JP2004323951A

JP2004323951A - High strength galvanized steel sheet having excellent hydrogen embrittlement resistance, weldability and hole expansibility, and its production method

Info

Publication number: JP2004323951A
Application number: JP2003123365A
Authority: JP
Inventors: Nobuhiro Fujita; 展弘藤田; Kunio Hayashi; 邦夫林; Akira Usami; 明宇佐見; Manabu Takahashi; 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: 2004-11-18
Anticipated expiration: 2023-04-28
Also published as: JP4317384B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength galvanized steel sheet in which the weldability and hole expansibility of a high strength steel sheet having a tensile strength of ≥900 MPa are simultaneously improved, and to provide its production method. <P>SOLUTION: The high strength galvanized steel sheet having excellent hydrogen embrittlement resistance, weldability and hole expansibility has a composition comprising, by mass, ≤0.25% C, ≤2.0% Si, ≤4.0% Mn, ≤0.05% P, ≤0.05% S, ≤3.0% Al and ≤0.01% N, and comprising one or more kinds of metals selected from ≤5.5% Ni, ≤3.0% Cu, ≤5.0% Cr, ≤5% Mo and ≤1.0% Nb, and the balance iron with inevitable impurities, has a structure comprising bainite and/or martensite of ≥70% in total by area and <3% residual austenite (Vγ), has a tensile strength (TS) of ≥900 MPa, and further satisfies the following inequalities (1-1) to (1-3): inequality (1-1): (3.0Nb+2.5Mo+1/10Si+Mn)-(2C<SP>0.5</SP>+2)>0, inequality (1-2): 0≤0.8×ä2Cu+20Mo+3Ni+Cr}-ä0.1-3.5×10<SP>7×</SP>(TS)<SP>-3.1</SP>}-0.3Vγ, and inequality (1-3): 0>Si+Al+2-(Mn+Ni+1.5Mo). <P>COPYRIGHT: (C)2005,JPO&NCIPI

