JP3983065B2 - Manufacturing method of thick steel plate having ultrafine grain structure and thick steel plate - Google Patents

Description

【００１３】
（式中、Ｒ_T,i，Ｒ_W,i，Ｒ_L,iは、それぞれ最終板形状の板厚方向、板幅方向、板長手方向を圧下方向とするｉ番目の圧下パスの圧下率（％）を示す）
でそれぞれ表される板厚方向、板幅方向、板長手方向の累積圧下歪ε_T，ε_W，ε_Lのうちの少なくとも２つが０．３以上であり、かつ、総累積圧下歪ε_T＋ε_W＋ε_Lが１．８以上となると共に、次式（４）
【数５】

（式中、ｔは圧延開始から終了までの時間（ｓ）、Ｔは圧延温度（℃）あるいは各パスの圧延温度の平均（℃）、Ｑは２５４０００を示す）
で表される圧延条件パラメータＺが１１以上となる多方向圧下温間多パス圧延を行い、厚さが１０ｍｍ以上の厚鋼板であって、板厚方向の中心部における結晶粒径が厚鋼板の全面に渡って１μｍ以下である厚鋼板を製造することを特徴とする超微細粒組織を有する厚鋼板の製造方法を提供する。
【００１８】
この出願の発明は、第２には、連続した同一圧下方向での圧延パスの累積圧下歪が０．３以上になるように圧延することを特徴とする超微細粒組織を有する厚鋼板の製造方法を提供する。
【００１９】
この出願の発明は、第３には、上記第１または第２の超微細粒組織を有する厚鋼板の製造方法によって製造される厚鋼板であって、厚さが１０ｍｍ以上であり、板厚方向の中心部における結晶粒径が厚鋼板の全面に渡って１μｍ以下であることを特徴とする厚鋼板を提供する。
【００２１】
【発明の実施の形態】
この出願の発明は、上記の通りの特徴を持つものであるが、以下にその実施の形態について説明する。
【００２２】
まず、この出願の発明が提供する超微細粒組織を有する厚鋼板の製造方法は、鋼材に対して、３５０〜７５０℃の温度範囲において、次式（１）〜（３）
【００２３】
【数７】

【００２４】
でそれぞれ表される、最終板形状の板厚方向、板幅方向、板長手方向の累積圧下歪ε_T，ε_W，ε_Lのうちの少なくとも２つが０．３以上であり、かつ、総累積圧下歪ε_T＋ε_W＋ε_Lが１．８以上となる多方向圧下温間多パス圧延を行うことを特徴としている。ここで、式中のＲ_T,i，Ｒ_W,i，Ｒ_L,iは、それぞれ板厚方向、板幅方向、板長手方向を圧下方向とするｉ番目の圧下パスの圧下率（％）を示している。
【００２５】
このように特徴づけられるこの出願の発明は、この出願の発明者らが鋭意研究を重ねて得るに至った、次に示す３つの斬新で、有用な知見を統合することにより実現されるものである。
（１）温間温度域での強加工
鋼材に対して、厚鋼板および薄鋼板等の製造に一般的に利用されている熱間温度域よりも低い温度である温間温度域において強加工を施すことによりある臨界歪よりも大きな歪を導入すると、この歪によるミクロな局所方位差が超微細結晶粒の起源となり、加工中あるいは加工後に起きる回復過程において粒内の転位密度が低下すると同時に結晶粒界が形作られ、超微細粒組織を形成することができる。すなわち、これまで再結晶温度の下限と見られていた７５０℃あるいはそれ以下の温度で加工しても、加工と同時に動的な再結晶が起こり、相変態を利用することなく結晶粒の超微細化を実現することができるのである。
（２）多パス圧延
この温間温度域での強加工によって超微細粒を生成させるには、鋼材に、ある臨界歪以上の歪を与えることが必要である。ここで、臨界歪を超える歪は結晶粒径にあまり影響を与えないこと、温間温度域での結晶粒成長速度は小さいために結晶粒径は冷却速度にあまり依存しなくなることなどを考慮すると、圧延後の冷却速度等に依存せずに、一般的な温間圧延を利用して臨界歪以上の歪を与えることにより、板厚中心部までほぼ一定の組織を得ることができる。また、この臨界歪以上の十分な歪は、温間圧延の累積圧下率を大きくとることによって導入することができ、最終的に得られる超微細粒組織は１パスあたりの圧下率にあまり依存しない。言い換えると、超微細粒組織を得るためには、１パス当たりの圧下率を大きくする必要はなく、１パス当たりの圧下量を小さくしてパス数を増やす多パス圧延とすることによりに実現できる。また、加工温度を低下することによる圧延荷重増大の問題も、この１パス当たりの圧下量を小さくしてパス数を増やす多パス圧延とすることにより回避できる。
