Welded member having excellent electrodeposition coating corrosion resistance at weld zone and automotive component having weld zone

Abstract

[Disclosure] [Technical Problem] An aspect of the present disclosure may provide a gas shielded arc welding member capable of securing excellent electrodeposition coating corrosion resistance of a weld zone in an automobile industrial field.SOLUTION: According to an aspect of the present disclosure, a high-strength hot-rolled steel sheet may include a base material and a weld zone, wherein the weld zone includes, by wt%, 0.001 to 0.30% of C, 1.00% or less (excluding 0%) of Si, 0.50 to 3.00% of Mn, 0.030% or less (excluding 0%) of P, 0.030% or less (excluding 0%) of S, 1.50% or less (excluding 0%) of Cr, 0.60% or less (excluding 0%) of Mo, less than 0.10% (excluding 0%) of Al, 0.40% or less (excluding 0%) of Ni, and less than 0.10% (excluding 0%) of Ti, in a normal weld zone excluding start and end portions of a weld bead corresponding to 25% of a total length of the weld bead or a length of a smaller value in a 15mm portion of the weld bead, an amount of Si in slag distributed along the end portion of the weld bead is 10% or less by wt% of the slag itself.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a welded component having excellent electrodeposition corrosion resistance at the welded portion, and more particularly to a welded component having excellent electrodeposition corrosion resistance at the welded portion of a gas-shielded arc weld, which can ensure excellent electrodeposition corrosion resistance at the welded portion and economic efficiency, which are essential when manufacturing parts, and an automotive part having such a welded portion. [Background technology]

[0002] In the automotive field, research into lightweighting technologies for car bodies and parts has emerged as a major issue due to fuel efficiency regulations related to environmental protection, including issues such as global warming. For these reasons, the application of high-strength steel materials to reduce the weight of chassis parts, which are important for the driving performance of automobiles, is also required.

[0003] To achieve such lightweight components, it is essential to increase the strength of the material, and it is important to ensure the durability of components made from high-strength steel in an environment where repeated fatigue loads are applied.

[0004] In the case of arc welding, which is mainly used to ensure strength when assembling automobile chassis parts, lap joints are formed between parts by welding a welding wire, so it is inevitable to give the joint a geometric shape, which acts as a repeated fatigue stress concentration point (notch effect) and becomes a fracture initiation point, resulting in a decrease in the durability of the part and a limitation in that the benefits of using high-strength steel are lost.

[0005] Therefore, to improve the fatigue properties of welds, it is most important to reduce the angle (toe angle) of the bead end, which is the area where stress is concentrated. In addition, controlling the material quality and stress of the toe area is also an important factor. As mentioned above, the trend toward higher strength and lighter weight components has led to thinner materials, which has increased demand for rust resistance to prevent through-hole corrosion. This has led to an increased use of plated steel. However, the weld metal of arc-welded parts, in particular, lacks a plated layer and has poorer corrosion resistance after painting than the base metal. This poses a problem: in the severe corrosive environment encountered during vehicle operation, premature corrosion occurs in the welds of chassis parts made of plated steel sheets, leading to reduced fatigue properties. Meanwhile, gas-shielded arc welding of plated steel involves the generation of zinc vapor, which can cause numerous porosity defects in the form of pits and blowholes in the weld bead, potentially reducing the strength of the weld and resulting in reduced welding productivity. Furthermore, in the case of general unplated steel materials, slag generated in the weld bead during gas-shielded arc welding causes coating defects and reduces corrosion resistance after coating. This poses a problem in that post-processing steps such as pickling or brushing to remove the slag after welding are required when manufacturing parts, resulting in increased costs.

[0006] Recently, there has been active development of environmentally friendly lightweight chassis parts for next-generation automobiles, and the development of welding technology that can improve the properties of welds while ensuring economic efficiency has become a particularly important issue. Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a gas-shielded arc-welded component capable of ensuring excellent electrodeposition corrosion resistance of welded portions in the automotive industry.

[0008] The object of the present invention is not limited to the above-mentioned matters. Further object of the present invention is described in the entire content of the specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the further object of the present invention from the content described in the specification of the present invention. [Means for solving the problem]

[0009] The present invention provides Includes the base material and the weld, The above weld zone contains, by weight, C: 0.001 to 0.30%, Si: 1.00% or less (excluding 0%), Mn: 0.50 to 3.00%, P: 0.030% or less (excluding 0%), S: 0.030% or less (excluding 0%), Cr: 1.50% or less (excluding 0%), Mo: 0.60% or less (excluding 0%), Al: less than 0.10% (excluding 0%), Ni: 0.40% or less (excluding 0%), Ti: less than 0.10%. The present invention relates to a welded component having excellent corrosion resistance to electrodeposition coating of a welded portion, in which the slag contains less than (excluding 0%) of Si, with the remainder consisting of Fe and other unavoidable impurities, and satisfies the following relational expressions 1 and 2, and in a normal weld, excluding the start and end portions of the weld bead corresponding to the smaller of 25% of the total length of the weld bead or 15 mm, the Si content of the slag distributed along the end of the weld bead is 10% or less by weight of the slag itself.

