TIG welding method and TIG welded joint

Abstract

Provided are a TIG welding method and a TIG welded joint which are excellent in terms of resistance to SCC by ammonia. This TIG welding method is for forming a weld metal on a steel material using an inert shielding gas and a welding wire, wherein the steel material and the welding wire have specific chemical compositions and welding conditions are set so that the carbon equivalent Ceq of the welding wire shown by expression (1) is 0.24-0.36, the welding heat input Q is 35.0 kJ / cm or less, and the index P shown by expression (2) using the plate thickness t and the welding heat input Q is 0.15-0.45. (1): Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15 (2): P=(Ceq+0.06)×t / Q

Description

TIG welding method and TIG welded joint

[0001] The present invention relates to a TIG welding method and a TIG welded joint using the same, and more particularly to a TIG welding method for forming a welded joint that has excellent SCC resistance to ammonia.

[0002] The TIG welding method uses tungsten, a non-consumable material, as an electrode rod, and involves melting a welding wire (i.e., filler metal) in an arc while blocking the atmosphere by spraying inert gas, typically argon gas or helium gas.

[0003] Ammonia is relatively easy to liquefy under atmospheric pressure, and thus has attracted attention as a hydrogen carrier for achieving a carbon-free society. Technologies for its co-firing with coal in thermal power plants and its use as marine fuel are being developed. Using ammonia as a fuel for thermal power plants requires larger ammonia storage tanks. Because ammonia is highly toxic, careful attention must be paid to preventing leakage from tanks. Carbon steel used in the construction of ammonia tanks is susceptible to stress corrosion cracking (SCC) in liquid ammonia, and the exact mechanism of cracking is unclear. However, it is believed that there is a correlation between the surface hardness of steel and weld metal and the occurrence of cracks. It is known that the occurrence of SCC is significantly reduced when the hardness of the liquid-contact surface is HV 210 or less. Welds, in particular, are prone to hardening due to thermal effects, and cracks are commonly observed in and near weld metals.

[0004] Until now, ammonia has mainly been used in the production of chemical fertilizers, and even the largest tanks in Japan have a capacity of around 15,000 tons. Due to the required strength, SLA325AN, a carbon steel plate for low-temperature pressure vessels that has undergone normalizing treatment, is often used. Welding materials with a typical strength of 50 kg / mm ​​are used depending on the strength of the steel plate. While welding is performed with care to reduce the hardness of the weld, the weld metal and weld heat-affected zones tend to harden, reaching around HV 250-280. Liquid ammonia tanks undergo periodic open inspections, and cracks are detected and repaired during the inspections.

[0005] To address these issues, 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 chemical components and manufacturing methods. However, no study has been conducted on a method for suppressing SCC in weld heat affected zones, where SCC due to ammonia is particularly likely to occur.

[0006] 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 composition.

[0007] Japanese Patent No. 5428999 Japanese Unexamined Patent Publication No. 159293 / 1983 Japanese Patent No. 6829111

[0008] However, in welded joints using these conventional steel materials and welding materials, the problem of SCC occurrence remains unresolved due to unavoidable hardening of the weld metal and weld heat-affected zone regardless of the welding conditions. Because it is difficult to completely prevent SCC occurrence, tanks must be periodically inspected, and welding repairs are currently performed whenever cracks are discovered during the inspection. It is expected that ammonia will be used as a fuel in the future, and large amounts of ammonia will need to be stored to ensure a stable energy supply. Therefore, larger ammonia land tanks are also required. If tanks are enlarged using conventional welding technology, defects such as SCC due to ammonia will increase, and welding repairs will require significant effort. Therefore, there is an urgent need to obtain welded joints with high SCC resistance.

[0009] An object of the present invention is to provide a TIG welding method and a TIG welded joint that can suppress the surface hardness of the weld metal surface layer under appropriate welding conditions and are less likely to cause ammonia-induced SCC.

[0010] The present inventors have conducted extensive research into the fabrication of welded joints using low-carbon equivalent (low-Ceq) steel produced by thermomechanical control technology (TMCP) and low-Ceq welding materials to reduce the hardness of welded joints to HV 210 or less as a means of improving SCC resistance. As a result, they discovered that the required tensile strength, Charpy impact properties, and surface hardness of the weld metal surface layer cannot all be satisfied by simply selecting the materials. Therefore, they discovered that an appropriate welding heat input value can be obtained by using parameters calculated from the plate thickness of the steel to be welded and the carbon equivalent (Ceq) of the welding material. Furthermore, they discovered that welding under the obtained heat input welding conditions suppresses the surface hardness of the weld metal surface layer and the surface hardness of the weld heat-affected zone of the steel, resulting in a welded joint that is less susceptible to SCC due to ammonia.

[0011] The present invention was completed based on these findings and further investigations.

[0012] The gist of the present invention is as follows. [1] A TIG welding method for forming a weld metal on a steel material using a shielding gas consisting of an inert gas and a welding wire, wherein the chemical composition of the steel material contains, in mass%, C: 0.030 to 0.090%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Al: 0.060% or less, N: 0.0010 to 0.0100%, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities, and the chemical composition of the welding wire contains, in mass%, C: 0.020 to 0.080%, Si: 0.30 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less, [2] A TIG welding method according to [1], further comprising, in addition to the chemical composition of the steel, one or more of the following elements selected from the group consisting of 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. [3] The TIG welding method according to any one of [1] to [3], wherein the welding wire further contains, in addition to the chemical composition thereof, one or more elements selected from the group consisting of Cu: 0.60% or less, Ni: 0.80% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.40% or less, Ti: 0.030 to 0.190%, and Zr: 0.01 to 0.15%, in mass %. [4] The TIG welding method according to any one of [1] to [3], wherein the welding wire has a carbon equivalent Ceq expressed by the following formula (1) of 0.24 to 0.36, a welding heat input Q in the TIG welding method is 35.0 kJ / cm or less, and an index P expressed by the following formula (2) of 0.15 to 0.45.Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 (1) Here, [element] in formula (1) represents the content (mass%) of the element, and if the element is not contained, the content is set to 0. P = (Ceq + 0.06) × t / Q (2) Here, Ceq in formula (2) is the carbon equivalent of the welding wire calculated from formula (1), and t is the plate thickness (mm) of the steel material. Also, Q in formula (2) is the welding heat input, and the welding heat input Q (J / cm) = welding current I (A) × arc voltage E (V) × 60 / welding speed V (cm / min). [5] A TIG welded joint of a steel material, wherein the chemical composition of the weld metal contains, in mass %, C: 0.030 to 0.070%, Si: 0.30 to 0.70%, Mn: 0.80 to 1.55%, P: 0.025% or less, S: 0.035% or less, Al: 0.040 to 0.150%, N: 0.010% or less, and O: 0.010% or less, with the balance being Fe and unavoidable impurities, the carbon equivalent Ceq of the weld metal, represented by the following formula (1), is 0.24 to 0.36, and the average Vickers hardness HV of a surface layer portion of the weld metal is 210 or less. Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 (1) Here, the [element] in formula (1) represents the content (mass%) of the element, and if the element is not contained, the content is set to 0. [6] The TIG welded joint according to [5] above, further containing, in addition to the chemical composition of the weld metal, one or more elements selected from, by mass%, Cu: 0.50% or less, Ni: 0.60% or less, Cr: 0.40% or less, Mo: 0.40% or less, V: 0.30% or less, Nb: 0.03% or less, Ti: 0.02 to 0.15%, Zr: 0.01 to 0.12%, and B: 0.010% or less. [7] The TIG welded joint according to [5] or [6], wherein the absorbed energy at −45° C. in the center of the weld metal is 27 J or more.