（３）多方向圧下圧延
また、たとえば板厚方向、板幅方向、板長手方向というような、およそ９０°程度ずつ異なる少なくとも２つの方向から圧下する圧延を組み合わせることにより、結晶粒の方位を分散させて大角粒界に囲まれた超微細粒を増加させることができ、鋼材の靭性を高めることができる。そして、たとえばこの板厚方向、板幅方向、板長手方向のそれぞれの方向の圧下をある程度以上連続して行うことにより、多方向圧下の効果はより有効に発揮される。
【００２６】
以上の知見をより実際的な厚鋼板の製造に適用するために、この出願の発明においては、温間温度域での強加工を、３５０〜７５０℃の温度範囲で行なうようにしている。強加工の温度を３５０℃よりも低くすると、回復が十分に起こらないために転位密度の高い加工組織が残存してしまう。また、強加工の温度が７５０℃よりも高いと、不連続再結晶あるいは通常の粒成長により結晶粒が粗大化して、１μｍ以下の超微細粒組織が得られなくなってしまう。
【００２７】
また、多方向圧下圧延による加工については、主たる圧下方向の加工に加えて、それとおよそ９０°程度の角度を成す別の方向からの圧下を組み合わせて、少なくとも２方向からの加工歪を与えることにようにしている。この主たる圧下方向は、一般的な厚鋼板については、最終板形状の板厚方向とすることが簡便であり、これとおよそ９０°程度の角度を成す別の方向は、板幅方向あるいは板長手方向とすることができるが、もちろんこれに限定されることはない。ここで、圧延方向に関するおよそ９０°程度の角度とは、９０±３０°程度の角度を含むものとして示すことができ、大角粒界の割合を増加するためには９０°であることがより好ましい。これによって、超微細結晶粒の方位を分散して、方位差角１５°以上の大角粒界の割合を、たとえば６０％以上にまで増加させることができる。このような多方向圧下圧延は、たとえば、図１に例示したような、通常のロール圧延機による板厚方向での圧延に、堅ロールによる板幅方向の圧延を組み合わせることや、図２に例示したように、通常のロール圧延機により、板厚方向での圧延の後、鋼材を９０°回転させて板厚方向での圧延を行なうこと等で実現することが考慮できる。また、この多方向での圧下圧延により、板厚中心部の結晶粒超微細化されやすくなるという効果も得ることができる。そして、この出願の発明においては、主たる圧下方向に加えてそれ以外の少なくとも１方向の累積圧下歪、すなわち、最終板形状の板厚方向、板幅方向、板長手方向の累積圧下歪ε_T，ε_W，ε_Lのうちの少なくとも２つを、０．３以上とするようにしている。この累積圧下歪が０．３未満の場合には、以上のような多方向圧下圧延の効果はほとんど得ることができずに亜結晶粒が増加してしまう。
【００２８】
さらにこの出願の発明においては、これら板厚方向、板幅方向、板長手方向についての総累積圧下歪ε_T＋ε_W＋ε_Lが、１．８以上となるようにしている。これは、温間温度域での圧延加工により扁平化した加工粒から生成する超微細結晶粒が臨界歪までの歪の増加に伴って増加すること、そして鋼材のほぼ全体が超微細結晶粒からなる組織を得るには総累積圧下歪が少なくとも１．８の歪が必要であることなどの理由からである。
【００２９】
以上のこの出願の発明の厚鋼板の製造方法における結晶粒の微細化の機構は、相変態を全く利用していないため、鋼の成分や冷却速度に依存することなく、広い成分範囲と板厚範囲の鋼板に適用可能となる。たとえば、フェライト単相鋼や、オーステナイト単相鋼等の相変態の存在しない鋼種にも適用可能となる。そして、そのような厚鋼板における結晶粒径は、出発鋼材の第２相粒子の分布や圧延温度、歪速度などにより変化し、従来の厚鋼板については実現されていなかった１μｍ以下、さらには０．４〜数μｍ程度の範囲で制御することができる。
【００３０】
またこの出願の発明の超微細粒組織を有する厚鋼板の製造方法は、より簡便かつ確実に所望の平均粒径の超微細粒結晶組織を得るための指標として、鋼材中のＦｅの結晶構造によって、次式（４）
【００３１】
【数８】

