JP3596509B2 - Manufacturing method of high strength hot rolled steel sheet - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、加工性に優れた高強度熱延鋼板の製造方法に関する
。
【０００２】
【従来の技術】
近年、高い強度と伸びフランジ性が要求される用途の熱延鋼板において、強度が高く加工性に優れている鋼板として、ベイナイト組織を主体とする高強度熱延鋼板が実用化されてきている。これらの高強度熱延鋼板は、自動車の軽量化等を目的として種々の構造部材や部品への適用が進められている。適用範囲の拡大に伴い、年々その仕様は厳しくなっており、さらなる加工性の向上が望まれている。
【０００３】
このようなベイナイト組織を主体とする変態組織（フェライト−ベイナイト、ベイナイト）を有する高強度熱延鋼板において、伸びフランジ性と強度を共に向上、即ち伸びフランジ性−強度バランスを向上させるには、組織を微細化することが有効である。熱延鋼板の組織制御は一般に仕上圧延およびその後の冷却を制御することにより可能である。そこで、これらの製造条件を限定することにより加工性の向上を図る技術が提案されている。
【０００４】
例えば、特開昭５４−６５１１８号公報には、急冷を２段に分けてその間に保持期間のある２段冷却を行う方法が提案されている。この場合、冷却速度は、１次、２次とも８０℃／ｓ以上として、粒成長を抑制している。特開昭５６−３３４２９号公報には、２段冷却の１次冷却開始温度を７２０〜８５０℃、冷却速度を、１次、２次とも３０〜２００℃／ｓとして、フェライト粒を微細化する技術が提案されている。
【０００５】
特開昭６０−１２１２２５号公報には、Ａｒ３〜Ａｒ３＋４０℃の温度範囲で累積圧下率４５％以上の圧下を加え、圧延後所定の温度まで徐冷し、３０℃／ｓ以上の冷却速度で冷却することにより、フェライト粒を微細に分散させ、マルテンサイトの微細化を図る技術が提案されている。またこの技術では、材質の均一性を図るための冷却条件として、冷却時の熱伝達係数を１０００Ｗ／ｍ２・Ｋ以下とすることが好ましいとしている。また、特許２８３１８５８号公報には、圧延後空冷し、その後５０〜１００℃／ｓで冷却して、穴拡げ性に有利なベイナイト主体の組織を得る技術が提案されている。
【０００６】
また、特開２０００−１０９９５１号公報には、Ｎｂ，Ｔｉ等の析出強化型元素を添加することにより組織を細粒化し、圧延後２０〜１５０℃／ｓで冷却することにより、高い強度と加工性を得る技術が提案されている。特公昭６２−３９２３０号公報は、加工誘起変態によるフェライトと焼入れ組織からなる二相高強度熱延鋼板が提案されている。この技術では、加工誘起等軸フェライト粒を５０〜７０％以上生成させることが必要としている。
【０００７】
【発明が解決しようとする課題】
しかしながら、上記特開昭５４−６５１１８号公報、特開昭５６−３３４２９号公報、特開昭６０−１２１２２５号公報に記載された圧延後の冷却条件では、実施してみると複合組織の微細化が十分に達成されず、第２相組織の微細分散化による伸びフランジ性の向上の点で限界があった。
【０００８】
特開２０００−１０９９５１号公報記載の技術は、高い強度を得るためには多量の添加元素を必要とする。さらに、材質の均一性の観点から冷却条件（熱伝達係数）を制限しているため、複合組織化するにはＭｎ等の合金元素を大量に添加する必要がある。同公報記載の実施例でも、発明例はＣ，Ｓｉを高くするか又はＭｎを２．５〜３．０％添加しており、溶接性や加工性に問題があると予想される。
【０００９】
特公昭６２−３９２３０号公報記載の技術は、仕上大圧下圧延による微細化技術であり、１パスでの圧下率は４０％以上が好ましいとしているが、通常の熱延仕上最終スタンドでこのような大圧下を行うことは困難であり、また板形状の観点から実用化には問題が多い。
【００１０】
本発明はこれらの課題を解決し、厳しいプレス加工用途にも適用可能であり、伸びフランジ性を始めとする加工性に優れた比較的低合金の高強度熱延鋼板の製造方法を提供することを目的とする。
【００１１】
【課題を解決するための手段】
上記の課題は次の発明により解決される。その発明は、化学成分として、mass%で、C:0.04〜0.12%、Si:2%以下、Mn:0.5〜2.5%、sol.Al:0.1%以下を含有する鋼を鋳造後、直接又は再加熱して熱間圧延を行い、Ar3変態点以上の仕上温度で熱間圧延を終了し、圧延終了後2秒以内に冷却を開始し、170℃以上の温度範囲にわたって冷却速度を200 ℃ /s 以上とするとともに、565℃以上650℃未満の温度で冷却を停止することを特徴とするTS ≧ 1000MPa の高強度熱延鋼板の製造方法である。
【００１２】
ここで、化学成分としては、上記元素に加えてさらにｍａｓｓ％で、Ｔｉ，Ｎｂ，Ｖ，Ｚｒの内１種以上を合計で０．０１〜０．２％含有する鋼を用いることもできる。また、これらの鋼の化学成分に加えてさらに、ｍａｓｓ％で、Ｃｒ：１％以下、Ｍｏ：０．５％以下の内１種以上を含有する鋼を用いることもできる。
【００１３】
