JP2004315919A

JP2004315919A - Galvanized steel sheet and method for manufacturing the same

Info

Publication number: JP2004315919A
Application number: JP2003113718A
Authority: JP
Inventors: Masaji Murase; 正次村瀬; Chiaki Kato; 千昭加藤
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2003-04-18
Filing date: 2003-04-18
Publication date: 2004-11-11

Abstract

<P>PROBLEM TO BE SOLVED: To provide a galvanized steel sheet which can be reutilized not only as product members for household electric appliance products, but also recovered as an iron resource and has end surface rust resistance even in case of a large thickness. <P>SOLUTION: The steel sheet is subjected to galvanizing on its surface. The galvanized steel sheet is formed by removing a surface thickened layer containing one or two or more selected from the group consisting of the Mn sulfide, Mn oxide and Si oxide of a steel sheet containing 0.001 to 0.1 mass% C, ≤0.1 mass% Si, 0.001 to 0.010 mass% S, 0.003 to 0.030 mass% Al, 0.05 to 0.15 mass% Mn, ≤0.02 mass% P, and the balance iron and inevitable impurities prior to plating, then applying plating thereto. Also, the galvanizing is recommended to be applied at ≥5 g/m<SP>2</SP>on at least the one surface or further an inorganic coating layer and/or organic coating layer is recommended to be formed on the galvanized surface for better results. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、亜鉛系めっき鋼板とその製造方法に係わり、主に室内で使用される電気製品、事務機器等の部材となる亜鉛系めっき鋼板において、切断端面、打ち抜き面等、亜鉛系めっきの所謂「犠牲防食」が及ばない部分の耐食性、つまり耐端面錆性の改善に関する。
【０００２】
【従来の技術】
鋼板の片面又は両面に亜鉛系めっきを施した表面処理鋼板（以下、亜鉛系めっき鋼板という）は、優れた耐食性を有しており、家電、事務機器等の製品部材として広く使用されている。これら製品部材は、該亜鉛系めっき鋼板にせん断加工、打ち抜き加工、穴あけ、トリミング等の機械的加工を行った後、所定のサイズ及び形状に切断してから成形することで製造されるが、使用後には、製品と共に廃却されるのが一般的である。ところが、家電リサイクル法が施行された平成１３年より、これら部材を再利用する動向がある。
【０００３】
しかしながら、家電製品等を回収し、その部材を再度同様な製品の部材として再利用しようとすると、部材を形成する亜鉛系めっき鋼板の切断面（例えば、せん断面、打ち抜き面、トリミング面等）に赤錆が発生しており、再利用し難いという問題がある。つまり、錆は電気伝導性を有するので、錆の剥落に起因した電気回路の損傷等が懸念されるためである。従来、家電製品分野では、亜鉛系めっき鋼板の表面は、亜鉛めっきの犠牲防食により十分に防食されるので、これら切断面の耐食性は、十分に検討されていなかった。
【０００４】
切断面の耐食性は、亜鉛めっきの犠牲防食性が到達しないために起きる現象であり、これを根本的に解決するには、下地鋼材の耐食性を向上させることが必要と考えられる。例えば、重量％で，Ｃ：０．１５％以下、酸可溶Ａｌ：０．００５〜１％，Ｃｒ：１．５〜２０％、Ｃｕ：０．８％以下、Ｐ：０．１５％以下、Ｎｉ：１０％以下の１種又は２種以上を含有し、さらにＴｉ，Ｎｂ，Ｖ，Ｚｒの１種又は２種以上を０．０３〜０．５０％含有する鋼板に亜鉛系めっきを施した亜鉛系めっき鋼板が開示されている（特許文献１参照）。しかしながら、この鋼板は、Ｃｕを含有しているので、鉄資源として回収し難く、またＣｒ及びＮｉを含有させるので、高価である等の問題があった。
【０００５】
また、重量％で、Ｃ：０．０１〜０．０８％，Ｓｉ：０．０４％以下、Ｍｎ：０．４％以下、Ｐ：０．０４％以下、Ｓ：０．０１５％以下を含み、且つＭｎとＳは、Ｍｎ（ｗｔ％）×Ｓ（ｗｔ％）≦０．００３を満足し、ＰとＳは、Ｓ（ｗｔ％）×Ｐ（ｗｔ％）≦０．０００３を満足する範囲で含有する鋼板に亜鉛めっきを施した亜鉛系めっき鋼板も開示されている。しかしながら、この鋼板は、必ずしも十分な耐端面錆性を有しておらず、特に、板厚が２．０ｍｍを超えて厚くなった場合には、耐端面錆性がまったく認められなかった。
【０００６】
【特許文献１】
特開昭６２−７８８８号公報
【特許文献２】
特開平１０−３３０８８３号公報
【０００７】
【発明が解決しようとする課題】
本発明は、かかる事情に鑑み、家電製品等の部材として再利用できるばかりでなく、鉄資源としても回収され、且つ板厚が厚い場合でも十分な耐端面錆性を有する亜鉛系めっき鋼板を提供することを目的としている。
【０００８】
【課題を解決するための手段】
本発明者らは、上記目的を達成するため鋭意研究を重ね、その成果を本発明に具現化した。
【０００９】
すなわち、本発明は、表面に亜鉛系めっきが施された鋼板であって、前記亜鉛系めっきを施される鋼板は、Ｃ：０．００１〜０．１質量％、Ｓｉ：０．１質量％以下、Ｓ：０．００１〜０．０１０質量％、Ａｌ：０．００３〜０．０３０質量％、Ｍｎ：０．０５〜０．１５質量％、Ｐ：０．０２質量％以下、その他鉄及び不可避不純物を含み、且つめっき前に、該鋼板の、Ｍｎ硫化物、Ｍｎ酸化物及びＳｉ酸化物からなる群より選ばれる１又は２以上を含有する表面濃化層を除去してなることを特徴とする亜鉛系めっき鋼板である。この場合、前記亜鉛系めっきを、少なくとも片面に５ｇ／ｍ^２以上施したり、あるいは前記亜鉛系めっき鋼板の表面に、さらに無機被覆層及び／又は有機被覆層を形成するのが好ましい。
【００１０】
