Method of manufacturing welded joints

Abstract

To provide a manufacturing method of a weld joint exhibiting a high fatigue strength.SOLUTION: A manufacturing method of a weld joint formed by welding a plurality of steel materials, the manufacturing method including: a welding step of welding the plurality of steel materials to obtain a weld joint in a state as welded; and a grinding step of grinding a region including a weld bead toe portion of the weld joint in a state as welded to expose both a heat-affected portion and a base material unaffected portion in a ground surface.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing a welded joint.

Background Art

[0002] In welded structures such as bridges, ships, buildings, and construction machinery, it is required to improve the fatigue strength of the welded joint from the viewpoint of improving the fatigue strength of the welded structure. As techniques for improving the fatigue strength of the welded joint, there are techniques for reducing welding residual stress, techniques for improving the toe shape, and the like. As the technique for reducing the welding residual stress, for example, in Patent Document 1, a method for improving the fatigue strength by introducing compressive residual stress by hammer peening or ultrasonic impact treatment along the weld bead is shown. Also, in Patent Documents 2 and 3, methods of welding using a low transformation temperature welding material are shown. As the technique for improving the toe shape, for example, in Patent Document 4, a method of grinding the welding termination portion of a fillet weld joint is shown, and in Patent Document 5, a method of melting and smoothing the welding termination portion by TIG arc heat is shown.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0004] As described above, there are various methods for improving the fatigue strength of welded joints, but these methods make it difficult to sufficiently increase the fatigue strength of welded joints, and further improvements are considered necessary. This disclosure has been made in view of the above circumstances, and its purpose is to provide a method for manufacturing a welded joint that exhibits high fatigue strength. [Means for solving the problem]

[0005] One aspect of the present invention is: A method for manufacturing a welded joint formed by welding multiple steel materials, A welding process in which the plurality of steel materials are welded together to obtain a welded joint in an as-welded state, A grinding process in which the area including the weld bead toe of a welded joint in an as-welded state is ground, exposing both the heat-affected zone and the base material raw material on the ground surface. This is a method for manufacturing welded joints, including [the specified element].

[0006] Aspect 2 of the present invention is, The method for manufacturing a welded joint according to embodiment 1, wherein at least one of the plurality of steel materials is fatigue-resistant steel.

[0007] A third aspect of the present invention is: After the welding process and before the grinding process, A penetration condition confirmation step is performed to check the penetration condition in a cross section perpendicular to the longitudinal direction of the weld bead of the welded joint in the as-welded state, The process includes determining grinding conditions in the aforementioned cross-section, including the distance from a perpendicular line to the base material surface at the weld bead toe to a perpendicular line to the base material surface where the center of the grinding wheel is located, and the grinding depth, according to the penetration status. The method for manufacturing a welded joint according to embodiment 1 or 2, wherein the grinding step is performed using the grinding conditions determined in the grinding condition determination step.

[0008] Aspect 4 of the present invention is In the grinding process, the center of the grinding wheel disk is on the perpendicular line to the base material surface at the end of the welded bead in a cross section perpendicular to the longitudinal direction of the welded bead, which is the method for manufacturing the welded joint according to any one of Aspects 1 to 3.

[0009] Aspect 5 of the present invention is In the grinding process, the center of the grinding wheel disk is on the perpendicular line to the base material surface at a position on the base material original part side rather than the end of the welded bead in a cross section perpendicular to the longitudinal direction of the welded bead, which is the method for manufacturing the welded joint according to any one of Aspects 1 to 3.

[0010] Aspect 6 of the present invention is In the grinding process, the grinding depth of the region is at least 0.2 mm, which is the method for manufacturing the welded joint according to any one of Aspects 1 to 5.

Effect of the Invention

[0011] According to the present disclosure, a method for manufacturing a welded joint showing high fatigue strength can be provided.

