Method for producing a steel component provided with a coating from a flat steel product

Abstract

The present invention relates to a method for producing a steel component by hot forming a flat steel product coated with an aluminium-based corrosion protection coating, a steel component produced by hot forming a flat steel product coated with an aluminium-based corrosion protection coating, and a flat steel product for producing such a steel component.The process comprises at least the following steps: a) providing a steel flat product coated with an aluminum-based corrosion protection coating; b) heating the coated steel flat product in a hot forming furnace whose furnace atmosphere contains at least 16% oxygen by volume and has a dew point of -20°C to +15°C; c) optionally holding the coated steel flat product in the hot forming furnace; d) transporting the heated steel flat product from the heating furnace to a forming tool and inserting the steel flat product into the forming tool; e) forming and simultaneously cooling the steel flat product in the forming tool.The flat steel product coated with an aluminum-based corrosion protection layer exhibits, after heating or optionally after holding, an aluminum oxide layer with an average thickness of at least 50 nm on the surface of the corrosion protection layer, wherein the aluminum oxide layer contains a maximum of 10 AlN rods per 1000 µm². The steel component obtained after forming exhibits, on the surface of the corrosion protection layer, an aluminum oxide layer with an average thickness of at least 50 nm, wherein the aluminum oxide layer contains a maximum of 10 AlN rods per 1000 µm².

Description

[0001] The invention relates to a method for producing a steel component by hot forming a flat steel product coated with an aluminum-based anti-corrosive coating. Furthermore, the invention relates to a steel component produced by hot forming a flat steel product coated with an aluminum-based anti-corrosive coating, as well as to a flat steel product for producing such a steel component.

[0002] For applications in automotive engineering or other industrial applications, materials with good corrosion resistance are required. Steel products coated with a corrosion-protective coating are suitable for this purpose. Such coatings are made of aluminum or aluminum alloys, or zinc or zinc alloys, for example, and protect the underlying steel substrate from corrosion, both through their mechanical barrier effect and through their cathodic protection.

[0003] When referring to flat steel products in this context, this refers to, for example, a steel strip or steel sheet, or a blank made from a steel sheet, such as a blank. Blanks are metal sheets that generally have more complex contours than the steel strips or steel sheets from which they are derived.

[0004] When we refer to a flat steel product coated with an aluminum-based corrosion protection coating, we mean a flat steel product coated with an aluminum-based corrosion protection coating on at least one side. A coated flat steel product therefore consists of a steel substrate, which can be, for example, a steel strip or a steel sheet, or a blank made from a steel sheet, such as a blank, and a corrosion protection coating present on at least one side of the steel substrate.

[0005] In hot forming, flat steel products are heated and then cooled in a tool. In principle, a distinction is made between two types of hot forming. In so-called direct hot forming, the flat steel product to be formed is first heated and immediately after heating placed in a forming tool, where it is formed and cooled simultaneously. In indirect hot forming, the flat steel product to be formed is first cold formed in a first forming tool, then heated and, after heating, cooled in a second forming tool, in which it is only slightly formed or not formed at all. Compared to direct hot forming, indirect hot forming offers the advantage of less tool wear when using uncoated flat steel products. The scale that forms on uncoated flat steel products during heating rubs against the surfaces of the forming tool, which therefore wears faster.Scale formation and thus tool wear can be reduced by coating the flat steel product with a corrosion protection coating.

[0006] WO 2008 / 053273 A1 discloses a method for producing a steel component coated with an aluminum-based anti-corrosive coating. The sheet metal to be formed, coated with aluminum or an aluminum alloy, is heated at a rate of 4 to 12 °C / s, held at temperatures of 880–940 °C for 3 to 13 minutes, then transferred to a forming tool, formed, and cooled at a rate of at least 30 °C / s. After forming, the component's coating has a multilayer structure with an interdiffusion layer, an intermediate layer, an intermetallic layer, and a surface layer. The interdiffusion layer consists of 86–95% Fe, 4–10% Al, and up to 5% Si. The intermediate layer consists of 39–47% Fe, 53–61% Al, and up to 2% Si. The intermetallic layer consists of 62-67% Fe, 30-34% Al, and 2-6% Si. The surface layer consists of 39-47% Fe, 53-61% Al, and up to 2% Si.The component should have good weldability and crack resistance.

[0007] However, even when using coated flat steel products, tool wear continues to be a problem that increases maintenance and repair costs. This is particularly true when forming flat steel products that are coated with an aluminum-based corrosion protection layer.

[0008] The invention is therefore based on the object of providing a method for producing a steel component provided with an aluminum-based anti-corrosive coating that minimizes the wear of a forming tool during direct hot forming. In particular, the invention is based on the object of providing a flat steel product for producing a steel component that is provided with an aluminum-based anti-corrosive coating and causes low hot-forming tool wear, as well as a steel component produced therefrom.

