JP3591502B2

JP3591502B2 - High-tensile steel sheet excellent in workability, and its manufacturing method and processing method

Info

Publication number: JP3591502B2
Application number: JP2001334464A
Authority: JP
Inventors: 毅塩崎; 義正船川; 孝信斉藤; 英司前田; 徹夫山本; 安浩村尾; 邦和冨田; 敬士山下; 博司益本
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2001-02-20
Filing date: 2001-10-31
Publication date: 2004-11-24
Anticipated expiration: 2021-10-31
Also published as: JP2002322543A

Description

【０００１】
【発明の属する技術分野】
本発明は、自動車用部材の素材に適した加工性に優れた高張力鋼板ならびにその製造方法および加工方法に関する。
【０００２】
【従来技術】
環境保全につながる燃費向上の観点から、自動車用鋼板の高強度薄肉化が強く求められている。自動車用部材はプレス成形により得られる複雑な形状のものが多く、高強度でありながら加工性の指標である伸びと伸びフランジ性がともに優れた材料が必要である。また、鋼板をより軽量化する観点からさらなる薄肉化が指向されており、板厚２．５ｍｍ以下の薄物に対する要望も強くなってきている。
【０００３】
従来、この種の鋼板は種々提案されており、例えば、特開平６−１７２９２４号公報には、転位密度の高いベイニティック・フェライト組織が生成した伸びフランジ性に優れる鋼板が提案されている。しかし、この鋼板は、転位密度の高いベイニティック・フェライト組織を含むため伸びが乏しいという欠点がある。また、ベイニティック・フェライト生成のためにランナウトテーブル上での強冷却が不可避であり薄物製造時にはランナウトテーブルでのストリップの走行性に問題が生じるため、板厚２．５ｍｍ以下といった薄物を生産するには不向きである。
【０００４】
特開平６−２００３５１号公報には、組織の大部分をポリゴナルフェライトとし、ＴｉＣを中心として析出強化および固溶強化した伸びフランジ性に優れる鋼板が提案されている。しかし、この鋼板に用いられている一般的によく知られた析出物で高張力化するには多量のＴｉ添加が必要とし、寸法の大きい析出物が生成しやすく、特性が不安定になりやすいという欠点がある。また、この鋼は特性向上のために圧延荷重を増大させるＳｉを積極的に用いているため、薄物の製造において圧延荷重が増大し、鋼板形状確保が難しい。
【０００５】
特開平７−１１３８２号公報には、微細なＴｉＣおよび／またはＮｂＣが析出したアシキュラー・フェライト組織を有した伸びフランジ性に優れる鋼板が提案されている。しかし、この鋼板も、先に述べた特開平６−１７２９２４号公報に提案された鋼板同様、アシキュラー・フェライトという転位密度の高い組織であるため十分な伸びが得られていない。また、この鋼は特開平６−２００３５１号公報に開示された鋼と同様に、特性向上のために圧延荷重を増大させるＳｉを積極的に用いているため、薄物の製造において圧延荷重が増大し、鋼板形状確保が難しい。
【０００６】
【発明が解決しようとする課題】
本発明はかかる事情に鑑みてなされたものであって、自動車用部材のようにプレス時の断面形状が複雑な用途に適した、加工性の指標である伸びと伸びフランジ性がともに優れた高張力鋼板ならびにその製造方法および加工方法を提供することを目的とする。
【０００７】
【課題を解決するための手段】
本発明者らは、上記目的を達成すべく鋭意検討を行った結果、以下の知見を得た。
（１）転位密度が低い組織とし、微細析出物で強化すると、強度−伸びバランスが向上する。
（２）実質的に単相組織とし、微細析出物で強化すると、強度−伸びフランジ性バランスが向上する。
（３）Ｍｏを含む複合析出物とすると、析出物が微細に析出する。
（４）複合析出物中のＭｏの割合が低くなると、析出物が粗大化するため、伸びと伸びフランジ性がともに低下する。
【０００８】
本発明はこれらの知見に基づいて完成されたものであり、以下の（１）〜（２２）を提供する。
【０００９】
（１）実質的にフェライト単相組織であり、平均粒径１０ｎｍ未満のＴｉおよびＭｏを含む炭化物が分散析出していることを特徴とする、引張強度が５９０ＭＰａ以上の加工性に優れた高張力鋼板。
【００１０】
（２）上記（１）において、ＴｉおよびＭｏを含む炭化物は、原子％で表されるＴｉ、Ｍｏが、Ｍｏ／（Ｔｉ＋Ｍｏ）≧０．２５を満たす組成を有することを特徴とする加工性に優れた高張力鋼板。
【００１１】
（３）上記（１）または（２）において、鋼中のＣ，Ｔｉ，Ｍｏを規定する以下の（１）式を満足することを特徴とする加工性に優れた高張力鋼板。
０．８≦（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｍｏ／９６）｝≦１．５ …（１）
ただし、上記（１）式中、Ｃ、Ｔｉ、Ｍｏは各成分の質量％を表す。
【００１２】
（４）上記（１）〜（３）のいずれかにおいて、鋼中に質量％で、Ｃ：０．０２〜０．０６％、Ｍｏ：０．０５〜０．５％、Ｔｉ：０．０３〜０．１４％を含むことを特徴とする加工性に優れた高張力鋼板。
【００１３】
（５）質量％で、Ｃ：０．０２〜０．０６％、Ｓｉ≦０．３％、Ｍｎ：０．５〜２．０％、Ｐ≦０．０６％、Ｓ≦０．００５％、Ａｌ≦０．０６％、Ｎ≦０．００６％、Ｍｏ：０．０５〜０．５％、Ｔｉ：０．０３〜０．１４％を含み、残部が実質的にＦｅであり、実質的にフェライト単相組織であり、原子％で表されるＴｉ、Ｍｏが、Ｍｏ／（Ｔｉ＋Ｍｏ）≧０．２５を満たす炭化物組成を有するＴｉおよびＭｏを含む炭化物が分散析出していることを特徴とする、引張強度が５９０ＭＰａ以上の加工性に優れた高張力鋼板。
【００１４】
（６）質量％で、Ｃ：０．０２〜０．０６％、Ｓｉ≦０．３％、Ｍｎ：０．５〜２．０％、Ｐ≦０．０６％、Ｓ≦０．００５％、Ａｌ≦０．０６％、Ｎ≦０．００６％、Ｍｏ：０．０５〜０．５％、Ｔｉ：０．０３〜０．１４％を含み、残部が実質的にＦｅであり、実質的にフェライト単相組織でかつ鋼中のＣ，Ｔｉ，Ｍｏを規定する以下の（１）式を満足し、ＴｉおよびＭｏを含む炭化物が分散析出していることを特徴とする、引張強度が５９０ＭＰａ以上の加工性に優れた高張力鋼板。
０．８≦（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｍｏ／９６）｝≦１．５ …（１）
ただし、上記（１）式中、Ｃ、Ｔｉ、Ｍｏは各成分の質量％を表す。
【００１５】
（７）上記（１）〜（３）のいずれかにおいて、質量％で、Ｃ：０．０２〜０．０６％、Ｓｉ≦０．３％、Ｍｎ：０．５〜２．０％、Ｐ≦０．０６％、Ｓ≦０．００５％、Ａｌ≦０．０６％、Ｎ≦０．００６％、Ｍｏ：０．０５〜０．５％、Ｔｉ：０．０３〜０．１４％を含み、残部が実質的にＦｅであることを特徴とする加工性に優れた高張力鋼板。
【００１６】
（８）上記（２）または（５）において、前記炭化物はＴｉ、Ｍｏに加え、ＮｂおよびＶの１種以上を含み、原子％で表されるＴｉ、Ｍｏが、前記Ｍｏ／（Ｔｉ＋Ｍｏ）≧０．２５を満たすことに代えて、原子％で表されるＴｉ、Ｍｏ、Ｎｂ、ＶがＭｏ／（Ｔｉ＋Ｎｂ＋Ｖ＋Ｍｏ）≧０．２５を満たす炭化物組成を有することを特徴とする加工性に優れた高張力鋼板。
【００１７】
（９）上記（１）において、前記炭化物はＴｉ、Ｍｏに加え、ＮｂおよびＶの１種以上を含むことを特徴とする、加工性に優れた高張力鋼板。
【００１８】
（１０）上記（８）または（９）において、鋼中のＣ、Ｔｉ、Ｎｂ、Ｖ、Ｍｏを規定する以下の（２）式を満足することを特徴とする加工性に優れた高張力鋼板。
０．８≦（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｎｂ／９３）＋（Ｖ／５１）＋（Ｍｏ／９６）｝≦１．５ …（２）
ただし、上記（２）式中、Ｃ、Ｔｉ、Ｎｂ、Ｖ、Ｍｏは各成分の質量％を表す。
【００１９】
（１１）上記（３）または（６）において、上記（１）式に代えて、鋼中のＣ、Ｔｉ、Ｎｂ、Ｖ、Ｍｏを規定する以下の（２）式を満足し、前記炭化物はＴｉ、Ｍｏに加え、ＮｂおよびＶの１種以上を含むことを特徴とする加工性に優れた高張力鋼板。
０．８≦（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｎｂ／９３）＋（Ｖ／５１）＋（Ｍｏ／９６）｝≦１．５ …（２）
ただし、上記（２）式中、Ｃ、Ｔｉ、Ｎｂ、Ｖ、Ｍｏは各成分の質量％を表す。
【００２０】
（１２）上記（９）〜（１１）のいずれかにおいて、前記炭化物は、原子％で表されるＴｉ、Ｍｏ、Ｎｂ、ＶがＭｏ／（Ｔｉ＋Ｎｂ＋Ｖ＋Ｍｏ）≧０．２５を満たす炭化物組成を有することを特徴とする加工性に優れた高張力鋼板。
【００２１】
（１３）上記（８）および（１０）〜（１２）のいずれかにおいて、鋼中に質量％で、Ｃ：０．０２〜０．０６％、Ｍｏ：０．０５〜０．５％、Ｔｉ：０．０３〜０．１４％を含み、さらに質量％でＮｂ≦０．０８％、Ｖ≦０．１５％のうち１種以上を含むことを特徴とする加工性に優れた高張力鋼板。
【００２２】
（１４）上記（８）〜（１２）のいずれかにおいて、質量％で、Ｃ：０．０２〜０．０６％、Ｓｉ≦０．３％、Ｍｎ：０．５〜２．０％、Ｐ≦０．０６％、Ｓ≦０．００５％、Ａｌ≦０．０６％、Ｎ≦０．００６％、Ｍｏ：０．０５〜０．５％、Ｔｉ：０．０３〜０．１４％を含み、Ｎｂ≦０．０８％、Ｖ≦０．１５％のうち１種以上を含有し、残部が実質的にＦｅであることを特徴とする加工性に優れた高張力鋼板。
【００２３】
（１５）上記（４）において、前記炭化物はＴｉ、Ｍｏに加え、ＮｂおよびＶの１種以上を含み、鋼中に質量％でＮｂ≦０．０８％、Ｖ≦０．１５％のうち１種以上を含むことを特徴とする加工性に優れた高張力鋼板。
【００２４】
（１６）上記（１）〜（１５）のいずれかにおいて、板厚２．５ｍｍ以下の薄物熱延鋼板であることを特徴とする加工性に優れた高張力鋼板。
【００２５】
（１７）上記（１）〜（１６）のいずれかにおいて、表面に溶融亜鉛系めっき皮膜を有することを特徴とする加工性に優れた高張力鋼板。
【００２６】
（１８）質量％で、Ｃ：０．０２〜０．０６％、Ｓｉ≦０．３％、Ｍｎ：０．５〜２．０％、Ｐ≦０．０６％、Ｓ≦０．００５％、Ａｌ≦０．０６％、Ｎ≦０．００６％、Ｍｏ：０．０５〜０．５％、Ｔｉ：０．０３〜０．１４％を含み、残部が実質的にＦｅからなる鋼を溶製し、仕上圧延終了温度８８０℃以上、巻取温度５７０℃以上の条件で熱間圧延を行い、実質的にフェライト単相組織であり、平均粒径１０ｎｍ未満のＴｉおよびＭｏを含む炭化物が分散析出している鋼板を得ることを特徴とする、引張強度が５９０ＭＰａ以上の加工性に優れた高張力鋼板の製造方法。
【００２７】
（１９）質量％で、Ｃ：０．０２〜０．０６％、Ｓｉ≦０．３％、Ｍｎ：０．５〜２．０％、Ｐ≦０．０６％、Ｓ≦０．００５％、Ａｌ≦０．０６％、Ｎ≦０．００６％、Ｍｏ：０．０５〜０．５％、Ｔｉ：０．０３〜０．１４％を含み、Ｎｂ≦０．０８％、Ｖ≦０．１５％のうち１種以上を含有し、残部が実質的にＦｅからなる鋼を溶製し、仕上圧延終了温度８８０℃以上、巻取温度５７０℃以上の条件で熱間圧延を行い、実質的にフェライト単相組織であり、平均粒径１０ｎｍ未満のＴｉおよびＭｏを含む炭化物が分散析出している鋼板を得ることを特徴とする、引張強度が５９０ＭＰａ以上の加工性に優れた高張力鋼板の製造方法。
【００２８】
（２０）上記（１）〜（１７）のいずれかの高張力鋼板からなる部材を準備する第１の工程と、前記部材にプレス成形を施して所望の形状のプレス成形品に加工する第２の工程とを有する高張力鋼板の加工方法。
【００２９】
（２１）上記（２０）において、前記プレス成形品は、自動車用部品、特に自動車用足廻り部材である高張力鋼板の加工方法。
【００３０】
（２２）上記（１）〜（１７）のいずれかの高張力鋼板により製造される自動車用部品。
【００３１】
なお、本発明において実質的にフェライト単相組織とは、本発明の析出物以外に、微量の他の相ないしは析出物を許容することをいい、好ましくはフェライトの面積比率が９５％以上である。
【００３２】
【発明の実施の形態】
以下、本発明について、金属組織、化学成分、および製造方法に分けて具体的に説明する。
【００３３】
［金属組織］
本発明に係る高張力鋼板は、実質的にフェライト単相組織であり、ＴｉおよびＭｏを含む炭化物が分散析出している。
【００３４】
・実質的にフェライト単相組織：
マトリックスを実質的にフェライト単相組織としたのは、伸びの向上には転位密度の低いフェライトが有効であり、また、伸びフランジ性の向上には単相組織とすることが有効であり、特に延性に富むフェライト単相組織でその効果が顕著であるためである。ただし、マトリックスは必ずしも完全にフェライト単相組織でなくともよく、実質的にフェライト単相組織、好ましくは面積比率で９５％以上フェライトであればよい。
【００３５】
・ＴｉおよびＭｏを含む炭化物：
ＴｉとＭｏとを含む炭化物は微細となるため鋼を強化するのに有効である。従来は、析出物としてＴｉＣを用いることが主流であったが、Ｔｉは析出物形成傾向が強いためＭｏを含まない場合、粗大化しやすく、強化に対する効果が低くなることから、必要な強化量を得るには加工性を劣化させるまでの析出物が必要となる。これに対し、ＴｉとＭｏとを含む複合炭化物は微細に析出して加工性を劣化させずに鋼を強化することができる。これは、Ｍｏの析出物形成傾向がＴｉと比べて弱いため、安定的に微細に存在できることで強化に対する効果が高く、加工性を良好に維持できる析出物量で必要な強化量が得られるためと考えられる。
【００３６】
炭化物が安定的に微細に存在できるためには、炭化物の組成が影響し、炭化物の組成が、原子％で表されるＭｏ、Ｔｉが、Ｍｏ／（Ｔｉ＋Ｍｏ）≧０．２５を満たすようになると、析出物の粗大化を抑制する効果が高くなり、所望の微細析出物を得ることができる。したがって、本発明の第１の観点では、原子％で表されるＴｉ、Ｍｏが、Ｍｏ／（Ｔｉ＋Ｍｏ）≧０．２５を満たす範囲でＴｉおよびＭｏを含む炭化物が分散析出していることを要件とする。
【００３７】
また、この複合炭化物の平均粒径を１０ｎｍ未満とすることで、析出物周囲の歪みが転位の移動の抵抗にとってより効果的となり、良好な鋼の強化が得られる。このため、本発明の第２の観点では、平均粒径１０ｎｍ未満のＴｉおよびＭｏを含む炭化物が分散析出していることを要件とする。さらに好ましくは、平均粒径５ｎｍ以下である。
【００３８】
さらに、鋼中のＣと（Ｔｉ＋Ｍｏ）との原子数比が０．８〜１．５となるように、Ｃ、Ｔｉ、Ｍｏの含有量を調整することにより、ＴｉとＭｏとを含む炭化物が微細に析出しやすくなり、１０ｎｍ未満の微細析出物の形成が容易となることから、本発明の第３の観点では、鋼中のＣと（Ｔｉ＋Ｍｏ）との原子数比が０．８から１．５であり、ＴｉおよびＭｏを含む炭化物が分散析出していることを要件とする。上記原子数比は０．８〜１．３がより好ましい。なお、上記Ｃと（Ｔｉ＋Ｍｏ）との原子数比０．８〜１．５を質量％換算すると、以下の（１）式を満たすこととなる。
０．８≦（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｍｏ／９６）｝≦１．５ …（１）
ただし、上記（１）式中、Ｃ、Ｔｉ、Ｍｏは各成分の質量％を表す。
【００３９】
炭化物がＴｉとＭｏに加え、ＮｂおよびＶの１種以上が複合して析出したものであっても、Ｍｏの析出物形成傾向はＮｂ、Ｖと比べて弱いため、その複合析出物はＴｉとＭｏの複合炭化物と同様に、安定的に微細に存在できる。このため、炭化物としては、ＴｉとＭｏの他にＮｂおよびＶの１種以上が複合析出したものであってもかまわない。
【００４０】
