A pipe having a dissimilar welded joint formed by ERW welding and SAW welding, and a method for manufacturing the same

Abstract

The present invention relates to pipe welding, and more specifically, to a SAW pipe that forms a heterogeneous weld joint by ERW welding and SAW welding, and a method for manufacturing the same. To this end, a SAW pipe that forms a heterogeneous weld joint by ERW welding and SAW welding is provided, comprising: a pipe (100) formed by roll forming a metal plate of a length member into a circular cross-section and then welding two adjacent sides, wherein an ERW weld joint (120) is formed in at least a portion between two adjacent sides; and a SAW weld joint (150) that fills a cutting groove (140) formed by cutting a portion of the ERW weld joint (120) and a portion of both sides between two adjacent sides.

Description

Technology Field

[0001] The present invention relates to pipe welding, and more specifically, to a SAW pipe that forms a heterogeneous weld joint using ERW welding and SAW welding, and a method for manufacturing the same. Background Technology

[0002] Generally, to mass-produce pipes such as steel tubes, steel plates are first bent into a circular shape using a roll forming device, and then completed by welding. Representative welding methods for manufacturing pipes include ERW (Electric Resistance Welding) and SAW (Submerged Arc Welding).

[0003] FIG. 1 is a perspective view showing the process of manufacturing a pipe by ERW welding. As shown in FIG. 1, the ERW welding device (10) contacts electrodes (14) to both side surfaces (11) of a bending plate (12) that is being transported in a round manner, and then applies current. At this time, the base material itself is heated and melted by the electrical resistance heat (Joule heat) generated in the base material, and the welded pipe (20) is completed by welding it by pressing it with a pair of welding rollers (16).

[0004] This ERW welding does not use filler material (welding rod, wire) and is characterized by the fact that the heat-affected zones (HAZ) on both sides of the weld (18) form symmetry. In addition, ERW welding is vulnerable to low-temperature impact toughness because the crystal structure becomes coarsened and oxides are easily generated due to exposure to high temperatures. Furthermore, ERW welding has stable mechanical properties, and by removing the bead at high temperatures immediately after welding, it has the advantage of a beautiful bead shape, such as the curvature of the inner and outer surfaces of the pipe. Therefore, it is suitable for manufacturing small-diameter (8-14 inches) or medium-diameter (14-20 inches) steel pipes that require thin thickness and watertightness.

[0005] FIG. 2 is a perspective view showing the process of manufacturing a pipe by SAW welding. As shown in FIG. 2, the SAW welding device manufactures a welded pipe by bending a thick plate into a circular shape in stages using a press device, and forming a weld joint in the longitudinal direction using an inner welder and an outer welder. SAW welding is a welding method in which the welding arc is completely submerged under flux powder, and the arc heat generated between the wire electrode (filler metal) and the base material is used for welding.

[0006] This type of SAW welding has the advantage of preventing oxidation and stabilizing weld quality because the arc is not exposed to the outside and the flux covers the molten pool. In addition, since the base metal and wire melt into a liquid state, penetration to deep grooves is achieved. Therefore, it is advantageous for welding thick plates and has the advantage of uniform weld quality and almost no spatter.

[0007] However, SAW welding has disadvantages such as the need to separately remove slag after cooling, its unsuitability for welding thin plates, and the requirement for flux management.

[0008] Therefore, SAW welding is mainly used for thick plates or large-diameter (20-inch to 60-inch) steel pipes, such as pressure vessels, ships, offshore structures, heavy equipment, bridges, and plant structures.

[0009] FIG. 3 is a conceptual diagram illustrating the roll forming process. As shown in FIG. 3, in the roll forming device (50), the bending plate (52) sequentially passes through the first, second, and third rollers (54, 56, 58) arranged in a row, forming the roll-formed product (60) with a circular cross-section.

