JP2010279991A - Method of lap-welding steel sheet by laser beam - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of lap-welding a steel sheet by a laser beam, by which the solidification crack is not caused even when the restriction is not provided on the structure of a flange and the position of a weld zone. <P>SOLUTION: In a method of welding a plurality of steel sheets 1A, 1B each other by superimposing them and emitting a laser beam L, at least one of the steel sheets 1A, 1B is a high-tensile steel sheet and, by forming a weld zone 2 in which the composition of ingredients of a weld metal is 0.05≤C≤0.08 mass%, or C<0.05 mass% and P+S≥0.03 mass%, the superimposed steel sheets 1A, 1B are welded each other. When taking the width of the superimposed part 5 of the steel sheets 1A, 1B as ≤8 mm, using the laser beam L the condensed beam diameter of which is ≤0.5 mm and expressing the laser output by p(w), a welding speed by v(mm/s), the total sheet thickness in a superimposed part to be welded by t(mm) and the condensed beam diameter by d(mm), a relationship which is expressed by next formula: äp/v/t<SP>1/2</SP>×d<SP>2</SP><12.5} is satisfied. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for laser lap welding of thin steel plates, and in particular, a plurality of thin steel plates having a thickness of 0.5 to 3.2 mm are stacked, and the stacked thin steel plates are irradiated while irradiating laser light from the overlapping direction. The present invention relates to a laser lap welding method for thin steel sheets in which laser light is moved along the edge of the steel sheets and these thin steel sheets are welded together. In addition, this invention is a method suitable mainly when laser welding the structure of a motor vehicle or household appliances.

  In recent years, high strength steel sheets with a tensile strength of 440 MPa or higher are often used for automobile bodies in order to meet the demands of improving the fuel efficiency and safety of automobiles, using laser welding. Therefore, it is required to weld these steel plates. Furthermore, there is a need for a laser welding method capable of obtaining stable joint strength in a method of superposing and welding high strength thin steel sheets.

  Since laser welding uses laser light as a heat source, control of the heat input is more reliable and easier than arc welding. For this reason, there is an advantage that thermal deformation can be reduced by appropriately setting welding conditions such as welding speed, laser beam irradiation output, and shielding gas flow rate. Laser welding is suitable for assembly welding of complicated members such as a car body because it can be welded from one side. Such laser welding is frequently used when welding a member obtained by forming a thin steel plate in various fields such as the automobile manufacturing industry and the electrical equipment manufacturing industry. In connection with this, many laser lap welding methods have been proposed that form lap joints with excellent weld joint strength.

  A typical example of a member formed from a thin steel plate and having a lap joint, which has been conventionally used for a vehicle body panel, is shown in FIGS. 2 (a), (b), (c), and (d). An example will be described with reference to the drawings. Like the example of illustration, it has the bending part 23 and the flange part 24, makes the cross-sectional substantially hat-shaped structural member the thin steel plates 21 (21A) mutually oppose, the flange part 24 is overlap | superposed, and the overlap part is formed. In addition to the frame members joined by spot welding or the like, as shown in FIGS. 2B and 2C, the thin steel plate 21B is interposed between the flange portion 24 and the thin steel plate 21B or the flange portion 24, and they are overlapped. The frame member which joined them similarly, and also the frame member which piled up the thin steel plate 21A which is a plurality of structural members in the same direction as shown in FIG.2 (d) are used. In particular, in recent years, in order to improve the fuel efficiency of automobiles due to global environmental problems, high-strength steel plates are frequently used for these members to reduce the weight of the vehicle body.

When laser welding is used to join the overlapped portions of the high-strength steel plates as described above, high welding strength can be obtained by continuous welding, and since the bead width is narrow, conventionally used spot welding and Since the degree of freedom of design of the joint is greater than that of arc welding, there is an advantage that the structural member can be reduced in size and weight by narrowing the width of the flange.
However, in a structural member made of high-tensile steel plate, for example, when the width of the flange portion is narrowed to reduce the weight of the member, the vicinity of the end of the overlapped portion is welded so as to be melted to the lower steel plate back surface by laser welding. It is clear from Patent Documents 1 and 2 that problems such as solidification cracking occur.

