JP2004083988A

JP2004083988A - HEAT RESISTANT HOT DIP Al BASED PLATED STEEL SHEET WORKED MATERIAL EXCELLENT IN OXIDATION RESISTANCE OF WORKED PART AND HIGH TEMPERATURE OXIDATION RESISTANT COATING STRUCTURE

Info

Publication number: JP2004083988A
Application number: JP2002245824A
Authority: JP
Inventors: Fukio Yoshizaki; 吉崎　布貴男; Yasunori Hattori; 服部　保徳; Atsushi Ando; 安藤　敦司
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 2002-08-26
Filing date: 2002-08-26
Publication date: 2004-03-18

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot dip Al based plated steel sheet worked material in which high temperature oxidation resistance of the worked part is improved, and to provide a high temperature oxidation resistant coating structure thereof. <P>SOLUTION: The heat resistant hot dip Al based plated steel sheet worked material is obtained by working a hot dip Al based plated steel sheet. The plating layer therein comprises, by mass, 0 to 13% Si and 0.5 to 8% Mg, and, if required, further comprises one or more of metals selected from 0.001 to 1% Sr, 0.001 to 1% Ca, 0.0001 to 0.1% Be and 0.001 to 1% Ba. An alloy layer is formed between the matrix and the plating layer. Cracks passing through the alloy layer are generated at the worked part. The material is used at 450 to 650°C. In the high temperature oxidation resistant coating structure of the worked part, Mg or Mg oxide is concentrated on the crack parts in the alloy layer grown by heating, and also, the abundance ratio of an Fe oxide phase in the vicinity of the boundary of the alloy layer/matrix is ≤50%. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、溶融Ａｌ基めっき鋼板を加工した材料であって、加工部の耐高温酸化特性に優れた耐熱用溶融Ａｌ基めっき鋼板加工材、および、溶融Ａｌ基めっき鋼板を加工した材料を高温で使用することによって形成された加工部の被覆構造であって、その後さらに長期間の耐高温酸化性を発揮する被覆構造に関するものである。
【０００２】
【従来の技術】
めっき層にＳｉを９〜１０質量％含有した溶融Ａｌ基めっき鋼板は概ね６００℃以下の温度において良好な耐高温酸化性を示し、かつ比較的安価であることから、低級耐熱用材料として広く使用されている。しかし、このめっき鋼板を構造物部材や機械部品等に加工した場合、その加工部において耐高温酸化性が劣化するという問題がある。加工度が大きい場合や使用環境が厳しい場合には、耐久性が著しく低下する。
【０００３】
溶融Ａｌ基めっき鋼板の表層部には、通常、最表層の「めっき層」の下に、めっき金属と鋼板素地中のＦｅ等が反応して生じた「合金層」が存在する。この合金層はＦｅとＡｌを主体とする金属間化合物であり、一般的にめっき層より脆い。溶融Ａｌ基めっき鋼板に「曲げ」等の加工を施すと、合金層にクラックが生じやすい。その多くは合金層を貫通する。加工度が大きくなるとめっき層にもクラックが発生し、最表面から鋼板素地までを貫くクラックが形成される。ただし、そのようなクラックの数は合金層のみを貫通するクラックに比べわずかである。
【０００４】
従来の溶融Ａｌ基めっき鋼板を加工した材料を約６００℃の高温大気に曝したとき、合金層に生じたクラックの鋼板素地に近い部分を起点としてＦｅ酸化物が生成しはじめる。めっき層と合金層の両方を貫くクラック、および合金層のみを貫くクラックにおいて、いずれもＦｅ酸化物の生成挙動はほぼ同様である。そして、合金層が成長するに伴ってＦｅ酸化物も量を増していき、数時間もすると、成長した合金層は多量のＦｅ酸化物に占められ、最表面にもＦｅ酸化物が現れるようになる。そうなると溶融Ａｌ基めっきによる耐高温酸化性は損なわれてしまう。つまり、溶融Ａｌ基めっき鋼板の加工部は、高温で使用しているうちに、めっきを施していない裸の鋼材と同様の状態になってしまう。
【０００５】
溶融Ａｌ基めっき鋼板の耐高温酸化性を改善する研究も行われている。例えば、特許第１５２０７７１号には母材（めっき原板）にＴｉ，Ｃｒを添加する技術が開示されている。また、特許１５７８５７９号には母材にＳｉ，Ｍｎ，Ｔｉを添加する技術が開示されている。これらの技術はいずれも実用化されており、溶融Ａｌ基めっき鋼板加工部の耐高温酸化性は多少改善されている。しかし、材料温度が６００℃近くまで上昇するような厳しい環境下では、加工部での酸化進行を阻止することはできず、このような高温で耐えうる耐高温酸化性は得られていない。
【０００６】
一方、材料温度が６００℃程度あるいはそれ以上になる用途には、一般的に耐熱鋼やステンレス鋼が使用される。例えばＳＵＨ４０９Ｌなどの規格鋼種がある。耐熱鋼やステンレス鋼の場合は加工部においても良好な耐高温酸化性が維持される。
【０００７】
【発明が解決しようとする課題】
ステンレス鋼は高価であるため、低コストが要求される低級耐熱部材には実際上採用しにくい。また、ステンレス鋼は通常の大気中での耐高温酸化性には優れるが、耐塩害腐食性については溶融Ａｌ基めっき鋼板よりかなり劣るという欠点がある。
他方、溶融Ａｌ基めっき鋼板は、本来、加工部以外の「平坦部」については大気中６００℃程度の加熱環境に長期間耐えうる性能を有している。しかし、加工部について抜本的に耐高温酸化性の改善を図ったものは知られておらず、まだ研究の余地が残されている。
【０００８】
本発明は、以上のような現状に鑑み、６００℃程度の高温大気中で長期間使用できる溶融Ａｌ基めっき鋼板の加工材を提供することを目的とする。
【０００９】
【課題を解決するための手段】
発明者らは、溶融Ａｌ基めっき鋼板加工部の高温酸化挙動について詳細に調査してきた。その結果、めっき層中に適量のＭｇを含有させたものにおいて、加工部での耐高温酸化性が飛躍的に向上する現象を見出した。
【００１０】
