WO2009047183A1

WO2009047183A1 - Method for the production of a steel component by thermoforming, and steel component produced by thermoforming

Info

Publication number: WO2009047183A1
Application number: PCT/EP2008/063139
Authority: WO
Inventors: Barbara Lupp; Sabine Hasenfuss; Ansgar Albers; Manfred Meurer; Wilhelm Warnecke
Original assignee: Thyssenkrupp Steel Ag
Priority date: 2007-10-02
Filing date: 2008-10-01
Publication date: 2009-04-16
Also published as: EP2045360A1; ATE535631T1; US20100294400A1; EP2045360B1

Abstract

The present invention relates to a method for the production of a steel component provided with a metallic coating which protects against corrosion, having the following work steps: coating a sheet steel product produced from low-alloy heat-treated steel with an Al coating containing at least 85% by weight Al and optionally up to 15% by weight Si; coating the sheet steel product provided with the Al coating with a Zn coating containing at least 90% by weight Zn; heating the sheet steel product to a thermoforming temperature of at least 750°C; thermoforming the heated steel component from the sheet steel product, and cooling sufficiently quickly to form a heat-treated or hardened structure of the thermoformed steel component.

Description

Process for producing a steel component by thermoforming and thermoforming

steel component

The invention relates to a method for producing a steel component provided with a metallic coating which protects against corrosion, in particular by a cathodic protective effect, by thermoforming a flat steel product produced from a low-alloy tempered steel. Moreover, the invention relates to a steel component produced by thermoforming a flat steel product and provided with a corrosion protection coating which protects against corrosion, in particular by a cathodic protective action.

When it comes to flat-rolled steel products, it refers to steel strips, steel sheets or blanks obtained therefrom and the steel substrate of the steel component obtained from such strips, sheets or blanks.

The rigidity and strength of components are becoming increasingly demanding, especially in vehicle construction. At the same time, however, in the interest of optimizing the energy consumption required for the drive of the respective vehicle, it is desirable to have the lowest possible body weight and correspondingly low material thicknesses. These can be fulfilled at first View contradictory requirements by high-strength and high-strength steel materials, which allow the use of suitable process steps, the production of components with very high strength at low Mateπaldicke.

A process that allows the production of correspondingly high-strength and at the same time thin-walled steel components is hot-press hardening. In hot pressing, a circuit board is first cut from a steel strip. This board is then heated to a thermoforming temperature that is typically above the Ar3 temperature of the particular steel material being processed. The thus heated board is then placed in the warm state in a ümformwerkzeug and brought into the desired component shape. Subsequently, or meanwhile, there is a cooling of the molded component, in which a Vergutungs- or Hartegefuge arises in the processed steel.

For compression molding, low alloyed steels are suitable. However, these steels are sensitive to corrosive attacks to which they are exposed, especially when they are used for the construction of vehicle bodies.

More recently, various attempts have been made to harness the benefits of hot working high strength steel suitable for hot press hardening, especially for these applications. A pioneer in this development is the prior art described in EP 0 971 044 B1. According to this known processes, a hot rolled steel sheet is processed, which in addition to iron and unavoidable impurities in (wt .-%) between 0.15 - 0.5% C, between 0.5 - 3% Mn, between 0.1 - 0.5% Si, between 0.01-1% Cr, less than 0.2% Ti, less than 0.1% each of Al and P, less than 0.05 S and between 0.0005-0.08% B. The steel assembled in accordance with this requirement is known in practice as 22MnB5.

The thus assembled steel strip is provided according to EP 0 971 044 Bl with a coating based on aluminum or an aluminum alloy. In particular, this coating is an AlSi coating which has Fe contents. The thus coated steel strip is heated to a temperature of more than 750 ⁰ C, formed into a component and then cooled at a Abkuhlgeschwindigkeit under which forms a Hartegefuge.

The steel component produced in the manner known from EP 0 971 044 B1 has, in addition to good strength properties, a fundamentally good resistance to corrosion. At the same time, steel components made available in accordance with this prior art can be used to produce steel components in just one hot stamping step without damaging the Al coating.