  That is, as shown in FIG. 7A, the flange portion 104 of the frame member 110 formed by superimposing the flange portions 104 provided on both sides of the structural member 100 having a substantially hat-shaped cross section is overlapped with the flange direction 104, that is, the flange. Laser light is irradiated from the direction intersecting the portion 104 and welding is performed so as to melt to the lower surface of the steel sheet. At this time, when welding is started from the longitudinal end portion of the flange portion 104 (see arrows in FIGS. 7A and 7B), as shown in FIG. There is a problem that deformation occurs to spread outward and cracks occur.

  Further, as shown in FIG. 8, even when the welding start point 105 is not set as the end portion in the longitudinal direction of the flange portion 102 and the point separated from the end portion by a predetermined distance is set as the welding start point, The central portion of 106 may bulge and cracks may occur. In FIG. 8, reference numeral 108 denotes a laser welding head. This is because when welding is performed by irradiating a laser beam so as to melt up to the back surface of the lower steel sheet, the melted part formed by laser light irradiation is solidified until the melted part is solidified. Since the part is in a state separated from the flange part 102 main body, if the width of this part is small, the part is deformed by thermal expansion due to heat conduction from the welded part 106, and the weld bead in the middle of solidification is pulled. It is considered that cracking occurs during solidification.

With respect to the problem of solidification cracking that occurs when laser welding is performed on the vicinity of the end of the overlapping portion of the conventional flange member, in Patent Documents 1 and 2, solidification cracking is prevented by the following method. Has been proposed.
First, in Patent Document 1, the occurrence of solidification cracks depends on the C, P, and S components of the weld metal, and as shown in FIG. 9, C <0.05% by mass and P + S <0.03% by mass. It is described that solidification cracking can be prevented by forming a welded portion so that 0.08% by mass <C <0.7% by mass and P + S <0.05% by mass. ing.

  Moreover, in patent document 2, when it is except the component range which does not produce the solidification crack shown by patent document 1, as shown in FIG. 10, first, the width | variety of the overlap part of the weld metal steel plates 111 and 114 shall be less than 8 mm. In addition, it is described that solidification cracking can be prevented by forming the welded portion 112 at a position where the distance B from the end of one steel plate (welded metal steel plate) 114 is 3.5 mm or more. Alternatively, as shown in FIG. 10, one steel plate 114 is overlapped so that the protrusion amount D from the other steel plate 111 is 5 mm or more, the width of the overlapping portion between the steel plates is within 8 mm, and the other It is described that solidification cracking can be prevented by forming the weld 112 at a position where the distance B from the end of the steel plate 111 is 1.5 mm or more.

JP 2007-229740 A JP 2007-229752 A

However, in the inventions described in Patent Documents 1 and 2, in order to avoid solidification cracking when the content of C (carbon), which is a component of the weld metal, is 0.05 ≦ C ≦ 0.08 mass%. There has been a problem that the structure of the overlapping portion of the flange and the formation position of the welded portion are restricted.
However, in laser welding of high-strength thin steel sheets, the width of the flange portion is shortened to reduce the weight of the member, and further, welding is performed so that the vicinity of the end of the overlapped portion is melted to the lower steel plate back surface. In order to avoid weld solidification cracking when the component C of the weld metal is 0.05 ≦ C ≦ 0.08 mass%, or C <0.05 mass% and P + S ≧ 0.03 mass%. As a method, a welding method that does not limit the structure of the overlapping portion of the flange and the formation position of the weld has not been proposed conventionally.

  The present invention has been made in view of the above problems, and it is possible to suppress the occurrence of solidification cracks and the like without restricting the structure of the overlapping portion of the flange and the formation position of the welded portion. An object is to provide a laser lap welding method.

As a result of extensive studies by the present inventors to solve the above problems, at least one thin steel plate is composed of a high-tensile steel plate, and the weld metal component C in the weld zone is 0.05 ≦ C ≦ 0. .08 mass%, or even when C <0.05 mass% and P + S ≧ 0.03 mass%, by optimizing various welding conditions, the structure of the overlapping part of the flange, The inventors have found that solidification cracking and the like can be suppressed without providing many restrictions on the formation position, and have completed the present invention.
That is, the gist of the present invention is as follows.

