JP4171296B2 - Steel sheet excellent in deep drawability, manufacturing method thereof and steel pipe manufacturing method excellent in workability - Google Patents
Steel sheet excellent in deep drawability, manufacturing method thereof and steel pipe manufacturing method excellent in workability Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、例えば自動車のパネル類、足廻り、メンバーなどに用いられる鋼板と鋼管およびそれらの製造方法に関するものである。本発明の鋼板および鋼管は、表面処理をしないものと、防錆のために溶融めっき、電気めっきなどの表面処理を施したものの両方を含む。めっきとは、純亜鉛のほか、主成分が亜鉛である合金のめっきの他、ＡｌやＭｇを主成分とするめっきなども含む。本発明によれば深絞り性に優れた高強度鋼板および鋼管を安価に得ることができるため地球環境保全に貢献しうるものと考えられる。また、ハイドロフォーム成形用としても好適である。
【０００２】
【従来の技術】
自動車の軽量化ニーズに伴い、鋼板の高強度化が望まれている。高強度化することで板厚減少による軽量化や衝突時の安全性向上が可能となる。しかしながら高強度で成形性特に深絞り性が優れた鋼板を得ようとすると、たとえば特許文献１に開示されているように、Ｃ量を著しく減じた極低炭素鋼にＳｉ，Ｍｎ，Ｐなどを添加して強化することが必須であった。Ｃ量を低減するためには製鋼工程で真空脱ガスを行わねばならず、製造過程でＣＯ2を多量に発生することになり、地球環境保全の観点で必ずしも最適なものとは言い難い。これに対してＣ量が比較的多く、かつ深絞り性の良好な鋼板についても開示されている。特許文献２〜７などに開示されている。しかしながらこれらについてもＣ量は実質的に０．０７％以下と低い。さらに特許文献８ではＣ量が０．１４％でも比較的良好なｒ値が得られることが示されている。しかしながらこれにはＰが多量に含有されており、２次加工性が劣化したり、溶接性や溶接後の疲労強度に問題を生ずる場合がある。本発明者らは、このような問題を解決するための技術を特許文献９に開示している。これに加えて、Ｍｎ量が多い場合には、良好な深絞り性を得るための特別な熱延条件が存在することを新たに見出した。
【０００３】
【特許文献１】
特開昭５６−１３９６５４号公報
【特許文献２】
特公昭５７−４７７４６号公報
【特許文献３】
特公平２−２０６９５号公報
【特許文献４】
特公昭５８−４９６２３号公報
【特許文献５】
特公昭６１−１２９８３号公報
【特許文献６】
特公平１−３７４５６号公報
【特許文献７】
特開昭５９−１３０３０号公報
【特許文献８】
特公昭６１−１００１２号公報
【特許文献９】
特開平２００２−２０６１３７号公報
【０００４】
【発明が解決しようとする課題】
本発明は、Ｃ量とＭｎ量が比較的多い鋼において深絞り性の良好な高強度鋼板および鋼管を高いコストをかけることなく、また、地球環境に過度の負荷をかけることなく得るものである。
【０００５】
【課題を解決するための手段】
本発明者らは、上記のような課題を解決すべく鋭意検討を行い、熱延板中の炭化物を均一かつ微細に分散させ、さらに熱延組織を均一にすることが、Ｃ量やＭｎ量の多い鋼における深絞り性向上に対して有用であるという従来にはない知見を得た。
【０００６】
本発明の要旨とするところは、
（１）質量％で、
Ｃ ：０．０４〜０．２５％
Ｓｉ：０．００１〜２．５％
Ｍｎ：０．８〜３．０％
Ｐ ：０．００１〜０．０６％
Ｓ ：０．０３％以下
Ｎ ：０．００１〜０．０１５％
Ａｌ：０．００８〜０．３％
を含有し、残部が鉄及び不可避的不純物からなり、平均ｒ値が１．２以上であり、フェライトと析出物からなる組織で構成されることを特徴とする深絞り性に優れた鋼板。
（２）圧延方向のｒ値（ｒＬ）が１．１以上、圧延方向に対して４５゜方向のｒ値（ｒＤ）が０．９以上、圧延方向と直角方向のｒ値（ｒＣ）が１．２以上であることを特徴とする（１）に記載の深絞り性に優れた鋼板。
（３）鋼板１／２板厚における板面の｛１１１｝、｛１００｝の各Ｘ線反射面強度比がそれぞれ４．０以上、２．５以下であることを特徴とする（１）または（２）に記載の深絞り性に優れた鋼板。
（４）鋼板を構成する結晶粒の平均結晶粒径が１５μｍ以上であることを特徴とする（１）〜（３）のいずれか１項に記載の深絞り性に優れた鋼板。
（５）鋼板を構成する結晶粒のアスペクト比の平均値が１．０以上５．０未満であることを特徴とする（１）〜（４）のいずれか１項に記載の深絞り性に優れた鋼板。
（６）０．２％耐力／引張最高強度で表される降伏比が０．７未満であることを特徴とする（１）〜（５）のいずれか１項に記載の深絞り性に優れた鋼板。
（７）Ａｌ／Ｎが３〜２５であることを特徴とする（１）〜（６）のいずれか１項に記載の深絞り性に優れた鋼板。
（８）Ｂを０．０００１〜０．０１質量％含むことを特徴とする（１）〜（７）のいずれか１項に記載の深絞り性に優れた鋼板板。
（９）ＺｒおよびＭｇの１種または２種を合計で０．０００１〜０．５質量％含むことを特徴とする（１）〜（８）のいずれか１項に記載の深絞り性に優れた鋼板板。
（１０）Ｔｉ，Ｎｂ，Ｖの１種又は２種以上を合計で０．００１〜０．２質量％含むことを特徴とする（１）〜（９）のいずれか１項に記載の深絞り性に優れた鋼板。
（１１）Ｓｎ、Ｃｒ、Ｃｕ、Ｎｉ、Ｃｏ、ＷおよびＭｏの１種又は２種以上を合計で０．００１〜２．５質量％含むことを特徴とする（１）〜（１０）のいずれか１項に記載の深絞り性に優れた鋼板。
（１２）Ｃａを０．０００１〜０．０１質量％含むことを特徴とする（１）〜（１１）のいずれか１項に記載の深絞り性に優れた鋼板。
