JP3559445B2

JP3559445B2 - Welding method of hot-dip galvanized steel sheet

Info

Publication number: JP3559445B2
Application number: JP07228398A
Authority: JP
Inventors: 国秀山根; 一浩児嶋; 英俊新頭; 孝志屋敷; 裕保石本
Original assignee: Idemitsu Kosan Co Ltd; Nippon Steel Corp
Current assignee: Idemitsu Kosan Co Ltd; Nippon Steel Corp
Priority date: 1998-03-20
Filing date: 1998-03-20
Publication date: 2004-09-02
Anticipated expiration: 2018-03-20
Also published as: JPH11267846A

Description

【０００１】
【発明の属する技術分野】
本発明は、溶融亜鉛系めっき鋼板で防食を施した鋼板の溶接方法に関するものである。
【０００２】
【従来の技術】
従来、溶融亜鉛系めっき鋼板は主として建築用材に使用されていたが、連続式亜鉛めっき設備が導入されて以来、外観、加工性などの品質が飛躍的に向上し、電気機器、自動車、土木など多方面に用いられるようになってきた。
【０００３】
溶融亜鉛系めっき鋼板を溶接する場合、鋼板表面から鉄より低い融点（４１９℃）と沸点（９０６℃）を持った亜鉛が、溶接時に溶滴ないし溶融池に侵入してその蒸気が突沸すると共に大気を巻き込み、溶接金属の凝固過程で浮上しきれずに気泡として残存し、ピットやブロ−ホ−ルを多発する。このような亜鉛の害を軽減する手段としては、予め溶接線上から亜鉛を機械的に除去することが有効であるが、手間がかかり非能率的である。
【０００４】
溶融亜鉛系めっき鋼板を使用した電気機器や自動車等の溶接にあたっては、屋内のロボット溶接が多くなっており、ガスシ−ルドア−ク溶接が多用され、専用のソリッドワイヤが開発されており、特開昭６３−７２４９８号公報には、ソリッドワイヤ中のＣ，Ｓｉ，Ｐ，Ｓを限定し、さらにＴｉ，Ａｌ，Ｎｉ，Ｃｕを適宜含有させた溶接材料が提案されている。更に、特開平２−２６３５９４号公報では、Ｃ，Ｓｉ，Ｍｎ，Ａｌを特定範囲とし、Ｃｕ、Ｎｂ，Ｖを特定範囲に限定することにより、ピットやブロ−ホ−ルが発生しにくく、高速溶接が可能なガスシ−ルドア−ク溶接ワイヤが開示されている。
【０００５】
一方、石油貯蔵タンクに代表される建築物における屋根板は、従来、熱延鋼板黒皮材を外側で溶接し、外面は手塗り塗装、内面は黒皮材のまま使用するのが一般的であった。このような、従来の黒皮材よりなるタンクにおいては、特に固定式屋根型石油貯蔵タンクにおいて、タンク内面の石油に接していない部分は、腐食が進行する。これに対して、本発明者等は、特開平７−１１２７９２号公報で開示したように屋根および胴体部の上部に溶融亜鉛めっき鋼板を配置することにより、耐食性を著しく向上させ、大幅にタンク寿命を延長可能とした。溶接施工は屋根になることから、風の影響を受けにくい被覆ア−ク溶接棒が使用される。溶接棒の系統としては、主にＪＩＳのＤ４３０１（イルミナイト系）、Ｄ４３０３（ライムチタニヤ系）およびＤ４３１３（高酸化チタン系）が使用されているが、溶融亜鉛系めっき鋼板の溶接においてピット・ブローホールが発生しにくく、欠陥の少ない溶接金属を得るには、充分に満足のゆくものではない。また、タンク外側からの溶接の熱影響部近傍でタンク内側の亜鉛めっきが溶接熱により消失しダメ−ジを受けて腐食する問題もあった。
【０００６】
【発明が解決しようとする課題】
本発明は、上記のように石油貯蔵タンクの屋根材として適用する溶融亜鉛系めっき鋼板の溶接において、ピットやブロ−ホ−ル等の欠陥発生の少ない被覆ア−ク溶接棒を用いて溶接し、溶接熱影響部近傍においても亜鉛めっきがダメ−ジを受けない溶接方法を提供することを目的とするものである。
【０００７】
【課題を解決するための手段】
本発明は、前述した要望に応えるため、溶接施工方法および被覆剤成分を種々検討して得られたものであって、その要旨とするところは、被覆剤と鋼心線とからなる被覆ア−ク溶接棒により、亜鉛めっき付着量が片面当たり６０〜４００ｇ／ｍ^２である溶融亜鉛系めっき鋼板をア−ク溶接する方法において、被覆剤中の炭素を被覆剤全体に対する重量％で０．０２〜０．５０％とし、式１で表される溶接入熱Ｗを０．５〜１．５ｋＪ／ｍｍとなるように調節して溶接し、更に、被覆剤が被覆剤全体の重量％で、Ｃ：０．０２〜０．５０％、ＴｉＯ_２：２５〜５５％、ＳｉＯ_２：１０〜２５％、ＣａＣＯ_３：４〜１６％、Ｆｅ−Ｍｎ：５〜１４％、鉄粉：１０〜４０％を含有し、残部がスラグ生成剤、ア−ク安定剤、ガス発生剤、脱酸剤、固着剤および不可避不純物からなることを特徴とする溶融亜鉛系めっき鋼板の溶接方法にある。
【０００８】
