Welded joint of welded structure and method for manufacturing same

Abstract

Provided are a welded joint having excellent tensile strength and proof stress without causing fracture in weld metal or a HAZ in a welded joint portion of a welded structure, and a welding method for obtaining the welded joint.　In this welded joint of a welded structure, the cross-sectional shape of the welded joint parallel to the surface of a base material is a shape in which the width of the HAZ increases and decreases repeatedly in a periodic manner, said shape being located 2 mm below the surface of the base material. Furthermore, the difference between the maximum value and the minimum value of the width of the HAZ at the position 2 mm below the surface of the base material is preferably 3 mm or more. In the welding method, during gas shielded arc welding, the tip of a wire is stopped immediately before coming into contact with one groove surface, the tip of the wire is brought into contact with the groove surface at the moment when the groove surface is melted, thereafter, the tip of the wire is quickly separated from the groove surface, the tip of the wire is moved to the other groove surface or the same groove surface, and the same operation is periodically repeated to perform the welding while manipulating the wire along the groove surface in the traveling direction.

Description

Welded joints for welded structures and their manufacturing method

[0001] This invention relates to welded joints for welded structures and methods for manufacturing the same, and more particularly to welded joints for welded structures that are large structures such as bridges and buildings, and to methods for manufacturing welded joints using gas shielded arc welding.

[0002] The welded structures targeted by this invention specifically include large structures such as bridges and buildings. The methods used for welding these structures include arc welding methods such as carbon dioxide shielded arc welding. In conventional carbon dioxide shielded arc welding, the welding method employs a rod-movement technique where the welding rod (hereinafter also referred to as "welding wire" or simply "wire") is moved alternately from one groove surface to the other during arc welding. This rod-movement technique is called the "weaving method."

[0003] As an example of this conventional weaving method, Patent Document 1 discloses a welding method in which the welding wire is moved in a crescent-shaped loop that protrudes in the welding direction using a downward welding method.

[0004] Japanese Patent Application Publication No. 6-71435

[0005] However, in the conventional welding method disclosed in Patent Document 1, the strength (tensile strength) of the welded joint consisting of the weld metal and the heat-affected zone (hereinafter, the heat-affected zone is also referred to as "HAZ") is low, and the elongation is also small. Furthermore, when the hardness of the HAZ is low, fracture often occurs along the HAZ, and this effect is particularly significant when the hardness (HV) of the weld metal is lower than that of the base metal (hereinafter, "undermatching"), resulting in a problem of reduced welded joint strength.

[0006] Furthermore, in conventional welding methods, when employing the aforementioned weaving method, the welding process was briefly stopped at both ends of the bead (hereinafter simply referred to as "both ends of the bead") in order to obtain deeper penetration at both ends of the bead in the width direction. However, with this method, sufficient penetration cannot be obtained because the molten metal is interposed between the arc and the base metal. On the other hand, if there is no stopping time at both ends of the bead, or if the stopping time is short, there is a problem that defects such as insufficient penetration and slag inclusion are likely to occur.

[0007] The present invention was made to solve the above-mentioned problems, and aims to provide a welded joint for a welded structure that has excellent tensile strength and yield strength, preventing fracture in the weld metal or HAZ, and a welding method for obtaining such a welded joint.

[0008] To achieve the above objectives, the inventors conducted various studies on welding methods and welding rod handling methods.

[0009] As a result, we found that by stopping the welding wire just before it contacts the groove surface and bringing the welding wire into contact with the groove surface at the moment the groove surface melts, penetration can be deepened, and by quickly removing it from the groove surface afterward, the width of the heat-affected zone (HAZ) due to arc heat can be narrowed. Furthermore, we found that by creating a weld metal with deep penetration, it is possible to prevent a decrease in the hardness of the HAZ.

[0010] Hereinafter, the rod handling method newly obtained from these results of the present invention will be referred to as the "horizontal whipping welding method" (hereinafter also referred to as the "new rod handling method").

[0011] This "horizontal whipping welding method" (new rod operation method) was further examined in comparison with the conventional weaving method (hereinafter also referred to as the "conventional method").