[0009] With regard to the method, this object is achieved by a method having the features specified in claim 1. Advantageous and preferred embodiments of the method according to the invention are specified in the claims that refer back to claim 1. With regard to the flat steel product, the object is achieved by a flat steel product having the features specified in claim 8. Advantageous and preferred embodiments of the flat steel product according to the invention are specified in the claims that refer back to claim 8. With regard to the steel component, the object is achieved by a steel component having the features specified in claim 12. Advantageous and preferred embodiments of the steel component according to the invention are specified in the claims that refer back to claim 12.

[0010] A method according to the invention for producing a steel component provided with an aluminum-based corrosion protection coating comprises at least the following process steps: a) Providing a flat steel product coated with an aluminium-based anti-corrosive coating; b) Heating the coated flat steel product in a hot forming furnace whose furnace atmosphere contains at least 16% oxygen by volume and has a dew point of -20 °C to +15 °C; c) Optionally holding the coated flat steel product in the hot forming furnace; d) Transporting the heated flat steel product from the heating furnace to a forming tool and placing the flat steel product in the forming tool; e) Forming and simultaneous cooling of the flat steel product in the forming tool.

[0011] In a process according to the invention, in process step a) a flat steel product is provided which is provided with an aluminum-based corrosion protection coating. The steel used for the steel substrate of the flat steel product is preferably a steel consisting of 0.1-0.4 wt.% C, preferably 0.15-0.3 wt.% C, particularly preferably 0.19-0.25 wt.% C, 0.5-3.0 wt.% Mn, preferably 0.9-1.6 wt.% Mn, 0.05-0.5 wt.% Si, preferably 0.15-0.4 wt.% Si, 0.01-0.2 wt.% Al, preferably 0.01-0.1 wt.% Al, 0.005-1.0 wt.% Cr, preferably 0.1-0.4 wt.% Cr, 0.0005-0.01 wt.% B, 0.001-0.2 wt.% V, preferably 0.001-0.010 wt.% V, particularly preferably 0.002 - 0.006 wt% V, 0.001 - 0.1 wt% Ti, 0.001 - 0.1 wt% Nb, optionally 0.01 - 0.4 wt% Ni, optionally 0.01 - 0.8 wt% Cu, optionally 0.002 - 1.0 wt% Mo, optionally 0.001 - 1.0 wt% W, up to 0.01 wt.-% N, and the remainder consists of iron and unavoidable impurities, with up to 0.1 wt.% P and up to 0.05 wt.% S being attributable to the unavoidable impurities.

[0012] Typically, steel substrates particularly suitable for the present invention have a thickness of 0.4 to 3 mm.

[0013] Where information is provided here regarding alloy contents and compositions of liquid or solid phases, this refers to weight or mass, respectively, unless expressly stated otherwise. Where information is provided here regarding atmospheric compositions, this refers to volume, unless expressly stated otherwise.

[0014] The anti-corrosive coating on the steel substrate consists of pure aluminum or an aluminum alloy. Suitable aluminum alloys typically consist of 3 - 15 wt.% Si, preferably 7 - 12 wt.% Si, particularly preferably 9 - 10 wt.% Si and optionally one or more elements selected from the group consisting of iron, transition metals other than iron, alkaline earth metals or mixtures thereof, in the following contents: 2 - 3.5 wt.% Fe, 0.05 - 2 wt.% transition metals other than Fe, preferably 0.1 - 0.5 wt.% transition metals other than Fe, particularly preferably 0.15 - 0.4 wt.% transition metals other than Fe, 0.05 - 2 wt.% alkaline earth metals, preferably 0.1 - 0.5 wt.% alkaline earth metals, particularly preferably 0.15 - 0.4 wt.% alkaline earth metals, and the remainder of aluminum and unavoidable impurities.Regarding transition metals, a distinction is made here between iron and other transition metals because iron can be present in higher concentrations than other transition metals. Iron, other transition metals, and also elements from the alkaline earth group lead to a dense, thin, and covering oxide layer that reduces the penetration of diffusible hydrogen. The following alkaline earth and transition metals have proven particularly suitable: Mg, Ca, Sr, Ba, Zr, and Ti.

[0015] The corrosion protection coating can be applied to the steel substrate in a conventional manner, for example, by means of a hot-dip coating process. Other application methods that allow the application of a corrosion protection layer are also conceivable. A suitable corrosion protection coating is typically a maximum of 30 µ m thick per side, especially 10 to 30 µ m thick per side.

[0016] In process step b), the coated flat steel product is heated for hot forming, which is carried out in process step e). The invention is based on the finding that the wear of the forming tool is influenced by the composition of the atmosphere in the heating furnace. The heating furnace is also referred to as a hot forming furnace because the coated steel substrate is heated therein before and for the hot forming process. A preferred example of a hot forming furnace is a roller hearth furnace. At an oxygen content of 6 vol% or more in the furnace atmosphere, an aluminum oxide layer forms on the aluminum-based corrosion protection coating. In the presence of nitrogen, aluminum nitride particles (AIN) in rod or needle form form on the surface of the coated flat steel product in addition to the aluminum oxide layer. AIN is generally very hard.The hardness of AlN is typically about 1230 HV1. If these AlN rods or needles grow beyond the aluminum oxide layer, they damage the surface of the forming tool during the forming process and cause increased wear. Particularly high levels of wear can be observed at oxygen concentrations of up to 13 vol. Tool wear decreases with increasing oxygen content.

