JP2004339553A

JP2004339553A - Method of producing hot dip metal plated steel strip

Info

Publication number: JP2004339553A
Application number: JP2003135533A
Authority: JP
Inventors: Nobuyuki Ishida; 信之石田; Norio Inoue; 紀夫井上; Tadashi Nara; 正奈良
Original assignee: JFE Steel Corp; NKK Steel Sheet and Strip Corp
Current assignee: JFE Steel Corp; NKK Steel Sheet and Strip Corp
Priority date: 2003-05-14
Filing date: 2003-05-14
Publication date: 2004-12-02

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of producing a hot dip metal plated steel strip which has excellent surface appearance by reducing the variation in the temperature of a steel strip dipped into a plating bath. <P>SOLUTION: As for the method of producing a hot dip metal plated steel strip, in continuous hot dip plating equipment provided with a heating furnace and a cooling furnace, the temperature of a steel strip subjected to annealing treatment is controlled to the one suitable for dipping into a plating bath, thereafter, the steel strip is dipped into a hot dip metal plating bath so as to be passed through and is subjected to hot dip metal plating, and successively, it is taken up from the hot dip metal plating bath to form a hot dip metal coating, the steel strip subjected to the annealing treatment is temporarily cooled to a temperature lower than that suitable for dipping into the plating bath, and is thereafter heated to a temperature suitable for dipping into the plating bath by induction heating. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は溶融金属めっき鋼帯の製造方法、特に、表面外観に優れる合金化溶融亜鉛めっき鋼帯の製造に好適な溶融金属めっき鋼帯の製造方法に関する。
【０００２】
【従来の技術】
連続溶融めっき設備で、溶融金属めっき鋼帯を製造する方法について、合金化溶融亜鉛めっき鋼帯を製造する場合を例に挙げて説明する。合金化溶融亜鉛めっき鋼帯は、例えば図８に示すような加熱炉、冷却炉を備えた連続溶融めっき設備を用いて製造される。
【０００３】
図８において、１は予熱炉、２は加熱炉で、直火加熱炉（ＤＦ）３と輻射管加熱炉（ＲＴ）４を備え、５は冷却炉で、ガスジェット冷却炉（ＪＣ）６と調整冷却炉７を備え、８はスナウト、９は溶融めっき槽、１０は溶融亜鉛めっき浴、１１はガスワイピングノズル、１２は合金化炉、１３は冷却装置である。ガスジェト冷却炉６は、低温の雰囲気ガスを鋼帯に直接吹き付けることで鋼帯を冷却する。調整冷却炉７は、空冷管と電気ヒータ（輻射式）を備える。
【０００４】
直火加熱炉３出側、輻射管加熱炉４出側、ガスジェット冷却炉６出側、スナウト８に、前記各部における鋼帯温度を検出する温度計１５、１６、１７、１８が設置されている。
【０００５】
図８の設備を用いて合金化溶融亜鉛めっき鋼板は次のようにして製造される。鋼帯Ｓは、予熱炉１で予熱され、直火加熱炉３で、温度計１５で検出する鋼帯温度Ｔ１が所定温度Ｔ１ａになるように制御され、輻射管加熱炉４で、温度計１６で検出する鋼帯温度Ｔ２が所定焼鈍温度Ｔ２ａになるように制御される。
【０００６】
次に、鋼帯Ｓは、スナウト８の温度計１８で検出される鋼帯温度Ｔ４がめっき浴に浸漬されるのに適した所定温度Ｔ４ａになるように、冷却炉５で冷却される。前記所定温度Ｔ４ａは、めっき品質に大きな影響を与える因子であり、通常、溶融亜鉛めっき浴温度に近似した温度に調整される。冷却炉５はガスジェット冷却炉６と調整冷却炉７を備えるが、調整冷却炉７は、加熱手段と冷却手段を備えるがその能力は小さく、鋼帯はほぼ一定温度に保熱される程度であり、または冷却されたとしてもその温度低下は少ない。そのため、冷却炉５における鋼帯温度は、実質的にガスジェット冷却炉６で制御され、ガスジェット冷却炉６出側の鋼帯温度Ｔ３の目標値Ｔ３ａは、スナウト８の温度計１８で検出する鋼帯温度が所定温度Ｔ４ａになるように、鋼帯サイズ、通板速度等に基き、調整冷却炉７及びスナウト８部分における鋼帯の温度低下量を考慮して、前記所定温度Ｔ４ａより高温に設定される。
【０００７】
冷却炉５で冷却された鋼帯Ｓは、スナウト８を経て溶融亜鉛めっき浴１０に浸漬通板されて亜鉛めっきされる。次いで溶融亜鉛めっき浴１０から引き上げられ、ガスワイピングノズル１１でめっき付着量が制御された後、合金化炉１２で加熱されてめっき皮膜が合金化処理され、冷却装置１３で冷却されて合金化溶融亜鉛めっき鋼帯が製造される。
【０００８】
