Coated arc welding rod, coated arc welding method, and method for manufacturing a welded joint

Abstract

To provide a coated electrode for high-tensile steel, which is superior in low-temperature toughness, and with which a welded zone restricted in high-temperature cracking can be obtained, while maintaining a target strength, even under a broad range of PWHT conditions in addition to As-welded conditions.SOLUTION: A coated electrode contains, in mass% to the total mass of a coating flux, 16%-27% of CO2, 4%-10% of F, 3%-11% of Si, 7.5%-13.3% of Ni, 1%- 11% of Fe, 0.3%-1.0% of Mo, 0.15%-1.2% of Cr; 1.5%-4.5% of Mg, and 1.5%-4.0% of Mn. When contents of Ni, Mn, and Si in the coating flux in mass% to the total mass of the coating flux is represented by [Ni], [Mn], and [Si] respectively, a value calculated according to [Ni] / ([Si]+[Mn]) is 0.85 to 1.45.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a covered arc welding rod used for welding high-tensile steel (hereinafter, also simply referred to as "welding rod"), a weld metal, a covered arc welding method, and a method for manufacturing a welded joint.

Background Art

[0002] Welded products such as offshore structures and tanks used for the excavation and production of oil, gas, etc. are required to be large-sized and operate in cold regions. The steel plates and welding materials used for these productions are required to have excellent properties of high strength and low-temperature toughness. In addition to these properties, in order to further improve the quality of welded products, post-weld heat treatment (PWHT) may be performed. By performing PWHT, for example, the residual stress in the welded part can be removed, so that the effect of suppressing cracks in the welded part can be obtained.

[0003] However, when this PWHT is performed, there is a risk that the toughness may decrease due to embrittlement of the welded part caused by precipitation hardening, temper embrittlement, etc. In particular, in the welded part of high-tensile steel, due to the influence of various elements added to improve the strength, embrittlement during PWHT is remarkable, and as the strength increases, it often becomes impossible to apply PWHT. Therefore, the development of a welding material for high-tensile steel that has excellent mechanical properties even after PWHT is required.

[0004] For example, in Patent Document 1, a covered arc welding rod that can obtain a weld metal having excellent low-temperature toughness and fracture toughness after stress relief annealing during welding of high-tensile steel, for example, high-tensile steel of 590 N / mm 2 or higher grade, has been proposed. In addition, Patent Document 2 proposes a low-hydrogen covered arc welding rod for 590 MPa grade high-tensile steel that has good welding workability and excellent strength and toughness at low temperature of the weld metal as-welded (AW) and after PWHT.

Prior Art Documents

Patent Document

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, for the above Patent Documents 1 and 2, only one type of PWHT condition is considered, and excellent toughness may not be obtained under other PWHT conditions. Also, it can be said that the greater the margin of the PWHT condition, the more applicable it is to all welded products, the easier the construction management is, and the more the quality stability of the welded part can be further improved. Furthermore, since neither of the above Patent Documents 1 and 2 considers high-temperature cracking, the requirements for welding rods capable of obtaining a welding metal with good high-temperature cracking resistance are even higher.

[0007] On the other hand, conventionally, it is known that PWHT may reduce toughness depending on the conditions, and the PWHT conditions are strictly determined by the type of material, thickness, welded joint, construction conditions, etc. Therefore, it is very difficult to provide a margin for the PWHT conditions. In particular, as the strength increases, more alloying elements are contained, so embrittlement phenomena such as temper embrittlement are likely to occur, and the PWHT conditions become more stringent.

[0008] The present invention has been made in view of such problems, and is a covered arc welding rod for high-tensile steel. It can obtain a welded joint that not only maintains the target strength but also has excellent low-temperature toughness and suppressed hot cracking during welding (hereinafter also referred to as "As-welded") and under a wide range of PWHT conditions. Another object is to provide a covered arc welding rod, a covered arc welding method, and a method for manufacturing a welded joint. Also, another object is to provide a weld metal that can maintain the target strength, has excellent low-temperature toughness, and can suppress hot cracking under a wide range of PWHT conditions.

Means for Solving the Problems

[0009] The above object of the present invention is achieved by the configuration of the following [1] related to a covered arc welding rod.

[0010] [1] A covered arc welding rod having a core wire and a coating agent that coats the core wire, The coating agent is based on the total mass of the coating agent, CO2: 16% by mass or more and 27% by mass or less, F: 4% by mass or more and 10% by mass or less, 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 or more and 11% by mass or less, Mo: 0.3% by mass or more and 1.0% by mass or less, Cr: 0.15% by mass or more and 1.20% by mass or less, Mg: 1.5% by mass or more and 4.5% by mass or less, Mn: 1.5% by mass or more and 4.0% by mass or less, and contains When the content of Ni in the coating agent is represented by [Ni] in mass% with respect to the total mass of the coating agent, When the content of Mn in the coating agent is represented by [Mn] in mass% with respect to the total mass of the coating agent, When the content of Si in the coating agent is represented by [Si] in mass% with respect to the total mass of the coating agent, A covered arc welding rod, characterized in that the value calculated by [Ni] / ([Si]+[Mn]) is 0.85 or more and 1.45 or less.

[0011] Preferred embodiments of the present invention relating to the covered arc welding electrode relate to the following [2] to [4].

[0012] [2] The coating agent further contains, based on the total mass of the coating agent, CaO: 20% by mass or more and 40% by mass or less, BaO: 2% by mass or more and 6% by mass or less, The covered arc welding electrode according to [1], characterized by containing these.

[0013] [3] The coating agent further contains, based on the total mass of the coating agent, Total amount of Na, K, and Li: 0.3% by mass or more and 4.0% by mass or less, Ti: 0.5% by mass or more and 4.0% by mass or less, and Al: 1.5% by mass or less, Zr: 0.8% by mass or less. The covered arc welding electrode according to [1] or [2], characterized by this.

