Bond flux and weld metal for submerged arc welding

Abstract

We provide a bond flux for submerged arc welding of high-Cr ferritic heat-resistant steel that can obtain weld metal with excellent tensile strength and toughness after PWHT. The bond flux for submerged arc welding of high-cr ferritic heat-resistant steel contains MgO, Ca, F, Al2O3, SiO2, CO2, Na, and C in predetermined ranges, and ZrO2 and Al have a predetermined content or less. When the content of MgO, Ca, F, Al2O3, SiO2, and CO2 in the flux is [MgO], [Ca], [F], [Al2O3], [SiO2], and [CO2], respectively, in mass% relative to the total mass of the flux, the value obtained by Equation (1): {[MgO]+1.4×([Ca]-1.055×[F])+2.055×[F]+0.5×[Al2O3]} / ([SiO2]+[CO2]) is 3.0 or more and 7.0 or less.

Description

Technology Field

[0001] The present invention relates to a bond flux for submerged arc welding and a weld metal suitable for welding high-Cr ferritic heat-resistant steel. Background Technology

[0002] Since thermal power boilers, turbines, pressure vessels for oil refining, and various heat-resistant and pressure-resistant steel pipes are used in high-temperature and high-pressure environments, various high-Cr ferritic heat-resistant steels are utilized depending on the operating conditions. Examples of such heat-resistant steels include A387Gr.92, which is specified in ASTM (American Society for Testing and Materials) and ASME (American Society of Mechanical Engineers) standards. Furthermore, many proposals have already been made regarding welding materials used for such high-Cr ferritic heat-resistant steels.

[0003] For example, Patent Document 1 discloses a submerged arc welding method for 9Cr-1Mo steel. The welding method described in Patent Document 1 is a method of welding by combining a wire with a controlled content of components and a flux, and it is described that excellent high-temperature strength and toughness can be obtained by preventing cracking. Prior art literature

[0004] Japanese Patent Publication No. Hei 1-258894 The problem to be solved

[0005] However, in recent years, further improvement in the mechanical performance of weld metals is required, and even if the welding method described in the above patent document 1 is used, the required mechanical performance cannot be satisfied. Specifically, there is a demand for a welding material that can obtain a weld metal with even better tensile strength and toughness after PWHT (Post Weld Heat Treatment).

[0006] The present invention, made in consideration of the aforementioned circumstances, aims to provide a bond flux for submerged arc welding of high-Cr ferritic heat-resistant steel capable of obtaining a weld metal having excellent tensile strength and toughness after PWHT, and a weld metal having excellent tensile strength and toughness. means of solving the problem

[0007] The above objective of the present invention is achieved by the following [1] configuration related to a bond flux for submerged arc welding.

[0008] [1] As a bond flux for submerged arc welding of high-Cr ferritic heat-resistant steel,

[0009] With respect to the total mass of the flux,

[0010] MgO: 24.0 mass% or more and 35.0 mass% or less,

[0011] Ca: 10.3 mass% or more and 21.9 mass% or less,

[0012] F: 7.8 mass% or more and 14.0 mass% or less,

[0013] Al2O3: 7.0 mass% or more and 25.0 mass% or less,

[0014] SiO2: 8.0 mass% or more and 22.0 mass% or less,

[0015] CO2: 1.0 mass% or more and 6.0 mass% or less,

[0016] Na: 0.5 mass% or more and 4.0 mass% or less,

[0017] C: Contains 0.02 mass% or more and 0.16 mass% or less, and

[0018] ZrO2: 4.0 mass% or less,

[0019] Al: 0.80 mass% or less,

[0020] A bond flux for submerged arc welding characterized in that, when the MgO content in the flux is [MgO] in mass% of the total mass of the flux, the Ca content in the flux is [Ca] in mass% of the total mass of the flux, the F content in the flux is [F] in mass% of the total mass of the flux, the Al2O3 content in the flux is [Al2O3] in mass% of the total mass of the flux, the SiO2 content in the flux is [SiO2] in mass% of the total mass of the flux, and the CO2 content in the flux is [CO2] in mass% of the total mass of the flux, the value obtained by the following formula (1) is 3.0 or higher and 7.0 or lower.

[0021] Equation (1): {[MgO]+1.4×([Ca]-1.055×[F])+2.055×[F]+0.5×[Al2O3]} / ([SiO2]+[CO2])

[0022] Preferred embodiments of the present invention related to bond flux for submerged arc welding are [2] to [5] below.

[0023] [2] Additionally, regarding the total mass of the flux,

[0024] A bond flux for submerged arc welding described in [1], characterized by containing Mn: 0.5 mass% or more and 2.5 mass% or less.

[0025] [3] Additionally, regarding the total mass of the flux,

[0026] K: Bond flux for submerged arc welding as described in [1] or [2], characterized by containing 0.5 mass% or more and 3.0 mass% or less.

[0027] [4] Additionally, regarding the total mass of the flux,

[0028] A bond flux for submerged arc welding described in any one of [1] to [3], characterized by containing Li: 0.05 mass% or more and 0.20 mass% or less.

[0029] [5] Used with submerged arc welding wire,

[0030] The above submerged arc welding wire, with respect to the total mass of the wire,

[0031] C: 0.07 mass% or more and 0.12 mass% or less,

[0032] Si: 0.10 mass% or more and 0.35 mass% or less,

[0033] Mn: 0.40 mass% or more, 0.80 mass% or less,

[0034] S: 0.001 mass% or more and 0.020 mass% or less,

[0035] Ni: 0.15 mass% or more and 0.40 mass% or less,

[0036] Cr: 8.0 mass% or more and 10.0 mass% or less,

[0037] Mo: 0.30 mass% or more and 0.60 mass% or less,

[0038] V: 0.15 mass% or more and 0.25 mass% or less,

[0039] Co: 0.30 mass% or more and 0.60 mass% or less,

[0040] B: 0.0003 mass% or more and 0.0030 mass% or less,

[0041] Nb: 0.020 mass% or more and 0.100 mass% or less,

[0042] W: 1.50 mass% or more and 2.00 mass% or less,

[0043] N: Contains 0.030 mass% or more and 0.070 mass% or less, and

[0044] P: 0.020 mass% or less,

[0045] Cu: 0.20 mass% or less,

[0046] Al: 0.020 mass% or less, and

[0047] A bond flux for submerged arc welding described in any one of [1] to [4], characterized in that the remainder is Fe and unavoidable impurities.

