JP4119758B2 - High-strength steel sheet excellent in workability and shape freezing property, and its production method - Google Patents

Description

【００４４】
飽和磁化測定における印加磁化（Ｈ_m）は、398000〜796000Ａ／ｍ［5000〜10000エルステッド（Ｏｅ）］であることが好ましい。また飽和磁化量は、測定温度の変化に影響を受けやすいことから、室温で測定する場合には、例えば23℃±3℃の範囲内で行うことが好ましい。
【００４５】
2％歪を加えることによりマルテンサイトへ変態する残留オーステナイトは、後述する様に、第一の連続焼鈍工程において、Ａ₁点以上Ａ₃点以下の温度で加熱保持した後、引き続いてＡ₃点超の温度で加熱保持することによって確実に生成させることができる。
【００４６】
次に、本発明鋼板を構成する基本成分について説明する。以下、化学成分の単位はすべて質量％である。
【００４７】
Ｃ： 0.06 〜 0.6 ％
Ｃは、高強度を確保し、且つ、残留オーステナイト量を確保するために必須の元素である。詳細には、オーステナイト相中に充分なＣ量を含み、室温でも所望のオーステナイト相を残留させるために重要な元素であり、全伸びを高めるのに有用である。この様な効果を得るには、Ｃを0.06％以上含有する必要がある。好ましくは0.10％以上である。但し、0.6％を超えて添加すると、その効果が飽和するのみならず、鋳造中への中心偏析などによる欠陥などが見られる。好ましくは0.25％以下にするのが良い。
【００４８】
Ｓｉ＋Ａｌ： 0.5 〜 3 ％
ＳｉやＡｌは、残留オーステナイトが分解して炭化物が生成するのを有効に抑える元素である。また特にＳｉは、固溶強化元素としても有用である。この様な作用を有効に発揮させるためには、ＳｉやＡｌを合計で0.5％以上添加することが必要である。好ましくは0.7％以上、より好ましくは1％以上である。但し、上記元素を合計で、3％を超えて添加しても上記効果は飽和してしまい、経済的に無駄である他、多量に添加すると、熱間脆性を起こす為、その上限を3％とする。好ましくは2.5％以下、より好ましくは2％以下である。なお、ＳｉとＡｌは併用して用いても良いし、Ｓｉ単独またはＡｌ単独で用いても良い。
【００４９】
Ｍｎ： 0.5 〜 3 ％
Ｍｎは、オーステナイト相を安定化し、所望の残留オーステナイトを得るために必要な元素である。この様な作用を有効に発揮させるためには、0.5％以上添加することが必要である。好ましくは0.7％以上、より好ましくは1％以上である。但し、3％を超えて添加すると、鋳片割れが生じる等の悪影響が見られる。好ましくは2.5％以下、より好ましくは2％以下である。
【００５０】
Ｐ： 0.15 ％以下（ 0 ％を含まない）
Ｐは、所望の残留オーステナイトを確保するのに有効な元素である。この様な作用を有効に発揮させるためには、0.03％以上（より好ましくは0.05％以上）添加することが推奨される。但し、0.15％を超えて添加すると二次加工性が劣化する。より好ましくは0.1％以下である。
【００５１】
Ｓ： 0.02 ％以下（ 0 ％を含む）
Ｓは、ＭｎＳ等の硫化物系介在物を形成し、割れの起点となって加工性を劣化させる元素であり、0.02％以下に抑制する必要がある。好ましくは0.015％以下である。
【００５２】
本発明の鋼は上記成分を基本的に含有し、残部は実質的に鉄及び不純物であるが、その他、本発明の作用を損なわない範囲で、以下の許容成分を添加することができる。
【００５３】
Ｍｏ： 1 ％以下（ 0 ％を含まない），Ｎｉ： 0.5 ％以下（ 0 ％を含まない），Ｃｕ： 0.5 ％以下（ 0 ％を含まない），Ｃｒ： 1 ％以下（ 0 ％を含まない）の少なくとも一種
これらの元素は、鋼の強化元素として有用であると共に、残留オーステナイトの安定化や所定量の確保に有効な元素である。この様な作用を有効に発揮させる為には、Ｍｏ：0.05％以上（より好ましくは0.1％以上）、Ｎｉ：0.05％以上（より好ましくは0.1％以上）、Ｃｕ：0.05％以上（より好ましくは0.1％以上）、Ｃｒ：0.05％以上（より好ましくは0.1％以上）を、夫々添加することが推奨される。但し、Ｍｏ及びＣｒは１％、Ｎｉ及びＣｕは0.5％を超えて添加しても上記効果が飽和してしまい、経済的に無駄である。より好ましくはＭｏ：0.8％以下、Ｎｉ：0.4％以下、Ｃｕ：0.4％以下、Ｃｒ：0.8％以下である。
【００５４】
Ｔｉ： 0.1 ％以下（ 0 ％を含まない），Ｎｂ： 0.1 ％以下（ 0 ％を含まない），Ｖ： 0.1 ％以下（ 0 ％を含まない）の少なくとも一種
これらの元素は、析出強化及び組織微細化効果があり、高強度化に有用な元素である。この様な作用を有効に発揮させるためには、Ｔｉ：0.01％以上（より好ましくは0.02％以上）、Ｎｂ：0.01％以上（より好ましくは0.02％以上）、Ｖ：0.01％以上（より好ましくは0.02％以上）を、夫々添加することが推奨される。但し、いずれの元素も0.1％を超えて添加すると上記効果が飽和してしまい、経済的に無駄である。より好ましくはＴｉ：0.08％以下、Ｎｂ：0.08％以下、Ｖ：0.08％以下である。
【００５５】
Ｃａ： 0.003 ％以下（ 0 ％を含まない）及び／又はＲＥＭ： 0.003 ％以下（ 0 ％を含まない）
Ｃａ及びＲＥＭ（希土類元素）は、鋼中硫化物の形態を制御し、加工性向上に有効な元素である。ここで、本発明に用いられる希土類元素としては、Ｓｃ、Ｙ、ランタノイド等が挙げられる。上記作用を有効に発揮させる為には、夫々、0.0003％以上（より好ましくは0.0005％以上）添加することが推奨される。但し、0.003％を超えて添加しても上記効果が飽和してしまい、経済的に無駄である。より好ましくは0.0025％以下である。
【００５６】
次に、上記要件を満足する鋼板を確実に製造できる方法について説明する。
【００５７】
本発明の要件を満足する鋼板は、少なくとも熱延工程、第一の連続焼鈍工程および第二の連続焼鈍工程またはめっき工程を経て製造される。
【００５８】
但し、熱延工程の条件は特に限定されず、通常実施される条件を適宜選択して採用することができる。本発明の製法は、熱延工程により所望の組織を確保するものではなく、熱延後に実施する第一の連続焼鈍工程、および第二の連続焼鈍工程またはめっき工程を制御することにより所望の組織を得るところに特徴があるからである。
【００５９】
従って本発明の製法では、熱延工程の後、第一の連続焼鈍工程に先立って冷延工程を含むものであっても構わず、この冷延工程の条件も上記熱延工程と同じく特に限定されない。
【００６０】
具体的には、上記熱延工程としては、Ａ_r3点以上で熱延終了後、平均冷却速度約30℃／ｓｅｃで冷却し、約500〜600℃の温度で巻取る等の条件を採用することができる。また、冷延工程では、約30〜70％の冷延率の冷間圧延を施すことが推奨される。勿論、これに限定する趣旨では決してない。
【００６１】
次に、熱延工程（または熱延工程後の冷延工程）で得られた鋼板を、第一の連続焼鈍工程、および第二の連続焼鈍工程またはめっき工程で熱処理する。
【００６２】
このうち第一の連続焼鈍（先の連続焼鈍）は、Ａ₁点以上Ａ₃点以下の温度で10〜300秒加熱保持する工程；引き続きＡ₃点超の温度で10〜300秒加熱保持する工程；及び30℃／ｓｅｃ以上の平均冷却速度で、Ｍｓ点以下の温度まで冷却する工程を包含する。これらの条件は、所望の母相組織（Ｃ含量のバラつきのある焼入マルテンサイト）を得るために設定されたものであり、この工程の説明図を図２に示す。
【００６３】
第二の連続焼鈍（後の連続焼鈍）またはめっきは、Ａ₁点以上Ａ₃点以下の温度で10〜600秒加熱保持する工程；3℃／ｓｅｃ以上の平均冷却速度で、Ｍｓ点以上Ｂｓ点以下の温度まで冷却する工程；及び該温度域で1秒以上保持する工程を包含する。これらの条件は、前記第一の連続焼鈍工程で生成した母相組織（焼入マルテンサイト）を焼戻して所望の焼戻マルテンサイトを得ると共に、Ｃγ_Rが低く不安定な残留オーステナイトとＣγ_Rが高く安定な残留オーステナイトの割合が適切に制御された第2相を得るために設定されたものである。この工程の説明図を図３に示す。
【００６４】
以下、前記図２および図３を参酌しつつ、第一の連続焼鈍工程、および第二の連続焼鈍工程またはめっき工程について説明する。
【００６５】
第一の連続焼鈍工程
上記熱延工程（または熱延工程後の冷延工程）で得られた鋼板を、Ａ₁点以上Ａ₃点以下の温度（図２中、Ｔ１）に加熱して均熱処理する（好ましくは1300℃以下）。均熱処理することによってフェライトを一部生成させて［フェライト＋オーステナイト］の2相組織とする。
【００６６】
Ａ₁点以上Ａ₃点以下の温度で加熱保持する時間（図２中、ｔ１）は、Ｃ含量の異なるオーステナイトを生成させるために極めて重要であり、10〜300秒とする必要がある。保持時間が10秒未満では、所望量のオーステナイトが生成しない。好ましくは30秒以上保持することが望ましい。しかし加熱保持時間が300秒を超えて長くなると、未溶解炭化物を核とするオーステナイト相の生成が進行してしまい、これをＡ₃点超の温度に加熱保持してもオーステナイト中のＣ含量のバラツキが小さくなると考えられる。この様な観点から保持時間は300秒以下にする必要があり、好ましくは120秒以下にするのが望ましい。
【００６７】
次に、上記2相組織とした鋼板を、引き続きＡ₃点超の温度（図２中、Ｔ２）に加熱し、フェライトをオーステナイト化する。Ａ₁点以上Ａ₃点以下の温度で均熱処理した鋼板を、さらにＡ₃点超に加熱するのは、前記フェライトをオーステナイト化することによって、上記Ａ₁点以上Ａ₃点以下の温度域で生成したオーステナイトよりも、Ｃ含量の少ないオーステナイトを生成させるためである。即ちＡ₁点以上Ａ₃点以下の温度域では、フェライトとオーステナイトの2相が生成するが、フェライトに含まれるＣ量は鋼中のＣ量よりも少ないので、過剰なＣはオーステナイト中へ拡散する。その結果、当該温度域で生成するオーステナイト中のＣ含量は、鋼中に含まれるＣ量よりも多くなる。これに対し、Ａ₃点超の温度域ではオーステナイトのみが生成するので、該温度域で生成するオーステナイト中のＣ含量は、鋼中に含まれるＣ量とほぼ等しくなる。
