EP1658390B1 - Method for producing a hardened steel part - Google Patents

Abstract

Method for production of a hardened profile part from a hardenable steel alloy having cathodic corrosion protection by: (a) application of a Zn coating to a hardenable steel alloy sheet; (b) roll profiling of the coated sheet with shaping into a roll-shaped profile strand; (c) heating of the coated steel sheet to the hardening temperature with admission of atmospheric oxygen, formation of a superficial oxide skin from oxygen-affine elements; and (d) cooling. Independent claims are included for: (1) a corrosion protection Zn layer for a steel sheet; (2) a hardened and profiled component in hardenable steel with a cathodically protected layer obtained as above.

Description

The invention relates to a method for producing a hardened steel component with cathodic corrosion protection, as well as a corrosion protection for steel sheets, as well as components made of steel sheets with the corrosion protection.

Low alloy steel sheets, especially for bodywork, are not resistant to corrosion after being produced by suitable forming steps, either by hot rolling or cold rolling. This means that after a relatively short time and due to the humidity at the surface, oxidation occurs.

It is known to protect steel sheets from corrosion with corresponding anti-corrosion layers. According to DIN-50900, Part 1, corrosion is the reaction of a metallic material with its environment, which causes a measurable change in the material and can lead to a deterioration in the function of a metallic component or an entire system. In order to avoid corrosion damage, steel is usually protected so that it can withstand the corrosion loads during the required service life. The avoidance of corrosion damage can be achieved by influencing the properties of the reactants and / or by changing the reaction conditions, separating the metallic material from the corrosive Medium done by applied protective layers and by electrochemical measures.

According to DIN 50902, a corrosion protection layer is a layer produced on a metal or in the near-surface region of a metal, which consists of one or more layers. Multi-layer coatings are also referred to as corrosion protection systems.

Possible corrosion protection layers are, for example, organic coatings, inorganic coatings and metallic coatings. The purpose of metallic corrosion protection layers is to transfer the properties of the support material to the steel surface for as long as possible. Accordingly, the choice of an effective metallic corrosion protection requires the knowledge of the corrosion-chemical relationships in the system steel / coating metal / attacking medium.

The coating metals can be electrochemically nobler or electrochemically less noble than steel. In the first case, the respective coating metal protects the steel only through the formation of protective layers. One speaks of a so-called barrier protection. As soon as the surface of the coating metal has pores or was injured, a "local element" forms in the presence of moisture, in which the base partner is attacked by the metal to be protected. The more noble coating metals include tin, nickel and copper.

Less precious metals form protective coatings on one side; on the other hand, being less noble than steel, they are additionally attacked by leaks in the layer. In the case of a breach of such a coating layer, the steel is accordingly not attacked, but by the formation of local elements first corrodes the less noble coating metal. One speaks of a so-called galvanic or cathodic corrosion protection. For example, zinc is one of the less noble metals.

Metallic protective layers are applied by various methods. Depending on the metal and process, the connection of the steel surface is chemical, physical or mechanical and ranges from alloy formation and diffusion to adhesion and mere mechanical clamping.

The metallic coatings are said to have similar technological and mechanical properties to steel as they do to steel, and to behave similarly to steel in terms of mechanical stress or plastic deformation. Accordingly, the coatings should not be damaged during forming and should not be affected by forming operations.

When applying hot-dip coatings, the metal to be protected is immersed in molten metal melts. As a result of the hot dip, corresponding alloy layers are formed at the phase boundary steel-coating metal. An example of this is the hot dip galvanizing.

In continuous hot dip galvanizing, the steel strip is passed through a zinc bath, the zinc bath having a temperature of about 450 ° C. The layer thickness - typically 6 - 20 microns - is set by stripping the excess, exhausted with the tape zinc with slot nozzles (air or nitrogen as Abstreifmedium). Hot-dip galvanized products have high corrosion resistance, good weldability and formability, and their main applications are the construction, automotive and household appliance industries.

In addition, the production of a coating of a zinc-iron alloy is known. For this purpose, these products are subjected to a diffusion annealing at temperatures above the zinc melting point, usually between 480 ° C and 550 ° C after hot-dip galvanizing. The zinc-iron alloy layers grow and absorb the overlying zinc layer. This process is called "galvannealing". The zinc-iron alloy thus produced also has a high corrosion resistance, good weldability and formability. Main applications are the automotive and home appliance industry. In addition, other coatings of aluminum, aluminum-silicon, zinc-aluminum and aluminum-zinc-silicon can be produced by hot dipping.

Furthermore, the production of electrodeposited metal coatings is known, i. the electrolytic, so under current passage deposition of metallic coatings of electrolytes.

The electrolytic coating is also possible with such metals, which can not be applied by hot dip process. Conventional layer thicknesses in electrolytic coatings are usually between 2.5 and 10 microns, they are thus generally lower than hot-dip coatings. Some metals, e.g. Zinc, also allow thick film coatings with electrolytic coating. Electrolytically galvanized sheets are mainly used in the automotive industry, because of the high surface quality, these sheets are used above all in the outer skin area. They have good formability, weldability and storability as well as good paintable and matt surfaces.

In particular, in the automotive industry there is an effort to make the body shell always easier. This depends on the one hand with it together that lighter vehicles consume less fuel, on the other hand vehicles are equipped with more and more additional functions and additional aggregates, which bring an increase in weight, which should be compensated by a lighter body shell.

At the same time, however, the safety requirements for motor vehicles are increasing, with the body being responsible for the safety of persons in a motor vehicle and their protection in the event of accidents. Accordingly, there is a requirement for lighter body heights to bring about increased safety in case of accident. This can only be achieved by using materials with increased strength, in particular in the area of the passenger compartment.

In order to achieve the required strengths, it is necessary to use steel grades which have improved properties of a mechanical nature or to treat the steel grades used so that they have the required mechanical properties.

In order to form steel sheets with increased strength, it is known to form steel components in one step and to harden at the same time. This process is also called "press hardening". Here, a steel sheet is heated to a temperature above the Austenitisierungstemperatur, usually above 900 ° C, and then formed in a cold tool. In this case, the tool deforms the hot steel sheet, which cools very rapidly due to the surface contact with the cold mold, so that the hardening effects known per se occur with steel. In addition, it is known to first reshape the steel sheet and then to cool and harden the formed sheet steel component in a sizing press. In contrast to the former method, it is advantageous that The sheet is cold formed and thus more complex shapes are possible. In both methods, however, the sheet is scaled on the surface by the heating, so that after forming and hardening the sheet surface must be cleaned, for example by sandblasting. Then the sheet is trimmed and, if necessary, necessary holes are punched. In this case, it is disadvantageous that the sheets have a very high hardness in the mechanical processing and therefore the processing is complicated and in particular a high tool wear exists.

