TIG welding method

Abstract

A TIG welding method with excellent SCC resistance to ammonia is provided. The TIG welding method of the present invention is a method for multi-layer welding of steel materials, in which a first welding wire having a specific chemical composition and a second welding wire having another specific chemical composition with a lower C content than the chemical composition of the first welding wire are used, and the first and subsequent layers are welded with the first welding wire, and the final layer is welded with the second welding wire.

Description

[Technical Field]

[0001] The present invention relates to a TIG welding method for steel materials, and more particularly to a TIG welding method for forming a welded joint that has excellent SCC resistance to ammonia. [Background technology]

[0002] The TIG welding method uses tungsten, a non-consumable material, as an electrode rod, and welds steel by melting the welding wire (i.e., filler metal) in the arc while blowing inert gases, generally argon gas or helium gas, onto the steel to block the atmosphere.

[0003] Ammonia is relatively easy to liquefy under normal pressure, and as such, it has attracted attention as a hydrogen carrier for realizing a decarbonized society. Technological development is underway to co-combine ammonia with coal in thermal power plants and as a fuel for ships. Using ammonia as a fuel for thermal power plants requires larger ammonia storage tanks. Because ammonia is highly toxic, extreme care must be taken to prevent leakage from the tanks. The carbon steel used in the construction of ammonia tanks is susceptible to stress corrosion cracking (hereinafter referred to as "SCC") in liquid ammonia, and the exact mechanism behind this cracking has not yet been elucidated.

[0004] However, it is believed that there is a correlation between the occurrence of cracks and the surface hardness (HV10), which is the Vickers hardness of the surface layer of steel and weld metal. When the average surface hardness (HV10) of these and the ammonia contact surface exceeds 220, the occurrence of SCC increases significantly, and it is known that cracks are particularly common near the weld metal.

[0005] Ammonia is mainly used in the production of chemical fertilizers. The largest tank in Japan has a capacity of about 15,000 tons, and due to the required strength level, SLA325AN, a carbon steel plate for low-temperature pressure vessels that has been normalized, is often used as the material for the tank. In addition, during welding, the general 50 kgf / mm 2 Welding materials with a strength of 490 MPa are used, and care is taken not to reduce the hardness of the weld. As a result, the weld metal tends to harden, with an average surface hardness (HV10) of around 250 to 280. For this reason, liquid ammonia tanks are subject to periodic open inspections, and cracks are detected and repaired during the inspections.

[0006] In response to this situation, for example, Patent Document 1 discloses an invention for a steel plate that aims to limit yield strength and improve low-temperature toughness by adjusting the chemical composition and improving the manufacturing method. However, Patent Document 1 does not consider a method for suppressing SCC in weld metal joints, where SCC caused by ammonia is particularly likely to occur.

[0007] Furthermore, as a TIG welding method using a low-carbon welding wire, for example, Patent Documents 2 and 3 disclose that the strength and toughness of the weld metal are improved by using a welding wire having a specific chemical composition.

[0008] Patent Document 4 also discloses that in welding austenitic stainless steel pipes, austenitic stainless steel weld metal is joined to the root side of the groove, and martensitic stainless steel weld metal is joined to the remaining groove above it. This joining hardens the surface layer of the weld metal, suppresses distortion on the inner surface of the pipe, and improves fatigue strength. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 5428999 [Patent Document 2] Japanese Patent Publication No. 57-159293 [Patent Document 3] Patent No. 6829111 [Patent Document 4] Japanese Patent Application Publication No. 53-108838 Summary of the Invention [Problem to be solved by the invention]

[0010] However, these welded joints using conventional carbon steel materials and welding wires inevitably harden the weld metal surface regardless of the welding conditions, resulting in the problem of SCC. Because it is difficult to completely prevent SCC, tanks must be periodically inspected, and welding repairs are performed whenever cracks are discovered during inspections. Ammonia is expected to become an important fuel in the future, and to ensure a stable energy supply, large amounts of ammonia will need to be stored, necessitating the need for larger ammonia land-based tanks. Increasing the tank size using conventional welding techniques will likely increase the risk of defects such as SCC due to ammonia, and welding repairs will require significant effort. Therefore, there is an urgent need to develop welded joints with high SCC resistance. Multi-layer welding using welding wire with low alloying elements reduces the hardness of the weld metal surface, but the heat input must be limited to prevent a decrease in joint strength and toughness, limiting construction efficiency.

[0011] The present invention aims to provide an economically advantageous TIG welding method that uses appropriate welding conditions to suppress the average surface hardness (HV10) of the weld metal in multi-layer welding to 220 or less, thereby making ammonia SCC less likely to occur. [Means for solving the problem]

[0012] The present inventors have conducted extensive research to achieve the above object and have obtained the following findings.

