JP6443319B2 - Lap laser spot welded joint and method of manufacturing the welded joint - Google Patents

Description

  The present invention relates to a lap laser spot welded joint and a method for manufacturing the welded joint.

  Conventionally, resistance spot welding is performed for welding structural members of automobiles having flange portions. However, resistance spot welding has a problem that it takes time to weld, a problem that the pitch cannot be narrowed due to the diversion, and a problem that there is a spatial restriction due to the welding machine body. For this reason, in recent years, laser welding has been studied in addition to conventional resistance spot welding.

  FIG. 8 shows a cross-sectional view of the joint when welded by a conventional laser welding method. FIG. 8 shows a conventional laser welded joint 21 in which a plurality of steel plates 22 and 23 are laminated, and a molten portion 24 is formed on the surface of each steel plate 22 and 23. In this laser welded joint 21, when the plate gap between the steel plates 22 and 23 to be laminated is large, a melt-off occurs as shown in FIG. 8, and the bonding strength decreases. Therefore, a laser welding technique capable of ensuring high joint strength without reducing productivity has been studied.

  Patent Document 1 is a lap laser welded joint in which a plurality of plate members overlapping in a non-long surface area in a railway vehicle structure are laser welded so that the penetration into the outermost plate penetrates or does not penetrate, A technique of a lap laser welded joint is disclosed in which a laser beam is continuously irradiated in a long surface area while scanning so as to draw a spiral line exceeding one layer, and laser welding is performed in a spiral line shape. ing.

  Patent Document 2 discloses a laser welding method in which a laser is oscillated from a laser oscillator, the laser is focused by a condensing lens, irradiated to a plurality of stacked plates, and the stacked plurality of plates are welded. , Irradiating a plurality of plates with a laser, forming a molten pool on the plurality of stacked plates, and welding the plurality of stacked plates by irradiating the laser on the molten pool and flowing the molten pool A technique of a laser welding method for a vehicle body or the like is disclosed.

  In Patent Document 3, laser spot welding by a laser processing machine is used to weld each adjacent ring fitting end portion of a plurality of axially arranged inner cylinder rings constituting a gas turbine combustor inner cylinder via a corrugated plate. The technique of the laser spot welding method of the gas turbine combustor which makes the bead pattern of a lap weld part into the square spiral shape which transfers to the exterior from the center using is disclosed.

JP 2009-154194 A JP 2012-228715 A JP-A-1-321084

  FIG. 9 shows a cross-sectional view of a joint when welding is performed by a conventional laser spot welding method such as the techniques described in Patent Documents 1 to 3. FIG. 9 shows an example of a conventional laser spot welded joint 31 in which a plurality of steel plates 32 and 33 are stacked and a melted portion 34 is formed on the surface of each steel plate 32 and 33.

  Certainly, in the techniques described in Patent Documents 1 to 3 above, joints have been proposed in which welding strength is not reduced even by a large plate gap by welding with a laser.

  However, as shown in FIG. 9, in the techniques of Patent Documents 1 to 3 described above, there is a problem that a welding defect occurs in the final solidified portion or the like of the weld center during welding with a plate assembly including a high-tensile steel plate. there were.

  Therefore, an object of the present invention is to provide a lap laser spot welded joint that has high joint strength and prevents the occurrence of welding defects, and a method for manufacturing the joint.

