JP2007002328A - Plated welded steel pipe excellent in corrosion resistance at weld zone and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a plated welded steel pipe having such excellent durability that it does not cause corrosion and white rust even when used for a long period under severe corrosive environments. <P>SOLUTION: The plated welded steel pipe is made from a plated steel strip having a Zn plating layer, a Zn-Al alloy plating layer, or a Zn-Al-Mg alloy plating layer 14 and is provided with a thermally sprayed repair layer 17 on the surface of a welded area and its adjacent area of the underground steel 15 where the plating layer is removed by bead cut after welding. The thermally sprayed repair layer 17 is made from a three-layered metal formed by thermal spray in the order of Al, Zn, Al beginning from the lower layer. Excellent adhesion to the underground steel can be ensured by providing the thermally sprayed Al layer as the lowermost layer. The sacrificial corrosion-protecting action of the Zn provided as the second layer is fully utilized. The Al layer formed thereon can prevent the formation of white rust due to Zn. As a consequence, the plated welded steel pipe is free from the formation of white rust and red rust and therefore excellent in durability. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a welded steel pipe having improved corrosion resistance without generating white rust in a welded portion and a method for manufacturing the same.

Weld-plated steel pipes made from a steel strip provided with a Zn plating layer, Zn-Al alloy plating layer or Zn-Al-Mg alloy plating layer utilize the properties of the material that are excellent in corrosion resistance, It is used as various structural members, pipes, exhaust gas pipes, etc. that are exposed to corrosive environments. Recently, it is also being applied to the framework of agricultural greenhouses and cable racks for electrical wiring.
The weld-plated steel pipe is manufactured, for example, by a pipe making line as shown in FIG. After the plated steel strip 1 is formed into an open pipe shape by roll forming, the surfaces of both ends in the width direction of the plated steel strip 1 are heated by the high-frequency coil 2 and thermocompression bonded by the squeeze roll 4. Although both ends in the width direction are fused by thermocompression bonding to form the weld-plated steel pipe 3, a bead 7 protruding in the radial direction of the steel pipe is formed in the welded portion. Accordingly, the peripheral surface of the weld-plated steel pipe 3 is smoothed by cutting the bead 7 with the bead cutters 5 and 6. However, since the plating layer is also removed by bead cutting in the welded part and the vicinity of the welded part, the surface of the base steel is exposed.

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 part by two-spray spraying of Al spraying and then Zn spraying or Zn-Al alloy spraying (Patent Document 1). In this method, an Al spray gun 10 and then a Zn spray or Zn-Al alloy spray gun 11 are arranged along the conveying direction of the weld-plated steel pipe 3, and the Al core wire 13, Zn fed into the spray guns 10, 11 or Zn A Zn—Al alloy core wire is heated by a frame and is applied to the weld as a molten metal.

In addition, the present applicants may use either Al-Mg alloy single spraying, Al-Mg alloy spraying and Al-Mg alloy spraying, or Al-Mg alloy spraying after Al spraying, or Patent Document 2 proposes that a welded steel pipe having excellent corrosion resistance can be obtained by repairing the welded part and the vicinity of the welded part by two-spraying Al spraying after Al-Mg alloy spraying.
This method is characterized by using Al-Mg alloy alone or Al together as a spray metal, and the Mg content of the Al-Mg alloy used in that case is 20 mass% or less. It is preferable to ensure workability.
JP-A-8-127855 JP 2003-328105 A

The technique described in Patent Document 1 for repairing a welded part and a welded vicinity part by continuous spraying of Al spraying and then Zn spraying or Zn-Al alloy spraying on the welded part and the welded part where the base steel is exposed is a thermal sprayed layer. Corrosion of the base steel is suppressed by sacrificial anticorrosive action of Zn contained in the iron. However, on the other hand, it is difficult to suppress the occurrence of white rust, which is a phenomenon of sacrificial anticorrosive action.
White rust is often a mixture of Zn hydroxide and oxide, and is literally a white corrosion product. White rust is a symbol that Zn exhibits a sacrificial anticorrosive action, but when the welded steel pipe is used for a structure, it may be evaluated as detrimental to the beauty and landscape. For this reason, depending on the application, it is also required to suppress the occurrence of white rust as much as possible.

