JP3715220B2 - Zn-Al-Mg hot-dip galvanized steel with excellent corrosion resistance - Google Patents

Description

【０００１】
【産業上の利用分野】
本発明は、建築用資材，各種構造材，機械構造部品，配管等として使用され、長期間にわたって優れた耐食性及び機械強度を維持するＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼材に関する。
【０００２】
【従来の技術】
溶融めっき鋼板は、優れた耐食性を活用し、腐食雰囲気に曝される屋根材，構造材，配管，部品等、広範な用途に使用されている。なかでも、Ｚｎ−Ａｌ−Ｍｇ系溶融めっき鋼板は、環境悪化が深刻な昨今において長期間にわたり優れた耐食性や機械強度を維持することから、従来の亜鉛めっき鋼板に代わる材料として注目されている。
Ｚｎ−Ａｌ−Ｍｇ系溶融めっき鋼板の表面に形成されているめっき層は、Ａｌ／Ｚｎ／Ｚｎ₂Ｍｇの三元共晶マトリックスに初晶Ａｌ相又は初晶Ａｌ相とＺｎ単相が分散した組織をもっており、Ａｌ及びＭｇにより耐食性が向上している。特にＭｇ由来の緻密で安定な腐食生成物がめっき層表面に均一に形成されることから、めっき層の耐食性が格段に向上する。
【０００３】
【発明が解決しようとする課題】
Ｚｎ−Ａｌ−Ｍｇ系溶融めっき鋼板は、本出願人が特開平１０−２２６８６５号公報，特開平１０−３０６３５７号公報等で紹介しためっき鋼板であり、高耐食性を活用して種々の分野における用途開発が進められている。その過程で、従来の溶融亜鉛めっき鋼板では問題にされていなかった割れが散見されることが判った。たとえば、オープンパイプ形状に成形したＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼板を溶接して溶接鋼管を製造するとき、熱影響部に割れが生じやすい。そこで、本出願人等は、溶接条件を制御することにより溶接割れを防止することを特願２０００−３０８０８３号で提案した。
【０００４】
溶接条件の制御によって溶接割れは確かに抑制されるが、この方法では溶接工程を経て製造される溶接鋼管に適用対象が限られる。同様な割れは、製品形状に組み立てたＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼板を張力負荷状態で使用する場合にも生じる。割れが発生すると、割れを介した下地鋼の露出部が腐食発生の起点となり、Ｚｎ−Ａｌ−Ｍｇ系めっき層の高耐食性が損なわれることは勿論、機械強度や疲労特性も低下する。
【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、下地鋼及び溶融めっき層の組成を特定の組合せとすることにより、Ｚｎ−Ａｌ−Ｍｇ系溶融めっき鋼板に生じがちな割れを抑え、長期間にわたり本来の高耐食性を活用できるＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼材を提供することを目的とする。
【０００６】
本発明のＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼材は、その目的を達成するため、Ｃ：０．０００５〜０．２５質量％，Ｎ：０．００７質量％以下，Ｓｉ：１．５質量％以下，Ｍｎ：０．０５〜２．０質量％，Ａｌ：０．００５〜０．１０質量％，Ｂ：０．００００２〜０．０１質量％を含み，必要に応じＴｉ，Ｎｂ，Ｖ，Ｚｒの１種又は２種以上：合計で０．０１〜１．２０質量％を含み、残部がＦｅ及び不可避的不純物からなる組成を有する下地鋼に、Ａｌ：４〜２２質量％，Ｍｇ：０．０５〜１０質量％，Ｂ：０．００１〜０．０４５質量％を含み、残部がＺｎ及び不可避的不純物からなる組成を有する溶融めっき層が形成されていることを特徴とする。
【０００７】
下地鋼は、更にＣｕ：０．０５〜２．０質量％，Ｎｉ：０．０２〜２．０質量％，Ｃｒ：０．０２〜１．０質量％，Ｐ：０．０３０〜０．１２質量％の１種又は２種以上を含むことができる。Ｚｎ−Ａｌ−Ｍｇ系溶融めっき層は、更にＴｉ：０．００２〜０．１質量％，Ｓｉ：２．０質量％以下の１種又は２種を含むことができる。
このＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼材は、連続搬送される鋼帯を溶融めっき浴に導入する連続溶融めっき法，所定形状に成形した鋼材を溶融めっき浴に浸漬するドブ漬け法の何れでも製造される。
【０００８】
【作用】
本発明者等は、Ｚｎ−Ａｌ−Ｍｇ系溶融めっき鋼板に生じる割れの発生メカニズムを種々の観点から調査・研究した。その結果、結晶粒界に沿った溶融金属脆化が割れの発生原因であることを究明した。溶融金属脆化は、下地鋼に接触した溶融めっき金属が凝固するまでの段階で溶融めっき金属又はその成分が下地鋼の結晶粒界に浸透し、結晶粒界を脆化させる現象である。同様な現象は、溶融めっき層が再溶融する溶接時においても生じる。
溶融亜鉛めっき鋼板を溶接する際に割れ発生原因となる溶融金属脆化の抑制には、下地鋼の低成分化，ＰやＳの添加，ＺｒやＴｉの添加が有効な対策であると報告されている（川崎製鉄技報第２５巻第１号第２０〜２６頁）。しかし、これらの対策は、Ｚｎ−Ａｌ−Ｍｇ系溶融めっき鋼板に生じる溶融金属脆化には有効でない。実際、本発明者等による調査・研究では、下地鋼の低成分化，Ｓの添加によって溶融金属脆化に起因する割れ発生に有意差が検出できなかった。また、割れ感受性に悪影響を及ぼすとされているＢがＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼板の割れ防止に有効なことが判った。
【０００９】
割れ防止策の相違は、溶融亜鉛めっき鋼板及びＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼板の溶接時に生じる割れは発生メカニズムが異なることを示唆している。本発明者等は、Ｚｎ−Ａｌ−Ｍｇ系溶融めっき鋼板について割れ発生メカニズムを次のように推察した。
