Method for producing a pre-coated metal sheet by removing the coating using a flexible laser beam, and the corresponding metal sheet

Abstract

The method of producing a pre-coated metal sheet for welding it to another pre-coated metal sheet comprises the following sequential steps:\n-\tProviding a pre-coated metal sheet (1) comprising the following parts:\n•\tA metal substrate (3) located on at least one of its faces (10),\n•\tA pre-coated layer (5),\n-\tRemoving (from at least one face (10) of the pre-coated metal sheet (1)) at least a portion of the pre-coated layer (5) so as to form a removal zone (7). The aforementioned removal is performed by striking the pre-coated layer (5) with a laser beam (15). The extraction step (during the extraction period) includes the following: • Relative displacement of the laser beam (15) relative to the metal sheet (1) in the forward direction (A). During extraction, the laser beam (15) is tilted relative to the face (10) of the metal sheet (1) such that the perpendicular projection of the laser beam (15) onto the face (10) of the metal sheet (1) is located in the region of the metal sheet (1) where the extraction has been performed and forms an inclination angle (α) that is between 12 and 50 degrees with respect to the normal direction (N) to the face (10) of the metal sheet (1).

Description

Method for producing a pre-coated metal sheet, by removing the coating using a flexible laser beam, and the corresponding metal sheet This invention relates to a method of producing a pre-coated metal sheet for welding to another pre-coated metal sheet, comprising the following steps: -Providing a pre-coated metal sheet containing a metal substrate provided on at least one of its faces (with a pre-coated layer) then -Removing (from at least one face of the pre-coated metal sheet) at least a portion of the pre-coated layer to form a removal zone. The removal operation is performed by the impact of a laser beam on the pre-coated layer. The removal step (during the removal period) involves the relative displacement of the laser beam relative to the metal sheet in the forward direction. Patent application EP 2 007 545 describes a method for producing a welded part of a sheet metal which has been pre-coated, the pre-coating comprising a layer of intermetallic alloy on which a layer of metal alloy is deposited. During this method, at least a portion of the pre-coated layer is removed by ablation using a laser beam before welding in order to limit the proportion of molten pre-coating during subsequent welding around the sheet metal, while at least a portion of the intermetallic alloy layer is retained to protect the areas on both sides of the weld joint from corrosion. Publication KR 10-1346317 describes a method in which an aluminum-silicon-based coating is removed from the periphery of a metal sheet before welding. KR 10-1346317 teaches tilting the laser head to the vertical at an angle of between 5 and 10 degrees to prevent the beam from being reflected by the metal sheet from the laser light receiver. KR 10-1346317 does not specify in which direction the laser head should be tilted relative to the direction of laser movement during removal. Having stated these methods, one of the objectives of this invention is to provide a method for preparing a pre-coated metal sheet for welding to another pre-coated metal sheet, which allows for obtaining a welded joint with suitable mechanical properties to reduce the wear time of the pre-coated sheet. To this end, the invention relates to a method of preparing the aforementioned pre-coated metal sheet, in which during the cutting, the laser beam is tilted relative to a face of the metal sheet such that the perpendicular projection of the laser beam on the face of the metal sheet is located in the region of the metal sheet where the cutting has been performed and forms an inclination angle of between 12 and 50 degrees with the direction normal to the face of the metal sheet. Given the specific features of this method: -The pre-coated layer is an aluminum layer, an aluminum-based layer, or an aluminum alloy layer; -The pre-coated layer is a layer of aluminum alloy that also includes silicon; -The inclination angle of the laser beam is between 15 and 45 degrees; -The inclination angle of the laser beam is between 20 and 40 degrees; -The inclination angle of the laser beam is between 25 and 40 degrees; -The inclination angle of the laser beam is between 25 and 35 degrees; -The laser beam is a pulsed laser beam. -In the harvesting stage, the distance between the laser head output lens and the larger metal sheet is equal to 150 mm, and specifically between 150 mm and 250 mm; -The extraction is done without blowing gas; -The extraction is done without suction; -The pre-coated layer contains a layer of intermetallic alloy on which a layer of metallic alloy is placed; -The harvesting area is completely free of metal alloy layer; -The cutting area is formed on the bottom face of the metal sheet; -The cutting zone is formed simultaneously on the bottom and top sides of the metal sheet; -The metal substrate is made of steel; -Substrate steel includes the following, by weight: 0.10% ≤ C ≤ 0.5% 0.5 ≤ Mn ≤ 3% 0.1 ≤ Si ≤ 1% 0.01 ≤ Si ≤ 1% Ti ≤ 0.2% Al ≤ 0.1% S ≤ 0.05% P ≤ 0.1% B ≤ 0.010% The residue is iron and impurities from smelting; -Substrate steel includes the following, by weight: 0.15% ≤ C ≤ 0.25% 0.8 ≤ Mn ≤ 1.8% 0.1% ≤ Si ≤ 0.35% 0.01 ≤ Cr ≤ 0.5% Ti ≤ 0.1% Al ≤ 0.1% S ≤ 0.05% P ≤ 0.1% B ≤ 0.005% The residue is iron and impurities from smelting; -Substrate steel includes the following, by weight: 0.040% ≤ C ≤ 0.100% 0.80 ≤ Mn ≤ 2.00% Si ≤ 0.30% S ≤ 0.005% P ≤ 0.030% 0.010% ≤ Al ≤ 0.070% 0.015% ≤ Nb ≤ 0.100% Ti ≤ 0.080% N ≤ 0.009% Cu ≤ 0.100% Ni ≤ 0.100% Cr ≤ 0.100% Mo ≤ 0.100% Ca ≤ 0.006%, The residue is iron and impurities from smelting; -The microstructure of the steel is fritto-pearlitic; -During the preparation step, two pre-coated metal sheets are prepared and placed side by side, a certain gap is placed between the two pre-coated metal sheets, then during the removal step, at least a portion of the pre-coated layer is removed simultaneously from both metal sheets to simultaneously create a removal area on each of said metal sheets; in the removal step, the laser beam is adjusted to the butt of the two metal sheets. -The harvesting area is located around the metal sheet; -The cutting area is not completely adjacent to the edge of the metal sheet; -In addition to the above, the method includes, after the cutting step to form the cutting region, cutting the metal sheet along the surface to form a metal sheet having a periphery that includes a region free of at least a portion of the pre-coated layer. The invention also relates to a metal sheet having a metal substrate provided