Solid wire for gas shielded arc welding

Abstract

To provide a solid wire for gas shield arc-welding, which restricts blow holes although a galvanized steel material is gas shield arc-welded, and secures mechanical characteristics as a weld metal of a high-strength steel material, and additionally, which is superior in a weld bead electrodeposition coating property, and to provide a method for manufacturing a weld joint using the solid wire for gas shield arc-welding.SOLUTION: A solid wire for gas shield arc-welding contains 0.15-0.25% of C, 0.30-1.50% of Si, 0.50-2.50% of Mn, 0.015% or less of P, 0.0060% or less of S, 0.120-0.300% of Ti, other optional chemical elements as needed, and the balance consisting of Fe and impurities. The solid wire for gas shield arc-welding satisfies the formula (1) and the formula (2) below. Formula (1): 0.27≤C+Mn / 10.1≤0.43. Formula (2): 0.05≤Si / Mn2≤1.20. In the Formula (1) and Formula (2), an element symbol indicates a content (mass%) of a corresponding chemical element.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This disclosure relates to a solid wire for gas shielded arc welding.

[0002] Gas shielded arc welding is widely used in various fields. For example, in the automotive sector, it is used for welding car bodies and car parts (such as suspension components). Solid wire is used in gas shielded arc welding in the automotive sector.

[0003] For example, Patent Document 1 discloses a solid wire for gas shielded arc welding using a shielding gas, and for welding galvanized steel sheets, wherein the solid wire contains predetermined amounts of C, Si, Mn, P, S, O, and Cr relative to its total mass, with the remainder being Fe and unavoidable impurities, satisfying 1.0 ≤ (Si mass% + Mn mass%) / {100 (S mass% + O mass%)} ≤ 4.0 and 0.50 ≤ Mn mass% / Si mass% ≤ 2.00, and the shielding gas is Ar gas containing 25-40% CO2 gas.

[0004] Furthermore, Patent Document 2 discloses "a welding wire used in gas shielded arc welding of galvanized steel sheets, which contains C: 0.02~0.05 mass, Si: 0.20~0.70 mass, Mn: 1.0~2.0 mass, Cr: 0.10~0.60 mass, P: 0.008~0.020 mass, S: 0.008 mass% or less, K: 0.0001~0.0030 mass, Ca: 0.0010 mass% or less, and satisfies the following conditions for Si content, Mn content, and Cr content: 1.5 ≤ [Si] + [Mn] ≤ 2.5, 0.6 ≤ [Si] + 3 [Cr] ≤ 2.0, and 2.0 ≤ [Mn] / [Si]."

[0005] Furthermore, Patent Document 3 discloses "a solid wire for welding galvanized steel sheets, containing C: 0.02 to 0.10 wt%, Si: 0.3 to 0.7 wt%, and Mn: 1.5 to 3.0 wt% as basic alloy components, for pulsed MAG arc welding of galvanized steel sheets."

[0006] Furthermore, Patent Document 4 discloses a "solid wire whose chemical composition, in mass%, is C: 0.03~0.15%, Si: greater than 0% and less than or equal to 0.29%, Mn: 0.5~2.8%, Ti: 0.10~0.30%, Al: 0.003~0.30%, Sn: 0.02~0.40%, P: greater than 0% and less than or equal to 0.015%, S: greater than 0% and less than or equal to 0.030%, B: 0~0.0100%, Cr: 0~1.5%, Ni: 0~3.0%, Mo: 0~1.0%, Nb: 0~0.3%, V: 0~0.3%, Cu: 0~0.50%, with the remainder being iron and impurities, and the content of Si, Mn, Ti, and Al satisfying Si×Mn≦0.30 and (Si+Mn / 5) / (Ti+Al)≦3.0."

[0007] Furthermore, Patent Document 5 discloses "a solid wire for gas shielded arc welding for joining multiple thin steel plates by gas shielded arc welding, wherein the solid wire contains, by mass % of the total wire mass, C: 0.05~0.20%, Si: 0.01~0.18%, Mn: 1.0~3.0%, Ti: 0.06~0.25%, Al: 0.003~0.10%, B: 0~0.0100%, P: greater than 0~0.015%, S: greater than 0~0.015%, and an arbitrary element, with the remainder being iron and impurities, satisfying Si×Mn≦0.30 and (Si+Mn / 5) / (Ti+Al)≦3.0, and further having a Ceq of 0.40~0.90%."

[0008] Furthermore, Patent Document 6 states that, in mass % of the total mass of the wire including plating, it contains C: 0.03~0.15%, Si: 0.2~0.5%, Mn: 0.3~0.8%, P: 0.02% or less, S: 0.02% or less, Al: 0.1~0.3%, Ti: 0.001~0.2%, Cu: 0~0.5%, Cr: 0~2.5%, Nb: 0~1.0%, V: 0~1.0%, with the remainder being Fe and impurities, and the following The disclosed solid wire for gas shielded arc welding has a value of X within the range of 1.5 to 3.5% by mass. The weld metal is a weld metal in which the value of X in the following formula is within the range of 1.0 to 4.0%. The disclosed invention also includes welded joints, welded members, welding methods, and methods for manufacturing welded joints using these solid wires or weld metals. The formula X = 2 × [Si] + [Mn] + 3 × [Ti] + 5 × [Al] is disclosed. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2013-184216 [Patent Document 2] Japanese Patent Publication No. 2004-136342 [Patent Document 3] Japanese Patent Application Publication No. 8-309533 [Patent Document 4] Japanese Patent Publication No. 2021-3717 [Patent Document 5] Japanese Patent Publication No. 2021-3732 [Patent Document 6] International Publication No. 2020 / 196869 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] In addition to high-strength steel sheets of 590 MPa and 780 MPa, ultra-high-strength steel sheets of 980 MPa, 1180 MPa, 1470 MPa, or even exceeding these strengths, are now being put into practical use as steel sheets for automobile bodies. Furthermore, to ensure corrosion resistance, steel sheets for automobile bodies are coated with zinc-based plating. In addition, electrocoating is applied to automobile bodies.

