Submerged arc welding joint

Abstract

The present invention provides a welded joint having a weld metal obtained by submerged arc welding at a linear energy of 300 kJ / cm or more. A submerged arc welded joint in which the weld metal has a specific composition, a specific yield strength, a specific tensile strength, a specific absorbed energy in a V-notch Charpy impact test at a test temperature of 0°C, and Ceq represented by the following formula (1) is in the range of 0.65 to 1.00, and α represented by the following formula (2) is in the range of 6.0 or less. Ceq = [C] + 0.17[Mn] + 0.04[Si] + 0.025[Ni] + 0.2[Cr] + 0.25[Mo] … (1) α = 30[C] + 0.7[Mn] + [Ni] - ([Si] + 0.5[Cr] + 1.5[Mo]) … (2) Here, [element] in formula (1), formula (2) is the content (mass %) of the element in the above-described weld metal.

Description

Technical Field

[0001] This invention relates to submerged arc welding joints, and more particularly to submerged arc welding joints having weld metal obtained by submerged arc welding of 780MPa grade high-tensile steel used in building structures under conditions of welding line energy of 300kJ / cm or more. Background Technology

[0002] In recent years, due to the increasing size and span of building structures, there has been a trend towards higher strength steel plates used in steel frames. Previously, the outer plates of box columns were primarily made of steel with strength grades up to 590 MPa; recently, 780 MPa grade steel has begun to be used. The joining of the outer plates of box columns generally employs submerged arc welding, and the strength of the weld metal is required to be equivalent to that of the base material.

[0003] As a welding material for submerged arc welding that can ensure a weld metal strength of 780 MPa or higher, for example, a solid welding wire for submerged arc welding is disclosed in Patent Document 1. It describes how, by specifying the C, Si, Mn, Ni, Cr, Mo, P, and S components in the total mass of the welding wire, and adjusting ([Mn]+[Ni]) / ([Cr]+[Mo]) to 1.4 to 4.0, the low-temperature toughness and resistance to hydrogen embrittlement of the weld metal can be significantly improved.

[0004] Furthermore, Patent Document 2 discloses a submerged arc welding method for 780MPa grade high-tensile steel. It describes how, by optimizing the amount of alloying elements and the particle size of the sintered flux, a good weld shape without slag contamination can be obtained. Furthermore, by limiting the chemical composition of the combined solid welding wire, a high-strength weld metal with a tensile strength of over 780MPa and good low-temperature toughness can be obtained.

[0005] Furthermore, Patent Document 3 discloses a welding wire for double-sided single-layer submerged arc welding. It describes how, in double-sided single-layer submerged arc welding with significant base metal dilution, the strength and toughness of the weld metal are ensured by limiting the composition of the welding wire; specifically, the tensile strength of the welding wire is limited to 1200 N / mm². 2 The following ensures the proper delivery of the welding wire.

[0006] Furthermore, Patent Document 4 discloses a high-strength welded steel pipe with excellent resistance to low-temperature cracking. The welded steel pipe, manufactured by submerged arc welding with one layer on each of the inner and outer surfaces, has a tensile strength of 800 MPa or higher for both the base material and the weld metal. It describes how the CS value, calculated from the contents of Mo, Ni, Mn, and C, determines the weld metal with excellent resistance to low-temperature cracking.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2015-110241

[0010] Patent Document 2: Japanese Patent Application Publication No. 2015-120175

[0011] Patent Document 3: Japanese Patent Application Publication No. 2004-337863

[0012] Patent Document 4: Japanese Patent Application Publication No. 2008-240096 Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] However, the welding heat inputs used in Patent Documents 1 through 4 are all below 50 kJ / cm. In the fabrication of box-shaped columns using steel of grade 590 MPa or lower for the outer plate, from a construction efficiency perspective, submerged arc welding with a welding heat input (hereinafter also referred to as "heat input") exceeding 300 kJ / cm is typically used. However, when using the welding materials disclosed in Patent Documents 1 through 4 with a heat input exceeding 300 kJ / cm, problems arise such as insufficient strength or toughness of the weld metal, or the formation of high-temperature cracks. It should be noted that the required weld metal strength here refers to a room-temperature yield strength (0.2% yield strength) of 630 MPa or higher and a tensile strength of 780 MPa or higher for weld metal manufactured according to JIS Z 3111. Furthermore, the required weld metal toughness refers to an absorbed energy vE0 of 47 J or higher in a V-notch Charpy impact test at a test temperature of 0°C for weld metal of welded joints manufactured according to JIS Z 3128.

[0015] The purpose of this invention is to solve the above-mentioned problems and provide a submerged arc welded joint having weld metal obtained by submerged arc welding with a line energy of 300 kJ / cm or more.