[0014] [4] The coating agent is regulated to Nb: 0.03% by mass or less, V: 0.03% by mass or less. The covered arc welding electrode according to any one of [1] to [3], characterized by this.

[0015] The above object of the present invention is achieved by the following configuration [5] relating to the weld metal.

[0016] [5] A weld metal characterized by being obtained by performing covered arc welding using the covered arc welding electrode according to any one of [1] to [4].

[0017] The above object of the present invention is achieved by the following configuration [6] relating to the covered arc welding method.

[0018] [6] A covered arc welding method characterized by performing covered arc welding using the covered arc welding electrode according to any one of [1] to [4].

[0019] The above object of the present invention is achieved by the following configuration [7] related to a method for manufacturing a welded joint.

[0020] [7] A method for manufacturing a welded joint, characterized in that high-tensile steel is used as a base material, and covered arc welding is performed using the covered arc welding rod described in any one of [1] to [4].

Effects of the Invention

[0021] According to the present invention, it is possible to provide a covered arc welding rod for high-tensile steel that can obtain a welded part that not only has the target strength in the as-welded state but also has excellent low-temperature toughness and can suppress high-temperature cracking under a wide range of PWHT conditions. Further, according to the present invention, it is possible to provide a weld metal that maintains the target strength and has excellent low-temperature toughness and can suppress high-temperature cracking under a wide range of PWHT conditions. Furthermore, according to the present invention, it is possible to provide a covered arc welding method and a method for manufacturing a welded joint that can obtain a welded part that maintains the target strength, has excellent low-temperature toughness, and can suppress high-temperature cracking under a wide range of PWHT conditions.

Embodiments for Carrying Out the Invention

[0022] The present inventors examined the mechanism of toughness reduction due to PWHT in high-strength welded parts, and also conducted intensive studies to obtain a welded part with excellent low-temperature toughness even when PWHT was performed under a wide range of conditions. As a result, the following findings were obtained, leading to the completion of the present invention. The above-mentioned welded part refers to the weld metal and the heat-affected zone (HAZ: Heat-Affected Zone), but in this specification, the weld metal will be described below.

[0023] First, the conventional toughness reduction mechanism in high-strength steel will be described. Conventionally, the following two causes have been cited as the main reasons for the reduction in the toughness of the weld metal after PWHT.

[0024] (Reason 1) When the contents of Cr, Mo, etc. in the weld metal are high, these components form carbides with C and precipitate, causing the weld metal to harden. (Reason 2) Brittleness phenomena such as temper embrittlement due to slow cooling from the PWHT temperature occur.

[0025] Conventionally, due to the influence of the above (Reason 1) and (Reason 2), it was mainly considered that the grain boundary strength of the weld metal decreased, and as a result, the toughness decreased. Therefore, conventionally, in order to ensure excellent toughness while maintaining the target strength after PWHT, the following countermeasures have been taken.

[0026] (Countermeasure 1) Suppress the carbides that precipitate and grow at the prior austenite grain boundaries. (Countermeasure 2) Suppress the segregation of impurity elements at the prior austenite grain boundaries.

[0027] However, depending on the PWHT conditions, simply implementing the above (Countermeasure 1) and (Countermeasure 2) is insufficient. Generally, the PWHT conditions consist of the holding temperature and holding time elements, and these elements can be organized by the Larson - Miller parameter (hereinafter referred to as "LMP") with these elements as parameters.

[0028] Therefore, in this specification, it is determined that a covered arc welding rod capable of obtaining a weld metal with excellent low - temperature toughness while maintaining the target strength under the following three conditions a to c has a wide range of PWHT conditions.

[0029] (Condition a) As - welded. (Condition b) As a PWHT condition with a high LMP, a condition of 620°C for 8 hours (hereinafter referred to as "high LMP condition"). (Condition c) As a PWHT condition with a low LMP, a condition of 580°C for 2 hours (hereinafter referred to as "low LMP condition").

[0030] It is known that, while welding under (Condition a) and under the high LMP condition of (Condition b), in addition to the above (Countermeasure 1) and (Countermeasure 2), by incorporating an appropriate amount of Ni in the welding rod, good low-temperature toughness can be obtained. On the other hand, under the low LMP condition of (Condition c), simply incorporating an appropriate amount of Ni cannot obtain good low-temperature toughness. The inventors have found the mechanism by which low-temperature toughness decreases when PWHT is carried out under the low LMP condition of (Condition c), and have also found a countermeasure that can obtain a weld metal with excellent low-temperature toughness while maintaining the target strength under a wide range of PWHT conditions. Below, the mechanism by which low-temperature toughness decreases and the countermeasure under the low LMP condition of (Condition c) will be described.

[0031] When a large amount of Ni is contained in the weld metal, a segregation zone containing Ni (hereinafter referred to as "Ni segregation zone"), that is, a region where Ni is concentrated, is formed on the weld metal structure. This Ni segregation zone does not affect the low-temperature toughness while welding under (Condition a) and under the high LMP condition of (Condition b). However, under the low LMP condition of (Condition c), the C content becomes high in the Ni segregation zone, and the formation, generation, and coarsening of island-shaped martensite and carbides are promoted in the Ni segregation zone. In particular, the formation of island-shaped martensite is remarkable, and due to the influence of these products, brittle fracture is likely to occur in the Ni segregation zone, and as a result, the low-temperature toughness decreases.

[0032] From these facts, the inventors have found that, in addition to (Countermeasure 1) and (Countermeasure 2), implementing the following (Countermeasure 3) as a countermeasure to ensure a weld metal with excellent low-temperature toughness while maintaining the target strength after PWHT is effective.

[0033] (Countermeasure 1) Suppress the carbides that precipitate and grow at the prior austenite grain boundaries. (Countermeasure 2) Suppress the segregation of impurity elements at the prior austenite grain boundaries. (Countermeasure 3) Suppress the formation of island-shaped martensite in the Ni segregation zone.

