JP4655366B2 - High strength alloyed hot dip galvanized steel sheet with excellent plating adhesion and corrosion resistance and method for producing the same - Google Patents
High strength alloyed hot dip galvanized steel sheet with excellent plating adhesion and corrosion resistance and method for producing the same Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、複雑なプレス成形加工にも充分に耐えうる高強度とめっき密着性及び耐食性に優れた高強度合金化溶融亜鉛めっき鋼板及びその製造方法に関するものである。
【０００２】
【従来の技術】
近年、地球環境の保全という観点から、自動車の燃費改善が要求されている。さらに加えて、衝突時に乗員を保護するため、自動車車体の安全性向上も要求されている。このようなことから、最近では自動車車体の軽量化及び自動車車体の強化が積極的に進められている。特に、自動車車体の軽量化のために、熱延鋼板及び冷延鋼板等の自動車用鋼板を高強度化し鋼板板厚を低減することが考えられている。一方、鋼板を素材とする自動車用部品の多くがプレス加工によって成形されるため、自動車用鋼板には優れたプレス成形性が要求される。また、合金化溶融亜鉛めっき鋼板は防錆性に優れ、安価に製造できるため、自動車車体用防錆表面処理鋼板として多用されている。
鋼板を高強度化するには、易酸化性元素であるＭｎ、Ｓｉ等の元素を添加し、固溶強化等を図る必要がある。また、最近の防錆性能の要求レベルの高まりから、従来使用されていた合金化溶融亜鉛めっき鋼板の耐食性をさらに向上することが求められている。
【０００３】
【発明が解決しようとする課題】
本発明は、上記問題を解決しようとするもので、下地鋼板がＳｉ、Ｍｎを多量に含んでいても、上層溶融亜鉛めっきの密着性に優れ、めっき鋼板として耐食性に優れた高強度合金化溶融亜鉛めっき鋼板及びその製造方法を提供することを目的とする。
【０００４】
【課題を解決するための手段】
すなわち、本発明は、次の（１）〜（６）に示す高強度合金化溶融亜鉛めっき鋼板及びその製造方法を提供するものである。
（１）Ｃ量０．０５〜０．２５％、Ｓｉ量０．１〜１．５％、Ｍｎ量０．５〜３．５％及びＢ量≦５ｐｐｍを満たす鋼板上に、めっき層中のＳｉ、Ｍｎ及びＢの含有量がそれぞれ
｛Ｆｅ％｝＊［Ｓｉ％］／１０（％）≧｛Ｓｉ％｝≧｛Ｆｅ％｝＊［Ｓｉ％］／１００（％）かつ
｛Ｆｅ％｝＊［Ｍｎ％］／１０（％）≧｛Ｍｎ％｝≧｛Ｆｅ％｝＊［Ｍｎ％］／１００（％）かつ
｛Ｂ｝（ｐｐｍ）≦１０（ｐｐｍ）を満たし、かつ
合金化度が７（％）≦｛Ｆｅ％｝≦１５（％）を満たす合金化溶融亜鉛めっき層を有することを特徴とするめっき密着性及び耐食性に優れた高強度合金化溶融亜鉛めっき鋼板。但し、｛ ｝はめっき層中の含有量、［ ］は鋼中の含有量、％は、以下、特に断らない限り質量％を表す。
【０００５】
（２）Ｃ量０．０５〜０．２５％、Ｓｉ量０．１〜１．５％、Ｍｎ量０．５〜３．５％及びＢ量≦５ｐｐｍを満たす鋼板を、加熱炉で７５０〜９５０℃に加熱し、Ｓｉ及びＭｎを含む厚さ０．０１〜０．３μｍの酸化皮膜を生成させた後、６０〜９０℃、１〜２０％の濃度の酸で１〜２０秒間酸洗を施し、酸化被膜が０．００１〜０．０５μｍになるまで除去した後、６５０〜８５０℃で焼鈍後、Ａｌ濃度が０．０８〜０．２０％である４４０〜４８０℃の亜鉛浴中にて溶融亜鉛めっきを施し、引き続き４５０〜６００℃で合金化度が７（％）≦｛Ｆｅ％｝≦１５（％）となるように合金化処理を施すことを特徴とするめっき密着性及び耐食性に優れた高強度合金化溶融亜鉛めっき鋼板の製造方法。
【０００６】
（３）さらに、前記鋼板中に、Ｃｕ、Ｎｉ及びＭｏからなる群から選択される少なくとも１種を０．０１〜１％含み、かつ前記合金化溶融亜鉛めっき層中に、Ｃｕ、Ｎｉ及びＭｏからなる群から選択される少なくとも１種を０．０１〜０．２％含むことを特徴とする前記（１）に記載のめっき密着性及び耐食性に優れた高強度合金化溶融亜鉛めっき鋼板。
【０００７】
（４）さらに、前記鋼板中に、Ａｌを０．０１〜１％含むことを特徴とする前記（１）又は（３）に記載のめっき密着性及び耐食性に優れた高強度合金化溶融亜鉛めっき鋼板。
【０００８】
（５）さらに、前記鋼板中に、Ｃｕ、Ｎｉ及びＭｏからなる群から選択される少なくとも１種を０．０１〜１％含むことを特徴とする前記（２）に記載のめっき密着性及び耐食性に優れた高強度合金化溶融亜鉛めっき鋼板の製造方法。
【０００９】
（６）さらに、前記鋼板中に、Ａｌを０．０１〜１％含むことを特徴とする前記（２）又は（５）に記載のめっき密着性及び耐食性に優れた高強度合金化溶融亜鉛めっき鋼板の製造方法。
【００１０】
【発明の実施の形態】