[0012] The results of these studies will be explained based on the drawings. Figure 1 schematically shows the arc conditions and the relationship between the base material and the molten area during welding using (a) the conventional method and (b) the new rod movement method. Figure 2 shows the direction of movement (movement) of the rod in the new rod movement method when welding is performed in one pass / layer, with (a) being a schematic plan view of the groove from above and (b) being a vertical cross-sectional view of the groove. Furthermore, Figure 3 shows the direction of movement (movement) of the rod in the new rod movement method when welding is performed in contact with the groove surface in two or more passes / layers, with (a) being a schematic plan view of the groove from above and (b) being a vertical cross-sectional view of the groove.

[0013] Further investigation was conducted into this new rod movement method. When performing gas shielded arc welding, by stopping just before the groove surface, checking the melting state, and then quickly moving the wire (torch), the arc can be brought into contact with the groove surface. This contact will hereafter be referred to as "engaging the arc." This arc penetration results in deep penetration, and it was found that when welding in one pass / layer, moving the wire (torch) alternately in a zigzag pattern within the groove in the welding direction results in a shape where the width of the heat zone (HAZ) periodically repeats its undulations. Furthermore, when welding in contact with the groove surface in two or more passes / layers, it was found that moving within the same groove surface in the welding direction results in a shape where the width of the HAZ periodically repeats its undulations.

[0014] As a result of welding using the new rod-operating method, a welded joint of the welded structure was obtained as shown in the external perspective view in Figure 4. With the new rod-operating method, we found that the width of the HAZ changes periodically, as shown in Figure 4, and a shape is formed that repeats an uneven zigzag pattern.

[0015] Therefore, further detailed examination of this uneven shape resulted in the shape shown in Figure 5. Figure 5(a) is a plan view of the HAZ and BOND (boundary between weld metal and HAZ) formed by implementing the new rod-moving method described above, where the shape of the BOND is connected in a convex arc toward the HAZ. Figure 5(b) shows a form in which the BOND shape is a combination of angular rectangular or trapezoidal unevenness.

[0016] As a method for forming the angular shape shown in Figure 5(b) above, we found that by making the groove surface a continuous uneven shape and performing a normal weaving method, the periodic uneven shape of the groove surface can be synchronized, resulting in a shape in which the width of the HAZ (Heat Edge Zone) periodically repeats its unevenness. We also confirmed that when a normal weaving method is performed on a groove whose surface shape is not uneven, the shape of the HAZ does not become uneven.

[0017] It was found that the tensile strength and yield strength of the welded joint are improved by making the width of the HAZ (Heat Absorption Zone) a shape that periodically repeats irregularities. Furthermore, it was discovered that there is an optimal range for the width of the HAZ and the period of rod movement.

[0018] The present invention was obtained by conducting diligent research on welding methods in addition to the above findings, and its gist is as follows: [1] A welded joint for a welded structure, characterized in that, in a cross-sectional shape parallel to the surface of the base material of the welded joint, the width of the heat-affected zone of the weld repeats periodically. [2] In [1], the welded joint for a welded structure, characterized in that the cross-sectional shape is the shape at a position 2 mm below the surface of the base material. [3] In [1] or [2], in the shape that repeats irregularities, the maximum value (H) of the width of the heat-affected zone of the weld at a position 2 mm below the surface of the base material. ＭＡＸ ) and minimum value (H ＭＩＮ ) difference (H ＭＡＸ -H ＭＩＮA welded joint for a welded structure, characterized in that the length is 3 mm or more. [4] A welded joint for a welded structure, characterized in that in any one of [1] to [3], one unit of the period of the repeating shape of the unevenness is a convex shape from the weld metal toward the base material, and the period (T) is 5 mm to 20 mm. [5] A method for manufacturing a welded joint of a welded structure as described in [2] above, wherein when welding is performed in one pass / layer using a welding wire, the tip of the welding wire is stopped just before it comes into contact with one groove surface (molten surface), the tip of the welding wire is brought into contact with the groove surface at the moment the groove surface melts, then quickly moves away from the groove surface and moves to the other groove surface, and the same operation is repeated periodically to weld both groove surfaces in the groove alternately, and when welding is performed in two passes / layer or more in contact with the groove surface, the tip of the welding wire is stopped just before it comes into contact with one groove surface (molten surface), the tip of the welding wire is brought into contact with the groove surface at the moment the groove surface melts, then quickly moves within the same groove surface in the welding direction, and the same operation is repeated periodically to weld the same groove surfaces in the groove sequentially. [6] A method for manufacturing a welded joint of a welded structure, characterized in that, in [5] above, the welding rod is moved such that the period (T) when the welding is performed periodically is 5 mm to 20 mm. [7] A method for manufacturing a welded joint of a welded structure, characterized in that, in [5] or [6] above, the volume percentage of carbon dioxide in the gas shielded arc welding is 60 volume% to 100 volume%, and the wire diameter of the welding wire is 0.6 mmφ to 2.0 mmφ.

