welded structure

Abstract

The present invention relates to a welded structure having a T-joint, which connects the end face of a joining component to the surface of a component to be joined with a plate thickness of 50 mm or more, thereby joining the joining component and the component to be joined. In addition, the weld metal of the T-joint has a specified weld metal composition and a weld metal microstructure with an austenitic phase of 80% or more in terms of area %.

Description

Technical Field

[0001] This invention relates to welded steel structures (hereinafter also referred to as welded structures) constructed using thick steel plates, such as large container ships and bulk carriers. In particular, this invention relates to welded structures with excellent brittle crack propagation stopping characteristics, capable of halting the propagation of brittle cracks originating from the base material of the thick steel plate or at the weld joint before reaching large-scale failure of the structure. Background Technology

[0002] To increase loading capacity and improve loading and unloading efficiency, container ships and bulk carriers, unlike tankers, have structures with enlarged openings on the upper part of the hull. Therefore, it is particularly necessary for container ships and bulk carriers to have high-strength or thick-walled hull plates.

[0003] Furthermore, container ships have become increasingly larger in recent years, with some reaching capacities of 6,000 to 24,000 TEU. It should be noted that TEU (Twenty-foot Equivalent Unit) represents the number of containers that can be loaded into a container ship, indicating its loading capacity. With the increasing size of these ships, the hull plating requires a thickness of 50mm or more and a yield strength of 390 N / mm². 2 The trend is towards thicker steel plates of grade 1 and above.

[0004] In recent years, from the perspective of shortening the construction period, steel plates used as hull plates for ships have mostly been butt-welded using high heat input welding methods such as arc welding. Such high heat input welding can easily lead to a significant reduction in the toughness of the heat-affected zone, becoming a cause of brittle cracking at the weld joint.

[0005] On the other hand, in ship hull structures, from a safety perspective, it has been believed that even in the event of brittle failure, the propagation of brittle cracks should be stopped before it reaches large-scale damage to prevent the hull from separating.

[0006] Accepting this idea, experimental results on the brittle crack propagation behavior of welded joints in shipbuilding steel plates with a thickness of less than 50 mm were reported in Non-Patent Literature 1.

[0007] Non-patent literature 1 experimentally investigated the propagation path and behavior of brittle cracks forcibly generated in the weld. It described how, if the fracture toughness of the weld is ensured to some extent, the brittle cracks mostly deviate from the weld towards the base material due to the influence of residual welding stress. Several examples of brittle cracks propagating along the weld were also confirmed. This suggests that the possibility of brittle fracture propagating linearly along the weld cannot be asserted.

[0008] However, apart from numerous examples of ships built using welding techniques equivalent to those used in Non-Patent Document 1 on steel plates with a thickness of less than 50 mm without any problems, the classification rules do not specifically require the brittle crack propagation cessation characteristics of welded parts of marine steel because it is recognized that steel base materials with good toughness (such as shipbuilding E-grade steel) have sufficient ability to stop brittle cracking.

[0009] Furthermore, in recent years, large container ships exceeding 6000 TEU have used steel plates with thicknesses sometimes exceeding 50 mm. In such cases, in addition to the decrease in breaking toughness due to increased plate thickness, there is a tendency for a further decrease in breaking toughness at the weld joint by employing high heat input welding with even higher welding heat inputs. In thick-walled high heat input welded joints obtained by performing high heat input welding on steel plates with thicknesses exceeding 50 mm, brittle cracks originating from the weld joint may propagate without deviating towards the base material, and may not stop at the steel plate base material such as the reinforcing steel. This is described, for example, in Non-Patent Document 2. Therefore, ensuring the safety of hull structures using thick-walled high-strength steel plates with a thickness of 50 mm or more has become a significant issue. In addition, Non-Patent Document 2 also points out that in order to stop the propagation of brittle cracks, thick steel plates with special brittle crack propagation stopping characteristics are needed.

[0010] To address this problem, for example, Patent Document 1 describes a welded structure, preferably a ship hull outer plate with a thickness of 50 mm or more. In this welded structure, reinforcement is arranged in a manner intersecting with butt welds and joined by fillet welds. In the technology described in Patent Document 1, by fabricating a structure in which steel plates with a specified microstructure are used as reinforcement materials for fillet welds, even if brittle cracking occurs at the weld head, the reinforcement materials can be prevented from brittle fracture, thus preventing fatal damage such as failure of the welded structure. However, in the technology described in Patent Document 1, complex processes are required to form the steel plate with the desired microstructure from the reinforcement material. This results in reduced productivity and difficulty in consistently ensuring steel plates with the desired microstructure.

[0011] Furthermore, Patent Document 2 describes a welded structure comprising a fillet weld joint formed by corner welding a joining component to a joined component. In the welded structure described in Patent Document 2, an unmelted portion remains at the mating surface of the joining component and the joined component in the cross-section of the fillet weld joint. The width of this unmelted portion is adjusted to satisfy a specific relationship with the brittle crack propagation stopping performance Kca of the joined component. Therefore, even if the joined component (flange) is a thick material with a plate thickness of 50 mm or more, the propagation of brittle cracks originating in the joining component can be stopped at the mating surface of the fillet weld joint, preventing the propagation of brittle cracks to the joined component. However, the technology described in Patent Document 2 lacks sufficient brittle crack propagation stopping characteristics in the joining component, and therefore cannot be considered a technology sufficient to stop the propagation of brittle cracks originating in the joined component using the joined component.

