Welded member having excellent fatigue strength of weld and method for manufacturing the same

Abstract

One embodiment of the present invention relates to a welded member obtained by overlapping a part of two base materials and performing fillet welding thereon using a welding material, and provides a welded member having excellent weld fatigue strength and a method for manufacturing the same, the welded member including a base material, a weld bead, and a root reinforcement weld metal, wherein the base material has a tensile strength of 780 MPa, a weld toe angle of the weld bead is 160 degrees or more, and the weld bead and the root reinforcement weld metal have a Vickers hardness of 280 Hv to 320 Hv and a fatigue strength of 350 MPa or more.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a welded part having excellent fatigue strength of a weld and a manufacturing method thereof. BACKGROUND

[0002] In the automotive field, since fuel efficiency regulation policies for environmental protection against problems such as global warming, research on lightweight technology for vehicle bodies and parts is becoming a major issue. According to this principle, high-strength steel materials also need to be applied to chassis parts important for the driving performance of a vehicle to reduce weight. In order to achieve weight reduction of such parts, the strength of the material must be increased, and it is an important factor to ensure the durability of parts made of high-strength steel materials in an environment where repeated fatigue loads are applied. In the case of arc welding mainly used to secure strength when assembling automotive chassis parts, since the lap joint welding between parts is performed by welding with a welding wire, it is inevitable to provide a joint portion having a certain geometry. However, since this acts as a repeated fatigue stress concentration portion (notch effect) and becomes a fracture initiation point, causing deterioration of the durability performance of the part, there is a limitation in losing the advantages of applying high-strength steel materials. As described above, for the fatigue characteristics of the welded portion, the most important thing is to reduce the angle of the end portion of the weld bead, which is mainly a stress concentration portion (weld toe angle), and it is reported that there is no direct correlation with the softening of the heat-affected zone (HAZ) due to heat input from welding.

[0003] Meanwhile, as a representative technology for solving such a problem, Patent Document 1 is provided. In Patent Document 1, a concept of material control for each temperature interval of the weld toe portion of the weld bead, i.e., the heat-affected zone (HAZ), is proposed to improve the fatigue characteristics of the arc welded portion of a steel material having a sheet thickness of 5 mm or less and a tensile strength of 780 MPa or more (for example, the position of the minimum hardness at a depth of 0.1 mm on the surface must be at least 0.3 mm from the fusion line), but the details of the specific welding method by which the fatigue characteristics can be improved by reducing the weld bead toe angle are insufficient.

[0004] As another technology, Patent Documents 2 and 3 are provided. Patent Document 2 proposes that the fatigue characteristics can be improved by applying a compressive stress by continuously hitting the end portion of the weld bead with a chisel (percussion pin) to form a plastic deformation zone, and Patent Document 3 discloses a remelting process method for the end portion of the weld bead by a plasma heat source after welding to reduce the weld toe angle of the arc weld bead between the subframe and the support of the chassis part for a vehicle. However, the above proposed methods have an inevitable limitation that the process cost can be increased when manufacturing the part since a post-welding process is added.

[0005] As another technique, Patent Literature 4 is provided. Patent Literature 4 proposes that a welded member can ensure excellent fatigue strength without a special post-processing treatment such as laser remelting after welding, but has a drawback that the level of fatigue strength is only at a level of up to 285 MPa.

[0006] [Related Art Documents]

[0007] (Patent Literature 1) Japanese Patent Laid-Open No. 2013-220431

[0008] (Patent Literature 2) Japanese Patent Laid-Open No. 2014-014831

[0009] (Patent Literature 3) Japanese Patent Laid-Open No. 2014-004609

[0010] (Patent Literature 4) Korean Patent Publication No. 10-2019-0103244 SUMMARY

[0011] TECHNICAL PROBLEM

[0012] One aspect of the present disclosure provides a welded member having excellent weld fatigue strength and a method for manufacturing the same.

[0013] TECHNICAL SOLUTION

[0014] According to one aspect of the present disclosure, a welded member having excellent weld fatigue strength thereof is obtained by lap joining a portion of two base materials and fillet welding using a welding material, the welded member including: a base material, a weld bead, and a reinforcement weld metal of a root portion, wherein the base material has a tensile strength of 780 MPa or more, the weld bead has a weld toe angle of 160° or more, and the reinforcement weld metal in the weld bead and the root portion has an average Vickers hardness of 280 Hv to 320 Hv and an average fatigue strength of 350 MPa or more.

