JP4317384B2

JP4317384B2 - High-strength galvanized steel sheet with excellent hydrogen embrittlement resistance, weldability and hole expansibility, and its manufacturing method

Info

Publication number: JP4317384B2
Application number: JP2003123365A
Authority: JP
Inventors: 展弘藤田; 邦夫林; 明宇佐見; 学高橋
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2003-04-28
Filing date: 2003-04-28
Publication date: 2009-08-19
Anticipated expiration: 2023-04-28
Also published as: JP2004323951A

Description

【０００１】
【発明の属する技術分野】
本発明は、自動車、建材、家電製品などに適する耐水素脆化、溶接性および穴拡げ性に優れた高強度亜鉛めっき鋼板およびその製造方法に関する。
【０００２】
【従来の技術】
従来、ボルト、ＰＣ鋼線やラインパイプといった用途には高強度鋼が多く使われており、９８０ＭＰａ以上の引張強さになると、鋼中への水素の侵入により遅れ破壊が発生することが知られている。これに対し、▲１▼薄鋼板は板厚が薄いため水素が侵入しても短時間で放出されること、▲２▼加工性の点で９８０ＭＰａ以上の鋼板の利用がほとんどなかったことなどから、遅れ破壊に対する問題意識は低かったと言える。
【０００３】
しかし、最近では自動車の軽量化や衝突安全性の向上の必要性から、９８０ＭＰａ以上の超高強度薄鋼板にプレス成形、パイプ成形、曲げ加工、端面加工、穴拡げ加工などを施して、バンパーやインパクトビーム等の補強材やシートレール等に使用に供する場合が急速に増えてきている。したがって、耐遅れ破壊性を備えた超高強度薄鋼板の開発が急務である。これまで、耐遅れ破壊を向上させる技術はほとんどがボルトや条鋼、厚板といった製品のままで、かつ耐力または降伏応力以下で使用されることの多い鋼材に対して開発されてきた。
【０００４】
例えば条鋼・ボルト用鋼においては、焼き戻しマルテンサイトを中心に開発が行われ、「遅れ破壊解明の新展開」（日本鉄鋼協会、１９９７年１月発行）
（非特許文献１）にＣｒ, ＭｏやＶといった焼き戻し軟化抵抗性を示す添加元素が耐遅れ破壊性向上に有効であることが報告されている。これは、合金炭化物を析出させて、これを水素のトラップサイトに活用することで遅れ破壊形態を粒界から粒内破壊へと移行させる技術である。しかし、これらの鋼はＣ量０．４％以上で合金元素も多く含むことから、薄鋼板で要求される加工性や溶接性が劣悪で、さらに、合金炭化物析出には数時間以上という析出熱処理が必要なため、製造性にも問題がある。
【０００５】
また、特開平１１―２９３３８３号公報（特許文献１）では、Ｔｉ、Ｍｇを主体とする酸化物が水素性欠陥を防ぐことに効果があるとされている。しかし、これは対象が厚鋼板であり、特に大入熱の溶接後の遅れ破壊については考慮されているものの、薄鋼板に要求される加工度の高い成形加工を受けたり、端面加工に伴うバリ発生等の遅れ破壊現象に及ぼす影響については一切考慮されていない。さらには、薄鋼板の基本的特性である加工性についての考慮もいっさい無い。
【０００６】
一方、薄鋼板の遅れ破壊に関しては、例えば、ＣＡＭＰ−ＩＳＩＪ，ｖｏｌ．５，１８３９〜１８４２頁、山崎ら、１９９２年１０、日本鉄鋼協会発行
（非特許文献２）に残留オーステナイト量の加工誘起変態に起因した遅れ破壊の助長について報告されている。これは、薄鋼板の成型加工を考慮したものであるが、耐遅れ破壊性を劣化させない残留オーステナイト量の規制について述べられている。すなわち、特定の組織を持つ高強度薄鋼板に関するものであり、根本的な耐遅れ破壊向上対策とは言えない。さらに、このような高強度材を用いて部材を組みあげる時には、延性、曲げ性、穴拡げ性や溶接性などが、引張強さが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。
【０００７】
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），ｐ３９５（非特許文献３）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形成性についても、第２相に残留オーステナイトを生成させることで現行の残留オーステナイト鋼並の張り出し性を示すことが開示されている。さらには、Ｍｓ温度以下でオーステンパ処理をすることで面積率２〜３％の残留オーステナイトを生成させると、引張強さ×穴拡率が最大となることも示されている。しかし、引張強さが８００ＭＰａを超えて顕在化する溶接性および溶接熱影響部での軟化挙動については考慮されていない。
【０００８】
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号公報（特許文献２）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軟化挙動抑制の効果も強度レベルが低く、８００ＭＰａ以上の極めて高強度な材料における溶接性や加工性について十分とはいえない。
【０００９】
特に、引張強さが８００ＭＰａ以上になると、溶接自体が困難になり、引張強さが９８０ＭＰａ以上でさらに顕著となる。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クラスの高強度材に比べ極めて顕著となる。また、高強度化へのマルテンサイトの活用は穴拡げ性や延性低下が助長されてしまう。
【００１０】
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），ｐ３９１（非特許文献４）｝。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。また、上記検討はすべて熱延および冷延に関するものであり、特にめっき工程での水素侵入やめっき工程前で侵入した水素のめっき工程での挙動などは一切考慮されていない。
【００１１】
【引用文献】
（１）非特許文献１｛「遅れ破壊解明の新展開」（日本鉄鋼協会、１９９７年１月発行）｝
（２）非特許文献２（ＣＡＭＰ−ＩＳＩＪ，ｖｏｌ．５，１８３９〜１８４２頁、山崎ら、１９９２年１０、日本鉄鋼協会発行）
（３）非特許文献３（ＣＡＭＰ−ＩＳＩＪ，ｖｏｌ．１３（２０００），ｐ３９５）
（４）非特許文献４（ＣＡＭＰ−ＩＳＩＪ，ｖｏｌ．１３（２０００），ｐ３９１）
（５）特許文献１（特開平１１―２９３３８３号公報）
（６）特許文献２（特開２０００−８７１７５号公報）
【００１２】
【発明が解決しようとする課題】
上記のように、特に自動車用薄鋼板の製造〜組み立て〜使用環境を十分考慮して水素脆化型の遅れ破壊に対する対策を講じかつ使用特性である溶接性や穴広げ性等を十分考慮した高強度めっき鋼板に関する開発事例はない。本発明は、前述のような従来技術の問題点を解決し、引張強さが９００ＭＰａ以上の高強度鋼板の溶接性および穴拡げ性を同時に改善した高強度亜鉛めっき鋼板およびその製造方法を提供することを目的とする。
【００１３】
【課題を解決するための手段】
本発明者らは、以上のような背景から、薄鋼板における使用環境および現状設備での製造方法を十分に考慮して、根本的に耐遅れ破壊性を向上させる方法を見出すに至った。すなわち、鋼板の組織および析出物制御に加えて鋼板中のトラップサイト制御および製造工程や使用環境から侵入し得る水素量を低減することで水素起因の耐遅れ破壊性を向上させることが可能なことを見出した。詳細は以下の通りである。
【００１４】
（１）質量％で、Ｃ：０．０１〜０．２５％、Ｓｉ：０．０１〜２．０％、Ｍｎ：０．０１〜４．０％、Ｐ：０．０００１〜０．０５％、Ｓ：０．０００１〜０．０５％、Ａｌ：０．０１〜３．０％、Ｎ：０．０００１〜０．０１％、Ｍｏ：０．００５〜５％、Ｎｂ：０．００１〜１．０％を含有し、残部が鉄および不可避的不純物からなり、ミクロ組織が、面積率でベイナイト、マルテンサイトの一方又は双方を合計で７０％以上、残留オーステナイト（Ｖγ）を３％未満含有し、引張強さ（ＴＳ）が９００ＭＰａ以上であり、更に下記式（１−１）〜（１−３）を満たすことを特徴とする耐水素脆化、溶接性および穴拡げ性に優れた高強度亜鉛めっき鋼板。
（３．０Ｎｂ＋２．５Ｍｏ＋１／１０Ｓｉ＋Ｍｎ）−（２Ｃ^0.5＋２）＞０ … （１−１）
０≦０．８×｛２Ｃｕ＋２０Ｍｏ＋３Ｎｉ＋Ｃｒ｝−｛０．１−３．５×１０⁷×（ＴＳ）^-3.1｝−０．３Ｖγ … （１−２）
０＞Ｓｉ＋Ａｌ＋２−（Ｍｎ＋Ｎｉ＋１．５Ｍｏ） … （１−３）
ここで、ＴＳ：引張強さ（ＭＰａ）、
元素記号は鋼中に含まれる各元素の質量％を示す。
