the present invention is as follows.
[0011]
[1] In a first aspect of the present invention, a first steel plate and a second steel plate are superimposed, and a fillet weld is formed between the end face of the first steel plate and the surface of the second steel plate. It is an underbody part for automobiles having a welded joint formed by welding. The chemical composition of the weld metal forming the welded joint is C: 0.02 to 0.20%, Si: more than 0 to less than 0.10%, Mn: 0.00%, in terms of % by mass with respect to the total mass of the welded metal. 3 to 2.0%, Al: 0.002 to 0.30%, Ti: 0.005 to 0.30%, P: more than 0% to 0.015%, S: more than 0% to 0.030% , Cu: 0-0.50%, Cr: 0-1.5%, Nb: 0-0.3%, V: 0-0.3%, Mo: 0-1.0%, Ni: 0- 2.5%, B: 0 to 0.005%, the balance being iron and impurities, satisfying the following formulas (1) and (2).
[Al]+[Ti]>0.05 Expression (1)
7×[Mn]−112×[Ti]−30×[Al]≦4.0 Expression (2)
　However, [Al], [Ti], and [Mn] mean the content in mass % of each component with respect to the total mass of the weld metal.
[2] The automobile underbody part according to [1] above, wherein the chemical composition of the weld metal is Cu: 0.05 to 0.50% and Cr: 0% by mass with respect to the total mass of the weld metal. .05-1.5%, Nb: 0.005-0.3%, V: 0.005-0.3%, Mo: 0.05-1.0%, Ni: 0.05-2.5 %, and B: 0.0005 to 0.005%.
[3] In the automobile underbody part according to [1] or [2] above, the thickness of the first steel plate on which the fillet weld is formed on the end face is 0.8 mm or more and 4.5 mm or less. There may be.
[4] In the automobile underbody part according to any one of [1] to [3] above, the tensile strength of the first steel plate and the second steel plate may be 780 MPa or less.
Effect of the invention
[0012]
　According to the underbody part of the present invention, the chemical composition of the weld metal is appropriately controlled, so that the weld metal in the welded joint can exhibit excellent strength and corrosion resistance.
Brief description of the drawing
[0013]
1] Fig. 1 is a perspective view of a vehicle undercarriage component according to an embodiment. [Fig.
[Fig. 2] Fig. 2 is a schematic cross-sectional view showing a welded joint of a vehicle underbody component according to the present embodiment.
MODE FOR CARRYING OUT THE INVENTION
[0014]
　The inventors of the present invention have obtained the following knowledge as a result of earnestly studying measures for solving the above-described problems.
(A) When the Si content of the weld metal is high, Si-based slag is generated, and coating failure occurs at the position where the Si-based slag is generated, and red rust is likely to occur.
(B) One of the causes of the decrease in the strength of the weld metal when the Si content of the weld metal is reduced is the generation of coarse ferrite due to insufficient deoxidation.
(C) When the Al content and the Ti content of the weld metal are increased more than before, the generation of coarse ferrite in the weld metal can be suppressed.
(D) When Al and Ti coexist in the weld metal, an effect of suppressing red rust can be obtained.
(E) Red rust can be suppressed by appropriately controlling the contents of Mn, Ti and Al.
[0015]
　The present invention is made based on the above findings. DETAILED DESCRIPTION OF THE INVENTION A vehicle undercarriage component according to an embodiment of the present invention will be described in detail below.
[0016]
　Automobile underbody parts according to the present embodiment include, for example, various lower arms, various upper arms, toe control arms, trailing arms, torsion beams, carriers, subframes, side rails, cabs, underrun protectors, wheels, and floor cloths. be. FIG. 1 is a perspective view of a lower arm, which is a vehicle underbody component. As shown in FIG. 1, the vehicle underbody component according to the present embodiment is a welded joint in which two steel plates are superimposed and welded together. A joint 1 is provided.
[0017]
　FIG. 2 is a schematic cross-sectional view showing the welded joint 1 of the automotive underbody part of FIG. As shown in FIG. 2, the welded joint 1 consists of two steel plates 2 and 3 (a first steel plate and a second steel plate) that are superimposed to form an end face of one steel plate 2 and a surface of the other steel plate 3. It is configured by forming a fillet welded portion 4 between them.
　The fillet weld 4 is formed by arc welding the two steel plates 2 and 3 using a welding wire.
[0018]
　The two steel plates 2 and 3 may be steel plates of the same type or steel plates of a different type. The plate thickness of the steel plate 2 having the fillet weld 4 formed on the end face is preferably 0.8 mm or more and 4.5 mm or less.
