KR102597121B1 - Lap laser spot weld joint and method for producing same, and automotive body structural member - Google Patents

Abstract

When manufacturing an overlap laser spot welded joint by overlapping a plurality of steel plates up and down and intermittently irradiating a laser beam on one surface of the overlapping steel plates to form a weld zone composed of joints arranged in series in a thermal image, at least: By spinning the laser beam to draw an elongated circle combining a semicircle and a straight line and scanning it spirally from the outside of the oval shape toward the inside, the shape of the joint is made into a large oval joint, and the short axis of the oval joint is formed. By controlling the welding conditions so that the width D ₁ , the major axis width D ₂ , the ratio of D ₂ to D ₁ (D ₂ /D ₁ ), and the minimum thickness u of the final solidified portion satisfy predetermined conditions, the weld end portion of the joint portion We propose an overlap laser spot welded joint with no cracks and excellent peel strength, a manufacturing method thereof, and a structural member for an automobile body having the welded joint.

Description

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

The present invention relates to an overlap laser spot weld joint, a method of manufacturing the same, and a structural member for an automobile body having the overlap laser spot weld 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, the welding pitch cannot be narrowed because the calorific value decreases due to the flow, and furthermore, a certain amount of space is required to set the welding gun, etc. , there are several problems. To solve these problems, in recent years, the application of overlap laser spot welding instead of conventional resistance spot welding has been examined and put into practice. Here, the overlap laser spot 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 spot welding joint is made by intermittently irradiating a laser beam to the surface of a plurality of overlapping steel sheets, melting and solidifying the steel sheet in the area irradiated with the laser beam, and forming a straight or circular joint in a continuous manner. A plurality of steel plates were joined by forming welded portions arranged in a row. However, there is a problem in overlap laser beam welding that, when the joint is straight, cracks are likely to occur in the final solidified portion on the weld termination side of the joint. Also, even when the joint is circular, there is a problem that cracks are likely to occur in the final solidified portion in the center of the joint. When a crack occurs, it propagates over the entire length of the joint, so not only does the static strength such as shear strength and peel strength of the weld joint decrease, but the fatigue strength also significantly decreases. Recently, in order to improve the strength and rigidity of the car body, high-strength steel sheets have been increasingly 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 obliquely to the ends of the 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 surroundings 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 an empty gap between 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. In addition, the method described in Patent Document 5 has a problem in that sufficient welding strength cannot be secured because the shape of the welded portion is limited to a circle or an elliptical shape close to a circle. Additionally, in the method described in Patent Document 6, there is a problem that cracks cannot be prevented from occurring at the weld end of a short straight joint because stress tends to concentrate at the end of the weld.

The present invention has been made in consideration of the above-mentioned problems faced by the prior art, and its purpose is to provide an overlap weld joint having a weld zone in which the joint (weld spot) is thermally formed by intermittently irradiating a laser beam, and the final solidification of the joint. To provide an overlap laser spot welded joint that does not cause cracks in the portion and has excellent peeling strength of the welded portion, proposes a manufacturing method thereof, and provides a structural member for an automobile body having this overlap laser spot welded joint. It's in the thing.

In order to solve the above problems, the inventors focused on the shape and size of the individual joints (weld spots) that make up the weld zone formed by laser welding and conducted extensive studies. As a result, in order to prevent cracking of the final solidified portion of the joint, it is effective to shape the joint into an oval shape larger than the conventional linear, circular, or oval shape, and to control the various dimensions of the oval joint to an appropriate range. After finding out this, we came to develop this invention.

The present invention based on the above knowledge is an overlap laser spot welding 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 0 to 0 of the total thickness T of the steel plates constituting the welded portion. It is within the range of 15%, the welded portion consists of oval joints arranged intermittently, and the oval joint portions are in the following (1) to (5);

1.0 ≤ T ≤ 6.0 ···(1)

2.0 ≤ D ₁ ≤ 8.0 ···(2)

6.0 ≤ D ₂ ≤ 15.0 ···(3)

1.1 ≤ D ₂ /D ₁ ≤ 5.0 ···(4)

0.6 ≤ u/T ≤ 1.0 ···(5)

Here, T: Total plate thickness of the steel plate constituting the weld zone (mm)

D ₁ : Width of the minor axis of the oval joint (mm)

D ₂ : Width of the long axis of the oval joint (mm)

u: Minimum thickness of the final solidified portion of the oval joint (mm)

It is an overlap laser spot welding joint characterized by satisfying all of the equations.

