EP3250727B1 - Component made of press-form-hardened, aluminum-based coated steel sheet, and method for producing such a component - Google Patents

Description

The invention relates to a component made of press-form-hardened sheet steel coated on the basis of aluminum, the coating having a coating which is applied in the hot-dip process and which contains aluminum and silicon. The invention also relates to a method for producing such a component. In particular, the coating relates to an aluminum-silicon coating.

It is known that hot-formed steel sheets are being used more and more, especially in automobile construction. The process, also known as press hardening, can be used to produce high-strength components that are primarily used in the bodywork area. Press hardening can in principle be carried out using two different method variants, namely using the direct or indirect method. While the process steps of forming and hardening run separately in the indirect process, they take place together in one tool in the direct process. In the following only the direct method is considered.

In the direct process, a steel sheet is heated above the so-called austenitizing temperature (Ac3). The steel sheet heated in this way is then transferred to a forming tool and formed into the finished component in a single-stage forming step and cooled by the cooled forming tool at a speed that is above the critical cooling rate of the steel, so that a hardened component is produced. The steel sheet itself is usually cut out of a steel strip, usually wound up as a coil, and then processed further. The sheet steel to be formed is often referred to as a blank.

Known hot-formable steels for this area of application are, for example, the manganese-boron steel "22MnB5" and recently also air-hardenable steels according to the European patent EP 2 449 138 B1 .

In addition to uncoated steel sheets, steel sheets with anti-scaling protection are also used for press hardening (e.g. for automotive body construction) used. In addition to the increased corrosion resistance of the finished component, the advantages here are that the blanks or components do not scale in the furnace, which reduces wear on the press tools due to flaked scale and the components often do not have to be blasted before further processing.

For press hardening, the following (alloy) coatings applied by hot dipping are currently known: aluminum-silicon (AS), zinc-aluminum (Z), zinc-aluminum-iron (ZF / Galvannealed), zinc-magnesium-aluminum (ZM ), as well as electrolytically deposited coatings made of zinc-nickel or zinc, the latter being converted into an iron-zinc alloy layer before hot forming. These anti-corrosion coatings are usually applied to the hot or cold strip in a continuous process.

The production of components by quenching preliminary products made of press-hardenable steels by hot forming in a forming tool is out of the German patent DE 601 19 826 T2 known. Here, a sheet metal blank previously heated above the austenitizing temperature to 800 - 1200 ° C and possibly provided with a metallic coating made of zinc or based on zinc is formed into a component in a tool that is cooled in some cases by hot forming, with rapid heat extraction during the forming process The sheet metal or component in the forming tool experiences quench hardening (press hardening) and the required strength properties are achieved through the resulting martensitic hardening structure.

The production of components by quenching precursors coated with an aluminum alloy made of press-hardenable steels by hot forming in a forming tool is out of the German patent DE 699 33 751 T2 known. Here, a sheet metal coated with an aluminum alloy is heated to over 700 ° C before forming, whereby an intermetallic alloyed compound based on iron, aluminum and silicon is formed on the surface and then the sheet metal is formed and cooled at a rate above the critical cooling rate .

From the published patent application US 2011/0300407 A1 is a method of manufacture a press hardened steel sheet for use in the automotive industry. In the hot-dip process, the steel sheet is provided with an aluminum-silicon (AS) coating with a layer of 20 to 80 g / m ² , heated to temperatures above 820 ° C. and the temperature is maintained for some time (approx. 3 minutes). Different intermetallic phases are formed in the coating, for example Fe ₃ Al, FeAl or Fe-Al ₂ O ₃ . After hot forming using a press, the product is still cooled in the press.

Also the European patent application EP 2 312 011 A1 describes a process for the production of metallic coatings on molded parts for use in automobile construction. For this purpose, the molded part is provided with an aluminum alloy in a molten bath and then subjected to a heat treatment in an oxidizing atmosphere to produce a high-temperature-resistant aluminum oxide layer. Anodic oxidation is also planned after the heat treatment.

