JP2017087263A - Welding method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for welding a high strength steel plate by which stress concentration in a heat affected zone can be suppressed.SOLUTION: This welding method includes: a nugget formation process S2 in which overlapped upper side steel plate and lower side steel plate are irradiated with laser beam from an irradiation device, and the upper side steel plate and the lower sided steel plate are welded to each other thereby forming a nugget; and a tempering process S3 in which a heat affected zone in an intermediate conjugate on which a nugget is formed is irradiated with laser beam, which has a spot diameter at an irradiation point larger than laser beam in the nugget formation process S2 and has an output lower than that of laser beam in the nugget formation process S2, from the irradiation device.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a welding method for joining high-tensile steel plates.

  Patent Document 1 discloses a welding method in which high-tensile steel plates are joined by laser screw welding (LSW). In this welding method, the laser is irradiated a plurality of times in a substantially concentric manner. The superposed high-tensile steel plates are melted by laser irradiation to form a cylindrical nugget, thereby joining the high-tensile steel plates together.

JP 2015-077610 A

  In the welding method as disclosed in Patent Document 1, heat propagates around the nugget formation planned position along with the irradiation of the laser for forming the nugget. Part is formed. In this way, the heat-affected zone formed around the nugget has a relatively reduced hardness compared to the portion not affected by heat. That is, the heat affected zone is softened. In the case of welding using a laser, since high energy can be concentrated in a narrow range, the range of the heat affected zone becomes narrow. In the case of LSW, since the same portion is irradiated with the laser a plurality of times as described above, the hardness is likely to decrease particularly in the heat affected zone. In a joined body in which a high-strength steel plate is welded by LSW, when a load is applied to the joined body, stress concentration occurs in the heat-affected zone where the hardness is locally reduced, and there is a risk that a crack starting from the location will occur. It is known that there is.

  This invention is made | formed in view of such a situation, The objective is to provide the welding method of the high strength steel plate which can suppress the stress concentration in a heat affected zone.

Hereinafter, means for solving the above-described problems and the effects thereof will be described.
A welding method for solving the above-mentioned problem is a welding method for joining a plurality of high-tensile steel plates by laser screw welding. A nugget forming step of forming high-strength steel sheets to form a nugget, the nugget, and a heat-affected zone formed around the nugget as the nugget is formed, in the intermediate joined body formed A tempering step in which a spot diameter of an irradiation point is larger than the laser in the nugget formation step and a laser having a lower output than the laser in the nugget formation step is irradiated from the irradiation device with respect to the heat affected zone. Including.

  According to the welding method, a nugget in which the superposed high-tensile steel plates are welded is formed by laser irradiation in the nugget forming step. And by irradiating a laser in the tempering process to the intermediate bonded body in which the heat affected zone is formed, it is possible to manufacture a bonded body that further expands the softened area around the nugget in the high-tensile steel plate. . The laser irradiated in the tempering process has a large spot diameter at the irradiation point and a low output. Therefore, the high-tensile steel plate can be heated so as not to melt the high-tensile steel plate. And the hardness of an irradiation point and the periphery of an irradiation point can be reduced by such heating.

  According to the above welding method, the hardness of the portion where the hardness is kept high is lowered in the heat-affected zone where the hardness is locally lowered by the laser irradiation in the tempering step. Therefore, the state where the hardness is locally reduced in a narrow range is eliminated. Thus, by intentionally expanding the range of the portion where the hardness is reduced, it is possible to suppress stress concentration in a narrow range and to suppress stress concentration in the heat affected zone. In other words, since the stress can be dispersed in a wider range and the ductility of the nugget peripheral portion can be improved, the yield strength of the joined body can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS Schematic which shows the steel plate which is the welding object of the welding method as one Embodiment, and the laser irradiation apparatus used for the welding method. The flowchart which shows the order of each process in the welding method of the embodiment. FIG. 3 is a view showing a cross-sectional structure of an intermediate joined body obtained by a nugget forming step in the welding method of the embodiment, and is a cross-sectional view of a portion indicated by line 3-3 in FIG. FIG. 4 is a cross-sectional view showing a cross-sectional structure of a joined body manufactured by the welding method of the embodiment, a cross-sectional view of a portion corresponding to the portion shown in FIG. 3, and hardness distribution in each portion of this portion. The figure which shows a relationship. The figure which shows the cross-section of the steel plate in the welding method as a comparative example.