[0010] [Equation 1] 3.5≦[Si]×100 / [Mn]≦34.0 (In the above Relational Formula 1, [Si] and [Mn] represent the weight percent content of each element in parentheses in the weld.)

[0011] [Equation 2] [Ti]+[Al]<0.10 (In the above relational expression 2, [Ti] and [Al] represent the weight percent content of each element in parentheses in the weld.)

[0012] The welded portion may further contain, by weight percent, one or more of Nb: 0.10% or less, V: 0.10% or less, Zr: 0.10% or less, and B: 0.01% or less.

[0013] The welded portion may further contain, by weight %, Cu: 0.50% or less.

[0014] The welded portion may have an area ratio of silicon-based oxide slag of 1% or less of the slag formed at the welded portion.

[0015] Even if the above welds are subjected to electrodeposition coating without undergoing pickling, there is a possibility that corrosion weight loss will not occur at the welds after a 15-year combined corrosion test in accordance with the GMW14872 test method.

[0016] The base metal may contain, by weight, C: 0.04 to 0.18%, Si: 2.0% or less (including 0%), Mn: 0.5 to 3.0%, Cr: 2.0% or less (including 0%), Mo: 2.0% or less (including 0%), Al: 0.01 to 0.10%, P: 0.05% or less (excluding 0%), S: 0.05% or less (excluding 0%), with the remainder being Fe and other unavoidable impurities.

[0017] The base material may further contain, by weight percent, one or more of Ti: 0.20% or less, Nb: 0.10% or less, and Cu: 0.10% or less.

[0018] The base material may have a thickness of 0.8 to 4.0 mm.

[0019] The present invention also relates to an automobile part having the above welded portion. [Effects of the Invention]

[0020] According to the present invention, a next-generation welding technology that ensures performance / cost competitiveness in line with the popularization of electric vehicles can be effectively provided, and gas-shielded arc welds with excellent electrodeposition coating corrosion resistance at the welds can be produced.

[0021] The various beneficial advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0022] [Figure 1] (a) and (b) are an enlarged photograph (a) of the appearance of the end of the weld bead of Example 2, and an image (b) of the Si content distribution of the slag distributed along the end of the weld bead, analyzed by SEM EDS analysis, in an embodiment of the present invention. [Figure 2] 1(a) and 1(b) are a magnified photograph (a) of the appearance of the end of the weld bead of Comparative Example 2 in an example of the present invention, and an image (b) of the Si content distribution of the slag distributed along the end of the weld bead analyzed by SEM EDS analysis. [Figure 3] 1(a) and 1(b) are photographs showing the appearance of the weld bead after a corrosion test of Inventive Example 4 (a) and the appearance of the weld bead after a corrosion test of Comparative Example 4 (b) in Examples of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention will be described below.

[0024] The present invention is characterized by controlling the Si and Mn contents of the weld in order to ensure excellent electrodeposition paintability and corrosion resistance of the weld. By controlling the Si and Mn contents in this manner, the proportion of silicon-based oxide slag that deteriorates the electrodeposition paintability of the weld can be reduced, particularly the proportion of silicon-based oxide slag that has a yellowish-brown color and a glass-like luster at the edge of the weld bead, thereby effectively improving the electrodeposition paintability and corrosion resistance of the weld.

[0025] In addition, the present invention requires that the Ti and Al contents be controlled to ensure the ability to reduce silicon-based oxide slag that distributes along the edges of the weld bead, which can degrade the electrodeposition paintability and corrosion resistance of the weld. By controlling the Ti and Al contents in this way, silicon-based oxide slag is distributed uniformly across the surface of the weld, reducing the possibility of continuous or intermittent band-like distribution along the edges of the weld bead, thereby improving properties.

[0026] On the other hand, the present invention can satisfy the requirement that the area fraction of silicon-based oxide slag in the weld be 1% or less (including 0%) relative to the total length of the weld, which may prevent the distribution of yellowish-brown slag with a glass-like luster at the end of the weld bead.