上述のように、母材中のＳｉ、Ｍｎ量が多いほど母材は高強度であり、めっき層中のＳｉ、Ｍｎ量が多いほどめっき層の耐食性は向上することが知られているが、母材中にＳｉ、Ｍｎを多く含むとめっき密着性が低下する。そこで、本発明者らは、この問題を解決するために以下の実験を行い、得られた知見から本発明を完成させるに至った。なお、軽元素であるＢは、Ｓｉ、Ｍｎより拡散しやすいため、焼鈍条件にもよるが、そのめっき層中の濃化量は、母材含有量の数十倍程度になりうると推定されている。
【００１１】
本発明は以下の実験事実に基づいて完成されたものである。
表２に記載の鋼種Ａの組成の厚さ３０ｍｍのシートバーを１２００℃で加熱し、５パスで厚さ２．０ｍｍの熱延板とし、６１０℃で巻き取った。次いで、酸洗により黒皮を除去し、焼鈍炉においてこの熱延板を７００〜９７０℃で６０秒間焼鈍した後、６０℃の５％ＨＣｌで１〜２０秒間酸洗した。その後、めっき装置にて、６００〜９００℃で２０秒間焼鈍し、浴中Ａｌ濃度０．１３％、浴温４６５℃の亜鉛浴中にて１秒間めっき処理した後、４４０〜５５０℃で合金化処理した。
得られた合金化溶融亜鉛めっき鋼板中の｛Ｆｅ％｝は１０％であり、｛Ｓｉ％｝は０．０１〜０．８％、｛Ｍｎ％｝は０．１〜２．２％であった。
【００１２】
ここで得られためっき鋼板のめっき密着性及び耐食性を調査した。めっき密着性は、めっき鋼板にセロファンテープを貼りテープ面を９０°内に曲げ、曲げ戻しをした後テープを剥したときの単位長さ当りのめっき剥離量を蛍光Ｘ線によりＺｎカウント数として測定し、表１の基準に照らしてランク１、２のものを良好、３以上のものを不良として評価した。耐食性は、軟鋼板の合金化溶融亜鉛めっき鋼板の塩水噴霧試験の結果における錆発生状況と比較して、錆発生面積率が同等以下であるものを良好、１０％超であるものを不良として評価した。図２は、耐食性に及ぼす酸洗前後の酸化被膜厚みの影響を示した図であり、耐食性が良好であったものを○とし、不良であったものを●として示した。
これらの結果を総合して、めっき密着性及び耐食性が共に良好であったものを○とし、耐食性は良好であるが、めっき密着性が不良であったものを◎とし、めっき密着性、耐食性が共に不良であったものを●として、図１に示した。
また、図３は、めっき密着性に及ぼす合金化溶融亜鉛めっき層中のＢの濃度の影響を示した図である。めっき層中のＢ量が６ｐｐｍ以下であると、めっき密着性は表１の基準に照らしてランク１（○）であり、また、めっき層中のＢ量が７〜１０ｐｐｍであるとランク２（△）である。一方、めっき層中のＢ量が１０ｐｐｍを超えると、ランク３以上（×）であった。
【００１３】
得られた結果から以下の知見を得た。
１）めっき層中のＳｉ、Ｍｎ量は、耐食性を向上させる効果がある。
２）Ｓｉ、Ｍｎの鋼材表面での濃化量は、母材含有量の数倍程度と推定されるため、めっき層中のＳｉ、Ｍｎ量は、母材そのものの含有量から理論的に考えられる取り込み量より多めになる。
３）めっき層中のＳｉ、Ｍｎの量は、母材のＳｉ、Ｍｎ量と、めっき層中の合金化後の｛Ｆｅ％｝に影響され、その範囲が
｛Ｆｅ％｝＊［Ｓｉ％］／１０（％）≧｛Ｓｉ％｝≧｛Ｆｅ％｝＊［Ｓｉ％］／１００（％）、
｛Ｆｅ％｝＊［Ｍｎ％］／１０（％）≧｛Ｍｎ％｝≧｛Ｆｅ％｝＊［Ｍｎ％］／１００（％）
であれば、めっき密着性と耐食性が共に良好な範囲が存在する。
４）図２の結果から、酸洗前後の酸化被膜厚みが特定範囲内である場合に、耐食性が良好となることがわかる。
５）図３の結果から、Ｂはめっき密着性を劣化させることがわかり、鋼中含有量のみならずめっき層中への取り込みを極力低減すべきである。さらには、めっき密着性が劣化することにより、不めっき、ピンホールなどの地鉄裸出部が生成するため、めっき層中にＳｉ等が取り込まれていても結果として耐食性が劣化するおそれがある。このため、めっき層中のＢ量は１０ｐｐｍ以下としなければならない。
【００１５】
以上の実験により知見したことから以下の本発明を完成した。
本発明の特徴は、めっき層中の｛Ｓｉ％｝、｛Ｍｎ％｝、鋼中の［Ｓｉ％］、［Ｍｎ％］及びめっき層中の｛Ｆｅ％｝を特定範囲として、めっき密着性、耐食性及び強度を併せて満足しためっき鋼板が得られることにある。すなわち、めっき層中のＳｉ、Ｍｎ、Ｂの含有量は、以下の式を満たす範囲が必要である。
｛Ｆｅ％｝＊［Ｓｉ％］／１０（％）≧｛Ｓｉ％｝≧｛Ｆｅ％｝＊［Ｓｉ％］／１００（％）かつ
｛Ｆｅ％｝＊［Ｍｎ％］／１０（％）≧｛Ｍｎ％｝≧｛Ｆｅ％｝＊［Ｍｎ％］／１００（％）かつ
｛Ｂ｝（ｐｐｍ）≦１０（ｐｐｍ）
｛Ｓｉ％｝、｛Ｍｎ％｝が上記範囲より少ない場合、充分な耐食性向上を望めない。一方、｛Ｓｉ％｝、｛Ｍｎ％｝が上記範囲を超える場合、耐食性は向上するが、溶融亜鉛めっき時に、めっき性が悪く、不めっきの部分は耐食性を劣化させる上、合金化後のめっき密着性を劣化させる。また、｛Ｂ｝が上記範囲を超える場合、めっき密着性を劣化させる。
【００１６】
また、合金化度｛Ｆｅ％｝を限定した理由は次のとおりである。本発明のめっき層中の｛Ｓｉ｝、｛Ｍｎ｝量の最適値は、合金化度と鋼中の［Ｓｉ］、［Ｍｎ］量によって決定される。
合金化度：７≦｛Ｆｅ％｝≦１５
７％未満だとζ相が多く残存するだけでなく、η相が残るため合金化が不十分となり摺動性が劣化するので、合金化度は７％以上が必要である。但し、１５％を超えるとΓ相が多量に生成するためにめっき密着性が劣化し、曲げ加工部の耐食性が劣化するため、上記範囲とした。
【００１７】
次に、本発明において鋼中の構成成分の含有量を限定した理由について説明する。
Ｃ量０．０５〜０．２５％
