Method for top welding of two steel sheets

Description

1 / 20 “METHOD FOR BUTT WELDING OF TWO STEEL PLATES” DIVIDED FROM BR 11 2024 011755-3, FILED ON 11 / 16 / 2022 Background of the Invention

[001] Laser-welded blanks are a widely used solution in the sheet metal forming industry, particularly in the automotive industry. It allows combining several blank sub-pieces of different qualities and thicknesses into a single blank. This has many advantages: the ideal material properties and thickness are used in each area of ​​the blank, leading to optimized performance of the final part in terms of safety, weight, environmental footprint, etc. It also allows maximizing the use of materials, thus reducing waste, costs, and environmental footprint. Overall, laser-welded blanks simplify the production process, improve part performance, and lead to increased passenger safety, productivity gains, cost savings, and reduced CO2 emissions.

[002] The emergence of new grades with very high strength and also very high formability presents new challenges for the manufacture of laser-welded raw parts. In fact, grades with higher strength and higher formability come with more alloying elements, which leads to new properties, new phenomena and risks of weld seam failure. This is particularly critical in the case of galvanized steel sheets, i.e., zinc-coated steel sheets. In fact, the zinc coating used for corrosion protection can cause liquid metal embrittlement (LME) during the welding operation. Brief Description of the Invention

[003] The present invention aims to address the issue of manufacturing laser-welded blank parts using high-strength zinc-coated steels, such that the resulting laser-welded blank parts have Petition 870260049675, dated 05 / 25 / 2026, page 14 / 64 2 / 20 reliable strength and formability, such that the weld seam does not present a structural weakness in the following part.

[004] The object of the present invention is achieved by providing a method for butt welding two steel plates according to claim 1, optionally comprising the features of claims 2 to 6. Another object of the present invention is a laser-welded blank according to claim 7, optionally comprising the features of claim 8. Another object of the present invention is a molded part according to claim 9. Brief Description of the Figures

[005] The invention will be described in more detail with reference to the following figures: Figure 1 is a diagram of a butt welding operation using a laser source; Figure 2A represents the latest generation method for testing welded assemblies; Figure 2B represents the recently developed method for testing welded assemblies; Figure 3A represents a cross-section of one possible embodiment of the preparation of the steel plate prior to butt welding according to the invention; Figure 3B represents a cross-section of one possible embodiment of the preparation of the steel plate prior to butt welding according to the invention; Figure 4 represents a cross-section of a welded assembly produced according to the invention. Detailed Description of the Invention

[006] In the following description, figures and claims, the Petition 870260049675, dated 05 / 25 / 2026, page 15 / 64 3 / 20 Spatial orientations and references are all made using an X, Y, Z coordinate system, where Z is the elevation direction, perpendicular to the upper and lower faces of the steel plates to be welded, and X and Y define the plane of the upper and lower faces of the steel plates. The coordinate system is represented in each figure. When the figure is a 2D planar representation, the axis that is outside the figure is represented by a point in a circle when it points towards the reader and by a cross in a circle when it points away from the reader, following established conventions.

[007] The directional terms “top”, “up”, “superior”, “above”, “lower”, “below”, “inferior”, “below”, etc. are defined according to the elevation direction Z. The directional terms “front” and “rear” are defined according to the direction X and more particularly according to the welding direction W parallel to the X axis as indicated in the figures. The terms “front” or “downstream” mean further ahead in the W direction, the terms “rear” or “upstream” mean further ahead in the opposite direction to the W direction.

[008] The “width” or “transverse” direction refers to the orientation parallel to the Y-axis.

[009] A steel plate refers to a flat sheet of steel. It has an upper and lower face, which are also called the upper and lower sides or upper and lower surfaces. The distance between these faces is designated as the thickness of the plate. The thickness can be measured, for example, using a micrometer, whose spindle and anvil are placed on the upper and lower faces. Similarly, the thickness can also be measured on a formed part. The thickness of the steel plates in the present invention is, for example, from 0.5 to 5.0 mm, preferably from 0.5 to 4.0 mm, and even more preferably from 0.5 to 3.5 mm.

