Welded member with excellent fatigue characteristics of the welded part

Abstract

The object of the present invention is to provide a gas shielded arc welding member capable of ensuring excellent fatigue characteristics in the field of the automobile industry. 【Solution means】The welding member of the present invention includes a base material and a welded portion. 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%), Ni: 0.40% or less (excluding 0%), Ti: less than 0.10% (excluding 0%), and the balance is composed of Fe and other inevitable impurities, satisfies the following relational expression 1, and the shape of the weld bead of the welded portion satisfies the following relational expression 2. [Relational expression 1] 0.30 ≦ [Si] + 0.25×([Mn] + [Cr]) ≦ 0.66 [Relational expression 2] e1 / b2 ≦ 0.35

Description

Technical Field

[0001] The present invention relates to a welded member having excellent fatigue characteristics of a welded portion, and more particularly, to a gas shielded arc welded portion capable of ensuring excellent durability performance and economy, which are essential requirements during component manufacturing.

Background Art

[0002] In the automotive field, due to policies on fuel consumption regulations for environmental protection such as the global warming problem, research on technologies for weight reduction of vehicle bodies and components has emerged as a major issue. For chassis components, which are important for vehicle driving performance, it is necessary to apply high-strength steel materials for weight reduction under such a general trend.

[0003] In order to achieve such component weight reduction, it is essential to increase the strength of the material, and it can be said that ensuring the durability performance of components made of high-strength steel materials in an environment where repeated fatigue loads are applied is an important factor.

[0004] However, in the case of arc welding, which is mainly used to ensure strength during the assembly of automotive chassis components, since the lap joint welding between components is performed by the deposition of welding wire, it is inevitable to impart the geometric shape of the joint. This acts as a concentration part (notch effect) of repeated fatigue stress and becomes a fracture initiation point, resulting in a decrease in the durability performance of the component. Therefore, there is a limit in losing the advantages of applying high-strength steel materials.

[0005] Therefore, in order to improve the fatigue characteristics of the welded part, it is most important to mainly reduce the angle (toe angle) of the bead tip, which is the stress concentration part. In addition to this, it can be said that controlling the material and stress of the toe part are important factors. Also, as described above, due to the thinning of the material based on the high strength and weight reduction of parts, the requirement for rust prevention properties to prevent through corrosion has increased, and the adoption of plated steel materials has been increasing. However, especially in the case of the welded metal of the arc welded part, there is no plating layer, and there is a limit to the deterioration of the corrosion resistance after painting with respect to the base material. As a result, in the severe corrosion environment during vehicle operation, there is a problem that it leads to the early occurrence of corrosion and the deterioration of fatigue characteristics in the welded part of the chassis parts made of plated steel sheets. On the other hand, during the gas shielded arc welding of plated steel materials, due to the generation of vapors such as zinc, a large amount of pore defects in the form of pits and blowholes are generated in the weld bead, which may lead to a decrease in the strength of the welded part, resulting in a problem of decreased welding productivity. Also, in the case of general non-plated steel materials, during gas shielded arc welding, the slag generated on the weld bead causes painting defects and becomes a factor in the deterioration of the corrosion resistance after painting. Therefore, there is a problem of cost increase due to post-treatment processes such as pickling or brushing for slag removal after welding during part manufacturing.

[0006] Recently, the development of lightweight chassis parts for next-generation eco-cars has been actively carried out, and in particular, the development of welding technologies that can improve the characteristics of welded parts while ensuring economic efficiency has become an important issue.

Summary of the Invention

Problems to be Solved by the Invention

[0007] An object of the present invention is to provide a gas shielded arc welding member that can ensure excellent fatigue characteristics in the automotive industry field.

[0008] The problems of the present invention are not limited to the above matters. Further problems of the present invention are described in the entire content of the specification, and those with ordinary knowledge in the technical field to which the present invention belongs will have no difficulty in understanding the further problems of the present invention from the entire content of the specification of the present invention.

