Method for butt welding steel parts to related steel parts

Abstract

1. A method for butt welding two steel sheets, comprising the steps of: providing two steel sheets (1, 2); removing at least a portion of the metal coating on all sides having a zinc-based metal coating thickness 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, to form a pre-weld ablation area (6) having a post-ablation zinc-based metal coating thickness Znab of 3.5 microns or less, and a post-weld ablation area (8) having a width of 0.5 mm or more; and butt welding the steel sheets (1, 2) using at least a laser source.

Description

Background Art

[0001] Laser welded blanks are a widely used solution in the steel sheet metal forming industry, particularly in the automotive industry. This makes it possible to combine several sub-blanks of the same blank, different grades and thicknesses. This has many advantages, with the optimal material properties and thickness being used in each area of the blank, resulting in optimized performance of the final part in terms of safety, weight, environmental footprint, etc. Furthermore, it enables designers to combine several parts into one. It also makes it possible to maximize the use of materials, thereby reducing scrap, costs and the environmental footprint. Overall, laser welded blanks simplify the production process, improve the performance of parts, lead to an improvement in passenger safety, an improvement in productivity, cost savings and a reduction in CO2 emissions.

Summary of the Invention

Problems to be Solved by the Invention

[0002] The emergence of new grades with very high strength and very high formability presents new challenges for manufacturing laser welded blanks. In fact, higher strength and higher formability grades involve more alloying elements, which bring new properties, new phenomena and a risk of failure to the weld seam. This is particularly important in the case of galvanized, 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.

Means for Solving the Problems

[0003] The present invention aims to address the problem of manufacturing laser welded blanks using high-strength zinc-coated steel such that the resulting laser welded blanks have high reliability resistance and formability and the weld seam does not present a structural weakness in subsequent parts.

[0004] An object of the present invention is achieved by providing a method for butt welding two steel plates according to claim 1, which optionally includes the features of claims 2 to 6. A further object of the present invention is a laser-welded blank according to claim 7, which optionally includes the features of claim 8. A further object of the present invention is a molded part according to claim 9.

[0005] The present invention will be further described with reference to the following figures. [Brief explanation of the drawing]

[0006] [Figure 1] This is a schematic diagram of a butt welding operation using a laser source. [Figure 2A] This diagram illustrates a state-of-the-art method for testing welded assemblies. [Figure 2B] This diagram illustrates a newly developed method for testing welded assemblies. [Figure 3A] This is a cross-sectional view of a possible embodiment of the steel plate preparation method before butt welding according to the present invention. [Figure 3B] This is a cross-sectional view of a possible embodiment of the steel plate preparation method before butt welding according to the present invention. [Figure 4] This is a cross-sectional view of a welded assembly produced according to the present invention. [Modes for carrying out the invention]

[0007] In the following description, drawings, and claims, all orientation and spatial references are made using reference X, Y, Z coordinates, where Z is the elevation direction perpendicular to the top and bottom surfaces of the welded steel sheet, and X and Y define the planes of the top and bottom surfaces of the steel sheet. The reference is shown in each figure. If the figure is a 2D planar representation, axes outside the figure are represented, according to established convention, by a dot in a circle when it is facing the reader, and by a cross in a circle when it is facing outward from the reader.

[0008] Directional terms such as "top," "up," "upper," "above," "bottom," "low," "lower," and "below" are defined according to the Z elevation angle direction. The directional terms "forward" and "backward" are defined according to the X direction, and more specifically according to the welding direction W parallel to the X axis, as shown in the figure. The terms "forward" or "downstream" mean further along the W direction, and the terms "backward" or "upstream" mean further along the opposite side of the W direction.

[0009] The "width" or "horizontal" direction refers to the direction parallel to the Y-axis.

[0010] A steel sheet refers to a flat steel sheet. It has an upper surface and a lower surface, also called an upper surface and a lower surface or upper and lower surface. The distance between the surfaces is indicated as the thickness of the sheet. The thickness can be measured using, for example, a micrometer, with its spindle and anvil positioned on the upper and lower surfaces. Similarly, the thickness can also be measured on a molded part. The thickness of the steel sheet in this invention is, for example, 0.5 to 5.0 mm, preferably 0.5 to 4.0 mm, and more preferably 0.5 to 3.5 mm.

