JP2012183542A - Fillet arc welding method of steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welding method that solves a problem in which fatigue fracture strength caused by a root part is increased in a lap fillet welded joint of steel plates, and to provide a welded joint obtained by the method.SOLUTION: A lap fillet welding method of steel plates is provided in which an upper plate and a lower plate are overlapped, welding is performed so that the overlapped part integrates into a weld metal, and in which an end face of the upper plate and the surface on the side of the upper plate of the lower plate are connected through the weld metal and an end face of the lower plate and the surface on the side of the lower plate of the upper plate are connected through the weld metal. The welded joint is obtained as the result that an overlap space (overlapped length) being the length of the overlapped part of the upper plate and the lower plate in the longitudinal direction of the steel plates satisfies the following expression. -0.26×t+ 2×t-2.4 ≤ overlap space ≤ 0.1×t-1.1×t+ 4.3×t-2.7, wherein tis the plate thickness of the upper plate.

Description

  The present invention relates to a lap fillet welding of a steel plate used as a structural member of an automobile or the like, and relates to a lap fillet arc welding method of a steel plate capable of preventing the occurrence of cracks and obtaining a welded joint having high fatigue strength. is there.

In recent years, in the automobile field, from the viewpoint of improving fuel consumption by reducing the weight of a vehicle body, the thickness of a steel material has been reduced by applying a high-tensile steel plate. Further, as the thickness of the steel material is reduced, there is a concern about the perforation of the steel material due to corrosion, and the application of a high-strength galvanized steel sheet has been studied from the viewpoint of preventing corrosion.
As shown in FIG. 1, the pile fillet arc welding of steel plates is made by stacking two steel plates, and stacking the end of one steel plate (the end face and the vicinity thereof) on the surface of the other steel plate, To do. A steel plate having a surface to be welded to the end portion of the upper plate is referred to as a lower plate. In general, when fillet welding (unless otherwise noted, refers to fillet arc welding; the same shall apply hereinafter), two steel plates are placed horizontally, the upper plate is placed vertically, and the lower plate is lowered. Thus, it is called in this way because it is piled up and down and the fillet welding is performed with the welding wire approaching downward. However, actual lap fillet welding is not limited to the vertical direction, but defines the upper plate and the lower plate based on the relative positional relationship of the steel plates that are stacked as described above.

  When a lap fillet welded joint of steel plates is applied to a structure such as an automobile, its fatigue fracture (fatigue strength) becomes a problem. In the case of lap fillet welded joints, fatigue cracks are welded toes on the lower plate side where stress concentrates due to shape discontinuity (on the welding wire side of the welded joint, the upper plate surface or end surface, and the lower plate surface And the weld metal interface) and the route (on the opposite side of the weld wire of the welded joint, which is the interface between the upper plate (or lower plate) and the weld metal), and progresses in the plate thickness direction to break.

  The failure starting from the weld toe is due to the concentration of stress on the toe shape such as the radius of curvature or flank angle of the toe. On the other hand, in the fracture starting from the root, the portion of the upper plate and the lower plate that are not welded is the portion that remains without being welded. Because the hardness of the weld metal is different from the upper plate, the lower plate, and the weld metal, stress is applied to the root portion of the weld metal (the portion corresponding to the tip of the root portion of the weld metal). This is to concentrate.

In general, since the degree of stress concentration is larger in the root portion than in the toe portion, a crack is easily generated from the root portion depending on how the load is applied. Therefore, the structural designer designs the arrangement of the welded parts in consideration of the arrangement of the joints and how to apply the load. However, the reality is that it is not possible to take this into account for every part of the weld.
Patent Document 1 (Japanese Patent Application Laid-Open No. 2010-120022) discloses a fillet welded joint for thin steel sheets that improves the shape of the toe portion and a welding method therefor. This has the effect of improving the shape of the weld toe, but not the root. That is, it cannot cope with fatigue fracture starting from the root portion.

