JP2004360046A

JP2004360046A - High-strength steel sheet and its production method

Info

Publication number: JP2004360046A
Application number: JP2003162703A
Authority: JP
Inventors: Shigeki Nomura; 茂樹野村; Norio Imai; 規雄今井
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 2003-06-06
Filing date: 2003-06-06
Publication date: 2004-12-24
Anticipated expiration: 2023-06-06
Also published as: JP4214840B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet having a tensile strength of 880 MPa or higher in the direction rectangular to the direction of rolling and a yield ratio of 0.80 or higher and suited for automobile underbody parts, and to provide a method for production thereof. <P>SOLUTION: The steel comprises 0.04 to 0.17% C, at most 1.1% Si, 1.6 to 2.6% Mn, at most 0.05% P, at most 0.02% S, 0.001 to 0.50% Al, at most 0.02% N, at most 0.11 to 0.30% V, and 0.07 to 0.25% Ti, and optionally contains at least one element selected from the group consisting of (1) at most 0.10% Nb, (2) at most 0.01% Ca, and (3) at least one of at most 1.0% Cr, at most 1.0% Mo, at most 1.0% Cu, and at most 1.0% Ni, with the balance being iron and unavoidable impurities. The steel is produced by heating bloom to 1,100°C or higher, subjecting the heated bloom to rough rolling, completing finish rolling at 750°C or higher, cooling the finish-rolled product at an average cooling rate of at least 5°C/sec, and winding the cooled product at 400 to 650°C. It is also possible that the rough rolling is performed after the heating of the bloom, the rough bar after the rough rolling is heated or kept at the same temperature, and the finish rolling is completed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、自動車足廻り部品などに使用され、その薄肉軽量化に寄与する高強度鋼板、特に熱延鋼板とその製造方法に関するものである。本発明にかかる鋼板は溶接性にも優れており、ホイール・リムなど溶接後成形されるような部品には最適である。
【０００２】
【従来の技術】
連続熱間圧延によって製造されるいわゆる熱延鋼板は、比較的安価な構造材料として、自動車をはじめとする各種の産業機器に広く使用されている。特に燃費低減に大きく寄与する自動車の足廻り部品への高強度熱延鋼板の適用が増加しつつある。最近は環境問題へのさらなる意識の高まりにより、熱延鋼板の高強度化の要望はさらに強くなり、超高強度である８８０ＭＰａ以上の引張強さを有する熱延鋼板の要望がでてきた。
【０００３】