The German patent DE 198 53 285 C1 presents a method for producing a protective layer on martensitic steel. In a protective gas atmosphere (argon with 5% H ₂ ), the steel to be coated is immersed in a melt of aluminum or an aluminum alloy, cooled and then hot isostatically pressed at the austenitizing temperature. The aluminum protective layer produced in this way is between 100 and 200 μm thick and should contain an approximately 1 μm thick aluminum oxide layer on its surface, no further information is given on its formation or preservation.

From the European patent application EP 2 017 074 A2 a motor vehicle pipeline made of a steel pipe with an aluminum layer is known, which is applied by means of hot-dip coating. A thickness of an aluminum oxide layer is set via the temperature of the aluminum and the oxygen concentration during the coating; it is between 4 and 30 nm.

The advantage of the aluminum-based coatings compared to the zinc-based coatings is that, in addition to a larger process window (e.g. with regard to the heating parameters), the finished components do not have to be processed further need to be blasted. In addition, there is no risk of liquid metal embrittlement with aluminum-based coatings and no microcracks can form in the near-surface substrate area at the former austenite grain boundaries, which can have a negative effect on fatigue strength at depths of more than 10 µm.

The disadvantage of using aluminum-based coatings, e.g. made of aluminum-silicon (AS), is the poor paint adhesion of the formed component in the cathodic dip painting (KTL) typical of automobiles if too short a heating time was used for press hardening. In the case of short heating times, the surface has too little roughness, so that sufficient paint adhesion is not achieved.

In contrast to zinc-based coatings, aluminum-based coatings cannot be phosphated or can only be phosphated insufficiently and thus no improvement in paint adhesion can be achieved through the phosphating step. For these reasons, up to now, minimum heating times have to be observed when processing circuit boards with aluminum-based coatings, as a result of which the coating is alloyed with iron and a rough surface topography is formed, which causes sufficient paint adhesion when painting the formed component.

The alloying of the coating with iron and the formation of a paintable surface topography, however, require a correspondingly long dwell time in the roller hearth furnace that is usually used, which significantly increases the cycle times and reduces the profitability of the press mold hardening. The minimum dwell time is thus determined by the coating and not by the base material, for which only the required austenitizing temperature would be required. In addition, the corrosion resistance is reduced by the stronger alloying with iron, since the aluminum content in the alloy layer decreases with the dwell time in the furnace and the iron content increases. Adapted, longer ovens are usually used for AS boards in order to achieve high cycle rates despite the necessary oven dwell time. However, these are more expensive to purchase and operate and also take up a lot of space.
Another disadvantage of AS coatings is that with very short annealing times, spot welding is extremely poor. This is expressed, for example, in only a very small welding area. One of the reasons for this is a very low contact resistance with short glow times.

The object of the invention is therefore to provide a component made of a press-hardened steel sheet coated on the basis of aluminum, which can be produced inexpensively and has excellent paintability and weldability, in particular resistance spot weldability. Furthermore, a method for producing such a component is to be specified.

gehorcht, auf der Interdiffusionszone I eine Zone mit verschiedenen intermetallischen Phasen mit einer mittleren Gesamtdicke zwischen 8 und 50 µm ausgebildet ist, auf der wiederum eine Aluminiumoxid und/oder -hydroxid enthaltende Deckschicht in einer mittleren Dicke von mindestens 0,05 µm bis höchstens 5 µm angeordnet ist.The teaching of the invention comprises a component made of press-form-hardened sheet steel coated on the basis of aluminum, the coating having a coating applied in the hot-dip process which contains aluminum and silicon, which is characterized in that the press-form-hardened component has an interdiffusion zone in the transition area between sheet steel and coating I, the thickness of the interdiffusion zone I having the following formula, depending on the layer thickness of the coating prior to heating and press hardening $$\mathrm{I.}\left[\mathrm{\mu m}\right]<\frac{1}{35}\times \mathrm{Edition}\mathrm{both\; sides}\left[\mathrm{G}/{\mathrm{m}}^2\right]+\frac{\mathrm{19th}}{\mathrm{7th}}$$

obeys, on the interdiffusion zone I a zone with different intermetallic phases with an average total thickness between 8 and 50 microns is formed, on which in turn a cover layer containing aluminum oxide and / or hydroxide with an average thickness of at least 0.05 microns to at most 5 microns is arranged.

