JP2004211140A - Hot-dip galvanized steel sheet and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-dip galvanized steel sheet which has a tensile strength of 780 MPa or higher, and superior formability for extension flange and brittleness resistance in fabricating, and to provide a method for stably manufacturing it. <P>SOLUTION: This galvanized steel sheet comprises 0.03-0.13% C, 0.7% or less Si, 2.0-4.0% Mn, 0.05% or less P, 0.005% or less S, 0.01-0.1% sol.Al, 0.005% or less N, 0.005-0.1% Ti, 0.0002-0.0040% B, while B, Ti and N satisfy the following expression of inequality: 0.0002≤B-(11/14)[N-(14/48)Ti]≤0.0030, (wherein if [N-(14/48)Ti]≤0, [N-(14/48)Ti] is nil), and the balance substantially iron; and has ferrite with an average grain size of 5 μm or smaller, and 15 to 80% by volume ratio of martensite. The steel sheet can further include one or more elements among Nb, Mo, V and Cr. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

ここで、式中の元素記号はそれぞれの元素のｍａｓｓ％を、ｔは加熱温度における保持時間（ｓｅｃ）を示し、［Ｎ−（１４／４８）Ｔｉ］≦０の場合は［Ｎ−（１４／４８）Ｔｉ］を０とする。
【００２４】
この発明は、伸びフランジ性および耐二次加工脆性に優れた高強度溶融亜鉛めっき鋼板を得るために、鋭意検討を重ねた結果、見出された知見に基づきなされた。それは、フェライトとマルテンサイトを含む複合組織の高強度鋼板において、伸びフランジ性の低下および耐二次加工脆性の劣化が、プレス成形前のブランキングおよびプレス成形時のフェライト／マルテンサイト界面近傍への応力集中に起因するということである。
【００２５】
プレス成形前のブランキングの際は、フェライト／マルテンサイト界面近傍への応力集中によって発生するマイクロボイドに起因して、伸びフランジ性が低下する。プレス成形時には、増大する結晶粒界への応力集中により、靭性（耐二次加工脆性）が劣化する。そこで、フェライトを細粒化するとともに、Ｂを適正添加して粒界強度を上昇させることにより、マイクロボイドの発生が抑制されて、伸びフランジ性の向上が図れると共に、細粒化と粒界強化により靭性が向上し、耐二次加工脆性の向上が図れるということである。
【００２６】
本発明は、７８０ＭＰａ以上の引張強度を有し、伸びフランジ性および耐二次加工脆性に優れた高強度溶融亜鉛めっき鋼板を得るため、上記の要件から構成されている。以下に、本発明の鋼成分の添加理由、成分限定範囲、組織形態および製造条件の限定理由について説明する。
【００２７】
（１）鋼成分の範囲
以下、％はｍａｓｓ％を示す。
【００２８】
Ｃ：０．０３〜０．１３％
Ｃは、鋼の強化に有効な元素であり、０．０３％以上の添加量を要する。一方、０．１３％を超えてＣを添加すると、圧延方向にバンド組織が発達し、プレス成形の際、このバンド組織への応力集中が発生するため靭性が劣化する。このため、Ｃ量は０．０３〜０．１３％の範囲内とする。
【００２９】
Ｓｉ：≦０．７％
Ｓｉは、鋼の強化に有効な元素であり、適宜添加することができる。しかし、Ｓｉ量が０．７％を超えると、焼鈍時の冷却過程において高温域でのフェライト変態が促進され、フェライト粒が粒成長するため、発明の意図する微細なフェライト粒が得られなくなる。さらに、Ｓｉ量が０．７％を超えると、溶融亜鉛めっきの密着性が劣化し、不均一なめっき皮膜が形成されるため、深絞り成形の際、鋼板表層への応力集中が大きくなり、成形後の耐二次加工脆性にとって好ましくない。このため、Ｓｉ量は０．７％以下とする。なお、Ｓｉ量を０．３％未満とすると、耐二次加工脆性が更に向上する。従って、Ｓｉ量を０．３％未満とすることが好ましい。
【００３０】
Ｍｎ：２．０〜４．０％
Ｍｎは、鋼の強化に有効な元素であり、２．０％以上の添加量を要する。一方、Ｍｎの添加量が４．０％を超えると、スラブの鋳造の際、Ｍｎの偏析が発生しやすくなる。圧延、溶融亜鉛めっき処理後、鋼板にはこの偏析に起因したバンド組織が発達するため、伸びフランジ性が著しく劣化する。このため、Ｍｎ量は２．０〜４．０％の範囲内とする。
【００３１】
Ｐ：≦０．０５％
Ｐは、鋼の強化に有効な元素であり、適宜添加することができる。しかし、Ｐの添加量が０．０５％を超えると、鋳造時のＰの偏析に起因した不均一組織が発達しやすくなり、延性が劣化する。従って、Ｐの添加量を０．０５％以下とする。
【００３２】
Ｓ：≦０．００５％
Ｓは、鋼中に過剰に存在すると、ＭｎＳが多量に形成されるため、鋼板の伸びおよび伸びフランジ性に好ましくない。特にＳ量が０．００５％を超えると、この悪影響が懸念される。このため、Ｓ量を０．００５％以下とする。
【００３３】
ｓｏｌ．Ａｌ：０．０１〜０．１％
Ａｌは鋼の脱酸のために、０．０１％以上必要である。しかし、Ａｌの添加量が０．１％を超えると、鋼中に酸化物等のＡｌ系介在物が多くなり、延性が著しく劣化する。このため、Ａｌ量は０．０１〜０．１％の範囲内とする。
【００３４】
Ｎ：≦０．００５％
Ｎは、鋼中に過剰に存在すると、スラブの鋳造の際、表面に割れが発生しやすくなる。特に、Ｎ量が０．００５％を超えると、この悪影響が顕著となる。このため、Ｎ量は０．００５％以下とする。
【００３５】
Ｂ：０．０００２〜０．００４０％
Ｂは、固溶状態で存在することにより、焼鈍時にオーステナイト粒界に偏析してオーステナイトを細粒化し、オーステナイトから変態するフェライトの細粒化に極めて有効である。また、Ｂは、オーステナイトからのマルテンサイト変態の促進にも極めて有効な元素であり、これらの効果を得るため０．０００２％以上添加する必要がある。しかし、Ｂの添加量が０．００４０％を超えると、これらの効果は飽和するばかりか、めっき表面外観が劣化する。このため、Ｂ量は０．０００２〜０．００４０％の範囲内とする。
【００３６】
Ｔｉ：０．００５〜０．１％
Ｔｉは、Ｎを析出固定することによりＢがＢＮとなるのを防止して、Ｂを固溶状態に保つことにより、上記Ｂの効果、即ちフェライトの細粒化およびマルテンサイト変態の促進に、大きく寄与する。このように固溶Ｂを確保するには、Ｔｉを０．００５％以上添加する必要がある。一方、Ｔｉの添加量が０．１％を超えると、めっき表面外観が劣化する。このため、Ｔｉ量は０．００５〜０．１％の範囲内とする。
【００３７】
固溶Ｂ量 Ｂ^＊＝Ｂ−（１１／１４）［Ｎ−（１４／４８）Ｔｉ］：０．０００２〜０．００３０％
ＢおよびＴｉによる上記の効果を得るには、固溶Ｂ量を適正に制御する必要がある。固溶Ｂ量の指標としては、Ｂ量から析出固定されていないＮの当量（１１／１４）［Ｎ−（１４／４８）Ｔｉ］を差し引いた量：
Ｂ^＊＝Ｂ−（１１／１４）［Ｎ−（１４／４８）Ｔｉ］