[0013] TIG welded joints manufactured by the TIG welding method according to the present invention exhibit high SCC resistance when used as welded joints for land-based ammonia tanks, and can reduce the frequency of repairs for cracks inside the tank, thereby providing significant industrial benefits.

[0014] Fig. 1 is a schematic cross-sectional view showing an example of a groove shape in the TIG welding method of the present invention. Fig. 2 is a schematic cross-sectional view showing an example of a sample position for a test piece for a Charpy impact test used in this example. Fig. 3 is a schematic cross-sectional view showing an example of a measurement position for a Vickers hardness test in this example. Fig. 4 is a distribution diagram showing the results of a measurement example of a Vickers hardness test in this example.

[0015] [TIG Welding] The TIG welding method of the present invention is a welding method in which a weld metal is formed on a steel material using a shielding gas consisting of an inert gas and a welding wire. As described above, TIG welding is a welding method in which a non-consumable material, tungsten, is used as an electrode rod, and another filler material (e.g., a welding wire) is melted in an arc while Ar gas or He gas is sprayed to block air. 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.

[0016] An example of a TIG welding method according to the present invention will be described. As shown in Figure 1, in accordance with JIS Z 3111, steel plates or steel materials (thickness t: 3 to 38 mm) that serve as base materials 1 are butted together, and a copper backing plate 3 is used to form a V-groove 2 with a groove angle θ. The groove angle θ is 10° to 60°.

[0017] A pure tungsten rod (3.2 mm diameter) is used as the electrode, and an arc is generated between the electrode and the base metal. The base metal is melted by the high-temperature arc, and a welding wire (1.2 mm diameter) is fed toward the molten metal using a wire feeder. The wire melts, solidifies, and becomes one with the base metal, forming a weld metal. The arc and weld metal are protected by an inert shielding gas during welding, resulting in high-quality weld metal.

[0018] Specific welding conditions include, for example, no preheating before welding and a downward position. Furthermore, it is preferable to perform welding under the following conditions: interpass temperature: 100°C, shielding gas: Ar, gas flow rate: 10 to 25 L / min. The welding current, voltage, welding speed, and welding heat input will be described later.

[0019] Under these welding conditions, weld metal is formed in the V-groove by single-layer or multi-layer welding. The weld metal is preferably formed by stacking multiple layers of weld metal. The number of layers is up to about 10, depending on conditions such as plate thickness, and the number of passes is preferably in the range of 1 to 2 passes per layer.

[0020] [Chemical Composition of Steel] The chemical composition of the steel used in the TIG welding method according to the present invention is as follows: Note that "%" in the following chemical compositions means "mass %" unless otherwise specified.

[0021] The basic composition of this steel sheet is 0.030 to 0.090% C, 0.50% or less Si, 0.50 to 2.00% Mn, 0.020% or less P, 0.010% or less S, 0.060% or less Al, 0.0010 to 0.0100% N, and 0.0100% or less O (oxygen). Furthermore, it may contain one or more elements selected from the group consisting of 2.00% or less Cu, 2.00% or less Ni, 1.00% or less Cr, 1.00% or less Mo, 1.00% or less V, 0.10% or less Nb, 0.005 to 0.100% Ti, 0.2000% or less Ca, 0.0200% or less Mg, and 0.0100% or less B, with the balance being Fe and unavoidable impurities.

[0022] [Basic Composition of Steel Material] First, the reasons for limiting the ranges of the basic composition of the steel material will be explained.

[0023] [C: 0.030 to 0.090%] C must be added in an amount of 0.030% or more to obtain the strength required for structural steel. However, adding more than 0.090% C can increase hardness and reduce toughness. Therefore, the C content is limited to the range of 0.030 to 0.090%. The C content is preferably in the range of 0.040 to 0.080%, and more preferably in the range of 0.050 to 0.070%.

[0024] [Si: 0.50% or less] Si acts as a deoxidizer, but at the same time, it also reduces toughness and weldability. Therefore, it is desirable to keep the content as low as possible, but since 0.50% or less is acceptable, the Si content is limited to 0.50% or less. Note that, since steel can be sufficiently deoxidized with Al, Ti, etc., the lower limit of the Si content is not particularly limited and may be 0%. Note that, from the viewpoint of toughness and weldability, the Si content is preferably 0.45% or less, more preferably 0.40% or less. On the other hand, when used as a deoxidizer, in order to maximize its effect, the Si content is preferably 0.01% or more, more preferably 0.10% or more.

[0025] [Mn: 0.50 to 2.00%] Mn is an element that increases the hardenability of steel and is one of the important elements that must be added to achieve high strength. To achieve this effect, the Mn content is limited to 0.50% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and achieving lower manufacturing costs, the Mn content is preferably 0.70% or more, and more preferably 0.90% or more. On the other hand, if the Mn content exceeds 2.00%, the strength becomes excessively high, and toughness and weldability decrease, and the alloy cost becomes excessively high. Therefore, the Mn content is limited to 2.00% or less. From the viewpoint of suppressing deterioration in toughness and weldability, the Mn content is preferably 1.80% or less, and more preferably 1.60% or less.