The US 6,564,604 B2 The object of the invention is to provide steel sheets which are subsequently subjected to a heat treatment, and a method for producing parts by press-hardening these coated steel sheets. In this case, it should be ensured despite the increase in temperature that the steel sheet is not decarburized and the O-surface of the steel sheet is not oxidized before, during and after the hot pressing or heat treatment. For this purpose, an alloyed intermetallic mixture should be applied to the surface before or after punching, which should provide protection against corrosion and decarburization and also can provide a lubricating function. In one embodiment, this document proposes to use a conventional, apparently electrolytically applied zinc layer, wherein this zinc layer is to convert with the steel substrate in a subsequent Austenitisieren the sheet substrate in a homogeneous Zn Fe Fe alloy layer. This homogeneous layer structure is confirmed by microscopic images. Contrary to previous assumptions, this coating is said to have a mechanical resistance that prevents it from melting. In practice, however, such an effect does not show. In addition, the use of zinc or zinc alloys is said to provide cathodic protection of the edges when Cuts are available. In this embodiment, however, is a disadvantage that with such a coating - contrary to the information in this document - but at the edges hardly a cathodic corrosion protection and in the region of the sheet surface, in violations of the layer, only a poor corrosion protection is achieved.

In the second example the US 6,564,604 B2 For example, a coating consisting of 50% to 55% aluminum and 45% to 50% zinc with possibly small amounts of silicon is specified. Such a coating is not new in itself and known under the brand name Galvalume®. It is stated that the coating metals zinc and aluminum with iron should form a homogeneous zinc-aluminum-iron alloy coating. In the case of this coating, it is disadvantageous that sufficient cathodic corrosion protection is no longer achieved here, but the predominant barrier protection which is achieved with this is not sufficient when used in the press hardening process, since partial surface damage to the surface is unavoidable. In summary, the method described in this document is unable to solve the problem that, in general, zinc-based cathodic corrosion coatings are not suitable for protecting steel sheets which are to be subjected to a heat treatment after coating and may also be subjected to a further shaping or forming step.

From the EP 1 013 785 A1 a method for producing a sheet metal component is known, wherein the sheet on the surface should have an aluminum layer or an aluminum alloy layer. A sheet provided with such coatings is to be subjected to a press hardening process, wherein possible coating alloys are mentioned Alloy with 9-10% silicon, 2-3.5% iron, balance aluminum with impurities and a second alloy with 2-4% iron and the balance aluminum with impurities. Such coatings are known per se and correspond to the coating of a hot-dip aluminized steel sheet. In such a coating is disadvantageous in that only a so-called barrier protection is achieved. The moment that such a barrier layer is damaged or cracked in the Fe-Al layer, the base material, in this case the steel, is attacked and corroded. A cathodic protective effect is absent.

Furthermore, it is disadvantageous that even such a hot-dip coated coating during the heating of the steel sheet to the austenitizing temperature and the subsequent press hardening step is so far chemically and mechanically claimed that the finished component has an insufficient corrosion protection layer. As a result, it can be stated that such a hot-dip aluminized layer is suitable for press-hardening complex geometries, i. for heating a steel sheet to a temperature higher than the austenitizing temperature is not well suited.

DE 10039375 A1 discloses a method for producing a corrosion protected steel sheet comprising the steps of: applying to a steel sheet a zinc coating by hot dipping in a zinc 5% aluminum melt, heating, alloying and curing (eg 950 ° C) in an atmosphere, wherein Oxide layer is formed on the surface and hot pressing of the coated steel sheet.

From the DE 102 46 614 A1 For example, a method for producing a coated structural component for vehicle construction is known. This method is intended to solve the problems of the aforementioned European patent application 1 013 785 A1 to solve. In particular, it is stated that the immersion process according to the European patent application 1 013 785 A would form an intermetallic phase already during the coating of the steel, this alloy layer being hard and brittle between the steel and the actual coating and being torn during cold forming. As a result, microcracks would form to a degree that the coating itself separates from the base material and thus loses its protective function. The DE 102 46 614 A1 proposes, therefore, to apply a coating as a metal or a metal alloy by means of a galvanic coating method in organic, non-aqueous solution, a particularly suitable and therefore preferred coating material being aluminum or an aluminum alloy. Alternatively, zinc or zinc alloys would be suitable. Such a coated sheet can then be cold preformed and hot finished molded. In this method, however, has the disadvantage that an aluminum coating, even if it was applied electrolytically, no longer offers corrosion protection in case of damage to the surface of the finished component, since the protective barrier has been broken. In the case of an electrodeposited zinc coating, it is disadvantageous that during heating for hot forming, the zinc is largely oxidized and no longer available for cathodic protection. Under a protective gas atmosphere, the zinc evaporates.

The object of the invention is to provide a method for producing a component from hardened steel sheet with an improved cathodic corrosion protection.

The object is achieved by a method having the features of claim 1.

Advantageous developments are characterized in the subclaims.

Another object is to provide a cathodic corrosion protection for steel sheets, which are subjected to forming and hardening.

The object is achieved with a corrosion protection with the features of claim 26. Advantageous developments are characterized in the dependent claims.

The inventive method provides, on a hardenable steel sheet, a coating of a mixture consisting essentially of zinc and one or more oxygen-affine elements, such as magnesium, silicon, titanium, calcium, aluminum, boron and manganese with a content of 0.1 to 15 Apply wt .-% of the oxygen affinity element and to heat the coated steel sheet at least partially with the access of oxygen to a temperature above the Austenitisierungstemperatur the sheet metal alloy and before or subsequently reshape, the sheet is cooled after sufficient heating and the cooling rate is measured in that hardening of the sheet metal alloy takes place. As a result, a hardened component is obtained from a steel sheet having a good cathodic corrosion protection.

The corrosion protection according to the invention for steel sheets, which are first subjected to a heat treatment and then reformed and thereby hardened, is a cathodic corrosion protection which is essentially based on zinc. According to the invention, 0.1% to 15% of one or more oxygen-containing elements such as magnesium, silicon, titanium, calcium, aluminum, boron and manganese or any mixture or alloy thereof are added to the zinc forming the coating. It has been found that such small amounts of an oxygen affinity element as magnesium, silicon, titanium, calcium, aluminum, boron and manganese cause a surprising effect in this particular application.

According to the invention, at least Mg, Al, Ti, Si, Ca, B, Mn are suitable as oxygen-affine elements. When aluminum is mentioned below, this is representative of the other elements mentioned.

The application of the coating according to the invention on a steel sheet can be done, for example, by so-called hot-dip galvanizing, i. a hot dip coating is performed wherein a liquid mixture of zinc and the oxygen-affine element (s) is applied. Furthermore, it is possible to electrolytically apply the coating, i. to deposit the mixture of zinc and the oxygen-affine element (s) collectively on the sheet surface, or first to deposit a zinc layer and then to deposit on the zinc surface one or more oxygen-affine elements in succession or any mixture or alloy thereof, or by vapor deposition or other suitable method deposit.

It has surprisingly been found that, despite the small amount of an oxygen-affine element, in particular aluminum, an essentially of AL ₂ O ₃ or an oxide of the oxygen-affine element (MgO, CaO, TiO, SiO ₂ , B ₂ O ₃ , MnO) existing, very effective and healing, superficial and opaque protective layer forms. This very thin oxide layer protects the underlying Zn-containing corrosion protection layer from oxidation even at very high temperatures. That is, during the special processing of the galvanized sheet in the press hardening process, an approximately two-layer corrosion protection layer is formed, which consists of a cathodically highly effective layer, with a high proportion of zinc and a very thin oxidation protection layer of one or more oxides (AL ₂ O ₃ , MgO , CaO, TiO, SiO ₂ , B ₂ O ₃ , MnO) to oxidation and Evaporation is protected. This results in a cathodic corrosion protection layer with a superior chemical resistance. This means that the heat treatment has to take place in an oxidizing atmosphere. Although under protective gas (oxygen-free atmosphere) oxidation can be avoided, the zinc would evaporate due to the high vapor pressure.

