JP5000578B2 - Laser welding method for thin steel sheets - Google Patents

Description

  In this invention, a plurality of thin steel sheets having a thickness of 0.5 to 3.2 mm are overlapped, and while irradiating laser light from this overlapping direction, the laser light is moved along the edge of the steel sheet, The present invention relates to a lap laser welding method for thin steel plates that weld these steel plates to each other. In addition, this invention mainly laser welds the structure of a motor vehicle and household appliances.

  A typical example of a member having a lap joint formed from a thin steel plate used for a vehicle body panel is shown in FIG. As shown in FIG. 1a, the cross section having the flange portion 2 and the bent portion 3 is a frame member in which a hat-shaped structural member 1 is opposed to each other and the flange portion 2 is overlapped, and the overlapped portion is joined by spot welding or the like. Alternatively, as shown in FIGS. 1b and 1c, the flange portion 2 and the steel plate 4 or a frame member in which the steel plate 4 is interposed between the flange portions and they are overlapped and joined in the same manner, as shown in FIG. 1d. In addition, a frame member in which a plurality of structural members 1 are overlapped in the same direction is used. Particularly, in recent years, in order to improve the fuel efficiency of automobiles due to global environmental problems, the use of high-tensile steel plates for these members has led to lighter vehicle bodies.

  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.

  However, in the structural member made of high-tensile steel plate, the width of the flange portion is reduced to reduce the weight of the member, and when trying to weld the vicinity of the overlapped portion end to the lower steel plate back surface by laser welding, It is shown in Patent Documents 1 and 2 that solidification cracking becomes a problem.

  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 both side flange portions of the structural member having a hat-shaped cross section are overlapped with each other. When starting welding from the longitudinal direction end of the flange and welding so as to melt to the lower steel plate back surface, as shown in FIG. Cracking occurs.

  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 up to the back surface of the lower steel plate, the flange on the end side from the melted part is solidified until the melted part formed by the laser light irradiation is solidified. The part is separated from the flange body. At this time, if the width of the part is small, the part expands due to heat conduction from the welded portion and deforms, pulling the weld bead in the middle of solidification, and solidifying. This is probably because cracks sometimes occur.

  In contrast to the problem of solidification cracking that occurs when laser welding is performed on the vicinity of the end of the flange overlap portion, in Patent Document 1, the occurrence of solidification cracking depends on the C component and P + S component of the weld metal. As shown, C <0.05 mass% and P + S <0.03 mass%, or 0.08 mass% <C <0.7 mass%, P + S <0.05 mass% Thus, an invention is described in which a weld is formed to prevent solidification cracking.

  Moreover, in patent document 2, when it is except the component range which does not produce the solidification crack shown by the said patent document 1, the width | variety of the overlap part of a weld metal steel plate shall be 8 mm or less, and 3. from the edge of the said steel plate. By forming a welded part at a position 5 mm or more apart, or overlapping so that one steel plate protrudes 5 mm or more from the other steel plate, the width of the overlapped portion of the steel plates at that time is within 8 mm, and An invention is described in which a solidified crack is prevented by forming a welded portion at a position 1.5 mm or more away from the end of the other steel plate.

JP 2007-229740 A JP 2007-229752 A

  However, when the steel plates are overlapped and laser-welded, the component C of the weld metal is C <0.05% by mass or 0.08% by mass <C as described in Patent Document 1 above. Can be welded even with a narrow flange width. However, in order to avoid solidification cracking when the component C of the weld metal is 0.05 ≦ C ≦ 0.08 mass%, it is described in Patent Document 2. As described above, there is 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 part is shortened to reduce the weight of the member, and further, the vicinity of the overlapped part end part is welded so as to be melted to the lower steel sheet back surface. In order to avoid weld solidification cracking when the component C of the weld metal is 0.05 ≦ C ≦ 0.08% by mass, welding that does not restrict the structure of the overlapping portion of the flange and the formation position of the welded portion The method has not been known so far.

  Therefore, in view of the situation as described above, the present invention is such that at least one steel plate is superposed from a plurality of steel plates made of high-tensile steel plates and is melted up to the back surface of the superposed lower steel plate. While irradiating the laser beam, the laser beam is moved along the end of the superposed steel plate and the superposed steel plates are welded to each other, and the component C of the weld metal is 0.05 ≦ C ≦ 0. In the case of 0.08% by mass, welding with a narrow flange width without the occurrence of solidification cracks as described above, even without many restrictions on the structure of the overlapping part of the flange and the formation position of the welded part It is an object of the present invention to provide a method of laminating laser welding of thin steel sheets that can contribute to weight reduction of members.