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軟化挙動を抑制の効果も強度レベルが低く、８００ＭＰａ以上の極めて高強度な材料における溶接性や加工性について十分とはいえない。
【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、スッポト溶接等の従来の溶接方法に加えてレーザー溶接なども一部適用される例もある。しかし、高強度故母材は特に溶接部および熱影響部での材質変動が５９０ＭＰａクラスの高強度材に比べ極めて顕著となる。また、高強度化にマルテンサイトの活用は穴拡げ性や延性低下が助長されてしまう。
【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％、を含有し、Ｎｉ：０．００１〜５．５％、Ｃｕ：０．００１〜３．０％、Ｃｒ：０．００１〜５．０％、Ｍｏ：０．００５〜５％、Ｎｂ：０．００１〜１．０％のうち１種以上を含有し、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｝−｛０．１−３．５×１０^７×（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。
【００１５】
（２）更に、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）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／５−３．５×１０^７×（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】
（３）更に、質量％にて、Ｓｅ：０．０００２〜０．０５％、Ａｓ：０．０００２〜０．０５％、Ｓｂ：０．０００２〜０．０５％、Ｓｎ：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）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薄鋼板。
【００１７】
（４）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１００〜１×１０^１３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）〜（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亜鉛めっき鋼板。
【００１８】
（５）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）〜（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。
（６）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）〜（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。
【００１９】
（７）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）〜（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。
【００２０】
（８）前記（１）〜（７）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）〜（７）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、Ａｒ_３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、Ａｃ_１＋３０（℃）以上、Ａｃ_３＋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】
（９）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（８）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。
（１０）前記（１）〜（７）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）〜（７）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、Ａｒ_３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Ａｃ_１＋３０（℃）以上、Ａｃ_３＋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、自信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、１００〜１×１０^１３個／ｍｍ^２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／ｍｍ^２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、１×１０^１３個／ｍｍ^２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Ａｒ_３＋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、Ａｃ_１＋３０〜Ａｃ_３＋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Ａｃ_３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、Ａｒ_３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Ａｃ_１およびＡｃ_３変態温度から計算される焼鈍温度に５〜１０％Ｈ_２−Ｎ_２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／１ＮＨ_４ＳＣＮ水溶液中で０．０１〜０．０２５ｍＡ／ｃｍ^２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]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability, and hole expandability 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 when the tensile strength exceeds 980 MPa, delayed fracture occurs due to intrusion of hydrogen into steel. ing. On the other hand, (1) thin steel sheets have a small thickness and are released in a short time even if hydrogen enters, and (2) there is almost no use of steel sheets 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, ultrahigh-strength steel sheets of 980 MPa or more are subjected to press forming, pipe forming, bending, end face processing, hole expanding processing, etc. The use of such materials as reinforcing materials such as impact beams, seat rails, and the like is rapidly increasing. Therefore, there is an urgent need to develop an ultra-high strength steel sheet having delayed fracture resistance. Until now, techniques for improving delayed fracture resistance have mostly been developed for steel products that are often used as bolts, steel bars, thick plates, or the like, and have a proof stress or yield stress or less.
[0004]
For example, in the development of bar steel and bolt steel, tempered martensite has been developed, and "new development of delayed fracture elucidation" (Iron and Steel Institute of Japan, published in January 1997).
(Non-Patent Document 1) reports that an additive element exhibiting tempering softening resistance, such as Cr, Mo or V, is effective for improving delayed fracture resistance. This is a technique in which an alloy carbide is precipitated and is used as a hydrogen trap site to shift a delayed fracture mode from a grain boundary to an intragranular fracture. However, since these steels have a C content of 0.4% or more and contain a lot of alloying elements, the workability and weldability required for thin steel sheets are inferior, and the precipitation heat treatment for alloy carbide precipitation is several hours or more. However, there is also a problem in manufacturability.
[0005]
Also, Japanese Patent Application Laid-Open No. H11-293383 (Patent Document 1) states that an oxide mainly composed of Ti and Mg is effective in preventing a hydrogen defect. However, this is intended for thick steel sheets, and although delayed fracture after welding with high heat input is taken into account in particular, it is subject to the high degree of processing required for thin steel sheets, No consideration is given to the effect on the delayed fracture phenomenon such as occurrence. Furthermore, there is no consideration regarding the workability, which is a basic property of a thin steel sheet.
[0006]
On the other hand, regarding delayed fracture of a thin steel sheet, for example, CAMP-ISIJ, vol. 5, 1839-1842, Yamazaki et al., October 1992, published by The Iron and Steel Institute of Japan
(Non-Patent Document 2) reports on the promotion of delayed fracture due to the work-induced transformation of the amount of retained austenite. This takes into account the forming of a thin steel sheet, but describes the regulation of the amount of retained austenite that does not degrade delayed fracture resistance. That is, it relates to a high-strength thin steel sheet having a specific structure, and cannot be said to be a fundamental measure for improving delayed fracture resistance. Furthermore, when assembling a member using such a high-strength material, ductility, bendability, hole-expandability, weldability, and the like become a more serious problem than a high-strength steel plate having a tensile strength of up to about 590 MPa. It is necessary to take measures against these.
[0007]
The following measures are taken for each characteristic.
For example, regarding the hole expandability, see CAMP-ISIJ, vol. 13 (2000), p395 (Non-Patent Document 3), the main phase is bainite to improve hole-expanding properties, and also to form overhanging properties by forming residual austenite in the second phase. It is disclosed that it exhibits an overhang property comparable to that of the existing retained austenitic steel. Further, it is also shown that when austempering is performed at an Ms temperature or lower to generate retained austenite having an area ratio of 2 to 3%, tensile strength × hole expansion ratio becomes maximum. However, no consideration has been given to the weldability in which the tensile strength becomes more than 800 MPa and the softening behavior in the heat affected zone.
[0008]
Regarding the weldability, there are many cases where the softening behavior (HAZ softening behavior) in the heat affected zone is regarded as a problem. On the other hand, for example, as disclosed in Japanese Patent Application Laid-Open No. 2000-87175 (Patent Document 2), it has been shown that the HAZ softening behavior is suppressed by precipitation of carbides of Nb and Mo (Nb, Mo) C. However, although this technique is considered in terms of fatigue strength, it does not sufficiently consider workability such as hole expandability. Further, the effect of suppressing the HAZ softening behavior also has a low strength level, and the weldability and workability of a very high-strength material of 800 MPa or more cannot be said to be sufficient.
[0009]
In particular, when the tensile strength is 800 MPa or more, the welding itself becomes difficult, and the tensile strength becomes even more remarkable when the tensile strength is 980 MPa or more. For this reason, in some cases, laser welding and the like are partially applied in addition to conventional welding methods such as spot welding. However, the high-strength base metal has much more remarkable material variation especially in the welded portion and the heat-affected zone than the 590 MPa class high-strength material. In addition, the use of martensite for increasing the strength promotes hole-expandability and ductility.
[0010]