  The inventors of the present invention have focused on the point that solid-solid cracking occurs at the center of the melted part when laser welding is performed on a high-alloy high-tensile steel plate. And as a result of intensive studies, the inventors adjust the lengths of the major axis and the minor axis of the molten part according to the total thickness of the steel plates to be welded, and adjust the length to a certain range. Thus, it has been found that a lap laser spot welded joint can be obtained which has high joint strength and prevents the occurrence of welding defects.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] a plurality of laminated steel plates;
A molten part that penetrates at least one of the plurality of steel plates and joins each of the plurality of steel plates;
With
The total thickness T of the plurality of steel plates is 2 mm ≦ T ≦ 4 mm,
The surface of the molten part on the outermost surface of the plurality of steel plates has an elliptical shape,
The length of the elliptical minor axis S is 2T ≦ S ≦ 3T,
The length of the elliptical major axis L is 3T ≦ L ≦ 4.5T,
A lap laser spot welded joint in which S / L, which is the ratio of S to L, is 0.50 or more and 0.80 or less.
[2] At least one of the plurality of steel plates is
% By mass
C: 0.07% to 0.25% or less,
P + S: less than 0.03%,
Mn: 1.8% to 3.0%,
Si: more than 1.2% and 1.8% or less, having a component composition consisting of the balance Fe and inevitable impurities,
The lap laser spot welded joint according to [1], wherein the tensile strength TS is 980 MPa or more.
[3] A method for producing a lap laser spot welded joint according to [1] or [2],
Laminating multiple steel plates,
A method of manufacturing a lap laser spot welded joint including a step of obtaining a joint by performing laser spot welding in which a laser beam is irradiated in a spiral manner on the surface of the outermost steel sheet so as to form an elliptical melted part in each steel sheet.

  According to the present invention, it is possible to obtain high joint strength and to prevent occurrence of welding defects.

It is a figure which shows an example of the overlap laser spot welded joint of this invention. It is a figure which shows an example of the overlap laser spot welded joint of this invention. (A) is a figure for demonstrating the fusion | melting part of the conventional overlap laser spot welding joint, (B) is a figure for demonstrating the fusion | melting part of the overlap laser spot welding joint of this invention. It is a figure for demonstrating the fusion | melting part of the overlap laser spot welding joint of this invention. It is AA sectional drawing of FIG. It is a figure for demonstrating the welding method of the overlap laser spot welding joint of this invention. It is a figure for demonstrating the position of the fusion | melting part of the overlap laser spot welding joint of this invention. Sectional drawing of the coupling at the time of welding by the conventional lap laser welding method is shown. Sectional drawing of the coupling at the time of welding by the conventional overlap laser spot welding method is shown.

  Hereinafter, the present invention will be described with reference to the drawings. In addition, this invention is not limited by this embodiment.

<Laminated laser spot welded joint>
1 and 2 are views showing an example of the lap laser spot welded joint 1 of the present invention. 1 and 2, as an example of a part to which the lap laser spot welded joint 1 of the present invention is applied, a flange part of a frame part having a substantially hat-shaped cross section and other parts arranged opposite to the flange part. Fig. 3 shows an automobile skeleton part in which a frame part or a panel part is welded to form a closed section. Hereinafter, the lap laser spot welded joint 1 of the present invention will be described using the expression that the steel plates 2 and 3 are laminated, including those in which such a flange portion and a frame component or a panel component are welded. To do. Moreover, although the case where the two steel plates 2 and 3 are laminated | stacked is demonstrated to the example as the overlap laser spot welding joint 1 of this invention below, three or more steel plates may be laminated | stacked.

  Below, the case where two steel plates are laminated is explained. As shown in FIGS. 1 and 2, the lap laser spot welded joint 1 of the present invention has a plurality of steel plates 2 and 3 laminated, and at least one of the plurality of steel plates 2 and 3 penetrates the steel plates. The molten part 4 which joins the some steel plates 2 and 3 is formed, and the dimension of the surface of the molten part 4 is adjusted to the specific range, It is characterized by the above-mentioned. Specifically, first, in the lap laser spot welded joint 1 of the present invention, the total thickness T of the laminated steel plates 2 and 3 is 2 mm ≦ T ≦ 4 mm. Moreover, about said fusion | melting part 4, the fusion | melting part 4 surface on the outermost layer surface of the some steel plates 2 and 3 has elliptical shape. For this ellipse, the minor axis S is 2T ≦ S ≦ 3T, the major axis L is 3T ≦ L ≦ 4.5T, and the ratio of S to L is 0.50 or more. 80 or less. Since it is such a structure, while obtaining high joint strength, generation | occurrence | production of a welding defect can be prevented. In the present invention, the high bonding strength refers to a peel strength based on an L-shaped tensile test in which steel plates bent into L-shapes are overlapped and welded as shown in FIG. 2 and a tensile load is applied from both sides. It means that it is 4.0 kN or more.