  In addition, Al-Mg alloy single spraying, Al-Mg alloy spraying and Al-Mg alloy spraying, or Al-Mg alloy spraying after Al spraying on the welded part where the base steel is exposed and in the vicinity of the welded part. In the technique of Patent Document 2 in which the welded part and the vicinity of the welded part are repaired by the two-part thermal spraying or the Al-Mg alloy thermal spraying and the two-part thermal spraying of Al, the Zn component is not included in the thermal sprayed layer. For this reason, depending on the mode of use, corrosion of the base steel may proceed starting from the pinhole defect formed in the base steel exposed part or the sprayed layer consisting of the unsprayed part.

  The present invention has been devised to solve such problems, and the thermal spraying of an Al-Zn-Al three-layer structure excellent in adhesion and corrosion resistance to the base steel and white rust resistance of the thermal spray layer itself. The purpose of the present invention is to provide a weld-plated steel pipe that has excellent durability with no occurrence of corrosion or white rust even when used in a severe corrosive environment for a long period of time by forming a repair layer on the welded part and the vicinity of the weld. To do.

  In order to achieve the object, a welded steel pipe excellent in corrosion resistance of the welded portion of the present invention is manufactured from a plated steel strip provided with a Zn plating layer, a Zn-Al alloy plating layer or a Zn-Al-Mg alloy plating layer. It is a welded steel pipe that has been piped, and a metal spray repair layer consisting of three layers of Al → Zn → Al in order from the lower layer is provided in the vicinity of the weld and the weld where the plating layer has been removed by the bead cut after welding. It is characterized by.

And when manufacturing a welded steel pipe from a plated steel strip provided with a Zn plated layer, a Zn-Al alloy plated layer or a Zn-Al-Mg alloy plated layer, the plated layer is removed after cutting the weld bead. It is manufactured by spraying in order of Al → Zn → Al from the lower layer in the vicinity of the welded portion and the welded portion and providing a thermal spray repair layer made of three layers of metal.
The thermal spray repair layer is formed in three layers in the order of Al → Zn → Al from the lower layer by triple spraying. It is desirable that Al is first deposited on the base steel, then Zn is deposited thereon, and that the outermost layer is formed so that Al covers Zn.

  Each layer of Al—Zn—Al is formed by three successive thermal sprays, and thus has three layers. If the thermal spray coating layer is relatively thick, for example, if it has a thickness of 20 μm or more, each layer of Al—Zn—Al is clearly formed in three layers, but each boundary surface is not necessarily a simple and clear surface. In general, an interface layer in which Al and Zn are mixed is formed. On the other hand, when the thermal spray coating layer is relatively thin, for example, when it is less than 20 μm, the boundary between Al and Zn tends to be interrupted, and a form in which Zn is dispersed in Al may occur. In the present specification, it is defined as a three-layer sprayed layer.

Mg may be added to the Al spraying applied as the first layer and the third layer described above for the purpose of further improving the corrosion resistance. For example, when spraying an Al—Mg alloy, the Mg content is preferably 20% by mass or less in order to ensure the workability of the welded steel pipe.
Similarly, Al may be added to the Zn spraying applied as the second layer. For example, when spraying a Zn—Al alloy, the Al content is preferably 30% by mass or less in consideration of the sacrificial anticorrosive effect of the pinhole portion and the workability of the welded steel pipe.

In the weld-plated steel pipe of the present invention, a repaired layer composed of an Al—Zn—Al triple sprayed layer is formed in the vicinity of the welded portion from which the plated layer has been removed by the bead cut after welding. The thermal spray repair layer adheres firmly to the weld due to the high affinity of Al to the exposed base steel of the weld and the oxide film on the surface of the plating layer, and suppresses corrosion of the base steel by sacrificial anticorrosive action against iron of Zn While having the effect, the third layer of Al also suppresses the occurrence of white rust and works as a protective film with a high barrier function.
Therefore, the weld-plated steel pipe of the present invention is free from the occurrence of corrosion and white rust starting from the welded part, and is a structural member, pipe, exhaust gas pipe, agricultural greenhouse, etc. that has a beautiful appearance over the long term. Preferably used.

