KR102603852B1 - Lap laser weld joint and method for producing same, and automotive body structural member - Google Patents

Abstract

When a laser beam is intermittently irradiated to one surface of a steel plate overlapping a plurality of steel plates to form a weld in which a linear first joint and a linear subsequent joint following the first joint are arranged in a row, at least the above The total gap G between the steel plates constituting the welded zone is set to be within the range of 0 to 15% of the total thickness T of the steel plates constituting the welded zone, and the moving direction of the welding head that irradiates the laser beam and the laser beam By reversing the scanning direction, the welding portion is formed so that the welding start end of the first joint and the welding termination portion of the subsequent joint adjacent to the first joint face each other, and the welding starting end portion and the welding termination portion of the subsequent joint portion face each other. In addition, by controlling the various dimensions of the above-mentioned joints to an appropriate range, there is no cracking at the weld end of the joint, and an overlap laser welded joint with excellent peeling strength, a manufacturing method thereof, and a structural member for an automobile body having the welded joint are provided. suggest.

Description

Overlap laser welded joint and its manufacturing method and structural member for automobile body {LAP LASER WELD JOINT AND METHOD FOR PRODUCING SAME, AND AUTOMOTIVE BODY STRUCTURAL MEMBER}

The present invention relates to an overlap laser welded joint, a method of manufacturing the same, and a structural member for an automobile body having the overlap laser welded joint.

Resistance spot welding is generally used to weld structural members (strength members) of an automobile body having a flange portion. However, resistance spot welding takes time to weld, and the heat generation amount decreases due to the flow, so the welding pitch cannot be narrowed, and furthermore, a certain amount of space is required to set the welding gun, etc. There are several problems. In order to solve these problems, the application of overlap laser welding instead of conventional resistance spot welding has been examined and put into practice in recent years. Here, the above-mentioned overlap laser welding refers to a welding method in which a laser beam is irradiated to one surface of a plurality of overlapping steel sheets to melt and join the steel sheets.

Conventionally, the overlap laser welding joint irradiates a laser beam intermittently on the surface of a plurality of overlapping steel sheets, melts and solidifies the steel sheet in the area irradiated with the laser beam, and forms a short-length straight joint continuously (列). ) A plurality of steel plates were joined by forming welds arranged in a shape. However, overlap laser beam welding has a problem in that cracks are likely to occur in the final solidified portion on the weld end side of the straight joint. When a crack occurs, it propagates over the entire length of the straight joint, so not only does the static strength such as shear strength and peel strength of the weld joint decrease, but also the fatigue strength significantly decreases. Recently, in order to improve the strength and rigidity of the car body, high-strength steel sheets have been widely used in automobile body members, especially structural members (strength members) that become skeleton members, and weld joints due to cracks occurring at the joints. A decrease in static strength or fatigue strength becomes a serious problem.

Therefore, various methods for preventing cracks at the weld ends of joints that occur when overlapping steel plates are laser beam welded are being studied. For example, Patent Document 1 discloses a technique for preventing weld cracks by protruding the steel plate on the lower side of the overlap weld and setting the welding start position away from the flange end. Additionally, Patent Document 2 discloses a technique for preventing weld cracks by irradiating a laser diagonally to the ends of overlapping surfaces. Additionally, Patent Documents 3 and 4 disclose a technique for preventing weld cracks by reheating or welding the once welded portion or the surrounding area of the welded portion. Additionally, Patent Document 5 discloses a technique for preventing the occurrence of weld cracks by welding the overlapping surfaces in an oval shape. Additionally, Patent Document 6 discloses a technique for preventing the occurrence of weld cracks by optimizing the steel sheet components and optimizing the ratio of the weld bead width and bead thickness.

Japanese Patent Publication No. 2007-229740 Japanese Patent Publication No. 2008-296236 Japanese Patent Publication No. 2012-240083 Japanese Patent Publication No. 2012-240086 Japanese Patent Publication No. 2017-113781 Japanese Patent Publication No. 2018-001197

However, in the method described in Patent Document 1, since the steel plate on the lower side of the overlap welding is protruded, there is a problem that the protruding portion becomes redundant and the part design is restricted. In addition, in the method described in Patent Document 2, since the laser is irradiated at an angle, when there is a gap in the overlapping plates, a fused zone is not formed well on the overlapping surface, resulting in insufficient penetration, and there is a problem in that it is difficult to secure sufficient strength. Additionally, in the methods described in Patent Documents 3 and 4, there is a problem that the welding time becomes longer because it is necessary to reheat or weld the once welded portion or the surrounding area of the welded portion. Additionally, the method described in Patent Document 5 welds the overlapping surfaces in an oval shape, and therefore cannot be applied to crack prevention at the weld end portion of a straight joint. In addition, in the method described in Patent Document 6, there is a problem that cracks cannot be prevented at the weld end of a short straight joint because stress tends to concentrate at the end of the weld.

The present invention was made in consideration of the above-mentioned problems faced by the prior art, and its purpose is to provide a weld joint having a short-length joint (weld point) formed in a row shape by intermittently irradiating a laser beam, Provides an overlap laser welded joint that does not cause cracks at the end of the weld and also has excellent peeling strength of the weld zone, proposes a manufacturing method thereof, and provides a structural member for an automobile body having this overlap laser welded joint. It's in the thing.

In order to solve the above problem, the inventors developed a linear joint formed initially (hereinafter referred to as “first joint”), which constitutes a welded portion of an overlap laser welding joint, and one or more linear joints formed subsequent to it. We paid careful attention to the relationship between the joints (hereinafter referred to as “subsequent joints”) and conducted extensive studies. In addition, in the present invention, the above-mentioned “first joint portion” and “subsequent joint portion” are collectively referred to as “joint portion”.

As a result, in order to prevent cracks occurring at the weld end of the joint, the shape of the first joint is made into a J shape, and the weld start end of the first joint and the weld termination of the subsequent joint adjacent to it are It is important to form a weld where the weld start end of a subsequent joint and the weld end of a subsequent joint formed adjacent to it face each other, that is, to form a weld so that the weld start end and the weld end between the joints face each other. In order to more reliably prevent cracking at the weld end of the joint and ensure sufficient strength of the weld zone, in addition to satisfying the above requirements, various dimensions of the first joint and subsequent joints must be adjusted. It was found that controlling within an appropriate range was effective, and the present invention was developed.

The present invention based on the above knowledge is an overlap laser welded joint having a welded portion formed by overlapping a plurality of steel plates, wherein the total gap (G) between the steel plates constituting the welded portion is equal to the total thickness (T) of the steel plates constituting the welded portion. ) is within the range of 0 to 15% of , the weld termination portions of subsequent joints adjacent thereto are opposed to each other, and the weld start portions and weld termination portions of the subsequent joint portions are opposed to each other, and the first joint is welded to a straight joint portion and the linear joint portion. It has a J-shape consisting of an arc-shaped or circular curved joint connected to the longitudinal end side, and the welded portion is of the following formulas (1) to (4).