In the following, aluminum-based coatings are understood to be metallic coatings in which aluminum is the main component (in percent by mass). Examples of possible aluminum-based coatings are aluminum-silicon (AS), aluminum-zinc-silicon (AZ), as well as the same coatings with admixtures of additional elements such as magnesium, transition metals such as manganese, titanium and rare earths. A coating of the steel sheet according to the invention is produced, for example, in a molten bath with an Si content of 8 to 12% by weight, an Fe content of 1 to 4% by weight, the remainder being aluminum.

Through the formation of a defined cover layer containing aluminum oxide and / or hydroxide on the aluminum-based coating of the steel sheet or the steel strip, the aforementioned negative aspects of aluminum-based coatings can be significantly reduced or even completely prevented.

The top layers containing aluminum oxide and / or hydroxide act on the component formed by compression molding due to their network-like structure as ideal adhesion promoters for subsequent painting, in particular cathodic dip painting (KTL). A lengthy alloying of the aluminum-based coating with iron in the furnace is no longer necessary, so that the throughput times in the furnace for heating the sheet steel to the forming temperature can be drastically reduced. While so far, for example, annealing times in a roller hearth furnace of at least 4 minutes at a furnace temperature of 950 ° C have been required for the alloying of the coating with iron and the formation of a paintable surface topography for sheet metal thicknesses of 1.5 mm Glowing times of only 2 - 3 minutes are required, which significantly reduces the glow time. The maximum possible furnace times do not change due to the top layer containing aluminum oxide and / or hydroxide. The heating process window is thus greatly expanded towards shorter furnace times.

For thicker sheets, the furnace time is extended accordingly due to the lower heating rate of the steel material. The typical oven temperatures between 900 and 950 ° C should also be adhered to here. For high cycle times, furnace temperatures between 930 and 950 ° C are advantageous.

In addition, the cover layer according to the invention made of aluminum oxides and / or hydroxides has an advantageous effect on resistance spot weldability in the case of short furnace times, since the contact resistance is increased and good resistance heating is thus achieved. For good weldability after short heating times, a thickness of this cover layer of at least 0.05 μm has therefore proven to be positive.

Tests have shown that the paint adhesion is better or the infiltration under corrosion due to a corrosive attack is less, the thicker the top layer containing aluminum oxide and / or hydroxide. On the other hand is at large thickness of this cover layer, the contact resistance in resistance spot welding is too high, which in turn would worsen the weldability. Therefore, a maximum thickness of the top layer of 5 µm should not be exceeded.

A thickness between 0.10 and 3 µm was found for the top layer as a good compromise between weldability and paint adhesion.

For excellent weldability with good paint adhesion, top layers with an average thickness between 0.15 and 1 µm are particularly advantageous.

• dass das Stahlblech oder Stahlband mit dem Überzug nach dem Schmelztauchprozess und vor dem Umformprozess einer Behandlung durch anodische Oxidation und/oder einer Plasmaoxidation und/oder einer Heißwasserbehandlung und/oder einer Behandlung in einer Atmosphäre, die mindestens variable Anteile von Sauerstoff, Wasserdampf unterzogen wird
• dass die Heißwasserbehandlung oder die Behandlung unter Wasserdampf bei Temperaturen von wenigstens 90 °C, vorteilhaft wenigstens 95 °C, erfolgt
• dass im Zuge der Behandlung auf der Oberfläche des Überzugs unter Ausbildung von Oxiden oder Hydroxiden eine Aluminiumoxid und/oder - hydroxid enthaltene Deckschicht mit einer Dicke von mindestens 0,05 µm bis höchstens 5µm ausgebildet wird
• dass das Stahlblech oder Stahlband zumindest bereichsweise auf eine Temperatur oberhalb der Austenitisierungstemperatur erwärmt wird
• dass das erwärmte Stahlblech oder Stahlband anschließend umgeformt und danach mit einer Geschwindigkeit abgekühlt wird, die zumindest bereichsweise oberhalb der kritischen Abkühlgeschwindigkeit liegt,