Brief Description of the Drawings

[0012] [Figure 1] It is a diagram for explaining the grinding surface of the welded joint. [Figure 2] It is a diagram for explaining another grinding surface of the welded joint. [Figure 3] It is a cross-sectional macro observation photograph of the welded joint A in the as-welded state of the general-purpose steel in Example 1. [Figure 4] It is a cross-sectional macro observation photograph of the welded joint A in the as-welded state of the fatigue-resistant steel in Example 1. [Figure 5] It is a cross-sectional macro observation photograph of the welded joint B in the as-welded state of the general-purpose steel in Example 1. [Figure 6] It is a cross-sectional macro observation photograph of the welded joint B in the as-welded state of the fatigue-resistant steel in Example 1. [Figure 7] It is a grinding image diagram of the welded joint B in the as-welded state of the fatigue-resistant steel in Example 1. [Figure 8]Another grinding image diagram of the welded joint B in the as-welded state of the fatigue-resistant steel in Example 1. [Figure 9] It is a diagram showing the fatigue test results of the welded joint A between the general-purpose steel and the fatigue-resistant steel in Example 1. [Figure 10] It is a cross-sectional macro-observation photograph showing the crack generation position after the fatigue test of the welded joint A of the fatigue-resistant steel in Example 1. [Figure 11] It is a diagram showing the fatigue test results of the welded joint B between the general-purpose steel and the fatigue-resistant steel in Example 1. [Figure 12] It is a diagram showing the fatigue test results of the welded joint B between the general-purpose steel and the fatigue-resistant steel in the as-welded state in Example 1. [Figure 13] It is a cross-sectional macro-observation photograph showing the crack generation position after the fatigue test of the welded joint B of the fatigue-resistant steel in Example 1. [Figure 14] It is a diagram showing the fatigue test results of the welded joint (gusset joint) between the general-purpose steel and the fatigue-resistant steel in Example 2.

Mode for Carrying Out the Invention

[0013] The inventors have intensively studied the manufacturing method of welded joints in order to manufacture welded joints showing high fatigue strength. In the manufacture of welded structures, the strength of the steel materials used is increasing. When the steel material is strengthened, the fatigue strength of the steel material itself is improved. However, even if the fatigue strength of the steel material itself is improved, the fatigue strength of the welded joint is not improved. For example, as is known from Osamu Watanabe et al., "Fatigue Strength of High-Strength Steel Welded Joints and Its Dominant Factors - Effects of Stress Concentration Coefficient and Welding Residual Stress -", Transactions of the Welding Research Council, 1995, Vol. 13, No. 3, p. 438-443. The reasons include that stress concentration caused by the shape of the stop end of the weld bead (also referred to as the "weld bead stop end") and the presence of tensile residual stress generated by welding make the fatigue crack likely to occur in the weld bead. As described above, in order to improve the fatigue strength of the welded joint, smoothing the shape of the stop end and reducing the welding residual stress have been cited as conventional techniques, but they have not led to a sufficient improvement in the fatigue life of the welded structure.

[0014] The inventors of the present invention considered that the reason why fatigue life has not been sufficiently improved in the prior art is that the crack initiation location is still in the weld metal. Therefore, the inventors conducted studies to realize a welded joint with a sufficiently improved fatigue life and a welded structure including the welded joint with a sufficiently improved fatigue life, and found that a method for manufacturing a welded joint made by welding multiple steel materials should include a welding step of welding the multiple steel materials to obtain a welded joint in an as-welded state, and a grinding step of grinding the region including the weld bead toe of the welded joint in the as-welded state, exposing both the heat-affected zone and the base material raw material on the grinding surface. According to the manufacturing method of the present disclosure, the crack initiation location can be in the base material region instead of within the weld metal. As a result, the inventors found that by selecting the type of steel material (base material), for example, selecting fatigue-resistant steel as the steel material (base material), it is possible to achieve high fatigue strength in the welded joint and further, high fatigue strength in the welded structure. The following describes each step. Here, "as-welded state" refers to the state in which the welded area has not undergone any material changes such as heat treatment or peening after welding.

[0015] [Welding Process] In the welding process, multiple steel materials are welded together to obtain a welded joint in an as-welded state. An "as-welded joint" refers to a welded joint that is in its as-welded state and before grinding, and is distinguished from the "welded joint" of this disclosure, i.e., a welded joint after grinding. [Steel material (base material)] The type of steel material (base material) used in the method for manufacturing welded joints according to this disclosure is not limited. Multiple types of steel materials may be the same or different. General-purpose steel as described in the examples below may also be used. From the viewpoint of further increasing the fatigue strength of welded joints and welded structures, it is preferable to use fatigue-resistant steel as at least one of the multiple types of steel materials. Preferably, fatigue-resistant steel can be used to improve the fatigue strength of the welded joint by increasing the fatigue resistance of the base material of the welded joint. Examples of fatigue-resistant steel include steel materials in which crack initiation resistance is improved by optimally arranging soft and hard phases, as shown in Japanese Patent No. 5104037 and Japanese Patent Application Publication No. 2009-202224, and steel materials in which crack initiation is suppressed by adjusting the alloy composition, as shown in Japanese Patent Application Publication No. 2022-13657. In this disclosure, any type of fatigue-resistant steel is not limited, and these fatigue-strengthened steel materials can be used. The shape of the steel material is not limited and may be steel plate, steel strip, steel bar, steel pipe, etc. One form of fatigue-resistant steel is a steel sheet having an alloy composition as shown in Japanese Patent Publication No. 2022-13657.

