JP6380672B2 - Welded joint and its manufacturing method - Google Patents

Description

  The present invention relates to a weld joint. More specifically, it relates to a welded joint of a steel structure.

  In welded joints where steel materials are welded together, it is common to use joints (welded joints) that have been welded so that the tensile strength of the weld metal exceeds the tensile strength standard lower limit of the steel material (overmatch) ). This is because the tensile strength of the weld metal of the welded portion is made stronger than the steel material to be welded, thereby preventing early breakage at the welded portion and sufficiently satisfying the predetermined design strength. As such an invention, the invention of the following Patent Document 1 has been proposed.

  In recent years, high strength steel materials have been developed and used. In order to use a high-strength steel material and perform a welding operation that is over-matched to the tensile strength of the steel material, it is difficult to manage the welding conditions, and the work efficiency is significantly reduced. In addition, for ultra-high-strength steel materials having a tensile strength of 780 MPa or more, there are currently few welding materials that can be used with peace of mind and it is difficult to obtain them.

  Regarding these problems when using high-strength steel materials, the inventions of Patent Documents 2 and 3 below have been proposed for fillet welding.

Japanese Patent No. 3756616 JP 2013-139047 A JP 2014-8515 A

  However, in the inventions described in Patent Documents 2 and 3, since stress concentration tends to occur at the toe portion of the reinforced weld metal in shape, there is a high possibility that brittle fracture will occur when tensile stress is applied. There is a problem.

  Note that, in a welded portion of high-strength steel, the amount of penetration tends to be small due to welding with low heat input. In addition, when the reinforced weld metal as in Patent Document 2 is formed with one weld bead by gas shield arc welding, problems such as a decrease in toughness of the base metal heat-affected zone in the vicinity of the toe portion also occur. However, there is a concern about early brittle fracture at the weld.

  An object of the present invention is to provide a welded joint that can secure weld joint strength even when the tensile strength of the weld metal is lower than the tensile strength of steel (undermatch) and prevent brittle fracture.

  In order to solve the above-mentioned problems, the present invention provides: 1) the strength of the welded portion (= total thickness of the surplus, steel plate thickness, back surface penetration depth, etc.) × weld so that the fracture of the steel material precedes the welded portion (2) Provide a reinforcement bead with a predetermined width on the surface of the steel material to ensure that the steel material breaks more reliably. 3) Reinforcement In order to prevent brittle fracture at the bead toe, the flank angle of the reinforcing bead toe is a predetermined value.

  With the above ideas, 1) weld metal has sufficient strength even if it has a lower tensile strength than steel, 2) stress / strain concentration at the toe of the reinforcing bead can be reduced, and 3) for ultra-high strength steel In addition, the construction conditions can be relaxed (reducing residual heat in the case of overmatch welding, enabling large heat input welding to simplify the welding process), and ensuring joint strength.

  The present inventors have further studied based on the above findings and completed the present invention. The gist of the present invention is as follows.

  [1] Both steel materials and their joints are provided with weld metal, and at least one surface is a weld joint having an overlay on the surfaces of both steel materials, and is built up on the surfaces of the steel materials. The width of each reinforcing bead, which is a surplus, is equal to or greater than the surplus thickness of the steel surface end, the flank angle θ of each reinforcing bead toe is 145 ° to 170 °, and the tensile strength of the weld metal is A welded joint that is smaller than the tensile strength of the steel and satisfies the condition of the following formula (1).

In Equation (1), a is the maximum thickness (mm) of the surplus, t is the thickness (mm) of the steel material, σ _uw is the tensile strength (MPa) of the weld metal, and α is the safety factor (no unit). The value defined by 1.01-1.20, T _sl is the tensile strength (MPa) of the steel material. d is 0 or more, and when both surfaces have extras, it is the maximum thickness (mm) of other extras.

  [2] The weld joint according to [1], wherein α is a safety factor (no unit) and is a value determined by 1.03 to 1.20.

  [3] A concave backing metal having a concave portion on a surface opposite to the surface including the surplus is provided, and d in the formula (1) is a filling depth (mm) in the concave backing metal. The weld joint according to [1] or [2].

  [4] A backing metal is provided on the surface opposite to the surface provided with the surplus, the backing metal has a penetration portion, and d in the formula (1) is a penetration depth (mm The weld joint according to [1] or [2].

