JP3945367B2 - Hot-rolled steel sheet and manufacturing method thereof - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、熱延鋼板及びその製造方法に関し、詳しくは、面内異方性が小さく加工性に優れ、タンデムミルによって製造可能な熱延鋼板及びその製造方法に関する。
【０００２】
【従来の技術】
近年、地球環境保護の観点から、自動車のＣＯ２ 排出量を低減するために車体の軽量化が求められ、鋼板を高強度化して板厚を低減して上記要求に応えることが進められている。
【０００３】
鋼板の高強度化手段のうちでも結晶粒微細化技術が大いに注目されている。これは、結晶粒微細化による高強度化が、環境保護という観点から排除することが望ましい合金元素の添加を必要としないため、或いはその添加量を削減できるためである。しかし、結晶粒の微細化を進めていくと、降伏点が極端に大きくなるとともに伸びが低くなって、加工性が低下してしまう。
【０００４】
高強度化に伴う加工性の低下を抑える目的で高強度鋼における組織の微細化及び第二相組織を制御する手法が、例えば、特許文献１に開示されている。すなわち、特許文献１には、ＮｂやＴｉを含有させた鋼を制御圧延して高強度化と組織微細化を同時に達成する技術が開示されている。しかし、この特許文献で提案された高強度鋼板は降伏比（すなわち、「降伏強度／引張強度」）が高く、このためプレス加工などで成形加工した後のスプリングバックが大きく形状凍結性に劣るものであり、しかも、機械的特性の異方性も大きいものである。
【０００５】
同様に、結晶粒の微細化を図る手段として高圧下率の熱間圧延を行う場合には、一般に、板面内における特性の異方性が大きくなってしまう。
【０００６】
自動車用高強度鋼板としては、高い強度を有することは勿論のこと、多様な部品形状に加工するために、高い成形性が要求され、又、こうした特性の板面内方向による差異（つまり、面内異方性）を低減することも強度の確保や良好な成形性を得るために重要である。しかし、従来の技術はいずれも上記の要求を確実に満たすものではなかった。
【０００７】
【特許文献１】
特開２０００−３２８１８６公報
【０００８】
【発明が解決しようとする課題】
本発明は、上記現状に鑑みてなされたもので、その目的は、タンデムミルによって製造可能な、面内異方性が小さく加工性に優れた板厚が５ｍｍ以下の熱延鋼板及びその製造方法を提供することである。
【０００９】
【課題を解決するための手段】
本発明の要旨は、下記（１）〜（３）に示す熱延鋼板、並びに（４）及び（５）に示す熱延鋼板の製造方法にある。
【００１０】
（１）化学組成が、質量％で、Ｃ：０．００１％を超えて０．３％まで、Ｓｉ：０．０１〜３．０％、Ｍｎ：０．０１〜１．５０％、Ｐ：０．００５〜０．５％及びＳ：０．０５％以下を含み、残部はＦｅ及び不純物からなる、体積割合で７５％以上のフェライトを含む板厚が５ｍｍ以下の熱延鋼板であって、板厚全体にわたるフェライトの平均粒径が１．１μｍ以上４μｍ未満、表面から板厚の１／８の深さの部位におけるフェライトの平均粒径ｄｓの板厚中心部におけるフェライトの平均粒径ｄｃに対する比率ｄｓ／ｄｃが０．４以上０．８未満、表面から板厚の１／８の深さの部位における圧延方向の＜１１１＞極密度が集合組織をもたないものの１．５倍以上及び、板厚中心部における板幅方向の＜１１１＞極密度が集合組織を持たないものの１．５倍以上である熱延鋼板。
【００１１】
（２）化学組成が、Ｆｅの一部に代えて、下記（ａ）群から選ばれる少なくとも１種以上の成分を含む上記（１）に記載の熱延鋼板。
（ａ）質量％で、Ｎｂ：０．３％以下、Ｔｉ：０．３％以下、Ｖ：０．３％以下、Ｃｕ：１．０％以下、Ｎｉ：１．０％以下、Ｃｒ：１．０％以下、Ｍｏ：１．０％以下及びＢ：０．００５％以下。
【００１２】
（３）化学組成が、Ｆｅの一部に代えて、下記（ｂ）群から選ばれる少なくとも１種以上の成分を、質量％で、合計で０．００５％以下含む上記（１）又は（２）に記載の熱延鋼板。
（ｂ）Ｃａ、ＲＥＭ（希土類元素）及びＭｇ。
【００１３】
（４）上記（１）から（３）までのいずれかに記載の化学組成を有する鋼塊若しくは鋼片をＡｃ3点以上の温度に加熱した後に、又は上記（１）から（３）までのいずれかに記載の化学組成を有する鋳造後の鋼塊若しくは熱間加工後の鋼片をＡｒ3点以下の温度域まで温度低下させることなしに、Ａｒ3点以上の温度域で粗圧延を行い、次いで、仕上げ圧延における合計圧下量を７０％以上、仕上げ温度をＡｒ3点〜「Ａｒ3点＋１００℃」の温度域の温度として仕上げ圧延を行い、前記仕上げ圧延を終了した後０．５秒以内に冷却を開始して、仕上げ温度から「Ａｒ3点−１００℃」までを４００℃／秒以上の平均冷却速度で冷却し、その後６００℃以下の温度で巻き取ることを特徴とする上記（１）から（３）までのいずれかに記載の熱延鋼板の製造方法。
【００１４】
（５）上記（１）から（３）までのいずれかに記載の化学組成を有する鋼塊若しくは鋼片をＡｃ3点以上の温度に加熱した後に、又は上記（１）から（３）までのいずれかに記載の化学組成を有する鋳造後の鋼塊若しくは熱間加工後の鋼片をＡｒ3点以下の温度域まで温度低下させることなしに、Ａｒ3点以上の温度域で粗圧延を行い、次いで、仕上げ圧延において最終の一段前のスタンドまでの仕上げ圧延の合計圧下量を７０％以上、最終の一段前のスタンド出側温度をＡｒ3点〜「Ａｒ3点＋１００℃」の温度域の温度として圧延し、最終の一段前のスタンドでの圧延を終了した後０．５秒以内に冷却を開始して、最終の一段前のスタンド出側温度から「Ａｒ3 点−１００℃」までを４００℃／秒以上の平均冷却速度で冷却し、次いで、最終スタンドで圧下量１０％以下の圧延を施し、その後６００℃以下の温度で巻き取ることを特徴とする上記（１）から（３）までのいずれかに記載の熱延鋼板の製造方法。
【００１５】
ここで、或る相の体積割合は面積割合に等しいことが知られており、したがって、上記フェライトが組織に占める体積割合は、例えば、通常の２次元的な評価方法によって求めたフェライトの面積割合から決定することができる。このため、本発明においては、光学顕微鏡で表面から板厚の１／４の深さの部位における任意の１０視野以上についていわゆる「メッシュ法」によってフェライトの面積割合を求め、それを平均した値を体積割合とする。
【００１６】
なお、本発明の熱延鋼板におけるフェライト以外の「相」は、パーライト、ベイナイト、マルテンサイトやオーステナイト（オーステナイトが変態せずに残ったいわゆる「残留オーステナイト」）などどんな相であってもよい。
【００１７】
「板厚全体にわたるフェライトの平均粒径」は、表面から板厚の１／８の深さの部位、１／４の深さの部位及び１／２の深さの部位（つまり、中心部）におけるそれぞれのフェライトの平均粒径を平均したものをいう。なお、上記の表面から板厚の１／８の深さの部位、１／４の深さの部位及び１／２の深さの部位におけるフェライトの平均粒径は、それぞれの部位で任意の１０視野以上について、いわゆる「切片法」で求めた平均切片長さを１．１２倍して得たものをいう。
【００１８】
「ＲＥＭ（希土類元素）」は、Ｓｃ、Ｙ及びランタノイドの合計１７元素の総称であり、ＲＥＭの含有量は上記元素の合計含有量を指す。
【００１９】
１パス当たりの圧下量（％）とは、ｎパス目の圧延前の被圧延材厚みをｔｎｉ、圧延後の被圧延材厚みをｔｎｏとしたとき｛（ｔｎｉ−ｔｎｏ）／ｔｎｉ｝×１００で求められるものをいう。又、ｎを２以上の整数として、ｎパスからなる圧延の合計圧下量（％）とは、当該圧延の１パス目の圧延前の被圧延材厚みをｔ１Ｉ、ｎパス目の圧延後の被圧延材厚みをｔｎｆとしたとき｛（ｔ１ｉ−ｔｎｏ）／ｔ１ｉ｝×１００で求められるものをいう。
【００２０】
