JP4453853B2

JP4453853B2 - Manufacturing method of hot-dip aluminized steel sheet with excellent workability, heat resistance and oxidation resistance

Info

Publication number: JP4453853B2
Application number: JP2000043732A
Authority: JP
Inventors: 和昭細見; 敦司安藤
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 2000-02-22
Filing date: 2000-02-22
Publication date: 2010-04-21
Anticipated expiration: 2020-02-22
Also published as: JP2001234318A

Description

【０００１】
【産業上の利用分野】
本発明は、加工性，耐熱性，耐酸化性に優れた溶融アルミめっき系鋼板及びその製造方法に関する。
【０００２】
【従来の技術】
溶融アルミめっき系鋼板は、溶融亜鉛めっき鋼板や溶融Ｚｎ−Ａｌ合金めっき鋼板に比較して耐食性，耐熱性，耐酸化性に優れているため、自動車の排気系部材，熱機器等の広範な分野で使用されている。この溶融アルミめっき系鋼板は、連続式溶融めっきラインでめっき原板をガス還元焼鈍した後、溶融アルミめっき浴に浸漬することにより製造されている。溶融アルミめっき浴には、通常、溶融アルミめっき層と下地鋼との合金化を抑制し、加工性，耐熱性，耐酸化性を向上させるため７〜１２質量％のＳｉが添加されている。
Ｓｉ濃度７〜１２質量％の溶融アルミめっき浴を用いて溶融めっきすると、溶融アルミめっき層１と下地鋼２との反応によって層厚３μｍ程度のＦｅ−Ａｌ−Ｓｉ合金層３が界面に生成する（図１）。溶融アルミめっき層１は、溶融アルミめっき浴と同様に７〜１２質量％のＳｉを含んでいる。Ｆｅ−Ａｌ−Ｓｉ合金層３は、ほぼＦｅ：３２質量％，Ａｌ：５９質量％，Ｓｉ：９質量％の組成になる。
【０００３】
【発明が解決しようとする課題】
Ｆｅ−Ａｌ−Ｓｉ合金層３は，硬質で靭性に劣るため、厚く成長すると加工時に溶融アルミめっき層１及びＦｅ−Ａｌ−Ｓｉ合金層３を貫通するクラックを発生させる原因になる。クラックの発生によって下地鋼２が露出すると、耐食性，耐熱性，耐酸化性が低下する。なかでも、深絞り等の高度加工が予定される溶融アルミめっき系鋼のめっき原板として使用される極低炭素Ｔｉ添加鋼やＴｉ-Ｎｂ添加鋼では、清浄度が高く合金化速度が大きいため、Ｆｅ−Ａｌ−Ｓｉ合金層３が成長しやすい。その結果、加工度が大きな部位に使用すると、溶融アルミめっき層１に発生したクラックに起因して耐食性，耐熱性，耐酸化性の低下が顕在化する。
【０００４】
また、Ｓｉによる合金成長抑制効果は、６００℃を超える加熱温度では著しく低下する。溶融アルミめっき系鋼板が６００℃を超える温度に長時間加熱されると、溶融アルミめっき層１と下地鋼２との合金化反応が促進されＦｅ−Ａｌ−Ｓｉ合金層３が成長すると共に、Ｆｅ−Ａｌ−Ｓｉ合金層３の下層にＦｅ−Ａｌ合金層が生成・成長する。溶融アルミめっき系鋼板製部材の使用形態にもよるが、合金成長が溶融アルミめっき層１の表層にまで達するＦｅの拡散が進行すると、耐熱性低下だけでなく耐酸化性も低下し、溶融アルミめっき系鋼板本来の特性が損なわれる。
そのため、高温での耐熱性や耐酸化性が要求される用途ではステンレス鋼が使用されている。しかし、ステンレス鋼は、溶融アルミめっき系鋼板に比較して高価であるため、価格面からの制約が加わる。
【０００５】
【課題を解決するための手段】
本発明は、このような問題を解消すべく案出されたものであり、溶融アルミめっき層と下地鋼との界面にＭｏを含む合金層（以下、Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層という）を形成することにより、Ｆｅ−Ａｌ合金層は勿論，Ｆｅ−Ａｌ−Ｓｉ合金層の生成を抑制し、加工性，耐熱性，耐酸化性に優れた溶融アルミめっき系鋼板を得ることを目的とする。
【０００６】
本発明の溶融アルミめっき系鋼板の製造方法は、その目的を達成するため、めっき原板の片面又は両面に、Ｍｏ濃度２０〜６０質量％、かつ膜厚０．１〜３μｍのＦｅ−Ｍｏプレめっきを施した後、めっき原板をＳｉ含有溶融アルミめっき浴に浸漬して溶融アルミめっき層を形成することを特徴とする。
【０００７】
【作用】