すなわち、合金層を貫通するクラックの鋼板素地に近い部分にＭｇまたはＭｇ酸化物が速やかに濃化する。その濃化は、６００℃の大気に５分程度曝した段階で、ＥＰＭＡによる検出が十分可能な程度となる。合金層のみを貫通するクラックに生じたＭｇまたはＭｇ酸化物の濃化は、その後、Ｆｅ酸化物の成長を顕著にくい止める作用を有するのである。めっき層をも貫通するクラック部分ではＦｅ酸化物の生成は進行する。しかし、そのようなクラックはもともと少ないため、成長した合金層が大量のＦｅ酸化物で占められる状態にはならない。研究の結果、高温の大気中に長時間（少なくとも３０分以上）曝すことによって、成長した合金層と鋼板素地との界面において、Ｆｅ酸化物相の存在割合が５０％以下となっている被覆構造を有するものでは、その温度でさらに長時間加熱しても、良好な耐高温酸化性が安定的に維持されることがわかった。本発明はこのような知見に基づいて完成したものである。
【００１１】
前記目的を達成するために、請求項１の発明は、溶融Ａｌ基めっき鋼板を加工した材料であって、ｉ）　溶融Ａｌ基めっき層がＳｉ：０（無添加）〜１３質量％，Ｍｇ：０．５〜８質量％を含有すること、ｉｉ）　４５０〜６５０℃の高温で使用されるものであること、を特徴とする加工部耐酸化性に優れた耐熱用溶融Ａｌ基めっき鋼板加工材である。
【００１２】
請求項２の発明は、さらに、ｉｉｉ）　鋼板素地と溶融めっき層の間に合金層が形成されていること、ｉｖ）　加工部には、合金層を貫通するクラックが生じていること、を特徴とするものである。
請求項３の発明は、さらに、ｖ）　大気中６００℃で５分加熱したとき、合金層中のクラック部にＭｇまたはＭｇ酸化物が濃化する性質を有していること、を特徴とするものである。
【００１３】
請求項４の発明は、上記の耐熱用溶融Ａｌ基めっき鋼板加工材において、特に溶融Ａｌ基めっき層がＳｉ：０（無添加）〜１３質量％，Ｍｇ：０．５〜８質量％を含有し、残部がＡｌおよび不可避的不純物からなる点を規定したものである。
請求項５の発明は、同様に溶融Ａｌ基めっき層がＳｉ：０（無添加）〜１３質量％，Ｍｇ：０．５〜８質量％を含有し、さらにＳｒ：０．００１〜１質量％，Ｃａ：０．００１〜１質量％，Ｂｅ：０．０００１〜０．１質量％，Ｂａ：０．００１〜１質量％のうち１種または２種以上を含有し、残部がＡｌおよび不可避的不純物からなる点を規定したものである。
【００１４】
請求項６の発明は、上記の耐熱用溶融Ａｌ基めっき鋼板加工材において、溶融Ａｌ基めっき層のＭｇ含有量が２〜８質量％である点を規定したものである。
請求項７の発明は、以上の耐熱用溶融Ａｌ基めっき鋼板加工材において、加工材が自動車排気系部材である点を規定したものである。
【００１５】
請求項８の発明は、溶融Ａｌ基めっき鋼板加工部に形成された耐高温酸化性に優れた被覆構造を特定したものである。すなわち、めっき層中にＳｉ：１３質量％以下，Ｍｇ：０．５〜８質量％を含む溶融Ａｌ基めっき鋼板の加工材を高温で使用することにより形成された「成長した合金層／鋼板素地」、または「残存めっき層／成長した合金層／鋼板素地」からなる加工部の被覆構造であって、成長した合金層中のクラック部分にＭｇまたはＭｇ酸化物が濃化しており、かつ、成長した合金層と鋼板素地との界面において、Ｆｅ酸化物相の存在割合が５０％以下となっていることを特徴とする加工部の耐高温酸化被覆構造である。
【００１６】
ここで、「成長した合金層」とは、高温に保持された結果、溶融めっき直後の初期のめっき層・合金層・鋼板素地が反応することにより領域を拡げていった合金層を意味し、成長過程にあるものも含む。「成長した合金層と鋼板素地との界面」および「Ｆｅ酸化物相」は、当該材料表面付近の断面を光学顕微鏡または走査型電子顕微鏡（ＳＥＭ）で観察することによって把握できる。「成長した合金層と鋼板素地との界面において、Ｆｅ酸化物相の存在割合が５０％以下となっている」とは、以下の状態をいう。すなわち、断面組織観察像（例えば顕微鏡写真やモニター上の映像など）の上で「界面」をトレースして一定長さ（例えば３００μｍ）の「測定線」を設定し、その測定線を一端から他端までたどっていき、Ｆｅ酸化物相が測定線上に存在するか、または測定線の片側に接して存在する部分の長さの総和を求め、これを「Ｆｅ酸化物相存在長さ」とする。このとき、「Ｆｅ酸化物相存在長さ」を測定線の長さで除して求めた割合が５０％以下である状態を意味する。
【００１７】
【発明の実施の形態】
本発明では、加工部の耐高温酸化性に優れた表面状態を実現するために、適量のＭｇを添加したＳｉ含有溶融Ａｌ基めっきを施した鋼板を使用する。
図１には、Ｍｇを含まない溶融Ａｌ基めっき（Ａｌ−９％Ｓｉ）を施した従来の鋼板と、Ｍｇを５質量％含む溶融Ａｌ基めっき（Ａｌ−９％Ｓｉ−５％Ｍｇ）を施した鋼板について、加工部の耐高温酸化特性を比較した断面組織観察像（光学顕微鏡像）を示す。いずれの材料も、めっき母材は板厚１．０ｍｍのＡｌキルド鋼であり、めっき付着量は鋼板片面当たり１００ｇ／ｍ^２とした。めっき後、内側の曲げ先端Ｒ＝２ｍｍで９０°曲げを行った試料を大気中６００℃で加熱した。図１は、加熱時間０分，５分，１０分，３０分のサンプルについて加工部（曲げ外側）の表面付近の断面を観察したものである。
【００１８】
Ａｌ−９％Ｓｉめっき材（以下、「０％Ｍｇ材」という）、およびＡｌ−９％Ｓｉ−５％Ｍｇめっき材（以下、「５％Ｍｇ材」という）のいずれにも、めっき層の下に合金層が形成されている。加熱前の断面を比べると、めっき層と合金層を貫くクラックは、０％Ｍｇ材より５％Ｍｇ材の方が多い。合金層のみを貫通するクラックの発生密度はいずれも同等である。
加熱時間５分の段階では、いずれも合金層のクラック部分に鋼板素地との界面付近から黒っぽく見える相が生成しはじめている。これは、ＥＰＭＡによる分析の結果、Ｆｅ酸化物であることがわかった。５％Ｍｇ材についてはそのＦｅ酸化物を取り囲むようにしてＭｇまたはＭｇ酸化物が濃化していた。
【００１９】
加熱時間１０分になると、０％Ｍｇ材では合金層が成長するに伴ってＦｅ酸化物が成長している。一方、５％Ｍｇ材ではＦｅ酸化物の成長はあまり見られない。ただ、別途詳細な調査によれば、めっき層をも貫くクラック部分ではＦｅ酸化物の成長が見られた。
加熱時間３０分になると、０％Ｍｇ材では成長した合金層中に多量のＦｅ酸化物が生成している。成長した合金層と鋼板素地との界面において、Ｆｅ酸化物相の存在割合は５０％を超えていることがわかる。一方、５％Ｍｇ材では成長した合金層中のＦｅ酸化物の存在状況は加熱時間１０分の場合とほとんど変わらず、良好な耐高温酸化特性を有することがわかる。
【００２０】
図２は、Ａｌ−９％ＳｉベースでＭｇ含有量が０〜５％の各溶融Ａｌ基めっきを施した材料について、上記と同様の曲げ加工を施し、大気中６００℃で１０００時間加熱した場合の加工部の断面組織観察像（光学顕微鏡像）である。０％Ｍｇ材では成長した合金層を貫くＦｅ酸化物が著しく成長し、最表面もＦｅ酸化物で覆われてしまっている。これに対し、１〜５％Ｍｇ材では、当初、めっき層と合金層を貫くクラックが生じていた箇所ではＦｅ酸化物が最表層まで成長しているが、それ以外の部分ではＦｅ酸化物の生成は顕著に抑制されている。これらのＭｇ含有材は、成長した合金層と鋼板素地との界面において、Ｆｅ酸化物相の存在割合が５０％以下に維持されている。このような被覆構造は長時間での高温酸化に対し優れた耐久性を示し、６００℃程度の大気に曝したとき、最表面がＦｅ酸化物で覆われるようなことはない。したがって、この被覆構造を「耐高温酸化被覆構造」と呼ぶことができる。
【００２１】
図３には、０％Ｍｇ材と５％Ｍｇ材を例に、大気中６００℃で加熱したときの加工部表層付近の初期酸化挙動を模式的に表した断面図を示す。
（ａ）は加熱前の状態であり、いずれも合金層にクラックが生じているが、めっき層を貫くクラックは０％Ｍｇ材より５％Ｍｇ材の方が多いことを表している。
（ｂ）は５分程度加熱した段階であり、いずれも合金層のクラック部分に鋼板素地との界面付近からＦｅ酸化物が生成しはじめているが、５％Ｍｇ材ではＦｅ酸化物を取り囲むようにしてＭｇまたはＭｇ酸化物の濃化が生じている様子を表している。
【００２２】
（ｃ）は１０分程度加熱した段階であり、０％Ｍｇ材では合金層の成長に伴ってＦｅ酸化物が横に幅を広げながら成長しているのに対し、５％Ｍｇ材ではめっき層を貫いていたクラック部分でＦｅ酸化物の成長が見られるものの、それ以外の部分ではＭｇまたはＭｇ酸化物の濃化がＦｅ酸化物の成長をくい止めている様子を表したものである。
