EP2290133B1 - Method for producing a steel component with an anti-corrosive metal coating and steel component - Google Patents

Abstract

The method for the production of a steel component provided with a metallic coating protected against corrosion, comprises providing a flat steel product that is produced from steel material containing 0.3-3 wt.% of manganese, where the steel material has an yield strength of 150-1100 MPa and a tensile strength of 300-1200 MPa, and coating the flat steel product with a corrosion protection coating that consists of zinc-nickel (ZnNi) alloy coating consisting of gamma -ZnNi-phase electrolytically deposited on the flat steel product. The method for the production of a steel component provided with a metallic coating protected against corrosion, comprises providing a flat steel product that is produced from steel material containing 0.3-3 wt.% of manganese, where the steel material has an yield strength of 150-1100 MPa and a tensile strength of 300-1200 MPa, coating the flat steel product with a corrosion protection coating that consists of zinc-nickel (ZnNi) alloy coating consisting of gamma -ZnNi-phase electrolytically deposited on the flat steel product, where the coating consists of nickel (7-15 wt.%) and also zinc and unavoidable impurities, heating a board formed from the flat steel product at 800[deg] C amounting to platinum temperature, forming the steel component from the platinum in a form tool and hardening the steel component through cooling at a temperature, in which the steel component exists itself in a condition suitable for forming compensation or hardening structure, with a cooling rate that suffices for forming the compensation structure or hardening structure. The formation of the steel component is carried out as pre-forming and the steel component is formed after heating. The zinc-nickel alloy coating consists of finished steel component of gamma -ZnNi and I ->zinc iron. A manganese-containing layer is present in the finished steel component on the corrosion protection coating, in which manganese is available in metallic or oxidic form. The manganese-containing layer has a thickness of 0.1-5 mu m and the manganese content of the manganese-containing layer is 0.1-18 wt.%. The corrosion protection coating comprises an additional zinc-layer before forming the steel component, where the zinc-layer is applied before forming the steel component on the zinc-nickel alloy coating. The thickness of the zinc-layer is 2.5-12.5 mu m. The corrosion protection coating of the finished steel component comprises a zinc-rich layer lying on the nickel-containing alloy coating. The formation of the steel component is carried out as hot forming and the forming and cooling of the steel component are carried out by a hot forming tool. The formation of the steel component and hardening are carried out in two separated processes. An independent claim is included for a steel component.

Description

The invention relates to a method of producing a steel component provided with a metallic corrosion protective coating by forming a mild steel Mn steel product which is provided with a ZnNi alloy coating prior to forming the steel component.

If this refers to "flat steel products", it means steel strips, steel sheets or boards made of them and the like.

In order to provide the combination of low weight, maximum strength and protective effect required in modern body construction, hot-formed components of high-strength steels are nowadays used in such areas of the body, which may be exposed to particularly high loads in the event of a crash.

In hot press hardening, steel blanks separated from cold or hot rolled steel strip are heated to a forming temperature generally above the austenitizing temperature of the respective steel and heated to a tool in a heated state Forming press laid. In the course of the subsequent transformation, the sheet metal blank or the component formed from it undergoes rapid cooling due to the contact with the cool tool. The cooling rates are set so that results in the component hardness structure.

A typical example of a steel suitable for hot press hardening is known as "22MnB5" and found in steel key 2004 under material number 1.5528.

In practice, the advantages of the known MnB steels which are particularly suitable for hot press hardening are counteracted by the fact that manganese-containing steels are generally not resistant to wet corrosion and are difficult to passivate. This compared to lower alloyed steels when exposed to elevated chloride ion concentrations great tendency to locally limited, but intense corrosion makes the use of the material group of high-alloy steel sheets belonging steels straight in the body construction difficult. In addition, manganese-containing steels tend to surface corrosion, which also limits the spectrum of their usability.

It is therefore looking for ways to provide even manganese-containing steels with a metallic coating that protects the steel from corrosive attack.

According to the in the EP 1 143 029 B1 described method for producing components by hot press hardening For this purpose, a steel sheet is first provided with a zinc coating and then heated before the hot deformation such that adjusts an intermetallic compound in the heating of the flat steel product by a transformation of the coating on the steel sheet. This is to protect the steel sheet against corrosion and decarburization and take over a lubricating function during the hot forming in the pressing tool.

When trying in the EP 1 143 029 B1 To realize the general approach proposed in practice, many problems arose. Thus, it proved difficult to apply the zinc coating to the steel substrate such that sufficient adhesion of the coating to the steel substrate after the formation of the intermetallic compound, sufficient coatability of the coating for subsequently applied coating and adequate resistance of both the coating itself and also the steel substrate is ensured against the formation of cracks in the hot forming.

A proposal on how to produce zinc coatings on steel strips, to which an organic coating can be applied particularly well, is in EP 1 630 244 A1 described. Accordingly, an up to 20 wt .-% Fe containing Zn layer is applied, for example, electrolytically or using another known coating method on the steel sheet to be processed. Subsequently, the thus coated steel sheet from room temperature Heated to 850-950 ° C and thermoformed at 700-950 ° C. Particularly suitable for the production of the Zn layer while the electrolytic deposition is mentioned. The Zn layer may also be formed as an alloy layer according to the known method. As possible alloy components of this layer are in the EP 1 630 244 A1 Mn, Ni, Cr, Co, Mg, Sn and Pb, and also mentioned Be, B, Si, P, S, Ti, V, W, Mo, Sb, Cd, Nb, Cu and Sr as additional alloying components.

Essential for that in the EP 1 630 244 A1 described method is that the present on him 1-50 microns thick Zn coating comprises an iron-zinc solid solution phase and has a zinc oxide layer whose thickness is limited on average to at most 2 microns. For this purpose, according to the known method, either the annealing condition when heating to the temperature required for the hot press forming is selected so that at best a controlled oxide formation occurs, or after hot forming the oxide layer present on the obtained steel component is or particle-removing process at least partially removed so far that the according to the EP 1 630 244 A1 maximum thickness of the oxide layer is maintained. Thus, this known procedure also requires complex measures in order to ensure, on the one hand, the desired anticorrosive effect of the Zn coating and, on the other hand, the required good coatability and lacquer adhesion in the case of coating taking place after thermoforming.

