JP2017221972A - Lap bonded joint and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bonded joint which has an excellent joint strength and an improved reliability.SOLUTION: In this bonded joint, a point-like junction has a melt solidified part 30 bridging over plural metal plates 20a, 20b, the melt solidified part has a re-melt solidified part 30a and a solidification re-heating part 30b, the re-melt solidified part has a point-like form which includes a circle equivalent central axis of the melt solidified part and bridges over the plural metal plates in a plan view of the melt solidified part, and the re-melt solidified part has a cylindrical shape and is positioned inside a melting boundary between the melt solidified part and a steel plate so as not to be overlapped on the melting boundary. The solidification re-heating part is positioned around the re-melt solidified part, and includes the melting boundary. Further, in the solidification re-heating part, a segregation of P is alleviated in a rectangular planar area of 100 μm×100 μm with a point on an extension line of a steel plate mating surface, which is separated from an intersection point between the mating surface and the melting boundary by 100 μm, as a center point.SELECTED DRAWING: Figure 3

Description

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

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

  Spot welding using resistance heating is widely used in the process of assembling the body and attaching parts made of high-strength steel sheets. However, in recent years, instead of spot welding, high power is used in some areas. A technique of joining in the form of spots (dots) using joining with light beams having a density (hereinafter referred to as light beams) is being applied to the manufacture of automobiles.

  Spot joints using light beams can be applied at higher speeds than spot welding, and are not affected by shunting unlike spot welding. Therefore, there is an advantage that the pitch of the joints can be shortened. Improvements are also possible.

On the other hand, the strength of a spot-like joint joint by light tends to be the same as or lower than that of conventional resistance spot welding. 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.
For this reason, when joining to a high strength steel plate with a light beam, the technique which improves joint strength, such as CTS, was desired.

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

  In such a technique of forming a joint portion by light rays in 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 the planar surface is viewed from the steel sheet surface side, which has a larger bonding area than the loop-shaped joint, the outer contour is substantially circular, and a joint is formed on the steel sheet by the light beam that has melted and solidified to the center. Thus, a technique for improving the joint strength has been reported (see Patent Document 4).

JP 2010-012504 A JP2012-240086A International Publication No. 2012/050097 JP 60-68185 A

The technique disclosed in Patent Document 4 is effective in improving the tensile shear strength (TSS) because the joint portion by light rays is dotted, but it is desired to further improve the cross tensile strength (CTS). It was.
Then, this invention makes it a subject to provide the lap joint joint which was excellent in joint strength and improved the reliability in view of such a situation.

The inventors of the present invention conducted diligent studies on means for further improving the CTS of a joint joint in which a steel sheet was joined with light rays to form a dotted joint formed by light rays.
When a load is applied to the joint in the peeling direction in the lap joint, stress is likely to concentrate near the melting boundary between the steel sheet and the melt-solidified part, leading to breakage. Therefore, it is considered that a sufficient CTS value cannot be secured. .
Accordingly, the inventors have conceived that the toughness of the portion is improved by heat-treating the vicinity of the melting boundary, and various investigations have been made on the heat-treatment location and heat-treatment method therefor.

As a result, the inside of the melt-solidified portion of the spot-like joint is irradiated in a ring shape to re-melt and solidify the irradiated portion, and P in the vicinity of the melting boundary between the steel plate and the melt-solidified portion is caused by heat at that time It was found that CTS is improved by reheating so as to alleviate such segregation of embrittlement elements.
The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) In a lap joint having a dotted joint formed of a plurality of superposed steel plates and formed by light rays,
The point-like joint formed by the light beam has a point-like melted and solidified portion straddling all the steel plates stacked,
The melt solidification part has a remelt solidification part and a solidification reheating part,
The remelted solidified part has a cylindrical shape and is positioned so as not to overlap the melted boundary inside the melted boundary between the melted solidified part and the steel plate,
The solidification reheating part is located around the remelting solidification part and includes the melting boundary,
Furthermore, in the solidification reheating part, a point separated by 100 μm on the extension line of the mating surface from the intersection of the steel sheet mating surface and the melting boundary is a center point,
In a rectangular plane region of 100 μm × 100 μm centered on the central point, parallel to the direction from the central point toward the central axis and perpendicular to the steel plate surface, the P concentration in mass% and on the steel plate surface Measured at a pitch of 1 μm along each of the parallel direction and the direction perpendicular to the steel sheet surface, thereby obtaining the measurement value of the P concentration at each of 100 × 100 measurement points,
Among the 100 measurement points × 100 measurement points, a partial average value of the measurement values of the P concentration at each of the 20 measurement points adjacent in a line in a direction parallel to the steel plate surface is applied to the steel plate surface. When repeatedly calculating by shifting one point along each of the parallel direction and the direction perpendicular to the steel sheet surface, thereby obtaining 81 × 100 partial average values,
Among the partial average values, the number of the partial average values that exceeds twice the total average value of the measured values of the P concentration at each of the 100 points × 100 measurement points is 0 or more and 100 or less. A characteristic lap joint.

(2) In the cross section in the plate thickness direction of the plurality of steel plates, the shortest distance from the intersection of the steel plate mating surface and the melting boundary to the remelted solidified portion is 0.5 to 1.0 mm. The lap joint according to (1).
(3) The lap joint according to (1) or (2) above, wherein the plurality of steel plates include one or more steel plates having a surface treatment film.