In the overlap laser spot welded joint of the present invention, at least one of the above steel plates 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, It is characterized by having a component composition containing Al: 0.08 mass% or less and N: 0.010 mass% or less, with the remainder being Fe and inevitable impurities.

In addition to the above chemical composition, the steel sheet in the overlap laser spot weld joint of the present invention further includes the following 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; It is characterized by containing at least one group of components selected from Cr: 1.0 mass% or less, Mo: 0.50 mass% or less, and B: 0.10 mass% or less.

Additionally, the overlap laser spot welded joint of the present invention is characterized in that at least one of the steel plates is a high-tensile steel plate 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 to one surface of the overlapped steel plates to form a weld zone consisting of oval joints arranged in succession in a thermal pattern, thereby performing overlap laser spot welding. In the method of manufacturing a joint, 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 oblong joint is formed by transmitting a laser beam in a semicircular manner. While spinning to draw an elongated circle combining straight lines, the oval shape is scanned spirally from the outside to the inside, and the oval joint is formed according to the following formulas (1) to (5).

1.0 ≤ T ≤ 6.0 ···(1)

2.0 ≤ D ₁ ≤ 8.0 ···(2)

6.0 ≤ D ₂ ≤ 15.0 ···(3)

1.1 ≤ D ₂ /D ₁ ≤ 5.0 ···(4)

0.6 ≤ u/T ≤ 1.0 ···(5)

Here, T: Total plate thickness of the steel plate constituting the weld zone (mm)

D ₁ : Width of the minor axis of the oval joint (mm)

D ₂ : Width of the long axis of the oval joint (mm)

u: Minimum thickness of the final solidified portion of the oval joint (mm)

We propose a method for manufacturing an overlap laser spot welding seam, characterized in that at least one of the laser power, focus position, welding speed, spin radius, and movement amount per spin and beam diameter is controlled to satisfy all of the above.

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

According to the present invention, the joint forming the welded portion of the overlap laser spot welded joint obtained by laser beam welding a plurality of overlapping steel plates is made into an oval joint larger than the conventional joint, thereby reliably suppressing the occurrence of cracks in the final solidified portion. In addition, it is possible to manufacture an overlap laser spot welded joint with excellent peel strength of the weld zone. In addition, the overlap laser spot welding joint of the present invention can form an oval joint with a wide major axis-minor axis ratio, thereby increasing the degree of freedom in component design and enabling the development of lighter, more rigid, and higher-strength members. . Therefore, the overlap laser spot welding 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 spot welding joint.
Figure 2 is a schematic diagram illustrating a welded portion of a conventional overlap laser spot welding 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 spot welding joint of the present invention, where (a) is a top view and (b) is a BB cross-sectional view of (a).
4 is a diagram explaining the welding method used for manufacturing the weld joint of the present invention.
FIG. 5 is a diagram illustrating an example of a scanning trajectory of a laser beam for obtaining an oval joint of the present invention.
Figure 6 is a view explaining the welding position of the overlap laser spot 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 spot welded joint used in an example of the present invention.

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

＜Overlapping laser spot welding joint＞

Fig. 1 is a perspective view showing an example of a conventional overlap laser spot welding joint. The overlap laser spot welding joint 1 is made 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. Two steel plates, a steel plate 2 having a roughly hat-shaped cross-sectional shape and a flat panel-shaped steel plate 3, are overlapped so that the flange portion 2b and the steel plate 3 face each other to form a joint surface, and the flange portion Welding is performed by irradiating a laser beam onto the surface of the flange portion 2b from above (2b), forming a molten portion (molten metal portion) that penetrates at least the steel plate 2, and solidifying it to form a joint portion (weld spot). This is being implemented. In addition, a heat-affected zone (HAZ) exists around the fusion zone, but the joint of the present invention refers only to the fusion zone excluding the heat-affected zone.

The welding portion of the overlap laser spot weld joint 1 applies a laser beam to the surface of the flange portion 2b while moving the welding head, which is the laser beam source, along the longitudinal direction (arrow direction in FIG. 1) of the vertical wall portion 2a. It is formed through intermittent investigation. As a result, as shown in FIG. 1, on the joint surface of the steel plate 2, an elliptical joint (weld spot) is formed continuously in a thermal pattern.