８８０ＭＰａ以上の引張強さを有する熱延鋼板としては、例えば特許文献１に、低温巻取りにて変態強化を利用する方法が開示されている。しかしながらこの方法では、熱間圧延後の冷却過程で平坦が不芳になるという問題と、降伏点／引張強さである降伏比（ＹＲ）が低いという問題があった。降伏比が低いと、そのような材料を、例えば、ホイール・リムの製造に用いた場合、縁石衝突等の時にリムが大きく変形するなど、高強度化の十分な効果が得られない。さらにホイール・リムなど溶接後成形される部品に適用する場合、溶接時にＨＡＺ部の軟化が生じるため、その後の成形性に問題があった。
【０００４】
一方、析出強化を主体に高強度化する方法は、例えば特許文献２、特許文献３、特許文献４、特許文献５が開示されている。いずれもＴｉを主体とした析出強化を図ったものであるが、引張強さで８８０ＭＰａ以上の高強度は得られていない。
【０００５】
【特許文献１】特開２０００−２８２１７５号公報
【特許文献２】特開平６−２００３５１号公報
【特許文献３】特開平６−２２８７０８号公報
【特許文献４】特開平８−１９９２９８号公報
【特許文献５】特開平１１−１９３４４３号公報
【０００６】
【発明が解決しようとする課題】
本発明は、前述したような従来技術の問題点を解決し、圧延直角方向の引張強さ８８０ＭＰａ以上を有し、かつ降伏比０．８０以上を有する鋼板とその製造方法を提供する。
【０００７】
【課題を解決するための手段】
本発明者らはかかる課題実現のため鋭意実験の結果、引張強度で８８０ＭＰａ以上、かつ降伏比０．８以上の高強度鋼板を得るのには、Ｖの多量の添加が有効であることを見出した。その際、Ｔｉの複合添加と適切なＭｎ添加が必要であることを見出したのである。
【０００８】
すなわち、高強度鋼板の製造方法の観点からは、熱間圧延後の巻取温度は鋼板の強度に大きく影響するが、析出強化が有効に働く巻取温度域と、変態強化で高強度化に寄与する巻取温度域とはその領域が異なり、同時に両方を有効に働かせることが困難であった。例えば、変態強化を利用するために低温巻取りを行えば、前述の平坦不良や降伏比が低いという問題が生じた。しかし、巻取温度を上げ析出強化を利用して高強度化を図っても、ＴｉやＮｂなどのみでは、ある程度以上添加しても効果が飽和することと、変態強化の寄与が減少することにより、引張強さで８８０ＭＰａ以上の高強度が得られないという問題があった。
【０００９】
本発明者らは鋭意実験の結果、Ｖ析出物の析出温度が比較的低く、変態強化を働かせながら、析出強化も働かせることが可能であることを見出した。その効果を発揮させるためには、Ｔｉとの複合添加により析出を促進させることが有効で、さらにＭｎ量で変態点を最適化することが有効であった。
【００１０】
これらの結果を総合することにより、降伏比０．８以上で引張強度で８８０ＭＰａ以上の高強度鋼板とその製造方法についての本発明を完成するに到ったのである。
本発明は、このような知見に基づき完成した高強度鋼板およびその製造方法であって、以下の通りである。
【００１１】
（１）質量％で、Ｃ：０．０４〜０．１７％、Ｓｉ：１．１％以下、Ｍｎ：１．６〜２．６％、Ｐ：０．０５％以下、Ｓ０．０２％以下、Ａｌ：０．００１〜０．５０％、Ｎ：０．０２％以下、Ｖ：０．１１〜０．３０％、Ｔｉ：０．０７〜０．２５％を含み、残部が鉄および不可避不純物の鋼組成を有し、圧延直角方向で８８０ＭＰａ以上の引張強さを有し、降伏比０．８以上を有する高強度鋼板。
【００１２】
（２）前記鋼組成が、質量％で、さらにＮｂ：０．１０％以下を含む上記（１）記載の高強度鋼板。
（３）前記鋼組成が、質量％で、さらにＣａ：０．０１％以下を含む上記（１）または（２）記載の高強度鋼板。
【００１３】
（４）前記鋼組成が、質量％で、さらにＣｒ：１．０％以下、Ｍｏ：１．０％以下、Ｃｕ：１．０％以下、およびＮｉ：１．０％以下から選ばれた１種または２種以上を含む上記（１）〜（３）のいずれかに記載の高強度鋼板。
【００１４】
（５）上記（１）〜（４）記載の鋼板が熱延鋼板であることを特徴とする高強度鋼板。
（６）上記（１）〜（４）のいずれかに記載の鋼組成を有する鋼片を、１１００℃以上に加熱後、粗圧延を実施し、その後、７５０ ℃以上の仕上温度で熱間圧延を完了し、次いで平均冷却速度５℃／ｓ以上で冷却を実施し、４００〜６５０ ℃で巻き取ることを特徴とする圧延直角方向で８８０ＭＰａ以上の引張強さを有し、降伏比０．８以上を有する高強度鋼板の製造方法。
【００１５】
（７）上記（１）〜（４）のいずれかに記載の鋼組成を有する鋼片を、１１００℃以上に加熱後、粗圧延を実施し、粗圧延完了後の粗バーを加熱または保温して、その後、７５０ ℃以上の仕上温度で熱間圧延を完了し、次いで平均冷却速度５℃／ｓ以上で冷却を実施し、４００〜６５０ ℃で巻き取ることを特徴とする圧延直角方向で８８０ＭＰａ以上の引張強さを有し、降伏比０．８以上を有する高強度鋼板の製造方法。
【００１６】
