T-shaped fillet-welded joint and steel structure

Abstract

To provide a T-shaped fillet-welded joint exhibiting high fatigue strength and a steel structure having the T-shaped fillet-welded joint.SOLUTION: A T-shaped welded joint has a standing plate fillet-welded to a flat plate, wherein a steel sheet used for the flat plate has a chemical composition satisfying a predetermined range of C, Si, Mn, P, S, Al, Cu, Ni, Nb, Ti, N, Ca, and Si+Cu, with the balance being Fe and inevitable impurities. Within the cross-sectional region in the thickness direction of the steel sheet up to 1 mm from the surface, the number density of non-metallic inclusions whose length exceeds 10 μm is 10 / mm2 or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to a fillet welded T-joint and a steel structure. [Background technology]

[0002] In welded structures such as bridges, ships, buildings, and construction machines, it is necessary to improve the fatigue strength of welded joints from the viewpoint of improving the fatigue strength of the welded structures. Techniques for improving the fatigue strength of welded joints include techniques for improving the toe shape and techniques for reducing welding residual stress.

[0003] As a technique for improving the shape of the weld toe, for example, Patent Document 1 discloses a method for grinding the weld toe of a fillet welded joint, and Patent Document 2 discloses a method for melting and smoothing the weld toe using TIG arc heat. As a technique for reducing the weld residual stress, for example, Patent Document 3 discloses a method for improving fatigue strength by introducing compressive residual stress along the weld bead by hammer peening or ultrasonic impact treatment. Furthermore, Patent Documents 4 and 5 disclose methods for welding using low transformation temperature welding materials.

[0004] When the welded structure is a bridge, for example, the welded joint is a fillet welded joint in which a rib material is attached to a steel deck plate of the bridge by fillet welding. As a technique for improving the fatigue strength of the fillet welded joint, for example, Patent Document 6 shows that in a substantially T-shaped fillet welded joint having a wedge groove (wedge-shaped gap) at the butt joint between a flat plate and a vertical plate, a wedge groove (wedge-shaped gap) with a groove angle of 20° to 35° is formed as the groove, and a back bead is formed by gas metal arc welding using a solid wire with a heat input of 5 kJ / cm to 10 kJ / cm per electrode and one pass.

[0005] Patent Document 7 also discloses a multi-pass welding method including a first welding step in which welding is performed using a welding material such that the transformation start temperature of the weld metal in the first pass is in the range of 175°C to 400°C, and then a second welding step in which welding is performed to build up the weld metal in one pass or two or more passes so that part of the weld metal formed in the first welding step becomes an unmelted part, and the second welding step in which the welding heat of the final pass retransforms all of the unmelted parts to austenite. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 5-69128 [Patent Document 2] Japanese Patent Application Publication No. 59-110490 [Patent Document 3] JP 2013-233590 A [Patent Document 4] Patent No. 3851953 [Patent Document 5] Patent No. 3752545 [Patent Document 6] JP 2008-290115 A [Patent Document 7] International Publication No. 2012 / 114532 Summary of the Invention [Problem to be solved by the invention]

[0007] When the strength of steel material is increased, the fatigue strength of the steel material itself improves. However, it is known that the fatigue strength of welded joints does not improve even if the fatigue strength of the steel material itself improves, for example, from Watanabe Osamu et al., "Fatigue Strength of High-Strength Steel Welded Joints and Its Controlling Factors - Effects of Stress Concentration Factor and Welding Residual Stress -", ​​Proceedings of the Japan Welding Society, Vol. 13, No. 3, pp. 438-443, 1995. The reason for this is thought to be that stress concentration due to the shape of the toe of the weld bead and the presence of tensile residual stress generated by welding make the weld bead prone to fatigue cracks, and the fatigue strength of the steel material does not affect these phenomena.

[0008] The technologies for improving the toe shape as in Patent Documents 1 and 2 and the technology for reducing welding residual stress as in Patent Document 3 contribute to improving the fatigue life of joints. However, for example, in the case of a fillet welded joint in which a rib is attached to a steel deck of a bridge by fillet welding, fatigue cracks initiate at the weld root, and it is not possible to take measures such as improving the toe shape or reducing the welding residual stress after welding.

[0009] Furthermore, techniques for improving fatigue at the weld root include welding using low transformation temperature welding materials as in Patent Documents 4 and 5, and performing reverse welding with mixed gas or multi-pass welding with special welding materials as in Patent Documents 6 and 7. However, all of these are special welding techniques, and do not improve fatigue properties over normal fillet welding (particularly fillet welding of a U-rib to a steel deck plate of a bridge, and more particularly one-sided fillet welding of a U-rib to a steel deck plate of a bridge).