[0034] Then, the inventors have found that by appropriately controlling the chemical composition of the covered arc welding rod and the parameters calculated from the contents of Ni, Si, and Mn, the above (Countermeasure 1) to (Countermeasure 3) can be achieved, and high-temperature cracking can be suppressed. That is, by using a welding rod in which (Countermeasure 1) to (Countermeasure 3) are realized, it is possible to obtain a weld metal with excellent low-temperature toughness while maintaining the target strength not only in the as-welded state but also after PWHT under a wide range of conditions.

[0035] Hereinafter, the embodiments for carrying out the present invention (hereinafter referred to as "the present embodiments") will be described in detail. It should be noted 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.

[0036] [1. Covered Arc Welding Rod] The covered arc welding rod according to the present embodiment is one in which a covering agent is coated on a steel core wire (hereinafter also simply referred to as "core wire").

[0037] [1-1. Coating Rate] The coating rate can be set to any value as long as the content of each element in the covering agent is within the scope of the present invention. The coating rate can be calculated by the formula: {[covering agent] / ([covering agent] + [core wire])} × 100, where [covering agent] is the mass (g) of the covering agent in the total mass of the welding rod and [core wire] is the mass (g) of the core wire in the total mass of the welding rod. In the present embodiment, it is preferable that the coating rate calculated by the above formula is 25% by mass or more and 40% by mass or less.

[0038] [1-2. Covering Agent] Hereinafter, the chemical composition contained in the covering agent of the covered arc welding rod according to the present embodiment and the reasons for the numerical limitations of the content will be described in more detail. The content in the present embodiment means mass% with respect to the total mass of the covering agent unless otherwise specified. In addition, unless otherwise noted, each of the following elements may be contained in the coating agent in the form of a metal, in the form of a compound, or in both the form of a metal and a compound. That is, regardless of the form in which each of the above elements is contained in the coating agent, it is defined by the conversion value converted to the elemental form. For example, when taking Si as an example, the Si content refers to the total of the Si conversion values of metallic Si and Si compounds. Note that metallic Si includes Si single substance and Si alloy.

[0039] (CO2: 16 to 27 mass%) In this embodiment, the content of carbonate in the coating agent is defined as the content of CO2. The carbonate decomposes into CO2 and oxide during welding and has the effect of preventing oxidation and nitridation of the weld metal. If the CO2 content in the coating agent is less than 16% by mass, sufficient gas will not be generated during welding, leading to nitridation and oxidation of the weld metal and deterioration of low-temperature toughness. Therefore, the CO2 content in the coating agent should be 16% by mass or more based on the total mass of the coating agent, preferably 17% by mass or more, and more preferably 18% by mass or more. On the other hand, if the CO2 content in the coating agent exceeds 27% by mass, since a large amount of carbonate as the CO2 source is contained, the fluidity of the molten slag increases, making it difficult to form a uniform and well-wrapping slag, and the slag detachability deteriorates. Therefore, the CO2 content in the coating agent should be 27% by mass or less based on the total mass of the coating agent, preferably 24% by mass or less, and more preferably 21% by mass or less. Note that examples of the CO2 source in the coating agent include carbonates such as CaCO3, BaCO3, MgCO3, MnCO3, FeCO3, Na2CO3, and K2CO3.

[0040] (F: 4 mass% or more and 10 mass% or less) Metal fluorides such as CaF2, MgF2, and AlF3 have the effect of lowering the melting point of the molten slag, improving the slag coating property, and enhancing the bead appearance. In addition, F can react with hydrogen during welding to reduce the hydrogen partial pressure in the weld metal, thus also having the effect of low-hydrogenizing the weld metal. If the F content in the coating agent is less than 4% by mass, the above-mentioned effects cannot be fully obtained. Therefore, the F content in the coating agent should be 4% by mass or more based on the total mass of the coating agent, preferably 5% by mass or more, and more preferably 6% by mass or more. On the other hand, when the F content in the coating agent exceeds 10% by mass, the arc becomes unstable and the amount of spatter generated increases. Therefore, the F content in the coating agent should be 10% by mass or less based on the total mass of the coating agent, preferably 9% by mass or less, and more preferably 8% by mass or less.

[0041] (Si: 3% by mass or more and 11% by mass or less) Si contained in metallic Si or alloys such as ferrosilicon has the effect of increasing the viscosity of the molten metal to adjust its fluidity and improving the bead appearance and bead shape. Oxides such as SiO2 act as slag formers. In addition, oxides such as SiO2 have the effect of increasing the viscosity of the molten slag to improve its fluidity and enhancing the bead appearance and bead shape. Thus, since metallic Si, Si alloys, and Si oxides in the coating agent each have various effects, in this embodiment, it is defined by the total Si content contained in metallic Si, Si alloys, and Si compounds in the coating agent.

[0042] If the Si content in the coating agent is less than 3% by mass, the viscosities of the molten metal and slag decrease, making it difficult to form beads during welding in the vertical position and unable to obtain good beads. Therefore, the Si content in the coating agent should be 3% by mass or more based on the total mass of the coating agent, preferably 4% by mass or more, and more preferably 5% by mass or more. On the one hand, when the Si content in the coating agent exceeds 11% by mass, hard island-shaped martensite is formed in the segregation band containing Ni, which promotes temper embrittlement and reduces low-temperature toughness. Therefore, the Si content in the coating agent should be 11% by mass or less, preferably 9% by mass or less, and more preferably 8% by mass or less, based on the total mass of the coating agent. Examples of the Si source in the coating agent include oxides of Si such as SiO2, silicates of Si, metallic Si, alloys such as ferrosilicon, and fixing agents such as water glass.

[0043] (Ni: 7.5% by mass or more and 13.3% by mass or less) Ni is a component that has the effect of improving the strength and low-temperature toughness of the weld metal by matrix strengthening. When the Ni content in the coating agent is less than 7.5% by mass, the desired tensile strength and low-temperature toughness cannot be obtained. Therefore, the Ni content in the coating agent should be 7.5% by mass or more, 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, based on the total mass of the coating agent. On the other hand, when the Ni content in the coating agent exceeds 13.3% by mass, low-melting-point impurity elements are concentrated in the segregation band, increasing the concern of high-temperature cracking. Therefore, the Ni content based on the total mass of the coating agent should be 13.3% by mass or less, preferably 12.0% by mass or less, and more preferably 11.0% by mass or less.