FIG. 2 is a schematic diagram showing a conventional welded portion formed on the flange portion 2b of the overlap laser spot welded joint shown in FIG. 1, where (a) shows the joint forming the welded portion from above the flange portion 2b. This plan view (b) is a cross-sectional view showing the A-A cross section shown in (a) above.

In conventional laser beam welding, when the joint 14 is formed in an elongated oval shape, that is, with a large short axis-major axis ratio of the melted zone, a weld crack 5 occurs in the center 14a, which becomes the final solidified zone, so that the short-major axis of the melted zone There was a problem that it was difficult to improve the welding strength because the ratio needed to be small. This is because, as shown in FIG. 2(b), in conventional laser beam welding, a large amount of sputtering occurs, and the central portion 14a, which becomes the final solidified portion, becomes excessively thin, so that the outer portion from the outer peripheral portion of the melted portion This is because tensile stress (force in the direction of arrow σa shown in (a) of FIG. 2) is applied intensively. On the other hand, when trying to increase the melt diameter of the joint in order to improve weld strength, another problem occurs in that melting occurs.

In addition, the cracks at the ends of the weld penetrate from the surface to the back surface of the final solidified portion of the joint, and the occurrence of cracks can be confirmed with the naked eye. However, to make a more certain determination, the width of the final solidified portion of the joint after welding must be wide. It is preferable to judge by cutting in each direction and observing the cut surface at a magnification of about 10 times with an optical microscope, for example.

Therefore, in order to reduce the stress concentration that occurs in the final solidified portion of the joint in laser beam welding, the inventors have repeatedly examined measures to prevent reduction of the thickness of the central portion that becomes the final solidified portion of the joint.

As a result, as will be described later, the laser beam is spun to draw an elongated circle combining a semicircle and a straight line, and is scanned spirally from the outside of the oval shape toward the inside, forming an oval shape as shown in FIG. 3(a). By forming a joint, the occurrence of sputtering can be suppressed, and further, as shown in FIG. 3(b), the minimum thickness u of the central portion 4a of the joint that becomes the final solidified portion can be thickened. Therefore, by forming the oval joint, the tensile stress (arrow σb shown in Fig. 3(a)) applied to the final solidified portion 4a of the joint and directed outward from the outer peripheral portion of the melted portion is significantly reduced compared to the conventional welding method. Since this can be reduced, it becomes possible to prevent cracking of the final solidified portion. Here, the “elliptical shape” in the present invention refers to a shape in which two circles with equal radii are connected by a common external tangent.

By employing the above-described oval-shaped joint larger than the conventional one, cracking in the final solidified portion of the joint can be significantly reduced. However, according to additional research by the inventors, in order to more reliably prevent cracking of the final solidified portion of the joint and to make the peeling strength of the welded zone sufficient, in addition to employing the above-described oval joint, The circular joint satisfies 0 ≤ G/T ≤ 0.15, which will be described later, and the following formulas (1) to (5);

1.0 ≤ T ≤ 6.0 ···(1)

2.0 ≤ D ₁ ≤ 8.0 ···(2)

6.0 ≤ D ₂ ≤ 15.0 ···(3)

1.1 ≤ D ₂ /D ₁ ≤ 5.0 ···(4)

0.6 ≤ u/T ≤ 1.0 ···(5)

Here, T: Total plate thickness of the steel plate constituting the weld zone (mm)

D ₁ : Width of the minor axis of the oval joint (mm)

D ₂ : Width of the long axis of the oval joint (mm)

u: Minimum thickness of the final solidified portion of the oval joint (mm)

I was able to see what was needed to satisfy everyone.

Hereinafter, it will be described in detail.

0 ≤ G/T ≤ 0.15

The overlap laser spot 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 ratio of the total gap G between the steel plates constituting the welded portion is 0 to 0.15. It is necessary that the ratio of the total gap G between the steel plates constituting the weld zone to the total thickness T 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%.

1.0 ≤ T ≤ 6.0

In addition, the overlap laser spot welding joint of the present invention requires that the total plate thickness T of the plurality of steel plates be in the range of 1.0 to 6.0 mm. When the total plate thickness is thinner than 1.0 mm or thicker than 6.0 mm, melting is likely to occur when irradiated with a laser beam, and it becomes difficult to perform overlap welding. Therefore, in the weld joint of the present invention, the total plate thickness T of the steel plates constituting the joint is in the range of 1.0 to 6.0 mm. Preferably it is in the range of 2.0 to 5.0 mm.