[0016] The aforementioned steel plate has a component composition that is C: 0.02~0.10% by mass, Si:0.10~0.60% by mass, Mn: 1.00~2.00% by mass, P: more than 0% by mass and 0.035% by mass or less, S: More than 0% by mass and 0.035% by mass or less, Cu: 0.10~0.60% by mass, Al: 0.010~0.060% by mass, Nb: more than 0% by mass and 0.050% by mass or less, Ti: more than 0% by mass and not more than 0.050% by mass, N: 0.0010~0.0100 mass%, and The remainder consists of iron and unavoidable impurities, and The total content of Si and Cu is 0.30% by mass or more.

[0017] The aforementioned steel plate may also contain, optionally, the following elements: (a) Ni: more than 0% by mass and not more than 1.00% by mass, Ca: more than 0% by mass and 0.0050% by mass or less, B: more than 0.0003% by mass and not more than 0.0050% by mass, V:0.003~0.500% by mass, Cr: 0.05~1.00 mass%, and Mo: One or more selected from the group consisting of 0.010% by mass or more and less than 0.05% by mass, and / or (b) REM: more than 0% by mass and not more than 0.0060% by mass, Zr: more than 0% by mass and 0.0050% by mass or less, Mg: 0.0005~0.0100 mass%, and Ta: One or more selected from the group consisting of 0.010 to 0.500 mass% It may further contain.

[0018] The welding conditions performed in the welding process include angle, heat quantity, welding speed, etc. However, according to the method disclosed herein, the region including the weld bead toe of the welded joint obtained in the as-welded state is ground, and both the heat-affected zone and the base metal raw material are exposed on the ground surface, so the welding conditions are not particularly limited. From the viewpoint of more easily exposing both the heat-affected zone and the base metal raw material on the ground surface during grinding, one embodiment is to weld under conditions such as shallow penetration and thin HAZ. One means of making the penetration shallow is to make the weld bead angle large, but this is not the only means.

[0019] The type of welded joint manufactured by the method of this disclosure is not particularly limited. In the examples described later, fillet welds and gusset welds were manufactured and evaluated, but the method of manufacture of this disclosure can be used to manufacture other welded joints such as butt joints.

[0020] [Grinding process] In the grinding process, the area including the weld bead toe of the welded joint in the as-welded state is ground, exposing both the heat-affected zone and the base material core on the ground surface. In this specification, the "base material" in a welded joint is divided into the "heat-affected zone (HAZ)," whose structure is different from that of the steel material (base plate, etc.) used for welding due to the effects of heat from welding, and the "base material core," which is not substantially affected by the heat from welding and whose structure is the same as that of the steel material used for welding.

[0021] The grinding surface will be explained using Figure 1. Figure 1 is a diagram illustrating grinding using a cross-sectional macro observation photograph showing a cross-section perpendicular to the longitudinal direction of the weld bead of a welded joint in an as-welded state. The aforementioned cross-sectional macro observation photograph is the same as that shown in Figure 7 right, which will be described later. The grinding surface refers to the surface exposed by grinding, and in the welded joint 1 shown in Figure 1, it refers to the curved surfaces A1 to A4. The grinding surface consists of the grinding surfaces A1 to A2 of the weld metal 2, the grinding surfaces A2 to A3 of the heat-affected zone (HAZ) 3, and the grinding surfaces A3 to A4 of the base metal raw material 4. In the manufacturing method of this disclosure, both the grinding surfaces A2 to A3 of the HAZ and the grinding surfaces A3 to A4 of the base metal raw material are observed by grinding. In the manufacturing method of this disclosure, by exposing the base metal raw material along with the heat-affected zone on the grinding surface by grinding, a welded joint that can avoid crack formation in the weld metal can be realized.

[0022] The grinding method is not limited to any particular type, as long as it ensures that the heat-affected zone and the raw material of the base material are reliably exposed on the grinding surface. For example, in addition to grinding using a burg grinder as demonstrated in the embodiments described later, grinding methods using rotary cutters or end mills can also be used. In the case of a grinder, the diameter of the grinding wheel can also be set as appropriate.