図９は、前記設備で合金化溶融亜鉛めっき鋼帯を製造する場合に、鋼帯Ｓが溶融亜鉛めっき浴１０に浸漬される前のヒートサイクルを説明する概念図である。図９中、Ｔ１は直火加熱温度（直火加熱炉（ＤＦ）３出側鋼帯温度）、Ｔ２は焼鈍温度（輻射管加熱炉（ＲＴ）４出側鋼帯温度）、Ｔ３は冷却温度（ジェット冷却炉（ＪＣ）６出側鋼帯温度）、Ｔ４はスナウト８部鋼帯温度である。図９では、鋼帯温度Ｔ３は鋼帯温度Ｔ４より高温である。
【０００９】
冷却炉５には、該炉体を構成する耐火物や該炉内に装備される搬送ロール等の付帯物がある。これらの熱慣性が大きいため、冷却炉５における鋼帯温度制御の応答性は劣る。そのため、ジェット冷却炉６出側の鋼帯温度Ｔ３の変動が大きく、その結果、冷却炉５で冷却後の鋼帯温度変動（スナウト８部の鋼帯温度Ｔ４の変動）が大きくなり、従って溶融亜鉛めっき浴１０に浸漬される鋼帯温度変動が大きくなる。この温度変動は、鋼帯サイズや通板速度が変化したときに一層顕著になる。前記温度変動が大きいと以下の問題が発生しやすくなる。
【００１０】
溶融亜鉛めっき浴１０に浸漬される鋼帯温度が溶融亜鉛めっき浴温度に対して高くなりすぎると、鋼帯幅方向中央部にフリージンク（合金化不足に起因する欠陥）が発生する。また、溶融亜鉛めっき浴１０に浸漬される鋼帯温度が溶融亜鉛めっき浴温度に対して低くなりすぎると、鋼帯端縁部にフリージンクが発生するようになる。浸漬される鋼帯温度が溶融亜鉛めっき浴温度に対して低くなりすぎることに起因して発生するフリージンクは、該鋼帯温度が高すぎることに起因して発生するフリージンクよりもより顕著であり、品質影響がより大きい。
【００１１】
また、鋼帯Ｓが溶融亜鉛めっき浴１０に浸漬される際、浸漬される鋼帯の温度変動が大きくなると、溶融めっき槽９に生成される浮遊ドロスに影響を来たし、鋼帯Ｓに付着してドロス欠陥が発生し易くなる。
【００１２】
前記問題の発生を軽減、防止するには、溶融亜鉛めっき浴１０に浸漬される鋼帯温度変動を小さくすることが必要である。
【００１３】
特許文献１には、焼鈍熱処理後の鋼帯を溶融亜鉛浴に連続して浸漬通板させて亜鉛めっきするに際し、スナウト部内に設けられた誘導加熱装置によって鋼帯を加熱し、溶融亜鉛浴に侵入する直前の鋼帯温度を微調整することで、溶融亜鉛めっき浴へ侵入する鋼帯板温を調整する方法が提案されている。
【００１４】
以下に先行技術文献情報について記載する。
【００１５】
【特許文献１】
特開平４−３２９８５６号公報
【００１６】
【発明が解決しようとする課題】
しかし、特許文献１の方法は、スナウト部に搬送される鋼帯温度変動が大きい場合、例えば搬送される鋼帯サイズや通板速度変更時等でスナウト部に搬送される鋼帯温度変動が大きくなると、該鋼帯温度が所定温度より高めに変動するようになるため、めっき浴に浸漬される鋼帯温度変動を確実に低減できない。またスナウト内のめっき浴面から蒸発した亜鉛蒸気が誘導加熱装置内で凝縮して誘導加熱コイルが短絡しやすく、操業安定性に問題がある。
【００１７】
本発明は、前記事情を考慮し、めっき浴に浸漬される鋼帯温度変動を小さくすることで、例えば合金化溶融亜鉛めっき鋼帯を製造する際に起こるフリージンクやドロス付着の問題を軽減できる、表面外観に優れる溶融金属めっき鋼帯の製造方法を提供することを目的とする。
【００１８】
また、本発明は、さらに、操業安定性に優れる溶融金属めっき鋼帯の製造方法を提供することを目的とする。
【００１９】
【課題を解決するための手段】
上記課題を解決するための本発明の手段は以下の通りである。
【００２０】
（１）加熱炉、冷却炉を備えた連続溶融めっき設備において、焼鈍処理した鋼帯をめっき浴に浸漬させるのに適した温度に調節した後溶融金属めっき浴に浸漬通板させて溶融金属めっきを行い、引き続き前記溶融金属めっき浴から引き上げて溶融金属被覆を形成する溶融金属めっき鋼帯の製造方法において、焼鈍処理した鋼帯を、一旦前記めっき浴に浸漬させるのに適した温度よりも低い温度に冷却した後、誘導加熱で前記めっき浴に浸漬させるのに適した温度に加熱することを特徴とする溶融金属めっき鋼帯の製造方法。
【００２１】
（２）冷却炉後部に誘導加熱装置を設けて誘導加熱することを特徴とする前記（１）に記載の溶融金属めっき鋼帯の製造方法。
【００２２】
（３）溶融金属は、溶融亜鉛であることを特徴とする前記（１）または（２）に記載の溶融金属めっき鋼帯の製造方法。
【００２３】
（４）前記（３）に記載の溶融金属めっき鋼帯の製造方法において、前記溶融金属めっき浴から引き上げて溶融金属被覆を形成した後、引き続き加熱して合金化処理を行うことを特徴とする溶融金属めっき鋼帯の製造方法。
【００２４】
【発明の実施の形態】
図１は、本発明の実施の形態の説明に参照する連続溶融めっき設備の要部を示す概略図である。図１において、説明済みの図８に示された部分と同じ作用の部分には同じ符号を付してその説明を省略する。図１では、図８に示した設備に対して、誘導加熱装置１４及び制御装置１９が追加されている。誘導加熱装置１４は、冷却炉５出側（調整冷却炉７出側）に設置されている。
【００２５】
以下、本発明の実施の形態について、図１の設備を用いて合金化溶融亜鉛めっき鋼帯を製造する場合を例に挙げて説明する。
【００２６】
図２は、図１の設備を用いて合金化溶融亜鉛めっき鋼帯を製造する場合に、鋼帯が溶融亜鉛めっき浴に浸漬される前のヒートサイクルを説明する概念図である。図２中、Ｔ１は直火加熱温度（直火加熱炉（ＤＦ）３出側鋼帯温度）、Ｔ２は焼鈍温度（輻射管加熱炉（ＲＴ）４出側鋼帯温度）、Ｔ３は冷却温度（ガスジェット冷却炉（ＪＣ）６出側鋼帯温度）、Ｔ４はスナウト８部鋼帯温度である。図２では、鋼帯温度Ｔ３は鋼帯温度Ｔ４より低温である。
【００２７】
鋼帯Ｓは、直火加熱炉（ＤＦ）３で、直火加熱炉３出側の温度計１５で検出する鋼帯温度Ｔ１が所定直火加熱温度Ｔ１ａになるように制御され、輻射管加熱炉（ＲＴ）４で、輻射管加熱炉４出側の温度計１６で検出する鋼帯温度Ｔ２が所定焼鈍温度Ｔ２ａになるように制御される。鋼帯温度Ｔ１ａ、Ｔ２ａは、鋼帯Ｓの鋼成分、材質規格、鋼帯寸法、鋼帯通板速度等に基いて定められる。
【００２８】
次に、鋼帯Ｓはガスジェット冷却炉（ＪＣ）６で、ガスジェット冷却炉６出側の温度計１７で検出する鋼帯温度Ｔ３が所定温度Ｔ３ｂになるように冷却制御される。前記鋼帯温度Ｔ３ｂは、スナウト８における鋼帯温度Ｔ４ａより低い温度に設定される。調整冷却炉７では、鋼帯温度はほぼ一定に保持するように制御され、または温度低下があってもその程度は少ないように制御される。