In order to solve the above problems, the present invention is characterized as follows.
(1) Superimposing a plurality of thin steel plates with a thickness of 0.5 to 3.2 mm and superimposing the laser beams while irradiating the laser beams from the superposition direction so as to melt to the back surface of the superposed lower steel plate In the laminating laser welding method for thin steel plates, a welded portion is formed by moving along the ends of the combined steel plates, and the stacked steel plates are welded to each other. At least one of the plurality of steel plates has a tensile strength. Forming a welded portion satisfying the relationship of 0.05 ≦ C ≦ 0.08 mass% and 4S + P <0.024 mass%. A method for laminating thin steel sheets, characterized by laser welding.
(2) The method of lap laser welding of thin steel plates according to (1) above, wherein welding is started at a position away from the welding direction end of the superposed steel plates.
(3) At least one steel plate of the plurality of steel plates is overlapped so as to protrude in a direction perpendicular to the welding direction from the other steel plates, and from a welding direction end of the stacked steel plates. The method of lap laser welding of thin steel sheets according to (1) above, wherein welding is started.
(4) At least one steel plate among the plurality of steel plates is a structural member having a bent portion and a flange portion subsequent thereto on at least one side of the steel plate end portions parallel to the welding direction or along the welding direction, The superposition laser of thin steel plates according to any one of (1) to (3) above, wherein the steel plate superimposing portion is a superposition of the flange of the structural member and another steel plate. Welding method.
(5) The at least one steel plate is a structural member having a hat-shaped cross section having a bent portion and a flange portion on both sides of a steel plate end portion parallel to the welding direction or along the welding direction. The method of lap laser welding of thin steel sheets as described in (4) above.
(6) The method according to (4) or (5) above, wherein the flange portion has a width of 8 mm or less.

  According to the first aspect of the present invention, at least one steel plate is sufficiently melted to the back surface of the superposed lower steel plate in the overlap laser welding of a structural member made of a high-strength steel plate, and the component C of the weld metal is 0. Even in the case of 0.05.ltoreq.C.ltoreq.0.08% by mass, the stacked steel sheets can be laser welded to each other without causing solidification cracks in the welded part. A welded portion having high strength can be formed, and the structural member can be reduced in size and weight.

  According to the second and third aspects of the invention, the laser welding method of the first aspect of the invention can be carried out in a form corresponding to the shape of the lap joint.

  According to the fourth and fifth aspects of the invention, the laser welding method according to the first to third aspects of the invention can be applied to the manufacture of automobile panel parts.

  According to the invention of claim 6, the structural member can be further reduced in size and weight.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to FIGS.

  The plate thickness of the steel plate targeted by the present invention is 0.5 to 3.2 mm. The lower limit of 0.5 mm of the plate thickness is defined because the plate thickness of the steel plate used in the structure of automobiles and household appliances targeted by the present invention is almost 0.5 mm or more. Moreover, the upper limit of 3.2 mm of plate | board thickness was prescribed | regulated from the welding capability of the laser welding machine. Note that the thickness of the entire overlapping portion is preferably 4.0 mm or less.

  A plate-like member made of a high-strength steel plate and having a flange at the end, such as a hat-shaped structural member shown in FIG. 5, is overlapped with a similar flange or steel plate and laser welded between the two to form a frame When manufacturing the member, as disclosed in Patent Document 2, for example, the entire frame member is lighter by using a structural member having a narrower flange width (width where the steel plates overlap) A, such as within 8 mm. In order to reduce the distance, the distance B from the welded portion to the flange end must be shorter than the range of 1.5 mm or more. Under such conditions, as shown in FIG. Even if welding is started at a position away from the end in the longitudinal direction, as described above, a portion deformed by heat conduction from the weld may pull the weld bead during solidification and solidification cracks may occur. .

  In order to reduce the weight of the structural member, it is more effective and preferable to make the flange width within 8 mm. If the flange width is less than 3 mm, the flange portion melts and cannot be welded, so the lower limit of the flange width is preferably 3 mm or more. Furthermore, in such a flange width, the distance from the weld bead to the flange end is preferably 1.5 mm or more. This is because if it is less than 1.5 mm, the flange end side melts to the flange end and is easily melted off as it is.

  Therefore, the present inventors investigated the cause of solidification cracking, and first examined the relationship between the weld metal component and solidification cracking.