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, CAMP-ISIJ, vol. 13 (2000), p391 (Non-Patent Document 4)}. 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. In addition, all of the above studies are related to hot rolling and cold rolling, and in particular, no consideration is given to hydrogen intrusion in the plating step or behavior of hydrogen invading before the plating step in the plating step.
[0011]
[References]
(1) Non-patent document 1 “New developments in elucidating delayed fracture” (Iron and Steel Institute of Japan, issued in January 1997)
(2) Non-Patent Document 2 (CAMP-ISIJ, vol. 5, pages 1839 to 1842, Yamazaki et al., October 1992, published by The Iron and Steel Institute of Japan)
(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 (JP-A-11-293383)
(6) Patent Document 2 (JP-A-2000-87175)
[0012]
[Problems to be solved by the invention]
As described above, in particular, the manufacturing, assembly, and use environment of thin steel sheets for automobiles are sufficiently considered to take measures against delayed fracture of the hydrogen embrittlement type, and to fully consider the use characteristics such as weldability and hole expandability. There are no development cases for high-strength plated steel sheets. The present invention solves the above-mentioned problems of the prior art, and provides a high-strength galvanized steel sheet having a tensile strength of 900 MPa or more and simultaneously improved weldability and hole expandability of a high-strength steel sheet, and a method for producing the same. The purpose is to:
[0013]
[Means for Solving the Problems]
In view of the above background, the present inventors have come to find a method for fundamentally improving delayed fracture resistance by sufficiently considering a use environment of thin steel sheets and a production method using existing facilities. That is, in addition to controlling the structure and precipitates of the steel sheet, it is possible to improve the delayed fracture resistance due to hydrogen by controlling the trap site in the steel sheet and reducing the amount of hydrogen that can enter from the manufacturing process and use environment. Was found. Details are as follows.
[0014]
(1) In 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: 0.0001-0.05%, Al: 0.01-3.0%, N: 0.0001-0.01%, Ni: 0.001-5.5%, Cu : 0.001 to 3.0%, Cr: 0.001 to 5.0%, Mo: 0.005 to 5%, Nb: 0.001 to 1.0%, the balance being Consists of iron and unavoidable impurities, has a microstructure containing at least 70% of bainite and / or martensite in total, less than 3% of retained austenite (Vγ), and has a tensile strength (TS) of less than 3%. Hydrogen embrittlement resistance, weldability and holes characterized by being not less than 900 MPa and further satisfying the following formulas (1-1) to (1-3). High-strength galvanized steel sheet excellent in the bottom of.
(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)
Here, TS: tensile strength (MPa),
Element symbols indicate mass% of each element contained in the steel.
[0015]
(2) Further, it contains V: 0.005 to 1%, the balance consists of iron and unavoidable impurities, and the microstructure is 70% or more in total of one or both of bainite and bainitic ferrite in area ratio. (1) characterized by containing less than 3% of retained austenite (Vγ), having a tensile strength (TS) of at least 900 MPa, and further satisfying the following formulas (2-1) to (2-3). A high-strength galvanized steel sheet with excellent hydrogen embrittlement resistance, weldability and hole expandability as described.
(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)
Here, TS: tensile strength (MPa),
Element symbols indicate mass% of each element contained in the steel.
[0016]
(3) Further, in mass%, Se: 0.0002 to 0.05%, As: 0.0002 to 0.05%, Sb: 0.0002 to 0.05%, Sn: 0.0002 to 0%. 0.05%, Pb: 0.0002-0.05%, Bi: 0.0002-0.05%, and the total thereof satisfies 0.05% or less. A high-strength steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to the above (1) or (2), characterized in that:
[0017]
(4) Further, the steel contains 0.001 to 5% by mass of Ti in mass%, and oxides, sulfides, nitrides, composite precipitates, and composite precipitates of Nb, V, Cr, Ti, and Mo are contained. Any one or more of the materials has an average particle diameter d: 0.001 to 5.0 μm, a density ρ: 100 to 1 × 10 per square mm. ^Thirteen , Distribution: a distribution form satisfying a ratio of standard deviation σ from average particle diameter to average particle diameter d: σ / d ≦ 1.0, and having a tensile strength of 900 MPa or more. The high-strength galvanized steel sheet excellent in hydrogen embrittlement resistance, weldability and hole expandability according to any one of the above (1) to (3).
[0018]
(5) Further, one type of W: 0.001 to 5%, Zr: 0.001 to 5%, Hf: 0.001 to 5%, and Ta: 0.001 to 5% by mass in steel. Or excellent in hydrogen embrittlement resistance, weldability, and hole expandability according to any one of the above (1) to (4), wherein two or more types are contained in a total of 0.001 to 5%. High strength galvanized steel sheet.
(6) The hydrogen embrittlement resistance according to any one of (1) to (5), wherein the steel further contains B: 0.0002 to 0.1% by mass%. Strength galvanized steel sheet with excellent weldability and hole expandability.
[0019]
(7) Further, REM: 0.0002-0.1%, Y: 0.0002-0.1%, Ca: 0.0002-0. Hydrogen embrittlement resistance and welding according to any one of the above (1) to (6), wherein one or two or more of 1% and Mg: 0.0002 to 0.1% are contained. High strength galvanized steel sheet with excellent heat resistance and hole spreadability.
[0020]
(8) A method for producing a steel sheet according to any one of (1) to (7), wherein a casting slab comprising the component according to any one of (1) to (7) is cast. As it is, or once cooled and then heated again, Ar ₃ Hot rolling is completed at a finishing temperature of not less than the point + 30 ° C., then cooled to a winding temperature at a cooling rate of 0.1 to 1000 / sec, and then a hot-rolled steel sheet wound at 400 to 700 ° C. is pickled and cooled. And then Ac ₁ +30 (℃) or more, Ac ₃ After annealing for 10 seconds to 30 minutes in a temperature range of +50 (° C.) or less, cooling at an average cooling rate of 0.1 to 100 ° C./second to 200 ° C. to a plating bath temperature of +100 (° C.), and then a Zn plating bath Hydrogen resistance, characterized by holding in a temperature range from temperature to Zn plating bath temperature + 100 (° C.) for 1 second to 3000 seconds including a subsequent plating immersion time, immersing in a Zn plating bath, and then cooling to room temperature. A method for producing a high-strength galvanized steel sheet with excellent embrittlement, weldability and hole expandability.
[0021]
(9) After dipping in a Zn plating bath, an alloying treatment is performed at 300 to 600 ° C., and the alloy is cooled to room temperature, and is excellent in hydrogen embrittlement resistance, weldability, and hole expandability as described in (8) above. Manufacturing method of high strength galvanized steel sheet.
(10) A method for producing a steel sheet according to any one of (1) to (7), wherein a casting slab comprising the component according to any one of (1) to (7) is cast. As it is, or once cooled and then heated again, Ar ₃ Hot rolling is completed at a finishing temperature of + 30 ° C. or higher, then cooled to a winding temperature at a cooling rate of 0.1 to 1000 ° C./sec, and then a hot-rolled steel sheet wound at 400 to 700 ° C. is pickled. , Cold rolling at a rolling reduction of 10 to 80%, and then Ac ₁ +30 (℃) or more, Ac ₃ After annealing in a temperature range of +50 (° C.) or less for 10 seconds to 30 minutes, the sample is cooled to a temperature range of 150 to 500 ° C. at a cooling rate of 0.1 to 200 ° C./sec at an average cooling rate, and is cooled to a temperature of 150 to 500 ° C. This is a method for producing a high-strength galvanized thin steel sheet excellent in hydrogen embrittlement resistance, weldability, and hole expandability, which is characterized by performing electroplating after holding for 2 to 3000 seconds.
[0022]
BEST MODE FOR CARRYING OUT 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 forming a hard phase such as martensite and increasing the strength. To obtain a strength of 900 MPa or more, 0.01% or more is necessary. , Hydrogen embrittlement is likely to occur. Therefore, the upper limit is set to 0.25%.
Si is a substitutional solid solution strengthening element that greatly hardens the material, is effective in increasing the strength of the steel sheet, and is an element that suppresses the precipitation of cementite. To 2.0% as the upper limit. Although the lower limit is not particularly defined, the extremely lowering causes an increase in the production cost, so it is desirable to add 0.005% or more.