[0012] Furthermore, Patent Documents 3-5 describe a welded structure formed by joining the end face of a joining member with the surface of a member to be joined by a fillet weld. In the technology described in Patent Documents 3-5, a welded structure is constructed such that the surface where the end face of the joining member is joined with the surface of the member to be joined has an unmelted portion, and at least one of the weld leg length or weld width is 16 mm or less. Furthermore, the fillet weld is configured such that the toughness of the fillet weld metal has a particularly good relationship with the thickness of the member to be joined; or further, it is configured such that the joining member is made of a steel plate with excellent brittle crack propagation prevention performance; or the weld metal of the weld joint has high toughness. This prevents the propagation of brittle cracks originating from the welded portion of the member from occurring at the fillet weld, the base material of the joining member, or the welded portion of the joining member or the member to be joined.

[0013] However, in the various technologies described in Patent Documents 3 to 5, the length of the weld leg (or the width of the weld deposit) needs to be limited to 16 mm or less. Therefore, from the viewpoint of ensuring the strength of the fillet weld, the maximum plate thickness that can be applied to the joining part (web) and the joined part (flange) is 80 mm.

[0014] To address this problem, for example, Patent Document 6 describes a welded structure comprising a fillet weld joint that abuts the end face of a joining member with the surface of a member to be joined, which has a plate thickness of 50 mm or more, and joins the joining member and the member to be joined. The welded structure described in Patent Document 6 has a fillet weld joint with a weld leg length and weld width exceeding 16 mm. On the surface where the end face of the joining member abuts the surface of the member to be joined, the cross-section of the fillet weld joint contains an unwelded portion of at least 95% of the plate thickness tw of the joining member. Furthermore, it employs fillet weld metal with toughness that satisfies a predetermined relationship between the smaller of the weld leg length and weld width (L) and the plate thickness tf of the member to be joined. Therefore, even if the plate thickness of the joining member is set to 65–120 mm, the fillet weld metal can prevent the propagation of brittle cracks generated in the member to be joined.

[0015] Furthermore, Patent Document 7 describes a welded structure with a doubler component at the joint between the web and the flange. The welded structure described in Patent Document 7 is as follows: the web and the doubler component are butt-welded, leaving unmelted portions at the joint surface; then, the doubler component and the flange are overlapped with a fillet weld, leaving unmelted portions at the overlap surface. In the technology described in Patent Document 7, if the doubler component is made of austenitic steel, the propagation of large brittle cracks can be prevented using the doubler component.

[0016] Existing technical documents

[0017] Patent documents

[0018] Patent Document 1: Japanese Patent Application Publication No. 2004-232052

[0019] Patent Document 2: Japanese Patent Application Publication No. 2007-326147

[0020] Patent Document 3: Japanese Patent Application Publication No. 5395985

[0021] Patent Document 4: Japanese Patent Application Publication No. 5365761

[0022] Patent Document 5: Japanese Patent Application Publication No. 5408396

[0023] Patent Document 6: Japanese Patent Application Publication No. 6744274

[0024] Patent Document 7: Japanese Patent Application Publication No. 6615215

[0025] Non-Patent Document 1: Research Division 147 of the Japan Shipbuilding Research Association: "Research on the Evaluation of Brittle Fracture Strength of High-Tension Steel Plate Welded Joints for Ship Hulls with High Heat Input Energy", No. 87 (February 1978), pp. 35-53, Japan Shipbuilding Research Association

[0026] Non-Patent Literature 2: Yamaguchi Shinya et al., “Development of Ultra-Large Container Ships - Practical Application of New High-Strength Ultra-Thick Steel Plates”, Journal of the Japan Society of Naval Architects and Marine Engineers, No. 3 (2005), pp. 70-76, November 2005. Summary of the Invention

[0027] However, in the technology described in Patent Document 6, strict construction management is required during welding to limit the weld leg length and weld width, resulting in reduced welding productivity and increased construction costs. Furthermore, in structures requiring partial penetration welding with a small unwelded portion, there is a problem of not being able to ensure sufficient brittle crack propagation cessation performance. Additionally, in the technology described in Patent Document 7, there is a problem of increased construction costs due to doubled component processing and welding; and when expensive austenitic steel plates are used for doubled components, there is an increase in material costs.

[0028] The purpose of this invention is to solve the problems of the prior art described above and to provide a welded structure with excellent performance in stopping the propagation of brittle cracks. This structure can prevent the propagation of brittle cracks originating in the flange (part of the welded component) with a plate thickness of 50 mm or more to the joining component (web) before large-scale failure occurs, without requiring strict construction management during welding. It should be noted that the welded structure of this invention is a welded structure with a T-joint, wherein the T-joint is formed by butt welding the end face of the joining component to the surface of the part being joined using fillet welds or partial penetration welds.

[0029] To achieve the aforementioned objectives, the inventors conducted in-depth research on various important factors affecting the toughness of brittle crack propagation stopping in T-joints. As a result, it was discovered that if the weld metal microstructure of the T-joint is primarily composed of austenite, the weld metal can exhibit high toughness. For example, even when the weld leg length and weld width are 16 mm or more, and the joint application is partially penetrated, a T-joint with excellent brittle crack propagation stopping performance can be produced. Furthermore, it was found that the brittle crack propagation stopping performance of the thick steel plate used in the joining component (web) can be specifically considered, and the weld metal of the T-joint can be used to prevent the propagation of brittle cracks generated in the joined component (flange) to the joining component (web).

[0030] This invention was completed based on the above-mentioned insights and further research. The main points of this invention are as follows.

[0031] [1] A welded structure comprising a T-joint, wherein the T-joint abuts the end face of a joining member with the surface of a member to be joined having a plate thickness of 50 mm or more, thereby joining the joining member with the member to be joined.