on at least one of its faces with a pre-coated layer. The metal sheet comprises, on at least one face, a removal zone in which the pre-coated layer is removed from a portion of its thickness. According to the specific characteristics of sheet metal: -In the harvesting area, the relative change Δ along the width of the harvesting area is considered to be the thickness of the part of the pre-coated layer remaining in the harvesting area and is defined as the ratio of the difference between the pre-coated thickness at half the width and the pre-coated thickness at one-third the width from the edge of the harvesting area to the pre-coated thickness at half the width, which is greater than 0% and less than or equal to 50%. -The pre-coated layer consists of a layer of intermetallic alloy on which a layer of metallic alloy is placed. -The harvesting area is completely free of metal alloy layer; -The harvesting area is located around the metal sheet; -The cutting area is not completely adjacent to the edge of the metal sheet. This invention also relates to a method of producing a welded chamber, which comprises the following steps: -Providing at least two metal sheets as described above, in which the cutting zone is located at the periphery of the metal sheet, or providing at least one metal sheet in which the cutting zone is not completely adjacent to the edge of the metal sheet, obtained by cutting in the cutting zone, thus obtaining a metal sheet which includes at its periphery an area free of at least a portion of the pre-coated layer or produced using the said production method. Then -Butt welding of these two metal sheets, the welding joint is performed on the edge including the cutting area. Based on the specific characteristics of this method, two metal sheets joined by impact welding have different thicknesses. This invention also relates to a method of producing a hot-pressed part, which includes the following sequential steps: -Presentation of the welded chamber obtained using the above method, -Heating the welded chamber so that a relatively or fully austenitic structure is created in the substrate layers of the metal sheets forming the aforementioned chamber, - Hot pressing said chamber to obtain a hot pressing piece; -Rapid cooling of the part to give it the desired mechanical properties. Due to the special characteristics of this method, the cooling rate is greater than the critical martensite quenching rate of the substrate steel of at least two metal sheets or the substrate steel of at least one metal sheet. The invention will be better understood by reading the following description, given by way of example only and with reference to the accompanying drawings, in which: -Figure 1 is a schematic view of the initial pre-coated metal sheet. - Figure 2 is a schematic view of a metal sheet obtained using the preparation method. -Figure 3 is a schematic view of a metal plate according to one of the options. -Figure 4 is a schematic view of the harvesting, i.e., the erosion stage of the production method. -Figure 5 is a graph showing the processing speed as a function of the laser beam inclination angle. - Figure 6 is a schematic cross-sectional view of the geometry of the ablation zone obtained under the conditions of the invention when the laser beam is inclined at an angle α between 25 and 50 degrees. -Figure 7 is a schematic view of the harvesting stage of the production method. This invention relates to a method of producing a pre-coated metal sheet for welding to another part, in particular to a secondary pre-coated metal sheet that has been produced in a similar manner. This method according to the invention includes the following steps: -Providing a primary pre-coated metal sheet 1 comprising a metal substrate 3 having a pre-coated layer 5 on at least one of its faces, -Removing at least a portion of the pre-coated layer 5 from at least one face of the initial pre-coated metal sheet 10. Figure 1 shows an example of an initial pre-coated metal sheet 1 in the preparation stage belonging to the method according to the present invention. In the context of this invention, the term "metal sheet" is understood broadly and refers in particular to any strip or any object obtained by cutting from a strip, coil or sheet. In the specific case shown in Figure 1, this metal sheet 1 comprises two faces 10 and four edges 13. However, the invention is not limited to this specific geometric shape. As shown in Figure 1, the pre-coated metal sheet 1 comprises a metal substrate 3 on at least one of its faces, the pre-coated layer 5 being located on the substrate 3 and in contact with it. Substrate 3 is a substrate made of steel. Substrate 3 steel is specifically a steel that has a fritto-pearlitic microstructure. The substrate 3 is preferably a heat treatable steel, particularly a steel with hard pressability, particularly a manganese and boron based steel such as MnB522 type steel. Based on an example, the 3-layer steel substrate includes the following, by weight: 0.10% ≤ C ≤ 0.5% 0.5 ≤ Mn ≤ 3% 0.1 ≤ Si ≤ 1% 0.01 ≤ Si ≤ 1% Ti ≤ 0.2% Al ≤ 0.1% S ≤ 0.05% P ≤ 0.1% B ≤ 0.010% The residue is iron and impurities from melting. Specifically, Tier 3 steel includes the following, by weight: 0.15% ≤ C ≤ 0.25% 0.8 ≤ Mn ≤ 1.8% 0.1% ≤ Si ≤ 0.35% 0.01 ≤ Cr ≤ 0.5% Ti ≤ 0.1% Al ≤ 0.1% S ≤ 0.05% P ≤ 0.1% B ≤ 0.005% The residue is iron and impurities from melting. Or rather, the 3-layer steel consists of the following, by weight: 0.040% ≤ C ≤ 0.100% 0.80 ≤ Mn ≤ 2.00% Si ≤ 0.30% S ≤ 0.005% P ≤ 0.030% 0.010% ≤ Al ≤ 0.070% 0.015% ≤ Nb ≤ 0.100% Ti ≤ 0.080% N ≤ 0.009% Cu ≤ 0.100% Ni ≤ 0.100% Cr ≤ 0.100% Mo ≤ 0.100% Ca ≤ 0.006%, The residue is iron and impurities from melting. For example, the metal substrate 3 is obtained by hot rolling or cold rolling followed by annealing, or by using any other suitable production method, depending on the desired thickness. The thickness of the substrate 3 is preferably between 0.5 mm and 4 mm, and in particular equal to 1.5 mm. The pre-coated layer 5 is a layer obtained by immersion, i.e. by immersion in a molten metal bath. This layer comprises a layer of intermetallic alloy 9 which is in contact with the substrate 3 located on the metal alloy layer 11. The intermetallic alloy layer 9 is formed by the reaction between the substrate 3 and the molten metal of the bath. This intermetallic alloy layer 9 comprises an intermetallic compound having at least one element of the metal alloy layer 11 and at least one element of the substrate 3. Its thickness is usually about a few micrometers. In particular, its average thickness is usually between 2 and 7 micrometers. The metal alloy layer 11 has a composition very close to that of the molten metal in the bath. This characteristic is due to the absorption of part of the molten metal from the