（１３）（１）〜（１２）の何れか１項に記載の鋼板を製造する方法であって、請求項１または（７）〜（１２）のいずれか１項に記載の化学成分を有する鋼をＡｒ３変態点以上で熱間圧延を完了し、熱延仕上げ温度から５５０℃までを平均冷却速度で３０℃／ｓ以上で冷却し、５５０℃以下の温度で巻き取り、圧下率３５％以上８５％未満の冷間圧延を施し、平均加熱速度４〜２００℃／ｈｒで加熱し、最高到達温度を６００〜８００℃とする焼鈍を行い、５〜１００℃／ｈｒの速度で冷却することを特徴する深絞り性に優れた鋼板の製造方法。
（１４）表面にメッキ層を有することを特徴とする（１）〜（１２）の何れか１項に記載の深絞り性に優れた鋼板。
（１５）（１４）記載の鋼板を製造する方法であって、焼鈍、冷却後の鋼板の表面に溶融メッキまたは電気メッキを施すことを特徴する（１３）記載の深絞り性に優れた鋼板の製造方法。
（１６）上記（１３）または（１５）記載の方法にしたがって製造した鋼板を接合して鋼管とすることを特徴とする加工性に優れた鋼管の製造方法。
【０００７】
【発明の実施の形態】
以下に本発明を詳細に説明する。
【０００８】
先ず、本発明の鋼板の鋼成分について説明する。
【０００９】
Ｃ：高強度化に有効で、また、Ｃ量を低減するためにはコストアップとなるので、０．０４質量％以上の添加とするが、良好なｒ値を得るためには過度の添加は好ましいものではなく上限を０．２５％とする。０．０８超〜０．１８％が望ましい範囲である。
【００１０】
Ｓｉ：安価に機械的強度を高めることが可能であり、要求される強度レベルに応じて添加する。また、Ｓｉは熱延板中の炭化物の微細化や組織の均一化に有用で、結果として深絞り性を向上させる効果を有するので０．２％以上の添加が好ましい。一方、過剰の添加はメッキのぬれ性、加工性さらには溶接性の劣化を招くので上限を２．５質量％とする。下限を０．００１％としたのは、これ未満とするのが製鋼技術上困難なためである。２．０％以下がより好ましい上限である。
【００１１】
Ｍｎ：Ｍｎは一般にｒ値を低下せしめる元素として知られている。その低下代はＣ量が多い鋼ほど顕著になる。本発明においては、このようなＭｎによるｒ値の劣化を抑制し、良好なｒ値を得るという技術課題に立脚しているので、Ｍｎの下限を０．８質量％とした。また、０．８質量％以上で強化効果が得られ易い。３．０質量％を上限としたのは、これを上回る添加は伸びやｒ値に悪影響を及ぼすためである。
【００１２】
Ｐ：高強度化に有効な元素であるので０．００１％以上添加する。０．０６％超を添加すると溶接性や溶接部の疲労強度、さらには耐２次加工脆性が劣化するのでこれを上限とする。好ましくは０．０４％未満である。
【００１３】
Ｓ：不純物であり、低いほど好ましく、熱間割れを防止するために０．０３％以下とする。好ましくは０．０１５％以下である。また、Ｍｎ量との関係において、Ｍｎ／Ｓ＞１０であることが好ましい。
【００１４】
Ｎ：良好なｒ値を得るためには０．００１％以上の添加が必須である。多すぎると時効性を劣化させたり、多量のＡｌ添加が必要となるため上限を０．０１５％とする。０．００２〜０．００７％がより好ましい範囲である。
【００１５】
Ａｌ：本発明において重要である。冷延後の徐加熱時にＮとのクラスターや析出物を形成することによって集合組織を発達せしめ、深絞り性が向上する。また、脱酸元素としても有用であるので０．００８質量％以上添加する。ただし、過度に添加するとコストアップとなり、表面欠陥を誘発し、ｒ値も低下する。したがって上限を０．３質量%とする。好ましくは０．０１〜０．１０質量%とする。
【００１６】
本発明によって得られる鋼板の平均ｒ値は１．２以上である。１．３以上であればより好ましい。
【００１７】
圧延方向のｒ値（ｒＬ）が１．１以上、圧延方向に対して４５゜方向のｒ値（ｒＤ）が０．９以上、圧延方向に対して直角方向のｒ値（ｒＣ）が１．２以上であることが好ましい。好ましくは、それぞれ、１．３以上、１．０以上、１．３以上である。
【００１８】
平均ｒ値は、（ｒＬ＋２×ｒＤ＋ｒＣ）／４で与えられる。ｒ値の測定はＪＩＳ１３号Ｂ試験片を用いた引っ張り試験を行い、１０％または１５％引っ張り後の標点間距離の変化と板幅変化からｒ値の定義にしたがって算出すればよい。
【００１９】
本発明の鋼板の組織はフェライトと析出物が主相でこれらによって９９％以上の体積率が占められる。析出物とは主に炭化物（多くの場合、セメンタイト）であることが通常であるが、化学成分によっては窒化物、炭窒化物、硫化物なども析出する。本発明の鋼板の組織中のマルテンサイトやベイナイトなど鉄の低温変態生成相および残留オーステナイトの量は体積分率で１％以下である。
【００２０】
本発明によって得られる鋼板は、少なくとも板厚中心における板面のＸ線反射面ランダム強度比が、{111}面、{100}面についてそれぞれ４．０以上、２．５以下である。ランダム強度比とはランダムサンプルのＸ線強度を基準としたときの相対的な強度である。板厚中心とは板厚の３／８〜５／８の範囲を指し、測定はこの範囲の任意の面で行えばよい。
【００２１】
鋼板を構成する結晶粒の平均結晶粒径は、１５μm以上である。これ以下の結晶粒経では良好なｒ値が得られない。また、これが１００μm以上となると成形時に肌荒れ等の問題になる場合があるため、１００μm未満であることが望ましい。結晶粒径は板面と垂直で圧延方向と平行な切断面（Ｌ断面）の板厚３／８〜５／８の範囲内について点算法などによって測定すればよい。なお、測定誤差を低減するためには結晶粒が１００個以上存在する領域について測定しなくてはならない。エッチングはナイタールが好ましい。
【００２２】
さらに鋼板を構成する結晶粒のアスペクト比の平均は、１．０以上５．０未満である。この範囲外であると良好なｒ値が得られない。アスペクト比とはＪＩＳＧ０５５２の方法によって測定される展伸度と同じである。すなわち、本発明の場合、板面と垂直で圧延方向と平行な切断面（Ｌ断面）における板厚３／８〜５／８の範囲内の圧延方向に垂直な一定長さの線分によって切断される結晶粒の数で圧延方向に平行な上記と同じ長さの線分によって切断される結晶粒の数を除したもので与えられる。好ましくは、１．５以上４．０未満である。
【００２３】