【００６８】
従って、Ａ₁点以上Ａ₃点以下の温度域で生成するオーステナイトよりも、Ａ₁点以上Ａ₃点以下の温度域で生成したフェライトをオーステナイト化したものの方がＣ含量は少なくなる。しかも本発明では、その後のＡ₃点超の温度域における加熱保持時間が適切に制御されているので、Ａ₃点超の温度では、Ｃ含量の異なるオーステナイトが存在することとなり、このオーステナイトを冷却することによって、Ｃ含量の異なる焼入マルテンサイトを生成できる。そして、この焼入マルテンサイトを後述する第二の連続焼鈍工程またはめっき工程において熱処理することで、最終的に得られる残留オーステナイトのＣ含量にバラツキが生じ、低降伏比化を達成できるのである。
【００６９】
ここで、Ａ₁点以上Ａ₃点以下の温度域で加熱保持した鋼板を、Ａ₃点超の温度で所定時間再加熱保持することによって、オーステナイト中のＣ含量にバラつきが生じる理由は、フェライトとオーステナイトの2相域（Ａ₁点以上Ａ₃点以下）では、オーステナイトはラスフェライト間に生成するのに対し、オーステナイト単相域（Ａ₃点超）では、未溶解炭化物が核となり、この核を中心にオーステナイト相が生成する。このためフェライトとオーステナイトの2相域と、オーステナイト単相域では、オーステナイトが生成するまでの時間に差が生じることとなり、この時間差が生成するオーステナイト中のＣ含量にバラつきをもたらすと考えられる。
【００７０】
なお、従来の鋼板では、熱延した鋼板（または熱延後に冷延した鋼板）をＡ₁点以上Ａ₃点以下の温度またはＡ₃点以上の温度に加熱保持された後、冷却されるが、この様に、Ａ₁点以上Ａ₃点以下の温度またはＡ₃点以上の温度のみで加熱保持したとしても、オーステナイト中のＣ含量にバラつきは生じない。従って、この様な製造条件では、本発明で規定する要件を満足する組織は得られないと考えられる。
【００７１】
Ａ₃点超の温度域で加熱保持する時間は、Ｃ含量の異なるオーステナイトを生成させるために極めて重要であり、10〜300秒とする必要がある。保持時間が10秒未満では、Ａ₁点以上Ａ₃点以下の温度域で生成したフェライトがオーステナイト化しない。好ましくは30秒以上、より好ましくは60秒以上保持することが望ましい。しかし加熱保持時間が300秒を超えて長くなると、Ｃの拡散が生じ、オーステナイト間でＣの均一拡散が進んでＣ含量が平均化してしまうため所望の効果を得ることができない。この様な観点から保持時間は300秒以下にする必要があり、好ましくは180秒以下、より好ましくは120秒以下にするのが望ましい。
【００７２】
次に、Ａ₃点以上の温度で所定時間加熱保持した鋼板は、30℃／ｓｅｃ以上の平均冷却速度（図２中、ＣＲ１）で、Ｍｓ点以下の温度（図２中、Ｔ３）まで冷却する。この冷却によってパーライト変態を避けながら、焼入マルテンサイトを得ることができる。このとき得られる焼入マルテンサイトは、Ｃ含量が不均一になっていると考えられる。オーステナイトから焼入マルテンサイトへの変態は、Ｃの拡散を伴わないので、Ｃ含量の不均一なオーステナイトを冷却することによって得られる焼入マルテンサイト中のＣ含量は、オーステナイト中のＣ含量とほぼ等しくなるからである。
【００７３】
上記平均冷却速度は、フェライトの生成のみならず、最終的に得られる残留オーステナイトの形態にも影響を与え、平均冷却速度が大きければ（好ましくは50℃／ｓｅｃ以上）、ラス状を呈することとなり一層好ましい。尚、平均冷却速度の上限は特に限定されず、大きければ大きい程良いが、実操業レベルとの関係で、適切に制御することが推奨される。
【００７４】
Ｍｓ点は、次に示す計算式から算出できる。
Ｍｓ＝561−474×［Ｃ］−33×［Ｍｎ］−17×［Ｎｉ］−17×［Ｃｒ］−21×［Ｍｏ］
式中、［ ］は各元素の質量％である。
【００７５】
第二の連続焼鈍工程またはめっき工程
上記第一の連続焼鈍工程で得られた鋼板は、Ａ₁点以上Ａ₃点以下の温度（図３中、Ｔ４）で10〜600秒（図３中、ｔ４）均熱することにより、所望の組織（焼戻マルテンサイト）を生成させる（2相域焼鈍）。この様な温度範囲で焼鈍する理由は、上記温度（Ａ₃点の温度）を超えると、第一の連続焼鈍工程で得られた焼入マルテンサイトがオーステナイト化してしまい、一方、上記温度（Ａ₁点の温度）を下回ると、所望の残留オーステナイトが得られないからである。更に、上記加熱保持時間（ｔ４）の制御は、所望の組織を得るために特に重要である。10秒未満では焼戻が不足し、所望の母相組織（焼戻マルテンサイト）が得られないからである。好ましくは20秒以上、より好ましくは30秒以上である。しかし、加熱保持時間が600秒を超えると、焼戻マルテンサイトの特徴であるラス状組織が維持できなくなり、機械的特性が劣化する。好ましくは500秒以下、より好ましくは400秒以下である。
【００７６】
次いで、平均冷却速度（図３中、ＣＲ２）を、3℃／ｓｅｃ以上（好ましくは5℃／ｓｅｃ以上）に制御し、パーライト変態を避けながら、Ｍｓ点以上Ｂｓ点以下の温度（ベイナイト変態：図３中、Ｔ５）まで冷却し、引き続きこの温度域で1秒以上（好ましくは5秒以上：図３中、ｔ５）保持する（オーステンパ処理）。これにより、残留オーステナイトへのＣ濃縮を、多量に且つ極めて短時間に得ることができる。
【００７７】
ここで、平均冷却速度が上記範囲を下回ると、所望の組織が得られず、パーライト等が生成する。尚、その上限は特に規定されず、大きければ大きい程良いが、実操業レベルとの関係で、適切に制御することが推奨される。
【００７８】
上記工程のうち、特にオーステンパ処理温度（Ｔ５）は、所望の組織を確保して本発明の作用を発揮させるのに重要である。上記温度範囲（Ｍｓ点以上Ｂｓ点以下）に制御すれば、多量の残留オーステナイトが得られ、これにより、残留オーステナイトによるＴＲＩＰ効果が発揮される。これに対し、Ｍｓ点未満ではマルテンサイト相が存在し、一方、Ｂｓ点を超えるとベイナイト相が多量に増加する。
【００７９】
上記保持時間（ｔ５）の上限は特に限定されないが、オーステナイトがベイナイトに変態する時間を考慮すると、好ましくは3000秒以下、より好ましくは2000秒以下に制御することが推奨される。
【００８０】
ここで、Ｂｓ点は次に示す計算式から算出できる。
Ｂｓ＝830−270×［Ｃ］−90×［Ｍｎ］−37×［Ｎｉ］−70×［Ｃｒ］−80×［Ｍｏ］
式中、［ ］は各元素の質量％である。
【００８１】
本発明におけるめっき条件は特に限定されず、公知のものを採用すれば良い。なお、合金化処理条件も公知のものを採用すれば良く、例えば550℃×60ｓｅｃ程度で行なえば良い。この温度域では、γ相はセメンタイトへ分解するが、この程度の時間であれば本発明の効果を損なわないことを本発明者らは確認している。
【００８２】
以上の様に、熱延工程、（必要に応じて冷延工程）、第一の連続焼鈍工程、および第二の連続焼鈍工程またはめっき工程を経て製造される鋼板は、本発明の要件を満足する組織を有しており、特に残留オーステナイトのＣ含量にはバラつきが生じている。従って、相対的にＣ含量の低い不安定な残留オーステナイトは、歪を加えることによって容易にマルテンサイトへ変態し、フェライトとの間における可動転位密度を高めることができ、低降伏比化（具体的には50％以下）を達成できる。また、相対的にＣ含量の高い安定な残留オーステナイトは、歪を加えても殆ど変態せず、さらに該残留オーステナイトはラス間に存在して微細のため、変形に対して安定であり、延性（特に全伸び）を高めることがでる。
【００８３】
よって、本発明の高強度鋼板は、加工性および形状凍結性に優れているので、例えば自動車や産業用機械等にプレス成形して使用される鋼板として好適に用いることができる。
【００８４】
【実施例】
以下、本発明を実施例によって更に詳細に説明するが、下記実施例は本発明を限定する性質のものではなく、前・後記の趣旨に適合し得る範囲で適当に変更して実施することも可能であり、それらはいずれも本発明の技術的範囲に含まれる。
【００８５】
実施例１：成分組成の検討
本実施例では、成分組成を変化させた場合における機械的特性の影響について調べた。具体的には、表１に記載の成分組成からなる供試鋼（表中の単位は質量％）を転炉溶製し、実験用スラブを得、各スラブを1200℃で30分間加熱した後、仕上温度（ＦＤＴ）を850℃として熱延し、次いで30℃／ｓｅｃの平均冷却速度で巻取り温度（ＣＴ）600℃まで冷却して、板厚2.5ｍｍの熱延鋼板を得た。
【００８６】
得られた熱延鋼板をそのまま又は酸洗後に常法に従って冷延し、板厚1.2ｍｍの冷延鋼板を得た。
【００８７】
得られた冷延鋼板を、前記図２および図３に示したパターンで処理した。具体的には、図２に示す第一の連続焼鈍工程については、Ｔ１：850℃、ｔ１：60秒、Ｔ２：930℃、ｔ２：60秒、ＣＲ１：50℃／ｓｅｃ、Ｔ３：室温、であり、図３に示す第二の連続焼鈍工程については、Ｔ４：850℃、ｔ４：120秒、ＣＲ２：25℃／ｓｅｃ、Ｔ５：420℃、ｔ５：120秒である。これらの製造条件は本発明の要件を満足している。
【００８８】
なお、本実施例では、第一の連続焼鈍の後、さらに第二の連続焼鈍を行なった場合について示したが、本発明はこれに限定されるものではなく、第二の連続焼鈍の代わりに上記パターンでめっきを行なっても構わない。
【００８９】
第一の連続焼鈍工程および第二の連続焼鈍工程を経て得られた鋼板について、各組織の存在率を次に示す様に測定した。まず、鋼板をレペラー腐食し、透過型電子顕微鏡観察（倍率：15000倍）により組織を同定した後、光学顕微鏡観察（倍率：1000倍）により顕微鏡写真から組織の面積率を測定した。各組織の面積率を表２に示す。
【００９０】
また、上記得られた鋼板について、残留オーステナイト全体のうち、2％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合を、前掲した飽和磁化測定法で測定した。測定時における印加磁化（Ｈｍ）は398000Ａ／ｍ［5000エルステッド（Ｏｅ）］、測定温度は23℃である。表２に、残留オーステナイト量（全体γ_R）および2％歪を加えることによりマルテンサイトへ変態する残留オーステナイト（変態γ_R）量を示すと共に、残留オーステナイト全体のうち2％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合を夫々示す。