(4) In a method for manufacturing a lap joint, wherein a plurality of steel plates are superposed, and the plurality of steel plates are joined by irradiating a light beam having a high power density,
High power having a point-like melt-solidified part by irradiating a light beam having a high power density in a limited region of one outer surface of the superposed steel plates, and melting and solidifying across all the superposed steel plates Forming a spot-like joint with light rays having a density,
Next, inside the melting boundary between the melt-solidified part and the steel sheet, reirradiate the light beam having a high power density in a ring shape so as not to overlap the melting boundary and not to penetrate the stacked steel sheets, 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 reheated portion including the melting boundary,
Furthermore, by adjusting the joining conditions when forming the remelted solidified portion,
A center point is a point of 100 μm from the melting boundary of the overlapping surface of the metal plate to the central axis side,
In the rectangular plane region of 100 μm × 100 μm centered on the center point, which is parallel to the direction from the center point toward the center axis and perpendicular to the metal plate surface, the P concentration is mass%, and the metal plate Measured at a pitch of 1 μm along each direction parallel to the surface and perpendicular to the metal plate surface, thereby obtaining a measurement value of the P concentration at each of 100 × 100 measurement points,
Of the 100 measurement points × 100 measurement points, the average value of the measurement values of the P concentration at each of the 20 measurement points adjacent to each other in a row in a direction parallel to the metal plate surface is calculated as the metal plate surface. When calculating a partial average value of 81 × 100 pieces by repeating the calculation while shifting one point along each of the direction parallel to the direction and the direction perpendicular to the metal plate surface,
Among the partial average values, the number of the partial average values that exceeds twice the total average value of the measured values of the P concentration at each of the 100 points × 100 measurement points is 0 or more and 100 or less. A method for manufacturing a lap joint.

(5) In the re-irradiation of the light beam, the shortest distance from the intersection of the steel plate mating surface and the melting boundary to the remelted solidified portion is 0.5 to 1.0 mm in the cross section in the plate thickness direction of the plurality of steel plates.
The method for producing a lap joint according to (4) above, wherein
(6) The method for producing a lap joint according to (4) or (5) above, wherein at least one steel plate having a surface treatment film formed thereon is used for the plurality of steel plates.

  Here, the point-like joint formed by the light beam means that the steel plates are mutually connected by the melt-solidified portion formed in a point shape by irradiating the light within a limited area of a part of the superposed steel plate surface. The joined part.

  According to the present invention, since the solidification reheating part having excellent toughness is provided in the vicinity of the melting boundary of the melt solidification part, the joint strength of the lap joint, particularly the cross tensile strength (CTS) can be improved, The reliability of the joint joint can be improved. And the reliability of a motor vehicle component can be improved by applying the joint joint of this invention to a motor vehicle component.

Sectional drawing of the joint joint which has a dotted | punctate junction part formed of a light beam is shown. (A) shows sectional drawing of the joint joint before re-irradiation of a light beam, (b) shows sectional drawing of the joint joint after re-irradiation of a light beam. It is a cross-sectional enlarged view of a melt-solidified part. It is a figure which shows the cross section of the joint joint of this invention. It is a figure which shows typically the joint joint which has the dotted | punctate junction part formed of a light beam, (a) shows the structure of the cross section of a joint joint, (b) shows the Vickers hardness distribution of a joint joint. It is a figure which shows typically similarly to FIG. 4 about the joint joint which irradiated the light ray cyclically | annularly inside the dotted | punctate junction part formed of the light ray. It is a figure which shows the schematic of the Vickers hardness distribution of a joint joint, (a) shows the outline of the Vickers hardness distribution of the joint joint which has the dotted | punctate junction formed with the light ray, (b) is the light ray The outline of the Vickers hardness distribution of the joint joint which irradiated the light beam cyclically | annularly to the inner side of the formed dotted joint part is shown. It is a perspective view which shows the outline | summary of formation of the dotted | punctate junction part formed with a light ray, (a) shows the outline | summary which irradiates a light beam with a different irradiation diameter, (b) widens a condensing area, and shows a light ray. The outline | summary which irradiates is shown, (c) shows the dotted | punctate junction part formed of the light ray. It is a perspective view which shows the outline | summary of the heat processing of the dotted | punctate joining part formed of the light ray, (a) shows the schematic diagram which irradiates a light beam, (b) has a remelting solidification part and a solidification reheating part. The perspective view of the dotted | punctate junction part formed of the light ray is shown.

First, the background of the study that led to the present invention and the basic configuration of the present invention will be described.
In a lap joint having a dotted joint formed by light rays, it has been desired to further improve the joint strength. In lap joints, when a load is applied to the joint in the peeling direction, stress concentrates near the boundary between the steel sheet and the melt-solidified part (melting boundary), particularly near the intersection of the steel sheet mating surface and the melting boundary. , Easy to break. Further, high strength steel sheets often contain an embrittlement element such as P, and if there is segregation of P, it is considered that this tends to be the starting point of fracture.
Therefore, it was studied to reduce the segregation of P by heat-treating the stress concentration portion near the intersection.

First, light is irradiated to one outer surface of two superposed steel plates 2a and 2b, the irradiated portion is melted and solidified, and the diameter is about 6 mm, and the two steel plates are penetrated in a column shape in the stacking direction. A joint joint 1a including a spot-like joint portion by a light beam having the melt-solidified portion 3 was formed (see FIG. 1 (a)).
Next, a beam with a beam diameter of 0.4 mm is re-irradiated along the circumference of the diameter of 4.0 mm inside the melting boundary 4 with the steel plate of the melt-solidified part 3 to re-melt and solidify the irradiated part. And having a cylindrical remelted and solidified part 3a inside the melted and solidified part and a solid-state reheated part 3b reheated by the heat of the remelted and solidified part 3a around it. A joint joint 1b including a portion was formed (see FIG. 1B).
At the time of re-irradiation, the front end portion 5 in the overlapping direction of the remelted solidified portion 3a is on the steel plate 2a side from the extension line of the steel plate mating surface 2c, and the intersection point 6 between the steel plate mating surface and the melting boundary is The irradiation condition of the light beam was adjusted so as to be a position heated to a temperature range of Ac3 point or higher.