[0016] Methods for solving problems

[0017] To solve the above problems, the inventors conducted repeated and in-depth research and found that, in order to ensure strength and toughness, it is effective to contain more than 2.5% by mass of Ni, which can improve strength without reducing toughness, and to adjust Ceq, represented by equation (1) described later, to 0.65 to 1.00. In addition, it was found that, in order to prevent high-temperature cracking, α, represented by equation (2) described later, can be adjusted to 6.0 or less.

[0018] This invention was completed based on further research into the above-mentioned insights.

[0019] The main points of this invention are as follows.

[0020] [1] A submerged arc welding head, comprising a weld metal obtained by combining welding wire and flux and performing submerged arc welding with a welding heat input of 300 kJ / cm or more, wherein the chemical composition of the weld metal, by mass%, contains C: 0.05-0.15%, Si: 0.2-0.9%, Mn: 0.5-1.3%, P: less than 0.015%, S: less than 0.015%, Cr: 0.10-0.45%, Mo: 0.5-2.0%, Ni: 2.5%. The content of the weld metal is 6.0%, O is 0.040% or less, N is 0.012% or less, and the balance is composed of Fe and unavoidable impurities. Furthermore, the Ceq represented by the following formula (1) is in the range of 0.65 to 1.00, the α represented by the following formula (2) is in the range of 6.0 or less, the yield strength of the weld metal in the tensile test is 630 MPa or more, and the tensile strength is 780 MPa or more. Moreover, the absorbed energy vE0 of the weld metal in the V-notch Charpy impact test at the test temperature of 0°C is 47 J or more.

[0021] Ceq=[C]+0.17[Mn]+0.04[Si]+0.025[Ni]+0.2[Cr]+0.25[Mo]…(1)

[0022] α=30[C]+0.7[Mn]+[Ni]-([Si]+0.5[Cr]+1.5[Mo])…(2)

[0023] Here, the [element] in Equations (1) and (2) refers to the content (mass%) of that element in the aforementioned weld metal.

[0024] [2] According to the submerged arc welding head described in [1], in addition to the chemical composition of the above-mentioned welding metal, it also contains, by mass %, one or more of the following: Cu: less than 0.8%, Al: less than 0.20% and Ti: less than 0.20%.

[0025] [3] According to [1] or [2], the submerged arc welding joint further contains, in mass %, one or more of the following: Nb: less than 0.10%, V: less than 0.10%, Ca: less than 0.010%, B: less than 0.010%, and REM: less than 0.020%.

[0026] Invention Effects

[0027] According to the present invention, when using submerged arc welding with a heat input of 300 kJ / cm or more, the weld metal of the weld joint can ensure a yield strength (0.2% yield strength) of 630 MPa or more, a tensile strength of 780 MPa or more, and a toughness of 47 J or more for the V-notch Charpy impact absorption energy at 0°C, and can prevent high-temperature cracking, thus playing a significant role in industry. Detailed Implementation

[0028] This invention relates to a submerged arc welding joint, which produces weld metal obtained by submerged arc welding with a high heat input of 300 kJ / cm or more. The weld joint of this invention is a submerged arc welding joint as described below, wherein the chemical composition of the weld metal, by mass%, contains: C: 0.05–0.15%, Si: 0.2–0.9%, Mn: 0.5–1.3%, P: less than 0.015%, S: less than 0.015%, Cr: 0.10–0.45%, Mo: 0.5–2.0%, Ni: 2.5–6.0%, O: less than 0.040%, and N: less than 0.012%. The balance consists of Fe and unavoidable impurities, and the Ceq represented by the following formula (1) is in the range of 0.65 to 1.00, the α represented by the following formula (2) is in the range of 6.0 or less, the yield strength (0.2% yield strength) of the weld metal in the tensile test is 630 MPa or more, and the tensile strength is 780 MPa or more, and the absorbed energy vE0 of the above weld metal in the V-notch Charpy impact test at the test temperature of 0°C is 47 J or more.

[0029] Ceq=[C]+0.17[Mn]+0.04[Si]+0.025[Ni]+0.2[Cr]+0.25[Mo]…(1)

[0030] α=30[C]+0.7[Mn]+[Ni]-([Si]+0.5[Cr]+1.5[Mo])…(2)

[0031] Here, the [element] in equations (1) and (2) refers to the content of that element in the aforementioned welding metal.

[0032] Submerged arc welding

[0033] Submerged arc welding (SAW) is a welding method in which an electrode wire is continuously supplied to a powdered flux pre-dispersed on a base material (in this invention, for example, high-strength steel), and an electric arc is generated between the tip of the electrode wire and the base material to perform continuous welding. This submerged arc welding has the advantage of increasing the deposition rate of the welding wire by applying a large current, thereby enabling highly efficient welding.

