Pre-coated steel sheet containing an additional coating to increase the mechanical strength of the weld metal area of ​​a welded steel piece prepared from that pre-coated sheet.

Abstract

Abstract\n\n\tThe invention relates mainly to a pre-coated steel sheet in which at least a portion of the periphery (7) of at least one of the (6a;6b) facing surfaces (6a;6b) of said pre-coated steel sheet (1,1') is provided with an additional coating (8) selected to increase the vapor pressure between the pre-coating (2) and said additional coating (8) during a laser welding process to a critical pressure, during which the pre-coating (2) is blown out of the weld (14).\n\tPreferably, the vaporization temperature of the additional coating (8) is higher than the vaporization temperature of the pre-coating (2) and the additional coating comprises Gamza elements such as carbon and / or nickel. The invention also relates to a steel piece obtained by laser welding, preferably butt-welding laser welding of at least one first and second steel piece (1,1') as described above.

Description

Pre-coated steel sheet containing an additional coating to increase the mechanical strength of the weld metal area of ​​a welded steel piece prepared from that pre-coated sheet. The present invention relates primarily to a pre-coated steel sheet comprising an additional coating for increasing the mechanical strength of the weld metal area of ​​a welded steel part prepared from that pre-coated sheet. The present invention also relates to a method for manufacturing said pre-coated steel sheet. The present invention further relates to a steel part obtained by laser welding at least one first and second pre-coated steel sheet, comprising an additional coating to improve the mechanical strength of the weld metal area of ​​the welded steel part. Finally, the present invention relates to a method for producing that steel part. The prior art discloses methods for manufacturing welded steel parts from steel sheets of different compositions and / or thicknesses which are continuously joined together by butt welding. In a first known manufacturing method, these welded sheets are cold rolled. In a second known manufacturing method, these welded sheets are heated to a temperature at which austenitization of the steel is possible and then hot rolled and then cooled rapidly in a die. The present invention relates to this second manufacturing method. The steel composition can be selected in a way that allows both subsequent heating and forming operations and also gives the welded steel part high mechanical strength, high impact strength and desirable corrosion resistance. In recent years, boron-containing hot forging steels (PHS) have attracted attention from automotive manufacturers due to their excellent tensile strength (1500 to 2000 MPa) in the hot forging condition. Thanks to the high specific strength of hot forging steels and their high flexibility in part design, such steels are widely used in automotive crash-prone components such as B-pillars, A-pillars and door rings. Typically, hot forging steels have a ferritic-pearlitic structure in the as-received condition and are then transformed into a fully martensitic structure during high-temperature austenitization and subsequently cooled to ambient temperature during hot forging with water-cooled dies at a critical cooling rate of about 30 °C / s. Hot forging steels are increasingly used by the automotive industry in various forms of corrosion-resistant alloy coatings, such as Al-Si, Zn and Zn-Ni. Among them, Al-Si coating has better corrosion resistance and more favorable high temperature oxidation resistance. A known method for manufacturing welded steel parts involves producing at least two steel sheets as described in EP 971044, butt-welding the two sheets to obtain a welded sheet, optionally cutting this welded sheet, and then heating the welded sheet before performing a hot forming operation to create the necessary shape for the intended use of the steel part. A well-known welding technology is laser beam welding. This technology has advantages in terms of flexibility, quality and efficiency compared to other welding technologies such as strip welding or arc welding. However, during the welding process, the aluminum-base primer containing an intermetallic alloy layer in contact with the steel substrate and overlaid with a metal alloy layer, is diluted with the steel substrate within the weld metal zone. This is the zone that is in a molten state during the welding process and hardens after the welding process to form the bond between the two sheets. In the range of pre-plated aluminum contents, aluminum, which is an alpha-gene element in solid solution in the matrix, prevents the transformation to austenite that occurs during the pre-molding stage. As a result, it is no longer possible to achieve martensite during cooling after hot forming and the welded joint will contain ferrite. The weld metal area will then exhibit a hardness and mechanical strength that is lower than that of the two adjacent sheets, which can lead to critical failure of the final part in the weld area. A parallel