KR102266643B1 - Overlap fillet arc welded seam and manufacturing method thereof - Google Patents

Abstract

Provides overlap fillet arc welded seams of high-strength steel sheets with excellent fatigue strength. An overlap fillet arc welding joint in which two steel plates are superimposed and welded so that the steel plate on the weld toe side becomes a high-strength steel plate at a position of 0.2 mm from the surface of the steel plate on the weld toe side in the plate thickness direction, from the weld toe to the base metal When the maximum value of Vickers hardness of a range is made into HU and the minimum value is made into HL, the relationship of following formula (1) is satisfy|filled. HU-HL≤300 (1)

Description

Overlap fillet arc welded seam and manufacturing method thereof

The present invention relates to a lap fillet arc welded seam and a method of making the same.

_{In the automobile field, social requests for suppression of CO 2} emission for the purpose of preventing global warming and improvement of safety (collision safety) of crew and pedestrians in the event of a collision have been increasing. _{Among these, the effect of reducing the body weight is great for reducing the CO 2} emission during vehicle driving, and by reducing the weight of 100 kg, it is possible to reduce the fuel consumption by about 1 km/l on average, while also reducing the _{CO 2 emission.} can do.

On the other hand, the standards for collision safety are getting stricter every year, and it is necessary to improve the strength and rigidity of the vehicle body and to ensure collision safety by optimal distribution of strength. In general, when the body strength is improved, the body weight increases and CO ₂ emissions increase. However, it is important to balance _{collision safety and reduction of body weight (ie suppression of CO 2} emission) by increasing the strength of the materials used for the body. It is possible. Steel material is a major material that accounts for about 70% of the weight of an automobile, and among them, the strength of steel sheet is being improved every year.

In order to reduce the weight of the vehicle body, that is, to realize weight reduction of body parts, it is necessary to secure the strength characteristics of not only the steel plate but also the welding part. In particular, improving the fatigue strength in the arc welding part of the chassis and frame parts of automobiles is an important task .

From the above background, various welding techniques aiming at the improvement of the fatigue strength of the arc welding joint of a high strength steel sheet are proposed.

For example, Patent Document 1 describes a welding method in which fatigue strength is improved by optimizing the component and structure of a hot-rolled steel sheet and the hardness distribution and structure of the welded portion. Moreover, Patent Document 2 describes a welding method in which fatigue strength is improved by optimizing the overlap length, which is the length in the longitudinal direction of the steel plate of the overlapping portion of the upper plate and the lower plate in overlap fillet welding. Moreover, in patent document 3, the hardness of the weld metal in the vicinity of a weld toe, and a heat-affected zone, and the fillet welded joint excellent in the fatigue-crack generation characteristic which optimizes those ratio are described.

Patent Document 1: Japanese Patent Publication No. 5909143 Patent Document 2: Japanese Patent Laid-Open No. 2012-183542 Patent Document 3: Japanese Patent Publication No. 5000476

However, in the welding method of Patent Document 1, the structure of the base metal and the heat-affected zone each needs to have ferrite in a certain ratio or more, and there is a problem that it cannot be applied to ultra-high-strength steel of 980 MPa or more, which is being considered for application expansion in recent years. .

Moreover, in the welding method of patent document 2, the overlap length of the joint which fatigue strength improves was limited, and there existed a subject that it could not respond with respect to various component shapes.

In the weld joint of Patent Document 3, since there is an upper limit on the hardness of the weld metal and the heat affected zone for which fatigue strength is improved, a method for suppressing the hardening of the heat affected zone will be especially considered when the alloy element content increases with the increase in strength of the steel sheet. it was necessary

The present invention has been made in view of the above problems, and an object of the present invention is to provide a high-strength steel sheet overlap fillet arc welded joint excellent in fatigue strength, and a method for manufacturing the same.

MEANS TO SOLVE THE PROBLEM The present inventors acquired the following knowledge, as a result of repeating earnest examination in order to achieve said objective.