【発明の実施の形態】
次に、本発明の実施の形態についてさらに具体的に詳細に説明する。なお、本明細書における鋼組成を示す「％」は特にことわりのない限り、「質量％」である。
【００１７】
（Ａ）鋼組成
Ｃ：Ｃは高強度化に必要な元素である。その含有量が０．０４％未満ではその効果が小さく、０．１７％を超えて含有すると熱間圧延後の冷却の際に平坦不良や特性バラツキ、さらに硬質第２相の増加により降伏比の低下が生じるため、０．０４〜０．１７％と定めた。好ましい下限は、０．０６％、さらに好ましくは０．０９％である。好ましい上限は０．１６％である。
【００１８】
Ｓｉ：Ｓｉは高強度化に有効な元素である。しかしながら多量の添加は化成処理性の低下や島状スケール疵と呼ばれる表面不良が著しくなるため、その含有量は１．１％以下と定めた。好ましくは０．６％以下、さらに好ましくは０．１％以下である。
【００１９】
Ｍｎ：Ｍｎは高強度化に必要な元素である。変態点を下げ、Ｖ析出物の析出状態を制御するのに寄与するとともに、変態強化によって高強度化にも寄与する。その効果は１．６％未満では得られない。また、２．６％を超えて含有すると熱間圧延での冷却での平坦不良や特性バラツキを生じさせるとともに、変態強化が主となって降伏比が低下する。したがって、その含有量を１．６〜２．６％と定めた。好ましくは下限が１．９％、上限が２．４％である。
【００２０】
Ｐ：Ｐは靱性を劣化させる好ましくない元素である。したがって、Ｐの含有量を０．０５％以下と定めた。ただし、Ｐにもスケールの剥離性を向上させる働きがあるので、０．００５〜０．０３％が好ましい範囲である。
【００２１】
Ｓ：ＳはＭｎＳ量を増加させ伸びフランジ性を低下させる好ましくない元素である。したがってその含有量は０．０２％以下と定めた。好ましくは０．００８％以下、さらに好ましくは０．００４％以下である。
【００２２】
Ａｌ：Ａｌは脱酸のため添加される。その効果は０．００１％未満では不十分で、また０．５０％を超えて含有すると溶接性が低下するため、０．００１〜０．５０％と定めた。好ましくは０．００１〜０．１％である。
【００２３】
Ｎ：Ｎは不純物元素である。したがって，含有量が高すぎると高温延性が低下し、表面疵の問題が発生するが、０．０２％までは許容される。
Ｔｉ：Ｔｉは本発明では重要な元素である。析出強化により溶接性の劣化を抑え、鋼板の高強度化に寄与するとともに、Ｖ析出物の析出核として働き、高強度化および析出による高降伏比達成に大きく寄与する。その効果は０．０７％未満では不十分で、添加量が多すぎても効果が飽和するので、その含有量を０．０７〜０．２５％と定めた。好ましくは０．０７〜０．１５％である。
【００２４】
Ｖ：Ｖは本発明で最も重要な元素である。比較的低い温度で析出し、高強度化に大きく寄与する。その効果は０．１１％未満では不十分である。また添加しすぎると、化成処理性が劣化してくるので、その含有量は０．１１〜０．３０％と定めた。好ましくは、０．１６〜０．３０％である。
【００２５】
Ｎｂ：Ｎｂは必要に応じて添加され、細粒化および析出強化で鋼板の高強度化に寄与する。添加しすぎても効果が飽和するので、その含有量は、０．１０％以下と定めた。好ましくは０．００１％以上である。さらに好ましくは０．００５％以上含有させるのが良い。
【００２６】
Ｍｏ、Ｃｒ、Ｃｕ、Ｎｉ：これらは必要に応じて１種または２種以上添加され、固溶強化、変態強化を通じて高強度化に寄与する。しかしながら、過剰に添加すると化成処理性の劣化や靱性の劣化をもたらすので、その含有量は、Ｍｏ：１．０％以下、Ｃｒ：１．０％以下、Ｃｕ：１．０％以下、Ｎｉ：１．０％以下と定めた。好ましくは、それぞれの下限は、Ｍｏ：０．００１％以上、Ｃｒ：０．００１％以上、Ｃｕ：０．００１％以上、Ｎｉ：０．００１％以上、さらに好ましくはＭｏ：０．０１％以上、Ｃｒ：０．０１％以上、Ｃｕ：０．０１％以上、Ｎｉ：０．０１％以上であり、少なくとも１種含有させるのが良い。
【００２７】
Ｃａ：Ｃａは必要に応じて添加され、硫化物の球状化を通じて、局部延性を向上させる。過剰に添加しても効果が飽和するので、その含有量を０．０１％以下と定めた。好ましくは０．０００１％以上、さらに好ましくは０．０００５％以上含有させるのが良い。
【００２８】
また，本発明にかかる鋼材をホイール・リムなど溶接を行ってから成形される部品に適用する場合は、式：Ｃ＋Ｓｉ／９０＋（Ｍｎ＋Ｃｒ）／１００の値を０．１８以下、さらに好ましくは０．１６以下にした鋼材を使用することが好ましい。
【００２９】
本発明における鋼組成は、残部はＦｅであり、不可避的不純物として、前述の窒素（Ｎ）、Ｐ、Ｓ等の他に、Ｂ、Ｓｎなどを合計０．０１％以下まではその存在は許容される。
【００３０】
