METHOD FOR PRODUCING COATED AND PRESS-FORMED STEEL PARTS AT A HIGH PRODUCTIVITY RATE

Abstract

The present invention relates to a pre-coated sheet or target comprising a heat-treatable steel substrate, overlaid on at least a portion of at least one of its main faces by a pre-coating having at least one layer of aluminum or aluminum alloy overlaid, on at least a portion of the aforementioned pre-coating, by a polymerized layer having a thickness between 2 and 30 µm composed of a polymer that does not contain silicone and whose nitrogen content is greater than 1% by weight expressed in relation to the aforementioned layer, wherein the aforementioned polymerized layer contains carbon pigments in an amount between 3 and 30% by weight expressed in relation to said layer.

Description

The present invention relates to a manufacturing method for pre-coated steel sheet parts that are heated, stamped, and then shaped during cooling achieved by holding them in a press tool. These parts are intended for use, among other things, as structural elements in motor vehicles for anti-intrusion or energy absorption functions. Such parts can also be used, for example, for the manufacture of tools or machinery parts for agriculture. In these types of applications, the goal is to produce steel parts that combine high mechanical strength, good corrosion resistance, and good weldability. It is also possible to manufacture these parts by hot stamping using high-productivity methods. These requirements are particularly relevant in the automotive industry, where the objective is to significantly reduce vehicle weight. Intrusion-resistant parts or parts that play a role in vehicle safety, such as bumpers, door reinforcements, or B-pillars, for example, require the qualities mentioned above. These qualities can be achieved particularly with steel parts that have a martensitic or bainitic-martensitic microstructure. The manufacture of parts of this type is known particularly from publications FR2780984 and FR2807447, according to which a blank cut from a heat-treatable steel sheet, previously coated with a metal or metal alloy, is heated in a furnace and then hot-formed. The pre-coating can be aluminum or an aluminum alloy, zinc, or a zinc alloy. During heating in the furnace, these pre-coating alloys bond with the steel substrate to form a compound that provides surface protection against decarburization and scale formation. This compound is suitable for hot forming. Holding the part in the tool after forming allows for rapid cooling, leading to the formation of microstructures in the steel substrate, which are associated with good mechanical properties of strength and hardness.This method is known as “press forming”. In this type of method, the blanks are generally heated in continuous furnaces, where they are moved forward on rollers. This phase comprises a heating stage followed by a holding temperature, typically around 900–950°C. The holding temperature and holding time are a function of, among other things, the thickness of the blanks and the type of pre-coating. For productivity reasons, it is desirable to use methods that shorten the heating stage in the furnace as much as possible. In this regard, publication EP2312005 describes a method in which a steel coil is pre-coated with aluminum and then rolled at 600–750°C for a period ranging from 1 hour to 200 hours.Diffusion of the steel from the substrate to the pre-coating occurs to the extent that a pre-alloyed product is obtained. After cutting, these pre-alloyed blanks can be heated more quickly, particularly due to the emissivity modification caused by the pre-alloying treatment. However, this method requires time-consuming pre-annealing of the coil. Document EP2463395 also proposes accelerating the heating phase kinetics by locally reducing the reflectivity of a target through various methods: pre-application of black paint, modification of surface roughness by shot blasting, rolling, laser etching, or etching by immersion in an acid solution. This document also describes examples in which pigments in the aqueous phase or solvent-based polyester / melamine black paint have been deposited onto pre-galvanized coatings. Considering the conventional mixing ratio of 90–92% polyester and 8–10% melamine (CisHsoNeOe) in the paint layer after drying, and a maximum pigment concentration by volume of 30%, the nitrogen content in the paint layer used in these tests is 1.7–2.4% after drying. However, this method completely omits certain essential problems related to the subsequent use of these parts.After hot stamping, the parts may be suitable for cataphoresis painting, weldable, and corrosion-resistant. However, as will be shown below, applying a conventional black paint that is resistant to high temperatures before hot stamping does not make it possible to obtain these properties. Attempts have been made to identify a method that simultaneously increases the productivity of the hot stamping process and controls the process in such a way that the hot stamped and press-formed parts obtained are compatible with conventional industrial production conditions; that is, they do not require, for example, modification of the existing settings of spot welding machines for the assembly of these parts. This method must also be compatible with the manufacture of welded targets pre-coated with an aluminum coating that requires prior ablation of a portion of the pre-coating on the periphery of the target, as described in EP2007545. Attempts have also been made to identify a method that is relatively insensitive to certain potential variations in manufacturing conditions. Specifically, attempts have been made to devise a method whose results are relatively insensitive to the preparation conditions of the pre-coated target. Furthermore, a method is being sought that will allow for excellent resistance to delayed cracking. It is known that press forming makes it possible to obtain parts with extremely high mechanical strength, whose microstructure can be susceptible to cracking due to the presence of hydrogen under stress, for example, those resulting from shearing the parts. Consequently, attempts have been made to define a method that does not present any increased risk of delayed cracking or that makes it possible to reduce the susceptibility to this risk. Additional attempts have been made to define a method that would make it possible to manufacture welded targets from sheets of different thicknesses that would not result in a significantly different interval in different parts of these welded targets. Finally, attempts have been made to devise a method in which the constituent stages and physical elements make it possible to implement the method without resulting in prohibitive cost increases. Surprisingly, the inventors have shown that the aforementioned problems can be economically solved by deposition prior to hot stamping of a polymerized layer on a pre-coating comprising at least one layer of aluminum or an aluminum alloy, wherein the polymerized