Method for welding coated steel plates

Abstract

The present invention provides a method for welding coated steel sheets, preferably for welding aluminum-silicon-based metal-coated steel sheets, comprising the following steps: providing a structure with two laser beams, said laser beams acting on the molten pool to be formed, at least one laser beam rotating about a rotation axis so that said laser beams move relative to each other, said laser beams being guided along the welding axis, in order to mix the molten pool, observing a defined stirring effect in conjunction with a defined welding speed, the following conditions applying to the stirring effect: wherein f rot is the rotation frequency, v w For welding speed, the following conditions apply:

Description

Technical Field

[0001] The invention relates to a method for welding coated steel sheets according to the preamble of claim 1 . Background Art

[0002] In automotive engineering, body components can be produced from highly hardenable steel sheets to ensure sufficient stability in the passenger compartment. High-strength steel offers advantages in this regard, as its high load-bearing capacity allows for reduced size and weight, which in turn reduces fuel consumption.

[0003] The type of the high-strength steel may be a manganese-boron alloy steel, such as 22MnB5. 22MnB5 is often used to increase strength.

[0004] These high-strength steels can be produced through a press-quenching process. In this process, they are heated to a high temperature to transform the original ferrite-pearlite steel structure into an austenite structure. When quenched at a rate exceeding the critical hardening rate, the iron's austenite high-temperature structure allows it to transform into a martensite structure. Due to the different carbon solubility in austenite and martensite, lattice distortion occurs in this process, and hardnesses exceeding 1500 MPa can be achieved. This quenching process is well known and commonly used.

[0005] In press hardening, two different processes are used to produce body parts. A distinction is made between the direct process and the indirect process.

[0006] In the direct process, the flat bar is austenitized and then formed and quenched one or more times in a press-quenching tool.

[0007] This direct process is relatively advantageous, but it is more difficult to remove products with highly complex geometries from the mold.

[0008] In the indirect process, flat steel strips are first cold-formed to produce components, which can have highly complex geometries. The formed components are then austenitized and then quenched in press-hardening tools. This process usually requires no extensive forming steps. The forming tools have a contour adapted to the formed component and are used solely for quenching.

[0009] This steel sheet for press hardening can simply be embodied in the form of a flat bar with an anti-corrosion coating. In this respect, conventional anti-corrosion coatings are zinc coatings, zinc alloy coatings, aluminum coatings, or aluminum alloy coatings.

[0010] It is also possible to use assembled strips, ie tailor-welded blanks, where different types of steel are welded together or steel of the same type but of different thicknesses is welded together.

[0011] Coated starting strips are also a natural option here. For example, two steel sheets made of 22MnB5 with different thicknesses and aluminum silicon coating are welded together.

[0012] However, particularly when welding aluminum-silicon-coated plates, it has been determined that the aluminum from the coating significantly migrates into the weld seam. Aluminum in the weld seam has a negative impact on martensitic transformation or martensite formation. Furthermore, aluminum can influence the formation of intermetallic phases, which are relatively hard but also brittle. Therefore, they can become a source of cracking.

[0013] To avoid this problem when welding tailor-welded blanks, the aluminum layer can be removed from certain areas on both sides of the intended butt weld joint before welding to prevent aluminum-silicon from penetrating into the weld.

[0014] The disadvantage here is that in this case there is no corrosion protection in the weld area and, in particular during quench heating, scaling and decarburization of the weld and the edges adjacent to the weld can subsequently occur.

[0015] A further disadvantage here is that these methods for removing the aluminum-silicon coating constitute additional processing steps that are not easily controllable.

[0016] DE102012111118B3 discloses a method for laser welding one or more press-hardenable steels, in particular manganese-boron steels. The welding is carried out in a butt joint, and the thickness of the one or more workpieces is at least 1.8 mm, and / or the thickness at the butt joint is increased by at least 0.4 mm. During the laser welding process, a filler wire is introduced into the molten pool generated by the laser beam. In order to ensure that the weld can reliably harden to a martensitic structure during the hot forming process, the document discloses that at least one of manganese, chromium, molybdenum, silicon and nickel can be added to the filler wire to promote the formation of austenite in the molten pool generated by the laser beam. The weight percentage of the at least one alloying element present in the filler wire is at least 0.1% higher than the weight percentage of the alloying element present in the press-hardenable steel workpiece.