Means for Solving the Problem

[0009] The welded member excellent in fatigue characteristics of the welded part of the present invention includes a base material and a welded part, wherein the above welded part 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% (excluding 0%), and the balance consists of Fe and other inevitable impurities, and satisfies the following relational expression 1. The shape of the weld bead of the above welded part is characterized by satisfying the following relational expression 2. [Relational Expression 1] 0.30 ≦ [Si] + 0.25 × ([Mn] + [Cr]) ≦ 0.66 (In the above relational expression 1, [Si], [Mn], and [Cr] represent the weight% content of each element in parentheses with respect to the welded part.) [Relational Expression 2] e1 / b2 ≦ 0.35 (In the above relational expression 2, as shown in FIG. 1, e1 represents the distance from the point where the line connected in the direction perpendicular to the lower plate from the welding route of the lap joint welded part and the line connected from the weld toe of the lower plate are in contact with each other to the weld toe of the upper plate, and b2 represents the distance from the weld toe of the lower plate to the welding route.)

[0010] The shape of the weld bead of the above welded part can further satisfy the following relational expression 3. [Relational Expression 3] θ2 - θ1 ≧ 34° (In the above relational expression 3, as shown in FIG. 2, θ1 represents the interior angle formed by the line parallel to the upper reference plane of the lower plate and the line connecting from the weld toe of the lower plate to the weld toe of the upper plate, and θ2 represents the interior angle formed by the line parallel to the lower reference plane of the upper plate of the lap joint welded part and the line connecting from the welding route to the weld toe of the upper plate.)

[0011] The above-mentioned welded part can further 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 by weight percentage.

[0012] The above-mentioned welded part can further contain Cu: 0.50% or less by weight percentage.

[0013] The above-mentioned welded part can satisfy a fatigue life of 245,000 Cycles or more at a maximum load of 10.1 kN (the ratio of the minimum load to the maximum load is 0.1 and the repeated load frequency is 15 Hz).

[0014] The above-mentioned base material contains, by weight percentage, 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%), and the balance is composed of Fe and other inevitable impurities.

[0015] The above-mentioned base material can be further composed of containing one or more of Ti: 0.20% or less, Nb: 0.10% or less, and Cu: 0.10% or less by weight percentage.

[0016] The above-mentioned base material can have a thickness of 0.8 to 4.0 mm.

[0017] Furthermore, the present invention further relates to an automotive part having the above-mentioned welding member.

Advantages of the Invention

[0018] According to the above-mentioned present invention, as a next-generation welding technology that secures performance / cost competitiveness in line with the era of popularization of electric vehicles, a gas shielded arc welded part with excellent fatigue characteristics of the welded part can be effectively provided.

[0019] The diverse and significant advantages and effects of the present invention are not limited to the above-described content and can be more easily understood during the process of explaining the specific embodiments of the present invention.

Brief Description of the Drawings

[0020]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0021] Hereinafter, the present invention will be described.

[0022] The present invention is characterized in that, in order to ensure excellent fatigue characteristics of the welded part, the Si, Mn, and Cr contents of the welded part are controlled. As a result of controlling the contents of such component elements, in gas shielded arc welding that normally performs constant voltage control, the flow of current through the arc column between the tip of the welding wire and the molten metal, that is, by strengthening the arc force, the position having the maximum penetration depth can be moved from the outside of the lower plate (in the direction of the weld toe) of the joint of the lap joint welding to the side of the welding route where the upper plate and the lower plate are in contact with the weld metal. Therefore, the effect of improving the low cycle fatigue life to which a relatively high fatigue load is applied can be obtained. Here, the low cycle fatigue life is defined as less than 245,000 Cycles, and the case where the value is equal to or more than this is defined as the high cycle fatigue life.