本発明者等は、溶融アルミめっき系鋼板の溶融アルミめっき層１と下地鋼２との間に生成・成長するＦｅ−Ａｌ−Ｓｉ合金層３に及ぼす各種元素の影響を調査した。その結果、めっき原板にＦｅ−Ｍｏプレめっきを施すことがＦｅ−Ａｌ−Ｓｉ合金層３の抑制に有効であることを見出した。
めっき原板に施されたＦｅ−Ｍｏプレめっき層は、Ｓｉ含有溶融アルミめっき浴にめっき原板を浸漬して溶融めっきすると、一部又は全部が溶融アルミめっき浴と反応し、Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４が溶融アルミめっき層１と下地鋼２との界面に生成する。Ｆｅ−Ｍｏプレめっき層の全部が溶融アルミめっき浴と反応する場合、従来のＦｅ−Ａｌ−Ｓｉ合金層３にＦｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４が置き換わる（図２）。Ｆｅ−Ｍｏプレめっき層の一部が溶融アルミめっき浴と反応する場合、Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４の下にＦｅ−Ｍｏ合金層５が残存する（図３）。
【０００８】
Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４及びＦｅ−Ｍｏ合金層５は、溶融アルミめっき層１と下地鋼２との拡散反応を抑制するバリアとして働く。そのため、溶融アルミめっき時にＦｅ−Ａｌ−Ｓｉ合金層３の成長が抑制され、溶融アルミめっき系鋼板の加工性が向上する。なお、Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４及びＦｅ−Ｍｏ合金層５は、好ましくは０．５μｍ以下の層厚にするとき、加工性に悪影響を及ぼすことはない。
高温雰囲気に曝される部材に溶融アルミめっき系鋼板を使用する用途でも、Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４及びＦｅ−Ｍｏ合金層５によってＦｅ−Ａｌ−Ｓｉ合金層３の成長が抑制され、溶融アルミめっき系鋼板本来の優れた耐食性，耐熱性，耐酸化性が維持される。
【０００９】
【実施の形態】
本発明で使用されるめっき原板は、組成，種類等の鋼種に制約を受けるものではなく、用途に応じて種種の鋼材が選択される。なかでも、溶融アルミめっき層１との間に合金化反応が生じやすい極低炭素Ｔｉ添加鋼，Ｔｉ-Ｎｂ添加鋼等の高清浄度鋼では、Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４による効果が顕著となる。
めっき原板は、電解脱脂で表面が清浄化された後、Ｆｅ−Ｍｏプレめっきされる。Ｆｅ−Ｍｏプレめっき層は、必ずしもめっき原板の両面に施す必要はない。たとえば、溶融アルミめっき系鋼板の片面のみに加工性，耐熱性，耐酸化性が要求される用途では、めっき原板の片面のみにＦｅ−Ｍｏプレめっき層を形成する。溶融アルミめっき系鋼板の両面に加工性，耐熱性，耐酸化性が要求される用途では、めっき原板の両面にＦｅ−Ｍｏプレめっき層を形成する。
【００１０】
Ｆｅ−Ｍｏプレめっき層は電気めっき，蒸着めっき，溶融めっき等で形成できるが、特殊な設備を必要とせず安価にＦｅ−Ｍｏプレめっき層が形成できる電気めっきが好ましい。電気めっきによるとき、形成されるＦｅ−Ｍｏプレめっき層の組成や層厚が電気めっき浴の浴組成，電解条件等によって容易に調整される。この点でも，電気めっきが有利である。Ｆｅ−Ｍｏプレめっき層は、既存の電気めっきラインで形成することもできるが、電気めっき設備を入側に付設した連続式溶融めっきラインを使用するとき、Ｆｅ−Ｍｏプレめっき及び溶融アルミめっきを連続化できるため生産性，製造コストの面でも有利になる。
電気めっきでは、Ｍｏは、単独で析出することなく、Ｆｅ族元素との誘起共析で析出する。電気めっき浴としては、具体的にはピロリン酸塩浴等のアルカリ浴やクエン酸浴等の酸性浴がある。
【００１１】
Ｆｅ−Ｍｏプレめっき層は、具体的にはＭｏ濃度２０〜６０質量％，層厚０．１〜３μｍが好ましい。２０質量％以上のＭｏ及び０．１μｍ以上の層厚でＦｅ−Ｍｏプレめっき層を形成するとき、溶融アルミめっき層１と下地鋼２との反応抑制に顕著なバリア作用を呈するＦｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４が形成される。しかし、６０質量％を超えるＭｏ濃度や３μｍを超える層厚は、製造コストを上昇させる原因となる。Ｆｅ−Ｍｏプレめっき層のＭｏ濃度は、溶融アルミめっき条件，溶融アルミめっき浴のＳｉ濃度等に応じて２０〜６０質量％の範囲で好適に定められる。