連続溶融めっきやドブ漬けめっき等の際、Ｚｎ−Ａｌ−Ｍｇ合金めっき浴がめっき原板に接触すると、活性度の高いＡｌがめっき原板のＦｅと先ず反応し、原板表面にＦｅ−Ａｌ合金層が形成される。Ｆｅ−Ａｌ合金層を介しＺｎ−Ａｌ−Ｍｇ系めっき層が形成されるが、Ｆｅ−Ａｌ合金層の形成に伴ってＺｎ−Ａｌ−Ｍｇ系めっき層のＡｌ濃度が低下し、反射効的にＭｇ濃度が増加する。Ｍｇ濃度の増加は、Ｚｎ−Ａｌ−Ｍｇ系めっき層を融点降下させる。因みに、Ｚｎ−３質量％Ｍｇでは凝固終了温度が３６０℃まで低下する。
【００１０】
Ｚｎ−Ａｌ−Ｍｇ系めっき層の生成・成長に伴う組成変化及び融点降下は、通常の溶融亜鉛めっき層の生成・成長過程ではみられない現象である。融点降下のため、溶融状態又は半溶融状態にあるＺｎ−Ａｌ−Ｍｇ合金が下地鋼に接触する時間が長くなり、下地鋼の結晶粒界に対するＺｎ−Ａｌ−Ｍｇ合金の浸透が進行する。
Ｚｎ−Ａｌ−Ｍｇ系めっき層の生成・成長過程を前提にし、本発明者等は、下地鋼の結晶粒界を強化する元素としてＢを使用することに想到した。結晶粒界を強化するＢの作用自体は従来から知られている技術であるが、Ｚｎ−Ａｌ−Ｍｇ系めっき層の生成・成長過程で溶融金属脆化の抑制にＢが有効なことは、本発明者等が見出した新規な知見である。また、溶融金属脆化に起因する割れがＢ添加で顕著に抑制されることから、結晶粒界の強化に留まらず、Ｚｎ−Ａｌ−Ｍｇ系めっき層の生成・成長にも何らかの影響をＢが及ぼしていることが窺われる。その結果、Ｚｎ−Ａｌ−Ｍｇ系めっき層では、「Ｂは悪影響を及ぼす」（川崎製鉄技報第２４頁右欄下から１１行）ことなく、溶融金属脆化が抑制されるものと推察される。
【００１１】
Ｂは、α結晶粒界に偏析して粒界強度を高め、γ結晶の粒界エネルギーを下げて焼入れ性を向上する作用を呈する。また、Ｚｎ−Ａｌ−Ｍｇ系溶融めっき鋼板では、下地鋼の結晶粒界にＺｎ−Ａｌ−Ｍｇ合金が浸透すること、溶融又は半溶融状態のＺｎ−Ａｌ−Ｍｇ合金に鋼成分が結晶粒界から溶け出すことを防止する作用を呈するものと考えられる。このようなＢの作用・効果を発現させる上で、下地鋼のマトリックスに固溶している有効Ｂ量を確保することが要求される。
下地鋼に含まれているＮは、フリーのＢと反応してＢＮ化合物となり、有効Ｂ量を大幅に低減する。したがって、下地鋼としてＮを低減し、或いはＴｉ，Ｎｂ，Ｖ，Ｚｒ等でＮを固定した鋼材を使用することにより、有効Ｂ量を低減するＮの影響が抑えられ、溶融金属脆化に起因する割れが抑制されたＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼板が得られる。
【００１２】
以上の考察から、本発明では、下地鋼及びＺｎ−Ａｌ−Ｍｇ系めっき層の組成を次のように特定した。
〔下地鋼〕
Ｃ：０．０００５〜０．２５質量％
フェライト相に固溶すると共に炭化物を形成することにより延性を低下させる合金成分であり、高い加工性が要求される用途ではＣ含有量が低いほど好ましい。他方、鋼材を強化する作用も呈することから、構造材等の用途ではＣ含有量を増加させて高強度化する必要がある。このような観点から、本発明ではＣ含有量の上限を０．２５質量％に設定した。加工性が特に要求される用途では、Ｃ含有量の上限を０．０１質量％と設定することにより固溶Ｂ量を確保することが好ましい。他方、０．０００５質量％未満にＣ含有量を下げることは、製造工程で過度の脱炭精錬を必要とし、製造コストを上げる原因となる。
【００１３】
Ｎ：０．００７質量％以下
Ｂと反応してＢＮを生成し、マトリックスに固溶している有効Ｂ量を消費する成分であるので、Ｎ含有量は可能な限り低いほうが好ましい。しかし、過度にＮ含有量を下げることは、鋼材の製造コストを上昇させる原因となる。そこで、Ｎ含有量の上限を０．００７質量％に設定する。固溶Ｂに与えるＮの悪影響は、Ｔｉ，Ｎｂ，Ｖ，Ｚｒ等でＮを固定することによっても抑制できる。しかし、０．００７質量％を超えるＮ含有量では、Ｎの固定に必要なＴｉ，Ｎｂ，Ｖ，Ｚｒの添加量が多くなり、鋼材清浄度や加工性に悪影響が現れやすくなる。
【００１４】
Ｓｉ：１．５質量％以下
フェライト相に固溶し鋼材の強度向上に有効な成分であるが、多量のＳｉが含まれる鋼材では鋼材表面にＳｉ濃化層が生じ、めっき性が低下する。そのため、Ｓｉ含有量の上限を１．５質量％に設定した。Ｓｉ含有量の増加に応じて延性が低下するので、加工性が要求される用途ではＳｉ含有量の上限を０．３０質量％に設定することが好ましい。
Ｍｎ：０．０５〜２．０質量％以下
熱延時にＳ起因の脆化を防止すると共に強度向上に有効な成分であり、０．０５質量％以上の含有量でＭｎの添加効果が顕著になる。しかし、加工性や溶接性が要求される用途では低いＭｎ含有量ほど望ましく、鋼材表面に濃化してめっき性にも悪影響を及ぼすことから、Ｍｎ含有量の上限を２．０質量％に設定した。
【００１５】
Ａｌ：０．００５〜０．１０質量％
製鋼時に脱酸剤として添加される成分であり、鋼中のＮをＡｌＮとして固定し、Ｎの時効硬化に起因した加工性の低下を防止すると共に、ＢＮとして消費されやすい有効Ｂ量を確保する上でも有効である。このような効果は、０．００５質量％以上のＡｌ含有量で顕著になる。ＡｌによるＮの固定を前提にすると、Ｎ≦Ａｌ×０．５２が満足されるようにＡｌ含有量を設定することが好ましい。しかし、０．１０質量％を超える過剰量のＡｌ含有では酸化物系介在物が増加し、加工性やめっき性に悪影響が現れる。
【００１６】
Ｂ：０．００００２〜０．０１０質量％
溶融金属脆化の抑制に有効な合金成分であり、０．００００２質量％以上でＢの添加効果が顕著になり、０．０１０質量％で添加効果が飽和する。０．０１０質量％を超える過剰量のＢ添加は、硼化物の生成，結晶粒の成長阻害等を促し、鋼板の加工性を劣化させる原因にもなる。結晶粒界へのＢ偏析は、結晶粒界の生成・移動及びＢの拡散速度に依存し、変態したままのα粒界等では有効なＢの粒界偏析が期待できないことがある。このような場合、Ｂを結晶粒界に十分拡散させる製造条件が冷延・焼鈍工程で採用される。
【００１７】
Ｔｉ，Ｎｂ，Ｖ，Ｚｒの１種又は２種以上：合計で０．０１〜１．２０質量％