[0015] According to another aspect of the present disclosure, a method for manufacturing a welded member having excellent weld fatigue strength thereof, which is obtained by lap joining a portion of two base materials and fillet welding using a welding material, in the method, wherein the base material has a tensile strength of 780 MPa or more, during welding, a shielding gas containing 5% to 10% of CO2 and the remaining portion of Ar in terms of vol% is used, during welding, a welding heat input (Q) defined by the following [Equation 1] satisfies 1.15t ≤ Q ≤ 1.6t (where t is the thickness (mm) of the base material, and the unit of Q is kJ / cm), a specific resistance (R) of the welding material defined by the following [Equation 2] satisfies 0.5 ≤ R ≤ 1.1, and X defined by the following [Equation 3] satisfies 0.6 ≤ X ≤ 3.4,

[0016] [Equation 1] Q = (I x E) x 0.048 / u

[0017] [Equation 2] R = [Si] + 0.25 x ([Mn] + [Cr])

[0018] [Equation 3] X = 28 x [Si] / [Mn] 2 - [Cr] / 3 + 4 x [Mo]

[0019] (However, in [Equation 1], I, E, and u respectively indicate welding current [A], welding voltage [V], and welding speed (cm / minute), and in [Equation 2] and [Equation 3], [Si], [Mn], [Cr], and [Mo] indicate each element content (wt%).

[0020] Advantages of the Invention

[0021] According to one aspect of the present disclosure, a welded member having excellent weld fatigue strength and a method for manufacturing the same can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 Photos captured with an optical microscope after etching the cross-sectional structure of the welded member according to one embodiment of the present disclosure with a nital solution are (a) for Inventive Example 1 and (b) for Comparative Example 5.

[0023] Figure 2 Hardness distributions of the welded member according to one embodiment of the present disclosure are shown, (a) for Inventive Example 1 and (b) for Comparative Example 5.

[0024] Figure 3 Image quality (IQ) and inverse pole figure (IPF) photos of Inventive Example 1 according to one embodiment of the present disclosure observed with EBSD are shown.

[0025] Figure 4 Image quality (IQ) and inverse pole figure (IPF) photos of Comparative Example 5 according to one embodiment of the present disclosure observed with EBSD are shown. DETAILED DESCRIPTION

[0026] Hereinafter, a welded member having excellent weld fatigue strength according to one embodiment of the present disclosure will be described.

[0027] The weldment of the present disclosure can be obtained by lap joining a portion of two base materials and fillet welding using a welding material. In this case, the weldment can include the base material, the weld bead, and the reinforcement weld metal of the root portion. The reinforcement weld metal in the root portion refers to additional weld metal formed during gas shielded arc welding according to the penetrability characteristics of the molten metal smoothly penetrating between the upper and lower plates of the lap joint portion. The root portion reinforcement weld metal exists between the rear end of the weld bead and the lap portion of the base material. By forming the reinforcement weld metal in the root portion in this area, reduction in fatigue strength due to stress concentration at the root portion in a typical fatigue environment can be effectively prevented.

[0028] The tensile strength of the base material is preferably 780 MPa or more. As described above, by using a high-strength base material, weight reduction can be achieved when applied to a portion of a vehicle body used in the automotive field. Meanwhile, in the present disclosure, the type is not particularly limited as long as it is a steel type having a high strength of 780 MPa or more as described above. However, it can preferably have an alloy composition similar to the alloy composition of the welding material applied to the present disclosure. For example, the base material can contain, by weight %, 0.02 to 0.08% of C; 0.01 to 0.5% of Si; 0.8 to 1.8% of Mn; 0.01 to 0.1% of Al; 0.001 to 0.02% of P; 0.001 to 0.01% of S; 0.001 to 0.01% of N; 0.01 to 0.12% of Ti; 0.01 to 0.05% of Nb, and the remainder of Fe and unavoidable impurities. In addition, the base material can further contain at least one of Mo, Cr, V, Ni, and B such that the total amount thereof is 1.5% by weight or less.

[0029] The thickness of the base material can be 1.0 to 2.0 mm. When the thickness of the base material is less than 1.0 mm, there can be a disadvantage that it is difficult to exhibit sufficient arc force to form the root portion reinforcement weld metal and be sensitive to melting during typical gas shielded arc welding. On the other hand, when the thickness thereof exceeds 2.0 mm, the thickness step of the lap joint portion becomes too large, so that it can be difficult to secure the weld bead toe angle for securing excellent fatigue strength.

[0030] The interval of the lap portion between the two base materials (the interval between the upper and lower plates to be welded) can be 0.5 mm or less (including 0 mm). When the interval of the lap portion between the two base materials exceeds 0.5 mm, it can be difficult to secure the weld bead toe angle for achieving excellent fatigue strength in the appropriate base material thickness range. The interval of the lap portion between the two base materials refers to the interval between the upper and lower plates to be welded.