[0032] For the aforementioned T-joints, the longer of the weld leg length and weld deposition width, L, is 16 mm or more.

[0033] The weld metal of the aforementioned T-joint has the following weld metal composition and microstructure:

[0034] The composition of the weld metal, by mass percent, is: C: 0.10–0.70%, Si: 0.10–1.00%, Mn: 15.00–28.00%, P: less than 0.030%, S: less than 0.015%, Ni: 1.00–5.00%, Cr: 0.50–4.00%, Mo: less than 2.00%, N: less than 0.150%, and O: less than 0.050%, with the remainder being Fe and unavoidable impurities.

[0035] The austenite phase in the above-mentioned weld metal microstructure is more than 80% by area%.

[0036] [2] According to the welded structure of [1], wherein the above-mentioned weld metal composition further contains, in mass percent, at least one of the following (a) and (b):

[0037] (a) Selected from one or more of the following: V: less than 0.10%, Ti: less than 0.10%, and Nb: less than 0.10%; and

[0038] (b) Selected from one or more of the following: Cu: less than 1.00%, Al: less than 0.10%, Ca: less than 0.010%, and REM: less than 0.020%.

[0039] [3] According to the welded structure described in [1], there is an unmelted portion on the surface where the end face of the joint member of the T-joint is abutted with the surface of the joint member, and the ratio of the width of the unmelted portion to the plate thickness of the joint member, i.e., the unmelted ratio Y, is 30% or more.

[0040] [4] According to the welded structure described in [2], there is an unmelted portion on the surface of the end face of the joint member of the T-joint that is in contact with the surface of the joint member, and the ratio of the width of the unmelted portion to the plate thickness of the joint member, i.e., the unmelted ratio Y, is 30% or more.

[0041] [5] According to the welding structure described in [1], the joined component has a welding head in a manner that intersects with the joined component.

[0042] [6] According to the welding structure described in [2], the joined component has a welding head in a manner that intersects with the joined component.

[0043] [7] According to the welding structure described in [3], the joined component has a welding head in a manner that intersects with the joined component.

[0044] [8] According to the welding structure described in [4], the joined component has a welding head in a manner that intersects with the joined component.

[0045] [9] According to the welding structure described in [5], the joining member has a welding head, and the joining member is arranged such that the welding head of the joining member intersects with the welding head of the joined member.

[0046]

[10] According to the welding structure described in [6], the joining member has a welding head, and the joining member is arranged such that the welding head of the joining member intersects with the welding head of the joined member.

[0047]

[11] According to the welding structure described in [7], the joining member has a welding head, and the joining member is arranged such that the welding head of the joining member intersects with the welding head of the joined member.

[0048]

[12] According to the welding structure described in [8], the joining member has a welding head, and the joining member is arranged such that the welding head of the joining member intersects with the welding head of the joined member.

[0049]

[13] The welded structure according to any one of [1] to

[12] , wherein the plate thickness of the joint member is 50 mm or more.

[0050]

[14] The welded structure according to any one of claims [1] to

[12] , wherein the gap between the joining member and the joined member is 10 mm or less.

[0051]

[15] According to the welded structure described in

[13] , the gap between the joining component and the joined component is 10 mm or less.

[0052] According to the present invention, the propagation of brittle cracks originating from thick-walled joined components (with a plate thickness of 50 mm or more) to the joined components can be prevented before large-scale damage occurs. According to the present invention, large-scale brittle fractures, particularly in large container ships and bulk carriers, such as hull separation, can be avoided, resulting in significant improvements in the safety of ship structures and having a notable industrial impact. Furthermore, according to the present invention, it is possible to manufacture welded structures with excellent brittle crack propagation prevention performance without using special steel or compromising safety, simply by selecting welding materials and adjusting welding conditions during welding construction. Attached Figure Description

[0053] Figure 1 This is an illustrative diagram schematically representing an example of the cross-section of a T-joint.

[0054] Figure 2 These are illustrations of another example of a T-joint. (a) is an external view, and (b) is a cross-sectional view.

[0055] Figure 3 This is an illustrative diagram schematically representing another example of a T-joint. (a) is an external view, and (b) is a cross-sectional view.

[0056] Figure 4 It is an explanatory diagram schematically representing the shape of an ultra-large structural model test object.

[0057] Figure 5 This is an illustrative diagram showing an example of the bevel shape of a T-joint. Detailed Implementation

[0058] One embodiment of the welded structure of the present invention is a T-joint that joins the joining member 1 and the joined member 2 by abutting the end face of the joining member 1 with the surface of the joined member 2. This welded structure of one embodiment of the present invention can be applied, for example, to hull structures in which the hull plating of a ship is the joined member, the bulkhead is the joined member, or the deck is the joined member and the hatch is the joined member. It should be noted that the aforementioned T-joint includes the joining member 1, the joined member 2, and the weld metal 5.

[0059] It should be noted that the joined component 2 is made of steel plate with a thickness of 50 mm or more, preferably 60 mm to 120 mm. Furthermore, the joining component 1 is preferably made of steel plate with a thickness of 50 mm or more, more preferably 60 mm to 120 mm. It should be noted that the type of steel used in the joining component 1 and the joined component 2 is not particularly limited; for example, a yield strength of 350 to 490 N / mm² is preferred. 2 Thick steel plate with a strength of (MPa).

