Butt joint structure, and manufacturing method for butt joint structure

Abstract

To prevent weld defects from occurring by focusing on a backing material for welding and without remaining of gas based on organic matters and the like contained in a rust-preventive agent applied to a bevel section between joint members in a weld metal when performing butt joint welding between joint members.SOLUTION: When joining bevel sections 41 of two joint members 4 by welding, a lining surface where irregularities composed of a protrusion 211 and a recess 212 are formed in a backing material 21 for welding is brought into contact with a reverse surface 42 of the joint members 4 to bridge bevel sections 41 and is welded.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a butt joint structure in which two joining members, such as steel pipe columns that constitute a steel structure, are joined to each other via a weld metal, and to a manufacturing method for the butt joint structure in which the grooves of the two joining members are joined to each other by welding. [Background technology]

[0002] Steel pipe columns and the like that constitute steel structures are manufactured in advance in factories and then transported to and assembled at the construction site. When these steel pipe columns are long, it is difficult to transport them to the construction site, so in many cases they are shortened to a length that falls within the transport limit before being transported to the construction site, and then butt-welded to obtain steel pipe columns of the desired length.

[0003] When actually performing butt welding, as shown in Fig. 14, welding is performed by abutting a welding backing material 10 against each back surface 7a, 8a of the butt portion 9 of each joint member 7, 8 constituting two steel pipe columns. At this time, groove portions 7b, 8b are formed by performing groove processing on each of the joint members 7, 8, and a rust inhibitor 75 is applied to the surfaces of the groove portions 7b, 8b. Since the rust inhibitor 75 contains organic matter, low melting point metal powder, etc., it is desirable to apply it once to the surfaces of the groove portions 7b, 8b and then remove it before welding.

[0004] However, it is difficult to completely remove this rust inhibitor from the grooves 7b and 8b, and it is particularly difficult to remove the rust inhibitor 75 that has entered the gap with the weld backing material 10. For this reason, from the viewpoint of construction efficiency, welding must be performed without completely removing this rust inhibitor 75 at present.

[0005] If welding is performed with the rust inhibitor 75 remaining, the organic matter and low melting point metals contained in the rust inhibitor 75 are decomposed by the welding heat, generating gas. The generated gas turns into bubbles 77 in the weld metal 76 as shown in Fig. 15, and ultimately remains as a welding defect such as a blowhole. If a welding defect occurs in the weld metal 76, it can impair the soundness of the butt joint in terms of the mechanical properties.

[0006] In addition, various techniques for discharging gas generated during welding of joints of steel frame members have been disclosed in the past. For example, Patent Document 1 discloses a welding toothed tab with alternating convex and concave portions. The technique disclosed in Patent Document 1 is intended to prevent initial welding defects by discharging gas generated during welding through the concave portions. Patent Document 2 discloses a welding end tab provided with a gas vent path for the purpose of promoting the discharge of gas generated by welding to the outside. However, the techniques disclosed in Patent Documents 1 and 2 are not techniques aimed at removing welding defects when forming weld metal at the butt joints 9 of each joint member 7 and 8 as shown in Figs. 14 and 15, and are not techniques focused on welding backing materials.

[0007] Patent Document 3 discloses a backing metal for welding used when butt-welding columns made of steel pipes, and discloses a technology in which a plurality of gas vent slits for releasing gas generated during welding are formed at intervals in the longitudinal direction of the backing metal body. However, the technology disclosed in Patent Document 3 is based on the premise that the backing metal body is formed to match the cross-sectional shape of the ring-shaped column so that the outer peripheral surface abuts against the inner peripheral surface of the column, and has the problem that it cannot be applied to butt joints between joining members, not limited to columns. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2001-30094 A [Patent Document 2] JP 2013-215759 A [Patent Document 3] Patent No. 4157429 Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, the present invention has been devised in consideration of the above-mentioned problems, and its object is to provide a butt joint structure and a manufacturing method for a butt joint structure that are suitable for preventing the occurrence of welding defects by preventing gases based on organic matter, etc. contained in the rust inhibitor applied to the groove between the joint members from remaining in the weld metal, focusing on a weld backing material when performing butt joint welding between any joint members, not limited to steel pipe columns that constitute steel structures, in a butt joint structure in which two joint members are joined to each other via a weld metal. [Means for solving the problem]

[0010] The present inventors have attempted to solve the above-mentioned problems by welding a backing surface having projections and recesses formed thereon by abutting the backing surface so as to straddle the groove portions of the joining members.