【００９１】
次に、第一の連続焼鈍工程および第二の連続焼鈍工程を経て得られた鋼板からＪＩＳ 5号試験片を作製し、引張試験によって引張強度（ＴＳ）、伸び（全伸びのこと：Ｅｌ）、及び降伏強度（ＹＰ）を測定した。測定されたＹＰとＴＳから降伏比（ＹＲ）を算出した。算出結果を表２に示す。なお、引張試験の歪速度は1ｍｍ／秒とした。また、Ｅｌは35％以上を合格、ＹＲは50％以下を合格とする。
【００９２】
【表１】

【００９３】
【表２】

【００９４】
これらの結果より、以下の様に考察できる。なお、以下No.は、全て表２中の実験No.を意味する。
【００９５】
No.2〜4、No.7〜15は、いずれも本発明で規定する成分を満足する例であり、良好な全伸び（Ｅｌ）と低降伏比（ＹＲ）を有する高強度鋼板が得られた。従って、この高強度鋼板は加工性および形状凍結性に優れている。
【００９６】
これに対し、本発明で規定する成分のいずれかを満足しない下記例は、夫々以下の不具合を有している。
【００９７】
No.1は、Ｃ含量が少ない例であり、所望の残留オーステナイトが得られないためＥｌ（全伸び）が低い。
【００９８】
No.5は、（Ｓｉ＋Ａｌ）の合計含量が少ない例であり、所望の残留オーステナイトが得られないためＥｌ（全伸び）が低い。
【００９９】
No.6は、Ｍｎ含量が少ない例であり、所望の残留オーステナイトが得られないためＥｌ（全伸び）が低い。
【０１００】
実施例２：製造条件の検討
本実施例では、前記表１の鋼No.4の実験用スラブを用い、1150℃で30分間の均質化焼鈍を行なったうえで仕上げ温度850℃の熱延して板厚3.2ｍｍとした後、冷延して板厚1.2ｍｍの冷延鋼板を得た。
【０１０１】
得られた冷延鋼板を、前記図２および図３に示したパターンで処理した。具体的な条件を表３に示す。
【０１０２】
第一の連続焼鈍工程、および第二の連続焼鈍工程またはめっき工程を経て得られた鋼板について、上記実施例１と同様に、各組織の存在率および残留オーステナイト全体のうち2％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合を測定し、結果を表４に示す。また、上記実施例１と同様に引張試験を行い、引張強度（ＴＳ）、伸び（全伸びのこと：Ｅｌ）、及び降伏強度（0.2％耐力のこと：ＹＰ）を測定すると共に、降伏比（ＹＲ）を算出し、これらの結果を表４に示す。
【０１０３】
【表３】

【０１０４】
【表４】

【０１０５】
これらの結果より、以下の様に考察できる。なお、以下No.は、全て表４中の実験No.を意味する。
【０１０６】
No.16およびNo.31は、いずれも本発明で規定する製造条件を満足する例であり、鋼の組成も本発明の要件を満足しているので、良好な全伸び（Ｅｌ）と低降伏比（ＹＲ）を有する高強度鋼板が得られた。従って、この高強度鋼板は加工性および形状凍結性に優れている。
【０１０７】
これに対し、本発明で規定する製造条件のいずれかを満足しない下記例は、夫々以下の不具合を有している。
【０１０８】
No.17は、第一の連続焼鈍工程におけるＡ₃点超の温度での加熱保持時間（ｔ２）が長過ぎるため、Ｃの均一拡散が進行してオーステナイト中のＣ含量が均一となる。従って、最終的に得られる残留オーステナイト中のＣ含量も均一となるので、安定な残留オーステナイトの割合が多くなり、残留オーステナイトのうち2％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が本発明の範囲を下回り、降伏比が高くなる。
【０１０９】
No.18は、第一の連続焼鈍工程におけるＡ₃点超の温度での加熱保持時間（ｔ２）が短過ぎるため、Ａ₁点以上Ａ₃点以下の温度で生成したフェライトがオーステナイト化しない。よって、焼入マルテンサイトの量が少なくなり、最終的に得られる焼戻マルテンサイト量が少なくなるため、全伸びが低くなると共に、降伏比が高くなる。
【０１１０】
No.19は、第一の連続焼鈍工程におけるＡ₁点以上Ａ₃点以下の温度を下回る範囲で加熱保持しているため、焼入マルテンサイト中のＣ含量が均一となっていると考えられ、最終的に得られる残留オーステナイト中のＣ含量が均一となる。そのため、残留オーステナイトのうち2％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が本発明の範囲を下回り、降伏比が高くなる。
【０１１１】
No.20は、第一の連続焼鈍工程におけるＡ₁点以上Ａ₃点以下の温度で加熱保持せず、Ａ₃点超の温度でのみ加熱保持した例である。従って、Ｃの均一拡散が進行してオーステナイト中のＣ含量が均一となり、最終的に得られる残留オーステナイト中のＣ含量が均一となる。よって、残留オーステナイトのうち2％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が本発明の範囲を下回り、降伏比が高くなる。
【０１１２】
No.21は、先願発明の方法を模した例であり、第一の連続焼鈍工程におけるＡ₁点以上Ａ₃点以下の温度でのみ加熱保持しており、Ａ₃点超の温度で加熱保持していない。従って、焼入マルテンサイトが少なくなるので、最終的に得られる焼戻マルテンサイトも少なくなる。また、残留オーステナイト中のＣ含量が均一となるため、残留オーステナイトのうち2％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が本発明の範囲を下回るため、降伏比が高くなる。
【０１１３】
No.22は、第一の連続焼鈍工程におけるＡ₃点超の温度からの平均冷却速度（ＣＲ１）が小さいため、パーライトが生成し、最終的に得られる焼戻マルテンサイトの量が少なくなる。従って、Ｅｌ（全伸び）が低く、且つ、降伏比も高い。
【０１１４】
No.23は、第一の連続焼鈍工程における冷却後の温度（Ｔ３）がＭｓ点を超えているので、最終的に焼戻ベイナイトが生成する。よって、焼戻マルテンサイトが生成せず、降伏比が高くなる。
【０１１５】
No.24は、第二の連続焼鈍工程における加熱保持温度（Ｔ４）がＡ₃点よりも高いため、最終的にベイナイトが多く生成し、焼戻しマルテンサイトが少なくなる。従って、延びが低くなる。
【０１１６】
No.25は、第二の連続焼鈍工程におけるＡ₁点以上Ａ₃点以下の温度（Ｔ４）での加熱保持時間（ｔ４）が長過ぎるため、フェライトが多く生成し、焼戻マルテンサイト量が少なくなる。そのためＥｌ（全伸び）が低くなる。また、残留オーステナイトのうち2％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が、本発明の要件を下回るので、降伏比が高くなる。
【０１１７】
No.26は、第二の連続焼鈍工程におけるＡ₁点以上Ａ₃点以下の温度（Ｔ４）での加熱保持時間（ｔ４）が短過ぎるため、オーステナイトの生成量が少なく、残留オーステナイトが少なくなる。従って、Ｅｌ（全伸び）が低くなる。また、加熱保持時間（ｔ４）が短いので、試料全体の温度が850℃に達せず、フェライトの生成量が多くなる。
【０１１８】
No.27は、第二の連続焼鈍工程におけるＡ₁点以上Ａ₃点以下の温度からの平均冷却速度（ＣＲ２）が小さいため、パーライトが多く生成する。従って、最終的に得られる焼戻マルテンサイトの量が少なくなり、Ｅｌ（全伸び）が低く、且つ、降伏比が高くなる。
【０１１９】
No.28は、第二の連続焼鈍工程における冷却後の温度（Ｔ５）が高過ぎるため、パーライトが多く生成する。従って、最終的に得られる焼戻マルテンサイトの量が少なくなり、Ｅｌ（全伸び）が低く、且つ、降伏比が高くなる。
【０１２０】
No.29は、第二の連続焼鈍工程における冷却後の温度（Ｔ５）が低過ぎるため、最終的に得られる残留オーステナイト量が1％と非常に少なく、Ｅｌ（全伸び）が低くなる。また、残留オーステナイトのうち2％歪を加えることによりマルテンサイトへ変態する残留オーステナイトは少ないので、降伏比が高くなる。
【０１２１】
No.30は、第二の連続焼鈍工程におけるＡ₁点以上Ａ₃点以下の温度（Ｔ４）で加熱保持した後、この温度域からＭｓ点未満の温度まで急冷しているので、不安定な残留オーステナイトが多く、残留オーステナイトのうち2％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が大きくなるので、破断し易くなり、Ｅｌ（全伸び）が低く、且つ、降伏比が高くなる。
【０１２２】
No.32は、第一の連続焼鈍工程におけるＡ₁点以上Ａ₃点以下の温度（Ｔ１）での加熱保持時間（ｔ１）が長過ぎるため、未溶解炭化物を核とするオーステナイト相の生成が進行してしまい、これをＡ₃点超の温度に加熱してもオーステナイト中のＣ含量のバラツキが小さくなると考えられる。従って、最終的に得られる残留オーステナイト中のＣ含量も均一となるので、安定な残留オーステナイトの割合が多くなり、残留オーステナイトのうち2％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が本発明の範囲を下回り、降伏比が高くなる。
【０１２３】
【発明の効果】
本発明によれば、500〜1400ＭＰａ級の高強度及び超高強度鋼板であっても、全伸びが35％以上で、且つ、降伏比が50％以下の加工性および形状凍結性に優れた高強度鋼板およびその鋼板を確実に製造できる方法を提供できた。
【図面の簡単な説明】
【図１】 飽和磁化測定に用いる装置の一部分を示す概略説明図である。
【図２】 第一の連続焼鈍工程を説明した図である。
【図３】 第二の連続焼鈍工程またはめっき工程を説明した図である。
【符号の説明】
１ 磁化検出用の４πＩコイル
２ 棒状試料
３ 磁界検出用のＨコイル
４ 導線[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength steel plate, and more particularly, to a high-strength steel plate having both workability and shape freezing property in a high-strength region of 500 to 1400 MPa class and an ultrahigh-strength region, and a method for producing the same.