[0011] Existing solid wires for gas shielded arc welding (hereinafter also referred to as "solid wires") include products designed for welding high-strength steel plates of the 980 MPa class. These solid wires are designed for multi-layer welding and often contain large amounts of alloying elements such as Mo and Ni.

[0012] Using these high-strength solid wires for welding high-strength steel sheets for automobiles can cause excessive hardening of the weld metal, leading to delayed fracture. Furthermore, the inclusion of alloying elements such as Mo and Ni increases the cost of the solid wire base. Additionally, the process of drawing the wire from the base to the solid wire requires more heat treatments, degrading manufacturability.

[0013] On the other hand, in gas shielded arc welding of automobile bodies, lap fillet welding is often employed. When zinc-plated steel sheets, such as alloyed hot-dip galvanized steel, are lap fillet-welded, pores are generated. The zinc on the surface adjacent to the root of the weld bead is heated to near the melting point of the steel during welding. Since the boiling point of zinc is lower than that of steel, low-boiling-point plating components such as zinc adjacent to the root of the weld evaporate instantly. If the two steel sheets are tightly stacked at this time, the evaporated metal vapors such as zinc cannot escape away from the molten pool between the steel sheets and enter the molten pool as bubbles. Then, as the supply of zinc and other vapors continues, the bubbles grow in the molten pool, leaving large pores after solidification is complete. In welded joints of high-strength steel sheets, these pores significantly reduce the mechanical properties of the welded joint compared to welded joints of mild steel sheets.

[0014] Furthermore, even when there is a gap between steel plates, if the molten steel flowing into the gap covers the plating surface, the low-boiling plating components sandwiched between the molten steel and the steel plate will instantly evaporate, forming bubbles in the melting pool. Therefore, the method of providing a gap between steel plates is not sufficient in terms of its effect. In order to obtain sufficient mechanical properties in the welded joints of high-strength steel plates, it is necessary to suppress such pores caused by metal vapor.

[0015] Solid wires capable of suppressing pores generated by such zinc plating are disclosed in Patent Documents 1 to 3. However, these solid wires are designed for welding mild steel plates. When used for welding high-strength steel plates, the welded metal becomes softer than the base material, and as a joint of high-strength steel plates, it cannot exhibit sufficient strength. That is, it becomes an undermatch joint and breaks at the weld bead in the tensile test.

[0016] By the way, in gas shielded arc welding, slag is generated on the surface and the stop end of the weld bead. The slag is an oxide formed by the reaction of deoxidizing components contained in the solid wire or the melting pool, such as Si and Mn, with the oxygen content in the shielding gas (oxygen content such as CO2 and O2). Among the slag, the slag of Si-Mn based oxide has poor conductivity and is difficult to be electrodeposited. Therefore, in the welded metal, the part where the slag of Si-Mn based oxide is generated becomes the starting point of rusting and reduces the corrosion resistance.

[0017] In order to address this issue, solid wires are disclosed in Patent Documents 4 and 5. However, the solid wire disclosed in Patent Document 4 is targeted at relatively low-strength steel plates, and when used for high-strength steel plates, it will become a strong undermatch joint. Patent Document 5 mainly targets hot-rolled steel plates for chassis with a reduced Si content, and discloses a solid wire containing Ti with a low Si content, which suppresses the generation of slag of Si-Mn based oxide with poor conductivity and enables electrodeposition. In addition, although the solid wire disclosed in Patent Document 5 is designed to be applicable to high-strength steel sheets, its applicability to galvanized steel sheets is insufficient, like the solid wire disclosed in Patent Document 4.

[0018] Furthermore, although Patent Document 6 discloses the composition of a weld metal suitable for automotive parts, its applicability to galvanized steel sheets is insufficient, and no solid wire capable of realizing such a weld metal has been clarified at all.

[0019] Therefore, an object of the present disclosure is to provide a solid wire for gas shielded arc welding that suppresses pores even when a zinc-based plated steel material is subjected to gas shielded arc welding, ensures mechanical properties as a weld metal for high-strength steel materials, and is excellent in electrodeposition coating properties of the weld bead.

Means for Solving the Problem

[0020] The means for solving the problem includes the following aspects. <1> By mass%, C: 0.15 to 0.25%, Si: 0.30 to 1.20%, Mn: 0.50 to 2.50%, P: 0.010% or less, S: 0.0060% or less, Ti: 0.120 to 0.300%, [[ID=3​​​​​​​​​​​​​​​​​​​​​​​​The remainder consists of Fe and impurities. A solid wire for gas shielded arc welding having a chemical composition that satisfies the following formulas (1) and (2). Formula (1) 0.27≦C+Mn / 10.1≦0.45 Formula (2) 0.10≦Si / Mn 2 ≤1.20 In formulas (1) and (2), the element symbols indicate the content (mass%) of the corresponding element. <2> In mass%, V: 0.001~0.070% Al: 0.001~0.005%, and Cu: 0.01~0.50%, Includes one or more of the following: <1> Solid wire for gas shielded arc welding as described above. <3> The content of C is 0.16 to 0.20%. <1> or <2> Solid wire for gas shielded arc welding as described above. <4> The Si content is 0.50 to 0.70%. <1> ~ <3> A solid wire for gas shielded arc welding as described in any one of the items. <5> The Mn content is 1.50 to 2.20%. <1> ~ <4> A solid wire for gas shielded arc welding as described in any one of the items. <6> The content of S is 0.0040% or less. <1> ~ <5> A solid wire for gas shielded arc welding as described in any one of the items. [Effects of the Invention]