From the DE 32 09 559 A1 Another method is known with which a zinc-nickel alloy coating can be deposited electrolytically on a steel strip. In the course of this process, the strip to be coated is subjected to an intensive electroless pre-treatment prior to the deposition of the ZnNi coating in order to produce a thin zinc-nickel-containing primary layer on it. Subsequently, the actual zinc-nickel coating is then applied electrolytically. In order for the electrolytic deposition of the alloy coating to be constant with a predetermined composition, separate anodes containing only one alloying element are used. These are connected to separate circuits in order to adjust the current flowing through them and thus the delivery of the respective metal in the electrolyte targeted.

The results of a systematic study of the properties of zinc alloy coatings on a steel sheet consisting of a hardenable steel are in the WO 2005/021822 A1 contain. The coating consisted essentially of zinc and additionally contained one or more oxygen-affine elements in a total amount of 0.1-15 wt .-% based on the total coating. The oxygen-affine elements are specifically named Mg, Al, Ti, Si, Ca, B and Mn. The steel sheet coated in this way was then brought to a temperature necessary for hardening under the admission of atmospheric oxygen. In this heat treatment, a superficial oxide layer of the oxygen-affine element (s) formed.

According to one of the in the WO 2005/021822 A1 experiments described on a sheet unspecified composition by electrochemical deposition of zinc and nickel ZnNi coating has been produced. The weight ratio of zinc to nickel in the anticorrosion layer was about 90/10 at a layer thickness of 5 μm. The thus coated sheet has been annealed for 270 s at 900 ° C. in the presence of atmospheric oxygen. The diffusion of the steel with the zinc layer resulted in a thin diffusion layer of zinc, nickel and iron. At the same time, most of the zinc oxidized to zinc oxide.

After the in the WO 2005/021822 A1 According to documented findings, the ZnNi coating thus obtained was a pure barrier protection without cathodic corrosion protection effect. Its surface showed a verdallert, green appearance with small local flaking to which the oxide layer does not adhere to the steel. These errors are in accordance with the WO 2005/021822 due to the fact that the coating itself did not contain a sufficiently oxygen-affine element.

Against this background, the object underlying the invention was to specify a method which is easy to carry out in practice and which makes it possible to produce a steel component with comparatively little effort, which is provided with a metallic coating which adheres well and reliably protects against corrosion. In addition, a corresponding procured steel component should be specified.

With regard to the method, this object has been achieved according to a first variant of the invention in that in the production of a steel component, the method steps indicated in claim 1 are passed.

An alternative, the above object in a corresponding manner-solving variant of the method according to the invention is given in claim 2.

The first variant of the method according to the invention comprises the shaping of the steel component in the so-called "direct" method, while the second method variant involves the shaping of the steel component in the so-called "indirect" method.

Advantageous embodiments of the method variants according to the invention are specified in the claims back to the claims 1 or 2 and explained below.

With regard to the steel component, the solution according to the invention of the abovementioned object is that such a component has the features specified in claim 13. Advantageous variants of the steel component according to the invention are specified in the claims related back to claim 13 and explained below.

In a method according to the invention for producing a steel component provided with a metallic coating which protects against corrosion, a flat steel product, ie a steel strip or steel sheet, is first of all made of a 0.3 to 3 wt .-% manganese-containing, higher-strength and hardenable steel material is produced. This has a yield strength of 150-1100 MPa and a tensile strength of 300-1200 MPa.

Typically, this steel material may be a high strength MnB steel of known composition. Accordingly, in addition to iron and unavoidable impurities (in% by weight), the steel processed according to the invention may contain 0.2-0.5% C, 0.5-3.0% Mn, 0.002-0.004% B, and optionally one or more elements of Group "Si, Cr, Al, Ti" contained in the following contents: 0.1-0.3% Si, 0.1-0.5% Cr, 0.02-0.05% Al, 0.025-0.04 % Ti.

The inventive method is suitable for the production of steel components both from conventionally only hot rolled hot strip or sheet as well as from conventionally cold rolled steel strip or sheet.

The correspondingly produced and provided flat steel product is coated with a corrosion protection coating, this coating according to the invention comprises an electrolytically applied to the steel substrate, one-phase consisting of γ-ZnNi phase zinc-nickel alloy coating. This ZnNi alloy coating alone can form the corrosion coating or be supplemented by further protective layers applied to it.

It is crucial that the γ-zinc-nickel phase of the resting on the steel substrate ZnNi alloy coating is already realized by the electrolytic coating. That is, unlike coating processes in which an alloy layer forms only as a result of heating to the temperature required for the subsequent hot forming and curing and the resulting diffusion processes, according to the invention is an alloy layer of certain composition and prior to heating on the steel flat product Structure composed of zinc and nickel. In this case, the proportions of Zn and Ni and the deposition conditions during the production of the ZnNi alloy layer are selected so that the ZnNi alloy layer is formed as a single-phase coating of Ni5Zn21 phase with a cubic lattice structure. It should be noted that this γ-ZnNi phase layer is deposited in an electrolyte deposition not in the stoichiometric composition, but at nickel contents in the range of 7-15%, with at Ni contents of up to 13 wt. %, in particular from 9 to 11% by weight, give particularly good properties of the coating.

