JP4016911B2 - Laser welding method of high strength steel - Google Patents

Description

  The present invention relates to a laser welding method for high-strength steel in which high-tensile steel sheets having a predetermined thickness are overlapped and welded with each other by a laser beam. In particular, the present invention relates to a laser welding method suitable for welding a flange portion such as a shock absorbing member for automobiles.

  In general, on both sides of the front part of a car body, a shock absorbing member having a closed cross-sectional structure called a front side member is provided, which is efficiently crushed at the time of a collision from the front and absorbs the energy of the collision to ensure the safety of the passenger. It has a structure. Conventionally, in order to enhance the energy absorption characteristics at the time of collision of such shock absorbing members, investigation of the weld shape required for controlling the crush mode at the time of collision, optimizing the welding position, or improving the weld strength Many proposals have been made. For example, in a method of laser welding a steel plate having a hat cross-sectional shape and a flat steel plate constituting a shock-absorbing member having a closed cross-sectional structure at a flange, a method of welding by limiting the melting width of laser welding to a certain range has been proposed. (See Patent Document 1). According to this method, there is no problem such as a failure of the crushing mode due to fracture from the welded part at the time of axial impact such as spot welding, and it is said that excellent shock absorption characteristics can be obtained. And it is supposed that an impact-absorbing characteristic can be improved further by manufacturing an impact-absorbing member using a steel plate of 590 MPa or more.

  However, the strength in the peeling direction of the superposed laser welded portion of a high-tensile steel plate of 440 Mpa or more decreases as the material strength increases. For example, when the impact absorbing member is crushed, the welded portion breaks, and sufficient impact absorption is achieved. There is a possibility that the characteristics cannot be exhibited. FIG. 7 shows the relationship between the material strength and the peel strength of the overlapping laser weld. FIG. 7 shows the peel strength of a welded test piece formed by stacking two 1.2 mm-thick steel plates and irradiating a laser beam from the upper surface, and performing a peel test by a test method described later. As plotted. With mild steel (SPCD) (◯) having a tensile strength of about 270 MPa, the peel strength per unit length is 270 N / mm (225 MPa), which is a value close to the base material strength. For example, the tensile strength is as high as 590 MPa. In tensile steel (SPFC590Y) (♦), the peel strength is 55 to 75 N / mm (46 to 63 MPa), which is only a very low value compared to the base material strength.

This is because a higher bending strength is applied to the weld bead portion than a low-strength material because the higher-strength material is less likely to be deformed. In FIG. 8, a low strength material (a) having a low tensile strength, such as mild steel, and a high strength material (b), such as a high strength steel, are overlapped and laser welded, and a peel test is performed with the same load F. The state of deformation of the peel test piece when performed is schematically shown. B is a weld bead portion, and C is a bent portion obtained by bending a weld test piece by 90 ° for a peel test. That is, under the same load condition, the length of the moment arm with the weld bead B as a fulcrum is as short as L _{1 in} the low-strength material (a) but as long as L _{2 in the} high-strength material (b). This is because a bending stress larger than that of the low-strength material (a) acts on the weld bead portion B in some cases. Further, since the vicinity of the laser-welded weld bead B is quenched and quenched and hardened, the hardness is higher than the base material and the ductility is reduced. Therefore, the tolerance for deformation is reduced at the weld bead boundary portion A that is most greatly deformed at the time of peeling deformation. Further, as shown in FIG. 9, undercut D occurs in the vicinity of the base material of the bead due to the shrinkage of the weld bead B due to cooling. For this reason, since the thickness t of the undercut D portion is smaller than the thickness t _{0 of the} base material, this portion often becomes the weakest portion E when subjected to a peeling load. For this reason, when the test piece is pulled to both sides in the peel test, the test piece is broken at the weakest portion E having a thin plate thickness (see FIG. 10), and this load becomes the peel strength. As described above, in the conventional high-strength steel superposition laser welding method, only a peel strength much lower than the material strength could be obtained.
JP 2002-79388 A

  An object of the present invention is to provide a laser welding method for improving the peel strength of a welded portion in superposition laser welding of high-strength steel in view of the above-described problems of the prior art.

  In superposition laser welding of high-tensile steel, the inventors eliminated the undercut formed in the weld bead and increased the thickness of the weld bead boundary, which is the most deformed at the time of delamination deformation. We focused on improving the peel strength of steel.

