JP6798359B2 - Laminated joint and its manufacturing method - Google Patents

Description

The present invention relates to a lap joint and a method for manufacturing the lap joint, and more particularly to a lap joint for a high-strength steel plate used for an automobile body and a method for manufacturing the same.

In recent years, in the automobile field, it has been required to reduce the weight of the vehicle body in order to reduce fuel consumption and CO ₂ emissions, and to increase the strength of the vehicle body member in order to improve collision safety. In order to satisfy these requirements, it is effective to use high-strength steel plates for vehicle body members and various parts.

In processes such as assembling a car body made of such high-strength steel plate and attaching parts, spot welding using resistance heat generation is widely used, but in recent years, some parts have high power instead of spot welding. A technique of spot welding using a bonding method using a light beam having a density (hereinafter referred to as a light beam) is being applied to the manufacture of automobiles.

Point-shaped joints using light rays can be performed at a higher speed than spot welding, and unlike spot welding, they are not affected by diversion, so they have the advantage of shortening the pitch of the joint points. Improvement is also possible.

On the other hand, the strength of the point-shaped joint joint by light rays tends to be about the same as or lower than that of conventional resistance spot welding, and in the case of a high-strength steel plate having a high carbon content of 980 MPa or more, the cross tension of the joint strength There was a problem that the strength decreased.
The joint strength, which is one of the quality indexes of the joint, includes the tensile shear strength (TSS) measured by applying a tensile load in the shear direction and the cross tensile strength measured by applying a tensile load in the peeling direction. There is (CTS). In joints formed by light rays, CTS tends to be as low as or lower than that of conventional resistance spot welding, and in the case of high-strength steel sheets having a large amount of carbon, CTS may be low.
Therefore, a technique for improving the joint strength such as CTS when the high-strength steel sheet is joined by light rays has been desired.

Under such circumstances, as a technique for improving the joint strength in joining with a light beam, a technique for forming another joint in the vicinity of the joint (see Patent Document 1), inside the closed loop-shaped main bead. , A technique for forming another bead for the purpose of tempering the present bead (see Patent Documents 2 and 3) is known.

In the technique of forming the joint portion by the light beam in such a loop shape, there is a concern that the joint area of the joint portion becomes small and the tensile shear strength (TSS) becomes low. On the other hand, when viewed from the surface side of the steel sheet, which has a larger joint area than the loop-shaped joint, the outer contour is substantially circular, and a joint formed by light rays that are melt-solidified to the center is formed on the steel sheet. (See Patent Document 4), a technique for improving joint strength has been reported.

Japanese Unexamined Patent Publication No. 2010-012504 Japanese Unexamined Patent Publication No. 2012-241986 International Publication No. 2012/05007 Japanese Unexamined Patent Publication No. 60-68185

The technique disclosed in Patent Document 4 is effective in improving the tensile shear strength (TSS) because the joints formed by light rays are dotted, but it is desired to further improve the cross tensile strength (CTS). Was there.
Therefore, in view of such circumstances, it is an object of the present invention to provide a lap joint with excellent joint strength and improved reliability.

The present inventors have diligently studied means for further improving the CTS of a joint joint in which a steel plate is joined by a light beam and a point-shaped joint portion is formed by the light beam.
When a load is applied to the joint portion in the peeling direction, the lap joint joint is considered to be unable to secure a sufficient CTS value because stress is concentrated in the vicinity of the fusion boundary between the steel plate and the melt-solidified portion and is likely to break. .. Therefore, the idea was to improve the toughness of the portion by heat-treating the vicinity of the melting boundary, and various investigations were conducted on the heat-treated portion and the heat-treated method for that purpose.

As a result, the inside of the melt-solidified portion of the point-shaped joint is irradiated with light rays in a ring shape to re-melt and solidify the irradiated portion, and the heat at that time is used to burn back the vicinity of the molten boundary between the steel sheet and the melt-solidified portion. It was found that CTS was improved by reheating as described above.
The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) In a lap joint joint composed of a plurality of lapped steel plates and having point-shaped joints by light rays.
The point-shaped joint portion formed by the light beam has a point-shaped melt-solidified portion straddling all the overlapped steel plates.
The melt-solidified portion has a remelt-solidified portion and a solidified / reheated portion.
The remelted solidified portion has a cylinder-like shape, and is positioned inside the molten boundary between the melted solidified portion and the steel plate so as not to overlap the molten boundary and not to penetrate the overlapped steel plates. And
A lap joint joint characterized in that the solidification / reheating portion is located around the remelt / solidification portion, includes the fusion boundary, and is softened from the remelt / solidification portion.

(2) Among the solidified and reheated portions, the average value of Vickers hardness in the range of 0.5 mm from the molten boundary of the mating surface of the steel sheet toward the remelted solidified portion is Hv390 or less and The lap joint according to (1) above, wherein the Vickers hardness of the remelted solidified portion is lower than the average value of Hv70 or more.

(3) The shortest distance from the intersection of the steel plate mating surface and the fusion boundary to the remelt solidification portion in the cross section of the plurality of steel plates in the plate thickness direction is 1.0 to 3.0 mm (1). The lap joint according to 1) or (2).

