Bonding flux for submerged arc welding

Abstract

To provide a bond flux for submerged arc welding of a high Cr ferritic heat-resistant steel which enables manufacture of weld metal having excellent tensile strength and toughness after PWHT.SOLUTION: A bond flux for submerged arc welding of a high Cr ferritic heat-resistant steel contains, by mass% to the total mass of the flux, 25.0-36.0% MgO, 7.0-18.0% Ca, 4.5-12.0% F, 9.5-21.7% Al2O3, 8.0-20.0% SiO2, 1.0-8.0% CO2, 0.5-4.0% Na, contains 4.0% or less ZrO2, 0.80% or less Al and 0.12% or less C, wherein when contents of MgO, F and CO2 in the flux are represented by [MgO], [F] and [CO2], a value obtained by expression (1): ([MgO]+2.055×[F]) / ([CO2]) is 6.0-45.0.SELECTED DRAWING: None

Description

Technical Field

[0005] , , ,

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

Background Art

[0002] The boilers, turbines, pressure vessels for petroleum refining, various heat-resistant and pressure-resistant steel pipes, etc. of thermal power generation are used in high-temperature and high-pressure environments, and various heat-resistant steel plates are used according to their usage environments. For example, as a pressure vessel alloy steel plate, A-387 Gr.91 defined in ASTM (American Society for Testing and Materials) standards or ASME (American Society of Mechanical Engineers) standards can be mentioned. And regarding welding materials used for such heat-resistant steel materials, many proposals have already been made.

[0003]

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

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in recent years, there has been a demand for further improvement in the mechanical performance of weld metal, and the welding method described in Patent Document 1 above cannot satisfy the required mechanical performance. Specifically, there is a demand for a welding material that can produce weld metal with even better tensile strength and toughness after PWHT (Post Weld Heat Treatment).

[0006] The present invention has been made in view of the above circumstances, and aims to provide a bond flux for submerged arc welding of high-Cr ferritic heat-resistant steel, which can obtain a weld metal with excellent tensile strength and toughness after PWHT, and a weld metal with excellent tensile strength and toughness. [Means for solving the problem]

[0007] The above objective of the present invention is achieved by the following configuration [1] relating to a bond flux for submerged arc welding. [1] A bond flux for submerged arc welding of high-Cr ferritic heat-resistant steel, With respect to the total mass of the flux, MgO: 25.0% by mass or more and 36.0% by mass or less, Ca: 7.0% by mass or more and 18.0% by mass or less, F: 4.5% by mass or more and 12.0% by mass or less, Al2O3: 9.5% by mass or more and 21.7% by mass or less, SiO2: 8.0% by mass or more and 20.0% by mass or less, CO2: 1.0% by mass or more and 8.0% by mass or less, It contains Na: 0.5% by mass or more and 4.0% by mass or less. ZrO2: 4.0% by mass or less, Al: 0.80% by mass or less, C: 0.12% by mass or less, A bond flux for submerged arc welding, characterized in that, when the MgO content in the flux is expressed as [MgO] in mass % of the total flux mass, the F content in the flux is expressed as [F] in mass % of the total flux mass, and the CO2 content in the flux is expressed as [CO2] in mass % of the total flux mass, the value obtained by the following formula (1) is 6.0 or more and 45.0 or less. Equation (1): ([MgO]+2.055×[F]) / ([CO2])

[0008] Preferred embodiments of the present invention relating to bond flux for submerged arc welding are described in the following [2] to [5].

[0009] [2] Furthermore, with respect to the total mass of the flux, A bond flux for submerged arc welding according to [1], characterized by containing Mn: 0.5% by mass or more and 2.5% by mass or less.

[0010] [3] Furthermore, with respect to the total mass of the flux, A bond flux for submerged arc welding according to [1] or [2], characterized by containing K: 0.5% by mass or more and 3.0% by mass or less.

[0011] [4] Furthermore, with respect to the total mass of the flux, A bond flux for submerged arc welding according to any one of [1] to [3], characterized by containing Li: 0.05% by mass or more and 0.20% by mass or less.

