JP2004244675A - Hot-dip galvanized high strength steel sheet with excellent bore expandabilty, and its manufacturing method - Google Patents
Hot-dip galvanized high strength steel sheet with excellent bore expandabilty, and its manufacturing method Download PDFInfo
- Publication number
- JP2004244675A JP2004244675A JP2003035291A JP2003035291A JP2004244675A JP 2004244675 A JP2004244675 A JP 2004244675A JP 2003035291 A JP2003035291 A JP 2003035291A JP 2003035291 A JP2003035291 A JP 2003035291A JP 2004244675 A JP2004244675 A JP 2004244675A
- Authority
- JP
- Japan
- Prior art keywords
- hot
- steel sheet
- dip galvanized
- mass
- temperature
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
Landscapes
- Coating With Molten Metal (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Abstract
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、穴拡げ性に優れた溶融亜鉛めっきおよび溶融合金化亜鉛めっき高強度鋼板とその製造方法に関する。本発明における亜鉛めっきとは、通常の溶融亜鉛めっきのみならず、合金化溶融亜鉛めっきも含む。めっき層には、純亜鉛の他、Ｆｅ、Ａｌ、Ｍｇ、Ｃｒ、などを含有しても構わない。
【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７８０ＭＰａ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。
【０００３】
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、穴拡げ性などの諸特性が重要となるが、引張り強度で５９０ＭＰａ程度までの高強度鋼板においてはこれらに対する対策が講じられている。
【０００４】
たとえば、穴拡げ性については、非特許文献１に開示されているように、主相をベイナイトとして穴拡げ性を向上させ、さらには張り出し成形性についても、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提案されている。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×穴拡げ率が最大となることも示されている。しかし、ここに開示されている技術は、５９０ＭＰａ程度の鋼板におけるものであり、９００ＭＰａを超えたものについては単純に成り立たない。
【０００５】
また、高強度材の高延性化を図るために、複合組織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、非特許文献２）。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。
【０００６】
また、引っ張り強度９００ＭＰａ以上の強度で、溶融亜鉛メッキを施したものとして、いくつかの開示例がある。これらを、以下にあげる。
【０００７】
（１） 特許文献１及び、特許文献２においては、高い強度を得られる技術が開示されているが、複合組織が主体であるために、相間の硬度差が出てしまい、穴拡げ率が３０％以下と低くなっている。
【０００８】
（２） 特許文献３においては、Ｃ濃度を０．１〜０．２％、Ｍｎ濃度を２〜３％とすることで、オーステナイト相を安定化させ、めっきラインで熱処理後、４８０℃〜５６０℃で低温保持することでオーステナイト相を残すことによって強度と加工性を得る方法が開示されている。しかしながら、残留オーステナイト相があるため、不均一な複合組織になりやすく、穴拡げ率は向上しない。
【０００９】
（３） 特許文献４においては、Ｃ：０．０５％、Ｓｉ：０．５５％、Ｍｎ：１．５９％に、Ｍｏ，Ｔｉ，Ｃｒ，Ｎｂ，Ｂ、Ｖ等を微量に添加し、９３％の穴拡げ率を得ているが，Ｍｎが低いため、熱処理時の加熱温度が低い場合や、熱処理後の冷却速度が低い場合等に不均一複合組織になってしまい、安定的に高い穴拡げ性を得ることが困難である。
【００１０】
（４） 特許文献５においては、Ｃ：０．１６％、Ｍｎ：２．３％、Ｔｉ：０．０１％にＮｂ，Ｂを微量に添加した例が開示されており、強度が１１８０ＭＰａ程度を得ているが、複合組織を基本としたものであるため、相間の硬度差のためにその穴拡げ率は、４０％以下にとどまっている。
【００１１】
従って、引っ張り強度において９００ＭＰａ以上の強度を持ち、穴拡げ率において４５％以上の値を持つ、穴拡げ性に優れた溶融亜鉛メッキ鋼板は、存在しない。
【００１２】
【非特許文献１】
ＣＡＭＰ−ＩＳＩＪ ｖｏｌ．１３ （２０００） ｐ．３９５
【非特許文献２】
ＣＡＭＰ−ＩＳＩＪ， ｖｏｌ．１３（２０００），ｐ．３９１
【特許文献１】
特許第２６０７９０６号公報
【特許文献２】
特許第２８６２１８７号公報
【特許文献３】
特開平１−１９８４５９号公報
【特許文献４】
特開２００１−３５５０４３号公報
【特許文献５】
特許第３０３７７６７号公報
【００１３】
【発明が解決しようとする課題】
前述のような問題の原因の一つに、連続溶融亜鉛めっき工程における冷却能力が挙げられる。即ち、同工程においては、一般に鋼板が最高温度に到達した後、０．１℃／秒〜２０℃／秒という、比較的低い冷却速度が採用されているため、組織が不均一になりやすい。その結果穴拡げ性が劣化することになる。さらに、合金化溶融亜鉛めっきを施したような鋼板においては、溶融亜鉛めっき槽に鋼板を浸漬した後に連続的に熱処理を施し、合金化させ、その後連続的に巻き取ることとなるが、この合金化のための熱処理過程において、それまでに造りこまれた鋼板中の組織において、鉄中に過飽和に固溶した炭素が鉄炭化物として析出し、鋼板中に微細に析出させた炭化鉄が粗大化するなどの現象が起こり、組織が変質してしまう。それに伴い、穴拡げ性が著しく劣化してしまうのである。
【００１４】
本発明は、このような従来技術の問題点を解決し、引張り強度が９００ＭＰａ以上で優れた穴拡げ性を有する溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき高強度鋼板、並びにそれらの製造方法を提供することを目的とする。
【００１５】
【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、化学成分を限定することで、引張り強度が９００ＭＰａ以上で優れた穴拡げ性を有する溶融亜鉛めっき鋼板を得ることができることを発見した。本発明による高強度鋼板は、〔（穴拡げ試験後の穴の内径／穴拡げ試験前の穴径）−１〕×１００で定義される穴拡げ率が４５％以上を有し、耐食性に優れることはもちろん、溶接熱影響部の軟化を抑制して溶接部の疲労耐久性にも優れる。
【００１６】
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。
【００１７】
（１） 質量％で、
Ｃ ：０．０３〜０．１２％、
Ｓｉ：０．００１〜０．８％
Ｍｎ：２．３〜３．３％、
Ｐ：０．００１〜０．１％、
Ｓ：０．０００１〜０．０１％、
Ａｌ：０．００１〜０．２％
を含有し、さらに、
Ｍｏ：０．１１〜１．０％、
Ｔｉ：０．００５〜０．３％
を含有し、残部をＦｅおよび不可避的不純物とし、引張強度が９００ＭＰａ以上であり、且つ穴拡げ率が４５％以上であることを特徴とする穴拡げ性に優れた溶融亜鉛めっき高強度鋼板。
【００１８】
（２） さらに、質量％で、Ｎｉ：０．０１〜２．０％、Ｃｕ：０．００１〜２．０％の１種または２種を含有することを特徴とする上記（１）に記載の穴拡げ性に優れた溶融亜鉛めっき高強度鋼板。
【００１９】
（３） さらに、質量％で、Ｂ：０．０００１〜０．０１％、Ｎｂ：０．００３〜０．３％の１種または２種を含有することを特徴とする上記（１）または（２）に記載の穴拡げ性に優れた溶融亜鉛めっき高強度鋼板。
【００２０】
（４） さらに、質量％で、Ｃｒ：０．０１〜２％、Ｃｏ：０．０１〜１％、Ｗ ：０．０１〜０．３％の１種または２種以上を含有することを特徴とする上記（１）〜（３）のいずれか１項に記載の穴拡げ性に優れた溶融亜鉛めっき高強度鋼板。
【００２１】
（５） さらに、質量％で、Ｚｒ、Ｈｆ、Ｔａ、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上記（１）〜（４）のいずれか１項に記載の穴拡げ性に優れた溶融亜鉛めっき高強度鋼板。
【００２２】
（６） さらに、質量％で、Ｃａ、Ｍｇ、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上記（１）〜（５）のいずれか１項に記載の穴拡げ性に優れた溶融亜鉛めっき高強度鋼板。
【００２３】
（７） 上記（１）〜（６）のいずれか１項に記載の成分からなる鋳造スラブを直接または一旦１０００℃以下まで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、その後雰囲気を酸素濃度が５０ｐｐｍ以下、露点を−２０℃以下とし、最高到達温度をＡｃ３ （℃）以上１１００℃以下とする熱処理をした後に、０．１〜２０℃／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引張強度が９００ＭＰａ以上であり、且つ穴拡げ率が４５％以上である穴拡げ性に優れた溶融亜鉛めっき高強度鋼板の製造方法。