複合炭化物が、Ｔｉ、Ｍｏに加え、ＮｂおよびＶの１種以上を含むものである場合、上記第１の観点では、その組成が、原子％で表されるＴｉ、Ｍｏが、前記Ｍｏ／（Ｔｉ＋Ｍｏ）≧０．２５を満たすことに代えて、原子％で表されるＴｉ、Ｍｏ、Ｎｂ、ＶがＭｏ／（Ｔｉ＋Ｎｂ＋Ｖ＋Ｍｏ）≧０．２５を満たすことが好ましい。この範囲であれば、複合析出物の粗大化を抑制する効果が高く、加工性を良好に維持することができる析出物量で必要な強化量を得ることができる。上記第２および第３の観点においても、Ｍｏ／（Ｔｉ＋Ｎｂ＋Ｖ＋Ｍｏ）≧０．２５を満たすことが好ましい。
【００４１】
このように、ＴｉとＭｏに加え、ＮｂおよびＶの１種以上が複合して析出した複合炭化物を微細に分散析出させやすくさせるためには、上記第３の観点における、鋼中のＣと（Ｔｉ＋Ｍｏ）との原子数比に代えて、Ｃと（Ｔｉ＋Ｎｂ＋Ｖ＋Ｍｏ）との原子数比が０．８〜１．５となるようにすることが好ましい。上記第１および第２の観点においても、Ｃと（Ｔｉ＋Ｎｂ＋Ｖ＋Ｍｏ）との原子数比が０．８〜１．５となるようにすることが好ましい。上記原子数比は０．８〜１．３がより好ましい。なお、上記Ｃと（Ｔｉ＋Ｎｂ＋Ｖ＋Ｍｏ）との原子数比０．８〜１．５を質量％換算すると、以下の（２）式を満たすこととなる。
０．８≦（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｎｂ／９３）＋（Ｖ／５１）＋（Ｍｏ／９６）｝≦１．５ …（２）
ただし、上記（２）式中、Ｃ、Ｔｉ、Ｎｂ、Ｖ、Ｍｏは各成分の質量％を表す。
【００４２】
［化学成分］
本発明では、上記金属組織さえ満たしていれば所望の伸びおよび伸びフランジ性および５９０ＭＰａ以上の強度が得られ、化学成分は特に限定されないが、質量％で、Ｃ：０．０２〜０．０６％、Ｓｉ≦０．３％、Ｍｎ：０．５〜２．０％、Ｐ≦０．０６％、Ｓ≦０．００５％、Ａｌ≦０．０６％、Ｎ≦０．００６％、Ｍｏ：０．０５〜０．５％、Ｔｉ：０．０３〜０．１４％を含み、残部が実質的にＦｅであることが好ましい。また、上述のように複合炭化物にＮｂおよびＶの１種以上を含有させる場合には、上記成分に加えＮｂ≦０．０８％、Ｖ≦０．１５％のうち１種以上を含有し、残部が実質的にＦｅであることが好ましい。以下、これら各成分について説明する。
【００４３】
Ｃ：０．０２〜０．０６％
Ｃは炭化物を形成し、鋼を強化するのに有効である。しかし、０．０２％未満では、鋼の強化が不十分であり、０．０６％を超えて添加するとパーライトが形成されることと析出物が粗大化することから伸びと伸びフランジ性を損なうおそれがある。このため、Ｃ含有量は０．０２〜０．０６％が好ましい。
【００４４】
Ｓｉ：０．３％以下
Ｓｉは固溶強化には有効な元素であるが、０．３％を超えて添加すると、フェライトからのＣ析出が促進されて粒界に粗大な鉄炭化物が析出しやすくなり、伸びフランジ性が低下する傾向となる。また、本発明においては、従来積極的に用いられてきたＳｉを低減することによりオーステナイトの圧延荷重を低減し、薄物の製造を容易化することができ、０．３％を超えて添加すると２．５ｍｍ以下の材料の圧延が不安定となる。また、Ｓｉ添加で圧延負荷が増大し、圧延材の形状が悪くなる。これらの理由により、Ｓｉ含有量は０．３％以下が好ましい。さらに好ましくは０．１５％以下であり、望ましくは０．０５％以下である。
【００４５】
Ｍｎ：０．５〜２．０％
Ｍｎは固溶強化により鋼を強化する観点からは０．５％以上が好ましいが、２．０％を超えて添加すると偏析し、かつ硬質相が形成され、伸びフランジ性が低下する。このため、Ｍｎ含有量は０．５〜２．０％が好ましい。
【００４６】
Ｐ：０．０６％以下
Ｐは固溶強化に有効であるが、０．０６％を超えて添加すると偏析して伸びフランジ性が低下するおそれがあるため、０．０６％以下とすることが好ましい。
【００４７】
Ｓ：０．００５％以下
Ｓは少ないほど好ましく、０．００５％を超えると伸びフランジ性を低下させるおそれがあるため、０．００５％以下が好ましい。
【００４８】
Ａｌ：０．０６％以下
Ａｌは脱酸剤として添加される。しかし、０．０６％を超えると伸びおよび伸びフランジ性がともに低下する傾向にあるため０．０６％以下が好ましい。
【００４９】
Ｎ：０．００６％以下
Ｎは少ないほど好ましく、０．００６％を超えると粗大な窒化物が増え、伸びフランジ性が低下する傾向にあるため０．００６％以下が好ましい。
【００５０】
Ｍｏ：０．０５〜０．５％
Ｍｏは本発明において重要な元素であり、０．０５％以上含有させることで、パーライト変態を抑制しつつ、Ｔｉとの微細な複合析出物、または、Ｔｉに加えＮｂおよびＶのうち１種以上を含む微細な複合析出物を形成し、優れた伸びおよび伸びフランジ性を確保し、かつ鋼を強化することができる。しかし、０．５％を超えて添加すると硬質相が形成され伸びフランジ性が低する傾向にある。このため、Ｍｏ含有量は０．０５〜０．５％が好ましい。
【００５１】
Ｔｉ：０．０３〜０．１４％
Ｔｉは本発明において重要な元素である。Ｍｏと複合析出物を形成することで、優れた伸びおよび伸びフランジ性を確保しつつ、鋼を強化することができる。しかし、０．０３％未満では、鋼を強化する効果が不十分であり、０．１４％を超えると伸びフランジ性が劣化する傾向にある。したがって、Ｔｉ含有量は０．０３〜０．１４％が好ましい。
【００５２】
Ｎｂ：０．０８％以下
Ｎｂは組織の細粒化に有効であり、かつＴｉおよびＭｏとともに複合析出して複合析出物を形成し、優れた伸びと伸びフランジ性を得ることに寄与するため、必要に応じて添加する。しかし、Ｎｂ量が０．０８％を超えると伸びが劣化する傾向にあるため、Ｎｂを含有させる場合には０．０８％以下が好ましい。なお、Ｎｂの組織の細粒化効果を得る観点からは０．００５％以上が好ましい。
【００５３】
Ｖ：０．１５％以下
Ｖは組織の微細化に有効であり、かつＴｉおよびＭｏとともに複合析出して複合析出物を形成し、優れた伸びと伸びフランジ性を得ることに寄与するため、必要に応じて添加する。しかし、Ｖ量が０．１５％を超えると伸びが劣化する傾向にあるため、Ｖを含有させる場合には０．１５％以下が好ましい。なお、Ｖの組織の細粒化効果を得る観点からは０．００１％以上が好ましい。
【００５４】
なお、Ｃｒ：０．１５％以下、Ｃｕ：０．１５％以下、Ｎｉ：０．１５％以下の１種類以上を含んでいても特性上問題はない。
【００５５】
［製造方法］
本発明では、上記高張力鋼を製造するに際し、熱間圧延を、仕上圧延終了温度８８０℃以上、巻取温度５７０℃以上の条件で行う。以下、これら条件について説明する。
【００５６】
・仕上圧延終了温度８８０℃以上
仕上圧延終了温度は伸びおよび伸びフランジ性と圧延荷重を低減するのに重要である。８８０℃未満では、表層が粗大粒となり伸びおよび伸びフランジ性が損なわれ、かつ未再結晶で圧延が進行するために起こる歪みの累積量が増大し、圧延荷重が著しく増大することで薄物の熱間圧延が困難となるため、８８０℃以上とする。
【００５７】
・巻取温度５７０℃以上
フェライト組織を得るため、およびランナウトテーブル上での注水量を抑えて薄物を安定的に通板させるため、巻取温度を５７０℃以上とする。これらに加えさらにランナウトテーブル上の鋼板の走行安定性を確保するには６００℃以上が好ましい。なお、パーライトの生成を抑制するためには巻取温度は７００℃以下とするのが望ましい。
【００５８】
本発明の高張力鋼板には、表面に溶融亜鉛系めっき皮膜を形成し、溶融亜鉛系めっき鋼板としたものも含む。本発明の高張力鋼板は良好な加工性を有することから、溶融亜鉛系めっき皮膜を形成しても良好な加工性を維持することができる。ここで、溶融亜鉛系めっきとは、亜鉛および亜鉛を主体とした溶融めっきであり、亜鉛の他にＡｌ、Ｃｒ等の合金元素を含んだものを含む。このような溶融亜鉛系めっきを施した本発明の高張力鋼板は、めっきままでもめっき後合金化処理を行ってもかまわない。めっき前焼鈍温度については、４５０℃未満ではめっきがつかず、７５０℃超えでは強度低下が生じやすい。そのため、焼鈍温度は４５０℃以上、７５０℃以下が好ましい。
【００５９】
なお、本発明の鋼板は、黒皮ままでも酸洗材でもその特性に差違はない。調質圧延についても通常行われているものであれば特に規定はない。また、上記溶融亜鉛めっきは酸洗後でも黒皮ままでも問題はない。亜鉛めっきについては電気めっきも可能である。化成処理についても特に問題はない。鋳造後直ちにもしくは補熱を目的とした加熱を施した後にそのまま熱間圧延を行う直送圧延を行っても本発明の効果に影響はない。さらに、粗圧延後に仕上圧延前で、圧延材を加熱しても、粗圧延後、圧延材を接合して行う連続圧延を行っても、さらには圧延材の加熱と連続圧延を同時に行っても本発明の効果は損なわれない。
【００６０】
本発明の高張力鋼板は、加工性に優れ、特に伸びフランジ性に優れているのでこれをプレス成形した場合、その特質が活かされ、自動車用部材、特にサスペンションアーム等の足廻り部材のようなプレス時の断面形状が複雑な部材を良好な品質で製造することができ、特に、プレス成形品の軽量化に資することができる。以下に具体的に、本発明に係る高張力鋼板の加工方法、換言すればプレス成形品の製造方法について説明する。
【００６１】
図２は、本発明に係る高張力鋼板の加工方法の作業フローの一例を示すフローチャートである。この作業フローは、通常、本発明に係る鋼板を製造することまたはその製造された鋼板を例えばコイルにして目的場所に搬送することを前工程としており、まず、本発明に係る高張力鋼板を準備することから始まる（Ｓ０、Ｓ１）。この鋼板に対してプレス加工を施す前に、鋼板に対して前処理的な加工を施すこともあれば（Ｓ２）、裁断機により所定の寸法や形状に加工することもある（Ｓ３）。前者のＳ２の工程では、例えば鋼板の幅方向の所定箇所に切り込みや穿孔を行い、引き続くプレス加工を終えた段階またはそのプレス加工の過程で、所定の寸法および形状のプレス成形品または被プレス加工部材として切り離すことができるようにしておく。後者のＳ３の工程では、最終的なプレス成形品の寸法、形状等を予め考慮して、所定の寸法および形状の鋼板部材に加工（したがって裁断）するようにしておく。その後、Ｓ２およびＳ３の工程を経由した部材には、プレス加工が施され、最終的に目的とする寸法・形状の所望のプレス成形品が製造される（Ｓ４）。このプレス加工は、通常は多段階で行われ、３段階以上７段階以下であることが多い。
【００６２】
Ｓ４の工程は、Ｓ２およびＳ３の工程を経由した部材に対してさらに所定の寸法や形状に裁断する工程を含む場合もある。この場合の「裁断」という作業は、例えば、少なくともプレス加工の過程で、Ｓ２およびＳ３の工程を経由した部材の端部のような最終的なプレス成形品には不要部分を切り離す作業であっても構わないし、また、Ｓ２の工程で設けられた鋼板の幅方向の切り込みや穿孔に沿って被プレス加工部材を切り離す作業であっても構わない。
【００６３】
なお、図２中、Ｎ１ないしＮ３は、鋼板、部材、プレス成形品を、機械的にあるいは作業員による搬送作業である場合がある。
【００６４】
こうして製造されるプレス成形品は、必要に応じて次工程に送られる。次工程としては、例えば、プレス成形品にさらに機械加工を施し、寸法や形状を調整する工程、プレス成形品を所定場所に搬送し、格納する工程、プレス成形品に表面処理を施す工程、プレス成形品を用いて自動車のような目的物を組み立てる組立工程がある。
【００６５】
図３は、図２に示した作業を実際に行う装置と鋼板、部材、プレス成形品の流れとの関係を示すブロック図である。この図においては、本発明に係る高張力鋼板はコイル状で準備されており、プレス加工機によりプレス成形品が製造される。プレス加工機は多段プレスを行う機種のものであるが、本件発明はこれに限定されない。
【００６６】
プレス加工機の前段に、裁断機その他の前処理機械を設置する場合（図３の（ａ））もあれば、設置しない場合（図３の（ｂ））もある。裁断機が設置される場合には、コイルから供給される長尺の本発明に係る鋼板から、必要な寸法または形状の部材を裁断し、この部材がプレス加工機においてプレス加工され、所定のプレス成形品となる。鋼板の幅方向に切り欠きや穿孔を施す前処理機械が設置される場合には、プレス加工機においてその切り欠きや穿孔に沿って裁断が行われても構わない。前処理機械を設置しない場合には、プレス加工機において鋼板がプレス加工される過程で、裁断が行われ、最終的に所定の寸法、形状を有するプレス成形品が製造される。なお、図３における「裁断」の意味は、図２における裁断と同じである。
【００６７】
こうして製造されるプレス成形品は、その原材料として加工性に優れ、特に伸びフランジ性に優れている本発明に係る高張力鋼板を使用しているので、プレス時の断面形状が複雑であっても、良好な品質で製造することができ、軽量なものとなる。このような特長は、プレス成形品が自動車用部材、特にサスペンションアーム等の足廻り部材である場合に特に有用である。
【００６８】
【実施例】
（実施例１）
表１に示す化学成分を有する鋼片を、１２５０℃に加熱し、通常の熱間圧延工程によって仕上温度８８０〜９３０℃で、板厚３．２ｍｍに仕上げた。この後、６００℃を超える巻取温度で、冷却速度と巻取温度を変化させて、種々の組織の鋼板を製造した。
【００６９】
得られた鋼板を酸洗後、鋼板から作製した薄膜を透過型電子顕微鏡（ＴＥＭ）によって組織観察を行うとともに析出物寸法を測定した。析出物中のＴｉ、Ｎｂ、Ｖ、Ｍｏの組成は、ＴＥＭに装備されたエネルギー分散型Ｘ線分光装置（ＥＤＸ）による分析から決定した。
【００７０】
また、得られた鋼板からＪＩＳ５号引張試験片および穴広げ試験片を採取した。引張試験片は圧延垂直方向から採取し、穴広げ試験は、１３０ｍｍ角の鋼板の中央に１０ｍｍφのポンチによりクリアランス１２．５％で打ち抜いた穴を有する試験片を準備し、６０°円錐ポンチにより打抜き穴のバリ側の反対方向から押し上げ、割れが鋼板を貫通した時点での穴径ｄを測定し、穴広げ率λを次式より算出した。
λ（％）＝［（ｄ−１０）／１０］×１００
【００７１】
表１に、組織、析出物平均粒径、析出物の組成（Ｍｏ比率）、引張強度（ＴＳ）、伸び（Ｅｌ）、穴広げ率（λ）を併記する。なお、表１中、Ａ値は、上記（１）式の（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｍｏ／９６）｝の値、または上記（２）式の（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｎｂ／９３）＋（Ｖ／５１）＋（Ｍｏ／９６）｝の値を示す。
【００７２】
表１に示す通り、本発明鋼のＮｏ．１〜１０はいずれもフェライト組織からなり、析出物の平均粒径は１０ｎｍ未満で、Ｍｏ／（Ｔｉ＋Ｎｂ＋Ｖ＋Ｍｏ）で表されるＭｏ比率（原子比）が０．２５以上となっているため、引張強度（ＴＳ）が５９０ＭＰａ以上で優れた伸びと伸びフランジ性を有している。なお、図１に、Ｎｏ．２の鋼板の透過型電子顕微鏡写真を示す。この写真から、微細析出物がフェライト単相組織中に均一に分散していることがわかる。
【００７３】
これに対し、比較鋼のＮｏ．１１はＣ量が多すぎることとＭｏ無添加のため、パーライトが生成し、かつ析出物が粗大化しており、伸びおよび伸びフランジ性がともに低く、特に伸びフランジ性が低い。また、Ｎｏ．１２はＭｏ無添加のため、析出物が粗大化しており、伸びおよび伸びフランジ性がともに低く、特に伸びフランジ性が低い。Ｎｏ．１３はＣ量が低いため、鋼の強化に必要な析出物量が少ないことから引張強度（ＴＳ）が５９０ＭＰａ未満となっている。Ｎｏ．１４はＭｎ量が多すぎるため偏析が顕著であり、かつ組織内にマルテンサイトが形成されているため、伸びおよび伸びフランジ性がともに低い。Ｎｏ．１５はＴｉ量が少ないため、鋼の強化に必要な析出物が不足して引張強度（ＴＳ）が５９０ＭＰａ未満となっている。Ｎｏ．１６はＴｉ量が多すぎるため、ＴｉとＭｏの複合析出物は存在するものの、複合析出物中のＭｏ比率が低く、またＳｉ量が多すぎるため、析出物が粗大化する傾向にあり、伸びと伸びフランジ性がともに低い。
【００７４】
【表１】

