JP3636872B2 - Method for producing high-tensile hot-rolled steel sheet having ultrafine structure - Google Patents

Description

なお、この発明において、Cu等の含有量 (または合計含有量) の下限が０wt％であるのは、それらの成分を添加しない場合があることを意味する。
【００１０】
【発明の実施の形態】
この発明の作用効果について、発明者らが行った実験結果を基に詳述する。
まず、動的再結晶微細化に関し、発明者らは、次の二つの考えの下に実験を行った。
第１は、スラブ加熱段階での初期オーステナイト粒を極度に微細化することである。この発明では、このための手段として加熱時に多量の未溶解TiＣ粒子を存在させ、この粒子によるピンニング作用によって初期オーステナイト粒の微細化を図る方法を採る。この観点からTi量が重要であり、また、加熱条件が重要であると考えた。
第２は，第１段階での動的再結晶ができるだけ容易に生起する成分条件に予め調整しておくことが重要であると考えた。
【００１１】
以上の考えの下に、発明者らは動的再結晶による微細化が最も進展する鋼の成分条件について研究した。そして、この発明を構成するに当たっての有益な知見を得たのでその研究結果について述べる。
表１，表２に示す化学成分が種々に異なる鋼について、950 〜1100℃の温度範囲で10min 以上加熱保持し、次いで950〜1100℃未満の温度範囲で１パス当たりの圧下量が20％以上となる圧下を少なくとも２回以上加える第１段階の圧延工程を施し、800 ℃以上950 ℃未満の温度範囲で５℃/s以上の冷却速度で冷却しながら１パス当たりの圧下量が20％以上で、累計圧下率が50％以上の圧延を施す第２段階の圧延工程を施したのち、この第２段階の圧延工程後に20℃/s以上の冷却速度で600 ℃以下の温度範囲まで冷却する工程を施して製造した熱延鋼板について、平均フェライト粒径を測定した結果を解析し、図１の結果を得た。
【００１２】
【表１】

