Method for manufacturing electric resistance welded steel pipe, and electric resistance welded steel pipe

Abstract

Provided is a method for manufacturing an electric-resistance-welded steel pipe, the method comprising: an end surface forming step for forming an end surface of an electric-resistance-welded steel pipe; on a first end face and a second end face on both sides in the width direction of the plated steel sheet, an outer inclined portion inclined from the outer surface side toward the inner surface side with respect to the plate thickness direction is formed. Or, an outer inclined part and an inner inclined part inclined relative to the plate thickness direction from the inner surface side to the outer surface side are respectively formed on a first end surface and a second end surface on two sides in the width direction of the plated steel plate, so that the plate thickness of the first end surface and the second end surface is reduced from the center in the width direction to the end surfaces; and a pipe manufacturing step in which the formed first end surface and second end surface are abutted and welded to manufacture a pipe. As a result, in an electric resistance welded steel pipe using a plated steel sheet as an original sheet, a decrease in corrosion resistance around the welded portion is suppressed.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing an electric resistance welded steel pipe and the electric resistance welded steel pipe. Background Art

[0002] Plated steel sheets exhibit high corrosion resistance due to the corrosion protection provided by the coating on the surface covered with the coating. However, when plated steel sheets are welded, the vicinity of the weld becomes locally hot, causing the coating components to evaporate and reduce corrosion resistance. Furthermore, when the weld bead formed by welding is removed, as in the case of electric resistance welded steel pipes, the plating near the weld is also partially removed along with the weld bead. This exposes the iron matrix around the weld, potentially further expanding the area of ​​reduced corrosion resistance.

[0003] Therefore, in the manufacturing process of electric resistance welded steel pipes using plated steel sheets as the base plate, it is usually necessary to perform post-repair such as spraying the main component of the coating (e.g., Zn) around the weld to compensate for the reduction in corrosion resistance around the weld. However, the corrosion resistance around the weld after the repair does not necessarily return to the same level as other parts. Due to the reduction in corrosion resistance around the weld, in fields requiring higher corrosion resistance, pre-plated products obtained by processing plated steel sheets are not used, but post-plated products obtained by immersing the plated steel sheets in hot dip coating after processing are used. In view of such circumstances, in order to improve productivity, it is required to improve the corrosion resistance around the weld of electric resistance welded steel pipes using plated steel sheets as the base plate.

[0004] As a technology related to the production of conventional electric-resistance-welded steel pipes, for example, Patent Document 1 discloses a technique in which a strip end is tapered with an inclined surface, and then resistance welding is performed with the butt joint angle of the vertical end face of the strip set within a range of ±1 degree. The inclined surface is connected from the vertical end face to both the outer and inner diameter ends of the strip in the thickness direction. Furthermore, Patent Document 2 discloses a technique in which both the outer and inner surfaces of an open pipe formed by forming a steel strip into a substantially tubular shape are tapered, and then resistance welding is performed with an upset depth of 40% to 75% of the strip thickness. The angle formed by the inclined surface with the vertical end face of the strip is 15° to 50°, and the length of the inclined surface from the strip surface in the thickness direction is 10% to 45% of the strip thickness.

[0005] Furthermore, as a technique for manufacturing welded pipes by butt-welding the widthwise ends of a plated steel strip, Patent Document 3, for example, discloses a method in which the plating at both widthwise ends of the welded portion is removed by grinding. By removing the plating, which adversely affects welding conditions and the structure of the welded portion, a welded portion having properties substantially similar to those of the base material can be obtained.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2009-45643

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2013-139051

[0010] Patent Document 3: Japanese Patent Application Laid-Open No. 4-197516

[0011] Patent Document 4: International Publication No. 2020 / 183883 Summary of the Invention

[0012] Problems to be solved by the invention

[0013] The technologies described in Patent Documents 1 and 2 above target electric resistance welded steel pipes used as oil well tubular goods or line pipes, which require even higher corrosion resistance, and do not envision the production of electric resistance welded steel pipes using plated steel sheets as base plates. Furthermore, the technology described in Patent Document 3 above removes the plating at both ends of the weld in the width direction, leading to concerns about reduced corrosion resistance around the weld.

[0014] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method for manufacturing an electric resistance welded steel pipe and an electric resistance welded steel pipe that can suppress a decrease in corrosion resistance around a weld portion in an electric resistance welded steel pipe using a plated steel plate as a base plate.

[0015] Solutions for solving problems

[0016] To solve the above-mentioned problems, according to a technical solution of the present invention, there is provided a method for manufacturing an electric-resistance-welded steel pipe, comprising: an end-face forming step in which an outer inclined portion inclined from the outer surface side toward the inner surface side relative to the plate thickness direction is formed on a first end face and a second end face on both sides in the width direction of a plated steel plate, or an outer inclined portion and an inner inclined portion inclined from the inner surface side toward the outer surface side relative to the plate thickness direction are formed on the first end face and the second end face on both sides in the width direction of the plated steel plate, so that the first end face and the second end face have a shape in which the plate thickness decreases from the center in the width direction toward the end face; and a pipe making step in which the formed first and second end faces are butted and welded to produce the pipe.

[0017] In the end surface forming step, at least a portion of the outer inclined portion may be covered with a plating layer continuous from the outer surface side.

[0018] In the pipe making step, the first and second end faces may be heated and melted, and press-bonded so that the ratio of the width of the weld bead on the outer surface side to the thickness of the plated steel sheet is 92% or less.