In addition, when the upper and lower steel plates are galvanized steel plates, a large amount of galvanization evaporates due to heat input associated with welding, so that the state of droplet transfer during welding becomes unstable, increasing spatter and melting. Blow holes and pits are generated when zinc vapor enters the metal. This increase in spatter and blowhole defects in the weld are problematic because the joint strength is reduced.
In Patent Document 2 (Japanese Patent Laid-Open No. 7-246465), as a technique for reducing spatter and blowhole during welding of a galvanized steel sheet, a gap of about 0.5 mm was provided between both members to be overlap welded. It is disclosed that it is effective to let gas escape to the opposite side of the weld.

However, providing a gap (gap) between the upper plate and the lower plate can be applied only to a structurally limited portion. Although it can be applied to a member that is long in the longitudinal direction such as a subframe of an automobile, it is difficult to accurately control a gap smaller than the plate thickness.
Usually, the overlap fillet welding of steel plates occupies the majority when the upper plate and the lower plate are overlapped without providing a gap. For example, an attachment such as a bracket or a stiffening plate (pad plate) cannot be welded with a gap.

  Patent Document 3 (Japanese Patent Application Laid-Open No. 7-1771679) describes a technique for eliminating the hardness difference between the weld metal and the steel plate and increasing the fatigue fracture strength. To be precise, this is a technique for reducing the hardness difference between the weld metal and the weld heat affected zone (HAZ) adjacent thereto. However, this relatively decreases the hardness of the weld metal, tends to cause cracks from the root portion, and tends to deteriorate fatigue fracture characteristics. Moreover, what was implemented by patent document 3 is the fillet welded joint which arranged the board orthogonally, and is not so effective in the fatigue fracture of the root part origin in the lap fillet welded joint of a steel plate.

JP 2010-120022 A Japanese Patent Laid-Open No. 7-246465 JP-A-7-171679

In order to increase the fatigue fracture strength of the lap fillet welded joint of steel plates, (a) increase the fatigue fracture strength due to the root part, (b) increase the fatigue fracture strength due to the weld toe part, (c) especially In lap fillet welding of galvanized steel sheets, there is a problem of preventing the occurrence of pore defects in pits and blowholes by zinc gas and increasing the fatigue fracture strength of the entire weld.
As described above, (B) Patent Document 1 for improving the shape of the weld toe, and (C) Patent Document 2 for pits and blowholes have proposed solutions.
However, in order to improve the fatigue fracture resistance of the lap fillet welded joint of steel plates, it is clear that it is necessary to increase the fatigue fracture strength due to the root portion.

  Then, this invention makes it a subject to raise the fatigue fracture strength resulting from a root part in the lap fillet welded joint of a steel plate, and it aims at providing the welding method which can solve it, and the welded joint as the result And

  As a result of intensive studies in order to solve the above problems, the inventors of the present invention have made contact portions between the upper plate and the lower plate that remain in a cutout shape in the root portion in the lap fillet welding of the steel plate (the unwelded overlap). Finding that the stress concentration in the root part can be alleviated by eliminating the "cutting part of the root part") and dissolving the overlapping part of the upper and lower plates in the weld metal. did.

  However, in this case, it is necessary to optimize the overlap of the upper plate and the lower plate (the overlap length of the upper plate and the lower plate before welding). If the overlap is large, the notch portion of the root portion remains, and if the overlap is small, the weld metal is pulled out to the back side of the lower plate, and welding defects such as perforations and dropouts occur. Therefore, the present inventors have devised a method for obtaining the optimum overlap allowance through experiments and the like.

Furthermore, by overlapping all the steel plates into the weld metal, the zinc gas generated when welding galvanized steel plates can be diffused and discharged out of the weld metal, reducing pits and blow holes. I found out that I can do it.
The present invention has been made based on these findings and ideas. The gist of the present invention is shown below.