[0026] [P: 0.020% or less] P is an element contained as an unavoidable impurity, and by segregating at grain boundaries, it has adverse effects such as reducing toughness and weldability. Therefore, it is desirable to keep the P content as low as possible, but since a P content of 0.020% or less is acceptable, the P content is limited to 0.020% 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%. Since P is usually an element inevitably contained in steel as an impurity, industrially, it may be more than 0%. Since excessive reduction leads to increased refining costs, the P content is preferably 0.001% or more.

[0027] [S: 0.010% or less] S is an element contained as an unavoidable impurity. It exists in steel as sulfide-based inclusions such as MnS, and has adverse effects, such as becoming the origin of fracture. Therefore, it is desirable to reduce the S content as much as possible. However, since a content of 0.010% or less is acceptable, 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%. Since S is usually an element inevitably contained in steel as an impurity, it may be more than 0% industrially. Since excessive reduction leads to an increase in refining costs, the S content is preferably 0.001% or more.

[0028] [Al: 0.060% or less] Al is an element that acts as a deoxidizer and also has the effect of refining crystal grains. However, if the Al content exceeds 0.060%, oxide-based inclusions increase, which reduces cleanliness and toughness. Therefore, the Al content is limited to 0.060% or less. The Al content is preferably 0.050% or less, and more preferably 0.040% or less. On the other hand, in order to fully exert the effect as a deoxidizer, the Al content is preferably 0.010% or more, and more preferably 0.020% or more.

[0029] [N: 0.0010 to 0.0100%] N combines with Ti and precipitates as TiN, contributing to refinement of the structure and improving toughness. To achieve this effect, the N content is limited to 0.0010% or more. The N content is preferably 0.0020% or more, and more preferably 0.0030% or more. On the other hand, an N content exceeding 0.0100% actually 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.

[0030] [O (oxygen): 0.0100% or less] O (oxygen) is an element contained as an inevitable impurity, and has adverse effects such as forming oxides and becoming the starting point of fracture. Therefore, 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%. Since O is usually an element that is inevitably contained in steel as an impurity, industrially, it may be more than 0%. Since excessive reduction leads to an increase in refining costs, from the viewpoint of cost, the O content is preferably 0.0020% or more.

[0031] [Optionally Selected Composition of Steel Material] Next, the reasons for specifying each composition range for the optionally selected composition of the steel material will be explained.

[0032] [Cu: 2.00% or less] Cu is an element that has the effect of increasing the hardenability of steel and improving the strength of the steel sheet, and can be added as desired. However, if the Cu content exceeds 2.00%, it will result in a deterioration in toughness and an increase in alloy costs. Therefore, when Cu is added, the Cu content is preferably 2.00% or less. The Cu content is more preferably 1.00% or less, and even more preferably 0.50% or less. On the other hand, when Cu is added, in order to further exert the strength-improving effect, the Cu content is preferably 0.01% or more, and more preferably 0.10% or more.

[0033] [Ni: 2.00% or less] Ni, like Cu, is an element that has the effect of improving the strength of the steel sheet and can be added as desired. However, if the Ni content exceeds 2.00%, it will result in deterioration of weldability and an increase in alloy costs. Therefore, when Ni is added, the Ni content is preferably 2.00% or less. The Ni content is more preferably 1.00% or less. On the other hand, when Ni is added, in order to further exert the strength-improving effect, the Ni content is preferably 0.01% or more, and more preferably 0.20% or more.

[0034] [Cr: 1.00% or less] Cr is an element that, like Cu, has the effect of improving the strength of the steel sheet and can be added as desired. However, a Cr content exceeding 1.00% will result in deterioration of weldability and an increase in alloy costs. Therefore, when Cr is added, the Cr content is preferably 1.00% or less. The Cr content is more preferably 0.50% or less. On the other hand, when Cr is added, in order to further exert the strength-improving effect, the Cr content is preferably 0.01% or more, more preferably 0.05% or more.

[0035] [Mo: 1.00% or less] Mo, like Cu, is an element that has the effect of improving the strength of steel sheet and can be added as desired. However, if the Mo content exceeds 1.00%, it will result in deterioration of weldability and an increase in alloy costs. Therefore, when Mo is added, the Mo content is preferably 1.00% or less. The Mo content is more preferably 0.50% or less. On the other hand, when Mo is added, in order to further exert the strength-improving effect, the Mo content is preferably 0.01% or more, more preferably 0.05% or more.

[0036] [V: 1.00% or less] V has the effect of suppressing grain coarsening by forming carbides and nitrides, but if it exceeds 1.00%, there is a risk of hardening the heat-affected zone and deteriorating toughness. V can be added arbitrarily. Therefore, when V is added, the V content is preferably 1.00% or less. The V content is more preferably 0.10% or less. On the other hand, when V is added, in order to further exert the effect of suppressing grain coarsening, the V content is preferably 0.01% or more, and more preferably 0.02% or more.

[0037] [Nb: 0.10% or less] Nb, like V, has the effect of suppressing grain coarsening by forming carbides and nitrides, but if it exceeds 0.10%, there is a risk of deteriorating the hardness and toughness of the heat-affected zone. Nb can be added arbitrarily. Therefore, when Nb is added, the Nb content is preferably 0.10% or less. The Nb content is more preferably 0.05% or less. On the other hand, when Nb is added, in order to further exert the effect of suppressing grain coarsening, the Nb content is preferably 0.01% or more, and more preferably 0.02% or more.

[0038] [Ti: 0.005 to 0.100%] Ti is an element that has a strong tendency to form nitrides and has the effect of fixing N and reducing solute N. Therefore, adding Ti can improve the toughness of the base metal and weld. Ti can be added arbitrarily. To achieve this effect, the Ti content is preferably 0.005% or more. The Ti content is more preferably 0.012% or more. On the other hand, if the Ti content exceeds 0.100%, the toughness actually decreases. Therefore, the Ti content is preferably 0.100% or less. The Ti content is more preferably 0.090% or less, and even more preferably 0.080% or less.

