Weld metal, welded joint, and welded structure

Abstract

To provide a weld metal exhibiting superior strength and toughness after both AW and SR and ensuring excellent toughness regardless of the position of the weld metal.SOLUTION: The weld metal contains, in mass%, C: 0.050% or more and 0.085% or less, and Mo: 0.10% or more and 0.35% or less, and further has specified contents of Si, Mn, S, Ni, Ti, B, O, Cu, Cr, P, and N, wherein when the contents of each component in the weld metal are expressed in mass% as [component], the value A1 calculated by formula (1): A1=[Si]+[Mn]+[Cr]-[Mo] is 1.84 or less, and the value A2 calculated by formula (2): A2=0.558+3.207×[C]+0.304×[Si]+0.294×[Mn]+0.119×[Ni]+1.011×[Mo]+4.093×[Ti]+15.364×[B] is 1.74 or more and 1.90 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a weld metal having excellent strength and toughness, a welded joint including the weld metal, and a welded structure including the welded joint. [Background technology]

[0002] Offshore structures used in the drilling and production of oil, gas, etc., and pipelines used to transport oil, gas, etc., are becoming larger and are increasingly operating in cold regions, and the steel plates and welding materials used in the construction of these welded structures are required to have high strength as well as excellent toughness at low temperatures.When constructing such welded structures, heat treatment for stress relief (SR) is sometimes performed after welding.

[0003] However, this SR reduces the strength and toughness of the weld, and the required properties may not be obtained. Therefore, in order to improve the strength and toughness after heat treatment, a weld metal containing a specified amount of Ni has been proposed.

[0004] For example, Patent Document 1 discloses a weld metal with improved strength and low-temperature toughness after stress relief annealing. The weld metal contains Mo, which has the effect of suppressing coarsening of grain boundary carbides and annealing softening, and the chemical composition of the weld metal is controlled to specify the average equivalent circle diameter of carbides of a predetermined size that are generated on the grain boundaries of the weld metal during welding.

[0005] Patent Document 2 discloses a flux-cored wire suitable for obtaining a weld metal that is excellent in strength and low-temperature toughness both as welded (AW) and after post-weld heat treatment (PWHT). Patent Document 2 describes that the low-temperature toughness after PWHT can be further improved by reducing the contents of V and Nb. It also describes that the contents of C, Si, Mn, Ni, B, Ti, Al, Mg, and Ti oxides can be appropriately adjusted to consistently obtain a weld metal that is excellent in strength and low-temperature toughness after AW and PWHT.

[0006] Furthermore, Patent Document 3 discloses a weld metal that has excellent tensile strength after SR and also has excellent low-temperature toughness regardless of the position within the weld metal. Patent Document 3 describes that if the weld metal contains an appropriate amount of Cu, the toughness after SR can be improved. It also describes that by appropriately controlling the value obtained from a predetermined formula using the contents of Si, Mn, Cr, and Mo in the weld metal, it is possible to prevent the toughness from becoming low depending on the measurement position. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2014-195832 [Patent Document 2] Japanese Patent Application Publication No. 2017-185521 [Patent Document 3] Japanese Patent Publication No. 2022-97256 Summary of the Invention [Problem to be solved by the invention]

[0008] However, in the above-mentioned offshore structures and pipelines, problems such as sulfide stress corrosion cracking (SSCC) and hydrogen embrittlement occur in a sour environment containing hydrogen sulfide. To address these problems, the standard (NACE MR0175) of the National Association of Corrosion Engineers (NACE) restricts the Ni content in weld metal to 1 mass% or less.

[0009] However, the Ni content is specified to be 0.20 to 2.00% in the weld metal described in Patent Document 1. The Ni content is also specified to be 0.1 to 3.0% in the flux-cored wire described in Patent Document 2, and it is thought that the weld metal also contains a similar amount of Ni, so it is not possible to fully meet the requirements of NACE.

[0010] Furthermore, the weld metal described in Patent Document 1 may have reduced mechanical properties depending on the test sampling position. Furthermore, the toughness in a temperature range below -40°C is not considered, and sufficient excellent low-temperature toughness may not be obtained. Furthermore, the flux-cored wire described in Patent Document 2 provides weld metal with excellent low-temperature toughness at -60°C, but the mechanical properties may be reduced depending on the measurement position of the weld metal.

[0011] Furthermore, the weld metal described in Patent Document 3 does not take into consideration toughness in the temperature range below -40°C, and there is a risk that excellent low-temperature toughness will not be sufficiently obtained.