wobei die Erwärm- und Verweilzeit während des Pressformhärtens so kurz ausgewählt werden, dass die Dicke der Interdiffusionszone I der Formel, die in Anspruch 1 erwähnt ist, gehorcht.According to the invention, the invention also comprises a method for producing a component, in particular according to claim 1, from press-form-hardened steel sheet coated on the basis of aluminum with particular suitability for painting and resistance spot welding, an aluminum-based coating being applied to the steel sheet as a coating in the hot-dip process, which thereby is marked,

• that the steel sheet or steel strip with the coating, after the hot-dip process and before the forming process, is subjected to a treatment by anodic oxidation and / or plasma oxidation and / or hot water treatment and / or treatment in an atmosphere that is subjected to at least variable proportions of oxygen and water vapor
• that the hot water treatment or the treatment under steam at temperatures of at least 90.degree. C., advantageously at least 95.degree. C., takes place
• that in the course of the treatment on the surface of the coating with the formation of oxides or hydroxides, an aluminum oxide and / or hydroxide-containing cover layer with a thickness of at least 0.05 µm to at most 5 µm is formed
• that the steel sheet or steel strip is heated at least in some areas to a temperature above the austenitizing temperature
• that the heated steel sheet or steel strip is then reshaped and then cooled at a rate that is at least partially above the critical cooling rate,

wherein the heating and dwell time during the compression molding hardening are selected to be so short that the thickness of the interdiffusion zone I of the formula shown in FIG Claim 1 is mentioned obeyed.

In connection with the invention, the term is to be understood at least in some areas in the sense of local sections of the treated steel sheet or steel strip, so that a steel sheet or steel strip is created with structures and properties that specifically differ from one another locally.

The cover layer is preferably applied to the surface of the coating in a continuous process.

The treatment advantageously takes place in an atmosphere which also contains proportions of basic components, preferably ammonia (NH ₃ ), primary, secondary or tertiary aliphatic amines (NH ₂ R, NHR ₂ ), NR ₃ ).

In terms of process technology, a thin oxidic top layer can advantageously be achieved by anodic oxidation (thin layer anodizing), plasma oxidation and a hydroxide-containing top layer by means of a hot water treatment of the aluminum-based coating at temperatures of at least 90 ° C, advantageously at least 95 ° C and / or a treatment in steam at temperatures of at least 90 ° C, advantageously at least 95 ° C can be produced.

As an alternative to anodizing, a gas phase treatment of the AS surface also leads to the same goal. For this purpose, the AS surface is treated with an atmosphere which can contain at least variable proportions of oxygen, water vapor, and optionally also proportions of basic components, in particular ammonia, primary, secondary or tertiary aliphatic amines. This treatment leads to a time- or temperature-controlled growth of a top layer containing aluminum oxide and / or hydroxide. Furthermore, the composition of the gas phase can be used to control the layer thickness growth of this cover layer. The treatment is carried out at a temperature of 40 ° C to 100 ° C, preferably 90 ° C to 100 ° C. Lower treatment temperatures lengthen the treatment time, treatment temperatures above 100 ° C may require pressure vessels.

Both anodization and gas phase treatment result in an aluminum oxide and / or hydroxide-containing cover layer, which has mesh-like or needle-like structures on its surface. The associated increase in surface area improves the adhesion of a subsequent KT coating.
Since longer heating times are no longer required to create a paintable surface topography, the corrosion protection of the coating is also increased. This can be explained by the fact that with only a short annealing time required in the roller hearth furnace, there is less diffusion of aluminum and iron. Among other things, this also leads to a relatively small interdiffusion zone. This is exemplary for an AS layer of the starting material of 150 g / m ² (AS150) below 7 μm.