layer is composed of a specific polymer containing carbon pigments in an amount of 3 and 30% by weight. To this end, a first object of the invention is a pre-coated sheet or target comprising a heat-treatable steel substrate 1 overlaid on at least a portion of at least one of its main faces by a pre-coating 2 comprising at least one layer of aluminum or aluminum alloy, overlaid on a portion of the pre-coating by a polymerized layer 3 having a thickness between 2 and 30 pm composed of a polymer that does not contain silicone and the nitrogen content being less than 1% by weight, expressed relative to the layer, wherein the polymerized layer contains carbon pigments in an amount between 3 and 30% by weight, expressed relative to the layer. Preferably, the polymer elements are selected from a list consisting of C, H, O, N. In a preferred embodiment, the polymerized layer is obtained from a resin in the form of an aqueous phase dispersion or emulsion. According to another preferred embodiment, the polymerized layer is obtained from a resin in solution form in a non-aqueous solvent. In another preferred embodiment, the polymerized layer consists of a film that is bonded by lamination to the substrate. Preferably, the polymerized layer is obtained from an acrylic-type resin. In a preferred embodiment, the polymerized layer is obtained from an epoxy or acrylic type resin in solution form in a non-aqueous solvent. In another preferred embodiment, the polymerized layer consists of a polyethylene terephthalate or polyethylene or polybutylene terephthalate or polypropylene film. The pigments preferably consist at least partially of activated carbon. In a preferred embodiment, the pigments are at least partially made of graphite. / αβρηη / ίζηζ / Ε / γίΛΐ The amount of activated carbon in the polymerized layer is preferably less than 5%, expressed by weight in relation to the layer. Preferably, the aluminum or aluminum alloy layer constitutes more than 50% of the previous coating thickness. An additional object of the invention is a sheet or a target according to one of the embodiments described above, characterized in that the pre-coating comprises an intermetallic alloy layer 4 in contact with the substrate 1 overlaid by an aluminum metal alloy layer 5 and wherein, on at least one pre-coated surface of the metal sheet, the polymerized layer and the metal alloy layer are not present in zone 6, where this zone is located on the periphery of the metal sheet or the target. An additional object of the invention is a welded target obtained by welding at least two targets, wherein at least one of the targets 7 is a target according to any of the above characteristics having a thickness e7 and at least one of the targets is a target 8 having a thickness e constituted by a steel substrate coated by a layer of aluminum or aluminum alloy identical to that of target 7, wherein the thickness of targets 7 and 8 is te— > 1. 6'b The white (7) is preferably a coated white over the complete previous coating of the polymerized layer 3 and the thicknesses e7 and es are such that 2.6 > — > 2.2.es A further object of the invention is a part 9 obtained by austenitizing, followed by hot stamping and forming while held in the stamping tool, from a sheet or blank having a pre-coating comprising at least one aluminum or aluminum-coated layer, wherein the substrate 10 microstructure of the part contains martensite and / or bainite, wherein the substrate is overlaid on at least one of the principal surfaces by a coating 11 resulting from inter-diffusion between the steel substrate and the pre-coating, wherein the coating 11 is overlaid by an oxide layer 12, wherein the average weight percentage of oxygen, between 0 and 0.01 pm below the surface of the part, is less than 25% and the average weight percentage of oxygen, between 0.1 and 0.2 pm below the surface, is less than 10%. An additional object of the invention is a method for manufacturing a press-formed part comprising the following successive steps: - a steel sheet or blank substrate is provided for heat treatment, - a pre-coating is applied comprising at least one layer of aluminum or aluminum alloy in contact with the steel substrate on at least one side of the sheet or target, then, - In the previous coating, a polymerized layer 3 is deposited which has a thickness between 2 and 30 pm, composed of a polymer that does not contain silicone and whose nitrogen content is less than 1% by weight expressed in relation to the layer, wherein the polymerized layer contains carbon pigments in an amount between 3 and 30% by weight expressed in relation to the layer, then - the blank or sheet is heated to obtain inter-diffusion between the steel substrate and the previous coating, to obtain a partially or completely austenitic structure in the sheet, then - the blank or sheet is hot-stamped to obtain a part, then - the part is cooled by holding it in a stamping tool so that the microstructure of the steel substrate contains, at least in a portion of the part, martensite and / or bainite. In a preferred embodiment, the thickness of the sheet or target is between 1 and 2 mm and the heating rate of the target or sheet between 50 and 500°C is between 15 and 35°C / s. Preferably, the aluminum or aluminum alloy layer occupies more than 50% of the thickness of the previous coating. In a preferred embodiment, the polymer elements are selected from a list consisting of C, N, O, N. A further object of the invention is a method for manufacturing a hot-stamped, press-formed welded target, comprising the following successive steps: - a welded target is provided, created by welding at least two targets, comprising - at least one white 7 according to one of the above characteristics, which has a thickness e7, - at least one target 8 having a thickness is made up of a steel substrate coated with a layer of aluminum or aluminum alloy of a previous coating identical to that of target 7,βΊ - where the thickness of the targets 7 and 8 is such thatb / αβρηη / ίζηζ / Ε / γι 0 - the welded target is heated to effect inter-diffusion between the steel substrate and the previous coating and to give it a partially or totally austenitic structure in the steel, then - the welded target is stamped to obtain a hot-stamped welded target, then - The hot-stamped welded target is cooled by holding it in the stamping tool to obtain, in at least a portion of the substrate of the hot-stamped welded target, martensite and / or bainite. Preferably, the welded, hot-stamped, and shaped white material [according to the invention] is characterized in that the white 7 is coated throughout the previous coating, by the polymerized layer 3 and in done2·6- -2·2· Additional features and advantages of the invention will become apparent in the following description, which are shown by example with reference to the accompanying figures, in which: Figure 1 illustrates a schematic example of a sheet or blank pre-coated according to the invention, prior to hot stamping. Figure 2 illustrates a schematic