【００３２】
で示される圧延条件パラメータＺの値を制御するようにしている。なお、式中のｔは圧延開始から終了までの時間（ｓ）を、Ｔは圧延温度（℃）あるいは各パスの圧延温度の平均（℃）を、ＱはＦｅの自己拡散の活性化エネルギーを示している。これまでの研究により、この出願の発明による温間温度域での強加工によって形成される超微細粒の平均粒径は、圧延温度（Ｔ）と歪速度（（ε_T＋ε_W＋ε_L）／ｔ）とにある程度依存することが明らかとなっている。この平均結晶粒径は、加工温度と歪速度の関数である上式（４）による加工条件パラメータＺの増加に伴って微細化させることができる。この出願の発明においては、出発鋼材中のＦｅの結晶構造ｂｃｃである場合、すなわち、フェライト、ベイナイト、マルテンサイトあるいはパーライト等を母相とする場合には、上式（４）におけるＱを２５４０００とし、圧延条件パラメータＺが１１以上の範囲で調整して多方向圧下温間多パス圧延を行うようにしている。鋼材中のＦｅの結晶構造がｆｃｃである場合、すなわち、オーステナイトを母相とするには、上式（４）におけるＱを３１００００とし、圧延条件パラメータＺが２０以上の範囲で調整して多方向圧下温間多パス圧延を行うことが可能である。この圧延条件パラメータＺがそれぞれ上記の臨界値のおよそ１１、およそ２０未満の場合には、平均粒径１μｍ以下の組織を得ることができない場合がある。
【００３３】
さらに、この出願の発明の超微細粒組織を有する厚鋼板の製造方法においては、鋼材として、第２相の分布状態を表すパラメータｆ/ｄが０．０３以上の複相組織を有する鋼材を用いることができる。ここで、ｆは第２相の分率を、ｄは第２相の平均直径（μｍ）を示す。また、第２相としては、セメンタイト等の炭化物、パーライト、マルテンサイト、ベイナイト、オーステナイト等を考慮することができ、したがって、出発材として、たとえば、「フェライト＋パーライト」、「フェライト＋セメンタイト」、「フェライト＋マルテンサイト」、「マルテンサイト、ベイナイト＋セメンタイト（焼き戻しマルテンサイト、ベイナイト）」等の組織を有する鋼材を用いることができる。このように、加工直前および加工中の組織に第２相が分散している鋼材を出発材として用いることにより、組成歪の分布がミクロに局在化し、歪導入に伴うミクロな局所方位差の形成が促進される。そしてこのミクロな局所方位差は、超微細粒の生成起源となるため、局所方位差を増大させることにより、超微細粒の形成促進、ひいては、大角粒界の割合の増加をより効率的に実現することができる。ここで、第２相は、なるべく多量かつ微細に分散していることが望ましい。また、第２相分率ｆと平均直径ｄを用いたパラメータｆ/ｄが０．０３未満の場合には、大角粒界の割合が低く、亜結晶が残存して機械的特性に優れた理想的な超微細粒組織が形成されにくいため、ｆ/ｄは０．０３以上とすることが望ましい。
【００３４】
加えてこの出願の発明が提供する超微細粒組織を有する厚鋼板の製造方法は、連続した同一圧下方向での圧延パスの累積圧下歪が０．３以上になるように圧延することを特徴としている。これは、１パスあたりの圧下率が小さい場合には、種々の方向の圧下を交互に行なうよりも、同一方向の圧下をある程度連続して行うほうが、多方向加工の効果が現われやすいことによるものである。このような連続する同一方向の圧下パスは、累積圧下歪が０．３以上になるようなパススケジュールで行うことが望ましいものとて例示される。
【００３５】
この出願の発明が提供する超微細粒組織を有する厚鋼板の製造方法は、以上に述べてきたように、一般的なロールを用いた圧延により行われる。
【００３６】
以上のようなこの出願の発明により得られる鋼板は、厚さが１０ｍｍ以上の厚鋼板であって、板厚方向の中心部における結晶粒径が、厚鋼板の全面に渡って１μｍ以下とされている。そして板厚方向の中心部まで大角粒界の割合の大きい、均一かつ微細な超微結晶粒組織からなる、高強度かつ延性・靭性に優れた超微細結晶粒組織が実現されることになる。このような超微細結晶組織を有する鋼材は、従来は小型試料についてのみ実現されていたが、この出願の発明においては、たとえば、厚さが１０ｍｍ以上で長さが２ｍ以上の板材をはじめ、その形状に制限されることなく、たとえば、線材、棒材、レール、Ｈ鋼材等の実用材としての提供が可能である。
【００３７】
また、この出願の発明の厚鋼板は、引っ張り強さが６５０ＭＰａ以上で、かつシャルピー破面遷移温度が−１７０℃以下という特性を有する。この６５０ＭＰａ以上という高強度は、結晶粒の超微細化の手法を基にして実現されるものであり、固溶強化、分散析出強化、相変態強化等の手法を全く対象としていない。したがって、この出願の発明の厚鋼板には、たとえば、Ｔｉ，Ｎｂ、Ｖ、Ｃｒ、Ｎｉ、Ｍｏ、Ｃｕ等の固溶強化、分散析出強化、相変態強化等を目的とした合金元素は、含まれていてもよいが、添加する必要は一切ない。
【００３８】
これらの各種の合金元素のうち、Ｃｒ、Ｎｉ、Ｍｏ等の希少資源元素は、分散析出強化や相変態強化の手法により鋼材の高強度化を実現できることが知られているが、その一方で、たとえばスクラップ材から再度分解、抽出することが困難であり、リサイクル性を損なうものであった。この出願の発明においては、このような希少資源元素を必要とせずに鋼材の高強度化を実現できる、リサイクル性に優れたものとして提供される。また、このようにこの出願の発明の厚鋼板は組成が限定されないことから焼入れ性が低下されることがない。たとえば、溶接可能な厚鋼板として多用されている「ＳＭ４９０」と同組成の鋼材についても、上記のような高強度、鋼靭性を実現することができる。
【００３９】
そして−１７０℃以下というシャルピー破面遷移温度は、実用上の脆化問題を完全に回避できる値であり、この出願の発明の厚鋼板における超微細結晶組織の６０％以上が大角粒界を構成していることから実現されるものである。
【００４０】
以上のように、この出願の発明の厚鋼材の高強度および高靭性の特性については、それぞれ、引っ張り強さが７００ＭＰａ以上、シャルピー破面遷移温度については−１９６℃以下等としての実現が可能とされる。そしてさらには、たとえば、「ＳＭ４９０」と同組成の鋼材について、引っ張り強さが１０１０ＭＰａ以上で、かつシャルピー破面遷移温度が−１７０℃以下である厚鋼材なども実現することができる。
【００４１】
以上のようなこの出願の発明の厚鋼材は、合金元素を添加せずに強度を高めることができ、リサイクルも可能となることから、資源とコストの節約の面で有益である。さらには、複雑な熱処理をすることなく、簡単な温間加工により製造できるため、エネルギー消費の低減につながり、さらには特別な圧延装置を必要とせずに既存の設備を利用することで製造することができ、実現性の高いものである。このようなこの出願の発明の厚鋼材は土木、建築、造船、ラインパイプ、貯槽、各種機械等の各分野で広く使用することができ、産業的にも極めて有益なものである。
【００４２】
以下に実施例を示し、この発明の実施の形態についてさらに詳しく説明する。
【００４３】
【実施例】
表１に示した化学組成で、第２相としてマルテンサイト、ベイナイト＋セメンタイト組織を有する鋼を供試鋼とし、図２に示したように圧延材を途中で９０°回転させて圧下方向を変化させる方法により、板厚方向に圧下する圧延と板幅方向に圧下する圧延とを様々に組み合わせた２方向圧下圧延を行なった。この圧延条件Ａ〜Ｌの詳細と、得られた鋼板の厚みを表２に示した。
【００４４】
【表１】