(1) Two steel plates are overlapped so that the end of the upper steel plate is positioned on the surface of the lower steel plate, and the upper plate and the lower plate are welded along the end of the upper plate. In the lap fillet arc welded joint of the steel plate, the upper plate end face and the upper plate side surface of the lower plate are connected via a weld metal, and the lower plate end face and the lower plate side surface of the upper plate are also connected with the weld metal. A lap fillet welded joint of steel plates, characterized by being connected via
That is, it is a lap fillet welded joint of steel plates characterized in that the overlapping portion of the upper plate and the lower plate is dissolved in the weld metal.
(2) The weld metal connecting the lower plate end surface and the lower plate side surface of the upper plate has a chamfered shape at the boundary between the upper plate and the lower plate side surface (1) ) The fillet welded joint of steel plates according to (1).
(3) The width of the chamfer of the weld metal connecting the lower plate end surface and the lower plate side surface of the upper plate is 25% or less of the upper plate thickness from the boundary with the lower plate side surface of the upper plate. The lap fillet welded joint of steel plates according to (2), characterized in that
(4) The lap fillet welded joint of steel plates according to any one of (1) to (3), wherein the upper plate and the lower plate are galvanized steel plates.
(5) The lap fillet welded joint for steel plates according to (4), wherein a gap is provided between the upper plate and the lower plate.
(6) Two steel plates are stacked so that the end of the upper steel plate is positioned on the surface of the lower steel plate, and the upper plate and the lower plate are welded along the end of the upper plate. In the overlapped fillet arc welded joint of steel plates, the upper plate and the lower plate are overlapped and welded so that the overlapped portion melts into the weld metal, and the upper plate end surface and the upper plate side surface of the lower plate are weld metal And the lower plate end face and the lower plate side surface of the upper plate are also connected via the weld metal.
(7) The steel sheet according to (6), wherein a superposition allowance (superposition length) which is a length in a steel plate longitudinal direction of the superposed portion of the upper plate and the lower plate satisfies the following formula: Lap fillet welding method.
−0.26 × t ₁ ² + 2 × t ₁ −2.4 ≦ Overlap ≦ 0.1 × t ₁ ³ −1.1 × t ₁ ² + 4.3 × t ₁ −2.7
t1: Plate thickness of the upper plate (8) Chamfering a boundary portion between the lower plate side surface of the upper plate and the weld metal connecting the lower plate end surface and the lower plate side surface of the upper plate (6) or (7).
(9) The width of the chamfer of the weld metal connecting the lower plate end surface and the lower plate side surface of the upper plate is 25% or less of the upper plate thickness from the boundary with the lower plate side surface of the upper plate. The method of welding fillet of steel sheets according to (8), characterized in that:
(10) The steel plate lap fillet welding method according to any one of (6) to (9), wherein the upper plate and the lower plate are galvanized steel plates.
(11) The method according to (10), wherein a gap is provided between the upper plate and the lower plate.

  According to the lap fillet welded joint of steel plates manufactured by the welding method according to the present invention, there is an effect of increasing the fatigue fracture strength due to the root portion. Furthermore, by applying to lap fillet welding of galvanized steel sheet, the effect of suppressing the generation of pits and blowholes is achieved.

It is a longitudinal cross-sectional view which shows the outline | summary of the lap fillet welded joint of a steel plate. FIG. 1 (a) shows a conventional lap fillet welded joint. FIG. 1 (b) shows a lap fillet welded joint according to the present invention. FIG.1 (c) is the lap fillet welded joint which concerns on this invention, Comprising: The outline | summary of the chamfering of a weld metal root part is shown. FIG.1 (d) shows the enlarged view of the route | root part of FIG.1 (c). It is a figure which shows the shape of the test piece with which the Example of this invention is provided. It is a figure which shows the application range of this invention from the relationship between the board thickness of an upper board, and a stacking allowance. It is a figure which shows the relationship between route part length and a blowhole rate. It is a figure which shows the relationship between the steel plate used as a base material, and the hardness of a weld metal. It is a figure which shows the relationship between the hardness of the steel plate used as a base material, and the fatigue strength (2 million times fatigue strength) of a welded joint.

