいずれもＥ値が20％以上であり、圧下率90％までの冷間圧延が可能である。さらに、圧下率40％のときのパス回数を見ると、Ｂ鋼では6回、Ｄ鋼では8回であり、比較例のNo.１，２やNo.１６〜２２に比べて少ない。
【００３８】
【発明の効果】
以上のように、本発明では、中・高炭素冷延鋼板の製造に際しＡ₁変態点を挟んだ特定条件の三段階焼鈍を一次焼鈍により炭化物を球状化させ、かつ、含有Ｃ量と炭化物粒径の関係を規定することにより、中・高炭素冷延鋼板の製造性が向上し、同時に製造コスト低減が可能となる。すなわち、高い冷延率まで１回の冷間圧延により行うことが可能となるので、従来の冷間圧延と焼鈍を何度か繰り返す製造方法に比べて、冷間圧延および焼鈍工程の繰り返し回数を低減することができる。また、パス回数を低減することも可能となる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a medium / high carbon cold-rolled steel sheet excellent in local ductility, and particularly relates to a production method which is inexpensive and has improved productivity.
[0002]
[Prior art]
The so-called medium / high carbon cold-rolled steel sheet with a C content of approximately 0.40 to 1.35% by mass in steel can be hardened and has a certain degree of workability in the annealed state before quenching. It is widely used as a material for various machine parts and bearing parts including automobile parts. In manufacturing parts, generally, punching and bending are performed, and relatively mild drawing and stretch flange molding may be performed. Further, when the part shape is complicated, it is often produced by welding two or three parts. These processed parts are finished into parts for various uses through heat treatment.
[0003]
Thus, since various processing is given to the medium and high carbon cold-rolled steel sheet as a raw material, the spheroidizing treatment of carbide is generally performed. Since the hot-rolled steel sheet of medium / high carbon steel containing 0.40 to 1.35% by mass of C is hard and has poor workability, it is softened by primary annealing and then cold-rolled and finish-annealed. In the case of cold rolling, if it cannot be reduced to a desired plate thickness by work hardening, cold rolling is once stopped and annealing is performed, then cold rolling is performed again, and finish annealing is performed. When the desired plate thickness is reduced, the total reduction ratio is increased, so that annealing and cold rolling are repeated, and the manufacturing process becomes complicated and the manufacturing cost increases.
[0004]
Under these circumstances, in JP-A-5-171288 and JP-A-7-17968, steel sheets are softened by devising hot rolling conditions and annealing methods in 0.80 to 1.30% by mass C steel, and after primary annealing. Technologies to improve the cold rolling rate are introduced. These are inferior in productivity because it is necessary to appropriately control the cooling rate after finish rolling during hot rolling.
[0005]
Japanese Patent Laid-Open No. 10-60540 discloses a technique for improving the cold rolling rate after primary annealing by devising an annealing method for applying a hot rolled steel sheet of 0.30 to 1.30% by mass C steel. It is shown. However, JP-A-5-171288, JP-A-7-17968, and JP-A-10-60540 are merely an increase in the rolling reduction due to softening, and breakage due to edge cutting or the like during cold rolling. An effective way to prevent this is not shown.
[0006]
[Problems to be solved by the invention]
Therefore, the present invention enables the steel sheet after the primary annealing to be cold-rolled without breaking up to a high rolling reduction, reduces the number of repetitions of annealing and cold rolling and the number of cold rolling passes, and is inexpensive. It aims at providing the manufacturing method of the medium and high carbon cold-rolled steel plate excellent in manufacturability.
[0007]
[Means for Solving the Problems]
The object is to subject the invention of claim 1, that is, a hot rolled steel sheet of steel containing C: 0.40 to 1.35% by mass to the three-stage annealing of the following (a), and the carbide particle size defined by the following (b): The carbide is spheroidized so that the relationship between (D) and the amount of contained C ([C%]) satisfies the following formula (c), and then cold-rolled at a cold rolling rate of 90% or less, and further Ac ₁ It can be achieved by a method for producing a medium / high carbon cold-rolled steel sheet characterized by performing annealing soaking at a temperature below the point or performing the following three-stage annealing (a).
[0008]
(a) (Ac ₁ -50) after 1-stage pressurized heat holding more than 0.5 hours at a temperature range of less than ° C. to Ac _1, 0.5 to 20 at a temperature range of _{_{Ac 1 ~ (Ac 1 +100)}} ℃ heating the second stage to hold time, and (Ar ₁ -80) performs heating of the third stage which holds 2 to 60 hours at a temperature range of ° C. to Ar ₁ in succession, and the second stage holding temperature The cooling rate to the third stage holding temperature is 5-30 ° C./h.