example of a welded target according to the invention in which the two targets do not have the same thickness. Figure 3 illustrates a schematic example of a target according to the invention intended to be laser welded and then hot stamped. Figure 4 is a schematic illustration of an example of the / αβρηη / ίζηζ / Ε / γίΛΐ 1. Constitution of the layers observed in a hot-stamped piece according to the invention. It should be noted that these diagrams do not attempt to reproduce the relative dimensions of different constituent elements to scale. Figure 5 illustrates the surface profile analysis of oxygen content after heating to 900°C, followed by hot stamping and press forming. Figure 6 illustrates the surface profile analysis of the carbon content, after heating to 900°C, followed by hot stamping and press forming. Figure 7 illustrates the surface profile analyses of carbon, oxygen, and silicon after treatment at 900°C, followed by customer stamping and press forming of a previously coated blank not in accordance with the invention. Figure 8 shows the surface appearance of a hot-stamped part according to the invention compared to a hot-stamped part that does not comprise the prior deposition of a carbon-pigmented polymer layer, the former being illustrated in Figure 9. The thickness of the steel sheet used in the method according to the invention is preferably between approximately 0.5 and 4 mm, a thickness range used particularly in the manufacture of structural or reinforcing parts for the automotive industry. The substrate sheet is a heat-treatable steel, i.e., a steel capable of hardening after austenitizing and rapid / αβρηη / ίζηζ / Ε / γίΛΐ 2 cooling by tempering. For example, steel advantageously contains the following elements, where the composition is expressed as a percentage by weight: - a carbon content between 0.07 and 0.5%, preferably between 0.09 and 0.38% by weight, and more preferably between 0.15 and 0.25% by weight. This element plays a major role in the hardenability and mechanical strength obtained after the cooling that follows the austenitizing treatment. Below a content of 0.07% by weight, the suitability for hardening is reduced, and the mechanical tensile strength after press forming is insufficient. A carbon content of 0.15% makes it possible to ensure sufficient hardenability in the hottest working areas. Above a content of 0.5% by weight, the risk of defect formation increases during hardening, particularly for the thickest parts. It also becomes difficult to ensure ductility during the joining of parts after press forming. A carbon content between 0.09 and 0.38% by weight...39% makes it possible to obtain a tensile strength Rm between approximately 1,000 and 2,050 MPa when the microstructure of the part is fully martensitic. In addition to its deoxidizing role, manganese also has a significant effect on forming capacity, particularly when present in amounts greater than 0.5% by weight and preferably greater than 0.8% by weight. However, it is preferable to limit its addition to 3% by weight and very preferably to 1.5% to prevent excessive segregation. The silicon content of the steel should be between 0.02 and 0.5% by weight, and preferably between 0.1 and 0.35%. Besides its role in deoxidizing the liquid steel, this element contributes to the steel's formability, although its concentration must be limited to prevent excessive oxide formation and any adverse effects on its hot-dip coating capabilities. Above a concentration greater than 0.01%, chromium increases formability and contributes to high strength after hot working. Above a concentration of 1%, preferably 0.3%, the effect of chromium on the uniformity of mechanical properties in the part becomes saturated. Aluminum is an element that promotes oxidation and nitrogen precipitation. In excessive amounts, coarse aluminates form during steelmaking, which tends to reduce ductility. As a result, the aluminum content is limited to 0.25% by weight. A minimum concentration of 0.001% allows for the oxidation of steel in the liquid state during processing. - In excessive quantities, sulfur and phosphorus result in increased embrittlement. Therefore, it is preferable to limit the levels of these elements to 0.05 and 0.1% by weight, respectively. Boron, whose content should be between 0.0005 and 0.010% by weight and preferably between 0.002 and 0.005% by weight, is an element that plays an important role in terms of forming capacity. Below / αβρηη / ίζηζ / E / γίΛΐ At a boron content of 0.0005%, a sufficient formability effect is not achieved. The full effect is obtained at a content of 0.002%. The maximum boron content should be less than 0.010% and preferably 0.005% to avoid an adverse effect on hardness. Titanium has a strong affinity for nitrogen. This protects boron, allowing it to be in its free form and thus fully contribute to formability. Above 0.2%, however, there is a risk of forming coarse titanium nitrides in the molten steel, which negatively affect hardness. The titanium content is preferably between 0.02% and 0.1%. Optionally, the steel may also contain tungsten in quantities between 0.001 and 0.3% by weight. In these quantities, this element increases formability and formability through the formation of carbides. Optionally, steel may also contain calcium in an amount between 0.0005% and 0.005%. By combining with oxygen and sulfur, calcium prevents the formation of large inclusions, which have an undesirable effect on the ductility of sheets or parts made from them. The composition of the sheet consists of iron and unavoidable impurities resulting from processing. Preferably, 22MnB5 steel will be used, which contains, in weight percentage: 0.20-0.25% C, 1.1-1.35% Mn, 0.15-0.35% Si, 0.02-0.06% Al, 0.02-0.05% Ti, 0.02-0.25% Cr, 0.002-0.004% B, the balance consisting of iron and unavoidable impurities. / αβρηη / ίζηζ / Ε / γίΛΐ 5 The steel substrate comprises a pre-coating of aluminum or an aluminum alloy. In the latter case, the coating therefore contains more than 50% aluminum by weight. This pre-coating, which is preferably applied by a continuous hot-dip process, is advantageously an aluminum-silicone alloy comprising 7–15% silicon by weight, 2–4% iron by weight, optionally 15–30 ppm calcium, the remainder consisting of aluminum and unavoidable impurities resulting from the process. The pre-coating can also be an aluminum alloy containing 40-45% Zn, 3-10% Fe, 1-3% Si, the balance consisting of aluminum and unavoidable impurities resulting from the process. The precoating can also consist of a superposition of layers deposited in successive stages, at least one of which is aluminum or an aluminum alloy. The aluminum or aluminum alloy layer (or, if there are multiple such layers, the sum of their thicknesses) preferably occupies more than 50% of the precoating thickness. This pre-coating is overlaid, on at least one of the main surfaces of the sheet, by a polymerized layer containing carbon pigments. This layer can be deposited over the entire pre-coating or only on a portion