Hereinafter, embodiments of the present invention will be described in detail according to examples.
FIG. 1A is a conceptual diagram of conventional lap fillet welding, and FIG. 1B is a conceptual diagram of lap fillet welding according to the present invention. Conventionally, the upper plate and the lower plate are overlapped with each other when the upper plate and the lower plate are overlapped (the overlapping length of the upper plate and the lower plate before welding). For this reason, even after fillet welding, the portion where the upper plate and the lower plate are not joined remains in a cutout shape when viewed in a longitudinal section as shown in FIG. This remaining notched portion is called a root portion, and its length (length in the longitudinal direction of the steel plate) is called a root portion length. The root portion also extends in the width direction of the upper plate and the lower plate, and this surface (the surface facing both the upper plate side and the lower plate side) is called a route surface.

In fillet welding, the interface between the weld metal on the side opposite to the welding wire and the upper plate or the lower plate is referred to as a route of the weld metal. When the upper plate and the lower plate are stacked so as to be in close contact with each other, the tip of the route portion becomes the route of the weld metal.
Therefore, the same effect as a notch is produced at the tip of the root portion, and stress concentration is likely to occur in the root of the weld metal due to the difference in hardness between the weld metal and the steel plate as the upper and lower plates.

FIG. 1B shows a conceptual diagram of a longitudinal section of a fillet welded joint according to the present invention.
It is characterized in that the root portion that is the source of stress concentration in the conventional fillet welded joint is eliminated. For this reason, the upper plate end surface of the welding wire side and the upper plate side surface of the lower plate are connected via the weld metal. Further, the lower plate end face and the lower plate side surface of the upper plate on the root side (that is, the side opposite to the welding wire side) are also connected via the weld metal. In other words, the overlapping portion of the upper plate and the lower plate is in a state of being melted into the weld metal.
Since the root portion does not remain, a notch effect does not occur in the welded joint, which contributes to an improvement in fatigue strength.

  The length of the route portion remaining after welding is referred to as the remaining route portion length. In order to investigate the influence of the remaining root length on the stress concentration, the relationship between various remaining root lengths and stress in the crack opening direction was examined by a finite element method (FEM). The results are shown in Table 1. The “crack opening direction stress” value in the table indicates the stress value when the average stress is 100 MPa.

  Table 1 shows the effect of reducing the stress concentration by chamfering the boundary between the lower plate side surface of the upper plate and the weld metal in the state where the root portion length according to the present invention is 0, that is, there is no remaining route portion after welding. The result verified by FEM is also shown. Even if the root portion is only 0 mm, the stress concentration is reduced by 5% compared to the conventional notch route.

  Further, chamfering the vicinity of the root of the weld metal, that is, as shown in FIGS. 1C and 1D, even if the root portion length is 0 mm, the weld metal is projected to the root, and the lower plate end surface and the upper surface It was confirmed that a significant stress concentration reducing effect was achieved by connecting the lower plate side surface of the plate with a weld metal (this is called “chamfering”). A chamfering width of 0.15 mm and 11% chamfering of 15 mm and 15% of the chamfering width of 0.15 mm, respectively, confirmed the effect of reducing stress in the crack (root) opening direction with respect to the conventional notched root. It was also confirmed that when the chamfer width in the vicinity of the root exceeds 0.5 mm, the effect of reducing crack opening direction stress does not change much.