[0011] Tailored blanks are fabricated by assembling several sheets of steel, or notched blanks known as subblanks, together, for example by laser welding, to optimize the performance of the part in its different areas, reduce the overall part weight, and lower the overall part cost.

[0012] Ultimate tensile strength, yield strength, elongation, and uniform elongation are measured according to the ISO standard ISO 6892-1, published in October 2009. Tensile test specimens are cut from a flat area. If necessary, small tensile test samples are taken to accommodate the entire available flat area on the part.

[0013] Hardness is a measure of a material's resistance to localized plastic deformation induced by mechanical indentation. It correlates well with the material's mechanical properties and is a useful local measurement method that eliminates the need to cut a sample for tensile testing. In this invention, hardness measurement is performed using a Vickers indenter in accordance with the ISO 6507-1 standard. Vickers hardness is expressed in units of Hv.

[0014] Referring to Figure 1, butt welding, or simply butt welding, is a specific type of welding operation in which two steel plates to be welded 1, 2 are positioned side by side along their respective edges E1, E2, and a weld seam 3 is created by melting the respective edges E1, E2 to form a molten pool containing a mixture of both steel plates and, optionally, additional material used to support the welding process. The molten pool then solidifies to form the weld seam 3. Figure 1 illustrates the case of laser butt welding in which a laser beam 10 emitting a laser beam 11 is used as the energy source to create the molten pool. The weld edges can be either straight or curved. The steel plates to be welded and the energy source used to melt the steel plates move relative to each other in a direction W during the welding operation at a speed known as the welding velocity. In specific embodiments, a gap 4 can be left between the steel plates 1, 2. This makes it possible to incorporate a considerable amount of additional material, such as filler wire, into the molten pool without, for example, causing an excess thickness of the weld seam. Such thickness excess is considered a geometric defect and is detrimental to further processing of the weld blank, for example, to stamping operations.

[0015] Laser welding refers to a welding operation that uses at least a laser source to provide the energy necessary to melt a steel plate. In certain embodiments, other energy sources such as electric arcs or infrared heating can be associated with the laser source to provide welding energy.

[0016] Laser welding generates heat in the vicinity of the edges E1 and E2 of the steel plates 1 and 2 to be welded. The region of steel plates 1 and 2 where the heat generated by the welding process induces a temperature rise is known as the heat-affected zone. An isotherm of a given temperature within the heat-affected zone indicates the point on the steel plate where the maximum temperature reaches the given temperature. For example, the 400°C isotherm of the heat-affected zone of steel plate 1 indicates all points on steel plate 1 near edge E1 where the highest temperature reached during welding was 400°C. Logically, any point closer to edge E1 than the 400°C isotherm will reach a temperature above 400°C during welding, while any point farther from E1 will reach a maximum temperature below 400°C during welding.

[0017] The quality of welded seams in terms of geometric defects is defined by the European standard EN10359:2015, entitled "Laser welded tailored blanks - Technical delivery conditions".

[0018] This invention relates to the welding of steel plates, at least one of which has a zinc coating on at least one side. A zinc coating means a coating having a chemical composition containing at least 80% zinc by weight. For example, zinc coatings include (but are not limited to) the following types of known coatings: - Electroplated pure zinc coating - Pure zinc coating with hot-dip galvanizing - A zinc / iron coating in which hot-dip galvanizing is applied directly, followed by an alloying step between the steel plate and the zinc coating, increasing the iron content of the coating to approximately 8-12% of Fe by weight. - Zinc, aluminum, magnesium alloy coating containing 1% to 6% aluminum, 0.5% to 5% magnesium by weight, with the remainder being Zn.

[0019] In the following description and claims, the zinc coating is characterized by its thickness. The thickness of the zinc coating can be measured using a standardized method as described in ISO standard 1463 "Metallic and oxide coatings - Measurement of coating thickness - Microscopical method". In order to clearly distinguish the metal coating from the substrate, it is possible to use nital etching as described in point 1 of appendix C of the aforementioned standard.