[0017] The invention is based on the finding that, at oxygen contents in the hot-forming furnace atmosphere of at least 16 vol.%, the formation of AlN rods is hindered to such an extent that significantly fewer AlN rods grow beyond the aluminum oxide layer than at oxygen contents of less than 16 vol. The heating of the coated flat steel product in process step b) therefore takes place in a hot-forming furnace whose atmosphere contains at least 16 vol.%, preferably at least 18 vol.%, and particularly preferably at least 19 vol.% oxygen.

[0018] In atmospheres with at least 16 vol.% oxygen, aluminum oxide layers with an average thickness of 50 nm or more regularly form. Since the thickness of the aluminum oxide layer can vary across the area covered by the aluminum oxide layer, the average thickness of the aluminum oxide layer is understood here to be the arithmetic mean of at least three thickness values ​​determined at different locations on the aluminum oxide layer. Aluminum oxide layers with an average thickness of at least 50 nm have proven beneficial for reducing tool wear. It is assumed that aluminum oxide layers with an average thickness of at least 50 nm have a smoothing effect. This means that AlN rods do not grow beyond this layer thickness.Furthermore, aluminum oxide layers with an average thickness of at least 50 nm, preferably at least 100 nm, have proven beneficial for preventing caking of the tool on the formed part and for reducing scoring on the tool. The aluminum oxide layer acts as a release agent between the forming tool and the surface of the flat steel product being formed. In the case of insufficiently annealed flat steel products, it separates the liquid phase components from the tool surface. Aluminum-based corrosion protection coatings on which aluminum oxide layers with an average thickness of at least 50 nm have formed typically contain Fe(Al1-xSix), τ1Fe3(Al,Si)5, Fe2Al5, and Fe3(Al1-xSix).

[0019] Covering aluminum oxide layers have proven particularly effective in reducing tool wear. Covering aluminum oxide layers, in this context, are aluminum oxide layers that completely cover the surface of the corrosion protection coating. At oxygen contents of less than 16 vol.%, however, oxide layers with an average thickness of less than 50 nm form. These are often not covering, which negatively impacts tool wear.

[0020] In principle, a furnace atmosphere containing up to 100 vol.% oxygen would be possible to reduce AlN formation. However, pure oxygen is highly explosive. For safety reasons, the oxygen content of the furnace atmosphere is preferably less than 50 vol.% and particularly preferably no more than 30 vol. For cost reasons and to improve weldability, painting, and bonding properties, the oxygen content of the furnace atmosphere can be limited to a maximum of 25 vol.%, preferably a maximum of 20 vol.%.

[0021] In a preferred embodiment, the atmosphere of the hot forming furnace contains at least 50 vol.% nitrogen in addition to oxygen to reduce the explosiveness of the furnace atmosphere. In accordance with the minimum oxygen content according to the invention, the nitrogen content of the furnace atmosphere is preferably less than 84 vol.%, particularly preferably less than 82 vol.%, and most preferably less than 81 vol.%. In this preferred embodiment, it is possible to provide a method for producing a steel component provided with an aluminum-based anti-corrosive coating, in which the wear of the forming tool can be minimized despite the presence of nitrogen in the furnace atmosphere.

[0022] In a further preferred embodiment, the atmosphere of the hot forming furnace consists of at least 16 vol.% oxygen, more preferably at least 18 vol.% oxygen, most preferably at least 19 vol.% oxygen, and the remainder in each case of nitrogen and unavoidable impurities.

[0023] In a further preferred embodiment, in addition to oxygen and nitrogen, one or more noble gases can be added to the atmosphere of the hot forming furnace in proportions of up to 1.5 vol.%, so that the atmosphere of the hot forming furnace consists of at least 16 vol.% oxygen, particularly preferably at least 18 vol.% oxygen, very particularly preferably at least 19 vol.% oxygen, up to 1.5 vol.% of one or more noble gases, and the remainder in each case of nitrogen and unavoidable impurities.

[0024] In a further preferred embodiment, one or more noble gases can be added to the atmosphere of the hot forming furnace without adding significant amounts of nitrogen, so that the atmosphere of the hot forming furnace consists of at least 16 vol.% oxygen, particularly preferably at least 18 vol.% oxygen, very particularly preferably at least 19 vol.% oxygen, and the remainder in each case of noble gases and unavoidable impurities.

[0025] In all embodiments, the furnace atmosphere may be contaminated with other gases in amounts of up to 1.1 vol.%. Furthermore, the atmosphere of the hot forming furnace may also contain water vapor with a water content of 0.5 to 15 g / m³.

[0026] For safety reasons, furnace atmospheres whose composition is non-explosive and not harmful to health are preferred.