[0031] The bead toe angle is preferably 160° or more. The bead toe angle refers to the angle between the bead and the base metal at the end of the bead, which is located at the lower portion of the two base metals. The reason for controlling the bead toe angle is to alleviate stress concentration of the welded portion in a usual fatigue environment. That is, by controlling the bead toe angle to be large, an effect of significantly improving the fatigue strength compared to a conventional welded portion can be obtained, whereas if the bead toe angle is less than 160°, it can be difficult to sufficiently obtain the effect.

[0032] The bead can include a microstructure of at least one of acicular ferrite and bainite, and the average effective grain size of the acicular ferrite and the bainite can be 5 μm or less. The acicular ferrite and the bainite are microstructures that are advantageous in securing the strength and toughness of the weld metal, i.e., the bead. Furthermore, in the present disclosure, an effect of simultaneously securing sufficient strength and toughness of the enhanced weld metal in the bead and the root portion can be obtained by refining the grains of the acicular ferrite and the bainite. If the average effective grain size of the acicular ferrite and the bainite exceeds 5 μm, it is difficult to simultaneously secure sufficient strength and toughness of the weld metal as described above. Meanwhile, the above-described average effective grain size refers to the average size of the grains converted from the number of grains per unit area.

[0033] Meanwhile, the welding material used during welding can include, by weight%, C: 0.06% to 0.1%, Si: 0.04% to 0.2%, Mn: 1.6% to 1.9%, Cr: 0.5% to 1.6%, Mo: 0.1% to 0.6%, and the remaining portion of Fe and other inevitable impurities.

[0034] Carbon (C): 0.06% to 0.1%

[0035] Carbon (C) is a beneficial element for the effect of stabilizing the arc to volumetric atomization. When the content of C is less than 0.06%, the volume becomes coarse and the arc becomes unstable, the spatter generation amount increases, and it can be difficult to secure sufficient strength of the weld metal, which is disadvantageous. On the other hand, when the content of C exceeds 0.1%, there can be disadvantages that the viscosity of the molten metal decreases, resulting in a poor bead shape, and the weld metal is over quenched, thereby reducing the toughness. The lower limit of the content of C is more preferably 0.062%, even more preferably 0.065%, and most preferably 0.07%. The upper limit of the content of C is more preferably 0.095%, even more preferably 0.09%, and most preferably 0.085%.

[0036] Silicon (Si): 0.04% to 0.2%

[0037] Silicon (Si) is an element (deoxidizing element) that promotes deoxidization of molten metal during arc welding, and is effective in suppressing the occurrence of pores. When the content of Si is less than 0.04%, there can be a disadvantage that deoxidization becomes insufficient and pores are easily generated, and when the content of Si exceeds 0.2%, there can be a disadvantage that non-conductive slag is significantly generated, causing a painting defect in the weld, and the permeability of the molten metal is reduced due to lack of surface activation of the weld caused by excessive deoxidization. The lower limit of the content of Si is more preferably 0.045%, even more preferably 0.05%, and most preferably 0.06%. The upper limit of the content of Si is more preferably 0.15%, even more preferably 0.1%, and most preferably 0.08%.

[0038] Manganese (Mn): 1.6% to 1.9%

[0039] Manganese (Mn) is a deoxidizing element and is an element that promotes deoxidization of molten metal during arc welding and suppresses pore generation. When the content of Mn is less than 1.6%, there can be a disadvantage that deoxidization becomes insufficient within the appropriate range of the content of Si described above and pores can be generated. When the content of Mn exceeds 1.9%, there can be a disadvantage that the viscosity of the molten metal becomes too high, and the welding speed is high, the molten metal cannot flow into the welding site properly, resulting in a raised bead, which can cause poor bead shape. The lower limit of the content of Mn is more preferably 1.65%, even more preferably 1.7%, and most preferably 1.75%. The upper limit of the content of Mn is more preferably 1.87%, even more preferably 1.85%, and most preferably 1.8%.

[0040] Chromium (Cr): 0.5% to 1.6%

[0041] Chromium (Cr) is a ferrite stabilizing element, and is an element that is advantageous in ensuring hardenability to improve the strength of the weld metal. When the content of Cr is less than 0.5%, there can be a disadvantage that it is difficult to ensure sufficient strength of the weld metal, and when the content of Cr exceeds 1.6%, there can be a disadvantage that in some cases, the brittleness of the weld metal is unnecessarily increased, making it difficult to ensure sufficient toughness. The lower limit of the content of Cr is more preferably 0.6%, even more preferably 0.7%, and most preferably 0.8%. The upper limit of the content of Cr is more preferably 1.55%, even more preferably 1.5%, and most preferably 1.45%.