[0034] [Chemical composition of welding metal]

[0035] Next, the rationale for specifying the chemical composition of the welding metal will be explained. It should be noted that, in the following, "%" in "chemical composition" refers to "mass %".

[0036] [C: 0.05~0.15%]

[0037] Carbon (C) is an element that significantly increases the strength of weld metal. When the C content in the weld metal is less than 0.05%, the strength is insufficient. Therefore, the C content is set to 0.05% or more. Preferably, the C content is set to 0.06% or more. On the other hand, when the C content exceeds 0.15%, the strength becomes excessively high, and the toughness deteriorates. Therefore, the C content is set to 0.15% or less. Preferably, the C content is 0.14% or less. More preferably, the C content is 0.12% or less.

[0038] [Si: 0.2-0.9%]

[0039] Si acts as a deoxidizing element in weld metal, helping to improve its toughness by reducing the amount of dissolved oxygen (O). Additionally, it also increases strength by lowering the phase transformation temperature. Therefore, the Si content is set to 0.2% or more. Preferably, the Si content is 0.3% or more. More preferably, it is 0.4% or more. On the other hand, when the Si content exceeds 0.9%, hardenability is excessive, and toughness decreases. Therefore, the Si content is set to 0.9% or less. Preferably, the Si content is 0.8% or less. More preferably, it is 0.7% or less. Even more preferably, it is 0.6% or less.

[0040] [Mn: 0.5-1.3%]

[0041] Mn is an element that improves hardenability, and to ensure strength, it needs to contain 0.5% or more. Therefore, the Mn content is set to 0.5% or more. The Mn content is preferably 0.6% or more. The Mn content is more preferably 0.7% or more. On the other hand, Mn is an element that easily segregates during solidification; when it exceeds 1.3%, high-temperature cracks are induced due to solidification segregation. Therefore, the Mn content is set to 1.3% or less. The Mn content is preferably 1.2% or less. The Mn content is more preferably 1.1% or less. The Mn content is even more preferably 1.0% or less.

[0042] [P: below 0.015%]

[0043] Phosphorus (P) is an element that segregates at grain boundaries and induces high-temperature cracks. In this invention, it is preferable to reduce it as much as possible, but it is permissible if it is below 0.015%. Therefore, the P content is set to 0.015% or less. The P content is preferably 0.012% or less. It should be noted that excessive reduction leads to a significant increase in refining costs. Therefore, the P content is preferably adjusted to 0.003% or more. The P content is more preferably 0.004% or more.

[0044] [S: less than 0.015%]

[0045] Sulfur (S) is an element that segregates at grain boundaries and induces high-temperature cracks. In this invention, it is preferable to reduce it as much as possible, but it is permissible if it is below 0.015%. Therefore, the S content is set to 0.015% or less. The S content is preferably 0.010% or less. The S content is more preferably 0.008% or less. The S content is even more preferably 0.007% or less. It should be noted that excessive reduction leads to a significant increase in refining costs. Therefore, the S content is preferably adjusted to 0.002% or more. The S content is more preferably 0.003% or more. The S content is even more preferably 0.004% or more.

[0046] [Cr: 0.10~0.45%]

[0047] Cr is an effective element for improving strength and needs to be present at a content of 0.10% or more. Therefore, the Cr content is set to 0.10% or more. The Cr content is preferably 0.15% or more. The Cr content is more preferably 0.18% or more. The Cr content is further preferably 0.22% or more. The Cr content is most preferably 0.30% or more. On the other hand, when the content exceeds 0.45%, carbide precipitation occurs, becoming the starting point for fracture and deteriorating the toughness of the weld metal. Therefore, the Cr content is set to 0.45% or less. The Cr content is preferably 0.40% or less. The Cr content is more preferably 0.38% or less. The Cr content is further preferably 0.36% or less.

[0048] [Mo: 0.5–2.0%]

[0049] Mo is an element effective in improving strength and, consequently, promotes the formation of δ-ferrite. Therefore, it is necessary to contain 0.5% or more. Thus, the Mo content is set to 0.5% or more. The Mo content is preferably 0.8% or more. The Mo content is more preferably 0.9% or more. The Mo content is even more preferably 1.0% or more. On the other hand, when the content exceeds 2.0%, carbide precipitation occurs, becoming the starting point for fracture, thus reducing toughness. Therefore, the Mo content is set to 2.0% or less. The Mo content is preferably 1.8% or less. The Mo content is more preferably 1.6% or less. The Mo content is even more preferably 1.4% or less.