[0023] Further, the present invention controls the weld bead of the welded part to a predetermined shape by controlling the Si, Mn, and Cr contents of the welded part described above, thereby reducing the sensitivity to fatigue fracture at the welding route and the toe of the lap joint weld part, improving the low cycle / high cycle fatigue life of the welded part, and further reducing the sensitivity to fatigue fracture at the weld toe, and the effect of being able to further improve the high cycle fatigue life under a relatively low fatigue load can be obtained.

[0024] First, a welding member according to one aspect of the present invention will be described. The welding member of the present invention includes a base material and a welded portion. At this time, the alloy composition of the welded portion will be described first. The contents of the alloy compositions described below are in wt%.

[0025] C:0.001~0.30% The above C is a main element that can lower the temperature at which acicular ferrite, bainite, and martensite transformations start through diffusionless transformation by continuous cooling in the high-temperature austenite phase during the solidification process of the weld metal. When the content of the above C is less than 0.001%, the hardening ability decreases, making it difficult to ensure sufficient strength of the weld metal. Moreover, according to the above principle, the low-temperature transformation start temperature does not become sufficiently low, and the canceling effect of the tensile residual stress in the welded portion due to the low-temperature transformation expansion effect during the cooling process is significantly reduced, and there may be a drawback that a high-hard-angle grain boundary structure with a large difference in azimuth angles between grains is not formed. On the other hand, when the content of C exceeds 0.30%, there may be drawbacks such as the viscosity of the molten metal becoming low, resulting in poor bead shape, and the weld metal being excessively hardened and the toughness decreasing.

[0026] Si: 1.00% or less (excluding 0%) The above Si is an element that promotes deoxidation of the molten metal during arc welding (deoxidizing element), which is advantageous for suppressing the generation of blowholes, and is an element that raises the low-temperature transformation start temperature. On the other hand, when the content of the above Si exceeds 1.00%, a large amount of non-conductive slag is generated, causing poor coating of the welded portion, and there may be drawbacks such as insufficient surface activation of the welded portion due to excessive deoxidation and a decrease in the meltability of the molten metal. Therefore, in the present invention, it is preferable to control the content of Si to 1.00% or less. The content of the above Si is more preferably 0.75% or less, even more preferably 0.50% or less, and most preferably 0.25% or less.

[0027] Mn: 0.50 - 3.00% The above Mn is a deoxidizing element, which promotes the deoxidation of the molten metal during arc welding and is beneficial for suppressing the generation of blowholes. It is an element that decreases the low-temperature transformation start temperature like C. If the content of the above Mn is less than 0.5%, there may be a drawback that the deoxidation effect is insufficient and the generation of blowholes becomes easy. On the other hand, if it exceeds 3.0%, the viscosity of the molten metal becomes excessively high, and when the welding speed is fast, the molten metal cannot flow appropriately into the welding part, and a humping bead is formed, so there may be a drawback that bead shape defects are likely to occur. More preferably, the content of the above Mn is limited to 2.50% or less.

[0028] Cr: 1.50% or less (excluding 0%) The above Cr is a ferrite-stabilizing element, which is beneficial for lowering the low-temperature transformation start temperature and improving the strength by ensuring the hardenability of the weld metal. If the content of the above Cr exceeds 1.50%, there may be a drawback that in some cases, the brittleness of the weld metal increases unnecessarily and it is difficult to ensure sufficient toughness. The content of the above Cr is more preferably 1.40% or less, even more preferably 1.30% or less, and most preferably 1.20% or less.

[0029] Mo: 0.60% or less (excluding 0%) The above Mo is a ferrite-stabilizing element, which is beneficial for ensuring the hardenability to improve the strength of the weld metal. If the content of the above Mo exceeds 0.60%, there may be a drawback that in some cases, the toughness of the weld metal decreases.

[0030] P: 0.030% or less (excluding 0%) The above P is an element that generally mixes into steel as an inevitable impurity and is also contained as a normal impurity in the solid wire for arc welding. If the content of the above P exceeds 0.030%, there may be a drawback that hot cracking of the weld metal becomes prominent.

