JP3854506B2 - High strength steel plate excellent in weldability, hole expansibility and ductility, and manufacturing method thereof - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、建材、家電製品、自動車などに適する溶接性、穴拡げ性および延性に優れ、引張強度が８００MPa以上の高強度鋼板およびその製造方法に関する。
【０００２】
【従来の技術】
近年、特に自動車車体において燃費向上や耐久性向上の観点を目的とした加工性の良い高強度鋼板の需要が高まっている。加えて、衝突安全性やキャビンスペースの拡大のニーズから引張り強度にして８００MPa級クラス以上の鋼板が、一部レインフォースなどの部材に使用されつつある。
このような高強度材を用いて部材を組みあげる時には、延性、曲げ性、穴拡げ性や溶接性などが、引張り強度で５９０ＭＰａ程度までの高強度鋼板以上に大きな問題となるため、これらに対する対策が必要となる。
各特性に対して、以下のような対策が各々講じられている。
【０００３】
たとえば、穴拡げ性については、CAMP-ISIJ vol.13 (2000) p.395にあるように、主相をベイナイトととして穴拡げ性を向上させ、さらには張り出し性形成性についても、第2相に残留オーステナイトを生成させることで現行の残留オーステナイト鋼並の張り出し性を示すことが開示されている。さらには、Ｍｓ温度以下でオーステンパ処理をすることで面積率２〜３％の残留オーステナイトを生成させると、引張り強度×穴拡率が最大となることも示されている。しかし、８００ＭＰａを超えて顕在化する溶接性および溶接熱影響部での軟化挙動については考慮されていない。
また、溶接性については、溶接熱影響部における軟化挙動（HAZ軟化挙動）が問題視されるケースが多い。これに対して、例えば特開2000−87175にあるようにNbおよびMoの炭化物（Ｎｂ，Ｍｏ）Ｃの析出によりHAZ軟化挙動を抑制することが示されている。しかし、この技術は、疲労強度に関して考慮されているものの穴拡げ性等の加工性について十分な考慮はない。また、HAZ軟化挙動を抑制の効果も強度レベルが低く、８００ＭＰａ以上の極めて高強度な材料における溶接性や加工性について十分とはいえない。特に、引張り強度が８００ＭＰa以上になると、溶接自体が困難になり、９８０ＭＰａ以上でさらに顕著となる。このため、スッポト溶接等の従来の溶接方法に加えてレーザー溶接なども一部適用される例もある。しかし、高強度故母材は特に溶接部および熱影響部での材質変動が５９０ＭＰaクラスの高強度材に比べ極めて顕著となる。また、高強度化にマルテンサイトの活用は穴拡げ性や延性低下が助長されてしまう。
【０００４】
【発明が解決しようとする課題】
本発明は、上記課題を解決し、引張り強度が８００ＭＰａ以上の高強度鋼板の溶接性、穴拡げ性および延性を同時に改善した高強度鋼板およびその製造方法を提供することを目的とする。
【０００５】
【課題を解決するための手段】
本発明者らは、種々検討を行った結果、引張り強度を８００ＭＰａ以上の領域で、溶接性、穴拡げ性および延性を同時に改善する手法として、ミクロ組織およびその硬度差などを規定することで、さらには、成分範囲や（Ａ）および（Ｂ）式による限定を行うことで、８００ＭＰａ以上の高強度を保ちつつ溶接熱影響部の軟化挙動を抑制して、さらには、穴拡げ性および延性を確保できることを見出した。
本発明は、上記知見に基づいて完成されたもので、その要旨とするところは以下の通りである。
【０００６】
（１）質量％で、
Ｃ ：０．０１〜０．３％、
Ｓｉ：０．００５〜２．５％、
Ｍｎ：０．０１〜３％、
Ｐ：０．００１０〜０．１％、
Ｓ：０．００１０〜０．０５％、
Ａｌ：０．００５〜２％、
Ｍｏ：０．０１〜０．３％、
Ｎｂ：０．００１〜０．１％を下記（Ａ）式を満たす範囲で含有し、
残部はＦｅおよび不可避的不純物からなり、
ミクロ組織が、主相をベイナイトまたはベイニティックフェライトとして面積率で合計５０〜９７％含有し、第２相をオーステナイトとして面積率で３〜５０％含有し、残部をフェライトまたはマルテンサイトとし、
前記ミクロ組織におけるオーステナイトの面積率（Ｖγ）とマルテンサイトの面積率（ＶαM）とが、下記（Ｂ）を満足し、
引張強度が８００MPa以上であることを特徴とする溶接性、穴拡げ性および延性に優れた高強度鋼板。
（3.0Nb+2.5Mo+2/3Si+Mn）−（2.3C0.5+1.80）＞０・・・（Ａ）
Ｖγ ／ ＶαM ＞ ２ ・・・（Ｂ）
（２）ミクロ組織における第２相と主相との硬度比である第２相の硬度／主相の硬度の値が０．５〜１．５であることを特徴とする（１）に記載の溶接性、穴拡げ性および延性に優れた高強度鋼板。
【０００７】
（３）さらに、質量％で、
Ｃｒ：０．０１〜５％、
Ｎｉ：０．０１〜５％、
Ｃｕ：０．０１〜５％、
Ｃｏ：０．０１〜５％、
Ｗ ：０．０１〜５％の１種または２種以上を含有することを特徴とする（１）または（２）に記載の溶接性、穴拡げ性および延性に優れた高強度鋼板。
（４）さらに、質量％で、
Zr、Hf、Ta、Ｔｉ、Ｖの１種または２種以上を合計で０．００１〜１％含有することを特徴とする（１）乃至（３）のいずれかに記載の溶接性、穴拡げ性および延性に優れた高強度鋼板。
【０００８】
（５）さらに、質量％で、Ｂ：０．０００１〜０．１％を含有することを特徴とする（１）乃至（４）のいずれかに記載の溶接性、穴拡げ性および延性に優れた高強度鋼板。
（６）さらに、質量％で、Ｃａ、Ｙ、Remの１種または２種以上を合計で０．００１〜０．５％含有することを特徴とする（１）乃至（５）のいずれかに記載の溶接性、穴拡げ性および延性に優れた高強度鋼板。
【０００９】
（７）（１）乃至（６）のいずれかに記載の成分からなる鋳造スラブを直接または一旦冷却した後に再度加熱し、熱延後巻取った熱延鋼板を酸洗後冷延し、その後焼鈍時の最高温度が０．８×（Ａｃ3 −Ａｃ1 ）＋Ａｃ1 （℃）以上Ａｃ3 ＋３０（℃）以下で焼鈍した後に、３〜１５０℃／秒の冷却速度で２００〜４５０℃の温度域に冷却し、引き続いて同温度域で１秒〜３０００秒保持することを特徴とする溶接性、穴拡げ性および延性に優れた高強度鋼板の製造方法。