can be drawn between the above-described detrimental interaction between the aluminum-base coating of hot-formed sheets and the laser welding operation, with the problems caused by the laser welding of third-generation zinc-based pre-coated cold-formed steels. These third-generation steels, which have very high ductility and strength, are used to manufacture complex structural parts by cold forming and are subject to liquid metal embrittlement during laser welding. This is due to the interaction between the pre-coated molten zinc and the retained austenite of the substrate. Several solutions have been developed to prevent the aforementioned harmful interaction. For example, document EP2007545 describes a solution that consists in removing the surface layer of the metal alloy on the peripheral surface of the sheets that are to be subjected to the welding operation, leaving only the intermetallic alloy layer. The removal can be carried out by brushing or by laser beam. The intermetallic alloy layer is maintained during the heat treatment that occurs before the forming operation to ensure corrosion resistance and to prevent decarburization and oxidation phenomena. The present invention aims to provide a novel solution for the base metal / precoat interaction during laser welding. The aim is to provide a precoat steel sheet that is easy to produce and improves the mechanical strength of the weld metal area of ​​a welded steel part from which the precoat sheet is prepared. To this end, a first object of the present invention includes a method for manufacturing a pre-coated steel sheet comprising at least the step of applying an additional coating to at least a portion around at least one of the facing surfaces of said pre-coated sheet, said additional coating being selected to increase the vapor pressure between the pre-coated and said additional coating during a laser welding process to a critical pressure at which the pre-coated pressure is ejected from the weld. The method according to the invention may also have the optional features listed below, considered individually or in combination: - The evaporation temperature of the additional coating is greater than the evaporation temperature of the precoat. - Additional coverage includes Gamza elements. - Additional coating includes carbon and / or nickel. Finally, the invention also includes a method for manufacturing a steel part comprising at least the step of laser welding at least one first and second pre-coated steel sheet, wherein at least a portion around at least one of the opposing surfaces of said first and second pre-coated steel sheets has previously been coated with an additional coating, said additional coating being selected to increase the vapor pressure between said pre-coating and said additional coating during said laser welding process to a pressure at which said pre-coating is ejected from said weld. The method according to the invention may also have the optional features listed below, which are contemplated individually or in combination: - Laser welding is a butt laser welding. - The application of additional coating to at least a portion of the circumference of one of the opposing surfaces of the first and second pre-coated steel sheets and the welding of those first and second pre-coated steel sheets are performed simultaneously. - The evaporation temperature of the additional coating is greater than the evaporation temperature of the precoat. - Additional coverage includes Gamza elements. - Additional coating includes carbon and / or nickel. Other features and advantages of the invention will be described in more detail in the next section. The invention will be better understood by considering the following description. This description is given for the purpose of illustration and explanation only and is not to be considered in any way limiting, the description being made with reference to the drawings in which: - Figure 1, which is a perspective and schematic view of a pre-coated steel sheet according to a construction of the invention, - Figure 2 which is a perspective and schematic view of a butt laser welding operation of the method according to an embodiment of the invention, - Figure 3 is a photograph of a laser butt welding operation of two pre-coated steel sheets with a zinc-based pre-coating and no additional coating, - Figure 4 is a photograph of a laser butt welding operation of two pre-coated steel sheets with a zinc-base pre-coating, in which the periphery of the steel sheets is coated with an additional coating according to the invention, - Figure 5 is a graph showing the total percentage of ferrite area in the welded area as a function of the thickness of the additional carbon-containing coating, including in the situation where there is no additional coating, - Figure 6 is a graph showing the weight percentage of aluminum in the welded area as a function of the thickness of the additional coating containing carbon, including in the situation where there is no additional coating. - Figure 7 is a graph showing the weight percent carbon in the welded area as a function of the thickness of the additional coating