Ｃ量は、必要強度を得るためと所望の組織を得るために不可欠である。少なくとも０．０５％が必要であるが、０．２５％を超えると溶接性が悪化するため、上記範囲とした。
【００１８】
Ｓｉ量０．１〜１．５％
Ｓｉ量は、固溶強化と所望の組織を得るために不可欠であり、延性を劣化させずに高強度化を図れる。また、Ｓｉ、Ｍｎ酸化物には耐食性を向上する効果があるため、めっき直前に適正量のＳｉ、Ｍｎ酸化物を残存させることにより、Ｓｉ、Ｍｎ酸化物及び鋼中の固溶Ｓｉ、Ｍｎを供給源とし、めっき密着性を維持したまま合金層中に適正量を取り込むことにより耐食性を向上する効果が得られる。所望の効果を得るためには０．１％が必要であるが、１．５％を超えるとめっき密着性が劣化するため、上記範囲とした。
【００１９】
Ｍｎ量０．５〜３．５％
Ｃと同様に必要強度を得るためと所望の組織を得るために０．５〜３．５％のＭｎ量が不可欠である。また、Ｓｉと同様に耐食性を向上する効果を得るためには少なくとも０．５％に満たないと効果に乏しいが、３．５％を超えると溶接性が悪化するため、上記範囲とした。
【００２０】
Ｂ量≦５ｐｐｍ
Ｂはめっき密着性を劣化させるため、鋼中含有量のみならずめっき層中への取り込みを極力低減するべきである。さらには、めっき密着性が劣化することにより不めっき、ピンホールなどの地鉄裸出部が生成するため、めっき層中にＳｉ等が取り込まれていても結果として耐食性が劣化するおそれがある。そのため上限を５ｐｐｍとした。
【００２１】
さらに、本発明の高強度合金化溶融亜鉛めっき鋼板の鋼中には、以下の元素を以下の量少なくとも１種類含んでもよい。その場合はさらに以下の効果を有する。
Ｃｕ量０．０１〜１％
Ｃｕはオーステナイト中に偏析し、所望の強度を得るためと所望の組織を得るために重要であるだけでなく、めっき密着性を向上する効果もある。めっき密着性が向上する理由は現時点では明らかになっていないが、これら所望の効果を得るためには、好ましくは０．０１％以上含んでいると効果的である。但し、１％を超えると経済性が劣化するため上限を１％とするのが好ましい。
【００２２】
Ｎｉ量０．０１〜１％
ＮｉはＣｕと同様オーステナイト中に偏析し、必要強度を得るためと所望の組織を得るために重要であるだけでなく、めっき密着性を向上する効果も有するので必要に応じて添加する。めっき密着性が向上する理由は現時点では明らかになっていないが、これら所望の効果を得るためには、Ｃｕと同様０．０１％以上含んでいるのが好ましい。但し、１％を超えると経済性が劣化するため上限を１％とするのが好ましい。
【００２３】
Ｍｏ量０．０１〜１％
Ｍｏは高価であるが、オーステナイト中に偏析し、必要強度を得るためと所望の組織を得るために重要であるだけでなく、めっき密着性を向上する効果も有する。めっき密着性が向上する理由は、Ｃｕ、Ｎｉと同様現時点では明らかになっていないが、これら所望の効果を得るためには、０．０１％以上含んでいるのが好ましい。但し、１％を超えると経済性が劣化するため上限を１％とするのが好ましい。
【００２４】
Ａｌ量０．０１〜１％
Ａｌは必要強度を得るためと所望の組織を得るために重要であり、結果としてＳｉ添加量を低減できるため、同等の引っ張り強度を有するＳｉ添加鋼よりめっき密着性の改善に有利であるので必要に応じて添加する。所望の効果を得るためには０．０１％以上含有するのが好ましいが、１％を超えると経済性が劣化するため上限を１％とするのが好ましい。
【００２５】
鋼中にＣｕ、Ｎｉ及びＭｏからなる群から選択される少なくとも１種を含む場合には、めっき層中にＣｕ、Ｎｉ及びＭｏからなる群から選択される少なくとも１種を０．０１〜０．２％含むことが好ましい。
めっき層中にこれらの元素の少なくともいずれか１種が含有されると、これらの元素とＦｅ−Ｚｎ合金層の複合効果により耐食性の向上がより効果的であるため好ましい。母材にＣｕ、Ｎｉ、Ｍｏを含む鋼板については、安定した耐食性向上効果を得るためには、めっき層中にそれぞれ０．０１％以上含まれることが好ましいが、０．２％を超えるためには鋼板母材中の含有量を増加させなければならなくなり経済性が悪化するため０．２％を上限とするのが好ましい。
また、本発明では、めっき層中の｛Ｆｅ％｝、｛Ｓｉ％｝及び｛Ｍｎ％｝と、鋼中の［Ｓｉ％］及び［Ｍｎ％］を、上記式で示すような関係とすることが必要である。
【００２６】
製造方法の特徴は、Ｓｉ、Ｍｎを含む酸化被膜を一旦つくり、次に、酸洗によりＢ量をコントロールし、めっき層中のＳｉ、Ｍｎ量を一定範囲とすることにある。
めっき層中のＢ量を低減する方法は、加熱炉での加熱後であって、連続溶融亜鉛めっき設備（ＣＧＬと表す。）炉での焼鈍前における酸洗で、Ｓｉ、Ｍｎ主体の酸化皮膜を酸洗によって適正量残存して合金化後に合金層中へ取り込み、それと同時にＢを除去させることであるが、ＢはＳｉ、Ｍｎより酸洗されやすいため、後述する酸洗条件によりほぼ完全に除去される。
【００２７】
以下、製造条件を限定した理由について説明する。
加熱炉での加熱温度
７５０℃を下回ると所望の組織が得られなくなり高強度化が図れないだけでなく、Ｓｉ、Ｍｎの表面濃化が不十分となる。この工程での表面濃化が不十分であると、合金化処理での表面濃化が多すぎてかえってめっき密着性が劣化する。９５０℃を超えると、効果が飽和するだけでなく操業上困難であり経済性に劣る。さらには多量の表面濃化物が生成するため、酸洗により適正量まで除去することができなくなり、不めっきが発生する。そのため、加熱炉での加熱温度は７５０〜９５０℃とする。