[0021] According to this disclosure, it is possible to provide a solid wire for gas shielded arc welding that suppresses porosity when zinc-plated steel is gas-shielded arc-welded, ensures the mechanical properties of high-strength steel as weld metal, and further exhibits excellent electrodeposition coating properties for the weld bead. [Modes for carrying out the invention]

[0022] An example of an embodiment of this disclosure will be described. In this specification, a numerical range represented by "~" means a range that includes the numbers before and after "~" as the lower and upper limits, unless those numbers are preceded by "greater than" or "less than". If the numbers before and after "~" are preceded by "greater than" or "less than", the numerical range means a range that does not include those numbers as the lower or upper limit. In the numerical ranges described stepwise in this specification, the upper limit of one stepwise numerical range may be replaced with the upper limit of another stepwise numerical range, or with the values ​​shown in the examples. Similarly, the lower limit of one stepwise numerical range may be replaced with the lower limit of another stepwise numerical range, or with the values ​​shown in the examples. Furthermore, regarding the content, "%" means "mass%". A percentage of "0" indicates that the component is optional and does not need to be included.

[0023] <Solid wire> The solid wire for gas shielded arc welding related to this disclosure (hereinafter also simply referred to as "solid wire") has a predetermined chemical composition as described below. The solid wire described herein suppresses porosity even when zinc-plated steel is gas-shielded arc welded, ensures the mechanical properties of high-strength steel as weld metal, and exhibits excellent electrodeposition coating properties for weld beads. The solid wire described herein was discovered through the following findings.

[0024] The inventors conducted welding tests using various commercially available and prototype solid wires and diligently studied the process.

[0025] Specifically, the inventors first investigated the mechanical strength of weld metal formed by welding. As a result, they obtained the following findings. For example, high-strength steel sheets used in automobile bodies contain many elements such as C and Mn that enhance hardenability. Therefore, if solid wire intended for mild steel is used to weld high-strength steel sheets, the martensite ratio in the weld metal will be low, and the mechanical strength of the weld metal cannot be ensured. Therefore, the inventors considered optimizing the martensite ratio of the weld metal and ensuring the mechanical strength of the weld metal constituting the welded joint by using a solid wire with increased C and Mn content, which has not been put into practical use in the past.

[0026] Strengthening elements such as C and Mn are effective in strengthening the weld metal structure, but they are tempered and lose strength when reheated. However, in fillet welding of thin steel plates using a single pass, for example, the cooling rate of the weld metal is fast and it is sufficiently hardened. Also, since there is no reheating by subsequent passes that occurs in multi-layer welding, it is not tempered. Therefore, by setting the carbon equivalent ("C + Mn / 10.1" value) within an appropriate range, along with the C and Mn content, sufficient hardenability can be ensured without including alloying elements such as Mo, Ni, Cr, and B, or with only small amounts of them, thereby optimizing the martensite ratio of the weld metal. This allows for the formation of a weld metal with a martensite-dominant structure mixed with bainite, ensuring the mechanical properties of the weld metal.

[0027] Next, the inventors investigated the formation of slag with poor conductivity, which causes deterioration of the electrodeposition coating properties of weld metal. As a result, they obtained the following findings, combined with prior knowledge. The Si and Mn content suitable for welding is within a range where Fe-Si-Mn oxides, which are formed as slag with poor conductivity, can float to the surface of the molten pool while agglomerating in the liquid phase, thereby sufficiently deoxidizing the molten steel. This range is Si / Mn 2 Expressed as the value of Si / Mn 2 If <0.40 and Mn is 1.00 or greater, then 0.05 <Si / Mn 2 That is the case. In the composition ranges of many commercially available solid wires (generally Si: 0.40 - 1.00%, Mn: 1.20 - 1.80), the "Si / Mn" 2 " value is generally 0.14 ≤ Si / Mn 2 ≤ 0.60, and it is made in a composition range where slag with poor cohesiveness mainly composed of Si - Mn oxides is likely to aggregate. Also, Mn assists the oxidation reaction of Si as a co - deoxidizer, and the smaller the "Si / Mn" 2 " value, the more likely the slag of Si - Mn oxides with poor conductivity aggregates and adheres to the weld bead surface as large slag, causing defects in electrodeposition coating. On the other hand, when the "Si / Mn" 2 " value exceeds the appropriate range and becomes large, solid - phase Si - based oxides are generated as slag with poor conductivity, and small slag with low conductivity is formed on the weld bead surface, causing defects in electrodeposition coating. Also, SiO₂ is likely to be left in the molten steel, which is not favorable for the mechanical properties of the weld metal.

[0028] By the way, for high - strength steel sheets for automobile bodies, when indicated using the "Si / Mn" 2 " value as an index, many fall within the range of 0.01 ≤ Si / Mn 2 ≤ 0.30. Although there is deoxidation consumption in the chemical composition of the weld metal, in one - pass welding widely used in automobile manufacturing, it is close to the average value of the chemical composition of the solid wire and the steel sheet composition. When using conventional solid wires, the "Si / Mn" 2 " value remains within the range where slag is likely to aggregate and float. By the way, the welding of high - strength steel sheets for automobile bodies is one - pass welding with a relatively low current and a shallow molten pool. Therefore, even if "Si / Mn" 2 " slightly exceeds 0.6, it is difficult for a large amount of SiO₂ to be contained in the weld metal. Rather, it is preferable not to form large - aggregated slag with inferior conductivity. Therefore, compared with conventional solid wires, Si is allowed to be contained within a range where the "Si / Mn" 2 " value does not become too large, so that slag with inferior conductivity does not aggregate too much. Furthermore, by limiting the component ranges of Si and Mn, and incorporating Ti into the solid wire, the formation of slag with good conductivity is promoted, while the formation of slag with poor conductivity is suppressed. This suppresses the formation of slag with poor conductivity while generating slag with high conductivity, thereby improving the electrodeposition coating properties of the weld metal.