【００１３】
【表２】

【００１４】
すなわち、図１からわかるように、この発明の範囲の熱延条件としたときの平均フェライト粒径の変化は、Ti量と成分パラメータＰmp、すなわち、Ｐmp＝1538−55×Ｃ（％）−80×｛Ｃ（％）｝² −13×Si（％）−4.8 ×Mn（％）−1.5 ×Cr（％）−4.3 ×Ni（％）−3.5 ×Cu（％）−1.2 ×Mo（％）によって整理することができ、Ti量が多く、かつ、Ｐmpが小さい側でフェライト粒径が微細化することがわかる。そして、平均フェライト粒径が２μm 未満の超微細フェライト粒が得られる条件は、Ti量が0.05wt％以上で、Ｐmpが1499未満であることがわかる。ここで、Ti量はTiＣ粒子の加熱段階での初期オーステナイト粒径の微細化作用を通じて影響したものであり、このことは、この研究の実験材についての電子顕微鏡による析出物観察結果によっても裏付けることができた。
【００１５】
一方、成分パラメータＰmpの影響の機構は必ずしも明確ではないものの次のように考えることができる。動的再結晶の難易を左右する要因の一つとして、固溶元素の影響があることが知られている。また、動的再結晶には自己拡散係数が大きいことが有利となることも知られている。Ｐmpは鋼の固溶元素の融点の変化に対する成分パラメータであるが、この融点と固相域でのFe原子の自己拡散のし易さの間には一定の関係があり、融点が低くなる成分条件のものはFe原子の自己拡散が容易化する方向となる。したがって、図１におけるＰmpの影響は、固溶元素による自己拡散のし易さの変化を通じて、動的再結晶に影響するものと考えることができる。
【００１６】
このように、加熱工程はTiＣ粒子による初期オーステナイト粒径を微細化し、この発明における微細化過程の初期条件を与える上で重要な工程であって、この段階での微細化が不足すれば、以降における圧延段階での動的再結晶が進行し難くなり、最終組織でのフェライト粒径の微細化も不十分となる。図２は、この発明の範囲の成分の鋼を用いて調査した初期オーステナイト粒径と加熱温度との関係であるが、加熱温度が1100℃を超えると急速に初期オーステナイト粒径が粗大化することがわかる。これは、初期オーステナイト粒界をピンニングしているTiＣ粒子が溶解し始めるからである。
【００１７】
また、第１段階の圧延工程は、上記加熱工程の初期微細オーステナイト粒に圧延による動的再結晶を生じさせ更に微細化させていく工程である。この段階での要点は、１パス当たりの圧下量が重要となる。図３は、この発明の範囲の成分鋼を用いて調査した、第１段階圧延工程における１パス当たりの圧下量と、最終組織における平均フェライト粒径との関係を示す図である。図３から、最終組織のフェライト粒微細化は、１パス当たりの圧下量を20％以上とする条件において明確であり、したがって、この圧下条件を採択することにより動的再結晶微細化が進展することが分かる。なお、この第１段階の圧延工程で微細化されたオーステナイト粒は、比較的安定で、通常の熱間圧延工程におけるパス間時間程度の滞留によっても急激な粒成長を起こし難いという特徴がある。これは、動的再結晶粒であることと、鋼中に分散するTiＣの存在によって起こる特有の現象と考えられ、この発明による微細化法の有利な点の一つである。
【００１８】
次に、静的再結晶微細化に必要な要件に関する実験結果について述べる。
第２段階の圧延工程は、第１段階の圧延工程までの動的再結晶で微細化したオースナイト粒に歪を蓄積し、静的再結晶による微細化を進展させる工程である。従来法においても静的再結晶微細化法は採られてはいるが、最終フェライト粒径を２μm 未満にすることはできない。しかし、この発明の特長は、その前工程において動的再結晶微細化を極限まで進めた上で、静的再結晶微細化を追加させるので、格段に微細化が進展するところにある。これは、後述するように、静的再結晶過程に入る前のオーステナイト粒径が20μm 以下と極めて微細粒であることと、動的再結晶粒を出発材としていることによるものと考えられる。
【００１９】
この工程での要件としては、累積圧下率と圧延中の冷却速度が重要である。図４は、この発明の範囲の成分鋼に関する第２段階圧延工程における累積圧下率と圧延中の冷却速度が、最終組織のフェライト粒径に及ぼす影響を示すものである。図４から明らかなように、累積圧下率50％以上で、かつ、圧延中の冷却速度が５℃/s以上の条件としたときに、最終組織のフェライト粒径が最も微細化することがわかる。ここでの累積圧下率の効果は、蓄積歪エネルギーを高め、静的再結晶粒数を増加させる作用を通じて最終組織の微細化に寄与するものと思われる。
なお、図４から、冷却速度が５℃/s未満の遅い条件では、累積圧下率を高めても最終組織のフェライト粒径の微細化が進展しなくなることがわかるが、これは、冷却速度が遅い場合には、圧延後の回復などが生じ、蓄積エネルギーがロスするためである。
【００２０】
次に、第２段階圧延後の冷却工程では、動的再結晶微細化と静的再結晶微細化を積み重ねて極限まで微細化させたオーステナイト粒をγ→α変態させ、変態微細化を図ることと変態後のα粒成長を抑制することが重要となる。また、ここは第２相を目標の組織に制御する工程でもある。図５は、この発明の範囲を成分鋼を用いて調査した、最終組織のフェライト粒径と圧延終了温度から600 ℃までの冷却速度の関係を示すものである。図５から、最終の平均フェライト粒径を２μm 未満にするためには、20℃/s以上の冷却速度が必要であることがわかる。
【００２１】
なお、20℃/s以上の冷却速度で冷却した場合でも、最終組織のフェライト分率は十分に70％以上にすることができる。これは、変態前のオーステナイト粒が極めて微細であることと、第２段階の圧延工程で蓄積された歪エネルギーによってα変態核数が著しく増大しており、速い冷却速度条件でもγ→α変態が加速的に進行するためである。しかし、極度に大きな冷却速度とした場合には、フェライト分率が低下する場合がある。好適な冷却速度は30〜200 ℃/sの範囲である。
【００２２】
また、フェライト粒微細化のために重要な温度区間は、圧延終了から600 ℃までであるが、600 ℃未満の温度区間は第２相を制御する上で重要な区間であり、目的の組織とするためにはこの区間での冷却停止温度を調整する必要がある。この目的のための冷却停止温度は、化学成分によって異なるものの、概略的には次ようである。
すなわち、第２相をパーライトとする場合の冷却停止温度は約550 〜590 ℃、ベイナイト及びアシキュラーフェライトとする場合には470 〜530 ℃、残留オーステナイト相を得たい場合には400 〜450 ℃、マルテンサイト相を得たい場合には400 ℃未満がそれぞれの冷却停止温度の好適範囲である。また、第２相が２種以上の異なる相を持つ複合相とする場合には、圧延終了温度から600 ℃まで20℃/s以上の冷却速度で冷却した後、600 ℃未満の温度範囲において上記したそれぞれの相での好適な温度範囲における冷却速度を調整することによって得ることができる。
【００２３】
かくして、この発明の範囲の成分鋼を用い、上記した各段階における好適な圧延条件を採択することにより、平均フェライト粒径が２μm 未満であり、ポリゴナルフェライト分率が70％以上でかつ、第２相が種々に異なるミクロ組織を持つ熱延鋼板を製造することができる。
【００２４】
次に、この発明において成分組成範囲について限定した理由について述べる。Ｃは、初期オーテナイト粒の微細化並びに圧延中及び圧延後の微細化を進展する上で必要なTiＣ粒子を生成するために必要であるばかりでなく、第２相を形成させ鋼の強度上昇を図るために必要である。前者の目的のために必要なＣ量は、重量比で少なくとも添加したTi量の1/4 倍以上であることが望ましく、かつ、後者の目的に対してはTiＣとして結合するＣ量を除いた余剰のＣ量が少なくとも0.03wt％以上は必要である。したがって、この発明の目的とする２μm 以上の超微細フェライト粒径でかつ490 MPa 級以上の強度を得るために必要なＣ量の下限は0.05wt％である。一方、Ｃ量が0.30wt％を超えるとフェライト分率を70％以上とすることができなくなり、延性、じん性、耐疲労特性、耐衝突特性が劣化する。最も好ましい範囲は0.07〜0.25％の範囲である。
【００２５】
Siは、固溶強化元素として、強度上昇に寄与すること、冷却工程でのフェライト変態を加速し、フェライト分率を高める上で効果的であるばかりでなく、この過程で生じる第２相中へのＣ濃化を進展させて強度−延性バランス並びに強度じん性バランスの改善に寄与すること、成分パラメータＰmpを低下させることを通じて動的再結晶を起こし易くさせ、最終組織のフェライト粒の微細化を助長し、延性、じん性、耐疲労特性、耐衝突特性を向上させる極めて有益な成分である。しかし、2.5 wt％以上になるとAr₃ 変態点が著しく高くなり、この発明における好適な圧延温度範囲が狭くなること及びTiＣ粒子を粗大化させる作用が大きくなり過ぎて逆に結晶粒の微細化が阻害される弊害が生じるので、上限を2.5 wt％とした。なお、最も好適な範囲は1.0 〜2.0 wt％の範囲である。
【００２６】
Mnは適量使用すればAr₃ 変態点を低下させる作用と、Pmp を低下させる作用を通じて結晶粒の微細化を助長する有益な作用を発揮する。また、第２相のマルテンサイト化および残留γ相化を進展させる作用を通じて、強度−延性バランス、強度−疲労強度バランスを高める作用を有する。この有益な作用を発揮させるためには少なくとも0.7 ％以上が必要である。しかし、3.0 ％を超えるとAr₃ 点が低下しすぎてフェライト分率70％以上を確保できなくなり、かえって強度−延性バランスが悪化するので上限を3.0 ％とした。もっとも好適な範囲は1.0 〜2.5 ％の範囲である。
【００２７】
Tiは、TiＣ粒子による微細化を利用するこの発明では必須の成分である。この作用を発揮させるうえで必要なTi量の下限は0.05％であり、これに満たない範囲では２μm 未満の超微細フェライト粒径を得ることはできない。また、このTiの作用は添加量が多くなってくると次第に飽和し、0.3 wt％を超えて添加してもそれ以上の効果を期待することができなくなる。そのため、添加量の範囲を0.05〜0.3 wt％の範囲とした。好ましい範囲は0.10〜0.20％である。なお、Ti添加量はＣ量との関係において留意すべき点がある。すなわち、この発明は、添加したTi量のほとんどをTiＣ粒子として存在させることを基本としている。TiＣ生成のためのＣ量に見合う以上に余剰のTiが存在する条件、すなわち、固溶状態のTiが存在する条件では、固溶Tiによる再結晶性の阻害作用が現れるので、動的再結晶及び静的再結晶微細化の積み上げにより超微細化を達成するこの発明では回避するのが望ましい。したがって、Ti量とＣ量は重量比でTi＜４×Ｃの関係を満たすことが望ましい。