15.0 ≤ L ₁ ≤ 30.0 … (One)

8.0 ≤ L ₂ ≤ 20.0 … (2)

1/8 ≤ w/b ≤ 1/2 … (3)

1/4 ≤ a / (L ₁ + L ₂ ) ≤ 1/2 or 1/2 ≤ a / L ₂ ≤ 1 … (4)

Here, L ₁ : Length of the first joint (mm)

L ₂ : Length of subsequent joint (mm)

b: Minimum thickness of molten metal at the joint (mm)

w: Width of molten metal at joint (mm)

a: Shortest distance between joints (mm)

It is an overlap laser welded joint that satisfies everyone.

In the overlap laser welded joint of the present invention, the first joint consists only of straight joints, and instead of the above formula (1), the following formula (1') is used;

30.0 < L _1' ≤ 40.0... (One')

Here, L _1': Length of straight joint (mm)

It is characterized by satisfying.

In addition, in the overlap laser welded joint of the present invention, at least one of the steel sheets has C: 0.07 to 0.4 mass%, Si: 0.2 to 3.5 mass%, Mn: 1.8 to 5.5 mass%, P+S: 0.03 mass% or less, and Al. : 0.08 mass% or less and N : 0.010 mass% or less, with the balance being Fe and inevitable impurities.

In addition, the steel sheet in the lap laser weld joint of the present invention has the following chemical compositions: Group A and Group B;

·Group A; One or two types selected from Ti: 0.0005 to 0.01 mass% and Nb: 0.005 to 0.050 mass%

·Group B; One or two or more selected from Cr: 1.0 mass% or less, Mo: 0.50 mass% or less, and B: 0.10 mass% or less

It is characterized by containing at least one group of ingredients.

Additionally, the overlap laser welded joint of the present invention is characterized in that at least one of the steel sheets is a high-tensile strength steel sheet with a tensile strength of 980 MPa or more.

In addition, in the present invention, a plurality of steel plates are overlapped vertically, and a laser beam is intermittently irradiated on one surface of the overlapped steel plates, so that a linear first joint and a linear subsequent joint following the first joint are formed in a row shape. When forming a welded portion arranged as follows, the total gap (G) between the steel plates constituting the welded portion is set to be within the range of 0 to 15% of the total thickness (T) of the steel plates constituting the welded portion, and the laser beam is irradiated. By reversing the direction of movement of the welding head and the scanning direction of the laser beam, the weld start end of the first joint and the weld end of the subsequent joint adjacent to the first joint face each other, and the weld start ends of the subsequent joints face each other. and the weld end portions are opposed to each other, and the shape of the first joint portion is set to be a J shape consisting of a straight joint portion and an arc-shaped or circular curved joint portion connected to the weld terminal side of the straight joint portion, and , the welded portion is represented by the following formulas (1) to (4);

15.0 ≤ L ₁ ≤ 30.0 … (One)

8.0 ≤ L ₂ ≤ 20.0 … (2)

1/8 ≤ w/b ≤ 1/2 … (3)

1/4 ≤ a / (L ₁ + L ₂ ) ≤ 1/2 or 1/2 ≤ a / L ₂ ≤ 1 … (4)

Here, L ₁ : Length of the first joint (mm)

L ₂ : Length of subsequent joint (mm)

b: Minimum thickness of molten metal at the joint (mm)

w: Width of molten metal at joint (mm)

a: Shortest distance between joints (mm)

We propose a method for manufacturing an overlap laser welded seam, which is characterized by controlling at least one of laser power, focus position, welding speed, and beam diameter to satisfy all.

In the method of manufacturing an overlap laser welded joint of the present invention, the first joint consists of only a straight joint, and instead of the above formula (1), the following formula (1') is used;

30.0 < L _1' ≤ 40.0... (One')

Here, L _1': Length of straight joint (mm)

Characterized by controlling at least one of laser power, focus position, welding speed, and beam diameter to satisfy.

Additionally, the present invention is a structural member for an automobile body having an overlapping laser welded joint according to any one of the above.

According to the present invention, not only can the occurrence of cracks in the weld end portion of the joint constituting the welded portion of an overlap laser welded joint obtained by laser beam welding a plurality of overlapping steel plates be reliably suppressed, but also the peeling strength of the welded portion can be reduced. Excellent overlap laser welded joints can be manufactured. In addition, since the overlap laser welded joint of the present invention can shorten the length of the joint, the degree of freedom in component design is increased and it is possible to develop a lighter, higher rigidity, and higher strength member. Therefore, the overlap laser welded joint of the present invention can be suitably applied to structural members (strength members) that form the skeleton of an automobile body.

1 is a perspective view showing an example of a conventional overlap laser welded joint.
FIG. 2 is a schematic diagram illustrating a welded portion of a conventional overlap laser welded joint, where (a) is a top view and (b) is a cross-sectional view AA of (a).
Figure 3 is a schematic diagram explaining the welded portion of the overlap laser welding joint of the present invention, (a) is a top view, and (b) is a BB cross-sectional view of (a).
Figure 4 is a perspective view showing an example of an overlap laser welded joint of the present invention.
Figure 5 is a perspective view explaining the welding method of the overlap laser welding joint of the present invention.
Figure 6 is a view explaining the welding position of the overlap laser welding joint of the present invention, (a) is a plan view, and (b) is a CC cross-sectional view of (a).
Figure 7 is a perspective view illustrating a peeling test piece with an overlapping laser welded joint used in an example of the present invention.

Hereinafter, the overlap laser welding joint of the present invention, its manufacturing method, and a structural member for an automobile body having the overlap laser welding joint will be described.

＜Overlap laser welded joint＞

1 is a perspective view showing an example of a conventional overlap laser welded joint. The overlapping laser welded joint 1 is formed by overlapping at least two steel plates, and in the example shown in Fig. 1, the vertical wall portion 2a and the flange portion 2b extending outward from the tip of the vertical wall portion 2a are formed. A steel plate 2 having a roughly hat-shaped cross-sectional shape and a steel plate 3 having a flat panel shape, two steel plates are overlapped so that the flange portion 2b and the steel plate 3 face each other. forming a joint surface, a laser beam is irradiated from above the flange portion 2b to the surface of the flange portion 2b, and a molten portion (molten metal portion) penetrating at least the steel plate 2 is formed and solidified. Welding is performed by forming a joint (welding point). In addition, a heat affected zone (HAZ) exists around the melted zone, but the joint of the present invention refers only to the melted zone excluding the heat affected zone.

