Coated electrode, welded metal, method of shielded metal arc welding and method of manufacturing a welded joint

Abstract

Provided are a coated electrode, a welded metal, a method of electrode arc welding, and a method of manufacturing a welded joint. A coated electrode for high-tension steel, which can obtain a welded portion that can maintain a target strength, has excellent low-temperature toughness, and has suppressed hot cracking, not only in the As-welded state but also under a wide range of PWHT conditions. The coated electrode contains CO2: 16% to 27%, F: 4% to 10%, Si: 3% to 11%, Ni: 7.5% to 13.3%, Fe: 1% to 11%, Mo: 0.3% to 1.0%, Cr: 0.15% to 1.2%, Mg: 1.5% to 4.5%, and Mn: 1.5% to 4.0%, and when the contents of Ni, Mn, and Si in the coating are represented by [Ni], [Mn], and [Si], respectively, the value calculated from [Ni] / ([Si]+[Mn]) is 0.85 to 1.45.

Description

Technical Field

[0001] This invention relates to flux-coated welding electrodes (hereinafter also simply referred to as "electrodes") used for welding high-tensile steel, welding metals, electrode arc welding methods, and methods for manufacturing welded joints. Background Technology

[0002] Welded structures and storage tanks used in oil and gas extraction and production require large-scale equipment and operation in cold regions. The steel plates and welding materials used in their manufacture must possess high strength and excellent low-temperature toughness. In addition to these properties, post-weld heat treatment (PWHT) is sometimes performed to further improve the quality of the welded products. By performing PWHT, for example, residual stress in the weld can be removed, thus inhibiting weld cracking.

[0003] However, if PWHT is performed, the weld joint may become embrittled due to precipitation hardening and temper embrittlement, potentially reducing toughness. This is especially true for weld joints of high-tensile steels, where the embrittlement caused by PWHT is particularly pronounced due to the influence of various elements added to increase strength. In most cases, the higher the strength, the less suitable PWHT should be.

[0004] Therefore, it is necessary to develop high-tensile steel welding materials that also possess excellent mechanical properties after PWHT.

[0005] For example, Patent Document 1 discloses a flux-coated welding electrode for welding high-tensile steel, such as 590 N / mm². 2 When using high-tensile steel of grade 1 or above, weld metals with excellent low-temperature toughness and fracture toughness after stress-relief annealing can be obtained.

[0006] In addition, Patent Document 2 proposes a low-hydrogen coated welding electrode for 590MPa high-tensile steel, which has good welding operability and excellent strength and low-temperature toughness of the weld metal after welding state (AW: as-welded) and PWHT.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 8-257791

[0010] Patent Document 2: Japanese Patent Application Publication No. 2017-64740 Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] However, the patent documents 1 and 2 mentioned above only consider one PWHT condition, and may not necessarily achieve excellent toughness under other PWHT conditions.

[0013] Furthermore, it can be said that PWHT conditions have a large tolerance, making them applicable to all welded products. Construction management is also easy, and the quality stability of the welded parts can be further improved.

[0014] Furthermore, since neither of the aforementioned patent documents 1 nor 2 takes hot cracking into account, the requirements for welding electrodes that can produce weld metal with good resistance to hot cracking are even higher.

[0015] On the other hand, it has always been known that PWHT may gradually lose toughness due to conditions, and the conditions for PWHT are strictly determined by the type of material, thickness, weld joint, construction conditions, etc.

[0016] Therefore, maintaining the tolerance of PWHT conditions is very difficult, especially since the higher the strength, the more added elements are contained, which makes it easier for temper embrittlement and other embrittlement phenomena to occur. Thus, PWHT conditions become more stringent.

[0017] This invention addresses these issues and aims to provide a flux-coated welding electrode for high-tensile steel that, not only in the welded state (hereinafter also referred to as "as-welded"), but also under a wide range of PWHT conditions, can produce a weld joint that maintains the target strength, exhibits excellent low-temperature toughness, and suppresses hot cracking. Furthermore, it provides a shielded metal arc welding method and a method for manufacturing the weld joint. Additionally, it aims to provide a weld metal that, even under a wide range of PWHT conditions, maintains the target strength, exhibits excellent low-temperature toughness, and suppresses hot cracking.

[0018] Problem-solving methods

[0019] The above-mentioned objective of the present invention can be achieved by the following [1] configuration of the flux-coated welding electrode.

[0020] [1] A flux-coated welding electrode, characterized in that it is a flux-coated welding electrode having a core and a coating covering the core, wherein the coating, relative to the total mass of the coating, contains

[0021] CO2: ≥16% by mass and ≤27% by mass

[0022] F: 4% by mass or more and 10% by mass or less

[0023] Si: 3% by mass or more and 11% by mass or less

[0024] Ni: 7.5% by mass or more and 13.3% by mass or less

[0025] Fe: 1% by mass or more and 11% by mass or less

[0026] Mo: ≥0.3% by mass and ≤1.0% by mass

[0027] Cr: ≥0.15% by mass and ≤1.20% by mass

[0028] Mg: ≥1.5% by mass and ≤4.5% by mass

[0029] Mn: ≥1.5% by mass and ≤4.0% by mass

[0030] The Ni content in the drug coating is expressed as [Ni], expressed as a percentage of mass relative to the total mass of the drug coating.

[0031] The content of Mn in the drug coating is expressed as [Mn], in terms of mass percentage relative to the total mass of the drug coating.

[0032] When the content of Si in the drug coating is expressed as [Si] as a percentage of the total mass of the drug coating,

[0033] The value calculated from [Ni] / ([Si]+[Mn]) is above 0.85 and below 1.45.

[0034] Preferred embodiments of the present invention of flux-coated welding electrodes relate to the following [2] to [4].

[0035] [2] According to the flux-coated welding electrode described in [1], the flux coating, relative to the total mass of the flux coating, further contains

[0036] CaO: ≥20% by mass, ≤40% by mass

[0037] BaO: 2% by mass or more, 6% by mass or less.

[0038] [3] According to [1] or [2] the flux-coated welding electrode, characterized in that the flux coating, relative to the total mass of the flux coating, further contains

[0039] The combined concentration of Na, K, and Li: ≥0.3% by mass and ≤4.0% by mass.