また、本発明は、表面に亜鉛系めっきが施された鋼板を製造するに際し、前記亜鉛系めっきが施される鋼板を、Ｃ：０．００１〜０．１質量％、Ｓｉ：０．１質量％以下、Ｓ：０．００１〜０．０１０質量％、Ａｌ：０．００３〜０．０３０質量％、Ｍｎ：０．０５〜０．１５質量％、Ｐ：０．０２質量％以下、その他鉄及び不可避不純物を含むものとし、めっき前に、該鋼板の、Ｍｎ硫化物、Ｍｎ酸化物及びＳｉ酸化物からなる群より選ばれる１又は２以上を含有する表面濃化層を、化学的手段又は物理的手段で除去してから、少なくとも片面に５ｇ／ｍ^２以上の亜鉛系めっきを施すことを特徴とする亜鉛系めっき鋼板の製造方法である。この場合、前記表面濃化層の除去を、該鋼板の最表面から厚み：０．１〜１５μｍを除去することとしたり、あるいは前記亜鉛系めっき鋼板の表面に、さらに無機被覆層及び／又は有機被覆層を形成するのが良い。
【００１１】
本発明では、亜鉛系めっきを施す前の被めっき鋼板の鋼成分を制御すると共に、主にＭｎ系析出物、Ｓ系析出物等からなる表面濃化層を除去するようにしたので、得られた亜鉛系めっき鋼板の切断端面における錆の発生起点が極力低減するようになる。その結果、該亜鉛系めっき鋼板で製造した製品部材は、再利用できるようになるばかりでなく、鉄資源としても回収されるようになる。
【００１２】
【発明の実施の形態】
以下、発明をなすに至った経緯をまじえ、本発明の実施の形態を説明する。
【００１３】
まず、発明者は、屋内環境で使用される家電製品等を構成する亜鉛系めっき鋼板の切断面に錆が発生する部位及び発生原因を、暴露試験を行って調査した。その結果、亜鉛系めっき鋼板は、図３に示すように、被めっき鋼板１、Ｍｎ硫化物、Ｍｎ酸化物及びＳｉ酸化物からなる群より選ばれる１又は２以上を含有する表面濃化層２及び亜鉛系めっき層３からなるが、その切断面のうち、破断面４ではなく、表面濃化層２が露出したせん断面５に錆（図示せず）が発生し易いことがわかった。なお、この錆の発生状況を面積率で評価し、図２に示す。また、鋼表面を酸洗（２０℃、１０質量％塩酸水溶液に１０秒浸漬、最表層から厚み０．０１〜０．０２μｍ除去）後にＧＤＳ（ＧｒｉｍＧｌｏｗＤｉｓｃｈａｒｇｅＳｐｅｃｔｒｏｓｃｏｐｙの略）分析を行ったところ、図１に示すように、鋼板表面には、Ｍｎ，Ｓ，Ｓｉが濃化していた。それらの濃化が錆の発生に大いに影響しているようであった。つまり、大気中、特に屋内使用環境下における鋼板の腐食は、きわめて腐食環境としてマイルドであり、腐食の起点が鋼板中の析出物及び環境から飛来するゴミ（該ゴミ中に存在する腐食性イオン形成成分、例えばＣｌ，Ｎａ，ＳＯ_４，ＮＯ_３，Ｃａ，Ｍｇ等）に起因するものと考えたのである。一般に、室内環境では結露等の水濡れ現象が起き難いが、表面に存在する析出物（例えば、Ｍｎ硫化物、Ｍｎ酸化物、Ｓｉ酸化物等）や環境から飛来するゴミ（イオン性物質）等が切断面に存在すると、それらが大気中の湿度から吸湿作用を起こし、腐食の起点になり得る。特に、せん断機の刃による亜鉛系めっき鋼板の強制的なせん断では、前記Ｍｎ硫化物、Ｍｎ酸化物、Ｓｉ酸化物等を含有する表面濃化層２をせん断面５側に引張るので、該せん断面は、鋼板の他の部分よりも腐食し易くなると考えられる。
【００１４】
そこで、発明者は、この原因を取り除けば、亜鉛系めっき鋼板の耐端面錆性を従来より向上させることができると考え、表面濃化層成分を始めとする鋼板成分の含有量を制限して、錆の発生起点を極力低減させることにした。
【００１５】
まず、亜鉛系めっきが施される鋼板の主な成分であるＣ，Ｓｉ，Ｓ，Ｐ，Ａｌ，Ｍｎは、いずれも製鋼段階や鋼板になった状態での機械的性質の面から有用な元素であるが、これらは、亜鉛系めっきが施された後の耐端面錆性を低下させる元素である。そこで、本発明では、下記のように含有量を限定する。
【００１６】
Ｃは、０．１質量％を超えると、鋼板を硬くし（硬くなると不具合になる理由は後述）、０．００１質量％未満になると、軟化し過ぎて切断時にバリ等の不具合が発生し易くなるので、０．００１〜０．１質量％とする。Ｓｉは、０．１質量％を超えると、鋼板の耐食性をかえって低下するので、０．１質量％以下とする。Ｓは、不可避的に存在する不純物元素であるが、これは低減できれば低威できるほど好ましい。ただし、その低減には精錬コストが嵩むので、０．００１〜０．０１０質量％とする。Ｐは、０．０２質量％を超えると、鋼板の耐食性が低下するので、０．０２質量％以下とする。Ａｌは、０．０３０質量％を超えると、鋼板中に非金属介在物が増加し、耐食性を劣化し、０．００３質量％未満では、Ａｌ除去のため精錬コストが著しく上昇し、さらには鋼板の機械的特性の低下を招くので、０．００３〜０．０３０質量％とする。Ｍｎは、０．１５質量％を超えると、ＭｎＳができ易くなり、耐食性を低下し、０．０５質量％を下回ると、鋼板が軟化し、切断時にバリ等の不具合が発生し易くなるので、０．０５〜０．１５質量％に抑える。
【００１７】
次に、発明者は、前記した調査結果に着眼し、せん断面の耐食性を劣化させる原因になっている表面濃化層を除去することにした。その除去には、種々の方法が考えられるので、本発明では、除去方法を特に限定しないが、化学的手段又は物理的手段が好ましい。ここで、化学的手段とは、表面濃化層を化学的に溶解する手段であり、酸洗、アルカリ溶解、化学研磨等が使用できるが、通常行われている酸洗の条件を変更して施すのが好ましい。また、物理的手段としては、表面濃化層を物理的に除去する手段であり、機械的方法（研磨、研削、ブラスト処理等）を利用するのが、技術的に問題が生じることがないので好ましい。その除去程度は、亜鉛系めっきが施される鋼板の所謂「地鉄（バルクともいう）に存在するＭｎ硫化物、Ｍｎ酸化物、Ｓｉ酸化物等の量に近いレベルまで減少させるのが良い。例えば、２０質量％の塩酸水溶液による酸洗では、温度を５０℃にして３０秒以上の酸洗をすれば良い。また、表面研磨や表面研削も含めて、鋼板の最表面からの厚みで０．１〜１５μｍを除去するのが良い。０．１μｍ未満では、除去が不十分な場合があり、１５μｍ超えでは、除去効果が飽和し、無駄になるからである。なお、０．１〜１．５μｍを除去することがさらに好ましい。
【００１８】
本発明で表面に施すめっきは、溶融めっき、電気めっき、蒸着めっき等のいかなる種類であっても良い。また、亜鉛系めっきとしては、犠牲防食性を発現するものであるならば、如何なるめっきでも良く、例えば、Ｚｎ単一系の他、Ｚｎ−Ａｌ，Ｚｎ−Ｎｉ，Ｚｎ−Ｃｒ，Ｚｎ−Ｆｅ，Ｚｎ−Ａｌ−Ｍｇ、Ｚｎ−Ａｌ−Ｍｇ−Ｓｉ等の多元系のめっきが利用できる。また、Ｚｎ−ＳｉＯ_２等の分散めっきでも良い。また、めっきの付着量は、片面あたり５ｇ／ｍ^２以上とすることが好ましい。５ｇ／ｍ^２未満では、切断面におけるＺｎの犠牲防食性が不十分である。なお、付着量の上限は、製造上の限界で決まるので、特に定めない。
【００１９】
引き続き、発明者は、本発明の効果をさらに向上させることについても研究を行った。本発明においては、平板部の耐食性向上、あるいは耐食性以外の耐指紋性の付与等のために、亜鉛系めっきを行った後に、さらに無機被覆層及び又は有機被覆層を形成するのが好ましい。無機被覆層とは、クロメート処理層、リン酸塩処理層、シラン処理層等である。また、有機被覆層とは、有機樹脂を含む被覆層である。特に、亜鉛系めっき層の表面に無機被覆層のみ、有機被覆層のみ、又は無機被覆層の上に有機被覆層を重ねて形成することが好ましい。とりわけ、前記無機被覆層の上に有機被覆層を重ねて複合皮膜を形成させると、さらに良好な耐端面錆性が得られると共に、平板部の各種特性を付与する上で望ましいことも見出し、このことも本発明に加えた。
【００２０】
【実施例】
被めっき鋼板の鋼種を表１に示す成分の鋼とし、板厚を１．６ｍｍに調整した後に、以下のような処理を施した。
【００２１】
【表１】