【００７５】
（実施例２）
表２に示す成分の鋼を溶製しスラブとした。次いで、オーステナイト域に加熱後、熱間圧延を行い、８８０℃以上で圧延を完了した。圧延後は巻取温度まで冷却し、表２に示す巻取温度で巻き取った。表２には板厚も同時に記載した。得られたコイル幅方向中央部からサンプルを採取し、引張方向が圧延方向と垂直になるようにＪＩＳ５号引張試験片を採取し、引張試験を行った。また、圧延後の板形状を目視で判定した。その結果も表２に示す。なお、圧延後の板形状の評価基準は、目視でフラットな板の場合を○、波うちが顕著な板を×とした。また、表２中のＡ値も表１と同様、（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｎｂ／９３）＋（Ｖ／５１）＋（Ｍｏ／９６）｝の値を示す。
【００７６】
表２のうち、Ｎｏ．１７〜Ｎｏ．２３は７８０ＭＰａ級鋼板において板厚を変化させた例と巻取温度を変化させてＭｏ／（Ｔｉ＋Ｎｂ＋Ｍｏ）比を変化させた例を示す。板厚２．０ｍｍであるＮｏ．１７、Ｎｏ．２１〜２３に注目すると巻取温度の変化にともないＭｏ／（Ｔｉ＋Ｎｂ＋Ｍｏ）比が変化しており、その値が０．２５未満のＮｏ．２２、２３では、急冷により強度は維持されたものの、低温変態相の増大により伸び（Ｅｌ）は低下した。また、形状も波打ちが顕著であった。Ｍｏ／（Ｔｉ＋Ｎｂ＋Ｍｏ）比が０．２５以上のＮｏ．１７〜２１では、緩冷却、高温巻取を行ってもＮｏ．２２、２３と比べて強度が維持された。また、板形状についても良好であった。
【００７７】
Ｎｏ．２４〜Ｎｏ．２９は５９０ＭＰａ級鋼板において板厚を変化させた例と巻取温度を変化させてＭｏ／（Ｔｉ＋Ｎｂ＋Ｍｏ）比を変化させた例を示す。板厚１．４ｍｍであるＮｏ．２６、２８、２９に注目すると巻取温度の変化にともないＭｏ／（Ｔｉ＋Ｎｂ＋Ｍｏ）比が変化しており、その値が０．２５未満のＮｏ．２８、２９では、急冷により強度は維持されたものの、低温変態相の増大により伸び（Ｅｌ）は低下した。また、形状も波うちが顕著であった。Ｍｏ／（Ｔｉ＋Ｎｂ＋Ｍｏ）比が０．２５以上のＮｏ．２４〜２７では、緩冷却、高温巻取を行ってもＮｏ．２８、２９とくらべて強度が維持された。また、板形状については、良好であった。
【００７８】
【表２】

