JP2004244650A

JP2004244650A - METHOD OF PRODUCING Zn-Al-Mg BASED ALLOY PLATED STEEL

Info

Publication number: JP2004244650A
Application number: JP2003032866A
Authority: JP
Inventors: Shintaro Yamanaka; 晋太郎山中; Masahiro Yamamoto; 正弘山本; Kunihiro Enomoto; 邦広榎本
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2003-02-10
Filing date: 2003-02-10
Publication date: 2004-09-02

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of producing a uniform Zn-Al-Mg based alloy plated steel which has a high coating weight and high corrosion resistance by a flux method. <P>SOLUTION: In the method of producing a Zn-Al-Mg based alloy plated steel, an oxide film on the surface of a steel is removed by mechanical descaling, thereafter, flux treatment is performed, and the steel is dipped into a Zn-Al-Mg based alloy plating bath heated at a bath temperature of 420 to 550°C and comprising, by mass, 0.1 to 5% Al and 0.1 to 5% Mg, and the balance Zn with inevitable impurities, so that a Zn-Al-Mg based plating layer is formed on the surface of the steel in a high coating weight. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、フラックス法による均一で高付着量かつ高耐食性のＺｎ−Ａｌ−Ｍｇ系合金めっき鋼材の製造方法に関する。
【０００２】
【従来の技術】
従来から、送電鉄塔、橋梁、道路用資材、建築金物、建築・土木用材料等の分野においては、経済性、耐久性、作業性等の観点から溶融Ｚｎめっき鋼材が多く用いられている。また、最近ではＺｎめっきよりも耐食性に優れるとのことからＺｎ−Ａｌ合金めっきも使われつつある。これらＺｎめっきやＺｎ−Ａｌ合金めっきにおいては、通常、めっきの付着量とその耐食性は比例関係にあり、めっきが厚いほど、地鉄の露出に至るまでに長時間を要すため、耐食性は向上する。すなわち、Ｚｎ−Ａｌ合金めっきの耐食性がＺｎめっきの２倍あったとしても、Ｚｎ−Ａｌ合金めっきの付着量がＺｎめっきの半分しかない場合、その耐食性は同等となる。
【０００３】
めっき付着量に影響を及ぼす因子としては、めっき浴の粘度や濡れ付着力、めっき時間、めっき温度、めっき浴からの素材の引き上げ速度、めっきのワイピング（たれきり）等が挙げられるが、これらの内、めっき浴の粘度や濡れ付着力の影響は、それほど大きくはない。また、めっき浴からの素材の引き上げ速度については、薄鋼板のめっきのような連続式においては、引き上げ速度の変更は容易であるが、フラックス式めっきのようなバッチ式のめっきにおいては、引き上げ速度の変更は難しい。めっきのワイピングについては、加工済の鋼材や溶接をした鋼材、また、小物等の複雑な形状の素材をめっきする場合が多い、フラックス式のめっきにおいては、均一なめっき付着量を得ることは難しく、そもそもワイピングは、めっきの付着量を減少させる方向に作用するため、高付着量で均一なめっきを得るには不向きである。
【０００４】
これらの理由のため、フラックス式めっきにおいては、めっき時間とめっき浴の温度によって、めっきの付着量を制御する場合が多い。めっき時間とめっき浴の温度は、地鉄とめっきの界面に生成するＦｅ−Ｚｎ系合金層の厚みに特に大きな影響を与え、めっき時間が長いか、めっき温度が高いと合金層は厚くなり、結果として、めっきの付着量は大きくなる。ところで、この合金層の厚みは、めっき浴の組成によっても大きく異なる。例えば、浴中のＡｌ量が０．０２質量％以下であるようなＺｎめっきにおいては、めっき時間を長くするか、めっき温度を高くすることで、Ｆｅ−Ｚｎ系合金層は成長し、６００ｇ／ｍ^２程度の厚いめっきも可能である。ところが、Ａｌを０．０５質量％以上含むようなＺｎ−Ａｌ系めっき浴では、Ｆｅ−Ａｌ系の合金層が初期に生成し、これがＦｅ−Ｚｎ系合金層の成長を妨げるために、めっき付着量が大きくなりにくい。
【０００５】
このＦｅ−Ａｌ系合金層は非常に安定で、例えば、５３０℃程度までめっき温度を高くしても安定で、Ｆｅ−Ｚｎ系合金層の成長を抑制するため、めっき付着量は大きくならない。比較的高濃度のＡｌを含むめっき浴の場合、例えば、質量％で、１０％以上Ａｌを含むめっき浴の場合は、めっき浴温度を高くすることで、Ｆｅ−Ａｌ系合金層自体が成長するため、めっき付着量は大きくなる。しかし、めっきが不均一、つまり、局所的に厚くなり、外観が不良となるため、製品として適さない。また、めっき時間を長くしたとしても、Ｆｅ−Ｚｎ系合金層は成長しにくい。かなり長い時間、例えば、１時間程度めっき浴に浸漬したときは、めっき付着量が大きくなる場合もあるが、やはり不均一なめっきで、外観が不良となるため、製品として適さない。このように、高付着量で均一なＺｎ−Ａｌ合金めっきを得ることは困難で、これまでに十分に検討されてはいない。
【０００６】
これまでに知られている高付着量のＺｎ−Ａｌ合金めっきを得る方法としては、次のようなものがある。例えば、通常のＡｌをほとんど含まない純Ｚｎめっきを行い、Ｆｅ−Ｚｎ系合金層を成長させた後に、Ｚｎ−Ａｌ合金めっきを行うという２段めっき処理が一部で採用されている。しかしながら、この方法では、めっき浴を２槽有する必要があること、また、２回のめっき操作による作業時間の増加や維持管理費、設置場所の増加といった種々の欠点がある。
【０００７】
特開平５−１０６００２号公報（特許文献１）では、Ｚｎ−Ａｌ合金めっき浴中に１．５〜１０質量％の銅を添加することで、比較的厚いＺｎ−Ａｌ合金めっきが得られる方法が提案されている。しかし、この方法では、めっき浴の組成が変動しやすく、めっき品質にばらつきが出るとことが懸念される。
また、特開平９−２５１３４号公報（特許文献２）では、鋼材を所定のフラックス処理した後に、Ａｌ濃度１０〜２０質量％からなるＺｎめっき浴に、めっき温度４５０〜５２０℃、めっき時間０．５〜１０分の条件でめっきすることで、めっき厚み（めっき付着量）４５０ｇ／ｍ^２以上の厚めっきが得られるとされている。ところが、この方法では、Ａｌ濃度が１０質量％以下のめっき浴の場合には適用できず、また、特殊なフラックスを使用する必要があるため、汎用性がない。
【０００８】
一方、特開平１１−３５００９５号公報（特許文献３）では、所定のフラックス処理をした後に、溶融フラックス層を有するＺｎ−Ａｌめっき浴に浸漬することで、高付着量のめっきが得られるとされている。ところが、この方法においても、Ａｌ含有量が３０〜７０質量％であるめっき浴を主対象としていることの他、特殊なフラックスを使用しており汎用性がないこと、また、溶融フラックス層を必要し、その劣化に対する交換、補充に対する作業に労力を要すること、また、フラックス中に弗化物を含んでいるため作業環境が懸念されること、等の種々の問題点がある。
【０００９】
さらに、特開平８−２８３９２５号公報（特許文献４）では、鋼材の表面粗度を一定の範囲内とし、さらにめっき浴中に一定の鉄分成分を含ませることによって、高付着量で均一なＺｎ−Ａｌ−Ｓｉめっきを得られるとされている。しかしながら、この方法は、Ａｌ−Ｓｉめっきを対象としており、めっき浴温が５８０℃以上と高いため、Ｚｎを主体とするめっきの場合には不適である。
以上のように高付着量のＺｎ−Ａｌ合金めっきを得る方法として、数種の方法が見出されているものの、作業性や品質、環境等といった面で欠点がある。また、高付着量のめっきができたとしても、安定的に均一な合金めっきを得ることは難しい。
【００１０】
【引用文献】
（１）特許文献１（特開平５−１０６００２号公報）
（２）特許文献２（特開平９−２５１３４号公報）
（３）特許文献３（特開平１１−３５００９５号公報）
（４）特許文献４（特開平８−２８３９２５号公報）
【００１１】
【発明が解決しようとする課題】
本発明は、このようなＺｎ−Ａｌ系合金めっきにおける従来の諸問題を解決し、フラックス法による均一で高付着量かつ高耐食性のＺｎ−Ａｌ−Ｍｇ系合金めっき鋼材の製造方法を提供することを目的とする。
【００１２】
【課題を解決するための手段】
本発明者らは、上記の従来の技術が抱える問題点を解決し、種々の組成のＺｎ−Ａｌ系合金めっきについて、均一に高付着量かつ高耐食性なめっきを安定的に得る方法について鋭意検討を重ねた結果、機械的な方法で鋼材表面の酸化膜を除去した後に、フラックス処理をし、一定量のＭｇを含むＺｎ−Ａｌめっき浴中に、一定のめっき浴温度範囲内で、鋼材にめっきすることで、均一で高付着量かつ高耐食性のＺｎ−Ａｌ−Ｍｇ系合金めっき鋼材を製造する方法を見出した。
【００１３】
すなわち、本発明は、
（１）機械的デスケーリングで鋼材表面の酸化膜を除去した後に、フラックス処理し、質量％で、Ａｌ：０．１〜５％、Ｍｇ：０．１〜５％、残部Ｚｎ及び不可避的不純物を含有する浴温４２０〜５５０℃のＺｎ−Ａｌ−Ｍｇ系合金めっき浴に浸漬し、鋼材表面にＺｎ−Ａｌ−Ｍｇ系めっき層を高付着量に形成することを特徴とするＺｎ−Ａｌ−Ｍｇ系合金めっき鋼材の製造方法、
（２）前記機械的デスケーリングで、鋼材表面の粗度Ｒａを０．０１〜５０μｍの範囲とする（１）に記載のＺｎ−Ａｌ−Ｍｇ系合金めっき鋼材の製造方法。
【００１４】
（３）前記機械的デスケーリング後に、質量％で、１〜３０％の塩酸又は１〜３０％硫酸の一方又は双方を少なくとも含有する水溶液に前記鋼材を浸漬する（１）または（２）に記載のＺｎ−Ａｌ−Ｍｇ系合金めっき鋼材の製造方法、
（４）前記フラックス処理前の鋼材表面に存在する酸化膜の厚みが１０ｎｍ以下であることを特徴とする（１）〜（３）のいずれかに記載のＺｎ−Ａｌ−Ｍｇ系合金めっき鋼材の製造方法、
（５）前記Ｚｎ−Ａｌ−Ｍｇ系合金めっき浴が、質量％で、さらにＳｉ：０．０５〜０．２％を含む（１）〜（４）のいずれかに記載のＺｎ−Ａｌ−Ｍｇ系合金めっき鋼材の製造方法である。
【００１５】
【発明の実施の形態】
以下、本発明を詳細に説明する。
本発明に用いられる被めっき材である鋼材の種類は限定されない。普通鋼、低合金鋼、高合金鋼等、その鋼成分には限定されるものではなく、また、その形状についても、板、鋼管、形鋼、線材や、溶接やボルト等で接合した組み合わせ鋼材、ボルト、ナット等の小物等、また、鉄塔、橋梁部材等の鋼製部材にも適用でき、めっき浴に浸漬し、めっき処理が可能なものであれば何でも良い。
【００１６】
本発明の製造方法では、まず、基材となる鋼材の表面酸化膜を機械的デスケーリングによって除去することが必須である。機械的デスケーリングとは、機械的に酸化皮膜を除去する方法のことで、例えば、サンドブラスト、ショットブラスト、グリットブラスト、切削、研削、研磨等の方法を挙げることができ、本発明においては、機械的に酸化皮膜を除去できる方法であれば良く、その具体的方法を限定するものではない。この機械的デスケーリングによって、高付着量で均一のめっきを得ることができる。
【００１７】
