JP2003160850A - Method for manufacturing hot-dip galvanized steel sheet - Google Patents
Method for manufacturing hot-dip galvanized steel sheetInfo
- Publication number
- JP2003160850A JP2003160850A JP2001357910A JP2001357910A JP2003160850A JP 2003160850 A JP2003160850 A JP 2003160850A JP 2001357910 A JP2001357910 A JP 2001357910A JP 2001357910 A JP2001357910 A JP 2001357910A JP 2003160850 A JP2003160850 A JP 2003160850A
- Authority
- JP
- Japan
- Prior art keywords
- steel sheet
- bath
- hot
- concentration
- molten zinc
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
Landscapes
- Coating With Molten Metal (AREA)
Abstract
Description
【発明の詳細な説明】
【０００１】
【発明が属する技術分野】本発明は溶融亜鉛めっき鋼
板、とくにスパングルを極力微細化した溶融亜鉛めっき
鋼板の製造方法に関する。
【０００２】
【従来の技術】溶融亜鉛めっき鋼板は、通常、素材鋼板
を還元雰囲気中で高温に加熱して、焼鈍すると共に鋼板
表面を還元して活性化し、その還元雰囲気中で亜鉛の融
点近くまで鋼板の温度を降下させ、溶融亜鉛浴を潜らせ
て表面にめっき皮膜を付着させて製造する。この鋼板表
面に付着した亜鉛が凝固する際、樹枝状に結晶が成長し
花模様のスパングルが形成される。しかしスパングル模
様は、その美しさから好まれる場合がある反面、表面を
塗装するときに下地の模様が塗装外観を損ねることや、
用途によっては平滑性を要求されたり、スパングル模様
を避けたい場合がある。
【０００３】たとえば、家電製品や電気機器などの装置
には、表面の平滑性がよく、目付量を少なくでき、加工
性や溶接性がすぐれていることにより、従来から電気亜
鉛めっき鋼板が多く使用されている。これに対し、目付
量を少なくする技術が進歩して、鋼板の加工性も向上し
コストも安い点から、溶融亜鉛めっき鋼板がそれに取っ
て代わって使用されるようになってきた。この場合、機
器内部などは塗装されずに用いられることも多く、表面
が電気亜鉛めっき鋼板に近い、スパングルを極力微細化
したミニマムスパングル、あるいはゼロスパングルと呼
ばれる溶融亜鉛めっき鋼板が適用される。
【０００４】ミニマムスパングル溶融亜鉛めっき鋼板の
製造方法としては、主に二種の方法が用いられる。一つ
は浴中亜鉛のＰｂの量をできるだけ低下させる方法であ
る。Ｐｂは溶融亜鉛の粘性を改善し、亜鉛の濡れ性を良
好にする上、スパングルの形成に大きく寄与するので、
スパングルが必要な場合は、通常、質量％にて0.2％以
下のＰｂが添加される。これに対し、スパングルを微細
にするには100ppm以下、好ましくは30ppm以下に低減す
る。
【０００５】もう一つは、溶融亜鉛浴を出た鋼板がめっ
き付着量調整直後の、亜鉛が未凝固状態にあるときに水
や蒸気などを噴射する方法で、スパングルが成長する前
に急速凝固させる。この場合、Ｐｂ量が多少多くてもミ
ニマムスパングル化が可能であるが、Ｐｂも低減できれ
ば、より安定して製造することができる。
【０００６】このようなミニマムスパングル溶融亜鉛め
っき鋼板を製造するとき、表面に鋼板の長さ方法に伸ば
された、正常な部分とは光沢度も色調も異なる、白色の
雲状のむらが発生することがある。この白色むらは、溶
融亜鉛めっき鋼板としての耐食性、塗装性、加工性また
は溶接性などには全く影響しないが、鋼板製品としての
外観が大きく損なわれる。
【０００７】ミニマムスパングル鋼板に関し、めっき皮
膜の鋼板との密着性や光沢度、あるいはスパングルの消
滅に関する改善は種々検討されている。しかし、このよ
うな白色むらの抑止対策については、これまでほとんど
知られていない。これに対し、めっきの付着量の不均一
や、水などの冷却剤噴射の不均一などの有無の影響を調
査したが、それによる改善効果はほとんど認められなか