[1] A plurality of thin steel plates having a thickness of 0.5 to 3.2 mm are superposed, and superposed while irradiating laser light from the superposition direction so as to melt to the back surface of the superposed thin steel plate. A laser lap welding method for thin steel sheets, in which the overlapped thin steel sheets are welded together by moving the laser beam along an edge of the thin steel sheets to form a welded portion. At least one of them is a plate made of high-strength steel, and the component composition of the weld metal is 0.05 ≦ C ≦ 0.08 mass%, or C <0.05 mass%, and P + S ≧ 0.03. By forming the welded portion in mass%, the superposed thin steel plates are welded to each other, the width of the superposed portion of the thin steel plates is within 8 mm, and the condensing diameter is 0.5 mm or less. Using light, the laser output is p (w), When the welding speed is v (mm / sec), the total plate thickness of the overlapped parts to be welded is t (mm), and the focused diameter is d (mm), the relationship expressed by the following formula (1) is satisfied. A method for laser lap welding of thin steel sheets.
p / v / t ^1/2 × d ² <12.5 (1)
[2] At least one thin steel plate of the plurality of thin steel plates includes a bent portion and a flange portion following the bent portion on at least one side of the steel plate end portions parallel to or along the welding direction. The thin steel plate according to [2] above, wherein the overlapping portion of the plurality of thin steel plates is a superposition of the flange portion and the other thin steel plate. Laser lap welding method.
[3] At least one thin steel plate of the plurality of thin steel plates has a substantially hat-shaped cross section having the bent portion and the flange portion on both sides of a steel plate end portion parallel to the welding direction or along the welding direction. The method of laser lap welding of thin steel sheets according to [2] above, wherein the method is a structural member.
[4] The method of laser lap welding of thin steel sheets according to [2] or [3] above, wherein the flange portion has a width of 8 mm or less.

According to the method of laser lap welding of thin steel plates according to claim 1 of the present invention, at least one of the plurality of thin steel plates is a high-tensile steel plate, and the width of the overlapping portion of the thin steel plates is within 8 mm. In addition, using a laser beam with a condensing diameter of 0.5 mm or less, the laser output is p (w), the welding speed is v (mm / sec), and the total thickness of the overlapped parts to be welded is t (mm). When the condensing diameter is d (mm), the welding metal component is a method of welding under conditions satisfying the relationship represented by the following formula {p / v / t ^1/2 × d ² <12.5}. The bottom surface of the lower thin steel plate formed by superimposing the welded portion having a composition of 0.05 ≦ C ≦ 0.08 mass% or C <0.05 mass% and P + S ≧ 0.03 mass%. Even if it is fully melted, the laminated thin steel does not cause solidification cracks in the weld. Can be laser welded together. Accordingly, a welded portion having high strength can be formed even if the width of the overlapping portion is narrow, and the structural member can be reduced in size and weight.
In addition, according to the laser lap welding method for thin steel plates according to claims 2 and 3 of the present invention, since at least one thin steel plate is a welding method that is a structural member having a flange portion, manufacture of automobile panel parts, etc. It becomes possible to apply to.
Further, according to the laser lap welding method for thin steel sheets according to claim 4 of the present invention, since the welding is performed by optimizing the width of the flange portion, the structural member can be further reduced in size and weight. Become.