[0044] (Fe: 1% by mass or more and 11% by mass or less) Fe is a component that affects the welding efficiency and weldability. When the Fe content in the coating agent is less than 1% by mass, the welding efficiency decreases, the arc flickers, and the weldability deteriorates. Therefore, the Fe content in the coating agent should be 1% by mass or more, preferably 3% by mass or more, and more preferably 5% by mass or more, based on the total mass of the coating agent. On the other hand, when the Fe content in the coating agent exceeds 11% by mass, the shielding effect decreases and the weldability deteriorates. Therefore, the Fe content in the coating agent should be 11% by mass or less, preferably 9% by mass or less, and more preferably 8% by mass or less, based on the total mass of the coating agent.

[0045] (Mo: 0.3% by mass or more and 1.0% by mass or less) Mo is a component that improves the strength of the weld metal and is effective in suppressing temper embrittlement. By the precipitation of Mo carbide into the grains of the weld metal, the precipitation of cementite at the grain boundaries can be suppressed, and the decrease in low-temperature toughness after PWHT can be suppressed. When the Mo content in the coating agent is less than 0.3% by mass, the precipitation of cementite at the grain boundaries due to PWHT cannot be suppressed, and the desired low-temperature toughness cannot be obtained. Therefore, the Mo content in the coating agent should be 0.3% by mass or more, preferably 0.5% by mass or more, and more preferably 0.6% by mass or more, based on the total mass of the coating agent. On the other hand, when the Mo content exceeds 1.0% by mass, the low-temperature toughness in AW decreases, and in PWHT, MoC2 precipitates excessively within the grains of the weld metal, resulting in a decrease in low-temperature toughness. Therefore, the Mo content in the coating agent should be 1.0% by mass or less, preferably 0.9% by mass or less, and more preferably 0.8% by mass or less, based on the total mass of the coating agent.

[0046] (Cr: 0.15% by mass or more and 1.20% by mass or less) Cr is a component that improves the strength of the weld metal and has the effect of suppressing the formation of coarse structures that precipitate at the grain boundaries. When the Cr content in the coating agent is less than 0.15% by mass, the formation of coarse structures that precipitate at the grain boundaries cannot be suppressed, and the desired tensile strength and low-temperature toughness after PWHT cannot be obtained. Therefore, the Cr content in the coating agent should be 0.15% by mass or more, preferably 0.30% by mass or more, and more preferably 0.40% by mass or more, based on the total mass of the coating agent. On the one hand, Cr is a component that promotes the precipitation and growth of mainly coarse grain boundary carbides by PWHT and reduces low-temperature toughness. When the Cr content in the coating agent exceeds 1.20% by mass, the low-temperature toughness after PWHT decreases. Therefore, the Cr content in the coating agent is 1.20% by mass or less based on the total mass of the coating agent, 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.

[0047] (Mg: 1.5% by mass or more and 4.5% by mass or less) Mg is a component that reduces the amount of oxides in the weld metal by deoxidation and improves low-temperature toughness. When the Mg content in the coating agent is less than 1.5% by mass, the desired deoxidation effect cannot be obtained. Therefore, the Mg content in the coating agent is 1.5% by mass or more based on the total mass of the coating agent, preferably 2.0% by mass or more, and more preferably 2.2% by mass or more. On the other hand, Mg has the effect of reducing the arc force during welding. When the Mg content in the coating agent exceeds 4.5% by mass, the arc becomes unstable and the bead shape deteriorates. Therefore, the Mg content in the coating agent is 4.5% by mass or less based on the total mass of the coating agent, preferably 3.8% by mass or less, and more preferably 3.0% by mass or less.

[0048] (Mn: 1.5% by mass or more and 4.0% by mass or less) Mn is a component that has the effect of improving the strength of the weld metal. When the Mn content in the coating agent is less than 1.5% by mass, the desired strength cannot be obtained. Therefore, the Mn content in the coating agent is 1.5% by mass or more based on the total mass of the coating agent, preferably 2.0% by mass or more, and more preferably 2.2% by mass or more. On the one hand, Mn, in particular, promotes temper embrittlement and reduces low-temperature toughness by generating hard island-like martensite in the segregation band containing Ni, and is also a component that causes high-temperature cracking. When the Mn content in the coating agent exceeds 4.0% by mass, not only does the low-temperature toughness after PWHT decrease, but the concern about high-temperature cracking also increases. Therefore, the Mn content in the coating agent should be 4.0% by mass or less, preferably 3.5% by mass or less, and more preferably 3.0% by mass or less, based on the total mass of the coating agent. Examples of the Mn source in the coating agent include oxides such as MnO, MnO2, Mn3O4, and Mn2O3, sulfides of Mn, carbonates of Mn, metallic Mn, and alloys such as ferromanganese.

[0049] (Value calculated by [Ni] / ([Si] + [Mn]): 0.85 or more and 1.45 or less) As described above, by appropriately controlling the value of this parameter calculated from the contents of Ni, Si, and Mn, while maintaining the target strength, the generation of island-like martensite in the Ni segregation band, which is the above (Countermeasure 3), can be suppressed, and a weld metal with excellent low-temperature toughness and suppressed high-temperature cracking can be obtained. When the value obtained by [Ni] / ([Mn] + [Si]) is less than 0.85, island-like martensite is likely to be generated in the Ni segregation band, and the low-temperature toughness after PWHT decreases. Therefore, the value calculated by [Ni] / ([Mn] + [Si]) should be 0.85 or more, preferably 0.90 or more, more preferably 0.95 or more, and even more preferably 1.00 or more. On the other hand, when the value obtained by [Ni] / ([Mn] + [Si]) exceeds 1.45, since the Ni content increases, the concern about high-temperature cracking increases. Therefore, the value calculated by [Ni] / ([Mn] + [Si]) should be 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.