った。また、冷却剤噴射の条件や、調質圧延のロール粗
度変更などにより多少は軽減されるが、本質的に防止す
ることはできなかった。
【０００８】
【発明が解決しようとする課題】本発明の目的は、ミニ
マムスパングル溶融亜鉛めっき鋼板表面に発生する、白
色むらを抑制する製造方法の提供にある。
【０００９】
【課題を解決するための手段】本発明者らは、ミニマム
スパングル溶融亜鉛めっき鋼板の表面に発生する白色む
らを抑止するため、その発生原因について種々調査をお
こなった。
【００１０】まずこの白色むらの発生状況は、客観的に
判定することが好ましいので、目視判定で白色むらのあ
る鋼板とない鋼板について、光沢度と白色度の計測によ
る判別を試みた。光沢度はJIS-Z-8741に準じ入射角60°
にて測定し、白色度（Ｌ値）をJIS-Z-8722により測定し
た。この場合、白色むらは健全な部分とそうでない部分
との相対的な差として検出される。
【００１１】鋼板の幅方向にこれらの測定をおこない、
最大値と最小値との差の平均値に対する比率をばらつき
の幅とし、このばらつきの幅の大きさと、目視による評
価とを対比してみると、図１に示す結果が得られた。図
１から、目視評価にて白色むらがなく良好と判断された
鋼板は、計器による光沢度のばらつきの幅が20％以下
で、かつ白色度のばらつきの幅が10％以下のものである
ことがわかる。
【００１２】表面の良好な鋼板および白色むらの発生し
た鋼板について、めっき皮膜の状態を断面観察などによ
り調査してみると、これらの鋼板間、あるいは白色むら
の部分と健全な部分とでは、亜鉛めっき皮膜が鋼板に付
着した境界層近傍の状態に違いがあるのではないかと思
われた。
【００１３】そこで、製造チャンスの種々異なる鋼板か
ら試料を採取し、発煙硝酸によりめっき皮膜のＺｎ部分
を溶かして取り除き、界面に生じているＡｌを多く含む
Ｆｅ−Ａｌ合金またはＦｅ−Ｚｎ−Ａｌ合金（以下単に
Ｆｅ−Ａｌ合金と言う）層を露出させ、次にこのＦｅ−
Ａｌ合金層を酸洗抑止剤を添加した塩酸溶液を用いて溶
解し、めっき皮膜のＺｎ部分のＡｌ濃度および界面のＦ
ｅ−Ａｌ合金層のＡｌ量を分析した。この分析結果と、
鋼板表面の白色むら発生との関係を調べると、図２に示
す結果が得られた。
【００１４】この図２から、めっき皮膜のＺｎ部分のＡ
ｌ濃度が高くなるほどＦｅ−Ａｌ合金層のＡｌ量が多く
なっており、白色むらもなくなることがわかる。めっき
皮膜のＡｌ濃度は、浴中溶融亜鉛のＡｌ濃度と相関があ
ると考えられるので、溶融亜鉛のＡｌ濃度と白色むらの
発生比率を調べてみると、図３の結果が得られた。この
場合、発生比率は鋼板コイル単位とし、コイル表面の一
部にでも発生しておれば不良として計上している。
【００１５】溶融亜鉛めっきをおこなうとき、浴中溶融
亜鉛には通常0.1〜0.2質量％程度のＡｌが添加される。
Ａｌの少量含有は、素材鋼板が溶融亜鉛浴に浸漬される
と優先的にＦｅと反応してＦｅ−Ａｌ合金層を形成し、
これがめっき皮膜の密着性を向上させ、硬くて脆いＦｅ
−Ｚｎ合金層の発達を抑制し、鋼板が加工されたときの
めっき皮膜剥離を防止する効果があるからである。
【００１６】図３の結果は、白色むらをなくすには、め
っき浴中溶融亜鉛のＡｌ濃度を高くすればよく、Ａｌ濃
度が0.16〜0.18％を超えると白色むらの発生は大きく減
少するが、これを下回る場合は白色むらが多発し、Ａｌ
濃度が低下するほどその発生頻度が増大することを示し
ている。したがって、ミニマムスパングル溶融亜鉛めっ
き鋼板の製造において白色むらをなくすには、溶融亜鉛
浴のＡｌ濃度を上げればよい。
【００１７】しかしながら多くの場合、同じめっきライ
ンまたは同じ溶融亜鉛浴槽を用いて、合金化溶融亜鉛め
っき鋼板も製造される。合金化溶融亜鉛めっき鋼板の製
造には、溶融亜鉛のＡｌ濃度を0.08〜0.14％に管理しな
ければならない。これは0.08％を下回ると合金化溶融亜
鉛めっき鋼板の耐パウダリング性が大きく低下するから
であり、0.14％を超えると、めっき皮膜の合金化が十分
できないからである。このように要求される浴中溶融亜
鉛のＡｌ濃度は、目的とする鋼板により異なるので、ミ
ニマムスパングル溶融亜鉛めっき鋼板の白色むら防止の
ためには、合金化溶融亜鉛めっき鋼板製造用とは別にし
て、それぞれの専用ラインで製造されることが望ましい
といえる。
【００１８】しかし現実には、一つのラインで両鋼板を
製造しなければならないことが多く、製造する鋼板によ
り浴中溶融亜鉛のＡｌ濃度を上げたり下げたりする必要
がある。しかしながら、このような浴中溶融亜鉛のＡｌ
濃度の変更には多大の時間を要し、その変更途中の間に