これらの発明は、上記の課題を解決すべく鋭意検討を重ね、高強度熱延鋼板の加工性、とりわけ伸びフランジ性に対する冷却条件の影響に着目して検討した。その過程で、圧延直後から一定時間以内に冷却を開始し、冷却開始後は少なくともある温度幅については急冷を行うことが、微細なベイナイトを主体とする組織の生成に有効であるという知見を得た。この発明は、この知見に基づきなされたものであり、以下その詳細について説明する。
【００１４】
まず、化学成分について説明する。
【００１５】
Ｃ： ０．０４〜０．１２％（ｍａｓｓ％、以下同じ）
Ｃは、焼入れ性を向上させ、低温変態相であるベイナイトを生成させて熱延鋼板の強度を確保するために必要な元素であり、そのためには最低０．０４％必要である。一方、０．１２％を超えるＣは、加工性および溶接性を劣化させる。従って、Ｃを０．０４〜０．１２％の範囲内とする。
【００１６】
Ｓｉ： ２％以下
Ｓｉは、固溶強化の効果と共に、ベイナイト変態におけるＣの拡散を促進して加工性の向上に寄与する元素であり、目標とする強度レベルに合わせて添加することができる。但し、Ｓｉが２％を超えると溶接性および表面性状を劣化させる。従って、Ｓｉを２％以下とする。
【００１７】
Ｍｎ： ０．５〜２．５％
Ｍｎは、焼入れ性を高める元素であり、固溶強化により鋼板の強度を確保するためには、０．５％は必要である。一方、Ｍｎ が２．５％を超えると、その効果が飽和するばかりか、バンド状組織を形成して加工性を劣化させる。従って、Ｍｎ を０．５〜２．５％の範囲内とする。
【００１８】
ｓｏｌ．Ａｌ： ０．１％以下
Ａｌは、脱酸剤として使用されると同時に、不可避的不純物として含有されるＮを固定して、加工性を向上させる効果を有する。しかし、０．１％を超えてＡｌを添加しても、その効果が飽和すると共に、清浄度を悪化させて加工性を劣化させる。従って、Ａｌをｓｏｌ．Ａｌで０．１％以下とする。
【００１９】
Ｔｉ，Ｎｂ，Ｖ，Ｚｒ： 添加する場合、合計で０．０１〜０．２％
本発明では、上記の化学成分に加えて、強度調整あるいは炭化物形成による固溶Ｃ，Ｎ低減を通した非時効化および深絞り性向上のため、必要に応じてＴｉ，Ｎｂ，Ｖ，Ｚｒを添加する。これらの元素は、合計の添加量が０．０１％未満では効果がなく、０．２％を超えても効果が飽和する。従って、Ｔｉ，Ｎｂ，Ｖ，Ｚｒを添加する場合は、これらの添加量を合計で０．０１〜０．２％とする。
【００２０】
Ｃｒ，Ｍｏ： 添加する場合、Ｃｒ≦１％，Ｍｏ≦０．５％
ＣｒおよびＭｏは、焼入れ性を高める元素であり、鋼板の強度に寄与する効果を有するので、必要に応じて添加することができる。しかし、これらの元素は、合金コストが高くコスト増を招くと共に、それぞれ１％および０．５％を超えると溶接性を劣化させる。従って、これらの元素を添加する場合は、Ｃｒを１％以下、Ｍｏを０．５％以下とする。
【００２１】
その他の元素については、本発明の効果を妨げない範囲で含まれていてもよい。また、Ｐ，Ｓ，Ｎ等の不可避的不純物については、低い方が好ましいが、通常の高強度熱延鋼板の範囲内であれば含まれていてもよい。
【００２２】
次に、本発明における製造条件について説明する。
【００２３】
仕上温度： Ａｒ３変態点以上
仕上温度がＡｒ３変態点未満になると、変態後の組織が加工され、延性の劣化、面内異方性の増加等、材質上好ましくない。従って、仕上温度をＡｒ３変態点以上とする。
【００２４】
圧延終了後の冷却開始時間： ２秒以内
圧延終了後の冷却開始までの期間は、オーステナイトの再結晶又は回復が進行する。冷却開始時間を２秒以内とすることにより、再結晶粒の成長又は回復の進行を防止し、変態後の組織をベイナイト組織主体の微細な組織とすることができる。
【００２５】
冷却条件： １７０℃以上の温度範囲にわたって冷却速度１５０℃／ｓ超
この発明では、冷却開始後は冷却速度と共に冷却前後の温度差を確保することにより、組織を制御する。冷却速度が１５０℃／ｓ以下では、変態温度が上昇してポリゴナルフェライトの生成が無視できなくなり、変態後のフェライト粒径も微細化しないため、強度が低下する。また、冷却前後の温度差が１７０℃未満では、やはりポリゴナルフェライトが生成するため強度が低下する。従って、冷却条件としては、１７０℃以上の温度範囲にわたって冷却速度１５０℃／ｓ超とする。
【００２６】
冷却終了温度： ４００℃以上６５０℃未満
冷却終了温度については、６５０℃を超えると、ポリゴナルフェライトの生成が顕著となり強度が低下する。一方、４００℃未満の低温域まで急冷すると、マルテンサイトが生成するため加工性が劣化する。従って、冷却終了温度は４００℃以上６５０℃未満とする。
【００２７】
基本的な製造条件は以上の通りであるが、以上の発明には、必要に応じて更に製造条件を加えることができる。
【００２８】
例えば、上記の発明において更に、仕上圧延機の入側又は仕上圧延機のスタンド間に誘導加熱装置を設置して、粗バー又は被圧延材を加熱することにより、仕上温度を調整することを特徴とする高強度熱延鋼板の製造方法とすることもできる。
【００２９】
