EP3250727B2 - Method for producing such a component made of press-form-hardened, aluminum-based coated steel sheet - Google Patents

Description

The invention relates to a method for producing a component made of press-hardened, aluminum-based steel sheet, the coating having a coating applied by hot-dipping that contains aluminum and silicon. In particular, the coating relates to an aluminum-silicon coating.

It is known that hot-formed steel sheets are being used more and more frequently, particularly 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 basically be carried out using two different process variants, namely the direct or indirect process. While in the indirect process the process steps of forming and hardening take place separately from each other, in the direct process they take place together in one tool. Only the direct method is considered below.

In the direct process, a steel sheet is heated above the so-called austenitization temperature (Ac3). The steel sheet heated in this way is then transferred to a mold and formed into the finished component in a single-stage forming step and simultaneously cooled by the cooled mold at a rate 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 further processed. The steel sheet to be formed is often referred to as a blank.

Well-known hot-formable steels for this area of application include the manganese-boron steel "22MnB5" and, more recently, air-hardenable steels in accordance with 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 oven, which reduces wear on the press tools caused by chipped scale and the components often do not have to be extensively blasted before further processing.

The following (alloy) coatings applied by hot-dipping are currently known for press hardening: 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 corrosion protection 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 from the German patent DE 601 19 826 T2 known. Here, a sheet metal blank that has previously been heated above the austenitizing temperature to 800 - 1200 °C and, if necessary, provided with a metallic coating made of zinc or based on zinc, is formed into a component by hot forming in a occasionally cooled tool, with rapid heat removal occurring during the forming Sheet metal or component undergoes quench hardening (press hardening) in the forming tool and the required strength properties are achieved through the resulting martensitic hardness structure.

The production of components by quenching pre-products coated with an aluminum alloy from press-hardenable steels by hot forming in a forming tool is from 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 created on the surface and the sheet metal is subsequently formed and cooled at a rate above the critical cooling rate .

From the disclosure document US 2011/0300407 A1 is a method of manufacturing 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 cooled in the press.

Also the European patent application EP 2 312 011 A1 describes a process for producing metallic coatings on cast parts for use in automobile construction. For this purpose, the cast part is coated with an aluminum alloy in a melt pool and then subjected to heat treatment in an oxidizing atmosphere to produce a high-temperature-resistant aluminum oxide layer. After the heat treatment, anodic oxidation is also provided.

The German patent specification DE 198 53 285 C1 presents a process for producing a protective layer on martensitic steel. Under a protective gas atmosphere (argon with 5% H ₂ ), the steel to be coated is dipped into a melt made of aluminum or an aluminum alloy, cooled and then hot isostatically pressed at the austenitizing temperature. The aluminum protective layer created in this way is between 100 and 200 µm thick and should contain an approx. 1 µm thick aluminum oxide layer on its surface, about the creation or maintenance of which no further information is provided.

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 hot-dip coating. A thickness of an aluminum oxide layer is adjusted via the temperature of the aluminum and the oxygen concentration during coating; it lies between 4 and 30 nm.

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

However, the disadvantage of using aluminum-based coatings, e.g. made of aluminum-silicon (AS), is the inadequate paint adhesion of the formed component in cathodic dip painting (KTL), which is typical for automobiles, if too short a heating time was used during press hardening. If the heating times are short, 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 only insufficiently phosphated and therefore the phosphating step cannot achieve any improvement in paint adhesion. For these reasons, minimum heating times have previously had to be adhered to when processing circuit boards with aluminum-based coatings, which causes the coating to alloy with iron and form a rough surface topography, which ensures sufficient paint adhesion when painting the formed component.

