JP3704306B2

JP3704306B2 - Hot-dip galvanized high-strength steel sheet excellent in weldability, hole expansibility and corrosion resistance, and method for producing the same

Info

Publication number: JP3704306B2
Application number: JP2001399396A
Authority: JP
Inventors: 展弘藤田; 裕一谷口; 賢一郎松村; 卓也原; 俊樹野中
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2001-12-28
Filing date: 2001-12-28
Publication date: 2005-10-12
Anticipated expiration: 2021-12-28
Also published as: JP2003193190A

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、穴拡げ性および耐食性に優れた溶融亜鉛めっき高強度鋼板および溶融合金化亜鉛めっき鋼板と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。これら部材の耐久性には、溶接部や加工部分の疲労耐久性や耐食性が挙げられる。
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。
【０００３】
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。
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して穴拡げ性を向上させ、さらには張り出し性成型性についても、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体積率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、800 ＭＰａ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。
【０００４】
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、800 ＭＰａ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800 ＭＰa 以上になると、溶接自体が困難になり、980MPa以上で更に顕著となる。このため、スポッ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、特に溶接部および熱影響部での材質変動が５９０ＭＰa クラス以上の高強度材に比べ極めて顕著となる。
【０００５】
また、高強度材の高延性化を図るために、複合組織化を積極的に活用することが一般的である。しかし、第２相にマルテンサイトや残留オーステナイトを活用した場合に、穴拡げ性が著しく低下してしまうという問題がある（例えば、CAMP-ISIJ, vol.13(2000),p.391）。また、本文献中には、主相をフェライト、第２相をマルテンサイトととし、両者の硬度差を減少させることで穴拡げ率が向上することが開示されているが、穴拡げ率で７０％未満と、著しく改善されているわけではない。
さらに、これらの鋼板には、Ｓｉなどが添加されたり、溶融亜鉛めっき工程での材質の確保や、めっき性やその後の耐食性の考慮が十分でない。
【０００６】
【発明が解決しようとする課題】
本発明は、前述のような従来技術の問題点を解決し、引張り強度が800 ＭＰａ以上の高強度鋼板の溶接性および穴拡げ性を改善し、溶融亜鉛めっきおよび溶融合金化亜鉛めっきを可能とした耐食性に優れ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。
【０００７】
【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800 ＭＰａ以上の領域で、溶接性、穴拡げ性を同時に改善する手法として、ミクロ組織およびを成分範囲や（Ａ）式による限定を行うことで、800 ＭＰａ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溶接部の疲労耐久性を向上させ、さらには、穴拡げ率：（穴拡げ試験後の穴の内径／穴拡げ試験前の穴径−１）×１００が７０％以上の穴拡げ性を確保できることを見出した。さらに、式（Ｂ）または（Ｃ）を満たすことでめっき性を改善し、耐食性の向上を図ることが可能なことを見出した。
【０００８】
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。
（１）質量％で、
Ｃ：０．０１〜０．２０％、
Ｓｉ：１．５％以下、
Ｍｎ：０．０１〜３％、
Ｐ：０．００１０〜０．１％、
Ｓ：０．００１０〜０．０５％、
Ａｌ：０．００５〜４％を含有し、さらに、
Ｍｏ：０．０１〜５．０％、
Ｎｂ：０．００１〜１．０％の１種または２種を下記（Ａ）および（Ｂ）式を同時に満たす範囲で含有し、残部をFeおよび不可避的不純物とし、ミクロ組織が、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７０％以上含有し、引張強度が800MPa以上であり、７０％以上の穴拡げ率を備えたことを特徴とする溶接性、穴拡げ性および耐食性に優れた溶融亜鉛めっき高強度鋼板。
3.0Nb+2.5Mo+Mn−5C^0.5 ＞０・・・（Ａ）
Al+15.0Si^２ +1.5Si-1.8 ＜０・・・（Ｂ）
【０００９】
（２）さらに、質量％で、
Ｃｒ：０．０１〜５％、
Ｎｉ：０．０１〜５％、
Ｃｕ：０．０１〜５％、
の１種または２種以上を含有することを特徴とし、さらに式（Ａ）および（Ｃ）を満たす（１）に記載（ただし本項では式（Ｂ）を満たす必要はない）の溶接性、穴拡げ性および耐食性に優れた溶融亜鉛めっき高強度鋼板。
3.0Nb+2.5Mo+Mn−5C^0.5 ＞０・・・（Ａ）
Al＋15.0Si²＋1.5Si ＋0.5Cr −(30Ni ＋10Cu) −1.8 ＜０・・・（Ｃ）
【００１０】
（３）さらに、質量％で、
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、穴拡げ性および耐食性に優れた溶融亜鉛めっき高強度鋼板。
【００１１】
（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）に記載の溶接性、穴拡げ性および耐食性に優れた溶融亜鉛めっき高強度鋼板。
【００１２】
（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）に記載の溶接性、穴拡げ性および耐食性に優れた溶融亜鉛めっき高強度鋼板。
【００１３】
（６）さらに、質量％で、Ｃａ、Ｙ、Rem の１種または２種以上を合計で０．０００１〜０．５％含有することを特徴とする（１）乃至（５）に記載の溶接性、穴拡げ性および耐食性に優れた溶融亜鉛めっき高強度鋼板。
【００１４】
（７）（１）乃至（６）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、その後昇温および焼鈍時の雰囲気が酸素濃度が５０ppm 以下で露点が−２０℃以下とし、焼鈍時の最高温度が０．３×（Ａｃ₃−Ａｃ₁）＋Ａｃ₁（℃）以上Ａｃ₃＋３０（℃）以下で焼鈍した後に、０．１〜１００℃／秒の冷却速度で亜鉛めっき浴温度−２０℃〜亜鉛めっき浴温度＋５０℃の温度域に冷却し、引き続いて同温度域でめっき浸漬を含めて１秒〜１０００秒保持を行うことを特徴とする溶接性、穴拡げ性および耐食性に優れた溶融亜鉛めっき高強度鋼板の製造方法。
【００１５】
（８）（７）に記載の製造方法において、亜鉛めっき浴温度−２０℃〜亜鉛めっき浴温度＋５０℃の温度域に冷却し、引き続いて同温度域でめっき浸漬を含めて１秒〜１０００秒保持後、合金化処理を４３０℃〜５８０℃を行うことを特徴とする溶接性、穴拡げ性および耐食性に優れた溶融亜鉛めっき高強度鋼板の製造方法。
【００１６】