（２）更に、質量％で、Ｎｉ：０．００１〜５．５％、Ｃｕ：０．００１〜３．０％、Ｃｒ：０．００１〜５．０％のうち１種または２種以上を含有することを特徴とする前記（１）記載の耐水素脆化、溶接性および穴拡げ性に優れた高強度亜鉛めっき鋼板。
【００１５】
（３）更に、質量％で、Ｖ：０．００５〜１％を含有し、残部が鉄および不可避的不純物からなり、ミクロ組織が、面積率でベイナイト、ベイニティックフェライトの一方又は双方を合計で７０％以上、残留オーステナイト（Ｖγ）を３％未満含有し、引張強さ（ＴＳ）が９００ＭＰａ以上であり、更に下記（２−１）〜（２−３）式を満たすことを特徴とする前記（１）又は（２）記載の耐水素脆化、溶接性および穴拡げ性に優れた高強度亜鉛めっき鋼板。
（３．０Ｎｂ＋２．５Ｍｏ＋１／１０Ｓｉ＋Ｍｎ）−（２Ｃ^0.5＋２）＞０ … （２−１）
０≦０．８×｛２Ｃｕ＋２０Ｍｏ＋３Ｎｉ＋Ｃｒ＋２０Ｖ｝−｛０．１−Ｖ／５−３．５×１０⁷×（ＴＳ）^-3.1｝−０．３Ｖγ … （２−２）
０＞Ｓｉ＋Ａｌ＋２−（Ｍｎ＋Ｎｉ＋１．５Ｍｏ） … （２−３）
ここで、ＴＳ：引張強さ（ＭＰａ）、
元素記号は鋼中に含まれる各元素の質量％を示す。
【００１６】
（４）更に、質量％にて、Ｓｅ：０．０００２〜０．０５％、Ａｓ：０．０００２〜０．０５％、Ｓｂ：０．０００２〜０．０５％、Ｐｂ：０．０００２〜０．０５％、Ｂｉ：０．０００２〜０．０５％、の１種または２種以上を含有し、かつ、それらの合計が０．０５％以下を満たすことを特徴とする前記（１）〜（３）の何れか１項に記載の耐水素脆化、溶接性および穴拡げ性に優れた高強度薄鋼板。
【００１７】
（５）さらに、鋼中に質量％で、Ｔｉ：０．００１〜５％を含有し、Ｎｂ、Ｖ、Ｃｒ、Ｔｉ、Ｍｏの酸化物、硫化物、窒化物、複合晶出物、複合析出物のいずれか１種以上が、平均粒子径ｄ：０．００１〜５．０μｍ、密度ρ：１平方ｍｍあたり１００〜１×１０¹³個、分布：平均粒子径からの標準偏差σと平均粒子径ｄの比：σ／ｄ≦１．０を満たす分布形態を有し、引張強度が９００ＭＰａ以上であることを特徴とする前記（１）〜（４）の何れか１項に記載の耐水素脆化、溶接性および穴拡げ性に優れた高強度亜鉛めっき鋼板。
【００１８】
（６）さらに、鋼中に質量％で、Ｗ：０．００１〜５％、Ｚｒ：０．００１〜５％、Ｈｆ：０．００１〜５％、Ｔａ：０．００１〜５％の１種または２種以上を合計で０．００１〜５％含有することを特徴とする前記（１）〜（５）の何れか１項に記載の耐水素脆化、溶接性および穴拡げ性に優れた高強度亜鉛めっき鋼板。
（７）さらに、鋼中に質量％で、Ｂ：０．０００２〜０．１％を含有することを特徴とする前記（１）〜（６）の何れか１項に記載の耐水素脆化、溶接性および穴拡げ性に優れた高強度亜鉛めっき鋼板。
【００１９】
（８）さらに、鋼中に質量％で、ＲＥＭ：０．０００２〜０．１％、Ｙ：０．０００２〜０．１％、Ｃａ：０．０００２〜０．１％、Ｍｇ：０．０００２〜０．１％の１種または２種以上を含有することを特徴とする前記（１）〜（７）の何れか１項に記載の耐水素脆化、溶接性および穴拡げ性に優れた高強度亜鉛めっき鋼板。
【００２０】
（９）前記（１）〜（８）のいずれか１項に記載の鋼板を製造する方法であって、（１）〜（８）のいずれか１項に記載の成分からなる鋳造スラブを鋳造まま、あるいは、一旦冷却した後に再度加熱し、Ａｒ₃点＋３０℃以上の仕上温度で熱間圧延を終了し、その後巻取温度まで０．１〜１０００／秒の冷却速度で冷却後４００℃〜７００℃で巻き取った熱延鋼板を酸洗後冷延し、その後、Ａｃ₁＋３０（℃）以上、Ａｃ₃＋５０（℃）以下の温度域で１０秒〜３０分焼鈍した後に、平均冷速で０．１〜１００℃／秒で２００℃〜めっき浴温度＋１００（℃）にまで冷却した後、Ｚｎめっき浴温度〜Ｚｎめっき浴温度＋１００（℃）の温度域で後続のめっき浸漬時間を含めて１秒〜３０００秒保持し、Ｚｎめっき浴に浸漬して、その後室温まで冷却することを特徴とする耐水素脆化、溶接性および穴拡げ性に優れた高強度亜鉛めっき鋼板の製造方法。
【００２１】
（１０）Ｚｎめっき浴に浸漬した後、３００〜６００℃で合金化処理を行い、室温まで冷却することを特徴とする前記（９）記載の耐水素脆化、溶接性および穴拡げ性に優れた高強度亜鉛めっき鋼板の製造方法。
（１１）前記（１）〜（８）のいずれか１項に記載の鋼板を製造する方法であって、（１）〜（８）のいずれか１項に記載の成分からなる鋳造スラブを鋳造まま、あるいは、一旦冷却した後に再度加熱し、Ａｒ₃点＋３０℃以上の仕上温度で熱間圧延を終了し、その後巻取温度まで０．１〜１０００℃／秒の冷却速度で冷却後４００℃〜７００℃で巻き取った熱延鋼板を酸洗後、圧下率１０〜８０％として冷間圧延し、その後焼鈍時にＡｃ₁＋３０（℃）以上、Ａｃ₃＋５０（℃）以下の温度域で１０秒〜３０分焼鈍した後に、平均冷速で０．１〜２００℃／秒の冷却速度で１５０〜５００℃の温度域に冷却し、同温度域で１秒〜３０００秒保持したのち、電気めっきを施すことを特徴とする耐水素脆化、溶接性および穴拡げ性に優れた高強度亜鉛めっき薄鋼板の製造方法である。
【００２２】
【発明の実施の形態】
まず、本発明における鋼の化学成分の限定理由について説明する。
Ｃは、鋼板の強度を上昇できる元素である。特にマルテンサイトなどの硬質相を生成し高強度化には必須の元素であり、９００ＭＰａ以上の強度を得るためには０．０１％以上が必要であるが、逆に多く含有すると、脆化傾向にあるため、水素脆性を生じ易くなる。従って、上限を０．２５％とした。
Ｓｉは、材質を大きく硬質化する置換型固溶体強化元素であり、鋼板の強度を上昇させることに有効なうえ、セメンタイト析出を抑制する元素であるが、特に溶融亜鉛めっきの濡れ性を確保する点から２．０％を上限とする。下限は特に定めないが、極低下は製造コストの高騰を招くことから、０．００５％以上の添加とすることが望ましい。
【００２３】
Ｍｎは、鋼板の強度上昇に有効な元素である。しかし、０．０１％未満ではこの効果が得られないので、下限値を０．０１％とした。逆に多いとＰ、Ｓとの共偏析を助長するだけでなく、加工性が劣化する場合があるため４．０％を上限値とする。
Ｐは、粒界偏析による粒界破壊の助長をする元素であり、低い方が望ましいが、極低化は製造コスト上好ましくないので０．０００１％以上とする。また耐食性を劣化させる元素であるため、上限を０．０５％とする。
【００２４】
Ｓは、腐食環境下での水素吸収を助長する元素であり、低い方が望ましいが、極低下は製造コスト上好ましくないので０．０００１％以上とする。特に加工性を高めるためには低い方が望ましく上限を０．０５％とする。
Ａｌは、脱酸のために０．０１％以上を添加するが、添加量が増加するとアルミナ等の介在物が増加し、加工性が劣化するため３．０％を上限とする。
Ｎは、加工性劣化や溶接時のブローホール発生にも寄与するため少ない方が良い。０．０１％を越えると加工性が劣化してくるので、０．０１％を上限とする。低Ｎ化は精錬コスト上昇を招くことから下限を０．０００１％とした。
【００２５】
Ｎｉは、水素侵入を抑制し遅れ破壊特性を向上させる効果や、鋼板の焼入れ性を高めることにより鋼板の強度を確保する効果がある。しかし、０．００１％未満ではこれらの効果が得られないため下限値を０．００１％とした。逆に、５．５％超では加工性が悪くなるため、上限値を５．５％とした。
Ｃｕは、水素侵入を抑制し遅れ破壊特性を向上させる効果や、強化に有効である上、自身の微細析出は遅れ破壊の向上にも寄与するため、０．００１％以上の添加とした。また、過剰添加は加工性の劣化を招くことから、上限を３．０％とした。
【００２６】
Ｃｒは、水素侵入を抑制し遅れ破壊特性を向上させる効果や、鋼板の強度上昇に有効な元素である。また、Ｃｒを含有する析出物および晶出物は水素トラップサイトとなるため非常に重要な元素で、Ｃｒ単独又はＮｂ，Ｖ、後述のＴｉ，Ｍｏの１種又は２種以上との複合添加が特に有効であるため、下限値を０．００１％とした。逆に、５％超含有すると加工性低下が生じるため、上限値を５％とした。
【００２７】
Ｍｏは、水素侵入を抑制し遅れ破壊特性を向上させる効果や、鋼板の焼入れ性を高め連続焼鈍設備で安定してマルテンサイトを得るために有効な元素であるだけでなく、粒界を強化して水素脆性の発生を抑制する効果がある。また、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。さらには、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−溶接性の極めて良好なバランスを得るために重要な添加元素である。しかし、０．００５％未満ではこれらの効果が得られないため、下限値を０．００５％とした。また、５％超ではこれらの効果が飽和するため、上限値を５％とした。
【００２８】
Ｎｂは、鋼板の強度上昇及び細粒化に有効な元素である。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。さらには、溶接熱影響部の軟化抑制にも効果的である。しかし、０．００１％未満ではこれらの効果が得られないため、下限値を０．００１％とした。逆に、１％超含有すると、炭窒化物の析出が多くなり加工性低下を生じるため、上限値を１％とした。