以上のような鋼組成の鋼は、例えば転炉、電気炉により溶製される。そのときの鋼種も、リムド鋼、キャップド鋼、セミキルド鋼またはキルド鋼のいずれでもよい。さらに、鋼片の製造は、造塊−分塊圧延あるいは連続鋳造のいずれの手段によってもよい。
【００３１】
このようにして用意された鋼片は、以下に述べる条件によって熱間圧延、冷却、巻取りを行う。
（Ｂ）熱間条件
熱間圧延は、加熱温度１１００℃以上、仕上温度は７５０ ℃以上で実施される。１１００℃以上の加熱温度により、ＴｉやＶの析出物等を固溶させ、その後の再析出にて鋼板の強化に寄与させるのである。いわゆる析出強化である。
【００３２】
加熱炉に装入する鋼片（スラブ）は高温の熱片であってもよいし、室温まで冷やした冷片であってもよい。加熱後粗圧延を実施し、その後仕上圧延が施される。仕上温度が７５０ ℃未満では、バンド組織の過度の生成や、変態して生じたフェライトへの過度の歪の導入で、加工性が劣化するので好ましくない。
【００３３】
加熱後は、粗圧延を行うが、この粗圧延後、仕上温度やコイル内での温度の均一化を図る目的で、粗バーを保温または加熱するのも有効な手段である。このときの粗バーの加熱または保温は、粗バーヒータや保温カバーで実施しても良いし、粗バーをコイル状に巻き取って、炉に装入しても構わない。なお、エッジなど一部のみ加熱するのも、有効な手段である。
【００３４】
また、仕上圧延開始前に前後の粗バーを接合して連続的に通板することで熱間圧延の効率化を図っても何ら問題はない。
仕上げ圧延後、冷却を行ってコイル状に巻取られる。本発明によれば、このときの冷却速度はＴｉやＶの析出物の粗大化を抑えるために平均で５℃／ｓ以上とする。冷却速度の上限は特に規定しないが、平均冷却速度が１００ ℃／ｓを超えると平坦不良が発生しやすい。
【００３５】
このようにして冷却してから巻取りを行うが、そのときの巻取温度も本発明では重要で、圧延直角方向での引張強さで８８０ＭＰａ以上、降伏比０．８０以上を確保するには、巻取り温度４００〜６５０ ℃とする。巻取り温度が、４００ ℃未満では析出強化の働きが不十分で、また６５０ ℃を超えると析出物の粗大化が生じ、引張強さや降伏比の低下が生じる。好ましい巻取温度は４５０〜６５０ ℃、さらに好ましくは４５０〜５８０ ℃である。
【００３６】
本発明にかかる鋼板は、その用途として、自動車などの足廻り部品を考えると、その薄肉軽量化を実現するために、圧延直角方向で８８０ＭＰａ以上の引張強さを有し、降伏比０．８以上を有するものとする。鋼板の引張強さおよび降伏比は自動車足回り部品の衝撃強度へ大きく影響する。したがって、引張強さは９３０ＭＰａ以上、さらには９８０ＭＰａ以上であるのが好ましく、降伏比は０．８５以上であることが好ましい。
【００３７】
本発明にしたがって製造された鋼板、例えば熱延鋼板については、通常はスキンパス圧延による形状矯正や酸洗によるスケールの除去が行われ、表面には防錆油が塗布されて出荷される。
【００３８】
もちろん、このようにして製造された熱延鋼板は、必要によりさらに冷間圧延などの冷間加工を行ってもよく、したがって、本発明にかかる鋼板としては、熱延鋼版、冷延鋼板、溶融亜鉛めっき鋼板、合金化溶融亜鉛めっき鋼板、亜鉛系電気めっき鋼板のいずれも含む。通常、これらの鋼板は板厚が０．４〜２５ｍｍである。もちろん、本発明はその特性が発揮される限り、そのような板厚によって制限されるものではない。
【００３９】
次に、本発明の作用効果を実施例によってさらに具体的に説明する。
【実施例】
表１に示す鋼組成の鋼を転炉にて溶製後、連続鋳造にてスラブとした。そのスラブで粗圧延および連続熱間仕上圧延を実施し、板厚２．４ｍｍの熱延鋼板を製造した。
【００４０】
本例において採用した熱延条件は表２に示す。また一部の試験材では粗圧延後の粗バーにおいて、粗バーヒータで加熱を行った。粗バーヒータ使用の有無も表２に合わせて示す。
【００４１】
次いで、得られた熱延鋼板について、スキンパスでの平坦矯正および酸洗でのスケール除去を行い、圧延直角方向にＪＩＳ５号引張試験片を採取して引張試験を行った。結果は表３に示す。
【００４２】
これらの結果からも分かるように、本発明鋼である試験番号１〜４、８〜１５の場合は、０．８０以上の降伏比と８８０ＭＰａ以上の引張強さを示した。加熱温度が低く仕上温度が低い試験番号５の場合は８８０ＭＰａ以上の引張り強さが得られないとともに、伸びが低かった。巻取温度が高い試験番号６と巻取温度が低い試験番号７では、析出強化と変態強化が十分に発揮できず、引張り強さが低いとともに降伏比が低い。また巻取温度が低い試験番号７は大きな平坦不良が発生した。Ｍｎ含有量が高い試験番号１６とＣ量が高い試験番号２０は強度は高くはなるが、降伏比が下がった。Ｍｎ量の低い試験番号１７とＶ量が低い試験番号１８と、Ｔｉ量が低い試験番号１９では、引張り強さが低くなった。
【００４３】
【表１】

