Laser welding method and device

Abstract

To reduce the risk of fracture in a laser-welded joint by smoothing a welded part step 5 and suppressing the formation of a groove 6 in a welded part 4 during laser welding that joins steel sheets of different thicknesses using a weld material.SOLUTION: A laser welding method includes: a step 100 of setting a gap center position C, a gap amount G, and an upper surface step amount D at opposing end faces of steel sheets 1, 2; a step 110 of measuring the set G and / or D; a step 120 of positioning a laser optical axis 8 at the gap center position C; a step 130 of deriving a shift direction and a shift amount S of the laser optical axis from the position C from each measured value or one measured value of G and D; and a step 140 of performing laser welding by shifting the laser optical axis 8 by the shift amount S in the derived shift direction.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a laser welding method and apparatus for joining two steel plates of different thicknesses on a continuous production line. [Background technology]

[0002] Generally, in continuous production lines (hereinafter simply referred to as "lines") such as steel sheet rolling lines, annealing lines, and plating lines, a leading sheet and a trailing sheet are butted together to join their opposing end faces and continuously processed as a single sheet in order to improve productivity. Laser welding is often used to join steel sheets in these continuous production lines, and a welding material (filler) is used to fill the gap in the welded portion.

[0003] Generally, the width and thickness dimensions of two steel plates passing through a line are not necessarily identical. When these two steel plates are laser welded together, the two steel plates (leading plate 1 and trailing plate 2) are usually aligned in the center (Fig. 2(a)) or bottom-side alignment (Fig. 2(b)). Here, "center alignment" refers to a configuration in which the thickness centers TC of the two steel plates 1 and 2 are aligned on the same plane, and "bottom-side alignment" refers to a configuration in which the bottom surfaces BS are aligned on the same plane. Therefore, the two steel plates 1 and 2 have a so-called thickness step 3, where their top surfaces are not on the same plane. Note that in Fig. 2 and the figures shown below, leading plate 1 is the thick side and trailing plate 2 is the thin side, but the reverse case (not shown) also has a thickness step.

[0004] In a laser-welded joint between two steel plates 1 and 2 with a thickness difference 3, as shown in Figure 3, the upper surface of the weld 4, where the welding material is primarily molten, is flattened by a swaging roll after welding to form a swaged surface 7, but there is a weld step 5 connected to it. This weld step 5 acts as a notch, reducing the fracture strength of the laser-welded joint. Note that the weld 4 in Figure 3 only shows the gap-filling portion and excess reinforcement between the steel plates 1 and 2 before joining; the molten and solidified portions of the steel plates 1 and 2 are not shown (the same applies below). Therefore, when it is predicted that the fracture strength of the laser-welded joint will decrease beyond a certain limit, a process is carried out in which the weld is removed, the welding conditions are changed, and the joint is re-welded (referred to as "rewelding").

[0005] However, rewelding can lead to problems such as reduced production efficiency, and if the reduction in fracture strength of the laser-welded joint is incorrectly predicted, the laser-welded joint can fracture on the line, resulting in a significant drop in production efficiency.

[0006] On the other hand, in laser welding for joining metal plates of different thicknesses without using welding material, a method is known in which all laser light (all laser beams) is irradiated onto the thicker plate, as shown in Fig. 9 (Patent Document 1). Note that 8 is the central axis of the laser beam, hereinafter referred to as the "laser optical axis."

[0007] Patent Document 1 claims that this method eliminates sharp steps on the upper surface of the weld and depressions on the lower surface, preventing die galling during press molding and mitigating stress concentration on the weld bead, resulting in a good press-molded product.