Ｖは、水素侵入を抑制し遅れ破壊特性を向上させる効果や鋼板の強度上昇及び粒径の微細化に加えて炭窒化物の形態制御により水素のトラップサイトととして活用可能であることからも耐水素脆化向上のための重要な添加元素である。しかし、０．００５％未満ではこの効果が得られないために、下限値を０．００５％とした。逆に、１％超含有すると炭窒化物の析出が顕著になり、延性低下が著しくなる。このため上限値を１％とした。
【００２９】
Ｓｅ，Ａｓ，Ｓｂ，Ｓｎ，Ｐｂ，Ｂｉは、１種又は２種以上の合計で０．０５％を超えて含有すると、耐遅れ破壊特性を著しく阻害するため、１種又は２種以上の合計で上限を０．０５％とした。一方で、極低化はリサイクル上の制限を狭める理由から、それぞれの元素は０．０００２％を下限とした。
Ｔｉは鋼板の強度上昇及び細粒化に有効な元素である。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。さらには、ＮｂやＭｏなどの元素との複合添加により耐遅れ破壊性向上にも寄与するため、Ｔｉの含有量は０．００１％以上とした。逆に、５％超では粗大析出または晶出物が多量に生成するために加工性が低下する。このため、上限値を５％とした。
【００３０】
Ｗ、Ｚｒ、Ｈｆ、Ｔａは、鋼板の強度上昇に有効である上、各元素を含有する析出物および晶出物は水素トラップサイトとなるため非常に重要な元素である。しかし、これらの元素の１種又は２種以上の合計が０．００１％未満ではこれらの効果が得られないため、下限値を０．００１％とした。逆に、５％超含有すると加工性低下が生じるため、上限値を５％とした。
Ｂは、鋼板の強度上昇に有効な元素である。しかし、０．０００２％未満ではこれらの効果が得られないため、下限値を０．０００２％とした。逆に、０．１％超含有すると熱間加工性が劣化するため、上限値を０．１％とした。
【００３１】
ＲＥＭ，Ｃａ，Ｍｇは、鋼板表面の腐食に伴う界面雰囲気の水素イオン濃度の上昇を抑制する、すなわち、ｐＨの低下を抑制するのに有効な元素である。しかし、それぞれ０．０００２％未満ではこれらの効果が得られないため、下限値を０．０００２％とした。逆に、それぞれ０．１％超含有すると加工性が劣化するため、上限値を０．１％とした。
Ｙは、介在物の形態制御に有効で、耐遅れ破壊性に寄与することから、０．０００２％以上の添加とした。一方、過剰添加は熱間加工性を劣化させるため、０．１％以下の添加とした。
【００３２】
次にミクロ組織、析出物について説明する。
焼き戻しマルテンサイト鋼において遅れ破壊は、旧オーステナイト粒界等に水素が集積することによってボイド等が発生し、その部分が起点となって破壊を生じると考えられている。そこで、水素のトラップサイトを均等かつ微細に分散させて、その部分に水素をトラップさせると、拡散性水素濃度が下がり、遅れ破壊の感受性が下がる。前出の特許文献１にあるように、ＭｇおよびＴｉを複合添加した厚鋼板における酸化物の分散形態制御で、水素起因の耐遅れ破壊性が向上することが分かっている。しかし、一般的に薄鋼板においては使用前に成形加工を受けるため、高い残留応力の発生や、加工端面におけるバリ等の存在が必然的に耐遅れ破壊性も劣化するため、これに伴う耐遅れ破壊特性の劣化を補足できない。このように、薄鋼板の使用形態を考慮した遅れ破壊特性に関する研究は少なく、ＭｇやＴｉの酸化物形態制御のみでは解決できない。
【００３３】
また、製造工程や環境から来る水素量が局部的にでも大量である場合を考えると、いくら鋼材内に水素のトラップサイトを分散させても必然的に水素起因の遅れ破壊は発生してしまう。このため、まず、▲１▼鋼材内にトラップサイトを分散させて鋼材自体の許容水素量を高めておくことおよび残留オーステナイトの抑制に加えて、▲２▼製造工程や置かれた環境から侵入し得る水素量を低減することが重要である。特にめっき鋼板の穴拡げ性向上を考える場合には、冷延鋼板の連続焼鈍の場合のように冷速制御等に制約が多く、後述する組織制御にも多くの制約があることからも、めっき工程およびめっき工程前での侵入水素量を極力抑える必要がある。
【００３４】
本発明者らは、上述の背景を踏まえて、めっき鋼板の製造〜使用環境において、耐遅れ破壊性を確保・向上させるため、種々の晶出物、析出物のトラップサイトの分散や鋼板の強度の影響に加えて、環境から入り得る水素量の低減について検討した。その結果、めっき鋼板の製造〜使用環境下（例えばプレス加工後の設計応力相当付加下）で、水素起因の耐遅れ破壊性を向上・確保するための技術を見出した。すなわち、
▲１▼鋼板の強度と成分による析出物および残留オーステナイト量の制御。
▲２▼鋼板の成分による耐侵入水素特性の制御。
をそれぞれ行うことで、自動車用薄鋼板の製造〜使用環境下での耐水素脆化を向上させることが出きる。これを満たすための条件として、式（１−２）および（２−２）を規定した。本式を満たすことで、高強度薄鋼板の対遅れ破壊性が確保できる。
【００３５】
次に、▲２▼鋼材の成分による水素侵入特性の制御、について述べる。
水素侵入の過程は、腐食や酸洗などによって、水分子（中性またはアルカリ性環境の場合）または水素イオン（酸性環境の場合）の還元反応が鋼板表面で生じると、鋼板表面に水素原子が生成、吸着する。この吸着水素原子は、（１）再結合して水素分子としてガス化するか、鋼板内部に侵入する。本発明者らはこれらの過程を鋭意研究した結果、水素侵入速度を低減するには、耐食性を向上させるほかに、（１）腐食反応の進行に伴う環境のｐＨ（水素イオン濃度）低下を極力抑えて、表面の吸着水素原子濃度を低くする、（２）再結合反応（水素発生反応）を加速することが有効であることを見出した。
【００３６】
（１）については、鋼中へのＲＥＭ，Ｃａ，Ｍｇ添加が有効であることを見出した。ここでＲＥＭはＲａｒｅＥａｒｔｈＭｅｔａｌの略でＬａから始まるランタノイド系元素の総称である。工業的な添加としてはミッシュメタルの形で添加する場合が多く、この場合にはＬａやＣｅの添加量が多くなる。腐食反応でＲＥＭ、Ｃａ，Ｙ、Ｍｇが溶出すると、水酸化物の平衡反応により、雰囲気をアルカリ化、すなわち腐食反応によるｐＨの低下を抑制する。
【００３７】
（２）については、二つの方法が見出された。第一の方法は、水素イオンまたは水の還元反応の交換電流密度を上昇させる方法である。Ｃｕ，Ｎｉ，Ｃｒ，Ｍｏが有効であり、０．１≦２Ｃｕ＋２０Ｍｏ＋３Ｎｉ＋Ｃｒ＋２０Ｖを満足した場合、水素透過速度は著しく抑制される。第二の方法は、上記の交換電流密度を低下させる。または水素発生過電圧を著しく上昇させる不純物元素を制限する方法である。該当する不純物元素として、Ｓｅ、Ａｓ，Ｓｂ，Ｐｂ，Ｂｉを制限すれば、水素透過速度の増加を抑制できる。
【００３８】
自動車用めっき鋼板において、水素侵入は次の過程で生じる。第一に、鋼板製造における酸洗工程など、第二に塗装やプレス加工などの加工組み立て工程、第三に使用環境での腐食である。いずれの環境でも、上記に述べた鋼材の成分による水素侵入特性の制御は有効である。自動車用鋼板の裸耐食性を向上させて水素侵入を抑制するには、高価な元素を大量に添加する必要があるが、これらの（１）および（２）の方法では、いずれも微量添加で顕著な効果が得られるという利点がある。
【００３９】
さらに、溶接性、穴広げおよび延性の確保については、ミクロ組織および成分範囲や（１−１）および（２−１）式や（１−３）および（２−３）式による限定を行うことで、引張強さ９００ＭＰａ以上の高強度を保ちつつ溶接熱影響部の軟化挙動を抑制して、さらには、穴拡げ率：（穴拡げ試験前の穴の内径／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。穴拡げ性を十分に確保するためにはベイナイト、マルテンサイトの一方又は双方とするのが有効で（さらなる高穴拡げ性化を指向する場合には一方が望ましい)、面積率で７０％以上含むこととした。この様な組織を形成させるためには式（１−３）および（２−３）式を満たす必要がある。
【００４０】
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、炭化物の無いベイニティックフェライトまたはマルテンサイトが主相で、面積率が９７％を超えることが望ましい。
一方、特にマルテンサイトが主相の場合には、溶接熱影響部での軟化が問題となる。これに対しては、後述するように成分を規定した（１−１）および（２−１）式を満たすことで、引張強さが９００ＭＰａ以上の高強度材の溶接性を確保するものとした。
【００４１】
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、残留オーステナ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。
また、（１−１）及び（２−１）式や（１−３）及び（２−３）式を満足しない場合には、引張強さが９００ＭＰａ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ないことに加えて穴拡げ性の確保も困難となる。
（３．０Ｎｂ＋２．５Ｍｏ＋１／１０Ｓｉ＋Ｍｎ）−（２Ｃ^0.5＋２）＞０
… （１−１）（２−１）
０＞Ｓｉ＋Ａｌ＋２−（Ｍｎ＋Ｎｉ＋１．５Ｍｏ） … （１−３）（２−３）