ミニマムスパングル鋼板を製造すると、白色むら不良が
発生しやすいとすれば、めっきラインの操業に大きな制
約を受ける。
【００１９】通常の溶融亜鉛めっき鋼板やミニマムスパ
ングル鋼板では、被膜の密着性確保の観点からは、浴中
溶融亜鉛のＡｌ濃度が合金化溶融亜鉛めっき鋼板製造の
ための上限に近い0.12〜0.14％程度以上あればよい。と
ころが合金化溶融亜鉛めっき鋼板では、浴中溶融亜鉛の
Ａｌ濃度が0.14％を超えるとほぼ製造不能になる。した
がって、溶融亜鉛のＡｌ濃度が0.14％から0.16％超ま
で、できれば0.18％以上になるまでの間において、ミニ
マムスパングル鋼板の白色むらの発生を抑止できれば、
めっきラインの操業上、製造する鋼板に制限を受けず、
きわめて好ましいと考えられる。
【００２０】先出の図２をみると、めっき皮膜のＺｎ部
分のＡｌ濃度が0.11％から0.14％の範囲、あるいはＦｅ
−Ａｌ合金層のＡｌ含有量が150mg／m２から200mg／m２
の範囲では白色むらのある鋼板とむらのない鋼板とが現
れ、これらの範囲を超える濃度あるいは含有量では白色
むらが発生していない。浴中溶融亜鉛のＡｌ濃度に対す
るめっき皮膜のＺｎ部分のＡｌ濃度は80％前後の比率に
なるので、めっき皮膜のＺｎ部分のＡｌ濃度が0.11％か
ら0.14％の範囲は、浴中溶融亜鉛のＡｌ濃度が0.14％か
ら0.18％に対応する。
【００２１】そこで、上記濃度範囲内で浴中溶融亜鉛の
Ａｌ濃度が同じであっても、白色むらが生じたり生じな
かったりする理由について、さらに調査を進めた。その
結果、めっき皮膜を除去した後に残ったＦｅ−Ａｌ合金
層を走査型電子顕微鏡で観察すると、めっき表面の健全
な部位ではＦｅ−Ａｌ合金層が鉄表面をほぼ均一に覆っ
ているように見えるのに対し、白色むらの発生した部位
ではＦｅ−Ａｌ合金層に黒い斑点状、またはまだら状の
部分が多くみられることがわかった。
【００２２】この黒色部分には正常なＦｅ−Ａｌ合金層
が形成されておらず、Ｆｅが現れていると推測されたの
で、黒色部分の面積率を計測し、黒色でない部分がＦｅ
−Ａｌ合金層であるとしてその比率を求め、これとＦｅ
−Ａｌ合金層のＡｌ含有量との関係をみると、図４の結
果が得られた。すなわち、白色むら部分は、めっき皮膜
と鉄との界面に形成されるＦｅ−Ａｌ合金層に面積率が
30％を超える欠陥が存在すると生じること、言い換えれ
ば、面積率が70％以上の十分なＦｅ−Ａｌ合金層が形成
されれば、白色むらは発生しないことが明らかになっ
た。
【００２３】界面のＦｅ−Ａｌ層が素材鋼板の表面を十
分に覆うには、鋼板が溶融亜鉛浴に侵入する時点におけ
る要因が大きく影響すると推測される。そこで、浴侵入
直前の諸製造条件を調査の結果、素材鋼板の溶融亜鉛浴
侵入直前の温度およびスナウト内の雰囲気の露点が強く
影響していることが明らかになった。 鋼板温度は溶融
亜鉛浴侵入直後の表面における合金層形成反応の進行を
大きく支配し、露点は素材鋼板表面の還元状態、または
活性度を決定するためと考えられる。
【００２４】以上の知見に基づき、さらに実験を繰り返
した結果、白色むら抑止に対する浴中溶融亜鉛のＡｌ濃
度と、鋼板温度および雰囲気露点との条件管理式を見出
すことができた。本発明はこの管理式に基づく製造方法
であって、その要旨は次のとおりである。
【００２５】素材鋼板が焼鈍過程を終えて冷却され、溶
融亜鉛浴に侵入する際、浴中溶融亜鉛のＡｌ濃度がＡ質
量％であるとき、鋼板の温度Ｔ（℃）および溶融亜鉛浴
侵入直前のスナウト内雰囲気ガスの露点Ｖ（℃）が、下
記式を満足する範囲とすることを特徴とするミニマム
スパングル溶融亜鉛めっき鋼板の製造方法。
（0.0025×Ｔ−0.012×Ｖ）×0.1≦Ａ ・・・・
【００２６】
【発明の実施の形態】本発明のミニマムスパングル溶融
亜鉛めっき鋼板の製造方法は、とくにめっき浴中溶融亜
鉛のＡｌ濃度が0.14質量％から0.18質量％の間にある場
合に、白色むら発生を抑止する。すなわち本方法では、
素材鋼板が焼鈍過程を終えて冷却され亜鉛溶融亜鉛浴に
侵入する際、溶融亜鉛浴中のＡｌ濃度がＡ質量％である
とき、鋼板の温度Ｔ（℃）および溶融亜鉛浴侵入直前の
スナウト内雰囲気ガスの露点Ｖ（℃）が下式を満足する
範囲とする。
（0.0025×Ｔ−0.012×Ｖ）×0.1≦Ａ ・・・・
【００２７】浴中溶融亜鉛のＡｌ濃度は、低くなりすぎ
ると、めっき鋼板の合金層を除く亜鉛めっき皮膜中のＡ
ｌ濃度が0.11質量％を下回ってしまい、白色むら発生を
抑止することができなくなるので、0.14質量％以上であ
ることが好ましい。また、浴中溶融亜鉛のＡｌ濃度は0.