本発明でいう温度は鋼板表面における温度をいい、「平均冷却速度」とは冷却前後の温度差を冷却時間で除したものをいう。
【００２１】
なお、本発明でいう「鋼板」とはコイル状に巻かれた「鋼帯」を含むものである。
【００２２】
以下、上記（１）〜（３）の熱延鋼板に係る発明、並びに（４）及び（５）のその製造方法に係る発明をそれぞれ（１）〜（５）の発明という。
【００２３】
本発明者らは、前記した目的を達成するために板厚全体にわたるフェライトの平均粒径、板厚方向の平均粒径の変化量や板厚方向の集合組織の変化と各種特性との関係及び鋼板の製造条件との関係などについて種々検討を行った。その結果、下記（ａ）〜（ｄ）の知見を得た。
【００２４】
（ａ）板厚方向における結晶粒組織の大きな変化が、加工性低下の原因の一つであり、大きな異方性を生じる原因でもある。
【００２５】
（ｂ）微細組織による高強度特性を維持しながら、加工性の低下や異方性が生じることを低減するには、板厚方向の組織変化を解消するよりも、これを制御して利用すればよい。
【００２６】
（ｃ）板厚方向の組織変化に関して制御すべき内容は、板厚全体にわたるフェライトの平均粒径、板厚方向のフェライトの平均粒径の変化量及び集合組織である。
【００２７】
（ｄ）熱間での仕上げ圧延条件及び仕上げ圧延直後の冷却条件を適正化することで、面内異方性が小さく加工性に優れた熱延鋼板が得られる。
【００２８】
前記（１）〜（５）の本発明は、上記の知見に基づいて完成されたものである。
【００２９】
【発明の実施の形態】
以下、本発明の各要件について詳しく説明する。
（Ａ）熱延鋼板の組織
本発明に係る熱延鋼板は、先ず第一に「体積割合で７５％以上のフェライトを含む板厚が５ｍｍ以下の熱延鋼板」とする。
【００３０】
本発明に係る異方性が小さく良好な加工性を有する熱延鋼板は、微細組織を有する熱延鋼板の製造時に生じる板表面近傍と内部の組織の違いを制御するだけではなく、積極的に利用して所望の特性を与えるものである。熱延鋼板の板厚が５ｍｍを超えると、所望の微細組織及び板表面近傍と内部の組織の違いを制御できなくなるので、熱延鋼板の板厚は５ｍｍ以下に限定する。
【００３１】
フェライトの体積割合は十分に大きく、しかもフェライト以外の他の相の合計割合よりも大きく７５％以上でなければならない。これは、フェライトの体積割合が７５％未満の場合には、延性が低下して良好な加工性が得られないためである。したがって、フェライトの体積割合を７５％以上とした。なお、フェライト以外の相（以下、このフェライト以外の相をまとめて第二相という）は、パーライト、ベイナイト、マルテンサイトやオーステナイトが変態せずに残ったいわゆる「残留オーステナイト」などどんな相であってもよい。第二相の種類と体積割合を調整することにより、用いられる分野や部品に一層適した、強度と加工性とのバランスを有する熱延鋼板を得ることができる。
【００３２】
なお、既に述べたように、或る相の体積割合は面積割合に等しいことが知られているので、本発明においては、光学顕微鏡で表面から板厚の１／４の深さの部位における任意の１０視野以上についていわゆる「メッシュ法」によってフェライトの面積割合を求め、それを平均した値を体積割合とする。
【００３３】
フェライト結晶粒を微細化することで、鋼板の強度を高めることができ、この効果は特に、板厚全体にわたるフェライトの平均粒径が４μｍ未満の場合に大きい。しかし、フェライト粒径が過度に小さくなると、降伏点と引張強度とがほぼ等しくなって降伏比が１に近づくので伸びが極端に低下してしまう。特に、上記板厚全体にわたるフェライトの平均粒径が１．１μｍを下回ると、伸びの低下が著しい。したがって、板厚全体にわたるフェライトの平均粒径を１．１μｍ以上４μｍ未満とした。なお、板厚全体にわたるフェライトの好ましい平均粒径は１．５μｍ以上、より好ましくは１．８μｍ以上である。
【００３４】
ここで、「板厚全体にわたるフェライトの平均粒径」は、表面から板厚の１／８の深さの部位、１／４の深さの部位及び１／２の深さの部位（つまり、中心部）におけるそれぞれのフェライトの平均粒径を平均したものをいい、上記の表面から板厚の１／８の深さの部位、１／４の深さの部位及び１／２の深さの部位におけるフェライトの平均粒径は、それぞれの部位で任意の１０視野以上について、いわゆる「切片法」で求めた平均切片長さを１．１２倍して得たものをいうことは既に述べたとおりである。
【００３５】
板厚方向におけるフェライトの平均粒径の変動は熱延鋼板の強度及び、強度と延性のバランスに影響し、この熱延鋼板の特性に及ぼす板厚方向におけるフェライトの平均粒径の変動の影響は、表面から板厚の１／８の深さの部位におけるフェライトの平均粒径ｄｓと表面から板厚の１／２の深さの部位（つまり、中心部）におけるフェライトの平均粒径ｄｃとの比である「ｄｓ／ｄｃ」の値によって整理できる。すなわち、ｄｓ／ｄｃの値が０．４未満の場合には、板厚表面近傍の降伏比が高くなって鋼板そのものの伸びが特に低下するとともに、伸びフランジ性の低下をきたす。一方、ｄｓ／ｄｃの値が０．８以上になると強度上昇の程度が小さい。したがって、ｄｓ／ｄｃを０．４以上０．８未満とした。
【００３６】
フェライトの＜１１１＞方向は原子が稠密に配列し、最もヤング率の高い方向であり、他の種々の機械特性にも影響を与える。そして、表面から板厚の１／８の深さの部位における圧延方向の＜１１１＞極密度（以下、＜１１１＞ｒｓと表記する）が集合組織をもたないものの１．５倍未満になるか、板厚中心部における板幅方向の＜１１１＞極密度（以下、＜１１１＞ｔｃと表記する）が集合組織を持たないものの１．５倍未満になれば、延性及び伸びフランジ性が低下するとともにに、強度の面内異方性が大きくなってしまう。したがって、＜１１１＞ｒｓ及び＜１１１＞ｔｃを、いずれも集合組織をもたないものの１．５倍以上と規定した。
【００３７】
以上述べたことから、（１）〜（３）の発明においては、体積割合で７５％以上のフェライトを含む板厚が５ｍｍ以下の熱延鋼板で、板厚全体にわたるフェライトの平均粒径が１．１μｍ以上４μｍ未満、前記ｄｓ／ｄｃが０．４以上０．８未満、＜１１１＞ｒｓ及び＜１１１＞ｔｃがいずれも集合組織をもたないものの１．５倍以上である熱延鋼板と規定した。
（Ｂ）熱延鋼板の化学組成
前記（１）の発明に係る熱延鋼板は、その化学組成に関しては次のとおり規定する。なお、前項（Ａ）で述べた組織を安定且つ確実に確保するとともに内面異方性が小さく加工性に優れた熱延鋼板を得るために、熱延鋼板の化学組成を（２）及び（３）の発明で規定するものとするのがよい。なお、以下の説明における各元素の含有量の「％」表示は「質量％」を意味する。
【００３８】
Ｃ：
Ｃは、鋼板の強度を高める好ましい成分であり、０．００１％を超える含有量とする必要がある。しかし、その含有量が０．３％を超えると加工性の低下や溶接性の劣化を招くことがある。したがって、Ｃの含有量は０．００１を超えて０．３％以下とする。
【００３９】
Ｓｉ：
Ｓｉは、固溶強化によって鋼板の強度を向上させる好ましい成分である。この効果を得るためにはＳｉの含有量は０．０１％以上が必要である。しかし、Ｓｉを３．０％を超えて含有させると鋼表面に酸化スケールが多量に生成され、製造上の困難を伴うことがある。したがって、Ｓｉの含有量は０．０１〜３．０％とする。
【００４０】
Ｍｎ：
Ｍｎは、鋼の強度を上昇させるのに有効な元素である。この効果を確実に得るには、Ｍｎは０．０１％以上の含有量が必要である。しかし、Ｍｎの含有量が１．５０％を超えると鋼塊に中心偏析を生じて熱間での圧延後にバンド組織を形成し伸びフランジ加工性の著しい低下を招くことがある。したがって、Ｍｎの含有量は０．０１〜１．５０％とする。
【００４１】
Ｐ：
Ｐは、固溶強化によって鋼板の強度を高める作用を有する。この効果を確実に得るには、Ｐは０．００５％以上の含有量が必要である。しかし、Ｐを０．５％を超えて含有させると粒界偏析による脆化をきたし、鋳造の際に鋼塊に割れを起こしやすく、製造上の困難を伴うことがある。したがって、Ｐの含有量は０．００５〜０．５％とする。