(4) The lap joint according to any one of (1) to (3) above, wherein the plurality of steel plates include one or more steel plates having a surface treatment film.

(5) In a method for manufacturing a lap joint, in which a plurality of steel plates are laminated and irradiated with a light beam having a high power density to join the plurality of steel plates.
A light beam having a high power density is irradiated into a limited area on one outer surface of one of the superposed steel sheets, and melt-solidified over all the superposed steel sheets to have a point-like melt-solidified portion. Forming a point-like joint by
Next, a light beam having a high power density is cyclically re-irradiated inside the melt boundary between the melt-solidified portion and the steel plate so as not to overlap the melt boundary and not to penetrate the overlapped steel plates. The irradiated portion is remelted and solidified to form a cylindrical remelted solidified portion, and the periphery of the remelted solidified portion is reheated to form a solidified and reheated portion including the melting boundary.
Further, a method for manufacturing a lap joint, which comprises adjusting the joining conditions when forming the remelted solidified portion to soften the solidified and reheated portion from the remelted solidified portion.

(6) In the re-irradiation of the light beam, the shortest distance from the intersection of the steel plate mating surface and the fusion boundary to the remelt solidification portion is 1.0 to 3.0 mm in the cross section of the plurality of steel plates in the plate thickness direction. The method for manufacturing a lap joint according to (5) above, wherein the lap joint is performed as described above.

(7) The method for manufacturing a lap joint according to (5) or (6) above, wherein one or more steel plates having a surface treatment film formed on the plurality of steel plates are used.

Here, the point-shaped joints formed by light rays are those in which the steel sheets are joined to each other by the melt-solidified parts formed in the form of dots by irradiating a part of the surface of the overlapped steel sheets with light rays. Refers to the part.

According to the present invention, since the solidification / reheating portion having excellent toughness is provided near the fusion boundary of the melt solidification portion, the joint strength of the lap joint, particularly the cross tensile strength (CTS) can be improved. The reliability of the joint can be improved. Then, by applying the joint of the present invention to an automobile part, the reliability of the automobile part can be improved.

It is a figure which shows typically the joint which has the point-like joint part by a light ray, (a) shows the structure of the cross section of a joint joint, and (b) shows the Vickers hardness distribution of a joint joint. It is a figure which shows typically the joint by irradiating the inside of the point-like joint part by a light ray in an annular shape, like FIG. 1. It is a figure which shows typically the example which is different from FIG. 2 by irradiating the inside of the point-like joint part by a light ray with the light beam in an annular shape, like FIG. It is a figure explaining the cross section of the joint joint of this invention. It is a perspective view which shows the outline of the formation of the point-like joint part by a light ray, (a) shows the outline figure which irradiates a light ray with a different irradiation diameter, and (b) is irradiates a light beam with a wide condensing area. A schematic view is shown, and (c) shows a perspective view of a point-shaped joint portion formed by the formed light rays. It is a perspective view which shows the outline of the heat treatment of a point-like joint part by a light ray, (a) shows the outline figure which irradiates a light ray, (b) is a point by a light ray which has a remelt solidification part and a solidification reheating part. A perspective view of the shaped joint is shown.

First, the background of the study leading to the present invention and the basic configuration of the present invention will be described.
It has been desired to further improve the joint strength in a lap joint having a point-shaped joint formed by light rays. In a lap joint, when a load is applied to the joint in the peeling direction, stress is concentrated near the boundary between the steel plate and the melt solidification part (melt boundary), especially near the intersection between the steel sheet mating surface and the melt boundary. , Easy to break.
Therefore, it was examined to heat-treat the part where the stress is concentrated near the melting boundary in the point-shaped joint by light rays.

First, the outer surface of one of the two stacked steel plates (tensile strength 1500 MPa class, plate thickness 1.6 mm) is irradiated with light rays to melt and solidify the irradiated portion, and the diameter is about 6.0 mm and 2 A point-shaped joint was formed by a light beam having a melt-solidified portion penetrating the steel plates in a columnar shape in the stacking direction (see FIG. 1 (a)).
Next, a light beam having a beam diameter of 0.4 mm is re-irradiated around the inside of the fusion boundary of the melt-solidified portion with the steel plate along the circumference of 3.0 mm in diameter to re-melt and solidify the irradiated portion. A cylindrical remelt solidification portion is provided inside the melt solidification portion, and a point-shaped joint portion formed by a light beam having a solidification reheat portion 3b reheated by the heat of the remelt solidification portion 3a is formed around the cylinder-shaped remelt solidification portion. (See FIG. 2 (a)).
At the time of re-irradiation, the inner end of the remelt solidification portion is on the steel plate 2a side from the extension line of the steel plate mating surface 2c, and the intersection of the steel plate mating surface and the melting boundary is the measurement point of Vickers hardness. The light irradiation conditions were adjusted so that the temperature was within the tempering temperature range.
Then, the change in Vickers hardness at the position near the steel plate mating surface was measured in the plate thickness direction cross section of the joint including the joint portion due to each light beam.
(B) of FIGS. 1 and 2 shows a schematic change in Vickers hardness.