【００４２】
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。なお、上記ミクロ組織の各相、フェライト、ベイナイト（ベイニティックフェライトを含む）、残留オーステナイト、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。
【００４３】
また、耐遅れ破壊性を確保・向上させるため、種々の晶出物、析出物の形態についても検討した。その結果、薄鋼板の使用環境下で、高い残留応力下や端面のバリ発生があっても、耐遅れ破壊性を向上・確保するための技術を見出した。
すなわち、
▲１▼Ｎｂ、Ｖ、Ｃｒ、Ｔｉ、Ｍｏの酸化物、硫化物、窒化物、複合晶出物、複合析出物の粒内の分散形態制御。
▲２▼鋼板のミクロ組織中の残留オーステナイト量。
をそれぞれ制御することで、有効に水素のトラップサイトであるＮｂ、Ｖ、Ｃｒ、Ｔｉ、およびＭｏの酸化物、硫化物、窒化物、複合晶出物および複合析出物を効果的に分散させ、加工後の耐遅れ破壊性の確保する事ができる。このためには、製造条件を制御する事によって、種々の元素の酸化物、硫化物、窒化物、複合晶出物および複合析出物が水素のトラップサイトになり得る形態制御を行った。
【００４４】
これは薄鋼板の加工により導入される転位や残留応力場とトラップサイトとなる粒子との相互作用が、厚鋼板での熱間圧延や溶接後冷却時に導入される転位や残留応力とのそれとは異なることや、薄鋼板と厚鋼板の熱処理方法の違いに起因すると考えられる。
詳細な限定については、以下のように限定する。
▲１▼の何れか１種以上の平均粒子径ｄ：平均粒子径は、０．００１〜５．０μｍに限定した。これは平均粒子径が５．０μｍを超えると、薄鋼板の機械的性質の劣化となる上製造も困難となり、加えて粗大粒子はトラップサイトとしての作用がなくなり、また破壊の起点となり得るためである。また平均粒子径が０．００１μｍ未満では水素のトラップサイトとしての効果が小さくなるからである。
【００４５】
密度ρ：粒子の存在密度は、１００〜１×１０¹³個／ｍｍ²とした。粒子密度が低いことは、トラップサイト数が少ないことを意味し、加工後の耐遅れ破壊性を確保できないため、下限を１００個／ｍｍ²とした。また、高密度の場合には、延性や成形加工性が劣化することおよび耐遅れ破壊性向上効果も飽和することから、１×１０¹³個／ｍｍ²を上限とした。
分布：粒子の分布を、平均粒子径からの標準偏差σ［μｍ］と平均粒子径ｄ［μｍ］の比が、σ/ｄ≦１．０を満たすこととした。σ/ｄ＞１．０とは、粒子分布が広範囲にわたることを意味し、耐遅れ破壊向上効果が同じ平均粒径に比べて小さくなり、延性劣化や破壊の起点数の増加にもつながることから、上限を決めて１．０以下とした。
【００４６】
ここで、Ｎｂ、Ｖ、Ｃｒ、Ｔｉ、Ｍｏの酸化物、硫化物、窒化物、複合晶出物、複合析出物を含む粒子の測定について述べる。平均粒子径の測定は、薄膜または抽出レプリカのサンプルを用いて、走査型または透過型電子顕微鏡にて、５０００〜５０００００倍の倍率で観察を行い、最低３０視野を測定することで得られる値とする。平均粒子径は、画像解析による円相当経にて評価する。平均粒子径は前記の方法により測定した粒子径を単純平均した値とし、標準偏差はこれらの粒子径から求めた値とする。
【００４７】
また、密度を求める際には、複合析出または晶出物は１ヶとして数える。組成分析は、ＥＤＸおよびＥＬＬＳを用い、構造解析はＤｉｆｆｒａｃｔｉｏｎｐａｔｔｅｒｎを解析することで行った。
複合晶出物とは、主にＭｇ、Ａｌ、Ｔｉ等を含有した単独あるいは複合酸化物であり、各複合化合物とは、Ｔｉ，Ｎｂ, Ｖ，Ｃｒ, Ｍｏ，Ｍｇなどを含有した化合物（炭化物、窒化物、酸化物や硫化物など）である。
上述したような、成分調整や組織制御を行うことで、溶融めっきおよび電気めっき製造時から大気環境においても、材質を確保しつつ耐水素脆化性を確保し得るものである。
【００４８】
次に、製造方法について説明する。製造方法は一般に行われているめっき鋼板の製造設備を使用することで構わない。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。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。これは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る。一方、低温加熱では圧延抵抗が高くなってしまうため、上記の温度範囲が望ましい。
【００４９】
次に、熱間圧延ではフェライト粒にひずみが過度に加わり加工性が低下するのを防ぐために熱間圧延をＡｒ₃＋３０℃以上で終了し、逆に高温すぎても焼鈍後の再結晶粒径およびＮｂ、Ｖ、Ｃｒ、Ｔｉ、およびＭｏ等の酸化物、硫化物、窒化物、複合晶出物および複合析出物が必要以上に粗大化するため、９５０℃以下で終了することが望ましい。
【００５０】
熱間圧延後４００℃〜７００℃の巻取温度域まで０．１〜１０００℃／秒の冷却速度で冷却する。冷却停止温度を４００℃より低くすることは操業上困難であり、冷却停止温度の上限が７００℃より高いとＮｂ、Ｖ、Ｃｒ、Ｔｉ、およびＭｏの酸化物、硫化物、窒化物、および複合析出物が粗大化することによりトラップ能が低下することが懸念される。また、冷却速度が０．１℃／秒より遅いと、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１０００℃／秒を超えることは操業上困難なため、これを上限とした。
【００５１】
熱間圧延後の冷却温度域は、トラップサイトとなるＮｂ、Ｖ、Ｃｒ、Ｔｉ、およびＭｏ等の酸化物、硫化物、窒化物、および複合析出物の析出しやすい４００〜７００℃とし、望ましくは６００〜７００℃が好ましい。またその温度域への冷却速度は、上記トラップサイトとなる析出物の析出を促進させるために、２０℃／秒以上の冷却速度が好ましい。
【００５２】
酸洗後の冷間圧延は、圧下率が低いと鋼板の形状矯正が難しくなるため下限値を１０％とすることが好ましい。また、８０％を超える圧下率で圧延すると、鋼板のエッジ部に割れの発生及び形状の乱れのため上限値を８０％とすることが好ましい。焼鈍温度は低すぎると未再結晶の状態になり硬質化し、逆に高すぎると粒が粗大化しプレス時に肌荒れを起こす場合があるという問題点があるため、Ａｃ₁＋３０〜Ａｃ₃＋５０℃とした。また、ベイナイトまたはマルテンサイトを生成させる観点からはＡｃ₃点以上が望ましい。この温度域での保持時間については十分に再結晶させるために最低でも１０秒の保持が必要で、保持時間が長時間になると結晶粒の粗大化を招くことから３０分以下とした。
【００５３】
焼鈍後、２００℃以上Ｚｎめっき浴温度＋１００℃の温度域まで０．１〜１００℃／秒の冷却速度で冷却し、引き続いてＺｎめっき浴温度〜Ｚｎめっき浴温度＋１００℃の温度域で後続のＺｎめっき浸漬時間を含めて１秒〜３０００秒保持を行う。冷却速度が０．１℃／秒より遅いと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℃／秒超の場合にはめっき密着性に問題が生じる事が懸念される。
【００５４】
また、最終的な鋼板中のベイナイトやマルテンサイトの相率を制御する目的から冷却停止温度および保持時間を調整する。ベイナイトを主体とする場合には冷却停止温度を３００℃以上としその温度域での保持時間を３０秒以上とすることが望ましい。一方、マルテンサイトを主体する場合には冷却停止温度を２００℃以上とすることが望ましい。冷却停止温度が２００℃未満になったり、保持時間が１秒未満になるとめっき性が劣化する。
【００５５】
一方、冷却停止温度がめっき浴温度＋１００℃を超えてもめっき性を損ねたり、炭化物の析出が生じてしまう。これは保持時間が３０００秒を超えても炭化物析出が生じてしまう懸念がある。また、合金化する場合にはめっき浴浸漬後３００〜６００℃にて合金化処理を行う。３００℃未満では未合金となり、６００℃を超えると炭化物が析出してしまう。更に電気めっきの場合には、焼鈍直後のめっき性劣化の懸念が少ないため、上記の溶融亜鉛めっきの工程における違いとして、焼鈍後の冷却速度は２００℃／ｓ以下とし、冷却停止温度も１５０℃を下限とする。操業上の容易さからこれらを限界値とした。
【００５６】
【実施例】
次に、本発明を実施例に基づいて説明する。
表１に示すような化学組成の鋼板を、１１５０〜１２５０℃に加熱し、Ａｒ₃点＋３０℃以上で熱延を完了し、冷却後巻き取った鋼帯を酸洗後、冷延して１．２ｍｍ厚とした。これらのＡｃ₁およびＡｃ₃変態温度から計算される焼鈍温度に５〜１０％Ｈ₂−Ｎ₂雰囲気中で昇温・保定したのち、溶融および電気めっきを施して各種試験に供した。これらの鋼板からＪＩＳ５号引張り試験片を採取して、機械的性質を測定した。さらに、鉄鋼連盟規格に準拠して穴拡げ試験を行い、穴拡げ率を求めた。溶接性については鋼板をつきあわせた各種溶接を行い、テフロン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。鋼板の耐遅れ破壊性の評価方法の詳細は以下の通りである。
【００５７】
【表１】

