Gas shielded arc welding method and method for manufacturing gas shielded arc welded joint

Abstract

Provided are a gas shielded arc welding method and a method for manufacturing a gas shielded arc welded joint. The present invention relates to a method for gas shielded arc welding a steel material using two types of flux cored wires having steel outer sheaths with different chemical compositions, wherein: a first welding wire and a second welding wire are used in the stated order; and gas shielded arc welding is performed using the first welding wire to form first weld metal, and then gas shielded arc welding is performed using the second welding wire overlapping the first weld metal to form second weld metal, thereby forming the weld metal of the welded joint.

Description

Gas shielded arc welding method and method for manufacturing gas shielded arc welded joints

[0001] This invention relates to a gas shielded arc welding method using flux-cored wires with two types of steel sheaths having different chemical compositions. In particular, it relates to a gas shielded arc welding method and a method for manufacturing gas shielded arc welded joints that have excellent SCC resistance to ammonia.

[0002] Gas shielded arc welding using flux-cored wire is a welding method in which a shielding gas is supplied between a consumable electrode (i.e., welding material) and the members to be joined, and an arc is generated in the gas atmosphere to melt the welding material.

[0003] Incidentally, ammonia is attracting attention as a hydrogen carrier for realizing a decarbonized society because it is relatively easy to liquefy under normal pressure, and development of technologies for its use in co-firing with coal in thermal power generation and as a marine fuel is underway. In order to use ammonia as fuel for thermal power generation, there is a need for larger ammonia storage facilities (hereinafter referred to as tanks). Furthermore, because ammonia is highly toxic, utmost care must be taken to prevent leakage from the tanks. The steel used in the construction of ammonia tanks is susceptible to stress corrosion cracking (hereinafter also referred to as ammonia SCC) in response to liquid ammonia, but the clear cause of the mechanism by which ammonia SCC occurs has not been clarified.

[0004] However, a correlation is thought to exist between the Vickers hardness (HV10) of the surface layer of steel and weld metal and the occurrence of ammonia SCC. Furthermore, it is known that when the average hardness (HV10) of the surface layer of steel and weld metal, which are the surfaces in contact with ammonia, exceeds 220, the occurrence of ammonia SCC increases significantly, and in particular, ammonia SCC is frequently observed near the weld metal.

[0005] The largest ammonia tanks in Japan have a capacity of approximately 15,000 tons, and given the required strength level, SLA325AN carbon steel sheets for low-temperature pressure vessels, which have undergone normalization treatment, are often used. Furthermore, during welding, a standard 50 kgf / mm² is applied depending on the strength of the steel sheet.２ Welding materials with a strength of 490 MPa are used. Furthermore, care is taken to ensure that the strength is greater than or equal to that of the base metal. As a result, the weld metal tends to harden, and the average hardness (HV10) of the surface layer of the weld metal is around 250 to 280. For this reason, liquid ammonia tanks are currently subjected to periodic open inspections, and if ammonia SCC is detected during the inspection, welding repairs are carried out.

[0006] In response to this situation, for example, Patent Document 1 discloses an invention of a steel sheet aimed at limiting yield strength and improving low-temperature toughness by adjusting the chemical composition and improving the manufacturing method. However, Patent Document 1 does not consider a method for suppressing ammonia SCC in the weld metal portion where ammonia SCC generation is significant.

[0007] Furthermore, as a gas shielded arc welding method using low-carbon welding wire, for example, there are Patent Documents 2 or 3. These documents disclose that the strength and toughness of the weld metal can be improved by using welding wire having a specific chemical composition.

[0008] Furthermore, Patent Document 4 discloses an invention for a welded joint of austenitic stainless steel pipe. In this welded joint, an austenitic weld metal is formed on the root side of the groove, and a martensitic weld metal is formed on the groove of the unwelded portion above it. This hardens the surface layer of the weld metal, suppressing the generation of distortion on the inner surface of the pipe and improving fatigue strength.

[0009] Patent No. 5428999 JP 04-309493 JP 09-201697 JP 53-108838

[0010] However, in conventional welded joints using steel materials and welding wires as described in these patent documents, hardening of the surface layer of the weld metal is unavoidable even with optimization of welding conditions. Therefore, the problem of ammonia SCC generation remains unresolved. Because it is difficult to completely suppress the generation of ammonia SCC, periodic tank opening inspections are necessary, and welding repairs are currently carried out whenever ammonia SCC is discovered during these inspections.

[0011] In the future, ammonia is expected to be used as an important fuel, and in order to supply stable energy, it will be necessary to store large quantities of ammonia, thus requiring larger onshore ammonia tanks. If tanks are enlarged using conventional welding techniques, defects such as ammonia SCC will increase, and welding repairs will require considerable effort. For this reason, there is an urgent need to obtain welded joints with high resistance to ammonia SCC. Multi-layer welding using welding wire with fewer alloying elements can lower the hardness of the surface layer of the weld metal. However, this necessitates considering the reduction in joint strength and toughness due to heat input limitations, and thus reduces welding efficiency.

[0012] Therefore, the present invention aims to provide an economically advantageous gas shielded arc welding method and a method for manufacturing a gas shielded arc welded joint using flux-cored wire, which controls the average hardness (HV10) of the surface layer of the weld metal in multilayer welding that comes into contact with ammonia to 220 or less by appropriate welding conditions. This control reduces the generation of ammonia SCC and also provides good joint toughness.

[0013] The inventors of this invention conducted diligent research to achieve the above objective and obtained the following findings.

[0014] In typical multilayer welding, unless the steel material is a combination of multiple steel materials such as clad steel or an austenitic stainless steel pipe (see, for example, Patent Document 4), welding is performed from the first layer to the final layer using a single type of welding wire selected according to the strength (grade) of the steel material.

[0015] Depending on their chemical composition, welding wires can either improve the mechanical properties of the weld metal upon reheating, or have good mechanical properties in their as-welded state but deteriorate upon reheating. On the other hand, we focused on the fact that the weld metal portion of a multi-layer welded joint has parts that are reheated and parts that remain as-welded and are not reheated. That is, from the first layer to the layer before the final layer in multi-layer welding, the weld metal from the welding wire melts and solidifies, cools to room temperature, and is then reheated by subsequent welding. In contrast, in the final layer of multi-layer welding, once the weld metal melts and solidifies and cools to room temperature, the weld metal is used as is in the structure without being reheated unless post-weld heat treatment is performed.

[0016] Because the final layer and the other layers (i.e., from the first layer to the layer before the final layer) undergo different thermal histories, using a single type of welding material to create a welded joint will inevitably include areas where the mechanical properties deteriorate due to the thermal history. While it is possible to obtain good mechanical properties in both areas that are not reheated and areas that are reheated by incorporating large amounts of alloying elements such as Ni and Mo into the welding wire, such alloying elements are expensive, leading to higher welding wire costs. Therefore, it is desirable to minimize the content of expensive alloying elements.

[0017] Based on the above-mentioned studies, the inventors have arrived at a welding method that can achieve both limiting the hardness of the surface layer of the weld metal and good joint toughness (hereinafter sometimes referred to as low-temperature toughness) by using two types of welding wires with different chemical compositions to produce weld metal that is reheated and weld metal that is not reheated during welding of steel materials. In other words, the gist of the present invention is as follows. [1] A gas shielded arc welding method for gas shielded arc welding steel materials using two types of flux-cored wires having different chemical compositions of steel sheaths, wherein the two types of flux-cored wires are a first welding wire and a second welding wire, and the first welding wire and the second welding wire are used in this order, and the first welding wire is used to gas shielded arc weld to form a first weld metal, and then the second welding wire is placed on top of the first weld metal and gas shielded arc weld to form a second weld metal, thereby forming the weld metal of a gas shielded arc welded joint, wherein the chemical composition of the steel sheath of the first welding wire is, in mass%, C: 0.05 to 0.11%, Si: 0.30 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04 to 0.19%, A gas shielded arc welding method comprising: a welding wire containing Cu: 0.60% or less, Ni: 1.60% or less, Cr: 0.50% or less, Mo: 0.50% or less, N: 0.0100% or less, and O: 0.0100% or less, with the remainder being Fe and unavoidable impurities; and a second welding wire steel sheath whose chemical composition, by mass%, contains C: 0.01 to 0.04%, Si: 0.02 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04 to 0.19%, N: 0.0100% or less, and O: 0.0100% or less, with the remainder being Fe and unavoidable impurities.[2] The gas shielded arc welding method according to [1], wherein the chemical composition of the steel sheath of the second welding wire further contains one or more selected by mass% from Cu: 0.30% or less, Ni: 0.40% or less, Cr: 0.20% or less, Mo: 0.20% or less, V: 0.40% or less, Ti: 0.190% or less, and Zr: 0.15% or less. [3] The gas shielded arc welding method according to [1] or [2], wherein at least one layer of the second weld metal is weaving weld. [4] A method for manufacturing a gas shielded arc welded joint produced by the gas shielded arc welding method described in any of [1] to [3] above, wherein the chemical composition of the first weld metal is, in mass%, C: more than 0.05% and 0.11%, Si: 0.30 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04 to 0.19%, Cu: 0.60% or less, Ni: 1.60% or less, Cr: 0.50% or less, Mo: 0.50% or less, N: 0.0100% or less, O: 0.0100% or less, with the remainder being Fe and unavoidable impurities, and the chemical composition of the second weld metal is, in mass%, C: 0.01 to 0.04%, A method for manufacturing a gas shielded arc welded joint, wherein the chemical composition of the second weld metal further contains, by mass%, one or more selected from Cu: 0.30% or less, Ni: 0.40% or less, Cr: 0.20% or less, Mo: 0.20% or less, V: 0.40% or less, Ti: 0.190% or less, and Zr: 0.15% or less, with the remainder being Fe and unavoidable impurities. [7] A method for manufacturing a gas shielded arc welded joint according to [6], wherein the chemical composition of the second weld metal further contains, by mass%, one or more selected from Cu: 0.30% or less, Ni: 0.40% or less, Cr: 0.20% or less, Mo: 0.20% or less, V: 0.40% or less, Ti: 0.190% or less, and Zr: 0.15% or less.