In tests, depending on the furnace dwell time, when using blanks with an AS layer of 150 g / m ² , the diffusion zone thicknesses of less than 5 µm and even less than 4 µm were achieved on the finished component.

When using boards with an AS layer of 80 g / m ² (AS80), it is known that the furnace time can be reduced slightly even if the coating is not according to the invention and that this also results in thinner diffusion layers of, for example, 5 μm. Tests have shown that with the solution according to the invention, the furnace times can be reduced even further in this case, and that the diffusion layers can be achieved with a thickness of less than 5 μm on the finished component. In further tests, by further shortening the heating time in the furnace, even thinner diffusion layers of less than 3 µm and even less than 2 µm were achieved on the finished component.

When using circuit boards with a layer between AS80 and AS150 and with layers smaller than AS80 or larger than AS150, the thicknesses of the interdiffusion layers I according to the invention for a layer of the starting material result from the linear relationship according to the following formulas for various sheet thickness-dependent Heating times:

According to the invention, the necessary heating time in the oven depends only on the sheet metal thickness, since the coating according to the invention does not require any holding time in the oven to produce a paintable surface. Thicker sheets therefore require longer heating times for heating than thinner sheets.

As an example for sheets with a thickness of 1.5 mm, table 1 lists short (220 seconds), very short (180 seconds) and extremely short (150 seconds) heating times compared to conventional heating times (360 seconds) in a roller hearth furnace.

Another positive effect of the short heating time is a significantly reduced proportion of pores in the alloy layer and in the diffusion zone. Pores arise with longer glow times, e.g. due to the Kirkendall effect. Tests have shown that the short-term annealing can reduce the total pore proportion to values of less than 6% and even to values of less than 4% or 2%. Which can, for example, have a beneficial effect on the suitability for welding.

In order to press mold hardening blanks with an aluminum-silicon coating, it is no longer necessary to keep the steel sheet in the furnace for long periods of time. The steel sheet only needs to be heated to the required forming temperature and, when the forming temperature is reached, it can be fed into the forming press immediately, formed and quenched.

As a result, shorter roller hearth furnaces than those previously used can advantageously be used. In addition, the use of other types of ovens, for example for inductive or conductive rapid heating, is possible without having to use the heated plates to form a paintable one

Surface topography must be kept at temperature.

In addition, it is now possible to only partially heat and harden the blanks, which means that spot weldability and KT paint adhesion are good even in areas with little heat influence.

The invention is described in more detail below with reference to the figures shown.

Figure 1 shows schematically the layer structure of the coating on a compression-molded component with a coating of AS and the usual long heating time according to the prior art in order to achieve a through-alloying of the coating with iron. A steel sheet with a coating of AS150, that is to say with a layer of 150 g / m ^{2 of} the coating, was used for the component. An interdiffusion zone Fe (Al, Si) with a thickness of 7 to 14 µm is formed on the martensitic steel base material, on which a zone with different intermetallic phases ( _{e.g. Fe 2} SiAl ₂ and FeAl ₂ ) has formed, the individual phases in this zone can be distributed in rows or clusters. As a result of the oxidation in the furnace and during the transfer to the press, an only very thin aluminum oxide layer with a thickness of less than 0.05 µm has formed. You can also see pores that have formed in the various zones.

Figure 2 In comparison, shows the layer structure of a coating according to the invention on a press-hardened component with an AS coating on which a cover layer according to the invention of at least 0.05 μm containing aluminum oxide and / or hydroxide is formed and which is produced with shorter heating times compared to the prior art has been. In the transition area between sheet steel and coating there is an interdiffusion zone in which aluminum and silicon have diffused into the steel Fe (Al, Si). Due to the very short heating time required in the furnace to the austenitizing temperature, this layer has an average thickness of less than 7 µm for AS150, for example. _{Another layer with different intermetallic phases (e.g. Fe 2} SiAl ₂ and FeAl ₂ ) is formed on this layer in the course of the heating process. The individual phases in this zone can appear in rows or in clusters and on which an aluminum oxide and / or -hydroxide-containing top layer in an average thickness of at least 0.05 µm to is arranged at most 5 µm.