[0042] Molybdenum (Mo): 0.1% to 0.6%

[0043] Molybdenum (Mo) is a ferrite stabilizing element and is an element that is advantageous to ensure hardenability to improve the strength of the weld metal. When the content of Mo is less than 0.1%, there can be a disadvantage that it is difficult to ensure sufficient strength of the weld metal within the above-mentioned appropriate component range, and when the content of Mo exceeds 0.6%, there can be a disadvantage that the toughness of the weld metal is reduced in some cases. The lower limit of the content of Mo is more preferably 0.15%, even more preferably 0.2%, and most preferably 0.25%. The upper limit of the content of Mo is more preferably 0.55%, even more preferably 0.52%, and most preferably 0.5%.

[0044] In addition, the welding material can further contain 0.015% or less of P, 0.005% or less of S, 0.10% or less of Ni, 0.25% or less of Cu, and 0.10% or less of Al.

[0045] Phosphorus (P): 0.015% or less

[0046] Phosphorus (P) is an element that is usually incorporated into steel as an unavoidable impurity, and is also an element that is contained in the solid welding wire for arc welding as a usual impurity. When the content of P exceeds 0.015%, there can be a disadvantage that high temperature cracking of the weld metal becomes excessive. The content of P is more preferably 0.014% or less, even more preferably 0.012% or less, and most preferably 0.01% or less.

[0047] Sulfur (S): 0.01% or less

[0048] S is also usually incorporated into steel as an unavoidable impurity, and is also an element that is contained in the solid welding wire for arc welding as a usual impurity. When the content of S exceeds 0.01%, there can be a disadvantage that the toughness of the weld metal is deteriorated in some cases, and the surface tension of the molten metal is insufficient during welding, so that the molten portion excessively flows downward due to gravity during high speed welding, resulting in poor shape of the bead. The content of S is more preferably 0.008% or less, even more preferably 0.006% or less, and most preferably 0.005% or less.

[0049] Nickel (Ni): 0.40% or less

[0050] Nickel (Ni) is an element that can improve the strength and toughness of the weld metal. However, when the content of Ni exceeds 0.40%, there can be a disadvantage that it becomes susceptible to cracking within the above-mentioned appropriate component range. The content of Ni is more preferably 0.30% or less, even more preferably 0.20% or less, and most preferably 0.10% or less.

[0051] Copper (Cu): 0.50% or less

[0052] Copper (Cu) is generally included in steel constituting a welding wire as an impurity at about 0.02%, and in a solid welding wire for arc welding, the content of Cu can be determined mainly due to copper plating performed on the surface of the welding wire. Cu is an element capable of stabilizing the feedability and electrical conductivity of the welding wire. However, when the content of Cu exceeds 0.50%, there can be a disadvantage of increasing the crack sensitivity of the weld metal. The content of Cu is more preferably 0.45% or less, even more preferably 0.40% or less, and most preferably 0.30% or less.

[0053] Aluminum (Al): 0.20% or less

[0054] Aluminum (Al) is a deoxidizing element and is an element that promotes deoxidation of molten metal during arc welding and improves the strength of the weld metal. When the content of Al exceeds 0.20%, the generation of Al-based oxides increases, so there can be a disadvantage that in some cases, within the above-described appropriate component range, the strength and toughness of the weld metal deteriorate, and the electrodeposition painting defect of the welded portion becomes sensitive due to non-conductive oxides. The content of Al is more preferably 0.15% or less, even more preferably 0.12% or less, and most preferably 0.10% or less.

[0055] Meanwhile, in the present disclosure, the type of the welding material is not particularly limited, but a solid welding wire or a metal cored welding wire can be preferably used. More preferably, a solid welding wire is used, which is more advantageous to secure the rigidity of the welding wire, so that the effect of improving the penetration of the molten metal can be obtained by securing the excellent feedability and straightness of the welding wire during welding.

[0056] In the welded member of the present disclosure provided as described above, the reinforced weld metal in the weld bead and the weld root can have an average Vickers hardness of 280 Hv to 320 Hv and an average fatigue strength of 350 MPa or more, so that a very excellent fatigue strength in the welded portion can be secured.

[0057] Hereinafter, a method for manufacturing a welded member having an excellent welded portion fatigue strength according to one embodiment of the present disclosure will be described.

[0058] According to the present disclosure, the method for manufacturing the welded member can be performed by lap joining a portion of two base materials and performing fillet welding using a welding material. It is preferable to use gas shielded arc welding during the fillet welding.

[0059] In this case, it is preferable to use a shielding gas containing 5 to 10% of CO2 and the balance of Ar in terms of vol% during welding. CO2 is a gas that is advantageous in ensuring the wettability of molten metal due to arc pinch force and surface activation effects caused by arc pinch during arc welding. When the fraction of CO2 is less than 5%, there is a disadvantage that the volume transfer of the welding wire is unstable and the wettability of the molten metal can be poor during arc welding, and when the fraction of CO2 exceeds 10%, there is a disadvantage that the arc pinch increases and the wettability increases, but it is difficult to ensure a sufficient angle of the weld toe to ensure excellent fatigue characteristics of the welded portion.