【００４５】
【表２】

【００４６】
この２方向圧下圧延により得られた鋼板について、板厚中央部の組織、引っ張り試験の結果、およびシャルピー試験の結果を表３に示した。なお、平均フェライト粒径としては、公称粒径（結晶粒１個の平均面積と等しい正方形の一辺の長さ）による表示を行なった。
【００４７】
【表３】

【００４８】
条件Ａ〜Ｇは、この出願の発明の方法であり、これらの方法により得られた鋼板は、板厚が１０ｍｍを超えて１５〜３０ｍｍの厚鋼板であっても、板厚中心部において平均粒径が１μｍ以下で、なおかつ粒界長さの６０％以上が方位差角１５°以上の大角粒界からなる超微細粒組織を有していることが確認された。そのため、シャルピー衝撃試験の結果においても、この出願の発明の鋼板は、およそ７００ＭＰａ以上の高い強度と、−１７０℃以下のシャルピー破断遷移温度を有し、優れた機械的特性を持ちあわせていることが確認された。
【００４９】
一方の条件Ｈは、板厚方向のみで圧下する通常の一方向圧下圧延である。条件Ｊは、板厚方向と板幅方向の圧下を組み合わせてはいるものの、その圧下率が十分でない。そのため、条件ＨおよびＪで得られた鋼板は、大角粒界の割合が５０％未満と少なく、亜結晶粒が多いため、シャルピー特性が劣ってしまっている。
【００５０】
また条件Ｉは、板厚方向と板幅方向の圧下を組み合わせてはいるものの、総累積圧下率が小さいため、組織微細化が十分ではなく、１μｍ以下の超微細粒が得られず、大角粒界の割合も５０％未満と低かった。条件Ｋは、板厚方向と板幅方向の圧下を組み合わせてはいるものの、圧延温度が適性な範囲よりも高いため結晶粒径が粗大化し、１μｍ以下の超微細粒組織が得られなかった。条件Ｌは、圧延温度が適性な範囲よりも低いため、加工硬化した組織のままであり、強度は高くなるものの、延性および靭性に劣るという結果になった。
【００５１】
もちろん、この発明は以上の例に限定されるものではなく、細部については様々な態様が可能であることは言うまでもない。
【００５２】
【発明の効果】
以上詳しく説明した通り、この発明によって、合金元素を添加することなく粒径を１μｍ以下に超微細化することで強度を高めることができ、環境的、リサイクル性にも貢献することのできる超微細粒組織を有する厚鋼板の製造方法と、粒径が１μｍ以下に超微細化され、大角粒界で囲まれた超微細粒組織を有する、高強度かつ高靭性な厚鋼板が提供される。
【図面の簡単な説明】
【図１】 ロール圧延機による板厚方向での圧延と、堅ロールによる板幅方向の圧延を組み合わせた、多方向圧下圧延を例示した図である。
【図２】 ロール圧延機による板厚方向での圧延後に鋼材を９０°回転させて板厚方向で圧延する多方向圧下圧延を例示した図である。[0001]
BACKGROUND OF THE INVENTION
The invention of this application relates to a method for producing a thick steel plate having an ultrafine grain structure and a thick steel plate. More specifically, the invention of this application can increase the strength by ultra-fine grain size to 1 μm or less without adding alloy elements, and can contribute to environmental and recyclability. The present invention relates to a method for producing a thick steel plate having a grain structure, and a high-strength and high-tough steel plate having a super-fine grain structure with a grain size of 1 μm or less and surrounded by a large-angle grain boundary.
[0002]
[Prior art and its problems]
The refinement of crystal grains has been considered to be an ideal technique for increasing the strength of steel materials, as it improves both the strength and toughness of steel materials. The heat treatment (TMCP: Thermo-Mechanical Controlled Processing) is widely used industrially as a method for miniaturization in crystals. At present, this method is used to reduce the average grain size to about 5 μm. Is realized relatively easily.
[0003]
As a result of research and development aimed at further refinement of crystal grains, in recent years, regarding thin steel sheets up to several millimeters in thickness, the grain size can be reduced to 1 μm or less. Several possible methods have been proposed. However, in a thick steel plate having a plate thickness exceeding 10 mm, it is difficult to sufficiently introduce processing strain to the vicinity of the central portion of the plate thickness and rapid heating / cooling. There was a problem that an ultrafine grain structure could not be obtained, and a problem that a sub-crystal grain having a small orientation difference angle between crystal grains and a small effect of improving toughness was increased.
[0004]
Therefore, several methods using multi-directional processing have been proposed as methods for suppressing the increase of sub-crystal grains and obtaining a structure mainly composed of ultrafine grains surrounded by large-angle grain boundaries. For example, Japanese Patent Laid-Open No. 11-95861 discloses a method for introducing large strains into a material from multiple directions by anvil compression. However, since this method uses a special processing method called anvil compression, there is a problem that it cannot be applied to the production of practical thick steel plates. In Japanese Patent Laid-Open No. 11-92825, the concept of multi-directional processing is adopted in hot rolling of a steel sheet in a hot strip mill by performing reduction in the sheet width direction, and dynamic recrystallization at the time of hot working is used. Thus, a method for obtaining an ultrafine grain structure is disclosed. However, in general thick steel plate rolling, since the rolling reduction per pass is smaller than that of a hot strip mill, it is difficult to obtain a fine structure by dynamic recrystallization in normal hot rolling. is there.
[0005]
As a technique for realizing ultra-fine grain refinement with a thick steel plate, for example, techniques disclosed in JP-A-11-21655, JP-A 2000-8123, JP-A 2000-144244, and the like There is. Japanese Patent Application Laid-Open No. 11-21655 discloses a method in which steel is heated to an α + γ2 phase temperature range and rolled, and then accelerated cooling is performed. However, in this method, the α + γ2 phase temperature range suitable for rolling is limited to a very narrow range that depends only on the steel components, so that the structure factors such as the second phase morphology and crystal grain size can be set independently. There was a problem that it became difficult to control. Moreover, since it is difficult to accelerate cooling at a sufficient cooling rate as the plate thickness increases, it is difficult to make the crystal grains ultrafine to the center of the plate thickness of the thick steel plate.
[0006]
Japanese Patent Application Laid-Open No. 2000-8123 discloses a method of performing controlled rolling with a large rolling reduction per pass using steel with limited hardenability. This method has problems such as hardenability, that is, the steel components are limited, and the load load of the rolling mill increases due to the necessity of increasing the rolling reduction per pass.
[0007]
Japanese Patent Application Laid-Open No. 2000-144244 discloses a method of using dynamic α → γ reverse transformation at the time of processing. However, in order to use dynamic reverse transformation, the components of steel and the processing temperature are limited. There is still a problem that it is difficult to freely control various tissue factors. Moreover, in the thick steel plate, since the temperature distribution in the plate thickness direction becomes remarkable, it is difficult to obtain a uniform structure.
[0008]
All of the conventional methods as exemplified above have a common point that they use a phenomenon that changes sensitively to a component or temperature change, such as γ → α transformation or α → γ transformation. The conditions for obtaining a fine grain structure are limited to a narrow range. Therefore, it is considered that the application of these methods becomes more difficult when the temperature in the plate thickness direction of the steel plate and the introduction of processing strain cannot be made uniform, that is, as the plate thickness increases. Above all, these conventional methods cannot be applied at all to steel types having no phase transformation such as high Cr ferritic steel and austenitic stainless steel.
[0009]
Therefore, the invention of this application has been made in view of the circumstances as described above, solves the problems of the prior art, and is not limited to the steel composition and manufacturing equipment, but has a thickness center exceeding 10 mm. It is an object of the present invention to provide a method of manufacturing a thick steel plate that realizes ultrafine refinement of crystal grains up to the part, and has high strength and excellent ductility and toughness, and the thick steel plate.
[0010]
[Means for Solving the Problems]
Therefore, the invention of this application provides the following invention as a solution to the above-mentioned problems.
[0011]
In the invention of this application, firstly, in a temperature range of 350 to 750 ° C. with respect to a steel material in which the crystal structure of Fe is bcc , the following formulas (1) to (3)
[0012]
[Expression 4]