[0013] In typical multi-layer welding, unless the steel is a combination of multiple steel materials such as clad steel or an austenitic stainless steel pipe (see, for example, Patent Document 4 mentioned above), welding is performed from the first layer to the last layer using a single type of welding wire selected according to the strength grade of the steel. Depending on the chemical composition of the welding wire, some welding wires improve the mechanical properties of the weld metal when reheated, while others have good mechanical properties in the as-welded state but deteriorate when reheated. Meanwhile, we focused on the fact that the weld metal portion of a multi-layer welded joint has both reheated and as-welded portions that are not reheated. That is, from the first layer to the layer before the last layer, the weld metal is cooled to room temperature after the weld metal from the welding wire melts and solidifies, and then reheated by subsequent welding. In contrast, for the final layer, once the weld metal melts and solidifies and cools to room temperature, the weld metal is used as is for the structure without reheating, unless post-weld heat treatment is performed.

[0014] Because the final layer and the other layers (i.e., the layers from the first layer to the layer before the final layer) are subjected to different thermal histories, when a joint is produced using a single type of welding material, there will inevitably be a portion in which the mechanical properties deteriorate due to the thermal history. By adding alloying elements such as Ni and Mo to the welding wire, it is possible to obtain good mechanical properties in both the as-welded portion that is not reheated and the portion that is reheated, but such alloying elements are expensive, which leads to a rise in the cost of the welding wire.

[0015] Based on the above-mentioned investigations, the present inventors have arrived at a welding method that can achieve both limited surface layer hardness and good low-temperature toughness in the weld metal zone by using different welding wires for the portion that is subjected to reheating in the welding of carbon steel and the final layer that is not subjected to reheating, that is, by using a combination of inexpensive welding wires. [1] A TIG welding method for forming a multi-layer weld metal on a steel material using a shielding gas made of an inert gas and a welding wire, the welding wire includes a first welding wire and a second welding wire having different chemical compositions; The chemical composition of the first welding wire is, in mass%, C: 0.05~0.11%, 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: 0.80% or less, Cr: 0.50% or less, Mo: 0.50% or less N: 0.0100% or less, and O: 0.0100% or less and the balance being Fe and unavoidable impurities, The chemical composition of the second welding wire is, in mass%, 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 and the balance being Fe and unavoidable impurities, The first welding wire is used for welding the first layer and subsequent layers of the multi-layer structure, the second welding wire is used to weld the final layer of the multi-layer structure; TIG welding method. [2] In addition to the chemical composition of the second welding wire, the second welding wire further contains, in mass%, 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: 0.030~0.190% Zr: 0.01 to 0.15%, and Nb: 0.001 to 0.015% Contains one or more selected from the following: The TIG welding method described in [1] above. [3] The TIG welding method according to [1] or [2], wherein the welding of at least one layer of the multiple layers is weaving welding. [4] The TIG welding method according to any one of [1] to [3], wherein at least one layer of the multiple layers is welded by hot wire welding. [5] The absorbed energy (vE -45 ) is 47 J or more. [6] The TIG welding method according to any one of [1] to [5], wherein the average surface hardness (HV10) of the weld metal is 220 or less. [Effects of the Invention]

[0016] The TIG welded joints obtained by the present invention can be used for the welded joints of land-based ammonia tanks, and they exhibit high mechanical properties and high SCC resistance while using inexpensive welding materials. This reduces the frequency of repairs for cracks inside the tank, and is an industrially significant advantage. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an example of a groove shape in the TIG welding method of the present invention. [Figure 2] FIG. 2 is a schematic cross-sectional view showing an example of a position where a test piece is taken for a Charpy impact test. [Figure 3] FIG. 3 is a schematic cross-sectional view showing an example of measurement positions in a Vickers hardness test. [Figure 4] FIG. 4 is a distribution diagram showing the results of a measurement example of the Vickers hardness test in this example. [Figure 5] FIG. 5 is a schematic diagram showing an example of weaving welding. [Figure 6]FIG. 6 is a schematic diagram showing an example of hot wire welding. DETAILED DESCRIPTION OF THE INVENTION

[0018] [Carbon steel] In the present invention, TIG welding is preferably performed on a base material made of carbon steel for an ammonia tank. The steel may be a steel plate, and preferred examples include carbon steel plates for low-temperature pressure vessels such as SLA325AN, SLA325ATMC, and SLA365.

[0019] The base material has a chemical composition that 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. It also 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 %" (hereinafter the same applies unless otherwise specified).

[0020] The base material may further contain one or more of the following component elements as needed. Since these may be contained as needed, the content of the following component elements may be 0%. That is, the base metal may contain one or more elements selected from 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, as needed.

[0021] The remainder of the base material's chemical composition other than the above-mentioned chemical composition consists of Fe and unavoidable impurities, such as Sn, Sb, As, Pb, and Bi, and the total content of these elements is allowable up to 0.10%.

[0022] [Multi-layer welded metal] In the present invention, the weld metal formed by welding the above-mentioned steel materials must be multi-layered, because if the weld metal is single-layered, there are no second or subsequent layers, and a second welding wire cannot be used, and therefore the effects of the present invention cannot be achieved.