It is a figure which illustrates typically an example of the laser lap welding method of the thin steel plate which concerns on this invention, and is a schematic perspective view which shows the relationship between the width | variety of a flange part, and the position of a welding part. It is a figure which illustrates typically an example of the laser lap welding method of the thin steel plate which concerns on this invention, and is a schematic perspective view which shows each example of the thin steel plate which makes the structural member which has a lap joint. It is a figure which illustrates typically an example of the laser lap welding method of the thin steel plate which concerns on this invention, and is a graph which shows the relationship between the temperature of the solidification process after welding of a welding part, and distortion. It is a figure which illustrates typically an example of the laser lap welding method of the thin steel plate which concerns on this invention, and is a figure which shows the relationship between a solidification brittle range and shrinkage displacement. It is a figure which illustrates typically an example of the laser lap welding method of the thin steel plate which concerns on this invention, and is a graph which shows the relationship between the density | concentration of each element in the weld metal which makes a welding part, and a solidification temperature range. It is a figure which illustrates typically an example of the laser lap welding method of the thin steel plate which concerns on this invention, and is a graph which shows the relationship between generation | occurrence | production of the solidification crack in a welding part, and laser welding conditions. It is a figure which illustrates typically the laser lap welding method of the conventional thin steel plate, and is a schematic perspective view which shows an example of the solidification crack which arises in a welding part. It is a figure which illustrates typically the laser lap welding method of the conventional thin steel plate, and is a schematic perspective view which shows the other example of the solidification crack which arises in a welding part. It is a figure which illustrates typically the laser lap welding method of the conventional thin steel plate, and is a graph which shows the relationship between generation | occurrence | production of the solidification crack in a welding part, and a component. It is a figure which illustrates typically the laser lap welding method of the conventional thin steel plate, and is a schematic perspective view which shows the relationship between the width | variety of a flange part, and the position of a welding part.

  Hereinafter, an embodiment of a laser lap welding method for a thin steel plate of the present invention will be described with reference to FIGS. 1 to 6 as appropriate (see also FIGS. 7 to 10 and the like). In addition, since this embodiment demonstrates in detail in order to make the meaning of the laser lap welding method of the thin steel plate of this invention better understood, unless otherwise specified, this invention is not limited.

In the laser lap welding method for thin steel sheets of the present invention, a plurality of thin steel sheets 1 (1A, 1B) having a thickness of 0.5 to 3.2 mm are superposed and melted to the back surface 1c of the superposed steel sheet 1 on the lower side. The superposed thin steel sheet 1 is formed by moving the laser light L along the end portion 1a of the superposed thin steel sheet 1 to form the welded portion 2 while irradiating the laser light L from the superimposing direction. Are welded to each other, at least one of the plurality of thin steel plates 1 is a high-tensile steel plate, and the component composition of the weld metal is 0.05 ≦ C ≦ 0.08 mass%, or C <0.05. By forming the welded portion 12 with mass% and P + S ≧ 0.03 mass%, the superposed thin steel plates 1A and 1B are welded together, and the width A of the overlapping portion 5 of the thin steel plates 1A and 1B is set. 8mm or less and condensing diameter is 0.5mm or less Using the laser beam L, the laser output is p (w), the welding speed is v (mm / sec), the total thickness of the overlapped portion (the overlapped welded portion) 5 is t (mm), and the focused diameter is When d (mm), the welding is performed under conditions that satisfy the relationship represented by the following formula (1).
p / v / t ^1/2 × d ² <12.5 (1)

(Material of thin steel plate)
In the present invention, as the material of the steel plate to be laser lap welded, all of the plurality of steel plates are high-tensile steel plates, and at least one of the thin steel plates is a high-tensile steel plate, and the other thin steel plates are high-tensile. In some cases, the steel has a tensile strength lower than that of the steel plate, and the present invention targets any of them.
As a range of the strength level of the high-tensile steel plate, the tensile strength is 440 MPa or more. This is because there is no problem of solidification cracking, which is a problem of the present invention, in the lap laser welding between thin steel sheets having a tensile strength of less than 440 MPa. Further, the upper limit of the steel plate strength level is not particularly limited as long as the weld metal component can satisfy the range of the present invention, but the component range of the weld metal is considered in consideration of the components of a general thin steel plate. In order to be within the scope of the present invention, it is preferable that the tensile strength is up to 980 MPa class.
Examples of the different combinations of thin steel plates include a combination of a thin steel plate having a tensile strength of 590 MPa and a thin steel plate 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 and the thickness of each superposed thin steel plate, unless a filler material such as a filler is added separately. The material of the structural member is selected so that the values of P, S satisfy the above-mentioned definition of the present invention. Moreover, when adding a filler material, it is necessary to calculate the value of C, P, and S in consideration of the addition amount.