【００１０】
【発明の実施の形態】
以下、本発明を詳細に説明する。
発明者らは、質量％で、Ｃ ：０．０１〜０．３％、Ｓｉ：０．００５〜２．５％、Ｍｎ：０．０１〜３％、Ｐ：０．００１０〜０．１％、Ｓ：０．００１０〜０．００５％、Ａｌ：０．００５〜２％を含有し、残部Ｆｅおよび不可避不純物からなる鋼板をベースに、各合金を添加した溶製し、鋳造まま又は一旦冷却した後に再度加熱し、熱延後巻取った熱延鋼板を酸洗後冷延し、その後焼鈍し、冷延焼鈍板を作成した。その鋼板について、ミクロ組織観察、鉄鋼連盟規定の穴拡げ試験、JISに準拠した引張り試験、鋼板をつきあわせてレーザー溶接を行い、その後球頭張り出し試験を行い、各特性を比較評価した。
その結果、最終的に得られるミクロ組織制御によりにおいて、８００ＭＰａ以上の引張り強度を得、溶接性、穴拡げ性および延性に優れた高強度鋼板が製造可能なことを見出した。
【００１１】
次に、基材鋼板の好ましいミクロ組織について述べる。
穴拡げ性を十分に確保するためには主組織をベイナイトまたはベイニティックフェライトとするのが有効で、面積分率で５０％以上含むことが望ましい。また、ここで言うベイナイトはラス境界に炭化物が生成しているいわゆる上部ベイナイトおよびラス内に微細炭化物が生成している下部ベイナイトの双方を含む。また、ベイニティックフェライトは炭化物のないベイナイトことを意味し、例えばアキュラーフェライトがその1例である。穴拡げ性向上には、炭化物が微細分散している下部ベイナイトもしくは炭化物の無いベイニティックフェライトで主相が構成されることが望ましい。しかし、この場合には延性の確保や溶接性、特に溶接熱影響部での軟化防止が問題となる。高延性化を指向する場合には第2相として、オーステナイト相を、面積率で３％以上残留させることが有効である。また、残部組織の一部としてポリゴナルフェライトを４０％以下の範囲で含んでも、引張り強度が８００ＭＰａ以上を確保しうる場合があり、この場合には本願発明の範囲とし、第２相はポリゴナルフェライトではなく、残留オーステナイトとする。また、溶接性については、後述するように成分の関係式およびオーステナイトとマルテンサイトの相率を規定した（Ａ）式および（Ｂ）式を満たすことで、高強度材の溶接性を確保するものとした。
【００１２】
高強度化の観点からすると、オーステナイトに加えてマルテンサイトを含んでも良い。しかしながら、マルテンサイトや安定化されたオーステナイトを含む場合には、穴拡げ性や溶接熱影響部の軟化挙動を助長する傾向にあることから、主相と第2相の硬度比：第２相の硬度／主相の硬度を０．５〜１．５の範囲とすること、また、鋼材成分で式（Ａ）を満たすこと、さらには、ミクロ組織における第2相の面積率に関して、オーステナイトの面積率をＶγ、マルテンサイトの面積率をＶαMとした時（Ｂ）式を満たすこととした。硬度比が０．５未満であったり、１．５を超えると穴拡げ性や延性が低下することに加えて、溶接熱影響部分の軟化が顕著になる。なお、硬度の測定はマイクロビッカース硬度計を使用し、組織の大きさに合わせて１〜１００ｇの荷重を用いて測定した。
また、式（Ａ）を満足しない場合には、引張り強度で８００ＭＰａ以上を確保できず、溶接熱影響部分の軟化を抑制できないことに加えて穴拡げ性および延性の確保も困難となる。また、式（Ｂ）を満たさずに、オーステナイトが少なく、マルテンサイト量が多くなると、強度は高くなるものの穴拡げ性および延性が低下する。また、特にマルテンサイト量が多くなると、穴拡げ性および延性が低下する傾向が顕著になることに加えて、溶接熱影響部分の軟化を抑制できなくなる。
【００１３】
（3.0Nb+2.5Mo+2/3Si+Mn）−（2.3C0.5+1.80）＞ ０・・・（Ａ）
Ｖγ ／ ＶαM ＞ ２ ・・・（Ｂ）
また、上記の他にミクロ組織の残部組織として、炭化物、窒化物、硫化物、酸化物などの１又は２以上を面積分率１％以下で含有する場合も本発明で用いることができる。なお、上記ミクロ組織の各相、フェライト（ベイニティックフェライト）、ベイナイト、オーステナイト、マルテンサイト、界面酸化相および残部組織の同定、存在位置の観察および占積率の測定は、ナイタール試薬および特開昭５９−２１９４７３号公報に開示された試薬により鋼板圧延方向断面または圧延直角方向断面を腐食して５００倍〜１０００倍の光学顕微鏡観察および１０００〜１０００００倍の電子顕微鏡（走査型および透過型）により定量化が可能である。各２０視野以上の観察を行い、ポイントカウント法や画像解析により各組織の面積率を求めることができる。
【００１４】
なお、ミクロ組織の各相の合計は１００％となるが、炭化物、酸化物、硫化物等の光学顕微鏡では観察・同定ができない相については主相の面積率に含めている。
次に、本発明における鋼板成分の好適な範囲の限定理由について述べる。
Ｃは、良好な強度延性バランスを確保するための主相および第２相の分率を制御する目的で添加する元素である。特に第２相がオーステナイトである場合には、面積分率のみならずその安定性向上にも寄与して延性を大きく向上させる。また、素地の微細均一化についても影響を与える。強度および各第２相の面積分率を確保するために下限を０．０１質量％（以下、同じ）とし、溶接性および穴拡げ性を保持可能な上限として０．３％とした。
【００１５】
Ｓｉは、強度延性バランスを劣化させる比較的粗大な炭化物の生成を抑制する目的で添加する元素であり、その下限を０．００５質量％とした。また、過剰添加は溶接性およびに悪影響を及ぼすため、上限を２．５質量％とした。
Ｍｎは、高強度化の目的で添加する。また、強度低下と延性劣化の１つの原因である炭化物析出やパーライト生成を抑制する目的で添加する。これらのことから、０．０１質量％以上とした。一方では、延性向上に寄与するベイナイト変態を遅滞させることや溶接性を劣化させることから３質量％を上限とした。
【００１６】
Ｐ量は強化元素であり極低化は経済的にも不利であることから０．００１０質量％を下限とした。また、多量添加では、溶接性や鋳造時や熱延時の製造性に悪影響を及ぼすため、０．１％を上限とした。