[0020] Figure 2A represents the state of the art of how the strength of a weld seam is currently evaluated. A tensile test specimen is prepared with a weld seam 3 joining two steel plates 1 and 2, and the weld seam 3 is placed at the center of the sample in the transverse direction for comparison with the tensile strength F. This established method gives a good evaluation of the relative strengths of the different elements constituting the sample, namely the two steel plates 1, 2 and the weld seam 3. According to this method, if the sample breaks outside the weld seam, the weld seam is considered to be strong enough. In other words, the weld seam is considered to be of good quality if it is at least as hard as the weakest of the two steel plates, which means that the weld is not the weak link in the assembly.

[0021] The above method gives a good evaluation of the pure mechanical strength of the weld seam, but in reality, it does not reflect the reality of the different deformation modes that the weld seam experiences under actual service conditions. When a laser-welded blank is punched, the weld seam deforms not only in the transverse direction but also in all directions.

[0022] The inventors have found that when laser welding steel plates in which at least one has high strength, for example, a tensile strength exceeding 590 MPa, small cracks may be initiated perpendicular to the weld seam in the region of the weld that undergoes deformation with a longitudinal component. Surprisingly, this type of crack is observed only in high-strength steel and not in lower grades. The risk of this type of crack occurring cannot be evaluated using the above methods because the behavior of the weld when deformed longitudinally is not tested at all by conventional test methods. Furthermore, there are statistical factors associated with this type of crack. For the same part shape with the same steel grade and the same laser welding parameters, some parts may be crack-free while others may develop small cracks. This is due to spontaneous variations in steel plate composition, welding process, stamping process, etc. Therefore, these small cracks are not entirely predictable and are difficult to detect by quality control, making them a further problem in industrial environments. Although these cracks may be small on a formed part, they represent a critical weakness of the part, leading to failure of the part during its lifespan and, in some cases, causing serious safety problems.

[0023] Therefore, the inventors have developed a new methodology for evaluating the risk of these small cracks occurring. As shown in Figure 2B, the inventors have found that when the weld seam is arranged in the longitudinal direction of a tensile specimen parallel to the tensile strength, there is a possibility of facing the occurrence of small transverse cracks in a laser welding assembly with at least one blank of high-strength steel. Furthermore, the inventors have found that a good criterion for ensuring that small cracks are less likely to occur in continuous production is that a series of 20 tensile tests are performed on specimens welded longitudinally, and the uniform elongation of the welded assembly is compared with the weighted average of the uniform elongations of each steel plate. The inventors have found that when the uniform elongation Uweld of the welded assembly is at least 50% of the weighted average (Usheet1*th1 + Usheet2*th2) / (th1 + th2) (where th1 and th2 are the thicknesses of the assembled steel plates 1 and 2, and Usheet1 and Usheet2 are their respective uniform elongations), the risk of crack occurrence is very low.

[0024] In the remaining part of the description, when Uweld is lower than (Usheet1*th1 + Usheet2*th2) / (th1 + th2), the test specimen is said to exhibit brittle fracture, while when Uweld is higher than the weighted average, the test specimen is said to exhibit ductile fracture.

[0025] Thanks to the newly developed statistical longitudinal test of the weld seam described above, the inventors were able to investigate the problem of small cracks in a large number of zinc-coated steel plates.

[0026] The inventors have demonstrated that there is a risk of small cracks occurring in a welded assembly comprising at least one zinc-coated steel sheet when the carbon content of the steel sheet exceeds 0.15% by weight and / or the silicon content exceeds 0.5%. The present invention applies to laser welding of zinc-coated steel sheets in which at least one of the steel sheets has a chemical composition comprising at least 0.15% by weight of carbon or 0.5% by weight of silicon, or both. For the sake of simplicity, the remainder of this specification assumes that at least steel sheet 1 has such a chemical composition.