本発明において、鋼板の表面に防錆を目的としてＺｎ、５％以下のＭｇを添加したＺｎ−Ｍｇめっき鋼板および５％以下のＡｌを添加したＺｎ−Ａｌめっき鋼板を施したものを総称して溶融亜鉛系めっき鋼板という。
【０００９】
【発明の実施の形態】
本発明者等は、亜鉛めっき鋼板の溶接において、ピットやブロ−ホ−ルの発生しやすい理由として、溶接時に亜鉛の突沸による空気の巻き込みを助長すると共に、溶融金属に入った亜鉛は鉄に溶解しにくく、かつ低沸点であるため溶融金属が凝固過程においても蒸気状態であり、これが残存して気泡となること、また被覆ア−ク溶接棒では、清浄な溶接金属を得るために被覆に脱酸剤を過剰に添加すると溶融状態の亜鉛は酸化されず気泡として残存し、ピット・ブロ−ホ−ルの発生をさらに助長することに起因すると考察した。また、溶接によりタンク内側の亜鉛めっきがダメージを受けるのは、亜鉛めっきにより溶接作業性が大きく低下し、溶接速度を遅くしなければならないことに起因すると考察した。
【００１０】
本発明者等は、これら亜鉛の影響を軽減するには、（１）有害な亜鉛を固定する、あるいはＺｎＯとしてスラグオフさせる、（２）溶融金属および溶融スラグの粘性を低下し、亜鉛蒸気を放出させる、（３）溶融スラグの追従性を良好にして溶接速度を速くすること、これらが有効であるとの観点から、被覆剤組成分について鋭意検討を行い、本発明を構成するに至ったものである。
【００１１】
すなわち、本発明は、（ａ）被覆剤の炭素量を限定することにより、溶融金属の酸化力を適正化し、亜鉛の酸化を促進すること、すなわち亜鉛蒸気の発生抑制に酸化反応熱による溶融金属の温度上昇を図り、さらには溶融金属の撹拌作用の強化によるガス放出の促進による亜鉛の影響の軽減を図ること、また、（ｂ）タンク外面からの溶接で生ずる溶接熱によるタンク内面側熱影響部の亜鉛めっき消失を低減させるため、水平すみ肉溶接は無論のこと、入熱が大きくなる傾斜部でも下進溶接が可能で、かつ低入熱溶接ができる被覆ア−ク溶接棒を開発したこと、に最大の特徴がある。更に、（ｃ）被覆剤中のＴｉＯ_２、ＳｉＯ_２、ＣａＣＯ_３の量を限定することによって、適正な粘性のスラグを形成し溶接作業性を確保すること、（ｄ）適正量のＭｎを添加することにより、溶接金属の酸化力を適正化し、亜鉛の酸化を促進すること、すなわち亜鉛蒸気の発生の抑制に、酸化反応熱による溶接金属の温度上昇と溶融金属の粘性の低下を図り、更には溶融金属の撹拌作用の強化によるガス放出の促進による亜鉛の影響の軽減を図ること、の複合作用により、さらにピットやブロ−ホ−ルの発生を抑制することができる。
【００１２】
以下に本発明における構成要件を説明する。
図２は、鋼材により製作された固定式屋根型石油貯蔵タンクの構造を示した図であって、基礎コンクリ−ト１の上面に、鋼製底板２が載置されて固定され、その底板２の周囲の上面に、鋼板からなる円筒状の胴体部３の下端部が載置されて溶接により固着され、前記底板２の上面の中央部に、鋼製支柱４の下端部が載置されて固着され、前記支柱４の上端部と胴体部３とに、鋼製屋根骨ラフタ−５が固着され、円錐形の鋼製屋根６は、前記屋根骨ラフタ−５と胴体部３の上端部と支柱４の上端部とに溶接により固着されて、石油貯蔵タンク７が構成され、その石油貯蔵タンク７内に、石油類８が貯蔵され、その石油類８と屋根６との間に気相部９が設けられている。また、図２において、胴体部３におけるハッチング部分が普通鋼板であり、胴体部３における黒い太線部分が、溶融亜鉛系めっき鋼板であり、屋根６にも溶融亜鉛系めっき鋼板が用いられている。
本発明において、耐食性を特に必要とする対象部位である屋根６の内面と胴体部３の上部は石油に接しておらず、気相部となっており、本部位での腐食環境は、ｐＨ５、３５℃の硫酸酸性水溶液噴霧試験で簡便な腐食シュミレ−ションが出来ることが、前述の特開平７−１１２７９２号公報に記載されている。そこで、本発明者等は、本腐食シュミレーション試験を用いて、溶融亜鉛系めっき鋼板継手部の耐食性を調査した。
【００１３】
その結果を図１に示す。図１は、タンク内面に溶融亜鉛系めっきを想定した場合の耐食性を示すグラフである。すなわち、Ｃ；０．１１〜０．３４重量％、ＳｉＯ_２；１５〜１９重量％、ＴｉＯ_２；２９〜３６重量％、ＣａＣＯ_３；５〜９重量％、Ａｌ_２Ｏ_３；０〜２重量％、有機物；１〜４重量％、Ｆｅ−Ｍｎ；６〜１１重量％、鉄粉；１２〜２４重量％、その他０〜６重量％からなる被覆剤を直径５ｍｍ、長さ４００ｍｍの炭素鋼心線に被覆率２３〜４６％になるように製造した後、乾燥し試験溶接棒を作製し、この溶接棒により０．５％Ｍｇ含有亜鉛めっき鋼板ＪＩＳＧ３３０２・ＳＧＨ４００Ｚ４５（板厚４．５ｍｍ、幅２５０ｍｍ、長さ５００ｍｍ）を図３に示すごとく２枚重ねて、１５度下進すみ肉溶接を行い、上記腐食試験を実施して、溶接部裏面の溶接熱影響部の残存めっき厚みを測定した。残存めっき厚みが１０μｍ以上あれば耐食性に問題はないと判定した。その結果、めっき付着量が片面当たり６０ｇ／ｍ^２未満では、耐食性が不十分となる。また、片面当たり４００ｇ／ｍ^２を超えると溶接性が劣化する。特に、ピットやブロ−ホ−ルの発生が顕著になる。更に溶融亜鉛系めっき鋼板の製造方法については、特に限定されず種々の方法を適用できる。