[0039] [Ca: 0.2000% or less] Ca is an element that has a toughness improving effect by fixing S. Ca can be added arbitrarily. However, when the Ca content exceeds 0.2000%, this effect saturates. Therefore, when Ca is added, the Ca content is preferably 0.2000% or less. The Ca content is more preferably 0.0300% or less. On the other hand, in order to further exert the toughness improving effect, it is preferable to contain 0.0005% or more. The Ca content is more preferably 0.0010% or more.

[0040] [Mg: 0.0200% or less] Like Ca, Mg is an element that bonds with S and suppresses the formation of MnS and other compounds that elongate in the rolling direction. Therefore, adding Mg can control the shape of sulfide-based inclusions so that they are 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 an increase in surface defects, which leads to deterioration of surface quality and reduced bending workability. Mg can be added at any concentration. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0100% or less. On the other hand, to further enhance the toughness-improving effect, when Mg is added, the Mg content is preferably 0.0005% or more. The Mg content is more preferably 0.0010% or more.

[0041] [B: 0.0100% or less] B is an element that has the effect of significantly improving hardenability even when added in small amounts. Therefore, it can improve the strength of the steel sheet, but if the B content exceeds 0.0100%, weldability will decrease. B can be added arbitrarily. Therefore, when B is added, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less, and even more preferably 0.0030% or less. On the other hand, in order to further exert the above-mentioned strength-improving effect, when B is added, the B content is preferably 0.0001% or more. The B content is more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0042] [Remainder of Steel Material] The remainder of the chemical composition of the steel material other than the above-mentioned chemical composition consists of Fe and unavoidable impurities. Examples of these unavoidable impurity elements include Sn, Sb, As, Pb, and Bi, and a total content of these elements of 0.10% or less is acceptable. Furthermore, as long as the above-mentioned basic chemical composition and optional chemical composition are satisfied, the inclusion of other unavoidable impurity elements is not precluded, and such embodiments are also within the technical scope of the present invention.

[0043] [Chemical Composition of Welding Wire] Next, the chemical composition of the welding wire used in the TIG welding method according to the present invention is as follows.

[0044] The basic composition includes C: 0.020-0.080%, Si: 0.30-0.90%, Mn: 0.50-1.80%, P: 0.025% or less, S: 0.035% or less, Al: 0.040-0.190%, N: 0.0100% or less, and O (oxygen): 0.0100% or less. Optionally, it may further include one or more of Cu: 0.60% or less, Ni: 0.80% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.40% or less, Ti: 0.030-0.190%, and Zr: 0.01-0.15%, with the balance being Fe and unavoidable impurities.

[0045] [Basic Composition of Welding Wire] First, the reasons for limiting the range of each composition of the basic composition of the welding wire will be explained.

[0046] [C: 0.020 to 0.080%] C must be contained in an amount of 0.020% or more to obtain the necessary strength for the weld metal, but if it exceeds 0.080%, the hardness increases and the toughness decreases. Therefore, the C content is limited to the range of 0.020 to 0.080%. The C content is preferably in the range of 0.030 to 0.060%, and more preferably in the range of 0.040 to 0.050%.

[0047] [Si: 0.30 to 0.90%] Si is an element necessary for ensuring the strength of the weld metal and for deoxidizing the weld metal. To achieve these effects, the Si content must be 0.30% or more. 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 0.40 to 0.60%.

[0048] [Mn: 0.50 to 1.80%] Mn is an element necessary for ensuring the strength of the weld metal and for deoxidizing the weld metal. To achieve these 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 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%.

[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 0.020% or less, and more preferably 0.010% or less. The lower limit of the P content is not particularly limited and may be 0%. Since P is usually an element that is inevitably contained in steel materials as an impurity, industrially, it may be more than 0%. Since excessive reduction leads to an increase in refining costs, the P content is preferably 0.001% or more.

[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 0.030% or less, and more preferably 0.020% or less. The lower limit of the S content is not particularly limited and may be 0%. Since S is usually an element that is inevitably contained in steel materials as an impurity, industrially, it may be more than 0%. Since excessive reduction leads to an increase in refining costs, the S content is preferably 0.001% or more.

[0051] [Al: 0.040 to 0.190%] The Al content must be 0.040% or more to deoxidize the weld metal. On the other hand, if the Al content exceeds 0.190%, the toughness of the weld metal deteriorates. Therefore, the Al content is limited to the range of 0.040 to 0.190%. The Al content is preferably 0.040 to 0.100%, and more preferably 0.040 to 0.060%.

[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 lead 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. Note that excessive reduction will lead to an increase in refining costs, so from the viewpoint of cost, the N content is preferably 0.0010% or more.

[0053] [O (oxygen): 0.0100% or less] O (oxygen) is an element contained as an inevitable impurity, and has adverse effects such as forming oxides and becoming the starting point of fracture. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0060% or less, and more preferably 0.0050% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0%. Since O is usually an element that is inevitably contained in steel materials as an impurity, industrially, it may be more than 0%. Since excessive reduction leads to an increase in refining costs, from the viewpoint of cost, the O content is preferably 0.0020% or more.

[0054] [Optional Composition of Welding Wire] Next, the reasons for specifying each composition range for optional compositions of the welding wire will be explained.

[0055] [Cu: 0.60% or less] Cu has the function of increasing the strength while maintaining high toughness of the weld metal, but if it exceeds 0.60%, it causes hot embrittlement and deteriorates the surface properties. Therefore, when Cu is added, the Cu content is preferably 0.60% or less. The Cu content is more preferably 0.30% or less. On the other hand, when Cu is added, in order to further exert the above-mentioned strength-improving effect, the Cu content is preferably 0.10% or more, and more preferably 0.15% or more.

[0056] [Ni: 0.80% or less] Ni has the same function as Cu, increasing the strength of the weld metal while maintaining high toughness, but if the Ni content exceeds 0.80%, the hardness increases too much. Therefore, when Ni is added, the Ni content is preferably 0.80% or less. The Ni content is more preferably 0.40% 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: 0.50% or less] Like Cu, Cr increases the strength of the weld metal while maintaining high toughness. However, if the Cr content exceeds 0.50%, the hardness increases too much. Therefore, when Cr is added, the Cr content is preferably 0.50% or less. The Cr content is more preferably 0.30% 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, and more preferably 0.10% or more.