[0027] Therefore, the welded member having a welded portion excellent in electrodeposition paintability of the present invention includes a base material and a welded portion, and the welded portion contains, by weight %, C: 0.001 to 0.30%, Si: 1.00% or less (excluding 0%), Mn: 0.50 to 3.00%, P: 0.030% or less (excluding 0%), S: 0.030% or less (excluding 0%), Cr: 1.50% or less (excluding 0%), Mo: 0.60% or less (excluding 0%), Al: less than 0.10% (excluding 0%), and the like. %, excluding Ni, 0.40% or less (excluding 0%), Ti: less than 0.10% (excluding 0%), with the balance consisting of Fe and other unavoidable impurities, satisfying relational formula 1 and relational formula 2, and excluding the start and end portions of the weld bead corresponding to the smaller of 25% of the total length of the weld bead or 15 mm, the Si content of the slag distributed along the end of the weld bead is 10% or less by weight of the slag itself.

[0028] The welded member of the present invention having a welded joint with excellent electrodeposition paintability will be described below. First, the reasons for adding each component and limiting the content in the welded joint that constitutes the welded member of the present invention will be described in detail. It should be noted that the contents of each component described below are all based on weight percent unless otherwise specified.

[0029] C:0.001~0.30% C is a key element that lowers the temperature at which diffusionless transformations of acicular ferrite, bainite, and martensite occur as the weld metal is continuously cooled from the high-temperature austenite phase during solidification. If the C content is less than 0.001%, not only is the hardening ability reduced, making it difficult to ensure sufficient weld metal strength, but also, due to the aforementioned principle, the low-temperature transformation start temperature is not sufficiently low, significantly reducing the effect of the low-temperature transformation expansion effect on the offset of tensile residual stress in the weld during cooling, and preventing the formation of a high-angle grain boundary structure with a large difference in orientation angle between grains. On the other hand, if the C content exceeds 0.30%, not only is the viscosity of the molten metal reduced, resulting in poor bead shape, but the weld metal is excessively hardened, resulting in reduced toughness.

[0030] Si: 1.00% or less (excluding 0%) The Si element promotes deoxidation of molten metal during arc welding (a deoxidizing element), which is advantageous in suppressing the occurrence of blowholes and raising the low-temperature transformation start temperature. On the other hand, if the Si content exceeds 1.00%, there may be drawbacks such as the generation of a large amount of non-conductive slag, which causes poor coating of the weld, and insufficient surface activation of the weld due to excessive deoxidation, which reduces the penetration of the molten metal. Therefore, in the present invention, it is preferable to control the Si content to 1.00% or less. The Si content is more preferably 0.85% or less, even more preferably 0.75% or less, and most preferably 0.65% or less.

[0031] Mn: 0.5 to 3.0% Mn is a deoxidizing element that promotes deoxidation of molten metal during arc welding, which is advantageous in preventing the occurrence of blowholes. Like C, it also lowers the low-temperature transformation start temperature. If the Mn content is less than 0.5%, the deoxidizing effect is insufficient, which can lead to the disadvantage of easily causing blowholes. On the other hand, if the Mn content exceeds 3.0%, the viscosity of the molten metal becomes excessively high, which can prevent the molten metal from properly flowing into the welded area at high welding speeds, resulting in the formation of humping beads, which can lead to poor bead shape. More preferably, the Mn content is limited to 2.50% or less.

[0032] Cr: 1.50% or less (excluding 0%) Cr is a ferrite stabilizing element that lowers the low-temperature transformation start temperature and is advantageous for improving strength by ensuring the hardenability of the weld metal. If the Cr content exceeds 1.50%, there may be a drawback in some cases in that the brittleness of the weld metal increases unnecessarily, making it difficult to ensure sufficient toughness. The Cr content is more preferably 1.40% or less, even more preferably 1.30% or less, and most preferably 1.20% or less.

[0033] Mo: 0.60% or less (excluding 0%) Mo is a ferrite stabilizing element and is an element advantageous in ensuring hardenability that improves the strength of the weld metal. If the Mo content exceeds 0.60%, there may be a drawback in some cases in that the toughness of the weld metal decreases.

[0034] P: 0.030% or less (excluding 0%) P is an element that is generally present in steel as an unavoidable impurity, and is also contained as a normal impurity in solid wire for arc welding. If the P content exceeds 0.030%, there may be a drawback in that hot cracking of the weld metal becomes significant.