  When the cross tensile strength (CTS) of the joint joints 1a and 1b having the joint portion by the light beam was investigated, the cross tension of the joint joint 1b obtained by re-irradiating the inner side of the melt-solidified portion 3 with the light beam. It was found that the strength was higher than the cross tensile strength of the joint joint 1a.

  Next, the present inventors conducted an analysis of segregation of P, which is an embrittlement element in the vicinity of the melting boundary, for the joint joints 1a and 1b. In the analysis of P segregation, the method disclosed in International Publication No. 2013/161937 was adopted. Since the method for analyzing the segregation of P is described in detail in this document, it will be briefly described here.

In FIG. 2, the cross-sectional enlarged view of a melt-solidification part is shown. FIG. 2 is an enlarged view of one side from the central axis C of the melt-solidified portion 3 in FIG.
First, in the analysis of segregation of P, the center point is a point that is separated from the intersection 6 between the mating surface 2c of the steel plates 2a and 2b and the melting boundary 4 by 100 μm on the center axis C side along the extension line of the mating surface. A rectangular planar region A of 100 μm × 100 μm centered on the central point, which is parallel to the direction from the central point toward the central axis C and perpendicular to the surfaces of the steel plates 2a and 2b, is set.

  In this rectangular plane region A, P concentration (mass%) is measured at a pitch of 1 μm in each of a direction parallel to and perpendicular to the mating surface 2c, and the P concentration (mass%) at each of 100 × 100 measurement points is measured. Obtain the measured value. The P concentration (% by mass) is measured with a field emission electron beam microanalyzer (FE-EPMA).

  Next, of the 100 measurement points × 100 measurement points, the partial average value of the measured values of P concentration at each of the 20 adjacent measurement points arranged in a line in a direction parallel to the mating surface 2c is applied to the mating surface 2c. Calculation is performed while shifting one point along each of the parallel direction and the vertical direction. This is repeated to obtain 81 × 100 partial average values.

  Among the partial average values, the number of partial average values exceeding twice the total average value of the measured values of P concentration at each of 100 × 100 measurement points is compared. When this number is 0 or more and 100 or less, the segregation of P is relaxed, and when it is 101 or more, it is judged that P is segregated.

  Using the analysis method for segregation of P, the segregation of P in the region A was performed in the joint joint 1a shown in FIG. 1A and the joint joint 1b shown in FIG. In 1a, P was segregated, but in the joint 1b, P segregation was relaxed.

  As a result, in the lap joint having a dotted joint formed by light rays, the cross tensile strength is reduced by relaxing the segregation of P in the vicinity of the intersection 6 of the joining surface of the dotted joint and the melting boundary. It was found that it improved. Further, it was confirmed that even if the combination of the steel plates was changed, the cross tensile strength was improved if the segregation of P was alleviated.

  The present invention has reached the invention described in the above (1) through the examination process as described above, and the necessary and preferred requirements will be further described in order.

[Joint joint]
As shown in the cross-sectional view of FIG. 3, the joint joint 10 of the present invention is formed by overlapping a plurality of steel plates 20 a and 20 b, irradiating a light beam in a limited region of the steel plate 20 a from the steel plate 20 a side. A plurality of steel plates 20a and 20b are joined together by light rays.

<Dotted joint formed by light rays>
The dotted joint formed by the light beam has a plurality of steel plates 20a and 20b, and has a dotted melt-solidified part 30 melted and solidified by irradiation of the light beam. It has a remelted and solidified part 30a in which the melted and solidified part is remelted and a solidified and reheated part 30b in which the melted and solidified part is reheated after solidification by the remelted and solidified part. In FIG. 3, the heat affected zone on the steel plate side is not shown.
Here, the point-like joint formed by the light beam means that the steel plates are mutually connected by the melt-solidified portion formed in a point shape by irradiating the light within a limited area of a part of the superposed steel plate surface. The joined part.

(Dot-shaped melt-solidified part)
The melted and solidified portion 30 formed at the spot-like joint by the light beam is a melted and solidified portion formed by irradiating a light within a limited region of a part of the surface of the stacked steel plates.
The width W of the melt-solidified portion 30 (equivalent circle diameter of the melt-solidified portion when the melt-solidified portion is viewed in plan from the light irradiation side) may be adjusted according to the joint strength and the like, and is not particularly limited. 5 to 12 mm. Preferably, it is 6-10 mm.
As long as the melt-solidified portion 30 is formed across the plurality of steel plates 20a and 20b forming the joint portion, it may or may not penetrate through to the outer surface of the steel plate on the side opposite to the light irradiation side. Good.

  Here, the dot shape means that when the molten solidified portion is viewed in plan from the irradiation side of the light beam, the outer peripheral contour of the molten solidified portion is limited to a limited region such as a circular shape or a polygonal shape, and the region. It means that it is melted and solidified to the center of. The circular shape includes a combination of semi-circles and semi-ellipses having different diameters in addition to the case where the melt-solidified portion is circular or elliptical when the melt-solidified portion is viewed in plan from the light irradiation side. Moreover, the shape of the melt-solidified part formed by irradiating the steel sheet with a light beam in a spiral shape from the outer peripheral side toward the central side or from the central side toward the outer peripheral side is also included in a dot shape.

(Remelt solidification part)
The remelted and solidified part 30a is a cylindrical shape obtained by reirradiating the melted and solidified part 30 and the melted and solidified part 30 inside the melting boundary 40 of the steel sheet along an annular trajectory and remelting and solidifying the irradiated part. It is a part having a shape, and has a structure that has been melted and solidified. The term “annular” means that the contour is circular or polygonal. The circular shape includes a combination of semi-circles and semi-ellipses having different diameters in addition to the case where the remelted solidified portion is circular or elliptical.