[0012] [5] Used with submerged arc welding wire, The aforementioned submerged arc welding wire is, with respect to the total mass of the wire, C: 0.030 mass% or more and 0.080 mass% or less, Si: 0.05% by mass or more and 0.30% by mass or less, Mn: 0.50 mass% or more and 2.20 mass% or less, Ni: 0.30 mass% or more and 1.00 mass% or less, Cr: 8.00 mass% or more and 10.50 mass% or less, Mo: 0.80 mass% or more and 1.20 mass% or less, V: 0.10 mass% or more and 0.40 mass% or less, Nb: 0.020 mass% or more and 0.095 mass% or less, N: 0.016 mass% or more and 0.065 mass% or less, and contains P: 0.015 mass% or less, S: 0.010 mass% or less, and the balance being Fe and inevitable impurities, the bonding flux for submerged arc welding according to any one of [1] to [4].

[0013] The above object of the present invention is achieved by the following configuration [6] related to the weld metal. [6] A weld metal, characterized in that it is formed using the bonding flux for submerged arc welding according to any one of [1] to [5].

Effects of the Invention

[0014] According to the present invention, there are provided a bonding flux for submerged arc welding of a 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.

Modes for Carrying Out the Invention

[0015] Hereinafter, modes for carrying out the present invention will be described in detail. Note that the present invention is not limited to the embodiments described below, and can be arbitrarily modified and implemented without departing from the gist of the present invention.

[0016] The inventors have intensively studied a bonding flux for submerged arc welding in order to obtain a weld metal with excellent tensile strength and toughness after PWHT. As a result, among the components in the flux, in particular, it has been found that it is effective to appropriately control the contents of MgO, Ca, F, CO2, Al and C, and to control the parameters using the contents of MgO, F and CO2.

[0017] Hereinafter, the reasons for adding components and the reasons for limiting the composition of the bonding flux for submerged arc welding according to the present embodiment will be described in detail.

[0018] [1. Bonding Flux for Submerged Arc Welding] The bonding flux for submerged arc welding according to the present embodiment is a bonding flux suitable for welding high Cr ferritic heat-resistant steel, and contains the following elements as essential components or may contain them as optional components.

[0019] <MgO: 25.0 mass% or more and 36.0 mass% or less> MgO is a slag-forming agent and is a component having the effect of improving the fluidity of the slag and adjusting the bead shape. MgO is also a component having the effect of reducing the amount of oxygen in the weld metal and ensuring toughness. When the MgO content in the flux is less than 25.0 mass%, the effect of reducing the amount of oxygen in the weld metal is insufficient, the toughness decreases, and the bead appearance deteriorates. Therefore, the MgO content with respect to the total mass of the flux is 25.0 mass% or more, preferably 25.5 mass% or more, and more preferably 26.0 mass% or more. On the other hand, when the MgO content in the flux exceeds 36.0 mass%, slag entrainment increases and welding workability deteriorates. Therefore, the MgO content with respect to the total mass of the flux is 36.0 mass% or less, preferably 35.5 mass% or less, and more preferably 35.0 mass% or less. In the present embodiment, the MgO content means a value obtained by converting all Mg contained in the flux into MgO.

[0020] <Ca: 7.0 mass% or more and 18.0 mass% or less> Ca acts as a deoxidizer and is a component having the effect of reducing the amount of oxygen in the weld metal. When the Ca content in the flux is less than 7.0 mass%, a sufficient deoxidation effect by Ca cannot be obtained, the toughness decreases, and the bead appearance deteriorates. Therefore, the Ca content with respect to the total mass of the flux is 7.0 mass% or more, preferably 8.0 mass% or more, and more preferably 9.0 mass% or more. On the other hand, when the Ca content in the flux exceeds 18.0 mass%, the slag detachability decreases. Therefore, the Ca content with respect to the total mass of the flux is 18.0 mass% or less, preferably 17.5 mass% or less, and more preferably 17.0 mass% or less. Note that Ca is contained in the flux in the form of fluoride and carbonate.

[0021] <F: 4.5 mass% or more and 12.0 mass% or less> F has the effect of reducing the diffusible hydrogen amount in the weld metal and improving the low-temperature crack resistance, and also has the effect of controlling the amount of oxygen in the weld metal and adjusting the bead shape. When the F content in the flux is less than 4.5 mass%, the amount of oxygen in the weld metal increases and the toughness decreases. Therefore, the F content with respect to the total mass of the flux is 4.5 mass% or more, preferably 5.2 mass% or more, and more preferably 5.8 mass% or more. On the other hand, when the F content in the flux exceeds 12.0 mass%, the arc becomes unstable and the bead shape and slag detachability decrease. Therefore, the F content with respect to the total mass of the flux is 12.0 mass% or less, preferably 11.5 mass% or less, and more preferably 11.0 mass% or less.