Ｓ量は、極低化は経済的に不利であることから、０．００１０質量％を下限とし、また、０．１質量％を上限としたのは、これを超える量の添加では、溶接性や鋳造時や熱延時の製造性に悪影響を及ぼすためである。
Ａｌは、脱酸元素として添加する。また、延性向上特に第２相がオーステナイトの場合に延性向上に寄与するベイナイト変態を促進させる効果があり、強度延性バランスを向上させる。このため、０．００５質量％以上の添加とした。一方過剰添加は溶接性およびめっき濡れ性を損なうため２％を上限とした。
【００１７】
Ｍｏは、強度延性バランスを劣化させる炭化物やパーライトの生成を抑制する目的で添加できる元素であり、良好な強度延性バランスを得るために重要な添加元素である。さらには、溶接熱影響部の軟化防止にも効果的であることから、その下限を０．０１質量％とした。また、過剰添加は、延性劣化を招くことから、上限を５％とした。
Nbは、微細な炭化物、窒化物または炭窒化物を形成して、鋼板の強化に極めて有効であること、ベイナイトおよびベイニティックフェライトの生成を助長する。また、溶接熱影響部の軟化抑制にも効果的であること、０．００１質量％以上の添加とした。一方で、過剰添加は、延性劣化や残留オーステナイト中へのＣの濃化を阻害することから、上限として０．１質量％とした。
【００１８】
また、８００ＭＰａ以上の強度レベルで、溶接性、穴拡げ性および延性の各特性をバランス良く、制御するためには、式（Ａ）を満足することとした。
（3.0Nb+2.5Mo+2/3Si+Mn）−（2.3C0.5+1.80）＞０・・・（Ａ）
さらに、本発明が対象とする鋼は、強度のさらなる向上を目的としてＣｒ、Ｎｉ、Ｃｕ、Ｃｏ、Ｗの１種または２種以上を含有できる。
Ｃｒは、強化目的および炭化物生成の抑制とベイナイトおよびベイニティックフェライト生成の目的から添加する元素で、０．０１％以上とし、５％を超える量の添加では、加工性に悪影響を及ぼすため、これを上限とした。
【００１９】
Ｎｉは、焼き入れ性の向上による強化目的で０．０１質量％以上とし、５質量％を超える量の添加では、加工性、特にマルテンサイトの硬度上昇寄与して悪影響を及ぼすため、これを上限とした。
Ｃｕは、強化目的で０．０１質量％以上の添加とし、５質量％を超える量の添加では、加工性および製造性に悪影響を及ぼす。
Ｃｏは、ベイナイト変態制御による強度延性バランスの向上のため、０．０１質量％以上の添加とした。一方、添加の上限は特に設けないが、高価な元素であるため多量添加は経済性を損なうため、５質量％以下にすることが望ましい。
【００２０】
Ｗは、０．０１質量％以上で強化効果が現れること、５質量％を上限としたのは、これを超える量の添加では、加工性に悪影響を及ぼすからである。
さらに、本発明が対象とする鋼は、強度のさらなる向上を目的として強炭化物形成元素であるＺｒ、Ｈｆ、Ta、Ti、Ｖの１種または２種を含有できる。これらの元素は、微細な炭化物、窒化物または炭窒化物を形成して、鋼板の強化にとって極めて有効であるため、必要に応じて１種または２種以上を合計で０．００１質量％以上の添加とした。一方で、延性劣化や残留オーステナイト中へのＣの濃化を阻害することから、１種または２種以上の合計添加量の上限として１質量％とした。
【００２１】
Ｂもまた、必要に応じて添加できる。Ｂは、０．０００１質量％以上の添加で粒界の強化や鋼材の高強度化に有効ではあるが、その添加量が０．１質量％を超えるとその効果が飽和するばかりでなく、加工性が低下するため、上限を０．１質量％とした。
Ｃａ、Ｙ、Ｒｅｍは、適量添加により介在物の形態制御、特に微細分散化の観点から０．００１％以上とし、一方で過剰添加は鋳造性や熱間加工性などの製造性および鋼板製品の延性を低下させるため０．５質量％を上限とした。
【００２２】
不可避的不純物として、例えばＮやＳｎなどがあるがこれら元素を０．０２質量％以下の範囲で含有しても本発明の効果を損なうものではない。
このような組織を有する溶接性、穴拡げ性および延性に優れた高強度鋼板およびの製造方法について以下に説明する。
熱延後冷延・焼鈍して本発明の鋼板を製造する場合には、所定の成分に調整されたスラブを鋳造ままもしくは一旦冷却した後再加熱して熱延を行う。このときの再加熱温度は１１００℃以上１３００℃以下とすることが望ましい。再加熱温度が高温になると粗粒化や厚い酸化スケールが形成され、一方、低温加熱では圧延抵抗が高くなってしまう。また熱延後は、高圧デスケーリング装置や酸洗することなどで表面スケール削除を行うと製品での表面清浄がよくなり、めっきを施す場合などに有利である。その後、冷延後焼鈍することで最終製品とする。また、電気めっきや溶融亜鉛めっき、溶融合金亜鉛めっきを施しても本願発明を阻害するものではない。また、熱延完了温度は鋼の化学成分によって決まるＡｒ3 変態温度以上で行うのが一般的であるが、Ａｒ3 から１０℃程度低温までであれば最終的な鋼板の特性を劣化させない。また、冷却後の巻取温度は鋼の化学成分によって決まるベイナイト変態開始温度以上とすることで、冷延時の荷重を必要以上に高めることがさけられるが、冷延の全圧下率が小さい場合にはこの限りでなく、鋼のベイナイト変態温度以下で巻き取られても最終的な鋼板の特性を劣化させない。また、冷延の全圧下率は、最終板厚と冷延荷重の関係から設定されるが、４０％以上であれば再結晶させるには十分で、最終的な鋼板の特性を劣化させない。
【００２３】
冷延後焼鈍する際に、焼鈍温度が鋼の化学成分によって決まる温度Ａｃ1 およびＡｃ3 温度（例えば「鉄鋼材料学」：Ｗ．Ｃ．Ｌｅｓｌｉｅ著、幸田成康監訳、丸善Ｐ２７３）で、表現される０．８×（Ａｃ3−Ａｃ1 ）＋Ａｃ1 （℃）未満の場合には、焼鈍温度で得られるオーステナイト量が少ないので、最終的な鋼板中に主にベイナイトまたはベイニティックフェライトを生成させることができない。また、第２相として残留オーステナイト相またはマルテンサイト相を十分な量、残すことができないためにこれを焼鈍温度の下限とした。また、焼鈍温度が高温となるほど結晶粒の粗大化や表面酸化が促進されるうえ、製造コストの上昇をまねくために、焼鈍温度の上限をＡｃ3 ＋30（℃）とした。この温度域での焼鈍時間は鋼板の温度均一化とオーステナイトの確保のために１０秒以上が必要である。しかし、３０分超では、粒界酸化相生成が促進されるうえ、コストの上昇を招く。
