JP4422302B2 - Low yield ratio high strength steel sheet with excellent shape freezing property and method for producing the same - Google Patents
Low yield ratio high strength steel sheet with excellent shape freezing property and method for producing the same Download PDFInfo
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Description
【０００１】
【発明の属する技術分野】
本発明は、自動車、建築、電気等の部材として有用な高強度鋼板及びその製造法に関し、特にプレス成形時の形状凍結性に優れる高強度鋼板、高強度合金化溶融亜鉛めっき鋼板及びその製造法に関するものである。
【０００２】
【従来の技術】
自動車は、近年の燃費節減の動向に対応すべく軽量化が検討されており、材料面では、軽量化のため薄肉化しても強度を確保できるように高強度化が進められている。ところが、一般に材料の形状凍結性は強度上昇に伴い劣化するので、形状凍結性と強度を両立する鋼板が求められている。形状凍結性の指標には引張試験における降伏応力（０．２％耐力）があり、この低いものが形状凍結性が良好となるが、高強度鋼板では最高強度が高いことが必要であるため、降伏応力と最高強度の比、いわゆる降伏比が低いことが重要である。
【０００３】
このため鋼板製造過程の熱処理工程において、金属組織中にマルテンサイト相を形成させた熱延鋼板及び冷延鋼板が開発されている。これは比較的軟質なフェライト相中に非常に硬質なマルテンサイト相が分散しているため、変形時にこれらの境界に変形が集中し降伏しやすくなると共に、熱処理後半の冷却過程におけるマルテンサイト変態に伴い鋼板中に多量の可動転位が導入されているため、変形時に転位の滑りが容易になり低い応力で変形が起きるものである。
【０００４】
従来この鋼板は、高価な合金元素を含まずに、０．０５〜０．１％程度のＣと０．３〜０．８％程度のＳｉ及び１．０〜２．５％程度のＭｎを基本的な合金元素とし、二相域でオーステナイトを生成させた後、冷却速度を比較的高くすることでマルテンサイト変態を起こし、室温で金属組織中にマルテンサイト相が形成するようにした鋼板であり、一般に「二相（デュアルフェイズ）鋼」、「ＤＰ （Dual Phase）鋼」などと呼ばれている。その技術は、例えば特開昭５５−１１０７３３号公報等で開示され、これらの延性を改善したものとして、マルテンサイト相に加えベイナイトやオーステナイト相を形成させた鋼板が特開昭６０−４３４６４号公報等に開示されている。
【０００５】
また近年、自動車の衝突安全性に関連して、衝突時のエネルギーを効率よく吸収する動的変形特性の優れた鋼板としてＤＰ鋼の適用が注目を集めており、特開平１１−８０８７８号公報などが開示されている。これら従来の鋼板は高価な合金元素を含まないとは言え、Ｓｉ，Ｍｎなどの添加量は多く、いわゆる普通鋼と比較すると合金による製造コストが高くなっているばかりでなく、高濃度のＳｉは自動車用鋼板で主流となりつつある亜鉛めっき鋼板への適用においては、亜鉛の付着性を阻害し、高濃度のＭｎ，Ｃは溶接性を劣化させている。同時に溶融めっき時の熱履歴によりマルテンサイトが炭化物に分解し、好ましいマルテンサイト量が得られなくなり特性が劣化する場合もある。
【０００６】
また、ＤＰ鋼用に特定の成分の鋼種を出鋼、通板することは工場全体の生産性を低下させることにもなるため、いわゆる普通鋼と同じ成分でＤＰ鋼を造ることが可能になれば、生産性が向上するばかりでなく、生産管理上の在庫削減、工程管理の簡素化など大きなメリットが期待できる。
【０００７】
【発明が解決しようとする課題】
本発明は製造コストを増加させ、または特性を劣化させる合金添加量を低減し、主要元素についていわゆる普通鋼と同程度の含有量で目的とするマルテンサイト組織を確保し、亜鉛めっきの付着性が良好で高耐食性表面処理鋼板への適用も可能な、形状凍結性の良好な高強度鋼板、及びその製造法を提供するものである。
【０００８】
【課題を解決するための手段】
本発明者らは、上記目的を達成できる高強度鋼板を提供するべく、特性と鋼板成分との関係について鋭意検討を行い、本発明を完成させたものである。
その趣旨は以下のとおりである。
【０００９】