[0027] To ensure the oxygen content in the hot forming furnace according to the invention, the dew point TP of the furnace atmosphere is set to values ​​between -20 °C and +15 °C, preferably between -15 °C and +15 °C. A dew point lower than -20 °C leads to a reduction in the oxygen content in the hot forming furnace, whereas values ​​higher than +15 °C can lead to increased hydrogen uptake.

[0028] In a preferred embodiment, a further reduction in tool wear but also an improvement in weldability as well as painting and bonding suitability can be achieved if, during heating in step b) in the hot forming furnace, the ratio of the volume fraction of nitrogen, V(N2), to the volume fraction of oxygen V(02) is 3.80 to 5.30: 3,80 ≤ V N2 / V O2 ≤ 5,30 where V(N2) = nitrogen volume in the hot forming furnace in vol%, V(O2) = oxygen volume in the hot forming furnace in vol%. Nitrogen-to-oxygen ratios of 4.0 to 5.2 have proven particularly effective.

[0029] In a further preferred embodiment, the steel flat product is heated in step b) in a hot forming furnace with furnace temperatures of 830 to 950 °C to a temperature T1 of between 830 and 950 °C, preferably between 900 and 950 °C. The heating preferably takes place in a continuous furnace, particularly preferably in a roller hearth furnace. The furnace temperature corresponds to the temperature to which the steel flat product is to be heated. Typically, the steel flat product is introduced into the hot forming furnace at room temperature. Residence times of the steel flat product in the hot forming furnace of 2 to 15 minutes have proven particularly favorable.

[0030] Optionally, in step c), the steel flat product can be held at this temperature in the hot forming furnace after reaching temperature T1. In this case, the residence time of the steel flat product in the hot forming furnace is also 2 to 15 minutes and includes the time for heating to furnace temperature as well as the time for optionally holding the steel flat product at furnace temperature. The furnace atmosphere during the holding period is the same as in step b) during heating.

[0031] The flat steel product provided with a corrosion protection coating obtained in process step b) after heating or optionally in process step c) after optional holding has an aluminum oxide layer on the surface of the corrosion protection coating with an average thickness of at least 50 nm, preferably at least 100 nm. Lower average thicknesses are often not covering the surface, which has a negative impact on tool wear. With regard to the influence of the aluminum oxide layer thickness on tool wear, there is no upper limit on the average thickness of the aluminum oxide layer. For operational reasons, average thicknesses of a maximum of 2300 nm are typically achieved. Frequently, the oxide layer thicknesses are only up to 1500 nm on average.However, if flat steel products with improved weldability and improved painting and bonding properties are to be produced, thinner aluminum oxide layers with an average thickness of no more than 150 nm have proven to be advantageous.

[0032] After heating or optional holding, AIN rods may be present on the aluminum oxide layer. After heating or optional holding, the aluminum oxide layer present on the surface of a flat steel product according to the invention contains µ m 2< a maximum of 10 AIN rods. A larger number of AIN rods per 1000 µm 2< has a negative impact on the rework and cleaning intervals within a forming campaign. A forming campaign is understood to be the totality of all blanks to be formed in a sequence under the same hot forming conditions. A forming campaign typically comprises up to 1500 blanks. The forming of a blank in the forming tool is also referred to as one stroke. While at a density of 11 or more AIN rods per 1000 µ m 2< area, a revision and cleaning of the hot forming tools must be carried out after 40 - 60 % of the component surface has been penetrated during a forming campaign, at a density of maximum 10 AIN rods per 1000 µm 2< area, however, a revision and cleaning of the hot forming tools should only be carried out after more than 60% of the component surface has been processed in a forming campaign, in particular after 70 - 80% of the component surface has been processed in a forming campaign. Tool wear can be further reduced to a revision and cleaning of the hot forming tools after more than 80% of the component surface has been processed in a forming campaign, if the density of the AIN rods per 1000 µ m 2< area is limited to a maximum of 5. Particularly low wear can be achieved with a rod density of a maximum of 2, in particular zero AIN rods per 1000 µ m 2< area can be achieved. Here, reworking and cleaning of the hot forming tools is only necessary after 100% of the component surface has been penetrated.

[0033] In this context, the term "component surface area of ​​a forming campaign" refers to the surface area of ​​the blanks that were hot-formed in the tool during a forming campaign. For forming campaigns with, for example, 1000 blanks, this means that a reworking and cleaning of the tool is not necessary at a density of 11 or more AIN rods per 1000 µ m 2< area after just 400-600 strokes, with a maximum of 10 AIN rods per 1000 µ m 2< area only after more than 600, especially after 700-800 strokes, with a maximum of 5 AIN rods per 1000 µ m 2< area only after more than 800 strokes and with a maximum of 2 AIN rods per 1000 µ m 2< area only after completion of the forming campaign.