[0019] According to the present invention, it is possible to obtain a welded joint for a welded structure that has excellent joint strength and does not experience fracture in the weld metal portion of the welded joint, resulting in significant industrial benefits. Furthermore, even when applying low-strength welding materials to high-strength steel materials, it is expected that fracture at the HAZ origin in the welded structure will be prevented, improving the performance of the welded joint.

[0020] These are schematic cross-sectional views of arc welding during welding. (a) Conventional method (weaving method), (b) New rod movement method (horizontal whipping welding method). Schematic diagram showing the movement of the welding wire in the new rod movement method when welding is performed in one pass / layer. (a) Plan view, (b) Cross-sectional view perpendicular to the welding direction. Schematic diagram showing the movement of the welding wire in the new rod movement method when welding is performed in contact with the groove surface in two or more passes / layers. (a) Plan view, (b) Cross-sectional view perpendicular to the welding direction. Schematic perspective view of the external appearance of a welded joint formed by the new rod movement method. Schematic plan view showing the shape of the boundary (BOND) between the weld metal and the HAZ. (a) Arc-shaped shape, (b) Square shape. Schematic plan view and perspective view showing the shape of the HAZ. (a) Conventional method, (b) New rod movement method. Plan view photograph of the welded joint. (a) Conventional method, (b) New rod movement method. This is a schematic plan view illustrating the repeating period T of the uneven shape of the boundary (BOND) between the weld metal and the HAZ. (a) One period for the case of an arc shape, (b) One period for the case of a square shape, and (c) How to determine the average period of the repeating pattern are shown. This is a plan view showing the shape of the test specimen for the tensile test of a welded joint. This is a plan view showing the sampling position of the test specimen for the tensile test of a welded joint. This is a perspective view showing the dimensions of the test specimen for the Vickers hardness test and a plan view showing the sampling position of the test specimen. This is a graph showing the hardness distribution results in the Vickers hardness test. These are electron microscope images of the microstructure of the weld metal. (a) Conventional method, (b) New rod operation method. This is a graph showing the measurement results of the welding thermal cycle. (a) Conventional method, (b) New rod operation method.

[0021] [Welded Structures and Welded Joints] The welded structures targeted by this invention are large structures such as bridges and buildings, and the general method used to manufacture these structures is to join steel materials (steel plates) that serve as the base material by welding. For example, when joining steel plates, a V-groove is created in two steel plates, i.e., the base material, and a welded joint is formed by performing gas shielded arc welding using carbon dioxide.

[0022] Between the weld metal and the base metal on both sides, a heat-affected zone (HAZ) is formed in which the structure of the base metal is altered by the arc heat during welding and the heat from the formed weld metal. In this invention, the weld metal, the HAZ on both sides, and the base metal together are referred to as the "welded joint," and the structure consisting of these is referred to as the "welded structure."

[0023] Examples of steel materials (steel grades) used for large-scale structures in this invention include YS325 to YS440. The plate thickness is set to 6 mm to 100 mm.

[0024] [Manufacturing Method for Welded Joints (Welding Method)] Here, we will illustrate the welding conditions when using the carbon dioxide shielded arc welding method. The volume percentage of carbon dioxide is 60% to 100% by volume, with the remainder being Ar gas, etc. The welding current I is 150A to 400A, the welding voltage V is 25V to 40V, the welding heat input Q is 15kJ / cm to 50kJ / cm, and the welding speed S is 10cm / min to 50cm / min. The wire diameter of the welding wire is 0.6mmφ to 2.0mmφ. Furthermore, since the method of the present invention is a rod movement method in which the welding wire is brought into contact with the groove surface and then quickly separated from the groove surface, it is performed by semi-automatic welding, but it is not limited to that and can also be performed by automatic welding.

[0025] [Bevel Shape] The bevel shape is a V-groove with a gap (spacing) at the bottom of the bevel. The bevel angle (θ) is 30° to 90°. The gap at the bottom of the bevel is 0 mm to 10 mm. In addition to V-grooves, this can also be applied to X-grooves, U-grooves, etc.