[0027] Small cracks appear near the weld seam, in the heat-affected zone, or within the weld seam itself. This points to the LME problem, which is related to a specific combination of metallurgy and microstructure in high-strength steel, associated with the presence of zinc that becomes liquid under the influence of the heat of the welding operation. In fact, the occurrence of LME in laser butt welding is surprising because it is known that LME is caused by a combination of three factors, one of which is not intended to occur in the case of laser butt welding: -Metallurgy of steel sheets containing large amounts of Si and / or C (this is actually the case with the aforementioned high-strength steel), - The presence of liquid metal, especially liquid zinc (which can actually be the case if zinc melts above 420°C under the influence of heat generated by a laser beam), - The presence of residual stress (which, on the other hand, does not seem to apply to laser butt welding, as this technique does not involve any mechanical pressure to create a weld, in contrast to spot welding, for example, where a specific welding force is applied).

[0028] Therefore, the inventors were surprised to observe LME-type cracks occurring in laser-welded assemblies of zinc-coated high-strength steel. The residual stress required to induce LME may be due to thermal strain and distortion induced by the phase transformation of the blank after the welding process.

[0029] Knowing that LME can actually occur when laser welding zinc-coated high-strength steel, the inventors attempted to apply a newly developed statistical longitudinal test of the weld seam to help develop countermeasures against LME occurrence.

[0030] The inventors attempted to remove the zinc coating near the weld seam 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 sheet to remove the zinc coating. It is also possible to use a pulsed laser beam to remove the zinc coating. Surprisingly, the inventors found that it is not necessary to completely remove the zinc coating near the weld seam to completely suppress the occurrence of LME-induced cracking. In fact, the authors found that the risk of LME disappears when the zinc coating thickness is 3.5 microns or less.

[0031] Referring to Figure 3A, the steel sheet 1 consists of a substrate 12 covered with a zinc coating 5 having a Zn coating thickness Znth expressed in microns. Note that, for the sake of simplicity, only the coating on the top surface of the steel sheet 1 is shown in the figure. The same applies to all other surfaces of the steel sheets 1 and 2, i.e., the bottom surface of the steel sheet 1 and the top and bottom surfaces of the steel sheet 2. In fact, the important point is that the present invention is applied and carried out by removing at least a portion of the zinc coating on all surfaces where the zinc coating thickness is greater than 3.5 microns and the steel sheet substrate 12 has a chemical composition expressed in weight percent having a carbon content of more than 0.15% or a silicon content of more than 0.5% or both.

[0032] The zinc coating 5 is at least partially removed over a width Wabini (initial ablation width), thus creating an ablated region 6. The coating thickness in the ablated region 6 is Znab (coating thickness in the ablated region). Note that the coating thickness in the ablated region refers to the maximum thickness in the ablated region. If the ablation process leaves a non-uniform coating thickness in the ablated region 6, Znab corresponds to the average amount of Zn in the ablated region, as measured by microscopic examination of the cross-section.

[0033] As explained earlier, the inventors found that it is necessary to control the ablation thickness of Znab to less than 3.5 microns in order to guarantee the absence of LME.

[0034] Referring now to Figure 4, Figure 4 is a cross-sectional view of a laser-welded blank 7 consisting of steel plates 1 and 2 and a weld seam 3.

[0035] The ablated region 8 after welding has a width Wabfin (final ablated width) that is smaller than the width Wabini of the ablated region 6 before welding. In fact, a portion of the edge E1 of the steel plate 1 is incorporated into the weld seam 3 by melting, and as a result, a portion of the ablated region 6 before welding is also incorporated into the weld seam 3.

[0036] Given that the melting point of zinc is 420°C and LME can only occur in the presence of liquid Zn, LME is a potential problem for laser-welded blanks 7 across the heat-affected zone in the region where the surface temperature exceeds 420°C. The inventors have found that when laser welding is applied, the location of the 420°C isotherm on the surface of the steel plate is always within a distance of 0.2 to 0.5 mm from the weld seam 3. Therefore, to prevent the occurrence of LME, the wabfin must be 0.5 mm or more.