[0023] One preferred grinding condition is that, in a cross-section perpendicular to the longitudinal direction of the weld bead, as shown in Figure 7 in the embodiment described later, the center of the grinding wheel (also called the "R center") lies on the perpendicular L1 to the base material surface at the weld bead toe Q. By grinding under these conditions, the region including the weld bead toe can be ground, and both the heat-affected zone and the base material can be easily exposed on the ground surface.

[0024] Another preferred grinding condition is, as shown in Figure 8 in the embodiment described later, that the center of the grinding wheel is on the perpendicular L2 to the base metal surface at position Z, which is on the base metal raw material side of the weld bead toe Q, in a cross section perpendicular to the longitudinal direction of the weld bead. By grinding under this condition, the area including the weld bead toe can be easily ground, and the heat-affected zone and the base metal raw material can be sufficiently exposed on the ground surface. In particular, more of the base metal raw material can be exposed on the ground surface than the heat-affected zone, and as a result, a welded joint can be realized that can sufficiently avoid crack formation in the weld metal.

[0025] When the center of the grinding wheel (R center) lies on the perpendicular L2 to the base material surface at position Z, which is closer to the base material core than the weld bead toe Q, the distance from the perpendicular L1 to the base material surface at the weld bead toe Q to the perpendicular L2 to the base material surface at position Z (d in Figure 8) can be, for example, greater than 0 mm and less than or equal to 1 mm, and even greater than or equal to 0.5 mm and less than or equal to 1 mm. By shifting the center of the grinding wheel (R center) in this way, closer to the base material core than the weld bead toe, it becomes easier to expose the base material core along with the HAZ in the cut surface. For example, in Figure 8 left, shown in the embodiment described later, the center of the grinding wheel (R center) lies on the perpendicular L2 to the base material surface at position Z, which is 1 mm closer to the base material core than the weld bead toe Q.

[0026] The grinding depth (removal depth) is not limited and can be determined according to the degree of penetration of the welded joint in the as-welded state obtained. In a cross section perpendicular to the welding direction, the grinding depth can be, for example, 0.2 mm or less depending on the penetration. Alternatively, depending on the penetration, the grinding depth may be at least 0.2 mm, more preferably 0.3 mm or more, or even greater than 0.3 mm. In a cross section perpendicular to the welding direction, the thickness of the HAZ can usually be 1 mm or more. In this case, even if grinding is performed to 0.3 mm, it is difficult to expose the base metal material. In this disclosure, in order to easily expose the base metal material material along with the HAZ on the ground surface, for example, the grinding depth may be at least 0.3 mm, or even greater than 0.3 mm, as one embodiment. Even in this case, for example, from the viewpoint of maintaining the strength of the welded joint, the grinding depth may be 0.5 mm or less.

[0027] The following embodiments can be considered from the viewpoint of exposing the base material core more and sufficiently suppressing crack generation in the weld metal. Figure 2 is a diagram illustrating grinding using a cross-sectional macro observation photograph showing a cross section perpendicular to the longitudinal direction of the weld bead of a welded joint in the as-welded state. The aforementioned cross-sectional macro observation photograph is the same as that of Figure 8 left, which will be described later. In Figure 2, the grinding surface refers to the curved surface from B1 to B4, and the grinding surface consists of the weld metal grinding surface B1 to B2, the HAZ grinding surface B2 to B3, and the base material core grinding surface B3 to B4. In Figure 2, the distance parallel to the base material surface of the base material core region (B3 to B4) on the grinding surface is represented by D1, and the distance parallel to the base material surface of the HAZ region (B2 to B3) on the grinding surface is represented by D2, and the relationship is that D1 is greater than D2 (D1 > D2). On the other hand, D1 shown in Figure 1 is smaller than D2. In this disclosure, as shown in Figure 2 above, it is believed that crack initiation in the weld metal can be sufficiently suppressed by making the base material region larger than the HAZ region within the base material region of the grinding surface.

[0028] The manufacturing method of the present disclosure further includes the following steps: A penetration condition confirmation step is performed to check the penetration condition in a cross section perpendicular to the longitudinal direction of the weld bead of the welded joint in the as-welded state, The process includes determining grinding conditions in the aforementioned cross-section, including the distance from a perpendicular line to the base material surface at the weld bead toe to a perpendicular line to the base material surface where the center of the grinding wheel is located, and the grinding depth, according to the penetration status. In the grinding step, grinding may be performed using the grinding conditions determined in the grinding condition determination step. By further including a step to confirm the penetration status and a grinding condition determination step, the heat-affected zone and the base material raw material can be reliably exposed on the ground surface.