  The above simulation is an example when the plate thickness is 2 mm. The inventors also performed the same simulation at plate thicknesses of 5 mm and 10 mm. As a result, it was confirmed that when the plate thickness was 5 mm, the chamfering width was 1.25 mm, and when the plate thickness was 10 mm, the effect of reducing crack opening direction stress was not significantly changed when the chamfering width was 2.5 mm. Therefore, the upper limit of the chamfer width may be 25% of the plate thickness. The lower limit of the chamfer width is more preferably 0.15 mm in order to surely exert the stress concentration reduction effect by 10% or more.

In addition, as shown in FIG. 1D (enlarged view of the route portion in FIG. 1C), the chamfer width is the width of the weld metal protruding in the route when the fillet weld cross section is taken, It is defined by the distance connecting the boundary (route) between the upper plate and the lower plate side surface on the weld metal and the boundary between the weld metal and the lower plate end surface.
The cross-sectional shape of the chamfered portion is not particularly limited, but the angle of the chamfered portion is preferably inclined approximately ± 30 degrees around 45 degrees with respect to the lower plate side surface of the upper plate. If the angle is too large, the chamfering effect is reduced, and the weld metal may penetrate through the lower plate and melt. On the other hand, if the angle is too small, stress concentration at the boundary between the weld metal and the end face of the lower plate increases. Therefore, the angle is preferably set to 45 ° ± 15 °. Further, the right-angled route means that the overlap margin is 0 mm and the chamfer width is 0 mm.

Next, the appropriate range of overlap allowance will be considered.
Unless an appropriate overlap margin is set for the plate thickness of the upper plate, the welding operation itself is not successful, and a welded joint cannot be obtained as well as a desired effect. That is, if the overlap margin is too large, the root portion remains, so stress concentration increases and the strength decreases. On the other hand, if the stacking allowance is too small, the weld metal is greatly pulled out to the back side (that is, the root side) of the upper plate, so that the upper plate is melted, and welding defects such as perforation and melt-down occur.

Therefore, there is an appropriate range for the stacking allowance with respect to the thickness of the upper plate. The results of investigation on this appropriate range are shown in FIG. In FIG. 3, ○ indicates a condition in which the root portion does not remain, ● indicates a condition in which the root portion remains, and × indicates a condition in which the weld metal has fallen off. As is clear from FIG. 3, it was found that an appropriate overlap margin range exists for the upper plate thickness. The present inventors have derived the following conditional expression for the proper overlap margin range (L) by polynomial function approximation of the plate thickness (t ₁ ) of the upper plate.

−0.26 × t ₁ ² + 2 × t ₁ −2.4 ≦ L
≦ 0.1 × t ₁ ³ −1.1 × t ₁ ² + 4.3 × t ₁ −2.7

If this equation is followed, an appropriate overlap can be easily obtained.
Next, the blowhole which becomes a problem in the lap fillet welding of the galvanized steel sheet was considered. As described in Patent Document 1, by providing a gap (gap) between the upper plate and the lower plate, zinc gas escapes to the route side of the weld metal, which is effective in suppressing blowholes.

 However, in the welded joint according to the present invention, since the route of the weld metal comes out on the back side (lower plate side) of the upper plate, the zinc gas escapes from the same as when the gap is provided, so that the blowhole suppressing effect is obtained. There is. Therefore, since the gap normally provided for avoiding a blowhole becomes unnecessary, the welded joint according to the present invention is not particularly limited with respect to the gap. It should be noted that providing a gap to some extent is advantageous from the viewpoint of fatigue strength because stress concentration at the root is mitigated, but if the gap becomes too large, a fracture form in which a crack is generated from the root portion is obtained. In some cases, gap management is important.

On the other hand, if the length of the root portion exists even a little, the weld metal does not come into contact with the atmosphere on the route side, so the zinc gas cannot escape and the occurrence of blowholes is not suppressed. If the blowhole in the weld metal is less than 20% by deposition, the static strength of the welded joint is not greatly reduced, but the fatigue strength is greatly affected. For this reason, it is desirable that the blowhole be 0% as much as possible.
Moreover, since the higher the strength of the steel plate, the more sensitive the blow hole to the strength, it is desirable to suppress the generation of blow holes.