bath by the strip during its movement through the bath. Its thickness is controlled by means of suitable control elements located at the bath outlet and in particular by means of gas jets and jets of nitrogen or air. Its average thickness is between 19 and 33 µm or an average thickness is between 10 and 20 µm. The pre-coated layer 5 is an aluminum layer, an aluminum alloy layer or an aluminum-based alloy layer. In this case, the intermetallic alloy layer 9 comprises intermetallic compounds of the Fex-Aly type, in particular Fe2Al5. "Aluminum alloy" refers to an alloy that contains more than 50% aluminum by weight. "Aluminum-based alloy" means an alloy in which aluminum is the major component by weight. In one example, the pre-coated layer is a 5-layer aluminum alloy that also includes silicon. For example, the metal alloy layer 11 includes the following, by weight: - 8% ≤ Si ≤ 11% - 2% ≤ Si ≤ 4%, The remainder is aluminum and possible impurities. The specific structure of the pre-coated layers 5 obtained by dip coating is described in particular in patent application EP 2 007 545. Desirably, and as shown in Figure 1, the substrate 3 has such a pre-coating 5 on both of its faces 10. Preferably, the initial pre-coated metal sheet 1 is obtained by cutting, in particular by slitting or laser cutting, a pre-coated strip having the above-mentioned properties. After the preparation step, the method according to the invention comprises removing from at least one face 10 of the initial precoated metal sheet 1 at least a portion of the precoated layer 5 in the removal zone 7. Figure 2 shows the metal sheet obtained after harvesting. In the example shown in Figure 2, the harvesting zone 7 is located around the perimeter 6 of the initial pre-coated metal sheet 1. Desirably, during this step, the metal alloy layer 11 is completely removed. It is advantageous to remove at least a portion of the pre-coated layer 5. In fact, without removal, during welding of a pre-coated metal sheet to another metal sheet, a portion of the pre-coated layer 5 is diluted with the substrate 3 in the molten zone, the zone that becomes liquid during the welding operation when the joint between the two metal sheets is formed and becomes solid after the welding operation. Then two phenomena occur: -According to the first phenomenon, the increase in the aluminum content in the molten metal due to the dilution of the pre-coated part in this area leads to the formation of intermetallic compounds. This feature causes the fracture of the starting point in the event of mechanical stress. -According to the second phenomenon, the solid solution of the alpha-gene element aluminum in the molten zone delays the austenite transformation of this zone during the heating stage before hot pressing. Then, it is not possible to achieve a completely cooled structure in the molten zone after cooling following hot forming, and the welded joint becomes ferrite-containing. Then, the molten zone has lower mechanical hardness and tensile strength than the two adjacent metal sheets. In the case of the preparation method according to the invention, it is desirable to reduce the amount of elements of the pre-coated layer 5 that enter the molten zone and negatively affect its mechanical properties. The inventors of the present invention have found that satisfactory results in terms of the mechanical properties of the welded joint are obtained when, in the case of the pre-coated layer 5 based on aluminum at the end of the removal step, the part of the pre-coated layer 5 that remains has an average thickness of a maximum of 5 µm. Considering the average thickness of the intermetallic alloy layer 9 that is usually obtained during the dip coating of a steel metal sheet with an aluminum-based coating, this thickness corresponds to the complete removal of the metal alloy layer 11 and possibly to the partial removal of the intermetallic alloy layer 9. During the removal step, the intermetallic alloy layer 9 preferably remains at least partially over the entire surface of the removal zone 7. Desirably, at the end of this step, the intermetallic alloy layer 9 remains with a thickness of at least 1 µm. It is advantageous for at least a portion of the intermetallic alloy layer 11 to remain in the removal zone 7. In fact, in order to account for any fluctuations in the width of the molten zone during the welding operation, the width of the removal zone 7 is greater than the width of the molten zone during welding. After the welding operation, regions remain on both sides of the welding joint from which the pre-coated layer 5 has been at least partially removed. However, the purpose of the aluminum-based pre-coated layer 5 is to protect the metal sheet 1 against corrosion after the hot press forming of the welding joint. Studies have shown that in the region of the removal zone 7 adjacent to the welding joint, at least a portion of the intermetallic alloy layer 9 remains over the entire surface of this region, allowing the welding assembly to benefit from adequate corrosion protection. In particular, an intermetallic alloy layer with a thickness of at least 1 µm is present at all points in the area 7 in the extraction area 7 to provide satisfactory corrosion resistance to the weld assembly in the areas adjacent to the weld joint. Alternatively, in cases where corrosion resistance is less important, the intermetallic alloy layer 11 in the removal region 7 can be completely removed. According to the invention, the removal of at least a portion of the pre-coated layer 5 is carried out using a laser beam 15 that is absorbed into the pre-coated layer 5 . Figures 4 to 6 schematically show, in side view, the harvesting step using a laser beam 15. The laser beam 15 is emitted by the laser head 17. The harvesting step, during the harvesting period, involves the relative displacement of the laser beam 15 with respect to the metal sheet 1 in the direction of laser advance, marked A. This direction of advance is indicated by arrows in Figures 4 and 6. The relative displacement of the laser beam 15 in the forward direction A, for example, is as follows: - Displacement of the laser beam 15 in direction A, the metal sheet 1 remains stationary; or -Moving the metal sheet 15 in the opposite direction A, the laser beam 15 remains stationary; or -Move the laser beam 15 along A and the metal sheet 1 in the direction opposite to A. For example, in the harvesting phase, a 15-inch laser beam is used with the following characteristics: The laser beam 15 preferably has a rectangular or square cross-section. For example, the laser beam 15 produces a focal spot on the face 10 of the metal sheet 1 being processed with a surface area of ​​between 0.4 mm² and 3 mm², and preferably between 0.7 mm² and 1.5 mm². The laser is preferably a pulsed