Coated electrode, method for manufacturing coated electrode, and method for manufacturing welded joint

Abstract

To obtain a welded joint excellent in weather resistance and coating peeling resistance even in an environment where corrosive substances such as a large amount of flying salt are present, and to provide a coated electrode excellent in mechanical properties of a welded metal, a method for producing the same, and a method for producing a welded joint.SOLUTION: A coated electrode according to one aspect of the present invention is a coated electrode including a steel core wire and a covering material disposed on a surface of the steel core wire, in which C and metal components of the steel core wire and the covering material contain, by mass% relative to the total mass of the coated electrode, C: 0.02 to 0.12%, Si: 0.50 to 2.50%, Mn: 0.50 to 2.50%, Sn: 0.05 to 0.50%, and Ti: 0.15 to 0.35%, with the balance being iron and impurities.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a covered electrode, a method for manufacturing a covered electrode, and a method for manufacturing a welded joint. [Background technology]

[0002] Weathering steel used for a long period of time in an atmospheric corrosive environment generally develops a protective rust layer on its surface. This rust layer shields the steel from corrosive substances from the outside, suppressing subsequent corrosion and providing weather resistance. For this reason, weathering steel is used in structures such as bridges as it can be used bare without painting.

[0003] However, in environments with high levels of airborne salt, such as coastal areas and inland areas where snow-melting agents are sprayed, it is difficult for a protective rust layer to form on the surface of weathering steel, making it difficult to demonstrate its corrosion-inhibiting effect. For this reason, weathering steel cannot be used in its bare form in these areas; it must be painted.

[0004] Furthermore, in environments with high levels of airborne salt, paint deterioration can cause scratches, which expose the steel directly below the scratched area to the corrosive environment, resulting in a nodular bulge of corrosion. This corrosion process progressively expands the paint scratches, causing continued corrosion of the structure. Therefore, in environments with high levels of airborne salt, steel is often repainted approximately every 10 years to extend the lifespan of structures. Because this repair process requires a significant amount of labor, several technical proposals have been made for new corrosion-resistant steel materials that can extend the lifespan of the paint and significantly extend the interval between repainting, thereby reducing maintenance costs.

[0005] Patent Document 1 describes a steel material for bridges that has excellent weather resistance and paint peeling resistance and can be used as a minimum maintenance material even in environments with high levels of airborne salt, such as coastal areas and areas where snow-melting salt is spread.

[0006] Patent Document 2 describes a coating film that is easily damaged mechanically and has a high resistance to SO4 2- and Cl - This paper describes a corrosion-resistant steel for coal and ore carrier holds that can extend the life of the coating and inhibit corrosion after the coating peels off, even in corrosive environments susceptible to both of these factors.

[0007] Patent Document 3 describes a welded joint with excellent corrosion resistance that exhibits excellent paint corrosion resistance even in severe seawater corrosive environments such as ballast tanks on ships, making it possible to extend the period until repair painting is required and ultimately reducing the amount of work required for repair painting.

[0008] The weathering steels and corrosion-resistant steels disclosed in the above Patent Documents 1 to 3 are, like other steel structures, mostly joined by welding before being used as structures. Therefore, in order to extend the coating life of the entire steel structure, extend the interval between repair coatings, and reduce maintenance costs, it is necessary to impart excellent weathering resistance and coating corrosion resistance to the weld metal as well. In particular, in welded joints, the reinforcement is the final layer (outermost layer) of the weld metal, and is often used in its protruding form above the base metal surface without being removed by cutting. Compared to coatings applied to the surrounding smooth base metal, coatings applied to the reinforcement area are relatively more susceptible to collisions and mechanical friction from loads during use and are more frequently and severely affected. Furthermore, because the reinforcement itself has a convex and complex shape, coating thickness tends to be thinner on the reinforcement compared to the surrounding base metal. For these reasons, the surface of the reinforcement is prone to peeling of the paint film, and is prone to becoming the starting point for a form of corrosion in which progressive paint film destruction progresses earlier than the base metal after the start of use of the steel structure. Therefore, it is extremely important to give the reinforcement, which is the outermost layer of the joint, i.e., the weld metal, weather resistance and paint corrosion resistance equivalent to or even better than that of the base metal in order to ensure the weather resistance and paint corrosion resistance of the entire structure, and new welding consumables are needed to achieve this.

[0009] However, when welding corrosion-resistant steels such as those disclosed in Patent Documents 1 and 2 using existing ordinary welding materials, the elements contained in the steel that ensure corrosion resistance migrate into the weld metal due to dilution of the steel. Therefore, unless the welding material also contains elements that ensure similar corrosion resistance, the concentration of elements effective for corrosion resistance will naturally be lower than that of the base metal, and sufficient corrosion resistance of the coating cannot be exhibited.

[0010] Furthermore, in the welded joint disclosed in Patent Document 3, the chemical composition of the base material and the chemical composition of the weld metal are specified to improve the corrosion resistance of coating, but information regarding the means, such as the formulation design and chemical composition of the welding material, required to realize the proposed weld metal is not disclosed, and is unclear. Patent Document 3 lists Sn as an element that improves the corrosion resistance of the welded joint, but does not disclose any means for transferring Sn to the weld metal.

[0011] Patent Document 4 discloses a covered metal arc welding electrode that exhibits natural slag separation even in narrow grooves and excellent hot cracking resistance. The covered metal arc welding electrode of Patent Document 4 contains Sn, which allows for the formation of a weld metal containing Sn. However, the function of Sn in the covered metal arc welding electrode disclosed in Patent Document 4 is to improve the removability of slag formed on the bead during welding, and Patent Document 4 does not intend to improve the corrosion resistance of the weld metal itself or the corrosion resistance of the coating using Sn. Furthermore, since Patent Document 4 is an element that improves slag removability, it does not consider any means for efficiently transferring Sn to the weld metal. Rather, Patent Document 4 states that Sn should be expelled from the weld metal along with the slag.