[0023] [TIG welding] As mentioned above, TIG welding is a method of welding steel materials by using tungsten, a non-consumable material, as the electrode rod, and by blowing Ar gas or He gas onto the steel material while cutting off the air and melting the filler material (welding wire) in the arc. This TIG welding can be applied to various alloy steels and non-ferrous metals, and can also weld complex shapes, resulting in excellent weld quality.

[0024] An example of a TIG welding method according to the present invention will be described. As shown in Fig. 1, steel plates or steel materials (plate thickness t: 3 to 38 mm) that serve as base materials 1 are butted together in accordance with JIS Z 3111, and a copper backing plate 3 is used to form a V-groove 2 with a groove angle θ of 50°. The groove angle θ is a value in the range of 10° to 60°.

[0025] A pure tungsten rod (e.g., 3.2 mm diameter) is used as the electrode, and an arc is generated between the base material and the electrode. The base material is melted by the high-temperature arc, and when a welding wire (e.g., 1.2 mm diameter) is fed into the molten pool using a wire feeder, the welding wire melts, solidifies, and becomes one with the base material, forming the weld metal. The arc and weld metal are protected by an inert shielding gas during welding, resulting in high-quality weld metal.

[0026] Specific welding conditions include, for example, no preheating before welding, a flat position, a current of 150-300 A (DCEN), a voltage of 8-15 V, a welding speed of 5-15 cm / min, and a welding heat input of 0.7-3.5 kJ / mm. Also, it is preferable to perform welding under conditions of an interpass temperature of 100-150°C, a shielding gas of Ar, and a gas flow rate of 10-25 L / min. The wire feed speed is preferably 600-5000 mm / min.

[0027] Multi-layer welding under these welding conditions forms multi-layer weld metal in the V-groove. The number of layers depends on conditions such as plate thickness, but is preferably up to 10 layers, and the number of passes is preferably in the range of 1 to 2 passes per layer.

[0028] [Welding wire] The welding wire used in the TIG welding method according to the present invention includes a first welding wire having a specific chemical composition and a second welding wire having another specific chemical composition with a lower content of C and alloying elements than the first welding wire. By using the first welding wire for welding the first layer and layers thereafter and using the second welding wire for welding the final layer, it is possible to ensure the mechanical properties of the weld metal and suppress SCC against ammonia without using expensive alloying elements. The chemical compositions of the first and second welding wires will be described below.

[0029] [First welding wire] In the present invention, the first welding wire is used for welding the first layer and layers thereafter, i.e., layers other than the final layer. First, the reasons for limiting the composition ranges of the chemical composition of the first welding wire will be explained.

[0030] [C: 0.05~0.11%] C must be present in an amount of 0.05% or more to obtain the necessary strength for the weld metal, but if it exceeds 0.11%, it increases hardness and reduces toughness. Therefore, the C content is limited to the range of 0.05 to 0.11%. Furthermore, the C content is preferably in the range of 0.06 to 0.10%, and more preferably in the range of 0.07 to 0.09%.

[0031] [Si: 0.30~0.90%] Silicon is an element necessary for ensuring the strength of the weld metal and for deoxidizing the weld metal, and a content of 0.30% or more is necessary to achieve these effects. On the other hand, if the Si content exceeds 0.90%, the toughness of the weld metal deteriorates. Therefore, the Si content is limited to the range of 0.30 to 0.90%. The Si content is preferably in the range of 0.40 to 0.80%, and more preferably in the range of 0.50 to 0.70%.

[0032] [Mn: 0.50~1.80%] Mn is an element necessary for ensuring the strength of the weld metal and for deoxidizing the weld metal, and a content of 0.50% or more is necessary to achieve these effects. On the other hand, if the Mn content exceeds 1.80%, the toughness of the weld metal will be significantly deteriorated. Therefore, the Mn content is limited to the range of 0.50 to 1.80%. The Mn content is preferably in the range of 0.70 to 1.50%, and more preferably in the range of 1.00 to 1.30%.

[0033] [P:0.025% or less] Since a P content exceeding 0.025% deteriorates the toughness of the weld metal, the P content is limited to 0.025% or less. The P content is preferably 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 the P content may be 0.000%. However, excessive reduction leads to an increase in the refining cost of the wire material, so from the viewpoint of cost, a P content of 0.001% or more is preferably acceptable.

[0034] [S:0.035% or less] If the S content exceeds 0.035%, the toughness of the weld metal deteriorates, so the S content is limited to 0.035% or less. The S content is preferably 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 the S content may be 0.000%. However, excessive reduction leads to an increase in the refining cost of the wire material, so from the viewpoint of cost, the S content is preferably permissible even if it is 0.001% or more.

[0035] [Al: 0.04~0.19%] An Al content of 0.04% or more is necessary for the deoxidation of the weld metal, but an Al content exceeding 0.19% deteriorates the toughness of the weld metal. Therefore, the Al content is limited to the range of 0.04 to 0.19%. The Al content is preferably 0.05 to 0.10%, and more preferably 0.06 to 0.08%.