[0022] <Al2O3: 9.5 mass% or more and 21.7 mass% or less> Al2O3 is a slag-forming agent and is a component that has the effect of improving the fluidity of the slag and adjusting the bead shape. When the Al2O3 content in the flux is less than 9.5% by mass, the bead shape deteriorates. Therefore, the Al2O3 content based on the total mass of the flux is 9.5% by mass or more, preferably 11.0% by mass or more, and more preferably 11.5% by mass or more. On the other hand, when the Al2O3 content in the flux exceeds 21.7% by mass, slag entrainment increases and welding workability decreases. Therefore, the Al2O3 content based on the total mass of the flux is 21.7% by mass or less, preferably 21.0% by mass or less, and more preferably 20.5% by mass or less.

[0023] <SiO2: 8.0% by mass or more and 20.0% by mass or less> SiO2 is a component that has the effect of improving the fluidity of the slag and adjusting the bead shape. When the SiO2 content in the flux is less than 8.0% by mass, the bead shape deteriorates. Therefore, the SiO2 content based on the total mass of the flux is 8.0% by mass or more, preferably 9.0% by mass or more, and more preferably 10.0% by mass or more. On the other hand, when the SiO2 content in the flux exceeds 20.0% by mass, slag entrainment increases and welding workability decreases. Therefore, the SiO2 content based on the total mass of the flux is 20.0% by mass or less, preferably 19.0% by mass or less, and more preferably 18.5% by mass or less. In this embodiment, the SiO2 content means the value obtained by converting all the Si contained in the flux into SiO2. Also, the SiO2 in the flux includes the SiO2 derived from the water glass used as a binder.

[0024] <CO2: 1.0% by mass or more and 8.0% by mass or less> CO2 has the effect of reducing the diffusible hydrogen content in the weld metal and improving the low-temperature cracking resistance, and also has the effect of controlling the oxygen content in the weld metal. When the CO2 content in the flux is less than 1.0% by mass, low-temperature cracking occurs. Therefore, the CO2 content with respect to the total mass of the flux should be 1.0% by mass or more, preferably 1.2% by mass or more. On the other hand, when the CO2 content in the flux exceeds 8.0% by mass, the oxygen content in the weld metal increases and the toughness decreases. Therefore, the CO2 content with respect to the total mass of the flux should be 8.0% by mass or less, preferably 7.0% by mass or less, and more preferably 6.0% by mass or less. Note that CO2 is contained in the flux in the form of metal carbonate. Examples of the metal carbonate include CaCO3, BaCO3, MgCO3, etc. As long as the converted value of these metal carbonates in terms of CO2 is within the above range, the same effect can be obtained.

[0025] <Na: 0.5% by mass or more and 4.0% by mass or less> Na is a component that has the effect of improving arc stability. When the Na content in the flux is less than 0.5% by mass, the arc stability decreases and welding defects occur. Therefore, the Na content with respect to the total mass of the flux should be 0.5% by mass or more, preferably 1.0% by mass or more, and more preferably 1.5% by mass or more. On the other hand, when the Na content in the flux exceeds 4.0% by mass, the moisture absorption amount of the flux increases, so the hydrogen content in the weld metal increases and low-temperature cracking occurs. Therefore, the Na content with respect to the total mass of the flux should be 4.0% by mass or less, preferably 3.6% by mass or less, and more preferably 3.2% by mass or less.

[0026] <ZrO2: 4.0% by mass or less (including 0% by mass)> ZrO2 is a slag-forming agent and has the effect of improving the fluidity of the slag and shaping the bead, but it does not necessarily need to be contained in the flux. When ZrO₂ is contained in the flux, the ZrO₂ content based on the total mass of the flux is preferably 0.01% by mass or more, and more preferably 0.1% by mass or more. On the other hand, when the ZrO₂ content in the flux exceeds 4.0% by mass, slag entrainment increases and welding workability deteriorates. Therefore, the ZrO₂ content based on the total mass of the flux is 4.0% by mass or less, preferably 3.5% by mass or less, and more preferably 3.0% by mass or less. In this embodiment, the ZrO₂ content means the value obtained by converting all Zr contained in the flux into ZrO₂.