(b) Average carbide particle diameter: The value obtained by averaging the equivalent circle diameters measured for individual carbides within the observation field for all the measured carbides in the observation of the metal structure of the cross section of the steel sheet. However, the field of view is the area where the total number of carbides is 300 or more.
(c) E value = (22 + 35 × D) −15 × [C%] ≧ 20
Here, Ac ₁ means the A ₁ transformation point (° C.) of the steel in the temperature rising process, and Ar ₁ means the A ₁ transformation point (° C.) in the temperature lowering process.
[0009]
In the invention of claim 2, the steel to be used in the above invention is, in mass%, C: 0.40 to 1.35%, Si: 0 to 0.40% (including no addition), Mn: 0 to 1.0% (no addition) In which P is 0.03% or less, S is 0.01% or less, T.Al is 0.1% or less, and the balance is Fe and inevitable impurities.
[0010]
Similarly, the invention of claim 3 includes C: 0.40 to 1.35%, Si: 0 to 0.40% (including no addition), Mn: 0 to 1.0% (including no addition), Cr: 0 to 1.6% ( Containing 0 to 0.3% (including no addition), Cu: 0 to 0.3% (including no addition), Ni: 0 to 2.0% (including no addition), P The steel is limited to 0.03% or less, S is 0.01% or less, and T.Al is 0.1% or less, with the balance being Fe and inevitable impurities. Here, 0% of the lower limit of Si, Cr, Mo, Cu, and Ni means that the element is not added. For example, as an example of steel targeted by claim 3, steel and the like in which only Si, Cr, and Mo are added within a specified range and other Cu and Ni are not added.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have studied in detail a method for producing a general medium-high carbon cold-rolled steel sheet, particularly a method for stably improving the critical rolling reduction in the cold rolling after the primary annealing. As a result, it has been found that breakage due to edge cutting or the like may not be avoided in cold rolling under high pressure only by softening by primary annealing. Result of further investigation in detail, when adopting a three-stage annealing of the particular condition sandwiching the A ₁ transformation point in the primary annealing, the steel sheet after primary annealing, satisfying the relationship with the content C content and carbide grain size, It has been found that the cold rolling ratio at the limit is improved, and breakage from the end of the steel plate during cold rolling can be stably avoided.
In addition, since the steel sheet after the primary annealing is also soft, it is possible to reduce the number of passes (number of passes through the cold rolling mill) necessary for cold rolling to the desired rolling reduction than before. Become.
[0012]
Hereinafter, matters for specifying the present invention will be described.
In the present invention, steel containing C: 0.40 to 1.35% by mass is targeted. C is an alloy element which is the most basic in carbon steel, and the quenching hardness and carbide content in the final product vary greatly depending on its content. In addition, the influence on the workability of the steel sheet in the manufacturing stage, that is, the rolling limit during cold rolling is great. Steel with a C content of 0.40% by mass or less has relatively good workability, and the rolling limit does not become a problem during cold rolling. On the other hand, even when the C content exceeds 1.35% by mass, the effect of increasing the rolling reduction by applying the present invention can be obtained, but the toughness after hot rolling is reduced and the steel strip is manufactured and handled. In addition, since it is not sufficiently softened even after annealing, a high reduction ratio of 90% cannot be provided in cold rolling. Therefore, in the present invention, steel having a C content in the range of 0.40 to 1.35% by mass is targeted.
[0013]
In the present invention, it is possible to omit or simplify the manufacturing process for a general commercial steel in which the contents of additive elements and impurity elements other than C are not specifically limited. It can also be applied to steels with limited contents of various elements for improvement.
S is an element that forms MnS inclusions. When the amount of inclusions increases, it leads to toughness deterioration of the product. Therefore, it is desirable to reduce the S content in the steel as much as possible. For this reason, it is desirable to use steel in which the S content is reduced to 0.01% by mass or less.
[0014]
Since P deteriorates ductility and toughness, the content is preferably 0.03% by mass or less. Al is added as a deoxidizer for molten steel, but if the amount of T.Al in the steel exceeds 0.1% by mass, the cleanliness of the steel is impaired and surface flaws are likely to occur on the steel sheet. The amount is desirably 0.1% by mass or less.