【００２８】
加熱炉での酸化皮膜（酸洗前の酸化被膜）
酸化被膜が０．０１μｍを下回ると、酸洗後に残存する量が減るため耐食性が劣化する。また、０．３μｍを超えると酸洗が困難である。そのため、加熱炉での酸化皮膜は０．０１〜０．３μｍとする。ここで、酸化皮膜の量はＧＤＳ、ＳＩＭＳ、ＡＥＳなど各種測定装置によりスパッタリング速度を調整しながら測定することにより見積もれる。
【００２９】
酸洗条件
酸洗温度が６０℃未満もしくは酸洗時間が１秒間未満では効果が得られにくく、酸洗温度が９０℃を超えるかもしくは酸洗時間が２０秒間を超えると表面が荒れ、めっき後の外観を損ねる。そのため、酸洗条件は１〜２０％の濃度の酸で、酸洗温度６０〜９０℃、酸洗時間１〜２０秒間施すこととする。酸は塩酸が経済的で好ましいが、その他の酸でも、硫酸、燐酸、硝酸など特に種類は問わない。
【００３０】
酸洗後の酸化皮膜量
酸洗後の酸化被膜量が０．００１μｍ未満では合金化後にめっき層中に取り込む量が少なくなるため耐食性向上効果が得られない。また、０．０５μｍを超えるとめっき密着性が劣化するだけでなく、不めっき部による耐食性の劣化が起こる。そのため、酸洗後の酸化被膜量は０．００１〜０．０５μｍとした。
【００３１】
還元加熱温度
好ましくはＣＧＬ（合金化）炉においてＨ2 を含む還元性雰囲気中で加熱して還元するが、加熱温度は、６５０℃未満では酸洗により生じた酸化皮膜が還元できず不めっきが発生する。また、８５０℃を超えるとＣＧＬ炉でのＳｉ、Ｍｎの表面濃化が多いため、同様に不めっきが発生する。そのためＣＧＬ炉での加熱温度は６５０〜８５０℃とした。
【００３２】
溶融亜鉛めっき浴
めっき層の合金化後の密着性を確保するため、Ａｌ濃度が０．０８％以上であることが必要である。但し、Ａｌ濃度が０．２０％を超えると合金化が困難になるため上限は０．２０％とした。浴温は４４０℃未満であるとめっき浴の浴温変動により凝固点（４２０℃）を下回る箇所が出てくる可能性があり、操業上安定性に欠ける。また、４８０℃を超えると加熱保持にかかるコストがかさむ。そのため浴温は４４０〜４８０℃とした。
【００３３】
合金化温度
合金化温度が４５０℃未満であるとζ相が生成しやすくなり、合金化溶融亜鉛めっき鋼板の摺動性に欠けるおそれがあるだけでなく、合金化に時間がかかるため生産性が劣化する。また、６００℃を超えるとΓ相が生成しやすくなり、合金化溶融亜鉛めっき鋼板のめっき密着性に欠けるおそれがある。そのため、合金化温度は４５０〜６００℃とした。また、合金化条件によってめっき層中のＦｅ量が決定されるため、めっき層中のＦｅ、Ｓｉ、Ｍｎ量と、母材中のＳｉ、Ｍｎ量を上記式で示す範囲とするためには合金化条件が重要である。
【００３４】
【実施例】
以下、実施例を挙げて本発明をより具体的に説明するが、本発明はこれらの実施例により何ら限定されるものではない。
実施例１〜９、比較例１〜８
表２に示した化学組成（Ｃ，Ｓｉ，Ｍｎ，Ａｌ，Ｐ，Ｓ，Ｃｒ，Ｃｕ，Ｎｉ，Ｍｏ，Ｔｉ，Ｎｂ，Ｂ）の厚さ３００ｍｍスラブを１２００℃で加熱し、熱間圧延により厚さ２．３ｍｍの熱延板とし、６２０℃で巻き取った。次いで、酸洗により黒皮を除去し、冷間圧延により５０％の圧下率で圧延し、連続焼鈍炉（ＣＡＬ）に通板した。続いて、ＣＧＬに通板して酸洗、亜鉛めっき、合金化処理を行った。めっき付着量は片面で４０ｇ／ｍ2 ずつであった。合金化温度は４５０〜６００℃、合金化時間は２０秒間とした。
めっき鋼板の製造条件（加熱炉での加熱温度、加熱炉生成被膜量、酸洗後残存被膜量、酸洗時間、ＣＧＬ焼鈍温度、めっき浴温、浴中Ａｌ濃度、合金化温度）を表３に、得られた合金化溶融亜鉛めっき鋼板のめっき層中のＳｉ、Ｍｎ、Ｂ、Ｃｕ、Ｎｉ、Ｍｏ含有量、めっき外観、めっき密着性及び耐食性の調査結果を表４に示した。製造条件が本発明範囲内のものはいずれもめっき密着性、耐食性が良好であるが、本発明範囲外である比較例では、めっき密着性、耐食性のいずれかもしくは両方が劣っていた。
【００３５】
【表１】
【表２】
【表３】
【発明の効果】
以上のように、本発明によれば、めっき密着性及び耐食性に優れた高強度合金化溶融亜鉛めっき鋼板が得られる。本発明の鋼板を適用することにより、自動車車体の軽量化及び低燃費化が可能となり、ひいては地球環境の改善にも大きく貢献する。
【図面の簡単な説明】
【図１】 めっき密着性及び耐食性に及ぼす合金化溶融亜鉛めっき層中のＳｉ及びＭｎの濃度の影響を示した図である。
【図２】 耐食性に及ぼす酸洗前後の酸化被膜の影響を示した図である。
【図３】 めっき密着性に及ぼす合金化溶融亜鉛めっき層中のＢの濃度の影響を示した図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength alloyed hot-dip galvanized steel sheet excellent in high strength, plating adhesion and corrosion resistance that can sufficiently withstand complex press-forming processes, and a method for producing the same.