[0012] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a weld metal that has excellent strength and toughness both after AW and SR and that can obtain excellent toughness regardless of the position of the weld metal, a welded joint including this weld metal, and a welded structure including this welded joint. [Means for solving the problem]

[0013] The above object of the present invention is achieved by the following configuration [1] relating to the weld metal.

[0014] [1] C: 0.050 mass% or more and 0.085 mass% or less, Si: 0.10% by mass or more and 0.40% by mass or less, Mn: 1.20 mass% or more and 1.60 mass% or less, S: more than 0% by mass and 0.020% by mass or less, Ni: 0.60% by mass or more and 1.00% by mass or less, Mo: 0.10% by mass or more and 0.35% by mass or less, Ti: 0.030 mass% or more and 0.080 mass% or less, B: 0.0010% by mass or more and 0.0045% by mass or less, and O: Contains 0.020% by mass or more and 0.080% by mass or less, Cu: 0.14% by mass or less, Cr: 0.20% by mass or less, P: 0.020% by mass or less, N: 0.0080% by mass or less, The balance is Fe and unavoidable impurities, and The Si content in the weld metal is expressed as [Si] in mass%. The Mn content in the weld metal is expressed as [Mn] in mass%. The Cr content in the weld metal is expressed as [Cr] in mass%. The Mo content in the weld metal is expressed as [Mo] in mass%. The carbon content in the weld metal is expressed as [C] in mass%. The Ni content in the weld metal is expressed as [Ni] in mass%. The Ti content in the weld metal is expressed as [Ti] in mass%. When the B content in the weld metal is expressed as [B] in mass%, The value A1 calculated by the following formula (1): 1.84 or less, A weld metal characterized in that the value A2 calculated by the following formula (2) is 1.74 or more and 1.90 or less. A1=[Si]+[Mn]+[Cr]-[Mo]...Equation (1) A2=0.558+3.207×[C]+0.304×[Si]+0.294×[Mn]+0.119×[Ni]+1.011×[Mo]+4.093×[Ti]+15.364×[B]...Formula (2)

[0015] A preferred embodiment of the present invention relating to the weld metal relates to the following [2].

[0016] [2] Furthermore, at least one selected from Al, Nb, and V, Al: 0.030% by mass or less, Nb: 0.020% by mass or less, The weld metal according to [1], characterized in that it contains V in the range of 0.020 mass % or less.

[0017] The above object of the present invention is achieved by the following configuration [3] relating to a welded joint.

[0018] [3] A welded joint comprising the weld metal according to [1] or [2].

[0019] The above object of the present invention is achieved by the following configuration [4] relating to a welded structure.

[0020] [4] A welded structure comprising the welded joint described in [3]. [Effects of the Invention]

[0021] According to the present invention, it is possible to provide a weld metal that has excellent strength and toughness both after AW and SR and that can obtain excellent toughness regardless of the position of the weld metal, a welded joint including this weld metal, and a welded structure including this welded joint. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present inventors have conducted extensive research to obtain weld metal that has excellent toughness even after SR. It has been known that increasing the content of alloying elements in the weld metal can suppress the precipitation of grain boundary ferrite, but on the other hand, if the grain boundary ferrite disappears, prior γ grain boundaries, which are embrittled areas after SR, remain, reducing the toughness of the weld metal. Therefore, the present inventors have discovered that by adding Mo, an element effective in suppressing grain boundary embrittlement, to the weld metal to suppress the precipitation of grain boundary ferrite, it is possible to improve the low-temperature toughness after SR.

[0023] The present inventors have also conducted extensive research to obtain excellent toughness regardless of the position in the weld metal. Here, the toughness of the weld metal decreases at positions shifted from the center compared to the center (pass intersection). This is because the effects of grain boundary ferrite and grain boundary embrittlement become more pronounced depending on the location in the weld metal. The present inventors have found that by using Mo to suppress the precipitation of grain boundary ferrite and by controlling the value obtained by the formula using the contents of Si, Cr, Mn, and Mo, the effects of grain boundary embrittlement can be suppressed, thereby solving the above-mentioned problem.

[0024] Furthermore, the present inventors have derived an equation using the contents of C, Si, Mn, Ni, Mo, Ti, and B in the weld metal, and have found that by controlling the values ​​obtained from this equation, it is possible to obtain a weld metal that has high strength even after SR. The present invention was made based on the above findings.

