Method for producing sheet metal component from flat steel product applied with corrosion protective coating

Abstract

To provide a method forming a sheet metal component from a flat steel product having optimal conditions for a coating with an organic coat, in particular for painting by satisfying the highest requirements for their weldability.SOLUTION: The method includes a) providing a flat steel product made of steel, b) annealing the flat steel product with a specified dew point temperature and annealing temperature under an annealing atmosphere comprising 0.1 to 15% hydrogen and nitrogen in a continuous furnace with four zones A, B, C, D, c) adhering a corrosion protective coating to the obtained flat steel product, f) heating the flat steel product to a hot forming temperature larger than a steel Ac3 temperature of the flat steel product and not exceeding 1000°C, g) hot-forming the flat steel product heated to a hot forming temperature and making a sheet metal component, and h) cooling at least one part of the component at cooling rate enough to generate hardening structure at the part of the sheet metal component.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing sheet metal components from flat steel products provided with a corrosion protective coating. [Background technology]

[0002] Flat steel products are understood here to mean rolled products whose length and width are each significantly greater than their thickness, including in particular steel strip and steel sheet.

[0003] Unless expressly stated otherwise, in this text information on the contents of alloy constituents is always given in wt. %.

[0004] On the other hand, the percentages of particular components of the atmosphere, especially the annealing atmosphere, are given in volume percent unless otherwise noted.

[0005] A method of the first type described is known from US Pat. No. 5,629,499. Flat steel products are used as starting products for this method, and the steel substrate consists of so-called "MnB steels." This type of steel is standardized in EN 10083-3 and has good hardenability. They allow reliable process control during hot pressing, thereby economically enabling martensite hardening to occur without additional cooling in the tool during hot forming. A typical example of such a steel is known under the name 22MnB5, which can be found in Key to Steel 2004 as material number 1.5528. Typically, commercially available fully consolidated 22MnB5 steel contains (in weight percent) in addition to iron and unavoidable impurities: 0.10-0.250% C, 1.0-1.4% Mn, 0.35-0.4% Si, max. 0.03% P, max. 0.01% S, max. 0.040% Al, max. 0.15% Ti, max. 0.1% Nb, max. 0.5% Cr+Mo in total, and max. 0.005% B. To protect flat steel products made from such composite steel from corrosion attacks and at the same time minimize the risk of hydrogen absorption during the heating required for hot forming, the flat steel products are provided by known methods with an aluminum-based corrosion-protective coating, which contains an effective content of at least one alkaline earth or transition metal as an additional alloying element in the range of 0.005-0.7% by weight. In addition, the coating may also have a Si content of 3-15 wt. % and an Fe content of up to 5 wt. %. As the at least one alkaline earth or transition metal of the protective coating, Mg is preferably used here in a content of 0.1-0.5 wt. %, with calcium, strontium, sodium, or barium also being possible alternatively or additionally. The Al-based protective coating can be applied to the steel substrate by hot-dip coating, also known in the art as "hot-dip aluminizing," or by gas separation processes, such as the well-known PVD (physical vapor deposition) or CVD (chemical vapor deposition).

[0006] No special requirements for the application of corrosion protection coatings to steel substrates made of MnB steel are mentioned in the prior art described above. When a board coated in the above manner is conventionally heated to a temperature of 900°C for a period of 360-800 s under normal atmosphere, at most minimal hydrogen absorption occurs in the steel substrate due to the presence of alkaline earth or transition metals in the coating, thus minimizing the risk of so-called "hydrogen embrittlement".

[0007] However, in practical use, it has been found that, despite this success, components formed from flat steel products manufactured in the above-described manner have optimized strength, but are not always able to meet the increasingly higher requirements for the behavior of sheet metal components manufactured from such flat steel products when resistance welding them to such components and when bonding organic layers, for example by coating.

[0008] Patent Document 2 also deals with a method for producing aluminum-based steel components coated with a metal corrosion-resistant coating. For this purpose, a flat steel product is provided, which consists, by weight, of 0.15-0.50% C, 0.50-3.0% Mn, 0.10-0.50% Si, 0.01-1.00% Cr, max 0.20% Ti, max 0.10% Al, max 0.10% P, max 0.1% Nb, max 0.01% N, max 0.05% S, and max 0.1% B, the balance being Fe and unavoidable impurities, and is coated with an Al coating, which consists, by weight, of 3-15% Si, 1-3.5% Fe, max 0.5% alkali and / or alkaline earth metals, the balance being Al and unavoidable impurities. The provided sheet metal is annealed in an oven at a certain temperature for a certain period of time, with the temperature and period being related to each other by parameters calculated by a complex formula. Depending on the furnace residence time and temperature, a so-called interdiffusion zone forms at the transition between the substrate and the coating, where no martensitic structure develops during press hardening and is not assigned to the Al coating. This interdiffusion zone extends from the center of the flat steel product to a thickness at which the component no longer exhibits martensitic structures, up to a thickness at which the Al coating has an iron content of ≦85 wt.% and an Al content of ≧10 wt.%. This prior art provides neither detailed information on how the interdiffusion zone can be designed nor an explanation of how its formation and composition can be controlled in a targeted manner with respect to specific surface properties of the coating. Instead, the focus here is on improving the deformation behavior of the Al coating, particularly the achievable bend angle. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] European Patent Application Publication No. 2 993 248(A1) [Patent Document 2] German Patent Application Publication No. 10 2017 210 201(A1) Summary of the Invention [Means for solving the problem]