The welding portion of the overlap laser welding joint 1 moves the welding head WH, which is a laser beam source, along the longitudinal direction of the vertical wall portion 2a (the arrow direction in FIG. 1) while directing the laser beam to the flange portion 2b. It is formed by intermittently irradiating the surface. As a result, as shown in FIG. 1, on the joint surface of the steel plate 2, a short straight first joint 4 and subsequent joint portions 5 are continuously formed. When the length of the flange portion is short, the subsequent joint portion 5 may be one, but when the flange portion is long, a plurality of the subsequent joint portions 5 are formed in a row along the longitudinal direction of the flange portion. As described above, in the present invention, the “first joint” and “subsequent joint” are collectively referred to as “joint”.

FIG. 2 is a schematic diagram showing a conventional weld formed on the flange portion 2b of the overlap laser welded seam shown in FIG. 1, where (a) shows the first joint 14 constituting the weld and the subsequent joint adjacent thereto ( 15) is a plan view seen from above the flange portion 2b, (b) is a cross-sectional view showing the A-A cross section shown at the subsequent joint portion 15 in (a) above.

In conventional laser beam welding, as shown in FIG. 2, when forming a weld consisting of a first joint 14 and a subsequent joint 15 in a short straight line, the welding direction (traveling direction of the welding head) and the The welding direction (scanning direction of the laser beam) of the first joint 14 and the subsequent joint 15 was in the same direction. Therefore, the central portions 14a and 15a of the weld end portion E (final solidification portion) of the first joint 14 and the subsequent joint 15 become crater-shaped and the plate thickness is minimized, so that there is Tensile stress (force in the direction of arrow Fa shown in Fig. 2(a)) directed outward is concentrated at the outer peripheral portion of the melted zone. Therefore, in the conventional overlap laser welded joint, a crack 8 occurs at the weld end portion E of the first joint 14 or the subsequent joint 15, and then the weld is formed from the weld end portion E. There is a problem that it propagates to the starting end (S) and significantly reduces the peeling strength and fatigue strength of the weld zone. Cracks that occur at the end of this weld can be prevented by making the first joint 14 or the subsequent joint 15 sufficiently long, but this method increases the welding time and causes a decrease in productivity. There is another problem.

In addition, the crack at the weld end portion occurs penetrating from the surface to the back surface at the weld end portion (final solidification portion) of the first joint 14 or the subsequent joint 15, and its occurrence can be confirmed with the naked eye. To determine more reliably, it is preferable to cut the weld end portion of the joint after welding in the width direction and observe the cut surface at a magnification of about 10 times with an optical microscope, for example, to make the decision.

Therefore, the inventors paid attention to the stress concentration that occurs at the weld end portion of the joint forming the weld zone formed by the conventional laser beam welding method, and proposed a measure to prevent cracks occurring at the weld end portion of the joint by reducing this. A thorough review was conducted. As a result, it was found that cracks occurring at the weld end portion of the joint could be prevented by employing the weld portion shown in FIG. 3 instead of the weld portion shown in FIG. 2 above. Here, FIG. 3 is a schematic diagram showing the welded portion of the present invention formed on the flange portion 2b of the same overlap laser welded joint as shown in FIG. 2, where (a) shows the first joint portion 4 constituting the welded portion and (b) is a plan view of the subsequent joint 5 adjacent to it as seen from above the flange portion 2b, and (b) is a cross-sectional view showing the B-B cross section of the subsequent joint 5 in (a).

As can be seen from FIG. 3, the characteristic of the weld of the present invention is that the weld start end of the first joint 4 and the weld termination of the subsequent joint 5 adjacent thereto face each other, and the weld end of the subsequent joint 5 adjacent thereto faces each other. The weld start end portion of and the weld termination portion of the subsequent joint portion 5 adjacent thereto face each other, that is, the weld start end portion and the weld termination portion of the adjacent joint portions face each other. By welding the welding start portion of the preceding joint and the welding termination portion of the succeeding joint portion as described above, cracking of the weld termination portion of the joint portion can be prevented. The reason is that, at the point when the weld end of the trailing joint has completed welding, the steel plate near the weld end is restrained by the preceding joint, so the outer portion of the melted zone applied to the center 5a of the final solidified zone is This is because the tensile stress (force in the direction of arrow Fb shown in Fig. 3(a)) is reduced.

Accordingly, when a welded portion is formed as shown in FIG. 3, cracking at the weld end portion of a subsequent joint can be prevented. However, since the steel plate around the weld end portion of the first joint is not constrained and is in a free state, cracking at the weld end E of the first joint cannot be prevented.

Therefore, the inventors further studied methods for suppressing cracking at the weld end portion of the first joint. As a result, as shown on the left side of FIG. 3(a), the shape of the first joint is a J shape consisting of a straight joint and an arc-shaped or circular curved joint formed by connecting to the terminal end side of the joint. found out that it was valid. By using this shape, the steel plate around the weld end of the first joint, that is, the weld end of the curved joint, is already restrained by the straight joint, so the outer periphery of the melted portion applied to the center 4a of the final solidified portion is The tensile stress directed outward from the portion (force in the direction of arrow Fc shown in Fig. 3(a)) is reduced, and cracking at the weld termination portion of the first joint can be prevented.

As shown in FIG. 3 above, the weld start end of the first joint 4 and the weld termination of the subsequent joint 5 adjacent thereto face each other, and the weld start end of the subsequent joint 5 and the subsequent weld terminal adjacent thereto are opposed to each other. In addition to forming the welded portion so that the welded ends of the jointed portion 5 face each other, by making the shape of the first jointed portion J-shaped, it is possible to prevent cracks in the welded terminal portions of the first jointed portion and subsequent jointed portions. Also, for reference, Fig. 4 shows an example of applying the weld part of the present invention shown in Fig. 3 to the overlap laser welded joint shown in Fig. 1.

However, according to additional research by the inventors, in order to more reliably prevent cracks occurring at the weld end portion of the joint and also provide sufficient peeling strength to the welded portion, in addition to forming the welded portion shown in FIG. 3, The total gap G between the steel plates constituting the welded portion is within the range of 0 to 15% of the total thickness T of the steel plates constituting the welded portion, and the first joint portion constituting the welded portion, that is, the welded portion Subsequent joints are in the following formulas (1) to (4):

15.0 ≤ L ₁ ≤ 30.0 … (One)

8.0 ≤ L ₂ ≤ 20.0 … (2)

1/8 ≤ w/b ≤ 1/2 … (3)

1/4 ≤ a / (L ₁ + L ₂ ) ≤ 1/2 or 1/2 ≤ a / L ₂ ≤ 1 … (4)

Here, L ₁ : Length of the first joint (mm)

L ₂ : Length of subsequent joint (mm)

b: Minimum thickness of molten metal at the joint (mm)

w: Width of molten metal at joint (mm)

a: Shortest distance between joints (mm)

I could see that it was necessary to satisfy everyone.