本発明の鋼板の引張試験で評価される降伏比（０．２％耐力／最高引張強度）は通常は０．７０未満である。形状凍結性の確保やプレス成形時の面歪みの発生を抑制する観点からは０．６５以下であることが好ましい。本発明では降伏比が低いので、ｎ値も良好である。特に低歪み域（１０％以下）でのｎ値が高い。降伏比の下限は特に定めないが、たとえばハイドロフォーム成形時の座屈を防止するためには０．４０以上であることが好ましい。
【００２４】
Ａｌ／Ｎは３〜２５の範囲であることが好ましい。この範囲外では良好なｒ値を得ることが困難となる。好ましくは５〜１５の範囲である。
【００２５】
Ｂはｒ値を向上させたり、耐２次加工性脆性の改善に有効であるので必要に応じて添加する。０．０００１％未満ではその効果はわずかで、０．０１％超添加しても格段の効果は得られない。０．０００２〜０．００２０％が好ましい範囲である。
【００２６】
ＺｒとＭｇは脱酸元素として有効である。一方、過剰の添加は酸化物、硫化物や窒化物の多量の晶出や析出を招き清浄度が劣化して、延性を低下させてしまう上、メッキ性を損なう。したがって、必要に応じてこれらの１種または２種を合計で０．０００１〜０．５０質量％とする。
【００２７】
Ｔｉ，Ｎｂ，Ｖも必要に応じて添加する。これらは、炭化物、窒化物もしくは炭窒化物を形成することによって鋼材を高強度化したり加工性を向上することができるので、１種又は２種以上を合計で０．００１％以上添加する。その合計が０．２％を越えた場合には母相であるフェライト粒内もしくは粒界に多量の炭化物、窒化物もしくは炭窒化物として析出して、延性を低下させる。また、焼鈍中のＡｌＮの析出を妨げ、本発明の特徴である深絞り性が損なわれることから、添加範囲を０．００１〜０．２質量％とした。より好ましくは０．０１〜０．０３％である。
【００２８】
Ｓｎ、Ｃｒ、Ｃｕ、Ｎｉ、Ｃｏ、Ｗ、Ｍｏは強化元素であり必要に応じてこれらの１種又は２種以上を合計で必要に応じて質量％で０．００１％以上添加する。特にＣｕはｒ値を向上せしめる効果を有するので、０．３％以上添加することが好ましい。過剰の添加は、コストアップや延性の低下を招くことから、２．５％以下とした。
【００２９】
Ｃａ：介在物制御のほか脱酸に有効な元素で、適量の添加は熱間加工性を向上させるが、過剰の添加は逆に熱間脆化を助長させるため、必要に応じて質量％で０．０００１〜０．０１％の範囲とする。
【００３０】
また、不可避的不純物として、Ｏ（酸素）、Ｚｎ、Ｐｂ、Ａｓ、Ｓｂなどをそれぞれ０．０２質量％以下の範囲で含んでも、本発明の効果を失するものではない。
【００３１】
次に本発明による鋼板の製造条件について説明する。
【００３２】
本発明鋼板の製造にあたっては、高炉、電炉等による溶製に続き各種の２次製錬を行いインゴット鋳造や連続鋳造を行い、連続鋳造の場合には室温付近まで冷却することなく熱間圧延するＣＣ−ＤＲなどの製造方法を組み合わせて製造してもかまわない。鋳造インゴットや鋳造スラブを再加熱して熱間圧延を行っても良いのは言うまでもない。熱間圧延の加熱温度は特に限定するものではないが、ＡｌＮを固溶状態とするために１１００℃以上とすることが好ましい。熱延の仕上げ温度はAr3変態点以上で行う。熱延仕上げ温度がAr3点を下回ると、高温で変態した粗大なフェライト粒、さらにはそれが加工され再結晶や粒成長により粗大化したフェライトと比較的低温域で変態した微細フェライト粒とが混在し、不均一な組織となる。熱延仕上げ温度の上限は特に設けないが、熱延組織を均一にするためには(Ar3+100)℃以下とすることが好ましい。
【００３３】
熱延後の冷却速度は重要である。すなわち熱延仕上げ後、巻き取り温度までの平均冷却速度を３０℃/s以上とする。本発明においては、熱延板における炭化物をできるだけ微細に分散させ、かつ組織を均一にすることが冷延焼鈍後のｒ値の向上に対して極めて重要である。上記の熱延冷却条件はこの観点から決定される。冷却速度が30℃/s未満となると、結晶粒径が不均一になるばかりでなく、パーライト変態が促進され、炭化物が粗大となる。上限は特に設けないが、あまり大きいと極度に硬質となる可能性があるので１００℃/s以下とすることが好ましい。
【００３４】
熱延板の組織として最も好ましいのは９７％以上のベイナイトによって構成される組織であり、下部ベイナイト組織であればさらに好ましい。ベイナイト単相であれば最良であることは言うまでもない。マルテンサイト単相組織でも良いが、硬質すぎて冷延が困難となる。フェライト単相またはフェライト、ベイナイト、マルテンサイト、残留オーステナイトのうちの２種類以上からなる複合組織を有する熱延板は冷延素材として好ましくない。
【００３５】
巻き取り温度は５５０℃以下とする。巻き取り温度が５５０℃超となるとＡｌＮの析出や粗大化、また炭化物が粗大化するため、ｒ値が劣化する。好ましくは５００℃未満である。熱間圧延の１パス以上について潤滑を施しても良い。また、粗圧延バーを互いに接合し、連続的に仕上げ熱延を行っても良い。粗圧延バーは一度巻き取って再度巻き戻してから仕上げ熱延に供してもかまわない。巻き取り温度の下限は特に設けないが、熱延板中の固溶Ｃを低減して、良好なｒ値を得るためには、１００℃以上とすることが好ましい。
【００３６】
熱間圧延後は酸洗することが望ましい。熱延後の冷間圧延の圧下率は高すぎても低すぎても良好な深絞り性を得るために好ましくないので３５〜８５％未満とする。５０〜７５％がより好ましい範囲である。
【００３７】
焼鈍は箱焼鈍が基本であるが、下記の要件を満たせばこの限りではない。良好なｒ値を得るためには、加熱速度を４〜２００℃／ｈｒとする必要がある。さらには１０〜４０℃/ｈｒが好ましい。最高到達温度もｒ値確保の観点から６００〜８００℃とすることが望ましい。６００℃未満では再結晶が完了せず加工性が劣化する。一方、８００℃超ではα＋γ域のγ分率の高い側に入るため、加工性が劣化する場合がある。なお、最高到達温度での保持時間は特に指定するものではないが、（最高到達温度−２０）℃以上での保持時間が２ｈｒ以上であることがｒ値向上の観点から好ましい。冷却速度は固溶Ｃを十分に低減する観点から決定される。すなわち、５〜１００℃／ｈｒの範囲とする。