【００２４】
（８） 上記（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℃にて行うことを特徴とする穴拡げ性に優れた溶融亜鉛めっき高強度鋼板の製造方法。
【００２５】
【発明の実施の形態】
以下、本発明を詳細に説明する。
【００２６】
先ず、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。
【００２７】
Ｃ：０．０３〜０．１２％
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相の分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０．０３質量％（以下、同じ）以上を必要とする。０．１２％を越えると、穴拡げ性が低下するのでこれを上限とする。更に、溶接部の強度が劣化しやすくなる。
０．０４〜０．１０％がより好ましい範囲である。
【００２８】
Ｓｉ：０．００１〜０．８％
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ので上限を０．８％とする。また、過剰添加は溶接性に悪影響を及ぼす。表面性状の観点から０．６％を上限とするのが好ましい。一方で、極低Ｓｉ化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０．００１％以上の添加とすることが望ましい。
【００２９】
Ｍｎ：２．３〜３．３％
Ｍｎは、本発明にとって重要である。即ち、前述した溶融亜鉛めっきラインにおける比較的遅い冷却速度に対してＭｎを所定の範囲に限定することが穴拡げ性の観点で極めて重要であることを見出した。フェライト変態を抑制して、面積率最大の相である主相をベイナイトまたはベイニティックフェライトとすることで均一組織を得る。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ため２．３質量％以上とする。好ましくは、２．５％超を下限とする。一方、過剰添加は、マルテンサイト生成を促進したり、偏析などによって延性や穴拡げ性の著しい低下を招くために３．３質量％を上限とした。
【００３０】
Ｐ：０．００１〜０．１％
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０．００１質量％を下限とした。また、多量添加は、溶接性や鋳造時や熱延時の製造性、さらには穴拡げ性にも悪影響を及ぼすため、０．１％を上限とした。
【００３１】
Ｓ：０．０００１〜０．０１％
Ｓは、低Ｓ化は穴拡げ性向上に有効である。一方、極低Ｓ化は経済的に不利であることから、０．０００１質量％を下限とし、また、０．０１質量％を上限としたのは、これを超える量の添加では、鋼板の穴拡げ性に悪影響を及ぼすためである。より好ましくは、０．００３％を上限とする。
【００３２】
Ａｌ：０．００１〜０．２％
Ａｌは、脱酸元素として有効である。このため、０．００１質量％を下限とした。一方過剰添加は穴拡げ性、溶接性およびめっき濡れ性を損なうため０．２％を上限とした。好ましくは、０．００５〜０．０８％の範囲である。
【００３３】
Ｍｏ：０．１１〜１％
Ｔｉ：０．００５〜０．３％
Ｍｏ、Ｔｉは、本発明にとって極めて重要である。即ち、ＭｏとＴｉを複合添加し、かつＭｎ量を上述のように２．３％以上とすることで初めて良好な穴拡げ性が得られる。Ｍｏの添加量は、０．１１％以上にてその効果が得られる。しかしながら、１％を越えるとコストの上昇が問題となるため、上限は、１．０％とする。Ｍｏは、その他に、焼入れ性を向上させ、かつ、溶接時の熱影響部において軟化を防止する効果も有する。
【００３４】
Ｔｉも上述の観点からその下限を０．００５質量％とする。また、過剰添加は、延性と穴拡げ性の劣化を招くことから、上限を０．３％とする。Ｔｉは、基本的に強度−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極めて良好なバランスを得るために有効である。しかし、Ｔｉだけを添加してもその効果は、本発明の目的に対して充分ではなく、上記の通り、ＭｎとＭｏと同時に添加することが必須である。より好ましくは、Ｍｏ：０．２０以上、Ｔｉ：０．０１５以上とする。ＭｏとＴｉの複合添加の効果をより助長するために、Ｍｏ（質量％）＋１０×Ｔｉ（質量％）≧０．３％とすることが望ましい。
【００３５】
さらに、本発明が対象とする鋼は、強度−穴拡げ性バランスに悪影響を与えずにめっき性を向上させることを目的として、Ｃｕ，Ｎｉを添加することができる。
【００３６】
Ｎｉは、めっき性向上以外には焼き入れ性の向上の目的もあり、０．０１質量％以上とし、２質量％を超えると、加工性、特にマルテンサイト生成に伴う硬度上昇に寄与して悪影響を及ぼすため、これを上限とした。
【００３７】
Ｃｕは、めっき性向上以外には強度の向上の目的もあり、０．００１質量％以上の添加とし、２質量％を超えると、加工性および製造性に悪影響を及ぼす。
【００３８】
特にＳｉ量が、０．３％以上添加されている場合には、Ｎｉを０．２％以上、Ｃｕを０．１％以上とすることがめっき性と合金化反応性の観点から望ましい。
【００３９】
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、Ｎｂ，Ｂを添加することができる。
【００４０】
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、０．００３質量％以上の添加とし。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０．３質量％とした。
【００４１】
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あるが、その添加量が０．０１質量％を超えるとその効果が飽和するばかりでなく、Ｎｂと同様に熱間加工性が低下するため、上限を０．０１質量％とした。
【００４２】
さらには、Ｃｒ、Ｃｏ、Ｗの１種または２種以上を含有できる。
【００４３】
Ｃｒは、強化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、２％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。
【００４４】
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、１質量％以下にすることが望ましい。
【００４５】
Ｗは、０．０１質量％以上で強化効果が現れ、０．３質量％を上限としたのは、これを超えると、加工性に悪影響を及ぼすためである。
【００４６】
さらに、本発明が対象とする鋼は、強度と穴拡げ性とのバランス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、Ｖの１種または２種以上を合計で０．００１質量％以上添加としてもよい。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。
【００４７】
Ｃａ、Ｍｇ、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、１種又は２種以上の合計で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。
【００４８】
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合計で０．２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。
【００４９】
次に、基材鋼板の好ましいミクロ組織について述べる。
【００５０】
９００ＭＰａ以上の引張り強度と、優れた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。優れた穴拡げ性を得るためには、面積率で８０％以上とする。
【００５１】
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、例えばアシキュラーフェライトがその１例である。
【００５２】
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８５％を超えることが望ましい。
【００５３】
その他残部組織として、フェライト、オーステナイト、マルテンサイト、上部ベイナイトの各組織の１種または２種以上の合計を、面積率として１５％未満含有しても良い。さらには１０％未満が好ましい。特に優れた穴拡げ性を得るためにはオーステナイト及びマルテンサイトは、面積率で５％以下、さらには３％以下とすることが好ましい。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、更には、これらの複合化合物を合計で面積率２％以下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。
【００５４】
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。
【００５５】
このような組織を有する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。
【００５６】
熱延後冷延・熱処理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１０００℃以下に冷却した後再加熱して熱延を行う。鋳造ままの鋼片をそのまま加熱して熱延することは加熱原単位の減少になり好ましく、また鋼片を１０００℃以下まで冷却すると最終製品の延性の観点から好ましい。
【００５７】
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、低温加熱では圧延時の変形抵抗が高くなってしまう。