2.0 ≤ D ₁ ≤ 8.0

In addition, the overlap laser spot welding joint of the present invention requires that the minor axis width D ₁ of the oval joint constituting the welded portion be in the range of 2.0 to 8.0 mm. If D ₁ is greater than 8.0 mm, melting occurs. Preferably it is 6.0 mm or less. On the other hand, D ₁ is set to 2.0 mm or more from the viewpoint of ensuring sufficient joint strength. Preferably it is 4.0 mm or more.

6.0 ≤ D ₂ ≤ 15.0

In addition, the overlap laser spot welding joint of the present invention requires that the major axis width D ₂ of the oval joint forming the welded portion be in the range of 6.0 to 15.0 mm. If D ₂ is greater than 15.0 mm, weld cracks occur. Preferably it is 13.0 mm or less. On the other hand, the lower limit of D ₂ is not specified, but is set to 6.0 mm from the viewpoint of ensuring sufficient joint strength. Preferably it is 8.0 mm or more.

1.1 ≤ D ₂ /D ₁ ≤ 5.0

In addition, the overlap laser spot welding joint of the present invention requires that the ratio (D ₂ /D ₁ ) of the major axis width D ₂ to the minor axis width D ₁ of the oval joint forming the weld zone be in the range of 1.1 to 5.0. If the ratio (D ₂ /D ₁ ) is greater than 5.0, weld cracks occur. Preferably it is 4.0 or less. On the other hand, even when the ratio (D ₂ /D ₁ ) is less than 1.1 times, weld cracks are likely to occur. Preferably it is 1.5 or more.

0.6 ≤ u/T ≤ 1.0

In addition, in the overlap laser spot welded joint of the present invention, the ratio (u/T) of the minimum thickness u of the melted zone occurring in the center 4a of the final solidified portion of the joint to the total plate thickness T of the steel plates constituting the joint is 0.6. It needs to be in the range of ~1.1. If the ratio (u/T) is less than 0.6, the tensile stress applied to the center 4a of the final solidified portion from the outer peripheral portion of the melted portion to the outside increases, making it impossible to prevent cracking. Preferably it is 0.7 or more. On the other hand, the ratio (u/T) is set to 1.0 or less because the thickness u of the melted portion at the center that becomes the final solidified portion by sputtering is usually smaller than the total plate thickness T of the plurality of steel plates.

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

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

plate thickness of steel plate

The total thickness of the steel plates constituting the overlap laser spot welded joint of the present invention has been described above, but the plate thickness of each steel plate is 0.5 to 3.2 mm, which is generally used as an outer plate or structural member (strength member) of an automobile body. It can be anything within the range, and there are no particular restrictions. 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 spot welding joint 1 of the shape shown in FIG. 1, the plate thickness t ₂ of the upper steel plate 2 is 0.6 to 1.8 mm, and the plate thickness t ₃ of the lower steel plate 3 is 0.6 to 1.8 mm. 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.

Composition of steel plate

In addition, the composition of the plurality of steel sheets constituting the overlap laser spot weld joint of the present invention is not particularly limited, but at least one steel sheet has C: 0.07 to 0.4 mass%, Si: 0.2 mass%, as described below. Contains ~ 3.5 mass%, Mn: 1.8 ~ 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 balance consisting of Fe and inevitable impurities. It is desirable.

C: 0.07 ~ 0.4 mass%

C is an element effective in improving the strength of steel, and by containing it at 0.07 mass% or more, the effects of precipitation strengthening and transformation strengthening can be obtained. Additionally, 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 ~ 3.5 mass%

Si is an element with excellent 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 ~ 5.5 mass%

Mn is an element effective in improving quenching properties and suppressing precipitation of coarse carbides, and is preferably contained in an amount of 1.8 mass% or more. Additionally, 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 deoxidizer 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 plate strength and the peeling strength of the weld zone. It is preferred that it contains ingredients from group 1.