従来よりＮはオーステナイト相を安定化させる元素として知られているが、従来の製造法のように溶鋼段階で高濃度のＮを含有させる方法では精錬が困難であり、また鋳造時に鋼片中にガスが発生し凝固後に気泡が残存して、良好な鋼片を得ることができない。このため本発明鋼が対象とする加工用鋼板への高Ｎ鋼の適用は検討されておらず、高温で形成されたオーステナイト相の温度降下に伴うマルテンサイト相への変態挙動はほとんど未解明であり、高Ｎ鋼でフェライトとマルテンサイトの二相組織である鋼材の特性については未知であった。
【００１０】
そこで本発明者はＮを、鋳造後、製品となる直前に含有させる方法を検討し、Ｎを多量に含有させることが本発明の目的とする特性向上に有効であることを見出した。本発明はこの知見をもとに、さらにＳｉ，Ｍｎ，Ｃ等の元素およびＣａ，Ｎａ，Ｍｇなどの微量元素の影響および窒化条件、さらに目的とする金属組織に制御するための熱履歴などを検討し、発明を完成させたものである。
【００１１】
即ち本発明の要点は、
（１）Ｎを高濃度に含有させる。
（２）窒化物を形成するＳｉ，Ａｌなどの含有量を適当な範囲に制御する。
（３）鉄窒化物の生成を制御するため、Ｃａ，Ｎａ，Ｍｇなどを必要に応じて添加する。
（４）金属組織を形成する各相の強度を調整し、鋼板としての強度と伸びを調整するため、Ｃ，Ｓｉ，Ｍｎ，Ｐなどの強化元素量を制御する。
ことにある。
【００１２】
この様な知見に基づく本発明は、以下の構成を要旨とする。
（１）質量％で、
Ｎ ：０．０５〜２．０％、 Ｓｉ：０．３３％以下、
Ｃ ：０．０６％以下、 Ｍｎ：１．１４％以下、
Ｐ ：０．０１３％以下、 Ａｌ：０．３％以下
を含有し、残部Ｆｅ及び不可避的不純物からなり、マルテンサイトの体積率が３〜３０％、残部フェライトであることを特徴とする形状凍結性に優れた低降伏比高強度鋼板。
（２）質量％で、Ｎｉ，Ｃｒ，Ｃａ，Ｎａ，Ｍｇ，Ｍｏのうち少なくとも１種以上をそれぞれ２．０％未満含むことを特徴とする前記（１）記載の形状凍結性に優れた低降伏比高強度鋼板。
（３）上記鋼板の上にＺｎ合金めっき層を有することを特徴とする前記（１）または（２）に記載の形状凍結性に優れた低降伏比高強度鋼板。
（４）熱間圧延、冷間圧延、及び焼鈍を含む工程を経て、請求項１または２に記載の低降伏比高強度鋼板を製造する方法であって、熱間圧延後焼鈍までの間に５５０〜８００℃の温度域でアンモニアを２％以上含む雰囲気中で２秒〜１０分保持する工程を含む処理を施し、また、最終製品となる直前に５８０℃以上の高温状態からの冷却過程において、５８０℃から１００℃に至るまでの時間が２００秒以下である処理を施すことを特徴とする形状凍結性に優れた低降伏比高強度鋼板の製造方法。
（５）前記（４）に記載の方法で得られた鋼板の上にＺｎ合金めっき層を施すことを特徴とする形状凍結性に優れた低降伏比高強度鋼板の製造方法。
【００１３】
【発明の実施の形態】
以下に本発明を詳細に説明する。
まず本発明における鋼板成分の限定理由を説明する。
Ｎは本発明の最も重要な元素である。ＮはＣと同様にオーステナイト生成元素であるが、Ｃよりもマルテンサイトの形成効率が安定している。これは焼鈍中の高温保定域においてＣ系の従来鋼に比べより低温までオーステナイトが安定で、冷却中の窒化物生成が抑制されることが一因と考えられる。このため、従来鋼で高温でのオーステナイト安定化または冷却中の炭化物析出抑制のため添加しているＭｎ，ＳｉやＡ１の含有量を減らすことができる。またマルテンサイト量の制御およびフェライト相中に存在する固溶Ｎにより、鋼板の強度調整にも用いることができる。Ｎ濃度が０．０５％未満ではその効果が見出せない。一方、Ｎ濃度を高めるにはＮ化処理時間が長くなることから、上限を２．０％とした。好ましくは０．０５〜１．０％である。
【００１４】
Ｃは、二相共存温度域でオーステナイト中に濃化することでオーステナイトを安定化し、冷却中のマルテンサイトヘの変態を制御する元素である。このため従来鋼では０．０８％程度含有させるが、本発明鋼ではＮによりオーステナイトの安定化を図っているためＣ含有量は特に限定しない。しかし、Ｃのオーステナイトからの変態挙動は変態温度によりパーライト、上部ベイナイト、下部ベイナイトなど複雑な挙動をとり、冷却中のマルテンサイトヘの変態を安定して制御する目的では厳格な温度制御が必要となる一因にもなる。さらに高濃度のＣ含有は鋼板の溶接性を劣化させる。しかし、いわゆる極低炭素域まで低減するには脱ガスによるコスト増加を招く。変態挙動の安定性と溶接性および製造コストを考慮すると、好ましい範囲は０．０６％以下、より好ましい範囲は０．０２〜０．０４％である。
【００１５】
Ｓｉは従来鋼では通常、セメンタイトの析出を抑制することでオーステナイトの安定性を高め、マルテンサイト変態を制御するため０．５％程度添加される。しかし、本発明鋼では窒化中に窒化物を形成しオーステナイトに濃化するＮ量を低減させるため、過剰な添加は好ましくない。一方、フェライト相を強化し鋼板の強度調整を行うには有効な元素である。好ましい範囲を０．３３％以下、さらに好ましくは０．０１〜０．１％以下とする。
