JP3986878B2

JP3986878B2 - Manufacturing method for welded steel pipe with excellent weld corrosion resistance

Info

Publication number: JP3986878B2
Application number: JP2002130504A
Authority: JP
Inventors: 博文武津; 幸弘守田; 正信藤井; 伸雄田中
Original assignee: Nisshin Steel Co Ltd; Nisshin Kokan Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd; Nippon Steel Nisshin Pipe Co Ltd
Priority date: 2002-05-02
Filing date: 2002-05-02
Publication date: 2007-10-03
Anticipated expiration: 2022-05-02
Also published as: JP2003328105A

Description

【０００１】
【産業上の利用分野】
本発明は、溶接部耐食性及び加工性に優れた溶接めっき鋼管の製造方法に関する。
【０００２】
【従来の技術】
Ｚｎめっき鋼帯，Ｚｎ−Ａｌ合金めっき鋼帯，Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼帯等のめっき鋼帯から造管された溶接めっき鋼管は、耐食性に優れている特性を活用し、過酷な腐食環境に曝される各種構造部材，配管，排ガス用管等として使用されており、最近では農業用ビニールハウスの骨組みや地中埋設管等への適用も進められている。
溶接めっき鋼管は、たとえば図１に示す造管ラインで製造される。めっき鋼帯Ｓをロールフォーミング又はロールレスフォーミングでオープンパイプ状に成形した後、めっき鋼帯Ｓの幅方向端部を高周波コイル１で加熱し、スクイズロール２で加熱圧着する。加熱圧着によって幅方向両端部が融合され溶接めっき鋼管Ｐになるが、溶接部に半径方向に突出したビードＢが形成される。そこで、ビードカッタ３，４でビードＢを切削することにより、溶接めっき鋼管Ｐの周面を平滑化する。しかし、溶接部は、ビード切削によってめっき層も除去されるため、下地鋼が露出した表面になっている。
【０００３】
溶接部耐食性を回復させるため、下地鋼が露出した溶接部に溶射補修層を設ける方法が採用されている。本出願人等も、Ａｌ溶射，Ｚｎ又はＺｎ−Ａｌ合金溶射の二連溶射で溶接部を補修する方法を紹介した（特開平８−１２７８５５号公報）。この方法では、溶接めっき鋼管Ｐの搬送方向に沿ってＡｌ溶射ガン５，Ｚｎ又はＺｎ−Ａｌ合金溶射ガン６を配置し、溶射ガン５，６に送り込まれたＡｌ芯線７，Ｚｎ又はＺｎ−Ａｌ合金芯線８をフレーム加熱し、溶融金属として溶接部に被着させる。
【０００４】
【発明が解決しようとする課題】
下地鋼が露出した溶接部がＺｎ−Ａｌ合金の溶射補修層で被覆されることにより溶接部耐食性は向上するが、過酷な腐食環境下で溶接めっき鋼管を使用するとき依然として溶接部又は溶接部近傍を起点にする腐食や白錆が散見される。腐食や白錆は、溶接めっき鋼管を用いた各種構造部材，配管，排ガス用管等の外観を著しく低下させ、場合によっては必要機能が損なわれることもある。環境悪化が深刻な最近では、腐食，白錆等の欠陥発生に対する対策が一層強く要求されている。
【０００５】
また、溶接めっき鋼管に対する溶射補修層の密着性が十分でないと、溶接鋼管の加工時や取扱い，施工の際に溶射補修層が亀裂，剥離，脱落することがある。亀裂，剥離，脱落の欠陥が生じると、欠陥部を介して腐食性成分が下地鋼に到達しやすくなり、溶射補修層の防食機能が損なわれる。形成した溶射補修層がポーラスな場合にも、腐食や白錆が発生しやすい。
【０００６】
【課題を解決するための手段】
本発明は、このような問題を解消すべく案出されたものであり、下地鋼に対する密着性及び耐食性に優れたＡｌ−Ｍｇ合金の溶射補修層を溶接部に形成することにより、過酷な腐食環境下で長期間使用されても腐食や白錆の発生がなく耐久性に優れた溶接めっき鋼管を提供することを目的とする。
【０００７】
本発明の溶接めっき鋼管の製造方法は、その目的を達成するため、Ｚｎ−Ａｌ−Ｍｇ合金めっき層が設けられているめっき鋼帯から溶接めっき鋼管を製造する際、溶接ビードをカットした後、４５０℃以上の状態にある溶接部にＡｌ−Ｍｇ合金→Ａｌ−Ｍｇ合金，Ａｌ→Ａｌ−Ｍｇ合金又はＡｌ−Ｍｇ合金→Ａｌの二連溶射により、ビードカットでめっき層が除去された溶接部表面にＡｌ−Ｍｇ合金の溶射補修層を設けることを特徴とする。
【０００９】
【作用】
溶接めっき鋼管は、ビードカットされた溶接部に溶射補修層が設けられている。溶接部１０及びその周辺では、溶接後のビードカットにより溶接ビード及びめっき層１１が除去されているが、露出した下地鋼１２及び溶接金属１３の上に溶射補修層１４が盛られている（図２）。溶射顕熱を利用して密着性の高い溶射補修層１４を形成するためには、溶接後の高温状態にある溶接部に溶射補修層１４を設けることが好ましい。厚い溶射補修層１４は、Ａｌ−Ｍｇ合金→Ａｌ−Ｍｇ合金，Ａｌ−Ｍｇ合金→Ａｌ又はＡｌ−Ｍｇ合金→Ａｌの二連溶射で形成できる。