【００２９】
ガスジェット冷却炉６出側の鋼帯温度Ｔ３の設定温度Ｔ３ｂを低くしすぎると、電力コストの上昇を招く。係る観点から、ガスジェット冷却炉６出側の鋼帯温度の設定温度Ｔ３ｂとスナウト８の鋼帯温度Ｔ４ａの差ΔＴ（＝Ｔ４ａ−Ｔ３ｂ）は３０℃以下とすることが好ましい。
【００３０】
誘導加熱装置１４を用いて、スナウト８の温度計１８で検出する鋼帯温度Ｔ４が所定温度Ｔ４ａとなるように加熱制御される。鋼帯温度Ｔ４ａは、良好なめっき品質を得る観点から、めっき浴に浸漬される鋼帯温度がめっき浴温度とほぼ同程度の温度になるように、スナウト温度検出部からめっき浴に浸漬されるまでの間の鋼帯温度低下量を考慮して設定される。亜鉛めっきの場合、めっき浴温度は４５０〜４８０℃程度に設定され、鋼帯温度はＴ４ａ、通常、めっき浴温度に対して＋２０〜３０℃程度である。
【００３１】
誘導加熱装置１４で所定温度Ｔ４ａに加熱された鋼帯Ｓはスナウト８を経て溶融亜鉛めっき浴１０に浸漬通板させて亜鉛めっきされる。次いで鋼帯は前記溶融亜鉛めっき浴１０から引き上げられ、ガスワイピングノズル１１でめっき付着量が制御された後、合金化炉１２で加熱されてめっき層が合金化処理され、冷却装置１３で冷却されて合金化溶融亜鉛めっき鋼帯が製造される。
【００３２】
本発明では、冷却炉５の鋼帯温度Ｔ３を、一旦スナウト８部の鋼帯温度Ｔ４ａよりも低温に調整した後、誘導加熱装置１４を用いてスナウト８部の鋼帯温度Ｔ４を所要温度Ｔ４ａに再加熱する。誘導加熱装置１４は温度制御の応答性に優れるので、前記鋼帯温度Ｔ３が変動していても、誘導加熱装置１４で再加熱後の鋼帯温度Ｔ４は所要温度Ｔ４ａに精度よく制御される。その結果、溶融亜鉛めっき浴１０に浸漬される鋼帯の温度変動が低減され、ドロス付着が低減され、また合金化処理時のフリージンクの発生が防止されることで、表面外観に優れた合金化溶融亜鉛めっき鋼帯が得られる。
【００３３】
連続溶融めっき設備では、種々のサイズの鋼帯が接続されて連続的に通板される。鋼帯同士が接続される鋼帯接続部前後で、冷却炉５において鋼帯を冷却するための熱負荷が急激に変わる。そのため、従来技術では、該接続部近傍部分において、鋼帯温度Ｔ３の変動が特に大きくなり、これに対応して鋼帯温度Ｔ４の変動が大きくなることが不可避であった。誘導加熱装置１４は、温度制御の応答性に優れるので、本発明では、鋼帯接続部近傍部分においても、鋼帯温度Ｔ４の変動を低減できることで、溶融亜鉛めっき浴１０に浸漬される鋼帯の温度変動を、従来技術に比べて顕著に低減できる。
【００３４】
また、接続部通過後冷却のための熱負荷が増加、例えば鋼帯厚が薄物から厚物に変更される場合、後行鋼帯の接続部近傍部分の鋼帯温度Ｔ３は上昇し、一方、前記熱負荷が減少、例えば鋼帯厚が厚物から薄物に変更される場合、後行鋼帯の接続部近傍部分の鋼帯温度Ｔ３は低下する傾向がある。従って、前記鋼帯温度Ｔ３の変動傾向を考慮し、接続部近傍部分について、予め設定した鋼帯温度Ｔ３の設定値Ｔ３ｂを変更することで、接続部近傍部分の鋼帯温度Ｔ４の温度変動をさらに低減することができる。例えば、鋼帯が薄物から厚物に移行するときは、先行鋼帯と後行鋼帯の接合部近傍部分について、ガスジェット冷却炉６出側の鋼帯温度Ｔ３の設定値を予め設定されている設定値Ｔ３ｂより低めに変更して冷却することで、接合部近傍部分におけるスナウト８での鋼帯温度Ｔ４の変動をより軽減できる。また、厚物から薄物に移行するときは、先行鋼帯と後行鋼帯の接合部近傍部分について、ガスジェット冷却炉６出側の鋼帯温度Ｔ３の設定値を予め設定されている設定値Ｔ３ｂより高めに変更して冷却するとよい。
【００３５】
図１に示した設備では、調整冷却炉７通過後、誘導加熱装置１４入側部における鋼帯温度を検出する温度計は設置されていない。該部分に温度計を設けてもよい。この場合、該温度計で検出する鋼帯温度が前記温度Ｔ３ｂになるように冷却炉５における温度制御を行っても良い。
【００３６】
溶融亜鉛めっきを行うと、スナウト８内では、溶融亜鉛めっき浴１０表面から亜鉛が蒸発する。蒸発した亜鉛が誘導加熱装置１４内で凝縮すると、該誘導加熱装置１４の誘導加熱コイルが短絡するおそれがある。
【００３７】
蒸発した亜鉛蒸気の多くはスナウト８内で凝縮する。図１に示した設備では、誘導加熱装置１４はスナウト８上流（鋼帯Ｓ通板方向に対して上流側）の冷却炉５出側（調整冷却炉７出側）に設けられているので、誘導加熱装置１４で亜鉛蒸気が凝縮することによる誘導加熱コイル短絡の問題は大幅に緩和されている。
【００３８】
誘導加熱装置１４内での亜鉛蒸気の凝縮の問題を解消する観点からは、溶融亜鉛めっき浴面から蒸発した亜鉛蒸気を誘導加熱装置１４側に流れないようにすることがより好ましい。
【００３９】
係る観点からは、めっき浴面と誘導加熱装置１４の間のスナウト８部分に排気口を設けて、溶融金属浴面から蒸発した亜鉛蒸気を該排気口からスナウト外に排出すること、また炉内を還元性雰囲気に保持するために供給される炉内ガス（Ｈ_２−Ｎ_２ガス）をスナウト内に設けられた排気口より上流側の炉内部分、より好ましくは冷却炉及びそれより上流側の炉内部分に供給することが好ましい。これによってめっき浴面から蒸発した亜鉛蒸気が誘導加熱装置１４側に流れなくなり、誘導加熱装置内で亜鉛蒸気が凝縮することによる誘導加熱コイル短絡の問題が解消される。
【００４０】
図３は、図１に示した連続溶融亜鉛めっき設備において、スナウト８下部のめっき浴面に近接した側の鋼帯表面に対面するスナウト壁面にめっき浴面から蒸発した亜鉛蒸気を排出するための排気口を設けて、めっき浴面から蒸発した亜鉛蒸気をスナウト外に速やかに排出させるようにしたものである。図３において、２１は排気口、２２は排気口に接続して立設された排気管、２３は弁である。図３には、温度計１８は記載されていない。また、炉内を還元性雰囲気に保持するために供給されるＨ_２−Ｎ_２ガスは、スナウト８内には供給されず、調整冷却炉７及びその上流の炉内部分に供給されている。
【００４１】