[0035] S: 0.030% or less (excluding 0%) S is an element that is generally found in steel as an unavoidable impurity, and is also found as a common impurity in solid wires for arc welding. If the S content exceeds 0.030%, the toughness of the weld metal may deteriorate in some cases, the surface tension of the molten metal may be insufficient during welding, and gravity may cause the molten metal to flow excessively downward during high-speed downward welding (vertical welding, welding from top to bottom), resulting in poor weld bead shape.

[0036] Al: Less than 0.10% (excluding 0%) Al is a deoxidizing element, and even trace amounts can promote deoxidation of molten metal during arc welding, thereby improving the strength of the weld metal. To ensure the above-mentioned effects, the lower limit of the Al content is set to 0%. If the Al content is 0.10% or more, the formation of Al-based oxides increases, which can lead to reduced strength and toughness of the weld metal and increased susceptibility to electrodeposition coating defects due to non-conductive oxides. More preferably, the Al content is limited to less than 0.07%.

[0037] Ti: Less than 0.10% (excluding 0%) Ti is a deoxidizing element, and even trace amounts can promote deoxidation of molten metal during arc welding, thereby improving the strength of the weld metal. It also facilitates the development of acicular ferrite, which can improve the toughness of the weld. To ensure the above-mentioned effects, the lower limit of the Ti content is set to 0%. If the Ti content is 0.10% or more, the formation of Ti-based oxides increases, which can lead to a decrease in the strength and toughness of the weld metal. More preferably, the Ti content is limited to less than 0.07%.

[0038] Ni: 0.40% or less (excluding 0%) Ni is an element that can improve the strength and toughness of the weld metal. To ensure the above-mentioned effects, 0% is excluded as the lower limit of the Ni content. However, if the Ni content exceeds 0.40%, there may be a drawback in that the weld metal becomes susceptible to cracking, so the Ni content is set to 0.40% or less. The Ni content is more preferably 0.30% or less, even more preferably 0.20% or less, and most preferably 0.10% or less.

[0039] Although not particularly limited, according to the present invention, the welded member may further selectively contain one or more of Nb: 0.10% or less, V: 0.10% or less, Zr: 0.10% or less, and B: 0.01% or less.

[0040] Nb: 0.10% or less Nb is an element that can enhance the hardening ability and densify the microstructure, thereby improving the strength and toughness of the weld metal. It also has the effect of improving the flow of molten metal during arc welding and stabilizing the arc. However, if the Nb content exceeds 0.10%, it may form low-melting point compounds at grain boundaries, which can lead to the disadvantage of hot cracking.

[0041] V: 0.10% or less V is an element that can enhance the hardening ability and densify the microstructure, thereby improving the strength and toughness of the weld metal. It is also a precipitation strengthening element that can improve the strength of the weld metal by forming carbonitrides. However, if the V content exceeds 0.10%, excessive precipitates can cause excessive strength, which can lead to a decrease in the toughness of the weld metal. Therefore, the V content is set to 0.10% or less.

[0042] Zr: 0.10% or less Zr is an element (deoxidizing element) that promotes deoxidation of molten metal during arc welding and is an element that is advantageous in suppressing the occurrence of blowholes. However, if the Zr content exceeds 0.10%, there may be a drawback in that the electrodeposition paintability of the welded portion decreases. Therefore, the Zr content is set to 0.10% or less.

[0043] B: 0.01% or less B is an element that can enhance the hardenability and improve the strength of the weld metal. However, if the B content exceeds 0.01%, the excessive hardenability may result in a decrease in the toughness of the weld metal. Therefore, the B content is set to 0.01% or less.

[0044] Furthermore, although not particularly limited, according to the present invention, the welded member may further selectively contain Cu: 0.50% or less by weight.

[0045] Cu: 0.50% or less The Cu is an element effective in improving the strength of the weld metal. However, if the Cu content exceeds 0.50%, there may be a drawback in that the weld metal becomes more susceptible to cracking. The Cu content is more preferably 0.45% or less, even more preferably 0.40% or less, and most preferably 0.30% or less. On the other hand, in order to obtain a sufficient strength-improving effect, the Cu content in the weld metal can be 0.01% or more.

[0046] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintended impurities may be inevitably mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since the above impurities are known to anyone skilled in the art, the present invention does not specifically mention all of them.

[0047] Equation 1 The present invention is characterized in that, in order to ensure excellent electrodeposition paintability of welds, the Si and Mn contents are controlled so that the value defined by the following relational expression 1 satisfies 3.5 to 34.0. By controlling the Si and Mn contents in this manner, the proportion of silicon-based oxide slag that deteriorates the electrodeposition paintability of welds is reduced, and in particular the proportion of silicon-based oxide slag that gives the edges of weld beads a yellowish-brown color and a glass-like luster is reduced, thereby effectively improving the electrodeposition paintability and corrosion resistance of welds. If the value defined by the following relational expression 1 is less than 3.5, the deoxidation of the welds is insufficient, making it easy for porosity defects to occur in the welds. Conversely, if it exceeds 34.0, the proportion of silicon-based oxide slag in the welds increases, which can cause problems such as a rapid deterioration in the electrodeposition paintability and corrosion resistance of the welds.