containing carbon, including in the situation where there is no additional coating. - Figure 8 shows a comparative profile of the ultimate tensile strength of the welded zone resulting from a laser butt welding operation of two pre-coated steel sheets in which the periphery of said steel sheets is coated with an additional coating comprising carbon and with an additional coating comprising nickel as a function of the thickness of the respective additional coating, including in the situation in which there is no additional coating. - Figure 9 is a photograph of a laser butt welding operation on two pre-coated steel sheets with an aluminum-base pre-coating and no additional coating. - Figure 10 is a photograph of a laser butt welding operation for two pre-coated steel sheets with an aluminum-based pre-coating, in which the periphery of said steel sheets is coated with an additional coating according to the invention. The pre-coated steel sheet of the invention is coated with a metallic coating which is generally designed to protect the steel substrate against corrosion. The metallic coating in the pre-coating can be, for example, aluminum-based, which is commonly used for hot-rolled steels. The metallic coating in the pre-coating can be, for example, zinc-based, which is commonly used for cold-rolled steels, where aluminum-based means that the coating contains at least 50% by weight of aluminum. Zinc-based means that the coating contains at least 50% by weight of zinc. The pre-coated steel sheet of the invention is coated and provided by immersion in a molten aluminum bath according to a method known as "dip coating" as described in EP971044. The term sheet is used in its broad sense and means any strip or object obtained by cutting a strip, coil or sheet. The aluminum bath which is the object of the dipping operation may also contain 8 to 11% silicon and 2 to 4% iron. Thus, the pre-coating in the pre-coated steel sheet is a steel alloy coating containing between 8 and 11% silicon and between 2 and 4% iron by weight. The steel forming the steel substrate of the sheets has the following composition, expressed in weight percentage: 0.10%≦C≦0.5% 0.5%≦Mn≦3% 0.1%≦Si≦1% 0.01%≦Cr≦1% Ti≦0.2% Al≦0.1% S≦0.05% P≦0.1% 0.0002%≦B≦0.010%, Residual iron and impurities from the process. The sheets to be welded together can be identical or of different compositions. Referring to Figure 1, the pre-coated steel sheet 1 of the invention comprises a steel alloy coating 2 which is in contact with a steel substrate 3. The steel alloy coating 2 has an intermetallic alloy layer 4 of the AlSiFe type which is in contact with the surface of the steel substrate 3. This intermetallic alloy layer 4 is the result of the reaction between the steel substrate 3 and the aluminum bath. This intermetallic alloy layer 4 is located under a metal alloy layer 5 which forms a surface layer of the pre-coating 2. The pre-coating 2 is present on two opposite surfaces 6a, 6b of the sheet 4. According to the invention, at least a portion of the periphery 7 of the upper surface 6a of the pre-coated steel sheet 1 is coated with an additional coating 8. According to Figure 1, which shows an embodiment of the invention, the additional coating 8 extends along a free edge 9 of the sheet 1. The features of the additional coating 8 will be described in more detail below. According to the invention, the additional coating 8 can be applied to the upper surface 6a or to both surfaces 6a, 6b by applying said additional coating 8 with an application tool, for example, spin coating or paint spraying or using a paint brush, which application methods are known to the person skilled in the art. The additional coating 8 is applied either in a separate step before the laser welding operation or in the same step of the process and simultaneously with the laser welding operation, according to Figure 2. Referring to Figure 2, a first sheet 1 and a second sheet 1' are placed edge-to-edge, known as a butt-welding configuration, in accordance with conventional laser welding methods, by contacting the free edges or by positioning those edges 9,9' at a very small distance. Figure 2 shows a part of a laser welding machine 10 comprising a welding head 11 comprising at least one implement 12 for ensuring the application of an additional coating 8,8' around the circumference of each sheet 1,1' and further comprising a laser beam 13. During the laser welding operation, a relative displacement takes place between the laser welding machine 10 and the sheets to be welded relative to the relative displacement of the welding machine 10 in the welding direction, as indicated by the arrow F. The additional coatings 8,8' are applied to the respective circumference of the pre-coated steel sheet 1,1' via the implement 12 located upstream of the laser beam 13. At the same time, the laser beam 13 carries out welding along the joint between the steel sheets 1,1' whose circumference has previously been coated with the additional coatings 8,8' and then a weld metal zone 14 is formed connecting the two steel sheets 1,1' to each other. Alternatively, the laser beam can be combined with a