【００１６】
Ｍｎは、従来鋼ではオーステナイトを安定化させマルテンサイトヘの変態を制御するため、Ｃと共に用いられ２％程度添加されていたが、本発明鋼ではオーステナイトが比較的低温でも安定であるＮでマルテンサイトヘの変態を制御しているため、それほど多くの添加は必要ではない。一方、フェライト相を強化し鋼板の強度調整に有効である。しかし、多量になるとバンド組織が顕著になり特性を劣化させるし、スポット溶接部がナゲット内で破断しやすくなり好ましくない。これらを考慮し、好ましい範囲を１．１４％以下とする。
【００１７】
Ａｌは脱酸材としても用いられると同時に、Ｓｉと同様にセメンタイトの析出を抑制しオーステナイトを安定化するため、従来鋼では積極的に用いられている。しかし、本発明鋼ではＮ化中に窒化物を形成しオーステナイトに濃化するＮ量を低減させるため、過剰な添加は好ましくない。好ましい範囲は０．３％以下、さらに好ましくは０．１％以下である。
【００１８】
ＰはＣ，Ｓｉ，Ｍｎと共に鋼板強度を調整するために添加するが、過剰な添加は加工性を顕著に劣化させると共に添加コストの上昇を招くので、上限は０．０１３％が望ましい。
【００１９】
本発明の鋼板は以上を基本成分とするが、これらの元素及びＦｅ以外に、高温域でのオーステナイトを安定化し窒化物への分解を抑制することでマルテンサイト変態を制御するため、Ｎｉ，Ｃｒ，Ｃａ，Ｎａ，Ｍｇ，Ｍｏのうち少なくとも１種以上を添加してもよい。過剰な添加は添加コストの増加になるだけでなく加工性を劣化させる場合もあるので、それぞれ２．０％未満に限定する。
【００２０】
また、従来の残留オーステナイト鋼に加工性、めっき性などを向上させるために添加されるＮｉ，Ｃｕ，Ｃｏなどは、従来鋼と同様に含有させても本発明の効果をなんら損なうものではない。
【００２１】
最終製品としての本発明鋼板の特性は、製品中に含まれるマルテンサイト相の体積率に左右される。マルテンサイト相の体積率が３％未満では望ましい効果 （低降伏比（０．６以下），高強度）が認められない。一方マルテンサイト相の体積率が３０％を超すと極度に厳しい成形を施した場合、二次加工性や衝撃性において問題を生じることがあるので、本発明では上限を３０％とした。
【００２２】
次に、本発明鋼板の製造方法について説明する。
本発明の特徴は、従来の加工用鋼板では考えられなかったほどの高濃度のＮを含有させることである。従来鋼のように溶鋼段階で成分調整し多くのＮを含有させることは困難であるが、鋼片または鋼板への窒化を適用すると、比較的容易に高濃度のＮを含有させることが可能になる。
【００２３】
ガスによる窒化の場合の条件としては、５５０〜８００℃の温度域でアンモニアを２％以上含む雰囲気中で２秒〜１０分保持することである。温度がこの範囲を外れると窒化効率が低下し、必要量のＮ化に長時間を要する。また、低温側に外れた場合は鉄窒化物を形成し、本発明鋼で必要とする高温でのオーステナイト形成に好ましい固溶Ｎを活用することができない。ガス組成は特に限定しないが、Ｎ化に必要なアンモニアの濃度を窒化効率の観点から２％以上に限定する。またＮ化に際しての本発明温度および本発明雰囲気中での保持時間は、必要Ｎ量との兼ね合いで決定されるが、操業性などを考慮し上記温度に保持する場合は、２秒〜１０分に限定する。
【００２４】
本発明の範囲内であれば、現状の焼鈍炉の一部の雰囲気中に僅かなアンモニアガスを流すことで窒化が可能になり、製造コストの増加はほとんどなく、従来鋼で行われている合金添加コストに比べると非常に小さく、製造コストの大幅な低減を図ることができる。
【００２５】
窒化のタイミングは鋳片から焼鈍板のどこでも可能であるが、窒化では表面から鋼内部へのＮの拡散を利用しているため、板厚は薄いほど高濃度の窒化が容易となる。このため熱間仕上げ圧延以降の工程で行うことが好ましい。
【００２６】
通常の冷延鋼板の製造においては、再結晶焼鈍工程中で焼鈍炉の一部または全部を本発明雰囲気にすることで窒化を行うことが生産上は都合がよい。工程の前半で高濃度のＮを含有させ、その後の高温処理または適当な温度での保定によりオーステナイト相の安定化を図る工程も可能であるし、焼鈍工程の最高温度への到達により再結晶および適当な延性を付与した後に窒化を行い、オーステナイト相を多く生成させるような工程も可能である。またこれらを組合わせたり、高温再結晶の後、本発明範囲内の低温で窒化を行い、その後再び高温に昇温し組織制御を行うような工程によっても、本発明の効果は何ら損なわれるものではない。
【００２７】
