METHOD OF MANUFACTURING A SHEET METAL COMPONENT FROM A FLAT STEEL PRODUCT PROVIDED WITH AN ANTICORROSIVE COATING.

Abstract

The invention relates to a method for manufacturing a sheet metal component wherein a) a flat steel product is provided made of a steel which (by weight) consists of 0.05 - 0.5% C, 0.5 - 3% Mn, 0.06 - 1.7% Si, 0.06% P, 0.01% S, 1.0% Al, 0.15% Ti, 0.6% Nb, 0.01% B, 1.0% Cr, 1.0% Mo, the total contents of Cr and Mo being 1.0%, 0.2% Ca, 0.1% V, the remainder being iron and impurities; b) the flat steel product is annealed in a continuous furnace with four zones A, B, C, D under an annealing atmosphere composed of 0.1 - 15% hydrogen and nitrogen with a dew point temperature TP and an annealing temperature GT, the following being applicable: Zone A TP: -10 - -25°C, GT: 800 - 950°C, Zone B TP: -27 - -41°C, GT: 800 - 930°C, Zone C TP: -30 - -80°C, GT: 800 - 950°C, Zone D: TP: -30 - -20°C, GT: 750 - 950°C; c) a corrosion protection coating composed of (by weight) < 15% Si, = 5% Fe, in total 0.1 - 5% of at least one alkaline earth or transition metal and as a remainder of Al and unavoidable impurities is applied to the flat steel product; optionally d) the flat steel product is rolled and e) a plate is separated from said flat steel product; f) the flat steel product is heated to a temperature >Ac3 of the flat steel product and = 1000°C for a holding time sufficient to introduce an amount of thermal energy Js > 100,000 - 800.000 kJs; g) the flat steel product is hot-formed into the sheet metal component; and h) at least one section of the component is cooled at a cooling rate sufficient to generate hardening structures. The invention also provides a component in which optimal adhesion of the corrosion protection layer to the steel substrate is ensured.