この発明は、粗圧延された粗バー又は仕上圧延中の被圧延材を誘導加熱装置により加熱する。これにより、圧延中の被圧延材の温度をより均一にでき、コイル内の機械的性質のより一層の均一化を図ることができる。
【００３０】
また、上記の発明において更に、冷却を停止する温度の変動幅を６０℃以内に冷却制御することを特徴とする高強度熱延鋼板の製造方法とすることもできる。
【００３１】
この発明は、冷却制御により冷却停止温度の変動幅を６０℃以内とする。これにより、後述のように引張強度の変動幅を１６％以内（中心値±８％以内）に抑えることができる。ここで、変動幅は最高値と最低値の差であり、変動幅６０℃以内というのは、中心値±３０℃以内と同じ意味である。
【００３２】
その他、上記の発明において更に、仕上圧延の最終スタンドの圧下率を３０％未満とすることを特徴とする高強度熱延鋼板の製造方法とすることもできる。
【００３３】
この発明は、仕上圧延時の最終スタンドの圧下率を３０％未満に制限しているので形状調整の効果があり、加工性のみならず板形状に優れた鋼板を得ることができる。
【００３４】
高強度熱延鋼板の発明としては、化学成分がこれらの発明の化学成分であり、組織がベイナイトを主体とする組織であるとともに、引張強度の変動が中心値±８％以内であることを特徴とする高強度熱延鋼板とすることもできる。
【００３５】
この発明は、前述の製造方法により冷却制御することにより製造可能である。化学成分を上記の発明の化学成分とし、圧延に引続き急冷を行うことにより、微細なベイナイトを主体とする組織が得られる。その後、所定温度で巻取ることにより、引張強度の変動幅を１６％以内（中心値±８％以内）に抑えることができる。この高強度熱延鋼板は、組織が均一であり、引張強度の変動幅が小さいので、曲げ加工時のスプリングバックが一定となる等、コイル内でのプレス加工性の変動を小さくできる。
【００３６】
【発明の実施の形態】
発明の実施に当たっては、連続鋳造から粗圧延まで直接圧延を行う直送圧延プロセス、又はスラブの再加熱を伴う製造プロセスにおいても、加工性を優れたレベルとするためには、化学成分を特定範囲に制御する必要がある。本発明では、Ｃ，Ｓｉ，Ｍｎ，ｓｏｌ．Ａｌの含有量を前述のように制御しているが、さらにＣａを適量添加することが好ましい。その場合、Ｃａの添加量は０．００５％以下とする。その他、本発明の効果を妨げない範囲で、例えば熱間加工性を向上させるため等の目的で、微量合金元素を添加することもできる。
【００３７】
連続鋳造スラブは、直送圧延プロセス又はスラブの再加熱を伴う製造プロセスにより粗圧延を行う。粗圧延後の粗バー又は仕上圧延中の被圧延材を、誘導加熱装置により加熱することにより、コイル内の機械的性質の均一化が図れる。また、仕上温度をＡｒ３変態点直上の狭い温度範囲に制御することにより、本発明の組織微細化の効果をより効果的に発揮させることができる。
【００３８】
本発明の効果は、原理的に、仕上圧延前の粗バーの加熱あるいは保熱の有無やその手法にはよらずに得られる。従って、誘導加熱に限らずコイルボックス等を用いた連続圧延プロセスに対しても、効果的に使用できる。また、仕上圧延直前または仕上圧延中に、被圧延材を誘導加熱装置により加熱するとき、エッジ加熱を行ってもよく、特に板厚２ｍｍ以下の薄鋼板を製造する場合に効果的である。
【００３９】
仕上圧延後は、前述の時間以内に急冷を開始することで、オーステナイトが再結晶する場合は、再結晶粒の粒成長を防止して組織の微細化が達成できる。同様に、オーステナイトの回復が起こる場合も、結晶粒内の変形帯密度を十分高いレベルに維持することができる。このようにして、オーステナイトの微細な再結晶粒の粒界、あるいは未再結晶粒の粒界と変形帯が、フェライト変態の核生成サイトとなり、オーステナイトが分断化され、変態後の組織をベイナイト組織主体の微細な組織とすることができる。
【００４０】
本発明で、ベイナイト組織を主体とするというのは、ほぼ全体がベイナイト組織であり、光学顕微鏡写真ではポリゴナルフェライト又はパーライトが認められないということである。仮に多少のポリゴナルフェライト又はパーライトが含まれていたとしても、強度を低下させるほど多くはなく、無視できる程度であればよい。
【００４１】
仕上圧延後の鋼板は急冷するが、冷却開始時期については、オーステナイトを再結晶させるためには、圧延加工後０．５秒以内では微細な再結晶粒が十分に生成せず、また冷却設備の設置においても、圧延機出側のスペースを考慮すると位置的に困難である。従って、冷却開始時期については、仕上圧延後０．５秒超とすることが望ましい。
【００４２】
なお、冷却速度の上限は特に規定しないが、冷却設備の能力により操業上は適宜上限が決まることになる。このようにして、冷却速度を１５０℃／ｓ超とする急冷を１７０℃以上の温度範囲について行った後は、そのままあるいは巻取温度まで更に冷却してコイルに巻取る。後者の場合、冷却停止温度は急冷を停止する温度となり、急冷後は巻取温度調節のための冷却（緩冷却可）を行う。 巻取温度は特に規定しないが、６５０℃以上ではポリゴナルフェライトが生成するので強度が低下し、４００℃未満ではマルテンサイトが生成するため加工性が劣化する。従って、巻取温度は４００℃以上６５０℃未満とすることが望ましい。