【００４４】
【表２】

【００４５】
【表３】

【００４６】
【発明の効果】
以上詳述したように、本発明により製造された鋼板は、圧延直角方向にて８８０ＭＰａ以上の引張強さと降伏比０．８０以上を有し、耐久性が問題となる自動車足回り部品、特に縁石強度が問題となるホイール・リムに最適で、かかる効果を有する本発明の意義は極めて著しい。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength steel sheet, particularly a hot-rolled steel sheet, and a method for producing the same, which are used in automobile undercarriage parts and the like and contribute to the reduction in thickness and weight. The steel sheet according to the present invention has excellent weldability and is optimal for parts such as wheels and rims that are formed after welding.
[0002]
[Prior art]
A so-called hot-rolled steel sheet manufactured by continuous hot rolling is widely used as a relatively inexpensive structural material in various industrial equipment including automobiles. In particular, the application of high-strength hot-rolled steel sheets to automobile undercarriage parts that greatly contributes to fuel efficiency reduction is increasing. Recently, due to further increase in awareness of environmental problems, the demand for higher strength of hot-rolled steel sheets has become stronger, and there has been a demand for hot-rolled steel sheets having a tensile strength of 880 MPa or more, which is ultra-high strength.
[0003]
As a hot-rolled steel sheet having a tensile strength of 880 MPa or more, for example, Patent Document 1 discloses a method using transformation strengthening by low-temperature winding. However, this method has a problem that flatness is unsatisfactory in the cooling process after hot rolling, and a yield ratio (YR) which is a yield point / tensile strength is low. If the yield ratio is low, when such a material is used, for example, in the manufacture of a wheel rim, a sufficient effect of increasing the strength cannot be obtained, for example, the rim is greatly deformed at the time of curb impact or the like. Furthermore, when applied to parts molded after welding, such as wheels and rims, the HAZ part is softened during welding, and there was a problem in subsequent formability.
[0004]
On the other hand, Patent Document 2, Patent Document 3, Patent Document 4, and Patent Document 5 are disclosed as methods for increasing the strength mainly by precipitation strengthening. In any case, precipitation strengthening mainly composed of Ti is attempted, but high strength of 880 MPa or more in tensile strength has not been obtained.
[0005]
[Patent Document 1] Japanese Patent Laid-Open No. 2000-282175 [Patent Document 2] Japanese Patent Laid-Open No. 6-200351 [Patent Document 3] Japanese Patent Laid-Open No. 6-228708 [Patent Document 4] Japanese Patent Laid-Open No. 8-199298 [Patent Document 3] Document 5 Japanese Patent Laid-Open No. 11-193443
[Problems to be solved by the invention]
The present invention solves the problems of the prior art as described above, and provides a steel sheet having a tensile strength of 880 MPa or more in the direction perpendicular to rolling and a yield ratio of 0.80 or more, and a method for producing the same.
[0007]
[Means for Solving the Problems]
As a result of diligent experiments, the present inventors have found that the addition of a large amount of V is effective for obtaining a high-strength steel sheet having a tensile strength of 880 MPa or more and a yield ratio of 0.8 or more. It was. At that time, it was found that Ti addition and appropriate Mn addition are necessary.
[0008]
That is, from the viewpoint of the manufacturing method of high-strength steel sheets, the coiling temperature after hot rolling greatly affects the strength of the steel sheet, but the coiling temperature range where precipitation strengthening works effectively, and transformation strengthening increases the strength. The contributing temperature range is different from the contributing temperature range, and it is difficult to make both work effectively at the same time. For example, if low-temperature winding is performed in order to use transformation strengthening, problems such as the above-described poor flatness and low yield ratio occur. However, even if the coiling temperature is increased and precipitation strengthening is used to increase the strength, only Ti or Nb or the like can saturate the effect even if it is added to a certain extent, and the contribution of transformation strengthening is reduced. There was a problem that a high strength of 880 MPa or more was not obtained in tensile strength.
[0009]
As a result of intensive experiments, the present inventors have found that the precipitation temperature of V precipitates is relatively low, and precipitation strengthening can be performed while transformation strengthening works. In order to exert the effect, it was effective to promote precipitation by combined addition with Ti, and it was also effective to optimize the transformation point with the amount of Mn.
[0010]
By combining these results, the present invention for a high-strength steel sheet having a yield ratio of 0.8 or more and a tensile strength of 880 MPa or more and its manufacturing method has been completed.
The present invention is a high-strength steel sheet completed based on such knowledge and a method for producing the same, as follows.
[0011]
(1) By mass%, C: 0.04 to 0.17%, Si: 1.1% or less, Mn: 1.6 to 2.6%, P: 0.05% or less, S 0.02% or less , Al: 0.001 to 0.50%, N: 0.02% or less, V: 0.11 to 0.30%, Ti: 0.07 to 0.25%, the balance being iron and inevitable impurities A high-strength steel sheet having a steel composition of 980 MPa, a tensile strength of 880 MPa or more in the direction perpendicular to the rolling, and a yield ratio of 0.8 or more.
[0012]
(2) The high-strength steel sheet according to (1), wherein the steel composition is mass% and further includes Nb: 0.10% or less.
(3) The high-strength steel sheet according to the above (1) or (2), wherein the steel composition is mass% and further contains Ca: 0.01% or less.
[0013]
(4) The steel composition is 1% selected by mass% and further selected from Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, and Ni: 1.0% or less. The high-strength steel sheet according to any one of the above (1) to (3), comprising seeds or two or more kinds.
[0014]
(5) A high-strength steel sheet, wherein the steel sheet described in (1) to (4) above is a hot-rolled steel sheet.