【００５８】
▲１▼調質圧延後、プレス時の歪を模擬する目的２％歪を鋼板に与える。
▲２▼鋼板より応力集中率３．２の切欠き板状引張り試験片を採取する。
▲３▼３％ＮａＣｌ−３ｇ／１ＮＨ₄ＳＣＮ水溶液中で０．０１〜０．０２５ｍＡ／ｃｍ²で定電流陰極チャージを施す。
▲４▼Ｃｄめっきを行う。
▲５▼引張り強度の０．８倍の一定荷重を付加する。
▲６▼１００ｈまで試験を行い、破断か未破断を判断する。
【００５９】
表２に各鋼のミクロ組織と残留γ量、機械的性質、遅れ破壊評価、および溶接性評価と各式の計算値について、また、表３及び表４に各鋼の材質特性に及ぼす各製造条件と材質について示す。本発明鋼は、溶接性、強度（引張強さで８００ＭＰａ以上）、穴拡げ性に優れていることがわかる。特に、穴広げ性については８０％と優れた値を示す。一方、比較例は、溶接部の球頭張り出し高さ、引っ張り強度および穴拡げ性、遅れ破壊評価の何れかが劣勢である。
【００６０】
【表２】

【００６１】
【表３】

【００６２】
【表４】

【００６３】
【発明の効果】
以上述べたように、本発明により、引張強さが９００ＭＰａ以上の高強度鋼板の溶接性、穴拡げ性を同時に改善し、さらに、耐遅れ破壊性を向上させた高強度めっき鋼板およびその製造方法を得ることができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expansibility suitable for automobiles, building materials, home appliances and the like, and a method for producing the same.
[0002]
[Prior art]
Conventionally, high-strength steel is often used for applications such as bolts, PC steel wires, and line pipes, and it is known that delayed fracture occurs due to the penetration of hydrogen into the steel when the tensile strength exceeds 980 MPa. ing. In contrast, (1) the thin steel plate is thin, so even if hydrogen enters, it is released in a short time, and (2) there is almost no use of a steel plate of 980 MPa or more in terms of workability. It can be said that the awareness of problems with delayed destruction was low.
[0003]
However, recently, due to the necessity of reducing the weight of automobiles and improving collision safety, press forming, pipe forming, bending, end face processing, hole expansion processing, etc. are applied to ultra-high strength thin steel sheets of 980 MPa or more, and bumpers and The number of cases where it is used for reinforcing materials such as impact beams and seat rails is rapidly increasing. Therefore, there is an urgent need to develop ultra high strength thin steel sheets with delayed fracture resistance. Until now, most technologies for improving delayed fracture resistance have been developed for steel materials that are often used as products such as bolts, strips, and thick plates, and that are often used below yield strength or yield stress.
[0004]
For example, in steel bars and bolt steels, development is centered on tempered martensite, and "new developments for delayed fracture elucidation" (Japan Steel Association, published in January 1997)
(Non-patent Document 1) reports that additive elements exhibiting temper softening resistance such as Cr, Mo and V are effective in improving delayed fracture resistance. This is a technique for precipitating alloy carbides and using them as hydrogen trap sites to shift the delayed fracture mode from grain boundaries to intragranular fracture. However, these steels have a C content of 0.4% or more and contain a large amount of alloy elements, so the workability and weldability required for thin steel sheets are inferior, and the precipitation of alloy carbide takes several hours or more. Therefore, there is a problem in manufacturability.
[0005]
Japanese Patent Laid-Open No. 11-293383 (Patent Document 1) states that an oxide mainly composed of Ti and Mg is effective in preventing hydrogen defects. However, this is intended for thick steel plates, especially considering delayed fracture after welding with high heat input, but undergoes high forming work required for thin steel plates and burrs associated with end face processing. No consideration is given to the effects on delayed fracture phenomena such as occurrence. Furthermore, no consideration is given to workability, which is a basic characteristic of thin steel sheets.
[0006]
On the other hand, regarding delayed fracture of thin steel plates, for example, CAMP-ISIJ, vol. 5, pp. 1839 to 1842, Yamazaki et al., 1992, 10 issued by the Japan Iron and Steel Institute
(Non-Patent Document 2) reports the promotion of delayed fracture due to processing-induced transformation of the retained austenite amount. This is in consideration of the forming process of a thin steel sheet, but describes the regulation of the amount of retained austenite which does not deteriorate the delayed fracture resistance. That is, it relates to a high-strength thin steel sheet having a specific structure, and is not a fundamental countermeasure for improving delayed fracture resistance. Furthermore, when assembling a member using such a high-strength material, ductility, bendability, hole expansibility, weldability, etc. become a big problem over high-strength steel sheets with a tensile strength of up to about 590 MPa, Measures against these are necessary.
[0007]
  The following measures are taken for each characteristic.
  For example, regarding hole expansibility, CAMP-ISIJ, vol. 13 (2000), p395 (Non-patent Document 3), the main phase is bainite.AsIt has been disclosed that the hole expandability is improved, and further, the stretchability is shown to be the same as that of the current retained austenitic steel by generating retained austenite in the second phase. Furthermore, it is also shown that when retained austenite having an area ratio of 2 to 3% is generated by austempering at a temperature equal to or lower than the Ms temperature, the tensile strength × the hole expansion ratio is maximized. However, the weldability that manifests the tensile strength exceeding 800 MPa and the softening behavior in the weld heat affected zone are not considered.
[0008]
  As for weldability, there are many cases where the softening behavior (HAZ softening behavior) in the weld heat affected zone is regarded as a problem. On the other hand, for example, as disclosed in JP 2000-87175 A (Patent Document 2), it is shown that the HAZ softening behavior is suppressed by precipitation of carbides (Nb, Mo) C of Nb and Mo. However, although this technique is considered with respect to fatigue strength, it does not sufficiently consider workability such as hole expansibility. HAZ softeningDepressionThe strength effect is also low and the weldability and workability of an extremely high strength material of 800 MPa or more are not sufficient.
[0009]
  In particular, when the tensile strength is 800 MPa or more, welding itself becomes difficult, and becomes more remarkable when the tensile strength is 980 MPa or more. For this reason,spotIn addition to conventional welding methods such as welding, there are examples in which laser welding or the like is partially applied. However, because of high strength,In particular, the material fluctuations at the welded part and the heat-affected zone are extremely remarkable as compared with the 590 MPa class high strength material. High strengthToThe use of martensite promotes hole expandability and ductility reduction.
[0010]
  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), p391 (nonpatent literature 4)}. In this document, the main phase is ferrite and the second phase is martensite.ageAlthough it is disclosed that the hole expansion rate is improved by reducing the hardness difference between the two, the hole expansion rate is not significantly improved to less than 70%. Further, all of the above examinations relate to hot rolling and cold rolling, and in particular, no consideration is given to hydrogen penetration in the plating process or behavior of hydrogen that has penetrated before the plating process in the plating process.
[0011]
[Cited document]
(1) Non-patent document 1 {"New development of delayed fracture elucidation" (Japan Steel Association, published in January 1997)}
(2) Non-Patent Document 2 (CAMP-ISIJ, vol. 5, pages 1839 to 1842, Yamazaki et al., 1992, 10, published by the Japan Iron and Steel Institute)
(3) Non-Patent Document 3 (CAMP-ISIJ, vol. 13 (2000), p395)
(4) Non-Patent Document 4 (CAMP-ISIJ, vol. 13 (2000), p391)
(5) Patent Document 1 (Japanese Patent Laid-Open No. 11-293383)
(6) Patent Document 2 (Japanese Patent Laid-Open No. 2000-87175)
[0012]
[Problems to be solved by the invention]
As mentioned above, especially in the manufacturing, assembly, and use environments of thin steel sheets for automobiles, measures against hydrogen embrittlement-type delayed fracture are taken, and the use characteristics such as weldability and hole expansibility are fully considered. There are no development examples related to high strength steel sheets. The present invention provides a high-strength galvanized steel sheet that solves the above-described problems of the prior art and simultaneously improves the weldability and hole expandability of a high-strength steel sheet having a tensile strength of 900 MPa or more, and a method for producing the same. For the purpose.
[0013]
[Means for Solving the Problems]
From the background as described above, the present inventors have come to find a method for fundamentally improving delayed fracture resistance by sufficiently considering the use environment in thin steel sheets and the manufacturing method using current equipment. That is, in addition to steel sheet structure and precipitate control, it is possible to improve delayed fracture resistance due to hydrogen by controlling trap sites in the steel sheet and reducing the amount of hydrogen that can enter from the manufacturing process and usage environment. I found. Details are as follows.
[0014]
  (1) By mass%, C: 0.01 to 0.25%, Si: 0.01 to 2.0%, Mn: 0.01 to 4.0%, P: 0.0001 to 0.05% , S: 0.0001 to 0.05%, Al: 0.01 to 3.0%, N: 0.0001 to 0.01%,Mo: 0.005 to 5%, Nb: 0.001 to 1.0%The balance is composed of iron and inevitable impurities, the microstructure contains bainite and martensite in an area ratio of 70% or more in total, and residual austenite (Vγ) is contained in less than 3%. (TS) is 900 MPa or more, and further satisfies the following formulas (1-1) to (1-3): high strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expansibility .
(3.0Nb + 2.5Mo + 1 / 10Si + Mn)-(2C^0.5+2)> 0 (1-1)
0 ≦ 0.8 × {2Cu + 20Mo + 3Ni + Cr} − {0.1−3.5 × 10⁷× (TS)^-3.1} -0.3Vγ (1-2)
0> Si + Al + 2- (Mn + Ni + 1.5Mo) (1-3)
Where TS: tensile strength (MPa),
The element symbol indicates mass% of each element contained in the steel.
(2) Further, by mass%, Ni: 0.001 to 5.5%, Cu: 0.001 to 3.0%, Cr: 0.001 to 5.0%, or one or more of them A high-strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability as described in (1) above.
[0015]
  (3In addition,% By massV: 0.005 to 1%, the balance is made of iron and inevitable impurities, and the microstructure is an area ratio of one or both of bainite and bainitic ferrite in total 70% or more, and retained austenite (Vγ ) In an amount of less than 3%, tensile strength (TS) is 900 MPa or more, and further satisfies the following formulas (2-1) to (2-3):Or (2)A high-strength galvanized steel sheet with excellent hydrogen embrittlement resistance, weldability and hole expandability.
(3.0Nb + 2.5Mo + 1 / 10Si + Mn)-(2C^0.5+2)> 0 (2-1)
0 ≦ 0.8 × {2Cu + 20Mo + 3Ni + Cr + 20V} − {0.1−V / 5−3.5 × 10⁷× (TS)^-3.1} -0.3Vγ (2-2)
0> Si + Al + 2- (Mn + Ni + 1.5Mo) (2-3)
Where TS: tensile strength (MPa),
The element symbol indicates mass% of each element contained in the steel.
[0016]
  (4) Further, in mass%, Se: 0.0002 to 0.05%, As: 0.0002 to 0.05%, Sb: 0.0002 to 0.05%, Pb: 0.0002 to 0.05%, Bi: 0.0002 to 0.05%, or one or two or more thereof, and the total of them satisfies 0.05% or less (1)To any one of (3)A high-strength thin steel sheet with excellent hydrogen embrittlement resistance, weldability and hole expandability.
[0017]
  (5) Further, the steel contains Ti: 0.001 to 5% by mass%, and is an oxide of Nb, V, Cr, Ti, Mo, sulfide, nitride, composite crystallized product, composite precipitate. Any one or more of them have an average particle diameter d: 0.001 to 5.0 μm, density ρ: 100 to 1 × 10 per square mm¹³Individual, distribution: ratio of standard deviation σ from average particle diameter and average particle diameter d: distribution form satisfying σ / d ≦ 1.0, and tensile strength is 900 MPa or more (1 ) ~ (4The high-strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to any one of the above.
[0018]
  (6) Further, in the steel by mass%, W: 0.001-5%, Zr: 0.001-5%, Hf: 0.001-5%, Ta: 0.001-5% (1) to (1) characterized by containing 0.001 to 5% in total of seeds or more.5The high-strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to any one of the above.
(7) Further, the above-mentioned (1) to (1), characterized in that the steel contains B: 0.0002 to 0.1% by mass%.6The high-strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to any one of the above.
[0019]
  (8) Further, in mass% in steel, REM: 0.0002 to 0.1%, Y: 0.0002 to 0.1%, Ca: 0.0002 to 0.1%, Mg: 0.0002 to 0 (1) to (1) containing 1% or two or more of 1%7The high-strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to any one of the above.
[0020]
  (9) (1) to (8Is a method for producing the steel sheet according to any one of (1) to (1).8The cast slab composed of the component described in any one of the above) is cast as it is, or once cooled and then heated again, Ar_ThreeHot rolling is finished at a finishing temperature of the point + 30 ° C. or higher, then cooled to a coiling temperature of 0.1 to 1000 / second and then hot rolled steel sheet wound at 400 ° C. to 700 ° C. is pickled and cooled. And then Ac₁+30 (° C) or higher, Ac_ThreeAfter annealing at a temperature range of +50 (° C.) or less for 10 seconds to 30 minutes and then cooling at an average cooling rate of 0.1 to 100 ° C./second to 200 ° C. to a plating bath temperature +100 (° C.), a Zn plating bath Hydrogen resistance, characterized by holding for 1 to 3000 seconds including the subsequent plating immersion time in the temperature range of temperature to Zn plating bath temperature +100 (° C.), immersed in a Zn plating bath, and then cooled to room temperature. A method for producing a high-strength galvanized steel sheet excellent in embrittlement, weldability and hole expandability.
[0021]
  (10) After being immersed in a Zn plating bath, alloying treatment is performed at 300 to 600 ° C., and cooling to room temperature is performed.9The method for producing a high-strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expansibility described in (1).