  [5] The weld joint according to any one of [1] to [4], wherein at least one of the steel materials has a groove having a bevel angle φ of 15 ° to 35 °.

  [6] A method for manufacturing a welded joint according to any one of [1] to [5], wherein a weld metal is formed in a root gap of both steel materials, and at least one surface is formed on the surfaces of both steel materials. A method for manufacturing a welded joint, which forms an overfill.

  [7] The method for manufacturing a welded joint according to [6], wherein the root gap is provided with a backing metal or a concave backing metal and welding is performed.

  According to the present invention, it is possible to provide a welded joint that secures weld joint strength at an undermatch weld joint using high-strength steel, can prevent brittle fracture, and can relax welding conditions.

FIG. 1 is a schematic diagram illustrating one embodiment of the present invention. FIG. 2 is a schematic diagram for explaining the dimensions and the like of the weld joint. FIG. 3 is a schematic diagram for explaining the dimensions and the like of the weld joint. FIG. 4 is a drawing showing the relationship between the flank angle of the reinforcing bead toe and the stress concentration factor. FIG. 5 is a schematic diagram illustrating one embodiment of the present invention. FIG. 6 is a schematic diagram illustrating one embodiment of the present invention. FIG. 7 is a schematic diagram illustrating one embodiment of the present invention. FIG. 8 is an example of a two-dimensional plane strain element model. FIG. 9 is a schematic diagram for explaining a gauge for examining a flank angle.

  Hereinafter, the present invention will be described in detail. The present invention is an undermatched weld joint in which the tensile strength of the weld metal is smaller than the tensile strength of the steel material. The present invention can also be applied to high-strength steel materials that have been difficult to apply in the past, and can also be applied to materials having a large difference in tensile strength between steel materials and weld metals. Specifically, in the present invention, the steel material is preferably applicable to a steel material having a tensile strength of 600 to 900 MPa, and the steel material preferably has a tensile strength of 600 to 800 MPa. The tensile strength of the weld metal is, for example, 550 MPa or more, and the upper limit is, for example, 800 MPa. From the viewpoint of welding workability and the like, the tensile strength of the weld metal is practically, for example, 550 to 700 MPa. Moreover, in this invention, the tensile strength difference of steel materials and a weld metal can also be over 200 MPa. Of course, the present invention can be applied to a steel material and a weld metal having a tensile strength difference of 200 MPa or less. The difference in tensile strength between the steel material and the weld metal is preferably 250 MPa or less. In addition, in this invention, the tensile strength of steel materials is calculated | required according to the JISZ2241 metal material tensile test method. The tensile strength of the weld metal is determined according to the JIS Z 2241 metal material tensile test method using JIS Z 3111 A0 test piece, A1 test piece or A2 test piece.

  FIG. 1 is a schematic cross-sectional view of a welded joint in butt welding, and is a schematic diagram illustrating an embodiment of the present invention. As shown in FIG. 1, the weld joint of the present invention is based on butt welding, and is based on complete penetration welding in which the entire thickness of the base material (steel material) is melted. The weld joint shown in FIG. 1 has a substantially symmetrical shape on the left and right. In the following description of the present invention, the illustration of the weld heat affected zone is omitted.

  The weld joint (welded joint) 1 includes both steel materials 4 and weld metals 3 at the joint portions thereof, that is, the weld joint (welded joint) 1 includes weld metal 3 and steel materials 4 on both sides thereof. In the weld metal 3, a region having a width represented by a dotted line (between the weld metal 3 side closest to the steel material 4) is the center portion 2 in the width direction. Such welding joints can be obtained by performing welding in a state in which the backing metal 6 is provided at the root gap portion between the steel materials 4. The “back metal” is a flat plate (for example, flat steel) that does not have a recess on the surface of the steel material to be welded before welding, and a shape that has a recess as shown in FIG. become.

  The upper surface in FIG. 1 has a surplus of the maximum thickness a, and is provided with a surplus (reinforcing bead 5) that is also built up on the surfaces of both steel materials 4. The extra thickness at the end of the steel surface is indicated by a double arrow 9. Further, the width of the reinforcing bead is from the steel surface end to the weld toe 8. The flank angle of the weld toe 8 is indicated by θ, and the bevel angle of the groove in the steel material 4 is indicated by φ. A backing metal 6 is provided on the surface opposite to the surface provided with the surplus, and the backing metal has a penetration portion 7 having a depth d.