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 precipitation of carbides or nitrides 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; Cr: 1.0 mass% or less, Mo: 0.50 mass% or less, and B: 0.10 mass% or less. One or two or more types selected from among

Cr, Mo, and B are elements effective in improving the hardenability of steel. 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 is added. It is desirable to contain it. 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, the range is Cr: 0.02 to 0.50 mass%, Mo: 0.010 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 sheets constituting the overlap laser spot weld joint of the present invention, it is preferable that at least one steel sheet is a high-tensile strength steel sheet 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 spot welded joint can achieve high joint strength, and even if welding defects occur in the conventional oval joint, if the oval joint of the present invention is the final joint, Since the stress concentration on the solidified portion is small, 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 spot weld joint of the present invention may be steel plates with the same composition and the same strength, or may be steel plates with different composition and different strengths.

＜Manufacturing method of overlap laser spot welding joint＞

Next, the manufacturing method of the overlap laser spot welding joint of the present invention will be explained using FIGS. 4 to 6.

The manufacturing method of the overlap laser spot welded joint of the present invention involves stacking a plurality of steel plates vertically and intermittently irradiating a laser beam to the surface of the uppermost steel sheet among the plurality of overlapping steel sheets to sequentially form the joint portion 4. By doing so, a welded portion is formed to manufacture a welded joint. In the example shown in Fig. 4, the overlap laser spot welding joint 1 of the present invention overlaps a plurality of steel sheets 2 and 3, and intermittently applies a laser beam 6 to the surface of the uppermost steel sheet 2. Through irradiation, overlap laser beam welding is performed by forming a thermal image continuously at the joint portion 4 of the steel plates 2 and 3.

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.

Here, an important point in the present invention is that the overlap laser spot 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. 0.15, that is, the ratio of the total gap G between the steel plates constituting the weld zone to the total thickness T of the steel plates constituting the weld zone 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%.

In addition, the most important point in the present invention is that, as shown in FIG. 5(a), the laser beam 6 is spun to draw an elongated circle combining a semicircle and a straight line, while forming a spiral from the outside of the oval shape toward the inside. By scanning, an oval-shaped joint larger than usual is formed. As described above, since the occurrence of sputtering can be suppressed by welding while spinning the laser beam, the thickness u of the central portion 4a, which becomes the final solidified portion of the joint, can be increased, as shown in FIG. 3(b). , it is possible to prevent excessive stress concentration applied to the same part, thereby preventing the occurrence of cracks.

Here, the size of the oval junction is, as shown in FIGS. 5(a) and 5(b), the spin radius r when spinning the laser beam, the advance amount c, which is the distance traveled per spin, and the laser beam. This drawing can be changed by adjusting the length L of the oval straight section and the radius R of the arcuate section. In addition, _the above-described spin radius r, advancement amount c _, length L of the oval straight portion, and radius R of the circular arc portion are, Equations (2) to (5) below;

2.0 ≤ D ₁ ≤ 8.0 ···(2)

6.0 ≤ D ₂ ≤ 15.0 ···(3)

1.1 ≤ D ₂ /D ₁ ≤ 5.0 ···(4)

0.6 ≤ u/T ≤ 1.0 ···(5)

It is necessary to adjust it to satisfy.

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 equations (2) to (5) above, the laser beam irradiation has an output of: 1.0 to 6.0 kW, and a focus position: -5 mm to the steel sheet surface + 5 mm from the surface of the steel sheet on which the laser beam is irradiated. , beam diameter: 0.2 to 0.6 mm, and laser beam scanning speed: 5.0 to 10.0 m/min. More preferably, laser power: 3.0 ~ 5.0 kW, focus position: steel plate surface irradiating the laser beam ~ steel plate surface + 5 mm, beam diameter: 0.3 ~ 0.5 mm, and scanning speed of the laser beam: 6.0 ~ 9.0 m/min. is the range.

In addition, in the above description, the case where the shape of the joint constituting the welded portion of the present invention is oval has been described, but it may be oval as long as it satisfies the equations (2) to (5) described above.

＜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 spot welding 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 described above is composed of a steel plate 2, which is a frame component with a roughly hat-shaped cross-sectional shape, and a steel plate 3, which is a panel component, and includes a flange portion 2b of the steel plate 2 and this plan. The steel plate 3 disposed opposite to the branch 2b is joined by a weld formed by the thermally continuous oval-shaped joint 4 formed by the laser beam welding described above, forming 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 welded portion is excellent from the viewpoint of ensuring collision safety. However, the overlap laser spot welded joint of the present invention is applied to the final solidified portion of the joint. Since it does not crack and has sufficient peel strength, it can be suitably used in structural members such as center pillars and roof rails of automobile bodies, for example.