[0002]
[Prior art]
Steel sheets used for press forming in automobiles, industrial machines and the like are required to have both excellent strength and ductility, and such required characteristics have been increasingly increased in recent years.
[0003]
However, as the strength of the steel plate increases, there is a problem that the spring back (elastic recovery) after the steel plate is processed increases and the shape freezeability deteriorates. As an index of shape freezing property, there is a yield stress (0.2% proof stress) in a tensile test, and if the yield stress is low, the shape freezing property is good. Since high strength steel plates are required to have high maximum strength, the ratio between yield stress and maximum strength (that is, yield ratio) needs to be low (see Non-Patent Document 1, for example).
[0004]
On the other hand, in order to improve the workability at the time of press forming, it is necessary to increase the ductility of the steel sheet, and in particular, an improvement in total elongation is required.
[0005]
As a technique aiming to provide a steel sheet having such a low yield ratio and a large total elongation, for example, Patent Document 1 defines the component composition of a cold-rolled steel sheet, and the structure of the steel sheet includes residual austenite, martensite, and ferrite. A technique for improving the properties of the cold-rolled steel sheet such as strength, weldability, ductility, low yield ratio and bake hardenability has been proposed. However, although the cold-rolled steel sheet obtained by this technique has achieved an elongation of about 35%, its yield ratio is about 55%, and further improvement in shape freezeability is desired.
[0006]
[Non-Patent Document 1]
See Thin Steel Sheet Forming Technology Study Group, “Press Forming Difficulty Handbook”, Nikkan Kogyo Shimbun, July 31, 1997 (2nd edition) p.178
[Patent Document 1]
Japanese Patent Laid-Open No. 3-77743 (see [Claims], Table 2, page 8)
[0007]
[Problems to be solved by the invention]
The present invention has been made in view of such a situation, and the object thereof is workability (particularly total elongation) and shape freezing property (particularly low) in a high-strength region and ultrahigh-strength region of 500 to 1400 MPa class. An object of the present invention is to provide a high-strength steel sheet having an excellent yield ratio and a method capable of reliably producing the steel sheet.
[0008]
[Means for Solving the Problems]
The high-strength steel sheet excellent in workability and shape freezing property according to the present invention that has solved the above problems is mass%, C: 0.06-0.6%, Si + Al: 0.5-3%. , Mn: 0.5 to 3%, P: 0.15% or less (excluding 0%), S: 0.02% or less (including 0%), and tempered martensite: all 15% or more in area ratio with respect to the structure, ferrite: 5 to 60% in area ratio with respect to the entire structure, retained austenite: 5% or more in volume ratio with respect to the entire structure, and further containing bainite and / or martensite. It has a structure that may be used, and the gist is that the ratio of residual austenite that transforms to martensite by applying 2% strain in the residual austenite is 20 to 50%.
[0009]
In the present invention, the mass% is
(1) Mo: 1% or less (not including 0%), Ni: 0.5% or less (not including 0%), Cu: 0.5% or less (not including 0%), Cr: 1% Containing at least one of the following (excluding 0%):
(2) At least one of Ti: 0.1% or less (not including 0%), Nb: 0.1% or less (not including 0%), V: 0.1% or less (not including 0%) Containing
(3) Ca: 0.003% or less (not including 0%) and / or REM: 0.003% or less (not including 0%)
thing,
Are all preferred embodiments.
[0010]
The method capable of reliably producing the high-strength steel sheet according to the present invention includes at least a hot rolling step, a first continuous annealing step, and a second continuous annealing step or a plating step, and the first continuous annealing step includes: A₁More than point A_ThreeHeating and holding at a temperature below the point for 10 to 300 seconds;_ThreeIncluding a step of heating and holding at a temperature above the point for 10 to 300 seconds; and a step of cooling to a temperature below the Ms point at an average cooling rate of 30 ° C./sec or more, and the second continuous annealing step or the plating step includes , A₁More than point A_ThreeA step of heating and holding at a temperature below the point for 10 to 600 seconds; a step of cooling to a temperature not lower than the Ms point and not higher than the Bs point at an average cooling rate of 3 ° C./sec or higher; and a step of subsequently holding for 1 second or longer in that temperature range In summary,
[0011]
In this invention, it is also preferable to include a cold rolling process prior to the first continuous annealing process after the hot rolling process.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In order to provide a high-strength steel sheet having a large total elongation and a low yield ratio, the present inventors have developed a TRIP steel sheet represented by Patent Document 1 [residual austenite (γ_R), And a steel plate in which the retained austenite is induced to undergo transformation (strain-induced transformation: TRIP) during work deformation to improve ductility] has been studied with particular attention paid to the retained austenite. As a result, after the TRIP steel sheet is hot-rolled (cold-rolled if necessary) and then manufactured by continuous annealing, the first continuous annealing (previous continuous annealing) is performed as “A₁More than point A_ThreeAfter heating and holding at a temperature below the point (two-phase region) for a predetermined time, A_ThreeIt is configured to “heat and hold for a predetermined time at a temperature exceeding the point (austenite single phase region)”, and the second continuous annealing (later continuous annealing) or plating is performed as “A₁More than point A_ThreeA high-strength steel plate with both of these characteristics by heat treatment as a structure that is heated and held at a temperature below the point (two-phase region) for a predetermined time [specifically, the matrix structure is a mixture of tempered martensite and ferrite. The structure is a TRIP steel plate in which the second phase is a retained austenite structure, and the retained austenite (hereinafter referred to as such retained austenite (γ_R), Γ_RC concentration (Cγ_RThe present invention has been completed by finding that a steel plate whose amount of) is sometimes referred to as low and unstable residual austenite) can be obtained. Hereinafter, the basic concept (basic idea) of the steel sheet of the present invention will be described in detail with the background to the present invention.
[0013]
First, regarding ductility (particularly total elongation) which is one of the required characteristics in the present invention, it is generally considered that retained austenite dominates the ductility in the structure of the TRIP steel sheet, and further improves the ductility. For that, Cγ_RIt is said that it is useful to increase the stability of retained austenite by increasing the A
[0014]
On the other hand, the shape freezing property (especially low yield ratio), which is another required characteristic in the present invention, is dominated by martensite existing at the beginning of deformation or unstable retained austenite that undergoes martensitic transformation near the yield point. It is said that
[0015]
However, the conventional TRIP steel sheet is Cγ from the viewpoint of emphasizing improvement in ductility._RThe fact is that the martensite useful for reducing the yield ratio and the unstable retained austenite are extremely reduced because the concentration is increased to increase the stability of retained austenite. As a result, although the local ductility is very excellent, the yield ratio is also high, and the shape freezeability is inferior, such as inferior overhang formability.
[0016]
In order to provide a TRIP steel sheet having a low yield ratio while ensuring excellent ductility, the inventors of the present invention have previously applied a new TRIP steel sheet [the mixed phase of tempered martensite and ferrite as the parent phase, Studies have been made based on the structure of a high-strength steel sheet having a fine retained austenite as a phase (Japanese Patent Application No. 2002-54606, hereinafter referred to as the prior invention). Although it was possible to provide a high-strength steel sheet with an excellent balance between strength and ductility by using a steel sheet with a unique structure as in the above-mentioned prior application invention, the yield ratio was slightly increased due to the improvement of ductility. This is because there is still a problem that the shape freezing property is still inferior (about 60%).
[0017]
As a result, if the above-described heat treatment unique to the present invention is adopted, Cγ_RThe inventors have found that a predetermined amount of retained austenite having different stability due to concentration variation (that is, retained austenite transformed to martensite by applying 2% strain) was obtained, and the present invention was completed.
[0018]
The reason why a high-strength steel sheet having a low yield ratio of 50% or less can be obtained while maintaining excellent elongation according to the present invention is the first continuous especially in the invention of the prior application in order to produce desired retained austenite. This is because the annealing process has been improved.
[0019]
That is, both the present invention and the prior application invention are common in that the TRIP steel sheet is manufactured through hot rolling (cold rolling if necessary) → first continuous annealing → second continuous annealing. Since the first continuous annealing process is immediately cooled after being heated to the two-phase region temperature, it is cooled to the austenite single-phase region after being heated and held at the two-phase region temperature as in the present invention. Compared to the case, the amount of C in the martensite (quenched martensite) generated in the process is considered to increase. As a result, according to the prior invention, an austenite phase having a high C concentration and high stability is obtained, and the C concentration (Cγ in the retained austenite generated by the second continuous annealing is obtained._R) And the retained austenite is in a fine form between the laths, and is stable against deformation. As a result, unstable retained austenite that transforms from retained austenite to martensite cannot be obtained. It is considered that the low yield ratio could not be realized.