[0002]
[Prior art]
In recent years, there has been a demand for improvement in fuel efficiency of automobiles from the viewpoint of conservation of the global environment. In addition, in order to protect passengers in the event of a collision, it is also required to improve the safety of automobile bodies. For these reasons, the weight reduction of automobile bodies and the reinforcement of automobile bodies have been actively promoted recently. In particular, in order to reduce the weight of an automobile body, it is considered to increase the strength of automobile steel sheets such as hot-rolled steel sheets and cold-rolled steel sheets to reduce the steel sheet thickness. On the other hand, since many automotive parts made of steel plates are formed by press working, the automotive steel plates are required to have excellent press formability. Moreover, since the galvannealed steel sheet is excellent in rust prevention and can be manufactured at low cost, it is frequently used as a rust-proof surface-treated steel sheet for automobile bodies.
In order to increase the strength of the steel sheet, it is necessary to add elements such as easily oxidizable elements such as Mn and Si to enhance the solid solution. Further, due to the recent increase in the required level of rust prevention performance, it has been demanded to further improve the corrosion resistance of conventionally used galvannealed steel sheets.
[0003]
[Problems to be solved by the invention]
The present invention is intended to solve the above problem, and even if the base steel sheet contains a large amount of Si and Mn, it has excellent adhesion to the upper layer hot dip galvanizing and is a high strength alloyed and melted alloy with excellent corrosion resistance as a plated steel sheet. It aims at providing a galvanized steel plate and its manufacturing method.
[0004]
[Means for Solving the Problems]
That is, the present invention provides a high-strength galvannealed steel sheet and a method for producing the same as shown in the following (1) to (6).
(1) On the steel sheet satisfying the C amount of 0.05 to 0.25%, the Si amount of 0.1 to 1.5%, the Mn amount of 0.5 to 3.5%, and the B amount of ≦ 5 ppm, The contents of Si, Mn and B are {Fe%} * [Si%] / 10 (%) ≧ {Si%} ≧ {Fe%} * [Si%] / 100 (%) and {Fe%} * [Mn%] / 10 (%) ≧ {Mn%} ≧ {Fe%} * [Mn%] / 100 (%) and {B} (ppm) ≦ 10 (ppm) and the degree of alloying is 7 A high-strength galvannealed steel sheet excellent in plating adhesion and corrosion resistance, characterized by having an alloyed galvanized layer satisfying (%) ≦ {Fe%} ≦ 15 (%). However, {} represents the content in the plating layer, [] represents the content in steel, and% represents mass% unless otherwise specified.
[0005]
(2) A steel sheet satisfying a C amount of 0.05 to 0.25%, a Si amount of 0.1 to 1.5%, a Mn amount of 0.5 to 3.5%, and a B amount of ≦ 5 ppm in a heating furnace After heating to 950 ° C. to form an oxide film having a thickness of 0.01 to 0.3 μm containing Si and Mn, pickling is performed for 1 to 20 seconds with an acid having a concentration of 1 to 20% at 60 to 90 ° C. And after removing the oxide film until it becomes 0.001 to 0.05 μm, after annealing at 650 to 850 ° C., in a zinc bath at 440 to 480 ° C. where the Al concentration is 0.08 to 0.20%. Applying hot dip galvanization, and then performing alloying treatment at 450 to 600 ° C. so that the degree of alloying is 7 (%) ≦ {Fe%} ≦ 15 (%). A method for producing excellent high-strength galvannealed steel sheets.
[0006]
(3) Further, 0.01 to 1% of at least one selected from the group consisting of Cu, Ni and Mo is contained in the steel plate, and Cu, Ni and Mo are contained in the galvannealed layer. The high-strength galvannealed steel sheet having excellent plating adhesion and corrosion resistance according to (1) above, containing 0.01 to 0.2% of at least one selected from the group consisting of:
[0007]
(4) The high strength alloyed hot dip galvanized steel having excellent plating adhesion and corrosion resistance according to (1) or (3), further comprising 0.01 to 1% of Al in the steel sheet. steel sheet.
[0008]
(5) Furthermore, 0.01 to 1% of at least one selected from the group consisting of Cu, Ni and Mo is included in the steel sheet, and the plating adhesion and corrosion resistance according to (2) above For producing high-strength galvannealed steel sheets with excellent strength.
[0009]
(6) The high strength alloyed hot dip galvanizing having excellent plating adhesion and corrosion resistance according to (2) or (5), further comprising 0.01 to 1% of Al in the steel sheet. A method of manufacturing a steel sheet.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
As described above, it is known that the larger the amount of Si and Mn in the base material, the higher the strength of the base material, and the greater the amount of Si and Mn in the plating layer, the more the corrosion resistance of the plating layer is improved. When a large amount of Si and Mn is contained in the base material, the plating adhesion deteriorates. Therefore, the present inventors have conducted the following experiment in order to solve this problem, and have completed the present invention from the obtained knowledge. B, which is a light element, diffuses more easily than Si and Mn, and depending on the annealing conditions, the concentration in the plating layer is estimated to be several tens of times the content of the base material. ing.
[0011]
The present invention has been completed based on the following experimental facts.
A 30 mm thick sheet bar having the composition of steel type A shown in Table 2 was heated at 1200 ° C. to form a hot rolled sheet having a thickness of 2.0 mm in 5 passes, and wound at 610 ° C. Next, the black skin was removed by pickling, and the hot-rolled sheet was annealed at 700 to 970 ° C. for 60 seconds in an annealing furnace, and then pickled with 5% HCl at 60 ° C. for 1 to 20 seconds. Then, annealing was performed at 600 to 900 ° C. for 20 seconds in a plating apparatus, and after plating for 1 second in a zinc bath having an Al concentration of 0.13% in the bath and a bath temperature of 465 ° C., alloying was performed at 440 to 550 ° C. Processed.
{Fe%} in the obtained galvannealed steel sheet was 10%, {Si%} was 0.01 to 0.8%, and {Mn%} was 0.1 to 2.2%. It was.