必要に応じて添加される合金成分であり、何れもＮを窒化物として固定し、溶融金属脆化抑制に有効な固溶Ｂ量を確保する作用を呈し、０．０１質量％以上で添加効果が顕著になる。Ｃの固定に有効でもある。しかし、１．２０質量％を超える過剰添加は、製造コストの上昇を招くことは勿論、鋼材の加工性を劣化させる。
下地鋼に含まれているＴｉ，Ｎｂ，Ｖ，Ｚｒ等は、溶融めっき浴に浸漬された鋼板からＦｅが溶出する際に同時に溶融めっき浴に溶出する。鋼板近傍の溶融めっき浴は鋼板が溶融めっき浴から引き上げられた後で溶融めっき層となるものであり、溶融めっき浴に溶解したＴｉ，Ｎｂ，Ｖ，ＺｒやＢの多くは溶融めっき層に取り込まれる。その結果、溶融めっき層の組織が微細化され、均一で美麗な外観をもつ溶融めっき層が形成される。
【００１８】
本発明で使用する下地鋼は、更にＣｕ，Ｎｉ，Ｃｒ，Ｐの１種又は２種以上を含むことができる。これら任意成分のうち、ＣｕはＰと複合して耐食性を改善し、強度向上にも有効であり、０．０５質量％以上で添加効果が顕著になるが、２．０質量％を超える過剰添加では延性の低下を招く。Ｎｉも強度向上に有効な成分であり、０．０２質量％以上で添加効果が顕著になるが、２．０質量％を超える過剰添加では延性の低下を招く。Ｃｒは強度向上及び母材耐食性の改善に有効な成分であり、０．０５質量％以上で添加効果が顕著になるが、１．０質量％を超える過剰添加は加工性の劣化を招く。Ｐを合金成分として添加する場合、０．０３０〜０．１２質量％の範囲でＰ含有量を選定する。
【００１９】
下地鋼には、製鋼上からＰ，Ｓ等が不純物として混入する。不純物としてのＰは延性に悪影響を及ぼすので、高い加工性が要求される用途ではＰ含有量を可能な限り低くすることが好ましい。しかし、強度改善にも働くことから高強度が要求される用途では、加工性やめっき性に悪影響を与えない０．１２質量％までの範囲でＰを含有させてもよい。Ｓは熱間脆化の原因であり、加工性，耐食性を劣化させる有害成分であるので、快削性等の特殊性能が要求される用途を除き、可能な限り低く（具体的には、０．０３質量％以下）に規制することが好ましい。
【００２０】
〔Ｚｎ−Ａｌ−Ｍｇ系めっき層〕
Ａｌ：４〜２２質量％
Ａｌは、めっき層からほとんど溶出することなく、当初のめっき層であった部分にＺｎ−Ａｌ系腐食生成物を形成する。Ｚｎ−Ａｌ系腐食生成物は、極めて固着性が強く、上層にあるＭｇ含有Ｚｎ系腐食生成物が腐食過程で消失しても、環境遮断機能のあるバリアとなって下地めっき層の腐食を抑制する。Ｚｎ−Ａｌ系腐食生成物の一部は、環境中のＳＯｘを取り込み、より強固な保護皮膜としても作用する。固着性が強く下地に対するバリアとして働くＺｎ−Ａｌ系腐食生成物を形成するためには、４質量％のＡｌ含有量が必要である。また、溶融めっき層形成時に鋼中Ｎと反応してＡｌＮとなり、固溶Ｂにとって有害なＮを低減する上でも有効である。しかし、２２質量％を超える過剰量のＡｌが含まれると、Ｚｎ−Ａｌ系腐食生成物による効果が飽和するばかりでなく、めっき層の加工性も低下する。
【００２１】
Ｍｇ：０．０５〜１０質量％
めっき層に含まれるＭｇは，めっき層最表層にＭｇを含むＺｎ系腐食生成物を形成し、屋外等の一般腐食環境下でめっき層の腐食速度を抑える効果を奏する。このような作用は、０．０５質量％以上のＭｇ含有量でみられ、Ｍｇ：１０質量％で飽和する。１０質量％を超える過剰量のＭｇ含有は、ドロスの多量発生等によって溶融めっき浴の安定性を低下させる。好ましくは、１．０〜１０質量％の範囲でＭｇ含有量を選定する。
【００２２】
Ｔｉ：０．００２〜０．１質量％，Ｂ：０．００１〜０．０４５質量％
共に任意成分として添加される合金成分であり、表面外観に悪影響を及ぼすＺｎ₁₁Ｍｇ₂相の生成を抑制し、めっき層中に晶出するＺｎ−Ｍｇ系金属間化合物を実質的にＺｎ₂Ｍｇのみにする作用を呈する。具体的には、０．００２質量％以上のＴｉを含ませると、Ｚｎ₁₁Ｍｇ₂相の生成が効果的に抑制される。しかし、０．１質量％を超える過剰量のＴｉが含まれると、めっき層中にＴｉ−Ａｌ系析出物が成長してめっき層に凹凸が生じ、外観が劣化しやすい。Ｚｎ₁₁Ｍｇ₂相の生成は、０．００１質量％以上のＢを含ませることによっても抑制される。Ｂ含有の場合でも、０．０４５質量％を超える過剰量ではＴｉ−Ｂ系，Ａｌ−Ｂ系析出物がめっき層中に析出し、同様に外観劣化の原因となる凹凸のあるめっき層が生じやすくなる。また、ＴｉやＢを含む鋼材を下地鋼に使用しているので、下地鋼から溶融めっき浴に溶出したＴｉ，Ｂによっても同様な効果が奏せられる。
【００２３】
Ｓｉ：０．００５〜２．０質量％
必要に応じて添加される合金成分であり、下地鋼／めっき層の界面にＦｅ−Ａｌ金属間化合物が厚く成長することを抑え、Ｚｎ−Ａｌ−Ｍｇ系溶融めっき鋼板の加工性を向上する作用を呈する。溶融めっき層の黒変化を防止し、表面の光沢性を維持する上でも有効な成分である。Ｆｅ−Ａｌ金属間化合物の生成抑制に及ぼすＳｉの作用は０．００５質量％以上の含有量でもみられるが、２．０質量％を超える過剰量のＳｉが含まれると、ポットに収容している溶融めっき金属に発生するドロス量が多くなる。
【００２４】
Ｚｎ−Ａｌ−Ｍｇ合金めっき層は、その他の成分として、めっき層表面におけるＭｇの酸化を防止する作用を呈するＣａ，Ｓｒ，Ｎａ，Ｙ，ミッシュメタルの１種又は２種以上、耐黒変性に有効なＮｉ，Ｃｏ，Ｓｎの１種又は２種以上、塗装後耐食性に有効なＣｕ，Ｃｒ，Ｍｎ，Ｚｒ，Ｍｏ，Ｗの１種又は２種以上を添加してもよい。
【００２５】
【実施例１】
表１の組成をもつ板厚０．８ｍｍの冷延鋼帯を７５０〜８５０℃で３０秒還元焼鈍した後、Ｚｎ−６．４質量％Ａｌ−３．１質量％Ｍｇ合金めっき浴（浴温：４００℃）にラインスピード１２５ｍ／分で送り込み、浸漬時間２秒でＺｎ−Ａｌ−Ｍｇ合金めっき浴から引き上げた後、めっき付着量を９０ｇ／ｍ²に調整した。
【００２６】

【００２７】
得られた各Ｚｎ−Ａｌ−Ｍｇ系溶融めっき鋼板から試験片を切り出し、溶融金属脆化に起因した割れが最も現れやすい溶接試験に供した。
溶接試験では、電流２５Ａ，電圧１２Ｖ，溶接速度２５０ｍｍ／分の条件で幅５０ｍｍの試験片を相互に溶接長４０ｍｍでＴＩＧ溶接し、溶接された試験片の両端を把持して張力を加えた。そして、溶接された試験片が破断したときの張力を測定することにより溶接強度を求めた。
【００２８】