めっき層１１には、Ｚｎ−６％Ａｌ−３％ＭｇのＺｎベース合金めっき層が用いられる。めっき層１１が形成されているめっき鋼帯Ｓのオープンパイプを溶接すると、溶接熱で加熱されためっき層１１の表面に薄い酸化皮膜が生成する。ビードカットでめっき層１１が除去された溶接部では、露出した下地鋼１２の表面に薄い酸化皮膜が生成している。
【００１０】
溶接部に盛られる溶射補修層は、酸素親和力の大きなＡｌ，Ｍｇを含む合金である。Ａｌ，Ｍｇは、露出した下地鋼１２及びめっき層１１の表層にある酸化皮膜の酸素と反応し、酸化物となって下地鋼１２，めっき層１１に強固に結合する。Ａｌ単独の溶射でも酸化皮膜との反応が生じるが、更に酸素親和力の大きく鉄酸化物との親和性が高いＭｇがあるため反応が促進され、めっき層１１，下地鋼１２の表面にＡｌ，Ｍｇの酸化物からなる緻密な皮膜が形成される。この皮膜を介して溶射層が形成されるため、めっき層１１，下地鋼１２に溶射補修層１４が強固に結合される。
溶射補修層１４のＡｌ，Ｍｇは、Ｚｎの腐食生成物に対しても影響を及ぼし、導電性で保護作用のない酸化亜鉛ＺｎＯの生成を抑え、緻密で保護作用のある塩基性塩化亜鉛Ｚｎ₅(ＯＨ)₈Ｃｌ₂・Ｈ₂Ｏの生成を促進させる。しかも、溶射補修層１４の表面にバリア作用や腐食防止機能のあるＡｌ，Ｍｇの酸化物皮膜が形成される。そのため、溶射補修層１４によって溶接部の耐食性が格段に向上する。
【００１１】
溶射補修層１４を好ましくは１０μｍ以上の膜厚で形成すると、ビードカットされた溶接部及び溶接部近傍が溶射補修層１４で均一に覆われ、溶接部耐食性の向上が確実になる。しかし、３０μｍを超える厚膜は、溶射金属を浪費するばかりでなく、下地鋼１２に対する溶射補修層１４の密着性にも悪影響が現れる。
密着性，耐食性に優れた溶射補修層１４で溶接部が覆われるため、曲げ，拡管，縮管等の加工によって溶接めっき鋼管Ｐを製品形状に加工した後でも溶射補修層１４が健全な状態に保たれ、溶接部を起点にする腐食や白錆の発生が抑えられる。なかでも、溶射補修層１４のＭｇ含有量を２０質量％以下にするとき、溶射補修層１４の加工性が確保され、成形加工時に亀裂，剥離等が溶射補修層１４に生じることが防止される。Ｍｇ含有量を２０質量％以下に規制することは、変色防止にも有効である。
【００１２】
二連溶射で溶射補修層１４を形成する場合、Ａｌ−Ｍｇ合金を２段階で溶射する方法，Ａｌ溶射後にＡｌ−Ｍｇ合金を溶射する方法，Ａｌ−Ｍｇ合金溶射後にＡｌを溶射する方法の何れも採用可能である。なかでも、Ａｌ−Ｍｇ合金溶射後にＡｌを溶射すると、溶射補修層１４の表層Ｍｇ濃度が低下し、溶射補修層１４の変色も抑えられる。何れの場合も、溶接後の高温状態（具体的には、４５０℃以上）にある溶接部に溶射補修層１４を設けることが、溶接部に対する溶射補修層１４の密着性を向上させる上で好ましい。
【００１３】
耐変色性は、農業用ビニールハウスの骨組み等に溶接めっき鋼管Ｐを使用する用途では重要な特性である。すなわち、農業用ビニールハウス内では農薬が頻繁に噴霧され、高温多湿な環境にあるため、溶射補修層１４の表層にＭｇが濃化していると、雰囲気の腐食性成分とＭｇとの反応によって溶射補修層１４が黒変化し、外観が著しく劣化する。この点、最初にＡｌ−Ｍｇ合金を、次いでＡｌを溶射することにより、表層のＭｇ濃度が抑えられ、耐変色性の良好な溶射補修層１４が形成される。
【００１４】
溶射補修層１４を形成した後、必要に応じ無機系又は有機系の化成処理液を用いて溶接部及び溶接部近傍を防錆処理する。無機系化成処理には、クロメート処理，リン酸塩処理，環境負荷の低減を目的としたクロムフリータイプの無機系防錆処理等がある。クロムフリー無機系防錆処理の一例として、バルブメタル（Ｚｒ，Ｖ，Ｍｏ，Ｔｉ等）の酸化物，水酸化物，フッ化物等の複合皮膜をめっき表層に形成させることで防錆性能が向上する。有機系化成処理には、ウレタン系樹脂，エポキシ樹脂，ポリエチレン，ポリプロピレン，エチレン−アクリル酸共重合体等のオレフィン系樹脂，ポリスチレン等のスチレン系樹脂，ポリエステル或いはこれらの共重合物又は変生物，アクリル系樹脂等を単独或いは複合添加した化成処理液を用いることが可能であり、更に、防錆添加剤としてリン系化合物や硫黄系化合物を用いることが有効である。
【００１５】
【実施例１】
目付け量９０ｇ／ｍ²でＺｎ−６％Ａｌ−３％Ｍｇ合金を溶融めっきした板厚１.２ｍｍのめっき鋼帯Ｓを直径２５.４ｍｍのオープンパイプに成形し、幅方向両端部を突合せ溶接した。溶接部には、溶接めっき鋼管Ｐの周面から最高で１.５ｍｍ突出した外面ビードが形成されていた。ビード突出部を切削した後、溶接めっき鋼管Ｐの表面を観察したところ、溶接線を中心として幅４.０ｍｍの範囲でめっき層１１が除去されており、溶接線両側４.０ｍｍの範囲でめっき層１１が薄くなっていた。
【００１６】
めっき層１１が除去された溶接部に、表１に示す二連溶射で幅１０ｍｍの溶射補修層１４を形成した。Ａｌ−Ｍｇ合金芯線としては、直径が１.４ｍｍで、５％のＭｇを含むものを使用した。Ａｌ芯線としては、直径が１.４ｍｍで純度９９．９％のＡｌ線材を使用した。比較例２では単一工程の溶射によってＺｎ溶射層を、比較例３では二連溶射によってＺｎ溶射層を形成した。溶射に際しては、何れの場合もＣ_２Ｈ_２＋Ｏ_２のフレームで芯線を加熱し、溶射ガン５，６から溶融金属として溶接部に被着させた。
【００１７】