Fatigue fracture in overlap fillet arc welded joints tends to occur in the vicinity of the weld toe where stress is concentrated. Therefore, by increasing the angle (flank angle: θ, Fig. 1) at the weld toe, stress concentration is relieved, and it is effective for improving the fatigue strength of the weld joint.

On the other hand, paying attention to the hardness distribution of the overlap fillet arc weld seam of the high-strength steel sheet, since the heat-affected zone near the weld toe has a large heat input, it is _{heated to Ac 3} or more of the steel sheet and then cooled, resulting in martensitic structure and bay The knight structure becomes the main structure, and hardening easily occurs with respect to the base material part (hereafter, _{the area heated to this Ac 3} point or more is called hardening area). In addition, in the region where the maximum attained temperature farther away from the base material side is Ac ₁ point or less, softening due to tampering of the martensitic structure may occur depending on the components and structure of the base material (hereinafter, this Ac ₁ point or less). The area heated to , is called the softening zone).

Therefore, even if the flank angle is increased to improve the fatigue strength of the overlap fillet arc welded joint of the high-strength steel sheet as described above, the hardness of the softening area outside (base metal side) is excessive with respect to the hardening area near the weld toe. When it decreases, the fatigue strength is adversely affected by stress concentration in the softening region, so that the effect of improving the fatigue strength cannot be sufficiently obtained. Conversely, even when the hardness in the hardening region increases excessively, it is considered that the effect of improving the fatigue strength cannot be sufficiently obtained because the stress concentration in the softening region increases.

From the above findings, the inventors have found that in order to improve the fatigue strength of the overlap fillet arc welded joint of high-strength steel sheet, the flank angle is increased, i.e., the shape of the weld toe is smoothed as much as possible, and the maximum value of hardness in the weld area within a predetermined range The knowledge that it is effective to relieve the stress concentration by suppressing the difference between the and the minimum value to a certain level or less was obtained.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[1] An overlap fillet arc weld joint in which two steel plates are overlapped and welded so that at least the steel plate on the weld toe side becomes a high strength steel plate, at a position of 0.2 mm from the steel plate surface on the weld toe side in the plate thickness direction, at the weld toe When the maximum value of Vickers hardness in the range of up to 30 mm in the plate width direction is HU and the minimum value is HL, the overlapping fillet arc welding joint, characterized in that the relationship of the following formula (1) is satisfied: HU-HL≤300 (One).

[2] For the steel sheet on the weld toe side, the sum of the area fractions of martensite and bainite structures with respect to the entire structure of the base material is MB (%), and the Vickers hardness in the weld heat affected zone represents the maximum value HU. The sum of the area fractions of martensite and bainite structures with respect to the entire structure within a radius of 0.1 mm centered on the position is MU (%), and the Vickers hardness in the weld heat affected zone represents the minimum value HL. When the sum of the area fractions of martensite and bainite structures with respect to the entire structure in a region within a radius of 0.1 mm centered on the position is ML (%), the relationship of the following formulas (2) to (4) is The overlap fillet arc welded seam according to [1], characterized in that it satisfies: MB>50 (2) MU>60 (3) ML>15 (4).

[3] The overlapping fillet arc welding joint according to [1] or [2], in which two steel plates are overlapped so that at least the steel plate on the weld toe side becomes a high-strength steel plate, and a shield gas is supplied using a welding wire for fillet arc welding. As a method, the shielding gas is composed of an inert gas and an oxidizing gas, and the shielding gas satisfies the relationship of the following formula (5), characterized in that: 1≤2x[O ₂ ]+[CO ₂ ]≤30 (5) (wherein [CO ₂ ] is _{the volume % of CO 2 in} the shielding gas, and [O ₂ ] is the volume % _{of O 2 in the shielding gas)}

[4] When the welding current is I (A), the arc voltage is V (V), and the welding speed is s (cm/min), the relationship of the following formula (14) is satisfied in [3], The manufacturing method of the overlapped fillet arc weld seam described: (I×V)/s≤100/(C+Mn/6+Si/24) (14) (wherein C, Mn, Si are the It is the mass% value of each element of the base material.)