を用いる。この量Ｂ^＊が０．０００２％未満では所望の微細組織が得られない。一方、この量が０．００３０％を超えると、これらの効果は飽和するばかりか、めっき表面外観が劣化する。このため、固溶Ｂ量 Ｂ^＊は０．０００２〜０．００３０％の範囲内とする。なお、［Ｎ−（１４／４８）Ｔｉ］の値が、負（ＴｉのＮ当量＞Ｎ）となる場合は０とする。この場合、Ｂ^＊＝Ｂとなり、固溶Ｂ量はＢ添加量に等しくなる。
【００３８】
本発明では、さらに必要に応じて、Ｎｂ，Ｍｏ，Ｖ，Ｃｒを次の範囲で添加することができる。
【００３９】
Ｎｂ： 添加する場合０．００５〜０．１％
Ｎｂは、Ｃと微細炭化物を形成することにより、組織の細粒化に寄与する元素である。この効果を得るには、０．００５％以上の添加を必要とする。一方、Ｎｂの添加量が０．１％を超えると、焼鈍時のフェライト、オーステナイトの再結晶は遅滞し、加工組織が残留し易くなり、鋼板の延性は著しく低下する。このため、Ｎｂを添加する場合は０．００５〜０．１％の範囲内とする。
【００４０】
Ｍｏ：０．０１〜１．０％
Ｍｏは、鋼の焼入性を向上させる元素であり、鋼の強化に有効である。Ｍｏの強化能を得るためには、０．０１％以上の添加を必要とする。一方、Ｍｏの添加量が１．０％を超えると、めっき表面外観が劣化する。このため、Ｍｏを添加する場合は０．０１〜１．０％の範囲内とする。
【００４１】
Ｖ：添加する場合０．０１〜０．５％
Ｖは、鋼の焼入性を向上させる元素であり、鋼の強化に有効である。Ｖの強化能を得るためには、０．０１％以上の添加を必要とする。一方、Ｖの添加量が０．５％を超えると、効果は飽和する。このため、Ｖを添加する場合は０．０１〜０．５％の範囲内とする。
【００４２】
Ｃｒ：添加する場合０．０１〜０．５％
Ｃｒは、Ｍｏ，Ｖ等と同様、鋼の焼入性を向上させる元素であり、鋼の強化に有効である。この効果を得るためには、０．０１％以上のＣｒの添加を必要とする。一方、Ｃｒの添加量が０．５％を超えると、強化能は飽和する。このため、Ｃｒを添加する場合は０．０１〜０．５％の範囲内とする。
【００４３】
上記の鋼成分以外の化学成分については、過剰に添加しなければ、本発明の効果を損なうことはない。例えば、Ｗ，Ｎｉは０．５％以下であれば、本発明の目的とする特性に悪影響を及ぼさない。なお、この発明で残部が実質的に鉄というのは、その他の合金元素あるいは不可避的不純物についても、本発明の目的とする特性に悪影響を及ぼさないかぎり、含有してもよいことを意味する。
【００４４】
（２）鋼板の組織形態
自動車の車体の軽量化の目的から、高強度鋼板の適用に対して、張出し性、伸びフランジ性等のプレス成形性が求められている。張出し性には、ｎ値、伸びなどの素材特性が要求されることから、フェライト、マルテンサイト主体の複合組織鋼板が望ましい。
【００４５】
しかし、この鋼板の場合、プレス成形前のブランキングの際、フェライトと硬質のマルテンサイトの界面近傍への応力集中によりマイクロボイドが多く発生するため、伸びフランジ性の低下が懸念されている。また、鋼の高強度化に伴い、素材自体の靭性が低下するため、プレス成形後の部品の靭性（耐二次加工脆性）の劣化が懸念される。このため、高強度材を実部品へ適用する際には、耐二次加工脆性の向上が重要となる。
【００４６】
そこで本発明では、フェライト、マルテンサイトを含有する７８０ＭＰａ以上の高強度鋼板において、伸びフランジ性とともに、プレス成形後の耐二次加工脆性を向上させるための組織因子を検討した。この結果、フェライトを平均粒径５μｍ以下に細粒化するとともに、粒界強度を上昇させるＢを適量化することにより、フェライト相と硬質のマルテンサイト相の界面の脆化に対する抵抗が増大し、伸びフランジ性および耐二次加工脆性の向上が可能であることが明らかとなった。
【００４７】
具体的な数値は、フェライト粒径とＢ量を種々に変化させた鋼板を用いて、６０゜円錐ポンチによる穴拡げ試験、および深絞り成形材の縦割れ試験を実施して求めた。用いた鋼板は、ｍａｓｓ％でＣ：０．０３５〜０．０７５％、Ｓｉ：０．０２〜０．２５％、Ｍｎ：２．０〜３．０％、Ｐ：０．０１〜０．０３％、Ｓ：０．００１〜０．００３％、ｓｏｌ．Ａｌ：０．０２〜０．０５％、Ｎ：０．００２０〜０．００３５％、Ｔｉ：０．０１〜０．０６％、Ｂ：０．００００（無添加）〜０．００４０％の化学成分を有し、ＴＳが８００〜８６０ＭＰａ、フェライトの平均粒径が２〜１５μｍ、マルテンサイト体積率が２７〜４２％である溶融亜鉛めっき鋼板（板厚：１．４ｍｍ）である。
【００４８】
この鋼板を用いて、ＪＦＳ ＴｌＯＯｌ（日本鉄鋼連盟規格）に準拠した穴拡げ試験により、伸びフランジ性の指標である穴拡げ率λを測定した。また、この鋼板から、図１に示すように、１２０ｍｍφのブランクを採取し、絞り比１．６（７５ｍｍφ）でカップに成形した後、カップ高さ２７ｍｍにトリムし、縦割れ試験用サンプルを作製した。このカップを用いて、冷媒中でカップの開口試験を実施し、カップの側壁部に縦割れ破壊が発生しない最低温度Ｔｃを測定した。
【００４９】
穴拡げ試験結果および縦割れ試験結果を、フェライトの平均粒径ｄ_Ｆ、固溶Ｂ量の指標であるＢ^＊＝Ｂ−（１１／１４）（Ｎ−（１４／４８）Ｔｉ）で整理して、図２に示す。フェライトの平均粒径ｄ_Ｆが大きくなるほど、穴拡げ率λは低く、縦割れ臨界温度Ｔｃは高くなり、ｄ_Ｆが５μｍを超えると（図中の△または●）、λは４０〜６６％（図中の△）、２０〜３８％（図中の●）と低くなり、伸びフランジ性は劣化する。
【００５０】
またこの図２で、Ｔｃは−１０〜−４０℃（図中の△）、０〜３０℃（図中の●）と高く、耐二次加工脆性が劣化する。また、Ｂ^＊＝Ｂ−（１１／１４）（Ｎ−（１４／４８）Ｔｉ）が０％の場合には、ｄ_Ｆが３μｍと小さくても（図中の△）、伸びフランジ性、耐二次加工脆性ともに劣化している。これらの結果は、いずれも穴拡げ試験前の穴打抜き時、および縦割れ試験前の深絞り成形時に、フェライト／マルテンサイトの界面への応力集中が大きいことに起因した特性劣化と考えられる。
【００５１】
一方、ｄ_Ｆが５μｍ以下であり、Ｂ^＊＝Ｂ−（１１／１４）（Ｎ−（１４／４８）Ｔｉ）が０．０００２〜０．００３０％の場合（図中○）の場合には、λは６５〜８６％と高く、Ｔｃは−７０〜−９０℃と低いことから、良好な伸びフランジ性と耐二次加工脆性が得られている。また、Ｂ−（１１／１４）（Ｎ−（１４／４８）Ｔｉ）が０．００３０％を超える場合（図中の×）は、良好な伸びフランジ性と耐二次加工脆性が得られるものの、めっき表面外観が劣化している。
【００５２】
このように、フェライトとマルテンサイトを有する７８０ＭＰａ以上の引張強度の溶融亜鉛めっき鋼板において、伸びフランジ性と耐二次加工脆性を向上させるには、フェライトを平均粒径で５μｍ以下に細粒化し、更に、粒界強度を上昇させるＢ量を適量化することが必要であることが明らかとなった。
【００５３】
本発明の溶融亜鉛めっき鋼板は、優れた伸びフランジ性と耐二次加工脆性を意図しており、上記（１）のように化学成分を調整し、また、上記（２）のようにフェライトを細粒化した鋼板であり、以下の方法にて製造することができる。
【００５４】
（３）鋼板の製造方法
上記（１）で述べた化学成分の鋼を溶製し、鋳造した後、熱間圧延を施す。鋼の溶製、鋳造は特に限定はなく、成分偏析等、特に組織が不均一でなければよい。また、熱間圧延は鋳造後、直ちに開始してもよいし、或いは一旦冷却し、加熱してから行なってもよい。粗圧延した後、仕上圧延を行ない、コイルに巻き取る。板厚方向の組織の均一化を図るためから、
仕上圧延はＡｒ_３点以上とし、コイル巻取温度は７００℃未満とするのが好ましい。
【００５５】
次に、得られた熱延板を酸洗し、冷間圧延した後、連続溶融亜鉛めっき処理を施す。冷間圧延率は特に限定する必要はない。焼鈍時の加熱は、固溶Ｂの粒界偏析によりオーステナイトを微細化し、これより変態するフェライトを平均粒径５μｍ以下に細粒化するため、適正制御する必要がある。つまり、加熱温度は、オーステナイト単相域であるＡｃ_３点以上とし、また、オーステナイトの粗大化を抑制するため、加熱温度の上限は９００℃以下とする。また、オーステナイト粒界へのＢの偏析を促進させるためには、加熱温度における保持時間を固溶Ｂ量により適正制御しなければならないことが明らかとなった。