【００２８】
Ｐは、結晶粒界で強い偏析を起こした場合、粒界強度を低下させるので特にじん性に悪影響が現れる。この発明の鋼は、結晶粒径が極めて微細であるために、従来の鋼よりもこのようなＰの粒界偏析を起こし難い。Ｐ量が0.05wt％を超えるとその悪影響を避け難くなるので、Ｐ量は0.05wt％以下に制限した。下限は、経済性の観点から 0.001wt％とするのが好ましい。さらに好ましい範囲は0.015 wt％未満である。
Ｓは、硫化物を形成して鋼の清浄度を悪化させ、耐疲労特性、じん性、耐衝突特性等に悪影響を及ぼす。この悪影響を避けるために上限は0.025 wt％とした。さらに好ましい範囲は0.015 wt％未満である。なお、耐サワー用のラインパイプなどに用いる場合には、硫化物の存在が耐ＨＩＣ特性に著しい悪影響を及ぼすので、これを避けるためにはＳ量は0.005 wt％以下とすることが望ましい。下限は経済性の観点から0.0005wt％とするのが好ましい。
Alは、脱酸成分として通常用いられる範囲で添加しても何の問題もない。一方、この発明の鋼はSi及びTiを比較的多く添加するので、その一部を利用してSi脱酸、あるいはAl−Si複合脱酸、もしくはSi−Ti脱酸を行うこともできる。したがって、このような場合には、Al添加量を極めて抑制しても清浄鋼を製造することができる。したがって、Alは0.0005wt％以上添加することとした。上限は経済性の観点から0.1 wt％に定めた。
【００２９】
以上がこの発明の鋼における基本成分であるが、この発明では、以上の成分の他に必要に応じCu、Ni、Cr、Moの一種又は二種以上、Nb、Zr、Taの一種又は二種以上、Ca、Ｂを含有させても良い。
Cu、Ni、Cr、Moは、Mnと同じようにAr₃ 変態点を低下させる作用及びＰmpを低下させる作用を通じてこの発明の効果を引き出す上で有効である。これらの添加量は一種又は二種を合計して2.5 wt％以下の範囲が望ましい。この範囲を超えて添加するとMnの上限を超えた場合と同様に悪影響が生じる。好ましい範囲は2.0wt％以下である。
Nb、Zr、Taは、TiＣと同様の炭化物と形成して結晶粒微細化に有効に作用するので、一種若しくは二種以上を合計して0.3 wt％以下の範囲で添加することができる。しかし、0.3 wt％を超えて添加するとTiの上限を超えた場合と同様に再結晶性に悪影響が生じる。好ましい範囲は0.2 wt％以下である。
Caは、硫化物の形状制御を通じて加工性を改善する作用があり、0.050 wt％以下の範囲で添加することができる。しかし、これを超えると却って清浄度を害する。好ましい範囲は0.01wt％以下である。
Ｂは、Ar₃ 変態点を低下させる作用を通じてこの発明の効果を助長する作用を有し、0.010 wt％以下の範囲で添加することができる。しかし、0.010 wt％を超えると再結晶性に悪影響が生じるので避けなければならない。好ましい範囲は５〜30wtppm である。
【００３０】
Ｐmpの規制は、この発明において極めて重要な点である。その理由は、先に述べたように、この発明が動的再結晶及び静的再結晶による微細化を極限まで進めることを目的として構成されたものであって、その基本的条件をこの因子の規制によって確保しているからである。Ｐmpが1499以上であると第１段階の圧延工程及び第２段階の圧延工程におけるオーステナイト粒の微細化が不十分となり、最終組織のフェライト粒の粒径を２μm 未満にすることができなくなる。一方、Ｐmpを1499未満の領域で低下していくと次第にその効果は飽和してくるので、必要以上に低下させる必要はない。最も好ましい範囲は1440〜1485の範囲である。
【００３１】
次に、圧延条件の数値規制理由について述べる。
スラブ加熱温度（ＳＲＴ）は、初期オーステナイト粒径を微細化するうえで1100℃以下とすることが必要である。この理由は、1100℃を超えるとTiＣ粒子の溶解が始まって初期オーステナイト粒の粗大化が生じ、以降の工程での再結晶微細化が有効に進展しなくなるためである。ＳＲＴの下限を950 ℃と定めたのは、次に述べる第１段階の圧延工程を動的再結晶温度範囲で行うために必要な要件である。また、加熱時間を10分以上とする理由は、鋼片内の温度を均一にし、最終組織の均一性を確保するために必要な条件だからである。なお、この発明の製造条件を満たした場合の加熱工程終了時点での初期オーステナイト粒径は20μm 以下を達成することができる。
【００３２】
第１段階の圧延工程は、動的再結晶によるオーステナイト粒の微細化を進行させる工程であり、このために望ましい圧延温度範囲は950 〜1100℃の領域である。1100℃を超える温度域での圧下によっても動的再結晶は生じるものの、オーステナイト粒を有効に微細化させることはできない。ただし、1100℃を超える温度域での圧下を予め行っても何か問題を生じることはない。一方、950 ℃未満の温度領域では動的再結晶が起こり難くなり、静的再結晶温度域となるので次に述べる第２段階の圧延工程に該当する。
【００３３】
第１段階の圧延工程において必要な要件は上記した950 〜1100℃の領域で１パス当たりの圧下量が20％以上である圧延を少なくとも１回以上施すことである。１パス当たりの圧下量が20％に満たない場合には動的再結晶微細化が進行せず最終組織のフェライト粒径を２μm 未満にすることができなくなる。なお、上述の条件で少なくとも１回の動的再結晶を生じさせると、オーステナイト粒の微細化は十分に進行する。これは、この発明が、加熱段階で初期オーステナイト粒を十分に微細化していることと、動的再結晶後のオーステナイト粒が安定で粒成長を起こし難いためである。なお、第１段階の圧延工程でこの発明の製造条件を満たす圧下を複数回行えば、更に微細化が進行するが、回数を重ねるに従って効果は飽和してくるので、むやみにパス数を多くする必要はなく、５パス以下で十分である。また、１パス当たりの圧下率は大きいほど微細化に有効であるが、この発明の鋼は、上述したＰmpの効果とTiＣ粒子による初期オーステイナイト粒の微細化効果によって従来の鋼よりもはるかに動的再結晶微細化が生起し易くしてあるので、従来鋼よりも低い圧下率範囲、大きい歪速度範囲、低い温度領域においても動的再結晶微細化が進行する。この発明の条件を満たした場合の第１段階の圧延工程終了時点でオーステナイト粒は20μm 以下に達する。
【００３４】
第２段階の圧延工程は、静的再結晶による微細化を進行させる過程である。この第２段階の圧延工程の意義は、上述した動的再結晶による微細化を繰り返しても到達できるオーステナイト粒に限界が生じるために、更に、超微細粒領域、すなわちオーステナイト粒として５μm 以下まで再結晶微細化を進行させる点にある。この工程は、750 ℃以上950 ℃未満の温度範囲で行うのが好適である。950 ℃以上は上述した動的再結晶範囲に該当するので第２段階圧延工程には含まれない。一方、750 ℃未満の温度範囲では難再結晶領域となるので回避しなければならない。また、この工程での必要な要件は、上記した温度範囲での累積圧下率であり、50％以上とすることが必要である。これに満たない場合には静的再結晶微細化が不十分となる。また、１パス当たりの圧下量を20％以上に定める理由は、この条件が満たされない時には歪エネルギーの蓄積効果が小さくなってたとえ累積圧下率が本発明範囲になったとしても第２段階圧延工程後の静的再結晶微細化が生じ難くなり最終フェライト粒径を２μm 未満にすることができなくなるからである。さらに、重要な要件はこの温度区間において施される圧延パスと次パスとの間における冷却速度であり、少なくとも５℃/s以上とすることが必要である。これに満たない冷却速度では、パス間において歪の回復が生じて歪エネルギー累積効果が生じない。したがって、この圧延工程内でパス間の冷却速度が５℃/sに満たない区間が生じた場合には、その冷却区間の直前の圧下分を累積圧下率に加えてはならない。パス間の冷却速度が大きくなると歪蓄積効果は大きくなって行くが、次第に飽和するので必要以上に大きくしなくても良い。最も好ましいのは、10℃/s〜50℃/sである。
【００３５】
第２段階圧延工程後の冷却工程では、平均冷却速度を20℃/s以上とすることが必要であり、これに満たない場合には最終組織のフェライト粒径を２μm 未満にすることはできない。
また、必要な冷却温度区間は、第２段階圧延工程終了後から６００℃以下の温度区間であり、この温度区間より高い領域で冷却を停止したならば変態後のα粒の成長が生じて最終フェライト粒径を２μm 未満にすることができない。なお、上記の冷却速度は圧延終了から600 ℃以下の温度区間での平均冷却速度であるから、この条件が満たされるならばこの区間中の短時間の20℃/s未満の区間があっても差し支えない。平均冷却速度の好ましい範囲は、30〜150 ℃/sの範囲であるが、200 ℃/s以上の冷却速度とするとフェライト変態の進行が抑制され、最終フェライト分率を70％以上とするこの発明の組織が得られなくなる。
【００３６】
600 ℃未満の温度領域での冷却速度並びに冷却停止温度、あるいは巻取温度などについてはこの発明では特に規制しない。この理由は、フェライト粒径２μm 未満、かつ、ポリゴナルフェライト分率70％以上の最終組織を得るための必要要件ではないためである。600 ℃未満の温度領域の温度履歴が影響するのは第２相であり、これを目的に応じて調整する場合に以上の因子を規制する。これらの方法については既に述べた。
【００３７】
【実施例】
（実施例１）
次に実施例によってこの発明をさらに具体的に説明する。表１、表２に示す種々の成分組成になるスラブを、1080℃で30min 加熱保持し、次いで、第１段階の圧延工程として950 〜1050℃の温度範囲で１パス当たりの圧下量が25〜40％の圧下を３回（この発明の条件を満たすパス回数が３回）加え、次いで、第２段階の圧延工程として800 〜925 ℃の温度範囲、パス後の最小冷却速度10℃/sで冷却しながら１パス当たりの圧下量が25〜40％で、累計圧下率が88％の圧延を施して（総パス回数７回、この発明の条件を満たすパス回数が７回）、800 ℃で仕上げ（仕上板厚3.0 mm）、その後は60℃/s以上の冷却速度で450 ℃まで冷却し、420 ℃で巻き取った。
かくして得られた3.0 mm厚さの熱延鋼板について、酸洗した後、ポリゴナルフェライトの体積率、フェライト結晶粒径を測定するとともに、JIS ５号試験片による引張り特性、両振り曲げ試験法による疲労特性、原厚まま２ｍｍＶノッチシャルピー試験片による延性一脆性遷移温度を調査した。これらの調査結果を表３、表４に示す。また、穴広げ性について調べ、その結果を表５、表６に示す。
【００３８】
【表３】