[0019] Furthermore, in the pipe making step, the plating layer may be applied to a weld portion formed by press-bonding the first end face and the second end face.

[0020] Alternatively, the plated steel sheet may be rolled into a coil after the first and second end faces are formed in the end face forming step, and the plated steel sheet rolled into a coil in the end face forming step may be unrolled and pipe-formed in the pipe-forming step.

[0021] Furthermore, to solve the above-mentioned problems, according to another technical aspect of the present invention, there is provided an electric resistance welded steel pipe, comprising a plated steel plate as a base plate, the pipe comprising an axially extending weld portion and heat-affected zones extending axially on both circumferential sides of the weld portion, wherein the ratio of the circumferential length of the outer surface of the weld portion to the plate thickness of the plated steel plate is 48% or greater, and for each heat-affected zone, the ratio of the circumferential length of the outer surface of the heat-affected zone to the plate thickness of the plated steel plate is 40% or less.

[0022] Furthermore, to address the aforementioned issues, according to another aspect of the present invention, there is provided an electric resistance welded steel pipe comprising a plated steel plate as a base plate, the pipe comprising a post-plated portion on its outer surface coated with a plating layer at least along a welded portion extending in the axial direction, wherein the ratio of the outer surface circumferential length of the post-plated portion to the plate thickness of the plated steel plate is 172% or less.

[0023] Effects of the Invention

[0024] As described above, according to the present invention, in an electric resistance welded steel pipe using a plated steel sheet as a base sheet, a decrease in corrosion resistance around the weld portion can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is an explanatory diagram showing an example of a welding process for welding both ends in the width direction of a steel plate formed into a cylindrical shape in the production of an electric resistance welded steel pipe.

[0026] Figure 2 The figures schematically show the shapes of the welded portion and the surrounding area of ​​an electric resistance welded steel pipe using a plated steel sheet as a base plate before and after welding, as viewed in the axial direction.

[0027] Figure 3 This is an explanatory diagram showing an example of a method for forming a first end surface and a second end surface of a plated steel sheet into shapes having an outer inclined portion and an inner inclined portion, respectively, in an end surface forming step.

[0028] Figure 4Yes through Figure 3 Schematic diagram of a side view of a cross section of a cut end surface of a plated steel sheet after being cut using the method.

[0029] Figure 5 This is an explanatory diagram showing an example of a method for forming the first end face and the second end face of a plated steel sheet into shapes each having an outwardly inclined portion in the end face forming step.

[0030] Figure 6 Yes through Figure 5 Schematic diagram of a side view of a cross section of a cut end surface of a plated steel sheet after being cut using the method.

[0031] Figure 7 These are explanatory diagrams schematically showing the shapes of the welded portion and its surroundings of a plated steel plate in a pipe-making process before and after welding, as viewed in the axial direction.

[0032] Figure 8 This is an explanatory diagram showing the definitions of weld width, heat-affected zone width, and weld bead width.

[0033] Figure 9 This is a schematic explanatory diagram showing the electric resistance welded steel pipe according to the present embodiment.

[0034] Figure 10 The following are simulation results showing an example of the relationship between the pressing amount and the weld bead width during welding of a steel pipe with an outer diameter of 114.3 mm.

[0035] Figure 11 The following are simulation results showing an example of the relationship between the pressing amount and the weld bead width during welding of a steel pipe with an outer diameter of 139.8 mm.

[0036] Figure 12 The following are simulation results showing an example of the relationship between the pressing amount and the weld bead width during welding of a steel pipe with an outer diameter of 159.8 mm.

[0037] Figure 13 This is a simulation result showing the state when the outer surface weld bead does not need to be removed. DETAILED DESCRIPTION

[0038] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In addition, in this specification and the accompanying drawings, components having substantially the same functional structure are denoted by the same reference numerals and repeated descriptions are omitted.

[0039] [1. Research on corrosion resistance around welds]

[0040] Figure 1This is an explanatory diagram showing an example of a welding process for welding both ends of a steel plate 3 formed into a cylindrical shape in the width direction, which is generally performed in the manufacture of electric resistance welded steel pipes. Figure 1 As shown, electric resistance welded steel pipe is manufactured by passing a cylindrically formed steel plate through a coil 15 through which a high-frequency current 10 flows. After heating the widthwise ends 3a and 3b by induction heating, the ends 3a and 3b are melted by pressurization with squeeze rolls 20 and then welded. Furthermore, the pressurization of the squeeze rolls 20 squeezes out molten steel on the outer and inner surfaces of the welded portion 3c where the widthwise ends 3a and 3b are joined, forming a weld bead. The weld bead formed on the outer surface is removed by, for example, a lathe tool 30.

[0041] In electric-resistance-welded steel pipes using plated steel sheets as the base sheet, corrosion resistance decreases around the welds. The inventors of this application conducted in-depth research on the main causes of this decline and determined that the main reasons for this decline are as follows:

[0042] First, in electric-resistance-welded steel pipe, the heat generated during welding causes localized high temperatures near both end surfaces of the plated steel sheet, causing the plating in these areas to evaporate. As a result, in the final product, the amount of plating remaining around the weld is reduced relative to other areas, or even no plating remains around the weld, resulting in reduced corrosion resistance only around the weld.