窒化はできるだけ最終工程に近い工程で行うことが、生産管理上の在庫削減および工程管理の上からは好ましい。例えば焼鈍工程で行うのであれば、それ以前の製鋼、熱延、冷延工程においてはＤＰ鋼用に特定のコイルの在庫を持つ必要がなくなり、メリットが大きい。
【００２８】
高温で形成したオーステナイト相の、冷却過程でのマルテンサイト相への変態を制御するのに有効なのは冷却速度である。Ｎを高濃度で含有する本発明鋼では、約５８０℃以上ではオーステナイト相が形成するので、その工程以降に５８０℃以上に昇温することのない工程での冷却条件を制御することが有効である。つまり、最終製品直前の５８０℃以上の高温状態からの冷却過程において、５８０℃から１００℃に至るまでの時間を２００秒以下とすることが、本発明で必要な組織の制御に好ましい。さらに好ましくは６０秒以下である。
【００２９】
本発明鋼は従来鋼と比較しＳｉ含有量が少ないため、亜鉛めっき鋼板用の原板として使用した場合のめっき性が良好となると同時に、高温でのオーステナイトが安定なため、めっき時の熱履歴によってもマルテンサイトヘの変態が影響を受け難く、めっきの有無に関わらず安定した特性を有するという特徴を有している。 Ｚｎめっき層厚みについては特に制約は設けないが、耐食性の観点から０．１μｍ以上、加工性の観点からすると１０μｍ以下であることが望ましい。
【００３０】
【実施例】
通常の熱延、冷延条件で得られた冷延鋼板について、焼鈍および一部のものについてはめっきを行い、０．６％で調質圧延し、鋼板またはめっき鋼板を製造した。成分を表１に示すが、本発明鋼においては焼鈍工程の最高到達温度からの冷却途中において、アンモニアガスを含む雰囲気中で保持することによりＮ化を行って高濃度にＮを含有させており、表１中のＮ量については最終製品での値である。鋼中Ｎ量はこの時の保持温度、保持時間、アンモニアガス濃度で調整した。Ｎ化条件を併せて表１に示す。めっきは１０％Ａｌ−Ｚｎ浴で行った。
【００３１】
鋼板中のマルテンサイトの体積率は、断面組織観察によるマルテンサイトの分布が奥行き方向にも同じ分布であると仮定し、断面観察から得られた面積率をそのまま体積率とした。これらの鋼板よりＪＩＳ５号引張試験片を採取し、ゲージ長さ５０mm、引張速度１０mm/minで常温引張試験を行った。
【００３２】
めっき性の評価は、不めっき発生とめっき密着性について行い、不めっきは目視で有無を判定し、めっき密着性は６０度Ｖ曲げ試験後のテープテストでテープ黒化度が２０％未満であれば合格とした。
【００３３】
また、溶接性は溶接電流：１０kA、加圧力：２２０kg、溶接時間：１２サイクル、電極径：６mm、電極形状：ドーム型、先端６φ−４０Ｒの溶接条件でスポット溶接を行い、ナゲット径が４√ｔ（ｔ：板厚）を切った時点までの連続打点数が１０００点を超えたものを合格とした。
材質およびめっき性の評価結果を表２に示す。
【００３４】
本発明鋼はめっきの有無に関わらずいずれも引張強度が５００ＭＰａ以上でありながら降伏比は０．６０以下であり、高強度と低降伏比を両立していると同時に、めっき性、溶接性も良好である。Ｎを高濃度に含有する本発明鋼の内でも、Ｓｉ，Ｃ，Ｍｎ，Ｐ，Ａｌなどが特定範囲にあるものは特に特性が良好である。またＮｉ，Ｃｒ，Ｃａ、Ｎａ、Ｍｇ，Ｍｏなどの微量元素の効果も確認できる。
【００３５】
これに対し、Ｎが本発明範囲にない従来鋼では、めっきなしでは低降伏比が実現できているが、めっき工程の熱履歴により可動転位が消失あるいはマルテンサイトが炭化物に分解してしまい、特性が劣化している。またＳｉやＡｌ含有量が高いためめっき性が劣り、ＣやＭｎが高いため溶接性が劣る。
【００３６】
【表１】
【表２】
【発明の効果】
以上説明したように、本発明は製造コストを増加させ、または特性を劣化させる合金添加量を低減せしめ、目的とするマルテンサイト組織を確保することにより、亜鉛めっきの付着性が良好で高耐食性表面処理鋼板への適用も可能な、形状凍結性の良好な高強度鋼板を得ることができる。[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to a high-strength steel sheet useful as a member for automobiles, architecture, electricity, and the like, and a method for producing the same, and in particular, a high-strength steel sheet excellent in shape freezing at the time of press forming, a high-strength galvannealed steel sheet and a method for producing the same. It is about.