[0036] [Cu:0.60% or less] Cu has the function of increasing the strength of the weld metal while maintaining its high toughness, but if its content exceeds 0.60%, it causes hot embrittlement and deteriorates the surface properties. Therefore, the Cu content is limited to 0.60% or less. The Cu content is preferably 0.40% or less. On the other hand, to further exert the above-mentioned strength-improving effect, the Cu content is preferably 0.10% or more, and more preferably 0.20% or more.

[0037] [Ni:0.80% or less] Ni has the same function as Cu, increasing the strength of the weld metal while maintaining its high toughness. However, if the Ni content exceeds 0.80%, the hardness increases and the toughness decreases. Therefore, the Ni content is limited to 0.80% or less. The Ni content is preferably 0.60% or less. On the other hand, to further exert the above-mentioned strength-improving effect, the Ni content is preferably 0.01% or more, and more preferably 0.10% or more.

[0038] [Cr:0.50% or less] Like Cu, Cr increases the strength of the 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. The Cr content is preferably 0.30% or less. On the other hand, to further exert the above-mentioned strength-improving effect, the Cr content is preferably 0.01% or more, and more preferably 0.10% or more.

[0039] [Mo: 0.50% or less] Like Cr, Mo also increases the strength of the weld metal while maintaining its high toughness, but 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. The Mo content is preferably 0.20% or less. On the other hand, to further exert the above-mentioned strength-improving effect, the Mo content is preferably 0.01% or more, and more preferably 0.05% or more.

[0040] [N:0.0100% or less] N is an element contained as an unavoidable impurity, and if the N content exceeds 0.0100%, it will result in a decrease in toughness. Therefore, from the viewpoint of suppressing the 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, excessive reduction will result in an increase in the refining cost of the wire material, so from the viewpoint of cost, the N content is preferably acceptable to be 0.0010% or more.

[0041] [O (oxygen): 0.0100% or less] O (oxygen) is an element contained as an unavoidable impurity and has adverse effects such as forming oxides and becoming the starting point of fracture, so the O content is limited to 0.0100% or less. The O content is preferably 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, excessive reduction leads to an increase in the refining cost of the wire material, so from the viewpoint of cost, the O content is preferably acceptable to be 0.0020% or more.

[0042] [Remainder of First Welding Wire] The remaining chemical composition of the first welding wire other than the above-described chemical composition of the first welding wire is composed of Fe and unavoidable impurities. Examples of the unavoidable impurity elements include Sn, Sb, As, Pb, and Bi, and the total content of these elements is acceptable as long as it is 0.10% or less. Furthermore, as long as the above-described chemical composition of the first welding wire is satisfied, unavoidable impurity elements other than these are not prevented from being contained in the first welding wire, and such embodiments are also included within the technical scope of the present invention.

[0043] Furthermore, as long as the above-mentioned composition ranges are satisfied, multiple types of welding wires may be used sequentially as the first welding wire. In this case, the layers other than the final layer may be welded using multiple types of welding wires.

[0044] [Second welding wire] In the present invention, the second welding wire is used for welding the final layer. Next, the chemical composition of the second welding wire will be described. The chemical composition of the second welding wire includes a basic composition and an optional selected composition. First, the reasons for limiting each composition range for the basic composition will be described, and then the reasons for specifying each composition range for the optional selected composition will be described.

[0045] [Basic composition of second welding wire] The reasons for limiting the ranges of the basic composition of the second welding wire are as follows.

[0046] [C: 0.01 to 0.04%] C must be present in an amount of 0.01% or more to obtain the necessary strength for the weld metal. However, in the as-welded state, if the C content exceeds 0.04%, the hardness increases and the toughness decreases. Therefore, the C content is limited to a range of 0.01 to 0.04%. It is essential that the upper limit of the C content be lower than the lower limit of the C content of the first welding wire in order to limit the surface hardness of the weld metal. The C content is preferably in the range of 0.02 to 0.04%, and more preferably in the range of 0.02 to 0.03%.

[0047] [Si: 0.02~0.90%] Silicon is an element necessary for ensuring the strength of the weld metal and for deoxidizing the weld metal, and to achieve these effects, a silicon content of 0.02% or more is required. On the other hand, if the silicon content exceeds 0.90%, the toughness of the weld metal deteriorates. Therefore, the silicon content is limited to the range of 0.02 to 0.90%. The silicon content is preferably in the range of 0.20 to 0.80%, and more preferably in the range of 0.40 to 0.70%.

[0048] [Mn: 0.50~1.80%] Mn is an element necessary for ensuring the strength of the weld metal and for deoxidizing the weld metal, and to achieve these effects, a Mn content of 0.50% or more is necessary. On the other hand, if the Mn content exceeds 1.80%, the toughness of the weld metal will be significantly reduced. Therefore, the Mn content is limited to the range of 0.50 to 1.80%. The Mn content is preferably in the range of 0.70 to 1.50%, and more preferably in the range of 1.00 to 1.30%.