• Die Art der Strömung am zu beschichtenden Substrat: Laminar oder turbulent; sowohl bei laminarer als auch bei turbulenter Strömung des Elektrolyten am zu beschichtenden Stahlflachprodukt stellen sich gute Beschichtungsergebnisse ein. Bei vielen in der Praxis zur Verfügung stehenden Beschichtungsanlagen wird jedoch aufgrund des intensiveren Austauschs zwischen Elektrolyt und Stahlsubstrat in der Praxis jedoch eine turbulente Strömung bevorzugt werden,
• Strömungsgeschwindigkeit des Elektrolyten: 0,1 - 6 m/s;
• Ni/Zn-Verhältnis des Elektrolyten: 0,4 - 4;
• Ausrichtung der Elektrolytströmung in Bezug auf das jeweils zu beschichtende Stahlsubstrat: Die Beschichtung des Stahlsubstrats kann sowohl in vertikal als auch in horizontal ausgerichteten Zellen erfolgen;
• Stromdichte: 10 - 140 A/dm2;
• Temperatur des Elektrolyten: 30 - 70°C;
• pH-Wert des Elektrolyten: 1 - 3,5.

Among the above-mentioned "deposition conditions" of the electrolytic coating are, for example, the type of flow on the substrate to be coated, the flow rate of the electrolyte, the Ni / Zn ratio of the electrolyte, the orientation of the electrolyte flow with respect to each coated steel substrate, the current density, the temperature and the pH of the electrolyte. According to the invention, these influencing variables are to be coordinated with one another in such a way that the desired single-phase ZnNi coating is established with the Ni contents predetermined according to the invention. For this purpose, the parameters mentioned can be varied as follows, depending on the respective available system technology:

• The type of flow on the substrate to be coated: laminar or turbulent; Both with laminar and turbulent flow of the electrolyte to be coated flat steel product set up good coating results. However, in many coating systems available in practice, a turbulent flow will be preferred in practice due to the more intensive exchange between electrolyte and steel substrate.
• Flow velocity of the electrolyte: 0.1 - 6 m / s;
• Ni / Zn ratio of the electrolyte: 0.4 - 4;
• Alignment of the electrolyte flow with respect to the steel substrate to be coated: The coating of the steel substrate can take place in both vertically and horizontally aligned cells;
• Current density: 10 - 140 A / dm2;
• Temperature of the electrolyte: 30 - 70 ° C;
• pH of the electrolyte: 1 - 3.5.

A particular advantage of the electrolytic coating according to the invention of the flat steel product with a ZnNi alloy layer of exactly predetermined composition and structure is also that the coating produced therefrom has a matt, rough surface which has a lower reflectivity than those typically produced in the course of known hot-press molding processes Zn-coatings. As a result, steel flat products coated in accordance with the invention have an increased heat absorption capacity, so that the subsequent heating to the respective board or component temperature can take place more quickly and with less expenditure of energy. The thus made shorter furnace lay times and energy savings make the inventive method particularly economical.

From the steel flat product coated in accordance with the invention, a steel plate is then formed. This can be divided in a conventional manner of the respective steel strip or steel sheet. It is also conceivable, however, that the flat steel product in the coating already has the shape required for the subsequent shaping of the component, that is to say corresponds to the printed circuit board.

The thus provided in accordance with the invention with a single-phase ZnNi alloy coating steel plate is then heated according to the first variant of the method according to the invention to a not less than 800 ° C amounting platinum temperature and then formed from the heated board, the steel component. According to the In the second variant of the method, on the other hand, the steel component is first at least preformed from the blank and only then is the heating carried out to a component temperature of at least 800 ° C.

In the course of heating to the board or component temperature no phase transformation takes place in the previously applied, single-phase γ-ZnNi coating. Instead, even at temperatures below 700 ° C in the ZnNi layer, a partial substitution of atoms, in which the intermetallic γ-zinc-nickel phase (Ni5Zn21) in a Γ-zinc-iron phase (Fe3Zn10) rearranges ,

Accordingly, regardless of the respective process variant, the coating produced according to the invention always consists of cubic intermetallic phases both in the undeformed flat steel product and in the finished steel component: in the starting state in the flat steel product of γ-ZnNi coated according to the invention and after the heating and hot working of the finished steel component made of γ-ZnNi and Γ-ZnFe.

According to the first variant of the method according to the invention, the inventively heated to a temperature of at least 800 ° C board is formed to the steel component. This can be done, for example, that the board is conveyed immediately after the heating to the mold used in each case. On the way to the mold, it is inevitable that the board will cool down, such that in the case of such hot forming following heating, the temperature of the board as it enters the mold is usually lower than the board temperature at the exit of the furnace. In the mold, the steel plate is formed in a conventional manner to the steel component.

If the shaping is carried out at temperatures which are sufficiently high for the formation of hardness or compensation structures, then the steel component obtained can be cooled, starting from the respective temperature, at a cooling rate sufficient for the formation of tempering or hardening structure in its steel substrate. This process can be carried out particularly economically in the thermoforming tool itself.

Accordingly, the method of the invention is due to the insensitivity of coated in accordance with the invention steel flat product against cracks in the steel substrate and abrasion especially for one-stage hot press molding, in which a hot forming and cooling of the steel component by utilizing the heat of the previously carried out heating to the board temperature in one go be done in a tool.

In the second variant of the method, first the circuit board is formed and then, without intermediary heat treatment, the steel component is formed from this circuit board. The forming of the steel component is typically carried out in a cold forming process, in which one or more cold forming operations are performed. The degree of cold forming can be so high that the steel component obtained is substantially completely finished. However, it is also conceivable to carry out the first shaping as preforms and to finish the steel component after heating in a mold. This finish forming can be combined with the hardening process by carrying out the hardening as a form hardening in a suitable molding tool. In this case, the steel component is placed in a finished final shape imaging tool and cooled sufficiently quickly for the formation of the desired hardness or tempering. The form hardening thus enables a particularly good dimensional stability of the steel component. The shape change during mold hardening is usually low.

Regardless of which of the two variants of the method according to the invention are used, neither the shaping nor the cooling necessary for the formation of the hardness or compensation structure must be carried out in a manner deviating from the prior art. Rather, known methods and existing devices can be used for this purpose. Due to the fact that an alloy coating is already produced on the board to be deformed in accordance with the invention, in the case of hot forming or molding at elevated temperatures, there is no risk of softening of the coating and, accordingly, adhesions of coating material to the coating in it Contact coming surfaces of the tool comes.