[0031] S: 0.030% or less (excluding 0%) The above S is an element that generally mixes into steel as an inevitable impurity and is also included as a normal impurity in solid wires for arc welding. When the content of the above S exceeds 0.030%, in some cases, the toughness of the weld metal deteriorates, the surface tension of the molten metal during welding is insufficient, and when performing high-speed downward welding (welding from top to bottom during vertical welding), the molten part may flow down excessively due to gravity, resulting in a defective bead shape.

[0032] Al: less than 0.10% (excluding 0%) The above Al is an element that can improve the strength of the weld metal by promoting the deoxidation of the molten metal during arc welding even in trace amounts as a deoxidizing element. To ensure the above-described effects, 0% is excluded as the lower limit of the Al content. When the content of the above Al is 0.10% or more, there may be a drawback that the formation of Al-based oxides increases, and in some cases, the strength and toughness of the weld metal decrease, and it becomes sensitive to defective electro-deposition painting of the welded part due to non-conductive oxides. More preferably, the content of the above Al is limited to less than 0.07%.

[0033] Ti: less than 0.10% (excluding 0%) The above Ti is an element that can improve the strength of the weld metal by promoting the deoxidation of the molten metal during arc welding even in trace amounts as a deoxidizing element. Also, it facilitates the development of acicular ferrite that can improve the toughness of the welded part. To ensure the above-described effects, 0% is excluded as the lower limit of the Ti content. When the content of the above Ti is 0.10% or more, there may be a drawback that the formation of Ti-based oxides increases and, in some cases, the strength and toughness of the weld metal decrease. More preferably, the content of the above Ti is limited to less than 0.07%.

[0034] Ni: 0.40% or less (excluding 0%) The above-mentioned Ni is an element that can improve the strength and toughness of the weld metal. To ensure the above-described effects, 0% is excluded as the lower limit of the Ni content. However, since there may be a drawback that the weld becomes sensitive to cracks when the Ni content exceeds 0.40%, 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.

[0035] Although not particularly limited, according to one embodiment of the present invention, the above-mentioned welding member can further 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, selectively.

[0036] Nb: 0.10% or less The above-mentioned Nb is an element that can enhance the hardening ability, make the microstructure dense, and improve the strength and toughness of the weld metal. Further, it has the effect of improving the flow of the molten metal during arc welding and stabilizing the arc. However, if the Nb content exceeds 0.10%, there may be a drawback that low-melting-point compounds are formed at the grain boundaries, making the weld prone to hot cracking.

[0037] V: 0.10% or less The above-mentioned V is an element that can enhance the hardening ability, make the microstructure dense, and improve the strength and toughness of the weld metal. Further, it is a precipitation strengthening element that can generate carbonitrides and improve the strength of the weld metal. However, if the V content exceeds 0.10%, there may be a drawback that the toughness of the weld metal may decrease in some cases due to excessive strength caused by excessive precipitates. Therefore, the V content is set to 0.10% or less.

[0038] Zr: 0.10% or less The above Zr is an element that promotes deoxidation of molten metal during arc welding (deoxidizing element) and is advantageous for suppressing the generation of blowholes. However, if the content of the above Zr exceeds 0.10%, there may be a drawback that the electrocoating property of the welded part deteriorates. Therefore, the content of the above Zr is set to 0.10% or less.

[0039] B: 0.01% or less The above B is an element that can enhance the hardening ability and improve the strength of the welded metal. However, if the content of the above B exceeds 0.01%, there may be a drawback that the toughness of the welded metal decreases in some cases due to excessive hardening ability. Therefore, the content of the above B is set to 0.01% or less.

[0040] Also, although not particularly limited, according to one embodiment of the present invention, the above welding member can further selectively contain, by weight%, Cu: 0.50% or less.