【００３８】
焼鈍後のスキンパスは形状強制や強度調整、さらには常温非時効性を確保する観点から必要に応じて行う。０．５〜５．０％が好ましい圧下率である。
【００３９】
このようにして製造した鋼板表面に種々のメッキを施しても良い。溶融メッキ、電気メッキのいずれでも良く、その種類も亜鉛やアルミを主成分とするメッキであれば良い。
【００４０】
このようにして製造された鋼板を接合して鋼管とすることができる。鋼板の圧延方向が管軸方向と一致することが望ましい。圧延方向以外、例えば、圧延方向と直角方向が管軸方向となるようにしてもハイドロフォーム用として特に劣るものではないが、鋼管製造の生産性が低下するためである。鋼管の製造にあたっては、通常は電縫溶接を用いるが、TIG、MIG、レーザー溶接、ＵＯや鍛接等の溶接・造管手法等を用いることも出来る。これらの溶接鋼管製造に於いて溶接熱影響部は必要とする特性に応じて局部的な固溶化熱処理を単独あるいは複合して、場合によっては複数回重ねて行っても良く、本発明の効果をさらに高める。この熱処理は溶接部と溶接熱影響部のみに付加することが目的であって、製造時にオンラインであるいはオフラインで施行できる。
【００４１】
鋼管のｒ値については鋼板のそれと同じ特徴を持つ。鋼管のｒ値の測定は、鋼管から試験片を切り出し、プレスによって平板とし、さらに引張試験片に加工して行う。鋼管の径や試験片の採取方向によってはＪＩＳ１３号Ｂ試験片を採取することが困難な場合があるが、その際にはＪＩＳ６号やＪＩＳ１４号Ｂ試験片等の小型試験片を用いて、均一伸びの範囲内で評価する。なお、鋼管から試験片を切り出す際には、鋼管の溶接部が引張試験片の平行部内に来ないように注意する。
【００４２】
Ｘ線測定は鋼管そのものでは測定することができないので、次のようにして行う。まず、鋼管を適当に切断して、プレス等により板状とする。これを測定板厚まで機械研磨などによって減厚し、最終的には１平均結晶粒径以上を目安に３０〜１００μm程度減厚させるよう化学研磨によって仕上げる。
【００４３】
本発明の鋼管は表面粗度が小さい。すなわち、ＪＩＳＢ０６０１で規定されるＲａが０．８μm以下であることが好ましい。高温縮経加工によって製造された鋼管が０．８μm超であるのとは対照的である。より好ましくは０．６μm以下である。
【００４４】
【実施例】
表１に示す成分の各鋼を溶製して1250℃に加熱後、仕上げ温度をAr3〜(Ar3+50)℃とする熱間圧延を行った後、表２に示す条件で巻き取った。得られた熱延板の組織も表２に示す。さらに表２に示す圧下率で冷延されたのち加熱速度２０℃/ｈｒ、最高到達温度を７００℃とする焼鈍をおこない、５時間保持後、１５℃/ｈｒで冷却した。さらに１．０％のスキンパスを施した。
【００４５】
得られた鋼板のｒ値をＪＩＳ１３号試験片を用いた引張試験により評価した。その他の引張特性についてはＪＩＳ５号試験片を用いて評価した。ここでｒ値は１０〜１５％引張変形後の板幅変化を測定することによって求めた。また、機械研磨によって板厚中心付近まで減厚し、化学研磨によって仕上げ、Ｘ線測定に供した。Ｘ線ランダム強度比を合わせて表２に示す。
【００４６】
この板を電縫溶接によって造管した。
【００４７】
得られた鋼管の加工性の評価は以下の方法で行った。前もって鋼管に１０ｍｍφのスクライブドサークルを転写し、内圧と軸押し量を制御して、円周方向への張り出し成形を行った。バースト直前での最大拡管率を示す部位（拡管率＝成形後の最大周長／母管の周長）の軸方向の歪εΦと円周方向の歪εθを測定した。この２つの歪の比ρ＝εΦ／εθと最大拡管率をプロットし、ρ＝−０．５となる拡管率Ｒｅをもってハイドロフォームの成形性指標とした。引張強度と伸びの評価はＪＩＳ１２号弧状試験片を用いて行った。
【００４８】
鋼管の機械的性質を表３に示す。
【００４９】
表２、表３より明らかなとおり、本発明例では本発明外の例に比較して、良好な特性が得られた。なお、鋼管の組織、Ｘ線ランダム強度比は鋼板のものとほとんど同じであった。
【００５０】
【表１】
【表２】
【表３】
【発明の効果】
本発明により、良好なｒ値を有する深絞り性に優れた高強度鋼板および鋼管が得られ、地球環境保全などに貢献するものである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel plate and a steel pipe used for, for example, automobile panels, suspensions, members, and the like, and methods for producing them. The steel plate and steel pipe of the present invention include both those not subjected to surface treatment and those subjected to surface treatment such as hot dipping and electroplating for rust prevention. In addition to pure zinc, the plating includes plating of an alloy whose main component is zinc, plating including Al or Mg as a main component. According to the present invention, a high-strength steel plate and a steel pipe excellent in deep drawability can be obtained at low cost, and it is considered that the present invention can contribute to global environmental conservation. It is also suitable for hydroform molding.