The Mn content of the inventively processed steel substrate of 0.3 to 3 wt .-%, in particular 0.5 to 3 wt .-%, comes in combination with the invention produced on the steel flat product one-phase television series γ-ZnNi alloy coating of particular importance to. Thus, the Mn present in the steel substrate contributes significantly to the good adhesion of the zinc-nickel alloy coating in the steel component produced according to the invention.

Before being heated to the board or component temperature, the anticorrosive coating applied according to the invention contains less than 0.1% by weight of manganese in each case. During the subsequent heating to the board or component temperature, diffusion of the manganese present in the steel substrate then begins in the direction of the free surface of the anticorrosive coating applied according to the invention.

On the one hand, the Mn atoms diffusing into the ZnNi alloy layer upon heating cause an intensive coupling of the coating to the steel substrate.

On the other hand, Mn reaches a substantial part of the surface of the corrosion protection coating produced according to the invention and is deposited there in metallic and / or oxidic form. The thickness of the Mn-containing layer present in this way on the coating produced according to the invention - hereinafter referred to simply as "Mn oxide layer" for the sake of simplicity - is typically 0.1-5 μm. The positive effects of the Mn oxide layer are particularly safe when their thickness is at least 0.2 .mu.m, in particular at least 0.5 μm. The Mn content of the anticorrosive coating is in this near-surface, adjacent to the surface Mn-containing layer at 1-18 wt .-%, in particular 4-7 wt .-%.

In addition to the coupling to the steel substrate described above, the pronounced Mn oxide layer present on the coating produced in accordance with the invention ensures particularly good adhesion of organic coatings applied to the anticorrosive coating. The procedure according to the invention is therefore particularly suitable for the production of parts for vehicle bodies, which are provided with a coating after their shaping.

Unlike the prior art described at the outset, it is not absolutely necessary to remove the pronounced oxide layer obtained according to the invention in accordance with the invention. Rather, a practice-oriented embodiment of the method variants according to the invention provides that the oxide layer obtained in the process according to the invention selectively remains on the anticorrosive coating, since this oxide layer not only a particularly good coatability, but because of their comparatively high conductivity beyond a total good weldability inventively produced and procured Steel components guaranteed.

Using steels with a Mn content of less than 0.3% by weight results in a yellowish-appearance coating, indicating that a coating consisting primarily of ZnO is present on the coating Oxide layer is present. The coating thus obtained shows after the thermoforming, similar to that in the WO 2005/012822 reported trial, local flaking and tindering. On the other hand, a coating produced according to the invention on a steel containing at least 0.3% by weight of Mn has a brownish surface which is free from tearing and flaking.

The ZnNi coating deposited on the flat steel product according to the invention is applied in practice with a thickness of 0.5-20 μm. A particularly good protective effect of the ZnNi coating produced according to the invention arises in this case if it is deposited more than 2 μm thick on the flat steel product. Typical thicknesses of a coating produced according to the invention are in the range from 2 to 20 μm, in particular from 5 to 10 μm.

A further optimized protection of the steel component according to the invention against corrosion can be achieved by the anticorrosive coating comprising, in addition to the ZnNi alloy coating applied to the flat steel product, a Zn layer which is likewise applied to the ZnNi layer before the heating step. It is then on the prepared for further processing to the component according to the invention flat steel product before heating to the respective board or component temperature before at least a two-layer anticorrosive coating whose first layer of the inventively embodied ZnNi alloy layer and its second layer of the formed thereon, consisting only of zinc Zn layer is formed.

The additionally applied, typically 2.5-12.5 μm thick Zn layer is present in the finished steel component according to the invention as a Zn-rich layer into which Mn and Fe of the steel substrate and Ni from the ZnNi layer can be alloyed. In this case, Zn partially reacts to form Zn oxide and forms with the Mn from the base material the Mn-containing layer lying on the anticorrosive coating produced according to the invention. The application of an additional Zn layer of the anti-corrosion coating before heating for the hot forming thus leads to a further improvement of the cathodic corrosion protection.

It has been found that in the finished thermoformed and cured state, even in the presence of the additional Zn layer on the surface of the anticorrosive coating, the Mn oxide layer described above in detail is present. This ensures the good weldability and the good suitability of a steel component produced and procured according to the invention for painting in the same way with a corrosion protection coating combined from a ZnNi and Zn layer.

The additional Zn layer of the anticorrosive coating can be deposited as well as the previously applied ZnNi layer electrolytically. This can be done, for example, in a continuous pass through, multi-stage device for electrolytic Coating in the first stages of ZnNi alloy coating on the respective steel substrate and in the steps passed through the Zn layer are deposited on the ZnNi layer.

As explained above, a steel member according to the present invention is made by hot press molding and has a steel substrate made of a 0.3-3 wt% manganese-containing steel substrate and an anticorrosive coating thereon comprising an electrodeposited ZnNi alloy plating composed of γ-ZnNi and Γ-ZnFe, and having on its free surface a Mn-containing layer in which Mn is present in metallic or oxidic form.

In addition, the anticorrosive coating in the manner already described above can comprise a Zn layer resting on the ZnNi layer, the Mn-containing layer also being present on the anticorrosive coating in this case as well.