With reference to FIG. 2, a preferred embodiment will be further described with respect to the surplus on the central portion 2 in the width direction, the surplus at the end portion of the steel material, and the reinforcing bead width. In FIG. 2, the horizontal axis is X, and the center of the center portion 2 in the width direction is X = 0. X ₁ is the distal end position of the groove, X ₂ is steel surface and edge position, X ₃ is a reinforcing bead toe position.

  In FIG. 2, a is the maximum thickness of the surplus, and it is preferable that the surplus becomes the maximum thickness on the center portion 2 in the width direction (just above the center portion 2 in the width direction). In addition, from the viewpoint of causing the steel material to break before the welded portion, the welded joint portion of the present invention satisfies the condition of the following formula (1). Moreover, the width | variety of each reinforcement bead is more than the extra thickness of the steel material surface edge part.

In Equation (1), a is the maximum thickness (mm) of the surplus, t is the thickness (mm) of the steel material, σ _uw is the tensile strength (MPa) of the weld metal, and α is the safety factor (no unit). The value defined by 1.01-1.20, T _sl is the tensile strength (MPa) of the steel material. d is 0 or more, and when both surfaces have extras, it is the maximum thickness (mm) of other extras. That is, d is the maximum thickness (mm) of the backside reinforcement of the steel material (in the case where a backing metal is provided on the backside, the depth of penetration of the backing metal), and 0 if there is no backfilling on the backside. . In FIG. 1, d is the penetration depth (mm) at the penetration portion. If the backing metal is not provided and there is no melted portion in the backing metal, d = 0 may be set. Moreover, in Formula (1), (alpha) is a safety factor (unitless), and it is preferable that it is a value defined by 1.03-1.20. Thus, when α is 1.03-1.20, the effect of the present invention, that is, the weld joint strength at the undermatch weld joint using the high-strength steel material is secured, and brittle fracture is prevented. It is possible to more reliably exhibit the effect of providing a welded joint that can relax welding conditions.

2, excess metal thickness at X ₂ position is weld reinforcement thickness of the steel material surface and edge. Groove portion excess Sheng _{a x} at an arbitrary position between the _X 1 to X ₂ in FIG. 2 is preferably satisfies the following formula (2).

Here, t is the thickness of the steel (mm), _{t x} is the thickness of the steel material of the groove portion (mm), sigma _uw tensile weld metal strength (MPa), alpha is met safety factor (unitless) The value defined by 1.01-1.20, T _sl is the tensile strength (MPa) of the steel material. In the formula (2), α is a safety factor (no unit) and is preferably a value determined by 1.03 to 1.20. Thus, the effect of the present invention can be more reliably exhibited when α is 1.03 to 1.20.

_X 3 -X ₂ in FIG. 2 is the width of the reinforcing bead (mm). The width of the reinforcing bead preferably satisfies the condition of the following formula (3).

In the above formula (3), a _x2 is the extra thickness (mm) of the steel material surface end, and θ is the flank angle.

FIG. 3 is a schematic diagram for explaining the dimensions and the like of the welded joint, and in the embodiment of the present invention, the height of the surplus height (thickness) at a plurality of plate thicknesses and the distance from the center of the central portion in the width direction are shown. It is a graph showing the relationship. The conditions are as follows. In addition, the graph of FIG. 3 was calculated | required by Formula (1)-(3) on the following conditions.
-Width direction distance of center part 2 in the width direction (interval between the weld metal parts of each steel material) = 7.0 mm, bevel angle φ = 35 °, flank angle θ = 150 °, steel thickness = 25 to 50 mm , D = 0mm
Steel material tensile strength standard lower limit value T _sl = 780 MPa, weld metal tensile strength σ _uw = 650 MPa
・ Safety factor α = 1.1
As shown in FIG. 3, as described above, it is preferable that the extra height is maximized at any one of the center portions in the width direction (0 to X ₁ in FIG. 3). The extra height (maximum extra thickness a) of the central portion in the width direction is 8.0 mm when t = 25 mm, 9.0 mm when t = 28 mm, 10.2 mm when t = 32 mm, 11.5 mm when t = 36 mm, 12.8 mm when t = 40 mm, 14.4 mm when t = 45 mm, and 16.0 mm when t = 50 mm.

