JP4841970B2 - Lap laser welding method - Google Patents

Description

  The present invention superimposes a plurality of plate materials, and irradiates laser light from the superposition direction in the vicinity of the end portion of the superposed plate material, while moving the laser light along the end portion, The present invention relates to a lap laser welding method for welding together.

As a body panel of an automobile, a cross section having a flange portion 2 and a bent portion 3 formed of a thin plate material made of high-strength steel, as shown in FIG. A frame member in which the overlapped portion is joined by spot welding or the like, and the flange portion 2 and the plate member 4 or the plate member 4 interposed between the flange portions as shown in FIGS. A frame member in which they are similarly joined, and a frame member in which a plurality of structural members 1 are stacked in the same direction as shown in FIG. 1d are used.
When laser welding is used to join the above-mentioned overlapping parts, the joint strength is high by continuous welding and the bead width is narrow. Therefore, the design of the joints is free compared to spot welding and arc welding used in the past. There are advantages such that the degree is large, the width of the flange portion is narrowed, and the structural member can be reduced in size and weight.

Conventionally, in laser welding of lap weld joints of plate materials, if the gap between the plate materials in the overlapped portion varies, the welding quality deteriorates, and therefore, the focus has been on optimizing the gap between the plate materials.
For example, pressing the roller from the laser irradiation side to the overlapping part, moving the roller together with the laser beam, pressing one plate against the other plate and adjusting the distance between them, or welding the plates together Patent Document 1 discloses that the flange portions are sandwiched from both sides by a pair of rollers and welded in the same manner.

  However, in structural members made of high-strength steel, the width of the flange portion is shortened to reduce the weight of the member, and the vicinity of the end of the overlapped portion is welded so as to melt to the back side of the lower plate material to further increase the joint strength If it tried to do so, it turned out that the weld solidification crack becomes a problem in the research of the present inventors.

That is, as shown in FIG. 2a, the laser beam is irradiated from the overlapping direction, that is, the direction intersecting the flange, to the flange portion of the frame member in which the both side flange portions of the structural member having a hat-shaped cross section are mutually overlapped. When starting welding from the longitudinal end of the flange while welding so as to melt up to the lower plate material back, as shown in FIG. Will occur.
Further, as shown in FIG. 3, even when the welding start point 5 is not the longitudinal end portion of the flange and the point separated from the end portion by a predetermined distance is the welding start point, the central portion of the welded portion 6 is not welded after welding. Swelling and cracks 7 may occur. In the figure, 8 is a laser welding head.

  This is because when welding is performed by irradiating a laser beam so as to melt to the back surface of the stacked lower plate material, the flange on the end side from the melted portion is solidified until the melted portion formed by the laser beam irradiation is solidified. The part is separated from the flange body. At this time, if the width of the part is small, it is considered that the part expands due to heat conduction from the welded portion, the part is deformed, the weld bead in the middle of solidification is pulled, and a crack occurs during solidification.

Conventionally, Patent Literature 2 and Patent Literature 3 are known as techniques for preventing cracking and deformation of a welded portion in laser welding.
In Patent Document 2, when laser welding a lap joint of a member made of high carbon steel and a member made of stainless steel or the like, shrinkage cracking occurs due to shrinkage stress generated at the time of melt solidification, and the cracking, It describes that the position of the joint portion is devised to prevent the tensile stress from being applied to the melted portion.

Further, in Patent Document 3, when one plate member is abutted with the other narrow plate member and the butt portion is laser-welded, the narrower plate member is deformed by heat and the butt portion is deformed. Since the interval is widened, it is described that the butt portions of both plate materials are temporarily attached to prevent deformation of the plate materials.
However, these documents do not mention at all the above solidification cracks and solutions for the same.