18質量％を超えると、鋼板の白色むらは発生せず、式
に示す規制は、とくには必要でなくなる。しかしなが
ら、この式にしたがって規制をおこなえば、浴中溶融亜
鉛のＡｌ濃度が0.14質量％以上の場合、ミニマムスパン
グル溶融亜鉛めっき鋼板における表面の白色むらを完全
になくすことができる。
【００２８】浴中溶融亜鉛のＡｌ濃度から、式によっ
て鋼板温度とスナウト内雰囲気ガスの露点を規制するの
は、素材鋼板の侵入時の温度を低くすること、および露
点を低下させることがいずれも白色むら発生に効果があ
るからである。
【００２９】ただし、式にて規制される範囲内であっ
ても、素材鋼板の侵入時の温度が低すぎると不めっきを
発生するようになり、高すぎると好ましくないＦｅ−Ｚ
ｎ合金層が過剰に発達するので、440〜500℃の範囲とす
るのがよい。また露点も低くしすぎるとスナウト内の溶
融亜鉛浴面からの亜鉛蒸気の発生が増して、これが内壁
面に堆積し素材鋼板面に落下してめっき皮膜の欠陥を生
じさせ、高くしすぎると不めっきを生じる。したがって
−40〜−15℃の範囲内で制御するとよい。
【００３０】
【実施例】板厚0.6mm、板幅914mm、炭素0.04質量％の通
常の低炭素鋼素材鋼板コイルにて連続式溶融亜鉛めっき
ラインを用い、洗浄後、空燃比0.95、板温600℃の無酸
化炉を経て、水素10％残部窒素の雰囲気中にて740℃に
加熱焼鈍後、素材鋼板温度を降下させて、460±5℃に保
持した溶融亜鉛浴に侵入させた。浴から引き上げ直後ガ
スワイピングによりめっき付着量を片面90g/m２とし、
直ちにミストスプレイによりミニマムスパングル化し
た。
【００３１】この溶融亜鉛めっきをおこなう際、浴中溶
融亜鉛が0.12％、0.14％、0.16％、0.18％および0.20％
である操業時に、それぞれ亜鉛のスナウト内雰囲気中露
点を−45〜−15℃の範囲で変え、素材鋼板の溶融亜鉛浴
侵入温度を440〜520℃の種々の温度に変えてめっきをお
こなった。
【００３２】得られた溶融亜鉛めっき鋼板コイルの各条
件が明確に確認されている部分よりそれぞれ鋼板試料を
切りだし、鋼板表面の幅方向の光沢度変動をJIS-Z-8741
に基づいて60°鏡面光沢法で測定し、白色度変動のＬ値
をJIS-Z8722に準じて測定して、白色むらを図１に示し
た基準により評価した。
【００３３】溶融亜鉛浴のＡｌ濃度を0.14％、0.16％ま
たは0.18％としたとき、それぞれ溶融亜鉛浴侵入時の鋼
板温度および露点を変えてミニマムスパングル鋼板を製
造した場合の、白色むら発生状況を図５、図６および図
７に示す。図中に示した点線は、式によるものであ
り、式を満足する素材鋼板温度およびスナウト内雰囲
気露点とすれば、白色むらを抑止できることがわかる。
【００３４】浴中溶融亜鉛のＡｌ濃度が0.12％のとき、
いずれのスナウト内雰囲気露点および素材鋼板の溶融亜
鉛浴侵入温度の組み合わせにおいても、白色むらが発生
していた。また浴中溶融亜鉛のＡｌ濃度が0.20％のとき
は、いずれの鋼板にも白色むらの発生は認められなかっ
た。
【００３５】
【発明の効果】ミニマムスパングル溶融亜鉛めっき鋼板
の表面の白色むらは鋼板の表面品質を悪くするが、この
白色むらは、めっき浴中溶融亜鉛のＡｌ濃度を高くする
ことにより抑止できる。しかしながら、合金化溶融亜鉛
めっき鋼板を製造するとき、浴中溶融亜鉛のＡｌ濃度を
低くしなければならず、このＡｌ濃度変更には、多大な
時間を要し、その間、ミニマムスパングル溶融亜鉛めっ
き鋼板の製造は避けざるを得ない。これに対し、別のめ
っきラインで製造するか、同じラインでは溶融亜鉛浴の
ポットを交換すればよいが、大規模な設備投資を要し、
容易には実施できない。
【００３６】本発明の製造方法では、浴中のＡｌ濃度を
合金化溶融亜鉛めっき鋼板に適した濃度に低下せるため
の低濃度範囲でも白色むらの発生を抑止することがで
き、新たな設備投資をおこなうことなく、同じめっきラ
インで、表面の良好なミニマムスパングル溶融亜鉛めっ
き鋼板と、合金化溶融亜鉛めっき鋼板との両方を容易に
製造することができる。Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet, and more particularly to a method for manufacturing a hot-dip galvanized steel sheet in which spangles are made as fine as possible. [0002] A hot-dip galvanized steel sheet is usually heated to a high temperature in a reducing atmosphere to be annealed and activated by reducing the surface of the steel sheet. The temperature of the steel sheet is lowered until the surface of the steel sheet is immersed in a molten zinc bath to deposit a plating film on the surface. When the zinc adhering to the surface of the steel sheet solidifies, crystals grow in a dendritic manner and spangles having a flower pattern are formed. However, spangles may be preferred for their beauty, but when painting the surface, the underlying pattern may damage the appearance of the paint,
Depending on the application, smoothness may be required or spangle patterns may be desired to be avoided. For example, electrogalvanized steel sheets have been widely used in appliances such as home electric appliances and electric appliances because of their good surface smoothness, small weight per unit area, and excellent workability and weldability. Have been. On the other hand, the technology for reducing the basis weight has been advanced, and the hot-dip galvanized steel sheet has come to be used instead of the steel sheet because the workability of the steel sheet is improved and the cost is low. In this case, the inside of the device is often used without being painted, and a hot-dip galvanized steel sheet called a minimum spangle, which has a surface close to that of an electrogalvanized steel sheet and has a spangle that is as fine as possible, or a zero spangle is applied. [0004] As a method for producing a minimum spangle hot-dip galvanized steel sheet, two methods are mainly used. One method is to reduce the amount of zinc Pb in the bath as much as possible. Pb improves the viscosity of molten zinc, improves the wettability of zinc, and greatly contributes to the formation of spangles.