[0010] Against this background, the objective emerged of showing a method that makes it possible to form sheet metal components from flat steel products of the type described above, which meet the highest requirements for weldability and, therefore, have optimal conditions for coating, in particular application, with organic coatings.

[0011] To this end, the invention proposes that at least the method steps set out in claim 1 are completed during the production of a sheet metal component.

[0012] It will be appreciated that when carrying out a method according to the present invention, a person skilled in the art will not only carry out the method steps referred to in the claims and described herein, but will also, if the need arises, carry out all other steps and operations that are typically carried out in the actual implementation of such methods in the prior art.

[0013] Advantageous embodiments of the invention are defined in the dependent claims and are explained in detail below, as well as the general concept of the invention.

[0014] In the method according to the invention for producing a sheet metal component from flat steel products provided with a corrosion-protective coating, therefore, at least the following work steps are carried out: a) providing a flat steel product made from steel consisting of (by weight): 0.05-0.5% C, 0.5-3% Mn, 0.06-1.7% Si, max 0.06% P, max 0.01% S, max 1.0% Al, max 0.15% Ti, max 0.6% Nb, max 0.01% B, max 1.0% Cr, max 1.0% Mo, wherein the sum of the Cr and Mo contents is not more than 1.0%, max 0.2% Ca, max 0.1% V, and the balance being iron and unavoidable impurities; b) annealing the flat steel products in a continuous furnace having four zones A, B, C, D, through which the flat steel products pass successively, in which they are annealed in an annealing atmosphere consisting in each case of 0.1 to 15% by volume of hydrogen and the remainder of nitrogen and technically unavoidable impurities, in which the annealing temperature GT A , G.T. B , G.T. C , G.T. D Dew point temperature TP A , T.P. B , T.P. C , T.P. D the annealing step, for which the following specifications apply:

[0015] [Table 0]

[0016] c) applying a corrosion protection coating to the flat steel product obtained in process step b), the corrosion protection coating consisting of (in % by weight) at most 15% Si, at most 5% Fe, a total of 0.1-5% of at least one alkaline earth or transition metal, and the remainder Al; d) optionally decorative rolling the corrosion-protective coated flat steel product; e) optionally separating the boards from the flat steel product; f) heating the flat steel product or board to a hot forming temperature above the Ac3 temperature of the steel of the flat steel product and not exceeding 1000°C for a holding time sufficient to introduce into the flat steel product or board an amount of thermal energy J greater than 100,000 kJ and not more than 800,000 kJ; g) hot forming the flat steel product heated to the hot forming temperature or the board heated to the hot forming temperature into a sheet metal component; h) cooling at least one portion of the component at a cooling rate sufficient to produce a hardened structure in that portion of the sheet metal component.

[0017] The invention is based on the knowledge that for the behavior of sheet metal components provided with aluminum-based ("Al-based") corrosion protection coatings, in the case of resistance welding to such sheet metal components and for the adhesion, in particular application, of organic coatings, it is not only the composition of the boundary layer between the corrosion protection coating and the ambient atmosphere that matters, but also parameters such as the overall roughness and conductivity of the coating in particular that play a decisive role. In this case, the method of carrying out annealing (work step b)) according to the invention before applying the corrosion protection coating (work step c)) creates the conditions for the components treated according to the invention to have an optimally homogeneous corrosion protection coating.

[0018] Thus, components manufactured according to the invention typically have a corrosion protection coating, which is formed by several layers of different compositions. By guiding the dew point and annealing temperature according to the invention during annealing in a continuous annealing furnace to prepare for the subsequent application of the corrosion protection coating, a significant reduction in the pores contained in the coating is achieved.

[0019] The annealing parameters selected according to the invention during the annealing prior to coating (work step b)) ensure that 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, due to the formation of an iron-aluminum layer ("Fe-Al layer") at the transition from the steel substrate to the corrosion protection coating. On the other hand, iron reaches the layer in a sufficient, homogeneous and uniform distribution, which improves the layer's conductivity and thereby optimizes its behavior during resistance welding.