[0012] Patent Document 5 discloses a weather-resistant low-hydrogen covered electrode for use in all-position welding using a steel core wire containing Cu and Ni. Patent Document 6 discloses a low-hydrogen covered electrode for highly weather-resistant steel, for welding Cu-Ni coastal-resistant steel in all positions.

[0013] However, even in the techniques described in Patent Documents 5 and 6, the coating applied to the final layer of a welded joint tends to be thinner than the coating applied to the surrounding flat base metal because the reinforcement has a convex and complex shape, making it more susceptible to peeling. Therefore, in environments with high levels of airborne salt, there was a problem in that the reinforcement, i.e., the weld metal, was prone to becoming a starting point for corrosion. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-163374 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-262555 [Patent Document 3] Japanese Patent Application Laid-Open No. 2012-77378 [Patent Document 4] Japanese Patent Application Laid-Open No. 2000-263286 [Patent Document 5] Japanese Patent Application Publication No. 5-293690 [Patent Document 6] Japanese Patent Application Laid-Open No. 2001-300769 Summary of the Invention [Problem to be solved by the invention]

[0015] In view of the above-described current situation, an object of the present invention is to provide a covered metal arc welding electrode that can provide a welded joint that is excellent in weather resistance and paint peeling resistance even in an environment where corrosive substances such as a large amount of airborne salt are present, and that produces a weld metal with excellent mechanical properties, a method for manufacturing the same, and a method for manufacturing a welded joint. [Means for solving the problem]

[0016] The gist of the present invention is as follows.

[0017] (1) A covered metal arc welding rod according to one embodiment of the present invention is a covered metal arc welding rod including a steel core wire and a coating disposed on the surface of the steel core wire, wherein the carbon and metal components of the steel core wire and the coating contain, in mass % relative to the total mass of the covered metal arc welding rod, 0.02 to 0.12% carbon, 0.50 to 2.50% silicon, 0.50 to 2.50% manganese, 0.05 to 0.50% tin, and 0.15 to 0.35% titanium, with the remainder being iron and impurities. (2) In the covered metal arc welding electrode described in (1) above, the coating preferably contains, in mass % relative to the total mass of the coating, Ti oxides: 0.50 to 2.00% in terms of TiO2; Si oxides: 1.00 to 3.00% in terms of SiO2; Mg oxides: 0.05 to 0.15% in terms of MgO; Al oxides: 0.50% or less in terms of Al2O3; metal carbonates: 10.0 to 25.0%; and metal fluorides: 2.0 to 8.0%. (3) Preferably, in the covered metal arc welding electrode according to (1) or (2) above, the metal components contained in the steel core wire and the coating further contain, in mass % relative to the total mass of the covered metal arc welding electrode, 0.80% or less of Ni. (4) Preferably, in the covered metal arc welding electrode according to any one of the above (1) to (3), the metal components contained in the steel core wire and the coating further contain, in mass % relative to the total mass of the covered metal arc welding electrode, 0.30% or less of Mo. (5) Preferably, in the covered electrode according to any one of (2) to (4) above, the metal carbonate is at least one selected from the group consisting of calcium carbonate, barium carbonate, magnesium carbonate, sodium carbonate, and manganese carbonate. (6) Preferably, in the covered electrode according to any one of (2) to (5) above, the metal fluoride is at least one selected from the group consisting of fluorite, magnesium fluoride, aluminum fluoride, and barium fluoride. (7) Preferably, in the covered electrode according to any one of the above (1) to (6), the coverage of the coating material is 22 to 37% of the total mass of the covered electrode.

[0018] (8) A method for producing a covered metal arc welding electrode according to another embodiment of the present invention includes the steps of applying a coating agent to a steel core wire and drying the steel core wire to which the coating agent has been applied, and the components of the steel core wire and the coating agent, as well as the coating rate of the coating agent, are selected so that the metal components contained in the steel core wire and the coating agent contain, in mass % relative to the total mass of the covered metal arc welding electrode, 0.02 to 0.12% C, 0.50 to 2.50% Si, 0.50 to 2.50% Mn, 0.05 to 0.50% Sn, and 0.15 to 0.35% Ti, with the remainder being iron and impurities. (9) Preferably, in the method for producing a covered metal arc welding electrode according to (8) above, the coating material contains, in mass % relative to the total mass of the coating material, Ti oxides: 0.50 to 2.00% in terms of TiO2 equivalent, Si oxides: 1.00 to 3.00% in terms of SiO2 equivalent, Mg oxides: 0.05 to 0.15% in terms of MgO equivalent, Al oxides: 0.50% or less in terms of Al2O3 equivalent, metal carbonates: 10.0 to 25.0%, and metal fluorides: 2.0 to 8.0%. (10) Preferably, in the method for producing a covered metal arc welding electrode according to (8) or (9) above, the metal components contained in the steel core wire and the coating further contain, in mass % relative to the total mass of the covered metal arc welding electrode, 0.80% or less of Ni. (11) Preferably, in the method for producing a covered metal arc welding electrode according to any one of the above (8) to (10), the metal components contained in the steel core wire and the coating further contain, in mass % relative to the total mass of the covered metal arc welding electrode, 0.30% or less of Mo. (12) Preferably, in the method for producing a covered electrode according to any one of (8) to (11) above, the coating agent contains water glass. (13) Preferably, in the method for producing a covered electrode according to any one of (9) to (12) above, the metal carbonate is at least one selected from the group consisting of calcium carbonate, barium carbonate, magnesium carbonate, sodium carbonate, and manganese carbonate. (14) Preferably, in the method for manufacturing a covered electrode according to any one of (9) to (13) above, the metal fluoride is at least one selected from the group consisting of fluorite, magnesium fluoride, aluminum fluoride, and barium fluoride.