【００４３】
このようにして、本発明では、コイル内での温度の変動を低減することにより、熱延鋼帯の幅方向及び長手方向における引張強さの変動（最大値と最小値の差）が、コイル内の引張強さの平均値の±８％以内であることを特徴とする熱延鋼板を得ることができる。このようなばらつきが狭小な鋼板は、曲げ加工時のスプリングバック等のプレス加工性のコイル内での変動が小さい。その結果、需要家においても、プレス加工後の形状矯正工程を省略でき、製品歩留りの向上も可能であり、工業的に非常に有用である。
【００４４】
また、熱延鋼帯の材質のばらつきを好ましいレベルまで低減するためには、上記の急冷停止温度を発明の範囲内とするとともに、さらに、引張強さの変動を±４％以内とすることにより、上記の需要家での性能を格段に向上し得る。これは、上記の急冷停止温度の変動を４０℃以内とすることにより、材質のばらつきをこのように狭小化できる。さらに、引張強さの変動を±２％以内とするには、上記の急冷停止温度の変動を２０℃以内とすればよい。
【００４５】
なお本発明におけるコイル幅方向の温度は、温度センサの測定方法も考慮して、コイル幅両エッジから３０ｍｍを除いた範囲を指す。また、引張特性については、コイル幅両エッジから３０ｍｍおよびコイル長手方向の両端から各５ｍを除いた位置より採取したサンプルを用いて調査し、総ての値の平均値をコイル内平均値とする。
【００４６】
【実施例】
表１に示す化学成分を有する鋼を溶製した。この鋼を、表２に示す製造条件で圧延し、板厚３．２ｍｍの熱延コイルＮｏ．１〜４を製造した。
【００４７】
【表１】
【表２】
これらの熱延コイルの機械的性質の測定結果を表３および図１に示す。
【００５０】
【表３】
本発明例の鋼板Ｎｏ．１，２は、比較例の鋼板Ｎｏ．３，４に比べて、同一化学組成でありながら強度が優れている。一方、穴拡がり率はいずれもほぼ同程度の値となっており、図１に示すように、穴拡がり率を保持しつつ大幅な強度上昇が得られている。なお、図中の破線は、強度（ＴＳ）と穴拡がり率の積が一定の曲線を示す。このように、本発明例の鋼板は、比較例に比べて穴拡がり率−強度バランスが優れていることがわかる。
【００５２】
図２に冷却速度と強度（ＴＳ）の関係を示す。冷却速度が１５０℃／ｓ超の領域で強度の上昇が見られ、２００℃／ｓ以上で１０００ＭＰａ以上の強度（ＴＳ）が安定して得られることが分かる。
【００５３】
表４に冷却停止温度の変動幅と引張強さの変動（ＴＳ変動）を示す。
【００５４】
【表４】
この表より、冷却停止温度変動を６０℃以内とすることで、引張強さの変動が±８％以内（表４の材質変動の欄）に抑えられていることが分かる。
【００５６】
【発明の効果】
本発明は、圧延直後から一定時間以内に冷却を開始し、冷却開始後は少なくともある温度幅にわたって急冷を行うことにより、金属組織を微細なベイナイトを主体とする組織としているので、伸びフランジ性等の加工性に優れた高強度熱延鋼板の製造が可能である。更に、冷却停止温度の変動幅を制御することにより、機械的性質が均一で、かつ板形状にも優れた高強度熱延鋼板の製造が可能である。また、連続鋳造から熱間圧延まで直接行う直送圧延プロセス、又はスラブの再加熱を伴う製造プロセスにおいても適用でき、優れた伸びフランジ性を有する工業的に有用な効果がもたらされる。
【図面の簡単な説明】
【図１】穴拡がり率−強度バランスを示す図。
【図２】冷却速度と強度（ＴＳ）の関係を示す図。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a high-strength hot-rolled steel sheet excellent in workability.
[0002]
[Prior art]
In recent years, among hot-rolled steel sheets for applications requiring high strength and stretch flangeability, high-strength hot-rolled steel sheets mainly composed of bainite structure have been put to practical use as steel sheets having high strength and excellent workability. These high-strength hot-rolled steel sheets are being applied to various structural members and parts for the purpose of reducing the weight of automobiles and the like. With the expansion of the application range, the specifications are becoming stricter year by year, and further improvement in workability is desired.