That is, the high-strength steel laser welding method of the present invention is a high-tensile steel laser welding method in which two high-strength steels of equal thickness are overlap welded with a laser beam, and a filler is supplied to the overlap weld. The surplus height of the weld bead cross section is 20 to 120% of the thickness of the high-tensile steel, and the surplus height is formed in the weld bead portion. Here, the height of the surplus is determined by the height up to the highest point of the bead protruding from the upper surface of the upper plate of the bead unit where the upper plate and the lower plate overlapped with each other and the lower surface of the lower plate. It is the sum of the height to the highest point of the bead.

By forming a surplus in the weld bead portion, it is possible to increase the thickness of the weld bead boundary portion that is deformed the most at the time of peeling deformation without causing a shape defect such as an undercut due to cooling in the bead portion. Therefore, the peel strength of high-strength steel can be improved. And if the extra height of a weld bead part exists in this range, the tensile strength of mild steel (SPCD), ie, the peel strength of 270 MPa or more, can be obtained.

  In the laser welding method for high strength steel of the present invention, the high strength steel is preferably a high strength steel having a tensile strength of 440 MPa or more.

  By using high-tensile steel having a tensile strength of 440 MPa or more, for example, the impact absorbing energy of the impact absorbing member can be increased, and the collision safety can be further improved along with the weight reduction of the automobile. FIG. 11 schematically shows a load-displacement curve measured when the impact absorbing member is axially crushed at high speed. In FIG. 11, the case where the impact-absorbing member having the same shape is formed of high-strength steel indicated by a solid line and the case where mild steel indicated by a dotted line is used are written and compared. The shock absorption energy is obtained by integrating the load with the displacement. The larger the value, the higher the shock absorption capacity of the shock absorption member, and it is possible to increase the shock absorption energy by using high strength steel instead of mild steel. I understand.

  The laser welding method for high-strength steel according to the present invention is a laser welding method for high-strength steel that is lap welded by a laser beam, wherein filler is supplied to the lap weld to form a surplus in the weld bead. And

  The laser welding method of the present invention can be carried out using a laser welding apparatus for welding a general steel plate as shown in FIG. In a laser welding apparatus 10 for superposing and welding high-tensile steel plates A and B, 11 is a laser welding torch of a laser oscillator R, and a condensing lens 12 for condensing a laser beam at the tip of the laser welding torch 11. Is provided. Further, the bracket 15 is provided so that the lead-out end of the linear filler wire 14 led out from the storage part 13 wound around and stored in the vicinity of the tip of the laser welding torch 11 is supplied to the tip of the condenser lens 12. It is supported so that it can be inserted through. Here, since the filler wire 14 is only required to be able to form a surplus in the cross-sectional shape of the weld bead, the filler material is not particularly limited as long as it is not extremely weaker than the base material. For example, YGW17, YGW23, etc. prescribed | regulated to JIS can be used conveniently.

FIG. 2 is a sketch of a cross section of a weld bead portion in which a surplus is formed on high-tensile steel (SPC440) using the laser welding apparatus 10. The high-tension steel plates A and B having a plate thickness t ₀ are fused with the weld bead B. By supplying the filler, a surplus of height h ₁ is formed on the upper surface of the bead portion B, and a surplus of height h ₂ is formed on the lower surface. The size of the surplus (hereinafter referred to as surplus height) can be set to a desired height by adjusting the welding conditions such as the welding speed and the filler wire supply speed. Here, when the total surplus height H of the weld bead portion B is the sum of the surplus heights, that is, H = h ₁ + h ₂ , the total surplus height H is obtained from the welded high-tensile steel. It is desirable that it is 20 to 120% of the plate thickness t ₀ . When the total surplus height H is less than 20%, it is not possible to obtain a sufficient improvement in peel strength due to surplus. On the other hand, if it exceeds 120%, the risk of dripping of the molten metal during laser welding increases, which is not preferable. More preferably, it is 75 to 120%.

  FIG. 3 is a sketch of a cross section of a weld bead portion in which the weld portion shown in FIG. 2 is pulled and broken by a peel test. It can be seen that the bead portion B is not deformed and is ductile fractured at the base material portion of the high-tensile steel plate B.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  SPFC440, SPFC590Y, and SPFC980Y having a thickness of 1.2 mm and a tensile strength of 440 MPa or more were prepared as high-tensile steels as test materials.

  For laser welding, a YAG laser was used, the laser oscillation output was 4 kW, the welding speed was 1 m / min, and a filler wire (YGW17) having a diameter of 1.2 mm was supplied to the tip of the condenser lens. Here, the supply speed of the filler wire was changed within a range of 0.5 to 2.0 m / min, and the extra height H of the beads of each test material was adjusted to a desired height. The laser beam was focused on the steel plate and the focused spot was 2.35 mm in diameter.