[0019] (15) A method for producing a welded joint according to another aspect of the present invention includes a step of arc-welding steel materials using the covered metal arc welding electrode according to any one of (1) to (7) above. [Effects of the Invention]

[0020] The method for producing a covered metal arc welding electrode and a weld joint according to the present invention can provide a weld metal that is excellent in weather resistance and paint peeling resistance even in environments with high levels of airborne salt, such as coastal areas, and can suppress the progression of corrosion due to the spread of paint peeling originating from the weld. This can extend the period until a repainting process is performed to extend the life of the structure, thereby reducing maintenance costs. Furthermore, the present invention can provide a covered metal arc welding electrode and a weld joint manufacturing method that produce a weld metal with excellent mechanical properties. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 2 is a cross-sectional schematic view of a covered electrode; [Figure 2] FIG. 1 is a diagram showing the locations where corrosion test specimen materials were collected for evaluating the corrosion resistance of welded parts. [Figure 3] FIG. 1 is a diagram showing the shape of a corrosion test piece for evaluating the coating corrosion resistance of a welded portion and an outline of a crosscut. [Figure 4] FIG. 1 is a diagram showing an outline of a corrosion test method (SAE J2334 test, implementation conditions per cycle). DETAILED DESCRIPTION OF THE INVENTION

[0022] (1. Coated electrode) The inventors have investigated the effects of various alloying elements in order to find the chemical compositions necessary for obtaining a covered electrode capable of achieving the above-mentioned object.

[0023] As a result, it was discovered that by incorporating tin (Sn) into a covered electrode and adding an appropriate amount of Sn to the weld metal, the corrosion resistance of the weld metal in an environment with a high amount of airborne salt can be improved.

[0024] The reason why Sn improves the corrosion resistance of weld metal is that metallic Sn in the weld metal reacts with tin ions (II) (Sn 2+ ) and exhibited inhibitory activity in the exposed area, i.e., in acidic chloride solutions, and suppressed corrosion at the anode where the pH was reduced. Furthermore, Sn contained iron(III) ions (Fe 3+ It has been found that this has the effect of reducing the concentration of HCl, thereby improving the corrosion resistance and paint peeling resistance of the weld metal in an environment with a high amount of airborne salt.

[0025] The present inventors also attempted to improve the welding workability and the mechanical properties of the weld metal in addition to the corrosion resistance of the welded joint, and as a result, the following points were clarified.

[0026] It has been found that adding appropriate amounts of Si, Ti, and metal carbonate to covered metal arc welding electrodes is effective in improving welding workability. These components stabilize the arc and produce a good bead shape. It has also been found that adding appropriate amounts of Ti oxide, Si oxide, and metal fluoride to covered metal arc welding electrodes produces a good bead shape in all-position welding. It has also been found that adjusting the Mg oxide content of covered metal arc welding electrodes is effective in preventing one-sided melting of the coating.

[0027] Regarding the mechanical properties of the weld metal, it was found that adding appropriate amounts of C, Si, and Mn to the covered metal arc welding rod provides adequate strength and improves toughness. It was also found that adding appropriate amounts of Ni to the covered metal arc welding rod improves the low-temperature toughness of the weld metal. Furthermore, it was found that adding Mo to the covered metal arc welding rod results in high-strength weld metal.

[0028] The application of the weld joint to which the covered electrode according to the present embodiment is applied is not particularly limited, but is preferably a weld joint constituting a structural steel material, particularly a steel structure such as a port facility, a bridge, an architectural / civil engineering structure, a tank, a ship / marine structure, a railway, or a container.

[0029] Furthermore, the steel material to which the covered electrode according to the present embodiment is applied is not particularly limited to a particular type of steel, and may be ordinary steel such as carbon steel, low alloy steel, etc. Weathering steel or low alloy steel containing Ni, Sn, etc. is advantageous from the viewpoints of weather resistance and coating corrosion resistance.

[0030] The covered metal arc welding rod 1 according to this embodiment, which was obtained based on the above findings, includes a steel core wire 11 and a coating 12 disposed on the surface of the steel core wire 11. The coating 12 is applied to the steel core wire 11 and baked to adhere to the surface of the steel core wire 11. For example, by baking the coating 12 to which water glass has been added, the coating 12 can be firmly adhered to the surface of the steel core wire 11.

[0031] The steel core wire 11 is a rod made of an iron alloy. The coating 12 contains metal powder and / or alloy powder. The components of the covered metal arc welding rod 1 are preferably controlled by the components and amounts of the metal powder and / or alloy powder contained in the coating 12. Changing the components of the steel core wire 11 is not easy from the viewpoint of manufacturing costs. In addition, the coating 12 may further contain oxides, carbonates, fluorides, etc. to improve welding workability.

[0032] (A) Metallic components The reasons for limiting the carbon and metal components contained in the steel core wire 11 and the coating material 12 are explained below. Here, "metal components" refers to components present in the steel core wire 11 or the coating material 12 in the form of a pure substance or alloy. Elements that constitute oxides, carbonates, and fluorides are not included in the metal components. Metal components dissolve into the molten pool to form the weld metal. On the other hand, oxides, carbonates, and fluorides become slag that covers the molten pool during welding, or are thermally decomposed and vaporized. Therefore, most of the metal elements that constitute oxides, carbonates, and fluorides are expelled from the weld metal.

[0033] The units of the contents of C and metal components described below are mass% relative to the total mass of the covered electrode. The value obtained by dividing the weight of the metal components contained in the steel core wire and coating by the weight of the covered electrode is the content of the metal components described below.