[0048] More preferably, the contents of Si and Mn are controlled so that the value defined by the following relational expression 1 satisfies 8.6 to 34.0.

[0049] [Equation 1] 3.5≦[Si]×100 / [Mn]≦34.0 (In the above Relational Formula 1, [Si] and [Mn] represent the weight percent content of each element in parentheses in the weld.)

[0050] Equation 2 In addition, the present invention requires that the Ti and Al contents be controlled so that the value defined by the following Relation 2 is less than 0.10 to ensure the characteristic of reducing silicon-based oxide slag distribution along the edge of the weld bead, which can degrade the electrodeposition paintability and corrosion resistance of the weld. Controlling the Ti and Al contents in this manner allows silicon-based oxide slag to be uniformly distributed across the entire surface of the weld and reduces its continuous or intermittent band-like distribution along the edge of the weld bead, thereby improving properties. If the value defined by the following Relation 2 is greater than 0.10, Ti and Al are strong deoxidizers that thermodynamically bond more easily with oxygen at high temperatures than Si during the cooling process during welding. Therefore, they preferentially form oxides and cover the surface, rather than Si, causing a change in the surface tension of the molten metal and allowing flow from the center to the periphery of the molten metal. This may also affect the slag distribution behavior on the surface of the molten metal, potentially resulting in the problem of additional silicon-based oxide slag formation along the edge of the weld bead.

[0051] More preferably, the value defined by the following relational expression 2 is controlled to be less than 0.07.

[0052] [Equation 2] [Ti]+[Al]<0.10 (In the above relational expression 2, [Ti] and [Al] represent the weight percent content of each element in parentheses in the weld.)

[0053] Furthermore, in the present invention, in a normal weld formed at a welded portion of a welded member, excluding the start and end portions of the weld bead corresponding to the smaller of 25% of the total length of the weld bead or 15 mm, the Si content of the slag distributed along the edge of the weld bead can be 10% or less by weight of the slag itself. That is, the present invention has confirmed through its research results that by controlling the Si content of the slag itself below a critical value, the amount of silicon-based oxide slag that is yellowish-brown and has a glass-like luster along the edge of the weld bead, which deteriorates the electrodeposition paintability and corrosion resistance of the weld, can be significantly reduced. However, if the Si content of the slag distributed along the edge of the weld bead exceeds 10% by weight of the slag itself, problems such as poor electrodeposition paintability and corrosion resistance of the weld may occur.

[0054] On the other hand, in the present invention, the start and end portions of the weld bead corresponding to the smaller of 25% or 15 mm of the total length of the weld bead are excluded because the arc usually deviates from a stable state in the sections where the arc starts or ends, and is therefore often excluded from the weld bead quality evaluation area by automobile manufacturers and others. In this case, if the total length of the weld bead is 60 mm or longer, there is no particular problem in reflecting the 15 mm long region from the start and end of the weld bead as the above-mentioned arc unstable area. For example, if the total length of the weld bead is 100 mm, the 15 mm long region, which corresponds to the smaller of 25 mm and 15 mm, which are 25% of the total length of the weld bead, is reflected as the start and end portions of the weld bead. If the total length of the weld bead is 30 mm or shorter, the 7.5 mm long region, which corresponds to the smaller of 7.5 mm and 15 mm, which are 25% of the total length of the weld bead, is reflected as the start and end portions of the weld bead.

[0055] Furthermore, in the present invention, the area fraction of the slag in the welded portion relative to the total length of the welded portion can be 1% or less (including 0%), which may prevent the distribution of slag that is yellowish-brown and has a glassy luster at the end of the weld bead. Therefore, the present invention can effectively provide automobile parts and other components having welded portions with excellent electrodeposition coating corrosion resistance at the welded portion, even if pickling treatment is omitted.

[0056] Meanwhile, the present invention does not particularly limit the alloy composition of the base material. However, as an example, the base material may contain, by weight, 0.04-0.18% C, 2.0% or less (including 0%) Si, 0.5-3.0% Mn, 2.0% or less (including 0%) Cr, 2.0% or less (including 0%) Mo, 0.01-0.10% Al, 0.05% or less (excluding 0%) P, and 0.05% or less (excluding 0%) S, with the balance being Fe and other unavoidable impurities. The base material may optionally further contain one or more of 0.20% or less (including 0%) Ti, 0.10% or less (including 0%) Nb, and 0.10% or less (including 0%) Cu. The base material may have a thickness of 0.8-4.0 mm.