[0050] In the above formula, [Ni] represents the value of the Ni content in the coating agent expressed as mass % with respect to the total mass of the coating agent, [Mn] represents the value of the Mn content in the coating agent expressed as mass % with respect to the total mass of the coating agent, and [Si] represents the value of the Si content in the coating agent expressed as mass % with respect to the total mass of the coating agent.

[0051] By including the above essential components in the coating agent of the covered arc welding rod according to this embodiment within a predetermined content range, it is possible to obtain a weld metal that not only has the target strength but also has excellent low-temperature toughness and suppressed high-temperature cracking under a wide range of PWHT conditions as well as AW. In the coating agent of the covered arc welding rod according to this embodiment, it is preferable that CaCO3 and BaCO3 are contained as carbonates as a CO2 source. This is because CaO, which is the decomposition product of CaCO3, and BaO, which is the decomposition product of BaCO3, form good slag and have the effect of obtaining an excellent bead shape. The preferable content of CaO and BaO will be described below.

[0052] (CaO: 20 mass % or more and 40 mass % or less) CaO is a slag-forming compound, enabling the formation of a uniform and well-encapsulating slag and having an effect of improving slag detachability. CaO also has the effect of ensuring the insulation of the coating agent. When the CaO content in the coating agent is 20 mass % or more, sufficient slag is formed, and a bead with a good shape can be obtained. Therefore, the CaO content in the coating agent is preferably 20 mass % or more, more preferably 25 mass % or more, and even more preferably 28 mass % with respect to the total mass of the coating agent. Moreover, when the CaO content in the coating agent is 40 mass % or less, the fluidity of the molten slag can be kept good, so that a uniform and well-encapsulating slag can be formed, and a good bead shape can be obtained. Also, it is possible to suppress the increase in arc strength and appropriately adjust the amount of spatter generated. Therefore, the CaO content in the coating agent is preferably 40 mass % or less, more preferably 38 mass % or less, and even more preferably 36 mass % with respect to the total mass of the coating agent.

[0053] Examples of the CaO source in the coating agent include CaO, Ca carbonates that thermally decompose during welding to generate CaO in the coating agent, and Ca silicates. In this specification, the CaO content is the value obtained by converting all the Ca contained in the coating agent into CaO.

[0054] (BaO: 2% by mass or more and 6% by mass or less) BaO is a main slag-forming compound and has the role of adjusting the basicity of the slag. When the BaO content in the coating agent is 2% by mass or more, it is possible to prevent the increase in the amount of oxygen in the weld metal and suppress the deterioration of the low-temperature toughness. Therefore, the BaO content in the coating agent is preferably 2% by mass or more, more preferably 3% by mass or more, based on the total mass of the coating agent. Also, when the BaO content in the coating agent is 6% by mass or less, it is possible to maintain good fluidity of the molten slag and form a good bead. Therefore, the BaO content in the coating agent is preferably 6% by mass or less, more preferably 5% by mass or less, based on the total mass of the coating agent.

[0055] Examples of the BaO source in the coating agent include BaO, Ba carbonates that thermally decompose during welding to generate BaO in the coating agent, and Ba silicates. In this specification, the BaO content is the value obtained by converting all the Ba contained in the coating agent into BaO.

[0056] In the coated arc welding rod according to this embodiment, further, in order to improve arc stability, the mechanical properties of the weld metal, bead shape, etc., Na, K, and Li, Ti, Al, and Zr may be contained in the coating agent within the ranges shown below. Also, when Nb and V are contained in the coating agent, it is preferable to regulate their contents within the ranges shown below. The content of each component that can be contained in the coating agent of the coated arc welding rod according to this embodiment and the reasons for its limitation will be further described.

[0057] (Total amount of Na, K and Li: 0.3% by mass or more and 4.0% by mass or less) Na, K and Li are components having the effect of stabilizing the arc. When the total amount of Na, K and Li in the coating agent is 0.3% by mass or more and 4.0% by mass or less, the arc stabilizing effect can be sufficiently obtained. Therefore, the total amount of Na, K and Li in the coating agent 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 with respect to the total mass of the coating agent. Also, the total amount of Na, K and Li in the coating agent 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 with respect to the total mass of the coating agent. Note that Na, K and Li in the coating agent are contained in oxides such as Na2O, K2O, Li2O, fixing agents such as metallic Na, metallic K, metallic Li, Na alloys, K alloys, Li alloys, and water glass.

[0058] (Ti: 0.5% by mass or more and 4.0% by mass or less) Metallic Ti and Ti contained in the alloy are deoxidizing elements and elements having the effect of improving the strength of the weld metal. Also, after acting as a deoxidizing agent, it exists in the weld metal as an oxide, and this oxide has the effect of refining the crystal grains. Oxides such as TiO2 act as slag formers and have the effect of improving the fluidity of the slag. When the Ti content in the coating agent is 0.5% by mass or more and 4.0% by mass or less, the deoxidizing effect, the effect of refining the crystal grains, and the effect of improving the fluidity of the slag can be sufficiently obtained. Therefore, the Ti content in the coating agent is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and even more preferably 1.3% by mass or more with respect to the total mass of the coating agent. Also, the Ti content in the coating agent is preferably 4.0% by mass or less, more preferably 3.0% by mass or less, and even more preferably 2.5% by mass or less with respect to the total mass of the coating agent. Note that examples of the Ti source in the coating agent include metallic Ti, alloys such as ferrotitanium, and compounds such as TiO2.