[0034] [C: 0.02-0.12% for steel core wire and coating] C is an important element for ensuring the strength and hardenability of the weld metal. If the C content is less than 0.02%, the necessary strength cannot be obtained in the weld metal. Furthermore, if the C content is less than 0.02%, the toughness of the weld metal decreases. On the other hand, if the C content exceeds 0.12%, hot cracking may occur more easily, and the strength of the weld metal increases while the toughness decreases. Furthermore, if the C content exceeds 0.12%, the arc blow becomes too strong, resulting in a large amount of spatter. Therefore, the total C content of the steel core wire and the coating material is set to 0.02 to 0.12%. Preferably, C is in the range of 0.03 to 0.10%. Note that C can be contained not only as a component contained in the steel core wire, but also via metal powder and alloy powder in the coating material.

[0035] [Si content of steel core wire and coating: 0.50-2.50%] Si is added as a deoxidizer during welding and also has a significant effect of stabilizing the arc, ensuring welding workability. If the Si content is less than 0.50%, deoxidation is insufficient, reducing the toughness of the weld metal and making blowholes more likely to occur. In this case, the arc becomes unstable, resulting in poor bead shape in all positions, and welding in vertical and overhead positions is particularly difficult to continue. Furthermore, in this case, slag removability may be insufficient. On the other hand, if the Si content exceeds 2.50%, low-melting-point oxides are precipitated at the grain boundaries of the weld metal, reducing the toughness of the weld metal. Therefore, the total Si content of the steel core wire and the coating material is set to 0.50 to 2.50%. The Si content is preferably in the range of 0.80 to 2.20%. In addition to being included in the steel core wire, Si can be included via metallic Si, Fe—Si, Fe—Si—Mn, etc. in the coating material.

[0036] [Mn content in steel core wire and coating: 0.50-2.50%] Mn, like Si, is an element that assists the deoxidation reaction during welding and is included to ensure the strength of the weld metal. If the Mn content is less than 0.50%, the strength of the weld metal decreases. In addition, in this case, deoxidation is insufficient, making blowholes more likely to occur in the weld metal. On the other hand, if the Mn content exceeds 2.50%, the strength of the weld metal becomes excessively high and the toughness decreases. Therefore, the total Mn content of the steel core wire and the coating material is set to 0.50 to 2.50%. The Mn content is preferably in the range of 0.60 to 2.00%. In addition to being included as a component in the steel core wire, Mn can be included via metallic Mn, Fe-Mn, Fe-Si-Mn, etc. in the coating material.

[0037] [Sn content in steel core wire and coating: 0.05-0.50%] Sn is an important element that is responsible for the weather resistance and paint peeling resistance of the weld metal. If the Sn content is less than 0.05%, the weather resistance and paint peeling resistance of the weld metal cannot be ensured. On the other hand, if the Sn content exceeds 0.50%, the toughness of the weld metal decreases due to segregation of Sn to the grain boundaries of the weld metal. Therefore, the Sn content is set to 0.05 to 0.50%. Preferably, the Sn content is in the range of 0.10 to 0.40%. More preferably, the Sn content is in the range of 0.10 to 0.30%. In addition to being a component contained in the steel core wire, Sn can be contained via metallic Sn in the coating material, etc.

[0038] [Ti: 0.15-0.35% in total for steel core wire and coating] Ti stabilizes the arc by reducing the potential gradient of the arc. It is also effective as a deoxidizer and is an element contained to ensure the toughness of the weld metal. If Ti is less than 0.15%, the arc becomes unstable and elongates, making it more likely that the coating will melt unevenly. In this case, the microstructure of the weld metal will not be refined, and the toughness of the weld metal will decrease. On the other hand, if Ti exceeds 0.35%, the crystallization of Ti oxides in the weld metal will increase, reducing the toughness of the weld metal. Therefore, the total Ti content of the steel core wire and the coating material should be 0.15 to 0.35%. Preferably, Ti is in the range of 0.18 to 0.33%. In addition to being contained in the steel core wire, Ti can be contained via metallic Ti, Fe—Ti, etc. in the coating material.

[0039] The remainder of the metal components of the steel core wire and the coating material include iron and impurities. The iron includes the Fe content of the steel core wire, the Fe content in the ferroalloy contained in the coating material, iron powder, etc. The impurities refer to components that are mixed in, for example, during the industrial production of steel materials, from raw materials such as ore or scrap, or various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the covered metal arc welding electrode according to this embodiment. In addition, the metal components of the steel core wire and the coating material may further contain elements that can further improve the properties of the covered metal arc welding electrode, such as those listed below.

[0040] [Ni: 0.80% or less in total for steel core wire and coating] Ni is added to improve the low-temperature toughness of the weld metal. If Ni exceeds 0.80%, hot cracking is likely to occur in the weld metal. Note that even if the Ni content is 0%, the toughness of the weld metal formed by the covered metal arc welding electrode according to this embodiment can be ensured, but in order to improve the low-temperature toughness of the weld metal, it is preferable to add 0.10% or more of Ni. Note that Ni can be added via metallic Ni in the coating material, and alloy powder such as Fe-Ni, in addition to being a component contained in the steel core wire.

[0041] [Mo: 0.30% or less in total for steel core wire and coating] Mo is added to improve the strength of the weld metal. If Mo exceeds 0.30%, the strength of the weld metal increases and the toughness of the weld metal decreases. Therefore, the total Mo content of the steel core wire and the coating is set to 0.30% or less. Note that even if the Mo content is 0%, the covered metal arc welding electrode according to this embodiment can ensure the strength of the weld metal, but to further improve the strength of the weld metal, it is preferable to set Mo to 0.01% or more. Note that Mo can be added not only as a component contained in the steel core wire, but also via metallic Mo or alloy powder such as Fe-Mo from the coating.