　If the plate thickness of the steel plate 2 is 0.8 mm or more, occurrence of poor welding during arc welding is suppressed, and if the plate thickness of the steel plate 2 is 4.5 mm or less, the effect of suppressing red rust is obtained. Preferably, the plate thickness of the steel plate 2 is 1.4 mm or more, more preferably 2.0 mm or more. Further, the plate thickness of the steel plate 2 is preferably 4.0 mm or less, more preferably 3.5 mm or less.
　In addition, it is more preferable that each of the two steel plates 2 and 3 has a plate thickness of 0.8 mm or more and 4.5 mm or less.
　If the plate thickness of the steel plates 2 and 3 is 0.8 mm or more, occurrence of poor welding during arc welding is suppressed, and if the plate thickness of the steel plates 2 and 3 is 4.5 mm or less, the weight can be reduced. Preferably, the plate thickness of steel plates 2 and 3 is 1.4 mm or more, more preferably 2.0 mm or more. Further, the plate thickness of steel plates 2 and 3 is preferably 4.0 mm or less, more preferably 3.5 mm or less.
　Moreover, the tensile strength of the steel plates 2 and 3 may be 780 MPa or less. On the other hand, the lower limit of the tensile strength of steel plates 2 and 3 is not particularly limited, but may be 440 MPa or more.
[0019]
　The composition of the weld metal 4 of the welded joint 1 can be adjusted by the composition of the steel plate and the composition of the welding wire. Each component composition in the weld metal 4 will be described below.
[0020]
　Satisfying corrosion resistance in the following description means that red rust does not occur after 50 cycles of a combined cycle test (CCT, 5% NaCl, wet rate of 50%) defined in JASO method M610.
　Satisfying the strength means that when a tensile test is performed on a welded test piece, the weld metal does not break, but the base material breaks.
　The term "weld metal" means the metal in which the steel plate base material and the welding wire are melted and mixed together.
　The chemical composition of the weld metal is represented by % by mass, which is the ratio to the total mass of the weld metal, and the description of the % by mass is simply described as %.
[0021]
　The chemical composition of the weld metal can be measured by optical emission spectroscopy using high frequency inductively coupled plasma (ICP). Specifically, (1) a region of the weld metal is specified in advance by visually observing a cross section perpendicular to the longitudinal direction in the central portion of the weld in the longitudinal direction, and (2) the region is cut with a drill. Weld metal shavings are collected, and (3) the shavings are used as a sample and measured by emission spectrometry using high-frequency inductively coupled plasma (ICP).
[0022]
[C: 0.02 to 0.20%]
　C has the effect of stabilizing the arc and refining the droplets. , and the amount of spatter generation increases. As a result, the bead shape becomes uneven and becomes defective, resulting in red rust. The reason why red rust occurs due to defective bead shape is that welding slag is likely to occur in the recessed portions caused by the defective bead shape, and water and mud containing moisture that cause red rust are likely to accumulate. On the other hand, if the C content is less than 0.02%, the tensile strength of the weld metal cannot be obtained, and the desired tensile strength cannot be obtained. Therefore, the lower limit of C is 0.02% or more, preferably 0.04% or more, and more preferably 0.06% or more.
　On the other hand, if the C content exceeds 0.20%, the weld metal hardens, resulting in a decrease in cracking resistance and a tendency for the weld metal to break. Therefore, the upper limit of C is 0.20%, preferably 0.15%.
[0023]
[Si: more than 0 to less than 0.10%]
　Si is contained in the welding wire or the base metal as a deoxidizing element. In particular, Si in the welding wire improves the tensile strength of the weld metal by promoting deoxidation of the weld pool. However, when it is contained excessively, non-conductive slag is formed and coating defects occur.
　Therefore, the upper limit of Si is less than 0.10%, preferably less than 0.09%, more preferably less than 0.08%. On the other hand, the lower limit may be more than 0%, but it is preferably 0.01% or more from the viewpoint of manufacturing cost and ensuring the stability of the bead shape during welding.
[0024]
[Mn: 0.3 to 2.0%]
　Like Si, Mn is also a deoxidizing element that accelerates the deoxidation of the molten pool during arc welding and improves the tensile strength of the weld metal. be. If the Mn content is low, the tensile strength of the weld metal cannot be sufficiently ensured, and the weld metal tends to break. Therefore, the lower limit of Mn is 0.3% or more, preferably 0.5% or more.