[0003]
In a high-strength hot-rolled steel sheet having such a transformation structure mainly composed of bainite structure (ferrite-bainite, bainite), to improve both stretch flangeability and strength, that is, to improve stretch flangeability-strength balance, it is necessary to improve the structure. It is effective to reduce the size. Control of the structure of the hot-rolled steel sheet is generally possible by controlling finish rolling and subsequent cooling. Therefore, a technique for improving workability by limiting these manufacturing conditions has been proposed.
[0004]
For example, Japanese Patent Laying-Open No. 54-65118 proposes a method in which rapid cooling is divided into two stages and two-stage cooling with a holding period therebetween is performed. In this case, the cooling rate is set to 80 ° C./s or more for both primary and secondary to suppress grain growth. JP-A-56-33429 discloses that ferrite grains are refined by setting the primary cooling start temperature of the two-stage cooling to 720 to 850 ° C and the cooling rate to 30 to 200 ° C / s for both the primary and secondary cooling. Technology has been proposed.
[0005]
Japanese Patent Application Laid-Open No. Sho 60-112225 discloses that rolling is performed at a cumulative reduction ratio of 45% or more in a temperature range of Ar 3 to Ar 3 + 40 ° C., and after rolling, the roll is gradually cooled to a predetermined temperature and cooled at 30 ° C./s or more. A technique has been proposed in which ferrite grains are finely dispersed by cooling at a high speed to reduce the size of martensite. Further, according to this technology, as a cooling condition for achieving uniformity of the material, it is preferable that a heat transfer coefficient at the time of cooling be 1000 W / m 2 · K or less. Further, Japanese Patent No. 2831858 proposes a technique for obtaining a bainite-based structure advantageous for hole expandability by air-cooling after rolling and then cooling at 50 to 100 ° C./s.
[0006]
Japanese Patent Application Laid-Open No. 2000-109951 discloses that a structure is refined by adding a precipitation strengthening element such as Nb or Ti, and then cooled at 20 to 150 ° C./s after rolling to obtain high strength and processing. A technique for obtaining the property has been proposed. Japanese Patent Publication No. Sho 62-39230 proposes a two-phase high-strength hot-rolled steel sheet comprising a ferrite and a quenched structure due to work-induced transformation. In this technique, it is necessary to generate 50 to 70% or more of the work-induced equiaxed ferrite grains.
[0007]
[Problems to be solved by the invention]
However, under the cooling conditions after rolling described in JP-A-54-65118, JP-A-56-33429, and JP-A-60-112225, it was found that the microstructure of the composite structure was reduced. Was not sufficiently achieved, and there was a limit in improving stretch flangeability due to fine dispersion of the second phase structure.
[0008]
The technique described in JP-A-2000-109951 requires a large amount of additional elements in order to obtain high strength. Furthermore, since cooling conditions (heat transfer coefficients) are limited from the viewpoint of material uniformity, it is necessary to add a large amount of alloying elements such as Mn to form a composite structure. Also in the examples described in the publication, the invention examples increase C and Si or add 2.5 to 3.0% of Mn, and it is expected that there is a problem in weldability and workability.
[0009]
The technique described in Japanese Patent Publication No. Sho 62-39230 is a fine-graining technique by large rolling under finish, and the rolling reduction in one pass is preferably 40% or more. It is difficult to perform large reduction, and there are many problems in practical use from the viewpoint of the plate shape.
[0010]
The present invention solves these problems and provides a method for producing a high-strength hot-rolled steel sheet of a relatively low alloy that is applicable to severe press working applications and has excellent workability including stretch flangeability. With the goal.
[0011]
[Means for Solving the Problems]
The above problem is solved by the following invention. According to the invention, a steel containing, as a chemical component, by mass%, C: 0.04 to 0.12%, Si: 2% or less, Mn: 0.5 to 2.5%, and sol. Heat and perform hot rolling, finish hot rolling at a finishing temperature not lower than the Ar 3 transformation point, start cooling within 2 seconds after the end of rolling, and increase the cooling rate to 200 ° C / over a temperature range of 170 ° C or higher. s or more and cooling is stopped at a temperature of 565 ° C. or more and less than 650 ° C. The method for producing a high-strength hot-rolled steel sheet with TS ≧ 1000 MPa .
[0012]
Here, as the chemical component, a steel containing, in addition to the above-mentioned elements, one or more of Ti, Nb, V, and Zr in total of 0.01 to 0.2% by mass% can also be used. Further, in addition to the chemical components of these steels, steels containing one or more of mass%, Cr: 1% or less, and Mo: 0.5% or less can also be used.
[0013]
In order to solve the above-mentioned problems, these inventions have been studied intensively, and studied by paying attention to the effect of cooling conditions on the workability of a high-strength hot-rolled steel sheet, particularly on stretch flangeability. In the process, it was found that starting cooling within a certain period of time immediately after rolling, and then quenching at least for a certain temperature range after the start of cooling, was effective in generating a microstructure mainly composed of fine bainite. Was. The present invention has been made based on this finding, and the details thereof will be described below.
[0014]
First, the chemical components will be described.
[0015]
C: 0.04 to 0.12% (mass%, the same applies hereinafter)
C is an element necessary for improving the hardenability and generating bainite, which is a low-temperature transformation phase, to secure the strength of the hot-rolled steel sheet. For that purpose, at least 0.04% is necessary. On the other hand, C exceeding 0.12% deteriorates workability and weldability. Therefore, C is set in the range of 0.04 to 0.12%.