[010] Custom-made welded raw parts are made by assembling, for example by laser welding, several sheets or parts. Petition 870260049675, dated 05 / 25 / 2026, p. 16 / 64 4 / 20 rough cut pieces of steel, known as rough sub-pieces, in order to optimize the performance of the part in its different areas, to reduce the total weight of the part and to reduce the total cost of the part.

[011] Ultimate tensile strength, yield strength, elongation, and uniform elongation are measured in accordance with ISO standard: ISO 6892-1, published in October 2009. Tensile test specimens are cut from flat areas. If necessary, small-sized tensile test samples are collected to accommodate the total flat area available on the part.

[012] Hardness is a measure of resistance to localized plastic deformation induced by mechanical indentation. It is well correlated with the mechanical properties of a material and is a useful local measurement method that does not require cutting a sample for tensile testing. In the present invention, hardness measurements are made using a Vickers indenter according to ISO 6507-1. Vickers hardness is expressed in the unit Hv.

[013] Referring to Figure 1, butt welding, or simply butt welding, is a specific type of welding operation in which the two steel plates to be welded (1, 2) are placed side by side along their respective edges (E1, E2) and the seam weld (3) is created by fusing the respective edges (E1, E2) to form a weld pool containing a mixture of both steel plates and possibly an additional material used to support the welding process. The weld pool then solidifies to form a weld seam (3). Figure 1 represents the case of a laser butt weld in which a laser source (10), emitting a laser beam (11), is used as the energy source to create the weld pool. The weld edges can be straight or curved. The steel plates to be welded and the power source used to fuse the steel plates move relative to each other during the welding operation in a direction W at a Petition 870260049675, dated 05 / 25 / 2026, page 17 / 64 5 / 20 speed known as welding speed. In a specific embodiment, a space (4) may be left between the steel plates (1, 2). This allows, for example, incorporating a significant amount of additional material, such as filler wire, into the weld pool, without creating an excessive thickness of the weld seam. Such excessive thickness is considered a geometric defect and would be detrimental to the further processing of the welded blank, for example, it would be detrimental to the stamping operation.

[014] Laser welding refers to a welding operation in which at least one laser source is used to provide the energy needed to fuse steel sheets. In particular embodiments, other energy sources, such as electric arcs, infrared heating, etc., may be associated with the laser source to provide the welding energy.

[015] Laser welding generates heat in the steel plates to be welded (1, 2) near their edges (E1, E2). The area of ​​the steel plates (1, 2) in which the heat generated by the welding process induces a temperature increase is known as the heat-affected zone. Isotherms of a given temperature in the heat-affected zone designate the points on the steel plates where the maximum temperature reached is given by that temperature. For example, the 400 °C isotherm of the heat-affected zone of plate (1) designates all points on the steel plate (1) near the edge (E1) where the maximum temperature reached during the welding operation was 400 °C. Logically, any point closer to the edge (E1) than the 400 °C isotherm will have reached a temperature higher than 400 °C during welding and conversely, any point further from (E1) will have reached a maximum temperature lower than 400 °C during welding.

[016] The quality of the weld seam in terms of geometric defects is defined by the European standard EN 10359:2015 entitled “Parts Petition 870260049675, dated 05 / 25 / 2026, page 18 / 64 6 / 20 custom laser-welded raw pieces - Technical delivery conditions.

[017] The present invention relates to the welding of steel sheets, at least one of which has a zinc coating on at least one side. By zinc coating is meant a coating having a chemical composition comprising at least 80% by weight of zinc. For example, zinc coatings include the following known types of coatings (the list is not exhaustive): - pure zinc coating applied by electrodeposition; - Pure zinc coating applied by hot-dip galvanizing; - Zinc / iron coating applied by hot-dip galvanizing followed directly by an alloying step between the steel sheet and the zinc coating to increase the iron content of the coating to values ​​around 8 to 12% by weight of Fe; and - Zinc, aluminum and magnesium alloy coatings containing, by weight percentage, 1% to 6% aluminum, 0.5% to 5% magnesium, the remainder being Zn.