[0018] According to the present invention, the resulting gas-shielded arc welded joint (hereinafter sometimes referred to as "welded joint") has toughness and strength equivalent to that of the base material, and furthermore, by reducing the hardness of the surface layer of the weld metal that comes into contact with ammonia, it has high ammonia-resistant SCC performance.

[0019] Figure 1 is a schematic cross-sectional view showing an example of groove shape in the gas shielded arc welding method using flux-cored wire of the present invention, where Figure 1(a) is a butt V groove shape and Figure 1(b) is a butt X groove shape. Figure 2 is a schematic cross-sectional view showing an example of the specimen sampling position for a Charpy impact test, where Figure 2(a) shows the weld metal obtained using a butt V groove shape and Figure 2(b) shows the weld metal obtained using a butt X groove shape. Figure 3 is a schematic cross-sectional view showing an example of the measurement position for a Vickers hardness test, where Figure 3(a) shows the weld metal obtained using a butt V groove shape and Figure 3(b) shows the weld metal obtained using a butt X groove shape. Figure 4 is a distribution map showing the results of an example of Vickers hardness test measurement. Figure 5 is a schematic diagram showing an example of weaving welding.

[0020] The following describes embodiments for carrying out the present invention.

[0021] [Steel Material] In this invention, gas shielded arc welding is preferably performed using steel material for ammonia tanks as the base material. The steel material may be a steel plate, and preferably examples include SLA325AN, SLA325ATMC, SLA365, etc., of carbon steel plates for low-temperature pressure vessels (JIS G3126:2015). The plate thickness range of the steel material for ammonia tanks is 9 mm to 38 mm.

[0022] The chemical composition of the above base material includes, for example, C: 0.030 to 0.090%, Si: 0.50% or less, Mn: 0.50 to 1.60%, P: 0.015% or less, and S: 0.010% or less. Furthermore, it contains Al: 0.060% or less, N: 0.0010 to 0.0100%, and O (oxygen): 0.0100% or less. Here, "%" in relation to the chemical composition means "mass%" (the same applies hereafter unless otherwise specified). The above base material further contains, if necessary, one or more elements selected from the following constituent elements.

[0023] The composition consists of one or more elements selected from the following: Cu: 2.00% or less, Ni: 2.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, V: 1.00% or less, Nb: 0.10% or less, Ti: 0.005 to 0.100%, Ca: 0.2000% or less, Mg: 0.0200% or less, and B: 0.0100% or less. Note that these component elements are optional and may be included as needed; therefore, the content of each component element may be 0%.

[0024] Furthermore, the remaining chemical composition of the base material, other than the chemical composition described above, consists of Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the purpose of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap. Examples of these unavoidable impurity elements include Sn, Sb, As, Pb, and Bi, and are permissible if their total is 0.10% or less. The reasons for limiting the compositional ranges for each chemical composition described above will be explained later.

[0025] [Multilayer Weld Metal] In this invention, the weld metal must be multilayered. If the weld metal is single-layered, there are no second or subsequent layers, and a second welding wire cannot be used, so the effects of this invention cannot be achieved. For this reason, it is preferable to use the second welding wire for 1 to 3 layers, including the final layer (i.e., the final layer, the layer before the final layer, and even the layer before that). For example, the second welding wire may be used only for the final layer, or it may be used for two layers, the final layer and the layer before the final layer.

[0026] In this invention, "first weld metal" refers to the weld metal formed by welding using a first welding wire, and "second weld metal" refers to the weld metal formed by welding using a second welding wire. Furthermore, the term "weld metal of the welded joint" is used without distinguishing between the first and second weld metals, and refers to the weld metal formed by the integration of both.

[0027] [Gas Shielded Arc Welding Using Flux-Cored Wire] Gas shielded arc welding using flux-cored wire is a welding method in which an arc is formed between the welding wire and the base metal in a shielding gas such as argon, carbon dioxide, or a mixture thereof, and the heat from this arc melts the base metal and the continuously supplied wire to weld it. Flux-cored wires of 1.0 mmφ to 1.6 mmφ can be used as electrodes for gas shielded arc welding. Preheating is not always necessary. By filling the inside of this flux-cored wire with a slag-forming agent, arc stabilizer, deoxidizer, etc., welding in all positions becomes possible, and the deposition rate is also high, resulting in high efficiency and, as a result, a significant reduction in welding costs. Details of this flux-cored wire will be explained in the welding wire section below.

[0028] Other welding conditions are as follows: current: 150A to 320A (DCEP) (DC positive electrode), voltage: 20V to 30V, welding speed: 10cm / min to 40cm / min, welding heat input: 10kJ / cm to 30kJ / cm.

[0029] This is because welding efficiency is significantly reduced when the current is less than 150A. On the other hand, when the current exceeds 320A, droplet transfer becomes unstable, resulting in an irregular bead shape and the generation of a large amount of spatter.

[0030] Furthermore, if the voltage is less than 20V, the arc becomes unstable, leading to a decrease in strength due to welding defects. On the other hand, if the voltage exceeds 30V, the arc length increases, the amount of spatter generated increases, and the corrosion resistance decreases due to spatter adhesion.

[0031] Furthermore, if the welding speed is less than 10 cm / min, excessive weld metal buildup occurs, resulting in an uneven bead, which can become a starting point for corrosion. On the other hand, if the welding speed exceeds 40 cm / min, the penetration becomes shallow, leading to a decrease in strength due to welding defects.

[0032] Also, when the welding heat input is less than 10 kJ / cm, the input heat is low, and the base metal and the weld metal do not fuse properly, resulting in a humping bead. On the other hand, when the welding heat input exceeds 30 kJ / cm, the bead shape becomes irregular. The welding heat input is more preferably 12.0 kJ / cm to 20.0 kJ / cm. The welding heat input is even more preferably 14.0 kJ / cm to 18.0 kJ / cm.

[0033] Also, the interpass temperature is set to 250°C or lower. This is because when the interpass temperature exceeds 250°C, embrittlement of the welded part occurs. Here, the "interpass temperature" means the temperature near the groove immediately before starting the welding of each pass.

[0034] Examples of the welding posture include all postures, specifically, downward, horizontal, upward, vertical upward progression, etc.

[0035] In addition to the V-groove shown in Fig. 1(a) and the X-groove shown in Fig. 1(b), the groove shape may also be an L-groove, a K-groove, etc. The preferred groove angle θ in the case of a V-groove or an X-groove is 10° to 60°.

[0036] By performing multi-layer welding under these welding conditions, multi-layer weld metals are formed in the groove. The number of layers depends on conditions such as the plate thickness, but is preferably up to 10 layers for each side. The number of passes per layer is preferably in the range of 1 to 2 passes.

[0037] [Welding Wire] The welding wire used in the gas shielded arc welding method using flux cored wires with different chemical compositions of two types of steel outer skins according to the present invention is the first welding wire and the second welding wire. In the welding method of the present invention, these wires are used in this order. First, gas shielded arc welding is performed using the first welding wire to form a first weld metal, and then, the second welding wire is used for gas shielded arc welding on top of the first weld metal to form a second weld metal, thereby forming the weld metal of the welded joint. Specifically, a second welding wire having a specific chemical composition different from that of the first welding wire, with lower contents of C and alloy elements than the first welding wire, is used. Thereby, it is possible to achieve both ensuring the mechanical properties of the weld metal and suppressing ammonia SCC without using expensive alloy elements.

[0038] The flux filled in the wire of the flux cored wire of the present invention is filled for the purpose of arc stability during welding and suppressing spatter. Specifically, as the flux, it contains Fe oxide, Si oxide, Mn oxide, Al oxide, Ti oxide, Ca oxide, Mg oxide, Zr oxide, and other fluorides and sulfides.

[0039] For example, when performing double-sided welding on the V-groove of FIG. 1(b) by the gas shielded arc welding method of the present invention, as described in the examples below, the first welding wire and the second welding wire may be used in this order during multi-layer welding on the front side of the steel material. The reason is that the side in contact with ammonia etc. (i.e., the inner surface side of the tank) corresponds to the front side of the steel material, so it is necessary to lower the hardness of the front side of the steel material in order to suppress ammonia SCC. In multi-layer welding on the back side of the steel material, it may be performed only with the first welding wire. The reason is that the outer surface side of the tank does not come into contact with liquid ammonia, so SCC does not occur.

[0040] Hereinafter, the chemical compositions of the first and second welding wires will be described.

[0041] [First Welding Wire] First, the reasons for limiting the chemical composition of the first welding wire to suitable composition ranges will be explained.

[0042] [C: 0.05-0.11%] C must be present in an amount of 0.05% or more to obtain the necessary strength as the first weld metal, but if it exceeds 0.11%, it will increase hardness and decrease toughness. Therefore, the C content is limited to the range of 0.05-0.11%. The C content is preferably in the range of 0.06-0.10%, and more preferably in the range of 0.07-0.09%.

[0043] [Si: 0.30-0.90%] Si is an element necessary to ensure the strength of the first weld metal and to deoxidize the weld metal. To obtain such effects, a Si content of 0.30% or more is required. On the other hand, if the Si content exceeds 0.90%, it will degrade the toughness of the first weld metal. Therefore, the Si content is limited to the range of 0.30-0.90%. Preferably, the Si content is in the range of 0.40-0.80%, and more preferably in the range of 0.50-0.70%.