【００１４】
亜鉛目付量が６０ｇ／ｍ^２以上の鋼板では、式（１）で表される溶接入熱Ｗが０．６ｋＪ／ｍｍと１．４ｋＪ／ｍｍでは良好な耐食性を有しているが、溶接入熱２．１ｋＪ／ｍｍでは、残存めっき厚みが１０μｍ以下であり、耐食性が劣化する。つまり溶接入熱を０．５〜１．５ｋＪ／ｍｍとしたのは、溶接入熱が０．５ｋＪ／ｍｍ未満の場合は、溶接欠陥が発生すること、１．５ｋＪ／ｍｍを超える場合は、外面からの溶接で生ずる溶接熱によるタンク内面側の熱影響部の亜鉛めっきが消失してしまい耐食性が劣るためである。
【００１５】
Ｗ＝６０ＩＶ／Ｓ（ｋＪ／ｍｍ）………………（１）
Ｉ：溶接電流（Ａ）
Ｖ：溶接電圧（Ｖ）
Ｓ：溶接速度（ｍｍ／ｓｅｃ）
次に、本発明者等は、亜鉛目付量が６０〜４００ｇ／ｍ^２の鋼板を溶接する場合において、ピット・ブロ−ホ−ルが極めて発生しにくい被覆アーク溶接棒を種々検討した結果、溶接棒被覆剤中の炭素量を限定することによりこの問題を解決できることを見いだした。図４は、被覆剤中の炭素量とピット・ブロ−ホ−ル発生数の関係を示すグラフである。すなわち、ＳｉＯ_２；１５〜１９重量％、ＴｉＯ_２；２９〜３６重量％、ＣａＣＯ_３；５〜９重量％、Ａｌ_２Ｏ_３；０〜２重量％、有機物；１〜４重量％、Ｆｅ−Ｍｎ；６〜１１重量％、鉄粉；１２〜２４重量％、その他０〜６重量％からなる被覆剤を、Ｃ；０．０６重量％、Ｓｉ；０．０１重量％、Ｍｎ；０．４９重量％を含有する直径５ｍｍ、長さ４００ｍｍの炭素鋼心線に被覆率２３〜４６％になるように製造した後、乾燥し試験溶接棒を作製し、この溶接棒により０．５％Ｍｇ含有亜鉛めっき鋼板ＪＩＳＧ３３０２・ＳＧＨ４００Ｚ４５（板厚４．５ｍｍ、幅２５０ｍｍ、長さ５００ｍｍ）を図３に示すごとく２枚重ねて、溶接入熱０．９〜１．４ｋＪ／ｍｍで１５度下進すみ肉溶接を行い、すみ肉溶接部に発生するピットとブロ−ホ−ルを比較した。計測は、ピット個数を目測で計測し、その後、放射線透過試験を行い計測した。図４から明らかなように被覆剤中の炭素量を限定することによって、ピットとブロ−ホ−ルが減少することが明らかである。炭素の添加量が０．０２重量％以上で効果がある。しかし、０．５０重量％を超えて添加すると、溶融状態の亜鉛が酸化されず気泡として溶接金属中に残り、そのまま凝固しピットやブロ−ホ−ルとなる。また、溶接金属が著しく硬化し、延性を損ない耐割れ性が劣化する。被覆剤に含有される炭素は、主にＦｅ−Ｍｎやグラファイトより添加される。ちなみに、鋼心線中の炭素量が０．０４〜０．０８重量％程度含有されていれば、前述の効果が得られる。
【００１６】
タンク屋根６の内面および胴体部３の上部内面に、溶融亜鉛系めっき鋼板を適用することにより、耐食性が向上する理由は、酸性腐食環境である硫酸性湿潤環境において亜鉛腐食生成物の防食効果による。また、溶接熱影響部の耐食性が良好であるのは、熱影響部に残存する亜鉛と周辺部に存在する亜鉛の電気防食効果による。また、石油タンクの種類についても特に限定されず、屋根固定型石油タンクのみならず、屋根浮揚式タンク等、他の種々のタンクにも適用可能である。その他に、浮き屋根タンク付属物の屋根シ−ル部の雨よけ板およびシ−ル材取付部の部材等にも適用が可能である。
【００１７】
ＴｉＯ_２は、下進溶接性に重要な成分であり、スラグ生成剤、粘性調整剤およびア−ク安定剤として使用される。添加量は、２５重量％未満ではスラグの粘性が低すぎ、下進溶接でスラグの被包性が悪く、アンダカットが発生しやすく、ビード形状も悪い。５５重量％を超えて添加するとスラグの粘性が高くなり、ビ−ド形状が劣化すると共にブロ−ホ−ルも増加する。
【００１８】
ＳｉＯ_２は、スラグ生成剤として使用される。添加量が１０重量％未満では被覆筒が浅く、ア−クが荒くなり、スパッタの発生が多くなる。２５重量％を超えて添加すると被覆剤の融点が高くなり、被覆筒が長くなりすぎ、不安定なア−ク状態となり、ビ−ド波形が悪くなる。
【００１９】
ＣａＣＯ_３は、下進溶接性に重要な成分であり、ガス発生剤、スラグ生成剤として使用される。添加量が４重量％未満では、スラグの粘性が低下し、ビ−ド形状が悪くなると共に、ガス発生剤としての効果が不十分で、ピット・ブロ−ホ−ルが発生する。１６重量％を超えるとア−ク電圧が低下し、ア−クが不安定となり短絡しやすく、ア−ク切れも発生する。
【００２０】
Ｆｅ−Ｍｎは脱酸剤および合金剤としての効果がある。また、前述の炭素源としても添加される。添加量が５重量％未満では脱酸不足となり、清浄な溶接金属が得られない。１４重量％を超えると脱酸過剰となり、亜鉛蒸気の影響が大きく働き、ピット・ブロ−ホ−ルが発生すると共に、溶接金属が硬化し、耐割れ性が劣化する。
【００２１】
鉄粉は、ア−ク安定剤および溶着金属量増加の目的で添加する。添加量が１０重量％未満では溶着量が不足し、すみ肉溶接でアンダカットが発生する。４０重量％を超えると被覆筒が弱くなり、短絡し易くなると共に、溶接金属の温度が低下しガスの放出が不十分になり、ピット・ブロ−ホ−ル発生の原因となる。