[0025] Hereinafter, a mode for carrying out the present invention (hereinafter referred to as "the present embodiment") will be described in detail. Note that the present invention is not limited to the embodiment described below, and can be carried out with any modifications within the scope of the gist of the present invention.

[0026] [Weld metal] The chemical components contained in the weld metal according to this embodiment and the reasons for the numerical limitations of their contents will be described in more detail. In this embodiment, regardless of the form in which each element whose content is defined below is contained in the weld metal, it shall be defined by the converted value converted to the elemental form alone.

[0027] <C: 0.050 mass% or more and 0.085 mass% or less> C is a component that segregates at grain boundaries and has the effect of suppressing the precipitation of grain boundary ferrite to ensure the strength of the weld metal after SR annealing. When the C content in the weld metal is less than 0.050 mass%, coarse grain boundary ferrite is formed, which causes a decrease in toughness and makes it impossible to obtain the desired strength after SR annealing. Therefore, the C content in the weld metal shall be 0.050 mass% or more, and preferably 0.055 mass% or more. On the other hand, when the C content in the weld metal exceeds 0.085 mass% and is excessively contained in the weld metal, the coarsening of grain boundary carbides is promoted, resulting in a decrease in toughness after SR. Therefore, the C content in the weld metal shall be 0.085 mass% or less, preferably 0.080 mass% or less, and more preferably 0.075 mass% or less.

[0028] <Si: 0.10 mass% or more and 0.40 mass% or less> Si is a component that has the effect of ensuring the strength of the weld metal after SR annealing. When the Si content in the weld metal is less than 0.10 mass%, the desired strength cannot be obtained after SR annealing. Therefore, the Si content in the weld metal shall be 0.10 mass% or more, and preferably 0.15 mass% or more. On the other hand, when the Si content in the weld metal exceeds 0.40 mass% and is excessively contained in the weld metal, the temper embrittlement caused by SR annealing is promoted, resulting in a decrease in toughness. Therefore, the Si content in the weld metal shall be 0.40 mass% or less, preferably 0.35 mass% or less, more preferably 0.30 mass% or less, and even more preferably 0.25 mass% or less.

[0029] <Mn: 1.20 mass% or more and 1.60 mass% or less> Mn is a component that forms oxides which serve as the starting points for the formation of fine structures during welding. Also, by segregating at grain boundaries, Mn can suppress the precipitation of grain boundary ferrite. Therefore, when Mn is contained in the weld metal at an appropriate content, the strength and toughness of the weld metal can be improved. If the Mn content in the weld metal is less than 1.20 mass%, the desired strength and toughness cannot be obtained. Thus, the Mn content in the weld metal should be 1.20 mass% or more, more preferably 1.25 mass% or more, and even more preferably 1.30 mass% or more. On the other hand, if the Mn content in the weld metal exceeds 1.60 mass% and is excessively contained in the weld metal, temper embrittlement due to SR annealing is promoted, leading to a decrease in toughness. Therefore, the Mn content in the weld metal should be 1.60 mass% or less, preferably 1.55 mass% or less.

[0030] <S: More than 0 mass% and 0.020 mass% or less> S is a component that forms MnS which serves as the starting point for the formation of fine structures during welding and has the effect of improving the strength and toughness of the weld metal. The above effect can be obtained even if the S content in the weld metal is trace. Therefore, the S content in the weld metal should be more than 0 mass%, preferably 0.002 mass% or more, and more preferably 0.004 mass% or more. On the other hand, if the S content in the weld metal exceeds 0.020 mass% and is excessively contained in the weld metal, temper embrittlement due to SR annealing is promoted, leading to a decrease in toughness. Therefore, the S content in the weld metal should be 0.020 mass% or less, preferably 0.015 mass% or less, and more preferably 0.010 mass% or less.

[0031] <Ni: 0.60 mass% or more and 1.00 mass% or less> Ni is a component that has the effect of improving the toughness of the matrix at low temperatures in the weld metal. If the Ni content in the weld metal is less than 0.60% by mass, the effect of improving toughness at low temperatures cannot be obtained. Therefore, the Ni content in the weld metal should be 0.60% by mass or more, preferably 0.65% by mass or more, more preferably 0.70% by mass or more, and even more preferably 0.75% by mass or more. On the other hand, if the Ni content in the weld metal exceeds 1.00% by mass, the Ni content in the weld metal is outside the range defined by NACE MR0175, and in a hydrogen sulfide environment, the susceptibility to sulfide stress corrosion cracking increases. Therefore, the Ni content in the weld metal should be 1.00% by mass or less, preferably 0.95% by mass or less.