[0041] Cu: 0.50% or less The above Cu is an element effective for improving the strength of the welded metal. However, if the content of the above Cu exceeds 0.50%, there may be a drawback that the crack sensitivity of the welded metal increases. The content of the above Cu is more preferably 0.45% or less, further preferably 0.40% or less, and most preferably 0.30% or less. On the other hand, in order to sufficiently obtain the effect of strength improvement, the above Cu can be contained in the welded metal at 0.01% or more.

[0042] In addition, the remaining components of the present invention are iron (Fe). However, in the normal manufacturing process, it is inevitable that unintended impurities may be mixed in from raw materials or the surrounding environment, so this cannot be excluded. Since the above impurities are understandable to any ordinary technician, all the contents thereof are not particularly mentioned in the present invention.

[0043] Relational Expression 1 The present invention also controls the contents of Si, Mn, and Cr so as to satisfy the following relational expression 1. As a result of controlling the contents of such component elements, in gas shielded arc welding that normally performs constant voltage control, the flow of current through the arc column between the tip of the welding wire and the molten metal, that is, by strengthening the arc force, the position having the maximum penetration depth moves from the outside of the lower plate of the joint of the fillet weld (in the direction of the weld toe) to the welding route side where the upper plate and the lower plate are in contact with the weld metal, and the effect of improving the low cycle fatigue life to which a relatively high fatigue load is applied is obtained. Here, the low cycle fatigue life is defined as less than 245,000 Cycles, and the case where it is equal to or more than this value is defined as the high cycle fatigue life. If the value defined by the following relational expression 1 is less than 0.30, despite the above-described effect, the welding part becomes sensitive to the generation of porosity defects in the welding part due to insufficient deoxidation of the welding part, and conversely, there may be a problem that the fatigue strength of the welding part decreases. On the contrary, if this value exceeds 0.66, there may be a problem that it is difficult to control the position having the maximum penetration depth from the above-described fillet weld part to move to the welding route side. More preferably, the contents of Si, Mn, and Cr are controlled so that the value defined by the following relational expression 1 satisfies 0.35 to 0.60.

[0044] [Relational Expression 1] 0.30 ≦ [Si] + 0.25 × ([Mn] + [Cr]) ≦ 0.66 (In the above relational expression 1, [Si], [Mn], and [Cr] indicate the weight % contents of the respective elements in parentheses with respect to the welded part.)

[0045] On the other hand, the weld bead of the welded part constituting the welded member of the present invention preferably has a predetermined shape. Specifically, it is preferable to control the weld ratio of the above-described welded part so that the value defined by the following relational expression 2 is 0.35 or less.

[0046] [Relational Expression 2] e1 / b2 ≦ 0.35 (In the above relational expression 2, as shown in FIG. 1, e1 indicates the distance from the point where the line connected in the direction perpendicular to the lower plate from the welding route of the lap joint welding part and the line connected from the welding toe of the lower plate are in contact with each other to the welding toe of the upper plate, and b2 indicates the distance from the welding toe of the lower plate to the welding route.)

[0047] The reason for restricting the shape of the weld bead of the lap joint welding part obtained by welding in the present invention as in the above relational expression 2 is that, based on the principle of the above relational expression 1, the position having the maximum penetration depth in the lap joint welding part moves appropriately from the outside of the lower plate (welding toe direction) to the welding route side to reduce the sensitivity of fatigue fracture at the welding route. At the same time, when the value of e1 is low at an appropriate ratio with respect to the value of b2 in relational expression 2, it is possible to effectively improve the high-cycle fatigue life with a relatively low fatigue load while preventing fatigue fracture at the welding toe. If the value defined by relational expression 2 exceeds 0.35, there may be a problem that the fatigue fracture of the welding toe becomes sensitive to a relatively low fatigue load and the high-cycle fatigue life becomes short. More preferably, the value defined by the above relational expression 2 is controlled to be 0.30 or less.)