As shown in FIGS. 1 and 2 (a), the Vickers hardness is measured over the measurement range L1 along the direction parallel to the steel plate surface at the position of the dotted line X (measurement position of Vickers hardness in the plate thickness direction). The hardness was measured. The dotted line X is a position 0.1 mm from the mating surface 2c of the steel plates 2a and 2b to the steel plate 2a side in the plate thickness direction. Further, L2 is a measurement range of Vickers hardness in the melt-solidified portion.

As shown in FIG. 1 (b), the Vickers hardness of the joint joint 1 before the heat treatment for re-irradiating the melt-solidified portion 3 with light rays is as hard as about HV460 inside the melt-solidified portion 3 (L2). It is almost constant. Just outside of the steel plate heat affected zone of L2 it is heated to a high temperature range of not lower than ₃ points A, because it is quenched, a large hardness. Further, the heat-affected zone of the steel sheet on the outer side is tempered to become a softened portion.

On the other hand, in the joint joint 1 having the solidification / reheating portion 3b around the remelting solidification portion 3a formed in a cylinder shape by re-irradiating the inside of the melt solidification portion 3 with a light beam in an annular shape, the Vickers hardness measurement point. However, since it is heated to the tempering temperature range by re-irradiation of light rays, the Vickers hardness is 0.5 mm inside the melting boundary because the range before and after the melting boundary is tempered as shown in FIG. 2 (b). The average hardness in the range of 1 was reduced to about HV300.

Then, when the cross tensile strength (CTS) of both joints was investigated, the cross tensile strength of the joint having the solidified and reheated portion formed by re-irradiating the inside of the molten solidified portion was found. It turned out to be higher. As a result, in the case where the point-shaped joint portion by the light beam shown in FIG. 2A has a solidification reheating portion softened from the remelt solidification portion, the periphery of the fusion boundary is tempered to reduce the hardness. It was found that the toughness was improved. Further, even if the combination of the steel plates was changed, the improvement of CTS was confirmed in the case where the point-shaped joint portion by the light beam had a solidification reheating portion softened from the remelt solidification portion.

The present invention has led to the invention described in (1) above through the above-mentioned examination process, and necessary requirements and preferable requirements will be sequentially described for such an invention.

[Joined joint]
As shown in the cross-sectional view of FIG. 4, the joint joint 10 of the present invention is formed by superimposing a plurality of steel plates 20a and 20b, irradiating a limited region of the steel plate 20a from the steel plate 20a side with light rays, and forming dots by the light rays. The joint portion of the above is formed, and a plurality of steel plates 20a and 20b are laminated and joined by a light beam.

<Point-shaped joints by light rays>
The point-shaped joint portion formed by the light beam has a point-shaped melt-solidified portion 30 in which a plurality of steel plates 20a and 20b are superposed and melt-solidified by irradiation with the light beam, and the melt-solidified portion 30 is internally said. It has a remelt solidification section 30a in which the melt solidification section is remelted, and a solidification reheating section 30b in which the melt solidification section is reheated after solidification by the remelt solidification section. Note that FIG. 4 does not show the heat-affected zone on the steel plate side.
Here, the point-shaped joints formed by light rays are those in which the steel sheets are joined to each other by the melt-solidified parts formed in the form of dots by irradiating a part of the surface of the overlapped steel sheets with light rays. Refers to the part.

(Dot-shaped melt-solidified part)
The melt-solidified portion 30 formed in the point-shaped joint portion formed by the light beam is a melt-solidified portion formed by irradiating a limited region of a part of the surface of the stacked steel plates with a light beam.
The width W of the melt-solidified portion 30 (the equivalent circle diameter of the melt-solidified portion when the melt-solidified portion is viewed in a plan view from the light irradiation side) may be adjusted according to the joint strength and the like, and is not particularly limited. , 5-12 mm, for example. It is preferably 6 to 10 mm.
If the melt-solidified portion 30 is formed so as to straddle the plurality of steel plates 20a and 20b forming the joint portion, the melt-solidified portion 30 may or may not penetrate to the outer surface of the steel plate on the opposite side to the light irradiation side. Good.

Here, the point shape means that when the melt-solidified portion is viewed in a plan view from the irradiation side of a light beam, the outer peripheral contour of the melt-solidified portion is limited to a limited region such as a circular shape or a polygonal shape, and that region. It means that it is melted and solidified to the center of. The circular shape includes not only the case where the melt-solidified portion is circular or elliptical when the melt-solidified portion is viewed in a plan view from the light irradiation side, but also a combination of semicircles or semi-ellips having different diameters. In addition, the shape of the melt-solidified portion formed by irradiating the steel sheet with light rays in a spiral shape from the outer peripheral side to the center side or from the center side to the outer peripheral side is also included in a dot shape.

(Remelt solidification part)
The remelt solidification portion 30a has a cylindrical shape obtained by re-irradiating the melt solidification portion 30 inside the fusion boundary 40 between the melt solidification portion 30 and the steel plate along an annular locus and remelting and solidifying the irradiated portion. It is a part having the shape of, and has a structure that remains melted and solidified. The circular shape means that the contour is circular or polygonal. The circular shape includes not only the case where the remelted solidified portion is circular or elliptical, but also a combination of semicircles or semicircles having different diameters.