【００１８】
溶射補修層１４が形成された溶接めっき鋼管Ｐから長さ１５０ｍｍの筒状試験片を切り出し、促進腐食試験，乾湿複合腐食試験に供した。
促進腐食試験では、温度３５℃の５％ＮａＣｌ水溶液を試験片の溶射補修層１４に噴霧し、塩水噴霧を所定時間経過した後で溶接部表面を観察し、白錆発生面積率を測定した。
乾湿複合腐食試験では、塩水噴霧（３５℃，５％ＮａＣｌ）２時間→乾燥（６０℃，３０％ＲＨ）４時間→湿潤（５０℃，９５％ＲＨ）２時間を１サイクルとし、所定サイクル数繰り返した後で溶接部表面を観察し、赤錆発生面積率を測定した。
【００１９】
表２の試験結果にみられるように、Ａｌ−Ｍｇ合金の二連溶射補修層１４を形成した本発明例１〜６では、Ｚｎ溶射層を設けた比較例２，３に比較して白錆発生が大幅に抑えられていた。Ａｌ−Ｍｇ合金の一連溶射層を設けた比較例１よりも乾湿複合腐食試験の結果が良好であった。また、塩水噴霧を長時間継続した後や、乾湿複合腐食試験を２５０サイクル繰り返した後でも赤錆は皆無であり、溶接部は当初の美麗な表面に維持されていた。更に、溶射補修層１４のＭｇ含有量が２０質量％以下に抑えられているので、溶射補修層１４の変色がなく、溶接めっき鋼管Ｐを曲げ加工しても曲げ外側の溶射補修層１４に亀裂や剥離が発生しなかった。
【００２０】