If spangles are required, typically 0.2% or less by weight of Pb is added. On the other hand, in order to make spangles fine, the amount is reduced to 100 ppm or less, preferably 30 ppm or less. [0005] The other is a method in which water or steam is injected when the zinc is in an unsolidified state immediately after the adjustment of the coating weight of the steel sheet that has exited the molten zinc bath, and is rapidly solidified before the spangle grows. Let it. In this case, the minimum spangle can be obtained even if the amount of Pb is somewhat large, but if Pb can be reduced, the production can be performed more stably. When such a minimum spangle hot-dip galvanized steel sheet is manufactured, white cloud-like unevenness having a glossiness and a color tone different from those of a normal part, which is stretched on the surface in a lengthwise manner, is generated. There is. The white unevenness has no effect on the corrosion resistance, paintability, workability, weldability, and the like of the hot-dip galvanized steel sheet, but significantly impairs the appearance of the steel sheet product. With respect to the minimum spangle steel sheet, various studies have been made to improve the adhesion and glossiness of the plating film to the steel sheet, or the disappearance of the spangle. However, little is known about such measures for suppressing white spots. On the other hand, the influence of the non-uniformity of the coating amount of the plating and the non-uniformity of the injection of the coolant such as water was investigated. However, the improvement effect was hardly recognized. In addition, although it was somewhat reduced by changes in coolant injection conditions and changes in the roughness of the roll in temper rolling, it could not be essentially prevented. [0008] An object of the present invention is to provide a manufacturing method for suppressing white unevenness occurring on the surface of a minimum spangle hot-dip galvanized steel sheet. Means for Solving the Problems The present inventors have conducted various investigations on the cause of the occurrence of white unevenness on the surface of a minimum spangle hot-dip galvanized steel sheet. First, since it is preferable to objectively determine the state of occurrence of the white unevenness, an attempt was made to visually determine whether the steel sheet had white unevenness or not by measuring the glossiness and whiteness. Gloss is 60 ° incident angle according to JIS-Z-8741
And the whiteness (L value) was measured according to JIS-Z-8722. In this case, white unevenness is detected as a relative difference between a healthy part and a non-healthy part. [0011] These measurements are made in the width direction of the steel sheet,
When the ratio of the difference between the maximum value and the minimum value to the average value is set as the width of the variation, and the magnitude of the width of the variation is compared with the visual evaluation, the result shown in FIG. 1 is obtained. From Fig. 1, the steel plate judged to be good without white unevenness by visual evaluation should have a variation in glossiness of 20% or less and a variation in whiteness of 10% or less due to the instrument. I understand. When the state of the plating film of a steel sheet having a good surface and a steel sheet having white unevenness is examined by cross-sectional observation and the like, it is found that zinc is found between the steel sheets or between the white uneven part and the healthy part. It was thought that there was a difference in the state near the boundary layer where the plating film adhered to the steel sheet. Therefore, samples were taken from steel sheets having various production opportunities, and the Zn portion of the plating film was dissolved and removed with fuming nitric acid, and an Fe-Al alloy or an Fe-Zn-Al alloy containing a large amount of Al generated at the interface was obtained. (Hereinafter simply referred to as Fe-Al alloy) layer,
The Al alloy layer is dissolved using a hydrochloric acid solution to which a pickling inhibitor is added, and the Al concentration in the Zn portion of the plating film and the F concentration at the interface are reduced.
The amount of Al in the e-Al alloy layer was analyzed. The result of this analysis,
When the relationship with the occurrence of white unevenness on the steel sheet surface was examined, the results shown in FIG. 2 were obtained. [0014] From FIG. 2, it can be seen from FIG.
It can be seen that the higher the 1 concentration, the greater the amount of Al in the Fe-Al alloy layer, and the less white unevenness. Since the Al concentration of the plating film is considered to have a correlation with the Al concentration of the molten zinc in the bath, the result of FIG. 3 was obtained by examining the Al concentration of the molten zinc and the generation ratio of white unevenness. In this case, the occurrence ratio is defined as a steel sheet coil unit, and if it occurs even on a part of the coil surface, it is counted as defective. When performing hot-dip galvanizing, Al in the bath is usually added with about 0.1 to 0.2% by mass of Al.