【００７９】
（実施例３）
表３に示す鋼を仕上げ温度９１０℃、巻き取り温度６３０℃で熱間圧延を行い、板厚約１．６ｍｍの熱延鋼板を作製した。これら熱延鋼板を酸洗後、合金化溶融亜鉛めっきを行った。得られた鋼板から作製した薄膜について透過型電子顕微鏡（ＴＥＭ）によって組織観察を行うとともに析出物の寸法を測定し、さらに析出物中のＴｉ、Ｎｂ、Ｖ、Ｍｏの組成をＴＥＭに装備されたＥＤＸによる分析から決定した。また、これらめっき鋼板からＪＩＳ５号引張試験片および穴広げ試験片を採取し、引張試験および穴広げ試験を行った。表３に、組織、析出物平均粒径、析出物の組成（Ｍｏ比率）、引張強度（ＴＳ）、伸び（Ｅｌ）、穴広げ率（λ）を併記する。なお、表３中のＡ値も表１と同様、（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｎｂ／９３）＋（Ｖ／５１）＋（Ｍｏ／９６）｝の値を示す。
【００８０】
表３に示すように、本発明例であるＮｏ．３０は、溶融亜鉛系めっきを行ってもＥｌおよびλとも良好な値を示すのに対し、比較例のＮｏ．３１は析出物にＭｏが含まれていないためλが低い値となった。
【００８１】
【表３】