[0027] <Al: 0.80% by mass or less (including 0% by mass)> Al combines with N to form AlN, reduces the precipitation amount of carbides and nitrides of Cr, Nb, and V which are essential for ensuring creep strength, and is a component that deteriorates creep strength. Therefore, it is preferable to reduce Al in the flux as much as possible. When the Al content in the flux exceeds 0.80% by mass, the bead becomes welded, and the slag detachability deteriorates. In addition, the yield of elements in the weld metal improves and the strength increases, so the toughness deteriorates. Therefore, the Al content based on the total mass of the flux is 0.80% by mass or less, preferably 0.75% by mass or less, and more preferably 0.70% by mass or less. Note that the Al defined as 0.80% by mass or less here is contained in the flux in the form of elemental Al and Al alloys, and does not include the oxide form.

[0028] <C: 0.12% by mass or less (including 0% by mass)> C is a component that increases the susceptibility to hot cracking. Therefore, it is preferable to reduce C in the flux as much as possible. If the carbon content in the flux exceeds 0.12% by mass, the susceptibility to high-temperature cracking increases, making cracks more likely to occur, especially in submerged arc welding in narrow gaps. Furthermore, 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 flux should be 0.12% by mass or less, preferably 0.11% by mass or less.

[0029] <Formula (1): 6.0 or more and 45.0 or less> Equation (1) below parameterizes the deoxidizing power of the flux components on the weld metal. When the value obtained from equation (1) is high, the deoxidizing power of the flux increases, reducing the oxygen content of the weld metal and thus improving toughness. However, if it becomes excessively high, the slag viscosity at each temperature becomes too high, degrading the weldability. In other words, by appropriately controlling the value obtained from equation (1), it is possible to achieve both improved toughness and improved weldability. If the value obtained by formula (1) is less than 6.0, the deoxidizing power will be weak, and the toughness of the weld metal will decrease. Therefore, the value obtained by formula (1) should be 6.0 or higher, preferably 11.0 or higher, and more preferably 15.0 or higher. On the other hand, if the value obtained by formula (1) exceeds 45.0, the shape of the weld bead deteriorates, such as the central part of the bead becoming convex. Therefore, the value obtained by formula (1) should be 45.0 or less, preferably 40.0 or less, and more preferably 35.0 or less.

[0030] Equation (1): ([MgO]+2.055×[F]) / ([CO2]) In formula (1) above, [MgO] is the MgO content in the flux expressed as a mass percentage of the total flux mass, [F] is the F content in the flux expressed as a mass percentage of the total flux mass, and [CO2] is the CO2 content in the flux expressed as a mass percentage of the total flux mass.

[0031] The bond flux for submerged arc welding according to this embodiment preferably contains the following components in predetermined contents in addition to the above components.

[0032] <Mn: 0.5 mass% or more and 2.5 mass% or less> Mn is a component having a deoxidizing effect. When Mn is contained in the flux, if the Mn content is 0.5 mass% or more, deoxidizing power is exerted and the weld metal becomes low in oxygen, so that toughness can be improved. Therefore, the Mn content with respect to the total mass of the flux is preferably 0.5 mass% or more, more preferably 0.6 mass% or more, and still more preferably 0.7 mass% or more. On the other hand, when 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 made good. Therefore, the Mn content with respect to the total mass of the flux is preferably 2.5 mass% or less, more preferably 2.3 mass% or less, and still more preferably 2.1 mass% or less.

[0033] <K: 0.5 mass% or more and 3.0 mass% or less> K is an arc stabilizer and is a component contained in the flux to improve arc stability. When K is contained in the flux, if the K content is 0.5 mass% or more, arc stability is improved, so that welding workability can be made good. Therefore, the K content with respect to the total mass of the flux is preferably 0.5 mass% or more, more preferably 0.6 mass% or more. On the other hand, when the K content in the flux is 3.0 mass% or less, the moisture absorption amount of the flux is suppressed and the amount of hydrogen in the weld metal is reduced, so that the occurrence of cold cracking can be suppressed. Therefore, the K content with respect to the total mass of the flux is preferably 3.0 mass% or less, more preferably 2.7 mass% or less, and still more preferably 2.4 mass% or less.

[0034] <Li: 0.05 mass% or more and 0.20 mass% or less> Lithium (Li) is an arc stabilizer and is an ingredient included in flux to improve arc stability. When Li is included in the flux, if the Li content is 0.05% by mass or more, arc stability is improved, and thus welding workability can be improved. Therefore, the Li content relative to the total mass of the flux is preferably 0.05% by mass or more, and more preferably 0.06% by mass or more. On the other hand, if the Li content in the flux is 0.20% by mass or less, the amount of moisture absorbed by the flux is suppressed, and the amount of hydrogen in the weld metal is reduced, thereby suppressing the occurrence of cold cracking. Therefore, the Li content relative to the total mass of the flux is preferably 0.20% by mass or less, more preferably 0.18% by mass or less, and even more preferably 0.17% by mass or less.