【００２４】
その後の一次冷却はオーステナイト相からフェライト相への変態をある程度抑しつつ、ベイナイトまたはベイニティックフェライトを生成させ、さらに未変態のオーステナイト相中にＣを濃化させてオーステナイトの安定化をはかるのに重要である。この冷却速度を３℃／秒未満にすることは、フェライトやパーライトの生成を促進して強度低下を招く懸念があることから、冷却速度の下限を３℃／秒とした。一方、冷却速度が１５０℃／秒超の場合には最終的な鋼板中のマルテンサイト相などの硬質相が多量になってしまうことや、操業上困難なため、これを上限とした。
この一次冷却が２００℃未満まで行われると、冷却中にマルテンサイトが多量に生成して、穴拡げ性や遅れ破壊を助長するため、冷却停止温度は３５０℃以上とした。また、冷却停止温度が４５０℃を超えると、その後の保持時に炭化物が短時間で生成してしまい、強度低下を招くため、これを上限とした。また、次にオーステナイトの安定化やマルテンサイトの硬度低下のため、この温度域での保持を行う。この停留時間が長時間になると生産性上好ましくないうえ、炭化物が生成してしまうことから３０００秒以内とすることが望ましい。また、鋼板中に残留しているオーステナイト相を室温で安定にするためには、その一部をベイナイト相へ変態させる事でオーステナイト中の炭素濃度を更に高めることが必須であることから、１秒以上保持し、好ましくは１５秒から２０分保持することが望ましい。２００℃未満ではベイナイト変態が起こりにくく、４５０℃を超えると炭化物が生じて十分な残留オーステナイト相を残すことが困難となる。
また、溶接方法については、通常行われる溶接方法、たとえばアーク、TIG, MIG、マッシュおよびレーザー等の溶接を行っても本願の範囲とする。
【００２５】
【実施例】
以下、実施例によって本発明をさらに詳細に説明する。
表１に示すような組成（質量％）の鋼板を、１２００℃に加熱し、Ａｒ3 変態温度以上で熱延を完了し、冷却後各鋼の化学成分で決まるベイナイト変態開始温度以上で巻き取った鋼帯を酸洗後、冷延して１．２ｍｍ厚とした。
その後、各鋼の成分（質量％）から下記式にしたがってＡｃ1 とＡｃ3 変態温度を計算により求めた。
【００２６】
これらの鋼板からJIS５号引張り試験片を採取して、機械的性質を測定した。さらに、鉄鋼連盟規格に準拠して穴拡げ試験を行い、穴拡げ率を求めた。溶接性については鋼板をつきあわせたレーザー溶接を行い、樹脂シート潤滑にて球頭張り出し試験を行い、母材に対する張り出し高さおよび破断位置を測定した。
表２にミクロ組織と各材質について、また表3に各製造条件と材質について示す。本願発明の要綱を満たす発明鋼は、溶接性、延性、強度（引張り強度で８００ＭＰａ以上）、穴拡げ性に優れていることがわかる。
一方、比較例はミクロ組織および式（Ａ）および（Ｂ）を同時に満たさないため、材質特性に劣る。例えば、比較例Ａ−３、Ｉ−３およびＣＥはミクロ組織の規定を見たさないため穴拡げ率；λが低い。また、式（Ａ）および（Ｂ）を同時に満たさないため溶接継ぎ手の球頭張り出し高さ比；Ｒが低い。 図1および２に、溶接継ぎ手の球頭張り出し高さ比；Ｒ、引っ張り強度；ＴＳ／ＭＰａ、伸び；Ｅｌ．／％および穴拡げ率；λを掛け合わせた値、すなわち、本願発明の特徴である溶接性と強度延性バランスおよび穴拡が性を同時に満たす時に高い値を示すパラメータと式（Ａ）および（Ｂ）の関連を示す。発明鋼は両式を同時に満たすためこのパラメータが高い値を示す。比較例は、何れかまたは両方を満たしていないため低い値を示す。
【００２７】
【発明の効果】
本発明により、引張り強度が８００ＭＰａ以上の高強度鋼板の溶接性、穴拡げ性および延性を同時に改善した高強度鋼板およびその製造方法を得ることができる。
【表１】
【図面の簡単な説明】
【図１】 Ｒ×ＴＳ×Ｅｌ×λと（Ａ）式の左辺の値との関係を示す図である。
【図２】 Ｒ×ＴＳ×Ｅｌ×λと（Ｂ）式の左辺の値との関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength steel sheet having excellent weldability, hole expansibility and ductility suitable for building materials, home appliances, automobiles, etc., and a tensile strength of 800 MPa or more, and a method for producing the same.
[0002]
[Prior art]
In recent years, there has been an increasing demand for high-strength steel sheets with good workability for the purpose of improving fuel efficiency and durability particularly in automobile bodies. In addition, steel plates with a tensile strength of 800 MPa class or higher are being used for some parts such as reinforcements due to the need for collision safety and increased cabin space.
When assembling members using such high-strength materials, ductility, bendability, hole expansibility, weldability, etc. become a major problem compared to high-strength steel sheets with a tensile strength of up to about 590 MPa. Is required.