[0020]
On the other hand, according to the present invention, in performing the first continuous annealing, first, after heating to the two-phase region temperature, the temperature is increased to the austenite single-phase region and then cooling is performed. Compared to the case, austenite having a lower C content is produced between the lath ferrites. Moreover, according to the present invention, since the heating time in the austenite single phase region is appropriately controlled (as short as possible), there is a time difference until the austenite is generated by the undissolved carbide as a nucleus. The C content in the austenite thus produced varies. Accordingly, an austenite phase having a different C concentration is produced in the steel, and as a result, the quenching martensite produced by the first continuous annealing can also be obtained with a C content variation. Become.
[0021]
Furthermore, according to the present invention, since the heat treatment is appropriately performed in the two-phase region even in the subsequent second continuous annealing step, the amount of C of residual austenite finally formed (Cγ_R) Is also likely to vary. As a result, Cγ_RA low amount of unstable austenite is obtained, and such retained austenite undergoes martensitic transformation after heat treatment or at the beginning of strain, increasing the dislocation density around ferrite, resulting in low yielding. It is thought that the ratio (YR ≦ 50%) can be secured.
[0022]
The basic idea for obtaining the steel sheet of the present invention has been described above. The requirements that characterize the present invention will be specifically described below.
[0023]
First, the structure characterizing the steel sheet of the present invention will be described.
[0024]
In the present invention, a mixed phase of tempered martensite and ferrite is used as a parent phase, the tempered martensite is 15% or more in area ratio with respect to the entire structure, and ferrite is in an area ratio of 5 to 60% with respect to the entire structure. It is necessary to contain. The reason for specifying such a range is as follows.
[0025]
Tempered martensite: by area ratio for all structures 15 %more than
The “tempered martensite” in the present invention means a soft structure with a low dislocation density and a lath-like structure. On the other hand, “martensite” is different from the tempered martensite in that it is a hard structure with a high dislocation density, and both can be distinguished by observation with a transmission electron microscope (TEM), for example. The conventional retained austenitic steel sheet is still different from the steel sheet of the present invention having the tempered martensite as a parent phase in that it has a soft block ferrite structure with a low dislocation density.
[0026]
As will be described later, the tempered martensite having such characteristics is obtained by applying hot-rolled steel sheets (or steel sheets cold-rolled after hot rolling) to A in the first continuous annealing step.₁More than point A_ThreeAfter heating and holding at a temperature below the point, A_ThreeBy heating and holding at a temperature exceeding the point, quenching from the temperature produces quenched martensite, and in the second continuous annealing step or plating step, A₁Above the point (above about 700 ℃) A_ThreeIt is obtained by annealing at a temperature below the point.
[0027]
The tempered martensite is necessary to increase the total elongation of the steel sheet and lower the yield ratio, and its content is 15% or more (preferably 20% or more) in terms of the area ratio with respect to the entire structure. It is important that The content of tempered martensite is determined by the balance with retained austenite, which will be described later, and may be appropriately controlled within a range where desired characteristics can be exhibited, but is preferably about 65% or less, more preferably Is less than 35%.
[0028]
Ferrite: in area ratio to the whole structure Five ~ 60 %
“Ferrite” in the present invention means polygonal ferrite, that is, ferrite having a low dislocation density, and the elongation characteristics (particularly total elongation) can be enhanced by containing this ferrite. In order to exert such effects, it is necessary to contain ferrite in an area ratio of 5% or more (preferably 10% or more) with respect to the entire structure. However, if it exceeds 60%, it will be difficult to ensure the required strength, so the upper limit is made 60%. More preferably it is 50% or less.
[0029]
In addition, the calculation procedure of the area ratio of each structure | tissue (except a retained austenite) is as follows. Ferrite and martensite are observed with an optical microscope by repeller corrosion, and bainite is observed with an optical microscope by picral corrosion, and the area ratio of each structure is calculated from the photograph. However, in the optical microscope observation by repeller corrosion, since martensite and retained austenite cannot be distinguished, in the present invention, from the total area ratio (area%) of martensite and retained austenite calculated by optical microscope observation by repeller corrosion, A value obtained by subtracting the amount of retained austenite (volume%) measured by the saturation magnetization measurement method described later is defined as the amount of martensite (area%).
[0030]
In the present invention, the retained austenite is used as the second phase, and the retained austenite needs to be contained in a volume ratio of 5% or more with respect to the entire structure. The reason for limiting to such a range is as follows.
[0031]
Residual austenite (γ _R ): Volumetric ratio for all tissues Five %more than
Residual austenite is useful for improving the total elongation and further fatigue properties, and in order to exert such an effect effectively, it should be present in a volume ratio of 5% (preferably 7% or more) with respect to the entire structure. is required. The content of retained austenite is determined by the balance with the tempered martensite and ferrite, and may be appropriately controlled within a range where desired properties can be exhibited, but is preferably about 30% or less, and Preferably it is 25% or less. A method for calculating the volume ratio of retained austenite will be described later.
[0032]
The steel sheet of the present invention may further contain bainite and / or martensite as the above-mentioned second phase structure in addition to the retained austenite as long as the action of the present invention is not impaired. This is because this structure can inevitably remain in the production process of the present invention. The content is not particularly limited, and the smaller the better, the better. However, it is recommended that the area ratio is preferably 5% or less, more preferably 3% or less with respect to the entire tissue.
[0033]
In the present invention, residual austenite that transforms to martensite by applying 2% strain of the retained austenite (ie, Cγ_RIt is important that the ratio of low and unstable retained austenite) is 20 to 50%.
[0034]
When the proportion of retained austenite that transforms to martensite by applying 2% strain is less than 20% of the entire retained austenite, the proportion of stable retained austenite increases to 80% or more, and 2 Even when% strain is applied, retained austenite hardly transforms into martensite. As a result, improvement of the movable dislocation density due to the transformation cannot be expected, and the yield ratio remains high. However, if the amount of unstable retained austenite in the entire retained austenite is 20% or more and the amount of unstable retained austenite increases, the unstable retained austenite is easily transformed into martensite by applying 2% strain. The dislocation density at the interface with the ferrite becomes higher. As a result, a low yield ratio of the steel sheet can be realized. However, if the ratio exceeds 50%, the amount of unstable retained austenite increases too much, and the amount of stable retained austenite that contributes to the improvement of ductility is insufficient, so that high ductility of the steel sheet cannot be realized. Therefore, the ratio needs to be 50% or less (preferably 35% or less).
[0035]
As described above, in the present invention, the proportion of residual austenite that transforms into martensite by adding 2% strain in the entire retained austenite is controlled within the above range, so that stable retained austenite and unstable retained austenite The balance of the amount is good. As a result, the effect of improving ductility (particularly total elongation) by stable retained austenite and the effect of improving low yield ratio by unstable retained austenite can be obtained in a well-balanced manner, and a low yield ratio and a large total elongation can be achieved.
[0036]
Here, the ratio of residual austenite that transforms to martensite by applying 2% strain in the entire retained austenite can be measured, for example, by the saturation magnetization measurement method previously proposed by the present inventors (Japanese Patent Application No. 2001-285750). reference). Hereinafter, the measurement principle and measurement procedure of this saturation magnetization measurement method will be described.
[0037]
The saturation magnetization measurement method is based on the following measurement principle. That is, a structure such as a ferrite phase or a martensite phase in a metal structure exhibits ferromagnetism at room temperature, whereas an austenite phase is paramagnetic. Therefore, the saturation magnetization amount (Is) per unit volume of a metal structure consisting only of a ferromagnetic structure such as a ferrite phase and a martensite phase is obtained in advance, and the saturation magnetization amount (I) of a sample containing an austenite phase is measured. By doing so, the proportion of the austenite (γ) phase can be obtained from the following equation (1). FIG. 1 schematically shows a part of an apparatus used for saturation magnetization measurement. For saturation magnetization, a magnetic field is generated using an electrode stone, a 4πI coil 1 for magnetization detection and an H coil 3 for magnetic field detection are mounted between electromagnets, and a rod-like sample 2 is inserted into the 4πI coil 1 to close the magnetism. This is obtained by measuring the magnetization curve in a state where a path is formed and the influence of the demagnetizing field is eliminated. In addition, 4 in FIG. 1 is a conducting wire.
γ (volume%) = (1−I / Is) × 100 (1)
[0038]
Next, the measurement procedure of the saturation magnetization measurement method will be described.
[0039]
A plurality of 4 mm × 30 mm test pieces are cut out from the steel type to be measured, and are stacked to produce a columnar test piece having a thickness of about 4 mm. Measured or calculated saturation magnetization (I) of the columnar specimen and the saturation magnetization (Is) when the amount of retained austenite is 0% in volume ratio, which is substantially the same component as the columnar specimen. The retained austenite in the columnar specimen (hereinafter referred to as “total γ”_RIs calculated).
Overall γ_R(Volume%) = (1−I / Is) × 100 (2)
[0040]
Next, a tensile test piece is produced from the steel type to be measured, strained by 2% with a tensile tester, and then a columnar test piece (4 mm × 3 mm × about 4 mm thick) is cut out from the parallel portion of the tensile test piece. From the saturation magnetization (It) of the columnar specimen and the saturation magnetization (Is), retained austenite (hereinafter referred to as “2% strain” in the columnar specimen after applying 2% strain according to the following equation (3): After γ_RIs calculated).
Γ after 2% strain_R(Volume%) = (1−It / Is) × 100 (3)
[0041]
Total γ calculated in this way_RAnd 2% strain after γ_RFrom the above, retained austenite that transforms to martensite by applying 2% strain (hereinafter referred to as “transformation γ”)_RThe amount can be calculated by the following equation (4).
Transformation γ_R(Volume%) = Overall γ_RΓ after −2% strain_R    ···(Four)
[0042]
Therefore, the entire retained austenite (total γ_R), Retained austenite (transformation γ) that transforms to martensite by applying 2% strain_R) Can be calculated by the following equation (5).
[0043]
[Expression 1]

[0044]
Applied magnetization in saturation magnetization measurement (H_m) Is preferably 398000-796000 A / m [5000-10000 Oersted (Oe)]. In addition, since the saturation magnetization is easily affected by changes in the measurement temperature, it is preferably performed within a range of, for example, 23 ° C. ± 3 ° C. when measured at room temperature.