[0008] In addition, in laser welding using welding materials, a welding method is used in which the laser beam is irradiated onto the thicker side of the plate material or the laser beam axis 8 is positioned so that it overlaps with the thicker side of the steel material. However, with such welding methods, as shown in Figure 4, a groove 6 occurs in the welded part 4, increasing the risk of fracture and forcing more frequent rewelding, making it difficult to improve production efficiency. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-218497 [Non-patent literature]

[0010] [Non-Patent Document 1] https: / / www.keyence.co.jp "Ultra-high-definition inline profile measuring instrument LJ-X series" Summary of the Invention [Problem to be solved by the invention]

[0011] In view of the above-mentioned problems, the present invention aims to reduce the risk of fracture of a laser welded joint by reducing the weld step 5 and suppressing the occurrence of grooves 6 in a weld 4 during laser welding, which joins steel plates of different thicknesses using a welding material. [Means for solving the problem]

[0012] The inventors conducted extensive research to achieve the above-mentioned object, and came to the following conclusion about the mechanism by which groove 6 occurs when all of the laser light is irradiated onto the thicker side of the plate material during laser welding using a welding material. That is, the molten liquefied portion on the thicker side is first liquefied due to the energy during welding. Then, the surface tension of the molten liquefied portion attracts the molten liquefied portion on the thinner side, which liquefies later, and a depression is formed on the surface of the molten liquefied portion from the thicker side to the thinner side. It was presumed that the molten liquefied portion solidifies while retaining the depression on its surface, and the depression becomes groove 6.

[0013] Based on the above assumption, as shown in Figure 5, we initially positioned the laser optical axis 8 at the gap center position C of the welded part, and attempted to shift it from position C to the thinner-wall side. Here, G is the gap amount, D is the upper surface step amount, and S is the shift amount. The gap amount G is the distance between the opposing end faces, the upper surface step amount D is the distance in the plate thickness direction of the plate thickness step 3, and the shift amount S is the distance between the laser optical axis 8 and the gap center position C.

[0014] It was then found that the appropriate shift (movement) range of the laser optical axis 8 should be within a specific region (hereinafter referred to as specific region 11) shown in Fig. 6, which will be described later, taking into account the relationship between the gap amount G and the upper surface step amount D. In other words, by shifting the laser optical axis 8 from the initially set gap center position C to within the range of position P, the occurrence of grooves 6 (Fig. 4) in the welded portion 4 is significantly suppressed. Furthermore, position P is the upper limit position S of specific region 11, and may also be within gap 10.

[0015] Also, an attempt was made to shift the position of the laser optical axis 8 from the gap center position C of the welded portion toward the thicker side, but this had little effect in suppressing the formation of the grooves 6.

[0016] The specific region 11 can be determined by organizing past laser welding operational results or experimental data, for example, as shown in Figure 6. The welding laser is a CO2 laser, YAG laser, semiconductor laser, fiber laser, or disk laser, with an output of 8 to 12 kW, a circular laser beam with a beam diameter of 20 to 30 mm, and high-tensile steel, carbon steel, or stainless steel, with a thickness of 1.8 to 6.0 mm and a width of 600 to 1850 mm. The welding material is a carbon or nickel alloy filler wire with a diameter of 0.9 to 1.2 mm.

[0017] FIG. 6 shows the following for gap amounts G of 0.01 mm, 0.10 mm, and 0.20 mm (FIGS. 6(a), 6(b), and 6(c)). Specifically, the specific region 11 was determined taking into account the distribution of ○, △, and × within the range of upper surface step D = 0.00 to 1.00 mm and shift amount S to the thinner side = 0.00 to 0.15 mm. Here, ○ indicates a case where there is no groove 6 as shown in FIG. 4 and the weld step 5 is gentle; △ indicates a case where there is either a groove 6 or a sharp weld step 5; and × indicates a case where there is a groove 6 and the weld step 5 is sharp. Note that when D = 0.00 mm, welding is possible without using welding material, and this is outside the scope of the present invention, and therefore is excluded from the specific region 11. In the specific region 11, grooves 6 are unlikely to form in the weld 4, and the weld step 5 is gentle. Shear cutting or laser cutting is preferably used for the cutting process to form the butt surfaces (opposing end surfaces) of steel plates 1 and 2. Due to cutting process errors, the processed surface may have inclination or unevenness from the vertical surface, and the value of G may vary depending on the position in the steel plate width direction. Therefore, the value of G at the center of the steel plate width direction was used.