[0044] [Mn: 0.50-1.80%] Mn is an element necessary to ensure the strength of the first weld metal and to deoxidize it. To obtain such effects, a Mn content of 0.50% or more is required. On the other hand, if the Mn content exceeds 1.80%, the toughness of the first weld metal will deteriorate significantly. Therefore, the Mn content is limited to the range of 0.50-1.80%. Preferably, the Mn content is in the range of 0.70-1.50%, and more preferably in the range of 1.00-1.30%.

[0045] [P: 0.025% or less] Since P content exceeding 0.025% degrades the toughness of the first weld metal, the P content is limited to 0.025% or less. Preferably, the P content is 0.010% or less, and more preferably 0.003% or less. On the other hand, the lower limit of the P content is not particularly limited and may be 0.000%. However, reducing excess P leads to a rise in the refining cost of the wire material, so from a cost standpoint, a P content of 0.001% or more is also acceptable.

[0046] [S: 0.035% or less] Since S content exceeding 0.035% degrades the toughness of the first weld metal, the S content is limited to 0.035% or less. Preferably, the S content is 0.010% or less, and more preferably 0.003% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0.000%. However, reducing excess S leads to a rise in the refining cost of the wire material, so from a cost standpoint, it is preferable that the S content be 0.001% or more.

[0047] [Al: 0.04-0.19%] From the viewpoint of deoxidizing the first weld metal, an Al content of 0.04% or more is necessary, while an Al content exceeding 0.19% will degrade the toughness of the weld metal. Therefore, the Al content is limited to the range of 0.04-0.19%. Preferably, the Al content is 0.05-0.10%, and more preferably 0.06-0.08%.

[0048] [Cu: 0.60% or less] Cu has the function of increasing the strength while maintaining the high toughness of the first weld metal, but if the Cu content exceeds 0.60%, hot brittleness occurs and the surface properties deteriorate. For this reason, the Cu content is limited to 0.60% or less. A Cu content of 0.40% or less is preferable. On the other hand, in order to further exhibit the above-mentioned strength-improving effect, a Cu content of 0.10% or more is preferable, and 0.20% or more is more preferable.

[0049] [Ni: 1.60% or less] Ni has a similar function to Cu, increasing the strength while maintaining the high toughness of the first weld metal. However, if the Ni content exceeds 1.60%, the hardness increases and the toughness decreases. Therefore, the Ni content is limited to 1.60% or less. It is preferable that the Ni content be 0.60% or less. On the other hand, in order to further exhibit the above-mentioned strength-improving effect, it is preferable that the Ni content be 0.01% or more, and more preferably 0.10% or more.

[0050] [Cr: 0.50% or less] Like Cu, Cr increases the strength of the first weld metal while maintaining its high toughness. However, if the Cr content exceeds 0.50%, the hardness increases and the toughness decreases. Therefore, the Cr content is limited to 0.50% or less. Preferably, the Cr content is 0.30% or less. On the other hand, to further enhance the aforementioned strength-improving effect, it is preferable to have a Cr content of 0.01% or more, and more preferably 0.10% or more.

[0051] [Mo: 0.50% or less] Like Cr, Mo increases the strength of the first weld metal while maintaining its high toughness. However, if the Mo content exceeds 0.50%, the hardness increases and the toughness decreases. Therefore, the Mo content is limited to 0.50% or less. It is preferable that the Mo content be 0.20% or less. On the other hand, in order to further exhibit the aforementioned strength-improving effect, it is preferable that the Mo content be 0.01% or more, and more preferable that it be 0.05% or more.

[0052] [N: 0.0100% or less] N is an element that is inevitably contained as an impurity, and if the N content exceeds 0.0100%, it leads to a decrease in toughness. Therefore, from the viewpoint of suppressing a decrease in toughness and weldability, the N content is limited to 0.0100% or less. The N content is preferably 0.0080% or less, and more preferably 0.0060% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0.0000%. However, reducing the excess leads to a rise in the refining cost of the wire material, so from the viewpoint of cost, the N content can preferably be 0.0010% or more.

[0053] [O (Oxygen): 0.0100% or less] O (oxygen) is an element that is contained as an unavoidable impurity and has adverse effects such as forming oxides and becoming the starting point for fracture. Therefore, the O content is limited to 0.0100% or less. Preferably, the O content is 0.0050% or less, and more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0.000%. However, reducing excess leads to a rise in the refining cost of the wire material, so from a cost standpoint, it is preferable that the O content be 0.0020% or more.

[0054] [Remaining Composition of the First Welding Wire] The remaining chemical composition of the first welding wire, other than the chemical composition of the first welding wire described above, consists of Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the objective of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap. Examples of these unavoidable impurity elements include Sn, Sb, As, Pb, and Bi, and are permissible if their total is 0.10% or less. Furthermore, as long as the chemical composition of the first welding wire described above is satisfied, there is no prejudice that other unavoidable impurity elements may be included in the first welding wire, and such embodiments are also included within the technical scope of the present invention.

[0055] Furthermore, multiple types of welding wires may be used sequentially as the first welding wire, provided that they satisfy the specified compositional range of the first welding wire.

[0056] [Second Welding Wire] Next, a suitable chemical composition for the second welding wire will be described. The chemical composition of the second welding wire includes a basic composition and optional compositions. First, the reasons for limiting the composition range for each basic composition will be explained, and then the reasons for defining the composition range for each optional composition will be explained.

[0057] [Basic Composition of the Second Welding Wire] The reasons for limiting the composition range for each component of the second welding wire are as follows.

[0058] [C: 0.01-0.04%] In order to obtain the necessary strength as a second weld metal, it is necessary to contain 0.01% or more carbon. However, in the as-welded state, if the carbon content exceeds 0.04%, it increases hardness and decreases toughness. Therefore, the carbon content is limited to the range of 0.01-0.04%. It is crucial that this upper limit of carbon content is less than the lower limit of carbon content of the first welding wire in order to limit the hardness of the surface layer of the second weld metal. The carbon content is preferably in the range of 0.02-0.04%, and more preferably in the range of 0.02-0.03%.

[0059] [Si: 0.02-0.90%] Si is an element necessary to ensure the strength of the second weld metal and to deoxidize the second weld metal, and to obtain such effects, a content of 0.02% or more is necessary. On the other hand, if the content exceeds 0.90%, the toughness of the second weld metal will deteriorate. Therefore, the Si content is limited to the range of 0.02-0.90%. The Si content is preferably in the range of 0.20-0.80%, and more preferably in the range of 0.40-0.70%.

[0060] [Mn: 0.50-1.80%] Mn is an element necessary to ensure the strength of the second weld metal and to deoxidize the second weld metal. To obtain such effects, a content of 0.50% or more is required. On the other hand, if the content exceeds 1.80%, the toughness of the second weld metal will deteriorate significantly. Therefore, the Mn content is limited to the range of 0.50-1.80%. Preferably, the Mn content is in the range of 0.70-1.50%, and more preferably in the range of 1.00-1.30%.

[0061] [P: 0.025% or less] Since P content exceeding 0.025% degrades the toughness of the second weld metal, the P content is limited to 0.025% or less. Preferably, the P content is 0.010% or less, and more preferably 0.003% or less. The lower limit of the P content is not particularly limited and may be 0.000%, however, since P is an element that is inevitably contained in steel material as an impurity, industrially it may exceed 0.000%. However, reducing the excess leads to a rise in the refining cost of the wire material, so from a cost standpoint, a P content of 0.001% or more is acceptable.

[0062] [S: 0.035% or less] Since S content exceeding 0.035% degrades the toughness of the second weld metal, the S content is limited to 0.035% or less. Preferably, the S content is 0.010% or less, and more preferably 0.003% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0.000%. However, excessive reduction leads to a rise in the refining cost of the wire material, so from a cost standpoint, an S content of 0.001% or more is acceptable.

[0063] [Al: 0.04-0.19%] From the viewpoint of deoxidizing the second weld metal, an Al content of 0.04% or more is necessary, while a content exceeding 0.19% will degrade the toughness of the weld metal. Therefore, the Al content is limited to the range of 0.04-0.19%. Preferably, the Al content is 0.05-0.10%, and more preferably 0.06-0.08%.

[0064] [N: 0.0100% or less] N is an element that is inevitably contained as an impurity, and if the N content exceeds 0.0100%, it leads to a decrease in the toughness of the second weld metal. Therefore, from the viewpoint of suppressing a decrease in the toughness and weldability of the second weld metal, the N content is limited to 0.0100% or less. The N content is preferably 0.0080% or less, and more preferably 0.0060% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0.0000%. However, reducing the excess leads to a rise in the refining cost of the wire material, so from the viewpoint of cost, an N content of 0.0010% or more is also acceptable.

[0065] [O (Oxygen): 0.0100% or less] O (oxygen) is an element that is contained as an unavoidable impurity and has adverse effects such as forming oxides and becoming the starting point for fracture. Therefore, the O content is limited to 0.0100% or less. Preferably, the O content is 0.0050% or less, and more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0.0000%. However, since excessive reduction leads to a rise in the refining cost of the wire material, from a cost standpoint, an O content of 0.0020% or more is also acceptable.

[0066] [Optional Composition of the Second Welding Wire] The reasons for specifying suitable composition ranges for the optional composition of the second welding wire are as follows.

[0067] [Cu: 0.30% or less] Cu has the function of increasing the strength while maintaining the high toughness of the second weld metal, but if the Cu content exceeds 0.30%, hot brittleness occurs and the surface properties deteriorate. For this reason, when Cu is included, it is preferable to keep the Cu content at 0.30% or less. It is more preferable to keep the Cu content at 0.25% or less. On the other hand, when Cu is included, in order to further exhibit the above-mentioned strength-improving effect, it is preferable to keep the Cu content at 0.10% or more, and more preferable to keep it at 0.20% or more.