It has also been found that the corrosion protection layer according to the invention for the press-hardening process also has such a high stability that a forming step following the austenitizing of the sheets does not destroy this layer. Even if microcracks occur on the cured component, however, the cathodic protection effect is at least significantly greater than the protective effect of the known corrosion protection layers for the press-hardening process.

To provide a sheet with the corrosion protection according to the invention, in a first step, a zinc alloy with a content of aluminum in weight percent of greater than 0.1 but less than 15%, in particular less than 10%, more preferably less than 5% on a Steel plate, in particular an alloyed steel sheet are applied, whereupon in a second step, parts of the coated sheet are machined and in particular cut out or punched out and heated on access of atmospheric oxygen to a temperature above the Austenitisierungstemperatur the sheet metal alloy and then cooled at an increased speed. A transformation of the cut out of the sheet metal part (the board) can be carried out before or after the heating of the sheet to the Austenitisierungstemperatur.

It is believed that in the first step of the process, when coating the sheet to the sheet surface or in the proximal region of the layer, a thin barrier phase of, in particular Fe ₂ Al ₅ -x Zn _{x is} formed, which impedes the Fe-Zn diffusion in a liquid metal coating process, which takes place in particular at a temperature up to 690 ° C. Thus, in the first process step, the sheet is formed with a zinc-metal coating with an addition of aluminum, which is effective only towards the sheet surface, as in the proximal region of the support an extremely thin barrier phase, which is effective against rapid growth of an iron-zinc compound phase, having. In addition, it is conceivable that only the presence of aluminum lowers the iron-zinc diffusion tendency in the region of the boundary layer.

If, in the second step, heating of the sheet provided with a zinc-aluminum-metal layer to the austenitizing temperature of the sheet metal material with access of atmospheric oxygen occurs, the metal layer on the sheet is liquefied for the time being. At the distal surface, the oxygen-containing aluminum from the zinc reacts with atmospheric oxygen to form solid oxide, thereby causing a decrease in the aluminum metal concentration, which causes a steady diffusion of aluminum towards depletion, that is to the distal region. This Tonerdeanreicherung, at the air exposed layer area now acts as oxidation protection for the layer metal and as Abdampfungssperre for the zinc.

In addition, during heating, the aluminum is withdrawn from the proximal blocking phase by continuous diffusion towards the distal region and is available there for the formation of the superficial Al ₂ O ₃ layer. Thus, the formation of a sheet metal coating is achieved, which leaves a cathodically highly effective layer with a high zinc content.

Well suited is, for example, a zinc alloy with a content of aluminum in weight percent of greater than 0.2 but less than 4, preferably greater than 0.26 but less than 2.5 wt .-%.

Conveniently, in the first step, the zinc alloy layer is applied to the sheet surface passing through a liquid metal bath at a temperature higher than 425 ° C, but lower than 690 ° C, especially at 440 ° C to 495 ° C, followed by cooling of the coated sheet, not only the proximal barrier phase can be effectively formed, or a very good diffusion inhibition can be observed in the region of the barrier layer, but it also takes place to improve the thermoforming properties of the sheet material.

An advantageous embodiment of the invention is given in a method in which a hot or cold rolled steel strip having a thickness of, for example, greater than 0.15 mm and a concentration range of at least one of the alloying elements within the limits in wt .-% carbon to 0.4, preferably 0.15 to 0.3 silicon until 19, preferably 0.11 to 1.5 manganese to 3.0, preferably 0.8 to 2.5 chrome to 1.5, preferably 0.1 to 0.9 molybdenum to 0, 9, preferably 0.1 to 0.5 nickel to 0, 9, titanium to 0.2 preferably 0.02 to 0.1 vanadium to 0.2 tungsten to 0.2, aluminum to 0.2, preferably 0.02 to 0.07 boron to 0.01, preferably 0.0005 to 0.005 sulfur Max. 0.01, preferably max. 0.008 phosphorus Max. 0.025, preferably max. 0.01 Rest iron and impurities
is used.

It has been found that the surface structure of the cathodic corrosion protection according to the invention is particularly favorable for a high adhesion of paints and varnishes.

The adhesion of the coating to the steel sheet article can be further improved if the surface layer has a zinc-rich intermetallic iron-zinc-aluminum phase and an iron-rich iron-zinc-aluminum phase, the iron-rich phase having a zinc to iron ratio of at most 0, 95 (Zn / Fe ≦ 0.95), preferably from 0.20 to 0.80 (Zn / Fe = 0.20 to 0.80), and the zinc rich phase has a zinc to iron ratio of at least 2.0 (Zn / Fe ≥ 2.0), preferably from 2.3 to 19.0 (Zn / Fe = 2.3 to 19.0).

Figur 1:

eine Aufheizkurve von Untersuchungsblechen beim Glühen in einem Strahlungsofen;

Figur 2:

mikroskopische Aufnahme des Querschliffs einer geglühten Probe eines nicht erfindungsgemäßen feueraluminierten Stahlblechs;

Figur 3:

den Potentialverlauf über die Messzeit bei einer galvanostatischen Auflösung für ein nicht erfindungsgemäßes feueraluminiertes Stahlblech;

Figur 4:

mikroskopische Aufnahme des Querschliffs einer geglühten Probe eines Stahlblechs mit einer nicht erfindungsgemäßen Beschichtung aus einer Aluminium-Zink-Silizium- Legierung;

Figur 5:

den Potentialverlauf über die Messzeit bei einem galvanostatischen Auflösungsversuch eines nicht erfindungsgemäßen Stahlblechs mit einer Beschichtung aus einer Aluminium-Zink-Silizium-Legierung;

Figur 6:

mikroskopische Aufnahme des Querschliffs einer geglühten Probe eines erfindungsgemäßen kathodischen korrosionsgeschützten Blechs;

Figur 7:

den Potentialverlauf für das Blech nach Figur 6;

Figur 8:

die mikroskopische Aufnahme des Querschliffs einer geglühten Probe eines erfindungsgemäßen mit einem kathodischen Korrosionsschutz versehenen Blechs;

Figur 9:

den Potentialverlauf des Blechs nach Figur 8;

Figur 10:

mikroskopische Aufnahmen der Oberfläche eines erfindungsgemäß beschichteten Blechs im ungehärteten - nicht thermisch behandelten - Zustand nach den Figuren 8 und 9 im Vergleich mit einem nicht erfindungsgemäß beschichteten und behandelten Blech;

Figur 11:

mikroskopische Aufnahme des Querschliffs eines nicht erfingdungsgemäß beschichteten und behandelten Blechs;

Figur 12:

den Potentialverlauf des nicht erfindungsgemäßem Blechs nach Figur 11;

Figur 13:

mikroskopische Aufnahme des Querschliffs eines erfindungsgemäß beschichteten und wärmebehandelten Blechs;

Figur 14:

den Potentialverlauf des Blechs nach Figur 13;