• Alkalische Entfettung des Stahlflachprodukts in einem Entfettungsbad. Typischerweise enthält das Entfettungsbad 5 - 150 g/l, insbesondere 10 - 20 g/l, eines Tensid-Reinigers. Die Temperatur des Entfettungsbades beträgt dabei 20 - 85 °C, wobei sich eine besonders gute Wirksamkeit bei einer Badtemperatur von 65 - 75 °C einstellt. Dies gilt insbesondere dann, wenn die Entfettung elektrolytisch erfolgt, wobei in diesem Fall besonders gute Reinigungsergebnisse erzielt werden, wenn mindestens ein Zyklus anodischer und kathodischer Probenpolung durchlaufen wird. Dabei kann es sich als vorteilhaft erweisen, wenn bei der alkalischen Reinigung nicht nur elektrolytisch tauchentfettet wird, sondern vor der elektrolytischen Reinigung schon eine Spritz-/Bürstreinigung mit dem alkalischen Medium durchgeführt wird.
• Spülen des Stahlflachproduktes, wobei diese Spülung mittels Klarwasser oder vollentsalztem Wasser durchgeführt wird.
• Dekapieren des Stahlflachproduktes. Beim Dekapieren werden die Flachprodukte durch ein Säurebad geleitet, das die Oxidschicht von ihnen abspült, ohne die Oberfläche des Stahlflachprodukts selbst anzugreifen. Durch den gezielt durchgeführten Schritt der Dekapierung wird der Oxidabtrag so gesteuert, dass man eine für die elektrolytische Bandverzinkung günstig eingestellte Oberfläche erhält. Nach dem Dekapieren kann ein erneutes Spülen des Stahlflachproduktes zweckmäßig sein, um Restbestände der beim Dekapieren eingesetzten Säure von dem Stahlflachprodukt zu entfernen.
• Sofern ein Spülen des Stahlflachprodukts durchgeführt wird, kann das Stahlflachprodukt währenddessen mechanisch gebürstet werden, um auch fest sitzende Partikel von seiner Oberfläche zu beseitigen.
• Auf dem vorbehandelten Stahlflachprodukt noch vorhandene Flüssigkeiten werden vor dem Eintritt in das Elektrolytbad üblicherweise mittels Abquetschrollen entfernt.

In order to ensure an optimum result of the electrolytic coating, the steel flat product may be subjected to a pre-treatment prior to the electrolytic coating in a conventional manner, in which the surface of the steel substrate is treated so that it optimally prepared for the subsequently performed coating with the corrosion layer Has surface state. For this purpose, one or more of the pre-treatment steps enumerated below can be run through:

• Alkaline degreasing of the flat steel product in a degreasing bath. Typically, the degreasing bath contains 5-150 g / l, especially 10-20 g / l, of a surfactant cleaner. The temperature of the degreasing bath is 20 - 85 ° C, with a particularly good effectiveness at a bath temperature of 65 - 75 ° C sets. This is especially true if the degreasing is carried out electrolytically, in which case particularly good cleaning results are achieved when at least one cycle of anodic and cathodic sample polarity is passed through. It may prove advantageous if in the alkaline cleaning is not only dipped electrolytically, but before the electrolytic cleaning already a spray / brush cleaning with the alkaline medium is performed.
• Rinsing the flat steel product, wherein this rinse is carried out by means of clear water or demineralized water.
• Picking the flat steel product. During pickling, the flat products are passed through an acid bath which rinses off the oxide layer 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. After picking, rinsing the flat steel product again may be expedient to remove residues of the acid used in pickling from the flat steel product.
• Meanwhile, if a rinse of the flat steel product is performed, the steel flat product may be mechanically brushed to also remove stuck particles from its surface.
• On the pretreated flat steel product still existing liquids are usually removed by means of squeezing rollers before entering the electrolyte bath.

As practical examples for leading to a particularly good result of the electrolytic coating pretreatments are the following variants:

Example 1:

A hood annealed cold strip is alkaline degreased and additionally degreased electrolytically. The degreasing bath at a concentration of 15 g / l contains a commercial cleaner available under the name "Ridoline C72" containing more than 25% sodium hydroxide, 1-5% of a fatty alcohol ether and 5-10% of an ethoxylated, propoxylated and methylated C12- 18 alcohol. The bath temperature is 65 ° C. The residence time in the spray degreasing is 5 s. This is followed by a brush cleaning. In the course of the strip is electrolytically degreased at a residence time of 3 s with anodic and cathodic polarity and a current density of 15 A / dm ^second This is followed by a multi-stage sink with demineralized water at room temperature with brush insert. The residence time in the sink is 3 s. In the following, a hydrochloric acid (20 g / l, temperature 35-38 ° C) is passed through with a residence time of 11 s. After an 8-s sink with demineralized water, the plate is transferred after passing through a squeezing in the electrolysis cell. In this, the coating according to the invention of the steel strip or sheet is explained below with reference to the embodiments in detail. The steel flat product emerging from the electrolytic coating line can be rinsed in several stages with water and demineralized water at room temperature. The total residence time in the sink is 17 s. Subsequently, the flat steel product then passes through a drying section.

Example 2:

Hot strip (pickled) of grade 22MnB5 (1.5528) is alkaline degreased and degreased electrolytically. In addition, the tape undergoes a brush cleaning in alkaline spray degreasing. The degreasing bath contains, at a concentration of 20 g / l, a commercial cleaner available under the name "Ridoline 1893" which contains 5-10% sodium hydroxide and 10-20% potassium hydroxide. The bath temperature is 75 ° C. The residence time in the spray degreasing is 2 s. In the course of the strip is electrolytically degreased at a residence time of 4 s with anodic and cathodic polarity at a current density of 15 A / dm ^second This is followed by a multi-stage sink with demineralized water at room temperature with upstream brush insert. The Length of stay is 3 s. In the following, a hydrochloric acid deacidification (90 g / l, temperature max 40 ° C) is carried out at a residence time of 7 s. After a five-stage cascade rinse with demineralized water, the sheet is transferred after passing through a squeezing into the electrolysis cell and there, as described in the following with reference to the embodiments, provided in accordance with the invention with a corrosion protection coating. After emerging from the plant for electrolytic coating, the steel flat product coated according to the invention is then rinsed in three stages with demineralized water at 50.degree. The sample then passes through a drying section with circulating air dryer, the air temperature being more than 100 ° C.