[0029] [Process for checking the degree of integration] In the process of checking the penetration status of the welded joint in the as-welded state obtained by the above welding, the penetration status can be checked in the following way, for example. However, the process of checking the penetration status is not limited to the method below and may be checked by other methods.

[0030] The welded joint is cut in its as-welded state so that a cross-section perpendicular to the welding direction (longitudinal direction of the weld bead) at the toe of the weld bead can be observed. As for the cutting method, methods that apply heat, such as gas cutting, are undesirable because heat other than that from welding can have an effect; cutting with a saw or band saw is preferred. At the cut surface, the weld metal, HAZ, and base metal can be distinguished, for example, by macroscopic observation (visual inspection) of the cross-section, based on the degree of discoloration. Alternatively, they may be distinguished by observation using a microscope such as an optical microscope.

[0031] When it is difficult to distinguish the degree of discoloration by visual inspection, or to make it easier to distinguish, one method is to corrode the cut surface and then distinguish the corroded cut surface by visual inspection, for example, by the degree of discoloration. As for the corrosion method, for example, one can use the method described in JIS G0553 "Method for Testing the Macrostructure of Steel". As for the corrosion method, for example, by using the nitric acid-ethanol method (Nital method), a nitric acid-ethanol solution with a volume fraction of 5-10% is prepared and used as the corrosive solution, making it easy to identify the area discolored by corrosion as the HAZ.

[0032] If it is difficult to distinguish between the heat-affected zone and the base metal core by corrosion, instead of corrosion, a heat transfer calculation using temperature simulation can be used to distinguish between weld metal, HAZ, and base metal core. For example, areas where the temperature rises above the transformation temperature (Ac1 point) can be identified as the HAZ, and the remaining base metal parts can be identified as the base metal core. As a heat transfer calculation method, for example, the relationship between the distance r from the instantaneous linear heat source and the maximum temperature Tp, as known from Nakao et al., "Microstructure Distribution of Heat-Affected Zone in Multilayer Welds of High-Tensile Steel," Transactions of the Japan Welding Society, 1985, Vol. 3, No. 4, pp. 766-773, can be used to determine the distance r at which Tp becomes the transformation temperature, thereby identifying the HAZ.

[0033] [Grinding condition determination process] In the process of checking the penetration status described above, after understanding the penetration status, grinding conditions, including the distance from the perpendicular line to the base material surface at the weld bead toe to the perpendicular line to the base material surface where the center of the grinding wheel is located, and the grinding depth, are determined according to the penetration status. Preferred grinding conditions are as described in the [Grinding Process] above.

[0034] In the manufacturing method disclosed herein, in actual operation, for example, when manufacturing multiple similar welded joints in succession, in the manufacturing process of the first welded joint, the grinding conditions are determined in the above-mentioned penetration confirmation step and grinding condition determination step, and after manufacturing the first welded joint, in the manufacturing of the second and subsequent welded joints, the penetration confirmation step and grinding condition determination step are omitted, and after the welding process, grinding is performed using the grinding conditions determined in the manufacturing of the first welded joint, thereby exposing both the heat-affected zone and the base material raw material on the ground surface.

[0035] The method for manufacturing a welded joint according to this disclosure may include steps other than those described above, as long as they do not impair the effects of this disclosure. [Examples]

[0036] The present invention will be described in more detail below with reference to examples. The present invention is not limited by the following examples, and can be implemented with appropriate modifications within the scope that is consistent with the spirit described above and below, and all such modifications are included within the technical scope of the present invention.

[0037] (Welding steel (base material)) Fatigue-resistant steel or general-purpose steel was used as the base material. Both the general-purpose steel and the fatigue-resistant steel had a tensile strength of 490 MPa, but the fatigue strength of the fatigue-resistant steel was greater than that of the general-purpose steel.

[0038] [Example 1 (Cross fillet weld joint)] 1. Welding Cross fillet welding (1 pass) was performed using the gas shielded arc welding method. As shown in Table 1 below, welding was performed under different conditions such as the thickness of the steel plates, and two types of welded joints A and B with different shapes (weld bead angle) were fabricated for both general-purpose steel and fatigue-resistant steel.