[0002]
[Prior art]
For automobiles, weight reduction is being studied in order to respond to recent trends in fuel efficiency savings, and in terms of materials, higher strength is being promoted so that strength can be secured even if the thickness is reduced due to weight reduction. However, since the shape freezing property of a material generally deteriorates as the strength increases, a steel sheet that satisfies both shape freezing property and strength is required. The index of shape freezing property is yield stress (0.2% proof stress) in the tensile test, and this low one has good shape freezing property, but high strength steel plate needs to have high maximum strength, It is important that the ratio of yield stress to maximum strength, the so-called yield ratio, is low.
[0003]
For this reason, hot-rolled steel sheets and cold-rolled steel sheets in which a martensite phase is formed in the metal structure have been developed in the heat treatment step of the steel sheet manufacturing process. This is because a very hard martensite phase is dispersed in a relatively soft ferrite phase, so that deformation concentrates on these boundaries during deformation and yields easily. Accordingly, since a large amount of movable dislocations are introduced into the steel sheet, the dislocations are easily slipped during deformation, and deformation occurs with low stress.
[0004]
Conventionally, this steel sheet contains about 0.05 to 0.1% C, about 0.3 to 0.8% Si and about 1.0 to 2.5% Mn without containing expensive alloy elements. A steel alloy in which martensite phase is formed in the metal structure at room temperature by generating martensite by making austenite in a two-phase region after making it a basic alloy element and then relatively increasing the cooling rate. It is generally called “Dual Phase Steel”, “DP (Dual Phase) Steel”, etc. The technique is disclosed in, for example, Japanese Patent Application Laid-Open No. 55-110733, and a steel sheet in which a bainite or austenite phase is formed in addition to the martensite phase is disclosed in Japanese Patent Application Laid-Open No. 60-43464. Etc. are disclosed.
[0005]
In recent years, DP steel has been attracting attention as a steel plate with excellent dynamic deformation characteristics that efficiently absorbs energy at the time of collision in relation to collision safety of automobiles, such as JP-A-11-80878. Is disclosed. Although these conventional steel plates do not contain expensive alloy elements, the amount of addition of Si, Mn and the like is large, and not only the manufacturing cost by the alloy is higher than so-called ordinary steel, but also high concentration of Si In application to galvanized steel sheets, which are becoming mainstream in automobile steel sheets, zinc adhesion is hindered, and high concentrations of Mn and C deteriorate weldability. At the same time, martensite is decomposed into carbides by the thermal history during hot dipping, and a preferable amount of martensite cannot be obtained and the characteristics may be deteriorated.
[0006]
In addition, it is possible to make DP steel with the same components as so-called ordinary steel because it is possible to reduce the productivity of the whole factory by putting out and passing through a steel grade of a specific component for DP steel. For example, not only productivity can be improved, but also significant benefits such as inventory reduction in production management and simplification of process management can be expected.
[0007]
[Problems to be solved by the invention]
The present invention increases the manufacturing cost or reduces the alloy addition amount that deteriorates the characteristics, secures the target martensite structure with the same content as the so-called ordinary steel for the main elements, and has the adhesion of galvanizing. The present invention provides a high-strength steel sheet having a good shape freezing property that can be applied to a good and highly corrosion-resistant surface-treated steel sheet, and a method for producing the same.
[0008]
[Means for Solving the Problems]
In order to provide a high-strength steel sheet that can achieve the above object, the present inventors have intensively studied the relationship between characteristics and steel sheet components, and completed the present invention.
The purpose is as follows.
[0009]
Conventionally, N is known as an element that stabilizes the austenite phase. However, refining is difficult with a method of containing a high concentration of N at the molten steel stage as in the conventional manufacturing method, and in the steel slab at the time of casting. Gas is generated and bubbles remain after solidification, and a good steel piece cannot be obtained. For this reason, the application of high-N steel to the steel sheet for processing, which is the subject of the present invention steel, has not been studied, and the transformation behavior of the austenite phase formed at high temperature into the martensite phase due to the temperature drop is almost unknown. In addition, the characteristics of the steel material, which is a high-N steel and has a two-phase structure of ferrite and martensite, was unknown.