However, the alloying of the coating with iron and the formation of a paintable surface topography require a correspondingly long residence time in the commonly used roller hearth furnace, which significantly extends the cycle times and reduces the economic efficiency of press mold hardening. The minimum residence time is therefore determined by the coating and not by the base material, for which only the necessary austenitization temperature would be necessary. In addition, the corrosion resistance is reduced by the increased alloying with iron, as the aluminum content in the alloy layer decreases with the furnace residence time 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 require a lot of space. Another disadvantage of AS coatings is that with very short annealing times, weldability using the spot welding process is extremely poor. This is expressed, for example, in only a very small welding area. The reason for this is, among other things, a very low contact resistance with short glow times.

The object of the invention is therefore to provide a method for producing a component from a press-hardened, aluminum-based steel sheet, which is cost-effective and leads to a component that has excellent paintability and weldability, in particular resistance spot weldability.

• 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 folgender Formel $$\mathrm{I}\left[\mathrm{\mu m}\right]<\frac{1}{35}\times \mathrm{Auflage}\mathrm{beidseitig}\left[\mathrm{g}/{\mathrm{m}}^2\right]+\frac{19}{7}$$

gehorcht, wobei darauf eine Zone mit verschiedenen intermetallischen Phasen mit einer mittleren Gesamtdicke zwischen 8 und 50 µm ausgebildet wird.According to the invention, the invention comprises a method for producing a component made of press-hardened, aluminum-based steel sheet with particular suitability for painting and resistance spot welding, an aluminum-based coating being applied to the steel sheet as a coating using the hot-dip process, which is characterized in that

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

whereby the heating and residence time during the press mold hardening are selected to be so short that the thickness of the interdiffusion zone I has the following formula $$\mathrm{I}\left[\mathrm{\mu m}\right]<\frac{1}{35}\times \mathrm{edition}\mathrm{bilaterally}\left[\mathrm{G}/{\mathrm{m}}^2\right]+\frac{19}{7}$$

obeys, with a zone with different intermetallic phases an average total thickness between 8 and 50 µm.

In the following, aluminum-based coatings are understood to mean metallic coatings in which aluminum is the main component (in mass percent). 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 melt pool with an Si content of 8 to 12% by weight, an Fe content of 1 to 4% by weight, and the balance being aluminum.

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

Due to their network-like structure, the cover layers containing aluminum oxide and/or hydroxide act as ideal adhesion promoters for subsequent painting, in particular cathodic dip painting (KTL), on the component formed by press-hardening. 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 steel sheet to the forming temperature can be drastically shortened. While previously, for example, for sheet thicknesses of 1.5 mm, annealing times in the roller hearth furnace of at least 4 minutes at a furnace temperature of 950 ° C were required for the alloying of the coating with iron and the formation of a paintable surface topography, in the method according to the invention, for a sheet thickness of 1.5 mm Annealing times of only 2 - 3 minutes are required, which means the annealing time is significantly reduced. The maximum possible oven times do not change due to the top layer containing aluminum oxide and/or hydroxide. This means that the heating process window is greatly expanded towards shorter oven times.

For thicker sheets, the oven time increases accordingly due to the lower heating rate of the steel material. The typical ones Oven temperatures between 900 and 950 °C should also be maintained here. For high cycle times, oven 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 the resistance spot weldability with 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 proven to be positive.

During tests it was found that the thicker the top layer containing aluminum oxide and/or hydroxide, the better the paint adhesion and the less infiltration due to corrosive attack. On the other hand, if this cover layer is too thick, the contact resistance during 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 of 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 of between 0.15 and 1 µm are particularly advantageous.

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 differ from one another in a targeted manner.

The top 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 oxide cover layer can advantageously be achieved by anodic oxidation (thin-film anodization), plasma oxidation and a cover layer containing hydroxide 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.

As an alternative to anodization, gas phase treatment of the AS surface also achieves 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 growth of the layer thickness of this top layer. The treatment is carried out at a temperature of 40 °C to 100 °C, preferably 90 °C to 100 °C. Lower treatment temperatures extend the treatment time; treatment temperatures above 100 °C may require pressure vessels.