[0059] (Al: 1.5 mass% or less) Metallic Al and Al contained in the alloy act as deoxidizing elements. Al oxides such as Al2O3 act as slag formers. Also, when Al2O3 or the like is contained in the coating agent, the viscosity of the molten slag can be increased, the fluidity can be improved, and the bead appearance and bead shape can be made good. In the present embodiment, in order to improve the bead appearance and bead shape, Al may be contained in the coating agent as necessary, and the Al content in the coating agent is preferably 0.02 mass% or more based on the total mass of the coating agent. On the other hand, when the Al content in the coating agent is 1.5 mass% or less based on the total mass of the coating agent, the viscosity of the molten slag can be appropriately adjusted, the fluidity can be controlled, and the bead appearance and bead shape can be kept good. Therefore, the Al content in the coating agent is preferably 1.5 mass% or less based on the total mass of the coating agent, more preferably 1.0 mass% or less, and even more preferably 0.7 mass% or less. Note that examples of the Al source in the coating agent include Al oxides such as Al2O3, metallic Al, alloys such as aluminomagnesium, and the like.

[0060] (Zr: 0.8 mass% or less) Metallic Zr and Zr contained in the alloy act as deoxidizing elements. Zr oxides such as ZrO2 act as slag formers. Also, when ZrO2 is contained in the coating agent, the wettability of the bead can be improved, and the effect of forming a flat bead can be obtained. In the present embodiment, in order to improve the wettability and bead shape of the bead, Zr may be contained in the coating agent as necessary, and the Zr content in the coating agent is preferably 0.01 mass% or more based on the total mass of the coating agent. On the one hand, when the Zr content in the coating agent is 0.8 mass% or less, while maintaining good slag detachability, the wettability of the bead can be improved, and a flat bead shape can be obtained. Therefore, the Zr content in the coating agent is preferably 0.8 mass% or less, more preferably 0.6 mass% or less, and even more preferably 0.3 mass% or less based on the total mass of the coating material. Examples of the Zr source in the coating agent include metallic Zr, Zr contained in an alloy, and Zr oxides such as ZrO2.

[0061] (Nb: 0.03 mass% or less) Nb is a component having the effect of improving the strength of the weld metal, but it is also a component that reduces the low-temperature toughness by precipitating carbides by PWHT. Therefore, in this embodiment, it is preferable to regulate the Nb content in the coating agent to a predetermined value or less, and it may be 0 mass%. When the Nb content in the coating material is 0.03 mass% or less, a decrease in low-temperature toughness after PWHT can be suppressed. Therefore, the Nb content in the coating agent is preferably 0.03 mass% or less based on the total mass of the coating material.

[0062] (V: 0.03 mass% or less) V is a component having the effect of improving the strength of the weld metal, but it is also a component that promotes the precipitation and growth of carbides and reduces the low-temperature toughness. Therefore, in this embodiment, it is preferable to regulate the V content in the coating agent to a predetermined value or less, and it may be 0 mass%. When the V content in the coating agent is 0.03 mass% or less, a decrease in low-temperature toughness after PWHT can be suppressed. Therefore, the V content in the coating agent is preferably 0.03 mass% or less based on the total mass of the coating agent.

[0063] (B: 0.10 mass% or less) B is a component that segregates at the prior austenite grain boundary and has the effect of improving the toughness of the weld metal by suppressing primary ferrite, but it is also a component that may cause hot cracking or SR cracking. In this embodiment, the lower limit of the B content in the coating agent is not particularly specified, and it may be 0 mass%. When the B content in the coating agent is 0.10 mass% or less, the occurrence of hot cracking and SR cracking can be suppressed in the weld metal. Therefore, the B content in the coating agent is preferably 0.10 mass% or less based on the total mass of the coating agent.

[0064] (Cu: 0.6 mass% or less) Cu is a component that has the effect of refining the structure of the weld metal and improving the low-temperature toughness while maintaining the strength. However, depending on the Cu content in the coating agent, it may promote the formation of precipitates and lower the low-temperature toughness. In this embodiment, Cu may be contained in the coating agent as necessary, but the lower limit of the Cu content in the coating agent is not particularly defined and may be 0 mass%. When the Cu content is 0.6 mass% or less, it does not promote the formation of precipitates and can suppress the decrease in low-temperature toughness. Therefore, the Cu content in the coating agent is preferably 0.6 mass% or less based on the total mass of the coating agent.

[0065] (Balance) In this embodiment, C is cited as another component that can be contained in the coating agent. When the C content in the coating agent is 0.30 mass% or less, the formation of carbides can be suppressed. Therefore, the C content in the coating agent is preferably 0.30 mass% or less based on the total mass of the coating agent.

[0066] In addition, in the covered arc welding according to this embodiment, the total content of CO2, F, Si, Ni, Fe, Mo, Cr, Mg, and Mn, which are essential components contained in the coating agent, is preferably 40 mass% or more, more preferably 45 mass% or more, and even more preferably 50 mass% or more based on the total mass of the coating agent. When further containing CaO and BaO, the total content of these is preferably 85 mass% or more, more preferably 87 mass% or more, and even more preferably 90 mass% or more based on the total mass of the coating agent. The coating agent further contains at least one of Na, K, and Li, and Ti. Whether it contains Al and Zr or not, the total of these contents is preferably 90% by mass or more, more preferably 93% by mass or more, and still more preferably 95% by mass or more based on the total mass of the coating agent.

[0067] (Impurities) Examples of the elements other than those described above that can be contained in the coating agent include inevitable impurities such as P, S, Sn, Sb, As, Pb, and N. From the viewpoint of ensuring the welding quality such as high temperature crack resistance, the inevitable contents of P, S, Sn, Sb, As, Pb, and N based on the total mass of the coating agent are preferably regulated to 0.5% by mass or less respectively. Also, the total value of the impurities based on the total mass of the coating agent is preferably regulated to 3% by mass or less.

[0068] <1-3. Core wire> Next, the components contained in the core wire of the coated arc welding rod according to the present embodiment and the preferable contents will be described below. In the present embodiment, as the core wire, for example, an iron-based core wire or a steel core wire mainly composed of Fe can be preferably used. As the steel core wire, a steel core wire composed of mild steel, high-tensile steel, and low-alloy steel can be preferably used.