[0023]
Mn is an element effective for increasing the strength of the steel sheet. However, if the content is less than 0.01%, this effect cannot be obtained, so the lower limit is set to 0.01%. Conversely, if the content is too large, not only will co-segregation with P and S be promoted, but also the workability may deteriorate, so the upper limit is set to 4.0%.
P is an element that promotes grain boundary destruction due to grain boundary segregation, and is preferably as low as possible. However, since extremely low level is not preferable in terms of manufacturing cost, the content is set to 0.0001% or more. Further, since it is an element that deteriorates corrosion resistance, the upper limit is set to 0.05%.
[0024]
S is an element that promotes hydrogen absorption in a corrosive environment, and is preferably as low as possible. However, since extreme reduction is not preferable in terms of manufacturing cost, the content of S is set to 0.0001% or more. In particular, in order to enhance the workability, a lower one is desirable, and the upper limit is made 0.05%.
Al is added in an amount of 0.01% or more for deoxidation. Increasing the amount of Al increases inclusions such as alumina and deteriorates workability, so the upper limit is 3.0%.
N should be small as it contributes to deterioration of workability and generation of blowholes during welding. If it exceeds 0.01%, workability deteriorates, so the upper limit is made 0.01%. Since lowering the N causes increase in the refining cost, the lower limit is set to 0.0001%.
[0025]
Ni has the effect of suppressing hydrogen intrusion and improving delayed fracture characteristics, and the effect of securing the strength of the steel sheet by increasing the hardenability of the steel sheet. However, if the content is less than 0.001%, these effects cannot be obtained, so the lower limit is set to 0.001%. Conversely, if the content exceeds 5.5%, the workability deteriorates, so the upper limit is set to 5.5%.
Cu is effective in strengthening delayed fracture characteristics by suppressing hydrogen penetration and strengthening, and is effective for strengthening. In addition, since fine precipitation of self contributes to improvement in delayed fracture, Cu is added in an amount of 0.001% or more. Moreover, since excessive addition causes deterioration of workability, the upper limit is set to 3.0%.
[0026]
Cr is an element that suppresses hydrogen intrusion and improves delayed fracture characteristics, and is effective for increasing the strength of steel sheets. In addition, precipitates and crystallized substances containing Cr are very important elements because they serve as hydrogen trap sites, and Cr alone or in combination with one or more of Nb, V, and Ti and Mo described below can be added. Since it is particularly effective, the lower limit is made 0.001%. Conversely, if the content exceeds 5%, the workability decreases, so the upper limit is set to 5%.
[0027]
Mo is an element that suppresses hydrogen intrusion and improves delayed fracture characteristics, and is an effective element for improving the hardenability of steel sheets and obtaining martensite stably with continuous annealing equipment, as well as strengthening grain boundaries. This has the effect of suppressing the occurrence of hydrogen embrittlement. In addition, the generation of carbides and pearlite that degrade the strength-hole expandability balance is suppressed. Furthermore, it is effective for suppressing the 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 expanding property-weldability. It is. However, if the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. 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 reducing the grain size of the steel sheet. By forming fine carbides, nitrides or carbonitrides, it is extremely effective in strengthening steel sheets. Furthermore, it is also effective in suppressing softening of the heat affected zone. However, if the content is less than 0.001%, these effects cannot be obtained, so the lower limit is set to 0.001%. Conversely, when the content exceeds 1%, the precipitation of carbonitride increases and the workability deteriorates. Therefore, the upper limit is set to 1%.
V can be used as a trap site for hydrogen by controlling the carbonitride shape, in addition to the effect of suppressing hydrogen intrusion and improving delayed fracture characteristics, increasing the strength and reducing the grain size of the steel sheet. It is an important additive element for improving hydrogen embrittlement. However, since the effect cannot be obtained with less than 0.005%, the lower limit is set to 0.005%. Conversely, when the content exceeds 1%, precipitation of carbonitride becomes remarkable, and ductility decreases remarkably. Therefore, the upper limit is set to 1%.
[0029]
If Se, As, Sb, Sn, Pb, and Bi are contained in an amount of 0.05% or more in total of one or more of them, the delayed fracture resistance is significantly impaired. And the upper limit was made 0.05%. On the other hand, each element has a lower limit of 0.0002% for the reason that ultra-lowering narrows restrictions on recycling.
Ti is an element effective for increasing the strength and reducing the grain size of the steel sheet. By forming fine carbides, nitrides or carbonitrides, it is extremely effective in strengthening steel sheets. Furthermore, the content of Ti is set to 0.001% or more because the composite addition with elements such as Nb and Mo contributes to the improvement of delayed fracture resistance. Conversely, if the content exceeds 5%, coarse precipitates or crystallized substances are generated in large amounts, so that workability is reduced. For this reason, the upper limit 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 amount of one or more of these elements is less than 0.001%, these effects cannot be obtained, so the lower limit was made 0.001%. Conversely, if the content exceeds 5%, the workability decreases, so the upper limit is set to 5%.
B is an element effective for increasing the strength of the steel sheet. However, if these effects are not obtained at less than 0.0002%, the lower limit is set to 0.0002%. Conversely, if the content exceeds 0.1%, the hot workability deteriorates, so the upper limit was set to 0.1%.
[0031]
REM, Ca, and Mg are effective elements for suppressing the increase in the hydrogen ion concentration in the interface atmosphere due to the corrosion of the steel sheet surface, that is, for suppressing the decrease in pH. However, these effects cannot be obtained if each is less than 0.0002%, so the lower limit was made 0.0002%. Conversely, if the content exceeds 0.1%, the workability deteriorates, so the upper limit was made 0.1%.
Y is effective for controlling the form of inclusions and contributes to delayed fracture resistance. Therefore, Y was added in an amount of 0.0002% or more. On the other hand, excessive addition deteriorates hot workability, so the addition was made 0.1% or less.
[0032]
Next, the microstructure and the precipitate will be described.
Delayed fracture in tempered martensitic steel is considered to cause voids and the like due to the accumulation of hydrogen at the former austenite grain boundaries and the like, and this part serves as a starting point to cause fracture. Therefore, if the hydrogen trap sites are evenly and finely dispersed and hydrogen is trapped in that portion, the concentration of diffusible hydrogen decreases, and the sensitivity to delayed fracture decreases. As described in Patent Document 1 mentioned above, it has been found that by controlling the dispersion form of oxides in a thick steel sheet to which Mg and Ti are added in combination, delayed fracture resistance due to hydrogen is improved. However, since thin steel sheets are generally subjected to forming before use, the occurrence of high residual stress and the presence of burrs and the like at the processing end face inevitably deteriorates the delayed fracture resistance. It cannot compensate for the degradation of fracture characteristics. As described above, there are few studies on the delayed fracture characteristics in consideration of the usage form of the thin steel sheet, 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, hydrogen-induced delayed fracture necessarily occurs. 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 to suppress the retained austenite, (2) invading from the manufacturing process and the placed environment. It is important to reduce the amount of hydrogen obtained. In particular, when considering the improvement of hole expandability of a plated steel sheet, there are many restrictions on cooling speed control and the like as in the case of continuous annealing of a cold-rolled steel sheet, and there are many restrictions on the structure control described later. It is necessary to minimize the amount of invading hydrogen before the process and the plating process.