[0043] Furthermore, the weld bead on the outer surface of the steel pipe is removed by cutting with a lathe tool or squeezing with rollers. This also partially removes the surrounding plating along with the weld bead, exposing the iron matrix metal around the weld, which can expand the area of ​​reduced corrosion resistance. After removing the weld bead, repairing the weld by spraying the main component of the plating around the weld can compensate for the reduced corrosion resistance around the weld. However, the corrosion resistance around the repaired weld does not necessarily return to the same level as that of the remaining areas.

[0044] In view of these main reasons, the present inventors have determined that by narrowing the width of the weld bead, the area where the iron base metal is exposed around the weld can be reduced, thereby suppressing the reduction in corrosion resistance around the weld. Furthermore, they have come to the conclusion that the weld bead can be narrowed by optimizing the shape of the end surfaces on both sides of the width direction of the plated steel sheet, thereby narrowing the width of the weld bead. Specifically, by forming the first and second end surfaces on both sides of the width direction of the plated steel sheet into shapes in which the corners between the surface and the end surfaces are chamfered only on the outer surface side, or on both the outer surface side and the inner surface side, respectively, the weld bead resulting from welding the first and second end surfaces is narrowed.

[0045] Figure 2 It is a diagram schematically showing the shapes before and after welding of the periphery of the welded portion of the electric resistance welded steel pipe using the plated steel plate as the base plate, observed axially. In the shape before welding, the state after heating the first end face 51 and the second end face 52 on both sides in the width direction of the plated steel plate 5 formed into a cylindrical shape is shown. The heated portion of the plated steel plate 5 becomes the welded portion Q1 and the heat-affected portion Q2 from the side close to the first end face 51 and the second end face 52. Here, the circumferential width of the welded portion Q1 and the heat-affected portion Q2 is assumed to be the same as the plate thickness. In addition, in the shape after welding, the first end face 51 and the second end face 52 of the plated steel plate 5 are butted and welded to produce an outer surface weld bead 53 on the outer surface 5o side and an inner surface weld bead 54 on the inner surface 5i side. In addition, in Figure 2 In the figure, the shape of the welded portion and its surroundings are exaggerated for easier understanding of the description.

[0046] exist Figure 2 In the conventional example, the first end face 51 and the second end face 52 of the plated steel sheet 5 substantially maintain the plate shape before being formed into a cylindrical shape, and are substantially perpendicular to the outer surface 5o and the inner surface 5i. When the first end face 51 and the second end face 52 of the plated steel sheet 5 are butted and welded, the base metal of the weld portion Q1 is squeezed toward the outer surface 5o and the inner surface 5i, resulting in an outer surface weld bead 53 and an inner surface weld bead 54.

[0047] Figure 2 The end face shape A and the end face shape B are examples of the shapes of the first end face 51 and the second end face 52 according to one embodiment of the present invention.

[0048] The end face shape A is a shape obtained by chamfering the corners of the outer surface 5o side and the inner surface 5i side. Figure 2 As shown, end face shape A is composed of an outer inclined portion 5s, which tilts relative to the plate thickness from the outer surface 5o side toward the inner surface 5i side, and an inner inclined portion 5t, which tilts relative to the plate thickness from the inner surface 5i side toward the outer surface 5o side, resulting in a shape in which the plate thickness decreases toward the end face. When the first and second end faces 51, 52 are configured as end face shape A, the amount of base metal in the weld portion Q1 is reduced compared to the conventional example by an amount corresponding to the corners on the outer and inner surfaces 5i sides. Therefore, when the first and second end faces 51, 52, having end face shape A, are butt-welded, both the outer and inner weld beads 53, 54, are smaller than those in the comparative example.

[0049] The end face shape B is a shape in which the corners on the outer surface 5o side are chamfered. Figure 2 As shown, the end face shape B includes an outer inclined portion 5s that is inclined relative to the plate thickness direction from the outer surface 5o side toward the inner surface 5i side, forming a shape in which the plate thickness decreases toward the end face. Figure 2In the example of end face shape B shown, a surface 5r, substantially perpendicular to inner surface 5i, exists on the inner surface 5i side. However, end face shape B may also consist solely of the outer inclined portion 5s. When the first and second end faces 51, 52 are configured as end face shape B, the amount of base metal in weld portion Q1 is reduced compared to the conventional example by an amount corresponding to the corner portion on the outer surface 5o side. Therefore, when the first and second end faces 51, 52, having end face shape B, are butt-welded, the outer surface weld bead 53 is smaller than in the comparative example.

[0050] By setting the shapes of the first end face 51 and the second end face 52 to end face shape A or end face shape B, the weld bead produced after welding can be narrowed, resulting in a narrower width for the weld bead to be removed. This also narrows the width of the repair by spraying the main component of the plating to compensate for the reduction in corrosion resistance, thereby improving the corrosion resistance around the weld.

[0051] Regarding the end face shape B, it is conceivable that the inner surface weld bead 54 has a size approximately the same as that of the conventional example. However, the inner surface weld bead 54, which is not visible from the outside, may not be removed. For example, in components used with the inner surface not exposed to the outside, such as guardrail support pipes and protective tubes, the presence of the inner surface weld bead does not pose a problem. Therefore, the present invention is not concerned with the presence or absence of the inner surface weld bead 54; its purpose is to narrow the outer surface weld bead 53.