【００１２】
Ｆｅ−Ｍｏプレめっき層が形成されためっき原板は、連続式溶融めっきラインのガス還元焼鈍炉に導入され、Ｈ₂−Ｎ₂混合ガス雰囲気中で還元焼鈍される。還元焼鈍条件は、特に制約されるものではなく、溶融アルミめっき系鋼板に要求される機械的性質に応じて加熱温度，時間等が設定される。
還元焼鈍されためっき原板は、溶融アルミめっき浴に導入され、溶融アルミめっきされる。溶融アルミめっき浴には、Ｆｅ−Ａｌ−Ｓｉ合金層３の成長を抑制するＳｉが添加されている。溶融アルミめっき浴に添加されたＳｉは、７質量％以上の濃度でＦｅ−Ａｌ−Ｓｉ合金層３に対する成長抑制効果が顕著になる。しかし、１２質量％を超えるＳｉ濃度では、溶融アルミめっき浴の融点が高くなるため溶融アルミめっき浴を高温保持するためのエネルギーコストが上昇すると共に、めっきポット，シンクロール等のめっき設備の侵食が激しくなる。
【００１３】
溶融アルミめっき時、溶融アルミめっき浴の浴温及びインレット温度を共に６５０〜６７０℃の温度域に維持することが好ましい。浴温及びインレット温度が６５０℃未満では不めっきが発生しやすく、６７０℃を超えるとＦｅ−Ａｌ−Ｓｉ合金層３が成長しやすくなる。
溶融アルミめっき浴からめっき原板を引き上げ、付着量を調整する。冷却後に溶融アルミめっき系鋼板の断面を観察すると、Ｆｅ−Ｍｏプレめっき層と溶融アルミめっき層１及び下地鋼２との反応生成物であるＦｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４が溶融アルミめっき層１と下地鋼２との界面に観察される。
【００１４】
Ｆｅ−Ｍｏプレめっき層が全て反応した場合、Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４の単相（図２）になる。たとえば、層厚０．１〜０．３μｍでＦｅ−Ｍｏプレめっき層を設けたものでは、Ｆｅ−Ｍｏプレめっき層の全量が溶融アルミめっき層１，下地鋼２と反応し、Ｍｏ濃度１０〜３０質量％，層厚０．２〜０．５μｍのＦｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４が形成される。
Ｆｅ−Ｍｏプレめっき層の一部が反応した場合、Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４及びＦｅ−Ｍｏ合金層５の複層（図３）となる。たとえば、層厚０．４〜３μｍでＦｅ−Ｍｏプレめっき層を設けたものでは、Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４の一部が溶融アルミめっき層１と反応し、Ｍｏ濃度１０〜３０質量％，層厚０．２〜０．５μｍのＦｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４が形成され、層厚０．１〜２．７μｍのＦｅ−Ｍｏ合金層５がＦｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４の下に残存する。
Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４単層型（図２）及びＦｅ−Ｍｏ合金層５が残存する複層型（図３）の何れにするかは、溶融アルミめっき系鋼板の用途に応じて適宜定められる。たとえば、高度の耐熱性，耐酸化性が要求される用途では、比較的厚いＦｅ−Ｍｏプレめっき層を設けることによりＦｅ−Ｍｏ合金層５を残存させた複層型が好ましい。
【００１５】
【実施例】
めっき原板として、板厚０．７ｍｍ，板幅１０００ｍｍの極低炭素Ｔｉ添加鋼帯（Ｃ：０．００３質量％，Ｓｉ：０．０２質量％，Ｍｎ：０．１４質量％，Ａｌ：０．０２質量％，Ｔｉ：０．０７質量％）を使用し、入側に電気めっき装置を付設した連続式溶融めっきラインで溶融アルミめっき系鋼板を製造した。製造条件は、次の通りである。
ラインスピード：７０ｍ／分
Ｆｅ−Ｍｏプレめっき：
表１の条件でＦｅ−Ｍｏプレめっき層を電気めっきによりめっき原板の両面に形成した。Ｆｅ−Ｍｏプレめっき層の層厚は、通電時間によって０．１〜３μｍの範囲に調整した。
【００１６】

【００１７】
還元焼鈍：
Ｆｅ−Ｍｏプレめっきしためっき原板を還元焼鈍炉に導入し、露点−４０℃の５０体積％Ｈ₂−Ｎ₂雰囲気中で８００℃に４０秒間加熱した。
溶融アルミめっき：
還元焼鈍されためっき原板をインレット温度６７０℃で浴温６６０℃，Ｓｉ濃度７〜１２質量％の溶融アルミめっき浴に３秒間浸漬し、溶融アルミめっき浴から引き上げた後、めっき付着量を片面当り４５ｇ／ｍ²に調整した。
【００１８】