The Al-based coated steel used in the known manner lacks an essential property which damages the steel upon injury cathodic protects against corrosion. This sensitivity proves to be particularly problematic when using the processed by the known method steels in the field of bodywork for automobiles.

In order to avoid this disadvantage, it has been proposed in WO 2005/021820 A1, WO 2005/021821 A1 and WO 2005/021822 A1 to apply a zinc-based coating to the steel substrate instead of an Al-based coating. Although sheet metal components produced from such coated flat steel products have cathodic corrosion protection. It must be accepted, however, that the deformation of the respective flat steel product to the component must be carried out in at least two stages, the first stage is a cold deformation, in which by far the largest part of the shaping takes place, and in the course of the hot deformation stage only one Calibration of the component with subsequent deterrence is possible. This leads to a limited economic usability of this known process.

An alternative attempt to use components produced from steels of the type described in EP 0 971 044 Bl more effectively for use in vehicle body construction is described in DE 103 33 166 A1. According to the method known from this publication, a steel component is produced in a manner known from EP 0 971 044 B1 and subsequently coated with an additional zinc layer. By this subsequent piece galvanizing is indeed the problem of cathodic corrosion protection solved, but this must be taken into account additional Nachtraglicher coating step, which leads not only to an increased expenditure of time in the manufacture of body parts, but also at an increased cost.

Against the background of the prior art explained above, the object of the invention was to provide an economical process for the production of high-strength steel components, which have optimized corrosion protection and are particularly suitable for use in automobile bodies. In addition, a suitably procured steel component should be created.

With regard to the method, this object has been achieved according to the invention in that in the production of a steel component, the operations specified in claim 1 are performed. Advantageous embodiments of this method are given in the claims based on claim 1.

With regard to the steel component, this object has been achieved according to the invention in that such a steel component is designed according to claim 27. Advantageous embodiments of this component are mentioned in the dependent from claim 27 claims.

According to the invention, a metallic coating is produced on a flat steel product produced from a low-alloy tempered steel, which consists of two successively applied in two process steps Layers is formed. The tempering steel may be, for example, a Mn-B steel, as it is already widely used in the prior art.

According to the invention, in the first step, the flat steel product produced from the suitably composed tempered steel is present with an Al coating containing at least 85% by weight of Al, with additional contents of up to 15% by weight being present in the Al coating applied according to the invention could be. Typical variants of the Al coating applied according to the invention are a coating consisting almost entirely of Al or an AlSi variant in which the Si content of the applied AlSi coating amounts to 8-12% by weight of Si.

Subsequently, a Zn coating is applied to this Al coating, which consists of at least 90 wt .-% of zinc.

Before the forming of the respective component, the two-layer coated steel flat product is then heated to a hot forming temperature of at least 750 ^° C. This results in the formation of an alloyed Al, Fe, Zn and Si base layer in which Al has the largest share, but Fe, Zn and Si emerge as essential ingredients. In practice, according to the invention typically thermoforming temperatures of 850 to 950 ⁰ C, in particular 850 - 900 ⁰ C, set.

The heated to the hot forming temperature flat steel product is thermoformed in a further step in a conventional manner to the respective component and in one for the desired training of Vergutungs- or Hartegefuges accelerated cooled.

The above-enumerated operations are the measures which are at least necessary to achieve the success achieved according to the invention. Of course, additional steps can be provided if this proves necessary from a production point of view.

For example, heating the thermoforming temperature to temperature may be preceded by dividing the flat product which has previously been present as a strip in the manner of the invention in two layers into sheets. In addition, each of the individual coating steps can be preceded by a cleaning of the surface of the flat steel product or of the coating applied thereto.

Surprisingly, it has initially been shown that the steel flat product coated in the manner according to the invention can be easily converted into a steel component. Thus, according to the invention coated flat steel product proved both for a direct, d. h as a one-step work step without previous cold forming performed thermoforming, as well as for an indirect, d. H. at least two-stage shaping characterized by a succession of cold working and hot working.