【００８２】
【発明の効果】
以上説明したように、本発明によれば、加工性の指標である伸びおよび伸びフランジ性に優れた高張力鋼板を提供することができ、自動車部材の軽量化に寄与する効果が顕著である。
【図面の簡単な説明】
【図１】本発明に係る高張力鋼板の金属組織を示す透過型電子顕微鏡写真。
【図２】本発明に係る高張力鋼板の加工方法の作業フローの一例を示すフローチャート。
【図３】図２に示した作業を実際に行う装置と鋼板、部材、プレス成形品の流れとの関係を示すブロック図。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-tensile steel sheet excellent in workability suitable for a material of a member for an automobile, and a method for manufacturing and processing the same.
[0002]
[Prior art]
From the viewpoint of improving fuel efficiency leading to environmental protection, there is a strong demand for high-strength and thin-walled automotive steel sheets. Many automotive members have a complicated shape obtained by press molding, and a material having high strength and excellent in both elongation and stretch flangeability, which are indicators of workability, is required. Further, from the viewpoint of further reducing the weight of the steel sheet, further reduction in thickness is aimed at, and demand for a thin object having a sheet thickness of 2.5 mm or less is increasing.
[0003]
Conventionally, various types of steel sheets of this kind have been proposed. For example, Japanese Patent Application Laid-Open No. 6-172924 proposes a steel sheet having a bainitic ferrite structure having a high dislocation density and having excellent stretch flangeability. However, this steel sheet has a drawback of poor elongation because it contains a bainitic ferrite structure having a high dislocation density. In addition, since strong cooling on the run-out table is inevitable due to the formation of bainitic ferrite, and there is a problem in the running property of the strip on the run-out table when manufacturing a thin product, a thin product having a thickness of 2.5 mm or less is produced. Not suitable for
[0004]
Japanese Patent Application Laid-Open No. Hei 6-200351 proposes a steel sheet excellent in stretch flangeability in which most of the structure is made of polygonal ferrite and precipitation strengthening and solid solution strengthening are performed mainly on TiC. However, in order to increase the tensile strength of a generally well-known precipitate used in this steel sheet, a large amount of Ti is required, a precipitate having a large size is easily generated, and the characteristics are likely to be unstable. There is a disadvantage that. Further, since this steel positively uses Si for increasing the rolling load in order to improve the properties, the rolling load is increased in the production of thin products, and it is difficult to secure the shape of the steel sheet.
[0005]
JP-A-7-11382 proposes a steel sheet having an acicular ferrite structure in which fine TiC and / or NbC is precipitated and having excellent stretch flangeability. However, this steel sheet, like the steel sheet proposed in JP-A-6-172924 described above, does not have sufficient elongation because it has a structure of acicular ferrite having a high dislocation density. In addition, as in the case of the steel disclosed in Japanese Patent Application Laid-Open No. Hei 6-200351, this steel positively uses Si for increasing the rolling load in order to improve the properties. It is difficult to secure the shape of the steel plate.
[0006]
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and is suitable for applications in which the cross-sectional shape at the time of pressing is complex, such as a member for automobiles. It is an object of the present invention to provide a high-strength steel sheet and a method for manufacturing and processing the same.
[0007]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to achieve the above object, and as a result, have obtained the following knowledge.
(1) Strengthening and elongation balance is improved when the dislocation density is low and the structure is strengthened with fine precipitates.
(2) When a substantially single-phase structure is formed and strengthened with fine precipitates, the strength-stretch flangeability balance is improved.
(3) When the composite precipitate contains Mo, the precipitate is finely precipitated.
(4) When the proportion of Mo in the composite precipitate is low, the precipitate is coarsened, so that both elongation and stretch flangeability are reduced.
[0008]
The present invention has been completed based on these findings, and the following (1) to (1)22)I will provide a.
[0009]
(1A high-strength steel sheet having a workability of a tensile strength of 590 MPa or more, which is substantially a ferrite single-phase structure and in which carbides containing Ti and Mo having an average particle size of less than 10 nm are dispersed and precipitated.
[0010]
(2)In the above (1), the carbide containing Ti and Mo is:Ti and Mo represented by atomic% satisfy Mo / (Ti + Mo) ≧ 0.25Having a compositionHigh tensile strength steel sheet with excellent workability.
[0011]
(3)In the above (1) or (2),In steelIt is necessary to satisfy the following equation (1) that defines C, Ti, and Mo.High strength steel sheet with excellent workability.
0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (1)
However, in the above formula (1), C, Ti, and Mo represent mass% of each component.
[0012]
(4)In any of the above (1) to (3), C: 0.02 to 0.06%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0. A high-tensile steel sheet excellent in workability characterized by containing 14%.
[0013]
(5)In mass%, C: 0.02-0.06%, Si ≦ 0.3%, Mn: 0.5-2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, the balance being substantially Fe, and substantially a ferrite single phase Tensile strength, characterized in that carbides containing Ti and Mo having a carbide composition satisfying Mo / (Ti + Mo) ≧ 0.25 are dispersed and precipitated, the structure being Ti and Mo expressed in atomic%. Is a high-strength steel sheet excellent in workability of 590 MPa or more.
[0014]
(6)In mass%, C: 0.02-0.06%, Si ≦ 0.3%, Mn: 0.5-2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, the balance being substantially Fe, and substantially a ferrite single phase Workability with a tensile strength of 590 MPa or more, characterized by satisfying the following formula (1) that defines the structure of C, Ti, and Mo in the steel and in which carbides containing Ti and Mo are dispersed and precipitated. Excellent high tensile steel sheet.
0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (1)
However, in the above formula (1), C, Ti, and Mo represent mass% of each component.
[0015]
(7In any one of the above (1) to (3), C: 0.02 to 0.06%, Si ≦ 0.3%, Mn: 0.5 to 2.0%, P ≦ 0 by mass%. 0.06%, S ≦ 0.005%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, the balance Is a high-tensile steel sheet excellent in workability, wherein is substantially Fe.
[0016]
(8)the above(2) or (5)In the above, the carbide contains one or more of Nb and V in addition to Ti and Mo, and Ti and Mo represented by atomic% satisfy the relation of Mo / (Ti + Mo) ≧ 0.25. %, Ti, Mo, Nb, and V satisfy Mo / (Ti + Nb + V + Mo) ≧ 0.25Having a carbide compositionA high-tensile steel sheet with excellent workability, characterized in that:
[0017]
(9)the above(1), Wherein the carbide includes one or more of Nb and V in addition to Ti and Mo.
[0018]
(10)the above(8) Or (9)Satisfies the following equation (2) that defines C, Ti, Nb, V, and Mo in steel.A high-tensile steel sheet with excellent workability, characterized in that:
0.8 ≦ (C / 12) / ｛(Ti / 48) + (Nb / 93) + (V / 51) + (Mo / 96)｝ ≦ 1.5 (2)
However, in the above formula (2), C, Ti, Nb, V, and Mo represent mass% of each component.
[0019]
(11)the above(3)Or (6)AtIn place of the above equation (1), the following equation (2) for defining C, Ti, Nb, V, and Mo in steel is satisfied.A high-tensile steel sheet excellent in workability, characterized in that the carbide contains one or more of Nb and V in addition to Ti and Mo.
0.8 ≦ (C / 12) / {(Ti / 48) + (Nb / 93) + (V / 51) + (Mo / 96)} ≦ 1.5 (2)
However, in the above formula (2), C, Ti, Nb, V, and Mo represent mass% of each component.
[0020]
(12)the above(9) ~ (11In any one of the above (1), (2), Ti, Mo, Nb, and V represented by atomic% satisfy Mo / (Ti + Nb + V + Mo) ≧ 0.25.Having a carbide compositionA high-tensile steel sheet with excellent workability, characterized in that:
[0021]
(13)In any one of the above (8) and (10) to (12), C: 0.02 to 0.06%, Mo: 0.05 to 0.5%, Ti: 0. A high-tensile steel sheet excellent in workability, characterized in that the high-strength steel sheet contains at least one of Nb ≦ 0.08% and V ≦ 0.15% by mass%.
[0022]
(14)the above(8) ~ (12), C: 0.02-0.06%, Si ≦ 0.3%, Mn: 0.5-2.0%, P ≦ 0.06%, S ≦ 0. 005%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, Nb ≦ 0.08%, V ≦ A high-tensile steel sheet excellent in workability, characterized in that it contains at least one of 0.15% and the balance is substantially Fe.
[0023]
(15)In the above (4), the carbide contains one or more of Nb and V in addition to Ti and Mo, and one or more of Nb ≦ 0.08% and V ≦ 0.15% by mass% in steel. High tensile strength steel sheet with excellent workability characterized by including
[0024]
(16) The above (1) to (Fifteen), A high-strength steel sheet excellent in workability, characterized in that it is a thin hot-rolled steel sheet having a thickness of 2.5 mm or less.
[0025]
(17) The above (1) to (164.) A high-strength steel sheet excellent in workability, characterized by having a hot-dip galvanized coating film on the surface in any one of the above.
[0026]
(18) In mass%, C: 0.02-0.06%, Si ≦ 0.3%, Mn: 0.5-2.0%, P ≦ 0.06%, S ≦ 0.005%, Melting steel containing Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.05-0.5%, Ti: 0.03-0.14%, with the balance being substantially Fe Then, hot rolling is performed under the conditions of a finish rolling end temperature of 880 ° C. or more and a winding temperature of 570 ° C. or more, and a carbide containing Ti and Mo having substantially a ferrite single phase structure and an average grain size of less than 10 nm is dispersed and precipitated. A method for producing a high-strength steel sheet having excellent workability with a tensile strength of 590 MPa or more, characterized in that a high-strength steel sheet is obtained.
[0027]
(19) In mass%, C: 0.02 to 0.06%, Si ≦ 0.3%, Mn: 0.5 to 2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, Nb ≦ 0.08%, V ≦ 0.15 %, And the remainder is substantially made of Fe, and hot-rolled under the conditions of a finish rolling end temperature of 880 ° C. or more and a winding temperature of 570 ° C. or more. High tensile strength excellent in workability with a tensile strength of 590 MPa or more characterized by obtaining a steel sheet having a ferrite single-phase structure and having carbides containing Ti and Mo having an average particle size of less than 10 nm dispersed and precipitated. Steel plate manufacturing method.
[0028]
(20) The above (1) to (17A) a first step of preparing a member made of any one of the high-tensile steel sheets, and a second step of press-forming the member to form a press-formed product having a desired shape. Method.
[0029]
(21)the above(20)), The press-formed product is a method for processing a high-strength steel sheet which is an automobile part, in particular, an automobile suspension member.
[0030]
(22) The above (1) to (17)) Automotive parts manufactured from any of the high-tensile steel sheets.
[0031]
In the present invention, a substantially ferrite single phase structure means that a small amount of other phases or precipitates are allowed in addition to the precipitates of the present invention. Preferably, the area ratio of ferrite is 95% or more. .
[0032]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be specifically described with respect to metal structures, chemical components, and manufacturing methods.
[0033]
[Metal structure]
The high-tensile steel sheet according to the present invention has a substantially ferrite single-phase structure, in which carbides containing Ti and Mo are dispersed and precipitated.
[0034]
・ Substantially ferrite single phase structure:
The fact that the matrix has a substantially ferrite single phase structure is that a ferrite having a low dislocation density is effective for improving elongation, and a single phase structure is effective for improving stretch flangeability. This is because the effect is remarkable in a ferrite single phase structure rich in ductility. However, the matrix does not necessarily have to be a completely ferrite single phase structure, but may be a substantially ferrite single phase structure, preferably a ferrite having an area ratio of 95% or more.
[0035]
-Carbide containing Ti and Mo:
Carbides containing Ti and Mo are effective in strengthening steel because they are fine. Conventionally, TiC has been mainly used as a precipitate. However, when Ti does not contain Mo because of a strong tendency to form a precipitate, it is likely to be coarsened and the effect on strengthening is low. To obtain it, precipitates are required until workability is deteriorated. On the other hand, a composite carbide containing Ti and Mo can strengthen steel without precipitating finely and deteriorating workability. This is because the precipitation tendency of Mo is weaker than that of Ti, so that it can be present stably and finely, and the effect on strengthening is high, and the required amount of reinforcement can be obtained with the amount of precipitate that can maintain good workability. Conceivable.
[0036]
In order for carbides to be present stably and finely, the composition of the carbides affects, and if the composition of the carbides is such that Mo and Ti expressed in atomic% satisfy Mo / (Ti + Mo) ≧ 0.25. In addition, the effect of suppressing the coarsening of the precipitates is enhanced, and a desired fine precipitate can be obtained. Therefore, in the first aspect of the present invention, it is required that carbides containing Ti and Mo are dispersed and precipitated in a range where Ti / Mo represented by atomic% satisfies Mo / (Ti + Mo) ≧ 0.25. And
[0037]
Further, by setting the average particle size of the composite carbide to less than 10 nm, the strain around the precipitate becomes more effective for the resistance of dislocation movement, and good steel strengthening can be obtained. Therefore, the second aspect of the present invention requires that carbides containing Ti and Mo having an average particle diameter of less than 10 nm are dispersed and precipitated. More preferably, the average particle size is 5 nm or less.
[0038]
Further, by adjusting the contents of C, Ti, and Mo so that the atomic ratio of C to (Ti + Mo) in the steel is 0.8 to 1.5, carbides containing Ti and Mo can be obtained. From the viewpoint of the third aspect of the present invention, the atomic ratio of C to (Ti + Mo) in steel is from 0.8 to 1 since fine precipitation is easy and formation of fine precipitates of less than 10 nm is easy. .5, and requires that carbides containing Ti and Mo are dispersed and precipitated. The atomic ratio is more preferably 0.8 to 1.3. When the atomic ratio of C to (Ti + Mo) of 0.8 to 1.5 is converted into mass%, the following formula (1) is satisfied.
0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (1)
However, in the above formula (1), C, Ti, and Mo represent mass% of each component.
[0039]
Even if the carbide is one in which at least one of Nb and V is precipitated in addition to Ti and Mo, the tendency of Mo to form a precipitate is weaker than that of Nb and V, so the composite precipitate is composed of Ti and Mo. Like the composite carbide of Mo, it can be present stably and finely. Therefore, the carbide may be one in which at least one of Nb and V is compositely precipitated in addition to Ti and Mo.
[0040]
In the case where the composite carbide contains one or more of Nb and V in addition to Ti and Mo, in the first aspect, the composition is such that Ti and Mo represented by atomic% are Mo / (Ti + Mo). Instead of satisfying ≧ 0.25, it is preferable that Ti, Mo, Nb, and V expressed by atomic% satisfy Mo / (Ti + Nb + V + Mo) ≧ 0.25. Within this range, the effect of suppressing the coarsening of the composite precipitate is high, and the necessary amount of reinforcement can be obtained with the amount of the precipitate capable of maintaining good workability. Also in the second and third aspects, it is preferable that Mo / (Ti + Nb + V + Mo) ≧ 0.25 is satisfied.
[0041]
As described above, in order to facilitate finely dispersing and precipitating the complex carbides precipitated by complexing one or more of Nb and V in addition to Ti and Mo, in the third viewpoint, C and ( It is preferable that the atomic ratio between C and (Ti + Nb + V + Mo) be 0.8 to 1.5 instead of the atomic ratio with Ti + Mo. Also in the first and second viewpoints, it is preferable that the atomic ratio of C to (Ti + Nb + V + Mo) is 0.8 to 1.5. The atomic ratio is more preferably 0.8 to 1.3. When the atomic ratio of C to (Ti + Nb + V + Mo) is 0.8 to 1.5 in terms of mass%, the following expression (2) is satisfied.
0.8 ≦ (C / 12) / {(Ti / 48) + (Nb / 93) + (V / 51) + (Mo / 96)} ≦ 1.5 (2)
However, in the above formula (2), C, Ti, Nb, V, and Mo represent mass% of each component.
[0042]
[Chemical composition]
In the present invention, desired elongation and stretch flangeability and strength of 590 MPa or more can be obtained as long as the above metal structure is satisfied, and the chemical components are not particularly limited, but C: 0.02 to 0.06% by mass%. , Si ≦ 0.3%, Mn: 0.5-2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0 0.05-0.5%, and Ti: 0.03-0.14%, and the balance is preferably substantially Fe. Also, as described above,carbideWhen one or more of Nb and V are contained in Nb, at least one of Nb ≦ 0.08% and V ≦ 0.15% is contained in addition to the above components, and the balance is substantially Fe. Is preferred. Hereinafter, each of these components will be described.
[0043]
C: 0.02 to 0.06%
C forms carbides and is effective in strengthening steel. However, if the content is less than 0.02%, the steel is insufficiently strengthened. If the content exceeds 0.06%, pearlite is formed and precipitates are coarsened, so that elongation and stretch flangeability may be impaired. There is. For this reason, the C content is preferably 0.02 to 0.06%.
[0044]
Si: 0.3% or less
Si is an effective element for solid solution strengthening, but if added in excess of 0.3%, precipitation of C from ferrite is promoted, and coarse iron carbide is easily precipitated at grain boundaries, and stretch flangeability is reduced. It tends to decrease. In the present invention, the rolling load of austenite can be reduced by reducing the amount of Si that has been actively used in the past, and the production of thin materials can be facilitated. Rolling of a material of 0.5 mm or less becomes unstable. In addition, the rolling load increases with the addition of Si, and the shape of the rolled material deteriorates. For these reasons, the Si content is preferably 0.3% or less. The content is more preferably 0.15% or less, and desirably 0.05% or less.
[0045]
Mn: 0.5 to 2.0%
Mn is preferably 0.5% or more from the viewpoint of strengthening the steel by solid solution strengthening, but when added in excess of 2.0%, segregation occurs, a hard phase is formed, and stretch flangeability decreases. Therefore, the Mn content is preferably 0.5 to 2.0%.
[0046]
P: 0.06% or less
P is effective for solid solution strengthening. However, if added in excess of 0.06%, segregation may occur and stretch flangeability may be reduced.
[0047]
S: 0.005% or less
S is preferably as small as possible, and if it exceeds 0.005%, the stretch flangeability may be reduced. Therefore, the content of S is preferably 0.005% or less.
[0048]
Al: 0.06% or less
Al is added as a deoxidizing agent. However, if it exceeds 0.06%, both elongation and stretch flangeability tend to decrease, so that 0.06% or less is preferable.
[0049]
N: 0.006% or less
N is preferably as small as possible. If it exceeds 0.006%, coarse nitrides increase and the stretch flangeability tends to decrease, so that 0.006% or less is preferable.
[0050]
Mo: 0.05-0.5%
Mo is an important element in the present invention, and by containing 0.05% or more, while suppressing pearlite transformation, a fine composite precipitate with Ti, or one or more of Nb and V in addition to Ti. To form fine composite precipitates, secure excellent elongation and stretch flangeability, and strengthen the steel. However, if it exceeds 0.5%, a hard phase is formed and the stretch flangeability tends to be low. Therefore, the Mo content is preferably 0.05 to 0.5%.
[0051]
Ti: 0.03 to 0.14%
Ti is an important element in the present invention. By forming a composite precipitate with Mo, steel can be strengthened while ensuring excellent elongation and stretch flangeability. However, if it is less than 0.03%, the effect of strengthening the steel is insufficient, and if it exceeds 0.14%, the stretch flangeability tends to deteriorate. Therefore, the Ti content is preferably 0.03 to 0.14%.
[0052]
Nb: 0.08% or less
Nb is effective for grain refinement of the structure, and is compounded with Ti and Mo to form a complex precipitate, thereby contributing to obtaining excellent elongation and stretch flangeability. Therefore, Nb is added as necessary. However, if the Nb content exceeds 0.08%, the elongation tends to deteriorate. Therefore, when Nb is contained, the content is preferably 0.08% or less. Note that 0.005% or more is preferable from the viewpoint of obtaining the effect of Nb structure refinement.
[0053]
V: 0.15% or less
V is added as necessary because it is effective for refining the structure, and also contributes to obtaining a composite precipitate by performing composite precipitation with Ti and Mo to obtain excellent elongation and stretch flangeability. However, if the V content exceeds 0.15%, the elongation tends to deteriorate. Therefore, when V is contained, the content is preferably 0.15% or less. In addition, 0.001% or more is preferable from the viewpoint of obtaining the effect of reducing the structure of V.
[0054]
It should be noted that there is no problem in characteristics even if one or more of Cr: 0.15% or less, Cu: 0.15% or less, and Ni: 0.15% or less is included.
[0055]
[Production method]
In the present invention, when producing the high-tensile steel, hot rolling is performed at a finish rolling end temperature of 880 ° C. or higher and a winding temperature of 570 ° C. or higher. Hereinafter, these conditions will be described.
[0056]
・ Finishing finish temperature 880 ℃ or more
The finish rolling finish temperature is important for reducing elongation and stretch flangeability and rolling load. If the temperature is less than 880 ° C., the surface layer becomes coarse grains, the elongation and stretch flangeability are impaired, and the amount of strain that occurs due to rolling progression due to non-recrystallization increases, and the rolling load increases significantly. Since the cold rolling becomes difficult, the temperature is set to 880 ° C. or higher.
[0057]
・ Take-up temperature of 570 ℃ or more
The winding temperature is set to 570 ° C. or higher in order to obtain a ferrite structure and to stably feed a thin material while suppressing the amount of water injected on the run-out table. In addition to these, in order to further ensure the running stability of the steel sheet on the run-out table, the temperature is preferably 600 ° C. or higher. In order to suppress the generation of pearlite, the winding temperature is preferably set to 700 ° C. or lower.
[0058]
The high-strength steel sheet of the present invention also includes a hot-dip galvanized steel sheet having a hot-dip galvanized coating film formed on the surface. Since the high-tensile steel sheet of the present invention has good workability, it can maintain good workability even if a hot-dip galvanized coating film is formed. Here, the hot-dip zinc-based plating is hot-dip plating mainly composed of zinc and zinc, and includes those containing alloy elements such as Al and Cr in addition to zinc. The high-strength steel sheet of the present invention that has been subjected to such hot-dip galvanizing may be subjected to an as-plated or post-plating alloying treatment. Regarding the pre-plating annealing temperature, if the temperature is lower than 450 ° C., plating is not performed, and if the temperature exceeds 750 ° C., the strength tends to decrease. Therefore, the annealing temperature is preferably 450 ° C. or more and 750 ° C. or less.
[0059]
In addition, the steel sheet of the present invention has no difference in its properties whether it is black scale or pickling material. There is no particular provision for the temper rolling as long as it is normally performed. In addition, there is no problem whether the hot-dip galvanizing is performed after pickling or as black scale. Electroplating is also possible for zinc plating. There is no particular problem regarding the chemical conversion treatment. The effect of the present invention is not affected by direct rolling, in which hot rolling is performed immediately after casting or after heating for the purpose of supplementary heat. Furthermore, even after the rough rolling and before the finish rolling, even if the rolled material is heated, after the rough rolling, even if the continuous rolling is performed by joining the rolled material, or even if the heating and the continuous rolling of the rolled material are simultaneously performed. The effect of the present invention is not impaired.
[0060]
The high-strength steel sheet of the present invention is excellent in workability, particularly excellent in stretch flangeability, so that when it is press-formed, its characteristics are utilized, such as automobile parts, particularly suspension members such as suspension arms. A member having a complicated cross-sectional shape at the time of pressing can be manufactured with good quality, and in particular, it can contribute to weight reduction of a press-formed product. Hereinafter, a method for processing a high-tensile steel sheet according to the present invention, in other words, a method for manufacturing a press-formed product will be specifically described.
[0061]
FIG. 2 is a flowchart illustrating an example of a work flow of the method for processing a high-tensile steel sheet according to the present invention. This work flow usually has a pre-process of manufacturing the steel sheet according to the present invention or transporting the manufactured steel sheet to a target location, for example, as a coil, and first prepares a high-tensile steel sheet according to the present invention. (S0, S1). Before performing the press working on the steel sheet, the steel sheet may be pre-processed (S2), or may be processed to a predetermined size or shape by a cutting machine (S3). In the former step S2, for example, a cut or perforation is made at a predetermined position in the width direction of the steel sheet, and at the stage where the subsequent press working is completed or in the process of the press working, a press-formed product having a predetermined size and shape or pressed work is performed. It should be able to be separated as a member. In the latter step S3, the dimensions and shape of the final press-formed product are considered in advance, and the steel sheet is processed (and thus cut) into a predetermined size and shape. Thereafter, the members that have gone through the steps of S2 and S3 are subjected to press working, and finally a desired press-formed product having a desired size and shape is manufactured (S4). This press working is usually performed in multiple stages, and often involves three to seven stages.
[0062]
The step of S4 may include a step of further cutting the member having undergone the steps of S2 and S3 into a predetermined size and shape. In this case, the operation of “cutting” is, for example, an operation of separating unnecessary portions from a final press-formed product such as an end portion of a member passing through the steps of S2 and S3 in at least a pressing process. Alternatively, the work may be an operation of separating the member to be pressed along the widthwise cut or perforation of the steel plate provided in the step S2.
[0063]
In FIG. 2, N1 to N3 may be a transfer operation of a steel plate, a member, or a press-formed product mechanically or by an operator.
[0064]
The press-formed product manufactured in this manner is sent to the next step as necessary. As the next step, for example, further processing the press-formed product, adjusting the size and shape, transporting the press-formed product to a predetermined location, storing the process, performing a surface treatment on the press-formed product, pressing There is an assembling process for assembling an object such as an automobile using a molded product.
[0065]
FIG. 3 is a block diagram showing the relationship between the apparatus for actually performing the operation shown in FIG. 2 and the flow of steel plates, members, and press-formed products. In this drawing, a high-tensile steel sheet according to the present invention is prepared in a coil shape, and a press-formed product is manufactured by a press machine. The press machine is of a type that performs a multi-stage press, but the present invention is not limited to this.
[0066]
In some cases, a cutting machine or other pre-processing machine is installed in front of the press working machine (FIG. 3 (a)), and in other cases it is not installed (FIG. 3 (b)). When a cutting machine is installed, a member having a required size or shape is cut from a long steel sheet according to the present invention supplied from a coil, and this member is pressed by a press machine, and a predetermined press is performed. It becomes a molded product. In the case where a pretreatment machine that cuts or perforates the steel sheet in the width direction is installed, cutting may be performed along the notch or perforation in the press machine. When a pre-processing machine is not installed, cutting is performed in the process of pressing a steel plate by a press working machine, and finally a press-formed product having a predetermined size and shape is manufactured. Note that the meaning of “cut” in FIG. 3 is the same as the cut in FIG.
[0067]
The press-formed product manufactured in this manner is excellent in workability as a raw material, and uses the high-tensile steel sheet according to the present invention, which is particularly excellent in stretch flangeability, so that the cross-sectional shape at the time of pressing is complicated. , It can be manufactured with good quality and is lightweight. Such a feature is particularly useful when the press-formed product is an automobile member, particularly a suspension member such as a suspension arm.
[0068]
【Example】
(Example 1)
A steel slab having the chemical components shown in Table 1 was heated to 1250 ° C and finished to a plate thickness of 3.2 mm at a finishing temperature of 880 to 930 ° C by a normal hot rolling process. Thereafter, at a winding temperature exceeding 600 ° C., the cooling rate and the winding temperature were changed to produce steel sheets having various structures.
[0069]
After pickling the obtained steel sheet, the structure of the thin film produced from the steel sheet was observed with a transmission electron microscope (TEM) and the size of the precipitate was measured. The composition of Ti, Nb, V, and Mo in the precipitate was determined by analysis using an energy dispersive X-ray spectrometer (EDX) equipped in a TEM.
[0070]
In addition, JIS No. 5 tensile test pieces and hole expanding test pieces were collected from the obtained steel sheets. Tensile test specimens were taken from the vertical direction of rolling, and in the hole expansion test, a test specimen having a hole punched at 12.5% clearance with a 10 mmφ punch in the center of a 130 mm square steel plate was prepared and punched with a 60 ° conical punch. The hole was pushed up from the direction opposite to the burr side of the hole, the hole diameter d at the time when the crack penetrated the steel plate was measured, and the hole expansion ratio λ was calculated by the following equation.
λ (%) = [(d−10) / 10] × 100
[0071]
Table 1 also shows the structure, the average particle size of the precipitate, the composition of the precipitate (Mo ratio), the tensile strength (TS), the elongation (El), and the hole expansion ratio (λ). In Table 1, the A value is the value of (C / 12) / {(Ti / 48) + (Mo / 96)} in the above equation (1) or (C / 12) in the above equation (2). / {(Ti / 48) + (Nb / 93) + (V / 51) + (Mo / 96)}.
[0072]
As shown in Table 1, the steel of the present invention has Each of Nos. 1 to 10 has a ferrite structure, the average particle size of the precipitate is less than 10 nm, and the Mo ratio (atomic ratio) represented by Mo / (Ti + Nb + V + Mo) is 0.25 or more. It has excellent elongation and stretch flangeability when (TS) is 590 MPa or more. Note that FIG. 2 shows a transmission electron micrograph of the steel sheet No. 2. This photograph shows that the fine precipitates are uniformly dispersed in the ferrite single phase structure.
[0073]
On the other hand, the comparative steel No. In No. 11, pearlite was formed and precipitates were coarsened due to an excessive amount of C and no Mo was added, and both elongation and stretch flangeability were low, and particularly stretch flangeability was low. No. In No. 12, since Mo was not added, the precipitate was coarsened, and both the elongation and the stretch flangeability were low, and particularly the stretch flangeability was low. No. Sample No. 13 has a low C content, so that the amount of precipitates necessary for strengthening the steel is small, so that the tensile strength (TS) is less than 590 MPa. No. In No. 14, segregation is remarkable because the amount of Mn is too large, and since martensite is formed in the structure, both elongation and stretch flangeability are low. No. No. 15 has a small amount of Ti, so that the precipitates necessary for strengthening the steel are insufficient and the tensile strength (TS) is less than 590 MPa. No. In No. 16, although the amount of Ti is too large, a composite precipitate of Ti and Mo is present, but the Mo ratio in the composite precipitate is low, and the amount of Si is too large, so that the precipitate tends to be coarse, And stretch flangeability are both low.
[0074]
[Table 1]