The remelting and solidifying portion 30a is formed so as not to overlap the melting boundary 40, and at that time, when the melting and solidifying portion is viewed in a plan view from the light irradiation side, the center corresponding to the circle of the melting and solidifying portion and the remelting portion are remelted. It does not have to coincide with the center of the solidified portion 30a corresponding to the circle.
Further, the remelt solidification portion 30a is formed so that the tip portion 50 in the stacking direction does not penetrate the stacked steel plates in the plate thickness direction of the joint joint 10. If it penetrates to the back surface of the lower plate, the strain of the joint becomes large and solidification cracks are likely to occur. Therefore, the joint is formed so as not to penetrate.

The position of the tip portion 50 in the overlapping direction of the remelt solidification portion 30a and the position of the outer contour of the remelt solidification portion 30a on the melting boundary 40 side are important for reheating the melt solidification portion near the steel sheet mating surface 20c. As will be described later, the melt-solidified portion near the intersection 60 of the steel sheet mating surface and the melting boundary is set to have a reheating temperature of 400 ° C. or higher and Ac _1- point temperature or lower.
FIG. 2 shows a case where the position of the tip portion of the remelted solidification portion 3a (30a) is on the light irradiation side (above) from the extension line of the steel sheet mating surface 2c (20c). The case where the position is below the extension line of the steel plate mating surface 2c is shown.

Since the remelt solidification portion 30a may reheat the melt solidification portion near the overlapping surface 20c of the steel sheet, the position of the tip portion 50 is on the light irradiation side (above) from the extension line of the steel sheet bonding surface 20c. However, it may be either downward, but in order to obtain a stable effect, it is preferable that the position of the tip portion is below the extension line of the steel sheet mating surface 20c. Further, in the case of stacking three steel plates, two mating surfaces 20c of the steel plates are formed. At this time, the remelt solidification portion 30a may have a depth that does not cross the extension line of the overlapping surface, but it is desirable that the remelting solidification portion 30a crosses the extension line of the overlapping surface at one place, and the extension of the two overlapping surfaces. It is even more desirable to cross the line.

The radial width Wb of the remelted solidified portion 30a formed in a cylindrical shape (the radial thickness of the annular remelted solidified portion when the molten solidified portion is viewed in a plan view from the light irradiation side) is in the vicinity of the intersection 60. In order to give the amount of heat required for the melt-solidified portion of the above to reach the reheating temperature of 400 ° C. or higher and Ac1 point temperature or lower, it is determined in relation to the position of the tip portion, but it may be 0.3 mm or higher. preferable. The upper limit of the width Wb is not particularly limited, but it is preferably 2.0 mm or less.

(Coagulation and reheating section)
The solidification / reheating portion 30b is a portion formed by re-irradiating the melt / solidification portion 30 of the point-shaped joint portion with light rays in a ring shape so as to include a melting boundary around the remelt / solidification portion 30a. It includes a portion softened from the melt-solidified portion 30a.
The solidification / reheating unit 30b has a different thermal effect depending on the distance from the remelting / solidifying unit 30a. At the portion adjacent to the remelting and solidifying portion 30a, the portion is reheated to a temperature equal to or higher than the melting point of the base metal at Ac1 point, resulting in a hardened structure. If it is separated from that, the reheating temperature becomes lower than the Ac1 point temperature, and the tempered structure is obtained.

In a lap joint, when a load is applied in the peeling direction of the steel sheet, stress concentrates on the molten boundary near the mating surface of the steel sheet, leading to fracture. Therefore, at least the intersection 60 between the steel sheet mating surface 20a and the molten boundary 40 The solidification / reheating portion 30b is formed so that the melt-coagulation portion located in the periphery has a reheating temperature of 400 ° C. or higher and Ac ₁ point temperature or lower, and the toughness of the portion is improved.

Specifically, since stress tends to concentrate in the melt-solidified portion near the steel plate mating surface 20c, the average value of the Vickers hardness is set to Hv390 or less, and the average value of the Vickers hardness of the remelt-solidified portion 30a is higher than the average value. It is preferable to lower Hv70 or more. In particular, in order to improve CTS, it is desirable that the range of at least 0.5 mm inward from the intersection 60 of the steel sheet mating surface 20c and the melting boundary is heated to a temperature of 400 ° C. or higher and Ac1 point temperature or lower. In the cross section of the steel plate in the plate thickness direction, the shortest distance Wc from the intersection 60 of the steel plate mating surface and the fusion boundary to the remelt solidification portion is preferably 1.0 to 3.0 mm.
Since the remelted solidified portion can be identified by observing the microstructure, Wc can be measured from the cross section of the joined portion after joining with light rays.