[0068] [Ni: 0.40% or less] Ni has a similar function to Cu, increasing the strength while maintaining the high toughness of the second weld metal. However, if the Ni content exceeds 0.40%, the hardness increases too much. Therefore, when Ni is included, it is preferable to keep the Ni content at 0.40% or less. It is more preferable to keep the Ni content at 0.30% or less. On the other hand, when Ni is included, in order to further exhibit the above-mentioned strength-improving effect, it is preferable to keep the Ni content at 0.01% or more, and more preferable to keep it at 0.10% or more.

[0069] [Cr: 0.20% or less] Like Cu, Cr increases the strength of the second weld metal while maintaining its high toughness. However, if the Cr content exceeds 0.20%, the hardness of the second weld metal increases too much. Therefore, when Cr is included, it is preferable to keep the Cr content at 0.20% or less. It is more preferable to keep the Cr content at 0.15% or less. On the other hand, when Cr is included, in order to further exhibit the aforementioned strength-improving effect, it is preferable to keep the Cr content at 0.01% or more, and more preferable to keep it at 0.10% or more.

[0070] [Mo: 0.20% or less] Like Cr, Mo increases the strength of the second weld metal while maintaining its high toughness, but if the Mo content exceeds 0.20%, the hardness increases too much. For this reason, when Mo is included, it is preferable to keep the Mo content at 0.20% or less. It is more preferable to keep the Mo content at 0.10% or less. On the other hand, when Mo is included, in order to further exhibit the above-mentioned strength-improving effect, it is preferable to keep the Mo content at 0.01% or more, and more preferable to keep it at 0.05% or more.

[0071] [V: 0.40% or less] V increases the strength and toughness of the second weld metal, but if present in excess, it forms carbides and increases hardness. Therefore, when V is included, it is preferable to keep the V content at 0.40% or less. It is more preferable to keep the V content at 0.20% or less. On the other hand, when V is included, in order to further exhibit the aforementioned strength and toughness improvement effect, it is preferable to keep the V content at 0.01% or more, and more preferable to keep it at 0.05% or more.

[0072] [Ti: 0.190% or less] Ti is TiO ２ Ti precipitates as a solid, and acts as a ferrite transformation nucleus in the second weld metal, contributing to increased toughness by suppressing the coarsening of austenite and providing a ferrite transformation nucleus. However, if the Ti content exceeds 0.190%, the amount of solid-solution Ti increases, degrading the toughness. Therefore, when Ti is included, it is preferable to keep the Ti content below 0.190%. On the other hand, if Ti is included, its effect is minimal if the Ti content is less than 0.030%, so it is preferable to keep the Ti content above 0.030%. A Ti content in the range of 0.050 to 0.100% is more preferable.

[0073] [Zr: 0.15% or less] Zr is an element that has the effect of suppressing grain growth and contributes to improving the toughness of the second weld metal. However, if the Zr content exceeds 0.15% and is excessive, the amount of solid-solution Zr increases, which degrades the toughness. Therefore, when Zr is included, it is preferable to keep the Zr content at 0.15% or less. On the other hand, when Zr is included, such an effect is difficult to obtain if the Zr content is less than 0.01%. Therefore, it is preferable to have a Zr content of 0.01% or more. A Zr content in the range of 0.03 to 0.10% is more preferable.

[0074] [Remaining Composition of the Second Welding Wire] The remaining chemical composition of the second welding wire, other than the chemical composition of the second welding wire described above, consists of Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the objective of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap. Examples of these unavoidable impurity elements include Sn, Sb, As, Pb, and Bi, and are permissible if their total is 0.10% or less. Furthermore, as long as the chemical composition of the second welding wire described above is satisfied, there is no prejudice that other unavoidable impurity elements may be included in the second welding wire, and such embodiments are also included within the technical scope of the present invention.

[0075] [Chemical Composition of the First Weld Metal] Although not shown in the diagram, the chemical composition of the first weld metal was determined by taking three samples from the center of the weld line 10 mm from the penetration tip of the first layer of weld metal, analyzing their chemical composition, and taking the average of the three samples as the chemical composition of the first weld metal. If stable arc welding is performed, the value will be very close to the chemical composition of the first welding wire described above. The reasons for limiting each composition range are as follows.

[0076] [C: Over 0.05% and up to 0.11%] As the first weld metal, carbon (C) must be present in a concentration of over 0.05% to obtain the necessary strength even when subjected to the heat effects of subsequent welding. However, if the C content exceeds 0.11%, it increases hardness and decreases toughness. Therefore, the C content is limited to a range of over 0.05% and up to 0.11%. Preferably, the C content is in the range of 0.06% to 0.10%, and more preferably in the range of 0.07% to 0.09%.

[0077] [Si: 0.30-0.90%] Si is an element necessary to ensure the strength of the first weld metal and to deoxidize the weld metal. To obtain such effects, a Si content of 0.30% or more is required. On the other hand, if the Si content exceeds 0.90%, the toughness of the first weld metal deteriorates. Therefore, the Si content is limited to the range of 0.30-0.90%. Preferably, the Si content is in the range of 0.40-0.80%, and more preferably in the range of 0.50-0.70%.

[0078] [Mn: 0.50-1.80%] Mn is an element necessary to ensure the strength of the first weld metal and to deoxidize it. To obtain such effects, a Mn content of 0.50% or more is required. On the other hand, if the Mn content exceeds 1.80%, the toughness of the first weld metal will deteriorate significantly. Therefore, the Mn content is limited to the range of 0.50-1.80%. Preferably, the Mn content is in the range of 0.70-1.50%, and more preferably in the range of 1.00-1.30%.

[0079] [P: 0.025% or less] Since P exceeding 0.025% degrades the toughness of the first weld metal, the P content is limited to 0.025% or less. Preferably, the P content is 0.010% or less, and more preferably 0.003% or less. On the other hand, the lower limit of the P content is not particularly limited and may be 0.000%. However, reducing the excess P leads to a surge in the refining cost of the welding wire material (hereinafter also simply referred to as "wire material"). Therefore, from a cost standpoint, a P content of 0.001% or more is preferable and acceptable.

[0080] [S: 0.035% or less] Since S content exceeding 0.035% degrades the toughness of the first weld metal, the S content is limited to 0.035% or less. Preferably, the S content is 0.010% or less, and more preferably 0.003% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0.000%. However, reducing excess S leads to a rise in the refining cost of the wire material. Therefore, from a cost standpoint, it is preferable that the S content be 0.001% or more.

[0081] [Al: 0.04-0.19%] From the viewpoint of deoxidizing the first weld metal, an Al content of 0.04% or more is necessary. On the other hand, if the Al content exceeds 0.19%, it will degrade the toughness of the weld metal. Therefore, the Al content is limited to the range of 0.04-0.19%. The Al content is preferably 0.05-0.10%, and more preferably 0.06-0.08%.

[0082] [Cu: 0.60% or less] Cu has the function of increasing the strength while maintaining the high toughness of the first weld metal, but if it is contained in a quantity exceeding 0.60%, hot brittleness will occur and the surface properties will deteriorate. For this reason, the Cu content should be 0.60% or less. Preferably, the Cu content is 0.30% or less. On the other hand, in order to further exhibit the above-mentioned strength-improving effect, the Cu content is preferably 0.10% or more, and more preferably 0.20% or more.

[0083] [Ni: 1.60% or less] Ni has a similar function to Cu, increasing the strength while maintaining the high toughness of the first weld metal. However, if the Ni content exceeds 1.60%, the hardness increases and the toughness decreases. Therefore, the Ni content should be 1.60% or less. Preferably, the Ni content is 1.00% or less. On the other hand, to further enhance the aforementioned strength-improving effect, it is preferable to have a Ni content of 0.01% or more, and more preferably 0.20% or more.

[0084] [Cr: 0.50% or less] Like Cu, Cr increases the strength of the first weld metal while maintaining its high toughness. However, if the Cr content exceeds 0.50%, the hardness increases and the toughness decreases. Therefore, the Cr content should be 0.50% or less. Preferably, the Cr content is 0.35% or less. On the other hand, to further enhance the aforementioned strength-improving effect, it is preferable to have a Cr content of 0.01% or more, and more preferably 0.10% or more.

[0085] [Mo: 0.50% or less] Like Cr, Mo increases the strength while ensuring high toughness of the first weld metal. However, if the Mo content exceeds 0.50%, the hardness increases and the toughness decreases. Therefore, the Mo content should be 0.50% or less. Preferably, the Mo content is 0.30% or less. On the other hand, to further enhance the strength-improving effect, it is preferable to have a Mo content of 0.01% or more, and more preferably 0.05% or more.

[0086] [N: 0.0100% or less] N is an element that is inevitably contained as an impurity, and if the N content exceeds 0.0100%, it leads to a decrease in toughness. Therefore, from the viewpoint of suppressing a decrease in toughness and weldability, the N content is limited to 0.0100% or less. Preferably, the N content is 0.0080% or less, and more preferably 0.0060% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0.0000%. However, reducing the excess N leads to a rise in the refining cost of the wire material. Therefore, from the viewpoint of cost, it is preferable that the N content be 0.0020% or more.

[0087] [O (Oxygen): 0.0100% or less] O (oxygen) is an element that is contained as an unavoidable impurity and has adverse effects such as forming oxides and becoming the starting point for fracture. For this reason, the O content is limited to 0.0100% or less. Preferably, the O content is 0.0080% or less, and more preferably 0.0070% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0.0000%. However, reducing the excess O leads to a rise in the refining cost of the wire material. Therefore, from the standpoint of cost, it is preferable that the O content be 0.0020% or more.