[0052] [2. Method for manufacturing electric resistance welded steel pipe using plated steel sheet as base sheet]

[0053] The following describes a method for manufacturing an electric-resistance-welded steel pipe using a plated steel sheet according to one embodiment of the present invention as a base sheet. The method for manufacturing an electric-resistance-welded steel pipe according to this embodiment includes an end-face forming step in which end faces (a first end face and a second end face) are formed on both sides of the width direction of the plated steel sheet serving as the base sheet; and a pipe-making step in which the formed first and second end faces are butted and welded to produce the pipe. In the following description, the plated steel sheet serving as the base sheet is a plated steel sheet containing Zn as the main component.

[0054] (End face forming process)

[0055] In the end surface forming step, the first end surface and the second end surface of the plated steel sheet as the base plate are formed into shapes in which the corners between the surface and the end surface are chamfered only on the outer surface side or on the outer surface side and the inner surface side. Specifically, the first end surface and the second end surface are formed into shapes in which the corners between the surface and the end surface are chamfered only on the outer surface side or on the outer surface side and the inner surface side. Figure 2The plate is formed into a shape having an outer inclined portion 5s or a shape having an outer inclined portion 5s and an inner inclined portion 5t as shown in the end face shape A or end face shape B. In this case, the plate thickness decreases from the center in the width direction of the plated steel plate toward the end face (first end face or second end face). The outer inclined portion 5s and the inner inclined portion 5t can be as shown in FIG. Figure 2 A flat surface is shown, but a curved surface is also possible.

[0056] The method for forming the end face is not particularly limited, but it is desirable that the coating layer on the surface of the plated steel sheet as the base plate covers the formed end face as much as possible. By making the coating layer cover the end face, the repair width of the main component of the sprayed coating layer can be minimized after welding the first and second end faces.

[0057] For example, Figure 2 In order to form the first end face and the second end face of the plated steel sheet into a shape having an outer inclined portion and an inner inclined portion, respectively, as in the end face shape A shown in FIG. Figure 3 A cutting device having a pair of annular blades 41 and 42 with V-shaped blade tips 41a and 42a is shown (e.g., Patent Document 4). The cutting device has the blade tips 41a and 42a with tip angles θ1 and θ2 set to 10° to 120°, and the tip radii R1 and R2 of the blade tips 41a and 42a set to 0.5% to 35.0% of the plate thickness. A plated steel sheet 5 is passed between the rotating annular blades 41 and 42 to cut its end. Consequently, the tensile force generated between the blade tips 41a and 42a of the pair of annular blades 41 and 42 and the plated steel sheet 5 causes the plating on the surface of the plated steel sheet 5 to penetrate the cut end surface, covering the cut end surface with the plating.

[0058] Figure 4 Shown by having Figure 3 An example of a cut end surface of a plated steel sheet 5 cut by a cutting device including a pair of annular blades 41 and 42 is shown. Figure 4 This is a schematic diagram of a side view of a cross section of a plated steel plate 5 whose two surfaces are covered with a plating layer 5b. Figure 4 As shown, the cut end surface of the plated steel sheet 5 is composed of a collapsed edge, an inclined surface, and a fracture surface. Figure 4 The collapsed edge and inclined surface on the upper side of Figure 3 The blade tip 41a of the annular blade portion 41 is formed, Figure 4 The collapsed edge and inclined surface on the lower side pass through Figure 3 The blade tip 42a of the annular blade portion 42 is formed. The inclined surface is covered by the plating layer 5b continuous from the surface of the plated steel plate 5. In addition, the fracture surface is generated by the crack generated by the blade tips 41a, 42a in the plated steel plate 5 as the starting point of the fracture.

[0059] For example, Figure 4 The upper inclined surface becomes the outer inclined portion 5s after the end surface is formed. Figure 4 The lower inclined surface of the end surface becomes the inner inclined portion 5t after the end surface is formed. Figure 3 The cutting device shown has a pair of annular blade portions 41 and 42 with blade tips 41a and 42a in the shape of the letter V, which cuts both sides of the plated steel plate 5 in the width direction, and can form an outer inclined portion 5s and an inner inclined portion 5t covered by the plating layer 5b on the first end face and the second end face respectively.

[0060] In addition, if Figure 2 In order to form the first end face and the second end face of the plated steel sheet into a shape having an outwardly inclined portion, as in the end face shape B shown in FIG. Figure 5 The cutting device shown is a pair of slit blades 61 and 62 having an inclined portion P1 and a protrusion P2 in the shape of the blade portions 61a and 62a. The inclined portion P1 of the blade portions 61a and 62a is inclined relative to the pressing direction of the plated steel sheet 5, and the protrusion P2 protrudes from the inclined portion P1 in the pressing direction. The plated steel sheet 5 is passed between the rotating slit blades 61 and 62, and its end is cut off. As a result, the inclined portion P1 of the blade portions 61a and 62a can be used to compress the coating 5b in the pressing direction (plate thickness direction) including the base material 5a and wrap it around the cut end face, and the cutting time can be delayed by applying compressive stress, so that the inclined surface can be covered with the coating. In addition, by completely cutting the plated steel sheet 5 using the protrusion P2 of the blade portions 61a and 62a, the generation of large burrs can be suppressed.

