steel pipe

Abstract

Ensuring corrosion resistance of the welded area and its vicinity, even in highly corrosive environments. [Solution] The steel pipe according to the present invention includes, in any cross-section, a first region where a first coating layer containing predetermined components exists, a second region where a second coating layer containing predetermined components exists, and a third region located between the first and second regions where a third coating layer exists. The third coating layer has a metallic structure consisting of phase A which is unevenly distributed at the interface of the third coating layer on the steel substrate side, phase B which is located on the steel substrate or on phase A, and phase C which contains a predetermined amount of Al, and phase B has phase B' which contains a predetermined amount of Mg. The area ratio of phase B' in the cross-section of phase B is 3.0 to 30.0% of the area of ​​phase B, and in the third region, the length of the region where the thickness of the third coating layer is less than 5.0 μm is less than 100.0 μm, and the proportion occupied by the region with a thickness of less than 5.0 μm is 40.0% or less in total with respect to the total length of the third coating layer.

Description

[Technical Field]

[0001] This invention relates to steel pipes. [Background technology]

[0002] Steel pipes that have been subjected to various zinc-based plating processes, such as hot-dip galvanizing, zinc-alloy plating, and zinc-alloy plating (including zinc-alloy-magnesium alloy plating), exhibit excellent corrosion resistance and are therefore widely used as materials for civil engineering and construction components, as well as for automotive parts and other materials.

[0003] There are several methods for manufacturing such plated steel pipes. One such method involves bending steel sheets coated with various zinc-based platings (so-called zinc-plated steel sheets) to form a cylindrical shape, and then welding the ends of the steel sheets together.

[0004] In the method of bending and welding plated steel sheets as described above, the welded portion may protrude above the surface of the plated steel sheet. Therefore, conventional methods have been employed to grind away these protruding portions. While this grinding process eliminates the protrusion of the welded portion, it also removes the plating layer near the welded portion, exposing the surface of the steel sheet. Since the exposed areas of the steel sheet have lower corrosion resistance than areas where the plating layer is properly maintained, it is common practice to repair the welded portion and the exposed area by thermal spraying with metallic aluminum (Al) or metallic zinc (Zn) to ensure the corrosion resistance of the plated steel pipe.

[0005] However, if the layer formed by thermal spraying is prone to detaching from welds, etc., then when the plated steel pipe is processed into the desired shape, the layer formed by thermal spraying may detach, making it impossible to guarantee the corrosion resistance of the various products manufactured. Therefore, it is important to ensure adhesion between the welds, etc., and the layer formed by thermal spraying. From this perspective, various technologies have been proposed to date.

[0006] For example, Patent Document 1 discloses a technique for improving the adhesion of a repaired area by sequentially forming a first thermal spray layer using an Al-Si alloy and a second thermal spray layer using a Zn-Al alloy on the upper layer of a welded joint when welding a hot-dip zinc-aluminum alloy plated steel material.

[0007] Furthermore, Patent Document 2 discloses a technique for ensuring corrosion resistance and workability of welded areas in welded galvanized steel pipes manufactured from plated steel strips having a Zn-Al-Mg alloy plating layer, by providing an Al-Mg alloy thermal spray repair layer in the welded area and its vicinity where the plating layer has been removed by bead cutting after welding.

[0008] Furthermore, Patent Document 3 discloses a technique for repairing the weld bead cutting portion of an electric resistance welded steel pipe having an Al plating layer, a Zn plating layer, or an alloy plating layer containing at least one of Al and Zn on its surface. This is achieved by performing arc spraying to form an arc sprayed layer, and then performing gas flame spraying on the arc sprayed layer to form a gas flame sprayed layer. Patent Document 3 also discloses an embodiment in which the alloy plating layer contains Al, Zn, and Mg, and an embodiment in which the arc sprayed layer contains Al and the gas flame sprayed layer contains Al and Zn. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2012-107324 [Patent Document 2] Japanese Patent Publication No. 2007-191800 [Patent Document 3] Japanese Patent Publication No. 2021-123781 [Overview of the initiative] [Problems that the invention aims to solve]

[0010] Zn-Al-Mg alloy plating, a type of zinc-based plating, is often used in highly corrosive environments such as underwater, in soil, and in concrete due to its high corrosion resistance. Therefore, in steel pipes manufactured from plated steel sheets with a Zn-Al-Mg alloy plating layer, the welded joints and other repaired areas formed during pipe manufacturing must also possess the same high corrosion resistance as the Zn-Al-Mg alloy plating layer.

[0011] In steel pipes with a Zn-Al-Mg alloy plating layer, due to the high corrosion resistance described above, the difference between the corrosion resistance of the Zn-Al-Mg alloy plating layer and the corrosion resistance of the repair layer formed on welds and repaired areas tends to be large. When the difference in corrosion resistance between the alloy plating layer and the repair layer is large, there is a concern that corrosion of the repair layer will progress more easily in highly corrosive environments. Therefore, it is preferable that the difference between the corrosion resistance of the alloy plating layer and the corrosion resistance of the repair layer be as small as possible.

[0012] The inventors' investigations revealed that even if welded joints are repaired using the techniques disclosed in Patent Documents 1 to 3, there is room for improvement in the differences in corrosion resistance, especially in highly corrosive environments such as concrete.

[0013] Therefore, the present invention has been made in view of the above problems, and the object of the present invention is to provide a steel pipe that can ensure corrosion resistance of repaired parts such as welded joints and their vicinity, even in highly corrosive environments. [Means for solving the problem]

[0014] To solve the above problems, the inventors conducted diligent research and found that it is possible to appropriately create a region in which Zn and Al exist in a specific state, as detailed below, between the welded portion of the steel pipe and the portion where the Zn-Al-Mg alloy plating layer exists. By creating such a specific region, it becomes possible to improve the difference in corrosion resistance between the repaired portion of the welded portion and the alloy plating layer. Based on these findings, the gist of the present invention is as follows:

[0015] (1) In any cross-section obtained by cutting the steel pipe in the radial direction perpendicular to the longitudinal direction, there is a first region in which a first coating layer exists that contains, by mass%, Al: 15.0~30.0% and Mg: 5.0~15.0%, with the remainder being Zn and impurities, and the thickness of the first coating layer is 5.0 μm or more, and this region extends continuously along the circumferential direction of the steel pipe for 1.0 mm or more; and an Al layer containing, by mass%, Zn: 0~20.0%, with the remainder being Al and impurities, and on the Al layer A second coating layer exists, located between the first region and the second region, and having a Zn layer, the Al layer having an average thickness of 10.0 to 100.0 μm and the Zn layer having an average thickness of 3.0 to 50.0 μm, wherein a region of the second coating layer having a thickness of 15.0 μm or more extends continuously along the circumferential direction for 500.0 μm or more, and a third coating layer mainly composed of Zn or Al. A third region in which exists, and the third coating layer comprises, as a metallic structure, phase A containing 0.1 to 40.0 mass% Al and 0.1 to 10.0 mass% Mg, with the remainder being Zn and impurities, and which is unevenly distributed at the interface portion on the steel substrate side of the third coating layer; phase B containing 0 to 20.0 mass% Zn, with the remainder being Al and impurities, and which is located on the steel substrate or on phase A; and phase C containing 0.1 to 20.0 mass% Al, with the remainder being Zn and impurities, and which is located on phase B, and further, A steel pipe wherein phase B has phase B' containing 0.1 to 10.0 mass% of Mg, the area ratio of phase B' in the cross-section of phase B is 3.0 to 30.0% of the area of ​​phase B, and in the third region, the length along the circumferential direction of the region where the thickness of the third coating layer is less than 5.0 μm is less than 100.0 μm, and the proportion occupied by the region with a thickness of less than 5.0 μm is 40.0% or less in total with respect to the total length along the circumferential direction of the third coating layer. (2) In any cross-section obtained by cutting the steel pipe in the radial direction perpendicular to the longitudinal direction, there is a first coating layer containing, by mass%, Al: 15.0~30.0%, Mg: 5.0~15.0%, and further containing one or more elements selected from the group consisting of element groups A to G below, with the remainder being Zn and impurities, and the thickness of the first coating layer is 5.0 μm or more, and there is a region of 1.0 mm or more that extends continuously along the circumferential direction of the steel pipe, and a first region containing, by mass%, Zn: 0~20.0%, and further selected from the group consisting of element groups H to J below A second coating layer exists, comprising an Al layer containing one or more of the following elements, with the remainder being Al and impurities, and a Zn layer located on the Al layer, containing Al: 0.1 to 20.0% by mass, and further containing one or more elements selected from the group consisting of element group H to element group J, with the remainder being Zn and impurities, wherein the average thickness of the Al layer is 10.0 to 100.0 μm, and the average thickness of the Zn layer is 3.0 to 50.0 μm, and a region of the second coating layer with a thickness of 15.0 μm or more exists continuously along the circumferential direction for 500.0 μm or more. The material comprises two regions, a third region located between the first region and the second region, and having a third coating layer mainly composed of Zn or Al, and further containing one or more elements selected from the group consisting of elements A to J, wherein the third coating layer has a metallic structure consisting of 0.1 to 40.0 mass% Al and 0.1 to 10.0 mass% Mg, with the remainder being Zn and impurities, and is unevenly distributed at the interface of the third coating layer on the steel substrate side, and a phase B containing 0 to 20.0 mass% Zn, with the remainder being Al and impurities, and is located on the steel substrate or on phase A, and contains 0.1 to 20 The third region comprises a phase C located on the phase B, containing 0.0 mass% of Mg, with the remainder being Zn and impurities. Furthermore, the phase B has a phase B' containing 0.1 to 10.0 mass% of Mg, the area ratio of the phase B' in the cross-section of the phase B is 3.0 to 30.0% of the area of ​​the phase B, and in the third region, the length along the circumferential direction of the region where the thickness of the third coating layer is less than 5.0 μm is less than 100.0 μm, and the proportion occupied by the region with a thickness of less than 5.0 μm is 40% of the total length along the circumferential direction of the third coating layer.A steel pipe with a content of 0% or less. [Element group A]: One or two selected from the group consisting of Si: 2.50% or less and Fe: 5.00% or less [Element group B]: One or more selected from the group consisting of Sb: 0.50% or less, Pb: 0.50% or less, and Sr: 0.50% or less [Element group C]: One or more selected from the group consisting of Cu: 0.25% or less, Ti: 0.25% or less, Cr: 0.25% or less, Nb: 0.25% or less, Ni: 0.25% or less, Mn: 0.25% or less, Mo: 0.25% or less, Co: 0.25% or less, and V: 0.25% or less [Element group D]: One or more selected from the group consisting of Sn: 2.00% or less, Bi: 0.50% or less, and In: 2.00% or less [Element group E]: One or more selected from the group consisting of Zr: 0.25% or less, Ag: 0.25% or less, and Li: 0.25% or less [Element group F]: One or more selected from the group consisting of Ca: 3.00% or less, La: 0.50% or less, Ce: 0.50% or less, and Y: 0.5% or less [Element group G]: B: 0.50% or less [Element group H]: One or more selected from the group consisting of C: 0.300% or less, S: 0.010% or less, and N: 0.006% or less [Element group I]: Fe: 5.00% or less [Element group J]: One or two selected from the group consisting of Mn: 1.50% or less and Ti: 0.10% or less (3) The total length of the third coating layer along the circumferential direction is 500 - 1500 μm in total. The steel pipe according to (1) or (2). (4) At the interface between the third coating layer and the steel substrate in the cross-section, the total length of the portion corresponding to the phase A at the position of the interface along the circumferential direction is 30 - 60% with respect to the total length of the interface along the circumferential direction. The steel pipe according to (3). (5) The average thickness of the third coating layer is 10 - 150 μm. The steel pipe according to (1) or (2). (6) The steel pipe is a welded galvanized steel pipe, and the second coating layer is located on the welded portion of the welded galvanized steel pipe, as described in (1) or (2). (7) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group A. (8) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group B. (9) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group C. (10) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group D. (11) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group E. (12) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group F. (13) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group G. (14) The steel pipe according to (2), wherein at least one of the second coating layer or the third coating layer has a chemical composition containing the element group H. (15) The steel pipe according to (2), wherein at least one of the second coating layer or the third coating layer has a chemical composition containing the element group I. (16) The steel pipe according to (2), wherein at least one of the second coating layer or the third coating layer has a chemical composition containing the element group J. [Effects of the Invention]

[0016] As described above, the present invention makes it possible to ensure corrosion resistance of the welded area and its vicinity even in highly corrosive environments. [Brief explanation of the drawing]

[0017] [Figure 1] This is a schematic cross-sectional view showing a cross-section obtained by cutting a steel pipe according to an embodiment of the present invention in the radial direction perpendicular to the longitudinal direction. [Figure 2] This is an explanatory diagram illustrating the first region having a first coating layer in a steel pipe according to the same embodiment. [Figure 3] This is an explanatory diagram illustrating the second region having a second coating layer in the steel pipe according to the same embodiment. [Figure 4] This is an explanatory diagram illustrating the third region having a third coating layer in a steel pipe according to the same embodiment. [Figure 5] This is an explanatory diagram illustrating the third region having a third coating layer in a steel pipe according to the same embodiment. [Modes for carrying out the invention]

[0018] Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. In this specification and drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant descriptions will be omitted.

[0019] (Regarding steel pipes) In the following, a steel pipe according to an embodiment of the present invention will be described in detail with reference to Figures 1 to 5.

[0020] <Regarding the overall structure of steel pipes> Figure 1 is a schematic cross-sectional view showing a cross-section obtained by cutting the steel pipe 1 according to this embodiment in the radial direction perpendicular to the longitudinal direction. For convenience, the following explanation will refer to the XYZ coordinate system shown in Figure 1, and will use this coordinate system as needed. In this coordinate system, the Z-axis direction is parallel to the longitudinal direction (pipe axis direction) of the steel pipe 1 according to this embodiment, and the X-axis and Y-axis directions are perpendicular to the Z-axis direction, respectively. Furthermore, the steel pipe in this embodiment may be not only tubular but also rectangular or irregularly shaped, and its cross-sectional shape is not limited. In the following, a tubular steel pipe with a circular cross-sectional shape will be described as an example.

[0021] The steel pipe 1 according to this embodiment is suitably used as a material for manufacturing various components such as solar power generation panel mounting frames and other structures, automobile parts, various building materials, pillars, signs, traffic lights, guardrails, etc. As schematically shown in Figure 1, the steel pipe 1 according to this embodiment has a raw steel pipe 10 and a coating layer 20 provided on the outer surface of the raw steel pipe 10.

[0022] The coating layer 20 of the steel pipe 1 according to this embodiment is a layer provided to ensure the corrosion resistance of the steel pipe 1 according to this embodiment, and has a specific configuration as detailed below. By having such a coating layer 20, the steel pipe 1 according to this embodiment exhibits excellent corrosion resistance even in highly corrosive environments such as concrete.

[0023] The steel pipe 1 according to this embodiment is manufactured by bending a plated steel sheet, which has a Zn-Al-Mg alloy plating layer formed on a base steel sheet, to form a cylindrical shape, welding the parts where the ends of the steel sheets butt together, and then performing post-processing such as grinding or repair on the protruding parts resulting from the welding.