[0060] It should be noted that in one embodiment of the welded structure of the present invention, the T-joint has a weld metal 5, and the longer of the weld leg length 3 and the weld width 13, L, is 16 mm or more. Furthermore, in one embodiment of the welded structure of the present invention, an unwelded portion 4 (width 16 of the unwelded portion) may exist at the mating surface of the joining member 1 and the joined member 2, serving as a structural discontinuity. In the presence of the unwelded portion 4, it is preferable to set the ratio of the width 16 of the unwelded portion to the plate thickness of the joining member 1, i.e., the unwelded ratio Y (=B / tw×100, B: width of the unwelded portion (mm), tw: plate thickness of the joining member (mm)), to 30% or more. By having the unwelded portion 4, brittle cracking propagating in the joined member 2 is more easily stopped at the mating surface. There is no particular upper limit to the unwelded ratio Y, but from the viewpoint of ensuring a specified strength, the unwelded ratio Y is preferably 98% or less. It should be noted that the weld leg length 3, the weld width 13, and the unwelded portion width 16 are in the joint cross-section of the T-joint (described later). Figure 1 The joint cross-section shown is a plane parallel to the xy plane when the thickness direction of the joining component 1 is set as the x-axis and the thickness direction of the joined component 2 is set as the y-axis. Measurements were performed on this plane.

[0061] This state is shown in the joint cross-section. Figure 1 . Figure 1 (a) indicates the case where the joining member 1 is joined vertically relative to the joined member 2, but is not limited to this. For example, Figure 1 As shown in (b), the joining member 1 can also be joined at an angle θ relative to the joined member 2. Additionally, as... Figure 1 As shown in (c), a gap 14 is provided between the joining member 1 and the joined member 2, thereby... Figure 1 As shown in (d), a spacer 15 can be inserted into the gap 14. Furthermore, from the viewpoint of reducing welding time, the gap 14 is preferably 10 mm or less.

[0062] Brittle cracking is extremely rare in the base material of steel plates with few defects; it mostly occurs in welded sections. In cases such as... Figure 2 , Figure 3 In the T-joint shown, brittle cracking originates from the weld head 11. To prevent the brittle cracking from propagating to the joint 1, a structural discontinuity is preferably present. As a structural discontinuity, for example as described above, an unmelted portion 4 is preferably present at the mating surface of the joined part 2 and the joint 1 of the T-joint. In the welded structure of one embodiment of the present invention, the weld metal of the T-joint has excellent toughness, so a structural discontinuity is not necessarily required. However, by having a structural discontinuity, preventing the propagation of brittle cracking becomes easier.

[0063] Figure 2 The welded structure shown is a structure in which the parts to be joined 2 are steel plates joined by a welding head 11, and the joining parts 1 are welded together in a manner that intersects with the welding portion 11 of the welding head. Furthermore, Figure 3 The welded structure shown is a welded structure in which the joining component 1 is a steel plate joined by a welding head 12, and the joined component 2 is a steel plate joined by a welding head 11, and the welding head 12 of the joining component 1 and the welding head 11 of the joined component 2 are welded in an intersecting manner.

[0064] Figure 2 and Figure 3 In this arrangement, the joining component 1 and the butt weld joint 11 are orthogonally arranged, but this is not a limitation. They can also be arranged at an angle. Furthermore, the manufacturing method of the weld joint is not particularly limited; commonly used manufacturing methods are applicable. For example, the components to be joined are butt welded together with steel plates, and the joining components are butt welded together with steel plates to obtain a joining component and a joined component with a butt weld joint. Then, the resulting joining component and joined component can be welded together to manufacture a T-joint. Alternatively, a set of joining components before butt welding can be temporarily welded to the joined component with steel plates, and then the joining components can be butt welded together with steel plates to obtain a joining component with a butt weld joint. Then, the resulting joining component can be permanently welded to the joined component to manufacture a T-joint.

[0065] In a welded structure according to one embodiment of the present invention, the longer of the weld leg length 3 and the weld deposition width 13 of the T-joint, i.e., L, is 16 mm or more. When L is less than 16 mm, i.e., both the weld leg length 3 and the weld deposition width 13 are less than 16 mm, it is advantageous to ensure the brittle crack propagation cessation performance. However, when the component plate thickness exceeds 80 mm, ensuring the strength of the weld becomes difficult. Furthermore, even when the component plate thickness is 80 mm or less, the risk of difficulty in ensuring the strength of the weld increases due to rework during construction. It should be noted that there is no particular upper limit to L; from the viewpoint of workability, L is preferably 30 mm or less.

[0066] Furthermore, in a welded structure according to one embodiment of the present invention, the microstructure of the weld metal of the T-joint (hereinafter also referred to as weld metal microstructure) is an austenitic phase comprising 80% or more in area percent (area ratio). There is no particular upper limit for the austenitic phase, and it can be 100% in area percent. Phases other than the austenitic phase (hereinafter also referred to as residual phases) comprise 0 to 20% in area percent, and ferrite phase can be exemplified as a residual phase.

[0067] By making the weld metal microstructure consist of an austenitic phase comprising 80% or more by area, the toughness of the weld metal is improved. Therefore, even when L is 16 mm or more, the weld metal of the T-joint can stop the propagation of brittle cracks originating in the joined parts and prevent the propagation of brittle cracks to the joined parts. It should be noted that, from the viewpoint of ensuring the strength of the welded structure, the weld metal having the above-described microstructure preferably has a hardness (strength) characteristic of 170–260 HV (390 MPa or more in yield strength and 490 MPa or more in tensile strength) on a Vickers hardness scale.

[0068] In addition, the weld metal of the T-joint has the following weld metal composition, wherein, by mass %, it is C: 0.10-0.70%, Si: 0.10-1.00%, Mn: 15.00-28.00%, P: less than 0.030%, S: less than 0.015%, Ni: 1.00-5.00%, Cr: 0.50-4.00%, Mo: less than 2.00%, N: less than 0.150%, and O: less than 0.050%, with the remainder consisting of Fe and unavoidable impurities.