[0034]
In view of the above-described background, the inventors of the present invention have developed various types of precipitates, dispersion of trap sites of precipitates, and strength of a steel sheet in order to secure and improve delayed fracture resistance in a production environment to a use environment of a plated steel sheet. In addition to the effects of, the reduction of the amount of hydrogen that can enter from the environment was studied. As a result, the present inventors have found a technique for improving and ensuring the delayed fracture resistance due to hydrogen under the manufacturing environment of the plated steel sheet to the use environment (for example, under the application of design stress after press working). That is,
(1) Control of the amount of precipitates and retained austenite by the strength and composition of the steel sheet.
{Circle around (2)} Control of the resistance to intrusion hydrogen by the composition of the steel sheet.
Respectively, it is possible to improve the hydrogen embrittlement resistance under the environment from the production of a thin steel sheet for an automobile to the use environment. Equations (1-2) and (2-2) are defined as conditions for satisfying this. By satisfying this expression, the delayed fracture resistance of the high-strength thin steel sheet can be secured.
[0035]
Next, (2) the control of the hydrogen penetration characteristics by the components of the steel material will be described.
In the process of hydrogen intrusion, 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) are recombined and gasified as hydrogen molecules, or penetrate into the steel plate. The present inventors have conducted intensive studies on these processes. As a result, 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 accompanying the progress of the corrosion reaction as much as possible. It has been found that it is effective to suppress the concentration and lower the concentration of adsorbed hydrogen atoms on the surface and (2) accelerate the recombination reaction (hydrogen generation reaction).
[0036]
Regarding (1), it was found that REM, Ca, and Mg addition to steel was effective. Here, REM is an abbreviation of Rare Earth Metal and is a general term for lanthanoid elements starting with La. As industrial addition, it is often added in the form of misch metal, and in this case, the added amount of La or Ce increases. 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]
Regarding (2), two methods were found. The first method is to increase the exchange current density of 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 significantly suppressed. The second method reduces the exchange current density described above. Alternatively, this is a method of restricting an impurity element that significantly increases the hydrogen generation overvoltage. By limiting Se, As, Sb, Pb, and Bi as the corresponding impurity elements, an increase in the hydrogen permeation rate can be suppressed.
[0038]
In the plated steel sheet for automobiles, hydrogen invasion occurs in the following process. First, there is a pickling process in steel plate manufacturing, second, a process assembly process such as painting and pressing, and third, corrosion in a use environment. In any environment, the control of the hydrogen penetration characteristics by the components of the steel material described above is effective. It is necessary to add a large amount of expensive elements in order to improve the bare corrosion resistance of steel sheets for automobiles and suppress hydrogen intrusion. However, in these methods (1) and (2), the addition of a very small amount is significant. There is an advantage that a special effect can be obtained.
[0039]
Furthermore, in order to ensure weldability, hole expansion, and ductility, the microstructure and the range of components, the expressions (1-1) and (2-1), and the expressions (1-3) and (2-3) should be limited. Thus, the softening behavior of the heat-affected zone is suppressed while maintaining a high tensile strength of 900 MPa or more, and the hole expansion ratio: (inner diameter of hole before hole expansion test / hole diameter before hole expansion test− 1) It has been found that x100 can secure 70% or more hole expandability. In order to ensure sufficient hole expandability, it is effective to use one or both of bainite and martensite (preferably in the case of further increasing hole expandability), the area ratio is 70% or more. I decided. In order to form such a structure, it is necessary to satisfy the expressions (1-3) and (2-3).
[0040]
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. Bainitic ferrite means bainite having no carbide, and for example, acicular ferrite is one example. In order to improve the hole expandability, it is desirable that lower bainite in which carbides are finely dispersed, bainitic ferrite or martensite having no carbides be the main phase, and the area ratio exceed 97%.
On the other hand, particularly when martensite is the main phase, softening in the heat affected zone becomes a problem. On the other hand, by satisfying the formulas (1-1) and (2-1) in which the components are specified as described later, the weldability of a high-strength material having a tensile strength of 900 MPa or more is secured. .
[0041]
From the viewpoint of ensuring ductility and increasing the strength, ferrite having an area ratio of less than 30% may be included. On the other hand, it is not desirable to include retained austenite from the viewpoint of hole expandability and softening behavior of the heat affected zone, but if the area ratio is less than about 3%, no remarkable deterioration in properties is observed. At less than 3%. Further, inclusions such as oxides and sulfides may be inevitably contained.
If the formulas (1-1) and (2-1) and the formulas (1-3) and (2-3) are not satisfied, a tensile strength of 900 MPa or more cannot be ensured or a welding heat-affected portion is not obtained. In addition to the fact that the softening of the steel cannot be suppressed, it is also difficult to ensure the hole expandability.
(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, a case where one or two or more of carbide, nitride, sulfide and oxide are contained at an area ratio of 1% or less as the remaining structure of the microstructure can be used in the present invention. It was included in the area ratio of the phase. In addition, each phase of the above microstructure, ferrite, bainite (including bainitic ferrite), retained austenite, martensite, interfacial oxidation phase and remaining structure, identification of the existing position, and measurement of the area ratio were carried out using a nital reagent and A reagent disclosed in JP-A-59-219473 corrodes a cross section in the rolling direction of a steel sheet or a cross section in a direction perpendicular to the rolling direction, and observes an optical microscope of 500 to 1000 times and an electron microscope of 1000 to 100,000 times (scanning and transmission types). ) Allows quantification. By observing at least 20 visual fields, the area ratio of each tissue can be determined by the point count method or image analysis.
[0043]
In addition, in order to secure and improve delayed fracture resistance, various forms of crystallized substances and precipitates were examined. As a result, they found a technique for improving and ensuring delayed fracture resistance even under high residual stress and burr generation on the end face under the usage environment of thin steel sheets.
That is,
{Circle around (1)} Control of intragranular dispersion morphology 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.
Respectively, effectively disperse the oxides, sulfides, nitrides, composite crystals and composite precipitates of hydrogen trap sites Nb, V, Cr, Ti, and Mo effectively, Delayed fracture resistance after processing can be ensured. For this purpose, by controlling the production conditions, morphological control was performed so that oxides, sulfides, nitrides, composite crystals, and composite precipitates of various elements could serve as hydrogen trap sites.
[0044]