【００３９】
【表４】

【００４０】
【表５】

【００４１】
【表６】

【００４２】
（実施例２）
表７に示す種々の成分組成になるスラブを、種々の条件で加熱保持し、次いで、第１段階の圧延工程を行い、加え、次いで、第２段階の圧延工程を施し、その後は冷却した。これらの加熱、第１段階の圧延、第２段階の圧延、冷却の各条件について表８に示す。
かくして得られた3.0 mm厚さの熱延鋼板について、酸洗した後、ポリゴナルフェライトの体積率、フェライト結晶粒径を測定するとともに、JIS ５号試験片による引張り特性、両振り曲げ試験法による疲労特性、原厚まま２ｍｍＶノッチシャルピー試験片による延性一脆性遷移温度を調査した。これらの調査結果を表９に示す。また、穴広げ性について調べ、その結果を表10に示す。
【００４３】
【表７】

【００４４】
【表８】

【００４５】
【表９】

【００４６】
【表１０】

【００４７】
（実施例３）
表７のＩ３の成分組成になるスラブを、種々の条件で加熱保持し、次いで、第１段階の圧延工程を行い、加え、次いで、第２段階の圧延工程を施し、その後は冷却した。これらの加熱、第１段階の圧延、第２段階の圧延、冷却の各条件について表１１に示す。
かくして得られた熱延鋼板について、酸洗した後、ポリゴナルフェライトの体積率、フェライト結晶粒径を測定するとともに、JIS ５号試験片による引張り特性、両振り曲げ試験法による疲労特性、原厚まま２ｍｍＶノッチシャルピー試験片による延性一脆性遷移温度を調査した。これらの調査結果を表12に示す。また、穴広げ性について調べ、その結果を表13に示す。
【００４８】
【表１１】

【００４９】
【表１２】

【００５０】
【表１３】

【００５１】
【発明の効果】
以上説明したように、この発明に従い得られた熱延鋼板は、ポリゴナルフェライトの平均粒径が２μm 未満と、従来よりも超微細であり、しかも異方性が少ないので、延性、じん性、疲労強度、耐衝突特性に優れるため、自動車用材、機械構造材、建築用材、パイプ材等としての工業的価値は大きい。また、この発明の熱延鋼板を得る製造方法は、一般のホットストリップミルで容易に実施可能であり、この点でも産業上の寄与は極めて大きい。
【図面の簡単な説明】
【図１】Ｐmpと鋼板の平均フェライト粒径との関係についてTi量を異ならせて調べた結果を示す図である。
【図２】初期オーステナイト粒径と加熱温度との関係を示す図である。
【図３】第１段階圧延工程における１パス当たりの圧下量と、最終組織における平均フェライト粒径との関係を示す図である。
【図４】第２段階圧延工程における累積圧下率と圧延中の冷却速度が、最終組織のフェライト粒径に及ぼす影響を示す図である。
【図５】最終組織のフェライト粒径と圧延終了温度から600 ℃までの冷却速度の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is a hot-rolled steel sheet having an ultrafine ferrite grain having an average grain size of less than 2 μm as it is hot-rolled and excellent in ductility, toughness, fatigue strength, and impact resistance characteristics, particularly in a high strength range of 490 MPa or more. High-tensile hot-rolled steel suitable for applications such as automotive materials, machine structural materials, building materials, pipe materials, etc., that exhibit superior properties Plank It relates to a manufacturing method.
[0002]
[Prior art]
In order to improve the mechanical properties of steel materials used in automobile materials, structural materials, pipe materials, etc., it is generally effective to make the metal structure finer, and more manufacturing methods aiming for an ultrafine structure have been sought. It has been. In particular, in recent years, there is a high need for high-tensile steel sheets that can achieve both low cost and high-functional properties, and these steel sheets have been used more frequently, resulting in deterioration of ductility, toughness, durability ratio, etc. associated with higher tensions. In order to suppress the above, miniaturization of the structure in high-tensile steel is an important issue.
[0003]
Here, when summarizing the means for refining the structure in the prior art, a controlled rolling method, a controlled cooling method, a large reduction rolling method, and the like are known. Of these methods, the most widely used for the purpose of simultaneously achieving high tension and microstructure refinement is precipitation strengthened high strength steel sheets containing Nb or Ti. The controlled rolling method and the controlled cooling method are used in the above-described structure refinement means. The reason why such precipitation-strengthened high-tensile steel sheets are widely used is that high tensile strength can be easily achieved using the precipitation strengthening action of Nb and Ti contained in these steel sheets, and austenite grains of Nb and Ti. This is because the ferrite crystal grains can be refined by the γ → α strain-induced transformation from the non-recrystallized deformed austenite grains when the low-temperature finish rolling is performed by utilizing the recrystallization suppressing action. However, the difficulty of the high-tensile hot-rolled steel sheet produced by this method is that the anisotropy of mechanical properties is large. For this reason, for example, in automotive steel sheets for press forming, the forming limit is determined by the characteristic level in the direction where the ductility is most inferior. is there. In addition, the difficulty of large anisotropy such as toughness and fatigue strength, which are important characteristics, in structural materials and pipe materials also leads to the same problem.
[0004]
On the other hand, there is a proposal represented by, for example, Japanese Patent Application Laid-Open No. 58-123823 as a method for refining a structure by large rolling. The main point of the refinement mechanism in these methods is to promote strain-induced transformation from γ to α by applying large pressure to austenite grains. In fact, the same mechanism is used. However, since the difference between the two is a strain-induced transformation from unrecrystallized austenite grains in precipitation strengthened steel, the method of the above-mentioned Japanese Patent Application Laid-Open No. 58-123823 has the advantage that the above-mentioned problem of anisotropy is less. is there. However, this method of refining the structure by large rolling is difficult to implement with a general hot strip mill, such as a reduction amount of 40% or more per pass, and difficult to implement with a general hot strip mill. Therefore, there has been a limit to the refinement of the final structure of the obtained steel sheet.
[0005]
As means for alleviating the large rolling reduction conditions which are the disadvantages of Japanese Patent Application Laid-Open No. 58-123823, Japanese Patent Application Laid-Open No. 9-143570 proposes a method utilizing the dynamic recrystallization phenomenon in the austenite rolling region. . This proposal has the advantages that it is easier to implement with a general hot strip than the large rolling method, and that the final structure is equiaxed and has less mechanical property anisotropy. The diameter is still limited to 2 μm.
[0006]
[Problems to be solved by the invention]
Since the improvement effect of the mechanical properties due to the refinement of the crystal grain size is inversely proportional to the square root of the crystal grain size, this improvement effect is only slowly improved in the region where the grain size is 2 μm or more, but 2 μm. If a crystal grain size of less than is realized, a significant property improvement is achieved.
[0007]
In order to solve the above-mentioned problems remaining in the prior art, the present invention can be easily implemented with a general hot strip mill, has little mechanical property anisotropy, and cannot be achieved with the prior art. Hot-rolled steel for machining that has achieved ultrafine grains with a final ferrite grain size of less than 2μm Plank Advantageous manufacturing method The The purpose is to propose.
[0008]
[Means for Solving the Problems]
In the present invention, in order to achieve ultrafine particles of less than 2 μm, the following two points are configured as the main points.
The first is to add up various refinement processes that occur in the hot rolling process. This means that the initial austenite grain size in the heating stage is made as fine as possible, the dynamic recrystallization refinement is advanced in the first stage rolling, and the above process is refined in the second stage rolling process. The austenite grains are further piled up with static recrystallization refinement to refine the austenite grains to the limit, and in the final cooling step, transformation refinement due to γ → α transformation is caused.
The second point is that the metallurgical design is performed so that the best effect is obtained in each of the above-described miniaturization processes. In particular, the latter is the most important element in the present invention, and is an important factor for extracting the effect.
[0009]
The present invention having the above-mentioned main part is as follows. , C : 0.05 to 0.30 wt%, Si: 1.0 to 2.5 wt%, Mn: 0.7 to 3.0 wt%, Al: 0.0005 to 0.1 wt% % And Ti: 0.05-0.3 wt%, and Cu, Ni, Cr, Mo, one or more of total, 0-2.5 wt%, Nb, Zr, Ta, one or more 0 to 0.3 wt%, Ca: 0 to 0.05 wt%, B: 0 to 0.010 wt%, P: 0.05 wt% or less, S: 0.025 wt% or less, and represented by the following formula A step of heating and holding a steel that satisfies the condition that the component parameter Pmp is less than 1499 and the balance is substantially an iron composition in a temperature range of 950 to 1100 ° C. for 10 minutes or more, and 950 to 1100 ℃ A first stage rolling process in which a rolling reduction of 20% or more per pass in the temperature range is applied one or more times; 700 ℃ or more A second stage rolling process in which rolling is performed at a cooling rate of 5 ° C./s or more in a temperature range of less than 950 ° C. and rolling at a reduction rate of 20% or more per pass and a cumulative reduction rate of 50% or more; The second stage rolling process is followed by a cooling process at a cooling rate of 20 ° C./s or more to a temperature range of 600 ° C. or less, and the polygonal ferrite fraction in the microstructure is 70% or more and the ferrite grains Method for producing high-tensile hot-rolled steel sheets with an ultrafine structure with an average particle size of less than 2 μm In law is there.