The extra height (the extra thickness a _x2 ) of the steel surface end (X ₂ in FIG. 3) is 3.0 mm when t = 25 mm, and 3.3 when t = 28 mm. 4 mm, 3.8 mm when t = 32 mm, 4.3 mm when t = 36 mm, 4.8 mm when t = 40 mm, 5.4 mm when t = 45 mm, and 6 when t = 50 mm. 0 mm.

  The distance from the center in the width direction to the toe end of the reinforcing bead is 26.2 mm when t = 25 mm, 28.9 mm when t = 28 mm, 32.6 mm when t = 32 mm, and t = 36 mm. 36.2 mm when t = 40 mm, 44.4 mm when t = 45 mm, 48.9 mm when t = 50 mm.

The width X ₃ -X ₂ of the reinforcing beads is 5.2 mm when t = 25 mm, 5.8 mm when t = 28 mm, 6.6 mm when t = 32 mm, and 7 when t = 36 mm. 0.5 mm, 8.3 mm when t = 40 mm, 9.4 mm when t = 45 mm, and 10.4 mm when t = 50 mm.

  In addition, in this invention, 7.0-20.0 mm is preferable and, as for the maximum value (maximum thickness a of a surplus), it is more preferable to set it as 15 mm-20 mm from a viewpoint of ensuring welded-part strength.

Further, from the viewpoint of preventing breakage at the toe end of the reinforcing bead, the surplus thickness a _x2 of the steel material surface end is preferably 2.5 to 10 mm, and more preferably 5 to 10 mm.

The width _X 3 -X ₂ reinforcing beads from the viewpoint of stress concentration relieving reinforcing bead toe portion is preferably set to 5.0Mm～50mm.

  Further, from the viewpoint of reinforcing bead welding workability, the thickness t of the steel material is preferably 12 to 80 mm, and more preferably 12 to 60 mm.

  Moreover, it is preferable that the width direction distance (root gap) of the width direction center part shall be 3.0 mm-10.0 mm from a viewpoint of ensuring the welding part penetration amount.

  Next, the flank angle will be described. In the present invention, the flank angle is determined by a radius gauge. The flank angle is an angle θ between the tangent line of the reinforcing bead 5 at the reinforcing bead toe 8 and the surface of the steel material 4.

  For the welded joint having the configuration shown in FIG. 1, the flank angle and stress concentration factor (how many times the average stress acting on the welded joint member is generated) of the reinforcing bead toe are analyzed in the FEM elastic-plastic analysis. And examined. The model was a two-dimensional plane strain element model and analyzed by changing the flank angle from 180 ° to 140 °. FIG. 8 is an example of an element model. Further, as an example of the analysis result, the result in the case of a steel plate thickness of 32 mm, a steel material strength of 780 MPa, and a weld metal strength of 700 MPa is shown in FIG.

  From the experiments so far, it has been confirmed that if the stress concentration coefficient in the FEM elastic analysis is 3 or less, the actual welded joint will retain almost the maximum proof stress due to the effect of plasticization. In the present invention, the flank angle θ is set so that the stress concentration factor is 2.5 or less, in view of a little safety.

  The flank angle θ is 145 ° or more in order to reduce stress / strain concentration at the toe portion of the reinforcing bead and prevent brittle fracture. On the other hand, from the viewpoint of welding workability, the flank angle θ is 170 ° or less. The flank angle is preferably 145 ° to 160 °.

  In the weld joint 1 of FIG. 1, each steel material 4 has a groove on one side. In order to prevent cracks and the like, the bevel angle φ is preferably 15 ° to 35 °.

  In the welded joint 1 of FIG. 1, the lower surface is provided with a backing metal 6, and the backing metal 6 has a penetration portion 7 having a depth d from the viewpoint of making the steel material break before the welded portion. doing. The depth d is preferably 2 mm or more, and more preferably 3 mm or more. When the steel material strength is 780 MPa or more, or when the steel material thickness (t) is 19 mm to 40 mm, the depth d is set to 5 mm to 10 mm from the viewpoint of avoiding excessive surplus with respect to the plate thickness (depth). More preferred is a melted part). In addition, when forming a deep penetration part, it is preferable to determine welding conditions beforehand and to confirm how much it melts. When determining the surplus height in consideration of the penetration depth d, it is desirable to use a material having a strength of 590 to 780 MPa as the backing metal 6 from the viewpoint of securing the weld metal strength.