(6) The steel slab having the steel composition described in any one of (1) to (4) above is heated to 1100 ° C. or higher, followed by rough rolling, and then hot rolled at a finishing temperature of 750 ° C. or higher. And then cooled at an average cooling rate of 5 ° C./s or more, and wound at 400 to 650 ° C. and has a tensile strength of 880 MPa or more in the direction perpendicular to the rolling, and a yield ratio of 0.8 The manufacturing method of the high strength steel plate which has the above.
[0015]
(7) The steel slab having the steel composition described in any one of (1) to (4) above is heated to 1100 ° C. or higher, followed by rough rolling, and the rough bar after the completion of rough rolling is heated or kept warm. Thereafter, hot rolling is completed at a finishing temperature of 750 ° C. or higher, then cooling is performed at an average cooling rate of 5 ° C./s or higher, and winding is performed at 400 to 650 ° C. The manufacturing method of the high strength steel plate which has the above tensile strength and has a yield ratio 0.8 or more.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described in more detail. In this specification, “%” indicating the steel composition is “% by mass” unless otherwise specified.
[0017]
(A) Steel composition C: C is an element necessary for increasing strength. If the content is less than 0.04%, the effect is small, and if it exceeds 0.17%, the yield ratio is reduced due to poor flatness and characteristic variations during cooling after hot rolling, and further, the increase in the hard second phase. Since the reduction occurs, it is determined to be 0.04 to 0.17%. A preferable lower limit is 0.06%, more preferably 0.09%. A preferable upper limit is 0.16%.
[0018]
Si: Si is an element effective for increasing the strength. However, when a large amount is added, the chemical conversion processability deteriorates and the surface defect called island-shaped scale wrinkles becomes remarkable, so the content is determined to be 1.1% or less. Preferably it is 0.6% or less, More preferably, it is 0.1% or less.
[0019]
Mn: Mn is an element necessary for increasing the strength. This contributes to lowering the transformation point and controlling the precipitation state of V precipitates, and also contributes to higher strength by strengthening transformation. The effect cannot be obtained at less than 1.6%. On the other hand, if the content exceeds 2.6%, flattening defects and characteristic variations are caused by cooling in hot rolling, and the yield ratio decreases mainly due to transformation strengthening. Therefore, the content was determined to be 1.6 to 2.6%. Preferably, the lower limit is 1.9% and the upper limit is 2.4%.
[0020]
P: P is an undesirable element that deteriorates toughness. Therefore, the content of P is set to 0.05% or less. However, P also has a function of improving the peelability of the scale, so 0.005 to 0.03% is a preferable range.
[0021]
S: S is an undesirable element that increases the amount of MnS and decreases the stretch flangeability. Accordingly, the content is determined to be 0.02% or less. Preferably it is 0.008% or less, More preferably, it is 0.004% or less.
[0022]
Al: Al is added for deoxidation. The effect is insufficient if it is less than 0.001%, and if it exceeds 0.50%, the weldability is lowered, so 0.001 to 0.50% was set. Preferably it is 0.001 to 0.1%.
[0023]
N: N is an impurity element. Therefore, if the content is too high, the high temperature ductility is lowered and the problem of surface flaws occurs, but up to 0.02% is allowed.
Ti: Ti is an important element in the present invention. Precipitation strengthening suppresses weldability deterioration and contributes to increasing the strength of the steel sheet, and also serves as a precipitation nucleus for V precipitates, greatly contributing to increasing the strength and achieving a high yield ratio by precipitation. The effect is insufficient if it is less than 0.07%, and the effect is saturated even if the addition amount is too large. Therefore, the content is determined to be 0.07 to 0.25%. Preferably it is 0.07 to 0.15%.
[0024]
V: V is the most important element in the present invention. It precipitates at a relatively low temperature and greatly contributes to high strength. The effect is insufficient if it is less than 0.11%. Moreover, since chemical conversion processability will deteriorate when it adds too much, the content was defined as 0.11-0.30%. Preferably, it is 0.16 to 0.30%.
[0025]
Nb: Nb is added as necessary, and contributes to increasing the strength of the steel sheet by refining and precipitation strengthening. Even if it is added too much, the effect is saturated, so the content was determined to be 0.10% or less. Preferably it is 0.001% or more. More preferably, 0.005% or more is contained.
[0026]
Mo, Cr, Cu, Ni: These are added in one or more as necessary, and contribute to high strength through solid solution strengthening and transformation strengthening. However, excessive addition causes deterioration of chemical conversion properties and toughness, so that the content thereof is Mo: 1.0% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: It was determined to be 1.0% or less. Preferably, the respective lower limits are Mo: 0.001% or more, Cr: 0.001% or more, Cu: 0.001% or more, Ni: 0.001% or more, more preferably Mo: 0.01% or more. Cr: 0.01% or more, Cu: 0.01% or more, Ni: 0.01% or more, and it is preferable to contain at least one kind.
[0027]
Ca: Ca is added as necessary to improve local ductility through spheroidization of sulfides. Even if added excessively, the effect is saturated, so the content was determined to be 0.01% or less. The content is preferably 0.0001% or more, more preferably 0.0005% or more.
[0028]
In addition, when the steel material according to the present invention is applied to a part formed after welding such as a wheel / rim, the value of the formula: C + Si / 90 + (Mn + Cr) / 100 is 0.18 or less, more preferably 0. It is preferable to use a steel material having a thickness of 16 or less.
[0029]
In the steel composition of the present invention, the balance is Fe, and in addition to the aforementioned nitrogen (N), P, S, etc., the presence of B, Sn, etc. is allowed up to a total of 0.01% or less as unavoidable impurities. Is done.