図１の設備のスナウト８に図３に示した亜鉛蒸気排気手段を設けた場合、次のようにして亜鉛蒸気がスナウト外に排出される。調整冷却炉７及びその上流の炉内部分に供給されたＨ_２−Ｎ_２ガスは、上流側（鋼帯通板方向と逆方向）に流れ、直火加熱炉３に流出する。また直火加熱炉３では直火バーナから鋼帯Ｓを加熱するために大量の燃料ガスと空気が送気されるため、直火加熱炉３〜誘導加熱装置１４が配置された炉内の各部及びスナウト８内の雰囲気はプラス圧に保持されている。さらに、立設された排気管２２のドラフト作用もあることから、弁２３を適度に開くことで、スナウト８内のめっき浴面から蒸発した亜鉛蒸気を含む雰囲気ガスは排気口２１から排気管２２を経てスナウト８外すなわち炉外に排気され、外気（大気）がスナウト８内に流入することもない。亜鉛蒸気が排気口２１からスナウト８外に排出されるので、誘導加熱装置１４での亜鉛蒸気凝縮の問題は解消される。また外気侵入に起因してめっき性が不良になることもない。
【００４２】
【実施例】
（実施例１）
合金化溶融亜鉛めっき鋼帯を製造時において、鋼帯サイズが薄物から厚物に変更されたときの接合部（溶接部）近傍部分における鋼帯温度の変動状況及びフリージンクの発生状況を示す。図４は本発明法による例、図５は従来法による例であり、いずれも鋼帯サイズが０．６ｍｍ×１２１９ｍｍ（先行鋼帯）から０．８ｍｍ×１２１９ｍｍ（後行鋼帯）に変更された場合である。
【００４３】
本発明法では、図１の設備を用い、図２に示されるヒートサイクルのように、ガスジェット冷却炉６の鋼帯温度Ｔ３の目標値Ｔ３ｂをスナウト８部における鋼帯温度Ｔ４の目標値Ｔ４ａ（＝４８０℃）より低温にして冷却した後、調整冷却炉７ではガスジェット冷却炉６出側鋼帯温度をほぼ保持するように炉温を設定し、誘導加熱装置１４でスナウト８部の鋼帯温度Ｔ４が前記所定温度Ｔ４ａになるように制御した。
【００４４】
従来法では、図８の設備を用い、図９に示されるヒートサイクルのように、ガスジェット冷却炉６の鋼帯温度Ｔ３の設定値Ｔ３ａを、スナウト８部の鋼帯温度Ｔ４が前記所要温度Ｔ４ａになるように、鋼帯温度Ｔ４ａより高めに設定して冷却制御した。なお、何れもめっき浴温度は４６０℃である。
【００４５】
従来法（図５）では、後行鋼帯の溶接部近傍部分で鋼帯温度Ｔ４の目標温度Ｔ４ａに対する偏差が大きくなり、図５中、ハッチングで示される領域でフリージンクが発生した。一方、本発明法（図４）では、後行鋼帯の溶接部近傍部分の鋼帯温度Ｔ４の目標温度Ｔ４ａに対する偏差は従来法に比べて小さく、フリージンクは発生しなかった。
【００４６】
（実施例２）
合金化溶融亜鉛めっき鋼帯を製造時において、鋼帯サイズが厚物から薄物に変更されたときの接合部（溶接部）近傍部分における鋼帯温度の変動状況及びフリージンクの発生状況を示す。図６は本発明法による例、図７は従来法による例であり、いずれも鋼帯サイズが１．２ｍｍ×９１４ｍｍ（先行鋼帯）から０．８ｍｍ×９１４ｍｍ（後行鋼帯）に変更された場合である。本発明法、従来法とも、実施例１同様に温度制御され、めっき浴温度も実施例１と同様である。
【００４７】
従来法（図７）では、後行鋼帯の溶接部近傍部分で鋼帯温度Ｔ４は目標温度Ｔ４ａに対して低下し、その偏差が大きかったことで、図７中、ハッチングで示される領域で鋼帯温度低下に起因するフリージンクが発生した。一方、本発明法（図６）では、後行鋼帯の溶接部近傍部分（図６中、ハッチングで示される領域）で不可避的に発生しやすいフリージンクが発生したが、従来法に比べて、その長さは短く、程度も軽かった。また、後行鋼帯の溶接部近傍部分の鋼帯温度Ｔ４は目標温度Ｔ４ａに対して低下したがその偏差は従来法に比べて小さかったため、鋼帯温度低下に起因するフリージンクは発生しなかった。
【００４８】
【発明の効果】
本発明によれば、めっき浴に浸漬される鋼帯温度の変動が低減され、めっき浴に浸漬されるのに適した温度に制御できるので、表面外観に優れる溶融金属めっき鋼帯が得られる。本発明は、フリージンクの発生やドロス付着の発生を防止し、表面外観に優れる合金化溶融亜鉛めっき鋼帯を製造する方法として好適である。
【００４９】
また、本発明では、誘導加熱コイル短絡の問題が解消され、操業安定性にも優れる。
【図面の簡単な説明】
【図１】本発明に係る合金化溶融亜鉛めっき鋼帯の製造方法の実施の形態の説明に参照した連続溶融めっき設備の要部を示す概略図。
【図２】本発明法で製造される合金化溶融亜鉛めっき鋼帯の溶融亜鉛めっき浴に浸漬される前の鋼帯のヒートサイクルを説明する概念図。
【図３】図１の連続溶融亜鉛めっき設備において、溶融亜鉛めっき浴面から蒸発した亜鉛蒸気をスナウト外に排出するための排気口をスナウト下部に設けた場合のスナウト近傍の要部配置を示す図。
【図４】本発明法において、鋼帯寸法が０．６ｍｍ×１２１９ｍｍ→０．８ｍｍ×１２１９ｍｍに変更されたときの溶接部近傍部分における鋼帯温度Ｔ３及びＴ４の変動状態を示す図。
【図５】従来法において、鋼帯寸法が０．６ｍｍ×１２１９ｍｍ→０．８ｍｍ×１２１９ｍｍに変更されたときの溶接部近傍部分における鋼帯温度Ｔ３及びＴ４の変動状態を示す図。
【図６】本発明法において、１．２ｍｍ×９１４ｍｍ→０．８ｍｍ×９１４ｍｍに変更されたときの溶接部近傍部分における鋼帯温度Ｔ３及びＴ４の変動状態を示す図。
【図７】従来法において、１．２ｍｍ×９１４ｍｍ→０．８ｍｍ×９１４ｍｍに変更されたときの溶接部近傍部分における鋼帯温度Ｔ３及びＴ４の変動状態を示す図。
【図８】従来技術の合金化溶融亜鉛めっき鋼帯の製造方法の説明に参照した連続溶融めっき設備の要部を示す概略図。
【図９】従来技術の合金化溶融亜鉛めっき鋼帯の製造方法において、溶融亜鉛めっき浴に浸漬される前の鋼帯のヒートサイクルを説明する概念図。
【符号の説明】
Ｓ鋼帯
１予熱炉
２加熱炉
３直火加熱炉（ＤＦ）
４輻射管加熱炉（ＲＴ）
５冷却炉
６ガスジェット冷却炉（ＪＣ）
７調整冷却炉
８スナウト
９溶融めっき槽
１０溶融亜鉛めっき浴
１１ガスワイピングノズル
１２合金化炉
１３冷却装置
１４誘導加熱装置
１５〜１８温度計
１９制御装置
２１排気口
２２排気管
２３弁[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a hot-dip galvanized steel strip, and more particularly to a method for producing a hot-dip galvanized steel strip suitable for producing an alloyed hot-dip galvanized steel strip having excellent surface appearance.