（ｄ）は数時間以上加熱した段階であり、０％Ｍｇ材では各クラック部分から成長したＦｅ酸化物が最表面まで成長して、まもなく表面を覆い尽くすようになるのに対し、５％Ｍｇ材ではめっき層を貫いていたクラック部分のＦｅ酸化物のみが表面まで成長し、それらの間隔が離れているため表面がＦｅ酸化物で覆われるような状態にはならないことを表している。
【００２３】
以下、本発明を特定するための事項について説明する。
本発明では、溶融Ａｌ基めっき鋼板を加工した材料、すなわち「溶融Ａｌ基めっき鋼板加工材」を対象とする。加工度が小さい場合でも、Ｍｇを適量含有しためっき組成とすることにより、耐高温酸化性は向上する。ただし、合金層に多数のクラックが生じるような加工度のとき、その向上効果は大きくなる。例えば、合金層のみを貫通するクラック数がめっき層をも貫通するクラック数の２倍以上になるような加工部では、Ｍｇによる耐高温酸化性向上効果が特に顕著に現れる。自動車排気系部材をはじめ、溶融Ａｌ基めっき鋼板が適用される多くの構造部材や機械部品の加工部がこれに相当する。
【００２４】
本発明の加工材において加工部の耐高温酸化性が向上する主たるメカニズムは、先に述べたように、加熱初期の段階において、合金層のクラック部分にＦｅ酸化物を取り囲むようにしてＭｇまたはＭｇ酸化物が濃化し、これがその後のＦｅ酸化物の成長をくい止めることによる。したがって、加熱開始から短時間で速やかにＭｇまたはＭｇ酸化物の濃化が生じなければＦｅ酸化物の成長を効果的に抑えることはできない。種々検討の結果、本発明で対象とする加工材は、大気中６００℃で加熱したとき、加熱時間５分で合金層中のクラック部にＭｇまたはＭｇ酸化物が濃化する性質を有しているものであることが好ましい。その濃化は、ＥＰＭＡによる分析で明らかに認められる程度のものを意味する。
【００２５】
合金層中のクラック部に濃化するＭｇは溶融めっき層中から供給されると考えられる。詳細な研究の結果、溶融Ａｌ基めっき層中のＭｇ含有量が０．５質量％以上のとき、前記の濃化現象が起こり、加工部の耐高温酸化性の向上効果が現れることがわかった。ただし、８質量％を超えるとめっき浴表面上にドロスが発生し易くなり、めっき浴表面を非酸化性雰囲気に保った場合においてもめっき鋼板の表面性状を良好にできない。したがって本発明ではめっき層中のＭｇ含有量を０．５〜８質量％に規定した。
【００２６】
発明者らは種々のＭｇ含有量の試験片について、種々の温度で大気中１０００時間の加熱実験を行い、加熱後の加工部外観を調査した。
それによると、加熱温度が４００℃の場合、めっき層中のＭｇ含有量が０〜８％のものにおいて、加工部外観の差異はほとんど認められなかった。
５００℃では、０％Ｍｇ材は加工部全体に褐色の変色が生じた。これに対し０．５〜８％Ｍｇ材では加工部の褐色化が防止された。
６００℃になると、０％Ｍｇ材は加工部全体が赤茶色にはっきり変色した。Ｍｇ含有材でも０．５〜１．５Ｍｇ材では加工部がわずかに褐色化するものがあったが、０％Ｍｇ材と比較すると顕著な耐高温酸化性を有していた。
【００２７】
発明者らは、加熱温度をさらに上げ、大気中７００℃で１０００時間保持する実験も行った。溶融Ａｌ基めっき鋼板は、通常７００℃もの高温で使用されることはない。実験の結果、めっき層中のＭｇ含有量が０．５〜１．５質量％のものは加工部全体がＦｅ酸化物に覆われ、０％Ｍｇ材とあまり変わらない状態となった。しかし、Ｍｇ含有量が２〜８質量％のものでは、表面全体がＦｅ酸化物に覆われるようなことはなかった。また断面観察の結果、Ｍｇ含有量が２％以上になると、成長した合金層と鋼板素地との界面におけるＦｅ酸化物相の存在割合が０．５〜１．５％Ｍｇ材と比べ大幅に減少し、５０％以下となることがわかった。したがって、２〜８％Ｍｇ材は、用途によっては７００℃での使用も可能であると考えられた。
以上のことから、特に厳しい環境下での信頼性を重視する場合、めっき層中のＭｇ含有量は２〜８質量％にするのが好ましい。
【００２８】
溶融Ａｌ基めっき層中のＳｉは、合金層を薄くして、めっき層に良好な加工性を付与するので、めっき浴にＳｉを添加することが望ましい。ただし、Ｓｉ含有量が１３質量％を超えるめっき組成では、めっき層中に塊状の析出物が生じ、加工を施したときにめっき層を貫通するクラックが極端に多くなり、加工部の耐高温酸化性は劣化する。このため、本発明ではめっき層中のＳｉ含有量を０（無添加）〜１３質量％に規定した。より好ましいＳｉ含有量の範囲は３〜１３質量％である。
【００２９】
めっき層中のＳｉ，Ｍｇ以外の残部は、Ａｌおよび不可避的不純物としてもよいし、あるいは、ＭｇおよびＳｉの前記効果を阻害しないような他の元素をさらに含有してもよい。ただし、「Ａｌ基」であるから、少なくとも５０質量％以上はＡｌである必要がある。
【００３０】
他の添加元素として、例えばＳｒ，Ｃａ，Ｂｅ，Ｂａ等が挙げられる。これらの元素は、Ｍｇを含有した溶融Ａｌ基めっき鋼板の表面性状を改善する効果がある。すなわち、めっき浴表面が大気雰囲気であるような一般的な溶融めっきラインでは、Ｍｇの添加によりめっき層表面にシワが発生し表面性状を劣化させる。Ｓｒ等の上記元素はシワの発生を抑制する作用を発揮するので、特別なシール構造を有しない設備では添加が有利である。
具体的には、溶融Ａｌ基めっき層の組成を、Ｓｉ：０（無添加）〜１３質量％，Ｍｇ：０．５〜８質量％を含有し、さらにＳｒ：０．００１〜１質量％，Ｃａ：０．００１〜１質量％，Ｂｅ：０．０００１〜０．１質量％，Ｂａ：０．００１〜１質量％のうち１種または２種以上を含有し、残部がＡｌおよび不可避的不純物からなるものとすることが好ましい。
【００３１】
その他の元素として、めっき層中にはＴｉ，Ｂ，Ｃｒ，Ｍｎ，Ｚｎ，Ｙ，Ｚｒ，Ｌａ，Ｃｅ等の元素を微量添加することができる。
なお、溶融めっき鋼板の製造においては、鋼板表面が溶融めっき金属と反応してわずかに溶けるため、めっき層中には鋼板に由来する元素（Ｆｅ等）が不可避的不純物として含有される。
【００３２】
めっきを施す母材（めっき原板）としては種々のものが適用できる。例えば、Ａｌキルド鋼，Ｔｉ添加ＩＦ鋼等が挙げられる。
【００３３】
本発明の耐熱用溶融Ａｌ基めっき鋼板加工材は、４５０〜６５０℃の温度範囲で使用したとき、加工部の耐高温酸化性改善効果が顕著に発揮される。したがって、本発明の加工材は、４５０〜６５０℃の高温で使用されるものを対象とする。なお、めっき層中のＭｇ含有量が特に２〜８質量％のものは、４５０〜７００℃の高温、好ましくは５５０〜７００℃の高温に曝される用途を対象とすることができる。
【００３４】
【実施例】
板厚１．０ｍｍのＡｌキルド鋼をめっき母材として、Ｓｉ：０〜１４質量％，Ｍｇ：０〜８質量％を含有し、場合によってはさらにＳｒ，Ｂｅ，Ｃａ，Ｂａの１種以上を所定量含有し、残部がＡｌおよび不可避的不純物である種々のめっき組成の溶融Ａｌ基めっき鋼板を製造した。めっき付着量は鋼板片面あたり１００ｇ／ｍ^２とした。Ｍｇを含有する場合において、Ｓｒ，Ｂｅ，Ｃａ，Ｂａのいずれかを含有するものはめっき浴表面が大気雰囲気の状態でめっきを行い、これらの元素を含有しないものはめっき浴表面を非酸化性雰囲気に保った状態でめっきを行った。これらのめっき鋼板に内曲げ半径２ｍｍの９０°曲げ加工を施して「溶融Ａｌ基めっき鋼板加工材」の試料を用意した。
【００３５】
これらの試料を用いて大気中で５００℃×７２０分，５５０℃×６０分，６００℃×３〜６分の加熱試験を行い、試験後の試料の加工部断面組織を走査型電子顕微鏡（ＳＥＭ）で観察し、また、ＥＰＭＡにより元素分析を行い、ＭｇまたはＭｇ酸化物の濃化状態を評価した。合金層のクラック部にＦｅ酸化物を取り巻くようにＭｇまたはＭｇ酸化物が濃化した組織となっているものを○、ＭｇやＭｇ酸化物の濃化が認められないものを×として評価した。
【００３６】
さらに、各加熱温度において大気中で１０００時間までの加熱試験を行い、断面組織観察により鋼板素地の酸化の程度（すなわち、成長した合金層中に存在するＦｅ酸化物の生成量）を評価した。具体的には、成長した合金層と鋼板素地との界面におけるＦｅ酸化物相の存在割合を調べた。評価基準は以下のとおりとした。
○：界面におけるＦｅ酸化物相の存在割合が３０％以下
△：界面におけるＦｅ酸化物相の存在割合が３０％を超え５０％以下
×：界面におけるＦｅ酸化物相の存在割合が５０％を超える
これらの結果を表１に示す。
【００３７】
【表１】