【００２２】
さらに、上記以外のア−ク安定剤、スラグ生成剤、ガス発生剤、脱酸剤とは、ＭｇＯやＦｅ_２Ｏ_３、Ａｌ_２Ｏ_３あるいは有機物等をさす。本発明溶接棒は、以上述べた配合フラックスを、珪酸ソ−ダ、珪酸カリで代表される水ガラス等の粘結剤により、鋼心線の周囲に被覆率が２０〜４５％となるように通常の押し出し式溶接棒塗装機により被覆塗装した後、水分を除去するために１２０〜２５０℃で乾燥して製造する。
【００２３】
【実施例】
次に実施例により本発明の効果を具体的に示す。各種溶融亜鉛系めっき鋼板を用いて、Ｃ；０．１１〜０．３４重量％、ＳｉＯ_２；１５〜１９重量％、ＴｉＯ_２；２９〜３６重量％、ＣａＣＯ_３；５〜９重量％、Ａｌ_２Ｏ_３；０〜２重量％、有機物；１〜４重量％、Ｆｅ−Ｍｎ；６〜１１重量％、鉄粉；１２〜２４重量％、その他０〜６重量％からなる被覆剤を直径５ｍｍ、長さ４００ｍｍの炭素鋼心線に被覆率２３〜４６％になるように製造した後、乾燥し試験溶接棒を作製したものを用いて溶接し、耐食性試験用試験片を製作し、実機タンク屋根に組込み３ヶ月適用した結果を表１に示す。腐食の評価は、地鉄浸食量が０ｍｍの場合を○、０．０１ｍｍ未満を△、０．０１ｍｍ以上を×とした。その結果、本発明のＡ−５〜Ａ−９、Ａ−１１、Ａ−１２、Ａ−１６、Ａ−１７は、本発明範囲内の溶接施工での溶融亜鉛系めっき鋼板製タンクであり、内面、外面共に耐食性が優れていることが明らかである。Ａ−１は、黒皮材であり、Ａ−２〜Ａ−４は、めっき厚みが薄いため、耐食性が劣った。Ａ−１０、Ａ−１５は、溶接入熱が低いため、またＡ−１４は、めっき厚みが厚いため溶接欠陥が多く発生し、腐食試験を実施しなかった。Ａ−４、Ａ−１３、Ａ−１８は、溶接入熱が大きいためタンク内面の熱影響部がダメ−ジを受けて耐食性が劣化した。
【００２４】
【表１】

【００２５】
【表２】

【００２６】
次に、耐ピット・ブロ−ホ−ルを調査するため試験を実施した。表２に本発明例溶接棒および比較のために用いた溶接棒の被覆剤組成並びに各種試験結果を示す。溶接棒の作製にあたっては、直径５．０ｍｍ、長さ４００ｍｍの炭素鋼心線に被覆率３１％になるように製造した後、乾燥し試験溶接棒を作製した。この溶接棒により、図３に示すごとく溶融亜鉛系めっき鋼板ＪＩＳＧ３３０２・ＳＧＨ４００Ｚ４５（板厚４．５ｍｍ、幅２５０ｍｍ、長さ５００ｍｍ）を２枚重ねて、１５度下進すみ肉溶接を行い、すみ肉溶接部に発生するピットとブロ−ホ−ルを比較した。なお、この際に溶接作業性を判定した。この溶接条件は、溶接電流２２０Ａ、溶接入熱０．９〜１．２ｋＪ／ｍｍで行った。溶接終了後、ビ−ド表面に発生するピットの個数を目視で計測した。その後、溶接ビ−ドを放射線透過試験を行い計測した。ピットとブロ−ホ−ルの個数が５個／ｍ以下を良好（○印）とし、それ以上を不良（×印）とした。溶接作業性の判定は、優れるものを○印とし、やや劣るものを△印として評価した。本発明溶接棒であるＢ−１からＢ−５は、適正の炭素量であり、他被覆剤も適正量含有されているため、ピットとブロ−ホ−ルの数が少なく、さらに溶接作業性も優れていた。
【００２７】
一方、Ｃ−１は、炭素量が少なくピットとブロ−ホ−ルが多く発生した。Ｃ−２は、炭素量が多すぎるためピットとブロ−ホ−ル発生した。Ｃ−３〜Ｃ−５は、本発明範囲内だが被覆剤の炭素以外の成分がより好ましい範囲からは外れる例である。溶接作業性がやや劣るものの、ピットとブローホールの発生はなく、総合的には良好な溶接を行うことができた。Ｃ−３は、ＳｉＯ_２が多く、またＴｉＯ_２が少ないため、Ｃ−４は、ＳｉＯ_２とＣａＣＯ_３が少ないため下進溶接性がやや劣った。Ｃ−５は、ＣａＣＯ_３とＦｅ−Ｍｎが多いため、ア−クが不安定となり溶接作業性がやや劣った。Ｃ−６は、鉄粉が多いため被覆筒が弱くなり溶接作業性がやや劣った。
【００２８】
【発明の効果】
以上のように、本発明によれば、溶融亜鉛めっき鋼板の施工において高耐食性を有し、ピットやブロ−ホ−ル等の欠陥の少ない優れた溶接方法であることから、その工業的意義は極めて大きい。
【図面の簡単な説明】
【図１】タンク内面に溶融亜鉛系めっきを想定した場合の耐食性を示すグラフである。
【図２】固定式屋根型石油貯蔵タンクの構造を示す縦断側面図である。
【図３】試験に用いられた試験板形状を示す斜視図である。
【図４】被覆剤中の炭素量とピット・ブロ−ホ−ル発生数の関係を示すグラフである。
【符号の説明】
１基礎コンクリ−ト
２鋼製底板
３胴体部
４支柱
５屋根骨ラフタ−
６屋根
７石油貯蔵タンク
８石油類
９気相部
Ｗ幅
Ｌ長さ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for welding a steel sheet which has been subjected to corrosion protection using a hot-dip galvanized steel sheet.