[018] In the following description and claims, the zinc coating is characterized by its thickness. The thickness of the zinc coating can be measured using standardized methods, such as that described in ISO 1463, “Metal and oxide coatings – Coating thickness measurement – ​​Microscopic method”. To clearly differentiate the metal coating from the substrate, Nital etching can be used, as described in point 1 of Annex C of the aforementioned standard.

[019] Figure 2a represents the state of the art of how the strength of a weld seam is currently evaluated. A tensile test sample is prepared in which the weld seam (3) joining the two steel plates (1 and 2) is placed in the middle of the sample in the transverse direction in Petition 870260049675, dated 05 / 25 / 2026, page 19 / 64 7 / 20 comparison with tensile strength F. This well-established method provides a good assessment of the relative tensile strength of the different elements that make up the samples, i.e., the two steel plates (1, 2) and the weld seam (3). According to this method, a weld seam is considered sufficiently strong if the sample breaks outside the weld seam. In other words, the weld seam is considered to be of good quality if it is at least harder than the weaker of the two steel plates, meaning that the weld is not the weakest link in the assembly.

[020] Although the method described above provides a good assessment of the pure mechanical strength of a weld seam, it does not actually reflect the reality of the different deformation modes that a weld seam will undergo under real conditions. When a laser-welded blank is stamped, the weld seam undergoes deformation in all directions and not just in the transverse direction.

[021] The inventors discovered that when laser welding steel sheets, at least one of which has high strength, for example, a tensile strength greater than 590 MPa, small cracks can be initiated perpendicular to the weld seam in areas where the weld is subjected to deformations having a longitudinal component. Surprisingly, this type of crack is observed only in high-strength steels and not in lower-quality steels. The risk of occurrence of this type of crack cannot be assessed using the method described above, because the behavior of the weld when deformed in the longitudinal direction is in no way tested in the traditional test method. Furthermore, there is a statistical element associated with this type of cracking. For the same part geometry involving the same types of steel and the same laser welding parameters, some parts may be free of cracks, while small cracks occur in other parts.This is due to variations that occur naturally. Petition 870260049675, dated 05 / 25 / 2026, page 20 / 64 8 / 20 in the composition of the steel sheet, in the welding process, in the stamping process, etc. These small cracks are therefore not entirely predictable, which is an additional problem in an industrial environment because they will be difficult to detect through quality control. Even if these cracks may be small-scale in the molded part, they represent a fatal weakness in the part and will lead to part failure during the part's service life, possibly causing serious safety problems.

[022] The inventors have therefore developed a new methodology for assessing the risk of occurrence of these small cracks. The inventors found that by placing the weld seam in the longitudinal direction of the tensile sample, parallel to the tensile strength, as shown in Figure 2b, it is possible to observe the occurrence of small transverse cracks in laser-welded assemblies involving at least one piece of high-strength steel. Furthermore, the inventors discovered that a good criterion for ensuring that small cracks are unlikely to occur in series production is to perform a series of 20 tensile tests on longitudinally welded samples and compare the uniform elongation of the welded assembly with the weighted average of the uniform elongation of each of the steel plates.The inventors discovered that the risk of cracking is very low if the uniform elongation of the welded assembly is at least 50% of the weighted average (Uplate1 * th1 + Uplate2 * th2) / (th1 + th2) in all 20 tensile tests performed, where th1 and th2 are the thicknesses of the steel plates (1, 2) that are assembled and Uplate1, Uplate2, their respective uniform elongations.

[023] In the rest of the description, when Usoldado is less than (Uchapa1 * th1 + Uchapa2 * th2) / (th1 + th2), the test sample will be considered as exhibiting a brittle failure - on the other hand, if Usoldado is greater than the aforementioned average weighted weight, the test sample will be considered as Petition 870260049675, dated 05 / 25 / 2026, page 21 / 64 9 / 20 exhibiting a ductile fault.

[024] Thanks to the newly developed statistical longitudinal tests of weld beads described above, the inventors were able to investigate the problem of small cracks in a large number of zinc-coated steel sheets.