[0069] As described above, by making the weld metal microstructure a microstructure with an austenitic phase content of 80% or more in area, the toughness of the weld metal is improved. Therefore, even when L is 16 mm or more, the weld metal of the T-joint can be used to stop the propagation of brittle cracks in the joined parts and prevent the propagation of brittle cracks to the joined parts.

[0070] Next, the reasons for limiting the composition of the weld metal as described above will be explained. Hereinafter, the mass percentage of the weld metal composition will only be expressed as a percentage.

[0071] C: 0.10~0.70%

[0072] Carbon (C) is an element that stabilizes austenite. Furthermore, C is an element that increases the strength of weld metal through solid solution strengthening. To achieve this effect, a C content of 0.10% or more is required. However, if the C content exceeds 0.70%, high-temperature cracking during welding is likely to occur. Therefore, the C content is 0.10% to 0.70%. It should be noted that the preferred C content is 0.20% to 0.60%.

[0073] Si: 0.10~1.00%

[0074] Si stabilizes austenite by inhibiting carbide precipitation, thereby allowing carbon to dissolve in the austenite. To achieve this effect, a Si content of 0.10% or higher is required. However, if the Si content exceeds 1.00%, Si segregates during solidification, forming a liquid phase at the solidification unit interface. This reduces high-temperature crack resistance and, consequently, toughness. Therefore, the Si content is 0.10–1.00%. It should be noted that the preferred Si content is 0.20–0.90%.

[0075] Mn: 15.00~28.00%

[0076] Mn is an element that stabilizes the austenitic phase at low cost. Therefore, a content of 15.00% or more of Mn is required. When the Mn content is less than 15.00%, the stability of austenite is insufficient. This results in the formation of a hard martensite phase in the weld metal, reducing toughness. On the other hand, if the Mn content exceeds 28.00%, excessive Mn segregation occurs during solidification, inducing high-temperature cracking. Therefore, the Mn content is 15.00% to 28.00%. It should be noted that the preferred Mn content is 17.00% to 26.00%.

[0077] P: below 0.030%

[0078] Phosphorus (P) is an element that induces high-temperature cracking due to grain boundary segregation. Therefore, P is preferably reduced as much as possible, but it is acceptable as long as it is below 0.030%. Therefore, the P content is set to below 0.030%. It should be noted that excessive reduction of P leads to an increase in refining costs. Therefore, the P content is preferably adjusted to above 0.002%.

[0079] S: below 0.015%

[0080] Sulfur (S) is an element that induces high-temperature cracking due to grain boundary segregation. Therefore, S content is preferably reduced as much as possible, but it is permissible if it is below 0.015%. Thus, the S content is 0.015% or less. It should be noted that excessive reduction of S leads to increased refining costs. Therefore, the S content is preferably adjusted to 0.001% or more.

[0081] Ni: 1.00~5.00%

[0082] Ni is an element that strengthens austenite grain boundaries, suppressing high-temperature crack formation by inhibiting grain boundary embrittlement. To achieve this effect, a Ni content of 1.00% or higher is required. Additionally, Ni also has a stabilizing effect on the austenite phase. However, Ni is a high-valence element, and a content exceeding 5.00% becomes economically unfavorable. Therefore, the Ni content is typically between 1.00% and 5.00%.

[0083] Cr: 0.50–4.00%

[0084] Cr has the effect of improving the strength of weld metal. When the Cr content is less than 0.50%, the above-mentioned effect cannot be ensured. On the other hand, if the Cr content exceeds 4.00%, the toughness and high-temperature crack resistance of the weld metal decrease. Therefore, the Cr content is 0.50% to 4.00%. It should be noted that the Cr content is preferably 0.70% to 3.00%.

[0085] Mo: 2.00% or less

[0086] Mo is an element that strengthens austenite grain boundaries, suppressing high-temperature cracking by inhibiting grain boundary embrittlement. Additionally, Mo also helps solidify weld metal, thereby improving wear resistance. To achieve this effect, a Mo content of 0.10% or more is preferred. On the other hand, if the Mo content exceeds 2.00%, the grains become too hard, the grain boundaries become relatively weak, and high-temperature cracking occurs. Therefore, a Mo content of 2.00% or less is preferred. It should be noted that a Mo content of 0.20% to 1.90% is more preferable.

[0087] N: below 0.150%

[0088] Nitrogen (N) is an unavoidable element. Like carbon (C), N effectively contributes to increasing the strength of the weld metal. Furthermore, N stabilizes the austenite phase and steadily improves low-temperature toughness. This effect is significant when the N content is 0.003% or more; therefore, an N content of 0.003% or more is preferred. However, if the N content exceeds 0.150%, nitrides form, reducing low-temperature toughness. Therefore, the N content is set to 0.150% or less. It should be noted that the N content is preferably 0.003% to 0.120%.

[0089] O: below 0.050%

[0090] Oxygen (O) is an unavoidable element that is incorporated into the weld metal. In the weld metal, O forms Al-based oxides and Si-based oxides, which help suppress the coarsening of the solidification structure. This effect becomes significant when the O content is 0.003% or more; therefore, the O content is preferably 0.003% or more. However, if the O content exceeds 0.050%, the coarsening of oxides becomes significant. Therefore, the O content is 0.050% or less. It should be noted that the O content is preferably 0.003% to 0.040%.