[0002]
[Prior art]
In the past, hot-dip galvanized steel sheets were mainly used for building materials, but since the introduction of continuous galvanizing equipment, the quality of appearance, workability, etc. has dramatically improved, and electrical equipment, automobiles, civil engineering, etc. It has been used in various fields.
[0003]
When welding a hot-dip galvanized steel sheet, zinc having a lower melting point (419 ° C) and a boiling point (906 ° C) than the iron from the steel sheet surface penetrates into the drip or molten pool at the time of welding, and the vapor is bumped. Entrained in the atmosphere, it does not float up during the solidification process of the weld metal and remains as bubbles, generating pits and blowholes frequently. As a means for reducing such harm of zinc, it is effective to mechanically remove zinc from the welding line in advance, but it is troublesome and inefficient.
[0004]
In welding electric equipment and automobiles using hot-dip galvanized steel sheets, indoor robot welding is increasing, gas shield arc welding is frequently used, and special solid wires are being developed. JP-A-63-72498 proposes a welding material in which C, Si, P, and S in a solid wire are limited and Ti, Al, Ni, and Cu are appropriately contained. Further, in Japanese Patent Application Laid-Open No. Hei 2-263594, pits and blowholes are less likely to be generated and high speed is achieved by limiting C, Si, Mn, and Al to a specific range and limiting Cu, Nb, and V to a specific range. A gas shielded arc welding wire capable of welding is disclosed.
[0005]
On the other hand, the roof shingles of buildings represented by oil storage tanks are conventionally welded with black-rolled hot rolled steel on the outside, hand-painted on the outside, and black-scale on the inside. there were. In such a conventional tank made of black scale material, particularly in a fixed roof type oil storage tank, the portion of the tank inner surface that is not in contact with the oil is corroded. On the other hand, the present inventors have remarkably improved the corrosion resistance by arranging the hot-dip galvanized steel sheet on the roof and the upper part of the body as disclosed in Japanese Patent Application Laid-Open No. 7-112792, and greatly improved the tank life. Can be extended. Since welding is performed on a roof, a covered arc welding rod that is not easily affected by wind is used. As welding rod systems, JIS D4301 (Illuminite type), D4303 (Lime titania type) and D4313 (High titanium oxide type) are mainly used. In order to obtain a weld metal with less occurrence of defects and few defects, it is not sufficiently satisfactory. In addition, there is also a problem that the zinc plating inside the tank disappears due to welding heat in the vicinity of the heat-affected zone of welding from the outside of the tank and is damaged due to damage.
[0006]
[Problems to be solved by the invention]
The present invention relates to the welding of a hot-dip galvanized steel sheet used as a roof material of an oil storage tank as described above, by welding using a covered arc welding rod with less occurrence of defects such as pits and blowholes. It is another object of the present invention to provide a welding method in which zinc plating is not damaged even in the vicinity of a heat affected zone.
[0007]
[Means for Solving the Problems]
The present invention has been obtained by variously examining a welding method and a coating material component in order to meet the above-mentioned demands. The gist of the present invention is to provide a coating material comprising a coating material and a steel core wire. In a method of arc-welding a hot-dip galvanized steel sheet having a galvanized coating amount of 60 to 400 g / m ² per one side with a welding rod, carbon in the coating material is reduced to 0.02% by weight based on the entire coating material. ０．５0.50%, and welding by adjusting the welding heat input W 1 represented by Formula 1 to be 0.5 to 1.5 kJ / mm. _{_{C: 0.02~0.50%, TiO 2:}} 25~55%, SiO 2: 10~25%, CaCO 3: 4~16%, Fe-Mn: 5~14%, iron powder: 10 to 40 %, The balance being slag forming agent, arc stabilizer, gas generating agent, deoxidizing , In the welding method of hot-dip galvanized steel sheet characterized by comprising the fixing agent and unavoidable impurities.