In addition, this In the present invention, the lower limit of the content (or total content) of Cu and the like is 0 wt% means that these components may not be added.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The operational effects of the present invention will be described in detail based on the results of experiments conducted by the inventors.
First, regarding dynamic recrystallization refinement, the inventors conducted experiments based on the following two ideas.
The first is to extremely refine the initial austenite grains in the slab heating stage. In the present invention, as a means for this purpose, a method is adopted in which a large amount of undissolved TiC particles are present during heating, and the initial austenite grains are refined by the pinning action of the particles. From this point of view, the Ti amount is important, and the heating conditions are considered important.
Second, it was considered important to adjust in advance to the component conditions at which dynamic recrystallization in the first stage occurs as easily as possible.
[0011]
Based on the above idea, the inventors have studied the component conditions of steel in which refinement by dynamic recrystallization is most advanced. And since the useful knowledge in constructing this invention was obtained, the research result is described.
Steels with different chemical compositions shown in Tables 1 and 2 are heated and held for 10 min or more in the temperature range of 950 to 1100 ° C, and then the reduction amount per pass in the temperature range of 950 to 1100 ° C But Apply the first rolling process to apply a reduction of 20% or more at least twice. ℃ or more A second stage rolling process is performed in which rolling is performed at a cooling rate of 5 ° C./s or more in a temperature range of less than 950 ° C., and rolling at a reduction rate of 20% or more per pass and a cumulative reduction rate of 50% or more. After that, the result of measuring the average ferrite grain size was analyzed for the hot-rolled steel sheet that was manufactured after the second stage rolling process by cooling to a temperature range of 600 ° C or less at a cooling rate of 20 ° C / s or more. The result of FIG. 1 was obtained.
[0012]
[Table 1]

[0013]
[Table 2]