Figur 15:

die mikroskopische Aufnahme des Querschliffs eines nicht erfindungsgemäßen elektrolytisch verzinkten Stahlblechs;

Figur 16:

den Potentialverlauf des Blechs nach Figur 15;

Figur 17:

die mikroskopische Aufnahme des Querschliffs einer geglühten Probe eines nicht erfindungsgemäßen Blechs mit einer Zink-Nickel-Beschichtung;

Figur 18:

den Potentialverlauf des nicht erfindungsgemäßen Blechs nach Figur 17;

Figur 19:

ein Vergleich der zum Auflösen erforderlichen Potentiale als Funktion der Zeit für die geprüften Materialien;

Figur 20:

ein Diagramm zeigend die zur Beurteilung des Korrosionsschutzes herangezogenen Fläche;

Figur 21:

ein Diagramm zeigend die unterschiedlichen Schutzenergien der geprüften Materialien;

Figur 22:

ein Diagramm zeigend die unterschiedlichen Schutzenergien eines erfindungsgemäßem Blechs bei zwei verschiedenen Aufheizbedingungen;

Figur 23:

qualitativ die Phasenausbildung als "Leopardenmuster" bei erfindungsgemäßen Beschichtungen;

Figur 24:

ein Diagramm zeigend mögliche erfindungsgemäße Verfahrensabläufe;

Figur 25:

ein Diagramm zeigend die Verteilung der Elemente Aluminium Zink und Eisen abhängig von der Tiefe der Oberflächenschicht vor dem Glühen des Blechs;

Figur 26:

ein Diagramm zeigend die Verteilung der Elemente Aluminium, Zink und Eisen abhängig von der Tiefe der O-berflächenschicht nach dem Glühen des Blechs als Nachweis der Bildung einer oberflächlichen Schutzhaut aus Aluminiumoxid.

The invention will be explained in more detail by way of examples, reference being made to drawings. It shows:

FIG. 1:

a heating curve of test sheets during annealing in a radiant furnace;

FIG. 2:

Microscopic image of the cross section of a calcined sample of a non-inventive hot-dip aluminized steel sheet;

FIG. 3:

the potential curve over the measuring time at a galvanostatic resolution for a non-inventive hot-dip aluminized sheet steel;

FIG. 4:

Microscopic image of the cross section of an annealed sample of a steel sheet with a non-inventive coating of an aluminum-zinc-silicon alloy;

FIG. 5:

the potential curve over the measuring time in a galvanostatic dissolution test of a non-inventive steel sheet with a coating of an aluminum-zinc-silicon alloy;

FIG. 6:

Microscopic image of the cross section of a calcined sample of a cathodic corrosion-protected sheet according to the invention;

FIG. 7:

the potential curve for the sheet after FIG. 6 ;

FIG. 8:

the micrograph of the cross section of a annealed sample of a cathodic corrosion protection sheet according to the invention;

FIG. 9:

the potential course of the sheet after FIG. 8 ;

FIG. 10:

Microscopic images of the surface of a coated sheet according to the invention in the uncured - not thermally treated - state after FIGS. 8 and 9 in comparison with a sheet not coated and treated according to the invention;

FIG. 11:

micrograph of the cross section of a sheet not coated and treated according to the invention;

FIG. 12:

the potential curve of the non-inventive sheet after FIG. 11 ;

FIG. 13:

Microscopic image of the cross section of a coated and heat treated sheet according to the invention;

FIG. 14:

the potential course of the sheet after FIG. 13 ;

FIG. 15:

the micrograph of the cross section of a non-inventive electrolytically galvanized steel sheet;

FIG. 16:

the potential course of the sheet after FIG. 15 ;

FIG. 17:

the micrograph of the cross section of a annealed sample of a non-inventive sheet with a zinc-nickel coating;

FIG. 18:

the potential curve of the sheet not according to the invention FIG. 17 ;

FIG. 19:

a comparison of the potentials required for dissolution as a function of time for the materials tested;

FIG. 20:

a diagram showing the area used for the assessment of corrosion protection;

FIG. 21:

a diagram showing the different protective energies of the tested materials;

FIG. 22:

a diagram showing the different protective energies of a sheet according to the invention at two different heating conditions;

FIG. 23:

qualitatively the phase formation as a "leopard pattern" in coatings according to the invention;

FIG. 24:

a diagram showing possible inventive procedures;

FIG. 25:

a diagram showing the distribution of the elements aluminum zinc and iron depending on the depth of the surface layer before the annealing of the sheet;

FIG. 26:

a diagram showing the distribution of the elements aluminum, zinc and iron depending on the depth of the O-surface layer after annealing of the sheet as evidence of the formation of a superficial protective skin of alumina.

It is about 1 mm thick steel sheets produced with a double-sided same corrosion protection layer of 15 microns layer thickness and examined. The panels are placed in a 900 ° C blast furnace for 4 minutes and 30 seconds and subsequently rapidly cooled between steel panels. The time between the furnace removal of the sheets and the cooling between the steel plates was 5 seconds. The heating curve of the sheets during annealing in the radiation furnace has approximately the course FIG. 1 ,

Subsequently, the obtained samples were analyzed for optical and electrochemical differences. Assessment criteria here were the appearance of the annealed steel sheets and the protection energy. The protection energy is the measure for the electrochemical protection of the layer, determined by galvanostatic dissolution.

The electrochemical method of galvanostatic dissolution of the metallic surface coatings of a material allows to classify the mechanism of corrosion protection of the layer. The potential-time behavior of a corrosion-protective layer is determined for a given constant current flow. For the measurements, a current density of 12.7 mA / cm ^{2 was} specified. The measuring arrangement is a three-electrode system. The counterelectrode used was a platinum network, the reference electrode consisting of Ag / AgCl (3M). The electrolyte consists of 100 g / l ZnSO ₄ .5H ₂ O and 200 g / l NaCl dissolved in deionized water.

If the potential required for dissolving the layer is greater than or equal to the steel potential, which can be easily determined by pickling or abrading the surface coating, this is called pure barrier protection without active cathodic corrosion protection. The barrier protection is characterized by the fact that it separates the base material from the corrosive medium.

The results of the coating examples will be described below.

Example 1 (not according to the invention)

A hot-dip aluminized steel sheet is made by passing a steel sheet through a liquid aluminum bath. Annealing to 900 ° C produces an aluminum-iron surface layer due to the reaction of the steel with the aluminum coating. The corresponding annealed sheet shows a dark gray appearance, the surface is homogeneous and visually shows no defects.

In the galvanostatic dissolution of the surface coating of the hot-dip aluminized sheet, a very high potential (+ 2.8 V) must be applied at the beginning of the measurement, in order to ensure the flow-circuit of 12.7 mA / cm ^2. After a short measuring time, the required potential drops to the steel potential. From this behavior, it can be seen that an annealed sheet with a layer obtained by fire aluminizing provides very efficient barrier protection. However, as soon as holes are formed in the coating, the potential drops to steel potential and it comes to the removal of the base material. Since the potential required for the resolution is never lower than the steel potential, there is a pure barrier protection without cathodic corrosion protection effect. The potential curve over the measuring time is in FIG. 3 shown a micrograph of a cross section in FIG. 2 ,

Example 2 (not according to the invention)

A steel sheet was hot dip galvanized with an aluminum-zinc layer, the melt consisting of 55% aluminum, 44% zinc and about 1% silicon. After surface coating and subsequent annealing at 900 ° C, a gray-bluish surface appears without defects. A cross section is in FIG. 4 shown.