[0049] [P:0.025% or less] If P exceeds 0.025%, the toughness of the weld metal deteriorates, so the P content is limited to 0.025% 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 0.000%. However, since P is an element that is usually inevitably contained in steel materials as an impurity, industrially, it may be greater than 0.000%. However, excessive reduction leads to an increase in the refining cost of the wire material, so from a cost perspective, a P content of preferably 0.001% or more is also acceptable.

[0050] [S:0.035% or less] If S exceeds 0.035%, the toughness of the weld metal deteriorates, so the S content is limited to 0.035% or less. The S content is preferably 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 an increase in the refining cost of the wire material, so from the viewpoint of cost, the S content is preferably permissible even if it is 0.001% or more.

[0051] [Al: 0.04~0.19%] An Al content of 0.04% or more is necessary for deoxidation of the weld metal, but an Al content exceeding 0.19% deteriorates the toughness of the weld metal. Therefore, the Al content is limited to the range of 0.04 to 0.19%. The Al content is preferably 0.05 to 0.10%, and more preferably 0.06 to 0.08%.

[0052] [N:0.0100% or less] N is an element contained as an unavoidable impurity, and if the N content exceeds 0.0100%, it will result in a decrease in toughness. Therefore, from the viewpoint of suppressing the 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, excessive reduction will result in an increase in the refining cost of the wire material, so from the viewpoint of cost, the N content is preferably acceptable to be 0.0010% or more.

[0053] [O (oxygen): 0.0100% or less] O (oxygen) is an element contained as an unavoidable impurity and has adverse effects such as forming oxides and becoming the starting point of fracture, so the O content is limited to 0.0100% or less. The O content is preferably 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, excessive reduction leads to an increase in the refining cost of the wire material, so from the viewpoint of cost, the O content is preferably permissible even if it is 0.0020% or more.

[0054] Optional Composition of Second Welding Wire The reasons for specifying each composition range for the optional optional composition of the second welding wire are as follows: The chemical composition of the second welding wire may contain, in addition to the basic composition, one or more elements selected from Cu, Ni, Cr, Mo, V, Ti, Zr, and Nb within each composition range shown below. The optional optional composition may be contained as needed, so its content may be 0%.

[0055] [Cu:0.30% or less] Cu has the function of increasing the strength of the weld metal while maintaining its high toughness, but a Cu content exceeding 0.30% causes hot embrittlement and deteriorates the surface properties. Therefore, when Cu is added, the Cu content is preferably 0.30% or less. The Cu content is more preferably 0.25% or less. On the other hand, when Cu is added, in order to fully exert the strength-improving effect, the Cu content is preferably 0.10% or more, more preferably 0.20% or more.

[0056] [Ni:0.40% or less] Ni has the same function as Cu, increasing the strength of the weld metal while maintaining its high toughness, but if the Ni content exceeds 0.40%, the hardness increases too much. Therefore, when Ni is added, the Ni content is preferably 0.40% or less. The Ni content is more preferably 0.30% or less. On the other hand, when Ni is added, in order to further exert the above-mentioned strength-improving effect, the Ni content is preferably 0.01% or more, and more preferably 0.10% or more.

[0057] [Cr:0.20% or less] Like Cu, Cr increases the strength of the weld metal while maintaining its high toughness, but if the Cr content exceeds 0.20%, the hardness increases too much. Therefore, when Cr is added, the Cr content is preferably 0.20% or less. The Cr content is more preferably 0.15% or less. On the other hand, when Cr is added, in order to further exert the above-mentioned strength-improving effect, the Cr content is preferably 0.01% or more, more preferably 0.10% or more.

[0058] [Mo: 0.20% or less] Like Cr, Mo also increases the strength of the weld metal while maintaining high toughness, but if the Mo content exceeds 0.20%, the hardness increases too much. Therefore, when Mo is added, the Mo content is preferably 0.20% or less. The Mo content is more preferably 0.10% or less. On the other hand, when Mo is added, in order to further exert the above-mentioned strength-improving effect, the Mo content is preferably 0.01% or more, more preferably 0.05% or more.

[0059] [V:0.40% or less] V increases the strength and toughness of the weld metal, but if added in excess, it forms carbides and increases hardness. Therefore, when V is added, the V content is preferably 0.40% or less. The V content is more preferably 0.20% or less. On the other hand, when V is added, in order to further exert the effect of improving the strength and toughness, the V content is preferably 0.01% or more, and more preferably 0.05% or more.

[0060] [Ti: 0.030~0.190%] Ti is an element that precipitates as TiO2 during solidification, suppresses coarsening of austenite in the weld metal, and contributes to high toughness by acting as a ferrite transformation nucleus. If the Ti content is less than 0.030%, this effect is small, and if the Ti content exceeds 0.190%, the amount of solute Ti increases, deteriorating toughness. Therefore, when Ti is added, the Ti content is preferably 0.030 to 0.190%. The Ti content is more preferably 0.050 to 0.100%.