The remelted and solidified portion 30a is formed so as not to overlap the melting boundary 40. At this time, when the molten and solidified portion is viewed in plan from the light irradiation side, the center corresponding to the circle of the molten and solidified portion is re-melted. It is not necessary to coincide with the center of the solidified portion 30a corresponding to the circle.
In the thickness direction of the joint 10, the remelted solidified portion 30 a is in the vicinity of the intersection 60 between the mating surface 20 c of the superposed steel plates and the melting boundary 40 as shown in FIG. 3. It is sufficient if it is formed up to the position, but it can also be a position that exceeds the extension line of the mating surface 20c or a position that penetrates the stacked steel plates.

  The position of the tip 50 in the overlapping direction of the remelting and solidifying part 30a and the position of the outer contour on the melting boundary 40 side of the remelting and solidifying part 30a are obtained by reheating the molten and solidifying part in the vicinity of the steel sheet mating surface 20a. It is important for alleviating segregation and, as will be described later, the molten solidified portion in the vicinity of the intersection 60 between the steel sheet mating surface and the melting boundary is set to have a reheating temperature not higher than the melting point of the base material and not lower than the Ac3 point temperature. Is done.

  The radial width Wb of the remelted and solidified portion 30a formed in a cylindrical shape (the radial thickness of the annular remelted and solidified portion when the melted and solidified portion is viewed in plan from the light irradiation side) is in the vicinity of the intersection 60. In order to give the amount of heat necessary for the melted and solidified portion to reach a reheating temperature equal to or higher than the Ac3 point temperature, it is determined in relation to the position of the tip portion, but is preferably 0.3 mm or more. The upper limit of the width Wb is not particularly limited, but is preferably about 0.3 W.

(Coagulation reheating part)
The solidification reheating part 30b is a part formed so as to include a melting boundary around the remelting and solidifying part 30a by reirradiating the melting and solidifying part 30 of the spot-like joint formed by the light beam in a ring shape. Yes, including a portion where segregation of P is reduced. 1 to 3, the region heated to the Ac3 point temperature or higher is shown as the solidification reheating unit 3b.

  In a lap joint, when a load is applied in the peeling direction of the steel sheet, stress concentrates on the melting boundary in the vicinity of the mating surface of the steel sheet, leading to fracture. Therefore, at least around the intersection 60 between the steel sheet mating surface 20c and the melting boundary 40 The solidification reheating part 30b is formed so that the melt solidification part located in the region has a reheating temperature equal to or higher than the Ac3 point temperature, and P segregation in the part is alleviated.

Specifically, since the stress is likely to concentrate in the melt-solidified portion in the vicinity of the steel sheet mating surface 20c, as described above, the number of partial average values exceeding twice the total average value of P concentration is zero. The number is 100 or less. In particular, in order to improve CTS, it is desirable that a range of at least 0.5 mm is heated inward from the intersection 60 of the steel sheet mating surface 20c and the melting boundary to a temperature equal to or higher than the Ac3 point temperature. In the cross section in the plate thickness direction, the shortest distance Wc from the intersection 60 between the steel plate mating surface and the melting boundary to the remelted solidified portion is preferably 0.5 to 1.0 mm.
In addition, since the remelted solidified part can be discriminated by observing the microstructure, Wc can be measured from the cross section of the bonded part after bonding with light.

  The analysis of the segregation of P in the solidification reheating part is performed as described above. However, the P concentration (mass%) can be measured by a field emission electron beam microanalyzer (FE-EPMA), and the following are exemplified as measurement conditions.

Acceleration voltage: 15 kV
Beam current: 0.5 μA
Beam dwell time per pixel: 60ms
Number of pixels: 250 × 250
Field of view: 100 μm × 100 μm

  Further, in the solidification reheating portion 30b, the portion reheated to the Ac3 point temperature or less below the melting point of the base material undergoes transformation in the heating process and the cooling process, respectively, and the crystal becomes finer. In such a portion, it is particularly preferable that the aspect ratio of crystal grains is polygonal ferrite of 1.5 or less.

(Vickers hardness of remelted solidified part and solidified reheated part)
Next, in the cross section in the plate thickness direction of the joint joint before and after re-irradiating the melt-solidified portion of the spot-like joint formed by the light beam, Vickers at the position near the steel plate mating surface and its extension line Changes in hardness were investigated.
4 and 5 show schematic views of the Vickers hardness distribution before and after re-irradiating an annular laser beam to the melt-solidified portion in a joint joint using a steel sheet having a tensile strength of 980 MPa. In each figure, (a) is a cross-sectional view of a joint joint, and (b) is a schematic diagram of the Vickers hardness distribution of the joint joint.

The Vickers hardness is the measurement range L1 of the Vickers hardness in the direction parallel to the surface of the steel plate, as indicated by the dotted line X (measurement position of the Vickers hardness in the cross section in the thickness direction) shown in FIGS. 4 (a) and 5 (a). Sought over.
A dotted line X is a position of 0.5 mm from the mating surface 20c of the steel plates 20a and 20b to the steel plate 20a side in the plate thickness direction. L2 is the measurement range of the Vickers hardness of the melt-solidified part.

  As shown in FIG. 4 (b), the Vickers hardness of the joint joint 10a before re-irradiating the melt-solidified portion 30 with light is as hard as about HV410 inside the melt-solidified portion 30 (L2) and is almost constant. It has become. The outer side of L2 is not a melt-solidified part, but is hard because it is heated to a high temperature region and quenched. In addition, although there exists a site | part with low hardness on the outer side, this is a HAZ softening part of the steel plate which is a base material.