(Thin steel plate thickness)
The sheet thickness of the thin steel plate targeted by the welding method of the present invention is set to 0.5 to 3.2 mm. The lower limit of 0.5 mm of the thickness of the thin steel plate is based on the fact that the thickness of the steel plate used in the structure of automobiles and household appliances targeted by the present invention is 0.5 mm or more. Stipulated. Moreover, the upper limit of 3.2 mm of the thickness of the thin steel plate was defined based on the welding ability of a commonly used laser welding machine.

(Flange part)
In the welding method of the present invention, as in the example shown in FIG. 1 and the like, at least one thin steel plate 1A among the plurality of thin steel plates 1 is parallel to the welding direction or is the end portion 1a along the welding direction. Is a structural member having a bent portion 3 and a flange portion 4 following the bent portion 3 on at least one side, and the overlapping portion 5 of the plurality of thin steel plates 1A and 1B is a flange portion 4 of the thin steel plate 1A which is a structural member. And the thin steel plate 1B. Further, in the illustrated example, the thin steel plate 1A is configured as a structural member having a substantially hat-shaped cross section having the bent portions 3 and the flange portions 4 on both sides of the end portion 1a parallel to the welding direction or along the welding direction. .

  Conventionally, a plate-like member made of a high-strength steel plate and having a flange at the end, such as a structural member having a substantially hat-shaped cross section shown in FIG. When manufacturing a frame member by welding, as disclosed in Patent Document 2, for example, using a structural member having a narrower flange width (width of the overlapping portion of steel plates) such as within 8 mm. In order to reduce the weight of the entire frame member, the distance B from the welded portion to the flange end portion must be a shorter distance within a range of 1.5 mm or more. Under such conditions, as shown in FIG. 8, even if welding is started at a position away from the longitudinal end of the flange portion 102, the portion deformed by heat conduction from the welded portion 106 is not detected as described above. In some cases, the weld bead during solidification was pulled to cause solidification cracking.

In the present invention, as described above, at least one of the plurality of thin steel plates 1A and 1B is a high-tensile steel plate, and a laser beam having a condensing diameter of 0.5 mm or less is used, and the laser output is p (w ), When the welding speed is v (mm / sec), the total thickness of the overlapped parts to be welded is t (mm), and the focused diameter is d (mm), the following formula {p / v / t ^{1 /} By performing laser lap welding under conditions satisfying the relationship represented by ² × d ² <12.5}, even if the width A of the overlapping portion 5 of the thin steel plate 1 is a narrow dimension of 8 mm or less, the welded portion It is possible to suppress the occurrence of solidification cracks. Accordingly, a welded portion having high strength can be formed even if the width of the overlapping portion is narrow, and the structural member can be reduced in size and weight.

In order to reduce the weight of the structural member, it is more effective and preferable that the width of the flange portion 4, that is, the width A of the overlapping portion 5 is within 8 mm. If the width A of the flange portion 4 is less than 3 mm, the flange portion 4 may melt and be unable to be welded. Therefore, the lower limit of the width A of the flange portion 4 is preferably 3 mm or more. Furthermore, in such a width A of the flange portion 4, it is preferable that the distance B from the weld bead (welded portion 2) to the end portion 1a is 1.5 mm or more. This is because if the length is less than 1.5 mm, the flange portion 4 is easily melted from the welded portion 2 to the end portion 1a. Further, the distance C from the welded portion 2 to the longitudinal end 1d of the flange portion 4 is preferably 1.5 mm or more for the same reason as described above.
Therefore, the present inventors investigated the cause of solidification cracking, and first examined the relationship between the weld metal component and solidification cracking.

  FIG. 3 is a graph showing the relationship between the temperature of the solidification process in laser lap welding of thin steel sheets and the strain generated around the weld. The graph of FIG. 3 shows mass%, C content of 0.06%, Si content of 0.5%, Mn content of 1.5%, plate thickness of 1.2 mm and tensile strength of 590 MPa. Obtained by using a sample that has been subjected to laser lap welding under the conditions of superposing tension steel plates, using a laser beam with a focused diameter d of 0.6 mm, a laser processing point output p of 2000 W, and a welding speed v of 33 mm / sec. It is what was done.