[0050] [Ni: 2.5–6.0%]

[0051] Ni is an effective element for increasing strength without reducing toughness, and in this invention, it needs to contain 2.5% or more. If the content is less than 2.5%, the toughness is insufficient. Therefore, the Ni content is set to 2.5% or more. Preferably, the Ni content is 2.8% or more. More preferably, the Ni content is greater than 3.0%. Even more preferably, the Ni content is 3.2% or more. On the other hand, Ni is an element that easily solidifies and segregates; therefore, when the content exceeds 6.0%, solidification segregation increases, inducing high-temperature cracking. Therefore, the Ni content is set to 6.0% or less. Preferably, the Ni content is 5.8% or less. More preferably, the Ni content is 5.5% or less. Even more preferably, the Ni content is 5.0% or less.

[0052] [O: below 0.040%]

[0053] Oxygen (O) is an element that inevitably mixes into the weld metal, forming oxides within it. These oxides become the starting point for fracture; therefore, O content is preferably reduced, but 0.040% or less is permissible. Thus, the O content is set to 0.040% or less. The O content is preferably 0.030% or less. The O content is more preferably 0.025% or less. On the other hand, there is no particular limitation on the lower limit, but for the purpose of preventing a decrease in toughness caused by excessive deoxidizing elements, it is preferably set to 0.010% or more.

[0054] [N: below 0.012%]

[0055] Nitrogen (N) is an element that inevitably mixes into the weld metal and reduces toughness. This effect is significant when the N content exceeds 0.012%, therefore, the N content is set to 0.012% or less. The N content is preferably 0.011% or less. More preferably, it is 0.010% or less. Even more preferably, it is 0.008% or less. On the other hand, N has the effect of increasing strength through solid solution strengthening, and this effect becomes significant when the content is 0.002% or more. Therefore, the N content is preferably 0.002% or more. More preferably, it is 0.003% or more.

[0056] [Optional Composition]

[0057] The above-mentioned components are the basic components of the welding metal of the present invention. In the present invention, based on the above-mentioned basic components, one or more of the following can be selected as optional components: Cu: less than 0.8%, Al: less than 0.20%, and Ti: less than 0.20%. Furthermore, one or more of the following can be selected as optional components: Nb: less than 0.10%, V: less than 0.10%, Ca: less than 0.010%, B: less than 0.010%, and REM: less than 0.020%.

[0058] [Cu: less than 0.8%]

[0059] Cu is an element that precipitates finely in weld metal and increases strength through precipitation strengthening. However, when its content exceeds 0.8%, it exhibits red-hot brittleness in the temperature range around 1100°C, inducing surface cracks in the weld. Therefore, when Cu is present, the Cu content is set to 0.8% or less. The Cu content is preferably 0.7% or less. More preferably, the Cu content is 0.6% or less. On the other hand, the aforementioned effect of increasing strength through precipitation strengthening becomes significant when the Cu content is 0.1% or more. Therefore, when Cu is present, the Cu content is preferably 0.1% or more. More preferably, the Cu content is 0.2% or more.

[0060] [Al: below 0.20%]

[0061] Al acts as a deoxidizing element in weld metal, contributing to improved weld metal toughness by reducing the amount of dissolved oxygen. However, when the content exceeds 0.20%, the Al oxides coarsen, becoming the starting point for fracture and thus deteriorating toughness. Therefore, when Al is present, the Al content is set to 0.20% or less. The Al content is preferably 0.18% or less. On the other hand, to obtain the aforementioned effect of improving weld metal toughness, when Al is present, the Al content is preferably 0.01% or more. The Al content is more preferably 0.02% or more. The Al content is even more preferably 0.03% or more.

[0062] [Ti: below 0.20%]

[0063] Ti is an element that functions as a deoxidizer in weld metal, helps improve the toughness of the weld metal by reducing the amount of dissolved oxygen, and reduces the amount of dissolved nitrogen by forming TiN. However, when its content exceeds 0.20%, the ductility and toughness decrease due to the increase in dissolved Ti. Therefore, when Ti is present, the Ti content is set to 0.20% or less. The Ti content is preferably 0.19% or less. The Ti content is more preferably 0.17% or less. On the other hand, in order to obtain the above-mentioned effects of improving the toughness of the weld metal and reducing the amount of dissolved nitrogen, when Ti is present, the Ti content is preferably 0.01% or more. The Ti content is more preferably 0.10% or more.

[0064] [Nb: below 0.10%]

[0065] Nitrogen (Nb) is an element that improves hardenability and contributes to increasing the strength of weld metal. However, when its content exceeds 0.10%, it forms carbides, which become the initiation point for fracture, thus reducing toughness. Therefore, when Nb is present, the Nb content is set to 0.10% or less. The Nb content is preferably 0.08% or less. More preferably, the Nb content is 0.05% or less. On the other hand, in order to obtain the aforementioned effects of improving hardenability and contributing to increasing the strength of weld metal, when Nb is present, it is preferable to contain 0.01% or more of Nb. More preferably, 0.02% or more.