[0088] [Chemical Composition of the Second Weld Metal] As shown in Figures 3(a) and 3(b), the chemical composition of the second weld metal was determined by taking three samples from 0.5 mm below the surface of the steel plate in the center of the weld line of the second weld metal, analyzing their chemical composition, and taking the average of the three samples as the chemical composition of the second weld metal. If stable arc welding is performed, the value will be very close to the chemical composition of the second welding wire described above. The reasons for limiting each composition range are as follows.

[0089] [C: 0.01-0.04%] C must be present in a concentration of 0.01% or more to obtain the necessary strength for the second weld metal. However, in the as-welded state, if the C content exceeds 0.04%, it increases hardness and decreases toughness. Therefore, the C content is limited to the range of 0.01-0.04%. It is crucial that this upper limit of C content is less than the lower limit of C content of the first welding wire in order to limit the hardness of the surface layer of the second weld metal. The C content is preferably in the range of 0.02-0.04%, and more preferably in the range of 0.02-0.03%.

[0090] [Si: 0.02-0.90%] Si is an element necessary to ensure the strength of the second weld metal and to deoxidize the second weld metal. To obtain such effects, the Si content must be 0.02% or more. On the other hand, if the Si content exceeds 0.90%, it will degrade the toughness of the second weld metal. Therefore, the Si content should be limited to the range of 0.02-0.90%. Preferably, the Si content is in the range of 0.20-0.80%, and more preferably in the range of 0.40-0.70%.

[0091] [Mn: 0.50-1.80%] Mn is an element necessary to ensure the strength of the second weld metal and to deoxidize the second weld metal. To obtain such effects, the Mn content must be 0.50% or more. On the other hand, if the Mn content exceeds 1.80%, the toughness of the second weld metal will deteriorate significantly. Therefore, the Mn content should be limited to the range of 0.50-1.80%. Preferably, the Mn content is in the range of 0.70-1.50%, and more preferably in the range of 1.00-1.30%.

[0092] [P: 0.025% or less] Since P content exceeding 0.025% degrades the toughness of the second weld metal, the P content is limited to 0.025% or less. Preferably, the P content is 0.010% or less, and more preferably 0.003% or less. The lower limit of the P content is not particularly limited and may be 0.000%. Normally, since P is an element that is inevitably contained in steel material as an impurity, it may exceed 0.000% industrially. However, reducing the excess leads to a rise in the refining costs of steel and wire materials, so from a cost standpoint, a P content of 0.001% or more is acceptable.

[0093] [S: 0.035% or less] Since sulfur content exceeding 0.035% degrades the toughness of the second weld metal, the sulfur content is limited to 0.035% or less. Preferably, the sulfur content is 0.010% or less, and more preferably 0.003% or less. On the other hand, the lower limit of the sulfur content is not particularly limited and may be 0.000%. However, reducing excess sulfur leads to increased refining costs for steel and wire materials, so from a cost perspective, a sulfur content of 0.001% or more is acceptable.

[0094] [Al: 0.04-0.19%] From the viewpoint of deoxidizing the second weld metal, an Al content of 0.04% or more is necessary, while an Al content exceeding 0.19% will degrade the toughness of the second weld metal. Therefore, the Al content is limited to the range of 0.04-0.19%. Preferably, the Al content is in the range of 0.05-0.10%, and more preferably in the range of 0.06-0.08%.

[0095] [N: 0.0100% or less] N is an element that is inevitably contained as an impurity, and if the N content exceeds 0.0100%, it leads to a decrease in the toughness of the second weld metal. Therefore, from the viewpoint of suppressing a decrease in the toughness and weldability of the second weld metal, the N content is limited to 0.0100% or less. The N content is preferably 0.0080% or less, and more preferably 0.0060% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0.0000%. However, reducing the excess leads to a rise in the refining costs of steel and wire materials, so from the viewpoint of cost, an N content of 0.0010% or more is also acceptable.

[0096] [O (Oxygen): 0.0100% or less] O (oxygen) is an element that is contained as an unavoidable impurity and has adverse effects such as forming oxides and becoming the starting point for fracture. For this reason, the O content is limited to 0.0100% or less. Preferably, the O content is 0.0050% or less, and more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0.0000%. However, since reducing excess leads to a rise in the refining costs of steel and wire materials, from a cost standpoint, an O content of 0.0020% or more is also acceptable.

[0097] [Optional Composition of Weld Metal for Second Weld] The reasons for specifying each composition range for the optional composition of the weld metal for the second weld are as follows.

[0098] [Cu: 0.30% or less] Cu has the function of increasing the strength while maintaining the toughness of the second weld metal, but if the Cu content exceeds 0.30%, hot brittleness occurs and the surface properties deteriorate. For this reason, when Cu is included, it is preferable to keep the Cu content at 0.30% or less. It is more preferable to keep the Cu content at 0.25% or less. On the other hand, when Cu is included, in order to further exhibit the above-mentioned strength-improving effect, it is preferable to keep the Cu content at 0.10% or more, and more preferable to keep it at 0.20% or more.

[0099] [Ni: 0.40% or less] Ni has a similar function to Cu, increasing the strength of the second weld metal while maintaining its high toughness. However, if the Ni content exceeds 0.40%, the hardness increases too much. Therefore, when Ni is included, it is preferable to keep the Ni content at 0.40% or less. It is more preferable to keep the Ni content at 0.30% or less. On the other hand, when Ni is included, in order to further exhibit the aforementioned strength-improving effect, it is preferable to keep the Ni content at 0.01% or more, and more preferable to keep it at 0.10% or more.

[0100] [Cr: 0.20% or less] Like Cu, Cr increases the strength of the second weld metal while maintaining its high toughness. However, if the Cr content exceeds 0.20%, the hardness of the second weld metal increases too much. Therefore, when Cr is included, it is preferable to keep the Cr content at 0.20% or less. It is more preferable to keep the Cr content at 0.15% or less. On the other hand, when Cr is included, in order to further exhibit the aforementioned strength-improving effect, it is preferable to keep the Cr content at 0.01% or more, and more preferable to keep it at 0.10% or more.

[0101] [Mo: 0.20% or less] Like Cr, Mo increases the strength of the second weld metal while maintaining its high toughness. However, if the Mo content exceeds 0.20%, the hardness of the second weld metal increases too much. Therefore, when Mo is included, it is preferable to keep the Mo content at 0.20% or less. It is more preferable to keep the Mo content at 0.10% or less. On the other hand, when Mo is included, in order to further exhibit the aforementioned strength-improving effect, it is preferable to keep the Mo content at 0.01% or more, and more preferable to keep it at 0.05% or more.

[0102] [V: 0.40% or less] V increases the strength and toughness of the second weld metal, but if present in excess, it forms carbides and increases hardness. Therefore, when V is included, it is preferable to keep the V content at 0.40% or less. It is more preferable to keep the V content at 0.20% or less. On the other hand, when V is included, in order to further exhibit the aforementioned strength and toughness improvement effect, it is preferable to keep the V content at 0.01% or more, and more preferable to keep it at 0.05% or more.

[0103] [Ti: 0.190% or less] Ti is TiO ２Ti precipitates as a solid, and acts as a ferrite transformation nucleus in the second weld metal, contributing to increased toughness by suppressing the coarsening of austenite and providing a ferrite transformation nucleus. However, if the Ti content exceeds 0.190%, the amount of solid-solution Ti increases, and the toughness deteriorates. Therefore, when Ti is included, it is preferable to keep the Ti content below 0.190%. On the other hand, if Ti is included, its effect is small if the Ti content is less than 0.030%. Therefore, it is preferable to keep the Ti content above 0.030%. A Ti content in the range of 0.050 to 0.100% is more preferable.

[0104] [Zr: 0.15% or less] Zr is an element that has the effect of suppressing grain growth and contributes to improving the toughness of the second weld metal. However, if the Zr content exceeds 0.15% and is excessive, the amount of solid-solution Zr increases, which degrades the toughness. Therefore, when Zr is included, it is preferable that the Zr content be 0.15% or less. On the other hand, when Zr is included, such an effect is difficult to obtain if the Zr content is less than 0.01%. Therefore, it is preferable that the Zr content be 0.01% or more. A Zr content in the range of 0.03 to 0.10% is more preferable.

[0105] [Remaining Composition of Weld Metal from the Second Weld] The remaining chemical composition of the weld metal from the second weld, other than the chemical composition of the weld metal from the second weld described above, consists of Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the objective of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap. Examples of these unavoidable impurity elements include Sn, Sb, As, Pb, and Bi, and are permissible if their total is 0.10% or less. Furthermore, as long as the chemical composition of the second weld metal described above is satisfied, there is no prejudice that other unavoidable impurity elements may be included in the second weld metal, and such embodiments are also included within the technical scope of the present invention.

[0106] [Weaving Welding] In the welding method using flux-cored wire according to the present invention, it is preferable that at least one layer of the first weld metal or the second weld metal among the above multilayers is weaving welded. That is, it is preferable to apply weaving welding to at least one layer of the first weld metal or at least one layer of the second weld metal. Here, weaving welding refers to a welding technique in which the torch 10 used in gas metal arc welding is moved left and right with respect to the weld line (center position in the groove width direction) while advancing in the welding direction (direction perpendicular to the plane of the paper), as shown in Figure 5 for example. The weaving width W in each layer (length in the groove width direction of the range in which the torch is moved left and right) is the root gap G in the case of the first layer, and the distance W between the intersection point of the weld metal surface of the previous layer and both walls of the groove in the case of other layers. The above "at least one layer" means one layer or two or more layers. The weaving width W is not particularly specified. When n layers of welding are performed on a groove, if the width of the intersection between the weld metal surface and the groove is defined as Xn, it is preferable to control the weaving width in the (n+1)th layer of welding to a range of Xn-1 (mm) or more and Xn-4 (mm) or less. This is because controlling it within this range ensures that the groove wall surface can be reliably melted.