  On the other hand, after re-irradiating the light beam in an annular shape, as shown in FIG. 5 (b), it receives the heat of the remelted solidified portion, and enters the melted solidified portion outside the melting boundary and inside the remelted solidified portion. A tempered portion with low Vickers hardness was formed. Further, the melt-solidified portion located in the vicinity of the re-melt solidified portion is heated again to a high temperature region by the re-melt solidified portion, hardened by being hardened, and has a width of 1 mm or more from the melting boundary toward the inside of the melt-solidified portion. It is getting harder. Note that the HAZ softened portion of the base material outside the melt-solidified portion 30 remains as it is.

Next, the steel plate of the member to be joined was changed to a 1500 MPa hot stamped steel plate, and the same investigation as described above was performed.
FIG. 6 shows a schematic diagram of the Vickers hardness distribution before and after re-irradiation with a laser beam in a joint joint using a hot stamped steel plate.

  In the hot stamped steel plate, the Vickers hardness of the joint 10a before re-irradiating the melted and solidified portion 30 with light is about HV450 on the inner side (L2) of the melted and solidified portion 30 as shown in FIG. Hard and almost constant. Although the outer side of L2 is not a melt-solidified part, it is heated to a high temperature region and quenched, so that the hardness is large. In addition, although there exists a site | part with low hardness on the outer side, this is a HAZ softening part of the steel plate which is a base material.

  Further, as shown in FIG. 6B, the Vickers hardness of the joint joint 10b after the re-irradiation with the light beam is as hard as about HV420 with a width of 1 mm or more from the melt boundary toward the melt-solidified portion. Further, a portion having a low Vickers hardness was formed outside the melting boundary. Note that the HAZ softened portion of the base material outside the melt-solidified portion 30 remains as it is.

Thus, in the joint joint of the present invention, even if the steel types of the steel plates are different, a high range of Vickers hardness is maintained over a width of 1 mm or more from the melting boundary toward the inside of the melt-solidified portion. Since there is no partial softening of the joining metal in the melt-solidified portion in the vicinity of the surface, a decrease in tensile shear strength is suppressed.
In order to suppress the decrease in the tensile shear strength, the melt boundary at the position (X position) 0.5 mm away from the extension line of the mating surface of the steel plates 20a and 20b in the overlapping direction in the solidification reheating part 30b. It is preferable that the average value of Vickers hardness in the range of 0.5 mm from the inside to the inside and the average value of Vickers hardness in the range of 0.5 to 1 mm are less than ± Hv70.
In the measurement of the average value of the Vickers hardness in the above range, the average value is obtained by measuring the Vickers hardness at equal intervals of 3 points or more including the center of each range and the vicinity of both ends.

  Further, when there are a plurality of overlapping surfaces of the steel plates, the measurement is performed on a line connecting the melting boundaries of the melt-solidified portion 30 in contact with the overlapping surfaces of the respective steel plates. The remelted solidified part and the solidified reheated part can be distinguished from each other by observing the microstructure.

<Multiple steel plates>
The steel plate used for the joint joint of the present invention is not particularly limited, and various steel plates can be used. The component composition of the steel sheet is not particularly limited, and may be a steel sheet having a component composition that can provide mechanical properties and the like according to the application. Moreover, when a high-strength steel sheet having a carbon content of 0.10 to 0.25% by mass is applied to the joint joint of the present invention, the improvement of the cross tensile strength is remarkable, and such a steel sheet may be targeted. preferable. Moreover, although P content is not specifically limited, 0.001% or more and 0.03 mass% or less in which the effect of P segregation improvement is anticipated is illustrated.

  The plate | board thickness of a steel plate is not specifically limited, It can be set as the range of 0.5-3.2 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 joint becomes small, The application range of the joint is limited. Moreover, even if the plate thickness is over 3.2 mm, an effect of improving the joint strength of the joint portion can be obtained, but the application range of the joint joint is limited from the viewpoint of reducing the weight of the member.

(Surface treatment film of steel sheet)
The steel plate may include at least one steel plate having a surface treatment film formed on both sides or one side of the joining portion. The surface treatment film includes a plating film, and may further include a coating film or the like. Examples of the plating film include zinc plating, aluminum plating, zinc / nickel plating, zinc / iron plating, zinc / aluminum / magnesium plating, and plating production methods include hot dipping and electroplating.

  The steel plate should just be a plate-shaped at least part which forms a joint, and the whole may not be a plate. For example, it includes a flange portion of a member press-molded into a specific shape having a cross-sectional hat shape. The number of steel plates to be stacked is not limited to two, and may be three or more. In addition, the types, component compositions, and plate thicknesses of the respective steel plates may be the same or different from each other. Moreover, it is not limited to what is comprised from a separate steel plate, The lap joint joint of what formed the shape of one steel plate in predetermined shapes, such as a tubular shape, and overlap | superposed the edge part may be sufficient.

Hereinafter, although not limited thereto, an example of a lap joint in an automobile will be shown.
In the case of A-pillar, a combination of 270 to 340 MPa class galvannealed steel sheet, 590 to 1800 MPa class non-plated steel sheet or hot stamped steel sheet, and 590 to 1800 MPa class non-plated steel sheet or hot stamped steel sheet A lap joint is illustrated.

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

  In the case of a side sill, a lap-jointed joint in a combination of three layers of a 270 to 340 MPa class galvannealed steel sheet, a 590 to 1800 MPa class galvannealed steel sheet, and a 590 to 1800 MPa class galvannealed steel sheet Is exemplified.

  In the case of a floor member, lap joining in a combination of two layers of a 270-590 MPa class alloyed hot dip galvanized steel floor panel and a 440-1800 MPa class non-plated steel sheet or an alloyed hot dip galvanized steel floor member A joint is illustrated.

[Manufacturing method of lap joint]
Next, the manufacturing method of the lap joint of this invention is demonstrated.
First, (a) superposing a plurality of steel plates, irradiating light within a limited region on one outer surface of the superposed steel plates, and melting and solidifying to the center of the region across all the superposed steel plates The formation of the point-like joint portion by the light beam having the point-like melted and solidified portion will be described with reference to FIG.