[0010] FIG. 4 is a flowchart illustrating the process of manufacturing a pipe using conventional SAW welding. As shown in FIG. 4, the steel plate is edge-milled according to dimensions (Step 1), then pre-bent with a press (Step 2), and then formed into a circle using a 10,000-ton press (Step 3). Next, tack welding (Step 4) is performed to maintain the circular cross-section, and the inner surface is first welded using SAW welding (Step 5). Next, the outer surface is long-seam milled to form a cutting groove (Step 6), and the outer surface is secondarily welded using SAW welding (Step 7). Then, an ultrasonic test (UT) is performed to check for defects in the weld (Step 8), and the pipe cross-section is corrected (Step 9) to complete the process.

[0011] FIG. 5a is a partial cross-sectional view of a pipe manufactured by conventional SAW welding as shown in FIG. 4, and FIG. 5b is a partial cross-sectional view of a pipe with the outer and inner beads removed from the cross-section of FIG. 5a. As shown in FIG. 5a and FIG. 5b, conventional SAW welding omits the removal of the outer and inner beads after the pipe is completed, but it has the disadvantage that if removal is required, an additional process is necessary because it is difficult for the cutting tool to access them. In particular, unlike the outer bead, the inner bead is difficult for the cutting tool to access, which has the disadvantage of requiring greater attention to quality control.

[0012] As such, conventional pipes used ERW welding for small diameters (8 to 14 inches) or medium diameters (14 to 20 inches), and SAW welding for large diameters (20 to 26 inches). Therefore, there was no welding process that could be uniformly applied regardless of the outer diameter across the range of 8 to 26 inches.

[0013] Furthermore, as ERW welding and SAW welding each have distinct advantages and disadvantages, there is a growing demand for a new pipe welding method that can maximize the mutual advantages while simultaneously compensating for the disadvantages of each. Prior art literature

[0014] 1. Korean Utility Model Registration No. 20-0216062 (Roll forming device for composite pipes), 2. Korean Patent Registration No. 10-1175389 (Method for manufacturing pipes), 3. Korean Patent Registration No. 10-2881626 (Roll forming method for high-strength functional steel sections). The problem to be solved

[0015] Accordingly, the present invention has been devised to solve the above-mentioned problems, and the objective of the present invention is to provide a SAW pipe and a method for manufacturing the same, which can uniformly apply a dissimilar weld joint formed by ERW welding and SAW welding, with an outer diameter of the pipe ranging from 8 inches to 26 inches.

[0016] Another objective of the present invention is to provide a SAW pipe and a method for manufacturing the same, which forms a heterogeneous weld joint using ERW welding and SAW welding, while maintaining the respective advantages of ERW welding and SAW welding to the maximum extent while simultaneously compensating for the respective disadvantages.

[0017] However, the technical problems to be solved by the present invention are not limited to those mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art to which the present invention belongs from the description below. means of solving the problem

[0018] To achieve the above technical objective, a SAW pipe is provided that forms a heterogeneous weld joint by ERW welding and SAW welding, characterized by comprising: a pipe (100) formed by roll forming a metal plate of a length member into a circular cross-section and then welding two adjacent sides, wherein an ERW (Electric Resistance Welding) weld joint (120) is formed in at least a portion between two adjacent sides; and a SAW (Submerged Arc Welding) weld joint (150) that fills a cutting groove (140) formed by cutting a portion of the ERW weld joint (120) and a portion of both sides between two adjacent sides.

[0019] Optionally, the metal plate of the length member is unwound from the carbon steel coil (32) and continuously supplied.

[0020] Optionally, the pipe (100) has an outer diameter of 8 to 26 inches.

[0021] Optionally, the ERW weld (120) forms part of the inner surface (102) of the pipe (100), and the SAW weld (150) forms part of the outer surface (104) of the pipe (100).

[0022] Optionally, the thickness ratio t3 : t4 of the SAW weld (140) thickness (t3) and the ERW weld (120) thickness (t4) along the radial direction of the pipe (100) is 9 : 1 to 6 : 4.

[0023] Optionally, the cross-section of the cutting groove (140) is in the shape of a U-groove or a V-groove.

[0024] Optionally, the exposed surface of the SAW weld (150) has the bead (155) removed to form a curved surface identical to the outer surface (104) of the pipe (100).