The following measures are taken for each characteristic.
[0003]
For example, with regard to hole expansibility, as shown in CAMP-ISIJ vol.13 (2000) p.395, the main phase is bainite to improve hole expansibility, and the overhanging formability is also the second phase. It is disclosed that by forming retained austenite, the present steel exhibits the same stretchability as that of the present retained austenitic steel. Furthermore, it is also shown that when retained austenite having an area ratio of 2 to 3% is generated by austempering at a temperature equal to or lower than the Ms temperature, the tensile strength × the hole expansion ratio is maximized. However, the weldability that manifests above 800 MPa and the softening behavior in the weld heat affected zone are not considered.
As for weldability, the softening behavior (HAZ softening behavior) in the weld heat affected zone is often regarded as a problem. On the other hand, for example, as disclosed in JP-A-2000-87175, it is shown that the HAZ softening behavior is suppressed by precipitation of carbides (Nb, Mo) C of Nb and Mo. However, although this technique is considered with respect to fatigue strength, it does not sufficiently consider workability such as hole expansibility. In addition, the effect of suppressing the HAZ softening behavior is also low in strength level, and it cannot be said that the weldability and workability in an extremely high strength material of 800 MPa or more are sufficient. In particular, when the tensile strength is 800 MPa or more, welding itself becomes difficult, and becomes more remarkable at 980 MPa or more. For this reason, in addition to the conventional welding methods such as spot welding, there is an example in which laser welding or the like is partially applied. However, in the case of a high-strength base metal, the material fluctuation particularly in the welded portion and the heat-affected zone is extremely remarkable as compared with a 590 MPa class high-strength material. In addition, the use of martensite for increasing the strength promotes hole expandability and ductility degradation.
[0004]
[Problems to be solved by the invention]
An object of the present invention is to solve the above-mentioned problems and to provide a high-strength steel sheet that simultaneously improves the weldability, hole expansibility and ductility of a high-strength steel sheet having a tensile strength of 800 MPa or more, and a method for producing the same.
[0005]
[Means for Solving the Problems]
As a result of various studies, the inventors have defined a microstructure and a hardness difference thereof as a technique for simultaneously improving weldability, hole expansibility, and ductility in a tensile strength of 800 MPa or more, Furthermore, by limiting by the component range and the formulas (A) and (B), the softening behavior of the weld heat affected zone is suppressed while maintaining a high strength of 800 MPa or more, and further, the hole expandability and ductility are reduced. It was found that it can be secured.
The present invention has been completed based on the above findings, and the gist thereof is as follows.
[0006]
(1) In mass%,
C: 0.01 to 0.3%
Si: 0.005 to 2.5%,
Mn: 0.01 to 3%
P: 0.0010 to 0.1%,
S: 0.0010 to 0.05%,
Al: 0.005 to 2%,
Mo: 0.01 to 0.3%,
Nb: 0.001 to 0.1% is contained in a range that satisfies the following formula (A),
The balance consists of Fe and inevitable impurities,
The microstructure contains the main phase as bainite or bainitic ferrite in a total area of 50 to 97%, the second phase as austenite in an area ratio of 3 to 50%, and the balance as ferrite or martensite,
The area ratio (Vγ) of austenite and the area ratio (VαM) of martensite in the microstructure satisfy the following (B):
A high-strength steel sheet excellent in weldability, hole expansibility and ductility, characterized by a tensile strength of 800 MPa or more.
(3.0Nb + 2.5Mo + 2 / 3Si + Mn)-(2.3C 0.5 +1.80)> 0 ... (A)
Vγ / VαM> 2 (B)
(2) The hardness of the second phase / the hardness of the main phase, which is the hardness ratio between the second phase and the main phase in the microstructure, is 0.5 to 1.5. High-strength steel sheet with excellent weldability, hole expansibility and ductility.
[0007]
(3) Furthermore, in mass%,
Cr: 0.01-5%
Ni: 0.01 to 5%,
Cu: 0.01 to 5%,
Co: 0.01-5%
W: High-strength steel sheet excellent in weldability, hole expansibility and ductility according to (1) or (2) , characterized by containing one or more of 0.01 to 5%.
(4) Furthermore, in mass%,
The weldability and hole expansion according to any one of (1) to (3), wherein one or more of Zr, Hf, Ta, Ti, and V are contained in a total amount of 0.001 to 1%. High-strength steel sheet with excellent properties and ductility.
[0008]
(5) Furthermore, it is excellent in weldability, hole expansibility, and ductility according to any one of ( 1) to (4), characterized by containing B: 0.0001 to 0.1% by mass%. High strength steel plate.
(6) Further, any one of (1) to (5) is characterized by containing 0.001 to 0.5% in total of one or more of Ca, Y, and Rem in mass%. High strength steel plate with excellent weldability, hole expansibility and ductility.
[0009]
(7) The cast slab composed of the component according to any one of (1) to (6) is directly or once cooled and then heated again, and the hot-rolled steel sheet wound after hot rolling is pickled and cold-rolled, and then The maximum temperature during annealing is 0.8 × (Ac 3 -Ac 1 ) + Ac 1 (° C.) or more and Ac 3 +30 (° C.) or less, and then the annealing temperature is 200 to 450 ° C. at a cooling rate of 3 to 150 ° C./second. A method for producing a high-strength steel sheet excellent in weldability, hole expansibility and ductility, characterized by cooling to a temperature range and subsequently maintaining the same temperature range for 1 second to 3000 seconds.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
The inventors, in mass%, C: 0.01 to 0.3%, Si: 0.005 to 2.5%, Mn: 0.01 to 3%, P: 0.0010 to 0.1% , S: 0.0010 to 0.005%, Al: 0.005 to 2%, based on a steel plate composed of the remaining Fe and inevitable impurities, melted by adding each alloy, as cast or once cooled Then, it was heated again, and the hot rolled steel sheet wound after hot rolling was pickled, cold rolled, and then annealed to prepare a cold rolled annealed sheet. The steel sheet was subjected to microstructural observation, hole enlargement test stipulated by the Federation of Iron and Steel, tensile test based on JIS, laser welding with the steel sheet, followed by ball head overhang test and comparative evaluation of each characteristic.
As a result, it has been found that a tensile strength of 800 MPa or more can be obtained by the microstructure control finally obtained, and a high-strength steel sheet excellent in weldability, hole expansibility and ductility can be produced.