  FIG. 7 is a perspective view showing an outline of formation of a dot-shaped joint portion by irradiating a light within a limited region (a planned irradiation position). FIG. 7 (a) shows an outline of irradiating light rays with different irradiation diameters, FIG. 7 (b) shows an outline of irradiating light rays with a wide condensing area, and FIG. The formed point-like junction is shown.

  FIG. 7A shows an example of a method of irradiating the light beam 70. The light beam 70 is irradiated with different irradiation diameters. In this figure, the irradiation planned portion 80a of the light beam 70 is indicated by a dotted line, and three irradiation planned portions 80a having different irradiation diameters are shown.

  In the formation of the point-like joining portion by the light beam, first, the plurality of steel plates 20a and 20b are overlapped, and the light beam 70 is irradiated from one steel plate 20a side to perform the joining by the light beam. In the irradiation of the light beam 70, when the irradiation scheduled portion 80a is viewed in plan from the irradiation side of the light beam 70, the light beam is scanned in a substantially circular shape as indicated by a white arrow. At that time, the irradiation of the light beam 70 is performed on the outer irradiation scheduled spot 80a, and then on the inner irradiation scheduled spot 80a, the inner irradiation scheduled spot 80a is performed, and then on the outer irradiation scheduled spot 80a. You may go. The scanning direction of the light beam is not particularly limited, and may be either clockwise or counterclockwise.

  In addition, when the irradiation target portion 80a from the irradiation side of the light beam 70 is viewed in plan, the outer peripheral shape of the irradiation target portion 80a of the light beam 70 is a circle, but an elliptical shape, a polygonal shape, a semicircle or a half ellipse having different diameters are used. A combined shape or a spiral shape may be used. When the irradiation target portion 80a of the light beam 70 is formed in a spiral shape, the irradiation of the light beam 70 is performed from the outer end portion of the spiral irradiation target portion toward the inner end portion or in a spiral irradiation schedule. The light beam is scanned in a spiral shape from the inner end portion to the outer end portion. The direction of the spiral is not particularly limited, and may be either clockwise or counterclockwise.

  In FIG. 7A, three irradiation planned places having different diameters are illustrated. However, depending on the focal area of the light beam and the joint area of the dotted joint formed by the light beam, The number can be increased or decreased.

  FIG. 7B 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 irradiation planned portion 80b of the light beam 70 is indicated by a dotted line. The irradiation of the light beam 70 is performed once with the light collection area widened.

  By irradiating the light beam 70 as shown in FIG. 7A and FIG. 7B, as shown in FIG.

  Moreover, when using one or more steel plates in which a surface treatment film is formed on a plurality of steel plates, it is preferable to irradiate the light beam 70 to the outer irradiation target portion 80a and then to the inner irradiation target portion 80a. Thereby, it becomes easy to collect and stir and remove the film that has become a gas causing a defect in the joint in the vicinity of the center of the melted part. Irradiation of the light beam 70 is performed on the inner irradiation planned portion 80a, and then the coating film that becomes a gas is formed regardless of whether the irradiation is performed on the outer irradiation planned portion 80a or the light collection area of the light beam 70 is increased. Since it can remove from a fusion | melting part, it does not exclude employ | adopting the irradiation method of these light rays.

  Next, (b) inside the melting boundary between the point-like melt-solidified portion and the steel plate, the light beam is re-irradiated in an annular shape so as not to overlap the melt boundary, and the irradiated portion is re-melted and solidified to form a cylinder. A remelted and solidified part of the solidified part, and re-heated (heat treated) around the remelted and solidified part to form a solidified and reheated part including the melting boundary. The adjustment of the bonding conditions to alleviate P segregation will be described.

In the heat treatment of the spot-like joint formed by the light beam, the process waits until the melt-solidified part of the spot-like joint obtained by the method shown in FIG. 7 is solidified, and then the melt-solidified part from the steel plate 20a side. This is performed by irradiating the inside 30 with the light beam 70. This is because the present invention utilizes diffusion in a solid at a high temperature, so that the effect cannot be obtained unless the melt-solidified portion 30 is solidified and irradiated with light.
The inner side of the melt-solidified part 30 refers to the inside of the melt-solidified part 30 excluding the melt boundary of the melt-solidified part 30.

Next, irradiation of the light beam 70 in the heat treatment of the spot-like joint formed by the light beam will be described with reference to FIG.
FIG. 8A shows an example of a method of irradiating the light beam 70 to the remelting planned portion, and the irradiation planned portion 90 of the light beam 70 is indicated by a dotted line.
In the irradiation of the light beam 70, the light beam is scanned in a substantially circular shape as indicated by an outline arrow. At that time, the scanning direction of the light beam is not particularly limited, and may be either clockwise or counterclockwise.

  In the portion 90 to be irradiated with the substantially circular light beam 70, the segregation of P around the intersection 60 between the melting boundary 40 and the steel sheet mating surface 20 c is mitigated by the cylinder-shaped remelted solidified portion 30 a formed by the irradiation of the light beam 70. The position is set.

  In addition, when the joint portion by the light beam is viewed in plan 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 is elliptical, polygonal, or diameter according to the outer contour of the melt-solidified portion. It is good also as a shape which combined the semicircle and semi-ellipse which differ. Further, the number of times of irradiation with the light beam 70 may be one or more times depending on the width Wb of the remelted solidified portion.

  By irradiating the light beam 70 as shown in FIG. 8 (a), as shown in FIG. 8 (b), when the molten and solidified portion 30 is viewed in plan from the irradiation side of the light beam 70, the remelted and solidified portion 30a is annularly formed. Is formed, and the solidification reheating part 30b is formed around it. At that time, the segregation of P in the vicinity of the intersection including the melting boundary is alleviated and the toughness is improved.