[Multiple steel plates]
In the lap laser spot welded joint 1 of the present invention, a plurality of steel plates 2 and 3 are laminated. For the plurality of steel plates 2 and 3, even if the tensile strength TS of at least one steel plate is 980 MPa or more, the lap laser spot welded joint 1 obtains high joint strength and prevents the occurrence of welding defects. Can do. Therefore, in the lap laser spot welded joint 1 of the present invention, it is preferable that at least one of the plurality of steel plates 2 and 3 is a high-tensile steel plate having a tensile strength TS of 980 MPa or more. The plurality of steel plates may be the same type and the same shape, or different types and different shapes.

  Moreover, the steel plate of the lap laser spot welded joint 1 of the present invention is, for example, in mass%, C: more than 0.07% and 0.25% or less, P + S: less than 0.03%, Mn: 1.8% or more 3 0.0% or less, Si: more than 1.2% and 1.8% or less, and having a component composition consisting of the balance Fe and inevitable impurities. Hereinafter,% in each component composition refers to mass%.

(C: 0.07% to 0.25% or less)
If the C content is 0.07% or less, the effect of precipitation strengthening may not be obtained. On the other hand, if the C content exceeds 0.25%, coarse carbides may be precipitated, and desired high strength and workability may not be ensured. Therefore, the C content is preferably 0.07% to 0.25% or less.

(P + S: less than 0.03%)
If the total amount (P + S) of the P content and the S content is 0.03% or more, the ductility may be lowered, and desired high strength and workability may not be ensured. Therefore, the total amount (P + S) of the P content and the S content is preferably less than 0.03%.

(Mn: 1.8% to 3.0%)
If the Mn content is less than 1.8%, sufficient hardenability cannot be ensured and coarse carbides may precipitate. On the other hand, if the Mn content is 3.0% or less, the grain boundary embrittlement susceptibility increases and the toughness and cold cracking resistance may deteriorate. Therefore, the Mn content is preferably 1.8% or more and 3.0% or less. The Mn content is more preferably 2.5% or less.

(Si: 1.2% to 1.8% or less)
If the Si content is 1.2% or less, the effect of increasing the strength of the steel by solid solution may not be sufficiently obtained. On the other hand, when the Si content exceeds 1.8%, the hardening of the weld heat-affected zone increases, and the toughness and cold crack resistance of the weld heat-affected zone may deteriorate. For this reason, the Si content is preferably 1.2% to 1.8%. The Si content is more preferably 1.5% or less.

(Remainder Fe and inevitable impurities)
The balance other than the above component composition is Fe and inevitable impurities. Inevitable impurities include Al: 0.0015 to 0.0050%, N: 0.002 to 0.005%, and the like.

[Melting part]
In the lap laser spot welded joint 1 of the present invention, a fusion zone 4 is formed that penetrates at least one of the steel plates 2 and 3 and joins the steel plates 2 and 3 together. FIG. 3 (A) is a view for explaining a melting portion 14 of a conventional lap laser spot welded joint, and FIG. 3 (B) is a view for explaining a melted portion 4 of the lap laser spot welded joint 1 of the present invention. FIG. FIG. 4 is a view for explaining the melting portion 4 of the lap laser spot welded joint 1 of the present invention, and FIG. 5 is a cross-sectional view taken along the line AA of FIG.

  As shown in FIG. 3A, when the surface shape of the melting portion 14 of the lap laser spot welded joint is circular, a tensile stress (from the outer peripheral side of the melting portion is applied to the central portion 15 which becomes the final solidified portion of the melting portion 14 (FIG. 3A). 3 (A), the force in the direction of arrow Fa) is concentrated and solidification cracks may occur and welding defects may occur.