[0066] [V: below 0.10%]

[0067] V is a carbide-forming element that helps improve the strength of weld metal by causing fine carbides to precipitate within the grains. However, when its content exceeds 0.10%, the excess carbides become the initiation point for fracture, thus reducing toughness. Therefore, when V is present, the V content is set to 0.10% or less. The V content is preferably 0.08% or less. More preferably, the V content is 0.05% or less. On the other hand, to obtain the aforementioned effect of improving the strength of weld metal, when V is present, the V content is preferably 0.01% or more. More preferably, it is 0.02% or more.

[0068] [Ca: below 0.010%]

[0069] Ca is an element that suppresses high-temperature cracking by combining with S in molten metal to form high-melting-point sulfides (CaS). However, when its content exceeds 0.010%, it segregates at the original austenite grain boundaries, causing grain boundary embrittlement and thus reducing toughness. Therefore, when Ca is present, the Ca content is set to 0.010% or less. The Ca content is preferably 0.008% or less. On the other hand, the above-mentioned effect of suppressing high-temperature cracking becomes significant when the content is 0.001% or more. Therefore, when Ca is present, the Ca content is preferably set to 0.001% or more. The Ca content is more preferably 0.005% or more.

[0070] [B: Below 0.010%]

[0071] Boron (B) is an element that improves hardenability and contributes to increasing the strength of weld metal. However, when its content exceeds 0.010%, hardenability becomes excessive, leading to the formation of martensitic structure and thus reducing toughness. Therefore, when B is present, the B content is set to 0.010% or less. The B content is preferably 0.008% or less. On the other hand, to obtain the aforementioned effect of improving the strength of weld metal, when B is present, the B content is preferably 0.001% or more. The B content is more preferably 0.002% or more.

[0072] [REM: below 0.020%]

[0073] REM (refined iron oxide) consists of rare earth elements such as Sc, Y, La, and Ce. It suppresses high-temperature cracking by combining with sulfur in molten metal to form high-melting-point sulfides. However, when its content exceeds 0.020%, it segregates at the original austenite grain boundaries, causing grain boundary embrittlement and thus reducing toughness. Therefore, when REM is present, the REM content is set to 0.020% or less. The REM content is preferably 0.018% or less. On the other hand, to achieve the aforementioned effect of suppressing high-temperature cracking, the presence of REM of 0.002% or more becomes significant. Therefore, when REM is present, the REM content is preferably set to 0.002% or more. The REM content is more preferably 0.003% or more. The REM content is even more preferably 0.005% or more.

[0074] [Composition of Balance]

[0075] The balance other than the above chemical composition consists of Fe and unavoidable impurities. It should be noted that unavoidable impurities include H, Mg, Zn, Re, Co, Sb, and Bi, as long as their total content is less than 0.0100%. Furthermore, as long as the above basic and selected compositions are satisfied, elements other than these may be included, and such embodiments are also within the scope of this invention.

[0076] [Ceq: 0.65~1.00]

[0077] In this invention, it is important that Ceq, represented by the following formula (1), is in the range of 0.65 to 1.00.

[0078] Ceq=[C]+0.17[Mn]+0.04[Si]+0.025[Ni]+0.2[Cr]+0.25[Mo]…(1)

[0079] Furthermore, in this invention, it is important that α, as represented by the following formula (2), is in the range of 6.0 or less.

[0080] α=30[C]+0.7[Mn]+[Ni]-([Si]+0.5[Cr]+1.5[Mo])…(2)

[0081] Here, [element] refers to the content (mass%) of that element in the aforementioned weld metal.

[0082] In order to solve the above problems, the inventors have conducted in-depth research and found that, in order to ensure strength and toughness, it is effective to adjust Ceq, represented by the above formula (1), to 0.65 to 1.00, and further adjust α, represented by the above formula (2), to 6.0 or less. The following is a detailed explanation.

[0083] Ceq, or carbon equivalent, is a numerical value obtained by converting the chemical composition of the weld metal to carbon (C). To ensure suitable tensile properties (0.2% yield strength and tensile strength) and toughness for use as 780 MPa grade steel, Ceq needs to be set in the range of 0.65 to 1.00. When Ceq is less than 0.65, the target tensile properties cannot be obtained. Therefore, Ceq is set to 0.65 or higher. Ceq is preferably 0.70 or higher. Ceq is more preferably 0.74 or higher. On the other hand, when Ceq exceeds 1.00, the tensile strength becomes too high, and the toughness decreases. Therefore, Ceq is set to 1.00 or lower. Ceq is preferably 0.95 or lower. Ceq is more preferably 0.90 or lower.