[0011] The butt joint structure according to the first invention is a butt joint structure in which the grooves of two joining members are joined via a weld metal, and is characterized in that it includes a welding backing material having an uneven backing surface that is abutted against the back surfaces of the joining members so as to span the grooves and is welded by the weld metal.

[0012] The butt joint structure according to a second aspect of the present invention is characterized in that, in the first aspect of the present invention, the welding backing material has a ratio of an area including a root gap between the grooves to the backing surface of 50% or more.

[0013] A butt joint structure according to a third aspect of the present invention is the first or second aspect of the present invention, wherein the depth of the recesses constituting the unevenness of the weld backing material is 0.2 to 3 mm.

[0014] The butt joint structure according to a fourth aspect of the present invention is the same as that of the first aspect of the present invention, wherein the convex portions constituting the unevenness of the weld backing material are formed by embossing.

[0015] A manufacturing method for a butt joint structure according to a fifth aspect of the present invention is a manufacturing method for a butt joint structure in which grooves of two joining members are joined by welding, and is characterized in that it includes a step of abutting an uneven backing surface of a welding backing material, which has an uneven surface formed thereon, against the back surfaces of the joining members so as to span the grooves, and welding the backing surface.

[0016] The manufacturing method of the butt joint structure of the sixth invention is characterized in that, in the fifth invention, the welding backing material is welded, the backing surface of which faces an area including a root gap between the grooves at a rate of 50% or more.

[0017] The manufacturing method for a butt joint structure according to the seventh invention is characterized in that, in the fifth or sixth invention, the welding backing material is welded, and the depth of the recesses constituting the unevenness is 0.2 to 3 mm.

[0018] The manufacturing method of the butt joint structure according to the eighth aspect of the present invention is characterized in that, in the fifth aspect, the welding backing material in which the convex portions constituting the unevenness are formed by embossing is welded. Effect of the Invention

[0019] According to the present invention having the above-mentioned configuration, in the process of forming the weld metal, the organic matter contained in the rust inhibitor is decomposed by the welding heat, and as a result, gas is generated. However, this generated gas is not taken into the molten pool, but is discharged to the outside through the recess. By disposing a part or the whole of the recess so as to include the root gap between the grooves, the molten pool comes into contact with the recess during welding, and the gas is not taken into the molten pool, but is discharged through the recess. As a result, it is possible to prevent welding defects such as blowholes caused by the generated gas from remaining in the obtained weld metal, and it is possible to ensure the soundness in terms of the mechanical properties of the butt joint. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1(a) is a perspective view of a configuration in which two steel pipe columns are joined to each other by butt welding, and FIG. 1(b) is a cross-sectional view of the butt welded structure. [Diagram 2] FIG. 2(a) is a perspective view of a configuration in which an H-shaped steel beam is joined to an external diaphragm provided on a steel pipe column by butt welding, and FIG. 2(b) is a cross-sectional view of the butt welded structure. [Diagram 3] FIG. 3(a) is a perspective view showing a state before welding in a butt-welded structure to which the present invention is applied, and FIG. 3(b) is a perspective view showing the state after welding. [Figure 4] FIG. 4 is a diagram for explaining various examples in which the recess is disposed so as to include only a part of the root gap between the grooves. [Diagram 5] FIG. 5 is a diagram for explaining the ratio of a region including a root gap between grooves to a backing surface of a welding backing material. [Figure 6] FIG. 6(a) is a diagram showing an example in which convex portions extending in the longitudinal direction are regularly arranged on the backing surface of a welding backing material, and FIG. 6(b) is a diagram showing an example in which the convex portions are formed in an approximately circular shape in a plan view. [Figure 7]FIG. 7 is a diagram for explaining an example of use of a backing material having a backing surface in which embossed convex portions are provided on concave portions as shown in FIG. [Figure 8] FIG. 8 is a diagram for explaining other examples of the cross-sectional shapes of the convex portion and the concave portion. [Figure 9] FIG. 9 is a diagram showing various examples of variations in the cross-sectional shapes of the convex and concave portions. [Figure 10] FIG. 10 is a diagram showing various examples of the planar shapes of the convex portions and the concave portions. [Figure 11] FIG. 11 is a diagram for explaining the ratio of the total area of ​​the convex portions to the total area of ​​the concave portions. [Figure 12] FIG. 12 is a diagram showing a cross-sectional shape of a joint in the embodiment. [Figure 13] FIG. 13 is a cross-sectional view taken along line XX′ in FIG. [Figure 14] FIG. 14 is a diagram for explaining a general method of butt welding. [Figure 15] FIG. 15 is a diagram for explaining problems with conventional butt welding. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, a butt joint structure to which the present invention is applied will be described in detail with reference to the drawings.