製造された溶融アルミめっき系鋼板から試験片を切り出し、走査型電子顕微鏡を用いて断面組織を観察し、Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４及びＦｅ−Ｍｏ合金層５の層厚を測定すると共に、各層を組成分析した。また、次の各試験で加工性，耐熱性，耐酸化性も調査した。比較のため、Ｆｅ−Ｍｏプレめっきを施さずに同じ条件下で溶融アルミめっきした従来の溶融アルミめっき系鋼板についても、同じ試験で断面組織を観察すると共に加工性，耐熱性，耐酸化性を調査した。
〔断面組織観察〕
幅１０ｍｍ，長さ２０ｍｍの試験片の断面を鏡面研磨した後、３質量％硝酸アルコール液でエッチングし、ランダムに選んだ視野を倍率１０⁴倍で撮影し、Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４及びＦｅ−Ｍｏ合金層５の層厚を測定した。また、走査型電子顕微鏡に付設されているエネルギー分散型Ｘ線マイクロアナライザでＦｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４及びＦｅ−Ｍｏ合金層５の組成をスポット分析した。
【００１９】
〔加工性試験〕
幅２０ｍｍ，長さ５０ｍｍの試験片を１８０度密着曲げ試験し、走査型電子顕微鏡を用いて曲げ部の外側を５００倍の倍率で観察し、クラックの発生状況を調査した。クラックの発生状況から加工性を判定し、クラックが観察されなかったものを○，中程度のクラックが発生したものを△，クラックが著しく発生しているものを×として加工性を３段階評価した。
〔耐熱性試験〕
幅５０ｍｍ，長さ５０ｍｍの試験片を大気雰囲気中で６００℃に５分間加熱した後、走査型電子顕微鏡を用いて倍率５００倍で断面組織を観察し、Ｆｅ−Ａｌ−Ｓｉ合金層及びＦｅ−Ａｌ合金層の生成有無を調査した。Ｆｅ−Ａｌ−Ｓｉ合金層及びＦｅ−Ａｌ合金層が検出できなかったものを○，観察されたものを×として耐熱性を評価した。
〔耐酸化性試験〕
幅３５ｍｍ，長さ６０ｍｍの試験片を大気雰囲気中で６００℃に１０００時間加熱し、試験片の酸化増量を測定した。この耐酸化性試験では、切断端面の酸化に起因する酸化増量の増加分は補正しなかった。
【００２０】
以上の調査結果を、Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４及びＦｅ−Ｍｏ合金層５の有無と併せて表２に示す。表２から明らかなように、Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層４及びＦｅ−Ｍｏ合金層５を介在させた試験番号１〜１８（本発明例）では、何れも優れた加工性及び耐熱性を示した。酸化増量も比較例の半分程度，或いはそれ以下であり、耐酸化性に優れていることが判る。
これに対し、Ｆｅ−Ｍｏプレめっきを施さずに溶融アルミめっきした試験番号１９（比較例）では、層厚約３．５ｍｍとＦｅ−Ａｌ−Ｓｉ合金層３が厚く成長しており、曲げ加工試験でクラックが著しく発生した。また、耐熱性及び耐酸化性も不良であった。
【００２１】

【００２２】
【発明の効果】
以上に説明したように、本発明の溶融アルミめっき系鋼板は、溶融アルミめっき層／下地鋼の界面にＦｅ−Ａｌ−Ｓｉ−Ｍｏ合金層，或いは更にＦｅ−Ｍｏ合金層を介在させている。Ｆｅ−Ａｌ−Ｓｉ−Ｍｏ合金層及びＦｅ−Ｍｏ合金層は、溶融アルミめっき時及び使用状態での高温雰囲気に溶融アルミめっき系鋼板が長時間曝されたとき、Ｆｅ−Ａｌ−Ｓｉ合金層の生成・成長を抑制するバリアとして働く。そのため、製造された溶融アルミめっき系鋼板は加工性が良好で、また当該溶融アルミめっき系鋼板から作られた部材は耐熱性，耐酸化性に優れた製品となる。
【図面の簡単な説明】
【図１】Ｆｅ−Ａｌ−Ｓｉ合金層が生成成長した従来の溶融アルミめっき系鋼板を示す断面図
【図２】本発明に従って溶融アルミめっき層／下地鋼の界面にＦｅ−Ａｌ−Ｓｉ−Ｍｏ合金層を介在させた溶融アルミめっき系鋼板
【図３】本発明に従って溶融アルミめっき層／下地鋼の界面にＦｅ−Ａｌ−Ｓｉ−Ｍｏ合金層及びＦｅ−Ｍｏ合金層を介在させた溶融アルミめっき系鋼板[0001]
[Industrial application fields]
The present invention relates to a hot-dip aluminized steel sheet excellent in workability, heat resistance, and oxidation resistance and a method for producing the same.