[0039] [Table 1]

[0040] 2. Confirmation of the penetration status of the welded joint and the image after grinding. First, before grinding the weld bead toe, the penetration status was confirmed by cross-sectional macroscopic observation photographs as follows. The above-mentioned cruciate fillet weld joint was cut so that a cross section perpendicular to the longitudinal direction of the weld bead could be observed. Next, the cut surface was etched with a nitric acid-ethanol solution to obtain cross-sectional macroscopic observation photographs in which the HAZ and the base material were distinguished. Figure 3 is a photograph of a cruciate fillet weld joint of general-purpose steel (reinforcement angle 43.0 degrees; hereafter, weld joints with this reinforcement angle less than 45 degrees are sometimes referred to as "general-purpose steel (as-welded) weld joint A"), and Figure 4 is a photograph of a cruciate fillet weld joint of fatigue-resistant steel (reinforcement angle 42.4 degrees; hereafter, weld joints with this reinforcement angle less than 45 degrees are sometimes referred to as "fatigue-resistant steel (as-welded) weld joint A"). These weld joints A had a reinforcement angle of less than 45 degrees and a smaller weld bead.

[0041] On the other hand, Figure 5 is a photograph of a cruciate fillet welded joint made of general-purpose steel (reinforcement angle 62.9 degrees; hereafter, welded joints with a reinforcement angle of 45 degrees or more are sometimes referred to as "general-purpose steel (as-welded) welded joint B"), and Figure 6 is a photograph of a cruciate fillet welded joint made of fatigue-resistant steel (reinforcement angle 56.3 degrees; hereafter, welded joints with a reinforcement angle of 45 degrees or more are sometimes referred to as "fatigue-resistant steel (as-welded) welded joint B"). These welded joints B had a reinforcement angle of 45 degrees or more and a large weld bead. The aforementioned "reinforcement angle" refers to the angle of the reinforcement portion of the weld bead, as defined in JIS Z 3001 (2008).

[0042] Next, we checked the image after grinding. We will explain the process of checking the image after grinding using fatigue-resistant steel welded joint B as an example. (Grinding image #1) Figure 7 is an illustrative diagram of grinding the weld bead toe with R as the center. Figure 7 left shows a welded joint B made of fatigue-resistant steel in an as-welded state, similar to Figure 6, with the grinding area drawn with a dashed line on a macroscopic cross-sectional photograph of this welded joint B made of fatigue-resistant steel in an as-welded state. Figure 7 right shows an image of the grinding area after grinding. As shown in the illustrative diagram on the right of Figure 7, it can be seen that grinding removes not only the HAZ but also the raw material of the base metal.

[0043] (Grinding image #2) Figure 8 shows a case where grinding is performed at a different position than in Figure 7, and is an illustrative diagram of grinding with a grinder with the R center set 1 mm from the weld bead toe towards the base material. As shown in Figure 7, when grinding is performed with the R center perpendicular to the base material surface at the weld bead toe, the exposure of the HAZ and base material after grinding is relatively small. In contrast, as shown in Figure 8, when grinding is performed with the R center perpendicular to the base material surface at a position 1 mm from the weld bead toe towards the base material, it can be seen that the HAZ and base material are more likely to be exposed near the deepest part of the grinding area.

[0044] As shown in Figures 7 and 8, the penetration status of the welded joint and the image after grinding can be confirmed in advance from the cross-sectional photograph of the weld penetration. From this image, for example, grinding conditions can be determined such as positioning the R center on a perpendicular line to the base material surface at a position shifted 0.5 to 1 mm from the weld bead toe towards the base material raw material side, and setting the grinding depth to at least 0.2 mm. By performing grinding with a grinder under these conditions, both the HAZ and the base material raw material can be reliably exposed on the ground surface.

[0045] 3. Grinding (Grinding the weld bead toe) Grinding of the weld bead toe was performed as follows. Specifically, for both the as-welded weld joint A of general-purpose steel and the as-welded weld joint A of fatigue-resistant steel, the toe shape was made smooth (toe radius 3 mm) by applying a grinder with the R center perpendicular to the base metal surface at the weld bead toe, as shown in Figure 7, to remove the weld metal at the weld bead toe and also remove a portion of the HAZ in the base metal. In other words, in weld joint A, neither the HAZ nor the base metal core was exposed after the grinding. Hereinafter, the weld joint A of general-purpose steel and the weld joint A of fatigue-resistant steel after grinding will be referred to as weld joint A of general-purpose steel and weld joint A of fatigue-resistant steel, respectively, to distinguish them from the as-welded weld joint A of general-purpose steel and weld joint A of fatigue-resistant steel that have not been ground.