[0025] Optionally, the pipe (100) may be an oil pipeline or a submarine oil pipeline or an oil well pipe.

[0026] Optionally, at least one of an inner coating layer (180) formed on the inner surface (102) of the pipe (100) and an outer coating layer (190) formed on the outer surface (104) of the pipe (100) may be further included.

[0027] The objective of the present invention as described above can also be achieved by a method for manufacturing a SAW pipe that forms a heterogeneous weld joint by ERW welding and SAW welding, characterized by comprising the following steps: unwinding a carbon steel coil (32) and pre-treating it into a metal plate (S100); roll forming the metal plate into a circular cross-section (S120); forming an ERW weld joint (120) by ERW welding between two adjacent sides of the circular cross-section (S140); removing the bead of the ERW weld joint (S150); forming a cutting groove (140) by cutting a part of the ERW weld joint (120) and a part of both sides between two adjacent sides (S160); and forming a SAW weld joint (150) by SAW welding inside the cutting groove (140) (S180).

[0028] Optionally, in the step (S160) of forming the cutting groove (140), the ratio t1 : t2 of the depth (t1) of the cutting groove (140) and the remaining thickness (t2) of the ERW weld (120) due to the cutting groove (140) along the radial direction of the pipe (100) is 5 : 5 to 5.5 : 4.5.

[0029] Optionally, in the step (S160) of forming the cutting groove (140), when the ratio t1 : t2 of the depth (t1) of the cutting groove (140) and the remaining thickness (t2) of the ERW weld (120) is 5 : 5, in the step (S180) of forming the SAW weld (150), the thickness ratio t3 : t4 of the thickness (t3) of the SAW weld (140) and the thickness (t4) of the ERW weld (120) is 9 : 1 to 8 : 2, or when the thickness ratio t1 : t2 is 5.5 : 4.5, the thickness ratio t3 : t4 is 6 : 4 to 7 : 3. Effects of the invention

[0030] According to one embodiment of the present invention, pipes can be manufactured using the same welding process even when the outer diameter of the pipe ranges from a small diameter (8 inches) to a large diameter (26 inches). Therefore, there is an advantage in being able to produce pipes of various diameters using a single manufacturing facility.

[0031] In addition, ERW welding is performed on the inner surface (102) of the pipe (100) and the bead is cut, so that no bead (155) remains on the inner surface (102). Therefore, the inner surface (102) maintains a smooth surface and is prepared to immediately form an internal coating layer (180). That is, the process of removing the bead (155) from the inner surface (102), which is performed in SAW welding, can be omitted.

[0032] In addition, since the ERW weld (120) is made of the same material as the pipe base material, it has the effect of increasing the watertightness of the pipe (100) and reducing the heat-affected zone (160) caused by the SAW weld (150).

[0033] In addition, since the SAW weld (150) is made of the same material as the pipe base material, it increases the watertightness performance of the pipe (100), and as the base material and wire melt into a liquid state in the SAW weld (150), penetration is achieved up to the deep cutting groove (140). Furthermore, due to the deep penetration, the SAW weld (150) does not produce defects, and its toughness against low-temperature impact and mechanical properties are superior to those of the ERW weld (120). As a result, it can be applied to the production of large-diameter pipes, and the weld adhesion of the pipe is increased. Therefore, the present invention can be applied to high-value pipes such as oil pipelines, submarine oil pipelines, or well pipes.

[0034] According to one embodiment of the present invention, pipes suitable for various design specifications can be efficiently produced by appropriately adjusting the thickness ratio of the ERW weld (120) and the SAW weld (150) and the welding heat input.

[0035] In addition, according to one embodiment of the present invention, pipes can be produced quickly and manufacturing costs can be reduced by applying a roll forming method instead of a conventional press forming method.