welding wire, not shown in Figure 2. The resulting steel part 100 essentially consists of two plates, called base metal 101, 101', which are connected by a weld metal region 14. The welding process is accomplished under conditions and with equipment that are well known to those skilled in the art. The additional coating 8 is initially selected based on its ability to increase the vapor pressure between the pre-coating 2 and the additional coating 8 during laser welding to a critical pressure at which the pre-coating 2 is ejected from the weld. When the pre-coating 2 is of the AlSiFe type, its ejection from the weld zone prevents or at least limits the aluminum content in the weld metal zone, which will be explained in more detail below. To provide this ejection, the additional coating 8 must remain in a state that allows a sufficient increase in vapor pressure between the pre-coating 2 and the additional coating 8 during laser welding. For this purpose, the evaporation temperature of the additional coating 8 is preferably higher than the evaporation temperature of the pre-coating 2 so that the evaporation of the pre-coating 2 due to the increase in temperature in the welding zone between the pre-coating 2 and the additional coating 8 can lead to an increase in vapor pressure to a critical pressure at which the additional coating 8 is ejected along the section of the pre-coating 2. Considering that the evaporation temperature of the pre-coating 2 of the AlSiFe type corresponds to the evaporation temperature of aluminum of about 2520 ° C, it is preferable to provide an additional coating 8 with an evaporation temperature of at least more than 2720 ° C. The additional coating 8 can also be preferably selected to bring the Gamza elements into the weld zone. For example, the additional coating 8 advantageously comprises carbon and / or nickel. Since carbon has a vaporization temperature of about 3500°C and nickel has a vaporization temperature of about 2913°C, both of these elements can be options for sufficiently increasing the vapor pressure between the pre-coating 2 and the additional coating 8 as described above. When the additional coating 8 is carbon-based, PELCO® conductive graphite isopropanol can be advantageously used. Referring to Figures 3, 4, and 9, 10, it can be seen that laser welding of a pre-coated steel sheet of the invention including an additional coating involves the ejection of material (aluminum) in the form of sparks (Figures 4 and 10) compared to laser welding of a pre-coated steel sheet without an additional coating (Figures 3 and 9). According to the invention, the additional coating 8 can be applied along the circumference on one surface of the pre-coated steel sheet 1 or on both opposite surfaces. When the additional coating 8 is applied to a surface of the pre-coated steel sheet 1 and when the additional coating 8 comprises pure nickel, the thickness of that additional coating 8 can be between 15 and 40 µm, preferably between 20 and 30 µm, most preferably about 25 µm. When the additional coating 8 is applied to a surface of the pre-coated steel sheet 1 and when the additional coating 8 comprises carbon (based on PELCO® conductive graphite isopropanol), the thickness of that additional coating 8 can be between 30 and 85 µm, preferably between 35 and 50 µm, most preferably about 40 µm. The width of the additional coating 8 is adjusted so that it covers at least the welding area. For this purpose, the width of the additional coating 8 can be between 2 and 5 mm. Example 1 In this example, the additional coating 8 is applied only to one surface (top surface) of each pre-coated steel sheet 1,1' that is to be welded together. Each 1.1' pre-coated steel sheet is Al-Si coated hot-dip galvanised steel (PHS) (USIBOR® 1500). The chemical composition of the hot-rolled forging steel used can be seen in Table 1 below: Table 1: Chemical composition of steel C Mn PS Si Cu Ni Mo Cr 0.23 1.22 0.013 0.001 0.27 0.02 0.037 0.02 0.20 Co V Al Sn Ti NB Fe 0.008 0.008 0.039 0.02 0.037 0.0054 0.0032 Other Exhibit 2 Al-Si contains 90% aluminum, 8% silicon and 2% iron. The thickness shown 2 is 15 micrometers. Referring to Figures 5, 6 and 7, the additional coating 8 is an isopropanol-based graphite-resistant dry film lubricant available under the trade name PELCO® Isopropanol-Based Conductive Graphite. In this example, the butt welding operation was first simulated using a single-grain-on-plate configuration. In this configuration, instead of using two separate pre-coated sheets that are placed side by side and welded (the butt welding configuration), the experiment was performed using a single sheet on which the laser welding operation was simulated by applying a laser beam to the sheet surface, with or without the previous application of an additional coating. Since this method uses the same type of laser and materials used in the butt welding process, the grain-on-plate configuration is a convenient way to simulate the physical phenomena associated with the energy effect created by the laser beam and the interactions between the substrate, pre-coated and additional coating. Because the method does