[0011]
Next, a preferable microstructure of the base steel sheet will be described.
In order to sufficiently ensure the hole expandability, it is effective to use bainite or bainitic ferrite as the main structure, and it is desirable that the area fraction is 50% or more. Moreover, the bainite said here includes both the so-called upper bainite in which carbide is generated at the lath boundary and the lower bainite in which fine carbide is generated in the lath. Bainitic ferrite means bainite without carbides, for example, acicular ferrite is one example. In order to improve the hole expansibility, it is desirable that the main phase is composed of lower bainite in which carbide is finely dispersed or bainitic ferrite without carbide. However, in this case, securing ductility and weldability, particularly prevention of softening in the heat affected zone is a problem. When aiming for high ductility, it is effective to leave the austenite phase at 3% or more in area ratio as the second phase. Further, even if polygonal ferrite is included in a range of 40% or less as a part of the remaining structure, the tensile strength may be able to ensure 800 MPa or more. In this case, the scope of the present invention is set, and the second phase is polygonal. Residual austenite, not ferrite. As for the weldability, as will be described later, by satisfying the formulas (A) and (B) that define the relational expression of the components and the phase ratio between austenite and martensite, the weldability of the high strength material is ensured. It was.
[0012]
From the viewpoint of increasing the strength, martensite may be included in addition to austenite. However, when it contains martensite and stabilized austenite, it tends to promote the hole expansion and softening behavior of the heat affected zone, so the hardness ratio of the main phase to the second phase: the second phase The area of austenite with respect to the hardness / main phase hardness being in the range of 0.5 to 1.5, and satisfying the formula (A) with the steel components, and the area ratio of the second phase in the microstructure When the rate is Vγ and the area ratio of martensite is VαM, the formula (B) is satisfied. When the hardness ratio is less than 0.5 or exceeds 1.5, hole expandability and ductility are lowered, and softening of the weld heat-affected portion becomes remarkable. The hardness was measured using a micro Vickers hardness meter and using a load of 1 to 100 g according to the size of the structure.
Moreover, when not satisfy | filling Formula (A), 800 MPa or more cannot be ensured by tensile strength, and also it becomes difficult to ensure hole expansibility and ductility in addition to not being able to suppress softening of a welding heat influence part. Moreover, when not satisfy | filling Formula (B) and there are few austenites and the amount of martensite increases, although a strength will become high, the hole expansibility and ductility will fall. In particular, when the amount of martensite increases, the tendency of the hole expandability and ductility to decrease becomes remarkable, and softening of the weld heat affected zone cannot be suppressed.
[0013]
(3.0Nb + 2.5Mo + 2 / 3Si + Mn)-(2.3C 0.5 +1.80)> 0 ... (A)
Vγ / VαM> 2 (B)
In addition to the above, the case where one or more of carbides, nitrides, sulfides, oxides, and the like are contained as the remaining microstructure of the microstructure with an area fraction of 1% or less can also be used in the present invention. In addition, each phase of the above microstructure, ferrite (bainitic ferrite), bainite, austenite, martensite, interfacial oxidation phase and remaining structure, identification of the existing position, and measurement of the space factor are measured by Nital reagent and JP The steel plate rolling direction cross section or the rolling perpendicular direction cross section is corroded with the reagent disclosed in Japanese Patent Publication No. 59-219473, and observed with an optical microscope of 500 to 1000 times and an electron microscope (scanning type and transmission type) of 1000 to 100,000 times. Quantification is possible. It is possible to obtain an area ratio of each tissue by observing 20 fields of view or more and using a point counting method or image analysis.
[0014]
The total of each phase of the microstructure is 100%, but phases that cannot be observed and identified with an optical microscope such as carbides, oxides, and sulfides are included in the area ratio of the main phase.
Next, the reason for limiting the preferable range of the steel plate component in the present invention will be described.
C is an element added for the purpose of controlling the fraction of the main phase and the second phase to ensure a good strength ductility balance. In particular, when the second phase is austenite, the ductility is greatly improved by contributing not only to the area fraction but also to the stability thereof. In addition, it has an effect on the fine uniformity of the substrate. In order to ensure the strength and the area fraction of each second phase, the lower limit was set to 0.01% by mass (hereinafter the same), and the upper limit capable of maintaining weldability and hole expandability was set to 0.3%.
[0015]
Si is an element added for the purpose of suppressing the formation of relatively coarse carbides that deteriorate the strength-ductility balance, and its lower limit was set to 0.005 mass%. Moreover, since excessive addition has a bad influence on weldability, the upper limit was made 2.5 mass%.
Mn is added for the purpose of increasing the strength. Further, it is added for the purpose of suppressing carbide precipitation and pearlite generation, which are one cause of strength reduction and ductility deterioration. From these things, it was set as 0.01 mass% or more. On the other hand, 3% by mass was made the upper limit because it delayed bainite transformation contributing to ductility improvement and deteriorated weldability.
[0016]
Since the amount of P is a strengthening element and extremely low is economically disadvantageous, 0.0010 mass% was made the lower limit. Moreover, since addition in a large amount adversely affects weldability and manufacturability during casting and hot rolling, the upper limit was set to 0.1%.
Since the extremely low S is economically disadvantageous, the lower limit is 0.0010% by mass, and the upper limit is 0.1% by mass. This is because it adversely affects manufacturability during casting and hot rolling.
Al is added as a deoxidizing element. Moreover, there exists an effect which accelerates | stimulates the bainite transformation which contributes to ductility improvement, especially when a 2nd phase is austenite, and a ductile strength balance is improved. For this reason, it was set as 0.005 mass% or more addition. On the other hand, excessive addition impairs weldability and plating wettability, so 2% was made the upper limit.
[0017]
Mo is an element that can be added for the purpose of suppressing the formation of carbide and pearlite that deteriorates the strength and ductility balance, and is an important additive element for obtaining a good strength and ductility balance. Furthermore, since it is also effective in preventing softening of the weld heat affected zone, the lower limit was made 0.01 mass%. Moreover, excessive addition causes ductility deterioration, so the upper limit was made 5%.
Nb forms fine carbides, nitrides or carbonitrides, is extremely effective in strengthening steel sheets, and promotes the formation of bainite and bainitic ferrite. Moreover, it is effective also in the softening suppression of a welding heat affected zone, and it was set as 0.001 mass% or more addition. On the other hand, excessive addition inhibits ductility deterioration and concentration of C in retained austenite, so the upper limit was made 0.1 mass%.