[0020] If the flat steel product provided in work step b) is already a blank suitable for directly forming components, work step e) can be omitted, whereas if the flat steel product provided is a steel strip or a larger steel sheet, boards of suitable size are separated therefrom in work step e).

[0021] The flat steel product or separated board (step e)) annealed and coated in accordance with the present invention (steps b) and c) is heated (step f) to the hot-forming temperature for hot-forming (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 metals provided in accordance with the present invention in the corrosion-protective coating diffuse to the surface due to their oxygen affinity and form an oxide layer there. Due to their similar atomic size, iron atoms can exchange places with alkaline earth or transition metal atoms in a 1:1 ratio and are thus incorporated into the metal grid, so that the diffusion of alkaline earth or transition metal atoms can at most result in a negligible number of defects. As a result of the reduction in defects achieved by the present invention, these defects cannot aggregate into pores in the corrosion protection coating of the component according to the present invention, and the component according to the present invention is therefore characterized by a significantly reduced number of pores compared to conventionally produced components, such as those produced according to the sample of EP 2 086 755 B1.

[0022] The effect of the use of the invention occurs particularly reliably when the additionally present alkaline earth or transition metal is magnesium ("Mg"), i.e. when Mg is present alone or in combination with other elements belonging to the group of alkaline earth or transition metals in the content provided according to the invention in the corrosion protective coating of the flat steel products treated according to the invention.

[0023] The method according to the invention is suitable for the production of components from flat steel products having a large thickness range, so that flat steel products having a thickness of 0.6 to 7 mm can be processed by the method according to the invention.

[0024] The production of the flat steel product provided in work step a) can be carried out in any manner known from the prior art. The method according to the invention is particularly suitable for processing flat steel products having a thickness of 0.8 to 4 mm, in particular 0.8 to 3 mm. Flat steel products having a greater thickness, above 3 mm, are usually processed in the hot-rolled state, while thinner sheets are usually provided in the cold-rolled state.

[0025] In operational step a), flat steel products having different thicknesses obtained by flexible or partial rolling over their length and / or width may be provided for the method according to the invention. Similarly, in operational step a) for the method according to the invention, flat steel products may be provided for the process according to the invention, which flat steel products consist of different sheet metal blanks welded together, or flat steel products consisting of similarly constructed flat steel products and steel strips welded together to form the flat steel product to be processed.

[0026] The flat steel products provided in each case according to the invention consist of steels having a composition typical for MnB steels, which usually have a yield strength of 250 to 580 MPa and a tensile strength of 400 to 720 MPa in the supplied condition.

[0027] Thus, the flat steel product provided according to the invention consists of: - 0.05 to 0.5 wt.% carbon ("C"), the C content being preferably 0.07 to 0.4 wt.%; - 0.5 to 3 wt.% manganese ("Mn"), the Mn content being preferably 0.8 to 2.5 wt.%, in particular 1.0 to 2.0 wt.%, - 0.06 to 1.7 wt.% silicon ("Si"), the Si content being preferably 0.06 to 1.1 wt.%, in particular 0.06 to 0.9 wt.%, - maximum 0.06% by weight of phosphorus ("P"), the P content being equal to or less than 0.03% by weight; - maximum 0.01% by weight of sulfur ("S"); - maximum 1.0 wt.% aluminum ("Al"), the Al content preferably being equal to or less than 0.5 wt.%, in particular equal to or less than 0.1 wt.%, - maximum 0.15% by weight of titanium ("Ti"); - max 0.6 wt.% niobium ("Nb"), the Nb content preferably being max 0.1 wt.%; - maximum 0.01% by weight of boron ("B"), the B content being preferably maximum 0.005% by weight; - maximum 1.0 wt.% chromium ("Cr"), the Cr content being preferably maximum 0.5 wt.%, in particular maximum 0.2 wt.%, - maximum 1.0 wt.-% molybdenum ("Mo"), the Mo content being preferably maximum 0.5 wt.-%, in particular maximum 0.2 wt.-%, - where the following applies for the Cr content %Cr and the Mo content %Mo: %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.

[0028] Due to the property profile of flat steel products, in particular because they allow the development of high strength in the finished component after hot forming and cooling, flat steel products which, in a manner known per se, consist of 0.07-0.4% by weight of C, 1.0-2% by weight of Mn, 0.06-0.4% by weight of Si, max. 0.03% by weight of P, max. 0.01% by weight of S, max. 0.1% by weight of Al, max. 0.15% by weight of Ti, max. 0.6% by weight of Nb, max. 0.005% by weight of B, max. 0.5% by weight of Cr, max. 0.5% by weight of Mo, in which the sum of the contents of Cr and Mo is less than 0.5% by weight, the remainder consisting of iron and unavoidable impurities, are of particular interest in practice.