[0061] Figure 6 Shown by having Figure 5 An example of a cut end surface of a plated steel sheet 5 cut by a cutting device using a pair of slit blades 61 and 62 is shown. Figure 6 This is a schematic diagram of a side view of a cross section of a plated steel plate 5 whose both surfaces are covered with a plating layer 5b. Figure 6 Indicates passing Figure 5 The inclined portion P1 of the slit blade 61 is pressed into the cutting end face on the side opposite to the inclined portion P1. Figure 6 As shown, the cut end surface of the plated steel sheet 5 has, in order from the top surface, an inclined surface, a shear surface, and a fracture surface. The inclined surface is formed along the inclined portion P1 of the blade portion 61a of the slit blade 61 and is inclined with respect to the plate thickness direction. The shear surface is a smooth surface formed by the movement of the slit blade 61 that penetrates the plated steel sheet 5 and is generated in small quantities. The fracture surface is the surface where the plated steel sheet 5 breaks, starting from a crack that has occurred in the plated steel sheet 5.

[0062] For example, Figure 6 The inclined surface becomes the outer inclined portion 5s after the end surface is formed. Figure 5The cutting device of the pair of slit blades 61 and 62 shown cuts both sides of the plated steel sheet 5 in the width direction, and can form outer inclined portions 5 s covered with the plating layer 5 b on the first end surface and the second end surface, respectively.

[0063] Thus, in the end surface forming step, the first end surface 51 and the second end surface 52 of the plated steel sheet 5 are formed to have Figure 4 The shape of the outer inclined portion 5s and the inner inclined portion 5t shown, or having Figure 6 The shape of the outer inclined portion 5s is shown.

[0064] Furthermore, the outer inclined portion 5s and the inner inclined portion 5t do not necessarily need to be covered with plating, but it is preferable to cover at least a portion of the outer inclined portion 5s with a continuous plating 5b extending from the outer surface. As described later, during the pipe forming process, if the first end face 51 and the second end face 52 are heated before welding, low-melting-point components such as Zn in the plating 5b at the welded portion where the base material 5a is semi-molten will evaporate. However, the plating 5b will remain molten in the heat-affected zone and solidify after cooling, covering the base material 5a. During the end-face forming process, by covering at least a portion of the outer inclined portion 5s of the first end face 51 and the second end face 52 with a continuous plating 5b extending from the outer surface, the area of ​​the base material 5a exposed to the outside, which reduces its corrosion resistance, can be reduced.

[0065] (Pipe making process)

[0066] Next, the first end face 51 and the second end face 52 formed in the end face forming step are butted and welded to form a pipe. Figure 1 The welding process is performed using the equipment shown in FIG. First, the first end face 51 and the second end face 52 are heated and melted. Then, the first end face 51 and the second end face 52 are press-joined. At this time, the ratio of the width of the weld bead on the outer surface 5o side (outer surface weld bead 53) (hereinafter also referred to as "weld bead width") to the plate thickness of the plated steel plate 5 is set to be 92% or less. The weld bead width is the portion that needs to be cut after welding and is the circumferential width of the portion that is discharged from the outer surface 5o of the steel pipe to the outside.

[0067] exist Figure 7 , regarding the welded portion and the periphery of the plated steel plate 5 in the pipe making process, the shapes before and after welding as viewed in the axial direction are schematically shown. Figure 7 As an example, the first end face 51 and the second end face 52 are shown as Figure 2 The case of end face shape A is shown.

[0068] When the first end face 51 and the second end face 52 of the plated steel sheet 5 formed into a cylindrical shape are heated, the heated portion of the plated steel sheet 5 becomes a weld Q1 and a heat-affected zone Q2 from the side close to the first end face 51 and the second end face 52. The weld Q1 is a portion where the iron of the base material softens and semi-melts. In the weld Q1, the base material reaches a high temperature of 900°C or higher, so low-melting-point components such as Zn in the plating components covering the surface of the plated steel sheet 5 evaporate. The heat-affected zone Q2 is a portion where the iron of the base material reaches a high temperature, causing the plating covering the surface of the plated steel sheet 5 to melt. That is, in the heat-affected zone Q2, the base material is 400°C to 900°C, and in part or all of this area, some or all of the plating components melt. Furthermore, a portion of the specific heat-affected portion Q2 away from the end portion (the first end surface 51 or the second end surface 52 ) remains the plated steel sheet 5 as the original sheet, and the outer surface 5 o and the inner surface 5 i are covered with the plating layer.

[0069] During the pipe-making process, the welded portion Q1 of the first end face 51 and the welded portion Q1 of the second end face 52 are pressed together to join the first and second end faces 51 and 52. Pressing the welded portions Q1 together prevents deformation of the triangular shapes of the first and second end faces 51 and 52, formed by the outer inclined portion 5s and the inner inclined portion 5t, when viewed from the side, and the plated steel sheet 5 becomes tubular (after welding 1). At this point, the molten base metal is expelled from the outer surface 5o and inner surface 5i, forming the outer surface weld bead 53 and the inner surface weld bead 54. Further pressing further expels the base metal from the outer surface 5o and inner surface 5i, and while the circumferential width of the joined welded portion Q1 decreases, the outer surface weld bead 53 and the inner surface weld bead 54 increase (after welding 2).

[0070] At least the outer surface weld bead 53, which is visible from the outside, is removed from the weld bead produced by welding. In the method for manufacturing electric resistance welded steel pipe of this embodiment, by narrowing the weld bead produced after welding, the width of the weld bead removed is reduced, thereby narrowing the repair width of the main component of the thermal spray coating and improving the corrosion resistance around the weld. Therefore, the ratio of the weld bead width of the outer surface weld bead 53 to the plate thickness of the plated steel plate 5 is set to 92% or less.