[0010] In particular, in the case of T-shaped single-sided fillet welded joints (hereinafter referred to as "steel deck plate U-rib welded joints") in which a U-rib is fillet welded to a steel deck plate of a bridge on one side, it is difficult to detect fatigue cracks that penetrate the flat plate (deck plate) from the weld joint side by visual inspection, so it is necessary to suppress the occurrence of fatigue cracks from the root of the U-rib welded joint.

[0011] As described above, there are various methods for improving the fatigue strength of a welded joint, but it is difficult to easily increase the fatigue strength of a welded joint using these methods, and further improvements are considered necessary. The present disclosure has been made in consideration of the above circumstances, and an object of the present disclosure is to provide a T-shaped fillet welded joint that exhibits high fatigue strength and a steel structure having the T-shaped fillet welded joint. [Means for solving the problem]

[0012] Aspect 1 of the present invention is A T-shaped welded joint formed by fillet welding a vertical plate to a flat plate, the steel plate used for the flat plate is: The chemical composition is C: 0.03~0.08% by mass, Si: 0.30~0.60% by mass, Mn: 1.00~2.00% by mass, P: more than 0 mass%, 0.020 mass% or less, S: More than 0% by mass, 0.002% by mass or less, Al: 0.01~0.05% by mass, Cu: 0.10~0.50% by mass, Ni: 0.10~0.50% by mass, Nb: more than 0 mass%, 0.030 mass% or less, Ti:0.005~0.025% by mass, N: 0.002~0.007% by mass, Ca: 0.0005 to 0.0030 mass%; and Si+Cu: 0.4% by mass or more; The balance is Fe and unavoidable impurities, and In the cross-sectional area of ​​the steel plate from the surface to 1 mm in the plate thickness direction, the number density of non-metallic inclusions exceeding 10 μm in length is 10 / mm 2 Below is a T-shaped fillet weld joint.

[0013] Aspect 2 of the present invention is 2. The T-shaped fillet welded joint according to claim 1, wherein the steel plate used for the flat plate has a yield strength of 500 MPa or less.

[0014] Aspect 3 of the present invention is The T-shaped fillet welded joint according to aspect 1 or 2, wherein the penetration rate of the weld is 50 to 90% of the plate thickness of the upright plate.

[0015] A fourth aspect of the present invention is A steel structure having the T-shaped fillet weld joint according to embodiment 1 or 2.

[0016] A fifth aspect of the present invention is A steel structure having the T-shaped fillet weld joint according to embodiment 3. Effect of the Invention

[0017] According to the present disclosure, it is possible to provide a T-fillet welded joint exhibiting high fatigue strength, and a steel structure having the T-fillet welded joint. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram for explaining how to determine the penetration rate. [Diagram 2] FIG. 2 shows a side view and a top view of the fatigue test specimen used in the fatigue test. [Diagram 3] FIG. 3 is a schematic side view of a fatigue test device used in the fatigue test. [Figure 4] FIG. 4 is a diagram explaining the positions of the strain gauges attached to the fatigue test specimen. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The present inventors have conducted extensive research to obtain a T-shaped welded joint (T-shaped fillet welded joint) formed by fillet welding a vertical plate to a flat plate, which exhibits high fatigue strength. As a result, they have found that such a welded joint can be obtained by controlling the chemical composition of the steel plate used for the flat plate and by controlling the morphology of nonmetallic inclusions in a cross-sectional region of the steel plate from the surface to 1 mm in the plate thickness direction (hereinafter sometimes referred to as the "surface layer region").

[0020] Furthermore, it has been found that a higher fatigue strength can be achieved by making the yield strength of the steel plate used for the flat plate preferably 500 MPa or less.

[0021] (Chemical composition of flat plate (steel plate) in T-shaped fillet welded joint) The chemical composition of the steel plate used for the flat plate is explained below.

[0022] C:0.03~0.08% by mass C is an important element mainly for ensuring the strength of the base material (i.e., steel plate). Therefore, the C content is set to 0.03 mass% or more. The C content is preferably 0.04 mass% or more. On the other hand, if the C content is excessive, not only will the strength be too high to obtain the desired tensile strength, but the hardenability will be excessive, the weld heat affected zone during low heat input welding will harden, and the weld crack resistance will decrease. Therefore, the C content is set to 0.08 mass% or less. It is preferably 0.07 mass% or less, and more preferably 0.06 mass% or less.

[0023] Si:0.30~0.60% by mass Silicon is an element that is necessary for ensuring the strength of the base material due to its large solid solution hardening effect, and is also an element that is effective in extending the fatigue crack initiation life (a life evaluated by the number of cycles from the start of stress loading until fatigue cracks occur) by suppressing the proliferation of dislocations. This action is also effective in the weld heat affected zone. In order to exert these effects, the Si content is set to 0.30 mass% or more. The Si content is preferably 0.35 mass% or more. However, if the Si content is excessive, there is a risk of deteriorating properties other than fatigue properties such as toughness. Therefore, the Si content is set to 0.60 mass% or less. The Si content is preferably 0.55 mass% or less.