[0057] In the present invention, the specific composition of the welding wire forming the weld is not limited, and as an example, a welding solid wire containing, by weight, C: 0.001 to 0.30%, Si: 0.25% or less (excluding 0%), Mn: 0.50 to 3.00%, P: 0.030% or less (excluding 0%), S: 0.030% or less (excluding 0%), Cr: 1.50% or less (excluding 0%), Mo: 0.60% or less (excluding 0%), Al: less than 0.10% (excluding 0%), Ni: 0.40% or less (excluding 0%), Cu: 0.50% or less (excluding 0%), Ti: less than 0.10% (excluding 0%), with the balance being Fe and other unavoidable impurities, can be used. The composition of the wire may optionally further contain one or more of Nb: 0.10% or less, V: 0.10% or less, and Zr: 0.10% or less, and may further contain B: 0.01% or less.

[0058] Meanwhile, in the present invention, the type of shielding gas used in welding the base metal is not particularly limited, and 100% CO2 gas, Ar + 20% CO2 gas, Ar + 10% CO2 gas, Ar + 5% CO2 gas, Ar + 2% O2 gas, etc. can be used as the shielding gas, but the effects of the present invention are particularly pronounced when Ar + 5 to 20% CO2 is used as the shielding gas. That is, in the present invention, in order to ensure the tensile strength of the weld without causing fracture of the weld metal or fusion line, it is preferable to use a mixture of Ar and 5 to 20% CO2 as the protective gas during the welding. [Example]

[0059] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are intended to illustrate and embody the present invention, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0060] (Example) Two hot-rolled pickled steel sheets were prepared as base materials, each having the alloy composition shown in Table 1 below, with a thickness of 2.0 mm, a length of 200 mm, a width of 150 mm, and tensile strengths of 380 MPa (Steel 1), 540 MPa (Steel 2), 670 MPa (Steel 3), 780 MPa (Steel 4), 980 MPa (Steel 5), and 1050 MPa (Steel 6). Seven types of solid wires for gas-shielded arc welding were then prepared, each having the composition shown in Table 2 below.

[0061] Next, the steel plates were lap-welded using the above-mentioned solid welding wires. Pulse DC (protective gas: Ar + 20% CO2) was used as the welding method, with a shielding gas flow rate of 20 l / min, a welding torch angle of 45° relative to the perpendicular direction of the base metal, a wire protrusion length of 15 mm, and welding current / voltage / speed conditions of 200 A, 20 V, and 80 cm / min. The gap at the lap joint was 0 mm, and the length of the lap joint was 10 mm.

[0062] Meanwhile, in the longitudinal direction of the welding base material, welding was started at a position 10 mm from the starting point, and after welding for a length of 180 mm, welding was finished at a position 10 mm from the end point on the opposite side from the welding start position.

[0063] [Table 1]

[0064] [Table 2] *In Table 2 above, the residual components are Fe and unavoidable impurities.

[0065] For each weld formed by the above welding, the microstructure of a cross section perpendicular to the longitudinal direction at the longitudinal center of the weld was observed with an optical microscope to confirm the weld metal region in advance, and the region was machined into the form of fine chips. Then, for each chip sample, the chemical composition of the weld was measured by emission spectroscopy using high-frequency inductively coupled plasma (ICP). The results are shown in Table 3 below.

[0066] To evaluate the corrosion resistance of the welds after electrodeposition coating, test specimens were divided into those that had been pickled using a neutral pickling solution and those that had not. They were then subjected to a 15-year North American combined corrosion test in accordance with the GMW14872 test method. After the test, the welds of each test specimen were shot blasted at a low pressure of approximately 0.5 bar to remove corrosion products from the welds. Areas with significant corrosion were then visually inspected, and the thickness loss of the corresponding areas after the corrosion test relative to the thickness of the original material before the corrosion test was measured. The corrosion weight loss of the welds was calculated using the following relational expression (3). The results are shown in Table 4.

[0067] [Equation 3] Corrosion weight loss of weld (%) = [(material thickness after corrosion test - material thickness before corrosion test) / material thickness before corrosion test] x 100

[0068] Meanwhile, the corrosion weight loss at the position where the most corrosion occurred was noted for each of the above-mentioned test specimens, and measurements were carried out for a total of five test specimens, and the results are shown in Table 4 (in particular, the maximum corrosion weight loss is underlined).