[0034] The flat steel product obtained in process step b) after heating or optionally in process step c) after optional holding has a corrosion protection coating whose thickness is typically 30 to 130 g / m 2< or 12 - 60 µ m per side. Beneath the aluminum oxide layer, the corrosion protection coating typically contains four intermetallic phases: Fe(Al1-xSix), τ1 Fe3(Al,Si)5, Fe2Al5 and Fe3(Al1-xSix).

[0035] In a preferred embodiment, tool wear can be further reduced if the AIN rods are rotated by a maximum of 6 µ m, preferably by a maximum of 2 µ m from the aluminum oxide layer, which means that they grow at most 6 µ m, preferably by a maximum of 2 µ m, particularly preferably not more than 1.0 µm from the aluminum oxide layer. If AlN rods occur, their height h, or the length with which the AlN rods protrude from the aluminum oxide layer, is typically 0.5 - 6 µ m. The height h of the AlN rods refers to their protrusion from the aluminum oxide layer. The height of an AlN rod is defined as the vertical line from the tip of the AlN rod to the surface of the aluminum oxide layer.

[0036] In a further preferred embodiment, the AIN rods have a diameter D of at most 2 µ m, preferably not more than 1.1 µ m, which also has a positive effect on tool wear. Their diameter is typically 1 - 2 µ m. The diameter of an AlN rod is the width of the AlN rod running halfway up the height of the AlN rod and parallel to the surface of the aluminum oxide layer.

[0037] The negative effects on tool wear can be reduced particularly effectively if the AIN rods are heated by a maximum of 6 µ m, preferably by a maximum of 2 µ m, protrude from the aluminum oxide layer, as well as a maximum of 2 µ m thick.

[0038] The steel flat product, coated with a corrosion protection coating obtained in process step b) after heating or, if applicable, in process step c) after optional holding, is transported from the hot forming furnace to a forming tool in process step d) and inserted into it. Transport from the hot forming furnace to the forming tool and insertion of the steel flat product into the tool takes place in a conventional manner. Transfer times of 3 to 15 seconds have proven particularly favorable. In this context, the transfer time is understood to be the period between furnace discharge and tool closure.

[0039] In process step e), the flat steel product is formed into a component in the forming tool using conventional methods and simultaneously cooled. Cooling also occurs conventionally. Forming preferably takes place immediately after the flat steel product is inserted into the tool. Cooling rates of 20 to 1000 K / s, preferably 25 to 500 K / s, have proven particularly favorable. These cooling rates have proven particularly suitable for hardening the flat steel product.

[0040] In a preferred embodiment, the method for producing a steel component provided with an aluminum-based corrosion protection coating consists only of the method steps a) to e), i.e. it does not comprise any further method steps beyond the method steps a) to e).

[0041] The steel component obtained by the process according to the invention after carrying out step e) is provided with an aluminum-based corrosion protection coating, in particular, it has an aluminum-based corrosion protection coating. During heating in step b) and optionally during the optional holding in process step c), iron atoms may diffuse from the steel substrate into the corrosion protection coating, which may increase the thickness of the corrosion protection coating. Thus, the thickness of the corrosion protection coating of the steel component after hot forming is typically 30 to 130 g / m² or 12 to 60 µm per side. On the surface of the corrosion protection coating, the steel component has an aluminum oxide layer with an average thickness of at least 50 nm, preferably at least 100 nm. The average thickness of the aluminum oxide layer is at most 2300 nm, preferably at most 1500 nm, particularly preferably at most 150 nm. Protruding from the aluminum oxide layer per 1000 µ m 2< area at most 10, preferably at most 5, particularly preferably at most 2, most preferably no AlN rods. This means that on the aluminum oxide layer in particular at most 10 AlN rods per 1000 µ m 2< , preferably at most 5, particularly preferably at most 2, very particularly preferably no AIN rods are present.

[0042] In a further preferred embodiment, the AIN rods protruding from the aluminum oxide layer of the steel component are at most 6 µ m, preferably no more than 2 µ m, particularly preferably not more than 1.0µ m high and have a maximum diameter of 2 µ m, preferably not more than 1.1 µ m. Below the aluminum oxide layer, the corrosion protection coating typically has four intermetallic phases: Fe(Al1-xSix), τ1 Fe3(Al,Si)5, Fe2Al5 and Fe3(Al1-xSix).

[0043] In particular, a steel component according to the invention can be produced by a method according to one of claims 1 to 7. In particular, a steel component according to the invention can be produced using a flat steel product according to one of claims 8 to 11.