laser, such as a Q-switched laser, a pulsed fiber laser, or a pulsed diode laser. The pulse duration is of the order of nanoseconds. This duration is typically between 1 ns and 300 ns, preferably between 10 ns and 150 ns, and most desirably between 30 ns and 80 ns. The rated power of the laser 15 is typically between 200 W and 1.7 kW, and preferably between 400 W and 1.7 kW. Preferably, in the harvesting step, the working distance corresponding to the distance between the output lens of the laser head 17 and the face 10 under processing of the metal sheet 1 is greater than or equal to 150 mm. In fact, during the harvesting step, the projections resulting from the erosion by the laser beam 15 are projected to a smaller height equal to 100 mm with respect to the face 10 being processed. This projection height is even smaller, equal to 50 mm, for the majority of the projections. As a result, the larger working distance equal to 150 mm makes it possible to avoid the risk of contamination of the laser head lenses by any projections resulting from the erosion by the laser beam 15. It should be noted that the projection is located on the elements of the laser head 17 between the output lens and the face 10 being processed and in particular on any protective elements intended to protect the output lens from the projections and does not pose a problem in the context of the method according to the invention. Ideally, this working distance is between 150 mm and 250 mm. In fact, using a working distance of more than 250 mm in particular causes a significant increase in the costs of the harvesting stage, as it requires the use of non-standard lenses in the laser lights, especially larger ones than those commonly used, as well as a greater number of lenses than are required when the working distance is within the aforementioned range. This working distance is preferably between 190 mm and 215 mm. According to this invention, in the harvesting step, the laser beam 15 is directed in a specific manner relative to the metal sheet 1. The specific orientation of the laser beam 15 during the harvesting phase is shown schematically in Figure 4. In order to facilitate the description of the subsequent directions, Figures 1 to 6 are presented with the coordinate system (x, y, z). As can be seen, the x axis of this coordinate system is oriented in the direction of travel A of the laser beam 15. The z axis of the coordinate system is oriented along the thickness of the metal sheet 1 and from half the thickness of the substrate 3 towards the erosion surface, that is, upwards in cases where erosion is carried out on the upper face 10 of the metal sheet 1 and downwards in cases where erosion is carried out on the lower face 10 of the metal sheet 1. The y axis of the coordinate system is perpendicular to the x and z axes and is oriented in such a way that the coordinate system forms a straight line with these two x, z axes. The bottom 10 is the face down during the execution of the withdrawal phase. The top 10 is the face up during the execution of the withdrawal phase. As shown in Figure 4, during the cutting, the laser beam 15 is inclined at an angle α with respect to the normal N towards the face 10 of the metal sheet 1 on which the cutting is performed. Said normal N is taken from the region of the metal sheet 1 on which the cutting is performed. The angle α is the angle between the axis L of the laser beam 15 and the normal N towards the face 10 of the metal sheet 1 on which the cutting is performed. Said normal N is taken from the region of the metal sheet 1 on which the cutting is performed. The slope angle α is an acute angle. An acute angle refers to an angle between 0 and 90 degrees that excludes boundaries. The laser beam 15 is further inclined so that the vertical projection of the laser beam 15 onto the face 10 of the metal sheet 1 being processed is located in the region of the metal sheet 1 where the removal has been carried out. The laser beam 15 is therefore emitted forward in the advancing direction A towards the face 10 of the metal sheet 1 being processed. In other words, the emission head 17 of the said laser beam 15 is located at the rear and in the advancing direction A, the impact area of ​​the laser beam 15 on the metal sheet 1. As shown in Figures 3 and 4, the laser beam 15 forms an obtuse angle with a region of the face 10 of the metal sheet 1 located below the impact zone of the laser beam 15 in the advancing direction A, i.e. with the region of the metal sheet remaining to be processed, and an acute angle is formed with a region of the face 10 of the metal sheet above the laser beam zone 15, i.e. with the region of the metal sheet remaining to be processed. Remember that the L axis of the laser beam 15 is processed entirely in the plane normal to the face 10 of the metal sheet 1 and includes the direction of advance A. According to the invention, the inclination angle α is between 12 and 50 degrees. In the context of the present invention, for the inclination angle α of the laser beam 15, Vm is defined as the advancing speed of the laser beam 15 at which the entire metal alloy layer 11 is removed and replaced in the entire intermetallic alloy layer 9. Therefore, for advancing speeds greater than Vm, parts of the metal alloy layer 11 remain in the region 7. The inventors of the present invention surprisingly found that when the laser beam 15 is oriented with the aforementioned orientation during harvesting at an inclination angle α between 12 and 50°, the velocity Vm(α) is at least 15% higher than its value for the angle α equal to 0°, which is designed by Vm(0°), i.e. the laser beam 15 is substantially perpendicular to the face 10 being processed. The angle α is therefore defined with respect to the direction N, normal to the face of the metal sheet 1, as shown in Figure 4. These results are shown in Figure 5, which shows the evolution of the ratio VM(α) / V(0°) as a function of the inclination angle α resulting from experiments carried out by the inventors, which will be described in detail below. The aforementioned orientation is therefore advantageous because it allows the laser beam 15 to be operated at a speed of at least 15% higher than the speed Vm allowed in typical cases where the laser beam is perpendicular to the face 10 being processed, and to obtain a quality result at least equal to the quality of the cut. However, increasing the speed of travel reduces the duration of processing the metal sheet 1 and, as a result, increases the effectiveness of the method, which in turn leads to a reduction in production costs. The inclination angle α is between 15 and 45 degrees. The inventors of the present invention have found that in this range of inclination angle α, the speed Vm(α) is at least 25% higher than its value for angle α equal