【００４２】
Ｓ：
Ｓは、Ｆｅ又はＭｎと結合して硫化物を形成し、加工条件が厳しい部位においてはボイド発生の起点となりやすく、加工性を低下させるため、その含有量は０．０５％以下に抑える必要がある。
【００４３】
前記（１）の発明に係る熱延鋼板は、上記のＣからＳまでの元素と、残部がＦｅ及び不純物からなる化学組成を有するものである。
【００４４】
前記（２）の発明に係る熱延鋼板は、前記（１）の発明に記載の成分中のＦｅの一部に代えて、下記（ａ）群から選ばれる少なくとも１種以上の成分を含む化学組成を有するものである。また、前記（３）の発明に係る熱延鋼板は、前記（１）又は（２）の発明に記載の成分中のＦｅの一部に代えて、下記（ｂ）群から選ばれる少なくとも１種以上の成分を合計で０．００５％以下含む化学組成を有するものである。
【００４５】
（ａ）Ｎｂ：０．３％以下、Ｔｉ：０．３％以下、Ｖ：０．３％以下、Ｃｕ：１．０％以下、Ｎｉ：１．０％以下、Ｃｒ：１．０％以下、Ｍｏ：１．０％以下及びＢ：０．００５％以下、
（ｂ）Ｃａ、ＲＥＭ（希土類元素）及びＭｇ。
【００４６】
ここで、上記（ａ）群に記載のＮｂからＢまでのいずれの元素も鋼の強度を一層高める作用を有するので、ＮｂからＢまでの元素は、以下に述べる範囲内でそれぞれを単独で含有させてもよいし、２種以上を複合して含有させてもよい。
【００４７】
又、上記（ｂ）群に記載のＣａからＭｇまでのいずれの元素もＭｎＳの形態を制御して熱延時に展伸しにくい介在物を形成し、加工性の低下を防止する作用を有するので、ＣａからＭｇまでの元素は、以下に述べる範囲内でそれぞれを単独で含有させてもよいし、２種以上を複合して含有させてもよい。
【００４８】
なお、ＲＥＭは、前述のとおりＳｃ、Ｙ及びランタノイドの合計１７元素を指し、ランタノイドの場合、工業的にはミッシュメタルの形で添加される。本発明でいうＲＥＭの含有量が上記元素の合計含有量を指すことは既に述べたとおりである。
【００４９】
（ａ）群（Ｎｂ、Ｔｉ、Ｖ、Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ及びＢ）：
Ｎｂ、Ｔｉ、Ｖ、Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ及びＢは、いずれも鋼の強度を一層高める作用を有する元素である。これらの元素のうちでＮｂ、Ｔｉ及びＶは、組織を微細化する作用も有する。前記の効果を確実に得るには、Ｎｂ、Ｔｉ、Ｖ、Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ及びＢはそれぞれ０．０１％以上、０．０１％以上、０．０１％以上、０．０５％以上、０．０５％以上、０．０５％以上、０．０５％以上及び０．０００１％以上の含有量とすることが好ましい。しかし、Ｎｂ、Ｔｉ及びＶはいずれも０．３％を超えて超えて含有すると異方性を大きくし、延性も低下させてしまう。Ｃｕ、Ｎｉ、Ｃｒ及びＭｏはいずれも１．０％を超えて、Ｂは０．００５％を超えてそれぞれ含有すると、延性の低下をきたす。したがって、Ｎｂ、Ｔｉ、Ｖ、Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ及びＢを添加する場合には、Ｎｂ、Ｔｉ及びＶの含有量はいずれも０．３％以下、Ｃｕ、Ｎｉ、Ｃｒ及びＭｏの含有量はいずれも１．０％以下、Ｂの含有量は０．００５％以下とするのがよい。
【００５０】
なお、Ｃｕを約０．３％を超えて含有させると熱間加工時に表面割れを起こすことがあるので、Ｃｕを約０．３％を超えて含有させる場合には、Ｃｕの半量以上のＮｉを同時に含有させることが望ましい。
【００５１】
（ｂ）群（Ｃａ、ＲＥＭ及びＭｇ）：
Ｃａ、ＲＥＭ及びＭｇは、いずれもＭｎＳの形態を制御して熱延時に展伸しにくい介在物を形成し、加工性の低下を防止する作用を有する元素である。この効果を確実に得るには、Ｃａは０．０００２％以上、ＲＥＭは０．０００２％以上、Ｍｇは０．０００２％以上の含有量とすることが好ましい。しかし、Ｃａ、ＲＥＭ及びＭｇのうちの１種又は２種以上の合計で０．００５％を超えて含有させても前記の効果が飽和してコストが嵩むばかりである。したがって、Ｃａ、ＲＥＭ及びＭｇを添加する場合には、Ｃａ、ＲＥＭ及びＭｇのうちの１種又は２種以上の合計で０．００５％以下の含有量とするのがよい。
（Ｃ）熱延鋼板の製造条件
これまでに述べた（１）〜（３）の発明に係る前記（Ａ）項で述べた組織及び（Ｂ）項で述べた化学組成を有する熱延鋼板は、例えば、「鋼塊若しくは鋼片をＡｃ3点以上の温度に加熱した後に、又は鋳造後の鋼塊若しくは熱間加工後の鋼片をＡｒ3点以下の温度域まで温度低下させることなしに、Ａｒ3点以上の温度域で粗圧延を行い、次いで、仕上げ圧延における合計圧下量を７０％以上、仕上げ温度をＡｒ3点〜「Ａｒ3点＋１００℃」の温度域の温度として仕上げ圧延を行い、前記仕上げ圧延を終了した後０．５秒以内に冷却を開始して、仕上げ温度から「Ａｒ3点−１００℃」までを４００℃／秒以上の平均冷却速度で冷却し、その後６００℃以下の温度で巻き取る」前記（４）の発明によって比較的容易に製造することができる。
【００５２】
すなわち、鋼塊又は鋼片をＡｃ３ 点以上の温度に再加熱することにより、合金元素をオーステナイト中に固溶させることができる。ここで、加熱炉や均熱炉など再加熱処理のための炉への装入は、鋳造後や熱間加工後の高温のままの状態で行ってもよいし、一旦室温近傍まで冷却した状態から行ってもよい。
【００５３】
上記のようにしてＡｃ３ 点以上の温度に再加熱した鋼塊若しくは鋼片、又はＡｒ３ 点以下の温度域まで温度低下していない鋳造後の鋼塊若しくは熱間加工後の鋼片に、タンデムミルによる圧延を施すのであるが、この際の粗圧延はオーステナイト領域であるＡｒ３ 点以上の温度で行うのがよい。粗圧延した後、仕上げ圧延はその合計圧下量を７０％以上、仕上げ温度をＡｒ３ 点〜「Ａｒ３ 点＋１００℃」の温度域の温度として行うのがよい。
【００５４】
仕上げ圧延の合計圧下量が７０％を下回ると、フェライト結晶粒が粗大化して、機械的性質が低下するし、異方性も大きくなる。なお、仕上げ圧延の合計圧下量は９０％以上とすることが一層よい。
【００５５】
仕上げ圧延における仕上げ温度が「Ａｒ３ 点＋１００℃」を超える場合も、フェライト結晶粒が粗大化して、機械的性質が低下するし、異方性も大きくなる。一方、前記仕上げ温度がＡｒ３ 点を下回ると加工フェライト粒が残存しやすくなって異方性が極めて強くなることがある。
【００５６】
前記仕上げ圧延を終了した後は、空冷の時間をほとんど与えることなく強冷却を行うことにより、フェライト粒が極めて細粒化するとともに、＜１１１＞ｒｓ及び＜１１１＞ｔｃが発達し、容易に前記（Ａ）項で述べた集合組織が形成されるとともに組織の微細化が促進される。なお、前記方位の発達は、平均冷却速度が速いほど、且つ、仕上げ圧延を終了してから冷却開始までの時間が短いほど大きい。このため、仕上げ圧延を終了した後は０．５秒以内に冷却を開始するのがよい。冷却開始までの時間は０．１秒以内とすることが一層好ましい。又、冷却する際の平均冷却速度は４００℃／秒以上とするのがよい。この平均冷却速度は６００℃／秒以上であれば一層好ましい。なお、上記の平均冷却速度で冷却するのは仕上げ温度から少なくとも「Ａｒ３ 点−１００℃」までである。これは「Ａｒ３ 点−１００℃」を超える温度までしか冷却しない場合には、フェライト粒が粗大化するとともに、所望の集合組織が得られない場合があるからである。「Ａｒ３ 点−１００℃」未満の温度まで冷却することを妨げないが、鋼によってはフェライトの体積割合が減少することがある。
【００５７】