Example 3:

Bonnet annealed cold strip of grade 22MnB5 (1.5528) is alkaline degreased and degreased electrolytically. The degreasing bath at a concentration of 20 g / l contains a cleaner containing 1-5% C12-18 fatty alcohol polyethylene glycol butyl ether and 0.5-2% potassium hydroxide. The bath temperature is 75 ° C. The residence time in the horizontal spray rinse is 12 s. This is followed by a double brush cleaning. In the course of the strip is electrolytically degreased at a residence time of 9 s with anodic and cathodic polarity and a current density of 10 A / dm ^second This is followed by a multi-stage sink with demineralized water at room temperature with brush insert. The residence time is 3 s. in the Subsequently, a hydrochloric acid (100 g / l, room temperature) is passed through with a residence time of 27 s. After a combined brushing and Spritzfrischwasserspüle the sheet is transferred after passing through a squeezing in the electrolytic cell. Therein, the electrolytic deposition of the anticorrosive coating according to the invention takes place as explained below with reference to the exemplary embodiments. Subsequent to the electrolytic coating, the steel flat product coated in accordance with the invention is then rinsed in two stages with water and demineralized water at 40.degree. Total residence time 18 s. Afterwards, the sample passes through a drying section with circulation fan with a recirculation air temperature of 75 ° C.

Optimal work results arise when the board or component temperature in a conventional manner maximum 920 ° C, in particular 830-905 ° C, is. This applies in particular if the shaping of the steel component is carried out as hot forming following the heating to the board or component temperature such that the heated board ("direct" method) or the heated steel component ("indirect" method) Acceptance of a certain temperature loss is placed in each subsequently used mold. The final thermoforming can be carried out particularly reliably when the board or component temperature is 850 - 880 ° C.

The heating to the board or component temperature can be done in a conventional manner in a continuous flow in a continuous furnace. Typical annealing times are in the range of 3 to 15 minutes. Alternatively, however, it is also possible to carry out the heating by means of an inductively or conductively operating heating device. This allows a particularly rapid and accurate heating to the respective predetermined temperature.

Fig. 1

das Ergebnis einer GDOS-Messung eines erfindungsgemäßen Überzugs nach der Warmformgebung für die Elemente O, Mn, Zn, Ni und Fe;

Fig. 2

das in Fig. 1 dargestellte Messergebnis isoliert für das Element Mn.

The invention of embodiments will be explained in more detail. Show it:

Fig. 1

the result of a GDOS measurement of a coating according to the invention after thermoforming for the elements O, Mn, Zn, Ni and Fe;

Fig. 2

this in Fig. 1 illustrated measurement result isolated for the element Mn.

Cold rolled and recrystallized annealed as well as temper rolled strip samples A-Z - hereafter referred to simply as "Samples A-V2" for simplicity - have been provided with a zinc nickel alloy layer in a continuous pass electrogalvanizing line. In addition, a sample "Z" has been hot-dip coated for comparison.

For the samples A - Z, which each consist of a hardenable steel, the essential Mn contents in the Column "Mn content" of Table 2 indicated. Thus, Samples A-Q and Z each contained Mn contents of more than 0.3 wt%, while the Mn contents of Samples V1, V2 were below the threshold of 0.3 wt%.

Each of the band-shaped samples A - V2 first went through a cleaning treatment in which the following work steps were completed successively:

First, the respective sample A - V2 was subjected in a 60 ° C warm alkaline cleaning bath at a residence time of 6 s spray cleaning with brush insert.

This was followed by an electrolytic degreasing at a current density of 15 A / dm ² for 3 s.

This was followed by a double clear water rinse with brush insert. The duration of this rinsing treatment was 3 s in each case.

In the following, a decantation with hydrochloric acid in a concentration of 150 g / l at room temperature was carried out for 8 seconds.

Finally, a three-stage cascade flushing took place.

The thus pretreated samples A - V2 have been subjected to an electrolytic coating in an electrolytic cell. In Table 1, for each of the samples A-V2, the respective set operating parameters "Zn" = Zn content of the electrolyte in g / l, "Ni" = Ni content of Electrolytes in g / l, "Na2SO4" = Na2SO4 content of the electrolyte in g / l, "pH" = pH value of the electrolyte, "T" = temperature of the electrolyte in ° C, "cell type" = orientation of the band flow through the electrolyte Electrolytes, "flow rate" = flow rate of the electrolyte in m / s and "current density" = current density in A / dm ² indicated.

For comparison, the sample Z has been conventionally hot-dip galvanized.

Table 2 lists, in addition to the Mn contents of the respective samples A-V2, the properties of the ZnNi coatings which have been electrodeposited under these conditions. It can be seen that in variants A-H and N-P a single-phase γ-ZnNi coating according to the invention has been obtained whereas in variants I - K η-Zn, i. elemental zinc, and γ-ZnNi coexistent.

In variants L and M, a thin layer of pure nickel has been applied to the steel substrate prior to application of the ZnNi layer (so-called "nickel flash"). These are pure nickel deposits, which are under the single-phase γ-ZnNi coating. Since such a multilayer structure has no positive effect on the properties to be achieved, these variants have also been described as not according to the invention, as are the samples obtained according to variants I-K.

The Ni content of the sample Q was too high, so that this too was not considered to be in accordance with the invention.

The samples V1 and V2 have been produced on a steel with a too low Mn content. Therefore, these samples are also not according to the invention, although they had a γ-ZnNi coating according to the invention.

From the electrolytically coated samples A-H and N-P to be regarded as being in the single-phase structure of their ZnNi alloy coating, boards 1 to 23 have been partitioned.

In addition, of the two-layer ZnNi coating with nickel-flashed samples L and M blanks 31-35, of which due to the high Ni content of their coating also as not according to the invention to be regarded sample Q a board 36, produced from the comparison Samples V1 and V2 boards 37-40 and from the comparison sample Z a board 41 has been divided.

The boards 1 to 41 are then heated to the specified in Table 3 board temperature "T-furnace" over a glow time "t-annealed" and hot-press-formed in a conventional hot press tool single stage to each steel component and cooled so quickly that in the steel substrate hardness structure established.