to 0 degrees, i.e. when the laser beam 15 is perpendicular to the face 10 of the metal sheet 1 being processed. This desirable range is advantageous because it further reduces the processing time of the metal sheet 1. Again advantageously, the inclination angle α is between 20 and 40 degrees. The inventors of the present invention have found that in this range of inclination angle α, the speed Vm(α) is at least 40% higher than its value for angle α equal to 0 degrees, i.e. when the laser beam 15 is perpendicular to the face 10 of the metal sheet 1 being processed. This desirable range is also advantageous because it results in a much greater reduction in the processing time of the metal sheet 1. These results are also shown in Figure 5. Desirably, the inclination angle α is between 25 and 35 degrees. In fact, as shown in Figure 5, in this range of inclination angle α, the speed Vm(α) is at least 75% higher than its value for angle α equal to 0 degrees, i.e. when the laser beam 15 is perpendicular to the face 10 of the metal sheet 1 being processed. This desirable range is also advantageous because it causes a much greater reduction in the processing time of the metal sheet 1. As shown in Figure 5, when the inclination angles α are between 30 and 35 degrees, the velocity Vm(α) is twice its value for the angle α equal to 0 degrees. Based on an example, a speed approximately equal to Vm is selected to perform the removal so that the metal alloy layer 11 in the removal zone 7 is completely removed and the intermetallic alloy layer 9 remains intact. Or, alternatively, the advancing speed of the laser beam 15 is selected lower than Vm in order to remove the intermetallic alloy layer 9 at least over the entire surface of the removal zone 7. In this case, the inclination of the laser beam 15 within the aforementioned range allows the selection of an advancing speed greater than the advancing speed of the laser beam 15 normal to the face 10 being processed, so that the removal result is at least the same. At the end of the harvesting stage, the prepared metal sheet '1 is obtained as schematically shown in Figure 2. This metal sheet '1 has the following characteristics. It comprises a metal substrate 3 which is covered on at least one of its faces 10) with a pre-coated layer 5 as previously defined and has a pick-up zone 7 around its periphery 6 free of at least a portion of the pre-coated layer 5. The metal sheet '1' is placed next to another metal sheet and then impact welded along the surface defined by the edge 13 of the metal sheet 1 located in the region 7. The width of the zone 7 is between 0.8 mm and 3 mm, and in particular between 0.8 mm and 2 mm. This width extends along at least one edge 13 of the metal sheet 1. In the example shown in Figure 2, the metal alloy layer 9 is completely removed from the region 7 while at least a portion of the intermetallic alloy layer 11 remains over the entire surface of the region 7. In particular, the intermetallic alloy layer 11 remains in the region 7. In this case, a feed rate of Vm is used during the removal step. Or alternatively, the metal alloy layer 9 is completely removed in region 7 and the intermetallic alloy layer 11 is partially removed over the entire surface of region 7. According to another example, the metal alloy layer 9 is completely removed in region 7 and the intermetallic alloy layer 11 is completely removed from the entire surface of region 7. In the example shown in Figure 2, the removal zone 7 extends around the periphery of the metal sheet 6. It therefore extends in the region adjacent to the edge 13 of the metal sheet. In this example, it extends parallel to the edge 13 over the entire surface of said edge 13. According to an alternative shown in Figure 3, the removal zone 7 is located in a region that is not completely adjacent to the edge 13 of the pre-coated metal sheet. This region extends parallel to the edge 13 of the metal sheet over the entire length of the edge 13 at a certain non-zero distance from said edge. According to this alternative, the metal sheet 1" obtained is cut along an axial plane 20 perpendicular to it and intersecting the cutting zone 7, in particular in its middle. This cutting is performed by a straightedge or laser cutting. The metal sheet '1' is obtained as shown in Figure 2. For example, according to the alternative, the aforementioned axial surface 20 passes through the extraction zone 7 and the width of the extraction zone 7 is 20% to 40% greater than the width of the molten zone formed by the welding operation performed along the aforementioned axial surface 20. Desirably, the width of the extraction zone 7 is chosen such that after the welding operation performed along the aforementioned axial surface 20, at least 0.1 mm of the extraction zone 7 remains on each side of the molten zone along the width of the extraction zone 7. Or alternatively, the width of the cutting zone 7 is between 0.4 mm and 30 mm. The minimum value of 0.4 mm corresponds to the width which, after cutting along the axial surface 20, produces two metal sheets with a very narrow cutting zone of 0.2 mm on each of the two metal sheets. The value of 30 mm corresponds to the cutting width which is suitable for industrial tools for such cutting. The subsequent cut can be made not on the axial surface 20 located in the middle of the cutting zone but at a suitable location to obtain a metal sheet whose cutting width is slightly larger than half the width of the molten zone resulting from the welding operation defined by the terms of the invention. When the pre-coated metal sheet 1 has a pre-coated layer 5 on each of its faces, the removal step is performed on each of its faces sequentially or simultaneously with the appropriate laser head 17. In this case, the metal sheet 1 has on each of its faces 10 a previously defined removal zone 7, these removal zones 7 being advantageously located next to each other along the normal N to the metal sheet '1. According to an example, the removal step of the pre-coated metal sheet manufacturing method described above is performed without gas blowing and / or without suction. In fact, the use of blowing and / or suction in conjunction with the laser beam inclination of 15 as described above compromises the stability of the removal method. In particular, the suction and blowing are performed in specific directions, even a small error in the position of the blower nozzle or the suction nozzle leads to the absence of a local minimum erosion, for erosion rates greater than Vm(0). Within the framework of the present invention, the inventors have carried out the following experiments which allowed them to obtain the curve shown in Figure 5. They began by cutting 1 sheet of metal from a pre-coated steel strip by dipping it in a molten bath of aluminum