【００３８】
Ｍｇを０．５％以上含有するめっき組成のものは、加熱初期段階で合金層中にＭｇまたはＭｇ酸化物の濃化が生じ、大気中１０００時間の加熱でも鋼板素地の酸化の程度は軽度であった。ただし、Ｎｏ．３６はめっき層中のＳｉ含有量が高いため、加工後にめっき層を貫通するクラックが極端に多くなり、その部分で酸化が進行して耐高温酸化性は劣った。なお、Ｎｏ．３６以外は、合金層のみを貫通するクラック数がめっき層をも貫通するクラック数の２倍以上になっていた。
【００３９】
【発明の効果】
めっき層中にＭｇを０．５〜８％含有させた本発明の溶融Ａｌ基めっき鋼板加工材は、従来の溶融Ａｌ基めっき鋼板加工材に比べ、加工部の耐高温酸化性が大幅に向上した。特に大気中４５０〜６５０℃の加熱においてその向上効果が顕著に現れる。また、めっき層中のＭｇ含有量が２〜８％と高いものは、大気中７００℃においても長時間の加工部耐久性を示す。７００℃という温度は従来の溶融Ａｌ基めっき鋼板加工材では適用が考えられなかった温度レベルである。
したがって、本発明は耐熱用溶融Ａｌ基めっき鋼板の用途拡大と、その加工部材の耐久性向上に寄与するものである。
【図面の簡単な説明】
【図１】溶融Ａｌ基めっき鋼板加工材について、Ａｌ−９％Ｓｉめっき材（０％Ｍｇ）とＡｌ−９％Ｓｉ−５％Ｍｇめっき材（５％Ｍｇ）を大気中６００℃で加熱したときの加工部の耐高温酸化特性を比較した断面組織観察像（光学顕微鏡像）である。
【図２】Ａｌ−９％ＳｉベースでＭｇ含有量が０〜５％の各溶融Ａｌ基めっき鋼板加工材を、大気中６００℃で１０００時間加熱したときの加工部の断面組織観察像（光学顕微鏡像）である。
【図３】０％Ｍｇ材と５％Ｍｇ材を例に、大気中６００℃で加熱したときの加工部表層付近の初期酸化挙動を模式的に表した断面図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a material obtained by processing a hot-dip Al-based coated steel sheet, which is a heat-treated hot-dip Al-based coated steel sheet having excellent resistance to high-temperature oxidation at a processed portion, and a high-temperature processed hot-dip Al-based steel sheet. The present invention relates to a coating structure of a processed part formed by using the same, and which exhibits a high-temperature oxidation resistance for a long time thereafter.
[0002]
[Prior art]
A hot-dip Al-based steel sheet containing 9 to 10% by mass of Si in a plating layer shows good high-temperature oxidation resistance at a temperature of approximately 600 ° C. or less and is relatively inexpensive, so is widely used as a low-grade heat-resistant material. Have been. However, when this plated steel sheet is processed into a structural member, a machine part, or the like, there is a problem that the high-temperature oxidation resistance is deteriorated in the processed portion. When the degree of processing is large or the use environment is severe, the durability is significantly reduced.
[0003]
In the surface layer portion of the hot-dip Al-based steel sheet, there is usually an “alloy layer” formed by a reaction between the plated metal and Fe in the base material of the steel sheet under the outermost “plating layer”. This alloy layer is an intermetallic compound mainly composed of Fe and Al, and is generally brittle than the plating layer. When processing such as “bending” is performed on a hot-dip Al-based plated steel sheet, cracks are likely to occur in the alloy layer. Most of them penetrate the alloy layer. When the degree of work increases, cracks also occur in the plating layer, and cracks penetrating from the outermost surface to the steel sheet base are formed. However, the number of such cracks is smaller than cracks penetrating only the alloy layer.
[0004]
When a material obtained by processing a conventional hot-dip Al-based steel sheet is exposed to a high-temperature atmosphere at about 600 ° C., Fe oxides begin to be generated starting from a portion of a crack formed in the alloy layer near the steel sheet base. In both cracks penetrating both the plating layer and the alloy layer and cracks penetrating only the alloy layer, the formation behavior of Fe oxide is almost the same. Then, as the alloy layer grows, the amount of Fe oxide also increases. After several hours, the grown alloy layer is occupied by a large amount of Fe oxide, and the Fe oxide appears on the outermost surface. Become. Then, the high-temperature oxidation resistance due to the hot-dip Al-based plating is impaired. That is, the processed part of the hot-dip Al-based plated steel sheet will be in the same state as the unplated bare steel material while being used at a high temperature.
[0005]
Research has also been conducted to improve the high-temperature oxidation resistance of hot-dip Al-based plated steel sheets. For example, Japanese Patent No. 1520771 discloses a technique in which Ti and Cr are added to a base material (plated original plate). Japanese Patent No. 1578579 discloses a technique of adding Si, Mn, and Ti to a base material. All of these techniques have been put to practical use, and the high-temperature oxidation resistance of the processed part of the hot-dip Al-based plated steel sheet has been somewhat improved. However, in a severe environment in which the material temperature rises to near 600 ° C., it is not possible to prevent the progress of oxidation in the processed portion, and high-temperature oxidation resistance that can withstand such high temperatures has not been obtained.
[0006]
On the other hand, heat-resistant steel or stainless steel is generally used for applications where the material temperature is about 600 ° C. or higher. For example, there is a standard steel type such as SUH409L. In the case of heat-resistant steel or stainless steel, good high-temperature oxidation resistance is maintained even in the processed part.
[0007]
[Problems to be solved by the invention]
Since stainless steel is expensive, it is practically difficult to use it for low-grade heat-resistant members requiring low cost. Further, stainless steel is excellent in high-temperature oxidation resistance in ordinary air, but has a disadvantage that it is considerably inferior to hot-dip Al-based steel sheet in salt damage corrosion resistance.
On the other hand, a hot-dip Al-based plated steel sheet originally has a performance that can withstand a heating environment of about 600 ° C. in the air for a long period of time in a “flat portion” other than the processed portion. However, no drastic improvement in high-temperature oxidation resistance has been known for the processed portion, and there is still room for research.
[0008]
The present invention has been made in view of the above circumstances, and has as its object to provide a work material for a hot-dip Al-based steel sheet that can be used for a long time in a high-temperature atmosphere of about 600 ° C.
[0009]
[Means for Solving the Problems]
The inventors have studied in detail the high-temperature oxidation behavior of a hot-dip Al-based plated steel sheet processed portion. As a result, it has been found that, in the case where an appropriate amount of Mg is contained in the plating layer, the high-temperature oxidation resistance in the processed portion is significantly improved.
[0010]
That is, Mg or Mg oxide is rapidly concentrated in a portion of the crack penetrating the alloy layer near the steel plate base. The concentration becomes sufficiently detectable by EPMA when exposed to the air at 600 ° C. for about 5 minutes. The concentration of Mg or Mg oxide generated in a crack penetrating only the alloy layer has a function of preventing the growth of Fe oxide from becoming extremely difficult. The generation of Fe oxide proceeds at cracks that also penetrate the plating layer. However, since such cracks are originally small, the grown alloy layer is not occupied by a large amount of Fe oxide. As a result of the research, the coating structure in which the proportion of the Fe oxide phase is 50% or less at the interface between the grown alloy layer and the steel sheet base after long-term exposure (at least 30 minutes or more) to the high-temperature atmosphere. It has been found that, in the case of having, the excellent high-temperature oxidation resistance is stably maintained even when the material is heated at that temperature for a long time. The present invention has been completed based on such findings.
[0011]
In order to achieve the above object, an invention according to claim 1 is a material obtained by processing a hot-dip Al-based plated steel sheet, wherein i) the hot-dip Al-based plating layer is Si: 0 (no addition) to 13% by mass, Mg: A hot-dip galvanized steel sheet with excellent oxidation resistance in a processed part, characterized by containing 0.5 to 8% by mass, and ii) being used at a high temperature of 450 to 650 ° C. It is.
[0012]
The invention of claim 2 is further characterized in that iii) an alloy layer is formed between the steel sheet base and the hot-dip coating layer, and iv) cracks penetrating the alloy layer in the processed portion. It is assumed that.
The invention according to claim 3 is further characterized in that, when heated at 600 ° C. for 5 minutes in the atmosphere, Mg or Mg oxide is concentrated in cracks in the alloy layer. Things.
[0013]
The invention according to claim 4 is the heat-treated hot-dip Al-based plated steel material, in which the hot-dip Al-based plating layer particularly contains Si: 0 (no addition) to 13% by mass and Mg: 0.5 to 8% by mass. However, the remaining portion is defined by a point composed of Al and inevitable impurities.
In the invention of claim 5, the hot-dip Al-based plating layer similarly contains Si: 0 (no addition) to 13% by mass, Mg: 0.5 to 8% by mass, and further Sr: 0.001 to 1% by mass. , Ca: 0.001 to 1% by mass, Be: 0.0001 to 0.1% by mass, Ba: 0.001 to 1% by mass, the balance being Al and inevitable. This is a point that defines a point composed of impurities.
[0014]