Figure 3 shows graphically the thickness I according to the invention of the interdiffusion zone for a layer of the starting material between 50 g / m ² and 180 g / m ² according to the following relationship: $$\mathrm{I.}\left[\mathrm{\mu m}\right]<\frac{1}{35}\times \mathrm{Edition}\mathrm{both\; sides}\left[\mathrm{G}/{\mathrm{m}}^2\right]+\frac{\mathrm{19th}}{\mathrm{7th}}$$

Table 1 summarizes tests on paint adhesion (phosphating treatment typical for automobiles and cathodic dip painting; testing after 72 hours of constant condensation climate in accordance with DIN EN ISO 6270-2: 2005 CH) and suitability for welding (resistance spot welding) of press-hardened AS150 samples at 940 ° C oven temperature and various heating times. The sheet thickness of the samples is 1.5 mm. It can be seen that there is only good paint adhesion and weldability with heating times of 220 s and less if a cover layer according to the invention containing aluminum oxide and / or hydroxide is present. In addition, short heating times of 220 s and less resulted in interdiffusion layers of less than 7 µm on the press-hardened component. In the case of the long heating times of 360 s according to the prior art, which are not according to the invention, on the other hand, due to the alloying of the coating with iron, good paint adhesion and weldability is given even in the samples without the cover layer according to the invention containing aluminum oxide and / or hydroxide. The thickness of the interdiffusion layers is well over 7 µm after a heating time of 360 s. Table 1 No. material thickness Edition Top layer Oven temperature Oven dwell time Welding area KT paint adhesion Thickness of the diffusion layer According to the invention 1 22MnB5 1.5mm AS150 No 940 ° C 150 s not ok not ok <7 µm No 2 22MnB5 1.5 mm AS150 Deposition time a 940 ° C 150 s > 1 kA (OK) OK <7 µm Yes 3 22MnB5 1.5 mm AS150 Deposition time b 940 ° C 150 s > 1 kA (OK) OK <7 µm Yes 4th 22MnB5 1.5 mm AS150 Deposition time c 940 ° C 150 s > 1 kA (OK) OK <7 µm Yes 5 22MnB5 1.5mm AS150 No 940 ° C 180 s not ok not ok <7 µm No 6th 22MnB5 1.5mm AS150 Deposition time a 940 ° C 180 s > 1 kA (OK) OK <7 µm Yes 7th 22MnB5 1.5 mm AS150 Deposition time b 940 ° C 180 s > 1 kA (OK) OK <7 µm Yes 8th 22MnB5 1.5mm AS150 Deposition time c 940 ° C 180 s > 1 kA (OK) OK <7 µm Yes 9 22MnB5 1.5mm AS150 No 940 ° C 220 s not ok not ok <7 µm No 10 22MnB5 1.5mm AS150 Deposition time a 940 ° C 220 s > 1 kA (OK) OK <7 µm Yes 11 22MnB5 1.5 mm AS150 Deposition time b 940 ° C 220 s > 1 kA (OK) OK <7 µm Yes 12th 22MnB5 1.5mm AS150 Deposition time c 940 ° C 220 s > 1 kA (OK) OK <7 µm ia 13th 22MnB5 1.5mm AS150 No 940 ° C 360 s > 1 kA (OK) OK > 7 µm No 14th 22MnB5 1.5 mm AS150 Deposition time a 940 ° C 360 s > 1 kA (OK) OK > 7 µm No 15th 22MnB5 1.5 mm AS150 Deposition time b 940 ° C 360 s > 1 kA (OK) OK > 7 µm No 16 22MnB5 1.5mm AS150 Deposition time c 940 ° C 360 s > 1 kA (OK) OK > 7 µm No

Claims (11)