Ｎ含有量が高いＡグループの鋼材を下地鋼とするめっき鋼板では、溶接強度が最高でも９０ＭＰａに留まっていた。また、張力が８５ＭＰａを超えた時点で亀裂の発生が検出された。亀裂は溶接熱影響部に集中しており、溶融金属脆化が原因であることが推測される。
Ｎ含有量を下げたＢグループの鋼材を下地鋼とするめっき鋼板では、破断に至る溶接強度が１００ＭＰａと若干向上したが、この場合にも張力が８５ＭＰａを超えた時点で亀裂の発生が検出された。
【００２９】
Ｎ含有量を下げてＢを添加したＣグループの鋼材を下地鋼とするめっき鋼板では、破断に至る溶接強度が１１０ＭＰａと更に向上した。また、めっき鋼板の下地鋼を顕微鏡観察したところ、ＢＮ等の化合物として析出しているＢが観察され、マトリックスに固溶している有効Ｂ量が減少していることが窺われるものの、亀裂の発生が抑制されていた。
【００３０】
Ｎ含有量を低下すると共にＴｉ，Ｎｂ，Ｖ，ＺｒでＮを固定し且つＢを添加したＤグループの鋼材を下地鋼とするめっき鋼板では、１４７ＭＰａの張力を加えた状態でも破断しなかった。また、張力付加中に亀裂の発生も検出されなかった。引張試験後に溶接部を観察すると、結晶粒界にめっき金属が浸透した痕跡が検出されず、溶接前の下地鋼／めっき層界面と同様な界面組織を維持していた。
以上の対比から明らかなように、Ｎ含有量を下げてＢを添加した鋼、或いは更にＴｉ，Ｎｂ，Ｖ，ＺｒでＮを固定した鋼材をめっき原板に使用するとき、溶融金属脆化に起因した割れが防止できることが判る。
【００３１】
【実施例２】
実施例１で好結果が得られたＤグループの鋼材Ｄ−１をめっき原板に使用し、連続溶融めっきラインで表２に示した組成のＺｎ−Ａｌ−Ｍｇ系めっき層を付着量９０ｇ／ｍ²で設けた。
【００３２】

【００３３】
各Ｚｎ−Ａｌ−Ｍｇ系溶融めっき鋼板から試験片を切り出し、実施例１と同じ条件下で溶接した。溶接された試験片を腐食試験した。腐食試験では、塩水噴霧（ＪＩＳ Ｚ２３７１）を１００時間継続し、めっき層の腐食減量から腐食速度を算出した。
また、溶接前の試験片を１８０度密着曲げ間加工し、曲げ戻しすることによって、めっき層の密着性を評価した。その結果、Ａｌ濃度２５．３質量％のめっき層（No.９）を設けためっき鋼板ではめっき層が著しく剥離したが、他のめっき層を設けためっき鋼板ではめっき層の著しい剥離が観察されなかった。Ａｌ濃度の高いめっき層で著しいめっき剥離が生じたことは、溶融めっき浴のＡｌ濃度の増加及び浴温の上昇に伴い、下地鋼／めっき層の界面に生成したＡｌ−Ｆｅ系合金層が厚く成長したことに原因があると考えられる。
【００３４】
表３の試験結果にみられるように、本発明で規定した組成条件を満足するめっき層No.１〜８を設けためっき鋼板では、耐食性，加工性の何れも優れていた。また、Ｔｉ，Ｂを含むめっき層No.５〜７を設けためっき鋼板では、凹凸のない美麗な外観が溶接後にも維持されていた。
【００３５】
他方、Ａｌを過剰に含むめっき層No.９を設けためっき鋼板では、Ｆｅ−Ａｌ合金層が過剰に成長し、加工後に下地鋼／めっき層の界面剥離が一部に検出された。逆にＡｌが不足するめっき層No.１０を設けためっき鋼板では、耐食性が不足していた。また、Ｍｇが過剰なめっき層No.１１を設けためっき鋼板ではめっき層成分が溶接熱影響部に浸透した組織が観察された。Ｍｇが不足するめっき層No.１２を設けためっき鋼板では、溶接熱影響に浸透しためっき層成分は検出されなかったが、Ｚｎ−Ａｌ−Ｍｇ系溶融めっき本来の高耐食性が発現されなかった。
【００３６】
以上の結果から、下地鋼の組成とめっき層の組成とを適正に組み合わせることによって、溶融金属脆化に起因した割れが抑えられ、溶接後においても引張強さ，疲労特性，高耐食性が発現される。
【００３７】

【００３８】
【実施例３】
板厚０．８ｍｍ，板幅３００ｍｍ，長さ２０００ｍｍの鋼材Ｄ−１を長手方向中心線に沿って曲げ加工し、Ｌ型材を作製した。Ｌ型材をフラックス処理した後、Ａｌ：６．４質量％，Ｍｇ：３．１質量％，Ｔｉ：０．０３質量％，Ｓｉ：０．０２９質量％，残部ＺｎのＺｎ−Ａｌ−Ｍｇ合金めっき浴（浴温：４００℃）に３０秒浸漬した。Ｚｎ−Ａｌ−Ｍｇ合金めっき浴から引き上げられたＬ型材に冷風を吹き付け、表面に付着している溶融めっき金属を冷却・凝固すると共に付着量を調整した。
【００３９】
溶融めっきされたＬ型材の断面を顕微鏡観察したところ、下地鋼の上にＦｅ−Ａｌ合金層を介して初晶Ａｌ相が分散したＡｌ／Ｚｎ／Ｚｎ₂Ｍｇ三元共晶組織をもつめっき層が形成されており、結晶粒界に浸透しためっき層成分は検出されなかった。比較のため、鋼材Ａ−１を下地鋼として同様にドブ漬けめっきしたものでは、下地鋼の結晶粒界に沿っためっき層成分の浸透が検出された。
【００４０】
【発明の効果】
以上に説明したように、本発明のＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼材は、下地鋼の組成及びめっき層の組成を適正に管理することにより、結晶粒界に沿ってめっき層成分が浸透することで生じる溶融金属脆化が抑制され、Ｚｎ−Ａｌ−Ｍｇ系溶融めっき本来の高耐食性が活用され、機械的強度，疲労特性，加工性にも優れためっき鋼板となる。また、溶融状態のめっき金属に下地鋼が接触する時間が比較的長いドブ漬けめっきにおいても、溶融金属脆化をもたらすめっき層成分が結晶粒界に浸透することが抑えられる。そのため、建築用資材，各種構造材，機械構造部品，配管等、広範な分野で使用されるめっき鋼材が提供される。[0001]
[Industrial application fields]
The present invention relates to a Zn—Al—Mg hot-dip plated steel material that is used as a building material, various structural materials, machine structural parts, piping, and the like and maintains excellent corrosion resistance and mechanical strength over a long period of time.