[0048] The above objective of the present invention is achieved by the following [6] configuration related to the weld metal.

[0049] [6] A weld metal characterized by being formed using a bond flux for submerged arc welding as described in any one of [1] to [5]. Effects of the invention

[0050] According to the present invention, a bond flux for submerged arc welding of high-Cr ferritic heat-resistant steel, which can obtain a weld metal having excellent tensile strength and toughness after PWHT, and a weld metal having excellent tensile strength and toughness can be provided. Specific details for implementing the invention

[0051] Hereinafter, embodiments for carrying out the present invention will be described in detail. Meanwhile, the present invention is not limited to the embodiments described below, and may be implemented with arbitrary modifications within the scope that does not deviate from the gist of the present invention.

[0052] The inventors have conducted a thorough investigation into a bond flux for submerged arc welding suitable for using ASTM A387Gr.92 as the welding base material, in order to obtain a weld metal with excellent tensile strength and toughness after PWHT. As a result, it was discovered that it is effective to appropriately control the content of each component in the flux, particularly MgO, Ca, F, Al2O3, SiO2, CO2, Al, and C, and to control parameters using the content of MgO, Ca, F, Al2O3, SiO2, and CO2.

[0053] Hereinafter, regarding the bond flux for submerged arc welding according to the present embodiment, the reasons for the addition of the components and the reasons for limiting the composition will be explained in detail.

[0054] [1. Bond Flux for Submerged Arc Welding]

[0055] The bond flux for submerged arc welding according to the present embodiment is a bond flux suitable for welding high-Cr ferritic heat-resistant steel, and may contain the following elements as essential components or as optional components.

[0056] <MgO: 24.0질량% 이상 35.0질량% 이하>

[0057] MgO is a slag-forming agent that improves slag fluidity and has the effect of shaping the weld bead. Additionally, MgO is a component that reduces the oxygen content in the weld metal, thereby ensuring toughness.

[0058] If the MgO content in the flux is less than 24.0 mass%, the effect of reducing the oxygen content in the weld metal is insufficient, resulting in reduced toughness and deterioration of the bead appearance. Therefore, the MgO content relative to the total mass of the flux should be 24.0 mass% or more, preferably 25.0 mass% or more, and more preferably 25.5 mass% or more.

[0059] Meanwhile, if the MgO content in the flux exceeds 35.0 mass%, the slag accumulation increases, and welding workability decreases. Therefore, the MgO content relative to the total mass of the flux is 35.0 mass% or less, preferably 34.5 mass% or less, and more preferably 34.0 mass% or less.

[0060] Meanwhile, in the present embodiment, the MgO content refers to the value obtained by converting all Mg contained in the flux into MgO.

[0061] <Ca: 10.3질량% 이상 21.9질량% 이하>

[0062] Ca is a component that acts as a deoxidizer and has the effect of reducing the amount of oxygen in the weld metal.

[0063] If the Ca content in the flux is less than 10.3 mass%, a sufficient deoxidation effect by Ca cannot be obtained, and the toughness decreases and the bead appearance deteriorates. Therefore, the Ca content relative to the total mass of the flux is 10.3 mass% or more, preferably 11.3 mass% or more, and more preferably 11.8 mass% or more.

[0064] Meanwhile, if the Ca content in the flux exceeds 21.9 mass%, the slag detachability decreases. Therefore, the Ca content relative to the total mass of the flux is 21.9 mass% or less, preferably 20.9 mass% or less, and more preferably 20.4 mass% or less.

[0065] Meanwhile, Ca is contained in the flux in the form of fluorides and carbonates.

[0066] <F: 7.8질량% 이상 14.0질량% 이하>

[0067] F is a component that has the effect of improving low-temperature crack resistance by reducing the amount of diffusible hydrogen in the weld metal, and also has the effect of organizing the bead shape by controlling the amount of oxygen in the weld metal.

[0068] If the F content in the flux is less than 7.8 mass%, the oxygen content in the weld metal increases, and the toughness decreases. Therefore, the F content relative to the total mass of the flux should be 7.8 mass% or more, preferably 8.3 mass% or more, and more preferably 8.6 mass% or more.

[0069] Meanwhile, if the F content in the flux exceeds 14.0 mass%, the arc becomes unstable, and the bead shape or slag detachability is reduced. Therefore, the F content relative to the total mass of the flux is 14.0 mass% or less, preferably 13.7 mass% or less, and more preferably 13.4 mass% or less.

[0070] <Al2O3: 7.0질량% 이상 25.0질량% 이하>

[0071] Al2O3 is a slag modifier and is a component that improves slag fluidity and has the effect of shaping the beads.

[0072] If the Al2O3 content in the flux is less than 7.0 mass%, the bead shape deteriorates. Therefore, the Al2O3 content relative to the total mass of the flux is 7.0 mass% or more, preferably 7.5 mass% or more, and more preferably 8.0 mass% or more.

[0073] Meanwhile, if the Al2O3 content in the flux exceeds 25.0 mass%, the slag accumulation increases, and welding workability decreases. Therefore, the Al2O3 content relative to the total mass of the flux is 25.0 mass% or less, preferably 24.5 mass% or less, and more preferably 24.0 mass% or less.

[0074] <SiO2: 8.0질량% 이상 22.0질량% 이하>

[0075] SiO2 is a component that improves the fluidity of the slag and has the effect of organizing the bead shape.