  FIG. 5 is a schematic diagram showing a modification of the embodiment shown in FIG. In the embodiment described in FIG. 5, the upper and lower sides of the steel material are provided with extras and each surface has the same configuration as the embodiment based on FIG. 1 described above. In the above formula (1), d is not the penetration depth at the penetration portion, but the height (mm) of the overfill. That is, in the above formula (1), the extra heights on the upper and lower surfaces are a and d, respectively.

  Other embodiments will be described below. Also in the following embodiment, the structure already described can be provided. In any of the embodiments, as shown in FIG. 5, extra reinforcement may be provided on both the upper and lower sides of the steel material.

  FIG. 6 is a schematic diagram for explaining an embodiment of the present invention. In this embodiment, a concave backing metal 6 having a concave portion is used. The recessed part should just be located on the center part 2 of the width direction. Within the recess is a weld metal fill 10. The depth d of the concave portion (filling portion 10) is not particularly limited, and is preferably 5 mm to 10 mm. It should be noted that the depth (filling depth) d of the filling portion 10 and the depth d of the penetration portion are common in that they are provided from the viewpoint of causing the steel material to break before the welded portion. The embodiment shown in FIG. 6 is preferably applied from the viewpoint of welding workability when the steel material strength is 780 MPa or more, or when the steel material thickness (t) is 40 mm to 80 mm.

FIG. 7 is a schematic diagram illustrating one embodiment of the present invention. The thickness of each steel material 4 is different, and one steel material 4 is a ladle groove not provided with a groove. Even in this case, the reinforcing beads 5 are provided on both sides. The surplus thickness a ₀ at the surface end of the left steel material (thicker steel material) is the surplus thickness a _X2 at the surface end of the right steel material (thin steel material), and the thicknesses t ₁ and t _{2 of} both steel materials (the thinner one) The thickness of the steel material is t ₁ , and the thickness of the thicker steel material is t ₂ ) (a ₀ = a _X2 − (t ₂ −t ₁ )). In both the left and right sides, the width (mm) of the reinforcing bead 5 is equal to or greater than the thickness of the steel material surface end. When one steel material does not have a groove, the bevel angle φ of the other steel material is preferably 25 ° to 35 ° from the viewpoint of weldability.

In the embodiment shown in FIG. 7, the extra thickness at the central portion 2 in the width direction is not uniquely determined. However, when the left steel material is determined as a reference, the surplus thickness in the width direction center portion 2 is equal to or greater than the surplus thickness a ₀ , and when the right steel material is determined as a reference, the surplus thickness in the width direction center portion 2 is If the surplus thickness a _X2 or more, it can be determined that “the surplus is maximum on the central portion 2 in the width direction”. In the above formula (1), the thinner plate thickness is adopted.

  In the present invention, the steel material can be appropriately selected and used. For example, as a combination of steel materials, a combination of a steel plate and a steel plate, a flange of H-shaped steel and a steel plate can be used. Moreover, you may apply this invention as a welding joint part of a UOE steel pipe or a spiral steel pipe.

  The manufacturing method of the welded joint according to the present invention, that is, the forming method (welding method) is not particularly limited. For example, it can be manufactured by the method for manufacturing a welded joint according to the present invention, in which a weld metal is formed in the root gap of both steel materials, and at least one surface is formed with a buildup on the surfaces of both steel materials. In that case, it is preferable to perform welding by providing a backing metal or a concave backing metal in the root gap. Specifically, for example, it can be formed by submerged arc welding. Further, the weld joint may be formed by one pass at a heat input of 50 to 200 kJ / cm, or the weld joint may be formed by a plurality of passes of two or more at a heat input of 10 to 40 kJ / cm. .