[0015]
Si is one of the elements having a great influence on the local ductility. If Si is added excessively, the ferrite is hardened by the solid solution strengthening action, which causes workability deterioration. Further, when the Si content is increased, scale flaws tend to be generated on the surface of the steel sheet during the production process, leading to a reduction in surface quality. Therefore, when Si is added, the content is made 0.40% by mass or less.
Mn is an additive element effective for improving the wear resistance of the steel sheet. If it is contained in a large amount exceeding 1.0% by mass, the ferrite is cured and the workability is deteriorated. Therefore, it is desirable to contain Mn in a range of 1.0% by mass or less.
[0016]
In the present invention, various characteristics can be improved by adding elements such as Cr, Mo, Cu, and Ni as required.
Cr is an element that improves hardenability and increases temper softening resistance. However, if a large amount of Cr exceeding 1.6% by mass is contained, even if the carbide is spheroidized by three-step annealing, it becomes difficult to soften and the workability deteriorates. Therefore, when adding Cr, it is made into the range of 1.6 mass% or less.
Mo contributes to the improvement of hardenability and temper softening resistance in the same manner as Cr when added in a small amount. However, if a large amount of Mo exceeding 0.3% by mass is contained, it becomes difficult to soften even if three-step annealing is performed, and the workability deteriorates. Therefore, when adding Mo, it is set as the range of 0.3 mass% or less.
[0017]
Cu improves the surface properties of the steel sheet because it improves the peelability of the oxide scale produced during hot rolling. However, if it is contained in an amount of 0.3% by mass or more, fine cracks are likely to occur on the surface of the steel sheet due to molten metal embrittlement, so Cu can be added in a range of 0.3% by mass or less. A preferable range of the Cu content is 0.10 to 0.15% by mass.
Ni is an alloy component that improves hardenability and prevents low temperature brittleness. In addition, since Ni has an action to counteract the adverse effect of molten metal embrittlement which is a problem due to the addition of Cu, especially when adding about 0.2% or more of Cu, it is extremely effective to add Ni of the same amount as Cu addition. Is. However, if a large amount of Ni exceeding 2.0% by mass is contained, it is difficult to soften even if three-stage annealing is performed, and press formability and workability before quenching deteriorate. Therefore, when adding Ni, it is made into the range of 2.0 mass% or less.
[0018]
In the production of medium- and high-carbon cold-rolled steel sheets, the present invention performs a “three-stage annealing” that sequentially heats the primary annealing in a three-stage temperature range sandwiching the A ₁ transformation point, thereby spheroidizing the carbide, The point of ([C%]) and the carbide particle size (D) satisfying the E value = (22 + 35 × D) −15 × [C%] ≧ 20, thereby omitting or simplifying the cold rolling process. Has major features.
[0019]
Generally, when steel is heated to a temperature of Ac ₁ point or higher, fine ones of carbides dissolve in austenite, and then cooled to a temperature of Ar ₁ point or lower, and again precipitates as carbides. At that time, if a large amount of undissolved carbide can remain in a temperature range of Ac ₁ point or higher, if the cooling rate is slowed, C dissolved in austenite does not generate pearlite, and undissolved carbide. Since it precipitates as a nucleus, the carbide after annealing is spheroidized. Also, in this case, the number of carbides decreases by ₁ or more points of heating than before annealing, and when the cooling rate is slow, new nucleation does not occur during cooling, and as a result, the number of carbides after annealing decreases from before annealing. To do. Since the total carbon content is constant, a decrease in the number of carbides means an increase in the average carbide particle size.
[0020]
However, in the temperature range of Ac ₁ point or higher, in equilibrium, all or part of the carbide is dissolved. For this reason, the number of undissolved carbides usually decreases when heated to a temperature range of Ac ₁ point or higher, and then C dissolved in austenite is regenerated with a wide lamellar spacing during the cooling process to a temperature of Ar ₁ point or lower. Precipitate as pearlite. As a result, the steel does not spheroidize and a workable steel plate cannot be obtained.
[0021]
Therefore, as a result of repeated studies, the present inventors have conducted a process of heating in a specific temperature range less than Ac ₁ point in advance for a certain period of time before heating the steel plate to Ac ₁ point or more, and then ac ₁ point or more. It has been found that an appropriate amount of undissolved carbide can remain in the temperature range. In addition, it was also found that the carbide can be spheroidized and coarsened by holding for a specific time in a specific temperature range after cooling to ₁ point or less of Ar. The conditions for the three-stage annealing of the present invention defined based on such knowledge will be described below.