[0014]
That is, as can be seen from FIG. 1, the change in the average ferrite grain size when the hot rolling condition is within the range of the present invention is the amount of Ti and the component parameter Pmp, that is, Pmp = 1538−55 × C (%) − 80. × {C (%)} ² −13 × Si (%) − 4.8 × Mn (%) − 1.5 × Cr (%) − 4.3 × Ni (%) − 3.5 × Cu (%) − 1.2 × Mo (%) It can be seen that the ferrite grain size becomes finer on the side where the amount is large and Pmp is small. It can be seen that the conditions for obtaining ultrafine ferrite grains having an average ferrite grain size of less than 2 μm are a Ti content of 0.05 wt% or more and a Pmp of less than 1499. Here, the amount of Ti was influenced through the refining action of the initial austenite grain size in the heating stage of TiC particles, which is supported by the observation of precipitates by electron microscope for the experimental materials in this study. I was able to.
[0015]
On the other hand, although the mechanism of the influence of the component parameter Pmp is not necessarily clear, it can be considered as follows. It is known that the influence of solid solution elements is one of the factors that determine the difficulty of dynamic recrystallization. It is also known that a large self-diffusion coefficient is advantageous for dynamic recrystallization. Pmp is a component parameter for the change in melting point of the solid solution element of steel, but there is a certain relationship between this melting point and the ease of self-diffusion of Fe atoms in the solid phase region, and the component that lowers the melting point. Under the condition, the self-diffusion of Fe atoms is facilitated. Therefore, it can be considered that the influence of Pmp in FIG. 1 affects the dynamic recrystallization through a change in the ease of self-diffusion by the solid solution element.
[0016]
Thus, the heating process is an important process for refining the initial austenite grain size by the TiC particles and giving the initial conditions of the refining process in the present invention. If the refining at this stage is insufficient, Thus, dynamic recrystallization at the rolling stage becomes difficult to proceed, and the ferrite grain size in the final structure becomes insufficient. FIG. 2 shows the relationship between the initial austenite grain size and the heating temperature investigated using the steels having the components in the range of the present invention. When the heating temperature exceeds 1100 ° C., the initial austenite grain size rapidly increases. I understand. This is because TiC particles pinning the initial austenite grain boundaries begin to dissolve.
[0017]
The first rolling process is a process in which the initial fine austenite grains in the heating process are dynamically recrystallized by rolling and further refined. The key point at this stage is the amount of reduction per pass. FIG. 3 is a diagram showing the relationship between the reduction amount per pass in the first stage rolling process and the average ferrite grain size in the final structure, investigated using the component steels within the scope of the present invention. From FIG. 3, the refinement of ferrite grains in the final structure is clear under the condition that the reduction amount per pass is 20% or more. Therefore, the dynamic recrystallization refinement progresses by adopting this reduction condition. I understand that. The austenite grains refined in the first stage rolling process are relatively stable, and have a feature that rapid grain growth is unlikely to occur due to residence for about the time between passes in a normal hot rolling process. This is considered to be a unique phenomenon caused by the dynamic recrystallized grains and the presence of TiC dispersed in the steel, and is one of the advantages of the refinement method according to the present invention.
[0018]
Next, the experimental results regarding the requirements necessary for static recrystallization refinement will be described.
The second stage rolling process is a process of accumulating strain in the austenite grains refined by dynamic recrystallization up to the first stage rolling process and progressing miniaturization by static recrystallization. Although the static recrystallization refinement method has been adopted in the conventional method, the final ferrite grain size cannot be made less than 2 μm. However, the feature of the present invention is that, since the dynamic recrystallization refinement is advanced to the limit in the previous process and the static recrystallization refinement is added, the refinement is remarkably advanced. As will be described later, this is considered to be due to the fact that the austenite grain size before entering the static recrystallization process is very fine as 20 μm or less and that the dynamic recrystallized grains are used as starting materials.
[0019]
As requirements in this step, the cumulative rolling reduction and the cooling rate during rolling are important. FIG. 4 shows the influence of the cumulative reduction ratio and the cooling rate during rolling on the ferrite grain size of the final structure in the second stage rolling process for the component steels within the scope of the present invention. As is apparent from FIG. 4, when the cumulative rolling reduction is 50% or more and the cooling rate during rolling is 5 ° C./s or more, the ferrite grain size of the final structure becomes the finest. . The effect of the cumulative rolling reduction here seems to contribute to the refinement of the final structure through the action of increasing the accumulated strain energy and increasing the number of static recrystallized grains.
Figure 4 From the results, it can be seen that under the slow cooling rate of less than 5 ° C / s, the refinement of the ferrite grain size in the final structure does not progress even if the cumulative rolling reduction is increased. But This is because, when it is slow, recovery after rolling occurs and stored energy is lost.
[0020]
Next, in the cooling process after the second stage rolling, dynamic recrystallization refinement and static recrystallization refinement are stacked and the austenite grains refined to the limit are subjected to γ → α transformation to achieve transformation refinement. It is important to suppress the growth of α grains after transformation. This is also a process of controlling the second phase to the target structure. FIG. 5 shows the relationship between the ferrite grain size of the final structure and the cooling rate from the rolling end temperature to 600 ° C., in which the scope of the present invention was investigated using component steel. FIG. 5 shows that a cooling rate of 20 ° C./s or more is necessary to make the final average ferrite grain size less than 2 μm.
[0021]
Even when cooling is performed at a cooling rate of 20 ° C./s or more, the ferrite fraction of the final structure can be sufficiently 70% or more. This is because the austenite grains before transformation are extremely fine, and the number of α transformation nuclei is remarkably increased due to the strain energy accumulated in the second stage rolling process, and the γ → α transformation does not occur even under fast cooling rate conditions. This is because it progresses at an accelerated rate. However, when the cooling rate is extremely high, the ferrite fraction may decrease. A suitable cooling rate is in the range of 30-200 ° C / s.
[0022]
The temperature range important for ferrite grain refinement is from the end of rolling to 600 ° C, but the temperature range below 600 ° C is important for controlling the second phase. In order to do this, it is necessary to adjust the cooling stop temperature in this section. Although the cooling stop temperature for this purpose varies depending on the chemical composition, it is roughly as follows.
That is, the cooling stop temperature when the second phase is pearlite is about 550 to 590 ° C., 470 to 530 ° C. when bainite and acicular ferrite are used, and 400 to 450 ° C. when the retained austenite phase is desired. When it is desired to obtain a martensite phase, less than 400 ° C. is a preferable range for each cooling stop temperature. When the second phase is a composite phase having two or more different phases, after cooling at a cooling rate of 20 ° C./s or higher from the rolling end temperature to 600 ° C., the above temperature is less than 600 ° C. It can be obtained by adjusting the cooling rate in a suitable temperature range in each phase.
[0023]
Thus, by using the component steels within the scope of the present invention and adopting suitable rolling conditions at each stage described above, the average ferrite grain size is less than 2 μm, the polygonal ferrite fraction is 70% or more, and Hot-rolled steel sheets having two different microstructures can be produced.
[0024]
Next, the reason for limiting the component composition range in this invention will be described. C is not only necessary for producing the fine TiC particles necessary for the refinement of the initial austenite grains and the progress of refinement during and after the rolling, but also increases the strength of the steel by forming a second phase. It is necessary to plan. The amount of C necessary for the former purpose is preferably at least 1/4 times the amount of Ti added by weight ratio, and for the latter purpose, the amount of C bound as TiC is excluded. The surplus C amount is required to be at least 0.03 wt% or more. Therefore, the lower limit of the amount of C necessary for obtaining the ultrafine ferrite grain size of 2 μm or more and the strength of 490 MPa class or more, which is the object of the present invention, is 0.05 wt%. On the other hand, when the C content exceeds 0.30 wt%, the ferrite fraction cannot be made 70% or more, and ductility, toughness, fatigue resistance, and impact resistance are deteriorated. The most preferred range is from 0.07 to 0.25%.
[0025]
As a solid solution strengthening element, Si not only contributes to increasing strength, accelerates the ferrite transformation in the cooling process, and increases the ferrite fraction, but also enters the second phase generated in this process. Contributing to the improvement of strength-ductility balance and strength-toughness balance by developing C concentration of steel, making dynamic recrystallization easy through reducing component parameter Pmp, and reducing the fineness of ferrite grains in the final structure It is a very useful component that promotes and improves ductility, toughness, fatigue resistance and impact resistance. However, at 2.5 wt% or more, Ar _Three The transformation point is remarkably high, the suitable rolling temperature range in this invention is narrowed, and the effect of coarsening the TiC particles becomes too large, and conversely, the adverse effect of inhibiting the refinement of crystal grains occurs. 2.5 wt%. The most preferred range is 1.0 to 2.0 wt%.
[0026]
If Mn is used in an appropriate amount, Ar _Three It exhibits the beneficial effect of promoting the refinement of crystal grains through the action of lowering the transformation point and the action of lowering Pmp. Moreover, it has the effect | action which raises a strength-ductility balance and an intensity | strength-fatigue-strength balance through the effect | action which advances martensite formation and residual gamma phase formation of a 2nd phase. In order to exert this beneficial effect, at least 0.7% is necessary. However, if it exceeds 3.0%, Ar _Three The point was too low to ensure a ferrite fraction of 70% or more. On the contrary, the strength-ductility balance deteriorated, so the upper limit was made 3.0%. The most preferred range is from 1.0 to 2.5%.
[0027]
Ti is an indispensable component in the present invention utilizing the refinement by TiC particles. The lower limit of the amount of Ti necessary for exerting this action is 0.05%, and an ultrafine ferrite particle size of less than 2 μm cannot be obtained in a range less than this. Further, the action of Ti gradually saturates as the addition amount increases, and even if it exceeds 0.3 wt%, no further effect can be expected. Therefore, the range of the addition amount is set to a range of 0.05 to 0.3 wt%. A preferable range is 0.10 to 0.20%. Note that the amount of Ti added should be noted in relation to the amount of C. That is, the present invention is based on the fact that most of the added Ti amount is present as TiC particles. Under conditions where there is excess Ti beyond the amount of C for TiC generation, that is, under conditions where solid solution Ti exists, the effect of inhibiting recrystallization by solid solution Ti appears. And it is desirable to avoid this in the present invention which achieves ultra-miniaturization by accumulating static recrystallization refinement. Therefore, it is desirable that the amount of Ti and the amount of C satisfy the relationship of Ti <4 × C by weight ratio.