　On the other hand, if Mn is excessively contained, the viscosity of the molten metal increases, and when the welding speed is high, the molten metal cannot properly flow into the welded portion, resulting in humping beads and poor bead shape. easier. As a result, the bead shape becomes uneven and becomes defective, resulting in red rust. Therefore, the upper limit of Mn is 2.0% or less, preferably 1.5% or less.
[0025]
[Al: 0.002 to 0.30%]
　Al is a strong deoxidizing element, and has the effect of promoting deoxidation of molten metal during arc welding and suppressing the formation of blowholes. Therefore, the lower limit of Al content is 0.002%, preferably 0.01%, more preferably 0.02%.
　On the other hand, when the Al content is excessive, Al-based slag increases, and red rust tends to occur between the slag and the weld metal. Therefore, the upper limit of the Al content of the weld metal is 0.30%, preferably 0.25%, more preferably 0.20%.
[0026]
[Ti: 0.005 to 0.300%]
　Since Ti is a deoxidizing element, it is effective in suppressing the occurrence of blowholes. Therefore, the lower limit of Ti content is 0.005%, preferably 0.01%, more preferably 0.05%.
　On the other hand, when Ti is contained excessively, the amount of Ti-based slag is increased, and the adhesion between the Ti-based slag and the weld metal is lowered, making them easy to separate. Therefore, red rust is likely to occur from the peeled portion. Therefore, the upper limit of the Ti content is 0.30%, preferably 0.25%, more preferably 0.20%.
[0027]
[Synergistic effect due to the coexistence of Al and Ti]
　Both Al and Ti are elements that suppress Si-based slag and generate Al-based slag and Ti-based slag, and contribute to the suppression of coating defects. . However, if only one of Al and Ti is contained above the lower limit, only Al-based slag or only Ti-based slag tends to agglomerate on the weld bead. If these slags agglomerate, even if there is no coating defect on the weld bead, gaps are likely to form between the weld metal and the slag, and red rust will occur from these gaps. That is, the generation of both Al-based slag and Ti-based slag suppresses aggregation of slag of the same type, and as a result, red rust is suppressed. Therefore, in the present invention, it is possible to obtain excellent corrosion resistance by using a composition system in which the weld metal contains both Al and Ti.
[0028]
[Al, Ti]
　The contents of Al and Ti satisfy the following formula (1).
[Al]+[Ti]>0.05 Expression (1)
　Both Al and Ti suppress the formation of coarsened ferrite, thereby sufficiently ensuring the strength of the weld metal. If the total content of Al and Ti is 0.05% or less, ferrite in the weld metal tends to coarsen even if blowholes do not occur, and sufficient strength cannot be obtained in the weld metal. Breakage is more likely to occur. Therefore, the lower limit of the total content of Al and Ti is over 0.05%, preferably 0.10%, and more preferably 0.15%.
　The upper limit of Al+Ti is not particularly limited, and may be 0.60% calculated from the respective upper limits of Al and Ti. However, when the upper limit of Al + Ti is 0.30% or less, the generation of Al-based slag and Ti-based slag is suppressed, and the generation of red rust from between the Al-based slag and Ti-based slag and the weld metal is suppressed. Therefore, it is preferable. The upper limit of Al+Ti is more preferably 0.20%.
[0029]
[Mn, Ti, Al]
　The contents of Mn, Ti and Al satisfy the following formula (2).
7×[Mn]−112×[Ti]−30×[Al]≦4.0 Expression (2)
　The inventors found that the As a result of investigating the presence or absence of red rust generation, when the value of 7 x [Mn] - 112 x [Ti] - 30 x [Al], which is an index for the generation of red rust, exceeds 4.0, red rust occurs early. It became clear that the corrosion resistance was poor. Therefore, in the above formula, the upper limit is set to 4.0.
　The lower limit is not particularly limited, and is -40.5 calculated from the lower limit of Mn and the upper limits of Al and Ti.
　In the above formulas (1) and (2), [Al], [Ti], and [Mn] mean the content of each component in mass % with respect to the total mass of the weld metal.
[0030]
[P: more than 0% to 0.015%]
　P is an element that is generally mixed into steel as an impurity. Also included in Here, since P is one of the main elements that cause hot cracking of the weld metal, it is desirable to suppress it as much as possible. If the P content exceeds 0.015%, hot cracking of the weld metal becomes conspicuous, so the upper limit of the P content of the weld metal is 0.015% or less.