[0024] Mn: 1.00~2.00% by mass Mn is an element that stabilizes austenite and lowers the transformation temperature. In addition, Mn is an element that is effective in ensuring impact properties due to the effect of refining crystal grain size by low-temperature transformation. Furthermore, Mn is also an element that is effective in improving hardenability and strength. In order to exert these effects, Mn is contained in an amount of 1.00 mass% or more. The Mn content is more preferably 1.20 mass% or more. However, if Mn is contained excessively, elongation properties, low-temperature toughness, and HAZ toughness deteriorate. Therefore, the upper limit of the Mn content is set to 2.00 mass%. The Mn content is preferably 1.80 mass% or less.

[0025] P: More than 0% by mass, 0.020% by mass or less P (phosphorus) is an element that is inevitably contained as an impurity during the manufacturing process, etc., and is an element that adversely affects the toughness and fatigue properties of the base material and HAZ. Therefore, the P content is set to 0.020 mass% or less. The less P, the better, with 0.015 mass% or less being preferred, and 0.010 mass% or less being most preferred. However, from the viewpoint of steelmaking capacity, it may usually be contained in an amount exceeding 0 mass%.

[0026] S: More than 0% by mass, 0.002% by mass or less Like P, S (sulfur) is an element that is inevitably contained as an impurity during the manufacturing process, and not only does it have a negative effect on the toughness of the base material and HAZ, but it also exists in the steel as non-metallic inclusions such as MnS and has a negative effect on fatigue properties. Therefore, it is necessary to limit the S content to a low level. The S content should be 0.002 mass% or less. The lower the S content, the better. However, from the perspective of steelmaking capacity, it can usually be contained in an amount of more than 0 mass%.

[0027] Al: 0.01~0.05% by mass Al is an element necessary for deoxidization, and is contained at 0.01 mass% or more. The Al content is preferably 0.015 mass% or more. On the other hand, if Al is contained excessively, coarse alumina-based inclusions are formed, which deteriorates the impact properties of the base material and HAZ. Therefore, the Al content is set to 0.05 mass% or less.

[0028] Cu:0.10~0.50% by mass Like Si, Cu is an effective element for extending the fatigue crack initiation life by suppressing the proliferation of dislocations, and is also an effective element for extending the fatigue crack initiation life in the weld heat affected zone. In order to effectively exert this effect, the Cu content is set to 0.10 mass% or more. The Cu content is preferably 0.15 mass% or more, more preferably 0.20 mass% or more. However, if the Cu content is excessive, not only will the hardenability become excessive, but cracks and the like will easily occur during hot working. Therefore, the Cu content is set to 0.50 mass% or less. The Cu content is preferably 0.45 mass% or less, more preferably 0.40 mass% or less.

[0029] Ni:0.10~0.50% by mass Ni has the effect of improving hardenability and refining the structure, and at the same time, has the effect of suppressing cracks during hot working that are easily caused by the addition of Cu. In order to exert such effects, the Ni content is set to 0.10 mass% or more. The Ni content is preferably 0.15 mass% or more, and more preferably 0.20 mass% or more. However, from the viewpoint of reducing raw material costs, it is better to have less Ni. Therefore, the Ni content is set to 0.50 mass% or less. The Ni content is preferably 0.45 mass% or less, and more preferably 0.40 mass% or less.

[0030] Nb: More than 0 mass%, 0.030 mass% or less Nb is an element effective for improving hardenability and refining the structure. In order to effectively exert these effects, the Nb content is set to more than 0 mass%. The Nb content is preferably 0.010 mass% or more, more preferably 0.015 mass% or more. However, if the Nb content is excessive, the hardenability becomes excessive, which adversely affects the toughness of the HAZ. Therefore, the Nb content is set to 0.030 mass% or less. The Nb content is preferably 0.025 mass% or less, more preferably 0.020 mass% or less.

[0031] Ti:0.005~0.025% by mass Ti has the effect of improving hardenability, and by forming TiN, it is an element that is useful for making the structure fine during slab heating or in the heat-affected zone during welding, and for preventing a decrease in toughness. Therefore, the Ti content is set to 0.005% by mass or more. The Ti content is preferably 0.008% by mass or more. However, if the Ti content is excessive, coarse TiN may be generated, which may deteriorate the properties such as the toughness of the base material and HAZ. Therefore, the Ti content is set to 0.025% by mass or less. The Ti content is preferably 0.020% by mass or less.