[0060] Further, it is preferable that the welding heat input (Q) defined by the following [Equation 1] during welding satisfies 1.15t ≤ Q ≤ 1.6t (where t is the thickness of the base material (mm), and the unit of Q is kJ / cm). If the welding heat input (Q) is less than 1.15t, there is a problem that the strength and toughness of the weld metal and the coarse-grained heat-affected zone can be insufficient, and if the welding heat input (Q) exceeds 1.6t, there is a problem that not only the strength of the weld metal is insufficient and the reduction in the strength of the heat-affected zone becomes too large, but also back gouging and melting easily occur in the welded portion, leading to defects.

[0061] [Equation 1] Q = (I x E) x 0.048 / v

[0062] (However, in [Equation 1], I, E, and v respectively represent the welding current [A], the welding voltage [V], and the welding speed (cm / minute)).

[0063] The welding piece of the present disclosure is a welding piece having a weld portion obtained by welding two or more base materials using a welding material, in which the surface of the weld portion can be activated by controlling deoxidation according to chemical components and reducing the specific resistance of the welding wire to increase the arc pinch force to improve the permeability of the molten metal. In particular, excessive deoxidation during arc welding can be prevented by controlling the content of Si, which is a main deoxidizing element in the alloy components of the welding material. At the same time, since the conventional gas shielded arc welding is controlled by the constant voltage method, as the specific resistance of the welding wire used as the anode for the flow of arc current decreases, the welding current (i.e., the arc pinch force) that increases the permeability of the molten metal can be increased. Therefore, it is preferable that the specific resistance (R) of the welding material used during welding, which is defined by the following [Equation 2], satisfies 0.5 ≤ R ≤ 1.1. At the same time, when the specific resistance (R) is less than 0.5, not only is the deoxidation of the molten metal during welding insufficient, but the specific resistance of the welding wire is too low such that it has the disadvantage of being difficult to obtain a good bead due to poor volume transfer at the tip of the welding wire, and when the specific resistance (R) exceeds 1.1, there is the disadvantage that according to the above principle, sufficient arc pinch force is not applied, and thus the permeability of the molten metal is insufficient.

[0064] [Equation 2] R = [Si] + 0.25 x ([Mn] + [Cr])

[0065] (However, in [Equation 2], [Si], [Mn], and [Cr] represent the content of each element (wt%).

[0066] Further, it is preferable that X defined by the following [Equation 3] satisfies 0.6 ≤ X ≤ 3.4. During the arc welding of the target thin plate in the present disclosure, the phase transformation structure according to the continuous cooling of the welded metal portion varies rapidly according to the value of X described above, and in this case, sufficient strength and toughness of the welded metal portion can be ensured by ensuring acicular ferrite and bainite structures, which are typical low-temperature transformation phases generated according to the transformation caused by the lattice deformation in the undiffused state. Thus, excellent fatigue strength in the welded portion can be achieved by ensuring a dense microstructure of the reinforced weld metal of the welded metal portion and the smoothly formed weld toe portion and root portion as described above. In this case, the nucleation of the acicular ferrite transformation starts from the complex oxides generated from the minute metal elements contained in the welding base material and the welding material, and in order to promote the transformation of the acicular ferrite phase, it is more effective when the CO2 fraction of the welding shielding gas is 5 to 10 vol%. When the amount of oxygen generated according to the dissociation reaction of CO2 during the arc welding is too large compared to the appropriate range described above, the number of oxides increases, but it cannot reach the critical oxide size for nucleation, so the acicular ferrite phase transformation is not easily generated. Otherwise, the transformation of the grain boundary ferrite, which is not conducive to ensuring toughness, increases. In contrast, when the amount of oxygen is insufficient compared to the appropriate range, the hardenability increases due to the reduction of the oxidation of the hardenable elements contained in the steel material of the welding base material and the welding wire, so that low-temperature transformations such as bainite and martensite occur mainly rather than acicular ferrite transformation. In order to obtain all effects of increasing the bead toe angle and increasing the phase fraction of the acicular ferrite, the CO2 fraction is preferably close to 5 vol%. Further, as described above, by appropriately controlling the content of Si, which is a strong deoxidizing element, in the chemical components of the welding material, it can contribute to the formation of complex oxides. Further, when the Vickers hardness (Hv, load of 500 gf, measured at 0.2 mm intervals) of the welded metal portion of the reinforced weld metal including the root portion becomes 280 or more, the fatigue strength in the welded portion can be significantly improved. Meanwhile, when the value of X is less than 0.6, there is a disadvantage that the martensite phase transformation is mainly promoted and the brittleness of the weld metal increases. When the value of X exceeds 3.4, there is a disadvantage that the strength of the weld metal decreases due to the lack of hardenability.