Description

METHOD FOR MANUFACTURING A SHEET METAL COMPONENT FROM A FLAT STEEL PRODUCT PROVIDED WITH AN ANTICORROSIVE COATING FIELD OF INVENTION The invention relates to a method for manufacturing a sheet metal component from a flat steel product provided with an anti-corrosive coating. Flat steel products here refer to rolled products whose length and width are significantly greater than their thickness. These include, above all, steel strips and steel sheets. Unless explicitly stated otherwise, in this text information on the content of alloy components is always provided in % by weight. In contrast, the proportions of certain components of an atmosphere, in particular an annealing atmosphere, are indicated in % by vol, unless otherwise indicated. BACKGROUND OF THE INVENTIONA method of the type described at the beginning is known from document EP 2 993 248 Al. The starting material for this method is a flat steel product whose steel substrate consists of so-called MnB steel. Steels of this type are standardized in EN 10083-3 and have good hardenability. They allow reliable process control during hot pressing, making it economically possible to continue inducing martensite hardening in the tool during hot forming without the need for additional cooling. A typical example of this type of steel is known by the designation 22MnB5, which can be found in the 2004 steel code under material number 1.5528. Generally, commercially available 22MnB5 steel contains, in addition to iron and unavoidable impurities, (by weight) between 0.10 and 0.250% C, between 1.0 and 1.4% Mn, between 0.35 and 0.4% Si, up to 0.03% P, up to 0.01% S, up to 0.040% Al, up to 0.15% Ti, up to 0.1% Nb, totaling up to 0.5% Cr + Mo and up to 0.005% B. To protect flat steel products formed from this composite steel against corrosive attack and, at the same time, minimize the risk of hydrogen absorption during the heating required for hot forming, the flat steel products are provided with an aluminum-based corrosion protection coating, manufactured according to a known method, containing effective contents of at least one alkaline earth or transition metal as an additional alloying component of 0.005–0.7% by weight. Furthermore, Si contents of 3–15% by weight and Fe contents of up to 5% by weight may be present in the coating. The minimum alkaline earth or transition metal for the protective layer is Mg, preferably used here in contents of 0.1 - 0.5% by weight, although calcium, strontium, sodium, or barium are also considered alternatives or additional elements. The protective Al-based layer can be applied to the steel substrate by hot-dip coating, also known technically as hot-dip aluminizing, or by a gas separation process, for example, the well-known PVD (Physical Vapor Deposition) or CVD (Chemical Vapor Deposition). The prior art described above does not mention any special requirements regarding the application of the corrosion protection coating to the steel substrate, which consists of MnB steel. When a coated plate is conventionally heated under a normal atmosphere for 360–800 seconds to a temperature of 900°C, the presence of the alkaline earth or transition metal in the coating results in minimal hydrogen absorption in the steel substrate, thus minimizing the risk of hydrogen embrittlement. However, in practical use it is observed that, despite this success, the components formed from the flat steel products produced in the manner described above have optimized strengths, but cannot always meet the increasingly high requirements demanded of the behavior of sheet metal components manufactured from such flat steel products in the case of resistance welding and the adhesion of organic layers, such as paint and the like, onto such sheet metal components. Document DE 10 2017 210 201 Al also deals with a method for manufacturing an aluminum-based steel component with an anti-corrosive metallic coating. For this purpose, a flat steel product is provided, consisting, by weight, of 0.15 to 0.50% C, of 0.50 to 3.0% of Min, 0.10 to 0.50% of Si, 0.01 to 1.00% of Cr, up to 0.20% of Ti, up to 0.10% of Al, up to 0.10% of P, up to 0.1% of Nb, up to 0.01% of N, up to 0.05% of S and up to 0.1% of B, the remainder being Fe and unavoidable impurities, and is coated with an Al coating consisting, by weight, of 3 to 15% of Si, 1 to 3.5% of Fe, up to 0.5% of alkali and / or alkaline earth metals, the remainder being Al and unavoidable impurities. The supplied metal sheet is annealed in a furnace at a specific temperature and for a specific period of time, which are linked by a parameter calculated according to a complex formula. Depending on the residence time in the furnace and the temperature, a so-called interdiffusion zone must form at the transition between the substrate and the coating. In this zone, no martensitic structure is produced during press hardening, but it should also not be attributed to the Al coating.This interdiffusion zone extends from the center of the flat steel product, from the thickness beyond which there is no longer a martensitic structure in the component, to the thickness beyond which the iron content of the Al coating is continuously < 85 wt% and the Al content is continuously > 10 wt%. This prior art also provides no information on how the interdiffusion zone could be designed in detail, nor instructions on how the formation and composition of the interdiffusion zone could be specifically controlled with respect to certain surface properties of the coating. Instead, it emphasizes considerations for improving the deformation behavior of the Al coating, particularly the achievable bending angle. BRIEF DESCRIPTION OF THE INVENTION In this context, the objective arises to indicate a method that allows forming sheet metal components from a flat steel product of the type explained above, which meet the most demanding requirements in terms of their weldability and which, therefore, have optimal conditions for a coating with an organic layer, in particular for paint. To achieve this objective, the invention proposes that at least the steps of the method indicated in claim 1 be completed during the manufacture of the sheet metal components. Not to mention that, in carrying out the method in accordance with the invention, the person skilled in the art not only carries out the steps of the method mentioned in the claims and explained herein, but also carries out all the other steps and activities that are usually carried out in the practical application of such methods in the prior art if the need arises. The advantageous embodiments of the invention are defined in the dependent claims and, like the general concept of the invention, are explained in detail below. In a method according to the invention for manufacturing a sheet metal component from a flat steel product that is provided with a protective coating against corrosion, the following work steps are therefore at least carried out: a) providing a flat steel product manufactured from a steel that (by weight) is composed of 0.05 - 0.5% C, 0.5 - 3% Mn, 