[0037] In conclusion, to prevent LME, it is necessary to reduce the thickness of the Zn coating near the edge across the width Wabini to a thickness of less than 3.5 microns of Znab, so that the width Wabfin of the ablated area after welding is 0.5 mm or more. By knowing the amount of material to be melted from the edge E1, a person skilled in the art can determine the required ablated width Wabini on the steel plate 1 before welding.

[0038] Therefore, it is possible to obtain a laser-welded blank that does not include LME near the weld seam 3, comprising first and second steel plates 1 and 2, each comprising a base material 12, at least one of which has a zinc-based metal coating 5 on at least one side, and all surfaces of the steel plate base material 12 having a zinc-based metal coating thickness Znth of 3.5 microns or more and a carbon content of 0.15% or a silicon content of 0.5% or more, or both, including a post-welded ablation region 8 having a post-welded ablation region thickness Znab of 3.5 microns or less and a post-welded ablated region width Wabfin of 0.5 mm or more.

[0039] Furthermore, by forming the aforementioned laser-welded blank, it is possible to manufacture parts without the risk of LME cracking near the weld. For example, the laser-welded blank can be manufactured into a part by cold stamping or hot forming.

[0040] In certain embodiments, zinc coating ablation is performed using an abrasive brush that rotates on a steel plate to remove at least a portion of the coating. For example, the brush is made of a polymer web encapsulating hard ceramic particles such as aluminum oxide or silicon carbide. In this case, the important parameters for brushing are the abrasive force of the brush, the force exerted on the coating by the brush, and the speed at which the brush moves across the surface. Depending on the material used and the zinc coating, it is necessary to adjust these parameters to obtain the desired abraded thickness of Znab. It is also possible to perform several brushing passes to remove more of the coating.

[0041] In certain embodiments, ablation is performed using a pulsed laser beam. The inventors have found that the interaction between a short laser pulse and the coating causes at least a portion of the coating to evaporate and be removed from the surface of the steel plate. For example, a laser output of 400W to 1500W can be used with a pulse frequency of 5 to 15kHz and an ablation rate of 2 to 15m / min.

[0042] In certain embodiments, as shown in Figure 3B, the ablated region 6 on the steel plate before welding does not begin at the edge E1 of the steel plate. Instead, there is an offset region 9 with a width Woffset where ablation does not occur. In other words, the coating thickness in the offset region 9 is the same as the bulk Znth of the material. The inventors have found that the presence of such an offset does not result in LME cracking, as long as the entire offset region 9 melts and is incorporated into the weld seam 3, and the Wabfin remains at least 0.5 mm. They have found that even if some zinc from the metal coating is incorporated into the weld seam, there is no LME in the weld seam itself. When an offset exists before welding and the entire offset region 9 is incorporated into the weld pool, the ablated region 8 after welding has the same characteristics as when there is no offset, as shown in Figure 4. Advantageously, the presence of such an offset can ensure better corrosion protection of the edge E1 before welding. Indeed, thanks to the sacrificial nature of zinc corrosion protection, the zinc coating in the offset region 9 is available to protect the uncoated exposed portion of the edge E1.

[0043] In certain embodiments, a decarburization step is applied to at least one of steel sheets 1, 2 to reduce the carbon content of the steel sheet near its surface. This decarburization step is performed before the application of the zinc coating. For example, the decarburization step is performed in a furnace used to anneal the steel sheet before coating it. For example, the decarburization step is performed by controlling the dew point in the annealing furnace to a value of -10°C or higher. Advantageously, the risk of LME is reduced by using steel sheets with a lower carbon content near the surface. For example, the carbon content at a depth of 20 microns from the surface of the steel sheet on each side is less than 0.15% by weight, preferably less than 0.10% by weight, and the carbon content of the steel sheet at the center (e.g., carbon content measured in a strip + / - 100 microns from the central thickness of the steel sheet) is greater than 0.15% by weight.