【００２３】
【発明の効果】
以上に説明したように、本発明の溶接めっき鋼管は、溶接後のビードカットでめっき層が除去された溶接部及び溶接部近傍にＡｌ−Ｍｇ合金の溶射補修層を形成している。溶射補修層は、溶接部の露出下地鋼やめっき層表面にある酸化皮膜に対するＡｌ，Ｍｇの親和力が高いことから溶接部に強固に付着し、バリア機能の高い保護膜として働く。そのため、溶接部を起点とする腐食や白錆の発生がなく、長期にわたって美麗な外観を呈する構造用部材，配管，排ガス用管，農業用ビニールハウスの骨組み等として好適な溶接めっき鋼管として使用される。
【図面の簡単な説明】
【図１】電縫鋼管を製造する造管ラインに組み込んだ溶接部の補修装置
【図２】溶射補修層が盛られた溶接部の断面を示す模式図
【符号の説明】
Ｓ：めっき鋼帯Ｐ：溶接めっき鋼管Ｂ：ビード
１：高周波コイル２：スクイズロール３，４：ビードカッタ５，６：溶射ガン７，８：溶射用芯線
１０：溶接部１１：めっき層１２：下地鋼１３：溶接金属１４：溶射補修層[0001]
[Industrial application fields]
The present invention relates to a method for manufacturing a weld-plated steel pipe excellent in welded portion corrosion resistance and workability.
[0002]
[Prior art]
Weld-plated steel pipes made from plated steel strips such as Zn-plated steel strip, Zn-Al alloy-plated steel strip, and Zn-Al-Mg alloy-plated steel strip, take advantage of their excellent corrosion resistance and cause severe corrosion It is used as various structural members exposed to the environment, pipes, exhaust gas pipes, etc., and recently applied to the framework of agricultural greenhouses and underground pipes.
The weld-plated steel pipe is manufactured by, for example, a pipe making line shown in FIG. After forming the plated steel strip S into an open pipe by roll forming or rollless forming, the widthwise end of the plated steel strip S is heated by the high-frequency coil 1, and thermocompression bonded by the squeeze roll 2. Although both ends in the width direction are fused to form a welded steel pipe P by thermocompression bonding, a bead B protruding in the radial direction is formed in the welded portion. Therefore, the peripheral surface of the weld-plated steel pipe P is smoothed by cutting the bead B with the bead cutters 3 and 4. However, since the plating layer is also removed by bead cutting, the welded portion has a surface on which the base steel is exposed.
[0003]
In order to recover the welded portion corrosion resistance, a method of providing a thermal spray repair layer on the welded portion where the base steel is exposed is employed. The present applicants also introduced a method of repairing a welded portion by two-spray spraying of Al spraying, Zn or Zn-Al alloy spraying (Japanese Patent Laid-Open No. 8-127855). In this method, an Al spray gun 5, Zn or Zn—Al alloy spray gun 6 is arranged along the conveying direction of the weld-plated steel pipe P, and the Al core wire 7, Zn or Zn—Al fed into the