Hereinafter, it will be described in detail.

0 ≤ G/T ≤ 0.15

The overlap laser welded joint of the present invention has a ratio (G/T) of the total gap (G) between the steel plates constituting the welded portion to the total thickness (T) of the steel plates constituting the welded portion (G/T) of 0 to 0.15, that is, the welded joint It is necessary that the ratio of the total gap (G) between the steel plates constituting the welded portion to the total thickness (T) of the constituting steel plates be within the range of 0 to 15%. If the ratio of G to T exceeds 15%, the depth of the crater at the end of the weld becomes deeper and stress becomes more likely to concentrate. Preferably it is in the range of 0 to 10%.

15.0 ㎜ ≤ L ₁ ≤ 30.0 ㎜

As described above, in order to prevent cracking of the weld end portion of the first joint constituting the welded portion, the overlap laser welded joint of the present invention changes the shape of the first joint into a straight joint and an arc-shaped or circular curved joint. It is important to form a J shape, but in order to more reliably prevent cracking at the weld end and ensure sufficient peel strength, the length (L ₁ ) of the first joint is in the range of 15.0 to 30.0 mm. It is necessary to do so. In addition, the length (L ₁ ) of the first J-shaped joint portion is the total length of the straight joint portion and the curved joint portion, as shown in FIG. 3 . If L ₁ is shorter than 15.0 mm, cracks occur at the weld termination portion of the first joint. Preferably it is 20.0 mm or more. On the other hand, the upper limit of L ₁ is not limited from the viewpoint of preventing cracking, but is set to 30.0 mm or less from the viewpoint of shortening the welding time. Preferably it is 25.0 mm or less.

8.0 ㎜ ≤ L ₂ ≤ 20.0 ㎜

In addition, the overlap laser welded joint of the present invention requires the length (L ₂ ) of the subsequent joint portion constituting the weld portion to be in the range of 8.0 to 20.0 mm. When L ₂ is shorter than 8.0 mm, even if the weld start end of the preceding joint and the weld termination of the trailing joint are opposed to each other, a welding area sufficient to relieve the stress applied to the weld end cannot be secured, so Cracks cannot be prevented reliably. Preferably it is 10.0 mm or more. On the other hand, the upper limit of L ₂ is not limited from the viewpoint of preventing cracking, but is set to 20.0 mm or less from the viewpoint of shortening the welding time. Preferably it is 15.0 mm or less.

1/8 ≤ w/b ≤ 1/2

In addition, the overlap laser welded joint of the present invention has a ratio (w/b) of the width (w) of the molten metal in the joint to the minimum thickness (b) that occurs at the weld termination (final solidification) of the joint that constitutes the weld. ) is required to be in the range of 1/8 to 1/2. If the ratio (w/b) is greater than 1/2, the outward tensile stress applied to the final solidification portion of the weld end of the first joint or subsequent joint increases at the outer peripheral portion of the melted zone, thereby suppressing the occurrence of weld cracks. I can't. On the other hand, if the ratio (w/b) is less than 1/8, sufficient strength of the joint cannot be secured. That is, in addition to making the weld start end portion of the preceding joint and the weld termination portion of the succeeding joint face to face, the ratio (w/b) at the joint portion is set to be in the range of 1/8 to 1/2, so that the weld termination portion of the joint portion cracking can be more reliably prevented, and the peeling strength of the weld zone can be sufficiently strengthened. A preferable ratio (w/b) is in the range of 1/6 to 1/3.

1/4 ≤ a / (L ₁ + L ₂ ) ≤ 1/2 or 1/2 ≤ a / L ₂ ≤ 1

In addition, in the overlap laser welded joint of the present invention, the shortest distance (a) between joints is, in the case between the first joint and the adjacent subsequent joint, (length of the first joint (L ₁ ) + length of the subsequent joint) The ratio of a to (L ₂ )) (a / (L ₁ + L ₂ )) is in the range of 1/4 to 1/2, and in the case between adjacent subsequent joints, the length of the subsequent joint (L ₂ ) It is necessary that the ratio of a to (a / L ₂ ) is in the range of 1/2 to 1. When the ratio (a / (L ₁ + L ₂ )) is greater than 1/2, or when the ratio (a / L ₂ ) is greater than 1, the subsequent joint is formed around the joint from the preceding joint of the joint. The effect of suppressing the deformation of the steel plate cannot be sufficiently obtained, and cracking at the weld end portion cannot be prevented. In addition, the lower limit of a is not limited from the viewpoint of preventing cracking, but from the viewpoint of shortening the welding time, the ratio (a / (L ₁ + L ₂ )) is 1/4 or more, and the ratio (a / L ₂ ) is set to 1/2 or more. In addition, the preferable ratio (a/(L ₁ + L ₂ )) is in the range of 1/3 to 2/5, and the preferable ratio (a/L ₂ ) is in the range of 3/5 to 9/10.

In addition, in the description of the present invention described above, a technique for preventing cracks occurring at the weld end of the first joint by forming the shape of the first joint into a J-shape was explained. However, as described above, cracking at the weld end of the joint can be prevented by increasing the length of the linear joint. In that case, the length (L _1' ) of the first joint is expressed by the following equation (1') instead of the above-mentioned equation (1);

30.0 < L _1' ≤ 40.0... (One')

Here, L _1' : Length of straight joint (mm)

It is necessary to satisfy. When the length (L _1' ) of the first joint consisting of only straight joints is 30.0 mm or less, a crack occurs at the terminal end of the first joint. Preferably it is 32.0 mm or more. On the other hand, the upper limit of L _1' is not limited from the viewpoint of preventing cracking, but is set to 40.0 mm or less from the viewpoint of shortening the welding time. Preferably it is 38.0 mm or less.

Next, the steel plate constituting the overlap laser welded joint of the present invention will be described.

1 to 4 show an example of configuring an overlapping laser welded joint by overlapping two steel plates, but it goes without saying that a welding joint can be configured by overlapping three or more steel plates.

plate thickness of steel plate

The steel plates constituting the overlap laser welded joint of the present invention may have a thickness within the range of 0.5 to 3.2 mm, which is generally used as an outer plate or structural member (strength member) of an automobile body, and is not particularly limited. In addition, the plurality of steel plates may all have the same plate thickness, or may have individually different plate thicknesses. For example, in the case of the overlap laser welded joint 1 of the shape shown in FIG. 4, the plate thickness t ₂ of the upper steel plate 2 is 0.6 to 1.8 mm, and the plate thickness of the lower steel plate 3 is ( t ₃ ) may be in the range of 1.0 to 2.5 mm, and the plate thickness (t ₂ ) of the upper steel plate 2 and the plate thickness (t 3 ) of the lower steel plate ₃ may be set to the same plate thickness of 0.5 to 3.2 mm. You can also do this.