[0002]
[Prior art]
Hot-dip aluminum-plated steel sheets are superior in corrosion resistance, heat resistance, and oxidation resistance compared to hot-dip galvanized steel sheets and hot-dip Zn-Al alloy-plated steel sheets. Used in. This hot-dip aluminum-plated steel sheet is manufactured by immersing a plating original plate in a hot-dip aluminum plating bath after subjecting the original plating plate to gas reduction annealing in a continuous hot-dip plating line. Usually, 7 to 12% by mass of Si is added to the molten aluminum plating bath in order to suppress alloying between the molten aluminum plated layer and the base steel and improve workability, heat resistance, and oxidation resistance.
When hot-dip plating is performed using a hot-dip aluminum plating bath having a Si concentration of 7 to 12% by mass, a Fe—Al—Si alloy layer 3 having a layer thickness of about 3 μm is generated at the interface by the reaction between the hot-dip aluminum plating layer 1 and the base steel 2. (FIG. 1). The molten aluminum plating layer 1 contains 7 to 12% by mass of Si similarly to the molten aluminum plating bath. The Fe—Al—Si alloy layer 3 has a composition of approximately Fe: 32 mass%, Al: 59 mass%, and Si: 9 mass%.
[0003]
[Problems to be solved by the invention]
Since the Fe—Al—Si alloy layer 3 is hard and inferior in toughness, if it grows thick, it causes cracks to penetrate the molten aluminum plating layer 1 and the Fe—Al—Si alloy layer 3 during processing. When the base steel 2 is exposed due to the occurrence of cracks, the corrosion resistance, heat resistance, and oxidation resistance deteriorate. Above all, in the ultra-low carbon Ti-added steel and Ti-Nb-added steel used as the plating base plate of the hot-dip aluminum-plated steel, which is planned for advanced processing such as deep drawing, the cleanliness is high and the alloying speed is high. The Fe—Al—Si alloy layer 3 is easy to grow. As a result, when it is used in a portion with a high degree of processing, a decrease in corrosion resistance, heat resistance, and oxidation resistance becomes obvious due to cracks generated in the molten aluminum plating layer 1.
[0004]
Further, the effect of suppressing the growth of the alloy by Si is remarkably reduced at a heating temperature exceeding 600 ° C. When the hot-dip aluminum-plated steel sheet is heated to a temperature exceeding 600 ° C. for a long time, the alloying reaction between the hot-dip aluminum plating layer 1 and the base steel 2 is promoted, and the Fe—Al—Si alloy layer 3 grows. A Fe—Al alloy layer is formed and grows under the Al—Si alloy layer 3. Depending on the type of use of the hot-dip aluminum-plated steel plate member, when the diffusion of Fe proceeds to reach the surface of the hot-dip aluminum plating layer 1, not only the heat resistance but also the oxidation resistance is lowered. The original properties of the plated steel sheet are impaired.
Therefore, stainless steel is used in applications that require heat resistance and oxidation resistance at high temperatures. However, since stainless steel is more expensive than a hot-dip aluminum-plated steel plate, there are additional restrictions on the price.
[0005]
[Means for Solving the Problems]
The present invention has been devised to solve such a problem, and an alloy layer containing Mo at the interface between the molten aluminum plating layer and the base steel (hereinafter referred to as an Fe—Al—Si—Mo alloy layer). The purpose of this invention is to suppress the formation of Fe-Al-Si alloy layers as well as Fe-Al alloy layers, and to obtain hot-dip aluminum-plated steel sheets with excellent workability, heat resistance, and oxidation resistance. To do.
[0006]
In order to achieve the object, the method for producing a hot-dip aluminum-plated steel sheet according to the present invention provides Fe-Mo pre-plating with a Mo concentration of 20 to 60% by mass and a film thickness of 0.1 to 3 μm on one side or both sides of a plating original plate. Then, the plating original plate is immersed in a Si-containing molten aluminum plating bath to form a molten aluminum plating layer .
[0007]
[Action]
The present inventors investigated the influence of various elements on the Fe—Al—Si alloy layer 3 formed and grown between the hot dip aluminum plating layer 1 and the base steel 2 of the hot dip galvanized steel sheet. As a result, it has been found that applying the Fe—Mo pre-plating to the plating original plate is effective for suppressing the Fe—Al—Si alloy layer 3.