As described above, in laser welding of a thin plate made of high-strength steel, the width of the flange portion is shortened in order to reduce the weight of the member, and the vicinity of the end of the overlapped portion is melted to the lower surface of the lower plate material. In the past, it has not been known that weld solidification cracking occurs when welding is performed so as to increase the joint strength.
JP-A-8-90264 Japanese Patent Laid-Open No. 11-245065 JP 59-215288 A Japanese Patent Laid-Open No. 03-281078

  Therefore, in the present invention, in view of the situation as described above, at least one plate material is formed by superimposing a plurality of plate materials made of high-strength steel, and in the vicinity of the end portion of the superposed plate material, Laser welding without solidification cracks as described above when laser beams are moved along the end portion while irradiating laser beams from the overlapping direction so as to melt to the back surface and the stacked plate materials are welded together. It is an object to provide a method.

In order to solve the above problems, the present invention is characterized as follows.
The invention of the lap laser welding method according to claim 1 is such that at least one of the plate members is made by superimposing a plurality of plate members made of high-strength steel, and the overlapped plate members are melted to the back surface of the stacked lower plate member. In the laser welding method, the laser beam is moved along the end portion of the superposed plate materials while continuously irradiating the laser beam from the superimposing direction to form a welded portion, and the superposed plate materials are welded together. ,
At least one plate material among the plurality of plate materials is formed of a thin plate material, and is formed of a structural member having a bent portion and a flange portion having a width of 8 mm or less following at least one side, and a weld metal is formed in the longitudinal direction of the flange portion. Is characterized in that the weld is formed such that C <0.05 mass% and P + S <0.03 mass%. In the following, the element content% is mass%.

The invention of the lap laser welding method according to claim 2 is such that at least one of the plate materials is made by superimposing a plurality of plate materials made of high-strength steel, and the overlapped plate material is melted to the back surface of the superimposed lower plate material. In the laser welding method, the laser beam is moved along the end portion of the superposed plate materials while continuously irradiating the laser beam from the superimposing direction to form a welded portion, and the superposed plate materials are welded together. ,
At least one plate material among the plurality of plate materials is formed of a thin plate material, and is formed of a structural member having a bent portion and a flange portion having a width of 8 mm or less following at least one side, and a weld metal is formed in the longitudinal direction of the flange portion. Is formed such that 0.08 mass% <C <0.7 mass% and P + S <0.05 mass%.

The invention of the lap laser welding method of claim 3 is characterized in that, as described in the claim, welding is started at a position distant from the end portion in the welding direction of the flange portion .
The invention of the overlap laser welding method of claim 4 is overlapped so that one plate member of the plurality of plate members protrudes from a flange portion of the other plate member, as described in the claim, Welding is started from an end portion in the welding direction of the flange portion .

According to a fifth aspect of the invention of the lap laser welding method, as described in the claim, the at least one plate member is a structural member having a hat-shaped cross section having a bent portion and a flange portion on both sides. It is characterized by .

According to the first and second aspects of the present invention, even if at least one plate material is sufficiently melted to the back surface of the lower plate material on which the structural members made of high-tensile steel are overlapped, solidification cracks are generated in the welded portion. Since the stacked plates can be laser welded together, a welded portion with high strength can be formed even if the width of the overlapped portion is narrow, and the structural member can be reduced in size and weight. Become.
According to the third and fourth aspects of the invention, the laser welding method of the first and second aspects of the invention can be carried out in a form corresponding to the shape of the lap joint.

According to the invention of claim 5, the laser welding method of the invention of claims 1 and 2 can be applied to the manufacture of automobile panel parts .

Hereinafter, an embodiment of the present invention will be described in detail with reference to FIGS.
A plate-like member made of high-strength steel and having a flange at the end, such as a hat-shaped structural member shown in FIG. 4, is overlapped with a similar flange or plate member, and laser-welded between the two to form a frame When manufacturing a member, using a structural member with a narrower flange width (width over which plate materials overlap) A, such as within 8 mm, to reduce the weight of the entire frame member, from the welded portion to the flange end The distance B must be shorter than the range of 1.5 mm or more, and under such conditions, welding is started at a position away from the flange longitudinal end as shown in FIG. Even if it did, as mentioned above, the site | part deform | transformed by the heat conduction from a welding part pulled the weld bead in the middle of solidification, and the solidification crack might generate | occur | produce.