[0030]
The steel having the above steel composition is produced by, for example, a converter or an electric furnace. The steel type at that time may be any of rimmed steel, capped steel, semi-killed steel or killed steel. Further, the steel slab may be manufactured by any means of ingot-splitting rolling or continuous casting.
[0031]
The steel slab thus prepared is subjected to hot rolling, cooling and winding under the conditions described below.
(B) Hot conditions Hot rolling is performed at a heating temperature of 1100 ° C or higher and a finishing temperature of 750 ° C or higher. By heating at 1100 ° C. or higher, Ti and V precipitates are dissolved, and subsequent reprecipitation contributes to strengthening of the steel sheet. This is so-called precipitation strengthening.
[0032]
The steel piece (slab) charged into the heating furnace may be a hot hot piece or a cold piece cooled to room temperature. After heating, rough rolling is performed, and then finish rolling is performed. If the finishing temperature is less than 750 ° C., the workability deteriorates due to excessive generation of band structure or introduction of excessive strain to the ferrite generated by transformation, which is not preferable.
[0033]
After the heating, rough rolling is performed, and after the rough rolling, it is also effective to keep or heat the rough bar for the purpose of uniformizing the finishing temperature and the temperature in the coil. The heating or heat retention of the coarse bar at this time may be performed with a coarse bar heater or a heat insulation cover, or the coarse bar may be wound into a coil and charged into a furnace. Note that it is also an effective means to heat only a part such as an edge.
[0034]
Moreover, there is no problem even if the efficiency of hot rolling is improved by joining the front and rear rough bars and continuously passing them before finishing rolling.
After finish rolling, it is cooled and wound into a coil. According to the present invention, the cooling rate at this time is 5 ° C./s or more on average in order to suppress coarsening of precipitates of Ti and V. The upper limit of the cooling rate is not particularly specified, but flatness is liable to occur when the average cooling rate exceeds 100 ° C./s.
[0035]
Winding is performed after cooling in this way, and the winding temperature at that time is also important in the present invention, and in order to ensure a tensile strength in the direction perpendicular to the rolling of 880 MPa or more and a yield ratio of 0.80 or more. The winding temperature is 400 to 650 ° C. When the coiling temperature is less than 400 ° C., the effect of precipitation strengthening is insufficient, and when it exceeds 650 ° C., coarsening of the precipitate occurs, and the tensile strength and the yield ratio are reduced. The winding temperature is preferably 450 to 650 ° C, more preferably 450 to 580 ° C.
[0036]
The steel plate according to the present invention has a tensile strength of 880 MPa or more in the direction perpendicular to the rolling and a yield ratio of 0.8 in order to realize thinning and light weight when considering an undercarriage part such as an automobile. It shall have the above. The tensile strength and yield ratio of steel sheets have a great influence on the impact strength of automobile undercarriage parts. Accordingly, the tensile strength is preferably 930 MPa or more, more preferably 980 MPa or more, and the yield ratio is preferably 0.85 or more.
[0037]
Steel sheets manufactured according to the present invention, for example, hot-rolled steel sheets, are usually shipped by applying a rust-preventive oil to the surface after being subjected to shape correction by skin pass rolling or removal of scale by pickling.
[0038]
Of course, the hot-rolled steel sheet produced in this way may be further subjected to cold working such as cold rolling if necessary. Therefore, as the steel sheet according to the present invention, a hot-rolled steel plate, a cold-rolled steel sheet, All of hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, and zinc-based electroplated steel sheet are included. Usually, these steel plates have a plate thickness of 0.4 to 25 mm. Of course, the present invention is not limited by such a plate thickness as long as the characteristics are exhibited.
[0039]
Next, the effects of the present invention will be described more specifically with reference to examples.
【Example】
Steels having the steel compositions shown in Table 1 were melted in a converter and then made into slabs by continuous casting. The slab was subjected to rough rolling and continuous hot finish rolling to produce a hot-rolled steel sheet having a thickness of 2.4 mm.
[0040]
Table 2 shows the hot rolling conditions employed in this example. In some of the test materials, the rough bar after the rough rolling was heated with a coarse bar heater. Table 2 also shows whether or not a coarse bar heater is used.
[0041]
Next, the obtained hot-rolled steel sheet was subjected to flattening with a skin pass and scale removal by pickling, and a JIS No. 5 tensile test piece was taken in the direction perpendicular to the rolling and subjected to a tensile test. The results are shown in Table 3.
[0042]
As can be seen from these results, the test numbers 1 to 4 and 8 to 15 which are the steels of the present invention showed a yield ratio of 0.80 or more and a tensile strength of 880 MPa or more. In the case of test number 5 where the heating temperature was low and the finishing temperature was low, a tensile strength of 880 MPa or more was not obtained, and the elongation was low. In Test No. 6 having a high coiling temperature and Test No. 7 having a low coiling temperature, precipitation strengthening and transformation strengthening cannot be sufficiently exhibited, and the tensile strength is low and the yield ratio is low. In test number 7 where the coiling temperature was low, a large flatness occurred. Test No. 16 having a high Mn content and Test No. 20 having a high C content have a high strength but a reduced yield ratio. In the test number 17 having a low Mn amount, the test number 18 having a low V amount, and the test number 19 having a low Ti amount, the tensile strength was low.
[0043]
[Table 1]