After each thermoforming carried out in a steel component according to the invention is a zinc alloy Surface having a zinc content of at least 60 wt .-%, in particular of at least 80 wt .-%, before. This results in a cathodic corrosion protection, which is clearly detectable electrochemically. Thus, it could be demonstrated in the accelerated corrosion test (salt spray fog test) that coatings produced according to the invention have a resistance to corrosion which is at least comparable to pure zinc excesses.

From the fact that up to 30% Al may be present in the top layer of the steel component obtained according to the invention, additional advantages with regard to corrosion protection result.

The high level of Al having, disposed between the Zn-dominated top layer and the respective steel substrate base layer of the inventive metallic Gesamtuberzugs protects this against excessive zinc and iron diffusion during the heat treatment in accordance with the invention preferably in the range of 750 to 900 ⁰ C in particular 850 to 900 ⁰ C chosen

Thermoforming temperatures. One of the advantages of the barrier effect of the base layer is the delayed formation of red rust on the surface. On the other hand, the base coat prevents zinc from reaching the grain boundaries of the steel substrate, which has the potential to cause hot working cracking. In addition, the Al-Fe-Zn-Si-containing base layer of the overall coating produced according to the invention protects the steel substrate particularly effectively against oxidation with the oxygen of the environment. With the procedure according to the invention, a particularly economically viable option of producing optimized corrosion-protected components made of high-strength hot-press-formable steel is thus available.

According to the findings summarized above, a steel component obtained in accordance with the invention carries a metallic coating which is formed by a base layer resting on the flat steel product and a cover layer lying on the base layer, the base layer containing at least 30% by weight Al, at least 20% by weight. % Fe and at least 3 wt .-% Si and the top layer at least 60 wt .-% Zn, in particular at least 80 wt .-%, and at least 5 wt% Al and up to 10 wt .-% Fe and up to 10 wt .-% Si has.

The Al coating can be applied to the respective flat steel product as a first coating layer by fireruminating in a particularly economical manner while at the same time providing optimum coating results.

The Zn coating can also be applied to the previously applied to the flat steel product AI layer by a hot dip galvanizing particularly economically in a comparable manner known per se and preserved in practice.

In addition, particularly good coating results can be achieved if the Zn coating is deposited electrolytically on the Al coating as an alternative to hot-dip galvanizing. When electrolytic galvanizing is preferably a layer with a Zn content of at least 99 wt .-% deposited.

Another alternative possibility of applying the Zn layer is that the Zn coating is deposited on the Al coating in a PVD process. The use of the PVD method (PVD = physical vapor reduction) for the application of the Zn layer allows a particularly exact adjustment of the layer thickness.

When hot-dip galvanizing and when applying by PVD method can be at least one other element of Al, Mg or Fe contained in addition to Zn. Advantageously, the contents should not exceed 5% by weight Al, 5% by weight Mg and / or 0.5% by weight Si.

The contents of other accompanying elements in the Zn coating, e.g. Pb, Bi, Cd, Ti, Cu, Cr or Ni, should not exceed 1 wt .-% in total.

To set a surface roughness which improves the wettability and bonding of the subsequently applied Zn layer, it may be expedient to subject the flat steel product provided with the Al coating to temper rolling before application of the Zn coating.

For the same purpose, it may be advantageous to decap the steel flat product provided with the Al coating before applying the Zn coating. During pickling, the flat products coated according to the invention are passed through an acid bath which unwinds the oxide layer from them, without attacking the surface of the flat steel product itself. By the deliberately carried out step of pickling the Oxidabtrag is controlled so that you get a favorable set for the electrolytic strip galvanizing surface.

In some cases, in particular in a non-continuous implementation of the method steps, it is advantageous to additionally perform an alkaline cleaning before picking.

The process according to the invention can be carried out in a particularly economical manner when the Al coating and then the Zn coating and all the work steps required between the respective coating steps are completed in a sequence of operations continuously following each other.

If a corresponding system technology is not available or if it proves to be expedient for other reasons, it is also easily possible to apply the Al coating and then the Zn coating in a fractional, discontinuous mode of operation.