[0045]
Residual austenite transformed to martensite by applying 2% strain is, as described later, in the first continuous annealing step, A₁More than point A_ThreeAfter heating and holding at a temperature below the point, A_ThreeIt can be reliably generated by heating and holding at a temperature above the point.
[0046]
Next, basic components constituting the steel plate of the present invention will be described. Hereinafter, all the units of chemical components are mass%.
[0047]
C: 0.06 ~ 0.6 %
C is an essential element for securing high strength and securing the amount of retained austenite. Specifically, it is an important element for containing a sufficient amount of C in the austenite phase and allowing the desired austenite phase to remain even at room temperature, and is useful for increasing the total elongation. In order to obtain such an effect, it is necessary to contain 0.06% or more of C. Preferably it is 0.10% or more. However, if added over 0.6%, not only the effect is saturated, but also defects due to center segregation during casting and the like are observed. Preferably it is 0.25% or less.
[0048]
Si + Al: 0.5 ~ Three %
Si and Al are elements that effectively suppress the decomposition of residual austenite and the formation of carbides. In particular, Si is also useful as a solid solution strengthening element. In order to effectively exhibit such an action, it is necessary to add 0.5% or more of Si or Al in total. Preferably it is 0.7% or more, More preferably, it is 1% or more. However, if the total amount of the above elements exceeds 3%, the above effect is saturated, and it is economically wasteful. If a large amount is added, it causes hot brittleness, so the upper limit is 3%. And Preferably it is 2.5% or less, More preferably, it is 2% or less. Si and Al may be used in combination, or Si alone or Al alone may be used.
[0049]
Mn: 0.5 ~ Three %
Mn is an element necessary for stabilizing the austenite phase and obtaining desired retained austenite. In order to exhibit such an action effectively, it is necessary to add 0.5% or more. Preferably it is 0.7% or more, More preferably, it is 1% or more. However, if it is added in excess of 3%, adverse effects such as slab cracking are observed. Preferably it is 2.5% or less, More preferably, it is 2% or less.
[0050]
P: 0.15 %Less than( 0 % Not included)
P is an element effective for securing desired retained austenite. In order to effectively exhibit such an action, it is recommended to add 0.03% or more (more preferably 0.05% or more). However, when it exceeds 0.15%, secondary workability deteriorates. More preferably, it is 0.1% or less.
[0051]
S: 0.02 %Less than( 0 %including)
S is an element that forms sulfide-based inclusions such as MnS and degrades workability as a starting point of cracking, and needs to be suppressed to 0.02% or less. Preferably it is 0.015% or less.
[0052]
The steel of the present invention basically contains the above components, and the balance is substantially iron and impurities, but the following allowable components can be added as long as the effects of the present invention are not impaired.
[0053]
Mo: 1 %Less than( 0 %), Ni: 0.5 %Less than( 0 %), Cu: 0.5 %Less than( 0 %), Cr: 1 %Less than( 0 % Not including)
These elements are useful elements for strengthening steel and are effective in stabilizing retained austenite and securing a predetermined amount. In order to effectively exert such actions, Mo: 0.05% or more (more preferably 0.1% or more), Ni: 0.05% or more (more preferably 0.1% or more), Cu: 0.05% or more (more preferably) 0.1% or more) and Cr: 0.05% or more (more preferably 0.1% or more) are recommended to be added. However, even if Mo and Cr are added in excess of 1% and Ni and Cu are added in excess of 0.5%, the above effect is saturated, which is economically wasteful. More preferably, Mo is 0.8% or less, Ni is 0.4% or less, Cu is 0.4% or less, and Cr is 0.8% or less.
[0054]
Ti: 0.1 %Less than( 0 %), Nb: 0.1 %Less than( 0 % Not included), V: 0.1 %Less than( 0 % Not including)
These elements have precipitation strengthening and microstructure refinement effects, and are useful elements for increasing the strength. In order to effectively exert such actions, Ti: 0.01% or more (more preferably 0.02% or more), Nb: 0.01% or more (more preferably 0.02% or more), V: 0.01% or more (more preferably 0.02% or more) is recommended. However, if any element is added in excess of 0.1%, the above effect is saturated, which is economically useless. More preferably, Ti is 0.08% or less, Nb is 0.08% or less, and V is 0.08% or less.
[0055]
Ca: 0.003 %Less than( 0 %) And / or REM: 0.003 %Less than( 0 % Not included)
Ca and REM (rare earth elements) are elements that control the form of sulfide in steel and are effective in improving workability. Here, examples of rare earth elements used in the present invention include Sc, Y, and lanthanoids. In order to effectively exhibit the above action, it is recommended to add 0.0003% or more (more preferably 0.0005% or more). However, even if added over 0.003%, the above effect is saturated and it is economically useless. More preferably, it is 0.0025% or less.
[0056]
Next, a method capable of reliably producing a steel sheet that satisfies the above requirements will be described.
[0057]
A steel sheet that satisfies the requirements of the present invention is manufactured through at least a hot rolling process, a first continuous annealing process, and a second continuous annealing process or a plating process.
[0058]
However, the conditions for the hot rolling step are not particularly limited, and the conditions that are usually carried out can be appropriately selected and employed. The production method of the present invention does not secure a desired structure by a hot rolling process, but a desired structure by controlling the first continuous annealing process and the second continuous annealing process or plating process to be performed after hot rolling. This is because there is a feature in obtaining.
[0059]
Therefore, in the production method of the present invention, after the hot rolling step, a cold rolling step may be included prior to the first continuous annealing step, and the conditions of the cold rolling step are also particularly limited as in the hot rolling step. Not.
[0060]
Specifically, as the hot rolling process, A_r3After completion of hot rolling at the point or more, conditions such as cooling at an average cooling rate of about 30 ° C./sec and winding at a temperature of about 500 to 600 ° C. can be employed. In the cold rolling process, it is recommended to perform cold rolling at a cold rolling rate of about 30 to 70%. Of course, this is not intended to be limited to this.
[0061]
Next, the steel plate obtained in the hot rolling process (or the cold rolling process after the hot rolling process) is heat-treated in the first continuous annealing process and the second continuous annealing process or plating process.
[0062]
Of these, the first continuous annealing (previous continuous annealing) is A₁More than point A_ThreeHeating and holding at a temperature below the point for 10 to 300 seconds;_ThreeA step of heating and holding at a temperature above the point for 10 to 300 seconds; and a step of cooling to a temperature below the Ms point at an average cooling rate of 30 ° C./sec or more. These conditions are set in order to obtain a desired matrix structure (quenched martensite with varying C content), and an explanatory diagram of this process is shown in FIG.
[0063]
The second continuous annealing (later continuous annealing) or plating is A₁More than point A_ThreeA step of heating and holding at a temperature below the point for 10 to 600 seconds; a step of cooling to a temperature not lower than the Ms point and not higher than the Bs point at an average cooling rate of 3 ° C./sec or higher; and a step of holding for 1 second or longer in the temperature range Includes. These conditions include tempering the matrix structure (quenched martensite) generated in the first continuous annealing step to obtain a desired tempered martensite, and Cγ_RLow and unstable retained austenite and Cγ_RIs set in order to obtain a second phase in which the ratio of high and stable retained austenite is appropriately controlled. An explanatory view of this process is shown in FIG.
[0064]
Hereinafter, the first continuous annealing step and the second continuous annealing step or the plating step will be described with reference to FIGS. 2 and 3.
[0065]
First continuous annealing process
The steel plate obtained in the hot rolling step (or the cold rolling step after the hot rolling step)₁More than point A_ThreeHeating to a temperature below the point (T1 in FIG. 2) and soaking (preferably 1300 ° C. or less). A part of ferrite is generated by soaking, and a two-phase structure of [ferrite + austenite] is obtained.
[0066]
A₁More than point A_ThreeThe time for heating and holding at a temperature below the point (t1 in FIG. 2) is extremely important for generating austenite having different C contents, and needs to be 10 to 300 seconds. When the holding time is less than 10 seconds, a desired amount of austenite is not generated. It is desirable to hold for 30 seconds or more. However, if the heating and holding time is longer than 300 seconds, the generation of an austenite phase having undissolved carbide as a nucleus proceeds, and this is_ThreeIt is considered that the variation in the C content in the austenite becomes small even when heated to a temperature above the point. From such a viewpoint, the holding time needs to be 300 seconds or less, and preferably 120 seconds or less.
[0067]
Next, the steel sheet having the above-mentioned two-phase structure is continuously changed to A._ThreeIt is heated to a temperature exceeding the point (T2 in FIG. 2), and ferrite is austenitized. A₁More than point A_ThreeA steel plate that has been soaked at a temperature below the point_ThreeHeating above the point is because the ferrite is austenitized, so that A₁More than point A_ThreeThis is because austenite having a lower C content than austenite generated in a temperature range below the point is generated. A₁More than point A_ThreeIn the temperature range below the point, two phases of ferrite and austenite are formed. However, since the amount of C contained in the ferrite is less than the amount of C in the steel, excess C diffuses into the austenite. As a result, the C content in the austenite produced in the temperature range is greater than the C content contained in the steel. In contrast, A_ThreeSince only austenite is generated in the temperature range above the point, the C content in the austenite generated in the temperature range is substantially equal to the amount of C contained in the steel.
[0068]
Therefore, A₁More than point A_ThreeThan austenite generated in the temperature range below the point₁More than point A_ThreeThe austenitized ferrite produced in the temperature range below the point has a lower C content. Moreover, in the present invention, the subsequent A_ThreeSince the heating and holding time in the temperature range above the point is appropriately controlled, A_ThreeAt temperatures above the point, austenite having a different C content exists, and quenching martensite having a different C content can be generated by cooling the austenite. And by heat-treating this quenched martensite in the second continuous annealing step or plating step to be described later, the C content of the residual austenite finally obtained varies, and a low yield ratio can be achieved.
[0069]
Where A₁More than point A_ThreeA steel plate heated and held in a temperature range below the point_ThreeThe reason why the C content in the austenite varies due to reheating and holding for a predetermined time at a temperature exceeding the point is that the two-phase region of ferrite and austenite (A₁More than point A_ThreeAt a point below), austenite is formed between lath ferrites, whereas austenite single phase region (A_ThreeIn (above point), undissolved carbide becomes a nucleus, and an austenite phase is generated around this nucleus. For this reason, in the two-phase region of ferrite and austenite and the austenite single-phase region, a difference occurs in the time until austenite is generated, and this time difference is considered to cause variation in the C content in the austenite generated.