[0029] This includes steels already in use, which consist of 0.07-0.4 wt% C, 1.0-1.5 wt% Mn, 0.3-0.4 wt% Si, max. 0.03 wt% P, max. 0.01 wt% S, max. 0.05 wt% Al, max. 0.15 wt% Ti, max. 0.6 wt% Nb, max. 0.005 wt% B, max. 0.5 wt% Cr, max. 0.5 wt% Mo, where the sum of the Cr and Mo contents is less than 0.5 wt%, with the remainder being iron and unavoidable impurities. Such composite steels achieve tensile strengths of up to 2000 MPa after hot forming and cooling.

[0030] As already mentioned, the annealing (work step b)) which is completed in four uninterrupted successive steps A, B, C, D on each treated flat steel product produces a surface that is almost completely, i.e. at least 70%, in particular at least 80% or at least 90%, covered with pure Fe. For this purpose, in zones A to D of the continuous annealing furnace used according to the invention, dew points and annealing temperatures that are specifically adapted in each case are set.

[0031] The annealing carried out in zones A to D in process step b) is carried out in an annealing atmosphere containing in each case 0.1 to 15% by volume of hydrogen, the remainder in each case consisting of nitrogen and unavoidable impurities, the total of which is usually not more than 5% by volume, in particular not more than 4% by volume, or preferably not more than 3% by volume.

[0032] The annealing temperature GT provided below and in the claims A , G.T. B , G.T. C , and G.T. D All information given indicates the average furnace chamber temperature during strip throughput.

[0033] Before entering zone A of the continuous furnace operated according to the invention, the flat steel products provided according to the invention have a wide range of oxide products on their surface, which have a negative effect on the quality of the coating, in particular on the formation of pores in the coating. The continuous annealing according to the invention converts these oxides so that after annealing only Fe is present in the technical sense on the surface of the flat steel products.

[0034] The dew point temperature TP in zone A of the continuous furnace A Set the temperature to -10°C to -25°C, and the annealing temperature GT A By setting the annealing temperature GT in zone A of the continuous furnace at 800-950°C, the oxides present on the flat steel products are covered with iron oxide. To achieve this in a specifically targeted manner, A The dew point temperature TP may be 810 to 940°C. A The temperature may be -15 to -25°C.

[0035] Due to the reduction of iron oxide in zones B and C, iron is present on the surface after zone C. In zone B, the dew point temperature TP of the annealing atmosphere prevailing there is B The annealing temperature GT is then reduced to -27 °C to -41 °C. B is maintained at 800-930 ° C, where in the case of the annealing completed in work step b), the annealing temperature GT in zone B of the continuous furnace B It has been proven that the desired effect is particularly reliable when the temperature is between 800 and 900°C.

[0036] In zone C, the dew point temperature TP of the annealing atmosphere prevailing there C The annealing temperature GT is then further reduced to -30 °C to -80 °C. C The temperature is maintained at 800-950 ° C to complete the reduction of iron oxide to iron. In the case of the annealing completed in zone C of the continuous furnace in work step b), the annealing temperature GT C is 800 to 920°C, and the dew point temperature TPC This effect can be achieved particularly reliably when the temperature is between -30°C and -50°C.

[0037] In zone D, the dew point temperature TP of the annealing atmosphere prevailing there D The annealing temperature GT is then raised to -20°C to -30°C. D is maintained at 750-950 °C, which allows the recrystallization of the steel flat product on the one hand, and on the other hand tempers the steel flat product in such a way that the previously achieved pure iron surface is maintained. In the case of the annealing completed in the zone D of the continuous furnace in the work step b), the annealing temperature GT D This effect can be achieved particularly reliably when the temperature is between 780 and 930°C.

[0038] The lambda value λ, which describes the ratio of the mass of air to fuel introduced into the continuous furnace, is typically between 0.95 and 1.1 in the annealing atmosphere maintained in zones A to D of the continuous furnace used according to the invention in the case of the annealing completed in working step b) of the method according to the invention.

[0039] A prerequisite for the effects achieved according to the present invention is the presence of at least one alkaline earth or transition metal in the aluminum (Al)-based corrosion protection coating applied after the annealing according to the present invention (step b)). Thus, after application of the corrosion protection coating (step c)) and before heating for hot forming (step f)), the coating of the flat steel product treated according to the present invention contains at least 0.1-5 wt. % of at least one alkaline earth or transition metal, with the remainder being Al and unavoidable impurities. An alkaline earth or transition metal content of at least 0.11 wt. % has proven particularly favorable in terms of reliability, allowing the positive effects of the presence of at least one alkaline earth or transition metal in the coating applied according to the present invention to be utilized. An alkaline earth or transition metal content of more than 5 wt. % can lead to a deterioration in surface quality due to increased oxide formation in the melting crucible. Excessive oxide formation can also occur during hot forming, which, on the one hand, promotes the splitting of water into hydrogen and oxygen, resulting in the risk of more hydrogen entering the steel. On the other hand, a thicker oxide layer can lead to increased contamination of the forming tool. To reliably avoid this effect, the content of alkaline earth or transition metals in the corrosion protection coating applied in process step c) can be limited to a total of 1.5% by weight or less, in particular 0.6% by weight or less. Thus, the content of alkaline earth or transition metals in the corrosion protection coating applied in process step c) is in particular 0.11 to 1.5% by weight, or in particular 0.11 to 0.6% by weight.