[0022]
[Heat retention at less than ₁ Ac]
The purpose of the first stage heating is to hold the steel plate at a temperature less than Ac ₁ point and to break up the pearlite produced by hot rolling to make the carbide (cementite) spherical. Although the divided carbide is relatively fine, the surface area per unit volume of carbide decreases as the spheroidization progresses. As a result, when heating at the second stage Ac ₁ point or higher, the carbide / austenite interface area is reduced. The solid solution of carbide can be delayed. In order to promote the division and spheroidization reaction of hot-rolled pearlite, it is desirable that the temperature be as high as possible within a range of less than Ac ₁ point. Spheroidization does not proceed sufficiently at temperatures lower than Ac ₁ -50 ° C. On the other hand, when Ac is ₁ or more, hot-rolled pearlite having a large interfacial area easily dissolves in austenite, and the object cannot be achieved. Therefore the heating temperature in the first stage was a temperature range of less than Ac ₁ -50 ° C. to Ac _1. Further, since the spheroidization cannot be sufficiently achieved when the holding time in the temperature range is less than 0.5 hours, the heating and holding time for the first stage is set to 0.5 hours or more. The upper limit of the retention time does not need to be specified, but it is preferably within 20 hours in consideration of industrial implementation.
[0023]
After the first stage heating, the temperature may be raised as it is, and the second stage heating may be performed. Alternatively, after cooling to room temperature, the temperature is raised again and used for the second stage heating. May be. When the holding time of 0.5 hour or more cannot be ensured by one heating due to the convenience of the equipment, etc., the first stage heating may be performed in a plurality of times. In such a case, the holding time within the above temperature range is set to 0.5 hours or more in total.
[0024]
[Heat retention at _one or more points of Ac]
The purpose of the second stage heating is to keep the steel sheet that has undergone the first stage heating at a temperature of Ac ₁ point or higher, so that fine carbides dissolve and disappear in the austenitized portion, and a relatively large spherical carbide is not removed. It is to leave it dissolved and to cause Ostwald growth of carbide in hypereutectoid steel. That is, it is a step of determining the number and dispersion state of undissolved carbides that should become nuclei for carbide precipitation by the subsequent third stage heating. If the heating temperature is less than Ac ₁ point, austenite is not generated. On the other hand, if the temperature exceeds Ac ₁ + 100 ° C., even if the carbides are spheroidized by heating in the first stage, many of them dissolve / disappear in the austenite, and the number of undissolved carbides becomes too small. Or disappear. Then, regenerated pearlite is generated in the cooling process to the third stage, and carbide spheroidization cannot be realized. If the heating and holding time is less than 0.5 hours, the solid carbide is not sufficiently dissolved in the austenite, and if the heating is continued for more than 20 hours, the number of undissolved carbides is reduced too much because it approaches an equilibrium state. Therefore, the second stage heating was to hold 0.5 to 20 hours at a temperature range of Ac _₁ ~Ac ₁ + 100 ℃.
[0025]
[Heat holding at ₁ point or less in the temperature lowering process]
The purpose of the third stage heating is to maintain the steel sheet that has undergone the first to second stage heating at a temperature of Ar ₁ point or lower, and from the austenite to the ferrite transformation by cooling from the second stage temperature. It is to cause the carbon to be discharged to precipitate with undissolved carbides as nuclei and to perform Ostwald growth of these carbides. That is, the number of carbides is a step of maintaining the number of undissolved carbides left by the second stage heating almost as it is and increasing the spheroidization rate of the carbides. If the holding temperature is not lower than Ar ₁ point, austenite → ferrite transformation does not occur. Further, when the holding temperature is lower than Ar ₁ -80 ° C. or when the holding time is less than 2 hours, the Ostwald growth does not proceed sufficiently. However, even if the holding time exceeds 60 hours, the effect is saturated and there is no industrial merit. Thus, the third stage heating was to hold 2 to 60 hours at a temperature range of Ar _₁ -80 ℃ ~Ar _1.
[0026]
[Cooling rate to hold temperature below ₁ point Ar]
When this cooling rate is high, the degree of supercooling of austenite increases and regenerated pearlite is easily generated. In order to sufficiently suppress the production of regenerated pearlite, the cooling rate needs to be 30 ° C./h or less. On the other hand, even if the cooling rate is lower than 5 ° C./h, the reproduction pearlite suppressing effect is saturated and there is no industrial merit. Therefore, the cooling rate is specified at 5 to 30 ° C./h.