Composition of steel plate

In addition, the component composition of the plurality of steel sheets constituting the overlap laser welded joint of the present invention is not particularly limited, but at least one steel sheet has C: 0.07 to 0.4 mass% and Si: 0.2 to 0.2 mass%, as described below. It contains 3.5 mass%, Mn: 1.8 to 5.5 mass%, P+S: 0.03 mass% or less, Al: 0.08 mass% or less, and N: 0.010 mass% or less, with the remainder being Fe and inevitable impurities. desirable.

C: 0.07 to 0.4 mass%

C is an element effective in improving the strength of steel, and by containing it in an amount of 0.07 mass% or more, the effects of precipitation strengthening and transformation strengthening can be obtained. In addition, by setting the C content to 0.4 mass% or less, the desired strength and processability can be secured without causing precipitation of coarse carbides. Therefore, the C content is preferably in the range of 0.07 to 0.4 mass%. More preferably, it is in the range of 0.15 to 0.3 mass%.

Si: 0.2 to 3.5 mass%

Si is an element excellent in solid solution strengthening ability, and the strength of steel can be increased by containing 0.2 mass% or more. In addition, by setting the Si content to 3.5 mass% or less, excessive hardening of the weld heat-affected zone can be suppressed and deterioration of the toughness and low-temperature cracking resistance of the weld heat-affected zone can be prevented. Therefore, the Si content is preferably in the range of 0.2 to 3.5 mass%. More preferably, it is in the range of 1.0 to 2.5 mass%.

Mn: 1.8 to 5.5 mass%

Mn is an element effective in improving quenching properties and suppressing precipitation of coarse carbides, and is preferably contained at 1.8 mass% or more. In addition, by setting the Mn content to 5.5 mass% or less, an increase in grain boundary embrittlement susceptibility can be suppressed and deterioration of toughness and low-temperature cracking resistance can be prevented. Therefore, the Mn content is preferably in the range of 1.8 to 5.5 mass%. More preferably, it is in the range of 2.0 to 3.5 mass%.

P+S: 0.03 mass% or less

P and S are harmful elements that adversely affect the ductility and toughness of steel, and by setting the total content of P and S to 0.03 mass% or less, a decrease in ductility and toughness can be prevented and desired strength and workability can be secured. Therefore, it is preferable that the total content of P and S is 0.03 mass% or less. More preferably, it is 0.02 mass% or less.

Al: 0.08 mass% or less

Al is an element added as a deoxidizing agent in the steelmaking stage, and is generally added in an amount of 0.01 mass% or more. However, when the Al content exceeds 0.08 mass%, inclusions such as alumina increase, and adverse effects on fatigue resistance become apparent. Therefore, the Al content is set to 0.08 mass% or less. Preferably it is in the range of 0.02 to 0.07 mass%.

N: 0.010 mass% or less

N is an element that significantly deteriorates the aging resistance of steel, and it is desirable to reduce it as much as possible. In particular, if N exceeds 0.010 mass%, the aging resistance deteriorates significantly, so the N content is set to 0.010 mass% or less. Additionally, the lower limit of N is not particularly limited, but is preferably set to about 0.001 mass% from the viewpoint of preventing an increase in manufacturing costs.

In addition, at least one steel plate constituting the weld joint of the present invention is, in addition to the above-mentioned component composition, at least one of the following group A and group B for the purpose of further improving the steel sheet strength and the peeling strength of the weld zone. It is preferred that it contains the components of the group.

Group A; One or two types selected from Ti: 0.0005 to 0.01 mass% and Nb: 0.005 to 0.050 mass%

Both Ti and Nb precipitate by forming carbides and nitrides, and have the effect of suppressing coarsening of austenite during annealing when manufacturing steel sheets. In order to obtain the above effect, it is preferable to contain 0.0005 mass% or more of Ti and 0.005 mass% or more of Nb of one or two types selected from Ti and Nb. However, even if Ti and Nb are contained excessively, the above effect is saturated and only causes an increase in raw material cost. In addition, because the recrystallization temperature is increased, the metal structure after annealing during the production of steel sheet becomes non-uniform, and there is a risk that the stretch flangeability may be impaired. In addition, the amount of carbide or nitride precipitated increases, which increases the yield ratio and there is a risk that shape freezing properties may deteriorate. Therefore, when containing Ti and/or Nb, it is preferable that Ti is set to 0.01 mass% or less and Nb is set to 0.050 mass% or less. A more preferable content is in the range of Ti: 0.0006 to 0.0080 mass% and Nb: 0.010 to 0.040 mass%.

Group B; One or two or more selected from Cr: 1.0 mass% or less, Mo: 0.50 mass% or less, and B: 0.10 mass% or less

Cr, Mo and B are elements effective in improving the hardenability of steel, and to obtain the above effect, at least one of Cr: 0.01 mass% or more, Mo: 0.004 mass% or more, and B: 0.0001 mass% or more It is desirable to contain . However, even if these elements are contained in excess, the above effect is saturated and only causes an increase in raw material costs. Therefore, when Cr, Mo, and B are added, it is preferable to add Cr: 1.0 mass% or less, Mo: 0.50 mass% or less, and B: 0.10 mass% or less. More preferably, it is in the range of Cr: 0.02 to 0.50 mass%, Mo: 0.01 to 0.10 mass%, and B: 0.001 to 0.03 mass%.

In at least one steel plate constituting the weld joint of the present invention, the remainder other than the above components is Fe and inevitable impurities.

tensile strength of steel plate

Moreover, among the plurality of steel plates constituting the overlap laser welded joint of the present invention, it is preferable that at least one steel plate is a high-tensile strength steel plate with a tensile strength TS of 980 MPa or more. If at least one steel plate is the above-mentioned high-strength steel plate, the overlap laser welded joint can achieve high joint strength, and even when weld cracks occur with the conventional welding method, in the present invention, the joint area around the joint by the preceding joint is used. Due to the effect of suppressing the deformation of the steel plate, the occurrence of weld cracks can be prevented. Therefore, for example, it is preferable that at least one of the plurality of steel sheets has the above-described component composition and has a tensile strength TS of 980 MPa or more. In addition, the plurality of steel plates constituting the overlap laser welded joint of the present invention may be steel plates with the same composition and the same strength, or may be steel plates with different components and different strengths.

＜Manufacturing method of overlap laser welded joint＞

Next, the manufacturing method of the overlap laser welded joint of the present invention will be described.