The annealed material is then subjected to galvanostatic dissolution. At the beginning of the measurement, the material shows a potential of about -0.92 V, which is necessary for the resolution, and is thus clearly below the steel potential. This value is comparable to the potential needed to dissolve a hot dip galvanized coating prior to the annealing process. However, this very zinc-rich phase ends after just about 350 seconds of measurement time. This is followed by a rapid increase to a potential that is now just below the steel potential lies. After breaking through this layer, the potential first drops to a value of about -0.54 V and then increases continuously to a value of about -0.35 V. Only then does it slowly sink to steel potential. This material shows some cathodic corrosion protection due to the very negative potential at the beginning of the measurement, which is well below the steel potential, in addition to the barrier protection. However, the part of the layer that provides cathodic protection against corrosion is used up after only about 350 seconds of measurement time. The remaining layer can only offer a low cathodic corrosion protection, since the difference between the required potential for the layer dissolution and the steel potential now only less than 0.12 V. In a poorly conductive electrolyte, this part of the cathodic corrosion protection is no longer usable. The potential-time diagram is in FIG. 5 shown.

Example 3 (according to the invention)

A steel sheet is hot-dip galvanized with a melt consisting essentially of 95% zinc and 5% aluminum. After annealing, the sheet shows a silvery-gray surface with no defects. In cross section ( FIG. 6 ) shows that the coating consists of a light phase and a dark phase, wherein the phases are Zn-Fe-Al-containing phases. The bright phases are more zinc-rich, the dark phases more iron-rich. Some of the aluminum reacted with atmospheric oxygen during the calcination and formed a protective Al ₂ O ₃ skin.

At the beginning of the measurement, the galvanostatic dissolution shows a potential of about -0.7 V required for the resolution. This value is significantly below the potential of the steel. After a measuring time of approx. 1,000 seconds it turns a potential of about -0.6V. This potential is also clearly below the steel potential. After a measurement time of approximately 3,500 seconds, this part of the layer is used up and the necessary potential for dissolving the layer approaches the steel potential. This coating thus offers after the annealing in addition to the barrier protection a cathodic corrosion protection. The potential is up to a measuring time of 3,500 seconds at a value of ≤ -0.6 V, so that a considerable cathodic protection is maintained over a long time, even if the sheet was fed to the austenitizing temperature. The potential-time diagram is in FIG. 7 shown.

Example 4 (according to the invention)

The sheet is passed through a melt or through a zinc bath, with a zinc content of 99.8% and an aluminum content of 0.2%. Aluminum present in the zinc coating reacts with atmospheric oxygen during the calcination and forms a protective Al ₂ O ₃ skin. Through constant diffusion of the oxygen-affinity aluminum to the surface, this protective skin is maintained and expanded. After annealing the sheet shows a silvery-gray surface without defects. From the originally about 15 microns thick zinc coating develops during the annealing due to diffusion, a about 20 to 25 microns thick layer, said layer ( FIG. 8 ) consists of a dark appearing phase with a composition Zn / Fe of about 30/70 and a bright area with the composition Zn / Fe of about 80/20. On the surface of the coating, an increased aluminum content is detectable. Due to the detection of oxides at the surface, it can be concluded that a thin Al ₂ O ₃ protective layer is present. At the beginning of the galvanostatic dissolution, the annealed material has a potential of approx. -0.75 V. After a measuring time of approx. 1,500 seconds, the potential required for the resolution increases to ≤ -0.6 V. The phase lasts up to a measuring time of approx. 2,800 seconds. Then the required potential increases to steel potential. In this case too, in addition to barrier protection, there is cathodic corrosion protection. The potential is up to a measurement time of 2,800 seconds at a value of ≤ -0.6 V. Thus, such a material has thus over a very long time a cathodic protection against corrosion. The potential-time diagram is FIG. 9 refer to.

Example 5 (not according to the invention)

The sheet is heated to a temperature of about 500 ° C after exiting the metal strip from the molten zinc (about 450 ° C strip temperature). Here, the zinc layer is completely converted into Zn-Fe phases. The zinc layer is thus wholly, i. converted to Zn-Fe phases to the surface. This results in zinc-rich phases on the steel sheet, all of which are formed with a Zn-Fe ratio of> 70% zinc. This anticorrosive layer contains some aluminum in the zinc bath, of the order of about 0.13%.

A 1 mm thick steel sheet with said heat treated and fully converted coating is heated for 4 minutes and 30 seconds in a 900 ° C oven. The result is a yellow-green surface.

The yellow-green surface indicates oxidation of the Zn-Fe phases during annealing. An aluminum oxide protective layer is undetectable. The reason for the absence of an aluminum oxide protective layer can be explained by the fact that in the Annealing treatment due to solid Zn-Fe phases, aluminum can not migrate to the surface so rapidly and protect the Zn-Fe coating from oxidation. When heating this material at temperatures around 500 ° C is still no liquid zinc-rich phase, because this forms only at higher temperatures of 782 ° C. If 782 ° C are reached, thermodynamically there is a liquid zinc-rich phase in which the aluminum is freely available. Nevertheless, the surface layer is not protected against oxidation.

Possibly at this time, the corrosion protection layer is already partially oxidized before and it can no longer form opaque alumina skin. The layer is wavy rugged in cross section and consists of Zn and Zn Fe oxides ( FIG. 11 ). In addition, the surface of the said material is much larger due to the highly crystalline acicular surface formation of the surface, which could also be disadvantageous for the formation of a covering and thicker aluminum oxide protective layer. The said non-inventive coating forms in the initial state, ie not in the thermally treated state, a brittle layer which is provided with numerous cracks, both transversely and longitudinally to the coating. ( FIG. 10 in comparison to the aforementioned inventive example (left in the picture)). As a result, in the course of the heating, both decarburization and oxidation of the steel substrates can take place, especially with cold preformed components.

In the galvanostatic dissolution of this material, a potential of approx. + 1V is applied for the resolution under constant current flow at the beginning of the measurement, which then settles to a value of approx. + 0.7V. Here, too, the potential lies well above the steel potential during the entire dissolution ( FIG. 12 ). Consequently, in these annealing conditions also be spoken of a pure barrier protection. Also in this case no cathodic corrosion protection could be determined.

Example 6 (according to the invention)

A sheet, as in the aforementioned example, is heat-treated immediately after hot-dip galvanizing at about 490 ° C to 550 ° C with the zinc layer only partially converted to Zn-Fe phases. The process is carried out in such a way that the phase transformation is only partially carried out and therefore not yet converted zinc with aluminum on the surface is present and thus free aluminum as oxidation protection for the zinc layer is available.