なお、本発明における高付着量とは、鋼材片側表面のめっき付着量が４００ｇ／ｍ^２以上であることを指す。機械的デスケーリングをすることで高付着量のめっきが得られるメカニズムについては、鋼材に与えられる残留応力の影響等が考えられるが、未だ推定の域を出ない。しかしながら、機械的デスケーリングを行うことで、Ｆｅ−Ｚｎ系合金層が容易に成長し、高付着量で均一のめっきが可能となる。このため、本発明においては、まず、鋼材表面を機械的にデスケーリングすることが必要である。
【００１８】
機械的デスケーリング後の鋼材の表面粗度Ｒａは、０．０１〜５０μｍの範囲とすることが望ましい。すなわち、表面粗度が０．０１μｍ未満では、めっきの付着量や外観は変わらないことがあり、ただコストを上昇させるだけである。表面粗度が５０μｍより大きいと、めっき表面の平滑性が劣る傾向にあり、外観が損なわれる。また、加工性も劣る傾向にある。従って、本発明における機械的デスケーリング後の鋼材表面粗度Ｒａを０．０１〜５０μｍの範囲とすることで、安定して良好なめっき外観を得ることができ、好ましくは０．１〜２０μｍの範囲、より好ましくは１．０〜１０μｍの範囲である。このような条件範囲のＲａにすることは、デスケーリングの方法を適宜選択することで可能である。例を挙げると、一般的には、アルミナ研磨、フライス盤による研削、サンドブラスト、グリットブラストの順番でＲａは大きくなる。
【００１９】
また、機械的デスケーリングの後に、質量％で、１〜３０％の塩酸又は１〜３０％硫酸の一方又は双方を少なくとも含有する水溶液に鋼材を浸漬しても良い。１〜３０％の塩酸又は１〜３０％硫酸の一方又は双方を少なくとも含有する水溶液に浸漬することで、デスケーリング後に鋼材表面に残存する鉄分や油分、汚れ、また、フラックス処理までの間に鋼材表面に付着する油分や汚れ等を除去することができ、めっき時に鋼材とめっきの反応性を高め、Ｆｅ−Ｚｎの合金化を活発にし、高付着量のめっきを得ることができる。
【００２０】
塩酸及び硫酸の濃度が１質量％未満では、鋼材表面の鉄分や汚れ、油分を除去するには不十分となることがあり、めっきを高付着量とする効果が小さくなる可能性がある。また、塩酸又は硫酸の濃度が３０質量％以上では、鋼材を溶解する能力が過剰となるため、機械的デスケーリングの効果が失われる可能性があり、均一で高付着量のめっきを得ることが困難となる場合がある。１〜３０質量％の塩酸又は１〜３０質量％の硫酸の一方、又は双方を少なくとも含有する水溶液に鋼材を浸漬する際、その水溶液の温度は特に規定しないが、２０〜７０℃の範囲であることが望ましい。
【００２１】
その理由は、２０℃以下では、鋼材表面の鉄分や汚れ、油分を除去するには不十分となることがあり、７０℃以上では、もはやその鋼材表面を清浄する効果は飽和し、また、塩化水素等の揮発により作業環境が悪化する可能性が高くなるからである。鋼材の浸漬時間は、１時間以下であることが望ましい。その理由は、１時間よりも長く浸漬すると、過剰に鋼材が溶解し、上記水溶液が汚染されるだけでなく、鋼材表面の肌荒れも顕著になるからである。
【００２２】
上記のような鋼材表面の前処理をした後に、フラックス処理をするが、さらに本発明では、フラックス前の鋼材表面に存在する酸化皮膜の厚みが１０ｎｍ以下であることが望ましい。ここでいう酸化皮膜とは、鉄系酸化皮膜、シリコン系酸化皮膜、アルミニウム系酸化皮膜、等の酸素との化合物であり、その種類を問わない。なお、ここでいう酸化皮膜の厚みは、ＡＥＳ（オージェ電子分光法）あるいはＸＰＳ（Ｘ線光電子分光法）等によって求めた酸化皮膜の厚みのことを指す。例えば、フラックス処理前の被めっき材の表面を、ＡＥＳ又はＸＰＳで酸素の深さ方向の強度を測定し、その強度が、最高強度と最低強度の平均値となるところの深さを、酸化皮膜の厚みとする。
【００２３】
通常、酸化皮膜の厚みは、鋼材の製造履歴と機械的デスケーリングの条件に依存されるため、上記酸化皮膜の計測は、同一条件で製造した小さな試験片によって決定することができる。
上記のように測定された酸化皮膜の厚みは、１０ｎｍ以下であることが望ましい。１０ｎｍ以下であれば、Ｆｅ−Ａｌ系及びＦｅ−Ｚｎ系の合金化反応が早急かつ均一に起こり、より密着性や加工性に富み、外観も良好なめっきが得られる。
【００２４】
本発明では、鋼材表面の前処理をした後、フラックス処理をする。ここで言うフラックス処理とは、鋼材表面にフラックスを塗布、付着させる処理のことであり、フラックスやその処理条件等については特に限定するものではなく、従来から知られている方法で良い。例えば、塩化亜鉛、塩化アンモニウム、塩化マグネシウム、塩化リチウム、塩化錫、塩化カリウム、塩化カルシウム、塩化鉛、塩化ナトリウム、塩化アンチモン、塩化インジウム、塩化ビスマス、塩化カドミウム等から選ばれる１種以上の塩化物と、アルカリ金属、アルカリ土類金属のフッ化物、ケイフッ化物、フッ化水素物等から選ばれる１種以上を少なくとも含有する水溶液に浸漬、乾燥する乾式フラックス法や、上記化合物からなるフラックスをめっき浴上に浮遊させた湿式フラックス法、あるいは、フラックス自体を溶融させた層に鋼材を浸漬する溶融フラックス法、等が適用できる。フラックスの組成、また、乾式フラックスの場合の水溶液濃度は、特に規定するものでなく、従来知られている組成、濃度で良い。
【００２５】
フラックス処理した鋼材は、質量％で、Ａｌ：０．１〜５％、Ｍｇ：０．１〜５％、残部Ｚｎ及び不可避的不純物を含有する浴温４２０〜５５０℃のＺｎ−Ａｌ−Ｍｇ系合金めっき浴に浸漬される。浴中のＡｌは、めっきを高耐食化させ、均一なめっき厚みで、かつ良好なめっき外観とする効果がある。Ａｌが０．１質量％未満では、耐食性の向上には不十分であり、また、５質量％よりも多いと、逆に不均一なめっきとなりやすい。このため、本発明のおけるめっき浴中のＡｌは、０．１〜５質量％の範囲に、好ましくは０．１５〜３質量％の範囲、より好ましくは０．２〜２質量％の範囲に規定する。
【００２６】
Ｍｇは、均一で高耐食性のめっきを得るのに必須な成分である。Ｍｇが０．１質量％未満では、めっきを高耐食化させるのに不十分であり、５質量％よりも多いと、めっきの加工性が損なわれるだけでなく、めっき浴中のドロスの発生量が増大し、不めっき等の原因となる。このため、本発明におけるめっき浴中のＭｇは、０．１〜５質量％の範囲に、好ましくは０．２〜３質量％の範囲、より好ましくは０．３〜２質量％の範囲に規定する。
【００２７】
また、本発明のＺｎ−Ａｌ−Ｍｇ系合金めっき浴には、さらに、質量％で、０．０５〜０．２％のＳｉを含んでいても良い。Ｓｉは、めっきを高耐食化する効果がある他、Ｆｅ−Ｚｎ系合金層を緻密にさせ、加工性を向上させる効果がある。Ｓｉが０．０５質量％未満では、上記の効果が小さく、０．２質量％以上では、逆に加工性が低下する傾向にある。このため、本発明におけるめっき浴中のＳｉは、０．０５〜０．２質量％の範囲、好ましくは０．１０〜０．１５質量％の範囲にすることが望ましい。また、このＺｎ−Ａｌ−Ｍｇ系めっき浴には、例えば、Ｆｅ、Ｐｂ、Ｃｄ、Ｓｎ、Ｃｕ、Ｓｂ、Ｂｉ、Ａｇ、Ｎｉ、Ａｓ等が不可避的不純物として混入していても、問題はない。
【００２８】
めっき浴温は、４２０〜５５０℃の範囲とすることが必須である。この温度範囲では、Ｆｅ−Ｚｎ系合金層が均一に成長し、高付着量かつ高耐食性のめっきを得ることができる。めっき浴温が４２０℃よりも低いと、Ｆｅ−Ｚｎの合金化反応が遅延し、また、その成長速度も遅いため、高付着量のめっきとならず、好ましくない。また、めっき浴温が５５０℃よりも高い場合、局所的に合金層が厚くなり、外観が不良となるだけでなく、めっき浴中に発生するドロスの量が多くなり、その除去に手間を要し、さらに、めっきした際のドロス巻き込み等で製品の外観が不良となるため、好ましくない。このため、本発明におけるＺｎ−Ａｌ−Ｍｇ系合金めっき浴の温度を４２０〜５５０℃の範囲に規定する。
【００２９】
被めっき材をめっき浴へ浸漬する速度は何ら規定するものではない。例えば、鋼管の連続めっきにおけるテーラーウィルソン方式のように、鋼管をめっき浴に落下させるような速い浸漬速度でも良いし、あるいは、鉄塔部材のような大型鋼製部材をめっきする場合のように、クレーンで被めっき材を吊り上げた後、ゆっくりとめっき浴に浸漬させるような遅い浸漬速度でもよい。
めっき浴への浸漬時間は特に規定するものでない。但し、５００℃以上のめっき浴に長時間、例えば、１時間以上浸漬すると、めっき付着量が極端に大きくなり、めっきの密着性に劣る場合があるだけでなく、めっきが不均一となる。このため、めっき時間は３０分以内とすることが望ましい。
【００３０】
被めっき材をめっき浴から取り出す速度も特に規定するものでない。テーラーウィルソン方式のように０．７〜２ｍ／ｓｅｃ程度の高速で取り出しても良いし、あるいは、クレーンを用いた場合のように０．２ｍ／ｓｅｃ程度の低速で取り出しても良い。さらに、めっき後の外観を良好にさせるため、必要に応じて、水冷や湯冷による冷却を行っても良い。
上記の条件でめっきを行うことによって、４００ｇ／ｍ^２以上の高付着量のめっきを得ることができる。めっき付着量の上限は特に規定しないが、８００ｇ／ｍ^２以下であることが望ましい。その理由は、めっき付着量が大きすぎると、めっきの密着性が劣る傾向にあるからである。
以上の手順、条件で処理することで、均一で高付着量かつ高耐食性のＺｎ−Ａｌ−Ｍｇ系合金めっきを得ることができる。
【００３１】
【実施例】
以下、本発明を実施例により、さらに詳細に説明する。
大きさが５０ｍｍ×１００ｍｍ×厚さ２．３ｍｍの黒皮付き普通鋼熱延鋼材を、機械研磨（表中にＫＭと表記）、機械研削（同じくＫＥと表記）、サンドブラスト（同じくＳＡと表記）、ショットブラスト（同じくＳＨと表記）、グリットブラスト（同じく、ＧＲと表記）、塩酸酸洗及び硫酸酸洗（同じくＡＰと表記）の種々の方法で、デスケーリングした後、乾式フラックス処理をした。また、一部の試験片は、機械的デスケーリングした後、さらに酸洗を行った。
【００３２】
なお、機械研磨は１５％塩酸酸洗でデスケーリングした後、鏡面状となるまでアルミナ研磨を行った。また、機械研削はフライス盤にて行った。サンドブラストには粒度範囲０．１〜０．３μｍのスタウロライト、ショットブラストには高炭素鋳鋼ショット、グリットブラストには高炭素鋳鋼グリットを用い、ＩＳＯ８５０１−１に記載のＳａ２．５の防錆度までデスケーリングした。塩酸酸洗及び硫酸酸洗は、表１及び表２に記載の濃度の水溶液（常温）に、鋼材を３０分浸漬した後に引き上げ、スケール残りがないことを確認した上で、次の工程に進んだ。これらのフラックス処理前の鋼材表面に存在する酸化皮膜の厚みは、同一の処理を行った試験材から小さな分析用試験材を切り出し、ＸＰＳ測定を行うことで、いずれも１０ｎｍ以下であることを確認した。
【００３３】
乾式フラックス処理とは、ＺｎＣｌ_２：２００ｇ／ｌ、ＮＨ_４Ｃｌ：４０ｇ／ｌ、ＮａＣｌ：１０ｇ／ｌからなる液温７０℃のフラックスに鋼材を１０秒間浸漬した後に引き上げ、２００℃に設定したオーブンの中で５分間乾燥させる処理である。その後、直ちに表１〜３に示す組成のめっき浴に２分間浸漬し、引き上げ後、湯冷して試験片を作製した。このようにして得られた試験片は、そのめっき付着量を、塩酸溶解法によって試験片の片面の付着量として求めた。
【００３４】
また、その外観を目視観察し、下記基準で評価した。可以上の評点を合格とした。
優；表面が平滑で、めっき欠陥が全く無い。
良；表面に凹凸が僅かに認められるが、めっき欠陥は無い。
可；凹凸がやや大きく、めっきやけが若干求められる。
劣；不めっきやピンホール、ドロス付着等が存在、又は、凹凸がやや多く、めっきやけが試験片面積の５０％以上存在する。
なお、ここでいうめっきやけとは、合金層がめっき表層まで成長していることである。
【００３５】
さらに、９０°折り曲げ試験を行い、その加工性を目視観察し、下記基準で評価した。可以上の評点を合格とした。
優；加工部にめっき割れが全く無い。
良；加工部に極めて微細な割れが１〜２個存在。
可；加工部に割れが僅かに存在。
劣；割れが加工部全面に存在、又は、加工部のめっきが剥離。
【００３６】
耐食性については、塩水噴霧試験（３５℃、５％ＮａＣｌ）を５００時間実施した後のめっき腐食量を求め、下記基準で評価した。良以上を合格とした。
優；めっき腐食量が、８０ｇ／ｍ^２未満。
良；めっき腐食量が、８０ｇ／ｍ^２以上１００ｇ／ｍ^２未満。
劣；めっき腐食量が、１００ｇ／ｍ^２以上。
これらの試験結果を表１〜３に示す。
【００３７】
【表１】