FIG. 5 is a diagram illustrating an example of a welding method used for manufacturing the overlap laser welded joint of the present invention. In the overlap laser welding joint 1 of the present invention, first, a plurality of steel plates are overlapped vertically, and a laser beam is intermittently irradiated to the surface of the uppermost steel plate among the plurality of overlapped steel plates to form the first joint 4. It is manufactured by forming a welded portion by sequentially forming a plurality of subsequent jointed portions 5 following it.

As described above, in the present invention, one-side welding is performed on a plurality of overlapping steel plates. By employing one-side welding, the work space required for welding can be reduced.

In addition, in one-side welding, from the viewpoint of preventing melting during welding, it is preferable to irradiate the laser beam from the side of the steel sheet with the larger sheet thickness among the plurality of overlapping steel sheets. On the other hand, from the viewpoint of preventing non-joining due to non-penetration, it is preferable to irradiate the laser beam from the side with a thinner plate thickness. Additionally, when the steel sheets have the same thickness, the laser beam may be irradiated from either side of the steel sheets.

In addition, in the overlap laser welding joint 1 shown in FIG. 5, two steel plates 2 and 3 are overlapped to form a joint surface, and a laser beam is irradiated intermittently on the joint surface to form a first J-shaped joint. A joint portion 4 is formed followed by a plurality of linear subsequent joint portions 5.

Here, what is important in the present invention is that the overlap laser welded joint of the present invention has a ratio (G/T) of the total gap (G) between the steel plates constituting the welded portion to the total thickness (T) of the steel plates constituting the welded portion. is set to 0 to 0.15, that is, the ratio of the total gap (G) between the steel plates constituting the welded portion to the total thickness (T) of the steel plates constituting the welded portion is set within the range of 0 to 15%. If the ratio of G to T exceeds 15%, the depth of the crater at the end of the weld becomes deeper and stress becomes more likely to concentrate. Preferably it is in the range of 0 to 10%.

Additionally, what is important in the present invention is that when the laser beam LB is irradiated to the surface of the steel sheet while moving the welding head WH, which irradiates the laser beam, in the welding direction (the arrow direction indicated by D in FIG. 5), the laser beam The scanning direction of (LB) is set to be opposite to the moving direction of the welding head WH (opposite direction to the arrow shown in D in FIG. 5). By reversing the moving direction of the welding head WH and the scanning direction of the laser beam LB as described above, the welding start end of the first joint 4 and the welding termination of the subsequent joint 5 adjacent thereto face each other. In addition, the welding portion can be formed so that the welding start end of the subsequent joint 5 and the welding termination of the subsequent joint 5 adjacent thereto face each other, that is, the welding start end and the welding termination of the adjacent joint face each other. Because of this, it becomes possible to prevent cracking at the weld end of the joint.

In addition, in the present invention, as shown in Fig. 5, the shape of the first joint 4 is composed of a straight joint and an arc-shaped or circular curved joint connected to the weld termination side of the straight joint. It is important to do it with a J-shaped image. Accordingly, cracks occurring at the weld end of the first joint can be prevented.

Furthermore, what is important in the present invention is that the welded portion (the first joint and the subsequent joint) formed as described above has the following formulas (1) to (4);

15.0 ≤ L ₁ ≤ 30.0 … (One)

8.0 ≤ L ₂ ≤ 20.0 … (2)

1/8 ≤ w/b ≤ 1/2 … (3)

1/4 ≤ a / (L ₁ + L ₂ ) ≤ 1/2 or 1/2 ≤ a / L ₂ ≤ 1 … (4)

Here, L ₁ : Length of the first joint (mm)

L ₂ : Length of subsequent joint (mm)

b: Minimum thickness of molten metal at the joint (mm)

w: Width of molten metal at joint (mm)

a: Shortest distance between joints (mm)

It is important to control the welding conditions of the laser beam, specifically, at least one of the laser power, focus position, welding speed and beam diameter, so that all are satisfied.

Here, as the type of laser beam used in the laser beam welding in the present invention, for example, a fiber laser, a disk laser, etc. can be used. In addition, in order to satisfy the above equations (1) to (4), the laser beam irradiation is: output: 2.0 to 6.0 kW, focus position: surface of the steel sheet to which the laser beam is irradiated ~ surface of the steel sheet + 30 mm, beam diameter: It is preferable to carry out the scan in the range of 0.4 to 1.0 mm and the scanning speed of the laser beam: 2.0 to 5.0 m/min. More preferably, laser power: 2.5 to 5.0 kW, focus position: surface of the steel sheet on which the laser beam is irradiated to surface of the steel sheet + 20 mm, beam diameter: 0.5 to 0.8 mm, and scanning speed of the laser beam: 2.5 to 4.5 m/min. is the range.

In addition, in the welding method of the present invention, the shape of the first joint 4 may be a shape consisting only of straight joints instead of the J-shape, but in that case, the length of the straight joint (L _1' ) is the formula (1') below, instead of formula (1);

30.0 ≤ L _1' ≤ 40.0... (One')

Here, L _1' : Length of straight joint (mm)

It is important to control it to satisfy. Accordingly, even if it is not a J-shaped wound, cracks occurring at the weld end portion of the first joint can be prevented.

＜Structural members for automobile bodies＞

Next, the structural member for an automobile body of the present invention will be described.

An example in which the overlap laser welded joint of the present invention can be preferably used is a structural member (strength member) that becomes a skeletal part of an automobile body. The member shown in FIG. 1 (FIG. 4) described above is composed of a steel plate 2, which is a frame component with a cross-sectional shape of approximately a hat shape, and a steel plate 3, which is a panel component, and the flange portion of the steel plate 2 (2b) and the steel plate (3) disposed opposite to the flange portion (2b) are joined by a welding portion consisting of the first joint portion (4) and the subsequent joint portion (5) formed by the laser beam welding described above, It constitutes a closed cross-section. In order to apply a member having such a shape to a strength member of an automobile body, it is important that the strength of the weld zone is excellent from the viewpoint of ensuring crash safety. However, the overlap laser welded joint of the present invention has no cracks at the weld end of the joint. Since it has no peeling strength and has sufficient peeling strength, it can be suitably used for structural members such as center pillars and roof rails of automobile bodies, for example.

Here, when applying the overlap laser welding joint of the present invention to manufacture structural members for automobile bodies, etc., the suitable position for forming the welded portion is an L-shaped cross section with flange portions 2b and 3b, as shown in FIG. 6. A case where two steel plates 2 and 3 having are overlapped so that the flange portions face each other and laser beam welding is performed from one side will be described as an example. FIG. 6(a) is a plan view of overlapping flange parts when viewed from above, showing that a welded portion consisting of a J-shaped first joint and a plurality of subsequent joints following the J-shaped first joint is formed in the flange portion. Figure 6 (b) is a cross-sectional view of the C-C cross section shown in (a) above.