【００２２】
まず、前記鋼板を、温度を５０℃にした２０質量％の塩酸水溶夜中に１分浸漬する処理（該鋼板の表面を０．１μｍ除去）を施すか、又は機械的研削により最表面から厚み１．０μｍを除去するか、又は機械的研磨により最表面から厚み１．０μｍを除去した。
【００２３】
そして、上記表面層を除去した鋼板に、下記の表面処理を施した。
【００２４】
▲１▼ 片面が２０ｇ／ｍ^２の付着量になるように、両面に「電気亜鉛めっき」する
▲２▼ 片面が５０ｇ／ｍ^２の付着量になるように、両面に「溶融亜鉛めっき」する
▲３▼ ５０ｇ／ｍ^２の付着量になるように、片面に「溶融Ｚｎ−５質量％Ａｌめっき」する
▲４▼１００ｇ／ｍ^２の付着量になるように、片面に「５５質量％Ａ１−Ｚｎめっき」する。
【００２５】
▲５▼▲１▼のめっき上に、シリカ添加クロメート処理（Ｃｒ還元率３７質量％，Ｓｉ／Ｃｒの質量比＝１．６）を、Ｃｒ換算で片面当たり４０ｍｇ／ｍ^２塗布し、１５０℃で焼き付ける
▲６▼ ▲１▼のめっき上に、シランカップリング剤（アミノプロピルトリメトキシラン）０．５質量％液を塗布し、１２０℃で焼き付ける
▲７▼ ▲５▼の上に、さらにクリアエポキシ樹脂（ジャパンエポキシキレジン株式会社製、エピコート８２８とエポメートＢ００２の混合硬化物）層を、保護層として７μｍ塗布し、１２０℃で２０分焼き付ける
▲８▼ ▲５▼の上に、さらにアクリル樹脂（ポリメタクリル酸メチル）層を保護層として３μｍ塗布し、１００℃で焼き付ける。
【００２６】
このように処理して得られた亜鉛系めっき鋼板を、それぞれせん断機にて切断し、その耐端面錆性を調査した。耐端面錆性の評価は、切断した供試サンプルを、実験室内に１２０日間暴露し、端面を観察した。その結果を、赤錆面積発生率が１０％以下は◎、１０〜２０％は○、２０〜３０％は△、３０％以上は×として表し、表２に一括して示す。
【００２７】
【表２】