ADVANTAGE OF THE INVENTION According to this invention, the fatigue strength of the overlapping fillet arc welding joint of a high strength steel sheet can be improved by making the difference between the maximum value HU and the minimum value HL of hardness in a welding part within a predetermined range a predetermined value or less. As described above, the overlap fillet arc welded seam of the present invention uses a high-strength steel sheet and has excellent fatigue strength, so it can be preferably used for parts requiring high strength and fatigue strength, for example, automobile chassis and frame parts, etc. have.

1 is a cross-sectional view of a lap fillet arc weld seam of the present invention.
2 is a plan view and a cross-sectional view showing a fatigue strength test piece of an Example.

The overlap fillet arc weld seam of the present invention is an overlap fillet arc weld joint in which two steel plates are overlapped and welded so that at least the steel plate on the weld toe side becomes a high strength steel plate, and is located 0.2 mm from the surface of the steel plate on the weld toe side in the plate thickness direction. In , when the maximum value of Vickers hardness in the range from the weld toe to 30 mm in the plate width direction is HU and the minimum value is HL, the relationship of the following formula (1) is satisfied.

HU-HL≤300 (1)

The overlap fillet arc welded seam of the present invention (hereinafter, simply referred to as "weld seam of the present invention") will be described in detail below using FIG. 1 which is an example of the overlap fillet arc welded seam of the present invention. 1 is a cross-sectional view of a lap fillet arc weld seam of the present invention. As shown in Fig. 1, the overlap fillet arc welded seam of the present invention overlaps two steel sheets 11 and 12, and along the cross section 13 of one steel sheet (the upper steel sheet 11 in Fig. 1). , obtained by fillet arc welding the end face 13 and the surface 14 of the other steel plate (lower steel plate 12 in FIG. 1 ). In addition, '16' is a weld metal (bead (welding line)) by fillet arc welding, and '17' is a weld toe.

As for the two steel plates constituting the overlapping fillet arc welded joint, at least the steel plate 12 on the weld toe side is a high-strength steel plate. In this specification, "high strength" refers to a case where the tensile strength TS is 780 MPa or more. There is a problem in that it is difficult to improve the fatigue strength in the overlap fillet arc welded joint using a high strength steel sheet. However, in the present invention, fatigue strength can be improved even in overlap fillet arc welded joints using ultra-high strength steel sheets of 780 MPa or more, further 980 MPa or more, or 1180 MPa or more, and a welded joint having excellent fatigue strength can be obtained. Tensile strength can be obtained by producing a JIS 5 tensile test piece from a steel sheet in a direction parallel to the rolling direction, and performing a tensile test in accordance with JIS Z 2241:2011.

The component composition of the steel plates 11 and 12 is not particularly limited. However, in order to set it as the weld joint which satisfy|fills the relationship of said Formula (1), it is necessary to suppress excessive hardening and softening of a weld heat affected zone. For this purpose, it is effective to optimize the alloying element content of the steel sheet as the base material, and C, Si and Mn are particularly important. It is preferable that C content sets it as 0.02 mass % - 0.3 mass %. When the C content is less than 0.02% by mass, the hardenability decreases and softening of the weld heat affected zone becomes remarkable, and conversely, when it exceeds 0.3% by mass, the effect of the present invention cannot be effectively obtained because hardening becomes remarkable. Similarly, from the viewpoint of suppressing a decrease in hardenability, the Si content of the steel sheet is preferably 0.01 mass% or more, and the Mn content is 0.5 mass% or more. In addition, a hot-rolled steel plate or a cold-rolled steel plate may be sufficient as a steel plate. Moreover, the steel plates 11 and 12 may be a plated steel plate which has a metal plating layer on the surface.

Although the plate|board thickness of the steel plates 11 and 12 is not specifically limited, For example, the effect of this invention can be acquired effectively when plate|board thickness sets it as the range of 1 mm - 5 mm.

In addition, the two steel plates 11 and the steel plate 12 may be the same or different, and the steel plate 11 and the steel plate 12 may be steel plates of the same type and the same shape, and may be steel plates of different types or different shapes.