[0035] <Total content of essential components> In this embodiment, the bond flux for submerged arc welding preferably contains a total content of MgO, Ca, F, Al2O3, SiO2, CO2, Na, ZrO2, Al, and C of 88% by mass or more, more preferably 91% by mass or more, and even more preferably 93% by mass or more, based on the total mass of the flux.

[0036] <Remainder> The bond flux for submerged arc welding according to this embodiment may contain, in addition to the above-mentioned components, for example, Fe, Mo, W, Cu, etc., to the extent that it does not hinder the effects of the present invention. These components may exist as individual elements or as compounds.

[0037] [2. Wire for submerged arc welding] The bond flux for submerged arc welding according to this embodiment is used together with a wire for submerged arc welding. Below, the reasons for the addition of components and the reasons for the compositional limitations of the wire, which is preferably used together with the bond flux for submerged arc welding according to this embodiment, will be explained in detail.

[0038] <C: 0.030 mass% or more and 0.080 mass% or less> C is a component that has a great influence on the hardenability and the amount of carbide precipitation in the weld metal, functions as an austenite stabilizing element, and has the effect of suppressing the remaining of the δ-ferrite phase in the weld metal. If the C content in the wire is less than 0.030 mass%, the C content in the weld metal becomes too low, resulting in insufficient carbide precipitation. Also, the δ-ferrite phase remains, and the desired creep strength cannot be obtained. Therefore, the C content with respect to the total mass of the wire is 0.030 mass% or more, preferably 0.032 mass% or more. On the other hand, if the C content in the wire exceeds 0.080 mass%, the high-temperature cracking susceptibility increases, and in particular, cracking is likely to occur in submerged arc welding with a narrow groove. Also, the amount of carbide precipitation increases, significantly increasing the strength of the weld metal and deteriorating the toughness. Therefore, the C content with respect to the total mass of the wire is 0.080 mass% or less, preferably 0.075 mass% or less, and more preferably 0.070 mass% or less.

[0039] <Si: 0.05 mass% or more and 0.30 mass% or less>[ Si is a component that improves the wettability of the weld bead, functions as a deoxidizer, and has the effect of improving the strength and toughness of the weld metal. If the Si content in the wire is less than 0.05 mass%, the Si content in the weld metal becomes too low, deteriorating the welding workability (e.g., wettability and fusion of the weld bead), and also deteriorating the toughness and creep strength. Therefore, the Si content with respect to the total mass of the wire is 0.05 mass% or more, preferably 0.06 mass% or more. On the other hand, if the Si content in the wire exceeds 0.30 mass%, the strength of the weld metal becomes significantly high and the toughness deteriorates. Therefore, the Si content with respect to the total mass of the wire is 0.30 mass% or less, preferably 0.25 mass% or less, and more preferably 0.21 mass% or less.

[0040] <Mn: 0.50 mass% or more and 2.20 mass% or less> Similar to Si, Mn functions as a deoxidizer and has the effect of improving the toughness of the weld metal. Also, Mn functions as an austenite stabilizing element and has the effect of suppressing the remaining of the δ-ferrite phase in the weld metal. Furthermore, as will be described later, Mn also has the effect of mitigating the adverse effect of S on hot cracking susceptibility. If the Mn content in the wire is less than 0.50 mass%, the Mn content in the weld metal becomes too low to obtain the desired toughness. Also, a soft δ-ferrite phase remains in the weld metal, deteriorating the creep strength. Furthermore, it becomes difficult to suppress hot cracking caused by S. Therefore, the Mn content with respect to the total mass of the wire should be 0.50 mass% or more, preferably 0.55 mass% or more, and more preferably 0.60 mass% or more. On the other hand, if the Mn content in the wire exceeds 2.20 mass%, the Mn content in the weld metal becomes too high, destabilizing the carbonitrides and reducing the creep strength. Therefore, the Mn content with respect to the total mass of the wire should be 2.20 mass% or less, preferably 2.00 mass% or less, and more preferably 1.50 mass% or less.