Hereinafter, an embodiment of a welding method will be described with reference to FIGS.
As shown in FIG. 1, the welding method of the present embodiment is used for joining an upper steel plate 30 and a lower steel plate 40. Both the upper steel plate 30 and the lower steel plate 40 are high-tensile steel plates. The upper steel plate 30 includes flanges 31 at both ends, and is formed into a hat shape in which a central portion 32 located between the flanges 31 is bent into a trapezoidal shape. The upper steel plate 30 and the lower steel plate 30 and the lower steel plate 40 are overlapped with a flat plate-like lower steel plate 40, and a plurality of overlapping portions are welded in a dotted manner to form a closed cross-sectional structure. The side steel plate 40 is joined.

  In the welding method of the present embodiment, welding is performed using a laser irradiated from the irradiation device 51. The irradiation device 51 is connected to the control device 61. The control device 61 transmits a command signal to the irradiation device 51 to control the output / spot diameter of the laser to be irradiated, laser scanning, and the like.

Each process of the welding method of this embodiment is demonstrated using FIG.2 and FIG.3.
As shown in FIG. 2, the welding method of the present embodiment includes an alignment step S1, a nugget forming step S2, and a tempering step S3.

  In the alignment step S1 performed first, the upper steel plate 30 and the lower steel plate 40 are overlapped as shown in FIG. Then, the positions of the upper steel plate 30 and the lower steel plate 40 are adjusted. Note that when the upper steel plate 30 and the lower steel plate 40 are melted in the next nugget forming step S2, the coating layers present on the surfaces of the upper steel plate 30 and the lower steel plate 40 are volatilized and gas is generated. Therefore, in this alignment process S1, alignment is performed so that a slight gap is generated between the flange 31 of the upper steel plate 30 and the lower steel plate 40 so that the gas generated at that time can be released. .

  In the welding method of the present embodiment, a nugget forming step S2 for welding the upper steel plate 30 and the lower steel plate 40 is performed after the alignment step S1. The nugget forming step S2 is a step of forming a plurality of nuggets 11 that join the upper steel plate 30 and the lower steel plate 40 together.

  As shown in FIG. 1, in the nugget forming step S <b> 2, the upper steel plate 30 and the lower steel plate are irradiated by irradiating a laser 52 from the irradiation device 51 to the flange 31 of the upper steel plate 30 superimposed on the lower steel plate 40. 40 is melted. As the molten upper steel plate 30 and lower steel plate 40 are solidified, the nugget 11 is formed at the irradiation point of the laser 52. In this way, the nugget 11 is formed with respect to a some location, and the some location is welded to dot shape, The intermediate joined body 10 which joined the upper side steel plate 30 and the lower side steel plate 40 is obtained.

  In the nugget forming step S2, each nugget 11 is formed by LSW. In the LSW, when one nugget 11 is formed, the laser 52 is irradiated a plurality of times from the irradiation device 51 toward the position where the nugget 11 is to be formed. Specifically, the laser 52 is irradiated so as to draw a circular scanning locus as the first irradiation. Then, the second irradiation and the third irradiation are performed so as to draw a scanning locus substantially the same as the first scanning locus. Next, the fourth irradiation is performed on the inner side in the radial direction of the scanning locus. Further, the fifth irradiation is performed on the inner side in the radial direction of the scanning locus in the fourth irradiation. In this way, the upper steel plate 30 and the lower steel plate 40 are melted. And the nugget 11 which joins the upper side steel plate 30 and the lower side steel plate 40 is formed because the fuse | melted upper side steel plate 30 and the lower side steel plate 40 solidify. The number of times of laser irradiation with the LSW can be changed as appropriate. In addition, an elliptical shape, a semicircular shape, a polygonal shape, a spiral shape, or the like can be adopted as the scanning locus.