[0018] The present invention was completed through further investigation based on the above findings, and has the following gist and configuration. [1] A laser welding method for joining the opposing end faces of two steel plates with different thicknesses using a welding material, a step 100 for setting a gap center position, a gap amount, and an upper surface step amount of the opposing end surfaces; a step 110 of measuring the set gap amount and / or upper surface step amount; Step 120 of positioning the laser optical axis at the center of the gap; a step 130 of deriving a shift direction and a shift amount of the laser optical axis from the gap center position from each measured value of the gap amount and the upper surface step amount, or from the measured value of either one and a set value of the other; and performing laser welding by shifting the laser optical axis in the derived shift direction by the shift amount (140). [2] The laser welding method according to [1], wherein the shift direction is toward the thinner of the two steel plates. [3] The laser welding method according to [1] or [2], further comprising a step of measuring at least one of the gap amount and the upper surface step amount using a sensor that travels ahead of the laser optical axis. [4] The laser welding method according to any one of [1] to [3], wherein the gap amount is set to a value of 0.01 to 0.50 mm. [5] The laser welding method according to any one of [1] to [4], wherein the two steel plates are a leading plate and a trailing plate on a continuous production line. [6] A laser welding device that joins the opposing end faces of two steel plates with different thicknesses using welding material, a means for setting a gap center position, a gap amount, and an upper surface step amount of the opposing end surfaces; a means for measuring the set gap amount and / or upper surface step amount; a means for positioning the laser optical axis at the center of the gap; a means for deriving a shift direction and a shift amount of the laser optical axis from the center position of the gap based on the measured values ​​of the gap amount and the upper surface step amount; a means for moving the laser beam axis in the derived shift direction by the shift amount to perform laser welding. [7] The laser welding device according to [6], further comprising a means for measuring the gap amount and / or the upper surface step amount, which travels ahead of the laser optical axis. [Effects of the Invention]

[0019] According to the present invention, in laser welding in which steel plates of different thicknesses are joined together using a welding material, the weld step 5 can be made gentler and the occurrence of grooves 6 in the weld 4 can be suppressed, thereby reducing the risk of fracture of the laser welded joint. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a process diagram illustrating an example of an embodiment of the present invention. [Figure 2] 10A and 10B are schematic diagrams showing thickness differences, where (a) shows the case of center alignment and (b) shows the case of bottom alignment. [Figure 3] FIG. 10 is a schematic diagram showing a step at a welded portion. [Figure 4] FIG. 10 is a schematic diagram showing a groove in a weld. [Figure 5] 10 is an explanatory diagram showing the shift direction, shift amount S, and specific area of ​​the laser optical axis. FIG. [Figure 6] FIG. 10 is a diagram showing a specific example of a specific region. [Figure 7] FIG. 1 is a schematic diagram showing a laser welding device. [Figure 8] FIG. 10 is a schematic diagram showing a process of measuring G and / or D by moving a shape sensor ahead of the laser optical axis. [Figure 9] FIG. 10 is a schematic diagram showing a method of irradiating all laser light onto the thicker side of the plate material. [Figure 10] 1 is a schematic diagram illustrating the definition of the step sharpness of a welded portion step 5 and the groove depth of a groove 6. FIG. [Figure 11] FIG. 1 is a schematic diagram showing a method of using a welding material. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, an embodiment of the present invention will be described. When the description is made with reference to the drawings, the same or corresponding components as those in the previous drawings will be given the same reference numerals, and the description thereof may be omitted.

[0022] The method of the present invention is a laser welding method for joining opposing end faces of two steel plates of different thicknesses using a welding material, and the apparatus of the present invention is a laser welding apparatus used to carry out the method of the present invention.