[0041] <Ni: 0.30 mass% or more and 1.00 mass% or less> Ni is a component that dissolves in the matrix of the weld metal and has the effect of improving the toughness of the ferrite itself. If the Ni content in the wire is less than 0.30 mass%, the effect of improving the toughness of the ferrite cannot be obtained. Therefore, the Ni content with respect to the total mass of the wire should be 0.30 mass% or more, preferably 0.34 mass% or more, and more preferably 0.38 mass% or more. On the one hand, when the Ni content in the wire exceeds 1.00% by mass, Ni concentrates in the final solidification part during welding, the solidification completion temperature decreases, and the high-temperature cracking susceptibility increases. Also, during creep deformation, the size of the carbonitrides coarsens and the creep strength decreases. Therefore, the Ni content with respect to the total mass of the wire should be 1.00% by mass or less, preferably 0.90% by mass or less, and more preferably 0.85% by mass or less.

[0042] <Cr: 8.00% by mass or more and 10.50% by mass or less> Cr is a component that has the effect of forming carbonitrides during PWHT and improving the creep strength of the weld metal. When the Cr content in the wire is less than 8.00% by mass, the precipitation amount of carbonitrides is insufficient and the desired creep strength cannot be obtained. Therefore, the Cr content with respect to the total mass of the wire should be 8.00% by mass or more, preferably 8.10% by mass or more, and more preferably 8.20% by mass or more. On the other hand, when the Cr content in the wire exceeds 10.50% by mass, the solidification completion temperature decreases, the high-temperature cracking susceptibility increases, and the δ-ferrite phase remains in the weld metal, resulting in a decrease in creep strength and toughness. Also, the slag detachability deteriorates significantly. Therefore, the Cr content with respect to the total mass of the wire should be 10.50% by mass or less, preferably 10.00% by mass or less, and more preferably 9.60% by mass or less.

[0043] <Mo: 0.80% by mass or more and 1.20% by mass or less> Mo is a component that has the effect of dissolving in the Cr-based carbides or the matrix during PWHT and improving the creep strength of the weld metal. When the Mo content in the wire is less than 0.80% by mass, the desired creep strength cannot be obtained. Therefore, the Mo content with respect to the total mass of the wire should be 0.80% by mass or more, preferably 0.82% by mass or more, and more preferably 0.85% by mass or more. On the one hand, when the Mo content in the wire exceeds 1.20% by mass, the amount of solid solution in the Cr-based carbides and the matrix phase increases excessively, resulting in a significant increase in the strength of the weld metal and deterioration of toughness. Therefore, the Mo content with respect to the total mass of the wire should be 1.20% by mass or less, preferably 1.15% by mass or less, and more preferably 1.10% by mass or less.

[0044] <V: 0.10% by mass or more and 0.40% by mass or less> V is a component that has the effect of forming carbonitrides during PWHT and improving the creep strength of the weld metal. When the V content in the wire is less than 0.10% by mass, the desired creep strength cannot be obtained. Therefore, the V content with respect to the total mass of the wire should be 0.10% by mass or more, preferably 0.14% by mass or more, and more preferably 0.18% by mass or more. On the other hand, when the V content in the wire exceeds 0.40% by mass, the amount of precipitation of carbonitrides increases significantly, resulting in an increase in the strength of the weld metal and deterioration of toughness. Therefore, the V content with respect to the total mass of the wire should be 0.40% by mass or less, preferably 0.35% by mass or less, and more preferably 0.30% by mass or less.

[0045] <Nb: 0.020% by mass or more and 0.095% by mass or less> Similar to V, Nb is a component that has the effect of forming carbonitrides during PWHT and improving the creep strength of the weld metal. When the Nb content in the wire is less than 0.020% by mass, the desired creep strength cannot be obtained. Therefore, the Nb content with respect to the total mass of the wire should be 0.020% by mass or more, preferably 0.040% by mass or more, and more preferably 0.060% by mass or more. On the one hand, when the Nb content in the wire exceeds 0.095% by mass, the amount of precipitation of carbonitrides increases significantly, resulting in an increase in the strength of the weld metal and deterioration of toughness. Also, the slag detachability deteriorates significantly. Therefore, the Nb content with respect to the total mass of the wire should be 0.095% by mass or less, preferably 0.093% by mass or less.