[0044] The invention is explained in more detail below with reference to drawings and exemplary embodiments. Fig. 1 : Schematic representation of the growth of the AIN rods and the aluminum oxide layer Fig. 2 : Schematic representation of the aluminum oxide layer and the AIN rods to explain the abbreviations used

[0045] In Figure 1 The growth of the AIN rods is shown schematically. An aluminum-based corrosion protection coating 2 is applied to a steel substrate 1. Initially, a thin aluminum oxide layer 3 forms on the aluminum-based corrosion protection coating 2 ( Fig. 1a The aluminum oxide layer is very brittle. During heating of the coated flat steel product, the corrosion protection coating beneath the aluminum oxide layer expands and melts at least partially. Due to the volume expansion, the brittle aluminum oxide layer 3 cracks in some places. In the presence of nitrogen, AlN nitrides can nucleate in the gaps of the aluminum oxide layer 3, which initially grow into small AlN rods 4 ( Fig. 1b )). Due to their elongated shape, the rods can also be called AIN needles. The AIN rods 4 typically have a round cross-section with a nearly constant diameter D ( Fig. 2 ). The AIN rods continue to grow in length and width during the heating process, while the thickness of the aluminum oxide layer increases ( Fig. 1c) and Fig. 1d )). At the end of the heating process, the AIN rods protrude from the aluminum oxide layer with a height h ( Fig. 2 ).

[0046] For testing, four forming campaigns were conducted, each with 1,000 blanks. Steel strips with the compositions listed in Table 1 were provided and coated on both sides using conventional hot-dip coating with aluminum-based corrosion protection coatings with the compositions listed in Table 2. Blanks were punched out of the coated steel strips using conventional methods and placed in a roller hearth furnace for heating. The blanks were heat-treated under the conditions listed in Table 3.

[0047] The blanks were heat-treated in hot-forming furnaces with furnace atmospheres containing oxygen contents of 15 to 20 vol.% and dew points of -15°C to +10°C. In principle, the furnace atmospheres can be adjusted in various ways. One possibility is to first allow the hot-forming furnace to cool down before introducing the furnace atmosphere according to the invention, which can typically take up to 20 hours, and to reheat it after the furnace atmosphere has been introduced. Another possibility is to introduce the furnace atmosphere according to the invention into a warm hot-forming furnace, which can have a beneficial effect on reducing the heating-up time. Another possibility, which contributes to further shortening the heating-up times, is to heat the furnace atmosphere according to the invention before introducing it into the warm hot-forming furnace.In experiments 1 to 4, the furnace atmospheres were adjusted by supplying the respective gases into the furnace via a gas line.

[0048] The 1.5 mm thick blanks were heated to flat steel product temperatures T1 of 925 °C (Test 1), 910 °C (Tests 2 and 4), and 930 °C (Test 3). The residence times t of the blanks in the hot forming furnace, which encompasses the time from heating up to leaving the furnace, were between 4 and 6 minutes. The heated blanks were removed from the hot forming furnace, samples were taken, and a portion of the blanks was placed in a press for forming within 3 to 10 s, formed into a component, and then sampled again. Several blanks were produced for each test condition until tool wear necessitated termination for cleaning and, if necessary, maintenance of the tool. For each test, one blank or one formed component was sampled before and after forming at the beginning, after 250 strokes, and after 500 strokes.Sampling of the formed blanks was carried out in flat areas of the components where the blanks were subjected to no or only minimal forming.

[0049] The number per area, the diameter, and the height of the AlN rods were determined using a scanning electron microscope at a magnification of V = 2000x on three flat sections per circuit board, each measuring 10 x 15 mm². Three areas of 100 mm² were measured, and the determined values ​​were arithmetically averaged. The thickness of the oxide layer was determined on three samples per circuit board using X-ray photoelectron spectroscopy (XPS). The samples were degreased with n-heptane, rinsed with propanol, and blown dry with air. The samples were each mounted on a sample carrier, introduced into the measuring chamber of the X-ray photoelectron spectroscope, and examined under high vacuum. The chamber pressure was < 5 x 10 ^ 8 mbar. Argon was used as the bombardment gas. The radiation was excited as Al K α with a bombardment voltage of 2 or 4 kV. At least one thickness measurement of the aluminum oxide layer was carried out on each sample.The thickness measurement results of all samples from a circuit board were arithmetically averaged and are referred to herein as the average aluminum oxide layer thickness or average thickness. A height was determined for each AlN rod. For AlN rods inclined toward the surface, the vertical line from the AlN rod tip to the surface was measured as height h. At half the height of each AlN rod, a width value was measured, which is referred to as the diameter of the AlN rod. For AlN rods inclined toward the surface, the width was measured along a section running parallel to the surface.

[0050] The condition of the forming tools was visually inspected on-site by expert personnel. The visual inspection of the forming tools was carried out after every 100 strokes.

[0051] In test 1, a maximum of 2 AIN rods per 1000 µ m 2< with a height of not more than 0.7 µ m and a maximum diameter of 1.1 µ m. The oxide layer had an average thickness of 120 nm before and after hot forming. Under the conditions of Test 1, 700 strokes could be performed before cleaning of the forming tool was necessary. The forming tool exhibited only minimal scoring, so no premature rework was necessary.

[0052] In test 2, no AIN rods were found on the samples taken before and after hot forming, and after hot forming a maximum of 2 AIN rods per 1000 µ m 2< with a height of not more than 0.5 µ m and a diameter of not more than 1 µm. The oxide layer had an average thickness of 103 nm before hot forming and an average thickness of 135 nm after hot forming. Under the conditions of Test 2, 1000 strokes could be performed before cleaning of the forming tool was necessary. The forming tool also exhibited only minimal scoring, so no premature rework was necessary in this case either.