[0048] Furthermore, in the present invention, it is more preferable that the shape of the weld bead of the above welding part is formed so as to satisfy the following relational expression 3.)

[0049] [Relational Expression 3] θ2 - θ1 ≧ 34° (In the above relational expression 3, as shown in FIG. 2, θ1 represents the inner angle formed by the line parallel to the upper reference plane of the lower plate and the line connecting from the welding toe of the lower plate to the welding toe of the upper plate, and θ2 indicates the inner angle formed by the line parallel to the lower reference plane of the upper plate of the lap joint welding part and the line connecting from the welding route to the welding toe of the upper plate.)

[0050] The reason for controlling the shape of the weld bead as in the above relational expression 3 is that, based on the principles of the above relational expressions 1 and 2, the sensitivity to fatigue failure of the weld root and toe of the lap joint weld is reduced, and the low-cycle / high-cycle fatigue life of the weld is improved. At the same time, the sensitivity to fatigue failure at the weld toe is further reduced to further improve the high-cycle fatigue life under a relatively low fatigue load. If the value of θ2 decreases, the concentration of fatigue stress at the weld root decreases, but the concentration of fatigue stress at the weld toe increases. Conversely, if the value of θ1 decreases, the concentration of fatigue stress at the weld toe decreases, but the concentration of fatigue stress at the weld root increases. Therefore, by controlling the values defined by the above relational expressions 1 and 2 to appropriate values based on the above principles and controlling the value defined by relational expression 3 to be 34° or more, it has been discovered through the research of the present invention that the low-cycle / high-cycle fatigue life of the lap joint weld can be effectively improved simultaneously. If the value defined by the above relational expression 3 is less than 34°, there may be a problem that the high-cycle fatigue life becomes shorter even if the low-cycle fatigue life of the weld increases.

[0051] On the other hand, in the present invention, the alloy composition of the base material is not particularly limited. However, as an example, the base material 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%), and the balance can consist of Fe and other inevitable impurities. Further, the base material can selectively further contain one or more of Ti: 0.20% or less (including 0%), Nb: 0.10% or less (including 0%), and Cu: 0.10% or less (including 0%). Furthermore, the base material can have a thickness of 0.8 to 4.0 mm.

[0052] In the present invention, the specific composition components of the welding wire for forming the welded part are not limited. As an example, in terms of weight percentage, it contains 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%), and the balance consists of Fe and other inevitable impurities. A solid welding wire for welding can be used, etc. Optionally, the above wire composition components can further contain one or more of Nb: 0.10% or less, V: 0.10% or less, and Zr: 0.10% or less, or can further contain B: 0.01% or less.

[0053] On the other hand, in the present invention, the type of shielding gas used for welding the base material 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. In particular, when Ar + 5 to 20% CO2 is used as the shielding gas, the remarkable effects of the present invention can be exerted. That is, in the present invention, in order to ensure the tensile strength of the welded part without generating weld metal or fusion line fracture during welding, it is preferable to use a protective gas during the above welding by mixing 5 to 20% of CO2 with Ar.

[0054] Hereinafter, the present invention will be described more specifically with reference to examples. However, it should be noted that the following examples are only for exemplifying and concretizing the present invention, and are not for limiting the scope of rights of the present invention. This is because the scope of rights of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

[0055] (Examples)

Table 1

[0056]

Table 2

[0057] Two base materials of each of four types of hot-rolled pickled steel sheets with a thickness of 2.0 mm, a width of 150 mm, lengths of 135 mm and 110 mm, and tensile strengths of 380 MPa (steel sheet 1), 540 MPa (steel sheet 2), 670 MPa (steel sheet 3), and 780 MPa (steel sheet 4) having the alloy composition as shown in Table 1 above were prepared (one pair with lengths of 135 mm and 110 mm). And a solid wire for gas shielded arc welding having the alloy composition as shown in Table 2 above was prepared.