[0036] However, the effects obtainable from the present invention are not limited to those mentioned above, and other unmentioned effects will be clearly understood by those skilled in the art from the description below. Brief explanation of the drawing

[0037] The following drawings attached to this specification illustrate preferred embodiments of the present invention and serve to further enhance understanding of the technical concept of the present invention together with the detailed description of the invention provided below; therefore, the present invention should not be interpreted as being limited only to the matters described in such drawings. FIG. 1 is a perspective view showing the process of manufacturing a pipe by ERW welding. FIG. 2 is a perspective view showing the process of manufacturing a pipe by SAW welding. Figure 3 is a conceptual diagram showing the roll forming process. FIG. 4 is a flowchart showing the process of manufacturing a pipe using conventional SAW welding. FIGS. 5A and 5B are partial cross-sectional views of a pipe manufactured by conventional SAW welding as shown in FIG. 4, FIG. 6 is a flowchart showing the process of manufacturing a SAW pipe using the manufacturing method according to the present invention. FIG. 7a is a partial cross-sectional view of the ERW welded pipe (100) in FIG. 6. FIG. 7b is a partial cross-sectional view of a pipe (100) having a cutting groove (140) formed in the pipe (100) of FIG. 7a. FIG. 7c is a partial cross-sectional view of a pipe (100) that has been SAW welded to the pipe (100) of FIG. 7b. FIG. 7d is a partial cross-sectional view of a pipe (100) with the bead (155) removed from the pipe (100) of FIG. 7c. FIG. 7e is a cross-sectional view of a pipe (100) having an inner coating layer (180) and an outer coating layer (190) formed on the pipe (100) of FIG. 7d. FIG. 8a is a partial cross-sectional photograph of the ERW welded pipe (100) in FIG. 6. FIG. 8b is a partial cross-sectional photograph of a pipe (100) having a cutting groove (140) formed according to the present invention. FIG. 8c is a partial cross-sectional photograph of a pipe (100) welded with SAW according to the present invention. FIG. 9 is a schematic flowchart illustrating a method for manufacturing a SAW pipe that forms a heterogeneous weld joint by ERW welding and SAW welding according to an embodiment of the present invention. Specific details for implementing the invention

[0038] Below, with reference to the attached drawings, embodiments of the present invention are described in detail so that those skilled in the art can easily implement the invention. However, since the description of the present invention is merely an example for structural or functional explanation, the scope of the present invention should not be interpreted as being limited by the embodiments described in the text. That is, since the embodiments are subject to various modifications and may take various forms, the scope of the present invention should be understood to include equivalents capable of realizing the technical concept. Furthermore, the objectives or effects presented in the present invention do not imply that a specific embodiment must include all of them or only such effects; therefore, the scope of the present invention should not be understood as being limited by them.

[0039] The meaning of the terms described in this invention should be understood as follows.

[0040] Terms such as "first" and "second" are intended to distinguish one component from another, and the scope of rights shall not be limited by these terms. For example, the first component may be named the second component, and similarly, the second component may be named the first component. When a component is referred to as being "connected" to another component, it should be understood that it may be directly connected to that other component, or that there may be other components in between. Conversely, when a component is referred to as being "directly connected" to another component, it should be understood that there are no other components in between. Meanwhile, other expressions describing the relationship between components, such as "between" and "exactly between," or "adjacent to" and "directly adjacent to," shall be interpreted in the same manner.

[0041] A singular expression should be understood to include a plural expression unless the context clearly indicates otherwise, and terms such as "include" or "have" are intended to specify the existence of the set-up features, numbers, steps, actions, components, parts, or combinations thereof, and should be understood not to preclude the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0042] Unless otherwise defined, all terms used herein have the same meaning as generally understood by those skilled in the art to which this invention pertains. Terms defined in commonly used dictionaries should be interpreted as having meanings consistent with the context of the relevant technology and should not be interpreted as having an ideal or overly formal meaning unless explicitly defined in this invention.

[0043] Composition of the embodiment

[0044] Hereinafter, the cross-sectional configuration of a pipe (100) according to an embodiment of the present invention will be described in detail with reference to the attached drawings. FIG. 7d is a partial cross-sectional view of a pipe (100) with the bead (155) removed from the pipe (100) of FIG. 7c, and FIG. 7e is a cross-sectional view of a pipe (100) with an inner coating layer (180) and an outer coating layer (190) formed on the pipe (100) of FIG. 7d.