[0107] This allows for sufficient melting of the groove surface in each layer, promoting fusion with the weld metal. Furthermore, it is preferable because it evenly distributes the weld metal of each layer across the entire width of the groove, promoting the suppression of welding defects and improvement of the weld bead shape. Particularly in automated welding, one-layer, one-pass welding by weaving is useful for saving labor in welding setup, as it only requires aligning the welding torch's tracing line with the center of the groove.

[0108] [Mechanical Properties of Weld Metal in Welded Joints] The mechanical properties of the weld metal of gas shielded arc welded joints using flux-cored wire obtained by the present invention satisfy the tensile strength of 440 MPa or more of carbon steel material for low-temperature pressure vessels used in ammonia tanks (JIS G3126:2015). Furthermore, the mechanical properties preferably satisfy either condition 1 or condition 2 below. More preferably, both conditions are satisfied.

[0109] As mentioned above, "weld metal of a welded joint" does not distinguish between the first weld metal and the second weld metal, but uses them as a single integrated weld metal. (Condition 1) Absorbed energy of the Charpy impact test of the weld metal of a welded joint at -45°C ( Ｖ E －４５ (Condition 2) The hardness of the surface layer of the second weld metal (HV10) is 47 J or higher.

[0110] By satisfying condition 1 above, the low-temperature toughness of the ammonia tank can be ensured. By satisfying condition 2 above, the suppression of ammonia SCC can be expected.

[0111] The Charpy impact test under condition 1 above shall be carried out in accordance with the provisions of JIS Z 3128. The sampling locations for the test specimens (V-notches) are shown in Figures 2(a) and 2(b). The direction of the V-notch of the test specimen 5 shall be perpendicular to the surface of the steel material 1, and the position a of the V-notch shall be such that the center line of the test specimen 5 is at the center of the weld line of the weld metal 4. Three test specimens 5 shall be taken from a position 0.5 mm below the surface of the steel material 1. The Charpy impact test shall be performed on the three test specimens 5, and the absorbed energy (vE) at a test temperature of -45°C shall be measured. －４５ The weld metal value of the weld joint is determined and its average value is taken as the value of the weld metal of the welded joint. Figure 2(b) shows the position where the test piece is taken from 0.5 mm below the surface (top surface) of the 38 mm thick steel material 1. This is because the top surface of the steel plate in Figure 2(b) is the inside of the tank and is the surface that comes into contact with ammonia.

[0112] Furthermore, the average value of the hardness (HV10) of the surface layer of the second weld metal under condition 2 above is determined using a Vickers hardness tester in the following manner. As shown in Figures 3(a) and 3(b), in the cross-section of the weld, measurement positions b are set at 1 mm intervals along a straight line parallel to the surface of the steel material, at a depth of 0.5 mm from the surface of the steel material. The Vickers hardness measured at measurement position b with a test force of 10 kgf is taken as the hardness (HV10) of the surface layer, and these measured values ​​are averaged to obtain the hardness value of the surface layer of the second weld metal. Note that in Figure 3(b), measurement positions b are set at 1 mm intervals along a straight line parallel to the surface of the steel material, at a depth of 0.5 mm from the surface (top surface) of the 38 mm thick steel material 1. This is because the top surface of the steel plate in Figure 3(b) is the inside of the tank and the surface that comes into contact with ammonia.

[0113] The reasons for limiting the composition ranges of the carbon steel materials for cryogenic pressure vessels (JIS G3126:2015) used in ammonia tanks, which are the subject of this invention, are as follows.

[0114] [Basic Composition of Steel Materials] The reasons for specifying the basic composition of steel materials are as follows.

[0115] [C: 0.030-0.090%] C must be present in a concentration of 0.030% or more to obtain the strength required for structural steel. However, a C content exceeding 0.090% can increase hardness, which may result in a decrease in toughness. Therefore, the C content should be limited to the range of 0.030-0.090%. A C content of 0.040-0.080% is preferred, and a range of 0.050-0.070% is more preferred.

[0116] [Si: 0.50% or less] Si is an element that acts as a deoxidizing agent, but it is also an element that causes a decrease in toughness and weldability. Therefore, it is desirable to keep the Si content as low as possible, but since a Si content of 0.50% or less is acceptable, the Si content is limited to 0.50% or less. In addition, since deoxidation of steel is also possible with Al and Ti, the lower limit of the Si content is not particularly limited and may be 0%. From the viewpoint of toughness and weldability, it is preferable that the Si content be 0.45% or less, and more preferably 0.40% or less. On the other hand, when used as a deoxidizing agent, it is preferable that the Si content be 0.01% or more, and more preferably 0.10% or more, in order to exert its effect more effectively.

[0117] [Mn: 0.50-1.60%] Mn is an element that increases the hardenability of steel and is one of the important elements that must be included in order to satisfy high strength. To obtain the above effect, the Mn content is limited to 0.50% or more. From the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost, it is preferable to have a Mn content of 0.70% or more, and more preferable to have a Mn content of 0.90% or more. On the other hand, if the Mn content exceeds 1.60%, the strength becomes excessively high, and toughness and weldability decrease, in addition to the alloy cost becoming excessively high. For this reason, the Mn content is limited to 1.60% or less. From the viewpoint of suppressing the decrease in toughness and weldability, it is preferable to have a Mn content of 1.50% or less, and more preferable to have a Mn content of 1.40% or less.

[0118] [P: 0.015% or less] P is an element that is inevitably contained as an impurity and has adverse effects such as reducing toughness and weldability by segregating at grain boundaries. For this reason, it is desirable to keep the P content as low as possible, but since it is acceptable if it is 0.015% or less, the P content is limited to 0.015% or less. The P content is preferably 0.010% or less, and more preferably 0.003% or less. The lower limit of the P content is not particularly limited and may be 0%, but since P is usually an element that is inevitably contained in steel as an impurity, it may be greater than 0% industrially. Reducing excess P leads to a rise in refining costs, so it is preferable to have a P content of 0.001% or more.

[0119] [S: 0.010% or less] S is an element that is inevitably contained as an impurity. It exists in steel as sulfide-based inclusions such as MnS and has adverse effects, such as being the starting point for fracture. Therefore, it is desirable to keep the S content as low as possible, but since it is acceptable if it is 0.010% or less, the S content is limited to 0.010% or less. The S content is preferably 0.008% or less, and more preferably 0.006% or less. The lower limit of the S content is not particularly limited and may be 0%, but since S is usually an element that is inevitably contained in steel as an impurity, it may be greater than 0% industrially. Reducing excess S leads to a rise in refining costs, so it is preferable to have an S content of 0.001% or more.

[0120] [Al: 0.060% or less] Al is an element that acts as a deoxidizing agent and also refines the crystal grains. However, if the Al content exceeds 0.060%, oxide inclusions increase, reducing cleanliness and thus toughness. Therefore, the Al content should be limited to 0.060% or less. The Al content should preferably be 0.050% or less, and more preferably 0.040% or less. On the other hand, in order to exhibit a greater deoxidizing effect, the Al content should preferably be 0.010% or more, and more preferably 0.020% or more.

[0121] [N: 0.0010 to 0.0100%] N combines with Ti to precipitate as TiN, contributing to the refinement of the structure and improving toughness. To obtain this effect, the N content should be limited to 0.0010% or more. Preferably, the N content should be 0.0020% or more, and more preferably 0.0030% or more. On the other hand, if the N content exceeds 0.0100%, it can actually lead to a decrease in toughness. Therefore, from the viewpoint of suppressing a decrease in toughness and weldability, the N content should be limited to 0.0100% or less. Preferably, the N content should be 0.0080% or less, and more preferably 0.0060% or less.

[0122] [O (Oxygen): 0.0100% or less] O (oxygen) is an element that is contained as an unavoidable impurity and has adverse effects such as forming oxides and becoming the starting point for fracture; therefore, the O content is limited to 0.0100% or less. Preferably, the O content is 0.0050% or less, and more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0%, however, since O is usually an element that is unavoidably contained in steel as an impurity, it may be more than 0% industrially. Reducing the excess O leads to a rise in the refining cost of steel materials; therefore, from a cost standpoint, it is preferable that the O content be 0.0020% or more.

[0123] [Optional Composition of Steel Materials] Next, we will explain the reasons for specifying each composition range for optional composition of steel materials.

[0124] [Cu: 2.00% or less] Cu is an element that increases the hardenability of steel and improves the strength of steel sheets. It can be included at will, but if the Cu content exceeds 2.00%, it will lead to a deterioration of toughness and an increase in alloy costs. For this reason, when Cu is included, it is preferable to keep the Cu content at 2.00% or less. It is more preferable to keep the Cu content at 1.00% or less, and even more preferable to keep it at 0.50% or less. On the other hand, when Cu is included, in order to further exhibit the aforementioned strength-improving effect, it is preferable to keep the Cu content at 0.01% or more, and more preferable to keep it at 0.10% or more.

[0125] [Ni: 2.00% or less] Ni, like Cu, is an element that improves the strength of steel plates and can be included as desired. However, if the Ni content exceeds 2.00%, it will lead to deterioration of weldability and an increase in alloy costs. Therefore, when Ni is included, it is preferable to keep the Ni content at 2.00% or less. It is more preferable to keep the Ni content at 1.00% or less. On the other hand, when Ni is included, in order to further exhibit the aforementioned strength-improving effect, it is preferable to keep the Ni content at 0.01% or more, and more preferable to keep it at 0.20% or more.