[0002]
[Prior art]
Hot-dip plated steel sheets are used in a wide range of applications such as roofing materials, structural materials, piping, and parts that are exposed to corrosive atmospheres by utilizing excellent corrosion resistance. Among these, Zn-Al-Mg hot-dip plated steel sheets are attracting attention as a substitute for conventional galvanized steel sheets because they maintain excellent corrosion resistance and mechanical strength over a long period of time in recent years when environmental degradation is serious.
In the plating layer formed on the surface of the Zn—Al—Mg hot-dip plated steel sheet, primary Al phase or primary Al phase and Zn single phase are dispersed in an Al / Zn / Zn ₂ Mg ternary eutectic matrix. It has a structure and its corrosion resistance is improved by Al and Mg. In particular, since a dense and stable corrosion product derived from Mg is uniformly formed on the surface of the plating layer, the corrosion resistance of the plating layer is remarkably improved.
[0003]
[Problems to be solved by the invention]
The Zn-Al-Mg hot-dip steel sheet is a plated steel sheet introduced by the present applicant in JP-A-10-226865, JP-A-10-306357, etc., and is used in various fields by utilizing high corrosion resistance. Development is underway. In the process, it was found that there were some cracks that were not considered a problem with conventional hot-dip galvanized steel sheets. For example, when a welded steel pipe is manufactured by welding a Zn—Al—Mg hot-dip plated steel sheet formed into an open pipe shape, cracks are likely to occur in the heat affected zone. Therefore, the present applicants proposed in Japanese Patent Application No. 2000-308083 to prevent weld cracking by controlling the welding conditions.
[0004]
Although welding cracks are certainly suppressed by controlling the welding conditions, this method limits the scope of application to welded steel pipes manufactured through a welding process. Similar cracks also occur when using a Zn-Al-Mg hot-dip galvanized steel sheet assembled in a product shape under tension load. When cracking occurs, the exposed portion of the base steel through the crack becomes a starting point of corrosion generation, and the high corrosion resistance of the Zn—Al—Mg-based plating layer is impaired, and mechanical strength and fatigue characteristics are also lowered.
[0005]
[Means for Solving the Problems]
The present invention has been devised to solve such problems, and tends to occur in a Zn-Al-Mg hot-dip plated steel sheet by using a specific combination of the composition of the base steel and the hot-dip plated layer. An object of the present invention is to provide a Zn—Al—Mg-based hot dipped galvanized steel material capable of suppressing cracking and utilizing the original high corrosion resistance over a long period of time.
[0006]
In order to achieve the object, the Zn—Al—Mg-based hot-plated steel material of the present invention has C: 0.0005 to 0.25 mass%, N: 0.007 mass% or less, Si: 1.5 mass% or less. , Mn: 0.05 to 2.0 mass %, Al: 0.005 to 0.10 mass%, B: 0.00002 to 0.01 mass% , Ti, Nb, V, Zr 1 type or 2 types or more: Al: 4 to 22% by mass, Mg: 0.05 to a base steel having a composition including 0.01 to 1.20% by mass in total , the balance being Fe and inevitable impurities 10% by mass , B: 0.001 to 0.045% by mass, and a hot- dip plated layer having a composition consisting of Zn and inevitable impurities is formed.
[0007]
The base steel is further Cu: 0.05 to 2.0 mass%, Ni: 0.02 to 2.0 mass%, Cr: 0.02 to 1.0 mass%, P: 0.030 to 0.12 1 type (s) or 2 or more types of the mass% can be included. The Zn—Al—Mg-based hot- plated layer can further contain one or two of Ti: 0.002 to 0.1 mass% and Si: 2.0 mass% or less.
This Zn-Al-Mg hot-dip galvanized steel is manufactured by either continuous hot dip plating, which introduces a continuously conveyed steel strip into a hot dip plating bath, or by dobbing, in which a steel material formed into a predetermined shape is immersed in the hot dip bath. Is done.
[0008]
[Action]
The present inventors investigated and studied the generation mechanism of cracks generated in Zn-Al-Mg hot-dip plated steel sheets from various viewpoints. As a result, it was found that molten metal embrittlement along the grain boundaries was the cause of cracking. The molten metal embrittlement is a phenomenon in which the molten plated metal or a component thereof permeates into the crystal grain boundary of the base steel and embrittles the crystal grain boundary until the molten plated metal in contact with the base steel is solidified. A similar phenomenon occurs during welding when the hot-dip plated layer is remelted.
It is reported that lowering the base steel, adding P and S, and adding Zr and Ti are effective measures to suppress molten metal embrittlement, which causes cracks when welding hot-dip galvanized steel sheets. (Kawasaki Steel Engineering Reports Vol. 25, No. 1, pp. 20-26). However, these measures are not effective for molten metal embrittlement that occurs in Zn-Al-Mg hot-dip steel sheets. In fact, in the investigations and researches by the present inventors, no significant difference was detected in the occurrence of cracks caused by molten metal embrittlement due to the lower component of the base steel and the addition of S. It was also found that B, which is said to have an adverse effect on cracking susceptibility, is effective in preventing cracking of Zn—Al—Mg hot-dip steel sheets.
[0009]
The difference in crack prevention measures suggests that the generation mechanism of cracks generated during welding of a hot-dip galvanized steel sheet and a Zn-Al-Mg hot-dip steel sheet is different. The present inventors have inferred the crack generation mechanism of the Zn—Al—Mg hot-dip steel sheet as follows.
When the Zn-Al-Mg alloy plating bath comes into contact with the plating original plate during continuous hot dipping or submerged plating, the highly active Al first reacts with Fe of the plating original plate, and the Fe-Al alloy layer is formed on the original plate surface. It is formed. A Zn-Al-Mg plating layer is formed through the Fe-Al alloy layer, but with the formation of the Fe-Al alloy layer, the Al concentration of the Zn-Al-Mg plating layer decreases, and the reflection is effectively performed. Mg concentration increases. Increasing the Mg concentration lowers the melting point of the Zn—Al—Mg plating layer. Incidentally, in the case of Zn-3 mass% Mg, the solidification end temperature decreases to 360 ° C.
[0010]
The change in composition and the melting point drop accompanying the generation / growth of the Zn—Al—Mg plating layer are phenomena that are not observed in the normal generation / growth process of the hot dip galvanization layer. Due to the melting point drop, the time for the Zn—Al—Mg alloy in the molten or semi-molten state to contact the base steel becomes longer, and the penetration of the Zn—Al—Mg alloy into the crystal grain boundary of the base steel proceeds.