【００５６】
具体的な数値は、加熱時の保持時間を変化させて焼鈍した鋼板を用いて、組織観察、穴拡げ試験、縦割れ試験を実施して求めた。組織観察では、走査型電子顕微鏡を用いて、圧延方向に平行で板面に垂直な断面において、無作為に抽出した２００個分のフェライトの平均粒径とマルテンサイトの体積率を測定した。また、穴拡げ試験、縦割れ試験は上記と同様の方法にて実施した。
【００５７】
用いた鋼板は、ｍａｓｓ％でＣ：０．０４５〜０．０７０％、Ｓｉ：０．１〜０．２５％、Ｍｎ：２．０〜３．０％、Ｐ：０．０１〜０．０３％、Ｓ：０．００１〜０．００３％、ｓｏｌ．Ａｌ：０．０２〜０．０５％、Ｎ：０．００２０〜０．００４０％、Ｔｉ：０．０１〜０．０６％、Ｂ：０．０００７〜０．００３０％、Ｎｂ：０．０２〜０．０４％の化学成分を有する冷延板（板厚：１．４ｍｍ）を、加熱温度８５０℃、加熱時間５０〜６００ｓｅｃにて焼鈍した溶融亜鉛めっき鋼板で、ＴＳが８１０〜８７０ＭＰａである。
【００５８】
穴拡げ試験結果、縦割れ試験結果、組織観察結果を、加熱時間、固溶Ｂ量の指標 Ｂ^＊＝Ｂ−（１１／１４）（Ｎ−（１４／４８）Ｔｉ）で整理して、図３に示す。なお、組織観察結果では、マルテンサイト体積率は３０〜４０％である。
【００５９】
図３に示すように、加熱時間ｔ（ｓｅｃ）が長くなるほど、また、Ｂ^＊が少ないほど、フェライトの平均粒径が増大している。これより、平均粒径５μｍ以下の微細フェライトを得るには、加熱時間ｔとＢ^＊＝Ｂ−（１１／１４）［Ｎ−（１４／４８）Ｔｉ］で規定される最適条件が存在することが明らかとなった。すなわち、加熱時間ｔ（ｓｅｃ）が４Ｂ^＊×１０^４＋３０（ｓｅｃ）以上かつ４Ｂ^＊×１０^４＋２８０以下の場合には（図中の○）、フェライトは平均粒径で２〜５μｍまで微細化し、λは６５〜８０％と高く、Ｔｃは−７０〜−９０℃と低くなり、良好な伸びフランジ性と耐二次加工脆性が得られている。
【００６０】
一方、加熱時間ｔが４Ｂ^＊×１０^４＋２８０（ｓｅｃ）を超えると（図中の□または×）、フェライト平均粒径が６〜９μｍ（□）、１０〜１６μｍ（×）と増大する。これに伴い、穴拡げ率λは前者（□）が４２〜５５％、後者（×）が２０〜３５％と低下し、伸びフランジ性は劣化するとともに、縦割れ臨界温度Ｔｃは、前者（□）が−１０〜−４０℃、後者（×）が１０〜４０℃と高くなり、良好な耐二次加工脆性は得られない。また、加熱時間が４Ｂ^＊×１０^４＋３０未満の場合には（図中の△）、未再結晶組織が残留しており、λは１０〜２０％と低く、Ｔｃは０〜３０℃と高くなり、伸びフランジ性、耐二次加工脆性ともに好ましくない。
【００６１】
続いて、加熱後の冷却条件、その後の溶融亜鉛めっき浴への浸漬条件は、特に限定する必要はなく、亜鉛めっき処理をした後、必要に応じてめっき層に合金化処理を施してもよい。
【００６２】
以上の製造工程を経て、本発明の意図する伸びフランジ性と耐二次加工脆性に優れた高強度溶融亜鉛めっき鋼板を製造することができる。また、このようにして得られた鋼板に電気めっきなどの表面処理を施しても所望の鋼板特性を損なうことはない。
【００６３】
【実施例】
表１に示す成分の鋼（鋼番１〜８：本発明鋼、鋼番９〜１３：比較鋼）を実験室にて溶製、鋳造して、板厚６０ｍｍのスラブを作製した。このスラブを板厚３０ｍｍまで分塊圧延した後、大気炉で１２７０℃×１ｈｒの加熱処理を施し、熱間圧延に供した。仕上圧延は８６０℃で実施し、５５０℃×１ｈｒの巻取相当の熱処理を施して、板厚４ｍｍの熱延板を作製した。
【００６４】
【表１】

【００６５】
次に、熱延板を酸洗し、板厚１．２ｍｍまで冷間圧延した後、焼鈍および溶融亜鉛めっき処理に供した。加熱は８５０℃×２００ｓｅｃとし、この後、平均冷却速度−５℃／ｓで冷却し、４６０℃の溶融亜鉛めっき浴中に浸漬した後、５５０℃で合金化処理を施した。続いて、得られた溶融亜鉛めっき鋼板に伸長率０．７％の調質圧延を施し、引張試験、組織観察、穴拡げ試験、縦割れ試験、めっき表面外観の評価を実施した。
【００６６】
引張試験は、ＪＩＳ Ｚ２２４１（日本工業規格）に準拠した方法にて実施した。組織観察は、走査型電子顕微鏡を用いて、圧延方向に平行で板面に垂直な断面において、無作為に抽出した２００個分のフェライトの平均粒径とマルテンサイトの体積率を測定した。穴拡げ試験は、ＪＦＳ ＴｌＯＯｌ（日本鉄鋼連盟規格）に準拠した方法にて実施し、穴拡げ率λにより、伸びフランジ性を評価した。縦割れ試験は、図１に示すカップ成形材の開口試験にて実施し、縦割れ臨界温度Ｔｃにより、耐二次加工脆性を評価した。
【００６７】
また、めっき表面外観は、幅×長さが１００ｍｍ×１５００ｍｍの範囲で、めっき表面を目視にて評価し、不めっき、点状およびすじ状の欠陥が認められた場合には、表面劣化（×）と判定した。これらの特性を評価した結果を表２に示す。
【００６８】
【表２】

【００６９】
表２に示すように、本発明例Ｎｏ．１〜８（鋼番１〜８）はいずれも本発明成分範囲にあり、ＴＳが７９０〜１０１２ＭＰａ、フェライトの平均粒径が２〜５μｍ、マルテンサイト体積率が２５〜６２％であり、７８０ＭＰａ以上の高い引張強度と平均粒径５μｍ以下の細粒組織を有する。
【００７０】
引張強度が７８０ＭＰａ級の本発明例Ｎｏ．１〜４、Ｎｏ．７、８では、縦割れ限界温度１ｔは−７５〜−１１０℃と低く、穴拡げ率λは６５〜８２％と高いことから、良好な伸びフランジ性と耐二次加工脆性が得られている。また、いずれもめっき表面性状は良好である。引張強度が９８０ＭＰａ級の本発明例Ｎｏ．５、６では、Ｔｃは−５０℃と低く、λは４５％と高いことから、伸びフランジ性、耐二次加工脆性ともに良好な特性が得られており、また、めっき表面性状も良好である。
【００７１】
一方、比較例Ｎｏ．９〜１３（鋼番９〜１３）はいずれも本発明の範囲外にあり、引張強度、伸びフランジ性、耐二次加工脆性、めっき表面性状を満足しない。比較例Ｎｏ．９は、ＴＳが８１４ＭＰａと７８０ＭＰａ以上の強度が得られているが、λが４０％と低く、Ｔｃが−４０℃と高いため、伸びフランジ性、耐二次加工脆性は好ましくない。引張強度が９８０ＭＰａ級の比較例Ｎｏ．１０では、λが１０％と低く、Ｔｃが−５℃と高くなり、伸びフランジ性、耐二次加工脆性は好ましくない。
【００７２】
比較例Ｎｏ．１１、１３は、ＴＳがそれぞれ８１８、８３３ＭＰａと所望の引張強度が得られているが、フェライトの平均粒径はいずれも８μｍと大きく、λが４２％、３４％と低く、Ｔｃが−１５℃、０℃と高くなり、いずれも伸びフランジ性と耐二次加工脆性は劣化している。また、比較例Ｎｏ．１１はめっき表面にすじ状欠陥が認められ、表面性状が好ましくない。比較例Ｎｏ．１２はＴＳが６７０ＭＰａであり、所望の引張強度が得られていない。
【００７３】
【発明の効果】
本発明によれば、鋼の化学成分を規定するとともに、Ｂ、Ｔｉ、Ｎで規定される成分量に応じて、焼鈍時の加熱時間を制御することにより、溶融亜鉛めっき処理後の鋼板組織を微細なフェライトとマルテンサイトを主体とする複合組織としている。このように、鋼の化学成分と製造条件を規定することにより、７８０ＭＰａ以上の強度を有する伸びフランジ性と耐二次加工脆性に優れた溶融亜鉛めっき鋼板を、安定して製造することが可能となり、厳しい伸びフランジ成形と低温での強靭性の求められる自動車の構造部品等へ適用できることから、自動車業界における利用価値は大きい。
【図面の簡単な説明】
【図１】鋼板の耐二次加工脆性の評価方法を示す図。
【図２】材質に及ぼすフェライトの平均粒径ｄ_Ｆおよび固溶Ｂ量 Ｂ^＊の 影響を示す図。（ Ｂ^＊＝Ｂ−（１１／１４）［Ｎ−（１４／４８）Ｔｉ］ ）