[0032] N:0.002~0.007% by mass N has the effect of forming nitrides with Al, Ti, etc. to refine the structure and improve the toughness of the base material and the welded heat affected zone. To realize this effect, the N content is set to 0.002 mass% or more. The N content is preferably 0.003 mass% or more. However, if the N content is excessive, the amount of nitrides precipitated in the base material increases, the base material toughness is significantly reduced, and further, coarse carbonitrides are formed in the welded heat affected zone, reducing the toughness. Therefore, the N content is set to 0.007 mass% or less. The N content is preferably 0.006 mass% or less.

[0033] Ca:0.0005~0.0030% by mass Ca is an element useful for controlling the shape of nonmetallic inclusions, such as suppressing the elongation of sulfide-based nonmetallic inclusions such as MnS. To achieve this effect, the Ca content is set to 0.0005% by mass or more. The Ca content is preferably 0.0010% by mass or more. However, an excessive Ca content leads to a decrease in cleanliness and a deterioration in toughness. Therefore, the Ca content is set to 0.0030% by mass or less. The Ca content is preferably 0.0025% by mass or less.

[0034] Si+Cu: 0.4% by mass or more Si and Cu have the effect of extending the fatigue crack initiation life by suppressing the proliferation of dislocations. This effect is also effectively exerted in the weld heat affected zone. That is, Si and Cu have the effect of suppressing the initiation of cracks in the weld heat affected zone and improving the fatigue properties of the welded joint. In order to effectively exert this effect, the total content of Si and Cu (Si+Cu) is set to 0.40 mass% or more. Si+Cu is preferably 0.45 mass% or more, and more preferably 0.50 mass% or more. The preferred upper limit of Si+Cu is the sum of the preferred upper limits of the contents of each of the elements Si and Cu.

[0035] Remainder: Fe and unavoidable impurities In a preferred embodiment, the balance is Fe and inevitable impurities. As inevitable impurities, the inclusion of trace elements (e.g., As, Sb, Sn, etc.) brought in due to the conditions of raw materials, materials, manufacturing facilities, etc. is permitted. Note that, for example, there are elements such as P and S, which are usually preferable to have a smaller content and are therefore inevitable impurities, but whose composition ranges are separately specified as described above. For this reason, in this specification, when referring to the "unavoidable impurities" constituting the balance, it is a concept excluding elements whose composition ranges are separately specified.

[0036] (Non-metallic inclusions in the surface layer of flat plates (steel plates) in T-shaped fillet welded joints) [Number density of nonmetallic inclusions with a length exceeding 10 μm in the cross-sectional area of ​​the steel plate from the surface to 1 mm in the plate thickness direction: 10 pieces / mm 2 below] When a vertical plate is fillet welded to a flat steel plate to produce a T-shaped fillet welded joint, fatigue cracks are generated from the weld toe or root of the flat plate on the surface of the flat steel plate. One of the causes of fatigue crack generation from the surface of the flat steel plate is the presence of nonmetallic inclusions in the surface layer of the flat steel plate. It is important to reduce the size and number of nonmetallic inclusions in the surface layer of the flat steel plate, and it is considered that reducing nonmetallic inclusions such as MnS that are elongated in the rolling direction by hot rolling, which are likely to cause the above-mentioned fatigue crack generation, is important for suppressing the above-mentioned fatigue crack generation.

[0037] In this embodiment, the number density of nonmetallic inclusions having a length exceeding 10 μm in a cross-sectional region from the surface to 1 mm in the sheet thickness direction of the steel sheet is set to 10 pieces / mm 2 The smaller the number density, the better. 2 It is preferable that the number density is equal to or less than 1000. The type of nonmetallic inclusions covered by the present disclosure is not limited, but may include, for example, the above-mentioned MnS, alumina, etc. The "length" of the nonmetallic inclusion refers to the circumscribed circle diameter of the nonmetallic inclusion. Note that, for example, MnS is elongated in the rolling direction during rolling, so the length direction of the nonmetallic inclusion may coincide with the rolling direction. Therefore, when the rolling direction of the steel plate is clear, the "length" of the nonmetallic inclusion can be said to be the linear length in the rolling direction of the steel plate. The number density is obtained by the method described in the examples described later. In addition, after taking a flat plate in the joint, it may be obtained by the method described in the examples.

[0038] (Yield strength of flat plate (steel plate) in T-shaped fillet welded joint) During welding of the vertical plate, the in-plane rotational deformation is restrained by the flat steel plate, so that the orthogonal residual stress changes depending on the yield strength of the flat steel plate. The smaller the orthogonal residual stress, the more effective it is in suppressing the occurrence of fatigue cracks. The occurrence of fatigue cracks is further suppressed by the steel plate used for the flat plate of the present disclosure satisfying the above-mentioned chemical composition and morphology of nonmetallic inclusions, and further having a yield strength of 500 MPa or less. The yield strength of the steel plate used for the flat plate is preferably 480 MPa or less, more preferably 450 MPa or less. However, from the viewpoint of ensuring the strength of the base material, the lower limit of the yield strength of the steel plate used for the flat plate is about 350 MPa.