前記条件で冷却した後は６００℃以下の温度で巻き取るのがよい。巻き取り温度が６００℃を超えると、上記の熱延及び冷却をすることにより得られた微細なフェライトが成長して粗大化するためである。
【００５８】
したがって、（４）の発明においては、上記（１）から（３）までのいずれかの発明に係る化学組成を有する鋼塊若しくは鋼片をＡｃ3点以上の温度に加熱した後に、又は上記（１）から（３）までのいずれかの発明に係る化学組成を有する鋳造後の鋼塊若しくは熱間加工後の鋼片をＡｒ3点以下の温度域まで温度低下させることなしに、Ａｒ3点以上の温度域で粗圧延を行い、次いで、仕上げ圧延における合計圧下量を７０％以上、仕上げ温度をＡｒ3点〜「Ａｒ3点＋１００℃」の温度域の温度として仕上げ圧延を行い、前記仕上げ圧延を終了した後０．５秒以内に冷却を開始して、仕上げ温度から「Ａｒ3点−１００℃」までを４００℃／秒以上の平均冷却速度で冷却し、その後６００℃以下の温度で巻き取ることとした。
【００５９】
（Ａ）項で述べた組織は、「鋼塊若しくは鋼片をＡｃ3点以上の温度に加熱した後に、又は鋳造後の鋼塊若しくは熱間加工後の鋼片をＡｒ3点以下の温度域まで温度低下させることなしに、Ａｒ3点以上の温度域で粗圧延を行い、次いで、仕上げ圧延において最終の一段前のスタンドまでの仕上げ圧延の合計圧下量を７０％以上、最終の一段前のスタンド出側温度をＡｒ3点〜「Ａｒ3点＋１００℃」の温度域の温度として圧延し、最終の一段前のスタンドでの圧延を終了した後０．５秒以内に冷却を開始して、最終の一段前のスタンド出側温度から「Ａｒ3点−１００℃」までを４００℃／秒以上の平均冷却速度で冷却し、次いで、最終スタンドで圧下量１０％以下の圧延を施し、その後６００℃以下の温度で巻き取る」ことによっても比較的容易に得られる。この方法は、最終スタンドでは被圧延材の表面に乗っている冷却水を除去し、且つ、冷却後に析出したフェライトに歪をほとんど与えない程度の軽い圧延を施すことにより、板厚方向の温度分布を緩和してｄｓ／ｄｃが過度に小さくなるのを防ぐものである。なお、最終スタンドにおける１０％を超える圧下は、加工フェライトを生成させるので最終スタンドの圧下量は１０％以下とするのがよい。一層好ましくは５％以下である。冷却水を除去するだけで圧下量は０％としてもよい。
【００６０】
したがって、（５）の発明においては、上記（１）から（３）までのいずれかの発明に係る化学組成を有する鋼塊若しくは鋼片をＡｃ3点以上の温度に加熱した後に、又は上記（１）から（３）までのいずれかの発明に係る化学組成を有する鋳造後の鋼塊若しくは熱間加工後の鋼片をＡｒ3点以下の温度域まで温度低下させることなしに、Ａｒ3点以上の温度域で粗圧延を行い、次いで、仕上げ圧延において最終の一段前のスタンドまでの仕上げ圧延の合計圧下量を７０％以上、最終の一段前のスタンド出側温度をＡｒ3点〜「Ａｒ3点＋１００℃」の温度域の温度として圧延し、最終の一段前のスタンドでの圧延を終了した後０．５秒以内に冷却を開始して、最終の一段前のスタンド出側温度から「Ａｒ3点−１００℃」までを４００℃／秒以上の平均冷却速度で冷却し、次いで、最終スタンドで圧下量１０％以下の圧延を施し、その後６００℃以下の温度で巻き取ることとした。
【００６１】
１パス当たりの圧下量（％）とは、ｎパス目の圧延前の被圧延材厚みをｔｎｉ、圧延後の被圧延材厚みをｔｎｏとしたとき｛（ｔｎｉ−ｔｎｏ）／ｔｎｉ｝×１００で求められるものをいい、又、ｎを２以上の整数として、ｎパスからなる圧延の合計圧下量（％）とは、当該圧延の１パス目の圧延前の被圧延材厚みをｔ１Ｉ、ｎパス目の圧延後の被圧延材厚みをｔｎｆとしたとき｛（ｔ１ｉ−ｔｎｏ）／ｔ１ｉ｝×１００で求められるものをいうこと、「平均冷却速度」が冷却前後の温度差を冷却時間で除したものを指し、本発明でいう温度が鋼板表面における温度をいうことは既に述べたとおりである。
【００６２】
以下、実施例により本発明を更に詳しく説明する。
【００６３】
【実施例】
（実施例１）
表１に示す化学組成を有するスラブを連続鋳造により製造し、連続鋳造終了後に室温まで空冷した。その後、上記のスラブを１２００〜１３００℃の温度域に再加熱した後、試験用小型タンデムミルを使用して、表２に示す条件で仕上圧延、冷却及び巻き取りして、板厚が１．８〜５．５ｍｍの鋼板を得た。なお、スラブ加熱温度は１２５０℃で粗圧延は通常の方法で行った。
【００６４】
【表１】
【表２】
得られた鋼板から試験片を採取し、組織、常温での引張特性及び伸びフランジ加工性を調査した。
【００６７】
組織は、光学顕微鏡と走査型電子顕微鏡を用いて、鋼板板厚の断面組織を観察し、又、＜１１１＞極密度は通常のＸ線測定方法により求めた。
【００６８】
常温での引張特性は、ＪＩＳ５号試験片を用いて調査した。なお、試験片は圧延方向を０゜として、０゜、４５゜及び９０゜の３方向から採取した。
【００６９】
伸びフランジ加工性は、縦横それぞれ１００ｍｍの正方形の試験片を採取し、その中央にポンチで直径が１０ｍｍの打ち抜き穴をあけ、先端角６０゜の円錐ポンチでこの穴を拡げて、穴の縁にクラックが貫通する限界の穴直径から計算される限界穴拡げ率によって評価した。
【００７０】
表３〜５に、前記の各調査結果をまとめて示す。この表において平均ｒ値はｒ0、ｒ45及びｒ90をそれぞれ試験片採取方向が０゜、４５゜及び９０゜の場合のｒ値として、「（ｒ0＋２ｒ45＋ｒ90）／４」の式で計算した値を指す。
【００７１】
【表３】
【表４】
【表５】
表３〜５から明らかなように、本発明の組織と化学組成の規定を満たす試験番号１〜３、１１、１３、１５、１６及び１８〜２０の熱延鋼板は異方性が小さく優れた加工性を有している。
【００７５】
これに対して、本発明の組織と化学組成の少なくともいずれかの規定から外れる試験番号４〜７、９、１０、１２、１４、１７及び２２の熱延鋼板は、延性、伸びフランジ加工性、面内異方性が劣り、加工性に劣るものである。
【００７６】
（実施例２）
連続鋳造により製造し、連続鋳造終了後に室温まで空冷した前記実施例１に示した鋼Ａのスラブを１２００〜１３００℃の温度域に再加熱した後、試験用小型タンデムミルを使用して、表６に示す条件で仕上圧延、冷却及び巻き取りして、板厚が１．９及び２．０ｍｍの鋼板を得た。なお、スラブ加熱温度は１２５０℃で粗圧延は通常の方法で行った。
【００７７】
【表６】
得られた鋼板から試験片を採取し、実施例１と同様にして組織、常温での引張特性及び伸びフランジ加工性を調査した。
【００７９】
表７及び表８に、前記の各調査結果をまとめて示す。なお表７における平均ｒ値も前記の「（ｒ0＋２ｒ45＋ｒ90）／４」の式で計算した値を指す。
【００８０】
【表７】
【表８】
表７及び表８から明らかなように、本発明の組織と化学組成の規定を満たす試験番号２４及び２５の熱延鋼板は異方性が小さく優れた加工性を有している。一方、本発明の化学組成の規定を満たす場合であっても、組織規定から外れる試験番号２６の熱延鋼板は加工性が低く異方性も極めて大きい。
【００８３】
【発明の効果】
本発明の熱延鋼板は、面内異方性が小さく加工性に優れるので、自動車に用いられる高強度構造部材の素材として利用することができる。本発明の熱延鋼板は本発明の方法によって比較的容易に製造することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot-rolled steel sheet and a method for producing the same, and more particularly to a hot-rolled steel sheet having a small in-plane anisotropy and excellent workability and capable of being produced by a tandem mill and a method for producing the same.