[0026] [Shape of HAZ] As mentioned above, the horizontal whipping welding method of the present invention involves stopping the welding arc just before it touches the groove surface. This is also referred to as "stopping just before contact" in the following explanation. Then, by bringing the welding wire into contact with (embedding into) the groove surface at the moment the groove surface melts, and then quickly (rapidly) separating it, the arc can be moved deep into the groove surface. In the case of one pass / layer welding, this operation is performed by moving the wire alternately in a zigzag pattern, repeating rapid heating and rapid cooling. In the case of two passes / layers or more, the above operation is performed for welding passes that touch the groove surface, and the welding progresses within the same groove surface. Here, the stopping time during the aforementioned stopping just before contact is preferably 50 msec to 500 msec on average. Also, the speed of separation when quickly separating is preferably 1 mm / sec to 10 mm / sec on average. In the following explanation, the case of one pass / layer will be described mainly.

[0027] As a result of the above, the microstructure of the weld metal becomes fine and dense, and the width of the heat-induced zone (HAZ) due to arc heat can be reduced, preventing a decrease in the hardness of the HAZ. When performing arc welding, by moving the weld metal alternately in a zigzag pattern within the groove in the welding direction, a shape in which the width of the HAZ periodically repeats irregularities can be obtained. This is illustrated in the schematic diagram in Figure 6 and the observation photograph in Figure 7.

[0028] Figure 6 shows a schematic plan view and perspective view of the shape of the HAZ, (a) using the conventional method and (b) using the new rod-operating method. Figure 7 is a plan view photograph of the welded joint, (a) using the conventional method and (b) using the new rod-operating method. These photographs were taken with a stereomicroscope after the surface of the formed welded joint was buffed and corroded with a 5% nitric acid alcohol solution. As can be seen from Figures 6 and 7 above, the shape of the HAZ formed by the conventional method is a smooth shape with continuous irregularities of almost the same width, while the shape of the HAZ formed by the new rod-operating method is a shape in which irregularities are periodically repeated.

[0029] Based on these results, it is preferable to define the uneven shape of the HAZ as follows.

[0030] First, regarding the width of the HAZ, that is, the distance from the boundary on the base material side of the HAZ to the boundary on the weld metal side (=BOND), the HAZ width at a position within 2 mm below the surface of the base material is specified. This is because when fracture occurs, the starting point is often within 2 mm below the surface of the base material, so it is preferable to specify the width of the HAZ at this position (depth from the surface). As the width of this HAZ, the maximum value of the HAZ width (H ＭＡＸ ) and the minimum value (H ＭＩＮ ), the difference between (=H ＭＡＸ -H ＭＩＮ ) is preferably specified to be 3 mm or more. This is because if this difference is less than 3 mm, the zigzag effect of the uneven shape will be reduced, and the hardness of the HAZ may decrease.

[0031] Next, the repeating period of the uneven shape of the HAZ will be described based on Fig. 8.

[0032] One period, for example, the period between a certain n-th convex portion and the next (n+1)-th convex portion is defined as "T ｎ ". To obtain the average period "T", as shown in Fig. 8(c), when the average distance between convex portions at five uneven parts at any position (within 100 mm) is taken as the period T, the period T can be obtained by the following formula (1). Period T = (T １ +T ２ +T ３ +T ４ ) / 4 ・・・ (1) The period T is preferably in the range of 5 mm to 20 mm. This is because if it deviates from this range, the hardness of the HAZ may decrease.

[0033] [Operating Conditions of Horizontal Whipping Welding Method] The specific operating method and operating conditions of the horizontal whipping welding method, which is a new electrode manipulation method, will be described.

[0034] As specific operating methods and operating conditions, first, the zigzag period (the timing at which the welding wire moves alternately) is preferably 1 Hz to 2 Hz. In addition, the movement interval is 5 mm to 10 mm, and the movement speed, that is, the welding speed, is preferably 10 cm / min to 50 cm / min.

[0035] Regarding the relationship with the groove shape, in the case of a V-groove, when the distance between the upper parts of the groove surfaces is 15 mm or more, two passes per layer of distributed welding are performed. The number of passes in multi-layer welding depends on the plate thickness of the base material, but when the plate thickness is 40 mm, it is preferably 8 passes in 5 layers, and in the case of double-sided welding, it is preferably 16 passes.