In the weld-plated steel pipe of the present invention, a bead-cut welded portion and a thermal spray repair layer are provided in the vicinity of the welded portion. In the thermal spray repair apparatus for a welded portion as shown in FIG. 1, a third-stage spray gun 12 is additionally provided, and the Al core wire 13 is connected from the first-stage and third-stage spray guns 10 and 12 to the second stage. A Zn core wire is heated by a flame from the thermal spray gun 11 at the stage and is deposited as a molten metal on the welded portion and in the vicinity of the welded portion.
Although the weld bead and the plating layer 14 are removed by the bead cut after welding in the welded part and the vicinity of the welded part, the thermal spray repair layer 17 is formed on the exposed base steel 15 and the weld metal 16 (FIG. 2). In order to form the thermal spray repair layer 17 having high adhesion using thermal spray sensible heat, it is preferable to provide the thermal spray repair layer 17 in a welded portion in a high temperature state after welding. As described above, the thick spray repair layer 17 can be formed by triple spraying of Al → Zn → Al.

  The plating layer 14 includes a Zn base alloy plating layer such as a Zn plating layer, a Zn-5% Al plating layer, a Zn-6% Al-3% Mg plating layer, a Zn-55% Al alloy plating layer, and the like. When the open pipe of the plated steel strip 1 on which the plating layer 14 is formed is welded, a thin oxide film is formed on the surface of the plating layer 14 heated by welding heat. A thin oxide film is formed on the exposed surface of the base steel 15 in the welded portion where the plating layer 14 is removed by bead cutting and in the vicinity of the welded portion.

The first layer of Al sprayed on the weld is a metal having an oxygen affinity greater than that of iron. The Al sprayed in the molten state reacts with the exposed underlying steel 15 and the oxide film on the surface of the plating layer 14, and combines with oxygen in the oxide film by a strong reducing action to form Al oxide.
Whether the Al oxide is replaced with the oxide film on the surface layer of the exposed base steel 15 and the plating layer 14 or dispersed in the sprayed molten Al, the sprayed Al as the first layer is Strongly coupled with the base steel.

Zn which is the second layer of thermal spraying exists between Al of the first layer and Al of the third layer. Vapor defects such as vacancies and shrinkage cavities inevitably occur in the sprayed coating, but Zn exerts a sacrificial anticorrosive action against iron in such unavoidable void defects in the Al layer, and corrodes the underlying steel. Suppress.
Al which is the third layer of thermal spraying prevents the second Zn layer from being directly exposed to the corrosive atmosphere, and exerts the action of suppressing the generation of white rust which is one end of the sacrificial anticorrosive action of Zn. . For this reason, it is desirable that Al sprayed on the Zn layer is formed so as to cover the entire surface of Zn.

The thermal spray repair layer 17 is preferably formed with a film thickness of 10 μm or more. When the thermal spray repair layer is thick, the occurrence of void defects is suppressed and the bead-cut welded part and the vicinity of the welded part are easily covered with the thermal sprayed repair layer, which improves the corrosion resistance near the welded part and the welded part. Be certain. However, a thick film exceeding 30 μm not only wastes the spray metal, but also adversely affects the adhesion of the spray repair layer 17 to the base steel 15.
Since the welded portion and the vicinity of the welded portion are covered with the thermal spray repair layer 17 having excellent adhesion and corrosion resistance, the thermal spray repair layer 17 is formed even after the welded steel pipe 1 is processed into a product shape by processing such as bending, pipe expansion, and contraction. It is maintained in a healthy state, and the occurrence of corrosion and white rust starting from the weld is suppressed. In particular, when the Mg content in the Al sprayed layer of the thermal spray repair layer 17 is set to 20% by mass or less, the workability of the thermal spray repair layer 17 is ensured, and cracks, peeling, and the like occur in the thermal spray repair layer 17 during the molding process. It is prevented. Regulating the Mg content to 20% by mass or less is also effective for preventing discoloration. Further, when the Al content in the Zn sprayed layer of the thermal spray repair layer 17 is set to 30% by mass or less, the processability of the thermal spray repair layer 17 is ensured, and cracks, delamination, etc. occur in the thermal spray repair layer 17 during the molding process. Is prevented. If it is contained at this level, the pinhole generation state during Zn spraying is not affected.

  After the thermal spray repair layer 17 is formed, the welded portion and the vicinity of the welded portion 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 an anticorrosive additive.

A plated steel strip 1 having a plate thickness of 1.2 mm and hot-plated with a Zn-6% Al-3% Mg alloy at a basis weight of 90 g / m ² was formed on a steel pipe having a diameter of 25.4 mm by a high-frequency pipe forming line. In the welded portion, an outer surface bead that protrudes up to 1.5 mm from the peripheral surface of the weld-plated steel pipe 3 was formed. After cutting the bead protrusion, the surface of the weld-plated steel pipe 3 was observed. As a result, the plating layer 14 was removed in a width range of 4.0 mm around the weld line, and plating was performed in a range of 3.0 mm on both sides of the weld line. Layer 14 was thin.