Here, when applying the overlap laser spot welding joint of the present invention to manufacture structural members for automobile bodies, etc., the suitable position for forming the welded portion is L-shaped with flange portions 2b and 3b, as shown in FIG. 6. An example will be given where two steel plates 2 and 3 having cross-sections are overlapped so that the flange portions face each other, and laser beam welding is performed from one side. Figure 6 (a) is a plan view of overlapping flange parts when viewed from above, showing that a welded portion consisting of a thermally continuous oval joint is formed in the flange part, and 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 the distance from there to the center of the width of the oval joint formed in the flange portion is welded. When defined as position

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, it is desirable for the welding position X to be 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 to 12.0 kN or more.

In addition, the equation (5) regarding the welding position , it can also be applied to an overlap laser spot welding joint in which a frame part (steel plate 2) and a panel part (steel plate 3) whose cross-sectional shape as shown in FIG. 1 is approximately a hat shape are laser beam welded, and the welding in this case The starting point (0 point) of 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 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. 6). Next, as shown in FIG. 7, after overlapping the short sides of the two L-shaped steel plates 7 so as to face each other, a laser beam is irradiated in the air to the overlapping portion to form an intermittently arranged oval joint, A T-shaped peeling test piece (8) was produced.

In addition, when performing the above-mentioned overlap laser beam welding, a fiber laser with a beam diameter of 0.4 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) ) of the surface), and then, as shown in Table 2, the gap G between the two steel plates, the output P of the irradiating laser beam, the scanning speed v, the radius r when the laser beam spins, and the amount of advancement per spin. c, and the length L of the oval straight part drawn by the laser beam and the radius R of the arcuate part are changed in various ways, and the minor axis width D ₁ and major axis width D ₂ of the oval joint part and the minimum thickness u of the final solidified part are shown in Table 2. As shown, it was changed in various ways. At this time, the welding position

For the T-shaped peeling test piece obtained in this way, the presence or absence of cracks and melting in the welded portion, especially the final solidified portion between the joint portion, was determined by visual inspection and penetrant inspection tests.

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 12.0 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, the test pieces (No. 1, 9, 17, 25, 33, 41, 49, 57, 65, and 73) welded with overlapped laser beams under conditions suitable for the present invention all showed cracks in the final solidified portion of the joint. There was no generation or melting, and the peeling strength was 12.0 kN or more.

In contrast, for the test pieces No. 2, 10, 18, 26, 34, 42, 50, 58, 66, and 74, the minimum thickness u of the final solidified area was less than 60% of the total plate thickness T, so melting occurred. However, cracks occurred in the final solidified portion of the joint, and the peeling strength was less than 12.0 kN.

In addition, in the test pieces No. 3, 11, 19, 27, 35, 43, 51, 59, 67, and 75, there was no melting because the gap G of the weld zone was larger than 15% of the total thickness T of the steel plate. Cracks occurred in the final solidified portion of the joint, and the peeling strength was less than 12.0 kN.

In addition, in the test pieces No. 4, 12, 20, 28, 36, 44, 52, 60, 68, and 76, the minor axis width D ₁ of the joint was all smaller than 2 mm, so there was no melting, but the final solidification of the joint was Cracks occurred in the part, and the peeling strength was less than 12.0 kN.

In addition, in the test specimens No. 5, 13, 21, 29, 37, 45, 53, 61, 69, and 77, the minor axis width D ₁ of the joint was greater than 8 mm, so there was no crack in the final solidified portion of the joint. , melting occurred.

In addition, in the test pieces No. 6, 14, 22, 30, 38, 46, 54, 62, 70, and 78, the minor axis width D ₁ of the joint was greater than 15 mm, so there was no melting, but the final solidification portion of the joint was A crack occurred in

In addition, the test specimens No. 7, 15, 23, 31, 39, 47, 55, 63, 71, and 79 all have a ratio (D ₂ /D ₁ ) of the major axis width D ₂ to the minor axis width D ₁ of the joint. Since it was greater than 5.0, there was no melting, but cracks occurred in the final solidified portion of the joint.

In addition, the test specimens No. 8, 16, 24, 32, 40, 48, 56, 64, 72, and 80 all have a ratio of the major axis width D ₂ to the minor axis width D ₁ of the joint (D ₂ /D ₁ ). Since it was smaller than 1.1, there was no melting, but cracks occurred in the final solidified portion of the joint.