[0024] In this embodiment, the size (outer diameter) of the steel pipe 1 is, for example, within the range of 10 to 160 mm.

[0025] Further details regarding the base steel pipe 10 and the coating layer 20 described above will be explained in more detail below.

[0026] <Regarding the unpainted steel pipe 10> As schematically shown in Figure 1, the base material of the steel pipe 1 according to this embodiment, the raw steel pipe 10, is composed of a steel base 11 and a welded joint 13.

[0027] Regarding steel substrate 11: The steel base material 11 in the base steel pipe 10 corresponds to the base steel sheet portion of the Zn-Al-Mg alloy plated steel sheet, which is the material of the steel pipe 1 according to this embodiment. The steel base material 11 according to this embodiment is not particularly limited, and various types of steel materials can be used depending on the mechanical strength (e.g., tensile strength) required for the steel pipe 1. Examples of such steel materials include various types of Al-killed steel, ultra-low carbon steel containing Ti, Nb, etc., high-strength steel further containing reinforcing elements such as P, Si, Mn in ultra-low carbon steel, and various other steel materials containing various components (Cr, N, Cu, B, Ni, Mg, Ca, V, Co, Zn, As, Y, Zr, Mo, Sn, Sb, Ta, W, Pb, Bi, REM, etc.).

[0028] Furthermore, a pre-plating layer may be provided on the surface of the steel substrate 11. Examples of metals that can be used for the pre-plating layer include Ni, Sn, or alloys combining these elements. Using a pre-plated steel sheet with these platings already applied eliminates unplated areas (areas where the plating metal is repelled by oxide films, etc.) when performing Zn-Al-Mg plating on the steel sheet. This is presumed to be because, when the pre-plated steel sheet is immersed in the molten plating bath, the metal elements in the molten plating bath react with the pre-plating layer to form Ni(Sn)-Al-Fe-Zn. The amount of pre-plating layer deposited per side is 0.2~2.0 g / m². 2 It is preferable that the amount of pre-plating layer is 0.2 g / m². 2 By doing so, it becomes possible to reliably achieve the pre-plating effect that suppresses non-plating. Also, the amount of pre-plating layer to adhere is 2.0 g / m 2 By doing the following, it becomes possible to achieve the above-mentioned non-plating suppression effect while preventing the dissolution of Fe by the pre-plating layer from being suppressed and thus making it difficult for Fe-Zn composite oxides to be formed.

[0029] Furthermore, the thickness of the steel substrate 11 is not particularly limited and can be set appropriately according to the mechanical strength and other requirements for the steel pipe 1 according to this embodiment.

[0030] Regarding the welded section 13: The welded portion 13 in the raw steel pipe 10 according to this embodiment is formed by bending a Zn-Al-Mg alloy plated steel sheet, which is the material of the steel pipe 1 according to this embodiment, into a cylindrical shape, and then welding the portion where the end faces of the steel sheets are joined together (butt joint). Here, the welding method used when welding the above-mentioned butt joint is not particularly limited, and various welding methods such as electric resistance welding including high-frequency welding, arc welding, laser welding, etc. can be used.

[0031] The components of the welded joint 13 vary depending on the chemical composition of the steel substrate 11, the chemical composition of the Zn-Al-Mg alloy plating layer provided on the steel substrate 11, and the chemical composition of the welding wire used as needed during welding. For example, the components of the welded joint 13 are generally mainly oxides of easily oxidized elements among the various elements that make up the steel substrate 11, the Zn-Al-Mg alloy plating layer, and the welding wire.

[0032] Here, in the cross-section of the steel pipe 1 according to this embodiment (the cross-section obtained when the pipe is cut radially so as to be perpendicular to the longitudinal direction of the steel pipe 1 (the Z-axis direction in Figure 1)), the length of the welded portion 13 along the circumferential direction of the steel pipe 1 is not specifically defined, but is generally 30 mm or less, regardless of the outer diameter of the steel pipe 1. Here, "circumferential direction" in this embodiment refers to the direction shown in Figure 2, which is perpendicular to the longitudinal direction of the steel pipe (the Z-axis direction in Figure 1, i.e., the imaginary line passing through the center of the cross-section of the steel pipe) and along the surface of the steel pipe.

[0033] Furthermore, when identifying the portion corresponding to the welded joint 13 in the cross-section of the steel pipe 1 of interest, it can be easily visualized by etching using an etching solution. For example, as an etching solution, it is possible to use Nital (composition: 95% ethanol, 5% sulfuric acid) or an etching solution prepared by mixing 60g of sodium dodecylbenzenesulfonate, 36g of picric acid, 60cc of ethanol, and 60cc of household detergent solution (for example, a common type such as dish soap) with 2400cc of water.

[0034] <Regarding the coating layer 20> The coating layer 20 of the steel pipe 1 according to this embodiment is a layer located on the outer surface of the base steel pipe 10 as described above, and is provided to ensure the corrosion resistance of the steel pipe 1. As schematically shown in Figure 1, the coating layer 20 is composed of three regions: a first region 21 that occupies most of the outer surface of the steel pipe 1, a second region 23 that is provided to at least cover the welded portion 13 of the base steel pipe 10, and a third region 25 located between the first region 21 and the second region 23.

[0035] The first region 21 of the coating layer 20 is the region in which the Zn-Al-Mg alloy plating layer that was present on the Zn-Al-Mg alloy plated steel sheet, which is the material of the steel pipe 1 according to this embodiment, remains even after it has become a steel pipe. In this first region 21, as will be described in detail below, there is a first coating layer that corresponds to the Zn-Al-Mg alloy plating layer of the plated steel sheet that served as the material.

[0036] Furthermore, the second region 23 of the coating layer 20 is a region where a layer exists that ensures the corrosion resistance of the welded portion 13, formed by a repair process after grinding the protruding parts generated during the manufacture of the steel pipe 1. In this second region 23, as will be described in detail below, there is a second coating layer formed by a repair process using, for example, thermal spraying.

[0037] Furthermore, the third region 25 of the coating layer 20 corresponds to the area where a portion of the Zn-Al-Mg alloy plating layer was removed during the grinding and repair processes when manufacturing the steel pipe 1. This third region 25 is the region where the third coating layer exists, formed by a reaction between the components of the remaining Zn-Al-Mg alloy plating layer and the components of the layer formed by the repair process (second coating layer) during the process for forming the second coating layer (thermal spraying, as detailed below).

[0038] The first to third regions 25 that constitute the coating layer 20 will be described in detail below. Note that in Figure 1, the illustration of the coating layer that may exist on the inner surface of the steel pipe 1 according to this embodiment is omitted. However, it is preferable that on the inner surface of the steel pipe 1 according to this embodiment, a first coating layer derived from a Zn-Al-Mg alloy plating layer, as detailed below, exists on the surface of the portion corresponding to the steel substrate 11.

[0039] <Regarding Area 1, Item 21> As described above, the first region 21 in this embodiment is a region in which the Zn-Al-Mg alloy plating layer, which was present in the Zn-Al-Mg alloy plated steel sheet that is the material of the steel pipe 1 in this embodiment, remains even after it has become a steel pipe. Below, this first region 21 will be described in detail with reference to Figures 1 and 2. Figure 2 is an enlarged view of the area near the boundary between the first region 21 and the third region 25 in Figure 1, and is an explanatory diagram for describing the first region 21 having the first coating layer 211 in the steel pipe according to this embodiment.

[0040] As shown in Figure 2, a first coating layer 211 is present in the first region 21. The first coating layer 211 contains, by mass%, Al: 15.0-30.0%, Mg: 5.0-15.0%, with the remainder being Zn and impurities. The first coating layer 211 has a thickness of 5.0 μm or more, and these regions extend continuously along the circumferential direction of the steel pipe 1 for a distance of 1.0 mm or more.

[0041] Regarding the method for identifying Area 1, Section 21: In any cross-section of the steel pipe 1 according to this embodiment, cut perpendicular to the diameter direction (i.e., perpendicular to the longitudinal direction of the pipe), as shown in Figure 1, the portion corresponding to the first region 21 can be easily identified by observing the cross-section of the steel pipe 1 of interest with a SEM equipped with an energy dispersive X-ray detector (EDX) (hereinafter abbreviated as "SEM-EDX") and performing elemental mapping analysis. In such elemental mapping analysis, it is sufficient to focus on at least the elements Zn, Al, and Mg, or all elements may be considered.

[0042] More specifically, in the elemental mapping analysis using SEM-EDX as described above, the region containing the Zn phase, Zn / Al phase, and Zn / Al / MgZn2 ternary eutectic phase can be identified as the region corresponding to the first region 21.

[0043] ≪Regarding the chemical composition of the first coating layer 211≫ The chemical composition of the first coating layer 211 according to this embodiment, in one embodiment, is as described above, containing Al: 15.0 to 30.0% and Mg: 5.0 to 15.0% by mass, with the remainder being Zn and impurities.

[0044] Furthermore, according to another embodiment, the chemical composition of the first coating layer 211 in this embodiment contains, by mass%, Al: 15.0 to 30.0%, Mg: 5.0 to 15.0%, and further contains one or more elements selected from the group consisting of element group A, element group B, element group C, element group D, element group E, element group F, and element group G, with the remainder being Zn and impurities.

[0045] [Element Group A]: One or two elements selected from the group consisting of Si: 2.50% or less and Fe: 5.00% or less. [Element Group B]: One or more elements selected from the group consisting of Sb: 0.50% or less, Pb: 0.50% or less, and Sr: 0.50% or less. [Element group C]: One or more elements selected from the group consisting of Cu: 0.25% or less, Ti: 0.25% or less, Cr: 0.25% or less, Nb: 0.25% or less, Ni: 0.25% or less, Mn: 0.25% or less, Mo: 0.25% or less, Co: 0.25% or less, V: 0.25% or less. [Element Group D]: One or more elements selected from the group consisting of Sn: 2.00% or less, Bi: 0.50% or less, and In: 2.00% or less. [Element Group E]: One or more elements selected from the group consisting of Zr: 0.25% or less, Ag: 0.25% or less, and Li: 0.25% or less. [Element Group F]: One or more elements selected from the group consisting of Ca: 3.00% or less, La: 0.50% or less, Ce: 0.50% or less, and Y: 0.5% or less. [Element group G]:B:0.50% or less

[0046] [Al:15.0~30.0% by mass] Al is an element necessary to constitute the main metal structure (Zn-Al-Mg metal structure) in the first coating layer 211 according to this embodiment, and is included in a predetermined amount or more to ensure corrosion resistance as a plated steel pipe. If the Al content in the first coating layer 211 is less than 15.0% by mass, the above-mentioned corrosion resistance cannot be guaranteed. Therefore, in the first coating layer 211 according to this embodiment, the Al content is 15.0% by mass or more. The Al content is preferably greater than 15.0% by mass, more preferably 16.0% by mass or more, even more preferably 17.0% by mass or more, and even more preferably 18.0% by mass or more. By having an Al content within the above range, it is possible to ensure corrosion resistance as a plated steel pipe.

[0047] On the other hand, if the Al content in the first coating layer 211 exceeds 30.0% by mass, the Al phase that functions as a cathode when placed in a corrosive environment increases excessively, making the corrosion of the steel pipe 1 more likely to progress, and thus the corrosion resistance of the plated steel pipe cannot be guaranteed. For this reason, in the first coating layer 211 according to this embodiment, the Al content is 30.0% by mass or less. The Al content is preferably 25.0% by mass or less, more preferably less than 25.0% by mass, and even more preferably 20.0% by mass or less.

[0048] [Mg:5.0~15.0% by mass] Mg is an element necessary to constitute the main metal structure (Zn-Al-Mg metal structure) in the first coating layer 211 according to this embodiment, and is included in a predetermined amount or more to ensure corrosion resistance as a plated steel pipe. If the Mg content in the first coating layer 211 is less than 5.0% by mass, the above-mentioned corrosion resistance cannot be guaranteed. Therefore, in the first coating layer 211 according to this embodiment, the Mg content is 5.0% by mass or more. The Mg content is preferably greater than 5.0% by mass, more preferably 5.5% by mass or more, and even more preferably 6.0% by mass or more. By having an Mg content within the above range, it is possible to ensure corrosion resistance as a plated steel pipe.

[0049] On the other hand, if the Mg content in the first coating layer 211 exceeds 15.0% by mass, the anode dissolution of the first coating layer 211 is more likely to progress when placed in a corrosive environment, which may make it impossible to guarantee the corrosion resistance of the plated steel pipe. Therefore, in the first coating layer 211 according to this embodiment, the Mg content is 15.0% by mass or less. The Mg content is preferably less than 12.5% ​​by mass, and more preferably 12.0% by mass or less. By keeping the Mg content within the above range, it is possible to guarantee the corrosion resistance of the plated steel pipe.

[0050] In the first coating layer 211 according to this embodiment, the remainder of the Al and Mg consists of Zn and impurities. Zn is an element necessary for constituting the main metal structure (Zn-Al-Mg metal structure) in the first coating layer 211 according to this embodiment, and is an important element for improving the corrosion resistance of the plated steel pipe. Furthermore, by containing Al and Mg within the above ranges, and also containing Zn, the corrosion resistance required for plated steel pipes can be ensured.

[0051] Next, in a preferred arrangement of the first coating layer 211 according to another embodiment of this product, the element groups A to G that the chemical composition of the first coating layer 211 may have will be described in detail.

[0052] In addition, in the first coating layer 211 according to another embodiment of this embodiment, if at least one of the elements belonging to element groups A to G below is included, it is preferable that at least one of the elements belonging to element groups A to G below is included within the following content range, and the total content is 21.50% by mass or less.

[0053] By keeping the total content of elements belonging to element groups A to G to 21.50% by mass or less, it becomes possible to enjoy the effects exhibited by the addition of each element, as detailed below, without interfering with each other. The total content of elements belonging to element groups A to G is preferably 15.00% by mass or less, and more preferably 10.00% by mass or less.

[0054] ◇Element group A In another embodiment of the first coating layer 211 according to this embodiment, the element group A that the first coating layer 211 may contain will be described. At least one of the elements of element group A shown below may be contained in the first coating layer 211 in place of a portion of the remainder of Zn. [Element Group A]: One or two elements selected from the group consisting of Si: 2.50% or less and Fe: 5.00% or less.

[0055] [Si:0~2.50% by mass] In another embodiment of the first coating layer 211 according to this embodiment, it is possible that it does not contain Si, so the lower limit of its content is 0 mass%. On the other hand, Si is an element that can suppress the excessive growth of the Fe-Al-based metal structure formed at the interface between the first coating layer 211 and the steel substrate 11, and further improve the adhesion between the first coating layer 211 and the steel substrate 11. When Si is included in the first coating layer 211, the Si content is preferably 0.05 mass% or more, and more preferably 0.20 mass% or more, in order to suppress the excessive growth of the Fe-Al-based metal structure. On the other hand, if the Si content exceeds 2.50 mass%, it may excessively form high-melting-point intermetallic compounds with Mg, which may inhibit the formation of Al-Mg oxides that have an effect of suppressing Zn evaporation during welding.