of it. In the latter case, the effects conferred by this layer, described below, are obtained in the areas where the layer is present. With regard particularly to the thermal effects of this layer, these also apply to a lesser extent in the areas that are / αβρηη / ίζηζ / E / γίΛΐ 6 locally adjacent to those in which the layer has been deposited. This polymerized layer can be obtained particularly through the following steps: - starting with a resin in the form of a dispersion or emulsion in the aqueous phase. In particular, an acrylic resin can be used. - starting with a resin in the form of a solution in a non-aqueous solvent. In particular, an epoxy-type resin can be used, for example, an epoxy-phenol or acrylic resin. - starting with a thermoplastic polymer film bonded by lamination to the substrate sheet. Specifically, polyethylene terephthalate or polyethylene or polybutylene terephthalate can be used. / αβρηη / ίζηζ / Ε / γίΛΐ For reasons of productivity and thickness consistency, this layer can be deposited primarily by lamination. After polymerization and / or drying, a polymerized layer is obtained with a thickness between 2 and 30 µm. A thickness less than 2 µm does not allow for a sufficient coverage rate to be achieved using this method. A thickness less than 30 µm leads to an increased risk of oven contamination during subsequent heating. The pre-coated sheet or target is illustrated schematically in Figure 1. Figure 1 illustrates the heat-treated steel substrate 1 overlaid by a pre-metallic coating 2, which is overlaid by a polymerized layer 3 containing carbon pigments. At this stage, the steel substrate has not yet hardened, that is, it has not 7 contains any constituent part, or at least some constituent parts, resulting from hardening, for example, less than 10% martensite. The sheet or target has an essentially flat shape. In the press-hardening method, the steel substrate is heated to a temperature Tγ, resulting in its austenitization, at least partially, to achieve a martensitic or bainitic transformation during subsequent cooling. If a painted sheet is to be heated to such a temperature, it would be natural to select a paint resistant to temperatures T higher than Ty, that is, one in which the binder retains its function relative to the pigments in the paint. It is known that pigments resistant to high temperatures are generally silicone or polysiloxane resins, which consequently contain silicone. In fact, these are based on Si-O-Si chains containing very stable bonds or are resistant to high temperatures. However, the inventors have surprisingly discovered that it is necessary to use polymers that do not contain silicone.The advantageous effects explained below are obtained when the constituent elements of the polymers are selected from carbon, hydrogen, oxygen, or nitrogen. During heating, these polymers break down at a temperature below Tγ and are partially combined with the oxygen in the oven atmosphere. Therefore, it should be expected that the paint pigments, devoid of a binder, will not adhere to the substrate afterward and will detach. Caution may also be necessary after heating, as a layer resulting from the decomposition of the paint will form. 8 will prevent the subsequent implementation of cataphoresis or resistance welding. However, the inventors have shown that, surprisingly, these undesirable consequences do not occur under the conditions of the invention. The nitrogen content of the polymerized layer 3 should be limited to 1%, preferably 0.5% and more preferably to 0.2% under penalty of formation of HCN-type compounds or excessive amounts of ammonia during heating to the temperature required for stamping. The weight content of the carbon pigments, expressed relative to the polymerized layer 3, is between 3 and 30%. Below 3%, the reduction in heating cycle time is insufficient. Above 30%, the mixture has an unsuitable viscosity for application. Within this content range, the part obtained after hot stamping has practically no carbon surface enrichment, as will be shown below. Carbon pigments can be in the form of graphite or activated carbon. The latter, obtained through a high-temperature carbonization process, has an amorphous structure and a large specific surface area, giving it high adsorption capacity. The weight content of activated carbon, expressed relative to the deposited layer, should be less than 5% to be suitable for blending with the polymer. The targets comprising the metallic pre-coating and the polymer layer containing the carbon particles are heated in a furnace under an ordinary atmosphere from room temperature to a temperature Tγ, conventionally close to 900°C, which makes possible / αβρηη / ίζηζ / Ε / ΥΐΛΐ 9. Subsequent hot stamping. During heating, the carbon in the layer remains present on the surface of the target for most of this heating stage; that is, its effect on reflectivity is exerted over the majority of this stage and, consequently, contributes to a very significant reduction in the duration of the stage. Under the conditions of the invention, the inventors have found that it gradually combines with the oxygen in the oven during heating and disappears almost completely when the target reaches temperature Tγ. Furthermore, the inventors have found that applying the paint according to the invention does not require subsequent shot blasting to remove a potential oxide layer, which could have an adverse effect on subsequent cataphoresis painting.Applying the paint according to the invention does not alter the resistance welding suitability of the parts after hot stamping, so it is not necessary to modify the welding machine settings. Furthermore, as will be shown below, the method according to the invention makes it possible to increase the delayed cracking resistance of the hot-stamped parts due to the reduction in diffusible hydrogen content. Applying the paint according to the invention does not reduce the corrosion resistance of the hot-stamped parts. The invention can be implemented in a particular manner illustrated in Figure 2, which schematically shows a welded blank comprising two blanks 7 and 8 having respective thicknesses e7 and 1. These two blanks comprise a steel substrate pre-coated with aluminum or aluminum alloy, wherein the pre-coating is identical on both blanks. The welded joint can be created by any suitable method, particularly by arc welding or laser welding. If such a welded seam is heated for hot stamping, the different thicknesses of the constituent parts result in different heating kinetics in the two parts, wherein blank 7 heats up less rapidly than the thinner blank 8.This, in turn, should lead to different microstructures and properties in the substrate and coating of parts 7 and 8 after hot stamping. In certain cases, it is not possible to determine satisfactory operating conditions, such as heating temperature and holding time, to obtain the desired properties in the different constituent zones of the welded joint. The invention reveals that a polymer layer