[0013]
(Where R _{T, i} , R _{W, i} , R _{L, i} are the reduction ratios of the i-th reduction path with the final plate shape in the plate thickness direction, plate width direction, and plate longitudinal direction, respectively) %))
Are at least two of the cumulative rolling strains ε _T , ε _W , ε _{L in} the plate thickness direction, the plate width direction, and the plate longitudinal direction, respectively, and the total cumulative rolling strain ε _T + ε _W + ε _L is 1.8 or more and the following equation (4)
[Equation 5]

(In the formula, t is the time (s) from the start to the end of rolling, T is the rolling temperature (° C.) or the average of the rolling temperatures in each pass (° C.), and Q is 254000)
In the rolling condition parameter Z performs multi-pass rolling multidirectional reduction temperature to be 11 or more expressed, a steel plate of more than 10mm thickness, grain size of the steel plate at the center in the thickness direction Provided is a method for producing a thick steel plate having an ultrafine grain structure, characterized by producing a thick steel plate having a thickness of 1 μm or less over the entire surface.
[0018]
The invention of this application, the second, the production of thick steel sheet having an ultrafine grain structure of cumulative reduction strain of rolling passes in a continuous same pressing direction, characterized in that the rolling to be 0.3 or more Provide a method.
[0019]
The invention of this application, the third, a thick steel plate manufactured by the manufacturing method of a steel plate having the first or second ultrafine grain structure, is not less 10mm or more thickness, the plate thickness direction A thick steel plate is provided in which the crystal grain size in the central portion of the steel plate is 1 μm or less over the entire surface of the thick steel plate.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
The invention of this application has the features as described above, and an embodiment thereof will be described below.
[0022]
First, the manufacturing method of the thick steel plate which has the ultra fine grain structure which the invention of this application provides has the following formulas (1) to (3) in the temperature range of 350 to 750 ° C. with respect to the steel material.
[0023]
[Expression 7]