[0071] Here, if Figure 8 The width of the weld Q1 (weld width) W1, the width of the heat-affected portion Q2 (heat-affected portion width) W2, and the weld bead width Wb of the outer surface weld bead 53 are defined as shown. The weld width W1 is set to the circumferential length of the weld Q1 at the outer surface 5o. The heat-affected portion width W2 is set to the circumferential length of the heat-affected portion Q2 at the outer surface 5o. The weld bead width Wb of the outer surface weld bead 53 is set to the circumferential length of the outer surface weld bead 53 at the outer surface 5o. In addition, Figure 8In FIG, the weld periphery on one end face side is shown. That is, the weld width W1, the heat-affected zone width W2 and the weld bead width Wb of the entire steel pipe are Figure 8 Twice the size.

[0072] Regardless of the shapes of the first end face 51 and the second end face 52 of the plated steel sheet 5, the areas heated in the pipe making process to form the welded portion Q1 and the heat-affected portion Q2 are substantially the same. However, if the welded portion Q1 is pressed with the same amount of pressure during welding, then Figure 2 When the end face shape is formed and chamfered like the end face shape A or the end face shape B, the amount of base metal at the end portion is reduced compared to the conventional example, and the pressing amount is increased accordingly.

[0073] Here, if the pressing amount increases, the amount of base metal of the weld Q1 that is discharged from the outer surface 5o of the steel pipe to the outside increases, and the weld bead width Wb of the outer surface weld bead 53 increases. However, in the case where the first end face 51 and the second end face 52 are chamfered, compared with the conventional example, even if the pressing amount increases corresponding to the decrease in the amount of base metal at the end, the amount of base metal of the weld Q1 that is discharged from the outer surface 5o of the steel pipe to the outside is less than that of the conventional example. Therefore, by chamfering the first end face 51 and the second end face 52, the pressing amount can be increased and the weld bead width Wb of the outer surface weld bead 53 can be reduced. If the pressing amount during welding is adjusted so that the ratio of the weld bead width Wb to the plate thickness t of the plated steel plate 5 is 92% or less, then Figure 2 Compared with the conventional example, the outer surface weld bead 53 can be narrowed.

[0074] Furthermore, due to the joining of the first end face 51 and the second end face 52, low-melting-point components such as Zn in the surface plating evaporate from the weld portion Q1, which is partially discharged to the outside of the steel pipe as a weld bead (outer surface weld bead 53 or inner surface weld bead 54). By increasing the amount of pressure between the first end face 51 and the second end face 52, the amount of weld portion Q1 remaining in the final product, which reduces corrosion resistance, can be reduced.

[0075] Furthermore, during the pipe manufacturing process, after the first end face 51 and the second end face 52 are press-joined, a coating may be applied to the weld portion Q1. If the outer surface weld bead 53 is not produced during welding and the outer surface of the steel pipe is not uneven, the outer surface weld bead 53 does not need to be removed. However, when the end faces (the first end face 51 and the second end face 52) are heated at the weld portion Q1, low-melting-point components such as Zn in the coating composition evaporate. Furthermore, if the outer surface weld bead 53 is produced during welding, the outer surface weld bead 53 is removed. In the area where the weld portion Q1 and the outer surface weld bead 53 are removed, the base metal is exposed, and the coating disappears. Therefore, by applying a coating to the area where the weld portion Q1 or the outer surface weld bead 53 is removed, using a spraying method using the main component of the coating, a reduction in corrosion resistance is suppressed.

[0076] In this manner, in the pipe manufacturing process, the first end face 51 and the second end face 52 formed in the end face forming process are butted and welded to produce the pipe. This allows the bead width of the outer surface weld bead 53 to be narrowed during welding, and the amount of pressure between the first end face 51 and the second end face 52 can be increased. This prevents a decrease in the corrosion resistance of the welded portion on the outer surface of the steel pipe after welding.

[0077] Furthermore, during the pipe-making process, when the first and second end faces 51, 52 are heated before welding, low-melting-point components such as Zn in the coating 5b evaporate in the weld zone Q1 where the base metal 5a is semi-molten. Therefore, the base metal and the coating do not mix in the weld zone Q1, and the residual penetrant (Japanese: ペネトレータ) generated during resistance welding, as feared in Patent Documents 1 and 2, does not occur. Furthermore, penetrants are often generated in special steels containing high amounts of Mn and Si, and are less likely to form in the steel sheet used as the base material for the plated steel sheet 5. Furthermore, during welding of the plated steel sheet 5, the base metal is covered by the coating, shortening the time the base metal is exposed to air during welding. This can suppress the formation of penetrants caused by oxidation of Mn and Si. Therefore, in the present embodiment, penetrants do not pose a problem.

[0078] The above describes the method for manufacturing electric-resistance-welded steel pipe according to this embodiment. According to this embodiment, after the end faces (first and second end faces) of the plated steel sheet serving as the base plate are chamfered and shaped, the formed first and second end faces are butted against each other and welded to produce the pipe. This narrows the weld bead produced after welding, reducing the width of the weld bead to be removed. As a result, the repair width of the main component of the sprayed plating is narrowed, which can suppress the degradation of corrosion resistance around the weld.

[0079] Furthermore, the end-surface forming step and the pipe-making step can be performed as a series of steps on the same production line or on separate production lines. For example, in the end-surface forming step, after forming the first end surface 51 and the second end surface 52 of the plated steel sheet 5 of the base plate, the plated steel sheet 5 is wound into a coil. Then, in the pipe-making step, the plated steel sheet 5 wound into a coil in the end-surface forming step can be unwound and pipe-made.