The invention of claim 6 specifies that in the above-mentioned heat-treated hot-dip Al-based plated steel sheet processed material, the Mg content of the hot-dip Al-based plating layer is 2 to 8% by mass.
The invention of claim 7 specifies that the processed material is a vehicle exhaust system member in the above-mentioned processed material for heat-resistant hot-dip Al-based steel sheet.
[0015]
The invention of claim 8 specifies a coating structure formed on a machined portion of a hot-dip Al-based plated steel sheet and having excellent high-temperature oxidation resistance. That is, the “grown alloy layer / steel base material” formed by using a hot-dip Al-based plated steel sheet containing 13% by mass or less of Si and 0.5 to 8% by mass of Mg in the plating layer at a high temperature. Or a coating structure of a processed portion composed of "residual plating layer / grown alloy layer / steel sheet base", wherein Mg or Mg oxide is concentrated in a crack portion in the grown alloy layer, and The high-temperature oxidation-resistant coating structure of the processed portion, characterized in that the Fe oxide phase is present at an interface of 50% or less at the interface between the alloy layer thus formed and the steel sheet substrate.
[0016]
Here, the `` grown alloy layer '' means an alloy layer that has been expanded at a high temperature, thereby expanding the area by the reaction of the initial plating layer, alloy layer, and steel sheet base immediately after hot-dip plating, Includes those that are in the process of growing. The “interface between the grown alloy layer and the steel sheet substrate” and the “Fe oxide phase” can be grasped by observing a cross section near the surface of the material with an optical microscope or a scanning electron microscope (SEM). "The percentage of the Fe oxide phase existing at the interface between the grown alloy layer and the steel sheet base is 50% or less" means the following state. That is, the "interface" is traced on a cross-sectional structure observation image (for example, a micrograph or an image on a monitor, etc.), and a "measurement line" of a fixed length (for example, 300 μm) is set. Trace to the end, find the sum of the lengths of the Fe oxide phase on the measurement line or the part in contact with one side of the measurement line, and use this as the “Fe oxide phase existing length” . At this time, the ratio obtained by dividing the “length of the Fe oxide phase existing” by the length of the measurement line is 50% or less.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, a steel sheet subjected to a Si-containing hot-dip Al-based plating to which an appropriate amount of Mg is added is used in order to realize a surface state excellent in high-temperature oxidation resistance of a processed portion.
FIG. 1 shows a conventional steel sheet subjected to a hot-dip Al-based plating (Al-9% Si) containing no Mg and a hot-dip Al-based plating (Al-9% Si-5% Mg) containing 5% by mass of Mg. A cross-sectional structure observation image (optical microscope image) of the processed steel sheet comparing the high-temperature oxidation resistance of the processed portion is shown. In each case, the plating base material was an Al-killed steel sheet having a thickness of 1.0 mm, and the amount of plating was 100 g / m ² per one side of the steel sheet. After plating, the sample which was bent 90 ° at the inner bending tip R = 2 mm was heated at 600 ° C in the atmosphere. FIG. 1 shows observations of cross sections near the surface of the processed portion (outside of the bend) of the samples for heating times of 0, 5, 10, and 30 minutes.
[0018]
Both the Al-9% Si plated material (hereinafter, referred to as “0% Mg material”) and the Al-9% Si-5% Mg plated material (hereinafter, referred to as “5% Mg material”) have a plating layer. An alloy layer is formed below. When the cross sections before heating are compared, cracks penetrating the plating layer and the alloy layer are more in the 5% Mg material than in the 0% Mg material. The cracks penetrating only through the alloy layer have the same density.
At the stage of the heating time of 5 minutes, a phase which looks blackish from near the interface with the steel sheet base is starting to be formed in the crack portion of the alloy layer. This was found to be Fe oxide as a result of analysis by EPMA. For the 5% Mg material, Mg or Mg oxide was concentrated so as to surround the Fe oxide.
[0019]
When the heating time reaches 10 minutes, the Fe oxide grows with the 0% Mg material as the alloy layer grows. On the other hand, in the case of the 5% Mg material, the growth of Fe oxide is hardly observed. However, according to a separate detailed investigation, the growth of Fe oxide was observed in cracks that also penetrated the plating layer.
When the heating time reaches 30 minutes, a large amount of Fe oxide is generated in the alloy layer grown with the 0% Mg material. It can be seen that the proportion of the Fe oxide phase at the interface between the grown alloy layer and the steel sheet base exceeds 50%. On the other hand, in the case of the 5% Mg material, the presence state of the Fe oxide in the grown alloy layer was almost the same as that in the case of the heating time of 10 minutes, and it was found that the alloy had good high-temperature oxidation resistance.
[0020]
FIG. 2 shows a case where the same bending process as described above is applied to a material obtained by applying each molten Al-based plating having an Mg content of 0 to 5% on an Al-9% Si base and heated at 600 ° C. in the air for 1000 hours. 3 is an observation image (optical microscope image) of a cross-sectional structure of the processed part. With the 0% Mg material, the Fe oxide penetrating the grown alloy layer remarkably grows, and the outermost surface is covered with the Fe oxide. On the other hand, in the case of the 1 to 5% Mg material, the Fe oxide grows up to the outermost layer at a portion where cracks have penetrated the plating layer and the alloy layer at first, but the Fe oxide of the Fe oxide is grown at other portions. Generation is significantly suppressed. In these Mg-containing materials, the proportion of the Fe oxide phase at the interface between the grown alloy layer and the steel sheet base is maintained at 50% or less. Such a coating structure shows excellent durability against high-temperature oxidation for a long time, and the outermost surface is not covered with Fe oxide when exposed to the air at about 600 ° C. Therefore, this coating structure can be called a “high temperature oxidation resistant coating structure”.
[0021]
FIG. 3 is a cross-sectional view schematically showing the initial oxidation behavior near the surface layer of the processed portion when the material is heated at 600 ° C. in the air, using 0% Mg material and 5% Mg material as examples.
(A) shows a state before heating, in which cracks are generated in the alloy layer, and the cracks penetrating the plating layer are more in the case of the 5% Mg material than in the case of the 0% Mg material.
(B) is a stage where heating is performed for about 5 minutes. In each case, the Fe oxide starts to be generated from the vicinity of the interface with the steel sheet base in the crack portion of the alloy layer. This shows that Mg or Mg oxide is concentrated.
[0022]
(C) is a stage where heating is performed for about 10 minutes. In the case of the 0% Mg material, the Fe oxide grows while expanding the width in accordance with the growth of the alloy layer, whereas in the case of the 5% Mg material, the plating layer is grown. The growth of Fe oxide is observed in the crack portion penetrating through, but Mg or Mg oxide is concentrated in other portions to prevent the growth of Fe oxide.
(D) is a stage where heating is performed for several hours or more. In the case of 0% Mg material, the Fe oxide grown from each crack portion grows to the outermost surface and soon covers the surface, whereas 5% Mg In the material, only the Fe oxide in the crack portion that penetrated the plating layer grew to the surface, and the distance between them was so large that the surface was not covered with the Fe oxide.
[0023]
Hereinafter, matters for specifying the present invention will be described.
In the present invention, a material obtained by processing a hot-dip Al-based plated steel sheet, that is, a “worked material of hot-dip Al-based plated steel sheet” is targeted. Even when the workability is small, the high-temperature oxidation resistance is improved by using a plating composition containing an appropriate amount of Mg. However, when the working ratio is such that a large number of cracks are formed in the alloy layer, the effect of the improvement is large. For example, in a processed portion in which the number of cracks penetrating only the alloy layer is twice or more the number of cracks penetrating the plating layer, the effect of improving the high-temperature oxidation resistance by Mg is particularly remarkable. The processed parts of many structural members and machine parts to which a hot-dip Al-based steel sheet is applied, such as automobile exhaust system members, correspond to this.