【００５８】
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 −１００℃程度の温度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。
【００５９】
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、めっきを施す際に有利である。その後、冷延後熱処理し、溶融亜鉛めっきを施すことで最終製品とする。また、後処理として更に電気めっきを施してもよい。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、３０％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。
【００６０】
冷延後加熱する際に、最高到達温度が鋼の化学成分によって決まる温度Ａｃ３ 温度（例えば「鉄鋼材料学」：Ｗ．Ｃ．Ｌｅｓｌｉｅ著、幸田成康監訳、丸善Ｐ２７３）未満の場合には、加熱時に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、最高到達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、最高到達温度の上限を１１００℃とした。Ａｃ３＋１００（℃）がより好ましい上限である。この温度域での熱処理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１秒以上が必要である。しかし、１０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ので１秒〜１０分とすることが好ましい。ここで、加熱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−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。
【００６１】
その後の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制しつつ、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、冷却速度の上限は高いほうが有利であるが、連続溶融亜鉛めっきライン通板時の緩冷却によっても高い穴拡げ性を確保するのが本発明の目的であるため、あえて上限を２０℃／秒とした。本発明鋼板は、１０℃／秒以下の領域の冷却速度、即ち、従来の鋼板においては強度、及びその穴拡げ性を安定的に確保しにくい冷却速度条件下でも、その目標とする９００ＭＰａ以上の強度及び４５％以上の穴拡げ性を充分に安定的に確保できるのである。更に、この冷却過程において、０．１℃／秒〜２０℃／秒の範囲で単一の冷却速度で目標とする温度まで冷却するのみならず、前記範囲内の冷却速度を複数個組み合わせることにより段階的に冷却速度を変えて目標とする温度まで冷却してもよい。
【００６２】
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。この温度域で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、ベイナイト変態を進行させたり、めっき濡れ性を確保するため１秒以上保持し、好ましくは１５秒から１０分保持する。
【００６３】
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。
【００６４】
本発明で得られる鋼板の引っ張り強度は、９００ＭＰａ以上である。好ましくは、９８０ＭＰａ以上である。上限はとくに限定しないが、１６００ＭＰａ以上とするのは困難であるのでこれを上限とする。穴拡げ性は４５％以上で、好ましくは６０％以上とする。２００％以上とすることは困難なのでこれが実質的な上限である。
【００６５】
また、本発明の鋼は、溶接性にも優れている。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、スポット、ＴＩＧ、ＭＩＧ、マッシュおよびレーザー等の溶接方法に適合する。
【００６６】
【実施例】
本発明になる鋼板について、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を行った。
【００６７】
以下、実施例によって本発明をさらに詳細に説明する。
＜実施例１＞
表１、表２（表１のつづき）に示すような組成の鋳造スラブを一端室温まで冷却した後、１２００℃に加熱し、Ａｒ３ 変態温度以上である８８０℃〜９１０℃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ある５５０℃で巻き取った厚さ２．５ｍｍの鋼帯を酸洗後、冷延して１．２ｍｍ厚の鋼板とした。
【００６８】
その後、各鋼の成分（質量％）から下記式にしたがってＡｃ３ 変態温度を計算により求めた。
【００６９】
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。各鋼の機械的性質及び穴拡げ性を表３に示す。本発明の要綱を満たす発明鋼は、穴拡げ性（４５％以上）強度（引張り強度で９００ＭＰａ以上）のバランスに優れていることがわかる。本発明鋼において、めっきの密着性などで特に問題は無く、めっき性は良好であるが、Ｎｉ及びＣｕが添加されている鋼種ＡＢ及びＡＣにおいては、めっき外観の仕上がりが更に良好であることが分かった。即ち、これらの添加によって更にめっきの密着性等が向上し、めっき性がより良好になっていることが分かる。
【００７０】
また、比較例である鋼種Ｑにおいては、Ｍｏ，Ｔｉが添加されておらず、他の元素組成は、本発明の範囲であり、穴拡げ性において劣っていることが分かる。即ち本発明におけるＭｏ，Ｔｉの効果を示す例である。鋼種Ｔ及びＵは、本発明鋼であり、強度、穴拡げ性ともに良好であるが、穴拡げ性が他の発明鋼に比較してやや劣る。
【００７１】
鋼種Ｔ及び鋼種Ｕにおいては、Ｍｏ（質量％）＋１０ｘＴｉ（質量％）が各々０．２３（％）、０．２１（％）である。この二鋼種以外の本発明になる鋼において、この量はすべて０．３（％）以上であり、発明実施の形態の項で述べたようにＭｏ（質量％）＋１０ｘＴｉ（質量％）≧０．３（％）が望ましいことが分かる。また、鋼種Ｊ及びＫは、発明鋼であり、各々穴広げ率が７６％、７１％と充分な穴拡げ性を示すが、鋼種Ｔ及びＵを除く本発明鋼と比較するとやや劣っている。これは、鋼種Ｊ及びＫにおいて、Ｎｂ及びＢが添加されていないため、冷却過程において組織に不均一性が生じやすくなっているためである。これは、実用に供するにあたって何の問題も生じせしめることはないが、Ｎｂ及びＢを本発明の組成範囲内で添加することが好ましいことを示すものである。
【００７２】
また、本発明の条件から外れる比較鋼は、すべて穴拡げ性が劣勢である。
【００７３】
【表１】
【表２】
【表３】
＜実施例２＞
表１、表２（表１のつづき）に示した鋼のうち、鋼種Ａ及び鋼種Ｒについて、冷延後の熱処理条件を変化させた。即ち、実施例１における熱処理条件において、最高到達温度及び、冷却過程における（最高到達温度−２０）（℃）から６５０℃までの冷却速度を変化させた。その他の条件は、実施例１における条件と同一とした。（但し、最高到達温度が異なるため、昇温過程において１０℃／秒の加熱速度で昇温する過程の到達温度であるところの（最高到達温度−１１０）（℃）は異なる。）それらの鋼について機械的性質及び穴拡げ性を測定した結果を表４に示す。本発明の条件を満たす発明鋼は、穴拡げ率（４５％以上）、強度（引張り強度で９００ＭＰａ以上）のバランスに優れていることがわかる。
【００７７】
一方、本発明の条件から外れる比較鋼は、穴拡げ性が劣勢である。
【００７８】
【表４】
＜実施例３＞
表１、表２（表１のつづき）に示した鋼のうち、鋼種Ｓについて鋳造スラブをそのまま実施例１と同じ条件で加熱〜冷延まで行い、冷延後に以下のように処理を行った。即ち、昇温速度５℃／ｓで最高到達温度まで昇温し、９０秒保持し、次に５℃／秒の冷却速度で６８０℃まで冷却し、その後冷却速度を変化させて４６０℃まで冷却した。引き続き４６０℃の溶融亜鉛めっき槽に浸漬し、その後３℃／秒の昇温速度で５００℃まで加熱し、３０秒保持して合金化処理を施した後、冷却した。以上の処理過程において、最高到達温度、及び６８０℃から４６０℃までの冷却速度を変化させた。処理終了後、機械試験及び穴拡げ性の測定を行った。結果を表５に示す。本発明の条件を満たす発明鋼は、穴拡げ性（４５％以上）強度（引張り強度で９００ＭＰａ以上）のバランスに優れていることがわかる。
【００８０】
一方、本発明の条件から外れる比較鋼は、穴拡げ性が劣勢である。
【００８１】
【表５】
【発明の効果】
本発明により、引張り強度が９００ＭＰａ以上であり、穴拡げ性が４５％以上である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。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to hot-dip galvanized and hot-dip galvanized high-strength steel sheets having excellent hole-expanding properties and suitable for building materials, home appliances, automobiles, and the like, and a method for producing the same. The zinc plating in the present invention includes not only ordinary hot-dip galvanizing but also alloyed hot-dip galvanizing. The plating layer may contain Fe, Al, Mg, Cr, etc. in addition to pure zinc.