[0076] If the SiO2 content in the flux is less than 8.0 mass%, the bead shape deteriorates. Therefore, the SiO2 content relative to the total mass of the flux is 8.0 mass% or more, preferably 8.5 mass% or more, and more preferably 9.0 mass% or more.

[0077] Meanwhile, if the SiO2 content in the flux exceeds 22.0 mass%, the slag accumulation increases, and welding workability decreases. Therefore, the SiO2 content relative to the total mass of the flux is 22.0 mass% or less, preferably 21.0 mass% or less, and more preferably 20.5 mass% or less.

[0078] Meanwhile, in the present embodiment, the SiO2 content refers to the value obtained by converting all Si contained in the flux into SiO2. In addition, the SiO2 in the flux includes SiO2 derived from water glass used as a binder.

[0079] <CO2: 1.0질량% 이상 6.0질량% 이하>

[0080] CO2 is a component that has the effect of improving low-temperature crack resistance by reducing the amount of diffusible hydrogen in the weld metal, and also has the effect of controlling the amount of oxygen in the weld metal.

[0081] If the CO2 content in the flux is less than 1.0 mass%, low-temperature cracking occurs. Therefore, the CO2 content relative to the total mass of the flux is 1.0 mass% or more, preferably 1.1 mass% or more, and more preferably 1.2 mass% or more.

[0082] Meanwhile, if the CO2 content in the flux exceeds 6.0 mass%, the oxygen content in the weld metal increases, and toughness decreases. Therefore, the CO2 content relative to the total mass of the flux is 6.0 mass% or less, preferably 5.5 mass% or less, and more preferably 5.0 mass% or less.

[0083] Meanwhile, CO2 is contained in the flux in the form of metal carbonates. Examples of metal carbonates include CaCO3, BaCO3, and MgCO3; however, if the equivalent value of these metal carbonates in terms of CO2 falls within the above range, a similar effect can be obtained.

[0084] <Na: 0.5질량% 이상 4.0질량% 이하>

[0085] Na is a component that has the effect of improving arc stability.

[0086] If the Na content in the flux is less than 0.5 mass%, arc stability is reduced, and welding defects occur. Therefore, the Na content relative to the total mass of the flux is 0.5 mass% or more, preferably 0.7 mass% or more, and more preferably 0.9 mass% or more.

[0087] Meanwhile, if the Na content in the flux exceeds 4.0 mass%, the hygroscopicity of the flux increases, which leads to an increase in the hydrogen content in the weld metal and causes low-temperature cracking. Therefore, the Na content relative to the total mass of the flux should be 4.0 mass% or less, preferably 3.5 mass% or less, and more preferably 3.1 mass% or less.

[0088] <C: 0.02질량% 이상 0.16질량% 이하>

[0089] C is a component that has the effect of improving tensile strength.

[0090] If the C content in the flux is less than 0.02 mass%, the effect of improving the tensile strength of the weld metal cannot be obtained. Therefore, the C content relative to the total mass of the flux is 0.02 mass% or more, preferably 0.03 mass% or more, and more preferably 0.04 mass% or more.

[0091] Meanwhile, if the C content in the flux exceeds 0.16 mass%, the strength of the weld metal becomes excessively high, making it prone to high-temperature cracking and reducing toughness. Therefore, the C content relative to the total mass of the flux should be 0.16 mass% or less, and it is preferable that it be 0.15 mass% or less.

[0092] <ZrO2: 4.0질량% 이하(0질량%를 포함한다)>

[0093] ZrO2 is a slag fluxing agent that improves slag fluidity and has the effect of shaping the beads, but it is not strictly necessary to include it in the flux.

[0094] When ZrO2 is included in the flux, it is preferable that the ZrO2 content relative to the total mass of the flux be 0.01 mass% or more, and more preferable that it be 0.1 mass% or more.

[0095] Meanwhile, if the ZrO2 content in the flux exceeds 4.0 mass%, the slag accumulation increases, and welding workability decreases. Therefore, the ZrO2 content relative to the total mass of the flux is 4.0 mass% or less, preferably 3.5 mass% or less, and more preferably 3.0 mass% or less.

[0096] Meanwhile, in the present embodiment, the ZrO2 content refers to the value obtained by converting all Zr contained in the flux into ZrO2.

[0097] <Al: 0.80질량% 이하(0질량%를 포함한다)>

[0098] Since Al combines with N to form AlN and is a component that degrades creep strength by reducing the precipitation of carbonitrides of Cr, Nb, and V—which are indispensable for securing creep strength—it is desirable to reduce the amount of Al in the flux as much as possible.

[0099] If the Al content in the flux exceeds 0.80 mass%, the beads are baked and the slag detachability deteriorates. In addition, the yield of elements in the weld metal improves, and as strength increases, toughness deteriorates. Therefore, the Al content relative to the total mass of the flux is preferably 0.80 mass% or less, 0.75 mass% or less, and more preferably 0.70 mass% or less.

[0100] Meanwhile, the Al specified here as 0.80 mass% or less is contained in the flux in the form of Al element, Fe-Al, and Al alloy, and is not included in the form of oxides.

[0101] <Equation (1): 3.0 or more and 7.0 or less>

[0102] The following equation (1) parameterizes the deoxidation power of the weld metal by the flux component. If the value obtained by equation (1) increases, the deoxidation power by the flux increases, and the oxygen content of the weld metal decreases, thereby improving toughness; however, if it becomes excessively high, the slag viscosity at each temperature becomes excessively high, which degrades weldability. In other words, by appropriately controlling the value obtained by equation (1), it is possible to achieve both improved toughness and improved weldability.

[0103] If the value obtained by Equation (1) is less than 3.0, the deoxidation power is weakened, and the toughness of the weld metal is reduced. Therefore, the value obtained by Equation (1) is 3.0 or higher, preferably 3.3 or higher, and more preferably 3.9 or higher.