[0084] [α: below 6.0]

[0085] The inventors investigated the causes of high-temperature cracking and found that solidification cracks occurred during austenite single-phase solidification. During austenite single-phase solidification, microstructural observation confirmed dendrite formation, particularly with increased solidification segregation of elements with low partition coefficients (Mn, P, S) that tend to remain in the liquid phase between dendrites. This solidification segregation, resulting in liquid phase retention at low temperatures, opens up during the shrinkage of the weld metal during solidification. Therefore, it is believed that limiting the presence of elements with low partition coefficients (Mn, P, S) and forming a δ-ferrite phase in the primary solidification crystals is effective in suppressing this solidification segregation. Specifically, by forming δ-ferrite in the primary solidification crystals, a solid δ-ferrite phase is formed between the austenite dendrites, thus mitigating solidification segregation. Furthermore, it was found that the formation of δ-ferrite leads to a more complex solidification morphology and a smaller original austenite grain size. It is speculated that the increase in grain boundary area due to the refinement of the original austenite grain size also leads to the mitigation of P and S segregation. Therefore, in order to make the primary crystals solidify into δ-ferrite, the balance between C, Mn, and Ni as austenite stabilizing elements and Si, Cr, and Mo as ferrite stabilizing elements is important. In order to control the content of these elements, α represented by equation (2) was proposed, and it was found that adjusting this value to below 6.0 is sufficient.

[0086] When α exceeds 6.0, high-temperature cracks occur, and a healthy joint without welding defects cannot be obtained. Therefore, α is set to 6.0 or less. α is preferably 5.5 or less. On the other hand, when α is 6.0 or less, there is a tendency for δ-ferrite to form as α decreases. Therefore, there is no particular lower limit, and in the embodiments described later, it was confirmed that no high-temperature cracks occurred in the range of 2.0 or more. Therefore, α is preferably 2.0 or more.

[0087] [Grain size (average original austenite grain size)]

[0088] P and S, which are unavoidable impurities, segregate at the austenite grain boundaries during the solidification of the weld metal, lowering the melting point of the weld metal and thus inducing high-temperature cracking. To reduce P and S segregation and prevent high-temperature cracking, refining the austenite grains and increasing the grain boundary area is effective. As mentioned above, it has been observed that when α is 6.0 or less, the solidification morphology becomes more complex due to the formation of δ-ferrite. Therefore, compared to the case of single-phase solidification in austenite, the original austenite grain size becomes smaller. Therefore, if α is 6.0 or less, it is presumed that P and S segregation is also reduced, thus preventing high-temperature cracking. In this invention, the grain size is not particularly limited. To obtain the above-mentioned effect, the average original austenite grain size is preferably 2.0 mm or less. More preferably, the average original austenite grain size is 1.5 mm or less. Even more preferably, the average original austenite grain size is 1.0 mm or less. The so-called average original austenite grain size here refers to the value obtained by taking the central part of the weld metal (5×5mm) as the measurement range, counting the number of grains that fall completely into the measurement range [nA] and the number of grains that are cut off by the boundary line of the measured area [nB], and calculating the average original austenite grain size [G] by the following formula (3).

[0089] [G] = {5 × 5 / ([nA] + 0.5[nB])} 1 / 2 …(3)

[0090] Mechanical properties of weld metals

[0091] Here, the preferred mechanical properties of the welded joint of the present invention will be described.

[0092] The preferred welding metal has the above-mentioned chemical composition, that is, the welding metal prepared according to JIS Z 3111 has a yield strength (0.2% yield strength) of 630 MPa or more at room temperature, and a tensile strength of 780 MPa or more, and the welding metal of the welded joint prepared according to JIS Z 3128 has an absorbed energy vE0 of 47 J or more in a V-notch Charpy impact test at a test temperature of 0°C.

[0093] [Material]

[0094] The base material for this invention is a 780MPa grade high-tensile steel plate or steel used in building structures. Hereinafter, a steel plate will be used as an example. Specific steel plate compositions (by mass%) can be listed, containing C: 0.06–0.12%, Si: less than 0.4%, Mn: 1.2–2.1%, P: less than 0.010%, S: less than 0.010%, Cr: less than 1.6%, Mo: less than 0.5%, Ni: 0.6–1.4%, O: less than 0.005%, N: less than 0.005%, and may contain one or more of Cu: less than 0.8%, Al: less than 0.20%, Ti: less than 0.20%, Nb: less than 0.10%, V: less than 0.10%, Ca: less than 0.010%, B: less than 0.010%, and REM: less than 0.020% as appropriate optional compositions, with the balance being Fe and unavoidable impurities.

[0095] In addition, it can also be applied to steel plates other than 780MPa grade, such as 590MPa grade or 980MPa grade.

[0096] It should be noted that, considering the linear energy is above 300kJ / cm, the thickness of the base material (steel plate) is preferably 30-100mm.