[0040] As already mentioned, Mg from the group of alkaline earth or transition metals has proven to be particularly suitable for the purposes according to the invention and may be present alone or in combination with other alkaline earth or transition metals, such as beryllium, calcium, strontium and barium, in the coatings applied according to the invention, in order to make available the effects sought by the invention.

[0041] Optionally, silicon ("Si") may also be present in the coating applied in step c) in a content of up to 15% by weight, in particular up to 11% by weight, to promote the formation of an iron-aluminum layer, which adheres well to the iron surface established in step b) and accounts for up to one-third of the total layer thickness of the coating. If the Si content is too high, an excessively large alloy layer thickness may result, which may lead to a loss of adhesion. A Si content of at least 3% by weight, in particular at least 8.5% by weight, has proven particularly favorable in this regard, so that a Si content of 3 to 15% by weight, in particular 3 to 11% by weight, in particular 8.5 to 11% by weight, allows the positive effect of Si to be used particularly reliably in practice.

[0042] Additionally, the coating applied in process step c) may optionally also contain Fe in an amount of up to 5% by weight, in particular up to 4% by weight, and in particular up to 3.5% by weight. The iron content in the coating may be set at this order of magnitude, since this is the saturation value for aluminum melts in the temperature range of 650-720°C. By specifically adding iron to the melt, the risk of dissolving iron components of the melting crucible in contact with the melt can be reduced. In this respect, an Fe content of at least 1% by weight has proven particularly favorable, so that in practice, the positive effect of Fe can be utilized particularly reliably with an Fe content of 1-5% by weight, in particular 1-4% by weight, and in particular 1-3.5% by weight.

[0043] The corrosion-protective coating can be applied in step c) of the method according to the invention in any known manner. Particularly preferred is the so-called "hot-dip aluminizing" process, in which the respective flat steel product is guided through a suitably heated molten bath configured according to the specifications of the invention. This hot-dip coating is particularly suitable for strip-shaped flat steel products with a thickness of up to 3 mm. For larger thicknesses, one of the vapor deposition processes mentioned at the beginning (PVD, CVD) can also be used to apply the corrosion-protective coating.

[0044] The load of the corrosion protection coating applied according to the invention in work step c) is typically 30 to 100 g / m² per side. 2 , especially 40-80g / m 2 Therefore, the total load on both sides of the coating is 60-200g / m 2 is.

[0045] After application of the corrosion protection coating (operation step c)), the flat steel product coated therewith can optionally be subjected to decorative rolling (operation step d)) to set the mechanical characteristics of the flat steel product and adjust its surface roughness or homogenize it. The degree of forming set for this (degree of forming = (thickness before decorative rolling - thickness after decorative rolling) / (thickness before decorative rolling)) is typically 0.1-5%.

[0046] After application of a corrosion protection coating (operational step c)) or optional decorative rolling (operational step d)), if necessary, the boards are separated from the flat steel product in a manner known per se and the dimensions of the boards are adapted in a manner known per se to the dimensions of the sheet metal component to be hot-formed therefrom (operational step e)).

[0047] The flat steel product itself or the board is then heated in step f) to a hot-forming temperature that is higher than the Ac3 temperature of the steel of the flat steel product but does not exceed 1000°C, in particular at least equal to the Ac3 temperature + 50°C and not more than 980°C, with a hot-forming temperature of 820-950°C proving particularly advantageous. The flat steel product is held at this temperature until a sufficient amount of heat has been introduced into the flat steel product or the board separated therefrom. The holding time and annealing temperature required in each case can be estimated on the basis that the amount of heat energy J introduced into the flat steel product or board in step f) should be higher than 100,000 kJ but not more than 800,000 kJ, where J can be calculated according to the following known equation: J[kJ]=[(T2-T1)xcxtxm] / 1000; where T2 is the final temperature of the component at the end of heating in K T1: Starting temperature of the component at the start of heating in K c: Heat capacity of steel (usually 460 J / kgK) t: Holding time of the flat steel product or board at the final temperature in s m: Mass of the flat steel product or board in kg

[0048] Heating can be carried out in any suitable manner. If a conventional continuous furnace is used for this purpose, the flat steel product or board is heated by radiation, and suitable holding times are usually 100 to 900 s, preferably 180 to 720 s, in particular 240 to 600 s. If a hot forming temperature of 850 to 930°C is selected, holding times of 180 to 600 s, in particular 240 to 600 s, are generally sufficient in practice. As an alternative to using a continuous furnace, heating can also be carried out in, for example, a conventional chamber furnace.