[0042] (a) Metal oxides, metal fluorides, metal carbonates) It is preferable that the covered electrode further contains one or more of metal oxides, metal fluorides, and metal carbonates. Most of the metal oxides, metal fluorides, and metal carbonates are discharged from the weld metal. However, the metal oxides, metal fluorides, and metal carbonates have the effect of improving weldability and reducing the amount of oxygen in the weld metal to improve the mechanical properties of the weld metal. The metal oxides, metal fluorides, and metal carbonates are contained in the coating material.

[0043] The metal oxides, metal fluorides, and metal carbonates contained in the coating agent 12 are described below. The units for the contents of the metal oxides, metal fluorides, and metal carbonates are different from the units for the contents of the metal components described above. The units for the contents of the metal oxides, metal fluorides, and metal carbonates are mass % relative to the total mass of the coating agent. The values ​​obtained by dividing the weights of the metal oxides, metal fluorides, and metal carbonates contained in the coating agent by the weight of the coating agent are the contents of the metal oxides, metal fluorides, and metal carbonates described below.

[0044] [Total TiO2 equivalent value of Ti oxide in coating material: 0.50-2.00%] Ti oxide is the main component of molten slag, and it adjusts the viscosity of the molten slag and stabilizes the arc. If the total TiO2 equivalent of Ti oxide is less than 0.50%, the arc becomes unstable, resulting in poor bead shape in all-position welding. On the other hand, if the total TiO2 equivalent of Ti oxide exceeds 2.00%, the viscosity of the molten slag increases and the slag fluidity decreases, resulting in a convex bead shape in vertical and overhead welding positions. Therefore, the total TiO2 equivalent of Ti oxide in the coating material is set to 0.50 to 2.00%. Preferably, the total TiO2 equivalent of Ti oxide in the coating material is in the range of 0.60 to 1.80%. Ti oxide can be contained in the coating material via rutile, titanium oxide, titanium slag, ilmenite, sodium titanate, potassium titanate, etc.

[0045] The "total TiO2 equivalent values ​​of Ti oxides in the coating agent" refers to the amount of TiO2 in the coating agent (mass % relative to the total mass of the coating agent) when all Ti oxides in the coating agent are considered to be TiO2. Similarly, the total SiO2 equivalent values, total MgO equivalent values, and total Al2O3 equivalent values ​​described below refer to the amounts of SiO2, MgO, and Al2O3 (mass % relative to the total mass of the coating agent) when all Si oxides, Mg oxides, and Al oxides in the coating agent are considered to be SiO2, MgO, and Al2O3, respectively.

[0046] [Total SiO2 equivalent value of Si oxide in coating material: 1.00-3.00%] Silicon oxides have the effect of adjusting the viscosity of the molten slag. If the total SiO2 equivalent of silicon oxides is less than 1.00%, the viscosity of the molten slag will be insufficient, resulting in poor bead shape in all-position welding. On the other hand, if the total SiO2 equivalent of silicon oxides exceeds 3.00%, the viscosity of the molten slag will increase and the slag fluidity will decrease, resulting in a convex bead shape during vertical and overhead welding. In this case, slag removability will also be poor. Therefore, the total SiO2 equivalent of silicon oxides in the coating material is set to 1.00 to 3.00%. Preferably, the total SiO2 equivalent of silicon oxides in the coating material is in the range of 1.20 to 2.60%. Silicon oxides can be contained in the coating material via silica sand, zircon sand, feldspar, sodium silicate, potassium silicate, etc. Water glass, which is added to the coating material to fuse the coating material to the steel core wire, is also considered to be silicon oxide.

[0047] [Total Mg oxides in coating material converted to MgO: 0.05-0.15%] Mg oxides have excellent heat resistance and are effective in suppressing one-sided dissolution of the coating material. If the total Mg oxides in terms of MgO is less than 0.05%, one-sided dissolution of the coating material is likely to occur. On the other hand, if the total Mg oxides in terms of MgO exceeds 0.15%, the viscosity of the molten slag increases and the slag fluidity decreases, resulting in a convex bead shape during welding in the vertical position and overhead position. Therefore, the total Mg oxides in terms of MgO is set to 0.05 to 0.15%. Preferably, the total Mg oxides in terms of MgO is in the range of 0.05 to 0.13%. Mg oxides can be contained in the coating material via magnesium oxide, magnesia clinker, etc.

[0048] [Total of Al oxides converted to Al2O3: 0.50% or less] Al oxides have the effect of stabilizing the arc and adjusting the viscosity of the molten slag. If the total Al2O3 equivalent of Al oxides exceeds 0.50%, the slag becomes glassy, ​​making it difficult to remove. In this case, slag entrapment is likely to occur. Therefore, the total Al2O3 equivalent of Al oxides should be 0.50% or less. In addition, to obtain the effect of improving the bead shape in all-position welding, it is preferable that the total Al2O3 equivalent of Al oxides be 0.01% or more. Al oxides can be added as alumina, feldspar, etc.

[0049] [Metal carbonate: 10.0-25.0%] Metal carbonate decomposes in the arc to generate CO2 gas, thereby shielding the molten slag and molten weld metal from the atmosphere and suppressing excessive oxidation reactions and the dissolution of oxygen and nitrogen into the weld metal. If the metal carbonate content is less than 10.0%, the shielding effect is insufficient, making blowholes more likely to occur. On the other hand, if the metal carbonate content exceeds 25.0%, the arc becomes unstable, resulting in a convex bead shape in all-position welding and poor slag removability. Therefore, the metal carbonate content is set to 10.0 to 25.0%. The metal carbonate content is preferably in the range of 10.0 to 22.0%. The metal carbonate may be, for example, one or more selected from the group consisting of calcium carbonate, barium carbonate, magnesium carbonate, sodium carbonate, and manganese carbonate in the coating, but is not limited thereto. When multiple types of metal carbonates are contained in a covered electrode, the total amount of the multiple types of metal carbonates may be within the above-mentioned range.