【００３８】
表１は、鋼材のデスケーリング方法を変えた場合の試験結果である。めっき組成としては、質量％で、Ａｌ：０．２％、Ｍｇ：０．５％、残部がＺｎ及び不可避的不純物となるように調整し、めっき浴温は４８０〜５００℃で行った。
デスケーリング方法が機械的方法である場合（Ｎｏ．１〜１３）は、その機械的デスケーリングの具体的手法によらず、めっきの付着量はいずれも５００ｇ／ｍ^２以上の高付着量であり、また、そのめっき外観も良好である。
【００３９】
特に、デスケーリング後の表面粗度Ｒａが１０μｍ以下である場合は、表面が極めて平滑である。また、加工性についても、Ｒａが小さい方が良好である。但し、Ｒａが小さすぎてもその効果が飽和することは、先にも述べた通りである。また、機械的デスケーリングの後に、塩酸酸洗をした場合（Ｎｏ．９〜１２）は、良好な外観かつ高付着量のめっきが得られた。塩酸の濃度が３０％より大きい場合（Ｎｏ．１２）は、塩酸の濃度が１〜３０％範囲である場合（Ｎｏ．９〜１１）よりも、ややめっき付着量が小さくなる傾向にある。
【００４０】
Ｎｏ．１３は、硫酸酸洗した場合である。塩酸酸洗と同様に、高付着量のめっきが得られる。デスケーリング方法が機械的手法ではなく、化学的手法の酸洗である場合（Ｎｏ．１４、１５）は、めっき外観は良好なものの、付着量は１００〜１５０ｇ／ｍ^２であり、高付着量のめっきが得られず、耐食性も劣る。以上から、本発明の均一で高付着量かつ高耐食性のＺｎ−Ａｌ−Ｍｇ系合金めっきを得るには、機械的デスケーリングを行う必要があり、また、その後に、質量％で、１〜３０％の濃度の酸洗をしてもよいことが分かる。さらに、鋼材表面粗度Ｒａは、小さい方がその外観が良好で、特に５μｍ以下では、その外観は極めて良好で、均一なめっきが得られる。
【００４１】
【表２】