[0080] [3. Electric resistance welded steel pipe using plated steel sheet as base plate]

[0081] exist Figure 9 An example of an electric resistance welded steel pipe 50 manufactured by the above-mentioned manufacturing method is shown in FIG. Figure 9 Schematic diagram showing an electric resistance welded steel pipe 50 using a plated steel sheet as a base plate. Figure 9As shown, the electric resistance welded steel pipe 50 has a weld portion Q1 extending in the axial direction (α direction) and heat-affected zones Q2 extending in the axial direction on both sides of the weld portion Q1 in the circumferential direction (β direction). In this electric resistance welded steel pipe 50, the ratio of the circumferential length W1 of the outer surface 5o of the weld portion Q1 to the plate thickness t of the plated steel sheet (W1 / t) is 48% or greater. Furthermore, the ratio of the circumferential length W2 of the outer surface 5o of the heat-affected zone Q2 to the plate thickness t of the plated steel sheet (W2 / t) is 40% or less (80% or less for the entire steel pipe).

[0082] The outer surface 5o of the electric resistance welded steel pipe 50 may also include a post-plated portion coated at least along the weld portion Q1 extending in the (α direction). The post-plated portion is a portion formed by applying a coating by spraying, etc., after the outer surface weld bead 53 is removed. The post-plated portion includes at least the welded outer surface weld bead 53 and may also include a portion of the heat-affected zone Q2 adjacent to the weld portion Q1 including the welded outer surface weld bead 53. When the circumferential length of the outer surface 5o of the post-plated portion is denoted as W3, the length W3 of the post-plated portion is greater than or equal to the length W1 of the weld portion Q1 and equal to the width Wb of the removed weld bead. The ratio of the length W3 of the post-plated portion to the plate thickness t of the plated steel sheet (W3 / t) is 172% or less.

[0083] Such an electric resistance welded steel pipe 50 suppresses a decrease in corrosion resistance around the weld portion Q1.

[0084] Example

[0085] The simulation verified that the end face shape of the steel plate is set to Figure 2 The relationship between the pressing amount and the weld width in the conventional example, end face shape A, and end face shape B is shown. In this verification, simulations were performed on steel pipes with three different outer diameters (114.3mm, 139.8mm, and 159.8mm) and a plate thickness of 4.5mm. In end face shape A, the outer inclined portion and the inner inclined portion have an inclination angle of 30° relative to the plate thickness direction, respectively, and are symmetrical in shape. In end face shape B, the outer inclined portion has an inclination angle of 45° relative to the plate thickness direction, and the ratio of the plate thickness of the outer inclined portion to the plate thickness of the portion roughly perpendicular to the inner surface is 7:3. The tensile strength of the steel plate is assumed to be 400MPa.

[0086] The simulations set up a 900°C region simulating the weld, a 300°C region simulating the heat-affected zone, and a 0°C region simulating the portion unaffected by heating, starting from the end face. The circumferential widths of the 900°C and 300°C regions were assumed to be the same as the plate thickness.

[0087] Figures 10 to 12 Indicates the simulation results. Figures 10 to 12In the figure, the three states of the end face shapes are shown: the pressing amount is -1mm (before welding), the pressing amount is 5mm (after welding), and the pressing amount is 9mm (after welding). Figures 10 to 12 As shown, regardless of the steel pipe diameter, the bead width of the outer surface weld bead after welding is smaller for both end face shape A and end face shape B than in the conventional example. Furthermore, with end face shape B, the chamfered portion of the end face is larger than with end face shape A, resulting in a smaller 900°C region on the outer surface. These results show that, when the pressing amount is kept constant, the bead width of the outer surface weld bead can be reduced with end face shape B compared to end face shape A, and the 900°C region remaining in the steel pipe can also be reduced.

[0088] Table 1 below shows the bead width, weld width, and heat-affected zone width of the outer surface weld bead when the pressing amount is 9 mm. Table 2 below also shows the ranges of the ratios of the bead width, weld width, and heat-affected zone width of the outer surface weld bead to the plate thickness, calculated based on Table 1. Tables 1 and 2 show values ​​for one end face; however, the values ​​are doubled for the steel pipe as a whole.

[0089] [Table 1]

[0090] Table 1

[0091]

[0092] [Table 2]

[0093] Table 2

[0094]

[0095] The greater the amount of pressure applied, the wider the weld bead. Therefore, the ratios of weld bead width to plate thickness shown in Table 2 represent the maximum possible ranges. According to Table 2, the maximum ratio of weld bead width to plate thickness for the conventional example ranges from 100% to 112% for the entire steel pipe. In contrast, for end face shapes A and B, the maximum ratios of weld bead width to plate thickness range from 58% to 92% for the entire steel pipe, which is smaller than the conventional example. Thus, by adopting end face shapes A or B, the bead width of the outer surface weld bead can be reduced.

[0096] The greater the amount of pressure, the smaller the weld width. Therefore, the ratio of the weld width to the plate thickness shown in Table 2 represents the minimum acceptable range. According to Table 2, the minimum ratio of the weld width to the plate thickness for the conventional example is 76% to 80% for the entire steel pipe. In contrast, for end face shapes A and B, the minimum ratio of the weld width to the plate thickness is 48% to 80% for the entire steel pipe, allowing for values ​​smaller than those of the conventional example.