[0024]
And at least two of the cumulative rolling strains ε _T , ε _W , ε _L in the plate thickness direction, plate width direction, and plate longitudinal direction of the final plate shape are each 0.3 or more and the total cumulative It is characterized by performing multi-directional rolling warm multi-pass rolling with a rolling strain ε _T + ε _W + ε _L of 1.8 or more. Here, R _{T, i} , R _{W, i} , R _{L, i} in the equation are the reduction ratio (%) of the i-th reduction path with the plate thickness direction, the plate width direction, and the plate longitudinal direction as the reduction direction, respectively. Is shown.
[0025]
The invention of this application characterized in this way is realized by integrating the following three novel and useful findings that the inventors of this application have obtained through extensive research. is there.
(1) Strong processing in the warm temperature range, which is lower than the hot temperature range generally used for the production of thick steel plates and thin steel plates, etc. When a strain greater than a certain critical strain is introduced, the micro local orientation difference due to this strain becomes the origin of ultrafine grains, and the dislocation density in the grains decreases at the same time during the recovery process that occurs during or after processing. Grain boundaries are formed and an ultrafine grain structure can be formed. That is, even if processing is performed at a temperature of 750 ° C. or lower, which has been regarded as the lower limit of the recrystallization temperature, dynamic recrystallization occurs at the same time as the processing, and ultrafine crystal grains are obtained without using phase transformation. Can be realized.
(2) Multi-pass rolling In order to generate ultrafine grains by strong processing in this warm temperature range, it is necessary to give the steel material a strain greater than a certain critical strain. Here, considering that strain exceeding the critical strain does not significantly affect the crystal grain size, and that the crystal grain growth rate in the warm temperature range is small, the crystal grain size becomes less dependent on the cooling rate. By applying a general warm rolling to a strain higher than the critical strain without depending on the cooling rate after rolling, a substantially constant structure can be obtained up to the center of the plate thickness. Further, sufficient strain above the critical strain can be introduced by increasing the cumulative rolling reduction of warm rolling, and the ultrafine grain structure finally obtained does not depend much on the rolling reduction per pass. . In other words, in order to obtain an ultrafine grain structure, it is not necessary to increase the reduction rate per pass, and it can be realized by reducing the amount of reduction per pass and increasing the number of passes. . Further, the problem of an increase in rolling load due to a reduction in the processing temperature can be avoided by reducing the amount of reduction per pass to increase the number of passes.
(3) Multi-directional reduction rolling Also, the orientation of crystal grains can be dispersed by combining rolling reductions in at least two directions that differ by about 90 °, such as the plate thickness direction, the plate width direction, and the plate longitudinal direction. It is possible to increase the number of ultrafine grains surrounded by the large-angle grain boundaries, and to increase the toughness of the steel material. For example, the effect of multi-directional reduction is more effectively exhibited by continuously performing the reduction in each of the plate thickness direction, the plate width direction, and the plate longitudinal direction to some extent.
[0026]
In order to apply the above knowledge to more practical production of thick steel plates, in the invention of this application, strong processing in the warm temperature range is performed in the temperature range of 350 to 750 ° C. When lower than 350 ° C. The temperature of the large deformation, intends want high processing tissue dislocation density remains for recovery does not occur sufficiently. Further, the strength when the temperature of the processing is higher than 750 ° C., discontinuous recrystallization or crystal grains are coarsened by conventional grain growth, want intends no longer 1μm following ultrafine grained structure is obtained.
[0027]
In addition, for processing by multi-directional rolling, in addition to processing in the main rolling direction, it is combined with rolling from another direction that forms an angle of about 90 ° to give processing strain from at least two directions. I am doing so. As for the main rolling direction, for a general thick steel plate, it is easy to make it the final plate-shaped plate thickness direction, and another direction forming an angle of about 90 ° with this is the plate width direction or the plate longitudinal direction. It can be a direction, but of course it is not limited to this. Here, the angle of about 90 ° with respect to the rolling direction can be shown as including an angle of about 90 ± 30 °, and 90 ° is more preferable in order to increase the proportion of large-angle grain boundaries. . This makes it possible to disperse the orientation of ultrafine crystal grains and increase the ratio of large-angle grain boundaries having an orientation difference angle of 15 ° or more to, for example, 60% or more. Such multi-directional rolling is performed by combining rolling in the plate thickness direction with a normal roll rolling mill as illustrated in FIG. 1 and rolling in the plate width direction with a hard roll, as illustrated in FIG. As described above, it can be considered that the rolling is performed in the thickness direction by rotating the steel material by 90 ° after the rolling in the thickness direction by a normal roll rolling mill. Moreover, the effect that it becomes easy to carry out the ultrafine refinement | miniaturization of the crystal grain of a plate | board thickness center part by the rolling in this multi-direction can also be acquired. In the invention of this application, in addition to the main rolling direction, the cumulative rolling strain in at least one other direction, that is, the cumulative rolling strain ε _T in the plate thickness direction, the plate width direction, and the plate longitudinal direction of the final plate shape, At least two of ε _W and ε _L are set to 0.3 or more. When the cumulative rolling strain is less than 0.3, the effect of the multi-directional rolling rolling as described above can hardly be obtained, and subgrains increase.
[0028]
Further, in the invention of this application, the total cumulative rolling strain ε _T + ε _W + ε _{L in the} plate thickness direction, the plate width direction, and the plate longitudinal direction is set to be 1.8 or more. This is because the ultrafine crystal grains generated from the processed grains flattened by rolling in the warm temperature range increase with increasing strain up to the critical strain, and almost the entire steel material is made up of ultrafine crystal grains. This is because a strain having a total cumulative rolling strain of at least 1.8 is required to obtain a desired structure.
[0029]
The mechanism of grain refinement in the manufacturing method of the thick steel plate of the invention of the above application does not use phase transformation at all, so it does not depend on the steel composition and cooling rate, and has a wide component range and plate thickness. Applicable to a range of steel plates. For example, the present invention can be applied to a steel type that does not have a phase transformation, such as a ferritic single phase steel or an austenitic single phase steel. The crystal grain size in such a thick steel plate changes depending on the distribution of the second phase particles of the starting steel material, the rolling temperature, the strain rate, etc., which is 1 μm or less, which is not realized for conventional thick steel plates, and further 0 It can be controlled in the range of about 4 to several μm.
[0030]
The method for producing a thick steel plate having an ultrafine grain structure according to the invention of this application is based on the crystal structure of Fe in the steel as an index for obtaining an ultrafine grain structure having a desired average grain size more easily and reliably. The following equation (4)
[0031]
[Equation 8]