[0008]
In the present invention, a steel sheet is provided with a Zn-Mg plated steel sheet containing 5% or less of Mg and a Zn-Al plated steel sheet containing 5% or less of Al for the purpose of rust prevention. It is called hot-dip galvanized steel sheet.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
The inventors of the present invention have found that pits and blowholes are liable to occur in welding galvanized steel sheets, while encouraging the entrainment of air due to bumping of zinc at the time of welding and zinc entering the molten metal to iron. Since it is difficult to dissolve and has a low boiling point, the molten metal is in a vapor state even during the solidification process, and this remains to form bubbles. It was considered that when an excessive amount of the deoxidizing agent was added, zinc in a molten state was not oxidized but remained as air bubbles, which further promoted the generation of pit blowholes. We also considered that the galvanization inside the tank was damaged by the welding because the galvanizing greatly reduced the welding workability and the welding speed had to be reduced.
[0010]
The present inventors reduce the effects of zinc by (1) fixing harmful zinc or slag-off as ZnO, (2) reducing the viscosity of molten metal and molten slag, and releasing zinc vapor. (3) From the viewpoint of improving the followability of the molten slag and increasing the welding speed, and from the viewpoint that these are effective, the present inventors have conducted intensive studies on the coating composition and came to constitute the present invention. It is.
[0011]
That is, the present invention relates to (a) limiting the amount of carbon in the coating agent to optimize the oxidizing power of the molten metal and promote the oxidation of zinc. (B) to reduce the effect of zinc by promoting gas release by enhancing the stirring action of the molten metal, and (b) the heat effect on the inner surface of the tank due to welding heat generated by welding from the outer surface of the tank In order to reduce the loss of galvanized area, of course, horizontal fillet welding has been developed, and a coated arc welding rod that can perform downward welding even on inclined parts where heat input is large and that can perform low heat input welding has been developed. This has the biggest feature. Further, (c) by limiting the amount of TiO _2, SiO _{2, and} CaCO _{3 in} the coating agent, to form a slag having an appropriate viscosity to secure welding workability, and (d) adding an appropriate amount of Mn. By optimizing the oxidizing power of the weld metal and promoting the oxidation of zinc, that is, to suppress the generation of zinc vapor, the temperature of the weld metal is increased by the heat of oxidation reaction and the viscosity of the molten metal is reduced. The effect of reducing the influence of zinc by promoting gas release by strengthening the stirring action of the molten metal is to further suppress the generation of pits and blowholes.
[0012]
Hereinafter, the constituent requirements in the present invention will be described.
FIG. 2 is a view showing the structure of a fixed roof type oil storage tank made of steel, in which a steel bottom plate 2 is placed and fixed on the upper surface of a base concrete 1, and the bottom plate 2 is fixed. The lower end of the cylindrical body 3 made of a steel plate is placed on the upper surface around the base plate and fixed by welding, and the lower end of the steel column 4 is placed on the center of the upper surface of the bottom plate 2. A steel roof bone rafter 5 is fixedly secured to the upper end of the column 4 and the body 3, and the conical steel roof 6 is connected to the roof bone rafter 5 and the upper end of the body 3. An oil storage tank 7 is formed by being fixed to the upper end of the support 4 by welding, and oils 8 are stored in the oil storage tank 7, and a gas phase section is provided between the oils 8 and the roof 6. 9 are provided. In FIG. 2, a hatched portion in the body 3 is a normal steel plate, a black thick line portion in the body 3 is a hot-dip galvanized steel plate, and the roof 6 is also a hot-dip galvanized steel plate.
In the present invention, the inner surface of the roof 6 and the upper part of the body 3, which are the target parts particularly requiring corrosion resistance, are not in contact with petroleum but are in a gas phase, and the corrosive environment at this part is pH 5, It is described in the aforementioned JP-A-7-112792 that a simple corrosion simulation can be performed by a sulfuric acid aqueous solution spray test at 35 ° C. Then, the present inventors investigated the corrosion resistance of the hot-dip galvanized steel sheet joint using the present corrosion simulation test.
[0013]
The result is shown in FIG. FIG. 1 is a graph showing the corrosion resistance when assuming hot-dip galvanizing on the inner surface of the tank. That, C; from .11 to 0.34 _wt%, SiO 2; _15~19 _wt%, TiO 2; _29~36 wt%, CaCO _3; 5~9 _wt%, _Al 2 O 3; _0~2 wt %, Organic matter; 1 to 4% by weight, Fe-Mn: 6 to 11% by weight, iron powder: 12 to 24% by weight, and a coating agent consisting of 0 to 6% by weight and a carbon steel core having a diameter of 5 mm and a length of 400 mm. After the wire was manufactured so as to have a coverage of 23 to 46%, it was dried to produce a test welding rod, and this welding rod was used to make a 0.5% Mg-containing galvanized steel sheet JISG3302 / SGH400 Z45 (with a thickness of 4.5 mm and a width of 4.5 mm). 3 were overlapped with each other as shown in FIG. 3, and the fillet welding was performed 15 degrees downward, the above corrosion test was performed, and the residual plating thickness of the weld heat affected zone on the back surface of the welded portion was measured. . It was determined that if the residual plating thickness was 10 μm or more, there was no problem in corrosion resistance. As a result, if the plating adhesion amount is less than 60 g / m ² per side, the corrosion resistance becomes insufficient. On the other hand, if it exceeds 400 g / m ² per one surface, the weldability deteriorates. In particular, generation of pits and blowholes becomes remarkable. Further, the method for producing the hot-dip galvanized steel sheet is not particularly limited, and various methods can be applied.
[0014]
A steel sheet having a zinc basis weight of 60 g / m ² or more has good corrosion resistance when the welding heat input W represented by the formula (1) is 0.6 kJ / mm and 1.4 kJ / mm. At a heat of 2.1 kJ / mm, the residual plating thickness is 10 μm or less, and the corrosion resistance deteriorates. That is, the reason why the welding heat input is set to 0.5 to 1.5 kJ / mm 2 is that when the welding heat input is less than 0.5 kJ / mm 2, a welding defect occurs, and when the welding heat input exceeds 1.5 kJ / mm 2, This is because galvanization of the heat-affected zone on the inner surface side of the tank due to welding heat generated by welding from the outer surface disappears, resulting in poor corrosion resistance.