[0002]
[Prior art]
A method for producing a hot-dip galvanized steel strip in a continuous hot-dip galvanizing facility will be described with reference to an example in which a galvannealed steel strip is produced. The alloyed hot-dip galvanized steel strip is manufactured using, for example, a continuous hot-dip galvanizing facility provided with a heating furnace and a cooling furnace as shown in FIG.
[0003]
8, reference numeral 1 denotes a preheating furnace, 2 denotes a heating furnace, which includes an open flame heating furnace (DF) 3 and a radiation tube heating furnace (RT) 4. Reference numeral 5 denotes a cooling furnace, and a gas jet cooling furnace (JC) 6 An adjustment cooling furnace 7 is provided, 8 is a snout, 9 is a hot-dip galvanizing bath, 10 is a hot-dip galvanizing bath, 11 is a gas wiping nozzle, 12 is an alloying furnace, and 13 is a cooling device. The gas jet cooling furnace 6 cools the steel strip by directly blowing a low-temperature atmosphere gas onto the steel strip. The conditioning cooling furnace 7 includes an air cooling tube and an electric heater (radiation type).
[0004]
Thermometers 15, 16, 17, and 18 for detecting the temperature of the steel strip in each of the above-mentioned parts are installed at the outlet of the direct fire heating furnace 3, the outlet of the radiation tube heating furnace 4, the outlet of the gas jet cooling furnace 6, and the snout 8. I have.
[0005]
An alloyed hot-dip galvanized steel sheet is manufactured using the equipment of FIG. 8 as follows. The steel strip S is preheated in the preheating furnace 1 and is controlled in the direct heating furnace 3 so that the steel strip temperature T1 detected by the thermometer 15 becomes the predetermined temperature T1a. Is controlled so that the steel strip temperature T2 detected by the above becomes the predetermined annealing temperature T2a.
[0006]
Next, the steel strip S is cooled in the cooling furnace 5 so that the steel strip temperature T4 detected by the thermometer 18 of the snout 8 becomes a predetermined temperature T4a suitable for being immersed in the plating bath. The predetermined temperature T4a is a factor that greatly affects the plating quality, and is usually adjusted to a temperature close to the hot-dip galvanizing bath temperature. The cooling furnace 5 includes a gas jet cooling furnace 6 and a conditioning cooling furnace 7, and the conditioning cooling furnace 7 includes heating means and cooling means but has a small capacity, and the steel strip is kept at a substantially constant temperature. Or, even if cooled, the temperature drop is small. Therefore, the steel strip temperature in the cooling furnace 5 is substantially controlled by the gas jet cooling furnace 6, and the target value T3a of the steel strip temperature T3 on the exit side of the gas jet cooling furnace 6 is detected by the thermometer 18 of the snout 8. The steel strip temperature is set to be higher than the predetermined temperature T4a so that the steel strip temperature becomes the predetermined temperature T4a, based on the steel strip size, the passing speed, etc., and taking into account the temperature reduction amount of the steel strip in the adjusted cooling furnace 7 and the snout 8 portion. Is set.
[0007]
The steel strip S cooled in the cooling furnace 5 is immersed in a hot-dip galvanizing bath 10 through a snout 8 and galvanized. Next, after being pulled up from the hot-dip galvanizing bath 10 and controlling the amount of coating by a gas wiping nozzle 11, the coating film is heated by an alloying furnace 12 to be alloyed, and cooled by a cooling device 13 to be alloyed and melted. Galvanized steel strip is manufactured.
[0008]
FIG. 9 is a conceptual diagram for explaining a heat cycle before the steel strip S is immersed in the hot-dip galvanizing bath 10 when the alloyed hot-dip galvanized steel strip is manufactured by the above-mentioned equipment. In FIG. 9, T1 is the direct heating temperature (the temperature of the steel strip on the exit side of the direct heating furnace (DF) 3), T2 is the annealing temperature (the temperature of the steel strip on the exit side of the radiation tube heating furnace (RT) 4), and T3 is the cooling temperature. (Jet cooling furnace (JC) 6 outlet steel strip temperature), T4 is the snout 8 part steel strip temperature. In FIG. 9, the steel strip temperature T3 is higher than the steel strip temperature T4.
[0009]
The cooling furnace 5 includes refractories constituting the furnace body and incidental objects such as transport rolls provided in the furnace. Due to the large thermal inertia, the response of the steel strip temperature control in the cooling furnace 5 is poor. Therefore, the fluctuation of the steel strip temperature T3 on the exit side of the jet cooling furnace 6 is large, and as a result, the fluctuation of the steel strip temperature after cooling in the cooling furnace 5 (the fluctuation of the steel strip temperature T4 of the snout 8 part) becomes large, and accordingly, the melting The temperature fluctuation of the steel strip immersed in the galvanizing bath 10 increases. This temperature fluctuation becomes more remarkable when the steel strip size or the passing speed changes. If the temperature fluctuation is large, the following problems are likely to occur.
[0010]
If the temperature of the steel strip immersed in the hot-dip galvanizing bath 10 is too high with respect to the hot-dip galvanizing bath temperature, free zinc (defect due to insufficient alloying) is generated at the center in the width direction of the steel strip. When the temperature of the steel strip immersed in the hot-dip galvanizing bath 10 is too low with respect to the hot-dip galvanizing bath temperature, free zinc is generated at the edge of the steel strip. The free zinc generated due to the steel strip temperature being immersed being too low with respect to the hot dip galvanizing bath temperature is more remarkable than the free zinc generated due to the steel strip temperature being too high. Yes, quality impact is greater.
[0011]
Further, when the steel strip S is immersed in the hot-dip galvanizing bath 10, if the temperature fluctuation of the steel strip immersed becomes large, it affects the floating dross generated in the hot-dip galvanizing bath 9 and adheres to the steel strip S. Dross defects easily occur.
[0012]
In order to reduce or prevent the above-mentioned problem, it is necessary to reduce the temperature fluctuation of the steel strip immersed in the hot-dip galvanizing bath 10.
[0013]
In Patent Literature 1, when the steel strip after the annealing heat treatment is continuously immersed and passed through a molten zinc bath to perform galvanization, the steel strip is heated by an induction heating device provided in a snout part, and the steel strip is heated. A method has been proposed in which the temperature of a steel strip entering a hot-dip galvanizing bath is adjusted by finely adjusting the temperature of the steel strip immediately before the penetration.