0.06 - 1.7% Si, up to 0.06% P, up to 0.01% S, up to 1.0% Al, up to 0.15% Ti, up to 0.6% Nb, up to 0.01% B, up to 1.0% Cr, up to 1.0% Mo, the total Cr and Mo content being at most 1.0%, up to 0.2% Ca, up to 0.1% V, and the remainder being unavoidable iron and impurities; b) Annealing of the flat steel product in a continuous furnace having four zones A, B, C, D, through which the flat steel product passes successively and in which the flat steel product is annealed under an annealing atmosphere composed in each case of 0.1-15 vol. of hydrogen and as a remainder of nitrogen, as well as technically unavoidable impurities, with a dew point temperature TPa, TPb, TPc, TPd at an annealing temperature GTa, GTb, GTc, GTd: ML / a / ZUZ 1 4001 Zone Dew Point Temperature TP Annealing Temperature GT A -10°C < TP a < -25°C 800°C < GTa < 950°C CB -27°C <TPb<-41°C 800°C < GT b<930°C C -30°C < TP c < -80°C 800°C < GT c<950°C D -30°C < TP D < -20°C 750°C < GTd < 950°C c) applying a corrosion protection coating to the flat steel product obtained in work step b), wherein the corrosion protection coating is composed (by weight) of up to 15% Si, up to 5% Fe, in total between 0.1 and 5% of at least one alkaline earth or transition metal and the remainder of Al; d) optionally: roll the flat steel product provided with the anti-corrosive coating; e) optionally: separate a plate from the flat steel product; f) heating the flat steel product or plate to a hot forming temperature higher than the Ac3 temperature of the steel of the flat steel product and not exceeding 1,000°C for a holding time sufficient to introduce into the flat steel product or plate an amount of thermal energy Js greater than 100,000 kJs and at most 800,000 kJs; g) hot forming of the flat steel product heated to the hot forming temperature or of the board heated to the hot forming temperature into the sheet metal component; h) cool at least one section of the component at a cooling rate sufficient to generate a hardening structure in the section of the sheet metal component. The invention is based on the knowledge that, for the behavior of sheet metal components provided with an aluminum-based (Al-based) corrosion protection coating, in the case of resistance welding and for the adhesion of an organic coating, particularly paint, to such sheet metal components, not only is the composition of the boundary layer between the corrosion protection coating and the ambient atmosphere important, but, in particular, parameters such as the roughness and conductivity of the overall layer also play a decisive role. In this case, the annealing method according to the invention (step b)) prior to applying the corrosion protection coating (step c)) creates the conditions for the component processed according to the invention to have an optimally homogeneous corrosion protection coating. Thus, components produced according to the invention typically have a corrosion-protective coating, which consists of a plurality of layers of different compositions. By guiding the dew point and annealing temperature according to the invention during annealing in the continuous annealing furnace to prepare for the subsequent application of the corrosion-protective coating, a significant reduction in the porosity of the coating is achieved. By using the annealing parameters selected according to the invention during the annealing (work step b)) prior to coating, pure iron (Fe) is present on at least 70% of the surface of the finished annealed flat steel product. This results in good adhesion of the subsequently applied Al-based coating by forming an iron-aluminum layer (Fe-Al layer) at the transition between the steel substrate and the anti-corrosion coating. Furthermore, the iron reaches the layer in sufficient quantity and with a homogeneous distribution, thus improving the layer's conductivity and optimizing its performance during resistance welding. If the flat steel product supplied in working step b) is already a blank directly suitable for shaping into the component, working step e) may be omitted. If, on the other hand, the flat steel product supplied is a steel strip or a larger steel sheet, a board of suitable size is cut from it in working step e). The annealed and coated flat steel product according to the invention (working steps b), c)) or the separate board (working step e)) is heated to the hot forming temperature (working step f)) for hot forming (working step g)). The iron already present in the homogeneous boundary layer of the corrosion-protective coating can diffuse uniformly into the coating without any significant defects. At the same time, the alkaline earth or transition metal provided according to the invention in the corrosion-protective coating diffuses to the surface due to its affinity for oxygen and forms an oxide layer there.Due to their comparable atomic size, iron atoms can exchange places in a 1:1 ratio with alkaline earth or transition metal atoms and are thus incorporated into the metallic lattice. Consequently, at most, a negligible number of defects can also occur due to the diffusion of alkaline earth or transition metal atoms. As a result of the defect reduction achieved according to the invention, these defects cannot accumulate in pores in the corrosion-protective coating of a component according to the invention. Therefore, a component according to the invention is characterized by a significantly reduced number of pores compared to conventionally produced components, for example, those produced according to the sample in EP 2 086 755 B1. The effects used by the invention are produced in a particularly reliable way if the additional alkaline earth or transition metal present is magnesium (Mg), hence if Mg is present alone or in combination with other elements belonging to the group of alkaline earth or transition metals in the content provided in accordance with the invention in the corrosion protection coating of a flat steel product processed in accordance with the invention. The method according to the invention is suitable for manufacturing flat steel product components with a wide range of thicknesses. Thus, flat steel products with a thickness of 0.6–7 mm can be processed using the method according to the invention. The production of the flat steel products provided in step a) can be carried out in any manner known in the prior art. The method according to the invention is particularly suitable for processing flat steel products with a thickness of 0.8–4 mm, in particular 0.8–3 mm. Flat steel products with thicknesses greater than 3 mm are typically processed in the hot-rolled condition, while thinner sheets are typically provided in the cold-rolled condition. In work step a), flat steel products may also be provided for the method according to the invention, having obtained different thicknesses by flexible or partial lengthwise and / or widthwise rolling. Likewise, in work step a) for the method according to the invention, flat steel products composed of different raw sheets welded together, or of flat steel