In addition, Nos. 81 and 82 show test results on test pieces obtained by overlapping laser beam welding two steel plates with different strength levels under conditions suitable for the present invention. Even if it is a combination of 590 MPa class and 980 MPa class, No. 81, whose steel composition is within the suitable range for the present invention, does not have weld cracks and excellent peeling strength is obtained, but No. 82, whose steel composition is outside the suitable range for the present invention, produces weld cracks and has poor peeling strength. It was also less than 12.0 kN.

As described above, in the present invention examples in which overlap laser beam welding was performed according to the present invention, good overlap laser spot welded joints having the characteristics targeted by the present invention were obtained. However, comparative examples that fall outside the conditions of the present invention In , a good overlap laser spot 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: Overlapping laser spot welded seam
2, 3: Steel plate
2a: Vertical wall portion of steel plate (2)
2b: Flange portion of steel plate (2)
4, 14: joint (weld spot)
4a, 14a: Final solidification part of the joint (center part)
5: Cracks in the final solidified portion of the joint
6: Laser beam
7: L-shaped steel plate
7a: Long side of L-shaped steel plate
7b: Width of L-shaped steel plate
8: Peeling test piece
S: Laser beam irradiation start point when forming a joint
E: Laser beam irradiation end point when forming a joint
σa, σb: Stress applied to the final solidified portion of the joint
T: Total thickness of steel plates constituting the joint
u: Minimum thickness of the final solidified portion of the joint
D ₁ : Minor width of the oval joint
D ₂ : Width of the long axis of the oval joint
G: Total gap between steel plates constituting the joint
X: welding position
0: Origin of welding position

Claims (7)

In the overlap laser spot welding 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 is made of oval joints arranged intermittently, and
An overlap laser spot welded joint, wherein the oval joint satisfies all of the following equations (1) to (5).
1.0 ≤ T ≤ 6.0 ···(1)
2.0 ≤ D ₁ ≤ 8.0 ···(2)
6.0 ≤ D ₂ ≤ 15.0 ···(3)
2.2 ≤ D ₂ /D ₁ ≤ 5.0 ···(4)
0.6 ≤ u/T ≤ 1.0 ···(5)
Here, T: Total plate thickness of the steel plate constituting the weld zone (mm)
D ₁ : Width of the minor axis of the oval joint (mm)
D ₂ : Width of the long axis of the oval joint (mm)
u: Minimum thickness of the final solidified portion of the oval 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. An overlap laser spot welded joint, characterized in that it has a composition containing mass% or less, and the balance consists of Fe and inevitable impurities. According to claim 2,
An overlap laser spot weld joint, wherein the steel sheet contains, in addition to the above component composition, at least one component 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; Cr: 1.0 mass% or less, Mo: 0.50 mass% or less, and B: 0.10 mass% or less. One or two or more types selected from among The method according to any one of claims 1 to 3,
An overlap laser spot 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 method of manufacturing an overlap laser spot welded joint by overlapping a plurality of steel plates up and down and intermittently irradiating a laser beam on one surface of the overlapping steel plates to form a weld zone consisting of oval joints arranged in a continuous row. Because,
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,
The oval junction part spins the laser beam to draw an elongated circle combining a semicircle and a straight line, and scans in a spiral form from the outside of the oval shape toward the inside,
The oval joint controls at least one of the laser power, focus position, welding speed, spin radius, movement amount per spin, and beam diameter so that all of the following equations (1) to (5) are satisfied. Method for manufacturing overlap laser spot welded joints.
1.0 ≤ T ≤ 6.0 ···(1)
2.0 ≤ D ₁ ≤ 8.0 ···(2)
6.0 ≤ D ₂ ≤ 15.0 ···(3)
1.1 ≤ D ₂ /D ₁ ≤ 5.0 ···(4)
0.6 ≤ u/T ≤ 1.0 ···(5)
Here, T: Total plate thickness of the steel plate constituting the weld zone (mm)
D ₁ : Width of the minor axis of the oval joint (mm)
D ₂ : Width of the long axis of the oval joint (mm)
u: Minimum thickness of the final solidified portion of the oval joint (mm) A structural member for an automobile body having an overlapping laser spot welded joint according to any one of claims 1 to 3. A structural member for an automobile body having an overlapping laser spot welded joint according to claim 4.

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