[0017] DE 10 2014 001 979 A1 discloses a method for laser welding one or more hardenable steel workpieces in a butt joint. The steel is preferably manganese-boron steel with a thickness between 0.5 and 1.8 mm and / or an increase in thickness of at least 0.2 to 0.4 mm at the butt joint. During laser welding, a filler wire is introduced into the molten pool. The molten pool is generated by a single laser beam. To ensure that the weld seam reliably hardens to a martensitic structure during hot forming, the document discloses that at least one of manganese, chromium, molybdenum, silicon, and nickel may be added to the filler wire to promote austenite formation.

[0018] EP2737971A1 discloses a tailor-welded blank and its manufacturing method. This method uses a method for joining sheets of different thicknesses or compositions to reduce quality issues in the weld area. A filler wire is also used, designed to avoid generating ferrite in the temperature range of 800-950°C. This method is particularly suitable for aluminum-silicon coated sheets. This filler wire should also contain a high content of austenite-stabilizing elements, particularly carbon or manganese.

[0019] EP1878531B1 discloses a method for laser-arc hybrid welding of workpieces coated with a metal surface, where the surface coating may include aluminum. A laser beam is combined with at least one arc to melt the metal and weld the part or parts. Prior to welding, an aluminum-silicon coating is deposited on a transversely cut surface of at least one part to be welded.

[0020] EP2942143B1 discloses a method for joining two blanks, each of which is a steel sheet coated with aluminum or an aluminum alloy. The two blanks are welded using a laser beam and an electric arc. The arc welding torch includes a filler wire electrode made of a steel alloy containing a stabilizing element. The laser and arc move in a welding direction, with the arc welding torch positioned in front of the laser beam in this direction.

[0021] EP2883646B1 discloses a method for joining two blanks, at least one of which includes an aluminum layer or an aluminum alloy layer. During the welding process, metal powder is added to the welding zone, and the metal powder is an iron-based powder containing a gamma-stabilizing element. The laser beam welding is a double-point laser beam welding.

[0022] EP2007545B1 discloses a method for producing welded components with excellent mechanical properties. A steel plate is coated with a coating comprising an intermetallic compound layer and a metal alloy layer disposed on the intermetallic compound layer. To weld the steel plate, the metal alloy layer, located on the intermetallic compound layer, is removed from the periphery of the steel plate (i.e., the area to be welded). This metal alloy layer is typically an aluminum alloy layer. This metal alloy layer is removed using a laser beam to evaporate the aluminum-silicon alloy layer before welding, thereby preventing the aluminum from adversely affecting the weld. The intermetallic compound layer should remain in place to potentially provide corrosion protection.

[0023] WO2017 / 103149A1 discloses a welding method in which two separately generated laser beams are guided along a predetermined weld seam. The first laser beam is used to melt the flux-cored welding wire, while the second laser beam ensures mixing of the molten pool through a rotational motion. This ensures both melting and mixing of the filler material. Furthermore, a method is disclosed in which a guide laser beam is generated to melt the flux-cored welding wire. Simultaneously, a follower laser beam is split into two laser beams, which are guided sequentially along the weld seam. It is noted that in this case, there is no need to move these laser beams.

[0024] WO 2019 / 030249 A1 discloses a welding method in which two aluminum-silicon coated plates are joined by a laser that rotates to mix the molten pool during the welding process.

[0025] A disadvantage of WO 2017 / 103149 A1 is that the equipment technology is extremely expensive. It has also been shown that with the disclosed serial bifocals with lines, sufficient homogenization does not occur.

[0026] DE102017120051A1 discloses a method for laser beam welding of one or more steel plates produced by press hardening, at least one of which has an aluminum coating. Laser beam welding is performed by adding a filler wire to the molten pool generated solely by the laser beam. The filler wire should contain at least one austenite-stabilizing alloying element. The purpose of the method is to obtain a weld with a strength comparable to that of the base material after hot forming with low energy consumption and high production efficiency. To achieve this, it is recommended to set the laser beam to an oscillating motion so that the laser beam oscillates transversely to the welding direction, the oscillation frequency of the laser beam being at least 200 Hz, preferably at least 500 Hz, in order to eliminate the need to remove the aluminum coating from the edges of the plates to be welded.

[0027] JP 20040001084 discloses a welding method that combines a laser welding method and an arc welding method to improve gap bridging capability and deep welding. The laser can be designed as a laser having two beams, and the welding device is designed to rotate the two beams around each other.