[0069] In addition, to observe the appearance of the bead of the welded part and to investigate the slag fraction, photographs were taken of the bead surface for a central 150 mm long portion of the bead, excluding the 15 mm portions at the beginning and end of the 180 mm weld bead.

[0070] Then, the slag portions were marked, the sum of the areas of the marked portions was calculated, and the slag area ratio was calculated from the following relational expression 4.

[0071] [Equation 4] Slag area ratio (%) = [total area of ​​slag parts / area of ​​entire image] x 100

[0072] In evaluating the slag generation status shown in Table 4, the standard value for the silicon-based oxide slag area ratio was set to 1%, with cases of 1% or less being evaluated as ○ (pass), and all other cases being evaluated as X (fail). On the other hand, when silicon-based oxide slag with a yellowish-brown color and a glass-like luster was distributed at the edge of the weld bead, it was evaluated as X (fail) regardless of the slag area ratio. This target standard was set to completely omit post-processing steps for removing slag from welds, such as separate pickling or mechanical abrasive brushing (both pickling and brushing were performed as necessary), in order to improve the paint adhesion and corrosion resistance of welds during part manufacturing.

[0073] In addition, energy dispersive X-ray spectroscopy (EDS) of a scanning electron microscope (SEM) was used to analyze the components of the normal bead in the central 150 mm long section of the 180 mm weld bead of each test piece, excluding the 15 mm sections at the beginning and end. The slag, which was distributed continuously or intermittently in thin bands along the edge of the bead, was irradiated with the above-mentioned X-rays, and the relative weight (wt%) of the Si component to the total weight of the slag was measured, and the maximum value of the measured values ​​is shown as the Fs value in Table 4 below.

[0074] [Table 3] *In Table 3 above, the residual components are Fe and unavoidable impurities.

[0075] [Table 4] *In Table 4 above, "-" indicates that there is no applicable information. Also, Fs (%) means the maximum weight percent of Si content relative to the slag components detected via SEM ESD, distributed continuously or intermittently in a thin band along the edge of the weld bead.

[0076] As shown in Table 1-4 above, in the case of Example 1-9, which has an alloy composition of the weld and satisfies Relational Formula 1-2, the silicon-based oxide slag area ratio before pickling treatment is 1% or less, and the Fs value is also 10% or less, confirming excellent electrodeposition coatability.

[0077] Generally, automotive parts are manufactured through a series of processes after welding and assembly: degreasing, water washing, pickling (including pickling in strong acid, weak acid, or neutral solutions), surface conditioning, conversion coating (including phosphate treatment), water washing, and electrodeposition coating. In particular, the introduction of the aforementioned pickling process or mechanical abrasive brushing is unavoidable in order to improve the electrodeposition coating properties and corrosion resistance of the welded parts after electrodeposition coating. Furthermore, pickling with a strong acid solution is difficult due to environmental issues. When pickling with a weak acid or neutral solution is performed, complete removal of slag from the weld is difficult, and in most cases, slag remains at the edges of the weld bead, which can reduce the corrosion resistance of the welded parts due to the electrodeposition coating.

[0078] In the present invention, by controlling the alloy composition of the weld as described above, the amount of silicon-based oxide slag formed in the weld can be reduced and the Fs value can be controlled to an appropriate value or less, thereby providing a welded component that has excellent electrodeposition coatability and does not impose a burden on the above-mentioned pickling treatment.

[0079] In contrast, in Comparative Examples 1-6, which do not satisfy Relational Formula 1, the silicon-based oxide slag area ratio before pickling exceeded 1%. In particular, in Comparative Examples 1-4, a large amount of silicon-based oxide slag remained even after pickling. This resulted in corrosion weight loss at the weld. On the other hand, Comparative Examples 7-8 did not satisfy Relational Formulas 2 and 1-2. In particular, Comparative Example 7 satisfied Relational Formula 1 and had a silicon-based oxide slag area ratio of 1% or less before pickling, but did not satisfy Relational Formula 2. The Fs value exceeded 10%, resulting in corrosion weight loss at the weld. Ti and Al are strong deoxidizers that thermodynamically bond more easily with oxygen at high temperatures during the cooling process during welding than Si. Therefore, they preferentially form oxides and cover the surface, causing a change in the surface tension of the molten metal and allowing the molten metal to flow from the center to the periphery. Because this may affect the slag distribution behavior on the molten metal surface, it was determined that silicon-based oxide slag was further formed along the edges of the weld bead. In addition, Comparative Example 8 did not satisfy all of the Relational Expressions 1-2, and not only did the silicon-based oxide slag area ratio exceed 1% both before and after pickling treatment, but the Fs value also exceeded 10%, confirming that corrosion weight loss occurred in the weld.