described above is deposited to obtain a polymerized layer containing 3 to 30% carbon pigments on the thicker white 7. The thinner white 8 does not have such a layer on the pre-coating of aluminum or aluminum alloy. Depending on the desired uniformity of properties on the hot-stamped part, it is possible to deposit the layer on the entire white 7 or only on a portion thereof. The welded white is then placed in an oven. The pre-application of the paint layer makes it possible to increase the emissivity of white 7 and reduce the difference in heating rate between the two whites 7 and 8 that results from their differences in thickness. / αβρηη / ίζηζ / Ε / γίΛΐ The inventors have shown that when the targets are provided in such a way that 2.6 > - > 2.2, the heating duration is practically the same in parts 7 and 8, because the application of the paint layer according to the invention modifies the emissivity to fully compensate for the effect of the difference in thickness between whites 7 and 8 in the heating cycle, which ensures excellent uniformity of the part properties after hot stamping and press hardening. Blanks 7 and 8 can be pre-coated, for example, with an aluminum-silicone alloy comprising 7–15 wt% silicon, 2–4 wt% iron, optionally 15–30 ppm calcium, the remainder being aluminum and unavoidable impurities resulting from the process. Under these conditions, as illustrated schematically in Figure 3, the pre-coating 2 comprises an intermetallic alloy layer 4 several micrometers thick, comprising mostly Fe₂Al₃, Fe₂Al₅, and FeₓAl₂Si₃, in contact with the steel substrate 1. This intermetallic layer 4 is overlaid by an Al-Si-Fe metallic alloy layer 5. In the thicker blank 7, this layer 5 is overlaid on a polymer layer 3 described above, containing 3–30% carbon pigments.To prevent the formation of brittle intermetallic compounds in the fusion zone during welding, the metallic alloy layer 5 is removed from the periphery of the targets, leaving the thin layer 4 in place. This local ablation can be carried out by any means, particularly by fusion and vaporization using a pulsed layer. The inventors have shown that the presence of the carbon-pigmented polymer layer does not interfere with the ablation, which can be performed under satisfactory productivity conditions. By way of example, the desired results can be obtained using a 50 W to 1.5 kW laser source, ablation rates of 3 to 6 m / min, and a wavelength of 300 to 1,500 nm. Figure 3 illustrates an example where paint layer 3 and a metallic alloy layer 5 have been removed from one edge of a previously coated painted target. The intermetallic coating 4 is therefore exposed on surface 6 after this ablation. Figure 3 illustrates an example where the ablation has been carried out on only one surface of the target. It is also possible to perform this ablation on both sides when it is desired to minimize the amount of aluminum introduced into the re-melted weld metal. The blanks are welded along the edge where the ablation has been performed. The blanks are then heated, hot stamped, and press-formed. The inventors have shown that the method according to the invention makes it possible to manufacture hot-stamped parts that have good stability for spot welding and cataphoresis painting, good corrosion resistance and delayed cracking. By means of non-restrictive examples, the following modalities illustrate the advantages achieved by the invention. / αβρηη / ίζηζ / Ε / γίΛΐ Example 1: One-mm-thick blanks made of steel were provided, having the following composition by weight percent: 0.228% C, 1.189% Mn, 0.014% P, 0.001% S, 0.275% Si, 0.028% Al, 0.034% Ti, 0.003% B, 0.177% Cr, the balance being iron and impurities resulting from the process. These blanks comprised a 24-pm-thick pre-coating on each surface, containing 9% by weight silicon, 3% by weight iron, the balance being aluminum, and unavoidable impurities. On some of these blanks, a layer was Test Polymer type of the layer deposited over the previous coating Weight percentage of activated carbon pigments expressed in relation to the layer after drying (%) Weight percentage of graphite carbon pigments expressed in relation to the layer after drying (%) Thickness of the layer deposited after drying (micrometers) 11 Phenoxy-acrylic resin 5 - 15 I2 Phenoxy-acrylic resin - 15 15 I3 Phenoxy-acrylic resin 1 12 15 I4 Phenoxy-acrylic resin 1 12 11 I5 Phenoxy-acrylic resin 1 12 26 R1 Polysiloxane 0 0 30 R2 No polymer coating, no carbon pigments - - - / αβρηη / ίζηζ / Ε / γΐΛ Table 1. Test conditions (I = Invention, R = Reference) Thermogravimetric analyses show that most acrylic resin decomposes at approximately 400°C. The steel blanks prepared under the conditions specified in the table above were heated from room temperature to 900°C in a furnace under ordinary atmosphere, held at this temperature for one minute, then hot-stamped and tempered by holding them in the stamping tool. The rapid cooling achieved in this way creates a martensitic structure in the steel substrate. The mechanical strength Rm is approximately 1,500 MPa. The thermal cycle causes an alloy of the substrate iron with the coating, thus creating inter-metallic alloys that essentially contain aluminum, iron, and silicon. Under each of the above conditions, thermocouples were used to measure the heating time, that is, the time that elapses between the moment when the target is at room temperature and when it reaches 900°C. The results are shown in Table 2. Test C (S) 11 70 I2 70 I3 70 I4 70 I5 70 R1 68 R2 165 / αβρηη / ίζηζ / Ε / γΐΛ Table 5 - Heating time from 20 to 900°C The application of a polymerized layer comprising carbon pigments (tests 11 to I5 and R) makes it possible to reduce the heating time by more than 50% compared to the previous metallic coating alone (test R2). The average rate obtained during heating between 50 and 500°C was also measured in the blanks prepared under conditions I3 and R2, which have a thickness between 1 and 2 mm. Table 3 presents the results obtained. Test Thickness (mm) V20-500°C (°C / S) I3 1 31 1.5 22 2 1 6 R2 1 1 2 1.5 1 0 2 7 Table 3. Heating rate between 50 and 500°C Under the conditions of the invention, therefore, it has been shown that it is possible to obtain a heating rate between 15 and 40°C / s between 50 and 500°C for the entire thickness range, while this rate remains below 12°C / s under the reference conditions R2. Figure 5 illustrates the oxygen analysis by Light Discharge Spectroscopy performed on targets heated to 900°C and then hardened. These analyses express the variation of oxygen content as a weight percentage with thickness, starting from the surface of the press-formed parts. In relation to reference tests R1 (polyisoloxane-based coating) and R2 (pre-coated metal without paint application), the application of resin and carbon pigments according to the invention (I3) results in a reduction of the oxide layer on the extreme surface. In the case of test I3, the average oxygen content measured between 0 and 0.01 pm below the surface is 16.7%, while it is 