In FIG. 6, the suitable position for forming the welded portion is the center line of the plate thickness of the steel plates 2 and 3 as the starting point (point 0), and extends from there to the width center portion of the joint 5(4) formed on the flange portion. When the distance is defined as the welding position (X), the welding position (X) is expressed by the following equation (5);

5t ≤ X ≤ 8t … (5)

Here, t: Thickness of the thickest steel plate among the steel plates constituting the weld zone (mm)

It is desirable to satisfy. For example, when the thickness (T) of the thickest steel plate is 2 mm, the welding position (X) is preferably in the range of 10 to 16 mm.

If the welding position On the other hand, if the welding position A more preferable range of X is the range of 6t ≤ X ≤ 7t. By forming the welded portion at the above position, the peeling strength of the welded joint of two overlapping steel sheets having a total plate thickness of 2 to 5 mm can be made 3.0 kN or more.

In addition, the equation (5) regarding the welding position For example, it can also be applied to an overlap laser welded joint where a frame part (steel plate 2) and a panel part (steel plate 3) having a roughly hat-shaped cross-sectional shape as shown in Figure 1 or Figure 4 are laser beam welded, In this case, the starting point (0 point) of the welding position

Example

A high-strength steel sheet having the component compositions A to J shown in Table 1, having a sheet thickness of 1.2 mm, 1.6 mm, or 2.0 mm, and having a tensile strength TS of 590 to 1180 MPa, width: 100 mm, length: 150 mm. A sample was taken, and this was bent into an L-shape with a long side of 120 mm and a short side of 30 mm to obtain an L-shaped steel sheet. Here, the long side of the L-shaped steel plate corresponds to the vertical wall 2a in FIG. 1 (FIG. 4), and the short side corresponds to the flange portion 2b in FIG. 1 (FIG. 4). Next, as shown in FIG. 7, after the two L-shaped steel plates 7 are overlapped so that their short sides face each other, a laser beam is irradiated in the air to the overlapped portion to form the first joint 4 and a plurality of subsequent joints. A welded portion consisting of the joint portion 5 was formed, and a T-shaped peeling test piece 9 was manufactured.

In addition, when performing the above-mentioned overlap laser beam welding, a fiber laser with a beam diameter of 0.6 mmϕ at the focal position is used as the laser beam, and the focal position is the upper surface of the overlapped steel sheet (the upper steel sheet shown in FIG. 7 (7) ) After setting the surface of ), as shown in Table 2, the gap (G) between the two steel plates, the output of the irradiating laser beam, and the scanning speed are changed in various ways to form a J-shaped first joint. Length (L ₁ ), length of the subsequent joint (L ₂ ), shortest distance between the first joint and the adjacent subsequent joint and between subsequent joints (a), minimum thickness of the molten metal portion of the joint (b) and melting of the joint The width (w) of the metal was varied as shown in Table 2. At this time, the welding position (X) forming the weld zone was set to 6.5 times (constant) the thickest plate thickness (T).

In addition, as the first joint, a welded portion employing a straight first joint having a long length (L _1' ) instead of the J-shaped first joint was manufactured in the same manner, and a T-shaped peeling test piece (9) was produced. was manufactured.

With respect to the T-shaped peeling test piece obtained in this way, the presence or absence of cracks at the welded portion, particularly the weld termination portion of the first joint and subsequent joints, was determined by visual inspection and penetrant testing.

Next, on the T-shaped peeling test piece, a tensile test was performed with the longitudinal direction of the long sides of the two L-shaped steel plates as the tensile direction at a speed of 10 mm/min, and the peeling strength (maximum load) was measured. In addition, in this example, the case where the peeling strength was 3.0 kN or more was judged as “pass”.

[Table 1]

[Table 2-1]

[Table 2-2]

[Table 2-3]

The results of determining the presence or absence of weld cracks and the measurement results of peeling strength are listed in Table 2.

From these results, all of the test pieces (Nos. 1, 8, 15, 22, 29, 36, 43, 50, 57, and 64) that were overlapped with laser beam welding under conditions suitable for the present invention showed cracks at the weld ends of the joints. There was no peeling strength, and the peeling strength was 3.0 kN or more.

In comparison, no. In the test pieces 2, 9, 16, 23, 30, 37, 44, 51, 58, and 65, the gap (G) of the weld zone was larger than 15% of the total thickness (T) of the steel plate, so all of the test pieces were at the weld end of the joint zone. Cracks occurred in the part, and the peeling strength was less than 3.0 kN.

Also, no. For the test specimens 3, 10, 17, 24, 31, 38, 45, 52, 59, and 66, the width (w) of the molten metal at the joint was greater than 1/2 of the minimum thickness (b) of the molten metal at the joint. , weld cracks occurred. Meanwhile, no. For the test specimens 4, 11, 18, 25, 32, 39, 46, 53, 60, and 67, the width (w) of the molten metal at the joint was less than 1/8 of the minimum thickness (b) of the molten metal at the joint. Therefore, no weld cracks occurred, but the peeling strength was less than 3.0 kN.

Also, no. In the test pieces 5, 12, 19, 26, 33, 40, 47, 54, 61, and 68, weld cracks occurred because the length (L ₁ ) of the first joint was shorter than 15.0 mm.

Also, no. In the test pieces 6, 13, 20, 27, 34, 41, 48, 55, 62, and 69, weld cracks occurred because the length (L ₂ ) of the subsequent joint was shorter than 8.0 mm.

Also, no. For the test pieces 7, 14, 21, 28, 35, 42, 49, 56, 63 and 70, the distance (a) between the first joint and adjacent subsequent joints and between subsequent joints is equal to the subsequent joint length (L ₂ ), weld cracks occurred, and the peeling strength was less than 3.0 kN.

Also, no. 71 and 72 show test results on test pieces obtained by overlapping laser beam welding of two steel plates with different strength levels under conditions suitable for the present invention. Even in the combination of 590 MPa class and 980 MPa class, the steel component composition No. within the suitability range of this invention. No. 71 has no weld cracks and excellent peeling strength is obtained, but the steel composition is outside the suitable range of the present invention. In case 72, weld cracks occurred and the peeling strength was less than 3.0 kN.

Also, no. 73 is a test piece employing a straight joint with a long length (L _1' ) as the first joint, and by overlapping laser beam welding under conditions suitable for the present invention, there are no weld cracks and peeling equivalent to that of the J-shaped first joint is achieved. It was seen that strength was achieved.