　The lower limit of the P content is not particularly limited, and is more than 0%, but may be 0.001% from the viewpoint of the cost and productivity of removing P.
[0031]
[S: more than 0% to 0.030%]
　S, like P, is an element that is generally mixed into steel as an impurity, and is also usually contained in welding wire as an impurity. Also included in metals. Here, S is an element that impairs the crack resistance of the weld metal, and is preferably suppressed as much as possible. If the S content exceeds 0.030%, the crack resistance of the weld metal deteriorates, so the S content of the weld metal is 0.030% or less.
　The lower limit of the S content is not particularly limited, so it is more than 0%, but it may be 0.001% from the viewpoint of the cost of removing S and productivity.
[0032]
　Cu, Cr, Nb, V, Mo, Ni and B are not essential elements, but one or more of them may be contained at the same time if necessary. The effect obtained by containing each element and the upper limit will be described. In addition, the lower limit when these elements are not contained is 0%.
[0033]
[Cu: 0 to 0.50%]
　Since Cu may be contained in the weld metal due to the copper plating of the welding wire, 0.05% or more may be contained. On the other hand, if the Cu content is excessive, weld cracks are likely to occur, so the upper limit of Cu is 0.50% or less.
[0034]
[Cr: 0 to 1.5%]
　Cr may be contained in an amount of 0.05% or more in order to improve the hardenability of the weld zone and improve the tensile strength. On the other hand, when Cr is contained excessively, the elongation of the weld zone is lowered. Therefore, the upper limit of Cr is 1.5% or less.
[0035]
[Nb: 0 to 0.3%]
　Nb may be contained in an amount of 0.005% or more in order to improve the hardenability of the weld zone and improve the tensile strength. On the other hand, when Nb is contained excessively, the elongation of the weld zone is lowered. Therefore, the upper limit of Nb is 0.3% or less.
[0036]
[V: 0 to 0.3%]
　V may be contained in an amount of 0.005% or more in order to improve the hardenability of the weld zone and improve the tensile strength. On the other hand, when V is contained excessively, the elongation of the weld zone is lowered. Therefore, the upper limit of V is 0.3% or less.
[0037]
[Mo: 0 to 1.0%]
　Mo may be contained in an amount of 0.05% or more in order to improve the hardenability of the weld zone and improve the tensile strength. On the other hand, when Mo is contained excessively, the elongation of the weld zone is lowered. Therefore, the upper limit of Mo is 1.0% or less.
[0038]
[Ni: 0 to 2.5%]
　Ni may be contained in an amount of 0.05% or more in order to improve the tensile strength and elongation of the weld zone. On the other hand, when Ni is contained excessively, weld cracks are likely to occur. Therefore, the upper limit of Ni is 2.5% or less. Preferably, it is 2.0% or less.
[0039]
[B: 0 to 0.005%]
　B may be contained in an amount of 0.0005% or more in order to improve the hardenability of the weld zone and improve the tensile strength. On the other hand, when B is contained excessively, the elongation of the weld zone is lowered. Therefore, the upper limit of B is 0.005%. Preferably, it is 0.003% or less.
[0040]
　The balance of the components described above consists of Fe and impurities. Impurities are components that are contained in raw materials, components that are mixed in during the manufacturing process, and are not components that are intentionally included in the weld metal, or components that adversely affect the automotive underbody parts according to the present embodiment. It refers to a component that is allowed as long as it does not give
[0041]
　The automobile underbody component according to the present embodiment has been described above. Although the type of steel plate forming the base material of the automobile underbody part according to the present embodiment is not particularly limited, C: 0.020 to 0.30%, Si: 0 to less than 0.05%, Mn: The steel sheet preferably contains 0.30 to 3.00%, P: less than 0.05%, and S: less than 0.010%. Moreover, in addition to the above components, the steel sheet may contain arbitrary components such as Al and Ti. In addition, the tensile strength of the steel plate is not particularly limited, but by using a steel plate of 440 MPa or more as the base material, the effect of ensuring the strength and corrosion resistance of the weld metal of the underbody part according to the present embodiment at a high level is further enhanced. played.
Example
[0042]
　EXAMPLES Hereinafter, the effects of the present invention will be specifically described with reference to examples.