30.3% for test R2. This reduction in the average surface oxygen content makes it possible to reduce contact resistance, which improves suitability for resistance spot welding.While not tied to a specific theory, it is thought that the deposition of the polymer layer according to the invention to some extent protects the underlying aluminum alloy layer and reduces the formation of alumina on the surface. In the other tests, I4 and I5, the variation of oxygen content as a function of depth has been found to be very similar to that illustrated for I3 in Figure 5. In the case of test R1, the use of a polysiloxane-based polymer results in the formation of a thin oxygen layer. The average oxygen content measured between 0.1 and 0.2 pm below the surface is 18%, while it is less than 10% under the conditions of the invention: 3.8% in test I3, 3% for test I4, and 4.8% for test I5. In the case of test R1, subsequent cataphoresis painting requires the removal of the oxide layer by costly shot blasting or other blasting treatments, whereas these treatments are not necessary in the case of the invention because the oxide layer is significantly thinner. In tests I3-I5 according to the invention, the oxygen content on the surface after press forming is relatively independent of the thickness of the polymerized layer deposited on the previous coating, as shown in the table below. / αβρηη / ίζηζ / Ε / γίΛΐ Test Thickness of the deposited layer after drying (micrometers) Average oxygen content between 0 and 0.01 micrometers below the surface (%) Average oxygen content between 0.1 and 0.2 micrometers below the surface (%) I4 1 1 1 7.5 3 I3 1 5 16.6 3.8 I5 26 14.4 4.8 Table 4: Characteristics of surface oxygen content as a function of polymer thickness deposited in the previous coating This means that the initial stage of deposition of the polymer layer with the carbon particles can be carried out with a certain tolerance in terms of thickness and, therefore, does not require the implementation of specific, expensive application methods. It will be observed in Figure 6 that the deposition of carbon pigments with the polymer under the conditions of the invention (tests I3 and I5) does not cause significant surface carbon enrichment compared to the reference situation R2. Contrary to what might be expected, the inventors have shown that the addition of carbon pigments further reduces the carbon content on the extreme surface after the press forming treatment. This indicates that a reaction of carbon with oxygen in the atmosphere occurs almost completely during the heating phase of the blank in the oven. Under the conditions of the invention, interrupted tests at different temperatures during heating show that carbon remains present on the target surface for most stages; that is, its effect on reflectivity occurs directly during a very large part of the heating cycle. However, as previously stated, the gradual combination of atmospheric oxygen with carbon results in the almost complete disappearance of the former when the target reaches a temperature of 900°C. In the case of a deposit made under the reference conditions (R1), Figure 7 shows the variations in the carbon, oxygen, and silicon content by weight measured in the parts obtained by heating to 900°C, holding them at this temperature for one minute, followed by hot stamping and press forming. In addition to a higher oxygen content than in the tests according to invention I3-I5, a significant increase in the surface silicon content is observed, which is present in the form of oxide. This modifies the suitability for resistance welding due to the sharp increase in contact resistance, which is greater than 1.5 milliohms. The suitability for resistance spot welding of the parts created according to conditions I2 and R2 was evaluated by performing spot welding at a welding force of 350 daN.The welding capacity range was evaluated by the difference between the minimum current lmin, which allows for a spot diameter of 6 mm with satisfactory mechanical strength, and the maximum current lmax, beyond which molten metal is expelled during welding. The welding capacity range (lmax-lmin) is approximately 1,500 A, equivalent to conditions I2 and R2. Furthermore, the results of transverse tensile tests for spot welding were found to be identical. For a welding current lmin, the mechanical strength is 3,370 N (condition R2) and 3,300 N (condition I2). For a welding current lmax, the mechanical strength is 4,290 N (condition R2) and 4,127 N (condition I2). Therefore, the application of polymer and carbon particles according to the invention does not alter the suitability for spot welding.The invention can be implemented without modifying the settings of the welding machines. It is possible to weld a sheet pre-coated with aluminum or aluminum alloy with a polymer and carbon deposit according to the invention to a sheet that has only been pre-coated, ensuring that the welding conditions are perfectly suitable for these two types of sheets. The surfaces of the hot-stamped and hardened parts under conditions 11-3 and R2 were observed in zenith terms using a scanning electron microscope. Under the reference condition (R2, Figure 8), the surface roughness is significant, which guarantees good suitability for subsequent cataphoresis painting. Figure 9 shows that the surface roughness of the part manufactured under condition I2 of the invention is similar. Identical observations were noted for conditions I1 and I3. As noted above, the surface of the parts manufactured according to the invention is not enriched with carbon, which ensures that the suitability for cataphoresis is not reduced by the prior application of the polymer and carbon pigments. Example 2: The resistance to different forms of corrosion of the manufactured parts, hot stamped and press hardened, was evaluated according to conditions I2 (pre-coating with AISi and painting according to the invention) and R2 (pre-coating with AISi only) of Table 1. The resistance to cosmetic corrosion was determined under the following conditions: scratches of varying depths were made affecting either only the coating (condition A) or also the substrate (condition B) of the hot-stamped parts. These parts were subjected to temperature and humidity cycling in a saline mist for six weeks under the conditions described in the well-known “New VDA Test 233-102.” The width of blistering at the rayon level was subsequently measured. The results are shown in Table 5. / αβρηη / ίζηζ / Ε / γίΛΐ Width of blisters (mm) Condition A Width of blisters (mm) Condition B I2 2.7 4 R2 2.6 4.2 Table 5 - Cosmetic Corrosion Results Compared to the reference test R2, the application of paint according to the invention has been found not to reduce resistance to cosmetic corrosion. Resistance to penetrating corrosion was evaluated by tests conducted over 12 weeks under the "NEW VDA" test conditions specified above. The measured weight losses for parts manufactured under conditions I2 and R2 are as follows: Weight loss (g / m2) I2 