  On the other hand, in the present invention, as shown in FIG. 3B, the surface shape of the melted portion 4 is a specific elliptical shape, so that the tensile stress from the outer peripheral side of the melted portion 4 (in FIG. 3B) Force in the direction of arrow Fb) can be dispersed without being concentrated in one central portion 5, the occurrence of solidification cracks can be suppressed, and the occurrence of welding defects can be prevented. Note that not only when welding is performed by irradiating the laser from the outer periphery toward the central portion 5, but also when welding is performed by irradiating the laser from the central portion 5 toward the outer periphery, cooling is performed from the metal on the outer peripheral side of the melting portion 4. Therefore, the central portion 5 becomes the final solidified portion.

  Under such a technical idea, in the lap laser spot welded joint 1 of the present invention, the total thickness T of the plurality of steel plates 2 and 3 is set to 2 mm ≦ T ≦ 4 mm, and the surface of the fusion zone 4 shown in FIG. The ellipse has a short axis S of 2T ≦ S ≦ 3T, a long axis L of 3T ≦ L ≦ 4.5T, and S / L of 0.50 or more and 0.80 or less. It is characterized by being. As a result, as shown in FIG. 5, in the lap laser spot welded joint 1 of the present invention, a high joint strength is obtained even when a large plate gap exists between the steel plates 2 and 3, and welding defects are generated in the fusion zone 4. Can be prevented.

(Short axis S: 2T ≦ S ≦ 3T)
If the length of the minor axis S of the ellipse is less than 2T, the molten metal is insufficient and a weld defect is likely to occur. That is, in laser welding, a lot of molten metal is lost as spatter at the start of welding. Therefore, when the area of the molten part 4 is small, the molten metal in the molten pool is insufficient, and welding defects are likely to occur. On the other hand, when the length of the short axis S is more than 3T, the amount of molten metal increases, and the burnout occurs. Therefore, the length of the short axis S is 2T or more and 3T or less.

(Long axis L: 3T ≦ L ≦ 4.5T)
If the length of the major axis L of the ellipse is less than 3T, the molten metal is insufficient, and welding defects are likely to occur. On the other hand, when the length of the long axis L exceeds 4.5T, the melt-out occurs. Therefore, the length of the long axis L is 3T or more and 4.5T or less.

(S / L: 0.50 to 0.80)
When the ratio (S / L) of the minor axis S to the major axis L is less than 0.50, the surface shape of the melted part 4 becomes too long and welding defects occur along the major axis direction of the melted part 4. To do. On the other hand, if the S / L exceeds 0.80, the melted part 4 becomes nearly circular, and a spot-like weld defect occurs at the center of the melted part 4. Such a weld defect also causes a poor appearance. Therefore, S / L is 0.50 or more and 0.80 or less. Preferably, S / L is 0.60 or more. Preferably, S / L is 0.70 or less.

<Manufacturing method of lap laser spot welded joint>
Next, the manufacturing method of the lap laser spot welded joint 1 of the present invention described above will be described. FIG. 6 is a view for explaining a welding method of the lap laser spot welded joint 1 of the present invention. As shown in FIG. 6, the lap laser spot welded joint 1 of the present invention is spirally formed on the surface of the outermost steel plate 2 so that a plurality of steel plates 2 and 3 are laminated and a molten portion 4 is formed in the steel plates 2 and 3. It can be obtained by performing laser spot welding to irradiate the laser beam L in a shape. The steel plates 2 and 3 have, for example, the above-described component composition and a tensile strength TS of 980 MPa or more.

  In the above laser spot welding, the laser beam L is continuously irradiated while scanning so as to draw a spiral line or an ellipse exceeding a single layer. FIG. 6 shows only the case of a spiral wire as an example. In this case, irradiating the laser beam L spirally toward the outer periphery after welding the center of the ellipse prevents excessive heat conduction to the center 5 (see FIG. 3B), This is preferable in order to prevent melting away.

  As the laser beam, a fiber laser, a disk laser, or the like can be used. For example, a beam diameter: 0.4 to 1.2 mm, a laser output: 2.0 to 5.0 kW, a focal position: on the outermost surface of the steel plate It can be 30 mm above the steel sheet outermost layer surface, welding speed: 2.0-5.0 m / min.