[0056] Furthermore, if the Si content in the plating bath for producing the first coating layer 211 is too high, the viscosity of the plating bath may increase excessively, potentially reducing the operability during the production of the plated steel sheet (hereinafter referred to as "plating operability"). Therefore, the Si content in the plating bath is adjusted from the viewpoint of plating operability. The Si content in the first coating layer 211 is preferably 1.50% by mass or less, and more preferably 1.00% by mass or less.

[0057] [Fe:0~5.00% by mass] The first coating layer 211 may contain elements from the base material, the steel substrate 11. In particular, in the hot-dip galvanizing method, elements constituting the steel substrate 11 are easily mixed into the first coating layer 211 due to interdiffusion of elements caused by solid-liquid reactions between the steel substrate 11 and the first coating layer 211. Due to such elemental mixing, the first coating layer 211 often contains a predetermined amount of Fe, and this content is often 0.01% by mass or more. If the above interdiffusion is promoted, the adhesion between the steel substrate 11 and the first coating layer 211 is improved. From the viewpoint of improving the adhesion between the steel substrate 11 and the first coating layer 211, it is preferable that the Fe content in the first coating layer 211 be 0.20% by mass or more.

[0058] Furthermore, within a range that does not impair the effects of the present invention, Fe may be intentionally added to the plating bath used when manufacturing the first coating layer 211. However, if the Fe content in the plating bath increases, high-melting-point intermetallic compounds of Fe and Al will form in the plating bath. In this case, such high-melting-point intermetallic compounds tend to adhere to the first coating layer 211 as dross, significantly degrading the appearance quality, which is undesirable. From this viewpoint, the Fe content in the plating bath is adjusted. The Fe content in the first coating layer 211 is preferably 5.00% by mass or less. The Fe content in the first coating layer 211 is more preferably 3.00% by mass or less, and even more preferably 2.00% by mass or less, 1.00% by mass or less, or 0.50% by mass or less.

[0059] ◇Element group B In another embodiment of the first coating layer 211 according to this embodiment, the element group B that the first coating layer 211 may contain will be described. At least one of the elements of element group B shown below may be contained in the first coating layer 211 in place of a portion of the remainder of Zn. [Element Group B]: One or more elements selected from the group consisting of Sb: 0.50% or less, Pb: 0.50% or less, and Sr: 0.50% or less.

[0060] [Sb:0~0.50% by mass] [Pb:0~0.50% by mass] [Sr:0~0.50% by mass] In another embodiment of the first coating layer 211 according to this embodiment, it is possible that it does not contain Sb, Pb, and Sr, so the lower limit of the content of these elements is 0 mass%. On the other hand, if at least one of Sb, Pb, and Sr is contained in the first coating layer 211, spangles are formed on the surface of the first coating layer 211, making it possible to improve the metallic luster. For this reason, from the viewpoint of further improving the design of the plated steel pipe, it is preferable that at least one of Sb, Pb, and Sr is contained in the first coating layer 211. This design improvement effect is manifested when the content of at least one of Sb, Pb, and Sr is 0.05 mass% or more. For this reason, when at least one of Sb, Pb, and Sr is contained in the first coating layer 211, it is preferable that the content of each of these elements be independently 0.05 mass% or more.

[0061] On the other hand, if the first coating layer 211 is formed in which the content of any of Sb, Pb, or Sr exceeds 0.5% by mass, the amount of dross generated in the plating bath used to form the first coating layer 211 will increase, and it may not be possible to manufacture a plated steel sheet with good plating properties. For this reason, it is preferable that the content of Sb, Pb, and Sr in the first coating layer 211 is 0.50% by mass or less, independently of each other. Preferably, the content of Sb, Pb, and Sr is 0.20% by mass or less, independently of each other.

[0062] ◇Element group C In another embodiment of the first coating layer 211 according to this embodiment, the group of elements C that the first coating layer 211 may contain will be described. At least one of the elements of the group of elements C shown below may be contained in the first coating layer 211 in place of a portion of the remainder of Zn. [Element group C]: One or more elements selected from the group consisting of Cu: 0.25% or less, Ti: 0.25% or less, Cr: 0.25% or less, Nb: 0.25% or less, Ni: 0.25% or less, Mn: 0.25% or less, Mo: 0.25% or less, Co: 0.25% or less, and V: 0.25% or less.

[0063] [Cu:0~0.25% by mass] [Ti:0~0.25% by mass] [Cr:0~0.25% by mass] [Nb:0~0.25% by mass] [Ni:0~0.25% by mass] [Mn:0~0.25% by mass] [Mo:0~0.25% by mass] [Co:0~0.25% by mass] [V:0~0.25% by mass] In another embodiment of the first coating layer 211 according to this embodiment, it is possible that it does not contain Cu, Ti, Cr, Nb, Ni, Mn, Mo, Co, or V, so the lower limit of the content of these elements is 0 mass%. On the other hand, if at least one of Cu, Ti, Cr, Nb, Ni, Mn, Mo, Co, or V is contained in the first coating layer 211, when such plated steel sheet is welded, these elements are incorporated into the Fe-Al metal structure generated by welding, making it possible to further improve the corrosion resistance of the welded part 13. This effect of improving the corrosion resistance of the welded part is exhibited when the content of at least one of Cu, Ti, Cr, Nb, Ni, Mn, Mo, Co, or V in the first coating layer 211 is 0.05 mass% or more. Therefore, when at least one of Cu, Ti, Cr, Nb, Ni, Mn, Mo, Co, or V is included in the first coating layer 211, it is preferable that the content of each of these elements be 0.05% by mass or more, independently.

[0064] On the other hand, when forming the first coating layer 211 such that the content of any of Cu, Ti, Cr, Nb, Ni, Mn, Mo, Co, or V exceeds 0.25% by mass, these elements may form various intermetallic compounds in the plating bath used to form the first coating layer 211, leading to an increase in the viscosity of the plating bath and potentially making it impossible to produce a plated steel sheet with good plating properties. Therefore, it is preferable that the content of Cu, Ti, Cr, Nb, Ni, Mn, Mo, Co, and V in the first coating layer 211 be 0.25% by mass or less, independently of each other. Preferably, the content of Cu, Ti, Cr, Nb, Ni, Mn, Mo, Co, and V is 0.20% by mass or less, independently of each other.

[0065] ◇Element group D In another embodiment of the first coating layer 211 according to this embodiment, the element group D that the first coating layer 211 may contain will be described. At least one of the elements of element group D shown below may be contained in the first coating layer 211 in place of a portion of the remainder of Zn. [Element Group D]: One or more elements selected from the group consisting of Sn: 2.00% or less, Bi: 0.50% or less, and In: 2.00% or less.

[0066] [Sn:0~2.00% by mass] [Bi:0~0.50% by mass] [In:0~2.00% by mass] In another embodiment of the first coating layer 211 according to this embodiment, it is possible that it does not contain Sn, Bi, and In, so the lower limit of the content of these elements is 0 mass%. On the other hand, Sn, Bi, and In form intermetallic compounds with Mg in the first coating layer 211, which can improve the weldability of the first coating layer 211. Furthermore, since these intermetallic compounds all have high melting points, when the plated steel sheet is welded, they remain as intermetallic compounds after welding without evaporating. The presence of these elements makes it possible to improve corrosion resistance and corrosion protection, and also improve the corrosion resistance of the welded part 13. This effect of improving corrosion resistance is exhibited when the content of at least one of Sn, Bi, and In in the first coating layer 211 is 0.05 mass% or more. Therefore, when at least one of Sn, Bi, and In is included in the first coating layer 211, it is preferable that the content of each of these elements be independently 0.05 mass% or more.

[0067] On the other hand, excessive Sn content may increase the amount of intermetallic compounds formed, potentially reducing the corrosion resistance of the first coating layer 211 after welding. Furthermore, excessive Bi and In content may make the first coating layer 211 brittle and prone to peeling, as well as reduce its corrosion resistance after welding. These phenomena become more pronounced when the Sn content exceeds 2.00 mass%, the Bi content exceeds 0.50 mass%, and the In content exceeds 2.00 mass%. Therefore, it is preferable that the Sn content be 2.00 mass% or less, the Bi content be 0.50 mass% or less, and the In content be 2.00 mass% or less. More preferably, the Sn content is 1.00 mass% or less, the Bi content is 0.30 mass% or less, and the In content is 1.00 mass% or less.

[0068] ◇Element group E Next, in another embodiment of the first coating layer 211 according to this embodiment, the group of elements E that the plating layer 103 may contain will be described. At least one of the elements of the group of elements E shown below may be contained in the first coating layer 211 in place of a portion of the remaining Zn. [Element Group E]: One or more elements selected from the group consisting of Zr: 0.25% or less, Ag: 0.25% or less, and Li: 0.25% or less.

[0069] [Zr:0~0.25% by mass] [Ag:0~0.25% by mass] [Li:0~0.25% by mass] In this embodiment, it is possible that the first coating layer 211 may not contain Zr, Ag, or Li; therefore, the lower limit of the content of these elements is 0% by mass. On the other hand, if at least one of Zr, Ag, or Li is included in the first coating layer 211, it is possible to further improve the plating operability. This improvement in plating operability is achieved when the content of at least one of Zr, Ag, or Li in the first coating layer 211 is 0.01% by mass or more. Therefore, when at least one of Zr, Ag, or Li is included, it is preferable that the content of each of these elements be independently 0.01% by mass or more.

[0070] On the other hand, when forming a first coating layer 211 in which the content of any one of Zr, Ag, or Li exceeds 0.25% by mass, a large amount of dross is likely to be generated in the plating bath used to form the first coating layer 211. For this reason, it is preferable that the content of at least one of Zr, Ag, or Li be 0.25% by mass or less, and more preferably 0.05% by mass or less, independently of each other.

[0071] ◇Element group F In another embodiment of the first coating layer 211 according to this embodiment, the group of elements F that the first coating layer 211 may contain will be described. At least one of the elements of the group of elements F shown below may be contained in the first coating layer 211 in place of a portion of the remainder of Zn. [Element group F]: One or more elements selected from the group consisting of Ca: 3.00% or less, La: 0.50% or less, Ce: 0.50% or less, and Y: 0.50% or less.

[0072] [Ca:0~3.00% by mass] In another embodiment of the first coating layer 211 according to this embodiment, it is possible that it does not contain Ca, so the lower limit of its content is 0 mass%. On the other hand, if Ca is included in the plating bath for manufacturing the first coating layer 211, it is possible to reduce the dross generated as the Mg concentration increases during plating operations, thereby improving plating operability.

[0073] Furthermore, when Ca is included in the first coating layer 211, it forms intermetallic compounds with Al and Zn. Moreover, when Si is included in the first coating layer 211 along with Ca, Ca forms intermetallic compounds with Si. These intermetallic compounds have high melting points and stable structures, making it possible to suppress liquid metal embrittlement (LME) cracking when the plated steel sheet is welded. When Ca is included in the first coating layer 211, this LME suppression effect during welding is achieved when the Ca content is 0.01% by mass or more. More preferably, the Ca content in the first coating layer 211 is 0.05% by mass or more.

[0074] On the other hand, if the Ca content in the first coating layer 211 exceeds 3.00% by mass, the corrosion resistance of the plated steel pipe may decrease. From this viewpoint, it is preferable that the Ca content in the first coating layer 211 be 3.00% by mass or less. More preferably, the Ca content in the first coating layer 211 is 2.00% by mass or less, and even more preferably 1.00% by mass or less.

[0075] [La:0~0.50% by mass] [Ce:0~0.50% by mass] [Y:0~0.50% by mass] In another embodiment of the first coating layer 211 according to this embodiment, it is possible that it does not contain La, Ce, and Y, so the lower limit of the content of these elements is 0 mass%. On the other hand, La, Ce, and Y are elements that exhibit almost the same effect as Ca. This is because the atomic radii of each element are close to the atomic radius of Ca, and when these elements are contained in the first coating layer 211, they substitute for Ca.

[0076] The effects of improving plating operability and suppressing LME during welding are achieved by setting the content of each of these elements to 0.01% by mass or more, independently. Therefore, when at least one of La, Ce, and Y is included, it is preferable that the content of each of these elements be 0.01% by mass or more, independently. The content of La, Ce, and Y in the first coating layer 211 is more preferably 0.05% by mass or more, independently.

[0077] On the other hand, if the La, Ce, and Y content in the plating bath for producing the first coating layer 211 is too high, the viscosity of the plating bath may increase excessively, potentially reducing the operability of the plating process. Therefore, the La, Ce, and Y content in the plating bath is adjusted from the viewpoint of operability. Preferably, the content of La, Ce, and Y is 0.50% by mass or less, independently of each other. More preferably, the content of La, Ce, and Y is 0.10% by mass or less, independently of each other.

[0078] ◇Element group G In another embodiment of the zinc-based plating layer 13 according to this embodiment, the group of elements G that the zinc-based plating layer 13 may contain will be described. The elements of the group of elements G shown below are elements that may be contained in the zinc-based plating layer 13 in place of a portion of the remaining Zn. [Element group G]:B:0.50% or less

[0079] [B:0~0.50% by mass] In another embodiment of the first coating layer 211 according to this embodiment, it is possible that B is not contained, so the lower limit of its content is 0 mass%. On the other hand, when B is contained in the first coating layer 211, it has the effect of further suppressing LME. This is presumed to be because when B is contained in the first coating layer 211, it combines with at least one of Zn, Al, Mg, and Ca to form various intermetallic compounds. Furthermore, it is thought that the presence of B in the first coating layer 211 causes B to diffuse from the first coating layer 211 to the steel substrate 11, and that this has the effect of further suppressing LME of the steel substrate 11 through grain boundary strengthening. Moreover, it is presumed that the various intermetallic compounds formed with respect to B have extremely high melting points and therefore also act to suppress Zn evaporation during welding. These improvement effects are achieved when B is contained at a concentration of 0.05 mass% or more. Therefore, when B is included, it is preferable that the B content be 0.05 mass% or more.

[0080] On the other hand, if an excessive amount of B is added to the plating bath in order to include B in the first coating layer 211, it can cause a rapid increase in the plating melting point, leading to a decrease in plating operability and potentially making it impossible to produce plated steel sheets with excellent plating properties. This decrease in plating operability becomes particularly noticeable when the B content exceeds 0.50% by mass, so it is preferable that the B content be 0.50% by mass or less. More preferably, the B content is 0.10% by mass or less.

[0081] [Method for measuring chemical components] The chemical composition of the first coating layer 211 can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) or ICP-MS (Inductively Coupled Plasma Mass Spectrometry). When analyzing chemical components down to 0.1 mass%, ICP-AES should be used, while for analyzing trace amounts of chemical components less than 0.1 mass%, ICP-MS should be used. In this embodiment, a sample measuring 30 mm x 30 mm in plan view is cut from a portion of the steel pipe 1 sufficiently far from the end of the welded portion 13 (for example, 15 mm or more away). The collected sample is immersed in a 10% HCl aqueous solution with an inhibitor for about 1 minute to peel off the portion of the first coating layer 211, and a solution is prepared in which this first coating layer 211 is dissolved. The obtained solution can be analyzed by ICP-AES or ICP-MS to obtain the overall average chemical composition of the first coating layer 211.