[0050] [Metal fluoride: 2.0~8.0%] Metal fluorides have the effect of reducing the viscosity of molten slag, improving slag fluidity, and improving bead shape. If the metal fluoride content is less than 2.0%, the appropriate viscosity of the molten slag cannot be obtained, resulting in poor bead shape in all-position welding. On the other hand, if the metal fluoride content exceeds 8.0%, slag removability becomes poor. Therefore, the metal fluoride content is set to 2.0 to 8.0%. The metal fluoride content is preferably in the range of 3.0 to 7.0%. The metal fluoride may be, for example, one or more selected from the group consisting of fluorite, magnesium fluoride, aluminum fluoride, and barium fluoride in the coating material, but is not limited thereto. When multiple types of metal fluorides are contained in a covered metal arc welding electrode, the total amount of the multiple types of metal fluorides may be within the above-mentioned range.

[0051] The covered electrode according to the present embodiment may contain one or more coating agents such as hectorite and mica in a total amount of 3% or less, and may also contain impurities. Note that hectorite and mica are not included in the above-mentioned silicon oxides and aluminum oxides.

[0052] The coverage of the steel core wire with the coating material is not particularly limited, but from the viewpoint of productivity, it is preferably 22 to 37% of the total mass of the covered electrode. The coverage is the value obtained by dividing the total mass of the coating material by the total mass of the covered electrode (i.e., the total mass of the coating material and the steel core wire).

[0053] (2. Manufacturing method of covered electrode) The method for manufacturing a covered metal arc welding rod according to this embodiment includes the steps of applying a coating to a steel core wire and drying the coated steel core wire. The covered metal arc welding rod must be manufactured so that the carbon and metal components contained in the steel core wire and the coating contain, in mass percent relative to the total mass of the covered metal arc welding rod, 0.02-0.12% carbon, 0.50-2.50% silicon, 0.50-2.50% manganese, 0.05-0.50% tin, and 0.15-0.35% titanium, with the remainder being iron and impurities. The steel core wire and the coating may contain, in mass percent relative to the total mass of the steel core wire and the coating, one or more elements selected from the group consisting of up to 0.80% nickel and up to 0.30% molybdenum.

[0054] The metal composition of the covered electrode can be controlled by selecting the components of the steel core wire and the coating, as well as the coverage of the coating. The components of the coating can be easily changed by adding metal powder or alloy powder. Therefore, it is preferable to control the metal composition of the covered electrode via the components and coverage of the coating.

[0055] Furthermore, the coating agent may contain, in mass % relative to the total mass of the coating agent, the sum of Ti oxides converted into TiO2: 0.50 to 2.00%, the sum of Si oxides converted into SiO2: 1.00 to 3.00%, the sum of Mg oxides converted into MgO: 0.05 to 0.15%, metal carbonates: 10.0 to 25.0%, metal fluorides: 2.0 to 8.0%, and the sum of Al oxides converted into Al2O3: 0.50% or less.

[0056] The metal carbonate may be one or more selected from the group consisting of calcium carbonate, barium carbonate, magnesium carbonate, sodium carbonate, and manganese carbonate. The metal fluoride may be one or more selected from the group consisting of fluorite, magnesium fluoride, aluminum fluoride, and barium fluoride. The reasons for limiting these components and the preferred contents thereof are the same as those described above regarding the components of the covered electrode.

[0057] The conditions for drying the steel core wire and the coating material are not particularly limited, and ordinary conditions may be selected as appropriate. The coating rate of the coating material on the outer periphery of the steel core wire is not particularly limited, but from the viewpoint of productivity, it is preferable that the amount of the coating material be 22 to 37% of the total mass of the covered electrode. It is also preferable that the coating material contains water glass, so that the coating material is firmly fixed to the steel core wire.

[0058] (3. Manufacturing method of welded joints) A method for manufacturing a welded joint according to another aspect of the present invention includes a step of arc-welding steel materials using a covered metal arc welding electrode according to the present embodiment. The type of steel material is not particularly limited, and can be ordinary steel materials such as carbon steel and low-alloy steel. If the steel material is weathering steel or low-alloy steel containing Ni, Sn, etc., it is advantageous from the viewpoint of weather resistance and coating corrosion resistance. In the method for manufacturing a welded joint according to another aspect of the present invention, the welding conditions are also not particularly limited, and can be appropriately selected within normal ranges. [Example]

[0059] The effects of the present invention will be explained in more detail below with reference to examples.

[0060] A coating agent was used to coat the outer periphery of a 4.0 mm diameter, 400 mm long steel core wire with the chemical composition shown in Table 1 at a coating rate of 22 to 35%, and the wire was then dried to produce prototype low-hydrogen coated metal arc welding electrodes made of corrosion-resistant steel with various compositions shown in Tables 2 and 3. The chemical composition of the steel core wire listed in Table 1 is expressed in mass% relative to the total mass of the steel core wire. In Tables 1, 2, and 3, the content of components that were not included in the raw materials or whose content was at the impurity level is indicated by "-". The remainder of the chemical composition of the steel core wire is iron and impurities. The remainder of the prototype coated metal arc welding electrodes was the Fe content of the steel core wire, the Fe content in the ferroalloy, iron powder, and one or more coating agents such as hectorite and mica totaling 3% or less, plus impurities.