[0104] Meanwhile, if the value obtained by Equation (1) exceeds 7.0, the shape of the weld bead deteriorates, such as the central part of the bead becoming a convex bead shape. Therefore, the value obtained by Equation (1) is preferably 7.0 or less, 6.5 or less, and more preferably 6.1 or less.

[0105] Equation (1): {[MgO]+1.4×([Ca]-1.055×[F])+2.055×[F]+0.5×[Al2O3]} / ([SiO2]+[CO2])

[0106] Meanwhile, in the above formula (1), [MgO] is a value representing the MgO content in the flux as a mass% relative to the total mass of the flux, [Ca] is a value representing the Ca content in the flux as a mass% relative to the total mass of the flux, [F] is a value representing the F content in the flux as a mass% relative to the total mass of the flux, [Al2O3] is a value representing the Al2O3 content in the flux as a mass% relative to the total mass of the flux, [SiO2] is a value representing the SiO2 content in the flux as a mass% relative to the total mass of the flux, and [CO2] is a value representing the CO2 content in the flux as a mass% relative to the total mass of the flux.

[0107] The bond flux for submerged arc welding according to the present embodiment preferably contains the following components in a predetermined amount in addition to the above components.

[0108] <Mn: 0.5질량% 이상 2.5질량% 이하>

[0109] Mn is a component that has a deoxidizing effect.

[0110] When Mn is included in the flux, if the Mn content is 0.5 mass% or more, it exerts deoxidation power, and since the weld metal becomes low-oxygen, toughness can be improved. Therefore, it is preferable that the Mn content relative to the total mass of the flux be 0.5 mass% or more, and more preferable that it be 0.6 mass% or more.

[0111] Meanwhile, if the Mn content in the flux is 2.5 mass% or less, the balance between the strength and toughness of the weld metal can be improved. Accordingly, the Mn content relative to the total mass of the flux is preferably 2.5 mass% or less, more preferably 2.2 mass% or less, and even more preferably 1.9 mass% or less.

[0112] <K: 0.5질량% 이상 3.0질량% 이하>

[0113] K is an arc stabilizer and is a component included in the flux to improve arc stability.

[0114] When K is included in the flux, if the K content is 0.5 mass% or more, arc stability is improved, which can improve welding workability. Therefore, it is preferable that the K content relative to the total mass of the flux be 0.5 mass% or more, and more preferable that it be 0.6 mass% or more.

[0115] Meanwhile, if the K content in the flux is 3.0 mass% or less, the moisture absorption of the flux is suppressed, and since the amount of hydrogen in the weld metal is reduced, the occurrence of low-temperature cracking can be suppressed. Accordingly, the K content relative to the total mass of the flux is preferably 3.0 mass% or less, more preferably 2.7 mass% or less, and even more preferably 2.4 mass% or less.

[0116] <Li: 0.05질량% 이상 0.20질량% 이하>

[0117] Li is an arc stabilizer and is a component included in the flux to ensure good arc stability.

[0118] When Li is included in the flux, if the Li content is 0.05 mass% or more, arc stability is improved, which can improve welding workability. Therefore, it is preferable that the Li content relative to the total mass of the flux be 0.05 mass% or more, and more preferable that it be 0.06 mass% or more.

[0119] Meanwhile, if the Li content in the flux is 0.20 mass% or less, the moisture absorption of the flux is suppressed, and since the amount of hydrogen in the weld metal is reduced, the occurrence of low-temperature cracking can be suppressed. Accordingly, the Li content relative to the total mass of the flux is preferably 0.20 mass% or less, more preferably 0.18 mass% or less, and even more preferably 0.17 mass% or less.

[0120] <Total Content of Essential Ingredients>

[0121] In the bond flux for submerged arc welding according to the present embodiment, the total content of MgO, Ca, F, Al2O3, SiO2, CO2, Na, C, ZrO2, and Al is preferably 88 mass% or more with respect to the total mass of the flux, more preferably 91 mass% or more, and even more preferably 93 mass% or more.

[0122] <Zanbu>

[0123] The bond flux for submerged arc welding according to the present embodiment may contain, in addition to the above components, for example, Fe, Mo, W, Cu, etc., within a range that does not interfere with the effects of the present invention. Meanwhile, these components may exist as individual elements or as compounds.

[0124] [2. Submerged Arc Welding Wire]

[0125] The bond flux for submerged arc welding according to the present embodiment is used together with a wire for submerged arc welding. Hereinafter, regarding a wire preferred for use with the bond flux for submerged arc welding according to the present embodiment, the reasons for adding the component and limiting the composition are explained in detail.

[0126] <C: 0.07질량% 이상 0.12질량% 이하>

[0127] C is a component that has a significant effect on the hardenability and carbonitride precipitation amount in the weld metal, and also functions as an austenite stabilizing element, thereby having the effect of suppressing the retention of the δ ferrite phase in the weld metal.

[0128] If the C content in the wire is less than 0.07 mass%, the C content in the weld metal becomes excessively low, resulting in insufficient precipitation of carbides. Additionally, δ-ferrite phases remain, making it impossible to obtain the desired creep strength. Therefore, the C content relative to the total mass of the wire should be 0.07 mass% or more, and it is preferable that it be 0.08 mass% or more.

[0129] On the other hand, if the carbon content in the wire exceeds 0.12 mass%, high-temperature cracking susceptibility increases, making it prone to cracking, particularly in submerged arc welding within narrow wires. Additionally, the amount of carbide precipitation increases, significantly increasing the strength of the weld metal and degrading its toughness. Therefore, the carbon content relative to the total mass of the wire should be 0.12 mass% or less, preferably 0.11 mass% or less, and more preferably 0.10 mass% or less.

[0130] <Si: 0.10질량% 이상 0.35질량% 이하>

[0131] Si is a component that improves the affinity of the weld bead and functions as a deoxidizer, thereby having the effect of enhancing the strength and toughness of the weld metal.