[0069] In the present embodiment, other components in the core wire are not particularly limited, but in addition to Fe, C, Si, Mn, P, S, N, Cu, etc. may be contained. Based on the total mass of the core wire, the C content in the core wire 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). Further, Nb, V, Cr, Ni, Mo, Ti, Al, and B may be further contained in the core wire. Among these components, the contents of Nb and V are preferably 0.02% by mass or less, respectively. Further, the total content of Cr, Ni, Mo, Ti, and Al is preferably 4.0% by mass or less. Furthermore, the B content is preferably 0.02% by mass or less.

[0070] In this embodiment, the outer diameter of the core wire is not particularly limited, but for example, it is preferably 2.6 mm or more and 5.0 mm or less.

[0071] [2. Method for manufacturing a covered arc welding rod] The covered arc welding rod according to this embodiment is produced by blending raw materials of the covering agent so that the covering agent has the above-described component composition, kneading with a predetermined fixing agent, and coating the surface of a predetermined core wire so that the mass of the covering agent is in the range of 25% by mass or more and 40% by mass or less with respect to the total mass of the covered arc welding rod, and then firing at 450°C to 550°C for about 1 hour. When manufacturing the covered arc welding rod according to this embodiment, the type of the core wire, the type of the fixing agent, the method for forming the covering agent, etc. are not particularly limited, and the normal specifications and conditions in the case of manufacturing a covered arc welding rod can be used.

[0072] [3. Weld metal] The weld metal according to this embodiment is obtained by performing covered arc welding using the covered arc welding rod according to this embodiment described in [1. Covered arc welding rod] above. In the weld metal according to this embodiment, conditions other than using the covered arc welding rod according to this embodiment are not particularly limited, and the type of the base material can be appropriately selected according to the required characteristics.

[0073] [4. Covered arc welding method] The covered arc welding method according to this embodiment is a method of welding using the covered arc welding rod according to this embodiment described in [1. Covered arc welding rod] above. In addition, in the covered arc welding method according to the present embodiment, various welding conditions other than using the covered arc welding rod according to the present embodiment are not particularly limited, and general conditions in the welding method using the covered arc welding rod can be used for the type of base material, welding voltage, welding current, welding posture, etc.

[0074] [5. Method for manufacturing welded joint] The method for manufacturing a welded joint according to the present embodiment is a method for manufacturing a welded joint by covered arc welding using high-tensile steel as a base material to be welded and the covered arc welding rod according to the present embodiment described in [1. Covered arc welding rod] above. In addition, in the method for manufacturing a welded joint, the welding conditions other than using high-tensile steel as a base material to be welded and performing covered arc welding using the covered arc welding rod according to the present embodiment are not particularly limited, and general conditions in the welding method using the covered arc welding rod can be used for the welding voltage, welding current, welding posture, etc. Moreover, the high-tensile steel that can be used as the base material is not limited, but it is preferably 720 MPa class or higher. For example, P690Q, P690QH, P690QL1, and P690QL2 of EN 10028-6:2017, KD620, KD690, KE620, and KE690 defined by the Nippon Kaiji Kyokai (NK), and VL690 defined by DNV, etc. can be mentioned.

Examples

[0075] Hereinafter, inventive examples and comparative examples of the covered arc welding rod according to the present embodiment will be described.

[0076] [Covered arc welding] (Manufacture of covered arc welding rod) Using a welding rod coating machine, the surface of a steel core wire with a diameter of 4.0 mm was coated with a coating agent having various component compositions, and then baked at 450 to 550 °C for about 1 hour to produce each covered arc welding rod of the inventive examples and comparative examples. The coating rate was made to be in the range of 25% by mass or more and 40% by mass or less with respect to the total mass of the covered arc welding rod.

[0077] Covered arc welding Next, using the obtained covered arc welding electrode, covered arc welding was performed on steel plates having the plate thicknesses and chemical components shown in Table 2 below under the welding conditions shown in Table 1 below to produce welded joints.

[0078] [Evaluation of mechanical properties] [Preparation of test pieces] The mechanical properties of the weld metal were evaluated for tensile performance and impact performance by sampling a tensile test piece (Type A2) and an impact test piece (V-notch test piece) from the central part in the plate thickness direction of the weld metal in accordance with the "Tensile and Impact Test Methods for Weld Metal" specified in JIS Z 3111:2005. In considering a wide range of PWHT conditions, a method of organizing the changes in mechanical properties when PWHT (post-weld heat treatment) is performed at various holding temperatures and holding times using the Larson-Miller parameter (hereinafter abbreviated as LMP) is often used. In this example, the PWHT conditions with a low LMP were set at a temperature of 580°C for 2 hours, and the conditions with a high LMP were set at a temperature of 620°C for 8 hours for evaluation.

[0079] [Tensile test] The tensile test was carried out on as-welded test pieces and test pieces subjected to PWHT at a temperature of 580°C for 2 hours and 620°C for 8 hours with the test temperature set at room temperature (about 20 ± 2°C), and the tensile performance was evaluated by measuring the yield stress and tensile strength. In the examples of the present invention, when the tensile strength (TS) in the as-welded state is 780 MPa or more, it is judged that the strength is good, and in each case after PWHT at a temperature of 580°C for 2 hours and 620°C for 8 hours, when the tensile strength (TS) is 750 MPa or more, it is judged that the strength is good.

[0080] [Impact test] The impact test was carried out on the as-welded specimens and the specimens subjected to PWHT at a temperature of 580°C for 2 hours and at a temperature of 620°C for 8 hours. The test temperatures were -40°C and -60°C. The Charpy absorption energy (vE-40°C, vE-60°C) was measured three times at each test temperature, and the toughness was evaluated using the minimum value among the three Charpy absorption energies. In the examples of the present invention, when the minimum values of the absorption energies at -40°C and -60°C after as-welded and PWHT were 100 J or more and 80 J or more, respectively, it was determined that the toughness was good. Those with good strength and toughness after as-welded and PWHT were considered qualified, and the others were considered unqualified.