[0040] Ti: ≥0.5% by mass and ≤4.0% by mass, and

[0041] Al: less than 1.5% by mass

[0042] Zr: less than 0.8% by mass.

[0043] [4] The flux-coated welding electrode according to any one of [1] to [3], characterized in that the flux coating, relative to the total mass of the flux coating, is limited to

[0044] Nb: less than 0.03% by mass

[0045] V: Less than 0.03% by mass.

[0046] The above-mentioned objective of the present invention can be achieved by the following [5] composition of the welding metal.

[0047] [5] A weldable metal, characterized in that it is obtained by using a flux-coated electrode as described in any one of [1] to [4] for shielded metal arc welding.

[0048] The above-mentioned objective of the present invention can be achieved by the following configuration of the shielded metal arc welding method [6].

[0049] [6] A method for shielded metal arc welding, characterized in that shielded metal arc welding is performed using any one of [1] to [4].

[0050] The above-mentioned objective of the present invention can be achieved by the following configuration of the method for manufacturing the welded joint [7].

[0051] [7] A method for manufacturing a welded joint, characterized in that high-tensile steel is used as the base material and a flux-coated electrode as described in any one of [1] to [4] is used for electrode arc welding.

[0052] Invention Effects

[0053] According to the present invention, a flux-coated welding electrode for high-tensile steel can be provided, which can produce welds that maintain target strength, exhibit excellent low-temperature toughness, and suppress hot cracking, not only under as-welded conditions but also under a wide range of PWHT conditions.

[0054] Furthermore, according to the present invention, a weld metal can be provided that maintains the target strength even under a wide range of PWHT conditions, while exhibiting excellent low-temperature toughness and suppressing hot cracking.

[0055] Furthermore, according to the present invention, a method for shielded metal arc welding and a method for manufacturing welded joints can be provided, which can obtain welds that maintain target strength, have excellent low-temperature toughness, and suppress hot cracking even under a wide range of PWHT conditions. Detailed Implementation

[0056] The inventors investigated the mechanism by which PWHT (Potassium Metal Heat Treatment) on high-strength welds causes a decrease in toughness, and conducted intensive research to obtain welds that still possess excellent low-temperature toughness when PWHT is performed under a wide range of conditions. As a result, the following insights were discovered, leading to the completion of this invention. Furthermore, the term "weld" as used above refers to both the weld metal and the heat-affected zone (HAZ), but in this specification, the following description refers specifically to the weld metal.

[0057] First, the existing mechanism of toughness reduction in high-strength steel will be explained.

[0058] The following two reasons have been previously cited as the main causes of the reduced toughness of weld metal after PWHT.

[0059] (Reason 1) When the content of Cr, Mo, etc. in the welding metal is high, these components form carbides with C and precipitate, which leads to the hardening of the welding metal.

[0060] (Reason 2) Slow cooling starting from the PWHT temperature leads to embrittlement phenomena such as tempering embrittlement.

[0061] It was previously believed that the main effect of the above-mentioned (reason 1) and (reason 2) was the reduction in grain boundary strength of the weld metal, which resulted in a reduction in toughness.

[0062] Therefore, the following measures have always been taken to maintain the target strength while ensuring excellent toughness after PWHT.

[0063] (Measure 1) Suppress the precipitation of carbides at the original austenite grain boundaries.

[0064] (Measure 2) Suppress the segregation of impurity elements at the original austenite grain boundaries.

[0065] However, according to PWHT conditions, implementing only (measure 1) and (measure 2) is insufficient. Furthermore, PWHT conditions generally consist of the elements of holding temperature and holding time, which can be organized using the Larson-Miller parameter (hereinafter referred to as "LMP"), which takes these elements as parameters.

[0066] Therefore, in this specification, the flux-coated welding electrode that can obtain weld metal with excellent low-temperature toughness while maintaining the target strength under the following three conditions a to c is judged to have a wide PWHT condition range.

[0067] (Condition a) Welded state (As-welded).

[0068] (Condition b) is the PWHT condition for high LMP, which is the condition of 8 hours at 620°C (hereinafter referred to as "high LMP condition").

[0069] (Condition c) is the PWHT condition for low LMP, which is a condition at 580°C for 2 hours (hereinafter referred to as the "low LMP condition").

[0070] Under the welding condition of (condition a) and the high LMP condition of (condition b), it can be seen that, in addition to (measure 1) and (measure 2) mentioned above, good low-temperature toughness can be obtained by making the welding electrode contain an appropriate amount of Ni.

[0071] On the other hand, under the low LMP conditions (condition c), a suitable amount of Ni is insufficient to achieve good low-temperature toughness. The inventors have discovered the mechanism by which low-temperature toughness decreases when PWHT is performed under the low LMP conditions (condition c), and have also discovered measures to obtain weld metals that maintain both target strength and excellent low-temperature toughness even under a wide range of PWHT conditions. The mechanism and measures for the decrease in low-temperature toughness under the low LMP conditions (condition c) will be explained below.

[0072] If the weld metal contains a large amount of Ni, Ni-containing segregation bands (hereinafter referred to as "Ni segregation bands") will form on the weld metal microstructure, which are Ni-thickened regions. These Ni segregation bands have no effect on low-temperature toughness under the weld condition (condition a) and the high LMP condition (condition b).

[0073] However, under the low LMP conditions (condition c), the C content in the Ni segregation zone increases, promoting the formation and coarsening of island martensite and carbides. In particular, the formation of island martensite is significant. Due to the influence of these products, brittle fracture easily occurs in the Ni segregation zone, resulting in reduced low-temperature toughness.

[0074] Accordingly, the inventors have discovered that, as a measure to ensure that the weld metal maintains both the target strength and excellent low-temperature toughness after PWHT, it is effective to implement (measure 3) as shown below, in addition to (measure 1) and (measure 2).

[0075] (Measure 1) Suppress the precipitation of carbides at the original austenite grain boundaries.

[0076] (Measure 2) Suppress the segregation of impurity elements at the original austenite grain boundaries.

[0077] (Measure 3) Suppress the formation of island martensite in Ni segregation bands.