A repair device provided with triple thermal spray guns 10, 11, and 12 behind the cutting tool shown in FIG. 1 is used in the triple thermal spraying shown in Table 1 in the welded part from which the plating layer 14 has been removed and in the vicinity of the welded part. A thermal spray repair layer 17 having a width of 10 mm was formed. The sprayed metal core wire with a diameter of 1.4 mm was used, and the thickness of the sprayed repair layer was changed by adjusting the spraying conditions.
As the Al core wire, an Al wire rod having a diameter of 1.4 mm and a purity of 99% and an Al—Mg alloy wire rod having a diameter of 1.4 mm and containing 5% Mg were used. As the Zn core wire, a Zn wire having a diameter of 1.4 mm and a purity of 99.9% and a Zn-Al alloy wire having a diameter of 1.4 mm and containing 5% Al were used. In each case, the core wire was heated with a C ₂ H ₂ + O ₂ frame in each case, and was applied to the welded portion as a molten metal from the thermal spray guns 10, 11, and 12.

A cylindrical test piece having a length of 150 mm was cut out from the weld-plated steel pipe 1 on which the thermal spray repair layer 17 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 thermal spray repair layer 17 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.
The results are shown in Table 2.

As can be seen from the test results in Table 2, all of the examples of the present invention subjected to the triple thermal spraying of Al—Zn—Al have good initial white rust resistance and red rust resistance after a long-term wet and dry combined corrosion test. Has been obtained. However, No. 4 of the example of the present invention in which the thickness of the thermal spray repair layer was less than 10 μm was slightly inferior to the other examples of the present invention in both white rust resistance and red rust resistance.
No. 6, which is a comparative example, was obtained by performing triple thermal spraying of only Al. However, although white rust resistance is sufficient, it does not contain Zn that exhibits sacrificial anticorrosive action in the pinhole portion or the like. The result was inferior red rust resistance. Similarly, in No. 7, since Zn was exposed on the surface of the thermal spray repair layer, the white rust resistance was remarkably inferior. No.8, which was sprayed with Zn after coating without coating Al on the lower layer, had lower adhesion to the base compared to the case without Al lower layer, so it was resistant to white rust and red rust. Both were not enough.
Further, in Comparative Examples Nos. 9 to 13 in which Al series double spraying was performed, sufficient red rust resistance was not obtained even if the thickness of the thermal spray repair layer was increased.

Repair equipment for welded parts incorporated in a pipe making line to manufacture ERW steel pipes Schematic showing the cross-section of the weld zone with the thermal spray repair layer Enlarged view of thermal spray repair department

Claims (6)

  This is a weld-plated steel pipe formed from a plated steel strip provided with a Zn plating layer, Zn-Al alloy plating layer or Zn-Al-Mg alloy plating layer, and the plating layer is removed by bead cutting after welding. A weld-plated steel pipe excellent in welded portion corrosion resistance, characterized in that a welded layer and a welded layer of metal consisting of three layers of Al → Zn → Al are provided in the vicinity of the welded portion in the vicinity of the welded portion.   The weld-plated steel pipe having excellent weld corrosion resistance according to claim 1, wherein Al in the first layer and / or the third layer contains 20% by mass or less of Mg.   The weld-plated steel pipe excellent in welded portion corrosion resistance according to claim 1 or 2, wherein Zn of the second layer contains 30% by mass or less of Al.   When a welded steel pipe is manufactured from a plated steel strip provided with a Zn plated layer, a Zn-Al alloy plated layer or a Zn-Al-Mg alloy plated layer, the weld bead is cut and then the plated layer is removed. A method for producing a weld-plated steel pipe excellent in weld corrosion resistance, characterized in that a thermal spray repair layer composed of three layers of metals is sprayed in the order of Al → Zn → Al from the lower layer in the vicinity of the weld and weld.   The method for producing a weld-plated steel pipe excellent in welded portion corrosion resistance according to claim 4, wherein Al sprayed as the first layer and / or the third layer contains 20% by mass or less of Mg.   The method for producing a weld-plated steel pipe excellent in weld corrosion resistance according to claim 4 or 5, wherein Zn sprayed as the second layer contains 30% by mass or less of Al.

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