[0058] Next, the above-mentioned molten zinc-plated steel sheets were lap joint welded using the above solid wires for welding respectively. At this time, Pulse DC (shielding gas: Ar + 20% CO2) was used as the welding method, the shielding gas flow rate was 20 l / min, the angle of the welding torch was 45° with respect to the vertical direction of the base material, the wire protruding length was 15 mm, and the welding current / voltage / speed conditions were 200 A - 20 V - 80 cm / min. Also, the gap of the lap joint was 0 mm and the length of the lap joint was 25 mm.

[0059] On the other hand, in the width direction of the welded base material, welding was started at a position 10 mm from the starting point, and after welding was carried out for a length of 130 mm, welding was terminated at a position 10 mm from the end point on the opposite side of the welding start position.

[0060] For each welded part formed by the above welding, the microstructure of the cross-sectional part perpendicular to the length direction at the center of the length direction of the welded part was observed with an optical microscope to preliminarily confirm the area of the weld metal, and the area was machined by cutting in the form of fine chips (chips). Next, with each chip sample, the chemical composition of the weld metal was measured by emission spectroscopy using high-frequency inductively coupled plasma (ICP) and shown in Table 3 below.

[0061] In addition, factors for specifying the shape of the weld bead of the welded portion formed by the above welding were determined, the values of the respective factors were shown in Table 4 below, and further, the values of Relational Expression 2 and Relational Expression 3 were calculated and described.

[0062] In Table 4 below, the units of a, b1, b2, e1, and e2 are mm, and the units of θ1 and θ1 are °, and each is defined as follows.

[0063] First, as shown in FIG. 1, a is a line connecting vertically to the lower plate at the boundary point (welding route) where the upper plate and the lower plate of the lap joint welded portion contact the weld metal, and the line connecting from the end of the weld metal of the lower plate (weld toe) to the point where the lines connected from the weld route intersect. The length of the line connecting vertically from this line to the weld route is shown, and this length can be 77% or more with respect to the base material thickness (mm).

[0064] b1 represents the length from the point where the line connecting vertically to the lower plate from the weld route and the line connecting from the weld toe intersect to the weld route, and this length can be 99% or more with respect to the base material thickness (mm).

[0065] b2 represents the length from the weld toe of the lower plate to the weld route, and this length can be 99% or more with respect to the base material thickness (mm).

[0066] e1 indicates the length from the point where the line connecting vertically to the lower plate from the weld route and the line connecting from the weld toe of the lower plate intersect to the end of the weld metal of the upper plate (weld toe), and this length can be 11% or more with respect to the base material thickness (mm).

[0067] e2 represents the maximum depth at which the weld metal penetrates in the vertical direction from the line parallel to the upper reference plane of the lower plate, and this length can be 11% or more with respect to the base material thickness (mm).

[0068] And, as shown in the attached FIG. 2, θ1 indicates the interior angle formed by the line parallel to the upper reference plane of the lower plate and the line connecting from the weld toe of the lower plate to the weld toe of the upper plate.

[0069] As shown in FIG. 2, θ2 represents the internal angle formed by the line parallel to the lower reference plane of the upper plate and the line connecting from the welding root to the weld toe of the upper plate.

[0070] In addition, the fatigue characteristics of the welded portion formed by welding as described above were measured, and the results are shown in Table 5 below. At this time, the specific method for measuring the fatigue characteristics is as follows. First, a rectangular test piece with a width of 50 mm and a length of 220 mm was taken from the central part of each welding test piece, and then rectangular test pieces with a width of 50 mm and a length of 40 mm were used as spacers and positioned at both ends in the length direction of the above fatigue test piece, and adhered by resistance spot welding at two points each. This is to make the test piece uniaxial during the tensile fatigue test. Next, tensile-tensile fatigue tests were conducted under three load conditions of maximum load values of 10.1 kN, 9.1 kN, and 8.1 kN, and the fatigue life (Cycles) was measured. At this time, the ratio of the minimum load to the maximum load (minimum load / maximum load) was 0.1, and the repeated load frequency was 15 Hz. Also, the minimum load means the minimum value of the repeated load having the above-described constant load application frequency, and the maximum load means the maximum value of the repeated load.