[0002]
[Prior art]
In recent years, from the viewpoint of protecting the global environment, automobile CO2 In order to reduce the amount of discharge, a reduction in weight of the vehicle body is required, and efforts are being made to meet the above requirements by increasing the strength of the steel plate to reduce the plate thickness.
[0003]
Of the means for increasing the strength of steel sheets, the crystal grain refinement technique has received much attention. This is because the increase in strength due to the refinement of crystal grains does not require the addition of an alloy element that is desirably excluded from the viewpoint of environmental protection, or the amount added can be reduced. However, as the grain refinement proceeds, the yield point becomes extremely large and the elongation becomes low, so that the workability deteriorates.
[0004]
For example, Patent Document 1 discloses a technique for miniaturizing a structure and controlling a second phase structure in high-strength steel for the purpose of suppressing a decrease in workability associated with an increase in strength. That is, Patent Document 1 discloses a technique for simultaneously achieving high strength and fine structure by controlled rolling of steel containing Nb and Ti. However, the high-strength steel plate proposed in this patent document has a high yield ratio (ie, “yield strength / tensile strength”), and therefore, the spring back after forming by pressing or the like is large and the shape freezeability is inferior. In addition, the anisotropy of mechanical properties is also large.
[0005]
Similarly, when hot rolling at a high pressure reduction is performed as a means for refining crystal grains, generally anisotropy of characteristics in the plate surface is increased.
[0006]
High-strength steel sheets for automobiles not only have high strength, but also require high formability in order to be processed into a variety of component shapes. It is also important to reduce the inner anisotropy) in order to ensure strength and obtain good moldability. However, none of the conventional techniques reliably satisfy the above requirements.
[0007]
[Patent Document 1]
JP 2000-328186 A
[0008]
[Problems to be solved by the invention]
The present invention has been made in view of the above-described present situation, and an object thereof is a hot-rolled steel sheet having a thickness of 5 mm or less, which has a small in-plane anisotropy and excellent workability, and can be produced by a tandem mill. Is to provide.
[0009]
[Means for Solving the Problems]
The gist of the present invention resides in the hot-rolled steel sheets shown in the following (1) to (3), and the hot-rolled steel sheets shown in (4) and (5).
[0010]
(1)Chemical composition is mass%, C: more than 0.001% to 0.3%, Si: 0.01-3.0%, Mn: 0.01-1.50%, P: 0.005 -0.5% and S: 0.05% or less, the balance consists of Fe and impurities,By volume ratio75% In a hot rolled steel sheet having a thickness of 5 mm or less, wherein the ferrite has an average grain size of 1.1 μm or more and less than 4 μm and a depth of 1/8 of the thickness from the surface. The ratio ds / dc of the average grain diameter ds of ferrite to the average grain diameter dc of ferrite in the center part of the sheet thickness is 0.4 or more and less than 0.8, and in the rolling direction at a depth of 1/8 of the sheet thickness from the surface. Hot rolling with <111> pole density not less than 1.5 times that without texture, and <111> pole density in the plate width direction at the center of the plate thickness being not less than 1.5 times that without texture steel sheet.
[0011]
(2) The hot rolled steel sheet according to (1), wherein the chemical composition includes at least one component selected from the following group (a) instead of a part of Fe.
(A)% By massNb: 0.3% or less, Ti: 0.3% or less, V: 0.3% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less and B: 0.005% or less.
[0012]
(3) Instead of a part of Fe, the chemical composition is at least one component selected from the following group (b)In mass%The hot rolled steel sheet according to the above (1) or (2), which contains 0.005% or less in total.
(B) Ca, REM (rare earth element) and Mg.
[0013]
(4)The chemical composition according to any one of (1) to (3) aboveSteel ingot or steel slab is AcThreeAfter heating to a temperature above the point, orThe chemical composition according to any one of (1) to (3) aboveA steel ingot after casting or a steel piece after hot workingThreeWithout reducing the temperature to a temperature range below the point, ArThreeRough rolling is performed in the temperature range above the point, then the total rolling reduction in the finish rolling is 70% or more, and the finishing temperature is ArThreePoint ~ "ArThreeThe finish rolling is performed at a temperature in the temperature range of “point + 100 ° C.”, and cooling is started within 0.5 seconds after finishing the finish rolling.ThreeThe heat according to any one of (1) to (3) above, wherein the point is cooled to “−100 ° C.” at an average cooling rate of 400 ° C./second or more and then wound up at a temperature of 600 ° C. or less. A method for producing rolled steel sheets.
[0014]
(5)The chemical composition according to any one of (1) to (3) aboveSteel ingot or steel slab is AcThreeAfter heating to a temperature above the point, orThe chemical composition according to any one of (1) to (3) aboveA steel ingot after casting or a steel piece after hot workingThreeWithout reducing the temperature to a temperature range below the point, ArThreeThe rough rolling is performed in a temperature range above the point, and then the total rolling reduction amount of the final rolling up to the last one stage in the final rolling is 70% or more, and the stand outlet temperature in the last one stage is ArThreePoint ~ "ArThreeRolling is performed at a temperature in the temperature range of “point + 100 ° C.”, cooling is started within 0.5 seconds after the rolling in the last stage stand is completed, and “Ar” is determined from the stand outlet temperature in the last previous stage.Three It is cooled to an average cooling rate of 400 ° C./second or higher until “point−100 ° C.”, then subjected to rolling at a reduction of 10% or less at the final stand, and then wound up at a temperature of 600 ° C. or lower. (1) The manufacturing method of the hot rolled sheet steel in any one of (3).
[0015]
Here, it is known that the volume ratio of a certain phase is equal to the area ratio. Therefore, the volume ratio of the ferrite in the structure is, for example, the area ratio of ferrite obtained by a normal two-dimensional evaluation method. Can be determined from For this reason, in the present invention, the area ratio of the ferrite is obtained by the so-called “mesh method” with respect to an arbitrary 10 fields of view or more in a region having a depth of ¼ of the plate thickness from the surface by an optical microscope, and an average value thereof is obtained. The volume ratio.
[0016]
The “phase” other than ferrite in the hot-rolled steel sheet of the present invention may be any phase such as pearlite, bainite, martensite, and austenite (so-called “residual austenite” in which austenite remains without transformation).
[0017]
“The average grain diameter of ferrite over the entire plate thickness” is a portion having a depth of ８, ¼ depth and ½ depth of the plate thickness from the surface (that is, the central portion). Means the average of the average grain size of each ferrite. Note that the average grain size of ferrite in the portion having a depth of ８, ¼ depth, and ½ depth from the surface is arbitrary 10 at each portion. For the field of view or more, the average section length obtained by the so-called “section method” is multiplied by 1.12.
[0018]
“REM (rare earth element)” is a general term for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM indicates the total content of the above elements.
[0019]
The amount of reduction (%) per pass is the thickness of the material to be rolled before rolling the n-th pass.niThe thickness of the rolled material after rolling is tno{(Tni-Tno) / Tni} It means what is required by x100. The total reduction amount (%) of rolling consisting of n passes, where n is an integer of 2 or more, is the thickness of the material to be rolled before rolling in the first pass of the rolling.1I, The thickness of the material to be rolled after rolling of the nth pass is tnf{(T1i-Tno) / T1i} It means what is required by x100.
[0020]
The temperature referred to in the present invention refers to the temperature on the steel sheet surface, and the “average cooling rate” refers to the temperature difference before and after cooling divided by the cooling time.
[0021]
The “steel plate” in the present invention includes a “steel strip” wound in a coil shape.
[0022]
Hereinafter, the inventions related to the hot-rolled steel sheets (1) to (3) and the inventions related to the manufacturing methods (4) and (5) are referred to as inventions (1) to (5), respectively.
[0023]
In order to achieve the above-described object, the present inventors have found that the average grain size of ferrite over the entire thickness, the amount of change in the average grain size in the thickness direction, the change in texture in the thickness direction, and the relationship between various properties and Various studies were made on the relationship with the manufacturing conditions of the steel sheet. As a result, the following findings (a) to (d) were obtained.
[0024]
(A) A large change in the crystal grain structure in the plate thickness direction is one of the causes of workability deterioration and also a cause of large anisotropy.
[0025]
(B) In order to reduce the deterioration of workability and the occurrence of anisotropy while maintaining the high strength characteristics due to the fine structure, this can be used by controlling this rather than eliminating the structural change in the thickness direction. That's fine.
[0026]
(C) The contents to be controlled regarding the structural change in the plate thickness direction are the average grain size of ferrite over the entire plate thickness, the amount of change in the average grain size of ferrite in the plate thickness direction, and the texture.