[0049] Heating of the flat steel products or boards can also be carried out in two steps, also in a manner known per se, to first achieve pre-alloying of the corrosion-protective coating and then bring the flat steel products or boards to the respective hot-forming temperature.

[0050] The board heated to the hot-forming temperature or the flat steel product heated to the hot-forming temperature is inserted into the hot-forming tool within a transition time of typically less than 15 seconds, in particular less than 10 seconds, and is then hot-formed there into the component (work step g)).

[0051] Subsequently or simultaneously, at least one portion of the resulting component is cooled in a controlled manner known per se, thereby generating the desired structure in the relevant portion of the component. The cooling rates required for this are typically 20-500 K / s, with cooling rates higher than 30 K / s, especially higher than 50 K / s, being particularly practical. Cooling "of at least one portion" naturally also includes the possibility of cooling the entire component in the above manner in order to generate a hardened structure throughout the component.

[0052] The method according to the invention allows the production of sheet metal components made from flat steel products, the steel substrate of which consists of steel containing (in % by weight): 0.05-0.5% C, 0.5-3% Mn, 0.06-1.7% Si, max 0.06% P, max 0.01% S, max 1.0% Al, max 0.15% Ti, max 0.6% Nb, max 0.01% B, max 1.0% Cr, max 1.0% Mo, where the sum of the Cr and Mo contents is not more than 1.0%, max 0.2% Ca, in particular max 0.1% V, with the balance being iron and unavoidable impurities, and coated with a corrosion-protective coating. the corrosion protection coating consists of (in weight percent) at most 15% Si, at most 5% Fe, a total of 0.1-5% of at least one alkaline earth or transition metal, and the remainder Al and unavoidable impurities, the layer of the corrosion protection coating adjacent to the steel substrate is an interdiffusion layer consisting of ferrite with an Al content of at most 50% by weight, in particular at least 1% by weight Al, in which in a cross section of this interdiffusion layer the proportion of the surface covered by pores with a diameter of ≥ 0.1 μm is less than 10%, in particular less than 5%, preferably less than 3%, and the surface covered by pores in the interdiffusion layer is < 300 μm over a measured length of 500 μm. 2 , especially 200 μm 2 less than 100 μm, particularly preferably 2 The thickness of the alloy layer here is 1 to 30 μm, preferably 2 to 20 μm, and particularly preferably 4 to 16 μm.

[0053] The invention is explained in more detail below using exemplary embodiments. [Brief explanation of the drawings]

[0054] [Figure 1] 1 shows, at a magnification of 500x, a cross section of a steel sheet of a sheet metal component produced according to the invention by hot forming, which cross section was prepared in the conventional manner by etching with 3% nital to reveal the layer structure present on the steel sheet. [Figure 2] FIG. 2 is a schematic diagram showing a cross section according to FIG. DETAILED DESCRIPTION OF THE INVENTION

[0055] Thus, the corrosion protective coating K formed on the steel substrate S comprises an interdiffusion layer D directly connected to the steel substrate S, which interdiffusion layer D consists essentially of alpha mixed crystals (i.e., ferrite) with an increased Al content, where Fe2Al5 is still present in the phase. The interdiffusion layer D is characterized by being homogeneously and uniformly formed and being substantially pore-free.

[0056] In the direction of the free surface O of the corrosion protection coating K, a first Si-rich layer S1 is formed on the diffusion layer D. At the boundary between the diffusion layer D and the Si-rich layer S1, a small number of pores P1 are present in the diffusion layer D that are spaced apart from one another.

[0057] A first intermediate layer Z1 is formed on the Si-rich layer S1 toward the free surface O. It consists of aluminum-iron, mostly aluminum. Traces of silicon, alkaline earth and / or transition metals, as well as unavoidable impurities, may also be present in layer S1. The intermediate layer Z1 is non-porous.

[0058] On the intermediate layer Z1 in the direction of the free surface O there is a second Si-rich layer S2.

[0059] The second intermediate layer Z2 is formed on the Si-rich layer S2 toward the free surface O. The layer Z2 is also composed of aluminum-iron, mostly aluminum, with alkaline earth and / or transition metals present. Traces of Si and unavoidable impurities may also be present. The intermediate layer Z2 is also non-porous.

[0060] The second intermediate layer Z2 is coated on its side facing the free surface O with an oxide layer OX, which consists essentially of oxides of aluminum, silicon, and alkaline earth and / or transition metals. On the hot-formed component, the oxide layer may have an average thickness of up to 1.5 μm. A small number of crater-shaped pores P2, open to the environment, are formed at a large distance from one another on the surface of the oxide layer OX, which forms the free surface O of the corrosion protection coating K.