[0071]

Table 3

[0072]

Table 4

[0073]

Table 5

[0074] As shown in Tables 1 to 5 above, in the cases of Invention Examples 1 to 4 in which the alloy components of the welded part were optimally controlled to satisfy Relational Expression 1, and thereby the shape of the weld bead was optimally controlled, that is, Relational Expressions 2 to 3 were satisfied, it was confirmed that welded members having relatively excellent fatigue characteristics could be obtained. On the other hand, in Comparative Examples 1 to 4, since the relational expression 1 of the alloy components of the welded part was not satisfied and the relational expressions 2 to 3 of the shape of the weld bead were not satisfied either, it can be seen that the low-cycle / high-cycle fatigue life deteriorated in all of the fatigue tests evaluated compared to Invention Examples 1 to 4 under the load conditions.

Claims

1. including a base material and a welded portion, 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%), Ni: 0.40% or less (excluding 0%), Ti: less than 0.10% (excluding 0%), the balance being composed of Fe and other inevitable impurities, and satisfies the following relational expression 1, a welded member excellent in fatigue characteristics of the welded portion, characterized in that the shape of the weld bead of the welded portion satisfies the following relational expression 2. [Relational Expression 1] 0.30 ≦ [Si] + 0.25×([Mn] + [Cr]) ≦ 0.66 (In the relational expression 1, [Si], [Mn], and [Cr] represent the weight% content of each element in parentheses with respect to the welded portion.) [Relational Expression 2] e1 / b2 ≦ 0.35 (In the relational expression 2, e1 represents the distance from the point where the line connected in the direction perpendicular to the lower plate from the weld root of the fillet weld portion and the line connected from the weld toe of the lower plate are in contact with each other to the weld toe of the upper plate, and b2 represents the distance from the weld toe of the lower plate to the weld root.)

2. a welded member excellent in fatigue characteristics of the welded portion according to claim 1, characterized in that the shape of the weld bead of the welded portion further satisfies the following relational expression 3. [Relational Expression 3] θ2 - θ1 ≧ 34° (In the relational expression 3, θ1 represents the inner angle formed by the line parallel to the upper reference plane of the lower plate and the line connecting from the weld toe of the lower plate to the weld toe of the upper plate, and θ2 represents the inner angle formed by the line parallel to the lower reference plane of the upper plate of the fillet weld portion and the line connecting from the weld root to the weld toe of the upper plate.)

3. a welded member excellent in fatigue characteristics of the welded portion according to claim 1, characterized in that the welded portion further contains 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 by weight%.

4. a welded member excellent in fatigue characteristics of the welded portion according to claim 1, characterized in that the welded portion further contains Cu: 0.50% or less by weight%.

5. The welded part has a fatigue life of 245,000 Cycles or more under a maximum load of 10.1 kN (ratio of minimum load to maximum load: 0.1 and frequency of repeated load: 15 Hz), and is characterized in that it is a welded member having excellent fatigue characteristics of the welded part according to claim 1.

6. The base material 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%), and the balance consists of Fe and other inevitable impurities, and is characterized in that it is a welded member having excellent fatigue characteristics of the welded part according to claim 1.

7. The base material further contains one or more of Ti: 0.20% or less, Nb: 0.10% or less, and Cu: 0.10% or less by weight%, and is characterized in that it is a welded member having excellent fatigue characteristics of the welded part according to claim 6.

8. The base material has a thickness of 0.8 to 4.0 mm, and is characterized in that it is a welded member having excellent fatigue characteristics of the welded part according to claim 1.

9. An automotive part characterized by including the welded member according to claim 1.

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