  In addition, in the formation of the spot-like joint portion by the light beam and the heat treatment of the melting boundary of the spot-like joint portion formed by the light beam, the light beam irradiation method may be the same irradiation method or a different irradiation method. For example, light beams may be irradiated with different irradiation diameters to form point-like joints, and light rays may be irradiated with different irradiation diameters to heat-treat the melting boundaries of the dot-like joints formed by the light beams. Then, widen the condensing area and irradiate with light rays to form spot-like joints by the light rays, irradiate light rays with different irradiation diameters, and melt the boundary of the spot-like joints formed by the light rays. You may heat-process.

Next, the heating temperature of the coagulation reheating unit 30b in the irradiation of the light beam 70 will be described.
The segregation of P in the region in the range of at least 0.5 mm from the intersection 60 between the steel plate mating surface 20c and the melting boundary 40 in the melt-solidified portion 30 of the spot-like joint formed by the light beam is relieved. It is good to heat.

  In order to alleviate the segregation of P in the region in the range of at least 0.5 mm from the intersection point 60, the light beam is used under the condition that the highest temperature in this range is not higher than the melting point of the base material and is not lower than the Ac3 point temperature (for example, not lower than 900 ° C.). 70 is irradiated to the inside of the melt-solidified part 30. As the temperature near the intersection 60, the temperature measured on the steel plate surface can be used as a representative value. The temperature can be measured using a radiation thermometer or a thermocouple.

  In order to achieve such a temperature, the relationship between the circle equivalent diameter Wa outside the remelted solidified portion or the width Wb of the remelted solidified portion to be formed, and the temperature in the above-described range during re-irradiation of light rays, Investigate the relationship between the re-irradiation time of the light beam and the temperature within the above range, and adjust the equivalent circle diameter Wa outside the re-melting and solidifying part, the width Wb of the re-melting and solidifying part, the re-irradiation time of the light beam, etc. Can be done. In order to set the region in the range of 0.5 mm from the intersection point 60 to the Ac3 point temperature or less below the melting point of the base material, the shortest distance Wc from the intersection point 60 to the remelted solidified portion is 0.5 to 1.0 mm. Thus, adjusting the irradiation of light rays is exemplified. Preferably, it is 0.6-0.9 mm.

Next, a description will be given of the light beam used in the formation of the dotted joint portion by the light beam and the heat treatment of the dotted joint portion formed by the light beam.
The bonding by light beam is not particularly limited, but remote laser bonding is preferable. In remote laser bonding, a galvano mirror attached to the tip of a robot arm is used for bonding by moving a light beam between bonding points at a high speed, and the bonding work time can be greatly reduced. As the light beam to be used for joining, for example, CO ₂ beam, YAG light, fiber light, DISK rays, can be used a light beam such as a semiconductor light.

  Moreover, the conventional conditions can be employ | adopted for the conditions of joining by a light ray. For example, it can be performed under the joining conditions of a light output of 2 to 30 kW, a beam diameter of the condensing surface of 0.1 to 8.0 mm, and a joining speed of 0.1 to 60 m / min.

  In addition, the assembly of automobiles consists of a plurality of joining processes, but when the manufacturing method of the present invention is carried out within one process, the joining and remelting by light irradiation is carried out for each joining point. However, in order to reduce the variation in temperature change, it is more preferable to perform a plurality of melt bonding by light irradiation, and then perform a plurality of remelting by light irradiation. Moreover, when implementing the manufacturing method of this invention in a some joining process, the waiting time of cooling can be eliminated by making the fusion | melting joining process by light irradiation, and the remelting process by light irradiation into a separate process.

  Next, examples of the present invention will be described. The conditions in the examples are one condition example adopted to confirm the feasibility and effects 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 a fiber laser beam using a remote beam joining device having a galvanometer mirror, to prepare a test piece having a dotted joint formed by the light beam. Table 2 shows the joining conditions when forming a spot-like joint with light rays. The beam diameter is the diameter of the light beam on the condensing surface.

Next, each test piece was heat-treated at the melt-solidified portion in the vicinity of the steel sheet mating surface 20c. In this heat treatment, the melt-solidified part and the cylinder-shaped remelt-solidified part are made to coincide with each other, and the remelt-solidified part is formed so that the lower end is located at the lower position (0.1 mm) of the steel sheet mating surface. went.
Table 3 shows the formation conditions (heat treatment conditions) of the remelted solidified portion. The beam diameter is the diameter of the light beam on the condensing surface. In the heat treatment, the same joining apparatus using a remote beam as that for forming the spot-like melted and solidified portion was used.

  Table 4 shows the shortest distance Wc from the intersection of the steel plate mating surface and the melting boundary to the remelted solidified portion, the average Vickers hardness A of the remelted solidified portion, and the extension of the steel plate mating surface. The average Vickers hardness B, the difference between the average Vickers hardness A and B, the P segregation, and the cross tensile strength (CTS) in the solidification reheated part from the boundary to the position 0.5 mm inside is shown. CTS used the method described in JIS Z3137 as a strength test method for spot bonding.

  P segregation is expressed as “◯” when the partial average of P concentration (mass%) is more than 0 and less than 100 in accordance with the analysis method of P segregation described above. The case where the number is 101 or more is represented as “×”. CTS used the method described in JIS Z3137 as a strength test method for spot bonding.

  No. 2 to 4, 6 to 8, and 10 to 13 are heat-treated on the spot-like joints formed by light rays, and satisfy all the configurations defined by the joints of the present invention. The toughness in the range of 0.5 mm is improved and the cross tensile strength (CTS) is high.