【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：０．０１〜０．２％、Ｓｉ：1.5 ％以下、Ｍｎ：０．０１〜３％、Ｐ：０．００１０〜０．１％、Ｓ：０．００１０〜０．０５％、Ａｌ：０．００５〜4 ％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って破断部位置を測定した。また、各溶接継ぎ手を作成して、引張り- 引張りモードの疲労試験を10⁵〜10⁶cyclesまで行い、母材との疲労強度を比較することでめっき材の溶接継ぎ手の疲労特性を比較評価した。ここで、溶接継ぎ手は、ビードオンで行い、溶接後、試験片端面および表面を研削して溶接チャンスによる継ぎ手形状のばらつき因子を極力少なくする方法を採った。また、めっき性の試験は、外観観察して５段階の評点をつけた。
評点５：不めっき等全くなく良好な外観。
評点４：めっき剥離が極めて少なく良好な外観。
評点３：不めっきやめっき剥離発生。
評点２：不めっきやめっき剥離多発。
評点１：めっき濡れず。
【００１７】
また、めっき後の耐食性試験には、板厚の３倍の曲率半径で板を曲げたあとで乾湿繰り返し試験を自動車技術会の規格（JASO）のサイクル腐食試験を100 回まで行った。腐食状況は、外観および断面を光学顕微鏡にて200 〜1000倍の20視野以上を観察し、内面への腐食の進行度合いを観察することで５段階評価とした。評点は以下のようである。
評点５：発錆等全くなく良好な外観。腐食の進行度合いは、めっき相のみまたは母材への腐食深さが50μｍ未満。
評点４：発錆が極めて少なく良好な外観。腐食の進行度合いは、母材への腐食深さが50μｍ〜100 μｍ未満。
評点３：錆発生が明確に認められる。
評点２：錆多発。
評点１：錆多発、一部に膨れや穴あき等あり。
【００１８】
その結果、最終的に得られるミクロ組織制御により、800 ＭＰａ以上の引張り強度と７０％以上の穴拡げ率を得、溶接性、穴拡げ性および耐食性に優れた溶融亜鉛めっき鋼板及び溶融合金化亜鉛めっき高強度鋼板が製造可能なことを見出した。
【００１９】
次に、基材鋼板の好ましいミクロ組織について述べる。
800 ＭＰａ以上の引張り強度を得、溶接性および穴拡げ性を両立させるためには、主相としてベイナイトまたはベイニティックフェライトが適している。溶接性および穴拡げ性を両立させるためには、面積率で７０％以上とする。
【００２０】
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が主相で、面積率が９７％を超えることが望ましい。
【００２１】
一方、溶接熱影響部での軟化防止が問題となる。これに対しては、後述するように成分を規定した（Ａ）式を満たすことで、引張り強度が800MPa以上の高強度めっき材の溶接性および溶接後疲労強度を確保できる。
【００２２】
延性確保や高強度化の観点からすると、面積率で３０％未満のフェライトを含んでも良い。一方、オーステナイトおよび／またはマルテンサイトを含むことは穴拡げ加工性や溶接熱影響部の軟化挙動の点から望ましくないが、面積率３％未満程度であれば、顕著な特性劣化が認められないことから、面積率で３％未満含んでも良い。さらに、酸化物や硫化物等の介在物を不可避的に含んでも良い。
【００２３】
また、式（Ａ）を満足しない場合には、めっき材での引張り強度で800 ＭＰａ以上を確保できなかったり、溶接熱影響部分の軟化を抑制できず、溶接継ぎ手の疲労特性が劣化してしまうだけでなく、穴拡げ性の確保も困難となる。
3.0Nb+2.5Mo+Mn−5C^0.5 ＞０・・・（Ａ）
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、酸化物の１又は２以上を面積率１％以下で含有する場合も本発明で用いることができ、これらは主相の面積率に含めた。
【００２４】
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面積率の測定は、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事ができる。
【００２５】
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−穴拡げ性バランスを確保するための主相および第２相の率を制御する目的で添加する元素である。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０．２０％とした。好ましくは、Ｃ：０．０５〜０．１５％とすることにより良好な強度−穴拡げ性バランスが得られる。
【００２６】
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るがめっき性を著しく劣化させる。このため１．５％以下とした。一方で、極低下は製造コストの高騰を招くことから、めっき性を大きくは悪化させない０．００５以上の添加とすることが望ましい。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１．５質量％とした。好ましくは、Ｓｉ：０．０５％以下とすることによりさらに著しい効果が得られる。
【００２７】
Ｍｎは、高強度化の目的で添加する。また、フェライト変態を抑制して、主相をベイナイトまたはベイニティックフェライトにするのに有効である。さらに、強度低下と穴拡げ性劣化の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、過剰添加は、マルテンサイト生成を促進したり、延性の著しい低下を招くために３質量％を上限とした。好ましくは、Ｍｎ：１．５〜３．０％とすることにより良好な強度−穴拡げ性バランスが得られる。
【００２８】
Ｐは、強化元素である。また、低Ｐ化は穴拡げ性を向上させるが、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。
【００２９】
Ｓは、低Ｓ化は穴拡げ性向上に有効である。一方、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、０．０５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。
【００３０】
Ａｌは、脱酸元素として添加する。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４％を上限とした。
【００３１】
Ｍｏは、強度−穴拡げ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。また、フェライト変態を抑制して、主相をベイナイトまたはベイニティックフェライトにするのに有効であり、良好な強度−穴拡げ性およびめっき材の溶接性や溶接後の疲労耐久性の確保の極めて良好なバランスを得るための添加元素であることから、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５．０％とした。
【００３２】
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。また、フェライト変態を遅滞させ、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。さらには、溶接熱影響部の軟化抑制にも効果的であることから、０．００１質量％以上の添加とし、０．０１％以上の添加が溶接性向上のためには好ましい。一方で、過剰添加は、延性や熱間加工性を劣化させることから、上限として１．０質量％とした。
【００３３】
また、800 ＭＰａ以上の強度レベルで、溶接性および穴拡げ性をバランス良く、制御するためには、式（Ａ）を満足しなければならない。
3.0Nb+2.5Mo+Mn−5C^0.5 ＞０・・・（Ａ）
【００３４】
Ｎｂ、ＭｏおよびＭｎは焼入れ性を向上させ、さらにＮｂおよびＭｏは熱影響部の軟化防止に効果的で、各係数はその程度を反映するものである。一方、Ｃは効果に有効なものの、溶接時の軟化挙動を助長する。その機構の詳細については、明確になっていないが、ＮｂおよびＭｏの軟化防止効果は溶接熱サイクル中に析出物形成やソリュウトドラッグ効果によると考えられる。一方、Ｃは焼入れ硬貨量が大きいうえ析出粗大化を助長することから、熱影響部の軟化を含めた溶接性を劣化させると考えられる。図１に示すように、式（Ａ）の左辺の値がゼロ以下の領域では、たとえベイナイトまたはベイニティックフェライトの面積率が70％以上としても、疲労強度比−引張り強度−穴拡げ率：λの両立した良好な材質は得られないことがわかる（図１中×印）。また、式（Ａ）の左辺の値がゼロ超の領域においても、製造方法の違いからベイナイトおよびベイニティックフェライトの面積率が低いものは、良好な材質が得られないこともわかる（図１中＊印）。すなわち、組織および式（Ａ）の条件を同時に満たすもののみ良好な材質が得られることがわかる。