[0075]
(Example 2)
Steels having the components shown in Table 2 were melted to form slabs. Next, hot rolling was performed after heating to the austenite region, and the rolling was completed at 880 ° C. or higher. After rolling, it was cooled to a winding temperature and wound at a winding temperature shown in Table 2. Table 2 also shows the plate thickness. A sample was taken from the center part of the obtained coil width direction, a JIS No. 5 tensile test piece was taken so that the tensile direction was perpendicular to the rolling direction, and a tensile test was performed. Further, the shape of the plate after rolling was visually determined. Table 2 also shows the results. In addition, the evaluation criteria of the plate shape after rolling were as follows: 板 in the case of a visually flat plate, and × in the case of a noticeably wavy plate. Also, the A value in Table 2 shows the value of (C / 12) / {(Ti / 48) + (Nb / 93) + (V / 51) + (Mo / 96)} similarly to Table 1.
[0076]
In Table 2, No. 17-No. 23 shows an example in which the thickness was changed in a 780 MPa class steel sheet and an example in which the Mo / (Ti + Nb + Mo) ratio was changed by changing the winding temperature. No. 2 having a plate thickness of 2.0 mm. 17, No. Paying attention to Nos. 21 to 23, the Mo / (Ti + Nb + Mo) ratio changes with the change of the winding temperature, and the value of No. In Nos. 22 and 23, the strength was maintained by quenching, but the elongation (El) decreased due to the increase in the low-temperature transformation phase. Also, the shape was remarkably wavy. The Mo / (Ti + Nb + Mo) ratio of No. In Nos. 17 to 21, even if slow cooling and high-temperature winding were performed, The strength was maintained as compared with 22 and 23. The plate shape was also good.
[0077]
No. 24-No. Reference numeral 29 denotes an example in which the thickness of the 590 MPa class steel sheet is changed, and an example in which the Mo / (Ti + Nb + Mo) ratio is changed by changing the winding temperature. No. having a board thickness of 1.4 mm. When attention is paid to Nos. 26, 28, and 29, the Mo / (Ti + Nb + Mo) ratio changes with the change in the winding temperature. In Nos. 28 and 29, the strength was maintained by quenching, but the elongation (El) decreased due to the increase in the low-temperature transformation phase. Also, the shape was noticeably wavy. The Mo / (Ti + Nb + Mo) ratio of No. In Nos. 24 to 27, no. The strength was maintained as compared with 28 and 29. In addition, the plate shape was good.
[0078]
[Table 2]