not involve placing two sheets together and welding them, it is easier to perform than butt welding, thus facilitating testing. The pre-coated steel sheets were welded in a grain-on-plate configuration using an IPG Photonics Ytterbium Fiber Laser System (Model: YLS-6000-S2) with a power and speed of 4 kW and 4 m / min, respectively. A detailed description of the welding system can be seen in Table 2 below. Table 2: Laser welding system Laser Type Laser Source Laser Head Fiber Beam Type Model Type Focal Length Spot Size Fiber Core Ytterbium Laser System IPG Photonics YLS-6000-S2 Laser Mechanism 200 mm 0.6mm Diameter 0.3mm Diameter After welding, the welded sheets are austenitized in a furnace at 930°C for 5 minutes and then quenched between flat matrices. The ferrite content (reference 15 in Figure 5), aluminum content (reference 16 in Figure 6), and carbon content (reference 17 in Figure 7) in the melting zone were measured by image analysis using Clemex Vision Lite software as a function of the thickness of the additional carbon coating 8. According to Figure 5, it can be seen that the percentage of ferrite area in the weld metal area is significantly reduced for a carbon coating thickness of 20 µm, and the largest reduction of 30% is observed for a carbon coating thickness of 40 µm compared to the percentage of ferrite area in the weld metal area when the welded sheets are not coated with additional coating. This reduction in the percentage of ferrite zone can be explained by the release of aluminum from the Al-Si pre-coating 2 during laser welding. This release is confirmed by Figure 6, where it can be seen that the weight percent of aluminum 16 in the weld metal zone is significantly reduced for a carbon coating thickness of 20 µm, with the largest reduction of 30% for a carbon coating thickness of 40 µm compared to the percentage of aluminum in the weld metal zone when the welded sheets are not coated with an additional coating. At the same time, as can be seen in Figure 7, the weight percentage of carbon in the weld metal area increases with increasing carbon coating thickness. Figure 8 shows a comparative profile of the ultimate tensile strength of the welded zone resulting from a laser welding operation in the butt welding configuration of two inventive pre-coated steel sheets, each coated with an additional coating of carbon (Ref. 19) and nickel (Ref. 18), respectively, as a function of the additional coating thickness. Reference 20 shows the ultimate tensile strength of 1543 MPa of the steel substrate. For an additional coating containing nickel (Ref. 19), the ultimate tensile strength reaches a maximum ultimate tensile strength of 1539 MPa for an additional coating thickness of 25 µm, then the fracture is transmitted from the weld metal zone to the base metal. In order to avoid a systemic fracture in the weld metal zone and with reference to the shape of the curve in Ref. 19, the thickness of the nickel coating can be between 15 and 40 µm, preferably between 20 and 30 µm. For an additional coating consisting of carbon (ref. 18), the ultimate tensile strength reaches a maximum ultimate tensile strength of 1555 MPa for an additional coating thickness of 40 µm, then the fracture is transmitted from the weld metal zone to the base metal. To avoid a systemic fracture in the weld metal zone and with reference to the shape of the curve in ref. 18, the thickness of the carbon coating can be between 30 and 85 µm, preferably between 35 and 50 µm. Example 2 In this example, the primer is a zinc-based primer typically used in conjunction with cold-rolled forming steels. The test was performed using a butt welding configuration. The additional coating used is a nickel-based coating. Figure 9 shows a photograph of a butt welding operation performed without additional coating. Significantly, no primer rejection occurs. On the other hand, in the case of sheets with additional coating on the edges to be welded, it can be seen in Figure 10 that the primer rejection occurs in the form of sparks. In conclusion, pre-coated steel sheets were successfully joined by laser butt welding by introducing an additional nickel or carbon coating that was deposited on at least one surface of a portion of the periphery of each pre-coated steel sheet before welding. The aluminum content in the weld metal zone was reduced below a critical value required for the formation of a soft delta-ferrite phase. Therefore, the formation of the delta-ferrite phase in the weld assembly was inhibited / eliminated. The microstructure of the weld metal zone was transformed from a ferritic-martensitic dual phase structure to a fully martensitic structure, which showed superior mechanical properties (both microhardness and tensile properties) compared to the unwelded base metal in the hot-formed condition. Similar ultimate tensile strength was achieved as compared to the unwelded base metal. The fracture path was transferred from the weld metal zone to the base metal. The ductility and strength of the welded joint after hot-formed pre-coated steel pieces were improved to the level of the unwelded and unwelded base hot-formed steels.