[0046] For both the general-purpose steel welded joint B and the fatigue-resistant steel welded joint B, as shown in the image in Figure 8, the R center was set to be perpendicular to the base material surface at position Z, 1 mm from position Q on the left side of Figure 8, and ground using a grinder to remove the weld metal at the weld bead toe and expose the base material HAZ and base material. The grinding depth at this time was set to at least 0.2 mm, and a burr grinder with a tip diameter of φ7 mm was used to achieve a finished toe radius of 3 mm. Hereinafter, the ground general-purpose steel welded joint B and the ground fatigue-resistant steel welded joint B will be referred to as general-purpose steel welded joint B and fatigue-resistant steel welded joint B, respectively, to distinguish them from the unground, as-welded general-purpose steel welded joint B and fatigue-resistant steel welded joint B.

[0047] In this embodiment, grinding was performed using a burr grinder, but the grinding method is not limited as long as both the HAZ and the raw material of the base metal can be exposed near the weld bead toe.

[0048] 4. Evaluation of fatigue life of welded joints Fatigue tests were conducted on load-nontransmission type cross fillet welds using each of the fabricated welded joints (general-purpose steel welded joint A, fatigue-resistant steel welded joint A, general-purpose steel welded joint B, and fatigue-resistant steel welded joint B). The fatigue test conditions were load-controlled bidirectional operation, with a test termination limit of 5 million cycles.

[0049] Figure 9 shows the fatigue test results for welded joint A made of general-purpose steel and welded joint A made of fatigue-resistant steel. In Figure 9, "toe finish" indicates that the joint has been ground. From Figure 9, in the case of welded joint A where neither the HAZ nor the base material was exposed by grinding, the difference in steel plate type did not affect the fatigue life, and the fatigue life of both welded joint A made of general-purpose steel and fatigue-resistant steel was almost the same.

[0050] Figure 10 is a macroscopic cross-sectional photograph of a welded joint A made of fatigue-resistant steel, showing the location of crack initiation after a fatigue test with arrows. In the case of welded joint A made of fatigue-resistant steel, neither the HAZ nor the base metal core was exposed, and as shown in Figure 10, the crack initiation location was in the weld metal. From the results in Figure 9 and the photograph in Figure 10, it can be seen that in the case of welded joint A, where neither the HAZ nor the base metal core is exposed, fatigue cracks occurred within the weld metal, and the result reflected the fatigue strength of the weld metal, not the fatigue strength of the steel plate (base metal). In this case, even if the fatigue strength of the steel plate (base metal) is increased, it is difficult to increase the fatigue strength of the welded joint itself.

[0051] Next, Figure 11 shows the fatigue test results for welded joint B made of general-purpose steel and welded joint B made of fatigue-resistant steel. In Figure 11, "finished toe" indicates that the joint has been ground. For reference, Figure 12 shows the fatigue test results for welded joint B made of general-purpose steel without grinding (as-welded) and welded joint B made of fatigue-resistant steel without grinding (as-welded). From Figure 12, it can be seen that in the case of welded joint B without grinding (as-welded), there was no difference in fatigue life between general-purpose steel and fatigue-resistant steel. This fatigue life roughly corresponds to the fatigue grade D line (JSSC-D grade, fatigue grade for welded joints with smooth toes and no load transmission) in the fatigue design guidelines and commentary for steel structures of the Japan Society of Steel Construction (JSSC), as shown in Figure 12. In other words, the fatigue strength of the base material did not affect the fatigue strength of the welded joint. The reason for using fatigue grade D as the standard in this example is as follows. In other words, the "non-load-transmitting cross fillet weld joint" targeted in this embodiment is specified in the Japan Society of Steel Construction (JSSC)'s Fatigue Design Guidelines and Commentary for Steel Structures to be designed to the following standards: (1) when it has a smooth toe: Grade D, (2) when the toe is finished: Grade D, and (3) when it is not finished: Grade E. There are two design options, Grade D or Grade E, but the higher of these, Grade D, was used as the standard.

[0052] On the other hand, as shown in Figure 11, by grinding both the HAZ and the base material raw material of the welded joint B made of general-purpose steel and the welded joint B made of fatigue-resistant steel, the fatigue life was significantly improved compared to the D grade line in Figure 11. Furthermore, in Figure 11, the fatigue life of the welded joint B made of fatigue-resistant steel was even better than that of the welded joint B made of general-purpose steel.