[0022] The butt joint structure to which the present invention is applied is applied to, for example, a steel pipe column 1 for a building structure shown in Figures 1 and 2. As shown in the perspective view of Figure 1(a), two steel pipe columns 1a and 1b that have been shortened and brought to a building site from the viewpoint of ease of transportation are joined to each other by butt welding, and as a result, a butt welded structure 2 is formed as shown in the cross-sectional view of Figure 1(b). The butt welded structure 2 is formed by welding a joint member 4a constituting the steel pipe column 1a and a joint member 4b constituting the steel pipe column 1b while abutting the welding backing material 21 to each other, and thereby fixing them to each other by a weld metal 25.

[0023] The butt welded structure 2 is formed by joining flanges 41 provided at both ends of a web 42 of an H-shaped steel beam 40 to an external diaphragm 3 provided on a steel pipe column 1 by butt welding, as shown in the perspective view of Fig. 2(a) and the cross-sectional view of Fig. 2(b), for example. The butt welded structure 2 is formed by welding a joining member 4a constituting the external diaphragm 3 and a joining member 4b constituting the web 42 together with a welding metal 25 by welding the joining member 4a and the joining member 4b by abutting the welding backing material 21 against each other.

[0024] FIG. 3 is an enlarged view of the area surrounded by the dotted lines in FIG. 2(a) and FIG. 2(b) in the butt-welded structure 2 to which the present invention is applied. In particular, FIG. 3(a) is a view showing the state before welding, and FIG. 3(b) is a view showing the state after welding.

[0025] The butt-welded structure 2 includes two joining members 4 a and 4 b , a weld backing material 21 , and a weld metal 25 .

[0026] The two joining members 4a, 4b are subjected to groove processing to form grooves 41a, 41b, respectively. A rust inhibitor 43 is applied to the surfaces of the grooves 41a, 41b. The rust inhibitor 43 contains, for example, organic matter such as resin or organic solvent, or low-melting-point metal powder such as aluminum.

[0027] The welding backing material 21 is made of a metal material, and it is preferable to use a steel plate with a C content of 0.5% by weight or less. If the C content is too high, cracks will occur in the backing material during welding, which is undesirable. The welding backing material 21 has an uneven surface formed with a convex portion 211 and a concave portion 212. The welding backing material 21 is disposed so that the backing surface 200 on which the convex portion 211 and the concave portion 212 are formed straddles the groove portions 41a and 41b. The welding backing material 21 is welded with the backing surface 200 in contact with the back surface 42a of the joining member 4a and the back surface 42b of the joining member 4b. More specifically, the welding is performed with the convex portion 211 on the backing surface 200 in contact with the back surface 42a of the joining member 4a and the back surface 42b of the joining member 4b, and the concave portion 212 spaced apart from the back surfaces 42a and 42b.

[0028] At this time, it is assumed that the recess 212 is disposed so as to straddle the root gap R between the grooves 41a and 41b. For example, as shown in Fig. 3(a), the recess 212 may be disposed so as to include the entire root gap R between the grooves 41a and 41b, but is not limited thereto.

[0029] The recess 212 may be disposed so as to include only a portion of the root gap R between the grooves 41a and 41b. Fig. 4(a) shows an example in which the recess 212 includes a portion of the root gap R between the grooves 41a and 41b in the depth direction X, and the protrusion 211 includes the other portion. Fig. 4(b) shows an example in which the recess 212 includes a portion of the root gap R between the grooves 41a and 41b in the width direction Y, and the protrusion 211 includes the other portion.

[0030] According to the present invention, as shown in Fig. 5, it is preferable that the ratio of the region 55 including the root gap R between the grooves 41a, 41b to the backing surface 200 of the welding backing material 21 is 50% or more. The region 55 here is a region that constitutes the root gap R at the tip of the grooves 41a, 41b, in other words, at the deepest part. That is, the region 55 is a region surrounded by the root gap R and the width of the grooves 41a, 41b.