The Fe—Mo pre-plated layer applied to the plating original plate is partially or entirely reacted with the molten aluminum plating bath when the plating original plate is immersed in a Si-containing molten aluminum plating bath and is subjected to hot-dip plating, and Fe—Al—Si— The Mo alloy layer 4 is formed at the interface between the molten aluminum plating layer 1 and the base steel 2. When the entire Fe—Mo pre-plated layer reacts with the molten aluminum plating bath, the Fe—Al—Si—Mo alloy layer 4 is replaced with the conventional Fe—Al—Si alloy layer 3 (FIG. 2). When a part of the Fe—Mo pre-plated layer reacts with the molten aluminum plating bath, the Fe—Mo alloy layer 5 remains under the Fe—Al—Si—Mo alloy layer 4 (FIG. 3).
[0008]
The Fe—Al—Si—Mo alloy layer 4 and the Fe—Mo alloy layer 5 function as a barrier that suppresses the diffusion reaction between the molten aluminum plating layer 1 and the base steel 2. Therefore, the growth of the Fe—Al—Si alloy layer 3 is suppressed during hot-dip aluminum plating, and the workability of the hot-dip aluminum-plated steel sheet is improved. The Fe—Al—Si—Mo alloy layer 4 and the Fe—Mo alloy layer 5 preferably do not adversely affect workability when the layer thickness is preferably 0.5 μm or less.
Even in applications where a hot-dip aluminum-plated steel sheet is used for a member exposed to a high temperature atmosphere, the growth of the Fe—Al—Si alloy layer 3 is suppressed by the Fe—Al—Si—Mo alloy layer 4 and the Fe—Mo alloy layer 5. In addition, the original excellent corrosion resistance, heat resistance, and oxidation resistance of the hot-dip aluminized steel sheet are maintained.
[0009]
Embodiment
The plating plate used in the present invention is not limited by the steel type such as composition and type, and various types of steel materials are selected according to the application. Among them, in the case of high cleanliness steel such as ultra-low carbon Ti-added steel and Ti-Nb-added steel, which is likely to cause an alloying reaction with the molten aluminum plating layer 1, the effect of the Fe-Al-Si-Mo alloy layer 4 Becomes prominent.
The plating original plate is subjected to Fe-Mo pre-plating after the surface is cleaned by electrolytic degreasing. The Fe—Mo pre-plated layer is not necessarily applied to both surfaces of the plating original plate. For example, in applications where workability, heat resistance, and oxidation resistance are required only on one side of a hot-dip aluminum plated steel sheet, an Fe—Mo pre-plated layer is formed only on one side of the plating original plate. In applications where workability, heat resistance, and oxidation resistance are required on both sides of a hot-dip aluminum-plated steel sheet, an Fe—Mo pre-plated layer is formed on both sides of the original plating plate.
[0010]
The Fe—Mo pre-plated layer can be formed by electroplating, vapor deposition plating, hot dipping, etc., but electroplating that does not require special equipment and can form the Fe—Mo pre-plated layer at low cost is preferable. When electroplating, the composition and thickness of the Fe-Mo pre-plated layer to be formed are easily adjusted by the bath composition of the electroplating bath, the electrolysis conditions, and the like. Also in this respect, electroplating is advantageous. The Fe-Mo pre-plating layer can be formed with an existing electroplating line, but when using a continuous hot-dip plating line with an electroplating facility on the inlet side, Fe-Mo pre-plating and hot-dip aluminum plating are used. Since it can be continuous, it is advantageous in terms of productivity and manufacturing cost.
In electroplating, Mo does not precipitate alone, but precipitates by induced eutectoid with Fe group elements. Specific examples of the electroplating bath include an alkaline bath such as a pyrophosphate bath and an acidic bath such as a citric acid bath.
[0011]
Specifically, the Fe—Mo pre-plated layer preferably has a Mo concentration of 20 to 60% by mass and a layer thickness of 0.1 to 3 μm. Fe-Al- exhibiting a remarkable barrier action for suppressing the reaction between the molten aluminum plating layer 1 and the base steel 2 when the Fe-Mo pre-plated layer is formed with Mo of 20 mass% or more and a layer thickness of 0.1 µm or more. A Si—Mo alloy layer 4 is formed. However, Mo concentration exceeding 60% by mass and layer thickness exceeding 3 μm cause an increase in manufacturing cost. The Mo concentration of the Fe—Mo pre-plated layer is suitably determined in the range of 20 to 60% by mass according to the molten aluminum plating conditions, the Si concentration of the molten aluminum plating bath, and the like.
[0012]
The plating original plate on which the Fe—Mo pre-plated layer is formed is introduced into a gas reduction annealing furnace of a continuous hot dipping plating line, and reduction annealing is performed in an H ₂ —N ₂ mixed gas atmosphere. The reduction annealing conditions are not particularly limited, and the heating temperature, time, etc. are set according to the mechanical properties required for the hot-dip aluminized steel sheet.