As shown in FIG. 3, it is understood that a tensile force is applied to the welded portion immediately below the liquidus temperature, and conversely, if the temperature is sufficiently lowered from the liquidus temperature, a compressive force is applied to the welded portion. . In the graph of FIG. 3, a large strain in the tensile direction is generated in the region (2) in the solidification process behind the laser light irradiation position, 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) that is highly susceptible to solidification cracking, as indicated by the hatched line in the graph of FIG. 4, between the temperature of the weld and the amount of displacement of the member accompanying solidification shrinkage. Along with this, the value of solidification shrinkage displacement (P) increases, and it is considered that solidification cracking occurs when it passes through the embrittlement 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 with the liquid phase. , Solidification cracking 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. Here, in the binary system for Fe, for example, C, P, and S as shown in FIG. 5 are known as elements that widen the solidification temperature range even when a small amount is added.
Further, 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. Therefore, the temperature at the final solidification position from the apparent solid phase temperature is indicated by a bold broken line shown in FIG. It is thought that it is an element which is easy to cause the solidification cracking easily. Further, in the weld metal, it is considered that there is a particularly sensitive component range in which the grain boundary strength in the solidification process is low in the component range, and the component ranges are C, P, and S amount ranges. It is thought that it is decided by. For this reason, in Patent Documents 1 and 2, as shown in FIG. 9, the component of the weld metal is C <0.05% and 0.03% ≦ P + S or 0.05 ≦ C ≦ 0.08%. In this range, it is shown that solidification cracking occurs.

  Therefore, the present inventors considered that there is a laser welding condition in which solidification cracking does not occur by reducing the tensile stress due to welding deformation and reducing the displacement amount P, and conducted a more detailed investigation. Here, as a laser welding condition for reducing the amount of displacement (welding deformation), the amount of heat input (processing point output p / welding speed v) may be reduced. Further, welding with a smaller amount of heat input (processing point output p / welding speed v) is possible as the laser focused diameter d is smaller. In addition, with respect to the total thickness t of the overlapped parts to be welded, the larger the thickness, the smaller the welding deformation for the same heat input.

  In the experiment, a steel plate having a thickness of 0.5 to 3.2 mm and a tensile strength of 270 MPa to 980 MPa is used, and the weld metal component has a C content in a range of 0.05 ≦ C ≦ 0.08%. Or adjusted so that C <0.05 mass% and P + S ≧ 0.03 mass%. The laser welding conditions are variously changed in the range of the laser beam condensing diameter d of 0.3 to 0.9 mm, the processing point output p of 1500 to 4500 w, and the welding speed v of 17 to 100 mm / sec. went.

The result of the above experiment is shown in FIG. The horizontal axis in FIG. 6 indicates the laser output as p (w), the welding speed as v (mm / sec), the total thickness of the overlapped parts to be welded as t (mm), and the focused diameter as d (mm). In this case, the welding condition is represented by the following expression {p / v / t ^1/2 × d ² }. As a result, as defined in the present invention, at least one of the plurality of thin steel plates 1 (1A, 1B) is made of a high-strength steel plate, and the plurality of thin steel plates 1A, 1B are superposed and this superposition is performed. While irradiating the laser beam L from the overlapping direction so as to melt to the back surface 1c of the lower thin steel sheet 1B, welding is started from a position away from the end 11 in the welding direction of the stacked thin steel sheets 1. By moving the laser beam L along the end portion 1a of the combined thin steel plates 1 to form the welded portion 2, when welding the stacked thin steel plates 1 to each other, the component of the weld metal is 0.05 ≦ C. ≦ 0.08 mass%, or C <0.05 mass% and P + S ≧ 0.03 mass%, laser output is p (w), welding speed is v (mm / sec), overlap Total of matching parts (superimposed welded parts) 5 The thickness t (mm), if the condensing diameter is d (mm), if the welding conditions represented by the following formula (1), it became clear that not cause solidification cracking.
p / v / t ^1/2 × d ² <12.5 (1)

  As described above, the details of the reason why the presence or absence of solidification cracks in the laser welded portion greatly differs depending on the laser welding conditions are not necessarily clear, but the present inventors consider that the reason is as follows.