[0080] 1(a) and 1(b) are an enlarged photograph of the appearance of the end of the weld bead of Example 2 (a) and an image of the Si content distribution in the slag distributed along the end of the weld bead analyzed by SEM EDS analysis (b).

[0081] Here, it can be seen that Si is more uniformly dispersed on the surface of the weld [light-colored area in Figure 1(b)], and that silicon-based oxide slag, which is yellowish-brown and has a glassy luster, is not distributed at the edge of the weld bead.

[0082] 2(a) and 2(b) are a magnified photograph of the appearance of the end of the weld bead of Comparative Example 2 in the example of the present invention (a), and an image of the Si content distribution in the slag distributed along the end of the weld bead analyzed by SEM EDS analysis (b).

[0083] Here, it can be seen that on the surface of the weld, Si is more concentrated at the edge of the weld bead [light-colored area in Figure 2(b)], and that silicon-based oxide slag, which is yellowish-brown in color and has a glass-like luster, is distributed at the edge of the weld bead.

[0084] 3(a) and 3(b) are photographs showing the appearance of the weld bead after a corrosion test of Inventive Example 4 (a) and the appearance of the weld bead after a corrosion test of Comparative Example 4 (b) in the examples of the present invention.

[0085] Here, it can be seen that in the inventive example, only a very small amount of red rust occurred on the surface of the weld bead and that the red rust did not spread to the base material, whereas in the comparative example, it can be seen that the red rust was very severe not only on the weld bead but also on the base material, and that a lot of corrosion had occurred.

Claims

1. Includes the base material and the weld, The welded portion contains, in weight percent, C: 0.001 to 0.30%, Si: 1.00% or less (excluding 0%), Mn: 0.50 to 3.00%, P: 0.030% or less (excluding 0%), S: 0.030% or less (excluding 0%), Cr: 1.50% or less (excluding 0%), Mo: 0.60% or less (excluding 0%), Al: less than 0.10% (excluding 0%), Ni: 0.40% or less (excluding 0%), Ti: less than 0.10% (excluding 0%), Cu: 0%. A welded component with excellent corrosion resistance to electrocoating of welded joints, characterized in that it contains 0.50% or less of S, B: 0.01% or less, and the remainder being Fe and other inevitable impurities, and satisfies the following relational expressions 1 and 2, and in a normal welded joint excluding the start and end portions of the weld bead corresponding to the smaller of 25% of the total length of the weld bead or 15 mm, the Si content of the slag distributed along the end of the weld bead is 10% or less by weight of the slag itself. [Relationship 1] 3.5≦[Si]×100 / [Mn]≦34.0 (In the above Relational Formula 1, [Si] and [Mn] represent the weight percent contents of each element in parentheses in the weld.) [Relationship 2] [Ti]+[Al]<0.10 (In the above Relational Formula 2, [Ti] and [Al] represent the weight percent contents of each element in parentheses in the weld.)

2. The welded member according to claim 1, wherein the welded portion further contains, by weight percent, one or more of Nb: 0.10% or less, V: 0.10% or less, and Zr: 0.10% or less.

3. 2. The welded member according to claim 1, wherein the area ratio of silicon-based oxide slag in the slag formed at the welded portion is 1% or less.

4. 2. The welded member having excellent corrosion resistance due to electrodeposition coating of welded parts according to claim 1, wherein even if the welded parts are subjected to electrodeposition coating without undergoing pickling treatment, no corrosion weight loss occurs at the welded parts after a 15-year combined corrosion test in accordance with the GMW 14872 test method.

5. 2. The welded component with excellent corrosion resistance to electrodeposition coating of welded parts according to claim 1, characterized in that the base metal contains, by weight%, C: 0.04 to 0.18%, Si: 2.0% or less (including 0%), Mn: 0.5 to 3.0%, Cr: 2.0% or less (including 0%), Mo: 2.0% or less (including 0%), Al: 0.01 to 0.10%, P: 0.05% or less (excluding 0%), S: 0.05% or less (excluding 0%), with the balance being Fe and other unavoidable impurities.

6. 6. The welded member according to claim 5, wherein the base material further contains, by weight percent, one or more of Ti: 0.20% or less, Nb: 0.10% or less, and Cu: 0.10% or less.

7. 2. The welded member according to claim 1, wherein the base material has a thickness of 0.8 to 4.0 mm.

8. An automobile part having the welded portion according to claim 1.

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