[0132] If the Si content in the wire is less than 0.10 mass%, the Si content in the weld metal becomes excessively low, which degrades weldability (e.g., weld bead affinity or fusion properties) and also degrades toughness and creep strength. Therefore, the Si content relative to the total mass of the wire should be 0.10 mass% or more, preferably 0.11 mass% or more, and more preferably 0.12 mass% or more.

[0133] Meanwhile, if the Si content in the wire exceeds 0.35 mass%, the strength of the weld metal increases significantly, and the toughness deteriorates. Therefore, the Si content relative to the total mass of the wire is 0.35 mass% or less, preferably 0.33 mass% or less, and more preferably 0.31 mass% or less.

[0134] <Mn: 0.40질량% 이상 0.80질량% 이하>

[0135] Mn, like Si, functions as a deoxidizer and is a component that has the effect of improving the toughness of the weld metal. In addition, Mn functions as an austenite stabilizing element and is a component that has the effect of suppressing the persistence of the δ-ferrite phase in the weld metal. Furthermore, as described below, Mn also has the effect of mitigating the adverse effects of high-temperature cracking caused by S.

[0136] If the Mn content in the wire is less than 0.40 mass%, the Mn content in the weld metal becomes excessively low, making it impossible to obtain the desired toughness, and furthermore, soft δ-ferrite phases remain in the weld metal, causing creep strength to deteriorate. Moreover, it becomes difficult to suppress high-temperature cracking caused by S. Therefore, the Mn content relative to the total mass of the wire should be 0.40 mass% or more, preferably 0.44 mass% or more, and more preferably 0.48 mass% or more.

[0137] Meanwhile, if the Mn content in the wire exceeds 0.80 mass%, the Mn content in the weld metal becomes excessively high, destabilizing carbonitrides and lowering creep strength. Therefore, the Mn content relative to the total mass of the wire is preferably 0.80 mass% or less, 0.78 mass% or less, and more preferably 0.77 mass% or less.

[0138] <S: 0.001질량% 이상 0.020질량% 이하>

[0139] Sulfur (S) is a component that lowers the surface energy of the molten pool, thereby improving weldability—particularly affinity on the weld surface—and having the effect of refining the bead appearance and toe shape. However, Sulfur combines with Fe during welding to form a low-melting-point Fe-FeS eutectic in the final solidified zone; this not only increases high-temperature cracking susceptibility but also embrittles the weld metal, thereby degrading its toughness.

[0140] If the S content in the wire is less than 0.001 mass%, the bead shape deteriorates. Therefore, the S content relative to the total mass of the wire is 0.001 mass% or more.

[0141] Meanwhile, if the S content in the wire exceeds 0.020 mass%, high-temperature cracking becomes more likely to occur, and toughness deteriorates. Therefore, the S content relative to the total mass of the wire is preferably 0.020 mass% or less, 0.018 mass% or less, and more preferably 0.016 mass% or less.

[0142] <Ni: 0.15질량% 이상 0.40질량% 이하>

[0143] Ni is a component that is dissolved in the matrix of the weld metal and has the effect of improving the toughness of the ferrite itself.

[0144] If the Ni content in the wire is less than 0.15 mass%, the effect of improving the toughness of the ferrite cannot be obtained. Therefore, the Ni content relative to the total mass of the wire is 0.15 mass% or more, preferably 0.17 mass% or more, and more preferably 0.20 mass% or more.

[0145] Meanwhile, if the Ni content in the wire exceeds 0.40 mass%, Ni becomes concentrated in the final solidification zone during welding, and the solidification completion temperature is lowered, increasing susceptibility to high-temperature cracking. In addition, the size of carbonitrides becomes coarsened during creep deformation, and creep strength decreases. Therefore, the Ni content relative to the total mass of the wire is preferably 0.40 mass% or less, 0.38 mass% or less, and more preferably 0.36 mass% or less.

[0146] <Cr: 8.0질량% 이상 10.0질량% 이하>

[0147] Cr is a component that forms carbonitrides during PWHT, which has the effect of improving the creep strength of the weld metal.

[0148] If the Cr content in the wire is less than 8.0 mass%, the amount of carbonitride precipitation is insufficient, and the desired creep strength cannot be obtained. Therefore, the Cr content relative to the total mass of the wire is 8.0 mass% or more, preferably 8.2 mass% or more, and more preferably 8.4 mass% or more.

[0149] Meanwhile, if the Cr content in the wire exceeds 10.0 mass%, the solidification completion temperature is lowered, increasing susceptibility to high-temperature cracking, and the δ-ferrite phase remains in the weld metal, thereby reducing creep strength and toughness. In addition, slag detachability is significantly deteriorated. Therefore, the Cr content relative to the total mass of the wire is preferably 10.0 mass% or less, 9.7 mass% or less, and more preferably 9.5 mass% or less.

[0150] <Mo: 0.30질량% 이상 0.60질량% 이하>

[0151] Mo is a component that is dissolved in Cr-based carbides or the matrix phase during PWHT, and has the effect of improving the creep strength of the weld metal.

[0152] If the Mo content in the wire is less than 0.30 mass%, the desired creep strength cannot be obtained. Therefore, the Mo content relative to the total mass of the wire is 0.30 mass% or more, preferably 0.32 mass% or more, and more preferably 0.33 mass% or more.

[0153] Meanwhile, if the Mo content in the wire exceeds 0.60 mass%, the solid solution content in the Cr-based carbides and the matrix increases excessively, significantly increasing the strength of the weld metal and deteriorating its toughness. Therefore, the Mo content relative to the total mass of the wire should be 0.60 mass% or less, preferably 0.57 mass% or less, and more preferably 0.53 mass% or less.

[0154] <V: 0.15질량% 이상 0.25질량% 이하>

[0155] V is a component that forms carbonitrides during PWHT, which has the effect of improving the creep strength of the weld metal.

[0156] If the V content in the wire is less than 0.15 mass%, the desired creep strength cannot be obtained. Therefore, the V content relative to the total mass of the wire should be 0.15 mass% or more, and it is preferable that it be 0.16 mass% or more.