[0044] In certain embodiments, the ablation step is performed such that the remaining zinc thickness Znab is greater than 0.5 microns, more preferably greater than 1.0 micron. As previously described, the inventors have found that there is no LME risk even if a small amount of zinc remains in the ablated area 6, as long as Znab is less than 3.5 microns. Advantageously, leaving some zinc in the ablated area 6 ensures some degree of corrosion protection of the ablated steel sheet before welding. Thanks to the sacrificial nature of the zinc coating, this corrosion protection extends to the uncoated bare areas at the edges on the sides of the steel sheet. Furthermore, applying an ablation process that leaves some zinc coating in the ablated area 6 means there is no risk of removing (by brushing) or melting (by laser ablation) the underlying substrate 12 of the steel sheet beneath the ablated area 6. This is interesting because ablating the substrate 12 itself beneath the coating would weaken the laser-welded blank by creating localized geometric defects and localized thickness deficiencies.

[0045] In certain embodiments, the laser welding process is performed less than one minute, preferably less than 30 seconds, after the ablation process has been completed. For example, the ablation and welding processes are performed on the same apparatus equipped with facilities for the ablation and welding steps. For example, the ablation and welding processes are performed under the same clamping operation, meaning that steel plates 1 and 2 are clamped to hold them in place for the ablation step and the same clamping is maintained in place for the welding process. Advantageously, performing ablation and welding in a rapid sequence, even under the same clamping operation in some cases, can increase productivity, reduce the amount of material handling steps, and reduce the risk of corrosion resulting from the ablation step.

[0046] In certain embodiments, the ablation process is performed in the center of the steel sheet rather than directly at the edge of the sheet, and the sheet is subsequently cut in the region where the ablation process was performed, so that the ablated region is located (possibly with an offset) at the edge of the cut sample.

[0047] In certain embodiments, at least one characteristic of the welded steel sheet relating to its chemical composition, microstructure, and mechanical properties corresponds to one of the rows in the following table (the chemical composition is expressed in weight percent, with the remainder being Fe and unavoidable impurities derived from the refining process; the % retained austenite in the microstructure of the steel sheet is expressed in surface percent of the cross-section; YP represents the yield point expressed in MPa; UTS represents the ultimate tensile strength expressed in MPa; and El% is the elongation measured according to the ISO 6892 standard mentioned above).

[0048] [Table 1]

[0049] The present invention will be explained below with reference to the following examples, but these are by no means limiting.

[0050] Table 1 shows the chemical composition of the steel plates used.

[0051] [Table 2]

[0052] The Si content was over 0.5% and the carbon content was over 0.15%, which is significant in terms of LME risk when steel sheets are welded with a zinc coating. The steel sheets were decarburized before applying the hot-dip pure zinc coating so that the carbon content, measured 20 microns from the surface of the steel sheet using glow discharge emission optical spectroscopy (GDOES), was 0.05 wt%.

[0053] The initial coating thickness of the steel sheet is 7 to 11 microns, depending on the sample (in practice, due to process variations that naturally occur on the production line, there may be variations in coating thickness when using industrially produced steel sheets).

[0054] The steel plate has a thickness of 1.0 mm. A homogeneous assembly of the same steel plate and thickness will be produced for this trial (in other words, steel plate 1 and steel plate 2 will have the same characteristics).

[0055] Table 2 lists the ablation process parameters used.

[0056] [Table 3]

[0057] The reference numeral for a sample begins with "I" to indicate the invention if the sample was produced according to the present invention, and begins with "R" to indicate a reference if it is a counterexample outside the scope of the present invention. Zn coating ablation was performed using a low-power pulsed laser or mechanical brushing.

[0058] Mechanical brushing was performed using a 3M Scotch-Brite "Deburr and Finish pro 4C MED+" brush mounted on a robot, with a diameter of 76.2 mm, a width of 12.7 mm, and rotating at 6000 rpm. The brushing speed corresponds to the speed at which the robot arm moves along the sheet being brushed.

[0059] Table 3 shows the results for the ablated region 6 before welding and the ablated region 8 after welding, as well as the results of the mechanical test of the weld.

[0060] [Table 4]

[0061] The underlined values ​​correspond to features that are outside the scope of the present invention in the case of the reference sample.