[0015]
W = 60 IV / S (kJ / mm) ... (1)
I: welding current (A)
V: welding voltage (V)
S: welding speed (mm / sec)
Next, the present inventors have conducted various studies on coated arc welding rods in which pits and blow holes are extremely unlikely to occur when welding a steel sheet having a zinc basis weight of 60 to 400 g / m ^2. It has been found that this problem can be solved by limiting the amount of carbon in the bar coating. FIG. 4 is a graph showing the relationship between the amount of carbon in the coating material and the number of pit blowholes generated. That is, SiO ₂ : 15 to 19% by weight, TiO ₂ : 29 to 36% by weight, CaCO ₃ : 5 to 9% by weight, Al ₂ O ₃ : 0 to 2% by weight, organic matter: 1 to 4% by weight, Fe -Mn: 6 to 11% by weight, iron powder: 12 to 24% by weight, and other coatings consisting of 0 to 6% by weight, C: 0.06% by weight, Si: 0.01% by weight, Mn: 0. A carbon steel core wire having a diameter of 5 mm and a length of 400 mm containing 49% by weight was manufactured so as to have a coverage of 23 to 46%, and then dried to prepare a test welding rod. As shown in FIG. 3, two galvanized steel sheets JIS G3302 and SGH400 Z45 (sheet thickness 4.5 mm, width 250 mm, length 500 mm) are superimposed as shown in FIG. 3, and the welding heat input is 0.9 to 1.4 kJ /

mm

2 and 15 degrees below. Performs fillet welding and advances the pitch generated in the fillet weld. And Bro - ho - were compared Lumpur. The measurement was carried out by measuring the number of pits by eye measurement, and then performing a radiation transmission test. As is evident from FIG. 4, the pits and blowholes are reduced by limiting the amount of carbon in the coating. The effect is obtained when the added amount of carbon is 0.02% by weight or more. However, if it is added in excess of 0.50% by weight, zinc in the molten state is not oxidized, remains in the weld metal as air bubbles, and solidifies as it is to form pits and blowholes. In addition, the weld metal hardens significantly, deteriorating ductility and deteriorating crack resistance. The carbon contained in the coating agent is mainly added from Fe-Mn or graphite. Incidentally, if the carbon content in the steel core wire is about 0.04 to 0.08% by weight, the above-described effects can be obtained.
[0016]
The reason why the hot-dip galvanized steel sheet is applied to the inner surface of the tank roof 6 and the upper inner surface of the body portion 3 is that the corrosion resistance is improved because of the anti-corrosion effect of zinc corrosion products in a sulfuric acid wet environment which is an acidic corrosion environment. . The good corrosion resistance of the weld heat-affected zone is due to the galvanic protection effect of zinc remaining in the heat-affected zone and zinc present in the peripheral portion. Further, the type of the oil tank is not particularly limited, and can be applied not only to the fixed roof type oil tank but also to various other tanks such as a roof floating type tank. In addition, the present invention can be applied to a rain shield plate of a roof seal part of a floating roof tank accessory, a member of a seal material mounting part, and the like.
[0017]
TiO ₂ is an important component for the downward weldability, and is used as a slag forming agent, a viscosity modifier and an arc stabilizer. If the addition amount is less than 25% by weight, the viscosity of the slag is too low, the encapsulation of the slag by the downward welding is poor, the undercut is easily generated, and the bead shape is also poor. If the addition exceeds 55% by weight, the viscosity of the slag increases, the bead shape deteriorates, and the blowhole increases.
[0018]
SiO ₂ is used as a slag generator. If the addition amount is less than 10% by weight, the coating cylinder becomes shallow, the arc becomes rough, and the generation of spatters increases. If added in excess of 25% by weight, the melting point of the coating agent will increase, the coating cylinder will be too long, the arc will be unstable, and the bead waveform will be poor.
[0019]
CaCO ₃ is an important component for downward weldability, and is used as a gas generating agent and a slag generating agent. If the addition amount is less than 4% by weight, the viscosity of the slag decreases, the bead shape deteriorates, the effect as a gas generating agent is insufficient, and pit blowhole is generated. If it exceeds 16% by weight, the arc voltage decreases, the arc becomes unstable, short-circuits easily occur, and the arc breaks.
[0020]
Fe-Mn has an effect as a deoxidizing agent and an alloying agent. It is also added as the above-mentioned carbon source. If the addition amount is less than 5% by weight, deoxidation becomes insufficient, and a clean weld metal cannot be obtained. If it exceeds 14% by weight, deoxidation will be excessive, and the influence of zinc vapor will work greatly, pit blowholes will be generated, and the weld metal will be hardened and crack resistance will deteriorate.
[0021]
Iron powder is added for the purpose of increasing the amount of an arc stabilizer and a deposited metal. If the addition amount is less than 10% by weight, the amount of welding is insufficient, and undercut occurs in fillet welding. If the content exceeds 40% by weight, the coating cylinder becomes weak and short-circuit is apt to occur, and the temperature of the weld metal lowers, gas is insufficiently released, and pit blowholes are generated.
[0022]
Further, the arc stabilizer, slag forming agent, gas generating agent and deoxidizing agent other than those mentioned above refer to MgO, Fe ₂ O ₃ , Al ₂ O ₃ or organic substances. The welding rod of the present invention is prepared by applying the above-described compounded flux to a steel wire with a binder such as water glass typified by sodium silicate and potassium silicate so that the coverage of the steel core wire becomes 20 to 45%. After coating with a conventional extrusion type welding rod coating machine, it is manufactured by drying at 120 to 250 ° C. to remove moisture.