【００４２】
表２は、めっき組成を変えた場合の試験結果である。鋼材のデスケーリングについては、ショットブラストによる機械的デスケーリングで行い、鋼材の表面粗度Ｒａは、２〜５μｍの範囲とし、めっきは４５０〜５００℃で行った。なお、フラックス処理前の鋼材表面に存在する酸化皮膜の厚みは、ＸＰＳによりいずれも１０ｎｍ以下であることを確認している。Ａｌ濃度を変化させた場合の実施例は、Ｎｏ．１６〜２４である。この場合、めっき浴中のＭｇは約０．５質量％、Ｓｉは無添加とした。いずれの試験片も高付着量で、外観、加工性、耐食性の良好なめっきとなることが分かる。特に、Ａｌが０．２〜２質量％の範囲の場合（Ｎｏ．１９〜２１）、外観、加工性、耐食性が極めて優れる。
【００４３】
Ａｌが本発明の範囲外である場合、すなわち、Ａｌが０．１質量％未満である場合（Ｎｏ．３８）は、高付着量であるものの、めっきやけが生じて不均一な外観で、加工部全面に割れが生じて加工性に劣り、耐食性にも劣るめっきとなる。一方、Ａｌが５質量％より大きい場合（Ｎｏ．３９及び４０）、高付着量で耐食性が良好であるものの、凹凸の激しい不均一なめっきとなり、加工部全面に割れが生じて加工性に劣る。Ｍｇ濃度を変化させた場合の実施例はＮｏ．２５〜３１である。この場合、めっき浴中のＡｌは約０．２質量％、Ｓｉは無添加とした。いずれも高付着量で外観、加工性、耐食性の良好なめっきとなることが分かる。
【００４４】
特に、Ｍｇが０．３〜２質量％の範囲である場合（Ｎｏ．２７〜２９）、外観、加工性、耐食性に優れる。Ｍｇが本発明の範囲外である場合、すなわち、Ｍｇが０．１質量％未満である場合（Ｎｏ．４１）、高付着量であるものの耐食性に劣る。一方、Ｍｇが５質量％より大きい場合（Ｎｏ．４２）、高付着量であるものの、ドロス付着やめっきやけが生じ、外観が劣化するだけでなく、加工部全面に割れが生じ、加工性も劣る。また、Ｎｏ．３２〜３６は、めっき浴中にＳｉを添加した場合の結果である。この場合、めっき浴中のＡｌは約３．０質量％、Ｍｇは約０．５質量％とした。Ｓｉを０．０５〜０．２質量％の範囲とすることで、加工性及び耐食性が向上する。特に、Ｓｉが０．１０〜０．１５質量％の範囲であるときは、大幅に加工性が向上する。Ｎｏ．３７は、機械的デスケーリング後に１０質量％塩酸で酸洗した場合の試験結果であり、６００ｇ／ｍ^２以上の高付着量のめっきが得られた。
【００４５】
【表３】