【００３５】
また、めっき性を確保して、より良好な耐食性を確保する目的から、式（Ｂ）を満たさなければならない。
Al+15.0Si² +1.5Si-1.8 ＜０・・・（Ｂ）
【００３６】
ＡｌおよびＳｉはめっきの濡れ性や合金化挙動に影響を及ぼす。特に濡れ性を劣化させ、各係数はその程度を反映するものである。その機構の詳細については、明確になっていないが、表面に形成される酸化物を形態に起因していると考えられる。図２に示すように、式（Ｂ）の左辺の値がゼロ以上の領域では、耐食性試験後の外観評点が低くく、耐食性に劣る。また、式（Ｂ）の左辺の値がゼロ未満の領域においても、製造方法の違いから耐食性に劣るものもある。
【００３７】
さらに、本発明が対象とする鋼は、強度−穴拡げ性バランスのさらなる向上を目的として、Ｃｒ、Ｎｉ、Ｃｕ１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、加工性やめっき性に悪影響を及ぼすため、これを上限とした。
Ｎｉは、焼き入れ性の向上およびめっき性向上の目的で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生成に伴う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、強化およびめっき性向上の目的で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。
【００３８】
また、めっき性の確保に関しては、より良好な耐食性を確保する目的から、式（Ｃ）を満たさなければならない。
Al＋15.0Si²＋1.5Si ＋0.5Cr −(30Ni ＋10Cu) −1.8 ＜０・・・（Ｃ）
【００３９】
上述のように、ＣｒはＡｌやＳｉと同様、表面の酸化物形態に影響するためめっき性を劣化させる。一方、ＮｉおよびＣｕはめっき性改善に有効であり、各係数はその程度を反映するものである。したがって、Ｃｒ、ＮｉやＣｕを含む場合には、（Ｂ）式でなく（Ｃ）式を満たさなければならない。図３に示すように、式（Ｃ）の左辺の値がゼロ以上の領域では、めっき外観評点が低く、一方、式（Ｃ）の左辺の値がゼロ未満の領域ではすべて良好なめっき外観評点が得られる。
【００４０】
さらには、Ｃｏ、Ｗの１種または２種以上を含有できる。
Ｃｏは、ベイナイト変態制御による強度−穴拡げ性の良好なバランスのため、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。
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ためである。
【００４１】
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種以上を含有できる。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、延性や熱間加工性の劣化を招くことから、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。
【００４２】
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。
【００４３】
Ｃａ、Ｙ、Ｒｅｍは、適量添加により介在物制御、特に微細分散化に寄与することから０．０００１％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。
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。
【００４４】
このような組織を有する溶接性、穴拡げ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直接もしくは一旦冷却した後再加熱して熱延を行う。
【００４５】
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。
【００４６】
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０．３×（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、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。ここで、昇温および焼鈍時の雰囲気が酸素濃度が５０ppm 以下で露点が−２０℃以下とした。酸素濃度が５０ppm を超えたり、露点がー２０℃を超えると、鋼板のめっき性、特に濡れ性が劣化し、不めっきの原因となる。
【００４７】
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生成させるのに重要である。この冷却速度を０．１℃／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０．１℃／秒とした。一方、冷却速度が１００℃／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。
【００４８】
この後の冷却がめっき浴温度−２０℃未満まで行われると、めっき浴浸入時の抜熱が大きいことなどの操業上の問題がある。また、冷却停止温度がめっき浴＋５０℃を超えると、操業上の問題に加え、その後の保持時に炭化物が生成してしまい、強度低下を招くため、これを上限とした。また、次にベイナイト変態の進行を促すため、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１０００秒以内とすることが望ましい。また、ベイナイト変態進行させるため、１秒以上保持し、好ましくは１５秒から１０分保持することが望ましい。めっき浴温度−２０℃未満ではベイナイト変態が起こりにくく、めっき浴温度−５０℃を超えると炭化物が生じて材質劣化してしまう。また、合金化処理を行う場合には、４３０℃以上５８０℃以下とした。合金化処理温度が４３０℃未満であると合金化の進行が遅く、生産性が悪い。また、５８０℃を超えると炭化物析出を伴い、材質劣化する。
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。
【００４９】
【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に示すような組成の鋼板を、１２００℃に加熱し、Ａｒ_３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Ａｃ_１とＡｃ_３変態温度を計算により求めた。
Ａｃ_１＝７２３−１０．７×Ｍｎ％＋２９．１×Ｓｉ％、
Ａｃ_３＝９１０−２０３×（Ｃ％）^１／２−１５．２×Ｎｉ％
＋４４．７×Ｓｉ％＋１０４×Ｖ％＋３１．５×Ｍｏ％
−３０×Ｍｎ％−１１×Ｃｒ％＋４００×Ａｌ％、
これらのＡｃ_１およびＡｃ_３変態温度から計算される焼鈍温度に１０％Ｈ_２−Ｎ_２雰囲気中で昇温・保定したのち、３〜１５０℃／秒の冷却速度で２００〜４５０℃まで冷却し、引き続いて１〜３０００秒保持した後、冷却した。なお、表３中の冷却停止温度はこの冷却工程の最終温度を示すものであるが、保持温度はこの冷却停止温度と必ずしも同一である必要はない。
【００５０】
これらの鋼板からJIS ５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にビードオンの形で各溶接を行い、弗素系樹脂（テフロン商品名）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。また、各溶接継ぎ手を作成して、引張り−引張りモードの疲労試験を10⁵〜10⁶cyclesまで行い、母材との疲労強度を比較することでめっき材の溶接継ぎ手の疲労特性を比較評価した。ここで、溶接継ぎ手は、ビードオンで行い、溶接後、試験片端面および表面を研削して溶接チャンスによる継ぎ手形状のばらつき因子を極力少なくする方法を採った。また、めっき性の試験は、外観観察して５段階の評点をつけた。
また、めっき後の耐食性試験には、板厚の３倍の曲率半径で板を曲げたあとで乾湿繰り返し試験を行い、その発錆の程度をやはり５段階で評価した。
【００５１】
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800 ＭＰａ以上）、穴拡げ性に優れていることがわかる。
一方、本発明の条件から外れる比較例は、溶接部の球頭張り出加工における破断位置が溶接熱影響部となり、疲労強度も低く、穴拡げ性やめっき性や耐食性も劣勢である。
【００５２】
【表１】