[0029] Next, the inventors investigated the mechanism of pore generation when zinc-plated steel is gas-shielded arc welded. As a result, they obtained the following findings. The boiling point of zinc is lower than the melting point of iron. Therefore, zinc vapor exists as bubbles in molten steel, attempting to rise vertically against gravity. At this time, the size of the bubbles is mainly determined by the balance between the internal pressure from the zinc vapor and the surface tension of the molten steel. The surface tension of molten steel is higher at lower temperatures, reaching its maximum just before solidification. However, when sulfur (S) dissolves in molten steel, its surface tension decreases, reducing the pressure that crushes the bubbles, resulting in larger zinc vapor bubbles. Conversely, lowering the sulfur content of the molten steel makes bubble growth more difficult.

[0030] There are two approaches to controlling porosity in molten steel by controlling its surface tension. One is to lower the surface tension as much as possible to allow bubbles to grow and easily detach from the molten pool, while the other is to raise the surface tension as much as possible to suppress the generation and growth of bubbles within the molten pool, thereby suppressing porosity. To maintain high surface tension in molten steel, the amount of surface-active elements such as O and S in the molten steel should be reduced. To reduce the amount of O dissolved in the molten steel, sufficient deoxidizing elements are needed. This can be easily achieved if there is no shortage of deoxidizing elements such as Si, Mn, and Ti. On the other hand, suppressing zinc vapor-induced porosity by reducing the S content has only yielded limited results when actually applied to steel sheets for automobile bodies. This is because low-strength steel sheets have a high S content, and even if the S content of the solid wire is reduced, the S content of the weld metal does not decrease significantly, and the effect of suppressing porosity in lap fillet welds is limited.

[0031] However, due to the demand for lighter automobile bodies, in addition to high-strength steel sheets of 590 MPa and 780 MPa, ultra-high-strength steel sheets of 980 MPa, 1180 MPa, 1470 MPa, or even stronger than these, have been put into practical use as steel sheets for automobile bodies. In these high-strength steel sheets, the amount of S decreases as the strength increases. Therefore, we have found that in ultra-high-strength steel sheets with a tensile strength exceeding 980 MPa and excellent ductility, reducing the sulfur content of the solid wire sufficiently lowers the sulfur content of the weld metal to a level where porosity can be suppressed, maintaining a high surface tension in the molten steel, and making it difficult for bubbles to form and grow.

[0032] Based on the above findings, it has been found that the solid wire relating to this disclosure suppresses porosity even when gas-shielded arc welding zinc-plated steel materials, ensures the mechanical properties of high-strength steel materials as weld metal, and further exhibits excellent electrodeposition coating properties for weld beads.

[0033] (Chemical composition of solid wire) The following provides a detailed explanation of the chemical composition of solid wire. In the chemical composition of solid wires, "%" means "mass % of the total mass of the solid wire's chemical composition" unless otherwise specified.

[0034] (C: 0.15~0.25%) Carbon (C) is a strengthening element that enhances hardenability and adds strength to the weld metal. If the carbon content is too low, sufficient hardenability cannot be obtained, and the mechanical properties of the weld metal will not be satisfactory. If the carbon content is too high, the weld metal will become a full martensitic structure and harden excessively. Furthermore, the martensitic transformation temperature (Ms point) will decrease, preventing auto-tempering from progressing, resulting in a weld metal with low toughness. Therefore, the amount of C should be set to 0.15 to 0.25. The lower limit of the C content is preferably 0.16% or higher. The upper limit for the amount of C is preferably 0.22% or less, or 0.20% or less.

[0035] (Si: 0.30~1.20%) Si is a deoxidizing element that generates Si oxide (i.e., slag with poor conductivity), which is the source of electrodeposition coating defects. However, simply reducing the amount of Si does not reduce electrodeposition coating defects because other deoxidizing elements such as Mn are also present. The slag with poor conductivity that forms on the surface of the weld bead tends to decrease as the amount of Si increases because it becomes less likely to aggregate, while the slag with poor conductivity that forms at the toe of the weld bead tends to increase as the amount of Si increases. In other words, if the amount of Si is too low, the amount of slag with poor conductivity increases on the surface of the weld bead, and if the amount of Si is too high, the amount of slag with poor conductivity increases at the toe of the weld bead. In addition, Si is a strong deoxidizing element that suppresses the CO reaction in molten droplets formed at the tip of solid wires, thereby preventing an increase in sputtering. Since sputtering tends to increase when the carbon content is high (0.15% or more), an appropriate amount of Si is necessary to suppress sputtering. Therefore, the Si content should be 0.30 to 1.20%. The lower limit of the Si content is preferably 0.40% or more, or 0.50% or more. The upper limit for the Si content is preferably 1.00% or less, or 0.70% or less.