[0002]
[Prior art]
In recent years, demand for high-strength steel sheets having good workability for the purpose of improving fuel efficiency and durability has been increasing, particularly in automobile bodies. In addition, steel plates having a tensile strength of 780 MPa class or higher have been used in some members such as reinforces in view of the need for collision safety and an increase in cabin space.
[0003]
When assembling a member using such a high-strength material, various characteristics such as ductility, bendability, and hole-expandability are important. For high-strength steel sheets with a tensile strength of up to about 590 MPa, countermeasures against these are taken. Have been.
[0004]
For example, as for the hole expandability, as disclosed in Non-Patent Document 1, the main phase is bainite to improve the hole expandability, and also for the stretch formability, it is necessary to form the retained austenite in the second phase. Has been proposed to exhibit the same overhang property as the existing retained austenitic steel. Furthermore, it is also shown that when austempering is performed at a temperature of Ms or lower to generate a retained austenite having a volume ratio of 2 to 3%, the tensile strength × the hole expansion rate is maximized. However, the technology disclosed herein is for a steel plate of about 590 MPa, and simply does not hold for a steel sheet exceeding 900 MPa.
[0005]
In addition, in order to increase the ductility of a high-strength material, it is common to actively use a composite structure. However, when martensite or retained austenite is used for the second phase, there is a problem that the hole expandability is significantly reduced (for example, Non-Patent Document 2). In addition, this document discloses that the main phase is ferrite and the second phase is martensite, and the hole expansion ratio is improved by reducing the difference in hardness between the two. %, There is no significant improvement.
[0006]
In addition, there are some disclosed examples of hot-dip galvanizing with a tensile strength of 900 MPa or more. These are listed below.
[0007]
(1) Patent Literature 1 and Patent Literature 2 disclose techniques capable of obtaining high strength. However, since the composite structure is mainly used, a hardness difference between the phases occurs, and the hole expansion rate is 30%. % Or less.
[0008]
(2) In Patent Document 3, by setting the C concentration to 0.1 to 0.2% and the Mn concentration to 2 to 3%, the austenite phase is stabilized, and after heat treatment in a plating line, 480 ° C. to 560. There is disclosed a method of obtaining strength and workability by maintaining an austenite phase by maintaining a low temperature at ℃. However, since there is a retained austenite phase, an uneven composite structure is likely to be formed, and the hole expansion rate is not improved.
[0009]
(3) In Patent Document 4, a small amount of Mo, Ti, Cr, Nb, B, V, etc. is added to C: 0.05%, Si: 0.55%, Mn: 1.59%, and 93 % Hole expansion ratio, but due to low Mn, when the heating temperature during heat treatment is low, or when the cooling rate after heat treatment is low, a heterogeneous composite structure is formed, resulting in a stably high hole. It is difficult to obtain expandability.
[0010]
(4) Patent Document 5 discloses an example in which Nb and B are added in trace amounts to C: 0.16%, Mn: 2.3%, and Ti: 0.01%, and has a strength of about 1180 MPa. However, the hole expansion rate is 40% or less because of the difference in hardness between the phases because of the composite structure.
[0011]
Therefore, there is no hot-dip galvanized steel sheet which has a tensile strength of 900 MPa or more and a hole expansion ratio of 45% or more and has excellent hole expandability.
[0012]
[Non-patent document 1]
CAMP-ISIJ vol. 13 (2000) p. 395
[Non-patent document 2]
CAMP-ISIJ, vol. 13 (2000), p. 391
[Patent Document 1]
Japanese Patent No. 2607906
[Patent Document 2]
Japanese Patent No. 2862187
[Patent Document 3]
JP-A-1-198459
[Patent Document 4]
JP 2001-355043 A
[Patent Document 5]
Japanese Patent No. 3037767
[0013]
[Problems to be solved by the invention]
One of the causes of the above-mentioned problem is the cooling capacity in the continuous hot-dip galvanizing process. That is, in the same process, after the steel sheet reaches the maximum temperature, a relatively low cooling rate of 0.1 ° C./sec to 20 ° C./sec is generally employed, so that the structure tends to be uneven. As a result, the hole expandability deteriorates. Furthermore, in the case of a steel sheet that has been subjected to alloyed hot-dip galvanizing, the steel sheet is immersed in a hot-dip galvanizing bath, then subjected to a continuous heat treatment, alloyed, and then continuously wound up. In the heat treatment process for carbonization, the supersaturated solid solution carbon in iron precipitates as iron carbide in the structure of the steel sheet built up to that point, and the finely precipitated iron carbide in the steel sheet coarsens. Phenomenon occurs, and the organization is deteriorated. Along with this, the hole expandability is significantly deteriorated.
[0014]
The present invention solves such problems of the prior art, and provides a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized high-strength steel sheet having excellent hole expandability with a tensile strength of 900 MPa or more, and a method for producing them. The purpose is to do.
[0015]
[Means for Solving the Problems]
As a result of various studies, the present inventors have found that by limiting the chemical components, a hot-dip galvanized steel sheet having a tensile strength of 900 MPa or more and excellent hole expandability can be obtained. The high-strength steel sheet according to the present invention has a hole expansion ratio defined by [(inner diameter of hole after hole expansion test / hole diameter before hole expansion test) −1] × 100 of 45% or more, and is excellent in corrosion resistance. Needless to say, the softening of the heat affected zone is suppressed and the fatigue durability of the welded portion is excellent.
[0016]
The present invention has been completed based on the above findings, and the gist thereof is as follows.
[0017]
(1) In mass%,
C: 0.03-0.12%,
Si: 0.001 to 0.8%
Mn: 2.3 to 3.3%,
P: 0.001-0.1%,
S: 0.0001-0.01%,
Al: 0.001 to 0.2%
Containing, further,
Mo: 0.11 to 1.0%,
Ti: 0.005 to 0.3%
A high-strength hot-dip galvanized steel sheet having excellent hole expandability, characterized by containing Fe and the remainder being Fe and inevitable impurities, having a tensile strength of 900 MPa or more, and a hole expansion ratio of 45% or more.
[0018]
(2) The above-mentioned (1), further comprising one or two of Ni: 0.01 to 2.0% and Cu: 0.001 to 2.0% by mass%. Hot-dip galvanized high-strength steel sheet with excellent hole expansion properties.
[0019]
(3) The above (1) or (1), further comprising one or two of B: 0.0001 to 0.01% and Nb: 0.003 to 0.3% by mass%. A hot-dip galvanized high-strength steel sheet excellent in hole expandability according to 2).
[0020]
(4) Further, one or more of Cr: 0.01 to 2%, Co: 0.01 to 1%, and W: 0.01 to 0.3% are contained by mass%. The hot-dip galvanized high-strength steel sheet excellent in hole expandability according to any one of the above (1) to (3).
[0021]
(5) Any one of the above (1) to (4), further comprising one or more of Zr, Hf, Ta and V in a mass% of 0.001 to 1% in total. 2. A hot-dip galvanized high-strength steel sheet having excellent hole expandability according to claim 1.
[0022]
(6) Any one of the above (1) to (5), further containing one or more of Ca, Mg, and REM in a mass% of 0.0001 to 0.5% in total. 2. A hot-dip galvanized high-strength steel sheet having excellent hole expandability according to claim 1.
[0023]
(7) The cast slab comprising the component described in any one of the above (1) to (6) is directly or temporarily cooled to 1000 ° C. or lower, and then heated again. After washing and cold rolling, the atmosphere is adjusted to have an oxygen concentration of 50 ppm or less, a dew point of -20 ° C or less, and a maximum temperature of Ac. 3 (° C) or more and 1100 ° C or less, and then cooled at a cooling rate of 0.1 to 20 ° C / sec to a temperature range of −20 ° C to + 50 ° C. High-strength hot-dip galvanized steel with excellent tensile strength of 900MPa or more and excellent hole-expansion rate of 45% or more, characterized by holding for 1 second to 1000 seconds including immersion in a temperature range. Steel plate manufacturing method.