[0012]
The plating adhesion and corrosion resistance of the plated steel sheet obtained here were investigated. Plating adhesion is measured by measuring the amount of plating peeling per unit length as a Zn count with fluorescent X-rays when cellophane tape is applied to a plated steel sheet, the tape surface is bent within 90 °, bent back, and then peeled off. Then, according to the criteria in Table 1, those with ranks 1 and 2 were evaluated as good, and those with 3 or more were evaluated as bad. Corrosion resistance is evaluated as good when the rust generation area ratio is less than or equal to the rust generation status in the salt spray test result of the alloyed hot-dip galvanized steel sheet of mild steel sheet, and is evaluated as good when it exceeds 10%. did. FIG. 2 is a diagram showing the influence of the thickness of the oxide film before and after pickling on the corrosion resistance. The case where the corrosion resistance was good was shown as ◯, and the case where it was poor was shown as ●.
By combining these results, ◯ indicates that both the plating adhesion and corrosion resistance are good, and ◯ indicates that the corrosion resistance is good, but the plating adhesion is poor, and the plating adhesion and corrosion resistance are Those which were both bad are shown in FIG. 1 as ●.
FIG. 3 is a diagram showing the influence of the concentration of B in the alloyed hot-dip galvanized layer on the plating adhesion. When the amount of B in the plating layer is 6 ppm or less, the plating adhesion is Rank 1 (◯) in light of the criteria in Table 1, and when the amount of B in the plating layer is 7 to 10 ppm, Rank 2 ( Δ). On the other hand, when the amount of B in the plating layer exceeded 10 ppm, the rank was 3 or more (x).
[0013]
The following findings were obtained from the obtained results.
1) The amounts of Si and Mn in the plating layer have the effect of improving the corrosion resistance.
2) Since the concentration of Si and Mn on the steel surface is estimated to be several times the content of the base material, the amount of Si and Mn in the plating layer is theoretically considered from the content of the base material itself. More than the amount taken up.
3) The amount of Si and Mn in the plating layer is affected by the amount of Si and Mn in the base material and {Fe%} after alloying in the plating layer, and the range is {Fe%} * [Si%] / 10 (%) ≧ {Si%} ≧ {Fe%} * [Si%] / 100 (%)
{Fe%} * [Mn%] / 10 (%) ≧ {Mn%} ≧ {Fe%} * [Mn%] / 100 (%)
If it is, there exists a range with favorable plating adhesiveness and corrosion resistance.
4) From the results of FIG. 2, it can be seen that the corrosion resistance is good when the thickness of the oxide film before and after pickling is within a specific range.
5) From the results of FIG. 3, it can be seen that B deteriorates the plating adhesion, and not only the content in steel but also the incorporation into the plating layer should be reduced as much as possible. In addition, since the plating adhesion deteriorates, bare metal exposed parts such as non-plating and pinholes are generated, and thus corrosion resistance may deteriorate as a result even if Si or the like is incorporated in the plating layer. . For this reason, the amount of B in a plating layer must be 10 ppm or less.
[0015]
The following present invention was completed from the findings of the above experiments.
The characteristics of the present invention are: {Si%} in plating layer, {Mn%}, [Si%] in steel, [Mn%] and {Fe%} in plating layer as specific ranges, plating adhesion, It is to obtain a plated steel sheet that satisfies both corrosion resistance and strength. That is, the content of Si, Mn, and B in the plating layer needs to satisfy the following formula.
{Fe%} * [Si%] / 10 (%) ≧ {Si%} ≧ {Fe%} * [Si%] / 100 (%) and {Fe%} * [Mn%] / 10 (%) ≧ {Mn%} ≧ {Fe%} * [Mn%] / 100 (%) and {B} (ppm) ≦ 10 (ppm)
When {Si%} and {Mn%} are less than the above ranges, sufficient improvement in corrosion resistance cannot be expected. On the other hand, when {Si%} and {Mn%} exceed the above ranges, the corrosion resistance is improved, but the plating performance is poor during hot dip galvanizing, and the unplated portion deteriorates the corrosion resistance and is plated after alloying. Degradation of adhesion. Moreover, when {B} exceeds the above range, plating adhesion is deteriorated.
[0016]
The reason why the degree of alloying {Fe%} is limited is as follows. The optimum value of the amount of {Si} and {Mn} in the plating layer of the present invention is determined by the degree of alloying and the amount of [Si] and [Mn] in the steel.
Alloying degree: 7 ≦ {Fe%} ≦ 15
If it is less than 7%, not only a large amount of ζ phase remains, but also the η phase remains, so that alloying becomes insufficient and slidability deteriorates. Therefore, the degree of alloying needs to be 7% or more. However, if it exceeds 15%, a large amount of Γ phase is generated, so that the plating adhesion is deteriorated and the corrosion resistance of the bent portion is deteriorated.
[0017]
Next, the reason why the content of the constituent components in the steel is limited in the present invention will be described.
C amount 0.05-0.25%
The amount of C is indispensable for obtaining the required strength and obtaining the desired structure. At least 0.05% is necessary, but if it exceeds 0.25%, weldability deteriorates, so the above range was adopted.
[0018]
Si amount 0.1-1.5%
The amount of Si is indispensable for solid solution strengthening and obtaining a desired structure, and can increase the strength without deteriorating ductility. In addition, since Si and Mn oxide have the effect of improving the corrosion resistance, by leaving an appropriate amount of Si and Mn oxide immediately before plating, Si, Mn oxide and solid solution Si and Mn in steel can be reduced. The effect of improving the corrosion resistance can be obtained by taking an appropriate amount into the alloy layer while maintaining the plating adhesion, as a supply source. In order to obtain a desired effect, 0.1% is necessary. However, if it exceeds 1.5%, the plating adhesion deteriorates, so the above range was adopted.
[0019]
Mn amount 0.5-3.5%
Similar to C, an Mn amount of 0.5 to 3.5% is indispensable for obtaining the required strength and obtaining the desired structure. Further, in order to obtain the effect of improving the corrosion resistance similarly to Si, the effect is poor unless it is at least 0.5%. However, if it exceeds 3.5%, the weldability deteriorates.
[0020]
B amount ≦ 5ppm
Since B deteriorates the plating adhesion, the incorporation into the plating layer as well as the content in steel should be reduced as much as possible. Furthermore, since the bare metal parts such as non-plating and pinholes are generated due to the deterioration of the plating adhesion, the corrosion resistance may be deteriorated as a result even if Si or the like is taken into the plating layer. Therefore, the upper limit was set to 5 ppm.