[0002]
[Prior art]
Along with the need for lighter automobiles, higher strength of steel sheets is desired. By increasing the strength, it becomes possible to reduce the weight by reducing the plate thickness and improve the safety at the time of collision. However, when trying to obtain a steel sheet having high strength and excellent formability, particularly deep drawability, for example, as disclosed in Patent Document 1, Si, Mn, P, etc. are added to an extremely low carbon steel with a significantly reduced C content. It was essential to add and strengthen. In order to reduce the amount of C, vacuum degassing must be performed in the steel making process, and a large amount of CO 2 is generated in the manufacturing process, which is not necessarily optimal from the viewpoint of global environmental conservation. On the other hand, a steel sheet having a relatively large amount of C and good deep drawability is also disclosed. It is disclosed in Patent Documents 2-7. However, also in these cases, the amount of C is substantially as low as 0.07% or less. Further, Patent Document 8 shows that a relatively good r value can be obtained even when the C content is 0.14%. However, this contains a large amount of P, which may deteriorate secondary workability or cause problems in weldability and fatigue strength after welding. The present inventors have disclosed a technique for solving such a problem in Patent Document 9. In addition to this, it has been newly found that there is a special hot rolling condition for obtaining a good deep drawability when the amount of Mn is large.
[0003]
[Patent Document 1]
JP-A-56-139654 [Patent Document 2]
Japanese Patent Publication No.57-47746 [Patent Document 3]
Japanese Patent Publication No. 2-20695 [Patent Document 4]
Japanese Patent Publication No. 58-49623 [Patent Document 5]
Japanese Patent Publication No. 61-12983 [Patent Document 6]
Japanese Patent Publication No. 1-337456 [Patent Document 7]
JP 59-13030 [Patent Document 8]
Japanese Patent Publication No. 61-10012 [Patent Document 9]
Japanese Patent Laid-Open No. 2002-206137
[Problems to be solved by the invention]
The present invention provides a high-strength steel sheet and a steel pipe with good deep drawability in steels having a relatively large amount of C and Mn, without high costs and without overloading the global environment. .
[0005]
[Means for Solving the Problems]
The present inventors have intensively studied to solve the problems as described above, and uniformly and finely disperse the carbides in the hot-rolled sheet, and further make the hot-rolled structure uniform. We have obtained an unprecedented finding that it is useful for improving the deep drawability of steel with a large amount of steel.
[0006]
The gist of the present invention is that
(1) In mass%,
C: 0.04 to 0.25%
Si: 0.001 to 2.5%
Mn: 0.8 to 3.0%
P: 0.001 to 0.06%
S: 0.03% or less N: 0.001 to 0.015%
Al: 0.008 to 0.3%
The balance is made of iron and inevitable impurities, the average r value is 1.2 or more, and is composed of a structure consisting of ferrite and precipitates, and is excellent in deep drawability.
(2) The r value (rL) in the rolling direction is 1.1 or more, the r value (rD) in the 45 ° direction with respect to the rolling direction is 0.9 or more, and the r value (rC) in the direction perpendicular to the rolling direction is 1. The steel sheet excellent in deep drawability according to (1), characterized in that it is 2 or more.
(3) The intensity ratio of {111} and {100} X-ray reflecting surfaces of the plate surface at a steel plate 1/2 thickness is 4.0 or more and 2.5 or less, respectively (1) or The steel plate excellent in deep drawability as described in (2).
(4) The steel sheet excellent in deep drawability according to any one of (1) to (3), wherein an average crystal grain size of crystal grains constituting the steel sheet is 15 μm or more.
(5) The average value of the aspect ratio of the crystal grains constituting the steel sheet is 1.0 or more and less than 5.0, and the deep drawability according to any one of (1) to (4) Excellent steel plate.
(6) The deep drawability according to any one of (1) to (5), wherein the yield ratio expressed by 0.2% yield strength / maximum tensile strength is less than 0.7. Steel plate.
(7) Al / N is 3-25, The steel plate excellent in deep drawability of any one of (1)-(6) characterized by the above-mentioned.
(8) The steel plate excellent in deep drawability according to any one of (1) to (7), wherein B is contained in an amount of 0.0001 to 0.01% by mass.
(9) Excellent in deep drawability according to any one of (1) to (8), characterized in that one or two of Zr and Mg are included in total of 0.0001 to 0.5 mass% Steel plate.
(10) The deep drawing according to any one of (1) to (9), wherein 0.001 to 0.2% by mass in total of one or more of Ti, Nb, and V is included. Steel sheet with excellent properties.
(11) Any one of (1) to (10) characterized by containing 0.001 to 2.5 mass% in total of one or more of Sn, Cr, Cu, Ni, Co, W and Mo A steel sheet excellent in deep drawability as set forth in claim 1.
(12) The steel sheet excellent in deep drawability according to any one of (1) to (11), characterized by containing 0.0001 to 0.01 mass% of Ca.
(13) A method for producing the steel sheet according to any one of (1) to (12), comprising the chemical component according to any one of claims 1 or (7) to (12). The steel was hot-rolled at the Ar 3 transformation point or higher, the hot rolling finish temperature to 550 ° C. was cooled at an average cooling rate of 30 ° C./s or higher, and the steel was wound at a temperature of 550 ° C. or lower. More than 85% cold rolling is performed, heating is performed at an average heating rate of 4 to 200 ° C./hr, annealing is performed with a maximum temperature of 600 to 800 ° C., and cooling is performed at a rate of 5 to 100 ° C./hr. A method for producing a steel sheet excellent in deep drawability characterized by
(14) The steel sheet excellent in deep drawability according to any one of (1) to (12), wherein the surface has a plating layer.
(15) A method for producing a steel sheet according to (14), characterized in that the surface of the steel sheet after annealing and cooling is subjected to hot dipping or electroplating. (13) Manufacturing method .
(16 ) A method for producing a steel pipe excellent in workability, comprising joining steel plates produced according to the method described in (13) or (15) above to form a steel pipe.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
[0008]
First, the steel component of the steel plate of this invention is demonstrated.