[0036] (Mn: 0.50~2.50%) Mn is a strengthening element that enhances hardenability and adds strength to the weld metal. However, Mn is also a deoxidizing element, and it is the element that generates Mn oxides (i.e., slag with poor conductivity) that result in defective areas in electrodeposition coatings. If the Mn content is too low, sufficient hardenability cannot be obtained, and the necessary mechanical properties of the weld metal cannot be achieved. If the Mn content is too high, the weld metal will form a full martensitic structure, the Ms point will decrease, and auto-tempering will not proceed, resulting in a weld metal with low toughness. In addition, the amount of slag with poor conductivity will increase on the surface of the weld bead. Therefore, the amount of Mn should be 0.50 to 2.50%. The lower limit of the Mn content is preferably 1.00% or more, or 1.50% or more. The upper limit of the Mn content is preferably 2.30% or less, or 2.20% or less.

[0037] (P:0.010% or less) P is an impurity that causes high-temperature cracking in weld metal, and it is an element that should be reduced as much as possible. Therefore, the amount of P should be 0.010% or less. The upper limit for the amount of P is preferably 0.008% or less, or 0.006% or less. However, while ideally the amount of phosphorus (P) should be 0%, it may be greater than 0% or 0.001% or more from the viewpoint of the cost and productivity of P removal.

[0038] (S :0.0060% or less) S is a surface-active element that lowers the surface tension of molten steel. In ultra-high-strength steel sheets, the high content of Si and Mn, which are strong deoxidizing agents, and the low content of S, tend to keep the amount of O, a surface-active element that lowers the surface tension of molten steel, low in the molten pool. Furthermore, reducing the S content of solid wire effectively reduces the S content in the molten pool, maintaining a high surface tension of the molten steel. As a result, bubbles are less likely to form and grow in the molten pool. Therefore, the amount of S should be 0.0060% or less. The upper limit of the S content is preferably 0.0040% or less, or 0.0030% or less. However, while ideally the sulfur content should be 0%, it may be greater than 0% or 0.0005% or more from the viewpoint of the cost and productivity of sulfur removal.

[0039] (Ti: 0.120~0.300%) Ti is a deoxidizing element and is the element that produces Ti oxide slag. However, Ti oxide has some degree of conductivity. Therefore, unlike Si-Mn oxides that do not contain Ti (i.e., slags with inferior conductivity), slags containing Ti produce slags with relatively high conductivity. Therefore, by incorporating Ti into the solid wire and generating Ti-containing oxides on the molten pool surface, the amount of Ti-free Si-Mn oxides produced decreases, and the amount of conductive slag increases, thereby improving the electrodeposition properties of the weld bead. On the other hand, if the amount of Ti is too high, the mechanical properties of the weld metal will deteriorate. Therefore, the Ti content should be 0.120 to 0.300%. The lower limit of the Ti content is preferably 0.130% or higher. The upper limit for the amount of Ti is preferably 0.280% or less.

[0040] (V : 0~0.100%) V is an optional element that may be included in the solid wire. In other words, the V content may be 0%. V is a deoxidizing element and is the element that produces V oxides. However, unlike Si-Mn oxides, which have inferior conductivity, V oxides form oxides with high conductivity. Furthermore, V improves the conductivity of Ti oxides by forming composite oxides with Ti oxides that have relatively low conductivity, such as Ti2O3. Therefore, when V is included in a solid wire along with Ti, V forms a highly conductive V-composite oxide, which can improve electrodeposition coating properties. On the other hand, if the amount of V is too high, the mechanical properties of the weld metal will deteriorate. Therefore, the amount of V is set to 0-0.100%. The lower limit of the V amount is preferably 0.010% or higher. The upper limit of the V amount is preferably 0.070% or less, or 0.050% or less.

[0041] (Al: 0~0.010%) Al is an optional element that may be included in solid wire. In other words, the Al content may be 0%. Al is a deoxidizing element and is the element that produces Al oxide slag. Furthermore, incorporating Al into a solid wire and generating Al oxide reduces the amount of O in the molten steel, thereby reducing the amount of Si-Mn oxides, which have poor conductivity. Although Al oxide also has poor conductivity, it does not aggregate, so it can improve the electrodeposition properties of the weld bead. However, if the amount of Al is too high, it will have the effect of reducing the electrodeposition properties of the weld bead. In addition, Al oxide that does not rise to the surface of the weld bead remains in the weld metal, reducing its mechanical properties. Therefore, the amount of Al should be 0-0.010%. The lower limit of the amount of Al is preferably 0.001% or higher. The upper limit for the amount of Al is preferably 0.005% or less, or 0.004% or less.

[0042] (Cu: 0~0.50%) Cu is an optional element that may be included in solid wire. In other words, the Cu content may be 0%. Solid wires are often copper-plated to stabilize their wire feeding and power supply properties. Therefore, copper plating results in a copper (Cu) content in the solid wire. However, too much Cu can increase the likelihood of welding cracks. Therefore, the amount of Cu should be between 0 and 0.50%. The lower limit of the Cu content is preferably 0.01% or more, or 0.02% or more. The upper limit for the amount of Cu is preferably 0.40% or less, or 0.30% or less.

[0043] (Se:0~0.004%, Bi:0~0.004, O:0~0.010%) Se, Bi, and O are optional elements that may be included in the solid wire. In other words, the content of Se, Bi, and O may each be 0%. Se, Bi, and O, like S, are surface-active elements that lower the surface tension of molten steel. Therefore, even when solid wire contains one or more of Se, Bi, and O, reducing the amounts of Se, Bi, and O within the above range maintains a high surface tension in the molten steel. This makes it difficult for bubbles to form and grow in the molten steel. Therefore, the amounts of Se and Bi are set to 0-0.004%, and the amount of O is set to 0-0.010%. The upper limits for Se and Bi are preferably 0.001% or less, and the upper limit for O is preferably 0.008% or less.