[0018]
Further, in order to control the weldability, hole expansibility, and ductility in a balanced manner at a strength level of 800 MPa or more, the formula (A) is satisfied.
(3.0Nb + 2.5Mo + 2 / 3Si + Mn)-(2.3C 0.5 +1.80)> 0 ... (A)
Furthermore, the steel targeted by the present invention can contain one or more of Cr, Ni, Cu, Co, and W for the purpose of further improving the strength.
Cr is an element added for the purpose of strengthening and suppressing the formation of carbides and the purpose of forming bainite and bainitic ferrite, and is 0.01% or more, and if added in an amount exceeding 5%, the workability is adversely affected. This was the upper limit.
[0019]
Ni is 0.01% by mass or more for the purpose of strengthening by improving the hardenability, and if added in an amount exceeding 5% by mass, the workability, particularly the martensite hardness increase, has an adverse effect. It was.
Cu is added in an amount of 0.01% by mass or more for the purpose of strengthening, and if it exceeds 5% by mass, the workability and manufacturability are adversely affected.
Co was added in an amount of 0.01% by mass or more in order to improve the strength ductility balance by controlling the bainite transformation. On the other hand, the upper limit of addition is not particularly set, but since it is an expensive element, addition of a large amount impairs economic efficiency, so it is desirable to make it 5% by mass or less.
[0020]
The reinforcing effect appears when W is 0.01% by mass or more. The reason why the upper limit is 5% by mass is that if the amount exceeds W, the workability is adversely affected.
Further, the steel targeted by the present invention can contain one or two of Zr, Hf, Ta, Ti, and V, which are strong carbide forming elements, for the purpose of further improving the strength. These elements form fine carbides, nitrides or carbonitrides and are extremely effective for strengthening the steel sheet. Therefore, if necessary, one or more of these elements may be added in a total amount of 0.001% by mass or more. It was set as addition. On the other hand, since it inhibits ductility deterioration and concentration of C in retained austenite, the upper limit of the total amount of one or more types is set to 1% by mass.
[0021]
B can also be added as needed. B is effective for strengthening grain boundaries and increasing the strength of steel by adding 0.0001% by mass or more, but when the added amount exceeds 0.1% by mass, the effect is saturated, and The upper limit was made 0.1% by mass because the properties deteriorated.
Ca, Y, and Rem should be added 0.001% or more from the viewpoint of the form control of inclusions, particularly fine dispersion, by adding appropriate amounts, while excessive addition can improve the manufacturability such as castability and hot workability and the steel sheet product. In order to reduce the ductility, the upper limit was made 0.5 mass%.
[0022]
Inevitable impurities include, for example, N and Sn. Even if these elements are contained in the range of 0.02% by mass or less, the effect of the present invention is not impaired.
A high strength steel sheet having such a structure and excellent in weldability, hole expansibility and ductility will be described below.
When the steel sheet of the present invention is manufactured by cold rolling and annealing after hot rolling, the slab adjusted to a predetermined component is cast as it is or once cooled and then reheated for hot rolling. In this case, the reheating temperature is desirably 1100 ° C. or higher and 1300 ° C. or lower. When the reheating temperature is high, coarse grains and thick oxide scales are formed. On the other hand, low temperature heating increases the rolling resistance. In addition, after hot rolling, removing the surface scale by using a high-pressure descaling device or pickling improves the surface cleanliness of the product, which is advantageous for plating. Then, it is set as a final product by annealing after cold rolling. Moreover, even if electroplating, hot dip galvanization, or hot dip galvanization is performed, the present invention is not hindered. The hot rolling completion temperature is generally higher than the Ar 3 transformation temperature determined by the chemical composition of the steel, but if it is from Ar 3 to about 10 ° C., the final steel sheet characteristics will not be deteriorated. In addition, the coiling temperature after cooling is higher than the bainite transformation start temperature determined by the chemical composition of the steel, so that the load during cold rolling can be increased more than necessary, but when the total rolling reduction of cold rolling is small Is not limited to this, and even if the steel sheet is wound at a temperature lower than the bainite transformation temperature of the steel, the properties of the final steel sheet are not deteriorated. The total rolling reduction of cold rolling is set based on the relationship between the final thickness and the cold rolling load, but if it is 40% or more, it is sufficient for recrystallization and does not deteriorate the properties of the final steel plate.
[0023]
When annealing after cold rolling, the annealing temperature is expressed by the temperatures Ac1 and Ac3 determined by the chemical composition of the steel (for example, “Steel Material Science” by W.C. Leslie, translated by Koyasu Naruyasu, Maruzen P273). When the temperature is less than 0.8 × (Ac 3 −Ac 1 ) + Ac 1 (° C.), the amount of austenite obtained at the annealing temperature is small, so that mainly bainite or bainitic ferrite is generated in the final steel sheet. I can't. Further, since a sufficient amount of retained austenite phase or martensite phase cannot be left as the second phase, this was set as the lower limit of the annealing temperature. In addition, as the annealing temperature becomes higher, the coarsening of the crystal grains and the surface oxidation are promoted, and the upper limit of the annealing temperature is set to Ac 3 +30 (° C.) in order to increase the manufacturing cost. The annealing time in this temperature range requires 10 seconds or more to make the temperature of the steel plate uniform and to secure austenite. However, if it exceeds 30 minutes, the generation of a grain boundary oxidation phase is promoted and the cost is increased.
[0024]
Subsequent primary cooling suppresses the transformation from the austenite phase to the ferrite phase to some extent, forms bainite or bainitic ferrite, and further stabilizes the austenite by concentrating C in the untransformed austenite phase. Is important to. Setting this cooling rate to less than 3 ° C./second may promote the formation of ferrite and pearlite and cause a decrease in strength, so the lower limit of the cooling rate was set to 3 ° C./second. On the other hand, when the cooling rate is higher than 150 ° C./sec, a hard phase such as a martensite phase in the final steel sheet becomes large, and it is difficult to operate, so this was set as the upper limit.