[025] The inventors were able to establish that the risk of small cracks occurring in a welded assembly including at least one zinc-coated steel sheet was present when the C content of said steel sheet, in % by weight, is greater than 0.15% and / or when the Si content is greater than 0.5%. The present invention applies to laser welding of zinc-coated steel sheets, of which at least one of the steel sheets has a chemical composition comprising at least 0.15% by weight of C or 0.5% by weight of Si, or both. For simplicity, we will consider that at least steel sheet (1) has such a chemical composition in the remainder of the description.

[026] Small cracks appear near the weld seam, in the heat-affected zone, or in the weld seam itself. This points to a LME problem related to the combination of specific metallurgical and microstructural properties of high-strength steels associated with the presence of zinc, which becomes liquid under the influence of the heat from the welding operation. In fact, the occurrence of LME in laser butt welding is surprising because it is known that LME results from the combination of three factors, one of which should not occur in the case of laser butt welding: - metallurgy of steel sheets containing a large amount of Si and / or C - which is in fact the case for the high-strength steels described earlier; - presence of liquid metal, in particular liquid zinc - this can indeed be the case when zinc melts above 420 °C under the influence Petition 870260049675, dated 05 / 25 / 2026, page 22 / 64 10 / 20 of the heat generated by the laser beam; and - presence of residual stress - this, on the other hand, does not seem to be the case in laser butt welding, because this technology does not involve any mechanical pressure to generate the weld, unlike spot welding, for example, where a specific welding force is applied.

[027] It was therefore surprising to the inventors to see LME-type cracks occurring in laser-welded assemblies of zinc-coated high-strength steels. It is possible that the residual stress required to induce LME comes from thermal deformation and deformation induced by the phase transformation of the blanks after the welding operation.

[028] Knowing that LME can in fact occur during laser welding of high-strength zinc-coated steels, the inventors sought to apply the newly developed statistical longitudinal test of weld beads to support the development of countermeasures against the occurrence of LME.

[029] The inventors proceeded to remove the zinc coating in the vicinity of the weld seam in order to suppress LME-induced cracking. Several possible methods can be applied to remove the zinc coating. It is possible to mechanically brush the surface of the sheets to remove the zinc coating. It is also possible to use a pulsed laser beam to ablate the zinc coating. The inventors found that surprisingly it was not necessary to completely remove the zinc coating in the vicinity of the weld seam to completely suppress the occurrence of LME-induced cracking. In fact, the authors found that once the thickness of the zinc coating was equal to or less than 3.5 microns, there was no longer a risk of LME.

[030] With reference to figure 3a, the steel sheet (1) consists of a substrate (12) covered with a zinc coating (5), having a Petition 870260049675, dated 05 / 25 / 2026, p. 23 / 64 11 / 20 coating thickness Zn Znth, expressed in microns. Note that in the Figure, for simplicity, reference is made only to the coating on the top face of the steel sheet (1). The same applies to all other faces of the steel sheets (1 and 2), i.e., the bottom face of the steel sheet (1) and the top and bottom faces of the sheet (2). In fact, what is important is to apply and implement the present invention by removing at least part of the zinc coating on all faces where the zinc coating thickness is greater than 3.5 microns and the substrate (12) of the steel sheet has a chemical composition in % by weight, with a carbon content greater than 0.15% or a silicon content greater than 0.5% or both.

[031] The zinc coating (5) is at least partially removed along a Wabini width (the initial ablation width), thus creating an ablated area (6). The coating thickness in the ablated area (6) is Znab (coating thickness in the ablated area). It should be noted that the coating thickness in the ablated area refers to the maximum thickness in the ablated area. If the ablation process leaves an uneven coating thickness in the ablated area (6), Znab corresponds to the average amount of Zn in the ablated area, measured by microscopic investigation of the cross-section.

[032] As explained earlier, the inventors found that to ensure the absence of LME, it is necessary to control the thickness of the Znab ablation below 3.5 microns.

[033] We now refer to figure 4, which is a cross-section of the laser welded raw piece (7) consisting of the steel plates (1 and 2) as well as the weld seam (3).

[034] The ablated area after welding (8) has a smaller Wabfin width (final ablation width) than the Wabini width of the ablated area before welding (6). In fact, part of the edge (E1) of the steel plate (1) was Petition 870260049675, dated 05 / 25 / 2026, page 24 / 64 12 / 20 incorporated by fusion into the weld seam (3) and as a consequence, part of the area ablated before welding (6) was also incorporated into the weld seam (3).