alloy, which contained 9.3% silicon and 2.8% iron, with the remainder being aluminum and unavoidable impurities. These sheets of metal contained the following on each side: - Pre-coated layer 5 comprising a layer of intermetallic alloy 9 having a majority of Fe2AI3, Fe2AI5 and FexAIySiz with a thickness of approximately 5 μm in contact with steel substrate 3 on which a layer 11 of Al-Si metal alloy with an average thickness of approximately 24 μm is placed. Substrate 3 has the following ingredients, in weight percent: C Mn Si Al Cr Ti BNSP Iron and unavoidable impurities 0.22 1.16 0.26 0.03 0.17 0.035 0.003 0.005 0.001 0.012 Residue Then, the metal alloy layer 11 was removed using a laser beam over a width of about 1.5 mm from the edge 13 of the metal sheets 1 using a laser beam 15 at different inclination angles α, while a layer of intermetallic alloy 9 remained intact. The extraction was performed using a pulsed fiber laser with a power rating of 1000 W, pulse delivery at a frequency of 10 kHz and a focal spot of approximately 1 mm². The pulse duration was approximately 70 ns. For each inclination angle α, the corresponding velocity Vm was measured. These experiments made it possible to obtain the curve of Figure 5, which has been analyzed previously. With other compositions of substrate 3 and in particular substrates 3 having the following composition, similar results have been obtained by the inventors in weight percentages: 0.04% ≤ C ≤ 0.1%, 0.3% ≤ Mn ≤ 2%, Si ≤ 0.3%, Ti ≤ 0.08%, 0.015% ≤ Nb ≤ 0.1%, Al ≤ 0.1%, S ≤ 0.05%, P ≤ 0.1%, Cu, Ni, Cr, Mo, and less than 0.1%, the rest being iron and impurities resulting from production. And also with the above-specified substrates 3 coated with the pre-coated layer 5 and having the above-mentioned composition, similar results have been obtained by the inventors, in which the total thickness of the pre-coated layer is about 35 μm. Similar results have been obtained by inventors with Q-switched lasers. It should be noted that the curve of Figure 5 also shows that the beneficial technical effect obtained from this invention is unattainable when the laser beam 15 is bent so that its perpendicular projection onto the face 10 under processing remains in the region of the face 10 in the processing state, not in the region of the face being processed, corresponding to the negative inclination angle α. The inventors of the present invention, without wishing to be bound by a theory, propose the following explanation for the observed beneficial effects of the inclination of the laser beam. They have observed that the impact of the laser beam 15 on the pre-coated layer 5 leads to an explosion of the pre-coated layer upon contact with the laser beam. This explosion causes the formation of a metal vapor which, in suspension, contains the pre-coated particles vertically above the impact zone of the laser beam 15. When the laser beam 15 is directed perpendicularly to the surface of the face 10, i.e. when α = 0°, it must traverse this cloud of particles of considerable height and part of its energy is dissipated in the cloud before any useful effect. On the contrary, when the laser beam 15 is bent in the manner described above, this cloud of particles is not traversed or at least traverses it less, which makes it more effective.When the laser beam 15 is bent so that its perpendicular projection is on the face 10 being processed and remains in the area of ​​the face 10 being processed and not in the area of ​​the face being processed, corresponding to the negative angle of inclination, it also traverses the cloud of particles of considerable height, so its effect is reduced, as with the beam 15 perpendicular to the surface of the face 10 being processed. In the context of the experiments carried out, the inventors of the present invention have found that when the laser beam 15 is bent with an inclination angle α between 25 and 50 degrees, the removal zone 7 obtained by erosion has a significant surface uniformity regardless of the advancing speed of the laser beam 15. For example, the table below shows the results of experiments conducted by the inventors of the present invention. Feed rate of the slope angle α at which the thickness non-uniformity Δ ≤ 50% of the pre-coated slope angle α at which the thickness non-uniformity Δ ≤ 70% of the pre-coated 10 m / min 22.5° 20° 11 m / min 22.5° 15° 14 m / min 25° 0° 17 m / min 22.5° 0° In this table, Δ for the cross-sectional area provided through the cut-off region 7, perpendicular to the edge 13 of the metal sheet '1' adjacent to the region 7, represents the relative difference between: - the remaining pre-coated thickness in one third of the width of the removal zone 7 from the edge of the removal zone 7 along the width of said zone. In this example, the edge corresponds to the edge 13 of the metal sheet 1, which is indicated by h1 / 3; and -The remaining pre-coated thickness at half the width of the cut-off zone 7, denoted by h1 / 2. Figure 6 shows a schematic view of these parameters. Specifically, Δ is obtained using the following formula: Therefore Δ constitutes the coordinate measurement of the remaining pre-coated thickness in the harvesting zone 7 at the end of the harvesting step. In the past, the pre-coated thickness referred to the subsequent thickness in the pick-up zone 7 measured from the substrate 3 in the direction normal to the face 10 of the metal sheet '1. In the table above Δ ≤ 50% means that the smaller relative difference is equal to 50%. Δ ≤ 70% means that the smaller relative difference is equal to 70%. After the inclined beam erosion, it is observed that h1 / 2>h1 / 3(or equivalently: h1 / 2-h1 / 3>0) i.e. the coating thickness at half width is greater than the thickness obtained in moving from this position. Therefore, Δ>0%. However, tests carried out by the inventors show that, across the width of the harvesting zone 7, a smaller difference Δ equal to 50% in the pre-coated thickness is observed, regardless of the harvesting speed used when the inclination angle α is greater than 25°. In contrast, for smaller inclination angles, the difference Δ is higher. The remaining pre-coated thickness at half the width is usually at least twice as much as at one-third the width as defined above. It should be noted that in practice, the pre-coated thickness h1 / 3 varies very little with the laser beam inclination 15. The improved matching of the remaining pre-coated thickness in the ablation zone 7 essentially results from the reduction of the coating thickness h1 / 2 with the laser beam inclination 15, the thickness h1 / 2 being close to h1 / 3 for increasing the laser beam inclination 15. The range of the inclination angle α between 25 and 50° therefore allows achieving both a significant efficiency of the harvesting method and a very good