[0028]
When P causes strong segregation at the crystal grain boundary, it lowers the grain boundary strength, and thus adversely affects toughness. The steel according to the present invention has a very fine crystal grain size, and therefore is less likely to cause such grain boundary segregation of P than conventional steel. P amount is If it exceeds 0.05wt%, it will be difficult to avoid the adverse effects. P amount is 0.05wt% or less Restricted to . The lower limit is 0.001 for economic reasons. wt % Do Is preferred. A more preferred range is less than 0.015 wt%.
S forms sulfides and deteriorates the cleanliness of the steel, and adversely affects fatigue resistance, toughness, impact resistance, and the like. To avoid this adverse effect, the upper limit is 0.025 wt%. did . A more preferred range is less than 0.015 wt%. In addition, when used for a sour-resistant line pipe or the like, the presence of sulfides has a significant adverse effect on the HIC resistance. Therefore, to avoid this, the amount of S is preferably 0.005 wt% or less. The lower limit is preferably 0.0005 wt% from the viewpoint of economy.
Al can be added in the range usually used as a deoxidizing component without any problem. On the other hand, since the steel of the present invention contains a relatively large amount of Si and Ti, it is possible to perform Si deoxidation, Al-Si composite deoxidation, or Si-Ti deoxidation using a part of the steel. Therefore, in such a case, clean steel can be manufactured even if the amount of Al added is extremely suppressed. Therefore, Al should be added 0.0005wt% or more. Was . The upper limit is 0.1 wt% from the viewpoint of economy Determined .
[0029]
The above is the basic component in the steel of the present invention, but in the present invention, in addition to the above components, one or more of Cu, Ni, Cr, Mo, and one or two of Nb, Zr, Ta as required. As described above, Ca and B may be contained.
Cu, Ni, Cr, Mo are just like Mn, Ar _Three This is effective in drawing out the effects of the present invention through the action of lowering the transformation point and the action of lowering Pmp. These addition amounts are desirably in the range of 2.5 wt% or less in total of one or two. If it is added beyond this range, an adverse effect occurs as in the case where the upper limit of Mn is exceeded. A preferred range is 2.0 wt% or less.
Nb, Zr, and Ta are formed with the same carbide as TiC and effectively act to refine crystal grains. Therefore, one or more of Nb, Zr, and Ta can be added in a range of 0.3 wt% or less. However, if added in excess of 0.3 wt%, the recrystallization property is adversely affected as in the case where the upper limit of Ti is exceeded. A preferable range is 0.2 wt% or less.
Ca has an effect of improving workability through shape control of sulfide, and can be added in a range of 0.050 wt% or less. However, if this is exceeded, the cleanliness is adversely affected. A preferable range is 0.01 wt% or less.
B is Ar _Three It has the effect of promoting the effect of the present invention through the action of lowering the transformation point, and can be added in the range of 0.010 wt% or less. However, if it exceeds 0.010 wt%, the recrystallization property is adversely affected and must be avoided. A preferred range is 5-30 wtppm.
[0030]
The regulation of Pmp is a very important point in the present invention. The reason for this is that, as described above, the present invention is configured for the purpose of minimizing miniaturization by dynamic recrystallization and static recrystallization. This is because it is secured by regulation. When Pmp is 1499 or more, the austenite grains are not sufficiently refined in the first rolling process and the second rolling process, and the grain size of the ferrite grains in the final structure cannot be made less than 2 μm. On the other hand, when Pmp is lowered in a region less than 1499, the effect gradually becomes saturated, and it is not necessary to lower it more than necessary. The most preferred range is from 1440 to 1485.
[0031]
Next, the reason for numerical regulation of rolling conditions will be described.
The slab heating temperature (SRT) needs to be 1100 ° C. or lower in order to refine the initial austenite grain size. This is because when the temperature exceeds 1100 ° C., dissolution of the TiC particles starts and coarsening of the initial austenite grains occurs, and recrystallization refinement in the subsequent steps does not progress effectively. The lower limit of SRT is set to 950 ° C. is a requirement necessary for performing the first stage rolling process described below in the dynamic recrystallization temperature range. The reason for setting the heating time to 10 minutes or more is that it is a necessary condition for making the temperature in the steel slab uniform and ensuring the uniformity of the final structure. Note that the initial austenite grain size at the end of the heating step when the production conditions of the present invention are satisfied can be 20 μm or less.
[0032]
The first rolling process is a process of advancing the refinement of austenite grains by dynamic recrystallization, and a desirable rolling temperature range for this purpose is a range of 950 to 1100 ° C. Although dynamic recrystallization occurs even when the temperature is over 1100 ° C., austenite grains cannot be made finer effectively. However, there is no problem even if the reduction in the temperature range exceeding 1100 ° C. is performed in advance. On the other hand, dynamic recrystallization hardly occurs in a temperature range of less than 950 ° C. and becomes a static recrystallization temperature range, which corresponds to the second stage rolling process described below.
[0033]
A necessary requirement in the rolling process of the first stage is to perform at least one rolling in which the reduction amount per pass is 20% or more in the above-described region of 950 to 1100 ° C. When the amount of reduction per pass is less than 20%, dynamic recrystallization refinement does not proceed and the ferrite grain size of the final structure cannot be made less than 2 μm. If at least one dynamic recrystallization is caused under the above-described conditions, the austenite grains are sufficiently refined. This is because the present invention sufficiently refines the initial austenite grains in the heating stage and the austenite grains after dynamic recrystallization are stable and hardly cause grain growth. In addition, if the reduction satisfying the manufacturing condition of the present invention is performed a plurality of times in the first stage rolling process, further miniaturization proceeds, but the effect becomes saturated as the number of times is increased, so the number of passes is increased unnecessarily. There is no need, and 5 passes or less is sufficient. Further, the larger the rolling reduction per pass, the more effective the refinement. However, the steel of the present invention is far more than the conventional steel due to the effect of Pmp described above and the refinement effect of the initial austenite grains by TiC particles. Therefore, dynamic recrystallization refinement proceeds even in a lower rolling reduction range, a larger strain rate range, and a lower temperature range than conventional steel. The austenite grains reach 20 μm or less at the end of the first rolling process when the conditions of the present invention are satisfied.
[0034]
The second stage rolling process is a process in which miniaturization by static recrystallization proceeds. The significance of this second stage rolling process is that there is a limit to the austenite grains that can be reached even if the above-described dynamic recrystallization is repeated, so that the ultrafine grain region, that is, austenite grains can be further reduced to 5 μm or less. It is in the point that crystal refinement proceeds. This process is 750 ℃ or more It is preferred to carry out in a temperature range below 950 ° C. Since 950 ° C. or higher corresponds to the above-mentioned dynamic recrystallization range, it is not included in the second stage rolling process. On the other hand, in the temperature range below 750 ° C., it becomes a difficult recrystallization region and must be avoided. In addition, a necessary requirement in this step is the cumulative rolling reduction in the above temperature range, and it is necessary to set it to 50% or more. When less than this, static recrystallization refinement becomes insufficient. The reason why the reduction amount per pass is set to 20% or more is that when this condition is not satisfied, even if the cumulative effect of strain energy is reduced and the cumulative reduction rate is within the range of the present invention, the second stage rolling process. This is because subsequent static recrystallization refinement is difficult to occur, and the final ferrite grain size cannot be made less than 2 μm. Furthermore, an important requirement is the cooling rate between the rolling pass and the next pass applied in this temperature interval, and it is necessary to set at least 5 ° C./s or more. If the cooling rate is less than this, strain recovery occurs between passes, and the strain energy accumulation effect does not occur. Therefore, when a section in which the cooling rate between passes is less than 5 ° C./s occurs in this rolling process, the reduction immediately before the cooling section must not be added to the cumulative reduction ratio. As the cooling rate between passes increases, the strain accumulation effect increases, but since it gradually saturates, it need not be increased more than necessary. Most preferred is 10 ° C./s to 50 ° C./s.
[0035]
In the cooling process after the second stage rolling process, it is necessary to set the average cooling rate to 20 ° C./s or more. If it is less than this, the ferrite grain size of the final structure cannot be less than 2 μm.
Further, the necessary cooling temperature interval is a temperature interval of 600 ° C. or less after the end of the second stage rolling process, and if the cooling is stopped in a region higher than this temperature interval, the growth of α grains after transformation occurs and the final The ferrite grain size cannot be less than 2 μm. The above cooling rate is an average cooling rate in a temperature section of 600 ° C. or less from the end of rolling, so if this condition is satisfied, even if there is a short section of less than 20 ° C./s in this section. There is no problem. The preferable range of the average cooling rate is 30 to 150 ° C./s. However, if the cooling rate is 200 ° C./s or more, the progress of ferrite transformation is suppressed, and the final ferrite fraction is 70% or more. No organization can be obtained.
[0036]
In the present invention, the cooling rate in the temperature region below 600 ° C., the cooling stop temperature, the coiling temperature, etc. are not particularly restricted. This is because it is not a necessary requirement for obtaining a final structure having a ferrite grain size of less than 2 μm and a polygonal ferrite fraction of 70% or more. The second phase is affected by the temperature history in the temperature region below 600 ° C, and the above factors are regulated when this is adjusted according to the purpose. These methods have already been described.
[0037]
【Example】
(Example 1)
Next, the present invention will be described more specifically with reference to examples. Slabs having various component compositions shown in Table 1 and Table 2 are heated and held at 1080 ° C. for 30 minutes, and then the first rolling process is performed at a temperature range of 950 to 1050 ° C. with a reduction amount of 25 to 1 per pass. Add 40% reduction 3 times (3 passes to satisfy the conditions of this invention), then in the second stage rolling process at a temperature range of 800-925 ° C, with a minimum cooling rate of 10 ° C / s after pass While cooling, rolling with a rolling reduction of 25 to 40% and a cumulative reduction ratio of 88% was performed (total number of passes 7 times, the number of passes satisfying the conditions of the present invention was 7 times) at 800 ° C. Finished (finished plate thickness: 3.0 mm), then cooled to 450 ° C. at a cooling rate of 60 ° C./s or more, and wound up at 420 ° C.
The hot-rolled steel sheet with a thickness of 3.0 mm thus obtained was pickled and then measured for the volume fraction of polygonal ferrite and the ferrite crystal grain size, as well as by the tensile properties by JIS No. 5 test piece, by the double swing bending test method. The ductility-brittle transition temperature was investigated using a 2 mm V-notch Charpy specimen with the same fatigue thickness as the original thickness. These survey results are shown in Tables 3 and 4. Further, the hole expandability was examined, and the results are shown in Tables 5 and 6.
[0038]
[Table 3]