[035] Given that the melting point of zinc is 420 °C and that LME can only occur in the presence of liquid Zn, LME is a potential problem in the laser welded blank (7) over the heat-affected zone in areas that have reached a surface temperature above 420 °C. The inventors found that when laser welding is applied, the location of the 420 °C isotherm on the surface of the steel sheets is always at a distance from the weld seam (3) of 0.2 to 0.5 mm. To avoid the occurrence of LME, it is necessary that the Wabfin be equal to or greater than 0.5 mm.

[036] In conclusion, to avoid LME, it is necessary to reduce the thickness of the Zn coating near the edge to a Znab thickness below 3.5 microns along a Wabini width, so that the width of the ablated area after Wabfin welding is equal to or greater than 0.5 mm. Knowing the amount of material that is melted from the edge (E1), the technician will be able to determine the required Wabini ablation width on the steel plate (1) before welding.

[037] It will thus be possible to obtain a laser-welded blank free of LME in the vicinity of the weld seam (3), comprising the first and a second steel sheet (1 and 2), each comprising a substrate (12) and at least one of them having a zinc-based metal coating (5) on at least one side, wherein all faces having a thickness of the zinc-based metal coating Znth greater than 3.5 microns and a steel sheet substrate (12) having a carbon content greater than 0.15% or a silicon content greater than 0.5% or both, comprising an ablation area after welding (8) having a thickness of the zinc-based metal coating after ablation Znab which is equal to or less than 3.5 microns and a Petition 870260049675, dated 05 / 25 / 2026, page 25 / 64 13 / 20 width of the ablated area after Wabfin welding equal to or greater than 0.5 mm.

[038] It will also be possible to manufacture parts by forming the laser-welded blank described above without the risk of LME cracks in the vicinity of the weld. For example, the laser-welded blank can be manufactured into a piece by cold stamping or hot forming.

[039] In one particular embodiment, zinc coating ablation is performed using abrasive brushes rotating over the steel sheet to remove at least part of the coating. For example, the brushes are made of a polymer web that encapsulates hard ceramic particles, such as, for example, aluminum oxides or silicon carbides. In this case, the critical parameters for brushing will be the abrasive power of the brush, the force exerted by the brush on the coating, and the speed at which the brush moves across the surface. Depending on the material and zinc coating used, it will be necessary to adjust these parameters to obtain the desired ablation thickness. It is also possible to perform multiple brushing passes to remove more coating.

[040] In one particular embodiment, ablation is performed using a pulsed laser beam. The inventors found that the interaction between the short laser pulses and the coating causes at least part of the coating to evaporate and be expelled from the surface of the steel plate. For example, a laser power of 400W to 1500W can be used with a pulse frequency of 5 to 15kHz and an ablation speed between 2 and 15 m / min.

[041] In a particular embodiment, as illustrated in figure 3b, the ablated area (6) on the steel plate before welding does not start exactly at the edge (E1) of the steel plate. Instead, there is a displacement area (9), having a width Wdisplacement, over which no ablation is performed. In other words, the coating thickness in the displacement area (9) is the same as Petition 870260049675, dated 05 / 25 / 2026, page 26 / 64 14 / 20 in most of the material, Znth. The inventors found that the presence of such a displacement does not lead to LME cracking, provided that the entire displacement area (9) is melted and incorporated into the weld seam (3) and provided that Wabfin remains equal to or greater than 0.5 mm. It was found that there is no LME in the weld seam itself, even if some zinc from the metal coating is incorporated into the weld seam. When a displacement is present before welding and the entire displacement region (9) is incorporated into the weld pool, the ablated region after welding (8) will have the same characteristics as if there were no displacement, as represented in Figure 4. Advantageously, the presence of such a displacement can ensure better protection against edge corrosion (E1) before welding.In fact, thanks to the sacrificial nature of zinc corrosion protection, the zinc coating in the deviation area (9) is available to protect the exposed uncoated parts of the edge (E1).