[0039] (Method of manufacturing flat plates (steel plates) for T-shaped fillet welded joints) In this embodiment, a steel slab having a predetermined chemical composition is obtained by melting and casting (e.g., continuous casting). Then, before hot rolling, a depth of about 1 to 2 mm on each side of the front and back surfaces of the steel slab is removed by laser cutting or grinding to obtain the steel sheet according to this embodiment.

[0040] The above-mentioned cutting or grinding removes nonmetallic inclusions such as MnS present in the surface layer of the steel billet obtained by casting, which tend to expand in the rolling direction by hot rolling. In addition, unavoidable steel billet cracks may occur on the extreme surface of the steel billet due to nitrides such as Nb and Ti (microalloy elements). Furthermore, foreign inclusions may also be present in the surface layer of the steel billet due to the inclusion of mold powder used during continuous casting. When foreign inclusions open up and become cracks due to rolling, they are generally ground and removed after rolling, but when the foreign inclusions are small and do not open up due to rolling and remain in the surface layer of the steel plate, they become a cause of fatigue cracks. Therefore, it is preferable to remove these steel billet cracks and foreign inclusions by cutting or grinding.

[0041] The heating temperature and hot rolling conditions during hot rolling are not particularly limited, and general manufacturing conditions for thick steel plates can be set so as to obtain the desired strength (generally tensile strength of 400 MPa to 600 MPa), toughness, and size. The plate thickness can be, for example, 6 to 100 mm, and the plate thickness can also be controlled by appropriately setting the hot rolling conditions. As a method for setting the yield strength of the steel plate to a preferred range of 500 MPa or less, for example, normalizing after hot rolling can be performed, as shown in the examples described later.

[0042] (T-shaped fillet weld joint) The T-shaped fillet welded joint of the present disclosure is a T-shaped welded joint formed by fillet welding a vertical plate to a flat plate, and the steel plate used for the flat plate satisfies the above-mentioned chemical composition and non-metallic inclusion form. On the other hand, the steel plate used for the vertical plate is not limited and is appropriately selected depending on the application of the T-shaped welded joint, etc. As the steel plate used for the vertical plate, a steel plate of the same strength class as the flat plate can generally be used.

[0043] The T-shaped fillet welded joint of the present disclosure is preferably a T-shaped one-sided fillet welded joint, and the penetration rate of the weld is 50 to 90% of the plate thickness of the vertical plate. As shown in FIG. 1, the penetration rate is calculated by (t / T)×100(%), where T (mm) is the plate thickness of the vertical plate 5 and t (mm) is the penetration depth in the plate thickness of the vertical plate 5. The penetration rate needs to be 50% or more of the plate thickness of the vertical plate in order to concentrate stress on the weld root side, and is more preferably 55% or more. If the penetration rate exceeds 90% of the plate thickness of the vertical plate, the stability of the weld may lead to localized weld loss, increasing the risk of not obtaining a sound welded joint. The upper limit of the penetration rate is preferably 85% or less.

[0044] The T-shaped fillet welded joint of the present invention, preferably welded by one-sided fillet welding so that the penetration rate is 50 to 90% of the plate thickness of the vertical plate, can more effectively suppress the occurrence of fatigue cracks, particularly when the plate is subjected to repeated cantilever bending amplitude at the weld root side. When the plate is subjected to repeated cantilever bending amplitude, the weld heat-affected zone of the plate at the weld root side becomes a stress concentration area, but if the plate is a steel plate with the above-mentioned chemical composition and nonmetallic inclusion form controlled, fatigue cracks are less likely to occur and high fatigue properties can be exhibited.

[0045] A T-shaped fillet welded joint can be a joint in which a vertical plate is fillet welded to a flat plate in a substantially T-shape. The vertical plate is not limited to an I-rib (flat plate) welded substantially perpendicularly to the flat plate, but includes a U-rib welded at an angle of approximately 77° to the flat plate. The T-shaped fillet welded joint of the present disclosure can exhibit excellent fatigue properties when a U-rib is used as the vertical plate. In the examples described below, a fatigue test specimen is prepared that simulates a T-shaped fillet welded joint using a U-rib as the vertical plate, and the vertical plate in the fatigue test specimen simulates a part of the U-rib.

[0046] (Manufacturing method for T-shaped fillet welded joints) In the present disclosure, the composition of the welding material, the welding method, and the welding conditions can be adjusted according to the steel plate used for the welded joint to form a weld metal of an appropriate composition. As a welding method for producing the T-shaped fillet welded joint of the present disclosure, various welding methods such as shielded arc welding, gas metal arc welding, submerged arc welding, and FCW (flux cored wire) can be suitably applied.