This is because the interaction between the dislocations and residual stress fields introduced by the processing of thin steel plates and the particles that become trap sites is different from that of the dislocations and residual stresses introduced during hot rolling and cooling after welding in thick steel plates. It is thought to be due to the difference and the difference in the heat treatment method between the thin steel plate and the thick steel plate.
The details are limited as follows.
Average particle diameter d of any one or more of {circle around (1)}: The average particle diameter was limited to 0.001 to 5.0 μm. This is because if the average particle diameter exceeds 5.0 μm, the mechanical properties of the thin steel sheet deteriorate and the production becomes difficult. In addition, the coarse particles do not function as trap sites and can be a 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: Density of particles is 100 to 1 × 10 ^Thirteen Pieces / mm ² And The low particle density means that the number of trap sites is small, and the delayed fracture resistance after processing cannot be secured. ² And When the density is high, the ductility and the formability are deteriorated, and the effect of improving delayed fracture resistance is saturated. ^Thirteen Pieces / mm ² Was set as the upper limit.
Distribution: The particle distribution was such that the ratio of the standard deviation σ [μm] from the average particle diameter to the average particle diameter d [μm] satisfied σ / d ≦ 1.0. σ / d> 1.0 means that the particle distribution is wide, and the effect of improving delayed fracture resistance is smaller than that of the same average particle size, leading 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, the measurement of particles including oxides, sulfides, nitrides, composite crystals, and composite precipitates of Nb, V, Cr, Ti, and Mo will be described. The average particle diameter is measured using a sample of a thin film or an extracted replica, using a scanning or transmission electron microscope, observing at a magnification of 5,000 to 500,000 times, and a value obtained by measuring at least 30 visual fields. I do. The average particle diameter is evaluated by a circle equivalent diameter by image analysis. The average particle diameter is a value obtained by simply averaging the particle diameters measured by the above method, and the standard deviation is a value obtained from these particle diameters.
[0047]
When obtaining the density, the number of composite precipitates or crystals is counted as one. The composition analysis was performed by using EDX and ELLS, and the structural analysis was performed by analyzing Diffraction pattern.
The composite crystal is a single oxide or a composite oxide mainly containing Mg, Al, Ti and the like, and each composite compound is a compound containing Ti, Nb, V, Cr, Mo, Mg and the like (carbide). , Nitrides, oxides and sulfides).
By performing the component adjustment and the structure control as described above, the hydrogen embrittlement resistance can be ensured while securing the material even in the atmosphere environment from the time of producing hot-dip plating and electroplating.
[0048]
Next, a manufacturing method will be described. The manufacturing method may be to use a commonly used equipment for manufacturing a plated steel sheet. 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 temporarily cooled and then reheated to perform hot rolling. It is desirable that the reheating temperature at this time be 1100 ° C. or more and 1300 ° C. or less. This is because when the reheating temperature becomes high, coarse grains and a thick oxide scale are formed. On the other hand, since the rolling resistance is increased by low-temperature heating, the above-mentioned temperature range is desirable.
[0049]
Next, in hot rolling, hot rolling is carried out by Ar to prevent the ferrite grains from being excessively strained and the workability is reduced. ₃ Finished at + 30 ° C. or higher, conversely, even if the temperature was too high, the recrystallized grain size after annealing and oxides, sulfides, nitrides, composite precipitates and composite precipitates such as Nb, V, Cr, Ti, and Mo Is desirably terminated at 950.degree.
[0050]
After hot rolling, it is cooled at a cooling rate of 0.1 to 1000 ° C / sec to a winding temperature range of 400 ° C to 700 ° C. It is difficult to make the cooling stop temperature lower than 400 ° C., and 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 are reduced. There is a concern that the trapping ability may decrease due to coarsening of the precipitate. Further, if the cooling rate is lower than 0.1 ° C./sec, there is a concern that the generation of pearlite may be promoted and the strength may be reduced. Therefore, the lower limit of the cooling rate is set to 0.1 ° C./sec. On the other hand, if the cooling rate exceeds 1000 ° C./sec, it is difficult to operate, so the upper limit was set.
[0051]
The cooling temperature range after hot rolling is set to 400 to 700 ° C. where oxides such as Nb, V, Cr, Ti, and Mo serving as trap sites, sulfides, nitrides, and composite precipitates are easily deposited. Is preferably 600 to 700 ° C. In addition, the cooling rate to the temperature range is preferably a cooling rate of 20 ° C./sec or more in order to promote the precipitation of the precipitate serving as the trap site.
[0052]
In cold rolling after pickling, it is preferable to set the lower limit to 10% because a low rolling reduction makes it difficult to correct the shape of the steel sheet. Further, when rolling is performed at a rolling reduction of more than 80%, the upper limit value is preferably set to 80% because of the occurrence of cracks at the edge portion of the steel sheet and the disorder of the shape. If the annealing temperature is too low, it becomes unrecrystallized and becomes hard, whereas if it is too high, there is a problem that the grains become coarse and the surface may be roughened during pressing. ₁ +30 to Ac ₃ + 50 ° C. From the viewpoint of forming bainite or martensite, Ac ₃ Is desirable. The holding time in this temperature range is required to be at least 10 seconds in order to sufficiently recrystallize, and if the holding time is long, the crystal grains are coarsened.
[0053]
After annealing, it is cooled at a cooling rate of 0.1 to 100 ° C./sec to a temperature range of 200 ° C. or more and a Zn plating bath temperature + 100 ° C., and subsequently, is cooled in a temperature range of Zn plating bath temperature to Zn plating bath temperature + 100 ° C. Hold for 1 second to 3000 seconds including the immersion time of Zn plating. If the cooling rate is lower than 0.1 ° C./sec, the lower limit of the cooling rate is set to 0.1 ° C./sec. On the other hand, when the cooling rate is higher than 100 ° C./sec, 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 sheet. When bainite is mainly used, the cooling stop temperature is preferably 300 ° C. or more, and the holding time in that temperature range is preferably 30 seconds or more. 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, and carbide precipitates. This may cause carbide precipitation even if the retention time exceeds 3000 seconds. In the case of alloying, an alloying treatment is performed at 300 to 600 ° C. after immersion in the plating bath. If it is lower than 300 ° C., it becomes unalloyed, and if it exceeds 600 ° C., carbides are precipitated. Further, in the case of electroplating, since there is little concern about deterioration in plating properties immediately after annealing, the difference in the above-described hot-dip galvanizing process is that the cooling rate after annealing is 200 ° C / s or less and the cooling stop temperature is 150 ° C. Is the lower limit. These were the limit values because of the ease of operation.
[0056]
【Example】
Next, the present invention will be described based on examples.
A steel sheet having a chemical composition as shown in Table 1 was heated to 1150 to 1250 ° C. ₃ Hot rolling was completed at a point of + 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 Ac ₁ And Ac ₃ 5-10% H for annealing temperature calculated from transformation temperature ₂ -N ₂ After the temperature was raised and maintained in an atmosphere, the samples were subjected to melting and electroplating and subjected to various tests. 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. Regarding the weldability, various weldings were performed with steel plates, and a ball head overhang test was performed using Teflon lubrication, and the overhang height and the fracture position with respect to the base metal were measured. The details of the method for evaluating the delayed fracture resistance of a steel sheet are as follows.
[0057]
[Table 1]