[0157] Meanwhile, if the V content in the wire exceeds 0.25 mass%, the amount of carbonitride precipitation increases significantly, the strength of the weld metal increases, and the toughness deteriorates. Therefore, the V content relative to the total mass of the wire should be 0.25 mass% or less, and it is preferable that it be 0.24 mass% or less.

[0158] <Co: 0.30질량% 이상 0.60질량% 이하>

[0159] Co is a component that has the effect of improving the room temperature strength and creep strength of weld metal.

[0160] If the Co content in the wire is less than 0.30 mass%, the effect of improving the strength of the weld metal cannot be obtained. Therefore, the Co content relative to the total mass of the wire is 0.30 mass% or more, preferably 0.32 mass% or more, and more preferably 0.33 mass% or more.

[0161] Meanwhile, if the Co content in the wire exceeds 0.60 mass%, the strength of the weld metal becomes excessively high, and the toughness decreases. Therefore, the Co content relative to the total mass of the wire is 0.60 mass% or less, preferably 0.56 mass% or less, and more preferably 0.51 mass% or less.

[0162] <B: 0.0003질량% 이상 0.0030질량% 이하>

[0163] B is a component that has the effect of stabilizing the toughness of the weld metal.

[0164] If the B content in the wire is less than 0.0003 mass%, the toughness of the weld metal is insufficient. Therefore, the B content relative to the total mass of the wire is 0.0003 mass% or more.

[0165] Meanwhile, if the B content in the wire exceeds 0.0030 mass%, the strength of the weld metal becomes excessively high, which not only leads to a decrease in toughness but also increases susceptibility to high-temperature cracking during welding. Therefore, the B content relative to the total mass of the wire is preferably 0.0030 mass% or less, 0.0029 mass% or less, and more preferably 0.0028 mass% or less.

[0166] <Nb: 0.020질량% 이상 0.100질량% 이하>

[0167] Like V, Nb is a component that forms carbonitrides during PWHT, thereby having the effect of improving the creep strength of the weld metal.

[0168] If the Nb content in the wire is less than 0.020 mass%, the desired creep strength cannot be obtained. Therefore, the Nb content relative to the total mass of the wire is 0.020 mass% or more, preferably 0.021 mass% or more, and more preferably 0.023 mass% or more.

[0169] Meanwhile, if the Nb content in the wire exceeds 0.100 mass%, the amount of carbonitride precipitation increases significantly, the strength of the weld metal increases, and the toughness deteriorates. In addition, slag detachability deteriorates significantly. Therefore, the Nb content relative to the total mass of the wire is preferably 0.100 mass% or less, 0.080 mass% or less, and more preferably 0.065 mass% or less.

[0170] <W: 1.50질량% 이상 2.00질량% 이하>

[0171] W is a component that has the effect of improving the room temperature strength and creep strength of the weld metal.

[0172] If the W content in the wire is less than 1.50 mass%, the effect of improving the strength of the weld metal cannot be obtained. Therefore, the W content relative to the total mass of the wire is 1.50 mass% or more, preferably 1.51 mass% or more, and more preferably 1.53 mass% or more.

[0173] Meanwhile, if the W content in the wire exceeds 2.00 mass%, the strength of the weld metal becomes excessively high, and the toughness decreases. Therefore, the W content relative to the total mass of the wire is 2.00 mass% or less, preferably 1.90 mass% or less, and more preferably 1.85 mass% or less.

[0174] <N: 0.030질량% 이상 0.070질량% 이하>

[0175] N is a component that combines with Cr, V, Nb, etc., during PWHT to form carbonitrides, which has the effect of improving the creep strength of the weld metal.

[0176] If the N content in the wire is less than 0.030 mass%, the desired creep strength cannot be obtained. Therefore, the N content relative to the total mass of the wire is 0.030 mass% or more, preferably 0.035 mass% or more, and more preferably 0.038 mass% or more.

[0177] Meanwhile, if the N content in the wire exceeds 0.070 mass%, the amount of carbonitride precipitation increases significantly, the strength of the weld metal increases, and the toughness deteriorates. In addition, N2 gas generated during the welding process tends to remain in the molten metal, causing blowholes to occur. Therefore, the N content relative to the total mass of the wire is preferably 0.070 mass% or less, 0.068 mass% or less, and more preferably 0.066 mass% or less.

[0178] <P: 0.020질량% 이하>

[0179] Since phosphorus forms low-melting point compounds in the final solidification zone during welding, which not only increases susceptibility to high-temperature cracking but also embrittles the weld metal and degrades its toughness, it is desirable to reduce the phosphorus content in the wire as much as possible.

[0180] If the P content in the wire exceeds 0.020 mass%, high-temperature cracking becomes more likely to occur, and toughness deteriorates. Therefore, the P content relative to the total mass of the wire is preferably 0.020 mass% or less, 0.016 mass% or less, and more preferably 0.011 mass% or less.

[0181] <Cu: 0.20질량% 이하>

[0182] Cu is a component that makes high-temperature cracking in weld metal more likely to occur.

[0183] If the Cu content in the wire exceeds 0.20 mass%, high-temperature cracking of the weld metal is likely to occur. Therefore, the Cu content relative to the total mass of the wire is 0.20 mass% or less, preferably 0.16 mass% or less, and more preferably 0.11 mass% or less.

[0184] <Al: 0.020질량% 이하>

[0185] Since Al combines with N to form AlN and reduces the precipitation of carbonitrides of Cr, Nb, and V—which are indispensable for securing creep strength—thereby degrading creep strength, it is desirable to reduce the Al content in the wire as much as possible.

[0186] If the Al content in the wire exceeds 0.020 mass%, the bead is baked and the slag detachability deteriorates. In addition, the yield of elements in the weld metal improves, and as strength increases, toughness deteriorates. Therefore, the Al content relative to the total mass of the wire is preferably 0.020 mass% or less, 0.016 mass% or less, and more preferably 0.012 mass% or less.