[0047] The present disclosure also includes a steel structure having the T-shaped fillet welded joint. Examples of steel structures include ships, marine structures, penstocks, bridges, storage tanks, construction machines, and the like. The steel structure of the present disclosure includes the above-mentioned T-shaped fillet welded joint of the present disclosure as a welded joint, and therefore can exhibit high fatigue properties. In particular, it is suitable as a T-shaped fillet welded joint of a bridge. As a deck plate (flat plate) that is effective in suppressing fatigue cracking from the root side of the U-rib weld of a bridge steel deck plate, a steel plate (base material) with a predetermined chemical composition and a controlled form of nonmetallic inclusions is used, so that compared to the case where a general-purpose steel plate is used for the flat plate, excellent fatigue properties are exhibited, and in particular fatigue cracking is sufficiently suppressed. EXAMPLES

[0048] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples, and can be practiced with appropriate modifications within the scope of the above and below-mentioned aims, and all such modifications are included in the technical scope of the present invention.

[0049] 1. Sample Preparation Steel slabs with the chemical composition shown in Table 1 were obtained by continuous casting. For Steel 1-1 and Steel 1-2, the front and back surfaces of the steel slabs were removed by about 1 to 2 mm on each side by scouring and grinding, and then the steel slabs were heated and hot rolled to obtain steel plates (thick steel plates) with a thickness of 16 mm. On the other hand, for Steel 2 and Steel 3, the steel slabs obtained by continuous casting were heated and hot rolled to obtain steel plates (thick steel plates) with a thickness of 16 mm. Here, controlled rolling was performed in the hot rolling of Steel 1-1, and the target finishing temperature for the hot rolling was 740°C. Controlled rolling was not performed in the hot rolling of Steel 1-2, and normalizing was performed with the furnace temperature set to 930°C after hot rolling. For Steel 2 and Steel 3, controlled rolling was performed with a target finishing temperature of 750°C, and accelerated cooling was performed immediately after hot rolling with a cooling rate of 5°C / s and a cooling stop temperature of 580°C. In Table 1, underlined values ​​indicate values ​​outside the range of the present disclosure. Steel 1-1 and Steel 1-2 are examples of the present invention, and Steel 2 and Steel 3 are comparative examples of general-purpose thick steel plates with tensile strength of 490 MPa class.

[0050] [Table 1]

[0051] 2. Observation of non-metallic inclusions Steel plates (thick steel plates) of Steel 1-1 and Steel 2 were used, and samples for observation were prepared by cutting the steel plates at a position near w / 4 in the plate width direction so that the cross section of each steel plate in the plate thickness direction and in the direction parallel to the rolling direction could be observed. The preparation of test pieces for observation was performed according to JIS G0555, a microscope test method for nonmetallic inclusions in steel. Nonmetallic inclusions, which are thought to elongate in the rolling direction by hot rolling and have a negative effect on fatigue performance, are easily observed in the above cross sections, and therefore it can be said that the control of the shape of nonmetallic inclusions in the cross sections is a strict control condition.

[0052] More specifically, nonmetallic inclusions were observed at 10 random locations in a region from the steel plate surface to a depth of 1 mm. Each location was observed at a magnification of 200x (size of one field of view: 470 μm×350 μm). In the observation, the number of nonmetallic inclusions whose circumscribed circle diameter exceeded 10 μm in length was counted. Note that in this example, the rolling direction of the steel plate was clear, and the "length" of the nonmetallic inclusions also corresponds to the linear length in the rolling direction of the steel plate. The number of nonmetallic inclusions whose length exceeded 10 μm in a total of 10 fields of view was tallied, and the number of nonmetallic inclusions whose length exceeded 10 μm in length was counted. 2 The number of nonmetallic inclusions per unit area (number density of nonmetallic inclusions) was calculated. The results are shown in Table 2.

[0053] [Table 2]

[0054] Although Steel 1-1 and Steel 1-2 were produced in separate charges, they were produced with the same chemical composition, and the difference in the chemical composition of the obtained steel billets was within the range of unavoidable component variation in production, so the steel sheets obtained from these steel billets were treated as steel sheets with the same chemical composition. In addition, since the difference in production conditions between Steel 1-1 and Steel 1-2 described above is considered not to affect the number density of the nonmetallic inclusions specified in this embodiment, Steel 1-1 and Steel 1-2 are considered to have the same number density of the nonmetallic inclusions. Therefore, the observation of the nonmetallic inclusions was carried out using Steel 1-1 out of Steel 1-1 and Steel 1-2.

[0055] Steels 2 and 3 were also manufactured using separate charges, but because they were manufactured with the same chemical composition in mind, the differences in the chemical compositions of the resulting steel billets were within the range of unavoidable component variations during manufacturing. Furthermore, because Steels 2 and 3 were manufactured under the same conditions, the steel plates obtained from these billets were treated as equivalent products. Therefore, the observation of nonmetallic inclusions was performed using Steel 2 of Steels 2 and 3, and the evaluation of fatigue properties described below was performed using Steel 3 of Steels 2 and 3.