[0024]
The main mechanism for improving the high-temperature oxidation resistance of the processed portion in the processed material of the present invention is, as described above, in the initial stage of heating, so that the crack portion of the alloy layer surrounds the Fe oxide in the crack portion of the Mg or Mg. The oxides thicken, which hinders the subsequent growth of Fe oxides. Therefore, the growth of Fe oxides cannot be effectively suppressed unless Mg or Mg oxides are rapidly and rapidly concentrated from the start of heating. As a result of various studies, the work material targeted in the present invention has a property that when heated at 600 ° C. in the air, Mg or Mg oxide is concentrated in cracks in the alloy layer in a heating time of 5 minutes. Preferably. The enrichment means that which is clearly observed in the analysis by EPMA.
[0025]
It is considered that Mg concentrated in the crack portion in the alloy layer is supplied from the hot-dip plating layer. As a result of detailed research, it has been found that when the Mg content in the hot-dip Al-based plating layer is 0.5% by mass or more, the above-described enrichment phenomenon occurs, and the effect of improving the high-temperature oxidation resistance of the processed portion appears. . However, if it exceeds 8% by mass, dross tends to be generated on the surface of the plating bath, and the surface properties of the plated steel sheet cannot be improved even when the surface of the plating bath is kept in a non-oxidizing atmosphere. Therefore, in the present invention, the Mg content in the plating layer is specified to be 0.5 to 8% by mass.
[0026]
The inventors conducted a heating experiment in the atmosphere at various temperatures for 1000 hours on test pieces having various Mg contents, and investigated the appearance of the processed portion after heating.
According to this, when the heating temperature was 400 ° C., when the Mg content in the plating layer was 0 to 8%, almost no difference in the appearance of the processed portion was observed.
At 500 ° C., the 0% Mg material caused brown discoloration in the entire processed portion. On the other hand, in the case of the 0.5 to 8% Mg material, browning of the processed portion was prevented.
At 600 ° C., the entire processed portion of the 0% Mg material was clearly discolored to reddish brown. In the case of the Mg-containing material, the 0.5-1.5Mg material slightly browned the processed portion, but had a remarkable high-temperature oxidation resistance as compared with the 0% Mg material.
[0027]
The inventors also conducted an experiment in which the heating temperature was further increased and the temperature was maintained at 700 ° C. in the atmosphere for 1000 hours. Hot-dip Al-based plated steel sheets are not usually used at temperatures as high as 700 ° C. As a result of the experiment, when the content of Mg in the plating layer was 0.5 to 1.5% by mass, the entire processed portion was covered with the Fe oxide, which was not much different from the 0% Mg material. However, when the Mg content was 2 to 8% by mass, the entire surface was not covered with the Fe oxide. Also, as a result of the cross-sectional observation, when the Mg content becomes 2% or more, the existence ratio of the Fe oxide phase at the interface between the grown alloy layer and the steel sheet substrate is significantly reduced as compared with the 0.5 to 1.5% Mg material. However, it was found that it became 50% or less. Therefore, it was considered that the 2 to 8% Mg material could be used at 700 ° C. depending on the application.
From the above, it is preferable to set the Mg content in the plating layer to 2 to 8% by mass when importance is placed on reliability in a particularly severe environment.
[0028]
Since Si in the hot-dip Al-based plating layer makes the alloy layer thin and imparts good workability to the plating layer, it is desirable to add Si to the plating bath. However, with a plating composition having a Si content of more than 13% by mass, massive precipitates are formed in the plating layer, and the number of cracks penetrating the plating layer becomes extremely large when the plating is performed. The property deteriorates. For this reason, in the present invention, the Si content in the plating layer is specified to be 0 (no addition) to 13% by mass. A more preferable range of the Si content is 3 to 13% by mass.
[0029]
The balance other than Si and Mg in the plating layer may be Al and inevitable impurities, or may further contain other elements that do not impair the effects of Mg and Si. However, since it is an “Al group”, at least 50% by mass or more needs to be Al.
[0030]
Examples of other additional elements include, for example, Sr, Ca, Be, and Ba. These elements have the effect of improving the surface properties of the Mg-containing hot-dip Al-based steel sheet. That is, in a general hot-dip plating line in which the plating bath surface is in the atmosphere, the addition of Mg causes wrinkles on the plating layer surface to deteriorate the surface properties. Since the above elements such as Sr exert an action of suppressing the generation of wrinkles, their addition is advantageous in equipment having no special seal structure.
Specifically, the composition of the hot-dip Al-based plating layer contains Si: 0 (no addition) to 13% by mass, Mg: 0.5 to 8% by mass, Sr: 0.001 to 1% by mass, One or more of Ca: 0.001 to 1% by mass, Be: 0.0001 to 0.1% by mass, Ba: 0.001 to 1% by mass, with the balance being Al and inevitable impurities It is preferable to be composed of
[0031]
As other elements, trace elements such as Ti, B, Cr, Mn, Zn, Y, Zr, La, and Ce can be added to the plating layer.
In the production of a hot-dip coated steel sheet, since the surface of the steel sheet reacts with the hot-dip coated metal and is slightly melted, an element (Fe or the like) derived from the steel sheet is contained as an inevitable impurity in the plated layer.
[0032]
Various materials can be applied as a base material (plated original plate) to be plated. For example, Al-killed steel, IF-added IF steel, and the like can be given.
[0033]
When used in a temperature range of 450 to 650 ° C, the hot-dip hot-dip Al-based plated steel sheet processing material of the present invention exhibits a remarkable effect of improving the high-temperature oxidation resistance of the processed portion. Therefore, the processed material of the present invention is intended for a material used at a high temperature of 450 to 650 ° C. In addition, the thing whose content of Mg in a plating layer is 2-8 mass% especially can be aimed at the use exposed to high temperature of 450-700 degreeC, Preferably it is 550-700 degreeC.
[0034]
【Example】
Using 1.0 mm Al-killed steel as a plating base material, it contains 0 to 14% by mass of Si and 0 to 8% by mass of Mg, and may further contain one or more of Sr, Be, Ca, and Ba. Hot-dip Al-based steel sheets having various plating compositions containing a predetermined amount and the balance being Al and inevitable impurities were produced. The coating weight was 100 g / m ² per one side of the steel sheet. In the case of containing Mg, those containing any of Sr, Be, Ca, and Ba perform plating with the plating bath surface in an air atmosphere, and those not containing these elements make the plating bath surface non-oxidizing. Plating was performed while maintaining the atmosphere. These plated steel sheets were subjected to a 90 ° bending process with an inner bending radius of 2 mm to prepare samples of “processed material for hot-dip Al-based plated steel sheets”.
[0035]
Using these samples, a heating test was performed in the air at 500 ° C. × 720 minutes, 550 ° C. × 60 minutes, and 600 ° C. × 3 to 6 minutes. ), And elemental analysis was performed by EPMA to evaluate the concentration state of Mg or Mg oxide. Those having a structure in which Mg or Mg oxide was concentrated so as to surround the Fe oxide in the crack portion of the alloy layer were evaluated as ○, and those in which no Mg or Mg oxide was observed were evaluated as ×.
[0036]
Furthermore, a heating test was performed in the air at each heating temperature for up to 1000 hours, and the degree of oxidation of the steel sheet base (that is, the amount of Fe oxides present in the grown alloy layer) was evaluated by observing the cross-sectional structure. Specifically, the existence ratio of the Fe oxide phase at the interface between the grown alloy layer and the steel sheet base was examined. The evaluation criteria were as follows.
：: the proportion of the Fe oxide phase at the interface is 30% or less △: the proportion of the Fe oxide phase at the interface exceeds 30% and 50% or less ×: the proportion of the Fe oxide phase at the interface exceeds 50% Table 1 shows the results.
[0037]
[Table 1]