[0021]
Furthermore, the steel of the high-strength galvannealed steel sheet of the present invention may contain at least one of the following elements in the following amounts. In that case, the following effects are further obtained.
Cu amount 0.01-1%
Cu segregates in austenite and is not only important for obtaining a desired strength and a desired structure, but also has an effect of improving plating adhesion. The reason why the plating adhesion is improved is not clarified at the present time. However, in order to obtain these desired effects, it is effective to contain 0.01% or more preferably. However, if it exceeds 1%, the economic efficiency deteriorates, so the upper limit is preferably made 1%.
[0022]
Ni content 0.01-1%
Ni is segregated in austenite like Cu and is not only important for obtaining the required strength and for obtaining the desired structure, but also has the effect of improving the plating adhesion, so it is added as necessary. The reason why the plating adhesiveness is improved is not clarified at the present time. However, in order to obtain these desired effects, it is preferable to contain 0.01% or more like Cu. However, if it exceeds 1%, the economic efficiency deteriorates, so the upper limit is preferably made 1%.
[0023]
Mo amount 0.01-1%
Although Mo is expensive, it segregates in austenite and is not only important for obtaining the required strength and for obtaining a desired structure, but also has an effect of improving plating adhesion. The reason why the plating adhesion is improved is not clarified at present as in the case of Cu and Ni. However, in order to obtain these desired effects, it is preferable to include 0.01% or more. However, if it exceeds 1%, the economic efficiency deteriorates, so the upper limit is preferably made 1%.
[0024]
Al content 0.01-1%
Al is important for obtaining the required strength and for obtaining the desired structure. As a result, the amount of Si added can be reduced, so it is advantageous because it is more advantageous for improving plating adhesion than Si-added steel having the same tensile strength. Add as appropriate. In order to obtain a desired effect, the content is preferably 0.01% or more. However, if it exceeds 1%, the economical efficiency deteriorates, so the upper limit is preferably made 1%.
[0025]
When the steel contains at least one selected from the group consisting of Cu, Ni and Mo, the plating layer contains at least one selected from the group consisting of Cu, Ni and Mo from 0.01 to 0.00. It is preferable to contain 2%.
When at least one of these elements is contained in the plating layer, it is preferable because the corrosion resistance is more effectively improved by the combined effect of these elements and the Fe—Zn alloy layer. For steel sheets containing Cu, Ni, and Mo in the base material, in order to obtain a stable effect of improving corrosion resistance, it is preferable that each of the plating layers contains 0.01% or more, but exceeds 0.2%. Is more preferably 0.2% because the content in the steel plate base material must be increased and the economic efficiency is deteriorated.
Further, in the present invention, {Fe%}, {Si%} and {Mn%} in the plating layer and [Si%] and [Mn%] in the steel have a relationship represented by the above formula. is required.
[0026]
The manufacturing method is characterized in that an oxide film containing Si and Mn is once formed, and then the amount of B is controlled by pickling to keep the amounts of Si and Mn in the plating layer within a certain range.
The method of reducing the amount of B in the plating layer is an oxide film mainly composed of Si and Mn by pickling after heating in a heating furnace and before annealing in a continuous hot dip galvanizing equipment (CGL) furnace. Is pickled into the alloy layer after alloying, and at the same time, B is removed, but since B is easier to pickle than Si and Mn, it is almost completely removed by pickling conditions described later. Removed.
[0027]
Hereinafter, the reason for limiting the manufacturing conditions will be described.
When the heating temperature in the heating furnace is lower than 750 ° C., a desired structure cannot be obtained and the strength cannot be increased, and the surface concentration of Si and Mn becomes insufficient. If the surface concentration in this step is insufficient, the surface concentration in the alloying process is too much, and the plating adhesion deteriorates. If it exceeds 950 ° C., the effect is not only saturated but also difficult to operate and inferior in economic efficiency. Furthermore, since a large amount of surface concentrate is generated, it cannot be removed to an appropriate amount by pickling, and non-plating occurs. Therefore, the heating temperature in a heating furnace shall be 750-950 degreeC.
[0028]
Oxide film in heating furnace (oxide film before pickling)
When the oxide film is less than 0.01 μm, the amount remaining after pickling is reduced, so that the corrosion resistance is deteriorated. Moreover, if it exceeds 0.3 μm, pickling is difficult. Therefore, the oxide film in a heating furnace shall be 0.01-0.3 micrometer. Here, the amount of the oxide film can be estimated by measuring while adjusting the sputtering rate by various measuring devices such as GDS, SIMS, and AES.
[0029]
Pickling conditions If the pickling temperature is less than 60 ° C or the pickling time is less than 1 second, the effect is difficult to obtain, and if the pickling temperature exceeds 90 ° C or the pickling time exceeds 20 seconds, the surface becomes rough and after plating Detracts from the appearance. Therefore, the pickling conditions are an acid having a concentration of 1 to 20%, a pickling temperature of 60 to 90 ° C., and a pickling time of 1 to 20 seconds. As the acid, hydrochloric acid is economical and preferable, but other acids such as sulfuric acid, phosphoric acid and nitric acid are not particularly limited.
[0030]
The amount of oxide film after pickling If the amount of oxide film after pickling is less than 0.001 μm, the amount taken into the plating layer after alloying is reduced, so the effect of improving corrosion resistance cannot be obtained. On the other hand, if it exceeds 0.05 μm, not only the plating adhesion deteriorates, but also the corrosion resistance deteriorates due to the non-plated portion. Therefore, the oxide film amount after pickling was set to 0.001 to 0.05 μm.
[0031]
Reduction heating temperature Preferably, reduction is performed by heating in a reducing atmosphere containing H 2 in a CGL (alloying) furnace. However, if the heating temperature is less than 650 ° C., the oxide film produced by pickling cannot be reduced and unplating may occur. appear. Further, when the temperature exceeds 850 ° C., the surface concentration of Si and Mn in the CGL furnace is large, so that non-plating occurs similarly. Therefore, the heating temperature in the CGL furnace was set to 650 to 850 ° C.