【００４６】
表３は、めっき浴温を変えた場合の試験結果である。鋼材のデスケーリングについては、ショットブラストによる機械的デスケーリングで行い、鋼材の表面粗度Ｒａは、２〜５μｍの範囲とした。また、一部の鋼材（Ｎｏ．４８及びＮｏ．４９）は、フラックス処理前に、濃硝酸に浸漬し、表面に約３０ｎｍの酸化皮膜を形成させた。めっき浴組成は、Ａｌ：約０．２質量％、Ｍｇ：約０．５質量％、残部がＺｎ及び不可避的不純物とした。
【００４７】
めっき浴温が本発明の範囲である４２０〜５５０℃の場合（Ｎｏ．４３〜Ｎｏ．４９）、高付着量で、外観、加工性、耐食性に優れるＺｎ−Ａｌ−Ｍｇ系合金が得られる。また、フラックス前の鋼材表面の酸化皮膜の厚みが１０ｎｍより大きい場合（Ｎｏ．４８及びＮｏ．４９）は、酸化皮膜厚みが１０ｎｍ以下である場合（Ｎｏ．４３〜Ｎｏ．４７）よりも、やや加工性や外観に劣ることが分かる。本発明の範囲外のめっき浴温である場合、すなわち、４２０℃未満である場合（Ｎｏ．４８、Ｎｏ．４９）は、Ｆｅ−Ｚｎ合金が成長せず、高付着量のめっきとならず、耐食性も劣る。また、５５０℃よりも高いめっき浴温の場合（Ｎｏ．５０、Ｎｏ．５１）は、ドロスの発生量が多く、試験片にドロスが付着したり、めっきやけが生じて、外観が劣るばかりか、合金層が不均一に成長し、加工部全面に割れが生じ、加工性に劣った。
【００４８】
【発明の効果】
以上述べたように、本発明の製造方法によれば、均一で高付着量かつ高耐食性のＺｎ−Ａｌ−Ｍｇ系合金めっき鋼材を製造することができる。すなわち、鋼材のデスケーリングを機械的手法により行い、さらに、めっき浴中のＡｌ、Ｍｇの組成及びめっき浴温を特定の範囲とすることで、従来は困難であった、均一で、高付着量のめっき層を有する加工性・耐食性に優れたＺｎ−Ａｌ−Ｍｇ系合金めっき鋼材を提供することができる。このため、長期耐久性が求められる送電鉄塔、橋梁、道路用資材、建築金物等、建築・土木用材料等にこの鋼材を適用することで、これら施設のメンテナンスフリーが実現でき、産業上の価値は極めて大きい。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a Zn—Al—Mg-based alloy-plated steel material having a uniform, high adhesion amount and high corrosion resistance by a flux method.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in the fields of power transmission towers, bridges, road materials, building hardware, construction and civil engineering materials, hot-dip galvanized steel materials have been widely used from the viewpoints of economy, durability, workability, and the like. In recent years, Zn-Al alloy plating is being used because it has better corrosion resistance than Zn plating. In these Zn plating and Zn-Al alloy plating, there is usually a proportional relationship between the amount of plating and its corrosion resistance. The thicker the plating, the longer it takes to reach the exposure of the base iron, the better the corrosion resistance. I do. That is, even if the corrosion resistance of the Zn—Al alloy plating is twice as large as that of the Zn plating, the corrosion resistance is equivalent if the amount of the Zn—Al alloy plating is only half that of the Zn plating.
[0003]
Factors that affect the amount of plating applied include the viscosity and wet adhesion of the plating bath, the plating time, the plating temperature, the speed at which the material is pulled up from the plating bath, and the wiping of the plating. Among them, the influence of the viscosity of the plating bath and the wet adhesion is not so large. Regarding the pulling speed of the material from the plating bath, it is easy to change the pulling speed in a continuous type such as plating of a thin steel sheet, but in a batch type plating such as a flux type plating, the pulling speed is not changed. Is difficult to change. Regarding wiping of plating, it is often the case that processed steel materials, welded steel materials, and materials with complicated shapes such as small items are plated.In flux type plating, it is difficult to obtain a uniform plating weight In the first place, wiping acts in a direction to reduce the amount of plating, and is not suitable for obtaining uniform plating with a large amount of plating.
[0004]
For these reasons, in flux plating, the amount of plating is often controlled by the plating time and the temperature of the plating bath. The plating time and the temperature of the plating bath have a particularly large effect on the thickness of the Fe-Zn-based alloy layer generated at the interface between the base iron and the plating, and when the plating time is long or the plating temperature is high, the alloy layer becomes thick, As a result, the deposition amount of plating increases. By the way, the thickness of the alloy layer greatly varies depending on the composition of the plating bath. For example, in Zn plating in which the amount of Al in the bath is 0.02% by mass or less, by increasing the plating time or increasing the plating temperature, the Fe-Zn-based alloy layer grows to 600 g / m ² Somewhat thick plating is also possible. However, in a Zn-Al-based plating bath containing 0.05% by mass or more of Al, an Fe-Al-based alloy layer is formed at an early stage, and this hinders the growth of the Fe-Zn-based alloy layer. It is difficult to increase the amount.
[0005]
This Fe-Al-based alloy layer is very stable, for example, is stable even when the plating temperature is increased to about 530 ° C. Since the growth of the Fe-Zn-based alloy layer is suppressed, the amount of plating does not increase. In the case of a plating bath containing a relatively high concentration of Al, for example, in the case of a plating bath containing 10% or more Al by mass%, the Fe-Al-based alloy layer itself grows by increasing the plating bath temperature. Therefore, the amount of plating increases. However, the plating is not uniform, that is, the plating becomes locally thick and the appearance is poor, so that it is not suitable as a product. Further, even if the plating time is lengthened, the Fe—Zn-based alloy layer does not easily grow. When immersed in a plating bath for a considerably long period of time, for example, about one hour, the amount of plating applied may increase, but it is also unsuitable as a product due to uneven plating and poor appearance. As described above, it is difficult to obtain a uniform Zn-Al alloy plating with a high adhesion amount, and it has not been sufficiently studied so far.
[0006]
The following are known methods for obtaining a Zn-Al alloy plating with a high adhesion amount. For example, a two-step plating process in which ordinary pure Zn plating containing almost no Al is performed, a Zn—Al alloy plating is performed after growing an Fe—Zn-based alloy layer is partially adopted. However, this method has various disadvantages such as the necessity of having two plating baths, an increase in work time, maintenance and management costs, and an increase in installation locations due to two plating operations.
[0007]
JP-A-5-106002 (Patent Document 1) discloses a method in which a relatively thick Zn-Al alloy plating can be obtained by adding 1.5 to 10% by mass of copper to a Zn-Al alloy plating bath. Proposed. However, in this method, the composition of the plating bath is likely to fluctuate, and there is a concern that the plating quality varies.
Further, in Japanese Patent Application Laid-Open No. 9-25134 (Patent Document 2), after a steel material is subjected to a predetermined flux treatment, a plating temperature of 450 to 520 ° C. and a plating time of 0.1 to 20% by mass are applied to a Zn plating bath having an Al concentration of 10 to 20% by mass. By plating under conditions of 5 to 10 minutes, the plating thickness (plating adhesion amount) is 450 g / m. ² It is said that the above thick plating can be obtained. However, this method cannot be applied to a plating bath having an Al concentration of 10% by mass or less, and has a lack of versatility since a special flux needs to be used.
[0008]
On the other hand, Japanese Patent Application Laid-Open No. 11-350095 (Patent Document 3) states that after performing a predetermined flux treatment, immersion in a Zn-Al plating bath having a molten flux layer enables plating with a high adhesion amount to be obtained. ing. However, also in this method, the plating bath having an Al content of 30 to 70% by mass is mainly used, a special flux is used, and there is no versatility, and a molten flux layer is required. However, there are various problems, such as the need for labor for replacement and replenishment for the deterioration, and concern about the working environment because the flux contains fluoride.
[0009]
Furthermore, in Japanese Patent Application Laid-Open No. 8-283925 (Patent Document 4), a high adhesion amount and a uniform Zn content are obtained by setting the surface roughness of a steel material within a certain range and further including a certain iron component in a plating bath. -It is said that -Al-Si plating can be obtained. However, this method is intended for Al-Si plating and the plating bath temperature is as high as 580 ° C. or higher, and is not suitable for plating mainly composed of Zn.
As described above, although several methods have been found as methods for obtaining a Zn—Al alloy plating with a high adhesion amount, there are drawbacks in terms of workability, quality, environment, and the like. Further, even if a plating with a high adhesion amount can be performed, it is difficult to stably obtain a uniform alloy plating.
[0010]
[References]
(1) Patent Document 1 (JP-A-5-106002)
(2) Patent Document 2 (Japanese Patent Application Laid-Open No. 9-25134)
(3) Patent Document 3 (Japanese Patent Application Laid-Open No. 11-350095)
(4) Patent Document 4 (Japanese Patent Laid-Open No. 8-283925)
[0011]
[Problems to be solved by the invention]
The present invention solves the conventional problems in such Zn-Al-based alloy plating and provides a method for producing a Zn-Al-Mg-based alloy-plated steel material having a uniform, high coating weight and high corrosion resistance by a flux method. With the goal.
[0012]
[Means for Solving the Problems]
Means for Solving the Problems The present inventors have solved the above-mentioned problems of the conventional technology, and have intensively studied a method for stably obtaining a uniform coating amount and high corrosion resistance plating on Zn-Al-based alloy platings of various compositions. As a result, after removing the oxide film on the surface of the steel material by a mechanical method, flux treatment is performed, and in a Zn-Al plating bath containing a certain amount of Mg, within a certain plating bath temperature range, the steel material is removed. By plating, a method for producing a Zn—Al—Mg-based alloy-plated steel material having a uniform, high adhesion amount and high corrosion resistance was found.
[0013]
That is, the present invention
(1) After removing the oxide film on the surface of the steel material by mechanical descaling, flux treatment is performed, and in mass%, Al: 0.1 to 5%, Mg: 0.1 to 5%, balance Zn and inevitable impurities. Immersion in a Zn-Al-Mg based alloy plating bath at a bath temperature of 420 to 550 [deg.] C. to form a Zn-Al-Mg based plated layer on the surface of a steel material with a high adhesion amount. Manufacturing method of Mg-based alloy-plated steel,
(2) The method for producing a Zn-Al-Mg-based alloy-plated steel material according to (1), wherein the mechanical descaling causes the roughness Ra of the surface of the steel material to be in a range of 0.01 to 50 µm.
[0014]
(3) The method according to (1) or (2), wherein after the mechanical descaling, the steel material is immersed in an aqueous solution containing at least one or both of 1% to 30% hydrochloric acid or 1% to 30% sulfuric acid in mass%. Production method of Zn-Al-Mg-based alloy-plated steel material,
(4) The Zn-Al-Mg-based alloy-plated steel material according to any one of (1) to (3), wherein the thickness of the oxide film existing on the surface of the steel material before the flux treatment is 10 nm or less. Production method,
(5) The Zn-Al-Mg alloy plating bath according to any one of (1) to (4), wherein the Zn-Al-Mg-based alloy plating bath further contains Si: 0.05 to 0.2% by mass%. This is a method for producing a base alloy plated steel material.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
The type of steel material to be plated used in the present invention is not limited. The steel components are not limited, such as ordinary steel, low alloy steel, and high alloy steel, and their shapes are also plates, steel pipes, shaped steels, wire rods, and combined steel materials joined by welding, bolts, etc. , Bolts, nuts, etc., and steel members such as steel towers, bridge members, etc., and anything that can be immersed in a plating bath and plated.
[0016]
In the manufacturing method of the present invention, first, it is essential to remove a surface oxide film of a steel material as a base material by mechanical descaling. The mechanical descaling is a method of mechanically removing an oxide film, for example, a method of sand blasting, shot blasting, grit blasting, cutting, grinding, polishing, and the like. Any method can be used as long as it can remove the oxide film, and the specific method is not limited. By this mechanical descaling, uniform plating can be obtained with a high adhesion amount.
[0017]
The high coating weight in the present invention means that the coating weight on one side of the steel material is 400 g / m2. ² It points out that it is above. Regarding the mechanism by which the mechanical descaling can provide a plating with a high coating weight, the effect of residual stress applied to the steel material and the like can be considered, but this has not yet been estimated. However, by performing mechanical descaling, the Fe—Zn-based alloy layer easily grows, and uniform plating with a large amount of deposition becomes possible. For this reason, in the present invention, it is necessary to first mechanically descaling the steel material surface.
[0018]
It is desirable that the surface roughness Ra of the steel material after the mechanical descaling is in the range of 0.01 to 50 μm. That is, if the surface roughness is less than 0.01 μm, the amount of plating and the appearance may not be changed, but only increase the cost. When the surface roughness is larger than 50 μm, the smoothness of the plating surface tends to be inferior, and the appearance is impaired. In addition, workability tends to be poor. Therefore, by setting the steel material surface roughness Ra after mechanical descaling in the present invention in the range of 0.01 to 50 μm, a good plating appearance can be obtained stably, and preferably 0.1 to 20 μm. It is more preferably in the range of 1.0 to 10 μm. Ra within such a condition range can be achieved by appropriately selecting a descaling method. To give an example, generally, Ra increases in the order of alumina polishing, grinding by a milling machine, sand blast, and grit blast.
[0019]
After the mechanical descaling, the steel material may be immersed in an aqueous solution containing at least one or both of hydrochloric acid and 1 to 30% sulfuric acid in mass%. By immersing in an aqueous solution containing at least one or both of 1 to 30% hydrochloric acid and 1 to 30% sulfuric acid, iron and oil remaining on the steel surface after descaling, dirt, Oil and dirt adhering to the surface can be removed, the reactivity between the steel material and the plating during plating can be increased, the alloying of Fe-Zn can be activated, and plating with a large amount of adhesion can be obtained.
[0020]
If the concentrations of hydrochloric acid and sulfuric acid are less than 1% by mass, it may be insufficient to remove iron, dirt, and oil on the surface of the steel material, and the effect of increasing the plating adhesion may be reduced. If the concentration of hydrochloric acid or sulfuric acid is 30% by mass or more, the ability to dissolve the steel material becomes excessive, and the effect of mechanical descaling may be lost. It can be difficult. When the steel material is immersed in an aqueous solution containing at least one of 1 to 30% by mass of hydrochloric acid or 1 to 30% by mass of sulfuric acid, the temperature of the aqueous solution is not particularly limited, but is in the range of 20 to 70 ° C. It is desirable.
[0021]
The reason is that if the temperature is lower than 20 ° C., it may be insufficient to remove iron, dirt and oil on the surface of the steel material, and if the temperature is higher than 70 ° C., the effect of cleaning the surface of the steel material is no longer saturated, This is because the working environment is more likely to be deteriorated due to volatilization of hydrogen and the like. The immersion time of the steel material is desirably 1 hour or less. The reason is that if immersed for more than one hour, the steel material is excessively dissolved, not only the aqueous solution is contaminated, but also the surface roughness of the steel material becomes remarkable.
[0022]
The flux treatment is performed after the above-described pretreatment of the steel material surface. In the present invention, it is desirable that the thickness of the oxide film existing on the steel material surface before the flux is 10 nm or less. The oxide film referred to here is a compound with oxygen, such as an iron oxide film, a silicon oxide film, and an aluminum oxide film, regardless of the type. Note that the thickness of the oxide film here refers to the thickness of the oxide film obtained by AES (Auger electron spectroscopy) or XPS (X-ray photoelectron spectroscopy). For example, the surface of the material to be plated before the flux treatment is measured by AES or XPS to measure the strength in the depth direction of oxygen, and the depth at which the strength becomes the average value of the highest strength and the lowest strength is determined as an oxide film. Thickness.
[0023]
Usually, the thickness of the oxide film depends on the manufacturing history of the steel material and the conditions of mechanical descaling, and thus the measurement of the oxide film can be determined by a small test piece manufactured under the same conditions.
The thickness of the oxide film measured as described above is desirably 10 nm or less. When the thickness is 10 nm or less, the alloying reaction of the Fe-Al-based and Fe-Zn-based alloys occurs promptly and uniformly, so that plating having more excellent adhesion and workability and good appearance can be obtained.
[0024]
In the present invention, the flux treatment is performed after the pretreatment of the steel material surface. The flux treatment referred to herein is a treatment for applying and attaching a flux to the surface of a steel material, and there is no particular limitation on the flux and the treatment conditions, and a conventionally known method may be used. For example, at least one chloride selected from zinc chloride, ammonium chloride, magnesium chloride, lithium chloride, tin chloride, potassium chloride, calcium chloride, lead chloride, sodium chloride, antimony chloride, indium chloride, bismuth chloride, cadmium chloride and the like. And a dry flux method of dipping and drying in an aqueous solution containing at least one kind selected from fluorides of alkali metals and alkaline earth metals, silicofluorides, hydrogen fluorides, etc. A wet flux method suspended above or a molten flux method in which a steel material is immersed in a layer in which the flux itself is melted can be applied. The composition of the flux and the concentration of the aqueous solution in the case of the dry flux are not particularly limited, and may be a conventionally known composition and concentration.
[0025]
The flux-treated steel material is a Zn-Al-Mg-based steel material having a bath temperature of 420 to 550C containing Al: 0.1 to 5%, Mg: 0.1 to 5%, the balance being Zn and inevitable impurities in mass%. It is immersed in an alloy plating bath. Al in the bath has the effect of making the plating highly corrosion resistant, having a uniform plating thickness, and providing a good plating appearance. If Al is less than 0.1% by mass, corrosion resistance is not sufficiently improved, and if it is more than 5% by mass, non-uniform plating is liable to occur. For this reason, Al in the plating bath of the present invention is in the range of 0.1 to 5% by mass, preferably in the range of 0.15 to 3% by mass, and more preferably in the range of 0.2 to 2% by mass. Stipulate.
[0026]
Mg is an essential component for obtaining uniform and highly corrosion-resistant plating. If the content of Mg is less than 0.1% by mass, it is insufficient to make the plating highly corrosion resistant. If the content is more than 5% by mass, not only the workability of the plating is impaired, but also the amount of dross generated in the plating bath. Increases, causing non-plating and the like. For this reason, Mg in the plating bath in the present invention is specified in the range of 0.1 to 5% by mass, preferably in the range of 0.2 to 3% by mass, and more preferably in the range of 0.3 to 2% by mass. I do.
[0027]
Moreover, the Zn-Al-Mg-based alloy plating bath of the present invention may further contain 0.05 to 0.2% by mass of Si. Si has the effect of increasing the corrosion resistance of the plating and the effect of increasing the density of the Fe-Zn-based alloy layer and improving the workability. If the content of Si is less than 0.05% by mass, the above effect is small, and if the content is 0.2% by mass or more, the workability tends to decrease. Therefore, the content of Si in the plating bath in the present invention is desirably in the range of 0.05 to 0.2% by mass, preferably in the range of 0.10 to 0.15% by mass. Further, there is no problem even if, for example, Fe, Pb, Cd, Sn, Cu, Sb, Bi, Ag, Ni, As, etc. are mixed as unavoidable impurities in the Zn-Al-Mg-based plating bath. .
[0028]
The plating bath temperature must be in the range of 420 to 550 ° C. In this temperature range, the Fe—Zn-based alloy layer grows uniformly, and it is possible to obtain plating with a high adhesion amount and high corrosion resistance. If the plating bath temperature is lower than 420 ° C., the alloying reaction of Fe—Zn is delayed, and the growth rate is low, so that plating with a high adhesion amount is not obtained, which is not preferable. When the plating bath temperature is higher than 550 ° C., the alloy layer is locally thickened, not only the appearance is deteriorated, but also the amount of dross generated in the plating bath increases, and it takes time to remove the dross. Further, the appearance of the product becomes poor due to dross entrainment at the time of plating, which is not preferable. For this reason, the temperature of the Zn—Al—Mg-based alloy plating bath in the present invention is specified in the range of 420 to 550 ° C.
[0029]
The speed at which the material to be plated is immersed in the plating bath is not specified at all. For example, as in the Taylor-Wilson method in continuous plating of steel pipes, a high immersion speed such as dropping a steel pipe into a plating bath may be used, or, as in the case of plating a large steel member such as a steel tower member, a crane. After the material to be plated is lifted by the method described above, a slow immersion speed such that the material is slowly immersed in the plating bath may be used.
The immersion time in the plating bath is not particularly specified. However, if the plating bath is immersed in a plating bath at 500 ° C. or higher for a long time, for example, 1 hour or more, the amount of plating adhesion becomes extremely large, and not only may the adhesion of the plating deteriorate, but also the plating becomes uneven. For this reason, it is desirable that the plating time be within 30 minutes.
[0030]
The speed at which the material to be plated is removed from the plating bath is not particularly specified. It may be taken out at a high speed of about 0.7 to 2 m / sec as in the Taylor-Wilson system, or may be taken out at a low speed of about 0.2 m / sec as in the case of using a crane. Further, in order to improve the appearance after plating, cooling by water cooling or hot water cooling may be performed as necessary.
400 g / m 2 by plating under the above conditions ² It is possible to obtain a plating with a high adhesion amount as described above. Although the upper limit of the coating weight is not particularly specified, 800 g / m ² It is desirable that: The reason for this is that if the plating adhesion amount is too large, the adhesion of the plating tends to be poor.
By performing the treatment under the above procedure and conditions, it is possible to obtain a Zn—Al—Mg-based alloy plating that is uniform, has a high adhesion amount, and has high corrosion resistance.
[0031]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples.
A hot-rolled steel sheet with black skin having a size of 50 mm x 100 mm x 2.3 mm in thickness is mechanically polished (expressed as KM in the table), mechanically grounded (also expressed as KE), and sandblasted (also expressed as SA). , Shot blasting (also denoted as SH), grit blasting (also denoted as GR), hydrochloric acid pickling, and sulfuric acid pickling (also denoted as AP), and then subjected to dry flux treatment. Some of the test pieces were further subjected to pickling after mechanical descaling.
[0032]
In mechanical polishing, after descaling with 15% hydrochloric acid pickling, alumina polishing was performed until a mirror surface was obtained. The mechanical grinding was performed by a milling machine. Using staurolite with a particle size range of 0.1 to 0.3 μm for sand blast, high carbon cast steel shot for shot blast, and high carbon cast steel grit for grit blast, up to the rust prevention of Sa2.5 described in ISO 8501-1 Descaled. In the hydrochloric acid pickling and the sulfuric acid pickling, the steel material is immersed in an aqueous solution (normal temperature) having a concentration shown in Table 1 and Table 30 for 30 minutes, pulled up, and after confirming that there is no scale residue, proceed to the next step. It is. The thickness of the oxide film existing on the surface of the steel material before the flux treatment was confirmed to be 10 nm or less by cutting a small analytical test material from the test material subjected to the same treatment and performing XPS measurement. did.
[0033]
Dry flux treatment means ZnCl ₂ : 200 g / l, NH ₄ This is a process in which a steel material is immersed in a flux consisting of 40 g / l of Cl and 10 g / l of NaCl at a liquid temperature of 70 ° C. for 10 seconds, then pulled up, and dried in an oven set at 200 ° C. for 5 minutes. Then, it was immediately immersed in a plating bath having the composition shown in Tables 1 to 2 for 2 minutes, pulled up, and cooled with hot water to produce a test piece. The test piece thus obtained was determined by measuring the amount of plating applied as the amount of adhesion on one side of the test piece by a hydrochloric acid dissolution method.
[0034]
The appearance was visually observed and evaluated according to the following criteria. A grade higher than or equal to acceptable was passed.
Excellent: The surface is smooth and there is no plating defect.
Good; slight irregularities were observed on the surface, but no plating defects.
Acceptable; unevenness is slightly large, and plating burn is slightly required.
Poor; non-plating, pinholes, dross adhesion, etc., or slight irregularities, 50% or more of plating or injury specimen area.
The term “burning” as used herein means that the alloy layer has grown to the plating surface layer.
[0035]
Further, a 90 ° bending test was performed, and the workability was visually observed and evaluated according to the following criteria. A grade higher than or equal to acceptable was passed.
Excellent: There is no plating crack in the processed part.
Good: One or two very fine cracks were present in the processed part.
OK: Cracks are slightly present in the processed part.
Poor; cracks were present on the entire surface of the processed part, or plating of the processed part was peeled off.
[0036]
Regarding corrosion resistance, the amount of plating corrosion after performing a salt spray test (35 ° C., 5% NaCl) for 500 hours was determined and evaluated according to the following criteria. Good or better was considered a pass.
Excellent; plating corrosion amount is 80 g / m ² Less than.
Good; plating corrosion amount is 80 g / m ² 100g / m or more ² Less than.
Poor; plating corrosion amount is 100 g / m ² that's all.
Tables 1 to 3 show the test results.
[0037]
[Table 1]