1 70 R2 1 80 Table 6 - Cosmetic corrosion results Compared to the reference test R2, the application of the paint according to the invention does not reduce the resistance to penetrating corrosion. The 1 mm thick parts, hot-stamped and manufactured according to conditions I2 and R2, were subjected to cataphoresis painting. The adhesion of this cataphoresis layer was measured after scratching in a checkered pattern, followed by immersion in water at 50°C for 10 days. It was found that applying the paint according to the invention does not reduce the adhesion of the cataphoresis layer. Example 3: The targets were made of 22MnB5 steel, 1, 5, and 2 mm thick, comprising on both sides a 23 gm thick pre-coating containing 9% Si and 3% Fe, the balance being aluminum and unavoidable impurities. On some of the targets, a layer composed of polymer and carbon pigments was deposited by roller coating over both entire surfaces under conditions I2 according to the invention indicated in Table 1. The other targets were not painted (condition R). The blanks were heated to 900°C, held at this temperature for one minute, then hot stamped and shaped by holding them in a stamping tool. The diffusible hydrogen content was measured using a thermal desorption analysis method, which is a known method. The diffusible hydrogen content of these blanks is shown in Table 7. / αβρηη / ίζηζ / Ε / γίΛΐ Fan thickness (mm) Diffusible hydrogen content (ppm) I2 1 .5 0.15 R 1 .5 0.21 I2 2 0.17 R 2 0.25 / αβρηη / ίζηζ / Ε / γΐΛ Table 7 - Diffusible hydrogen content The application of the specific paint according to the invention makes it possible to significantly reduce the diffusible hydrogen content. Without being limited to a single theory, the inventors believe that applying the coating according to the invention reduces the length of time during which hydrogen can be absorbed during the heating stage prior to stamping. The invention, therefore, makes it possible to significantly reduce the susceptibility to delayed cracking of hot-stamped and formed parts. The implementation of the invention thus makes it possible to use steels containing higher levels of alloying elements and results in higher mechanical strength after hot stamping, all without an increased risk of delayed cracking. Example 4: The targets were made of 22MnB5 steel, 1, 2, and 2.5 mm thick, comprising on both sides a 23 µm thick pre-coating containing 9% Si and 3% Fe, the remainder being aluminum and unavoidable impurities. The pre-coating consists of a thin layer of intermetallic alloy, mostly composed of Fe2Al3, Fe2Al3, and Fe2Al3Si3, approximately 4 µm thick, in contact with the steel substrate. This intermetallic layer is overlaid by a 19 µm thick Al-Si metallic alloy layer. In some of the targets, a layer composed of polymer and carbon pigments was deposited by roller coating over the two complete main surfaces under the I2 conditions indicated in Table 1. These targets were heated to 900°C. Thermal cycles measured by thermocouples made it possible to determine the thermal emissivity. For the sheets painted according to the invention, the thermal emissivity decreases during the heating cycle from approximately 0.6 to 0.3. For the unpainted reference sheets, the emissivity decreases from approximately 0.2 to 0.1 during the heating cycle. With reference to schematic Figure 2, the targets were assembled by laser welding. These targets consisted of two targets of different thicknesses. The thinner target, identified as 8, with a thickness es, is made of a sheet pre-coated with the aforementioned aluminum alloy. The thicker target, identified as 7, with a thickness e7, is made of a sheet pre-coated with the same aluminum alloy and a polymerized layer according to condition I2 above. To prevent the formation of intermetallic compounds in the fusion zone during welding, a pulsed laser was used to remove the Al-Si-Fe metal alloy coating by ablation from both surfaces to a width of 1.1 mm from the periphery of the targets, leaving the thin intermetallic alloy layer in place.In the case of previously coated, unpainted targets, only the metallic alloy layer was removed; in the case of targets previously coated with a polymerized layer, both the metallic alloy layer and the polymerized layer were removed to the width indicated above. An ablation rate of 3 m / min made it possible to obtain the desired results. These welded blanks were subsequently heated to 900°C by measuring the temperature using thermocouples placed on each of the two parts 7 and 8 of the welded blanks (blanks with thicknesses e7 and e8). The heating time ΔV20° on each of parts 7 and 8 was therefore determined as the temperature difference between these two parts at each instant of the heating. The maximum temperature difference during the heating cycle is shown in Table 8. The welded blanks were then hot stamped and press formed. / αβρηη / ίζηζ / Ε / γίΛΐ Test ID e? (mm) blank without polymerized layer e8 (mm) blank with polymerized layer aC(7) (s) Δ<®(8) (s) ΛΘ; fC) I4 2 1 2 160 170 44 I5 2.5 1 2.5 175 170 2 Table 8 In comparison, Table 9 presents the results obtained during the heating of a welded target consisting of the two targets previously coated with Al-Si alloy, unpainted, with respective thicknesses of 2 mm and 1 mm. Test ID e7 (mm) unpainted white is (mm) unpainted white e7 Δ^°(7) (S) Δ?^(8) (S) ΔΘ| ( C) R3 2 1 2 360 170 155 Table 9 Under the conditions of the invention (tests I4 and I5), the heating time is very similar in the two parts 7 and 8 that make up the weld joint. This ensures that the microstructures of the steel substrate after austenitization and the coating obtained by interdiffusion will be very similar in parts 7 and 8 of the welded joint. In comparison, the reference test R3 results in a situation where the thicker part of the weld target heats up to 900°C much more slowly than the thinner part. Consequently, the thinner part of the welded joint must be held at 900°C for 190 seconds for the thicker part to reach the temperature of 900°C, which can lead to undesirable austenitic grain growth in the steel substrate of the thinner part or excessive interdiffusion between the previous coating and the substrate in this part. The method according to the invention makes it possible to prevent these problems. In test I5, where the ratio is equal to 2.5, the temperature difference between the two welded parts at any point in the heating cycle is particularly low, less than 2°C, while it is equal to 44°C in test I4. This preferred mode in which the thickness range is between 2.2 and 2.6 will be selected when the objective is the greatest possible thermal uniformity during the heating of the welded targets. Therefore, the invention describes the manufacture of hot-stamped parts under conditions of increased productivity, exhibiting good suitability for spot welding and cataphoresis painting, high corrosion resistance, and delayed cracking. These parts can be advantageously used as structural components or reinforcements in the automotive industry.