  In order to reduce the weight of the structural member, it is more effective to set the flange width within 8 mm. In such a flange width, the distance from the weld bead to the flange end is set to 1.5 mm or more. This is because if the length is less than 1.5 mm, the flange end portion side melts to the flange end and is easily melted down.

Thus, the present inventors investigated the cause of solidification cracking, and first examined the relationship between the weld metal component and solidification cracking.
FIG. 5 shows the relationship between the temperature of the solidification process in the laminating laser welding of thin plates and the strain generated around the weld. The figure shows a steel plate with a tensile strength of 590 MPa with a plate thickness of 1.2 mm consisting of 0.06% carbon, 0.5% silicon, and 1.5% manganese. It was obtained by using a sample obtained by laser welding at a speed of 2 m / min.
According to FIG. 5, a tensile force acts on the welded portion immediately below the liquidus temperature, and a compressive force acts on the welded portion when the temperature is sufficiently lowered from the liquidus temperature. In the region (2) in the solidification process behind the laser light irradiation position, a large strain in the tensile direction is generated, which can be said to lead to solidification cracking.

The relationship between the occurrence of strain and solidification cracking can be explained from the relationship between the generally known solidification temperature brittleness range (BTR) and shrinkage displacement (P) as shown in FIG.
That is, there is an embrittlement region (D) having a high solidification cracking sensitivity indicated by hatching in the figure between the temperature of the weld and the displacement amount of the member accompanying solidification shrinkage, and the solidification shrinkage displacement (P) as the temperature decreases. ) Increases, and it is believed that solidification cracking occurs when it passes through the embrittled region. Also, just below the liquidus temperature, the minimum ductility value (Dmin) is small and the embrittlement region is wide, but the liquidus ratio is high, so even if solidification cracks occur between columnar crystals, they are filled by the liquid phase and solidification cracks are not Does not occur.

In general, one of the factors affecting solidification cracking is the solidification temperature range between the liquid phase and the solid phase. In the binary system for Fe, C, P, and S are known as elements that widen the solidification temperature range even when a small amount is added (for example, Matsuda, “Welding Metallurgy”, published by Nikkan Kogyo Shimbun, 1972, No. 158). Page).
C, P, and S have a small equilibrium partition coefficient, and the solute is discharged into the molten metal and remains between the columnar crystals. It is considered that it is an element that tends to expand to a BTR and easily cause solidification cracking.
Furthermore, it is considered that there is a component range that is particularly sensitive to cracking, such that the grain boundary strength in the solidification process is low in that component range, and that the component range is determined by the range of C amount and P + S amount. It is done.

Therefore, the range of the amount of C and P + S in which solidification cracking occurred was examined.
In the experiment, steel sheets having various C amounts and P + S amounts and tensile strengths in the range of 270 MPa to 1470 MPa were used, and the same type or different types were combined to form a hat-shaped structural member and plate as shown in FIG. The shaped members were overlapped so that the flange width A was 8 mm.
And the distance B from the weld bead center to the end of the plate material: 3.0 mm, the distance C from the flange longitudinal end to the welding start point, 5 mm, laser processing point output: 3.5 kW, welding speed: 2 m / Laser welding was performed under the condition of min.
FIG. 7 shows the result of arranging the occurrence of cracks after welding according to the C content and P + S amount of the weld metal. In FIG. 7, (1) and (3) are regions where solidification cracks do not occur, and (2) is a region where solidification cracks occur.

From this result, a plurality of plate materials made of steel plates having a tensile strength in the range of 270 MPa to 1470 MPa are superposed, and superposed so as to melt to the back surface of the superposed lower plate material in the vicinity of the end of the superposed plate member. Laser beam is irradiated from the direction, welding is started from a position away from the welding direction end of the overlapped plate material, and the laser beam is moved along the end to form a welded portion. When welding plate materials to each other, if a weld metal with C <0.05%, P + S <0.05%, or 0.08% <C is formed in the welded portion, solidification cracking occurs. It was found that laser welding was possible without
In particular, it has also been found that when at least one plate material is made of a high-tensile steel plate of 440 MPa or more, solidification cracking is likely to occur, and even in that case, solidification cracking can be prevented by forming the above weld metal.