[0126] [Cr: 1.00% or less] Cr, like Cu, is an element that improves the strength of steel plates and can be included at will. However, if the Cr content exceeds 1.00%, it will lead to deterioration of weldability and an increase in alloy costs. Therefore, when Cr is included, it is preferable to keep the Cr content at 1.00% or less. It is more preferable to keep the Cr content at 0.50% or less. On the other hand, when Cr is included, in order to further exhibit the aforementioned strength-improving effect, it is preferable to keep the Cr content at 0.01% or more, and more preferable to keep it at 0.05% or more.

[0127] [Mo: 1.00% or less] Mo, like Cu, is an element that improves the strength of steel plates and can be included as desired. However, if the Mo content exceeds 1.00%, it will lead to deterioration of weldability and an increase in alloy costs. Therefore, when Mo is included, it is preferable to keep the Mo content at 1.00% or less. It is more preferable to keep the Mo content at 0.50% or less. On the other hand, when Mo is included, in order to further exhibit the aforementioned strength-improving effect, it is preferable to keep the Mo content at 0.01% or more, and more preferable to keep it at 0.05% or more.

[0128] [V: 1.00% or less] V can be included as desired. V has the effect of suppressing grain coarsening by forming carbides and nitrides, but if the V content exceeds 1.00%, there is a risk of hardening the heat-affected zone and degrading the toughness. For this reason, when V is included, it is preferable to keep the V content at 1.00% or less. It is more preferable to keep the V content at 0.10% or less. On the other hand, when V is included, in order to further exhibit the grain coarsening suppression effect mentioned above, it is preferable to keep the V content at 0.01% or more, and more preferable to keep it at 0.02% or more.

[0129] [Nb: 0.10% or less] Nb can be included as desired. Like V, Nb has the effect of suppressing grain coarsening by forming carbides and nitrides, but if the Nb content exceeds 0.10%, there is a risk of hardening the heat-affected zone and degrading the toughness. For this reason, when Nb is included, it is preferable to keep the Nb content at 0.10% or less. It is more preferable to keep the Nb content at 0.05% or less. On the other hand, when Nb is included, in order to further exhibit the grain coarsening suppression effect mentioned above, it is preferable to keep the Nb content at 0.01% or more, and more preferable to keep it at 0.02% or more.

[0130] [Ti: 0.005 to 0.100%] Ti can be included as desired. Ti is an element that has a strong tendency to form nitrides and has the effect of fixing N and reducing solid-solution N. Therefore, the inclusion of Ti can improve the toughness of the base material and the weld. To obtain this effect, when Ti is included, it is preferable that the Ti content be 0.005% or more. It is more preferable that the Ti content be 0.012% or more. On the other hand, if the Ti content exceeds 0.100%, the toughness will actually decrease. Therefore, when Ti is included, it is preferable that the Ti content be 0.100% or less. It is more preferable that the Ti content be 0.090% or less, and even more preferable that be 0.080% or less.

[0131] [Ca: 0.2000% or less] Ca can be included as desired. Ca is an element that has the effect of improving toughness by fixing S. However, if the Ca content exceeds 0.2000%, this effect saturates. Therefore, when Ca is included, it is preferable that the Ca content be 0.2000% or less. It is more preferable that the Ca content be 0.0300% or less. On the other hand, in order to further exhibit the toughness improvement effect mentioned above, when Ca is included, it is preferable that the Ca content be 0.0005% or more. It is more preferable that the Ca content be 0.0010% or more.

[0132] [Mg: 0.0200% or less] Mg can be included as desired. Like Ca, Mg is an element that combines with S and suppresses the formation of MnS and other elements that elongate in the rolling direction. Therefore, by including Mg, the morphology of sulfide inclusions can be controlled so that they appear spherical, thereby improving the toughness of welds and other parts. However, if the Mg content exceeds 0.0200%, the cleanliness of the steel decreases. A decrease in cleanliness leads to deterioration of surface properties due to an increase in surface defects and a decrease in bendability. For this reason, when Mg is included, it is preferable to keep the Mg content at 0.0200% or less. It is more preferable to keep the Mg content at 0.0100% or less. On the other hand, in order to further exhibit the toughness improvement effect mentioned above, when Mg is included, it is preferable to keep the Mg content at 0.0005% or more. It is more preferable to keep the Mg content at 0.0010% or more.

[0133] [B: 0.0100% or less] B can be included as desired. B is an element that significantly improves hardenability even in trace amounts. Therefore, it can improve the strength of the steel plate, but if the B content exceeds 0.0100%, the weldability decreases. For this reason, when B is included, it is preferable to keep the B content at 0.0100% or less. It is more preferable to keep the B content at 0.0050% or less, and even more preferable to keep it at 0.0030% or less. On the other hand, in order to further exhibit the above-mentioned strength-improving effect, when B is included, it is preferable to keep the B content at 0.0001% or more. It is more preferable to keep the B content at 0.0005% or more, and even more preferable to keep it at 0.0010% or more.

[0134] [Remaining Composition of Steel Material] The remaining chemical composition of the steel material, other than the chemical composition described above, consists of Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap. Examples of these unavoidable impurity elements include Sn, Sb, As, Pb, and Bi, and are permissible if their total is 0.10% or less. Furthermore, as long as the above-mentioned basic composition and optional selected composition are satisfied, there is no prejudice to the inclusion of other unavoidable impurity elements, and such embodiments are also included within the technical scope of the present invention.

[0135] [Method for Manufacturing Gas Shielded Arc Welded Joints] In the present invention, gas shielded arc welded joints can be manufactured using the gas shielded arc welding method of the present invention described above. The welded joint obtained by this manufacturing method has the weld metal described above. As a result, the welded joint has the mechanical properties of the weld metal described above, and can therefore ensure toughness and strength equivalent to that of the base material. Furthermore, by reducing the hardness of the surface layer of the weld metal that comes into contact with ammonia, it has high ammonia SCC resistance performance.

[0136] The present invention will be described below based on examples. However, the following examples are merely illustrative and intended to explain the present invention in more detail, and do not limit the scope of the rights of the present invention.

[0137] In this example, two types of 15 mm thick steel material 1, as shown in Table 1, were used: steel material symbols a and b. A V-groove 2 (groove angle θ: 50°) and a root gap G of 3 mm to 5 mm were provided for the steel material 1 as shown in Figure 1(a), and the two pieces were butted together in accordance with JIS Z 3111:2005. A copper backing plate 3 was used, and gas shielded arc welding was performed using flux-cored wire.

[0138] Furthermore, in the example of the 38 mm thick steel material 1, the steel material symbol c shown in Table 1 was used. The X groove 2 (groove angle θ on the front side of the steel material) of the steel material 1 is shown in Figure 1(b). １ : 50°, bevel angle θ on the back side of the steel material ２ The material was processed to a 60° angle, and double-sided welding was performed using a flux-cored wire. Here, as shown in Figure 1(b), the groove depth t1 on the front side of the steel material was 20 mm, and the groove depth t2 on the back side of the steel material was 17 mm. In this example of the present invention, the final layer refers to the final layer of the multi-layer welding on the side that comes into contact with ammonia, etc. (i.e., the front side of the steel plate, corresponding to the inner surface of the tank), as shown in Figures 2(b) and 3(b). For the final layer of the multi-layer welding on the back side of the steel material (corresponding to the outer surface of the tank), the first welding wire was used as is, and the second welding wire was not used.

[0139] Steel material symbol a is TS440MPa grade steel, and steel material symbol b is TS490MPa grade steel. The tensile strength of the welded joints of these steels is required to be the same as that of the base material, 440MPa and 490MPa grade, respectively. Steel material symbol c is TS440MPa grade steel, and its welded joint is required to be the same as that of the base material, 440MPa grade. The tensile strength of the welded joints can be measured by the method described in the examples below.

[0140]

[0141] Next, three types of flux-cored wire (1.2 mm in diameter) shown in Table 2 were used as welding wires. The chemical composition in Table 2 refers to the chemical composition of the steel sheath of the flux-cored wire. As mentioned above, the flux filled into the flux-cored wire is used to stabilize the arc during welding and suppress sputtering. Therefore, it contains Fe oxide, Si oxide, Mn oxide, Al oxide, Ti oxide, Ca oxide, Mg oxide, Zr oxide, and other fluorides and sulfides. Furthermore, the above flux does not contain metal powder.

[0142] Welding wire symbol A is generally used for steel materials with strength levels of steel material symbols a and b, and corresponds to the first welding wire. Welding wire symbols B and C correspond to the second welding wire, and their carbon content is lower than that of welding wire symbol A. Welding wire symbols B and C have the basic composition and optional composition (Mo, Ti, Zr, etc.) of the second welding wire.

[0143]

[0144] Using the steel materials and welding wires described above, multi-layer welding was performed from one side without preheating and in a downward position, under the various welding conditions shown in Tables 3 to 5-2. The conditions for weaving welding are also shown in Tables 3 to 4-2.

[0145] Specifically, for steel plates with a thickness of 15 mm, symbols a and b, welded joints (welded joints No. 1 to 8) were fabricated by arc welding the surface side of the steel material (corresponding to the inner surface side of the tank) using various welding conditions shown in Tables 3 and 5-1, without preheating and in a downward position.

[0146] Also, for the steel material symbol c with a plate thickness of 38 mm, without preheating and in a downward posture, the back side of the steel material (corresponding to the outer side of the tank) was first arc welded under the various welding conditions shown in Tables 4-1, 4-2, and 5-2. At that time, the first welding wire symbol A was used from the first layer to the final layer (the sixth layer). Next, the front side of the steel material (corresponding to the inner side of the tank) was arc welded. At that time, the first welding wire symbol A was continuously applied from the first layer to the seventh layer. In the example of the present invention, the second welding wire symbol B was applied to the final layer (the eighth layer) (weld joint No. 10). In the comparative example, the first welding wire symbol A was directly applied to the final layer (the eighth layer) as it was (weld joint No. 9). The number of welding passes was 1 or 2 passes for each layer. Note that the number of layers of weld joints No. 9 and 10 was 14 layers in total as shown in Tables 4-1 and 4-2.