[0009]
C: Effective for increasing the strength and increasing the cost in order to reduce the amount of C. Addition of 0.04% by mass or more, but in order to obtain a good r value, excessive addition is It is not preferable and the upper limit is 0.25%. A desirable range is from over 0.08 to 0.18%.
[0010]
Si: It is possible to increase the mechanical strength at low cost, and it is added according to the required strength level. Further, Si is useful for refining carbides in the hot-rolled sheet and homogenizing the structure, and as a result, has an effect of improving deep drawability, so addition of 0.2% or more is preferable. On the other hand, excessive addition causes deterioration of the wettability, workability, and weldability of the plating, so the upper limit is made 2.5 mass%. The reason why the lower limit is set to 0.001% is that it is difficult to make it lower than this in terms of steelmaking technology. 2.0% or less is a more preferable upper limit.
[0011]
Mn: Mn is generally known as an element that lowers the r value. The reduction cost becomes more prominent as the amount of C increases. In the present invention, the lower limit of Mn is set to 0.8 mass% because it is based on the technical problem of suppressing such deterioration of the r value due to Mn and obtaining a good r value. Moreover, the reinforcement | strengthening effect is easy to be acquired at 0.8 mass% or more. The reason why the upper limit is set to 3.0% by mass is that addition exceeding the above has an adverse effect on elongation and r value.
[0012]
P: 0.001% or more is added because it is an element effective for increasing the strength. If over 0.06% is added, the weldability, fatigue strength of the welded portion, and secondary work brittleness resistance deteriorate, so this is the upper limit. Preferably it is less than 0.04%.
[0013]
S: Impurity, preferably as low as possible, and 0.03% or less to prevent hot cracking. Preferably it is 0.015% or less. Further, in relation to the amount of Mn, it is preferable that Mn / S> 10.
[0014]
N: Addition of 0.001% or more is essential to obtain a good r value. If the amount is too large, the aging property is deteriorated or a large amount of Al is required, so the upper limit is made 0.015%. 0.002 to 0.007% is a more preferable range.
[0015]
Al: important in the present invention. By forming clusters and precipitates with N at the time of slow heating after cold rolling, a texture is developed and deep drawability is improved. Moreover, since it is useful also as a deoxidation element, 0.008 mass% or more is added. However, excessive addition increases costs, induces surface defects, and decreases the r value. Therefore, the upper limit is set to 0.3% by mass. Preferably it is 0.01-0.10 mass%.
[0016]
The average r value of the steel sheet obtained by the present invention is 1.2 or more. It is more preferable if it is 1.3 or more.
[0017]
The r value (rL) in the rolling direction is 1.1 or more, the r value (rD) in the 45 ° direction with respect to the rolling direction is 0.9 or more, and the r value (rC) in the direction perpendicular to the rolling direction is 1. It is preferable that it is 2 or more. Preferably, they are 1.3 or more, 1.0 or more, and 1.3 or more, respectively.
[0018]
The average r value is given by (rL + 2 × rD + rC) / 4. The r value is measured by performing a tensile test using a JIS No. 13 B test piece and calculating according to the definition of the r value from the change in distance between gauge points and the change in plate width after 10% or 15% pull.
[0019]
The structure of the steel sheet of the present invention is mainly composed of ferrite and precipitates, and these account for a volume ratio of 99% or more. The precipitates are usually carbides (in many cases, cementite), but nitrides, carbonitrides, sulfides and the like are also precipitated depending on chemical components. The amount of low-temperature transformation transformation phase of iron such as martensite and bainite and residual austenite in the structure of the steel sheet of the present invention is 1% or less in volume fraction.
[0020]
In the steel plate obtained by the present invention, the X-ray reflecting surface random intensity ratio of the plate surface at least in the center of the plate thickness is 4.0 or more and 2.5 or less for the {111} plane and {100} plane, respectively. The random intensity ratio is a relative intensity based on the X-ray intensity of a random sample. The center of the plate thickness refers to a range of 3/8 to 5/8 of the plate thickness, and the measurement may be performed on any surface within this range.
[0021]
The average crystal grain size of the crystal grains constituting the steel plate is 15 μm or more. If the crystal grain size is less than this, a good r value cannot be obtained. Further, if this is 100 μm or more, it may cause problems such as rough skin at the time of molding. Therefore, it is preferably less than 100 μm. The crystal grain size may be measured by a point method or the like within the range of the plate thickness 3/8 to 5/8 of the cut surface (L cross section) perpendicular to the plate surface and parallel to the rolling direction. In order to reduce the measurement error, it is necessary to measure a region where 100 or more crystal grains exist. Etching is preferably nital.
[0022]
Further, the average aspect ratio of the crystal grains constituting the steel sheet is 1.0 or more and less than 5.0. If it is out of this range, a good r value cannot be obtained. The aspect ratio is the same as the degree of expansion measured by the method of JISG0552. That is, in the case of the present invention, cutting is performed by a line segment of a certain length perpendicular to the rolling direction within the range of the thickness 3/8 to 5/8 in the cutting plane (L cross section) perpendicular to the plate surface and parallel to the rolling direction. It is given by dividing the number of crystal grains by the number of crystal grains to be cut by a line segment of the same length as described above parallel to the rolling direction. Preferably, it is 1.5 or more and less than 4.0.
[0023]
The yield ratio (0.2% proof stress / maximum tensile strength) evaluated in the tensile test of the steel sheet of the present invention is usually less than 0.70. From the viewpoint of securing the shape freezing property and suppressing the occurrence of surface distortion during press molding, it is preferably 0.65 or less. In the present invention, since the yield ratio is low, the n value is also good. In particular, the n value in the low strain region (10% or less) is high. The lower limit of the yield ratio is not particularly defined, but is preferably 0.40 or more in order to prevent buckling during hydroforming, for example.
[0024]
Al / N is preferably in the range of 3-25. Outside this range, it is difficult to obtain a good r value. Preferably it is the range of 5-15.
[0025]
B is effective for improving the r value and improving the secondary workability brittleness resistance, and is added as necessary. If it is less than 0.0001%, the effect is slight, and even if added over 0.01%, a remarkable effect cannot be obtained. 0.0002 to 0.0020% is a preferred range.
[0026]
Zr and Mg are effective as deoxidizing elements. On the other hand, excessive addition causes a large amount of crystallization and precipitation of oxides, sulfides and nitrides, which deteriorates cleanliness and lowers ductility and impairs plating properties. Therefore, these 1 type or 2 types shall be 0.0001-0.50 mass% in total as needed.