[0010]
Therefore, the present inventor studied a method of containing N immediately after casting and immediately before becoming a product, and found that containing a large amount of N is effective in improving the target properties of the present invention. Based on this knowledge, the present invention further affects the influence of elements such as Si, Mn, and C and trace elements such as Ca, Na and Mg, nitriding conditions, and the heat history for controlling the target metal structure. It has been studied and completed the invention.
[0011]
That is, the gist of the present invention is
(1) N is contained in a high concentration.
(2) Control the content of Si, Al, etc. that form nitrides in an appropriate range.
(3) In order to control the formation of iron nitride, Ca, Na, Mg or the like is added as necessary.
(4) In order to adjust the strength of each phase forming the metal structure and adjust the strength and elongation as a steel sheet, the amount of strengthening elements such as C, Si, Mn, and P is controlled.
There is.
[0012]
The gist of the present invention based on such knowledge is as follows.
(1) In mass%,
N: 0.05 to 2.0%, Si: 0.33% or less,
C: 0.06% or less, Mn: 1.14% or less,
P: 0.013% or less, Al: 0.3% or less, consisting of the balance Fe and inevitable impurities, the volume fraction of martensite being 3 to 30% , and the balance freezing Low yield ratio high strength steel plate with excellent properties.
(2) Low in shape freezing property according to the above (1), characterized by containing at least one of Ni, Cr, Ca, Na, Mg, and Mo in less than 2.0% by mass. High yield strength steel plate.
(3) The low yield ratio high strength steel plate having excellent shape freezing property according to (1) or (2), wherein a Zn alloy plating layer is provided on the steel plate.
(4) hot rolling, cold rolling, and through a process including the annealing, a method of producing a low yield ratio high-strength steel sheet according to claim 1 or 2, until after hot rolling annealing In a cooling process from a high temperature state of 580 ° C. or more immediately before the final product is obtained , a treatment including a step of holding for 2 seconds to 10 minutes in an atmosphere containing 2% or more of ammonia in a temperature range of 550 to 800 ° C. The manufacturing method of the low yield ratio high strength steel plate excellent in the shape freezing property characterized by performing the process for 200 seconds or less from 580 degreeC to 100 degreeC.
(5) A method for producing a low-yield-ratio high-strength steel sheet excellent in shape freezing, characterized in that a Zn alloy plating layer is applied on the steel sheet obtained by the method described in (4).
[0013]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
First, the reasons for limiting the steel plate components in the present invention will be described.
N is the most important element of the present invention. N is an austenite-forming element like C, but martensite formation efficiency is more stable than C. This is considered to be due to the fact that austenite is more stable at a lower temperature in the high temperature holding region during annealing than the conventional C-based steel, and the formation of nitride during cooling is suppressed. For this reason, it is possible to reduce the contents of Mn, Si and A1 added for stabilizing austenite at high temperatures or suppressing carbide precipitation during cooling in the conventional steel. It can also be used to adjust the strength of the steel sheet by controlling the amount of martensite and solute N present in the ferrite phase. If the N concentration is less than 0.05 %, the effect cannot be found. On the other hand, in order to increase the N concentration, the N treatment time becomes longer, so the upper limit was made 2.0%. Preferably it is 0.05 to 1.0%.
[0014]
C is an element that stabilizes austenite by concentrating in austenite in a two-phase coexisting temperature range and controls transformation to martensite during cooling. For this reason, the conventional steel contains about 0.08%, but in the steel of the present invention, the austenite is stabilized by N, so the C content is not particularly limited. However, the transformation behavior of C from austenite has complex behavior such as pearlite, upper bainite, and lower bainite depending on the transformation temperature, and strict temperature control is required for the purpose of stably controlling the transformation to martensite during cooling. It also contributes. Furthermore, the high concentration of C deteriorates the weldability of the steel sheet. However, in order to reduce to a so-called extremely low carbon region, an increase in cost due to degassing is caused. In consideration of stability of transformation behavior, weldability, and production cost, the preferable range is 0.06% or less, and the more preferable range is 0.02 to 0.04%.
[0015]
In conventional steels, Si is usually added in an amount of about 0.5% in order to improve the austenite stability by suppressing the precipitation of cementite and to control the martensitic transformation. However, in the steel of the present invention, excessive addition is not preferable because it reduces the amount of N that forms nitrides during nitriding and concentrates to austenite. On the other hand, it is an effective element for strengthening the ferrite phase and adjusting the strength of the steel sheet. A preferable range is 0.33 % or less, more preferably 0.01 to 0.1% or less.
[0016]
Mn is used together with C in order to stabilize austenite and control the transformation to martensite in conventional steels, and has been added in an amount of about 2%. Therefore, the addition of so much is not necessary. On the other hand, it is effective in adjusting the strength of the steel sheet by strengthening the ferrite phase. However, when the amount is large, the band structure becomes remarkable and the characteristics are deteriorated, and the spot welded portion is easily broken in the nugget, which is not preferable. Taking these into consideration, the preferable range is made 1.14 % or less.