[0023]
【Example】
Next, the effects of the present invention will be specifically described with reference to examples. Using various hot-dip galvanized steel sheets, C: 0.11 to 0.34% by weight, SiO ₂ : 15 to 19% by weight, TiO ₂ : 29 to 36% by weight, CaCO ₃ : 5 to 9% by weight, Al ₂ O ₃ ; 0 to 2% by weight, organic matter: 1 to 4% by weight, Fe-Mn: 6 to 11% by weight, iron powder: 12 to 24% by weight, and a coating agent of 5 to 6% by weight having a diameter of 5 mm. After being manufactured so as to have a coverage of 23 to 46% on a carbon steel core wire having a length of 400 mm, it was dried and welded using a test welding rod, and a test piece for corrosion resistance test was manufactured. Table 1 shows the results of three months of installation on the roof. The corrosion was evaluated as follows: 場合 when the amount of ground iron erosion was 0 mm, Δ when less than 0.01 mm, and × when 0.01 mm or more. As a result, A-5 to A-9, A-11, A-12, A-16, and A-17 of the present invention are hot-dip galvanized steel sheet tanks for welding within the scope of the present invention, It is clear that both the inner and outer surfaces have excellent corrosion resistance. A-1 is a black scale material, and A-2 to A-4 were inferior in corrosion resistance because of a small plating thickness. In A-10 and A-15, the welding heat input was low, and in A-14, the plating thickness was large, so that many welding defects were generated, and the corrosion test was not performed. In A-4, A-13, and A-18, the heat-affected zone on the inner surface of the tank was damaged due to large welding heat input, and the corrosion resistance was deteriorated.
[0024]
[Table 1]

[0025]
[Table 2]

[0026]
Next, a test was conducted to investigate the pit blowhole resistance. Table 2 shows the coating composition of the welding rod of the present invention and the welding rod used for comparison and the results of various tests. In producing the welding rod, a carbon steel core wire having a diameter of 5.0 mm and a length of 400 mm was manufactured so as to have a coverage of 31%, and then dried to produce a test welding rod. As shown in FIG. 3, two hot-dip galvanized steel sheets JIS G3302 and SGH400 Z45 (sheet thickness 4.5 mm, width 250 mm, length 500 mm) are overlapped with this welding rod, and the fillet welding is performed 15 degrees downward, The pits and blowholes generated in the fillet weld were compared. At this time, the welding workability was determined. The welding conditions were a welding current of 220 A and a welding heat input of 0.9 to 1.2 kJ / mm 2. After welding, the number of pits generated on the bead surface was visually measured. Thereafter, the weld bead was measured by performing a radiation transmission test. When the number of pits and blowholes was 5 / m or less, it was regarded as good (印), and when it was more than 5 / m, it was poor (印). The evaluation of welding workability was evaluated as "good" with a circle and "slightly poor" with a triangle. Since the welding rods B-1 to B-5 of the present invention have an appropriate amount of carbon and contain an appropriate amount of other coating agent, the number of pits and blowholes is small, and the welding workability is further improved. Was also excellent.
[0027]
On the other hand, C-1 had a small amount of carbon and generated many pits and blowholes. C-2 had pits and blowholes due to too much carbon. C-3 to C-5 are examples within the range of the present invention, but components other than carbon of the coating agent are out of the more preferable range. Although welding workability was slightly inferior, no pits and blowholes were generated, and good welding could be performed overall. C-3, since SiO ₂ is large and also TiO ₂ is less, C-4 is ShitaSusumu weldability for SiO ₂ and CaCO ₃ is less was slightly inferior. C-5 has a large amount of CaCO ₃ and Fe—Mn, so that the arc becomes unstable and the welding workability is slightly inferior. C-6 had a large amount of iron powder, so the coating cylinder was weak and welding workability was slightly inferior.
[0028]
【The invention's effect】
As described above, according to the present invention, since it is an excellent welding method having high corrosion resistance and less defects such as pits and blow holes in the construction of hot-dip galvanized steel sheet, its industrial significance is Extremely large.
[Brief description of the drawings]
FIG. 1 is a graph showing the corrosion resistance when assuming hot-dip galvanizing on the inner surface of a tank.
FIG. 2 is a vertical sectional side view showing a structure of a fixed roof type oil storage tank.
FIG. 3 is a perspective view showing a test plate shape used for a test.
FIG. 4 is a graph showing the relationship between the amount of carbon in a coating agent and the number of pit blowholes generated.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Basic concrete 2 Steel bottom plate 3 Body part 4 Prop 5 Roof raft
6 Roof 7 Oil storage tank 8 Oil 9 Gas phase W Width L Length

Claims

In a method of arc-welding a hot-dip galvanized steel sheet having a galvanized coating amount of 60 to 400 g / m ² per one side with a coated arc welding rod composed of a coating agent and a steel core wire, A hot-dip galvanizing method characterized in that carbon is set to 0.02 to 0.50% by weight based on the whole coating agent, and welding is performed by adjusting welding heat input to 0.5 to 1.5 kJ / mm. Steel plate welding method.

The coating agent is a weight% of the whole coating agent,
C: 0.02 to 0.50%
TiO _2: 25~55%
SiO ₂ : 10 to 25%
CaCO ₃ : 4 to 16%
Fe-Mn: 5 to 14%
Iron powder: 10-40%
The method for welding a hot-dip galvanized steel sheet according to claim 1, characterized in that it comprises: a slag forming agent, an arc stabilizer, a gas generating agent, a deoxidizing agent, a fixing agent and inevitable impurities.