[0031] The proportion of the backing surface 200 facing this region 55 is uniquely determined as the facing proportion, regardless of the convex portions 211 and concave portions 212 of the facing backing surface.

[0032] If the proportion of the backing surface 200 of the welding backing material 21 facing the region 55 is 50% or more, the opportunity for the molten pool to come into contact with the recess 212 during welding increases, making it possible to efficiently discharge gas into the recess 212 and reducing the number of bubbles remaining in the weld metal 25.

[0033] The welding backing material 21 may be disposed before welding by, for example, temporarily welding a portion of it to the back surface 42a of the joining member 4a and the back surface 42b of the joining member 4b.

[0034] The backing surface 200 having the projections 211 and recesses 212 is brought into contact with the back surface 42a of the joining member 4a and the back surface 42b of the joining member 4b, and the grooves 41a and 41b are welded together to form a weld metal 25 as shown in FIG. 3(b). In the process of forming the weld metal 25, the organic matter contained in the rust inhibitor 43 is decomposed by the welding heat, and gas is generated. However, the generated gas is not taken into the molten pool, but is discharged to the outside through the recesses 212. As described above, by disposing a part or the whole of the recesses 212 so as to include the root gap R between the grooves 41a and 41b, the molten pool comes into contact with the recesses 212 during welding, and the gas is not taken into the molten pool, but is discharged through the recesses 212.

[0035] As a result, it is possible to prevent welding defects such as blowholes caused by the generated gas from remaining in the obtained weld metal 25, and it is possible to ensure the soundness of the butt joint in terms of the mechanical properties.

[0036] In particular, the present invention is suitable for butt welding of materials ranging from mild steel to high-strength steel having a strength level of about 1200 MPa. The butt welding position performed in the present invention can be applied to any position, including downward, horizontal, vertical, and upward.

[0037] In the above-mentioned embodiment, the butt welding of two steel pipe columns 1a, 1b and the butt welding of the flange 41 of the H-shaped steel beam 40 to the external diaphragm 3 provided on the steel pipe column 1 are taken as examples to describe the case of forming a butt joint between the connecting members 4, but the present invention is not limited thereto. The present invention is applicable to butt welding between connecting members 4 made of steel in any object other than the steel pipe column 1.

[0038] The welding backing material 21 is not limited to the above-mentioned example. Fig. 6 is a plan view showing the unevenness of the backing surface 200 of the welding backing material 21 in plan view. Fig. 6(a) shows an example of using a checkered steel plate with embossed convex parts 211 for slip prevention for the backing surface 200. The backing surface 200 made of the checkered steel plate has embossed convex parts 211 of the same planar shape extended in the longitudinal direction regularly arranged on the concave parts 212. Fig. 6(b) shows an example of the embossed convex parts 211 formed in a substantially circular shape in plan view.

[0039] However, the convex portions 211 are not limited to being regularly arranged, but may be irregularly arranged. Also, the convex portions 211 are not limited to having the same planar shape, but may be different planar shapes.

[0040] In a backing material 21 having a backing surface 200 with embossed convex portions 211 provided on such concave portions 212, the proportion of the area of ​​the convex portions 211 to the entire backing surface 200 is significantly smaller than that of the concave portions 212.

[0041] Similarly, when using a backing material 21 having a backing surface 200 with an embossed convex portion 211 on such a concave portion 212, welding is performed by abutting the convex portion 211 against the back surface 42a of the joining member 4a and the back surface 42b of the joining member 4b as shown in Fig. 7(a), and the concave portion 212 is spaced apart from the back surfaces 42a and 42b. As described above, the embossed convex portion 211 has a much smaller area than the concave portion 212 in the entire backing surface 200, so only a small proportion of the convex portion 211 contacts the back surfaces 42a and 42b, and the majority of the concave portion 212 is spaced apart from the back surfaces 42a and 42b.

[0042] When welding is performed in this state, weld metal 25 is formed as shown in Fig. 7(b). Gas generated by welding heat in the process of forming this weld metal 25 is not taken into the molten pool, but is similarly discharged to the outside through recess 212. In particular, as described above, by disposing part or all of recess 212 so as to include root gap R between groove portions 41a, 41b, the molten pool comes into contact with recess 212 during welding, and gas is not taken into the molten pool but is discharged through recess 212. Since the proportion of the area occupied by recess 212 is large, the proportion of the molten pool that comes into contact with recess 212 during welding is also increased accordingly, and as a result, gas can be discharged through recess 212 with high efficiency.