The small content of Al reacts preferentially with Fe when the steel sheet is immersed in a molten zinc bath to form an Fe-Al alloy layer,
This improves the adhesion of the plating film and makes the hard and brittle Fe
-It is because there is an effect of suppressing the development of the Zn alloy layer and preventing peeling of the plating film when the steel sheet is processed. FIG. 3 shows that the white unevenness can be eliminated by increasing the Al concentration of the molten zinc in the plating bath. When the Al concentration exceeds 0.16 to 0.18%, the occurrence of white unevenness is greatly reduced. If less than this, white unevenness frequently occurs and Al
This indicates that the frequency of occurrence increases as the concentration decreases. Therefore, in order to eliminate white unevenness in the production of the minimum spangle hot-dip galvanized steel sheet, the Al concentration in the hot-dip zinc bath may be increased. However, in many cases, the same galvannealed steel sheet is manufactured using the same plating line or the same hot dip galvanizing bath. In manufacturing an alloyed hot-dip galvanized steel sheet, the Al concentration in the hot-dip zinc must be controlled to 0.08 to 0.14%. This is because if it is less than 0.08%, the powdering resistance of the galvannealed steel sheet is greatly reduced, and if it exceeds 0.14%, alloying of the plating film cannot be sufficiently performed. Since the required Al concentration of the hot-dip zinc in the bath varies depending on the target steel sheet, in order to prevent the white unevenness of the minimum spangled hot-dip galvanized steel sheet, it is necessary to separate it from that for manufacturing the alloyed hot-dip galvanized steel sheet. Therefore, it can be said that it is desirable to manufacture each dedicated line. However, in practice, both steel sheets must be manufactured in one line, and it is necessary to raise or lower the Al concentration of the molten zinc in the bath depending on the steel sheets to be manufactured. However, in such baths the molten zinc Al
It takes a great deal of time to change the concentration, and if a minimum spangled steel sheet is manufactured during the change, if white unevenness is likely to occur, the operation of the plating line is greatly restricted. In a normal hot-dip galvanized steel sheet or a minimum spangled steel sheet, from the viewpoint of ensuring the adhesion of the coating, the Al concentration of the hot-dip zinc in the bath is 0.12 to 0.14%, which is close to the upper limit for the production of a galvannealed steel sheet. It is sufficient if it is at least about. However, in the case of an alloyed hot-dip galvanized steel sheet, if the Al concentration of the hot-dip zinc in the bath exceeds 0.14%, it becomes almost impossible to manufacture. Therefore, if the occurrence of white unevenness of the minimum spangle steel sheet can be suppressed until the Al concentration of the molten zinc is from 0.14% to more than 0.16%, preferably 0.18% or more,
Due to the operation of the plating line, there is no restriction on the steel plate to be manufactured.
It is considered very favorable. Referring to FIG. 2, the Al concentration in the Zn portion of the plating film is in the range of 0.11% to 0.14%,
The Al content of the Al alloy layer is from 150 mg / m 2 to 200 mg / m 2
In the range, a steel plate having white unevenness and a steel plate without unevenness appear, and when the concentration or the content exceeds these ranges, white unevenness does not occur. Since the Al concentration of the Zn portion of the plating film relative to the Al concentration of the molten zinc in the bath is about 80%, the range of the Al concentration of the Zn portion of the plating film in the range of 0.11% to 0.14% is the range of the Al concentration of the molten zinc in the bath. The concentration corresponds to 0.14% to 0.18%. Therefore, even if the Al concentration of the molten zinc in the bath was the same within the above concentration range, further investigation was conducted on the reason why white unevenness was generated or not. As a result, when the Fe-Al alloy layer remaining after removing the plating film is observed with a scanning electron microscope, it appears that the Fe-Al alloy layer covers the iron surface almost uniformly at a healthy portion of the plating surface. On the other hand, it was found that black spot-like or mottled portions were often observed in the Fe-Al alloy layer in the portion where white unevenness occurred. Since a normal Fe-Al alloy layer was not formed in the black portion and it was presumed that Fe appeared, the area ratio of the black portion was measured, and the non-black portion was determined to be Fe.
-Al alloy layer, and its ratio is determined.
FIG. 4 shows the relationship between the Al content and the Al content of the Al alloy layer. That is, the area ratio of the white non-uniform part is reduced by the area ratio of the Fe-Al alloy layer formed at the interface between the plating film and iron.
It has been clarified that the occurrence of a defect exceeding 30% occurs, in other words, if a sufficient Fe-Al alloy layer having an area ratio of 70% or more is formed, white unevenness does not occur. It is presumed that the factor at the time when the steel sheet enters the molten zinc bath greatly affects the Fe—Al layer at the interface to sufficiently cover the surface of the material steel sheet. As a result of investigation of various manufacturing conditions immediately before the bath intrusion, it was found that the temperature immediately before the steel plate intrusion of the molten zinc bath and the dew point of the atmosphere in the snout were strongly affected. It is considered that the steel sheet temperature largely governs the progress of the alloy layer forming reaction on the surface immediately after entering the molten zinc bath, and the dew point is considered to determine the reduction state or activity of the material steel sheet surface. Based on the above findings, as a result of repeating the experiment, a condition control formula for the Al concentration of the molten zinc in the bath, the temperature of the steel sheet and the dew point of the atmosphere for suppressing white unevenness was found. The present invention is a manufacturing method based on this control formula, and its gist is as follows. When the material steel sheet is cooled after the annealing process and enters the molten zinc bath, when the Al concentration of the molten zinc in the bath is A mass%, the temperature T (° C.) of the steel sheet and immediately before entering the molten zinc bath. Wherein the dew point V (° C.) of the atmosphere gas in the snout is in a range satisfying the following expression. (0.0025 × T−0.012 × V) × 0.1 ≦ A The method for producing a minimum spangle hot-dip galvanized steel sheet according to the present invention is particularly applicable to a method of manufacturing a hot-dip galvanized steel sheet having an Al concentration of hot-dip zinc in a plating bath. When the content is between 0.14% by mass and 0.18% by mass, white unevenness is suppressed. That is, in this method,
When the material steel sheet is cooled after the annealing process and enters the zinc molten zinc bath, when the Al concentration in the molten zinc bath is A mass%, the temperature T (° C.) of the steel sheet and the snout immediately before the molten zinc bath enters. The dew point V (° C.) of the atmosphere gas is set to satisfy the following expression. (0.0025 × T−0.012 × V) × 0.1 ≦ A When the Al concentration of the molten zinc in the bath is too low, A in the galvanized film excluding the alloy layer of the galvanized steel sheet.