[0046] <N: Not less than 0.016% by mass and not more than 0.065% by mass> N is a component that combines with Cr, V, Nb, etc. during PWHT to form carbides and nitrides, and has the effect of improving the creep strength of the weld metal. If the N content in the wire is less than 0.016% by mass, the desired creep strength cannot be obtained. Therefore, the N content based on the total mass of the wire should be not less than 0.016% by mass, preferably not less than 0.025% by mass, and more preferably not less than 0.035% by mass. On the other hand, if the N content in the wire exceeds 0.065% by mass, the precipitation amount of carbides and nitrides significantly increases, the strength of the weld metal becomes high, and the toughness deteriorates. Also, N2 gas generated during the welding process is likely to remain in the molten metal, resulting in blowholes. Therefore, the N content based on the total mass of the wire should be not more than 0.065% by mass, preferably not more than 0.063% by mass, and more preferably not more than 0.061% by mass.

[0047] <P: Not more than 0.015% by mass (including 0% by mass)> P is a component that forms a low-melting-point compound at the final solidification part during welding, not only increases the susceptibility to hot cracking, but also embrittles the weld metal and deteriorates the toughness. Therefore, it is preferable to reduce P in the wire as much as possible. If the P content in the wire exceeds 0.015% by mass, hot cracking is likely to occur and the toughness deteriorates. Therefore, the P content based on the total mass of the wire should be not more than 0.015% by mass, preferably not more than 0.012% by mass, and more preferably not more than 0.009% by mass.

[0048] <S: Not more than 0.010% by mass (including 0% by mass)> S is a component that combines with Fe during welding to form a Fe-FeS eutectic with a low melting point at the final solidification part, not only increases the hot cracking susceptibility, but also embrittles the weld metal and deteriorates the toughness. Therefore, it is preferable to reduce S in the wire as much as possible. If the sulfur content in the wire exceeds 0.010% by mass, high-temperature cracking becomes more likely, and toughness deteriorates. Therefore, the sulfur content relative to the total mass of the wire should be 0.010% by mass or less, and preferably 0.009% by mass or less.

[0049] <Remainder> In the wire preferably used with the bond flux for submerged arc welding according to this embodiment, the remainder consists of Fe and unavoidable impurities. Examples of unavoidable impurities include Sn, As, Sb, Pb, and Bi. Of these impurities, Sn, As, and Sb are preferably present in amounts of, for example, 0.005% by mass or less of the total mass of the wire, and the total is preferably 0.015% by mass or less. Furthermore, it is preferable that Pb and Bi are each present in amounts of, for example, 0.001% by mass or less relative to the total mass of the wire. In addition, elements other than those listed above as unavoidable impurities may be present in the wire as unavoidable impurities if they are each present in amounts of 0.10% by mass or less relative to the total mass of the wire, preferably 0.08% by mass or less, and more preferably 0.06% by mass or less.

[0050] [3. Weld metal] The weld metal according to this embodiment is a weld metal formed using the bond flux described in [1. Bond flux for submerged arc welding] above. In this embodiment, the welding conditions other than using the bond flux for submerged arc welding according to this embodiment are not particularly limited, and the usual conditions for submerged arc welding methods can be used for the type of base metal, welding voltage, welding current, welding position, etc. [Examples]

[0051] The present invention will be specifically described below with reference to examples and comparative examples demonstrating the effects of the present invention.

[0052] [Submerged arc welding] First, a wire with a diameter of 4 mm was prepared, and bond fluxes containing various chemical components were manufactured. The chemical components in the wire and their content are shown in Table 1 below, and the chemical components in the flux and their content are shown in Table 2 below. The remainder of the chemical composition in the wire shown in Table 1 below consists of Fe and unavoidable impurities. In addition, in the chemical composition of the bond flux shown in Table 2 below, Al is not actively added, so it is represented as "<0.80" (less than 0.80). Next, submerged arc welding was performed using the above wire and bonding flux, followed by post-weld heat treatment (PWHT) under two different conditions. The welding and PWHT conditions are shown below.

[0053] (Welding conditions) Base material: ASTM A387 Gr.91 Base material thickness: 25mm Bevel angle, shape: 10°, V type Root spacing: 20mm Polarity: Alternating Current (AC) Wire diameter: 4.0mm Welding position: Downward Current: 450~550A Voltage: 30~32V Welding speed: 30-36 cm / min Preheating / pass-through temperature: 250~300℃ Lamination method: 5-7 layers, 10-14 passes

[0054] (PWHT condition) A: 750~765℃, 40~44hr B: 735~750℃, 9~10hr

[0055] [evaluation] After the submerged arc welding described above, heat treatment was performed under the PWHT conditions described above, and the room temperature strength and toughness of the resulting weld metal were evaluated. The specific temperature (°C) and time (hr) of the PWHT conditions, as well as the evaluation results of each test, are shown in Table 3 below.