[0056] 3. Tensile test of steel plate A tensile test was carried out according to JIS Z2241 to determine the yield strength of the steel plate. The test piece shape was No. 1A, and the test piece was taken so that the width direction of the steel plate (direction perpendicular to the rolling direction) was the longitudinal direction of the test piece. The obtained yield strength is shown in Table 3.

[0057] [Table 3]

[0058] 4. Evaluation of fatigue properties Fatigue properties were evaluated using steel plates (plate thickness 16 mm) of Steel 1-1, Steel 1-2, and Steel 3. In detail, fatigue test specimens (small welded joint test models) simulating the welded part of the steel deck plate and U-rib, as shown in Figure 2, were fabricated using steel plates of Steel 1-1, Steel 1-2, and Steel 3 as deck plates (flat plates). The lower part of Figure 2 is a top view of fatigue test specimen 1, and the upper part of Figure 2 is a side view of fatigue test specimen 1. In the side view in the upper part of Figure 2 and Figure 3 shown later, the bolt 9 shown in the top view in the lower part of Figure 2 is omitted.

[0059] As shown in Figure 2, a steel plate with a thickness of 6 mm was assembled as a vertical plate 5 at an angle to the deck plate 3 to simulate a U-rib, and one-sided fillet welding was performed so that the penetration rate was about 75% of the plate thickness of the vertical plate 5. The conditions for one-sided fillet welding were CO 2The welding current was 330A, the arc voltage was 34V, the welding speed was 45cm / min, and the heat input was 15.0kJ / cm. Next, a 12mm thick steel plate was assembled as a transverse rib simulating steel plate 7 so that it intersected with the U-rib to simulate the transverse rib, and double-sided fillet welding was performed to obtain a welded body. Next, as shown in the lower part of Fig. 2, the circumference of the welded body was cut so that the transverse rib was located in the center of the width with a width of 200mm, and a fatigue test specimen 1 was obtained. In Fig. 2, reference numeral 11 indicates a one-sided fillet welded portion, and reference numeral 13 indicates a two-sided fillet welded portion. The penetration ratios of the one-sided fillet welds (ratio of the penetration depth to the plate thickness of the vertical plate) of Steel 1-1 and Steel 3 were measured at two points of the fatigue test specimens and the average value was calculated. As a result, the penetration ratio of Steel 1-1 was 72.3%, and the penetration ratio of Steel 3 was 65.7%. The penetration ratio of Steel 1-2 was not measured. However, the penetration rate is governed by the welding current, arc voltage, welding speed, and welding heat input, and the one-sided fillet welding of Steel 1-2 was performed under the same welding conditions as Steel 1-1 and Steel 3. Therefore, it can be said that the fatigue test specimen obtained with Steel 1-2 also had a penetration rate equivalent to that of the fatigue test specimens of Steel 1-1 and Steel 3.

[0060] A fatigue test was performed using the obtained fatigue test specimen 1 as follows. FIG. 3 is a schematic side view of a fatigue test device 20. As shown in FIG. 3, the fatigue test was performed using a portable actuator 21 by a cantilever bending method, the stress ratio was set to 0.1 by load control, and the cutoff limit was set to 10 million times. As shown in FIG. 3, the distance from the vertical plate 5 to the load application position was set to 200 mm. A plurality of strain gauges were attached to the deck plate 3 of the fatigue test specimen 1 in FIG. 2 in advance, and the fatigue test was performed. FIG. 4 is a top view of the fatigue test specimen 1 showing the strain gauge attachment positions. The strain gauges were attached to about half of the area from the vertical plate 5 of the deck plate 3 to the portable actuator 21. Of these, the strain gauges ST02, ST03, ST05, ST06, ST07, and ST08 located 50 mm away from the horizontal rib simulation steel plate 7 correspond to general parts that are not affected by the intersection with the horizontal rib simulation steel plate 7.

[0061] In the fatigue test, the value of the attached strain gauge was measured to investigate the strain variation range during the fatigue test. A fatigue crack was defined as occurring when the strain variation range measured by the strain gauge dropped by 5% from the initial value, and the number of cycles at which the strain dropped by 5% from the initial value was defined as the fatigue crack initiation life N5% and evaluated. The strain gradient according to the bending moment was then confirmed in the general part not affected by the cross rib intersection, and the nominal stress range Δσ was defined as the value extrapolated to the simulated U-rib position. The cutoff limit strength σw was taken as the average value of the minimum Δσ at which a fatigue crack occurred by the cutoff limit, and the maximum Δσ at which it did not occur.