[0058] [Mo: 0.50% or less] Like Cr, Mo also increases the strength of the weld metal while maintaining high toughness, but if the Mo content exceeds 0.50%, the hardness increases too much. Therefore, when Mo is added, the Mo content is preferably 0.50% or less. The Mo content is more preferably 0.20% 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, and 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 to 0.190%] Ti is an element that precipitates as TiO2 during solidification, suppresses coarsening of austenite grains in the weld metal, and contributes to high toughness by acting as ferrite transformation nuclei. 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, degrading toughness. Therefore, when Ti is added, the Ti content is preferably 0.030 to 0.190%. The Ti content is more preferably 0.040 to 0.100%.

[0061] [Zr: 0.01 to 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 achieve such an effect. Furthermore, if the Zr content is excessive, exceeding 0.15%, the amount of solute Zr increases, deteriorating 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] [Remainder Composition of Welding Wire] The remaining chemical composition of the welding wire other than the above-mentioned chemical composition consists of Fe and inevitable impurities. Examples of these inevitable impurity elements include Sn, Sb, As, Pb, and Bi, and a total content of these elements of 0.10% or less is acceptable. Furthermore, as long as the basic chemical composition and optional chemical composition described above are satisfied, the inclusion of other inevitable impurity elements is not precluded, and such embodiments are also within the technical scope of the present invention.

[0063] [Relationship between Carbon Equivalent of Welding Wire and Welding Heat Input] As described above, the present inventors investigated TIG welded joints using low-Ceq steel produced by thermomechanical control technology (TMCP) and low-carbon, low-Ceq welding wire in order to reduce the hardness of the welded joint to HV 210 or less as a means of improving SCC resistance. As a result, they found that material selection alone does not necessarily satisfy all of the required tensile strength, Charpy impact properties, and surface hardness of the weld metal surface layer. Further investigations revealed that an appropriate welding heat input can be obtained using parameters calculated from the steel plate thickness and the Ceq of the welding wire. Furthermore, they found that welding under the obtained welding heat input conditions suppresses the surface hardness of the weld metal surface layer and produces a welded joint that is less susceptible to ammonia-induced SCC.

[0064] Here, the above-mentioned "plate thickness of steel material" is defined as "t (unit: mm)", and the steel material in question refers to the steel material used for TIG welded joints. Furthermore, the above-mentioned "welding heat input" is defined as "Q (unit: J / cm)", and this welding heat input is a value calculated using the calculation formula described below. Furthermore, the above-mentioned "Ceq" is the carbon equivalent, and is a value calculated using formula (1) described below.

[0065] The carbon equivalent (Ceq) of the welding wire and the parameter (index P), which are the basis for adjusting the welding heat input (Q), will be described below.

[0066] [Index P] First, the technical significance of index P will be explained. Index P, which is derived as a parameter for obtaining optimal welding conditions, is expressed by the following formula (2): P = (Ceq + 0.06) × t / Q (2) Here, Ceq in formula (2) is the carbon equivalent of the welding wire obtained from formula (1), and t is the plate thickness [mm] of the steel material. Also, Q in formula (2) is the welding heat input, and the welding heat input Q [J / cm] = welding current I [A] × arc voltage E [V] × 60 / welding speed V [cm / min].

[0067] In the course of the above-mentioned investigations, the inventors found that it is preferable to adjust the welding heat input Q within the range of the following formula (3), which is obtained from the relationship between the Ceq of the welding wire and the thickness t of the steel material. From formula (3), formula (2) for the above index P was derived, and thereby a welding condition was arrived at that allows for easy adjustment of the welding heat input: (Ceq + 0.06) × t / 0.45 ≦ Q ≦ (Ceq + 0.06) × t / 0.15 ... (3)

[0068] By modifying the above equation (3) to find the index P in equation (2), it is found that the index P is preferably in the range of 0.15 to 0.45. If the index P is less than 0.15, the welding heat input Q is excessively large, and the cooling rate is slow relative to the hardenability of the weld metal and the weld heat-affected zone. As a result, the ferrite side plate becomes coarse, the structure becomes embrittled, and the absorbed energy decreases, which is undesirable. Furthermore, if the index P exceeds 0.45, the welding heat input Q decreases, and the cooling rate becomes excessive relative to the hardenability of the weld metal and the weld heat-affected zone. This hardens the weld metal and the weld heat-affected zone, and the surface hardness of the weld metal exceeds HV210, which is undesirable. The index P is more preferably 0.17 or more and more preferably 0.41 or less. The index P is even more preferably 0.20 or more and even more preferably 0.38 or less.

[0069] [Welding Heat Input Q] A supplementary explanation regarding the welding heat input Q is provided. The welding heat input Q in the TIG welding method is preferably 35.0 kJ / cm or less. If the welding heat input Q exceeds 35.0 kJ / cm, the cooling rate becomes slow relative to the hardenability of the weld metal and the weld heat-affected zone, resulting in coarsening of the ferrite side plates, embrittlement of the structure, and reduced absorbed energy. The welding heat input Q is more preferably 30.0 kJ / cm or less. On the other hand, if the welding heat input Q is less than 15.0 kJ / cm, the cooling rate becomes excessive relative to the hardenability of the weld metal and the weld heat-affected zone when the steel plate thickness t is large. This hardens the weld metal and the weld heat-affected zone, resulting in the surface hardness of the weld metal exceeding HV 210, which is undesirable. Therefore, the welding heat input Q is more preferably 15.0 kJ / cm or more. It is more preferable that the welding heat input Q is 16.0 kJ / cm or more.

[0070] [Carbon equivalent Ceq] Next, a supplementary explanation will be given regarding the carbon equivalent Ceq of the welding wire. First, the carbon equivalent (Ceq) will be explained. The carbon equivalent (Ceq) of general carbon steel in the present invention is expressed by the following formula (1): Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 (1) Here, the [element] shown in formula (1) represents the content (mass%) of the element, and if the element is not contained, the content is set to 0.

[0071] The Ceq expressed by the above formula (1) can also be applied to steel materials, welding wire, and the weld metal of the formed weld joint. However, in the present invention, it is the Ceq of the welding wire that determines the suitable welding heat input Q, and this Ceq is an important factor when setting the welding conditions. That is, to obtain an appropriate welding heat input Q, it is preferable that the Ceq of the welding wire be in the range of 0.24 to 0.36. This is because a Ceq of less than 0.24 results in insufficient strength, and a Ceq of more than 0.36 is undesirable because the surface hardness of the weld metal exceeds the appropriate value. The Ceq of the welding wire is more preferably 0.26 or more and more preferably 0.32 or less.