[0043]
　The following steel sheets a, the following steel sheets b, the following steel sheets c, the following steel sheets d, or the following steel sheets e are welded by lap fillet arc welding using various welding wires. was manufactured and the weld metal was evaluated. One and the other steel plates were brought into close contact with each other with an overlapping margin of 15 mm, and the plate thickness of the steel plates was set from 2.6 mm to 5.0 mm. As for the welding posture, the weld line was horizontal, and the inclination angle α of the other steel plate was 0°. The welding method was a pulse MAG arc welding method, the inclination angle (raising angle) of the welding torch was 60°, and the shielding gas used was Ar gas containing mainly 20 vol% CO 2 . Ar gas containing 3% O 2 and Ar gas containing 20% ​​CO 2 and 2% O 2 were also used as the shield gas . The tip of the wire was aimed at the corner formed by the end face of one steel plate and the surface of the other steel plate.
　The composition of the weld metal was adjusted by using solid wires of various composition systems as the welding wire.
[0044]
　The tensile strength, plate thickness and major components of each steel plate are as follows.
(Steel plate a)
Tensile strength: 270 MPa Plate
thickness: 3.2 mm
Components: C = 0.040%, Si = 0.01%, Mn = 0.30%, P = 0.010%, S = 0 .005%
(Steel plate b)
Tensile strength: 440 MPa Plate
thickness: 3.2 mm
Components: C = 0.150%, Si = 0.01%, Mn = 0.50%, P = 0.010%, S = 0.005%, Al = 0.02%
(Steel plate c)
· Tensile strength: 590 MPa · Plate
thickness: 2.9 mm
· Components: C = 0.050%, Si = 0.02%, Mn = 1.0%. 25%, P = 0.010%, S = 0.005%, Al = 0.30%, Ti = 0.05%
(steel plate d)
Tensile strength: 780 MPa Plate
thickness: 2.6 mm
Composition: C = 0.045%, Si = 0.02%, Mn = 1.55%, P = 0.010%, S = 0.005%, Al = 0.30%, Ti = 0.13%
(steel plate e )
Tensile strength: 780 MPa Plate
thickness: 5.0 mm
・Components: C = 0.045%, Si = 0.08%, Mn = 1.50%, P = 0.007%, S = 0.005%, Al = 0.05%, Ti = 0.07 %
[0045]
　The welded joint thus obtained was measured for the chemical composition of the weld metal.
　Specifically, (1) a region of the weld metal is specified in advance by visually observing a cross section perpendicular to the longitudinal direction in the central portion of the welded portion in the longitudinal direction, and (2) the region is cut with a drill. Chips of the weld metal were collected, and (3) the chemical components of the weld metal were measured by using the chips as a sample and measuring by emission spectrometry using high frequency inductively coupled plasma (ICP).
　Tables 1 to 3 show the content of each component and the values ​​of formulas (1) and (2). Numerical values ​​outside the scope of the present invention are underlined. Ingredients that were not added were left blank in the table.
[0046]
　Tables 1 to 3 also show evaluation results of strength (breaking position) and red rust for each experimental example.
[0047]
[table 1]

[0048]
[Table 2]

[0049]
[Table 3]

[0050]
(Evaluation of Strength) The
　strength was evaluated at the fracture position in the joint tensile test. In the tensile test, two steel plates of 25 mm×100 mm were overlapped with each other by 15 mm at the ends in the longitudinal direction and lap fillet-welded, and the tensile test was performed in the longitudinal direction at a tensile speed of 10 mm/min. When the fracture position was in the base metal, it was evaluated as OK, and when the fracture position was in the weld metal, it was evaluated as NG.
[0051]
(Evaluation of red rust)
　A combined cycle test (CCT, 5% NaCl, wet rate of 50%) specified in JASO method M610 was performed for 50 cycles and 100 cycles, and the presence or absence of red rust was evaluated. When red rust was not generated, it was judged as OK, and when red rust was generated, it was judged as NG.
　Corrosion resistance was evaluated in the following three grades of A, B and C. It is assumed that A and B satisfy the corrosion resistance and C does not satisfy the corrosion resistance.
A: Both 50 cycles and 100 cycles are OK.
B: 50 cycles is OK, 100 cycles is NG.
C: Both 50 cycles and 100 cycles are NG.
[0052]
　Experiment No. according to the example of the present invention. In Nos. 1 to 27 and 39, the chemical composition of the weld metal was appropriately controlled, and therefore excellent strength and corrosion resistance could be obtained in the weld metal. However, in Experiment No. 1, the plate thickness of the steel plate on which the fillet weld was formed on the end face exceeded 4.5 mm. In No. 39, red rust did not occur at 50 cycles in the combined cycle test, but red rust occurred at 100 cycles.