[0045] As shown in FIG. 7d, the pipe (100) is formed by roll-forming a metal plate of a length member into a circular cross-section as in FIG. 3 and then welding the adjacent sides. The pipe (100) can be applied from a small diameter of 8 inches to a large diameter of 26 inches. For diameters less than 8 inches, precise welding quality control in the thickness direction is difficult, and for diameters greater than 26 inches, separate handling equipment is required to handle large, heavy pipes. At this time, the metal plate of the length member is unwound from a carbon steel coil (32) and continuously supplied, thereby enabling rapid continuous production.

[0046] The ERW weld (120) is formed on at least a portion of the adjacent sides, specifically forming a certain thickness (t4) from the inner surface (102) of the pipe (100). This ERW weld (120) can be manufactured in the same manner as described in FIG. 1. Since the ERW weld (120) forms a relatively smooth or gently concave curved surface on the inner surface (102), it can be seen as forming the same curved surface as the inner surface (102) or on an extension of the inner surface (102). As a result, it can be prepared to form an inner coating layer (180) immediately without an additional bead (155) removal process on the inner surface (102).

[0047] The SAW weld (150) is formed by filling the cutting groove (140) formed by cutting the upper part and the parts of both sides of the ERW weld (120) between the adjacent sides of the pipe (100).

[0048] The cutting groove (140) is cut along the longitudinal direction by long seam milling, and the cross-section has a U-groove, V-groove, or a combined shape thereof.

[0049] As shown in FIG. 7d, the thickness ratio t3 : t4 of the thickness of the SAW weld (140) (t3) and the thickness of the ERW weld (120) (t4) along the radial direction of the pipe (100) is 9 : 1 to 6 : 4. If the ratio is less than 9 : 1, the effect of ERW welding is negligible and there is a possibility of it being mixed into the molten metal during SAW welding. Also, if the ratio exceeds 6 : 4, there is a risk of strength reduction in large-diameter steel pipes.

[0050] Optionally, as illustrated in FIG. 7e, an inner coating layer (180) may be formed on the inner surface (102) of the pipe (100) or an outer coating layer (190) may be formed on the outer surface (104) of the pipe (100). These inner coating layer (180) and outer coating layer (190) may be a paint layer, an anti-corrosion coating layer, a synthetic resin coating layer, an insulating layer, etc.

[0051] Manufacturing method of the example

[0052] Hereinafter, a method for manufacturing a pipe (100) according to an embodiment of the present invention will be described in detail with reference to the attached drawings. FIG. 6 is a flowchart showing the process of manufacturing a pipe using the manufacturing method according to the present invention, and FIG. 9 is a schematic flowchart showing a method for manufacturing a SAW pipe that forms a heterogeneous weld joint by ERW welding and SAW welding according to an embodiment of the present invention.

[0053] As illustrated in FIGS. 6 and 9, first, a pretreatment process is performed to form a metal plate (coil plate) by unwinding a carbon steel coil (32) (S100). Specifically, after uncoiling (Step 1), a leveling process is performed to remove residual bending stress and form a smooth plate (Step 2, S102). Then, both sides of the metal plate (coil plate) are cut to the correct dimensions (Step 3, S104).

[0054] Next, the metal sheet is formed into a circular cross-section by roll forming using a roll forming device (50) as shown in FIG. 3 (Step 4, S120).

[0055] Next, the adjacent sides of the circular cross-section are brought close together, and then ERW welding is performed to form an ERW weld (120) (Step 5, S140). FIG. 7a is a partial cross-sectional view of the ERW welded pipe (100), and FIG. 8a is a partial cross-sectional photograph of the ERW welded pipe (100). The ERW weld (120) is symmetrical with respect to both sides and is symmetrical up and down, and since it is made of the same material as the base material, the boundary between the base material and the weld is not clear.

[0056] Next, the bead of the ERW weld (120) is removed (S150).