  Here, the intermediate joined body 10 is demonstrated using FIG. A plurality of cylindrical nuggets 11 are formed on the intermediate bonded body 10 by LSW in the nugget forming step S2. When each nugget 11 is formed, the heat of the laser 52 propagates around each nugget 11. Therefore, the heat affected zone 12 is formed in the intermediate joined body 10 so as to surround each nugget 11. The heat-affected zone 12 is a portion where the structure of the upper steel plate 30 and the lower steel plate 40 is changed and softened by the propagation of the heat of the laser 52. That is, the heat affected zone 12 is a portion where the hardness is reduced in the intermediate bonded body 10.

  As shown in FIG. 2, in the welding method of the present embodiment, a tempering step S3 is performed after the nugget forming step S2. In the tempering step S3, as shown in FIG. 3, the intermediate bonded body 10 is heated again by irradiating the intermediate bonded body 10 with a laser. Here, of the heat affected zone 12 formed around the nugget 11 in the intermediate joined body 10, the portion closer to the central portion 32 (for example, the range shown on the right side of the heat affected zone 12 in FIG. 3). Irradiate laser. In the tempering step S3, the laser is irradiated from the irradiation device 51 as in the nugget forming step S2. At this time, the control device 61 makes the output of the laser 53 lower than that of the laser 52 in the nugget formation step S2 so that the spot diameter of the irradiation point of the laser 53 becomes larger than that of the laser 52 in the nugget formation step S2. The irradiation device 51 is controlled. The laser 53 irradiated in the tempering step S3 has a low output as compared with the laser 52 in the nugget forming step S2, and has a larger spot diameter. Therefore, the energy is dispersed as compared with the laser 52 that concentrates the energy in a narrow range. In this way, the irradiation with the laser 53 in the tempering step S3 performs heating in a temperature range that does not exceed the melting point of the upper steel plate 30. By such heating, the structure of the upper steel plate 30 is changed and the hardness is lowered.

As described above, the joined body 20 in which the upper steel plate 30 and the lower steel plate 40 are joined is manufactured through the steps S1 to S3.
Next, the effect | action of the welding method concerning this embodiment is demonstrated.

  By the irradiation of the laser 52 in the nugget forming step S2, the nugget 11 in which the superposed upper steel plate 30 and the lower steel plate 40 are welded is formed. And the joined body which further expanded the softened range around the nugget 11 in the intermediate joined body 10 by irradiating the intermediate joined body 10 in which the heat affected zone 12 is formed with the laser 53 in the tempering step S3. 20 is manufactured.

  The joined body 20 manufactured by the welding method of this embodiment and the distribution of hardness corresponding to each part of the joined body 20 will be described with reference to FIG. In FIG. 4, regarding the hardness distribution, the hardness distribution in the bonded body 20 is shown by a solid line, and the hardness distribution in the intermediate bonded body 10 before undergoing the tempering step S3 is shown by a broken line for comparison.

  As shown by a broken line in FIG. 4, in the intermediate bonded body 10, the hardness locally decreases in the heat affected zone 12. On the other hand, as shown by the solid line, in the joined body 20 that has undergone the tempering step S3, the range of the portion where the hardness is reduced in the portion closer to the central portion 32 than the nugget 11 is enlarged.

  In FIG. 4, the range in which the hardness of the upper steel plate 30 is reduced by the irradiation of the laser 53 is shown as the softened region A. The softened region A includes a range where the laser 53 is irradiated and a range where heat is propagated along with the laser 53 irradiation.