[0067] [Equation 3] X = 28 x [Si] / [Mn] 2 -[Cr] / 3 + 4 x [Mo]

[0068] (However, in [Equation 3], [Si], [Mn], [Cr], and [Mo] represent the content of each element (wt%).

[0069] Embodiment of Invention

[0070] Hereinafter, the present disclosure will be described in detail through examples. However, the following examples are merely examples for more specifically illustrating the present disclosure, and do not limit the scope of the present disclosure.

[0071] (Example 1)

[0072] After dissolving ingots having the alloy compositions shown in Table 1 below, welding wires were prepared by annealing after drawing at room temperature via hot rolling. Then, Cu plating layers were formed on the surfaces of the welding wires, at which time, they were plated so that the copper content was in the range of 0.12 to 0.50% by mass with respect to the entire welding wire including the plating layer. Then, the copper-plated welding wires were drawn, and manufactured as welding solid wires having a diameter of 0.9 to 1.2 mm.

[0073] Using the welding solid wires manufactured as described above, two Pickled & Oiled (PO) steel sheets having the alloy compositions shown in Table 2 below were fillet-welded (lap joint welding) using the welding conditions shown in Table 3 below. In this case, the PO steel sheets had a tensile strength of 780 MPa, an average hardness of 260 Hv, and a thickness of 2.0 mm. During welding, the interval between the lap portions between the two base materials was fixed to 0.5 mm or less by clamping, and as a lap joint portion, pulsed MAG welding was performed under the conditions of a wire protrusion length of 15 mm, a welding speed of 80 cm / minute.

[0074] For the welded joints manufactured by the above welding, whether or not reinforcement weld metal in the root portion was formed, the bead toe angle, the average Vickers hardness of the bead and the reinforcement weld metal in the root portion, and the average fatigue strength thereof, and the microstructure and the average effective grain size thereof were measured, and the results thereof are shown in Table 4 below.

[0075] Whether or not reinforcement weld metal in the root portion was formed was determined by the presence of additional weld metal between the rear end portion of the bead and the lap portion of the base material, that is, in the region beyond the molten boundary line (the boundary between the weld metal and the heat-affected zone).

[0076] The bead toe angle was measured as an external angle formed by the contact of the normal line of the curved surface of the toe portion of the bead with the reference plane of the lower plate of the welding base material.

[0077] The average Vickers hardness of the bead and the reinforcement weld metal in the root portion was measured at intervals of 0.2 mm in the width direction using a Vickers hardness tester under the condition that the load was 500 gf, and the average value thereof was measured.

[0078] The average fatigue strength of the enhanced weld metal in the weld bead and the weld root was measured by taking a sample from the welded part and then performing a fatigue test to satisfy 2 x 10 6 cycles as the fatigue strength. In this case, the fatigue strength was described as the average value of three samples. In the fatigue test, a tensile-tensile high cycle fatigue test was used for each load to measure the fatigue life (cycles) of the welded part, and in this case, the ratio of the minimum load to the maximum load was 0.1, the repetition load frequency was 15 Hz, and in addition, the fatigue life corresponding to the converted strength (MPa) was obtained by dividing the load (kN) by the area according to the width and thickness of each sample.

[0079] After the cross-sectional structure of the welded part was micro-polished and etched with a nitric acid ethanol solution, the microstructure was observed with an optical microscope. In addition, Kikuchi patterns were analyzed by Electron Backscattered Diffraction (EBSD) to obtain an image quality (IQ) and an inverse pole figure (IPF) map that visualizes grain boundary and grain orientation information.

[0080] The average effective grain size was measured by classifying the grains by referring to the IQ and IPF maps of the EBSD and the microstructure photographs observed with the optical microscope described above, and then calculating the average size of the grains converted from the number of grains per unit area.

[0081] [Table 1]

[0082]

[0083] [Table 2]

[0084]

[0085] [Table 3]

[0086]

[0087] [Table 4]

[0088]

[0089] As can be seen from Tables 1 to 4, by securing the weld bead toe angle, the microstructure, and the average effective grain size obtained by the present disclosure, the welded part manufactured to satisfy the manufacturing conditions proposed by the present disclosure has excellent average Vickers hardness and average fatigue strength compared to the base material.

[0090] On the other hand, Comparative Example 1 shows a level of CO2 fraction of the shielding gas lower than that proposed by the present disclosure, so that the reinforcement weld metal in the root portion is not formed, and thus it can be seen that the average fatigue strength is at a low level.

[0091] Comparative Example 2 shows a level of CO2 fraction of the shielding gas exceeding that proposed by the present disclosure, so that the bead toe angle is small, and thus it can be seen that the average fatigue strength is at a low level.