Since the 1 concentration is lower than 0.11% by mass and the occurrence of white unevenness cannot be suppressed, the concentration is preferably 0.14% by mass or more. In addition, the Al concentration of the molten zinc in the bath was 0.1.
When the content exceeds 18% by mass, white unevenness of the steel sheet does not occur, and the regulation shown in the equation is not particularly required. However, if the regulation is performed according to this formula, when the Al concentration of the molten zinc in the bath is 0.14% by mass or more, it is possible to completely eliminate the white unevenness of the surface of the minimum spangled galvanized steel sheet. From the Al concentration of the molten zinc in the bath, the temperature of the steel sheet and the dew point of the atmospheric gas in the snout are regulated by the formulas in order to lower the temperature at the time of intrusion of the steel sheet and to lower the dew point. This is because white unevenness is effective. However, even within the range regulated by the formula, if the temperature at the time of intrusion of the base steel sheet is too low, non-plating occurs, and if the temperature is too high, it is not preferable that Fe-Z
Since the n-alloy layer develops excessively, the temperature is preferably set in the range of 440 to 500 ° C. Also, if the dew point is too low, the generation of zinc vapor from the molten zinc bath surface in the snout increases, which deposits on the inner wall surface and falls on the surface of the material steel sheet, causing defects in the plating film. Plating occurs. Therefore, it is preferable to control the temperature within the range of −40 to −15 ° C. EXAMPLE A continuous low-temperature galvanizing line was used for a normal low-carbon steel material coil having a thickness of 0.6 mm, a width of 914 mm, and a carbon content of 0.04% by mass. After washing, an air-fuel ratio of 0.95 and a plate temperature of 600 were used. After heating and annealing at 740 ° C. in an atmosphere of 10% hydrogen and remaining nitrogen through a non-oxidizing furnace at ℃, the temperature of the material steel sheet was lowered and the steel sheet was introduced into a molten zinc bath maintained at 460 ± 5 ° C. The coating weight and one side 90 g / m 2 by immediately lifted from the bath gas wiping,
Immediately, it was minimum spangled by mist spray. When performing the hot-dip galvanizing, the molten zinc in the bath is 0.12%, 0.14%, 0.16%, 0.18% and 0.20%.
During the operation, the plating was performed by changing the dew point of the zinc in the atmosphere in the snout within the range of -45 to -15 ° C and changing the temperature of the material steel sheet into the molten zinc bath at various temperatures of 440 to 520 ° C. A steel sheet sample was cut out from each part of the obtained hot-dip galvanized steel sheet coil where each condition was clearly confirmed, and the gloss variation in the width direction of the steel sheet surface was measured according to JIS-Z-8741.
Was measured by a 60 ° specular gloss method, and the L value of whiteness variation was measured according to JIS-Z8722, and white unevenness was evaluated according to the standard shown in FIG. When the Al concentration of the molten zinc bath was 0.14%, 0.16% or 0.18%, the occurrence of white unevenness when the minimum spangled steel sheet was manufactured by changing the steel sheet temperature and dew point when entering the molten zinc bath was changed. These are shown in FIG. 5, FIG. 6 and FIG. The dotted line shown in the figure is based on the equation, and it can be seen that white unevenness can be suppressed if the temperature of the material steel sheet and the dew point in the snout are satisfied. When the Al concentration of the molten zinc in the bath is 0.12%,
In any combination of the atmosphere dew point in the snout and the temperature of the molten steel bath entering the steel sheet, white unevenness occurred. When the Al concentration of the molten zinc in the bath was 0.20%, no white unevenness was observed in any of the steel sheets. The white unevenness on the surface of the minimum spangled hot-dip galvanized steel sheet deteriorates the surface quality of the steel sheet. This white unevenness can be suppressed by increasing the Al concentration of the hot-dip zinc in the plating bath. However, when manufacturing an alloyed hot-dip galvanized steel sheet, it is necessary to lower the Al concentration of the hot-dip zinc in the bath, and changing this Al concentration requires a great deal of time. The production of unavoidable. On the other hand, it is only necessary to manufacture on a different plating line or replace the pot of the molten zinc bath on the same line, but it requires a large-scale capital investment,
Not easily implemented. According to the production method of the present invention, the generation of white unevenness can be suppressed even in a low concentration range for lowering the Al concentration in the bath to a concentration suitable for an alloyed hot-dip galvanized steel sheet. , Both the minimum spangle hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet having good surfaces can be easily produced on the same plating line.