[0027]
Although the carbides are spheroidized and coarsened by the above annealing, the C content ([C%]) and the carbide particle size (D) satisfy E value = (22 + 35 × D) −15 × [C%] ≧ 20. When satisfied, the critical rolling reduction in cold rolling can be improved without causing breakage due to cutting off of the ears or the like.
The E value is a value obtained by evaluating the local deformability of the steel sheet, and the larger the E value, the less likely it is to cause a trouble of breakage due to edge cutting or the like during cold rolling. Moreover, it means that productivity is improved by omission or simplification of a cold rolling process, so that C content is small and a carbide | carbonized_material particle size (D) is large. That is, even in steel with a high C content, which is inferior in workability, the limit rolling reduction in cold rolling is improved by spheroidizing the carbide and increasing its particle size.
In the present invention, cold rolling up to 90% can be performed by performing three-stage annealing and adjusting the relationship between the C content and the carbide particle size within an appropriate range.
[0028]
Here, the average carbide particle diameter means a value obtained by averaging the equivalent circle diameters measured for individual carbides in the observation field for all the measured carbides in the observation of the metal structure of the cross section of the steel sheet (for example, observation with a scanning electron microscope). However, the observation visual field is an area where the total number of carbides is 300 or more.
[0029]
After the primary annealing, as annealing for eliminating the strain and work hardening of the cold-rolled steel sheet and obtaining a soft steel sheet with good workability, annealing may be performed with soaking of less than Ac ₁ point. However, when the carbide is further coarsened, it is effective to perform three-stage annealing under the same conditions as the primary annealing.
[0030]
When the total cold rolling rate exceeds 90%, the primary annealing according to claim 1 can be performed, and the subsequent cold rolling and annealing can be repeated twice or more. Even in this case, if the three-stage annealing is performed for the primary annealing, the number of passes in one cold rolling process can be reduced and the process of cold rolling / annealing can be omitted, so that the productivity is improved.
For example, when producing a 0.32 mm cold-rolled annealed material from a 5 mm hot-rolled sheet, the conventional method can perform cold-rolling / annealing even when cold rolling up to 60% reduction is possible after primary annealing. Will be repeated. On the other hand, by applying the three-stage annealing of the present invention to the primary annealing, cold rolling with a cold rolling rate of 90% is performed to obtain a sheet thickness of 0.5 mm, and after annealing, a cold rolling rate of 36% is further applied. Since the sheet thickness can be reduced to 0.32 mm by rolling, the process of cold rolling and annealing can be omitted once.
[0031]
【Example】
〔Example〕
Table 1 shows the chemical composition, Ac ₁ transformation point, and Ar ₁ transformation point of the test steel sheet. The Ac ₁ transformation point and the Ar ₁ transformation point are as follows: a test steel specimen having a diameter of 5 mm and a length of 10 mm was “temperature raised at 10 ° C./h→held at 900 ° C. for 10 minutes to completely austenite → 10 ° C./h The shrinkage / expansion of the test piece was measured while heating / cooling with a heat pattern of “cooled by” and obtained from the change of the shrinkage / expansion curve.
[0032]
[Table 1]

[0033]
Thickness of steel in Table 1 after hot rolling under conditions of finishing pass temperature 850 ℃, total rolling reduction 80% in finishing hot rolling, final pass rolling reduction 10% in finish rolling, winding temperature 600 ℃ After producing a 5.0 mm hot-rolled steel strip and pickling, each steel plate was subjected to primary annealing under various conditions. About the steel plate after primary annealing, the average carbide particle size was measured. Further, cold rolling was performed up to a cold rolling rate of 90%, and the number of passes when the rolling reduction rate was 40% and the cold rolling rate at the time of occurrence of the ear break were investigated.
[0034]
The average carbide particle diameter is observed within a certain region of the cross section of the steel sheet with a scanning electron microscope, and an equivalent circle diameter of each carbide is calculated using an image processing device (manufactured by Nireco Corporation, LUZEX IIIU). The average was obtained for the measured carbides. At that time, the total number of measured carbides was in the range of 300-1000.
These test results are shown in Table 2 together with primary annealing conditions, E value, and hardness after primary annealing.
[0035]
[Table 2]

[0036]
The H steel with a high C content was annealed within the specified range of the present invention, and even when the E value was set to 20 or more, the hardness was hard and the ear breakage occurred at a reduction rate of 55%. In steels B and D, No. 1 and 2 subjected to one-step soaking at a temperature of Ac ₁ point or less outside the scope of the present invention have a small carbide particle size, so the E value is smaller than 20, 60% or less. Ear loss occurred at a rolling reduction of. Even if steel D is subjected to three-stage annealing, No. 21 where the temperature of the third stage is low and No. 22 where the holding time of the third stage is short also have a small carbide particle size and an E value of less than 20. Ear loss occurred at a reduction rate of 55%.