【００５３】
【表２】

【００５４】
【表３】

【００５５】
【発明の効果】
本発明により、引張り強度が800 ＭＰａ以上の高強度鋼板の溶接性、穴拡げ性および耐食性を同時に改善した溶融亜鉛めっき高強度鋼板およびその製造方法を得ることができる。
【図面の簡単な説明】
【図１】疲労強度比×引張り強度×λ（穴拡げ率）と（Ａ）式の左辺の値との関係を示すグラフである。
【図２】乾湿繰り返し腐食試験後の外観評点と（Ｂ）式の左辺の値との関係を示すグラフである。
【図３】めっき外観評点と（Ｃ）式の左辺の値との関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot-dip galvanized high-strength steel sheet and hot-dip alloyed galvanized steel sheet excellent in weldability, hole expansibility and corrosion resistance suitable for building materials, home appliances, automobiles, and the like, 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 800MPa class or higher are being used for some parts such as reinforcement due to the need for collision safety and expansion of cabin space. The durability of these members includes fatigue durability and corrosion resistance of welded parts and processed parts.
When assembling members using such high-strength materials, ductility, bendability, hole expansibility, etc. become a major problem over high-strength steel sheets with a tensile strength of up to about 590 MPa, so measures for these are necessary. Become.
[0003]
The following measures are taken for each characteristic.
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 overhang formability is also the second phase. It is disclosed that by forming the retained austenite, it shows the same stretchability as the current retained austenitic steel. Furthermore, it is also shown that when retained austenite having a volume 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.
[0004]
As for weldability, 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 of Nb and Mo (Nb, Mo) C. 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 has a low strength level, and it cannot be said that the weldability and workability of 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, due to the high strength, the material fluctuation particularly in the welded portion and the heat affected zone becomes extremely remarkable as compared with a high strength material of 590 MPa class or higher.
[0005]
Further, in order to increase the ductility of a high-strength material, it is common to actively utilize composite organization. However, when martensite or retained austenite is used in the second phase, there is a problem that the hole expandability is significantly lowered (for example, CAMP-ISIJ, vol. 13 (2000), p. 391). Further, this document discloses that the hole expansion rate is improved by setting the main phase as ferrite and the second phase as martensite and reducing the hardness difference between the two, but the hole expansion rate is 70. Less than%, it is not a significant improvement.
Further, Si and the like are added to these steel plates, and securing of materials in the hot dip galvanizing process and consideration of plating properties and subsequent corrosion resistance are not sufficient.
[0006]
[Problems to be solved by the invention]
The present invention solves the problems of the prior art as described above, improves the weldability and hole expansibility of high-strength steel sheets having a tensile strength of 800 MPa or more, and enables hot dip galvanizing and hot galvannealed galvanizing. An object of the present invention is to provide a high-strength steel sheet excellent in corrosion resistance and a method for producing the same.
[0007]
[Means for Solving the Problems]
  As a result of various studies, the inventors have determined that the microstructure and the component range and the formula (A) are limited as a technique for simultaneously improving the weldability and the hole expandability in a region where the tensile strength is 800 MPa or more. By doing so, it suppresses the softening behavior of the weld heat-affected zone while maintaining a high strength of 800 MPa or more, and improves the fatigue durability of the weld zone. Furthermore, the hole expansion rate: (After hole expansion testIt has been found that a hole expandability of 70% or more can be secured when the inner diameter of the hole / the diameter of the hole before the hole expansion test-1) × 100. Further, it has been found that satisfying the formula (B) or (C) can improve the plating property and improve the corrosion resistance.
[0008]
  The present invention has been completed based on the above findings, and the gist thereof is as follows.
(1) In mass%,
        C: 0.01-0.20%,
        Si: 1.5% or less,
        Mn: 0.01 to 3%
        P: 0.0010 to 0.1%,
        S: 0.0010 to 0.05%,
        Al: 0.005 to 4%, further,
        Mo: 0.01 to 5.0%,
        Nb: One or two of 0.001 to 1.0% is contained within a range that simultaneously satisfies the following formulas (A) and (B), the balance is Fe and inevitable impurities, and the microstructure is bainite or bay Nitric ferrite contains 70% or more area ratio and has a tensile strength of 800MPa or more.With a hole expansion rate of 70% or moreHot-dip galvanized high-strength steel sheet with excellent weldability, hole expansibility and corrosion resistance.
  3.0Nb + 2.5Mo + Mn−5C^0.5    > 0 (A)
  Al + 15.0Si² + 1.5Si-1.8 <0 (B)
[0009]
(2) Furthermore, in mass%,
Cr: 0.01-5%
Ni: 0.01 to 5%,
Cu: 0.01 to 5%,
The weldability according to (1) satisfying formulas (A) and (C) (however, it is not necessary to satisfy formula (B) in this section), Hot-dip galvanized high-strength steel sheet with excellent hole expansion and corrosion resistance.
  3.0Nb + 2.5Mo + Mn−5C^0.5  > 0 (A)
  Al + 15.0Si²+ 1.5Si + 0.5Cr-(30Ni + 10Cu) -1.8 <0 (C)
[0010]
(3) Furthermore, in mass%,
Co: 0.01-5%
W: 0.01 to 5%
The hot-dip galvanized high-strength steel sheet having excellent weldability, hole expansibility and corrosion resistance according to (1) or (2), characterized by containing one or more of the following.
[0011]
(4) Furthermore, in mass%,
The weldability, hole expansibility, and corrosion resistance according to (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%. Hot-dip galvanized high-strength steel sheet.
[0012]
(5) The molten zinc having excellent weldability, hole expansibility and corrosion resistance according to (1) to (4), further comprising B: 0.0001 to 0.1% by mass% Plated high-strength steel sheet.
[0013]
(6) The welding according to any one of (1) to (5), further including 0.0001 to 0.5% in total of one or more of Ca, Y, and Rem in terms of mass%. Hot-dip galvanized high-strength steel sheet with excellent properties, hole expansibility and corrosion resistance.
[0014]
(7) The cast slab composed of the components described in (1) to (6) is directly or once cooled and then heated again, the hot-rolled steel sheet wound after hot rolling is pickled and cold-rolled, and then heated and The atmosphere during annealing is an oxygen concentration of 50 ppm or less and a dew point of −20 ° C. or less, and the maximum temperature during annealing is 0.3 × (Ac_Three-Ac₁) + Ac₁(℃) or more Ac_ThreeAfter annealing at +30 (° C.) or lower, cool to a temperature range of galvanizing bath temperature −20 ° C. to zinc plating bath temperature + 50 ° C. at a cooling rate of 0.1 to 100 ° C./second, and subsequently plating in the same temperature range A method for producing a hot-dip galvanized high-strength steel sheet excellent in weldability, hole expansibility and corrosion resistance, characterized by holding for 1 second to 1000 seconds including immersion.