[0147] Note that the shielding gas was CO ２ gas at 100% by volume, and the gas flow rate was set to 10 L / min to 25 L / min. The inter-pass temperature was 200°C.

[0148]

[0149]

[0150]

[0151]

[0152]

[0153] Also, Tables 6-1 and 6-2 show the evaluation results of the mechanical properties of the following described weld metals in addition to the same welding conditions. Note that in the "Welding Wire Symbol" column shown in Table 6-2, the type of welding wire used for welding the front side of the steel material is indicated. Tables 7 and 8 show the chemical compositions of the first and second weld metals.

[0154]

[0155]

[0156]

[0157]

[0158] The methods for conducting tensile tests on the joints, Charpy impact tests on the weld metal, and measuring the Vickers hardness of the surface layer of the second weld metal, which were performed on each of the obtained weld joints, are as follows.

[0159] The joint tensile test was conducted in accordance with JIS Z 3121:2013, using full-thickness test specimens with the excess weld material removed. Three such full-thickness test specimens were taken. The average value of the tensile tests from these three joints was calculated and used as the tensile strength value of the welded joint.

[0160] The Charpy impact test was conducted in accordance with the provisions of JIS Z 3128:2017. The sampling locations of the test specimens (V-notches) are shown in Figures 2(a) and 2(b). In Figure 2(a), the direction of the V-notch of test specimen 5 is perpendicular to the front surface of the steel material 1. In the X-groove shown in Figure 2(b), the direction of the V-notch of test specimen 5 is perpendicular to the front surface of the steel material (i.e., corresponding to the inside of the tank). Furthermore, the position a of the V-notch was set to be at the center of the weld line of the weld metal 4 of the welded joint on the center line of test specimen 5, and three test specimens 5 were taken from a position 0.5 mm below the front surface of the steel material 1. The Charpy impact test was performed on all three specimens, and the absorbed energy (vE) at the test temperature: -45°C was measured. －４５ The values ​​of the weld metal for the welded joint were calculated and their average values ​​were taken as the weld metal values ​​for the welded joint.

[0161] Furthermore, a Vickers hardness tester was used for the Vickers hardness test of the surface layer. As shown in Figures 3(a) and 3(b), in the cross-section of the weld, measurement positions b were set at 1 mm intervals along a straight line parallel to the surface of the front side of the steel material 1, at a depth of 0.5 mm from the front surface of the steel material 1. Note that the "surface" in the X groove of Figure 3(b) refers to the surface of the front side of the steel material (i.e., corresponding to the inside of the tank). The Vickers hardness measured at measurement position b with a test force of 10 kgf was taken as the hardness of the surface layer (HV10). An example of the measurement results of the hardness of the surface layer (HV10) is shown in Figure 4. Figure 4 shows the individual values ​​measured for welded joint No. 4 (example of the present invention) and welded joint No. 1 (comparative example). The average value of these measured values ​​was calculated and taken as the average value of the hardness of the surface layer (HV10). In joint No. 4, the average hardness (HV10) of the surface layer was 211, which is below 220, so it is considered a joint that is less likely to generate ammonia SCC. In welded joint No. 1, the average hardness (HV10) of the surface layer was 255, which is above 220, so there is concern about the generation of ammonia SCC.

[0162] [Evaluation Results] As shown in Tables 6-1 and 6-2, the fracture location of the joint tensile test specimens is in the base metal in all welded joints, indicating that the weld metal of the welded joint has a tensile strength exceeding that of the base metal.

[0163] Welded joints No. 4, 7, 8, and 10 (examples of the present invention) are joints in which the first layer to the layer before the final layer is welded with welding wire symbol A (first welding wire), and the final layer is welded with welding wire symbol B (second welding wire).

[0164] Furthermore, welded joint No. 8 (example of the present invention) is a joint in which the first layer to the layer before the final layer is welded with welding wire symbol A (first welding wire), and the final layer is welded with welding wire symbol C (second welding wire). In all of these cases, the average value of the hardness of the surface layer (HV10) satisfies the standard value (220 or less), and the absorbed energy (vE －４５ The value (47 J or higher) met the standard.

[0165] On the other hand, in welded joints No. 1 and 5 (comparative example), welding was performed from the first layer to the final layer using welding wire symbol A (first welding wire), so the average value of the hardness (HV10) of the surface layer and the absorbed energy ( Ｖ E －４５ None of the above met the standard values.

[0166] Welded joints No. 2 and 6 (comparative examples) were welded from the first to the final layer using welding wire symbol B (second welding wire) with low levels of carbon and alloying elements, resulting in an average hardness (HV10) of 220 or less at the surface. However, the reheated areas became brittle, resulting in a significant decrease in absorbed energy, which did not meet the standard value.

[0167] Welded joint No. 3 (comparative example) has the welding wire combination reversed from welded joint No. 4 (example of the present invention), but the average value of the hardness of the surface layer (HV10) and the absorbed energy ( Ｖ E －４５ None of the above met the standard values.

[0168] Welded joint No. 9 (comparative example) was welded in two passes for the final layer, resulting in a mixture of passes that received reheating and passes that remained as welded. As a result, the average hardness (HV10) of the surface layer exceeded 220, failing to meet the standard value. Also, absorbed energy ( Ｖ E －４５ None of the above met the standard values.

[0169] 1. Base material (steel) 2. V-groove, X-groove 3. Backing plate 4. Weld metal 5. Charpy impact test specimen 10. Torch a. Notch position b. Measurement position for Vickers hardness test t. Plate thickness t １ ,t ２ Groove depth R, root length θ, θ １ , θ ２ Bevel angle W Weaving width

Claims

1. A gas shielded arc welding method for gas-shielded arc welding steel materials using two types of flux-cored wires with different chemical compositions of steel sheaths, wherein the two types of flux-cored wires are a first welding wire and a second welding wire, and the first welding wire and the second welding wire are used in this order, and the first welding wire is used to gas-shielded arc weld to form a first weld metal, and then the second welding wire is placed on top of the first weld metal and gas-shielded arc weld to form a second weld metal, thereby forming the weld metal of a gas-shielded arc welded joint, wherein the chemical composition of the steel sheath of the first welding wire is, in mass%, C: 0.05 to 0.11%, Si: 0.30 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04 to 0.19%, A gas shielded arc welding method comprising: a welding wire containing Cu: 0.60% or less, Ni: 1.60% or less, Cr: 0.50% or less, Mo: 0.50% or less, N: 0.0100% or less, and O: 0.0100% or less, with the remainder being Fe and unavoidable impurities; and a second welding wire steel sheath whose chemical composition, by mass%, contains C: 0.01 to 0.04%, Si: 0.02 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04 to 0.19%, N: 0.0100% or less, and O: 0.0100% or less, with the remainder being Fe and unavoidable impurities.

2. The gas shielded arc welding method according to claim 1, wherein the chemical composition of the steel sheath of the second welding wire further contains one or more selected by mass% from Cu: 0.30% or less, Ni: 0.40% or less, Cr: 0.20% or less, Mo: 0.20% or less, V: 0.40% or less, Ti: 0.190% or less, and Zr: 0.15% or less.

3. The gas shielded arc welding method according to claim 1 or 2, wherein at least one layer of the second weld metal is weaving weld.

4. A method for manufacturing a gas shielded arc welded joint produced by the gas shielded arc welding method described in claim 1 or 2, wherein the chemical composition of the first weld metal is, in mass%, C: more than 0.05% and 0.11%, Si: 0.30 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04 to 0.19%, Cu: 0.60% or less, Ni: 1.60% or less, Cr: 0.50% or less, Mo: 0.50% or less, N: 0.0100% or less, O: 0.0100% or less, with the remainder being Fe and unavoidable impurities, and the chemical composition of the second weld metal is, in mass%, C: 0.01 to 0.04%, A method for manufacturing a gas shielded arc welded joint, comprising: Si: 0.02-0.90%, Mn: 0.50-1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04-0.19%, N: 0.0100% or less, O: 0.0100% or less, with the remainder being Fe and unavoidable impurities.

5. A method for manufacturing a gas shielded arc welded joint produced by the welding method described in claim 3, wherein the chemical composition of the first weld metal is, in mass%, C: more than 0.05% and 0.11%, Si: 0.30 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04 to 0.19%, Cu: 0.60% or less, Ni: 1.60% or less, Cr: 0.50% or less, Mo: 0.50% or less, N: 0.0100% or less, O: 0.0100% or less, with the remainder being Fe and unavoidable impurities, and the chemical composition of the second weld metal is, in mass%, C: 0.01 to 0.04%, Si: 0.02 to 0.90%, A method for manufacturing a gas shielded arc welded joint, comprising: Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04 to 0.19%, N: 0.0100% or less, O: 0.0100% or less, with the remainder being Fe and unavoidable impurities.

6. The method for manufacturing a gas shielded arc welded joint according to claim 4, wherein the chemical composition of the second weld metal further contains one or more selected by mass% from Cu: 0.30% or less, Ni: 0.40% or less, Cr: 0.20% or less, Mo: 0.20% or less, V: 0.40% or less, Ti: 0.190% or less, and Zr: 0.15% or less.

7. The method for manufacturing a gas shielded arc welded joint according to claim 5, wherein the chemical composition of the second weld metal further contains one or more selected by mass% from Cu: 0.30% or less, Ni: 0.40% or less, Cr: 0.20% or less, Mo: 0.20% or less, V: 0.40% or less, Ti: 0.190% or less, and Zr: 0.15% or less.

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