Thus, in the welding method according to the present embodiment, the joined body 20 in which the state where the hardness is locally reduced in a narrow range is eliminated is manufactured.
By the way, in a joined body manufactured by welding high-strength steel plates, in order to avoid cracks starting from the heat-affected zone, a tempering process in which heat is applied to the high-strength steel plates before welding may be performed in advance. Generally known. As tempering performed before such welding, there are tempering partially performed on the planned welding position of the high-strength steel sheet and tempering performed on the entire high-strength steel sheet. The tempering performed before such welding has the following problems.

  First, the problem which arises when performing partial tempering before welding is demonstrated using FIG. When the first steel sheet 130 that has been partially tempered in advance in the tempering area B1 and the second steel sheet 140 that has been partially tempered in advance in the tempering area B2, the tempering area B1 and the tempering area B2 are the same. Even if it is a shape, the following deviations (a), (b), and (c) occur. And there exists a possibility that a nugget and a heat affected zone may be formed in the range which has not been tempered.

  (A) The alignment error between the steel plates is a factor, and the tempering region B1 and the tempering region B2 are shifted. That is, a deviation D1 occurs between the tempering region B1 and the tempering region B2, and the region where the tempering region B1 and the tempering region B2 overlap is narrowed.

  (B) Since the steel plate that has been partially tempered in advance is welded, it is necessary to perform the welding aiming at the tempered region, but the nugget actually formed deviates from the planned welding position. There is a fear. For example, if welding should be performed at the position of the axis C1 but shifted to the position of the axis C2, there is a possibility that the shift D2 occurs.

(C) There is a possibility that the range of the nugget and the heat-affected zone may be shifted due to the formation of the nugget having a larger diameter than expected.
When the nugget or the heat-affected zone is formed in a range where tempering is not performed due to the deviations shown in (a), (b), and (c), the effect is obtained despite the tempering process. Cannot be obtained sufficiently. In addition, in consideration of the influence of such deviation, it is conceivable to set a range of partial tempering performed in advance, but in that case, the range of the softened portion in each steel plate is expanded. As a result, the rigidity of the joined body is lowered. Further, when partial tempering is performed, each steel plate to be welded is individually tempered, which increases the number of man-hours until the joined body is manufactured.

  On the other hand, when tempering the entire steel sheet, since the entire steel sheet is tempered, there is no possibility that a nugget or a heat-affected zone is formed in a range where tempering is not performed. However, when tempering is performed on the entire steel sheet, tempering is also performed on the portion that is not the planned welding position. That is, even a portion that does not need to be softened is softened. Therefore, there exists a possibility that the rigidity of the joined body manufactured using the said steel plate may fall.

  Compared with such a conventional welding method, in the welding method in the present embodiment, the laser is irradiated from the irradiation device 51 in both the nugget formation step S2 and the tempering step S3. Therefore, the nugget forming step S2 and the tempering step S3 can be continuously performed using the same apparatus. Therefore, the welding process is simplified as compared with a conventional welding method in which welding is performed after partial tempering is performed on each steel sheet to be joined.

  Moreover, in the welding method in this embodiment, tempering process S3 is performed with respect to the intermediate joined body 10 after nugget 11 formation. Therefore, there is no possibility that the deviation exemplified in the above (a), (b), and (c) occurs. That is, the formation of nuggets and heat-affected zones in a range where tempering is not performed is suppressed. Furthermore, since it is a partial tempering while eliminating the possibility of the deviation exemplified in (A), (B), and (C), it is possible to suppress a decrease in rigidity of the joined body 20.

According to the welding method of the present embodiment described above, the following effects can be obtained.
(1) The laser 53 irradiated in the tempering step S3 performed subsequent to the nugget forming step S2 has a larger spot diameter at the irradiation point and a lower output than the laser 52 irradiated in the nugget forming step S2. Therefore, the intermediate bonded body 10 can be heated so as not to melt the intermediate bonded body 10. Such heating can reduce the irradiation point of the laser 53 and the hardness around the irradiation point.