[0027]
(D) A hot-rolled steel sheet having small in-plane anisotropy and excellent workability can be obtained by optimizing hot finish rolling conditions and cooling conditions immediately after finish rolling.
[0028]
The present inventions (1) to (5) have been completed based on the above findings.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, each requirement of the present invention will be described in detail.
(A) Structure of hot-rolled steel sheet
The hot-rolled steel sheet according to the present invention is firstly "by volume ratio.75% Is a hot rolled steel sheet having a thickness of 5 mm or less.
[0030]
The hot-rolled steel sheet with small anisotropy and good workability according to the present invention not only controls the difference between the structure near and inside the sheet surface that occurs during the production of a hot-rolled steel sheet having a fine structure, but also actively It is used to give desired characteristics. If the thickness of the hot-rolled steel sheet exceeds 5 mm, the desired fine structure and the difference between the vicinity of the plate surface and the internal structure cannot be controlled, so the thickness of the hot-rolled steel sheet is limited to 5 mm or less.
[0031]
The volume ratio of ferrite is sufficiently large, and is larger than the total ratio of other phases other than ferrite.Over 75%There must be. This is because the volume fraction of ferrite is75If it is less than%, the ductility is lowered and good workability cannot be obtained. Therefore, the volume fraction of ferrite75% Or moreIt was. NaThe phase other than ferrite (hereinafter, the phases other than ferrite are collectively referred to as the second phase) is any phase such as so-called “residual austenite” in which pearlite, bainite, martensite and austenite remain without transformation. Also good. By adjusting the type and volume ratio of the second phase, it is possible to obtain a hot-rolled steel sheet having a balance between strength and workability, which is more suitable for the field and parts used.
[0032]
As already described, it is known that the volume ratio of a certain phase is equal to the area ratio. Therefore, in the present invention, an optical microscope is used in an arbitrary part at a depth of 1/4 of the plate thickness from the surface. The area ratio of the ferrite is obtained by the so-called “mesh method” for 10 or more fields of view, and the average value is defined as the volume ratio.
[0033]
By refining the ferrite crystal grains, the strength of the steel sheet can be increased, and this effect is particularly great when the average grain diameter of ferrite over the entire plate thickness is less than 4 μm. However, if the ferrite grain size becomes excessively small, the yield point and the tensile strength are almost equal, and the yield ratio approaches 1, so the elongation is extremely lowered. In particular, when the average particle diameter of ferrite over the entire plate thickness is less than 1.1 μm, the decrease in elongation is remarkable. Therefore, the average particle diameter of ferrite over the entire plate thickness is set to 1.1 μm or more and less than 4 μm. In addition, the preferable average particle diameter of the ferrite over the whole plate thickness is 1.5 μm or more, more preferably 1.8 μm or more.
[0034]
Here, “the average grain diameter of ferrite over the entire plate thickness” means a portion having a depth of 1/8, a portion having a depth of 1/4, and a portion having a depth of 1/2 (that is, a portion having a depth of 1/2 from the surface (that is, The average grain size of each ferrite in the center part) is the average, and the above-mentioned surface is a portion having a depth of 1/8, a portion having a depth of 1/4, and a depth having a depth of 1/2. As described above, the average grain size of ferrite in each part is obtained by multiplying the average intercept length obtained by the so-called “intercept method” by 1.12 for any 10 or more visual fields in each part. It is.
[0035]
Variation in the average grain size of ferrite in the thickness direction affects the strength of the hot-rolled steel sheet and the balance between strength and ductility, and the effect of variation in the average grain size of ferrite in the thickness direction on the properties of this hot-rolled steel sheet is The average particle diameter ds of ferrite in a portion having a depth of 1/8 of the plate thickness from the surface and the average particle size dc of ferrite in a portion having a depth of 1/2 of the plate thickness from the surface (that is, the central portion) It can be arranged by the value of the ratio “ds / dc”. That is, when the value of ds / dc is less than 0.4, the yield ratio in the vicinity of the surface of the plate thickness is increased, and the elongation of the steel plate itself is particularly lowered and the stretch flangeability is lowered. On the other hand, when the value of ds / dc is 0.8 or more, the degree of strength increase is small. Therefore, ds / dc was set to 0.4 or more and less than 0.8.
[0036]
The <111> direction of ferrite is a direction in which atoms are densely arranged and has the highest Young's modulus, and affects other various mechanical properties. And, the <111> pole density in the rolling direction (hereinafter referred to as <111> rs) at a site having a depth of 1/8 of the plate thickness from the surface is less than 1.5 times that without the texture. Or, if the <111> pole density in the plate width direction at the center of the plate thickness (hereinafter referred to as <111> tc) is less than 1.5 times that of no texture, ductility and stretch flangeability are reduced. In addition, the in-plane anisotropy of strength increases. Therefore, <111> rs and <111> tc were both defined as 1.5 times or more of those having no texture.
[0037]
From the above, (1)~ (3)InventionInIs by volume ratio75% Of a ferrite sheet containing 5% or more of ferrite and having an average particle diameter of ferrite of 1.1 μm or more and less than 4 μm, and the ds / dc is 0.4 or more and less than 0.8, <111 > Rs and <111> tc were defined as hot-rolled steel sheets that were 1.5 times or more of those having no texture.
(B) Chemical composition of hot-rolled steel sheet
The hot-rolled steel sheet according to the invention of (1) is related to its chemical composition.It is defined as follows. In addition,Secure the organization described in (A) above in a stable and reliable mannerAs well asTo obtain hot-rolled steel sheets with small internal anisotropy and excellent workabilityIn addition,It is good to prescribe | regulate the chemical composition of a hot-rolled steel plate by invention of (2) and (3). In the following description, “%” of the content of each element means “mass%”.
[0038]
C:
C is a preferable component for increasing the strength of the steel sheet, and the content exceeds 0.001%.It is necessary toHowever, if its content exceeds 0.3%, workability and weldability may be deteriorated. Therefore, the C content exceeds 0.001 and is 0.3% or less.The
[0039]
Si:
Si is a preferred component that improves the strength of the steel sheet by solid solution strengthening. This effectTo getSi contentIs0.01% or moreIs necessaryThe However, if Si is contained in an amount exceeding 3.0%, a large amount of oxide scale is generated on the steel surface, which may be difficult to manufacture. Therefore, the Si content is 0.01-3.0%.The
[0040]
Mn:
Mn is an element effective for increasing the strength of steel. In order to reliably obtain this effect, the Mn content is 0.01% or more.is required.However, the Mn content is1.50If the content exceeds 50%, center segregation occurs in the steel ingot, and a band structure may be formed after hot rolling to cause a significant decrease in stretch flangeability. Therefore, the Mn content is 0.01 to1.50%The
[0041]
P:
P has the effect of increasing the strength of the steel sheet by solid solution strengthening. To obtain this effect with certainty, the P content is 0.005% or more.is required.However, when P is contained in excess of 0.5%, embrittlement occurs due to grain boundary segregation, and the steel ingot is easily cracked during casting, which may be accompanied by manufacturing difficulties. Therefore, the P content is 0.005 to 0.5%.The
[0042]
S:
S forms a sulfide by combining with Fe or Mn, and tends to be the starting point of void generation at sites where the processing conditions are severe, and reduces the workability, so the content is suppressed to 0.05% or less.There is a need.
[0043]
Said(1)The hot-rolled steel sheet according to the invention has a chemical composition consisting of the above-described elements from C to S, with the balance being Fe and impurities.
[0044]
Said(2)The hot-rolled steel sheet according to the invention is(1)In place of a part of Fe in the component described in the invention of (a)FlockIt has a chemical composition containing at least one component selected from the above.Moreover, the hot-rolled steel sheet according to the invention of (3) is at least one selected from the following group (b) instead of a part of Fe in the component described in the invention of (1) or (2). It has a chemical composition containing 0.005% or less of the above components in total.
[0045]
(A) Nb: 0.3% or less, Ti: 0.3% or less, V: 0.3% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less Mo: 1.0% or less and B: 0.005% or less,
(B) Ca, REM (rare earth element) andMg.
[0046]
Here, since any element from Nb to B described in the group (a) has an effect of further increasing the strength of the steel, each element from Nb to B contains each independently within the range described below. You may make it contain, and may contain 2 or more types in combination.
[0047]
In addition, since any element from Ca to Mg described in the group (b) controls the form of MnS to form inclusions that are difficult to expand during hot rolling, and has the effect of preventing deterioration of workability. The elements from Ca to Mg may be contained alone within the range described below, or two or more elements may be combined and contained.
[0048]
Note that REM refers to a total of 17 elements of Sc, Y, and lanthanoid as described above. In the case of lanthanoid, it is added industrially in the form of misch metal. As described above, the REM content in the present invention indicates the total content of the above elements.