【００２８】
表２より、本発明に係る亜鉛系めっき鋼板は、いずれも優れた耐端面錆性を有していることが明らかである。なお、これらの実施例における耐端面錆の促進試験は、実機の端面錆発生傾向と良好な相関があることを予め確認して採用したものである。このような条件下で耐端面錆の促進試験を行っても、本発明に係る亜鉛系めっき鋼板は、いずれも優れた耐端面錆性を発揮しているので、実際に家電製品や事務機器等の部材として採用すれば、優れた耐端面錆性を示し、使用後に回収部材の再利用が可能となることも明らかである。
【００２９】
【発明の効果】
以上述べたように、本発明により、従来より耐端面錆性が一層優れた亜鉛系めっき鋼板及びその製造方法が提供される。従って、部品の再利用が可能となるばかりでなく、製品コストや材料使用量の大幅削減が達成できるので、本発明は、地球環境の保護に貢献すること大である。
【図面の簡単な説明】
【図１】被めっき鋼板の表面を，酸洗後にＧＤＳ分析した結果を示す図である。
【図２】切断後の暴露試験で得られた亜鉛系めっき鋼板の暴露日数と、せん断面及び破断面の錆発生面積率との関係を示す図である。
【図３】亜鉛系めっき鋼板の切断面を説明する断面図である。
【符号の説明】
１被めっき鋼板
２表面濃化層
３亜鉛系めっき層
４破断面
５せん断面[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a galvanized steel sheet and a method for producing the same, and mainly relates to electrical products used indoors, in a galvanized steel sheet that is a member of office equipment, etc., a cut end face, a punched face, etc., so-called zinc-based plating. It relates to the improvement of corrosion resistance, that is, the rust resistance of the end face, which does not reach “sacrificial corrosion protection”.
[0002]
[Prior art]
BACKGROUND ART A surface-treated steel sheet in which one or both surfaces of a steel sheet is galvanized (hereinafter, referred to as a galvanized steel sheet) has excellent corrosion resistance and is widely used as a product member of home appliances, office equipment, and the like. These product members are manufactured by subjecting the galvanized steel sheet to mechanical processing such as shearing, punching, drilling, and trimming, and then cutting to a predetermined size and shape, followed by molding. Later, it is generally disposed of with the product. However, since 2001 when the Home Appliance Recycling Law was enforced, there has been a trend to reuse these members.
[0003]
However, when home appliances and the like are collected and the members are to be reused as members of similar products again, the cut surfaces (for example, shearing surfaces, punched surfaces, trimming surfaces, etc.) of the zinc-based plated steel sheets forming the members are used. There is a problem that red rust is generated and it is difficult to reuse. That is, since rust has electric conductivity, there is a concern that electric circuits may be damaged due to rust peeling. Conventionally, in the field of home electric appliances, the surface of a galvanized steel sheet is sufficiently protected from corrosion by the sacrificial corrosion protection of galvanization, so that the corrosion resistance of these cut surfaces has not been sufficiently studied.
[0004]
The corrosion resistance of the cut surface is a phenomenon that occurs because the sacrificial corrosion resistance of zinc plating does not reach, and it is considered that to fundamentally solve this problem, it is necessary to improve the corrosion resistance of the base steel material. For example, in weight%, C: 0.15% or less, acid-soluble Al: 0.005 to 1%, Cr: 1.5 to 20%, Cu: 0.8% or less, P: 0.15% or less , Ni: 10% or less of one or more kinds, and further, one or more of Ti, Nb, V, Zr and 0.03-0.50%. A disclosed zinc-based plated steel sheet is disclosed (see Patent Document 1). However, since this steel sheet contains Cu, it is difficult to recover it as an iron resource, and since it contains Cr and Ni, it is expensive.
[0005]
Also, by weight%, C: 0.01 to 0.08%, Si: 0.04% or less, Mn: 0.4% or less, P: 0.04% or less, S: 0.015% or less. And Mn and S satisfy Mn (wt%) × S (wt%) ≦ 0.003, and P and S satisfy S (wt%) × P (wt%) ≦ 0.0003. There is also disclosed a galvanized steel sheet obtained by subjecting a steel sheet contained in the above to galvanization. However, this steel sheet did not necessarily have sufficient end face rust resistance, and especially when the sheet thickness was greater than 2.0 mm, no end face rust resistance was observed at all.
[0006]
[Patent Document 1]
JP-A-62-7888 [Patent Document 2]
JP-A-10-330883
[Problems to be solved by the invention]
In view of such circumstances, the present invention provides a galvanized steel sheet that can be reused not only as a member of home electric appliances and the like, but also recovered as an iron resource, and has sufficient edge rust resistance even when the sheet thickness is large. It is intended to be.
[0008]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to achieve the above object, and have embodied the results in the present invention.
[0009]
That is, the present invention relates to a steel sheet having a surface zinc-plated, wherein the steel sheet subjected to the zinc plating is C: 0.001 to 0.1% by mass, Si: 0.1% by mass. Hereinafter, S: 0.001 to 0.010% by mass, Al: 0.003 to 0.030% by mass, Mn: 0.05 to 0.15% by mass, P: 0.02% by mass or less, other iron and It is characterized in that a surface-concentrated layer containing unavoidable impurities and containing one or more selected from the group consisting of Mn sulfide, Mn oxide and Si oxide is removed from the steel sheet before plating. It is a zinc-based plated steel sheet. In this case, it is preferable to apply the zinc-based plating on at least one surface at 5 g / m ² or more, or to further form an inorganic coating layer and / or an organic coating layer on the surface of the zinc-based plated steel sheet.
[0010]
In addition, in the present invention, when producing a steel sheet having a zinc-plated surface, the steel sheet to be subjected to the zinc-based plating is prepared by adding C: 0.001 to 0.1% by mass, Si: 0.1% by mass. %, S: 0.001 to 0.010% by mass, Al: 0.003 to 0.030% by mass, Mn: 0.05 to 0.15% by mass, P: 0.02% by mass or less, other iron Before plating, the steel sheet is provided with a surface-enriched layer containing one or two or more selected from the group consisting of Mn sulfide, Mn oxide and Si oxide before plating by chemical means or physical means. A method for producing a galvanized steel sheet, characterized in that at least one surface is zinc-plated at 5 g / m ² or more after removal by a standard means. In this case, the removal of the surface-concentrated layer may be performed by removing a thickness of 0.1 to 15 μm from the outermost surface of the steel sheet, or may further include an inorganic coating layer and / or an organic layer on the surface of the galvanized steel sheet. It is preferable to form a coating layer.
[0011]
In the present invention, the steel composition of the steel sheet to be plated before the zinc-based plating is controlled and the surface-concentrated layer mainly composed of Mn-based precipitates, S-based precipitates, etc. is removed, so that the resulting steel sheet is obtained. The starting point of rust generation on the cut end face of the galvanized steel sheet is reduced as much as possible. As a result, product members manufactured from the galvanized steel sheet can not only be reused but also be recovered as iron resources.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described, taking into account the circumstances that led to the invention.
[0013]