(11) (1) to (8Is a method for producing the steel sheet according to any one of (1) to (1).8The cast slab composed of the component described in any one of the above) is cast as it is, or once cooled and then heated again, Ar_ThreeAfter hot rolling is finished at a finishing temperature of point + 30 ° C. or higher, and after cooling at a cooling rate of 0.1 to 1000 ° C./second to the coiling temperature, the hot rolled steel sheet wound up at 400 ° C. to 700 ° C. is pickled. , Cold-rolled at a rolling reduction of 10 to 80% and then Ac during annealing₁+30 (° C) or higher, Ac_ThreeAfter annealing at a temperature range of +50 (° C.) or less for 10 seconds to 30 minutes, it is cooled to a temperature range of 150 to 500 ° C. at a cooling rate of 0.1 to 200 ° C./second at an average cooling rate, and 1 in the same temperature range. A method for producing a high-strength galvanized sheet steel excellent in hydrogen embrittlement resistance, weldability and hole expansibility, characterized by performing electroplating after holding for 2 seconds to 3000 seconds.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
First, the reasons for limiting the chemical components of steel in the present invention will be described.
C is an element that can increase the strength of the steel sheet. In particular, it is an essential element for increasing the strength by generating a hard phase such as martensite. To obtain a strength of 900 MPa or more, 0.01% or more is necessary. Therefore, hydrogen embrittlement is likely to occur. Therefore, the upper limit was made 0.25%.
Si is a substitutional solid solution strengthening element that hardens the material greatly, and is effective in increasing the strength of the steel sheet, and is an element that suppresses cementite precipitation, but particularly ensures the wettability of hot dip galvanizing. To 2.0%. Although the lower limit is not particularly defined, it is desirable to add 0.005% or more because the extreme decrease leads to an increase in manufacturing cost.
[0023]
Mn is an element effective for increasing the strength of the steel sheet. However, since this effect cannot be obtained if the content is less than 0.01%, the lower limit is set to 0.01%. On the contrary, if the amount is too large, not only co-segregation with P and S is promoted, but also workability may be deteriorated, so 4.0% is made the upper limit.
P is an element that promotes grain boundary fracture due to grain boundary segregation. A lower value is desirable, but an extremely low value is not preferable in terms of manufacturing cost, so 0.0001% or more. Moreover, since it is an element which degrades corrosion resistance, the upper limit is made 0.05%.
[0024]
S is an element that promotes hydrogen absorption in a corrosive environment, and a lower value is desirable. However, an extremely low reduction is not preferable in terms of manufacturing cost, so it is 0.0001% or more. In particular, in order to improve the workability, a lower value is desirable and the upper limit is set to 0.05%.
Al is added in an amount of 0.01% or more for deoxidation, but inclusion increases such as alumina and the workability deteriorates as the addition amount increases, so 3.0% is made the upper limit.
N is better because it contributes to workability deterioration and blowhole generation during welding. If it exceeds 0.01%, workability deteriorates, so 0.01% is made the upper limit. Lowering N increases the refining cost, so the lower limit was made 0.0001%.
[0025]
  Ni has an effect of suppressing hydrogen penetration and improving delayed fracture characteristics, and an effect of ensuring the strength of the steel sheet by enhancing the hardenability of the steel sheet. However, if less than 0.001%, these effects cannot be obtained, so the lower limit was set to 0.001%. On the other hand, if it exceeds 5.5%, the workability deteriorates, so the upper limit was set to 5.5%.
  Cu is effective for suppressing hydrogen penetration and improving delayed fracture characteristics, and is effective for strengthening.HimselfSince the fine precipitation contributes to the improvement of delayed fracture, the addition amount is 0.001% or more. Moreover, since excessive addition causes deterioration of workability, the upper limit was made 3.0%.
[0026]
Cr is an element that is effective for suppressing hydrogen intrusion and improving delayed fracture characteristics, and for increasing the strength of the steel sheet. In addition, precipitates and crystallized substances containing Cr are very important elements because they become hydrogen trap sites, and Cr alone or combined addition with Nb, V, one or more of Ti and Mo described later can be added. Since it is particularly effective, the lower limit is set to 0.001%. On the other hand, if the content exceeds 5%, the workability deteriorates, so the upper limit was made 5%.
[0027]
Mo is not only an effective element to suppress hydrogen intrusion and improve delayed fracture characteristics, and to improve the hardenability of steel sheets and to obtain martensite stably in continuous annealing equipment, but also strengthens grain boundaries. This has the effect of suppressing the occurrence of hydrogen embrittlement. Moreover, the production | generation of the carbide | carbonized_material and pearlite which degrade a strength-hole expansibility balance is suppressed. Furthermore, it is effective for suppressing ferrite transformation and making the main phase bainite or bainitic ferrite, and is an important additive element for obtaining a very good balance of good strength-hole expansibility-weldability It is. However, since these effects cannot be obtained at less than 0.005%, the lower limit is set to 0.005%. Further, if the content exceeds 5%, these effects are saturated, so the upper limit is set to 5%.
[0028]
  Nb is an element effective for increasing the strength and refining of the steel sheet. Forming fine carbides, nitrides or carbonitrides is extremely effective for strengthening steel sheets. Furthermore, it is effective for suppressing softening of the weld heat affected zone. However, since these effects cannot be obtained at less than 0.001%, the lower limit is set to 0.001%. On the other hand, if the content exceeds 1%, precipitation of carbonitrides increases and the workability decreases, so the upper limit was made 1%.
  V can also be used as a hydrogen trap site by controlling the form of carbonitride in addition to the effects of suppressing hydrogen intrusion and improving delayed fracture characteristics, increasing the strength of steel sheets, and reducing the grain size. It is an important additive element for improving hydrogen embrittlement. However, since this effect cannot be obtained at less than 0.005%, the lower limit is set to 0.005%. On the other hand, when the content exceeds 1%, the precipitation of carbonitrides becomes remarkable, and the ductility decreases remarkably. For this reason, the upper limit is set to 1%.
[0029]
When Se, As, Sb, Sn, Pb, Bi is contained in a total of more than 0.05% in one or more types, the delayed fracture resistance is significantly inhibited. The upper limit was made 0.05%. On the other hand, for the extremely low level, the limit of recycling is narrowed, so each element has a lower limit of 0.0002%.
Ti is an element effective for increasing the strength and refining of the steel sheet. Forming fine carbides, nitrides or carbonitrides is extremely effective for strengthening steel sheets. Furthermore, the composite addition with elements such as Nb and Mo contributes to the improvement of delayed fracture resistance, so the Ti content is set to 0.001% or more. On the other hand, if it exceeds 5%, a large amount of coarse precipitates or crystallized products are formed, so that workability is lowered. For this reason, the upper limit value was set to 5%.
[0030]
W, Zr, Hf, and Ta are very important elements because they are effective in increasing the strength of the steel sheet, and precipitates and crystallized substances containing each element serve as hydrogen trap sites. However, if the total of one or more of these elements is less than 0.001%, these effects cannot be obtained, so the lower limit was set to 0.001%. On the other hand, if the content exceeds 5%, the workability deteriorates, so the upper limit was made 5%.
B is an element effective for increasing the strength of the steel sheet. However, since these effects cannot be obtained if the content is less than 0.0002%, the lower limit is set to 0.0002%. On the other hand, if the content exceeds 0.1%, the hot workability deteriorates, so the upper limit was made 0.1%.
[0031]
REM, Ca, and Mg are effective elements for suppressing the increase in the hydrogen ion concentration in the interface atmosphere accompanying the corrosion of the steel sheet surface, that is, for suppressing the decrease in pH. However, since these effects cannot be obtained if the content is less than 0.0002%, the lower limit is set to 0.0002%. On the other hand, if the content exceeds 0.1%, the workability deteriorates, so the upper limit was set to 0.1%.
Y is effective for controlling the form of inclusions and contributes to delayed fracture resistance, so 0.0002% or more was added. On the other hand, excessive addition deteriorates hot workability, so 0.1% or less was added.
[0032]
Next, the microstructure and precipitates will be described.
In tempered martensitic steel, delayed fracture is considered to be caused by the accumulation of hydrogen at the prior austenite grain boundaries and the like, resulting in the formation of voids and the like as the starting point. Therefore, if the hydrogen trap sites are dispersed evenly and finely and hydrogen is trapped there, the concentration of diffusible hydrogen is lowered and the susceptibility to delayed fracture is lowered. As described in the above-mentioned Patent Document 1, it has been found that delayed fracture resistance due to hydrogen is improved by controlling the oxide dispersion form in a thick steel plate to which Mg and Ti are added in combination. However, since thin steel sheets are generally subjected to forming before use, the occurrence of high residual stress and the presence of burrs on the processed end faces inevitably deteriorate delayed fracture resistance. It cannot supplement the deterioration of fracture characteristics. Thus, there are few studies on delayed fracture characteristics in consideration of the usage pattern of thin steel sheets, and it cannot be solved only by controlling the oxide morphology of Mg or Ti.
[0033]
Considering the case where the amount of hydrogen coming from the manufacturing process and the environment is large even locally, no matter how many hydrogen trap sites are dispersed in the steel material, delayed fracture due to hydrogen will inevitably occur. For this reason, first, in addition to (1) dispersing trap sites in the steel material to increase the allowable hydrogen amount of the steel material itself and suppressing retained austenite, (2) intrusion from the manufacturing process and the surrounding environment. It is important to reduce the amount of hydrogen obtained. In particular, when considering improving the hole expandability of plated steel sheets, there are many restrictions on cold speed control, etc., as in the case of continuous annealing of cold-rolled steel sheets. It is necessary to minimize the amount of invading hydrogen before the process and the plating process.
[0034]
In light of the above-mentioned background, the present inventors ensured and improved delayed fracture resistance in the production and use environments of plated steel sheets in order to disperse various crystallized substances, precipitate trap sites, and the strength of the steel sheets. In addition to the effects of, the reduction of the amount of hydrogen that could enter from the environment was examined. As a result, the present inventors have found a technique for improving and ensuring the delayed fracture resistance due to hydrogen in the production to use environment of a plated steel sheet (for example, with the addition of design stress after pressing). That is,
(1) Control of precipitates and residual austenite amount by strength and composition of steel sheet.
(2) Control of intrusion-resistant hydrogen characteristics by steel plate components.
By performing each of the above, it is possible to improve hydrogen embrittlement resistance under the environment of production and use of thin steel sheets for automobiles. Formulas (1-2) and (2-2) were defined as conditions for satisfying this. By satisfying this formula, the delayed fracture property of the high strength thin steel sheet can be secured.
[0035]
Next, (2) control of hydrogen intrusion characteristics by steel components will be described.
During the hydrogen intrusion process, when a reduction reaction of water molecules (in a neutral or alkaline environment) or hydrogen ions (in an acidic environment) occurs on the steel sheet surface due to corrosion or pickling, hydrogen atoms are generated on the steel sheet surface. Adsorb. The adsorbed hydrogen atoms (1) recombine and gasify as hydrogen molecules or enter the steel plate. As a result of intensive studies on these processes, the present inventors have found that in order to reduce the hydrogen penetration rate, in addition to improving the corrosion resistance, (1) reducing the pH (hydrogen ion concentration) of the environment as the corrosion reaction progresses as much as possible. It was found that it is effective to suppress the concentration of adsorbed hydrogen atoms on the surface and to accelerate the recombination reaction (hydrogen generation reaction).
[0036]
About (1), it discovered that REM, Ca, and Mg addition to steel were effective. Here, REM is an abbreviation for Rare Earth Metal and is a general term for lanthanoid elements starting from La. As industrial addition, it is often added in the form of misch metal. In this case, the amount of La and Ce added is increased. When REM, Ca, Y, and Mg are eluted in the corrosion reaction, the atmosphere is alkalized by the equilibrium reaction of the hydroxide, that is, a decrease in pH due to the corrosion reaction is suppressed.
[0037]
Two methods have been found for (2). The first method is a method of increasing the exchange current density in the reduction reaction of hydrogen ions or water. Cu, Ni, Cr, and Mo are effective, and when 0.1 ≦ 2Cu + 20Mo + 3Ni + Cr + 20V is satisfied, the hydrogen permeation rate is remarkably suppressed. The second method reduces the exchange current density. Alternatively, the impurity element that significantly increases the hydrogen generation overvoltage is limited. By limiting Se, As, Sb, Pb, and Bi as corresponding impurity elements, an increase in hydrogen permeation rate can be suppressed.
[0038]