[0038]
In the plating composition containing 0.5% or more of Mg, Mg or Mg oxide is concentrated in the alloy layer at the initial stage of heating, and the degree of oxidation of the steel sheet substrate is slight even by heating for 1000 hours in the air. there were. However, no. In No. 36, since the Si content in the plating layer was high, cracks penetrating the plating layer after processing became extremely large, and oxidation proceeded in that portion, resulting in poor high-temperature oxidation resistance. In addition, No. Except for 36, the number of cracks penetrating only the alloy layer was twice or more the number of cracks penetrating the plating layer.
[0039]
【The invention's effect】
The hot-dip Al-based coated steel sheet processed material of the present invention in which 0.5 to 8% of Mg is contained in the plating layer has significantly improved high-temperature oxidation resistance in the processed part as compared with the conventional hot-dip Al-based steel sheet processed material. did. In particular, the effect of improvement is remarkable when heated at 450 to 650 ° C. in the atmosphere. In addition, those having a high Mg content of 2 to 8% in the plating layer exhibit long-term processed part durability even at 700 ° C. in the atmosphere. The temperature of 700 ° C. is a temperature level that could not be considered in a conventional hot-dip Al-based plated steel sheet material.
Therefore, the present invention contributes to expanding the use of the heat-resistant hot-dip Al-based steel sheet and improving the durability of the processed member.
[Brief description of the drawings]
FIG. 1 A hot-dip Al-based plated steel sheet processed material was heated at 600 ° C. in the air by heating an Al-9% Si plated material (0% Mg) and an Al-9% Si-5% Mg plated material (5% Mg). It is a cross-sectional structure observation image (optical microscope image) comparing the high temperature oxidation resistance of the processed part at the time.
FIG. 2 is a cross-sectional structure observation image (optical image) of a processed portion of a hot-dip Al-based plated steel sheet having an Mg content of 0 to 5% based on Al-9% Si when heated at 600 ° C. for 1000 hours in the air. (Microscopic image).
FIG. 3 is a cross-sectional view schematically showing an initial oxidation behavior near the surface layer of a processed portion when heated at 600 ° C. in the air, using a 0% Mg material and a 5% Mg material as examples.