[0091] The above-mentioned components are the basic components of weld metal, but in addition to the above-mentioned basic components, as optional components, any one of the following may be included:

[0092] (a) Selected from one or more of V: ​​less than 0.10%, Ti: less than 0.10%, and Nb: less than 0.10%, and

[0093] (b) Selected from one or more of the following: Cu: less than 1.00%, Al: less than 0.10%, Ca: less than 0.010%, and REM: less than 0.020%.

[0094] (a) Selected from one or more of the following: V: less than 0.10%, Ti: less than 0.10%, and Nb: less than 0.10%.

[0095] V, Ti, and Nb are all carbide-forming elements that contribute to the strength of weld metal by precipitating fine carbides within the grains. One or more of these elements can be present in the weld metal.

[0096] V: below 0.10%

[0097] V is a carbide-forming element that contributes to increased strength of weld metal by precipitating fine carbides within the grains. To achieve this effect, a V content of 0.001% or more is preferred. However, if the V content exceeds 0.10%, excessive carbides become the initiation point for fracture, thus reducing low-temperature toughness. Therefore, when V is present, the V content is preferably 0.10% or less. It should be noted that a V content of 0.002% to 0.050% is more preferred.

[0098] Ti: below 0.10%

[0099] In addition, Ti, like V, is a carbide-forming element, causing fine carbide precipitation, which helps improve the strength of the weld metal. To achieve this effect, it is preferable to contain 0.001% or more Ti. However, if the Ti content exceeds 0.10%, the excessive carbides become the starting point for damage, thus reducing low-temperature toughness. Therefore, when Ti is present, the Ti content is preferably 0.10% or less. It should be noted that the Ti content is more preferably 0.002% to 0.050%.

[0100] Nb: below 0.10%

[0101] Furthermore, Nb, like V and Ti, is a carbide-forming element, causing the precipitation of fine carbides, which contributes to the improvement of the strength of the weld metal. To achieve this effect, a Nb content of 0.001% or more is preferred. However, if the Nb content exceeds 0.10%, the excessive carbides become the starting point for damage, thus reducing low-temperature toughness. Therefore, when Nb is present, the Nb content is preferably 0.10% or less. It should be noted that an Nb content of 0.002% to 0.090% is more preferred.

[0102] (b) Selected from one or more of the following: Cu: less than 1.00%, Al: less than 0.10%, Ca: less than 0.010%, and REM: less than 0.020%.

[0103] Cu is an element that helps stabilize austenite. Al acts as a deoxidizer. In addition, Ca and REM are elements that help suppress high-temperature cracking. Cu, Al, Ca, and REM can be present in any combination of one or more of them.

[0104] Cu: below 1.00%

[0105] Cu is an element that stabilizes the austenitic phase. To achieve this effect, it is preferable to contain 0.01% or more Cu. However, if the Cu content exceeds 1.00%, a low-melting-point liquid phase is formed at the grain boundaries, thus causing high-temperature cracks. Therefore, when Cu is present, the Cu content is preferably 1.00% or less. It should be noted that the Cu content is more preferably 0.02% to 0.80%.

[0106] Al: below 0.10%

[0107] Al acts as a deoxidizer. Furthermore, Al plays an important role in increasing the viscosity of molten metal, stably maintaining its bead-like shape, and reducing sputtering. Moreover, Al lowers the solid-liquid coexistence temperature range, which helps suppress the formation of high-temperature cracks in the weld metal. This effect becomes significant when the Al content is 0.001% or more; therefore, the Al content is preferably 0.001% or more. However, if the Al content exceeds 0.10%, the viscosity of the molten metal becomes excessively high, conversely increasing defects such as increased sputtering and poor weld bead propagation and fusion. Therefore, in the case of Al content, the Al content is preferably 0.10% or less. It should be noted that the Al content is more preferably 0.002% to 0.090%.

[0108] Ca: below 0.010%

[0109] Ca is an element that helps suppress high-temperature cracking. Furthermore, Ca suppresses high-temperature cracking by combining with S in molten metal to form high-melting-point sulfides, CaS. This effect becomes significant when the Ca content is 0.001% or higher. On the other hand, if the Ca content exceeds 0.010%, arc interference occurs during welding, making stable welding difficult. Therefore, when Ca is present, the Ca content is preferably 0.010% or less. It should be noted that the Ca content is more preferably 0.002% to 0.008%.

[0110] REM: below 0.020%

[0111] Like Ca, REM is an element that helps suppress high-temperature cracking. Furthermore, REM is a powerful deoxidizer, existing in the weld metal as REM oxides. REM oxides contribute to suppressing high-temperature cracking by altering the solidification morphology of the weld metal through their role as nucleation sites during solidification. This effect is significant when the REM content is 0.001% or higher. However, if the REM content exceeds 0.020%, the stability of the arc decreases. Therefore, when REM is present, the REM content is preferably 0.020% or less. It should be noted that a REM content of 0.002 to 0.016% is more preferred.

[0112] The remaining components besides those mentioned above are Fe and unavoidable impurities. It should be noted that unavoidable impurities include Bi, Sn, Sb, etc., which are permissible if their total content is less than 0.2%.

[0113] Furthermore, the weld metal of a T-joint having the aforementioned weld metal composition and structure can be formed, for example, by multi-layer welding with adjustments to the welding materials and welding conditions.

[0114] As a welding method, the commonly used gas metal arc welding method is preferred.

[0115] In order to form a T-joint weld metal having the above-described weld metal composition and weld metal structure, the solid welding wire used preferably has the following welding wire composition:

[0116] By mass%, C: 0.10–0.70%, Si: 0.10–1.00%, Mn: 15.00–28.00%, P: less than 0.030%, S: less than 0.015%, Ni: 1.00–5.00%, Cr: 0.50–4.00%, Mo: less than 2.00%, N: less than 0.150%, and O: less than 0.050%.