[0017]
Al is also used as a deoxidizing material, and at the same time, like Si, suppresses cementite precipitation and stabilizes austenite. Therefore, Al is actively used in conventional steels. However, in the steel according to the present invention, excessive addition is not preferable because it reduces the amount of N that forms nitrides and concentrates to austenite during Nation. A preferable range is 0.3% or less, more preferably 0.1% or less.
[0018]
P is added together with C, Si, and Mn to adjust the strength of the steel sheet. However, excessive addition significantly degrades workability and increases the cost of addition, so the upper limit is preferably 0.013 %.
[0019]
The steel sheet of the present invention has the above as the basic components. In addition to these elements and Fe, Ni, Cr are used to control the martensitic transformation by stabilizing austenite at high temperatures and suppressing decomposition into nitrides. , Ca, Na, Mg, Mo may be added at least one or more. Excessive addition not only increases the addition cost but also may degrade the workability, so each is limited to less than 2.0%.
[0020]
Moreover, even if Ni, Cu, Co and the like added to improve the workability and plating properties of the conventional retained austenitic steel are contained in the same manner as in the conventional steel, the effect of the present invention is not impaired.
[0021]
The properties of the steel sheet of the present invention as a final product depend on the volume ratio of the martensite phase contained in the product. When the volume ratio of the martensite phase is less than 3%, desirable effects (low yield ratio (0.6 or less), high strength) are not observed. On the other hand, when the volume ratio of the martensite phase exceeds 30%, when extremely severe molding is performed, problems may occur in secondary workability and impact resistance. Therefore, in the present invention, the upper limit is set to 30%.
[0022]
Next, the manufacturing method of this invention steel plate is demonstrated.
The feature of the present invention is to contain N at a concentration as high as could not be considered in conventional steel sheets for processing. Although it is difficult to adjust the components at the molten steel stage and contain a large amount of N as in conventional steel, it is possible to contain a high concentration of N relatively easily by applying nitriding to a steel piece or steel plate. Become.
[0023]
As a condition in the case of nitriding with a gas, it is to hold in an atmosphere containing 2% or more of ammonia in a temperature range of 550 to 800 ° C. for 2 seconds to 10 minutes. If the temperature is out of this range, the nitriding efficiency is lowered, and it takes a long time for N conversion. Moreover, when it remove | deviates to a low temperature side, an iron nitride is formed and the solid solution N preferable for austenite formation at the high temperature required by this invention steel cannot be utilized. The gas composition is not particularly limited, but the concentration of ammonia necessary for N is limited to 2% or more from the viewpoint of nitriding efficiency. Further, the temperature of the present invention and the holding time in the atmosphere of the present invention at the time of Nation are determined in consideration of the necessary N amount, but 2 seconds to 10 minutes when maintaining the above temperature in consideration of operability and the like. Limited to.
[0024]
If it is within the scope of the present invention, nitriding can be performed by flowing a slight amount of ammonia gas in a part of the atmosphere of the current annealing furnace, and there is almost no increase in manufacturing cost, and an alloy made of conventional steel Compared to the addition cost, it is very small, and the manufacturing cost can be greatly reduced.
[0025]
The timing of nitriding can be anywhere from the slab to the annealed plate. However, since nitriding uses diffusion of N from the surface to the inside of the steel, nitriding with a higher concentration becomes easier as the plate thickness is thinner. For this reason, it is preferable to carry out in the process after hot finish rolling.
[0026]
In the production of a normal cold-rolled steel sheet, it is convenient in production to perform nitriding by making the atmosphere of the present invention part or all of the annealing furnace in the recrystallization annealing process. It is possible to include a high concentration of N in the first half of the process, and to stabilize the austenite phase by subsequent high-temperature treatment or holding at an appropriate temperature, and recrystallization and crystallization by reaching the maximum temperature in the annealing process. A process in which nitriding is performed after imparting appropriate ductility to generate a large amount of austenite phase is also possible. In addition, the effects of the present invention may be impaired even by a process in which these are combined or nitridized at a low temperature within the scope of the present invention after high-temperature recrystallization and then heated to a high temperature again to control the structure. is not.
[0027]
Nitriding is preferably performed in a process as close to the final process as possible from the viewpoint of inventory reduction and process control in production management. For example, if it is performed in the annealing process, it is not necessary to have a specific coil stock for DP steel in the steel making, hot rolling and cold rolling processes before that, and there is a great merit.
[0028]
The cooling rate is effective in controlling the transformation of the austenite phase formed at a high temperature into the martensite phase during the cooling process. In the steel of the present invention containing N at a high concentration, an austenite phase is formed at about 580 ° C. or higher. Therefore, it is effective to control the cooling conditions in a step where the temperature does not rise to 580 ° C. or higher after that step. is there. That is, in the cooling process from a high temperature state of 580 ° C. or more immediately before the final product, the time from 580 ° C. to 100 ° C. is preferably 200 seconds or less in order to control the structure required in the present invention. More preferably, it is 60 seconds or less.