[0097] [Manufacturing method of welded joint]

[0098] Next, the manufacturing method of the welded joint of the present invention will be described using examples.

[0099] First, a 780MPa grade steel plate with the desired thickness is prepared as the material to be welded. Then, the prepared steel plates are beveled in a manner that forms a specified bevel shape between them. For the bevel shape formed, a V-groove or a serrated bevel, which is commonly used in fillet welds of box columns, is preferred.

[0100] Next, the beveled steel plates are butted together, and submerged arc welding with a heat input of 300 kJ / cm or more is carried out using welding wire and flux to form a layer of welding metal and weld together to make a submerged arc welded joint.

[0101] As for welding conditions, it is preferable to perform the welding with a current value of 1200–2100 A and a voltage value of 30–40 V for the leading electrode, and a current value of 1000–1800 A and a voltage value of 36–45 V for the trailing electrode. Furthermore, it is preferable to perform the welding with a welding speed of 12–35 cm / min and a linear energy of 300–600 kJ / cm.

[0102] For the welding wire and flux used, the composition is adjusted to achieve the above-mentioned composition, taking into account base metal dilution. It should be noted that the welding metal composition only needs to be within the above-mentioned range; there are no particular limitations on the type of welding wire or flux. It is acceptable for the welding wire to be either a solid wire or a metal-core wire. Similarly, it is acceptable for the flux to be either a molten flux or a bonding flux. It should be noted that when using a flux-cored welding wire with flux encased inside, the welding material is manufactured with the total composition of the steel sheath, metal powder, and flux powder as the target composition.

[0103] [Manufacturing method of welding wire]

[0104] The manufacturing methods of solid core welding wire and metal core welding wire are described as welding wires for welding.

[0105] Solid welding wire is preferably produced by the following steps: molten steel with the target composition is melted in a commonly used smelting furnace such as an electric furnace or a vacuum melting furnace, and then cast in a mold of a specified shape; next, the resulting steel ingot is heated to a specified temperature; and then the heated steel ingot is hot-rolled to form a steel bar of a specified shape; finally, the steel bar is cold-rolled multiple times (cold drawing) and annealed at a temperature of 900–1200°C as needed to produce a welding wire of the desired size.

[0106] Examples of welding wire chemical compositions include those containing C: 0.03–0.15%, Si: 0.2–1.2%, Mn: 0.5–1.4%, P: less than 0.025%, S: less than 0.010%, Cr: 0.10–0.55%, Mo: 0.5–3.0%, Ni: 2.0–9.0%, O: less than 0.080%, N: less than 0.020%, and may contain one or more of Cu: less than 0.8%, Al: less than 0.40%, Ti: less than 0.50%, Nb: less than 0.20%, V: less than 0.20%, Ca: less than 0.030%, B: less than 0.030%, and REM: less than 0.050% as appropriate optional compositions, with the balance consisting of Fe and unavoidable impurities.

[0107] Furthermore, regarding metal-core welding wire, for example, a thin steel sheet (0.5 mm thick) with a composition of 0.01% C, 0.01% Si, 0.30% Mn, 0.010% P, 0.010% S, and the balance Fe is used as the steel outer sheath material. This material is then cold-bent in the width direction to form a U-shape. Next, alloy powder with adjusted composition is filled into the resulting steel outer sheath in a manner consistent with the target welding wire composition. Finally, a cold-drawing process is performed to produce a welding wire with a diameter of φ3.2 mm.

[0108] The alloy powder is an alloy powder whose composition, relative to the composition of the steel outer sheath material, includes metallic components that supplement the total composition of the welding wire used for welding.

[0109] [Welding Flux]

[0110] The welding flux is not particularly limited, and generally known fusion flux or binder flux can be used. Examples of the chemical composition of a binder flux include powder materials containing SiO2: 15%, CaO: 15%, MgO: 30%, Al2O3: 25%, CaF2: 10%, CaCO3: 5%, etc. However, in this invention, the welding flux is not limited to these.

[0111] Example

[0112] The following examples and comparative examples illustrate the effects of the present invention. It should be noted that the present invention is not limited to the contents described in the examples.

[0113] The steel plate uses 780MPa grade steel with a thickness of 50mm as shown in Table 1, and is set with a V-shaped bevel angle of 35°, a root face of 1mm, and a root gap of 0mm.

[0114]

[0115] In this embodiment, a metal-core welding wire is fabricated. The steel sheath is made of a steel plate containing 0.01% C, 0.01% Si, 0.30% Mn, 0.010% P, 0.010% S, with the balance being Fe. Alloy powder is filled into this steel sheath to produce a metal-core welding wire with a wire diameter of φ3.2 mm and the composition shown in Table 2, which is used as the welding wire.

[0116]

[0117] The flux used is a bonding flux with the composition shown in Table 3.