  Under such manufacturing conditions, the total thickness T of the plurality of steel plates 2 and 3 is 2 mm ≦ T ≦ 4 mm, and the short axis S of the elliptical shape of the surface of the melted part 4 is 2T ≦ S ≦ 3T. A lap laser spot welded joint in which the axis L is 3T ≦ L ≦ 4.5T and the S / L is 0.50 or more and 0.80 or less can be obtained.

  Next, a preferred welding position in the lap laser spot welded joint of the present invention will be described with reference to FIG. FIG. 7 is a view for explaining the position of the melting portion 4 of the lap laser spot welded joint 1 of the present invention. FIG. 7A is a top view showing a combination of two steel plates 2 and 3, and FIG. 7B is a cross-sectional view taken along the line BB in FIG. 7A. In the description of FIG. 7, the two steel plates 2 and 3 are also referred to as a flange portion and other frame components or panel components, respectively.

As shown in FIGS. 7A and 7B, the position coordinates of the end portion of the contact position between the flange portion and another frame component or panel component are set to 0, and the flange outer end side of the flange portion is set to (−), The vertical wall side in a substantially hat shape (only a part of the shape is shown in the drawing) is represented by a coordinate system with (+), and the thickness of the substantially hat-shaped frame component is t (mm). At this time, by applying the one-side welding method at the welding position X represented by the following formula (1) and performing welding, the total plate thickness as shown in FIGS. 1 and 2 is 2 to 4 mm. The peel strength of the L-shaped tensile test piece having a flange length of 50 mm can be made 4.0 kN or more by stacking two sheets.
-2t ≧ X ≧ −4t (1)
The reason for setting X as in the above equation is as follows.

  If the welding position X is closer to the flange contact end than -2t, the weld metal part is more likely to break during the tensile test, and the peel strength may be lowered. On the other hand, if the welding position X is further away from the flange end than −4t, the moment applied to the melted portion tends to increase, and the peel strength may decrease. Therefore, it is preferable to set the welding position X as shown in the above formula (1).

  As described above, according to the present invention, a laser spot that does not cause a weld defect on the front and back surfaces of a laminated steel sheet including a high-strength steel sheet does not deteriorate even in a large sheet gap. A welded joint can be obtained. By using such a joint, it is possible to obtain an automotive framework component having high corrosion resistance.

  As described above, as an example of a part to which the lap laser spot welded joint 1 of the present invention is applied, a flange part of a frame part having a substantially hat-shaped cross section, and other parts arranged opposite to the flange part. FIGS. 1 and 2 show frame parts for automobiles in which the frame parts or panel parts are welded to form a closed cross section.

  Hereinafter, the present invention will be described based on examples.

  Tensile strength is 980 MPa class (C: 0.13%, Si: 1.40%, Mn: 2.2%, P: 0.015%, S: 0.002%), 1180 MPa class (C: 0.00%). 13%, Si: 1.40%, Mn: 2.2%, P: 0.012%, S: 0.001%), plate thickness of 1.2 mm, 1.6 mm, 2.0 mm, 50 mm width Using a steel plate, the L-shaped cross-sectional shape was bent as shown in FIG. Two L-shaped steel plates of the same steel type and the same plate thickness were overlapped, and then laser-welded at a plurality of locations in the longitudinal direction of the overlapping portion of the flange to prepare a test piece. The board gap was 0.4 mm. The above-mentioned 980 MPa class steel sheet has a mass composition of C: 0.13%, P: 0.015%, S: 0.002%, Mn: 2.2%, Si: 1.40%. The 1180 MPa class steel sheet is composed of the balance Fe and inevitable impurities, and the component composition is C: 0.13%, Si: 1.40%, Mn: 2.2%, P: It has 0.012%, S: 0.001%, and consists of the balance Fe and inevitable impurities.

  At this time, as shown in Table 1, the welding bead conditions for forming the flange portion by laser welding are set such that the welding position coordinate is 0 at the end of the contact position between the flange portions of the test piece, and the outside of the flange of the test piece. The welding position X, the minor axis S, and the major axis L of the surface of the melted part were variously changed in the coordinate system where the end side was (−) and the vertical wall side of the test piece was (+).