[0015]
(8) In the production method described in (7), the temperature is cooled to a temperature range of galvanizing bath temperature −20 ° C. to galvanizing bath temperature + 50 ° C., followed by plating immersion in the same temperature range for 1 second to 1000 seconds. A method for producing a hot-dip galvanized high-strength steel sheet excellent in weldability, hole expansibility and corrosion resistance, characterized by performing an alloying treatment at 430 ° C. to 580 ° C. after holding.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
The inventors, in mass%, C: 0.01 to 0.2%, Si: 1.5% or less, Mn: 0.01 to 3%, P: 0.0010 to 0.1%, S: 0.00. 0010 ~ 0.05%, Al: 0.005 ~ 4% contained, based on the steel sheet consisting of the balance Fe and inevitable impurities, melted by adding each alloy, heated again as cast or once cooled The hot-rolled steel sheet wound after hot rolling was pickled, cold-rolled, and then annealed to create 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 compliant with JIS, laser welding with the steel sheet, and then the ball head overhang test was performed to determine the position of the fracture part. In addition, each weld joint is created and a fatigue test in tension-tensile mode is performed.^Five~Ten⁶The fatigue characteristics of the plated joints were compared and evaluated by comparing the fatigue strength with the base metal. Here, the welding joint was bead-on, and after welding, the end face and the surface of the test piece were ground to reduce the joint shape variation factor due to the welding chance as much as possible. In addition, the plating test was evaluated by observing the appearance and giving a five-point score.
Score 5: Good appearance without any plating.
Score 4: Good appearance with very little plating peeling.
Score 3: Non-plating or plating peeling occurred.
Score 2: Non-plating and frequent plating peeling.
Score 1: No plating wet.
[0017]
In addition, in the corrosion resistance test after plating, the plate was bent with a radius of curvature three times the plate thickness, and then the wet and dry repeated test was conducted up to 100 times of the automobile engineering society standard (JASO) cycle corrosion test. The corrosion situation was evaluated on a five-point scale by observing the appearance and cross section of 20 or more fields of 200 to 1000 times with an optical microscope and observing the progress of corrosion on the inner surface. The score is as follows.
Score 5: Good appearance with no rusting. The degree of corrosion is less than 50 μm when only the plating phase or the depth of corrosion on the base material is reached.
Score 4: Good appearance with very little rusting. The degree of corrosion progression is that the depth of corrosion on the base metal is 50 μm to less than 100 μm.
Rating 3: Rust generation is clearly recognized.
Score 2: Rust occurs frequently.
Score 1: Frequent rusting, some blisters and holes.
[0018]
  As a result, the tensile strength of 800 MPa or more andHole expansion rate of 70% or moreIt was found that a hot-dip galvanized steel sheet and a hot-dip galvanized high-strength steel sheet excellent in weldability, hole expansibility and corrosion resistance can be produced.
[0019]
Next, a preferable microstructure of the base steel sheet will be described.
  In order to obtain a tensile strength of 800 MPa or more and achieve both weldability and hole expandability, bainite or bainitic ferrite is suitable as the main phase. In order to achieve both weldability and hole expandability, the area ratio is set to 70% or more.
[0020]
Moreover, the bainite said here contains both the upper bainite in which the carbide | carbonized_material has produced | generated in the lath boundary, and the lower bainite in which the fine carbide | carbonized_material has produced | generated in the lath. Bainitic ferrite means bainite having no carbide, and for example, acicular ferrite is one example.
In order to improve hole expansibility, it is desirable that the lower phase bainite in which carbide is finely dispersed or bainitic ferrite without carbide is the main phase and the area ratio exceeds 97%.
[0021]
On the other hand, prevention of softening in the weld heat affected zone becomes a problem. On the other hand, by satisfying the formula (A) that defines the components as described later, the weldability and post-weld fatigue strength of a high-strength plating material having a tensile strength of 800 MPa or more can be secured.
[0022]
From the viewpoint of ensuring ductility and increasing strength, ferrite with an area ratio of less than 30% may be included. On the other hand, the inclusion of austenite and / or martensite is undesirable from the viewpoint of hole expansion workability and softening behavior of the weld heat-affected zone, but if the area ratio is less than 3%, no significant deterioration in properties is observed. Therefore, the area ratio may include less than 3%. Furthermore, inclusions such as oxides and sulfides may be inevitably included.
[0023]
Further, if the formula (A) is not satisfied, the tensile strength of the plating material cannot be ensured to be 800 MPa or more, the softening of the heat affected zone cannot be suppressed, and the fatigue characteristics of the welded joint deteriorate. In addition, it is difficult to ensure hole expandability.
3.0Nb + 2.5Mo + Mn−5C^0.5  > 0 (A)
In addition to the above, as the remaining microstructure of the microstructure, one or more of carbides, nitrides, sulfides, and oxides can be used in the present invention. It was included in the phase area ratio.
[0024]
In addition, each phase of the above microstructure, ferrite (bainitic ferrite), bainite, austenite, martensite, interfacial oxidation phase and remaining structure, identification of existing positions, and measurement of area ratio are performed using Nital reagent and The steel plate rolling direction cross section or the rolling perpendicular direction cross section is corroded with the reagent disclosed in Japanese Patent No. 59-219473, and quantified by observation with an optical microscope of 500 to 1000 times and an electron microscope (scanning type and transmission type) of 1000 to 100,000 times. 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.
[0025]
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 ratio of the main phase and the second phase to ensure a good strength-hole expansibility balance. It will also affect the fine uniformity of the substrate. In order to ensure the strength and the area ratio 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 0.20%. Preferably, C: 0.05 to 0.15% makes it possible to obtain a good strength-hole expansibility balance.
[0026]
Si is an element added for the purpose of suppressing the formation of relatively coarse carbides that deteriorate the strength and ductility balance, but significantly deteriorates the plating properties. For this reason, it was made 1.5% or less. On the other hand, since extreme reduction leads to an increase in manufacturing cost, it is desirable to add 0.005 or more that does not greatly deteriorate the plating property. Moreover, since excessive addition has a bad influence on weldability, the upper limit was made 1.5 mass%. Preferably, a more remarkable effect can be obtained by setting Si: 0.05% or less.