[0023] [Steel plate] The steel types of the steel plates for both the leading and trailing plates include hot-rolled mild steel plates and strips, hot-rolled steel plates and strips for automotive structures, hot-rolled high-tensile steel plates and strips for automotive workability, and hot-rolled stainless steel plates and strips, as specified in JIS. Joining of different steel types is also possible. In the case of a continuous line, the plate thickness at the time of joining for both the leading and trailing plates is 1.8 to 6.0 mm. Examples of cutting processes for forming the opposing end faces of the leading plate and the trailing plate include shear cutting and laser cutting.

[0024] [Use of welding materials] In the present invention, a welding material is used to easily accommodate changes in the gap between the opposing end faces and changes in the required joint strength. Examples of welding materials include carbon wire and nickel alloy wire, and they are appropriately selected and used depending on the steel type, gap size, and other factors. When used, the tip of a wire- or rod-shaped welding material 15 with a diameter of 0.9 to 1.2 mm is preferably positioned above the gap 10, with a height h of 30 to 60 mm from the top surface of the thicker steel sheet 1 to the wire tip. The welding material 15 is aligned with the laser optical axis 8 as shown in FIG. 11 , and melted by the heat of the laser, resulting in a molten zone 16 that drips into the gap. If the height h is less than 30 mm, the welding material 15 and the steel sheet may come into contact, deteriorating the weld shape and reducing the weld strength. If the height h exceeds 60 mm, the laser beam may become unstable, resulting in insufficient heat input to the welding material and steel sheet, leading to poor penetration. A more preferable height h is 40 to 50 mm.

[0025] [Laser welding machine] The method of the present invention is carried out using a laser welding machine. The laser welding machine has a clamp 21 and a welding head 22 (Fig. 7). The clamp 21 holds the steel sheets 1 and 2, aligns their widthwise centers, butts their opposing end faces, and sets the coordinates of the gap center position C, the gap amount G, and the top surface step amount D to target values. The welding head 22 translates the laser optical axis 8 in a direction perpendicular to the opposing end faces to align it with the target position, and performs welding under conditions such as predetermined laser output and welding speed. The welding head 22 may also be equipped with a means (e.g., a wire feeder, not shown) for feeding the welding material to the intersection with the laser optical axis. The welding conditions will be described later.

[0026] [Steps according to the present invention] In the method of the present invention, steps 100, 110, 120, 130 and 140 are carried out in this order after the pre-step shown in the flow chart of FIG. (pre-process) The step preceding step 100 is a cutting process for forming the opposing end faces of the leading plate and the trailing plate. As a cutting method, shear cutting or laser cutting is preferred from the viewpoint of making the cut surfaces flat and vertical. (Step 100) In step 100, the gap center position C of the opposing end faces, gap amount G (hereinafter simply referred to as G), and upper surface step amount D (hereinafter simply referred to as D) are set. Regarding this setting method, in the case of center alignment (FIG. 2(a)), the leading sheet 1 and the trailing sheet 2 are held by clamps 21, and the positions of the center of the sheet width and the center of the sheet thickness TC of both sheets are aligned, and the calculated value of D = |thickness of leading sheet - thickness of trailing sheet| / 2 is set as the set value for D. In the case of bottom surface alignment (FIG. 2(b)), the positions of the center of the sheet width and the bottom surface BS of both sheets are aligned, and the calculated value of D = |thickness of leading sheet - thickness of trailing sheet| is set as the set value for D. Then, the distance between the opposing end faces of both sheets is adjusted to the target value and set as the set value for G, and the center position of the distance between the opposing end faces is set as the set value for gap center position C.