And, in the present invention, the Vickers hardness at a position of 0.2 mm from the surface of the steel plate 12 on the weld toe 17 side in the plate thickness direction is from the weld toe in the plate width direction (horizontal direction in Fig. 1). When HU is the maximum value of Vickers hardness in the range up to 30 mm, and HL is the minimum value of Vickers hardness in the range from the weld toe to 30 mm in the plate width direction, the relationship of Formula (1) is satisfied.

In the present invention, by satisfying the relationship of the formula (1), the stress concentration in the softening region of the overlap fillet arc weld joint having the weld toe can be relieved, and the overlap fillet arc weld joint excellent in fatigue strength is obtained. can be obtained

The present invention can be applied to welded joints using ultra-high-strength steel sheets of 980 MPa or more, and further, 1180 MPa or more. In addition, the present invention is not particularly limited to the overlap length of the steel sheet of the weld joint, and can respond to various parts shapes. In addition, in Patent Document 3, in order to improve the fatigue strength of the weld joint, there is an upper limit on the hardness of the weld metal and the heat-affected zone, and especially when the content of alloying elements increases so that the steel sheet becomes high-strength, a method of suppressing the hardening of the heat-affected zone is examined. There is a problem that needs to be done. However, in this invention, the hardness of a welding heat affected zone may be high, for example, the maximum value HU of Vickers hardness may be 450 or more. Further, the present invention relates to lap fillet welding, and in butt welding, even if the above formula (1) is satisfied, the effect of improving the fatigue strength is small. This is because, in butt welding, the stress concentration due to the bead shape is smaller than that in lamination fillet welding, so that the fatigue strength decreases significantly due to the presence of the softened portion.

Here, the Vickers hardness in the range from the weld toe to 30 mm in the plate width direction will be described. In the range from the weld toe to 30 mm in the sheet width direction, the weld heat affected zone (HAZ), in which the steel structure or hardness has changed due to the heat input from the welding, and the steel sheet before welding, in which the steel structure and hardness are hardly changed ( It is an area including a part of the base metal (not affected by welding heat input). And, in the present invention, the martensitic structure or the bainite structure is basically the main steel structure, and the maximum value HU and the minimum value HL of the Vickers hardness in the range from the weld toe to 30 mm in the sheet width direction are the effects of welding heat. It exists in the part, and the Vickers hardness of the base material is a value between the maximum value HU and the minimum value HL of the weld heat affected zone. Therefore, by satisfying the above formula (1), the difference between the maximum value HU and the minimum value HL of the Vickers hardness in the weld heat affected zone, which has not been considered conventionally, can be suppressed to 300 or less, and stress concentration can be relieved. and can improve fatigue strength.

When the tensile strength of the steel sheet used exceeds 1150 MPa, it is preferable to satisfy the relationship of the following formula (6), and it is more preferable to satisfy the relationship of the following formula (7). In addition, since softening of the heat-affected zone due to tempering does not occur as in the case of using a high-strength steel sheet when the tensile strength is low and not a high-strength steel sheet, the present invention is applied to a high-strength steel sheet having a tensile strength of at least 780 MPa or more.

HU-HL≤250 (6)

HU-HL≤200 (7)

As shown in FIG. 1, the said Vickers hardness measuring method is a plate from the surface 14 of the steel plate 12 on the side of the weld toe part 17 in the plate thickness direction cross section perpendicular to the bead (weld line) of a weld joint. At a position of 0.2 mm in the thickness direction, 30 mm from the weld toe to the base material in the direction perpendicular to the plate thickness (the range from the weld toe to 30 mm in the plate width direction, in FIG. 1, "Vickers hardness measurement range" ') at a measurement interval of 0.2 mm and a measurement load of 200 g, the Vickers hardness test described in JIS Z 2244 may be performed, and the maximum value of the Vickers hardness may be HU and the minimum value may be HL. In addition, the Vickers hardness of the base material is substantially uniform throughout the sheet thickness direction, the longitudinal direction, and the width direction of the steel sheet.