[0117] It may contain at least one of the following: (a) and (b)

[0118] (a) Selected from one or more of the following: V: less than 0.10%, Ti: less than 0.10%, and Nb: less than 0.10%; and

[0119] (b) Selected from one or more of the following: Cu: 1.00% or less, Al: 0.10% or less, Ca: 0.010% or less, and REM: 0.020% or less.

[0120] The remainder consists of Fe and unavoidable impurities.

[0121] Furthermore, it is preferable to use a welding wire with the above-described welding wire composition, and to perform gas metal arc welding in a shielding gas to form a multilayer weld metal. It should be noted that the welding conditions are preferably set to a downward orientation, while simultaneously satisfying the following conditions: current: 150–450 A (DCEP), voltage: 20–40 V, welding speed: 15–60 cm / min, inter-pass temperature: 100–200 °C, and shielding gas: 80 vol% Ar – 20 vol% CO2. It should be noted that, in order to adjust the strength of the weld metal, it is preferable to adjust the welding heat input per pass to the range of 1.0–3.0 kJ / mm.

[0122] In addition, during welding, it is also possible to... Figure 5 The connecting component 1 shown is provided with a bevel having a specified angle (40°).

[0123] The present invention will be further described below based on embodiments.

[0124] Example

[0125] The yield strength of the plate thickness tw shown in Table 2 is 355~460 N / mm. 2 (MPa) grade thick steel plate is used as joint component 1, with the yield strength of plate thickness tf as shown in Table 2: 355~460N / mm. 2 (MPa) grade thick steel plate is used as the joined component 2. The end face of the joining component 1 is mated with the surface of the joined component 2, and they are welded together to form a... Figure 4 Large welded joints 9 with actual structural dimensions of the shapes shown in (a), (b), and (c). Additionally, the joined components are made of thick steel plates (base material only, denoted as "base material" in Table 2) Figure 4 (a)) or thick steel plates with butt welds (the types in Table 2 are marked as "joints") Figure 4 (b) and (c)), the joining parts are made of thick steel plates (base material only, the types in Table 2 are marked as "base material") Figure 4 (a) and (b)), or thick steel plates with butt welds (the types in Table 2 are marked as "joints") Figure 4 (c) It should be noted that the weld joints are fabricated using single-pass high-energy electric arc welding (SEGARC and dual-electrode SEGARC) or multi-layer CO2 gas surfacing, as shown in Table 2.

[0126] Furthermore, a T-joint was fabricated by welding the joint component 1 and the joined component 2 using gas metal arc welding (GMAW), achieving the weld metal composition shown in Table 1 and the weld metal microstructure, hardness, and L shown in Table 2. The welding materials, welding heat input, and shielding gas were varied to create the weld metal. The welding material was adjusted to achieve the desired weld metal composition, using a solid wire with a diameter of 1.2 mm. The welding conditions were set as follows: downward orientation, current: 150–450 A (DCEP), voltage: 20–40 V, welding speed: 15–60 cm / min, inter-pass temperature: 100–200 °C, and shielding gas: 80 vol% Ar – 20 vol% CO2. Additionally, to ensure the weld metal hardness within the specified range, the heat input per pass was adjusted to a range of 1.0–3.0 kJ / mm.

[0127] It should be noted that in some welded joints (T-joints), a gap 14 is provided between the joining member 1 and the joined member 2. Additionally, in some welded joints (T-joints), a gap 14 is provided in the joining member 1. Figure 5 Welding is performed using the bevel shown.

[0128] Test specimens were collected from the weld metal of the obtained T-joint. The weld metal composition was determined using chemical analysis following standard methods on the collected specimens. The results are shown in Table 2.

[0129] In addition, using the collected test pieces, the austenitic and ferrite phases were identified by phase analysis based on the EBSD method according to conventional methods, and the area ratio of each phase in the weld metal microstructure was calculated. The results are shown in Table 2.

[0130] In addition, the hardness of the weld metal was determined using the collected test pieces according to JIS Z 2244-1 (2020). The results are shown in Table 2.

[0131] Next, using the obtained large welded joint 9, fabrication was carried out. Figure 4 The ultra-large structural model test specimen shown underwent a brittle crack propagation cessation test. A steel plate of the same thickness as the joined component 2 was welded to the underside of the joined component 2 via a temporary weld 8. Additionally, a mechanical notch 7 was provided in the joined component 2.

[0132] in addition, Figure 4 In the ultra-large structural model test specimen shown in (b), the butt weld head 11 of the joined component 2 is fabricated orthogonally to the joining component 1. Furthermore, in Figure 4In the ultra-large structural model test body shown in (c), the butt welding head 11 of the joined component 2 is made to intersect with the butt welding head 12 of the joined component 1. Then, the front end of the mechanical notch 7 is machined into the BOND portion of the butt welding head 11 or the weld metal WM.

[0133] In addition, a brittle crack propagation cessation test was conducted by impacting the mechanical notch 7 to induce brittle cracking, and investigating whether the propagating brittle crack stopped in the weld metal (WM). All tests were conducted at stresses ranging from 243 to 283 N / mm². 2 Temperature: Implemented at -10℃. Stress: 243 N / mm² 2 It is the yield strength of 355 N / mm² applied to ship hulls. 2 The maximum allowable stress equivalent to that of grade steel plate is 257 N / mm². 2 It is the yield strength of 390 N / mm² applied to ship hulls. 2 The maximum allowable stress equivalent to that of grade steel plate is 283 N / mm². 2 It is the yield strength of 460 N / mm² applied to ship hulls. 2 The maximum allowable stress of the grade steel plate is equivalent to the test stress, which is set based on the yield strength of the joint components and is equivalent to the maximum allowable stress. Temperature -10℃ is the design temperature of the ship.