When this primary cooling is performed to less than 200 ° C., a large amount of martensite is generated during cooling to promote hole expansibility and delayed fracture, so the cooling stop temperature is set to 350 ° C. or higher. In addition, if the cooling stop temperature exceeds 450 ° C., carbides are generated in a short time during subsequent holding, leading to a decrease in strength. Next, in order to stabilize austenite and lower the hardness of martensite, the temperature is maintained. If the retention time is long, it is not preferable from the viewpoint of productivity, and carbides are generated. Further, in order to stabilize the austenite phase remaining in the steel sheet at room temperature, it is essential to further increase the carbon concentration in the austenite by transforming a part of the austenite phase into the bainite phase. It is desirable to hold the above, preferably 15 seconds to 20 minutes. If it is less than 200 ° C., bainite transformation hardly occurs, and if it exceeds 450 ° C., carbides are generated and it is difficult to leave a sufficient residual austenite phase.
In addition, the welding method is within the scope of the present application even if a commonly performed welding method, for example, arc, TIG, MIG, mash, laser, or the like is performed.
[0025]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples.
A steel sheet having a composition (% by mass) as shown in Table 1 is heated to 1200 ° C., and hot rolling is completed at an Ar 3 transformation temperature or higher, and after cooling, it is wound at a bainite transformation starting temperature or higher determined by the chemical composition of each steel. The steel strip was pickled and then cold rolled to a thickness of 1.2 mm.
Thereafter, it was determined by calculating the Ac 1 and Ac 3 transformation temperature according to the following equation from a component of the steel (mass%).
[0026]
JIS No. 5 tensile test specimens were collected from these steel plates and measured for mechanical properties. In addition, a hole expansion test was performed in accordance with the Steel Federation standard, and the hole expansion rate was obtained. As for weldability, laser welding was performed with steel plates attached, a ball head overhang test was performed with resin sheet lubrication, and the overhang height and fracture position of the base material were measured.
Table 2 shows the microstructure and materials, and Table 3 shows the manufacturing conditions and materials. It can be seen that the invention steel satisfying the outline of the present invention is excellent in weldability, ductility, strength (tensile strength of 800 MPa or more), and hole expansibility.
On the other hand, since the comparative example does not satisfy the microstructure and the formulas (A) and (B) at the same time, the material properties are inferior. For example, Comparative Examples A-3, I-3 and CE do not see the definition of the microstructure, so the hole expansion rate; λ is low. Further, since the expressions (A) and (B) are not satisfied at the same time, the height ratio R of the ball head overhang of the weld joint is low. FIGS. 1 and 2 show the weld head's ball head overhang height ratio; R, tensile strength; TS / MPa, elongation; El. /% And the hole expansion ratio; a value obtained by multiplying λ, that is, a parameter and formulas (A) and (B ). Invented steel satisfies both equations at the same time, so this parameter shows a high value. Since a comparative example does not satisfy either or both, a low value is shown.
[0027]
【The invention's effect】
According to the present invention, it is possible to obtain a high-strength steel sheet having improved weldability, hole expansibility and ductility of a high-strength steel sheet having a tensile strength of 800 MPa or more and a method for producing the same.
[Table 1]
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a relationship between R × TS × El × λ and a value on a left side of equation (A).
FIG. 2 is a diagram illustrating a relationship between R × TS × El × λ and a value on the left side of the equation (B).
Claims (7)
Ｃ ：０．０１〜０．３％、
Ｓｉ：０．００５〜２．５％、
Ｍｎ：０．０１〜３％、
Ｐ：０．００１０〜０．１％、
Ｓ：０．００１０〜０．０５％、
Ａｌ：０．００５〜２％、
Ｍｏ：０．０１〜０．３％、
Ｎｂ：０．００１〜０．１％を下記（Ａ）式を満たす範囲で含有し、
残部はＦｅおよび不可避的不純物からなり、
ミクロ組織が、主相をベイナイトまたはベイニティックフェライトとして面積率で合計５０〜９７％含有し、第２相をオーステナイトとして面積率で３〜５０％含有し、残部をフェライトまたはマルテンサイトとし、
前記ミクロ組織におけるオーステナイトの面積率（Ｖγ）とマルテンサイトの面積率（ＶαM）とが、下記（Ｂ）を満足し、
引張強度が８００MPa以上であることを特徴とする溶接性、穴拡げ性および延性に優れた高強度鋼板。
（3.0Nb+2.5Mo+2/3Si+Mn）−（2.3C0.5+1.80）＞０・・・（Ａ）
Ｖγ ／ ＶαM ＞ ２ ・・・（Ｂ）% By mass
C: 0.01 to 0.3%
Si: 0.005 to 2.5%,
Mn: 0.01 to 3%
P: 0.0010 to 0.1%,
S: 0.0010 to 0.05%,
Al: 0.005 to 2%,
Mo: 0.01 to 0.3%,
Nb: 0.001 to 0.1% is contained in a range satisfying the following formula (A),
The balance consists of Fe and inevitable impurities,
The microstructure contains the main phase as bainite or bainitic ferrite in a total area of 50 to 97%, the second phase as austenite in an area ratio of 3 to 50%, and the balance as ferrite or martensite,
The area ratio (Vγ) of austenite and the area ratio (VαM) of martensite in the microstructure satisfy the following (B):
A high-strength steel sheet excellent in weldability, hole expansibility and ductility, characterized by a tensile strength of 800 MPa or more.
(3.0Nb + 2.5Mo + 2 / 3Si + Mn)-(2.3C 0.5 +1.80)> 0 ... (A)
Vγ / VαM> 2 (B)
Ｃｒ：０．０１〜５％、
Ｎｉ：０．０１〜５％、
Ｃｕ：０．０１〜５％、
Ｃｏ：０．０１〜５％、
Ｗ ：０．０１〜５％の１種または２種以上を含有することを特徴とする請求項１または請求項２に記載の溶接性、穴拡げ性および延性に優れた高強度鋼板。Furthermore, in mass%,
Cr: 0.01-5%
Ni: 0.01 to 5%,
Cu: 0.01 to 5%,
Co: 0.01-5%
The high-strength steel sheet excellent in weldability, hole expansibility and ductility according to claim 1 or 2, characterized by containing one or more of W: 0.01 to 5%.
Zr、Hf、Ta、Ｔｉ、Ｖの１種または２種以上を合計で０．００１〜１％含有することを特徴とする請求項１乃至請求項３のいずれかに記載の溶接性、穴拡げ性および延性に優れた高強度鋼板。Furthermore, in mass%,
The weldability and hole expansion according to any one of claims 1 to 3, characterized by containing 0.001 to 1% of one or more of Zr, Hf, Ta, Ti, and V in total. High-strength steel sheet with excellent ductility and ductility.
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