[0044]
[Table 2]

[0045]
[Table 3]

[0046]
【The invention's effect】
As described above in detail, the steel sheet produced according to the present invention has a tensile strength of 880 MPa or more and a yield ratio of 0.80 or more in the direction perpendicular to the rolling direction, and especially an automobile underbody part, particularly curbstone, in which durability is a problem. The significance of the present invention, which is optimal for a wheel rim in which strength is a problem and has such an effect, is extremely remarkable.

Claims

In mass%, C: 0.04 to 0.17%, Si: 1.1% or less, Mn: 1.6 to 2.6%, P: 0.05% or less, S 0.02% or less, Al: Steel composition containing 0.001 to 0.50%, N: 0.02% or less, V: 0.11 to 0.30%, Ti: 0.07 to 0.25%, the balance being iron and inevitable impurities A high-strength steel sheet having a tensile strength of 880 MPa or more in the direction perpendicular to the rolling and a yield ratio of 0.8 or more.

The high-strength steel sheet according to claim 1, wherein the steel composition includes mass% and further includes Nb: 0.10% or less.

The high-strength steel sheet according to claim 1 or 2, wherein the steel composition includes mass% and further contains Ca: 0.01% or less.

The steel composition is 1% or 2 selected by mass% and further selected from Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, and Ni: 1.0% or less. The high-strength steel sheet according to any one of claims 1 to 3, comprising at least a seed.

A high-strength steel sheet, wherein the steel sheet according to claim 1 is a hot-rolled steel sheet.

The steel slab having the steel composition according to any one of claims 1 to 4 is heated to 1100 ° C or higher, and then subjected to rough rolling, and then hot rolling is completed at a finishing temperature of 750 ° C or higher, and then averaged A high-strength steel sheet having a tensile strength of 880 MPa or more in the direction perpendicular to the rolling and having a yield ratio of 0.8 or more, characterized by cooling at a cooling rate of 5 ° C./s or more and winding at 400 to 650 ° C. Manufacturing method.

The steel slab having the steel composition according to any one of claims 1 to 4 is heated to 1100 ° C or higher, followed by rough rolling, and the rough bar after the completion of rough rolling is heated or kept warm, and then 750 ° C. Hot rolling is completed at the above finishing temperature, then cooling is performed at an average cooling rate of 5 ° C./s or more, and winding is performed at 400 to 650 ° C., and a tensile strength of 880 MPa or more is obtained in the direction perpendicular to rolling. A method for producing a high strength steel sheet having a yield ratio of 0.8 or more.