First, two test pieces having the shape shown in FIG. 4 (thickness 1.2 mm × 100 mm × 60 mm, chuck width 40 mm) were cut out from each steel plate. Next, these two test pieces are overlapped so as not to cause a gap, and a bead B is formed by laser welding in parallel to the end K at a position 15 mm away from the end K having a width of 60 mm under the above conditions. A weld specimen TP ₁ was obtained. The laser welding length was 50 mm.

Each welded test piece TP ₁ obtained was formed into a peel test piece TP ₂ shown in FIG. The peel test piece TP ₂ is a T-shape that can be pulled by bending two steel plates joined to the weld test piece TP ₁ outward at 90 ° with a bend radius of 3 mm at a position 21 mm from the weld bead B. This is a test piece. It was determined tensile fracture load peeling direction at the peeling test piece TP ₂ tensile tester. The breaking load was divided by the length of the welded part (50 mm) to obtain peel strength F (N / mm). The tensile speed of the peel test was constant at 6 mm / min. Note that the number of repetitions under the same conditions was 3, and the average value was evaluated. The results are shown in FIG.

  In FIG. 6, the horizontal axis is the extra height H of each weld specimen, and the vertical axis is the peel strength F (N / mm) per unit of the weld. 6 represents the type of steel sheet, a (■) is SPFC 440 (tensile strength 440 MPa), b (♦) is SPFC 590Y (590 MPa), and c (Δ) is a reference example. It is a measurement result of SPFC980Y (980 MPa). Here, a dotted line f having a peel strength F of 259 N / mm is a peel strength of mild steel (SPCD) when no surfacing with a filler is performed.

  It can be seen that the peel strength is improved in proportion to the increase in surplus height for any steel plate. In particular, with respect to SPFC440 of a and SPFC590Y of b, the improvement in peel strength F due to the increase in the height H is remarkable. In SPFC440 of a, when the surplus height H exceeds 0.3 mm (25% of the steel plate thickness), a peel strength higher than that of mild steel can be obtained, and in SPFC590Y of b, 0.9 mm (75 %), A peel strength higher than that of mild steel can be obtained.

  By using the laser welding method of the present invention, for example, even if the impact absorbing member formed of mild steel (SPCD) is reduced in weight using thin high-strength steel, the peel strength of the welded portion is reduced. The peel strength of the mild steel material can be made higher, and the impact absorption characteristics can be further improved.

  According to the laser welding method of the high strength steel of the present invention, the peel strength of the laser welded portion can be improved, so that the weight of the shock absorbing member such as the front side panel of the automobile body is reduced and the shock absorbing characteristics are also improved. Can be made.

It is a principal part block diagram explaining the welding apparatus of superposition laser welding. It is a cross-sectional sketch of the welding bead part of the superposition laser welding which formed the surplus in the welding bead part. It is a cross-sectional sketch which shows the weld bead part of FIG. 2 fractured | ruptured by the peeling test. It is a figure which shows the shape of the welding test piece of superposition laser welding. It is a figure which shows the shape of a peeling test piece. It is a figure which shows the relationship between the surplus height and peeling strength of each high-tensile steel. It is a figure which shows the relationship between the tensile strength and peeling strength of each steel material. It is a figure which shows the mode of a deformation | transformation of the test piece at the time of a peeling test. (A) shows the case of a low-strength material, and (b) shows the case of a high-strength material. It is a cross-sectional sketch of the weld bead part of the superposition laser welding used as the prior art which does not use a filler. 10 is a cross-sectional sketch showing the weld bead portion of FIG. 9 broken by a peel test. It is a schematic diagram of the load-displacement curve of an impact-absorbing member.

Explanation of symbols

10: laser welding apparatus 11: laser welding torch 12: condenser lens 14: filler wire A: weld bead boundary B: weld bead C: bend D: undercut E: weakest portion h _1, h _2: excess weld Height R: Laser oscillator

Claims (2)

In a high-strength steel laser welding method in which two high-strength steels of equal thickness are overlap-welded with a laser beam,
A filler is supplied to the overlap welded portion, and the surplus height is formed in the weld bead portion so that the surplus height of the weld bead cross section is 20 to 120% of the thickness of the high-tensile steel. Laser welding method for high strength steel.
Here, the height of the surplus is the height from the upper surface of the upper plate of the bead portion where the upper plate and the lower plate that are overlapped are fused to the highest point of the bead and the lower surface of the lower plate. Is the sum of the height of the bead protruding from the height to the highest point.   The high-strength steel laser welding method according to claim 1, wherein the high-tensile steel is a high-tensile steel having a tensile strength of 440 MPa or more.

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