【図３】材質に及ぼす加熱時間ｔおよび固溶Ｂ量 Ｂ^＊の 影響を示す図。（ Ｂ^＊＝Ｂ−（１１／１４）［Ｎ−（１４／４８）Ｔｉ］ ）[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a hot-dip galvanized steel sheet having excellent stretch flangeability and secondary work brittleness resistance, and a method for producing the same.
[0002]
[Prior art]
In recent years, from the viewpoint of prevention of global warming, CO emitted from various industries such as automobiles, electricity and chemical manufacturers ₂ Reduction of gas is required. As a specific initiative, automakers are developing electric vehicles and reducing the weight of gasoline-powered vehicles to reduce the fuel ratio. To reduce the weight of an automobile body, it is effective to reduce the thickness of a steel sheet applied to various parts of an automobile.
[0003]
However, while the weight is reduced by reducing the plate thickness, there is a concern that the rigidity of the vehicle body may be reduced. To reduce the weight while maintaining the rigidity of the vehicle body, the use of high-strength steel sheets is promising, and the application of high-strength steel sheets to various automobile structural parts such as rockers and members is being studied. On the other hand, when it is formed into an actual part, deterioration of press formability such as elongation and stretch flangeability becomes a problem. In addition, since the toughness of the material itself decreases as the strength increases, it is required to improve the toughness (secondary work brittleness resistance) of the formed part.
[0004]
To meet such demands, various high-strength steel sheets have been developed. For example, Japanese Patent Application Laid-Open No. 4-173946 discloses a method for producing a high ductility and high strength alloyed hot-dip galvanized steel sheet. According to this technique, when cooling Nb-added steel having a C content of 0.06 to 0.30% at the time of annealing, the cooling rate in a predetermined temperature range is weighted with the Mn, Mo, Cr, Si, and P contents. By specifying the total value, 655 to 877 MPa (66.8 to 89.5 kg / mm) having good elongation and stretch flangeability can be obtained. ² ) Can be obtained.
[0005]
JP-A-6-93340 discloses a method and a facility for producing a high-strength galvannealed steel sheet having excellent stretch flangeability. This technology controls the temperature of cooling and reheating from heating to galvanizing during the continuous annealing hot-dip galvanizing process to obtain tempered martensite. MPa (53-81 kg / mm ² ) Can be obtained.
[0006]
JP-A-6-57373 discloses a high r-value and high tensile strength cold rolled steel sheet having excellent secondary work brittleness resistance and a method for producing the same. This technology uses a P-added Ti-Nb-B steel in which the B content is adjusted within a predetermined range determined by a weighted sum of the Si, Mn, and P contents, so that secondary resistance is reduced. 368-502 MPa (37.5-51.2 kg / mm) with good processing brittleness ² ) Can be obtained.
[0007]
JP-A-2001-192768 discloses a high-strength hot-dip galvanized steel sheet having excellent ductility, stretch flangeability, and fatigue resistance, and a method for producing the same. This technology contains C: 0.05 to 0.20%, Si: 0.3 to 1.8%, Mn: 1.0 to 3.0%, etc., has a composite structure, and contains ferrite. 30% or more by volume, 20% or more by volume of tempered martensite, 2% or more by volume of retained austenite, and the average crystal grain size of the ferrite and tempered martensite is 10 μm or less. It is characterized by.