[0072] The preferred range for the welding current I is I≧100 [A]. This is necessary to sufficiently melt the wire. If the welding current I is less than 100 [A], the wire will not melt sufficiently, which may cause the wire to get stuck and damage the wire feeder. The upper limit of the welding current I is not particularly specified. Preferably, the welding current I is 250 [A] or less. The preferred range for the arc voltage E is E≧8 [V]. This is necessary to generate an arc. If the arc voltage E is less than 8 [V], the arc will not generate. The upper limit of the arc voltage E is not particularly specified. Preferably, the arc voltage E is 15 [V] or less. The preferred range for the welding speed V is V≦50 [cm / min]. This is to prevent residual fusion in the groove. If the welding speed V is more than 50 [cm / min], a welding defect known as insufficient fusion will occur. The lower limit of the welding speed V is not particularly specified. Preferably, the welding speed V is 4 cm / min or more.

[0073] [TIG welded joint] The TIG welded joint of the present invention is manufactured by the TIG welding method described above. The TIG welded joint comprises a base material and a weld metal. The basic composition of this base material is the same as the "basic composition of the steel plate" described above, so a description thereof will be omitted. The chemical composition of the base material may further include an optional selected composition in addition to the basic composition. This optional selected composition is the same as the "optionally selected composition of the steel material" described above, so a description thereof will be omitted. The remaining chemical composition of the steel plate other than the above-described chemical composition is Fe and unavoidable impurities.

[0074] [Chemical Composition of Weld Metal] The chemical composition of the weld metal in the TIG welded joint according to the present invention is as follows.

[0075] The basic composition includes C: 0.030-0.070%, Si: 0.30-0.70%, Mn: 0.80-1.55%, P: 0.025% or less, S: 0.0035% or less, Al: 0.040-0.150%, N: 0.010% or less, and O (oxygen): 0.010% or less. Furthermore, as needed, one or more elements selected from Cu: 0.50% or less, Ni: 0.60% or less, Cr: 0.40% or less, Mo: 0.40% or less, V: 0.30% or less, Nb: 0.03% or less, Ti: 0.02-0.15%, Zr: 0.01-0.12%, and B: 0.010% or less are contained. The balance is Fe and unavoidable impurities.

[0076] [Basic Composition of Weld Metal] First, the reasons for limiting the ranges of the basic composition of the weld metal will be explained.

[0077] [C: 0.030 to 0.070%] C is an element that provides the necessary strength for the weld metal, and to achieve this effect, a C content of 0.030% or more is required. However, if the C content exceeds 0.070%, the hardness increases and the toughness decreases. Therefore, the C content is limited to the range of 0.030 to 0.070%. The C content is preferably in the range of 0.035 to 0.060%, and more preferably in the range of 0.040 to 0.050%.

[0078] [Si: 0.30 to 0.70%] Si is an element necessary for ensuring the strength of the weld metal and for deoxidizing the weld metal, and to obtain these effects, a content of 0.30% or more is required. On the other hand, if the Si content exceeds 0.70%, the toughness of the weld metal deteriorates. Therefore, the Si content is limited to the range of 0.30 to 0.70%. The Si content is preferably in the range of 0.40 to 0.60%.

[0079] [Mn: 0.80 to 1.55%] 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 content of 0.80% or more is required. On the other hand, if the Mn content exceeds 1.55%, the toughness of the weld metal will be significantly deteriorated. Therefore, the Mn content is limited to the range of 0.80 to 1.55%. The Mn content is preferably in the range of 0.90 to 1.40%, and more preferably in the range of 1.00 to 1.20%.

[0080] [P: 0.025% or less] If the P content 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%. Since P is usually an element that is inevitably contained in steel materials as an impurity, it may be more than 0% industrially. The P content is preferably 0.001% or more.

[0081] [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. The lower limit of the S content is not particularly limited and may be 0%. Since S is usually an element that is inevitably contained in steel materials as an impurity, industrially, it may be more than 0%. The S content is preferably 0.001% or more.

[0082] [Al: 0.040 to 0.150%] Al must be contained in an amount of 0.040% or more to deoxidize the weld metal. On the other hand, if the Al content exceeds 0.150%, the toughness of the weld metal deteriorates. Therefore, the Al content is limited to the range of 0.040 to 0.150%. The Al content is preferably 0.060 to 0.120%, and more preferably 0.080 to 0.100%.

[0083] [N: 0.010% or less] N is an element contained as an unavoidable impurity, and if the N content exceeds 0.010%, it will lead to a decrease in toughness. Therefore, from the viewpoint of suppressing a decrease in the toughness and weldability of the weld metal, the N content is limited to 0.010% or less. The N content is preferably 0.008% or less, and more preferably 0.006% or less. Note that excessive reduction will lead to an increase in refining costs, so from the viewpoint of cost, the N content is more preferably 0.001% or more.

[0084] [O (oxygen): 0.010% 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. Therefore, the O content is limited to 0.010% or less. The O content is preferably 0.005% or less, and more preferably 0.003% or less. The O content may be 0%, and is more preferably 0.002% or more.

[0085] [Optionally Selected Composition of Weld Metal] Next, the reasons for specifying each composition range for the optionally selected composition of the weld metal will be explained.

[0086] [Cu: 0.50% or less] Cu has the function of increasing the strength while maintaining high toughness of the weld metal, but if it exceeds 0.50%, it causes hot embrittlement and deteriorates the surface properties. Therefore, when Cu is added, the Cu content is preferably 0.50% or less. The Cu content is more preferably 0.30% or less. On the other hand, when Cu is added, in order 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.

[0087] [Ni: 0.60% or less] Ni has the same function as Cu, increasing the strength of the weld metal while maintaining high toughness, but if the Ni content exceeds 0.60%, the hardness increases too much. Therefore, when Ni is added, the Ni content is preferably 0.60% or less. The Ni content is more preferably 0.40% 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.

