Method for manufacturing sheet metal components from flat steel products provided with a corrosion-protective coating - Patents.com

Abstract

The present invention relates to a method for manufacturing a sheet metal component, in which a) a flat product made of steel is provided, the steel consisting of (in % by weight) 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, where the sum of the Cr and Mo contents is ≦1.0%, ≦0.2% Ca, ≦0.1% V, the balance being iron and impurities; and b) the flat product is annealed in a continuous furnace having four zones A, B, C, D at a dew point temperature TP and an annealing temperature GT under an annealing atmosphere consisting of 0.1-15% hydrogen and nitrogen, wherein the following applies: Zone A TP: -10 to -25°C, GT: 800 to 950°C, Zone B TP: -27 to -41°C, GT: 800 to 930°C, Zone C TP: -30 to -80°C, GT: 800 to 950°C, Zone D TP: -30 to -20°C, GT: 750 to 950°C; c) applying a corrosion-protective coating consisting of (by weight) <15% Si, ≦5% Fe, a total of 0.1 to 5% of at least one alkaline earth or transition metal, the balance being Al and unavoidable impurities, to a flat steel product; optionally d) decoratively rolling the flat steel product; e) separating boards from the flat steel product; f) heating the flat steel product to > the Ac3 temperature of the steel of the flat steel product and ≦1000°C for a holding time sufficient to introduce a thermal energy amount >100,000 to 800,000 kJ; g) hot-forming the flat steel product into a sheet metal component; h) cooling at least one portion of the component at a cooling rate sufficient to produce a hardened structure. The present invention also provides a component in which optimized adhesion of the corrosion-protective coating to the steel substrate is ensured.

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. Al-based protective coatings can be applied to steel substrates by hot-dip coating, also known under the technical term "hot-dip aluminizing", or by gas separation processes, such as, for example, the well-known PVD (physical vapor deposition) or CVD (chemical vapor deposition). Execution It can be done.

[0006] Corrosion protection coating on steel substrate made of MnB steel give No special requirements for the process are mentioned in the prior art discussed above. When a board coated with the process described above 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 a flat steel product 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 work step b). give The corrosion protection coating comprises (in weight percent): 3~ 15% Si, 1~ 5% Fe, 0.1-5% total of at least one alkaline earth similar gold The remainder consists of Al. Apply a corrosion protection coating Step, d) optionally decorative rolling the corrosion-protective coated flat steel product; e) optionally separating the boards from the flat steel product; f) Flat steel products or If step e) is performed100,000kJ on board s Higher than 800,000kJ s The amount of heat energy J s heating the flat steel product or board to a hot forming temperature above the Ac3 temperature of the steel of the flat steel product but not exceeding 1000°C for a holding time sufficient to introduce g) Flat steel products heated to hot forming temperatures or If step e) is performed hot forming the board heated to a 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 present 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 surrounding atmosphere that matters, but also parameters such as the overall roughness and conductivity of the coating play a decisive role here. give The manner in which the annealing according to the invention (work step b)) is carried out before (work step c)) creates the conditions for the components treated according to the invention to have an optimally homogeneous corrosion-protective coating.

[0018] Thus, components manufactured according to the invention usually have a corrosion protection coating, which is formed by several layers of different compositions. During annealing in a continuous annealing furnace, the dew point and annealing temperature are regulated according to the invention to prevent subsequent corrosion protection coating. To carry out By providing a coating with a significantly reduced pore content, a significant reduction in the pore content of the coating is achieved.

[0019] The annealing parameters selected according to the invention during the annealing before 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 subsequent Execution This results in good adhesion of the applied Al-based coating due to the formation of an iron-aluminium layer ("Fe-Al layer") at the transition from the steel substrate to the corrosion protection coating. On the other hand, iron reaches the layer with a sufficient, homogeneous and uniform distribution, which improves the conductivity of the layer and therefore 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 present invention is particularly reliably achieved when alkaline earth metals are present in addition to the metals. similar gold When the metal is magnesium ("Mg"), that is, when Mg is present alone or in the presence of alkaline earth metals in the content provided according to the invention in the corrosion protective coating of the flat steel product treated according to the invention, similar gold When present in combination with other elements belonging to the same group.