[0044] (N: 0~0.0030%) N is an optional element that may be included in the solid wire. In other words, the N content may be 0%. N is often included in small amounts due to degassing costs and production considerations, and in small amounts, it is a harmless element. However, too much nitrogen can embrittle the weld metal. Therefore, the amount of N is set to 0 to 0.0030%. The lower limit of the amount of N is preferably 0.0010% or more. The upper limit for the amount of N is preferably 0.0025% or less.

[0045] (Ni:0~0.010%, Cr:0~0.010%, Mo:0~0.010%, Nb:0~0.001, B:0~0.0060%) Ni, Cr, Mo, Nb, and B are optional elements that may be included in the solid wire. In other words, the content of Ni, Cr, Mo, Nb, and B may each be 0%. Ni, Cr, Mo, Nb, and B are strengthening elements that enhance hardenability and impart strength to the weld metal. However, since the solid wire according to this disclosure has a high carbon and manganese content and high hardenability, it is not necessary to actively include Ni, Cr, Mo, Nb, and B to improve the mechanical properties of the weld metal. Even when Ni, Cr, Mo, Nb, and B are included, it is preferable that the amounts of Ni, Cr, Mo, Nb, and B are small. On the other hand, if the amounts of Ni, Cr, Mo, Nb, and B are too high, the weld metal will become a full martensitic structure, and the martensitic transformation temperature will decrease, resulting in a weld metal with low toughness. Therefore, the amounts of Ni, Cr, Mo, and Nb should each be 0-0.010%, and the amount of B should be 0-0.0060%. The upper limits for Ni, Cr, Mo, and Nb are preferably 0.005% or less, and the upper limit for B is preferably 0.0020% or less.

[0046] (Remainder: Fe and impurities) The remainder of the chemical composition of the solid wire relating to this disclosure is Fe and impurities. The remaining impurities refer to components that are introduced during the industrial manufacturing of solid wire, either originating from the raw materials or due to various factors in the manufacturing process, and which are acceptable as long as they do not adversely affect the solid wire.

[0047] (Formula (1)) The solid wire relating to this disclosure satisfies the following formula (1). Formula (1) 0.27≦C+Mn / 10.1≦0.45 In formula (1), the element symbols indicate the content (mass%) of the corresponding element.

[0048] In equation (1), the value "C + Mn / 10.1" is the carbon equivalent that gives the hardness, i.e., tensile strength, of the weld metal of the ultra-high-strength steel sheet, which is the main target, assuming a single-pass weld. Even if the amount of C and Mn are restricted individually, for example, when welding ultra-high-strength steel sheets of 980 MPa class or higher, it is not always possible to form weld metal with appropriate mechanical properties. Therefore, by limiting the carbon equivalent value "C+Mn / 10.1" to an appropriate range as an indicator of the hardness, or strength, of the weld metal, it is ensured that weld metal with appropriate mechanical properties can be formed. Generally, when ultra-high-strength steel plates are welded, softening occurs in the heat-affected zone (HAZ). However, if the "C+Mn / 10.1" value is too small, the weld metal will be softer than the softened HAZ, resulting in a large undermatch and insufficient joint efficiency. On the other hand, if the "C+Mn / 10.1" value is too large, the weld metal will become too hard, raising concerns about delayed fracture. Therefore, the value of "C+Mn / 10.1" should be between 0.27 and 0.45. In other words, C and Mn are included in the solid wire in such a way that equation (1) is satisfied. The lower limit of the "C+Mn / 10.1" value is preferably 0.30% or higher, or 0.33% or higher. The upper limit of the "C+Mn / 10.1" value is preferably 0.40% or less, or 0.37% or less.

[0049] (Formula (2)) The solid wire relating to this disclosure satisfies the following formula (2). Formula (2) 0.10≦Si / Mn 2 ≤1.20 In formula (2), the element symbols indicate the content (mass%) of the corresponding element.

[0050] In equation (2), "Si / Mn 2 The "value" is an indicator of the intensity of the oxidation reaction between Si and Mn. Mn acts as a coordinating deoxidizer, assisting in the oxidation reaction of Si. 2 If the value is small, slag with poor conductivity will aggregate. On the other hand, "Si / Mn 2 A higher value suppresses slag aggregation. However, solid-phase Si oxide is generated, which can lead to the formation of small, low-conductivity slag particles on the weld bead surface or make them more likely to remain in the weld metal. Therefore, "Si / Mn 2 The value should be between 0.10 and 1.20. In other words, Si and Mn should be included in the solid wire to satisfy equation (2). "Si / Mn 2 The lower limit of the value is preferably 0.15 or higher. "Si / Mn 2 The upper limit of the value is preferably 0.60 or less.

[0051] [Method for manufacturing welded joints] Using the solid wire for gas shielded arc welding according to this disclosure, porosity can be suppressed even when welding zinc-plated steel materials using gas shielded arc welding, ensuring the mechanical properties of high-strength steel materials as weld metal, and furthermore, a weld bead with excellent electrodeposition coating properties can be obtained. In particular, because the carbon and manganese content of the weld metal falls within an appropriate range depending on the strength of the steel plate, a welded joint can be obtained that has a weld metal that is neither under-matched nor excessively over-matched.

[0052] It is well known that reducing the oxygen source in the shielding gas is an effective way to reduce slag. When performing gas shielded arc welding using the solid wire according to this disclosure, it is preferable to use an Ar gas as the shielding gas, which is a mixture of one or two types from 5 to 20 volume% CO2 or 1 to 5 volume% O2, in order to reduce slag and further improve electrodeposition coating properties. When gas shielded arc welding is performed using a shielding gas with a high Ar mixing ratio, the generation of slag with poor conductivity is further suppressed. As a result, the electrodeposition coating properties of the weld bead can be further improved. In addition, the arc widens and the weld bead becomes flatter, reducing stress concentration.