On the premise of the formation / growth process of the Zn—Al—Mg plating layer, the present inventors have come up with the idea of using B as an element for strengthening the grain boundary of the base steel. Although the action itself of B for strengthening the grain boundary is a conventionally known technique, B is effective in suppressing molten metal embrittlement during the formation and growth process of a Zn—Al—Mg-based plating layer. This is a novel finding found by the present inventors. Further, since cracks due to molten metal embrittlement are remarkably suppressed by addition of B, B has some influence on the formation and growth of the Zn—Al—Mg based plating layer as well as the strengthening of the crystal grain boundary. It is redeemed for the effect. As a result, in the Zn—Al—Mg based plating layer, it is surmised that the molten metal embrittlement is suppressed without “B has an adverse effect” (11 lines from the lower right column on page 24 of Kawasaki Steel Technical Report). The
[0011]
B segregates at the α crystal grain boundary to increase the grain boundary strength, and lowers the grain boundary energy of the γ crystal to improve the hardenability. Further, in the Zn-Al-Mg hot-dip steel sheet, the Zn-Al-Mg alloy penetrates into the crystal grain boundary of the base steel, and the steel component is contained in the molten or semi-molten Zn-Al-Mg alloy. It is thought that it exhibits the action of preventing dissolution. In order to express such an action and effect of B, it is required to secure an effective amount of B dissolved in the matrix of the base steel.
N contained in the base steel reacts with free B to become a BN compound, and the amount of effective B is greatly reduced. Therefore, by using a steel material in which N is reduced as the base steel or N is fixed with Ti, Nb, V, Zr, etc., the effect of N that reduces the effective B amount is suppressed, resulting in molten metal embrittlement. Zn-Al-Mg hot-dip plated steel sheet with suppressed cracking is obtained.
[0012]
From the above consideration, in the present invention, the composition of the base steel and the Zn—Al—Mg based plating layer was specified as follows.
[Base steel]
C: 0.0005 to 0.25% by mass
It is an alloy component that lowers the ductility by forming a carbide while forming a solid solution in the ferrite phase, and in applications where high workability is required, the lower the C content, the better. On the other hand, since the effect | action which strengthens steel materials is also exhibited, it is necessary to increase C content and to make high intensity | strength in uses, such as a structural material. From such a viewpoint, in the present invention, the upper limit of the C content is set to 0.25% by mass. In applications where workability is particularly required, it is preferable to secure the solid solution B amount by setting the upper limit of the C content to 0.01% by mass. On the other hand, reducing the C content to less than 0.0005% by mass requires excessive decarburization refining in the production process, which increases the production cost.
[0013]
N: 0.007% by mass or less N is a component that reacts with B to produce BN and consumes an effective amount of B dissolved in the matrix. Therefore, the N content is preferably as low as possible. However, excessively reducing the N content causes an increase in the manufacturing cost of the steel material. Therefore, the upper limit of the N content is set to 0.007% by mass. The adverse effect of N on the solid solution B can also be suppressed by fixing N with Ti, Nb, V, Zr or the like. However, when the N content exceeds 0.007% by mass, the amount of Ti, Nb, V, Zr necessary for fixing N increases, and the steel cleanliness and workability tend to be adversely affected.
[0014]
Si: 1.5% by mass or less Si is a component that is dissolved in the ferrite phase and is an effective component for improving the strength of the steel material. However, in a steel material containing a large amount of Si, a Si-concentrated layer is formed on the surface of the steel material and the plating property is lowered. Therefore, the upper limit of the Si content is set to 1.5% by mass. Since the ductility decreases as the Si content increases, it is preferable to set the upper limit of the Si content to 0.30% by mass in applications where workability is required.
Mn: 0.05 to 2.0% by mass or less Mn is a component that prevents embrittlement due to S during hot rolling and is effective for improving the strength. The content of 0.05% by mass or more has a remarkable effect of adding Mn. Become. However, in applications where workability and weldability are required, a lower Mn content is desirable, and it concentrates on the steel surface and adversely affects plating properties. Therefore, the upper limit of the Mn content is set to 2.0% by mass. .
[0015]
Al: 0.005 to 0.10% by mass
It is a component added as a deoxidizer during steelmaking, fixing N in the steel as AlN, preventing deterioration of workability due to age hardening of N, and securing an effective B amount that is easily consumed as BN. It is also effective above. Such an effect becomes remarkable when the Al content is 0.005% by mass or more. Assuming that N is fixed by Al, it is preferable to set the Al content so that N ≦ Al × 0.52 is satisfied. However, if an excessive amount of Al exceeds 0.10% by mass, oxide inclusions increase, which adversely affects workability and plating properties.
[0016]
B: 0.00002-0.010 mass%
It is an alloy component effective for suppressing molten metal embrittlement. When 0.00002% by mass or more, the addition effect of B becomes remarkable, and when 0.010% by mass, the addition effect is saturated. Addition of an excessive amount of B exceeding 0.010% by mass promotes formation of borides, inhibition of crystal grain growth, and the like, and causes deterioration of the workability of the steel sheet. The B segregation to the grain boundary depends on the generation and movement of the grain boundary and the diffusion rate of B, and effective grain boundary segregation of B may not be expected at the α grain boundary as transformed. In such a case, manufacturing conditions for sufficiently diffusing B into the crystal grain boundaries are employed in the cold rolling / annealing step.
[0017]
One or more of Ti, Nb, V, and Zr: 0.01 to 1.20 mass% in total
It is an alloy component that is added as necessary, and in each case, N is fixed as a nitride and exhibits an effect of ensuring a solid solution B amount effective for suppressing molten metal embrittlement. Becomes prominent. It is also effective for fixing C. However, excessive addition exceeding 1.20% by mass not only causes an increase in production cost but also deteriorates the workability of the steel material.
Ti, Nb, V, Zr, and the like contained in the base steel are simultaneously eluted into the hot dipping bath when Fe is eluted from the steel plate immersed in the hot dipping bath. The hot dip plating bath in the vicinity of the steel plate becomes a hot dip coating layer after the steel plate is pulled up from the hot dip plating bath, and most of Ti, Nb, V, Zr and B dissolved in the hot dip plating bath are taken into the hot dip plating layer. It is. As a result, the structure of the hot dip plating layer is refined, and a hot dip plating layer having a uniform and beautiful appearance is formed.
[0018]
The base steel used in the present invention can further contain one or more of Cu, Ni, Cr, and P. Of these optional components, Cu is combined with P to improve corrosion resistance and is effective in improving strength. The addition effect becomes remarkable at 0.05% by mass or more, but excessive addition exceeding 2.0% by mass Then, ductility is reduced. Ni is also an effective component for improving the strength, and the effect of addition becomes remarkable when the content is 0.02% by mass or more, but excessive addition exceeding 2.0% by mass causes a decrease in ductility. Cr is an effective component for improving the strength and the corrosion resistance of the base metal, and the effect of addition becomes remarkable when it is 0.05% by mass or more, but excessive addition exceeding 1.0% by mass causes deterioration of workability. When adding P as an alloy component, the P content is selected in the range of 0.030 to 0.12% by mass.