[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 and steel strips of similar construction 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 effect achieved according to the invention is that after the annealing according to the invention (work step b)) Execution At least one alkaline earth metal is present in the aluminum (Al)-based corrosion protection coating similar gold Therefore, corrosion protection coating was given (Work step c) )rear and before heating for hot forming (operation step f)), the coating of the flat steel product treated according to the invention contains at least 0.1 to 5% by weight of at least one alkaline earth metal. similar gold The remainder is Al and unavoidable impurities. similar gold A content of at least 0.11% by weight of the metal has proven to be particularly favorable in terms of reliability, which allows the production of the metal according to the invention. Execution The applied coating contains at least one alkaline earth metal. similar goldThe positive effects of the presence of the genus can be exploited. similar gold If the metal content exceeds 5% by weight, an increased oxide formation in the melting crucible can occur, leading to a deterioration in the surface quality. Excessive oxides can also be formed during hot forming, which on the one hand can promote the splitting of water into hydrogen and oxygen, with the result that there is a risk of more hydrogen entering the steel. On the other hand, a thicker oxide layer can lead to increased contamination in the forming tool. In order to reliably avoid this effect, in work step c) Execution Alkaline earth metals in corrosion protection coatings Genus The total content can be limited to not more than 1.5% by weight, in particular not more than 0.6% by weight. Execution Corrosion protection coating alkaline earth similar gold The content of the element is in particular 0.11 to 1.5% by weight, or in particular 0.11 to 0.6% by weight.

[0040] As already mentioned, alkaline earth similar gold Of the group Mg has proven to be particularly suitable for the purposes according to the invention and is used in accordance with the invention to enable the effects sought by the invention to be used. Execution Mg may be present alone in the resulting coating or in combination with other alkaline earth elements such as beryllium, calcium, strontium, and barium. similar gold May be present in combination with the genera.

[0041] Optionally, in step c) to promote the formation of the iron-aluminum layer Execution During the coating 3~ 15% by weight, especially 3~ 11% by weight , especially 8.5 to 11% by weightA silicon ("Si") content of 0.01 wt. % may also be present, and this iron-aluminum layer adheres well to the iron surface established in process 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 wt. %, in particular at least 8.5 wt. %, has proven particularly favorable in this respect, so that a Si content of 3 to 15 wt. %, in particular 3 to 11 wt. %, in particular 8.5 to 11 wt. % allows the positive effect of Si to be used particularly reliably in practice.

[0042] In addition, in step c) Execution In the coating to be applied, optionally 1~ 5% by weight, especially 1~ 4% by weight, especially 1~ An Fe content of 3.5% by weight may also be present. 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 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 protection coating can be applied in any known manner in process step c) of the method according to the invention. Execution In this case, the so-called "hot dip aluminizing" is particularly suitable, in which the respective flat steel product is guided through a suitably heated molten bath constructed in accordance with the specifications of the invention. Such hot dip coating is particularly suitable for flat steel products in strip form with a thickness of up to 3 mm. For greater thicknesses, one of the first-mentioned vapor deposition processes (PVD, CVD) can also be used to provide a corrosion protection coating. give It can be used for

[0044] In work step c) according to the invention Execution Corrosion protection coating Coating Weight is usually 30 to 100 g / m per side 2 , especially 40-80g / m 2 Therefore, both sides of the coating Coating Weight Total weight is 60-200g / m 2 is.

[0045] Corrosion Protection Coating was given (Work step c) )rear The coated flat steel product 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] Corrosion Protection Coating was given (Work step c) rear After optional decorative rolling (operational step d)), if necessary, 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 components 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 and 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 to be 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 depend on the amount of heat energy J introduced into the flat steel product or board in step f). s is 100,000kJ s Higher than 800,000kJs can be estimated based on the condition that s can be calculated according to the following known equation: J s [kJ s ]=[(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 beginning 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 less 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, which corrosion-protective coating consists of (in % by weight): 3~ 15% Si, 1~ 5% Fe, 0.1-5% total of at least one alkaline earth similar gold The layer of the corrosion protection coating adjacent to the steel substrate is an interdiffusion layer consisting of ferrite with an Al content of up to 50% by weight, in particular at least 1% by weight, of 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 less than 300 μm over a measurement 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 in the direction of the free surface O. The layer Z2 is also made of aluminum iron, the majority of which is aluminum, and the alkaline earth metal. similar gold Trace amounts of silicon and unavoidable impurities may also be present. The intermediate layer Z2 is also non-porous.