For each of the steel components produced from the boards 1 to 41, this is in the course of the hot-pressing deformation determined hot forming behavior has been assessed and tested whether it has come in the hot press forming to cracking in the respective steel substrate. The results of this assessment and testing are also listed in Table 3.

The steel components formed from the boards 1 to 36 and 41 were then subjected to a salt spray test according to DIN EN ISO 9227. If corrosion of the base metal has been detected after 72h or 144h, the column "Base metal corrosion 72h" and "Base metal corrosion 144h" of Table 3 is noted.

It was found that the steel components which had from 9 to 23 boards, the Ni contents of 9 to 13 wt .-% in their ZnNi alloy coating, in addition to optimum forming behavior had superior corrosion resistance.

In the steel component formed from the conventionally coated board 41 obtained from Sample Z, good hot working performance was exhibited. However, it did not meet the requirements for preventing the cracking of its steel substrate.

The steel components made from Blanks 37-40 divided from Comparative Samples V1 and V2 showed flaking and insufficient corrosion resistance of the coating. Since this was an exclusion criterion, is at these Steel components no further testing has been carried out.

GDOS (Glow Discharge Optical Emission Spectrometry) measurement is a standard technique for rapidly detecting a concentration profile of coatings. It is for example in the VDI Lexicon Materials, ed. by Hubert Gräfen, VDI-Verlag GmbH, Dusseldorf 1993.

In Fig. 1 the typical result of the GDOS measurement of the corrosion protection coating of a steel component produced and obtained in accordance with the invention is shown. The contents of Mn (short dashed line), O (dotted line), Zn (long dashed line), Fe (dash-dotted line) and Ni (solid line) are plotted over the layer thickness of the coating. It turns out that there is a high concentration of Mn on the surface of the coating, which has diffused from the steel substrate through the coating on its surface where it is oxidized with the ambient oxygen. In the ZnNi alloy layer of the coating, by contrast, the Mn content is significantly lower and only increases again in the steel substrate. This is particularly clear on the basis of Fig. 2 , By contrast, the Ni content of the coating is essentially constant over its entire thickness.

In a further experiment, a recrystallized cold strip is initially electrolytically with a, as in the above-described inventive samples single-phase γ-ZnNi phase ZnNi alloy coating has been coated. The layer thickness of the γ-ZnNi alloy coating was 7 μm at a Ni content of 10%. Subsequently, a 5 μm thick pure zinc Zn layer was also electrolytically applied to this ZnNi alloy coating.

From the thus obtained, provided with a two-layer anti-corrosion coating cold strip boards have been divided, which have been heated to a platinum temperature of 880 ° C within 5 minutes. After hot working and hardening, a corrosion protection layer was present on the resulting steel component. There was also a distinct Mn oxide layer on its surface under which a Zn-rich layer existed, under which was again a ZnNi layer resting on the steel substrate. Table 1 sample Zn [g / l] Ni [g / l] Na2SO4 [g / l] PH value Temp. [° C] cell type Flow velocity [m / s] Current density [A / dm ² ] A 42 126 28 1.6 65 horizontal 0.3 10 B 42 126 28 1.6 65 horizontal 0.3 10 C 42 126 28 1.6 65 horizontal 0.3 10 D 75 70 23 1.4 60 vertical 4 40 e 75 79 23 1.4 60 vertical 4 40 F 75 75 23 1.4 60 vertical 4 40 G 75 85 23 1.4 60 vertical 4 40 H 75 90 25 1.4 63 vertical 4 40 I 75 79 23 1.4 60 horizontal 3.5 40 J 105 75 23 1.4 60 horizontal 4.4 40 K 75 79 23 1.4 60 horizontal 3.5 40 L 42 126 28 1.6 65 vertical 3.5 40 M 42 126 28 1.6 65 vertical 3.5 40 N 62 75 27 1.6 65 horizontal 0.5 20 O 62 75 27 1.6 65 horizontal 0.5 20 P 62 75 27 1.6 65 horizontal 0.5 20 Q 36 144 25 1.5 69 horizontal 0.3 10 V1 75 70 23 1.4 60 vertical 4 40 V2 75 79 23 1.4 60 vertical 4 40 Z Hot-dip coating - Conventional hot dip galvanized sample Mn content in the base material [mass%] coating According to the invention? Thickness of the Ni flash layer [μm] Thickness of ZnNi coating [μm] Ni content of ZnNi coating [mass%] crystallographic structure of the ZnNi coating A 1.3 - 6 14 γ Yes B 1.3 - 8th γ Yes C 1.3 - 10 γ Yes D 1 - 10 9 γ Yes e 2 - 10 12 γ Yes F 1 - 15 11 γ Yes G 1.4 - 8th 12 γ Yes H 1.4 - 7 13 γ Yes I 1.5 - 5 10 η + γ No J 1.5 - 8th 9 η + γ No K 1.5 - 10 11 η + γ No L 1.5 1 8th 14 γ No M 1.25 2 7 γ No N 1.25 - 6 13 γ Yes O 1.25 - 8th γ Yes P 2.2 - 9 γ Yes Q 1.3 - 8th 16 γ No V1 0.1 - 10 9 γ No V2 0.2 - 10 12 γ No Z 1.2 η No sample circuit board coating T-oven [° C] t annealing [min] Warmumformverhalten cracking Base metal corrosion 72h ²⁾ Base metal corrosion 144h ²⁾ According to the invention Thickness [μm] Ni content [% by weight] A 1 6 14 880 5 Good No No Yes Yes B 2 8th 880 4 Good No No Yes Yes B 3 8th 880 5 Good No No Yes Yes C 4 10 880 6 Good No No Yes Yes C 5 10 880 4 Good No No Yes Yes C 6 10 880 5 Good No No Yes Yes C 7 10 860 7 Good No No Yes Yes C 8th 10 860 5 Good No No Yes Yes D 9 10 9 880 5 Good No No No Yes D 10 10 880 8th Good No No No Yes e 11 10 12 880 5 Good No No No Yes e 12 10 860 8th Good No No No Yes F 13 15 10.5 880 5 Good No No No Yes F 14 15 880 5 Good No No No Yes H 15 7 13 880 5 Good No No No Yes N 16 6 860 7 Good No No No Yes N 17 6 880 6 Good No No No Yes O 18 8th 860 10 Good No No No Yes O 19 8th 880 8th Good No No No Yes O 20 8th 900 6 Good No No No Yes P 21 9 860 12 Good No No No Yes P 22 9 880 10 Good No No No Yes P 23 9 900 8th Good No No No Yes L 31 (1) 8 ¹⁾ 14 880 3 Good No Yes Yes No L 32 (1) 8 ¹⁾ 880 4 Good No Yes Yes No L 33 (1) 8 ¹⁾ 880 5 Good No Yes Yes No M 34 (2) 7 ¹⁾ 860 4 Good No Yes Yes No M 35 (2) 7 ¹⁾ 860 5 Good No Yes Yes No Q 36 8th 16 880 7 Good No Yes Yes No V1 37 10 9 860 8th bad No further evaluation due to poor hot forming behavior (local flaking) No V1 38 10 880 5 bad No V2 39 10 12 880 5 bad No V2 40 10 860 8th bad No Z 41 10 - 880 5 Good Yes No No No ¹⁾ Values in () = thickness of Ni flash
²⁾ salt spray test acc. DIN EN ISO 9227