  In contrast, no. 1, no. 5 and No. In No. 14, since the heat treatment was not performed on the dotted joint formed by the light beam, the toughness of the melt boundary of the melt-solidified portion was not improved, and the cross tensile strength (CTS) was low. 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 reheating part having excellent toughness is provided in the vicinity of the melting boundary of the dotted joint formed by the light beam, the joint strength of the lap joint, in particular, the cross tensile strength (CTS). The reliability of the joint joint can be improved. And the reliability of a motor vehicle component can be improved by applying the joint joint of this invention to a motor vehicle component. Therefore, the present invention has high industrial applicability.

DESCRIPTION OF SYMBOLS 1, 10 Joint joint 2a, 20a Steel plate 2b, 20b Steel plate 2c, 20c Steel plate mating surface 3, 30 Melt-solidified part 3a, 30a of the point-like junction formed by light rays Remelt-solidified part formed in the melt-solidified part 3b, 30b Solidification reheating part formed in the melt solidification part 4, 40 Melting boundary between the steel sheet and the melt solidification part 5, 50 Tip part 6 in the overlapping direction of the remelt solidification part 6, 60 Steel sheet mating surface and melting boundary 70 Light beam 80a, 80b Planned irradiation point 90 Planned irradiation point A Rectangular plane region X Measurement position of Vickers hardness in the plate thickness direction L1 Measurement range of Vickers hardness in the direction parallel to the steel plate surface L2 Range C Central axis W Equivalent circle diameter of melt-solidified portion Wa Equivalent circle diameter outside re-melt-solidified portion Wb Radial width of re-melt-solidified portion Wc Intersection of steel sheet mating surface and melting boundary The shortest distance from the point to the remelted solidified part

Claims (6)

In a lap joint having a dotted joint formed by a plurality of steel plates stacked and formed by light rays,
The point-like joint formed by the light beam has a point-like melted and solidified portion straddling all the steel plates stacked,
The melt solidification part has a remelt solidification part and a solidification reheating part,
The remelted solidified part has a cylindrical shape and is positioned so as not to overlap the melted boundary inside the melted boundary between the melted solidified part and the steel plate,
The solidification reheating part is located around the remelting solidification part and includes the melting boundary;
Furthermore, in the solidification reheating part, a point separated by 100 μm on the extension line of the mating surface from the intersection of the steel sheet mating surface and the melting boundary is a center point,
In a rectangular plane region of 100 μm × 100 μm centered on the central point, parallel to the direction from the central point toward the central axis and perpendicular to the steel plate surface, the P concentration in mass% and on the steel plate surface Measured at a pitch of 1 μm along each of the parallel direction and the direction perpendicular to the steel sheet surface, thereby obtaining the measurement value of the P concentration at each of 100 × 100 measurement points,
Among the 100 measurement points × 100 measurement points, a partial average value of the measurement values of the P concentration at each of the 20 measurement points adjacent in a line in a direction parallel to the steel plate surface is applied to the steel plate surface. When repeatedly calculating by shifting one point along each of the parallel direction and the direction perpendicular to the steel sheet surface, thereby obtaining 81 × 100 partial average values,
Among the partial average values, the number of the partial average values that exceeds twice the total average value of the measured values of the P concentration at each of the 100 points × 100 measurement points is 0 or more and 100 or less. A characteristic lap joint. The shortest distance from the intersection of the steel sheet mating surface and the melting boundary to the remelted solidified portion in a cross section in the plate thickness direction of the plurality of steel plates is 0.5 to 1.0 mm.
The lap joint joint described in 1.   The lap joint according to claim 1 or 2, wherein the plurality of steel plates include at least one steel plate having a surface treatment film. In the method of manufacturing a lap joint, which superimposes a plurality of steel plates and irradiates a light beam having a high power density to join the plurality of steel plates,
High power having a point-like melt-solidified part by irradiating a light beam having a high power density in a limited region of one outer surface of the superposed steel plates, and melting and solidifying across all the superposed steel plates Forming a spot-like joint with light rays having a density,
Next, inside the melting boundary between the melt-solidified part and the steel sheet, reirradiate the light beam having a high power density in a ring shape so as not to overlap the melting boundary and not to penetrate the stacked steel sheets, 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 reheated portion including the melting boundary,
Furthermore, by adjusting the joining conditions when forming the remelted solidified portion,
A center point is a point of 100 μm from the melting boundary of the overlapping surface of the metal plate to the central axis side,
In the rectangular plane region of 100 μm × 100 μm centered on the center point, which is parallel to the direction from the center point toward the center axis and perpendicular to the metal plate surface, the P concentration is mass%, and the metal plate Measured at a pitch of 1 μm along each direction parallel to the surface and perpendicular to the metal plate surface, thereby obtaining a measurement value of the P concentration at each of 100 × 100 measurement points,
Of the 100 measurement points × 100 measurement points, the average value of the measurement values of the P concentration at each of the 20 measurement points adjacent to each other in a row in a direction parallel to the metal plate surface is calculated as the metal plate surface. When calculating a partial average value of 81 × 100 pieces by repeating the calculation while shifting one point along each of the direction parallel to the direction and the direction perpendicular to the metal plate surface,
Among the partial average values, the number of the partial average values that exceeds twice the total average value of the measured values of the P concentration at each of the 100 points × 100 measurement points is 0 or more and 100 or less. A method for manufacturing a lap joint. The re-irradiation of the light beam is performed so that the shortest distance from the intersection of the steel sheet mating surface and the melting boundary to the remelted solidified portion is 0.5 to 1.0 mm in the cross section in the plate thickness direction of the plurality of steel plates. The method for producing a lap joint according to claim 4.   The method for producing a lap joint according to claim 4 or 5, wherein at least one steel plate having a surface treatment film formed thereon is used for the plurality of steel plates.

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