First, the inventor conducted an exposure test to investigate a part where rust is generated on a cut surface of a galvanized steel sheet constituting a home electric appliance or the like used in an indoor environment and the cause of the rust by performing an exposure test. As a result, as shown in FIG. 3, the galvanized steel sheet has a surface-concentrated layer 2 containing one or more selected from the group consisting of a steel sheet to be plated 1, Mn sulfide, Mn oxide and Si oxide. And the zinc-based plating layer 3, but it was found that rust (not shown) was apt to occur on the sheared surface 5 where the surface concentrated layer 2 was exposed, not on the fractured surface 4 among the cut surfaces. The occurrence of rust was evaluated by area ratio and is shown in FIG. Further, the steel surface was pickled (immersed in a 10% by mass aqueous solution of hydrochloric acid at 20 ° C. for 10 seconds, and the thickness of the outermost layer was removed from 0.01 to 0.02 μm), and then subjected to GDS (Grim Glow Discharge Spectroscopy) analysis. As shown in FIG. 1, Mn, S, and Si were concentrated on the steel sheet surface. Their thickening appeared to have a large effect on rust generation. That is, the corrosion of a steel sheet in the atmosphere, particularly in an indoor use environment, is extremely mild as a corrosive environment, and the starting point of the corrosion is a precipitate in the steel sheet and dust flying from the environment (corrosive ion formation existing in the dust). Components, for example, Cl, Na, SO ₄ , NO ₃ , Ca, Mg, etc.). Generally, in an indoor environment, water wetting phenomenon such as dew condensation hardly occurs, but precipitates (for example, Mn sulfide, Mn oxide, Si oxide, etc.) existing on the surface and dust (ionic substance) flying from the environment, etc. When present on the cut surface, they can absorb moisture from atmospheric humidity and become a starting point for corrosion. In particular, when the galvanized steel sheet is forcibly sheared by a blade of a shearing machine, the surface-concentrated layer 2 containing the Mn sulfide, Mn oxide, Si oxide and the like is pulled toward the shear surface 5 side. It is believed that the cross section is more likely to corrode than other parts of the steel sheet.
[0014]
Therefore, the inventor believes that by removing this cause, it is possible to improve the end face rust resistance of the galvanized steel sheet more than before, and by limiting the content of the steel sheet component including the surface concentrated layer component. In addition, the starting point of rust generation was reduced as much as possible.
[0015]
First, C, Si, S, P, Al, and Mn, which are the main components of the steel sheet to be subjected to zinc plating, are all useful elements in terms of the mechanical properties in the steelmaking stage and in the state of the steel sheet. However, these are elements that reduce the end surface rust resistance after the zinc-based plating is applied. Therefore, in the present invention, the content is limited as follows.
[0016]
When C exceeds 0.1% by mass, the steel sheet is hardened (the reason why the problem is caused when the steel is hardened will be described later), and when it is less than 0.001% by mass, the steel sheet is excessively softened and defects such as burrs are likely to occur during cutting. Therefore, the content is set to 0.001 to 0.1% by mass. If the content of Si exceeds 0.1% by mass, the corrosion resistance of the steel sheet is rather deteriorated. Therefore, the content of Si is set to 0.1% by mass or less. S is an unavoidable impurity element, and it is more preferable to reduce S if it can be reduced. However, since the refining cost increases for the reduction, the content is set to 0.001 to 0.010% by mass. If P exceeds 0.02% by mass, the corrosion resistance of the steel sheet is reduced. If Al exceeds 0.030% by mass, nonmetallic inclusions increase in the steel sheet, and the corrosion resistance is deteriorated. If it is less than 0.003% by mass, the refining cost for removing Al increases significantly, Since the mechanical properties of the steel powder deteriorate, the content is set to 0.003 to 0.030% by mass. When Mn is more than 0.15% by mass, MnS is easily formed and the corrosion resistance is reduced. When the content is less than 0.05% by mass, the steel sheet is softened and defects such as burrs are easily generated at the time of cutting. It is suppressed to 0.05 to 0.15% by mass.
[0017]
Next, the inventor paid attention to the above-mentioned investigation results, and decided to remove the surface-concentrated layer that causes deterioration of the corrosion resistance of the shear surface. Since various methods can be considered for the removal, the removal method is not particularly limited in the present invention, but a chemical means or a physical means is preferable. Here, the chemical means is a means for chemically dissolving the surface-concentrated layer, and pickling, alkali dissolution, chemical polishing, and the like can be used. It is preferred to apply. The physical means is a means for physically removing the surface-concentrated layer, and the use of a mechanical method (polishing, grinding, blasting, or the like) does not cause any technical problems. preferable. The degree of removal is preferably reduced to a level close to the amount of Mn sulfide, Mn oxide, Si oxide, and the like present in the so-called "ground iron (also called bulk)" of the steel plate to be subjected to zinc-based plating. For example, in pickling with a 20% by mass aqueous hydrochloric acid solution, pickling may be performed for 30 seconds or more at a temperature of 50 ° C. In addition, the thickness from the outermost surface of the steel sheet may be 0, including surface polishing and surface grinding. If the thickness is less than 0.1 μm, the removal may be insufficient, and if the thickness exceeds 15 μm, the removal effect is saturated and wasteful. It is more preferable to remove 0.5 μm.
[0018]
The plating applied to the surface in the present invention may be any type such as hot-dip plating, electroplating, vapor deposition plating and the like. As the zinc-based plating, any plating may be used as long as it exhibits sacrificial corrosion resistance. For example, Zn-based, Zn-Al, Zn-Ni, Zn-Cr, Zn-Fe, Multi-system plating such as Zn-Al-Mg and Zn-Al-Mg-Si can be used. In addition, it may be a dispersion plating such as Zn-SiO _2. Further, the amount of plating is preferably 5 g / m ² or more per side. If it is less than 5 g / m ² , the sacrificial corrosion protection of Zn on the cut surface is insufficient. It should be noted that the upper limit of the adhesion amount is not particularly defined because it is determined by a manufacturing limit.
[0019]
Subsequently, the inventor also conducted research on further improving the effects of the present invention. In the present invention, it is preferable to further form an inorganic coating layer and / or an organic coating layer after performing zinc-based plating in order to improve the corrosion resistance of the flat plate portion or to impart fingerprint resistance other than corrosion resistance. The inorganic coating layer is a chromate treatment layer, a phosphate treatment layer, a silane treatment layer, or the like. Further, the organic coating layer is a coating layer containing an organic resin. In particular, it is preferable to form only the inorganic coating layer, only the organic coating layer on the surface of the zinc-based plating layer, or to overlap the organic coating layer on the inorganic coating layer. In particular, it has been found that, when an organic coating layer is overlaid on the inorganic coating layer to form a composite coating, even better end face rust resistance is obtained and it is desirable to impart various properties to the flat plate portion. This is also added to the present invention.
[0020]
【Example】
The steel type of the steel plate to be plated was steel having the components shown in Table 1, and after adjusting the plate thickness to 1.6 mm, the following treatment was performed.
[0021]
[Table 1]