In the measurement of the average value of the Vickers hardness of the remelting and solidifying portion 30a and the solidifying and reheating portion 30b, the intersection points 60 of the steel plate mating surface 20c and the melting boundary 40 are connected in the cross section in the plate thickness direction including the central axis C. Measure on the parallel line. Then, the Vickers hardness is measured at three or more points including the central position of the melt-solidified portion 30 and the vicinity of the intersection 60, and the average value is obtained. As an example of specific measurement conditions, the hardness is measured at a pitch of 0.15 mm from both ends with a test force of 0.3 kg, and the average value of the hardness at 0.15 mm, 0.30 mm, and 0.45 mm is obtained. When there are a plurality of overlapping surfaces of the steel plates, the measurement is performed on the line connecting the intersections of the overlapping surfaces of the respective steel plates.

<Multiple steel plates>
The steel sheet used for the joint of the present invention is not particularly limited, and may be a steel sheet having a chemical composition or structure that can obtain mechanical properties and the like according to the intended use. Further, when a high-strength steel sheet having a carbon content of 0.10% by mass or more is applied to the joint of the present invention, the cross tensile strength is remarkably improved, and such a steel sheet is preferably targeted.

The thickness of the steel plate is not particularly limited and may be in the range of 0.5 to 3.2 mm. Even if the plate thickness is less than 0.5 mm, the effect of improving the joint strength of the joint can be obtained, but since the joint strength is affected by the plate thickness, the effect of improving the strength of the entire joint is reduced, and the joint is joined. The scope of application of joints is limited. Further, even if the plate thickness exceeds 3.2 mm, the effect of improving the joint strength of the joint portion can be obtained, but the applicable range of the joint joint is limited from the viewpoint of weight reduction of the member.

The steel sheet may include at least one steel sheet having a surface treatment film formed on both sides or one side of the joint. The surface treatment film includes a plating film, and may further include a coating film and the like. Examples of the plating film include zinc plating, aluminum plating, zinc / nickel plating, zinc / iron plating, zinc / aluminum / magnesium-based plating, and the plating manufacturing method includes hot-dip plating and electroplating.

The steel plate may have at least a plate-like portion forming a joint, and may not be entirely a plate. For example, it includes a flange portion of a member press-molded into a specific shape having a hat-shaped cross section. The number of steel plates to be stacked is not limited to two, and may be three or more. Further, the type, composition and thickness of each steel sheet may be the same or different from each other. Further, the joint is not limited to those composed of separate steel plates, and may be a lap joint joint formed by molding one steel plate into a predetermined shape such as a tubular shape and overlapping the ends.

Hereinafter, an example of a lap joint in an automobile is shown, although not limited to this.
In the case of A-pillar, it is a combination of 270-340 MPa class alloyed hot-dip galvanized steel sheet, 590-1800 MPa class non-plated steel sheet or hot stamped steel sheet, and 590-1800 MPa class non-plated steel sheet or hot stamped steel sheet. A lap joint is exemplified.

In the case of B-pillar, it is a combination of three layers of alloyed hot-dip galvanized steel sheet with tensile strength of 270-340 MPa class, 590-1800 MPa class non-plated steel sheet or hot stamped steel sheet, and 440-980 MPa class non-plated steel sheet. A lap joint is exemplified.

In the case of side sill, a lap joint joint consisting of a combination of 270-340 MPa class alloyed hot-dip galvanized steel sheet, 590-1800 MPa class alloyed hot-dip galvanized steel sheet, and 590-1800 MPa class alloyed hot-dip galvanized steel sheet. Is exemplified.

In the case of floor members, the floor panel of 270-590 MPa class alloyed hot-dip galvanized steel sheet and the floor member of 440-1800 MPa class non-plated steel sheet or alloyed hot-dip galvanized steel sheet are lap-bonded in a double-layered combination. A joint is exemplified.

[Manufacturing method of lap joint]
Next, the method for manufacturing the lap joint of the present invention will be described.
First, (a) a plurality of steel plates are superposed, a light beam is irradiated into a limited region of one outer surface of the superposed steel plates, and the steel plates are melt-solidified to the center of the region across all the superposed steel plates. It will be described with reference to FIG. 5 that the point-shaped joint portion is formed by the light beam having the point-shaped melt-solidified portion.

FIG. 5 is a perspective view showing an outline of formation of a point-shaped joint portion by irradiating a limited area (scheduled irradiation location) with light rays. FIG. 5A shows an outline of irradiating light rays with different irradiation diameters, FIG. 5B shows an outline of irradiating light rays with a wide condensing area, and FIG. 5C is formed. It shows a point-like joint due to the light beam.

FIG. 5A shows an example of a method of irradiating the light beam 70, which irradiates the light ray 70 with different irradiation diameters. In this figure, the planned irradiation points 80a of the light beam 70 are shown by dotted lines, and three planned irradiation points 80a having different irradiation diameters are shown.

In the formation of the point-shaped joint portion by the light beam, first, a plurality of steel plates 20a and 20b are overlapped, and the light ray 70 is irradiated from one of the steel plates 20a side to perform the joint by the light beam. In the irradiation of the light beam 70, when the planned irradiation portion 80a is viewed in a plan view from the irradiation side of the light ray 70, the light ray is scanned in a substantially circular shape as shown by a white arrow. At that time, the light beam 70 is irradiated to the outer scheduled irradiation portion 80a, and then to the inner scheduled irradiation portion 80a, but also to the inner scheduled irradiation portion 80a, and then to the outer scheduled irradiation portion 80a. You may go. The scanning direction of the light beam is not particularly limited, and may be clockwise or counterclockwise.