[0038]
Table 1 shows the test results when the descaling method of the steel material was changed. The plating composition was adjusted so that Al: 0.2%, Mg: 0.5%, and the balance was Zn and unavoidable impurities by mass%, and the plating bath temperature was 480 to 500 ° C.
When the descaling method is a mechanical method (Nos. 1 to 13), the deposition amount of the plating is 500 g / m2 regardless of the specific method of the mechanical descaling. ² The coating amount is high as described above, and the plating appearance is also good.
[0039]
In particular, when the surface roughness Ra after descaling is 10 μm or less, the surface is extremely smooth. Also, as for workability, the smaller the Ra, the better. However, the effect is saturated even if Ra is too small, as described above. In addition, when hydrochloric acid pickling was performed after the mechanical descaling (Nos. 9 to 12), a plating having a good appearance and a high adhesion amount was obtained. When the concentration of hydrochloric acid is more than 30% (No. 12), the amount of plating tends to be slightly smaller than when the concentration of hydrochloric acid is in the range of 1 to 30% (Nos. 9 to 11).
[0040]
No. No. 13 is a case where washing with sulfuric acid was performed. As in the case of pickling with hydrochloric acid, plating with a high adhesion amount can be obtained. When the descaling method is not a mechanical method but a chemical method of pickling (Nos. 14 and 15), the plating appearance is good, but the adhesion amount is 100 to 150 g / m. ² Therefore, plating with a high adhesion amount cannot be obtained, and the corrosion resistance is poor. From the above, it is necessary to perform mechanical descaling in order to obtain a uniform, high deposition amount and high corrosion resistance Zn—Al—Mg-based alloy plating of the present invention, and thereafter, 1 to 30% by mass. It can be seen that pickling at a concentration of% may be performed. Further, the smaller the surface roughness Ra of the steel material, the better its appearance. Particularly, when the surface roughness Ra is 5 μm or less, the appearance is extremely good and uniform plating can be obtained.
[0041]
[Table 2]