When the laser welding conditions are such that the heat input is limited in order to reduce the tensile stress due to welding deformation as in the present invention, the amount of displacement decreases as shown by the one-dot chain line in FIG. It is considered that solidification cracking did not occur even when the BTR was large and the BTR was wide. Here, in order to reduce the welding deformation, it is preferable that the laser processing point output p is small, the welding speed v is large, and the focused diameter d is small. Further, when the plate thickness is thick, welding deformation is difficult to occur. Considering these, the presence or absence of solidification cracks can be arranged by analyzing the above experimental results, the laser output is p (w), the welding speed is v (mm / sec), and the overlapped welded parts A laser satisfying the following formula {p / v / t ^1/2 × d ² <12.5} {formula (1)} where t is the total thickness and d is the condensing diameter. It became clear that solidification cracking was not caused by using welding conditions.

  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.

As described above, according to the laser lap welding method of the thin steel plate 1 according to the present invention, at least one of the plurality of thin steel plates 1A, 1B is a high-tensile steel plate, and the thin steel plates 1A, 1B are overlapped. The width A of the mating portion 5 (flange portion 4) is set to within 8 mm, and the laser beam L with a focused diameter of 0.5 mm or less is used, the laser output is p (w), and the welding speed is v (mm / sec). When the total plate thickness of the overlapped portion 5 is t (mm) and the light collection diameter is d (mm), it is represented by the following formula {p / v / t ^1/2 × d ² <12.5}. Since welding is performed under conditions that satisfy the relationship, the weld metal component composition is 0.05 ≦ C ≦ 0.08 mass%, or C <0.05 mass%, and P + S ≧ 0.03 mass%. Even when the part 2 is formed and fully melted up to the back surface 1c of the laminated thin steel sheet 1B, Without solidification cracking occurs in the welded portion 2, superimposed steel sheet 1A, can be laser welded together 1B. Therefore, even if the width W of the overlapping portion 5 is narrow, the welded portion 2 having high strength can be formed, and the structural member can be reduced in size and weight.

In addition, according to the laser lap welding method for thin steel sheets of the present invention, at least one thin steel sheet 1A can be applied to the manufacture of automobile panel parts and the like by using a structural member having a flange portion 4. It becomes.
Further, according to the present invention, the structural member can be further reduced in size and weight by making the width A of the flange portion 4 suitable for welding.

  In addition, in description of the laser lap welding method of the thin steel plate 1 (1A, 1B) of this embodiment, the example applied when manufacturing a structural member as shown in FIG. 1 (refer also FIG.2 (b)). Although described, the present invention is not limited to this. For example, as shown in FIG. 2 (a), a thin steel plate 21 (21A) that is a structural member having a bent portion 23 and a flange portion 24 and having a substantially hat-shaped cross section is opposed to each other, and the flange portion 24 is overlapped. In addition to the frame member in which the overlapped portion is joined by spot welding or the like, a thin steel plate 21B is interposed between the two flange portions 24 as shown in FIG. It is also possible to apply to manufacture of the joined frame member. Furthermore, as shown in FIG. 2 (d), it is also possible to apply to the manufacture of a frame member in which thin steel plates 21A, which are a plurality of structural members, are overlapped in the same direction. By applying the method of laser lap welding of thin steel sheets of the present invention to the manufacturing process of structural members as described above, for example, it becomes possible to reduce the size and weight of an automobile member, and thus, to reduce the weight of the vehicle body. Excellent effects such as improved fuel efficiency can be obtained.

  Hereinafter, examples of the laser lap welding method for thin steel sheets according to the present invention will be given and the present invention will be described more specifically, but the conditions adopted in the examples are for confirming the feasibility and effects of the present invention. The present invention is not limited to this one condition example. 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.

  In the present example, as a test material, a structural member having a cross-sectional hat shape made of a thin steel plate having a thickness of 1.2 mm and a tensile strength of 270 to 980 MPa is aligned with the thin steel plate on the flat plate. Overlaid on. At this time, the width of each overlapping portion of the thin steel plates was 8 mm. In these steel plates, the amount of C (carbon) in the weld metal is 0.05 ≦ C ≦ 0.08%, or C <0.05 mass%, and P + S ≧ 0.03% mass. A sample was used.