coordination of the remaining pre-coated thickness in the harvesting zone 7. This very good coordination is beneficial. In fact, such coordination minimizes the amount of aluminum in the welded joint and provides very good corrosion resistance in the areas of the metal sheets 1 adjacent to the welded joint while maximizing the thickness of the intermetallic alloy layer 9 remaining in these areas. As previously explained, the range of the inclination angle α between 20 and 40 degrees allows for better efficiency. Thus, the range of the inclination angle α between 25 and 40 degrees allows for both significant efficiency of the removal method and very good uniformity of the remaining pre-coated thickness in the removal zone 7. Similar results were obtained for the metal sheet '1' where the cut-off region 7 is not adjacent to the edge 13. In this case, the edge of the cut-off region 7 corresponds to one of the two edges of the cut-off region along the width of the cut-off region. The cutting method according to the invention thus makes it possible to obtain metal sheets '1, "1 as previously described. In the cutting zone 7, the relative difference Δ, along the width of the cutting zone 7 of the thickness of the part of the pre-coated layer 5 remaining in the cutting zone 7, is taken to be the ratio of the difference between the pre-coated thickness at half the width h1 / 2 and the pre-coated thickness h1 / 3 at one third of the width from the edge of the cutting zone 7 to the pre-coated thickness at half the width h1 / 2, which is strictly greater than 0% and smaller than 50%. This invention also relates to a method of producing a welded chamber, which comprises the following steps: -Provide at least two metal sheets '1' produced according to the above method; -Impact welding of these two metal sheets '1, the welding joint is carried out on the edge 13 which includes the removal zone 7 and in which the pre-coated layer 5 is at least partially removed and in particular free from the metal alloy layer 11. Depending on the structure of the metal substrate and the desired mechanical properties of the welded joint, the welding method is preferably a laser welding method with or without filler wire, or an electric arc welding method. The metal sheets '1' have the same thickness. Or they have different thicknesses. At the end of this method, a welded housing is obtained, consisting of two pre-coated metal sheets joined together by impact welding. According to one example, during the preparation step at least two metal sheets '1 are provided, obtained from at least one metal sheet '1, as previously described by cutting in the cutting zone '7 so that a metal sheet '1 is obtained which includes at its periphery a zone 7 free of at least a part of the pre-coated layer 5. The invention also relates to a method of producing a sequential part, comprising the following steps: -Preparation of the welded chamber obtained using the above-mentioned method; -Heating the chamber so as to impose a relatively or fully austenitic structure on the substrates of the metal sheets constituting the aforementioned chamber; -Hot forming said chamber in order to obtain the part; -Cooling the part at a rate that can give it the desired mechanical properties. Ideally, during the cooling stage, the cooling rate is greater than the critical martensitic quenching rate. Therefore, the part is produced as a structural or safety part for a motor vehicle. According to an example not shown, in the preparation step, two pre-coated metal sheets 1 (as described above) are prepared and placed next to each other. A certain gap is maintained between the two metal sheets. Then, in the removal step, at least a portion of the pre-coated layer 5 is removed simultaneously from both metal sheets 1 to simultaneously create a removal area 7 on each of the metal sheets 1. The laser beam 15 is adjusted so that the two metal sheets 1 overlap in the removal step.

Claims

CLAIMS 1.- A method for preparing a pre-coated metal sheet (1’; 1”) for welding thereof to another pre-coated metal sheet, comprising the following successive steps: - providing a pre-coated metal sheet (1) comprising a metal substrate (3) provided, on at least one of its faces (10), with a pre-coating layer (5), then - removing, on at least one face (10) of said pre-coated metal sheet (1), at least part of said pre-coating layer (5) so as to form a removal zone (7), said removal being done by the impact of a laser beam (15) on said pre-coating layer (5), the removal step comprising, over the course of the removal, the relative displacement of said laser beam (15) with respect to the metal sheet (1) in a direction of advance (A), characterized in that during the removal, the laser beam (15) is inclined relative to the face (10) of the metal sheet (1) such that the orthogonal projection of the laser beam (15) on said face (10) of the metal sheet (1) is located in the zone of the metal sheet (1) in which the removal has already been done, and in that it forms an angle of inclination (α) comprised between 12° and 50° with the direction normal (N) to the face (10) of the metal sheet (1).2.- The method according to claim 1, wherein the pre-coating layer (5) is a layer of aluminum, an aluminum-based layer or a layer of aluminum alloy.3.- The method according to any one of claims 1 or 2, wherein the pre-coating layer (5) is a layer of aluminum alloy further comprising silicon.4.- The method according to any one of claims 1 to 3, wherein the angle of inclination (α) of the laser beam (15) is comprised between 15° and 45°.5.- The method according to any one of claims 1 to 4, wherein the angle of inclination (α) of the laser beam (15) is comprised between 20° and 40°.6.- The method according to any one of claims 1 to 5, wherein the angle of inclination (α) of the laser beam (15) is comprised between 25° and 40°.7.- The method according to any one of claims 1 to 6, wherein the angle of inclination (α) of the laser beam (15) is comprised between 25° and 35°.8.- The method according to any one of claims 1 to 7, wherein the laser beam (15) is a pulsed laser beam.9.- The method according to any one of claims 1 to 8, wherein the pre-coating layer (5) comprises a layer of intermetallic alloy (9) topped by a layer of metal alloy (11).10.- The method according to claim 9, wherein the removal zone (7) is completely free of the layer of metal alloy (11).11.- The method according to any one of claims 1 to 10, wherein the removal zone (7) is formed on a lower face (10) of the metal sheet (1).12.- The method according to any one of claims 1 to 11, wherein a removal zone (7) is formed simultaneously on a lower face (10) and on an upper face (10) of the metal sheet (1).13.- The method according to any one of the preceding claims, wherein the removal is done without suction.14.