spray gun

5, 6. The alloy core wire 8 is heated by a frame and is deposited as a molten metal on the weld.
[0004]
[Problems to be solved by the invention]
Corrosion resistance of the welded part is improved by coating the welded part where the base steel is exposed with a thermal spray repair layer of Zn-Al alloy, but it is still in the welded part or the vicinity of the welded part when the welded steel pipe is used in a severe corrosive environment. Corrosion and white rust are sometimes found. Corrosion and white rust remarkably deteriorate the appearance of various structural members, pipes, exhaust gas pipes and the like using weld-plated steel pipes, and in some cases, necessary functions may be impaired. In recent years, when environmental degradation is serious, countermeasures against defects such as corrosion and white rust have been increasingly demanded.
[0005]
Moreover, if the adhesion of the thermal spray repair layer to the welded steel pipe is not sufficient, the thermal spray repair layer may crack, peel or fall off during processing, handling or construction of the welded steel pipe. When cracks, delamination, and drop-off defects occur, the corrosive components easily reach the base steel through the defect portions, and the anticorrosion function of the thermal spray repair layer is impaired. Corrosion and white rust are likely to occur even when the formed thermal spray repair layer is porous.
[0006]
[Means for Solving the Problems]
The present invention has been devised to solve such problems, and by forming a sprayed repair layer of an Al-Mg alloy having excellent adhesion and corrosion resistance to the base steel on the welded portion, severe corrosion is caused. An object of the present invention is to provide a weld-plated steel pipe that is excellent in durability and does not generate corrosion or white rust even when used for a long time in an environment.
[0007]
In order to achieve the object, the method for producing a welded steel pipe according to the present invention cuts a weld bead when producing a welded steel pipe from a plated steel strip provided with a Zn-Al-Mg alloy plating layer, A welded part in which the plating layer is removed by bead cut by two-step thermal spraying of Al-Mg alloy-> Al-Mg alloy, Al-> Al-Mg alloy or Al-Mg alloy-> Al on the welded part at 450 ° C or higher An Al—Mg alloy thermal spray repair layer is provided on the surface.
[0009]
[Action]
The weld-plated steel pipe is provided with a thermal spray repair layer in a welded portion that is bead cut. Although the weld bead and the plating layer 11 are removed by the bead cut after welding in the welded part 10 and its periphery, the thermal spray repair layer 14 is piled up on the exposed base steel 12 and the weld metal 13 (FIG. 2). In order to form the thermal spray repair layer 14 with high adhesion using thermal spray sensible heat, it is preferable to provide the thermal spray repair layer 14 in a welded portion in a high temperature state after welding. The thick spray repair layer 14 can be formed by two-step spraying of Al—Mg alloy → Al—Mg alloy, Al—Mg alloy → Al or Al—Mg alloy → Al.
As the plating layer 11, a Zn-6% Al-3% Mg Zn-based alloy plating layer is used. When the open pipe of the plated steel strip S on which the plating layer 11 is formed is welded, a thin oxide film is generated on the surface of the plating layer 11 heated by welding heat. In the welded portion from which the plating layer 11 has been removed by bead cutting, a thin oxide film is formed on the exposed surface of the base steel 12.
[0010]
The thermal spray repair layer deposited on the welded portion is an alloy containing Al and Mg having a large oxygen affinity. Al and Mg react with the exposed base steel 12 and oxygen in the oxide film on the surface of the plating layer 11 to form an oxide and firmly bond to the base steel 12 and the plating layer 11. Although reaction with an oxide film occurs even by thermal spraying of Al alone, the reaction is further promoted due to Mg having high oxygen affinity and high affinity with iron oxide, and Al, Mg on the surface of the plating layer 11 and the base steel 12. A dense film made of the oxide is formed. Since the thermal spray layer is formed through this film, the thermal spray repair layer 14 is firmly bonded to the plating layer 11 and the base steel 12.
Al and Mg in the thermal spray repair layer 14 also affect the corrosion product of Zn, suppress the formation of conductive and non-protective zinc oxide ZnO, and a dense and protective basic zinc chloride Zn ₅ Promotes the formation of (OH) ₈ Cl ₂ .H ₂ O. In addition, an Al / Mg oxide film having a barrier action and a corrosion preventing function is formed on the surface of the thermal spray repair layer 14. Therefore, the thermal spray repair layer 14 significantly improves the corrosion resistance of the welded portion.
[0011]