[0088] [Cr: 0.40% or less] Like Cu, Cr increases the strength of the weld metal while maintaining high toughness. However, if the Cr content exceeds 0.40%, the hardness increases too much. Therefore, when Cr is added, the Cr content is preferably 0.40% or less. The Cr content is more preferably 0.30% 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, and more preferably 0.10% or more.

[0089] [Mo: 0.40% or less] Like Cr, Mo also increases the strength of the weld metal while maintaining high toughness, but if the Mo content exceeds 0.40%, the hardness increases too much. Therefore, when Mo is added, the Mo content is preferably 0.40% or less. The Mo content is more preferably 0.20% 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, and more preferably 0.05% or more.

[0090] [V: 0.30% 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 0.30% or less. The V content is more preferably 0.10% 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.

[0091] [Nb: 0.03% or less] Nb improves the strength of the weld metal by precipitation strengthening, but if added in excess, the toughness of the weld metal deteriorates, so when Nb is added, the Nb content is preferably 0.30-0.03% or less. On the other hand, when Nb is added, in order to further exert the effect of improving strength, the Nb content is preferably 0.01% or more, and more preferably 0.02% or more.

[0092] [Ti: 0.02 to 0.15%] 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 ferrite transformation nuclei. If the Ti content is less than 0.02%, this effect is small, but if the Ti content exceeds 0.15%, the amount of solute Ti increases, degrading toughness. Therefore, if Ti is added, the Ti content is preferably 0.02 to 0.15%. The Ti content is more preferably 0.06 to 0.10%.

[0093] [Zr: 0.01 to 0.12%] Zr is an element that has a grain growth suppression effect and contributes to improving the toughness of the weld metal. However, if the Zr content is less than 0.01%, it is difficult to achieve such an effect. Furthermore, if the Zr content is excessively high, exceeding 0.12%, the amount of solute Zr increases, deteriorating toughness. Therefore, when Zr is added, the Zr content is preferably 0.01 to 0.12%. The Zr content is more preferably 0.02 to 0.10%.

[0094] [B: 0.010% or less] B improves hardenability, thereby suppressing the formation of coarse grain boundary ferrite in the weld metal and improving the strength and toughness of the weld metal. However, if the B content exceeds 0.010%, weldability and toughness deteriorate, so the upper limit of the B content is set to 0.010%. Therefore, when B is added, the B content is preferably 0.010% or less. The B content is preferably 0.0006 to 0.005%.

[0095] [Remaining Composition of Weld Metal] The remaining chemical composition of the weld metal other than the above-mentioned chemical composition consists of Fe and unavoidable impurities. Examples of these unavoidable impurity elements include Sn, Sb, As, Pb, and Bi, and a total content of these elements of 0.10% or less is acceptable. Furthermore, as long as the basic chemical composition and optional chemical composition described above are satisfied, the inclusion of other unavoidable impurity elements is not prohibited, and such embodiments are also within the technical scope of the present invention.

[0096] [Carbon equivalent of weld metal] The carbon equivalent Ceq of the weld metal can be calculated from the following formula (1) described above: Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 (1) Here, the [element] in formula (1) represents the content (mass%) of the element, and if the element is not contained, the content is set to 0.

[0097] The Ceq of the weld metal is preferably in the range of 0.24 to 0.36. If the Ceq is less than 0.24, the strength becomes insufficient, and if it exceeds 0.36, the surface hardness exceeds the appropriate value.

[0098] [Average Vickers Hardness of Weld Metal Surface Layer] One of the characteristics of the weld metal in the TIG welded joint according to the present invention that is of interest is the average Vickers hardness HV (HV10) of the surface layer of the weld metal. Here, the average Vickers hardness HV refers to the average value of the hardness measured at multiple locations on the surface layer of the weld metal in a surface layer hardness test using a Vickers hardness tester, which will be described later. If this average value exceeds 210, stress corrosion cracking (SCC) due to ammonia may occur, so the average Vickers hardness HV is preferably set to 210 or less. There is no particular lower limit for the average Vickers hardness HV. If the HV is too low, the strength of the weld will decrease, so it is more preferable that it be 180 or more.

[0099] [Absorbed Energy of Weld Metal] The absorbed energy of the weld metal is a value used to evaluate low-temperature toughness. From the viewpoint of preventing brittle fracture in a welded structure to which the TIG-welded joint of the present invention is applied, the absorbed energy may be specified as a property of the weld metal to be 27 J or more. This absorbed energy can be measured by a Charpy impact test specified in JIS Z 2242:2018. If the absorbed energy is less than 27 J, there is a concern about the occurrence of brittle fracture, which is rapid fracture without plastic deformation. There is no particular lower limit for the absorbed energy. In order to reduce the probability of brittle fracture, it is more preferable that the absorbed energy be 47 J or more.

[0100] [Tensile Strength of Weld Metal] The tensile strength of the weld metal is a value used to evaluate the strength of the weld joint. The TIG welded joint of the present invention can fully exhibit the properties of the steel material if it has a strength equal to or greater than that of the steel plate, so the tensile strength may be specified as a property of the weld metal to be 440 MPa or more. This tensile strength can be measured using the tensile test method for butt-welded joints specified in JIS Z3121:2013. If the tensile strength is less than 440 MPa, the weld will fracture before the steel material fractures, preventing the steel material from fully exhibiting its performance. There is no particular upper limit for the tensile strength. If the tensile strength is too high, there is a risk of impairing the toughness of the weld, so it is more preferable to set it to 550 MPa or less.

[0101] 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.

[0102] The welding method was as follows: The steel materials (four types) shown in Table 1 were processed into the groove shape 2 (groove angle θ: 50°) shown in Figure 1, and TIG welding was performed with a root gap G of 3 to 5 mm. A copper plate was used as the backing plate 3.

[0103]

[0104] As the welding wire of the welding material, four types of solid wire (diameter 1.2 mm) shown in Table 2 were used.

[0105]

[0106] Using the above steel material and welding wire, multi-layer welding was performed from one side without preheating in a flat position under the various welding conditions shown in Tables 3 and 4. The shielding gas was 100% by volume of Ar gas, and the gas flow rate was 10 to 25 L / min. Depending on conditions such as plate thickness, the number of weld metal layers was limited to 10 layers, and the number of passes was set appropriately within the range of 1 to 2 passes per layer.