[0062] These results are shown in Table 4. As described above, in this example, the steel plates obtained from the billets of Steel 2 and Steel 3 were treated as equivalent products, and the evaluation results of the fatigue properties of Steel 3 can also be the evaluation results of the fatigue properties of Steel 2. In this example, a case in which the cut-off limit strength σw was 10% or more higher than the cut-off limit strength σw of Steels 2 and 3, which are general-purpose thick steel plates with a tensile strength of 490 MPa, was evaluated as showing high fatigue strength and excellent fatigue properties.

[0063] [Table 4]

[0064] The results in Tables 1 to 4 reveal the following: Because the welded joints of the present disclosure satisfy the prescribed chemical composition and morphology of nonmetallic inclusions, the cutoff limit strength σw is improved by 22% or more compared to Steel 2 and Steel 3, and it has been found that they exhibit better fatigue properties than conventional ones.

[0065] Furthermore, as described above, when welding vertical plates, the in-plane rotational deformation is restrained by the flat steel plate, so the orthogonal residual stress changes depending on the yield strength of the flat steel plate. The smaller the orthogonal residual stress, the more effective it is in suppressing fatigue crack generation, so the lower the yield strength of the flat steel plate is, the better. Steels 2 and 3, which have lower yield strengths than Steel 1-1, do not satisfy the specified chemical composition and the morphology of nonmetallic inclusions, so their fatigue properties are not improved. On the other hand, as in Steel 1-2, it was found that the occurrence of fatigue cracks is further suppressed by using a steel plate used for a flat plate that satisfies the specified chemical composition and the morphology of nonmetallic inclusions, and further, by setting the yield strength of the flat steel plate to 500 MPa or less.

[0066] The disclosure of this specification may include the following aspects. (Aspect 1) A T-shaped welded joint formed by fillet welding a vertical plate to a flat plate, the steel plate used for the flat plate is: The chemical composition is C: 0.03~0.08% by mass, Si: 0.30~0.60% by mass, Mn: 1.00~2.00% by mass, P: more than 0 mass%, 0.020 mass% or less, S: More than 0% by mass, 0.002% by mass or less, Al: 0.01~0.05% by mass, Cu: 0.10~0.50% by mass, Ni: 0.10~0.50% by mass, Nb: more than 0 mass%, 0.030 mass% or less, Ti:0.005~0.025% by mass, N: 0.002~0.007% by mass, Ca: 0.0005 to 0.0030 mass%; and Si+Cu: 0.4% by mass or more; The balance is Fe and unavoidable impurities, and In the cross-sectional area of ​​the steel plate from the surface to 1 mm in the plate thickness direction, the number density of non-metallic inclusions exceeding 10 μm in length is 10 / mm 2 Below is a T-shaped fillet weld joint. (Aspect 2) 2. The T-shaped fillet welded joint according to aspect 1, wherein the T-shaped fillet welded joint has a penetration rate of 50 to 90% of the plate thickness of the upright plate. (Aspect 3) 3. A steel structure having the T-shaped fillet weld joint according to claim 1 or 2. [Explanation of symbols]

[0067] 1. Fatigue test specimen 3 Deck plate (flat plate) 5. Vertical plate (U-rib imitation steel plate) 7. Transverse rib simulated steel plate 9 Volts 11 One-sided fillet weld 13 Double-sided fillet weld 20 Fatigue Testing Equipment 21 Portable actuator 23 Support stand

Claims

1. A T-shaped welded joint formed by fillet welding a vertical plate to a flat plate, the steel plate used for the flat plate is: The chemical composition is C: 0.03 to 0.08% by mass, Si: 0.30 to 0.60% by mass, Mn: 1.00 to 2.00% by mass, P: more than 0 mass%, 0.020 mass% or less, S: more than 0% by mass, 0.002% by mass or less, Al: 0.01 to 0.05% by mass, Cu: 0.10 to 0.50% by mass, Ni: 0.10 to 0.50% by mass, Nb: more than 0 mass%, 0.030 mass% or less, Ti: 0.005 to 0.025% by mass, N: 0.002 to 0.007% by mass, Ca: 0.0005 to 0.0030% by mass, and Si+Cu: 0.4% by mass or more; The balance is Fe and unavoidable impurities, and In a cross-sectional region of the steel plate from the surface to 1 mm in the plate thickness direction, the number density of non-metallic inclusions having a length exceeding 10 μm is 10 pieces / mm ２ Below is a T-shaped fillet weld joint.

2. 2. The T-shaped fillet welded joint according to claim 1, wherein the yield strength of the steel plate used for the flat plate is 500 MPa or less.

3. The T-shaped fillet welded joint according to claim 1 or 2, wherein the penetration rate of the weld is 50 to 90% of the plate thickness of the vertical plate.

4. A steel structure having the T-shaped fillet welded joint according to claim 1 or 2.

5. A steel structure having the T-shaped fillet weld joint according to claim 3.

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