[0053] In Experiment 3, the furnace atmosphere did not contain only 15 vol.% oxygen according to the invention. On average, 13 AIN rods per 1000 were found in the samples taken before and after hot forming. µ m 2<. The height of the AIN rods was between 3 and 5.5 µ m, the diameter was between 1.5 µ m and 2 µm. The oxide layer had an average thickness of 45 nm after hot forming. Under the conditions of test 3, there was massive dust formation in the tool and abrasive damage to the radii, so the tool had to be cleaned after 500 strokes and the radii had to be reworked after just 1000 strokes.

[0054] In test 4, the furnace atmosphere contained 21 vol.% oxygen, 78% nitrogen, the remainder noble gases and unavoidable impurities. A maximum of 1 AIN rod per 1000 µ m 2< with a height of not more than 0.6 µ m and a maximum diameter of 1.1 µm. The oxide layer had an average thickness of 2100 nm. Under the conditions of Test 1, 800 strokes could be performed before cleaning of the forming tool was necessary. The forming tool exhibited only minimal scoring, so no premature rework was necessary. However, the increased oxide layer formation led to a deterioration in weldability, painting, and bonding properties. Table 1. Data in wt.%, balance iron and unavoidable impurities Steel C Si Mn P S Al Nb Ti B Other A 0,09 0,33 0,9 0,025 0,020 0,010 0,08 0,010 0,004 - B 0,22 0,35 1,35 0,022 0,008 0,010 - 0,03 0,004 Cr+Mo 0.4 C 0,37 0,3 1,2 0,022 0,007 0,013 - 0,11 0,003 Cr+Mo 0.38 Table 2. Data in wt.%, remainder aluminum coating Si Fe unavoidable contamination K1 9 2,5 1 K2 10 3,5 1 K3 8,5 3 1 Table 3. Data in wt.%, remainder aluminum Attempt Steel coating Coating thickness before heating [g / m 2 per side] T1 [°C] TP [°C] Oven atmosphere t [min] 1 A K1 70 925 -5 18% by volume 02, 1% by volume noble gases, balance N2 5 2 B K2 40 910 +10 20% by volume 02, 0.8% by volume noble gases, balance N2 6 3 C K3 75 930 -15 15% by volume 02, 0.8% by volume noble gases, balance N2 4 4 B K1 70 925 -5 21% by volume 02, 0.8% by volume noble gases, balance N2 5

[0055] The invention further relates to the embodiments of the invention described in the following sentences: 1. A method for producing a steel component provided with an aluminum-based anti-corrosive coating, comprising the following method steps: a) providing a flat steel product provided with an aluminum-based anti-corrosive coating; b) heating the coated flat steel product in a hot-forming furnace whose furnace atmosphere contains at least 16% oxygen by volume and has a dew point of -20 °C to +15 °C; c) optionally holding the coated flat steel product in the hot-forming furnace; d) transporting the heated flat steel product into a forming tool; e) forming and simultaneously cooling the flat steel product in the forming tool. 2. Method according to sentence 1 characterized in thatthe steel used for the steel substrate of the flat steel product consists of 0.1 - 0.4 wt% C, 0.5 - 3.0 wt% Mn, 0.05 - 0.5 wt% Si, 0.01-0.2 wt% Al, 0.005 - 1.0 wt% Cr, 0.0005 - 0.01 wt% B, 0.001 - 0.2 wt% V, 0.001 - 0.1 wt% Ti, 0.001 - 0.1 wt% Nb, optionally 0.01 - 0.4 wt% Ni, optionally 0.01 - 0.8 wt% Cu, optionally 0.002 - 1.0 wt% Mo, optionally 0.001 - 1.0 wt% W, up to 0.01 wt% N, and the remainder of iron and unavoidable impurities, with up to 0.1 wt.% P and up to 0.05 wt.% S are to be attributed to the unavoidable impurities. 3. Process according to sentence 1 or 2, characterized in that the corrosion protection coating present on the steel substrate consists of 3 - 15 wt.% Si and optionally one or more elements selected from the group consisting of iron, transition metals other than iron, alkaline earth metals and mixtures thereof, in the following contents: 2 - 3.5 wt.% Fe, 0.05 - 2 wt.-% of transition metals other than Fe, 0.05 - 2 wt.% alkaline earth metals, and the remainder of aluminum and unavoidable impurities. 4. A process according to any one of the preceding sentences, characterized in that the atmosphere of the hot-forming furnace consists of at least 16 vol.% oxygen, up to 1.5 vol.% of one or more noble gases, and the remainder of nitrogen and unavoidable impurities. 5. A process according to any one of the preceding sentences, characterized in that in the hot-forming furnace, the ratio of the volume fraction of nitrogen to the volume fraction of oxygen is between 3.80 and 5.30. 6. A process according to any one of the preceding sentences, characterized in that the coated flat steel product is heated to a temperature between 830 and 950°C. 7. A process according to any one of the preceding sentences, characterized in that the residence time of the flat steel product in the hot-forming furnace is between 2 and 15 minutes.Flat steel product with an aluminum-based anti-corrosive coating. characterized in that it has an aluminium oxide layer with an average thickness of at least 50 nm on the surface of the corrosion protection coating, with a maximum of 10 AIN rods per 1000 µ m 2< are present. 9. Steel flat product according to sentence 8, characterized in that the aluminum oxide layer has an average thickness of at most 2300 nm. 10. Steel flat product according to sentence 8 or 9, characterized in that the AIN rods are at most 6 µ m protrude from the aluminum oxide layer. 11. Steel flat product according to one of sentences 8 to 10, characterized in that the AIN rods have a diameter of at most 2 µm. 12. A steel component provided with an aluminium-based corrosion protection coating, characterized in that it has an aluminium oxide layer with an average thickness of at least 50 nm on the surface of the corrosion protection coating, with a maximum of 10 AIN rods per 1000 µ m 2< are present. 13. Steel component according to sentence 12, characterized in that the average thickness of the aluminum oxide layer is at most 2300 nm. 14. Steel component according to sentence 12 or 13, characterized in that it is produced by a method according to one of sentences 1 to 7. 15. Steel component according to one of sentences 12 to 14, characterized in that it is produced using a flat steel product according to one of sentences 8 to 11.