[0061] [Table 1]

[0062] [Table 2]

[0063] [Table 3]

[0064] For the welding workability study, T-shaped fillet specimens were prepared using 12 mm thick JIS G 3106:2015 SM490A steel using the covered electrodes shown in Tables 2 and 3. The welding current was 140 A for vertical welding and 170 A for horizontal fillet welding. Arc stability, spatter generation, slag removability, bead shape, and the presence or absence of one-sided melting were visually inspected for each welding position. X-ray transmission testing (JIS Z 3104:1995) was also used to determine the presence or absence of flaws (defects) and the type of weld defect based on the flaw's size and shape.

[0065] The methods for evaluating the mechanical properties and corrosion resistance of the weld metal are as follows. Weld metal tests were conducted in accordance with JIS Z 3111:2005, followed by tensile tests (JIS Z 2201:2011), impact tests (JIS Z 2242:2005), and the corrosion resistance evaluation test described below. The base metal used was a steel plate containing 0.11% C, 0.18% Si, 1.44% Mn, 0.011% P, 0.002% S, and 0.12% Sn. The welding conditions were a welding current of 140 to 170 A, an arc voltage of 25 V, a welding speed of 15 cm / min, and an interpass temperature of 100 to 130°C.

[0066] The criteria for determining whether a weld metal passes the mechanical properties test are, in principle, that the tensile strength is 510 to 660 MPa in a tensile test and that the absorbed energy is 60 J or more in an impact test at 0°C. For those containing Ni to ensure low-temperature toughness, those with a tensile strength of 510 to 660 MPa and an absorbed energy of 60 J or more at -20°C were deemed to have passed. Note that for covered electrodes not containing Ni, the absorbed energy at -20°C of the deposited metal obtained from these electrodes was not measured, and the value is indicated by the symbol "-". For those containing Mo, a tensile strength of 590 to 720 MPa and an absorbed energy of 60 J or more at 0°C were deemed acceptable.

[0067] The corrosion resistance was evaluated using the following procedure. First, a corrosion test specimen material 4 (3 mm thick × 60 mm wide × 150 mm long) shown in Figure 2 was taken from a position 1 mm deep from the surface of the base material 2, with the weld metal 3 at the center. After shot blasting, it was heated and dried at a furnace temperature of 80°C. A corrosion test specimen was prepared by painting both sides of the corrosion test specimen material 4 with either paint A (Banno #200, manufactured by Chugoku Paint Co., Ltd.) or paint B (Neo Gosei Primer HB, manufactured by Shinto Paint Co., Ltd.) to a thickness of 200 to 350 μm. A cross-cut 5 was made on the above test specimen across the weld metal 3, as shown in Figure 3, to simulate a coating scratch. The cross-cut 5 was made with a utility knife, creating a scratch from the coating to the underlying steel surface.

[0068] Thereafter, the obtained corrosion test specimens were subjected to SAE (Society of Automotive Engineers) J2334 testing.

[0069] Here, we will explain the SAE J2334 test. The SAE J2334 test is an accelerated test conducted under repeated wet-dry conditions, with one cycle (total 24 hours) consisting of three steps: wetting (50°C, 100% RH, 6 hours), salt deposition (immersion in an aqueous solution of 0.5% NaCl, 0.1% CaCl2, and 0.075% NaHCO3 by mass, 0.25 hours), and drying (60°C, 50% RH, 17.75 hours). An outline of one cycle is shown in Figure 4. This corrosion test simulates a severe corrosive environment with an airborne salt load exceeding 1 mdd, and the corrosion pattern is said to be similar to that of atmospheric exposure tests (see Hiroo Nagano, Masato Yamashita, and Hitoshi Uchida, Environmental Materials Science, Kyoritsu Shuppan (2004), p. 74).

[0070] After 80 cycles of the SAE J2334 test, the paint peeling and blistering area ratio of each test specimen was measured. The paint peeling and blistering area ratio of a test specimen was calculated by dividing the area of ​​the test specimen where paint peeling and blistering occurred by the paint film area of ​​the test specimen. The remaining paint film and the resulting rust layer on the surface were then removed from the test specimen. The corrosion depth of the scratches in the paint film was measured using a laser focus depth meter. The reference surface was the uncorroded area of ​​each test specimen, and measurement positions were standardized for all test specimens, at nine locations near cross-cut 5. The corrosion depths obtained from the nine measurement points on each test specimen were averaged to calculate the average corrosion depth.

[0071] The weather resistance and paint peeling resistance were evaluated as passing if the paint peeling / blistering area ratio was less than 50% and the average corrosion depth of the paint film scratches was less than 0.5 mm. These results are summarized in Tables 4 and 5.

[0072] [Table 4]

[0073] [Table 5]

[0074] Welding rods No. 1 to No. 15 and welding rods No. 26 to No. 35 in Tables 2, 3, 4, and 5 are examples of the present invention, and welding rods No. 16 to 25 and welding rods No. 36 to No. 38 are comparative examples.

[0075] Welding rods No. 1 to No. 15 and welding rods No. 26 to No. 35, which are examples of the present invention, had appropriate amounts of C, Si, Mn, Sn, and Ti compared to covered metal arc welding rods. Therefore, in the welded joints obtained with these welding rods, the paint peeling / blistering area ratio was less than 50%, and the average corrosion depth of the paint defects was less than 0.5 mm. According to the examples of the present invention, welded joints with excellent weather resistance and paint peeling resistance were obtained even in environments where corrosive substances such as high amounts of airborne salt were present.

[0076] In addition, for welding rods No. 1 to No. 15, the total Ti oxides (equivalent to TiO2), total Si oxides (equivalent to SiO2), total Mg oxides (equivalent to MgO), total metal carbonates, total metal fluorides, and total Al oxides (equivalent to Al2O3) in the coating were within the preferred ranges. This resulted in a stable arc, low spatter generation, good slag removability and bead shape, no uneven melting, no welding defects detected in X-ray transmission tests, and excellent tensile strength and absorbed energy of the weld metal, resulting in extremely satisfactory results. For welding rods No. 2, 3, 7, 9, 12, and 14 containing Ni, the absorbed energy of the weld metal at -20°C was 60 J or more. Furthermore, for welding rods No. 3, 5, 6, 7, 8, and 12 containing Mo, the tensile strength of the weld metal was 590 MPa or more.