The particular advantage of the invention, as already explained, is that the deformation of the

Flat steel product can be made to the steel component in a single thermoforming step. Thus, coated steel strip according to the invention proves to be particularly insensitive to those in hot forming in one Traces occurring even if the respective component is given a complex shape.

Equally, however, it is also possible to carry out the forming of the flat product coated according to the invention in several stages, wherein in each case at least one forming stage is carried out as the thermoforming step following the heating to the thermoforming temperature. Accordingly, if this proves to be advantageous from a production point of view, the flat steel product may undergo at least one cold forming step before being heated to the hot forming temperature. In this case, the deformation can take place almost completely during the cold forming, so that in this case the hot forming step carried out after the cold forming represents rather a warm calibration with subsequent quenching in the tool.

Particularly good results are obtained when the Al coating applied to the flat steel product before heating to the thermoforming temperature has a thickness of 5 to 25 μm, in particular 5 to 15 μm, and the Zn coating applied to the AlSi coating Heating to the thermoforming temperature has a thickness of 2 - 10 microns. In this case, investigations have shown that especially when the AlSi coating is applied by fire aluminizing, prior to heating to the thermoforming temperature between the flat steel product and the correspondingly applied AlSi coating a 2-5 .mu.m thick, Al, Si and Fe containing alloy boundary layer available is. Taking into account the above-mentioned thicknesses of its individual layers, one according to the invention has two work procedures deforming flat product applied metallic coating typically has a total thickness of 7 - 35 microns on.

As explained, in a finished part according to the invention, a base layer lying directly on the flat steel product and consisting predominantly of Al and additional Fe, Zn and Si is present, on which predominantly Zn and additional contents are present Al, Si and Fe existing cover layer is located. The base layer has at least 30 wt .-% Al, at least 20 wt .-% Fe, at least 3 wt .-% Si and at most 30 wt .-% Zn, while in the top layer at least 60 wt .-%, in particular at least 80 wt .-%, Zn at least 5 wt .-% Al and at most 10 wt .-% Fe and at most 10 wt .-% Si are present.

The thickness of the base layer of the finished molded component according to the invention is typically 10 to 50 μm, in particular 15 to 25 μm, while the thickness of the cover layer is typically in the range of 5 to 20 μm, in particular 3 to 10 μm.

The invention will be explained in more detail with reference to exemplary embodiments. Show it:

Fig. 1 is a schematic representation of a first inventive workflow in the coating of a flat steel product; 2 is a schematic representation of a second inventive workflow in the coating of a flat steel product.

Fig. 3 is a schematic representation of a third process according to the invention in the coating of a flat steel product;

4 shows a schematic illustration of a fourth workflow according to the invention in the coating of a flat steel product;

5 is a schematic representation of a fifth inventive workflow in the coating of a flat steel product.

FIG. 6 shows a comparison of the layer structure on a flat steel product coated according to the invention before and after heating to thermoforming temperature; FIG.

7 shows the result of a quiescent potential measurement on different samples;

8 shows a detail of a microsection of a steel flat product coated according to the invention before heating to forming temperature;

9 shows a detail of a micrograph of a steel flat product coated according to the invention after heating to deformation temperature. In the figures 1 to 5 various possibilities of practical implementation of the erfmdungsgemaßen method are exemplified. In each case, the respective examples are based on a cold-rolled steel strip which is produced, for example, from the known 22MnB5 steel.

In the procedure indicated in Fig. 1, the operations of cleaning, fire aluminizing (i.e., annealing, passing through an AlSi hot-dip bath), tempering and electrolytic coating are completed in a continuous operation.