[0079]
(Example 3)
The steel shown in Table 3 was hot-rolled at a finishing temperature of 910 ° C and a winding temperature of 630 ° C to produce a hot-rolled steel sheet having a thickness of about 1.6 mm. After pickling these hot-rolled steel sheets, galvannealing was performed. The structure of the thin film produced from the obtained steel sheet was observed with a transmission electron microscope (TEM), the dimensions of the precipitate were measured, and the composition of Ti, Nb, V, and Mo in the precipitate was mounted on the TEM. Determined from analysis by EDX. Further, JIS No. 5 tensile test pieces and hole expanding test pieces were collected from these plated steel sheets, and a tensile test and a hole expanding test were performed. Table 3 also shows the structure, the average particle size of the precipitate, the composition of the precipitate (Mo ratio), the tensile strength (TS), the elongation (El), and the hole expansion ratio (λ). Note that the A value in Table 3 also indicates the value of (C / 12) / {(Ti / 48) + (Nb / 93) + (V / 51) + (Mo / 96)} as in Table 1.
[0080]
As shown in Table 3, No. 1 of the present invention example. No. 30 shows good values for both El and λ even when hot-dip galvanizing is performed, whereas No. 30 of Comparative Example shows good values. In No. 31, λ was a low value because Mo was not contained in the precipitate.
[0081]
[Table 3]