products of similar composition and steel strips welded together to form the flat steel product to be manufactured, may be provided for the process according to the invention. ML / a / ZUZ 1 4001 process. The flat steel product provided according to the invention in each case consists of a steel having a composition typical for MnB steels. Such steels typically have a yield strength of 250-580 MPa and a tensile strength of 400-720 MPa in the as-supplied condition. Thus, a flat steel product supplied in accordance with the invention is composed of - 0.05 - 0.5% by weight of carbon (C), where the C content is preferably 0.07 - 0.4% by weight, - 0.5 - 3% by weight of manganese (Mn), the Mn content being preferably 0.8 - 2.5% by weight, in particular 1.0 - 2.0% by weight, - 0.06 - 1.7% by weight of silicon (Si), the Si content being preferably 0.06 - 1.1% by weight, in particular 0.06 - 0.9% by weight, - up to 0.06 of phosphorus (P), with the maximum P content being 0.03% by weight - up to 0.01% by weight of sulfur (S), - up to 1.0% by weight of aluminum (Al), the Al content preferably not exceeding 0.5% by weight, in particular not exceeding 0.1% by weight, - up to 0.15% by weight of titanium (Ti) - up to 0.6% by weight of niobium (Nb), where the Nb content is preferably up to 0.1% by weight, - up to 0.01% by weight of boron (B), the B content preferably being up to 0.005% by weight - up to 1.0% by weight of chromium (Cr), the Cr content preferably being up to 0.5% by weight, in particular up to 0.2% by weight, - up to 1.0% by weight of molybdenum (Mo), the Mo content preferably being up to 0.5% by weight, in particular up to 0.2% by weight - where for the %Cr content of Cr and the %Mo content of Mo the following applies: %Cr + %Mo < 1 % by weight, - optionally up to 0.2% by weight, in particular up to 0.1% by weight, of calcium (Ca), - optionally up to 0.1% by weight of vanadium (Va) and the remainder iron and unavoidable impurities. Due to their property profile, particularly their potential for developing high strengths in the hot-formed and cooled finished component, flat steel products, which are known to consist of 0.07–0.4 wt% C, 1.0–2 wt% Mn, 0.06–0.4 wt% Si, up to 0.03 wt% P, up to 0. In practice, steels with a total Cr and Mo content of at most 0.5 wt%, while the remainder consists of iron and unavoidable impurities, are of particular interest. This includes steels already in mass production that are composed of 0.07–0.4 wt% C, 1.0–1.5 wt% Mn, 0.3–0.4 wt% Si, up to 0.03 wt% P, up to 0.01 wt% S, up to 0.05 wt% Al, up to 0.15 wt% Ti, up to 0.6 wt% Nb, up to 0.005 wt% B, up to 0.5 wt% Cr, and up to 0.5 wt% Mo, where the total Cr and Mo content is at most 0.5 wt% and the remainder consists of iron and unavoidable impurities. These composite steels achieve tensile strengths of up to 2,000 MPa after hot forming and quenching. As already mentioned, the annealing (working step b)) carried out in four successive uninterrupted steps A, B, C, D on the respectively processed flat steel product produces a surface that is largely covered, i.e., up to at least 70%, and in particular at least 80% or at least 90%, by pure Fe. For this purpose, in zones A-D of the continuous annealing furnace used according to the invention, specially adjusted dew point and annealing temperatures are set in each case. The annealing carried out in the working stage b) in zones A - D takes place in each case under an annealing atmosphere containing 0.1-15% vol. of hydrogen, the remainder of which consists of nitrogen and unavoidable impurities in each case, where the total impurities are typically at most 5% vol., in particular at most 4% vol. or preferably at most 3% vol. All information provided below and in the claims for annealing temperatures GTa, GTb, GTc and GTd refers to the average temperature of the furnace chamber during strip performance. Before entering zone A of the continuous furnace operated according to the invention, there is a wide range of oxide products on the surface of the flat steel product provided according to the invention, which have a negative effect on the coating quality and, in particular, on the formation of pores in the coating. Through continuous annealing according to the invention, these oxides are converted so that, in the technical sense, only Fe is present on the surface of the flat steel product after annealing. By adjusting the dew point temperature (TPa) to -10°C to -25°C and the annealing temperature (GTa) to 800–950°C in zone A of the continuous furnace, the oxides present in the flat steel product are coated with iron oxides. For particularly selective results, the annealing temperature (GTa) can be set at 810–940°C and the dew point temperature (TPa) at -15–-25°C in zone A of the continuous furnace. In zones B and C, the iron oxides are reduced, so that iron is present on the surface after zone C. In zone B, the dew point temperature TPb of the prevailing annealing atmosphere is then reduced to -27° to -41°C and the annealing temperature GTb is maintained at 800-930°C, which has been shown to be particularly reliable in terms of the desired effect if the annealing temperature GTb in zone B of the continuous furnace is 800-900°C in the case of annealing completed in work stage b). In zone C, the dew point temperature TPc of the prevailing annealing atmosphere is further reduced to -30°C to -80°C, and the annealing temperature GTc is maintained at 800–950°C to complete the reduction of iron oxide to iron. This effect can be achieved particularly reliably if the annealing temperature GTc is 800–920°C and the dew point temperature TPc is -30°C to -50°C, as in the case of annealing completed in working phase b) in zone C of the continuous furnace. In zone D, the dew point temperature TPd of the prevailing annealing atmosphere is then increased to -30°C to -20°C, and the annealing temperature GTd is maintained at 750–950°C to temper the flat steel product so that, on the one hand, recrystallization can occur, and on the other hand, the previously achieved pure iron surface is preserved. This effect can be achieved particularly reliably if the annealing temperature GTd is 780–930°C in the case of annealing carried out in working phase b) in zone D of the continuous furnace. The lambda value λ describes the ratio between the masses of air and fuel introduced into the continuous furnace and in the annealing atmosphere maintained in zones A - D of a continuous furnace used in accordance with the invention is typically 0.95 - 1.1 in the case of annealing completed in the working step b) of the method in accordance with the invention. The prerequisite for the effects achieved according to the invention is the presence of at least one alkaline earth or transition metal in the aluminum (Al)-based corrosion protection coating applied after annealing according to the invention (step b). Thus, in the coating of a flat steel product processed according to the invention, after applying the corrosion protection coating (step c)) and before heating for hot forming (step f)), at least 