In the plated steel sheet for automobiles, hydrogen intrusion occurs in the following process. The first is a pickling process in steel plate manufacturing, the second is a process assembly process such as painting or pressing, and the third is corrosion in the use environment. In any environment, it is effective to control the hydrogen penetration characteristics by the steel components described above. In order to improve the bare corrosion resistance of automobile steel plates and suppress hydrogen intrusion, it is necessary to add a large amount of expensive elements. In these methods (1) and (2), both are remarkable by adding a small amount. There is an advantage that an advantageous effect can be obtained.
[0039]
Furthermore, with respect to ensuring weldability, hole expansion, and ductility, the microstructure and the component range, and (1-1) and (2-1) formulas and (1-3) and (2-3) formulas should be used. In addition, the softening behavior of the weld heat-affected zone is suppressed while maintaining a high strength of 900 MPa or more, and further, the hole expansion ratio: (the inner diameter of the hole before the hole expansion test / the hole diameter before the hole expansion test− 1) It was found that x100 can secure a hole expandability of 70% or more. In order to ensure sufficient hole expandability, it is effective to use one or both of bainite and martensite (one is preferable when aiming at further high hole expandability), and the area ratio is 70% or more. It was decided. In order to form such a structure, it is necessary to satisfy the expressions (1-3) and (2-3).
[0040]
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 the hole expansibility, it is desirable that the lower phase bainite in which carbide is finely dispersed, bainitic ferrite without carbide or martensite is the main phase, and the area ratio exceeds 97%.
On the other hand, especially when martensite is the main phase, softening at the weld heat affected zone becomes a problem. For this, the weldability of a high strength material having a tensile strength of 900 MPa or more is ensured by satisfying the formulas (1-1) and (2-1) that define the components as described later. .
[0041]
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 retained austenite 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 characteristic deterioration is observed. And less than 3% may be included. Furthermore, inclusions such as oxides and sulfides may be inevitably included.
Further, when the formulas (1-1) and (2-1) and the formulas (1-3) and (2-3) are not satisfied, the tensile strength cannot be ensured to be 900 MPa or more, or the weld heat affected part. In addition to being unable to suppress softening, it is difficult to ensure hole expansibility.
(3.0Nb + 2.5Mo + 1 / 10Si + Mn)-(2C^0.5+2)> 0
... (1-1) (2-1)
0> Si + Al + 2- (Mn + Ni + 1.5Mo) (1-3) (2-3)
[0042]
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. In addition, each phase of the above microstructure, ferrite, bainite (including bainitic ferrite), retained austenite, martensite, interfacial oxidized phase and residual structure, observation of the existing position and measurement of area ratio, The steel sheet rolling direction cross section or the rolling perpendicular direction cross section is corroded by the reagent disclosed in Japanese Patent Application Laid-Open No. 59-219473, and observed with an optical microscope of 500 to 1000 times and an electron microscope of 1000 to 100,000 times (scanning type and transmission type). ) Can be quantified. 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.
[0043]
In addition, in order to ensure and improve delayed fracture resistance, various forms of crystallized substances and precipitates were also examined. As a result, we have found a technique for improving and ensuring delayed fracture resistance even under high residual stress and end face burrs in the environment where thin steel sheets are used.
That is,
{Circle around (1)} Control of the dispersion form in the grains of oxides, sulfides, nitrides, composite crystals, and composite precipitates of Nb, V, Cr, Ti, and Mo.
(2) Amount of retained austenite in the microstructure of the steel sheet.
By effectively controlling each of these, oxides, sulfides, nitrides, composite crystallized substances and composite precipitates of Nb, V, Cr, Ti, and Mo that are hydrogen trap sites are effectively dispersed, It is possible to ensure delayed fracture resistance after processing. For this purpose, by controlling the production conditions, a form control was performed in which oxides, sulfides, nitrides, composite crystals and composite precipitates of various elements could become hydrogen trap sites.
[0044]
This is because the interaction between dislocations and residual stress fields introduced by processing thin steel sheets and particles that become trap sites is different from those introduced during hot rolling and cooling after welding on thick steel sheets. It is thought that it originates in a difference and the difference in the heat processing method of a thin steel plate and a thick steel plate.
About detailed limitation, it limits as follows.
(1) One or more average particle diameters d: The average particle diameter was limited to 0.001 to 5.0 μm. This is because when the average particle diameter exceeds 5.0 μm, the mechanical properties of the thin steel sheet deteriorate and the manufacturing becomes difficult, and in addition, the coarse particles can no longer act as trap sites and can be the starting point of destruction. is there. Further, when the average particle diameter is less than 0.001 μm, the effect as a hydrogen trap site is reduced.
[0045]
Density ρ: The density of particles is 100 to 1 × 10¹³Piece / mm²It was. Low particle density means that the number of trap sites is small, and delayed fracture resistance after processing cannot be secured, so the lower limit is 100 particles / mm.²It was. Further, in the case of a high density, the ductility and molding processability deteriorate, and the effect of improving delayed fracture resistance is saturated.¹³Piece / mm²Was the upper limit.
Distribution: The ratio of the standard deviation σ [μm] from the average particle diameter to the average particle diameter d [μm] is determined to satisfy σ / d ≦ 1.0. σ / d> 1.0 means that the particle distribution covers a wide range, and the effect of improving delayed fracture resistance is smaller than that of the same average particle diameter, which leads to ductility deterioration and an increase in the number of starting points of fracture. The upper limit was determined to be 1.0 or less.
[0046]
Here, measurement of particles including oxides, sulfides, nitrides, composite crystallized substances, and composite precipitates of Nb, V, Cr, Ti, and Mo will be described. Measurement of the average particle diameter is a value obtained by observing at least 5000 visual fields with a scanning or transmission electron microscope at a magnification of 5,000 to 500,000 using a sample of a thin film or an extraction replica. To do. The average particle diameter is evaluated by the equivalent circle by image analysis. The average particle size is a value obtained by simply averaging the particle sizes measured by the above method, and the standard deviation is a value obtained from these particle sizes.
[0047]
Moreover, when calculating | requiring a density, a composite precipitation or a crystallization thing is counted as one piece. The composition analysis was performed by using EDX and ELLS, and the structural analysis was performed by analyzing the diffraction pattern.
A composite crystallized substance is a single or composite oxide mainly containing Mg, Al, Ti or the like, and each composite compound is a compound (carbide) containing Ti, Nb, V, Cr, Mo, Mg or the like. Nitrides, oxides, sulfides, etc.).
By performing the component adjustment and the structure control as described above, the hydrogen embrittlement resistance can be secured while securing the material even in the atmospheric environment from the time of the hot dip plating and electroplating production.
[0048]
Next, a manufacturing method will be described. The manufacturing method may be performed by using a plating steel sheet manufacturing facility that is generally used. 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. In this case, the reheating temperature is desirably 1100 ° C. or higher and 1300 ° C. or lower. As the reheating temperature becomes higher, coarse grains and thick oxide scales are formed. On the other hand, since the rolling resistance becomes high at low temperature heating, the above temperature range is desirable.
[0049]
Next, in hot rolling, hot rolling is performed in order to prevent the ferrite grains from being excessively strained and lowering the workability._ThreeFinished at + 30 ° C or higher, conversely, even if the temperature is too high, the recrystallized grain size after annealing and oxides such as Nb, V, Cr, Ti, and Mo, sulfides, nitrides, composite crystallized products, and composite precipitates However, it is desirable to end at 950 ° C. or lower.
[0050]
After hot rolling, it is cooled at a cooling rate of 0.1 to 1000 ° C./second to a coiling temperature range of 400 ° C. to 700 ° C. It is difficult to operate the cooling stop temperature lower than 400 ° C. When the upper limit of the cooling stop temperature is higher than 700 ° C, oxides, sulfides, nitrides, and composites of Nb, V, Cr, Ti, and Mo There is a concern that the trapping ability is reduced due to the coarsening of the precipitate. Further, if the cooling rate is slower than 0.1 ° C./second, there is a concern that the generation of pearlite is promoted and the strength is lowered, so the lower limit of the cooling rate is set to 0.1 ° C./second. On the other hand, since it is difficult in operation that the cooling rate exceeds 1000 ° C./second, this is set as the upper limit.
[0051]
The cooling temperature range after hot rolling is preferably 400 to 700 ° C. where oxides such as Nb, V, Cr, Ti, and Mo, sulfides, nitrides, and composite precipitates that are trap sites are likely to precipitate. Is preferably 600 to 700 ° C. Further, the cooling rate to the temperature region is preferably a cooling rate of 20 ° C./second or more in order to promote the precipitation of the precipitate that becomes the trap site.
[0052]
In cold rolling after pickling, if the rolling reduction is low, it becomes difficult to correct the shape of the steel sheet, so the lower limit is preferably 10%. In addition, when rolling at a rolling reduction exceeding 80%, it is preferable to set the upper limit value to 80% because of the occurrence of cracks in the edge portion of the steel sheet and the disorder of the shape. If the annealing temperature is too low, it becomes a non-recrystallized state and hardens, whereas if it is too high, the grains become coarse and rough skin may occur during pressing.₁+30 to Ac_Three+ 50 ° C. From the viewpoint of generating bainite or martensite, Ac_ThreeA point or higher is desirable. The holding time in this temperature range is set to 30 minutes or less because holding for at least 10 seconds is necessary in order to sufficiently recrystallize, and if the holding time becomes long, crystal grains become coarse.
[0053]
After annealing, cooling is performed at a cooling rate of 0.1 to 100 ° C./second to a temperature range of 200 ° C. or higher to a temperature range of Zn plating bath temperature + 100 ° C., and subsequently in a temperature range of Zn plating bath temperature to Zn plating bath temperature + 100 ° C. Hold for 1 to 3000 seconds including Zn plating immersion time. If the cooling rate is slower than 0.1 ° C./second, the lower limit of the cooling rate is set to 0.1 ° C./second because there is a concern that the formation of ferrite and pearlite is promoted and the strength is lowered. On the other hand, when the cooling rate exceeds 100 ° C./second, there is a concern that a problem may occur in plating adhesion.
[0054]
Further, the cooling stop temperature and the holding time are adjusted for the purpose of controlling the phase ratio of bainite and martensite in the final steel plate. When bainite is mainly used, it is desirable that the cooling stop temperature is 300 ° C. or higher and the holding time in that temperature range is 30 seconds or longer. On the other hand, when mainly martensite is used, the cooling stop temperature is desirably 200 ° C. or higher. When the cooling stop temperature is less than 200 ° C. or the holding time is less than 1 second, the plating property is deteriorated.
[0055]
On the other hand, even if the cooling stop temperature exceeds the plating bath temperature + 100 ° C., the plating property is impaired or the precipitation of carbides occurs. This may cause carbide precipitation even if the holding time exceeds 3000 seconds. Moreover, when alloying, an alloying process is performed at 300-600 degreeC after plating bath immersion. If it is less than 300 degreeC, it will become an alloy, and if it exceeds 600 degreeC, a carbide | carbonized_material will precipitate. Furthermore, in the case of electroplating, since there is little concern about deterioration of the plating properties immediately after annealing, as a difference in the hot dip galvanizing process, the cooling rate after annealing is set to 200 ° C./s or less, and the cooling stop temperature is also 150 ° C. Is the lower limit. These were set as limit values for ease of operation.
[0056]
【Example】
Next, this invention is demonstrated based on an Example.
A steel plate having a chemical composition as shown in Table 1 was heated to 1150 to 1250 ° C., and Ar_ThreeHot rolling was completed at the point + 30 ° C. or higher, and the steel strip wound up after cooling was pickled and then cold rolled to a thickness of 1.2 mm. These Ac₁And Ac_Three5-10% H to annealing temperature calculated from transformation temperature₂-N₂After raising the temperature and holding in the atmosphere, it was subjected to various tests by melting and electroplating. 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, various welds were performed with steel sheets attached, and a ball head overhang test was conducted with Teflon lubrication, and the overhang height and fracture position of the base material were measured. The details of the evaluation method of delayed fracture resistance of the steel sheet are as follows.
[0057]
[Table 1]