[0022]
First, the steel plate is immersed in a 20% by mass aqueous solution of hydrochloric acid at a temperature of 50 ° C. for 1 minute (removing the surface of the steel plate by 0.1 μm), or a thickness of 1 mm from the outermost surface by mechanical grinding. 1.0 μm, or 1.0 μm in thickness was removed from the outermost surface by mechanical polishing.
[0023]
Then, the steel sheet from which the surface layer was removed was subjected to the following surface treatment.
[0024]
▲ 1 ▼ so one side is adhesion amount of 20 g / m ^2, so as to "galvanized" on both sides ▲ 2 ▼ one side is adhesion amount of 50 g / m ^2, to "galvanized" on both sides {Circle around (3)} "Molten Zn-5 mass% Al plating" is performed on one side so that the adhesion amount is 50 g / m ^2. {Circle around (4)} "55 mass% A1 is coated on one side so that the adhesion amount is 100 g / m ^2. -Zn plating ".
[0025]
▲ 5 ▼ ▲ 1 ▼ plating on the silica added chromate treatment (Cr reduction ratio 37 mass%, mass ratio = 1.6 Si / Cr) and, 40 mg / ^{m 2} was coated per side with Cr terms 150 Baking at ℃ 6) Apply a 0.5% by mass solution of a silane coupling agent (aminopropyltrimethoxysilane) on the plating of ▲ 1), and bake at 120 ° C. Further, a clear epoxy resin (mixed cured product of Epicoat 828 and Epomate B002, manufactured by Japan Epoxy Kiresin Co., Ltd.) is applied as a protective layer to a thickness of 7 μm and baked at 120 ° C. for 20 minutes. An acrylic resin (polymethyl methacrylate) layer is applied as a protective layer to a thickness of 3 μm and baked at 100 ° C.
[0026]
The galvanized steel sheets obtained by the treatment as described above were cut by a shearing machine, respectively, and their end face rust resistance was investigated. For the evaluation of the end face rust resistance, the cut test sample was exposed to a laboratory for 120 days, and the end face was observed. The results are shown as ◎ when the red rust area occurrence rate is 10% or less, ○ when 10 to 20%, 20 △ when 30 to 30%, and X when 30% or more.
[0027]
[Table 2]