Further, when the planned irradiation portion 80a from the irradiation side of the light beam 70 is viewed in a plan view, the outer peripheral shape of the planned irradiation portion 80a of the light ray 70 is a circle, but a semicircle or a semicircle having an elliptical shape, a polygonal shape, or a different diameter is used. It may be a combined shape or a spiral shape. When the planned irradiation point 80a of the light beam 70 has a spiral shape, the light beam 70 is scheduled to be irradiated from the outer end of the planned spiral irradiation point toward the inner end or in a spiral shape. The light beam is scanned in a spiral shape from the inner end of the portion toward the outer end. The direction of the spiral is not particularly limited, and may be clockwise or counterclockwise.

In FIG. 5A, three planned irradiation points having different diameters are illustrated, but the number of planned irradiation points having different diameters is increased or decreased depending on the focal area of the light beam and the joint area of the point-shaped joints formed by the light beam. Can be made to.

FIG. 5B shows another example of the method of irradiating the light beam 70, which irradiates the light beam 70 with a wide condensing area. In this figure, the planned irradiation portion 80b of the light beam 70 is shown by a dotted line. Then, the light beam 70 is irradiated at one time by widening the light collecting area.

By irradiating the light beam 70 as shown in FIGS. 5 (a) and 5 (b), as shown in FIG. 5 (c), the melt-solidified portion 30 of the point-shaped joint portion by the light beam can be formed.

Further, when one or more steel plates having a surface treatment film formed on the plurality of steel plates are used, it is preferable that the light beam 70 is irradiated to the outer scheduled irradiation portion 80a and then to the inner scheduled irradiation portion 80a. As a result, it becomes easy to collect the film which has become a gas that causes defects in the joint portion near the center of the molten portion and remove it by stirring. Even if the light beam 70 is irradiated to the inner planned irradiation portion 80a and then to the outer scheduled irradiation portion 80a, or if the light collecting area of the light ray 70 is widened, the film that has become a gas is formed. Since it can be removed from the molten portion, it does not exclude the adoption of these light beam irradiation methods.

Next, (b) the inside of the melt boundary between the melt-solidified portion and the steel plate is re-irradiated in a ring shape with light rays so as not to overlap the melt boundary and not to penetrate the overlapped steel plates. The portion is remelted and solidified to form a cylindrical remelted solidified portion, and the periphery of the remelted solidified portion is reheated (heated) to form a solidified and reheated portion including the molten boundary.
Further, it will be described that the joining condition at the time of forming the remelted solidified portion is adjusted to soften the solidified reheated portion from the remelted solidified portion.

In the heat treatment of the punctate joints with light rays, the temperature of the melt-solidified portion 30 of the punctate joints obtained by the method shown in FIG. 5 is equal to or lower than a predetermined temperature, for example, Ms points-50 ° C. (Ms points) for steel sheets. : Wait until the temperature becomes lower than the martensitic transformation start temperature), and then irradiate the inside of the melt-solidified portion 30 with light rays 70 from the steel plate 20a side. The inside means the inside of the melt-solidified portion 30 excluding the melt boundary of the melt-solidified portion 30.

When the temperature of the melt-solidified portion 30 is Ms point −50 ° C. or lower, a certain amount or more of martensite is generated in the steel sheet. Therefore, by heat-treating the melt-solidified portion 30 of the point-shaped joint, the martensite is formed. Is tempered and softened, improving joint strength. Further, it is more preferable that the temperature of the melt-solidified portion 30 at the start of the heat treatment of the melt-solidified portion 30 of the point-shaped joint portion is Ms point −250 ° C. or lower. This is because a general steel sheet completes martensitic transformation at an Ms point of −250 ° C.

Next, the irradiation of the light beam 70 in the heat treatment of the point-shaped joint portion by the light beam will be described with reference to FIG.
FIG. 6A shows an example of a method of irradiating the planned remelting portion with the light beam 70, and the planned irradiation portion 90 of the light beam 70 is shown by a dotted line.
In the irradiation of the light beam 70, the light ray is scanned in a substantially circular shape as indicated by the white arrow. At that time, the scanning direction of the light beam is not particularly limited, and may be clockwise or counterclockwise.

The portion 90 to be irradiated with the substantially circular light beam 70 is located at a position where the circumference of the intersection 60 between the melting boundary 40 and the steel plate mating surface 20c is tempered by the cylinder-shaped remelt solidification portion 30a formed by the irradiation of the light beam 70. Set.

Further, when the joint portion formed by the light beam is viewed in a plan view from the irradiation side of the light beam 70, the outer peripheral shape of the planned irradiation portion 90 of the light beam 70 is a circle, but the elliptical shape, polygonal shape, and diameter are matched to the outer contour of the melt-solidified portion. The shape may be a combination of different semicircles and semi-ellipses. Further, the number of irradiations of the light beam 70 may be once or a plurality of times depending on the width Wb of the remelted solidification portion.