[0024]
(8) In the manufacturing method described in the above (7), the temperature is cooled to a temperature range of zinc plating bath temperature −20 ° C. to zinc plating bath temperature + 50 ° C., and subsequently 1 second to 1000 seconds including plating immersion at the same temperature range. A method for producing a hot-dip galvanized high-strength steel sheet having excellent hole expandability, wherein alloying treatment is performed at 430 ° C. to 580 ° C. after holding for seconds.
[0025]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
[0026]
First, the reason for limiting the preferred range of the steel sheet component in the present invention will be described.
[0027]
C: 0.03 to 0.12%
It is an element added for the purpose of controlling the fractions of the main phase and the second phase for securing a good strength-hole expanding property balance. It also affects the fine uniformity of the substrate. To secure the strength and the area ratio of each second phase, 0.03% by mass (hereinafter the same) is required. If it exceeds 0.12%, the hole expandability is reduced, so this is made the upper limit. Further, the strength of the welded portion is likely to deteriorate.
0.04 to 0.10% is a more preferable range.
[0028]
Si: 0.001 to 0.8%
Si is an element added for the purpose of suppressing the formation of relatively coarse carbides that degrade the strength-ductility balance, but since the plating property is significantly deteriorated, the upper limit is set to 0.8%. Excessive addition has a bad influence on weldability. From the viewpoint of surface properties, the upper limit is preferably set to 0.6%. On the other hand, since ultra-low Si causes an increase in manufacturing cost, it is desirable to add 0.001% or more that does not greatly deteriorate the plating property.
[0029]
Mn: 2.3 to 3.3%
Mn is important for the present invention. That is, it has been found that it is extremely important to limit Mn to a predetermined range for a relatively slow cooling rate in the hot-dip galvanizing line from the viewpoint of hole expandability. A uniform structure is obtained by suppressing the ferrite transformation and setting the main phase, which is the phase having the largest area ratio, to bainite or bainitic ferrite. Further, the content is set to 2.3% by mass or more in order to suppress carbide precipitation and pearlite generation, which are one cause of a decrease in strength and a deterioration in hole expandability. Preferably, the lower limit is more than 2.5%. On the other hand, excessive addition promotes the formation of martensite and causes remarkable decrease in ductility and hole expandability due to segregation and the like, so the upper limit was set to 3.3% by mass.
[0030]
P: 0.001-0.1%
P is a strengthening element. Further, lowering the P improves the hole-expanding property, but the extremely lowering is economically disadvantageous, so the lower limit was made 0.001% by mass. Further, the addition of a large amount has an adverse effect on weldability, manufacturability during casting or hot rolling, and also on hole expandability, so the upper limit was made 0.1%.
[0031]
S: 0.0001-0.01%
As for S, lowering S is effective in improving hole expandability. On the other hand, since the extremely low S is economically disadvantageous, the lower limit of 0.0001% by mass and the upper limit of 0.01% by mass are that the addition of an amount exceeding this lowers the hole in the steel sheet. This is because it has an adverse effect on the spreadability. More preferably, the upper limit is 0.003%.
[0032]
Al: 0.001 to 0.2%
Al is effective as a deoxidizing element. Therefore, the lower limit is 0.001% by mass. On the other hand, excessive addition impairs hole expandability, weldability and plating wettability, so the upper limit was made 0.2%. Preferably, it is in the range of 0.005 to 0.08%.
[0033]
Mo: 0.11 to 1%
Ti: 0.005 to 0.3%
Mo and Ti are extremely important for the present invention. That is, good hole expandability can be obtained only by adding Mo and Ti in a combined manner and setting the Mn content to 2.3% or more as described above. The effect is obtained when the amount of Mo added is 0.11% or more. However, if it exceeds 1%, an increase in cost becomes a problem, so the upper limit is made 1.0%. Mo also has the effect of improving the hardenability and preventing softening in the heat-affected zone during welding.
[0034]
The lower limit of Ti is set to 0.005% by mass from the above viewpoint. Further, excessive addition causes deterioration of ductility and hole expandability, so the upper limit is made 0.3%. Ti basically suppresses the formation of carbides and pearlite that deteriorate the strength-hole expandability balance. In addition, it is effective for suppressing ferrite transformation and making the main phase bainite or bainitic ferrite, and has excellent strength-hole expandability and weldability of plated material and extremely good fatigue durability after welding. It is effective for obtaining balance. However, the effect of adding only Ti is not sufficient for the purpose of the present invention, and as described above, it is essential to add Mn and Mo at the same time. More preferably, Mo: 0.20 or more and Ti: 0.015 or more. In order to further promote the effect of the composite addition of Mo and Ti, it is desirable that Mo (% by mass) + 10 × Ti (% by mass) ≧ 0.3%.
[0035]
Further, the steel targeted by the present invention may be added with Cu and Ni for the purpose of improving the plating property without adversely affecting the strength-hole expandability balance.
[0036]
Ni has the purpose of improving the hardenability in addition to the improvement of the plating property, and is set to 0.01% by mass or more. , The upper limit was set.
[0037]
Cu also has the purpose of improving the strength in addition to the improvement of the plating property. When Cu is added in an amount of 0.001% by mass or more, if it exceeds 2% by mass, workability and manufacturability are adversely affected.
[0038]
In particular, when the amount of Si is 0.3% or more, it is desirable to make Ni 0.2% or more and Cu 0.1% or more from the viewpoint of plating property and alloying reactivity.
[0039]
Furthermore, Nb and B can be added to the steel targeted by the present invention for the purpose of further improving the strength-hole expandability balance.
[0040]
Nb forms fine carbides, nitrides or carbonitrides and is extremely effective in strengthening steel sheets. It also slows down ferrite transformation and promotes the formation of bainite and bainitic ferrite. Further, it is effective in suppressing the softening of the heat affected zone by welding, so 0.003% by mass or more is added. On the other hand, since excessive addition deteriorates ductility and hot workability, the upper limit is set to 0.3% by mass.
[0041]
B is effective for strengthening grain boundaries and increasing the strength of steel when added in an amount of 0.0001% by mass or more. However, when the amount exceeds 0.01% by mass, not only the effect is saturated but also Nb Since the hot workability is reduced as in the case of the above, the upper limit is set to 0.01% by mass.
[0042]
Further, one or more of Cr, Co, and W can be contained.
[0043]
Cr is an element added for the purpose of strengthening, suppressing the formation of carbides, and forming bainite and bainitic ferrite. When Cr is added in an amount of 0.01% or more, workability and plating property are adversely affected. Therefore, this was set as the upper limit.
[0044]
Co was added in an amount of 0.01% by mass or more for a good balance between strength and hole expandability by controlling bainite transformation. On the other hand, the upper limit of the addition is not particularly set. However, since it is an expensive element, the addition of a large amount impairs the economic efficiency, so that it is preferable to set the amount to 1% by mass or less.
[0045]
The effect of strengthening appears when W is 0.01% by mass or more, and the upper limit is set to 0.3% by mass.
[0046]
Further, in the steel targeted by the present invention, in order to further improve the balance between strength and hole expandability, one or more of Zr, Hf, Ta, and V, which are strong carbide forming elements, are added in a total amount of 0.1%. 001% by mass or more may be added. On the other hand, since the ductility and the hot workability are deteriorated, the upper limit of the total amount of one or two or more kinds is set to 1% by mass.
[0047]
Ca, Mg, and REM contribute to inclusion control, particularly fine dispersion, when added in appropriate amounts. Therefore, the total amount of one or more of them is 0.0001% or more. In order to reduce the productivity such as workability and the ductility of the steel sheet product, the upper limit is 0.5% by mass.
[0048]
Inevitable impurities include, for example, N and Sn. However, even if these elements are contained in a total amount of 0.2% by mass or less, the effect of the present invention is not impaired.
[0049]
Next, a preferred microstructure of the base steel sheet will be described.
[0050]
In order to achieve both a tensile strength of 900 MPa or more and excellent hole expandability, bainite or bainitic ferrite is suitable as the main phase. In order to obtain excellent hole expandability, the area ratio is set to 80% or more.