Then, the overlapped portion of the thin steel plates was laser lap welded under the conditions described below.
First, a YAG laser is used as the laser, the laser beam condensing diameter d is 0.3 to 0.9 mm, the laser processing point output p is 1500 to 4500 w, and the welding speed v is 17 to 100 mm / sec. Various changes were made within the range shown in Table 1 below. At this time, the welding process was performed while supplying Ar gas as the shielding gas.
The laser irradiation position was in the range of 2.5 to 4.0 mm from the end of the flange portion.
The presence or absence of solidification cracks in the welded portion when laser lap welding of a plurality of thin steel sheets under the above conditions is shown in Table 1 below as “◯ (no occurrence)” and “× (occurrence)”. .

  Table 1 below shows a list of welding conditions for each of the inventive examples and the comparative examples, and also shows a list of occurrence of solidification cracks in the welds obtained by welding from the longitudinal ends of the flanges. .

As in the results shown in Table 1, in the present invention examples (invention examples 1 to 6) in which laser lap welding was performed under the welding conditions satisfying the provisions of the present invention using the thin steel sheets defined in the present invention, solidification cracks were observed. It can be seen that a plurality of thin steel plates can be satisfactorily stacked and laser-welded without occurring.
On the other hand, in the comparative examples (comparative examples 1 to 4) in which any one of the welding conditions is outside the specified range of the present invention, solidification cracks occurred in the welded parts in all examples.

  From the results of the examples described above, by applying the laser lap welding method for thin steel sheets of the present invention, solidification cracks are generated in the welded part without any restrictions on the structure of the flange part and the position of the welded part. It is clear that a plurality of thin steel plates can be well welded.

DESCRIPTION OF SYMBOLS 1, 1A, 1B ... Thin steel plate, 1a ... End part (flange part), 1c ... Back surface, 1d ... End part in longitudinal direction (flange part) 2 ... Welded part, 3 ... Bending part, 4 ... Flange part, 5 ... Overlap Alignment part, A ... Width (width of overlapping part, width of flange part) L ... Laser beam, p ... Laser output, t ... Total plate thickness of flange part (overlapping part: welded part), v ... Welding speed , D ... Condensing diameter

Claims (4)

A plurality of thin steel plates having a thickness of 0.5 to 3.2 mm are superposed, and laser light is irradiated from the superposition direction so as to melt to the back surface of the superposed lower thin steel plate. A laser lap welding method for thin steel sheets, in which the overlapped thin steel sheets are welded together by moving the laser beam along an end portion to form a welded portion,
At least one of the plurality of thin steel plates is a high-tensile steel plate, and the component composition of the weld metal is 0.05 ≦ C ≦ 0.08 mass%, or C <0.05 mass%, and P + S ≧ By forming the welded portion that is 0.03% by mass, the stacked thin steel plates are welded together,
The width of the overlapped portion of the thin steel plates is 8 mm or less, and a laser beam with a focused diameter of 0.5 mm or less is used, the laser output is p (w), the welding speed is v (mm / sec), and the overlap is performed. Laser lap welding of thin steel plates, characterized by satisfying the relationship represented by the following formula (1) when the total thickness of the welded parts is t (mm) and the focused diameter is d (mm) Method.
p / v / t ^1/2 × d ² <12.5 (1)   At least one thin steel plate of the plurality of thin steel plates is a structural member having a bent portion and a flange portion following the bent portion on at least one side of the end portions of the steel plate parallel to the welding direction or along the welding direction. 2. The method of laser lap welding of thin steel sheets according to claim 1, wherein the overlapping portions of the plurality of thin steel plates are obtained by superimposing the flange portions and the other thin steel plates.   At least one thin steel plate of the plurality of thin steel plates is a structural member having a substantially hat-shaped cross section having the bent portion and the flange portion on both sides of a steel plate end portion parallel to the welding direction or along the welding direction. The laser lap welding method for a thin steel sheet according to claim 2, wherein the laser lap welding method is provided.   The laser lap welding method for thin steel sheets according to claim 2 or 3, wherein a lateral width of the flange portion is within 8 mm.

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