[0027] [Preferable setting range for gap amount G] The gap amount G set value in step 100 is preferably 0.01 to 0.50 mm. If G is less than 0.01 mm, laser welding may be possible without using welding material. On the other hand, if G exceeds 0.50 mm, the joint strength may be insufficient even if welding material is used. More preferably, G is 0.01 to 0.20 mm. Note that when G is set to 0.20 mm, as shown in FIG. 6(c), specific region 11 does not exist when D is 0.50 mm or more, so it is preferable to set D to more than 0.00 mm and less than 0.50 mm. Furthermore, when G exceeds 0.20 mm, it is preferable to separately and similarly determine in advance the specific region of the upper surface step amount D and shift amount S where no grooves will be generated. (Step 110) In step 110, G and / or D are measured. In the setting method described above, there may be a large error between the set values ​​of G and / or D and the actual values. Therefore, measured values ​​of either one or both of G and D are used to derive the shift amount S described below. Preferably, measured values ​​are used for both G and D. An example of a measurement location is the central position in the width direction of the steel sheet. An example of a method for measuring G and / or D is a method using an optical triangulation type shape sensor using a laser beam for distance measurement (for example, Non-Patent Document 1).

[0028] [Measurement of G and / or D by running the shape sensor ahead] When measuring G and / or D using this shape sensor, it is preferable to measure G and / or D by moving the shape sensor 30 ahead of the laser optical axis 8, as shown in Fig. 8. This provides information on the correspondence between a plurality of positions in the welding direction 40 and G and / or D, and enables the shift amount S to be derived with higher accuracy. Note that Fig. 8 shows a case where the shape sensor 30 measures both G and D, and the calculation means 50 derives S as the median value of the range of S within the specific region 11 in Fig. 6 from the measured values ​​of G and D by the shape sensor 30, and transmits this to the welding head 22. (Step 120) In step 120, the laser optical axis 8 is positioned at the gap center position C (FIG. 7). This positioning is performed as follows: The position of the laser optical axis 8 before the positioning is stored as the position at the end of the previous welding, and the welding head 22 is displaced from there to the gap center position C by the direction and magnitude of the displacement vector 60. (Step 130) In step 130, the shift direction and shift amount S of the laser optical axis 8 from the gap center position C are derived from the measured values ​​of G and D or the measured value of either one and the set value of the other. For this derivation, the specific area 11 shown in Fig. 6 is used. As for the shift direction, since the shift direction in Fig. 6 is the direction toward the thinner side, the shift direction can be derived as the direction toward the thinner side.

[0029] As mentioned above, shifting the laser optical axis 8 toward the thicker side is less effective in suppressing the occurrence of grooves 6 (FIG. 4), so it is preferable to shift it toward the thinner side.

[0030] The shift amount S can be estimated by linear interpolation and used as the median value of the range of the specific region 11 in Fig. 6 corresponding to the gap amount G and the upper surface step amount D. For example, when G = 0.15 mm and D = 0.42 mm, S = 0.10 mm according to Figs. 6(b) and (c). (Step 140) Step 140 is a step of performing laser welding (traveling while irradiating laser light) by shifting the laser optical axis 8 in the derived shift direction by the shift amount S in the welding head 22. The welding conditions are preferably as follows.

[0031] The laser beam diameter is preferably 10 to 40 mm. If the laser beam diameter is less than 10 mm, the joining strength may be insufficient, and if it exceeds 40 mm, the heat-affected zone becomes excessively large. The laser beam diameter is more preferably 20 to 30 mm.

[0032] The laser output is preferably 8 to 12 kW. If the laser output is less than 8 kW, the welding material and steel sheet will not penetrate sufficiently, and if it exceeds 12 kW, the welding material and steel sheet will break down into metal particles and scatter during welding, resulting in insufficient joint strength. The laser output is more preferably 8 to 10 kW.

[0033] The diameter of the welding material is preferably 0.5 to 1.5 mm. If the diameter of the welding material is less than 0.5 mm, the joint strength will be insufficient, and if it exceeds 1.5 mm, the excess filler will be unnecessarily large. The diameter of the welding material is more preferably 0.9 to 1.2 mm.