[0043] Incidentally, the cross-sectional shape of the top of the protrusion 211 is desirably finished to be flat so that it can stably contact the back surface 42a of the joining member 4a and the back surface 42b of the joining member 4b, but is not limited thereto. For example, the top of the protrusion 211 may be curved as shown in Fig. 8(a). That is, by making the top of the protrusion 211 curved, it comes into point contact or line contact with the back surface 42, but it is possible to separate the recess 212 from the back surfaces 42a and 42b in the same way, and it is possible to exhaust gas from the molten pool.

[0044] In addition, the cross-sectional shape of the recess 212 is not limited to the above-mentioned embodiment, and is not limited to the case where the bottom surface is formed flat. For example, as shown in FIG. 8(b), the cross-sectional shape of the bottom surface of the recess 212 may be inclined like a curved surface.

[0045] 9(a) to (f) show examples of various variations in the cross-sectional shapes of the convex portion 211 and the concave portion 212. The cross-sectional shapes of the convex portion 211 and the concave portion 212 may be formed in any shape, and as long as at least the convex portion 211 is formed so as to protrude relative to the concave portion 212, the above-mentioned desired effect can be achieved. Furthermore, the convex portion 211 and the concave portion 212 are not limited to being formed in a regular shape, and may be finished in an irregular shape, for example, as shown in FIG. 9(f).

[0046] 10(a) to (e) show examples of various variations in the planar shapes of the convex portion 211 and the concave portion 212. The planar shapes of the convex portion 211 and the concave portion 212 are not limited to these, and may be configured in any regular or irregular shape. However, the concave portion 212 surrounded by the convex portion 211 must have at least an air passage formed therein leading to the outside. For example, as shown in FIG. 10(a) and (b), it can be seen that the concave portion 212 surrounded by the convex portion 211 has an air passage formed therein leading to the outside. This allows gas generated during welding to be discharged to the outside through the air passage of the concave portion 212. EXAMPLES

[0047] Next, an embodiment of a butt joint structure to which the present invention is applied will be described.

[0048] It is desirable that the proportion of the backing surface 200 of the weld backing material 21 facing the region 55 including the root gap R between the grooves 41a, 41b is 50% or more.

[0049] 11, when convex portions 211 and concave portions 212 are formed in the plan view of backing surface 200, the area of ​​the portion facing region 55 is M1, and the total area of ​​backing surface 200 is M, the ratio of backing surface 200 facing region 55 including root gap R between groove portions 41a, 41b can be expressed as M1 / M×100(%). If this ratio is 50% or more, the opportunity for the molten pool to come into contact with concave portions 212 during welding increases, making it possible to efficiently discharge gas into concave portions 212 and reducing the amount of bubbles remaining in weld metal 25.

[0050] The effects of the present invention will be specifically described below with reference to examples. The solid wire has a product diameter of 1.4 mm, and its chemical composition is shown in Table 1. The backing material has a plate thickness of 6 mm, and its chemical composition is shown in Table 2. A commercially available groove rust inhibitor was applied to the groove surface of a test steel plate having the groove shape shown in FIG. 12 until the color of the steel plate was hidden by the color of the rust inhibitor, and horizontal welding was performed under the welding conditions shown in Table 3.

[0051] The steel plate was JIS G3136:2022 SN490C, with a plate thickness of 20 mm, a gap of 7 mm, and a groove angle of 35° with a backing metal. In Fig. 12, the above-mentioned groove angle, gap, and plate thickness are shown when the backing metal 21 is brought into contact with the steel plate 4a and the steel plate 4b. The presence or absence of welding defects was confirmed by a radiographic test according to JIS Z3104:1995 to check for the presence or absence of porosity defects such as blowholes, with types 1 and 2 being passed and types 3 and 4 being failed.

[0052] In the experimental verification, in addition to invention examples 1 to 5 in which the ratio of the area facing region 55 was set, comparative example 1 in which no irregularities were formed on contact surface 200 but a smooth flat surface was also produced and verified. Table 1 shows the experimental verification results.