  FIG. 6 shows the relationship between the temperature of the solidification process in the overlap laser welding of thin steel plates and the strain generated around the weld. The figure uses a high-tensile steel plate with a tensile strength of 590 MPa having a thickness of 1.2 mm and containing, by mass%, C content of 0.06%, Si content of 0.5%, and Mn content of 1.5%. This was obtained by using a sample obtained by laser welding at a laser processing point output of 2 kW and a welding speed of 2 m / min.

  FIG. 6 shows that 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 beam irradiation position, a large strain in the tensile direction occurs, 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, immediately 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 and solidification cracks. 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 increase the solidification temperature range even when added in a small amount, as shown in FIG.

  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 as shown by the bold broken line in FIG. It is considered that it is an element that tends to decrease, easily spread 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 the component range, and the component range is determined by each range of C, P, and S amount. it is conceivable that.

  Therefore, in Patent Documents 1 and 2, the components of the weld metal as shown in FIG. 4 are C <0.05% and 0.03% ≦ P + S or 0.05 ≦ C ≦ 0.08%. It is assumed that solidification cracks occur in the range of.

  However, as shown in FIG. 10, the relationship between the solute concentration and the solidification temperature range is different for C, P, and S, and S and P are easy to widen the solidification temperature range for C. From this, the present inventors considered that it was insufficient to arrange the easiness of solidification cracking with C alone.

  And even if the composition of the weld metal which is easy to generate the solidification crack shown in Patent Documents 1 and 2 is in the range of 0.05 ≦ C ≦ 0.08%, the solidification crack occurs in the region of lower P and lower S. Considering that there is no P and S range, the range of the amount of component C in the weld metal, where solidification cracking is likely to occur, is 0.05 ≦ C ≦ 0.08%. I did it.

In the experiment, a steel sheet having a thickness of 0.5 to 3.2 mm and a tensile strength of 270 MPa to a tensile strength of 780 MPa was used, and the weld metal content was in the range of C amount 0.05 ≦ C ≦ 0.08%. The hat-type structural members as shown in FIG. 5 were superposed under the following superposition conditions by combining the same species or different species so as to contain various amounts of P and S.
Flange width A: 8mm
Distance B: 3.0mm from the center of the weld bead to the edge of the steel plate
Distance from flange longitudinal end to welding start point C: 5 mm

  Then, laser welding was performed under the welding conditions of a laser processing point output of 3.5 kW and a welding speed of 2 m / min.

  The results are shown in FIG. 8, organized by the presence or absence of cracks. A region below the straight line (4S + P = 0.024) indicated by (1) at the lower left in FIG. 8 is a region where solidification cracks do not occur, and (2) is a region where solidification cracks occur.

  From this result, at least one steel plate is superposed with a plurality of steel plates made of high-tensile steel plates, and irradiated with laser light from the superposition direction so as to be melted to the back surface of the superposed lower steel plate. When welding is started from a position away from the welding direction end of the combined steel plates, the laser beam is moved along the ends of the superposed steel plates to form welds, and the superposed steel plates are welded together It was found that laser welding can be performed without causing solidification cracks if a weld metal with 0.05 ≦ C ≦ 0.08% and 4S + P <0.024% is formed in the weld.

  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 is not necessarily clear, but the present inventors believe that the reason is as follows. .

  The low P, S region (4S + P <0.024%) shown in (1) of FIG. 8 has a high solid phase temperature, and solidification occurs before the tensile force increases as shown by region (1) in FIG. Complete. In this case, a tensile force is generated, but since it is considered to be within the range of Dmin shown in FIG. 7, solidification cracks do not occur.

  Further, in the case of a relatively high P and S region (4S + P ≧ 0.024%) shown by (2) in FIG. 8, the solid phase temperature is relatively low and the tensile force is maximized (2 in FIG. Solidification is completed in the area of Since the tensile force in this case is considered to pass through the embrittlement region shown in FIG. 7, solidification cracks occur.

  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 portion of the overlapped steel plates was examined, but when welding is started from the welding direction end portion of the overlapped steel plates, As shown in FIG. 2, it is necessary to restrain the end portion with a clamp jig so that the end portion does not open. If it does in this way, as shown in Drawing 3, it can weld without swelling in the middle.

  Further, as shown in FIG. 9, one steel plate 4 is overlapped with the other steel plate 1 so as to protrude in a direction perpendicular to the welding direction, and the overlapping portion is parallel to the welding direction or in the welding direction. In the case where the end portions are not aligned, if the distance D from which one steel plate 4 protrudes from the other steel plate 1 and the distance B from the end portion to the welded portion are increased to some extent, the end in the welding direction of the stacked steel plates Even if welding is started from the part, welding can be performed without generating solidification cracks by forming the weld metal as described above. 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, as shown in FIG. 5, when starting welding with a distance C from the flange longitudinal end, is the distance C the same as the distance B from the weld bead center of the weld to the flange width end? It is better to make the distance more than that.