Both anodization and gas phase treatment lead to a cover layer containing aluminum oxide and/or hydroxide, which has network or needle-like structures on its surface. The associated increase in surface area improves the adhesion of subsequent KT painting.
Since longer heating times are no longer necessary to form a surface topography that can be painted, 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, less diffusion of aluminum and iron takes place. This also leads, among other things, to a relatively small interdiffusion zone. This is an example for an AS layer of the starting material of 150 g/m ² (AS150) below 7 µm.

In tests, depending on the length of time in the oven when using boards with an AS layer of 150 g/m ^{2 ,} thicknesses of the diffusion zone were also found to be below 5 µm, and even achieved below 4 µm on the finished component.

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

(kurze Ewärmzeit) $$\mathrm{I}\left[\mathrm{\mu m}\right]<\frac{1}{35}\times \mathrm{Auflage}\mathrm{beidseitig}\left[\mathrm{g}/{\mathrm{m}}^2\right]+\frac{5}{7}$$

(sehr kurze Ewärmzeit) $$\mathrm{I}\left[\mathrm{\mu m}\right]<\frac{1}{35}\times \mathrm{Auflage}\mathrm{beidseitig}\left[\mathrm{g}/{\mathrm{m}}^2\right]-\frac{2}{7}$$

(äußerst kurze Erwärmzeit)When using boards with a layer layer between AS80 and AS150 and with layer layers that are smaller than AS80 or larger than AS150, after press hardening, the thicknesses of the interdiffusion layers I according to the invention for a layer layer of the starting material result from the linear relationship according to the following formulas for various sheet thickness-dependent Heating times: $$\mathrm{I}\left[\mathrm{\mu m}\right]<\frac{1}{35}\times \mathrm{edition}\mathrm{bilaterally}\left[\mathrm{G}/{\mathrm{m}}^2\right]+\frac{19}{7}$$

(short heating time ) $$\mathrm{I}\left[\mathrm{\mu m}\right]<\frac{1}{35}\times \mathrm{edition}\mathrm{bilaterally}\left[\mathrm{G}/{\mathrm{m}}^2\right]+\frac{5}{7}$$

(very short heating time ) $$\mathrm{I}\left[\mathrm{\mu m}\right]<\frac{1}{35}\times \mathrm{edition}\mathrm{bilaterally}\left[\mathrm{G}/{\mathrm{m}}^2\right]-\frac{2}{7}$$

(extremely short heating time )

According to the invention, the necessary heating time in the oven depends only on the sheet thickness, since the coating according to the invention does not require any holding time in the oven to produce a surface that can be painted. Thicker sheets therefore require longer heating times 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 usual heating times (360 seconds) in the 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 during longer annealing times, for example due to the Kirkendall effect. Tests have shown that short-term annealing can reduce the total pore content to values of less than 6% and even to values of less than 4% or 2%. Which, for example, can have a beneficial effect on the suitability for welding.

For the press-hardening of circuit boards with an aluminum-silicon coating, it is no longer necessary to maintain long residence times for the steel sheet in the oven. The steel sheet only needs to be heated to the required forming temperature and can be immediately fed to the forming press, formed and quenched when the forming temperature is reached.

This means that shorter roller hearth ovens than those previously used can also be advantageously used. In addition, the use of other types of ovens, for example for inductive or conductive rapid heating, is possible without the heated boards having to be kept at temperature to form a surface topography that can be painted.

Furthermore, it is now possible to only partially heat and harden circuit boards, which ensures good spot weldability and KT paint adhesion 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 press-hardened component with a coating made of AS and the usual long heating time according to the prior art to achieve alloying of the coating with iron. A steel sheet with a coating of AS150, i.e. with a coating layer of 150 g/m ^2, 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 being in This zone can occur in rows or clusters.

Due to the oxidation in the oven and during transfer into the press, a very thin layer of aluminum oxide with a thickness of less than 0.05 µm was formed. Pores that have formed in the different zones can also be seen.