[0092] Comparative Example 3 shows a level of welding heat input lower than that proposed by the present disclosure, so that it can be seen that the average Vickers hardness is high due to an increase in hardenability, but the average fatigue strength is low at a low level due to a decrease in the bead toe angle.

[0093] Comparative Example 4 shows a level of welding heat input exceeding that proposed by the present disclosure, so that it can be seen that the average Vickers hardness is low due to a decrease in hardenability, but the average effective grain size is increased and the bead toe angle is small, so that it can be seen that the average fatigue strength is at a low level.

[0094] In Comparative Example 5, since the R value and the X value of the welding material do not satisfy those proposed by the present disclosure, the reinforcement weld metal in the root portion is not formed and the bead toe angle is small, so that it can be seen that the average Vickers hardness and the average fatigue strength are at low levels.

[0095] Figure 1 For the photographs observed with an optical microscope after etching the cross-sectional structure of the welded member with a nitric acid ethanol solution, (a) is the photograph of Inventive Example 1, and (b) is the photograph of Comparative Example 5. As can be seen from Figure 1 in the case of Inventive Example 1, the bead toe angle is 164°, which is very smooth, and it can be seen that the formation of the reinforcement weld metal in the root portion is remarkable due to an increase in the penetrability of the molten metal. On the other hand, in the case of Comparative Example 5, the bead toe angle is 157°, and the radius of curvature is relatively small, which is disadvantageous for fatigue resistance, and it can be seen that the reinforcement weld metal in the root portion is not formed at all.

[0096] Figure 2 The hardness distribution of the welded member is shown, (a) shows the hardness distribution of Inventive Example 1, and (b) shows the hardness distribution of Comparative Example 5. As can be seen from Figure 2 in the case of Inventive Example 1, the hardness of the welded metal portion including the reinforcement weld metal in the root portion is at a level of 280 Hv to 320 Hv, which is higher than 260 Hv, which is the average hardness of the base material, but in the case of Comparative Example 1, it can be seen that it is at a level similar to the average hardness of the base material.

[0097] Figure 3 and Figure 4are images quality (IQ) and inverse pole figure (IPF) photographs of inventive example 1 and comparative example 5 observed with EBSD, respectively. As shown in FIGS. 1 and 2, it can be seen that inventive example 1 has a relatively dense microstructure compared to comparative example 5. Figure 3 and Figure 4 As shown in FIGS. 1 and 2, it can be seen that inventive example 1 has a relatively dense microstructure compared to comparative example 5.

[0098] (Example 2)

[0099] To evaluate the fatigue life of the welded portion, a specimen for fatigue test was prepared from the weldment corresponding to inventive example 1 and comparative example 5 described in example 1. In this case, after the base metal was cut to a width of 150 mm and a length of 120 mm, the width of the lap portion was 25 mm so that both sides of the joint portion were welded by about 100 mm. Thereafter, after a specimen having a width of 50 mm was taken from the center portion of the weldment, a specimen for fatigue test was prepared by spot welding both ends of the specimen with a lap portion having a width of 50 mm and a length of 40 mm, i.e., a compensator equal to the thickness of the base metal, to apply a uniaxial load. Then, a tensile-tensile high cycle fatigue test was performed for each load to measure the fatigue life (cycles) of the welded portion, and the results are shown in Table 5 below. In this case, the ratio of the minimum load to the maximum load was 0.1, and the repeated load frequency was 15 Hz. Further, the fatigue life corresponding to the converted strength (MPa) was obtained by dividing the load (kN) by the area according to the width and thickness of each specimen, and in this case, the fatigue life satisfying 2 x 107cycles was 360 MPa. 6 The maximum added load at 2 x 107cycles was defined as the fatigue strength.

[0100] [Table 5]

[0101]

[0102] As can be seen from Table 5 above, inventive example 1 showed a fatigue strength of 360 MPa, which is three times the fatigue strength of 120 MPa in comparative example 5. Meanwhile, in the case of comparative example 5, when the maximum load was 180 MPa or more, the fatigue life showed a sharp decline. Further, when the maximum load was 320 MPa or more, the fatigue failure occurred in the root portion, not the toe portion of the weld bead, whereas the welded portion of inventive example 1 had an excellent fatigue strength of up to 360 MPa with no fatigue failure at all in the toe portion and the root portion.