  Furthermore, as the material of the steel plates to be lap welded, all of the plurality of steel plates are high-tensile steel plates, and at least one of the steel plates is a high-tensile steel plate, and the other steel plates are tensile compared to the high-tensile steel plates. In some cases, the strength of the steel is low, and the present invention is intended for 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 lap laser welding of steel plates having a tensile strength of less than 440 MPa. The upper limit of the steel plate strength level is not particularly limited as long as the components of the weld metal 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 achieve the scope of the invention, the tensile strength is preferably up to 980 MPa class.

  Examples of different combinations of steel plates include a combination of a steel plate having a tensile strength of 590 MPa and a steel plate having a 270 MPa class.

  In laser welding, when a filler metal such as a filler is not added separately, the weld metal component is an average component calculated from the base material component value and the plate thickness of each superposed steel plate, and calculated C, The material of the structural member is selected so that the values of P and S satisfy the above-mentioned definition of the present invention.

  When adding a filler material, it is necessary to calculate the values of C, P, and 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 a steel plate having a thickness of 1.2 mm and a tensile strength of 370 to 980 MPa was superimposed on a steel plate on a flat plate so that the end portions were aligned. The width of each overlapped portion was 8 mm. For these steel plates, samples with different amounts of P and S were used when the C amount of the weld metal was in the range of 0.05 ≦ C ≦ 0.08%. 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 a point separated by 5.0 mm from the flange longitudinal end was set as the start position.

  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 typically the example of the member which has a lap joint formed from the thin steel plate with a perspective view. It is a figure which shows typically an example of the solidification crack observed with the laser welding lap joint of a thin steel plate with a perspective view. It is a figure which shows typically the other example of the solidification crack observed with the laser welding lap joint of a thin steel plate with a perspective view. It is a figure which shows the relationship (relationship of C and P + S) of generation | occurrence | production of the solidification crack observed with the laser welding lap joint of a thin steel plate, and a weld metal component. It is a figure which illustrates typically the flange width and welding part position of a laser welding lap joint of a thin steel plate with a perspective view. 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 the generation | occurrence | production of the solidification crack observed with the laser welding lap joint of a thin steel plate, and the component of a weld metal (relationship of P and S). It is a figure which illustrates typically the example of the joint in the case of starting welding from the edge part of a flange in the case of welding of the laser welding lap joint of a thin steel plate with a perspective view. It is a figure explaining the relationship between the solute density | concentration of a Fe-2 binary system alloy phase diagram, and the solidification temperature range.

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 Steel plate 5 Welding start point 6 Welded part (weld bead)
7 Solidification cracking portion 8 Laser welding head A Flange width B Distance from welded portion to flange end C Distance from flange longitudinal end to welding start point D Distance from which one steel plate protrudes from the other steel plate

Claims (6)

A steel plate in which a plurality of thin steel plates having a thickness of 0.5 to 3.2 mm are superposed, and the laser light is superimposed while irradiating laser light from the superposition direction so as to melt to the back surface of the superposed lower steel plate. In the method of laminating laser welding of thin steel plates, welding the stacked steel plates to each other, forming a weld by moving along the end of
At least one of the plurality of steel plates is a high strength steel plate having a tensile strength of 440 MPa or higher, and the weld metal in the weld zone is 0.05 ≦ C ≦ 0.08 mass%, 4S + P <0. A method of forming a welded portion satisfying a relationship of 024% by mass, a method for laminating laser welding of thin steel plates.   The method of laminating a thin steel plate according to claim 1, wherein welding is started at a position away from an end portion in the welding direction of the superposed steel plates.   At least one steel plate of the plurality of steel plates is overlapped so as to protrude in a direction perpendicular to the welding direction from the other steel plates, and welding is performed from an end portion in the welding direction of the stacked steel plates. The method of lap laser welding of thin steel sheets according to claim 1, characterized by starting.   At least one steel plate of the plurality of 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 steel plate end portions parallel to the welding direction or along the welding direction. 4. The method of laminating a thin steel plate according to claim 1, wherein the mating portion is obtained by superimposing the flange of the structural member and another steel plate. 5.   The cross section of the at least one steel plate is a hat-shaped structural member having a bent portion and a flange portion on both sides of an end portion of the steel plate parallel to the welding direction or along the welding direction. The method of laminating laser welding of thin steel plates as described in 1.   6. The method of laminating a thin steel sheet according to claim 4, wherein the flange portion has a width of 8 mm or less.

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