[0061] For comparison, components were formed from flat steel products coated with an AlSi coating according to the prior art sample described in EP 2 086 755 B1, which coating consisted (in weight percent) of 9.5% Si, 3.5% Fe, and the remainder aluminum and unavoidable impurities, and therefore did not contain alkaline earth or transition metals of the type added according to the invention.

[0062] The steel substrate of the flat steel product consisted of (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.

[0063] Before applying the metal coating and forming it into flat steel products, the comparatively processed flat steel products were subjected to an annealing treatment in a four-zone continuous furnace, where the dew point temperature TP and the annealing temperature GT were set as shown in Table 6. The air ratio λ in the continuous furnace was 0.98.

[0064] For comparison, a five-layer corrosion protection coating was also produced for a conventionally produced component. However, the number of pores P2 in the oxide layer OX of the component produced according to the invention was reduced by at least 25% compared to the number of pores in the coating of the conventionally produced component for comparison, and the number of pores P1 in the diffusion layer D was reduced by at least 40% compared to the pores present in the corresponding layer of the corrosion protection coating of the conventionally produced component for comparison. After a residence time in the furnace of 600 s, the area covered by pores P1 in a measured length of 500 μm of layer D was 300 μm 2 It was.

[0065] The reduction in pores in P2 reduces paint craters and improves adhesion and weldability. The pores in P2 have openings of a few nanometers toward the atmosphere. When components are further processed after hot forming, as is typically done for cars, they undergo cathodic dip coating in addition to multiple cleaning steps. Contact with aqueous solutions is unavoidable. During cleaning, surfactants added to the cleaning water improve wetting and significantly reduce the surface tension of the water, allowing water to penetrate the pores P2 of the layer. In the cathodic dip coating process, water can also penetrate the open pores P2. In this particular case, the cleaning water also separates paint particles that cannot penetrate the pores P2 due to the size of the openings. The water present in the pores P2 then reaches its boiling point as the paint layer is baked, creating a gas phase that explosively leaks through the paint into the environment in a kind of delayed boiling. As a result of this reaction, so-called paint craters form, which, in addition to their visual impact, significantly reduce the paint's corrosion protection. Corrosion and paint penetration can occur at these points, especially in the case of aluminum-based coatings. The appearance of visually noticeable red rust formed by the high iron content of the coating is particularly problematic for further processors.

[0066] In addition, on surfaces with many open pores P2, the adhesive has a higher viscosity and is therefore unable to penetrate the pores P2, which can result in incomplete coverage of the surface with the adhesive. Cavities can also form in the area of ​​the pores, resulting in impaired adhesion.

[0067] The pores P2 present in the layer OX also cause changes in the current path in the material during resistance spot welding, which has a negative effect on weldability.

[0068] A high pore count also increases the surface area over which water can split during oxidation in the hot forming process, allowing the diffused hydrogen to penetrate into the material, which is known to increase the risk of hydrogen-induced cracking.

[0069] By minimizing the frequency with which pores P2 occur during the manufacture of sheet metal components according to the invention, the risks associated with pore formation in conventionally manufactured components can be effectively reduced.

[0070] Reducing the number of pores P1 in the diffusion layer D also results in an increase in the transferable force of the adhesive and improved weldability.

[0071] The pores P2 represent cavities in the corrosion protection coating K. If the number of pores were too large, there would be a risk of the corrosion protection coating K breaking down in the boundary region between the diffusion layer D and the first Si-rich layer S1, and as a result the adhesive seam would also fail at an early stage. The reduction in the number of pores P1 achieved according to the invention increases the area over which the adhesive forces are transmitted by more than 60%, thus reducing the risk of delamination failure accordingly.

[0072] To demonstrate the effectiveness of the present invention, conventionally cold-rolled steel sheets, each having a thickness of 1.5 mm, were produced from six steels ST1 to ST6, the compositions of which are shown in Table 1 (work step a) of the method according to the invention).

[0073] The steel sheets provided in this manner were subjected to successive annealing G1, G2 or G3 in nine tests V1 to V9 in a continuous furnace having in each case four successive zones A, B, C, D. Table 2 shows the dew point temperatures TP set in zones A to D for the annealing variants G1 to G3. A ~TP D , annealing temperature GT A ~GT D , as well as the hydrogen content H2 and nitrogen content N2 of the respective annealing atmosphere, the remainder of which consisted of technically unavoidable impurities (working step b) of the method according to the invention).

[0074] Each of the samples annealed in this way is conventionally coated with an Al-based corrosion protection coating Z1 to Z5 with a load AG, the compositions of which are shown in Table 3 (work step c) of the method according to the invention).

[0075] Samples, each coated with one of the corrosion protection coatings Z1 to Z5, were subjected to hot forming at the temperature T WU and hold it at that temperature for t WU (working step f) of the method according to the invention).