When the thermal spray repair layer 14 is formed with a film thickness of preferably 10 μm or more, the welded part cut in the bead and the vicinity of the welded part are uniformly covered with the thermal spray repair layer 14, and the corrosion resistance of the welded part is reliably improved. However, a thick film exceeding 30 μm not only wastes the sprayed metal, but also adversely affects the adhesion of the sprayed repair layer 14 to the base steel 12.
Since the welded portion is covered with the thermal spray repair layer 14 having excellent adhesion and corrosion resistance, the thermal spray repair layer 14 is in a healthy state even after the weld-plated steel pipe P is processed into a product shape by processing such as bending, pipe expansion and contraction. Therefore, corrosion and white rust starting from the weld are suppressed. In particular, when the Mg content of the thermal spray repair layer 14 is set to 20% by mass or less, the processability of the thermal spray repair layer 14 is ensured, and cracks, peeling, and the like are prevented from occurring in the thermal spray repair layer 14 during molding. . Regulating the Mg content to 20% by mass or less is also effective for preventing discoloration.
[0012]
When the thermal spray repair layer 14 is formed by two-step thermal spraying, any one of a method of spraying an Al-Mg alloy in two stages, a method of spraying an Al-Mg alloy after Al spraying, and a method of spraying Al after Al-Mg alloy spraying Can also be adopted. In particular, when Al is sprayed after Al-Mg alloy spraying, the surface Mg concentration of the sprayed repair layer 14 is reduced, and discoloration of the sprayed repair layer 14 is also suppressed. In any case, it is preferable to improve the adhesion of the thermal spray repair layer 14 to the welded portion by providing the thermal spray repair layer 14 in a welded portion in a high temperature state (specifically, 450 ° C. or higher) after welding. .
[0013]
Discoloration resistance is an important characteristic in applications in which welded steel pipes P are used for the framework of agricultural greenhouses and the like. That is, since agricultural chemicals are frequently sprayed in a greenhouse for agriculture and are in a hot and humid environment, if Mg is concentrated on the surface layer of the thermal spray repair layer 14, thermal spraying is caused by the reaction between the corrosive component of the atmosphere and Mg. The repair layer 14 changes to black and the appearance is remarkably deteriorated. In this regard, by spraying the Al—Mg alloy first and then Al, the Mg concentration of the surface layer is suppressed, and the thermal spray repair layer 14 with good discoloration resistance is formed.
[0014]
After the thermal spray repair layer 14 is formed, the welded part and the vicinity of the welded part are subjected to rust prevention treatment using an inorganic or organic chemical conversion treatment liquid as necessary. Examples of inorganic chemical conversion treatment include chromate treatment, phosphate treatment, and chromium-free inorganic rust prevention treatment for the purpose of reducing environmental impact. As an example of chromium-free inorganic rust prevention treatment, rust prevention performance is improved by forming a composite film of valve metal (Zr, V, Mo, Ti, etc.) oxide, hydroxide, fluoride, etc. on the plating surface layer. To do. For organic chemical conversion treatment, urethane resin, epoxy resin, olefin resin such as polyethylene, polypropylene, ethylene-acrylic acid copolymer, styrene resin such as polystyrene, polyester or copolymer or metabolite thereof, acrylic It is possible to use a chemical conversion treatment solution to which a single resin or the like is added alone or in combination, and it is also effective to use a phosphorus compound or a sulfur compound as a rust preventive additive.
[0015]
[Example 1]
A plated steel strip S with a basis weight of 90 g / m ² and hot-plated Zn-6% Al-3% Mg alloy is formed into an open pipe with a diameter of 25.4 mm and both ends in the width direction are butt welded did. In the welded portion, an outer bead that protrudes up to 1.5 mm from the peripheral surface of the weld-plated steel pipe P was formed. After cutting the bead protrusion, the surface of the weld-plated steel pipe P was observed. As a result, the plating layer 11 was removed in the range of width 4.0 mm around the weld line, and plating was performed in the range of 4.0 mm on both sides of the weld line. Layer 11 was thin.
[0016]
A thermal spray repair layer 14 having a width of 10 mm was formed by double spraying shown in Table 1 on the welded portion from which the plating layer 11 was removed. As the Al—Mg alloy core wire, one having a diameter of 1.4 mm and containing 5% Mg was used. As the Al core wire, an Al wire rod having a diameter of 1.4 mm and a purity of 99.9% was used. In Comparative Example 2, a Zn sprayed layer was formed by single-step spraying, and in Comparative Example 3, a Zn sprayed layer was formed by double spraying. In each case, the core wire was heated with a frame of C ₂ H ₂ + O ₂ in each case, and was applied to the welded portion as a molten metal from the