[0027]
Ti, Nb, and V are also added as necessary. These can increase the strength of the steel material or improve the workability by forming carbides, nitrides or carbonitrides, so one or two or more of them are added in a total amount of 0.001% or more. When the total exceeds 0.2%, it precipitates as a large amount of carbide, nitride, or carbonitride in the ferrite grains or grain boundaries as the parent phase, and the ductility is lowered. Moreover, since precipitation of AlN during annealing is hindered and the deep drawability which is a feature of the present invention is impaired, the addition range is set to 0.001 to 0.2% by mass. More preferably, it is 0.01 to 0.03%.
[0028]
Sn, Cr, Cu, Ni, Co, W, and Mo are strengthening elements, and if necessary, one or more of these may be added in a total amount of 0.001% or more as required. In particular, since Cu has an effect of improving the r value, it is preferable to add 0.3% or more. Excessive addition causes an increase in cost and a decrease in ductility, so the content was made 2.5% or less.
[0029]
Ca: An element effective for inclusion control as well as deoxidation. Addition of an appropriate amount improves hot workability, but excessive addition conversely promotes hot embrittlement. The range is 0.0001 to 0.01%.
[0030]
Moreover, even if it contains O (oxygen), Zn, Pb, As, Sb, etc. in the range of 0.02 mass% or less as an unavoidable impurity, the effect of this invention is not lost.
[0031]
Next, manufacturing conditions for the steel sheet according to the present invention will be described.
[0032]
In the production of the steel sheet of the present invention, ingot casting and continuous casting are performed by performing various secondary smelting following smelting by a blast furnace, electric furnace, etc., and in the case of continuous casting, hot rolling is performed without cooling to near room temperature. A manufacturing method such as CC-DR may be combined. Needless to say, the cast ingot or cast slab may be reheated for hot rolling. The heating temperature for hot rolling is not particularly limited, but is preferably 1100 ° C. or higher in order to make AlN into a solid solution state. The finishing temperature for hot rolling is above the Ar 3 transformation point. When the hot rolling finishing temperature is lower than the Ar 3 point, coarse ferrite grains transformed at high temperature, ferrite that has been processed and coarsened by recrystallization and grain growth, and fine ferrite grains transformed at a relatively low temperature range are formed. Mixed and non-uniform organization. The upper limit of the hot rolling finish temperature is not particularly set, but is preferably (Ar 3 +100) ° C. or lower in order to make the hot rolled structure uniform.
[0033]
The cooling rate after hot rolling is important. That is, after hot rolling finish, the average cooling rate up to the coiling temperature is set to 30 ° C./s or more. In the present invention, it is extremely important for the improvement of the r value after cold rolling annealing to disperse the carbides in the hot rolled sheet as finely as possible and to make the structure uniform. The above hot rolling cooling conditions are determined from this viewpoint. When the cooling rate is less than 30 ° C./s, not only the crystal grain size becomes nonuniform, but also the pearlite transformation is promoted and the carbide becomes coarse. There is no particular upper limit, but if it is too large, it may become extremely hard, so it is preferable to set it to 100 ° C./s or less.
[0034]
The most preferable structure of the hot-rolled sheet is a structure composed of 97% or more of bainite, and a lower bainite structure is more preferable. It goes without saying that a bainite single phase is the best. A martensite single-phase structure may be used, but it is too hard to be cold-rolled. A hot-rolled sheet having a composite structure composed of two or more of ferrite single phase or ferrite, bainite, martensite, and retained austenite is not preferable as a cold-rolled material.
[0035]
The winding temperature is 550 ° C. or lower. When the coiling temperature exceeds 550 ° C., precipitation and coarsening of AlN and coarsening of the carbides cause deterioration of the r value. Preferably it is less than 500 degreeC. Lubrication may be performed for one or more passes of hot rolling. Alternatively, the rough rolling bars may be joined to each other and finish hot rolled continuously. The rough rolled bar may be wound once and then rewound again before being subjected to finish hot rolling. Although there is no particular lower limit for the coiling temperature, it is preferably 100 ° C. or higher in order to reduce the solid solution C in the hot-rolled sheet and obtain a good r value.
[0036]
It is desirable to pickle after hot rolling. If the rolling reduction ratio after cold rolling is too high or too low, it is not preferable in order to obtain good deep drawability, so it is less than 35 to 85%. 50 to 75% is a more preferable range.
[0037]
Although annealing is basically box annealing, it is not limited as long as the following requirements are satisfied. In order to obtain a good r value, the heating rate needs to be 4 to 200 ° C./hr. Furthermore, 10-40 degreeC / hr is preferable. The maximum temperature reached is preferably 600 to 800 ° C. from the viewpoint of securing the r value. If it is less than 600 ° C., recrystallization is not completed and workability deteriorates. On the other hand, if it exceeds 800 ° C., it falls on the side with a higher γ fraction in the α + γ region, so the workability may deteriorate. The holding time at the highest temperature is not particularly specified, but the holding time at (highest temperature −20) ° C. or higher is preferably 2 hours or higher from the viewpoint of improving the r value. The cooling rate is determined from the viewpoint of sufficiently reducing the solid solution C. That is, the range is 5 to 100 ° C./hr.
[0038]
The skin pass after annealing is performed as necessary from the viewpoint of shape forcing, strength adjustment, and securing non-aging at room temperature. 0.5 to 5.0% is a preferable rolling reduction.
[0039]
Various platings may be applied to the surface of the steel plate thus manufactured. Either hot-dip plating or electroplating may be used, and the type of plating may be zinc or aluminum as a main component.
[0040]
The steel plates thus produced can be joined to form a steel pipe. It is desirable that the rolling direction of the steel plate coincides with the tube axis direction. For example, even if the direction perpendicular to the rolling direction is the tube axis direction other than the rolling direction, it is not particularly inferior for hydroforming, but this is because the productivity of steel pipe production is reduced. In the production of steel pipes, electric seam welding is usually used, but welding and pipe making techniques such as TIG, MIG, laser welding, UO and forge welding can also be used. In the production of these welded steel pipes, the weld heat affected zone may be subjected to local solution heat treatment alone or in combination depending on the required properties, and may be repeated several times in some cases. Increase further. This heat treatment is intended to be applied only to the weld zone and the weld heat affected zone, and can be performed online or offline at the time of manufacture.