Moreover, in the weld joint of this invention, the flank angle of the weld toe part 17 is 120 degrees or more, for example. The flank angle θ of the weld toe 17 is, as shown in FIG. 1, among the angles formed by the tangent of the weld metal 16 in the weld toe 17 and the surface of the steel plate 12, the weld metal and is the opposite angle.

In addition, the steel structure of the steel plate 12 on the side of the weld toe 17 side is the sum of the area fractions of the martensite structure and the bainite structure with respect to the entire structure of the base material is MB (%), and The sum of the area fractions of the martensitic structure and the bainite structure with respect to the entire structure of the region within a radius of 0.1 mm centered on the position at which the Vickers hardness in the Vickers hardness shows the maximum value HU is MU (%), and the welding heat When the sum of the area fractions of martensitic and bainite structures with respect to the entire structure of the region within a radius of 0.1 mm centered on the position where the Vickers hardness in the affected zone shows the minimum value HL is ML (%) , it is preferable to satisfy the relation of the following formulas (2) to (4). When the steel structure of the steel sheet satisfies the relationship of formulas (2) to (4), the effect of improving fatigue strength, which is the effect of the present invention, can be obtained more effectively.

MB>50 (2)

MU>60 (3)

ML>15 (4)

In addition, when the steel structure of the weld metal is the sum of the area fractions of the martensite structure and the bainite structure with respect to the entire structure of the weld metal as MW (%), in addition to the above formulas (2) and (3), It is preferable to satisfy the relationship of the following formulas (8) and (9), and it is more preferable to satisfy the relationship of the formulas (10) and (11). In addition to formulas (2) and (3), by satisfying the relationships of formulas (8) and (9), and further formulas (10) and (11), the hardness of the weld metal is applied to the weld heat affected zone. The excellent fatigue strength, which is the effect of the present invention, can also be sufficiently secured while preventing a decrease in static strength due to excessively lowering (excessive under-matching).

ML>25 (8)

MW>25 (9)

ML>35 (10)

MW>35 (11)

In order to satisfy the area fraction of such a steel structure, it becomes effective to optimize the area fraction of the steel structure of the steel plate used as the base material of a weld joint before welding. It is preferable to set the area fraction of the bainite structure to 40% to 95% and the area fraction of the martensite structure to 3% to 60% with respect to the entire structure of the steel sheet before welding, that is, the base material of the weld joint. If the area fraction of the bainite structure is less than 40%, softening of the weld heat affected zone due to tempering of the martensitic structure becomes remarkable, and it becomes difficult to satisfy the relationship of the formula (1) in the weld joint. Conversely, when the area fraction of the bainite structure exceeds 95%, it becomes difficult to obtain a tensile strength of 980 MPa or more, and the scope of application of the present invention is greatly limited. Similarly, when the area fraction of the martensitic structure with respect to the entire structure of the steel sheet before welding is less than 3%, it becomes difficult to obtain a tensile strength of 980 MPa or more. Conversely, when the area fraction of the martensitic structure exceeds 60%, the effect of welding heat Wealth softening becomes remarkable. In addition, since the base material of a welded joint is an area|region in which the steel structure and hardness are hardly changed by welding, the steel structure of the base material of a welded joint is substantially equal to the steel structure of the steel plate (steel plate used) before welding.

Such a steel structure (steel structure of the base material, the steel structure of the region within a radius of 0.1 mm centered on the position where the Vickers hardness in the weld heat affected zone shows the maximum value HU, the Vickers hardness in the weld heat affected zone is The steel structure of the area within a radius of 0.1 mm centered on the position showing the minimum value HL, the steel structure of the weld metal) is observed by the following method, and the area fraction of each structure with respect to the entire structure and the sum thereof can be obtained. . First, it is polished so that the cross section in the plate thickness direction perpendicular to the bead of the weld joint becomes an observation surface, nital corrosion is performed, and by scanning electron microscope (SEM: 1000 times magnification), for each area, random 5 A location is photographed, and the area fraction of each tissue with respect to the whole tissue is calculated|required by the dotted-scattering method. In addition, about the steel structure of a base material, arbitrary 5 places away from a weld metal by 30 mm or more are image|photographed. Then, the calculated area fraction of the martensitic structure and the area fraction of the bainite structure are summed, MB (%) for the base material, and the position where the Vickers hardness in the weld heat affected zone shows the maximum value HU is centered MU (%) for an area within a radius of 0.1 mm where , ML (%) for an area within a radius of 0.1 mm centered on the position at which the Vickers hardness in the heat-affected zone shows the minimum value HL, For the weld metal, find the MW (%).