[0061] [Zr: 0.01~0.15%] Zr is an element that has the effect of suppressing grain growth and contributes to improving the toughness of the weld metal. However, if the Zr content is less than 0.01%, it is difficult to obtain such an effect. Moreover, if the Zr content is excessively greater than 0.15%, the amount of dissolved Zr increases, deteriorating the toughness. Therefore, when Zr is added, the Zr content is preferably 0.01 to 0.15%. The Zr content is more preferably 0.03 to 0.10%.

[0062] [Nb: 0.001 to 0.015%] Nb is an element that contributes to improving the strength of the weld metal through precipitation hardening. However, if the Nb content is less than 0.001%, it is difficult to obtain such an effect. If the Nb content exceeds 0.015%, carbides become coarse, deteriorating the toughness of the weld metal. Therefore, when Nb is added, the Nb content is preferably 0.001 to 0.015%. The Nb content is more preferably 0.010% or less. On the other hand, when Ni is added, in order to further exert the above-mentioned strength-improving effect, the Ni content is more preferably 0.002% or more, and even more preferably 0.004% or more.

[0063] [Remainder of second welding wire composition] The remaining chemical composition of the second welding wire other than the above-described chemical composition of the second welding wire is composed of Fe and unavoidable impurities. Examples of the unavoidable impurity elements include Sn, Sb, As, Pb, and Bi, and the total content of these elements is acceptable as long as it is 0.10% or less. Furthermore, as long as the above-described chemical composition of the second welding wire is satisfied, it is not prohibited for the second welding wire to contain unavoidable impurity elements other than these, and such embodiments are also included within the technical scope of the present invention.

[0064] [Weaving welding] In the TIG welding method according to the present invention, it is preferable that at least one layer of the multiple layers be weaved. Here, weaving refers to a welding technique in which a torch 10 used in TIG welding is moved back and forth relative to the weld line (i.e., the center position in the groove width direction) while advancing in the welding direction (i.e., perpendicular to the paper surface), as shown in FIG. 5, for example. A welding wire (not shown in FIG. 5) is moved in conjunction with the torch 10. The weaving width for each layer can be the root gap G for the first layer, or the gap at the intersection between the weld metal surface of the previous layer and both walls of the groove for other layers. This "weaving width" refers to the length of the groove width direction over the range of left and right movement of the torch.

[0065] This allows the groove surface of each layer to be sufficiently melted and promotes fusion with the weld metal. Furthermore, the weld metal of each layer can be evenly distributed across the entire width of the groove, which is advantageous because it helps to suppress welding defects and improve the weld bead shape. In particular, in automated welding, one-layer, one-pass welding using weaving only requires aligning the tracing line of the welding torch with the center of the groove, which is also useful for reducing the labor required for setup.

[0066] [Hot wire welding] In the TIG welding method of the present invention, it is preferable that at least one layer of the multilayer structure be hot wire welded. Here, hot wire welding refers to a welding method in which a welding wire 11 is electrically heated and fed into a molten pool 9 generated by an arc 8, as shown in FIG. 6 . The arc 8 is generated between the tungsten electrode 20 at the tip of the torch 10 and the base material 1 by supplying power from a welding power source 14 to the torch 10. The welding wire 11 is electrically heated by supplying power from a hot wire power source 15 through a contact tube 13 while being fed from a wire feeder 12 to the molten pool 9. This method enables high deposition and high-speed welding by adjusting the wire current and wire feed speed, which is preferable because it improves welding efficiency. The welding wire may be fed from either the front or rear of the welding direction.

[0067] [Composition of welded joints] The TIG welded joint obtained by the present invention is an arc welded joint including a base metal and a weld metal. The base metal has the above-mentioned chemical composition. The weld metal for the first layer and subsequent layers contains 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%, Cu: 0.60% or less, Ni: 0.80% 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 balance being Fe and unavoidable impurities. The final layer of the weld metal contains 0.01-0.04% C, 0.02-0.90% Si, 0.50-1.80% Mn, 0.025% or less P, 0.035% or less S, 0.04-0.19% Al, 0.0100% or less N, and 0.0100% or less O. Optionally, it also contains one or more elements selected from 0.30% or less Cu, 0.40% or less Ni, 0.20% or less Cr, 0.20% or less Mo, 0.40% or less V, 0.030-0.190% Ti, 0.01-0.15% Zr, and 0.001-0.015% Nb. The remainder of the final layer of the weld metal consists of Fe and unavoidable impurities.

[0068] [Mechanical properties of welded joints] The mechanical properties of the TIG welded joint obtained by the present invention preferably satisfy either one of the following conditions 1 and 2. More preferably, the mechanical properties satisfy both conditions. (Condition 1) Absorbed energy (vE -45 ) is 47J or more. (Condition 2) The average surface hardness (HV10) of the weld metal is 220 or less, and preferably 210 or less.