1. Component made of press-form-hardened steel sheet coated on the basis of aluminium, the coating having an overcoat which is applied by a hot-dip process and contains aluminium and silicon, characterised in that the press-form-hardened component has an interdiffusion zone I in the transition region between the steel sheet and the overcoat, wherein, depending on the coating weight of the overcoat before heating and press hardening, which for the starting material is between 50 g/m² and 180 g/m², the thickness of the interdiffusion zone I obeys the following formula $$\mathrm{I}\left[\mathrm{\mu m}\right]<\frac{1}{35}\times \mathrm{weight}\mathrm{on}\mathrm{both}\mathrm{sides}\left[\mathrm{g}/{\mathrm{m}}^2\right]+\frac{19}{7}$$

   

   and, on the interdiffusion zone I, a zone having different intermetallic phases with an average total thickness between 8 and 50 µm is formed, on which zone in turn a cover layer containing aluminium oxide and/or hydroxide is arranged in an average thickness of at least 0.05 µm to at most 5 µm.
2. Component according to claim 1, characterised in that, depending on the current coating weight of the starting material, the thickness of the interdiffusion zone I is designed according to the following formula $$\mathrm{I}\left[\mathrm{\mu m}\right]<\frac{1}{35}\times \mathrm{weight}\mathrm{on}\mathrm{both}\mathrm{sides}\left[\mathrm{g}/{\mathrm{m}}^2\right]+\frac{5}{7}.$$

   
3. Component according to claim 1, characterised in that, depending on the current coating weight of the starting material, the thickness of the interdiffusion zone I is designed according to the following formula $$\mathrm{I}\left[\mathrm{\mu m}\right]<\frac{1}{35}\times \mathrm{weight}\mathrm{on}\mathrm{both}\mathrm{sides}\left[\mathrm{g}/{\mathrm{m}}^2\right]-\frac{2}{7}.$$

   
4. Component according to at least one of claims 1 to 3, characterised in that the average layer thickness of the cover layer is at least 0.10 µm and at most 3.0 µm.
5. Component according to at least one of claims 1 to 3, characterised in that the average layer thickness of the cover layer is at least 0.15 µm and at most 1.0 µm.
6. Component according to at least one of claims 1 to 5, characterised in that the overcoat has a total porosity of less than 6%, advantageously less than 4% and optimally less than 2%.
7. Component according to at least one of claims 1 to 6, characterised in that the overcoat of the steel sheet was produced in a molten bath with an Si content from 8 to 12 wt.%, an Fe content from 1 to 4 wt.%, the remainder aluminium and unavoidable impurities.
8. Method for producing a component, in particular according to claim 1, from press-form-hardened steel sheet or steel strip coated on the basis of aluminium, with particular suitability for painting and resistance spot welding, with an aluminium-based overcoat being applied to the steel sheet or steel strip as a coating by a hot-dip process, characterised in that

   - the steel sheet or steel strip having the overcoat is subjected, after the hot-dip process and before the shaping process, to treatment by anodic oxidation and/or plasma oxidation and/or hot water treatment and/or treatment in an atmosphere containing at least variable proportions of oxygen and water vapour,

   - the hot water treatment or the treatment under water vapour takes place at temperatures of at least 90°C, advantageously at least 95°C,

   - in the course of the treatment, a cover layer containing aluminium oxide and/or hydroxide and having a thickness of at least 0.05 µm to at most 5 µm is formed on the surface of the overcoat with the formation of oxides or hydroxides

   - the steel sheet or steel strip is heated at least in some regions to a temperature above the austenitisation temperature

   - the heated sheet steel or steel strip is then shaped and then cooled at a rate that is above the critical cooling rate at least in some regions,

   wherein the heating time and dwell time during the press form hardening are selected to be short enough that the thickness of the interdiffusion zone I obeys the formula mentioned in claim 1.
9. Method according to claim 8, characterised in that the cover layer is applied to the surface of the overcoat in a continuous process.
10. Method according to either claim 8 or claim 9, characterised in that the treatment takes place in an atmosphere which also contains proportions of basic components, preferably ammonia (NH₃), primary, secondary or tertiary aliphatic amines (NH₂R, NHR₂).
11. Use of a component according to claims 1 to 7 for the manufacture of motor vehicles.

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