[0082] Regarding the amount of the first coating layer 211 attached: The amount of the first coating layer 211 adhering to the outer surface of the steel pipe 1, as described above, is, for example, 10.0 to 200.0 g / m². 2 It is preferable that the concentration be around 50.0 to 150.0 g / m². 2 It is more preferable that the amount of the first coating layer 211 is within the above range. By having the amount of the first coating layer 211 adhere within the above range, the steel pipe 1 according to this embodiment can exhibit sufficient corrosion resistance. Furthermore, by having the amount of the first coating layer 211 adhere within the above range, the average thickness of the first coating layer 211 will be approximately 5 to 40 μm.

[0083] The amount of the first coating layer 211 can be measured as follows. First, a sample measuring 30 mm x 30 mm in plan view is cut from a point sufficiently far from the end of the welded joint 13 (for example, a point 15 mm or more away), and the mass of the sample is measured. Then, protective tape, such as a tape-like seal, is applied to the surface of the sample corresponding to the inner surface of the steel pipe 1 to prevent the first coating layer 211 that may be present on this surface from dissolving in the next step. The type of protective tape is not limited as long as it adheres closely to the inner surface of the steel pipe 1 and does not allow the 10% HCl aqueous solution to penetrate. Then, the sample is immersed in a 10% HCl aqueous solution to which an inhibitor that suppresses corrosion of the base steel material (for example, Hibilon manufactured by Sugimura Chemical Industry Co., Ltd.) has been added, and the first coating layer 211 on the side without the protective tape is pickled and removed, and the mass of the sample after pickling is measured. The amount of the first coating layer 211 can be determined from the change in mass of the sample before and after pickling.

[0084] ≪Regarding the circumferential length of the first region 21≫ Let's focus on an arbitrary cross-section obtained by cutting the steel pipe 1 according to this embodiment in the radial direction so as to be perpendicular to the longitudinal direction of the steel pipe 1 (the Z-axis direction in Figure 1). In such a cross-section, the first region 21, which is composed of the first coating layer 211 having the chemical composition described above, has a region where the thickness of the first coating layer 211 is 5.0 μm or more, and this region is continuously present for 1.0 mm or more along the circumferential direction of the steel pipe 1.

[0085] Here, if the circumferential length of the region where the thickness of the first coating layer 211 is 5.0 μm or more is continuously less than 1.0 mm, the coating state of the first coating layer 211 on the steel pipe 1 is insufficient, and corrosion resistance as a plated steel pipe cannot be guaranteed. The longer the circumferential length of the region where the thickness of the first coating layer 211 is 5.0 μm or more, the better, and there is no particular upper limit.

[0086] The circumferential length of the first coating layer 211, as described above, is determined by observing an arbitrary cross-section of interest using a scanning electron microscope (SEM) (for example, a JSM-7000F manufactured by JEOL Ltd.) at a magnification of approximately 500x, and measuring the length at the interface between the first coating layer 211 and the steel substrate 11 using the length-measuring function implemented in the SEM. In such measurement, it is sufficient that there is a continuous region of 1.0 mm or more in which the thickness of the first coating layer 211 is 5.0 μm or more.

[0087] <Regarding Area 23> Next, with reference to Figures 1 and 3, the second region 23 of the coating layer 20 according to this embodiment will be described in detail. Figure 3 is an explanatory diagram for describing the second region having the second coating layer in the steel pipe according to this embodiment, and is an enlarged view of the area near the boundary between the second region 23 and the third region 25 shown in Figure 1.

[0088] As previously mentioned, the second region 23 in this embodiment is a region formed by a repair process after grinding off protruding portions generated during the manufacturing of the steel pipe 1. The second region 23 contains a layer to ensure the corrosion resistance of the welded portion 13. As schematically shown in Figure 3, the second region 23 contains a second coating layer 231 formed by a repair process using thermal spraying.

[0089] More specifically, the second region 23 in this embodiment is a region composed of a second coating layer 231 consisting of an Al layer 233 provided on the surface of the welded portion 13 and its vicinity, and a Zn layer 235 on the Al layer 233, so as schematically shown in Figure 3, that at least covers the welded portion 13 of the base steel pipe 10.

[0090] Regarding the method for identifying area 23 in the second domain: In any cross-section of the steel pipe 1 according to this embodiment, as shown in Figure 1, obtained by cutting the pipe radially, the portion corresponding to the second region 23 can be easily identified by observing the cross-section of the steel pipe 1 of interest using SEM-EDX and performing elemental mapping analysis, similar to how the portion corresponding to the first region 21 is identified. In such elemental mapping analysis, it is sufficient to focus on at least the elements Zn, Al, Mg, and Fe, or all elements may be considered.

[0091] More specifically, in the elemental mapping analysis using SEM-EDX as described above, a region with a two-layer structure consisting of an Al phase layer and a Zn phase layer can be identified as the region corresponding to the second region 23.

[0092] Furthermore, the second region 23 in this embodiment is a region having a second covering layer 231 that at least covers the welded portion 13 of the raw steel pipe 10, as described above. Therefore, by identifying the portion of the cross section of interest that corresponds to the welded portion 13 of the raw steel pipe 10, it is possible to roughly identify the location where the second region 23 exists.

[0093] ≪Regarding the chemical composition of the second coating layer 231≫ The second coating layer 231 of the second region 23 according to this embodiment, according to one embodiment, comprises an Al layer 233 containing Zn: 0 to 20.0% by mass, with the remainder being Al and impurities, and a Zn layer 235 containing Al: 0.1 to 20.0% by mass, with the remainder being Zn and impurities.

[0094] Furthermore, according to another embodiment, the second coating layer 231 of this embodiment comprises an Al layer 233 containing Zn: 0 to 20.0% by mass, and further containing one or more elements selected from the group consisting of element groups H to J below, with the remainder being Al and impurities, and a Zn layer 235 containing Al: 0.1 to 20.0% by mass, and further containing one or more elements selected from the group consisting of element groups H to J below, with the remainder being Zn and impurities.

[0095] [Element Group H]: One or more elements selected from the group consisting of C: 0.300% or less, S: 0.010% or less, and N: 0.006% or less. [Element group I]:Fe:5.00% or less [Element group J]: One or two elements selected from the group consisting of Mn: 1.50% or less, Ti: 0.10% or less.

[0096] ◇Regarding the chemical composition of Al layer 233 As described above, the Al layer 233 according to this embodiment is a layer containing Al, which is the main component (a component with a content of 50% by mass or more), and impurities, and may further contain Zn, which is 20.0% by mass or less, and one or more elements selected from the group consisting of element group H to element group J below.

[0097] Al has a higher melting point compared to Zn, which is the main component of the Zn layer 235 described later (melting point of Al: approximately 660°C, melting point of Zn: approximately 420°C). By positioning the Al layer 233, which has Al as its main component, on the welded part 13, it is possible to ensure adhesion between the welded part 13 and the Al layer 233.

[0098] Furthermore, the Al layer 233 according to this embodiment may contain Zn in an amount of 20.0% by mass or less, replacing a portion of the main component Al. Since the Al layer 233 may not contain Zn, the lower limit of the Zn content is 0% by mass. By containing more than 0% by mass of Zn in the Al layer 233, it is possible to further improve the corrosion resistance of the Al layer 233, as well as to further improve the adhesion between the Zn layer 235 and the Al layer 233, which will be described later. When the Al layer 233 contains Zn, the Zn content is more preferably 5% by mass or more.

[0099] On the other hand, when the Al layer 233 contains Zn, by setting the Zn content to 20.0% by mass or less, it is possible to improve both the corrosion resistance of the Al layer 233 and the adhesion between the Al layer 233 and the Zn layer 235 while ensuring adhesion between the Al layer 233 and the welded part 13.

[0100] Next, in a preferred arrangement of the second coating layer 231 according to another embodiment of this model, the element groups H to J that the Al layer 233 may have in its chemical composition will be described in detail.

[0101] In addition, in the second coating layer 231 according to another embodiment of this embodiment, if at least one of the elements belonging to element groups H to J below is included, it is preferable that at least one of the elements belonging to element groups H to J below is included within the following content range, and the total content is 10,000% by mass or less.

[0102] By keeping the total content of elements belonging to element groups H to J to 10,000 mass% or less, it becomes possible to enjoy the effects exhibited by the addition of each element, as detailed below, without compromising each other. The total content of elements belonging to element groups H to J is preferably 8,000 mass% or less, and more preferably 5,000 mass% or less.

[0103] ◇Element group H In another embodiment of the second coating layer 231 according to this embodiment, the group of elements H that the Al layer 233 may contain will be described. At least one of the elements of the group of elements H shown below may be contained in the Al layer 233 in place of a portion of the remaining Al. [Element Group H]: One or more elements selected from the group consisting of C: 0.300% or less, S: 0.010% or less, and N: 0.006% or less.

[0104] [C:0~0.300%] [S: 0~0.010%] [N: 0~0.006%] In another embodiment of the Al layer 233 according to this embodiment, it is possible that it does not contain C, S, and N, so the lower limit of the content of these elements is 0 mass%. On the other hand, C, S, and N are elements that can be mixed into the Al layer 233 from the base steel pipe 10 side when forming the second coating layer 233. By including these elements in the Al layer 233, the adhesion between the Al layer 233 and the base steel pipe 10 can be further improved.

[0105] This improved adhesion effect is achieved when the C content is 0.100% or more, the S content is 0.001% or more, or the N content is 0.001% or more. Therefore, when at least one of C, S, and N is included, it is preferable that the C content be 0.100% or more, the S content be 0.001% or more, and the N content be 0.001% or more. In the Al layer 233, it is more preferable that the C content be 0.200% or more, the S content be 0.003% or more, and the N content be 0.002% or more.

[0106] On the other hand, if the carbon content in the Al layer 233 exceeds 0.300%, the sulfur content exceeds 0.010%, or the nitrogen content exceeds 0.006%, the corrosion resistance of the Al layer 233 may decrease. Therefore, it is preferable that the carbon content in the Al layer 233 is 0.300% or less, the sulfur content is 0.010% or less, and the nitrogen content is 0.006% or less. It is even more preferable that the carbon content in the Al layer 233 is 0.250% or less, the sulfur content is 0.007% or less, and the nitrogen content is 0.003% or less.

[0107] ◇Element group I In another embodiment of the second coating layer 231 according to this embodiment, the element group I that the Al layer 233 may contain will be described. The elements of element group I shown below are elements that may be contained in the Al layer 233 in place of a portion of the remaining Al. [Element group I]:Fe:5.00% or less

[0108] [Fe:0~5.00% by mass] Fe is an element that can be mixed into the Al layer 233 from the base steel pipe 10 side when forming the second coating layer 233. Due to such mixing, the Al layer 233 may contain a predetermined amount of Fe, and this content is often 0.01% by mass or more. If such mixing is promoted, the adhesion between the base steel pipe 10 and the Al layer 233 will improve. From the viewpoint of improving the adhesion between the base steel pipe 10 and the Al layer 233, it is preferable that the Fe content in the Al layer 233 be 0.20% by mass or more.

[0109] Furthermore, within the limits that do not impair the effects of the present invention, Fe may be intentionally added to the thermal spray material (also called thermal spray core wire or wire) used in the thermal spraying process for forming the Al layer 233. However, if the Fe content in the thermal spray material becomes too high, the content of Al, which is the main component, may become too low, potentially reducing the corrosion resistance of the second coating layer 231. From this viewpoint, the Fe content in the Al layer 233 is preferably 5.00% by mass or less. The Fe content of the Al layer 233 is more preferably 3.00% by mass or less, and even more preferably 2.00% by mass or less, 1.00% by mass or less, or 0.50% by mass or less. ◇Element group J In another embodiment of the second coating layer 231 according to this embodiment, the group of elements J that the Al layer 233 may contain will be described. At least one of the elements of the group of elements J shown below may be contained in the Al layer 233 in place of a portion of the remaining Al. [Element group J]: One or two elements selected from the group consisting of Mn: 1.50% or less, Ti: 0.10% or less.

[0110] [Mn: 0~1.50%] [Ti: 0~0.10%] In another embodiment of the Al layer 233 according to this embodiment, it is possible that it does not contain Mn or Ti, so the lower limit of the content of these elements is 0 mass%. On the other hand, if at least one of Mn or Ti is contained in the Al layer 233, these elements are incorporated into the weld 13, making it possible to further improve the corrosion resistance of the weld 13. This effect of improving the corrosion resistance of the weld is manifested when the content of at least one of Mn or Ti in the Al layer 233 is 0.05 mass% or more. Therefore, when at least one of Mn or Ti is contained in the Al layer 233, it is preferable that the content of each of these elements be 0.05 mass% or more, independently.

[0111] On the other hand, if the Mn and Ti content in the Al layer 233 becomes too high, the content of Al, which is the main component, may become too low, potentially reducing the corrosion resistance of the second coating layer 231. From this viewpoint, the Mn content in the Al layer 233 is preferably 1.50% by mass or less, and the Ti content is preferably 0.10% or less. The Mn content of the Al layer 233 is more preferably 0.20% by mass or less. Furthermore, the Ti content of the Al layer 233 is more preferably 0.08% by mass or less.

[0112] ◇Regarding the chemical composition of Zn layer 235 As described above, the Zn layer 235 according to this embodiment is a layer containing Zn, which is the main component (a component whose content is 50% by mass or more), and impurities, and may further contain Al, which is 20.0% by mass or less, and one or more elements selected from the group consisting of element group H to element group J.

[0113] Zn is an element that possesses so-called sacrificial corrosion protection properties. Therefore, by positioning a Zn layer 235, which has Zn as its main component, on the Al layer 233, it is possible to improve the corrosion resistance of the second coating layer 231.

[0114] Furthermore, the Zn layer 235 according to this embodiment may contain Al in an amount of 20.0% by mass or less, replacing a portion of the main component Zn. Since the Zn layer 235 may not contain Al, the lower limit of the Al content is 0% by mass. By containing more than 0% by mass of Al in the Zn layer 235, it is possible to further improve the corrosion resistance of the Zn layer 235, as well as to further improve the adhesion between the Zn layer 235 and the Al layer 233. When the Zn layer 235 contains Al, the Al content is more preferably 15% by mass or more.

[0115] On the other hand, when the Zn layer 235 contains Al, by limiting the Al content to 20.0% by mass or less, it is possible to improve both the corrosion resistance of the Zn layer 235 and the adhesion between the Al layer 233 and the Zn layer 235 while suppressing a decrease in sacrificial corrosion protection.

[0116] In addition, in a preferred arrangement of the second coating layer 231 according to another embodiment of this model, the elements H to J that the Zn layer 235 may have are the same as described above, so a detailed explanation will be omitted below.

[0117] [Method for measuring chemical components] The chemical composition of the Al layer 233 and Zn layer 235 described above can be determined by performing quantitative analysis using SEM-EDX on the portion of the second coating layer 231 (the portion with a two-layer structure) that was identified by elemental mapping analysis using SEM-EDX as explained earlier. More specifically, in the portion corresponding to the second coating layer 231, quantitative analysis using SEM-EDX is performed at arbitrary locations in both the Al-based layer located closer to the weld 13 and the Zn-based layer located above this Al-based layer. Such quantitative analysis is performed at five arbitrary locations in each layer, and the average of the obtained measurements can be treated as the chemical composition of each layer.