[0028] DE102014107716B3 discloses a welding method, wherein in order to reduce the occurrence of welding spatter, during the welding process, the laser beam performing the welding is set to a superimposed three-dimensional oscillating motion while moving forward, and the oscillating motion is parallel or perpendicular to the butt joint.

[0029] A disadvantage of the above methods is that, for unknown reasons, stable load-bearing welds cannot be reliably produced in practice and the ultimate strength of the welds often varies widely. Summary of the Invention

[0030] The object of the present invention is to provide a welding method and to produce welds having reproducible mechanical properties.

[0031] This object is achieved by a method having the features of claim 1 .

[0032] The dependent claims also disclose advantageous modifications.

[0033] According to the present invention, it has been found that in methods where one laser beam melts the welding wire and two subsequent laser beams introduce additional energy into the weld pool, for unknown reasons the weld seams often have very different properties.

[0034] The inventors also discovered that even with optimal mixing of aluminum from the coating into the weld pool using a single laser for stirring and a welding wire, there was still enough aluminum, mathematically speaking, to prevent the weld from hardening sufficiently, especially in thinner steel sheets. It was also determined that an oscillating single-point laser clearly cannot guarantee consistent weld quality.

[0035] According to the present invention, the naturally existing weld pool current is superimposed with an additional forced current generated by the rotation of the dual laser beams in order to evenly distribute the filler wire and the aluminum from the coating in the weld seam.

[0036] The present inventors have discovered that a relationship exists between rotation frequency and welding speed, without which the desired stirring effect cannot be achieved.

[0037] In order to further enhance the desired effect, it has also proven advantageous to set the spot diameter and the spot spacing in a defined manner.

[0038] According to the present invention, at least two laser beams, in particular two sub-laser beams, are provided that rotate within the molten pool. In this regard, the welding laser can, for example, rotate symmetrically or asymmetrically around the center of the molten pool. One welding beam or laser beam rotates around the center of the molten pool at a smaller radius, while the second welding beam or laser beam rotates around the center of the molten pool at a larger radius.

[0039] In another possible embodiment, one laser beam moves along the center of the weld pool, while a second welding beam or laser beam oscillates or rotates along an orbit around the first beam. The present inventors discovered that the advance speed and stirring effect must be matched. The stirring effect in this case is defined as the number of revolutions divided by the advance distance. An incorrect combination of advance speed and stirring effect can lead to negative effects such as bulging, strong spatter, and even weld perforation. Excessively low advance speeds can negatively impact the economic viability of the process.

[0040] To this end, the present invention provides a method for welding coated steel sheets, preferably aluminum-silicon coated steel sheets, comprising: providing a structure with two laser beams, said laser beams acting on a molten pool to be formed, at least one laser beam rotating about a rotation axis so that said laser beams move relative to one another, said laser beams being guided along a welding axis, in order to mix the molten pool, observing a defined stirring effect in conjunction with a defined welding speed, the following conditions applying to the stirring effect:

[0041] ,

[0042] Where η is the stirring effect, frot is the rotation frequency, v w For welding speed, the following conditions apply:

[0043] .

[0044] In one embodiment of the present invention, the laser beams are symmetrically arranged around the rotation axis and rotate around the rotation axis at completely opposite positions, or one laser beam is guided along the welding axis and the other laser beam rotates around the first laser beam, or the first laser beam rotates around the rotation axis with a smaller first radius and the second laser beam rotates around the rotation axis with a larger radius, or the movement mode of the laser beams is a combination of the above movement modes.

[0045] In one embodiment, the symmetrical rotation of the laser beams, or more precisely the symmetrical rotation of the projection areas or spots of the laser beams (2, 3), each of which is spaced from the center by a spot spacing x df , spot diameter or laser beam diameter d f The total coverage area of ​​the laser beam is 0.1 mm to 1 mm, and the width of the total coverage area of ​​the laser beam is the sum of the spacing between the centers of the light spots and the diameter of a light spot, and the sum is 0.5 mm to 2.5 mm.

[0046] In this embodiment, the spot spacing x df Suitable for the following conditions:

[0047] .

[0048] In a subsequent embodiment, in a laser beam structure, two laser beams are positioned on a track, the first laser beam is maintained on the central axis of the molten pool along the welding forward direction, that is, on the welding axis, and the second laser beam or the second light spot rotates around the rotation axis, and the rotation axis is located on the welding axis or swings around the welding axis to constitute the center point of the first light spot.