[0049]
(A) Group (Nb, Ti, V, Cu, Ni, Cr, Mo and B):
Nb, Ti, V, Cu, Ni, Cr, Mo, and B are all elements that have an action of further increasing the strength of steel. Among these elements, Nb, Ti, and V also have an effect of refining the structure. In order to reliably obtain the above effects, Nb, Ti, V, Cu, Ni, Cr, Mo, and B are 0.01% or more, 0.01% or more, 0.01% or more, 0.05% or more, respectively. , 0.05% or more, 0.05% or more, 0.05% or more, and 0.0001% or more is preferable. However, if Nb, Ti and V are contained in excess of 0.3%, anisotropy is increased and ductility is also lowered. If Cu, Ni, Cr and Mo all exceed 1.0% and B exceeds 0.005%, the ductility is lowered. Therefore, when Nb, Ti, V, Cu, Ni, Cr, Mo and B are added, the contents of Nb, Ti and V are all 0.3% or less, and the contents of Cu, Ni, Cr and Mo are contained. The amount is preferably 1.0% or less, and the B content is preferably 0.005% or less.
[0050]
When Cu is contained in excess of about 0.3%, surface cracking may occur during hot working. Therefore, when Cu is contained in excess of about 0.3%, Ni is more than half of Cu. It is desirable to contain at the same time.
[0051]
(B) Group (Ca, REM and Mg):
Ca, REM, and Mg are all elements that have the effect of controlling the form of MnS to form inclusions that are difficult to expand during hot rolling, and prevent deterioration of workability. In order to reliably obtain this effect, it is preferable that the Ca content is 0.0002% or more, the REM content is 0.0002% or more, and the Mg content is 0.0002% or more. However, even if the total content of one or more of Ca, REM, and Mg exceeds 0.005%, the above effects are saturated and the cost is increased. Therefore, when adding Ca, REM, and Mg, it is good to set it as content of 0.005% or less in total of 1 type, or 2 or more types of Ca, REM, and Mg.
(C) Manufacturing conditions for hot-rolled steel sheet
The organization described in the item (A) according to the inventions (1) to (3) described aboveAnd the chemical composition described in (B)For example, a hot-rolled steel sheet having “ThreeAfter heating to a temperature equal to or higher than the point, or steel ingot after casting or steel slab after hot working, ArThreeWithout reducing the temperature to a temperature range below the point, ArThreeRough rolling is performed in the temperature range above the point, then the total rolling reduction in the finish rolling is 70% or more, and the finishing temperature is ArThreePoint ~ "ArThreeThe finish rolling is performed at a temperature in the temperature range of “point + 100 ° C.”, and cooling is started within 0.5 seconds after finishing the finish rolling.ThreeIt can be manufactured relatively easily according to the invention of the above (4), which is cooled to “point−100 ° C.” at an average cooling rate of 400 ° C./second or more and then wound at a temperature of 600 ° C. or less.
[0052]
That is, the steel ingot or steel piece is3 By reheating to a temperature above the point, the alloy element can be dissolved in austenite. Here, charging into a furnace for reheating treatment such as a heating furnace or a soaking furnace may be performed in a state of high temperature after casting or hot working, or once cooled to near room temperature You may go from.
[0053]
Ac as above3 Steel ingot or billet reheated to a temperature above the point, or Ar3 The steel ingot after casting or the slab after hot working which has not been lowered to a temperature range below the point is subjected to rolling by a tandem mill. In this case, rough rolling is performed in Ar which is an austenite region.3 It is better to perform at a temperature above the point. After rough rolling, finish rolling has a total rolling reduction of 70% or more and finishing temperature of Ar.3 Point ~ "Ar3 The temperature is preferably in the temperature range of “point + 100 ° C.”.
[0054]
When the total rolling reduction of the finish rolling is less than 70%, the ferrite crystal grains are coarsened, the mechanical properties are lowered, and the anisotropy is also increased. Note that the total rolling reduction of finish rolling is more preferably 90% or more.
[0055]
The finishing temperature in finish rolling is “Ar3 When the temperature exceeds the point + 100 ° C., the ferrite crystal grains are coarsened, the mechanical properties are lowered, and the anisotropy is also increased. On the other hand, the finishing temperature is Ar3 Below the point, the processed ferrite grains tend to remain and the anisotropy may become extremely strong.
[0056]
After finishing the finish rolling, by performing strong cooling with almost no air cooling time, the ferrite grains become extremely fine, and <111> rs and <111> tc develop,easilyThe texture described in the item (A) is formed and the refinement of the structure is promoted. In addition, the development of the azimuth increases as the average cooling rate increases and the time from finishing finish rolling to starting cooling decreases. For this reason, it is preferable to start cooling within 0.5 seconds after finishing rolling. More preferably, the time to start cooling is within 0.1 seconds. The average cooling rate during cooling is preferably 400 ° C./second or more. This average cooling rate is more preferably 600 ° C./second or more. Note that cooling at the above average cooling rate is at least “Ar from the finishing temperature.3 It is up to "point-100 ° C". This is "Ar3 This is because when the cooling is performed only to a temperature exceeding “point−100 ° C.”, the ferrite grains become coarse and a desired texture may not be obtained. "Ar3 Although it does not prevent cooling to the temperature below "point-100 degreeC", the volume fraction of a ferrite may reduce depending on steel.
[0057]
After cooling under the above conditions, it is preferable to wind up at a temperature of 600 ° C. or lower. This is because when the coiling temperature exceeds 600 ° C., fine ferrite obtained by the above hot rolling and cooling grows and becomes coarse.
[0058]
Therefore, in the invention of (4),It has a chemical composition according to any of the inventions (1) to (3) aboveSteel ingot or steel slab is AcThreeAfter heating to a temperature above the point, orIt has a chemical composition according to any of the inventions (1) to (3) aboveA steel ingot after casting or a steel piece after hot workingThreeWithout reducing the temperature to a temperature range below the point, ArThreeRough rolling is performed in the temperature range above the point, then the total rolling reduction in the finish rolling is 70% or more, and the finishing temperature is ArThreePoint ~ "ArThreeThe finish rolling is performed at a temperature in the temperature range of “point + 100 ° C.”, and cooling is started within 0.5 seconds after finishing the finish rolling.Three“Point to −100 ° C.” was cooled at an average cooling rate of 400 ° C./second or higher, and then wound at a temperature of 600 ° C. or lower.
[0059]
The structure described in the section (A) is “steel ingot or steel slab is Ac.ThreeAfter heating to a temperature equal to or higher than the point, or steel ingot after casting or steel slab after hot working, ArThreeWithout reducing the temperature to a temperature range below the point, ArThreeThe rough rolling is performed in a temperature range above the point, and then the total rolling reduction amount of the final rolling up to the last one stage in the final rolling is 70% or more, and the stand outlet temperature in the last one stage is ArThreePoint ~ "ArThreeRolling is performed at a temperature in the temperature range of “point + 100 ° C.”, cooling is started within 0.5 seconds after the rolling in the last stage stand is completed, and “Ar” is determined from the stand outlet temperature in the last previous stage.ThreeIt is comparatively also possible to “cool to a point of −100 ° C.” at an average cooling rate of 400 ° C./second or higher, then rolling at a final stand with a reduction amount of 10% or less, and then winding at a temperature of 600 ° C. or less. Easy to get. This method removes the cooling water on the surface of the material to be rolled in the final stand, and performs a light rolling that does not give any distortion to the ferrite deposited after cooling, thereby providing a temperature distribution in the thickness direction. To prevent ds / dc from becoming excessively small. In addition, since the reduction exceeding 10% in the final stand generates processed ferrite, the reduction amount of the final stand is 10%The following is recommended. More preferably, it is 5% or less. The reduction amount may be 0% simply by removing the cooling water.
[0060]
Therefore, in the invention of (5),It has a chemical composition according to any of the inventions (1) to (3) aboveSteel ingot or steel slab is AcThreeAfter heating to a temperature above the point, orIt has a chemical composition according to any of the inventions (1) to (3) aboveA steel ingot after casting or a steel piece after hot workingThreeWithout reducing the temperature to a temperature range below the point, ArThreeThe rough rolling is performed in a temperature range above the point, and then the total rolling reduction amount of the final rolling up to the last one stage in the final rolling is 70% or more, and the stand outlet temperature in the last one stage is ArThreePoint ~ "ArThreeRolling is performed at a temperature in the temperature range of “point + 100 ° C.”, cooling is started within 0.5 seconds after the rolling in the last stage stand is completed, and “Ar” is determined from the stand outlet temperature in the last previous stage.Three“Point to −100 ° C.” was cooled at an average cooling rate of 400 ° C./second or higher, and then rolled with a reduction amount of 10% or less at the final stand, and then wound at a temperature of 600 ° C. or lower.