[0028]
From Table 2, it is clear that the galvanized steel sheets according to the present invention all have excellent edge rust resistance. The acceleration test of end face rust resistance in these examples was adopted after confirming in advance that there was a good correlation with the end face rust occurrence tendency of the actual machine. Even under the accelerated test of end face rust resistance under such conditions, the zinc-based plated steel sheets according to the present invention exhibit excellent end face rust resistance, and thus are actually used in home electric appliances and office equipment. It is also clear that adoption as a member of the above shows excellent end face rust resistance and that the recovered member can be reused after use.
[0029]
【The invention's effect】
INDUSTRIAL APPLICABILITY As described above, the present invention provides a galvanized steel sheet having more excellent end face rust resistance than before and a method for producing the same. Therefore, not only can parts be reused, but also product cost and material usage can be significantly reduced, and the present invention greatly contributes to protection of the global environment.
[Brief description of the drawings]
FIG. 1 is a diagram showing the results of GDS analysis of the surface of a steel plate to be plated after pickling.
FIG. 2 is a graph showing the relationship between the number of days of exposure of a zinc-based plated steel sheet obtained in an exposure test after cutting and the rust generation area ratio of a shear surface and a fracture surface.
FIG. 3 is a cross-sectional view illustrating a cut surface of a galvanized steel sheet.
[Explanation of symbols]
Reference Signs List 1 steel plate to be plated 2 surface thickened layer 3 zinc-based plated layer 4 fracture surface 5 shear surface