[0032]
The value of the rolling condition parameter Z indicated by is controlled. In the formula, t is the time (s) from the start to the end of rolling, T is the rolling temperature (° C.) or the average of the rolling temperature of each pass (° C.), and Q is the activation energy for self-diffusion of Fe. Show. According to the research so far, the average particle size of the ultrafine grains formed by the strong processing in the warm temperature range according to the invention of this application is calculated as follows: rolling temperature (T) and strain rate ((ε _T + ε _W + ε _L ) / It is clear that it depends to some extent on t). This average crystal grain size can be refined as the processing condition parameter Z is increased by the above equation (4), which is a function of the processing temperature and strain rate. In the invention of this application, when the crystal structure of Fe in the starting steel material is bcc, that is, when ferrite, bainite, martensite, pearlite, or the like is used as a parent phase, Q in the above formula (4) is set to 254000. The rolling condition parameter Z is adjusted in the range of 11 or more to perform multi-directional rolling warm multi-pass rolling . If the crystal structure of Fe in the steel material is fcc, i.e., to the parent phase austenite, the Q in the above formula (4) and 310000, the rolling condition parameter Z multi adjusted in the range of 20 or more It is possible to perform directional reduction warm multi-pass rolling. The rolling condition parameter Z of approximately 11 each above the critical value, in the case of less than about 20, when it is impossible to obtain an average particle size 1μm or less tissue there Ru.
[0033]
Furthermore, in the method for producing a thick steel plate having an ultrafine grain structure according to the invention of this application, a steel material having a multiphase structure in which the parameter f / d representing the distribution state of the second phase is 0.03 or more is used as the steel material. Can Here, f represents the fraction of the second phase, and d represents the average diameter (μm) of the second phase. Further, as the second phase, carbides such as cementite, pearlite, martensite, bainite, austenite and the like can be considered. Therefore, as a starting material, for example, “ferrite + pearlite”, “ferrite + cementite”, “ Steel materials having a structure such as “ferrite + martensite”, “martensite, bainite + cementite (tempered martensite, bainite)” can be used. In this way, by using a steel material in which the second phase is dispersed in the structure immediately before and during processing as a starting material, the distribution of composition strain is localized microscopically, and microscopic local misorientation due to strain introduction is reduced. Formation is promoted. And since this micro local misorientation is the origin of the formation of ultrafine grains, by increasing the local misorientation, the formation of ultrafine grains can be promoted and consequently the proportion of large angle grain boundaries can be increased more efficiently. it is Ru can be. Here, it is desirable that the second phase is dispersed as much and as finely as possible. In addition, when the parameter f / d using the second phase fraction f and the average diameter d is less than 0.03, the ratio of large-angle grain boundaries is low, and sub-crystals remain and are ideal for excellent mechanical properties. F / d is preferably 0.03 or more because a typical ultrafine grain structure is difficult to form.
[0034]
In addition, the method of manufacturing a thick steel plate having an ultrafine grain structure provided by the invention of this application is characterized in that rolling is performed so that the cumulative rolling strain of a rolling pass in the same rolling direction is 0.3 or more. Yes. This is because when the rolling reduction per pass is small, the effect of multi-directional machining is more likely to occur when rolling in the same direction is continued to some extent rather than alternately in various directions. It is. Such a continuous reduction pass in the same direction is exemplified as a pass schedule in which the cumulative reduction strain is 0.3 or more.
[0035]
As described above, the method for producing a thick steel plate having an ultrafine grain structure provided by the invention of this application is performed by rolling using a general roll .
[0036]
The steel plate obtained by the invention of this application as described above is a thick steel plate having a thickness of 10 mm or more, and the crystal grain size at the center in the thickness direction is 1 μm or less over the entire surface of the thick steel plate. Yes. As a result, an ultrafine crystal grain structure having a high strength and excellent ductility and toughness, which is composed of a uniform and fine ultrafine crystal grain structure with a large proportion of large-angle grain boundaries up to the center in the plate thickness direction, is realized. Conventionally, the steel material having such an ultrafine crystal structure has been realized only for a small sample. In the invention of this application, for example, a plate material having a thickness of 10 mm or more and a length of 2 m or more is used. Without being limited to the shape, for example, it can be provided as a practical material such as a wire, a bar, a rail, or an H steel.
[0037]
Further, steel plate of the invention of this application, a tensile strength of at least 650 MPa, and the Charpy fracture appearance transition temperature has the characteristic that -170 ° C. or less. This high strength of 650 MPa or more is realized based on a technique for ultrafine crystal grains, and does not cover methods such as solid solution strengthening, dispersion precipitation strengthening, and phase transformation strengthening. Therefore, the steel plate of the invention of this application, for example, Ti, Nb, V, Cr , Ni, Mo, solid solution strengthening of Cu and the like, dispersion precipitation strengthening, an alloy elemental aimed at phase transformation strengthening is Although it may be contained, it is not necessary to add at all.
[0038]
Among these various alloy elements, rare resource elements such as Cr, Ni, and Mo are known to be able to achieve high strength of steel materials by means of dispersion precipitation strengthening and phase transformation strengthening, For example, it is difficult to decompose and extract from scrap material again, and the recyclability is impaired. In the invention of this application, it is provided as an excellent recyclable material that can achieve high strength of steel without requiring such a rare resource element. Moreover, since the composition of the thick steel plate of the invention of this application is not limited, the hardenability is not lowered. For example, the above-described high strength and steel toughness can be realized even for a steel material having the same composition as “SM490”, which is frequently used as a weldable thick steel plate.
[0039]
The Charpy fracture surface transition temperature of −170 ° C. or less is a value that can completely avoid the practical embrittlement problem, and 60% or more of the ultrafine crystal structure in the thick steel plate of the invention of this application constitutes a large-angle grain boundary. It is realized by doing.
[0040]
As described above, the high strength and high toughness characteristics of the thick steel material of the invention of this application can be realized as a tensile strength of 700 MPa or more and a Charpy fracture surface transition temperature of −196 ° C. or less, respectively. Is done. Furthermore, for example, a thick steel material having a tensile strength of 1010 MPa or more and a Charpy fracture surface transition temperature of −170 ° C. or less can be realized for a steel material having the same composition as “SM490”.
[0041]
The thick steel material of the invention of this application as described above can increase the strength without adding an alloy element and can be recycled, which is advantageous in terms of saving resources and costs. Furthermore, since it can be manufactured by simple warm working without complicated heat treatment, it leads to reduction of energy consumption, and furthermore, it is manufactured by using existing equipment without requiring special rolling equipment. Can be realized and is highly feasible. Such a thick steel material of the invention of this application can be widely used in various fields such as civil engineering, construction, shipbuilding, line pipes, storage tanks, various machines, etc., and is extremely useful industrially.
[0042]
Examples will be shown below, and the embodiments of the present invention will be described in more detail.
[0043]
【Example】
Steel having martensite and bainite + cementite structure as the second phase with the chemical composition shown in Table 1 was used as the test steel, and the rolling direction was changed by rotating the rolled material by 90 ° in the middle as shown in FIG. Depending on the method, two-way rolling was carried out by variously combining rolling rolling in the sheet thickness direction and rolling rolling in the sheet width direction. Details of the rolling conditions A to L and the thickness of the obtained steel sheet are shown in Table 2.
[0044]
[Table 1]