[0082]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a high-tensile steel sheet excellent in elongation and stretch flangeability, which are indicators of workability, and the effect of contributing to weight reduction of automobile members is remarkable.
[Brief description of the drawings]
FIG. 1 is a transmission electron micrograph showing the metal structure of a high-tensile steel sheet according to the present invention.
FIG. 2 is a flowchart showing an example of a work flow of a method for processing a high-tensile steel sheet according to the present invention.
FIG. 3 is a block diagram showing a relationship between an apparatus for actually performing the operation shown in FIG. 2 and flows of steel plates, members, and press-formed products.

Claims

A high-strength steel sheet having a tensile strength of 590 MPa or more and excellent in workability, which is substantially a ferrite single-phase structure and in which carbides containing Ti and Mo having an average particle size of less than 10 nm are dispersed and precipitated.

The carbide containing Ti and Mo has a composition in which Ti and Mo represented by atomic% satisfy Mo / (Ti + Mo) ≧ 0.25, and the carbide having excellent workability according to claim 1, wherein: Tension steel plate.

The high-tensile steel sheet excellent in workability according to claim 1 or 2, wherein the following formula (1) for defining C, Ti, and Mo in the steel is satisfied .
0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (1)
However, in the above formula (1), C, Ti, and Mo represent mass% of each component.

2. The steel according to claim 1, wherein C: 0.02 to 0.06%, Mo: 0.05 to 0.5%, and Ti: 0.03 to 0.14% by mass% in the steel. A high-tensile steel sheet having excellent workability according to claim 3.

In mass%, C: 0.02-0.06%, Si ≦ 0.3%, Mn: 0.5-2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, the balance being substantially Fe, and substantially a ferrite single phase Tensile strength, characterized in that carbides containing Ti and Mo having a carbide composition satisfying Mo / (Ti + Mo) ≧ 0.25 are dispersed and precipitated, the structure being Ti and Mo expressed in atomic%. Is a high-strength steel sheet excellent in workability of 590 MPa or more.

In mass%, C: 0.02-0.06%, Si ≦ 0.3%, Mn: 0.5-2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, the balance being substantially Fe, and substantially a ferrite single phase Workability with a tensile strength of 590 MPa or more, characterized by satisfying the following expression (1) that defines the structure of C, Ti, and Mo in the steel and in which carbides containing Ti and Mo are dispersed and precipitated. Excellent high-tensile steel plate.
0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (1)
However, in the above formula (1), C, Ti, and Mo represent mass% of each component.

In mass%, C: 0.02-0.06%, Si ≦ 0.3%, Mn: 0.5-2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, the balance being substantially Fe. The high tensile strength steel sheet excellent in workability according to any one of claims 1 to 3.

The carbide contains one or more of Nb and V in addition to Ti and Mo. Instead of Ti and Mo expressed in atomic% satisfying the condition of Mo / (Ti + Mo) ≧ 0.25, the carbide is expressed in atomic%. The high-tensile steel sheet excellent in workability according to claim 2 or 5 , wherein Ti, Mo, Nb, and V represented have a carbide composition satisfying Mo / (Ti + Nb + V + Mo) ≧ 0.25.

The high-tensile steel sheet with excellent workability according to claim 1 , wherein the carbide includes one or more of Nb and V in addition to Ti and Mo.

The high tensile strength steel sheet excellent in workability according to claim 8 or 9 , wherein the following formula (2) for defining C, Ti, Nb, V, and Mo in the steel is satisfied .
0.8 ≦ (C / 12) / {(Ti / 48) + (Nb / 93) + (V / 51) + (Mo / 96)} ≦ 1.5 (2)
However, in the above formula (2), C, Ti, Nb, V, and Mo represent mass% of each component.

Instead of the above (1), C in the steel satisfy Ti, Nb, V, the following equation (2) that the Mo to provisions, the carbides of Ti, in addition to Mo, 1 kind of Nb and V The high-tensile steel sheet excellent in workability according to claim 3 or claim 6 , comprising the above.
0.8 ≦ (C / 12) / {(Ti / 48) + (Nb / 93) + (V / 51) + (Mo / 96)} ≦ 1.5 (2)
However, in the above formula (2), C, Ti, Nb, V, and Mo represent mass% of each component.

The carbides, Ti, expressed in atomic%, Mo, Nb, V is Mo / (Ti + Nb + V + Mo) any one of claim 11 claim 9, characterized in that it comprises a carbide composition satisfying ≧ 0.25 High-tensile steel sheet with excellent workability described in 1.

In steel, C: 0.02 to 0.06%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14% by mass%, and Nb ≦ 0. The high-tensile steel sheet excellent in workability according to any one of claims 8 and 10 to 12, wherein the steel sheet contains at least one of 08% and V ≤ 0.15%.

In mass%, C: 0.02-0.06%, Si ≦ 0.3%, Mn: 0.5-2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, Nb ≦ 0.08%, V ≦ 0.15% The high-tensile steel sheet excellent in workability according to any one of claims 8 to 12 , comprising one or more kinds, and the balance being substantially Fe.

The carbide contains at least one of Nb and V in addition to Ti and Mo, and contains at least one of Nb ≦ 0.08% and V ≦ 0.15% by mass in steel. A high-tensile steel sheet having excellent workability according to claim 4.

The high tensile strength steel sheet excellent in workability according to any one of claims 1 to 15 , which is a thin hot-rolled steel sheet having a thickness of 2.5 mm or less.

The high-tensile steel sheet excellent in workability according to any one of claims 1 to 16 , wherein the high-strength steel sheet has a hot-dip zinc-based plating film on a surface.

In mass%, C: 0.02-0.06%, Si ≦ 0.3%, Mn: 0.5-2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, the balance being substantially molten Fe Hot rolling is performed under the conditions of a rolling end temperature of 880 ° C. or more and a winding temperature of 570 ° C. or more, and substantially a ferrite single phase structure, in which carbides containing Ti and Mo having an average particle diameter of less than 10 nm are dispersed and precipitated. A method for producing a high-tensile steel sheet having excellent workability with a tensile strength of 590 MPa or more, characterized by obtaining a steel sheet.

In mass%, C: 0.02-0.06%, Si ≦ 0.3%, Mn: 0.5-2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, Nb ≦ 0.08%, V ≦ 0.15% A steel containing one or more kinds and a balance substantially consisting of Fe is melted, and hot-rolled at a finish rolling end temperature of 880 ° C. or more and a winding temperature of 570 ° C. or more, thereby substantially forming a ferrite single phase. A method for producing a high-tensile steel sheet excellent in workability having a tensile strength of 590 MPa or more, wherein the steel sheet has a structure and carbides containing Ti and Mo having an average particle size of less than 10 nm are dispersed and precipitated.

A first step of preparing a member made of the high-tensile steel sheet according to any one of claims 1 to 17 , and a second step of performing press forming on the member to form a pressed product having a desired shape. And a method for processing a high-tensile steel sheet having the following.

21. The method for processing a high-tensile steel sheet according to claim 20 , wherein the press-formed product is an automobile part.

An automotive component manufactured from the high-tensile steel sheet according to any one of claims 1 to 17 .