[0042]
Table 2 shows the test results when the plating composition was changed. The descaling of the steel material was performed by mechanical descaling by shot blast, the surface roughness Ra of the steel material was in the range of 2 to 5 μm, and the plating was performed at 450 to 500 ° C. The thickness of the oxide film existing on the surface of the steel material before the flux treatment was confirmed to be 10 nm or less by XPS. An example in which the Al concentration is changed is described in No. 16 to 24. In this case, Mg in the plating bath was about 0.5% by mass, and Si was not added. It can be seen that all of the test pieces have a high adhesion amount and are excellent in appearance, workability, and corrosion resistance. In particular, when Al is in the range of 0.2 to 2% by mass (Nos. 19 to 21), the appearance, workability, and corrosion resistance are extremely excellent.
[0043]
When Al is out of the range of the present invention, that is, when Al is less than 0.1% by mass (No. 38), although the coating amount is high, plating and burn are generated, and the appearance is uneven. Cracking occurs on the entire surface, resulting in poor workability and poor corrosion resistance. On the other hand, when Al is more than 5% by mass (Nos. 39 and 40), although the corrosion resistance is good with a large amount of adhesion, uneven plating with severe unevenness occurs, and cracks occur on the entire processed portion, resulting in poor workability. . An example in which the Mg concentration is changed is No. 25 to 31. In this case, Al in the plating bath was about 0.2% by mass, and Si was not added. It can be seen that all of them have high adhesion and good plating appearance, workability and corrosion resistance.
[0044]
In particular, when Mg is in the range of 0.3 to 2% by mass (Nos. 27 to 29), the appearance, workability, and corrosion resistance are excellent. When Mg is out of the range of the present invention, that is, when Mg is less than 0.1% by mass (No. 41), the corrosion resistance is inferior although the adhesion amount is high. On the other hand, when Mg is more than 5% by mass (No. 42), although the amount of adhesion is high, dross adhesion and plating are generated, and not only the appearance is deteriorated, but also the entire processed portion is cracked and workability is deteriorated. Inferior. No. 32 to 36 are the results when Si was added to the plating bath. In this case, Al in the plating bath was about 3.0% by mass, and Mg was about 0.5% by mass. By setting the content of Si in the range of 0.05 to 0.2% by mass, workability and corrosion resistance are improved. In particular, when Si is in the range of 0.10 to 0.15% by mass, workability is greatly improved. No. 37 is a test result in the case of pickling with 10% by mass hydrochloric acid after mechanical descaling, and is 600 g / m 2. ² The plating with the above-mentioned high adhesion amount was obtained.
[0045]
[Table 3]