Claims

1. A part obtained by austenitizing followed by hot stamping and forming by holding in the stamping tool a blank or sheet comprising at least a portion of the part: a steel substrate; a pre-coating having at least one layer containing aluminum or aluminum alloy; a microstructure of a steel substrate including martensite and / or bainite; a coating on at least one face of the steel substrate resulting from interdiffusion between the steel substrate and the pre-coating; and an oxide layer overlying the coating; a weight-average oxygen content between 0 and 0.01 pm below the surface of the part that is less than 25%, and a weight-average oxygen percentage between 0.1 and 0.2 pm below the surface being less than 10%.

2. The part according to claim 1, wherein the weight-average oxygen content between 0 and 0.01 pm below a surface of the part is 17.5% or less, and the weight-average oxygen content between 0.1 and 0.2 pm below a surface is 4.8% or less. / QArnn / Lznz / E / YiAi 3. The part according to claim 1, wherein, prior to austenitization, a polymerized layer having a thickness from 2 pm to 30 pm and comprising a polymer that does not contain silicon, a nitrogen content of less than 1% by weight, and carbon pigments in an amount from 3 to 30% by weight.

4. The part according to claim 3, wherein the polymer consists of C, H, O or N.

5. The part according to claim 3, wherein the polymerized layer is obtained from a resin in the form of an aqueous phase dispersion or emulsion.

6. The part according to claim 3, wherein the polymerized layer is obtained from a resin in the form of a solution in a non-aqueous solvent.

7. The part according to claim 3, wherein the polymerized layer is a film roller-bonded to the substrate.

8. The part according to claim 5, wherein the polymerized layer is obtained from an acrylic-type resin.

9. The part according to claim 6, wherein the polymerized layer is obtained from an epoxy or acrylic resin.

10. The part according to claim 7, wherein the polymerized layer is a film of polyethylene terephthalate or polyethylene or polybutylene terephthalate or propylene. / αβρηη / ίζηζ / Ε / γίΛΐ 11. The part according to claim 3, wherein the carbon pigments include activated carbon.

12. The part according to claim 3, wherein the carbon pigments include graphite.

13. The part according to claim 11, wherein the amount of activated carbon in the polymerized layer is less than 5% by weight.

14. The part according to claim 1, wherein the aluminum or aluminum alloy layer is greater than 50% of a previous coating thickness.

15. The part according to claim 1, wherein the precoating is applied by a continuous hot-dip process and wherein the precoating is an aluminum-silicon alloy comprising 7-15 wt% silicon, 2-4 wt% iron, optionally 15-30 ppm calcium, the remainder consisting of aluminum and unavoidable impurities resulting from processing.

16. The part according to claim 1, wherein the pre-coating is an aluminum alloy containing 40-45% Zn, 3-10% Fe, 1-3% Si, and the remainder consisting of aluminum and the unavoidable impurities resulting from processing.

17. The part according to claim 3, wherein the austenitizing includes heating to an austenitizing temperature Ty and where the heating includes heating at a heating rate between 15 and 40 SC and between 50 and 500 SC.

18. The part according to claim 17, wherein the austenitizing temperature is at least 900 eC.

19. The part according to claim 3, wherein the austenitization includes heating to an austenitization temperature Ty and wherein the carbon of the carbon pigment gradually combines with atmospheric oxygen during said heating.

20. The part according to claim 19, wherein the heating includes heating at a heating rate between 15 and 40 °C and between 50 and 500 °C.

21. The part according to claim 20, wherein the carbon in the carbon pigment remains present during a major part of said heating and substantially disappears when the austenitizing temperature Tγ is reached.

22. A manufacturing method for a hot-formed part comprising the steps of: providing a sheet or blank of a raw steel substrate for heat treatment; heating the blank or sheet in a furnace, including the pre-coating and polymerized layer, to an austenitizing temperature Tg of the steel substrate to decompose the polymerized layer and combine the carbon of the polymerized layer with oxygen in the furnace, and to obtain interdiffusion between the steel substrate and the pre-coating and to give the steel a partially or fully austenitic structure; then hot-stamping the blank or sheet to obtain a part; then cooling the part by holding the part in a stamping tool so that the microstructure of the steel substrate includes, at least in a portion of the part, martensite or bainite.

23. The method of claim 22, wherein the steel substrate target is a welded target.

24. The method of claim 23, further comprising welding at least a first and second preform to form the welded preform, the first preform having a first thickness e7, the second preform having a thickness e8, the thicknesses of the first and second preforms being such that e7 / e8> 1.

25. The method of claim 24, wherein the polymerized layer, when present, forms an upper surface of the first target.

26. The method of claim 23, wherein the second target does not have a polymerized layer over the previous coating.

27. The method according to claim 22, wherein the carbon content of the steel substrate is between 0.07 and 0.5% by weight.

28. The method of claim 22, wherein the carbon content of the steel substrate is between 0.09 and 0.38% by weight.

29. The method of claim 22, wherein the carbon content of the steel substrate is between 0.15 and 0.25% by weight.

30. The method of claim 22, wherein the sheet or preform has a thickness of between 1 mm and 2 mm and the heating includes heating at a heating rate between 15 and 35 °C / s between 50 and 500 °C.

31. The method of claim 30, wherein the heating includes heating the pre-coating and the polymerized layer to 900 °C.