Fig. 2 shows an example in which the operations indicated in Fig. 1 are run through, but not in a continuous, but m a broken sequence. Thus, in the example given in FIG. 2, after the formerly aluminum-coated steel strip has been tempered, the strip is wound into a coil, transferred to an electrolytically working coating machine, cleaned and de-capped there and then electrolytically provided with the Zn coating applied to the AlSi coating ,

In the example given in Fig. 3, the operations of cold strip cleaning, fire aluminizing (ie, annealing and passing through an AlSi melt bath), hot dip galvanizing (i.e., cooling to the bath inlet temperature and passing through a Zn melt bath) are carried out go through a continuous Arbeitabiauf, while in the example shown in Fig. 4, the same steps so far be completed discontinuously, as there after the fire aluminizing provided with the AlSi coating tape is cooled to room temperature and placed in a hot-dip galvanizing plant before being annealed there and passed through the melt bath.

Finally, Fig. 5 gives an example of a procedure in which the steel strip is first cleaned, then fire-aluminized (ie, annealed and passed through an AlSi melt bath), then finish rolled, then cleaned, and finally by application of a PVD Process is coated with the Zn layer.

Fig. 6 shows in its left half the layer structure of a coating, as it is present in accordance with the invention procedure before heating to thermoforming temperature. Thus, between the steel substrate and the overlying AlSi layer, typically containing 90 wt% and 10 wt% Si, is one

Alloy layer formed containing Al, Si and Fe. The AlSi layer ("first layer") and the alloy layer together form the "base layer" of the overall coating. The Zn layer ("second layer"), which typically consists of 99% by weight of Zn and less than 1% by weight of Al, is applied to the "base layer" as the "top layer".

In the right half of Fig. 6, the layer structure of Gesamtabszugs is shown, which is at a temperature of more than five minutes extending heating of the layer structure shown in the left half of 900 ⁰ C. Accordingly, after this heating on the steel substrate, a base layer consisting of 40% by weight of Al, 30% by weight of Fe, 20% by weight of Zn and 5% by weight of Si, on which a covering layer consisting of 80% by weight of Zn, 16% by weight of Al, 2% by weight of Si and 2% by weight of Fe. Covering layer and base layer together form the overall coating there as well.

Based on quiescent potential measurements, the results of which are summarized in Fig. 7, the cathodic protective effect of the inventively generated, after heating to a thermoforming temperature of 880 ⁰ C obtained coating has been detected (curve "AS + Zn, annealed 880 ⁰ C"). It was found that the cathodic protective effect of the coating inventively produced is better than the effect of a conventional Zn Uberzugs after heating at 880 ⁰ C (curve "Z annealed 880 ⁰ C") and the protective effect of a ungegluhten Zn Uberzugs (curve " Z ungegluht ") comes approximately the same. Also, the resting potential measurements have confirmed that a conventional AlSi Uberzug (curve "AS annealed 950 ⁰ C") is not coated on the in this case required for thermoforming hot-forming temperature of 950 ⁰ C no improvement in the cathodic protection over a after heating, ungegluhten Sheet (curve "sheet unglazed") yields.

To test the method according to the invention, a large number of experiments have been carried out, of which three are explained by way of example below: Trial 1:

A steel strip made of a hardenable steel having a carbon content of 0.22%, a Mn content of 1.2%, a Cr content of 0.20% and a B content of 0.003% is used as a cold-rolled strip in annealed in a continuous hot dip coating line and coated with an AlSi melt. For this purpose, the tape was first cleaned in a cleaning of the Schmutzruckstände from the cold rolling process and then went through an annealing furnace by it has been heated to 750 ⁰ C.

At this temperature, the strip has been annealed recrystallizing in the annealing furnace in a protective gas atmosphere with 10% H ₂ and balance N ₂ .

After cooling to a temperature of 680 ⁰ C (also still under inert gas 10% H ₂ , balance N ₂ ), the band has entered an aluminum bath with a temperature of 660 ⁰ C. In addition to Al, the aluminum bath additionally contained about 10% by weight of silicon.

After the tape has been pulled out of the molten bath, a coating thickness of 18 μm has been set by means of the Dusen scraping system.

After cooling the strip to <50 ⁰ C is performed by skin-pass rolling in a Dressiergerust the adjustment of the surface roughness of the tape provided with the AlSi-coating. In a subsequent section of the production line, the strip was then first chemically treated in an aqueous solution with 80 g / l HCl (hydrochloric acid) for 10 s at 40 ⁰ C.