[042] In a particular embodiment, a decarburization step is applied to at least one of the steel sheets (1, 2) in order to reduce the carbon content of said steel sheet near the surface of said steel sheet. This decarburization step is performed before the application of the zinc coating. For example, the decarburization step is performed in the furnace used to anneale the steel sheet before coating it. For example, the decarburization step is performed by controlling the dew point in said annealing furnace to a value equal to or greater than -10 °C. Advantageously, by using a steel sheet with a lower carbon content near the surface, the risk of LME is reduced.For example, the carbon content at a depth of 20 microns from the surface of the steel plate on each side is less than 0.15% by weight, preferably less than 0.10% by weight, and the carbon content of the steel plate in the center of said steel plate (e.g., the carbon content measured in a range of + / - 100 microns from the average thickness of the steel plate) is greater than 0.15% by weight. Petition 870260049675, dated 05 / 25 / 2026, page 27 / 64 15 / 20

[043] In a particular embodiment, the ablation step is carried out in such a way that the remaining thickness of the zinc Znab is greater than 0.5 microns, more preferably greater than 1.0 microns. As explained earlier, the inventors found that there is no risk of LME even if there is a small amount of zinc remaining in the ablated area (6), provided that the Znab is below 3.5 microns. Advantageously, by leaving some zinc in the ablated area (6), it is possible to ensure a certain amount of protection against corrosion of the ablated steel sheet before welding. Thanks to the sacrificial nature of the zinc coating, this corrosion protection extends to the uncoated areas of the edges, on the sides of the steel sheets.Furthermore, applying an ablation process that leaves some zinc coating on the ablated area (6) means that there is no risk of removal (by brushing) or fusion (by laser ablation) of the underlying substrate (12) of the steel sheet below the ablated area (6). This is interesting because ablation of the substrate itself (12) below the coating would weaken the laser welded blank, creating local geometric imperfections and a local underthickness.

[044] In a particular embodiment, the laser welding process is carried out less than 1 minute after the ablation process has been carried out, preferably even less than 30 seconds. For example, the ablation process and the welding process are carried out in the same device, which comprises equipment for the ablation step and for the welding step. For example, the ablation process and the welding process are carried out under the same clamping operation, meaning that the steel plates (1, 2) are clamped to be held in place for the ablation step and the same clamping is held in place for the welding process. Advantageously, by performing ablation and welding in a fast sequence, possibly even under the same clamping operation, it will be possible to increase productivity, reduce the number of handling steps, Petition 870260049675, dated 05 / 25 / 2026, page 28 / 64 16 / 20 material and also reduce the risk of corrosion occurring due to the ablation step.

[045] In one particular embodiment, the ablation process is not performed directly on the edge of the steel plate, but in the middle of the steel plate - said steel plate being subsequently cut in the area where the ablation process was performed so that the ablated area is located on the edge of the cut sample (possibly with displacement).

[046] In a particular embodiment, the characteristics of at least one of the steel plates to be welded in terms of chemical composition, microstructure and mechanical properties correspond to one of the lines in the following table (chemical composition is expressed as % by weight and the balance is Fe and unavoidable impurities from the manufacturing process, % residual austenite in the microstructure of the steel plate is expressed as % of the surface of a cross-section, YP means the yield point expressed in MPa, UTS means the ultimate tensile strength expressed in MPa, % El is the elongation measured according to the ISO 6892 standard mentioned above): Table 1 % C % Mn % Si % Al % Cr % Nb YP UTS % El 0.11 to 0.15 1.1 to 1.8 0.5 to 0.9 <0.05 330 to 440 590 to 700 >26% 0.13 to 0.18 1.1 to 1.8 0.5 to 0.9 0.6 to 1 330 to 440 590 to 700 >26% 0.13 to 0.18 1.1 to 1.8 0.5 to 0.9 0.6 to 1 440 to 550 780 to 900 >18% 0.15 to 0.25 2 to 2.8 0 to 0.4 0.2 to 1.2 0.01 to 0.5 700 to 850 980 to 1130 >13% 0.1 to 0.5 1 to 3.4 0.5 to 2.5 0.03 to 1.5 0.05 to 1 600 to 750 980 to 1130 >19% 0.15 to 0.23 1.4 to 2.6 0.6 to 1.5 0.02 to 1 <0.3 <0.035 600 to 750 980 to 1130 >19% 0.18 to 0.25 1.5 to 2.5 0.9 to 1.8 0.02 to 1 0.1 to 0.4 0.01 to 0.035 600 to 750 980 to 1130 >19% 0.15 to 0.25 1.8 to 3 1.2 to 2 <0.1 <0.5 700 to 820 1050 to 1180 >14% 0.19 to 0.24 1.5 to 2.5 1.2 to 2 0.01 to 0.06 0.2 to 0.5 850 to 1080 1180 to 1330 >13% Petition 870260049675, dated 05 / 25 / 2026, page 29 / 64 17 / 20 % C % Mn % Si % Al % Cr % Nb YP UTS % El 0.13 to 0.22 2.4 to 3 1.2 to 2.3 0.02 to 1 <0.05 850 to 1080 1180 to 1330 >13% 0.13 to 0.22 2.4 to 3 1.2 to 2.3 0.02 to 1 <0.05 850 to 1080 1180 to 1330 >13%