0.1–5 wt% of at least one alkaline earth or transition metal is present, with the remainder being Al and unavoidable impurities. In this case, the alkaline earth or transition metal content is at least 0.1–5 wt%.Contents of 11% by weight have proven particularly favorable in terms of reliability, allowing the positive effects of the presence of at least one alkali or transition metal in the coating applied according to the invention to be fully utilized. If the alkali or transition metal content exceeds 5% by weight, increased oxide formation would occur in the melting crucible, reducing surface quality. Excessive oxide formation would also occur during hot forming, which, on the one hand, would promote the fission of water into hydrogen and oxygen, consequently creating a risk of increased hydrogen penetration into the steel. On the other hand, the thicker oxide layer could lead to greater contamination of the forming tool. To reliably avoid this effect, the alkaline earth or transition metal content in the anti-corrosion coating applied in the working phase c) can be limited to a maximum of 1.5% by weight, in particular to a maximum of 0.6% by weight. The content of alkaline earth or transition metals in the corrosion protection coating applied in working phase c) is therefore, in particular, from 0.11 to 1.5%. ML / a / ZUZ 1 4001 by weight or, in particular, from 0.11 to 0.6% by weight. As already mentioned, Mg from the alkaline earth or transition metal group has proven to be particularly suitable for the purposes according to the invention, and may be present in the coating applied according to the invention alone or in combination with other alkaline earth or transition metals, such as beryllium, calcium, strontium, and barium, in order to achieve the desired effects according to the invention. Optionally, silicon (Si) can also be present in the layer, which is applied in working step c), at contents of up to 15 wt%, particularly up to 11 wt%, in order to promote the formation of an iron-aluminum layer that adheres well to the iron surface established in working step b) and therefore occupies at most one-third of the total layer thickness. If the Si content is too high, an excessively large alloy layer thickness would result, which in turn could lead to a loss of adhesion. Si contents of at least 3 wt%, particularly at least 8.5 wt%, are especially favorable in this respect, so that with a Si content of 3–15 wt%, particularly 3–11 wt%, and especially 8.5–11 wt%, the positive influences of Si can be used particularly reliably in practice. Furthermore, in the coating applied in step c), Fe can also be optionally present in contents of up to 5% by weight, particularly up to 4% by weight, and especially up to 3.5% by weight. The iron would be fixed in the coating at this order of magnitude because this is the saturation value of an aluminum melt in the temperature range of 650–720°C. By specifically adding iron to the melt, the risk of dissolution of ferrous components from the melting crucible that come into contact with the melt can be reduced. In this respect, Fe contents of at least 1% by weight are particularly advantageous, so that in practice the positive influences of Fe can be reliably utilized with an Fe content of 1–5% by weight, particularly 1–4% by weight, and especially 1–3.5% by weight. The anti-corrosion coating can be applied in step c) of the method according to the invention in any known way. In this case, hot-dip aluminizing is particularly suitable, where the respective flat steel product is guided through a suitably heated and compounded molten bath according to the specifications of the invention. This hot-dip coating is especially suitable for flat, strip-shaped steel products with a thickness of up to 3 mm. For greater thicknesses, one of the vapor deposition (PVD, CVD) processes mentioned at the beginning can also be used to apply the anti-corrosion coating. The load of a corrosion protection coating applied according to the invention in working step c) is typically 30–100 g / m², in particular 40–80 g / m² per side. The load on both sides of the coating is therefore 60–200 g / m² in total. After applying the anti-corrosion coating (work step c)), the flat steel product with the corresponding coating can optionally undergo rolling (work step d)) to adjust the mechanical properties of the flat steel product, to adjust its surface roughness, or to homogenize it. The rolling degrees set for this purpose (rolling degree = (thickness before rolling - thickness after rolling) / (thickness before rolling)) are typically from 0.1 to 5%. After applying the anti-corrosion coating (work step c)) or after optionally carrying out hot rolling (work step d)), a plate is separated, if necessary, from the flat steel product in a manner known to itself, the dimensions of which are adapted in a known manner to the dimensions of the sheet metal component to be hot-formed from it (work step e)). The flat steel product or sheet metal is then heated in working stage f) to a hot forming temperature that is higher than the Ac3 temperature of the flat steel product and does not exceed 1000°C; in particular, it is at least equal to Ac3 + 50°C and at most 980°C, where hot forming temperatures of 820–950°C have proven to be particularly advantageous. The flat steel product is held at this temperature until a sufficient amount of heat is introduced into the flat steel product or into the separate sheet.The holding time and annealing temperature required in each case can be estimated on the basis of the condition that the amount of thermal energy Js introduced into the flat steel product or cardboard at work stage f) must be greater than 100,000 kJs and at most 800,000 kJs, wherein Js can be calculated according to the following known equation. Js [kJs] = [(T2-T1) xcxtxm] / 1000; with T2: Final temperature of the component at the end of heating in K TI: Initial temperature of the component at the beginning of heating in K c: Heat capacity of steel (normally 460 J / kgK) t: Holding time of the flat steel product or board at the final temperature in K sm: Mass of the flat steel product or board in kg. Heating can be carried out in any suitable manner. If a conventional continuous furnace is used, where the flat steel product or board is heated by radiant heat, the appropriate holding time is typically 100–900 s, preferably 180–720 s, and particularly 240–600 s. If a hot forming temperature of 850–930°C is selected, holding times of 180–600 s, and particularly 240–600 s, are generally sufficient in practice. As an alternative to using a continuous furnace, heating can also be carried out in a conventional chamber furnace, for example. The heating of the flat steel product or board can also take place in two stages, in a manner also known per se, to initially achieve a preheating of the corrosion protection coating and subsequently bring the flat steel product or board to the respective hot forming temperature. The board heated to the hot forming temperature or the flat steel product