[0039]
[Table 4]

[0040]
[Table 5]

[0041]
[Table 6]

[0042]
(Example 2)
The slabs having various component compositions shown in Table 7 were heated and held under various conditions, then subjected to the first stage rolling process, added, then subjected to the second stage rolling process, and then cooled. Table 8 shows these heating, first-stage rolling, second-stage rolling, and cooling conditions.
The hot-rolled steel sheet with a thickness of 3.0 mm thus obtained was pickled and then measured for the volume fraction of polygonal ferrite and the ferrite crystal grain size, as well as by the tensile properties by JIS No. 5 test piece, by the double swing bending test method. The ductility-brittle transition temperature was investigated using a 2 mm V-notch Charpy specimen with the same fatigue thickness as the original thickness. Table 9 shows the results of these investigations. Further, the hole expandability was examined, and the result is shown in Table 10.
[0043]
[Table 7]

[0044]
[Table 8]

[0045]
[Table 9]

[0046]
[Table 10]

[0047]
(Example 3)
The slab having the component composition of I3 in Table 7 was heated and held under various conditions, then subjected to the first stage rolling process, added, and then subjected to the second stage rolling process, and then cooled. Table 11 shows these heating, first stage rolling, second stage rolling, and cooling conditions.
The hot-rolled steel sheet thus obtained is pickled and then measured for the volume fraction of polygonal ferrite and the ferrite crystal grain size, as well as the tensile properties with JIS No. 5 test piece, the fatigue properties with the double-bending test method, the original thickness The ductile one brittle transition temperature was investigated using a 2 mm V notch Charpy test piece. Table 12 shows the results of these surveys. Further, the hole expandability was examined, and the results are shown in Table 13.
[0048]
[Table 11]

[0049]
[Table 12]

[0050]
[Table 13]

[0051]
【The invention's effect】
As described above, the present invention Obtained according to Hot-rolled steel sheet has an average grain size of polygonal ferrite of less than 2μm, which is ultrafine and less anisotropic, and has excellent ductility, toughness, fatigue strength, and impact resistance. Industrial value as machine structural materials, building materials, pipe materials, etc. is great. Moreover, the manufacturing method for obtaining the hot-rolled steel sheet according to the present invention can be easily carried out with a general hot strip mill, and in this respect, the industrial contribution is extremely large.
[Brief description of the drawings]
FIG. 1 is a graph showing the results of examining the relationship between Pmp and the average ferrite grain size of a steel sheet by varying the amount of Ti.
FIG. 2 is a diagram showing the relationship between the initial austenite grain size and the heating temperature.
FIG. 3 is a diagram showing the relationship between the amount of reduction per pass in the first stage rolling process and the average ferrite grain size in the final structure.
FIG. 4 is a diagram showing the influence of the cumulative reduction ratio and the cooling rate during rolling on the ferrite grain size of the final structure in the second stage rolling process.
FIG. 5 is a graph showing the relationship between the ferrite grain size of the final structure and the cooling rate from the rolling end temperature to 600 ° C.

Claims (1)

C: 0.05~0.30wt%, Si: 1.0 ~2.5 wt%, Mn: 0.7 ~3.0 wt%, Al: 0.0005~0.1 wt% 及 Beauty Ti: 0.05 to 0.3 comprises wt%, and, Cu, Ni, Cr , One or more of Mo in total 0 to 2.5 wt%, one or more of Nb, Zr, Ta in total 0 to 0.3 wt%, and Ca: 0 to 0.05 wt%, B: In addition to containing 0 to 0.010 wt%, P: 0.05 wt% or less, S: 0.025 wt% or less, and satisfying the condition that the component parameter Pmp represented by the following formula is less than 1499, the remainder is substantially Heating and holding the steel with the iron composition at a temperature range of 950 to 1100 ° C for 10 minutes or more, and applying a reduction of 20% or more per pass at a temperature range of 950 to 1100 ° C at least once The rolling process of the first stage and the reduction rate per pass is 20% or more and the cumulative reduction rate is 50% or more while cooling at a cooling rate of 5 ° C / s or more in the temperature range of 700 ° C or more and less than 950 ° C. Second stage for rolling And the step of cooling to a temperature range of 600 ° C. or lower at a cooling rate of 20 ° C./s or higher after the second rolling step, and the polygonal ferrite fraction in the microstructure is 70% or higher. And the manufacturing method of the high-tensile-strength hot-rolled steel plate which has the ultra-fine structure whose average particle diameter of the ferrite grain is less than 2 micrometers.

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