[0058]
{Circle around (1)} After temper rolling, a steel sheet is given a 2% strain for the purpose of simulating the strain during pressing.
{Circle around (2)} A notched plate-like tensile test piece having a stress concentration rate of 3.2 is sampled from a steel sheet.
(3) 3% NaCl-3g / 1NH ₄ 0.01 to 0.025 mA / cm in SCN aqueous solution ² To apply a constant current cathode charge.
(4) Cd plating is performed.
(5) A constant load of 0.8 times the tensile strength is applied.
(6) The test is performed up to 100 hours, and it is determined whether the sample is broken or not broken.
[0059]
Table 2 shows the microstructure and residual γ content, mechanical properties, delayed fracture evaluation, and weldability evaluation of each steel, and the calculated values of each formula. Tables 3 and 4 show the production effects on the material properties of each steel. The conditions and materials are shown. It is understood that the steel of the present invention is excellent in weldability, strength (800 MPa or more in tensile strength), and hole expandability. In particular, the hole expanding property shows an excellent value of 80%. On the other hand, the comparative example is inferior in any of the 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】
INDUSTRIAL APPLICABILITY As described above, according to the present invention, a high-strength galvanized steel sheet having a tensile strength of 900 MPa or more, which simultaneously improves weldability and hole expandability of a high-strength steel sheet, and further has improved delayed fracture resistance, and a method for producing the same. Can be obtained.