[0032] <Mo: 0.10% by mass or more and 0.35% by mass or less> Mo is a component that segregates at grain boundaries and has the effect of suppressing the precipitation of grain boundary ferrite, as well as the effect of suppressing temper embrittlement and the growth of grain boundary carbides. Also, Mo is a component that has the effect of suppressing annealing softening. In this embodiment, as described above, by utilizing Mo, which is an element effective in suppressing grain boundary embrittlement, to suppress grain boundary ferrite, a weld metal having excellent strength and toughness can be obtained. If the Mo content in the weld metal is less than 0.10% by mass, the suppression of the precipitation of grain boundary ferrite becomes insufficient, and tempering may cause embrittlement, and the low-temperature toughness after SR may deteriorate. Therefore, the Mo content in the weld metal should be 0.10% by mass or more, preferably 0.15% by mass or more. On the other hand, if the Mo content in the weld metal exceeds 0.35% by mass, a large amount of fine carbides will precipitate due to SR annealing, so the strength of the weld metal will increase excessively, and as a result, the low-temperature toughness may decrease. Therefore, the Mo content in the weld metal should be 0.35% by mass or less, preferably 0.30% by mass or less, and more preferably 0.25% by mass or less.

[0033] <Ti: 0.030% by mass or more and 0.080% by mass or less> Ti is a component that forms oxides which serve as the nuclei for the formation of fine structures during welding. Therefore, if Ti is contained in the weld metal at an appropriate content, the toughness of the weld metal can be improved. If the Ti content in the weld metal is less than 0.030% by mass, the desired toughness cannot be obtained. Therefore, the Ti content in the weld metal should be 0.030% by mass or more, preferably 0.040% by mass or more, and more preferably 0.045% by mass or more. On the other hand, if the Ti content exceeds 0.080% by mass and is excessively contained in the weld metal, fine carbides will precipitate, resulting in an excessive increase in the strength of the weld metal and a decrease in low-temperature toughness. Therefore, the Ti content in the weld metal should be 0.080% by mass or less, preferably 0.070% by mass or less, and more preferably 0.065% by mass or less.

[0034] <B: 0.0010% by mass or more and 0.0045% by mass or less> B is a component that has the effect of suppressing the formation of grain boundary ferrite, which has an adverse effect on the strength and toughness of the weld metal. If the B content in the weld metal is less than 0.0010% by mass, the effect of suppressing the decrease in the strength and toughness of the weld metal cannot be sufficiently obtained. Therefore, the B content in the weld metal should be 0.0010% by mass or more, preferably 0.0015% by mass or more, and more preferably 0.0018% by mass or more. On the other hand, if the B content in the weld metal exceeds 0.0045% by mass, it will prevent the formation of fine structures from oxides during welding and excessive precipitates will form after SR, resulting in deterioration of toughness. Therefore, the B content in the weld metal should be 0.0045% by mass or less, preferably 0.0040% by mass or less, and more preferably 0.0030% by mass or less.

[0035] <O: 0.020% by mass or more and 0.080% by mass or less> O is a component that forms oxides which serve as the starting points for the formation of fine structures during welding and can improve the toughness of the weld metal. If the O content in the weld metal is less than 0.020% by mass, the desired toughness cannot be obtained. Therefore, the O content in the weld metal should be 0.020% by mass or more, preferably 0.030% by mass or more, and more preferably 0.040% by mass or more. On the other hand, if the O content in the weld metal exceeds 0.080% by mass, it may cause coarsening of the oxides and potentially reduce the toughness of the weld metal. Therefore, the O content in the weld metal should be 0.080% by mass or less, preferably 0.070% by mass or less, and more preferably 0.060% by mass or less.

[0036] <Cu: 0.14% by mass or less> Cu is a component that has the effect of improving the toughness of the low-temperature matrix. However, in this embodiment, it is not necessarily required for the weld metal to contain Cu, and it may be 0% by mass. On the other hand, if the Cu content in the weld metal exceeds 0.14% by mass and is excessively contained in the weld metal, it will promote temper embrittlement and cause a reduction in toughness. Therefore, the Cu content in the weld metal should be 0.14% by mass or less, preferably 0.10% by mass or less, and more preferably 0.05% by mass or less.