This was followed in electrolysis cells, the electrolytic Äbscheidung of 7 microns zinc from a zinc sulfate electrolyte at a current density of approx. 50 A / dm ² and an electrolyte temperature of approx. 60 ⁰ C on the surface of the AlSi coating.

Finally, the tape has been wound into a finished coil.

From the coated tape boards have been cut and first cold preformed in a forming press. The preformed parts were then heated in an oven to a thermoforming temperature of 880 ⁰ C for 5 min. The layer structure present before heating to the hot forming temperature is shown in FIG. 8.

Subsequently, the heated blanks are transferred by means of a manipulator in a hot forming press and formed there to a finished component and cooled quickly in the tool in a known manner. FIG. 9 shows the overall coating present on the component produced in this way.

Trial 2:

A steel strip made of a hardenable steel is as a cold-rolled strip in a continuous Hot dip coating line annealed and coated. The tape has first been cleaned and annealed as in Example 1. Subsequently, it has undergone an aluminum-silicon bath (Si content 10%) whose temperature was 660 ⁰ C. The subsequently set by Abstreifdusen thickness of the resulting AlSi coating was 15 microns. After a Kuhlstrecke, over which the tape has been cooled to 480 ⁰ C, the tape is immersed in a second molten bath of zinc, which was provided with an addition of 0.2% Al. With the subsequent Abstreifdusen a zinc coating thickness of 5 microns has been set. After cooling the strip to <50 ⁰ C, the setting of the surface roughness was carried out in a temper mill. Finally, the tape has been wound into a finished coil.

From the strip thus coated with a first AlSi layer and a second Zn layer coated thereon, blanks were cut for the hot forming process and heated in an oven at 900 ^° C. for 5 minutes. Subsequently, the boards are transferred by means of a manipulator in a forming press and here converted into a component and cooled in the tool.

Also for the thus obtained component corrosion protection properties could be detected, which corresponded to the properties that have been determined for the component produced in accordance with Experiment 1. Trial 3:

A steel strip of a hardenable steel has been annealed and coated as a cold-rolled strip in a continuous hot dip coating line. The tape is first cleaned as in Example 1, annealed and provided with an AlSi coating. The coating thickness set by the stripping nozzles amounts to 20 μm in this case. After cooling the strip to <50 ⁰ C was carried out by temper rolling in a temper rolling the adjustment of the surface roughness.

In a subsequently run through section, the tape was first cleaned alkaline, then to be coated in a PVD module with a zinc coating of 3 microns. Finally, the tape has been wound into a finished coil.

Blanks were cut from the coated strip for the hot forming process and heated in an oven at 900 ⁰ C for 5 min. Subsequently, the boards are converted by means of manipulator in a forming press and here converted into a component and cooled in the tool accelerated.

Also for the thus obtained component corrosion protection properties could be detected, which corresponded to the properties that have been determined for the component produced in accordance with Experiment 1.

Claims

PATENT AT SPRU

1. A method for producing a steel component provided with a metallic coating which protects against corrosion, comprising the following steps:

Coating a flat steel product made from a low alloy tempered steel with an Al coating containing at least 85% by weight of Al and optionally up to 15% by weight of Si;

Coating of the aluminum flat product provided with the Al coating with a Zn coating containing at least 90% by weight of Zn,

- warming of the flat steel product on a forming amount of at least 750 ⁰ C hot-forming temperature,

- thermoforming of the heated steel component from the flat steel product, and

- To form Vergutungs- or Hartegefuge sufficiently fast cooling cooling of the thermoformed steel component.

2. The method according to claim 1, characterized in that the thermoforming temperature is 850 to 950 ⁰ C, in particular 850-900 ⁰ C.

3. Method according to one of claims 1 or 2, characterized in that the Al coating is applied by fire aluminizing.

4. A process according to any one of the preceding claims, wherein the Al coating comprises 5-12% by weight of Si.

5. Method according to one of the preceding claims, characterized in that the Zn coating is applied by hot-dip galvanizing to the Al layer previously applied to the flat steel product.