[0019]
In the base steel, P, S and the like are mixed as impurities from the steel making. Since P as an impurity adversely affects ductility, it is preferable to make the P content as low as possible in applications where high workability is required. However, in applications where high strength is required because it also works to improve strength, P may be contained in a range of up to 0.12% by mass that does not adversely affect workability and plating properties. Since S is a cause of hot embrittlement and is a harmful component that degrades workability and corrosion resistance, it is as low as possible except for applications that require special performance such as free-cutting properties (specifically, 0 0.03 mass% or less).
[0020]
[Zn-Al-Mg-based plating layer]
Al: 4-22 mass%
Al hardly elutes from the plating layer, and forms a Zn-Al based corrosion product in the portion that was the original plating layer. The Zn-Al corrosion product has extremely strong adhesion, and even if the Mg-containing Zn corrosion product in the upper layer disappears during the corrosion process, it becomes a barrier with an environmental barrier function and suppresses corrosion of the underlying plating layer To do. Some of the Zn-Al based corrosion products take in SOx in the environment and act as a stronger protective film. In order to form a Zn—Al based corrosion product that has a strong adhesion and acts as a barrier against the base, an Al content of 4 mass% is required. Moreover, it reacts with N in the steel at the time of forming the hot dip plating to become AlN, which is also effective in reducing N harmful to solute B. However, when an excessive amount of Al exceeding 22% by mass is contained, not only the effect of the Zn—Al based corrosion product is saturated, but also the workability of the plating layer is lowered.
[0021]
Mg: 0.05 to 10% by mass
Mg contained in the plating layer forms a Zn-based corrosion product containing Mg in the outermost layer of the plating layer, and has an effect of suppressing the corrosion rate of the plating layer in a general corrosive environment such as outdoors. Such an effect is observed at an Mg content of 0.05% by mass or more, and is saturated at Mg: 10% by mass. The excessive Mg content exceeding 10% by mass decreases the stability of the hot dipping bath due to the generation of a large amount of dross. Preferably, the Mg content is selected in the range of 1.0 to 10% by mass.
[0022]
Ti: 0.002-0.1% by mass, B: 0.001-0.045% by mass
Both are alloy components that are added as optional components, and the formation of Zn ₁₁ Mg ₂ phase that adversely affects the surface appearance is suppressed, and Zn—Mg-based intermetallic compounds that crystallize in the plating layer are substantially Zn ₂ Mg. It exhibits only the effect. Specifically, when 0.002 mass% or more of Ti is included, the generation of the Zn ₁₁ Mg ₂ phase is effectively suppressed. However, when an excessive amount of Ti exceeding 0.1% by mass is contained, Ti—Al-based precipitates grow in the plating layer, resulting in unevenness in the plating layer, and the appearance tends to deteriorate. The formation of the Zn ₁₁ Mg ₂ phase is also suppressed by including 0.001% by mass or more of B. Even in the case of containing B, Ti-B-based and Al-B-based precipitates are deposited in the plating layer at an excess amount exceeding 0.045% by mass, resulting in an uneven plating layer that similarly causes appearance deterioration. It becomes easy. Moreover, since the steel material containing Ti and B is used for the base steel, the same effect can be obtained by Ti and B eluted from the base steel into the hot dipping bath.
[0023]
Si: 0.005 to 2.0 mass%
It is an alloy component added as necessary, and it suppresses the growth of a thick Fe-Al intermetallic compound at the interface between the base steel and the plating layer and improves the workability of the Zn-Al-Mg hot-dip plated steel sheet. Presents. It is an effective component for preventing the black change of the hot dip plating layer and maintaining the gloss of the surface. The effect of Si on the suppression of the formation of Fe-Al intermetallic compounds is observed even at a content of 0.005% by mass or more, but when an excessive amount of Si exceeding 2.0% by mass is contained, it is accommodated in a pot. The amount of dross generated in the hot dip plated metal increases.
[0024]
As other components, the Zn—Al—Mg alloy plating layer is one or more of Ca, Sr, Na, Y, and Misch metal that exhibits an action of preventing oxidation of Mg on the surface of the plating layer, and is resistant to blackening. One or more kinds of effective Ni, Co, and Sn, and one or more kinds of Cu, Cr, Mn, Zr, Mo, and W effective for post-coating corrosion resistance may be added.
[0025]
[Example 1]
A cold rolled steel strip having a thickness of 0.8 mm having the composition shown in Table 1 was subjected to reduction annealing at 750 to 850 ° C. for 30 seconds, and then a Zn-6.4 mass% Al-3.1 mass% Mg alloy plating bath (bath temperature). : 400 [deg.] C.) at a line speed of 125 m / min, and after lifting from the Zn-Al-Mg alloy plating bath with a dipping time of 2 seconds, the plating adhesion was adjusted to 90 g / m < ² >.
[0026]

[0027]
A test piece was cut out from each of the obtained Zn—Al—Mg hot-dip plated steel sheets and subjected to a welding test in which cracks caused by molten metal embrittlement were most likely to appear.
In the welding test, test pieces having a width of 50 mm were TIG welded to each other with a weld length of 40 mm under conditions of current 25 A, voltage 12 V, and welding speed 250 mm / min, and both ends of the welded test pieces were gripped and tension was applied. And the welding strength was calculated | required by measuring the tension | tensile_strength when the welded test piece fractured | ruptured.
[0028]
In the plated steel sheet in which the steel material of group A having a high N content is used as the base steel, the welding strength remains at 90 MPa at the maximum. Moreover, the occurrence of cracks was detected when the tension exceeded 85 MPa. The cracks are concentrated in the weld heat-affected zone, which is presumed to be caused by molten metal embrittlement.
In the plated steel sheet that uses the steel of Group B with a lower N content as the base steel, the weld strength leading to fracture was slightly improved to 100 MPa. In this case, however, cracks were detected when the tension exceeded 85 MPa. It was.
[0029]
In the plated steel sheet using the C group steel material with the N content reduced and B added as the base steel, the weld strength leading to the fracture was further improved to 110 MPa. In addition, when the base steel of the plated steel sheet was observed with a microscope, B precipitated as a compound such as BN was observed, and although it seems that the amount of effective B dissolved in the matrix is reduced, Occurrence was suppressed.
[0030]
In the plated steel sheet in which the N content was lowered and N was fixed with Ti, Nb, V, and Zr and the steel material of the D group added with B was used as the base steel, it did not break even when a tension of 147 MPa was applied. In addition, the occurrence of cracks was not detected during tension application. When the weld was observed after the tensile test, no trace of plating metal permeating into the crystal grain boundary was detected, and the interface structure similar to the base steel / plated layer interface before welding was maintained.
As is clear from the above comparison, when steel with lower N content and added B, or steel with N fixed with Ti, Nb, V, Zr is used for the plating plate, it is caused by molten metal embrittlement. It can be seen that cracking can be prevented.
[0031]
[Example 2]
The steel material D-1 of Group D obtained with good results in Example 1 was used as a plating base plate, and a Zn-Al-Mg-based plating layer having the composition shown in Table 2 on the continuous hot-plating line was applied in an amount of 90 g / m. ² provided.