The overlap fillet arc welding seam of the present invention is, for example, overlapping fillet arc welding seam in which two steel sheets are overlapped so that at least the steel sheet on the weld toe side becomes a high-strength steel sheet, a welding wire is used, and a shield gas is supplied to perform fillet arc welding. can be manufactured by a manufacturing method characterized in that the shielding gas is composed of an inert gas and an oxidizing gas, and the relationship of the following formula (5) is satisfied.

1≤2×[O ₂ ]+[CO ₂ ]≤30 (5)

(wherein [CO ₂ ] is _{the volume % of CO 2 in} the shielding gas, and [O ₂ ] is the volume % _{of O 2 in the shielding gas)}

In the shielding gas, when 2×[O ₂ ]+[CO ₂ ] is less than 1%, the arc directivity is bad, so the weld bead meanders, and when 2×[O ₂ ]+[CO ₂ ] is more than 30%, When the arc shrinks, the weld bead tends to be convex, and when the flank angle of the weld toe is reduced, the fatigue strength of the weld joint is remarkably lowered in some cases. In the case of using a welding wire for high-strength steel sheets, it is preferable to satisfy the relationship of the following formula (12), and it is more preferable to satisfy the relationship of the following formula (13).

3≤2×[O ₂ ]+[CO ₂ ]≤20 (12)

3≤2×[O ₂ ]+[CO ₂ ]≤15 (13)

It may be made of Ar, etc. As the inert gas, as the oxidizing gas may include CO _2, O ₂ or the like.

In addition to this, excessive hardening and softening of the weld heat affected zone can be suppressed by preventing excessive heat input at the time of welding. For this, it is important to optimize the welding conditions, and when the welding current is I(A), the arc voltage is V(V), and the welding speed is s(cm/min), it is necessary to satisfy the relationship of the following formula (14) desirable. Preferred ranges of each welding condition are welding current: 100 A to 300 A, arc voltage: 10 V to 30 V, and welding speed: 50 cm/min to 200 cm/min. The welding current and arc voltage in Formula (14) are average values rather than peak values. Moreover, there is no restriction|limiting in the power supply characteristic of the welding wire used or a welding machine.

(I×V)/s≤100/(C+Mn/6+Si/24) (14)

(Wherein, C, Mn, and Si are the mass% values of each element of the base metal of the steel sheet on the weld toe side.)

In addition, in this specification, said each formula prescribes|regulates the relationship only of numerical values.

Example

Hereinafter, although an Example is used for further understanding of this invention, it demonstrates using an Example, but an Example does not limit this invention at all.

(Invention Examples and Comparative Examples)

First, using the hot-rolled steel sheet (all without plating) shown in Table 1 as a test steel sheet, as shown in Fig. 1 under the welding conditions shown in Table 2, the steel sheet 11 and the steel sheet 12 were welded by a pulse-mag welding method. The overlap fillet arc welding was performed, and the overlap fillet arc welded seam was obtained. As the steel plate 11 and the steel plate 12, the same steel plate was used. All welding wires used a solid wire for MAG welding having a diameter of 1.2 mm, and a mixed gas obtained by adding _{CO 2 and O 2 to Ar was used as the shielding gas.} The average value of the pulsed welding current was taken as the average value of the current during welding measured with an oscilloscope.