6. The method of claim 1, wherein the Zn coating is deposited electrolytically on the Al coating.

7. The method according to claim 6, characterized in that the Zn coating contains at least 99 wt .-% Zn.

8. The method of claim 1, wherein the Zn coating is deposited on the Al coating in a PVD process.

9. The process according to claim 5 or 8, wherein in the Zn coating, in addition to Zn, at least one element from the group "Al, Mg, Si" is contained in the Zn coating.

10. Method according to one of the preceding claims, characterized in that the flat steel product provided with the Al coating is subjected to skin pass rolling before application of the Zn coating.

11. Method according to one of the preceding claims, characterized in that the flat steel product provided with the Al coating is subjected to pickling before the Zn coating is applied.

12. The method according to any one of the preceding claims, characterized in that the Al coating and then the Zn coating are applied in continuously successive steps.

13. The process as claimed in claim 1, wherein the Al coating is applied and then the Zn coating is applied in discontinuously successive working steps.

14. Method according to one of the preceding claims, characterized in that the conversion of the flat steel product to the steel component takes place in a single thermoforming step.

15. The method according to any one of claims 1 to 13, d a d u r c h e c e n e c e s e, in which the forming is carried out in several stages, wherein at least one forming step is performed as following the heating to thermoforming temperature thermoforming step.

16. The method of claim 15, wherein the flat steel product passes through at least one cold forming step prior to heating to the thermoforming temperature.

17. The method according to any one of the preceding claims, characterized in that the applied to the flat steel product Al coating before heating to the Mold temperature has a thickness of 5 - 25 microns.

18. The method according to claim 1, wherein the Zn coating applied to the Al coating before heating to the thermoforming temperature has a thickness of 2-10 μm.

19. Method according to one of the preceding claims, characterized in that between the flat steel product and the Al coating before heating to the thermoforming temperature a

2 - 5 microns thick, Al, Si and Fe containing alloy boundary layer is present.

20. Method according to one of claims 17 to 19, wherein the total thickness of the metallic coating present on the flat steel product before heating to the hot forming temperature amounts to 7 to 35 [mu] m.

21. The method according to any one of the preceding claims, characterized in that the finished molded steel component has a directly on the flat steel product from which the steel component is formed, lying mainly consisting of Al and additional Fe, Zn and Si existing base layer, on the one to consists predominantly of Zn and additional contents of Al, Si and Fe existing cover layer.

22. The method of claim 21, wherein the base layer has at least 30 wt% Al, at least 20 wt% Fe, and at least 3 wt% Si.

23. The method according to claim 21, wherein the cover layer contains at least 60% by weight of Zn, at least one of claims 21 or 22, wherein said cover layer is at least 60% by weight Zn

5% by weight of Al and up to 10% by weight of Fe and up to 10% by weight of Si.

24. The method according to claim 21, wherein the thickness of the base layer is 15-25 μm.

25. The method according to claim 21, wherein the thickness of the cover layer is 3-10 μm.

26. The method according to any one of the preceding claims, characterized in that the flat steel product is produced from a manganese-boron steel.

27 steel component produced by at least one hot forming step forming a low alloy tempered steel produced, coated with a corrosion-resistant metallic coating steel flat product, characterized in that the metallic coating formed by a resting on the steel flat product base layer and lying on the base layer in that the base layer contains at least 30% by weight of Al, at least 20% by weight of Fe, at least 3% by weight of Si and at most 30% by weight of Zn, and in that the outer layer contains at least 60% by weight of Zn, at least 5% by weight Al, up to 10% by weight Fe and up to 10% by weight Si.

28. Steel component according to claim 27, wherein the thickness of the base layer amounts to 10-50 μm, in particular 15-25 μm.

29. Steel component according to claim 27 or 28, wherein a thickness of the cover layer is 5-20 μm, in particular 3-10 μm.

30. The steel component according to claim 27, wherein the flat steel product of a manganese-boron steel is produced.