[0032]

[0033]
A test piece was cut out from each Zn—Al—Mg hot-dip steel sheet and welded under the same conditions as in Example 1. The welded specimen was subjected to a corrosion test. In the corrosion test, salt spray (JIS Z2371) was continued for 100 hours, and the corrosion rate was calculated from the corrosion weight loss of the plating layer.
Moreover, the adhesiveness of the plating layer was evaluated by subjecting the test piece before welding to 180 ° contact bending and bending back. As a result, while the plated steel sheet provided with the plating layer (No. 9) having an Al concentration of 25.3 mass% was significantly peeled off, the plated steel sheet provided with other plated layers was observed to be significantly peeled off. There wasn't. Remarkable plating peeling occurred in the plating layer with a high Al concentration. The increase in the Al concentration in the hot dip plating bath and the rise in bath temperature resulted in a thicker Al-Fe alloy layer formed at the base steel / plating layer interface. This is thought to be due to the growth.
[0034]
As seen from the test results in Table 3, the plated steel sheets provided with the plating layers No. 1 to 8 satisfying the composition conditions defined in the present invention were excellent in both corrosion resistance and workability. Moreover, in the plated steel plate provided with plating layers No. 5 to 7 containing Ti and B, a beautiful appearance without irregularities was maintained after welding.
[0035]
On the other hand, in the plated steel sheet provided with the plating layer No. 9 containing excessive Al, the Fe—Al alloy layer grew excessively, and the interface steel / plating layer peeling was partially detected after processing. On the other hand, the plated steel sheet provided with the plating layer No. 10 in which Al is insufficient has insufficient corrosion resistance. Further, in the plated steel sheet provided with the plating layer No. 11 containing excessive Mg, a structure in which the plating layer component penetrated into the weld heat affected zone was observed. In the plated steel sheet provided with the plating layer No. 12 lacking Mg, the plating layer component penetrating the influence of welding heat was not detected, but the original high corrosion resistance of the Zn—Al—Mg hot-dip plating was not expressed.
[0036]
From the above results, by properly combining the composition of the base steel and the plating layer, cracking due to molten metal embrittlement can be suppressed, and tensile strength, fatigue properties, and high corrosion resistance can be achieved even after welding. The
[0037]

[0038]
[Example 3]
A steel material D-1 having a plate thickness of 0.8 mm, a plate width of 300 mm, and a length of 2000 mm was bent along the longitudinal center line to produce an L-shaped material. After flux-treating the L-shaped material, Al: 6.4% by mass, Mg: 3.1% by mass, Ti: 0.03% by mass, Si: 0.029% by mass, balance Zn—Al—Mg alloy plating It was immersed in a bath (bath temperature: 400 ° C.) for 30 seconds. Cold air was blown onto the L-shaped material pulled up from the Zn—Al—Mg alloy plating bath to cool and solidify the hot-plated metal adhering to the surface and adjust the amount of adhesion.
[0039]
When the cross section of the hot-plated L-shaped material was observed with a microscope, a plated layer having an Al / Zn / Zn ₂ Mg ternary eutectic structure in which the primary Al phase was dispersed on the base steel via the Fe—Al alloy layer As a result, a plating layer component penetrating into the crystal grain boundary was not detected. For comparison, penetration of the plating layer component along the crystal grain boundary of the base steel was detected in the case where the steel material A-1 was similarly dipped and plated as the base steel.
[0040]
【The invention's effect】
As explained above, in the Zn-Al-Mg hot-dip plated steel material of the present invention, the plating layer component penetrates along the crystal grain boundary by appropriately managing the composition of the base steel and the composition of the plating layer. As a result, molten metal embrittlement is suppressed, the high corrosion resistance inherent in Zn-Al-Mg hot-dip plating is utilized, and a plated steel sheet having excellent mechanical strength, fatigue characteristics, and workability is obtained. In addition, even in the dipping plating in which the base steel is in contact with the molten plated metal for a relatively long time, it is possible to suppress the penetration of the plating layer component that causes the molten metal embrittlement into the crystal grain boundary. Therefore, plated steel materials used in a wide range of fields such as building materials, various structural materials, machine structural parts, piping, and the like are provided.

Claims (4)

Ｃ：０．０００５〜０．２５質量％，Ｎ：０．００７質量％以下，Ｓｉ：１．５質量％以下，Ｍｎ：０．０５〜２．０質量％，Ａｌ：０．００５〜０．１０質量％，Ｂ：０．００００２〜０．０１質量％を含み、残部がＦｅ及び不可避的不純物からなる組成を有する下地鋼に、Ａｌ：４〜２２質量％，Ｍｇ：０．０５〜１０質量％，Ｂ：０．００１〜０．０４５質量％を含み、残部がＺｎ及び不可避的不純物からなる組成を有する溶融めっき層が形成されていることを特徴とする耐食性に優れたＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼材。 C : 0.0005-0.25 mass%, N: 0.007 mass% or less, Si: 1.5 mass% or less, Mn: 0.05-2.0 mass %, Al: 0.005-0. 10% by mass, B: 0.00002 to 0.01% by mass , with the base steel having a composition consisting of Fe and inevitable impurities as the balance , Al: 4 to 22% by mass, Mg: 0.05 to 10% by mass % , B: Zn-Al-Mg excellent in corrosion resistance, characterized in that a hot-dip plated layer having a composition containing 0.001 to 0.045 mass% with the balance being Zn and inevitable impurities is formed. Hot-dip galvanized steel. 下地鋼が更にＴｉ，Ｎｂ，Ｖ，Ｚｒの１種又は２種以上：合計で０．０１〜１．２０質量％を含む請求項１記載のＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼材。  The Zn-Al-Mg hot-dip galvanized steel material according to claim 1, wherein the base steel further contains one or more of Ti, Nb, V, and Zr: 0.01 to 1.20 mass% in total. 下地鋼が更にＣｕ：０．０５〜２．０質量％，Ｎｉ：０．０２〜２．０質量％，Ｃｒ：０．０２〜１．０質量％，Ｐ：０．０３０〜０．１２質量％の１種又は２種以上を含む請求項１又は２記載のＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼材。  The base steel is further Cu: 0.05 to 2.0 mass%, Ni: 0.02 to 2.0 mass%, Cr: 0.02 to 1.0 mass%, P: 0.030 to 0.12 mass% % Zn-Al-Mg based hot-dip galvanized steel material according to claim 1 or 2, which comprises 1% or more. 溶融めっき層が更にＴｉ：０．００２〜０．１質量％，Ｓｉ：２．０質量％以下の１種又は２種を含む請求項１〜３何れかに記載のＺｎ−Ａｌ−Ｍｇ系溶融めっき鋼材。The Zn-Al-Mg based melt according to any one of claims 1 to 3, wherein the hot-dip plating layer further contains one or two of Ti: 0.002 to 0.1% by mass and Si : 2.0% by mass or less. Plated steel.