A 1 mm thick steel sheet is rapidly inductively heated to 900 ° C with the inventive heat-treated and only partially converted into Zn-Fe phase coating. The result is a surface that is gray and without defects. A SEM / EDX examination of the cross section ( FIG. 13 ) shows an approximately 20 microns thick surface layer, wherein from the originally about 15 microns thick zinc coating of the coating has formed in the inductive annealing due to diffusion, an about 20 microns Zn-Fe layer, said layer with the typical for the invention two-phase structure a "leopard pattern" shows, with a dark phase in the image with a composition Zn / Fe of about 30/70 and bright areas with the composition Zn / Fe of about 80/20. In addition, individual areas with zinc contents ≥ 90% zinc are present. On the surface a protective layer of alumina is detectable.

In the galvanostatic detachment of the surface coating of a rapidly heated metal sheet with the hot-dip galvanized layer according to the invention and in contrast to Example 5 only incompletely before the press hardening is located at the beginning of the measurement necessary for the resolution potential at about -0.94 V and is comparable with the potential necessary for the dissolution of an unannealed zinc coating. After a measuring time of approx. 500 seconds, the potential rises to a value of -0.79 V, far below the steel potential. After approx. 2,200 seconds measuring time, ≤ -0.6 V are necessary for the detachment, with the potential subsequently rising to -0.38 V and then approaching the steel potential ( Figure. 14 ). In the case of the material according to the invention which is heated up quickly and incompletely heat-treated prior to press-hardening, it is therefore possible to form both a barrier protection and a very good cathodic corrosion protection. Even with this material, the cathodic protection against corrosion can be maintained over a very long measuring time.

Example 7 (not according to the invention)

A sheet is electrolytically galvanized by electrochemical deposition of zinc on steel. During annealing, the diffusion of the steel and the zinc layer creates a thin Zn-Fe layer. Most of the zinc oxidizes to zinc oxide, which appears green by the simultaneous formation of iron oxides. The surface shows a green appearance with local scale marks where the zinc oxide layer does not adhere to the steel.

A REM / EDX examination ( FIG. 15 ) of the sample sheet in transverse section confirms that a large part of the coating consists of zinc-iron-oxide deposits. With galvanostatic dissolution, the potential required for the current flow is included about +1 V and thus well above the steel potential. In the course of the measurement, the potential fluctuates between +0.8 and -0.1 V, but is above the steel potential throughout the entire dissolution of the coating. It follows that the corrosion protection of a annealed, electrolytically galvanized sheet is a pure barrier protection, but which is less efficient than with fumed sheet, since the potential is lower at the beginning of the measurement with electrolytically coated sheet than with hot-dip aluminized sheet. The potential required for the dissolution lies above the steel potential throughout the entire dissolution. Thus, even with a annealed, electrolytically coated metal sheet there is no cathodic corrosion protection at any time. The potential-time diagram is FIG. 16 refer to. The potential is fundamentally above steel potential, but varies in detail depending on the experiment under identical experimental conditions.

Example 8 (not according to the invention)

A sheet is made by electroplating zinc and nickel on the steel surface. The weight ratio of zinc to nickel in the anticorrosion layer is about 90/10. The deposited layer thickness is 5 μm.

The sheet is annealed with the coating for 4 minutes and 30 seconds at 900 ° C in the presence of atmospheric oxygen. During annealing, the diffusion of the steel and the zinc layer creates a thin diffusion layer of zinc, nickel and iron. However, due to the absence of aluminum, most of the zinc oxidizes again to zinc oxide. The surface shows a scaled, green appearance with small local flaking to which the oxide layer does not adhere to the steel.

A SEM / EDX examination of a cross section ( FIG. 17 ) shows that the majority of the coating has been oxidized and is therefore not available for cathodic corrosion protection.

At the beginning of the measurement, the potential required for the resolution of the layer is 1.5 V, far above the steel potential. After approx. 250 seconds it sinks to approx. 0.04 V and oscillates between + 0.25 V. After approx. 1.700 seconds measuring time, it finally settles to a value of - 0.27 V and remains until the end of the Measurement at this value. The potential required for the resolution of the layer is well above the steel potential throughout the entire measurement time. Consequently, this coating has a pure barrier protection after annealing, without any cathodic corrosion protection (Figure 18).

9. Detection of the aluminum oxide layer by GDOES analysis

Using a GDOES (Glow Discharge Optical Emission Spectroscopy) study, the formation of the aluminum oxide layer during annealing (and the migration of the aluminum to the surface) can be detected.

For GDOES measurement:

A 1 mm thick, coated according to Example 4 steel sheet with a layer thickness of 15 microns was placed for 4 min 30 s in a 900 ° C hot air blast furnace, then rapidly cooled between two 5 cm thick steel plates and the surface with a GDOES measurement analyzed.

In FIGS. 25 and 26 the GDOES analyzes of the coated sheet according to Example 4 are shown before and after the annealing. Before hardening ( Fig. 25 ) is reached after about 15 microns, the transition zinc layer steel, after curing, the layer is about 23 microns thick.

After hardening ( Fig. 26 ) shows the increased aluminum content at the surface compared to the unannealed sheet.

10. Summary

The examples show that only the corrosion-protected sheets used according to the invention for the press-hardening process still provide cathodic corrosion protection even after annealing, in particular with a cathodic corrosion protection energy> 4 J / cm ² . The potentials required for dissolution as a function of time are in FIG. 19 compared to each other.

For the evaluation of the quality of the cathodic corrosion protection, not only the time during which the cathodic corrosion protection can be maintained must be considered, but also the difference between the potential required for the dissolution and the steel potential must be considered. The larger this difference, the more effective is the cathodic protection against corrosion even with poorly conducting electrolytes. The cathodic corrosion protection is negligible with a voltage difference of 100 mV to the steel potential in poorly conducting electrolytes. Although a smaller difference to the steel potential is in principle still a cathodic corrosion protection, if a current flow is detected when using a steel electrode, but this is negligible for practical aspects, since the corrosive medium must conduct very well, so this contribution to the cathodic corrosion protection can be used. This is practically not the case under atmospheric conditions (rainwater, humidity, etc.). Therefore, the difference between the potential required for the dissolution and the steel potential was not used for the evaluation, but a threshold value of 100 mV below the steel potential was used. Only the difference up to this threshold was taken into account for the evaluation of the cathodic protection.

As an evaluation criterion for the cathodic protection of the respective surface coating after annealing, the area between the potential curve at the galvanostatic dissolution and the specified threshold value of 100 mV was set below the steel potential ( FIG. 20 ). Only the area below the threshold is taken into account. The overlying surface contributes negligibly little or not at all to the cathodic corrosion protection and is therefore not included in the evaluation.

The area thus obtained is multiplied by the current density, the protection energy per unit area with which the base material can be actively protected against corrosion. The greater this energy, the better the cathodic corrosion protection. In FIG. 21 the calculated protective energies per unit area are compared. While a sheet with the known aluminum-zinc layer of 55% aluminum and 44% zinc, as it is also known from the prior art, only a protection energy per unit area of about 1.8 J / cm ² , which is Protection energy per unit area in accordance with the invention coated sheets 5.6 J / cm ² and 5.9 J / cm ² .

As cathodic corrosion protection in the context of the invention, it is subsequently specified that coatings of 15 μm thickness are used and the illustrated process and experimental conditions at least a cathodic corrosion protection energy of 4 J / cm ^{2 is} present.