[0118] [Table 3]

[0119]

[0120] Submerged arc welding was performed under the welding conditions in Table 4 to fabricate submerged arc welded joints.

[0121] [Table 4]

[0122]

[0123] After welding, an analytical test piece is cut from the center of the weld metal and elemental analysis is performed using wet analysis.

[0124] In addition, a macroscopic cross-section of the weld metal is cut, mirror-polished, and then observed using an optical microscope to determine the presence or absence of weld cracks. If cracks are found in the weld metal, it is evaluated as "present" weld crack. If no cracks are found, it is evaluated as "absent" weld crack.

[0125] In addition, after the macroscopic cross section after mirror polishing is exposed by saturated aqueous solution of picric acid, the original austenite grain boundaries are observed using an optical microscope to evaluate the austenite grain size. Regarding the calculation of the austenite grain size, the central part of the weld metal (5×5mm) is used as the measurement range. The number of grains that fall completely into the measurement range [nA] and the number of grains that are cut off by the boundary line of the measured area [nB] are counted. The average original austenite grain size [G] is calculated by the following formula (3).

[0126] [G] = {5 × 5 / ([nA] + 0.5[nB])} 1 / 2 …(3)

[0127] In addition, a tensile test piece (parallel part diameter of 12.5 mm φ) of A1 size was cut from the center of the weld metal 20 mm from the back of the steel plate of the obtained joint in accordance with the provisions of JIS Z 3111, and a tensile test was carried out.

[0128] In addition, according to JIS Z 3128, a Charpy impact test piece (V-notch) was cut from the center of the weld metal 10 mm from the back of the joint steel plate, and an impact test was performed. The test was conducted at a temperature of 0°C.

[0129] Tensile tests were performed on three specimens each at room temperature, and the average value of the obtained values ​​(0.2% yield strength and tensile strength) was taken as the tensile property of the weld metal. Charpy impact tests were also performed on three specimens each, and the absorbed energy vE0 at the test temperature of 0°C was calculated. The average value of this value was taken as the cryogenic impact toughness of the weld metal of the weld joint.

[0130] Regarding the target values ​​of the present invention, as described above, the yield strength (0.2% yield strength) of the weld metal at room temperature is 630 MPa or more, its tensile strength is 780 MPa or more, and the absorbed energy vE0 of the weld metal in the V-notch Charpy impact test at a test temperature of 0°C is 47 J or more. The results obtained are shown in Table 5.

[0131]

[0132] The examples of this invention are all high-strength and high-toughness welded joints with no cracks found, a yield strength (0.2% yield strength) of 630 MPa or more at room temperature, a tensile strength of 780 MPa or more, and a Charpy impact test absorption energy vE0 of 47 J or more at a test temperature of 0°C.

[0133] On the other hand, in comparative examples outside the scope of the present invention, cracks occurred, or the strength or impact toughness of the weld metal was insufficient, and the target weld joint was not obtained.

Claims

1. A submerged arc welding head, comprising a weld metal obtained by combining welding wire and flux and performing submerged arc welding with a welding heat input of 300 kJ / cm or higher, wherein, The chemical composition of the weld metal, by mass%, contains C: 0.05–0.15%, Si: 0.2–0.9%, Mn: 0.5–1.3%, P: less than 0.015%, S: less than 0.015%, Cr: 0.10–0.45%, Mo: 0.5–2.0%, Ni: 2.5–6.0%, O: less than 0.040%, N: less than 0.012%, with the balance consisting of Fe and unavoidable impurities. Furthermore, Ceq, as expressed in formula (1), is in the range of 0.65–1.00, and α, as expressed in formula (2), is in the range of 6.0 or less. The weld metal has a yield strength of 630 MPa or more in a tensile test, a tensile strength of 780 MPa or more, and an absorbed energy vE0 of 47 J or more in a V-notch Charpy impact test at a test temperature of 0°C. Ceq=[C]+0.17[Mn]+0.04[Si]+0.025[Ni]+0.2[Cr]+0.25[Mo] … (1) α=30[C]+0.7[Mn]+[Ni]-([Si]+0.5[Cr]+1.5[Mo]) … (2) Here, [element] in equations (1) and (2) refers to the mass percentage content of that element in the weld metal.

2. The submerged arc welding joint according to claim 1, wherein, Based on the chemical composition of the weld metal, it also contains, by mass %, one or more of the following: Cu: less than 0.8%, Al: less than 0.20%, and Ti: less than 0.20%.

3. The submerged arc welding joint according to claim 1 or 2, wherein, Based on the chemical composition of the weld metal, it further contains, by mass%, one or more of the following: less than 0.10% Nb, less than 0.10% V, less than 0.010% Ca, less than 0.010% B, and less than 0.020% REM.

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