[0057] Next, a cutting groove (140) is formed by cutting the middle and upper portions of the ERW weld (120) and parts of both sides of the base material between the adjacent sides (step 6, S160). FIG. 7b is a partial cross-sectional view of the pipe (100) with the cutting groove (140) formed for the pipe (100) of FIG. 7a, and FIG. 8b is a partial cross-sectional photograph of the pipe (100) with the cutting groove (140) formed according to the present invention. As shown in FIG. 7b and FIG. 8b, the ratio t1 : t2 of the depth (t1) of the cutting groove (140) and the remaining thickness (t2) of the ERW weld (120) due to the cutting groove (140) along the radial direction of the pipe (100) is 5 : 5 to 5.5 : 4.5. At a ratio of less than 5:5, the toughness and mechanical properties of low-temperature (-45℃) impact are reduced, and there is a risk of strength reduction in large-diameter steel pipes. Also, at a ratio greater than 5:5, the depth of the cutting groove (140) is too deep, so the ERW weld (120) may become excessively thin or the ERW welding characteristics may disappear during SAW welding.

[0058] Accordingly, in a more specific embodiment, when the ratio t1 : t2 of the depth (t1) of the cutting groove (140) and the remaining thickness (t2) of the ERW weld (120) is 5 : 5, in the step (S180) of forming the SAW weld (150), the thickness ratio t3 : t4 of the thickness (t3) of the SAW weld (140) and the thickness (t4) of the ERW weld (120) is 9 : 1 to 8 : 2. If the thickness ratio t1 : t2 is 5.5 : 4.5, the thickness ratio t3 : t4 is 6 : 4 to 7 : 3.

[0059] Next, the interior of the cutting groove (140) is SAW welded to form a SAW weld (150) (Step 7, S180). The SAW welding can be a one-pass longitudinal submerged arc welding (LSAW), but if the thickness of the pipe (100) is thick, it may be a multi-pass welding. FIG. 7c is a partial cross-sectional view of the pipe (100) that has been SAW welded with respect to the pipe (100) of FIG. 7b, and FIG. 8c is a partial cross-sectional photograph of the pipe (100) that has been SAW welded according to the present invention. As shown in FIG. 7c and FIG. 8c, a convex bead (155) is formed on the upper part of the SAW weld (150), and a heat-affected zone (160) appears as the base material is subjected to high-temperature heat.

[0060] FIG. 7d is a partial cross-sectional view of a pipe (100) with the bead (155) removed from the pipe (100) of FIG. 7c.

[0061] Next, a post-processing step is performed on the pipe (S220). Specifically, the post-processing step involves performing an Ultrasonic Test (UT) to check for defects in the weld (Step 8, S222), and straightening, shrinking, or expanding the pipe cross-section (Step 9, S224).

[0062] Optionally, an outer coating layer (190) may be formed on the outer surface (104) or an inner coating layer (180) may be formed on the inner surface (102). FIG. 7e is a cross-sectional view of a pipe (100) having an inner coating layer (180) and an outer coating layer (190) formed on the pipe (100) of FIG. 7d.

[0063] Experimental Example

[0064] Hereinafter, experimental examples of preferred embodiments will be described in detail with reference to the attached drawings. [Table 1] is a data table summarizing the experimental results of Comparative Examples 1, 2, and 3 and Experimental Examples 1, 2, 3, and 4.

[0065] division electric current voltage Long seam milling SAW welding Impact value (-45℃) verdict ERW Residual Ratio (Relative to Total Thickness) SAW / ERW thickness ratio ERW - - - - 30 Insufficient Comparative Example 1 700 30 30% - - Yongrak Comparative Example 2 560 30 35% 4 : 6 54J Insufficient Comparative Example 3 600 30 35% 5 : 5 66J Insufficient Experimental Example 1 680 30 45% 6 : 4 73J Good Experimental Example 2 700 30 45% 7 : 3 104J Good Experimental Example 3 750 30 50% 8 : 2 228J excellence Experimental Example 4 800 30 50% 9 : 1 249J excellence

[0066] As can be seen in [Table 1], in Experimental Example 1, when the ratio of the depth (t1) of the cutting groove (140) by long seam milling and the remaining thickness (t2) of the ERW weld (120) is 55% : 45%, and the ratio of the thickness (t3) of the SAW weld (140) and the thickness (t4) of the ERW weld (120) is 6 : 4, the low temperature (-45℃) impact value is 73J and is judged to be good.