[0118] Regarding the thickness of the second coating layer 231: [Regarding the average thickness of Al layer 233] The average thickness of the Al layer 233 in the second coating layer 231 according to this embodiment (thickness d1 in Figure 3) is in the range of 10.0 to 100.0 μm. If the average thickness of the Al layer 233 is less than 10.0 μm, the Al layer 233 constituting the second coating layer 231 is too thin, and therefore the second coating layer 231 cannot maintain sufficient corrosion resistance. By making the average thickness of the Al layer 233 10.0 μm or more, the second coating layer 231 according to this embodiment can achieve sufficient corrosion resistance. The average thickness of the Al layer 233 is preferably 15.0 μm or more, and more preferably 20.0 μm or more.

[0119] On the other hand, if the average thickness of the Al layer 233 exceeds 100.0 μm, while the corrosion resistance exhibited by the second coating layer 231 is excellent, the manufacturing cost for forming the Al layer 233 increases excessively, which is undesirable. Therefore, from the viewpoint of suppressing manufacturing costs, the average thickness of the Al layer 233 is set to 100.0 μm or less. The average thickness of the Al layer 233 is preferably 60.0 μm or less, and more preferably 40.0 μm or less.

[0120] [Regarding the average thickness of Zn layer 235] The average thickness of the Zn layer 235 in the second coating layer 231 according to this embodiment (thickness d2 in Figure 3) is in the range of 3.0 to 50.0 μm. If the average thickness of the Zn layer 235 is less than 3.0 μm, the Zn layer 235 constituting the second coating layer 231 is too thin, and therefore the second coating layer 231 cannot maintain sufficient corrosion resistance. By making the average thickness of the Zn layer 235 3.0 μm or more, the second coating layer 231 according to this embodiment can achieve sufficient corrosion resistance. The average thickness of the Zn layer 235 is preferably 10.0 μm or more, and more preferably 15.0 μm or more.

[0121] On the other hand, if the average thickness of the Zn layer 235 exceeds 50.0 μm, while the corrosion resistance exhibited by the second coating layer 231 is excellent, the manufacturing cost for forming the Zn layer 235 increases excessively, which is undesirable. Therefore, from the viewpoint of suppressing manufacturing costs, the average thickness of the Zn layer 235 is set to 50.0 μm or less. The average thickness of the Zn layer 235 is preferably 25.0 μm or less, and more preferably 20.0 μm or less.

[0122] Here, the Al layer 233 and Zn layer 235, as well as the welded portion 13 of the base steel pipe 10, can be easily distinguished by observing a cross-section of the steel pipe 1 cut in the radial direction according to this embodiment using a scanning electron microscope (SEM). Therefore, the average thickness of the Al layer 233 and the average thickness of the Zn layer 235 can be measured by cross-sectional observation using an SEM as follows.

[0123] More specifically, we focus on an arbitrary cross-section obtained by cutting the steel pipe 1 according to this embodiment in the radial direction perpendicular to the longitudinal direction of the steel pipe 1 (the Z-axis direction in Figure 1). The obtained cross-section is observed at a magnification of approximately 500 times using a SEM, and the thickness of the Al layer 233 and the thickness of the Zn layer 235 are measured at any 10 locations within the observation field using the length measurement function implemented in the SEM. By averaging the 10 measurements obtained in this way over the number of measurement locations, the average thickness of the Al layer 233 and the average thickness of the Zn layer 235 can be calculated.

[0124] [Circumferential length of the second coating layer 231, which has a thickness of 15.0 μm or more] In the second coating layer 231 according to this embodiment, we focus on the region where the thickness of the second coating layer 231 (the thickness from the interface between the welded portion 13 and the Al layer 233 to the surface of the Zn layer 235) is 15.0 μm or more. The circumferential length of such a region with a thickness of 15.0 μm or more is 500.0 μm or more, and it extends continuously along the circumferential direction.

[0125] If the circumferential continuous length of the region where the thickness of the second coating layer 231 is 15.0 μm or more is less than 500.0 μm, then the length of this region is too short, and as a result, the welded portion 13 of the base steel pipe 10 and its vicinity cannot be covered with the second coating layer 231 of sufficient thickness, making it impossible to guarantee the corrosion resistance of the welded portion 13. By ensuring that the circumferential continuous length of the region where the thickness of the second coating layer 231 is 15.0 μm or more is 500.0 μm or more, the corrosion resistance of the welded portion 13 can be guaranteed.

[0126] For regions where the thickness of the second coating layer 231 is 15.0 μm or more, the longer the continuous length in the circumferential direction, the better, and there is no particular upper limit specified. However, if such a length is too long, the effect of improving the corrosion resistance of the welded joint 13 will saturate, and the manufacturing cost for producing the second coating layer 231 may become too high. From this perspective, for regions where the thickness of the second coating layer 231 is 15.0 μm or more, the practical upper limit for the continuous length in the circumferential direction is approximately 20.0 mm (20000.0 μm).

[0127] Furthermore, the circumferential length of the region where the thickness of the second coating layer 231 is 15.0 μm or more, as described above, can be determined by observing an arbitrary cross-section of interest using a scanning electron microscope (SEM, for example, a JEOL Ltd. scanning electron microscope JSM-7000F) at a magnification of approximately 500x. The circumferential length at the interface between the welded joint 13 and the second coating layer 231 can be measured using the length measuring function implemented in the SEM.

[0128] [Regarding the average thickness of the second coating layer 231] Furthermore, in the steel pipe 1 according to this embodiment, the overall average thickness of the second coating layer 231 is preferably in the range of 13.0 to 110.0 μm. By having an average thickness of the second coating layer 231 in the range of 13.0 to 110.0 μm, the second coating layer 231 according to this embodiment exhibits better corrosion resistance while suppressing an increase in manufacturing costs. The average thickness of the second coating layer 231 is more preferably in the range of 35.0 to 60.0 μm.

[0129] Here, the average thickness of the second coating layer 231, as described above, can be measured by observing an arbitrary cross-section obtained by cutting the steel pipe 1 according to this embodiment in the radial direction perpendicular to the longitudinal direction of the steel pipe 1 (the Z-axis direction in Figure 1) using a SEM, similar to the average thickness of the Al layer 233 and the Zn layer 235. More specifically, the obtained cross-section is observed at a magnification of approximately 500 times using an SEM, and the length from the interface between the welded part 13 and the Al layer 233 to the surface of the Zn layer 235 is measured at any 10 locations within the observation field using the length measuring function implemented in the SEM. The average thickness of the second coating layer 231 can be calculated by averaging the 10 measurements obtained in this way over the number of measurement locations.

[0130] <Regarding Area 3, Item 25> Next, with reference to Figures 1 and 4 to 6, the third region 25 of the coating layer 20 according to this embodiment will be described in detail. Figures 4 to 6 are explanatory diagrams for describing the third region 25 having the third coating layer in the steel pipe according to this embodiment. Of these, Figure 4 is an enlarged view of the third region 25 shown in Figure 1, the boundary between the third region 25 and the first region 21, and the area near the boundary between the second region 23 and the third region 25.

[0131] As previously mentioned, the third region 25 in this embodiment corresponds to the region where a portion of the first coating layer 211 was removed during the grinding and repair processes when manufacturing the steel pipe 1. A third coating layer 251 is present in this third region 25. The third coating layer 251 is formed by a reaction between the components of the first coating layer 211 remaining in the third region 25 during the grinding process and the thermal spray material sprayed during the repair process.

[0132] More specifically, the third region 25 according to this embodiment is located on the steel substrate 11 of the raw steel pipe 10, as schematically shown in Figure 4, and is located between the first region 21 and the second region 23 described earlier. As the portion corresponding to the first region 21 and the portion corresponding to the second region 23 can be easily identified by the method described earlier, the portion corresponding to the third region 25 can be easily identified as the portion located between the first region 21 and the second region 23.

[0133] ≪Regarding the chemical composition of the third coating layer 251≫ The third coating layer 251 of the third region 25 according to this embodiment is, in one embodiment, mainly composed of Zn or Al, and further composed of impurities. Here, "mainly composed of Zn or Al" means that the Zn content or Al content is 50% by mass or more.

[0134] Furthermore, according to another embodiment, the third coating layer 251 in this embodiment is mainly composed of Zn or Al by mass%, and further comprises one or more elements selected from the group consisting of element groups A to J mentioned earlier, and impurities.

[0135] The third coating layer 251 according to this embodiment is formed by a reaction between components of the first coating layer 211 that remain partially on the steel substrate 11 during grinding and repair processes, and the thermal spray material that is thermal sprayed during the repair process. Furthermore, during such repair processes, components of the first coating layer 211 contained in the first region 21 may be mixed in by diffusion.

[0136] Therefore, the chemical composition of the third coating layer 251 according to this embodiment will be mainly Zn or Al, and may also contain one or more elements selected from the group consisting of element groups A to J that may be contained in the first coating layer 211 and the second coating layer 231.

[0137] As explained earlier, the elements in element groups A to J that the third coating layer 251 may contain are as described above, so a detailed explanation will be omitted below.

[0138] <<Regarding the metallographic structure of the third coating layer 251>> The third coating layer 251 according to this embodiment includes, as schematically shown in Figure 4, a phase A (reference numeral 253 in Figure 4) which is unevenly distributed at the interface portion of the third coating layer 251 on the steel substrate 11 side, a phase B (reference numeral 255 in Figure 4) located on the steel substrate 11 or on phase A of the third coating layer 251, and a phase C (reference numeral 259 in Figure 4) located on the above-mentioned phase B.

[0139] Phase A, which is unevenly distributed at the interface of the third coating layer 251 on the steel substrate 11 side, is a metallic structure containing 0.1 to 40.0 mass% Al and 0.1 to 10.0 mass% Mg, with the remainder being Zn and impurities. This phase A is a metallic structure mainly produced by the reaction of components derived from the Al layer 233 constituting the second coating layer 231 and components of the first coating layer 211 that remain on the surface of the steel substrate 11 during the grinding and repair processes performed when manufacturing the steel pipe 1. The presence of such phase A at the interface of the third coating layer 251 on the steel substrate 11 side allows the third coating layer 251 in this embodiment to exhibit excellent corrosion resistance.

[0140] Here, in the above-mentioned phase A (reference numeral 253), if the Al content is less than 0.1% by mass, or if the Mg content is less than 0.1% by mass, the content of Al and Mg, which are elements that contribute to the corrosion resistance of the third coating layer 251, is too low, and the third coating layer 251 cannot exhibit sufficient corrosion resistance. For this reason, in phase A (reference numeral 253), the Al content is 0.1% by mass or more, and the Mg content is 0.1% by mass or more. The Al content in phase A (reference numeral 253) is preferably 10.0% by mass or more, and more preferably 15.0% by mass or more. Also, the Mg content in phase A (reference numeral 253) is preferably 1.5% by mass or more, and more preferably 2.5% by mass or more.

[0141] On the other hand, in the third coating layer 251 according to this embodiment, if the Al content in phase A (reference numeral 253) exceeds 40.0% by mass, or if the Mg content in phase A (reference numeral 253) exceeds 10.0% by mass, this cannot occur. The Al content in phase A (reference numeral 253) is preferably 30.0% by mass or less, and more preferably 25.0% by mass or less. The Mg content in phase A (reference numeral 253) is preferably 8.0% by mass or less, and more preferably 6.0% by mass or less.

[0142] In the third coating layer 251, phase B (reference numeral 255), located on the steel substrate 11 or on phase A, has a metallic structure containing 0 to 20.0 mass% of Zn, with the remainder being Al and impurities. This phase B (reference numeral 255) is a metallic structure mainly derived from the components of the Al layer 233 that constitutes the second coating layer 231.

[0143] The above-mentioned phase B (reference numeral 255) may not contain Zn depending on the composition of the Al layer 233 of the second coating layer 231; therefore, the lower limit of the Zn content is 0 mass%. Furthermore, the inclusion of Zn in phase B makes it possible to further improve the corrosion resistance of the third coating layer 251. The Zn content in phase B is preferably 5.0 mass% or more, and more preferably 8.0 mass% or more.

[0144] On the other hand, in the third coating layer 251 according to this embodiment, if the Zn content in phase B exceeds 20.0% by mass, this cannot occur. Furthermore, the Zn content in phase B is preferably 15.0% by mass or less, and more preferably 13.0% by mass or less.

[0145] Furthermore, in the third coating layer 251 according to this embodiment, as schematically shown in Figure 4, a phase B' (reference numeral 257) containing Mg is present in the phase B described above. The first coating layer 211 according to this embodiment contains a large amount of Mg, ranging from 5.0% to 15.0% by mass, as described above. The Mg originating from the first coating layer 211 is dissolved in the phase B described above and exists as a phase B' (reference numeral 257) containing 0.1% to 10.0% by mass of Mg. In this phase B' (reference numeral 257), the remainder of the Mg consists of Al, Zn, and impurities. The presence of phase B' (reference numeral 257) in phase B (reference numeral 255) makes it possible to improve the corrosion resistance of the third coating layer 251.

[0146] Here, if the Mg content in phase B' (reference numeral 257) is less than 0.1% by mass, the Mg content in phase B' is too low, and the third coating layer 251 cannot exhibit sufficient corrosion resistance. Therefore, the Mg content in phase B' (reference numeral 257) is 0.1% by mass or more. The Mg content in phase B' (reference numeral 257) is preferably 0.5% by mass or more, and more preferably 1.0% by mass or more.

[0147] On the other hand, in the third coating layer 251 according to this embodiment, this cannot occur if the Mg content in phase B' exceeds 10.0% by mass. Furthermore, the Mg content in phase B' is preferably 8.0% by mass or less, and more preferably 6.0% by mass or less.

[0148] Furthermore, in the third coating layer 251 according to this embodiment, the area ratio of phase B' in the cross-section of phase B (for example, the area ratio of phase B' in phase B when observing the cross-section obtained by cutting the third coating layer 251 in the diameter direction perpendicular to the longitudinal direction of the steel pipe 1 (the Z-axis direction in Figure 1)) is 3.0 to 30.0%. If the area ratio of phase B' in the cross-section of phase B' is less than 3.0%, the amount of phase B' is too small, and the third coating layer 251 cannot exhibit sufficient corrosion resistance. Therefore, the area ratio of phase B' in the cross-section of phase B' is 3.0% or more. Preferably, the area ratio of phase B' in the cross-section of phase B' is 5.0% or more, and more preferably 8.0% or more.

[0149] On the other hand, if the area ratio of phase B' in the cross-section of phase B' is 30.0% or less, the corrosion resistance of the third coating layer 251 can be sufficiently maintained. Therefore, the area ratio of phase B' in the cross-section of phase B' is 30.0% or less. Preferably, the area ratio of phase B' in the cross-section of phase B' is 25.0% or less, and more preferably 20.0% or less.