  (2) By deliberately expanding the range of the portion where the hardness is lowered, it is possible to suppress stress concentration in a narrow range and to suppress stress concentration in the heat affected zone 12. That is, since the stress can be dispersed in a wider range and the ductility of the peripheral portion of the nugget 11 can be improved, the yield strength of the joined body 20 can be improved.

(3) The nugget forming step S2 and the tempering step S3 can be continuously performed using the same apparatus. Therefore, the welding process can be shortened or simplified.
(4) A tempering step S3 is performed on the intermediate bonded body 10 after the nugget 11 is formed. Therefore, tempering for suppressing the concentration of stress in a narrow range can be accurately performed at a necessary location.

In addition, the said embodiment can also be implemented with the following forms which changed this suitably.
The joined body 20 has a closed cross-section structure by welding the upper steel plate 30 and the lower steel plate 40 as shown in FIG. Therefore, when a load is applied to the joined body 20, the load is applied to the central portion 32 side portion of each nugget 11 provided on the flange 31. In the said embodiment, the laser 53 in the tempering process S3 was irradiated only to the part close | similar to the center part 32 among heat-affected parts in consideration of the three-dimensional structure of such conjugate | zygote 20. FIG. On the other hand, the laser 53 can be irradiated to the entire heat affected zone 12.

  In the tempering step S <b> 3, the laser 53 was irradiated to the heat affected zone 12. You may irradiate the laser 53 to the area | region containing the heat affected zone 12, ie, the range wider than the heat affected zone 12. Also in this case, the ductility can be improved and the yield strength of the joined body 20 can be improved as in the above embodiment.

  In the above embodiment, the plurality of nuggets 11 are formed in the nugget forming step S2, and the laser 53 is irradiated to the heat affected zone 12 formed around each nugget 11 in the tempering step S3. For each of the nuggets 11, a tempering step can be performed following the nugget forming step. In this case, after the nugget formation step and the tempering step, the nugget formation step and the tempering step are performed again at another nugget formation scheduled position. As described above, the joined body 20 can also be manufactured by repeating the nugget formation step and the tempering step according to the number of nuggets 11 to be formed.

  -In the said embodiment, although the example which joins the two high-tensile steel plates of the upper steel plate 30 and the lower steel plate 40 was shown, the welding method of the said embodiment is applied when joining three or more high-tensile steel plates. You can also

  In the alignment step S1, an example is shown in which the position is adjusted so that a gap for releasing gas is formed between the flange 31 of the upper steel plate 30 and the lower steel plate 40, but such a gap is not generated. Alternatively, the flange 31 of the upper steel plate 30 and the lower steel plate 40 may be overlapped.

  -The upper side steel plate 30 and the lower side steel plate 40 are not restricted to the shape illustrated to the said embodiment. By irradiating the intermediate bonded body 10 with a laser 53 having a large spot diameter and a low output so as to improve the ductility by reducing the hardness of a portion where stress concentration may occur in the bonded body 20. The same effects as in the above embodiment can be obtained.

  DESCRIPTION OF SYMBOLS 10 ... Intermediate joined body, 11 ... Nugget, 12 ... Heat affected zone, 20 ... Joined body, 30 ... Upper steel plate, 31 ... Flange, 32 ... Center part, 40 ... Lower steel plate, 51 ... Irradiation device, 52 ... Laser, 53 ... Laser, 61 ... Control device, A ... Softening region.

Claims (1)

A welding method for joining a plurality of high-tensile steel plates by laser screw welding,
A nugget forming step of irradiating a laser from an irradiation device to the superposed high-tensile steel plates to melt the high-tensile steel plates to form a nugget,
Irradiation of the heat-affected zone in the intermediate joint formed with the nugget and the heat-affected zone formed around the nugget as the nugget is formed rather than the laser in the nugget formation step. A tempering step in which a spot diameter of a spot is large and a laser having a lower output than the laser in the nugget forming step is irradiated from the irradiation device.

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