As described above, in all of the present invention examples in which overlap laser beam welding was performed according to the present invention, good overlap laser welded joints having the characteristics targeted by the present invention were obtained, while comparative examples outside the conditions of the present invention were obtained. In , a good overlap laser welded seam could not be obtained.

Since the technology of the present invention enables high-speed and low-strain welding, it can be suitably applied to structural members for automobiles having a flange portion.

1: Overlap laser welded seam
2, 3: Steel plate
2a: Vertical wall part
2b: Flange part
4, 14: first joint
4a, 14a: Center of the weld end of the first joint
5, 15: Subsequent joints
5a, 15a: Center of weld end of subsequent joint
6: welding part
7: L-shaped steel plate
7a: Long side of L-shaped steel plate
7b: Width of L-shaped steel plate
8: Cracks at the weld end of the joint
9: Peeling test piece
WH: Welding head
LB: Laser beam
D: moving direction of welding head
d: scanning direction of the laser beam
L ₁ , L _1': Length of the first joint
L ₂ : Length of subsequent joint
a: Shortest distance between adjacent joints
b: Minimum thickness of molten metal at the joint
w: Width of molten metal at joint
G: Gap between steel plates
S: Weld start end of joint
E: Welded end of joint
Fa: Stress applied to the weld end of a conventional joint
Fb: Stress applied at the initial weld end of the subsequent joint
Fc: Stress applied to the weld end of the first joint
0: Reference point of welding position (X)

Claims (11)

In an overlap laser welded joint having a weld formed by overlapping a plurality of steel plates,
The total gap (G) between the steel plates constituting the welded portion is within the range of 0 to 15% of the total thickness (T) of the steel plates constituting the welded portion,
The welded portion consists of a linear first joint and straight subsequent joints arranged in a row following the first joint,
The weld start end of the first joint and the weld termination of the subsequent joint adjacent thereto face each other, and the weld start and weld termination of the subsequent joints face each other,
The first joint has a J-shape consisting of a straight joint and an arc-shaped or circular curved joint connected to the weld end side of the straight joint,
Additionally, the overlap laser welded joint is characterized in that the welded portion satisfies all of the following equations (1) to (4).
15.0 ≤ L ₁ ≤ 30.0 … (One)
8.0 ≤ L ₂ ≤ 20.0 … (2)
1/8 ≤ w/b ≤ 1/2 … (3)
1/4 ≤ a / (L ₁ + L ₂ ) ≤ 1/2 or 1/2 ≤ a / L ₂ ≤ 1 … (4)
Here, L ₁ : Length of the first joint (mm)
L ₂ : Length of subsequent joint (mm)
b: Minimum thickness of molten metal at the joint (mm)
w: Width of molten metal at joint (mm)
a: Shortest distance between joints (mm) According to claim 1,
An overlap laser welded joint, characterized in that the first joint consists only of straight joints and satisfies the following equation (1') instead of the equation (1) above.
30.0 < L _1' ≤ 40.0... (One')
Here, L _1' : Length of straight joint (mm) According to claim 1,
At least one of the above steel sheets has C: 0.07 to 0.4 mass%, Si: 0.2 to 3.5 mass%, Mn: 1.8 to 5.5 mass%, P+S: 0.03 mass% or less, Al: 0.08 mass% or less, and N: 0.010 mass%. An overlap laser welded joint characterized by having a composition containing the following and the remainder being Fe and inevitable impurities. According to claim 2,
At least one of the above steel sheets has C: 0.07 to 0.4 mass%, Si: 0.2 to 3.5 mass%, Mn: 1.8 to 5.5 mass%, P+S: 0.03 mass% or less, Al: 0.08 mass% or less, and N: 0.010 mass%. An overlap laser welded joint characterized by having a composition containing the following and the remainder being Fe and inevitable impurities. According to claim 3,
An overlap laser weld joint, characterized in that the steel sheet contains, in addition to the above component composition, at least one group of components from the following groups A and B.
·Group A; One or two types selected from Ti: 0.0005 to 0.01 mass% and Nb: 0.005 to 0.050 mass%
·Group B; One or two or more selected from Cr: 1.0 mass% or less, Mo: 0.50 mass% or less, and B: 0.10 mass% or less According to claim 4,
An overlap laser weld joint, characterized in that the steel sheet contains, in addition to the above component composition, at least one group of components from the following groups A and B.
·Group A; One or two types selected from Ti: 0.0005 to 0.01 mass% and Nb: 0.005 to 0.050 mass%
·Group B; One or two or more selected from Cr: 1.0 mass% or less, Mo: 0.50 mass% or less, and B: 0.10 mass% or less The method according to any one of claims 1 to 6,
An overlap laser welded joint, characterized in that at least one of the steel plates is a high-tensile strength steel plate with a tensile strength of 980 MPa or more. A plurality of steel plates are overlapped up and down, and a laser beam is intermittently irradiated on one surface of the overlapped steel plates to form a weld zone in which a linear first joint and linear subsequent joints following the first joint are arranged in a row. when doing,
The total gap (G) between the steel plates constituting the welded portion is set to be within the range of 0 to 15% of the total thickness (T) of the steel plates constituting the welded portion,
By reversing the moving direction of the welding head that irradiates the laser beam and the scanning direction of the laser beam, the welding start end of the first joint and the welding termination of the subsequent joint adjacent to the first joint face each other, and the subsequent joint In addition to making the welding start and welding ends face each other,
The shape of the first joint is a J shape consisting of a straight joint and an arc-shaped or circular curved joint connected to the weld end side of the straight joint,
In addition, a method of manufacturing an overlap laser welding joint, characterized in that at least one of laser power, focus position, welding speed, and beam diameter is controlled so that the welding part satisfies all of the following equations (1) to (4).
15.0 ≤ L ₁ ≤ 30.0 … (One)
8.0 ≤ L ₂ ≤ 20.0 … (2)
1/8 ≤ w/b ≤ 1/2 … (3)
1/4 ≤ a / (L ₁ + L ₂ ) ≤ 1/2 or 1/2 ≤ a / L ₂ ≤ 1 … (4)
Here, L ₁ : Length of the first joint (mm)
L ₂ : Length of subsequent joint (mm)
b: Minimum thickness of molten metal at the joint (mm)
w: Width of molten metal at joint (mm)
a: Shortest distance between joints (mm) According to claim 8,
The first joint is made up of only straight joints, and at least one of the laser power, focus position, welding speed, and beam diameter is controlled to satisfy the following equation (1') instead of the equation (1) above. Method for manufacturing an overlap laser welded seam, characterized by:
30.0 < L _1' ≤ 40.0... (One')
Here, L _1' : Length of straight joint (mm) A structural member for an automobile body having an overlapping laser welded joint according to any one of claims 1 to 6. A structural member for an automobile body having an overlapping laser welded joint according to claim 7.

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