[0034] The welding speed is preferably 2 to 8 m / min. If the welding speed is less than 2 m / min, the excess metal will increase unnecessarily, and if it exceeds 8 m / min, the heat input will tend to be insufficient. The welding speed is more preferably 3 to 6 m / min.

[0035] [Application to continuous production lines] When applied to a continuous production line, the present invention reduces the risk of fracture of the laser welded joint, reduces the frequency of rewelding, and is therefore effective in improving production efficiency. Therefore, in the present invention, the two steel sheets 1 and 2 to be welded are preferably a leading sheet and a trailing sheet on a continuous production line. Examples of continuous production lines include rolling, annealing, and plating lines. Fracture of the laser welded joint is likely to occur in sections of the line where a relatively large tension is applied to the steel sheets, such as between upstream and downstream rolling rolls, between pinch rolls, or between bridle rolls.

[0036] [Device of the present invention] The apparatus of the present invention includes means for performing steps 100, 110, 120, 130, and 140. An example of a means for performing step 100 (setting the gap center position C, gap amount G, and upper surface step amount D) is a clamp 21 (FIG. 7). An example of a means for performing step 110 (measuring G and / or D) is a shape sensor 30 (FIG. 8). An example of a means for performing step 120 (positioning the laser optical axis 8 at the gap center position C) is a welding head 22 (FIG. 7). The welding head 22 also performs step 140 (shifting the laser optical axis by the shift amount S in the derived shift direction and performing welding). An example of a means for performing step 130 (deriving the shift direction and shift amount S) is a calculation means 50 (FIG. 8) equipped with logic that operates a specific region 11 (FIG. 6), determines the shift direction as the direction toward the thinner wall, and derives the shift amount S as the median value of the range of S within the specific region 11.

[0037] In order to improve the operability of measuring G and / or D by the preceding travel of the shape sensor, the device of the present invention is provided with a means for measuring G and / or D by traveling ahead of the laser optical axis 8. Such a means includes a shape sensor 30 (FIG. 8) and its traveling mechanism (not shown). The shape sensor 30 is used not only to measure G and / or D before welding, but also to measure the shape of the welded part after welding. [Example]

[0038] Hereinafter, the embodiments of the present invention will be described in more detail with reference to examples.

[0039] The present invention was implemented in a continuous rolling line in which hot-rolled steel sheets are joined together and cold-rolled, and this serves as an example of the present invention. While the preceding diagram illustrates only the case where the leading sheet 1 is the thick side and the trailing sheet 2 is the thin side, the reverse case is also included in the present invention. Steel sheets are rolled into a coil, unrolled into a strip, joined, and continuously rolled in a tandem rolling stand. After rolling, they are cut and wound into a coil, so the unit of measurement is called a coil. The unit weight of a coil (the weight of one coil) is 7 to 42 tons. Fracture of laser-welded joints is likely to occur between the stands of the tandem rolling stand.

[0040] The steel plate is high-tensile steel. The plate thickness at the time of joining is 4.5 mm on the thick side and 3.5 mm on the thin side. The plate width is the same, 1100 to 1150 mm. The gap amount G is 0.01 to 0.20 mm, and the upper surface step amount D is 0.01 to 1.00 mm.

[0041] The laser was a CO2 laser with an output of 8-10 kW, a beam diameter of 30 mm, and a welding speed of 4.0 m / min.

[0042] A carbon rod (0.9 mm in diameter) was used as the welding material 15. The tip of the carbon rod was placed above the gap, at a height h of 40 mm from the top surface of the thick-walled steel plate, and intersected with the laser optical axis 8, and the molten part 16 was dropped into the gap (Fig. 11).

[0043] The shift direction of the laser optical axis 8 was toward the thinner side, and a specific area 11 (Fig. 6) was used to derive the shift amount S. Note that a shape sensor 30 (Fig. 8) that travels ahead of the laser optical axis 8 was used to measure G and D. G and D were measured in correspondence with multiple positions in the welding direction 40.