[0067] In Experimental Example 2, when the ratio of the depth (t1) of the cutting groove (140) by long seam milling to the remaining thickness (t2) of the ERW weld (120) is 55% to 45%, and the ratio of the thickness (t3) of the SAW weld (140) to the thickness (t4) of the ERW weld (120) is 7 to 3, the low temperature (-45℃) impact value is 104J and is judged to be good.

[0068] In Experimental Example 3, when the ratio t1 : t2 of the depth (t1) of the cutting groove (140) by long seam milling and the remaining thickness (t2) of the ERW weld (120) is 50% : 50%, and the ratio t3 : t4 of the thickness (t3) of the SAW weld (140) and the thickness (t4) of the ERW weld (120) is 8 : 2, the low temperature (-45℃) impact value is judged to be excellent at 228J.

[0069] In Experimental Example 4, when the ratio t1 : t2 of the depth (t1) of the cutting groove (140) by long seam milling and the remaining thickness (t2) of the ERW weld (120) is 50% : 50%, and the ratio t3 : t4 of the thickness (t3) of the SAW weld (140) and the thickness (t4) of the ERW weld (120) is 9 : 1, the low temperature (-45℃) impact value is judged to be excellent at 249J.

[0070] In the experimental example, the current and voltage can be adjusted according to the thickness of the pipe.

[0071] As described above, the detailed description of the preferred embodiments of the present invention disclosed is provided to enable those skilled in the art to implement and practice the present invention. Although the present invention has been described with reference to preferred embodiments, those skilled in the art will understand that various modifications and changes can be made to the present invention without departing from the scope of the invention. For example, those skilled in the art may utilize each configuration described in the embodiments described above in combination with one another. Accordingly, the present invention is not intended to be limited to the embodiments shown herein, but to be given the broadest scope consistent with the principles and novel features disclosed herein.

[0072] The present invention may be embodied in other specific forms without departing from the spirit and essential features of the invention. Accordingly, the above detailed description should not be interpreted restrictively in all respects but should be considered exemplary. The scope of the invention shall be determined by a reasonable interpretation of the appended claims, and all modifications within the equivalent scope of the invention are included within the scope of the invention. The invention is not intended to be limited to the embodiments shown herein, but to be given the broadest possible scope consistent with the principles and novel features disclosed herein. Furthermore, embodiments may be constructed by combining claims that are not explicitly related in the claims, or by including them as new claims through amendments made after filing. Explanation of the symbols

[0073] 10 : ERW (Electric Resistance Welding) welding device, 12 : Bending plate, 14 : Electrode, 16 : Welding roller, 18 : Welded part, 20 : Welding pipe, 32 : Thick plate, 37 : Welding pipe, 40 : External weld, 42 : Internal weld, 44 : Heat-affected zone, 46 : External processing part, 47 : Inner Process Study, 50 : Roll forming device, 52 : Bending plate, 54, 56. 58: 1st, 2nd, and 3rd rollers, 60 : Roll-formed products, 100 : pipe, 102 : Inner self, 104 : Turn away, 120 : ERW weld, 140 : Cutting groove, 150 : SAW weld, 155 : Bead, 160 : Heat-affected zone, 170 : Exterior processing part, 180 : Internal coating layer, 190 : External coating layer, t1 : depth of the cutting groove (140), t2: Remaining thickness of the ERW weld due to the cutting groove (140) t3 : Thickness of the SAW weld, t4: Thickness of the ERW weld.