A zinc layer which has been deposited electrolytically on the steel sheet surface is not in itself capable of providing a corrosion protection according to the invention, even after a heating step above the austenitizing temperature. According to the invention, however, the invention can also be achieved with an electrodeposited coating. For this purpose, the zinc can be deposited simultaneously with the oxygen-affine elements or elements in an electrolysis step on the sheet surface simultaneously, so that on the sheet surface, a coating with a homogeneous structure is formed containing both zinc and the oxygen-affine or the elements. When heated to the austenitizing temperature, such a coating behaves like a coating of the same composition applied to the sheet surface in the hot-dip galvanizing process.

In a further advantageous embodiment, in a first electrolysis step only zinc is deposited on the sheet surface and in a second electrolysis step, the oxygen-affine element (s) is deposited on the zinc layer. The second coating of the oxygen-affine elements may be significantly thinner than the zinc coating. When such a coating according to the invention is heated, the outer layer located on the zinc layer oxidizes from the oxygen-affine element (s) and protects the underlying zinc with an oxide skin. Of course, the oxygen affinity element or elements are selected so that they do not evaporate from the zinc layer or are oxidized in a manner that does not leave a protective oxide skin. In a further advantageous embodiment, first a zinc layer is deposited electrolytically and then a layer of the oxygen-affine element (s) is applied by vapor deposition or other suitable non-electrolytic coating methods.

It is typical of the coatings according to the invention that in addition to the superficial protective layer of an oxide of the oxygen-affine element (s) used, in particular Al ₂ O ₃ after the heat treatment for press-hardening, the layers according to the invention exhibit a typical "leopard pattern" consisting of a zinc-rich, Fe-Zn-Al intermetallic phase and an iron-rich Fe-Zn-Al phase, wherein the iron-rich phase has a zinc to iron ratio of at most 0.95 (Zn / Fe≤0.95), preferably from 0.20 to 0.50 (Zn / Fe = 0.20 to 0.80) and the zinc-rich phase has a zinc to iron ratio of at least 2.0 (Zn / Fe> 2.0), preferably from 2.3 to 19.0 ( Zn / Fe = 2.3 to 19.0). It has been found that only when such a biphasic construction is achieved is sufficient cathodic protection still present. Such a biphasic structure, however, only arises if the formation of an Al ₂ O ₃ protective layer on the surface of the coating has previously taken place. In contrast to a known coating according to the US 6,564,604 B2 , which has a homogeneous structure in terms of structure and texture, where Zn Fe needles are to be present in a zinc matrix, an inhomogeneous structure of at least two different phases is achieved here.

In the invention it is advantageous that a continuously and thus economically produced steel sheet for producing press-hardened components is created, which has a cathodic protection against corrosion, which is also reliable in the Heating the sheet over the Austenitisierungstemperatur and the subsequent forming is maintained.

Claims (39)

1. A method for manufacturing a hardened steel component with cathodic corrosion protection, wherein:

   a) a coating is applied in a continuous coating process onto a sheet composed of a hardenable steel alloy;

   b) the coating is essentially composed of zinc, and

   c) the coating also contains one or more high oxygen affinity elements in a total quantity of 0.1 wt.% to 15 wt.% in relation to the overall coating, and

   d) then the coated steel sheet, at least in some areas, is brought, through the entry of atmospheric oxygen, to an austenitization temperature required for the hardening and heated until a structural change required for the hardening occurs;

   e) on the coating, a surface skin composed of an oxide of the high oxygen affinity element or elements is formed, and

   f) the sheet metal is formed before or after the heating;

   g) the sheet metal is cooled after the sufficient heating; the cooling rate is dimensioned so that a hardening of the sheet metal alloy is achieved;

   h) magnesium and/or silicon and/or titanium and/or calcium and/or aluminum and/or manganese and/or boron are used as the high oxygen affinity elements in the mixture; and

   i) the coating mixture is selected so that during the heating, the surface of the layer develops an oxide skin composed of oxides of the high oxygen affinity element or elements and the coating comprises at least two phases; and a zinc-rich phase and an iron-rich phase are formed.
2. The method according to claim 1, characterized in that the coating is applied by means of the hot dip coating process, using a mixture composed essentially of zinc with the high oxygen affinity element or elements.
3. The method according to claim 1 or 2, characterized in that 0.2 wt.% to 5 wt.% of the high oxygen affinity elements is used.
4. The method according to one of the preceding claims, characterized in that 0.26 wt.% to 2.5 wt.% of the high oxygen affinity elements is used.
5. The method according to one of the preceding claims, characterized in that the iron-rich phase has a zinc to iron ratio of at most 0.95 (Zn/Fe ≤ 0.95), preferably from 0.20 to 0.80 (Zn/Fe = 0.20 to 0.80), and the zinc-rich phase has a zinc to iron ratio of at least 2.0 (Zn/Fe ≥ 2.0), preferably from 2.3 to 19.0 (Zn/Fe = 2.3 to 19.0).
6. The method according to one of the preceding claims, characterized in that the iron-rich phase has a zinc to iron ratio of approximately 30:70 and the zinc-rich phase is embodied with a zinc to iron ratio of approximately 80:20.
7. The method according to one of the preceding claims, characterized in that the layer also has individual regions with zinc percentages of > 90% zinc.
8. The method according to one of the preceding claims, characterized in that the coating is embodied so that with a starting thickness of 15 µm after the hardening process, it produces a cathodic protective action of at least 4 J/cm².
9. The method according to one of the preceding claims, characterized in that the coating with the mixture of zinc and the high oxygen affinity element or elements is produced in a pass through a liquid metal bath at a temperature of 425°C to 690°C with a subsequent cooling of the coated sheet.
10. The method according to one of the preceding claims, characterized in that the coating with the mixture of zinc and the high oxygen affinity element or elements is produced in a pass through a liquid metal bath at a temperature of 440°C to 495°C with a subsequent cooling of the coated sheet.
11. The method according to one of the preceding claims, characterized in that the sheet is heated inductively.
12. The method according to one of the preceding claims, characterized in that the sheet is heated inductively in the tool.
13. The method according to one of the preceding claims, characterized in that the sheet is heated in the radiation furnace.
14. The method according to one of the preceding claims, characterized in that the cooling takes place in the forming tool.
15. The method according to one of the preceding claims, characterized in that the cooling is carried out during the forming procedure, by means of cooled forming tools.
16. The method according to one of the preceding claims, characterized in that the cooling takes place after the forming procedure in the forming tool.
17. The method according to one of the preceding claims, characterized in that the cooling takes place in a form-hardening tool into which the formed sheet is inserted after the heating process and in which a form-fitting engagement is produced between the formed sheet and correspondingly formed, cooled form-hardening tools.
18. The method according to one of the preceding claims, characterized in that the heating and cooling take place in the form-hardening tool; the heating is carried out inductively and after the inductive heating, the form is cooled.
19. The method according to one of the preceding claims, characterized in that the forming and hardening of the component takes place with a roll-forming device; the coating sheet is at least partially heated to the austenitization temperature, is roll-formed before, during, and/or after this, and after the roll-forming, is cooled at a cooling rate that causes a hardening of the sheet metal alloy.
20. A method for manufacturing a hardened steel component with cathodic corrosion protection, wherein:

    a) a coating is applied in a continuous coating process onto a sheet composed of a hardenable steel alloy;