[0053]
　Experiment No. according to the comparative example. In No. 28, since the Si content of the weld metal was excessive, non-conductive slag was formed and coating defects occurred.
[0054]
　Experiment No. according to the comparative example. In No. 29, the chemical composition of the weld metal did not satisfy the formula (2), so the occurrence of red rust could not be suppressed.
[0055]
　Experiment No. according to the comparative example. In No. 30, since the C content in the weld metal was excessive, the weld metal hardened and the desired strength could not be obtained.
[0056]
　Experiment No. according to the comparative example. In No. 31, the chemical composition of the weld metal did not satisfy the formula (1), so the effect of suppressing the formation of coarse ferrite could not be sufficiently obtained, and the strength of the weld metal could not be ensured.
[0057]
　Experiment No. according to the comparative example. In No. 32, the C content in the weld metal was too small, so the tensile strength in the weld metal could not be obtained, and the desired tensile strength could not be obtained. Also, red rust was generated due to the defective bead shape.
[0058]
　Experiment No. according to the comparative example. In No. 33, since the Mn content of the weld metal was excessive, red rust occurred due to the defective bead shape.
[0059]
　Experiment No. according to the comparative example. In No. 34, since the Al content of the weld metal was excessive, Al-based slag increased, and red rust occurred between the Al-based slag and the weld metal.
[0060]
　Experiment No. according to the comparative example. In 35, the Ti content of the weld metal was too small, and Al and Ti did not coexist, so only the Al-based slag aggregated on the weld bead, creating a gap between the weld metal and the Al-based slag. Red rust was generated from the voids.
[0061]
　Experiment No. according to the comparative example. In No. 36, since the Mn content of the weld metal was too small, the strength of the weld metal could not be ensured, and fracture occurred in the weld metal.
[0062]
　Experiment No. according to the comparative example. In No. 37, since the Ti content of the weld metal was excessive, the amount of Ti-based slag increased, and the adhesion of the Ti-based slag decreased, resulting in separation. Therefore, red rust was generated at the peeled portion.
[0063]
　Experiment No. according to the comparative example. In 38, the Al content of the weld metal was too small, and Al and Ti did not coexist, so only the Ti-based slag aggregated on the weld bead, creating a gap between the weld metal and the Ti-based slag. Red rust was generated from the voids.
Industrial applicability
[0064]
　INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a vehicle underbody part having a welded joint having excellent weld metal strength and corrosion resistance, which is highly industrially useful.
Code explanation
[0065]
1 welded joint
2 first steel plate
3 second steel plate
4 fillet weld, weld metal
The scope of the claims
[Claim 1]
　An automobile underbody part comprising a weld joint in which a first steel plate and a second steel plate are superimposed and a fillet weld is formed between an end face of the first steel plate and a surface of the second steel plate. The chemical composition of
　the weld metal forming the welded joint is, in mass% with respect to the total mass of the weld metal,
C: 0.02 to 0.20%,
Si: more than 0 to less than 0.10%,
Mn: 0.3 to 2.0%,
Al: 0.002 to 0.30%,
Ti: 0.005 to 0.30%,
P: more than 0% to 0.015%,
S: more than 0% to 0.030%,
Cu: 0-0.50%,
Cr: 0-1.5%,
Nb: 0-0.3%,
V: 0-0.3%,
Mo: 0-1.0%,
Ni: 0 to 2.5%,
B: 0 to 0.005%
, the balance being iron and impurities,
　and satisfying the following formulas (1) and (2). .
[Al]+[Ti]>0.05 Expression (1)
7×[Mn]−112×[Ti]−30×[Al]≦4.0 Expression (2)
　However, [Al], [Ti], and [Mn] mean the content in mass % of each component with respect to the total mass of the weld metal.
[Claim 2]
　The chemical components of the weld metal are
Cu: 0.05 to 0.50%,
Cr: 0.05 to 1.5%,
Nb: 0.005 to 0.3 , in mass% with respect to the total mass of the weld metal. %,
V: 0.005 to 0.3%,
Mo: 0.05 to 1.0%,
Ni: 0.05 to 2.5%,
B: 0.0005 to 0.005
% or two or more kinds thereof.
[Claim 3]
　3. The vehicle underbody part according to claim 1, wherein the thickness of the first steel plate having the fillet weld on the end face is 0.8 mm or more and 4.5 mm or less.
[Claim 4]
　4. The automobile underbody part according to any one of claims 1 to 3, wherein the tensile strength of the first steel plate and the second steel plate is 780 MPa or less.