[0061]
The amount of reduction (%) per pass is the thickness of the material to be rolled before rolling the n-th pass.niThe thickness of the rolled material after rolling is tno{(Tni-Tno) / Tni} X100 is calculated, and n is an integer of 2 or more, and the total rolling reduction (%) of n passes is the thickness of the rolled material before rolling in the first pass of the rolling. t1I, The thickness of the material to be rolled after rolling of the nth pass is tnf{(T1i-Tno) / T1i} Means what is obtained by × 100, “average cooling rate” refers to the difference between the temperature difference before and after cooling divided by the cooling time, and the temperature in the present invention refers to the temperature on the surface of the steel sheet as already described It is.
[0062]
Hereinafter, the present invention will be described in more detail with reference to examples.
[0063]
【Example】
Example 1
A slab having the chemical composition shown in Table 1 was produced by continuous casting, and air-cooled to room temperature after completion of continuous casting. Thereafter, the above slab was reheated to a temperature range of 1200 to 1300 ° C., and then subjected to finish rolling, cooling and winding under the conditions shown in Table 2 using a small test tandem mill. A steel plate of 8 to 5.5 mm was obtained. In addition, the slab heating temperature was 1250 degreeC and rough rolling was performed by the normal method.
[0064]
[Table 1]
[Table 2]
Test pieces were collected from the obtained steel plates and examined for the structure, tensile properties at room temperature, and stretch flangeability.
[0067]
As for the structure, the cross-sectional structure of the steel plate thickness was observed using an optical microscope and a scanning electron microscope, and the <111> pole density was determined by a normal X-ray measurement method.
[0068]
The tensile properties at room temperature were investigated using JIS No. 5 test pieces. The test specimens were collected from three directions of 0 °, 45 °, and 90 °, with the rolling direction being 0 °.
[0069]
Stretch flange workability is to take a square test piece of 100 mm in length and width, punch a hole with a diameter of 10 mm with a punch in the center, widen this hole with a conical punch with a tip angle of 60 °, and place it on the edge of the hole. The critical hole expansion rate calculated from the critical hole diameter through which the crack penetrates was evaluated.
[0070]
Tables 3 to 5 collectively show the results of the above investigations. In this table, the average r value refers to a value calculated by the formula “(r0 + 2r45 + r90) / 4”, where r0, r45, and r90 are r values when the specimen collection directions are 0 °, 45 °, and 90 °, respectively.
[0071]
[Table 3]
[Table 4]
[Table 5]
As is apparent from Tables 3 to 5, the structure of the present inventionAnd chemical compositionTest number 1 to meet the regulations3, 11, 13, 15, 16as well as18 ~20The hot-rolled steel sheet has small anisotropy and excellent workability.
[0075]
In contrast, the organization of the present inventionAnd / or chemical compositionThe hot-rolled steel sheets with test numbers 4 to 7, 9, 10, 12, 14, 17 and 22 that deviate from the specifications are inferior in ductility, stretch flange workability and in-plane anisotropy, and inferior in workability.
[0076]
(Example 2)
The steel A slab shown in Example 1 manufactured by continuous casting and air-cooled to room temperature after completion of continuous casting was reheated to a temperature range of 1200 to 1300 ° C., and then a small tandem mill for testing was used. Finishing rolling, cooling and winding were performed under the conditions shown in Fig. 6 to obtain steel plates having thicknesses of 1.9 and 2.0 mm. In addition, the slab heating temperature was 1250 degreeC and rough rolling was performed by the normal method.
[0077]
[Table 6]
Test pieces were collected from the obtained steel plates, and the structure, tensile properties at room temperature and stretch flange workability were investigated in the same manner as in Example 1.
[0079]
Tables 7 and 8 collectively show the results of the above investigations. It should be noted that the average r value in Table 7 also indicates the value calculated by the above-mentioned formula “(r0 + 2r45 + r90) / 4”.
[0080]
[Table 7]
[Table 8]
As is clear from Tables 7 and 8, the structure of the present inventionAnd chemical compositionThe hot-rolled steel sheets with test numbers 24 and 25 that satisfy the regulations have small anisotropy and excellent workability. On the other hand, the present inventionEven if it meets the chemical composition regulations,The hot-rolled steel sheet of test number 26 that deviates from the structure regulations has low workability and extremely large anisotropy.
[0083]
【The invention's effect】
Since the hot-rolled steel sheet of the present invention has small in-plane anisotropy and excellent workability, it can be used as a material for high-strength structural members used in automobiles. The hot-rolled steel sheet of the present invention can be produced relatively easily by the method of the present invention.
Claims (5)
（ａ）質量％で、Ｎｂ：０．３％以下、Ｔｉ：０．３％以下、Ｖ：０．３％以下、Ｃｕ：１．０％以下、Ｎｉ：１．０％以下、Ｃｒ：１．０％以下、Ｍｏ：１．０％以下及びＢ：０．００５％以下The hot rolled steel sheet according to claim 1, wherein the chemical composition includes at least one component selected from the following group (a) instead of a part of Fe.
(A) By mass%, Nb: 0.3% or less, Ti: 0.3% or less, V: 0.3% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1 0.0% or less, Mo: 1.0% or less, and B: 0.005% or less
（ｂ）Ｃａ、ＲＥＭ（希土類元素）及びＭｇ3. The hot rolling according to claim 1, wherein the chemical composition contains at least one component selected from the following group (b) in mass% in total of 0.005% or less in place of part of Fe. steel sheet.
(B) Ca, REM (rare earth element) and Mg
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| WO2006011503A1 (en) | 2004-07-27 | 2006-02-02 | Nippon Steel Corporation | High young’s modulus steel plate, zinc hot dip galvanized steel sheet using the same, alloyed zinc hot dip galvanized steel sheet, high young’s modulus steel pipe, and method for production thereof |
| JP4581665B2 (en) * | 2004-12-08 | 2010-11-17 | 住友金属工業株式会社 | High-strength hot-rolled steel sheet and its manufacturing method |
| JP4670538B2 (en) * | 2005-08-05 | 2011-04-13 | 住友金属工業株式会社 | Method for producing hot-rolled steel sheet having fine ferrite structure |
| JP4867257B2 (en) * | 2005-09-29 | 2012-02-01 | Ｊｆｅスチール株式会社 | High-strength thin steel sheet with excellent rigidity and manufacturing method thereof |
| JP5228447B2 (en) * | 2006-11-07 | 2013-07-03 | 新日鐵住金株式会社 | High Young's modulus steel plate and method for producing the same |
| US8404060B2 (en) | 2007-02-02 | 2013-03-26 | Nippon Steel & Sumitomo Metal Corporation | Method for manufacturing hot-rolled sheet having fine-grained ferrite, and hot-rolled sheet |
| KR101344645B1 (en) | 2011-10-28 | 2013-12-26 | 현대제철 주식회사 | Method of manufacturing oil tubular country goods |
| US9903004B2 (en) | 2012-12-19 | 2018-02-27 | Nippon Steel & Sumitomo Metal Corporation | Hot-rolled steel sheet and method for manufacturing the same |
| JP5765411B2 (en) * | 2013-12-12 | 2015-08-19 | 新日鐵住金株式会社 | Cold rolled steel sheet manufacturing method |
| JP5708775B2 (en) * | 2013-12-12 | 2015-04-30 | 新日鐵住金株式会社 | Structural member |
| WO2017168991A1 (en) * | 2016-03-31 | 2017-10-05 | Ｊｆｅスチール株式会社 | Thin steel sheet, plated steel sheet, hot-rolled steel sheet manufacturing method, cold-rolled full hard steel sheet manufacturing method, thin steel sheet manufacturing method, and plated steel sheet manufacturing method |
| KR101797367B1 (en) | 2016-06-21 | 2017-11-13 | 현대제철 주식회사 | High carbon hot-rolled steel sheet and method of manufacturing the same |
| KR102237488B1 (en) * | 2019-12-16 | 2021-04-08 | 주식회사 포스코 | High hardness steel sheet having excellent punching formability and manufacturing method for the same |
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| WO2019088104A1 (en) | 2017-10-30 | 2019-05-09 | 新日鐵住金株式会社 | Hot-rolled steel sheet and manufacturing method therefor |
| KR20200019966A (en) | 2017-10-30 | 2020-02-25 | 닛폰세이테츠 가부시키가이샤 | Hot rolled steel sheet and its manufacturing method |
| US11198929B2 (en) | 2017-10-30 | 2021-12-14 | Nippon Steel Corporation | Hot rolled steel sheet and method for producing same |
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