[0078] Furthermore, the inventors have discovered that by appropriately controlling the chemical composition of the flux-coated welding electrode and the parameters calculated based on the content of Ni, Si, and Mn, the above-mentioned measures (1) to (3) can be achieved, and hot cracking can be suppressed. That is, by using welding electrodes that achieve (1) to (3), not only as-welded, but also after PWHT under a wide range of conditions, weld metal that maintains the target strength and has excellent low-temperature toughness can be obtained.

[0079] The following describes in detail the method for implementing the present invention (hereinafter referred to as "this embodiment"). Furthermore, the present invention is not limited to the embodiments described below, and can be implemented in any way without departing from the spirit of the invention.

[0080] [1. Flux-coated welding rods]

[0081] The flux-coated welding electrode of this embodiment is made by coating a steel core (hereinafter also referred to as "core") with flux.

[0082] <1-1. Coverage Ratio>

[0083] As long as the content of each element in the coating is within the range of this invention, the coating rate can be set to any value. Furthermore, the coating rate, when the mass (g) of the coating in the total mass of the electrode is [coating], and the mass (g) of the core in the total mass of the electrode is [core], can be calculated using the formula: {[coating] / ([coating]+[core])}×100. In this embodiment, the coating rate calculated by the above formula is preferably 25% by mass or more and 40% by mass or less.

[0084] <1-2. Drug coating>

[0085] The following explains in more detail the chemical composition of the flux coating of the flux-coated welding electrode according to this embodiment and the reasons for limiting its content. Unless otherwise specified, the content in this embodiment refers to a percentage of the total mass of the flux coating.

[0086] Furthermore, unless otherwise specified, the elements listed below may be contained in the coating in metallic form or in compound form, or in both metallic and compound forms. That is, regardless of the form in which the above elements are contained in the coating, their contents are defined by their conversion values ​​to individual elemental values. For example, when listing Si, the Si content refers to the sum of the conversion values ​​of metallic Si and Si compounds. Also, metallic Si includes both Si monomers and Si alloys.

[0087] (CO2: 16-27% by mass)

[0088] In this embodiment, the carbonate content in the flux coating is defined as the CO2 content. Carbonates decompose into CO2 and oxides during welding, effectively preventing oxidation and nitriding of the weld metal. If the CO2 content in the flux coating is less than 16% by mass, sufficient gas cannot be generated during welding, leading to nitriding and oxidation of the weld metal, and deterioration of low-temperature toughness. Therefore, the CO2 content in the flux coating is 16% by mass or more relative to the total mass of the flux coating, preferably 17% by mass or more, and more preferably 18% by mass or more.

[0089] On the other hand, if the CO2 content in the coating exceeds 27% by mass, the fluidity of the molten slag increases due to the large amount of carbonate containing CO2 sources, making it difficult to uniformly form a well-coated slag and deteriorating the slag's peelability. Therefore, the CO2 content in the coating is preferably 24% by mass or less, and more preferably 21% by mass or less, relative to the total mass of the coating.

[0090] Furthermore, carbonates such as CaCO3, BaCO3, MgCO3, MnCO3, FeCO3, Na2CO3, and K2CO3 can be listed as sources of CO2 in the drug coating.

[0091] (F: 4% by mass or more and 10% by mass or less)

[0092] Metal fluorides such as CaF2, MgF2, and AlF3 lower the melting point of the molten slag, improving slag coating properties and resulting in a better weld appearance. Furthermore, fluorine (F) reacts with hydrogen during welding, lowering the hydrogen partial pressure in the weld metal, thus reducing the hydrogen content of the weld metal. If the F content in the coating is less than 4% by mass, the above-mentioned effects cannot be fully achieved. Therefore, the F content in the coating is 4% by mass or more relative to the total mass of the coating, preferably 5% by mass or more, and more preferably 6% by mass or more.

[0093] On the other hand, if the F content in the coating exceeds 10% by mass, the arc becomes unstable and the amount of spatter increases. Therefore, the F content in the coating is preferably 9% by mass or less, and more preferably 8% by mass or less, relative to the total mass of the coating.

[0094] (Si: 3% by mass or more and 11% by mass or less)

[0095] The Si contained in metals such as Si and ferrosilicon increases the viscosity of molten metal and adjusts its fluidity, resulting in a good weld appearance and weld shape.

[0096] Oxides such as SiO2 act as slag-forming agents. Furthermore, oxides such as SiO2 increase the viscosity of the molten slag, improving its fluidity and resulting in a better weld bead appearance and shape. Since metallic Si, Si alloys, and Si oxides in the coating each possess various effects, in this embodiment, the total Si content contained in metallic Si, Si alloys, and Si compounds in the coating is specified.

[0097] If the Si content in the flux coating is less than 3% by mass, the viscosity of the molten metal and slag decreases, making weld bead formation difficult when welding in a vertical welding posture, and resulting in poor weld bead quality. Therefore, the Si content in the flux coating is 3% by mass or more relative to the total mass of the flux coating, preferably 4% by mass or more, and more preferably 5% by mass or more.

[0098] On the other hand, if the Si content in the coating is higher than 11% by mass, hard island-like martensite will form in the Ni-containing segregation zone, thereby promoting temper embrittlement and reducing low-temperature toughness. Therefore, the Si content in the coating is preferably 9% by mass or less, and more preferably 8% by mass or less, relative to the total mass of the coating.

[0099] Furthermore, as sources of Si in the drug coating, examples include Si oxides such as SiO2, Si silicates, metallic Si, alloys of ferrosilicon, and binders such as water glass.

[0100] (Ni: 7.5% by mass or more and 13.3% by mass or less)

[0101] Ni is a component that enhances the strength and low-temperature toughness of weld metal through matrix strengthening. If the Ni content in the coating is less than 7.5% by mass, the required tensile strength and low-temperature toughness cannot be obtained. Therefore, the Ni content in the coating is 7.5% by mass or more relative to the total mass of the coating, preferably 8.0% by mass or more, more preferably 8.5% by mass or more, and even more preferably 9.0% by mass or more.

[0102] On the other hand, if the Ni content in the coating is higher than 13.3% by mass, low-melting-point impurity elements thicken in the segregation zone, increasing the likelihood of hot cracking. Therefore, the Ni content relative to the total mass of the coating is 13.3% by mass or less, preferably 12.0% by mass or less, and more preferably 11.0% by mass or less.