[0014]
The prior art document information is described below.
[0015]
[Patent Document 1]
JP-A-4-329856
[Problems to be solved by the invention]
However, when the temperature of the steel strip conveyed to the snout portion is large, the temperature of the steel strip conveyed to the snout portion is large when, for example, the size of the conveyed steel strip or the passing speed is changed. Then, since the steel strip temperature fluctuates higher than the predetermined temperature, the fluctuation of the temperature of the steel strip immersed in the plating bath cannot be reliably reduced. In addition, zinc vapor evaporated from the plating bath surface in the snout is condensed in the induction heating device, and the induction heating coil is likely to be short-circuited, resulting in a problem in operational stability.
[0017]
The present invention, in consideration of the above circumstances, reduces the temperature fluctuation of a steel strip immersed in a plating bath, thereby reducing the problem of free zinc and dross adhesion that occurs, for example, when manufacturing an alloyed hot-dip galvanized steel strip. Another object of the present invention is to provide a method for producing a hot-dip metal-plated steel strip having excellent surface appearance.
[0018]
Another object of the present invention is to provide a method for producing a hot-dip metal-plated steel strip having excellent operation stability.
[0019]
[Means for Solving the Problems]
Means of the present invention for solving the above problems are as follows.
[0020]
(1) In a continuous hot-dip plating facility equipped with a heating furnace and a cooling furnace, the annealed steel strip is adjusted to a temperature suitable for immersion in a plating bath, and then immersed in a hot-dip metal plating bath to pass hot-dip metal plating. Performing, in the method of manufacturing a hot-dip metal-plated steel strip to subsequently withdraw from the hot-dip metal plating bath to form a hot-dip metal coating, the annealed steel strip is lower than a temperature suitable for once immersing in the hot-dip bath. A method for producing a hot-dip galvanized steel strip, comprising cooling to a temperature, and then heating to a temperature suitable for being immersed in the plating bath by induction heating.
[0021]
(2) The method for producing a hot-dip galvanized steel strip according to the above (1), wherein an induction heating device is provided at a rear portion of the cooling furnace to perform induction heating.
[0022]
(3) The method according to (1) or (2), wherein the molten metal is molten zinc.
[0023]
(4) The method for producing a hot-dip metal-plated steel strip according to the above (3), wherein the hot-dip-metal coating is pulled up from the hot-dip metal plating bath to form a hot-dip metal coating. Manufacturing method of hot-dip metal-plated steel strip
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a schematic diagram showing a main part of a continuous hot-dip plating facility referred to in describing an embodiment of the present invention. In FIG. 1, the same reference numerals are given to the portions having the same operations as the portions shown in FIG. 8 already described, and the description thereof will be omitted. 1, an induction heating device 14 and a control device 19 are added to the equipment shown in FIG. The induction heating device 14 is installed on the outlet side of the cooling furnace 5 (the outlet side of the regulated cooling furnace 7).
[0025]
Hereinafter, an embodiment of the present invention will be described with reference to an example in which an alloyed hot-dip galvanized steel strip is manufactured using the equipment shown in FIG.
[0026]
FIG. 2 is a conceptual diagram illustrating a heat cycle before the steel strip is immersed in a hot-dip galvanizing bath when a galvannealed steel strip is manufactured using the equipment of FIG. 1. In FIG. 2, T1 is an open flame heating temperature (direct fire furnace (DF) 3 outlet steel strip temperature), T2 is an annealing temperature (radiation tube heating furnace (RT) 4 outlet steel strip temperature), and T3 is a cooling temperature. (Gas jet cooling furnace (JC) 6 outlet steel strip temperature), T4 is the steel strip temperature of the snout 8 part. In FIG. 2, the steel strip temperature T3 is lower than the steel strip temperature T4.
[0027]
The steel strip S is controlled by a direct heating furnace (DF) 3 so that the steel strip temperature T1 detected by the thermometer 15 on the outlet side of the direct heating furnace 3 becomes a predetermined direct heating temperature T1a. In the furnace (RT) 4, the steel strip temperature T2 detected by the thermometer 16 on the exit side of the radiation tube heating furnace 4 is controlled so as to become a predetermined annealing temperature T2a. The steel strip temperatures T1a and T2a are determined based on the steel composition of the steel strip S, material specifications, steel strip dimensions, steel strip passing speed, and the like.
[0028]
Next, the steel strip S is controlled to be cooled by the gas jet cooling furnace (JC) 6 so that the steel strip temperature T3 detected by the thermometer 17 on the outlet side of the gas jet cooling furnace 6 becomes the predetermined temperature T3b. The steel strip temperature T3b is set to a temperature lower than the steel strip temperature T4a in the snout 8. In the controlled cooling furnace 7, the steel strip temperature is controlled so as to be kept substantially constant, or the temperature is controlled so that even if there is a temperature drop, the degree thereof is small.
[0029]
If the set temperature T3b of the steel strip temperature T3 on the outlet side of the gas jet cooling furnace 6 is too low, an increase in power cost is caused. From this viewpoint, the difference ΔT (= T4a−T3b) between the set temperature T3b of the steel strip temperature on the outlet side of the gas jet cooling furnace 6 and the steel strip temperature T4a of the snout 8 is preferably set to 30 ° C. or less.
[0030]
The heating is controlled using the induction heating device 14 so that the steel strip temperature T4 detected by the thermometer 18 of the snout 8 becomes the predetermined temperature T4a. From the viewpoint of obtaining good plating quality, the steel strip temperature T4a is immersed in the plating bath from the snout temperature detection unit so that the steel strip temperature immersed in the plating bath is substantially the same as the plating bath temperature. The temperature is set in consideration of the steel strip temperature decrease during the period. In the case of zinc plating, the plating bath temperature is set to about 450 to 480 ° C., and the steel strip temperature is T4a, usually about +20 to 30 ° C. with respect to the plating bath temperature.
[0031]
The steel strip S heated to the predetermined temperature T4a by the induction heating device 14 is immersed in the hot-dip galvanizing bath 10 through the snout 8 and galvanized. Next, the steel strip is pulled up from the hot-dip galvanizing bath 10, and the amount of coating is controlled by a gas wiping nozzle 11. Then, the steel strip is heated in an alloying furnace 12 to alloy the plated layer, and cooled in a cooling device 13. To produce an alloyed hot-dip galvanized steel strip.
[0032]
In the present invention, after the steel strip temperature T3 of the cooling furnace 5 is once adjusted to be lower than the steel strip temperature T4a of the eight parts of the snout, the steel strip temperature T4 of the eight parts of the snout is changed to the required temperature T4a by using the induction heating device 14. Reheat. Since the induction heating device 14 has excellent responsiveness in temperature control, even if the steel strip temperature T3 fluctuates, the steel strip temperature T4 after reheating by the induction heating device 14 is accurately controlled to the required temperature T4a. As a result, the temperature fluctuation of the steel strip immersed in the hot-dip galvanizing bath 10 is reduced, the dross adhesion is reduced, and the generation of free zinc during the alloying treatment is prevented. A galvannealed steel strip is obtained.