[0053] An example of the main application of the solid wire relating to this disclosure is arc welding of automobile bodies. Specifically, examples of welded joints include fillet joints, butt joints, etc., manufactured in a single pass of welding. The number of steel plates to be welded may be one or multiple.

[0054] The steel material to be welded is preferably a relatively thin steel plate (thin steel sheet) with a thickness of about 0.8 mm to 3.6 mm, which allows for the manufacture of welded joints in a single pass of welding.

[0055] In particular, zinc-plated high-strength steel sheets with high amounts of elements that enhance hardenability, such as C and Mn, and low amounts of S (for example, ultra-high-strength steel sheets with tensile strengths of 980 MPa, 1180 MPa, 1470 MPa, or higher, in accordance with JIS Z2241:2011) are preferred as the steel material. By using high-strength steel sheets as the steel sheet, welded joints with excellent mechanical properties can be obtained. Of course, the application to lower-strength steel sheets or unplated steel sheets is not excluded.

[0056] Specifically, suitable examples of zinc-plated high-strength steel sheets for use as steel materials include steel sheets having the following chemical composition. In mass%, C: 0.13~0.30%, Si: 0.30~2.00%, Mn: 1.50~2.80%, P: 0.004~0.015%, S: 0.0007~0.0040%, N: 0.0010~0.0070%, O: 0.001~0.008%, Al: 0-1.00%, Ti: 0~0.050%, B: 0~0.0050%, Cr: 0~1.00%, Mo: 0~0.50%, Cu: 0~0.50%, Ni: 0~0.50%, Co: 0~0.50%, W: 0~0.50%, Sn: 0~0.50%, Sb: 0~0.50%, Nb: 0~0.050%, V: 0~0.50%, Ca: 0~0.010%, Mg: 0~0.010%, Ce: 0~0.010%, Zr: 0~0.010%, La: 0~0.010%, Hf: 0~0.010%, Bi: 0~0.010%, REM: 0~0.010%, and Remainder: Steel sheet having a chemical composition consisting of Fe and impurities.

[0057] The main zinc-plated high-strength steel sheets used in automobiles include steel sheets having a zinc-plated layer, specifically, hot-dip galvanized steel sheets (i.e., steel sheets having a plating layer mainly composed of zinc, so-called GI steel sheets) and alloyed hot-dip galvanized steel sheets (i.e., steel sheets having a plating layer in which a zinc-plated layer mainly composed of zinc is formed, and then heated to alloy the iron of the steel sheet with the plating layer, so-called GA steel sheets).

[0058] Furthermore, examples include hot-dip galvanized steel sheets with zinc-aluminum-magnesium plating, zinc-aluminum-magnesium-silicon plating, zinc-aluminum plating, zinc-aluminum-silicon plating, etc., and electro-galvanized steel sheets having a plating layer mainly composed of zinc.

[0059] The welding targets using the solid wire described herein are not limited to zinc-plated steel sheets, but may also include plated steel sheets other than zinc-plated steel sheets (e.g., aluminum-plated steel sheets, pre-Ni plated steel sheets), unplated steel sheets, etc. Typical steel types include composite structure steel and TRIP steel, with typical tensile strengths ranging from 980 MPa to 1470 MPa. Furthermore, the material to be welded using the solid wire according to this disclosure may be martensitic steel. In martensitic steel and hot-stamped steel sheets, arc welding causes significant softening in the heat-affected zone. Therefore, the strength of the weld metal does not need to be equal to or greater than that of the base metal; it only needs to be higher than the strength of the softened area. In high-strength steel sheets used in automobiles, softening occurs in the heat-affected zone during welding. Therefore, by selecting appropriate welding conditions, it is sometimes possible to avoid fracture in the weld metal, or high joint strength may not be required at all. Consequently, the tensile strength of the steel sheet is not limited to 1470 MPa or less.

[0060] With the solid wire described herein, even when welding plated steel sheets other than zinc-plated steel sheets, or unplated steel sheets, a welded joint can be obtained that has excellent weld metal properties while ensuring the mechanical properties of the weld metal and having excellent electrodeposition coating properties for the weld metal. Furthermore, the welding target is not limited to steel plates; other steel materials may also be used. These steel materials may include steel pipes, civil engineering and construction materials (such as fences, corrugated pipes, drainage ditch covers, sand-drift prevention plates, bolts, wire mesh, guardrails, and watertight walls), home appliance components (such as air conditioner outdoor unit casings), and automobile parts (such as undercarriage components), which are steel materials formed from steel plates. Examples of forming processes include various plastic deformation methods such as press working, roll forming, bending, and hot stamping. [Examples]

[0061] Next, the feasibility and effects of this disclosure will be described in more detail with reference to examples and comparative examples. However, the following examples are not limiting to this disclosure, and any design modifications made in accordance with the spirit of the preceding and following descriptions fall within the technical scope of this disclosure.

[0062] The raw materials were vacuum melted, forged, rolled, drawn, and annealed to form a solid wire with a product diameter of 1.2 mm. A portion of the solid wire surface was then copper-plated, and a 20 kg spool was used as a prototype. Table 1 shows the chemical composition and calculated values ​​of the prototype solid wire. Values ​​outside the scope of this disclosure are underlined. Components that are not present (or present in amounts too small to analyze) are left blank in the table.

[0063] [Table 1-1] [Table 1-2] [Table 1-3] [Table 1-4]

[0064] Next, alloyed hot-dip galvanized steel sheets having the chemical compositions shown in Table 2 were prepared. The sheet thickness was 1.6 mm, and the sheets were either composite structure steel or TRIP steel.