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 containing aluminum oxide and / or hydroxide of at least 0.05 μm is formed and which is produced with shortened heating times compared to the prior art became. In the transition area between the steel sheet and the coating, an interdiffusion zone is formed in which aluminum and silicon have diffused into the steel Fe(Al, Si). Due to the very short heating time required in the oven to the austenitization temperature, this layer, for example for AS150, has an average thickness of less than 7 µm. In the course of heating, a further layer with different intermetallic phases (eg Fe ₂ SiAl ₂ and FeAl ₂ ) forms on this layer, whereby the individual phases in this zone can appear in rows or clusters and on which an aluminum oxide and/or -hydroxide-containing cover layer is arranged with an average thickness of at least 0.05 μm to a maximum of 5 μm.

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

Table 1 summarizes tests on paint adhesion (phosphating treatment typical for automobiles and cathodic dip painting; testing after 72 hours of condensation water constant climate according to DIN EN ISO 6270-2:2005 CH) and weldability (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 good paint adhesion and weldability are only achieved with heating times of 220 s and less a cover layer containing aluminum oxide and/or hydroxide according to the invention is present. Short heating times of 220 s and less also resulted in interdiffusion layers of less than 7 µm on the press-hardened component. However, with the long heating times of 360 s according to the prior art, which are not according to the invention, good paint adhesion and weldability are ensured even in the samples without the top layer according to the invention containing aluminum oxide and / or hydroxide due to the through-alloying of the coating with iron. 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 residence time Welding area KT paint adhesion Thickness of the diffusion layer According to the invention 1 22MnB5 1.5mm AS150 no 940°C 150s not ok not ok <7µm no 2 22MnB5 1.5mm AS150 Separation time a 940°C 150s >1 kA (OK) OK <7µm Yes 3 22MnB5 1.5mm AS150 Separation time b 940°C 150s >1 kA (OK) OK <7µm Yes 4 22MnB5 1.5mm AS150 Deposition time c 940°C 150s >1 kA (OK) OK <7µm Yes 5 22MnB5 1.5mm AS150 no 940°C 180s not ok not ok <7µm no 6 22MnB5 1.5mm AS150 Separation time a 940°C 180s >1 kA (OK) OK <7µm Yes 7 22MnB5 1.5mm AS150 Separation time b 940°C 180s >1 kA (OK) OK <7µm Yes 8th 22MnB5 1.5mm AS150 Deposition time c 940°C 180s >1 kA (OK) OK <7µm Yes 9 22MnB5 1.5mm AS150 no 940°C 220s not ok not ok <7µm no 10 22MnB5 1.5mm AS150 Separation time a 940°C 220s >1 kA (OK) OK <7µm Yes 11 22MnB5 1.5mm AS150 Separation time b 940°C 220s >1 kA (OK) OK <7µm Yes 12 22MnB5 1.5mm AS150 Deposition time c 940°C 220s >1 kA (OK) OK <7µm ia 13 22MnB5 1.5mm AS150 no 940°C 360s >1 kA (OK) OK >7 µm no 14 22MnB5 1.5mm AS150 Separation time a 940°C 360s >1 kA (OK) OK >7 µm no 15 22MnB5 1.5mm AS150 Separation time b 940°C 360s >1 kA (OK) OK >7 µm no 16 22MnB5 1.5mm AS150 Deposition time c 940°C 360s >1 kA (OK) OK >7 µm no

Claims (3)

1. Method for producing a component 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 following formula $$\mathrm{I}\left[\mathrm{\mu m}\right]<\frac{1}{35}\times \mathrm{weight}\mathrm{on}\mathrm{both}\mathrm{sides}\left[\mathrm{g}/{\mathrm{m}}^2\right]+\frac{19}{7}$$

   

   wherein thereon a zone having different intermetallic phases with a thickness between 8 and 50 µm is formed.
2. Method according to claim 1, characterised in that the cover layer is applied to the surface of the overcoat in a continuous process.
3. Method according to either claim 1 or claim 2, 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).

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