[0103] (Fe: 1% by mass or more and 11% by mass or less)

[0104] Fe is a component that affects deposition efficiency and welding operability. If the Fe content in the coating is less than 1% by mass, the welding efficiency decreases, the arc vibrates, and the welding operability decreases. Therefore, the Fe content in the coating is 1% by mass or more relative to the total mass of the coating, preferably 3% by mass or more, and more preferably 5% by mass or more.

[0105] On the other hand, if the Fe content in the coating is higher than 11% by mass, the protective effect is reduced and the weldability is decreased. Therefore, the Fe content in the coating is preferably 9% by mass or less, and more preferably 8% by mass or less, relative to the total mass of the coating.

[0106] (Mo: ≥0.3% by mass and ≤1.0% by mass)

[0107] Mo increases the strength of weld metal and is effective in suppressing temper embrittlement. The precipitation of Mo carbides into the weld metal grains can inhibit the precipitation of cementite at grain boundaries, thus suppressing the decrease in low-temperature toughness after PWHT.

[0108] If the Mo content in the coating is less than 0.3% by mass, the precipitation of cementite to the grain boundaries caused by PWHT cannot be suppressed, and the required low-temperature toughness cannot be obtained. Therefore, the Mo content in the coating is 0.3% by mass or more relative to the total mass of the coating, preferably 0.5% by mass or more, and more preferably 0.6% by mass or more.

[0109] On the other hand, if the Mo content is higher than 1.0% by mass, the low-temperature toughness under AW decreases, and in PWHT, MoC2 also excessively precipitates within the grains of the weld metal, further reducing low-temperature toughness. Therefore, the Mo content in the coating is preferably 0.9% by mass or less, and more preferably 0.8% by mass or less, relative to the total mass of the coating.

[0110] (Cr: ≥0.15% by mass and ≤1.20% by mass)

[0111] Cr increases the strength of the weld metal and is a component that inhibits the precipitation of coarse microstructures at grain boundaries. If the Cr content in the coating is less than 0.15% by mass, the precipitation of coarse microstructures at grain boundaries cannot be inhibited, and the required tensile strength and low-temperature toughness after PWHT cannot be obtained. Therefore, the Cr content in the coating is 0.15% by mass or more relative to the total mass of the coating, preferably 0.30% by mass or more, and more preferably 0.40% by mass or more.

[0112] On the other hand, Cr, through PWHT, mainly promotes the precipitation and growth of coarse grain boundary carbides, and is a component that reduces low-temperature toughness. If the Cr content in the coating is higher than 1.20% by mass, the low-temperature toughness after PWHT decreases. Therefore, the Cr content in the coating is preferably 1.10% by mass or less, more preferably 1.00% by mass or less, and even more preferably 0.80% by mass or less, relative to the total mass of the coating.

[0113] (Mg: ≥1.5% by mass and ≤4.5% by mass)

[0114] Mg reduces the amount of oxides in the weld metal through deoxidation, thus improving low-temperature toughness. If the Mg content in the coating is less than 1.5% by mass, the desired deoxidation effect cannot be achieved. Therefore, the Mg content in the coating is 1.5% by mass or more relative to the total mass of the coating, preferably 2.0% by mass or more, and more preferably 2.2% by mass or more.

[0115] On the other hand, Mg has the effect of reducing the arc force during welding. If the Mg content in the coating is higher than 4.5% by mass, the arc becomes unstable and the weld bead shape is poor. Therefore, the Mg content in the coating is preferably 3.8% by mass or less, and more preferably 3.0% by mass or less, relative to the total mass of the coating.

[0116] (Mn: ≥1.5% by mass and ≤4.0% by mass)

[0117] Mn is a component that improves the strength of weld metal. If the Mn content in the coating is less than 1.5% by mass, the required strength cannot be obtained. Therefore, the Mn content in the coating is 1.5% by mass or more relative to the total mass of the coating, preferably 2.0% by mass or more, and more preferably 2.2% by mass or more.

[0118] On the other hand, Mn forms hard island-like martensite, particularly in Ni-containing segregation zones, which promotes temper embrittlement. Besides reducing low-temperature toughness, it is also a major cause of hot cracking. If the Mn content in the coating exceeds 4.0% by mass, not only does the low-temperature toughness after PWHT decrease, but the likelihood of hot cracking also increases. Therefore, the Mn content in the coating is preferably 3.5% by mass or less, and more preferably 3.0% by mass or less, relative to the total mass of the coating.

[0119] Furthermore, as sources of Mn in the drug coating, examples include oxides of MnO, MnO2, Mn3O4, and Mn2O3, Mn sulfides, Mn carbonates, metallic Mn, and alloys of manganese iron, etc.

[0120] (Values ​​calculated from [Ni] / ([Si]+[Mn]): ≥0.85 and ≤1.45)

[0121] As mentioned above, by properly controlling the value of this parameter calculated based on the contents of Ni, Si and Mn, it is possible to obtain a weld metal that can maintain the target strength, suppress the formation of island martensite in the Ni segregation zone as described in (measure 3), exhibit excellent low-temperature toughness, and suppress hot cracking.

[0122] If the value obtained from [Ni] / ([Mn]+[Si]) is less than 0.85, island-like martensite is easily formed in the Ni segregation bands, resulting in reduced low-temperature toughness after PWHT. Therefore, the value calculated from [Ni] / ([Mn]+[Si]) is 0.85 or more, preferably 0.90 or more, more preferably 0.95 or more, and even more preferably 1.00 or more.

[0123] On the other hand, if the value obtained from [Ni] / ([Mn]+[Si]) is higher than 1.45, the Ni content becomes higher, thereby increasing the possibility of hot cracking. Therefore, the value calculated from [Ni] / ([Mn]+[Si]) is 1.45 or less, preferably 1.30 or less, more preferably 1.20 or less, even more preferably 1.10 or less, and particularly preferably 1.07 or less.

[0124] Furthermore, in the above formula, [Ni] represents the Ni content in the drug coating as a percentage of mass relative to the total mass of the drug coating, [Mn] represents the Mn content in the drug coating as a percentage of mass relative to the total mass of the drug coating, and [Si] represents the Si content in the drug coating as a percentage of mass relative to the total mass of the drug coating.