[0016]
Si: 2% or less Si is an element that contributes to the improvement of workability by promoting the diffusion of C in the bainite transformation together with the effect of solid solution strengthening, and can be added in accordance with the target strength level. However, if Si exceeds 2%, the weldability and the surface properties deteriorate. Therefore, the content of Si is set to 2% or less.
[0017]
Mn: 0.5 to 2.5%
Mn is an element that enhances the hardenability, and 0.5% is required to secure the strength of the steel sheet by solid solution strengthening. On the other hand, when Mn exceeds 2.5%, not only the effect is saturated, but also a band-like structure is formed to deteriorate workability. Therefore, Mn is set in the range of 0.5 to 2.5%.
[0018]
sol. Al: 0.1% or less Al is used as a deoxidizing agent and has an effect of fixing N contained as an unavoidable impurity and improving workability. However, even if Al is added in excess of 0.1%, the effect is saturated and the cleanliness is deteriorated to deteriorate the workability. Therefore, Al was added to sol. Al is set to 0.1% or less.
[0019]
Ti, Nb, V, Zr: When added, 0.01 to 0.2% in total
In the present invention, in addition to the above-mentioned chemical components, Ti, Nb, V, and Zr may be added as necessary for non-aging and deep drawability through strength adjustment or reduction of solid solution C and N by carbide formation. Added. These elements have no effect when the total added amount is less than 0.01%, and the effect is saturated when the total amount exceeds 0.2%. Therefore, when Ti, Nb, V, and Zr are added, the total amount of these additives is 0.01 to 0.2%.
[0020]
Cr, Mo: When added, Cr ≦ 1%, Mo ≦ 0.5%
Cr and Mo are elements that enhance hardenability and have an effect of contributing to the strength of the steel sheet, and thus can be added as necessary. However, these elements increase the cost of the alloy and cause an increase in cost, and when the content exceeds 1% and 0.5%, respectively, the weldability is deteriorated. Therefore, when these elements are added, the content of Cr is set to 1% or less, and the content of Mo is set to 0.5% or less.
[0021]
Other elements may be included as long as the effects of the present invention are not hindered. Also, inevitable impurities such as P, S, and N are preferably as low as possible, but may be contained within the range of a normal high-strength hot-rolled steel sheet.
[0022]
Next, the manufacturing conditions in the present invention will be described.
[0023]
Finishing temperature: the Ar 3 transformation point or more finishing temperature is lower than Ar 3 transformation point, processed tissue after transformation, deterioration of ductility, an increase of in-plane anisotropy, the material is not preferable. Therefore, the finishing temperature is set to the Ar 3 transformation point or higher.
[0024]
Cooling start time after the end of rolling: Within 2 seconds During the period from the end of rolling to the start of cooling, austenite recrystallization or recovery proceeds. By setting the cooling start time within 2 seconds, the progress of growth or recovery of recrystallized grains can be prevented, and the structure after transformation can be a fine structure mainly composed of bainite structure.
[0025]
Cooling conditions: Cooling rate over 150 ° C./s over a temperature range of 170 ° C. or more In the present invention, the structure is controlled by securing the cooling rate and the temperature difference before and after cooling together with the cooling rate. If the cooling rate is 150 ° C./s or less, the transformation temperature rises and the formation of polygonal ferrite cannot be ignored, and the ferrite grain size after transformation does not become fine, so that the strength decreases. If the temperature difference before and after cooling is less than 170 ° C., polygonal ferrite is also formed, so that the strength is reduced. Therefore, the cooling condition is set to a cooling rate of more than 150 ° C./s over a temperature range of 170 ° C. or more.
[0026]
Cooling end temperature: 400 ° C. or higher and lower than 650 ° C. When the cooling end temperature is higher than 650 ° C., the formation of polygonal ferrite becomes conspicuous and the strength decreases. On the other hand, if the material is rapidly cooled to a low temperature range of less than 400 ° C., martensite is generated, thereby deteriorating workability. Therefore, the cooling end temperature is 400 ° C. or more and less than 650 ° C.
[0027]
Although the basic manufacturing conditions are as described above, further manufacturing conditions can be added to the above invention as needed.
[0028]
For example, in the above invention, furthermore, an induction heating device is installed between the entrance of the finishing mill or the stand of the finishing mill, and the finishing temperature is adjusted by heating the rough bar or the material to be rolled. And a method for producing a high-strength hot-rolled steel sheet.
[0029]
According to the present invention, a rough-rolled rough bar or a material to be rolled during finish rolling is heated by an induction heating device. Thereby, the temperature of the material to be rolled during rolling can be made more uniform, and the mechanical properties in the coil can be made more uniform.
[0030]
Further, in the above invention, a method of manufacturing a high-strength hot-rolled steel sheet may be further characterized in that the fluctuation range of the temperature at which cooling is stopped is controlled to be within 60 ° C.
[0031]
According to the present invention, the fluctuation range of the cooling stop temperature is set to 60 ° C. or less by the cooling control. As a result, the fluctuation range of the tensile strength can be suppressed to within 16% (center value ± 8%) as described later. Here, the fluctuation width is the difference between the highest value and the lowest value, and the fluctuation width within 60 ° C has the same meaning as the central value within ± 30 ° C.