[0107] Table 3 shows the manufacturing conditions of the welded joints (specifically, the combinations of steel materials, welding wires, and welding conditions) and the chemical compositions of the obtained weld metals.

[0108]

[0109] Table 4 also shows the results of the index P according to the present invention under the same welding conditions, as well as the evaluation results of the mechanical properties and stress corrosion cracking of the weld metal described below.

[0110]

[0111] A joint tensile test and a Charpy impact test (vE) were performed on each of the obtained welded joints. -45 The Vickers hardness of the weld metal surface layer was measured as follows.

[0112] The Charpy impact test was carried out in accordance with the provisions of JIS Z 3128. The position from which the test specimens (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 each of the three specimens, and the absorbed energy (vE -45 ) was calculated, and the average value was used as the value of the weld metal of the welded joint.

[0113] The Vickers hardness of the surface layer was measured using a Vickers hardness tester at a depth of 0.5 mm from the surface of the steel and weld metal in the cross-sectional shape of the weld, at measurement positions b spaced 1 mm apart along a line parallel to the surface of the weld metal and steel, under a load of 10 kgf (HV10), as shown in Figure 3 . An example of the Vickers hardness measurement results is shown in Figure 4 . Figure 4 shows the individual values ​​measured for weld joint No. 1 (an example of the present invention) and weld joint No. 16 (a comparative example). The average of these measurements was used as the Vickers hardness value of the weld metal surface layer. For weld joint No. 1, the average value was 209, which was below 210 and therefore considered to be a joint less susceptible to stress corrosion cracking. For weld joint No. 16, the average value was 242, raising concerns about the occurrence of stress corrosion cracking in this joint.

[0114] [Evaluation Results] In all of the inventive examples of welded joints Nos. 1, 4, 5, 6, 8, 9, 10, 13, 14, 15, 17, 18, 19, 23, 24, 26, 27, and 28, the index P calculated from the welding heat input Q, the steel plate thickness t, and the welding wire Ceq values ​​using formula (2) satisfied the range of 0.15 to 0.45. Furthermore, the weld metal tensile strength was 440 MPa or more, the average weld metal surface hardness was HV210 or less, and the absorbed energy (vE -45 ) was 27 J or more, indicating excellent mechanical properties, and a welded joint having a hardness of the weld metal surface layer with suppressed hardening could be obtained.

[0115] On the other hand, in the comparative examples of welded joints Nos. 2, 3, 11, 12, 20, and 21, the index P was below the preferable lower limit, so the welding heat input was large and the cooling rate was slow relative to the hardenability of the weld metal. As a result, the ferrite side plates became coarse, embrittling the structure and significantly reducing the absorbed energy.

[0116] In the comparative examples of welded joints Nos. 7, 16, 22, and 25, the index P exceeded the preferable upper limit, resulting in a small welding heat input and an excessive cooling rate relative to the hardenability of the weld metal. As a result, the weld metal hardened, and the surface hardness of the weld metal exceeded HV210.

[0117] REFERENCE SIGNS LIST 1 Base material (steel material) 2 Groove portion 3 Backing plate 4 Weld metal 5 Test piece for Charpy impact test a Notch position b Measurement position for Vickers hardness test t Plate thickness θ Groove angle

Claims

1. A TIG welding method for forming a weld metal on a steel material using a shielding gas consisting of an inert gas and a welding wire, wherein the chemical composition of the steel material contains, in mass%, C: 0.030 to 0.090%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Al: 0.060% or less, N: 0.0010 to 0.0100%, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities; and the chemical composition of the welding wire contains, in mass%, C: 0.020 to 0.080%, Si: 0.30 to 0.90%, Mn: 0.50 to 1.80%, P: 0.025% or less, S: 0.035% or less. A TIG welding method comprising: Al: 0.040 to 0.190%, N: 0.0100% or less, and O: 0.0100% or less, with the balance consisting of Fe and unavoidable impurities.

2. The TIG welding method according to claim 1, wherein in addition to the chemical composition of the steel material, the steel material further contains, by mass%, 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.

3. The TIG welding method according to claim 1 or 2, wherein the welding wire further contains, in addition to the chemical composition thereof, one or more elements selected from the group consisting of Cu: 0.60% or less, Ni: 0.80% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.40% or less, Ti: 0.030 to 0.190%, and Zr: 0.01 to 0.15%, in mass%.

4. The TIG welding method according to any one of claims 1 to 3, wherein the welding wire has a carbon equivalent Ceq, expressed by the following formula (1), of 0.24 to 0.36, the welding heat input Q in the TIG welding method is 35.0 kJ / cm or less, and the index P, expressed by the following formula (2), is 0.15 to 0.45: Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 ... (1) Here, the [element] in formula (1) represents the content (mass%) of the element, and if the element is not contained, the content is set to 0. P = (Ceq + 0.06) × t / Q (2) Here, Ceq in formula (2) is the carbon equivalent of the welding wire obtained from formula (1), t is the plate thickness (mm) of the steel material, and Q in formula (2) is the welding heat input, where Q (J / cm) = welding current I (A) × arc voltage E (V) × 60 / welding speed V (cm / min).

5. A TIG welded joint of steel, wherein the chemical composition of the weld metal contains, in mass %, C: 0.030 to 0.070%, Si: 0.30 to 0.70%, Mn: 0.80 to 1.55%, P: 0.025% or less, S: 0.035% or less, Al: 0.040 to 0.150%, N: 0.010% or less, and O: 0.010% or less, with the balance being Fe and unavoidable impurities, the carbon equivalent Ceq of the weld metal, expressed by the following formula (1), is 0.24 to 0.36, and the average Vickers hardness HV of the surface layer of the weld metal is 210 or less. Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15 (1) Here, the [element] in formula (1) represents the content (mass%) of the element, and if the element is not contained, the content is set to 0.

6. A TIG welded joint according to claim 5, further containing, in addition to the chemical composition of the weld metal, one or more elements selected from, by mass%, Cu: 0.50% or less, Ni: 0.60% or less, Cr: 0.40% or less, Mo: 0.40% or less, V: 0.30% or less, Nb: 0.03% or less, Ti: 0.02 to 0.15%, Zr: 0.01 to 0.12%, and B: 0.010% or less.

7. A TIG welded joint as set forth in claim 5 or 6, wherein the absorbed energy at the center of the weld metal at -45°C is 27 J or more.

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