[0097] The HAZ widths varied slightly between the conventional example, end face shape A, and end face shape B, and therefore remained the same, as shown in Tables 1 and 2. Furthermore, Tables 1 and 2 show the results when the pressing amount was set to 9 mm. At least the conventional example shows slightly overheated results compared to the state where the end portion is sufficiently heated to avoid poor welding. In actual operation, sufficient heating of the end portion is also necessary to avoid poor welding, but to avoid excessive heating, the HAZ width is assumed to be the same as or smaller than the results of this verification. Therefore, it is assumed that the ratio of the HAZ width to the plate thickness is 40% or less for each HAZ (80% or less for the entire steel pipe).

[0098] Furthermore, if a post-plated portion is present, the ratio of the post-plated portion to the plate thickness is considered to be 172% or less, based on the overall steel pipe. The post-plated portion includes at least the outer surface weld bead after welding and may also include a portion of the heat-affected zone adjacent to the weld portion including the outer surface weld bead. Therefore, the ratio of the post-plated portion to the plate thickness is considered to be at least the maximum value (92%) of the ratio of the width of the outer surface weld bead after welding to the overall steel pipe thickness. This ratio is considered to be 172% or less, taking into account the maximum value (80%) of the ratio of the width of the heat-affected zone to the plate thickness.

[0099] in addition, Figure 13 The figure shows a state where the outer surface weld bead does not need to be removed when three steel pipes with different outer diameters (114.3mm, 139.8mm, and 159.8mm) are set to the conventional example, end face shape A, and end face shape B. This state is a state in which the base metal of the weld portion is discharged from the outer surface after welding, but the quality of the product is not affected and the weld bead width is minimized. The amount of pressure required to achieve this state is 0mm in the conventional example, 5.5mm in the case of end face shape A, and 10.5mm in the case of end face shape B, regardless of the outer diameter of the steel pipe. In other words, by setting the amount of pressure to 5.5mm in the case of end face shape A and 10.5mm in the case of end face shape B, the width of the outer surface weld bead can be minimized.

[0100] While preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. Anyone with ordinary knowledge in the technical field to which the present invention pertains can conceive of various variations or modifications within the scope of the technical concepts set forth in the claims, and such variations or modifications are naturally considered to fall within the scope of protection of the present invention.

[0101] Description of Reference Numerals

[0102] 5. Plated steel sheet; 5i. Inner surface; 5o. Outer surface; 5s. Outer inclined portion; 5t. Inner inclined portion; 10. High-frequency current; 15. Coil; 20. Extrusion roller; 30. Turning tool; 41, 42. Annular blade portion; 41a, 42a. Blade tip; 50. Electric resistance welded steel pipe; 51. First end face; 52. Second end face; 53. Outer surface weld bead; 54. Inner surface weld bead; 61, 62. Slit blade; 61a, 62a. Blade portion; Q1. Welding portion; Q2. Heat-affected zone.

Claims

1. A method for manufacturing an electric resistance welded steel pipe, comprising: an end surface forming step, in which an outer inclined portion inclined from the outer surface side toward the inner surface side relative to the plate thickness direction is formed on a first end surface and a second end surface on both sides in the width direction of the plated steel plate, or an outer inclined portion and an inner inclined portion inclined from the inner surface side toward the outer surface side relative to the plate thickness direction are formed on the first end surface and the second end surface on both sides in the width direction of the plated steel plate, respectively, so that the first end surface and the second end surface have a shape in which the plate thickness decreases from the center in the width direction toward the end surface; as well as A pipe making step is performed by butting and welding the formed first end surface and the formed second end surface to form a pipe.

2. The method for manufacturing an electric resistance welded steel pipe according to claim 1, wherein: In the end surface forming step, at least a portion of the outer inclined portion is covered with a plating layer continuous from the outer surface side.

3. The method for manufacturing an electric resistance welded steel pipe according to claim 1 or 2, wherein: In the pipe making step, the first end face and the second end face are heated and melted, and are press-joined so that the ratio of the width of the weld bead on the outer surface side to the plate thickness of the plated steel plate becomes 92% or less.

4. The method for manufacturing an electric resistance welded steel pipe according to claim 3, wherein: In the pipe manufacturing step, a plating layer is applied to a weld portion formed by press-bonding the first end surface and the second end surface.

5. The method for manufacturing an electric resistance welded steel pipe according to claim 1 or 2, wherein: In the end face forming step, after forming the first end face and the second end face, the plated steel sheet is rolled into a coil. In the pipe making step, the plated steel sheet wound into a coil in the end surface forming step is unwound to form a pipe.

6. An electric resistance welded steel pipe, comprising a plated steel plate as a base plate, comprising a weld portion extending in the axial direction and heat-affected zones extending in the axial direction on both circumferential sides of the weld portion. The ratio of the circumferential length of the outer surface of the welded portion to the thickness of the plated steel plate is 48% or more. Regarding each of the heat-affected portions, a ratio of an outer surface circumferential length of the heat-affected portion to a plate thickness of the plated steel plate is 40% or less.

7. An electric resistance welded steel pipe, comprising a plated steel plate as a base plate, and having a post-plated portion on the outer surface of the pipe coated with a plated layer at least along a weld portion extending in the axial direction. A ratio of an outer surface circumferential length of the post-plating portion to a plate thickness of the plated steel plate is 172% or less.

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