[0044] A shape sensor 30 was used to evaluate the shape of the weld 4, and image processing was performed from the shape measurement results of the weld step 5 and groove 6 to determine the step sharpness H / W and groove depth Z shown in Figure 10, which were then controlled based on threshold values. Here, W is the dimension of the weld step 5 in the plate length direction, H is the dimension of the weld step 5 in the plate thickness direction, and Z is the dimension of the groove 6 in the plate thickness direction from the upper end of the thin-wall side to the groove bottom. If the step sharpness H / W and groove depth Z were below their respective threshold values ​​and / or if another cause (such as a welding defect) occurred, it was determined that rewelding was necessary; otherwise, it was determined that rewelding was not necessary.

[0045] As a result, in the conventional example, rewelding was required for 31 coils out of 667 coils, a rewelding rate of 4.6%. In contrast, in the example of the present invention, rewelding was required for 14 coils out of 597 coils, a rewelding rate of 2.3%, which is half that of the conventional example. In the example of the present invention, groove 6 did not occur, and H / W was below the threshold, so the need for rewelding was due to other factors. [Explanation of symbols]

[0046] 1, 2 steel plates (leading plate 1, trailing plate 2) 3 Plate thickness difference 4 Welded parts 5. Welding step 6 grooves 7 Swaging surface 8 Laser optical axis 10. Gap 11 Specific areas 15 Welding materials 16 Welding section 21 Clamp 22 Welding head 30 Shape sensor (shape sensor using optical triangulation method with laser beam for distance measurement) 40 Welding direction 50 Calculation means 60 Displacement Vector BS bottom side D Top surface step amount G gap amount S Shift amount TC thickness center

Claims

1. A laser welding method for joining opposing end surfaces of two steel plates having different plate thicknesses using a welding material, comprising: a step 100 of setting a gap center position, a gap amount, and an upper surface step amount of the opposing end surfaces; a step 110 of measuring the set gap amount and / or upper surface step amount; Step 120: positioning the laser optical axis at the gap center position; a step 130 of deriving a shift direction and a shift amount of the laser optical axis from the gap center position from each measured value of the gap amount and the upper surface step amount, or from the measured value of either one and a set value of the other; and performing laser welding by shifting the laser optical axis in the derived shift direction by the shift amount (140).

2. 2. The laser welding method according to claim 1, wherein the shift direction is a shift direction toward the thinner of the two steel plates.

3. 3. The laser welding method according to claim 1, further comprising the step of measuring at least one of the gap amount and the upper surface step amount with a sensor that travels ahead of the laser optical axis.

4. 3. The laser welding method according to claim 1, wherein the gap amount is set to a value in the range of 0.01 to 0.50 mm.

5. 4. The laser welding method according to claim 3, wherein the gap amount is set to a value of 0.01 to 0.50 mm.

6. 3. The laser welding method according to claim 1, wherein the two steel plates are a leading plate and a trailing plate on a continuous production line.

7. 4. The laser welding method according to claim 3, wherein the two steel plates are a leading plate and a trailing plate on a continuous production line.

8. 5. The laser welding method according to claim 4, wherein the two steel plates are a leading plate and a trailing plate on a continuous production line.

9. 6. The laser welding method according to claim 5, wherein the two steel plates are a leading plate and a trailing plate on a continuous production line.

10. A laser welding device that joins opposing end surfaces of two steel plates having different plate thicknesses using a welding material, a means for setting a gap center position, a gap amount, and an upper surface step amount of the opposing end surfaces; a means for measuring the set gap amount and / or upper surface step amount; a means for positioning the laser optical axis at the center of the gap; a means for deriving a shift direction and a shift amount of the laser optical axis from the center position of the gap based on the measured values ​​of the gap amount and the upper surface step amount; a means for moving the laser beam axis in the derived shift direction by the shift amount to perform laser welding.

11. 11. The laser welding apparatus according to claim 10, further comprising a means for measuring the gap amount and / or the upper surface step amount, the means traveling ahead of the laser beam axis.

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