heated to the hot forming temperature is introduced into the hot forming tool in a transfer time typically less than 15 seconds, in particular less than 10 seconds, and then the component is hot formed there (working stage g)). Subsequently, or simultaneously, at least one section of the resulting component is cooled in a controlled and known manner to generate the desired structure in the corresponding section. The cooling rates required for this are typically from 20 to 500 K / s, with rates above 30 K / s, and particularly above 50 K / s, being especially practical. Cooling at least one section also includes, of course, the possibility of cooling the entire component in the manner described above to generate hardening structures throughout the component. According to the method of the invention, a sheet metal component is produced from a flat steel product, the steel substrate of which consists of a steel (by weight percent) comprising 0.05–0.5% C, 0.5–3% Mn, 0.06–1.7% Si, up to 0.06% P, up to 0.01% S, up to 1.0% Al, up to 0.15% Ti, up to 0.6% Nb, up to 0.01% B, up to 1.0% Cr, up to 1.0% Mo, the total Cr and Mo content being at most 1.0%, up to 0.2% Ca, in particular up to 0.1% V, and the remainder being iron and unavoidable impurities, and which is coated with a corrosion-protective coating comprising (by weight percent) up to 15% Si, up to 5% of Fe, totaling 0.1-5% of at least one alkaline earth or transition metal and the remainder being Al and unavoidable impurities, wherein the anti-corrosion coating layer adjacent to the steel substrate is an interdiffusion layer formed by ferrite with an Al content of up to 50% by weight, in particular at least 1% by weight of Al, wherein in a cross-section of the interdiffusion layer, the proportion of the surface covered by pores with a diameter > 0.1 pm is less than 10%, in particular less than 5%, preferably less than 3%, and wherein the surface covered by pores in the interdiffusion layer is < 300 pm², in particular less than 200 pm², particularly preferably less than 100 pm² over a measuring length of 500 pm. The thickness of the alloy layer here is 1-30 pm, preferably 2-20 pm, in particular 4-16 pm. BRIEF DESCRIPTION OF THE FIGURES The invention is explained in more detail below using exemplary embodiments. Figure 1 shows a cross-section of a steel sheet from a sheet metal component manufactured according to the invention by hot forming at a magnification of 500x. The cross-section was conventionally prepared by etching with 3% Nital to clarify the layered structure present in the steel sheet. Fig. 2 shows a schematic representation of the cross section in accordance with Fig. 1. DETAILED DESCRIPTION OF THE INVENTION Consequently, the corrosion protection coating K formed on the steel substrate S comprises an interdiffusion layer D directly connected to the steel substrate S, consisting substantially of a mixed alpha crystal (i.e., ferrite) with a higher Al content. Fe2Al15 is still present here in phases. The interdiffusion layer D is characterized by being homogeneously and uniformly formed and by being practically pore-free. In the direction of the free surface O of the anti-corrosive coating K, a first Si Si rich layer has formed on the diffusion layer D. At the boundary between the diffusion layer D and the Si Si rich layer, the P1 pores are present in the diffusion layer D in small numbers and widely separated from each other. In the direction of the free surface O in the Si-rich layer, a first intermediate layer Zi has formed, consisting of aluminum iron, with the majority being aluminum. Traces of Si, alkaline earth metals, and / or transition metals, as well as unavoidable impurities, may also be present in the Si layer. The intermediate layer Zi is pore-free. In the direction of the free surface O of the intermediate layer Zi there is a second Si-rich layer S2. In the direction of the free surface O of the Si-rich layer S2, a second intermediate layer Z2 forms. The Z2 layer is also composed of iron-aluminum, with aluminum being the majority and alkaline earth and / or transition metals possibly also present. Traces of Si, as well as unavoidable impurities, may also be present. The intermediate layer Z2 is also pore-free. The second intermediate layer Z2 is coated on its side facing the free surface O with an oxide layer OX, consisting substantially of aluminum, silicon, and alkaline earth and / or transition metal oxides. The thickness of the oxide layer can average up to 1.5 pm in a hot-formed component. Crater-like pores P2, open to the environment, have formed on the surface of the oxide layer OX that forms the free surface O of the anti-corrosion coating K, in small numbers and at a considerable distance from one another. By way of comparison, a component was formed from a flat steel product that was coated with an AISi coating in accordance with the prior art sample described in EP 2 086 755. Its coating consisted of (by weight) 9.5% Si, 3.5% Fe and, as the remainder being aluminum and unavoidable impurities, was therefore free of alkaline earth or transition metals of the type added in accordance with the invention. The steel substrate of the flat steel product had (by weight) 0.224% C, 0.25% Si, 1.16% Mn, 0.014% P, 0.002% S, 0.039% Al, 0.0034% N, 0.2% Cr, 0.03% Ti and 0.0026% B. Before applying the metallic coating and forming the flat steel product, the processed flat steel product for comparison was subjected to an annealing treatment in a continuous furnace with four zones in which the dew point temperatures TP and the annealing temperatures GT indicated in Table 6 were set. The air ratio λ in the continuous furnace was 0.98. A five-layer structure of the corrosion protection coating was also created for the conventionally produced component for comparison. However, compared to the number of pores in the layer of the conventionally produced component for comparison, in the component produced according to the invention, the number of P2 pores in the oxide layer OX was reduced by at least 25% and the number of P1 pores in the diffusion layer D by at least 40%, compared to the pores present in the corresponding layers of the corrosion protection coating of the conventionally produced component for comparison. The surface area covered by P1 pores was 300 pm² after a furnace residence time of 600 s with a measured length of 500 pm in layer D. Reducing the number of pores in P2 leads to fewer paint craters and improves adhesion and weldability. P2 pores have openings facing the atmosphere, a few nanometers in size. If a component undergoes further processing after hot forming, as is typical in automobiles, it will be subjected to dip coating, in addition to several cleaning steps. In this case, contact with water-based solutions is unavoidable. During cleaning, water can penetrate the P2 pores of the coating because the surfactants added to the cleaning water improve wetting and significantly reduce the surface tension of the water. Water can also penetrate the P2 pores opened during the dip coating process. In