Next, based on an Example, this invention is demonstrated.
In Examples, a fatigue test was performed on a test piece obtained by laminating fillet welding of thin steel plates, and the effect was verified. Table 2 shows the test steel sheets, and Table 3 shows the test welding wires. As the test steel plates, hot dip galvanized steel plates were used. Except for the effect of blowholes, galvanized steel sheets and non-plated steel sheets can be considered to have the same weld joint characteristics. Here, in order to evaluate the influence with respect to the blowhole of the welded joint which concerns on this invention, a galvanized steel plate is used.

FIG. 2 shows the shape of a test piece subjected to a fatigue test. The weld bead toe of the lap fillet arc welded joint was placed at the center of the test piece. The welding conditions are as follows.
・ Welding method Consumable electrode welding / Welding power source DP350 (Daihen Co., Ltd.)
・ Welding mode DC-Pulse (Ar + 20% CO2 gas)
・ Welding posture Downward horizontal overlap fillet welding ・ Distance between tip base materials (extrusion length) 15mm
・ Torch angle 55 °
・ Target position 0.5mm away from overlapped corner ・ Shield gas flow 20L / min
・ Welding speed 100cm / min
・ The wire feed speed was set to an appropriate value to prevent undercutting on the upper plate side.

The test piece was subjected to an electrohydraulic fatigue test apparatus, subjected to an axial tension fatigue test with a constant stress amplitude and a stress ratio of 0.1, and the fatigue strength was measured. Fatigue strength was evaluated with a stress amplitude at which the test piece did not break even when stress was repeatedly applied 2 million times. The joint with a root length of 0 mm was produced by setting the welding conditions so that the chamfer width was about 20% of the upper plate thickness.
The evaluation results are shown in Table 4.

In the present example, the feeding speed of the welding wire was set to an appropriate value so as not to cause an undercut on the upper plate side. J10 to J18 are examples of the present invention, and J1 to J9 are comparative examples in which the root portion length is set to about 5 to 8 mm. Further, “Fatigue Strength Improvement Rate (vs. Comparative Example)” in Table 4 indicates the fatigue strength improvement rate with respect to a comparative material having the same steel plate and the same welding wire but having different root part lengths. Similarly, "Fatigue strength improvement rate (vs. soft wire)" is a group of the same steel type and the same route length, and is a relative comparison (fatigue strength ratio) with an example using the softest wire in the group. Show. Similarly, the “fatigue strength improvement rate (vs. J17)” is represented by a relative comparison (fatigue strength ratio) with respect to J17 having the lowest fatigue strength among joints having a root portion length of 0 mm and a blowhole ratio of 0%. The “fatigue strength improvement rate (vs. blowhole 0%)” is expressed as a relative comparison (fatigue strength ratio) with respect to the same steel type and the same welding wire with no blowhole.
As can be seen from the results, all of the inventive examples had an effect of improving the fatigue strength by 13 to 36% with respect to the comparative example in which the root length was set to about 5 to 8 mm with the same steel plate and the same welding wire.

  Furthermore, it was confirmed that the inventive example has a blowhole rate of 0% and a high blowhole suppression effect. FIG. 4 shows the relationship between the blowhole rate (volume fraction) and the route length. It can be seen that the blowhole rate increases if the route length exists even a little. Therefore, it was also confirmed that the blowhole suppression effect of the present invention is large.

  It is known that when there is a blowhole, the fatigue strength decreases, but the decrease rate increases as the base material hardness increases. That is, the higher the base material hardness, the higher the blowhole sensitivity. It was also found that if the overmatch rate (the ratio of the hardness of the weld metal to the base material) is 120% or more, the fatigue strength reduction rate due to blowholes can be suppressed to 20% or less. Also from this point of view, it is possible to suppress blowhole sensitivity regardless of the steel type of the base material by setting the length of the root portion to 0 as in the present invention example.