[0056] <Intensity at room temperature> For weld metal after PWHT under condition A, tensile tests were conducted at room temperature in accordance with the tensile and impact test methods for weld metal specified in JIS Z3111:2005, and the 0.2% yield stress (YS) and tensile strength (TS) were measured. A strength equivalent to that of the base material, A387 Gr.91, was considered acceptable. Specifically, for TS, values ​​between 585 and 760 MPa were considered acceptable, while values ​​below 585 MPa or above 760 MPa were considered unacceptable. For YS, values ​​of 415 MPa or higher were considered acceptable, while values ​​below 415 MPa were considered unacceptable.

[0057] <Toughness> Three test specimens were taken from the weld metal after PWHT under condition B, and a Charpy impact test was performed at 0°C in accordance with the Charpy impact test method for metallic materials specified in JIS Z2242. The average value of the absorbed energy vE (J) measured for each test specimen was calculated, and the toughness was evaluated. A specimen was deemed to have passed if the average absorbed energy obtained from the measurement was 54 J or higher, and failed if it was less than 54 J.

[0058] [Table 1]

[0059] [Table 2]

[0060] [Table 3]

[0061] As shown in Tables 2 and 3, in Invention Examples No. 1 to 6, the chemical content of the bonding flux for submerged arc welding is within the range of the present invention. As a result of performing submerged arc welding using these fluxes, weld metal with excellent tensile strength and toughness after PWHT was obtained.

[0062] On the other hand, in Comparative Examples No. 1 to 3, the values ​​calculated by formula (1) based on the MgO, F, and CO2 content in the flux were below the lower limit of the range of the present invention, resulting in decreased room-temperature strength and toughness. In particular, Comparative Examples No. 1 and 2 had lower toughness compared to Comparative Example No. 3 because the CO2 content in the flux exceeded the upper limit of the range of the present invention.

Claims

1. A bond flux for submerged arc welding of high-cr ferritic heat-resistant steel, With respect to the total mass of the flux, MgO: 25.0% by mass or more and 36.0% by mass or less, Ca: 7.0% by mass or more and 18.0% by mass or less, F: 4.5% by mass or more and 12.0% by mass or less, Al ２ O ３ : 9.5% or more by mass, 21.7% or less by mass SiO ２ : 8.0% or more by mass and less than 20.0% by mass CO ２ : 1.0% or more by mass, 8.0% or less by mass Na: 0.5% by mass or more and 4.0% by mass or less, Mn: 0.5% by mass or more and 2.5% by mass or less, K: 0.5% by mass or more and 3.0% by mass or less, Contains Li: 0.05% by mass or more and 0.20% by mass or less, ZrO ２ Less than 4.0% by mass Al: 0.80% by mass or less, C: 0.12% by mass or less, The total content of MgO, Ca, F, Al₂O₃, SiO₂, CO₂, Na, ZrO₂, Al, and C is 91% by mass or more relative to the total mass of the flux. The MgO content in the flux is expressed as [MgO] in mass % of the total mass of the flux, the F content in the flux is expressed as [F] in mass % of the total mass of the flux, and the CO content in the flux is expressed as [F] in mass % of the total mass of the flux. ２ The content is expressed as a mass % relative to the total mass of flux [CO ２ A bond flux for submerged arc welding, characterized in that, when the following conditions are met, the value obtained by formula (1) is 6.0 or more and 45.0 or less. Formula (1): ([MgO] + 2.055×[F]) / ([CO ２ )

2. Used with submerged arc welding wire, The aforementioned submerged arc welding wire is, with respect to the total mass of the wire, C: 0.030% by mass or more and 0.080% by mass or less, Si: 0.05% by mass or more and 0.30% by mass or less, Mn: 0.50% by mass or more and 2.20% by mass or less, Ni: 0.30% by mass or more and 1.00% by mass or less, Cr: 8.00% by mass or more and 10.50% by mass or less, Mo: 0.80% by mass or more and 1.20% by mass or less, V: 0.10% by mass or more and 0.40% by mass or less, Nb: 0.020 mass% or more and 0.095 mass% or less, N: Contains 0.016% by mass or more and 0.065% by mass or less, P: 0.015% by mass or less, S: 0.010 mass% or less, The bond flux for submerged arc welding according to claim 1, characterized in that the remainder is Fe and unavoidable impurities.

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