- The method according to any one of the preceding claims, wherein the removal is done without gas blowing.15.- The method according to any one of the preceding claims, wherein, during the removal step, the distance between the output lens of the laser head (17) and the metal sheet (1) is greater than or equal to 150 mm, and in particular comprised between 150 mm and 250 mm.16.- The method according to any one of claims 1 to 15, wherein the metal substrate (3) is made up of steel.17.- The method according to claim 16, wherein the steel of the substrate (3) comprises, by weight: 0.10% ≤ C ≤ 0.5% 0.5 ≤ Mn ≤ 3% 0.1 ≤ Si ≤ 1% 0.01 ≤ Si ≤ 1% Ti ≤ 0.2% Al ≤ 0.1% S ≤ 0.05% P ≤ 0.1% B ≤ 0.010% the rest being iron and impurities from smelting.18.- The method according to claim 16, wherein the steel of the substrate (3) comprises, by weight: 0.15% ≤ C ≤ 0.25% 0.8 ≤ Mn ≤ 1.8% 0.1% ≤ Si ≤ 0.35% 0.01 ≤ Cr ≤ 0.5% Ti ≤ 0.1% Al ≤ 0.1% S ≤ 0.05% P ≤ 0.1% B ≤ 0.005% the rest being being iron and impurities from smelting .19.- The method according to claim 16, wherein the steel of the substrate (3) comprises, by weight: 0.040% ≤ C ≤ 0.100% 0.80 ≤ Mn ≤ 2.00% Si ≤ 0.30% S ≤ 0.005% P ≤ 0.030% 0.010% ≤ Al ≤ 0.070% 0.015% ≤ Nb ≤ 0.100% Ti ≤ 0.080% N ≤ 0.009% Cu ≤ 0.100% Ni ≤ 0.100% Cr ≤ 0.100% Mo ≤ 0.100% Ca ≤ 0.006%, the rest being iron and impurities from smelting.20.- The method according to any one of claims 16 to 19, wherein the microstructure of said steel is ferrito-pearlitic.21.- The method according to any one of claims 1 to 20, wherein, during the provision step, two pre-coated metal sheets (1) are supplied and they are arranged side by side, leaving a predetermined gap between the two pre-coated metal sheets (1), then, during the removal step, at least part of the pre-coating layer (5) is simultaneously removed from each of the two metal sheets (1) in order to simultaneously form a removal zone (7) on each of said metal sheets (1), the laser beam (15) being arranged overlapping the two metal sheets (1) during the removal step.22.- The method according to any one of claims 1 to 21, wherein the removal zone (7) is located at the periphery (6) of the metal sheet (1).23.- The method according to any one of claims 1 to 21, wherein the removal zone (7) is not completely adjacent to the edge (13) of the metal sheet (1).24.- The method according to claim 23, further comprising, after the removal step to form the removal zone (7), cutting of the metal sheet (1”) along a plane (20) so as to form a metal sheet (1’) comprising, at its periphery, a zone (7) free of at least part of the pre-coating layer (5).25.- A metal sheet (1’, 1”) comprising a metal substrate (3) bearing, on at least one of its faces (10), a pre-coating layer (5), the metal sheet (1’) comprising, on said at least one face (10), a removal zone (7) where the pre-coating layer (5) has been removed over part of its thickness, characterized in that, in the removal zone (7), the relative variation Δ, considered along the width of the removal zone (7), of the thickness of the part of the pre-coating layer (5) remaining in the removal zone (7), defined by the ratio of the difference between the pre-coating thickness at half-width (h1 / 2) and the pre-coating thickness (h1 / 3) at one third of the width considered from the edge of the removal zone (7) to the thickness of the pre-coating at half-width (h1 / 2) is strictly greater than 0% and less than or equal to 50%.26.- The metal sheet (1’, 1”) according to claim 25, wherein the pre-coating layer (5) comprises a layer of intermetallic alloy (9) topped by a layer of metal alloy (11).27.- The metal sheet (1’, 1”) according to claim 26, wherein the removal zone (7) is completely free of the layer of metal alloy (11).28.- The metal sheet (1’) according to any one of claims 25 to 27, wherein the removal zone (7) is located at the periphery (6) of the metal sheet (1’).29.- The metal sheet (1”) according to any one of claims 25 to 27, wherein the removal zone (7) is not completely adjacent to the edge (13) of the metal sheet (1).30.- A method for manufacturing a welded blank, comprising the following successive steps: - providing at least two metal sheets (1’) according to any one of claims 25 to 28 or obtained from at least one metal sheet (1”) according to claim 29 by cutting in the removal zone (7) so as to obtain a metal sheet (1’) comprising, at its periphery, a zone (7) free of at least part of the pre-coating layer (5) or manufactured according to the method according to any one of claims 1 to 22 and 24, then - butt welding these two metal sheets (1’), the welded connection being done on the edge (13) comprising the removal zone (7).31.- The method according to claim 30, wherein the two butt welded metal sheets (1’) have different thicknesses.32.- A method for manufacturing a hot pressed part comprising the following successive steps: - providing a welded blank obtained using the method according to claim 30 or according to claim 31, then - heating said welded blank so as to impart a partially or fully austenitic structure to the substrates (3) of the metal sheets (1) making up said blank, then - hot press-forming said blank to obtain a hot pressed part; - cooling the part with a speed able to give it targeted mechanical properties.33.- The method according to claim 32, wherein the cooling speed is greater than the critical martensitic quenching speed of the steel of the substrate (3) of said at least two metal sheets (1’) or the steel of the substrate (3) of said at least one metal sheet (1”). ABSTRACT Method for producing a pre-coated metal sheet This method for preparing a pre-coated metal sheet for welding thereof to another pre-coated metal sheet, comprises the following successive steps: - providing a pre-coated metal sheet (1) comprising a metal substrate (3) provided, on at least one of its faces (10), with a pre-coating layer (5), then - removing, on at least one face (10) of said pre-coated metal sheet (1), at least part of said pre-coating layer (5) so as to form a removal zone (7), said removal being done by the impact of a laser beam (15) on said pre-coating layer (5), the removal step comprising, over the course of the removal, the relative displacement of said laser beam (15) with respect to the metal sheet (1) in a direction of advance (A). During the removal, the laser beam (15) is inclined relative to the face (10) of the metal sheet (1) such that the orthogonal projection of the laser beam (15) on said face (10) of the metal sheet (1) is located in the zone of the metal sheet (1) in which the removal has already been done, and it forms an angle of inclination (α) of between 12° and 50° with the direction normal (N) to the face of the metal sheet (1). Figure 4

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