[0036] [Wire manipulation method] The wire manipulation method for welding wires is described while comparing a conventional weaving method with the horizontal whipping welding method of the new wire manipulation method.

[0037] First, in the conventional method, even if the welding wire is brought close to the groove surface during welding, arc heat is not sufficiently transferred, so the groove surface remains in an unmelted state, and penetration is achieved through heat conduction via the weld metal (about 1500° C. or lower). Therefore, penetration becomes shallow, resulting in a slow temperature rise and slow cooling treatment. As a result, the HAZ becomes a wide width, making it difficult to prevent a decrease in yield strength of the HAZ.

[0038] In contrast, in the new wire manipulation method, when the welding wire is brought close to the groove surface, a molten film is formed on the groove surface by arc heat. The welding wire is continuously brought close to the groove surface, and the approach is stopped once immediately before the welding wire comes into contact with the molten surface, that is, the movement is stopped dimensionally. Then, at the moment when it is confirmed that the groove surface has melted, the welding wire is forced into the groove surface. By doing this, the welding arc directly hits the groove surface, and rapid heating to 5000° C. or higher enables deep penetration. Thereafter, the welding wire is quickly separated from the groove surface. This deep penetration and quick separation enable rapid heating and rapid cooling treatment. As a result, the width of the HAZ in the portion where such operation is performed is narrowed, and a decrease in hardness of the HAZ can be prevented, so fracture in the HAZ is less likely to occur.

[0039] Here, the microstructure of the HAZ, for example, when the base material is of YS325 class, becomes a finer structure compared with the conventional method. Regarding joint strength, since the width of the HAZ is narrow, the reduction in joint strength is reduced due to the plastic constraint effect at locations adjacent to the HAZ. In addition, the fracture position is also located away from the HAZ. Furthermore, regarding the hardness of the HAZ, rapid heating and rapid cooling reduce the cooling rate between 800° C. and 500° C., which affects hardness, resulting in a narrow HAZ width and also reduced softening of the HAZ.

[0040] [Irregular shape of HAZ width (angular shape, not arc-shaped)] As described above, the shape of the HAZ width formed by the new rod-operating method is an arc-shaped irregular shape, and the HAZ width H ＭＡＸ and H ＭＩＮ We found that when the difference is 3 mm or more, it exhibits excellent effects. Based on this finding, further investigation revealed that even if the shape is not arc-shaped, H ＭＡＸ and H ＭＩＮ I realized that if the uneven shape has a difference of 3 mm or more, an equivalent excellent effect can be obtained. In other words, even if the uneven shape is not arc-shaped but has angular parts that are close to a square or trapezoid shape, an equivalent effect can be obtained if the welded joint is a connected zigzag shape. An example of such a shape is shown in Figure 5(b). Note that in Figure 5(b), the corners are made right angles and the whole is shown as linear, but this is a schematic illustration of the shape, and the actual shape after welding will be more rounded.

[0041] A welding method to obtain such a "rounded angular shape" involves grinding the groove surface alternately with a grinding jig beforehand to form a groove surface with an angular, uneven shape having a set period of irregularities, width, and depth of recesses. Here, the interval (period) of the irregularities is preferably in the range of 5 mm to 20 mm. By welding the formed groove surface using the conventional weaving method, the irregularities of the groove surface are synchronized (transferred), and a welded joint with a rounded, angular, uneven shape can be formed.

[0042] The present invention will be further described below based on examples. However, the following examples are merely illustrative and intended to illustrate the present invention in more detail, and do not limit the scope of the rights of the present invention.

[0043] [Example 1] Comparison of the conventional method and the new rod method by various characteristic tests (a) Tensile test of welded joints (Test method) - Preparation of test piece... Equivalent to JIS Z3121 No. 1 test piece (parallel section modified), see Figure 9. - Base material (steel)... SA440B (plate thickness 36 mm) - Welding material (wire)... YGW11 (wire diameter 1.2 mm) - Welding method... Semi-automatic welding using carbon dioxide shielded arc welding - Method and location of test piece sampling... See Figure 10.

[0044] (Test Results) In the case of undermatching in joints welded using the conventional method (weaving method), fracture occurred in the weld metal portion. However, in the case of undermatching in joints welded using the new rod-operating method (horizontal whipping welding method), fracture did not occur in the weld metal portion, but rather in the base metal portion.