The reduction-annealed plating plate is introduced into a molten aluminum plating bath and subjected to molten aluminum plating. Si that suppresses the growth of the Fe—Al—Si alloy layer 3 is added to the molten aluminum plating bath. Si added to the hot dip aluminum plating bath has a remarkable growth suppression effect on the Fe—Al—Si alloy layer 3 at a concentration of 7% by mass or more. However, when the Si concentration exceeds 12% by mass, the melting point of the molten aluminum plating bath increases, so that the energy cost for maintaining the molten aluminum plating bath at a high temperature increases, and the erosion of the plating equipment such as the plating pot and sink roll occurs. Become intense.
[0013]
At the time of molten aluminum plating, it is preferable to maintain both the bath temperature and the inlet temperature of the molten aluminum plating bath in a temperature range of 650 to 670 ° C. When the bath temperature and the inlet temperature are less than 650 ° C., non-plating is likely to occur, and when it exceeds 670 ° C., the Fe—Al—Si alloy layer 3 is likely to grow.
Pull up the plating plate from the molten aluminum plating bath and adjust the amount of adhesion. When the cross section of the hot-dip aluminum-plated steel sheet is observed after cooling, the Fe-Al-Si-Mo alloy layer 4 which is a reaction product of the Fe-Mo pre-plated layer, the hot-dip aluminum plated layer 1 and the base steel 2 is hot-dip aluminum plated. Observed at the interface between the layer 1 and the base steel 2.
[0014]
When all of the Fe—Mo pre-plated layers have reacted, the single phase of the Fe—Al—Si—Mo alloy layer 4 (FIG. 2) is obtained. For example, in the case where the Fe—Mo pre-plated layer is provided with a layer thickness of 0.1 to 0.3 μm, the entire amount of the Fe—Mo pre-plated layer reacts with the molten aluminum plated layer 1 and the base steel 2, and the Mo concentration is 10 to 10. An Fe—Al—Si—Mo alloy layer 4 of 30% by mass and a layer thickness of 0.2 to 0.5 μm is formed.
When a part of the Fe—Mo pre-plated layer reacts, it becomes a multilayer (FIG. 3) of the Fe—Al—Si—Mo alloy layer 4 and the Fe—Mo alloy layer 5. For example, in the case where the Fe—Mo pre-plated layer is provided with a layer thickness of 0.4 to 3 μm, a part of the Fe—Al—Si—Mo alloy layer 4 reacts with the molten aluminum plated layer 1 and the Mo concentration is 10 to 30. The Fe—Al—Si—Mo alloy layer 4 having a mass% of 0.2 to 0.5 μm is formed, and the Fe—Mo alloy layer 5 of 0.1 to 2.7 μm is formed of Fe—Al—Si—. It remains under the Mo alloy layer 4.
Depending on the use of the hot-dip aluminum-plated steel sheet, the Fe-Al-Si-Mo alloy layer 4 single-layer type (FIG. 2) or the multilayer type in which the Fe-Mo alloy layer 5 remains (FIG. 3) is used. As appropriate. For example, in applications where high heat resistance and oxidation resistance are required, a multilayer type in which the Fe—Mo alloy layer 5 is left by providing a relatively thick Fe—Mo pre-plated layer is preferable.
[0015]
【Example】
As an original plating plate, an ultra-low carbon Ti-added steel strip having a plate thickness of 0.7 mm and a plate width of 1000 mm (C: 0.003 mass%, Si: 0.02 mass%, Mn: 0.14 mass%, Al: 0.00%). 02 mass%, Ti: 0.07 mass%) was used, and a hot dip galvanized steel sheet was produced on a continuous hot dip plating line provided with an electroplating apparatus on the inlet side. The manufacturing conditions are as follows.
Line speed: 70 m / min Fe-Mo pre-plating:
Under the conditions shown in Table 1, Fe-Mo pre-plated layers were formed on both surfaces of the plating original plate by electroplating. The layer thickness of the Fe—Mo pre-plated layer was adjusted to a range of 0.1 to 3 μm depending on the energization time.
[0016]

[0017]
Reduction annealing:
The plating base plate plated with Fe—Mo was introduced into a reduction annealing furnace and heated to 800 ° C. for 40 seconds in a 50 vol% H ₂ —N ₂ atmosphere having a dew point of −40 ° C.
Hot dipped aluminum plating:
The plated plate after reduction annealing is immersed for 3 seconds in a molten aluminum plating bath with an inlet temperature of 670 ° C., a bath temperature of 660 ° C., and a Si concentration of 7 to 12% by mass, and then pulled up from the molten aluminum plating bath. Adjusted to 45 g / m ² .