[0125] The flux-coated welding electrode of this embodiment, by containing the aforementioned essential components in the coating within a specified range, can produce weld metal that maintains the target strength, exhibits excellent low-temperature toughness, and suppresses hot cracking, not only under AW (all-weather) conditions but also under a wide range of PWHT (partially-wheat-heat-hydrogen) conditions. In the coating of the flux-coated welding electrode of this embodiment, the carbonates serving as CO2 sources preferably contain CaCO3 and BaCO3. This is because CaO, as a decomposition product of CaCO3, and BaO, as a decomposition product of BaCO3, form a good slag, resulting in an excellent weld bead shape. The preferred contents of CaO and BaO will be explained below.

[0126] (CaO: ≥20% by mass and ≤40% by mass)

[0127] CaO is a slag-forming compound that can uniformly form slag with good coating properties, improving slag peeling properties. In addition, CaO also ensures the insulation of the coating. If the CaO content in the coating is 20% by mass or more, sufficient slag can be formed, resulting in a weld bead with a good shape. Therefore, the CaO content in the coating is preferably 20% by mass or more relative to the total mass of the coating, more preferably 25% by mass or more, and even more preferably 28% by mass.

[0128] Furthermore, if the CaO content in the flux coating is 40% by mass or less, the fluidity of the molten slag can be well maintained, a uniformly formed slag with good coating properties can be formed, and a good weld bead shape can be obtained. Additionally, it can suppress arc intensification and appropriately adjust the amount of spatter. Therefore, the CaO content in the flux coating is preferably 40% by mass or less relative to the total mass of the flux coating, more preferably 38% by mass or less, and even more preferably 36% by mass or less.

[0129] Examples of sources of CaO in the flux coating include CaO, Ca carbonates that generate CaO in the flux coating due to thermal decomposition during welding, and Ca silicates.

[0130] Furthermore, in this instruction manual, the so-called CaO content is the value obtained by converting all the Ca contained in the drug coating into CaO.

[0131] (BaO: 2% by mass and less than 6% by mass)

[0132] BaO is the main slag-forming compound and plays a role in adjusting the basicity of the slag. If the BaO content in the coating is 2% by mass or more, it can prevent the oxygen content in the weld metal from increasing and inhibit the deterioration of low-temperature toughness. Therefore, the BaO content in the coating is preferably 2% by mass or more, and more preferably 3% by mass or more, relative to the total mass of the coating.

[0133] Furthermore, if the BaO content in the coating is 6% by mass or less, the fluidity of the molten slag can be well maintained, and a good weld bead can be formed. Therefore, the BaO content in the coating is preferably 6% by mass or less relative to the total mass of the coating, and more preferably 5% by mass or less.

[0134] BaO sources in the flux coating can include BaO, Ba carbonates that generate BaO in the flux coating due to thermal decomposition during welding, and Ba silicates.

[0135] Furthermore, in this instruction manual, the so-called BaO content is the value obtained by converting all Ba contained in the drug coating into BaO.

[0136] In the flux-coated welding electrode of this embodiment, in order to improve arc stability, the mechanical properties of the weld metal, and the weld bead shape, the coating may also contain Na, K, and Li, as well as Ti, Al, and Zr within the ranges shown below. Furthermore, when the coating contains Nb and V, their content is preferably limited to the ranges shown below. The content of each component that can be contained in the coating of the flux-coated welding electrode of this embodiment and the reasons for limiting them will be further explained.

[0137] (Total concentration of Na, K, and Li: ≥0.3% by mass and ≤4.0% by mass)

[0138] Na, K, and Li are components that have an arc-stabilizing effect.

[0139] If the combined amount of Na, K, and Li in the coating is 0.3% by mass or more and 4.0% by mass or less, sufficient arc stabilization effect can be achieved. Therefore, the combined amount of Na, K, and Li in the coating is preferably 0.3% by mass or more, more preferably 0.8% by mass or more, and even more preferably 1.5% by mass or more, relative to the total mass of the coating. Furthermore, the combined amount of Na, K, and Li in the coating is preferably 4.0% by mass or less, more preferably 3.5% by mass or less, and even more preferably 3.0% by mass or less, relative to the total mass of the coating.

[0140] Furthermore, the Na, K, and Li in the drug coating are contained in oxides such as Na2O, K2O, and Li2O, as well as in metallic Na, metallic K, metallic Li, Na alloys, K alloys, Li alloys, water glass, and other binders.

[0141] (Ti: ≥0.5% by mass and ≤4.0% by mass)

[0142] Metallic Ti, or Ti contained in alloys, is a deoxidizing element that improves the strength of weld metal. Furthermore, after acting as a deoxidizer, it also acts as an oxide within the weld metal, which has the effect of refining grain size. Oxides such as TiO2 act as slag-forming agents, improving the fluidity of the molten slag.

[0143] If the Ti content in the coating is 0.5% by mass or more and 4.0% by mass or less, sufficient deoxidation, grain refinement, and slag fluidity improvement effects can be achieved. Therefore, the Ti content in the coating is preferably 0.5% by mass or more relative to the total mass of the coating, more preferably 1.0% by mass or more, and even more preferably 1.3% by mass or more. Furthermore, the Ti content in the coating is preferably 4.0% by mass or less relative to the total mass of the coating, more preferably 3.0% by mass or less, and even more preferably 2.5% by mass or less.

[0144] Furthermore, as Ti sources in the drug coating, compounds such as metallic Ti, alloys of titanium iron, and TiO2 can be listed.

[0145] (Al: less than 1.5% by mass)

[0146] Metallic Al, or Al contained in alloys, acts as a deoxidizing element. Al oxides such as Al₂O₃ act as slag-forming agents. Furthermore, if the coating contains Al₂O₃, it can increase the viscosity of the molten slag, improve its fluidity, and result in a better weld appearance and shape. In this embodiment, to improve the weld appearance and shape, the coating may contain Al as needed; the Al content in the coating is preferably 0.02% by mass or more relative to the total mass of the coating.