[0150] In the third coating layer 251 according to this embodiment, the remaining metallic structure after phases A (reference numeral 253), B (reference numeral 255), and B' (reference numeral 257) described above is mainly a Zn-based phase. More specifically, it is phase C (reference numeral 259 in Figure 4) which contains 0.1 to 20.0 mass% Al and the remainder consists of Zn and impurities. As schematically shown in Figure 4, this phase C (reference numeral 259) is located on phase B (reference numeral 255).

[0151] In this case, if the Al content in phase C (reference numeral 259) is less than 0.1% by mass, the Al content is too low, and the third coating layer 251 cannot exhibit sufficient corrosion resistance. Therefore, the Al content in phase C (reference numeral 259) is 0.1% by mass or more. The Al content in phase C (reference numeral 259) is preferably 10.0% by mass or more, and more preferably 15.0% by mass or more.

[0152] On the other hand, in the above-mentioned phase C (reference numeral 259), if the Al content exceeds 20.0 mass%, the sacrificial corrosion protection by Zn decreases, which is undesirable. In phase C (reference numeral 259), the Al content is 20.0 mass% or less. The Al content in phase C (reference numeral 259) is preferably 30.0 mass% or less, and more preferably 25.0 mass% or less.

[0153] Here, phases A (reference numeral 253), B (reference numeral 255), B' (reference numeral 257), and C (reference numeral 259) described above can be easily identified by observing a cross-section of the steel pipe 1 according to this embodiment, cut in the radial direction, using SEM-EDX and performing elemental mapping analysis. In such elemental mapping analysis, it is sufficient to focus on at least the elements Zn, Al, and Mg, or all elements may be considered.

[0154] More specifically, in the elemental mapping analysis using SEM-EDX as described above, the region located near the interface of the third coating layer 251 on the steel substrate side, mainly composed of Zn and containing Mg, can be identified as the portion corresponding to phase A (reference numeral 253). Furthermore, the region located on the steel substrate or on phase A, and composed of a phase mainly composed of Al, can be identified as the portion corresponding to phase B (reference numeral 255). In addition, the region within the portion corresponding to phase B, where Mg is present, can be identified as the portion corresponding to phase B' (reference numeral 257). Moreover, the portion other than phases A, B, and B' identified in this manner can be identified as the portion corresponding to phase C (reference numeral 259).

[0155] ◇Distribution state of phase A (indicated by 253) in the longitudinal direction of steel pipe 1 Furthermore, in the third coating layer 251 according to this embodiment, we will focus on the distribution state of phase A, which is unevenly distributed near the interface on the steel substrate 11 side, in the longitudinal direction (Z-axis direction) of the steel pipe 1 (for example, the distribution state in the direction perpendicular to the plane of the paper in Figure 4). In this case, it is preferable that the above-mentioned phase A (reference numeral 253) exists continuously for a certain length, substantially parallel to the longitudinal direction (Z-axis direction). In other words, when the distribution state of phase A present in the third coating layer 251 is viewed from above in the diameter direction of the pipe, it is preferable that phase A exists in a streaky manner substantially parallel to the longitudinal direction (Z-axis direction). The corrosion resistance of the third coating layer 251 according to this embodiment is further improved by the streaky distribution of phase A.

[0156] The area ratio of phase B' within phase B can be determined by observing a cross-section where the location of phase B is known using a scanning electron microscope (SEM). An arbitrary location in the region of the third coating layer 251 present in the cross-section of interest is observed within an observation field of view of 150 μm × 200 μm. Here, a person skilled in the art can easily determine the location of phase B' within phase B from its appearance. The area of ​​each phase B and phase B' present in the observation field is measured using functions implemented in the SEM. From the obtained measurements, the area ratio of phase B' within phase B in the observation field of interest can be calculated. Such measurements are performed in any 10 observation fields, and the obtained area ratios are averaged by the number of observation fields. In this way, the area ratio of phase B' within phase B can be determined.

[0157] [Regarding the circumferential length of the third coating layer 251] In the cross-section of the third region 25 according to this embodiment, schematically shown in Figure 4, the length a along the circumferential direction of the region where the thickness of the third coating layer 251 is less than 5.0 μm is less than 100.0 μm. Furthermore, the proportion occupied by the region with a thickness of less than 5.0 μm is 40.0% or less in total with respect to the total length along the circumferential direction of the third coating layer 251.

[0158] If the circumferential length a of the region where the thickness of the third coating layer 251 is less than 5.0 μm exceeds 100.0 μm, then the region where the third coating layer 251 is thin (less than 5.0 μm) exists over too long a distance, and the corrosion resistance of the third coating layer 251 cannot be guaranteed. Also, if the proportion of the region where the thickness is less than 5.0 μm exceeds 40.0% of the total circumferential length of the third coating layer 251, then the region where the third coating layer 251 is thin (less than 5.0 μm) exists over too long a distance, and the corrosion resistance of the third coating layer 251 cannot be guaranteed. The length of each region with a thickness of less than 5.0 μm along the circumferential direction is less than 100.0 μm, and the proportion occupied by such regions with a thickness of less than 5.0 μm is 40.0% or less in total with respect to the total length of the third coating layer 251 along the circumferential direction. In this embodiment, the third coating layer 251 can achieve excellent corrosion resistance.

[0159] The circumferential length a of the region with a thickness of less than 5.0 μm is preferably 90.0 μm or less, and more preferably 70.0 μm or less. On the other hand, the smaller the circumferential length a of the region with a thickness of less than 5.0 μm, the better, and there is no particular lower limit. However, the circumferential length a of the region with a thickness of less than 5.0 μm is practically limited to about 10.0 μm.

[0160] Furthermore, the proportion of regions with a thickness of less than 5.0 μm is preferably 35.0% or less in total, and more preferably 30.0% or less in total. On the other hand, the smaller the proportion of regions with a thickness of less than 5.0 μm, the better, and there is no specific lower limit. However, in practice, the lower limit of the proportion of regions with a thickness of less than 5.0 μm is approximately 5.0%.

[0161] In the cross-section shown in Figure 4, the total length of the third coating layer 251 along the circumferential direction is preferably 500 to 1500 μm. By having the circumferential length of the third coating layer 251 within the above range, the third region 25 is positioned between the first region 21 and the second region 23 at a more favorable size, making it possible to further smooth the difference between the corrosion resistance exhibited by the first region 21 and the corrosion resistance exhibited by the second region 23. As a result, the corrosion resistance of the steel pipe 1 according to this embodiment can be further improved. More preferably, the total length of the third coating layer 251 along the circumferential direction is 600 to 1200 μm.

[0162] Furthermore, the length of the third coating layer 251 along the circumferential direction, and the length of the region where the thickness of the third coating layer 251 is less than 5.0 μm, can be measured by observing any cross-section of interest using a scanning electron microscope (SEM, for example, JEOL Ltd.'s JSM-7000F) at a magnification of approximately 500x, and using the length measuring function implemented in the SEM, paying attention to the thickness of the third coating layer 251, at the interface between the steel substrate 11 and the third coating layer 251.

[0163] [Regarding the circumferential length of the portion corresponding to phase A] Let us focus on the cross-section of the third region 25 according to this embodiment, as schematically shown in Figure 5. In the third coating layer 251 according to this embodiment, at the interface between the third coating layer 251 and the steel substrate 11 in the cross-section, the total length along the circumferential direction of the portion corresponding to phase A (reference numeral 253) at the interface is preferably 30 to 60% of the total length along the circumferential direction of the interface. By keeping the total length along the circumferential direction of the portion corresponding to phase A at the interface within the above range, the third coating layer 251 according to this embodiment exhibits even better corrosion resistance. The total length along the circumferential direction of the portion corresponding to phase A at the interface is more preferably 35% or more. Furthermore, the total length along the circumferential direction of the portion corresponding to phase A at the interface is more preferably 50% or less.

[0164] The ratio regarding the length as described above may be measured as follows. First, observe a cross-section as shown in FIG. 5 at a magnification of about 500 times, and using the length measurement function or the like installed in the SEM, for the interface between the third coating layer 251 and the steel base 11, measure the overall length along the circumferential direction (L C ) in FIG. 5. At the same time, measure the lengths of the portions corresponding to phase A at the position of the above interface, and calculate the total thereof. For example, in the case of FIG. 5, three phases A exist in the third coating layer 251. In this case, if the lengths along the circumferential direction of phase A at the position of the interface are L C1 , L C2 , L C3 respectively, then (L C1 + L C2 + L C3 ) becomes the total length of the portions corresponding to phase A at the position of the interface. Using the obtained respective values, the value obtained by {(L C1 + L C2 + L C3 ) / L C} × 100 [%] is the ratio of the length as described above.

[0165] [Regarding the average thickness of the third coating layer 251] The average thickness of the third coating layer 251 according to the present embodiment is preferably within the range of 10 to 20 μm. By making the average thickness of the third coating layer 251 within the above range, it becomes possible to further improve the corrosion resistance of the third coating layer 251 according to the present embodiment. The average thickness of the third coating layer 251 is more preferably 15 μm or more. Also, the average thickness of the third coating layer 251 is more preferably 18 μm or less.

[0166] Here, the average thickness of the above third coating layer 251 can be measured as follows by cross-section observation using SEM. More specifically, we focus on an arbitrary cross-section obtained by cutting the steel pipe 1 according to this embodiment in the radial direction perpendicular to the longitudinal direction of the steel pipe 1 (the Z-axis direction in Figure 1). The obtained cross-section is observed at a magnification of approximately 500 times using a SEM, and the thickness of the third coating layer 251 is measured at 10 arbitrary locations within the observation field using the length measuring function implemented in the SEM. The average thickness of the third coating layer 251 can be calculated by averaging the 10 measurements obtained in this way over the number of measurement locations.

[0167] The steel pipe 1 according to this embodiment has been described in detail above with reference to Figures 1 to 5.

[0168] (Regarding the manufacturing method of steel pipes) Next, an example of a manufacturing method for the steel pipe 1 according to this embodiment will be briefly described below.

[0169] <Manufacturing method for Zn-Al-Mg alloy plated steel sheet, which is the raw material> The Zn-Al-Mg alloy plated steel sheet used as the material for the steel pipe 1 according to this embodiment is manufactured by using the steel material described above as the base material and forming a Zn-Al-Mg alloy plating layer on the surface of the steel material.

[0170] In this process, the Zn-Al-Mg alloy plating layer can be formed using methods such as hot-dip galvanizing, thermal spraying, cold spraying, sputtering, and vapor deposition. However, hot-dip galvanizing is the most preferable method in terms of cost.

[0171] The following describes in detail an example of a manufacturing method for obtaining a Zn-Al-Mg alloy plating layer using a hot-dip galvanizing method. In the manufacturing process of such a Zn-Al-Mg alloy plating layer, first, a steel sheet, as an example of the steel material to be used as the base material, is rolled to the desired thickness using the Zenzimir method, then wound into a coil, and placed on the hot-dip galvanizing line.

[0172] In a hot-dip galvanizing line, steel sheets are continuously fed through a coil. At this time, the steel sheets are heated and reduced at 800°C in an annealing facility installed on the line, for example, in an environment where oxidation is unlikely to occur, such as an oxygen concentration of 20 ppm or less, in an N2-5%H2 gas atmosphere. After that, they are air-cooled with N2 gas to approximately 20°C above the temperature of the subsequent plating bath, and then immersed in the plating bath.

[0173] Here, a molten plating alloy having the chemical composition of the first coating layer 211 as described above is prepared in the plating bath. The temperature of the plating bath is set to be above the melting point of the plating alloy (for example, around 460-660°C).

[0174] When preparing the material for the plating alloy, it is preferable to use pure metal (purity of 99% or higher) as the alloying material. First, a predetermined amount of alloying metal is mixed to obtain the composition of the plating layer as described above, and then completely melted into an alloy using a high-frequency induction furnace or arc furnace under vacuum or inert gas displacement conditions. Furthermore, the alloy mixed with the predetermined components (composition of the plating layer described above) is melted in the atmosphere, and the resulting molten material is used as the plating bath.

[0175] Furthermore, there are no particular restrictions on using pure metals when producing the plating alloys described above; existing Zn alloys, Mg alloys, and Al alloys can be melted and used. In this case, as long as an alloy with a predetermined composition and low impurity levels is used, there will be no problems.

[0176] After immersing the steel plate in the plating bath described above, it is withdrawn at a predetermined speed. At this time, the amount of plating deposited is controlled, for example, by N2 wiping gas, so that the zinc-based plating layer formed reaches the desired thickness. Here, conditions other than the bath temperature can be those of general plating operation, and no special equipment or conditions are required.

[0177] Furthermore, various heat treatments may be applied to the molten plated alloy located on the steel plate, as needed.

[0178] <Manufacturing of welded galvanized steel pipes> The Zn-Al-Mg alloy plated steel sheet manufactured as described above is subjected to bending to bring the widthwise ends of the plated steel sheet together, and the joined ends are welded. Here, the conditions for bending the plated steel sheet and the welding conditions are not particularly specified, and various known conditions and methods can be used as appropriate. As a result, a welded plated steel pipe can be obtained in which the Zn-Al-Mg alloy plated layer (i.e., the first coating layer 211) is located on the surface of the steel base 11 in the base steel pipe 10 composed of the steel base 11 and the welded part 13.

[0179] <Grinding process for protruding parts formed by welding> The welded and plated steel pipes manufactured as described above have protruding portions formed by the welding process, as explained earlier. Therefore, these protruding portions are ground down using various known grinding methods. There are no specific requirements for the grinding method or grinding conditions of the protruding portions; any known method may be used.

[0180] <Repair treatment of welded joints> In order to ensure the corrosion resistance of the welded portion whose protruding part has been ground down as described above, a repair treatment as described below is performed to form a second coating layer 231 that at least covers the welded portion.

[0181] [Fume removal treatment] Prior to the formation of the second coating layer 231, fumes that may be present in the ground portion of the protruding part and its surrounding area are removed. Various methods exist for removing such fumes, including physical and chemical methods, but in the manufacturing method of the steel pipe 1 according to this embodiment, a physical removal method (mechanical removal method) is employed.

[0182] Here, the physical removal method (mechanical removal method) of fumes is not particularly limited, and various known methods such as grinding using a grinder, grinding using a wire brush, and blasting can be used.

[0183] As described above, the grinding process removes fumes from the ground portion, but along with the fumes, a portion of the first coating layer 211 that was present around the ground portion is also removed. In the manufacturing method of the steel pipe 1 according to this embodiment, the processing conditions in this fume removal process are controlled to control the remaining state of the first coating layer 211 that remains around the welded portion 13 to a desired state. By achieving a region around the welded portion 13 where the first coating layer 211 has been partially removed to a desired state, a third region 25 of the steel pipe 1 according to this embodiment is formed in the region located around the welded portion 13 after the thermal spraying process described later.

[0184] As a method for removing fumes, for example, by using a grinder or blasting process, it is possible to leave the first coating layer 211 distributed in a dotted pattern when viewed from a plane perpendicular to the surface of the steel pipe. Alternatively, as a method for removing fumes, for example, by using a wire brush, it is possible to leave the first coating layer 211 distributed in a streaky pattern when viewed from above on the surface of the steel pipe.