By irradiating the light beam 70 as shown in FIG. 6 (a), as shown in FIG. 6 (b), when the melt-solidified portion 30 is viewed in a plan view from the irradiation side of the light beam 70, the remelt-solidified portion 30a is annularly remelted and solidified. Is formed, and a solidification reheating portion 30b is formed around the solidification / reheating portion 30b. Then, at that time, the vicinity of the intersection including the melting boundary is tempered, and the toughness is improved.

Further, in the formation of the point-shaped joint portion by the light beam and the heat treatment of the molten boundary of the point-shaped joint portion by the light beam, the light beam irradiation method may be the same irradiation method or a different irradiation method. For example, even if light rays are irradiated with different irradiation diameters to form point-shaped joints, and light rays are irradiated with different irradiation diameters to heat the melting boundary of the point-shaped joints, the condensing area is widened. Then, the point-shaped joint may be formed by irradiating the light beam, and the molten boundary of the point-shaped joint may be heat-treated by irradiating the light with different irradiation diameters.

Next, the heating temperature of the solidification reheating unit 30b in the irradiation of the light beam 70 will be described.
It is preferable to reheat the region of at least 0.5 mm from the intersection 60 of the steel plate mating surface 20a and the melting boundary 40 in the melt-solidified portion 30 of the point-shaped joint portion by the light beam so as to be reheated.

In order to burn back the range of the intersection 60 to 0.5 mm or more, the light beam 70 is melt-coagulated under the condition that the maximum temperature reached in this range is a predetermined temperature of ₁ Ac or less (for example, 400 ° C or more and 700 ° C or less). Irradiate the inside of the portion 30. As the temperature near the intersection 60, the temperature measured on the surface of the steel sheet can be used as a representative value. The temperature can be measured using a radiation thermometer or a thermocouple.

In order to obtain such a temperature, the relationship between the circle-equivalent diameter Wa outside the remelting and solidifying portion or the width Wb of the formed remelting and solidifying portion and the temperature in the above range during reirradiation of light rays is determined in advance. Investigate the relationship between the re-irradiation time of the light beam and the temperature in the above range, and adjust the outer circle-equivalent diameter Wa of the remelt-solidified portion, the width Wb of the remelt-solidified portion, the re-irradiation time of the light beam, and the like. Can be done with. Further, in order to set the region in the range of the intersection 60 to 0.5 mm to 400 ° C. or higher and 700 ° C. or lower, the light beam is set so that the shortest distance Wc from the intersection 60 to the remelted solidification portion is 1.0 to 3.0 mm. It is exemplified to adjust the irradiation of. It is preferably 1.0 to 2.0 mm.

Next, the light rays used in the formation of the point-shaped joints by the light rays and the heat treatment of the point-shaped joints by the light rays will be described.
The bonding by light rays is not particularly limited, but a remote laser bonding is preferable. In remote laser joining, a galvano mirror attached to the tip of a robot arm moves light rays between joining points at high speed for joining, which makes it possible to significantly reduce the joining work time. Further, as the light beam used for bonding, for example, a light ray such as a CO ₂ ray, a YAG ray, a fiber ray, a DISK ray, or a semiconductor ray can be used.

In addition, conventional conditions can be adopted as the conditions for joining with light rays. For example, the light output can be 2 to 30 kW, the beam diameter of the condensing surface is 0.1 to 8.0 mm, and the bonding speed is 0.1 to 60 m / min.

Further, assembling an automobile consists of a plurality of joining steps, but when the manufacturing method of the present invention is carried out in one step, joining and remelting by light beam irradiation are carried out at each joining point. However, in order to reduce the waiting time for cooling from the joining to the start of remelting, more preferably, a plurality of melt joinings are carried out by light beam irradiation, and then a plurality of remelting is carried out by light beam irradiation. Is preferable. Further, when the manufacturing method of the present invention is carried out in a plurality of joining steps, the waiting time for cooling can be eliminated by setting the melt joining step by light irradiation and the remelting step by light irradiation as separate steps.

Next, examples of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to this one condition example.

Two steel plates shown in Table 1 were superposed and joined by using a remote joining device having a galvanometer mirror and using a fiber laser for light rays to prepare a test piece having a point-shaped joint portion by light rays. Table 2 shows the conditions for forming point-shaped joints by light rays. The beam diameter is the diameter of the light beam on the condensing surface.

Next, each test piece was heat-treated in the melt-solidified portion near the steel sheet mating surface 20a. In this heat treatment, the center of the melt-solidified portion and the cylinder-shaped remelt-solidified portion are aligned, and the remelt-solidified portion is formed so that the lower end is located at the lower position (0.4 mm) of the steel plate mating surface. went.
Table 3 shows the conditions for forming the remelted solidified portion (heat treatment conditions). The beam diameter is the diameter of the light beam on the condensing surface. In the heat treatment, the same remote joining device as for forming the point-shaped melt-solidified portion was used.