A tensile test of the welded joint (welded joint) was performed. Specifically, as shown in FIG. 1, first, two same steel plates (plate thickness 25 mm × width 400 mm × length 200 mm, tensile strength 780 MPa class steel plate) are set to a surplus thickness target value of 10 mm (safety factor α = 1.1 aim), 8 mm (safety factor α = 1.01 aim), butt welding was performed so as to be 4 mm to obtain a welded joint (welded member). And it cut out by machining so that a test site | part might be set to 75 mm in width from the obtained welded joint as a test body. From the macro test (test for observing the cross section of the welded portion) of the obtained test specimen, the test specimen No. 1-No. In all cases, the flank angle θ of the reinforcing bead toe was 145 °. In addition, about the flank angle (theta), it can also be easily confirmed whether it is an angle within a predetermined range by using the stainless steel gauge 20 as shown in FIG. FIG. 8 is a schematic diagram illustrating a gauge for examining a flank angle. For example, by using a gauge having an angle Z of 145 ° and a gauge having an angle Z of 170 °, the flank angle θ is 145 ° to 170 °. It can be easily determined whether or not it is within the range. The groove and welding conditions are as follows.
<Cave>
Groove shape: V-shaped groove, groove angle (bevel angle φ): 35 °, route gap (RG): 5 mm, backing metal: thickness 9 mm × width 25 mm
<Welding conditions>
Welding material: 680 N / mm class ² solid wire, diameter = 1.4 mm
Current: 280-300 (A), Voltage: 28-30 (V), Speed: 18-30 (cm / min), Temperature between passes: Maximum 250 ° C.
Table 1 shows the tensile strength, the measurement results of the welds, and the tensile test results. The tensile strengths of the base material (steel material) and the weld metal were determined by the methods described above. The yield strength of the base material (steel material) and the 0.2% proof stress of the weld metal were determined in accordance with JIS Z 2241, respectively. In addition, No. In 3, there was no reinforcing bead.

  As shown in Table 1, the specimen manufactured according to the present invention, that is, the width of the reinforcing bead is equal to or greater than the extra thickness of the steel surface end, and the flank angle of the reinforcing bead toe is not less than 145 ° and not more than 170 °. The tensile strength of the weld metal is smaller than the tensile strength of each steel material, and the conditions of the above formula (1) are satisfied. No. 1 specimen and No. 1 No. 1 with safety factor α slightly lower than 1 specimen. In the two specimens, it was confirmed that the base material (steel material) was preceded by breakage even when the weld metal strength was weak. On the other hand, no. In the three specimens, the weld metal breakage preceded. From the test results, the effect of the present invention was confirmed.

DESCRIPTION OF SYMBOLS 1 Welded joint part 2 Width direction center part 3 Weld metal 4 Steel material 5 Reinforcement bead 6 Back metal 7 Melting part 8 Reinforcement bead toe part 9 Extra thickness 10 of steel surface end part Filling part

Claims (7)

Both steel materials and their joints are provided with weld metal, and at least one surface is a weld joint having a surfacing on the surface of both steel materials,
The width of each reinforcing bead that is a surplus piled up on the surface of the steel material satisfies the condition of the following formula (3) ,
The flank angle θ of each reinforcing bead toe is 145 ° to 170 °,
The tensile strength of the weld metal is smaller than the tensile strength of each steel material,
A welded joint that satisfies the following formula (1).
In Equation (1), a is the maximum thickness (mm) of the surplus, t is the thickness (mm) of the steel material, σ _uw is the tensile strength (MPa) of the weld metal, and α is the safety factor (no unit). The value defined by 1.01-1.20, T _sl is the tensile strength (MPa) of the steel material. d is 0 or more, and when both surfaces have extras, it is the maximum thickness (mm) of other extras.
In the formula _(3), x 3 is reinforcing bead toe position, x ₂ is the steel surface and edge positions of the groove, a _x2 are excess metal thickness of the steel surface and edge of the groove (mm), θ is a flank angle is there.   The welded joint according to claim 1, wherein α is a safety factor (no unit) and is a value determined by 1.03 to 1.20.   2. A concave backing metal having a concave portion on a surface opposite to the surface including the surplus is provided, and d in the formula (1) is a filling depth (mm) in the concave backing metal. The weld joint according to 2.   A backing metal is provided on the surface opposite to the surface provided with the surplus, the backing metal has a penetration portion, and d in the formula (1) is a penetration depth (mm) in the penetration portion. The welded joint according to claim 1 or 2.   The weld joint according to any one of claims 1 to 4, wherein at least one of the steel materials has a groove having a bevel angle φ of 15 ° to 35 °. A method for manufacturing a welded joint according to any one of claims 1 to 5,
A method for manufacturing a welded joint, wherein a weld metal is formed in a root gap of both steel materials, and further, at least one surface forms an overfill on the surfaces of both steel materials.   The method for manufacturing a welded joint according to claim 6, wherein welding is performed by providing a backing metal or a concave backing metal in the route gap.