[0185] Here, it is preferable to control the processing conditions for the removal treatment so that, for example, the area ratio of the remaining first coating layer 211 in a plan view (hereinafter sometimes simply referred to as the "remaining area ratio") is 5 to 20% of the total area of ​​the first coating layer 211 that has undergone the removal treatment. The area of ​​the first coating layer 211 that has undergone the removal treatment is defined as the area of ​​the first coating layer 211 that has been ground, that is, the area from the boundary between the ground portion of the fume and the first coating layer 211 until the exposure of the steel substrate 11 disappears. Considering the fume removal method described above, this area of ​​the first coating layer 211 that has undergone the removal treatment is thought to be approximately 3 to 10 mm from the end of the weld, moving away from the weld.

[0186] Here, the remaining area ratio of the first coating layer 211 in the "region of the first coating layer 211 that has undergone fume removal treatment" can be measured by utilizing the fact that the steel substrate 11 is exposed by the fume removal treatment. In other words, the remaining area ratio of the first coating layer 211 can be measured by performing various color tests that produce a color reaction in the area of ​​the first coating layer 211 that has undergone fume removal treatment on the "region of the first coating layer 211 that has undergone fume removal treatment".

[0187] More specifically, the area for the color test is defined as the range from the end of the weld to any point on the outer perimeter of the "area of ​​the first coating layer 211 that has undergone fume removal treatment" (a range of distance from the end of the weld to the outer perimeter × 30 mm). The results of the color test obtained are then analyzed by image processing to calculate the area ratio in plan view. Similarly, the same color test is performed at two other locations randomly selected in the circumferential direction of the steel pipe 1, and the area ratio in plan view is calculated for each of these locations. The average of the area ratios obtained at these three locations is defined as the "area ratio of the remaining first coating layer 211 in plan view."

[0188] By keeping the remaining area ratio of the first coating layer 211 within the range described above, the Al and Mg contained in the remaining first coating layer 211 can be sufficiently dispersed within the thermal spray structure formed by the thermal spray treatment described later. Furthermore, because the remaining area ratio of the first coating layer 211 is within the range described above, phase A, described later, is sufficiently formed at the interface on the steel substrate side. With the formation of these structures, the third coating layer 251 according to this embodiment exhibits excellent corrosion resistance.

[0189] The above color test is not particularly limited, and various color tests can be applied. However, it is preferable to use, for example, the ferroxyl test specified in JIS H8626:1995, or the pinhole test disclosed in non-patent literature (Shozo Matsuda, Tadashi Tanaka, Keiichi Tanikawa, "A New Pinhole Test Method for Galvanized Steel Sheets," Metal Surface Technology, Vol. 23 (1972), pp. 142-146). In the above two types of color tests, a reagent for the color reaction is applied to the surface of the test specimen to induce a color reaction, and then the surface of the test specimen is covered with test paper to transfer the area where the color reaction occurred to the test paper. Therefore, by using these color tests, it is possible to identify the area where the steel substrate 11 is exposed, regardless of the shape of the steel pipe 1.

[0190] Furthermore, in the region of the first coating layer 211 that has undergone the fume removal treatment described above, the amount of fume adhering to the first coating layer 211 is 6.5 to 25.0 g / m². 2 It is preferable to keep it within this range. By leaving the first coating layer 211 with such an amount of adhesion, the Al, Mg, and phase A contained in the first coating layer 211 can be sufficiently dispersed in the thermal spray structure formed by the thermal spray treatment described later, and the corrosion resistance of the formed second coating layer 231 can be further improved. This amount of adhesion can be determined by using the "area of ​​the first coating layer 211 that has undergone fume removal treatment" as the measurement target area and performing the same method as the "method for measuring the amount of adhesion of the first coating layer 211" described earlier.

[0191] Furthermore, when the first coating layer 211 is left in a streaky pattern, it is preferable that the spacing between the remaining streaks of the first coating layer 211 be within the range of, for example, 10 to 100 μm. By setting the spacing between the streaks within this range, the Al and Mg contained in the first coating layer 211 can be sufficiently dispersed within the thermal spray structure formed by the thermal spray treatment described later, thereby further improving the corrosion resistance of the formed second coating layer 231. Here, the spacing between the remaining streaks of the first coating layer 211 as described above can be measured by observing an arbitrary cross-section with a scanning electron microscope (for example, a scanning electron microscope JSM-7000F manufactured by JEOL Ltd.) at a magnification of approximately 500x and using the length measuring function implemented in the SEM.

[0192] Furthermore, depending on the distribution state of the first coating layer 211 that remains as described above, it is also possible to control the distribution state of phase A in the third coating layer 251 formed by the thermal spraying treatment described later to a desired state.

[0193] Here, the remaining state of the first coating layer 211 as described above can be controlled to a desired state by selecting the cutting method to be used and adjusting the cutting conditions adopted (for example, the pressing pressure of the cutting tool, the cutting speed, the material of the abrasive, etc.).

[0194] [Thermal spraying treatment] After the fumes have been removed from the weld and the surrounding area as described above, thermal spraying is performed. This thermal spraying process forms the second coating layer 231 as previously explained, and also forms a third coating layer 251 in the area surrounding the weld.

[0195] In the thermal spraying treatment of the welded area, first, an Al layer 233 is formed using a desired thermal spraying material mainly composed of Al, and then a Zn layer 235 is formed using a desired thermal spraying material mainly composed of Zn. The thermal spraying method is not particularly limited, and known thermal spraying methods such as arc spraying and gas flame spraying can be used. Furthermore, specific thermal spraying conditions are not specified.

[0196] However, in the steel pipe 1 according to this embodiment, in order to achieve a more favorable second coating layer 231, it is preferable that the Al layer 233 be formed by arc spraying and the Zn layer 235 be formed by gas flame spraying.

[0197] Here, "arc spraying" refers to a thermal spraying method in which a DC arc discharge is generated at the tips of two spray materials (spray cores, wires) continuously fed from a spray gun, and the molten metal is blown away with compressed air or the like. "Gas flame spraying," on the other hand, refers to a thermal spraying method in which spray materials (spray cores, wires) are continuously supplied from a spray gun into a combustion flame filled with fuel and oxygen gas, and the molten material within the flame is blown away with compressed air or the like.

[0198] Arc sprayed layers, compared to gas flame sprayed layers, have a higher temperature of molten metal, resulting in higher adhesion strength to the weld. Therefore, forming an Al layer 233 on the weld by arc spraying can further improve the adhesion between the weld 13 and the Al layer 233. In addition, arc spraying has a higher energy density for melting the spray material and a faster film formation rate compared to gas flame spraying. Therefore, arc spraying makes it easy to form a thick Al layer 233.

[0199] On the other hand, in arc spraying, the metal molten by the intermittent arc discharge at the tip of the spray wire becomes particles and is welded to the surface of the weld, so the thickness of the sprayed material tends to vary along the longitudinal direction of the weld. Therefore, by forming a Zn layer 235 by gas flame spraying on the Al layer 233 by arc spraying, the variation in the thickness of the material along the longitudinal direction described above can be reduced, and the corrosion resistance of the entire second coating layer 231 can be stably improved.

[0200] Furthermore, generally speaking, layers formed by arc spraying tend to have a rougher surface than layers formed by gas flame spraying. Therefore, by forming a Zn layer 235 by gas flame spraying on an Al layer 233 formed by arc spraying, it is possible to improve the adhesion between the Al layer 233 and the Zn layer 235 through a so-called anchoring effect.

[0201] Here, for the thermal spray material used in the thermal spraying process described above, a metal element or alloy containing the desired element may be used as appropriate in order to achieve the desired chemical composition of the second coating layer 231.

[0202] ≪Thermal spray temperature≫ In the manufacturing method of the steel pipe 1 according to this embodiment, the temperature of the steel pipe during thermal spraying is one of the important factors in order to achieve the desired distribution of the metal structure of phases A to C in the third coating layer 251 as described above.

[0203] Specifically, prior to carrying out the thermal spraying treatment described above, it is important to control the steel pipe temperature to, for example, within the range of 300.0 to 600.0°C. If the steel pipe temperature is below 300.0°C, the structure created by the thermal spraying and the first coating layer (plating layer) will not melt sufficiently, resulting in a layered distribution of Al and Zn, making it highly likely that the phase A to phase C distribution state described earlier cannot be achieved. Therefore, in order to achieve the desired phase A to phase C distribution state, it is preferable to set the steel pipe temperature to 300.0°C or higher.

[0204] On the other hand, if the steel pipe temperature during thermal spraying exceeds 600.0°C, phases B and B' do not exist in an appropriate state, and phase A, which originates from the Zn-Al-Mg plating layer, does not concentrate near the interface on the steel substrate 11 side, but instead becomes dispersed throughout the third coating layer 251.

[0205] In particular, by keeping the steel pipe temperature during thermal spraying within the range of 350.0 to 500.0°C, it becomes possible to achieve a more favorable distribution state for phases A and B.

[0206] ≪Cooling rate of steel pipes to room temperature after thermal spraying≫ Furthermore, in order to achieve a more favorable distribution state of phases A to C in the third coating layer 251, it is preferable to set the cooling rate of the steel pipe to room temperature after thermal spraying to a desired state. For example, if the cooling rate of the steel pipe to room temperature after thermal spraying is too fast, there is a high possibility that the second coating layer produced by thermal spraying will not obtain the desired structure. On the other hand, if the cooling rate of the steel pipe to room temperature after thermal spraying is too slow, the plating structure present near the sprayed area may change, potentially reducing corrosion resistance. From the above viewpoint, it is preferable that the cooling rate of the steel pipe to room temperature after thermal spraying is, for example, in the range of 20 to 60°C / min. More preferably, the cooling rate of the steel pipe to room temperature after thermal spraying is in the range of 30 to 50°C / min. Note that there are no particular restrictions on the cooling method for achieving the above cooling rates, and various known cooling methods such as air cooling and water cooling can be employed.

[0207] The above briefly describes an example of a method for manufacturing the steel pipe 1 according to this embodiment. [Examples]

[0208] The steel pipes according to the present invention will be described in more detail below, with reference to examples and comparative examples. Note that the following examples are merely examples of steel pipes according to the present invention, and the steel pipes according to the present invention are not limited to the examples below.

[0209] (Example test) <Manufacturing of plated steel pipes> In the following test examples, plated steel pipes were manufactured using plated steel materials having the three types of plating layers shown in Table 1. For the eight types of plated steel materials shown in Table 1, cold-rolled steel sheets were used as the base material. Furthermore, the average thickness of the plating layer with the following plating composition for each plated steel material was 20 μm.

[0210] [Table 1]

[0211] Each of the above-mentioned plated steel materials was formed into an open pipe, butted together, and then plated steel pipes were created by high-frequency welding.

[0212] In each plated steel pipe obtained as described above, the plating composition shown in Table 1 is the plating composition of the first coating layer described earlier. Furthermore, when the circumferential length of the region in each plated steel pipe where the thickness of the plating layer (i.e., the first coating layer) having the above-described plating composition is 5.0 μm or more was measured separately, it was confirmed that it existed continuously along the circumferential direction for 1.0 mm or more.

[0213] <Cutting of protruding parts> After high-frequency welding, the protruding portions of the welded areas formed by the high-frequency welding were machined off each of the resulting plated steel pipes.

[0214] <Fume Removal> After cutting off the protruding portions of each plated steel pipe, fumes generated by the cutting process were removed using a grinder or wire brush. Following fume removal, the remaining state of the first coating layer near the fume removal area was confirmed using the method described earlier. The results are summarized in Table 4 below.

[0215] <Repair treatment by thermal spraying> Thermal spraying was performed on the welded area and its surrounding region from which the fumes had been removed, using the thermal spray materials (welding wires) shown in Tables 2 and 3 below. This formed the second coating layer as described above, and also formed a third coating layer in the region located between the second and first coating layers.

[0216] In the thermal spraying process described above, the plated steel pipes, after fume removal, were preheated to the temperatures shown in Table 4 below. The heated plated steel pipes were then sprayed with thermal spraying material for the Al layer by arc spraying, and thermal spraying material for the Zn layer by gas flame spraying. Afterward, the sprayed steel pipes were cooled to room temperature at the cooling rates shown in Table 4 below. The thermal spraying conditions other than the steel pipe temperature and cooling rate for arc spraying and gas flame spraying are as follows.

[0217] In the thermal spraying process, the transport speed of the plated steel pipe was set to 80 m / min. For arc spraying, a high-speed inverter drive thermal spraying machine (ARC-BOY A400 manufactured by D-Tech Co., Ltd.) was used, with a welding wire feed speed of 13.0 m / min, compressed air pressure of 0.52 MPa, voltage of 21.4 V, current of 170 A, and a distance of 30 mm between the spray gun and the weld. For gas flame spraying, a gas-fired metallizing spraying machine (CD-3 type gas flame metal wire spraying machine manufactured by Koken Techno Co., Ltd.) was used, with a wire feed speed of 4.2 m / min, compressed air pressure of 0.40 MPa, and a distance of 30 mm between the spray gun and the weld.

[0218] [Table 2]

[0219] [Table 3]

[0220] [Table 4]

[0221] As described above, each of the steel pipes shown in Table 4 was manufactured. In this test example, multiple steel pipes were manufactured for each level shown in Table 4 and used for measuring and evaluating various properties.

[0222] For each steel pipe obtained as described above, various characteristics related to the second region (second coating layer) and the third region (third coating layer) were measured using the method previously described. The results obtained are summarized in Table 5 below. Note that the impurities in the chemical composition of the third region and third coating layer in Table 5 are omitted because they are present in trace amounts and of multiple types.

[0223] <Evaluation Method> Each steel pipe obtained as described above was evaluated from the viewpoint of corrosion resistance. The evaluation method and evaluation criteria are as follows.

[0224] [Corrosion resistance] Each obtained steel pipe was subjected to a salt spray test (SST) as specified in JIS Z2371:2015, with each test lasting 72 hours, three times to evaluate its corrosion resistance. After the above tests, the steel pipes were pickled with hydrochloric acid to remove the first to third coating layers and corrosion products. Subsequently, the corrosion status of the surface of the raw steel material (more specifically, the surface in the area from the center of the weld to a position 5 cm away from the weld) was investigated using a shape measuring instrument (Keyence Corporation VR-5000). The evaluation criteria were as follows: Scores of AAA, AA, A, and B were considered pass. The results obtained are summarized in Table 5 below. Rating AAA: No pits formed in the steel material even after three consecutive tests. AA: For steel materials subjected to three consecutive tests, 1 to 3 pits were formed within the above range. A: For steel materials subjected to three consecutive tests, 4 to 10 pits were formed within the above-mentioned range. B: No pits formed in the steel material that underwent testing once, but in the steel material that underwent testing three consecutive times, pits formed in 11 or more locations within the above range. C: Pits were formed in the steel material that underwent one test.

[0225] [Table 5]

[0226] As is clear from Table 5 above, the steel pipes corresponding to the examples of the present invention exhibit excellent corrosion resistance, while the steel pipes corresponding to the comparative examples of the present invention had insufficient corrosion resistance.

[0227] As described above, the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that those having ordinary knowledge in the technical field to which the present invention pertains can conceive of various modification examples or correction examples within the scope of the technical idea described in the claims, and it is naturally understood that these also belong to the technical scope of the present invention.