[0065] [Table 2]

[0066] Then, using the prototype solid wire under the conditions shown in Table 3, welded test specimens including lap fillet joints were fabricated by overlapping the ends of two steel plates and performing gas shielded arc welding. A DC power supply was used for welding, with a shielding gas flow rate of 20 L / min, a welding current of 160 Amp, a voltage of 17.4 V, and a welding speed of 60 cm / min. The following tests were performed on the fabricated welded test specimens.

[0067] (Tensile strength of welded joints) Tensile test specimens conforming to JIS Z2241:2011 were cut from the above welded test specimens and tensile tests were performed. The weld bead was positioned in the center of the parallel section of the tensile test specimen, with the tensile direction and the longitudinal direction of the bead perpendicular to each other. Furthermore, considering the conditions under which the product would actually be used in an automobile, the excess weld bead was not ground down. In the tensile test, the maximum load was measured, the joint efficiency (tensile strength of the joint / tensile strength of the base material (%)) was determined, and the fracture location was recorded. Specimens were classified as good if the fracture location was in the base material or heat-affected zone, and as bad if the fracture location was in the weld bead.

[0068] (Upper limit of hardness of weld metal) If the weld metal becomes too hard, the joint structure can strongly restrict the weld bead, potentially leading to delayed fracture due to hydrogen dissolved in the weld bead during welding. For this reason, small pieces for cross-sectional observation were cut from the weld test specimen, embedded in resin, and polished, and a micro-Vickers hardness test was performed on the weld metal. Delayed fracture is more likely to occur the stronger the weld bead restriction and the greater the amount of dissolved hydrogen. Therefore, it does not necessarily occur when the hardness of the weld metal exceeds a certain value. Based on experience, we classified weld metal hardness exceeding 470HV0.5 as exceeding the upper limit, indicating a possibility of delayed fracture. Furthermore, 430HV0.5, which is less prone to delayed fracture, was set as the baseline value, and weld metal hardnesses of 470HV0.5 or less, and above 430HV0.5 were classified as within the upper limit, while those of 430HV0.5 or less were classified as within the baseline value.

[0069] (Electrodeposition coating properties) After degreasing and chemical treatment of the weld test specimens, electrodeposition coating was applied to the entire weld bead to a calculated film thickness of 20 μm, and the coating was baked. The weld bead was then photographed, and the ratio of the area of ​​electrodeposition defects to the weld bead area was measured from the images. The weld bead length of the test specimen was 120 mm, and the electrodeposition defect rate was determined from a 90 mm length of the weld bead, excluding the 15 mm at the start and end of the weld. Gray paint was used for electrodeposition to identify areas of electrodeposition defects where reddish-brown or black slag was exposed. If the electrodeposition defect rate exceeded 5% by area, it was judged to be poor electrodeposition performance, and if it was 5% or less but greater than 3%, it was judged to be good. Furthermore, if the electrodeposition defect rate was 3% or less by area, it was judged to be excellent.

[0070] (Porrosion resistance) Welded joints were radiographed using X-ray radiography, and pores within a 90mm length of the weld bead (excluding the 15mm sections at the start and end of the weld) were measured. Pores were classified into three categories based on their longitudinal length: 0.5mm or less, between 0.5mm and 1.5mm, and 1.5mm. The number of pores was then counted. Representative lengths of 0.3mm, 1mm, and 1.5mm were then set, and the ratio of pore length to weld bead length (%) was calculated. Pores with a ratio exceeding 10% were classified as poor, those with a ratio of 10% or less and exceeding 7.5% were classified as acceptable, those with a ratio of 7.5% or less and exceeding 5% were classified as good, and those with a ratio of 5% or less were classified as excellent.

[0071] [Table 3-1] [Table 3-2]

[0072] From the above results, it can be seen that the solid wire of this embodiment suppresses porosity even when gas-shielded arc welding zinc-plated steel, ensures the mechanical properties of high-strength steel as weld metal, and exhibits superior electrodeposition coating properties for weld beads compared to the solid wire of the comparative example.

Claims

1. In mass percent, C: 0.16-0.25%, Si: 0.30-1.20%, Mn: 0.50 to 2.50%, P: 0.010% or less, S: 0.0060% or less, Ti: 0.120-0.300%, V: 0 to 0.100%, Al: 0.001-0.010%, Cu: 0 to 0.50%, Se: 0 to 0.004%, Bi: 0 to 0.004%, O: 0 to 0.010%, N: 0 to 0.0030%, Ni: 0 to 0.010%, Cr: 0 to 0.010%, Mo: 0 to 0.010%, Nb: 0 to 0.010%, B: 0-0.0060%, and, The remainder consists of Fe and impurities. A solid wire for gas shielded arc welding having a chemical composition that satisfies the following formulas (1) and (2). Formula (1) 0.27≦C+Mn / 10.1≦0.45 Equation (2) 0.10≦Si / Mn ２ ≤1.20 In formulas (1) and (2), the element symbols indicate the content (mass%) of the corresponding element.

2. In mass percent, V: 0.001~0.070% Al: 0.001-0.005%, and Cu: 0.01 to 0.50%, A solid wire for gas shielded arc welding according to claim 1, comprising one or more of the above.

3. The solid wire for gas shielded arc welding according to claim 1 or claim 2, wherein the content of C is 0.16 to 0.20%.

4. The solid wire for gas shielded arc welding according to any one of claims 1 to 3, wherein the Si content is 0.50 to 0.70%.

5. The solid wire for gas shielded arc welding according to any one of claims 1 to 4, wherein the Mn content is 1.50 to 2.20%.

6. The solid wire for gas shielded arc welding according to any one of claims 1 to 5, wherein the content of S is 0.0040% or less.

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