this specific case, the cleaning water also causes paint particles to separate, as they cannot penetrate the P2 pores due to the size of the openings.The water present in the P2 pores reaches its boiling point when the paint layers bake, resulting in vapor phases that, in a sort of delayed boiling, escape explosively through the paint into the surrounding environment. This reaction creates what are known as paint craters, which, in addition to their visual impact, also significantly reduce the paint's corrosion protection. In the case of aluminum-based coatings, in particular, corrosion and paint infiltration can occur at these points. The resulting red rust, which forms due to the coating's high iron content and is visually prominent, is especially problematic for subsequent processors. Furthermore, on a surface with many open P2 pores, adhesives cannot penetrate the P2 pores due to their higher viscosity. This can result in incomplete surface coverage with adhesive. Additionally, cavities form in the pore area, further compromising adhesion. The P2 pores present in the OX layer also cause changes in the current paths in the material during resistance spot welding, which negatively affects weldability. In the case of a high pore count, there is also an increased surface area where water can diffuse during oxidation in the hot forming process. This allows diffuse hydrogen to penetrate the material, which is known to increase the risk of hydrogen-induced cracking. By minimizing the frequency with which P2 pores occur during the manufacture of a sheet metal component according to the invention, the risks associated with pore formation in conventionally produced components can be effectively reduced. Reducing the number of Pl pores in the diffusion layer D also leads to an increase in the transferable strength of adhesive bonds and an improvement in weldability. The pores in P2 represent cavities within the corrosion protection coating K. If the number of pores is too high, there is a risk that the corrosion protection coating K will break down in the boundary region between the diffusion layer D and the first Si-rich layer, resulting in early failure of the adhesive bond. By reducing the number of pores Pl according to the invention, the surface area over which the adhesive bond forces are transferred is increased by more than 60%, and the risk of delamination fracture is correspondingly reduced. To demonstrate the effect of the invention, steel sheets 1.5 mm thick each and conventionally cold-rolled from six ST1 - ST6 steels, whose compositions are indicated in Table 1 (work step a) of the method according to the invention). The steel sheets thus obtained were subjected in nine tests VI - V9 in each case to continuous annealing G1, G2 or G3 in a continuous furnace, which had four consecutive zones A, B, C, D. Table 2 shows the dew point temperatures TPa - TPd established in zones A - D for the G1 - G3 variants of the annealing, the annealing temperatures GTa - GTd as well as the hydrogen content H2 and the nitrogen content N2 of the respective annealing atmosphere, the remainder of which was formed by technically unavoidable impurities (working step b) the method in accordance with the invention). The annealed samples are each conventionally coated with an Al-based Z1-Z5 anti-corrosion coating with an AG filler. The compositions of the Z1-Z5 corrosion protection coatings are indicated in Table 3 (working step c) of the method according to the invention). The samples, each provided with one of the anti-corrosion coatings Z1-Z5, were heated in each case in tests VI-V9 in the continuous furnace to a hot forming temperature TWU, at which they were held for a holding time twu (working stage f) of the method according to the invention. The ST1-ST6 steel, from which the samples used in tests VI-V9 are made, the annealing variants G1-G3 used in tests VI-V9, the compositions Zi-Z5 of the anti-corrosion coatings produced in tests VI-V9 and their respective AG charges, as well as the hot forming temperatures Twu and the holding times twu selected in tests VI-V9 are indicated in Table 4. The samples heated in this manner were removed from the continuous furnace after a transfer time of 3–7 s in each case and placed in a conventional hot forming tool where they were hot formed into a component. Cooling then took place at 270 K / s in each case to room temperature (working steps g) and h) of the method according to the invention. From the components obtained in tests VI-V9, three cross-sections of known shape were produced and etched with 3% Nital to clarify the layer structure. Illustrations of the cross-sections were generated at 500x magnification, as shown in Figure 1. In the respective illustration, the Pl and P2 pores present in the OX and D layers were counted over a section with a length of 550 pm. The arithmetic mean was calculated from the count results determined for the three cross-sections of one sample in each case. This arithmetic mean of the Pl and P2 pore counts was compared with the comparative values ​​determined in the same way for a control sample. The relative reduction in the number of Pl and P2 pores resulting from this comparison and achieved by the invention is shown in Table 5. Table 5 also shows the proportion of paint craters on the total surface of the respective sample, the reduction of the delamination zone, and the weld region determined according to the SEP 1220-2 steel-iron test sheet. Weld regions above 1 kA have been classified as OK. ML / a / ZUZ 1 4001 Table 1 Steel C Si Mn PS Al Nb Ti BA 0.08 0.33 0.95 0.025 0.02 0.013 0.09 0.01 0.005 B 0.23 0.38 1.3 0.02 0.007 0.013 - 0.03 0.004 C 0.38 0.37 1.38 0.02 0.008 0.013 - 0.1 0.005 D 0.2 0.35 1.35 0.02 0.008 0.012 - 0.02 0.004 E 0.14 0.25 1.07 0.1 0.001 0.08 0.025 0.01 0.002 F 0.24 0.3 1.3 0.022 0.008 0.012 - 0.02 0.004 Information in % by weight, the remainder being Fe and unavoidable impurities Table 2 Annealing Lambda Value Dew Point Temperature TP Annealing Temperature GT Annealing Atmosphere GA [°C] [°C] [vol%] ABCDABCD H2 N2 G1 1.05 -25 -40 -40 -20 880 880 850 800 7 91 G2 1.1 -20 -40 -45 -25 890 890 830 800 10 87 G3 0.95 -12 -30 -47 -22 890 900 900 820 5 92 Table 3 Corrosion protection coating Mg Si Fe Zi 0.3 9.5 3 z2 0.5 8 3.5 z3 0.1 10 3 Z4 2 8 2 z5 0.8 8 3 Information in % by weight, remaining Al and unavoidable impurities Table 4 Test Annealed Steel Corrosion Protection Coating AG Twu twu [g / m2] [°C] [S] VI A G1 z3 69 920 300 V2 B G2 z2 70 920 180 V3 C G1 z3 75 925 360 V4 D G3 z5 65 920 420 V5 E G1 Zi 70 900 300 V6 F G3 Z4 71 920 360 V7 H G2 Zi 65 925 360 V8 B G1 z3 72 920 300 V9 ​​D G3 z2 71 925 300 Table 5 Test Pore Reduction P1 Pore Reduction P2 Paint Craters Decrease in Fracture Area due to Delamination Weld Region [%] [kA] VI 25 50 8.4 67 1.1 V2 30 75 10.5 70 1 V3 28 43 9.5 62 1.2 V4 35 52 10.9 60 1.1 V5 33 60 11.6 65 1.2 V6 25 58 7.8 63 1 V7 37 57 12.5 60 1.1 V8 33 70 11.6 70 1 V9 28 65 9.5 69 1.2 Table 6 TP TG zone [°C] [°C] A -30 750 B -23 780 C -25 780 D -35 740 ΙνΙΛ / α / ZυΖΊ / U 14001