[0033]
In continuous hot-dip plating equipment, steel strips of various sizes are connected and continuously passed. The heat load for cooling the steel strip in the cooling furnace 5 changes rapidly before and after the steel strip connection part where the steel strips are connected. Therefore, in the related art, the fluctuation of the steel strip temperature T3 becomes particularly large in the vicinity of the connection portion, and it is inevitable that the fluctuation of the steel strip temperature T4 becomes large correspondingly. Since the induction heating device 14 is excellent in responsiveness of temperature control, in the present invention, the fluctuation of the steel strip temperature T4 can be reduced even in the vicinity of the steel strip connection part, so that the steel strip immersed in the hot-dip galvanizing bath 10 can be reduced. Can be remarkably reduced as compared with the prior art.
[0034]
Further, when the heat load for cooling after passing through the connection portion increases, for example, when the steel strip thickness is changed from a thin material to a thick material, the steel strip temperature T3 in the vicinity of the connection portion of the following steel strip increases, while When the heat load decreases, for example, when the thickness of the steel strip is changed from a thick one to a thin one, the steel strip temperature T3 in the vicinity of the connection part of the subsequent steel strip tends to decrease. Therefore, by taking into account the fluctuation tendency of the steel strip temperature T3, by changing the preset value T3b of the steel strip temperature T3 in the vicinity of the connection part, the temperature fluctuation of the steel strip temperature T4 in the vicinity of the connection part is reduced. It can be further reduced. For example, when the steel strip shifts from a thin to a thick one, the set value of the steel strip temperature T3 on the exit side of the gas jet cooling furnace 6 is set in advance for the vicinity of the joint between the preceding steel strip and the following steel strip. By changing the temperature to a value lower than the set value T3b and cooling, the fluctuation of the steel strip temperature T4 in the snout 8 in the vicinity of the joint can be further reduced. When shifting from a thick material to a thin material, the set value of the steel strip temperature T3 on the exit side of the gas jet cooling furnace 6 is set to a predetermined set value in the vicinity of the joint between the preceding steel strip and the following steel strip. It is preferable to change the temperature to be higher than T3b and cool it.
[0035]
In the equipment shown in FIG. 1, a thermometer for detecting the temperature of the steel strip at the inlet side of the induction heating device 14 after passing through the conditioning cooling furnace 7 is not installed. A thermometer may be provided in this part. In this case, the temperature in the cooling furnace 5 may be controlled so that the steel strip temperature detected by the thermometer becomes the temperature T3b.
[0036]
When hot dip galvanizing is performed, zinc evaporates from the surface of the hot dip galvanizing bath 10 in the snout 8. If the evaporated zinc condenses in the induction heating device 14, the induction heating coil of the induction heating device 14 may be short-circuited.
[0037]
Most of the evaporated zinc vapor condenses in the snout 8. In the equipment shown in FIG. 1, the induction heating device 14 is provided on the outlet side of the cooling furnace 5 (the outlet side of the regulated cooling furnace 7) upstream of the snout 8 (upstream side with respect to the steel strip S passing direction). The problem of induction heating coil short circuit due to the condensation of zinc vapor in the induction heating device 14 is greatly reduced.
[0038]
From the viewpoint of solving the problem of the condensation of zinc vapor in the induction heating device 14, it is more preferable to prevent the zinc vapor evaporated from the hot-dip galvanizing bath from flowing to the induction heating device 14 side.
[0039]
From this viewpoint, an exhaust port is provided in the snout 8 between the plating bath surface and the induction heating device 14 to discharge zinc vapor evaporated from the molten metal bath surface to the outside of the snout from the exhaust port. Furnace gas (H ₂ -N ₂ gas) supplied to maintain the gas in a reducing atmosphere, a furnace section upstream of an exhaust port provided in the snout, more preferably a cooling furnace and an upstream side thereof Is preferably supplied to a portion inside the furnace. As a result, the zinc vapor evaporated from the plating bath surface does not flow to the induction heating device 14 side, and the problem of the induction heating coil short circuit due to the condensation of the zinc vapor in the induction heating device is solved.
[0040]
FIG. 3 shows the continuous hot-dip galvanizing equipment shown in FIG. 1 for discharging zinc vapor evaporated from the plating bath surface to the snout wall surface facing the steel strip surface on the side close to the plating bath surface below the snout 8. An exhaust port is provided to quickly discharge zinc vapor evaporated from the plating bath surface to the outside of the snout. In FIG. 3, 21 is an exhaust port, 22 is an exhaust pipe connected to the exhaust port, and 23 is a valve. FIG. 3 does not show the thermometer 18. In addition, the H ₂ -N ₂ gas supplied to maintain the inside of the furnace in a reducing atmosphere is not supplied into the snout 8 but is supplied to the conditioning cooling furnace 7 and a furnace part upstream thereof.
[0041]
When the zinc vapor exhaust means shown in FIG. 3 is provided in the snout 8 of the facility shown in FIG. 1, zinc vapor is discharged out of the snout as follows. The H ₂ -N ₂ gas supplied to the conditioning cooling furnace 7 and the upstream part of the furnace flows upstream (in a direction opposite to the steel strip passing direction) and flows out to the direct heating furnace 3. Further, since a large amount of fuel gas and air are supplied from the direct fire burner to heat the steel strip S from the direct fire burner, each part in the furnace where the direct fire heating furnace 3 to the induction heating device 14 are arranged is provided. The atmosphere in the snout 8 is maintained at a positive pressure. Furthermore, since there is also a draft function of the exhaust pipe 22 that is erected, by opening the valve 23 appropriately, the atmospheric gas containing zinc vapor evaporated from the plating bath surface in the snout 8 can be discharged from the exhaust port 21 through the exhaust pipe 22. Then, the air is exhausted outside the snout 8, that is, outside the furnace, and the outside air (atmosphere) does not flow into the snout 8. Since the zinc vapor is discharged out of the snout 8 from the exhaust port 21, the problem of the zinc vapor condensation in the induction heating device 14 is eliminated. Further, the plating property does not become poor due to the invasion of the outside air.
[0042]
【Example】
(Example 1)
FIG. 4 shows the fluctuation of the steel strip temperature and the occurrence of free zinc in the vicinity of the joint (weld) when the size of the steel strip is changed from thin to thick when manufacturing the alloyed hot-dip galvanized steel strip. FIG. 4 is an example according to the method of the present invention, and FIG. 5 is an example according to the conventional method. In each case, the steel strip size was changed from 0.6 mm × 1219 mm (leading steel strip) to 0.8 mm × 1219 mm (following steel strip). Is the case.