[0134] The results are shown in Table 3.

[0135]

[0136] [Table 2]

[0137]

[0138] Table 2 (continued)

[0139]

[0140] *)Y(%)-(B / tw)×100

[0141] **) Does the weld have a bevel? The shape of the bevel is shown in Figure 5

[0142] ***)γ: Austenitic phase α: Ferrite phase

[0143] ****)L: The value of the longer of the weld leg length and weld width

[0144] [Table 3]

[0145]

[0146] In the inventive examples, brittle cracks propagate within the joined parts 2 and then penetrate the weld metal 5, where they cease. In contrast, in the comparative examples, the brittle cracks do not stop at the weld metal 5 but propagate to the joined parts 1. In the comparative examples, the weld metal 5 fails to prevent the propagation of brittle cracks.

[0147] Symbol Explanation

[0148] 1: Joining components

[0149] 2: The part being joined

[0150] 3: Solder leg length

[0151] 4: Unwelded area

[0152] 5: Welding metal

[0153] 7: Mechanical gap

[0154] 8: Temporary welding

[0155] 9: Large welded joints

[0156] 11: Butt weld joint of the joined parts

[0157] 12: Butt weld joints of mating components

[0158] 13: Weld width

[0159] 14: Gap

[0160] 15: Isolation material

[0161] 16: Width of the unwelded portion

Claims

1. A welded structure comprising a T-joint, wherein the T-joint mates the end face of a joining component with the surface of a component to be joined, the joining component being joined to the component to be joined. For the T-joint, the longer of the weld leg length and the weld deposition width, L, is 16 mm or more. The weld metal of the T-joint has the following weld metal composition and weld metal microstructure: The weld metal composition consists of, by mass%, C: 0.10–0.70%, Si: 0.10–1.00%, Mn: 15.00–28.00%, P: less than 0.030%, S: less than 0.015%, Ni: 1.00–5.00%, Cr: 0.50–4.00%, Mo: less than 2.00%, N: less than 0.150%, and O: less than 0.050%, with Fe as the remainder and unavoidable impurities. The austenite phase in the weld metal microstructure is more than 80% by area percentage.

2. A welded structure comprising a T-joint, wherein the T-joint mates the end face of a joining component with the surface of a component to be joined, having a plate thickness of 50 mm or more, thereby joining the joining component with the component to be joined. For the T-joint, the longer of the weld leg length and the weld deposition width, L, is 16 mm or more. The weld metal of the T-joint has the following weld metal composition and weld metal microstructure: The weld metal composition comprises, by mass%, at least one of (a) and (b) below, with Fe as the remainder, and unavoidable impurities, C: 0.10–0.70%, Si: 0.10–1.00%, Mn: 15.00–28.00%, S: 0.015% or less, Ni: 1.00–5.00%, Cr: 0.50–4.00%, Mo: 0.00% or less, N: 0.150% or less, and O: 0.050% or less, and by mass%, at least one of (a) and (b) below, with Fe as the remainder. (a) Selected from one or more of the following: V: less than 0.10%, Ti: less than 0.10%, and Nb: less than 0.10%; and (b) Selected from one or more of the following: Cu: 1.00% or less, Al: 0.10% or less, Ca: 0.010% or less, and REM: 0.020% or less. The austenite phase in the weld metal microstructure is more than 80% by area percentage.

3. The welded structure according to claim 1, wherein, In the T-joint, there is an unmelted portion on the surface where the end face of the joining member aligns with the surface of the joined member, and the ratio of the width of the unmelted portion to the thickness of the joining member, i.e., the unmelted ratio Y, is 30% or more.

4. The welded structure according to claim 2, wherein, In the T-joint, there is an unmelted portion on the surface where the end face of the joining member aligns with the surface of the joined member, and the ratio of the width of the unmelted portion to the plate thickness of the joining member, i.e., the unmelted ratio Y, is 30% or more.

5. The welded structure according to claim 1, wherein, The joined component has a butt weld head in a manner that intersects with the joining component.

6. The welded structure according to claim 2, wherein, The joined component has a butt weld head in a manner that intersects with the joining component.

7. The welded structure according to claim 3, wherein, The joined component has a butt weld head in a manner that intersects with the joining component.

8. The welded structure according to claim 4, wherein, The joined component has a butt weld head in a manner that intersects with the joining component.

9. The welded structure according to claim 5, wherein, The joining component has a butt welding head, and the joining component is configured such that the butt welding head of the joining component intersects with the butt welding head of the joined component.

10. The welded structure according to claim 6, wherein, The joining component has a butt welding head, and the joining component is configured such that the butt welding head of the joining component intersects with the butt welding head of the joined component.

11. The welded structure according to claim 7, wherein, The joining component has a butt welding head, and the joining component is configured such that the butt welding head of the joining component intersects with the butt welding head of the joined component.

12. The welded structure according to claim 8, wherein, The joining component has a butt welding head, and the joining component is configured such that the butt welding head of the joining component intersects with the butt welding head of the joined component.

13. The welded structure according to any one of claims 1 to 12, wherein, The thickness of the joining component is 50 mm or more.

14. The welded structure according to any one of claims 1 to 12, wherein, The gap between the joining component and the joined component is less than 10 mm.

15. The welded structure according to claim 13, wherein, The gap between the joining component and the joined component is less than 10 mm.

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