[0027]
Mn is added for the purpose of increasing the strength. Further, it is effective for suppressing the ferrite transformation and making the main phase into bainite or bainitic ferrite. Furthermore, it is added for the purpose of suppressing carbide precipitation and pearlite formation, which are one cause of strength reduction and hole expandability deterioration. From these things, it was set as 0.01 mass% or more. On the other hand, excessive addition promoted martensite formation or caused a significant decrease in ductility, so the upper limit was 3% by mass. Preferably, a good balance between strength and hole expandability can be obtained by setting Mn to 1.5 to 3.0%.
[0028]
P is a strengthening element. Moreover, although lowering P improves hole expansibility, since extremely lowering is also economically disadvantageous, 0.0010 mass% was made into the minimum. Moreover, since addition in a large amount adversely affects weldability, manufacturability at the time of casting or hot rolling, the upper limit was made 0.1%.
[0029]
As for S, lowering S is effective for improving hole expansibility. On the other hand, since extremely low is economically disadvantageous, the lower limit is set to 0.0010% by mass, and the upper limit is set to 0.05% by mass. This is because it adversely affects manufacturability during time and hot rolling.
[0030]
Al is added as a deoxidizing element. 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 4% was made the upper limit.
[0031]
Mo suppresses the formation of carbides and pearlite that degrade the strength-hole expansibility balance. It is also effective in suppressing ferrite transformation and making the main phase into bainite or bainitic ferrite, and ensures excellent strength-hole expansibility, plating material weldability and fatigue durability after welding. Since it is an additive element for obtaining a good balance, the lower limit was set to 0.01% by mass. Further, excessive addition causes ductile deterioration, so the upper limit was made 5.0%.
[0032]
Nb forms fine carbides, nitrides or carbonitrides and is extremely effective for strengthening steel sheets. It also retards ferrite transformation and promotes the formation of bainite and bainitic ferrite. Furthermore, since it is also effective in suppressing softening of the weld heat affected zone, 0.001% by mass or more is added, and 0.01% or more is preferable for improving weldability. On the other hand, excessive addition deteriorates ductility and hot workability, so the upper limit was made 1.0 mass%.
[0033]
Further, in order to control the weldability and the hole expandability in a well-balanced manner at a strength level of 800 MPa or more, the formula (A) must be satisfied.
3.0Nb + 2.5Mo + Mn−5C^0.5  > 0 (A)
[0034]
Nb, Mo and Mn improve hardenability, and Nb and Mo are effective in preventing softening of the heat affected zone, and each coefficient reflects the degree. On the other hand, although C is effective, the softening behavior during welding is promoted. Although the details of the mechanism are not clear, it is considered that the softening prevention effect of Nb and Mo is due to the formation of precipitates and the solution drag effect during the welding heat cycle. On the other hand, C is thought to deteriorate weldability including softening of the heat-affected zone because C has a large amount of hardened coins and promotes precipitation coarsening. As shown in FIG. 1, in the region where the value of the left side of the formula (A) is zero or less, even if the area ratio of bainite or bainitic ferrite is 70% or more, the fatigue strength ratio-tensile strength-hole expansion rate: It can be seen that a good material having both λ cannot be obtained (marked with x in FIG. 1). Further, even in a region where the value of the left side of the formula (A) is more than zero, it can be seen that a good material cannot be obtained if the area ratio of bainite and bainitic ferrite is low due to the difference in the manufacturing method (FIG. 1). Medium *). That is, it can be seen that only a material satisfying the conditions of the structure and the formula (A) can be obtained.
[0035]
Moreover, for the purpose of ensuring plating properties and ensuring better corrosion resistance, the formula (B) must be satisfied.
Al + 15.0Si² + 1.5Si-1.8 <0 (B)
[0036]
Al and Si affect the wettability and alloying behavior of the plating. In particular, the wettability is deteriorated, and each coefficient reflects its degree. Although the details of the mechanism are not clear, it is considered that the oxide formed on the surface is caused by the form. As shown in FIG. 2, in the area | region where the value of the left side of Formula (B) is zero or more, the external appearance score after a corrosion resistance test is low, and it is inferior to corrosion resistance. Even in the region where the value of the left side of the formula (B) is less than zero, there are some inferior in corrosion resistance due to the difference in the manufacturing method.
[0037]
Furthermore, the steel which is the object of the present invention can contain Cr, Ni, Cu, or two or more kinds for the purpose of further improving the strength-hole expansibility balance.
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 it is 0.01% or more, and if added in an amount exceeding 5%, workability and plating properties are adversely affected. This is the upper limit.
Ni is 0.01% by mass or more for the purpose of improving hardenability and plating properties, and if added in an amount exceeding 5% by mass, the workability, particularly the hardness increase accompanying martensite formation, has an adverse effect. This is the upper limit.
Cu is added in an amount of 0.01% by mass or more for the purpose of strengthening and improving the plating property, and if added in an amount exceeding 5% by mass, workability and manufacturability are adversely affected.
[0038]
Moreover, regarding the securing of the plating property, the formula (C) must be satisfied for the purpose of securing better corrosion resistance.
  Al + 15.0Si²+ 1.5Si + 0.5Cr-(30Ni + 10Cu) -1.8 <0 (C)
[0039]
As described above, Cr, like Al and Si, affects the oxide form on the surface and thus degrades the plating properties. On the other hand, Ni and Cu are effective for improving plating properties, and each coefficient reflects the degree. Therefore, when Cr, Ni, or Cu is included, the formula (C) must be satisfied instead of the formula (B). As shown in FIG. 3, the plating appearance score is low in the area where the value of the left side of the formula (C) is zero or more, while the plating appearance score is all good in the area where the value of the left side of the expression (C) is less than zero. Is obtained.
[0040]
Furthermore, one or more of Co and W can be contained.
Co was added in an amount of 0.01% by mass or more for a good balance between strength and hole expansibility by bainite transformation control. 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 mass% or less.
The reinforcing effect appears when W is 0.01% by mass or more, and the reason why the upper limit is 5% by mass is that if the amount exceeds this, the workability is adversely affected.
[0041]