[0041]
The r value of a steel pipe has the same characteristics as that of a steel plate. The r value of the steel pipe is measured by cutting out a test piece from the steel pipe, forming a flat plate by pressing, and further processing it into a tensile test piece. Depending on the diameter of the steel pipe and the sampling direction of the test piece, it may be difficult to collect the JIS No. 13 B test piece. In that case, use a small test piece such as the JIS No. 6 or JIS No. 14 B test piece. Evaluation is made within the range of elongation. When cutting out the test piece from the steel pipe, care should be taken so that the welded portion of the steel pipe does not come into the parallel part of the tensile test piece.
[0042]
Since X-ray measurement cannot be performed with the steel pipe itself, it is performed as follows. First, the steel pipe is appropriately cut and formed into a plate shape by a press or the like. The thickness is reduced by mechanical polishing or the like to the measurement plate thickness, and finally finished by chemical polishing so as to reduce the thickness by about 30 to 100 μm with reference to one average crystal grain size or more.
[0043]
The steel pipe of the present invention has a small surface roughness. That is, Ra specified by JISB0601 is preferably 0.8 μm or less. This is in contrast to steel pipes produced by high-temperature warping that are greater than 0.8 μm. More preferably, it is 0.6 μm or less.
[0044]
【Example】
Each steel having the components shown in Table 1 is melted and heated to 1250 ° C., then hot rolled to a finishing temperature of Ar 3 to (Ar 3 +50) ° C., and then wound under the conditions shown in Table 2. It was. Table 2 also shows the structure of the obtained hot-rolled sheet. Further, after being cold-rolled at the rolling reduction shown in Table 2, annealing was performed at a heating rate of 20 ° C./hr and a maximum attained temperature of 700 ° C., held for 5 hours, and then cooled at 15 ° C./hr. Further, a 1.0% skin pass was applied.
[0045]
The r value of the obtained steel plate was evaluated by a tensile test using a JIS No. 13 test piece. Other tensile properties were evaluated using JIS No. 5 test pieces. Here, the r value was determined by measuring the change in the plate width after 10-15% tensile deformation. Further, the thickness was reduced to the vicinity of the center of the plate thickness by mechanical polishing, finished by chemical polishing, and used for X-ray measurement. The X-ray random intensity ratio is shown together in Table 2.
[0046]
This plate was made by electro-welding.
[0047]
The workability of the obtained steel pipe was evaluated by the following method. In advance, a scribed circle of 10 mmφ was transferred to the steel pipe, and the inner pressure and the axial push amount were controlled to perform the overhang forming in the circumferential direction. Strain εΦ in the axial direction and strain εθ in the circumferential direction of the portion showing the maximum tube expansion rate immediately before the burst (tube expansion rate = maximum circumferential length after molding / circumferential length of the mother tube) were measured. The ratio of these two strains ρ = εΦ / εθ and the maximum tube expansion ratio were plotted, and the tube expansion ratio Re at which ρ = −0.5 was used as the formability index of the hydroform. Evaluation of tensile strength and elongation was performed using a JIS No. 12 arc specimen.
[0048]
Table 3 shows the mechanical properties of the steel pipe.
[0049]
As is clear from Tables 2 and 3, in the examples of the present invention, good characteristics were obtained as compared with the examples outside the present invention. Note that the structure of the steel pipe and the X-ray random strength ratio were almost the same as those of the steel sheet.
[0050]
[Table 1]
[Table 2]
[Table 3]
【The invention's effect】
According to the present invention, a high-strength steel plate and a steel pipe excellent in deep drawability having a good r value can be obtained, which contributes to global environmental conservation and the like.
Claims (16)
Ｃ ：０．０４〜０．２５％
Ｓｉ：０．００１〜２．５％
Ｍｎ：０．８〜３．０％
Ｐ ：０．００１〜０．０６％
Ｓ ：０．０３％以下
Ｎ ：０．００１〜０．０１５％
Ａｌ：０．００８〜０．３％
を含有し、残部が鉄及び不可避的不純物からなり、平均ｒ値が１．２以上であり、フェライトと析出物からなる組織で構成されることを特徴とする深絞り性に優れた鋼板。% By mass
C: 0.04 to 0.25%
Si: 0.001 to 2.5%
Mn: 0.8 to 3.0%
P: 0.001 to 0.06%
S: 0.03% or less N: 0.001 to 0.015%
Al: 0.008 to 0.3%
The balance is made of iron and inevitable impurities, the average r value is 1.2 or more, and is composed of a structure consisting of ferrite and precipitates, and is excellent in deep drawability.
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| JP2002368878A JP4171296B2 (en) | 2002-05-27 | 2002-12-19 | Steel sheet excellent in deep drawability, manufacturing method thereof and steel pipe manufacturing method excellent in workability |
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| JP2002368878A JP4171296B2 (en) | 2002-05-27 | 2002-12-19 | Steel sheet excellent in deep drawability, manufacturing method thereof and steel pipe manufacturing method excellent in workability |
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| JP4537865B2 (en) * | 2005-02-14 | 2010-09-08 | 新日本製鐵株式会社 | High-strength cold-rolled steel sheet excellent in deep drawability and manufacturing method thereof |
| JP5402869B2 (en) * | 2010-07-30 | 2014-01-29 | 新日鐵住金株式会社 | High-strength cold-rolled steel sheet excellent in deep drawability and manufacturing method thereof |
| JP5811818B2 (en) * | 2011-12-06 | 2015-11-11 | 新日鐵住金株式会社 | Steel plate manufacturing method |
| KR101585739B1 (en) | 2013-12-25 | 2016-01-14 | 주식회사 포스코 | Cold rolled steel sheet having high yield ratio and excelent impact property and method for manufacturing the same |
| KR101611762B1 (en) | 2014-12-12 | 2016-04-14 | 주식회사 포스코 | Cold rolled steel sheet having excellent bendability and crash worthiness and method for manufacturing the same |
| KR101657820B1 (en) * | 2014-12-23 | 2016-09-20 | 주식회사 포스코 | Cold rolled steel sheet having excellent resistance delamination and crash worthiness and method for manufacturing the same |
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