[0228] The embodiments disclosed this time are illustrative in all respects and not restrictive. The above embodiments may be omitted, substituted, or changed in various forms without departing from the scope of the claims and the configuration and the gist thereof belonging to the technical scope of the present invention as described later. For example, the constituent elements of the above embodiments can be arbitrarily combined within a range that does not impair their effects. Also, from such an arbitrary combination, the actions and effects of each constituent element involved in the combination can be naturally obtained, and other actions and other effects that are obvious to those skilled in the art from the description of this specification can be obtained.

[0229] Also, the effects described in this specification are merely explanatory or illustrative and not restrictive. That is, the technology according to the present invention can exhibit other effects that are obvious to those skilled in the art from the description of this specification, together with or instead of the above effects.

[0230] In addition, the following configurations also belong to the technical scope of the present invention. (1) In any cross-section obtained by cutting the steel pipe in the pipe diameter direction so as to be orthogonal to the longitudinal direction, in mass%, Al: 15.0 to 30.0%, Mg: 5.0 to 15.0% A first coating layer exists which contains and the remainder consists of Zn and impurities. A first region in which the thickness of the first coating layer is 5.0 μm or more extends continuously along the circumferential direction of the steel pipe for 1.0 mm or more, In mass%, Zn: 0-20.0% It contains an Al layer, the remainder being Al and impurities, Located on the aforementioned Al layer, by mass%, Al: 0.1-20.0% A Zn layer containing Zn and impurities, the remainder being Zn and impurities, It has, A second coating layer exists in which the average thickness of the Al layer is 10.0 to 100.0 μm and the average thickness of the Zn layer is 3.0 to 50.0 μm. A second region in which the thickness of the second coating layer is 15.0 μm or more extends continuously along the circumferential direction for 500.0 μm or more, A third region located between the first region and the second region, in which a third coating layer mainly composed of Zn or Al exists, Includes, The third coating layer is As a metallic structure, Phase A, which is predominantly located at the interface on the steel substrate side of the third coating layer, contains 0.1 to 40.0 mass% Al and 0.1 to 10.0 mass% Mg, with the remainder being Zn and impurities, and is predominantly located at the interface on the steel substrate side of the third coating layer. Phase B, which contains 0 to 20.0 mass% Zn and the remainder being Al and impurities, is located on the steel substrate or on the aforementioned phase A. It contains 0.1 to 20.0 mass% of Al, with the remainder being Zn and impurities, and is located on phase B, It has, and further, The aforementioned phase B has a phase B' containing 0.1 to 10.0% by mass of Mg, The area ratio of phase B' in the cross-section of phase B is 3.0 to 30.0% of the area of ​​phase B. A steel pipe in which, in the third region, the length along the circumferential direction of the region in which the thickness of the third coating layer is less than 5.0 μm is less than 100.0 μm, and the proportion occupied by the region in which the thickness is less than 5.0 μm is 40.0% or less in total with respect to the total length along the circumferential direction of the third coating layer. (2) In any cross-section obtained by cutting a steel pipe in the radial direction perpendicular to the longitudinal direction, In mass%, Al: 15.0-30.0% Mg: 5.0~15.0% It contains, and further contains one or more elements selected from the group consisting of element groups A to G below, with the remainder being Zn and impurities, and there is a first coating layer. A first region in which the thickness of the first coating layer is 5.0 μm or more extends continuously along the circumferential direction of the steel pipe for 1.0 mm or more, In mass%, Zn: 0-20.0% It contains, and further contains one or more elements selected from the group consisting of element groups H to J below, with the remainder being an Al layer consisting of Al and impurities, Located on the aforementioned Al layer, by mass%, Al: 0.1-20.0% It contains, and further contains one or more elements selected from the group consisting of element groups H to J below, with the remainder being a Zn layer consisting of Zn and impurities, It has, A second coating layer exists in which the average thickness of the Al layer is 10.0 to 100.0 μm and the average thickness of the Zn layer is 3.0 to 50.0 μm. A second region in which the thickness of the second coating layer is 15.0 μm or more extends continuously along the circumferential direction for 500.0 μm or more, A third region is located between the first region and the second region, and contains a third coating layer mainly composed of Zn or Al, and further containing one or more elements selected from the group consisting of element groups A to J below, Includes, The third coating layer is As a metal structure, containing 0.1 to 40.0% by mass of Al, 0.1 to 10.0% by mass of Mg, with the balance being composed of Zn and impurities, and a phase A unevenly distributed in the interface portion on the steel substrate side of the third coating layer, containing 0 to 20.0% by mass of Zn, with the balance being composed of Al and impurities, and a phase B located on the steel substrate or on the phase A, containing 0.1 to 20.0% by mass of Al, with the balance being composed of Zn and impurities, and a phase C located on the phase B, having, and further, the phase B has a phase B' containing 0.1 to 10.0% by mass of Mg, the area ratio of the phase B' in the cross-section of the phase B is 3.0 to 30.0% with respect to the area of the phase B, in the third region, the length along the circumferential direction of the region where the thickness of the third coating layer is less than 5.0 μm is less than 100.0 μm respectively, and the ratio occupied by the region where the thickness is less than 5.0 μm is 40.0% or less in total with respect to the entire length along the circumferential direction of the third coating layer, steel pipe. [Element group A]: One or two selected from the group consisting of Si: 2.50% or less, Fe: 5.00% or less [Element group B]: One or more selected from the group consisting of Sb: 0.50% or less, Pb: 0.50% or less, Sr: 0.50% or less [Element group C]: One or more selected from the group consisting of Cu: 0.25% or less, Ti: 0.25% or less, Cr: 0.25% or less, Nb: 0.25% or less, Ni: 0.25% or less, Mn: 0.25% or less, Mo: 0.25% or less, Co: 0.25% or less, V: 0.25% or less [Element group D]: One or more selected from the group consisting of Sn: 2.00% or less, Bi: 0.50% or less, In: 2.00% or less [Element group E]: One or more selected from the group consisting of Zr: 0.25% or less, Ag: 0.25% or less, Li: 0.25% or less [Element group F]: One or more selected from the group consisting of Ca: 3.00% or less, La: 0.50% or less, Ce: 0.50% or less, Y: 0.5% or less [Element group G]:B:0.50% or less [Element Group H]: One or more elements selected from the group consisting of C: 0.300% or less, S: 0.010% or less, and N: 0.006% or less. [Element group I]:Fe:5.00% or less [Element group J]: One or two elements selected from the group consisting of Mn: 1.50% or less, Ti: 0.10% or less. (3) The steel pipe according to (1) or (2), wherein the total length of the third coating layer along the circumferential direction is 500 to 1500 μm. (4) The steel pipe according to (3), wherein at the interface between the third coating layer and the steel substrate in the cross-section, the total length along the circumferential direction of the portion corresponding to phase A at the location of the interface is 30 to 60% of the total length along the circumferential direction of the interface. (5) The steel pipe described in any one of (1) to (4), wherein the average thickness of the third coating layer is 10 to 150 μm. (6) The steel pipe is a welded-plated steel pipe. The second coating layer is located on the welded portion of the welded plated steel pipe, and is the steel pipe according to any one of (1) to (5). (7) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group A. (8) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group B. (9) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group C. (10) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group D. (11) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group E. (12) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group F. (13) The steel pipe according to (2), wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group G. (14) The steel pipe according to (2), wherein at least one of the second coating layer or the third coating layer has a chemical composition containing the element group H. (15) The steel pipe according to (2), wherein at least one of the second coating layer or the third coating layer has a chemical composition containing the element group I. (16) The steel pipe according to (2), wherein at least one of the second coating layer or the third coating layer has a chemical composition containing the element group J. [Explanation of symbols]

[0231] 1 Steel pipe 10 Untreated steel pipes 11 Steel base 13. Welded section 20 Coating layer 21 First area 211 1st coating layer 23 Second area 231 Second coating layer 233 Al layer 235 Zn layer 25 Third area 251 Third coating layer 253 Phase A 255 Phase B 257 Phase B' 259 Phase C

Claims

1. In any cross-section obtained by cutting a steel pipe in the radial direction perpendicular to the longitudinal direction, In mass percent, Al: 15.0-30.0%, Mg: 5.0-15.0% A first coating layer exists which contains and the remainder consists of Zn and impurities. A first region in which the thickness of the first coating layer is 5.0 μm or more extends continuously along the circumferential direction of the steel pipe for 1.0 mm or more, In mass percent, Zn: 0-20.0% It contains an Al layer, the remainder of which consists of Al and impurities, Located on the aforementioned Al layer, in mass% of, Al: 0.1-20.0% It contains a Zn layer, the remainder of which consists of Zn and impurities, It has, A second coating layer exists in which the average thickness of the Al layer is 10.0 to 100.0 μm and the average thickness of the Zn layer is 3.0 to 50.0 μm. A second region in which the thickness of the second coating layer is 15.0 μm or more extends continuously along the circumferential direction for 500.0 μm or more, A third region located between the first region and the second region, wherein a third coating layer mainly composed of Zn or Al exists, Includes, The third coating layer is As a metallic structure, Phase A, which is predominantly located at the interface on the steel substrate side of the third coating layer, contains 0.1 to 40.0% by mass of Al and 0.1 to 10.0% by mass of Mg, with the remainder being Zn and impurities, Phase B, which contains 0 to 20.0% by mass of Zn and the remainder being Al and impurities, is located on the steel substrate or on the aforementioned phase A. It contains 0.1 to 20.0% by mass of Al, with the remainder being Zn and impurities, and is located on the aforementioned phase B, It has, and further, The aforementioned phase B has a phase B' containing 0.1 to 10.0% by mass of Mg, The area ratio of phase B' in the cross-section of phase B is 3.0 to 30.0% of the area of ​​phase B. A steel pipe in which, in the third region, the length along the circumferential direction of the region in which the thickness of the third coating layer is less than 5.0 μm is less than 100.0 μm, and the proportion occupied by the region in which the thickness is less than 5.0 μm is 40.0% or less in total with respect to the total length of the third coating layer along the circumferential direction.

2. In any cross-section obtained by cutting a steel pipe in the radial direction perpendicular to the longitudinal direction, In mass percent, Al: 15.0-30.0%, Mg: 5.0-15.0% It contains, and further contains one or more elements selected from the group consisting of elements A to G below, with the remainder being Zn and impurities, and there is a first coating layer. A first region in which the thickness of the first coating layer is 5.0 μm or more extends continuously along the circumferential direction of the steel pipe for 1.0 mm or more, In mass percent, Zn: 0-20.0% It contains, and further contains one or more elements selected from the group consisting of element groups H to J below, with the remainder being an Al layer consisting of Al and impurities, Located on the aforementioned Al layer, in mass% of, Al: 0.1-20.0% It contains, and further contains one or more elements selected from the group consisting of element group H to element group J below, with the remainder being a Zn layer consisting of Zn and impurities, It has, A second coating layer exists in which the average thickness of the Al layer is 10.0 to 100.0 μm and the average thickness of the Zn layer is 3.0 to 50.0 μm. A second region in which the thickness of the second coating layer is 15.0 μm or more extends continuously along the circumferential direction for 500.0 μm or more, A third region is located between the first region and the second region, and contains a third coating layer that is mainly composed of Zn or Al and further contains one or more elements selected from the group consisting of element groups A to J below, Includes, The third coating layer is As a metallic structure, Phase A, which is predominantly located at the interface on the steel substrate side of the third coating layer, contains 0.1 to 40.0% by mass of Al and 0.1 to 10.0% by mass of Mg, with the remainder being Zn and impurities, Phase B, which contains 0 to 20.0% by mass of Zn and the remainder being Al and impurities, is located on the steel substrate or on the aforementioned phase A. It contains 0.1 to 20.0% by mass of Al, with the remainder being Zn and impurities, and is located on the aforementioned phase B, It has, and further, The aforementioned phase B has a phase B' containing 0.1 to 10.0% by mass of Mg, The area ratio of phase B' in the cross-section of phase B is 3.0 to 30.0% of the area of ​​phase B. A steel pipe in which, in the third region, the length along the circumferential direction of the region in which the thickness of the third coating layer is less than 5.0 μm is less than 100.0 μm, and the proportion occupied by the region in which the thickness is less than 5.0 μm is 40.0% or less in total with respect to the total length of the third coating layer along the circumferential direction. [Element Group A]: One or two elements selected from the group consisting of Si: 2.50% or less, Fe: 5.00% or less. [Element Group B]: One or more elements selected from the group consisting of Sb: 0.50% or less, Pb: 0.50% or less, and Sr: 0.50% or less. [Element Group C]: One or more elements selected from the group consisting of Cu: 0.25% or less, Ti: 0.25% or less, Cr: 0.25% or less, Nb: 0.25% or less, Ni: 0.25% or less, Mn: 0.25% or less, Mo: 0.25% or less, Co: 0.25% or less, V: 0.25% or less. [Element Group D]: One or more elements selected from the group consisting of Sn: 2.00% or less, Bi: 0.50% or less, and In: 2.00% or less. [Element Group E]: One or more elements selected from the group consisting of Zr: 0.25% or less, Ag: 0.25% or less, and Li: 0.25% or less. [Element Group F]: One or more elements selected from the group consisting of Ca: 3.00% or less, La: 0.50% or less, Ce: 0.50% or less, and Y: 0.5% or less. [Element group G]: B: 0.50% or less [Element Group H]: One or more elements selected from the group consisting of C: 0.300% or less, S: 0.010% or less, and N: 0.006% or less. [Element group I]: Fe: 5.00% or less [Element group J]: One or two elements selected from the group consisting of Mn: 1.50% or less, Ti: 0.10% or less.

3. The steel pipe according to claim 1 or 2, wherein the total length of the third coating layer along the circumferential direction is 500 to 1500 μm.

4. The steel pipe according to claim 3, wherein, at the interface between the third coating layer and the steel substrate in the cross-section, the total length along the circumferential direction of the portion corresponding to phase A at the interface is 30 to 60% of the total length along the circumferential direction of the interface.

5. The steel pipe according to claim 1 or 2, wherein the average thickness of the third coating layer is 10 to 20 μm.

6. The steel pipe is a welded-plated steel pipe. The steel pipe according to claim 1 or 2, wherein the second coating layer is located on the welded portion of the welded plated steel pipe.

7. The steel pipe according to claim 2, wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group A.

8. The steel pipe according to claim 2, wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group B.

9. The steel pipe according to claim 2, wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group C.

10. The steel pipe according to claim 2, wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group D.

11. The steel pipe according to claim 2, wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group E.

12. The steel pipe according to claim 2, wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group F.

13. The steel pipe according to claim 2, wherein at least one of the first coating layer or the third coating layer has a chemical composition containing the element group G.

14. The steel pipe according to claim 2, wherein at least one of the second coating layer or the third coating layer has a chemical composition containing the element group H.

15. The steel pipe according to claim 2, wherein at least one of the second coating layer or the third coating layer has a chemical composition containing the element group I.

16. The steel pipe according to claim 2, wherein at least one of the second coating layer or the third coating layer has a chemical composition containing the element group J.

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