Claims

1. A flat steel product provided with an aluminum-based corrosion protection coating, wherein the steel used for the steel substrate of the flat steel product consists of 0.1 - 0.4 wt% C, 0.5 - 3.0 wt% Mn, 0.05 - 0.5 wt% Si, 0.01 - 0.2 wt% Al, 0.005 - 1.0 wt% Cr, 0.0005 - 0.01 wt% B, 0.001 - 0.2 wt% V, 0.001 - 0.1 wt% Ti, 0.001 - 0.1 wt% Nb, optionally 0.01 - 0.4 wt% Ni, optionally 0.01 - 0.8 wt% Cu, optionally 0.002 - 1.0 wt% Mo, optionally 0.001 - 1.0 wt% W, up to 0.01 wt% N, and the remainder being iron and unavoidable impurities, with up to 0.1 wt% P and up to 0.05 wt% S being attributable to the unavoidable impurities, characterized in that the flat steel product has an aluminium oxide layer with an average thickness of at least 50 nm on the surface of the corrosion protection coating, with a maximum of 10 AIN rods per 1000 µ m 2 are present.

2. Flat steel product according to claim 1 characterized in that the aluminum oxide layer has an average thickness of not more than 2300 nm.

3. Flat steel product according to claim 1 or 2 characterized in that the aluminum oxide layer has an average thickness of not more than 150 nm.

4. Flat steel product according to one of claims 1 to 3 characterized in that the AIN rods by a maximum of 6 µ m protrude from the aluminum oxide layer.

5. Flat steel product according to one of claims 1 to 4 characterized in that the AIN rods have a diameter of no more than 2 µ m.

6. Flat steel product according to one of claims 1 to 5, characterized in that the thickness of the anti-corrosive coating 12-60 µ m per side.

7. Flat steel product according to one of claims 1 to 6, characterized in that The corrosion protection coating below the aluminum oxide layer contains four intermetallic phases: Fe(Al 1-x Si x), τ1 Fe3(Al,Si)5, Fe2Al5 and Fe3(Al 1-x Si x ) contains.

8. Steel component coated with an aluminum-based corrosion protection coating characterized in that it has an aluminium oxide layer with an average thickness of at least 50 nm on the surface of the corrosion protection coating, with a maximum of 10 AIN rods per 1000 µ m 2 are present.

9. Steel component according to claim 8 characterized in that the average thickness of the aluminum oxide layer is at most 2300 nm.

10. Steel component according to one of claims 8 or 9, characterized in that the aluminum oxide layer has an average thickness of not more than 150 nm.

11. Steel component according to one of claims 8 to 10, characterized in that the thickness of the anti-corrosive coating 12-60 µ m per side.

12. Steel component according to one of claims 8 to 11, characterized in thatThe corrosion protection coating below the aluminum oxide layer contains four intermetallic phases: Fe(Al 1-x Si x ), τ1 Fe3(Al,Si)5, Fe2Al5 and Fe3(Al 1-x Si x ) contains.

13. Steel component according to one of claims 8 to 12 characterized in that the AIN rods by a maximum of 6 µ m protrude from the aluminum oxide layer.

14. Steel component according to one of claims 8 to 13 characterized in that the AIN rods have a diameter of no more than 2 µ m.

15. Steel component according to one of claims 8 to 14, characterized in that the AIN rods protruding from the aluminium oxide layer of the steel component are not more than 2 µ m high and a maximum diameter of 1.1 µ m have.

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