[0045] The above-mentioned undermatching refers to a case where the strength of the weld metal is lower than that of the base metal. The base metal used, SA440, has JIS Z 3312 G59JA1UC3M1T (YP ≥ 500 N / mm) as the welding material. ２ ,TS≧590N / mm ２ ) is recommended as an overmatch. However, in this test, JIS Z 3312 YGW11 (YP≧400N / mm ２ ,TS≧490N / mm ２ ) was a welding material two ranks lower, resulting in undermatching.

[0046] In the case of undermatching, the conventional method (weaving method) causes strain to occur from the weld metal to the heat-affected zone (HAZ), which can lead to fracture in the weld metal. However, with the new rod-operated method, strain is less likely to occur in the weld metal, and fracture in the weld metal does not occur. The reason for this is that in the conventional method, the softening range and degree of softening in the HAZ are large, so strain concentrates in the HAZ and becomes the starting point for fracture. It is thought that the weld metal is the path through which this strain propagates. On the other hand, in the new rod-operated method, the softening is small in all of the above cases, and the reason for this is that in the case of soft joints, fracture is thought to occur in the base metal.

[0047] (i) Vickers hardness distribution (Test method) In accordance with JIS Z 2244, the welded specimen was cut with a saw and then cut into small pieces with a microcutter. These small pieces were polished to a diamond paste (1 μm) and then etched in a 5% nitric acid alcohol solution for 1 second to prepare test specimens. The dimensions and sampling locations of the test specimens are shown in Figure 11.

[0048] SA440B (plate thickness 36 mm) was used as the base material (steel) to be welded, and YGW11 (wire diameter 1.2 mm) was used as the welding material (wire). For the welding method, the standard method was performed using the conventional carbon dioxide shielded arc welding method, while the new rod-operated method was performed using semi-automatic welding with carbon dioxide shielded arc welding.

[0049] (Test Results) In the conventional method, the HAZ hardness has a wide range and a small value. In the new rod-operated method, the HAZ width is narrower and the hardness is higher. This difference is due to the operation of the new rod-operated method, and as a result, in the tensile strength test, fracture did not occur in the HAZ portion, but fracture occurred in the base material portion.

[0050] Figure 12 shows a graph comparing the test results for hardness distribution. The tests here were conducted using a load of 0.2 kg, based on "JIS Z 2244: Low Test Force Vickers Hardness Test". The hardness measurement position was 6 mm below the surface of the steel material.

[0051] According to Figure 12, it was found that the new rod-operating method resulted in less softening and a narrower width of the HAZ near the BOND.

[0052] (c) Microstructure of the weld metal The microstructure of the obtained weld metal is observed using an electron microscope (microscopic image) and the result is shown in Figure 13.

[0053] The electron microscope used was a JSM-7001F manufactured by JEOL Ltd. The test specimens for observation were buffed and then corroded with a 5% nitric acid alcohol solution before observation.

[0054] Comparing the microstructure of the weld metal, it can be seen that the new rod-operating method results in a finer and denser structure than the conventional method. Because of this finer and denser structure, it is presumed that the Vickers hardness measurement of the weld metal will show less softening and less strain concentration. Thus, the formation of a finer and denser structure in the weld metal is due to the fact that, as shown in the welding thermal cycle measurement results described next, the HAZ is heated and cooled in a short time by the arc heat during welding when using the new rod-operating method.

[0055] (E) Welding Thermal Cycle A comparative test of the welding thermal cycle will be explained based on Figure 14. Note that the welding thermal cycle test in Figure 14 is the result of measuring the welding thermal cycle of the HAZ at a depth of 2 mm from the base material surface and 5 mm away from the BOND.

[0056] In the new rod-moving method of the present invention, the arc can be brought directly close to the groove surface, and by moving the rod alternately in a zigzag pattern, rapid heating and cooling are repeated. As a result, compared to the welding heat cycle of the conventional method shown in Figure 14(a), the welding heat cycle is larger and higher temperatures can be reached, as shown in Figure 14(b) for the new rod-moving method. Consequently, the metal structure of the heat-affected zone (HAZ) becomes finer and denser, as described above, and the HAZ width becomes narrower.

[0057] [Example 2] An example investigating the variation in characteristic values ​​of welded joints when welding conditions are changed. Next, we fabricated welded joints by changing the welding conditions for the new rod welding method and the conventional welding method, and the results of tensile tests and impact tests were performed on the obtained welded joints are shown. The tensile tests performed here are the same as those described in Example 1 above. The impact tests were measured using a Charpy impact tester manufactured by JT Toshi Co., Ltd.