[0081] The chemical composition of the core wire is shown in Table 3 below, and the chemical composition of the coating agent is shown in Tables 4 and 5 below. The evaluation results of the mechanical properties are shown in Table 6 below. The remainder of the components of the core wire shown in Table 3 below is Fe and impurities. The remainder of the components of the coating agent shown in Tables 4 and 5 below is impurities. In addition, in Table 4 below, [Ni] is the value representing the Ni content in the coating agent as a mass % with respect to the total mass of the coating agent, [Mn] is the value representing the Mn content in the coating agent as a mass % with respect to the total mass of the coating agent, and [Si] is the value representing the Si content in the coating agent as a mass % with respect to the total mass of the coating agent.

[0082] Furthermore, in the description of the content in Table 5 below, those described as "-" indicate that the content was below the quantification limit value. Moreover, for those for which the evaluation of the mechanical properties when subjected to PWHT at a temperature of 580°C for 2 hours was not performed, in Table 6, it was indicated as "-" in the evaluation result column.

[0083]

Table 1

[0084]

Table 2

[0085]

Table 3

[0086]

Table 4

[0087]

Table 5

[0088]

Table 6

[0089] As shown in Tables 4 to 6 above, in Invention Examples No. 1 to 7 where the content of each component in the coating agent was within the range defined in the present invention, the as-welded tensile strength (TS) was 780 MPa or more as intended, and the tensile strengths (TS) after two different types of PWHTs were 750 MPa or more as intended, and a weld metal having excellent strength could be obtained. Further, the absorbed energy at -40°C was 100 J or more, and the absorbed energy at -60°C was 80 J or more, and excellent low-temperature toughness could be obtained. From this, not only in the as-welded state but also under a wide range of PWHT conditions, a weld metal having excellent low-temperature toughness and suppressed generation of high-temperature cracks while maintaining the intended strength could be obtained. In addition, Invention Examples No. 1 to 7 were able to produce a welded joint having a welded portion with excellent low-temperature toughness and suppressed generation of high-temperature cracks while maintaining the intended strength under a wide range of PWHT conditions, not only in the as-welded state.

[0090] On the other hand, in Comparative Example No. 1, since the Cr content in the coating agent was less than the lower limit of the range defined in the present invention, the tensile strength after as-welded and after long-term PWHT decreased, and the toughness at -60°C after short-term PWHT also decreased. In Comparative Examples No. 2 and 5, since the value obtained by the formula [Ni] / ([Mn]+[Si]) was less than the lower limit of the range defined in the present invention, the low-temperature toughness after short-term PWHT and after long-term PWHT decreased. In Comparative Example No. 3, the Mo content in the coating agent exceeded the upper limit of the range defined in the present invention, and the value obtained by the formula [Ni] / ([Mn]+[Si]) was less than the lower limit of the range defined in the present invention. Therefore, the low-temperature toughness after as-welded, after short-term PWHT, and after long-term PWHT decreased.

[0091] In Comparative Example No. 4, the Mo content in the coating agent exceeded the upper limit of the range defined in the present invention, and the Cr content in the coating agent was less than the lower limit of the range defined in the present invention. Therefore, the strength did not decrease, but since the value obtained by the formula [Ni] / ([Mn]+[Si]) was less than the lower limit of the range defined in the present invention, the low-temperature toughness after short-term PWHT and after long-term PWHT decreased. In Comparative Examples No. 6 to 9, the Ni content in the coating agent was less than the lower limit of the range defined in the present invention, and the value obtained by the formula [Ni] / ([Mn]+[Si]) was less than the lower limit of the range defined in the present invention. Therefore, at least one of the strength after as-welded, the toughness at -60°C, the toughness at -40°C, the strength after long-term PWHT, the toughness at -60°C, and the toughness at -40°C decreased.

[0092] In Comparative Example No. 10, since the value obtained by the formula [Ni] / ([Mn]+[Si]) exceeded the upper limit of the range defined in the present invention, the concern about the occurrence of hot cracking increased. Further, in Comparative Example No. 11, since both the Ni content in the coating agent and the value obtained by the formula [Ni] / ([Mn]+[Si]) exceeded the upper limit of the range defined in the present invention, the concern about the occurrence of hot cracking increased as in Comparative Example No. 10.

Claims

1. A covered arc welding rod having a core wire and a covering agent covering the core wire, wherein the covering agent, based on the total mass of the covering agent, CO ２ : 16% by mass or more and 27% by mass or less, F: 4% by mass or more and 10% by mass or less, 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 or more and 11% by mass or less, Mo: 0.3% by mass or more and 1.0% by mass or less, Cr: 0.15% by mass or more and 1.20% by mass or less, Mg: 1.5% by mass or more and 4.5% by mass or less, Mn: 1.5% by mass or more and 4.0% by mass or less, CaO: 20% by mass or more and 40% by mass or less, BaO: 2% by mass or more and 6% by mass or less, Total amount of Na, K and Li: 0.3% by mass or more and 4.0% by mass or less, Ti: 0.5% by mass or more and 4.0% by mass or less, and contains, Al: 1.5% by mass or less, Zr: 0.8% by mass or less, Nb: 0.03% by mass or less, V: 0.03% by mass or less, C: 0.30% by mass or less, and when the content of Ni in the covering agent is represented as [Ni] in mass% based on the total mass of the covering agent, the content of Mn in the covering agent is represented as [Mn] in mass% based on the total mass of the covering agent, and when the content of Si in the covering agent is represented as [Si] in mass% based on the total mass of the covering agent, a covered arc welding rod, characterized in that the value calculated by [Ni] / ([Si]+[Mn]) is 0.85 or more and 1.45 or less.

2. A covered arc welding method, characterized by performing covered arc welding using the covered arc welding rod according to Claim 1.

3. A method for manufacturing a welded joint, characterized by using the covered arc welding rod according to Claim 1 and performing covered arc welding with high-tensile steel as the base material.

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