[0051]
In addition, the bainite referred to here includes both upper bainite in which carbide is generated at the lath boundary and lower bainite in which fine carbide is generated in the lath. Further, bainitic ferrite means bainite without carbide, and for example, acicular ferrite is one example.
[0052]
To improve the hole expandability, it is desirable that the lower bainite in which carbide is finely dispersed or the bainitic ferrite without carbide is the main phase and the area ratio exceeds 85%.
[0053]
Other than the remaining structure, one or more of the respective structures of ferrite, austenite, martensite, and upper bainite may be contained in an area ratio of less than 15%. More preferably, it is less than 10%. In order to obtain particularly excellent hole expandability, the area ratio of austenite and martensite is preferably 5% or less, more preferably 3% or less. In addition to the above, the present invention can also be used in the present invention in cases where the remaining microstructure includes carbides, nitrides, sulfides, oxides, and further, these composite compounds have a total area ratio of 2% or less. These were included in the area ratio of the main phase.
[0054]
The identification of each phase of the above microstructure, ferrite (bainitic ferrite), bainite, austenite, martensite, interfacial oxide phase and the remaining structure, observation of the existing position, and measurement of the area ratio were carried out by the Nital reagent and the method disclosed in Corrosion of the cross section in the rolling direction of the steel sheet or the cross section in the direction perpendicular to the rolling by the reagent disclosed in JP-A-59-219473, and quantification by observation with an optical microscope of 500 to 1000 times and an electron microscope of 1000 to 100,000 times (scanning type and transmission type). Is possible. By observing at least 20 visual fields, the area ratio of each tissue can be determined by the point count method or image analysis.
[0055]
A method for producing a high-strength steel sheet having such a structure and excellent in hole expandability and a method for producing the same will be described below.
[0056]
When the steel sheet of the present invention is manufactured by cold rolling and heat treatment after hot rolling, a slab adjusted to a predetermined component is directly or temporarily cooled to 1000 ° C. or lower and then reheated to perform hot rolling. It is preferable to heat the as-cast steel slab and hot-roll it as it is, because it reduces the heat intensity, and it is preferable to cool the slab to 1000 ° C. or less from the viewpoint of the ductility of the final product.
[0057]
It is desirable that the reheating temperature at this time be 1100 ° C. or more and 1300 ° C. or less. When the reheating temperature is high, coarse grains and a thick oxide scale are formed, while on the other hand, when the reheating temperature is low, the deformation resistance during rolling increases.
[0058]
The hot-rolling completion temperature is determined by the chemical composition of steel. 3 Generally, the reaction is performed at a temperature higher than the transformation temperature. 3 Up to a temperature of about −100 ° C., the properties of the final steel sheet are not deteriorated. In addition, by setting the winding temperature after cooling to be higher than the bainite transformation start temperature determined by the chemical composition of the steel, the load during cold rolling can be increased more than necessary, but when the total draft of the cold rolling is small, Is not limited thereto, and does not deteriorate the properties of the final steel sheet even if it is wound at a temperature lower than the bainite transformation temperature of the steel.
[0059]
After hot rolling, if the surface scale is removed by a high-pressure descaling apparatus or pickling, the surface of the product can be cleaned well, which is advantageous when plating. After that, heat treatment is performed after cold rolling, and hot-dip galvanizing is performed to obtain a final product. Further, electroplating may be further performed as a post-treatment. Further, the total rolling reduction of the cold rolling is set from the relationship between the final sheet thickness and the cold rolling load, but if it is 30% or more, it is sufficient for recrystallization and does not deteriorate the properties of the final steel sheet.
[0060]
When heating after cold rolling, the maximum temperature, Ac, is determined by the chemical composition of the steel. 3 If the temperature is lower than the temperature (for example, “Steel Materials Science”: WC Leslie, translated by Shigeyasu Koda, Maruzen P273), the amount of austenite obtained during heating is small, so that bainite or bayite is mainly contained in the final steel sheet. Nitic ferrite cannot be generated. Further, as the maximum temperature reaches a higher temperature, the coarsening of the crystal grains and the surface oxidation are promoted, and the upper limit of the maximum temperature is set to 1100 ° C. in order to increase the production cost. Ac 3 +100 (° C.) is a more preferred upper limit. The heat treatment time in this temperature range requires 1 second or more in order to make the temperature of the steel sheet uniform and to secure austenite. However, if the time exceeds 10 minutes, the formation of the grain boundary oxidized phase is promoted and the cost is increased. Therefore, the time is preferably 1 second to 10 minutes. Here, the atmosphere at the time of heating was set to have an oxygen concentration of 50 ppm or less and a dew point of -20 ° C or less. If the oxygen concentration exceeds 50 ppm or the dew point exceeds −20 ° C., the plating property, particularly the wettability, of the steel sheet deteriorates, which causes non-plating.
[0061]
Subsequent cooling is important for forming bainite or bainitic ferrite while suppressing the transformation of the austenite phase to the ferrite phase to some extent. When the cooling rate is set to less than 0.1 ° C./sec, there is a concern that the formation of ferrite or pearlite is promoted and the strength is reduced, so the lower limit of the cooling rate is set to 0.1 ° C./sec. On the other hand, it is advantageous that the upper limit of the cooling rate is higher. However, since the object of the present invention is to secure high hole expandability even by slow cooling during continuous hot-dip galvanizing line passing, the upper limit is intentionally set at 20 ° C. / Seconds. The steel sheet of the present invention has a cooling rate in the region of 10 ° C./sec or less, that is, the target steel sheet has a target cooling rate of 900 MPa or more even under a cooling rate condition in which it is difficult to stably secure strength and hole expandability in the conventional steel sheet. Strength and hole expandability of 45% or more can be sufficiently and stably secured. Further, in this cooling process, not only cooling to a target temperature at a single cooling rate in the range of 0.1 ° C./sec to 20 ° C./sec, but also combining a plurality of cooling rates within the above-mentioned range. The cooling rate may be changed stepwise to cool to a target temperature.
[0062]
If cooling is performed to a plating bath temperature of less than −20 ° C., there is an operational problem such as a large heat removal when the plating bath enters. Further, when the cooling stop temperature exceeds + 50 ° C. in the plating bath, in addition to the problem in operation, carbides are generated at the time of subsequent holding, resulting in a decrease in strength. If the residence time in this temperature range is long, it is not preferable from the viewpoint of productivity, and carbide is generated. In order to promote bainite transformation and to ensure plating wettability, the temperature is maintained for 1 second or more, preferably for 15 seconds to 10 minutes.
[0063]
In the case of performing the alloying treatment, the temperature was set to 430 ° C. or more and 580 ° C. or less. When the alloying temperature is lower than 430 ° C., the progress of alloying is slow, and the productivity is poor. On the other hand, if the temperature exceeds 580 ° C., carbides are precipitated and the material is deteriorated.
[0064]
The steel sheet obtained by the present invention has a tensile strength of 900 MPa or more. Preferably, it is 980 MPa or more. The upper limit is not particularly limited, but it is difficult to increase the pressure to 1600 MPa or more. The hole spreadability is 45% or more, preferably 60% or more. This is a practical upper limit because it is difficult to make the content 200% or more.
[0065]
Further, the steel of the present invention is excellent in weldability. As for the welding method, it is compatible with welding methods usually performed, for example, welding methods such as arc, spot, TIG, MIG, mash and laser.
[0066]
【Example】
The steel sheet according to the present invention was subjected to microstructure observation, a hole expansion test prescribed by the Iron and Steel Federation, and a tensile test in accordance with JIS.
[0067]
Hereinafter, the present invention will be described in more detail with reference to examples.
<Example 1>
A cast slab having a composition as shown in Tables 1 and 2 (continued from Table 1) was once cooled to room temperature and then heated to 1200 ° C. 3 Hot rolling is completed at 880 ° C to 910 ° C, which is higher than the transformation temperature, and after cooling, a 2.5mm thick steel strip wound at 550 ° C, which is higher than the bainite transformation start temperature determined by the chemical composition of each steel, is pickled. Thereafter, the steel sheet was cold-rolled into a steel sheet having a thickness of 1.2 mm.