Claims (19)

1. Method of producing a steel component which is provided with a metallic coating which gives protection against corrosion, comprising the following operating steps:

   a) making available of a flat steel product which is produced from a steel material containing 0.3 - 3 wt.-% manganese, which steel material has a yield point of 150 - 1100 MPa and a tensile strength of 300 - 1200 MPa,

   b) coating of the flat steel product with an anti-corrosion coating which comprises a ZnNi alloy coating comprising a single γ-ZnNi phase which is electrolytically deposited on the flat steel product and which contains, as well as zinc and unavoidable impurities, 7 - 15 wt.-% nickel,

   c) heating of a blank formed from the flat steel product to a blank temperature of at least 800°C,

   d) forming of the steel component from the blank in a forming die, and

   e) hardening of the steel component by cooling from a temperature at which the steel component is in a state suitable for the formation of tempered or hardened microstructure, at a cooling rate which is sufficient for the formation of the tempered or hardened microstructure.
2. Method of producing a steel component which is provided with a metallic coating which gives protection against corrosion, comprising the following operating steps:

   a) making available of a flat steel product which is produced from a steel material containing 0.3 - 3 wt.-% manganese, which steel material has a yield point of 150 - 1100 MPa and a tensile strength of 300 - 1200 MPa,

   b) coating of the flat steel product with an anti-corrosion coating which comprises a ZnNi alloy coating comprising a single γ-ZnNi phase which is electrolytically deposited on the flat steel product and which contains, as well as zinc and unavoidable impurities, 7 - 15 wt.-% nickel,

   c) forming of the steel component from a blank formed from the flat steel product in a forming die,

   d) heating of the steel component to a component temperature of at least 800°C,

   e) hardening of the steel component by cooling from a temperature at which the steel component is in a state suitable for the formation of tempered or hardened microstructure, at a cooling rate which is sufficient for the formation of the tempered or hardened microstructure.
3. Method according to Claim 2, characterised in that the forming of the steel component (operating step c)) is performed as pre-forming and in that the steel component is formed to a finished state after the heating (operating step d)).
4. Method according to one of the preceding claims, characterised in that coating of ZnNi alloy on the finished steel component is composed of γ-ZnNi and Γ-ZnFe.
5. Method according to one of the preceding claims, characterised in that, in the case of the finished steel component, an Mn-containing layer in which Mn is present in metallic or oxidic form is present on the anti-corrosion coating.
6. Method according to Claim 5, characterised in that the Mn-containing layer is 0.1 - 5 µm thick.
7. Method according to Claim 5 or 6, characterised in that the Mn content of the Mn-containing layer is 0.1 to 18 wt.-%.
8. Method according to one of the preceding claims, characterised in that, before the forming of the steel component, the anti-corrosion coating comprises an additional layer of Zn which is likewise applied to the coating of ZnNi alloy before the forming of the steel component.
9. Method according to Claim 8, characterised in that the layer of Zn is 2.5 to 12.5 µm thick.
10. Method according to either of claims 8 and 9, characterised in that the anti-corrosion coating of the finished steel component comprises a Zn-rich layer lying on the nickel-containing alloy coating.
11. Method according to one of the preceding claims, characterised in that the forming of the steel component is performed as hot forming and the forming and cooling of the steel component are performed in a single operation in a hot-forming die.
12. Method according to one of claims 1 to 11, characterised in that the forming of the steel component and the hardening are performed in succession to one another in two separate steps.
13. Steel component having a steel substrate comprising a steel containing 0.3 - 3 wt.-% manganese, and having an anti-corrosion coating applied to the steel substrate which comprises an electrolytically deposited coating of ZnNi alloy which lies on the steel substrate, which is composed of γ-ZnNi and Γ-ZnFe, and which has at its free surface an Mn-containing layer in which the Mn is present in metallic or oxidic form.
14. Steel component according to Claim 13, characterised in that the coating of ZnNi alloy is more than 2 µm thick.
15. Steel component according to either of claims 13 and 14, characterised in that the coating of ZnNi alloy contains 1 - 15 wt.-% Ni.
16. Steel component according to one of claims 13 to 15, characterised in that the Mn content of the Mn-containing layer is 1 - 18 wt.-%.
17. Steel component according to one of claims 13 to 16, characterised in that the thickness of the Mn-containing layer is 0.1 - 5 µm.
18. Steel component according to one of claims 13 to 17, characterised in that the anti-corrosion coating comprises a zinc-rich layer lying on the coating of ZnNi alloy.
19. Steel component according to one of claims 13 to 18, characterised in that an organic coating is applied to the Mn-containing layer.

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