[0076] Steels ST1 to ST6 constituting the samples used in tests V1 to V9, respectively, annealing variants G1 to G3 used in tests V1 to V9, compositions Z1 to Z5 of corrosion protection coatings produced in tests V1 to V9, respectively, and their respective loads AG, and hot forming temperatures T selected in tests V1 to V9, respectively. WU and retention time t WU is shown in Table 4.

[0077] The samples heated in this way were removed from the continuous furnace after a transition time of in each case 3-7 s and placed in a conventional hot forming tool, where they were hot formed into components, followed by cooling to room temperature at in each case 270 K / s (work steps g) and h) of the method according to the invention).

[0078] Cross sections of three of the components obtained in tests V1 to V9 were produced in a manner known per se and etched with 3% nital to clarify the layer structure. As shown by way of example in FIG. 1, views of these cross sections were produced at a magnification of 500x. In each view, the pores P1, P2 present in layers OX and D were counted over a section having a length of 550 μm. An arithmetic mean was formed from the counting results determined for the three cross sections of each sample. This arithmetic mean of the numbers determined for pores P1 and P2 was compared with a comparative value determined in the same way for a comparative sample.

[0079] The resulting comparison shows the relative reduction in pore counts P1 and P2 achieved by the present invention, as shown in Table 5. Table 5 also shows the percentage of paint craters in the total area of ​​each sample, the reduction in spalled area, and the weld area determined according to steel test sheet SEP 1220-2. Weld areas greater than 1 kA are classified as "OK."

[0080] [Table 1]

[0081] [Table 2]

[0082] [Table 3]

[0083] [Table 4]

[0084]

Table 5

[0085]

Table 6

Claims

1. A sheet metal component manufactured from a flat steel product, wherein the steel substrate of the flat steel product is made of steel, and the steel consists of (by weight %) 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, where the total content of Cr and Mo is 1.0% or less, up to 0.2% Ca, up to 0.1% V, and the remainder being iron and unavoidable impurities, and a corrosion protection coating The corrosion-protective coating is coated with (in weight percent) 8-10% Si, 2-3.5% Fe, 0.1-2% Mg, and the remainder Al and unavoidable impurities, and the layer of the corrosion-protective coating adjacent to the steel substrate is an interdiffusion layer (D) made of ferrite having a maximum Al content of 50% by weight, and in the cross-section of the interdiffusion layer (D), the proportion of the surface covered by pores having a diameter of ≥0.1 μm is less than 10%, and the surface covered by pores in the interdiffusion layer (D) has a diameter of <300 μm over a measurement length of 500 μm ２ It is a sheet metal component.

2. The sheet metal component according to claim 1, characterized in that the interdiffusion layer (D) has a thickness of 1 to 30 μm.

3. The sheet metal component according to claim 2, characterized in that the interdiffusion layer (D) has a thickness of 2 to 20 μm.

4. The sheet metal component according to claim 3, characterized in that the interdiffusion layer (D) has a thickness of 4 to 16 μm.

5. In the aforementioned interdiffusion layer (D), the surface covered by pores is 200 μm over a measurement length of 500 μm. ２ A sheet metal component according to claim 1, which is less than [amount missing].

6. In the aforementioned interdiffusion layer (D), the surface covered by pores is 100 μm over a measurement length of 500 μm. ２ The sheet metal component according to claim 5, which is less than [amount missing].

7. The sheet metal component according to claim 1, characterized in that the layer of the corrosion protection coating adjacent to the steel substrate is an interdiffusion layer (D) made of ferrite with an Al content of at least 1% by weight and up to 50% by weight.

8. The sheet metal component according to claim 1, characterized in that, in the cross-section of the interdiffusion layer (D), the proportion of the surface covered by pores having a diameter of ≥ 0.1 μm is less than 5%.

9. The sheet metal component according to claim 8, characterized in that, in the cross-section of the interdiffusion layer (D), the proportion of the surface covered by pores having a diameter of ≥ 0.1 μm is less than 3%.

10. The sheet metal component according to claim 1, wherein the steel substrate of the flat steel product for manufacturing the sheet metal component consists of 0.07 to 0.4 wt% C, 1.0 to 2 wt% Mn, 0.06 to 0.4 wt% Si, up to 0.03 wt% P, up to 0.01 wt% S, up to 0.1 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 content of Cr and Mo is 0.5 wt% or less, and the remainder consists of iron and unavoidable impurities.

11. The sheet metal component according to claim 10, wherein the steel substrate of the flat steel product for manufacturing the sheet metal component consists of 0.07 to 0.4 wt% C, 1.0 to 1.5 wt% Mn, 0.3 to 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 content of Cr and Mo is 0.5 wt% or less, and the remainder consists of iron and unavoidable impurities.

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