[047] The invention will now be illustrated by the following examples, which are by no means limiting.

[048] Table 2 shows the chemical composition of the steel sheet used. Table 2: Composition of Steel Sheet Expressed as % by Weight %C %Mn %P %S %Si %Cu %Ni %Cr %AI %Mo %V %Nb %B %Ti 0.22 1.96 0.0045 0.0013 1.12 0.01 0.01 0.19 0.56 0.004 0.004 0.032 0.0004 0.007

[049] The Si content is greater than 0.5% and the carbon content is greater than 0.15%, which means that the steel plate is critical in terms of LME risk when welding with zinc coating. The steel plate was decarburized before the application of a pure zinc coating by hot dip, so that the carbon content measured at 20 microns from the surface of the steel plate using luminous discharge optical emission spectrometry (GDOES) is 0.05% by weight.

[050] The initial coating thickness of the steel sheet is 7 to 11 microns, depending on the sample (in fact, there may be a variation in coating thickness when using industrially produced steel sheets due to natural process variations in production lines).

[051] The steel plates are 1.0 mm thick. Homogeneous sets of the same steel plate and thickness are produced for the current tests (in other words, the characteristics of steel plate (1) and steel plate (2) are the same).

[052] Table 3 lists the ablation process parameters that were used. Petition 870260049675, dated 05 / 25 / 2026, page 30 / 64 18 / 20 Table 3: Ablation Process Parameters Ref. Zn Removal Method Laser Power (W) Pulse Frequency (KHz) Laser Speed ​​(m / min) Brushing Force (N) Brushing Speed ​​(m / min) Brushing Passes R1 no ablation - - - - R2 laser 200 12 10 R3 laser 350 12 10 R4 laser 500 12 10 l1 laser 490 10 2 l2 laser 490 10 2 l3 laser 900 12 8 l4 mechanical - - 30 2.5 3 l5 mechanical - - 20 2.5 1 l6 mechanical - - 20 5 1 l7 mechanical - - 30 2.5 1 l8 mechanical - - 10 2.5 1 l9 mechanical - - 10 5 1

[053] Sample references begin with I, for invention, in the case of samples produced according to the present invention and with R, for reference, in the case of counter-examples, outside the present invention. Ablation of the Zn coating was performed with low-power pulsed laser or mechanical brushing.

[054] Mechanical brushing was performed using 3M Scotch-Brite “Deburr and Finish pro 4C MED+” brushes mounted on a robot, with a diameter of 76.2 mm, a width of 12.7 mm and a rotation speed of 6,000 rpm. The brushing speed corresponds to the speed at which the robotic arm moves along the sheet to be brushed.