[0018]
A test piece is cut out from the manufactured hot-dip aluminum-plated steel sheet, the cross-sectional structure is observed using a scanning electron microscope, and the layer thicknesses of the Fe—Al—Si—Mo alloy layer 4 and the Fe—Mo alloy layer 5 are measured. At the same time, the composition of each layer was analyzed. In addition, workability, heat resistance, and oxidation resistance were investigated in the following tests. For comparison, with regard to conventional hot-dip aluminum-plated steel sheets that were hot-dip aluminum plated under the same conditions without applying Fe-Mo pre-plating, the cross-sectional structure was observed in the same test and the workability, heat resistance, and oxidation resistance were improved. investigated.
[Cross-sectional structure observation]
After width 10 mm, the cross section of 20mm long test piece was mirror-polished, etched with 3% by weight nitric acid alcohol solution, and photographs a view chosen randomly magnification ¹⁰⁴ times, Fe-Al-Si-Mo alloy layer 4 and the thickness of the Fe—Mo alloy layer 5 were measured. Further, the composition of the Fe—Al—Si—Mo alloy layer 4 and the Fe—Mo alloy layer 5 was spot analyzed with an energy dispersive X-ray microanalyzer attached to the scanning electron microscope.
[0019]
[Workability test]
A test piece having a width of 20 mm and a length of 50 mm was subjected to a 180-degree close contact bending test, and the outside of the bent portion was observed at a magnification of 500 times using a scanning electron microscope to investigate the occurrence of cracks. The workability was judged from the occurrence of cracks, and the workability was evaluated in three stages, with ○ indicating that no crack was observed, Δ indicating that a moderate crack was generated, and × indicating that cracks were significantly generated. .
[Heat resistance test]
A test piece having a width of 50 mm and a length of 50 mm was heated to 600 ° C. for 5 minutes in an air atmosphere, and then a cross-sectional structure was observed at a magnification of 500 times using a scanning electron microscope to obtain an Fe—Al—Si alloy layer and an Fe— Whether or not an Al alloy layer was formed was investigated. The heat resistance was evaluated by assuming that the Fe—Al—Si alloy layer and the Fe—Al alloy layer could not be detected as ◯ and the observed one as x.
[Oxidation resistance test]
A test piece having a width of 35 mm and a length of 60 mm was heated to 600 ° C. for 1000 hours in an air atmosphere, and the oxidation increase of the test piece was measured. In this oxidation resistance test, the increase in oxidation increase due to the oxidation of the cut end face was not corrected.
[0020]
The above investigation results are shown in Table 2 together with the presence or absence of the Fe—Al—Si—Mo alloy layer 4 and the Fe—Mo alloy layer 5. As is apparent from Table 2, in test numbers 1 to 18 (examples of the present invention) in which the Fe—Al—Si—Mo alloy layer 4 and the Fe—Mo alloy layer 5 were interposed, all had excellent workability and heat resistance. showed that. The increase in oxidation is about half or less than that of the comparative example, indicating that the oxidation resistance is excellent.
On the other hand, in test number 19 (comparative example) in which hot-dip aluminum plating was performed without applying Fe-Mo pre-plating, the layer thickness of about 3.5 mm and the Fe-Al-Si alloy layer 3 grew thickly, and bending work was performed. Cracks remarkably occurred in the test. Moreover, heat resistance and oxidation resistance were also poor.
[0021]

[0022]
【The invention's effect】
As described above, the hot-dip aluminum-plated steel sheet of the present invention has a Fe—Al—Si—Mo alloy layer or a Fe—Mo alloy layer interposed at the hot-dip aluminum plating layer / underlying steel interface. The Fe-Al-Si-Mo alloy layer and the Fe-Mo alloy layer are formed when the hot-dip aluminum plating steel sheet is exposed to a high temperature atmosphere during hot-dip aluminum plating and in a use state for a long time. Works as a barrier to suppress generation and growth. Therefore, the manufactured hot-dip aluminum-plated steel sheet has good workability, and a member made from the hot-dip aluminum-plated steel sheet becomes a product excellent in heat resistance and oxidation resistance.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a conventional hot-dip aluminum-plated steel sheet on which an Fe—Al—Si alloy layer has been formed and grown. FIG. 2 shows an Fe—Al—Si—Mo at the hot-dip aluminum plating layer / underlying steel interface according to the present invention. Hot-dip aluminized steel plate with alloy layer interposed [FIG. 3] Hot-aluminum plated steel plate with Fe—Al—Si—Mo alloy layer and Fe—Mo alloy layer at the interface between the hot-dip aluminum plated layer and the base steel according to the present invention Steel plate

Claims

One side or both sides of the plating original plate is subjected to Fe-Mo pre-plating with a Mo concentration of 20 to 60% by mass and a film thickness of 0.1 to 3 μm, and then the plating original plate is immersed in a Si-containing molten aluminum plating bath to obtain molten aluminum. A method for producing a hot-dip aluminum-plated steel sheet with excellent workability, heat resistance and oxidation resistance, characterized by forming a plating layer.