[0058]
(1) After temper rolling, the steel sheet is given 2% strain for the purpose of simulating strain during pressing.
(2) A notched plate-like tensile test piece having a stress concentration rate of 3.2 is taken from the steel plate.
(3) 3% NaCl-3g / 1NH_Four0.01-0.025 mA / cm in SCN aqueous solution²A constant current cathode charge is applied.
(4) Cd plating is performed.
(5) Apply a constant load 0.8 times the tensile strength.
(6) The test is conducted up to 100 hours, and it is judged whether the rupture or not.
[0059]
Table 2 shows the microstructure and residual γ content of each steel, mechanical properties, delayed fracture evaluation, weldability evaluation, and calculated values of each formula. Tables 3 and 4 show the effects on material properties of each steel. It shows about conditions and materials. It turns out that this invention steel is excellent in weldability, intensity | strength (800 MPa or more in tensile strength), and hole expansibility. In particular, the hole expandability shows an excellent value of 80%. On the other hand, the comparative example is inferior in any of the ball head overhang height, tensile strength and hole expandability, and delayed fracture evaluation of the welded portion.
[0060]
[Table 2]

[0061]
[Table 3]

[0062]
[Table 4]

[0063]
【The invention's effect】
As described above, according to the present invention, a high-strength plated steel sheet that simultaneously improves the weldability and hole expansibility of a high-strength steel sheet having a tensile strength of 900 MPa or more and further has improved delayed fracture resistance, and a method for producing the same Can be obtained.

Claims

% By mass
C: 0.01 to 0.25%
Si: 0.01 to 2.0%,
Mn: 0.01 to 4.0%,
P: 0.0001 to 0.05%,
S: 0.0001 to 0.05%,
Al: 0.01 to 3.0%,
N: 0.0001 to 0.01%
Mo: 0.005 to 5%,
Nb: 0.001 to 1.0%
The balance is composed of iron and inevitable impurities, the microstructure contains bainite and martensite in an area ratio of 70% or more in total, and residual austenite (Vγ) is contained in less than 3%. (TS) is 900 MPa or more, and further satisfies the following formulas (1-1) to (1-3): high strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expansibility .
(3.0Nb + 2.5Mo + 1 / 10Si + Mn) − (2C ^0.5 +2)> 0 (1-1)
0 ≦ 0.8 × {2Cu + 20Mo + 3Ni + Cr} − {0.1−3.5 × 10 ⁷ × (TS) ^−3.1 } −0.3Vγ (1-2)
0> Si + Al + 2- (Mn + Ni + 1.5Mo) (1-3)
Where TS: tensile strength (MPa),
The element symbol indicates mass% of each element contained in the steel.

Furthermore, in mass%,
Ni: 0.001 to 5.5%,
Cu: 0.001 to 3.0%,
Cr: 0.001 to 5.0%
The high-strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expansibility according to claim 1, comprising one or more of them.

Furthermore, in mass%, V: 0.005 to 1% is contained, the balance is composed of iron and inevitable impurities, and the microstructure is 70% in total of one or both of bainite and bainitic ferrite by area ratio. The residual austenite (Vγ) is contained in an amount of less than 3%, the tensile strength (TS) is 900 MPa or more, and the following formulas (2-1) to (2-3) are satisfied. Or a high-strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expansibility described in 2 .
(3.0Nb + 2.5Mo + 1 / 10Si + Mn) − (2C ^0.5 +2)> 0 (2-1)
0 ≦ 0.8 × {2Cu + 20Mo + 3Ni + Cr + 20V} − {0.1−V / 5−3.5 × 10 ⁷ × (TS) ^−3.1 } −0.3Vγ (2-2)
0> Si + Al + 2- (Mn + Ni + 1.5Mo) (2-3)
Where TS: tensile strength (MPa),
The element symbol indicates mass% of each element contained in the steel.

Furthermore, in mass%,
Se: 0.0002 to 0.05%,
As: 0.0002 to 0.05%,
Sb: 0.0002~0.05%,
P b: 0.0002~0.05%,
Bi: 0.0002 to 0.05%,
The hydrogen embrittlement resistance and weldability according to any one of claims 1 to 3, characterized in that one or more of the above are contained and the total thereof satisfies 0.05% or less. A high-strength thin steel plate with excellent hole expandability.

Further, the steel contains Ti: 0.001 to 5% by mass, and any of oxides, sulfides, nitrides, composite crystallized products, and composite precipitates of Nb, V, Cr, Ti, and Mo Or more, the average particle diameter d: 0.001 to 5.0 μm, density ρ: 100 to 1 × 10 ¹³ particles per square mm, distribution: standard deviation σ from average particle diameter and average particle diameter d Ratio: a distribution form satisfying σ / d ≦ 1.0, and tensile strength is 900 MPa or more, hydrogen embrittlement resistance according to any one of claims 1 to 4 , weldability and High-strength galvanized steel sheet with excellent hole expandability.

Furthermore, in steel,
W: 0.001 to 5%,
Zr: 0.001 to 5%,
Hf: 0.001 to 5%,
Ta: 0.001 to 5%
1 type or 2 types or more are contained in total 0.001-5%, It was excellent in hydrogen embrittlement resistance, weldability, and hole expansibility of any one of Claims 1-5 characterized by the above-mentioned. High strength galvanized steel sheet.

Further, the steel contains B: 0.0002 to 0.1% by mass% in the steel, hydrogen embrittlement resistance, weldability and hole expansion according to any one of claims 1 to 6. High strength galvanized steel sheet with excellent properties.

Furthermore, in steel,
REM: 0.0002 to 0.1%
Y: 0.0002 to 0.1%
Ca: 0.0002 to 0.1%,
Mg: 0.0002 to 0.1%
The high-strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability, and hole expansibility according to any one of claims 1 to 7 , characterized by containing at least one of the following.

A method of manufacturing a steel sheet according to any one of claims 1-8, while casting a cast slab containing components according to any one of claims 1-8, or after once cooled Heated again, finished hot rolling at a finishing temperature of Ar ₃ point + 30 ° C. or higher, then cooled to a coiling temperature at a cooling rate of 0.1 to 1000 / second and then rolled up at 400 ° C. to 700 ° C. The steel sheet is cold-rolled after pickling, and then annealed for 10 seconds to 30 minutes in a temperature range of Ac ₁ +30 (° C.) or higher and Ac ₃ +50 (° C.) or lower, and then at an average cold speed of 0.1 to 100 ° C. / After cooling from 200 ° C. to plating bath temperature +100 (° C.) in a second, hold for 1 second to 3000 seconds including the subsequent plating immersion time in the temperature range of Zn plating bath temperature to Zn plating bath temperature +100 (° C.). Soak in a Zn plating bath and then cool to room temperature A method for producing a high-strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability.

The high-strength zinc excellent in hydrogen embrittlement resistance, weldability and hole expansibility according to claim 9 , characterized in that after being immersed in a Zn plating bath, an alloying treatment is performed at 300 to 600 ° C and cooling to room temperature. Manufacturing method of plated steel sheet.

A method of manufacturing a steel sheet according to any one of claims 1-8, while casting a cast slab containing components according to any one of claims 1-8, or after once cooled Heat again, heat rolling finished at a finishing temperature of Ar ₃ point + 30 ° C. or higher, then cooled to a winding temperature at a cooling rate of 0.1 to 1000 ° C./second and then wound up at 400 ° C. to 700 ° C. After pickling the cold rolled steel sheet, it is cold-rolled at a reduction rate of 10 to 80%, and then annealed for 10 seconds to 30 minutes in a temperature range of Ac ₁ +30 (° C.) or higher and Ac ₃ +50 (° C.) or lower during annealing. It is cooled to a temperature range of 150 to 500 ° C. at an average cooling rate of 0.1 to 200 ° C./sec, held for 1 to 3000 seconds in the same temperature range, and then subjected to electroplating. High-strength galvanized thin steel with excellent hydrogen embrittlement, weldability and hole expandability A manufacturing method of a board.