[0187] <Zanbu>

[0188] In a wire preferred for use with a bond flux for submerged arc welding according to the present embodiment, the remainder is Fe and unavoidable impurities. Examples of unavoidable impurities include Sn, As, Sb, Pb, Bi, etc. Among these impurities, Sn, As, and Sb are each preferably 0.005 mass% or less with respect to the total mass of the wire, for example, and are preferably 0.015 mass% or less in total.

[0189] In addition, it is preferable that Pb and Bi each have an amount of, for example, 0.001 mass% or less with respect to the total mass of the wire. Furthermore, even if an element other than those listed as unavoidable impurities is present in the wire as an unavoidable impurity, if each has an amount of 0.10 mass% or less with respect to the total mass of the wire, it is preferable that it be 0.08 mass% or less, and more preferable that it be 0.06 mass% or less.

[0190] [3. Welded Metal]

[0191] The weld metal according to the present embodiment is a weld metal formed using the bond flux described in [1. Bond flux for submerged arc welding] above.

[0192] Meanwhile, regarding the weld metal according to the present embodiment, various welding conditions other than using the bond flux for submerged arc welding according to the present embodiment are not particularly limited, and normal conditions in the submerged arc welding method can be used with respect to the type of base material, welding voltage, welding current, welding position, etc.

[0193] Examples

[0194] The contents of the present invention will be explained in detail below with reference to inventive examples and comparative examples demonstrating the effects of the present invention.

[0195] [Submerged Arc Welding]

[0196] First, a wire with a diameter of 2.4 mm and various chemical compositions was manufactured, along with a bond flux having various chemical compositions. The chemical composition and content of the wire are shown in Table 1 below, and the wire No. used, and the chemical composition and content of the flux are shown in Table 2 below.

[0197] Meanwhile, the remainder of the chemical composition in the wire shown in Table 1 below is Fe and unavoidable impurities. Also, regarding the chemical composition in the bond flux shown in Table 2 below, Al is a component that is not actively added, so it was set to “<0.80” (less than 0.80).

[0198] Next, submerged arc welding was performed using the above wire and bond flux, and then post-weld heat treatment (PWHT) was performed under the conditions shown below. The welding conditions and PWHT conditions are shown below. Meanwhile, as the welding base material, a 9Cr-based steel or mild steel was used, and a welding material simulating the chemical composition of ASTM A387Gr.92 was buttered.

[0199] (Welding conditions)

[0200] Base material plate thickness: 25mm

[0201] Improvement angle, shape: 30°, V-shaped

[0202] Root spacing: 13mm

[0203] Polarity: Direct Current Electrode Positive (DCEP)

[0204] Wire diameter: 2.4mm

[0205] Welding position: Downward

[0206] Current: 350–450A (Target: 400A)

[0207] Voltage: 27–30V (Target: 29V)

[0208] Welding speed: 33–39 cm / min (target 36 cm / min)

[0209] Preheating / Inter-pass temperature: 210–260°C

[0210] Layering method: 8 layers, 16–17 passes

[0211] (PWHT conditions)

[0212] Approx. 760℃, approximately 2 hours

[0213] [evaluation]

[0214] After the above submerged arc welding, heat treatment was performed under the above PWHT conditions, and the room temperature strength and toughness of the obtained weld metal were evaluated. The specific temperatures (°C) and times (hr) of the PWHT conditions, and the evaluation results of each test are shown in Table 3 below.

[0215] Room temperature strength

[0216] For the weld metal after PWHT, a tensile test at room temperature was performed in accordance with the tensile and impact test methods for weld metals specified in JIS Z3111:2005, and the 0.2% yield stress (YS) and tensile strength (TS) were measured. Meanwhile, a strength level equivalent to that of the A387 Gr.92 base metal was obtained, and acceptance criteria were established based on the tensile performance classification specified in AWS 5.23. Specifically, for YS, a value of 540 MPa or higher was accepted, and a value of less than 540 MPa was rejected. In addition, for TS, a value between 620 and 775 MPa was accepted, and a value of less than 620 MPa or greater than 775 MPa was rejected.

[0217] Character

[0218] Three test specimens were taken from the weld metal after PWHT, and a Charpy impact test was performed at 20°C in accordance with the Charpy impact test method for metallic materials specified in JIS Z2242. Then, the average value of the absorbed energy vE (J) of each measured test specimen was calculated to evaluate toughness. Meanwhile, a value of 47J or higher was considered acceptable, and a value of less than 47J was considered unacceptable.

[0219]

[0220]

[0221]

[0222] As shown in Tables 2 and 3 above, since the chemical content of the bond flux for submerged arc welding in Invention Examples No. 1 to 7 is within the range of the present invention, when submerged arc welding was performed using these fluxes, a weld metal having excellent tensile strength and toughness after PWHT could be obtained.

[0223] Meanwhile, Comparative Examples No. 1 to 5 had lower toughness of the weld metal because the value calculated by Equation (1), based on the content of MgO, Ca, F, Al2O3, SiO2, and CO2 in the flux, was less than the lower limit of the range of the present invention.

[0224] In addition, for Comparative Examples No. 2 and 3, the tensile strength exceeded the desired range and became a high value.

[0225] In Comparative Example No. 6, the toughness of the weld metal was reduced because the content of ZrO2 in the flux exceeded the upper limit of the scope of the present invention.

[0226] Comparative Example No. 7 had a reduced toughness of the weld metal because the MgO content exceeded the upper limit of the range of the present invention and the F content was below the lower limit of the range of the present invention.

[0227] Although various embodiments have been described above, it goes without saying that the present invention is not limited to these examples. It is clear to those skilled in the art that various modifications or variations can be made within the scope described in the claims, and these are naturally understood to fall within the technical scope of the present invention. Furthermore, the components of the above embodiments may be combined arbitrarily within the scope that does not deviate from the spirit of the invention.