[0032]
In addition, in the above invention, a method for producing a high-strength hot-rolled steel sheet, characterized in that the rolling reduction of the final stand of finish rolling is less than 30%.
[0033]
According to the present invention, since the rolling reduction of the final stand at the time of finish rolling is limited to less than 30%, there is an effect of shape adjustment, and a steel sheet excellent not only in workability but also in sheet shape can be obtained.
[0034]
The invention of the high-strength hot-rolled steel sheet is characterized in that the chemical components are the chemical components of these inventions, the structure is mainly bainite, and the variation in tensile strength is within a central value of ± 8%. High-strength hot-rolled steel sheet.
[0035]
The present invention can be manufactured by controlling the cooling by the above-described manufacturing method. By making the chemical component the chemical component of the invention described above and performing rapid cooling after rolling, a structure mainly composed of fine bainite can be obtained. Thereafter, by winding at a predetermined temperature, the fluctuation range of the tensile strength can be suppressed to within 16% (within ± 8% of the central value). This high-strength hot-rolled steel sheet has a uniform structure and a small fluctuation range in tensile strength, so that a change in press workability in a coil, such as a constant springback during bending, can be reduced.
[0036]
BEST MODE FOR CARRYING OUT THE INVENTION
In the practice of the invention, in a direct rolling process in which direct rolling is performed from continuous casting to rough rolling, or in a manufacturing process involving reheating of the slab, in order to achieve a high level of workability, the chemical composition is limited to a specific range. You need to control. In the present invention, C, Si, Mn, sol. Although the content of Al is controlled as described above, it is preferable to further add an appropriate amount of Ca. In that case, the addition amount of Ca is set to 0.005% or less. In addition, as long as the effects of the present invention are not impaired, a trace alloy element can be added for the purpose of, for example, improving hot workability.
[0037]
The continuous casting slab is subjected to rough rolling by a direct rolling process or a manufacturing process involving reheating of the slab. By heating the rough bar after the rough rolling or the material to be rolled during the finish rolling by the induction heating device, the mechanical properties in the coil can be made uniform. Further, by controlling the finishing temperature to a narrow temperature range just above the Ar 3 transformation point, the effect of the structure refinement of the present invention can be more effectively exerted.
[0038]
In principle, the effects of the present invention can be obtained without depending on the presence or absence of the method of heating or keeping the rough bar before the finish rolling and the method thereof. Therefore, the present invention can be effectively used not only for induction heating but also for a continuous rolling process using a coil box or the like. In addition, when the material to be rolled is heated by an induction heating device immediately before or during finish rolling, edge heating may be performed. This is particularly effective when a thin steel plate having a thickness of 2 mm or less is manufactured.
[0039]
After finish rolling, quenching is started within the above-mentioned time, and when austenite is recrystallized, grain growth of recrystallized grains can be prevented to achieve a finer structure. Similarly, when austenite recovery occurs, the deformation zone density in the crystal grains can be maintained at a sufficiently high level. In this way, the grain boundaries of fine recrystallized grains of austenite or the grain boundaries of unrecrystallized grains and deformation zones become nucleation sites for ferrite transformation, austenite is fragmented, and the transformed structure is transformed into a bainite structure. The main structure can be a fine structure.
[0040]
In the present invention, the phrase "having a bainite structure as a main component" means that almost the entire structure is a bainite structure, and no polygonal ferrite or pearlite is observed in an optical microscope photograph. Even if a small amount of polygonal ferrite or pearlite is contained, it is not so large as to decrease the strength and may be negligible.
[0041]
Although the steel sheet after finish rolling is rapidly cooled, the cooling start time is such that fine recrystallized grains are not sufficiently generated within 0.5 seconds after rolling to recrystallize austenite. Also in installation, it is difficult to position in consideration of the space on the exit side of the rolling mill. Therefore, it is desirable that the cooling start time be more than 0.5 seconds after the finish rolling.
[0042]
The upper limit of the cooling rate is not particularly defined, but the upper limit is appropriately determined in operation depending on the capacity of the cooling facility. After the rapid cooling at a cooling rate of more than 150 ° C./s in the temperature range of 170 ° C. or higher in this manner, the coil is wound as it is or further cooled to the winding temperature. In the latter case, the cooling stop temperature is a temperature at which the rapid cooling is stopped, and after the rapid cooling, cooling for adjusting the winding temperature (slow cooling is possible) is performed. Although the winding temperature is not particularly limited, if the temperature is 650 ° C. or higher, the strength decreases because polygonal ferrite is generated, and if the temperature is lower than 400 ° C., martensite is generated, resulting in deterioration in workability. Therefore, it is desirable that the winding temperature be 400 ° C. or more and less than 650 ° C.
[0043]
Thus, in the present invention, by reducing the temperature fluctuation in the coil, the fluctuation (the difference between the maximum value and the minimum value) of the tensile strength in the width direction and the longitudinal direction of the hot-rolled steel strip is reduced. A hot-rolled steel sheet characterized by being within ± 8% of the average value of the tensile strength in the steel sheet. Such a steel sheet having a small variation has a small variation in a press-workable coil such as a springback during bending. As a result, even a customer can omit the shape correction step after press working, improve the product yield, and is industrially very useful.