Claims

A steel plate with a zinc-based plating on the surface,
The steel sheet subjected to the zinc-based plating is as follows: C: 0.001 to 0.1% by mass, Si: 0.1% by mass or less, S: 0.001 to 0.010% by mass, Al: 0.003 to 0.030% by mass, Mn: 0.05 to 0.15% by mass, P: 0.02% by mass or less, other iron and unavoidable impurities, and Mn sulfide and Mn oxidation of the steel sheet before plating. A zinc-based plated steel sheet obtained by removing a surface-concentrated layer containing one or more selected from the group consisting of an oxide and a Si oxide.

The galvanized steel sheet according to claim 1, wherein the zinc-based plating is applied to at least one surface at 5 g / m ² or more.

The galvanized steel sheet according to claim 1 or 2, further comprising an inorganic coating layer and / or an organic coating layer formed on the surface of the galvanized steel sheet.

When producing steel sheets with zinc-based plating on the surface,
The steel sheet to be subjected to the zinc-based plating is prepared as follows: C: 0.001 to 0.1% by mass, Si: 0.1% by mass or less, S: 0.001 to 0.010% by mass, Al: 0.003 to 0.030% by mass, Mn: 0.05 to 0.15% by mass, P: 0.02% by mass or less, other iron and unavoidable impurities, and Mn sulfide and Mn oxidation of the steel sheet before plating. After removing the surface-concentrated layer containing one or more selected from the group consisting of a product and a Si oxide by chemical or physical means, a zinc-based plating of 5 g / m ² or more is formed on at least one surface. A method for producing a galvanized steel sheet.

The method for producing a galvanized steel sheet according to claim 4, wherein the removal of the surface-concentrated layer is performed by removing a thickness: 0.1 to 15 m from the outermost surface of the steel sheet.

The method according to claim 4, further comprising forming an inorganic coating layer and / or an organic coating layer on the surface of the galvanized steel sheet.