  However, in the range of 0.08% <C, if the C content is 0.7% or more, martensite transformation-induced cracking occurs as shown in Patent Document 4, so the upper limit of the C content is less than 0.7%. In addition, if the total content of P and S increases, the embrittlement region shown in FIG. 6 is greatly widened and solidification cracks are likely to occur. Therefore, P + S <0.05% is necessary.

As described above, the details of the reason why the crack susceptibility of the laser weld varies greatly depending on the amount of C, the amount of P, and the amount of S are not necessarily clear, but the present inventors believe that the reason is as follows.
The low C and low P, S region (C <0.05%, P + S <0.03%) shown in (1) of FIG. 7 has a high liquidus temperature, and as shown in region (1) of FIG. Solidification is completed before the force increases. In this case, a tensile force is generated, but since it is considered to be within the range of Dmin shown in FIG. 6, no solidification cracking occurs.

Further, the low C region (C <0.05%) and the high P, S region (P + S> 0.03%), and middle C region (0.05 <C <0.08%) shown in FIG. ), Solidification is completed in the region (2) of FIG. 5 where the solid phase temperature is relatively low and the tensile force is maximum. Since the tensile force in this case is considered to pass through the embrittlement region shown in FIG. 6, solidification cracks occur.
On the other hand, in the case of the high C region (0.08% <C) shown in (3) of FIG. 7, the solid phase temperature is sufficiently low, and the liquid phase is sufficient in the region of (2) in FIG. Therefore, even if a crack occurs, it is filled with the liquid phase, so that no solidification crack occurs. Solidification is completed in the region of (3) in FIG. 5. In this region, the tensile force is sufficiently small and within the range of Dmin shown in FIG. It is thought not to.

In addition, in the case of the above laser welding, the case where welding is started from a position away from the welding direction end of the overlapped plate material was examined, but when welding is started from the welding direction end portion of the overlapped plate material, It is necessary to restrain the end with a clamp jig so that the end does not open as shown in FIG. If it does in this way, as shown in Drawing 3, it can weld without swelling in the middle.
In addition, as shown in FIG. 8, one plate member 4 is overlapped so as to protrude from the other plate member 1, and when the end portions of the overlapped portion are not aligned, one plate member 4 is If the distance D protruding from the plate material 1 and the distance B from the end portion to the welded portion are increased to some extent, the weld metal is formed as described above even if welding is started from the end portion in the welding direction of the overlapped plate materials. Therefore, welding can be performed without causing solidification cracks. In that case, in order to reliably prevent solidification cracking, the distance D is preferably 5 mm or more, and the distance B is preferably 2 mm or more.

In the present invention, for example, a YAG laser, a fiber laser, a DISK laser, or the like can be used as the laser oscillator, and examples of the welding conditions include a machining point output of 3.5 kW and a welding speed of 2.0 m / min. .
In addition, when welding is started at a distance C from the flange longitudinal end, the distance C is equal to or greater than the distance B from the weld bead center of the weld to the flange width end. Is good.

As a material of the plate material to be lap welded, the case where a high strength steel having a tensile strength of 440 MPa or more, at least one of which is likely to cause solidification cracking, is an object of the present invention.
As an example in the case of using a steel other than the high-tensile steel, for example, there is a combination of a steel plate having a tensile strength of 590 MPa class and a steel sheet having a 270 MPa class.

In laser welding, the component of the weld metal is an average component calculated from the base material component value of each overlapped plate and the plate thickness when a filler material such as a filler is not added separately, and calculated as C and The material of the structural member is selected so that the value of P + S satisfies the above definition of the present invention. In addition, in the case where three or more plate materials are overlapped, in some cases, it can be expressed by an average component calculated from the base material component value and the plate thickness in all combinations of two adjacent plates. Similarly, the material of the structural member is selected so that the calculated value satisfies the above-mentioned definition of the present invention.
When a filler material is added, it is necessary to calculate the values of C and P + S in consideration of the amount added.