[0147] On the other hand, if the Al content in the coating is 1.5% by mass or less relative to the total mass of the coating, the viscosity of the molten slag can be appropriately adjusted, the fluidity can be controlled, and the appearance and shape of the weld bead can be well maintained. Therefore, the Al content in the coating is preferably 1.5% by mass or less relative to the total mass of the coating, more preferably 1.0% by mass or less, and even more preferably 0.7% by mass or less.

[0148] Furthermore, as sources of Al in the drug coating, examples include Al oxides such as Al2O3, metallic Al, and alloys of aluminum and magnesium.

[0149] (Zr: less than 0.8% by mass)

[0150] Metallic Zr, or Zr contained in alloys, acts as a deoxidizing element. Zr oxides such as ZrO2 act as slag-forming agents. Furthermore, if the coating contains ZrO2, the weld bead fusion is improved, resulting in a flat weld bead. In this embodiment, to improve weld bead fusion and shape, the coating may contain Zr as needed; the Zr content in the coating is preferably 0.01% by mass or more relative to the total mass of the coating.

[0151] On the other hand, if the Zr content in the coating is less than 0.8% by mass, good slag removal properties can be maintained, and weld fusion can be improved, resulting in a flat weld shape. Therefore, the Zr content in the coating is preferably less than 0.8% by mass relative to the total mass of the coating, more preferably less than 0.6% by mass, and even more preferably less than 0.3% by mass.

[0152] Furthermore, Zr sources in the coating can include metallic Zr, Zr contained in alloys, Zr oxides such as ZrO2, etc.

[0153] (Nb: less than 0.03% by mass)

[0154] Nitrogen (Nb) is a component that improves the strength of weld metal, but it also causes carbide precipitation after PWHT, which reduces low-temperature toughness. Therefore, in this embodiment, it is preferable to limit the Nb content in the coating to a specified value, or even 0% by mass. If the Nb content in the coating is 0.03% by mass or less, the reduction in low-temperature toughness after PWHT can be suppressed. Therefore, the Nb content in the coating is preferably 0.03% by mass or less relative to the total mass of the coating.

[0155] (V: less than 0.03% by mass)

[0156] V is a component that improves the strength of weld metal, but it also promotes the precipitation and growth of carbides, reducing low-temperature toughness. Therefore, in this embodiment, it is preferable to limit the V content in the coating to a specified value or less, or even 0% by mass. If the V content in the coating is 0.03% by mass or less, the reduction in low-temperature toughness after PWHT can be suppressed. Therefore, the V content in the coating is preferably 0.03% by mass or less relative to the total mass of the coating.

[0157] (B: less than 0.10% by mass)

[0158] Boron segregates at the original austenite grain boundaries, suppressing proeutectoid ferrite, thus improving the toughness of weld metals. However, it is also a component that can cause hot cracking and SR cracking. In this embodiment, the lower limit of the boron content in the coating is not specifically specified, and it can also be 0% by mass.

[0159] If the boron content in the flux coating is 0.10% by mass or less, hot cracking and SR cracking can be suppressed in the weld metal. Therefore, the boron content in the flux coating is preferably 0.10% by mass or less relative to the total mass of the flux coating.

[0160] (Cu: less than 0.6% by mass)

[0161] Cu is a component that maintains strength while refining the microstructure of the weld metal and improving low-temperature toughness. However, depending on the Cu content in the coating, it may promote the formation of precipitates, thus reducing low-temperature toughness. In this embodiment, the coating can contain Cu as needed, but there is no specific limit to the lower limit of Cu content in the coating; it can also be 0% by mass.

[0162] If the Cu content is below 0.6% by mass, it will not promote the formation of precipitates and can suppress the decrease in low-temperature toughness. Therefore, the Cu content in the coating is preferably below 0.6% by mass relative to the total mass of the coating.

[0163] (margin)

[0164] In this embodiment, C can be listed as one of the other components that can be contained in the drug coating. If the C content in the drug coating is 0.30% by mass or less, the formation of carbides can be suppressed. Therefore, the C content in the drug coating is preferably 0.30% by mass or less relative to the total mass of the drug coating.

[0165] Furthermore, in the shielded metal arc welding of this embodiment, the total content of CO2, F, Si, Ni, Fe, Mo, Cr, Mg and Mn, which are essential components contained in the coating, is preferably 40% by mass or more, more preferably 45% by mass or more, and even more preferably 50% by mass or more, relative to the total mass of the coating.

[0166] Furthermore, when it further contains CaO and BaO, the total content of the above elements is preferably 85% by mass or more, more preferably 87% by mass or more, and even more preferably 90% by mass or more, relative to the total mass of the drug coating.

[0167] The coating also contains at least one of Na, K and Li, and Ti. Furthermore, whether it contains Al and Zr or not, the total content of the above elements is preferably 90% by mass or more, more preferably 93% by mass or more, and even more preferably 95% by mass or more, relative to the total mass of the coating.

[0168] (Impurities)

[0169] Other elements besides those mentioned above that can be included in the flux coating include unavoidable impurities such as P, S, Sn, Sb, As, Pb, and N. From the viewpoint of ensuring weld quality such as resistance to hot cracking, the unavoidable content of P, S, Sn, Sb, As, Pb, and N relative to the total mass of the flux coating is preferably limited to 0.5% by mass or less for each. Furthermore, the total value of impurities relative to the total mass of the flux coating is preferably limited to 3% by mass or less.

[0170] <1-3. Welding Core>

[0171] Next, the composition and preferred content of the core of the flux-coated welding electrode of this embodiment will be described below.

[0172] In this embodiment, iron-based welding cores or steel welding cores with Fe as the main component can be used as suitable welding cores. As steel welding cores, steel welding cores made of mild steel, high-tensile steel, and low-alloy steel are preferred.

[0173] Furthermore, in this embodiment, there are no particular limitations on other components in the welding core, but in addition to Fe, it may contain C, Si, Mn, P, S, N, Cu, etc. Relative to the total mass of the welding core, it is preferable that the C content in the welding core is 0.13% by mass or less, the Si content is 0.3% by mass or less (including 0% by mass), the Mn content is 0.2% by mass or more and 1.0% by mass or less, the P content is 0.040% by mass or less (including 0% by mass), the S content is 0.035% by mass or less (including 0% by mass), and the Cu content is 0.2% by mass or less (including 0% by mass).