[0046]
Table 3 shows the test results when the plating bath temperature was changed. The descaling of the steel material was performed by mechanical descaling by shot blasting, and the surface roughness Ra of the steel material was in the range of 2 to 5 μm. Further, some steel materials (No. 48 and No. 49) were immersed in concentrated nitric acid before the flux treatment to form an oxide film of about 30 nm on the surface. The plating bath composition was about 0.2% by mass of Al, about 0.5% by mass of Mg, and the balance was Zn and unavoidable impurities.
[0047]
When the plating bath temperature is within the range of 420 to 550 ° C. in the range of the present invention (No. 43 to No. 49), a Zn—Al—Mg-based alloy having a high adhesion amount and excellent in appearance, workability, and corrosion resistance can be obtained. In addition, when the thickness of the oxide film on the surface of the steel material before the flux is larger than 10 nm (No. 48 and No. 49), it is slightly higher than when the thickness of the oxide film is 10 nm or less (No. 43 to No. 47). It turns out that it is inferior in workability and appearance. When the plating bath temperature is out of the range of the present invention, that is, when the plating bath temperature is lower than 420 ° C. (No. 48 and No. 49), the Fe—Zn alloy does not grow, and the plating with a high adhesion amount does not occur. Poor corrosion resistance. In the case of a plating bath temperature higher than 550 ° C. (No. 50 and No. 51), the amount of dross generated was large, and not only the appearance was inferior due to the dross adhering to the test piece, but also plating and burns occurring. In addition, the alloy layer grew non-uniformly, cracks occurred on the entire processed portion, and the workability was poor.
[0048]
【The invention's effect】
As described above, according to the production method of the present invention, it is possible to produce a Zn—Al—Mg-based alloy-plated steel material having a uniform, high adhesion amount and high corrosion resistance. That is, the descaling of the steel material is performed by a mechanical method, and furthermore, by setting the composition of Al and Mg in the plating bath and the plating bath temperature to a specific range, a uniform and high adhesion amount which has been difficult in the past. And a Zn-Al-Mg-based alloy-plated steel material having excellent workability and corrosion resistance having a plating layer of (i). For this reason, by applying this steel material to power transmission towers, bridges, road materials, building hardware, construction and civil engineering materials, etc., which require long-term durability, maintenance-free of these facilities can be realized, and industrial value Is extremely large.

Claims

After removing the oxide film on the surface of the steel material by mechanical descaling, it is subjected to flux treatment, and contains Al: 0.1 to 5%, Mg: 0.1 to 5%, and the balance Zn and unavoidable impurities by mass%. A Zn-Al-Mg-based alloy characterized by being immersed in a Zn-Al-Mg-based alloy plating bath at a bath temperature of 420 to 550 [deg.] C to form a Zn-Al-Mg-based plating layer on a steel material surface with a high adhesion amount. Manufacturing method of plated steel.

The method for producing a Zn-Al-Mg-based alloy-plated steel material according to claim 1, wherein the mechanical descaling causes the roughness Ra of the steel material surface to be in a range of 0.01 to 50 m.

The Zn-Al- according to claim 1 or 2, wherein the steel material is immersed in an aqueous solution containing at least one or both of 1% to 30% hydrochloric acid or 1% to 30% sulfuric acid by mass% after the mechanical descaling. Manufacturing method of Mg-based alloy plated steel.

The method for producing a Zn-Al-Mg-based alloy-plated steel material according to any one of claims 1 to 3, wherein the thickness of the oxide film existing on the surface of the steel material before the flux treatment is 10 nm or less.

The Zn-Al-Mg-based alloy plating steel material according to any one of claims 1 to 4, wherein the Zn-Al-Mg-based alloy plating bath further contains Si: 0.05 to 0.2% by mass%. Method.