Claims

In mass%,
C: 0.01-0.25%,
Si: 0.01 to 2.0%,
Mn: 0.01 to 4.0%,
P: 0.0001-0.05%,
S: 0.0001-0.05%,
Al: 0.01 to 3.0%,
N: 0.0001 to 0.01%,
Containing
Ni: 0.001 to 5.5%,
Cu: 0.001 to 3.0%,
Cr: 0.001 to 5.0%,
Mo: 0.005 to 5%,
Nb: 0.001 to 1.0%
And the balance consists of iron and unavoidable impurities, and the microstructure is 70% or more in total in one or both of bainite and martensite, and the residual austenite (Vγ) is less than 3% in area ratio. It is excellent in hydrogen embrittlement resistance, weldability and hole expandability characterized by having a tensile strength (TS) of 900 MPa or more and satisfying the following formulas (1-1) to (1-3). High strength galvanized steel sheet.
(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)
Here, TS: tensile strength (MPa),
Element symbols indicate mass% of each element contained in the steel.

Further, V: 0.005 to 1%, the balance being iron and unavoidable impurities, and the microstructure is 70% or more in total of one or both of bainite and bainitic ferrite in area ratio, and retained austenite. The hydrogen resistant composition according to claim 1, wherein the composition contains (Vγ) less than 3%, has a tensile strength (TS) of 900 MPa or more, and further satisfies the following formulas (2-1) to (2-3). High strength galvanized steel sheet with excellent embrittlement, 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 × 10 ⁷ × (TS) ^−3.1 } −0.3Vγ (2-2)
0> Si + Al + 2- (Mn + Ni + 1.5Mo) (2-3)
Here, TS: tensile strength (MPa),
Element symbols indicate mass% of each element contained in the steel.

Furthermore, in mass%,
Se: 0.0002-0.05%,
As: 0.0002-0.05%,
Sb: 0.0002-0.05%,
Sn: 0.0002-0.05%,
Pb: 0.0002-0.05%,
Bi: 0.0002-0.05%,
3. Hydrogen embrittlement resistance, weldability and hole expandability according to claim 1 or 2, wherein one or more of the following are contained, and the total of them satisfies 0.05% or less. High strength steel sheet.

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

Furthermore, in mass% in steel,
W: 0.001 to 5%,
Zr: 0.001 to 5%,
Hf: 0.001 to 5%,
Ta: 0.001 to 5%
5. A hydrogen embrittlement resistance, a weldability and a hole expansion property according to any one of claims 1 to 4, wherein one or two or more of the above are contained in total. High strength galvanized steel sheet.

Furthermore, B: 0.0002-0.1% is contained by mass% in steel, The hydrogen embrittlement resistance, weldability, and hole expansion of any one of Claims 1-5 characterized by the above-mentioned. High strength galvanized steel sheet with excellent properties.

Furthermore, in mass% in steel,
REM: 0.0002-0.1%,
Y: 0.0002-0.1%,
Ca: 0.0002-0. 1%,
Mg: 0.0002-0. 1%
The high-strength galvanized steel sheet having excellent hydrogen embrittlement resistance, weldability and hole expandability according to any one of claims 1 to 6, characterized by containing one or more of the following.

A method for producing a steel sheet according to any one of claims 1 to 7, wherein a cast slab composed of the component according to any one of claims 1 to 7 remains as-cast, or after once cooled. It is heated again, hot rolling is completed at a finishing temperature of ₃ points of Ar + 30 ° C. or higher, then cooled to a winding temperature at a cooling rate of 0.1 to 1000 / sec, and then rolled at 400 to 700 ° C. After the steel sheet is pickled and cold rolled, and then annealed in a temperature range of Ac ₁ +30 (° C.) or more and Ac ₃ +50 (° C.) or less for 10 seconds to 30 minutes, the average cooling rate is 0.1 to 100 ° C. / After cooling to 200 ° C. to plating bath temperature + 100 (° C.) in seconds, the temperature is maintained in a temperature range of Zn plating bath temperature to Zn plating bath temperature + 100 (° C.) for 1 second to 3000 seconds including a subsequent plating immersion time. Immersed in Zn plating bath, then cooled to room temperature DOO hydrogen embrittlement resistance characterized by, the method of producing a high strength galvanized steel sheet having excellent weldability and hole expandability.

9. A high-strength zinc excellent in hydrogen embrittlement resistance, weldability and hole-expanding properties according to claim 8, wherein after dipping in a Zn plating bath, an alloying treatment is performed at 300 to 600 ° C., and the alloy is cooled to room temperature. Manufacturing method of plated steel sheet.

A method for producing a steel sheet according to any one of claims 1 to 7, wherein a cast slab composed of the component according to any one of claims 1 to 7 remains as-cast, or after once cooled. It is heated again, the hot rolling is completed at a finishing temperature of ₃ points of Ar + 30 ° C. or more, then cooled to a winding temperature at a cooling rate of 0.1 to 1000 ° C./sec, and then heated at 400 ° C. to 700 ° C. After pickling the rolled steel sheet, cold rolling is performed at a rolling reduction of 10 to 80%, and after annealing in a temperature range of Ac ₁ +30 (° C.) or more and Ac ₃ +50 (° C.) or less during annealing 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./sec at an average cooling rate, held at the same temperature range for 1 to 3000 seconds, and then subjected to electroplating. High strength galvanized with excellent hydrogen embrittlement, weldability and hole spreadability Method of manufacturing a steel plate.