[0062] Sample R1, which was not ablated and served as an initial reference, represents the extremely poor mechanical performance of a welded assembly for which no measures were taken to prevent LME. 40% of the samples tested longitudinally using the innovative test method described above failed due to LME cracking.

[0063] Samples R2 to R4 underwent ablation, but the remaining zinc thickness (Znab) after ablation was too thick in order to prevent the occurrence of LME.

[0064] Samples I1 to I9 demonstrate the technical effects of the present invention. By removing the zinc coating to less than 3.5 microns in width of 0.5 mm or more after welding, LME is prevented, and a structurally sound assembly is produced.

[0065] In the case of I2, ablation was performed leaving a 0.3 mm offset region at the edge of the steel plate. Despite the presence of this unablated region at the edge of the steel plate, the entire offset region was absorbed into the weld seam, and since the ablated width after welding exceeded 0.5 mm, the final assembly does not contain LME.

[0066] In the case of sample I9, the remaining zinc coating thickness after ablation is 3.2 microns. Despite the presence of a remaining zinc coating thickness, no ablation is observed in the resulting assembly.

[0067] In conclusion, LME will not occur if the steel plate is ablated before welding, producing an ablated region on the final welded assembly with a width exceeding 0.5 mm and a remaining Znab thickness of 3.5 microns or less.

Claims

1. A method for butt welding two steel plates (1, 2), wherein each steel plate (1, 2) comprises a base material (12), at least one of the steel plates (1, 2) has a zinc-based metal coating (5) of thickness Znth on at least one side, and at least one of the steel plates has a chemical composition of the base material (12) expressed in weight percent, having a carbon content of more than 0.15% or a silicon content of more than 0.5% or both. The method described above is - The step of providing the two steel plates (1, 2), - A step of forming an ablation region (6) before welding on all surfaces of a steel plate substrate (12) having a zinc-based metal coating thickness Znth exceeding 3.5 microns, and a carbon content exceeding 0.15% or a silicon content exceeding 0.5%, or both, by removing at least a portion of the metal coating, thereby creating an ablation region (6) before welding such that the ablation region (8) width Wavefin is 0.5 mm or more, and the ablation region (8) width Wavefin is 0.5 mm or more. - The step of butt welding the steel plates (1, 2) using at least a laser source, Methods that include...

2. The method according to claim 1, wherein the metal coating removal step is performed using a pulsed laser beam.

3. The method according to claim 1, wherein the metal coating removal step is performed using mechanical brushing.

4. The method according to claim 1, wherein an offset region (9) is left between the pre-weld ablation region (6) and the edge of the steel plate, the pre-weld ablation region (6) having a zinc-based metal coating thickness Znab of 3.5 microns or less, and the offset region (9) having a zinc-based metal coating thickness equal to the pre-weld zinc-based metal coating thickness Znth of 3.5 microns or more.

5. The method according to claim 1, wherein in at least one ablation region (6), the thickness Znab of the zinc-based metal coating after ablation is 0.5 microns or more.

6. The method according to any one of claims 1 to 5, wherein at least one of the steel plates (1, 2) is decarburized before the application of the zinc-based metal coating such that the surface carbon content at a depth of 20 microns is less than 0.15% by weight.

7. A laser-welded blank (7) comprising first and second steel plates (1, 2), each comprising a base material (12), at least one of which has a zinc-based metal coating (5) on at least one side, and a weld seam (3), wherein all surfaces of the steel plate base material (12) having a zinc-based metal coating thickness Znth of 3.5 microns or more and a carbon content of 0.15% or a silicon content of 0.5% or both, include a post-welded ablation region (8) having a post-welded ablation region width Wavefin of 0.5 mm or more with a post-welded ablation region thickness Znab of 3.5 microns or less.

8. The laser-welded blank (7) according to claim 7, wherein the thickness Znab of the zinc-based metal coating is 0.5 microns or more for at least one post-weld ablation region (8).

9. A molded part manufactured by forming a laser-welded blank according to claim 7 or 8.

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