[0008]
[Patent Document 1]
JP-A-4-173946
[0009]
[Patent Document 2]
JP-A-6-93340
[0010]
[Patent Document 3]
JP-A-6-57373
[0011]
[Patent Document 4]
JP 2001-192768 A
[0012]
[Problems to be solved by the invention]
In the technology disclosed in Japanese Patent Application Laid-Open No. 4-173946, a characteristic value of 44 to 86% is obtained for the hole expansion ratio λ, which is an index of stretch flangeability. No stable high characteristic value of 44 to 61% was obtained. Further, in this technique, a hot-dip galvanized steel sheet having a strength of 980 MPa or more has not been stably obtained.
[0013]
On the other hand, according to the technique disclosed in Japanese Patent Application Laid-Open No. 6-93340, a good hole expansion rate of 85 to 86% is obtained in a steel sheet having a strength of 780 MPa. However, in this technique, in a continuous hot-dip galvanizing line, a forced cooling facility and a new heating furnace must be newly installed between the soaking zone and the hot-dip galvanizing bath, so that the production cost is extremely high.
[0014]
Further, even with this technique, a hot-dip galvanized steel sheet having a strength of 980 MPa or more has not been obtained. Furthermore, all of the above-mentioned prior arts focus on improving the stretch flangeability of a high-strength steel sheet, and it is considered that the toughness of the steel sheet cannot be improved. For this reason, in the steel sheet obtained by these techniques, there is a concern that the toughness (resistance to secondary working embrittlement) of the part during press forming is deteriorated.
[0015]
On the other hand, according to the technique disclosed in Japanese Unexamined Patent Publication No. Hei 6-57373, a steel sheet having a strength of up to about 500 MPa and having good secondary working embrittlement resistance can be obtained. It is considered difficult to manufacture.
[0016]
The technique described in Japanese Patent Application Laid-Open No. 2001-192768 is aimed at improving fatigue resistance characteristics, but does not disclose secondary work brittleness resistance at all.
[0017]
In addition, steel sheets used for undercarriage parts of automobiles such as rockers are required to have good secondary work brittleness and a strength of 780 MPa or more. Since the stress concentration at the crystal grain boundary sometimes increases, the situation becomes more severe for the resistance to secondary working embrittlement. Therefore, any of the above-mentioned conventional techniques has a problem that stretch flangeability, resistance to secondary working brittleness, and tensile strength of 780 MPa or more cannot be satisfied.
[0018]
Therefore, the present invention solves the above problems and provides a hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more, excellent stretch flangeability and secondary work brittleness resistance, and a method for stably producing the same. The purpose is to:
[0019]
[Means for Solving the Problems]
The above problem is solved by the following invention. In the invention, the chemical components are mass% and C: 0.03 to 0.13%, Si ≦ 0.7%, Mn: 2.0 to 4.0%, P ≦ 0.05%, S ≦ 0. 005%, Sol. Al: O. 01-0.1%, N ≦ 0.005%, Ti: 0.005-0.1%, B: 0.0002-0.0040%, B, Ti, N satisfy the following inequality A molten zinc excellent in stretch flangeability and secondary work brittleness resistance, characterized in that the balance substantially comprises iron, ferrite having an average particle size of 5 μm or less and martensite having a volume ratio of 15 to 80%. It is a plated steel sheet.
[0020]
0.0002 ≦ B− (11/14) [N− (14/48) Ti] ≦ 0.0030 (1)
Here, the element symbols in the formula indicate mass% of each element, and when [N− (14/48) Ti] ≦ 0, [N− (14/48) Ti] is set to 0.
[0021]
Further, in the hot-dip galvanized steel sheet of the present invention, Nb: 0.005 to 0.1%, Mo: 0.01 to 1.0%, and V: 0.01 to 0. A hot-dip galvanized steel sheet having excellent stretch flangeability and secondary work brittleness resistance, characterized in that it contains at least one of 5% and Cr: 0.01 to 0.5%.
[0022]
The invention of the manufacturing method relating to the hot-dip galvanized steel sheet of these inventions is a step of smelting and casting steel having the above chemical composition, a step of hot-rolling the cast slab, and then pickling. And a step of heating to a temperature of not less than Ac 3 points and not more than 900 ° C. after the cold rolling, holding for a time t (sec) satisfying the following formula, cooling, and performing a hot-dip galvanizing process. A method for producing a hot-dip galvanized steel sheet having excellent stretch flangeability and secondary work brittleness resistance.
[0023]

Here, the symbol of the element in the formula represents the mass% of each element, t represents the holding time (sec) at the heating temperature, and [N- (14/48) Ti] ≦ [N- (14 / 48) Ti] is set to 0.
[0024]
The present invention has been made based on the findings found as a result of intensive studies in order to obtain a high-strength hot-dip galvanized steel sheet having excellent stretch flangeability and secondary work brittleness resistance. This is because, in a high-strength steel sheet with a composite structure containing ferrite and martensite, a decrease in stretch flangeability and a deterioration in secondary work brittleness resistance are caused by blanking before press forming and near the ferrite / martensite interface during press forming. This is due to stress concentration.
[0025]
During blanking before press molding, stretch flangeability is reduced due to microvoids generated by stress concentration near the ferrite / martensite interface. At the time of press molding, toughness (resistance to secondary working embrittlement) is degraded due to increasing stress concentration on crystal grain boundaries. Therefore, by reducing the size of ferrite and increasing the grain boundary strength by adding B appropriately, the generation of microvoids is suppressed, the stretch flangeability can be improved, and the grain refinement and grain boundary strengthening are achieved. Thereby, the toughness is improved and the secondary work brittleness resistance can be improved.
[0026]
The present invention has the above requirements in order to obtain a high-strength hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more, and excellent stretch flangeability and secondary work brittleness resistance. The reasons for adding the steel components of the present invention, the ranges for limiting the components, the morphology, and the reasons for limiting the manufacturing conditions will be described below.
[0027]
(1) Range of steel components
Hereinafter,% indicates mass%.
[0028]
C: 0.03 to 0.13%
C is an element effective for strengthening steel, and requires an addition amount of 0.03% or more. On the other hand, if C is added in excess of 0.13%, a band structure develops in the rolling direction, and stress concentration occurs in the band structure during press forming, so that toughness deteriorates. For this reason, the C content is in the range of 0.03 to 0.13%.
[0029]
Si: ≦ 0.7%
Si is an element effective for strengthening steel, and can be added as appropriate. However, when the Si content exceeds 0.7%, ferrite transformation in a high temperature range is promoted in the cooling process during annealing, and ferrite grains grow, so that fine ferrite grains intended by the present invention cannot be obtained. Further, when the Si content exceeds 0.7%, the adhesion of hot-dip galvanized coating deteriorates, and an uneven plating film is formed, so that during deep drawing, stress concentration on the surface layer of the steel sheet increases, It is not preferable for the secondary work brittleness resistance after molding. Therefore, the amount of Si is set to 0.7% or less. If the Si content is less than 0.3%, the secondary work brittleness resistance is further improved. Therefore, it is preferable that the amount of Si be less than 0.3%.