[0058] First, Table 1 shows the chemical composition of the base material (steel type) used.

[0059]

[0060] Next, welding conditions such as steel type, welding wire, and groove were selected, and welded joints No. 1 to 8 were fabricated using the new rod-operating method and the conventional welding method. Welding was performed in three layers, one pass per layer. The conditions and the measurement results of the HAZ width of the resulting welded joints are summarized in Table 2.

[0061]

[0062] Furthermore, tensile and impact tests were performed on the obtained welded joints, and the measurement results are shown in Table 3.

[0063] From the obtained welded joints, tensile test specimens and Charpy impact test specimens (V-notches) were taken at a position 1 mm below the surface of the weld metal, bond, and HAZ, in accordance with the provisions of JIS Z 3111, and tensile and impact tests were performed.

[0064] Tensile tests were performed on three samples at room temperature, and the average of the obtained values ​​(0.2% yield strength and tensile strength) was used as the characteristic value of the welded joint. In addition, it was confirmed whether the fracture occurred in the base metal or the weld metal during the tensile test.

[0065] In addition, Charpy impact tests were performed on three samples each, and the absorbed energy J (VE) at a test temperature of 0°C was measured. ０℃ The impact absorption energy of the welded joint was calculated and its average value was taken as the impact absorption energy of the welded joint.

[0066]

[0067] From the above results, it was possible to obtain a welded joint with excellent tensile strength that does not cause fracture in the weld metal or HAZ by the welding method using the new rod-operated welding method, which is an example of the present invention.

[0068] 1. Base metal (steel) 2. Welding wire 3. Arc 4. Weld pool 5. Groove 6. Weld metal 7. HAZ (Heat-Affected Zone) 8. BOND (Boundary between weld metal and HAZ)

Claims

1. A welded joint for a welded structure, characterized in that, in the cross-sectional shape of the welded joint parallel to the surface of the base material, the width of the heat-affected zone of the weld has a shape in which irregularities repeat periodically.

2. The welded joint of the welded structure according to claim 1, characterized in that the cross-sectional shape is the shape at a position 2 mm below the surface of the base material.

3. In the shape with repeating irregularities, the maximum width of the welding heat-affected zone at a position 2 mm below the surface of the base material (H ＭＡＸ ) and minimum value (H ＭＩＮ ) difference (H ＭＡＸ -H ＭＩＮ A welded joint of a welded structure according to claim 1 or 2, characterized in that the thickness of the joint is 3 mm or more.

4. A welded joint for a welded structure according to claim 1 or 2, characterized in that one unit of the period of the repeating uneven shape is a convex shape from the weld metal toward the base material, and the period (T) is 5 mm to 20 mm.

5. A welded joint for a welded structure according to claim 3, characterized in that one unit of the period of the repeating uneven shape is a convex shape from the weld metal toward the base material, and the period (T) is 5 mm to 20 mm.

6. A method for manufacturing a welded joint of a welded structure according to claim 2, wherein when performing gas shielded arc welding in a groove using a welding wire, when welding is performed in one pass / layer, the tip of the welding wire is stopped just before it comes into contact with one groove surface (molten surface), the tip of the welding wire is brought into contact with the groove surface at the moment the groove surface melts, then quickly moves away from the groove surface and moves to the other groove surface, and the same operation is repeated periodically to weld both groove surfaces in the groove alternately, and when welding is performed in two passes / layer or more in contact with the groove surface, the tip of the welding wire is stopped just before it comes into contact with one groove surface (molten surface), the tip of the welding wire is brought into contact with the groove surface at the moment the groove surface melts, then quickly moves within the same groove surface in the welding direction, and the same operation is repeated periodically to weld the same groove surfaces in the groove sequentially.

7. The method for manufacturing a welded joint of a welded structure according to claim 6, characterized in that the rod is moved such that the period (T) when the process is repeated periodically is 5 mm to 20 mm.

8. The method for manufacturing a welded joint of a welded structure according to 6 or 7, characterized in that the volume percentage of carbon dioxide in the gas shielded arc welding is 60% to 100% by volume, and the wire diameter of the welding wire is 0.6 mmφ to 2.0 mmφ.

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