  Examples of the present invention will be described below, but the conditions adopted in the examples are one example of conditions for confirming the feasibility and effects of the present invention, and the present invention is limited to this one example of conditions. It is not something. This invention is prescribed | regulated only by the matter described in a claim, Embodiment other than the above can also be implemented. As long as the object of the present invention is achieved without departing from the present invention, various conditions can be adopted.

As a test material, a structural member having a cross-sectional hat shape made of high-strength steel (SPFC590) having a thickness of 1.2 mm and a tensile strength of 590 MPa or more is aligned with a plate material on a flat plate made of high-tensile steel. The width of the overlapped portion was 8 mm. As high-tensile steel, samples having different amounts of C and P + S were used. The overlapped portion was laser welded under the following conditions.
For laser welding, a YAG laser was used, the laser processing point output was 3.5 kW, and the welding speed was 2.0 m / min. The laser beam was focused on the steel plate and the focused spot was 0.6 mm in diameter. The laser irradiation position was 2.5 to 4.0 mm from the flange end, and the point separated from the flange longitudinal end by 5.0 mm was taken as the start position. (Inventive Examples 1-3, Comparative Examples 1-3)
Further, as shown in FIG. 8, the same steel plates as described above were overlapped with D being 5 mm, B was 2.5 mm, and laser welding was performed in the same manner as described above from the flange end. (Invention Example 4)
The obtained results are shown in Table 1. It can be seen from Table 1 that welding can be performed without causing cracks when the conditions of the present invention are satisfied.

It is a figure which shows the example of the member which has a lap joint. It is a figure which shows one example of a solidification crack. It is a figure which shows the other example of a solidification crack. It is a figure explaining a flange width and a welding part position. It is a figure which shows the relationship between the temperature of the solidification process after welding, and distortion. It is a figure which shows the relationship between a solidification brittle range and shrinkage displacement. It is a figure which shows the relationship between generation | occurrence | production of a solidification crack, and a component. It is a figure explaining the example of the joint in the case of starting welding from a flange edge part.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Structural member of hat-shaped cross section 2 Flange part of structural member 3 Bending part of structural member 4 Plate material 5 Welding start point 6 Welded part (weld bead)
7 Solidification crack 8 Laser welding head A Flange width B Distance from weld to flange end C Distance from flange longitudinal end to welding start point D Distance from which one plate protrudes from the other plate

Claims (5)

At least one of the plate materials is a stack of a plurality of plate materials made of high-tensile steel, while irradiating a laser beam from the overlay direction so as to melt to the back surface of the stacked lower plate material, In the laser welding method in which the laser beam is moved along the edge of the overlapped plate material to continuously form a weld, and the overlapped plate material is welded together,
At least one plate material among the plurality of plate materials is formed of a thin plate material, and is formed of a structural member having a bent portion and a flange portion having a width of 8 mm or less following at least one side, and a weld metal is formed in the longitudinal direction of the flange portion. The lap laser welding method is characterized in that the weld is formed such that C <0.05 mass% and P + S <0.03 mass%. At least one of the plate materials is a stack of a plurality of plate materials made of high-tensile steel, while irradiating a laser beam from the overlay direction so as to melt to the back surface of the stacked lower plate material, In the laser welding method in which the laser beam is moved along the edge of the overlapped plate material to continuously form a weld, and the overlapped plate material is welded together,
At least one plate material among the plurality of plate materials is formed of a thin plate material, and is formed of a structural member having a bent portion and a flange portion having a width of 8 mm or less following at least one side, and a weld metal is formed in the longitudinal direction of the flange portion. The lap laser welding method is characterized in that the weld is formed such that 0.08 mass% <C <0.7 mass% and P + S <0.05 mass%.   The lap laser welding method according to claim 1 or 2, wherein welding is started at a position away from an end portion in the welding direction of the flange portion.   The one plate member of the plurality of plate members is overlapped so as to protrude from the flange portion of the other plate member, and welding is started from an end portion in the welding direction of the flange portion. The lap laser welding method described in 1.   The lap laser welding method according to any one of claims 1 to 4, wherein the at least one sheet material is a structural member having a hat-shaped cross section having a bent portion and a flange portion on both sides.

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