[0228] Meanwhile, this application is based on Japanese patent application filed on September 7, 2021 (JP 2021-145757), the contents of which are incorporated by reference during this application.

Claims

Claim 1 As a bond flux for submerged arc welding of high-Cr ferritic heat-resistant steel, with respect to the total mass of the flux, it contains MgO: 24.0 mass% or more and 35.0 mass% or less, Ca: 10.3 mass% or more and 21.9 mass% or less, F: 7.8 mass% or more and 14.0 mass% or less, Al2O3: 7.0 mass% or more and 25.0 mass% or less, SiO2: 8.0 mass% or more and 22.0 mass% or less, CO2: 1.0 mass% or more and 6.0 mass% or less, Na: 0.5 mass% or more and 4.0 mass% or less, C: 0.02 mass% or more and 0.16 mass% or less, ZrO2: 4.0 mass% or less, and Al: 0.80 mass% or less, and the MgO content in the flux is the total mass of the flux A bond flux for submerged arc welding characterized in that, when [MgO] is set in mass% relative to mass, the Ca content in the flux is set in mass% relative to the total mass of the flux as [Ca], the F content in the flux is set in mass% relative to the total mass of the flux as [F], the Al2O3 content in the flux is set in mass% relative to the total mass of the flux as [Al2O3], the SiO2 content in the flux is set in mass% relative to the total mass of the flux as [SiO2], and the CO2 content in the flux is set in mass% relative to the total mass of the flux as [CO2], the value obtained by the following formula (1) is 3.0 or higher and 7.0 or lower. Formula (1): {[MgO]+1.4×([Ca]-1.055×[F])+2.055×[F]+0.5×[Al2O3]} / ([SiO2]+[CO2]) Claim 2 A bond flux for submerged arc welding according to claim 1, further characterized by containing Mn: 0.5 mass% or more and 2.5 mass% or less with respect to the total mass of the flux. Claim 3 A bond flux for submerged arc welding according to claim 1 or 2, further characterized by containing K: 0.5 mass% or more and 3.0 mass% or less with respect to the total mass of the flux. Claim 4 A bond flux for submerged arc welding according to claim 1 or 2, further characterized by containing Li: 0.05 mass% or more and 0.20 mass% or less with respect to the total mass of the flux. Claim 5 A bond flux for submerged arc welding according to claim 3, further characterized by containing Li: 0.05 mass% or more and 0.20 mass% or less with respect to the total mass of the flux. Claim 6 In claim 1 or 2, used together with a submerged arc welding wire, wherein the submerged arc welding wire comprises, with respect to the total mass of the wire, C: 0.07 mass% or more and 0.12 mass% or less, Si: 0.10 mass% or more and 0.35 mass% or less, Mn: 0.40 mass% or more and 0.80 mass% or less, S: 0.001 mass% or more and 0.020 mass% or less, Ni: 0.15 mass% or more and 0.40 mass% or less, Cr: 8.0 mass% or more and 10.0 mass% or less, Mo: 0.30 mass% or more and 0.60 mass% or less, V: 0.15 mass% or more and 0.25 mass% or less, Co: 0.30 mass% or more and 0.60 mass% or less, B: A bond flux for submerged arc welding characterized by containing 0.0003 mass% or more and 0.0030 mass% or less, Nb: 0.020 mass% or more and 0.100 mass% or less, W: 1.50 mass% or more and 2.00 mass% or less, N: 0.030 mass% or more and 0.070 mass% or less, P: 0.020 mass% or less, Cu: 0.20 mass% or less, Al: 0.020 mass% or less, and the remainder being Fe and unavoidable impurities. Claim 7 In claim 3, used together with a submerged arc welding wire, said submerged arc welding wire comprises, with respect to the total mass of the wire, C: 0.07 mass% or more and 0.12 mass% or less, Si: 0.10 mass% or more and 0.35 mass% or less, Mn: 0.40 mass% or more and 0.80 mass% or less, S: 0.001 mass% or more and 0.020 mass% or less, Ni: 0.15 mass% or more and 0.40 mass% or less, Cr: 8.0 mass% or more and 10.0 mass% or less, Mo: 0.30 mass% or more and 0.60 mass% or less, V: 0.15 mass% or more and 0.25 mass% or less, Co: 0.30 mass% or more and 0.60 mass% or less, and B: 0.0003 mass% or more. A bond flux for submerged arc welding characterized by containing 0.0030 mass% or less, Nb: 0.020 mass% or more and 0.100 mass% or less, W: 1.50 mass% or more and 2.00 mass% or less, N: 0.030 mass% or more and 0.070 mass% or less, P: 0.020 mass% or less, Cu: 0.20 mass% or less, Al: 0.020 mass% or less, and the remainder being Fe and unavoidable impurities. Claim 8 In claim 4, used together with a submerged arc welding wire, said submerged arc welding wire comprises, with respect to the total mass of the wire, C: 0.07 mass% or more and 0.12 mass% or less, Si: 0.10 mass% or more and 0.35 mass% or less, Mn: 0.40 mass% or more and 0.80 mass% or less, S: 0.001 mass% or more and 0.020 mass% or less, Ni: 0.15 mass% or more and 0.40 mass% or less, Cr: 8.0 mass% or more and 10.0 mass% or less, Mo: 0.30 mass% or more and 0.60 mass% or less, V: 0.15 mass% or more and 0.25 mass% or less, Co: 0.30 mass% or more and 0.60 mass% or less, and B: 0.0003 mass% or more. A bond flux for submerged arc welding characterized by containing 0.0030 mass% or less, Nb: 0.020 mass% or more and 0.100 mass% or less, W: 1.50 mass% or more and 2.00 mass% or less, N: 0.030 mass% or more and 0.070 mass% or less, P: 0.020 mass% or less, Cu: 0.20 mass% or less, Al: 0.020 mass% or less, and the remainder being Fe and unavoidable impurities. Claim 9 A weld metal characterized by being formed using a bond flux for submerged arc welding as described in claim 1 or 2.

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