[0029]
Since the steel of the present invention has a lower Si content than the conventional steel, the plating properties when used as an original plate for a galvanized steel sheet are good, and at the same time, the austenite at high temperature is stable. Also, the transformation to martensite is not easily affected, and has the characteristics of having stable characteristics regardless of the presence or absence of plating. Although there is no particular restriction on the thickness of the Zn plating layer, it is preferably 0.1 μm or more from the viewpoint of corrosion resistance and 10 μm or less from the viewpoint of workability.
[0030]
【Example】
Cold-rolled steel sheets obtained under normal hot rolling and cold rolling conditions were annealed and partly plated and temper rolled at 0.6% to produce steel sheets or plated steel sheets. Ingredients are shown in Table 1. In the steel of the present invention, during the cooling from the highest temperature reached in the annealing process, N is formed by holding it in an atmosphere containing ammonia gas to contain N in a high concentration. The N amount in Table 1 is the value in the final product. The amount of N in the steel was adjusted by the holding temperature, holding time, and ammonia gas concentration at this time. Table 1 also shows the Nation conditions. Plating was performed in a 10% Al—Zn bath.
[0031]
As for the volume ratio of martensite in the steel sheet, it was assumed that the distribution of martensite by cross-sectional structure observation was the same distribution in the depth direction, and the area ratio obtained from cross-sectional observation was directly used as the volume ratio. JIS No. 5 tensile test specimens were collected from these steel plates and subjected to a room temperature tensile test at a gauge length of 50 mm and a tensile speed of 10 mm / min.
[0032]
Plating property is evaluated for non-plating occurrence and plating adhesion. Whether or not unplating is visually determined, the plating adhesion should be less than 20% in the tape blackness in the tape test after 60 degree V bending test. Was accepted.
[0033]
Also, the weldability is spot welding under the welding conditions of welding current: 10 kA, pressure: 220 kg, welding time: 12 cycles, electrode diameter: 6 mm, electrode shape: dome shape, tip 6φ-40R, and the nugget diameter is 4√. A sample in which the number of continuous hit points up to the time when t (t: plate thickness) was cut exceeded 1000 points was accepted.
Table 2 shows the evaluation results of the materials and plating properties.
[0034]
The steel according to the present invention has a tensile strength of 500 MPa or more regardless of the presence or absence of plating, and a yield ratio of 0.60 or less. At the same time, it has both high strength and low yield ratio. It is good. Among the steels of the present invention containing N at a high concentration, those having Si, C, Mn, P, Al, etc. in a specific range have particularly good characteristics. Further, the effects of trace elements such as Ni, Cr, Ca, Na, Mg, and Mo can be confirmed.
[0035]
In contrast, in conventional steels where N is not within the scope of the present invention, a low yield ratio can be realized without plating, but the dislocations disappear or martensite decomposes into carbides due to the thermal history of the plating process. Has deteriorated. Moreover, since Si and Al content is high, plating property is inferior, and since C and Mn are high, weldability is inferior.
[0036]
[Table 1]
[Table 2]
【The invention's effect】
As described above, the present invention increases the manufacturing cost or reduces the amount of alloy addition that degrades the characteristics, and secures the target martensite structure, so that the adhesion of galvanization is good and the surface has high corrosion resistance. It is possible to obtain a high-strength steel sheet having good shape freezing property that can be applied to the treated steel sheet.
Claims (5)
Ｎ ：０．０５〜２．０％、
Ｓｉ：０．３３％以下、
Ｃ ：０．０６％以下、
Ｍｎ：１．１４％以下、
Ｐ ：０．０１３％以下、
Ａｌ：０．３％以下
を含有し、残部Ｆｅ及び不可避的不純物からなり、マルテンサイトの体積率が３〜３０％、残部フェライトであることを特徴とする形状凍結性に優れた低降伏比高強度鋼板。% By mass
N: 0.05-2.0%
Si: 0.33% or less,
C: 0.06% or less,
Mn: 1.14% or less,
P: 0.013% or less
Al: 0.3% or less, consisting of the balance Fe and inevitable impurities, the volume ratio of martensite is 3 to 30% , and the balance is ferrite. Strength steel plate.
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2000204975A JP4422302B2 (en) | 2000-07-06 | 2000-07-06 | Low yield ratio high strength steel sheet with excellent shape freezing property and method for producing the same |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2000204975A JP4422302B2 (en) | 2000-07-06 | 2000-07-06 | Low yield ratio high strength steel sheet with excellent shape freezing property and method for producing the same |
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| Publication Number | Publication Date |
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| JP2002020834A JP2002020834A (en) | 2002-01-23 |
| JP4422302B2 true JP4422302B2 (en) | 2010-02-24 |
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| Country | Link |
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