[0053] Figure 13 is a macroscopic cross-sectional photograph of a fatigue-resistant steel welded joint B after a fatigue test, with arrows indicating the location of crack initiation. In the case of fatigue-resistant steel welded joint B, both the heat-affected zone (HAZ) and the base metal core were exposed, and as shown in Figure 13, the crack initiation location was not in the weld metal but in the base metal (base metal core).

[0054] By comparing Figure 11 with Figures 9 and 12, the following was found: As shown in Figure 11, by exposing both the HAZ and the base metal core through grinding, the crack initiation location could be made in the base metal (base metal core) rather than in the weld metal. As a result, the fatigue life of the welded joint could be increased compared to the as-welded case without grinding and the case where only the HAZ was ground (welded joint A). By using fatigue-resistant steel as the base metal (base metal core), the fatigue life of the welded joint can be further improved compared to the case where general-purpose steel is used, and consequently, the fatigue strength of the welded structure including the welded joint can be increased.

[0055] [Example 2 (Gusset Fitting)] In Example 2, a gusset joint different from the welded joint (welded joint B) in Example 1 was fabricated using general-purpose steel or fatigue-resistant steel, similar to Example 1, and its fatigue characteristics were evaluated.

[0056] First, an out-of-plane gusset joint was fabricated by fillet welding a 100mm high add-on plate of the same thickness to a 490MPa class steel plate with a joint plate thickness of 12mm. FAMILIARC(registered trademark) MX-Z210 was used as the welding material. In both cases, whether general-purpose steel or fatigue-resistant steel was used, the add-on plate was made of the same material as the general-purpose steel.

[0057] The toe of the fillet weld of the out-of-plane gusset joint obtained by welding was ground with a grinder under the same grinding conditions as for welded joint B in Example 1 (Figure 8).

[0058] A bending fatigue test was performed on the joint after grinding. The fatigue life in this test was defined as the number of cycles N5% at which the value of the strain gauge near the toe of the weld decreased by 5% from the start of the test, with the point at which a crack occurred at the toe of the weld joint being the point at which the strain gauge value near the toe decreased by 5% from the start of the test. The results are shown in Figure 14. In Figure 14, "Toe Finished" indicates that the joint has been ground. From Figure 14, it was confirmed that even when a gusset joint was fabricated as a weld joint different from that of Example 1, the fatigue life of the weld joint was further improved compared to when general-purpose steel was used, by using fatigue-resistant steel for the steel plate (base material), similar to Figure 11. [Industrial applicability]

[0059] The method for manufacturing a welded joint according to the embodiment of the present invention is suitable for the manufacture of welded structures such as bridges, ships, buildings, and construction machinery, because the resulting welded joint has excellent fatigue properties. [Explanation of Symbols]

[0060] 1. Welded joint (part of it) 2. Weld metal 3 Heat affected zone (HAZ) 4 Base material

Claims

1. A method for manufacturing a welded joint formed by welding multiple steel materials, A welding process in which the plurality of steel materials are welded together to obtain a welded joint in an as-welded state, A grinding process in which the area including the weld bead toe of a welded joint in an as-welded state is ground, exposing both the heat-affected zone and the base material raw material on the ground surface. A method for manufacturing welded joints, including [the specified part of the method].

2. The method for manufacturing a welded joint according to claim 1, wherein at least one of the plurality of steel materials is fatigue-resistant steel.

3. After the welding process and before the grinding process, A penetration condition confirmation step is performed to check the penetration condition in a cross section perpendicular to the longitudinal direction of the weld bead of the welded joint in the as-welded state, The process includes determining grinding conditions in the aforementioned cross-section, including the distance from a perpendicular line to the base material surface at the weld bead toe to a perpendicular line to the base material surface where the center of the grinding wheel is located, and the grinding depth, according to the penetration status. The method for manufacturing a welded joint according to claim 1, wherein the grinding step is performed using the grinding conditions determined in the grinding condition determination step.

4. The method for manufacturing a welded joint according to claim 1, wherein in the grinding step, the center of the grinding wheel is on a perpendicular line to the base material surface at the toe of the welded bead in a cross section perpendicular to the longitudinal direction of the welded bead.

5. The method for manufacturing a welded joint according to claim 1, wherein in the grinding step, the center of the grinding wheel is on a perpendicular line to the base material surface at a position on the base material raw material side of the weld bead toe in a cross section perpendicular to the longitudinal direction of the weld bead.

6. The method for manufacturing a welded joint according to claim 1, wherein the grinding step involves grinding the region to a depth of at least 0.2 mm.

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