【図面の簡単な説明】
【図１】ミニマムスパングル溶融亜鉛めっき鋼板表面の
白色むらについて、計測器による光沢度ばらつきおよび
白色度ばらつきと、目視観察による評価との関係を示す
図である。
【図２】白色むら発生と、鋼板の亜鉛めっき皮膜のＡｌ
濃度およびめっき皮膜−素材鋼板間の界面に形成された
Ｆｅ−Ａｌ合金層のＡｌ量との関係を示す図である。
【図３】溶融亜鉛の溶融亜鉛浴中のＡｌ濃度と、白色む
らの発生比率との関係を示す図である。
【図４】健全部分および白色むら部分と、Ｆｅ−Ａｌ合
金層のＡｌ含有量およびその合金層の面積率との関係を
示す図である。
【図５】溶融亜鉛浴のＡｌ濃度が0.14％のときの、溶融
亜鉛浴に侵入する素材鋼板の温度およびスナウト内雰囲
気の露点と、白色むら発生との関係を示す図である。
【図６】溶融亜鉛浴のＡｌ濃度が0.16％のときの、溶融
亜鉛浴に侵入する素材鋼板の温度およびスナウト内雰囲
気の露点と、白色むら発生との関係を示す図である。
【図７】溶融亜鉛浴のＡｌ濃度が0.18％のときの、溶融
亜鉛浴に侵入する素材鋼板の温度およびスナウト内雰囲
気の露点と、白色むら発生との関係を示す図である。BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a relationship between gloss variation and whiteness variation by a measuring instrument and evaluation by visual observation regarding white unevenness on the surface of a minimum spangle hot-dip galvanized steel sheet. FIG. 2 Generation of white unevenness and Al of galvanized film on steel plate
It is a figure which shows the relationship between the density | concentration and the amount of Al of the Fe-Al alloy layer formed in the interface between a plating film and a material steel plate. FIG. 3 is a graph showing the relationship between the Al concentration of molten zinc in a molten zinc bath and the occurrence ratio of white unevenness. FIG. 4 is a diagram showing a relationship between a healthy portion and an uneven white portion, an Al content of an Fe—Al alloy layer, and an area ratio of the alloy layer. FIG. 5 is a graph showing the relationship between the temperature of a steel sheet entering a molten zinc bath, the dew point of an atmosphere in a snout, and the occurrence of white unevenness when the molten zinc bath has an Al concentration of 0.14%. FIG. 6 is a graph showing the relationship between the temperature of a steel sheet entering a molten zinc bath, the dew point of an atmosphere in a snout, and the occurrence of white unevenness when the Al concentration of the molten zinc bath is 0.16%. FIG. 7 is a graph showing the relationship between the temperature of a steel sheet entering a molten zinc bath, the dew point of an atmosphere in a snout, and the occurrence of white unevenness when the molten zinc bath has an Al concentration of 0.18%.
───────────────────────────────────────────────────── フロントページの続き Ｆターム(参考） 4K027 AA02 AA22 AB09 AB42 AB44 AC62 AD10 AE03 AE12 AE25 AE34 ────────────────────────────────────────────────── ─── Continuation of front page F term (reference) 4K027 AA02 AA22 AB09 AB42 AB44 AC62 AD10 AE03 AE12 AE25 AE34
Claims (1)
亜鉛浴に侵入する際、浴中溶融亜鉛のＡｌ濃度がＡ質量
％であるとき、鋼板の温度Ｔ（℃）および溶融亜鉛浴侵
入直前のスナウト内雰囲気ガスの露点Ｖ（℃）が、下記
式を満足する範囲とすることを特徴とするミニマムス
パングル溶融亜鉛めっき鋼板の製造方法。 （0.0025×Ｔ−0.012×Ｖ）×0.1≦Ａ Claims: 1. When a steel sheet is cooled after entering an annealing process and enters a molten zinc bath, when the Al concentration of the molten zinc in the bath is A mass%, the temperature of the steel sheet is T (° C.). ) And the dew point V (° C.) of the atmospheric gas in the snout immediately before entering the hot dip zinc bath is in a range satisfying the following expression. (0.0025 × T−0.012 × V) × 0.1 ≦ A
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2001357910A JP3636132B2 (en) | 2001-11-22 | 2001-11-22 | Method for producing hot-dip galvanized steel sheet |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2001357910A JP3636132B2 (en) | 2001-11-22 | 2001-11-22 | Method for producing hot-dip galvanized steel sheet |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JP2003160850A true JP2003160850A (en) | 2003-06-06 |
| JP3636132B2 JP3636132B2 (en) | 2005-04-06 |
Family
ID=19169181
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP2001357910A Expired - Fee Related JP3636132B2 (en) | 2001-11-22 | 2001-11-22 | Method for producing hot-dip galvanized steel sheet |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP4828544B2 (en) * | 2004-10-20 | 2011-11-30 | アルセロールミタル・フランス | Method for hot dip plating of iron-carbon-manganese steel strip in a zinc bath |
| CN114250430A (en) * | 2020-09-21 | 2022-03-29 | 宝山钢铁股份有限公司 | Furnace nose inner atmosphere temperature control method and heating device beneficial to zinc ash inhibition |
-
2001
- 2001-11-22 JP JP2001357910A patent/JP3636132B2/en not_active Expired - Fee Related
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP4828544B2 (en) * | 2004-10-20 | 2011-11-30 | アルセロールミタル・フランス | Method for hot dip plating of iron-carbon-manganese steel strip in a zinc bath |
| CN114250430A (en) * | 2020-09-21 | 2022-03-29 | 宝山钢铁股份有限公司 | Furnace nose inner atmosphere temperature control method and heating device beneficial to zinc ash inhibition |
| CN114250430B (en) * | 2020-09-21 | 2024-01-09 | 宝山钢铁股份有限公司 | Furnace nose internal atmosphere temperature control method and heating device beneficial to inhibiting zinc ash |
Also Published As
| Publication number | Publication date |
|---|---|
| JP3636132B2 (en) | 2005-04-06 |
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