[0053] [Table 1]

[0054] [Table 2]

[0055] [Table 3]

[0056] [Table 4]

[0057] As a result of experimental verification, in invention examples 1 to 6, the ratio (%) facing region 55 was 10% or more, and so the results of the radiographic test were pass. In invention examples 4 to 6, in which the ratio (%) facing region 55 was 50% or more, no blowholes occurred, and the results of the radiographic test were pass class 1, verifying that they exhibited superior behavior.

[0058] On the other hand, Comparative Example 1, in which the proportion (%) facing region 55 is 0%, is an example in which backing surface 200 is configured as a smooth flat surface without forming any irregularities. However, the gas components generated by the thermal decomposition of rust inhibitor 43 could not be sufficiently discharged outside the system, causing numerous blowholes, and the result of the radiation transmission test was Class 3, resulting in failure. EXAMPLES

[0059] In the welding backing material 21, the depth of the recesses 212 constituting the unevenness is preferably 0.2 to 3 mm. Fig. 13 is a cross-sectional view taken along line XX' in Fig. 11, and the depth of the recesses 212 refers to a depth T from the top of the projection 211 to the bottom of the recess 212. If the top of the projection 211 and the bottom of the recess 212 are not flattened but are curved, or are sharpened in cross section, the depth from the top of the projection 211 to the bottom of the recess 212 is taken as T.

[0060] In the experimental verification, in addition to invention examples 7 to 12 in which the depth T was set, comparative example 2 in which the contact surface 200 was not formed with irregularities but was formed of a smooth plane was also produced and verified. Table 5 shows the experimental verification results.

[0061] [Table 5]

[0062] As a result of experimental verification, in invention examples 7 to 12, which form the convex portion 211 and the concave portion 212 and have a depth T of 0.01 mm or more, there were few blowholes and the results of the radiographic test were acceptable. In particular, in invention examples 8 to 11, which have a depth T of 0.2 to 3 mm, no blowholes were generated and the results of the radiographic test were acceptable as Class 1, verifying that they exhibited superior behavior. In invention example 12, which has a depth of more than 3 mm, the expected function required of the welding backing material 21 during welding was not fully exhibited, and molten metal dripping occurred on the weld backside.

[0063] On the other hand, in Comparative Example 2 in which the depth T is 0 mm, the backing surface 200 is configured as a smooth plane without forming any irregularities. However, the gas components generated by the thermal decomposition of the rust inhibitor 43 could not be sufficiently discharged outside the system, causing many blowholes, and the result of the radiation transmission test was Class 3, which was a failure. [Explanation of symbols]

[0064] 1 steel pipe column 2. Welded structure 3 Outer diaphragm 4 Joining materials 21 Welding backing material 24 Welding metal 25 Welding metal 40 H-shaped steel beam 41 Flange 41a Bevel section 42 Web 43 Rust inhibitor 55 areas 200 sides 211 Convex 212 Recess

Claims

1. In a butt joint structure in which the grooves of two joining members are joined via a weld metal, The backing surface having the projections and recesses is abutted against the back surface of the joining member so as to span the groove portion, and the backing material is welded by the weld metal. A butt joint structure characterized by:

2. The ratio of the area including the root gap between the grooves to the backing surface of the welding backing material is 50% or more.

2. The butt joint structure according to claim 1,

3. The depth of the recesses constituting the unevenness of the welding backing material is 0.2 to 3 mm.

3. The butt joint structure according to claim 1 or 2, characterized in that

4. The welding backing material has convex portions constituting the unevenness formed by embossing.

2. The butt joint structure according to claim 1,

5. In a manufacturing method of a butt joint structure in which groove portions of two joining members are joined by welding, The method includes a step of welding the backing surface of the welding backing material, on which the unevenness is formed, by contacting the back surface of the joining member so as to span the groove portion. A manufacturing method for a butt joint structure comprising:

6. The welding backing material is welded, the backing surface of which faces a region including a root gap between the grooves at a rate of 50% or more. The method for manufacturing a butt joint structure according to claim 5, further comprising the steps of:

7. The depth of the recesses constituting the unevenness is 0.2 to 3 mm.

7. A method for manufacturing a butt joint structure according to claim 5 or 6, characterized in that

8. The welding backing material in which the convex portions constituting the unevenness are formed by embossing is welded. The method for manufacturing a butt joint structure according to claim 5, further comprising the steps of:

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