[0068]
Then, from the components (% by mass) of each steel, Ac 3 The transformation temperature was determined by calculation.
[0069]
JIS No. 5 tensile test pieces were collected from these steel sheets, and their mechanical properties were measured. Further, a hole expansion test was performed in accordance with the standards of the Iron and Steel Federation to determine the hole expansion ratio. Table 3 shows the mechanical properties and hole expandability of each steel. It can be seen that the invention steel satisfying the requirements of the present invention has an excellent balance of hole expandability (45% or more) and strength (tensile strength of 900 MPa or more). In the steel of the present invention, there is no particular problem in the adhesion of the plating and the like, and the plating properties are good. However, in the steel types AB and AC to which Ni and Cu are added, the finish of the plating appearance is more favorable. Do you get it. That is, it can be seen that the addition of these further improves the plating adhesion and the like, and further improves the plating properties.
[0070]
In addition, in the steel type Q as a comparative example, Mo and Ti were not added, and the other element compositions were within the scope of the present invention, and it was found that the hole expandability was poor. That is, this is an example showing the effects of Mo and Ti in the present invention. Steel types T and U are steels of the present invention and have good strength and hole expandability, but are slightly inferior in hole expandability to other invention steels.
[0071]
In steel type T and steel type U, Mo (% by mass) + 10 × Ti (% by mass) is 0.23 (%) and 0.21 (%), respectively. In the steels according to the present invention other than these two steel types, the amounts are all 0.3 (%) or more, and as described in the section of the embodiment of the invention, Mo (% by mass) + 10 × Ti (% by mass) ≧ 0. It is understood that 3 (%) is desirable. Further, steel types J and K are invention steels, and have sufficient hole expandability of 76% and 71%, respectively, but are slightly inferior to the steels of the present invention except steel types T and U. This is because in the steel types J and K, since Nb and B are not added, the structure tends to be non-uniform during the cooling process. This does not cause any problem in practical use, but indicates that it is preferable to add Nb and B within the composition range of the present invention.
[0072]
All of the comparative steels that do not satisfy the conditions of the present invention have inferior hole expandability.
[0073]
[Table 1]
[Table 2]
[Table 3]
<Example 2>
Of the steels shown in Tables 1 and 2 (continued from Table 1), the heat treatment conditions after cold rolling were changed for steel types A and R. That is, under the heat treatment conditions in Example 1, the maximum attained temperature and the cooling rate from (maximum attained temperature−20) (° C.) to 650 ° C. in the cooling process were changed. Other conditions were the same as those in Example 1. (However, since the maximum temperature is different, (the maximum temperature-110) (° C.) which is the temperature reached in the process of raising the temperature at a heating rate of 10 ° C./sec in the temperature raising process is different.) Table 4 shows the results of measuring the mechanical properties and the hole expandability of the sample. It can be seen that the invention steel satisfying the conditions of the present invention has an excellent balance between the hole expansion rate (45% or more) and the strength (tensile strength of 900 MPa or more).
[0077]
On the other hand, the comparative steels deviating from the conditions of the present invention are inferior in hole expandability.
[0078]
[Table 4]
<Example 3>
Among the steels shown in Tables 1 and 2 (continued from Table 1), for the steel type S, the cast slab was directly subjected to heating to cold rolling under the same conditions as in Example 1, and after the cold rolling, the following treatment was performed. . That is, the temperature is raised to the highest temperature at a rate of 5 ° C./s, held for 90 seconds, then cooled to 680 ° C. at a cooling rate of 5 ° C./sec, and then changed to a cooling rate of 460 ° C. did. Subsequently, it was immersed in a hot-dip galvanizing bath at 460 ° C., heated to 500 ° C. at a rate of 3 ° C./second, held for 30 seconds to perform an alloying treatment, and then cooled. In the above process, the maximum temperature and the cooling rate from 680 ° C. to 460 ° C. were changed. After the treatment, a mechanical test and a measurement of hole expandability were performed. Table 5 shows the results. It can be seen that the invention steel satisfying the conditions of the present invention has an excellent balance of hole expandability (45% or more) and strength (tensile strength of 900 MPa or more).
[0080]
On the other hand, the comparative steels deviating from the conditions of the present invention are inferior in hole expandability.
[0081]
[Table 5]
【The invention's effect】
According to the present invention, a hot-dip galvanized high-strength steel sheet having a tensile strength of 900 MPa or more and a hole expandability of 45% or more and a method for producing the same can be obtained.
Claims (8)
Ｃ ：０．０３〜０．１２％、
Ｓｉ：０．００１〜０．８％、
Ｍｎ：２．３〜３．３％、
Ｐ：０．００１〜０．１％、
Ｓ：０．０００１〜０．０１％、
Ａｌ：０．００１〜０．２％
を含有し、さらに、
Ｍｏ：０．１１〜１．０％、
Ｔｉ：０．００５〜０．３％
を含有し、残部をＦｅおよび不可避的不純物とし、引張強度が９００ＭＰａ以上であり、且つ穴拡げ率が４５％以上であることを特徴とする穴拡げ性に優れた溶融亜鉛めっき高強度鋼板。In mass%,
C: 0.03-0.12%,
Si: 0.001 to 0.8%,
Mn: 2.3 to 3.3%,
P: 0.001-0.1%,
S: 0.0001-0.01%,
Al: 0.001 to 0.2%
Containing, further,
Mo: 0.11 to 1.0%,
Ti: 0.005 to 0.3%
A high-strength hot-dip galvanized steel sheet having excellent hole expandability, characterized by containing Fe and the remainder being Fe and inevitable impurities, having a tensile strength of 900 MPa or more, and a hole expansion ratio of 45% or more.
Ｎｉ：０．０１〜２．０％、
Ｃｕ：０．００１〜２．０％、
の１種または２種を含有することを特徴とする請求項１に記載の穴拡げ性に優れた溶融亜鉛めっき高強度鋼板。Furthermore, in mass%,
Ni: 0.01 to 2.0%,
Cu: 0.001 to 2.0%,
The hot-dip galvanized high-strength steel sheet having excellent hole expandability according to claim 1, comprising one or two of the following.
Ｂ：０．０００１〜０．０１％、
Ｎｂ：０．００３〜０．３％、
の１種または２種を含有することを特徴とする請求項１または請求項２に記載の穴拡げ性に優れた溶融亜鉛めっき高強度鋼板。Furthermore, in mass%,
B: 0.0001 to 0.01%,
Nb: 0.003 to 0.3%,
The hot-dip galvanized high-strength steel sheet excellent in hole expandability according to claim 1 or 2, which comprises one or two of the following.