    b) the coating is essentially composed of zinc, and

    c) the coating also contains one or more high oxygen affinity elements in a total quantity of 0.1 wt.% to 15 wt.% in relation to the overall coating, and

    d) then the coated steel sheet, at least in some areas, is brought, through the entry of atmospheric oxygen, to an austenitization temperature required for the hardening and heated until a structural change required for the hardening occurs;

    e) on the coating, a surface skin composed of an oxide of the high oxygen affinity element or elements is formed, and

    f) the sheet metal is formed before or after the heating;

    g) the sheet metal is cooled after the sufficient heating; the cooling rate is dimensioned so that a hardening of the sheet metal alloy is achieved;

    h) magnesium and/or silicon and/or titanium and/or calcium and/or aluminum and/or manganese and/or boron are used as the high oxygen affinity elements in the mixture; and

    i) the coating mixture is selected so that during the heating, the surface of the layer develops an oxide skin composed of oxides of the high oxygen affinity element or elements and the coating comprises at least two phases; a zinc-rich phase and an iron-rich phase are formed; and

    j) the coating is electrolytically applied.
21. The method according to claim 20, characterized in that in the electrolytic coating process, first a zinc layer is deposited and in a subsequent second step, the high oxygen affinity element or elements is/are deposited onto the deposited zinc layer.
22. The method according to claim 20 or 21, characterized in that first, a zinc layer is electrolytically deposited onto the surface of the sheet and then a coating composed of the high oxygen affinity element or elements is deposited onto the zinc surface.
23. The method according to one of claims 20 through 22, characterized in that the high oxygen affinity element or elements are vapor deposited or are deposited with other suitable methods.
24. The method according to one of claims 20 through 23, characterized in that 0.26 wt.% to 2.5 wt.% of the high oxygen affinity elements is used.
25. A corrosion protection layer on steel plates that have undergone a hardening step, wherein after being deposited onto the steel sheet, the corrosion protection layer is subjected to a heat treatment with the entry of oxygen; the coating is essentially composed of zinc and also one or more high oxygen affinity elements in a total quantity of 0.1 wt.% to 15.0 wt.% in relation to the overall coating; the surface of the corrosion protection layer has an oxide skin composed of oxides of the high oxygen affinity element or elements and the coating comprises at least two phases; a zinc-rich phase and an iron-rich phase are formed.
26. A corrosion protection layer on steel plates that have undergone a hardening step, wherein after being deposited onto the steel sheet, the corrosion protection layer is subjected to a heat treatment with the entry of oxygen; the coating is essentially composed of zinc and also one or more high oxygen affinity elements in a total quantity of 0.1 wt.% to 15.0 wt.% in relation to the overall coating; the surface of the corrosion protection layer has an oxide skin composed of oxides of the high oxygen affinity element or elements and the coating comprises at least two phases; a zinc-rich phase and an iron-rich phase are formed; the corrosion protection layer is a corrosion protection layer deposited by means of an electrolytic depositing method; the corrosion protection layer has been produced through electrolytic depositing of essentially zinc and at the same time, one or more high oxygen affinity elements; or the corrosion protection layer has been produced through first, the electrolytic depositing of essentially zinc and the subsequent vapor deposition or application using other suitable means, of one or more high oxygen affinity elements.
27. The corrosion protection layer according to claim 25 or 26, characterized in that the corrosion protection layer contains magnesium and/or silicon and/or titanium and/or calcium and/or aluminum and/or boron and/or manganese as high oxygen affinity elements in the mixture.
28. The corrosion protection layer according to claim 25 or 27, characterized in that the corrosion protection layer is a corrosion protection layer that is deposited by means of a hot dip coating process.
29. The corrosion protection layer according to one of claims 25 through 28, characterized in that the coating is composed of a mixture of essentially zinc and the mixture also includes one or more high oxygen affinity elements.
30. The corrosion protection layer according to one of claims 25 through 29, characterized in that it contains the high oxygen affinity elements in a total quantity of 0.1 wt.% to 15.0 wt.% in relation to the overall coating.
31. The corrosion protection layer according to one of claims 25 through 30, characterized in that it contains these high oxygen affinity elements in a total quantity of 0.02 wt.% to 0.5 wt.% in relation to the overall coating.
32. The corrosion protection layer according to one of claims 25 through 31, characterized in that it contains the high oxygen affinity elements in a total quantity of 0.6 wt.% to 2.5 wt.% in relation to the overall coating.
33. The corrosion protection layer according to one of claims 25 through 32, characterized in that essentially aluminum is contained as a high oxygen affinity element.
34. The corrosion protection layer according to one of claims 25 through 33, characterized in that the iron-rich phase has a zinc to iron ratio of at most 0.95 (Zn/Fe ≤ 0.95), preferably from 0.20 to 0.80 (Zn/Fe = 0.20 to 0.80), and the zinc-rich phase has a zinc to iron ratio of at least 2.0 (Zn/Fe ≥ 2.0), preferably from 2.3 to 19.0 (Zn/Fe = 2.3 to 19.0).
35. The corrosion protection layer according to one of claims 25 through 34, characterized in that the iron-rich phase has a zinc to iron ratio of approximately 30:70 and the zinc-rich phase is embodied with a zinc to iron ratio of approximately 80:20.
36. The corrosion protection layer according to one of claims 25 through 35, characterized in that the corrosion protection layer also has individual regions with zinc percentages of ≥ 90 wt.% zinc.
37. The corrosion protection layer according to one of claims 25 through 36, characterized in that the corrosion protection layer, with a starting thickness of 15 µm, has a cathodic protective energy of at least 4 J/cm².
38. A hardened steel component with cathodic corrosion protection, composed of a hotrolled or cold-rolled steel band with a thickness of ≥ 0.15 mm, wherein the hardness is achieved by heating it to an austenitization temperature required for the hardening and until a structural change required for the hardening occurs and a cooling that is carried out after the sufficient heating; the cooling rate is dimensioned so that a hardening of the sheet metal alloy is achieved; a coating essentially composed of zinc is present on the surface; the coating contains one or more high oxygen affinity elements in a total quantity of 0.1 wt.% to 15 wt.%; the cooling has been achieved during the forming procedure, by means of cooled forming tools; in particular, a hardened steel component that has been manufactured with a method according to one of claims 1 through 24 and with a corrosion protection layer according to one of claims 25 through 37.
39. The hardened steel component according to claim 38, characterized in that the component is embodied with a concentration range of at least one of the alloy elements within the following limits, expressed in wt.%: carbon up to 0.4, preferably 0.15 to 0.3 silicon up to 1.9, preferably 0.11 to 1.5 manganese up to 3.0, preferably 0.8 to 2.5 chromium up to 1.5, preferably 0.1 to 0.9 molybdenum up to 0.9, preferably 0.1 to 0.5 nickel up to 0.9, titanium up to 0.2, preferably 0.02 to 0.1 vanadium up to 0.2, tungsten up to 0.2, aluminum up to 0.2, preferably 0.02 to 0.07 boron up to 0.01, preferably 0.0005 to 0.005 sulfur max. 0.01, preferably max. 0.008 phosphorus max. 0.025, preferably max. 0.01 and the rest, iron and impurities.

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