[0037] <Cr: 0.20% by mass or less> Cr is a component that has the effect of ensuring the strength of the weld metal. However, in this embodiment, it is not necessarily required for the weld metal to contain Cr, and it may be 0% by mass. On the other hand, if the Cr content in the weld metal exceeds 0.20% by mass, it will promote temper embrittlement after SR and cause a reduction in toughness. Therefore, the Cr content in the weld metal should be 0.20% by mass or less, preferably 0.10% by mass or less, and more preferably 0.05% by mass or less.

[0038] <P: 0.020% by mass or less> P is a component that segregates at grain boundaries, causes embrittlement of grain boundaries, and thus leads to a decrease in toughness. It is an inevitable component in the weld metal. When the P content in the weld metal exceeds 0.020 mass%, and it is excessively contained in the weld metal, temper embrittlement due to SR annealing is promoted, resulting in a decrease in toughness. Therefore, the P content in the weld metal should be 0.020 mass% or less, preferably 0.015 mass% or less, and more preferably 0.010 mass% or less. Since P is an inevitable impurity, the lower the P content, the better, and the lower limit is not particularly limited. That is, the P content in the weld metal may be 0 mass%, but setting it to 0 mass% is industrially impossible, so it is actually more than 0 mass%.

[0039] <N: 0.0080 mass% or less> N is a component that excessively increases the strength of the weld metal and causes a decrease in low-temperature toughness. It is an inevitable component in the weld metal. When the N content in the weld metal exceeds 0.0080 mass%, and it is excessively contained in the weld metal, the strength of the weld metal may increase excessively, thereby potentially reducing the low-temperature toughness. Therefore, the N content in the weld metal should be 0.0080 mass% or less, preferably 0.0070 mass% or less, and more preferably 0.0060 mass% or less. Since N is an inevitable impurity, the lower the N content, the better, and the lower limit is not particularly limited. That is, the N content in the weld metal may be 0 mass%, but setting it to 0 mass% is industrially impossible, so it is actually more than 0 mass%.

[0040] <Value A1 calculated by formula (1): 1.84 or less> In weld metal, toughness tends to decrease at positions shifted from the center compared to the center (pass intersection). This is because the effects of grain boundary ferrite and grain boundary embrittlement become more pronounced depending on the location in the weld metal. In this embodiment, by adding Mo to the weld metal within the above range and controlling the value A1 calculated by the following formula (1) based on the contents of Si, Cr, Mn, and Mo, the effects of grain boundary embrittlement can be suppressed, and excellent toughness can be obtained regardless of the position in the weld metal.

[0041] If the value A1 calculated by the following formula (1) exceeds 1.84, the toughness at positions shifted from the center of the weld metal decreases. Therefore, the value A1 calculated by the following formula (1) is set to 1.84 or less, preferably 1.70 or less, and more preferably 1.60 or less. On the other hand, the value A1 calculated by the following formula (1) is not particularly limited to a lower limit, but from the viewpoint of ensuring strength, it is preferably 1.00 or more, and more preferably 1.30 or more.

[0042] A1=[Si]+[Mn]+[Cr]-[Mo]...Equation (1) However, in the above formula (1), [Si] is a value representing the Si content in the weld metal in mass %. [Mn] is a value representing the Mn content in the weld metal in mass %. [Cr] is a value representing the Cr content in the weld metal in mass %. [Mo] is a value representing the Mo content in the weld metal in mass %.

[0043] <Value A2 calculated by formula (2): 1.74 or more and 1.90 or less> Generally, SR reduces the strength of the weld metal, but in this embodiment, by controlling the contents of the above-mentioned components and the value A2 calculated by the following formula (2), high strength can be obtained even after SR. If the value A2 calculated by the following formula (2) is less than 1.74, the strength after SR will decrease. Therefore, the value A2 calculated by the following formula (2) is set to 1.74 or more, and preferably 1.76 or more.

[0044] On the other hand, if the value A2 calculated by the following formula (2) exceeds 1.90, the strength increases excessively and the toughness after SR decreases. Therefore, the value A2 calculated by the following formula (2) is set to 1.90 or less, preferably 1.88 or less, and more preferably 1.86 or less.