[0069] By satisfying the above condition 1, the low-temperature toughness of the ammonia tank can be ensured. By satisfying the above condition 2, it is expected that SCC caused by ammonia can be suppressed. In addition, in condition 1, the absorbed energy (vE -45 ): The reason for setting it at 47J or more is that this is the value required by the steel standards for welded structures.

[0070] The Charpy impact test under condition 1 above is carried out in accordance with the provisions of JIS Z 3128. The position at which the test specimens are taken is shown in Figure 2. The notch shape of the test specimen is a V-notch. The direction of the V-notch of the test specimen 5 is perpendicular to the surface of the steel material 1, and position a of the V-notch is the midpoint of the fusion line of the weld metal 4 on the center line of the test specimen 5. Three test specimens 5 are taken from a position 2 mm below the surface of the steel material 1. The Charpy impact test is carried out using three test specimens 5, and the absorbed energy (vE -45 ) are calculated for each, and the average value is taken as the absorbed energy value of the weld metal of the welded joint.

[0071] The average surface hardness (HV10) of the weld metal under the above condition 2 is determined using a Vickers hardness tester in the following manner. As shown in Figure 3, measurement positions b are set at 1 mm intervals on a line parallel to the surface of the steel material at a depth of 0.5 mm from the surface of the steel material in the cross section of the weld. The Vickers hardness measured at each measurement position b with a test force of 10 kgf is defined as the surface hardness (HV10), and these measured values ​​are averaged to define the surface hardness value of the weld metal of the welded joint. [Example]

[0072] The present invention will be further described below with reference to examples. However, the following examples are merely intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0073] In the examples, 15 mm thick steel materials (three types) shown in Table 1 were machined into a V-groove 2 (groove angle θ: 50°) as shown in Figure 1, and a root gap G of 3 to 5 mm was provided, followed by TIG welding of the steel materials. A copper plate was used as the backing plate 3. Steel material No. a was TS440 MPa class steel, steel material No. b was TS490 MPa class steel, and steel material No. c was TS440 MPa class steel, and the tensile strength of each welded joint was required to be 440 MPa class and 490 MPa class, equivalent to that of the base metal.

[0074] [Table 1]

[0075] Next, four types of solid wire (1.2 mm diameter) shown in Table 2 were used as welding wire. Welding wire No. B is generally used for steel materials with strengths of steel materials No. a and No. b, and corresponds to the first welding wire. Welding wire No. A, No. C, and No. D correspond to the second welding wire, and have a lower C content than No. B. Welding wire No. A and No. D have the basic composition and optional selected compositions of the second welding wire, and welding wire No. C has only the basic composition of the second welding wire. The remaining chemical composition of each welding wire is Fe and unavoidable impurities.

[0076] [Table 2]

[0077] Using the above steel material and welding wire, multi-layer welding was performed from one side without preheating and in a flat position under the various welding conditions shown in Tables 3-1, 3-2, and 4. Tables 3-1 and 3-2 also list the conditions for weaving welding and hot wire welding. In the case of "welding condition 1" shown in Table 3-1, the first layer was weaving welded, and the second to fifth layers were weaving welded and hot wire welded. In the case of "welding condition 2" shown in Table 3-2, the first to fifth layers were weaving welded and hot wire welded. The "welding condition" column in Table 4 lists the number of the welding condition that was applied. The number of welding passes was one for each layer. The shielding gas was 100% Ar gas by volume, and the gas flow rate was 10 to 25 L / min.

[0078] [Table 3-1]

[0079] [Table 3-2]

[0080] [Table 4]

[0081] Table 5 also shows the same welding conditions as well as the evaluation results of the mechanical properties of the weld metal described below.

[0082] [Table 5]

[0083] The methods for the joint tensile test, Charpy impact test at the center of the weld metal, and Vickers hardness measurement at the surface layer of the weld metal were as follows:

[0084] The joint tensile test was carried out in accordance with the provisions of JIS Z 3121:2013.

[0085] The Charpy impact test was carried out in accordance with the provisions of JIS Z 3128. The position from which the test specimens (notch shape: V notch) were taken is shown in Figure 2. The direction of the V notch of the test specimen 5 was perpendicular to the surface of the steel material 1, and the position a of the V notch was set to the midpoint of the fusion line of the weld metal 4 on the center line of the test specimen 5. Three test specimens 5 were taken from a position 2 mm below the surface of the steel material 1. The Charpy impact test was carried out on three specimens taken from each weld joint, and the absorbed energy (vE -45 ) was calculated for each weld joint, and the average value was used as the absorbed energy value of the weld metal of each weld joint.