[0044]
In addition, in order to reduce the variation in the material of the hot-rolled steel strip to a preferable level, the above-mentioned quenching stop temperature is set within the range of the present invention, and the fluctuation of the tensile strength is set within ± 4%. Thus, the performance at the consumer can be significantly improved. This is because the variation of the material can be narrowed in this way by keeping the fluctuation of the quenching stop temperature within 40 ° C. Further, in order to keep the variation in tensile strength within ± 2%, the variation in the above-mentioned rapid cooling stop temperature may be made within 20 ° C.
[0045]
In the present invention, the temperature in the coil width direction indicates a range excluding 30 mm from both edges of the coil width in consideration of the measurement method of the temperature sensor. The tensile properties were investigated using samples taken from positions excluding 30 mm from both edges of the coil width and 5 m from both ends in the longitudinal direction of the coil, and the average value of all values was taken as the average value in the coil. .
[0046]
【Example】
Steel having the chemical components shown in Table 1 was melted. This steel was rolled under the manufacturing conditions shown in Table 2 to obtain a hot-rolled coil No. 3.2 mm thick. 1-4 were produced.
[0047]
[Table 1]
[Table 2]
The measurement results of the mechanical properties of these hot rolled coils are shown in Table 3 and FIG.
[0050]
[Table 3]
In the steel sheet No. of the present invention example. 1 and 2 are steel sheet Nos. Of Comparative Examples. Compared with Nos. 3 and 4, the strength is excellent while having the same chemical composition. On the other hand, the hole expansion ratios are almost the same values, and as shown in FIG. 1, a large increase in strength is obtained while maintaining the hole expansion ratio. The broken line in the figure indicates a curve in which the product of the strength (TS) and the hole expansion rate is constant. Thus, it can be seen that the steel sheet of the present invention example has a better hole expansion rate-strength balance than the comparative example.
[0052]
FIG. 2 shows the relationship between the cooling rate and the strength (TS). It can be seen that the strength increases in the region where the cooling rate exceeds 150 ° C./s, and that the strength (TS) of 1000 MPa or more can be stably obtained at 200 ° C./s or more.
[0053]
Table 4 shows the fluctuation width of the cooling stop temperature and the fluctuation of the tensile strength (TS fluctuation).
[0054]
[Table 4]
From this table, it can be seen that by setting the cooling stop temperature fluctuation within 60 ° C., the fluctuation in tensile strength is suppressed to within ± 8% (the column of material fluctuation in Table 4).
[0056]
【The invention's effect】
The present invention starts cooling within a certain period of time immediately after rolling, and quenches at least over a certain temperature range after the start of cooling, so that the metal structure is a structure mainly composed of fine bainite, so that stretch flangeability and the like are obtained. It is possible to manufacture a high-strength hot-rolled steel sheet having excellent workability. Further, by controlling the fluctuation range of the cooling stop temperature, it is possible to manufacture a high-strength hot-rolled steel sheet having uniform mechanical properties and excellent sheet shape. In addition, the present invention can be applied to a direct rolling process in which continuous casting to hot rolling is performed directly, or a manufacturing process involving reheating of a slab, and an industrially useful effect having excellent stretch flangeability is provided.
[Brief description of the drawings]
FIG. 1 is a diagram showing a hole spreading rate-strength balance.
FIG. 2 is a diagram showing a relationship between a cooling rate and strength (TS).
Claims (5)
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2001308650A JP3596509B2 (en) | 2001-10-04 | 2001-10-04 | Manufacturing method of high strength hot rolled steel sheet |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2001308650A JP3596509B2 (en) | 2001-10-04 | 2001-10-04 | Manufacturing method of high strength hot rolled steel sheet |
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| Publication Number | Publication Date |
|---|---|
| JP2003112204A JP2003112204A (en) | 2003-04-15 |
| JP3596509B2 true JP3596509B2 (en) | 2004-12-02 |
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| JP2001308650A Expired - Fee Related JP3596509B2 (en) | 2001-10-04 | 2001-10-04 | Manufacturing method of high strength hot rolled steel sheet |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JP3954411B2 (en) * | 2002-02-28 | 2007-08-08 | 日新製鋼株式会社 | Manufacturing method of high-strength hot-rolled steel sheet with excellent material uniformity and hole expandability |
| JP2008266792A (en) * | 2008-05-28 | 2008-11-06 | Sumitomo Metal Ind Ltd | Hot-rolled steel sheet |
| CN105324505B (en) | 2013-06-27 | 2017-04-05 | 杰富意钢铁株式会社 | High tensile hot rolled steel sheet and its manufacture method |
| JP6213098B2 (en) * | 2013-09-25 | 2017-10-18 | 新日鐵住金株式会社 | High-strength hot-rolled steel sheet with excellent fatigue characteristics and method for producing the same |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JP3332172B2 (en) * | 1993-06-30 | 2002-10-07 | 日本鋼管株式会社 | High strength hot rolled steel sheet with excellent strength-stretch flange balance and strength-ductility balance |
| JP2000290748A (en) * | 1999-04-08 | 2000-10-17 | Kawasaki Steel Corp | Hot rolled steel sheet for working excellent in notch fatigue resistance and its production |
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