[0049] In this embodiment, the spot diameter is 0.1-1 mm, and the following conditions apply:

[0050] and .

[0051] In another embodiment, a laser beam configuration includes two laser beams or two laser spots rotating about a rotation axis, wherein the first laser beam or the first laser spot rotates about the rotation axis at a first radius, and the second laser beam or the second laser spot rotates about the rotation axis at a second radius, wherein one radius is greater than the other radius, and the following conditions apply:

[0052] , , .

[0053] Among them, x off is the distance of the first laser beam from the axis of rotation and thus defines the eccentricity of the laser beams relative to each other.

[0054] In the above embodiment, it is advantageous if a laser with a power of 2 to 10 kW, in particular 3 to 8 kW, preferably 4 to 7 kW is used for welding.

[0055] In mm -1 As a unit, if the stirring effect η is 4~30 mm -1 , is also beneficial.

[0056] In another embodiment, the welding speed v w 5~12m / min, especially 6~10m / min.

[0057] The optimal process window for stirring effect also depends on the welding speed v w .

[0058] Advantageously, boron manganese steel is used as the base material, which can be hardened by austenitizing and quenching processes, and particularly preferably has a tensile strength greater than 900 MPa, in particular steel belonging to the CMnB steel group, such as 22MnB5 or 20MnB8 steel.

[0059] In this respect, it is advantageous if a steel with the following general alloy composition (in percentages by mass) is used as the base material:

[0060]

[0061] Residual iron and unavoidable smelting-related impurities.

[0062] Steel having the following general alloy composition (in mass %) may be advantageously used as the base material:

[0063]

[0064] Residual iron and smelting related impurities.

[0065] The present invention also relates to a sheet metal strip comprising a first steel plate and a second steel plate welded to each other according to the above method.

[0066] In one embodiment, it is advantageous if the first steel sheet and the second steel sheet have different alloy compositions. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] The present invention will be further described below with reference to the accompanying drawings, in which:

[0068] FIG1 is a schematic diagram of symmetrical, asymmetrical, and orbitally rotating welding laser beams;

[0069] Figure 2 is the process window related to the stirring effect according to the present invention;

[0070] Figure 3 Described Figure 2 , and provides the meaning of the peripheral area;

[0071] Figure 4 is a comparison table of 16 different tests according to an embodiment of the present invention and not according to an embodiment of the present invention;

[0072] Figure 5 Schematic diagram of a symmetrical stirring device with spot spacing and spot diameter functions, wherein the spot spacing and the spot diameter are related;

[0073] Figure 6 It is a process window with a symmetrical stirring device;

[0074] Figure 7 This is a schematic diagram of an orbital stirring device with spot spacing and spot diameter functions;

[0075] Figure 8 It is the process window of the orbital mixing device;

[0076] Figure 9 Schematic diagram of an asymmetric stirring device with spot spacing, spot diameter, and centrifugal function;

[0077] Figure 10 shows a hardened weld from a polished micrograph according to test T1 in the table;

[0078] Figure 11 A micrograph of the polish according to test T2 in the table is shown;

[0079] Figure 12 shows a polished photomicrograph of a weld tested according to T4 in the table; and

[0080] Figure 13 Shown are cross-sectional views of welds from polished micrographs according to the T16 test in the table. DETAILED DESCRIPTION

[0081] Figure 1 shows three different laser beam structures. These three different laser beam structures are the main possible situations and can be combined with each other. Among these three different laser beam structures, the one with a symmetrical structure (see Figure 1a ), the laser beams are arranged symmetrically with respect to the axis of rotation and in this case can also rotate in opposite positions around the axis of rotation.

[0082] The symmetrical structure advantageously achieves a maximum stirring effect.

[0083] Asymmetric structure (refer to Figure 1b ), one laser beam is closer to the axis of rotation than the other, resulting in eccentricity. Asymmetric structures can positively influence weld seam geometry.

[0084] In the track arrangement (see FIG. 1 c ), a central laser beam is provided which moves along the welding advance direction, while a second laser beam is spaced apart from the central laser beam and rotates around the central laser beam and the rotation axis.

[0085] The rail arrangement advantageously compensates for possible differences in the thickness of the sheet materials.

[0086] Figure 5 Detailed view of the symmetrical stirring device.