[0077] Among the comparative examples, welding rod No. 16 had a low C content, and therefore the tensile strength and absorbed energy of the deposited metal were low. Welding rod No. 17 had an excess of carbon, which resulted in high tensile strength of the deposited metal and low absorbed energy. Welding rod No. 18 had a low Si content, so the absorbed energy of the deposited metal was low. Welding rod No. 19 had an excess of Si, and the absorbed energy of the deposited metal was low. Welding rod No. 20 had a low Mn content, so the tensile strength of the deposited metal was low. Welding rod No. 21 contained excessive Mn, and the absorbed energy of the deposited metal was low. Welding rod No. 22 contained less Sn, so the area ratio of paint peeling and swelling on the weld metal was large, and the average corrosion depth of the paint film scratches was also deep. Welding rod No. 23 contained excessive Sn, and the absorbed energy of the deposited metal was low. Welding rod No. 24 contained a small amount of Ti, so the absorbed energy of the deposited metal was low. Welding rod No. 25 contained excessive Ti, and the absorbed energy of the deposited metal was low. Welding rod No. 37 contained excessive Ni, causing crater cracks. Welding rod No. 38 contained excessive Mo and had a low absorbed energy. [Explanation of symbols]

[0078] 1. Covered metal arc welding rod 11 Steel core 12 Coating 2 Base material (steel) 3 Weld metal 4. Corrosion test specimen material 5 Crosscut

Claims

1. A steel core wire; a coating agent disposed on the surface of the steel core wire; A covered metal arc welding rod comprising: The C and metal components of the steel core wire and the coating are expressed as mass% relative to the total mass of the covered electrode. C: 0.02-0.12%, Si: 0.50 to 2.50%, Mn: 0.50 to 2.50%, Sn: 0.05 to 0.50%, and Ti: 0.15-0.35% Contains The balance is iron and impurities A covered electrode for arc welding characterized by:

2. The coating agent comprises, in mass % based on the total mass of the coating agent: Ti oxide: TiO ２ The total conversion value is 0.50 to 2.00%, Si oxide: SiO ２ The total conversion value is 1.00 to 3.00%, Mg oxides: 0.05 to 0.15% in total in terms of MgO, Al oxide: Al ２ O ３ Total conversion value is 0.50% or less Metal carbonate: 10.0 to 25.0%, and Metal fluoride: 2.0-8.0% 2. The covered electrode according to claim 1, further comprising:

3. The metal components contained in the steel core wire and the coating are expressed in mass% relative to the total mass of the covered electrode, Ni: 0.80% or less 2. The covered electrode according to claim 1, further comprising:

4. The metal components contained in the steel core wire and the coating are expressed in mass% relative to the total mass of the covered electrode, Mo: 0.30% or less 2. The covered electrode according to claim 1, further comprising:

5. 3. The covered electrode according to claim 2, wherein the metal carbonate is at least one selected from the group consisting of calcium carbonate, barium carbonate, magnesium carbonate, sodium carbonate, and manganese carbonate.

6. 3. The covered electrode according to claim 2, wherein the metal fluoride is at least one selected from the group consisting of fluorite, magnesium fluoride, aluminum fluoride, and barium fluoride.

7. 2. The covered electrode according to claim 1, wherein a coverage of the coating material is 22 to 37% of the total mass of the covered electrode.

8. applying a coating to the steel core wire; drying the steel core wire coated with the coating; A covered metal arc welding rod comprising: The components of the steel core wire and the coating material, and the coating rate of the coating material are selected so that the metal components contained in the steel core wire and the coating material contain, in mass % relative to the total mass of the covered electrode, C: 0.02 to 0.12%, Si: 0.50 to 2.50%, Mn: 0.50 to 2.50%, Sn: 0.05 to 0.50%, and Ti: 0.15 to 0.35%, with the remainder being iron and impurities. A method for manufacturing a covered electrode.

9. The coating agent comprises, in mass % based on the total mass of the coating agent: Ti oxide: TiO ２ The total conversion value is 0.50 to 2.00%, Si oxide: SiO ２ The total conversion value is 1.00 to 3.00%, Mg oxides: 0.05 to 0.15% in total in terms of MgO, Al oxide: Al ２ O ３ Total conversion value is 0.50% or less Metal carbonate: 10.0 to 25.0%, and Metal fluoride: 2.0-8.0% 9. The method for producing a covered electrode according to claim 8, further comprising the step of:

10. The metal components contained in the steel core wire and the coating are expressed in mass% relative to the total mass of the covered electrode, Ni: 0.80% or less The method for producing a covered electrode according to claim 8 or 9, further comprising:

11. The metal components contained in the steel core wire and the coating are expressed in mass% relative to the total mass of the covered electrode, Mo: 0.30% or less The method for producing a covered electrode according to claim 8 or 9, further comprising:

12. 10. The method for manufacturing a covered electrode according to claim 8, wherein the coating material contains water glass.

13. 10. The method for producing a covered electrode according to claim 9, wherein the metal carbonate is at least one selected from the group consisting of calcium carbonate, barium carbonate, magnesium carbonate, sodium carbonate, and manganese carbonate.

14. 10. The method for producing a covered electrode according to claim 9, wherein the metal fluoride is at least one selected from the group consisting of fluorite, magnesium fluoride, aluminum fluoride, and barium fluoride.

15. A method for manufacturing a welded joint, comprising a step of metal arc welding steel materials using the metal arc welding rod according to any one of claims 1 to 7.

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