Table 4 shows the test pieces after heat treatment in the range of the shortest distance Wc from the intersection of the steel plate mating surface and the fusion boundary to the remelt solidification portion, the average Vickers hardness A of the remelt solidification portion, and 0.5 mm from the intersection. The average Vickers hardness B, the difference between the average Vickers hardnesses A and B, and the cross tensile strength (CTS) in the melt-solidified portion are shown.
For CTS, the method described in JIS Z3137 as a method for testing the strength of spot bonding was used.

No. In 2-4, 6-8, 10 to 13, the point-shaped joints are heat-treated by light rays, and 0.5 mm from the intersection of the melt-solidified parts in order to satisfy all the configurations specified by the joints of the present invention. The toughness of the range is improved and the cross tensile strength (CTS) is increased.

On the other hand, No. 1, No. 5 and No. In No. 14, since the point-shaped joint portion by the light beam was not heat-treated, the toughness of the melt boundary of the melt-solidified portion was not improved, and the cross tensile strength (CTS) was low. In addition, No. In No. 9, since the entire melt-solidified portion was heat-treated, the toughness of the melt boundary of the melt-solidified portion was not improved, and the cross tensile strength (CTS) was low.

According to the present invention, since the solidification and reheating portion having excellent toughness is provided near the melting boundary of the point-shaped joint portion by the light beam, the joint strength of the lap joint, particularly the cross tensile strength (CTS) is improved. be able to. Then, by applying the joint of the present invention to an automobile part, the reliability of the automobile part can be improved. Therefore, the present invention has high industrial applicability.

1, 10 Joined joints 2a, 20a Steel plates 2b, 20b Steel plates 2c, 20c Steel plate mating surfaces 3, 30 Remelting and solidifying parts 3a, 30a formed in the melting and solidifying parts of point-shaped joints by light rays 3b, 30b Solidification reheating part formed in the melt solidification part 40 Melting boundary between the steel plate and the melt solidification part 50 Tip part in the overlapping direction of the remelt solidification part 60 Intersection between the steel plate mating surface and the fusion boundary 70 Rays 80a, 80b Scheduled to be irradiated Locations 90a, 90b Scheduled irradiation location X Vickers hardness measurement position in the plate thickness direction L1 Vickers hardness measurement range in the direction parallel to the steel plate surface L2 Range of melt-solidified part in L1 C Central axis W Corresponding to the circle of melt-solidified part Diameter Wa Corresponding circle diameter on the outside of the remelt solidification part Wb Radial width of the remelt solidification part Wc The shortest distance from the intersection of the steel plate mating surface and the fusion boundary to the remelt solidification part

Claims (7)

In a lap joint that is composed of a plurality of lapped steel plates and has point-like joints by light rays.
The point-shaped joint portion formed by the light beam has a point-shaped melt-solidified portion straddling all the overlapped steel plates.
The melt-coagulation section has a remelt-coagulation section and a coagulation / reheating section.
The remelted solidified portion has a cylinder-like shape, and is positioned inside the molten boundary between the melted solidified portion and the steel plate so as not to overlap the molten boundary and not to penetrate the overlapped steel plates. And
A lap joint joint characterized in that the solidification / reheating portion is located around the remelt / solidification portion, includes the fusion boundary, and is softened from the remelt / solidification portion. Among the solidified and reheated portions, the average value of Vickers hardness in the range of 0.5 mm from the molten boundary of the mating surface of the steel sheet toward the remelted solidified portion is Hv390 or less, and the reheating The lap joint according to claim 1, wherein the Vickers hardness of the melt-solidified portion is lower than the average value of Hv70 or more. Claim 1 or 2 is characterized in that the shortest distance from the intersection of the steel plate mating surface and the melting boundary to the remelted solidified portion is 1.0 to 3.0 mm in the cross section of the plurality of steel plates in the plate thickness direction. Laminated joints described in. The lap joint according to any one of claims 1 to 3, wherein the plurality of steel plates include one or more steel plates having a surface treatment film. In a method for manufacturing a lap joint joint in which a plurality of steel plates are laminated and irradiated with a light beam having a high power density to join the plurality of steel plates.
A light beam having a high power density is irradiated into a limited area on one outer surface of one of the superposed steel sheets, and the light beam is melt-solidified across all the superposed steel sheets to have a punctate melt-solidified portion. Form a dotted joint and
Next, a light beam having a high power density is cyclically re-irradiated inside the melt boundary between the melt-solidified portion and the steel plate so as not to overlap the melt boundary and not to penetrate the overlapped steel plates. The irradiated portion is remelted and solidified to form a cylindrical remelted solidified portion, and the periphery of the remelted solidified portion is reheated to form a solidified and reheated portion including the melting boundary.
Further, a method for manufacturing a lap joint, which comprises adjusting the joining conditions when forming the remelted solidified portion to soften the solidified and reheated portion from the remelted solidified portion. The re-irradiation of the light beam is performed so that the shortest distance from the intersection of the steel plate mating surface and the fusion boundary to the remelt solidification portion is 1.0 to 3.0 mm in the cross section of the plurality of steel plates in the plate thickness direction. The method for manufacturing a lap joint according to claim 5, wherein the lap joint is made. The method for manufacturing a lap joint according to claim 5 or 6, wherein one or more steel plates having a surface treatment film formed on the plurality of steel plates are used.

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