[0045]
[Table 2]

[0046]
Table 3 shows the structure of the center of the plate thickness, the result of the tensile test, and the result of the Charpy test for the steel plate obtained by the two-way rolling. In addition, as an average ferrite particle size, the display by nominal particle size (The length of one side of a square equal to the average area of one crystal grain) was performed.
[0047]
[Table 3]

[0048]
Conditions A to G are the methods of the invention of this application, and the steel plates obtained by these methods have an average grain size in the center of the plate thickness even if the plate thickness is 15 to 30 mm exceeding 10 mm. It was confirmed that a diameter of 1 μm or less and 60% or more of the grain boundary length has an ultrafine grain structure composed of large angle grain boundaries having an orientation difference angle of 15 ° or more. Therefore, even in the result of the Charpy impact test, the steel sheet of the invention of this application has a high strength of about 700 MPa or more and a Charpy fracture transition temperature of −170 ° C. or less, and has excellent mechanical properties. Was confirmed.
[0049]
One condition H is normal unidirectional reduction rolling that reduces only in the thickness direction. Condition J combines the reduction in the plate thickness direction and the plate width direction, but the reduction rate is not sufficient. Therefore, the steel plates obtained under conditions H and J have a low ratio of large-angle grain boundaries of less than 50% and a large number of sub-crystal grains, so that the Charpy characteristics are inferior.
[0050]
Condition I is a combination of sheet thickness direction and sheet width direction reduction, but because the total cumulative reduction ratio is small, the structure is not sufficiently refined, and ultrafine grains of 1 μm or less cannot be obtained. The ratio of the boundaries was also low, less than 50%. In condition K, although the reduction in the plate thickness direction and the plate width direction is combined, the rolling temperature is higher than the appropriate range, so the crystal grain size becomes coarse and an ultrafine grain structure of 1 μm or less cannot be obtained. Condition L had a result that the rolling temperature was lower than the appropriate range, so that the work-hardened structure remained as it was and the strength was high, but the ductility and toughness were inferior.
[0051]
Of course, the present invention is not limited to the above examples, and it goes without saying that various aspects are possible in detail.
[0052]
【The invention's effect】
As explained in detail above, according to the present invention, it is possible to increase the strength by making the particle size to 1 μm or less without adding an alloy element, and to contribute to environmental and recyclability. There are provided a method for producing a thick steel plate having a grain structure, and a high strength and high tough steel plate having a super fine grain structure in which a grain size is ultrafinened to 1 μm or less and surrounded by a large angle grain boundary.
[Brief description of the drawings]
FIG. 1 is a diagram exemplifying multi-directional reduction rolling in which rolling in a sheet thickness direction by a roll mill and rolling in a sheet width direction by a hard roll are combined.
FIG. 2 is a diagram exemplifying multi-directional reduction rolling in which a steel material is rotated 90 ° after rolling in a sheet thickness direction by a roll rolling mill and rolled in the sheet thickness direction.

Claims (3)

In a temperature range of 350 to 750 ° C. with respect to a steel material in which the crystal structure of Fe is bcc , the following formulas (1) to (3)

(Where R _{T, i} , R _{W, i} , R _{L, i} are the reduction ratios of the i-th reduction path with the final plate shape in the plate thickness direction, plate width direction, and plate longitudinal direction, respectively) %))
Are at least two of the cumulative rolling strains ε _T , ε _W , ε _{L in} the plate thickness direction, the plate width direction, and the plate longitudinal direction, respectively, and the total cumulative rolling strain ε _T + ε _W + ε _L is 1.8 or more and the following equation (4)

(In the formula, t is the time (s) from the start to the end of rolling, T is the rolling temperature (° C.) or the average of the rolling temperatures in each pass (° C.), and Q is 254000)
In the rolling condition parameter Z performs multi-pass rolling multidirectional reduction temperature to be 11 or more expressed, a steel plate of more than 10mm thickness, grain size of the steel plate at the center in the thickness direction A method for producing a thick steel plate having an ultrafine grain structure, comprising producing a thick steel plate having a thickness of 1 μm or less over the entire surface. 2. The method for producing a thick steel plate having an ultrafine grain structure according to claim 1 , wherein rolling is performed so that a cumulative rolling strain of a rolling pass in a continuous same rolling direction is 0.3 or more . A thick steel plate produced by the method for producing a thick steel plate having an ultrafine grain structure according to claim 1, wherein the thickness is 10 mm or more, and the crystal grain size at the center in the thickness direction is a thick steel plate. thick steel plate you wherein a is 1μm or less over the entire surface.

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