Claims

1. A method for manufacturing a sheet metal component from a flat steel product provided with an anti-corrosive coating, characterized in that it comprises the following work steps: a) providing a flat steel product produced from a steel which (by weight) is composed of 0.05 - 0.5% C, 0.5 - 3% Mn, 0.06 - 1.7% Si, up to 0.06% P, up to 0.01% S, up to 1.0% Al, up to 0.15% Ti, up to 0.6% Nb, up to 0.01% B, up to 1.0% Cr, up to 1.0% Mo, the total content of Cr and Mo being at most 1.0%, up to 0.2% Ca, up to 0.1% V, and the remainder being iron and unavoidable impurities; b) Annealing of the flat steel product in a continuous furnace having four zones A, B, C, D, through which the flat steel product passes successively and in which the flat steel product is annealed under an annealing atmosphere composed in each case of 0.1-15 vol.of hydrogen and as a residue of nitrogen, as well as technically unavoidable impurities, with a dew point temperature TPa, TPb, TPc, TPd at an annealing temperature GTa, GTb, GTc, GTd: Zone Dew point temperature TP Annealing temperature GT A -10°C < TP a < -25°C 800°C < GTa < 950°C CB -27°C <TPB<-41°C 800°C < GT B< 930°C C -30°C < TP c < -80°C 800°C < GT c<950°C D -30°C < TP D < -20°C 750°C < GTd < 950°C c) aplicar un revestimiento de protección contra la corrosión al producto de acero plano obtenido en la etapa de trabajo b), en donde el revestimiento de protección contra la corrosión está compuesto (en peso) por hasta un 15% de Si, hasta un 5% de Fe, en total entre un 0.1 and 5% of at least one alkaline earth or transition metal and the remainder being Al and unavoidable impurities; d) optionally: rolling the flat steel product provided with the anti-corrosion coating, e) optionally: separating a plate from the flat steel product, f) heating the flat steel product or the plate to a hot forming temperature higher than the Ac3 temperature of the steel of the flat steel product and not exceeding 1000°C for a holding time sufficient to introduce an amount of thermal energy Js greater than 100,000 kJs and at most 800.000 kJs in the flat steel product or plate; g) hot forming of the flat steel product heated to the hot forming temperature or of the plate heated to the hot forming temperature in the sheet metal component; h) cooling at least one section of the component at a cooling rate sufficient to generate a hardening structure in the section of the sheet metal component.

2. The method according to claim 1, further characterized in that the thickness of the flat steel product provided in the working stage a) is 0.6 - 7 mm.

3. The method in accordance with any of the preceding claims, further characterized in that the annealing temperature GTa is 810 - 940°C and the dew point temperature TPa is -15°C to -25°C in zone A of the continuous furnace in the case of annealing carried out in work stage b).

4. The method in accordance with any of the preceding claims, further characterized in that the annealing temperature GTb is 800 - 900°C in zone B of the continuous furnace in the case of annealing completed in work stage b).

5. The method in accordance with any of the preceding claims, further characterized in that the annealing temperature GTc is 800 - 920°C and the dew point temperature TPc is -30°C to -50°C in zone C of the continuous furnace in the case of annealing completed in work stage b).

6. The method in accordance with any of the preceding claims, further characterized in that the annealing temperature GTd is 780 - 930°C in zone D of the continuous furnace in the case of annealing completed in work stage b).

7. The method in accordance with any of the preceding claims, further characterized in that the lambda value of the annealing atmosphere maintained in zones A - D is 0.95 - 1.1 in the case of annealing completed in work stage b).

8. The method in accordance with any of the preceding claims, further characterized in that the Si content of the anti-corrosive coating applied to the flat steel product in the working stage c) is at least 3% by weight.

9. The method in accordance with any of the preceding claims, further characterized in that the Fe content of the corrosion protection coating applied to the flat steel product in the working stage c) is at least 1% by weight.

10. The method in accordance with any of the preceding claims, further characterized in that the corrosion protection coating applied to the flat steel product in the working stage c) contains in total at least 0.11% by weight of alkaline earth or transition metals.

11. The method in accordance with any of the preceding claims, further characterized in that the alkaline earth or transition metal content in the corrosion protection coating applied to the flat steel product in the working stage c) is in total a maximum of 0.6% by weight.

12. The method in accordance with any of the preceding claims, further characterized in that the corrosion protection coating applied to the flat steel product in the working stage c) contains magnesium as the at least one alkaline earth or transition metal.

13. The method in accordance with any of the preceding claims, further characterized in that the load of the anti-corrosive coating applied to the flat steel product in the work stage c) is 30 - 100 g / m2 per coated side of the flat steel product.

14. The method in accordance with any of the preceding claims, further characterized in that the application of the anti-corrosive coating in the work stage c) takes place by hot-dip coating.

15. The method in accordance with any of the preceding claims, further characterized in that the heating of the flat steel product or board in the working stage f) takes place in a continuous furnace by radiant heat and the holding time is 100 - 900 s.

16. A sheet metal component characterized in that it is manufactured from a flat steel product, the steel substrate of which is formed from a steel that (in wt%) is composed of 0.05–0.5% C, 0.5–3% Mn, 0.06–1.7% Si, up to 0.06% P, up to 0.01% S, up to 1.0% Al, up to 0.15% Ti, up to 0.6% Nb, up to 0.01% B, up to 1.0% Cr, up to 1.0% Mo, wherein the total Cr and Mo content is at most 1.0%, up to 0.2% Ca, up to 0.1% V, and the remainder being iron and unavoidable impurities, and which is coated with a corrosion protection consisting of (by weight) up to 15% Si, up to 5% Fe, in total 0.1-5% by weight of at least one alkaline earth or transition metal and the remainder being Al and unavoidable impurities, wherein the anti-corrosion coating layer adjacent to the steel substrate is an interdiffusion layer (D) formed by ferrite with an Al content of up to 50% by weight, wherein in a cross-section of the interdiffusion layer (D), the proportion of the surface covered by pores with a diameter >0.1 pm is less than 10% and wherein the surface covered by pores in the interdiffusion layer (D) is < 300 pm2 in a measuring length of 500 pm.

17. The metal foil component according to claim 16, further characterized in that the interdiffusion layer (D) has a thickness of 1 - 30 pm.

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