[0045] A2=0.558+3.207×[C]+0.304×[Si]+0.294×[Mn]+0.119×[Ni]+1.011×[Mo]+4.093×[Ti]+15.364×[B]...Formula (2) However, in the above formula (2), [C] is a value representing the C content in the weld metal in mass %. [Si] is a value representing the Si content in the weld metal in mass %. [Mn] is a value representing the Mn content in the weld metal in mass %. [Ni] is a value representing the Ni content in the weld metal in mass %. [Mo] is a value representing the Mo content in the weld metal in mass %. [Ti] is a value representing the Ti content in the weld metal in mass %. [B] is a value representing the B content in the weld metal in mass %.

[0046] The weld metal according to this embodiment preferably further contains at least one element selected from Al, Nb, and V, each within the content ranges shown below. The reasons for limiting the content of each element when these elements are contained in the weld metal are explained below.

[0047] <Al: 0.030 mass% or less> Al is not an essential component in the weld metal according to this embodiment, but it is a component that forms oxides acting as the transformation starting point of acicular ferrite and has the effect of improving the strength and toughness of the weld metal. Therefore, it is preferable that Al is contained in the weld metal. Even if the Al content in the weld metal is trace, the effect of improving the strength and toughness of the weld metal can be obtained. However, when Al is contained in the weld metal, the Al content in the weld metal is preferably 0.002 mass% or more, and more preferably 0.004 mass% or more. On the other hand, when the Al content in the weld metal exceeds 0.030 mass%, the oxides become coarsened, and instead, the toughness decreases. Therefore, when Al is contained in the weld metal, the Al content in the weld metal is preferably 0.030 mass% or less, more preferably 0.020 mass% or less, and even more preferably 0.010 mass% or less.

[0048] <Nb: 0.020 mass% or less> Nb is not an essential component in the weld metal according to this embodiment, but it is a component that forms fine carbides after SR and has the effect of ensuring strength. Therefore, it is preferable that Nb is contained in the weld metal. Even if the Nb content in the weld metal is trace, the effect of ensuring the strength of the weld metal can be obtained. However, when Nb is contained in the weld metal, the Nb content in the weld metal is preferably 0.001 mass% or more, and more preferably 0.002 mass% or more. On the other hand, when the Nb content in the weld metal exceeds 0.020 mass%, a large amount of fine carbides precipitate, which may cause the strength of the weld metal to rise excessively, and as a result, the low-temperature toughness may decrease. Therefore, when Nb is contained in the weld metal, the Nb content in the weld metal is preferably 0.020 mass% or less, more preferably 0.015 mass% or less, even more preferably 0.010 mass% or less, and particularly preferably 0.005 mass% or less.

[0049] <V: 0.020 mass% or less> Although V is not an essential component in the weld metal according to this embodiment, it is a component that forms fine carbides after SR and has the effect of ensuring strength, so it is preferable that the weld metal contain V. Even if the V content in the weld metal is small, the effect of ensuring the strength of the weld metal can be obtained, but when V is contained in the weld metal, the V content in the weld metal is preferably 0.002 mass% or more, and more preferably 0.005 mass% or more. On the other hand, if the V content in the weld metal exceeds 0.020 mass%, a large amount of fine carbides will precipitate, causing the strength of the weld metal to increase excessively, which may result in a decrease in low-temperature toughness. Therefore, when V is contained in the weld metal, the V content is preferably 0.020 mass% or less, more preferably 0.015 mass% or less, and even more preferably 0.010 mass% or less.

[0050] <Remainder> In this embodiment, the balance other than the above components is Fe and unavoidable impurities. The total amount of unavoidable impurities in the weld metal is preferably regulated to 0.01 mass% or less. In addition to Mg, Na, K, and F, unavoidable impurities in the weld metal that remain without being discharged as slag include As, Sb, Sn, Co, and Zn.

[0051] [Weld metal manufacturing method] The weld metal according to this embodiment can be produced by, for example, gas-shielded arc welding using a welding wire selected so that the components in the weld metal fall within the above-mentioned ranges. There are no particular limitations on the welding conditions, and as long as the components in the weld metal fall within the above-mentioned ranges, general conditions can be used, such as the type of base metal, welding voltage, welding current, and welding position. There are also no limitations on the shielding gas, but a more preferable example is 80% by volume Ar-20% by volume CO.

[0052] The welding material for producing the weld metal according to this embodiment may have a composition corresponding to the components of the weld metal. However, by appropriately adjusting the components that are likely to be oxidized and consumed during welding, it is possible to obtain a weld metal having the desired components.

[0053] [Welded joints] The weld joint according to this embodiment includes the weld metal according to this embodiment. Note that the weld joint according to this embodiment is not particularly limited in terms of the conditions other than the inclusion of the weld metal according to this embodiment. For example, the base metal may be a weld metal having a tensile strength of 490 N / mm 2 It is preferable to use steel of the grade.