[0032]
In order to ensure adhesion after alloying of the hot dip galvanizing bath plating layer, the Al concentration needs to be 0.08% or more. However, if the Al concentration exceeds 0.20%, alloying becomes difficult, so the upper limit was made 0.20%. If the bath temperature is lower than 440 ° C., a portion below the freezing point (420 ° C.) may appear due to the bath temperature fluctuation of the plating bath, and the operation is not stable. Moreover, when it exceeds 480 degreeC, the cost concerning heating holding will increase. Therefore, the bath temperature was set to 440 to 480 ° C.
[0033]
Alloying temperature If the alloying temperature is less than 450 ° C, the ζ phase is likely to be generated, and not only the slidability of the alloyed hot-dip galvanized steel sheet may be lacking, but also the time required for alloying increases productivity. to degrade. Moreover, when it exceeds 600 degreeC, it will become easy to produce | generate a (GAMMA) phase and there exists a possibility that the plating adhesiveness of an galvannealed steel plate may be missing. Therefore, the alloying temperature was set to 450 to 600 ° C. In addition, since the amount of Fe in the plating layer is determined depending on the alloying conditions, the amount of Fe, Si, and Mn in the plating layer and the amount of Si and Mn in the base material must be in the range indicated by the above formula. Conditioning is important.
[0034]
【Example】
EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated more concretely, this invention is not limited at all by these Examples.
Examples 1-9, Comparative Examples 1-8
A 300 mm thick slab having the chemical composition shown in Table 2 (C, Si, Mn, Al, P, S, Cr, Cu, Ni, Mo, Ti, Nb, B) is heated at 1200 ° C. and hot rolled. A hot-rolled sheet having a thickness of 2.3 mm was taken up at 620 ° C. Next, the black skin was removed by pickling, rolled at a reduction rate of 50% by cold rolling, and passed through a continuous annealing furnace (CAL). Subsequently, pickling, galvanizing, and alloying treatment were performed by passing through CGL. The amount of plating was 40 g / m 2 on one side. The alloying temperature was 450 to 600 ° C., and the alloying time was 20 seconds.
Table 3 shows the production conditions of the plated steel sheet (heating temperature in heating furnace, amount of coating film produced in heating furnace, amount of remaining coating after pickling, pickling time, CGL annealing temperature, plating bath temperature, Al concentration in bath, alloying temperature) Table 4 shows the results of investigations on the Si, Mn, B, Cu, Ni, and Mo contents, plating appearance, plating adhesion, and corrosion resistance in the plated layer of the obtained galvannealed steel sheet. Any of the production conditions within the range of the present invention have good plating adhesion and corrosion resistance, but in Comparative Examples outside the scope of the present invention, either or both of the plating adhesion and corrosion resistance were inferior.
[0035]
[Table 1]
[Table 2]
[Table 3]
【The invention's effect】
As described above, according to the present invention, a high-strength galvannealed steel sheet excellent in plating adhesion and corrosion resistance can be obtained. By applying the steel plate of the present invention, it is possible to reduce the weight and fuel consumption of an automobile body, and thus greatly contribute to the improvement of the global environment.
[Brief description of the drawings]
FIG. 1 is a graph showing the influence of Si and Mn concentrations in an alloyed hot-dip galvanized layer on plating adhesion and corrosion resistance.
FIG. 2 is a diagram showing the effect of an oxide film before and after pickling on corrosion resistance.
FIG. 3 is a diagram showing the influence of the concentration of B in the alloyed hot-dip galvanized layer on the plating adhesion.
Claims (6)
｛Ｆｅ％｝＊［Ｓｉ％］／１０（％）≧｛Ｓｉ％｝≧｛Ｆｅ％｝＊［Ｓｉ％］／１００（％）かつ
｛Ｆｅ％｝＊［Ｍｎ％］／１０（％）≧｛Ｍｎ％｝≧｛Ｆｅ％｝＊［Ｍｎ％］／１００（％）かつ
｛Ｂ｝（ｐｐｍ）≦１０（ｐｐｍ）を満たし、かつ
合金化度が７（％）≦｛Ｆｅ％｝≦１５（％）を満たす合金化溶融亜鉛めっき層を有することを特徴とするめっき密着性及び耐食性に優れた高強度合金化溶融亜鉛めっき鋼板。但し、｛ ｝はめっき層中の含有量、［ ］は鋼中の含有量、％は、以下、特に断らない限り質量％を表す。On the steel sheet satisfying the C amount of 0.05 to 0.25%, the Si amount of 0.1 to 1.5%, the Mn amount of 0.5 to 3.5%, and the B amount ≦ 5 ppm, Si and Mn in the plating layer And B content is {Fe%} * [Si%] / 10 (%) ≧ {Si%} ≧ {Fe%} * [Si%] / 100 (%) and {Fe%} * [Mn% ] / 10 (%) ≧ {Mn%} ≧ {Fe%} * [Mn%] / 100 (%) and {B} (ppm) ≦ 10 (ppm) and the degree of alloying is 7 (%) A high-strength galvannealed steel sheet excellent in plating adhesion and corrosion resistance, characterized by having an alloyed galvanized layer satisfying ≦ {Fe%} ≦ 15 (%). However, {} represents the content in the plating layer, [] represents the content in steel, and% represents mass% unless otherwise specified.
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| RU2572901C9 (en) | 2011-07-29 | 2016-06-20 | Ниппон Стил Энд Сумитомо Метал Корпорейшн | Annealed layer of galvanic coating, and steel plate with such coating, and method of its producing |
| MX352497B (en) | 2011-09-30 | 2017-11-28 | Nippon Steel & Sumitomo Metal Corp | Alloyed hot-dip galvanized steel sheet. |
| JP5852690B2 (en) * | 2013-04-26 | 2016-02-03 | 株式会社神戸製鋼所 | Alloyed hot-dip galvanized steel sheet for hot stamping |
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| JP2001140022A (en) * | 1999-08-27 | 2001-05-22 | Nippon Steel Corp | Method of manufacturing high strength galvanized steel sheet excellent in press-formability |
| JP2001207236A (en) * | 2000-01-26 | 2001-07-31 | Kawasaki Steel Corp | High tensile strength hot dip galvanized steel plate and producing method therefor |
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| JP2001207236A (en) * | 2000-01-26 | 2001-07-31 | Kawasaki Steel Corp | High tensile strength hot dip galvanized steel plate and producing method therefor |
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