[0174] In addition, the core may contain Nb, V, Cr, Ni, Mo, Ti, Al, and B. Among these components, the content of Nb and V is preferably 0.02% by mass or less each. Furthermore, the combined content of Cr, Ni, Mo, Ti, and Al is preferably 4.0% by mass or less. Moreover, the B content is preferably 0.02% by mass or less.

[0175] In this embodiment, the outer diameter of the welding core is not particularly limited, but it is preferably 2.6 mm or more and 5.0 mm or less.

[0176] [2. Manufacturing method of flux-coated welding electrodes]

[0177] The flux-coated welding electrode of this embodiment can be manufactured by mixing raw materials of flux coating in such a way that the flux coating is a component of the above-mentioned composition, mixing them together with a specified adhesive, coating the mixture onto a specified welding core surface, such that the mass of the flux coating is in the range of 25% to 40% by mass relative to the total mass of the flux-coated welding electrode, and firing it at 450°C to 550°C for about 1 hour.

[0178] Furthermore, when manufacturing the flux-coated welding electrode of this embodiment, there are no particular limitations on the type of core, the type of adhesive, or the method of forming the coating, and the usual specifications and conditions for manufacturing flux-coated welding electrodes can be used.

[0179] [3. Welding Metal]

[0180] The weld metal in this embodiment is obtained by performing shielded metal arc welding using the flux-coated welding electrode described in [1. flux-coated welding electrode] of this embodiment.

[0181] Furthermore, in the weld metal of this embodiment, there are no particular limitations on conditions other than using the flux-coated welding rod of this embodiment, and the type of base material can be appropriately selected according to the required characteristics.

[0182] [4. Shielded Metal Arc Welding Method]

[0183] The shielded metal arc welding method of this embodiment is a welding method using the flux-coated welding electrode described in [1. flux-coated welding electrode] of this embodiment.

[0184] Furthermore, in the shielded metal arc welding method of this embodiment, there are no particular limitations on various welding conditions other than those using the flux-coated welding electrode of this embodiment. Regarding the type of base material, welding voltage, welding current, welding posture, etc., general conditions in welding methods using flux-coated welding electrodes can be adopted.

[0185] [5. Manufacturing method of welded joint]

[0186] The method for manufacturing a welded joint in this embodiment is to use high-tensile steel as the base material and the flux-coated electrode described in [1. flux-coated electrode] above to manufacture a welded joint by electrode arc welding.

[0187] Furthermore, in the method for manufacturing the welded joint, there are no particular limitations on welding conditions other than using high-tensile steel as the base material and performing shielded metal arc welding using the flux-coated electrode of this embodiment. Regarding welding voltage, welding current, welding posture, etc., general conditions in welding methods using flux-coated electrodes can be adopted.

[0188] In addition, there are no limitations on the high-tensile steels that can be used as base materials, but those with a strength of 720 MPa or higher are preferred. Examples include P690Q, P690QH, P690QL1 and P690QL2 of EN10028-6:2017, KD620, KD690, KE620 and KE690 as specified by the Japan Maritime Association (NK), and VL690 as specified by DNV.

[0189] Example

[0190] Hereinafter, an inventive example and a comparative example of the flux-coated welding electrode of this embodiment will be described.

[0191] [Shielded Metal Arc Welding]

[0192] (Preparation of flux-coated welding rods)

[0193] Using a welding electrode coating machine, the surface of a 4.0 mm diameter steel welding core is coated with a flux composed of various components, and then fired at 450–550°C for approximately 1 hour to produce the flux-coated welding electrodes of the invention example and comparative example. The coating rate relative to the total mass of the flux-coated welding electrode is in the range of 25% by mass or more and 40% by mass or less.

[0194] (Shielded Metal Arc Welding)

[0195] Next, using the obtained flux-coated welding electrode, a shielded metal arc welding (SMAW) was performed on a steel plate with the plate thickness and chemical composition shown in Table 2 below, according to the welding conditions shown in Table 1 below, to produce a welded joint.

[0196] [Evaluation of Mechanical Properties]

[0197] (Production of the experimental film)

[0198] The mechanical properties of the weld metal were evaluated according to the "Tensive and Impact Test Methods for Weld Metals" specified in JIS Z 3111:2005. Tensile test specimens (A2 size) and impact test specimens (V-notch test specimens) were extracted from the center of the weld metal along its thickness direction to assess tensile and impact properties. When studying a wide range of PWHT conditions, the changes in mechanical properties during PWHT (post-weld heat treatment) at various holding temperatures and times are often analyzed using the Larson-Miller parameter (LMP). In this example, the PWHT condition with a low LMP was 580°C for 2 hours, and the condition with a high LMP was 620°C for 8 hours.

[0199] (Tension test)

[0200] Tensile tests were conducted on As-welded specimens at 580°C for 2 hours and on PWHT specimens at 620°C for 8 hours, with the test temperature set to room temperature (approximately 20±2°C). Tensile properties were evaluated by measuring yield stress and tensile strength.

[0201] Furthermore, in this invention, when the tensile strength (TS) under as-welded conditions is 780 MPa or higher, it is judged to be of good strength. After PWHT at 580°C for 2 hours and at 620°C for 8 hours, respectively, when the tensile strength (TS) is 750 MPa or higher, it is judged to be of good strength.

[0202] (Impact test)

[0203] Impact tests were conducted on As-welded test pieces and PWHT test pieces that had undergone 2 hours at 580°C and 8 hours at 620°C. The test temperatures were -40°C and -60°C, and the pendulum impact absorption energy (vE-40°C, vE-60°C) was measured three times at each temperature. The minimum value of the three pendulum impact absorption energies was used to evaluate toughness. Furthermore, in this invention example, when the minimum absorption energy at -40°C and -60°C after As-welding and PWHTing were ≥100J and ≥80J respectively, it was judged to be of good toughness.

[0204] Furthermore, those with good strength and toughness after as-welded and PWHT are considered qualified, while others are considered unqualified.