[0054] [Welded structures] The welded structure according to this embodiment includes the welded joint according to this embodiment. Note that the welded structure according to this embodiment is not particularly limited in terms of conditions other than the inclusion of the welded joint according to this embodiment. Examples of the welded structure according to this embodiment include offshore structures used in the drilling and production of oil, gas, etc., and pipelines used to transport oil, gas, etc. [Example]

[0055] Hereinafter, examples of the weld metal according to this embodiment and comparative examples will be described.

[0056] [Evaluation of mechanical properties of weld metal] <Wire production> First, a strip-shaped steel sheath was prepared using an SPCC steel sheet having the composition shown in Table 1 below, and this steel sheath was filled with flux to prepare a flux-cored wire with a diameter of 1.2 mm. The chemical components and their contents in the flux-cored wire are shown in Table 2 below. The remainder of the components shown in Table 2 includes O, Mg, Na, K, and F derived from oxides, as well as unavoidable impurities. The flux filling rate was set to be in the range of 10 to 20 mass %.

[0057] <Gas shielded arc welding> Next, the obtained flux-cored wire was used to perform gas-shielded arc welding on base metals having thicknesses and chemical compositions shown in Table 3 below. In this example, a V-groove was formed, and gas-shielded arc welding was performed under the welding conditions shown in Table 4 below to form a weld metal. The chemical composition of the obtained weld metal is shown in Table 5 below, and the value A1 calculated using formula (1) and the value A2 calculated using formula (2) based on the contents of specific components in the weld metal are shown in Table 6 below. The remainder of the chemical composition of the weld metal shown in Table 5 is Fe and unavoidable impurities.

[0058] <Evaluation of mechanical properties> The mechanical properties of the weld metal were evaluated in accordance with the "Tensile and impact test method for weld metal" specified in JIS Z 3111:2005. A tensile test piece (A0 type) and an impact test piece (V-notch test piece) were taken from the center of the obtained weld metal in the plate thickness direction, and the tensile performance and impact performance were evaluated.

[0059] (Collection of test specimens) Tensile test specimens were taken from the as-welded weld metal and from weld metal that had been subjected to SR at 620°C for 6.2 hours, and subjected to tensile testing. Impact test specimens were taken from the as-welded weld metal and from weld metal that had been subjected to SR at 620°C for 6.2 hours. Two types of specimens were taken: one with a notch formed in the center of the structure consisting mainly of the reheated zone (center notch), and the other with a notch formed 4 mm away from the center of the structure consisting mainly of the original zone (center + 4 mm notch).

[0060] (Tensile test) The tensile test was carried out on each test piece at room temperature (about 20±2°C), and the tensile strength was measured to evaluate the tensile performance. In the present invention examples, all test pieces were judged to have good strength when the tensile strength TS was 580 MPa or more.

[0061] (Impact test) The impact test was carried out on each test piece at a test temperature of -60°C, and the toughness was evaluated by measuring the Charpy absorbed energy (vE-60°C). In the present invention, the toughness was determined to be good when the average value of the Charpy absorbed energy (vE-60°C) at -60°C of three test pieces (center notch) in which a notch was formed at a central position where the structure mainly consisted of the reheated portion, and the average value of the Charpy absorbed energy (vE-60°C) at -60°C of three test pieces (center +4mm notch) in which a notch was formed at a position shifted 4 mm from the center where the structure mainly consisted of the original portion, were 50 J or more. In all of the as-welded test pieces and test pieces that had been subjected to SR at a temperature of 620°C for 6.2 hours, those that had good strength and toughness were deemed to have passed, and those that did not were deemed to have failed.

[0062] The evaluation results of the mechanical properties of the weld metal are also shown in Table 6 below. In Table 6, the value A1 according to the formula (1) and the value A2 according to the formula (2) represent the following values. A1=[Si]+[Mn]+[Cr]-[Mo]...Equation (1) A2=0.558+3.207×[C]+0.304×[Si]+0.294×[Mn]+0.119×[Ni]+1.011×[Mo]+4.093×[Ti]+15.364×[B]...Formula (2)

[0063] However, in the above formulas (1) and (2), [Si] is the value of the Si content in the weld metal expressed in mass %. [Mn] is the Mn content in the weld metal expressed in mass %. [Cr] is the Cr content in the weld metal expressed in mass %. [Mo] is the Mo content in the weld metal expressed in mass %. [C] is the C content in the weld metal expressed in mass %. [Ni] is the Ni content in the weld metal expressed in mass %. [Ti] is the value of the Ti content in the weld metal expressed in mass %. [B] is the B content in the weld metal expressed in mass %.