[0087] With this laser beam configuration 1, there are two laser beams 2, 3, each spaced approximately the same distance from an idealized melt pool center 4. Preferably, the idealized melt pool center 4 also coincides with the rotation axis 5, about which the two laser beams 2, 3 rotate according to rotation directions 6, 7. Thus, sequential positions 2', 3' of the sample are shown offset by 90°. The laser beams 2, 3, or more precisely, the projected areas (spots) of the laser beams 2, 3, have a given diameter d corresponding to the expanded arrows 8, 9. f .

[0088] The two laser beams 2 and 3, or more precisely, the projection areas (spots) of the two laser beams 2 and 3 as viewed from the center, are respectively centered at a spot spacing x. df Therefore, the theoretical molten pool width is equal to the sum of the spot spacing and half the diameter of each spot. The welding forward movement is along the idealized molten pool center 4 according to arrow 10 at a welding advancement speed v w conduct.

[0089] For this structure of symmetrical rotating device, the spot diameter d f Preferably it is 0.1~1mm.

[0090] The sum of the distance between the light spot centers and the light spot diameter is preferably 0.5 to 3 mm, particularly preferably 0.9 to 2.5 mm. df , preferably the following conditions apply: x df ≥0.8 * d f .

[0091] Figure 6 For a suitable process window with a symmetrical stirring device, the relationship between the spot spacing and the spot diameter is shown. As mentioned above, the spot diameter d f Usually 0.1~1mm.

[0092] In another advantageous laser beam configuration 11 (cf. Figure 7), the two laser beams 2 and 3 are positioned along a track, which means that the first light spot 2 remains on the welding axis 4 according to the welding forward direction 10, while the second light spot 3 rotates around the rotation axis 5, which is located on the welding axis 4 and constitutes the center point of the first light spot 2.

[0093] The rotation of the second light spot 3 is accordingly carried out along a rotation direction 7 which is around the rotation axis 5 with a specific radius. Figure 7 The second laser spot 3 can be located at different positions, shown here as a position 3' rotated by 180°. However, during welding, a full rotation is performed along the weld seam advancing direction 10.

[0094] The welding axis 4 also constitutes the idealized weld pool center 4.

[0095] In this advantageous embodiment, the spot diameter is 0.1 to 1 mm and the following conditions apply: .

[0096] Condition x df ≥ 0.8 * d f Also applies here. Figure 8 The function of the spot spacing to the spot diameter is displayed in the process window, and the corresponding area according to the present invention is located in the closed area.

[0097] In the laser beam structure 1 (ref. Figure 9 ), the two laser spots 2, 3 are again rotated about the rotation axis 5, but the first laser beam 2 or the first laser spot 2 first rotation direction 6 is positioned closer to the rotation axis than the second rotation direction 7 of the second laser beam 3. The center of the spot spacing is thus spaced apart from the melt pool center 4, or more precisely, the center of the spot spacing is positioned offset from the melt pool center 4.

[0098] In this advantageous embodiment, the spot diameter d f Again, it lies between 0.1 and 1 mm, for which the following conditions are additionally met:

[0099] , , .

[0100] Figure 2 is the process window related to the stirring effect according to the present invention. In this regard, Figure 3 This is the corresponding effect when the process is executed with inappropriate parameters (i.e., parameters outside the process window).

[0101] Choosing a too strong stirring effect combined with high welding speeds can lead to ridges (instability of the welding process), increased spatter, and even perforation of the laser weld.

[0102] Surprisingly, even if a stirring effect that is too weak is selected, the tendency to splash increases dramatically.

[0103] Laser welding speed less than 4 m / min (v w ) is actually technically possible, but it is no longer economically worthwhile.

[0104] Figure 4 The table shows 16 welding tests performed at different welding feed speeds, different stirring effects, and different power levels. After hardening, the welds were inspected and classified based on weld uniformity and process stability. The abbreviation "na" stands for "not evaluable" because a stable weld could not be produced in these tests.

[0105] Figure 10 For the weld structure after hardening ( Figure 4 Example T1 in the table does not adhere to the parameters of the present invention. Visual inspection alone reveals a clear lack of uniformity in the weld microstructure after hardening. The welding speed was 6 m / min. The spot diameter was 0.3 mm, but the spot spacing was zero, meaning only a single laser was used. Clearly, achieving a satisfactory quality result using this conventional method is impossible.