Claims

A material obtained by processing a hot-dip Al-based steel sheet,
The hot-dip Al-based plating layer contains Si: 0 (no addition) to 13% by mass, and Mg: 0.5 to 8% by mass;
Being used at a high temperature of 450 to 650 ° C.,
A hot-dip hot-dip Al-based plated steel material with excellent oxidation resistance.

A material obtained by processing a hot-dip Al-based steel sheet,
The hot-dip Al-based plating layer contains Si: 0 (no addition) to 13% by mass, and Mg: 0.5 to 8% by mass;
That an alloy layer is formed between the steel sheet base and the hot-dip coating layer,
In the processed part, there is a crack penetrating the alloy layer,
Being used at a high temperature of 450 to 650 ° C.,
A hot-dip hot-dip Al-based plated steel material with excellent oxidation resistance.

A material obtained by processing a hot-dip Al-based steel sheet,
The hot-dip Al-based plating layer contains Si: 0 (no addition) to 13% by mass, and Mg: 0.5 to 8% by mass;
That an alloy layer is formed between the steel sheet base and the hot-dip coating layer,
In the processed part, there is a crack penetrating the alloy layer,
When heated at 600 ° C. for 5 minutes in the atmosphere, it has a property that Mg or Mg oxide is concentrated at cracks in the alloy layer;
Being used at a high temperature of 450 to 650 ° C.,
A hot-dip hot-dip Al-based plated steel material with excellent oxidation resistance.

The hot-dip Al-based plating layer contains Si: 0 (no addition) to 13% by mass and Mg: 0.5 to 8% by mass, and the balance consists of Al and inevitable impurities. 2. A heat-resistant hot-dip Al-based plated steel sheet processing material according to item 1.

The hot-dip Al-based plating layer contains Si: 0 (no addition) to 13% by mass, Mg: 0.5 to 8% by mass, Sr: 0.001 to 1% by mass, and Ca: 0.001 to 1% by mass. %, Be: 0.0001 to 0.1% by mass, Ba: 0.001 to 1% by mass, and the balance consists of Al and unavoidable impurities. Item 6. A heat-resistant hot-dip Al-based plated steel sheet processed material according to any one of Items 1 to 3.

The hot-dip galvanized steel sheet processing material for heat resistance according to claim 1, wherein the Mg content of the hot-dip Al-based plating layer is 2 to 8% by mass.

The heat-treated hot-dip Al-based plated steel sheet processed material according to any one of claims 1 to 6, wherein the processed material is an automobile exhaust system member.

“Growed alloy layer / steel base” formed by using a hot-dip Al-based plated steel sheet containing 13% by mass or less of Mg and 0.5 to 8% by mass of Mg in a plating layer at a high temperature; Or a coating structure of a processed portion consisting of “residual plating layer / grown alloy layer / steel plate base”, wherein Mg or Mg oxide is concentrated in a crack portion in the grown alloy layer, and the grown alloy A high-temperature oxidation-resistant coating structure in a processed portion, wherein an Fe oxide phase is present at an interface of 50% or less at an interface between the layer and the steel sheet base.