[0064] [Table 1]

[0065] [Table 2]

[0066] [Table 3]

[0067] [Table 4]

[0068] [Table 5]

[0069] [Table 6]

[0070] As shown in Tables 5 and 6 above, in Invention Examples Nos. 1 to 7, the contents of each component in the weld metal and the values ​​A1 and A2 calculated by Equation (1) and Equation (2), respectively, were within the ranges specified in the present invention. Therefore, the strength and toughness were excellent both after AW and SR, and excellent toughness could be obtained regardless of the position of the weld metal.

[0071] On the other hand, in Comparative Example No. 1, the C content in the weld metal was below the lower limit of the range specified in the present invention, and therefore the strength after SR was reduced. In Comparative Example No. 2, the Mo content in the weld metal was less than the lower limit of the range specified in the present invention, the value A1 according to formula (1) exceeded the upper limit of the range specified in the present invention, and the value A2 according to formula (2) was less than the lower limit of the range specified in the present invention. Therefore, the toughness at positions shifted from the center of the weld metal after SR and the strength after SR decreased. In Comparative Examples 3 and 4, the Mo content in the weld metal was below the lower limit of the range specified in the present invention, and the value A2 according to formula (2) was below the lower limit of the range specified in the present invention, so the strength of the weld metal was reduced.

[0072] In Comparative Example No. 5, the C and Mn contents in the weld metal were less than the lower limit of the range specified in the present invention, and the value A2 calculated by formula (2) was less than the lower limit of the range specified in the present invention, so both the strength and toughness of the weld metal were reduced. In Comparative Example No. 6, the value A2 obtained by formula (2) was less than the lower limit of the range defined in the present invention, and therefore the strength of the weld metal after SR was reduced. In Comparative Example No. 7, the value A2 obtained by formula (2) exceeded the range specified in the present invention, and therefore the toughness after SR was reduced. In Comparative Examples 8 to 11, the Cu content in the weld metal exceeded the upper limit of the range specified in the present invention, and therefore the toughness after SR was reduced.

Claims

1. C: 0.050% by mass or more and 0.085% by mass or less, Si: 0.10% by mass or more and 0.40% by mass or less, Mn: 1.20% by mass or more and 1.60% by mass or less, S: more than 0% by mass and not more than 0.020% by mass, Ni: 0.60% by mass or more and 1.00% by mass or less, Mo: 0.10% by mass or more and 0.35% by mass or less, Ti: 0.030 mass% or more and 0.080 mass% or less, B: 0.0010% by mass or more and 0.0045% by mass or less, and O: 0.020% by mass or more and 0.080% by mass or less, Cu: 0.14% by mass or less, Cr: 0.20% by mass or less, P: 0.020% by mass or less, N: 0.0080% by mass or less; The balance is Fe and unavoidable impurities, and The Si content in the weld metal is expressed as [Si] in mass%, The Mn content in the weld metal is expressed in mass% as [Mn], The Cr content in the weld metal is expressed as [Cr] in mass %, The Mo content in the weld metal is expressed as [Mo] in mass%, The C content in the weld metal is expressed as [C] in mass%, The Ni content in the weld metal is expressed as [Ni] in mass%, The Ti content in the weld metal is expressed as [Ti] in mass%, When the B content in the weld metal is expressed as [B] in mass%, A value A1 calculated by the following formula (1): 1.84 or less, A weld metal characterized in that the value A2 calculated by the following formula (2) is 1.74 or more and 1.90 or less. A1=[Si]+[Mn]+[Cr]-[Mo]...Formula (1) A2=0.558+3.207×[C]+0.304×[Si]+0.294×[Mn]+0.119×[Ni]+1.011×[Mo]+4.093×[Ti]+15.364×[B]...Formula (2)

2. Furthermore, at least one selected from Al, Nb, and V, Al: 0.030% by mass or less, Nb: 0.020% by mass or less, The weld metal according to claim 1, characterized in that it contains V in the range of 0.020 mass % or less.

3. A welded joint comprising the weld metal according to claim 1 or 2.

4. A welded structure comprising the weld joint of claim 3.

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