[055] Table 4 presents the results of the ablated area before welding (6) and after welding (8), as well as the result of the mechanical tests of the welds. Table 4: Characteristics of Ablated Regions and Weld Properties Ref Znab (microns) Wabini (mm) Displacement (mm) Wabfin (mm) Occurrence of LME? % of brittle failure in 20 longitudinal test samples R1 93 0 0 0 Yes 40% R2 36 1.2 0 0.7 Yes 5% R3 94 1.2 0 0.7 Yes 10% Petition 870260049675, dated 05 / 25 / 2026, p. 31 / 64 19 / 20 Ref Znab (microns) Wabini (mm) Wdisplacement (mm) Wabfin (mm) Occurrence of LME? Percentage of brittle failure in 20 longitudinal test samples R4 83 1.2 0 0.7 Yes 5% l1 0.1 1.3 0 0.8 No 0% l2 0.1 0.9 0.3 0.6 No 0% l3 0.1 1.2 0 0.7 No 0% l4 0.1 6.4 0 5.8 No 0% l5 0.2 6.4 0 5.8 No 0% l6 0.5 6.4 0 5.8 No 0% l7 0.5 6.4 0 5.8 No 0% l8 0.6 6.4 0 5.8 No 0% l9 3.2 6.4 0 5.8 No 0%

[056] The underlined values ​​correspond to the characteristics that are outside the invention in the case of reference samples.

[057] Sample R1 is not ablated and serves as an initial reference, representing the very poor mechanical performance of a welded assembly in which no countermeasures are taken to prevent LME. 40% of the samples tested longitudinally according to the innovative test method described above failed due to cracks in the LME.

[058] Samples R2 to R4 are cases where ablation was performed, but the remaining zinc thickness after Znab ablation is too high to prevent LME occurrence.

[059] Samples I1 to I9 illustrate the technical effect of the invention. By removing the zinc coating below 3.5 microns in a width equal to or greater than 0.5 mm after welding, LME is avoided and structurally sound assemblies are produced.

[060] In the case of I2, ablation was performed leaving a displacement area of ​​0.3 mm at the edge of the steel plates. Despite the presence of this non-ablated area at the edge of the steel plates, the final assembly is free of LME because the entire displaced area is absorbed in the weld seam and the ablated width after welding is greater than 0.5 mm.

[061] In the case of samples I9, the remaining thickness of Petition 870260049675, dated 05 / 25 / 2026, page 32 / 64 20 / 20 coating thickness after Znab ablation is 3.2 microns. No ablation is observed in the resulting assembly, despite the presence of the remaining zinc coating thickness.

[062] In conclusion, when steel plates are ablated before welding to produce an ablated area with a remaining Znab thickness that is equal to or less than 3.5 microns over a width greater than 0.5 mm in the final weld assembly, LME does not occur. Petition 870260049675, dated 05 / 25 / 2026, page 33 / 64

Claims

1 / 2 Claims 1. METHOD FOR BUTT WELDING OF TWO STEEL PLATES (1, 2), each comprising a substrate (12) and at least one steel plate (1, 2) having a zinc-based metal coating (5) on at least one side with a thickness Znth, at least one steel plate (1, 2) having a chemical composition of the substrate (12) expressed in % by weight with a carbon content greater than 0.15% or a silicon content greater than 0.5% or both, method comprising the steps of: - providing the two steel plates (1, 2);- on all faces having a zinc-based metal coating thickness (5) Znth greater than 3.5 microns and a steel sheet substrate (12) with a carbon content greater than 0.15% or a silicon content greater than 0.5% or both: removing at least part of the metal coating to form an ablation area before welding (6) with a zinc-based metal coating thickness (5) after ablation Znab that is equal to or less than 3.5 microns and such that the Wabfin width of the ablation area after welding (8) is equal to or greater than 0.5 mm; the method characterized by: - ​​in at least one ablation area (6) the zinc-based metal coating thickness (5) after ablation Znab being equal to or greater than 0.5 microns; - butt welding of the steel sheets (1,2) using at least one laser source (10).

2. METHOD, according to claim 1, characterized in that the metal coating removal step is performed using a pulsed laser beam (11).

3. METHOD, according to claim 1, characterized in that the metal coating removal step is carried out using mechanical brushing.

4. METHOD, according to any one of claims 1 to 3, characterized in that at least one of the steel plates (1,2) is subjected to a decarburization treatment before the application of the zinc-based metal coating (5), such that the surface carbon content at a depth of 20 microns is less than 0.15% by weight. Petition 870260049675, dated 05 / 25 / 2026, p. 35 / 64

Images