[0070]
In the conventional steel plate, a hot-rolled steel plate (or a steel plate cold-rolled after hot rolling) is used as A.₁More than point A_ThreeTemperature below point or A_ThreeAfter being heated and held at a temperature above the point, it is cooled.₁More than point A_ThreeTemperature below point or A_ThreeEven if it is heated and held only at a temperature above the point, the C content in the austenite does not vary. Therefore, it is considered that such a production condition cannot provide a structure that satisfies the requirements defined in the present invention.
[0071]
A_ThreeThe time of heating and holding in the temperature range above the point is extremely important for generating austenite having different C contents, and it is necessary to set it to 10 to 300 seconds. If the holding time is less than 10 seconds, A₁More than point A_ThreeThe ferrite formed in the temperature range below the point does not become austenite. It is desirable to hold for 30 seconds or more, more preferably 60 seconds or more. However, if the heating and holding time is longer than 300 seconds, C diffusion occurs and the C content is averaged between austenite and the C content is averaged, so that a desired effect cannot be obtained. From such a point of view, the holding time needs to be 300 seconds or less, preferably 180 seconds or less, more preferably 120 seconds or less.
[0072]
Next, A_ThreeThe steel sheet heated and held at a temperature above the point for a predetermined time is cooled to a temperature below the Ms point (T3 in FIG. 2) at an average cooling rate of 30 ° C./sec or more (CR1 in FIG. 2). Quenching martensite can be obtained while avoiding pearlite transformation by this cooling. It is considered that the quenching martensite obtained at this time has an uneven C content. Since the transformation from austenite to hardened martensite is not accompanied by C diffusion, the C content in hardened martensite obtained by cooling austenite with non-uniform C content is almost equal to the C content in austenite. Because it becomes equal.
[0073]
The average cooling rate affects not only the formation of ferrite but also the form of residual austenite finally obtained. If the average cooling rate is large (preferably 50 ° C./sec or more), it will have a lath shape. Even more preferred. The upper limit of the average cooling rate is not particularly limited, and the larger the better, the better. However, appropriate control is recommended in relation to the actual operation level.
[0074]
The Ms point can be calculated from the following calculation formula.
Ms = 561−474 × [C] −33 × [Mn] −17 × [Ni] −17 × [Cr] −21 × [Mo]
In the formula, [] is mass% of each element.
[0075]
Second continuous annealing process or plating process
The steel plate obtained in the first continuous annealing step is A₁More than point A_ThreeA desired structure (tempered martensite) is generated by soaking at a temperature below the point (T4 in FIG. 3) for 10 to 600 seconds (t4 in FIG. 3) (two-phase annealing). The reason for annealing in such a temperature range is that the temperature (A_ThreeIf the temperature exceeds the point), the quenched martensite obtained in the first continuous annealing step becomes austenite, while the temperature (A₁This is because the desired retained austenite cannot be obtained when the temperature is lower than the point temperature. Furthermore, the control of the heating and holding time (t4) is particularly important for obtaining a desired structure. This is because if the time is less than 10 seconds, tempering is insufficient and a desired matrix structure (tempered martensite) cannot be obtained. Preferably it is 20 seconds or more, more preferably 30 seconds or more. However, if the heating and holding time exceeds 600 seconds, the lath-like structure that is characteristic of tempered martensite cannot be maintained, and the mechanical properties deteriorate. Preferably it is 500 seconds or less, more preferably 400 seconds or less.
[0076]
Next, the average cooling rate (CR2 in FIG. 3) is controlled to 3 ° C./sec or more (preferably 5 ° C./sec or more) to avoid the pearlite transformation, and the temperature from the Ms point to the Bs point (bainite transformation: In FIG. 3, it is cooled to T5), and then kept in this temperature range for 1 second or longer (preferably 5 seconds or longer: t5 in FIG. 3) (austempering). Thereby, C concentration to a retained austenite can be obtained in a large amount and in an extremely short time.
[0077]
Here, when the average cooling rate is below the above range, a desired structure cannot be obtained, and pearlite or the like is generated. The upper limit is not particularly defined, and the larger the better, the better. However, it is recommended to control appropriately in relation to the actual operation level.
[0078]
Among the above steps, the austempering temperature (T5) is particularly important for securing the desired structure and exerting the action of the present invention. When the temperature is controlled within the above temperature range (Ms point or more and Bs point or less), a large amount of retained austenite is obtained, and thereby, the TRIP effect due to retained austenite is exhibited. On the other hand, if it is less than Ms point, a martensite phase exists, and if it exceeds Bs point, a bainite phase increases in large quantities.
[0079]
The upper limit of the holding time (t5) is not particularly limited, but it is recommended to control the austenite to 3000 seconds or less, more preferably 2000 seconds or less, considering the time for austenite to transform into bainite.
[0080]
Here, the Bs point can be calculated from the following calculation formula.
Bs = 830−270 × [C] −90 × [Mn] −37 × [Ni] −70 × [Cr] −80 × [Mo]
In the formula, [] is mass% of each element.
[0081]
The plating conditions in this invention are not specifically limited, What is necessary is just to employ | adopt a well-known thing. Note that known alloying conditions may be employed, for example, about 550 ° C. × 60 sec. In this temperature range, the γ phase decomposes into cementite, but the present inventors have confirmed that the effect of the present invention is not impaired if this time is reached.
[0082]
As described above, the steel sheet manufactured through the hot rolling process, (optionally cold rolling process), the first continuous annealing process, and the second continuous annealing process or plating process satisfies the requirements of the present invention. In particular, the C content of the retained austenite varies. Therefore, unstable retained austenite having a relatively low C content can be easily transformed into martensite by applying strain, and the movable dislocation density between ferrite and the ferrite can be increased. Less than 50%). In addition, stable retained austenite having a relatively high C content hardly transforms even when strain is applied, and further, the retained austenite is present between the laths and is fine, so that it is stable against deformation and has ductility ( In particular, the total elongation) can be increased.
[0083]
Therefore, since the high-strength steel sheet of the present invention is excellent in workability and shape freezing property, it can be suitably used as a steel sheet that is used by being press-formed, for example, in an automobile or industrial machine.
[0084]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.
[0085]
Example 1: Examination of component composition
In this example, the influence of mechanical characteristics when the component composition was changed was examined. Specifically, test steels having the composition shown in Table 1 (units in the table are mass%) were melted in a converter to obtain experimental slabs, and each slab was heated at 1200 ° C. for 30 minutes. The steel sheet was hot-rolled at a finishing temperature (FDT) of 850 ° C. and then cooled to a coiling temperature (CT) of 600 ° C. at an average cooling rate of 30 ° C./sec to obtain a hot-rolled steel plate having a thickness of 2.5 mm.
[0086]
The obtained hot-rolled steel sheet was cold-rolled as it was or after pickling according to a conventional method to obtain a cold-rolled steel sheet having a thickness of 1.2 mm.
[0087]
The obtained cold-rolled steel sheet was processed with the pattern shown in FIG. 2 and FIG. Specifically, for the first continuous annealing step shown in FIG. 2, T1: 850 ° C., t1: 60 seconds, T2: 930 ° C, t2: 60 seconds, CR1: 50 ° C / sec, T3: room temperature. Yes, the second continuous annealing step shown in FIG. 3 is T4: 850 ° C., t4: 120 seconds, CR2: 25 ° C./sec, T5: 420 ° C., t5: 120 seconds. These manufacturing conditions satisfy the requirements of the present invention.
[0088]
In addition, in the present Example, although it showed about the case where 2nd continuous annealing was performed after 1st continuous annealing, this invention is not limited to this, Instead of 2nd continuous annealing Plating may be performed with the above pattern.
[0089]
About the steel plate obtained through the 1st continuous annealing process and the 2nd continuous annealing process, the existence rate of each structure | tissue was measured as shown next. First, the steel sheet was repeller-corroded and the structure was identified by observation with a transmission electron microscope (magnification: 15000 times), and then the area ratio of the structure was measured from the micrograph by observation with an optical microscope (magnification: 1000 times). Table 2 shows the area ratio of each tissue.
[0090]
Further, with respect to the obtained steel sheet, the ratio of retained austenite that transforms to martensite by applying 2% strain in the entire retained austenite was measured by the saturation magnetization measurement method described above. The applied magnetization (Hm) during measurement is 398000 A / m [5000 oersted (Oe)], and the measurement temperature is 23 ° C. Table 2 shows the amount of retained austenite (total γ_R) And 2% strain to transform the retained austenite (transformation γ) to martensite_R) And the ratio of residual austenite that transforms to martensite by applying 2% strain in the entire residual austenite.
[0091]
Next, a JIS No. 5 test piece was prepared from the steel sheet obtained through the first continuous annealing step and the second continuous annealing step, and tensile strength (TS) and elongation (total elongation: El) by a tensile test. , And yield strength (YP) were measured. The yield ratio (YR) was calculated from the measured YP and TS. Table 2 shows the calculation results. The strain rate in the tensile test was 1 mm / second. In addition, El passes 35% or more, and YR passes 50% or less.
[0092]
[Table 1]

[0093]
[Table 2]

[0094]
From these results, it can be considered as follows. In the following, “No.” means the experiment No. in Table 2.
[0095]
Nos. 2 to 4 and Nos. 7 to 15 are examples satisfying the components defined in the present invention, and a high-strength steel sheet having good total elongation (El) and low yield ratio (YR) is obtained. It was. Therefore, this high-strength steel plate is excellent in workability and shape freezing property.
[0096]
On the other hand, the following examples that do not satisfy any of the components defined in the present invention have the following problems.
[0097]
No. 1 is an example in which the C content is low, and the desired retained austenite cannot be obtained, so El (total elongation) is low.
[0098]
No. 5 is an example in which the total content of (Si + Al) is small, and El (total elongation) is low because desired retained austenite cannot be obtained.
[0099]
No. 6 is an example with a low Mn content, and the desired retained austenite cannot be obtained, so El (total elongation) is low.