Again, it is known that the suppression of blowholes has a large effect of improving static strength and fatigue strength, and the effect is greater as the strength of the steel plate becomes higher. Therefore, excellent fatigue fracture resistance can be obtained by applying the present invention to a galvanized steel sheet.
FIG. 5 shows the relationship between the base metal hardness and the weld metal hardness, and FIG. 6 shows the relationship between the base material hardness and the fatigue strength. The fatigue strength increases in proportion to the hardness of the base material, but the increase rate is small when the overmatch rate is low. Further, if the overmatch rate is 120% or more, the fatigue strength is improved by 120% or more with respect to a so-called even match joint having an overmatch rate of about 100%.

In the above, according to the Example, the welded joint and the welding method which concern on this invention were demonstrated.
The embodiments of the present invention are not limited to the embodiments described in the above examples. Those substantially satisfying the requirements of the invention fall within the scope of the invention.

  The present invention can be used not only in the machine industry but also in industries that can apply lap fillet welding of steel plates.

Claims (11)

  Two steel plates are overlapped so that the end of the upper steel plate is positioned on the surface of the lower steel plate, and the upper plate and the lower plate are welded along the end of the upper plate. In a steel plate lap fillet arc welded joint, the upper plate end surface and the upper plate side surface of the lower plate are connected via a weld metal, and the lower plate end surface and the lower plate side surface of the upper plate are also connected via the weld metal. A lap fillet welded joint for steel plates, characterized in that   The boundary portion between the lower plate side surface of the upper plate and the weld metal connecting the lower plate end surface and the lower plate side surface of the upper plate has a chamfered shape. Steel plate lap fillet welded joint.   The width of the chamfer of the weld metal connecting the lower plate end surface and the lower plate side surface of the upper plate is 25% or less of the upper plate thickness from the boundary with the lower plate side surface of the upper plate. The lap fillet welded joint of steel plates according to claim 2.   The lap fillet welded joint according to any one of claims 1 to 3, wherein the upper plate and the lower plate are galvanized steel plates.   The lap fillet welded joint for steel plates according to claim 4, wherein a gap is provided between the upper plate and the lower plate.   Two steel plates are overlapped so that the end of the upper steel plate is positioned on the surface of the lower steel plate, and the upper plate and the lower plate are welded along the end of the upper plate. In a lap fillet arc welded joint of steel plates, the upper plate and the lower plate are overlapped and welded so that the overlapped portion melts into the weld metal, and the upper plate end surface and the upper plate side surface of the lower plate are connected via the weld metal. A method for welding fillet of steel sheets, wherein the lower plate end surface and the lower plate side surface of the upper plate are also connected via the weld metal and a series of weld metals. The overlap fillet welding method for steel sheets according to claim 6, wherein an overlap margin, which is a length in a longitudinal direction of the steel sheet, of the overlapping portion of the upper plate and the lower plate satisfies the following expression.
−0.26 × t ₁ ² + 2 × t ₁ −2.4 ≦ Overlap allowance
≦ 0.1 × t ₁ ³ −1.1 × t ₁ ² + 4.3 × t ₁ −2.7
t1: Thickness of the upper plate   The boundary portion between the lower plate side surface of the upper plate and the weld metal connecting the lower plate end surface and the lower plate side surface of the upper plate is chamfered. Lap fillet welding method for steel plates.   The width of the chamfer of the weld metal connecting the lower plate end surface and the lower plate side surface of the upper plate is 25% or less of the upper plate thickness from the boundary with the lower plate side surface of the upper plate. A method for welding the fillet of steel sheets according to claim 8.   10. The method according to claim 6, wherein the upper plate and the lower plate are galvanized steel plates. 11.   The method according to claim 10, wherein a gap is provided between the upper plate and the lower plate.

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