[0043]
In the method of the present invention, the target value T3b of the steel strip temperature T3 of the gas jet cooling furnace 6 is set to the target value T4a of the steel strip temperature T4 in the snout 8 part by using the equipment of FIG. 1 and the heat cycle shown in FIG. (= 480 ° C.), and after cooling at a lower temperature, the furnace temperature was set in the controlled cooling furnace 7 so as to substantially maintain the temperature of the steel strip on the exit side of the gas jet cooling furnace 6. The zone temperature T4 was controlled so as to be the predetermined temperature T4a.
[0044]
In the conventional method, the set value T3a of the steel strip temperature T3 of the gas jet cooling furnace 6 is changed by using the equipment shown in FIG. 8 and the steel strip temperature T4 of the snout 8 is set to the required temperature as in the heat cycle shown in FIG. The cooling was controlled by setting the steel strip temperature to be higher than T4a so as to be T4a. Note that the plating bath temperature is 460 ° C. in each case.
[0045]
In the conventional method (FIG. 5), the deviation of the steel strip temperature T4 from the target temperature T4a becomes large in the vicinity of the welded portion of the succeeding steel strip, and free zinc occurs in the area indicated by hatching in FIG. On the other hand, in the method of the present invention (FIG. 4), the deviation of the steel strip temperature T4 in the vicinity of the welded portion of the subsequent steel strip from the target temperature T4a was smaller than that of the conventional method, and free zinc did not occur.
[0046]
(Example 2)
The manufacturing situation of the steel strip temperature in the vicinity of a joint part (welding part) when the steel strip size is changed from a thick thing to a thin thing at the time of manufacturing an alloyed hot-dip galvanized steel strip, and the generation situation of free zinc are shown. FIG. 6 shows an example according to the method of the present invention, and FIG. 7 shows an example according to the conventional method. In each case, the steel strip size was changed from 1.2 mm × 914 mm (leading steel strip) to 0.8 mm × 914 mm (following steel strip). Is the case. In both the method of the present invention and the conventional method, the temperature is controlled as in Example 1, and the plating bath temperature is also the same as in Example 1.
[0047]
In the conventional method (FIG. 7), the steel strip temperature T4 is lower than the target temperature T4 a in the vicinity of the welded portion of the succeeding steel strip, and the deviation is large. Free zinc was generated due to a drop in steel strip temperature. On the other hand, in the method of the present invention (FIG. 6), free zinc, which is inevitably generated in the vicinity of the welded portion of the succeeding steel strip (the area indicated by hatching in FIG. 6), occurred. , Its length was short and light. In addition, the steel strip temperature T4 in the vicinity of the welded portion of the succeeding steel strip decreased with respect to the target temperature T4a, but the deviation was smaller than in the conventional method, so that free zinc caused by the decrease in the steel strip temperature did not occur. Was.
[0048]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, since the fluctuation | variation of the temperature of the steel strip immersed in a plating bath is reduced and it can control to the temperature suitable for immersion in a plating bath, the hot-dip metal-plated steel strip excellent in surface appearance is obtained. INDUSTRIAL APPLICABILITY The present invention is suitable as a method for producing an alloyed hot-dip galvanized steel strip which prevents the occurrence of free zinc and the attachment of dross and has an excellent surface appearance.
[0049]
Further, in the present invention, the problem of the induction heating coil short circuit is solved, and the operation stability is excellent.
[Brief description of the drawings]
FIG. 1 is a schematic view showing a main part of a continuous hot-dip galvanizing facility referred to for describing an embodiment of a method for producing a galvannealed steel strip according to the present invention.
FIG. 2 is a conceptual diagram illustrating a heat cycle of an alloyed hot-dip galvanized steel strip manufactured by the method of the present invention before the steel strip is immersed in a hot-dip galvanizing bath.
FIG. 3 shows a main portion arrangement near the snout when an exhaust port for discharging zinc vapor evaporated from the hot dip galvanizing bath to the outside of the snout is provided at the lower portion of the snout in the continuous galvanizing equipment of FIG. FIG.
FIG. 4 is a diagram showing the fluctuation state of the steel strip temperatures T3 and T4 in the vicinity of the weld when the steel strip dimensions are changed from 0.6 mm × 1219 mm → 0.8 mm × 1219 mm in the method of the present invention.
FIG. 5 is a view showing a variation state of steel strip temperatures T3 and T4 in a portion near a weld when a steel strip dimension is changed from 0.6 mm × 1219 mm → 0.8 mm × 1219 mm in a conventional method.
FIG. 6 is a view showing the fluctuation state of the steel strip temperatures T3 and T4 in the vicinity of the weld when the diameter is changed from 1.2 mm × 914 mm → 0.8 mm × 914 mm in the method of the present invention.
FIG. 7 is a diagram showing the fluctuation state of the steel strip temperatures T3 and T4 in the vicinity of the weld when the diameter is changed from 1.2 mm × 914 mm → 0.8 mm × 914 mm in the conventional method.
FIG. 8 is a schematic view showing a main part of a continuous hot-dip galvanizing facility referred to for describing a method for producing a galvannealed steel strip of the prior art.
FIG. 9 is a conceptual diagram illustrating a heat cycle of a steel strip before being immersed in a hot-dip galvanizing bath in a conventional method for producing a galvannealed steel strip.
[Explanation of symbols]
S Steel strip 1 Preheating furnace 2 Heating furnace 3 Open flame heating furnace (DF)
4 Radiation tube heating furnace (RT)
5 Cooling furnace 6 Gas jet cooling furnace (JC)
Reference Signs List 7 Regulated cooling furnace 8 Snout 9 Hot-dip galvanizing bath 10 Hot-dip galvanizing bath 11 Gas wiping nozzle 12 Alloying furnace 13 Cooling device 14 Induction heating device 15-18 Thermometer 19 Control device 21 Exhaust port 22 Exhaust pipe 23 Valve

Claims

In a continuous hot-dip plating equipment equipped with a heating furnace and a cooling furnace, after the steel strip subjected to the annealing treatment is adjusted to a temperature suitable for immersing in the plating bath, the steel strip is immersed in the hot-dip metal plating bath to perform hot-dip metal plating, Subsequently, in the method for producing a hot-dip metal-plated steel strip which is pulled up from the hot-dip metal plating bath to form a hot-dip metal coating, the annealed steel strip is cooled to a temperature lower than a temperature suitable for temporarily immersing the steel strip in the hot-dip bath. And then heating to a temperature suitable for being immersed in the plating bath by induction heating.

The method for producing a hot-dip metal-plated steel strip according to claim 1, wherein an induction heating device is provided at a rear portion of the cooling furnace to perform induction heating.

3. The method according to claim 1, wherein the molten metal is molten zinc.

4. The method for producing a hot-dip metal-coated steel strip according to claim 3, wherein the hot-dip metal-coated steel strip is pulled up from the hot-dip metal plating bath to form a hot-dip metal coating, and then is heated and alloyed. The production method of the belt.