Claims

Claim 1 A pipe (100) formed by roll forming a metal plate of length to form a circular cross-section and then welding adjacent sides, comprising: an ERW (Electric Resistance Welding) weld (120) formed in at least a portion between the adjacent sides; and a SAW (Submerged Arc Welding) weld (150) that fills a cutting groove (140) formed by cutting a portion of the ERW weld (120) and a portion of the adjacent sides between the adjacent sides, wherein the exposed surface of the SAW weld (150) forms a curved surface identical to the outer surface (104) of the pipe (100), thereby forming a dissimilar weld by ERW welding and SAW welding. Claim 2 A SAW pipe that forms a heterogeneous weld joint by ERW welding and SAW welding, characterized in that, in claim 1, the metal plate of the length member is unwound from a carbon steel coil (32) and continuously supplied. Claim 3 A SAW pipe that forms a heterogeneous weld joint by ERW welding and SAW welding, characterized in that the pipe (100) has an outer diameter of 8 inches to 26 inches in claim 1. Claim 4 A SAW pipe that forms heterogeneous welds by ERW welding and SAW welding, characterized in that, in claim 1, the ERW weld (120) forms a part of the inner surface (102) of the pipe (100), and the SAW weld (150) forms a part of the outer surface (104) of the pipe (100). Claim 5 A SAW pipe forming a heterogeneous weld joint by ERW welding and SAW welding, characterized in that, in claim 4, the thickness ratio t3 : t4 of the thickness of the SAW weld joint (140) and the thickness (t4) of the ERW weld joint (120) along the radial direction of the pipe (100) is 9 : 1 to 6 :

4. Claim 6 A SAW pipe that forms a heterogeneous weld joint by ERW welding and SAW welding, characterized in that, in claim 1, the cross-section of the cutting groove (140) is in the shape of a U-groove or a V-groove. Claim 7 delete Claim 8 A SAW pipe that forms a heterogeneous weld joint by ERW welding and SAW welding, characterized in that, in claim 1, the pipe (100) is an oil pipeline, a submarine oil pipeline, or an oil well pipe. Claim 9 A SAW pipe that forms a heterogeneous weld joint by ERW welding and SAW welding, characterized in that, in claim 1, it further comprises at least one of an inner coating layer (180) formed on the inner surface (102) of the pipe (100) and an outer coating layer (190) formed on the outer surface (104) of the pipe (100). Claim 10 A step of pre-treating a carbon steel coil (32) by unwinding it into a metal plate (S100); a step of forming the metal plate into a circular cross-section by roll forming (S120); a step of forming an ERW weld (120) by ERW welding between two adjacent sides of the circular cross-section (S140); a step of removing the bead of the ERW weld (120) (S150); a step of forming a cutting groove (140) by cutting a part of the ERW weld (120) and a part of the two adjacent sides between the two adjacent sides (S160). A method for manufacturing a SAW pipe that forms a heterogeneous weld joint by ERW welding and SAW welding, comprising the step (S180) of forming a SAW weld joint (150) by SAW (Submerged Arc Welding) the interior of the cutting groove (140); and wherein the exposed surface of the SAW weld joint (150) forms a curved surface identical to the outer surface (104) of the pipe (100). Claim 11 A method for manufacturing a SAW pipe that forms a heterogeneous weld joint by ERW welding and SAW welding, characterized in that, in the step (S160) of forming the cutting groove (140), the ratio t1 : t2 of the depth (t1) of the cutting groove (140) and the remaining thickness (t2) of the ERW weld joint (120) due to the cutting groove (140) along the radial direction of the pipe (100) is 5 : 5 to 5.5 : 4.

5. Claim 12 A method for manufacturing a SAW pipe that forms a heterogeneous weld by ERW welding and SAW welding, characterized in that, in the step (S160) of forming the cutting groove (140), when the ratio t1 : t2 of the depth (t1) of the cutting groove (140) and the remaining thickness (t2) of the ERW weld (120) is 5 : 5, in the step (S180) of forming the SAW weld (150), the thickness ratio t3 : t4 of the thickness (t3) of the SAW weld (140) and the thickness (t4) of the ERW weld (120) is 9 : 1 to 8 : 2, or when the thickness ratio t1 : t2 is 5.5 : 4.5, the thickness ratio t3 : t4 is 6 : 4 to 7 : 3.

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