[0086] A Vickers hardness tester was used to test the Vickers hardness of the weld metal surface. As shown in Figure 3, measurement positions b were set at 1 mm intervals on a line parallel to the surface of the steel material in the weld metal at a depth of 0.5 mm from the surface of the steel material in the cross section of the weld. The Vickers hardness measured at each measurement position b with a test force of 10 kgf was taken as the surface hardness (HV10). An example of the surface hardness (HV10) measurement results is shown in Figure 4. Figure 4 shows the individual values ​​measured for welded joint No. 3 (an example of the present invention) and welded joint No. 2 (a comparative example). The average of these measurements was taken as the average surface hardness (HV10). For welded joint No. 3, the average surface hardness (HV10) was 209, which was below 220 and is therefore considered to be a joint that is less susceptible to stress corrosion cracking. In welded joint No. 2, the average surface hardness (HV10) was 238, exceeding 220, raising concerns about the occurrence of stress corrosion cracking. Note that measurement position b is within the weld metal, so the number of points at measurement position b varies depending on the width of the weld metal.

[0087] [Evaluation results] As shown in Table 5, the fracture location of the joint tensile test specimens was in the base metal in all welded joints, so the weld metal has strength that exceeds the strength of the base metal.

[0088] Welded joints Nos. 3 and 7 (invention examples) are joints in which the first layer to the layer just before the final layer was welded with welding wire No. B, and the final layer was welded with welding wire No. A. Welded joints Nos. 8 and 10 (invention examples) are joints in which the first layer to the layer just before the final layer was welded with welding wire No. B, and the final layer was welded with welding wire No. C. Welded joint No. 9 (invention example) is a joint in which the first layer to the layer just before the final layer was welded with welding wire No. B, and the final layer was welded with welding wire No. D. All of these joints had an average surface hardness (HV10) that met the standard value (220 or less) and an absorbed energy (vE -45 ) met the standard value (47J or more).

[0089] On the other hand, in welded joints No. 1 and 5 (comparative examples), welding was performed from the first layer to the last layer using welding wire No. A, which has low C and alloying elements. Therefore, the average surface hardness (HV10) was 220 or less, but the absorbed energy was significantly reduced due to embrittlement of the reheated area. Welded joints No. 2 and 6 (comparative examples) were welded from the first layer to the last layer using welding wire No. B, but the average surface hardness (HV10) and absorbed energy (vE -45 ) did not satisfy the standard values. Welded joint No. 4 (comparative example) had the welding wire combination reversed to that of welded joint No. 3 (invention example), but the average surface hardness (HV10) and absorbed energy (vE -45 ) did not meet the standard values. [Explanation of symbols]

[0090] 1 Base material (steel material) 2 V bevel 3 Backing plate 4 Weld metal 5. Charpy impact test specimen 8. Arc 9 Molten pool 10. Torch 11 Welding wire 12 Wire Feeder 13 Contact Tube 14 Welding power source 15 Hot Wire Power Supply 20 Tungsten electrodes a Notch position b Measurement position of Vickers hardness test t Plate thickness θ Bevel angle

Claims

1. A TIG welding method for forming a multi-layer weld metal on a steel material using a shielding gas made of an inert gas and a welding wire, the welding wire includes a first welding wire and a second welding wire having different chemical compositions; The chemical composition of the first welding wire is, in mass%, C: 0.05-0.11%, Si: 0.30-0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04-0.19%, Cu: 0.60% or less, Ni: 0.80% or less, Cr: 0.50% or less, Mo: 0.50% or less, N: 0.0100% or less, and O: 0.0100% or less and the balance being Fe and unavoidable impurities, The chemical composition of the second welding wire is, in mass%, C: 0.01-0.04%, Si: 0.02-0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.04-0.19%, N: 0.0100% or less, and O: 0.0100% or less and the balance being Fe and unavoidable impurities, The first welding wire is used for welding the first layer and subsequent layers of the multi-layer structure; the second welding wire is used to weld the final layer of the multi-layer structure; TIG welding method.

2. In addition to the chemical composition of the second welding wire, the second welding wire further comprises, in mass %, 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: 0.030 to 0.190%, Zr: 0.01 to 0.15%, and Nb: 0.001-0.015% Contains one or more selected from the following: The TIG welding method according to claim 1.

3. The welding of at least one layer of the multiple layers is a weaving welding. The TIG welding method according to claim 1.

4. The welding of at least one layer of the multilayer structure is a weaving welding. The TIG welding method according to claim 2.

5. At least one layer of the multilayer structure is welded by hot wire welding. The TIG welding method according to claim 1.

6. At least one layer of the multilayer structure is welded by hot wire welding. The TIG welding method according to claim 2.

7. At least one layer of the multilayer structure is welded by hot wire welding. The TIG welding method according to claim 3.

8. At least one layer of the multilayer structure is welded by hot wire welding. The TIG welding method according to claim 4.

9. The absorbed energy (vE -45 9. The TIG welding method according to claim 1, wherein the welding strength is 47 J or more.

10. The TIG welding method according to any one of claims 1 to 8, wherein an average value of surface hardness (HV10) of the weld metal is 220 or less.

11. A TIG welding method as described in Claim 9, wherein the average surface hardness (HV10) of the weld metal is 220 or less.

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