[0106] Figure 11 The results of the embodiment according to the present invention are as follows ( Figure 4 (Example T2 in the table) The polished micrograph after hardening is uniform. The spot diameter here is 0.3 mm. A symmetrical stirring device was used, with a spot spacing of 0.9 mm.

[0107] The laser power is 4.3kW, the welding speed is 6 m / min, and the stirring effect η is 4.125 mm. -1 , the stirring effect is the quotient of the rotation frequency and the welding speed (or more accurately the quotient of the welding advance distance).

[0108] The distance between the center of the light spot and the rotation axis is 0.45 mm, which means that several light spots run around the rotation axis with a radius.

[0109] Figure 12This is test T4, which is not in accordance with the present invention. The spot diameter and spot spacing are indeed within the ranges according to the present invention. The welding speed is also within the ranges according to the present invention, at 6 m / min, corresponding to the speed of test T2, and the distance between the center of the spot and the axis of rotation is indeed the same. However, as a function of the rotation frequency and the welding speed, the stirring effect is so weak that the polished micrographs show a noticeable lack of uniformity in the hardened weld.

[0110] Figure 13 The results of test T16 according to the present invention are shown. It is clear that a uniform weld structure is present. In this case, the spot spacing was 0.4 mm, the spot diameter was 0.3 mm, and the forward speed corresponded to the forward speed of the other tests. The stirring effect η was 4.125 mm. -1 , within the scope of the present invention. The distance between the center of the light spot and the rotation axis is 0.2 mm.

[0111] According to the present invention, two steel sheets of different thicknesses can be welded together using a welding filler wire, preferably CMn steel, in particular hardenable CMnB steel, in particular 22MnB5 steel. In particular, aluminum-silicon coated steel sheets having a tensile strength of more than 900 MPa after hardening can be joined by welding without ablation.

Claims

1. A method for welding coated steel plates, characterized in that: The method comprises the following steps: A structure is provided with two laser beams, which act on the molten pool to be formed, at least one laser beam being rotated about an axis of rotation so that the laser beams move relative to one another, the laser beams being guided along the welding axis, in order to mix the molten pool, a defined stirring effect being observed in conjunction with a defined welding speed, the following conditions applying to the stirring effect: ， Where η is the stirring effect, f rot is the rotation frequency, v w For welding speed, the following conditions apply: and , Wherein, the stirring effect η is 4~30 mm -1 , In a laser beam configuration, two laser beams or two laser spots rotate about a rotation axis, a first laser beam or a first laser spot rotates about the rotation axis at a first radius, and a second laser beam or a second laser spot rotates about the rotation axis at a second radius, wherein one radius is greater than the other, and the following conditions apply: ， ， ， Among them, x df is the spot spacing, x off The distance of the first laser beam from the axis of rotation defines the eccentricity of the laser beams relative to each other, d f is the spot diameter.

2. The method according to claim 1, wherein Welding is performed using a laser with a power of 2 to 10 kW.

3. The method according to claim 1, wherein The welding speed v w 5~12 m / min.

4. The method according to claim 1, wherein Boron-manganese steel is used as the base material. The base material is hardened by austenitizing and quenching processes, and its tensile strength is greater than 900 MPa.

5. The method according to claim 1, wherein The steel containing the following components as the matrix material is calculated by mass percentage: Residual iron and smelting related impurities.

6. The method according to claim 1, wherein The steel containing the following components as the matrix material is calculated by mass percentage: Residual iron and smelting related impurities.

7. The method according to claim 1, wherein Used for welding aluminum-silicon metal coated steel plates.

8. The method according to claim 1, wherein Welding is performed using a laser with a power of 3 to 8 kW.

9. The method according to claim 1, wherein Welding is performed using a laser with a power of 4 to 7 kW.

10. The method according to claim 1, wherein The welding speed v w 6~10 m / min.

11. The method according to claim 1, wherein The coated steel sheet uses boron-manganese steel, and the boron-manganese steel uses steel belonging to the CMnB steel group.

12. The method according to claim 1, wherein The coated steel plate uses boron manganese steel, and the boron manganese steel uses 22MnB5 steel or 20MnB8 steel.

13. A lath, characterized in that The strip comprises a first steel plate and a second steel plate, and the first steel plate and the second steel plate are welded together by the method according to any one of claims 1 to 12.

14. The slat according to claim 13, wherein The first steel plate and the second steel plate have different alloy compositions.

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