Claims

Claims 1. A method for manufacturing a pre-coated steel sheet comprising a steel substrate (3) and a metal pre-coating (2) in contact with the surface of said steel substrate (3), said method comprising at least the step of applying an additional coating (8,8') to said metal alloy coating (2) by spin coating, paint spraying or using a paint brush, at least on a portion of the periphery (7) of at least one (6a;6b) of said facing surfaces (6a,6b) of said pre-coated sheet (1,1'), said additional coating (8,8') having a higher evaporation temperature than said metal alloy coating (2) and being selected to increase the vapor pressure between said pre-coating (2) and said additional coating (8,8') during a butt welding process to a critical pressure at which the pressure of said pre-coating (2) is ejected from the weld (14), wherein: - said additional coating comprises nickel and the additional coating thickness is at least 15 μm or - said additional coating comprises carbon and the thickness is The additional coating is at least 30 micrometers.

2. The method according to the preceding claim, wherein the additional coating comprises Gamza elements.

3. A method for manufacturing a steel part comprising at least the step of laser butt welding at least first (1) and second (1') pre-coated steel sheets (1'), each comprising a steel substrate (3) and a metal pre-coating (2) in contact with the surface of said steel substrate (3), wherein at least a portion of the periphery (7) of at least one of the (6a;6b) opposing surfaces (6a;6b) of said first (1) and second (1') pre-coated steel sheets (1') has been previously coated with an additional coating (8,8') applied to said metal alloy coating (2) by spin coating, or by spraying with paint or by using a paint brush, said additional coating (8,8') having a higher evaporation temperature than said metal alloy coating (2) and selected to increase the vapor pressure between said pre-coating (2) and said additional coating (8,8') during the butt welding process to a critical pressure at which the pressure of said pre-coating (2) from the weld (14) is thrown out, where: - the additional coating comprises nickel and the additional coating thickness is at least 15 μm or - the additional coating comprises carbon and the additional coating thickness is at least 30 μm.

4. A method according to the preceding claim, wherein the application of the additional coating (8,8') to at least a portion of the periphery (7) of one of the (6a) opposing surfaces (6a,6b) of said first (1) and second (1') pre-coated steel sheets and the laser butt welding of said first (1) and second (1') pre-coated steel sheets are carried out simultaneously.

5. A method according to claim 3 or 4, wherein the additional coating (8,8') comprises Gammaza elements.

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