  A fiber laser was used for laser welding, the laser output was 4.5 kW, the beam diameter at the focal position was fixed at 0.6 mmφ, the welding speed and the processing point distance were adjusted, and the penetration was adjusted. The welding was performed in the atmosphere.

  Table 1 shows the results of the tensile test and the appearance determination when the test piece size is a test piece width of 50 mm, a lateral wall length of 120 mm, and a flange width of 30 mm.

  The tensile test was performed at a speed of 10 mm / min based on JIS Z3136, and the appearance was determined by measuring the irregularities on the front and back surfaces of the steel plates laminated with a 3D measuring instrument.

  Further, the peel strength was measured by an L-shaped tensile test in which steel plates bent in an L shape were overlapped and welded as shown in FIG. 2 and a tensile load was applied from both sides. When the peel strength was 4.0 kN or higher, it was judged as having passed the test as having high bonding strength.

  Moreover, the welding defect was evaluated by the result of the unevenness | corrugation measurement by the 3D measuring device of the laminated steel plate front and back, and the penetration test.

  As shown in Table 1, the test piece of the example of the present invention had a peel strength of 4.0 kN or more, and no welding defect occurred.

  On the other hand, among the test pieces of the comparative examples, No. In No. 2, the length of the minor axis S was less than 2T, the peel strength was less than 4.0 kN, and welding defects occurred.

  No. In No. 3, the length of the long axis L exceeded 4.5T, and a welding defect occurred.

  No. In No. 6, S / L exceeded 0.80, and welding defects occurred.

  No. In No. 8, the length of the short axis S was less than 2T, the peel strength was less than 4.0 kN, and welding defects occurred.

  No. In No. 9, the length of the long axis L exceeded 4.5T, and a welding defect occurred.

  No. In No. 10, the length of the long axis L was less than 3T, the S / L was more than 0.8, the peel strength was less than 4.0 kN, and welding defects occurred.

  No. In No. 14, the length of the short axis S was less than 2T, and a weld defect occurred.

  No. No. 16 had an S / L exceeding 0.80, and a weld defect occurred.

  No. No. 17 had an S / L of less than 0.50, and a weld defect occurred.

  No. No. 20, S / L exceeded 0.80, and a weld defect occurred.

  No. In No. 22, the length of the long axis L was less than 3T, the S / L exceeded 0.80, the peel strength was less than 4.0 kN, and a weld defect occurred.

  No. No. 24 had an S / L exceeding 0.80, a peel strength of less than 4.0 kN, and a weld defect occurred.

1, 21, 31 Lap laser spot welded joint 2, 3, 22, 23, 32, 33 Steel plate 4, 14, 24, 34 Melting part 5, 15 Center part L Laser beam

Claims (3)

A plurality of laminated steel plates;
A molten part that penetrates at least one of the plurality of steel plates and joins each of the plurality of steel plates;
With
The total thickness T of the plurality of steel plates is 2 mm ≦ T ≦ 4 mm,
The surface of the molten part on the outermost surface of the plurality of steel plates has an elliptical shape,
The length of the minor axis S of the ellipse is 2T ≦ S ≦ 3T;
The length of the major axis L of the ellipse is 3T ≦ L ≦ 4.5T;
A lap laser spot welded joint in which S / L, which is the ratio of S to L, is 0.50 or more and 0.80 or less. At least one of the plurality of steel plates is
% By mass
C: 0.07% to 0.25% or less,
P + S: less than 0.03%,
Mn: 1.8% to 3.0%,
Si: more than 1.2% and 1.8% or less, having a component composition consisting of the balance Fe and inevitable impurities,
The lap laser spot welded joint according to claim 1, wherein the tensile strength TS is 980 MPa or more. A method for producing a lap laser spot welded joint according to claim 1 or 2,
Laminating multiple steel plates,
Manufacture of lap laser spot welded joints, including a step of obtaining joints by performing laser spot welding that irradiates laser in a spiral shape or an elliptical shape exceeding a single layer on the surface of the outermost steel sheet so as to form a fusion zone in each steel sheet Method.

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