[0100]
Example 2: Examination of manufacturing conditions
In this example, the steel No. 4 experimental slab in Table 1 was used, and after homogenization annealing at 1150 ° C. for 30 minutes and hot rolling at a finishing temperature of 850 ° C. to a sheet thickness of 3.2 mm Then, a cold rolled steel sheet having a thickness of 1.2 mm was obtained by cold rolling.
[0101]
The obtained cold-rolled steel sheet was processed with the pattern shown in FIG. 2 and FIG. Specific conditions are shown in Table 3.
[0102]
For the steel sheet obtained through the first continuous annealing step and the second continuous annealing step or the plating step, 2% strain is applied to the abundance of each structure and the entire retained austenite as in Example 1 above. The ratio of retained austenite that transforms to martensite was measured by Table 1, and the results are shown in Table 4. In addition, a tensile test was performed in the same manner as in Example 1, and the tensile strength (TS), elongation (total elongation: El), and yield strength (0.2% proof stress: YP) were measured, and the yield ratio ( YR) was calculated and the results are shown in Table 4.
[0103]
[Table 3]

[0104]
[Table 4]

[0105]
From these results, it can be considered as follows. In the following, “No.” means the experiment No. in Table 4.
[0106]
No. 16 and No. 31 are both examples satisfying the production conditions specified in the present invention, and the steel composition also satisfies the requirements of the present invention, so that the total elongation (El) and low yield are low. A high-strength steel sheet having a ratio (YR) was obtained. Therefore, this high-strength steel plate is excellent in workability and shape freezing property.
[0107]
On the other hand, the following examples that do not satisfy any of the manufacturing conditions defined in the present invention have the following problems.
[0108]
No. 17 is A in the first continuous annealing process._ThreeSince the heating and holding time (t2) at a temperature exceeding the point is too long, the uniform diffusion of C proceeds and the C content in the austenite becomes uniform. Therefore, since the C content in the finally obtained retained austenite becomes uniform, the proportion of stable retained austenite increases, and the proportion of retained austenite that transforms to martensite by applying 2% strain in the retained austenite is increased. Below the range of the present invention, the yield ratio becomes high.
[0109]
No. 18 is A in the first continuous annealing process._ThreeSince the heating and holding time (t2) at a temperature above the point is too short, A₁More than point A_ThreeFerrite formed at a temperature below the point does not become austenitic. Therefore, the amount of quenched martensite is reduced, and the amount of tempered martensite finally obtained is reduced, so that the total elongation is reduced and the yield ratio is increased.
[0110]
No. 19 is A in the first continuous annealing process.₁More than point A_ThreeIt is considered that the C content in the quenched martensite is uniform because the temperature is kept below the temperature below the point, and the C content in the residual austenite finally obtained is uniform. Therefore, the ratio of the retained austenite that transforms to martensite by applying 2% strain in the retained austenite falls below the range of the present invention, and the yield ratio becomes high.
[0111]
No. 20 is A in the first continuous annealing process.₁More than point A_ThreeDo not hold heated at a temperature below the point, A_ThreeThis is an example of heating and holding only at a temperature above the point. Therefore, the uniform diffusion of C proceeds, the C content in the austenite becomes uniform, and the C content in the residual austenite finally obtained becomes uniform. Therefore, the ratio of residual austenite that transforms to martensite by applying 2% strain in the residual austenite falls below the range of the present invention, and the yield ratio becomes high.
[0112]
No. 21 is an example simulating the method of the invention of the prior application, and A in the first continuous annealing step₁More than point A_ThreeHeated and held only at a temperature below the point, A_ThreeIt is not heated and maintained at a temperature above the point. Therefore, since quenching martensite is reduced, the final tempered martensite is also reduced. Further, since the C content in the retained austenite becomes uniform, the ratio of the retained austenite that transforms to martensite by applying 2% strain in the retained austenite is less than the range of the present invention, so that the yield ratio becomes high.
[0113]
No. 22 is A in the first continuous annealing process._ThreeSince the average cooling rate (CR1) from the temperature above the point is small, pearlite is generated, and the amount of tempered martensite finally obtained is reduced. Therefore, El (total elongation) is low and the yield ratio is also high.
[0114]
In No. 23, since the temperature (T3) after cooling in the first continuous annealing process exceeds the Ms point, tempered bainite is finally generated. Therefore, tempered martensite is not generated and the yield ratio is increased.
[0115]
In No. 24, the heating and holding temperature (T4) in the second continuous annealing process is A._ThreeSince it is higher than the point, eventually a lot of bainite is generated and tempered martensite is reduced. Accordingly, the elongation is reduced.
[0116]
No. 25 is A in the second continuous annealing process.₁More than point A_ThreeSince the heat holding time (t4) at a temperature below the point (T4) is too long, a large amount of ferrite is generated, and the amount of tempered martensite is reduced. Therefore, El (total elongation) is lowered. Moreover, since the ratio of the retained austenite which transforms to martensite by applying 2% strain in the retained austenite is below the requirement of the present invention, the yield ratio becomes high.
[0117]
No. 26 is A in the second continuous annealing process.₁More than point A_ThreeSince the heat holding time (t4) at a temperature below the point (T4) is too short, the amount of austenite produced is small and the residual austenite is small. Therefore, El (total elongation) is lowered. Further, since the heating and holding time (t4) is short, the temperature of the entire sample does not reach 850 ° C., and the amount of ferrite produced increases.
[0118]
No. 27 is A in the second continuous annealing process.₁More than point A_ThreeSince the average cooling rate (CR2) from the temperature below the point is small, a large amount of pearlite is generated. Therefore, the amount of tempered martensite finally obtained is reduced, the El (total elongation) is low, and the yield ratio is high.
[0119]
In No. 28, since the temperature (T5) after cooling in the second continuous annealing process is too high, a large amount of pearlite is generated. Therefore, the amount of tempered martensite finally obtained is reduced, the El (total elongation) is low, and the yield ratio is high.
[0120]
In No. 29, since the temperature (T5) after cooling in the second continuous annealing step is too low, the amount of retained austenite finally obtained is as very low as 1%, and El (total elongation) is low. In addition, the yield ratio is increased because there is little residual austenite that transforms to martensite by applying 2% strain in the residual austenite.
[0121]
No. 30 is A in the second continuous annealing process.₁More than point A_ThreeAfter heating and holding at a temperature below the point (T4), it is rapidly cooled from this temperature range to a temperature below the Ms point, so there is a lot of unstable retained austenite, and by adding 2% strain of the retained austenite, martensite Since the proportion of retained austenite that transforms into a large proportion increases, it becomes easy to break, the El (total elongation) is low, and the yield ratio is high.
[0122]
No. 32 is A in the first continuous annealing process.₁More than point A_ThreeSince the heating and holding time (t1) at a temperature below the point (T1) is too long, generation of an austenite phase with undissolved carbide as a nucleus proceeds, and this is_ThreeEven when heated to a temperature above the point, the variation in the C content in the austenite is considered to be small. Therefore, since the C content in the finally obtained retained austenite becomes uniform, the proportion of stable retained austenite increases, and the proportion of retained austenite that transforms to martensite by applying 2% strain in the retained austenite is increased. Below the range of the present invention, the yield ratio becomes high.
[0123]
【The invention's effect】
According to the present invention, even a high-strength and ultra-high-strength steel sheet of 500 to 1400 MPa class is excellent in workability and shape freezing property with a total elongation of 35% or more and a yield ratio of 50% or less. It was possible to provide a strength steel plate and a method capable of reliably manufacturing the steel plate.
[Brief description of the drawings]
FIG. 1 is a schematic explanatory diagram showing a part of an apparatus used for saturation magnetization measurement.
FIG. 2 is a diagram illustrating a first continuous annealing step.
FIG. 3 is a diagram illustrating a second continuous annealing step or a plating step.
[Explanation of symbols]
1 4πI coil for magnetization detection
2 Rod sample
3 H coil for magnetic field detection
4 conductors

Claims (6)

% By mass
C: 0.06 to 0.6%,
Contains both Si and Al, 0.5-3% in total ,
Mn: 0.5-3%,
P: 0.15% or less (excluding 0%),
S: each containing 0.02% or less (including 0%),
The balance consists of iron and inevitable impurities,
Tempered martensite: 15% or more in area ratio with respect to the whole structure,
Ferrite: 10 to 60% in area ratio with respect to the entire structure,
Residual austenite: 5% or more by volume with respect to the whole structure,
Furthermore, it has a structure that may contain bainite and / or martensite, and
A high-strength steel sheet excellent in workability and shape freezing property, characterized in that the ratio of residual austenite that transforms to martensite by applying 2% strain in the residual austenite is 20 to 50%. Furthermore, in mass%,
Mo: 1% or less (excluding 0%),
Ni: 0.5% or less (excluding 0%),
Cu: 0.5% or less (excluding 0%),
Cr: 1% or less (excluding 0%)
The high-strength steel sheet according to claim 1, which contains at least one of the following. Furthermore, in mass%,
Ti: 0.1% or less (excluding 0%),
Nb: 0.1% or less (excluding 0%),
V: 0.1% or less (excluding 0%)
The high-strength steel plate according to claim 2, which contains at least one of the following. Furthermore, in mass%,
Ca: 0.003% or less (excluding 0%), and / or
REM: 0.003% or less (excluding 0%)
The high-strength steel sheet according to claim 2 or 3, comprising: A method for producing the high-strength steel sheet according to any one of claims 1 to 4,
Including at least a hot rolling step, a first continuous annealing step, and a second continuous annealing step or plating step,
The first continuous annealing step is
A step of heating and holding at a temperature of A ₁ point or more and A ₃ point or less for 10 to 300 seconds;
A step of heating and holding at a temperature exceeding A ₃ point for 10 to 300 seconds; and a step of cooling to a temperature below the Ms point at an average cooling rate of 30 ° C./sec or more,
The second continuous annealing step or plating step is
A step of heating and holding at a temperature of A ₁ point or more and A ₃ point or less for 10 to 600 seconds;
Workability and shape freezing characteristics, including a step of cooling to a temperature of not less than Ms point and not more than Bs point at an average cooling rate of 3 ° C./sec or more; and subsequently maintaining in that temperature range for 1 second or more. A high-strength steel sheet with excellent strength. The manufacturing method of the high strength steel plate of Claim 5 including a cold rolling process prior to a 1st continuous annealing process after the said hot rolling process.

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