[0060] The side of the second intermediate layer Z2 facing the free surface O is covered with an oxide layer OX, which is essentially composed of aluminum, silicon, and alkaline earth metals. similar gold On hot-formed components, the oxide layer can be up to 1.5 μm thick on average. On the surface of the oxide layer OX, which forms the free surface O of the corrosion protection coating K, a small number of crater-shaped pores P2 open to the environment are formed at a large distance from one another.

[0061] For comparison, components were made from flat steel products coated with an AlSi coating according to the prior art example 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 contained alkaline earth metals of the type added according to the invention. similar gold No genus was included.

[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] Metal coating Covering Before being formed 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 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 (working 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 manner Coating Weight The aluminum-based corrosion protection coatings Z1 to Z5 with AG are coated in a conventional manner, 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 Coating Weight AG, and the hot forming temperature T selected in tests V1 to V9 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. 1. A method for manufacturing sheet metal components from flat steel products provided with a corrosion-protective coating, comprising the following work steps: a) providing a flat steel product made from steel consisting of (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, 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 product in a continuous furnace having four zones A, B, C, D, through which the flat steel product passes successively, in which it is annealed under an annealing atmosphere consisting in each case of 0.1 to 15% by volume of hydrogen and the remainder nitrogen and technically unavoidable impurities, wherein 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, wherein the following specifications apply, and the combination of the dew point temperature and the annealing temperature is set differently in successive zones; Table 0 c) applying a corrosion protection coating to the flat steel product obtained in process step b), said corrosion protection coating consisting of (in % by weight) 8-10% Si, 2-3.5% Fe, 0.1-2% Mg, the balance being Al and unavoidable impurities; d) optionally decorative rolling the flat steel product provided with the corrosion protective coating; e) optionally, separating boards from the flat steel product, the boards being blanks that are cut from the flat steel product to form the sheet metal component, the dimensions of the boards being adapted to the dimensions of the sheet metal component; f) heating the flat steel product or the 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, if step e) has been carried out, into the board an amount of thermal energy Js of more than 100,000 kJs and not more than 800,000 kJs, Js is expressed by the following known equation: Js[kJs]=[(T2-T1)x c x t x m] / 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 beginning 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 the heating step, g) hot forming the flat steel product heated to the hot forming temperature or, if step e) was performed, the board heated to the hot forming temperature into the sheet metal component; h) cooling at least one portion of said component at a cooling rate sufficient to produce a hardened structure in said portion of said sheet metal component; A method comprising:

2. 2. The method according to claim 1, characterized in that the thickness of the flat steel product provided in work step a) is between 0.6 and 7 mm.

3. In the case of the annealing completed in work step b), the annealing temperature GT in zone A of the continuous furnace Ａ is 810 to 940°C, and the dew point temperature TP Ａ The method according to claim 1 or 2, characterized in that the temperature is -15°C to -25°C.

4. In the case of the annealing completed in work step b), the annealing temperature GT in zone B of the continuous furnace Ｂ The method according to any one of claims 1 to 3, characterized in that the temperature is 800 to 900 °C.

5. In the case of the annealing completed in work step b), the annealing temperature GT in the zone C of the continuous furnace Ｃ is 800 to 920°C, and the dew point temperature TP Ｃ The method according to any one of claims 1 to 4, wherein the temperature is between -30°C and -50°C.

6. In the case of the annealing completed in work step b), the annealing temperature GT in the zone D of the continuous furnace Ｄ The method according to any one of claims 1 to 5, characterized in that the temperature is 780 to 930 °C.

7. 7. The method according to claim 1, wherein the lambda value λ, representing the ratio by mass of air to fuel in the annealing atmosphere introduced into the continuous furnace and maintained in zones A to D, is between 0.95 and 1.1 for the annealing completed in work step b).

8. 8. The method according to claim 1, wherein the corrosion protection coating applied to the flat steel product in work step c) has a Mg content of 0.11 to 0.6% by weight.

9. The coating weight of the corrosion protection coating applied to the flat steel product in work step c) is between 30 and 100 g / m² per coated side of the flat steel product. ２ The method according to any one of claims 1 to 8, characterized in that

10. 10. Method according to any one of the preceding claims, characterized in that in work step c) the corrosion protection coating is applied to the flat steel product by means of hot dip coating.

11. 11. The method according to any one of claims 1 to 10, characterized in that the heating of the flat steel products or boards in work step f) is carried out by radiant heat in a continuous furnace, and the holding time is between 100 and 900 s.

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