Furthermore, the steel targeted by the present invention can contain one or more 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 steel sheets. Therefore, if necessary, one or more 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 causes deterioration of ductility and hot workability, the upper limit of the total amount of one kind or two or more kinds is set to 1% by mass.
[0042]
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.
[0043]
Ca, Y, and Rem contribute to inclusion control, particularly fine dispersion, by adding appropriate amounts, so 0.0001% or more is added, while excessive addition is required for productivity such as castability and hot workability, and for steel sheet products. In order to reduce the ductility, the upper limit was made 0.5 mass%.
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.
[0044]
A high-strength steel plate having such a structure and excellent in weldability and hole expandability and a method for producing the same 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 directly or once cooled and then reheated for hot rolling.
[0045]
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 determined by the chemical composition of the steel._ThreeGenerally, it is carried out at the transformation temperature or higher, but Ar_ThreeIf it is from about 10 degreeC to about 10 degreeC low temperature, the characteristic of the final steel plate 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.
[0046]
Temperature Ac determined by the chemical composition of steel when annealing after cold rolling₁And Ac_Three0.3 × (Ac expressed by temperature (for example, “steel material science”: W. C. Leslie, translated by Koyasu Naruyasu, Maruzen P273)_Three-Ac₁) + Ac₁If it is less than (° C.), the amount of austenite obtained at the annealing temperature is small, so that bainite or bainitic ferrite cannot be produced mainly in the final steel sheet. In addition, as the annealing temperature becomes higher, the coarsening of the crystal grains and the surface oxidation are promoted, and in order to increase the manufacturing cost, the upper limit of the annealing temperature is set to Ac._Three+30 (° C). 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. Here, the atmosphere during the temperature rise and annealing was such that the oxygen concentration was 50 ppm or less and the dew point was −20 ° C. or less. If the oxygen concentration exceeds 50 ppm or the dew point exceeds −20 ° C., the plateability of the steel sheet, in particular the wettability, deteriorates and causes non-plating.
[0047]
Subsequent primary cooling is important for producing bainite or bainitic ferrite while suppressing the transformation from the austenite phase to the ferrite phase to some extent. Setting this cooling rate to less than 0.1 ° 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 0.1 ° C./second. On the other hand, when the cooling rate is higher than 100 ° C./second, the hard steel phase such as the martensite phase in the final steel sheet becomes large and operation is difficult.
[0048]
If the subsequent cooling is performed to a plating bath temperature of less than −20 ° C., there are operational problems such as a large heat removal when the plating bath enters. Further, if the cooling stop temperature exceeds the plating bath + 50 ° C., in addition to operational problems, carbides are generated during the subsequent holding, leading to a decrease in strength, so this was made the upper limit. Next, in order to promote the progress of the bainite transformation, the temperature range is maintained. If the retention time is long, it is not preferable in terms of productivity, and carbides are generated. Further, in order to cause the bainite transformation to proceed, it is desirable to hold for 1 second or longer, preferably 15 seconds to 10 minutes. If the plating bath temperature is lower than −20 ° C., the bainite transformation hardly occurs, and if the plating bath temperature exceeds −50 ° C., carbides are generated and the material is deteriorated. Moreover, when performing an alloying process, it was set as 430 degreeC or more and 580 degrees C or less. When the alloying treatment temperature is lower than 430 ° C., the progress of alloying is slow and the productivity is poor. Moreover, when it exceeds 580 degreeC, a carbide | carbonized_material precipitation will accompany and material quality will deteriorate.
The welding method is also within the scope of the present application even when a commonly used welding method, for example, arc, TIG, MIG, mash, laser, or the like is used.
[0049]
【Example】
  Hereinafter, the present invention will be described in more detail with reference to examples.
  A steel plate having a composition as shown in Table 1 was heated to 1200 ° C., and Ar₃Hot rolling was completed above the transformation temperature, and after cooling, the steel strip wound up above the bainite transformation start temperature determined by the chemical composition of each steel was pickled and then cold rolled to a thickness of 1.2 mm.
  Then, according to the following formula from the component (mass%) of each steel, Ac₁And Ac₃The transformation temperature was determined by calculation.
Ac₁  = 723-10.7 × Mn% + 29.1 × Si%,
Ac₃  = 910-203 × (C%)^1/2-15.2 x Ni%
          + 44.7 × Si% + 104 × V% + 31.5 × Mo%
          −30 × Mn% −11 × Cr% + 400 × Al%,
  These Ac₁And Ac₃10% H in the annealing temperature calculated from the transformation temperature₂-N₂After raising the temperature and holding in the atmosphere, it was cooled to 200 to 450 ° C. at a cooling rate of 3 to 150 ° C./second, subsequently held for 1 to 3000 seconds, and then cooled.The cooling stop temperature in Table 3 indicates the final temperature of this cooling step, but the holding temperature is not necessarily the same as this cooling stop temperature.
[0050]
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, each weld was made in the form of a bead on a steel plate, and a ball head overhang test was conducted by lubrication with a fluorine-based resin (Teflon trade name), and the overhang height and fracture position of the base material were measured. In addition, each weld joint is created and a fatigue test in tension-tensile mode is performed.^Five~Ten⁶The fatigue characteristics of the plated joints were compared and evaluated by comparing the fatigue strength with the base metal. Here, the welding joint was bead-on, and after welding, the end face and the surface of the test piece were ground to reduce the joint shape variation factor due to the welding chance as much as possible. In addition, the plating test was evaluated by observing the appearance and giving a five-point score.
In addition, in the corrosion resistance test after plating, the plate was bent with a radius of curvature three times the plate thickness, and then a wet and dry repeated test was performed, and the degree of rusting was also evaluated in five stages.
[0051]
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 expandability.
On the other hand, in the comparative example that deviates from the conditions of the present invention, the fracture position in the ball head overhanging process of the welded portion becomes the weld heat affected zone, the fatigue strength is low, and the hole expandability, plating property and corrosion resistance are also inferior.
[0052]
[Table 1]