Ｃｒ：０．０１〜２．０％
Ｃｏ：０．０１〜１％、
Ｗ ：０．０１〜０．３％
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〜請求項３のいずれか１項に記載の穴拡げ性に優れた溶融亜鉛めっき高強度鋼板。Furthermore, in mass%,
Cr: 0.01 to 2.0%
Co: 0.01-1%,
W: 0.01 to 0.3%
The hot-dip galvanized high-strength steel sheet having excellent hole expandability according to any one of claims 1 to 3, wherein the hot-dip galvanized steel sheet contains one or more of the following.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2003035291A JP4288085B2 (en) | 2003-02-13 | 2003-02-13 | Hot-dip galvanized high-strength steel sheet excellent in hole expansibility and method for producing the same |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2003035291A JP4288085B2 (en) | 2003-02-13 | 2003-02-13 | Hot-dip galvanized high-strength steel sheet excellent in hole expansibility and method for producing the same |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JP2004244675A true JP2004244675A (en) | 2004-09-02 |
| JP4288085B2 JP4288085B2 (en) | 2009-07-01 |
Family
ID=33020755
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP2003035291A Expired - Fee Related JP4288085B2 (en) | 2003-02-13 | 2003-02-13 | Hot-dip galvanized high-strength steel sheet excellent in hole expansibility and method for producing the same |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JP4288085B2 (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2007119842A (en) * | 2005-10-27 | 2007-05-17 | Jfe Steel Kk | Method for producing high-strength galvanized steel sheet excellent in stretch-flanging property |
| WO2008078912A1 (en) * | 2006-12-27 | 2008-07-03 | Posco | Zn-coated steel sheet having excellent surface quality and the method for manufacturing the same |
| WO2008082146A1 (en) * | 2006-12-28 | 2008-07-10 | Posco | High strength zn-coated steel sheet having excellent mechanical properites and surface quality and the method for manufacturing the same |
-
2003
- 2003-02-13 JP JP2003035291A patent/JP4288085B2/en not_active Expired - Fee Related
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2007119842A (en) * | 2005-10-27 | 2007-05-17 | Jfe Steel Kk | Method for producing high-strength galvanized steel sheet excellent in stretch-flanging property |
| WO2008078912A1 (en) * | 2006-12-27 | 2008-07-03 | Posco | Zn-coated steel sheet having excellent surface quality and the method for manufacturing the same |
| WO2008082146A1 (en) * | 2006-12-28 | 2008-07-10 | Posco | High strength zn-coated steel sheet having excellent mechanical properites and surface quality and the method for manufacturing the same |
| JP2010502845A (en) * | 2006-12-28 | 2010-01-28 | ポスコ | High strength galvanized steel sheet with excellent mechanical properties and surface quality and method for producing the same |
Also Published As
| Publication number | Publication date |
|---|---|
| JP4288085B2 (en) | 2009-07-01 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| JP6635236B1 (en) | High strength cold rolled steel sheet and method for producing the same | |
| JP6525114B1 (en) | High strength galvanized steel sheet and method of manufacturing the same | |
| KR101130837B1 (en) | High-strength steel sheets which are extreamely excellent in the balance between burring workability and ductility and excellent in fatigue endurance, zinc-coated steel sheets, and processes for production of both | |
| JP4072090B2 (en) | High-strength steel sheet with excellent stretch flangeability and manufacturing method thereof | |
| CN109072380B (en) | Steel sheet, plated steel sheet, and method for producing same | |
| JP5162836B2 (en) | High-strength cold-rolled steel sheet excellent in hydrogen embrittlement resistance of welds and method for producing the same | |
| JP6544494B1 (en) | High strength galvanized steel sheet and method of manufacturing the same | |
| JP3704306B2 (en) | Hot-dip galvanized high-strength steel sheet excellent in weldability, hole expansibility and corrosion resistance, and method for producing the same | |
| JP2007211279A (en) | Ultrahigh strength steel sheet having excellent hydrogen brittleness resistance, method for producing the same, method for producing ultrahigh strength hot dip galvanized steel sheet and method for producing ultrahigh strength hot dip alloyed galvanized steel sheet | |
| WO2012002565A1 (en) | High-strength hot-dip galvanized steel sheet with excellent processability and process for producing same | |
| CN108779536B (en) | Steel sheet, plated steel sheet, and method for producing same | |
| JP2005105367A (en) | High yield ratio and high strength cold-rolled steel plate and high yield ratio and high strength galvanized steel plate excellent in weldability and ductility, and high yield ratio and high strength alloyed galvanized steel plate and its manufacturing method | |
| JP2007231369A (en) | High-strength cold rolled steel, high-strength hot dip galvanized steel sheet and high-strength galvannealed steel sheet having excellent formability and weldability, method for producing high-strength cold rolled steel sheet, method for producing high-strength hot dip galvanized steel sheet and method for producing high-strength galvannealed steel sheet | |
| WO2017168958A1 (en) | Thin steel sheet, plated steel sheet, method for producing hot-rolled steel sheet, method for producing cold-rolled full hard steel sheet, method for producing thin steel sheet, and method for producing plated steel sheet | |
| JP4050991B2 (en) | High-strength steel sheet with excellent stretch flangeability and manufacturing method thereof | |
| JP4528135B2 (en) | High strength and high ductility hot dip galvanized steel sheet excellent in hole expansibility and method for producing the same | |
| JP3698046B2 (en) | High-strength hot-dip galvanized steel sheet excellent in workability and plating property and method for producing the same | |
| KR102217100B1 (en) | High-strength steel sheet and its manufacturing method | |
| EP4180547A1 (en) | Hot-pressed member and manufacturing method therefor | |
| JP2006265607A (en) | High strength cold rolled steel sheet, high strength hot dip galvanized steel sheet, high strength alloyed hot dip galvannealed steel sheet, production method of high strength cold rolled steel sheet, production method of high hot dip galvannealed steel sheet, and production method of high strength alloyed galvannealed steel sheet | |
| JP4500197B2 (en) | Manufacturing method of high-strength cold-rolled steel sheet, high-strength hot-dip galvanized steel sheet, and high-strength galvannealed steel sheet with excellent formability and weldability | |
| JP2010043360A (en) | High-strength and high-ductility hot-dip galvanized steel sheet superior in hole expandability, and manufacturing method therefor | |
| JP2006274378A (en) | High yield ratio high strength cold rolled steel sheet, high yield ratio high strength hot dip galvanized steel sheet, high yield ratio high strength alloyed hot dip galvanized steel sheet, and method for producing them | |
| JP2012031466A (en) | High strength steel sheet, and method of manufacturing the same | |
| JP6947327B2 (en) | High-strength steel sheets, high-strength members and their manufacturing methods |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| A621 | Written request for application examination |
Free format text: JAPANESE INTERMEDIATE CODE: A621 Effective date: 20050913 |
|
| A977 | Report on retrieval |
Free format text: JAPANESE INTERMEDIATE CODE: A971007 Effective date: 20071119 |
|
| A131 | Notification of reasons for refusal |
Free format text: JAPANESE INTERMEDIATE CODE: A131 Effective date: 20080401 |
|
| A521 | Request for written amendment filed |
Free format text: JAPANESE INTERMEDIATE CODE: A523 Effective date: 20080514 |
|
| TRDD | Decision of grant or rejection written | ||
| A01 | Written decision to grant a patent or to grant a registration (utility model) |
Free format text: JAPANESE INTERMEDIATE CODE: A01 Effective date: 20090317 |
|
| A01 | Written decision to grant a patent or to grant a registration (utility model) |
Free format text: JAPANESE INTERMEDIATE CODE: A01 |
|
| A61 | First payment of annual fees (during grant procedure) |
Free format text: JAPANESE INTERMEDIATE CODE: A61 Effective date: 20090330 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20120403 Year of fee payment: 3 |
|
| R151 | Written notification of patent or utility model registration |
Ref document number: 4288085 Country of ref document: JP Free format text: JAPANESE INTERMEDIATE CODE: R151 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20120403 Year of fee payment: 3 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20120403 Year of fee payment: 3 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20130403 Year of fee payment: 4 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20130403 Year of fee payment: 4 |
|
| S531 | Written request for registration of change of domicile |
Free format text: JAPANESE INTERMEDIATE CODE: R313531 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20130403 Year of fee payment: 4 |
|
| R350 | Written notification of registration of transfer |
Free format text: JAPANESE INTERMEDIATE CODE: R350 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20130403 Year of fee payment: 4 |
|
| S533 | Written request for registration of change of name |
Free format text: JAPANESE INTERMEDIATE CODE: R313533 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20130403 Year of fee payment: 4 |
|
| R350 | Written notification of registration of transfer |
Free format text: JAPANESE INTERMEDIATE CODE: R350 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20140403 Year of fee payment: 5 |
|
| S533 | Written request for registration of change of name |
Free format text: JAPANESE INTERMEDIATE CODE: R313533 |
|
| R350 | Written notification of registration of transfer |
Free format text: JAPANESE INTERMEDIATE CODE: R350 |
|
| LAPS | Cancellation because of no payment of annual fees |