[0030]
Mn: 2.0-4.0%
Mn is an element effective for strengthening steel, and requires an addition amount of 2.0% or more. On the other hand, when the added amount of Mn exceeds 4.0%, segregation of Mn tends to occur during casting of the slab. After rolling and hot-dip galvanizing, a band structure due to the segregation develops in the steel sheet, so that the stretch flangeability is significantly deteriorated. For this reason, the Mn content is set in the range of 2.0 to 4.0%.
[0031]
P: ≦ 0.05%
P is an element effective for strengthening steel and can be added as appropriate. However, if the added amount of P exceeds 0.05%, a non-uniform structure due to segregation of P at the time of casting tends to develop, and ductility deteriorates. Therefore, the addition amount of P is set to 0.05% or less.
[0032]
S: ≦ 0.005%
If S is excessively present in steel, a large amount of MnS is formed, which is not preferable for elongation and stretch flangeability of the steel sheet. In particular, when the S content exceeds 0.005%, there is a concern about this adverse effect. For this reason, the S content is set to 0.005% or less.
[0033]
sol. Al: 0.01 to 0.1%
Al is required to be 0.01% or more to deoxidize steel. However, when the addition amount of Al exceeds 0.1%, Al-based inclusions such as oxides increase in steel, and the ductility is significantly deteriorated. For this reason, the Al content is in the range of 0.01 to 0.1%.
[0034]
N: ≦ 0.005%
If N is excessively present in steel, cracks tend to occur on the surface during casting of the slab. In particular, when the N content exceeds 0.005%, this adverse effect becomes remarkable. Therefore, the N content is set to 0.005% or less.
[0035]
B: 0.0002-0.0040%
Since B is present in a solid solution state, it segregates at the austenite grain boundary during annealing to make austenite finer, and is extremely effective in making ferrite transformed from austenite finer. B is also an extremely effective element for promoting martensitic transformation from austenite, and it is necessary to add 0.0002% or more to obtain these effects. However, when the added amount of B exceeds 0.0040%, these effects are not only saturated, but also the plating surface appearance deteriorates. For this reason, the amount of B is made into the range of 0.0002 to 0.0040%.
[0036]
Ti: 0.005 to 0.1%
Ti prevents B from becoming BN by precipitating and fixing N, and by keeping B in a solid solution state, the above-mentioned effect of B, namely, the promotion of finer ferrite and martensitic transformation, Contribute greatly. In order to secure solid solution B as described above, it is necessary to add 0.005% or more of Ti. On the other hand, when the addition amount of Ti exceeds 0.1%, the plating surface appearance deteriorates. Therefore, the amount of Ti is set in the range of 0.005 to 0.1%.
[0037]
Solid solution B amount B ^* = B- (11/14) [N- (14/48) Ti]: 0.0002-0.0030%
In order to obtain the above effects by B and Ti, it is necessary to appropriately control the amount of solute B. As an index of the amount of solid solution B, the amount obtained by subtracting the equivalent (11/14) [N− (14/48) Ti] of N not precipitated and fixed from the amount of B:
B ^* = B- (11/14) [N- (14/48) Ti]
Is used. This quantity B ^* Is less than 0.0002%, a desired fine structure cannot be obtained. On the other hand, if this amount exceeds 0.0030%, these effects are not only saturated, but also the plating surface appearance deteriorates. Therefore, the amount of solid solution B ^* Is in the range of 0.0002 to 0.0030%. If the value of [N− (14/48) Ti] is negative (N equivalent of Ti> N), it is set to 0. In this case, B ^* = B, and the amount of solute B becomes equal to the amount of B added.
[0038]
In the present invention, if necessary, Nb, Mo, V, and Cr can be added in the following ranges.
[0039]
Nb: 0.005 to 0.1% when added
Nb is an element that contributes to the refinement of the structure by forming fine carbides with C. In order to obtain this effect, 0.005% or more must be added. On the other hand, when the added amount of Nb exceeds 0.1%, recrystallization of ferrite and austenite during annealing is delayed, a work structure is likely to remain, and the ductility of the steel sheet is significantly reduced. Therefore, when Nb is added, the content is in the range of 0.005 to 0.1%.
[0040]
Mo: 0.01 to 1.0%
Mo is an element that improves the hardenability of steel and is effective for strengthening steel. In order to obtain Mo strengthening ability, addition of 0.01% or more is required. On the other hand, when the addition amount of Mo exceeds 1.0%, the plating surface appearance deteriorates. For this reason, when adding Mo, it is made into the range of 0.01-1.0%.
[0041]
V: 0.01 to 0.5% when added
V is an element that improves the hardenability of steel and is effective for strengthening steel. In order to obtain the strengthening ability of V, 0.01% or more must be added. On the other hand, when the added amount of V exceeds 0.5%, the effect is saturated. Therefore, when V is added, the content is in the range of 0.01 to 0.5%.
[0042]
Cr: 0.01 to 0.5% when added
Cr, like Mo and V, is an element that improves the hardenability of steel, and is effective for strengthening steel. In order to obtain this effect, 0.01% or more of Cr must be added. On the other hand, when the added amount of Cr exceeds 0.5%, the strengthening ability is saturated. Therefore, when Cr is added, the content is in the range of 0.01 to 0.5%.
[0043]
The effects of the present invention will not be impaired unless chemical components other than the above steel components are added in excess. For example, if W and Ni are 0.5% or less, there is no adverse effect on the characteristics aimed at by the present invention. In the present invention, the fact that the balance is substantially iron means that other alloying elements or unavoidable impurities may be contained as long as the desired properties of the present invention are not adversely affected.
[0044]
(2) Microstructure of steel sheet
For the purpose of reducing the weight of an automobile body, press formability such as stretchability and stretch flangeability is required for application of a high-strength steel sheet. Since material properties such as n value and elongation are required for the overhang property, a composite structure steel sheet mainly composed of ferrite and martensite is desirable.
[0045]
However, in the case of this steel sheet, during blanking before press forming, many microvoids are generated due to stress concentration near the interface between ferrite and hard martensite, and there is a concern that the stretch flangeability is reduced. Further, since the toughness of the material itself decreases with the increase in the strength of steel, there is a concern that the toughness (resistance to secondary working embrittlement) of the part after press forming is deteriorated. For this reason, when applying a high-strength material to an actual component, it is important to improve the resistance to secondary working brittleness.
[0046]
Therefore, in the present invention, in a high-strength steel sheet containing 780 MPa or more containing ferrite and martensite, a structural factor for improving the resistance to secondary working brittleness after press forming together with the stretch flangeability was examined. As a result, the ferrite is refined to an average particle size of 5 μm or less, and the amount of B that increases the grain boundary strength is appropriately adjusted, whereby the resistance to embrittlement at the interface between the ferrite phase and the hard martensite phase increases, It has been clarified that the stretch flangeability and the resistance to secondary working brittleness can be improved.
[0047]
Specific numerical values were obtained by conducting a hole expansion test using a 60 ° conical punch and a vertical cracking test of a deep drawn material using steel sheets having variously changed ferrite grain size and B content. The steel sheet used was: mass: C: 0.035 to 0.075%, Si: 0.02 to 0.25%, Mn: 2.0 to 3.0%, P: 0.01 to 0.03. %, S: 0.001 to 0.003%, sol. Al: 0.02 to 0.05%, N: 0.0020 to 0.0035%, Ti: 0.01 to 0.06%, B: 0.0000 (no addition) to 0.0040% of chemical components A hot-dip galvanized steel sheet (sheet thickness: 1.4 mm) having a TS of 800 to 860 MPa, an average ferrite particle size of 2 to 15 μm, and a martensite volume ratio of 27 to 42%.