[0053]
[Table 2]

[0054]
[Table 3]

[0055]
【The invention's effect】
According to the present invention, it is possible to obtain a hot-dip galvanized high-strength steel sheet and a method for producing the same, in which the weldability, hole expansibility, and corrosion resistance of a high-strength steel sheet having a tensile strength of 800 MPa or more are simultaneously improved.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between fatigue strength ratio × tensile strength × λ (hole expansion rate) and the value on the left side of equation (A).
FIG. 2 is a graph showing the relationship between the appearance score after a dry and wet repeated corrosion test and the value on the left side of equation (B).
FIG. 3 is a graph showing the relationship between the plating appearance score and the value on the left side of equation (C).

Claims

% By mass
C: 0.01-0.20%,
Si: 1.5% or less,
Mn: 0.01 to 3%
P: 0.0010 to 0.1%,
S: 0.0010 to 0.05%,
Al: 0.005 to 4%, further,
Mo: 0.01 to 5.0%,
Nb: One or two of 0.001 to 1.0% is contained within a range that simultaneously satisfies the following formulas (A) and (B), the balance is Fe and inevitable impurities, and the microstructure is bainite or bay contained in an area ratio of 70% or more bainitic ferrite, the tensile strength of Ri der least 800 MPa, weldability, molten zinc excellent in hole expandability and corrosion resistance characterized by having a 70% or more hole expansion ratio Plated high-strength steel sheet.
3.0Nb + 2.5Mo + Mn -5C ^0.5 > 0 (A)
Al + 15.0Si ² + 1.5Si-1.8 <0 (B)

Furthermore, in mass%,
Cr: 0.01-5%
Ni: 0.01 to 5%,
Cu: 0.01 to 5%,
The weldability according to claim 1, characterized in that it contains one or more of the following, and further satisfies the formulas (A) and (C) (however, it is not necessary to satisfy the formula (B) in this section): Hot-dip galvanized high-strength steel sheet with excellent hole expansion and corrosion resistance.
3.0Nb + 2.5Mo + Mn-5C ^0.5 > 0 (A)
^{Al + 15.0Si 2 + 1.5Si + 0.5Cr} - (30Ni + 10Cu) -1.8 <0 ··· (C)

Furthermore, in mass%,
Co: 0.01-5%
W: 0.01 to 5%
The hot-dip galvanized high-strength steel sheet excellent in weldability, hole expansibility, and corrosion resistance according to claim 1 or 2, characterized by containing at least one of the following.

Furthermore, in mass%,
The weldability, hole expansibility, and corrosion resistance according to claims 1 to 3, characterized by containing one or more of Zr, Hf, Ta, Ti, and V in a total amount of 0.001 to 1%. Hot-dip galvanized high-strength steel sheet.

The hot dip galvanized high strength excellent in weldability, hole expansibility and corrosion resistance according to claim 1, further comprising, by mass%, B: 0.0001 to 0.1%. steel sheet.

The weldability and hole according to claim 1, further comprising 0.0001 to 0.5% in total of one or more of Ca, Y, and Rem in terms of mass%. Hot-dip galvanized high-strength steel sheet with excellent spreadability and corrosion resistance.

The cast slab composed of the components according to claim 1 to claim 6 is directly or once cooled and then heated again, the hot-rolled steel sheet wound after hot rolling is pickled, cold-rolled, and then heated and annealed. After annealing at an oxygen concentration of 50 ppm or less, a dew point of −20 ° C. or less, and a maximum temperature during annealing of 0.3 × (Ac ₃ −Ac ₁ ) + Ac ₁ (° C.) or more and Ac ₃ +30 (° C.) or less. , Cooling at a cooling rate of 0.1 to 100 ° C./second to a temperature range of galvanizing bath temperature −20 ° C. to galvanizing bath temperature + 50 ° C., followed by plating immersion in the same temperature range for 1 second to 1000 seconds A method for producing a hot-dip galvanized high-strength steel sheet excellent in weldability, hole expansibility and corrosion resistance, characterized by holding.

In the manufacturing method according to claim 7, after cooling to a temperature range of galvanizing bath temperature -20 ° C to galvanizing bath temperature + 50 ° C, and subsequently holding the plating immersion in the same temperature range for 1 second to 1000 seconds, A method for producing a hot-dip galvanized high-strength steel sheet excellent in weldability, hole expansibility and corrosion resistance, characterized in that the alloying treatment is performed at 430 ° C to 580 ° C.