Claims

1. A weld having excellent weld fatigue strength, the weld being obtained by overlapping a portion of two base materials and performing fillet welding using a welding material, the weld comprising: a base material, a weld bead, and a reinforced weld metal of a root portion, wherein the base material has a tensile strength of 780 MPa or more, wherein the reinforced weld metal of the root portion is present between a rear end portion of the weld bead and an overlapping portion of the base material, a weld toe angle of the weld bead is 160° or more, and the weld bead and the reinforced weld metal in the root portion have an average Vickers hardness of 280 Hv to 320 Hv and an average fatigue strength of 350 MPa or more. 2.The weld having excellent weld fatigue strength according to claim 1, wherein the base material comprises, by weight%: 0.02% to 0.08% of C, 0.01% to 0.5% of Si, 0.8% to 1.8% of Mn, 0.01% to 0.1% of Al, 0.001% to 0.02% of P, 0.001% to 0.01% of S, 0.001% to 0.01% of N, 0.01% to 0.12% of Ti, 0.01% to 0.05% of Nb, and a remainder of Fe and other inevitable impurities. 3.The weld having excellent weld fatigue strength according to claim 2, wherein the base material further comprises: at least one of Mo, Cr, V, Ni, and B such that a total amount thereof is 1.5% by weight or less. 4.The weld having excellent weld fatigue strength according to claim 1, wherein a thickness of the base material is 1.0 mm to 2.0 mm. 5.The weld having excellent weld fatigue strength according to claim 1, wherein a spacing of an overlapping portion between the two base materials is 0.5 mm or less (including 0 mm). 6.The weld having excellent weld fatigue strength according to claim 1, wherein the weld bead comprises a microstructure of at least one of acicular ferrite and bainite, the acicular ferrite and the bainite having an average effective grain size of 5 μm or less. 7.The weld having excellent weld fatigue strength according to claim 1, wherein the welding material comprises, by weight%: 0.06% to 0.1% of C, 0.04% to 0.2% of Si, 1.6% to 1.9% of Mn, 0.5% to 1.6% of Cr, 0.1% to 0.6% of Mo, and a remainder of Fe and other inevitable impurities. 8.The weld having excellent weld fatigue strength according to claim 7, wherein the welding material further comprises: 0.015% or less of P, 0.01% or less of S, 0.40% or less of Ni, 0.50% or less of Cu, and 0.20% or less of Al. 9.The weld having excellent weld fatigue strength according to claim 1, wherein the welding material is a solid wire or a metal-cored wire.

10. A method for manufacturing a welded member having excellent weld portion fatigue strength, the welded member being obtained by lap joining a portion of two base materials and performing fillet welding using a welding material, in the method, wherein the base material has a tensile strength of 780 MPa or more, during welding, a shielding gas containing 5 to 10% of CO2 and the remainder of Ar in terms of vol% is used, during the welding, a welding heat input (Q) defined by the following [Equation 1] satisfies 1.15t < Q < 1.6t wherein, t is the thickness of the base material (mm), and Q has units of kJ / cm, and for the welding material, a specific resistance (R) defined by the following [Equation 2] satisfies 0.5 < R < 1.1, and X defined by the following [Equation 3] satisfies 0.6 < X < 3.4, [Equation 1] Q = (I x E) x 0.048 / u [Equation 2] R = [Si] + 0.25 x ([Mn] + [Cr]) [Equation 3] X = 28 x [Si] / [Mn] 2 -[Cr] / 3 + 4 x [Mo] However, in the [Equation 1], I, E, and u respectively represent welding current [A], welding voltage [V], and welding speed (cm / minute), and in the [Equation 2] and the [Equation 3], [Si], [Mn], [Cr], and [Mo] represent the content of each element (in terms of wt%).

11. The method for manufacturing a welded member having excellent weld portion fatigue strength according to claim 10, wherein the base material contains, in terms of wt%: 0.02 to 0.08% of C, 0.01 to 0.5% of Si, 0.8 to 1.8% of Mn, 0.01 to 0.1% of Al, 0.001 to 0.02% of P, 0.001 to 0.01% of S, 0.001 to 0.01% of N, 0.01 to 0.12% of Ti, 0.01 to 0.05% of Nb, and the remainder of Fe and other inevitable impurities.

12. The method for manufacturing a welded member having excellent weld portion fatigue strength according to claim 11, wherein the base material further contains: at least one of Mo, Cr, V, Ni, and B such that the total amount thereof is 1.5 wt% or less.

13. The method for manufacturing a welded member having excellent weld portion fatigue strength according to claim 10, wherein the welding material contains, in terms of wt%: 0.06 to 0.1% of C, 0.04 to 0.2% of Si, 1.6 to 1.9% of Mn, 0.5 to 1.6% of Cr, 0.1 to 0.6% of Mo, and the remainder of Fe and other inevitable impurities.

14. The method for manufacturing a welded member having excellent weld portion fatigue strength according to claim 13, wherein the welding material further contains: 0.015% or less of P, 0.01% or less of S, 0.40% or less of Ni, 0.50% or less of Cu, and 0.20% or less of Al.

15. The method for manufacturing a welded member having excellent weld portion fatigue strength according to claim 10, wherein the welding material is a solid wire or a metal cored wire.

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