[0048]
Using this steel sheet, a hole expansion ratio λ, which is an index of stretch flangeability, was measured by a hole expansion test based on JFS TOOL (Japan Iron and Steel Federation Standard). Also, as shown in FIG. 1, a 120 mmφ blank was sampled from this steel plate, formed into a cup with a drawing ratio of 1.6 (75 mmφ), and then trimmed to a cup height of 27 mm to prepare a sample for a vertical crack test. did. Using this cup, an opening test of the cup was performed in a refrigerant, and a minimum temperature Tc at which a vertical crack did not occur on the side wall of the cup was measured.
[0049]
The results of the hole expansion test and the vertical cracking test were calculated using the average ferrite grain size d. _F , B which is an index of the amount of solute B ^* = B- (11/14) (N- (14/48) Ti) and shown in FIG. Average particle size d of ferrite _F Is larger, the hole expansion ratio λ is lower, the vertical crack critical temperature Tc is higher, and d _F Exceeds 5 μm (△ or ● in the figure), λ decreases to 40 to 66% (％ in the figure) and 20 to 38% (％ in the figure), and the stretch flangeability deteriorates.
[0050]
In FIG. 2, Tc is as high as −10 to −40 ° C. (Δ in the figure) and 0 to 30 ° C. (● in the figure), and the secondary working brittleness is deteriorated. Also, B ^* = B- (11/14) (N- (14/48) Ti) when 0%, d _F Is as small as 3 μm (△ in the figure), both the stretch flangeability and the resistance to secondary working brittleness are degraded. These results are considered to be attributed to the characteristic deterioration caused by a large stress concentration at the ferrite / martensite interface at the time of hole punching before the hole expansion test and at the time of deep drawing before the longitudinal cracking test.
[0051]
On the other hand, d _F Is 5 μm or less, and B ^* When = B- (11/14) (N- (14/48) Ti) is 0.0002 to 0.0030% (in the figure), λ is as high as 65 to 86% and Tc is Since it is as low as −70 to −90 ° C., good stretch flangeability and secondary work brittleness resistance are obtained. When B- (11/14) (N- (14/48) Ti) exceeds 0.0030% (x in the figure), good stretch flangeability and secondary work brittleness resistance are obtained. In addition, the plating surface appearance is deteriorated.
[0052]
As described above, in a hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more having ferrite and martensite, in order to improve stretch flangeability and secondary work brittleness resistance, ferrite is refined to an average particle size of 5 μm or less, Further, it became clear that it was necessary to optimize the amount of B for increasing the grain boundary strength.
[0053]
The hot-dip galvanized steel sheet of the present invention is intended for excellent stretch flangeability and secondary work brittleness resistance, and adjusts the chemical composition as described in (1) above, and ferrite as described in (2) above. It is a fine-grained steel sheet and can be manufactured by the following method.
[0054]
(3) Steel plate manufacturing method
After melting and casting the steel having the chemical composition described in the above (1), hot rolling is performed. The smelting and casting of steel is not particularly limited, and it is sufficient that the structure such as component segregation is not particularly non-uniform. Hot rolling may be started immediately after casting, or may be performed after cooling and heating. After rough rolling, finish rolling is performed and wound into a coil. In order to equalize the structure in the thickness direction,
Finish rolling is Ar ₃ And the coil winding temperature is preferably lower than 700 ° C.
[0055]
Next, the obtained hot-rolled sheet is pickled, cold-rolled, and then subjected to continuous hot-dip galvanizing. The cold rolling reduction need not be particularly limited. Heating during annealing needs to be appropriately controlled in order to refine austenite due to segregation of solid solution B at the grain boundaries and to refine ferrite transformed therefrom to an average grain size of 5 μm or less. That is, the heating temperature is set to the austenite single phase region Ac ₃ The upper limit of the heating temperature is 900 ° C. or less in order to suppress the austenite coarsening. Further, it has been clarified that in order to promote the segregation of B at the austenite grain boundaries, the holding time at the heating temperature must be properly controlled by the amount of solid solution B.
[0056]
Specific numerical values were obtained by performing a structure observation, a hole expansion test, and a vertical crack test using a steel sheet annealed while changing the holding time during heating. In the microstructure observation, the average grain size and the volume fraction of martensite of 200 randomly extracted ferrites were measured in a cross section parallel to the rolling direction and perpendicular to the plate surface using a scanning electron microscope. The hole expansion test and the vertical cracking test were performed in the same manner as described above.
[0057]
The steel sheet used was: mass: C: 0.045 to 0.070%, Si: 0.1 to 0.25%, Mn: 2.0 to 3.0%, P: 0.01 to 0.03. %, S: 0.001 to 0.003%, sol. Al: 0.02 to 0.05%, N: 0.0020 to 0.0040%, Ti: 0.01 to 0.06%, B: 0.0007 to 0.0030%, Nb: 0.02 to A hot-dip galvanized steel sheet obtained by annealing a cold-rolled sheet (sheet thickness: 1.4 mm) having a chemical composition of 0.04% at a heating temperature of 850 ° C. and a heating time of 50 to 600 seconds, and has a TS of 810 to 870 MPa.
[0058]
The results of the hole expansion test, the vertical cracking test, and the microstructure observation are converted into the heating time and the solid solution B index B ^* = B- (11/14) (N- (14/48) Ti) and shown in FIG. In addition, according to the structure observation result, the martensite volume ratio is 30 to 40%.
[0059]
As shown in FIG. 3, as the heating time t (sec) becomes longer, ^* The smaller the value, the larger the average particle size of the ferrite. Thus, to obtain fine ferrite having an average particle size of 5 μm or less, the heating time t and B ^* = B- (11/14) [N- (14/48) Ti]. That is, the heating time t (sec) is 4B ^* × 10 ⁴ +30 (sec) or more and 4B ^* × 10 ⁴ In the case of +280 or less (○ in the figure), the ferrite is refined to an average particle size of 2 to 5 μm, λ is as high as 65 to 80%, Tc is as low as −70 to −90 ° C., and good elongation is obtained. Flangeability and secondary work brittleness resistance are obtained.
[0060]
On the other hand, when the heating time t is 4B ^* × 10 ⁴ If it exceeds +280 (sec) (□ or × in the figure), the average ferrite grain size increases to 6 to 9 μm (□) and 10 to 16 μm (×). Along with this, the hole expansion ratio λ is reduced to 42 to 55% for the former (□) and 20 to 35% for the latter (×), and the stretch flangeability is deteriorated. ) Is as high as −10 to −40 ° C., and the latter (×) is as high as 10 to 40 ° C., and good secondary working brittleness resistance cannot be obtained. The heating time is 4B ^* × 10 ⁴ In the case of less than +30 (△ in the figure), an unrecrystallized structure remains, λ is as low as 10 to 20%, Tc is as high as 0 to 30 ° C., stretch flangeability, secondary working resistance. Both brittleness are not preferred.
[0061]
Subsequently, the cooling conditions after the heating and the subsequent immersion conditions in the hot-dip galvanizing bath are not particularly limited, and after the galvanizing treatment, the plating layer may be subjected to an alloying treatment if necessary. .
[0062]
Through the above manufacturing steps, a high-strength hot-dip galvanized steel sheet excellent in stretch flangeability and secondary work brittleness resistance intended by the present invention can be manufactured. Further, even if the steel sheet thus obtained is subjected to a surface treatment such as electroplating, desired properties of the steel sheet are not impaired.