spray guns

5 and 6.
[0017]

[0018]
A cylindrical test piece having a length of 150 mm was cut out from the weld-plated steel pipe P on which the thermal spray repair layer 14 was formed, and subjected to an accelerated corrosion test and a wet and dry combined corrosion test.
In the accelerated corrosion test, a 5% NaCl aqueous solution having a temperature of 35 ° C. was sprayed on the sprayed repair layer 14 of the test piece, and after spraying the salt water for a predetermined time, the surface of the welded portion was observed to measure the white rust generation area ratio.
In the wet and dry combined corrosion test, salt spray (35 ° C, 5% NaCl) 2 hours → dry (60 ° C, 30% RH) 4 hours → wet (50 ° C, 95% RH) 2 hours is one cycle, and the number of cycles After repeating, the surface of the weld was observed and the area ratio of red rust occurrence was measured.
[0019]
As seen in the test results of Table 2, the inventive examples 1 to 6 in which the double thermal spray repair layer 14 of Al-Mg alloy was formed had white rust as compared with the comparative examples 2 and 3 in which the Zn thermal spray layer was provided. Occurrence was greatly suppressed. The result of the wet and dry combined corrosion test was better than that of Comparative Example 1 in which a series of sprayed layers of Al-Mg alloy was provided. In addition, no red rust was observed after the salt spray was continued for a long time or after the dry / wet combined corrosion test was repeated 250 cycles, and the weld was maintained at the original beautiful surface. Furthermore, since the Mg content of the thermal spray repair layer 14 is suppressed to 20% by mass or less, the thermal spray repair layer 14 is not discolored, and even if the weld-plated steel pipe P is bent, the thermal spray repair layer 14 on the outer side of the bending is cracked. No peeling occurred.
[0020]

[0023]
【The invention's effect】
As described above, the weld-plated steel pipe of the present invention forms a sprayed repair layer of an Al—Mg alloy in the welded portion from which the plated layer has been removed by the bead cut after welding and in the vicinity of the welded portion. The thermal spray repair layer adheres firmly to the welded portion because it has a high affinity for Al and Mg with respect to the exposed underlying steel and the oxide film on the surface of the plated layer, and acts as a protective film with a high barrier function. Therefore, it is used as a welded steel pipe suitable for structural members, pipes, exhaust gas pipes, agricultural greenhouses, etc. that have no appearance of corrosion and white rust starting from the weld and have a beautiful appearance over the long term. The
[Brief description of the drawings]
[Fig. 1] Repair equipment for welded parts incorporated in a pipe making line for producing ERW steel pipes [Fig. 2] Schematic diagram showing a cross section of a welded portion with a thermal spray repair layer [Explanation of symbols]
S: plated steel strip P: welded steel pipe B: bead 1: high frequency coil 2: squeeze roll 3, 4: bead cutter 5, 6: thermal spray gun 7, 8: core wire for thermal spray 10: weld 11: plated layer 12: underlayer Steel 13: Weld metal 14: Thermal spray repair layer

Claims

Zn-Al-Mg when alloy plating layer to produce a welded steel pipe from the plated steel strip which is provided, after cutting the weld bead, Al-Mg alloy welds in the state of more than 450 ℃ → Al-Mg Welding characterized in that a sprayed repair layer of Al-Mg alloy is provided on the surface of the welded part from which the plating layer has been removed by bead cut by double spraying of alloy, Al-> Al-Mg alloy or Al-Mg alloy-> Al Of welded steel pipe with excellent corrosion resistance.