About the obtained lap fillet weld seam, the measurement of Vickers hardness and the structure|tissue observation were performed by the said method. Further, the flank angle θ of the weld toe 17 was measured. A result is shown in Table 3.

Further, as shown in Fig. 2, by machining from the obtained overlap fillet weld joint, a fatigue strength test piece having a parallel portion width of 22 mm was obtained with the weld toe 17 (bead toe) as the center in the longitudinal direction. 2 is a plan view (FIG. 2(a)) and cross-sectional view (FIG. 2(b)) showing a fatigue strength test piece, in FIG. 2, the unit of the numerical value indicated by the bidirectional arrow is mm, R in the drawing is The radius of curvature of the curved portion indicated by the arrow. As a fatigue test of the produced fatigue strength test piece, a pulsating bending fatigue test was employed. The load applied to the fatigue strength test piece was 100 to 500 MPa, the repetition frequency was 20 Hz, and the number of repetitions was 1,000,000 times. The fatigue strength test result was judged as the following A, B, C, and F from joint fatigue strength. A result is shown in Table 3.

A: 250MPa≤joint fatigue strength

B: 200 MPa ≤ joint fatigue strength < 250 MPa

C: 150 MPa ≤ joint fatigue strength < 200 MPa

F: Joint fatigue strength <150 MPa

As shown in Table 3, the determination of the fatigue strength test piece of the example of the present invention was any one of A to C, and the effect of the present invention was effectively obtained.

[Table 1]

[Table 2]

[Table 3]

11, 12; grater 16; weld metal
17; weld toe θ; flank angle

Claims (4)

An overlap fillet arc weld joint in which two steel plates are overlapped and welded so that at least the steel plate on the weld toe side becomes a steel plate having a tensile strength TS of 780 MPa or more,
When the maximum value of Vickers hardness in the range of 0.2 mm in the sheet thickness direction from the surface of the steel sheet on the weld toe side to 30 mm in the sheet width direction from the weld toe end is HU and the minimum value is HL,
Vickers hardness in the above range in the plate width direction from the weld toe satisfies the relationship of the following formula (1),
An overlapping fillet arc welded seam, characterized in that the flank angle of the weld toe is not less than 120°:
HU-HL≤300 (1). The method of claim 1,
For the steel plate on the weld toe side, the sum of the area fractions of martensite and bainite structures with respect to the entire structure of the base material is MB (%), and the Vickers hardness in the weld heat affected zone is centered at the position at which the maximum value HU is indicated. MU (%) the sum of the area fractions of martensite and bainite structures with respect to the entire structure of a region within a radius of 0.1 mm where MU (%), and the position where the Vickers hardness in the weld heat affected zone indicates the minimum value HL is the center When the sum of the area fractions of martensite and bainite structures with respect to the entire structure of the region within a radius of 0.1 mm in which ML (%) is ML (%), the relationship of the following formulas (2) to (4) is satisfied. Overlap Fillet Arc Welded Seams Features:
MB>50 (2)
MU>60 (3)
ML>15 (4). The overlap fillet arc welding joint according to claim 1 or 2, wherein two steel sheets are overlapped so that at least the steel sheet on the weld toe side becomes a steel sheet having a tensile strength TS of 780 MPa or more, and a shield gas is supplied using a welding wire for fillet arc welding. As a manufacturing method of
The method for manufacturing an overlap fillet arc welded joint, wherein the shielding gas is composed of an inert gas and an oxidizing gas, and the shielding gas satisfies the relationship of the following formula (5):
1≤2×[O ₂ ]+[CO ₂ ]≤30 (5)
(Wherein, [CO ₂ ] is _{the volume % of CO 2 in} the shielding gas, and [O ₂ ] is the volume % _{of O 2 in the shielding gas).} 4. The method of claim 3,
When the welding current is I (A), the arc voltage is V (V), and the welding speed is s (cm/min), the relationship of the following formula (14) is satisfied. Way:
(I×V)/s≤100/(C+Mn/6+Si/24) (14)
(Wherein, C, Mn, and Si are the mass% values of each element of the base metal of the steel sheet on the weld toe side.).

Images