[0063]
【Example】
Steels having the components shown in Table 1 (steel numbers 1 to 8: steels of the present invention, steel numbers 9 to 13: comparative steels) were melted and cast in a laboratory to produce slabs having a plate thickness of 60 mm. After slab-rolling the slab to a plate thickness of 30 mm, the slab was subjected to a heat treatment at 1270 ° C. × 1 hr in an air furnace and subjected to hot rolling. The finish rolling was performed at 860 ° C., and a heat treatment corresponding to winding at 550 ° C. × 1 hr was performed to prepare a hot-rolled sheet having a thickness of 4 mm.
[0064]
[Table 1]

[0065]
Next, the hot-rolled sheet was pickled, cold-rolled to a sheet thickness of 1.2 mm, and then subjected to annealing and galvanizing. The heating was performed at 850 ° C. × 200 sec. Thereafter, the alloy was cooled at an average cooling rate of −5 ° C./s, immersed in a hot-dip galvanizing bath at 460 ° C., and then subjected to an alloying treatment at 550 ° C. Subsequently, the resulting hot-dip galvanized steel sheet was subjected to temper rolling at an elongation of 0.7%, and a tensile test, a structure observation, a hole expansion test, a vertical crack test, and an evaluation of the plating surface appearance were performed.
[0066]
The tensile test was performed by a method based on JIS Z2241 (Japanese Industrial Standard). Microstructure observation was performed using a scanning electron microscope to measure the average grain size and volume fraction of martensite of 200 randomly extracted ferrites in a cross section parallel to the rolling direction and perpendicular to the plate surface. The hole expansion test was performed by a method based on JFS TOOL (Japan Iron and Steel Federation), and the stretch flangeability was evaluated by the hole expansion ratio λ. The vertical cracking test was carried out by an opening test of the cup molding material shown in FIG. 1, and the secondary working brittleness resistance was evaluated by the vertical cracking critical temperature Tc.
[0067]
Further, the plating surface appearance was evaluated by visual observation of the plating surface in a range of width × length of 100 mm × 1500 mm, and when non-plating, dot-like and streak-like defects were recognized, surface deterioration (× ). Table 2 shows the results of evaluating these characteristics.
[0068]
[Table 2]

[0069]
As shown in Table 2, the present invention example No. 1 to 8 (Steel Nos. 1 to 8) are all within the range of the present invention, TS is 790 to 1012 MPa, ferrite average particle size is 2 to 5 μm, martensite volume ratio is 25 to 62%, and 780 MPa or more. Has a high tensile strength and a fine grain structure with an average particle size of 5 μm or less.
[0070]
Inventive Example No. having a tensile strength of 780 MPa class. 1 to 4, No. In Nos. 7 and 8, since the vertical crack limit temperature 1t is as low as -75 to -110 ° C and the hole expansion ratio λ is as high as 65 to 82%, good stretch flangeability and secondary work brittleness resistance are obtained. . In addition, all have good plating surface properties. Inventive Example No. having a tensile strength of 980 MPa class. In Nos. 5 and 6, since Tc was as low as −50 ° C. and λ was as high as 45%, good properties were obtained in both stretch flangeability and secondary work brittleness resistance, and the plating surface properties were also good. .
[0071]
On the other hand, in Comparative Example No. Nos. 9 to 13 (Steel Nos. 9 to 13) are outside the scope of the present invention, and do not satisfy tensile strength, stretch flangeability, secondary work brittleness resistance, and plating surface properties. Comparative Example No. In No. 9, TS has a strength of 814 MPa or 780 MPa or more, but since λ is as low as 40% and Tc is as high as −40 ° C., stretch flangeability and secondary work brittleness resistance are not preferred. Comparative Example No. having a tensile strength of 980 MPa class. In No. 10, λ is as low as 10% and Tc is as high as −5 ° C., and the stretch flangeability and the resistance to secondary working embrittlement are not preferred.
[0072]
Comparative Example No. In Nos. 11 and 13, TS was 818 and 833 MPa, respectively, and desired tensile strength was obtained. However, the average grain size of ferrite was as large as 8 μm, λ was as low as 42% and 34%, and Tc was −15 ° C. , 0 ° C., and the stretch flangeability and the resistance to secondary working brittleness are all degraded. In Comparative Example No. In No. 11, streaky defects were observed on the plating surface, and the surface properties were not preferable. Comparative Example No. In No. 12, the TS was 670 MPa, and the desired tensile strength was not obtained.
[0073]
【The invention's effect】
According to the present invention, the steel composition after hot-dip galvanizing is controlled by controlling the heating time during annealing while controlling the chemical components of the steel and controlling the heating time during annealing according to the component amounts defined by B, Ti, and N. It has a composite structure mainly composed of fine ferrite and martensite. Thus, by defining the chemical composition of the steel and the production conditions, it becomes possible to stably produce a hot-dip galvanized steel sheet having a stretch flangeability having a strength of 780 MPa or more and an excellent secondary work brittleness resistance. Since it can be applied to structural parts and the like of automobiles that require strict stretch flange forming and toughness at low temperatures, its utility value in the automobile industry is great.
[Brief description of the drawings]
FIG. 1 is a view showing a method for evaluating secondary work brittleness resistance of a steel sheet.
FIG. 2 shows the effect of ferrite on average grain size d _F And the amount of solute B ^* The figure which shows the influence of. (B ^* = B- (11/14) [N- (14/48) Ti])
FIG. 3 shows the heating time t and the amount of dissolved B on the material B ^* The figure which shows the influence of. (B ^* = B- (11/14) [N- (14/48) Ti])

Claims (3)

Chemical component is mass%, C: 0.03-0.13%, Si ≦ 0.7%, Mn: 2.0-4.0%, P ≦ 0.05%, S ≦ 0.005%, Sol . Al: O. 01-0.1%, N ≦ 0.005%, Ti: 0.005-0.1%, B: 0.0002-0.0040%, B, Ti, N satisfy the following inequality A molten zinc excellent in stretch flangeability and secondary working brittleness resistance, characterized in that the balance substantially comprises iron, ferrite having an average particle size of 5 μm or less and martensite having a volume ratio of 15 to 80%. Plated steel sheet.
0.0002 ≦ B− (11/14) [N− (14/48) Ti] ≦ 0.0030
Here, the element symbols in the formula indicate mass% of each element, and when [N− (14/48) Ti] ≦ 0, [N− (14/48) Ti] is set to 0. 2. The hot-dip galvanized steel sheet according to claim 1, further comprising Nb: 0.005 to 0.1%, Mo: 0.01 to 1.0%, and V: 0.01 to 0. A hot-dip galvanized steel sheet excellent in stretch flangeability and secondary work brittleness resistance, characterized by containing at least one of 5% and Cr: 0.01 to 0.5%. A step of melting and casting the steel having the chemical composition according to claim 1 or 2, a step of hot rolling the cast slab, and a step of cold rolling by pickling. After the cold rolling, heating is performed to a temperature of not less than ₃ points and not more than 900 ° C. for a time t (sec) that satisfies the following inequality, followed by cooling and hot-dip galvanizing. A method for producing a hot-dip galvanized steel sheet having excellent stretch flangeability and secondary work brittleness resistance.
4 × {B− (11/14) [N− (14/48) Ti]} × 10 ⁴ + 30 ≦ t ≦ 4 × {B− (11/14) [N− (14/48) Ti]} × 10 ⁴ +280
Here, the symbol of the element in the formula represents the mass% of each element, t represents the holding time (sec) at the heating temperature, and [N- (14/48) Ti] ≦ [N- (14 / 48) Ti] is set to 0.

Images