[0205] The chemical composition of the core electrode is shown in Table 3 below, and the chemical composition of the coating electrode is shown in Tables 4 and 5 below. Furthermore, the evaluation results of the mechanical properties are shown in Table 6 below. Also, the balance of the core electrode composition shown in Table 3 below is Fe and impurities. The balance of the coating electrode composition shown in Tables 4 and 5 below is impurities.

[0206] In addition, in Table 4 below, [Ni] is the value of Ni content in the drug coating as a mass percentage relative to the total mass of the drug coating, [Mn] is the value of Mn content in the drug coating as a mass percentage relative to the total mass of the drug coating, and [Si] is the value of Si content in the drug coating as a mass percentage relative to the total mass of the drug coating.

[0207] Furthermore, in the content descriptions in Table 5 below, "-" indicates values ​​below the quantitative limit. Additionally, for PWHT that have not undergone mechanical property evaluation at 580°C for 2 hours, the evaluation result column in Table 6 is displayed as "-".

[0208] Table 1

[0209]

[0210] Table 2

[0211]

[0212] Table 3

[0213]

[0214] Table 4

[0215]

[0216] Table 5

[0217]

[0218] Table 6

[0219]

[0220] As shown in Tables 4 to 6 above, in Invention Examples No. 1 to 7, where the content of each component in the coating is within the range specified in this invention, the tensile strength (TS) of the as-welded weld metal is 780 MPa or higher, achieving the target. Furthermore, the tensile strength (TS) after two different PWHT processes is 750 MPa or higher, achieving the target, thus obtaining weld metal with excellent strength. In addition, the absorbed energy at -40°C is 100 J or higher, and the absorbed energy at -60°C is 80 J or higher, resulting in excellent low-temperature toughness. Therefore, it can be seen that not only with as-welded weld metal, but also under a wide range of PWHT conditions, weld metal that maintains the target strength, exhibits excellent low-temperature toughness, and suppresses the occurrence of hot cracking can be obtained.

[0221] Furthermore, the invention examples No. 1 to 7 can manufacture welded joints that maintain the target strength and exhibit excellent low-temperature toughness not only under as-welded conditions but also under a wide range of PWHT conditions, and suppress the occurrence of hot cracking.

[0222] On the other hand, in Comparative Example No. 1, because the Cr content in the coating is lower than the lower limit of the range specified in this invention, the tensile strength after as-welded and long-term PWHT decreases, and the toughness at -60°C after short-term PWHT also decreases. In Comparative Examples No. 2 and 5, because the value obtained from formula [Ni] / ([Mn]+[Si]) is lower than the lower limit of the range specified in this invention, the low-temperature toughness after both short-term and long-term PWHT decreases. In Comparative Example No. 3, because the Mo content in the coating is higher than the upper limit of the range specified in this invention, and the value obtained from formula [Ni] / ([Mn]+[Si]) is lower than the lower limit of the range specified in this invention, the low-temperature toughness after as-welded, both short-term and long-term PWHT decreases.

[0223] Comparative Example No. 4 shows that the strength was not reduced because the Mo content in the coating was higher than the upper limit of the range specified in this invention and the Cr content in the coating was lower than the lower limit of the range specified in this invention. However, the low-temperature toughness decreased after short-term PWHT and long-term PWHT because the value obtained by formula [Ni] / ([Mn]+[Si]) was lower than the lower limit of the range specified in this invention.

[0224] In Comparative Examples No. 6 to 9, because the Ni content in the coating is lower than the lower limit of the range specified in this invention, the value obtained by formula [Ni] / ([Mn]+[Si]) is lower than the lower limit of the range specified in this invention. Therefore, at least one of the strength of As-welded, the toughness at -60°C and the toughness at -40°C, and the strength, toughness at -60°C and the toughness at -40°C after long-term PWHT are reduced.

[0225] Comparative Example No. 10 shows an increased likelihood of hot cracking because the value obtained from formula [Ni] / ([Mn]+[Si]) is higher than the upper limit of the range specified in this invention. Similarly, Comparative Example No. 11 also shows an increased likelihood of hot cracking because both the Ni content in the coating and the value obtained from formula [Ni] / ([Mn]+[Si]) are higher than the upper limit of the range specified in this invention.

Claims

1. A flux-coated welding electrode, characterized in that it comprises a core and a coating covering the core, wherein the coating contains, relative to the total mass of the coating, a certain percentage of the core. CO2: ≥16% by mass and ≤27% by mass F: 4% or more by mass and less than 10% by mass Si: 3% by mass or more and 11% by mass or less Ni: 7.5% by mass or more and 13.3% by mass or less Fe: ≥1% by mass and ≤11% by mass Mo: ≥0.3% by mass and ≤1.0% by mass Cr: ≥0.15% by mass and ≤1.20% by mass Mg: 1.5% or more by mass and less than 4.5% by mass Mn: ≥1.5% by mass and ≤4.0% by mass CaO: ≥20% by mass and ≤40% by mass BaO: ≥2% by mass and ≤6% by mass The combined concentration of Na, K, and Li: ≥0.3% by mass and ≤4.0% by mass. Ti: ≥0.5% by mass and ≤4.0% by mass Al: less than 1.5% by mass Zr: less than 0.8% by mass C: less than 0.30% by mass, and The weight of the drug coating relative to the total weight of the drug coating is limited to a certain limit. Nb: less than 0.03% by mass V: less than 0.03% by mass The content of Ni in the drug coating is expressed as [Ni], expressed as a percentage of mass relative to the total mass of the drug coating. The content of Mn in the drug coating is expressed as [Mn], expressed as a percentage of the total mass of the drug coating. When the content of Si in the drug coating is expressed as [Si] as a percentage of the total mass of the drug coating, The value calculated from [Ni] / ([Si]+[Mn]) is above 0.85 and below 1.

45.

2. A welding metal, characterized in that, It is obtained by using the flux-coated welding electrode of claim 1 with high-tensile steel as the base material for shielded metal arc welding.

3. A method for shielded metal arc welding, characterized in that, Electrode welding is performed using the flux-coated electrode as described in claim 1.

4. A method for manufacturing a welded joint, characterized in that, Using high-tensile steel as the base material, and employing the flux-coated welding electrode as described in claim 1, shielded metal arc welding is performed.

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