No. 16 having a low first stage holding temperature, No. 17 having a high second stage temperature, No. 18 having a long second stage, and No. 19 having a fast cooling rate from the second stage are regenerated perlite. As a result, the carbides were not spheroidized, and the ears were cut off at a reduction rate of 50% or less despite the hardness equivalent to that of the three-step annealing specified in the present invention (No. 4, 9, 15). Occurred. In No. 20 where the temperature of the third stage exceeds the Ar ₁ point, pearlite was generated in the cooling stage from the third stage, and thus the ear break occurred at 45%.
[0037]
On the other hand, Nos. 3 to 12, 14 and 15 in which the C amount is within the range specified in the present invention and subjected to three-stage annealing under appropriate conditions, all have an E value of 20% or more, and are reduced. Cold rolling up to 90% is possible. Furthermore, looking at the number of passes when the rolling reduction is 40%, it is 6 times for the B steel and 8 times for the D steel, which is smaller than the comparative examples No. 1, 2 and No. 16-22.
[0038]
【The invention's effect】
As described above, in the present invention, carbides were spheroidized by a primary annealing three stages annealing specific condition sandwiching the A ₁ transformation point in the production of medium and high carbon cold-rolled steel sheet, and the content C content and carbide grains By defining the diameter relationship, the productivity of the medium / high carbon cold-rolled steel sheet can be improved, and at the same time, the manufacturing cost can be reduced. That is, since it is possible to carry out by a single cold rolling up to a high cold rolling rate, the number of repetitions of the cold rolling and annealing steps can be reduced compared to the conventional manufacturing method in which cold rolling and annealing are repeated several times. Can be reduced. It is also possible to reduce the number of passes.

Claims

C: The hot rolled steel sheet containing 0.40 to 1.35% by mass is subjected to the three-stage annealing of the following (a), and the carbide particle diameter (D) and the C content ([C%] defined by (b) below) ) After spheroidizing the carbide so that the relationship of (c) satisfies the following formula (c), is cold rolling performed at a cold rolling rate of 90% or less, and is annealing annealed at a temperature less than Ac ₁ point? Or a method for producing a medium / high carbon cold-rolled steel sheet, characterized by performing the following three-stage annealing (a).
(a) (Ac ₁ -50) after 1-stage pressurized heat holding more than 0.5 hours at a temperature range of less than ° C. to Ac _1, 0.5 to 20 at a temperature range of _{_{Ac 1 ~ (Ac 1 +100)}} ℃ heating the second stage to hold time, and (Ar ₁ -80) performs heating of the third stage which holds 2 to 60 hours at a temperature range of ° C. to Ar ₁ in succession, and the second stage holding temperature The cooling rate to the third stage holding temperature is 5-30 ° C / h.
(b) Average carbide particle diameter: The value obtained by averaging the equivalent circle diameters measured for individual carbides within the observation field for all the measured carbides in the observation of the metal structure of the cross section of the steel sheet. However, the field of view is the area where the total number of carbides is 300 or more.
(c) E value = (22 + 35 × D) −15 × [C%] ≧ 20

In mass%, C: 0.40 to 1.35%, Si: 0 to 0.40% (including no addition), Mn: 0.3 to 1.0%, P is 0.03% or less, S is 0.01% or less, T. Al is contained. A medium-high carbon cold-rolled steel sheet, characterized by being subjected to the annealing and cold-rolling according to claim 1 to a steel hot-rolled steel sheet of which the content is limited to 0.1% or less and the balance is Fe and inevitable impurities. Manufacturing method.

In mass%, C: 0.40 to 1.35%, Si: 0.15 to 0.40%, Mn: 0.3 to 1.0%, Cr: 0 to 1.6% (including no addition), Mo: 0 to 0.3% (including no addition) Cu: 0 to 0.3% (including no addition), Ni: 0 to 2.0% (including no addition), P is 0.03% or less, S is 0.01% or less, and T.Al is 0.1% or less Production of medium-high carbon cold-rolled steel sheet, characterized by subjecting the hot-rolled steel sheet of steel limited to the content and the balance of Fe and unavoidable impurities to annealing and cold-rolling according to claim 1 Method.